Volume 5 biomass and biofuel production 5 11 – biomass to liquids technology

Volume 5 biomass and biofuel production 5 11 – biomass to liquids technology Volume 5 biomass and biofuel production 5 11 – biomass to liquids technology Volume 5 biomass and biofuel production 5 11 – biomass to liquids technology Volume 5 biomass and biofuel production 5 11 – biomass to liquids technology Volume 5 biomass and biofuel production 5 11 – biomass to liquids technology Volume 5 biomass and biofuel production 5 11 – biomass to liquids technology Volume 5 biomass and biofuel production 5 11 – biomass to liquids technology

5.11 Biomass to Liquids Technology G Evans and C Smith, NNFCC, Biocentre, Innovation Way, Heslington, York, UK © 2012 Elsevier Ltd All rights reserved 5.11.1 5.11.1.1 5.11.1.1.1 5.11.1.1.2 5.11.2 5.11.3 5.11.4 5.11.4.1 5.11.4.1.1 5.11.4.1.2 5.11.4.1.3 5.11.4.2 5.11.4.2.1 5.11.4.2.2 5.11.4.2.3 5.11.4.2.4 5.11.4.2.5 5.11.4.3 5.11.4.3.1 5.11.4.4 5.11.4.5 5.11.4.5.1 5.11.4.5.2 5.11.4.5.3 5.11.5 5.11.5.1 5.11.5.2 5.11.5.3 5.11.5.4 5.11.5.5 5.11.5.6 5.11.5.6.1 5.11.5.6.2 5.11.5.7 5.11.6 5.11.7 5.11.7.1 5.11.7.2 5.11.7.3 5.11.7.4 5.11.7.5 5.11.7.6 5.11.7.7 5.11.7.8 5.11.8 5.11.8.1 5.11.8.2 5.11.8.3 5.11.8.4 5.11.9 5.11.9.1 Introduction Biofuels Drivers and Issues Drivers Issues The BtL Process A Brief History of FT Steps in Biomass Conversion to Liquids via FT Feedstock Preparation and Pretreatment Size reduction (comminution) Drying Feedstock pretreatment Gasifiers for FT Entrained flow gasifiers Fluidized bed gasifiers Plasma gasifiers Scales of operation Conclusion Syngas Cleanup for FT Syngas contaminants Syngas Conditioning for FT FT Process FT catalysts Reactors for FT Summary Alternative BtL Fuel Options Methanol Dimethyl Ether Gasoline Mixed Alcohols (via Catalysts) Alcohols (via Fermentation) Hydrogen and BioSNG Hydrogen BioSNG Summary Timescales and Development of BtL Processes BtL Implementation Progress BioMCN Enerkem Choren Range Fuels INEOS Bio NSE Biofuels Chemrec Summary Outline of BtL Economics Capital Costs Feedstock Cost Analysis Economies of Scale Summary Environmental Issues GHG Savings Comprehensive Renewable Energy, Volume doi:10.1016/B978-0-08-087872-0.00515-1 156 157 157 158 159 160 162 162 162 162 163 168 169 170 171 172 172 172 172 177 177 178 180 183 183 183 184 185 185 186 186 186 186 187 187 188 189 190 190 190 190 191 191 191 191 191 194 196 196 197 197 155 156 Technology Solutions – New Processes 5.11.9.2 Comparative Technology GHG Saving Effectiveness 5.11.9.3 Comparative Land Use Effectiveness 5.11.9.4 Summary 5.11.10 Summary and Outlook References Further Reading Glossary Advanced biofuel A biofuel that is produced by a novel method and/or that gives a better product than current biofuels Advanced pretreatment In the context of thermochemical conversion of biomass, this refers to torrefaction, pyrolysis, and pelletizing, which convert biomass to a more dense energy carrier and can result in the production of a feedstock that is more amenable for use in gasification systems Biomass to liquids An advanced process to produce biofuels, in which biomass is converted to liquid fuels or chemicals via syngas, including dimethyl ether, methanol, ethanol, synthetic natural gas, and synthetic diesel BioSNG (biosynthetic natural gas) A methane-rich gas derived from gasification of biomass materials and that has been upgraded to a quality similar to that of natural gas Fischer–Tropsch A high-temperature and high-pressure catalytic process that converts syngas into heavy hydrocarbon wax This can be subsequently converted into finished fuels, typically by hydrocracking (a standard refinery procedure) The Fischer–Tropsch process can 200 200 201 203 204 204 produce a high-quality diesel and/or other valuable hydrocarbon products including kerosene and naphtha Gasification Production of a combustible gas via the partial combustion of a feedstock Pyrolysis Transformation of a substance by the action of heat, in the absence of an oxidant (e.g., air, oxygen) This procedure always yields solid, liquid, and gaseous products, the yields of which depend on the conditions (e.g., heating rate, temperature, pressure) applied to the feedstock It can be used as a densification strategy to allow biomass to be transported to a refinery, or, if upgraded, as a drop-in (or direct replacement) fuel Syndiesel (synthetically manufactured diesel) It can be derived from biomass via the biomass-to-liquids process using the Fischer–Tropsch process, and is also known as FT diesel Syngas A designed mixture of carbon dioxide and hydrogen produced by gasification, also known as synthesis gas Torrefaction A low-temperature slow pyrolysis carried out at around 300 °C that completely dries and devolatilizes biomass to give a dry and friable form of biomass 5.11.1 Introduction Increasing concerns over the environmental impact of using fossil fuels, security of supply, and price volatility have led to a growing interest in the production of alternative fuels from renewable resources Greenhouse gas (GHG) emissions from the transport fuel sector are a particular concern because currently they account for a quarter of annual worldwide carbon dioxide (CO2) emissions and are growing, while emissions from other sectors are declining or stable Biofuels are the only near-term option to decarbonize the transport system, particularly for the existing car fleet, yet concerns have been raised over the sustainability of current ‘first­ generation’ biofuels made from oilseeds, starch, and sugar crops Advanced biofuels, which are produced from biomass using a novel method of processing or which offer a superior product compared to existing biofuel routes, can be produced from a range of lignocellulosic materials including wood, agricultural residues, and the organic fractions of waste streams such as municipal solid wastes (MSWs) Compared with the feedstocks used to produce first-generation biofuels, these lignocellulosic feedstocks are typically cheaper and less subject to price fluctuations, have the potential to avoid competition with food resources, and yield greater GHG savings For these reasons, legislation in many countries is increasingly favoring the production and utilization of advanced biofuels from lignocellulosic biomass and wastes Advanced processes for producing fuels from biomass can be broadly split into either biological (e.g., lignocellulosic ethanol production, anaerobic digestion) or thermochemical routes (e.g., pyrolysis, gasification) The various routes for the production of biofuels from biomass to end product are summarized in Figure Thermochemical routes have a number of advantages over biochemical processes, in particular the ability to produce diesel substitutes and jet fuel Furthermore, thermochemical methods have a greater flexibility with regard to feedstocks compared to biochemical methods, which is of significant advantage in terms of supply, and can convert all organic compounds in the biomass feedstock Finally, thermochemical processes can produce a range of end products This is a key advantage allowing a plant to tailor end products to demand, known as the biorefinery concept These products can be exactly the same as are used today Biomass to Liquids Technology 157 Pure plant oil FAME/FAEE Hydrogenated vegatable oil Vegetable oil +m icr oo rg an ism s Oil seeds/cleaned waste oil, algae Synthetic diesel Diesel engine Methane Butanol Furanics Cereals Ethanol Sugarcane and beet Lignocellulose including bio­ derived wastes Sugars Syngas ETBE Upgraded pyrolysis oil Petrol engine DME Pyrolysis oil MTBE Methanol Torrefied biomass Pelletized biomass Fuel cells Hydrogen Figure Potential biofuel pathways Light blue lines indicate existing pathways for the production of biofuels; dark blue lines indicate alternative pathways to current biofuel products that may become commercially feasible in the near to mid-term (in particular before 2020); and dashed lines indicate pathways to biofuel products not currently in use but potentially feasible in the near to mid-term Redrawn with permission from LowCVP and NNFCC (2010) Biofuels Pathways LowCVP Report 5.11.1.1 Biofuels Drivers and Issues The biofuels industry is a relatively new industry that shows considerable potential for the future in terms of meeting various policy objectives and technology requirements Uptake is being driven by a number of drivers as shown in Figure However, there are a parallel range of issues that need to be addressed to ensure that the emerging biofuels industry builds into a robust and sustainable industry 5.11.1.1.1 Drivers Biofuels have the potential to address a wide range of environmental, socio-economic, political, and technological drivers The relative importance of each of these factors varies according to geographical location, and can change with time Drivers Climate change Issues Customer acceptance Energy security Environmental considerations Rural economy Feedstock sourcing Technology developments Supply and logistics Product specifications Government policies Pricing competitiveness Engine compatibility Figure Biofuels drivers and issues Red shading indicates drivers and issues of very high importance, orange indicates high importance, purple indicates medium importance, and white indicates low importance Redrawn from Nexant Inc (2007) Feasibility of Second Generation Biodiesel Production in the United Kingdom NNFCC Report 158 Technology Solutions – New Processes The principal strategic driver for the adoption of biofuels in the European Union (EU) is to mitigate climate change Given that transport accounts for almost a quarter of worldwide GHG emissions, and since these emissions are rising, this is a key sector for any climate change policy to focus on Most biofuels have been shown to provide GHG savings, but these are highly dependent on a range of factors including the crop used, processing methodology, and the parameters used in calculation An increasing focus is on quantifying the GHG emissions associated with biofuels production and this is becoming an increasingly important part of complying with biofuel incorporation targets The second most significant driver for biofuels uptake in the EU is energy security – in the United States and China, this is the primary driver for biofuels use The EU typically does not produce enough mid-distillate fuels and an opportunity exists for biomass-to-liquids (BtL) technologies to produce additional quantities of road diesel and jet fuel to meet increasing demands for these products Improving competitiveness and rural economics is the third driver for developing biofuels in the EU The production of biofuels requires large amounts of biomass and this provides a way to diversify and rejuvenate the rural sector by providing an alternative income stream for farmers and landowners The development of a biofuels industry also provides an opportunity to drive increased R&D in a wide range of sectors and to expand industrial infrastructure In addition to these well-known drivers, new technology developments and new fuel specifications also drive an increasing need for alternative fuels including biofuels For example, Ricardo and Lotus are currently developing E85 (85% ethanol/15% petrol mix) engines that can take advantage of ethanol’s high octane level with the potential to produce vehicles that will give close to diesel economy when fueled with E85 Such a development could drive the increased use of E85 and hence the increased production of ethanol Engines that utilize synthetic fuels are being investigated as a potential avenue by Volkswagen (Fischer–Tropsch (FT) diesel-type fuels) and by Volvo Trucks (dimethyl ether (DME)) BtL fuels can also deliver benefits to refiners who will increasingly need to source fuels from crude oils that are more difficult to access and refine Synthetic fuels such as FT diesel can potentially help refiners to meet increasingly stringent fuel specifications such as on sulfur content In addition to maintaining the status quo, high-quality blendstocks such as BtL diesel with high cetane and low sulfur can be used to increase margins by allowing more lower grade diesel blending components to be incorporated into higher value diesel streams The day-to-day use of biofuels is generally driven by government policies In the United States, the Renewable Fuels Standard (RFS2) was established for this In the EU, the Renewable Energy Directive (RED) will require that 10% of the energy of transport fuels is provided from renewable sources by 2020 Increasingly, there are also added incentives such as capital cost loans, duty differentials, and/or tradable certificates, which promote the use of biofuels 5.11.1.1.2 Issues During the development of the biofuels industry in the 2000s, a number of issues were highlighted and continue to be addressed These include, for example, biofuels sustainability, engine compatibility, and economic competitiveness (Figure 2) While biofuels are now an integral and growing part of the fuel mix in many countries, many customers are not aware that they are using biofuels, albeit in low blends with fossil fuels Conservatism among consumers, who often stick to products they are familiar with, may hinder the future larger scale uptake of biofuels This is exacerbated by the often negative press coverage of biofuels with respect to perceived issues of environmental damage and those affecting the quality of biofuels In particular, there has been concern over formulation issues and the effect this could have on engine performance Sustainability concerns include concerns over potential direct and indirect effects of biofuel production These effects include possible impacts on food prices and deforestation A considerable desire is on the part of many biofuel producers is to ensure sustainable sourcing of biomass and the use of waste materials and residues so that undesirable impacts are avoided Thus, the prospects for advanced biofuels such as BtL are potentially beneficial, as BtL plants can utilize waste materials, residues and crops grown on marginal and degraded lands However, BtL plants require around tonnes of biomass for tonne of output and given the current expected size of commercial plants is around 200 000 tonnes per annum (tpa) of products, this brings with it considerable logistic issues to ensure economic and sustainable supply of biomass to a plant, especially given that in many countries, biomass supply chains are undeveloped Research is ongoing to address this issue; this research includes looking at smaller scales of operation and increasing conversion efficiency Biofuel production is a rapidly developing, but still largely immature sector, especially with regard to advanced biofuels Capital costs of advanced biofuel production are high, and provide a considerable barrier to investment, although this is, to a certain extent, offset by the potential to use lower cost feedstocks This is in contrast to fatty acid methyl ester (FAME) and first-generation ethanol plants where the capital cost of a plant may be lower, but this is offset by higher and more volatile feedstock pricing Finally, engine compatibility must be considered Introducing a new fuel into that sold at forecourts brings challenges within both the existing and the future vehicle fleet Ethanol, for example, acts as an oxygenate at low concentrations but at higher blends may begin to exhibit incompatibilities with some materials of construction in some vehicles Although many of the challenges with respect to ethanol utilization at low ethanol concentrations in petrol have been resolved, R&D continues to establish and address other effects, which may for example arise during combustion, particularly with respect to higher ethanol blends in petrol such as E10, E20, and so on Even where drop-in BtL fuels such as FT diesel are incorporated into the existing fuel infrastructure, they may still exhibit differences during use and these will need to be identified and addressed through R&D These differences could manifest themselves, for example, in atomization and in cold flow performance Biomass to Liquids Technology 159 5.11.2 The BtL Process BtL is a member of the XtL (X to liquids) family where X can represent C for coal, G for gas, or B for biomass It is one of a range of thermochemical technologies that can be used to produce valuable products from biomass These products include liquid transport fuels, methane, heat, electricity, chemicals, and materials, or a mixture of two or more of these as shown in Figure In general, however, BtL is typically focused toward liquid fuels production In particular, BtL is often associated with the production of synthetic diesel via the FT process In this chapter, BtL is defined as any process that utilizes biomass gasification to produce a syngas, which is subsequently converted into a fuel However, BtL will refer mainly to the FT example – alternatives will be discussed in Section 5.11.5 For each of the XtL processes, the first stage is to convert the feedstock into a clean syngas having a designed mixture of carbon monoxide and hydrogen; FT processes using a cobalt catalyst typically require a ratio of two moles of hydrogen to one mole of carbon monoxide (other downstream fuel synthesis processes will require different ratios) Once this has been achieved, the feedstock from which the syngas was derived effectively becomes irrelevant A key feature of this technology, therefore, is that whatever feedstock is used, the final fuel, for which a number of options are available, is the same Syngas is produced from biomass by gasification Gasification is a very flexible process with respect to feedstock and, in principle, many different carbon sources can be used to produce syngas These include natural gas, coal, wood, straw, and refuse The syngas is subsequently converted into usable fuel (or chemical) forms such as synthetic diesel or ethanol A key advantage of BtL is the potential for the end fuels to be used in a range of existing end-use sectors with little or no modification; this includes the marine, rail, and aviation sectors This range of products is a long-term advantage, especially for sectors where there is no ready drop-in alternative, such as in aviation FT is a technologically mature process for converting coal and natural gas into liquid fuels For coal, the process has been used since before the Second World War, primarily to provide a source of liquid fuels when crude oil was not available More recently, gas-to-liquids (GtL) technology has been developed to produce synthetic diesel from gas in large stranded gas fields For biomass, however, FT experience is limited with just a few projects currently moving from the pilot stage to the demonstration stage across the world Furnace/boiler Methane Engine/turbine Fuel cell Syngas Chemical synthesis Mixed alcohols synthesis Hydrogen Dimethyl ether (DME) MTO Methanol synthesis Formaldehyde Carbon monoxide Acetyls Ammonia Fertilizers Chemicals and materials Gasification Ethanol fermentaion Fuels n-Paraffins Direct combustion Power Diesel/jet fuel Heat Fischer tropsch Figure Derivative fuels, energy, and chemicals from biomass gasification Dotted lines represent alternative gasification pathways not discussed in this chapter Reproduced from NNFCC presentation (2010) Inputs, technology and biomethane utilization: An overview Biorenewable Fuel and Fertiliser: Realising the Potential, 24 March York: FERA http://www.soci.org/ 160 Technology Solutions – New Processes 5.11.3 A Brief History of FT The formation of hydrocarbons from carbon monoxide and hydrogen using transition metal catalysts has been known since the experiments of Sabatier and Sanderens in 1902 These experiments, at the beginning of the twentieth century, showed that methane could be produced from carbon monoxide and hydrogen using a nickel catalyst It was not until 1923, though, that the production of liquid hydrocarbons from coal feedstocks was demonstrated by Fischer and Tropsch (Figure 4) using iron, cobalt, and ruthenium catalysts at high pressure This was followed in 1936 by the development of a process that could be carried out at medium pressure by Fischer and Pilcher A timeline of FT development is presented in Figure The uptake of FT processes has historically been linked to the need to develop liquid fuels when access to domestic natural resources or imports has been limited such as in Germany during the 1940s and in South Africa during the 1950s, 1960s, and 1970s The first commercial FT plants were commissioned in the late 1930s in Germany, with a combined output of 660 000 tpa of FT products While these plants closed down after the Second World War due to the competition with crude oil, they still provide the first examples of commercial production of FT products via coal-to-liquids (CtL) approaches The modern development of FT technologies based on CtL has been pioneered by Sasol in particular The first CtL plant in South Africa was commissioned in 1955 in Sasolburg, South Africa (Sasol I) and had a capacity to produce million tpa FT products from coal, before it converted to the use of natural gas in 2004 Despite tough economic competition with the newly discovered oil reserves in the Middle East in the 1950s, the success of the Sasolburg operation was attributed to the production of several high-quality linear waxes and the development of an associated chemical processing infrastructure to utilize the FT products to produce value-added chemicals Both Germany and South Africa remain the leaders in FT technology today In recent decades, there has been a renaissance of interest in the FT process due to successive oil crises coupled to concerns over security of supply and price Both CtL and GtL projects have been developed in this period In 1980 and 1982, Sasol followed the Sasolburg plant with the commissioning of two plants at Secunda, known as Sasol II and III The Secunda plants have a total capacity of 150 000 barrels (bbl) day−1 (6 million tpa), and there are plans to expand this to 180 000 bbl day−1 (7.25 million tpa) by 2014 Sasol has plans for expansion of CtL to China, with plans for a plant to be commissioned in 2013 and potentially in Indonesia at 80 000 bbl day−1 (3.22 million tpa) The identification of ‘stranded’ gas reserves, which have little or no local use, promoted interest in GtL technologies in the 1970s and 1980s The Mossgas GtL project at Mossel Bay in South Africa (now operated by PetroSA) was commissioned in 1992, and was the first large-scale GtL plant to be commissioned The plant, which has an estimated capacity of 36 000 bbl day−1 (1.45 million tpa), converts natural gas via a high-temperature, iron catalyst FT process developed by Sasol to a wide range of FT products, including unleaded gasoline, ultra-low-sulfur diesel, kerosene, low middle distillates, liquefied petroleum gas (LPG), and waxes The Mossgas project was followed in 1993 by the commissioning of the Shell plant at Bintulu, Malaysia The Bintulu plant uses a low-temperature GtL process in conjunction with a proprietary cobalt catalyst to specifically target middle distillate products The capacity is 13 000 bbl day−1 (0.5 million tpa) Like the Mossgas plant, the Bintulu plant uses FT to produce a range of automotive fuels, specialty chemicals, and waxes, and is based on the use of offshore gas reserves The BP GtL demonstration plant at Nikiski, Alaska was commissioned in 2002, and has an output of 300 bbl day−1 (12 000 tpa) More recently, the Sasol Oryx project in Qatar was commissioned in 2006 at an estimated cost of around $1 billion (bn) The Oryx project has a capacity of 34 000 bbl day−1 (1.37 million tpa) and uses the Sasol slurry phase distillate process, based on low-temperature FT Both Sasol and Shell are now building upon their experiences and developing further GtL projects using FT technology The Pearl GtL project is a joint venture between Qatar Petroleum and Shell, which will convert natural gas, using the same process used in Bintulu, to produce 140 000 bbl day−1 (5.64 million tpa) liquid fuels and other products This is expected to be commissioned in 2010, with a total capital cost of $12–$18 bn In Nigeria, Sasol has formed a partnership with Chevron to develop a plant with a Figure Fischer and Tropsch Reproduced with kind permission from Max Planck Institute of Coal Research Biomass to Liquids Technology 161 1902: Methane formed over Ni from CO and H2 (Sabatier and Senderens) 1900 1923: Fischer & Tropsch report their work using CO, Fe, and Ru catalysts 1920 1936: First plants commissioned in Germany; 200 000 tpa total capacity 1940 1944: German capacity increased to 700 000 tpa from nine plants 1950: 5000 bbl day−1 Hydrocol plant in Texas operated briefly Syngas made from methane Mid-1950s: Interest in FT declining as use of crude oil increases except in South Africa where CtL development continues into early 1990s 1970s and 80s: Renewed interest in FT due to increasing oil prices and energy security 1980 CtL 1960 2006: Qatar Petroleum-Sasol start-up 34 000 bpl day−1 GtL plant in Qatar using Sasol FT technology 2009: Ch oren Beta 15 000 tpa BtL plant starts up Enerkem 000 tpa BtL plant for production of bioethanol starts up in Canada 2010: Shell Pearl GtL project (140 000 bbl day−1, Shell MDS FT technology) expected to start operating 2020 BtL 2000 GtL emerging 1990s: Further revival of FT (also known now as GtL) due to discoveries of huge ‘stranded’ gas fields and need for cleaner fuels 1992: Mossgas plant commissioned First large GtL plant using natural gas combined with Sasol FT technology 1992/93: Startup of Shell middle distilate 13 000 bbl day−1 GtL plant in Bintulu, Malaysia Numbers of BtL plants start up including: INEOS Bio (UK, ethanol), BA/Solena (UK, jet fuel), Range Fuels (USA, ethanol), Enerkem (Canada, ethanol), Chemrec (Sweden, DME), Neste Oil/Stora Enso (Finland, FT fuels), Xynergo (Norway, FT fuels), Choren (Germany, FT fuels), BioTFuel (France, FT fuels) Figure Timeline of biomass-to-liquids (BtL) development capacity of 34 000 bbl day−1 (1.37 million tpa) at Escravos, which will combine Sasol’s slurry phase distillate FT technology with Chevron’s proprietary biocracking process This plant is expected to be commissioned in 2011 at an estimated cost of $1.7 bn Both CtL and GtL are now established technologies with developments progressing throughout the world BtL processes have begun to develop more recently, principally due to environmental concerns including the need to reduce GHG emissions from the transport sector BtL (utilizing FT) development is most advanced in Germany, where, for example, Choren Industries is in the process 162 Technology Solutions – New Processes of commissioning its new 15 000 tpa synthetic diesel demonstration plant Elsewhere, Neste in Finland is building an FT demonstra­ tion facility at Stora Enso’s Varkaus paper mill Several companies have announced their intention to build demonstration facilities such as Flambeau Paper Mill in Wisconsin, USA, which has announced its plan to build a plant with a capacity of 51 000 tpa for 2012, and Xynergo in Norway, which has plans for a 100 000 tpa diesel plant Others, not based around FT, include Range Fuels (wood to mixed alcohols, primarily ethanol), Enerkem in Canada (MSW to ethanol), INEOS Bio in the United Kingdom (MSW to ethanol), and Chemrec in Sweden (wood to DME) All these examples are explained in more detail in Section 5.11.7 5.11.4 Steps in Biomass Conversion to Liquids via FT BtL processes consist of a number of distinct but highly integrated steps as shown in Figure The first stage is feedstock preparation This is followed by the production of a syngas from the biomass by a process known as gasification The syngas is cleaned of contaminants and conditioned before it is catalytically converted to, in the case of low-temperature FT combined with a cobalt catalyst, a waxy hydrocarbon by the FT process This wax can then be hydrocracked, a standard refinery process, to create the desired product Each of the steps in the conversion of biomass to FT diesel is described in more depth in the following section 5.11.4.1 Feedstock Preparation and Pretreatment The type and degree of biomass preparation needed, in particular size reduction and drying, will vary depending on the require­ ments of the gasifier and the biomass used Biomass varies in its physical characteristics including its moisture content, resistance to crushing, and bulk density, and this affects the type of processing and the amount of processing required Feedstock preparation, and any subsequent pretreatment, has a significant effect on the efficiency of the BtL process and thus the importance of careful and correct feedstock preparation should not be understated 5.11.4.1.1 Size reduction (comminution) Biomass, as received, is often highly irregular in its size and shape Most gasification systems require some degree of size reduction to ensure that the biomass has the correct size and shape for subsequent conversion Size reduction can be carried out using chopping or shredding, hammer mills, or disk and knife milling These are all well-established techniques The tolerance to variations in size varies significantly between different gasifiers As shown in Table 1, entrained flow gasifiers have the most stringent size require­ ment, whereas for plasma gasifiers size is less important Size reduction is an energy-intensive step, and thus can be costly both economically and in terms of GHG balance of the fuel Often a trade-off is needed between the cost of size reduction and the benefits accrued in gasification After size reduction, sieving and screening are necessary to ensure a homogeneous feedstock prior to gasification Screening is also used to ensure that any contaminants, such as metals, are removed although other separation processes will also be used if such tramp material is likely 5.11.4.1.2 Drying As received, biomass can contain high levels of moisture For example, fresh wood has a moisture content of around 40–50%, and straw around 15% Most gasification systems require a relatively dry feedstock, although as shown in Table 1, systems vary in the acceptable moisture content Biomass with high moisture contents will lower gasification efficiencies and increase the amounts of methane and tars in the syngas However, some moisture can be beneficial in the gasification process to help control gasification temperatures and to modify the syngas composition Biomass drying can improve gasification by: Reducing feedstock moisture contents to an acceptable level for a particular gasifier Aiding long-term storage and reducing the costs of biomass transportation Biomass drying is typically carried out at 100–120 °C, and is costly due to the amount of energy required to drive off excess moisture While gasification improves with drier biomass feedstocks, drying beyond a specified level may be too costly and impractical Feedstock eedstock preparation preparation Gasification Gas cleaning Synthesis • Established for larger • Defined by feedstock • Range of technologies • Established technologies for large- scale (e.g., GtL) and gasifier suited • Emergent at smaller • Subset more suited for scale, high-intensity • ‘Standard’ gasifiers scale technology, but critical synthesis • Emergent at smaller to success • Few yet ‘bankable’ scale suited for • Oxidant selection biomass Fuels wo workup • Established technology Figure Steps in the Fischer–Tropsch (FT) process Reproduced with kind permission of Progressive Energy Limited Biomass to Liquids Technology Table 163 Specifications of biomass for gasification Gasifier Size Entrained flow Moisture (%) Composition Other 15 Should not change over time Limited proportion of high-ash agricultural residues Pretreatment steps being used 10–55 Can change over time Care needed with some agricultural residues 5–60 Can change over time Care needed with some agricultural residues Not important Not important, can change over time Higher energy content feedstocks preferred