Gas clean-up

Gasification or partial oxidation is likely to be practiced with one of two purposes: to supply gaseous feed for an IGCC plant, or to use the carbon monoxide/hydrogen mixture as synthesis gas. Neither application uses raw gas with no downstream treatment. The desirable components of the product gas are carbon monoxide, hydrogen, methane, and other light hydrocarbons. If the gas were to be used in combustion applications on site, carbon dioxide and water vapor would be considered neutral, i.e. having neither a positive nor negative effect, except for their effect as diluents of the combustible gases. If the gas is to be upgraded, further processed, or shipped by pipeline, then both of these components are undesirable. Processing units would have to be larger to handle these “extra” components that make no contribution to the calorific value of the gas. Some heat and compression work would be wasted on diluents of the desired components of the gas. Components of the raw gas that are always undesirable include particulate matter, droplets of tar, hydrogen chloride, ammonia, and hydrogen sulfide and other sulfur-containing compounds.

A first treatment step involves removal of particulate matter, which may consist of fine particles of ash or of partially reacted feedstock. This can be accomplished using a cyclone separator. Cyclones can be designed to operate at temperatures to 1000 °C, and pressures to about 50 MPa. They work well for particle sizes above 5 µm. If it is necessary to remove finer particles, baghouses with fabric filters or electrostatic precipitators can be used.