Page 41 - Energy from Toxic Organic Waste for Heat and Power Generation

Energy Extraction From Toxic Waste Originating From Food Processing Industries 29

55%–75% carbohydrates by dry weight. Cellulose, like starch, is a polymer
of glucose. However, unlike starch, the specific structure of cellulose favors
the ordering of the polymer chains into tightly packed, highly crystalline
structures that are water-insoluble and resistant to depolymerization [41].
Crystalline cellulose comprises the major proportion of cellulose, whereas a
small percentage of unorganized cellulose chains form amorphous cellulose.
Cellulose is more susceptible to enzymatic degradation in its amorphous
form [38].
Zheng et al. [42] and Hashem et al. [43] documented their research
work on the biogas production from a biomass-based feedstock and the
removal of heavy metal from agricultural waste. They have indicated that
hemicelluloses were mainly derived from the chains of pentose sugar and
acted as the cement material holding together the cellulose micells and fi-
ber. The backbone of the chains of hemicelluloses can be a homopolymer
(generally consisting of a single sugar repeat unit) or a heteropolymer (mix-
ture of different sugars). Among the most important sugars of the hemi-
cellulose’s component is xylose. In hard wood xylan, the backbone chain
consists of xylose units which were linked by β-(1 → 4)-glycosidic bonds
and branched by α-(1 → 2)-glycosidic bonds with 4-O-methylglucuronic
acid groups [43]. In addition, O-acetyl groups sometime replaced the OH
groups in positions C 2 and C 3 .
Demirbas [44] has reported that, for the soft biomass, the acetyl groups
were fewer in the backbone chain. However, soft biomass xylan has addi-
tional branches consisting of arabinofuranose units linked by α-(1 → 3)-gly-
cosidic bonds to the backbone. Hemicelluloses are largely soluble in alkali
and, as such, are more easily hydrolyzed. Lignin is covalently linked with
xylans in the case of hard organic biomass and with galactoglucomannans in
soft biomass. The basic chemical phenyl propane units of lignin (primarily
syringyl, guaiacyl, and p-hydroxy phenol) are bonded together by a set of
linkages to form a very complex matrix. This matrix comprises a variety of
functional groups, such as hydroxyl, methoxyl, and carbonyl, which impart
a high polarity to the lignin macromolecule [45]. These functional groups
and different types of sugars with carbon in the form of cellulose, xylan,
and lignin are the main base for quality biogas production and stimulating
anaerobic digestion.

3.5.3.2 Factors That Influence Biogas Production
Factors such as temperature, pH balance, carbon/nitrogen ratio, re-
tention time, and feedstock input rate to the digester affect the rate of