Typical biomass gasification takes place in the presence of injected air (or oxy­gen) and steam under high pressure at an elevated temperature, T > 850°C. In this regard, typical biomass gasification is very similar to advanced coal gasification process technologies [5, 28, 29]. The chemical reactions taking place in a biomass gasifier are very complex and they include: (1) pyrolytic decomposition of hydrocarbons and oxygenated organics such as carbo­hydrates (or saccharides) and cellulose, (2) further decomposition of frag­mented hydrocarbons (of reduced molecular weights), (3) recombination of methylene and methyl radicals, (4) partial oxidation of hydrocarbons and oxygenates, (5) steam gasification of hydrocarbons and oxygenates, (6) water gas shift reaction, (7) formation of polycyclic aromatic hydrocarbons (PAHs) and potential coking precursors, (8) carbon dioxide gasification of carbona­ceous materials, and more.

CxHy ^ CaHb + CcHd + e • H2

CxHy ^ CfHg + h • CH4 + j • H2

CuHvOw ^ CkHlOw1 + CmHnOw2 + P • H2O + Ц • H2 + Г • CO2 + S • CO

CxiHyl ^ fHgi + K • (• CH2 •)
(• CH2 •) + H2 ^ CH4

(• CH2 •) + (• CH2 •) ^ C2H4

CO + H2O ~ H2 + CO2

CxHy + x ■ CO2 ^ 2x ■ CO+y ■ H2

The first five reactions represent pyrolytic decomposition reactions of hydrocarbons and oxygenates, which provide some explanation for the for­mation of methane and lighter hydrocarbon species. The last five reactions explain the formation of carbon oxides and hydrogen, principal ingredients of biomass syngas. One can also notice that the above reactions are anal­ogous to the four classical gasification reactions of carbon and concurrent water gas shift reaction as shown below [14].

Cs + H2O ^ CO + H2
Cs + CO2 ^ 2 ■ CO
Cs + 2 ■ H2 ^ CH4
Cs + O2 ^ CO/CO2
CO + H2O ~ H2 + CO2

where Cs denotes carbon on the solid surface.

The reactions listed above are called steam gasification of carbon, Boudouard reaction, hydrogasification of carbon, partial oxidation of car­bon, and water gas shift reaction, respectively. It should be clearly noted that the last three reactions, as written, are exothermic, whereas the first two are endothermic at their typical operating conditions. The water gas shift reaction can proceed either in the forward or reverse direction depending upon the temperature and imposed/developed reaction environment. The forward water gas shift reaction is mildly exothermic, whereas the reverse water gas shift reaction is mildly endothermic.

Chemical equilibrium constants for a wide range of temperatures for selected chemical reactions that are of significance to the gasification of car­bon are listed in Table 5.9. Although the values are for reactions of carbon, nei­ther of biomass nor of coal char, they still provide general ideas for reactions involving carbonaceous matters. As the C/H ratio of solid materials increases or the number of carbon atoms in a hydrocarbon molecule increases, their thermodynamic equilibrium values become closer to those of carbon reac­tions. Furthermore, if biomass is pretreated before any gasification reactions,

then the equilibrium values in the table would be more relevant and closer to the actual values.

The temperature where Kp = 1 (i. e., ln Kp = 0) has some extra significance, by indicating the general location of chemical equilibrium shift. The tem­peratures where Kp = 1 for steam gasification, Boudouard reaction, hydro­gasification, and water gas shift reaction are 947 K (674°C), 970 K (697°C), 823 K (550°C), and 1,087 K (814°C), respectively. For example, it may be said that for the steam gasification of carbon to proceed in the forward direction, the gasification temperature must be higher than 674°C. Among the reactions listed in Table 5.9, the temperature-dependent variation of the equilibrium constant is the weakest for the water gas shift reaction, thus exhibiting an easily reversible nature of the chemical equilibrium for a very wide range of temperatures. This is the reason why the water gas shift reaction equilib­rium becomes a player in nearly all syngas reaction systems under widely varying process conditions.

When the coal gasification reaction is explained or modeled, most tech­nologists denote and simplify coal more or less as carbon, that is, C(s), based on the fact that the hydrogen content of coal is much lower than that for most hydrocarbons. However, such practice in the case of biomass gasification would become an oversimplification, inasmuch as the oxygen and hydrogen content in biomass feedstock are much higher than those of high rank coal. The abundance of oxygenated functional groups such as hydroxyl (-OH) groups in biomass makes most decomposition and transformation reactions proceed more easily.