Solid fuel in the presence of a gasifying agent (air, oxygen, steam) under thermal action undergoes chemical decomposition to produce the useful gas. According to the type of gasifying agent used, the heating value of the product gas obtained will also be different.

The conversion of gasification feedstocks can be divided into several gross stages: (1) decomposition of the original feedstock into volatile matter and char,

(2) conversion of the volatile matter by secondary reactions (combustion and reforming), and (3) conversion of the char by ‘char gasification’ reactions with H2O and CO2 to produce fuel gases (CO, H2, CH4), in addition to char combustion when oxygen is present. Devolatilization produces a broad spectrum of products, ranging from light gases to tars. The products are strongly dependent on the identity of the feedstock and process conditions, such as heating rate. These products may contain valuable species. Partial reforming of these products by contact with components of the char bed may result in improved gas quality. For example, if fuel gas is the desired product, such conversion could preserve methane while reforming undesirable tars. The progress of such reforming reactions is dependent on the nature of the char, including the inorganic (ash) components, and the type of reactor. The conversion of the entire feedstock to fuel gases by gasification reactions is generally endothermic, and air or oxygen is typically added to heat balance the process. In general, the solid fuel, during gasification, undergoes the following four processes that are more distinct in the case of the moving bed gasifier (such as an up and down draft gasifier) than in the case of fluidized bed gasification. The mechanism of gasification is shown in Fig. 16.3 and explained in detail below.

1 Drying: In this pricess, the moisture in the feedstock is vaporized. The feedstock does is not decompose because the temperature is not high enough to cause any chemical reaction.

2 Pyrolysis: During pyrolysis or devolatization, the volatile content of the matter is released from the feedstock and char is left. This reaction occurs in the absence of oxygen and at a temperature around 300-500°C. The reaction occurring in this process is endothermic in nature, thus the heat required is provided by the combustion of the feedstock during the oxidation process.

Feedstock = char + volatiles + energy (kJ/kg). [16.1]

3 Oxidations: In this process, the feedstock is combusted with the air supplied. As gasification is an endothermic process, the overall heat required is produced during this process. To maintain a favorable temperature in the gasifier and also avoid the excess dilution of the product gas, an equivalent ratio (actual air supply/stoichiometric air required for complete combustion) is maintained between 0.2-0.4. The reactions taking place in this process are:

C + O2 ^ CO2, [16.2]

2H2 + O2 ^ 2H2O. [16.3]

4 Reduction: In this process, several reactions take place. The product from this process is mainly the gas, consisting of carbon dioxide, hydrogen, methane and carbon monoxide. The following reactions take place:

The aforementioned reactions are the major gasification reactions. Depending upon the operating conditions, one reaction dominates over another and, thus, the product composition changes accordingly. For example, if steam gasification is used then the reactions [16.5] and [16.7] would be major ones and it can be seen that increasing temperature increases reaction [16.5] while decreasing reaction [16.7]. Similarly, if it is air or oxygen gasification, then reaction [16.4] will be the major one that increases with rise in temperature. Thus, air/oxygen gasification will have a higher concentration of CO.