Effects of acid and metal salt additives on product characteristics of biomass microwave pyrolysis

Abstract

To study the influence of additives on the pyrolysis behavior of microwave-heated biomass, cotton stalk pyrolysis was performed using both a particular microwave pyrolysis reactor and an electric heating system with the addition of acid (H3PO4) and metal salts (NaCl, K2CO3, and MgCl2) at 550 °C. The derived products were evaluated by environmental scanning electron microscopy, gas chromatography with mass spectrometric detection, and temperature adsorption analysis. The results indicated that microwave heating with additives was beneficial to enhance the quality of the pyrolysis products. The addition of H3PO4 was favorable for liquid oil formation with higher furfural and acetic acid contents. The highest yield of oil was achieved with 8% H3PO4, while the maximum char yield and surface area were obtained at 12% H3PO4, as higher percentages caused the formation of more porous solid char. The addition of metal salts promoted the formation of char at the cost of decreasing the liquid oil production. The maximum yields of bio-oil and char were achieved with the addition of MgCl2, while the minimum yields were obtained by adding K2CO3. The addition of MgCl2 (8%) promoted the formation and pore growth of char during the pyrolysis process; moreover, it was helpful for the production of furfural and acetic acid substances as well as a small amount of naphthalene. The addition of K2CO3 (8%) increased the gas production significantly but decreased the specific surface area of char. The addition of NaCl (8%) increased the char yield but decreased the oil and gas yields slightly. The comparative results showed that microwave pyrolysis with H3PO4 and MgCl2 additives were beneficial for the preparation of activated char with a large specific surface area and optimal formation of bio-oil.