CO and CO₂ emissions from spontaneous heating of coal under different ventilation rates

Abstract

Carbon monoxide (CO) and carbon dioxide (CO₂) emissions during a spontaneous heating event in a coal mine are important gases to monitor for detecting the spontaneous heating at an early stage. However, in underground coal mines, the CO and CO₂ concentrations and their related fire ratios may be affected by mine ventilation. In this study, CO and CO₂ emissions from spontaneous heating of a U.S. coal sample were evaluated in an isothermal oven under different airflow ventilation rates ranging from 100 to 500 cm³/min. Laboratory experiments were conducted at oven temperatures of 70, 90, and 100 °C. The temperature at the center of the coal sample was continually monitored, while the CO, CO₂, and oxygen (O₂) concentrations of the exit gas were continually measured. The results indicate that CO was generated immediately after the airflow passed through the coal, while CO₂ was generated in a late phase. The amounts of CO generated under different airflow rates were approximately the same at the initial temperature of 70 °C, while the amounts of CO generated increased significantly as the airflow rates and initial temperatures increased. The ratio of CO/CO₂ was found to be independent of airflow rate and initial temperature, approaching a constant value of 0.2 quickly if there was no thermal runaway. The value tended to decrease when a thermal runaway took place. The CO/O₂ deficiency ratio was dependent on both airflow rates and the initial temperature. The experimental results are in qualitative agreement with some large-scale test and field monitoring results.

Introduction

Coal mine fires caused by spontaneous heating generally occur by slow oxidation in the coal seams or gob areas and happen most frequently with low-rank coals. Spontaneous heating is a low-temperature coal oxidation reaction which takes place when coal is exposed to air. Coal oxidation is an irreversible exothermic reaction and its reaction rate increases with temperature. When the heat produced by the coal oxidation is not adequately dissipated by conduction or convection, the temperature in the coal mass increases. This increase in temperature leads to an increase in the coal oxidation rate. If not averted with appropriate action, this process results in thermal runaway and a fire ensues.

Spontaneous heating has long been a problem in the mining, storage, and transport of coal. Much research has been done to understand the effects of different parameters on the spontaneous heating process (Akgun and Arisoy, 1994, Beamish and Hamilton, 2005, Carras and Young, 1994, Kucuk et al., 2003, Monazam et al., 1998, Nugroho et al., 2000, Ren et al., 1999, Smith and Glasser, 2005, Smith and Lazzara, 1987, Wang et al., 2003a). The petrographic and geochemical properties of coal waste during self-heating process were investigated by Misz et al., 2007, Misz-kennan and Fabianska, 2010. Some works have been done to study the gas emissions from spontaneous combustion of coal and their impact on the environment (Carras et al., 2009, Pone et al., 2007). However, the chemical reaction between coal and O₂ at low temperatures is complex and still not well understood. Generally, three types of processes are believed to occur (Carras and Young, 1994): (i) physical adsorption of O₂; (ii) chemical adsorption which leads to the formation of coal–O₂ complexes and oxygenated carbon compounds; and (iii) oxidation in which the coal and O₂ react with the release of gaseous products, typically CO, CO₂, and water vapor (H₂O).

Although a number of methods have been proposed and used in laboratory experiments in an attempt to predict the spontaneous combustion tendencies of coals, spontaneous heating events still occur in underground coal mines because of complicated parameters such as ventilation airflow, geological conditions, and mining practices that are not considered in laboratory experimental conditions. However, it is well understood that if a spontaneous heating event is detected early enough, some prevention measures can be taken to control and suppress the heating. Therefore, early detection is critical in the prevention and control of fires caused by spontaneous combustion in underground coal mines.

The primary method used for the detection of spontaneous combustion in underground coal mines is the analysis of the gaseous products of coal oxidation using a gas analyzer or a gas chromatograph. The gaseous products of low-temperature oxidation at the very early stages are CO, CO₂, and H₂O. Methane (CH₄), hydrogen (H₂), and other low-molecular hydrocarbons are released as the temperature rises. Chamberlain and Hall (1973) found that CO is the most sensitive indicator of the early stages of coal oxidation, and the continuous monitoring of this gas provides the earliest detection of self-heating. Other gases have also been investigated, such as CO₂, CH₄, H₂, and higher hydrocarbons (Xie et al., 2011). CO₂ production increases with increasing temperature and is useful in determining the state of a fire. However, several sources of CO₂ can be present in mines, making its use unreliable. CH₄ is usually present in large background quantities and, as with H₂ and other hydrocarbons, is not produced until much higher temperatures are reached. Since many of the combustion product gases, including CO, are encountered in the normal mining of the coal seam, and because these gas concentrations fluctuate with changes in ventilation, the reliability of these gases as indicators of spontaneous combustion diminishes.

To better detect spontaneous heating in underground coal mines, some fire ratios, such as Graham's ratio (CO/O₂ deficiency) and Trickett's ratio (CO/CO₂) are used. These ratios were originally developed and are often used for assessing the status of a sealed-off fire with the gas samples usually collected behind the seal, as investigated by Singh et al. (2007). The application of fire ratios to detect a spontaneous fire in underground coal mines has had limited success because the gas concentrations are dependent not only upon the state of development of the heating but also upon the mass of coal involved in the heating, airflow flux through the heating zone, and the flow of air into which the gaseous products of the heating are mixed. Different fire ratios have also been used to detect the spontaneous heating at the early stage.

Cliff et al. (2000) conducted large-scale tests for detection and monitoring of spontaneous combustion of coal. They found that the best indicators of spontaneous combustion are those that are independent of airflow, such as the amount of CO released and Graham's ratio, but that even these indicators have some limitations. Chakravorty and Woolf (1979) found that the absolute level of CO in the mine air, whether high or low, is not of great significance but that an increasing trend is indicative of heating. In the U.S., the Graham's ratio (CO/O₂ deficiency) in the mine atmosphere has become the most widely used indicator of the occurrence of spontaneous combustion (Mitchell, 1996).

In this paper, laboratory-scale experiments were conducted to investigate the effect of ventilation on the CO and CO₂ emissions from a spontaneous heating. The influence of ventilation on two commonly used fire ratios is also examined. The experimental results have implications for the early detection of the spontaneous heating in underground coal mines.