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Syngas fermentation to bioethanol 209

quantities, such as Hg, As, Se, Cu, Pd, Cd, and Zn. It is more difficult to
eliminate oxidized Hg in the form of HgS.
HCl: Hydrochloric acid is a corrosive, strong mineral acid that some-
times forms during gasification with the syngas. Hydrogen halides concen-
tration in syngas is affected by temperature. As a result, removal of this
impurity is very important to diminish corrosion and slagging of the
equipment surfaces (filters, turbine blades, and heat exchanger).

6.5 Syngas purification

The syngas quality is very much depending on the presence of these
impurities in the produced syngas. The syngas cleaning is required for
chemical-production process, while combustion on high-ranked coal-fired
power stations almost requires no cleaning. It is expected that the syngas
for biological fermentation process toward the ethanol production will
require some cleaning but not very stringent. The syngas-cleaning pro-
cesses are as follows:
Wet scrubbing: This is one of the most effective techniques for remov-
ing particles from syngas. A wet scrubber acts by introducing the syngas
with a scrubbing liquid (i.e., water). Purified syngas is separated, and parti-
cles are collected with the scrubbing liquid.
Catalytic tar removal: The tar concentration is removed by catalytic
cracking of biomass over the char-supported Ni catalyst in a lab-scale
fixed-bed reactor. Recent studies have investigated the effect of catalytic
cracking temperature, Ni loading, and residence time of gas on product
distribution and gas composition, as reported by Hu et al. [50]. They also
found the optimum conditions for catalytic cracking a catalytic cracking
temperature of 800°C, 6 wt.% Ni loading, and a gas residence time of
0.5 s. Baidya et al. [51] also studied on the high-performance Ni Fe Mg
catalyst for tar removal in producer gas.
Thermal tar removal: Tar is eliminated by thermal decomposition com-
bined with physical adsorption, using a reformer as first step and a fixed-
bed absorber as second step. The required temperature for thermal tar
decomposition is about 800°C. The operational temperature has a signifi-
cant influence on tar decomposition. The gasifying tar was efficiently
decomposed by improving the efficiency of tar reduction. To this scope,
either steam or air was introduced into the reactor as a reforming agent.
Tar decomposition leads to the reduction of tar from the syngas that is
required to prevent damage to downstream equipment.