00/03254 Characterization of alkali metal emission from fuels and samples collected from fluidized bed gasification

Abstract

A novel technique for the characterization of the alkali metal release from various types of solid samples is presented. The alkali detection technique is based on surface ionization. A sample is heated in a laboratory unit, and alkali metals emitted from the sample are ionized at a hot platinum filament, giving rise to an ion current which is proportional to the arrival rate of alkali metal compounds onto the filament. The alkali metal release may be studied at temperatures up to 1,000 C, at pressures up to 10 bar and in either oxidizing or reducing atmospheres. To demonstrate the technique, some fuels, ash samples and bed materials collected from fluidized bed gasification have been analyzed at atmospheric pressure. For the fuels, two characteristic temperature intervals for alkali metal emission are generally identified. A small fraction of the alkali content is released below 500 C and is attributed to the decomposition of the organic structure. Another fraction of alkali compounds is released from the char residue at temperatures above 500 C. The difference in alkali release from samples of coal and biomass is demonstrated. For biomass samples with varying chlorine contents, differences in the alkali release profiles are observed. A highmore » chlorine content is shown to enhance the alkali metal emission from the char residue. It is demonstrated how the technique can be used for studying the effect of changing the operating conditions during fluidized bed gasification. The effects of different filter cleaning intervals and operating hours are discussed for samples of filter ash and fluidized bed material, respectively.« less