In-situ pyrolysis is widely regarded as a promising low-carbon pathway for the utilization of tar-rich coal resources. Tar-rich coal is characterized by a high tar yield during thermal decomposition; however, its large-scale underground application is constrained by fundamental challenges associated with inefficient coal-seam heating, limited heat transfer within deep coal seams, high external energy demand, and difficulties in process control and subsurface safety management. In this study, existing research on underground in-situ coal conversion and pyrolysis technologies is systematically synthesized, with a focus on their heating mechanisms, energy supply modes, and intrinsic technical limitations. Particular attention is given to the physicochemical behavior of tar-rich coal during pyrolysis and the oxidation and spontaneous combustion characteristics of the resulting pyrolytic semi-coke. The analysis indicates that the exothermic oxidation of semi-coke can be conceptually integrated as an internal heat source, forming a coupled pyrolysis–oxidation process that enhances thermal efficiency while reducing dependence on external energy input. From a coal-seam-scale perspective, key coupling mechanisms involving heat transfer in coal-bearing porous media, combustion stability, and thermal regulation are critically examined. Based on these insights, a conceptual technical framework for controlled in-situ pyrolysis of tar-rich coal is developed, emphasizing the integration of internal heat generation, enhanced heat transfer, system sealing, and real-time subsurface monitoring. The framework further incorporates CO2 geological sequestration as a complementary pathway for emission mitigation. Overall, this conceptual framework provides a unified perspective for advancing efficient, safe, and low-carbon in-situ conversion technologies for tar-rich coal and related unconventional solid carbon resources.

As a strategic coal-based oil and gas resource, tar-rich coal is abundant in China. The development of oil and gas resources rich in oil and coal is of great strategic value to ensure China’s energy security and help the “double carbon” goal and energy transformation. Underground in-situ pyrolysis extraction of oil and gas is one of the important ways for low-carbon green development and efficient utilization of tar-rich coal. Using the spontaneous combustion of pyrolysis semi-coke as the heat source of in-situ pyrolysis can improve the efficiency of tar-rich coal resource exploitation, but it is necessary to focus on breaking through the problems of controllable spontaneous combustion of pyrolysis semi-coke, efficient mass and heat transfer, reliable geological guarantee system and controllable underground environmental pollution. The scientific and technological conception of in-situ pyrolysis mining of controllable and spontaneous combustion of tar-rich coal is expounded. The mechanical theory of mining rock mass, the theory of controllable and spontaneous combustion of pyrolysis semi-coke, the visualization theory of “four fields” and the theory of multi-physical field coupling heat transfer are put forward. The coupling damage mechanism of underground coal and surrounding rock under mining disturbance, the exothermic characteristics and controllable conditions of spontaneous combustion of pyrolysis semi-coke, the mechanical behavior and energy evolution characteristics of multi-physical field coupling rock mass are revealed, which provides a theoretical basis for the construction of in-situ pyrolysis mining technology system of controllable and spontaneous combustion of tar-rich coal. For the key links in the mining technology, the pyrolysis semi-coke controlled spontaneous combustion technology, in-situ mass and heat transfer enhancement technology, underground closed system technology and in-situ mining geological guarantee technology were established, which provided technical support for the in-situ pyrolysis mining of tar-rich coal controlled spontaneous combustion. On this basis, the in-situ pyrolysis mining theory and technology system of controllable and spontaneous combustion of tar-rich coal was constructed, the difficulties and key points of future research were clarified, and the technical path and research direction of high-efficiency, low-carbon and safe development engineering, industrialization and scale of tar-rich coal in China were put forward.

Realistic failure process analysis, a thermal software simulation, was used to explore the scale effect of thermal cracking of rock under the thermal–mechanical coupling loading. The patterns and characteristics of thermal destruction were analyzed by simulating the thermal cracking of rocks with the same diameter different lengths, the same length but different diameters, and the same size ratio but different sizes (same length/diameter ratio but with different diameters). The acoustic emission and energy changes were also studied during thermal destruction. The results represented that the main forms of thermal cracking are tensile failure and shear failure. The smaller the scale is (length, diameter, and size), the more complex the pattern of thermal damage exhibited as failure patterns of inverted “S” or “V.” With the increasing scale, thermal damage models were simpler. The elastic modulus was determined by the diameter of specimens, and the peak stress was determined by the length of specimens. Overall, as the scale increased, the stress intensity decreased, but the number of acoustic emissions and acoustic emission energy and the corresponding accumulation increased.

The study of temperature effect on thermal destruction of rock using physical experiments can't reflect movement of the stress field and temperature field of heat damage inside the rock.The RFPA-thermal software is employed to build a thermo-mechanical coupled mathematical model to simulates the thermal destruction of rocks on setting simulation program in this paper.The pattern of thermal damage,the threshold temperature,acoustic emission (AE) characteristics of rock sample are obtained from the simulation.The results show that as the temperature increase under fixed pressure,the evolution of crack and fracture gradually accelerate,which mainly continue extends and widens along the existing crack and fracture.Meanwhile,there some more new fractures generate during the process.Cracks develop rapidly accelerated around 300 ℃ that is the temperature threshold.AE account and its energy represent exponentially growth with increasing temperature,and the corresponding fitting formula is got.

To definitely investigate the uature of fracture generation and the mechanism of temperature-pressure coupling in the cracks evolution process of coal mass,we analyze the effect of temperature,pressure,temperature-pressure coupling on coal fracture in this paper;combining damage theory and fractture criterion,we buill temperature-pressure coupling model,and illustrate the coupling mechanism fiom theory and mathematical relationship.The nature of coal under temperature-pressure coupling failure can be described using the principle of first damage and then following the failure criterion.

The present paper has done a comprehensive review and analysis of the basic characteristics and regularities of the extremely severe accidents and casualties of the coal mines in China during the period of 2002-2014.Statistically speaking,the total number of such extremely severe accidents and casualties in the field of coal mines stands for 421 in the above said period,which were scattering in the three dimensions in accordance with the region,the size and type,and then the ownership.The results show that Shanxi,Guizhou,Henan,Heilongjiang and Hunan are the five key provinces should account for 56.1％ of all the accident cases and 56.8％ of the total death casualty,respectively.The paper has also analyzed the causes leading to the frequency of the severe and great casualties in the provinces of Shanxi,Henan and Heilongjiang,who enjoy large numbers of small and middlesized coal mines,which experience longstanding excavation with out-of-date mining conditions and backward production equipment and mining technologies,not speaking of automation and informationalization in the coal mining routine.The paper has also analyzed in detail the status-in-situ of the inadequacy of safety super.vision and consultation institutions and the low-level coal production facility maintenance.For them,gas and flooding crash accidents are the key types beyond the realistic management control,for the severe casualties in the gas and flooding frustration may account for 83.3％ of the accidents and 86.3％ of the total industry sphere,respectively.Nevertheless,the state-owned key coal mines have still been emphasizing prevention as the chief targets,though the average casualty rate accounts for 23.7 persons in each accident.What is more,severe casualty frequency has been substantially rising in the recent two years.However,it is a great pleasure for us to see that there has been a declining tendency of accidents and casualty size for the township-owned coal mines year by year,they are still the focus of the targets for the state and the government to strive for reducing the absolute number of accidents and casualty to the minimalist level.

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.

针对煤田火区发展演化多场耦合作用过程,分析了热-流-固耦合机理及不同时期煤体燃烧状态及产物.揭示了煤田火区发展演化是由煤氧复合化学反应而放热升温,产生热应力及烧空区致使上覆岩层失稳塌陷,形成煤岩体裂隙网络产生裂隙场,从而为氧气及燃烧产物对流循环提供通道,进一步促使火区向深部扩展延伸的灾变机理.因此,煤田火区温度场、裂隙应力场、渗流场及化学场之间的耦合作用是加速煤体燃烧的非线性动力循环过程.

This paper combined the thermogravimetry TG209F1 Libra with Netzsch LFA 457 to study the thermogravimetric behaviours from 25°C-650°C and measured thermal properties from 25°C-300°C for bituminous coals taken from Wuda coalfield. The characteristic temperatures and different stages of reaction were determined using thermogravimetry (TG-DTG). The thermal diffusivity of the bituminous coal decreases with increasing temperature (from 0.11 ± 0.03 mm2 × s−1 to 0.078 + 0.02 mm2 × s−1, depending on the material). Moreover, the specific heat capacity of coal samples presented closely a linear increase with increasing temperature up to 300°C, thermal conductivity of coal appeared a cubic polynomial growth (0.117 - 0.175 W / (m × K)). It might happen because that the thermal expansion plays a more important role than the release of gases and volatile on the quantity of pores. [Received: April 1, 2015; Accepted: September 20, 2015]

为研究煤体一次和二次氧化过程中自燃极限参数的变化规律,运用程序升温试验台,对弱黏煤、1/3焦煤、贫煤进行一次氧化升温,经绝氧降温后进行二次氧化升温,对比分析3种烟煤两次氧化过程中的耗氧速率、放热强度、自燃极限参数变化规律.结果表明:对于同一种煤,二次氧化时的耗氧速率、放热强度、自燃极限参数曲线在某一温度范围内与一次氧化时的对应曲线发生交叉;对于不同变质程度的煤,交叉温度范围不同,变质程度越低,交叉温度范围越高;煤的变质程度越低,二次氧化时的自燃极限参数极值变化率越大.

In order to study the spontaneous combustion characteristics of the weathered coal, the temperature programmed experiments of weakly caking coal and 1/3 coking coal were carried out with the temperature programmed experiment table, and then the anaerobic cooling treatment of these coals was conducted by injecting N2 , this was regarded as the secondary temperature programmed of the weathered coal. By measuring O2 concentration in the process of the temperature programmed of the raw coal and weathered coal, the reaction activation energy and preexponential factor at different stages were calculated, and the variation regularity of spontaneous combustion characteristics of the weathered coal was studied. The experiment results showed that the reaction activation energy and pre-exponential factor of the weakly caking coal were lower than that of 1/3 coking coal when the temperature was below the critical temperature; the reaction activation energy and pre-exponential factor of the weakly caking coal were higher than that of 1/3 coking coal when the temperature was over the critical temperature;the reaction activation energy and pre-exponential factor of the weathered coal were clearly lower than that of the raw coal when temperature was below the dry temperature, and when the temperature was over the dry temperature, the reaction activation energy and pre-exponential factor of the weathered coal of the weakly caking coal were lower than that of the raw coal, and the reaction activation energy and pre-exponential factor of weathered coal of 1/3 coking coal were higher than that of the raw coal;the reaction activation energy and pre-exponential factor of both raw coal and weathered coal tended to increase with the temperature rise.

为了研究不同低变质煤种的自然升温过程中自燃特性参数的变化规律,选用低变质程度的长焰煤、不粘煤、弱粘煤3种煤样为研究对象,运用XK-Ⅶ型煤自然发火实验装置对3种松散煤体进行煤自然发火的特性参数测定,运用气相色谱仪对其不同温度下的气体进行分析,对煤自然发火参数进行测算.分析了耗氧速率、放热强度、CO、CO2、CH4的生成速率以及C2H6/CH4、C2H4/CH4比值的变化规律.实验结果表明,相同温度下CH4的产生速率随煤质加深增大;80℃以前,相同温度下放热强度随煤质变质程度加深降低,在100℃后,相同温度下放热强度随煤质变质程度加深反而逐渐增大.

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.

•A 15-t experimental furnace of coal spontaneous combustion was constructed.•Characteristics of coal spontaneous combustion and anaerobic cooling were studied.•Temperature variations of coal spontaneous combustion and anaerobic cooling were obtained.•Quantitative calculation method of oxygen consumption rate and heat intensity was created.•Corresponding relationship between gas ratios and coal temperature was established.

为研究空气相对湿度对煤自燃特性的影响,运用自制空气湿度控制装置和程序升温试验台,通入湿度不同的空气,对砚石台矿煤样进行程序升温,测定不同温度下所产生气体的浓度,分析耗氧速率、CO和CO2产生率、放热强度以及自燃极限参数的变化规律.试验结果表明:低温氧化前期,煤体的耗氧速率、放热强度和CO产生率与空气相对湿度成正比.随着反应的进行,85％或32％的湿度均会对耗氧速率、放热强度和CO产生率产生一定的抑制作用;CO2产生率随空气相对湿度的增大先升高后降低,最小浮煤厚度和下限氧浓度随空气相对湿度的增大先减小后增大,上限漏风强度先增大后减小.增大空气相对湿度对煤低温氧化前期有促进作用,其自燃环境条件更易被满足,自燃危险性升高.

•Thermal properties of coal and rocks changing with temperature are analyzed.•As increasing temperature, thermal diffusivity and thermal conductivity decrease.•Thermal conductivity appears to linearly decrease with increasing temperature.•At high temperature, thermal diffusivity and conductivity approach constant.•Volatile composition determines thermal conductivity of bituminous coal.

The paper is engaged in an experimental study of the impact of high moisture on the specific features of the coal spontaneous combustion by taking the samples from the Dananhu Coal Mine. In order to accomplish our study,we have carried out the said experiment with Dananhu coal sample by using a so-called XK-Ⅳ coal spontaneous combustion experiment device in this paper. When the coal sample was crushed into pieces,the samples can be put into a laboratory furnace,whose temperature has to be kept through the impermeable insulation with the tap water. And,then,the heating process can be simulated of the coal spontaneous combustion while the experiment was going on. At the same time,the concentrations of O2,CO,CO2 could be tested and measured via a gas chromatographer so as to work out the generation rate of the CO,CO2 and the exothermal intensity by using the thermal balance equation. Meanwhile,analyses could be done with the impact degree of the high moisture content on the generation rate,the exothermal intensity and other characteristic parameters. The results of our experiment have shown that the high moisture content rate produced by the coal spontaneous combustion would in turn have a significant effect,say,on the process. In the process of the experiment with the water temperature at temperature of 80℃,we had to promote the formation of peroxo-complex,with the oxygen consumption rate and the generation rate of CO and CO2 used to make the evaporation of water. And,then,water gasification and latent evaporation heating may inhibit the generation of complex peroxide,which is expected to follow at the temperatures between 80 ℃ and 110 ℃,resulting in the higher vapor pressure in the coal,which is likely to obstruct the contact of loose coal with oxygen. In so doing,the experiment would result in lower oxygen consumption rate and the lower production rates of CO,CO2.After 110 ℃ and 120 ℃,and,next,ended in the water evaporation and the inhibition effect on the reaction of coal and oxygen,which is likely to disappear gradually,resulting in the coal porosity so as to enlarge the contact area of coal with oxygen. And,finally,with the increase of the reaction temperature,the oxygen consumption rate,the generation rate of CO,CO2 would naturally grow exponentially. Hence,the above described calculation and exploration with the spontaneous combustion characteristic parameters can be expected to be able to provide basic data and theoretical backgrounds for forecasting and prevention of Dananhu coal spontaneous combustion.