Scottish Government

2. Biomass Sources, End-Uses and Conversion Technologies

2.1 Background

In this report, the term 'biomass' is used to refer to any organic matter available on a renewable basis, including that originating from forestry, agricultural crops or animal sources. The biomass sources used to generate energy will be referred to as 'biomass feedstocks'. Some biomass feedstocks, such as wood, can be converted directly into useful energy, while others need to be processed into fuels such as bioethanol or biodiesel that can in turn be combusted to produce energy. In this report, the term 'biofuels' refers only to transport biofuels. The energy produced from biomass feedstocks, be it used to generate electricity and/or heat or used in transport, is termed 'bioenergy'.

Globally, there are hundreds of different possible biomass feedstocks, many of which are not suitable for establishment in Scotland. Crops such as sugar cane and palm oil are major biomass feedstocks in Brazil and Indonesia respectively, but are not suited to the Scottish climate. This report will focus therefore on feedstocks that are already in place, or possess the potential to establish in Scotland. A summary of biomass feedstocks of relevance to Scotland, including an analysis of strengths and weaknesses and recommendations for the development of each biomass production chain was recently provided by Towers et al. (2004). The biomass feedstocks covered in this report are limited to those in the Towers et. al. report. Municipal solid waste, sewage sludge and other other waste feedstocks besides those listed above are beyond the scope of this study.

In general terms, biomass feedstocks can be divided into three broad categories: 1) wood and woody residues from forestry and wood-processing industries, 2) agricultural residues and by-products and 3) purpose-grown energy crops. It is possible to generalise further and state that there are three major energy end-uses of biomass feedstocks: 1) heat, 2) electricity/ CHP and 3) liquid transport fuels. Many feedstocks can have more than one end-use, such as tallow, that can be burned to generate heat or can be esterified to produce biodiesel. There are multiple pathways by which biomass feedstocks can be converted into their end-products, with significant variations in efficiency, costs and practicality. The production pathway selected can influence key environmental variables such as greenhouse gas emissions or air quality, so an understanding of the diversity of fuel production pathways facilitates interpretation of subsequent chapters on greenhouse gas emissions (Chapter 4) and environmental impacts of bioenergy production systems (Chapter 5).

A Note on Fuel Properties and Units Used

There are several properties of biomass feedstocks that influence their potential as fuels, the most important being their net calorific value (amount of heat energy released upon combustion of a given quantity of feedstock) and moisture content. If a biomass feedstock has a high moisture content, considerable energy will be expended to dry the feedstock for effective combustion. It is therefore desirable that moisture content be as low as possible. To facilitate comparison, calorific values will be expressed, whenever possible, on an oven dry tonne (odt) basis, where moisture content is taken to be zero.

In this report, calorific values of solid fuels will be expressed in terms of GJ (Gigajoules)/odt, where one GJ equals 1 x 10 ⁹ J (joules) or 1x10 ³MJ (megajoules). Energy generation will be addressed in terms of watts, with a watt being defined as the transfer of 1 joule of energy over the period of 1 second. One watt-hour is defined as the maintenance of a production rate of 1 Watt over a period of 1 hour ( RCEP 2004). To convert from MW to MWh yr ^-1, it is necessary to multiply by 8760 (the number of hours in one year), although this assumes that the maximum amount of energy possible was generated during that year. To be accurate, this figure needs to be multiplied by the load factor, which is the ratio of the amount of energy produced in a given period of time to the amount possible for that period. This varies dramatically among different end-uses. Table 2.1 introduces the major conversion factors used in this chapter and in the next chapter of this report.

Table 2.1: Important Conversion Factors Used in this Report

2.2 Biomass Feedstocks

2.2.1 Wood Fuels from Forestry and Wood Processing Industries

Background

In the European Union, woody biomass accounted for 3.2% of the total primary energy production in 2004 ( EC 2005). Much of this was used for space heating in Northern European countries with well developed district-heating schemes, but wood and wood products can also play an important role in generating bioelectricity. Wood and wood residues can be co-fired with coal at utility stations and therefore could provide considerable environmental benefits without needing to alter power station infrastructure.

Woody biomass feedstocks can be obtained from a wide range of sources, including low-grade timber from forestry thinning and clearfell, forestry residues, arboricultural arisings from municipal tree and woodland management processes, sawdust and offcuts from sawmills and recycled wood from processing/manufacturing facilities. Although some of these products are sought after by competing markets ( e.g. pulp, paper and chipboard), there is often a surplus of the resource that can be exploited to generate heat/electricity. In the case of arboricultural wood, for example, it is estimated that 10-18% of arboricultural arisings in the UK end up in landfill sites ( SDC 2005). It is possible therefore that the opportunity to sell forest arisings could make forest management more economically viable.

Combustion of wood generates heat that can be used directly for small-scale domestic heating or medium-scale district heating, or can be used to generate electricity on larger scales. The form of the wood needs to be adapted to the appliance it will feed into. Current heating systems are available that utilise whole logs, wood chips, wood pellets or wood briquettes. Logs are typically used in open-fires, closed stoves and mutually-fed boilers. Wood chips are used to fuel automatically fed boilers of set specifications and range in size from 0.5 cm to 5 cm ² ( SDC 2005). Wood pellets, ranging in size from 5-40 mm in length with a diameter of 5-12 mm, are produced by compressing shavings and sawdust in a pellet mill and are becoming increasingly popular in several European countries, most notably in Austria and Sweden.

Properties

The calorific value of dry wood (<15% moisture) is 17-22 GJ odt ^-1 and is therefore comparable to most other biofuels, but substantially lower than that of fossil fuels such as coal, which has a calorific value of approximately 30 GJ t ^-1 or crude oil which has a calorific value of 45.5 GJ t ^-1. Moisture content varies largely with the origin of the wood product and, in the case of forest residues, on the time of year at which they are harvested. The moisture content of freshly harvested wood tends to be between 50-60%. Most sawmill co-products have a moisture content similar to harvested wood (50-60%), although treated timber ( e.g. kiln-dried) and recycled material can have a moisture content as low as 15%. For most current small-scale applications, 30-35% is an acceptable normal moisture content standard, although larger-scale schemes can accept much higher moisture content (Forestry Comission Scotland, personal communication). Pellets must have a moisture content of 8-10% and require dry sawdust produced either from kiln-dried timber or from woody material that has been broken down to sawdust and artificially dried. Table 2.2 summarises some important properties of a selection of the fuels introduced in this chapter.

Generic advantages and constraints

An attractive feature of wood and woody residues is that they represent a resource that is already available as a co-product of other industries, and do not therefore need to be planted. In that sense, there are no establishment costs involved. On the other hand, there are considerable harvesting and transportation costs that need to be accounted for. Removal of residues from forest floors may lead to associated negative environmental impacts such as soil damage (see Chapter 5). In cases where woody biomass is planted for energy purposes, there will be additional impacts arising from afforestation. These include reduction in groundwater supplies through increased rainfall interception and reduced groundwater recharge rates as well as possible increases in acidification and nitrification impacts due to scavenging of atmospheric pollutants by forest canopies (Allen and Chapman 2001).

Recycled wood from the manufacturing industry is often mixed with processed materials that may introduce contaminants ( e.g. heavy metals or chlorinated organic compounds) that could jeopardize their use as feedstocks and necessitate their incineration under the Waste Incineration Directive ( WID).

Table 2.2: Properties of selected biomass feedstocks used for heat/ CHP/electricity production

The classification of wood arisings as waste is currently an area of much debate, with the key issue being whether wood residues are a product of the activity or whether they have been discarded. Forestry residuesand untreated sawmill co-products are not currently treated as waste amd are tthus excluded from the WID. For forestry arisings and recycled wood products, there are also issues of fuel heterogeneity that makes them less suitable for utilisation in small-scale heat plants. Pellets provide a more consistent, convenient and energy-dense fuel although they are more expensive. As pellets a processed fuel, there are additional primary energy inputs required for their production.

2.2.2 Agricultural Residues and By-products

2.2.2.1 Straw

Background

Cereal straw, a co-product of grain and seed production, has been used to generate heat or electricity at a number of scales. On average, one part straw is produced per part of grain produced, although this depends very much on the crop variety. Cereal straws are usually baled for handling and storage. The dimensions of the bales are typically 1.2 m x 1.3 m x 2.4 m and each bale weighs between 450 and 500 kg. Provided the straw is dry when baled, it can be stored outside for significant periods of time without deterioration in quality or excessive losses of dry matter resulting from microbial respiration ( DTI 2005).

Straw can be co-fired with coal or used directly to generate electricity/heat. Recently, the Elean plant in East Anglia, established in 2000 and utilizing 200, 000 tonnes of straw per year, became the world's first power station to produce electricity exclusively from straw (Bioener 2005). It is the Danes, however, that have made the most notable advances in straw bioenergy production, with Danish utilities being obliged to co-fire straw. This has led to considerable technological advances for more efficient energy conversion, including the design of systems which minimize fouling of combustion chambers and the development of straw pellets that allow for greater fuel standardisation and improved energy density (Pedersen 2005).

Properties

The chemical composition of straw varies with the crop, but the DTI (2005) report an average gross calorific value of 15 GJ t ^-1. Compared to other biofuels, cereal straw has a high ash content that can foul the boiler system and must be disposed of, presenting additional costs. Straw also has relatively high chlorine content (0.75%) and potassium content (0.5-2%). The concentration of chlorine can be reduced, however, by leaving the straw to lie for longer before harvesting, effectively allowing rain to wash out the chlorine (Centre for Biomass Technology 2002). As straw is usually harvested dry, its moisture content is usually below 20%.

Generic Advantages and Constraints

Straw has the advantage of being produced from traditional crops and therefore does not require farmers to develop new crops. However, there are several difficulties associated with the storage and burning of straw. Straw is usually harvested over a 70-day period in the summer / autumn and must be stored for the remainder of the year. The chemical constituents of straw, especially its high chlorine concentration, can lead to problems with corrosion, slagging and fouling in the combustion chamber. If co-fired with the coal, chemical interactions with coal constituents can exacerbate these effects. Another complicating factor with straw utilisation is that there may be competition for use as animal feed or bedding or as a means of improving soil organic matter content (Smith et al. 2000).

2.2.2.2 Dry Poultry Litter

Background

Dry litter, especially poultry litter, can be burnt to generate heat and/or power. Litter is a mixture of different substances with poultry litter being a combination of chicken droppings and the bedding material from broiler chickens. At the Eye Plant in southeast England, horse bedding and chicken feathers are added to the poultry litter mix before it is combusted to generate electricity. Traditionally, poultry litter is disposed of by spreading over the land, for use as a fertilizer. This, however, has implications for water quality, as some of its constituents may leach out into adjacent watercourses. Use of poultry litter to generate energy, may, therefore, provide environmental benefits, but this depends upon the source of nutrients used to replace the poultry manure on the field.

The UK has been at the forefront of power generation from chicken litter with four plants currently in operation: Eye, Glanford, and Thetford in England and Westfield, in Fife, Scotland. Although it is possible to co-fire poultry litter with coal, there are few examples of this described in the literature.

Properties

Dry manures have relatively low energy content, with the exact value depending on the litter moisture content, which is usually between 20-50%. The DTI (2005) cites an average gross calorific value of 8.8 GJ t ^-1 for poultry litter. The average ash content is also relatively high at 17.5% (Towers et al. 2004).

Generic Advantages and Constraints

Like forest residues, animal litter represents a resource that is readily available, without the need to allocate land specifically for its production. Like other by-product fuels, however, poultry litter availability is susceptible to fluctuations in the market of the main product (in this case poultry) and is thus an inherently a less stable fuel source. Utilisation of dry litter to generate energy may yield environmental benefits, as the traditional method of disposing it is spreading on land as a fertilizer, which can lead to leaching into water courses. On the other hand, its withdrawal from land application may lead to poor soil fertility or equally high leaching losses if replaced by other organic residues or mineral fertiliser. Furthermore, if replaced by mineral fertiliser, there is a significant energy (fossil fuel carbon) cost associated with producing the additional mineral fertilizer that may offset any benefits of using the poultry manure as a feedstock. There are also worries about trace concentrations of arsenic and other chemicals that could have adverse effects on health (Christen 2001).

2.2.2.3 Wet Manures (Slurries)

Background

Wet slurries or manure can be processed by anaerobic digestion ( AD) to give a biogas that can be used to generate heat and/or electricity. The main driver for this is environmental, being heavily motivated by concerns about reducing pollution arising from spreading slurries on land, as well as associated issues of odour control. The high water content of wet slurries means that large amounts of energy are required to burn them, rendering them inefficient for power production through combustion. Under anaerobic conditions, bacteria digest organic matter in the absence of oxygen to produce a gas consisting of methane (40-60%) and carbon dioxide, with a liquid digestate as a co-product. This is processed into an organic fertiliser and recycled onto agricultural land. This biogas can then be used to generate heat and/or electricity or, alternatively, can be used as a transport fuel. Anaerobic digesters vary greatly in complexity, ranging from simple covered lagoons to batched digestors for municipal solid waste.

The main feedstock for anaerobic digestion is pig slurry, but cattle slurry can also be utilised. Anaerobic digestion is usually used to provide heat/electricity on a farm-scale rather than on a larger scale. In Europe, however, community-level scales have become increasingly popular, where farmers pool their manure resources into a central anaerobic digestion plant. In the Dutch province of Frysland, a farming cooperative is using anaerobic digestion to supply power to a community of 25,000 people (Gorter 2005).

Properties

The calorific value of biogas is 18-22 MJ/m ³ (Towers et al. 2004) with resulting methane concentrations and biogas yields varying according to the kind of slurry that is used. Pig slurries produce biogas with the highest methane content although chicken slurry gives highest yields of biogas.

Generic Advantages and Constraints

AD systems can bring environmental benefits related to recycling potentially polluting slurry/sludge. Application of manures on land could lead to significant ammonia emissions, for example, which could be mitigated through use of manures as fuel. They are, however, very expensive and labour-intensive to run and there are very few examples in the literature of plants that have proved economically sustainable.

2.2.2.4 Animal By-products

Background

Once an animal is slaughtered, meat and offal are removed, with the remaining material being referred to as 'animal by-products'. These by-products represent a substantial fraction of the body weight of slaughtered animals and are further processed at animal rendering plants. The melted animal fat from these operations is referred to as tallow, while the solid protein fraction is usually ground into a powder such as meat meal or meat and bone meal ( MBM). Both of these products from the rendering industry have potential as feedstocks for bioenergy production, with MBM currently used for electricity production and tallow for both electricity and biodiesel production. In the UK, tallow is already being used on a commercial scale to produce biodiesel at the Argent plant in Motherwell (see section 3.2.4.1).

The MBM industry suffered greatly following the BSE crisis and currently a significant proportion of MBM in the UK is destroyed (Towers et al. 2004). The disposal of MBM is a costly process and the opportunity to generate a new market for this material through energy would provide the industry with a much-needed boost. The EU animal by-product regulation 1774/2002 splits animal raw material into three groups: 1) high risk material, including transmissible spongiform encephalopathy suspects, 2) medium risk material presenting a risk related to disease or residues of veterinary drugs 3) low risk material derived from animal parts fit for human consumption. Risk material is used as process heat in rendering plants, with the material of interest for biodiesel production being low-grade, non-risk material (Hamelinck et al. 2004).

The use of tallow for fuel in the United Kingdom is now a subject of strong debate, but current UK guidance is that combustion of tallow does come under the Waste Incineration Directive (Defra 2005). This differs from the situation in 22 of the 25 EU countries, which have decided against applying the WID to tallow as this would incur considerable economic and environmental impacts, with plants having to adopt fossil fuels to replace the tallow they would usually burn ( PDM Group 2005). The EC has commissioned a study to determine how member states apply WID to tallow combustion and investigate whether tallow should be taken out of the scope of the WID, the results of which will be published in 2006/7 ( UK Parliament 2006).

Properties

Meat and bone meal has a gross calorific value of 18.6 GJ t ^-1 ( DTI 2004), being similar therefore to that of other dry biomass feedstocks while tallow has a calorific value just over 90% that of fuel oil. There are slight technical issues that need to be addressed for tallow use in biodiesel production, relating primarily to its high viscosity. This usually requires ensuring that tallow is kept at high temperatures so as to not interfere with plant performance.

Generic Advantages and Constraints

In the UK, MBM and low-grade tallow are readily available at negligible costs, as they have few competing industries, although MBM has recently begun to be used in the cement industry (see Chapter 3). Besides the possible classification as a waste discussed above, the issue of public perception may be a constraining factor in the use of tallow, as animal by-products are still associated with BSE and their utilisation for energy seen as risky (Towers et al. 2004).

2.2.3 Purpose-Grown Energy Crops

2.2.3.1 Short Rotation Coppice ( SRC)

Background

Short-rotation coppice ( SRC) is viewed as one of the most promising energy crops to meet renewable energy targets in Europe. Coppicing is an old silvicultural management system where trees are planted close together and cut back after one year to encourage the growth of multiple stems from the base. Most of the coppice grown so far in the UK has been willow ( Salix spp.), but there has been an increasing amount of interest in growing poplar ( Populus spp.) for energy as well. The stems are cut every 2-4 years, depending on the growth rate of the site and the managed woodland is cut on a rotational basis, guaranteeing a steady supply of coppice. Willow has an average harvesting cycle of about 4 years and an average willow coppice can be harvested for up to 20 years. There are two main methods for harvesting SRC willow: direct cut and harvesting cut. In the former method, coppice is cut and chipped in one operation while in the latter, coppice is cut and transported and stored for subsequent chipping.

Like other biomass feedstocks, SRC can be used to produce heat and/or power at multiple scales. Sweden has made the most noticeable advances in growing willow as an energy crop, with over 15000 hectares planted (Verwijst 2005). In Sweden, harvesting is done during the winter when the ground freezes over using conventional heavy machinery. In Scotland, however, winter frosting is not common and lighter machinery is necessary as use of the heavy machinery can lead to soil degradation. Poplar has less frequently been trialled than willow and is more difficult to grow since it is not easily propagated from cuttings ( RCEP 2004).

Fuel Properties

SRC willow has a similar calorific value to other wood fuels, but may contain more bark and water at the time of harvesting, with a typical moisture content of 40-50% (Towers et al. 2004). Ash, sulphur and chlorine content are similar to other woody fuels.

Generic Advantages and Constraints

Willow grows well on most soils except those prone to drought risk and can therefore be used on marginal agricultural lands, where other arable crops are less successful. Nevertheless, there is likely to be a yield penalty associated with marginal sites. Due to its mosaic structure, SRC willow can also improve the general biodiversity of the landscape (see Chapter 5). As with most energy crops, however, there is an element of risk involved for the farmer, who must wait four years before the first harvest. SRC crops, especially poplar, can soak up much groundwater due to their deep woody roots, and these roots may make it more difficult for land to be returned to agricultural use. Due to the machinery used for harvesting, the minimum viable area tends to be greater than 10ha, which can limit farmers' options (Forestry Commission Scotland, personal communication). The regular coppicing required may also increase soil disturbance and susceptibility to erosion.

2.2.3.2 Short Rotation Forestry ( SRF)

Background

Short rotation single-stem forests, grown specifically for energy, represent another possible biomass feedstock for the future. Unlike willow and poplar, there is no coppicing involved and rotation periods are longer, meaning that these are treated more like conventional forests than agricultural crops. Rotation periods are still shorter (8-20 years), however, than those of conventional forest species. Ash ( Fraxinus spp.) is a prime European candidate for SRF, given its fast incremental growth, but alder ( Alnus spp.) has more potential for poorer, wetter sites. Exotic species such as eucalyptus, can yield 10-15 t ha ^-1, whereas the average yield for native species is 2-6 t ha ^-1 (Towers et al. 2005).

Short rotation forestry has not received as much interest as short-rotation coppice, possibly as a result of lower yields and much longer harvesting cycles, although interest is increasing (Scottish Parliament 2006, Hardcastle 2006). It is possible to grow mixed species stands where fast growing species, such as birch and alder, are grown in a matrix with conventional forestry species such as spruce or pine. This practise, common in Scandinavia, can provide nursery benefits besides energy benefits (Forestry Commission Scotland, personal communication). Short-rotation forestry still requires more research into optimal silvicultural treatments for yield information, but should be easily adaptable to already-existing technologies for processing SRC and forest by-products.

Properties

Calorific value, moisture content and chemical composition vary according to species, but are generally similar to those of short-rotation willow and other woody fuels.

Generic Advantages and Constraints

Planting short rotation forests could lead to several environmental benefits, such as improved hydrological cycling and carbon storage, but these are likely to be overshadowed by the difficult economics of SRF, where there could be no financial returns for up to 15 years (Towers et al. 2005). In relation to SRC, SRF will also result in reduced soil disturbance. Unlike SRC, however, most SRF species require productive sites and are difficult to grow on marginal agricultural land, thereby limiting their usefulness to sites for which there is already competition from other crops with faster returns. Some species, such as birch and alder, grow more easily on marginal land, meaning that they could be grown without major competition with other land uses. Furthermore, short rotation forestry may present the opportunity to bring underutilised farm woodland into economic activity.

2.2.3.3 Energy Grasses

Background

Giant perennial grasses have attracted much interest as bioenergy feedstocks due to their fast growth and high yields. Miscanthus giganteus, a member of genus of 20 grasses in Asia and Africa, is the species whose potential has been most closely studied, with several trials having been undertaken across the UK to ascertain yield information under different climatic constraints. Other grasses that have received attention as candidates for energy production are switchgrass ( Panicum virgatum), reed canary grass ( Phalaris arundinacea) and Spartina. Miscanthus and switchgrass are C4 grasses, which use a very efficient photosynthetic pathway that allows higher yields under optimal conditions compared to C3 plants.

Although there have been many field trials with Miscanthus across Europe, commercial planting of the crop has yet to occur on a large scale. In Sweden, however, large-scale planting of reed canary grass has begun and there are currently about 5000 ha dedicated to the crop (Verwijst 2005). Results so far seem to indicate that reed canary grass has a higher tolerance for lower temperatures than miscanthus, making this grass a more suitable candidate for northern European countries. Other advances in the utilisation of energy grasses include the current development of miscanthus pellets by Wood Energy Ltd. ( RCEP 2004).

Properties

The energy density of Miscanthus is 18.2 GJ odt ^-1, with the energy density of other energy grasses being very similar (Christian and Riche 1999). Miscanthus has lower ash and silica content than cereal straw, thereby presenting lower risk to combustion equipment, but the ash content of reed canary grass is very similar to that of straw. Moisture content varies largely with geographical location, tending to be higher in northern latitudes. Field trials in Aberdeen revealed an average moisture content of 33% over the first five years of establishment (cited in Towers et al. 2004). Energy grasses may yield additional environmental benefits beyond contributing to emission reductions. There is evidence, for example, that miscanthus is associated with nitrogen fixing bacteria, possibly reducing the need for nitrate fertiliser use (Eckert et al. 2005).

Generic Advantages and Constraints

As miscanthus is harvested each winter, it allows farmers to spread their workload evenly throughout the year (Towers et al. 2004). Energy grasses also have an advantage over forestry species in that it is easier to return the land to arable production if the farm decides to do so. One major drawback is that miscanthus can only establish by rhizomes, which may be more costly to establish than crops grown from seed, though potato planters have been used to establish the rhizomes more cheaply (Defra 2004). Miscanthus has generally shown higher yields than other energy grasses in field trials across the UK, but most of these have been in England. Reed canary grass generally results in lower yields than Miscanthus, but appears to be more suitable to harsher climatic conditions. Reed canary grass possesses the additional advantage over Miscanthus that it can be established from seed, as can switchgrass. Another drawback at present is that energy crops are still at an early stage of introduction in Europe and there are, as yet, no alternative market for them beyond the energy market, leaving farmers subject to energy market fluctuations. There is some evidence that C4 plants such as miscanthus may be marginal crops in most of Scotland due to climatic conditions.

2.2.4 Liquid Biofuel Feedstocks

Liquid biofuels could significantly reduce reliance on fossil fuels for transport and in so doing could provide substantial cuts in greenhouse gas emissions. There are two main transport biofuels under production currently: biodiesel and bioethanol. Biodiesel can be blended with diesel or be used unmodified in diesel engines while bioethanol is used a petrol additive in blends of up to 20% for use in petrol automotive engines. Use of 100% bioethanol requires engine modifications but was common practice in Brazil, where there was extensive production of sugar cane for fuel use until Brazil's petrochemical industry displaced bioethanol as the primary transport fuel (Sims et al. 2006).

Biodiesel is produced by transesterification of vegetable oils and animal fats, while bioethanol is produced by fermentation of sugars and starches. Both of these processes will be discussed in more detail in sections 2.3.3.1 and 2.3.3.2 respectively. The main feedstock for biodiesel production in Europe is oilseed rape, with lesser amounts being produced from used vegetable oils, tallow and imported oilseeds such as palm kernel and soybean. Sugar crops and starches serve as the main substrates for bioethanol production, with sugar beet and wheat being the primary crops used for this purpose in Europe. Bioethanol can be made from other starches and cereals, such as potatoes and barley, but the amount of bioethanol produced from weight equivalents of these is much lower.

With the Renewable Obligation on Transport Fuels obliging fuel suppliers to deliver 5% renewable transport fuels by 2010, increased production of biodiesel and bioethanol is necessary. Already some European countries have made notable advances in biofuel production in the last decade or so. In Germany, for example, the installed biodiesel production capacity in 2003 was almost 1,000,000 tonnes yr ^-1 (Bockey 2005). Liquid biofuel production also leads to the formation of several co-products that can supply various markets. Biodiesel production from rapeseed, for example, is accompanied by the production of rapeseed cake, used a cattle feed for its high protein content.

Properties

Table 2.3 summarises the properties of biodiesel, mineral diesel, bioethanol and petroleum. Biodiesel has an energy content of 37.2 GJ t ^-1, about 10% less than that of fossil-fuel derived diesel at 42.9 GJ t ^-1. Unlike diesel, biodiesel does not contain sulphur, although it has 10-11% oxygen, completely absent in regular diesel. Several comparisons have been made between the efficiency of biodiesel and diesel, but these are influenced largely by engine test cycle, engine design and biodiesel quality. The properties of the biodiesel produced are directly dependent on the feedstocks used for its production. Biodiesel derived from oil palm, for example, has a much lower freezing point than that derived from rapeseed and is therefore less suitable for utilisation in colder climates.

Bioethanol has a higher octane content than petroleum (109 for bioethanol and 97 for petroleum), a characteristic which increases engine efficiency. The Reid Vapour pressure, a measure of the volatility of a fuel, is considerably lower for bioethanol than for petrol (16.5 k PA vs 76 k PA). This can be a disadvantage when used in low temperature conditions, resulting in poor engine starts at temperatures below 20°C for cars running on pure bioethanol (Van Thuijl et al. 2003). Furthermore, the calorific value of bioethanol is only about 2/3 that of petrol (26.4 vs 41.3 GJ t ^-1).

Table 2.3: Comparison of Fuel Properties of Transport Biofuels with Fossil Diesel and Petrol

Source: van Thuijl et al. (2003)

Generic Advantages and Constraints

As well as being carbon neutral and thereby reducing CO₂ emissions by displacing fossil fuels, liquid biofuels may have associated positive impacts on environmental quality such as reduced emissions of selected air pollutants (see Chapter 5). The cereal and oilseed crops used to produce biodiesel can be cultivated on marginal agricultural lands and are crops with a long history of cultivation and management in Europe that have well-established supply chains. On the other hand, both bioethanol and biodiesel may require considerably more energy to produce than their fossil fuel equivalents, although the particularities of the production process are very important in determining this (see Chapter 4). These energy costs are reflected in the price of liquid biofuels in the UK, with production costs of biodiesel and bioethanol in the UK being approximately twice those of diesel and bioethanol respectively (see Chapter 6).

2.3 Biomass End-Uses and Conversion Technologies

2.3.1 Background

It is important to understand the variety of options available for producing power, heat/ CHP and transport fuels from biomass, as the technology chosen will have implications on costs, carbon mitigation potential, air quality and other environmental and economic impacts. It is beyond the scope of this report, however, to provide detailed technical descriptions of each technology and the many variants that may exist therein. Rather, this section aims to provide an introduction to conversion technologies in general that will be referred to in subsequent chapters. A more complete analysis of technical systems for generation of power/heat from biomass can be found in Howes et al. (2002), while a thorough review of transport biofuel production systems can be found in van Thuijl et al. (2003).

2.3.2 Heat

Biomass can be used to generate heat at various scales, ranging from domestic to industrial heating systems. Countries such as Sweden, Germany and Austria have been making use of fully-automated heating systems for over 30 years, although the first demonstrations of these in the UK have only come into being over the last 5-10 years. Biomass heating systems have generally relied on wood and woody residues, but as noted in the previous section, just about every biomass source can be used to produce heat.

A recent report for the DTI by Future Energy Solutions (2005) highlighted some of the major differences between heat supply and electricity generation that should be considered when evaluating the effectiveness of biomass options for heat production. These are:

• Plant location: heat needs to be produced at the point of heat use, while electricity does not
• Efficiency: the conversion of biomass to heat is very efficient, with most of the energy being converted to heat (up to 90% efficiency) while electricity production is a very inefficient process (20-30%), even with the most advanced combustion technologies available
• Cost of fuel conversion: cost of heat production is low, while electricity conversion is expensive as the fuel must be converted initially to a gas or to steam before electricity can be generated. This results in power-generating equipment being more capital-intensive than heat boilers.
• Product market: heat is a relatively low value product while electricity is relatively high value

The technology for biomass conversion to heat is well-established. Typically, a heat generation plant consists of a combustion chamber where the fuel is mixed with hot air, a heat exchanger where the hot gases from combustion transfer heat to another liquid (usually water) and a gas cleaning unit where cooled combustion gases are treated to remove pollutants before being emitted back to the atmosphere. The heated water or steam is pumped from the heat exchanger for distribution to its endpoint. The mainly mineral part of the fuel which is not combusted forms the residual bottom ash in the combustor, while finer particles are collected in the fly ash after gas cleaning.

2.3.3 Electricity

Electricity can be produced from biomass through three primary conversion routes: direct combustion, gasification and pyrolysis. Each of these systems is essentially a variation of a common process: the energy contained in biomass is released by oxidation to produce heat, which then drives a turbine/engine coupled to a generator or drives a generator through steam production to produce electricity. The differences between the systems result almost entirely from differences in the oxidation step, specifically in the amount of the oxidant, normally air, that is applied.

Under high temperatures, biomass decomposes into volatile and char fractions. In direct combustion systems, an excess of air is supplied so that both volatile and char components burn completely. In other words, the full energy value of the fuel is released into the reactor. Pyrolysis and gasification systems lead to the production of a fuel intermediate which can be gaseous or liquefied and, crucially, allows for a greater control over the combustion process. In gasification systems, a limited amount of oxidant is supplied so that oxidation and reduction reactions occur in the same reactor. The outcome is the production of a hot combustible gas, composed mainly of carbon monoxide and methane, which can then be burned to generate power. Pyrolysis differs from direct combustion and gasification systems in that biomass is heated in the absence of oxygen to drive off volatile gases, leaving a carbon-rich tar that can either be burned or gasified. Depending on the heating rate applied, pyrolysis can lead to the formation of either liquid or gas intermediates that can then be burned to generate electricity.

Bioelectricity production plants typically consist of separate chambers with distinct functions. The combustion chamber is the location of fuel oxidation, where direct combustion occurs or where the fuel intermediates are formed, in the case of pyrolysis and gasification. Prior to entry into the combustion chamber, however, biomass fuels often need to undergo a pre-processing step to ensure optimal oxidation. Normally this would consist of drying the fuel to reduce moisture content. Following combustion, the hot gases produced then enter a cleaning chamber, where pollutant particles are removed for compliance with legislation on emission regulation.

The main advantage of gasification and pyrolysis in electricity production over direct combustion is undoubtedly the efficiency of these systems. The steam cycle used to generate electricity in direct combustion systems operates at an efficiency of typically no more than 30%, being heavily constrained by temperatures in the steam cycle, especially the temperature at which steam enters the turbine (Howes et al. 2002). The fuel intermediates formed during pyrolysis and gasification allow for the use of gas turbine cycles with a far greater efficiency than conventional steam cycles, with the most advanced gasification systems estimated to have an overall efficiency in excess of 40%. These systems are likely to play an important role in future large-scale bioenergy production schemes, but this is not to say that direct combustion systems are without any benefits. On the contrary, while gasification and pyrolysis technologies are still at a very early stage of development, direct combustion is a proven technology with a long track record. Compared to its more modern counterparts, direct combustion systems are much cheaper and require less technical knowledge to operate.

A Note on Co-firing

Many biomass options can be co-fired with coal to produce electricity. Wood chips, wood waste, sewage sludge, palm kernels, straw and peat are some of the biomass sources being co-fired worldwide in a range of blend proportions on either a trial or commercial scale. A list of power plants all over the world currently co-firing biomass can be found on the Internet at the IEA Bioenergy Task 32 website ( http://www.ieabcc.nl/database/cofiring.php).

Before biomass can be fired with coal, several pre-processing steps are required, normally washing or cleaning to remove tramp material, drying and comminution (breaking down into smaller fragments) in the case of wood residues ( DTI 2005). Co-firing can have multiple benefits, viz:

• It makes use of currently existing plants, therefore not requiring infrastructure changes;
• It allows for immediate reductions in greenhouse gas emissions;
• It can utilise readily available 'waste' products;
• It makes use of the high steam parameters and technical efficiency improvement measures available in coal-powered plants, which are considerably than more efficient than most biomass only conversion plants currently available (Baxter and Koppejan 2004).

Co-firing benefits do not compare with the environmental benefits that would be achieved by complete substitution of coal with biomass but cofiring serves as an outlet for biomass feedstocks using the current power generation infrastructure. Co-firing encourages the formation of biomass supply chains to be used in increasing proportions in the future. Policy until 2015 explicitly encourages the use of purpose-grown energy crops (over other sources of biomass fuels) as the biomass component of co-firing.

2.3.4 Combined Heat and Power ( CHP)

Combined heat and power ( CHP) systems are used to produce both heat and electricity and consist of the basic heat unit described above coupled to an electrical generator that is driven by combustion gases or another working fluid. The hot gases from the combustion chamber enter a boiler where steam is generated that can then be used to turn a turbine coupled to a generator. Although the overall efficiency of biomass-operated CHP units can be quite high (up to 80%), the efficiency of the electrical component is very low (~10%), especially in small-scale units. For higher efficiency, the steam cycle can be replaced with a gas turbine or gas engine, where the fuel is gasified or pyrolysed rather than burned, but the overall efficiency and benefits to the user vary greatly among individual applications. In small scale CHP, the generator is usually not driven by a turbine, but by a reciprocating gas engine, such as a modified diesel engine ( RCEP 2004).

Biomass CHP has made significant breakthroughs in many Scandinavian countries and there are currently a number of schemes in development in the UK (see Chapter 3 for details of schemes under development in Scotland). For CHP projects to be viable, there needs to be a market for the heat output, therefore most plants are located in factories or buildings that utilize the heat produced or are located next to a heat distribution network ( RCEP 2004).

2.3.5 Transport Biofuels

Transport is not only one of the most energy-demanding sectors, it is also the most heavily dependent on oil and its derivatives. Hence, the utilisation of renewable transport biofuels will increase energy security by decreasing reliance on oil, as well as reducing greenhouse gas emissions. This section details the production of biodiesel and bioethanol, the two main currently available transport biofuels, and also the use of straight vegetable oil as a transport fuel.

Biodiesel

As mentioned in the previous section on biomass feedstocks, biodiesel can be produced from a range of different oilseeds and fats. In the UK, the most relevant feedstock is oilseed rape (section 2.2.4.4), with used cooking oil (section 2.2.4.4), and tallow from the animal rendering industry (section 2.2.3.3) also currently being used. Internationally, biodiesel is also being produced in large scales from palm oil and soybean oil.

The process of producing biodiesel from oilseed rape can be divided into three main parts: crushing, refining and esterification. Crushing extracts oil from the rapeseed, a process that involves the following steps: seed cleaning, tempering (pre-heating), dehulling (seed coat removal), flaking (increase in surface area to facilitate extraction), conditioning (heating of flaked seed) and extraction (Booth et al. 2005). The actual extraction step can be performed mechanically or through the application of a solvent. Solvent extraction systems utilize equipment that is much more expensive than mechanical crushing equipment and are normally restricted to large-scale production systems. The extraction efficiency of solvent-based systems is, however, significantly greater than that of mechanical crushing systems. Before the extracted oil can be converted to biodiesel, some further refining steps are necessary. These include degumming and neutralization. Degumming is the removal of phosphorous-based gums that can lead to engine problems while neutralisation is the removal of free fatty acids which can disrupt the functioning of the catalyst during biodiesel conversion. Further refining steps can include bleaching to improve oil colour and deodorising to improve odour, but these are largely unnecessary for biodiesel production. The esterification step ultimately converts the pre-processed rapeseed oil into rape methyl ester by mixing the oil with methanol in the presence of a catalyst, usually potassium hydroxide. For other biodiesel feedstocks, the mode of production is essentially the same as with rapeseed oil. Tallow and used vegetable oil have a greater concentration of free fatty acids than rapeseed oil. These can lead to engine problems and requires a second esterification reaction to overcome (Woods and Bauen 2003).

Bioethanol

As noted in section 2.2.4.4, bioethanol can be produced from a range of different materials including sugar beet, cereals such as wheat and barley and starchy crops such as potatoes. The process is relatively simple, consisting essentially of yeast fermentation of polymers or monomers of six-carbon sugars, producing ethanol and carbon dioxide as a by-product. Starches require an additional hydrolysis step, which can be acid or enzyme-mediated, before fermentation. The technology for conventional bioethanol production systems from sugars and starches is mature, but new modes of ethanol production are being trialled for the conversion of lignocellulosics to bioethanol (Kampman et al. 2005). Production of ethanol from lignocelulosics is technically more difficult than conventional production routes as woody materials are a complex mixture of glycopolymers that need to be broken down to release the simple sugars that will enable fermentation to proceed. Lignocellulose is comprised of cellulose, hemicellulose and lignin, each one of which requires a different hydrolysis and fermentation technology (Woods and Bauen 2003).

Pure vegetable oil

Some engines, such as Volvo and Mercedes, can run on pure vegetable oil without modification (Bio Power News 2004). The high viscosity of the oil does mean, however, that it cannot be used unmodified in many engine types without risks of engine malfunction. The process of producing pure vegetable oils is the same as that of biodiesel production, but without the esterification step (section 2.3.3.1). Commonly used vegetable oils include rapeseed oil, sunflower oil and palm oil.

2.4 Future Options

2.4.1 Biomass to Liquid ( BTL) Fuels

While bioethanol and biodiesel options currently represent the main means of reducing greenhouse gas emissions in transport sector in the near future, new technologies are being developed that are capable of converting biomass into a range of different products, including transport fuels. These technologies are gasification-based and utilise biomass to produce a product gas known as syngas (synthetic gas), consisting primarily of carbon monoxide and drogen. Rather than using the syngas to directly produce heat and electricity, as gasification systems designed for the power industry do, gas-to-liquid ( GTL) systems use syngas as a feedstock from which a variety of different products can be derived. Hence, GTL plants operate in a manner analogous to petroleum refineries and are sometimes described as Biorefineries (Sims et al., 2006). The fuels that can be produced in this manner are known as biomass to liquid ( BTL) fuels and include F-T naptha, FT-diesel, methanol, dimethyl ether ( DME) and hydrogen. These products can be used to supply diesel engines, petrol engines and/or fuel cells. At present, there are major BTL plant demonstrations underway in Germany, Sweden and Austria (Kavalov and Peteves 2005).

2.4.2 Biomaterials

There has been a steadily increasing interest in the extraction of high value products from biomass before its use for energy (Sims et al. 2006). A range of different products can be derived from biomass including paints, lubricants, biodegradable plastics and adhesives. If such processes develop into large-scale commercial operations, the economics of bioenergy production could potentially become much more viable. This option will not be discussed further in this report, but is mentioned here to draw a more holistic picture of biomass utilisation beyond energy production.

2.5 Summary of Biomass Utilisation Pathways

Figure 2.1 Summarises the main bioenergy production systems described in this chapter in terms of feedstock group, conversion technology and end product.

Figure 2.1: Summary of Common Bioenergy Pathways (Only currently commercially viable technologies are included.)

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