Dependence of alkali emissions in PFB combustion on coal composition

The dependence of alkali release in fluidised bed combustion on fuel composition was studied using excimer laser fragmentation fluorescence (ELIF), an in-situ method that determines gas-phase alkali species with sub-ppb sensitivity. The PFB-reactor at DMT was operated under combustion conditions and ELIF-signals were detected in the flue gas after the cyclone. Within each experiment, the coal composition was varied systematically by doping the coal with known amounts of additives. The chlorine and sodium contents were increased separately, by respectively doping methylene chloride into the air feed of the fluidised bed or sodium acetate into the coal. Sodium chloride was used as dopant in some runs, but this raised both components simultaneously. The addition of chlorine alone leads to overproportional increases in the alkali concentration whereas that of sodium acetate lead to relatively moderate increases. Thus the amount of chlorine in the coal and in the FB reactor is decisive for the level of alkali emitted. In contrast, the presence of clay minerals e.g. kaolin in the coal or the fluidised bed is found to strongly suppress alkali release, especially that of potassium species.