Process Energy Needs for Biofuel Production
Biofuel production has a variety of process heating needs depending on the conversion route adopted (Figure 11.1). In overall, the biomass thermochemical conversion processes are endothermic, and the heat required can be supplied by concentrated solar energy in such a way that the energy evolved from the fuel produced ideally represents the sum of energy stored during the photosynthesis and the direct thermal collection (Lede, 1998). Some of the reaction steps are exothermic in nature, which can provide part of the energy needed for the other steps in the plant. Nonetheless, if extra energy is supplied from solar and wind resources, then more of the biomass carbon can be converted into the liquid fuels.
Drying of biomass can put a heavy heat load on the process. For example, the external heating needs are about 3000–4000 kJ/kg of water removed (Lede, 1998). Hence, for a 1-kg water + 1-kg dry biomass feedstock, it will cost about 3000–4000 kJ in drying to get dry biomass with a heating value of 15,000 kJ; therefore, the drying needs are about 20–27% of the energy content of biomass.
The electrical (or mechanical) energy needs of the process are for conveying, grinding, and pumping. Energy needed for biomass grinding depends on the initial biomass size, final particle size, moisture content, material properties, scale of operation, and machine variables (Mani et al., 2004). Some of these values for four biomasses are shown in Table 11.1.