Sheng Xue

Numerical Analyses of the Major Parameters Affecting the Initiation of Outbursts of Coal and Gas

List of Symbols bi Body force per unit mass (N/m ) C Average matrix gas concentration (m/m) g Gravitational acceleration (m/s) I Unit tensor (dimensionless) DJ 0 1 The first stress invariant (N/m) K Absolute permeability (md) krn Relative permeability (dimensionless) P Pore pressure (Pa) PL Langmuir pressure (Pa) qg The well flowrate for gas (m /day) qw The well flowrate for water (m /day) qm Diffusion and desorption of gas between the matrix and the cleat (m/day) Rsw Gas solubility in water (m /m) s Gas or water shrinkage factor (m/m) S Degree of saturation (dimensionless) v Velocity (m/s) V Volume of gas adsorbed at pressure Pg (m ) VL Langmuir volume (m /m) Vm Bulk volume of a matrix element (m ) rij Stress tensor (N/m ) e Strain tensor (dimensionless) a Biot’s effective stress parameter ln Viscosity (Pa s) U Effective fracture porosity (dimensionless) s Sorption time (day) q Mass per unit volume of the medium (kg/m)

Investigation of Intrinsic and External Factors Contributing to the Occurrence of Coal Bumps in the Mining Area of Western Beijing, China

An investigation has been made to relate the occurrence of coal bumps to specific geological and mining conditions to the mining area of western Beijing. This investigation demonstrates that the high frequency of coal bumps in this area is due to four localized conditions, namely intrinsic coal properties, the presence of overturned strata and thrust faults, high in situ stress and the extraction of coal from island mining faces. Laboratory tests of coal samples indicated that the coals have a short duration of dynamic fracture, high bursting energy and high elastic strain energy, indicating that the coal is intrinsically prone to the occurrence of coal bumps. This investigation has also revealed that there are overturned strata and well-developed large- and medium-scale thrust faults in this area, and the presence of these structures results in plastic flow, severe discontinuities, rapid changes in overburden thickness and dipping of the coal seams. Well-developed secondary fold structures are also present in the axes and limbs of the primary folds. The instability of thrust faults, in combination with large-scale intrusion of igneous rocks, is closely associated with sudden roof breaking and induces sharp variations in electromagnetic radiation (EMR) and micro-seismic signals, which could be used to help predict coal bumps. In situ stress tests in the mining area demonstrate that the maximum and minimum principal stresses are nearly horizontal and that the intermediate principal stress is approximately vertical. The in situ stress level in the area is higher than the average in the Beijing area, North China and mainland China. In addition to the presence of overturned strata and thrust faults and high in situ stress levels, another external factor contributing to the frequency of coal bumps is coal extraction from island mining faces in this area. Island mining faces experience intermittent mining-induced abutment stress when a fault exists at one side of the island mining face due to reactivation of the fault, and this stress redistribution increases the likelihood of coal bumps during coal extraction from island mining faces.

Evaluation of the susceptibility of coal to spontaneous combustion by a TG profile subtraction method

It is imperative to have an in-depth understanding of the intrinsic reaction between coal and oxygen during low-temperature oxidation, as the reaction is the main source responsible for the self-heating and spontaneous combustion of coal. As low-temperature oxidation of coal involves a series of physical and chemical process and many parallel reactions, it is difficult to directly investigate the intrinsic reaction between coal and oxygen by conventional analytical method. Thermogravimetric analysis (TGA) was used to investigate the intrinsic reaction between coal and oxygen based on the mass change. By means of the subtraction analysis method of TGA, the TG-subtraction curves were obtained by subtracting the TG-N2 curves from the TG-air curves. The results indicate that a TG-subtraction curve can better reflect the intrinsic reaction of coal oxidation than a TG-air curve by eliminating the influence of evaporation of water and thermal decomposition of inherent oxygen-containing groups. In terms of the rate of mass increase, the intrinsic reactions can be divided into three stages: slow oxidation stage, advanced oxidation stage and rapid oxidation stage. The activation energy at each of the stages, obtained by Coats and Redfern’s model, can be used to as a technical parameter to evaluate the proneness of coal spontaneous combustion. The optimum experiment conditions were also developed to study low-temperature coal oxidation with the subtraction method of TGA.

Laboratorijska analiza spontanog izgaranja ugljena na niskoj temperaturi u zraku s reduciranim kisikom i niskom koncentracijom metana

Izvorni znanstveni clanak Provedena su laboratorijska ispitivanju u svrhu proucavanja spontanog izgaranja ugljena na niskoj temperaturi (30 ÷ 200 °C) u zraku s reduciranim kisikom (10 ÷ 21 %) i niskim koncentracijama metana (0 ÷ 3 %). Analiziralo se stvaranje ugljicnog monoksida kod spontanog izgaranja ugljena. Rezultati ispitivanja pokazuju da i koncentracije kisika i metana djeluju na spontano izgaranje ugljena. Pri datoj koncentraciji kisika koncentracija metana u zraku je imala pozitivan ucinak na spontano izgaranje ugljena, a ucinak se povecao s koncentracijom metana. Taj je ucinak oslabio i smanjio se porastom temperature. Pocetna je temperature ugljicnog dioksida, kod koje se ugljicni monoksid poceo javljati, porasla smanjenjem koncentracije kisika u zraku, a porast je donekle smanjen povecanjem koncentracije metana u zraku. Međutim, kako se koncentracija kisika smanjivala do određene vrijednosti, koncentracija metana je pokazivala malo ili nikakvog utjecaja na spontano izgaranje ugljena. Uzete su u obzir karakteristike ugljena kod niske temperature sorpcije kisika i metana kako bi se objasnili rezultati. Dobiveni rezultati su od posebne koristi u predviđanju i pracenju spontanog izgaranja ugljena u plinovitim ugljenokopima. Kljucne rijeci: koncentracija kisika; niska koncentracija metana; niska temperatura; spontano izgaranje ugljena; ugljenokop

A New Coal Sampling System for Measurement of Gas Content in Soft Coal Seams

Core samples or drill cuttings from boreholes drilled in coal seams are used in direct measurements of gas content in coal. In soft coal seams, core samples are difficult or sometimes impossible to obtain due to poor borehole stability such as large borehole deformation and borehole collapse, drill cuttings have to be taken and used. Due to their faster initial gas desorption rates, drill cuttings need to be taken rapidly and accurately at given positions during borehole drilling to ensure the accuracy of gas content measurements. To meet these sampling requirements of drill cuttings, a new sampling-while-drilling (SWD) system has been developed. The SWD system is based on a special design of double-tubing drill rods and a reversed circulation of pressurized air. A field test of the SWD system was carried out with satisfactory results. This paper presents the principle and results of the field test of the SWD system.

A Direct Measurement System to Estimate Gas Content in Coal

Gas content in a coal seam is used in mine safety such as gas emission estimation and the assessment of the risk of outbursts of coal and gas in underground coal mines. A direct measurement system has been developed to estimate the gas content. The main components of the system include an innovative sampling-while-drilling unit for coal sampling, a portable gas desorbing meter for initial gas flow, a desktop fixed surface-based unit for measuring gas flow while the sample is crushed, and computation program for gas content estimation. The system has been successfully applied in some underground coal mines in Huainan. This paper describes the system and its main components.

A Study on the Outburst Threshold Value of Gas Content in Coal

An outburst of coal and gas is a major hazard in underground coal mining. It occurs when certain conditions of coal stress, strength and gassiness are met. Based mainly on field observations related to the conditions, a number of indices are used in prediction of the outburst. Some of the indices are considered to be inadequate, inappropriate or impractical in some coal seam conditions. Significant progresses have been made in development of the technologies over the last few years. One of the major achievements is an analytical method in determination of an outburst threshold value of the gas content in a coal seam. The method combines field measurements and numerical modelling. The method is illustrated in this paper in a case study for determination of the outburst threshold value of gas content for a coal seam.