Effects of the endogenous potassium and calcium metal ions on the yields and characteristics of products drived from pyrolysis of corn stalk

Endogenous alkali and alkaline earth metals (AAEMs) in biomass ash and pyrolysis temperature have a significant impact on the properties of pyrolysis polygeneration products. In this study, alkali metal (potassium) and alkaline earth metal (calcium) were selected as representative metals of AAEMs. Potassium ions (K+) and calcium ions (Ca2+) at different concentrations (2%, 5%, 7%) were added to corn stover via the impregnation method, and fixed-bed pyrolysis experiments were conducted at 400, 500 and 600 ℃ to systematically investigate the yields and compositions of pyrolysis gas, liquid, and solid products. The results showed that in terms of the effect of concentration, with the increase of metal ion concentration, the biochar yield increased significantly—when the concentration of Ca2+ was 7%, the biochar yield reached 24.96%—while the bio-oil yield generally decreased. Due to its strong Lewis acidity, Ca2+ significantly promoted the formation of H2 (accounting for 32.49% in the gas phase at 7% concentration) and facilitated the enrichment of furan compounds to 65.88%. For K+, low concentrations were conducive to the formation of phenolic substances, while high concentrations promoted the generation of ketone compounds and intensified bio-oil cracking. In terms of the effect of temperature, as the pyrolysis temperature increased from 400 to 600 ℃, the biochar yield decreased, while the gas yield increased continuously. In addition, high temperatures promoted the secondary cracking and reforming reactions of volatiles, significantly increasing the yields of H2 and CH4 while inhibiting the formation of oxygen-containing components. Specifically, at 600 ℃, catalysis by K+ increased the content of acidic substances to 39.41%, whereas catalysis by Ca2+ increased the content of furan compounds to 65.89%. This study realized the directional regulation of high-value components in bio-oil and high-energy gases by adjusting the concentration of metal ions and pyrolysis temperature, providing a theoretical basis for the optimized utilization of biomass pyrolysis products.