The carbon dioxide produced during the synthesis of diesel and biodiesel combined with carbon dioxide produced when the biodiesel is burnt is given in Fig. 8.18. Diesel produces around 80 g CO2/MJ compared with 43.7 g CO2/MJ for rapeseed biodiesel which is a reduction of 45%. In the case of diesel produced by the FT process by the

gasification of coal, natural gas and biomass, the carbon dioxide produced varies considerably. Both natural gas and coal FT diesel produce more carbon dioxide than diesel, 98 g CO2/MJ and 233 g CO2/MJ, respectively. The use of biomass in the syn­thesis of FT diesel yields only 5 g CO2/MJ which represents a 94% reduction in car­bon dioxide compared with mineral diesel.

The carbon dioxide fixed during growth and its distribution in products during the production of biodiesel from rapeseed is shown in Fig. 8.19. The rapeseed plant

Rapeseed + atmospheric
carbon dioxide

і

2.85 kg CO2 / kg biodiesel
2227 kg CO2

Fig. 8.19. The distribution of carbon dioxide in the production of biodiesel from rapeseed. (From Peterson and Hustrulid, 1998; Mortimer et a/., 2003.)

fixes a total of 15,736 kg CO2/ha, where 13,336 kg was retained in the plant and the remaining 2400 kg stayed in the soil sequestered by the soil microorganisms. The yield of the seed was 2240 kg/ha, 28.5% of the total plant material, containing 4800 kg carbon dioxide/ha. The rest of the plant, the straw, contains 8514 kg CO2/ha, 63.8% of the carbon dioxide fixed, which is often ploughed into the soil. The oil extracted from the seed contains 840 kg/ha which is an oil yield of 37.5% leaving the meal or cake containing 2389 kg CO2/ha to be used as feed or fuel. On transesterification the oil is converted into 840 kg biodiesel and 78 kg glycerol. The biodiesel represents some 9.2% of the whole plant and yields 2227 kg CO2/ha when used as a fuel. Figure 8.19 also includes the energy content of the biodiesel and the co-products where it can be seen that the meal and straw contain more energy than the biodiesel. For these reasons, it may be more efficient to convert the whole plant or biomass into a biofuel rather than just the oil extracted from the seed. This should be the nature of the second-generation biofuels.

The unit operations used in the production of biodiesel have been evaluated for their carbon dioxide and GHG production and energy input (Fig. 8.20a, b,c). It is clear that the major carbon dioxide-producing stages were esterification, use of nitro­gen fertilizer and to a lesser extent solvent extraction of the oil. A second process has been included where solvent extraction has been replaced by cold pressing and low nitrogen growing conditions. This results in more than 50% reduction in energy input, carbon dioxide and GHG emissions. Thus, the processes of biofuel production can be made more environmentally suitable by modifications to the key stages of the process.