Second generation biofuels, also known as advanced biofuels, are fuels that can be manufactured from various types of biomass. Biomass is a wide-ranging term meaning any source of organic carbon that is renewed rapidly as part of the carbon cycle. Biomass is derived from plant materials but can also include animal materials.
First generation biofuels are made from the sugars and vegetable oils found in arable crops, which can be easily extracted using conventional technology. In comparison, second generation biofuels are made from lignocellulosic biomass or woody crops, agricultural residues or waste, which makes it harder to extract the required fuel.
Contents |
Second generation biofuel technologies have been developed because first generation biofuels manufacture has important limitations.[1] First generation biofuel processes are useful but limited in most cases: there is a threshold above which they cannot produce enough biofuel without threatening food supplies and biodiversity. Many first generation biofuels are dependent of subsidies and are not cost competitive with existing fossil fuels such as oil, and some of them produce only limited greenhouse gas emissions savings. When taking emissions from production and transport into account, life cycle assessment from first generation biofuels frequently approach those of traditional fossil fuels.[2]
Second generation biofuels can help solve these problems and can supply a larger proportion of our fuel supply sustainably, affordably, and with greater environmental benefits.
First generation bioethanol is produced by fermenting plant-derived sugars to ethanol, using a similar process to that used in beer and wine-making (see Ethanol fermentation). This requires the use of 'food' crops, such as sugar cane, corn, wheat, and sugar beet. These crops are required for food, so, if too much biofuel is made from them, food prices could rise and shortages might be experienced in some countries. Corn, wheat, and sugar beet also require high agricultural inputs in the form of fertilizers, which limit the greenhouse gas reductions that can be achieved. Biodiesel produced by transesterification from rapeseed oil, palm oil, or other plant oils is also considered a first generation biofuel.
The goal of second generation biofuel processes is to extend the amount of biofuel that can be produced sustainably by using biomass consisting of the residual non-food parts of current crops, such as stems, leaves and husks that are left behind once the food crop has been extracted, as well as other crops that are not used for food purposes (non food crops), such as switchgrass, grass, jatropha, whole crop maize, miscanthus and cereals that bear little grain, and also industry waste such as woodchips, skins and pulp from fruit pressing, etc.[3]
The problem that second generation biofuel processes are addressing is to extract useful feedstocks from this woody or fibrous biomass, where the useful sugars are locked in by lignin, hemicellulose and cellulose. All plants contain lignin, hemicellulose and cellulose. These are complex carbohydrates (molecules based on sugar). Lignocellulosic ethanol is made by freeing the sugar molecules from cellulose using enzymes, steam heating, or other pre-treatments. These sugars can then be fermented to produce ethanol in the same way as first generation bioethanol production. The by-product of this process is lignin. Lignin can be burned as a carbon neutral fuel to produce heat and power for the processing plant and possibly for surrounding homes and businesses.
The following subsections describe the main second generation routes currently under development.
Carbon-based materials can be heated at high temperatures in the absence (pyrolysis) or presence of oxygen, air and/or steam (gasification).
These thermochemical processes both yield a combustible gas and a solid char. The gas can be fermented or chemically synthesised into a range of fuels, including ethanol, synthetic diesel or jet fuel.[4]
Gasification technologies are well established for conventional feedstocks such as coal and crude oil. Second generation gasification technologies include gasification of forest and agricultural residues, waste wood, energy crops and black liquor.[5] Output is normally syngas for further synthesis to e.g. Fischer-Tropsch products, biomethanol, BioDME or biomethane (synthetic natural gas). Syngas can also be used in heat production and for generation of mechanical and electrical power via gas motors or gas turbines.
Pyrolysis is a well established technique for decomposition of organic material at elevated temperatures in the absence of oxygen. In second generation biofuels applications forest and agricultural residues, wood waste and energy crops can be used as feedstock to produce e.g. bio-oil for fuel oil applications.
Torrefaction is a form of pyrolysis at temperatures typically ranging between 200-320°C. Feedstocks and output are the same as for pyrolysis.
Chemical and biological processes that are currently used in other applications are being adapted for second generation biofuels. Biochemical processes typically employ pre-treatment to accelerate the hydrolysis process, which separates out the lignin, hemicellulose and cellulose. Once these ingredients are separated, the cellulose fractions can be fermented into alcohols.[4]
Feedstocks are energy crops, agricultural and forest residues, food industry and municipal biowaste and other biomass containing sugars. Products include alcohols (such as ethanol and butanol) and other hydrocarbons for transportation use.
The following second generation biofuels are under development, although it must be noted that most or all of these biofuels are synthesized from intermediary products such as syngas using methods that are identical in processes involving conventional feedstocks, first generation and second generation biofuels. The distinguishing feature is the technology involved in producing the intermediary product, rather than the ultimate off-take.
A process producing liquid fuels from gas (normally syngas) is called a Gas-to-Liquid (GtL) process.[6] When biomass is the source of the gas production the process is also referred to as Biomass-To-Liquids (BTL).
The Fischer-Tropsch (FT) process is a Gas-to-Liquid (GtL) process.[6] When biomass is the source of the gas production the process is also referred to as Biomass-To-Liquids (BTL).[10][11] A disadvantage of this process is the high energy investment for the FT synthesis and consequently, the process is not yet economic.
Second generation biofuel feedstocks include cereal and sugar crops, specifically grown energy crops, agricultural and municipal wastes, cultivated and waste oils, and algae.[14]
Plants are made from lignin, hemicellulose and cellulose; second generation technology uses one, two or all of these components. Common lignocellulosic energy crops include wheat straw, Miscanthus, short rotation coppice poplar and willow. However, each offers different opportunities and no one crop can be considered 'best' or 'worst'.[15]
Municipal Solid Waste comprises a very large range of materials, and total waste arisings are increasing. In the UK, recycling initiatives decrease the proportion of waste going straight for disposal, and the level of recycling is increasing each year. However, there remains significant opportunities to convert this waste to fuel via gasification or pyrolysis.[16]
Green waste such as forest residues or garden or park waste[17] may be used to produce biofuel via different routes. Examples include Biogas captured from biodegradable green waste, and gasification or hydrolysis to syngas for further processing to biofuels via catalytic processes.
Black liquor, the spent cooking liquor from the kraft process that contains concentrated lignin and hemicellulose, may be gasified with very high conversion efficiency and greenhouse gas reduction potential[18] to produce syngas for further synthesis to e.g. biomethanol or BioDME.
Producing second generation biofuels offers greater greenhouse gas emissions savings than those obtained by first generation biofuels.
Lignocellulosic biofuels can reduce greenhouse gas emissions by around 90% when compared with fossil petroleum, in contrast first generation biofuels offer savings of only 20-70%[19]
An operating lignocellulosic ethanol production plant is located in Canada, run by Iogen Corporation.[20] The demonstration-scale plant produces around 700,000 litres of bioethanol each year. A commercial plant is under construction. Many further lignocellulosic ethanol plants have been proposed in North America and around the world.
The Swedish specialty cellulose mill Domsjö Fabriker in Örnsköldsvik, Sweden develops a biorefinery using Chemrec's black liquor gasification technology.[21] When commissioned in 2015 the biorefinery will produce 140,000 tons of biomethanol or 100,000 tons of BioDME per year, replacing 2% of Sweden's imports of diesel fuel for transportation purposes.
In the UK, comapnies like INEOS Bio and British Airways are developing advanced biofuel refineries, which are due to be built by 2013 and 2014 respectively. Under favourable economic conditions and strong improvements in policy support, NNFCC projections suggest advanced biofuels could meet up to 4.3 per cent of the UK's transport fuel by 2020 and save 3.2 million tonnes of CO2 each year, equivalent to taking nearly a million cars off the road.[15]