Effects of Steam on the Release of Potassium, Chlorine, and Sulfur during Char Conversion, Investigated under Dual-Fluidized-Bed Gasification Conditions

Abstract

The corrosion and fouling of heat-transfer surfaces and the agglomeration of bed materials in fluidized beds are some of the ash-related problems caused by the transformation and release to the gas phase of ash-forming elements from biomass during thermochemical conversion processes. The magnitudes of these problems are largely dependent upon the release of potassium (K), chlorine (Cl), and sulfur (S) from the biomass. We investigated the effects of steam on the release of K, Cl, and S during char conversion, under conditions relevant for dual-fluidized-bed gasification (DFBG). The study was carried out with wheat straw in a laboratory-scale bubbling fluidized-bed reactor in the temperature range of 800–900 °C. The release of K, Cl, and S from wheat straw during devolatilization, char gasification, and char combustion was quantified with a mass balance that linked the masses of these elements in the wheat straw to the mass of the solid residue obtained at the end of each experiment. To facilitate analyses of the experimental results, leaching and the Brunauer–Emmett–Teller surface area measurement of the wheat straw and some of the solid residues were carried out. The results show that, during devolatilization, the release of volatile salts, e.g., KCl, is significantly limited by intraparticle diffusion resistance, owing to a compact char matrix (i.e., negligible porosity). However, during char gasification, steam renders the char less compact by expanding and/or creating new pores in the char. As a result, intraparticle diffusion resistance decreases, thereby facilitating the evaporation of volatile salts of K and S from the char matrix. The conversion of the char is also conducive to the release of char-bound K and S, especially at 900 °C. At temperatures of >800 °C, the relative proportions of the elements released and char gasified indicate that the release of K can somewhat be decoupled from the release of S and Cl by maximizing the extent of char conversion in the gasification chamber. The results also show that, during char combustion, the proportions of the char that can be combusted and the extent of the release of the elements are influenced by the extent to which the char is gasified in the gasification chamber.