Page 29 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

8 Biomass Gasification, Pyrolysis, and Torrefaction

FIGURE 1.4 Bus with on-board gasifier during Second World War. Source: http://www.wood-
gas.com/history.htm.

A brief description of the biochemical and thermochemical routes of
biomass conversion is presented in the following sections.

1.2.1 Biochemical Conversion

In biochemical conversion, biomass molecules are broken down into smaller
molecules by bacteria or enzymes (Figure 1.5). This process is much slower
than thermochemical conversion process but does not require much external
energy. The three principal routes for biochemical conversion are as follows:
1. Digestion (anaerobic and aerobic)
2. Fermentation
3. Enzymatic or acid hydrolysis.
The main products of anaerobic digestion are methane and carbon diox-
ide in addition to a solid residue. Bacteria take oxygen from the biomass
itself instead of from ambient air.
Aerobic digestion, or composting, is also a biochemical breakdown of
biomass, except that it takes place in the presence of oxygen. It uses different
types of microorganisms that access oxygen from the air, producing carbon
dioxide, heat, and a solid digestate.
In fermentation, part of the biomass is converted into sugars using acids
or enzymes. The sugar is then converted into ethanol or other chemicals with
the help of yeast. The lignin is not converted and is left either for combus-
tion or for thermochemical conversion into chemicals. Unlike anaerobic
digestion, the product of fermentation is liquid.
Fermentation of starch- and sugar-based feedstock (e.g., corn and sugar-
cane) into ethanol (Figure 1.5A) is a fully commercial process, but this is not