Page 388 - Refining Biomass Residues for Sustainable Energy and Bioproducts

Bioenergy generation from agricultural wastes and enrichment of end products 351

Thus it needs to be treated for an efficient use. The products obtained from different
processes require different technologies for its enrichment.

For pyrolysis, the major problem is associated with the biooil. Their direct applications as
fuels are limited by the problems of high viscosity, high oxygen content and corrosion,
and their thermal instability. Therefore biooils should be upgraded using proper methods
before they can be used in diesel or gasoline engines. Hydrotreating, hydrocracking, ester-
ification, emulsification, and catalytic pyrolysis are various techniques that can improve
the quality of biooil (Xiu and Shahbazi, 2012). Hydrotreating is a nondestructive, simple
hydrogenation process that removes the oxygen from the biooil and the common catalysts
used for this process are sulfide CoMo/Al 2 O 3 and NiMo/Al 2 O 3 systems. Hydrocracking is
another thermal process accompanied by cracking at high temperature at relatively high
pressure. Dual function catalysts are used where zeolites provide the cracking function,
whereas platinum and tungsten oxide catalyze the reaction. Polar solvents reduce the vis-
cosity of the biooil and also increase the heating value. Also, biooil can be emulsified
with other fuel source. Biooil with the help of surfactants can be emulsified with the die-
sel oil. Thus upgraded biooil can be used in engines or in other fields as a source of
energy.
For gasification, hydrogen-enriched fuel gas has gathered attention due to the use of syn-
gas after gasification in gas engine or gas turbine based power generation applications.
The enrichment of syngas can be done by catalytic gasification (metal oxides) or with
some modification in gasification zone of the gasifier for better interaction of the reac-
tants. End product of gasification contains CO, H 2 ,CH 4 , and CO 2 and traces of NH 3 ,
HCl, H 2 S, HCN, CS 2 , tar, particulate maters, etc. as impurities. To use it as synthesis gas,
these impurities must be removed so that CO and H 2 enriched. For this the exit gas from
gasification chamber passed through cyclone separator to remove particulate materials fol-
lowed by water wash, where NH 3 and HCl washed away with water. The exit gas, thereaf-
ter, should be passed through ZnO guard for H 2 S followed by active carbon filter for
traces of NH 3 , HCl, and CS 2 . Finally, it should be passed through silica gel for moisture
trap and can be utilized effectively.
In AD, raw biogas is a mixture of CH 4 (40% 65%), CO 2 (35% 55%), traces of H 2 S, N 2 ,
H 2 , water vapor, and other components (siloxanes, volatile hydrocarbons, chlorinated and
fluorinated hydrocarbons, etc.). The fraction of CH 4 is the combustible part of biogas and
its concentration determines the quality of biogas. Biogas upgradation is the removal of
contaminants (H 2 S and other toxic components, CO 2 , and water vapor) from biogas
stream to provide a methane rich ( . 95%) gas. Biogas upgradation techniques are mainly
divided into adsorption, absorption (physical and chemical), and membrane separation
(Miltner et al., 2017). Pressure swing adsorption is used to adsorb the CO 2 ,H 2 S, N 2 , and
O 2 from biogas stream on the basis of molecular size and thus providing 96% 98% pure
methane. Removal of H2S prior to this process is recommended as it irreversibly binds to
adsorbent material causing toxicity to it. Zeolites and activated carbon are the commonly
used adsorbents for biogas upgradation. Physical water scrubbing method is the most
commonly used technique for biogas purification based on the increased solubility of CO 2
and H 2 S in water in comparison to CH 4 . The pressurized biogas stream (6 10 bar) is
injected into an absorption column that is filled with packing material to increase the
gas liquid mass transfer, from the bottom side of the column, while the water is injected
from the top part of the column. Amine scrubbing, caustic solvent scrubbing, and amino
acid salt solutions are the separation techniques used for CO 2 and H 2 S removal from bio-
gas stream. Commonly used amines for chemical absorption are monoethanolamine,