Page 254 - Lignocellulosic Biomass to Liquid Biofuels

210 Lignocellulosic Biomass to Liquid Biofuels

Tar removal by oil washing: This is one of the techniques for the removal
of tar from syngas [50]. Unyaphan et al. [52] reported the improvement
of tar-removal performance of oil scrubber by producing microbubbles.
Chemical absorption: This technique is applied for the absorption process
where atoms, molecules, or ions enter some bulk phase-liquid or solid
materials. Pallozzi et al. [53] stated on the combined gas conditioning and
cleaning for the reduction of tars during biomass gasification.

6.6 Factors affecting syngas fermentation
6.6.1 Effect of organic source

The substrate containing organic source is biomass-based syngas. The car-
bonic acid and carbonate formation are depending on the concentrations
of carbon dioxide and high acetate, which may potentially inhibit micro-
biological actions in the fermentation media. Biomass-based syngas can
constrain hydrogen production and adapt product distribution of ethanol
and acetate. It has also potential to retain microbial cells in an inactive
stage during the bioethanol production.

6.6.2 pH level of the medium

The pH level is one of the most important factors, which is affected on
syngas fermentation. It depends on the various types of medium, nutrient,
or substrate. As a result, the productivity of ethanol or acetic acid from
biomass-based syngas depends on the pH level. It also affects the substrate
metabolism and other critical factors, such as pH, membrane, and proton
motive force. The biological actions can be affected by small changes of
pH. Obviously, large changes of pH can lead to the death of microbes or
at least can inhibit the generation of the desired products. The acetogenic
bacteria are widely used in syngas fermentation, where the ethanol pro-
duction level is high at lower pH levels. The optimum pH value for the
generation of acetate is from 5 to 7 (growth level), whereas for ethanol it
is 4 4.5 (nongrowth level) by using specific types of bacteria.

6.6.3 Temperature of the medium
This parameter is also one of the key factors that affect the production of
yield. The specific type of microorganisms is survived on specific tempera-
ture. The optimum temperature for mesophilic microorganisms is needed