A common set of reactions, given in Table 15.1, occurs when carbonaceous solids undergo thermal processing. Whether the aim is pyrolysis, gasification, or combustion, each of these reactions occurs in some parts of the reactor because of gas–solids contacting. In this chapter, we consider, in order, gasification as an endothermic process to generate H2 and CO mixtures for fuel or synthesis gas, pyrolysis as a process to generate useful tars (or liquids), and combustion as a process to produce heat.
Most commercial gasifiers use coal as feed, and may be classified by the type of solids–gas contacting (moving, entrained, fluidized, or spouted bed), by the state of the ash (dry, agglomerated, or molten), and by the oxidant (air, air–steam, or oxygen–steam). A low-calorific-value gas results from air–steam gasification, and a medium-calorific-value gas from using steam or steam–oxygen mixtures. The carbonaceous feedstock may be fed as a dry solid, a sludge, or a slurry.
Performance measures can be identified for comparing gasifiers of different designs and operating conditions. For production of fuel gases, the heating value of the produced gas is important and is usually reported on a dry gas basis. For synthesis gas or pure hydrogen production, the molar ratio of H2∕CO leaving the gasifier is critical. Low tar yields are usually beneficial, unless a raw fuel gas is desired. For sizing scaled-up gasification processes, throughput of solids feed per unit cross-section of reactor (kg/m2s) is of major importance.