This section provides a brief overview of the thermochemical processes, namely, direct com­bustion, pyrolysis, and gasification, for converting biomass to useful energy, chemicals, and fuels.

2.1.1 Direct biomass combustion

Direct biomass combustion has traditionally been used to supply heat and power in the process industry. However, such systems for electricity generation have low overall efficiency and emit significant pollutants (Caputo et al., 2005). Systems utilizing direct combustion of agricultural waste include kilns and boilers for generating steam used for various industrial applications includ­ing electricity production. Werther et al. (2000) provide a review on direct biomass combustion. Figure 2.2 from their paper shows a schematic of processes associated with the combustion of wood or straw.

The sequence of events which a lump of solid fuel undergoes during combustion includes heat­ing up, drying, devolatilization, ignition and combustion of volatiles, and finally the combustion of char. As discussed by Werther et al. (2000), the fundamental information required to charac­terize the combustion of agricultural residues include temperatures at the start of devolatilization and char combustion, the influence of drying on the devolatilization process, the composition of devolatilization products, and the effect of volatile release and combustion on the overall combustion process.

There are many operational and environmental challenges associated with the biomass com­bustion technology. These include the low bulk density of agricultural waste (~5—10 times lower than coal), high moisture content, low melting point of the ash, and high content of volatile matter. The low density leads to problems such as high volume required for storage, low energy output on a volume basis, and high transportation costs. Densification is often used to address these problems. Similarly, the low melting temperature of the ash leads to problems such as bed agglomeration in a fluidized bed, and fouling, scaling and corrosion of heat transfer surfaces. The higher content of volatile matter implies significant differences between the combustion and emission characteristics of agriculture biomass and fossil fuels (Ogada et al., 1996). For instance, the presence of volatile matter enhances the biomass ignitability and reactivity, but the combus­tion process becomes difficult to control. This presents challenges in using agriculture biomass in the existing combustion devices. Moreover, due to the presence of sulfur, nitrogen, chlorine etc., the biomass combustion leads to the formation of gaseous pollutants such as SOX, NOX, N2O and HCl. Many of these issues can be addressed in biomass co-fired combustion systems (Backreedy,

2005) , but the amount of biomass is generally limited to 5-10% of the total feedstock due to concern about the plugging of existing feed systems (Yoshioka et al., 2005). Further discussion of these issues can be found in Werther el al. (2000).

Figure 2.2. A schematic of various processes associated with the combustion process of a lump of straw or wood chip (Werther et al., 2000).

There are few fundamental studies, experimental or theoretical, dealing with the biomass combustion and emission characteristics. This may partly be due to the lack of information about the physical and chemical properties of various biomass feed stocks. Consequently, there has not been as much work on the development of reliable kinetic and thermo-transport models for investigating biomass combustion and emissions. Such information is of critical importance for the design and efficient operation of biomass-based combustion systems. The lack of this information has also been a factor in low utilization of direct biomass combustion compared to biomass pyrolysis and gasification. Thus, there is a need for more fundamental research on biomass combustion, and the development of a database on the physical and chemical properties of biomass feed stocks. As such information becomes available, along with appropriate kinetic and thermo-transport models, the existing software, which has developed for the simulation of coal combustion (Smith et al., 1990), may be modified for predicting the biomass combustion and emission characteristics. Subsequently, these tools can be further refined for optimizing and improving the performance of direct biomass combustion systems.