Dynamic investigation on potassium migration and transformation during biochar combustion and its correlation with combustion reactivity

For agricultural biomass combustion, K migration and transformation is a key factor affecting boiler operation and reaction characteristics. In this study, dynamic K migration and transformation characteristics during rice straw char combustion process under different O2 concentrations were studied using chemical fractionation analysis (CFA), X-ray diffraction (XRD) and scanning electron microscopy coupled with energy dispersive X-ray (SEM-EDX). Furthermore, the correlation between K migration/transformation and char combustion reactivity was discussed. The results showed that most of K in rice straw char was H2O-soluble K. And the main forms of H2O-soluble K and insoluble K are KCl and KAlSi3O8, respectively. As reaction proceeded, the contents of H2Osoluble K and insoluble K decreased but those of NH4Ac-soluble K and HCl-soluble K increased. Most of the released K is H2O-soluble K. H2O-soluble K would be transformed into NH4Ac-soluble K and HCl-soluble K, and some insoluble K would be transformed into H2O-soluble K. The increase of O2 concentration would stimulate the transformation of H2O-soluble K into NH4Ac-soluble K, HCl-soluble K and released K. However, the reaction to form K-aluminosilicate was hardly found during the combustion process at 400 celcius. Ultimately, it is found that H2O-soluble K is the key occurrence form K affecting the combustion characteristics of rice straw char.