Coal burning disasters

To prevent spontaneous coal burning at a goaf in the People's Republic of China, the effectiveness of a grouting system was simulated (PRC). The colliery has been in operation for more than 27 years, and it is known to have a complicated air circulation network with airflow leakages that could lead to spontaneous combustion of at goaves. To begin, MIVENA, a coalmine aeration simulator, was used to test the mine aeration network's ability to control airflows into and out of goaves and working faces. In the second technique, FLUENT simulation to forecast spontaneous burning of coal with seepage flow of air in the goaf #3205. It was seen that the goaf could be subdivided into three different regions based on the concentration of oxygen in the goaf. Lastly, the arithmetical simulation outcomes indicate that the slurry grouting approach can be effective as well as an economical approach to reducing permeability in the area of the goaf to prevent residual coal from spontaneous combustion.

Keywords: spontaneous combustion, Simulation, FLUENT, MIVENA, goaf areas

Introduction

It is clear that goaf areas are the key areas when looking at the underground coal mines fire. In these areas, it is possible to induce a spontaneous fire in the goaf areas. Following the data about coal production in China, Ninety percent of total coal production is from underground mines, and there are about six hundred state-owned main coal mines out of which 25.1 percent are highly gaseous mines, and approximately 17 percent are fume out-burst coalmines. In Chinese coal mines, fatal accidents happened, especially in the past ten years. Since the mines yielded a large volume of coal that meets local demand, it caused insufficient efforts to keep safety operation at goaf areas and working faces. Precisely, most accidents were indirectly or directly associated with spontaneous combustion of coal in goaf mine areas (Wang et al, 2015.55).

Chen and Qi looked at the distributing circumstance of abutment force as well as the way airflow in goaf, and later they came up with the theoretical method to predict the velocity of airflow in goaf region as well as the spontaneous combustion region where the velocity of airflow is below 0.24 m/min. In 2013, Pan and others studied a goaf and subdivided it into three sectors linked to impulsive coal combustion using the distribution of oxygen and concentration of carbon monoxide. They categorized the goaf zone to asphyxiation and radiating zones by oxygen concentrations less than 7 percent and larger than 18 percent, respectively (Wang et al, 2015.55). In this study, a preventing approach for the spontaneous burning of coal at a goaf area in Haizi Colliery China has been analyzed, for it is a distinctive lignite coal mine in PRC and can experience spontaneous combustion because of fusain found in coal beds.

The mine area of Haizi Colliery is approximately 6.44 km2 comprising 1,200-5,200 m alongside the strike as well as tendency of 0 - 1,700 m. The coal layers are divided by three normal lines that form an isolated block region like a triangle zone bearing coal. The current depth that represent working face is about −320 m as from the surface level (Hu et al, 2015. 462). The self-ignition temperature of the yielded lignite coal is around 275°C. And the time from when the coal contacted air to the time when spontaneous combustion occur has been estimated to be roughly 1 - 1.5 month. According to the mine’s operational record of 2002 - 2013, spontaneous ignition occurrence has been regularly studied at a working face. The features of coal from Haizi are recorded in Table 1. Proximate and density examination of Haizi.



Density

Fixed carbon

Ash

Volatile matter

Moisture

(kg/cm3)

(%)

(%)

(%)

(%)



1.29

47.51

5.23

35.07

11.6





2. Analysis of Mine Ventilation Network

The centralized aeration system in Haizi Colliery is utilized with a key intake channel for fresh airflow as well as an exhausting channel used only for draining airflow. The intake tube has a diameter of about 6 m which is equal to 28.3 m2 in surface area. It is also is utilized for hoisting workers and elevating coal, and the exhaust channel is about 9.6 m2 in surface area. There are two key centrifugal fans; one is standby, and other is running (Hu et al, 2015. 462). They were fitted out to provide sufficient ventilation. Each fan is operated by a 220 kW motor with a rotating speed of 740 rpm. It has a total airflow intake of 3564 meters cubed per minute operated by an aggregate loss of pressure Pa.

In the current study, MIVENA, the coal mine ventilation system simulator has been used to forecast airflows and aeration characteristics centered on the system data. To be specific, the air circulation of #3205 working face, is the total-mechanized residual coal cutting faces, which was dedicated by altering conditions with the operation of working face. Later, the simulated statistics was used to regulate air movement in the working face bearing in mind the whole aeration in the system (Hu et al, 2015. 462).

According to the safety regulations for coal mines ventilation in China, the velocity of airflow should meet the range of 0.25- 4 m/s. Here, Haizi Colliery assessed the amount of air thru the goaf. It was about 480 m3/min which meets the requirements indicated above, for the air seepage was estimated to be 368 m3/min (Wang et al, 2015.55). The challenge which was experienced was that several shortcuts or bypass airways are present at the bottom of the mine with comparatively large aeration resistance in the left wing. The arithmetical simulations were done for the aeration cases, using several useful approaches to solve the challenge (Hu et al, 2015. 462). Based on results of the simulation, the effective way out was to put an additional intake channel freshly excavated in this region, for the newly dug airway can result in a decrease in air movement resistance of 30 percent at the #3205 working face. It was projected that the pressure reduces from 319 Pascal to 270 Pascal at the face; so it was also anticipated to control airflow leakage easily into the goaf zone than before.

Using the process indicated above, air seepage can be decreased to 200 meters cubed per minute from 338 meters cubed per min at the #3205 operational face. At working faces, once excavation is done (such as #1709 and #2207 working faces), stopping barriers were erected immediately to avoid leakage of the air into goaf regions as well as to improve the efficiency of fresh-air usage in the coal mine (Hu et al, 2015. 462). After optimizing coal mine ventilation air movements as well as lowering pressure near working faces, authors concentrated on studying the goaf area as well as preventing impulsive combustion of coal in the goaf area.

Zones at Goaf Area

The state of goaf area changes steps by step with the advancing of the working face. Air seepage at the working faces provides O2 to the coal in the goaf zone as well as coal joints close by the goaf, and it can result in spontaneous combustion of coal in the goaf zone that is categorized into three zones. The zones are oxidized, radiating, as well as asphyxiation zone. These zones can be described by flow speed of air seepage, the concentration of oxygen in the goaf region, as well as the rate of temperature increase that are assessed by the approach elaborated in the Coal Mine Safety Standards of China (Wang et al, 2015.55).

The table below represent the Chinese safety Regulations for different goaf zones.



Zone

Oxygen concentration

Airflow velocity

Temperature gradient



Radiating

—

 m/s

—

Oxidized

O2 ≥ 8%

0.02 m/s ≥ ≥ 0.001 m/s

> 1°C/day

Asphyxiation

O2 < 8%

 m/s

—











Detecting Oxygen Concentration in Goaf Area

The tube bundle air checking system has been continuously applied to study the spontaneous combustion procedure of coal in the goaf area. It is able to sense the primary stage of spontaneous coal combustion through monitoring of oxidation products like CO, and hence raise the alarm to alert the mine engineers (Zhang et al, 2013. 515). At #3205 working face, fume sample is drawn into the tube bundle using a gas pump. Several air contents like O2, N2, CH4, CO, CO2, C2H6, C2H4, and others, are checked to detect the combustion of coal at the checking point. The changes in concentration and distribution of gas in the goaf region were utilized to divide the area into three different zones in the goaf region, and its cons and pros were debated to govern the spontaneous burning of coal.





About fifty gas checking points are fitted underground, particularly in the goaf zones by the monitoring structure. Continuous checking of gas contents for three months was scrutinized to identify useful evidence on identifying spontaneous combustion of coal (Zhang et al, 2013. 515). The circulation of oxygen concentration measured in the goaf area is indicated in the figure below where the horizontal axis represents the distance alongside the #3205 working face from the upstream corner while the vertical axis represents the distance from the working face in the goaf area.



Goaf Air Flow Simulation

The FLUENT, a numerical simulator is an efficiency tool to study spontaneous combustion of residual coal in the goaf region. This is because monitoring statistics are narrowed to reproduce its heating course since spontaneous fire is a comprehensive phenomenon including many chemical and physical factors. The radiating zone border is challenging to determine by use of the oxygen circulation curve. In this study, the arithmetical simulation by FLUENT was taken considered to help in marking the boundary (Zhang et al, 2013. 515).

In the model for current 2-dimensional arithmetical simulations by FLUENT, the goaf area was presumed to be a zone of absorbent media composed of unmined coal and collapsed rock. At first, standards of goaf permeability, as well as differential pressure, are needed to simulate the air movement seepage along the working faces (Zhang et al, 2013. 515). The arithmetical simulations on airflow seepage distribution in goaf zone were done using the above air movement conditions by the FLUENT utilizing the grid lump size of 0.5 m × 0.5 m as illustrated in the figure below.



The region where the speed of air leakage is above 0.001 m/s representing 7 percent oxygen is situated 97 meters from the #1011 inlet airway as well as #2205 outlet roadway, whereas this section was 79 m from the mid of #3205 working faces. The boundary to oxidation zone is situated about 74-92 m away from the working faces (Zhang et al, 2013. 515). The pressure decrease in the #3205 operational face that fitted coal cutting device, conveyors, and supports is much greater than that of the key airways. The recorded pressure decrease was 319 Pascal alongside the working faces 134 m in lengths. Air movement in the working faces was taken as stormy flow, for airflow roughness, as well as velocity, were big enough. In FLUENT, the average-model that is a semi-experimental model centered on model movement equations for the turbulence dynamic energy, as well as its dissipation level, was selected to simulate the air movement in the working faces.

Preventive Method for Goaf Fire

Injecting Grouting Slurry into Goaf Area

Some effective approaches have been analyzed to prevent combustions in the #3205 working faces. These methods include grouting, nitrogen filling, gelatin injecting, as well as inhibitor spraying (Zhang et al, 2013. 515). Their effectiveness and costs were assessed centered on the present situations of the coal mines. The grouting filling method utilizing pulp slurry was selected as the best approach to thwart spontaneous burning of coal in the goaf region. In grouting filling scheme, the slurry was moved by natural hydraulic head between the goafs and surface.

Grouting Method and Result

The goaf region was filled with the slurry to thwart the spontaneous coal burning of residual coal by connecting the tube end at the point 40 m from both terminals of the #3205 working face. The space between the working face and the filling area was measured to be larger than 10 m since grouting slurry transported from 15-20 m in the direction of the working faces. When spontaneous combustion is noticed in the goaf area, the working face, as well as goaf area, should be filled with pulp slurry immediately (Zhang et al, 2013. 515). Following Chinese regulations, the pulp slurry spill should cover the section 5 m from the working faces in the goaf region.

Grouting Result Simulation Verification

When the goaf area is presumed to be filled consistently with grouting pulp slurry, the oxidation region happens in an insufficient zone behind the #3205 working faces. These outcomes indicate that the pulp slurry grouting is an effective and useful system to prevent spontaneous combustion of coal in the goaf area. The oxidation region is affected by absorbency in the goaf region (Zhang et al, 2013. 515). The figure below indicates the arithmetical simulation outcome of the connection between the goaf porosity and the area of oxidation zone.



Conclusions

In this research, an effective, as well as economical approach to averting spontaneous burning of coal in a goaf region, was studied based on arithmetical predictions by flow simulator FLUENT and ventilation simulator MIVENA. The features of temperature are rising, and oxidation in the goaf region in the Haizi Colliery China have been investigated by the combination of tube bundle gas checking results as well as arithmetical simulation results. Also, subdividing the goaf region into three different zones that are asphyxiation, radiating, and oxidized zones were looked at when considering the spontaneous burning of coal. The filling absorbency in the goaf region by adding grouting slurry functions to avert the spontaneous burning of coal in the goaf region.



Work cited

Hu, Xincheng, et al. "Coal spontaneous combustion prediction in gob using chaos analysis on gas indicators from upper tunnel." Journal of Natural Gas Science and Engineering 26 (2015): 461-469.

Wang, Yongjun, et al. "A study on preventing spontaneous combustion of residual coal in a coal mine goaf." Journal of Geological Research 2015 (2015).

Zhang, M., et al. "Spontaneous heating and gas drainage on a coal face with" four-gateroad" and overlying drainage tunnel." Zhongguo Kuangye Daxue Xuebao/Journal of China University of Mining and Technology 42.4 (2013): 513-518.