Page 252 - Lignocellulosic Biomass to Liquid Biofuels

208 Lignocellulosic Biomass to Liquid Biofuels

pellets and chips in entrained-flow gasifiers and fluidized-bed by fragmen-
tation. Moreover, temperature expressively alters its chemical properties
and finally increases the residual solid content. The level of undesirable
components of tars, dust, and ash produced during gasification depends
on some factors. These factors depend on the gasifying feedstocks, type of
reactors, and various operational parameters, such as temperature, pressure,
and gasifying agents [49].
Tar: Tar is a dark, oily, and viscous flammable liquid distilled from
biomass, charcoal, or coal. It is the mixture of hydrocarbons, resins, alco-
hols, and other compounds. It is the most important by-product that pro-
duced during biomass gasification. Sometimes, these components are
mixed with syngas which clog the transportation engines, when it is used
directly. This by-product also used for road-making and for coating and
preserving the timbers. The quality of the tar is affected by temperature,
residence time, and equivalent ratios that promote the thermal cracking.
Among these, temperature plays a vital role for the reduction of tar con-
tent in producer gas by promoting thermal cracking. Usually, with the
increase in the temperature, the concentration of tar decreases. Equivalent
ratio is another significant effect that promotes the oxidation reactions
during char volatilization. As a result, tar concentration increases by
increasing equivalent ratios [48].
NH 3 : Ammonia is a compound of nitrogen and hydrogen with the
formula NH 3 . During biomass gasification it is formed along with the pro-
duction of syngas. This type of impurity occurs because of the presence of
nitrogen in the feedstocks (biomass or coal). It also depends on the con-
centration of nitrogen in various feedstocks. Moreover, operating para-
meters impact the yield of nitrogenous impurities. The existence of steam
during gasification promotes the formation of NH 3 . These syngas impuri-
ties are generally produced due to the decomposition of proteins or het-
erocyclic aromatic structures. Moreover, nitrogen-containing compounds
in syngas may deactivate catalytic activity, and ultimately it may cause of
air pollution. Therefore the quantity of sulfur-based impurities in biomass
producer gas is lower in comparison to that of coal-based producer gas.
H 2 S, COS: These are the poisonous and flammable syngas impurities
that affect the quality of syngas. During gasification, primary gases are
involved in different reactions with H 2 S and other sulfur impurities. As a
result, these gases are influencing the yield.
Mercury: This is another type of toxic impurities that exists the pro-
ducer syngas. The producer gas contains several heavy metals in trace