Characteristics of pyrolysis products of biomass under cryogenic pretreatment using liquid nitrogen

Biomass is a valuable and sustainable energy source that can be converted into bio-oil and combustible gases through pyrolysis. Despite being a widely used method for biomass conversion, pyrolysis has been plagued by the issue of low yield and poor quality of pyrolysis products. To address these challenges, a novel approach involving cryogenic pretreatment with liquid nitrogen was explored for wheat straw and corn straw. This method, which combines cryogenic pretreatment with fast pyrolysis, aims to enhance product yield and quality. Results from the study show that compared to traditional pyrolysis, this method increased bio-oil yields by 14.16~23.73 wt%. The highest bio-oil yield of 22.3~26.3 wt% was achieved at a pretreatment temperature of -90 °C, with a direct relationship observed between cryogenic temperature and bio-oil yield. Cryogenic pretreatment with liquid nitrogen has a significant transformation effect on the composition distribution of bio-oil, and there is a significant increase in BTEX and phenolic substances in the bio-oil composition. Cryogenic pretreatment with liquid nitrogen inhibits the release of CO2 and CO in the biomass pyrolysis gas, and the content ratio of the components decreases by 7.2% to 29.7%; it promotes the generation of CH4 and H2, and the content ratio of the components increases by 0.2% to 14.7%. In addition, cryogenic pretreatment increases the content ratio of C elements in the elemental composition of the solid product by 0.2% to 2.98%, while reducing the content ratio of O elements by 0.28% to 2.52%, which is beneficial to improving the combustibility of bio-char and reducing its risk of spontaneous combustion. Thus, cryogenic pretreatment significantly enhances both the yield and composition of bio-oil, providing a promising pathway for efficient biomass conversion into liquid fuels.