Page 240 - Lignocellulosic Biomass to Liquid Biofuels

Syngas fermentation to bioethanol 197

species. The anaerobic acetogenic, hydrogenogenic, and methanogenic
microorganisms are able to produce different chemicals from biomass-
derived syngas. Drzyzga et al. [17] reported that in most of the cases,
expensive chemical catalyst is used for syngas fermentation for the conver-
sion of syngas (C1 components) into various types of multicarbon com-
pounds. Therefore research is still undergoing for syngas fermentation for
the enhancement of microorganism’s productivity and their metabolisms
process.
Microorganisms are unable to degrade the biomass entirely through
the direct fermentation of biomass, which contained lignin. The gasifica-
tion is the technique where lignin-based biomass converted into syngas by
using fixed-bed (downdraft and updraft gasifier), fluidized-bed, and
entrained bed gasifier [9,18 21]. Therefore cogasification technique is
employed for enhancing the quality of syngas by using biomass, natural
coal, and by-product charcoal [9,22 24]. Produced syngas contains some
impurities, such as tar (higher hydrocarbon) and small particles [25 27],
which hinder the metabolic functions of microorganisms.
Currently, syngas fermentation is concerning to enhance the yield by
optimizing the bioreactor operational parameters [17]. Mesophilic and
thermophilic type microbes are used for this process based on the fermen-
tation conditions. The common microorganisms, employed for syngas fer-
mentation, are Clostridium autoethanogenum, Clostridium carboxidivorans,
Acetobacterium woodii, Clostridium ragsdalei, Butyribacterium methylotropphicum,
Clostridium ljungdahlii, etc. [4]. Heiskanen et al. [28] also reported that the
bioethanol production from syngas is more efficient by using biological
catalysts (A. woodii and C. carboxidivorans) than chemical catalysts (copper,
cobalt, or iron). The detailed list of common microorganisms that are
involved in syngas fermentation with their optimum parameters of tem-
perature, pH, and yield is shown in Table 6.1.

6.1.2 Microbial culture medium

The microbial culture media is the necessary part for syngas fermentation.
The fermentation medium is prepared by the mixture of nutrients, sub-
strates, and microorganisms. The ratio and type of nutrient are depending
on the specific type of microorganisms that are responsible for the produc-
tion of ethanol or other products. Reinforced clostridial medium (RCM)
is usually used as a medium for syngas fermentation. The composition of
the medium is yeast extract (3.0 g/L), lab-lemco powder (10.0 g/L),