Simulation of spontaneous combustion, to study the causes of coal fires in the Rujigou Basin

doi:10.1016/S0016-2361(00)00065-X

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Volume 80, Issue 1, January 2001, Pages 7-16

https://doi.org/10.1016/S0016-2361(00)00065-X Get rights and content

Abstract

The paper discusses the development of a numerical simulation model “COALTEMP” to study the oxidation and possible spontaneous combustion of coal, that is exposed to the atmosphere and the daily cycle of solar irradiation. First the differential equations that describe heat flow, oxygen flow and oxidation in the coal matrix, and the equations describing the exchange of heat, radiation and oxygen with the atmosphere, are presented. Then the numerical solution is briefly discussed. The model is then used to study the spontaneous combustion of coal in general, and in the Rujigou Coal Basin in particular. The role of spontaneous combustion and mining activities in the development of coal fires is analysed and conclusions are drawn with respect to coal fire prevention.

Introduction

In the Rujigou Coal Basin (Ningxia Hui Autonomous region, People's Republic of China), coal fires cause yearly coal losses in the order of 300,000 tons with an estimated value, in the international market, of 15 million dollars. In co-operation with the Beijing Remote Sensing Corporation (BRSC) and several Dutch partners, we are developing a coal fire monitoring system for the Ningxia Bureau of Coal Industry, based on remote sensing and GIS. As a part of this project a model has been developed which simulates the transport of heat and oxygen within a coal matrix as well as the conversion between the two by the oxidation of coal.

Several papers have addressed the subject of spontaneous heating of coal [1], [2] as well as the modelling of spontaneous combustion [3], [4], [5]. The models are usually two-dimensional and simulate the heating and combustion in stockpiles.

The model presented in this paper (COALTEMP) is intended to be used as a practical tool in relation to the above mentioned coal fire monitoring system. It is one-dimensional and needs relatively little input data to be specified by the user. It maintains, however, all essential elements to study spontaneous combustion of coal. The model is more elaborate than earlier published models with respect to its description of the exchange of heat, radiation and oxygen with the atmosphere. In fact it allows the simulation of daily cycles under solar radiation at a given wind speed, for any day of the year and any exposition of the surface. Besides this, the user can also specify an airflow rate through the coal matrix in order to study the effect of convective flow through the coal matrix. An underground situation is simulated by setting the net radiation income at the surface equal to zero.

The model, in a slightly modified version, may also be used to simulate the course of the surface temperature for different rock types. In relation to the coal fire monitoring system, COALTEMP has the following functions:

1.
To study the possibility of spontaneous combustion of different coal types under different atmospheric conditions and different exposure, and on this basis draw conclusions in terms of coal fire fighting and prevention.
2.
To study the temperature contrast between different rock formations at different times of the day and on this basis determine a strategy for remote sensing data acquisition.

The present paper addresses the first function. We will briefly discuss the properties of Rujigou coal and their oxidation. Subsequently the set-up of the model in terms of coal matrix properties, flow equations and exchange with the atmosphere is presented. The solution of the system of differential equations and the input and output of the simulation model are briefly discussed. Thereafter the influence of coal properties, air infiltration and solar radiation on spontaneous combustion are studied. Also long term heating experiments covering several years are presented. Finally the conclusions are drawn.

Section snippets

Properties of Rujigou coal

The properties of the Rujigou coals may be characterised by their proximate and ultimate analysis. These have been carried out according to the standard NEN procedures. The results of four coal samples referred to in the present paper are presented in the following tables:

Results of proximate analysis

Sample	Moisture (mass%)	Ash (mass%)	Volatiles (mass%)	Caloric value (kcal/kg, daf)
960316	5.45	6.50	18.62	7558
960344	0.89	4.56	8.03	8386
970990	0.57	4.69	10.70	8392
971078	1.11	4.38	8.96	8296

Results of ultimate analysis

Coal oxidation rate

The oxidation of coal is a chemical reaction that very simply may be represented as $COAL+O_{2} →CO_{2} +heat$ In reality the reaction is complicated and may consist of various stages and pathways, depending also on the presence of other substances such as water and pyrite. For the overall chemical reaction of dry coal, Schmall [4], referring to Kok [6] presents the following overall reaction equation: $C_{100} H_{74} O_{11} +113O_{2} →100CO_{2} + 37H_{2} O+4.2×10^{8} J/kmol O_{2}$ For the first part of the reaction, which consists of the

Heat and oxygen transport

In the previous section an equation which describes the rate of the oxidation of coal was presented. This reaction requires oxygen from the air and generates heat. The heat that is generated will tend to increase the temperature and, as one can see from Eq. (3), this would increase again the reaction rate. In this way the reaction may be accelerated and the coal may finally ignite. However, there may be other circumstances that prevent this scenario. The heat generated may flow into the

Coal matrix properties

The calculation of the thermal conductivity of the coal matrix (λ_M) as a function of the porosity is done by the method of De Vries [8]. This method was developed for sediments, but may as well be applied to coal dust or litter. The equations are as follows: $λ_{M} =[ϵλ_{A} +f(1−ϵ)λ_{C}]/[ϵ+f(1−ϵ)]$ where $f=0.66/(1+0.125λ_{C} /λ_{A})+0.33/(1+0.75λ_{C} /λ_{A})$ The values taken for the thermal conductivies of (solid) coal and air are $λ_{C} =3.4 (W/mK) (Guan [9], [10])$ $λ_{A} =0.0235 (W/mK)$ The volumetric heat capacity of the coal matrix can

Exchange with the atmosphere

The formulation of the interaction of the coal surface with the atmosphere is largely based on Rosema [11].

Solving the system of equations

The analytical solutions for the system of , with the , are not known. Therefore we will have to find the solutions T(z,t) and Z(z,t) in a numerical way. The numerical solution used is known as the implicit finite differences method with expanding grid. In this approach the differentials in the previous equations are replaced by finite differences. This is illustrated in Fig. 1, which shows a grid representing the z and t co-ordinates of our problem. Let us consider grid point j at depth z_j

COALTEMP input and output

The formulations and calculations described in 2 Properties of Rujigou coal, 3 Coal oxidation rate, 4 Heat and oxygen transport, 5 Coal matrix properties have been programmed in computer code. Versions in FORTRAN and DELPHI (object oriented PASCAL) have been produced, which run on a PC under DOS and Windows, respectively. These computer programs are referred to as the COALTEMP simulation model. This model enables the simulation of coal oxidation and the related heat and oxygen flow in a coal

Simulations for a susceptible coal

In this section we will investigate if we can simulate spontaneous combustion. Coal which is susceptible to spontaneous combustion has a low activation energy (E) and a high frequency factor (F). We have chosen the most sensitive coal presented by Banerjee [7, p. 17]. This is the coal sample from Talcher (Orissa) with $E=26.88×10^{6} J/kmol$ and $F=6 s^{−1} .$ Like in the previous simulation we assume a coal matrix porosity of 50%, which may be representative of slack or coal dust. Further the same input

Conclusions

It can be concluded that spontaneous combustion is a likely cause of the coal fires in the Rujigou coal basin. As long as the coal is solid and not intensively exposed to air, spontaneous combustion will not occur. Mining activities, however, cause accumulations of coal litter and dust in combination with exposure to air. Under these conditions internal heating up of coal litter will easily occur. A disturbance of the pile may cause the fire to start. Also air infiltration may set the coal on

Acknowledgements

This simulation study has been carried out in the framework of the project “Development and Implementation of a Coal Fire Monitoring and Fighting System in China”. This project was carried out in co-operation between the Beijing Remote Sensing Corporation (principal), EARS Remote Sensing Consultants (main contractor), NITG and ITC (sub-contractors). The project was financially enabled by The Netherlands and the Chinese Governments through MILIEV funding.

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Simulation of spontaneous combustion, to study the causes of coal fires in the Rujigou Basin

Abstract

Introduction

Section snippets

Properties of Rujigou coal

Coal oxidation rate

Heat and oxygen transport

Coal matrix properties

Exchange with the atmosphere

Solving the system of equations

COALTEMP input and output

Simulations for a susceptible coal

Conclusions

Acknowledgements

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Spontaneous heating of stored coal