Gasification of biomass or biomass-containing streams offers a means to produce syngas that can be combusted directly or converted to higher value products. Facilities of three gasification technologies that have been put into operation have encountered issues with degradation of structural components. Our laboratory has investigated the causes of degradation of structural components of these gasifiers. In all cases, the likely causes of degradation have been determined and alternate materials identified. In some cases, these alternate materials have been put into use and found to provide improved performance. Results from examination of two types of gasifiers have been reported previously, but they are summarized in this report in order to show the parallels in materials issues for biomass gasifiers.
More extensive use of our abundant biomass resources offers a means to reduce our use of fossil fuels and the associated production of greenhouse gases. Utilization techniques include biochemical and thermochemical processing with gasification being one means to produce a syngas for a gaseous fuel or conversion to a liquid product. However, the high temperatures required for biomass gasification can result in problems with corrosion/degradation of structural components of the gasification system. We have examined components from three totally different biomass gasification systems. Analyses have been conducted to determine the degradation mechanisms, and laboratory corrosion tests have been conducted in simulated gasifier environments. Alternate materials have been identified and, in some cases, put into use. Although serious material problems were encountered in all three technologies, material solutions were also identified.
Two of the biomass gasification systems that have been studied involved processing of black liquor, a paper mill aqueous waste stream that contains dissolved salts as well as organic material. The usual method of processing black liquor is to burn it in a chemical recovery boiler which generates steam for use in the mill as well as reducing sodium sulfate contained in the black liquor to sodium sulfide. This is not a particularly efficient process, and there are materials issues associated with the tubes that comprise the floor, walls and roof of the boiler as well as the heat recovery tubes in the superheater area. Rupture of any of these tubes could result in a catastrophic explosion if the water released by the tube failure reaches the molten salt on the boiler floor. In the superheater, condensation of low melting point salts limits the maximum operating temperature of these tubes although it has been demonstrated that it would be economical to use tubes of more expensive alloys that would be resistant to degradation by the salts thus enabling superheater operation at considerably higher temperatures.1,2 However, this would require a very significant change in boiler design as well as operating practices. Gasification offers a totally different approach to the processing of black liquor and is an unique alternative to conventional technology.
One of the black liquor gasification methods blended preheated black liquor with steam and a substoichiometric amount of air to produce syngas and molten salts. These molten salts were found to be very aggressive toward the refractory lining of the gasifier while some gaseous reaction products were found to penetrate the refractory lining and condense on the relatively cool gasifier shell where other materials degradation issues were encountered.3