Changes in the reaction regime during low-temperature oxidation of coal in confined spaces

Highlights

• Low-temperature oxidation of coal is studied in a batch reactor.

• CO release and O₂ consumption rates are described mathematically.

• Reaction order and reaction rate are obtained from the reaction rate equation.

• Reaction regime switches during coal oxidation in confined spaces.

Abstract

A study of the initial stage of the low-temperature oxidation of coal is important, not only for the prevention for fires in coal industry, but also in reducing emissions of hazardous gases. A batch reactor was introduced and a series of simulated experiments of coal oxidation was carried out in this paper. The time-dependent rates of CO emission and oxygen consumption during oxidation experiments of coal samples with different particle size ranges were obtained simultaneously from the measurements of CO and O₂ concentration in the reactor. The experimental results show that the rate of CO emission presents three stages in the duration of coal oxidation. Based on the rates of oxygen consumption, the reaction order and reaction rate were obtained from the reaction rate equation. These results indicate that the reaction regime switches during coal oxidation in confined spaces. The initial decrease of reaction rate is due to the decrease of the active sites in coal. While the decrease of reaction rate in the following period of coal oxidation is attributed to the reaction-inhibition mechanism of oxidation products. And it is mainly represented by the chemisorption sequence of coal oxidation. The oxidative activity of coal can be restored when the oxidation products are evacuated. A mechanism of low-temperature oxidation of coal in confined spaces was also suggested.

Introduction

When fresh coal is exposed to air it undergoes low-temperature oxidation which is followed by formation of surface oxides and emission of toxic and fire hazardous gases (Green, Aizenshtat, Hower, Hatch, & Cohen, 2011; Grossman, Davidi, & Cohen, 1994). Emission of carbon oxides from fossil fuels has an enormous environmental and economical impact on coal utilization as a fuel for utility plants (Davidi, Grossman, & Cohen, 1995; Xie, Xue, Cheng, & Wang, 2011). The oxidation of coal at low temperature under atmospheric conditions can also lead to self-heating and may even result in the spontaneous combustion of coal in certain cases, especially when planning the storage of coal in confined spaces (silos, ship holds) or even in underground mining (Sipilä et al., 2012; Wang, Dlugogorski, & Kennedy, 1998; Zhu, Song, Tan, & Hao, 2013). Both low-temperature oxidation and spontaneous combustion are serious economic, environmental and safety problems.

Although extensive investigations were carried out to gain a deep understanding of this hazard, this multistage reaction mechanism is quite complicated and even today it is not yet fully understood (Xie et al., 2011). It is generally accepted that two parallel reaction sequences, namely chemisorption sequence and direct burn-off reactions, contribute to emission of carbon oxides during oxidation. The chemisorption sequence consists of multiple steps: the physical adsorption and chemisorption of oxygen on the surfaces of pores; the formation of intermediate oxy-coal complexes; the decomposition of unstable solid oxygenated intermediates to gaseous products and stable solid complexes. While the so-called direct burn-off occurs at specific sites in a coal's aromatic or aliphatic structure, resulting in the direct formation of gaseous products (Carras & Young, 1994; Kam, Hixson, & Perlmutter, 1976; Karsner & Perlmutter, 1982; Krishnaswamy, Gunn, & Agarwal, 1996; Wang, Dlugogorski, & Kennedy, 2003a).

A study of the initial stage of the room-temperature oxidation of coal is important, not only for the prevention for fires in coal industry, but also in reducing emissions of hazardous gases (Shi, Wang, Deng, & Wen, 2005). The effect of ventilation rate on spontaneous heating coal was investigated by Yuan and Smith (2012). Some works have been done to study the oxygen consumption rate of coal during self-heating process (Qin et al., 2012; Zhu, He, & Li, 2012). However, few works have been devoted to investigate the oxidation of coal under ambient temperature, which is the initial stage of coal self-heating. The transformation of active groups during coal oxidation at the room-temperature has been obtained from simulated calculation (Shi et al., 2005), but the oxygen consumption and the formation of carbon oxides have not been studied experimentally. The mechanisms of the oxygen consumption and the formation of carbon oxides during this process are hardly studied, mostly because this process is very slow and oxidation production is very little. Flow reactors were used in experiments by numerous researchers, but this leads to dilution of the oxidation products by the carrier gas.

A batch reactor was used and a series of simulated experiments of coal oxidation was carried out in this paper. The major parameters which characterize low-temperature oxidation of coal include the rate of oxygen consumption and the rates of formation of carbon-containing gaseous products (Wang, Dlugogorski, & Kennedy, 2003b). Previous studies show that appreciable amount of the CO₂ formed at the surface during coal oxidation is adsorbed within the coal pore structure at low temperature while CO is not expected to be absorbed by the coals (Aizenshtat, Green, Stark, Weidner, & Cohen, 2010; Cao et al., 2011). In addition, CO is a toxic gas which is disadvantageous from both the economic and safety standpoint. Therefore, the time-dependent rates of CO emission and O₂ consumption were obtained simultaneously from the measurements of CO and O₂ concentration in the reactor, just after the onset of the oxidation of coal, using CO and oxygen transducer, respectively. The oxygen concentration in oxidizing medium and the particle size of coal samples, which are the most important variables affecting the oxidation of coal, are evaluated in this paper. A comprehensive mechanism for low-temperature oxidation of coal in confined spaces is also suggested on the basis of the experimental findings.