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Based on the particle size evolution of tectonic coal in formation process, the differences between tectonic coal and intact coal in pore structure and initial gas desorption characteristics were studied. Then, the influences of particle size reduction on pore structure and initial gas desorption characteristics were investigated; and the initial gas energy release model was derived, which was used to calculate the gas expansion energy of tectonic coals with different particle sizes. This study concludes that the particle size of tectonic coal will be reduced under the pulverization effect during the formation process, and the particle sizes of these types of tectonic coal reach millimeter level or even smaller scale under pulverized state. The pore volume and specific surface area of minipores (pore size: 10–100 nm), mesopores (pore size: 100–1000 nm) and macropores (pore size: 1000–10000 nm) in tectonic coal are larger than those in intact coal by over 10 times. Tectonic coal’s average gas desorption rate in the first 10 s reaches up to 11.47 times of that of intact coal. Within the 5 particle size greater than 0.074 mm, the pulverization obviously transforms the minipores and mesopores in coals, but its effect on micropores (pore size: 0.9–10 nm) is limited. When the particles are pulverized to blow 0.074 mm, the micropores are obviously transformed. The initial gas energy release model for coal samples with different particle sizes is derived and verified through the experimental data. As calculated using the initial gas energy release model, the gas expansion energy will be reduced sharply with the particle size reduction when the particle size is smaller than 1 mm. Therefore, the particle size of tectonic coal decides its initial gas desorption capacity, which is of critical significance to the gas energy release in the outburst process. This study will be of great scientific significance for exploring the outburst mechanism and prevention.

Seam gas pre-drainage, is widely used as an effective method to control gas and coal outburst in underground coal mines. However, in CO2 abundant low permeable seams, this technology seems to be less efficient due to the CO2 sorption characteristics and the lower safe mining threshold limit for CO2 applied in many outburst risk management plans. This paper presents the experimental investigations of the applicability of nitrogen (N2) injection to improve gas drainage in CO2 rich seams. Core specimens were obtained from Bulli coal seam, Sydney Basin and N2 flood tests were conducted under different permeability conditions (0.3 mD and 0.06 mD). A triaxial permeability test rig equipped with a back pressure regulator was used to conduct the test. The variation of gas composition, gas outlet flow rate, N2 flushing efficiency, and permeability variation were analyzed. In addition, a comparative study between N2 flushing coal seam CO2 and N2-ECBM was conducted and it was observed that CO2 was much more difficult to drain out and with longer time compared with CH4. Based on the test results, a two-stage flushing mechanism was obtained. In the first stage, the original free phase coal seam gas accounted for the large percentage and this stage last for a shorter time. In the second stage, desorption time governed the flushing efficiency and desorbed gas was the primary gas. It took much longer time than the first stage. The final recovery rate in 0.3 mD and 0.06 mD scenarios were 90.5% and 87.7%, and the N2 consumption/CO2 production ratios were 15 and 11.2, respectively. Through these laboratory experiments, we concluded that N2 injection can significantly decrease coal seam CO2 content level and increase gas drainage efficiency.

Lean-oxygen environment caused by methane in high gassy coal mine goaf has a significant influence on the prediction index gases of residual coal spontaneous combustion. The coal sample from Xishan coalfield in China was used to conduct the low-temperature oxidation experiments under different lean-oxygen environments. The experimental results show that different lean-oxygen environments had a significant delay effect on the formation of CO, CO2, H2 and C2H6, and the lower the O2 concentration, the more obvious the delay effect, and the delay effect of methane was greater than that of N2. A lean-oxygen environment caused by methane also had a significant influence on the values of CO/ΔO2 ratio and CO2/ΔO2 ratio. However, when the temperature was T can be accurately expressed by exponential function. The absolute amount of CO, CO2 and C2H6 generated by 1 k g residual coal ( C CO , C CO 2 and C C 2 H 6 ), which can reduce the influence of air leakage and residual coal mass on prediction index gases, were introduced to predict the spontaneous combustion in high gassy coal mine goaf. Then, the distribution diagrams of C CO , C CO 2 and C C 2 H 6 with temperature and O2 concentration were obtained, so as to accurately predict the degree of spontaneous combustion in high gassy coal mine goaf. The research results are of great significance to the prevention of spontaneous combustion in goaf of high gassy mine.

Determining gas diffusion coefficient from experimental data is a key step of reproducing and predicting the diffusion process in coal. Previously analytical solution, including the unipore diffusion model and the bidisperse diffusion model, has been used extensively to estimate the gas diffusion coefficient(s) in coal. The utilization of analytical solution is convenient, however, there are some defects which may affect the accuracy of the results. For example, it is not suitable for fitting manometric sorption data, and the assumption of linear adsorption isotherm is not true. In this paper, we present a numerical solution to determine the gas diffusion coefficients. Three models were developed based on different assumptions of pore system and diffusion forms, i.e., unipore model assuming one kind of pore and diffusion, bidisperse model I (BM I) assuming independent macropore diffusion and micropore diffusion, bidisperse model II (BM II) assuming dependent macropore diffusion and micropore diffusion. Nitrogen diffusion experiment was conducted and the adsorption isotherm was measured. Sphere geometry was built for numerical simulation and the proposed models were used to fit the experimental data to determine the diffusion coefficients. Results show that neither the analytical unipore model nor the numerical unipore model can describe the diffusion process perfectly. By giving the same diffusion coefficient, the modelled fractional uptake ratio of numerical unipore model is smaller than the result of analytical unipore modeling at early stage while greater at later stage, which is due to the different assumptions of adsorption isotherm. Both BM I and BM II can describe the diffusion process well. The determined macropore diffusion coefficients of the two models are similar, while the micropore diffusion coefficient and the macropore adsorption ratio of BM II are greater than that of BM I. These can be explained by the different roles of the macropore diffusion in the two models. The gas pressure change at the center of the coal sphere was examined, from the modeling result of BM I, the macropore pressure increases sharply and then drops along with the external gas pressure, while the initial increasing rate of macropore pressure of BM II is much smaller and tends to be stable at later stage. No apparent impacts of initial gas pressures on diffusion coefficients can be observed, the change of diffusion coefficients and macropore adsorption ratio are actually small with increasing gas pressure. The numerical solution of determining gas diffusion coefficients can easily relax the assumptions and restrictions of the analytical solution. It can be used to test different kinds of coal samples, investigate different diffusion mechanisms and match all kinds of experimental data. The measured sorption isotherm and coal properties can also be incorporated into the modeling, which makes the determined diffusion coefficients more reliable. This paper is a preliminary attempt and we hope it can bring the researchers some new ideas about studying the gas diffusion characteristics in coal.

Pre-gas drainage plays a significant role in the prevention of coal and gas outburst in underground coal mines. However, difficulties of reducing the content of coal seam gas, especially localized coal seam CO 2 , to be below threshold values within a given drainage lead time have been encountered in numerous coal mines in Australia. As the proneness of nitrogen outburst is way smaller than CO 2 or methane outburst, nitrogen injection would be an attractive technology for the underground gas drainage management if it is really able to enhance the drainage efficiency. From this point of view, this paper aims at investigating the feasibility of implementing this technology in Bulli seam, Sydney Basin through numerical modelling. Four parts of work were conducted in this paper. Firstly, a binary gas transport model was developed through analysing the binary gas transport mechanism in both coal matrix and coal cleat, and also the matrix-cleat interaction due to binary gas sorption swelling. A real gas mixture viscosity was introduced and its change depends on the molar fractions of both components and their own viscosities. Secondly, the characteristics of the Bulli seam were studied to collect the parameters required by the binary gas transport model. Through laboratory experiment, field investigation and reviewing literature, the coal seam gas (methane, CO 2 ) and nitrogen adsorption and diffusion properties, in-situ seam properties such as permeability, porosity, cleat spacing and virgin pressure are either measured or estimated. Thirdly, the regular drainage effects in six scenarios with different initial permeability and coal seam gas type were examined and three of them are found to have drainage difficulties, i.e., the moderate permeability (0.5 mD) CO 2 case, the low permeability (0.005 mD) methane case and the low permeability (0.005 mD) CO 2 case. The effects of nitrogen injection on enhancing gas drainage efficiency, especially reducing the gas content, are studied for these scenarios. Results show that a satisfactory result is obtained for the moderate permeability CO 2 case, the CO 2 content can be reduced largely and an apparent increase of CO 2 production rate can be seen. However, the effect of nitrogen injection in low permeability methane scenario is even worse than the regular drainage using two boreholes, although it can increase the methane flow rate of single borehole. For low permeability CO 2 case, nitrogen injection can accelerate the reduction ratio of CO 2 content in part of the injection area and increase the total CO 2 flow rate, but the enhanced effect is limited and the CO 2 content is still high after 12 months of injection. Last but not least, the feature of the movement of coal seam gas content peak in the injection area is discussed and the reasons inducing the different injection effects between methane and CO 2 scenarios are analysed. A combination of lateral movement and drop of coal seam gas content peak is found, and permeability is believed to have great impacts on the movement speed. There is no sign indicating either diffusion or permeability change controls the different injection effects between methane and CO 2 scenarios, while the different competitive adsorption characteristics and the increase of mixture gas viscosities may have influences on this difference.

Self-heating or even spontaneous combustion of stockpiled coal, which is likely to outbreak under favourable circumstances during its transport, process, and storage, is a long-standing thermal dynamic hazard. This hazard is harmful in diverse aspects: causing loss of coal resource and caking property, raising safety concerns upon occurrence of open fire, and giving off noxious/greenhouse effect gases. Due to the complexity of involved physical process (e.g. heat and mass transport) and chemical process (e.g. coal oxidation), formulating an analytical solution to the problem with or even without a transient approach would be a daunting task and the problem is thus more often addressed numerically. So far many numerical models to self-heating of coal have been developed and to summarise these erratic findings, this work critically reviewed theses numerical solutions since the last four decades. Mechanism of self-heating on coal mass and low temperature coal oxidation especially kinetic modelling of coal oxidation is firstly investigated to clarify the involved physical and chemical processes. On basis of the mechanistic understanding, theoretical derivations and progressive advances on governing equations like energy, mass, and momentum conservation are reviewed and compiled in details. Through parametric studies or sensitivity check these models produced fruitful but slightly inconsistent findings. Therefore to provide industry more unbiased and comprehensive guides, the present work examined the influences of various contributors including wind flow, stockpile dimensions, coal particle size, moisture content, and packing porosity on the self-heating behaviour of stockpiled coal. Last not the least, major challenges and perspectives this subject may have are briefly discussed.

Klinkenberg effect is an important phenomenon for gas flow in low permeability reservoirs, and its influence increases with the reduction of gas pressure. Unlike conventional gas reservoirs, coal seam is unique with its high compressibility and sorption-induced-swelling features. During the coalbed methane (CBM) recovery process, coal cleat width varies due to the net effect of effective stress and coal matrix shrinkage, thus the Klinkenberg coefficient cannot be treated as a constant like rock reservoirs. A brief review of previous studies shows that the influence of coal seam characteristics on Klinkenberg effect was ignored by other researchers. By using the bundled matchstick conceptual model of coal, two improved models are proposed in this paper, one is under constant effective stress and the other is under reservoir condition. The former shows that the proportion of permeability change due to Klinkenberg effect is greater than the result from original model, and the Klinkenberg coefficient varies substantially albeit the influence of effective stress is eliminated. The latter links apparent permeability and coal porosity together, and the results show good agreement with field data, especially when the gas pressure is relative low. It can be concluded that apparent permeability model is crucial for explicit prediction of gas permeability changes during CBM recovery process, which may be underestimated without due consideration of Klinkenberg effect. The development of the improved apparent permeability models has significant value in the accurate prediction of CBM production as a result of permeability changes.

This paper presents the results of adiabatic oxidation studies on the propensity of 18 pulverised coals to spontaneous combustion. All the coal samples were tested at an initial temperature of 40°C and three samples at 60°C to mimic the condition in a coal mill. Their propensities to spontaneous combustion were ranked according to their initial rate of heating (IRH) and total temperature rise (TTR) values. The influence of initial temperature, moisture content, particle size and coal ageing on spontaneous combustion were also studied. The results demonstrated that air humidity is an important factor in determining whether a heating will progress rapidly or not. The particle size distribution of the coal affects the IRH and TTR values, with relatively smaller particles tending to be more reactive. Aged and pre-oxidised coals have higher IRH and lower TTR values, and the coal becomes less reactive. The magnitude of the temperature rises (TTR) increases with increasing initial temperature. No direct correlation was observed between the adiabatic oxidation values and the individual coal properties.