18.3 Biomass as Feedstock for Biohydrogen

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(i) Agricultural waste includes both agricultural food crop residues and animal

wastes.

(ii) Forest residues which encompass all terrestrial trees/shrubs and aquatic plants,

wood, and logging residues.

(iii) Municipal waste involves both household and industrial waste and sewage

sludge.

All these categorized biomasses are mainly composed of three elements, i.e.

cellulose, hemicellulose, and lignin. Cellulose and hemicellulose are polymers

of C6 and C5 sugars, respectively. Various researchers have evaluated the direct

conversion of raw lignocellulosic biomass to hydrogen using microbial fermen-

tation to avoid cost-intensive pretreatment processes. However, this resulted in

very low yield and efficiency, mainly due to the inaccessibility of biocatalyst to

sugars embedded inside the lignin and cellulose content of the biomass. It has,

in turn, streamlined research toward the application of pretreatment methods for

efficient sugar release from lignocellulosic biomass. The pretreatment methods

are preferable over lignocellulosic biomass since it improves hydrolysis, solu-

bilization, and recovery of high sugar yields for fermentative H2 production.

Many research groups have investigated various forms of pretreatment, which

again depends on the type of biomass used for energy conversion [8]. Several

techniques that include mechanical, chemical, enzymatic, thermo-chemical, and

thermal processes are used to enhance the solubilization of organic matter. The

lignin-rich biomass requires harsh treatment, which includes a combination of a

few of these pretreatment techniques for breaking the recalcitrant polysaccharides

into small sugar molecules that can be easily metabolized by microorganisms

for their growth and hydrogen production. Mechanical grinding and sieving are

essential steps of pretreatment of lignocellulosic agricultural and forest residues

to produce uniform particle size and increase the surface area for fermentation.

This step is followed by a chemical or thermo-chemical pretreatment method using

alkali, acids, oxidizing agents, solvents, ionic liquids, and sometimes combined

with thermal treatment at different temperatures. Cornstalk waste, which is a

major component of agricultural waste, has been used to produce hydrogen after

pretreatment with alkali and acid, followed by heat treatment [9]. This resulted in

maximum cumulative H2 yield ranging from 57 to 150 ml-H2/g volatile solid (VS),

which was many-fold higher than the initial value, thus supporting the efficiency

of pretreatment methods.

Similar to lignocellulosic biomass, municipal solid waste (MSW) also requires

pretreatment for biological hydrogen production. The composition of MSW varies

from place to place, and it is the nature and composition of MSW, which significantly

affects hydrogen production yields. Numerous studies have been conducted by

researchers to evaluate the effect of composition and sources of MSW on hydrogen

yield, which concluded that MSW rich in simple carbohydrates or sugars such

as food waste and vegetable waste give high hydrogen yields [10]. It is further

corroborated that MSW rich in fat and protein act as low-grade feedstock for H2

production.