Steam Gasification

There are principally two approaches for the thermochemical conversion of coal into syngas. Autothermal gasifiersprovide the necessary heat-of-reaction by means of partial oxidation within the gasification reactor. This process applies commonly pure oxygen in order to avoid the dilution of the gas with nitrogen. Allothermal or indirect gasificationnecessitates getting the required heat-of reaction from an external heat source into the gasifier.

The main advantages of allothermal gasification systems are significantly higher H2/CO ratios which are particularly favourable for the synthesis of any liquid or gaseous energy carrier. Allothermal gasification systems are usually bubbling or circulating fluidized bed gasification systems. The main technical challenge for allothermal gasifiers is the heat transfer into the fluidized bed. Especially steam reforming requires high heat fluxes at high temperatures. Two concepts are discussed to solve the heat transfer problem: The first idea is to circulate hot solids (i.e. fluidized bed material) from a combustion zone to a gasifier zone. The most popular concept is the Güssing gasifier. It consists of two connected fluidized bed reactors - one for gasification and one for combustion. The bed material
circulates between the combustion chamber and the gasifier to transfer the required heat into the gasifier (FICFB Gasifier, Güssing [2]).

A second concept suggests indirectly heated fluidized bed gasifier systems with integrated heat exchanger tubes. The main problem with indirect heating is the generation of sufficiently high heat fluxes from the heat carrier gas, i.e. flue gas, into the fluidized bed. The heat transfer between the fluidized bed and the heat exchanger tubes is excellent, but the inner heat transfer between tube wall and the gaseous heat carrier is normally quite poor. Previous research projects have focussed on improving the heat transfer to the gasifier in the German Coal industries DMT / MBG process [3]. Another concept comprises pulse combustors [4]. The most effective technology for an indirectly heated gasification has been developed successfully within the European FP5 project “Biomass Heatpipe-Reformer” [5]. The application of heatpipes improves the heat transfer into the fluidized bed by magnitudes. First demonstration plants are currently under construction and in demonstration. The company AGNION is currently commercializing this concept for the production of syngas from biomass.

 

[1]  Karl, J. Frank, N., Karellas, S. Saule, M., Hohenwarter, U., Conversion of Syngas from biogas in solid oxide fuel cells, Proceedings of FUELCELL2006, June 19-21, 2006, Irvine, CA
[3]  Hofbauer. H. Conversion Technoliogies: Gasification overview, Proc. 15th Europ. Biomass Conf. Berlin, May 2007
[4]  Kubiak et. al.”Fluidized Bed Generator for allothermic Gasification of Coal”, US-Patent No. 5439491, 1995
[5]  Thomas. M. Grace, ”Bring on the gasifiers” PIMA Magazine, April 1996 Pp 42-43