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Research Papers: Energy Systems Analysis

Forced Convective Heat Extraction in Underground High-Temperature Zones of Coal Fire Area

[+] Author and Article Information
Yan Tang

Key Laboratory of Gas and Fire Control for
Coal Mines (China University of Mining
and Technology),
Ministry of Education,
Xuzhou 221116, China;
School of Safety Engineering,
China University of Mining and Technology,
Xuzhou 221116, China

Xiaoxing Zhong

Key Laboratory of Gas and Fire Control for Coal
Mines (China University of Mining
and Technology),
Ministry of Education,
Xuzhou 221116, China;
School of Safety Engineering,
China University of Mining and Technology,
Xuzhou 221116, China
e-mail: zhxxcumt@cumt.edu.cn

Guangyu Li, Xinhao Zhang

Key Laboratory of Gas and Fire Control for Coal
Mines (China University of Mining
and Technology),
Ministry of Education,
Xuzhou 221116, China;
School of Safety Engineering,
China University of Mining and Technology,
Xuzhou 221116, China

1Corresponding author.

Contributed by the Advanced Energy Systems Division of ASME for publication in the JOURNAL OF ENERGY RESOURCES TECHNOLOGY. Manuscript received September 20, 2017; final manuscript received February 28, 2018; published online March 29, 2018. Assoc. Editor: Ronald Breault.

J. Energy Resour. Technol 140(7), 072008 (Mar 29, 2018) (9 pages) Paper No: JERT-17-1502; doi: 10.1115/1.4039615 History: Received September 20, 2017; Revised February 28, 2018

Coal fires exist in almost every coal-producing country and generate huge amounts of heat energy every year. In this paper, forced convective heat-extraction is presented as a method to exploit the potential heat in coal fire zones as an energy resource. A geological model of coal fire zones and a combustion model for underground coal in an O2-depleted atmosphere are established. The borehole layouts, the heat transfer medium (HTM) injection rates, and the cooling effect of the HTM on the coal and rock are analyzed using a three-dimensional (3D) simulation software (fluent). The results show that a borehole layout of multihole injection and oriented type proves to be suitable for coal fire zones. The simulation predicts that the temperature of the extracted HTM and the rate of heat extraction decrease as extraction time increases. The simulation further predicts that the temperature of the extracted HTM can be increased by reducing the rate at which the HTM injected. Additionally, the heat-extraction rate is more stable for relatively low HTM injection rates. The temperature of the coal fire zones can be reduced effectively by using forced convective heat-extraction, with the maximum temperature of the coal fire zones and the average temperature in the residual coal zone being cubic and quadratic function relationship of the heat-extraction time, respectively. This research provides a reference for waste-energy exploitation in coal fire areas.

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Figures

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Fig. 2

Geometric model showing the zones into which an underground coal fire is divided

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Fig. 1

Diagrammatic sketch of forced convective heat extraction method. “HTM” stands for “heat transfer medium”.

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Fig. 5

Temperature along section X = 45 m for the 264th day of the temperature evolution

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Fig. 11

Heat extraction rates at different extraction times for three different N2 injection rates

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Fig. 6

Three-dimensional (3D) color-coded contour map of N2 concentrations in the underground coal fire zones on the tenth day of extraction for single-hole injection and single-hole extraction method

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Fig. 7

Mass fraction of N2 and O2 along line l on the tenth day of extraction. The injection and extraction points are at y = 66 m (injection) and y = 50 m (extraction).

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Fig. 8

Three-dimensional color-coded contour map of N2 concentrations in the fire zone on the tenth day of heat extraction for multihole injection and oriented method

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Fig. 9

Temperature profiles along the left side of line l (y < 50 m) on the tenth day of extraction for both the single hole injection and four-hole injection methods. The “original data” line shows the temperature profile for no N2 injection.

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Fig. 10

Gas temperatures at the extraction borehole outlet for three different N2 injection rates

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Fig. 3

Temperature distributions in the residual coal zone along line l. The lines show the temperature for different days after the fire was initiated.

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Fig. 4

Temperature along section X = 45 m for the 27th day of the temperature evolution

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Fig. 15

Temperature–time graph showing the maximum temperature in the coal fire zone and the average temperature in the residual coal zone for the entire course of the 173-day heat extraction simulation

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Fig. 12

Temperatures along line l for (a) the 284th day of temperature evolution with N2 injection having been initiated 20 days earlier, and (b) the 284th day of the temperature evolution with no N2 injection. The temperature curve for the 264th day of the temperature evolution, before any N2 injection had begun, is shown for comparison.

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Fig. 13

Temperatures along line l on the 114th day of extraction for different N2 injection rates. The injection points are located at the coordinates y = 34 m and y = 66 m.

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Fig. 14

Three-dimensional color-coded contour maps showing temperatures on the 14th, 64th, 114th, and 173rd days of extraction. The maps show how the shapes and sizes of the hot and cooler zones change as heat is extracted by the HTM: (a) the 14th day, (b) the 64th day, (c) the 114th day, and (d) the 173rd day.

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