Disasters of gas-coal spontaneous combustion in goaf of steeply inclined extra-thick coal seams

In light of the escalating global energy imperatives,mining of challenging-to-access resources,such as steeply inclined extra-thick coal seams(SIEC),has emerged as one of the future trends within the domain of energy advancement.However,there is a risk of gas and coal spontaneous combustion coupling disasters(GCC)within the goaf of SIEC due to the complex goaf structure engendered by the unique mining methodologies of SIEC.To ensure that SIEC is mined safely and efficiently,this study conducts research on the GCC within the goaf of SIEC using field observation,theoretical analysis,and numerical modeling.The results demonstrate that the dip angle,the structural dimensions in terms of width-to-length ratio,and compressive strength of the overlying rock are the key factors contributing to the goaf instability of SIEC.The gangue was asymmetrically filled,primarily accumulating within the central and lower portions of the goaf,and the filling height increased proportionally with the advancing caving height,the expansion coefficient,and the thickness of the surrounding rock formation.The GCC occurs in the goaf of SIEC,with an air-return side range of 41 m and an air-intake side range of 14 m,at the intersection area of the“<”-shaped oxygen concentration distribution(coal spontaneous combustion)and the“>”-shaped gas concentration distribution(gas explosion).The optimal nitrogen flow rate is 1000 m3/h with an injection port situated 25 m away from the working face for the highest nitrogen diffusion efficacy and lowest risk of gas explosion,coal spontaneous combustion,and GCC.It has significant engineering applications for ensuring the safe mining of SIEC threatened by the GCC.

Effect and Mechanism of Rare Earth Hydrotalcite Inhibiting Coal Spontaneous Combustion

A hydrotalcite(layered double hydroxide, LDH) inhibitor which is suitable for the whole process of coal spontaneous combustion and a LDH inhibitor containing rare earth lanthanum elements were prepared. The inhibition effect and mechanism were analyzed by scanning electron microscopy(SEM),X-ray diffraction(XRD), thermal performance analysis, in-situ diffuse reflectance infrared spectroscopy and temperature-programmed experiment. The results have shown that the inhibitor containing lanthanum can play a good inhibitory role in every stage of coal oxidation. During the slow oxidation of coal samples, the inhibitor containing lanthanum ions can slow down the oxidation process of coal and increase the initial temperature of coal spontaneous combustion. At the same time, because the hydroxyl groups in LDHs are connected with-COO-groups on the coal surface through hydrogen bonds, the stability of coal is improved. With the increase of temperature, LDHs can remove interlayer water molecules and reduce the surface temperature of coal. CO release rate of coal samples decreases significantly after adding inhibitor containing lanthanum element, and the maximum inhibition rate of the inhibitor is 58.1%.

Basic theory research of coal spontaneous combustion

Discussed latest research results of basic theory research of coal spontaneous combustion in detail,with quantum chemical theory and method and experiment syste- matically studied chemical structure of coal molecule,adsorption mechanism of coal sur- face to oxygen molecule and chemical reaction mechanism and process of spontaneous combustion of organic macromolecule and low molecular weight compound in coal from microcosmic view,and established complete theoretical system of the mechanism of coal spontaneous combustion.

Presenting an engineering classification system for coal spontaneous combustion potential

The phenomenon of coal spontaneous combustion is one of the common hazards in coal mines and also one of the important reasons for the loss of coal in piles and mines. Based on previous researches, different types of coals have different spontaneous combustion characteristics. For coal loss prevention, a measure is necessary for prediction of coal spontaneous combustion. In this study, a new engineering classification system called ＂Coal Spontaneous Combustion Potential Index （CSCPI）＂ is presented based on the Fuzzy Delphi Analytic Hierarchy Process （FDAHP） approach. CSCPI classifies coals based on their spontaneous combustion capability. After recognition of the roles of the effective parameters influencing the initiation of a spontaneous combustion, a series of intrinsic, geological, and mining characteristics of coal seams are investigated. Then, the main stages of the implementation of the FDAHP method are studied and the weight of each parameter involved is calculated. A classification list of each parameter is formed, the CSCPI system is described, and the engineering classifying system is subsequently presented. In the CSCPI system, each coal seam can be rated by a number from 0 to 100; a higher number implies a greater ease for the coal spontaneous combustion capability. Based on the CSCPI system, the propensity of spontaneous combustion of coal can be classified into three potential levels： low, medium, and high. Finally, using the events of coal spontaneous combustion occurring in one of the Iranian coal mines, Eastern Alborz Coal Mines, an initial validation of the mentioned systematic approach is conducted. Comparison of the results obtained in this study illustrate a relatively good agreement.

Numerical comparison of coal spontaneous combustion danger influenced by different methane drainage patterns in gob area

The influence of gas drainage on float coal spontaneous combustion in the work face with ＂U＂ style ventilation was studied. Numerical simulation was used to compare the mutative law of steady flow and density field in the gob area under different drainage conditions by solving the equation set, including mass, momentum, and component transition. Consequently, the sequence of drainage effect and safety was obtained. The result manifests that the more effective the drainage pattern is, the easier float coal spontaneous combustion is caused due to air being guided into the depth of the gob area when the drainage position is arranged in the gas accumulation area. If the widened scope of oxidation zone exceeds the upper limit of the work face advancing speed, nitrogen injection should be applied to decrease the probability of spontaneous combustion. Then, the pipe laying drainage in the upper angle is most economical and safe compared with other drainage patterns when only the situation of gas accumulation is controlled in the upper angle. Finally, drainage pressure must not be too great. Otherwise the drainage density will decrease even if hazard is caused by back flow possibly happening in the return outlet when the drainage position is arranged near the work face.

Experimental simulation and numerical analysis of coal spontaneous combustion process at low temperature

The characteristic of coal spontaneous combustion includes oxidative property and exothermic capacity. It can really simulate the process of coal spontaneous combustion to use the large scale experimental unit loading coal 1 000 kg. According to the field change of gas concentration and coal temperature determined through experiment of coal self ignite at low temperature stage, and on the basis of hydromechanics and heat transfer theory, some parameters can be calculated at different low temperature stage, such as, oxygen consumption rate, heat liberation intensity. It offers a theoretic criterion for quantitatively analyzing characteristic of coal self ignite and forecasting coal spontaneous combustion. According to coal exothermic capability and its thermal storage surroundings, thermal equilibrium is applied to deduce the computational method of limit parameter of coal self ignite. It offers a quantitative theoretic criterion for coal self ignite forecasting and preventing. According to the measurement and test of spontaneous combustion of Haibei coal, some token parameter of Haibei coal spontaneous combustion is quantitatively analyzed, such as, spontaneous combustion period of coal, critical temperature, oxygen consumption rate, heat liberation intensity, and limit parameter of coal self ignite.

Experiment study of optimization on prediction index gases of coal spontaneous combustion

The coal of Anyuan Mine has the characteristic of easy spontaneous combustion. Conventional method is difficult to predict it. Coal samples from this mine were tested in laboratory. The data obtained from laboratory determination were initialized for the value which was defined as "K". The ratio of each index gas and value of "K", and the ratio of combination index gases and value of "K", were analyzed simultaneously. The research results show that for this coal mine, if there is carbon monoxide in the gas sample, the phenomenon of oxidation and temperature rising for coal exists in this mine; if there is C_2H_4 in the gas sample, the temperature of coal perhaps exceeds 130 °C. If the coal temperature is between 35 °C and 130 °C, prediction and forecast for coal spontaneous combustion depend on the value of Φ(CO)/K mainly; if the temperature of coal is between 130 °C and 300 °C, prediction and forecast for coal spontaneous combustion depend on the value of Φ(C_2H_6)/Φ(C_2H_2) and Φ(C_2H_6)/K. The research results provide experimental basis for the prediction of coal spontaneous combustion in Anyuan coal mine, and have better guidance on safe production of this coal mine.

Analysis and Design Innovation on Underground Gasifier for Medium-Deep Coal Seam

Over the past 80 years,dozens of underground coal gasification(UCG)mine field tests have been carried out around the world.However,in the early days,only a small number of shallow UCG projects in the former Soviet Union achieved commercialised production.In this century,a few pilot projects in Australia also achieved short-term small-scale commercialised production using modern UCG technology.However,the commercialisation of UCG,especially medium-deep UCG projects with good development prospects but difficult underground engineering conditions,has not progressed smoothly around the world.Considering investment economy,a single gasifier must realise a high daily output and accumulated output,as well as hold a long gasification tunnel to control a large number of coal resources.However,a long gasification tunnel can easily be affected by blockages and failure,for which the remedial solutions are difficult and expensive,which greatly restricts the investment economy.The design of the underground gasifier determines the success or failure of UCG projects,and it also requires the related petroleum engineering technology.Combining the advantages of the linear horizontal well(L-CRIP)and parallel horizontal well(P-CRIP),this paper proposes a new design scheme for an“inclined ladder”underground gasifier.That is to say,the combination of the main shaft of paired P-CRIP and multiple branch horizontal well gasification tunnels is adopted to realise the control of a large number of coal resources in a single gasifier.The completion of the main shaft by well cementation is beneficial for maintaining the integrity of the main shaft and the stability of the main structure.The branch horizontal well is used as the gasification tunnel,but the length and number of retracting injection points are limited,effectively reducing the probability of blockage or failure.The branch horizontal well spacing can be adjusted flexibly to avoid minor faults and large cracks,which is conducive to increasing the resource utilisation rate.In addition,for multi-layer thin coal seams or ultra-thick coal seams,a multi-layer gasifier sharing vertical well sections can be deployed,thereby saving investment on the vertical well sections.Through preliminary analysis,this gasifier design scheme can be realised in engineering,making it suitable for largescale deployment where it can increase the resource utilisation rate and ensure stable and controllable operations.The new gasifier has outstanding advantages in investment economy,and good prospects for application in the commercial UCG projects of medium-deep coal seams.

Minimum secure speed of fully mechanized coal face based on critical temperature of coal spontaneous combustion

The critical temperature theory of spontaneous combustion of coal and the numerical simulation method are used to explore the minimum secure speed of fully mechanized coal face to prevent the spontaneous combustion in goaf. Combined with the actual situation of workface 31005 in a coal mine, the highest temperatures in goal at different advancing speeds were obtained by the numerical simulation of spontaneous combustion in goal, and then a power function equation between the highest temperature and the advancing speed was achieved by regression analysis. The advancing speed corresponding to the critical temperature value was taken as the minimum safe speed of workface based on the equation. Finally, the accuracy and reliability of the speed were verified by the actual advancing process of workface 31005. The results of this research show that the new judgment method of the minimum safety speed has a higher value to be applied in the field.

Bifurcation characteristics of coal spontaneous combustion and analysis of critical state of gaseous reaction in a packed bed

The numerical model was presented for the coal combustion in the packed bed. The bifurcation characteristic of the ignition-extinction of solid-phase smoldering and tran- sition to flaming was studied for the packed bed of coal.One of the Frank-Kamenetskii parameter β_1 was selected as the control parameter.The computed results show that the bifurcation curve is obviously divided into two zones of solid-phase reaction and gas- phase reaction,and the total process of ignition-extinction presents twice bifurcation cha- racteristic.Moreover,the vanishing of critical state of ignition-extinction is studied.One of the transition points,ε_2=0.05,is numerically solved for the vanishing of critical state.The larger the value of ε_2 is,the easier the gas-phase can react.However,the combustion temperature will decrease with increasing ε_2.The other transition point α_2=0.53 is also ob- tained.With increasing the value of α_2,the combustion temperature of gas-phase reaction is close to the smoldering temperature of coal.When α_2 is infinite,the only reaction occur- ring is the smoldering combustion of solid-phase,and the gas-phase cannot react.

Time-shift effect of spontaneous combustion characteristics and microstructure difference of dry-soaked coal

The physical and chemical properties of the air-dried residual coal after soaking in the goaf will change,resulting in an increase in its spontaneous combustion tendency.This study aimed to look into the features and mechanism of soaked-dried coal's spontaneous combustion.Five samples of coal were dried to various degrees,and the weight loss features during thermal processing were examined.Based on this,the pore structure and chemical structure characteristics of the coal samples with the highest tendency to spontaneous combustion were quantitatively examined,and the mechanism by which soaking-drying afected the spontaneous combustion heating process of the remaining coal in goaf was investigated in turn.The results show that T1 decreases with the increase of drying time,T2–T6 shows a fuctuating change,and the ignition activation energy of 36-S-Coal is smaller than that of other coal samples.The pore type of 36-S-Coal changes from a oneend closed impermeable pore to an open pore,and the pore group area is large.During the 36 h drying process,the internal channels of the coal were dredged,and a large number of gravels and minerals were precipitated from the pores with the air fow.A large number of gravels were around the pores to form a surface structure that was easy to adsorb various gases.Furthermore,infrared spectroscopy was used to analyze the two coal samples.It was found that soaking and drying did not change the functional group types of coal samples,but the fatty chain degree of 36-S-Coal was reduced to 1.56.It shows that the aliphatic chain structure of coal is changed after 36 h of drying after 30 days of soaking,which leads to the continuous shedding of aliphatic chain branches of residual coal,and the skeleton of coal is looser,which makes the low-temperature oxidation reaction of 36-S-Coal easier.Based on the above results,the coal-oxygen composite mechanism of water-immerseddried coal is obtained,and it is considered that the key to the spontaneous combustion oxidation process of coal is to provide oxygen atoms and accelerate the formation of peroxides.

Study on Predicting Coal Spontaneous Combustion in Qingshan Mine

Based on the measurement experiment of oxidation gas products in the process of temperature-up, with the intention of improving the sensitivity of each sign gas for predicting coal spontaneous combustion and operatingness, this paper has researched carefully the change curves which show oxidation gas products of the coal samples of Qingshan mine changing with coal temperature. According to the actual condition of mine, the initial value k is led into, which is the ratio of the series of sums of carbon monoxide and carbon dioxide in the process of temperature-up to the initial value of carbon dioxide, and it raises the quality and sensitivity of forecast curve of sign gas and auxiliary index, and determines the forecast curve and the corresponding fitting curve of the sign gas for predicting coal spontaneous combustion of Yongzi coal of Qingshan mine. So the experiment data can be provided to forecast and predict the mine fires caused by coal spontaneous combustion. Most of all, this is a perfect guide to the safety production in the mine areas according to the sign gases.

Method for prevention and control of spontaneous combustion of coal seam and its application in mining field

Spontaneous combustion of coal seam has been and continues to be a big problem in coal mines. It could pose great threat to the safety of the whole mine and all miners, especially when it occurs in or nearby coal mines. Besides, environment of area surrounded mines during combustion can be threatened where large amount of toxic gases including CO_2, CO, SO_2 and H_2S can be leased by fire in mine. Hence, it is important and significant for scholars to study the controlling and preventing of the coal seam fire. In this paper, the complicated reasons for the occurrence and development of spontaneous combustion in coal seam are analysed and different models under various air leakage situations are built as well. Based on the model and approximately calculation, the difficulty of fire extinguishment in coal seam is pointed out as the difficulty and poor effect to remove the large amount of heat released. Detailed measurements about backfilling and case analyses are also provided on the basis of the recent ten years' practice of controlling spontaneous combustion in coal seams in China. A technical fire prevention and control method has been concluded as five steps including detection, prevention, sealing, injection and pressure adjustment. However, various backfill materials require different application and environmental factors, so in this paper, analyses and discussion about the effect and engineering application of prevention of spontaneous combustion are provided according to different backfilling technologies and methods. Once the aforementioned fire prevention can be widely applied and regulated in mines, green mining will be achievable concerning mine fire prevention and control.

Forecast of coal spontaneous combustion with artificial neural network model based on testing and monitoring gas indices

Forecast is very important for preventing and controlling the disaster of spontaneous combustion （sponcom）. Gaseous products of coal, such as carbon monoxide, ethylene, propane and hydrogen, are commonly used as indicators to reflect its status quo of sponcom in coal mines. Nevertheless, since the corresponding relationship between the temperature and the indicators is non-linear and can＇t be depicted with simple mathematical formula, it is very difficult to diagnose and forecast coal sponcom by monitoring indicator gases＇ distribution. A forward feeding 3-layer artificial neural network （ANN） model is employed to express the corresponding relation between temperature and index gases of coal sponcom more accurately. A large amount of data from programmed temperature oxidation experiments were employed to train the network to gain the connection strength between nerve cells and to accomplish the model. It proved in real coal productions that the ANN model can forecast coal sponcom accurately.

Study and application of plasticity plaster-slurry for preventing coal spontaneous combustion

Introduced the modulation scheme, function and mechanism of plasticity plaster slurry preventing coal spontaneous combustion. The applications show that the plasticity plaster slurry has good hygroscopicity and adsorptivity. To spray it on the coal wall of tunnel can shut off leakage wind fast and effectively. To press it into the coal body can absorb the heat and descend the temperature, surround the coal pieces, eliminate the possibility of the fiery district resuming combustion.

Spontaneous combustion of coals and coal-shales

Spontaneous combustion of coal is a well-known phenomena around the globe. Apart from the coal itself,burning coal-shales is becoming a problem in the South African coal mines. Serious incidents of spontaneous combustion have been reported as a result of self-heating of reactive coal-shales. The intrinsic properties and spontaneous combustion tests of 28 selected coal and coal-shale samples were conducted and a relationship between the two has been established. Intrinsic properties were obtained by using the proximate and ultimate analysis, and spontaneous combustion liability tests results were obtained by using the Wits-Ehac and Wits-CT indices. The experimental results show that intrinsic properties of these materials complement to the spontaneous combustion liability tests results. Comparative analyses of intrinsic properties and spontaneous combustion characteristics indicate similarities between the mechanism of coal oxidation and that of the oxidative processes undergone by coal-shales. For the tested samples, coal samples have a higher intrinsic spontaneous combustion reactivity rating than the coal-shales.Furthermore, an increase in carbon, moisture, hydrogen, volatile matter, nitrogen and a decrease in ash content indicate an increased proneness to self-heating. The concentration of pyrite found in the coal-shales accelerates self-heating. The event of spontaneous combustion can occur if coal-shales absorb sufficient oxygen when subjected to atmospheric conditions.

Risk assessment of gas control and spontaneous combustion of coal under gas drainage of an upper tunnel

The adjustment of the gas drainage rate has an immediate impact on air leakage in gob,thus resulting in the change of self-heating of coal.While regulating the gas drainage parameters,the risk of spontaneous combustion of coal should be considered.The risk assessment of gas control and spontaneous combustion of coal under gas drainage in a tunnel was investigated at different gas drainage rates.The distributions of the air volume along the working face,the gas management effects and the width of the oxidation zone were subjected to risk analysis.As the simulation results showed,with increasing gas drainage rate,although the safety of gas dilution by ventilation was assured,the intensifying air leakage caused the oxidation zone to move into the deeper gob and led to an increase in the width of the oxidation zone.A risk assessment method was proposed to determine a suitable gas drainage rate for the upper tunnel.The correctness of the risk assessment and the validity of the numerical modelling were confirmed by the field measurements.

Comparative Experimental Analysis on Coal Spontaneous Combustion

The goal of this study was to investigate coal quality features and their relationship to coal spontaneous combustion characteristics in multi-seam coal mines to better predict when coal spontaneous combustion is likely to occur.To that end,coal samples of various particle sizes were obtained from five coal seams(Nos.6,8,9,12 and 20)in the Shuangyashan City Xin’an Coal Mine.The samples were then respectively heated using a temperature programming system to observe and compare similarities and differences in the sponta-neous combustion process of different particle sizes in response to rising temperature.The experimental results show,that in all five coal seams,the concentration of CO,C_(2)H_(4),and C_(2)H_(6) increased with a certain degree of regularity as a function of rising temperature.However,of these three gasses,only CO and C_(2)H_(4) can be used as indicators to predict coal mine spontaneous combustion.The critical temperature for samples from all five coal seams ranged from 50–85℃,while the dry cracking temperature of coal seams 8 and 12(80–100℃)were lower than those of 6,9,and 20(100–120℃).Furthermore,the production rate of CO,C_(2)H_(4),and C_(2)H_(6) is related to both coal particle size and temperature.The smaller the particle size,the faster the production rate;and the higher the temperature,the more gas that gets produced.All five coal seems are mainly com-posed of long flame coal.However,they differ in that the No.12 coal seam contains weak cohesive coal;the No.8 coal seam contains lean and gas coal;and the Nos.6,9,and 20 coal seams contain a certain amount of anthracite.During the programmed coal heating,the CO,C_(2)H_(4),and C_(2)H_(6) release trend for the coal seams was No.12>No.8>Nos.6,9,and 20.These results demonstrate that the presence of weak cohesive coal and anthracite highly influence the concentration of CO,C_(2)H_(4),and C_(2)H_(6) released during coal spontaneous combustion.

Spontaneous Combustibility Characterisation of the Chirimiri Coals, Koriya District, Chhatisgarh, India

Representative coal samples were collected from different coal seams of the Chirimiri coalfield which covered the entire stratigraphic sequence. These samples were tested for Chemical analysis, Crossing Point Temperature (CPT), Petrography, Infrared studies (IR) and Differential Thermal Analysis (DTA). All the test results vindicated that the aforesaid parameters had a definite relationship with the stratigraphic disposition or the ranks of coal. The low rank coals found as younger seams in the stratigraphic sequence were more prone to spontaneous combustion whereas the higher rank coals found at the bottom of stratigraphic sequence were less prone to spontaneous combustion. Through combustibility characterisation by different tests, it was found that the upper Duman and Kaperti seams placed as younger seams in the stratigraphic sequence are highly prone to spontaneous combustion whereas the lower Karakoh and Sonawani seams seem to be least prone to spontaneous combustion.

Critical Value of CO of Forecasting Coal Spontaneous Combustion

CO has been used widely in the production process of colliery as an index gas to predict spontaneous combustion of coal. But in some collieries there are CO gas in the upper corner of the face all the times, sometime CO gas even exceeds the regulated critical index. This phenomenon is much more obvious in the fully-mechanized longwall face and fully-mechanized longwall and top-coal caving face. Although many measures of fire-proof and fire-extinguishing have been adopted, the flowing amount of CO gas can be only decreasd, but can not be eliminated completely. Using the different kinds of coal, the experiment of coal oxidation was made at the low temperature. The experiment indicates that some kinds of coal can produce CO under the condition of normal temperature oxidation, sometime the CO consistency is very high, and the intension of CO can be decreased with oxidation time prolonging. Selecting rational critical value of CO is the kev to predicting spontaneous combustion of coal correctly and reliably. The problem of selecting retional critical value of CO was studied. Finally, the amount of CO gas released by different kinds of coal was obtained under normal temperature condition.