Effects of different types of inorganic potassium in biomass on volatile-char interactions for co-pyrolysis of coal and biomass

Co-gasification of biomass and coal has many advantages. However, the volatile-char interaction during co-pyrolysis, which is the first step of co-gasification, significantly changes the properties and the structure of biochar, thus affecting gasification. The present work investigated the effect of the interaction between the volatile from a coal and the biochar of K-salt-doped (K2CO3, K2SO4 and KCl) pine wood in a lab-scale, fixed-bed reactor at 600, 700 and 800 degrees C to understand how the occurrence mode of inorganic K in biomass alters the volatile-char interaction during co-pyrolysis. The volatile-char interaction has insignificant effects on the biochar of pine wood without the doping of K salts while it has significant effects on the biochar of K-salt-doped pine wood, especially for K2CO3 and K2SO4. The volatile-char interaction significantly increases the formation of C-O-K bonds, the content of oxygen containing compounds and the aromaticity of the biochar for K2CO3-doped and K2SO4-doped pine wood. In contrast, the evident presence of C-O-K bonds can only be found for KCl-doped pine wood at 800 degrees C after the volatile-char interaction. The increase in the pyrolysis temperature and the presence of the volatile-char interaction make the carbon structure of the biochar of K2SO4-doped and KCl-doped pine wood less ordered and generate more active sites. However, although the char order is lower and the number of active sites is higher for the biochar of K2CO3-doped pine wood than for that of K2SO4-doped and KCl-doped pine wood, the volatile-char interaction has insignificant effects on improving the activity of its biochar. Lastly, the volatile-char interaction causes insignificant additional release of K from the biochar. This may indicate that significant amounts of stable C-O-K bonds have been formed in the biochar to retain K.