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• The design process of a well-type underground gasification ignition system was introduced. • The influence of ignition source on ignition process was studied. • The effect of exhaust borehole resistance on the ignition stage was obtained. • The longitudinal expansion and transverse expansion behaviors of the hot zone during the ignition stage were obtained. To investigate temperature field expansion behavior during the ignition stage, the variation of temperature fields of 12 sites versus time were studied using thermocouples during the ignition process. The results showed that the temperatures measured, and combustion processes, were not uniform. At the position where the igniter had started successfully, igniter 10 showed the smoothest flow in its exhaust holes, followed by igniters 4 and 1, successively. At the position of igniter 10, the ignition materials burned well and the temperature curve was smooth, but the highest combustion temperature of oil-containing split wood layer and coke layer was only 393 °C, the highest combustion temperature of block coal layer and solid coal wall was only 591 °C and 936 °C respectively. At the position of igniter 1, the ignition materials burned inadequately and the temperature curve was rough, but the highest combustion temperatures of oil-containing split wood layer, coke layer, block coal layer and solid coal wall were as high as 960 °C, 983 °C, 899 °C and 1280 °C respectively. The presence of the ignition source ensured, but did not determine, the occurrence of ignition. The temperature field in the gasified area could expand transversely by spreading combustion of the burning front and transverse movement of high-temperature gases to ignite the ignition materials away from fire sources. In locations with high exhaust resistance, the temperature field expanded primarily through the spreading of the burning front, while in the area with low exhaust resistance, the temperature field mainly through expanded the transverse movement of high-temperature gases. [ABSTRACT FROM AUTHOR]