Production of liquid aromatics by two-stage co-pyrolysis of sawdust and low-density polyethylene

Biomass can be converted into light aromatic hydrocarbons (such as benzene, toluene, and xylene) through catalytic pyrolysis to produce high-value chemicals, enabling its efficient utilization. However, biomass possesses the characteristic of being “rich in oxygen and deficient in hydrogen,” resulting in the generation of a large amount of oxygen-containing compounds during the pyrolysis process. This leads to easy oxidation, high acidity, and viscosity, as well as poor quality of the liquid products, with low aromatic hydrocarbon content, necessitating further modification and upgrading for utilization. Co-pyrolysis of plastics and biomass holds promise for improving the quality of biomass pyrolysis liquid products by utilizing the hydrogen-rich properties of plastics, reducing the oxygen content in the pyrolysis liquid products, and increasing the yield of aromatic hydrocarbons. In this study, a two-stage pyrolysis process, consisting of a preliminary pyrolysis stage and a synergistic conversion stage, was designed to synergistically convert pine wood sawdust and low-density polyethylene (LDPE) into liquid aromatic hydrocarbons. The liquid products obtained from the catalytic pyrolysis process naturally separate into an organic phase and an aqueous phase. When using the two-stage pyrolysis system, the organic phase yield was 11.4% higher than that of traditional single-stage pyrolysis. By comparing the separate pyrolysis and co-pyrolysis of sawdust and LDPE, it was found that there was a synergistic effect based on hydrogen transfer during the co-pyrolysis process, which promoted the effective deoxygenation of oxygen-containing compounds generated during the initial pyrolysis of sawdust into aromatic hydrocarbons, thereby increasing the yield of the target product, aromatic hydrocarbons, and this synergistic effect was more significant in the two-stage process. The proportion of LDPE in the feedstock influenced the strength of the synergistic effect during co-pyrolysis; when the mass ratio of sawdust to LDPE was 1∶1, the hydrogen supply from LDPE during the pyrolysis process was more balanced with the hydrogen demand of the sawdust pyrolysis products. At this ratio, the synergistic effect during co-pyrolysis was most significant, with an organic phase yield of 47.2% and an aromatic hydrocarbon content of 93.8%. Increasing the temperatures in the preliminary pyrolysis stage and the synergistic conversion stage can promote the volatilization of sawdust and LDPE and enhance the aromatization reaction, but excessively high reaction temperatures can lead to excessive cracking of intermediate products and promote gas generation; the highest yield of aromatic hydrocarbons from co-pyrolysis was achieved when the temperature in the preliminary pyrolysis stage was set at 600 ℃ and in the synergistic conversion stage at 500 ℃. The alkaline oxide MgO can enhance the preliminary pyrolysis of sawdust and LDPE, producing higher-quality intermediate products for subsequent synergistic conversion processes, thereby increasing the yield of aromatic hydrocarbons. Using MgO as the catalyst in the preliminary pyrolysis stage resulted in a 5.2% increase in the organic phase yield compared to the acidic oxide ZrO₂; further enhancing the synergistic conversion effect, co-loading the HZSM-5 catalyst in the synergistic conversion stage with Ga₂O₃ using the incipient wetness impregnation method led to an organic phase yield of 51.5%, with an aromatic hydrocarbon content of 98.9%