Removal of ash in biochar from carbonization by CO₂-enhanced water leaching and its mechanism

Ash content is an important factor affecting the quality and combustion performance of biochar. In this paper, a method of ash removal from biomass is proposed by carbonization followed by CO₂-enhanced water leaching. The effects of the carbonization temperature of bagasse, the temperature and time of CO₂-enhanced water leaching on the deashing were investigated. The results show that the deashing rate firstly increases and then decreases with the carbonization temperature, while the opposite trend is obtained with increasing the water leaching temperature and time. For bagasse biochar carbonized at 300 ℃, the deashing rate reaches 57% at the water leaching temperature of 40 ℃ for 4 h. Compared with water leaching without carbonization, the proposed method can increase the content of fixed carbon and the char yield by 7% and 3%, respectively. It is because  in the process of deashing, CO₂ diffuses and dissolves into water to form carbonic acid which reacts with part of metal salts to form water-soluble salts, resulting in the removal rate of K, Na and Ca up to above 50%, and part removal of calcite and dolomite. Also, the proposed process shows higher deashing efficiency and universality, but the deashing degree is closely related to the ash composition and kinds in biochar. As to peanut shell and poplar, the deashing rate exceeds 30% by carbonization at 300 ℃ and CO₂-enhanced water leaching at 40 ℃ for 4 h.