Biomass is a renewable energy source, which presents the advantage of storing energy.
We can expect to see an increase in the role of biomass for energy production in the future. Biomass offers the advantage of being a renewable energy which is also storable. In the European Union, energy from biomass reached 61 Mtoe in 2006, an increase of 3.1 Mtoe compared with 2004. In the world, biomass represents the main source of renewable energy, amounting to 1186 Mtoe in 2006 [1, 66].
The production of heat by combustion represents the main energy use of biomass, wood being the primary renewable energy source in Europe. Wood can be burnt in individual or collective boilers. It can also be used to produce steam, generate electricity as well as heat in cogeneration.
Anaerobic methane fermentation in the presence of waste rich in organic matter produces biogas. At least some of the methane produced can also be recovered in open refuse tips by aerobic fermentation. The main disadvantage of the biogas produced in this way is that it contains numerous contaminants, and in particular highly corrosive acid compounds. In general therefore, biogas requires intensive treatment. Biogas can also be used as a compressed gas fuel (NGV). Although this application is faced with the difficulty of distributing a gaseous fuel, it offers a highly interesting environmental balance.
Production of biofuels currently represents the main alternative to petroleum fuels in the field of transport. The production of biofuels has grown very rapidly, generating strong controversies about its negative impact on food supply. The world production in 2006 reached 24.4 Mtoe, as compared with 10.3 Mtoe in 2000.
Biofuels offer the advantage of reducing the dependence of the consumer countries on oil while at the same time improving the CO2 balance. The CO2 emitted by combustion of biomass is seen as neutral with respect to the greenhouse gas balance since it can be considered as being recycled during photosynthesis, as indicated earlier. We must nevertheless take into account all emissions generated during the production, transport and transformation of biomass (life cycle analysis), which may in some cases significantly reduce, or even completely cancel out, this advantage.
The European Union member states have set an initial goal of incorporating at least 5.75% of biofuels in 2010 and 10% by 2020 in the fuels of fossil origin (gasoline and diesel). Within the present context of economic crisis and controversy about the role of biofuels, these figures might be revised.
The USA announced an extremely ambitious objective of 30% biofuels in the transport sector in 2030.
Ethanol is by far the most extensively used biofuel worldwide. It is produced by fermentation of sugar, obtained from plants such as sugar cane and sugar beet. Production of ethanol from sugar cane is widespread in Brazil, a country which has considerably developed the use of ethanol as a fuel. It can also be produced from starch derived from cereals such as maize and wheat. The sugar required for the fermentation step is obtained beforehand from starch under the effect of an enzyme. The production of ethanol from cereals, in particular maize, is mainly carried out in the USA, which is the second major ethanol producer.
Ethanol can be used in gasoline engines. In Europe, however, it is widely used as ETBE, a compound formed with isobutene, so that it can be more easily incorporated into gasoline. In the future, direct use of ethanol as fuel is likely to take off with the use of flex-fuel engines, already widespread in Brazil, the USA and Sweden, and launched officially in France at the beginning of 2007: flex-fuel vehicles can run on traditional gasoline, E85 fuel containing 85% ethanol (superethanol), or a mixture of the two in any proportions after filling up in different service stations.
The global production of ethanol reached 40 Mt in 2006, including 75% for applications as fuel, the largest share of the production being concentrated in Brazil and the USA. Its share in energy terms represents more than 80% of the biofuels market (83% in 2006).
In Europe, vegetable oil methyl ester (VOME), obtained from rape seed oil or sunflower oil, is currently the most widely used biofuel, as it can be incorporated in diesel fuel, for which the demand in the European Union is higher than for gasoline.
Up to 5% VOME can be incorporated in diesel fuel without the need for any significant engine modifications. Higher incorporation levels are possible, but the engine must be modified accordingly.
An area equivalent to 30-40% of the current agricultural land , whether in Europe or the USA, would have to be dedicated to biofuel if production is to reach a level equivalent to 10% of fuel consumption, which seems neither possible nor acceptable.
The competition for land use with crops needed for food production has generated much debate about biofuels. It is therefore essential to be able to produce biofuels from a feedstock which cannot be used for food applications.
Since it is not processed for human food, the use of lignocellulosic biomass (wood, agricultural waste such as straw from cereals and oleaginous plants, fast rotation crops on agricultural areas, etc.) as raw material would considerably increase the biofuel production potential.
In order to produce liquid fuels from lignocellulosic biomass, we will nevertheless need to develop 'second generation' technologies, which have not yet been proven either industrially or economically. Two main conversion pathways are used:
• The thermochemical pathway, under development, consists of converting biomass under the effect of heat into a gaseous or liquid phase. Liquid fuels can be obtained through gasification followed by Fischer-Tropsch synthesis. The BTL (biomass to liquids) process is similar to GTL (gas to liquids) and CTL (coal to liquids) processes used to produce synthetic fuels from natural gas or coal, which are described in Chapter 7. It offers the advantage of producing high-quality liquid fuels, in particular diesel, which can be used directly in current engines.
Pyrolysis of lignocellulosic biomass in an oxygen-free atmosphere produces a liquid phase called 'bio-oil'. This product is rich in oxygenated compounds, making it immiscible with hydrocarbons. It can be converted into liquid fuels by gasification. Its conversion into fuel by hydrogenation is also one of the research pathways being investigated.
• The biochemical pathway requires fractionation of the lignocellu-losic biomass into its three fractions: cellulose, hemicelluloses and lignin. Glucose can be obtained from cellulose under the effect of enzymes. The glucose then undergoes a fermentation step to obtain ethanol. The aim of current studies is to improve the performance of this conversion process. The research work also concerns the conversion of hemicelluloses to obtain other sugars, the pentoses, whose subsequent conversion into ethanol by fermentation is also being studied. This conversion, which is more difficult than the case of the cellulose fraction, has not reached the industrial stage . Lignin is separated and can be used as a fuel to supply energy. Production of cellulosic ethanol is still currently more expensive than production of ethanol from other sugar plants. In view of the extensive research and development work in progress we can expect production to become more competitive.
Large-scale development of biofuels is only acceptable if compatible with sustainable development criteria. The risks associated with the production of biomass intended for use as biofuels must be carefully analysed
(competition with food uses, consumption of water, fertilisers and pesticides).
If we restrict ourselves to the current first generation processes, competition with food uses appears when the incorporated rate in fuels exceeds 5-10%. The new processes, whose production is based on lignocellulosic biomass, must therefore be implemented to reach higher penetration levels.
Care must be taken in this case to ensure that the biomass is produced under conditions acceptable for the environment and, in particular, does not result in deforestation or irreversible soil degradation.
Improvement of the CO2 balance, a critical selection criterion, must be assessed by performing a complete analysis of the life cycle, from the biomass production step through to use of the biofuel in the engine. It is absolutely essential to perform this life cycle analysis, since some cereal-based ethanol production processes, requiring large quantities of fossil energies to convert the biomass, offer little or even zero benefit.
In contrast, the more recent lignocellulosic biomass conversion pathways currently being explored reduce the global CO2 emissions by 70-90% over the entire biofuel production and utilisation cycle.
Emissions of greenhouse gases other than CO2, in particular nitrous oxide (N2O), resulting from the use of nitrogen-containing fertilisers, must also be taken into account.
In order to achieve high penetration rates of more than 10%, while respecting sustainable development criteria and without compromising the production of biomass for food use, new biofuel production processes based on lignocellulosic biomass must be developed.
These criteria must always be respected, no matter which process is implemented. This is an essential condition if biofuels are to make a significant contribution to solving energy and climate change problems.
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