Biomass-Based Energy Production - Introduction to Chemicals from Biomass - page 265

shown in Figure 7.3. It is noteworthy that hydrogen is produced during several
steps of the methane pathway, indicating that totally hydrogen and methane
formation coexists in nature [15].

7.3.7.2 Methods for Enhancing Biogas Production

Scientists have proposed a variety of ways to enhance biogas production as
reviewed by Yadvika et al. [37]. These include the use of additives, gas enhance-
ment by the recycling of digested slurry, variation of operational parameters, and
the use of fixed films. For the sake of brevity, we only discuss additives and the
variation of process conditions (e.g. the temperature and C/N ratio) in this chap-
ter. Additives are primarily used to promote microbial activity and to maintain
favorable conditions in terms of pH in order to minimize the inhibition of ace-
togenesis and methanogenesis. The parameter that has the strongest impact on
biogas production is the temperature. Biogas can be produced at temperatures
ranging from <30°C (psychrophilic) to 30-40°C (mesophilic) or even 40-50°C
(thermophilic). Anaerobic bacteria typically function best in the mesophilic and
thermophilic temperature ranges, so these temperatures yield more biogas. The
reaction can also occur at psychrophilic temperatures, but it proceeds much more
slowly under such conditions. It is also important to maintain an appropriate sub-
strate composition in terms of the C/N ratio. The anaerobic bacteria active in
biogas production use carbon 25-30 faster than nitrogen [37], so the substrate
C/N ratio should be between 20:1 and 30:1 in order to establish optimal condi-
tions for the bacteria.

7.4 Thermochemical Processes
Thermochemical processes for biomass conversion are known to be more efficient
than the alternatives in terms of reaction times (which range from a few seconds
to minutes) as well as the percent of utilization of different parts and degradation
of plant biomass including lignins. They can be used to convert biomass into
energy products such as heat and electricity, transport fuels such as methanol and
dimethyl ether (DME), and other chemicals. Thermochemical processes can be
divided into four main groups, as described in Chapter 3:

1. gasification of biomass, that is, its conversion into gaseous hydrocarbons such
as methane;
2. pyrolysis , that is, the heating of biomass in the absence of oxygen to produce
various energy products;
3. direct liquefaction of biomass by high-temperature pyrolysis or by high-
pressure-to-liquid products; and
4. combustion in the presence of air to convert chemical energy into heat or
mechanical power.