Agglomerating Fluid-Bed Processes
The idea behind agglomerating fluid-bed processes is to have a localized area of higher temperature where the ash reaches its softening point and can begin to fuse. The purpose of this concept is to allow a limited agglomeration of ash particles that, as they grow, become too heavy to remain in the bed and fall out at the bottom. This preferential separation of low-carbon ash particles is designed to permit a higher carbon conversion than conventional fluid-bed processes.
A potential advantage for such processes over conventional fluid beds is that the problem of a leachable ash is less serious because of the ash agglomeration step incorporated near the burner(s) in the bottom of the reactor. The burner(s) in these gasifiers are in fact oxygen/air lances that have two functions: that of introducing the fluidizing gas, and also creating a hot region where ash agglomeration occurs. As mentioned before, such two-in-one features are nice but always put restrictions on the operation as one tries to operate in the “no-go” temperature range between the ash softening point and the ash melting point.
Two processes have been developed using this principle: the U-gas technology developed by the Institute of Gas Technology (IGT), now offered by Carbona, and the Kellogg Rust Westinghouse (KRW) process.
Several U-gas gasifiers operating at about 4 bar have been installed in China. A description of the process is contained in Reimert and Schaub (1989).
The 100-MWIGCC Pinon Pine Plant near Reno, Nevada, in the United States uses the KRW process. This plant could not be started up successfully, primarily because of difficulties in the hot gas filtering section (U. S. Department of Energy 2002).