Greenhouse Gas Balance

The most frequently used criterion for evaluating the impact of biofuels on the environment is the GHG balance. This balance represents the difference between the emissions of GHGs during production and utilisation of a biofuel and the saving of GHGs due to the substitution of a fossil fuel. Thus, the result of a balance, the reduction of GHG emissions, depends considerably on the substituted fuel and the technology used. Recent balances calculate not only CO2 (1), but also CH4 (18, ..., 21) and N2O (296, ..., 310) in CO2 equivalents which are denoted here in brackets (e.g. Beer tit al. 2001; ADEME 2002; Patyk and Reinhardt 2002; CONCAWE 2006). Some of them consider further gases such as HFC (140, ..., 11,700), PFC (6, 500, ..., 9,200) and SF6 (23,900). Surveys and analyses of the results are given by Quirin et al. (2004), Arnold et al. (2006), Ramesohl et al. (2006), Nitsch (2007), Hill (2007), Fehrenbach et al. (2007) and others.

Most of the GHG emissions result from the energy input (CED) during production and conversion of the energy crops and from N2O emissions on the field. Thus, highly fertilised crops such as rape, maize and sugar beets as well as energy-intensive conversion processes such as ethanol and BtL production have a priori unfavourable results. However, high energy yields and the consideration of CO2 credits may improve them. Such credits result, for example, from the utilisation of by-products and/or residues as fertiliser or energy source. Moreover the carbon sequestration in soil may also influence the result. Depending on the detail of a balance, these shares are more or less completely considered in these calculations (Fig. 5.6).

In spite of the wide span of these calculation results, it can be summarised that oil seeds and grain converted to straight oil, biodiesel or bioethanol have the lowest CO2 saving potential, less than 5 t CO2eq ha-1 year-1. Ethanol as gasoline substitute produced from sugar beets and potato shows a very wide range from -0.7 to 11 t CO2eq ha-1 year-1 which is caused by different conversion technologies, different credit assumptions and different calculation methods. This is also true for biomethanol from whole crop cereals. With approximately 10-14 t CO2eq ha-1 year-1 the best figures in total are achieved by ligno-cellulosic crop species such as whole crop cereals and SRCs which are used to generate heat or combined heat and power with a boiler or gasifier.

Hence assuming that appropriate energy crop species were cultivated on 17.5 million hectares (15% of arable land of EU) and used in appropriate energy technologies with an average saving potential of 10 t CO2eq ha-1 year-1, more than 20% of the CO2 reduction target of the EU (843 Mt CO2eq "year-1 respectively 20% from 1990 to 2020 according to AEBIOM 2007) could be fulfilled.

Fig. 5.6 CO2-savings by use of biofuels made from energy crops according to Schmitz (2003), Quirin et al. (2004), CONCAWE (2006), Hill (2007) and BMELV (2007b). Conversion pathways: 1 Straight oil ^ Drive; 2 Biodiesel ^ Drive; 3 Grain ^ Heat; 4 Ethanol ^ Drive; 5 Ethanol ^ Heat & Power; 6 Ethanol ^ Drive; 7 Bales ^ Heat & Power; 8 BtL ^ Drive; 9 Methanol ^ Drive; 10 Ethanol ^ Drive; 11 Biogas ^ Drive; 12 Biogas ^ Heat & Power; 13 Chips ^ Heat; 14 Chips ^ Heat & Power; 15 BtL ^ Drive

Fig. 5.6 CO2-savings by use of biofuels made from energy crops according to Schmitz (2003), Quirin et al. (2004), CONCAWE (2006), Hill (2007) and BMELV (2007b). Conversion pathways: 1 Straight oil ^ Drive; 2 Biodiesel ^ Drive; 3 Grain ^ Heat; 4 Ethanol ^ Drive; 5 Ethanol ^ Heat & Power; 6 Ethanol ^ Drive; 7 Bales ^ Heat & Power; 8 BtL ^ Drive; 9 Methanol ^ Drive; 10 Ethanol ^ Drive; 11 Biogas ^ Drive; 12 Biogas ^ Heat & Power; 13 Chips ^ Heat; 14 Chips ^ Heat & Power; 15 BtL ^ Drive

Most important GHGs are carbon dioxide, methane and nitrous oxide, and the less important ones are fluorinated compounds, although their greenhouse warming potential ranges in the thousands of CO2 equivalents. GHG emissions correspond, in general, with the energy balance of energy conversion paths, i.e. liquid biofuels have the lowest GHG reduction potential in comparison to mineral fuels, whereas solid fuels save most GHG emissions compared to fossil fuels. GHG reduction ranges from 0 to 14 t ha-1 year-1 CO2 equivalents. The cultivation of energy crops on 15% of total arable land would significantly contribute to the EU CO2 reduction target.

5.10 Conclusion

The cultivation and supply of energy crops is, in general, sustainable, although there are differences between species. Ligno-cellulosic plants are more favourable in this context than oil seeds or grains. In addition to a great many usable species, which result in better biodiversity, the most important advantages of this species group consist of using the whole plant, releasing generally fewer environmentally harmful substances into the environment, as well as reducing the possibility of fertiliser and pesticide input without any loss in net energy gain. Ligno-cellulosic plants also include some promising perennial species such as poplar and willow, which need low inputs, cause low N2O emissions, sequestrate carbon in soil and have a high potential for GHG reduction. However, broad practical experiences and validated yields are lacking so far.

In addition, the conversion pathways have a strong influence on the results of sustainability assessments. Here, it is favourable to apply technologies which use the whole crop rather than only parts of it. So combustion, gasification and anaerobic digestion are, in general, more favourable than producing liquid biofuels, although the production of ethanol from high-yielding species like sugar beet or the production of BtL from whole crops, i.e. using also the ligno-cellulosic plant parts, can substantially increase the sustainability of this pathway.

Hence, the assessment of sustainability of bioenergy has to integrate both the cultivation of the energy crops and the route of conversion and utilisation of the bioenergy carrier. Although there is already a broad range of publications on this issue, more integrating research is required for a sustainable development in order to reach the targets set by the EU.

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