Many fluid-bed gasification processes have been and are being developed, but none of them incorporate the use of a heat carrier (sand and/or ash and/or char) in such a way that the tars are combusted and char reacts with the gasifying agent to produce synthesis gas and/or fuel gas. Such a system could produce pure syngas without the need for an air separation unit (ASU). Moreover, the gas is free of tars and the carbon conversion is virtually complete.
An example for a simplified process scheme in which the problems with tar are circumvented is shown in Figure 5-12 (Holt and van der Burgt 1997). The feed coal is fed to a bubbling fluid-bed pyrolyser, which is fluidized with a small amount of air and/or steam and uses a relatively small part of the hot heat carrier from the top of the riser (entrained bed) as a heat source. The complete or partial combustion of all gases and tars from the pyrolyser takes place with air in the bottom of the riser reactor. Moreover all residual carbon left on the heat carrier leaving the gasifier proper is (partially) combusted in the riser. The hot gases leaving the top of the riser via cyclones have the typical composition of a low Btu fuel gas or of a flue gas, depending on whether partial or complete combustion is used. Ash that is virtually free of carbon is removed from the system as a bleed from the cyclones in the top of the riser. In principle, two modes of operation are possible.
When synthesis gas is the required product, most of the hot heat carrier leaving the top of the riser is used in the endothermic gasifier section where the char leaving the pyrolyser reacts with steam according to the water gas reaction. When insufficient pyrolysis products are available for (partial) combustion in the riser, some additional coal and/or oil may be injected into the bottom of the riser or the char slip from the gasifier to the riser may be increased. Where fuel gas is the required product the gasifier reactor may be omitted.