Volatiles Combustion
The devolatilization of coal produces a variety of species, including tars, hydrocarbon liquids, and gases, including methane, CO, C02, H2, H20, HCN, and so forth. (Smoot and Smith 1985). This material reacts with the oxidant surrounding the coal particle. The extent to which the oxidant is completely or only partially depleted depends on the amount of volatiles produced.
In a combustion environment, where there is an overall excess of oxygen, the combustion of the volatiles is complete. In a gasification environment, this is not necessarily the case, especially where the coal has a high volatiles content. There is a recirculation of synthesis gas in many gasification reactors, not only in fluid-bed but also to some degree in entrained-flow reactors. To the extent that this occurs in the vicinity of the burner, the effects are very different from the combustion situation. Recirculated combustion flue gas consists mainly of carbon dioxide, water vapor and (in the air-blown situation) nitrogen. The carbon dioxide and water vapor have
a moderating effect, thus reducing temperature. In the case of gasification the recycled gas contains significant quantities of carbon monoxide and hydrogen (up to 90% for an oxygen-blown gasifier) and will cause locally very high temperatures, should it come into contact with the oxidant.
There is not much kinetic data available on volatiles combustion. It is, however, clearly established that this process, being a reaction between gases, is much more rapid than the heterogeneous char gasification where mass transport limitations play a more important role.
