Potassium transformation and release during biomass combustion

Abstract To study the transformation and release of fuel K, the combustion of corn stalk and wheat straw were performed in a reactor with a fixed bed between 400–1000°C. Measurements of weight and elemental composition, chemical fractionation analysis, and low‐temperature ashing coupled with X‐ray diffraction (XRD) analysis were conducted on biomass and residual samples. The effects of biomass species, temperature and duration time of combustion, oxygen concentration, and pretreatment of water washing were evaluated. The results show that compared with wheat straw, corn stalk has a lower K release, which is much more sensitive to the combustion temperature than to the oxygen concentration. The inorganic potassium occurs as KClO 3 and KClO 4 in wheat straw and KCl and KClO 4 in corn stalk. K 2 SO 4 appears through sulphation of KCl as combustion happens at 800°C or above. When the combustion temperature reached 900°C for corn stalk and 1000°C for wheat straw, some KCl changed into K 2 Si 2 O 5 , which contains insoluble K. K 2 Ca(SO 4 ) 2 appears as corn stalk and wheat straw are burnt at 1000 and 900°C, respectively. Along with the proceeding of wheat straw combustion, more ion‐exchangeable and water‐soluble K transform to the insoluble K with the increasing temperature. The pretreatment of water washing removes nearly all the water‐soluble K from the corn stalk and significantly decreases the K release from 3.26–0.27 mg g −1 in quantity and from 26.74%–14.84% in ratio at 1000°C, respectively.