Increase of bio-char yield by adding potassium salt during biomass pyrolysis

Abstract

Low yield is one of the key factors restricting the preparation of bio-char. Some studies suggested that the bio-char yield could be increased by adding potassium salts during biomass pyrolysis. Up to now this process has not been well understood. Therefore, this study focused on analyzing the pyrolysis of walnut shell impregnated with KCl at 400–800 °C to further reveal its mechanism. Results indicate that the effective temperature range of KCl is from 400 to 600 °C, during which the yield and free radical concentration of bio-char are both increased. This phenomenon is caused by the deposition of volatiles on KCl. It is found that there is a positive linear relationship between the increase of bio-char yield and the content of KCl covered by volatiles. Moreover, the KCl increases the defective structure of bio-char that may be related to the formation of quinone group. In addition, we analyzed the effect of different potassium salts, which follow the order: KCl > K2CO3 ≈ KOH > K2SO4 > KAc.