SYNTHESIS GAS TREATING
Although the number of gasification processes for any particular feedstock is limited, the range of choice for acid gas removal (AGR) processes and other gas treating tasks appears at first sight somewhat bewildering. Their selection requires a basic knowledge of the different classes of process and a systematic approach to defining the task in hand. Although the approach developed in this chapter cannot replace the expertise of the gas purification specialist, it can help to ensure that he or she has all the facts available to make an optimum selection.
The term acid gas removal is a general term that is often used as a synonym for desulfurization, but strictly speaking in the synthesis gas environment it includes also C02 removal, and it is in this inclusive sense that the term is used here.
A large number of different processes have been and are used for acid gas removal, but they are all based on one of the following principles:
• Absorption (physical or chemical) in a liquid solvent with a subsequent desorption step.
• Adsorption (again physical or chemical) onto a mass of solid particles.
• Diffusion through a permeable or semipermeable membrane.
• Chemical conversion, generally on a catalyst, often as a preparatory step to one of the above three methods.
Practically all commercially available processes operate with the syngas “cold,” that is, at ambient temperatures or lower (chemical adsorption of trace H2S on zinc oxide is the prominent exception). The loss of efficiency in IGCC applications, which is associated with gas cooling between gasifier and gas turbine, has generated considerable interest in the possibilities of hot desulfurization processes, and these are treated separately as a topic in its own right.