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Steam Gasification and Reforming Technologies
greater than 2 at the inlet of the HTS reactor was needed. Due to low tar content in
the inlet gas to the HTS reactor, a significant less deactivation of the catalyst in the
shift reactor occurred.
4.5.1.3 mixed Feedstock
In recent years, significant efforts have been made to gasify the mixtures of coal and
biomass, coal and waste, or biomass and waste in the presence of steam. Seo et al.
[30,88,89] used the successful two-stage fluidized bed model described earlier for
coal–biomass blend in the temperature range of 750°C–900°C and the steam/fuel
ratio of 0.5–0.8. Biomass-to-coal ratio was varied from 0 to 1. The study showed
that the product gas yield, carbon conversion, and cold gas efficiency increased with
increasing temperature and steam/fuel ratio. These parameters were higher for biomass gasification than those for coal gasification. A synergistic effect on gas yields
was observed with a larger surface area, pore volume, and presence of micropores
at a biomass/total feed ratio of 0.5. The calorific values of the product gas at 800°C
were 9.89–11.15  MJ/m 3 with the coal, 12.10–13.19  MJ/m 3 with the biomass, and
13.77–14.39 MJ/m 3 with the coal–biomass blend. The maximum cold efficiency was
0.45 with a biomass/total feed ratio of 0.5. Sun et al. (2001, pers. comm.) examined
various kinetic models for the gasification of biomass blended with waste filter carbon at temperatures around 850°C. Once again, high temperature and high flow
of steam increased the gasification rate; the gasification rate of filter carbon was
lower than that of wood chip. The data were taken for the steam pressure of 0.5 atm.
A modified volume reaction kinetic model best fit all the data.
Kumabe et al. [27] showed that at 900°C, the mixture of woody biomass and
coal in the presence of steam and air gave favorable results. The results of this study
were similar to those described above; increase in biomass gave more gases and
more hydrogen was produced at higher steam-to-feedstock ratio. Higher amount
of biomass also gave lower amount of char and tar. The study produced gas with
composition that was favorable to methanol, hydrocarbon fuels, and dimethyl ether
(DME) under high biomass feed conditions. The co-gasification was carried out in a
downdraft fixed-bed reactor and it provided cold gas efficiency ranging from 65% to
85%. Demirbas et al. [90], Demirbas and Caglar [91], and Demirbas [92,93] studied
hydrogen production from various biomass samples, black liquor, biomass/coal, and
biomass/heavy oil mixtures. In a most recent study, Demirbas [94] studied the effects
of co-firing MSW with pulverized coal in a bubbling fluidized bed combustor. The
results showed that the mixture produced less NO x and SO x in direct proportions to
the MSW concentration in the mixture. Similarly, mixture produced less CO 2 than
coal alone. The mixture burning can, however, bring the problems with chlorine
impurities in MSW that can lead to corrosion problems and inorganic impurities
such as Si, Al, Ti, Fe, Ca, Mg, Na, K, S, and P that can significantly change the composition of ash and its melting and agglomeration characteristics. This change in ash
characteristics may limit the market for its downstream use.
Numerous other investigators have also examined the steam gasification (some
in the presence of air or oxygen) of a variety of coal–biomass mixtures. Chmielniak
and Sciazko [95] produced syngas from steam gasification of coal–biomass mixture that was subsequently transformed to methanol, DME, ethylene, and gasoline.