Key messages:

• Poplar is among the fastest growing of temperate trees and is therefore of considerable interest as a bioenergy crop.
• Poplar can be grown as a Short Rotation Forestry (SRF) or Short Rotation Coppice (SRC).
• Poplar is a multipurpose crop with several environmental benefits such as reducing erosion, phytoremediation of contaminated soils, windbreaks, and increasing biodiversity.
• Poplar can be planted in densities of 1,500 up to 1,800 trees per hectare for bioenergy purposes.
• Yields of 20-25m³ of wood/ha/yr are achievable in the UK.
• Harvesting can be carried out every 2-3 years over a period of more than 20 years.

Introduction

Poplar (Populus spp.) belong to the family Salicaeae and are native to the Northern hemisphere. There are about 30 species present in the genus with many able to naturally hybridise. The species most under investigation for breeding of commercial varieties include P. nigra, P. deltoides, P. maximowiczii and P. trichocarpa. Poplar is among the fastest growing of temperate trees and is therefore of considerable interest as a bioenergy crop.

Mature Poplar Plantation

Poplar can be grown as a Short Rotation Forestry (SRF) or Short Rotation Coppice (SRC). The plant grows to a height of 15–50m with trunk diameter of 2.5m. They can grow around 1.5 to 3m a year depending on the variety and locations. When planted for forestry, poplar has the potential to reach a fully mature height of 30–50 meters. Once established, SRC poplar can be harvested every 2-5 years over a life span of more than 20 years. The plant grows to a height of about 2.5m after 3-4 years.

The leaves are oval to heart shaped, with serated edges. Poplar flowers are dioecious, meaning, male and female flowers are found on separate trees.

The flowers appear early in spring before the leaves emerge, providing pollen and nectar for pollinators.

The tree grows well on marginal land and is able to control erosion. Poplar is a multipurpose crop with several environmental benefits such as reducing erosion, phytoremediation of contaminated soils, windbreaks, and biodiversity.

Poplar Leaves

​Site Suitability

Poplar is suited to most soil types including clay, sand, loam, and humus soils. It grows particularly well on well drained and fertile loam with a wide range pH of 5-8. Poplar generally prefers soils with good drainage and no pooling of water. Certain poplar clones do not tolerate waterlogged soils as the roots require adequate oxygenation. Poplar is not suitable for areas where the water table depth is below 50 to 60cm.

Establishment

Poplar is planted from dormant hardwood cuttings or unrooted stems (~2m rods or 20-25cm cuttings). The cuttings and rods are harvested and placed in cold storage (0-4^oC), to stop them drying out. Planting is done in March and April.

Planting can be done by hand or machinery. The cutting is inserted deeply into the soil leaving about 20cm showing above the soil surface. The plant can grow up to about 1.5m in height during the first year and from 3-5m for each of the following years depending on the cultivar, site conditions, spacing and management practices.

For large-scale plantations, it is recommended to plant mixtures of poplar varieties with about 100 trees of a single cultivar in each block, and different cultivars in neighbouring blocks. If considering mechanised harvesting, plant at a spacing of 0.6m apart in twin rows with 0.75m between rows and a 1.5m alley between each twin-row. Poplar can be planted in densities of 1,500 up to 1,800 trees per hectare for bioenergy purposes. Yields of 20-25m³ of wood/ha/yr are achievable in the UK.

Plant for manual harvesting at stocking densities of ~6,600 stems per hectare (2,671/acre), planting single rows at 0.5m spacing, leaving 3m between rows to allow for better access. In smaller sites with less than 100 plants, plant at a spacing of 5m x 5m.

Timely weed control during the establishment of the poplar is important as poplar is intolerant to shade so does not compete well with weeds during the early stages of growth. Use of geotextile membranes can remove/reduce the need for herbicides.

Management, Pests, and Diseases

Young growth is attractive to deer, hare, and rabbits. Appropriate fencing may be required in areas where these animals are prevalent. Common pests include the small poplar leaf beetle (Phratora vitellinae), and the large leaf beetle (Chrysomela populi). The larvae and mature stages feed on the poplar leaf and can cause shoot dieback. Other known pests of poplar are the white-satin moth (Leucoma salicis) and poplar shoot borer (Gypsonoma aceriana). Insecticides may be required to control insect pests.

Some diseases observed in poplar include bacterial canker (Xanthomonas populi) which causes wilting, necrosis, rot, and injury of poplar trees. Poplar leaf rust (Melampsora larci-populina) is another common disease which causes the formation of yellow spots on the upper surface of the leaves and orange/rust-coloured pustules containing masses of spores on the underside of the leaves. These diseases cause premature death of the leaves. It is recommended to manage diseases by planting different mixtures of poplar clones that are genetically selected for resistance.

Harvesting

Harvesting is done between November and March after leaf fall and before bud swell in early spring. Harvesting can be done every 2-3 years over a period of more than 20 years. Harvesting can be done manually or mechanically. When established commercially, harvesting can be done with commercial scale modified forage harvesters used to cut and chip straight to a trailer.

Smaller-scale harvesting can be performed using a chainsaw or brush cutter. Biomass can be stored and seasoned as whole rods in stacks/bundles or cut down to billets.

Mechanical Harvesting of Poplar

Additional uses and benefits

Poplar has a wide range of uses. These include:

• Feedstock for biofuels.
• Planting as windbreaks, reducing wind speed.
• Prevention of soil erosion.
• Phytoremediation of contaminated soil.
• Improving biodiversity – by providing nesting opportunity for birds and bats.
• High quality wood for use in manufacturing and construction.
• Making cardboards, boxes, crates, paper, veneer and pellets.

Key messages:

• Reed canary grass (RCG) is a perennial, lignocellulosic crop that is native to the UK and has multiple uses.
• It is established from seed, making set-up costs lower than for many other perennial biomass crops.
• RCG is a shorter-term crop (5-10 years) than Miscanthus and willow, with average yields of 4-7 tonnes of dry matter per ha, depending on management.
• It is tolerant of a range of land types and conditions including flooding, drought, and freezing.
• RCG can be grazed in early summer (depending on the variety) and harvested for diverse other uses from late summer until early spring. This extends the annual window of biomass availability and supports multiple end uses (e.g. anaerobic digestion, combustion, animal bedding, seed supply, game cover).

Introduction

Reed canary grass (Phalaris arundinacea L., hereafter RCG) is a multipurpose, lignocellulosic perennial crop native to Europe, Asia, and North America. It can be grown on poorly productive land and tolerates a wide range of conditions. RCG is found in diverse habitats including wetlands and riparian zones but also drier areas like roadsides, forest margins, pastures, and disturbed areas. It is a fast-growing crop reaching heights of 1.5-2 m in a growing season, and, depending on management, can remain productive for 5-10 years.

Reed Canary Grass

Uses and Benefits

• Reed canary grass can be utilized in diverse energy conversion processes like combustion, gasification, pyrolysis, and anaerobic digestion.
• Reed canary grass has a high gross calorific value (dry ~18.2 MJ kg^-1), exceeding that of wheat and Miscanthus. Ash content (~3-8 %) depends particularly on harvest, management, and site.
• Reed canary grass has carbon sequestration potential and can be used for a prolonged ‘fallow’ period in a rotation where soil health can recover from more intensive arable practices and organic matter can be accrued.
• Reed canary grass creates habitat for diverse wildlife including birds and small mammals.
• Reed canary grass can mitigate greenhouse gas emissions and reduce nitrate leaching, acting as a buffer crop.
• Reed canary grass can be integrated into multifunctional landscapes as a biomass crop providing benefits of supporting ecosystem services and mitigating environmental impacts.
• Reed canary grass can be used for livestock feeding provided it is low in alkaloids (eg. Marathon or Palaton variety). It has good absorption properties and is a viable alternative to straw as bedding material for livestock.
• Reed canary grass has potential to remove contaminants from soil and water.

Establishment

Reed canary grass is planted by seed. It requires a firm, smooth seedbed and timely weed control during establishment, with fertilizer where nitrogen is limiting. Seed can be broadcasted or planted using a conventional seed drill. Conventional seed drilling provides more uniform stand development and seed depth. The ideal time to plant RCG is between March and April when soils are moist, and the root system has plenty of time to establish. The recommended seeding rate for forage production is 5-10 kg ha^-1 with row spacing of 12.5 cm, at a depth of about 2 cm. Rolling is recommended both before and after sowing to conserve the moisture in the seedbed followed by application of broadleaf herbicide to control weeds. Growth can be slow during the first year of cultivation while the root and rhizome system establishes. Harvesting can usually commence from year two. Nutrients translocate to the rhizomes over the autumn and winter period, providing energy for new shoots in spring, and improving the sustainability of the crop and quality of the biomass for combustion purposes.

Site Suitability

Reed canary grass is highly tolerant to waterlogging (established stands can tolerate extended periods of inundation) and a wide pH range from pH 4.9 to pH 8.2. It can thrive on marginal land unsuitable for food or feed production. Once established, Reed canary grass is one of the most productive temperate grasses during drought due to its deep rhizome and root system. As a temperate grass used extensively in Scandinavia, it is also highly tolerant of freezing conditions.

Biomass production and harvesting

Average yields of 5-7 tonnes per hectare are achievable in the UK, depending on management, although ‘pushing’ the crop may decrease crop overall persistence. Yields will be improved by fertilization where nutrients are limiting although over fertilizing may promote lodging. Harvesting the crop green will reduce sustainability, as nutrients not yet translocated to the below-ground biomass will be removed.

Reed canary grass under two harvesting regimes (autumn, LHS; spring RHS) on shallow, stony soil in West Wales.

Harvesting can be conducted with conventional grass harvesting machinery and usually commences from the second year after sowing. The timing of harvest depends on the intended market/end-use. Forage quality is best in spring and early summer, while harvest for anaerobic digestion can extend from summer until autumn senescence begins.  For combustion and animal bedding, harvesting is best delayed until late winter or early spring, where combustion quality will be improved through nutrient translocation below ground, leading to lower contents of sodium, potassium, chlorine and moisture (ca. 10-15%). Spring-harvested biomass requires little or no drying. The crop is typically mown first before being baled and transported. Alternatively, the grass can be cut and chipped using a forage harvester. Eliminating the crop after productive years will be easiest using herbicides e.g., glyphosate, although repeated cutting and conventional tillage may suffice.

Harvesting RCG whilst green will remove nutrients, increasing the need for fertilizer, but research has shown costs can be reduced using sewage sludge or nitrogen-fixing legumes.

Pests and Diseases

RCG disease levels are relatively low and are not considered a cause for concern (they include brown rust, mildew, buff spot, powdery mildew, and Rhynchosporium (leaf scald)). However, RCG can be attacked by insect larvae, which feed inside stem bases and can occasionally significantly reduce yield. Damage from rabbits and slugs can occur during the first year of establishment.

Economics

Returns during the first year are low, as establishment costs will not be balanced by revenue from biomass production. A study conducted in the UK found the average gross margin of £153.89 ha^-1 for RCG grown in multiple sites. The gross margins ranged from a minimum of £110.60 ha^-1 to a maximum of £219.40 ha^-1 across the sites studied. The differences in the gross margin were attributed to the variations in yield among different sites and prices of the biomass.

Take home messages:

• Biomass gasification technologies are thermochemical conversion processes enabling the conversion of biomass feedstocks into fuels, chemicals, electricity, and process heat.
• Various biomass feedstocks are currently used in fuelling gasification technologies including clean wood, energy crops, forestry, and agricultural residues.
• Biomass feedstocks are more suitable feedstock to fuel gasification technologies for achieving significant impact on the transition to net zero emission.
• Gasification technologies should be accompanied by carbon capture storage systems to reduce carbon dioxide emissions.

Introduction

Biomass gasification technologies are thermochemical conversion processes enabling the conversion of biomass into fuels, chemicals, electricity, and process heat. These conversion methods have some benefits over direct combustion by significantly reducing emissions of pollutants into the atmosphere and generating high heat efficiency. Biomass gasification technologies are projected to play an important role in meeting the UK’s net zero emission targets especially when targeted towards use in sectors (heat, industrial, and transport) where options for decarbonisation are difficult or expensive. This article provides an overview of biomass gasification technologies and biomass feedstock suitability for fuelling gasification technologies. The department for energy security and net zero report titled “Advanced gasification technologies: review and benchmarking” provides a comprehensive overview of biomass gasification technologies for generation of energy products.

Biomass gasification technology

Gasification is the thermal conversion of any carbon containing materials at elevated temperatures (650-1200^oC) in the presence of gasification agents (air, oxygen, carbon dioxides, or a combination of these). The resulting product is synthesis gas “syngas” which is suitable for producing heat, power generation, industrial applications, and liquid fuels production. Syngas consists primarily of carbon dioxide (CO₂), carbon monoxide (CO), hydrogen (H₂), methane (CH₄), and other contaminants including ash, char, tar, gaseous metals, sulphur compounds, and chlorine traces. The type of feedstock composition, gasification process, and gasification agent determines the main component and contaminants in the syngas. Contaminants in syngas are treated and removed to ensure high-quality syngas for use in fuel and power generation.

Gasification is considered to be the most efficient biomass to fuel conversion method. Biomass gasification aims at producing gas, whiles maximizing hydrogen concentration and minimizing the tar content and other contaminants. Air gasification produces syngas with a higher heating value of up to 7 MJ/nm³ which can be burned in boilers, turbines and in gas engines after treatment. Gasification by use of oxygen produces a syngas with gross calorific value (GCV) of up to 12 MJ/nm³ suitable for producing power or heat or converted into long-chain hydrocarbons. The use of steam in gasification yields higher gross calorific value of 15-20 MJ/nm³.  Syngas produced from steam or steam-oxygen gasification is preferred for fuels production due to the high concentration of hydrocarbons.

The biomass gasification process (figure 1) typically consists of key component systems involving feedstock input and preparation, and combustion at high temperatures in gasification reactors to produce syngas. The syngas produced is cleaned to remove tar and other contaminants to produce syngas of acceptable quality for use in syngas upgrading systems. These systems involve water-gas shift reactors which adjust the hydrogen to carbon ratio to produce low carbon hydrogen and hydrocarbon products.

Figure 1: Biomass gasification process (Adapted from DESNZ report – Task 2, 2021)

Decarbonisation potential of biomass gasification technology

Carbon dioxide is produced during the process of conversion of biomass feedstock using gasification technology to biofuels and other products. Carbon dioxide is produced during the stages of gasification, syngas cleaning, syngas reforming and product upgrading (figure 1). As such, gasification technology should be accompanied by carbon capture storage systems to reduce carbon dioxide emissions. To decarbonise, the gasification production process will require integrating carbon capture storage system for cost effective carbon dioxide reduction.  Also, by using renewable sources or carbon in the biomass feedstock to generate power to operate the processing systems. Carbon capture storage integrated gasification combined cycle have the advantages of reducing anthropogenic emissions as compared to combustion in boilers, reduces energy losses during separation and capture of carbon dioxide from syngas, and production of valuable by-products such as sulphur, nitrogen, and carbon dioxide. However, the integration of carbon capture technology in the gasification process involves considerable investment.

Biomass feedstock for fuelling gasification technologies

Various biomass feedstocks are currently used in fuelling gasification technologies including clean wood, energy crops, forestry, and agricultural residues. Bioenergy crops such as short rotation forestry, short rotation coppice, and perennial grasses, are carbon neutral and produce high amounts of biomass which is regularly harvested over a short period and therefore can provide a reliable supply of feedstock for fuelling AGTs. Bioenergy crops have low ash content but can contain high amounts of contaminants such as chlorine, sodium and potassium depending on the feedstock. The syngas produced needs cleaning to reduce the contaminants to improve the quality of the product. Energy crops as feedstock for fuelling gasification plants are however relatively expensive but can be available in large quantities if growers have confidence in price and demand from the market.

Clean wood can be sourced in large quantities in the form of wood pellets, chips, and logs, and is easily burnt generating good energy from combustion. Wood is however an expensive feedstock. High quality wood should be sourced for fuelling gasification technologies for high efficiency in production. Wood has the advantage of low ash content which reduces the amount of residual deposit during processing, thus reducing the cost of ash disposal.

Waste wood or recycled wood is cheaper than clean wood and can be sourced from municipal waste, and household waste recycling centres and construction and demolition waste. Waste wood can contain increased amounts of contaminants such as heavy metals, nitrogen, and chlorine, which increases technical risks of slagging, corrosion and fouling during processing. Boilers should be designed to match the waste wood quality for efficient operation.

Agricultural residues mainly used for energy production include straw, poultry litter, meat, and bone meal. Agricultural residues may contain high amounts of contaminants such as chlorine, sodium and potassium which contain variable amounts of ash. Moisture content can be high depending on the quality of the material. Straw contains significant amounts of chlorine, sodium, and potassium. Poultry litter for instance can be very wet containing high amounts of urine.

Table 1: Biomass feedstock suitability for gasification. Source: DESNZ report – Task 2 (2021)

Feedstock properties to consider for efficiency of the gasification process

When choosing the type of biomass feedstock, certain conditions need to be met to maximize the efficiency of the gasification process. Table 2 presents some feedstock properties to consider for design purpose.  

Moisture content affects the quality of the fuel as more energy is required to evaporate the moisture in the feedstock.  As a first step in the gasification process, the moisture content should be reduced to acceptable levels. The moisture content affects the hydrogen level in the producer gas which enhances gasification. Another component of the feedstock to consider is the heating value. This is the maximum amount of energy potentially derived from the feedstock. The higher the heating value, the more rapid the gasification.

A low ash content is favourable for gasification. Ash content can form slag, reducing efficient operation and increasing operational cost and the total cost of biomass feedstock conversion. Also, consider the cellulose to lignin ratio. Preferably, select feedstock with higher content of cellulose and lower content of lignin. Cellulose decomposes at a lower temperature than lignin does. Biomass with higher cellulose content produces higher quality syngas product which makes energy crops suitable feedstock.

Furthermore, consider the proportion of bound carbon and volatiles. High content of volatile and carbon combined with a low content of oxygen is suitable. Lower content of alkali metals (such as (Na, K, Mg, P, and Ca) is recommended to avoid formation of slag.

Table 2: Typical feedstock properties for design basis. Source: DESNZ report – Task 2 (2021)

Gasification technology Plants

Most gasification projects have aimed to produce electricity, and to a limited extent produce fuels and chemical products using mainly feedstock from biomass, refused derived fuel, plastic waste, municipal solid waste, and solid recovered fuel (Table 3). Gasification technology using biomass as feedstock are at various stages of development or early stages of deployment. Biomass gasification technologies have the potential to produce large volumes of fuel and electricity.

Source: DESNZ report – Task 2 (2021)

The future role of gasification technologies using biomass in the UK’s energy system should consider sustainable supply of biomass feedstock. Also, the integration of carbon capture technology would give gasification technologies the potential to operate with a net negative release of carbon dioxide which is an added value over existing combustion technologies.

Further information:

Department for Business, Energy and Industrial Strategy (2021). Advanced gasification technologies: review and benchmarking. BEIS Research Paper Number 2021/038

Related Biomass Connect Articles:

Bioenergy with Carbon Capture and Storage (BECCS)

Take home messages:

• Miscanthus is regarded as one of the most promising energy crops as it can provide a number of environmental benefits in addition to biomass and can enhance local biodiversity
• Miscanthus plantations are especially useful in nitrate vulnerable zones, the limited requirement for nitrogen makes it a low input crop with low potential for nutrient pollution
• Miscanthus can provide substantially improved habitat for many forms of native wildlife, due to the low intensity of the agricultural management system and the untreated headlands

Introduction

The use of plant biomass for production of heat, power and liquid transport fuels is gaining momentum on a global scale. This is because the energy from green plants has much to offer being renewable and largely carbon neutral compared to combustion of fossil fuels.  The UK is one of the countries dedicated to developing a low carbon economy.  It was the first government in the world to make a legal commitment to reduce CO₂ concentrations. However cultivation of crops, be it for food or biomass, creates disturbances in nature by fostering the growth of species desired by man and stressing unwanted species through soil tillage and pesticide application. Secondly there is impact on ecosystems and associated functions particularly land use.

Extensive cultivation of bioenergy crops will have implications on the landscape, and the social and environmental effects of this shift are not yet well understood. In fact, there is a great deal of debate regarding how the UK’s land resources could support a significant expansion in specialised bioenergy crops given the present and, more importantly, the future demands for food production in the face of climate change.

Few studies have examined the food-agriculture-environment trilemma and have suggested that dedicated biomass crops must have a high yield potential and an additional environmental benefit if they are to be used as feedstock for the production of biofuels. Additionally, biomass crops ought to be produced on marginal agricultural land where climatic, geographic, geological, or economic conditions make it difficult to produce food.

Miscanthus giganteus

The perennial C4 grasses of the genus Miscanthus attracted a lot of attention as a possible biomass crop in Europe during the 1990s and is now regarded as one of the most promising biomass crops for the establishment of a socially and environmentally sound bioeconomy. Studies suggests that the UK could offset 2-13 Mt of oil equivalent per year by planting Miscanthus. The species from this genus are among the C4 plants that are most tolerant to extreme cold and retain strong CO₂ assimilation at temperatures below 15 ^oC. It is a rhizomatous grass that, with the right growth conditions and cultivation methods, has a high biomass output potential of up to 40 tonnes dry matter (DM) per hectare per year in Europe. However, the production potential is estimated by UK producers to be lower for the UK at around 15 tonnes per hectare. Despite being a strong biomass candidate the impact of cultivation of Miscanthus in light of the concerns raised above must be evaluated. This technical article summarises the studies on the impact of cultivation of Miscanthus on biodiversity and the environment.

Environmental impacts of Miscanthus

Soil Carbon

Compared to vegetation or the atmosphere, the amount of carbon stored in the world’s soils is more than twice as great. It is crucial to comprehend how large-scale agricultural land use change may affect these storage reservoirs. Any type of soil disturbance, including cultivation and ploughing, is likely to cause temporary losses of soil organic carbon that are broken down by increased soil microbial activity. This recurrent annual disturbance under arable agriculture lowers the amount of soil organic carbon (SOC). The occasional disturbance losses in perennial agricultural systems, like grasslands, have time to be replaced, which can lead to increased steady-state soil carbon levels.

On the impact of Miscanthus plants on SOC sequestration, published scientific investigations have produced mixed results. This variation is brought on by the fact that carbon sequestration is sensitive to a variety of variables, such as climate, yearly precipitation, soil texture, initial soil carbon content, and the depth of soil sampling. Despite these differences, there is broad agreement that converting arable land to Miscanthus will boost carbon sequestration, whereas converting grassland may not be as advantageous. Additionally, it is crucial to remember that, in every situation, soil carbon concentrations will not keep rising indefinitely. Instead, a new, higher carbon equilibrium will ultimately be reached, a level which will be dependent on a range of site-specific characteristics.

Soil Condition

Increases in SOC associated with Miscanthus plantations also lead to other enhancements in soil quality, including enhanced soil texture, water retention, and fertility as a result of reduced tillage, as well as increases in litter inputs and soil organic matter (SOM). For instance, a study of four Miscanthus plantations in Germany found that over the course of four years, the SOM storage in the topsoil increased by 11.7 t per hectare. The soil also had more soil aggregates, better hydrophobicity (water repelling) and physical properties. Along with improving soil texture, the extensive root systems of Miscanthus result in significant below-ground biomass storage, which enhances the ability of these plantations to mitigate climate change. Estimated levels of carbon storage per hectare range between 5 and 12 t C over the crop’s 25-year life cycle.

Soil Nitrogen

There is a lot of interest in how Miscanthus plantations may affect nitrogen (N) cycling, leaching, and related changes in water quality because of their long growth seasons, high evapotranspiration rates, and vast root systems. One study reported that after establishment, Miscanthus can result in less nitrate leaching than arable crops, albeit this effect may depend on the N status of the soil (grassland or arable, microbial activity, N mineralisation). Even considering these factors, the establishment of energy grasses would be able to reduce N leaching by 50% according to a Swedish study, with additional benefits arising if these plantations are used as buffer strips alongside watercourses. This suggests that in the UK, these crops may have a similar impact.

Another study examined the leaching over the first three years of M. giganteus production on 38 fields with a variety of soils. In comparison to other crops, minimal to no leaching at all sites was recorded on Miscanthus plantations. The biggest nitrate losses occurred in the first winter following planting, on sites with sandy, shallow soils, and on sites where establishment failed, indicating that better management and site selection could further reduce these losses.

Phytoremediation

Phytoremediation strategies with the use of grasses

Phytoremediation is the approach of using plants for the in-situ treatment of soils or waters polluted with different inorganic chemicals or heavy metals. Phytoremediation using energy crop is a fast-growing field that offers significant environmental benefits. Miscanthus has a high water efficiency. Miscanthus uses a lot of water to generate big crops of standing biomass when sunshine availability is high and temperatures are suitable. It is therefore a helpful phytoremediation tool. A common use would be to absorb leachate from a landfill’s down gradient or as a cover crop on a closed landfill. Miscanthus has good potential for growth at marginal sites that are not heavily polluted as well as contaminated ones.

An important consideration in case of energy crops is safe management of ash or volatile elements such as mercury or selenium after combustion. But as aerial biomass growth only takes up a little amount of contaminants the biomass can be used to provide energy. In certain instances, reported biomass growth increased when contamination was present. While being produced for its energy value, Miscanthus has the ability to stabilise and possibly eliminate metal pollutants gradually over time. Water use and surface stabilisation by Miscanthus helps prevent metal carried away from the site by wind, soil erosion, or water flow.

Green House Gas Balance

Emissions of greenhouse gases (GHG), especially methane and nitrous oxide from the agriculture or other sectors have a high global warming potential. First-generation energy crops like maize necessitate intensive N fertilisation, and maize farming may be detrimental to the humus equilibrium in the soil. The total greenhouse gas balance may even be negative as a result of the nitrous oxide emission brought on by the use of N fertilisers, meaning that net GHG emissions from first generation biofuels may be higher than those brought on by the use of fossil fuels. Since perennial energy crops produce large amounts of biomass while using few resources, particularly N fertilisation, they are considered to be competitive alternatives.

Methane emissions from Miscanthus plantations are negligible. In fact, because of the significant soil-C sequestration, it really serves as a weak methane sink. Since it is a rhizomatous grass, the N-fixing bacteria in its rhizomes provide the required N. As a result, it produces good yields even in fields that are either unfertilized or only mildly N-fertilized while emitting little nitrous oxide. In fact, a US study found that using N fertilisers increased nitrous oxide emissions, inorganic N fluxes through the soil (mainly nitrate), and harvested N without significantly increasing biomass production. Moreover, as it is a perennial grass CO₂ emissions from soil disturbances due to agricultural practices are also negligible.

Impact on biodiversity

Biofuel production drives land-use change, a major cause of biodiversity loss, but there is limited knowledge of how different biofuel crops affect local biodiversity. The Royal Commission on Environmental Pollution stated “the impact of energy crops on biodiversity has not been a topic of significant research in the UK, partly due to the absence of large-scale plantations”. In general, the crop architecture of perennial grasses offers additional cover for wildlife, especially during the winter when other crops and swards are at their shortest, and non-crop vegetation may offer some birds opportunities for feeding and breeding.

Published studies on the impact of Miscanthus on biodiversity, have compared it to existing land use or other energy or annual crops, and both positive and negative impacts have been reported. On the positive impacts, even though Miscanthus does not produce nectar or fruit for wildlife, when integrated into the landscape, its fast-growing three meter tall canes provide continuous habitat cover for woodland and farmland birds, and invertebrate populations in different seasons. Also, the low input management in Miscanthus improve headland and field boundary quality for wildlife.  A study in Herefordshire, UK investigated ground flora, small mammal and bird species diversity in young Miscanthus plantations and observed considerably more open-ground bird species such as skylarks (Alauda arvensis), lapwings (Vanellus vanellus) and meadow pipits (Anthus pratensis) within Miscanthus than within reed canary-grass fields. In addition, richer diversity in weed vegetation was observed on Miscanthus fields compared to reed canary-grass or wheat. Likewise, a study in Cambridgeshire reported greater abundance and diversity of farmland birds in Miscanthus fields during the establishment phase of the crop. It was observed that most of the frequently occurring species in winter were woodland birds, whereas in summer farmland birds were more numerous. 

Despite these studies having indicated benefits of Miscanthus on biodiversity, it is important to note that these impacts were predominantly occurring in young Miscanthus plantations. These advantages diminish as the crop matures, develops denser and closer canopies, and also depends on size, and crop management. Many of these benefits will be lost if Miscanthus is managed solely to maximize crop yields. Other studies have noted negative impacts of Miscanthus in matured plantations. A study investigated nesting and foraging opportunities for Lapwings (Vanellus vanellus – a priority conservation species in the UK) in and around Miscanthus and Short Rotation Coppice Willow and found that despite Miscanthus providing nesting habitat for Lapwings, there was reduced hatching success compared to arable crops. Also, birds nesting in matured Miscanthus may be more vulnerable and suffer higher predation rates than those in arable crops. This suggest that nesting and foraging opportunities for birds such as skylarks and lapwings may be curtailed in Miscanthus plantations.

Some studies have observed increased earthworm species diversity due to lower ground disturbance and low input use in Miscanthus plantations, although earthworms feeding on Miscanthus leaf litter have showed reduced biomass due to the poor quality of Miscanthus leaf litter. Miscanthus leaf litter has low nitrogen and high carbon which makes it a poor food resource for earthworm feeding.

In an agricultural landscape the majority of wildlife and their food sources are found in interspersed woodland and field margins. Miscanthus can play an important role in this landscape by preventing chemical leaching into these important habitats, removing annual ground disturbance and soil erosion, improving water quality, and providing heterogeneous structure and overwinter cover. Integration of Miscanthus into landscapes should be considered in terms of its potential to increase or decrease biodiversity in land-use change.

Summary

Bioenergy crops are set to increase in the UK and the wider landscape. To develop a sustainable biomass market the impacts of biomass crops on environment and biodiversity must be considered. Any agricultural production is primarily based on human demand and there will always be a trade-off between nature and humans or one benefit and another. Miscanthus is a promising energy crop can provide a range of benefits while minimising environmental harm.

Miscanthus plantation increases soil organic carbon sequestration which in turn improves, soil texture, water retention and fertility. It can be used to recover marginal or contaminated land through phytoremediation. Being a rhizomatous grass, it needs little to no N fertiliser, which means very little N leaching or nitrous oxide emissions from Miscanthus fields. It serves as a weak methane sink thus helping to mitigate global warming potential of this gas. Miscanthus plantations are generally associated with greater biodiversity of invertebrates, insects, birds and small mammals although this effect may decline as the crop matures and its canopy closes.

Take home messages:

• The food versus fuel debate calls into question the ethics of diverting land from food to biofuel production.
• Bioenergy production from first generation (edible) biofuel feedstock may compete with food production and as consequence affect food security.
• A shift towards increasing production of second generation (non-edible) biofuel feedstocks and third generation biofuels along with advanced technologies of genetically engineered algae biomass would avoid the competition for food.
• Food and bioenergy need not compete for land. Biofuel crops should be integrated into existing landscape and agricultural lands in a multifunctional approach to improve use as food, fuel and for other ecosystem services.

Introduction

Biofuels have the potential to reduce reliance on fossil fuels whiles reducing environmental impacts and greenhouse gas emissions. Biofuel makes a small contribution to our energy demand whiles fossil fuels contributing approximately 87 percent of energy having a negative impact of increase in global warming. To limit global warming, biofuel production should be increased from 9.7 × 106 GJ d⁻¹ to 4.6 × 107 GJ d⁻¹ between 2016 and 2040. This would require large scale deployment of biofuel feedstock production and advanced technologies in biofuel production. However, there are trade-offs related to the expansion of bioenergy feedstock production by diverting land use from food to biofuel production, which poses a threat to food security especially in developing countries, and directly or indirectly leading to deforestation and other changes of land use that have a negative effect on greenhouse gas emissions. This calls into question the ethics of diverting land from food to energy and whether it implies less food production. In this article, we provide a review of the food versus fuel debate, whether a shift to other alternative biofuel feedstocks would avoid this competition for food.

Food vs Fuel

Food versus fuel debate

The food versus fuel debate calls into question the ethics of diverting land from food to energy production. The opinion piece written by the Guardian columnist, George Monbiot in 2004, brought attention to the issue of devoting land to fuel production in his article entitled ‘Feeding cars, not people’. He argued that the expansion of biofuel production is at the expense of food which is much needed to feed a growing population especially the food insecure in developing countries. A wide range of perspectives on the debate of food versus fuel further emerged following increases in world crop prices for cereals and vegetable oils which are raw materials for producing biofuels. Biofuels are produced from the fermentation of biological feedstocks containing fermentable sugars, lipids, or carbohydrates. Biofuels are categorized into first, second, third and fourth generation biomass feedstocks (see figure 1).

Figure 1: Biofuel feedstocks conversion to biofuels

What are the implications of use of first generation feedstocks for biofuels?

First generation biofuel feedstocks have been at the centre of the food versus fuel debate. First generation biofuels are produced from easily accessible and edible fractions of food crops. They produce large amounts of lipids or carbohydrates that are easily converted into biofuels. Bioethanol is produced from sugar crops (e.g., sugarcane, sugar beet and sweet sorghum) and starch crops (e.g., wheat, maize, and cassava), while oil seed crops such as rapeseed, sunflower, oil palm, and soybean may be used for production of biodiesel. These edible feedstocks have been mainly used for producing biofuels globally and have been highly commercialised, contributing over 50 billion litres of the total fuels produced per year. However, there are trade-offs with producing such edible feedstocks for biofuel production.

First, increasing production of first generation biofuel feedstock impacts food security, due to competition for direct land use. Thus, any dedicated use of productive land for production of biofuel feedstock inherently comes at a cost of not using the land for food or feed production. This claim is however based on the assumption that land is a limiting factor for global food production and to meet the rising demand for food and fuel will require newly cleared land leading to deforestation and increases in greenhouse gas (GHG) emissions. On the other hand, others have argued that global land is not a limiting factor when land cover, land use, and productive potential are considered.

Another facet to the debate is that the demand for biofuels has an impact on food prices, which disproportionately affect poor people in the global south who spend roughly half of their household incomes on food. The increased demand for production of biofuels from food grains and oilseeds in the U.S. and the EU contributed to this increase in food price, much of which was due to policies that incentivised production of biofuels. On the other hand, some studies have shown that biofuel prices do not seem to affect food commodity prices and have not been the most important contributing factor for food price inflation and that, different biofuels have different impacts on prices of food commodities. Such studies have found no strong evidence that biofuels such as ethanol would drive food commodity price.

Furthermore, there is evidence that these biomass feedstocks may not have a direct impact on food production since they are grown for different purposes. For instance, from the data on global use of major food commodities (see figure 2), the volume of use of maize, a major biofuel feedstock is mostly used for animal feed. The OECD-FAO projections on global use of maize is largely more towards animal feed production. Thus, the type of biomass production may not have a direct impact on food production since they are grown and used for different purposes. Hence, there is the need to examine the interdependencies between the multiple end-uses of biofuel feedstocks.

Figure 2: Global use of major commodities

Lastly, the studies have shown that first generation biofuels are poor crops for biodiversity and compete for water resources. For example, production of maize presents the risk of soil erosion, soil damage and runoff water which affects soil health. Limits should be set on bioenergy support schemes to regulate or control the amount of food and feed crops that can be used for production of biofuels.

Would second generation biomass crops avoid the competition for food?

In view of the negative trade-offs of first generation biofuel crops, second generation biofuel feedstocks which are derived from non-edible biomass have been proposed as more suitable for production of biofuels, as these feedstocks do not compete with food production. Second generation biofuels are based on lignocellulosic feedstocks including perennial energy crops (e.g., Miscanthus, short rotation coppice willow, poplar), Short rotation forestry (e.g., eucalyptus, black locust, paulownia), agricultural residues (e.g., forest thinning, sawdust, sugarcane bagasse, rice husk, rice bran, corn stover, wheat straw, and wheat bran), and forestry residues (e.g., small branches and bark from forest thinning operations, conservation management operations, wood pellets, or wastes from wood processing industries such as sawdust).

Compared to first generation biofuel feedstock, these non-edible feedstocks do not compete for food, can grow on marginal land, do not require much input such as fertilizers, and provide additional ecosystem benefits such as increasing biodiversity and soil carbon and reducing greenhouse gas emissions. They produce high amounts of biomass and can be harvested regularly over a long period and be processed for use as biofuels. These biomass crops have a potential to play a key role in decarbonisation of economies, both as a substitute to fossil fuel energy reducing greenhouse gas emissions, as well as to generate negative emissions using carbon capture and storage (BECCS).

However, the competition for land and food may still persist if productive land for food production is used for growing second generation biomass feedstocks. Planting second generation biomass feedstocks on marginal land avoids the competition for land use and food. Also, bioenergy crops should be integrated into existing landscape and agricultural land in a multifunctional approach to offer multiple benefits including use as fuel and other ecosystem services.

Shifting to third generation bioenergy feedstock and more advanced technologies?

Third and fourth generation biofuels are more promising as they do not create such land use and food competition. Third generation biofuels are non-food marine biomass derived from algae including, seaweeds (green, red, and brown macroalgal species) and microalgae.  These algae biomass have higher growth rates and do not require an arable or big land area for production and hence do not compete with land for food and also act as carbon sink.

With advanced technologies of genetic modification of microalgae, fourth generation biofuels are seen as more promising sustainable biofuel feedstock due to their high biofuel productivity and ability to capture large amounts of carbon dioxide. Research is still advancing in the use of genetically modified algae. As these technologies become more advanced, efficient, and economical, they could replace uses of first generation biofuels and fossil fuels for sustainable energy production.

Sustainable biofuel production in the UK

In the UK, bioenergy plays an important role in decarbonising the energy system by reducing greenhouse gas (GHG) emissions, expanding renewable energy and moving towards a low-carbon economy. Domestic biofuel crop production however constitutes a small component of existing agricultural land. Less than 1.4 percent of 18.6 million ha of agricultural land in the UK is currently used for bioenergy crops. In 2020, 121,000 ha of agricultural land were used to grow bioenergy crops. Of this, 92 percent constituted edible food crops for biofuels (diesel and bioethanol) and biogas, and about 8 percent constituted non-food biofuels used for electricity and heat generation. Future projections to reach net zero ambitions estimates planting about 30,000 ha yearly through to 2035 and estimated cultivation of 70,000 ha of bioenergy crops by 2050. This will certainly require large scale expansion of land for biofuel feedstock production.

A shift towards increasing production of second generation (non-edible) biofuel feedstocks and third generation biofuels along with advanced technologies of genetically engineered algae biomass would avoid the competition for food. The second generation biofuel crops may not necessarily compete directly with food production if grown on marginal lands including under-used, inaccessible, inconvenient, land prone to flooding, or degraded land.  It is estimated that 1.4 million hectares of marginal land in the UK could potentially be planted with perennial energy crops by 2050 avoiding competition for arable land for food production. Planting should also be prioritised on low quality agricultural lands (Grade 5‐4‐3b), public access land, contaminated land and land identified by the Forestry Commission as low risk for woodland creation. However, planting should be avoided on UK Biodiversity Action Plan (BAP) priority habitats, land with cultural value, peatlands and other high carbon soils >120 t C ha^‐1.

Food and bioenergy need not compete for land and, instead, should be integrated to improve resource management. Bioenergy crops should be incorporated into existing landscape and agricultural land in a multifunctional approach to deliver multiple outputs from the land including food, biodiversity, climate change, waste absorption, and other societal purposes.

Conclusion

The dilemma of diverting land from food to energy production has made bioenergy crops controversial with wide range of perspectives. Biofuels produced from using edible biomass stocks have been at the centre of the food versus fuel debate, with serious criticism that it may lead to competition with food, increase food prices, deforestation and other changes of land use that have a negative effect on greenhouse gas emissions. A shift towards increasing production of second generation (non-edible) biofuel feedstocks and third generation biofuels along with advanced technologies of genetically engineered algae biomass would avoid the competition for food. Planting second generation biomass feedstocks on marginal land avoids the competition for food production. Food and bioenergy need not compete for land, instead, biofuel crops should be integrated into existing landscape and agricultural lands in a multifunctional approach to offer multiple benefits including use as fuel and for other ecosystem services.

Take home messages:

• Industrial hemp has greater versatility and profitability than many other biomass crops like giant miscanthus, willow, poplar and switch grass.
• It yields high biomass (12- 15 t/ha of air-dried biomass) with minimum input of water, fertilizers and pesticides.
• The best applications for industrial hemp include co-production of hemp products like insulation material for buildings, hemp paper, textiles etc with biofuels.
• It is an excellent rotation crop that fits well with food and feed crops and improves the yield of the subsequent crops due to the beneficial effects of hemp on the soil like improving soil structure and reducing parasitic nematodes and fungi.
• The UK lags behind its European counterparts in hemp production due to the regulatory forces and unless changes to these regulations are made, farmers may be less attracted to the produce hemp.

Introduction

The conversion of biomass to biofuels has gained heightened interest due to the increasing demand for sustainable and eco-friendly sources of energy. Traditionally, biofuels have been produced from starchy or sugar crops such as corn, wheat, sugar beets and sugar cane. However, this gives rise to competition between food versus energy. Therefore, there is a need for dedicated non-food energy crops, especially those where the whole plant biomass can be used for energy production resulting in potentially higher land use efficiency. Moreover, if such crops could be fitted into a crop rotation system with food and feed crops land use efficiency can be further increased. Energy crops are projected to play a significant role in the future of bioenergy due to the paucity of biomass from forestry output and the scarcity of acceptable “waste” streams.

Hemp Plant

Industrial hemp (Cannabis sativa L.) is one of the oldest crops in the world traditionally grown for its fibre. Hemp fibre was the main fibre for maritime ropes and sails and was used extensively as a raw material for cordage and textiles. However, its production declined in the middle of the 19^th century due to the disappearance of the sailing navy, competition from natural fibres like cotton and jute and later due to intensive development of synthetic fibres.

In the 1930s cultivation was forbidden in most Western countries and in North America due to its genetic closeness to marijuana which created a lot of confusion and social, political and moral controversies. The 1990s marked renewal of hemp cultivation for agricultural, industrial and scientific reasons throughout the world mainly due to increasing consideration of natural resources, energy conservation and biomass conversion to bioproducts and biofuels.

Since 1992, France, the Netherlands, England, Spain and Germany have passed legislation allowing for the commercial cultivation of industrial hemp. Two years later, Canada  followed in the footsteps of the EU passing regulations that allowed farming of hemp. The area dedicated to hemp cultivation in the EU (fig 2) has increased significantly from 19,970 hectares (ha) in 2015 to 34,960 ha in 2019 (a 75% increase). In the same period, the production of hemp escalated 62.4% from 94,120 tonnes to 152,820 tonnes. France is the leading producer in Europe, accounting for more than 70% of EU production, followed by the Netherlands (10%) and Austria (4%).

Fig 2. EU land used for hemp Fig 2. EU land used for hemp cultivation from 2015- 2021cultivation from 2015- 2021

Recently there has been renewed interest in hemp as an insulating material as well as a feedstock for specialist paper. It is also an excellent rotational crop. It uses a fraction of the water needed to grow cotton, absorbs more carbon dioxide per hectare than other crops and most trees and its every part is beneficial. Its extensive root system improves soil structure and subsequent crops have less weed pressure. Studies in winter wheat crops grown after hemp have reported yield increases of 10-20%. This may be attributed to decreased parasitic nematodes and pathogenic fungi and reduction in soil water loss and soil erosion due to hemp growth.

Fig 3. Hemp fibres used in building insulation

Hemp is also highly efficient in ecological reconstruction and land reclamation owing to its phytoremediation capacity i.e its ability to remove heavy metals from the ground. Overtime it can remove heavy metals and other contaminating substances from the deeper layers of the soil as its root system develops. Finally, biochar produced from hemp for soil applications could potentially improve soil carbon sequestration and reduce greenhouse gas emissions.

Fig 4. Hemp stalks (left) used for production of hemp fibre (right)

Important factors in growing hemp for Energy

Soil and Climate

Hemp cultivars adapt well to a temperate climate therefore they can be farmed in most countries in Europe including UK. During the growing season, they require 200–300 mm of rainfall. The amount of rain that falls throughout the summer is less crucial for development than enough soil moisture. A strong tap root system effectively lessens the impact of a brief drought. As a result, seed germination can take up to 8–12 days at a soil temperature of 8–10^oC. Young plants may withstand brief frosts of up to -6^oC degrees. The duration of the day has a significant impact on how hemp develops. Hence, the recommended time for planting hemp for biomass is at the end of April.

A well-established soil that is nutrient- and organic-rich is necessary for hemp. The soil should have a pH that is neutral or slightly alkaline. A robust root system ensures that plants can absorb water and nutrients from the soil’s deeper layers. In crop rotations with a large proportion of cereals, hemp is a desirable preceding crop because it enhances the aggregative structure of the soil.

Tillage and Cultivar Selection

Hemp requires winter ploughing and spring tillage to prevent soil dryness. The amount of nutrients in the soil and the previous crop should be taken into consideration when deciding how much mineral fertiliser to apply before planting. The amounts of specific fertilisers are as follows: approximately 80 kg/ha of N, 50 kg/ha of P₂O₅, and 90 kg/ha of K₂O. In general, hemp doesn’t need chemical pesticides or weed killers to control weeds, diseases, or pests because it has one of the intensive early growth rates of any agricultural crop. Hemp should be planted at a density of 40–50 kg/ha with a row spacing of 20–30 cm.

There are 75 varieties listed in the EU common catalogue of varieties of agricultural plant species. The main factors that influence the choice of the hemp variety include the suitability for growth in the region and the end product for which it is grown. Most cultivars of hemp are capable of yielding 12–15 t of air-dried mass per hectare genetic and breeding work is carried out to obtain new lines yielding over 20 t/ha of biomass with a high fibre and cellulose content and trace amounts of THC.

What makes hemp a potential bioenergy crop?

The demand for biomass for conversion to energy is rapidly increasing and is expected to accelerate in the coming years. Industrial hemp is valuable due to its high biomass and energy yield per hectare. Several studies have claimed that hemp could be used in energy production as a fuel source with no sulphur emissions either by direct combustion or converted into liquid fuels such as bioethanol. Hemp oil, which was traditionally used as lamp oil, can be used to produce renewable biofuels which emit less carbon monoxide on combustion to replace gasoline for diesel engines thus helping in relieving global warming.

The whole plant can be used to make a variety of products in a biorefinery, including feedstock and biochemicals like succinic acid, heat from briquettes or pellets, electricity from baled biomass and vehicle fuels like biogas from anaerobic digestion or bioethanol from fermentation. In terms of biofuels, hemp can provide, biodiesel made from the pressed seed and bioethanol and methanol from the fermented stalk, all of which are clean renewable alternatives to petroleum-based fuels. The possible pathways for bioenergy production from hemp biomass are illustrated in figure 5.

Fig 5. Bioenergy pathways of hemp biomass conversion

Hemp is a rapidly growing plant that can withstand high planting density and its total biomass per hectare is similar to other energy crops like the giant miscanthus, poplar, switchgrass and willow. However, the significant advantage of hemp over other energy crops is that the concentration of digestible cellulose and hemicellulose is higher in hemp bast fibres. They contain 73-77% cellulose, 7-9% hemicelluloses, 2-6% lignin compared to 48%, 21-25% and 17-19% respectively in the hurd. On the other hand the cellulose and hemicellulose of hemp hurd are comparable to that in the stems of giant miscanthus, poplar, switch grass and willow (table 1). Notably, 20–30% of the stem biomass in hemp consists of high cellulose fibre, resulting in a higher ratio of digestible sugars to lignin in hemp than in other similar-yielding biofuel crops. These characteristics make hemp an above-average energy crop for some biochemical-based biofuel production and greenhouse gas mitigation applications.

Table 1. Fibre yield and composition of hemp compared to other proposed biomass crops.

Fuel Quality of Hemp

The most important characteristics when evaluating any fuel are the calorific value, ash content, melting behaviour, physical handling characteristics, and any negative effects on heat exchanger corrosion or stack emissions. The following section assess the fuel quality of hemp. Comparison to other fuel crops is made where data is available.

Calorific value

This is the most important property of any fuel. Biomass from both the whole hemp plant as well the by-products (shives and straw) can be a good raw material for energy production. A study at the Teagasc biofuel laboratory reported that, provided hemp can be dried in the field to a moisture of 20%, its net calorific value is higher than many other forms of biomass and much higher compared to that of peat (table 2). Thus, hemp can be an excellent alternative to plants operating on peat fired energy. However, differences in gross caloric value are small.

Table 2. Gross and net caloric values of some biomass fuels.

*heat available with no further drying and no recovery of latent heat

Another study reported the gross calorific values of hemp whole plant and by products compared to other biomass and found them to be comparable or better (fig 6). This study also reported hemp to be resilient to conditions of draught and the energy efficiency per hectare of hemp exposed to water shortage was 10% higher than that of maize under the same conditions confirming the suitability of industrial hemp for energy production.

Fig 6. Calorific values of hemp whole plant and by products compared to other biomass crops.

Ash and Melting Point

The ash content of hemp samples reported in scientific literature varies between 3-4% which is much higher than wood but lower than peat and similar to many non-wood biomass materials. Hemp’s high ash content unavoidably extends boiler maintenance times and causes ash disposal issues but how much longer depends on the application. For instance, it might render the fuel unusable as a pellet feedstock for home heating stoves or boilers in urban areas, be anticipated to have little impact on the rural boiler market and be welcomed by generating stations already using peat with 6-7% ash.

The melting behaviour of ash is also crucial, particularly in applications requiring high combustion temperatures. Low melting temperatures can cause ash to clump together and obstruct grate openings and ash removal systems. The melting behaviour depends on the composition of ash but even with variations the calculated values for hemp ash melting point is higher than most other non-wood biomass. Therefore, under normal combustion conditions ash melting problems should not be an issue with hemp.

Corrosivity and Emissions

One of the major issues with non-wood material is the propensity of the fuel to corrode heat exchanger surfaces and is mainly a function of chlorine, potassium and sodium contents of the material. Hemp has low Cl and Na levels, while its K content is higher than that of wood but similar to that of many straws. Hemp typically has superior qualities to cereal straw, which is effectively burned in power plants in Denmark and the UK, and boilers and Combined Heat Power (CHP) plants in Denmark.

Stack emissions (gases released into the air from boiler stack, chimneys or diesel generator set stack) depend on the plant design, operational factors as well as fuel composition. A fuel with high volatile content and low in sulphur, chlorine, potassium and heavy metals generally tend to have low stack emissions. Hemp stems contain <80% volatile compounds higher than most other plant biomass and moderate levels of S, Cl and K. Although no excessive heavy metals have been measured in hemp, but as hemp as the ability to take up heavy metals from the soil, emissions may be a problem if hemp is grown on contaminated soils.

Profitability analysis

The main factors to consider when determining profitability include crop yields, production costs, energy costs, and the support available for renewable energy. The following section analyses the profitability of hemp grown for energy based on these factors.

Yield

Hemp’s ability to produce significant biomass yields with relatively little water input is one of its main advantages. Hemp requires less water than many other crops, including alfalfa and maize, and may produce 15 t of dry matter with only 250–400 mm of water when grown using modern production techniques. The ambient temperature is another factor that influences hemp development; it can be grown most successfully between the ranges of 13 and 22 °C. As hemp has natural defences against disease and insect attack, growing it requires just a little amount of biocides. These factors make hemp relatively inexpensive to grow.

A 3-year study in Ireland showed that under Irish climatic conditions early sowing has a big impact on yield. Furthermore, seed rate can be chosen to optimize yield and seed cost without any concern about fibre quality. This study also reported that a seeding rate of 30kg/ha gave the highest yields. The study concluded that a harvested yield of 11 t/ha of dry matter, corresponding to about 13.7 t/ha at 20% moisture is achievable and profitable.

Harvesting

The cutting and windrowing procedure is expensive and has a significant impact on field losses, in-field drying, and baling efficiency. There is a need to develop new harvesting machine designs that enable the co-production of improved hemp products, such as high-quality seed and straw which may in turn help to improve the viability of industrial hemp production.

Economics of growing hemp

Hemp grown for energy must be profitable for farmer to produce biomass for energy. In order to ensure encouraging prices for the finished product based on contracts, lower production costs are required. The low-cost production of fibre, seeds, or essential oil for technical uses can slightly increase the profitability of hemp produced for energy. The residual biomass (straw and shives) can subsequently be sold as briquettes for use in the generation of energy.

Hemp production varies greatly on an annual basis, and therefore the profitability of growing hemp inevitably changes as well. In Canada, one of the top hemp producers in the world, the revenue from hemp production is estimated to be $510/acre for fibre, $400/acre for grain, and $580/acre for both fibre and grain hemp production. This is comparable to the suggested returns from a US study.

Another US study which compared hemp with kenaf, switch grass and sorghum found that hemp generated the greatest revenue of these four crops, yielding $2632/ha ($2462/ha from grain and $170/ha from ethanol) compared to $908 for kenaf, 4803/ha for switchgrass and $1725 for sorghum when used for coproduction of ethanol and grain, though hemp had lower biomass yield. These values include the yield and value of hemp and sorghum grain as well as the growing cost for each type of biomass. One study looked at the profitability of hemp by pricing it as an energy commodity with the same price as oil. It found that industrial hemp had a net yearly profit of between €2000 to €3500/ha, which was higher than the earnings from both canola and sugar beetroot. Using organic fertilisers also enhanced profits, though less significantly than for canola and sugar beetroot.

In EU countries, farmers who grow hemp are eligible for area-based direct payments under the Common Agricultural Policy (CAP) provided the hemp variety being cultivated has THC (Tetrahydrocannabinol, the main psychoactive compound in marijuana) content of no more than 0.3% and is listed in the EU common catalogue of varieties of agricultural plant species. In the UK very little hemp farming has taken place in the recent years, in spite of a long hemp farming history,  and the nation today falls behind its European rivals significantly. Only 20 farmers in the UK currently cultivate hemp on an area of 800 hectares. The main cause of this is the onerous regulations. Industrial hemp cultivation is legal in the UK with a licence since it is considered a “special purpose” under the Misuse of Drugs Act of 1971 (MDA). As a result, hemp farmers must adhere to strict guidelines for the licensing of controlled substances.

Conclusion

Hemp has great potential to become a promising commodity crop for producing both biofuels and value-added products. It is an excellent rotation crop that fits well with food and feed crops and improves the yield of the subsequent crops due to the beneficial effects of hemp on the soil. The best applications for industrial hemp include co-production of hemp products with biofuels. Its high yield, higher ratio of cellulose to hemicellulose and digestible sugars to lignin and economics of production give it a competitive advantage over other biomass crops like giant miscanthus, willow, poplar and switch grass.

To promote the development of hemp as a biomass crop in the UK, changes to the regulations which require licensing to produce hemp are required. Subsidies for investments in processing facilities and equipment would greatly increase the viability of hemp in existing crop cycles in the grasslands, where hemp biochar can be co-fired with coal to reduce emissions. Hemp producers can benefit from investment in processing equipment that can valorise many potential products from hemp.

Take home messages:

• Willow is harvested every three to four years depending on the plant growth. Harvesting is done during the dormant season in winter (mid-October to early March) after leaf fall and before bud break.
• Harvesting is the single largest cost component of willow biomass production accounting for about 32 – 60% of costs over the life cycle of the crop (20 plus years).
• There are multiple ways of harvesting willow including the whole stem, cut and chip, and bundle harvesting system.
• Efforts to reduce harvesting costs by improving the performance and reliability of the harvesters and chip collection system are essential for the profitability of willow production.

Introduction

Willow is a short rotation woody crop that rapidly produces large amounts of biomass. The harvested willow biomass can be processed into renewable energy, heat, and other products such as livestock and poultry beddings, biochemicals, bioplastics and many more. Willow is harvested in a three-to-four-year rotation cycle after establishment and can be harvested six to seven times before replanting and requires minimal crop maintenance between harvest.

Harvesting is the single largest cost component of willow production, accounting for about 32 – 60% of costs over the life cycle of the crop (20 plus years). As such there should be efforts aimed at reducing the harvest cost by having reliable high-performing harvest systems to improve the harvesting efficiency and increase the profitability of willow biomass production.

Timing of willow harvest

Willow harvesting is ideally carried out on a three-to-four-year rotation cycle. The plants should be ready for harvesting three years after the first cutback (coppice) or four years after planting depending on the plant development. Harvesting can be delayed for a year or two if the growth of the plant was poor due to drought or competition from weeds, insects, or pests. Willow harvesting is ideally done during the dormant season in winter (mid-October to early March) after leaf fall and before bud break in early spring. Willow can be harvested with a few leaves on the plant, but this increases the moisture and ash content and the harvested plant biomass removed from the field deprives the soil of carbon transfer and nutrients recycled into the soil. The recommended optimal condition for harvesting willow is when the ground is frozen or covered with little or no snow, to avoid excessive loads on the soil together with the formation of deep furrows in the field by the harvesting machines. However, such optimal conditions are not always feasible under field conditions as the winter season has wet weather conditions and the ground condition is wet and soils are susceptible to damage from heavy harvesting machinery.

Harvesting Systems

There are various ways of harvesting willow involving either a single step or two-step method of harvesting. With the single step, the willow crop is cut directly from the stump and chipped in one operation. While with the two-step harvesting, the willow crop is first cut, stored, and naturally air-dried before later chipping and processing into the desired end product. The willow plants can be harvested manually or mechanically depending on the level of automation and the type of machinery.

Manual harvesting

Manual harvesting involves the felling of the willow plants with a chainsaw or brush cutter and collecting the logs manually or with a tractor, followed by direct feeding of the logs into a chipper.  Alternatively, the logs can be collected and stored and chipped later. Manual harvesting is labour intensive requiring a minimum of two persons. Manual harvesting is best suited for harvesting willow on small scale plantations.  Manual harvesting is less costly and an affordable option for small scale farmers who lack the resources to purchase expensive harvesting machinery. Manual harvesting is also used in situations where it is the sole available option when commercial operated machinery is not available when needed for harvesting. It is however less efficient and less productive when compared with the use of mechanical harvesters. A report  indicated that, it takes on average 45 hours to manually harvest 1 hectare of willow containing about 18,000 plants.  A study reported harvesting cost varying from €16.3 ha⁻¹ to €23.2 ha⁻¹, suggesting manual harvesting an affordable option for small scale farmers even though gross production rates is very low (0.10 – 0.11ha/h). Another study showed that manual harvesting exposes workers to noise, uncomfortable work postures, and high cardiovascular loads. This study suggested that motor manual harvesting operations should consider the compatibility of equipment and operational conditions to the workers undertaking such tasks.

Mechanical Harvesting

Mechanical harvesting is more economical when used for harvesting largescale willow plantations, where the large capacity of the machines can be fully utilized, and the high capital cost can be spread over large harvest volumes. Studies have shown that in order to increase profitability, it is necessary to have efficient machinery available with which high-quality wood chips can be produced at low cost and easily handled during storage and transport. Commercial harvesters and chip-collection services are provided by contractors to willow growers, and this comes at a high cost and the challenge of availability of machinery when needed. Different modified forage harvesters and corn choppers are used for harvesting and chipping willow.

There are also specialized machinery designed for harvesting willow. Harvesting systems for willow includes whole stem harvesters, small and large single-pass cut-and-chip systems,  and bundle systems. These harvesting systems are either self-propelled or tractor pulled harvesters.

Whole stem harvest system

Whole stem harvesting involves the use of forest harvesters or chainsaws for felling willow plants. The cut stems are collected with a tractor or forwarder and transported from the field to a storage site for drying and chipping later or the harvested stems chipped and sent for immediate use in processing plants.  Harvesting willow as stems or in large pieces has the advantage of providing year-round supply of natural-dried willow stem for fuel processing plants. A study showed that, storing willow as whole stems produces a lower moisture and ash content fuel.

Cut and chip harvest system

With the cut and chip harvester, the willow plants are harvested in a single operation and directly chipped and transported to the end user or storage site. The harvester either pulls its own trailer to collects the harvested material or use a tractor-trailer combination, which travels alongside the harvester and receives the chips blown from the harvester. To optimize efficiency, the harvester downtime should be reduced by keeping the harvester moving and harvesting the willow crop on a continuing basis, while the trailer continuously collect the chips moving the chips to the load staging area or end user. The plantation design and staging area should be well laid out to facilitate machinery movement and turning at the end of the rows. A study showed that cut-and-chip harvesters were faster than the whole stem harvester, and the self-propelled harvester was faster than the tractor-pulled harvester. This study further showed that harvesting costs differed depending on the machinery used for harvesting. Chips produced from the cut and chip harvester however have high moisture content and may require further drying to reach lower moisture content for efficient thermal combustion into energy, which involves further handling cost and may cause emission problems and dry matter losses.  Dry chips can attract higher pricing at the power plant than wet chips.

Cut and Bundle system

Cutter-bundler harvesters are used to cut and fell willow plants. The harvested stems are gathered as bundles that are loose or tied with wire or yarn. The bundled stems are stored and dried to reduce the moisture content before supply to processing plants and other end users. Studies have shown that the cut and bundle system compared with the cut and chip system, allows for efficiently reducing the moisture content of the willow biomass before chipping and a more cost-effective alternative.

Some considerations in deciding on the choice of harvesting system

In deciding on the method for harvesting willow, land managers and landowners should consider the following factors that influence the harvesting operations of willow and create a harvesting plan considering these factors.

• Willow species and variety: Different species and varieties of willow have different growth forms, branching pattern (upright or arching stem), number of stems, stem thickness, stem diameters and stem height. The diversity of willow in growth form and stem structure affects the ease of harvesting and logistics required for harvesting.
• Diameter of the willow stem: Willow plants should not be allowed to overgrow beyond four years before harvesting. Overgrown stems can cause inefficiencies in the harvesting machinery when they exceed the mechanical specifications of the harvesting machinery.
• Age of the willow plant: The annual biomass production begins to decline as the plant ages, as such it is best to harvest the willow plants every three to four years. A study showed that stem age influenced the initial biomass composition and found that two-year-old stems contained more extractives and three-year-old stems contained more structural sugars. Also, heat production varied with stem age which impacted the final biomass composition.
• End product: Depending on the end product whether chips, pellets, or whole stems, different machinery and harvesting methods will result in different end products.
• Quality of the willow biomass: The quality of the harvested willow biomass is influenced by characteristics such as ash, moisture, energy and elemental content, and particle size distribution.
• Availability of machinery: The availability of harvest machinery at farm or at least in the region where willow plantation is located is very important for the reduction of harvest costs.
• Condition of the soil: It is best to harvest under dry or frozen ground conditions. Avoid very wet soils which affects machinery mobility and damages the soil. Large tires on harvesting machinery minimizes soil damage when harvesting in wet conditions.
• Size and layout of the plantation: Efficient plantation design with double rows or single rows, and headlands (unplanted areas around the edge of a crop field) to accommodate the efficient turning of machinery and reduce harvester downtime.
• Scale of plantation: Mechanical harvesting of large-scale willow plantation is more efficient. Manual harvesting is best suited for small scale willow plantations.
• Quantity of willow plants to be harvested: Land managers should consider the labour and other logistic requirements for the quantity of willow stems to be harvested for efficient harvest operation.

Conclusion

Willow biomass is harvested after three to four years depending on the plant development. Harvesting is done during the dormant season in winter (mid-October to early March) after leaf fall and before bud break in early spring. Harvesting is the single largest cost component of willow biomass production, accounting for about 32 – 60% of costs over the life cycle of the crop (20 plus years). There are multiple ways of harvesting willow including the whole stem harvesters, cut and chip system, and the bundle system. Harvesting can be done in a single step where willow crop is cut directly from the stump and chipped in one operation, or two-step where the willow crop is first cut, stored, and naturally air-dried before later chipping and processing into desired end product. Efforts to reduce harvesting costs by improving the performance and reliability of the harvesters and chip collection system are essential for the profitability of the willow biomass production.

Key messages:

• Eucalyptus species display rapid growth characteristics, high water and nutrient-use efficiency, and have been identified as having great potential for use in short rotation forestry (SRF) or coppice (SRC) in the UK.
• Low temperatures and unseasonal temperature fluxes are the main limiting factor to Eucalyptus growth.
• Eucalyptus species and provenance must be selected and matched appropriately on a site-by-site basis.

Introduction

The Eucalyptus genus is native to Australia where species have adapted to grow under a wide range of climates and challenging environmental conditions. A number of Eucalyptus species have been identified as having great potential as a short rotation forestry (SRF) or coppice (SRC) species in the UK. Eucalyptus is suited to short rotation forestry due to its fast growth rate and high biomass yield, which can exceed that of other native species. When planted in favourable locations growth rate can be 2 – 3 m per year and can be harvested from 4-6 years onward. Based on an 8-10 year rotation, data from early UK trials identified yields in the range of 20-30 m³ ha/yr, on a bark-free basis, equivalent to 10-15 oven dry tonnes per hectare per year. Yields of up to 48 m³ ha/yr have been reported from a commercial plantation in Cornwall. Eucalyptus produces a high-density wood, with a net calorific value of 18 GJ t, which seasons quickly and is suitable for use as a biomass fuel or timber product.

Site Suitability

Low temperatures and unseasonal temperature fluxes are the main limiting factor to Eucalyptus growth. Frost damage can be a problem during establishment, but risk can be mitigated by proper species selection and silvicultural management.

In general, there is an inverse relationship between frost hardiness and growth rate. E. nitens, for example, displays exceptionally fast growth rate but is more susceptible to frost, this restricts its growth-range to southern Britain. Species such as E. glaucescens and E. gunnii are more cold- tolerant. Mountain-type E. coccifera and E. urnigera have been shown to perform well in exposed conditions and have been established successfully as far north as Moray in Scotland. Other species such as E. rodwayi are swamp varieties which are suited to wetter regions and ground conditions.

A table listing some of the main species suitable for use in the UK, including indications of growth rate and frost hardiness is included below; adapted from a review of Eucalyptus in the British Isles.

Eucalyptus species and provenance must be selected and matched appropriately on a site-by site basis. Factors of water availability, elevation, likelihood of unseasonal frosts, soil type and light levels must all be considered.

Establishment

Ground can be prepared via traditional cultivation or planting using a no-till approach. Physical or chemical weed suppression is required post-planting. Alternatively, planting can also be performed through a geotextile membrane to suppress weeds.

Planting is performed from seedlings at stocking densities of between 1,600-2,500 stems per hectare at (2.5 x 2.5m – 2.0 x 2.0 m spacing). 2-3 ha is considered the minimum area for viable commercial production. Tree-shelters and stakes are generally required to protect young plants and support early growth. Due to rapid growth rates, close monitoring is essential for the first few years until trees successfully establish.

The primary objective is to achieve canopy closure as soon as possible and create a suitable microclimate for trees to thrive. Mixed species stands, or planting with other tree species in shelterbelts, can improve resilience and offset risk. For example, more cold tolerant species can be included on exposed stand edges. Mixed-age stands can also reduce risk from frost damage and exposure by providing shelter for younger trees by providing a positive microclimate and protected environment. Staggered planting and harvesting can be performed to provide continuous self-supply or be adapted to suit farmland available to fit around core farm business and provide other benefits.

Eucalyptus light canopy can afford options for growth in the understory. This means stands are well suited to agroforestry use (silvo-arable/pastural) as well as in shelterbelts, riparian buffer strips and sight screens.

Management, pests and diseases

Leaf loss is a common sign of stress in Eucalyptus trees and should be watched out for. Few pests are currently prevalent in UK. Psyllids are a common infestation on establishing trees but generally have minimal impact. The leaf beetle, Paropsisterna selmani has posed problems in Ireland and eucalyptus gall wasp (Ophelimus maskelli) could pose a threat in the UK. Eucalypts have displayed good resistance to Phytophthora and other foliar pathogens. Young growth can be palatable to deer but the oils in the foliage deter extensive browsing of the foliage. Fencing may be required in areas where these animals are prevalent.

Harvesting

Harvesting is performed using conventional forestry methods. First thinning can be done from year 4-6. At higher stocking densities, and under suitable growing conditions, around 16 t ha can be expected at 1^st thinning. Commercial data suggests an annual sustainable yield of around 40m³ ha yr can be achieved, producing consistent roundwood for chipping or firewood use from year 4-5 onward.

Coppicing – most Eucalypts will coppice well and their capability to do so also mitigates risk if damage to the main stem occurs due to frost or browsing. Coppicing can be performed from years 2–7 and stools will remain productive for 3-4 rotations of 4-8 years, after which the stool viability will drop.

If chipping wood: harvest time, storage conditions and moisture content of the chip needs careful consideration. Higher moisture content can lead to increased fungal growth (composting) in chip stack and under some conditions can also present a fire risk which needs to be accounted for in management of the chip.

Eucalyptus wood can retain higher concentrations of chlorine in the biomass which can cause corrosion in boiler systems if significant quantities are burned. However, this risk can be offset by biomass mixing or pre-treatment. There is also evidence chlorine content decreases with stand age.

​Additional uses and benefits

• Eucalyptus foliage is widely used in floristry.
• Eucalyptus attracts pollinators.
• Eucalyptus honey production
• Volatile oils from foliage have potential for multiple applications in nutraceutical and pharmaceutical products such as antiseptic and anti-inflammatory products.

Summary of promising Eucalyptus species for SRF in the UK

Further information:

Leslie, A. D., Mencuccini, M., & Perks, M. (2012). The potential for Eucalyptus as a wood fuel in the UK. Applied Energy, 89(1), 176-182.

Forestry Commission –  Grant funding for woodland

Forest Research – Wood fuel supply guidance

This article was produced in collaboration with Brian Elliott of Eucalyptus Renewables Ltd.

Watch Bryan Elliott’s Biomass connect webinar: Eucalyptus as a short-rotation forestry crop for the UK

Key messages:

• Buffer strips provide an effective land management option to reduce flood risk, soil erosion, and groundwater pollution from agricultural land.
• Buffer strips consist of planting strips of either grasses, other herbaceous perennials or tree species in 6-20 m wide strips along field margins or beside watercourses.
• Perennial biomass crops such as energy grasses (e.g. miscanthus, switchgrass, reed canary grass) and short-rotation trees (e.g. willow coppice and poplar) can all be used to create buffer strips. Their suitability needs to be matched to the buffer functions required, the site conditions, and planting location.
• For harvesting biomass, site logistics such as access and operating space for harvesting machinery needs to be considered in planning the locations of biomass buffer strips.

Introduction

A buffer strip is an area of land which is either left uncultivated or planted with perennial grasses, shrubs and trees. Planting strips or alleys of perennial biomass crops on existing agricultural land can provide a number of important benefits in terms of flood management, soil recovery and improvements in biodiversity, in addition to providing a harvestable resource. Wider adoption could help better manage landscapes to be more resilient to the effects of climate change, mitigate flood risk, and reduce the environmental impacts of intensive agriculture.

How are buffer strips used?

Buffer strips have been proven to be effective at reducing soil erosion, leaching and run-off of agricultural chemicals which is known to result in pollution of watercourses.

Perennial buffer strips planted adjacent to arable land have been shown to reduce soil sediment, nutrient loss, and pesticide drift while improving soil health and farmland biodiversity. Agroforestry systems have also been shown to provide effective windbreaks, provide wildlife corridors that can connect fragmented habitats and can also enhance the natural regulation of pests.  Provided the biomass crops can be safely and effectively harvested, their inclusion in buffer zones adds value by providing an additional harvestable resource which requires little or no maintenance. The biomass crops can make use of nutrient run-off, providing higher biomass yields while simultaneously reducing the nutrient load entering water courses which is a major source of pollution.

Broadly speaking, based on their location, buffer strips fall into one of two categories:

• In-field strips – across the slope of the field, along land contours and field margins.
• Riparian strips – flanking the banks of watercourses.

The current Environmental Land Management (ELM) scheme in England supports the adoption of buffer strips and there are woodland grants and incentives available for establishing and maintaining woodland for nature recovery, riparian buffers, improving water quality and reducing flood risk but biomass crops and short-rotation forestry (<8 yr rotation) are not currently eligible under current rules.

Use of buffer strips was piloted under the Catchment Sensitive Farming partnership, co-ordinated by Natural England and the Environment Agency, 3836 riparian buffer strips were established on participating farms in England between 2006-2018 and proved a popular measure, with 1 in 6 farms adopting their use. In-field and riparian buffer strips were both within the top-five most impactful catchment-sensitive farming measures identified. Use of biomass crops in buffer strips has great potential, but their adoption has so far been limited.

The main functions of buffer strips.

The main functions of buffer strips can be summarised as: Natural flood management, nutrient and pollution control, protection of soils and nature conservation.

Natural flood management – Buffer strips containing tall grasses, trees or shrubs act as a physical barrier which slows the run-off of surface water following heavy rains making them effective natural flood management strategies which are likely to be of growing importance. Buffer strips can help achieve this by acting in the following ways:

• Increasing the interception of rainfall – leaves catch rain before it reaches the ground slowing the rate of flow and allowing a proportion of the rainfall to evaporate from the leaf surface.
• Slowing the flow of surface water – above-ground vegetation provides a physical barrier which slows the flow of water running off the soil surface.
• Increasing absorption of water into the soil – Established buffer strips can improve soil structure allowing water to penetrate deeper into the soil, increasing the volume that can be stored in the soil and reduce the likelihood of saturation which leads to greater run-off.
• Larger perennial crops and tree species take up significant quantities of water from the soil, can help reduce soil saturation and ameliorate areas prone to waterlogging. Conversely, in drier conditions, buffers can also help to conserve soil moisture and reduce effects of drought.

Pollution control – In areas where agricultural land is more intensively managed, nitrate-N (NO₃^–) and phosphorous (P) leaching and run-off is known to be a main source of pollution resulting in eutrophication of lakes and waterways. Deep rooting perennial plants and tree species can act as a biological filter which take up and utilise nutrients before they enter watercourses. Studies have shown that riparian buffers can remove between 30-99% of nitrate and 20-100% of phosphorous depending on site conditions and the buffer composition, with combinations of grasses and trees tending to be most effective. In situations where frequent incidences of fertiliser run-off are common, some riparian buffers have been shown to reach saturation after a few years if the biomass is not regularly removed. For this reason, using biomass crops in buffers provides an added benefit as the leaching fertiliser can result in higher harvestable yields of biomass and also help maintain the effectiveness of the buffer strip in terms of N and P uptake. In addition to nutrient run-off, buffer strips can also be effective for wastewater management. Biomass crops such as willow short-rotation coppice have been shown to be particularly effective, permanent nutrient removal of 55 kg PO₄³⁻eq ha yr has been reported based on field trials conducted in Northern Ireland.

Pesticides are another potential source of pollution from agricultural practice. The pesticides applied in the UK have a wide range of different properties and modes of action, but in general most pesticides will preferentially adsorb to dead organic material such as leaf litter. Incorporation of tall, dense and fast-growing species in the buffer strip can also act as a windbreak or shelter belt which can create a physical barrier to prevent pesticide drift. Biomass crops which create deep leaf litter layers before the biomass is harvested, such as miscanthus and willow, may also be particularly effective if protection from fertiliser run-off and pesticide drift are both required. Poplar and willow have been shown to take up and immobilise a wide range of pollutants, including pesticides.

Source: Environment Agency (2020). 3D buffer strips: Designed to deliver more for the environment. Environment Agency, Bristol.

Soil health – The above ground vegetation of the buffer strip helps catch and retain soil that has been eroded by wind or water. On degraded arable soils, uncultivated buffer strips will also allow the soil to recover over time. Deep rooting plants can help stabilise soils at higher risk of erosion, such as riverbanks and areas around water bodies, often collectively referred to as riparian zones.

Buffer strips have also been shown to enhance soil carbon through sequestration, root turnover, leaf litter, and simply by leaving the area unploughed for a prolonged period which allows the soil time to recover and reduces incidences of soil compaction.  The total carbon stocks under short rotation forestry have been reported as 1.5 times higher than alternative land uses when planted on agricultural land.

 Nature conservation – Biodiversity and wildlife corridors – Buffer strips provide important habitats and can provide cover for wildlife to allow them to move freely between different habitats which are often fragmented by field boundaries. Tall grass and tree species incorporated into buffer strips also provide shade which can be of benefit to aquatic life in water courses. Perennial biomass crops also provide important winter cover. As they are harvested in late winter, they provide vegetative cover when adjacent agricultural land is often bare which also poses even greater risk of flooding and erosion.

Perennial biomass crops best suited to use in buffer strips

A number of different biomass crops may be suitable for use in buffer strips. Perennial grasses bred specifically for high-yield and energy content are one option. Stiff-stemmed grassland species that are resistant to high run-off rates are most suitable and can be selected as a feedstock for multiple purposes: Combustion, anaerobic digestion, forage/grazing or animal bedding. Wildflower mixtures are also an option for inclusion. Some energy grasses, such as miscanthus, may require a minimum planting area and an established local market in order to be economically viable to harvest on smaller scales, unless used on-farm as animal bedding.

Short-rotation coppice (SRC), e.g. tree species such as willow and poplar, which are cut to a low stump (stool) and produce multiple stems that are harvested every 2-5 years, are also an option where land is not too steep. Alternatively, short-rotation forestry (SRF) may be suitable. SRF uses high-yielding tree species grown at high stocking densities for 8-20 year rotations. Wood is harvested when stems reach approximately 20cm diameter. If space permits, conventional woodland management of wooded buffer zones is a long-term option.

Types of buffer strip

In terms of composition, buffers can be classed as: Vegetated, incorporating grasses and wildflower species; Wooded, incorporating tree and shrub species; or integrated/engineered, incorporating a mixture of species and/or earthworks. Integrated or engineered buffer strips are recommended as the best designs for incorporating biomass crop species. With reference to riparian buffer strips, a three-zone structure has been recommended as most effective for use with biomass crops:

1. Grass strip – This strip reduces the flow rate of surface run-off, filters sediment and reduces erosion. Nutrients are taken up by the plants during the growing season. The strip may be composed of any perennial grasses, including energy grasses. Depending on the grass used, the strip can also be used for extensive grazing, silage/hay production, animal bedding, or for bioenergy.
2. Woodland strip – This strip improves interception of rainfall, deep rooting tree species help stabilise banks and filter nutrient and pollutant run-off via uptake and immobilisation in the biomass. Taller species also act as windbreak and prevent erosion and pesticide drift. This strip can be composed of mixtures of short-rotation coppice (SRC) or short-rotation forestry species (SRF). The harvested biomass can be chipped or cut as billets for firewood.
3. Undisturbed strip – helps to protect planted strips during flooding events, reduces bank erosion and provides a buffer between planted biomass crops and water courses to allow space for harvesting and maintenance operations.

Slope can be used as one of the key considerations for planning a biomass buffer

Other considerations

Depending on where buffers are located, existing land drains can negate the effects of buffer strips due to providing a bypass for surface water run-off. In cases where extensive networks of land drains are already present, redirecting flow to a drainage ditch between the field and the buffer could provide a solution. Where this is not possible, a constructed wetland area could be another option for environmental protection and conservation.

The greater the level of engineering that goes into buffer strips, the greater the cost, but also the greater the benefit. Planning needs to take account of biomass crop establishment and harvesting requirements both in terms of cost and management considerations.

Where livestock are also present, or in areas where browsing damage from rabbits/deer is a problem, fencing may be required to prevent grazing of establishing buffer strips and to keep livestock away from water courses.

Biomass Connect Media:

Dr Chris Johnston of the Agri-Food and Biosciences Institute (AFBI), a Biomass Connect partner, gave talk at the UK Low Carbon Agriculture Show, February 2023, showing results of field trials into the effectiveness of short-rotation coppice willow for environmental protection and wastewater management.

Current UK policy

The current Environmental Land Management (ELM) scheme in England supports the adoption of buffer strips and there are woodland grants and incentives available for establishing and maintaining woodland for nature recovery, riparian buffers, improving water quality and reducing flood risk. Biomass crops and short-rotation forestry, with rotation length less than 8 years, are not yet eligible for ELMs or woodland creation (EWCO) incentives.

Conclusion – Main considerations when establishing biomass buffer strips.

The optimum design, composition and width of a buffer strip depends on a wide range of factors which include:

• The environmental and conservation issues being addressed.
• The previous and adjacent land use.
• The soil type and condition.
• The slope, topography and hydrology of the site.
• The desired biomass products and yields.
• Harvesting technology required and local markets available for the biomass.
• Cost of measures required and government payments available to support their use.

Further information:

Environment Agency (2020). 3D buffer strips: Designed to deliver more for the environment. Environment Agency, Bristol.

Related Biomass Connect Articles:

How does Short Rotation Coppice (SRC) willow affect biodiversity?

What effect does planting biomass crops have on soil carbon?

Other resources:

Funding for landowners – Nature based solutions to reduce flood risk

Examples and case studies – How nature-based solutions can mitigate flood risk.

Riparian forest management on freshwater environment – Best management practice guide.

Natural Flood Management Handbook – A practical guide for farmers.

Take home messages:

• Biomass pelletisation is the process of condensing biomass material into energy-dense pellets by forcing the biomass material under high pressure and temperature through a pellet mill to produce small cylindrical pellets.
• Biomass pellets have uniform size and shape, high bulk and energy density, and low moisture content, all contributing positively to the supply chain management and logistics cost.
• The quality and characteristics of biomass pellets are determined based on key parameters such as the type of biomass feedstock, moisture content, particle size, binding material, pressure, and temperature.
• A high-quality pellet should be dry, hard, durable, and with low ash content. The ENplus certification scheme outlines pellet properties and related threshold values.

Introduction

The global demand for biomass is increasing in response to the need to reduce dependency on fossil fuels, concerns about environmental problems related to its use, and the increasing desire to mitigate climate change. Biomass raw materials are widely regarded as a significant renewable energy source and an alternative to fossil fuels. However, there are limitations with the use of biomass feedstock in its natural form for energy conversion systems. Biomass feedstocks are bulky, have low bulk density, non-uniform properties, low energy density, and high storage, handling, and transport costs. One way to overcome the limitations is using densification to convert biomass feedstock into pellets before use for energy purposes.

Biomass Pellets

Biomass pellets are biomass materials compressed into high energy dense pellets, characterised by; homogenous shape and size, low moisture content, higher bulk density, thus reducing the storage, handling and transportation cost and lower environmental impacts. Biomass pellets could be used directly as fuel for residential heating stoves, heating boilers and large-scale power plants. Biomass use in the form of pellets generates a biofuel that is more cost-effective than the direct use of non-modified biomass residues for energy production. This article explores the critical issues in the biomass pelletisation process, biomass pellets characteristics and standardisation of high-quality biomass pellets for energy production.

Pellet production process

Biomass pelletisation is the process of condensing biomass material into energy-dense pellets by forcing the biomass material under high pressure and temperature through a pellet mill consisting of a die with cylindrical press channels and rollers, to produce small cylindrical pellets. The friction between the biomass and the press channel generates a force compressing the biomass into pellets that are dense and cut at a uniform size. The biomass pelletisation process consists of multiple steps including pre-treatment (particle size reduction, drying, and conditioning), pelletising, and post-treatment (cooling, screening, packaging).

Pre-treatment involves reducing the biomass particle size and drying to reduce the moisture content. The harvested biomass is processed into chips and fed into a mill, further reducing the particle size in order to avoid blockage in the pellet mill. Studies have found that biomass materials with small particle sizes and a large surface area during pelletisation result in a higher density, and stronger pellets.

Following the particle size reduction, the biomass material goes through a drying phase where the material is dried in a dryer. The moisture content affects the overall quality of the final pellet produced. Depending on the biomass material and the lignin content, additives or binding agents may be added to improve the quality of the final product. Studies have shown the need to incorporate additives to biomass feedstock when pelleting biomass materials with zero or low lignin content. Lignin is one of three major components of lignocellulosic biomass with binding attributes. Additives such as starch, corn flour, potato flour, vegetable oils, etc., which are materials intentionally added into the pellet production or added after production should not exceed two percent of the total pellet biomass. Additives have the benefit of improving the quality of pellets, reducing emissions, increasing production efficiency, or marking the pellets.

The pelletisation process takes place in a pellet mill. The pellet mill has basic components of a die and cylindrical press channel and rollers. The friction between the biomass and the press channels causes pressure to build and increases the temperature of the biomass material which softens the lignin content, binding the biomass material and compressing the biomass into small cylindrical pellets.

After pelletisation, the biomass pellets are allowed to cool to solidify into more durable forms. Once cooled, the biomass pellets are stored or packaged, ready for use. The fine materials that are not formed into pellets are recycled back into the pelletisation process.

Pellitisation

Pellet quality requirement

Pellets produced from biomass should meet international pellet standards. The UK Pellet Council (UKPC) manages the license for ENplus pellet quality certification in the UK, ensuring adherence to the standards and the production and delivery of high-quality wood pellets for heating. The ENplus certification scheme defines three pellet quality classes (ENplus A1, A2 & B) which are based on the ISO 17225-2. The ENplus certification scheme outlines pellet properties and related threshold values.

Biomass feedstock has an influence on the quality of the pellets produced. A high-quality pellet should be dry, hard, durable, and with a low ash content of less than two percent. According to the ENplus standards, the length of the pellet varies ranging from 3-40mm and a diameter of 6-8mm. Biomass pellets should have a moisture content of less than 10 percent. Dust produced during the processing stages of biomass collection, size reduction and pelletisation should be minimal (no more than 1 percent). Dust is highly combustible, leading to explosions and imposing health hazards.

Furthermore, quality pellets should be durable with high bulk density. Particle bonding by the introduction of additives is considered a vital event in biomass pelleting. The production of durable pellets is a function of the lasting attraction between individual particles during biomass pelleting. Where additives are added into the pellet production or added after production, it should not exceed two percent of the total pellet biomass.

Parameters affecting the quality of produced biomass pellets

The quality and characteristics of biomass pellets are determined based on key parameters such as the type of biomass feedstock, moisture content, particle size, binding material, pressure, and temperature.

The type of biomass feedstock influences the quality of the pellet.  Different feedstocks have different characteristics and energy requirements for pelletisation, which directly impacts production cost and production capabilities. Biomass materials often used for pelleting are wood from forestry, followed by agricultural residues. The limited availability of wood resources and increasing global demand for biomass pellets has resulted in efforts to broaden the raw material base used for pellet production, including short rotation coppice biomass crops and herbaceous perennial biomass such as Poplar (Poplar spp.), Miscanthus (Miscanthus giganteus), and switch grass (Panicum vigatum). For instance, a study found poplar as a suitable material for producing pellets that meet many of the pellet quality standards (see Table 1) required on the market. Studies on Miscanthus pelletisation have however often found the use of Miscanthus biomass producing pellets of low durability and high ash content and requiring high energy inputs. The lower lignin content and poor bonding between particles have been identified to account for the low pellet quality. The recommendation to improve the pellet quality of biomass with such characteristics is the addition of binders and mixtures of different raw materials as they affect the quality of the pellets.

Biomass feedstock with optimal moisture content is essential for producing quality pellets with better mechanical durability, strength, and thermal conversion performance. Recommended moisture content ranges from 10-15%. Moisture content affects the quality of pellets especially if pellets are to be stored before use. Research has shown that decreasing moisture content will increase the density and durability of pellets. Increasing the moisture content above the recommended optimum values however, has negative influence on the pellets mechanical durability and reduces the pellets density.

The applied pressure during the pelletisation process affects the natural binders such as lignin, starch, protein, and water-soluble carbohydrates present in the biomass material. As pressure increases up to a certain point, these binding components are pressed out causing the biomass particles to bond together. Studies have shown a positive effect of increasing pressure on various biomass feedstocks quality by increasing the durability and hardness of the produced pellets. Also, research has shown optimal die temperature for pelletising biomass feedstock is close to 100 °C.

Furthermore, the particle size of the biomass feedstock depends on the biomass characteristics, particle size reduction methods during pre-treatment and the pelletising equipment. Decreasing the biomass particle size increases friction in the press channel of a pellet mill resulting in increasing pellet density.

Conclusion

The global demand for pellets is increasing in response to the need to reduce dependency on fossil fuels and concerns about environmental problems related to their use. Biomass pellets are produced from woody and non-woody biomass feedstocks through a process of condensing biomass material into energy-dense pellets by forcing the biomass material under high pressure and temperature through a pellet mill to produce small cylindrical pellets. Biomass pellets have uniform size and shape, high bulk and energy density, and low moisture content, all contributing positively to the supply chain management and logistics cost. The quality and characteristics of biomass pellets are determined by key parameters such as the type of biomass feedstock, moisture content, particle size, binding material, pressure, and temperature. A high-quality pellet should be dry, hard, durable, and with low ash content. The ENplus certification scheme outlines pellet properties and related threshold values.