Biomass gasification reaction

The resulting synthesis gas can subsequently be converted into methanol (Reaction 3) or polymerized to a mixture of hydrocarbons via the Fischer-Tropsch synthesis (Reaction 4) [37, 38]. These conversions usually require a H2/CO molar ratio close to 2 (Reactions 3 and 4), which contrasts with the H2/CO ratio of 0.5 that is delivered upon biomass gasification (Reaction 2). It can therefore be suitable to adjust the H2/CO ratio through the water-gas shift reaction (Reaction 5) ... 

Reed. T.B. Markson, M. (1985). Biomass gasification reaction velocities. 

Figure 1.2 shows a generally agreed sequence of gasification reactions for coal and biomass.29 The gasification reaction... 

In the first reaction, pyrolysis, the dissociated and volatile components of the fuel are vaporized at temperatures as low as 600°C (1100°F). Included in the volatile vapors are hydrocarbon gases, hydrogen, carbon monoxide, carbon dioxide, tar, and water vapor. Because biomass fuels tend to have more volatile components (70 to 86% on a dry basis) than coal (30%), pyrolysis plays a larger role in biomass gasification than in coal gasification. 

As discussed above, the pyrolysis of biomass at high temperature (>1000 °C) results in the formation of synthesis gas, a valuable mixture of CO and H2. The decomposition of carbohydrate to synthesis gas is an endothermic reaction since the heating value of product is —125% of that of the feedstock (Reaction 1). The reaction becomes nearly thermo-neutral upon burning about 1/4 of the products. Since the thermodynamics favors the combustion of H2 over CO, the gasification reaction resemble the theoretical Reaction (2). Indeed numerous gasification processes feed 02 or air to drive the gasification reaction. 

Depending on the reason for converting the produced gas from biomass gasification into synthesis gas, for applications requiring different H2/CO ratios, the reformed gas may be ducted to the water-gas shift (WGS, Reaction 4) and preferential oxidation (PROX, Reaction 5) unit to obtain the H2 purity required for fuel cells, or directly to applications requiring a H2/CO ratio close to 2, i.e., the production of dimethyl ether (DME), methanol, Fischer-Tropsch (F-T) Diesel (Reaction 6) (Fig. 7.6). 

Table 2.2. Energy change for idealised cellulose thermal conversion reactions. (Source T. Reed (1981)., Biomass Gasification, reproduced with permission. Copyright 1981, Noyes Data Corporation.)...

The pyrolytic gasification of biomass has been interpreted to involve the decomposition of carbohydrates by depolymerization and dehydration followed by steam-carbon and steam-carbon fragment reactions. So the chemistries of coal and biomass gasification are quite similar in terms of the steam-carbon chemistry and are essentially identical after a certain point is reached in the gasification process. Note, however, that biomass is much more reactive than most coals. Biomass contains more volatile matter than coal, and the pyrolytic chars from biomass are more reactive than pyrolytic coal chars. 

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