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Other than traditional forest crops (see Chapter 4), a number of annual and perennial species have been identified as having high efficiency properties when converting solar energy into stored biomass which can then be converted into heat, electricity or transport fuels with zero or very low carbon emissions (Veenendal et al., 1997). (Conversion of biomass is described in Section 3.8.4.3.2 and biofuels for transport in Section 3.4.4).
High yielding short rotation forest crops or C4 plants (e.g., sugar cane and sorghum) can give stored energy equivalents of over 400 GJ/ha/yr at the commercial scale, leading to very positive input/output energy balances of the overall system (El Bassam, 1996). Ethanol production from maize and other cereals in the USA, from sugar cane in Brazil, and biodiesel from oilseed rape in Europe, are being commercially produced but are subject to commodity price fluctuations and government support. The relatively low energy yields per hectare for many oil crops (around 60 to 80GJ/ha/yr for oil) compared with crops grown for cellulose or starch/sugar (200–300GJ/ha/yr), has led to the US National Research Council advising against any further research investment (NRC, 1999b).
Liquid biofuels (see Section 3.4.4.6) when substituted for fossil fuels will directly reduce CO2 emissions. Therefore, a combination of bioenergy production with carbon sink options can result in maximum benefit from mitigation strategies. This can be achieved by planting energy crops such as short rotation coppice into arable or pasture land, which increases the carbon density of that land, while also yielding a source of biomass. Converting the accumulated carbon in the biofuels for energy purposes, and hence recycling it, alleviates the critical issue of maintaining the biotic carbon stocks over time as for a forest sink. Increased levels of soil carbon may also result from growing perennial energy crops (IEA Bioenergy, 1999), but a detailed life cycle assessment is warranted for specific crops and regions.
Land needed to grow energy crops competes directly with food and fibre production unless grown on marginal or degraded land, or unless surplus land is available. For the USA and Europe. Hall and Scrase (1998) calculated there to be sufficient land physically available to grow crops to supply all human needs of food, fibre, and energy at current population levels, though the study did not include social, economic, and logistical constraints. Sufficient labour, water, and nutrients must also be available if a sustainable and economic bioenergy industry is to be developed. On marginal lands, such as the increasingly saline soils of Australia, growing short rotation eucalyptus in strips between blocks of cereal crops can help lower the water table and hence, under certain circumstances, reduce the soil saline levels to bring back the natural fertility. However, water demands can be high for short rotation forest crops so the resulting overall effects are yet to be determined. An additional benefit is that the decentralization of energy production using energy crops to supply local conversion plants creates employment in rural areas (Grassi, 1998; El Bassam et al., 1998; Moreira and Goldemberg, 1999).
Certain woody crops and also perennial grasses grown to produce biomass have theoretically high dry matter yields, but commercial yields are often lower than expected from those produced in small plot research trials. In Sweden, for example, where 16,000 ha of coppice Salix species have been planted, around 2000ha were harvested for the first time during the winters of 1996 to 1998 to yield only 4.2 oven dry t/ha/yr on average (Larsson et al., 1998). With better management, genetic selection, and grower experience once viable markets for the product are established, it had been anticipated that commercial yields closer to 10 oven dry t/ha/yr would result.
Correct species selection to meet specific soil and climatic site conditions is necessary in order to maximize yields in terms of MJ/ha/yr (Sims et al., 1999). For example, the saccharose yield of Brazilian sugar cane has increased 10% to 143kg/t of fresh cane (70% moisture content wet basis) since 1990. Methods of identifying appropriate species based on non-destructive yield measurements and species fuelwood characteristics have been developed (Senelwa and Sims, 1998). Energy balance ratios for each unit of energy input required to produce solid fuels from short rotation forest crops are up to 1: 30, and can be even higher when crop residues are also utilized (Scholz, 1998). Woody crops normally require less energy inputs per hectare than food crops.
Forest sinks are covered in Chapter 4 and also in the Special Report on Land Use, Land-Use Change and Forestry (IPCC, 2000), but there is a link between these low cost sinks and eventually using some of the biomass grown for energy purposes. Once the limited area of available land is covered in forest sinks, no more planting will be possible and recycling of the carbon to displace fossil fuels may then become feasible. Economic mechanisms to link a forest sink project with a biofuel project have been suggested (Read, 1999).
Crop residues such as straw, bagasse, and rice husks, if not returned to the land for nutrient replenishment and soil conditioning, could be used more in the future for heat and power generation, at times in co-combustion with coal, and in appropriate conversion equipment now that the technology is well proven. Wood residues used in small-scale biomass gasifiers will become reliable and more cost effective in time, but at present have some operational risk attached, particularly under developing country conditions (Senelwa and Sims, 1999).
Animal manures and industrial organic wastes are currently used to generate biogas. For example, in Denmark there are 19 decentralized community scale biogas plants for electricity generation (Nielsen et al., 1998). Biogas can also be used for cogeneration, direct heating or as a transport fuel.
Many farmers in both developed and developing countries will remain unlikely to change their traditional production methods in the short term unless there are clear financial incentives to do so. Behavioural changes as a result of advisors educating members of farming communities to adopt new measures have rarely succeeded to date. Cultural factors have a strong influence on the general unwillingness to accept inappropriate development and hence new ideas. Changing attitudes are unlikely to occur unless farmers can also perceive personal co-benefits such as increased profitability, time saving, cost reductions, improved animal health, increased soil fertility, and less arduous tasks. Regulations in some form are the alternative (OECD, 1998a) but would probably be difficult to monitor, particularly in developing countries. Education of local extension officers is needed to encourage the uptake of new methods and more rapid implementation into the field. These barriers are discussed in Section 5.4.5.
Dietary changes from meat to fish or vegetables could help reduce emissions by 55MtCeq in Europe alone (Gielen et al, 1999) and possibly release land for energy cropping.
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