Environmental Engineering Reference
In-Depth Information
A water-gas shift reaction can be applied in order to increase the hydrogen
2 .
Pyrolysis of biomass is a complex function of the experimental conditions,
under which the pyrolysis process proceeds. The important factors, which
affect the yield and composition of the volatile fraction liberated, are biomass
species, chemical and structural composition of biomass, particle size, tem-
perature, heating rate, residence time, atmosphere, pressure and reactor con-
iguration [24]. Yield of products resulting from biomass pyrolysis include
charcoal, from a low temperature and low heating rate process, liquid prod-
ucts, from a low temperature, high heating rate, and short gas residence time
process, and fuel gas, from a high temperature, low heating rate, and long
gas residence time process.
The yields of hydrogen-rich gases via pyrolysis were related to the tem-
perature. Increasing the pyrolysis temperature resulted in an increase in the
hydrogen yield as a percentage of the total gases evolved [25]. The percent-
age of hydrogen in gaseous products by pyrolysis from the samples of
hazelnut shell, tea waste, and spruce wood increased from 36.8% to 43.5%,
41.0% to 53.9%, and 40.0% to 51.5% by volume, respectively, when the inal
pyrolysis temperature was increased from 700 to 950 K. One of the methods
to increase the hydrogen yield is to apply catalytic pyrolysis. Three types of
biomass feedstock, olive husk, cotton cocoon shell and tea waste were pyro-
lyzed at about 775-1025 K in the presence of ZnCl 2 and catalyst-to-biomass
ratios of 6.5-17 by weight [26]. The highest yield of hydrogen-rich gas
(70.3%) was achieved from olive husk using about 13% ZnCl 2 at about
1025 K. The K 2 CO 3 and Na 2 CO 3 as catalysts also affected on yield of gaseous
products from various biomass species with pyrolysis. The effect of K 2 CO 3
and Na 2 CO 3 on pyrolysis depends on the biomass species. The catalytic effect
of Na 2 CO 3 was greater than that of K 2 CO 3 for the cotton cocoon shell and
tea factory waste, but the catalytic effect of K 2 CO 3 was greater for the olive
husk [26]. The yields of hydrogen-rich gas from pyrolysis of agricultural
residues in the presence of Na 2 CO 3 were different at different temperatures.
Results shown in Figure 4.2 indicate that the highest yield of hydrogen-rich
gas was obtained from the walnut shell sample [27]. Among the different
metal oxides, Al 2 O 3 and Cr 2 O 3 exhibit better catalytic effect than the others
[23]. The use of noble metals (Rh, Ru, and Pt) in large-scale industrial steam
reforming is not common because of their relative high cost [28].
Hydrogen can also be produced by catalytic steam reforming of bio-oil or
its fractions [29, 30]. Hydrogen production from bio-oil provides a new route
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