Report: APP CMHS Project 1




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3.3.3. Gas and Outbursts Control

Gas is the main safety hazard in Chinese coal mines. In 2007, gas incidents occurred 273 times and resulted in 1,084 fatalities. The hazards associated with gas include high gas emission, outbursts of coal and gas, and gas explosion. To reduce the risk of the hazards, the gas levels and potential outbursts have to be predicted and effective technologies must be applied to reduce the gas level and eliminate the risk of outbursts occurring. With over 18,000 outbursts occurring in Chinese coal mines since 1950, a national guideline on prediction and control of outbursts was issued in 1995 and is currently being updated. The guideline is quite comprehensive. It specifies the technologies for prediction, prevention, and control of outbursts.

Current Technologies

Prediction of gas levels and potential outbursts are conducted during coal exploration, mine development and coal production. The prediction is divided into regional prediction and coal face prediction.

Regional prediction is undertaken during the coal exploration and mine development phases. The aim is to identify the outburst risk of a coal seam and divide an outburst-prone coal seam into three zones: outburst free zone, outburst prone zone, and outburst zone. The methods for regional prediction include single index, composite index, geostatistical and other methods. In the single index method, four indices are used: coal damage classification, initial gas desorption rate from coal, Protodyakonov strength and seam gas pressure. In the composite index method, two indices D and K are used to predict the outburst proneness of a coal seam (the definitions of D and K are described in next page). Use of D and K is meant to consider the composite effect of rock stress, gas pressure, coal strength and initial gas desorption rate. The geostatistical method is based on comparison studies of geological and outburst data from mined areas and geological conditions in these areas in order to roughly predict the risk of outbursts. Other methods such as electromagnetic method and coal metamorphic method are sometimes used for the regional prediction of outbursts.

Coal face outburst risk prediction is undertaken for developing roadways progressing from rock strata to coal seams (cross-measure roadway development) and for roadway development in coal seams. In cross-measure roadway development, three indices are used to predict the risk of outbursts: composite index D and K (the same as that in the regional prediction); gas desorption index Δh2; and gas desorption index K1. In seam roadway development, the risk of outbursts is predicted using four indices derived from: initial gas flow rate from boreholes (q); coal cutting index Smax; and the gas desorption indices Δh2 and K1.

Regional Prediction (RP)

Regional prediction is undertaken during the coal exploration and mine development phases. The aim is to identify the outburst risk of a coal seam and divide an outburst-prone coal seam into three zones: outburst zone, no-outburst zone and outburst threatening zone. The methods for regional prediction include single index, composite index, geostatistical and other methods.

In the single index method, four individual indices are used: coal damage classification, initial gas desorption rate from coal, Protodyakonov strength and seam gas pressure.

  • Based on the degree of damage, coal is classified into five types: I, II, III, IV and V based on its brightness, structure and structural features, cleats and strength. Type I coal is defined as undamaged coal and is bright or semi-bright, of clear bedding structure, one or two sets of cleat and strong. Type V coal is defined as fully pulverised coal and is dark, no structure or muddy-like, no cleat, pulverised and soft. Characteristics of Type II, III and IV coals lie between Type I coal and Type V coal.

  • Initial gas desorption rate from coal. The value of this index is governed by physical and mechanical properties of coal. For coal with the same gas content, the higher the desorption rate is, the more severe the coal is damaged, the more outburst-prone the coal is. The threshold value of the index is 10 mmHg.

  • Protodyakonov strength (f) is a simple test meant to determine the degree of pulverisation of coal upon impact. The lower the value of (f); the more outburst-prone the coal is.

  • Seam gas pressure (P) is the in-situ seam gas pressure. The higher the pressure, the more outburst-prone coal becomes.

Each index has a threshold value. Figure 31 shows the threshold value of each index. A coal seam is classified as an outburst seam only if all indices reach or exceed their respective threshold values.

Figure 31 Threshold values: single index method

Index


Coal damage type

Initial gas desorption rate

mmHg

Protodyakonov strength

f

Seam gas pressure

MPa

Threshold

III, IV, V

>= 10

<= 0.5

> 0.74

In the composite index method, two composite indices D and K are used to predict the outburst proneness of a coal seam. Use of D and K is meant to consider the composite effect of rock stress, gas pressure, coal strength and initial gas desorption rate. The key to the composite method is the determination of initial gas desorption rate, Protodyakonov strength and seam gas pressure. Values of D and K are calculated as follows:

Each composite index has a threshold value. Figure 32 shows the threshold value of each index. A coal seam is classified as an outburst seam only if all indices reach or exceed their respective threshold values.

Figure 32 Threshold values: composite index method

Index

D

K

Anthracite

Bituminous coal

Threshold

>= 0.25

>= 20

>= 15



The geostatistical method is based on comparison studies of geological and outburst data from mined areas; and geological conditions in areas to be mined in order to roughly predict the outburst-prone zone in the areas to be mined.

Other methods such as electromagnetic method and coal metamorphic method are also used sometimes for the regional prediction of outbursts.

Coal Face Prediction

Coal face outburst risk prediction is undertaken for developing roadways progressing from rock strata to coal seams (cross-measure roadway development) and for roadway development in coal seams.

For cross-measure roadway development, three indices are used to predict the risk of outbursts: composite index D and K (the same as that in the regional prediction); gas desorption index Δh2 and gas desorption index K1.

For roadway development in coal seams, the risk of outbursts is predicted using four indices derived from: initial gas flow rate from boreholes (q), coal cutting index Smax and the gas desorption indices Δh2 and K1.

  • Gas desorption index Δh2. The value of Δh2 refers to gas pressure in the coal and the degree of coal damage. Δh2 is measured with a MD-1 gas desorption meter developed by China Coal Research Institute (CCRI) Fushan. If the value of Δh2 is equal to or greater than 200 Pa for dry coal, or 160 Pa for wet coal, then the coal is defined as outburst prone.

  • Gas desorption index K1. The index is similar to Δh2 and derived from the characteristic value of gas desorption from coal. K1 is measured with a WTC outburst prediction device developed by CCRI Chongqing. If the value of K1 is equal to or greater than 0.5 ml/ (g·min1/2) for dry coal or 0.4 ml/ (g·min1/2) for wet coal, then the coal is defined as outburst prone.

  • Initial gas flow rate from boreholes (q) and flow decay coefficient α. This method considers gas pressure (content), coal permeability and the degree of coal damage. If the value of q is equal to or greater than 5 L/ (min·m) and α is equal to or less than 0.75, then the coal is defined as outburst prone.

  • R index. The value of R is derived from the maximum initial gas flow rate from boreholes (qmax, L/ (min·m)) and the maximum coal cuttings per unit length (Smax, L/m).

    If the value of R is equal to or greater than 6 the coal is predicted as outburst prone.

  • Combination of Smax and Δh2 and K1. If the value of Smax is equal to or greater than 5.4 L/m and Δh2 is equal to or greater than 200 Pa, then the coal is defined as outburst prone. If the value of Smax is equal to or greater than 5.4 L/m and K1 is equal to or greater than 0.5 ml/ (g·min1/2), then the coal is defined as outburst prone.

Outburst prediction is an integral part of underground coal mining in China, as shown in Figure 33).

Figure 33 Outburst management process

If coal is predicted as high gas and outburst prone, the key control measures and technologies are taken. These are shown in Figure 34 and include:

  • in-seam pre-drainage

  • cross-measure drainage

  • pre-mining protective coal seams – a seam of low gas and outburst risk is mined prior to the targeted high gas seam. This results in de-stressing and degassing of the adjacent seams

  • goaf gas drainage – surface goaf wells and in-seam pipelines directly into the goaf

  • gas drainage tunnel above and under working seams with or without cross measure boreholes

  • flushing boreholes with high pressure water jet

  • water injection into coal seams to stimulate gas flow

  • blasting to induce an outburst prior to mining.

Figure 34 Outburst management technologies

Application Sites

Many above-mentioned technologies have been successfully applied in many mines of China, such as those in the Huainan, Jincheng, Yangquan and Tiefa Groups.

Technology Gaps/Needs

Current methods of gas and outburst prediction and control have been developed in relatively shallow coal mining environments. As mining depth increases, gas level and outburst risk are expected to increase due to high rock stress, gas pressure and low permeability. These changes pose challenges to current prediction and control technologies and new technologies need to be developed. The key developments required include:

  • techniques for accurate prediction of gas emissions into mine workings

  • remote sensing for detection of geological structures ahead of working faces

  • gas drainage technologies in soft and deep coal seams

  • sampling technologies for gas conditions in soft and deep coal seams

  • technologies to enhance coal seam permeability – such as hydrofracture and blasting ahead of the mining face.


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