Synergistic effects of biomass and polyurethane co-pyrolysis on the yield, reactivity, and heating value of biochar at high temperatures

Abstract Biochar is a valuable solid product from the pyrolysis of solid wastes including biomass and plastic. In this study, the synergistic effects of biomass and polyurethane co-pyrolysis on the yield, reactivity, and heating value of char were investigated in a fixed-bed reactor. The reactivity and heating value were measured by a thermogravimetric analyzer. The effects of pyrolysis temperature, mixing ratio, and biomass type were studied. For individual pyrolysis, the biomass char presents porous structures while the plastic char shows blocking structures. The fragmenting and grinding abilities of plastic char are enhanced at higher temperatures. In addition, while the pyrolysis temperature increases from 1100 °C to 1250 °C the chlorine in biomass disappears. During co-pyrolysis, due to the wrapping of melted plastic on biomass particle surfaces, the porous structures of biomass char are blocked, thereby inhibiting the volatile release. As a result, higher char yields and heating values are achieved than theoretically expected ones. This synergistic effect on char yield from wood co-pyrolysis with plastic is stronger than that from straw co-pyrolysis with plastic, at lower pyrolysis temperatures, and in the biomass ratio range of 25–50 wt%. The char reactivity increases with the decrease of pyrolysis temperature and the increase of biomass ratio. Because of the catalysis of KCl in biomass and the intensified fragmentation of plastic char, synergistic effects are observed in promoting the char oxidation. The heating values of char products from the individual pyrolysis of all three samples, first increase and then decrease with the increment of pyrolysis temperature. As a result, the maximum heating values are obtained at 900 °C. However, the co-pyrolysis significantly changes the depending relationship between the heating value and the pyrolysis temperature. E.g., at a 50 wt% biomass ratio, with the increment of pyrolysis temperature, the heating value increases for wood co-pyrolysis while decreases for straw co-pyrolysis.