Biohydrogen Generation and Other Methods - Hydrogen Generation, Storage, and Utilization - page 57

CO and CH 4 in the product gas. In addition, thermal cracking that occurs
parallel to reforming produces carbonaceous deposits [34].

4.3.2 Hydrogen from Biomass via Gasification

Gasification of biomass has been identified as a possible system for produc-
ing renewable hydrogen, which is beneficial to exploiting biomass resources
and developing a highly efficient clean way for large-scale hydrogen produc-
tion, with less dependence on insecure fossil energy sources [35]. In general,
the gasification temperature is higher than that of pyrolysis and the yield of
hydrogen from the gasification is higher than that of the pyrolysis.
Biomass gasification can be considered as a form of pyrolysis, which takes
place at higher temperatures and produces a mixture of gases with H 2 content
ranging 6-6.5% [36]. The synthetic gas produced by the gasification of
biomass is made up of H 2 , CO, CH 4 , N 2 , CO 2 , and O 2 ; and tar is also formed
that is often removed with a physical dust removal method [37]. The product
distribution and gas composition depend on many factors, including the
gasification temperature and the reactor type. The most important gasifier
types are fixed bed (updraft or downdraft fixed beds), fluidized bed, and
entrained flow gasifiers. All these gasifiers need to include significant gas
conditioning along with the removal of tar and inorganic impurities and the
subsequent conversion of CO to H 2 by water gas shift reaction, as discussed
in the pyrolysis section. Table 4.4 shows typical gas composition data as
obtained from commercial wood and charcoal downdraft gasifiers operated
on low to medium moisture content fuels [38].
Gasification technologies provide the opportunity to convert renewable
biomass feedstocks into clean fuel gases or synthesis gases. The synthesis
gas includes mainly hydrogen and carbon monoxide (H 2 + CO), which is
also called bio-syngas [39, 40]. Table 4-5 shows the composition of bio-
syngas from biomass gasification [36]. Hydrogen production is the largest
use of syngas. Biomass can be converted to bio-syngas by non-catalytic,
catalytic, and steam gasification processes.

TABLE 4.4 Typical Gas Composition Data as Obtained from Commercial
Wood and Charcoal Downdraft Gasifiers Operated on Low to Medium
Moisture Content Fuels (Wood 20%, Charcoal 7%)

Heating Value (MJ·m − 3 )

Component H 2 (%) CO 2 (%) CH 4 (%) CO (%) N 2 (%)

Wood gas 12-20 9-15 2-3 17-22 50-54 5-5.9

Charcoal gas 4-10 1-3 0-2 28-32 55-65 4.5-5.6

Source : Reproduced with permission from Stassen and Knoef [38].