Page 262 - Lignocellulosic Biomass to Liquid Biofuels

218 Lignocellulosic Biomass to Liquid Biofuels

advanced and effective technologies for the production of liquid transpor-
tation fuels. In recent year, Fischer Tropsch (FT) process has received
the scientific attention for the production of sulfur less diesel fuel to offset
the fossil-fuel demand. FT synthesis represents one of the most promising
and sustainable solutions for the production of ultraclean fuel at economi-
cally feasible cost [6 21]. The FT fuels can be used in combustion
engines, which exhibit lower greenhouse gas emission levels compared to
petroleum-based fuels, because FT fuels are sulfur free and contain a very
low concentration of aromatics and nitrogen [6,7,22 24].
FT synthesis was developed by Franz Fischer and Hans Tropsch in
1925 at Kaiser Wilhelm Institute, Germany. FT is a heterogeneous cata-
lytic process that converts syngas into a variety of products, such as alco-
hols, aldehydes, olefins, paraffins, and especially liquid transportation fuels.
The syngas could be derived from different sources, such as biomass, coal,
coal-bed gas, natural gas or shale gas, through steam reforming, partial or
auto thermal oxidation, or gasification process [1,6,25,26]. The large-scale
FT plants do not have major technical issues and biomass gasification cou-
pled with FT process assures the production of green liquid fuels. The
biomass is gasified to produce syngas that is further transformed into green
liquid transportation fuels through FT synthesis [6,14,15,17]. Therefore
metal catalysts are necessary to carry out FT catalytic reactions at the
temperatures of 150°C 300°C and selective pressures, in the range of
5 60 bar, produce water as major by-product of the process
[6,22 24,27 31]. Moreover, cobalt (Co) and iron (Fe) are more vital
catalytic components for industrial-scale synthesis of FT process. The Fe
catalysts could operate under a wider range of temperatures and H 2 /CO
ratios with low CH 4 selectivity compared to Co catalysts. Particularly, Fe
catalysts show a higher activity for the water gas shift (WGS) reaction
under high temperature, which is more helpful for the conversion of syn-
gas with lower H 2 /CO ratios resulting from biomass or coal gasification
[1,6,10,28], although the syngas derived from biomass gasification could
be H 2 deficient, which will be demanding the Co catalysts for FT synthe-
sis. The Co catalysts-based FT process shows greater productivity than Fe-
based catalysts [6,32]. Moreover, certain chemical promoters, such as basic
and transition metals (K, Cu, and Mn), as well as structure promoters (Al,
Si, and Zn) are added into the Fe catalysts to improve FT synthesis perfor-
mances [2,6,13,24,25,30,33 47]. The overall schematic representation of
the FT synthesis technology is shown in Fig. 7.1. However, the industrial-
ization of FT process was started by using a Ruhrchemie atmospheric