Experimental study on physical-chemical characteristics of three-phase products during simulated in-situ pressurized pyrolysis of tar-rich coal

As a special coal-based hydrocarbon resource, the efficient and clean utilization of tar-rich coal is of great significance to realizing the "dual carbon" target in China. In-situ pyrolysis of tar-rich coal is a brand-new method to convert coal into oil and gas resources and contribute to the classification and utilization. At present, this technology is still in its infancy, and the research on the physical and chemical properties of pyrolysis products of tar-rich coal is still limited. The common temperature range of in-situ pyrolysis of tar-rich coal, the particle size samples from micron to millimeter, and three pyrolysis atmospheres of N₂, CO₂ and 50% N₂+50% CO₂ were selected. The effects of these three impacting factors on the physical and chemical properties of three-phase products of tar-rich coal pyrolysis were focused on through a self-built experimental system of pressurized pyrolysis. To better understand the thermal impact of the tar-rich coal pyrolysis process, the distribution features of the "pyrolysis-oxidation" three-phase products of tar-rich coal are examined using micron and millimeter samples, ignoring the effects of mechanical action. The results show that the mass loss of coal samples with large particle sizes is higher after pyrolysis, showing stronger pyrolysis reactivity. The volume fraction of CH₄ increased from 17.64% to 37.92% when the particle size of the coal sample increased from 2.00 mm to 10.00 mm. The increase in pyrolysis temperature improves the yield of gas and tar. meanwhile promotes the polycondensation of aromatic nuclei, however, excessive temperature may lead to the intensification of secondary reaction and the decrease of tar yield. In the pressure pyrolysis conditions, the generation of pyrolysis water in the CO₂ atmosphere consumes more H radicals, so the yield of H₂ in CO₂ atmosphere (14.64 mL/g, 8.54%) is lower than that in N₂ atmosphere (23.01 mL/g, 16.37%). The CO₂ atmosphere promotes the oxidation of monocyclic aromatic hydrocarbons and the cleavage of ether bonds. Simultaneously, it inhibits the polycondensation and cracking of aliphatic hydrocarbons so that the contents of aromatic hydrocarbons and phenols decrease and the contents of aliphatic hydrocarbons increase. Data support for the industrial application of in-situ pyrolysis of tar-rich coal is helpful in being provided.