Page 239 - Lignocellulosic Biomass to Liquid Biofuels

196 Lignocellulosic Biomass to Liquid Biofuels

6.1 Introduction

Syngas fermentation is one of the most favorable biochemical conversion
techniques for the production of biofuels [1 4]. In this process, syngas is
used as a substrate for microorganisms [2], which is produced through a
thermochemical process from biomasses [5 8]. Commonly, carbon-
containing lignocellulosic biomass (forest residue, coconut shell, empty
fruit bunch of palm oil, municipal solid waste, etc.) is converted into
gases, such as carbon dioxide (CO 2 ), carbon monoxide (CO), and meth-
ane (CH 4 ) [5,6,9 11], and it is further converted into biofuels by utilizing
carbon-fixing microbes [1,2,12].
Biomass-derived syngas fermentation from gasification of carbonaceous
feedstocks is the most promising conversion technologies of biomass to
liquid biofuels. Bioethanol along with acetate, butanol, butyrate, formal-
dehyde, peptone, and methane (produced from chemical catalytic and
biosynthetic processes) is converted to clean and sustainable transportation
fuel produced from the lignocellulosic biomasses, such as forest or agricul-
tural biomass [4,5]. Syngas comprises various mixture of CO, CO 2 ,H 2 ,
and CH 4 , which can be produced through gasification of lignocellulosic
biomass [6,13]. The composition of syngas varies with the type of biomass
used as the feedstock. Different types of gasifiers, such as downdraft,
fluidized-bed, and fixed-bed, are used to produce syngas, and it goes
through several cleaning stages before entering to the fermenter. Up to
this time, it is an on-going research at laboratory scale and novel concepts
are integrating to develop the commercial scale.
The acetogenic bacteria reduce H 2 CO 2 /CO to acetate in their met-
abolic pathway [14], which is the primary stage of syngas fermentation to
bioethanol. Commonly, the conversion of this substrate (syngas) results
into organic acid (acetic acid and butyric acid) and alcohols, such as etha-
nol, hexanol, and butanol [15]. These products are usually used as the
generation of electricity, transportation fuels, and commodity chemicals
[16]. Therefore syngas fermentation has a broad interest both in the scien-
tific and industrial fields as an alternative technology to produce renew-
able bioenergy over the last decade.

6.1.1 Microbiology of syngas fermentation
There are various types of microorganisms that are involved in syngas fer-
mentation. They have the capabilities of utilizing CO, H 2 , and CO 2 as
metabolic building blocks, both in the case of aerobic or anaerobic