Release and transformation of alkali metals during co-combustion of coal and sulfur-rich wheat straw

Abstract Co-combustion of coal and biomass is a low-cost, large-scale, and efficient way to utilize biomass energy, which has a wide range of potential applications. However, biomass, especially herbaceous fuels, contains high levels of volatile K, Na, and Cl, the use of which may result in ash-related operational problems, such as corrosion, fouling, and slagging during thermal utilization. The aim of this study is to investigate the effects of wheat straw and temperature on the release and transformation of alkali metal species during co-combustion of coal and S-rich wheat straw. Results indicate that the amounts of K and Na released during co-combustion could be reduced by the effects of Fe, Ti, S, Si, and Al from blended fuels. At lower wheat straw shares, the release of K decreased due to reactions of KCl with Fe species, and Ti species, forming K 2 Fe 2 O 4 and K 2 TiO 3 . At high wheat straw shares, the release of K could be mainly captured in the form of K 2 SO 4 ; small amounts of KAlSiO 4 were also observed in the bottom ash. When the wheat straw share was 80 wt.%, increasing temperatures enhanced the release of KCl(g) and NaCl(g) at 600–800 °C. By contrast, in the range of 800–1000 °C, the amounts of these gases released exhibited no apparent association with temperature. Compared with the release of K, fuel K was mainly retained in the bottom ash. The K 2 SO 4 content in the bottom ash decreased with increasing temperature in the range of 600–1000 °C, whereas the fraction of K in the form of KAlSiO 4 and KAlSiO 6 increased with increasing temperature, especially at temperatures between 800 and 1000 °C.