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This paper presents the application of composite fly ash gel injection technology, along with several other methods to help extinguish outcrop coal fires (excavation, blasting and sealing). The Haibaoqing coal fires extended for 6000-7000 m across *1000-m-wide area, part of which was simulated by Fluent to determine the distribution of high temperature and the situation of leakage. The radon-test method was used to accurately detect the extent of the burning area, along with the degree and tendency of burning. The areas were then separated into three distinct regions: north, middle and south. The gel injecting and monitoring boreholes were then installed in the regions. A comprehensive application of composite fly ash gel injection technology, excavation, blasting and sealing was used to extinguish the outcrop coal fire. After subsurface blasting, the composite gel was injected into the coal seams to extinguish and prevent re-ignition of the fires by filling in new areas created by the blasting and forming an effective composite coverage in order to prevent combustion. Boreholes monitored temperature and CO concentrations during the entire process to indicate when the fire was extinguished, along with extended monitoring to verify no combustion reoccurred. Using this combination of methods has proven to effectively extinguish the coal fires at reasonable costs in the region.Keywords Composite fly ash gel Á Radon-test Á Borehole Á Excavation Á Blasting Á Sealing Á Outcrop coal fire List of symbols q mThe density of coal (kg m -3 ) k mThe conductivity of coal (W m -1 K -1 ) c mThe specific heat capacity of coal (J kg -1 K -1 ) q gThe density of air (kg m -3 ) k gThe conductivity of air (W m -1 K -1 ) c gThe specific heat capacity of air (J kg -1 K -1 ) q yThe density of rock (kg m -3 ) k yThe conductivity of rock (W m -1 K -1 ) c yThe specific heat capacity of rock (J kg -1 K -1 ) HComprehensive pressure of dynamic pressure, static pressure and thermal potential pressure (Pa) C Molar concentration of oxygen (mol m -3 ) q e Integrated density (kg m -3 ) k e Effective thermal conductivity (W m -1 K -1 ) V(T) Oxygen consumption rate of coal (mol cm -3 s -1 ) D 0The diffusion coefficient(m 2 s -1 ) k 1The seepage coefficient k 2The seepage coefficientThe convective heat transfer coefficient of rock (W m -2 K -1 ) q tThe density of soil (kg m -3 ) k tThe conductivity of soil (W m -1 K -1 ) c tThe specific heat capacity of soil (J kg -1 K -1 ) n i Permeability coefficient of i direction (m 3 s kg -1 ) D e Comprehensive diffusion coefficient (m 2 s -1 ) c e Effective specific heat capacity (J kg -1 K -1 ) Q j Air-leakage intensity of outcrop coal in direction j (m 3 m -2 s -1 ) q(T) Heat liberation intensity of coal (J m -3 s -1 )

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The coalfield fire is determined by fractures of coal and rock that provide tunnel for gases and heat exchange. To study fracture propagation at high temperatures, high-resolution X-ray computed tomography (CT) was used to scan anthracite and mudstone samples collected from the Qinshui coalfield, Shanxi Province, northern China. The samples were scanned at 100°C intervals as they were subjected to temperatures of up to 500°C. Three-dimensional images were reconstructed by the CT software to analyze changes in the fractures and pores in the samples. The experimental results show that fracturing of anthracite began at 200°C. The generation rate of fractures in the coal samples increases slowly below 300°C, but above 300°C there is a sharp increase in fracture development. This indicates that the thermal fracturing temperature threshold for anthracite is 300°C. During the experiment, it was found that preexisting fractures, voids, and regenerative fractures formed around the hard portions of anthracite particles or along the weak boundaries between particles. Some regenerative fractures developed along the fabric of the relatively crystalline particles within the particle and terminate at the edge of the particle or where the fracture encounters a harder portion of coal. Some fractures even expanded enough to be transformed into voids as temperatures rose. In the mudstone, the porosity changed suddenly at 300°C. This indicated that there was a void generated at 200°C, but the void expanded when the temperature was increased. However, changes in the void were not obvious from 200 to 300°C.