Mechanisms underlying responses of pore structures in continental oil shale to microwave pyrolysis

Objective and Methods  Microwave pyrolysis technology holds great potential to achieve clean energy conversion and efficient energy utilization of oil shale resources. This study investigated oil shale in the Triassic Yanchang Formation, Ordos Basin. Through microwave pyrolysis experiments under different power values (600 W, 800 W, and 1000 W) conducted using a microwave equipment monitoring system, this study tested the gas products generated during pyrolysis. Using the nitrogen adsorption method, this study compared and analyzed the pore structure characteristics between the original oil shale and samples after microwave heating under different power values. Additionally, the mechanisms behind damage to oil shale during microwave heating were thoroughly explored.

Results After microwave pyrolysis, primary gas products of oil shale included CO₂, CH₄, H₂, and CO, as well as small amounts of free hydrocarbons such as n-butane and isobutane. With an increase in the microwave power, the pyrolysis time of the oil shale significantly shortened, and the pyrolysis rate greatly increased. Compared to the original sample, oil shale after microwave heating exhibited significant changes in the pore structure. Specifically, the ratio of the micropore volume to the macropore volume increased significantly, and the total pore volume increased with the microwave power. However, the proportion of mesopores decreased, leading to a decrease in their connectivity, and the proportion of micropores increased, resulting in more complex microporous structures. Under the action of microwaves, the water molecules inside the oil shale and the polar functional groups in organic matter were polarized and rapidly vibrated, promoting the rapid heating and pyrolysis of the oil shale. An increase in the temperature induced the formation and propagation of microfractures, leading to increased volume of some primary pores. Kerogen cracking and organic matter decomposition promoted the gradual propagation and interconnection of original pores and fractures, leading to the formation of fracture channels. Concurrently, the production and emission of gases such as CH₄, CO₂, and CO accelerated the development and interconnection of pores and fractures.

Conclusions Microwave pyrolysis can significantly change the pore structures of oil shale and increase pore volume, further promoting the rapid pyrolysis of oil shale. The results of this study will provide a solid theoretical foundation and experimental basis for the development of microwave pyrolysis technology for oil shale.