Research on the retorting pyrolysis mechanism and rational control conditions in underground coal gasification

The retorting pyrolysis zone of underground coal gasification directly affects syngas quality. However, current research suffers from an unclear pyrolysis mechanism and blind control strategies. Using coal samples from the Hancheng Magouqu Mine, thermogravimetric and fixed-bed experiments were conducted to analyze the evolution of pyrolysis products and to clarify the pyrolysis mechanism. Finally, rational control conditions were proposed based on different gasification processes. The results show that: (1) The retorting pyrolysis consists of three stages – decarboxylation, active pyrolysis, and polycondensation – with the active pyrolysis stage occurring in the temperature range of 370–540 °C. (2) In the early stage (150–370 °C), decarboxylation predominates, and CO2 is the main gaseous product. During the middle stage (370–540 °C), free-radical dehydrogenation and aromatic ring condensation dehydrogenation take place, generating large amounts of tar, with H2 being the dominant gas component. In the late stage (540–600 °C), polycondensation prevails, and the CH4 content is relatively high. (3) Considering both the carbon conversion rate and calorific value, it is recommended that the temperature in the retorting pyrolysis zone be maintained at 450–500 °C for forward combustion gasification and at 540–600 °C for injection point backward gasification. These findings provide a theoretical basis for the process design of underground coal gasification.