Distributions of products from the pyrolysis of tar-rich coals for tar extraction in northern Shaanxi Province, China

Objective Tar-rich coals stand as a special type of coal resource that integrates coal, oil, and gas properties. These coals, boasting abundant hydrogen-rich structures such as side chains and bridge bonds with aliphatic structures, are prone to crack and produce tar and gas during pyrolysis. These establish tar-rich coals as ideal raw materials that produce oil and gas through thermal conversion. Presently, the research into tar extraction from tar-rich coals through pyrolysis remains in its initial stage, and exploring the impacts of technological conditions, including heat carrier, heating type, and pyrolysis temperature, on the pyrolysis process holds critical significance for optimizing the pyrolysis for oil and gas extraction.

Methods Focusing on the tar-rich coals from the Shenmu mining area in northern Shaanxi Province, this study delved into the production characteristics of oil and gas from the pyrolysis of tar-rich coals under gaseous heat carriers CO₂ and N₂. Using the response surface methodology, this study constructed regression models to explore the impacts of several critical factors on both tar yield and light tar yield obtained from the pyrolysis of tar-rich coals. Using these regression models, this study investigated the impacts of the interactions among the heating type, gaseous heat carrier type, and pyrolysis temperature on the tar yield and light tar yield, along with the optimal reaction conditions for the maximum yields.

Results and Conclusions The results of this study are as follows: (1) In the case of slow pyrolysis, under the N₂ atmosphere, the tar yield increased first and then decreased as the temperature rose, reaching a maximum of 10.50% at 550 ℃, while the light tar yield kept increasing, reaching 60.67% at 600 ℃. In contrast, under the CO₂ atmosphere, both the tar yield and the gas yield were elevated. Meanwhile, the light tar yield further increased under the CO₂ atmosphere, reaching a maximum of 63.67%, with the contents of aromatic hydrocarbons and oxygen-containing compounds in the tar also increasing. (2) In the case of rapid pyrolysis, the tar yield obtained under both gaseous heat carriers rose, reaching a maximum of 11.71% at 600 ℃ under the CO₂ atmosphere, with the carbolic and anthracene oil contents in the tar increasing. In contrast, the light tar yield decreased, with the contents of phenols, aliphatic hydrocarbons, and oxygen-containing compounds in the tar increasing. Additionally, the H₂ and CH₄ concentrations in the gas products decreased somewhat. (3) The response surface methodology reveals that the impacts of the three factors on both tar yield and light tar yield decrease in the order of pyrolysis temperature, gaseous heat carrier type, and heating type. The optimization using the regression models allows for the identification of two optimal pyrolysis processes for the maximum tar yield and light tar yield: rapid pyrolysis at 595 ℃ in the CO₂ atmosphere and slow pyrolysis at 558 ℃ in the CO₂ atmosphere, with the optimal tar yield and light tar yield predicted at 11.72% and 60.75%, respectively. Experimental verification indicates that the tar yield and light tar yield under the optimal conditions were 11.94% and 60.67%, respectively, roughly aligning with the predicted results.