Research progress and conceptual framework of in-situ (catalytic) pyrolysis based on coal structure regulation and process intensification

The contradiction between the urgent demand for clean and efficient utilization of coal resources and the difficulty in overcoming technical bottlenecks in coal utilization (pyrolysis) has become increasingly prominent. It is thus crucial to systematically explore methods and a conceptual framework for the in-situ (catalytic) pyrolysis of low-rank coal, aimed at directional regulation of product yield and composition by influencing its physical/chemical structure and reaction processes. Investigating coal structure can reveal the regularities of product distribution and reaction pathways during pyrolysis. Intensifying the pyrolysis process can regulate reaction activity and mass transfer behavior, influencing free radical reaction pathways and the secondary transformation processes. Methods such as solvent swelling/dissolution, in-situ metal ions loading, solvent swelling coupled with metal catalysis, and co-pyrolysis can realize in-situ regulation of coal structure and intensification of the pyrolysis process through reconstructing the coal matrix, promoting directional bond cleavage, and supplying hydrogen, thereby optimizing the composition and distribution of products. Based on a multi-level/scale coupled regulation strategy, following a “point-line-surface” progression and taking co-pyrolysis as the anchor, coal-derived oil as an additive, benefiting from its “like dissolves like” and hydrogen-rich characteristics, can simultaneously exert swelling/dissolution and hydrogen-donating effects (point). Meanwhile, loading metal ions onto coal can establish a coupled system of “oil swelling/dissolution combined with in-situ catalysis” (line), thereby proposing a conceptual framework of co–(catalytic) pyrolysis of coal and oil (surface). In addition, the current limitations of coal pyrolysis research and the key issues that require further attention are also highlighted.