Experimental study on the thermal properties of coal during pyrolysis, oxidation, and re-oxidation

Applied Thermal Engineering, Volume 110, 5 January 2017, Pages 1137-1152

Jun Deng, Qing-Wei Li, Yang Xiao, Chi-Min Shu

Abstract:During heat transfer in coal, the thermal properties parameters (thermal diffusivity, specific heat capacity, and thermal conductivity) play a key role. The thermal properties of coal during pyrolysis, oxidation, and re-oxidation were investigated by laser-flash apparatus LFA457 from 30 to 300 °C. Thermogravimetric (TG) experiments were conducted to analyze the variation of coal mass. The results showed that during oxidation, the thermal properties of coal presented stage characteristics. As temperature increased, the thermal diffusivity decreased first and then increased; the specific heat capacity rose first and then performed steadily; the thermal conductivity presented a slow increment first and then rapidly grew. During pyrolysis and re-oxidation, the thermal diffusivity decreased as temperature increased, while the specific heat capacity and thermal conductivity increased. The thermal diffusivity and thermal conductivity during re-oxidation were higher than those during primary oxidation, while the specific heat capacity was lower. Meanwhile, the sensitivity of thermal diffusivity to temperature during re-oxidation decreased, while the sensitivity of thermal conductivity increased compared with that during pyrolysis and oxidation. The results are meaningful for an understanding of heat transfer in coal seam during spontaneous combustion of coal and fire spread.

Thermal analysis of spontaneous combustion behavior of partially oxidized coal

Process Safety and Environmental Protection, Volume 104, Part A, November 2016, Pages 218-224

Jun Deng, Jingyu Zhao, Yanni Zhang, Anchi Huang, Xiangrong Liu, Xiaowei Zhai, Caiping Wang

Abstract:Research on partially oxidized coal helps in the early detection of spontaneous combustion due to secondary oxidation of the coal remaining in mined-out areas. Three types of coal samples were used in this study. A self-designed temperature-programmable experimental system was developed to investigate the spontaneous combustion characteristics of partially oxidized coal. In addition, the differences between the oxidation characteristics of a fresh and a partially oxidized coal sample were studied. The CO concentration and its production rate, the oxygen consumption rate and the heat release rate were used as macro-characteristic parameters to show the oxidation characteristics. The results show that the characteristic parameters increased faster with temperature for partially oxidized coal compared to a fresh sample up to a temperature of 110–140 °C. Above this temperature the opposite is observed.

Experimental studies of spontaneous combustion and anaerobic cooling of coal

Fuel, Volume 157, 1 October 2015, Pages 261-269

Jun Deng, Yang Xiao, Qingwei Li, Junhui Lu, Hu Wen

Abstract:To study the spontaneous combustion and anaerobic cooling of coal, a 15-t experimental furnace was constructed, and large-scale spontaneous combustion tests were conducted. The coal sample came from No. 3 seam in Dongtan mine in Yanzhou coalfield, Shandong province, China. The variations in temperature, gas ratios, oxygen consumption rate, and heat intensity were studied. The experimental results show that for coal spontaneous combustion below 70 °C, the temperature increases slowly with time and the oxygen consumption rate and heat intensity increase slowly with temperature and accelerate when the temperature exceeds 100 °C. In anaerobic cooling, the cooling rate decreases with the decrease in temperature. When the coal temperature goes from 452.7 to 250.0 °C, the cooling rate decreases rapidly, whereas below 250.0 °C, the cooling rate decreases slowly. The variation trends of the CO2/CO, CH4/C2H6, C2H4/C2H6, and C3H8/C2H6 gas ratios in the spontaneous combustion and anaerobic cooling of coal in a certain range of temperature can be used to predict the temperature of coal.

Effects of pyrite on the spontaneous combustion of coal

International Journal of Coal Science & Technology, December 2015, Volume 2, Issue 4, pp 306–311

Jun Deng Xiaofeng Ma Yutao Zhang Yaqing Li Wenwen Zhu

Abstract:Pyrite has a significant effect on the spontaneous combustion of coal. The presence of pyrite can change the propensity of coal towards spontaneous combustion. The influences of various pyrite contents on the parameters of spontaneous combustion, such as index gases, temperature and released heat etc., were investigated in this study. Coal samples with different pyrite contents (0 %, 3 %, 5 %, 7 % and 9 %) were made by mixing coal and pyrite. The oxidation experiments under temperature-programmed condition were carried out to test the release rate of gaseous oxidation products at different temperatures. Differential scanning calorimeter (DSC) was employed to measure the intensity of heat release during coal oxidation for various pyrite contents. The results indicate that pyrite can nonlinearly accelerate the process of spontaneous combustion. The coal sample with a pyrite content of 5 % has the largest CO release rate and oxygen adsorption as well. However, the coal sample with a pyrite content of 7 % has the largest rate of heat flow according to the results from the DSC tests. Pyrite contents of 5 %–7 % in coal has the most significant effects on spontaneous combustion within the range of this study. The conclusions are conducive to the evaluation and control for the spontaneous combustion of coal.

Experimental study on the corresponding relationship between the index gases and critical temperature for coal spontaneous combustion

Journal of Thermal Analysis and Calorimetry, 18 August 2016

Yang Xiao Qing-Wei Li Jun Deng Chi-Min Shu Wei Wang

Abstract:The critical temperature is a key parameter in coal spontaneous combustion. To acquire variation of the critical temperature and the corresponding relationship between the index gases and critical temperature, we gleaned the coal samples from No. 3 seam in Yanzhou coalfield, Shandong Province, China. Thermogravimetry (TG) was used to analyze the effects of particle size, heating rate, and oxygen concentration on critical temperature. The index gases were determined by 15-t experimental furnace of coal spontaneous combustion. The results showed that the critical temperature measured by TG experiment indicates a trend that is augmented with the increase in particle size. When the heating rate was set up with 5 °C min−1 and the oxygen concentration was 14.9 mass% (13 vol%), the critical temperatures of coal samples with different particle sizes reached the maximal values. When the heating rate rose to 20 °C min−1 and the oxygen concentration was 14.9 mass% (13 vol%), the critical temperatures of coal samples with particle sizes <0.105 mm reached maximums, while they reached minimums when the particle size was >0.105 mm. As the particle was smaller than 0.098 mm, the difference in critical temperatures during two kinds of heating rate tended to first increase, then decrease, and reached a maximum under the condition that the oxygen concentration was 14.9 mass% (13 vol%), while it reached a minimum when the particle size was larger than 0.098 mm and presented a style of “M.” The critical temperature as measured by TG experiment was in the range of 45.4–103.2 °C. The critical temperature is, respectively, achieved in coal spontaneous combustion by the 15-t experimental furnace, in which the concentrations of CO and CO2 increased obviously and the value of CO2/CO reached its maximum.