Characteristics of coal dust ignited by a hot particle

https://doi.org/10.1016/j.psep.2021.07.021Get rights and content

Abstract

Ignition of coal dust caused by hot particles (ICDHP) will lead to serious combustion or explosion. In this work, the coupling details of multi–physical fields of the ICDHP were established. Two different chemical reactions of flaming combustion of volatiles and smoldering of coal dust were established to understand the mechanism and process of the ICDHP. The evolution of temperature, volatiles molar concentration and heat release characteristics under different particle temperatures and contact conditions between the hot particle and coal dust were investigated. Results show that there are two ways of the ICDHP, which are volatiles ignition and coal smoldering ignition. The ignition delay time (IDT) increases exponentially with the decrease of the hot particle temperature. The temperature of the hot particle is 1100 K, and thermal runaway occurs in the way of coal smoldering ignition. When the temperature of the hot particle is higher than 1100 K, thermal runaway occurs in the way of volatiles ignition. In addition, it is found that the IDT decreases exponentially with the increase of the buried depth (lp) of the hot particle in coal dust for the hot particle temperature of 1186 K. The IDT shows a trend of decreasing – increasing – decreasing with the increase of lp for the hot particle temperatures of 1271 K and 1369 K. If the hot particle is completely embedded in the coal dust, the smoldering combustion of the coal dust is found. In addition, the volatiles first react on the side of the hot particle, and the thermal runaway occurs at the position where the concentration and temperature of volatiles are high enough.

Introduction

Coal is still one of the main energy sources for a long time due to abundant storage and easy availability (Shi et al., 2021a; Zhang et al., 2016; Lin et al., 2019; Long et al., 2021). Pulverized coal is the main form of coal material in coal bunker, which can be better used for production and transportation (Zhang et al., 2021c). In addition, as an important raw material and process material, pulverized coal appears in a large number of industrial scenarios, such as power plants, coal preparation plants and chemical plants (Demirbaş, 2003; Schmal et al., 1987; Ma et al., 2019; Li et al., 2020). Inevitably, pulverized coal will be deposited in the relevant industrial sites to form coal dust (Song et al., 2019; Deng et al., 2021a, Deng et al., 2021b). Coal dust is a kind of material that is easy to be ignited, and the research on its ignition mechanism and prevention is an important topic in the current process safety and risk engineering field (Azam and Mishra, 2019; Copelli et al., 2019; Janès et al., 2019).

In the industrial environment, hot contact surface and hot particles are very easy to cause the ignition of coal dust, resulting in uncontrolled combustion or explosion (Beamish and Theiler, 2019; Janes et al., 2008; Yuan et al., 2019). Fire and explosion accidents caused by coal dust have occurred in large numbers (Essenhigh et al., 1989; Zhang et al., 2018; Zakharevich and Bogomolov, 2015). Some studies have characterized the ignition process and key parameters of coal dust using the hot contact surface or hot oven for the industrial process safety (Schmidt et al., 2003; Joshi et al., 2012; Park et al., 2009). The spontaneous combustion tendency and risk assessment under different coal body conditions and environmental conditions are concerned (Sahu et al., 2015; Zhang et al., 2021a, Zhang et al., 2021b; Xu et al., 2021a). The study of key parameters can effectively characterize the phenomenon and mechanism of coal ignition (Song et al., 2014, Song et al., 2020). Sun et al. have obtained the spontaneous combustion point and ignition point by analyzing time-temperature curve of coal oxidation (Sun et al., 2019). The main factors affecting the ignition characteristics of coal include coal body conditions and environmental conditions (Li et al., 2019; Su et al., 2017). Coal body conditions include coal type, coal particle size, etc., while environmental conditions include gas atmosphere, ambient temperature, etc (Shah et al., 2004; Lü et al., 2019; Zhang et al., 2019; Wu et al., 2015; Zhang et al., 2020; Yin et al., 2021). Xu et al. have studied the standard oxygen consumption rate and spontaneous combustion tendency of different coal rank and coal particle size (Xu et al., 2021b). Wen et al. used different experimental methods to test the key parameters of the coal spontaneous combustion process (Wen et al., 2020).

Hot particles can ignite coal dust and cause serious accidents (Glushkov et al., 2018). Hot particles are produced in the process of industrial production welding, mechanical friction and wire short circuit, etc. It is found that hot particles with high temperature can cause coal dust to burn violently in an instant. Therefore, compared with coal dust ignited by hot contact surface or hot oven, falling hot particles in coal dust may have more dangerous consequences (Yang et al., 2016a, b). In general, the coal dust particles are small, the pores are relatively high, and the spontaneous combustion tendency is large (Li et al., 2020). From the perspective of industrial process safety phenomenon and mechanism, studying the evolution of basic multi-physical coupling parameters can effectively understand the characteristics and mechanism of the ignition and combustion propagation under the influence of local heat source on coal dust in industry. It is helpful to reveal the cause of coal dust combustion and explosion. From the perspective of industrial process safety management and protection, the study of the hot particles igniting coal dust will help to establish more complete safety protection measures and safety management measures in related industries to prevent similar hazardous accidents (Xu et al., 2020; Lü et al., 2021). Urban et al. have studied the characteristics of solid fuel ignited by metal hot particles, and distinguished the ignition boundary to understand the ignition and spread characteristics of wildfire (Urban et al., 2019). Glushkov et al. have carried out the experimental and numerical simulation research on the ignition of pulverized coal by the hot particles, with the purpose of formulating fire prevention guidelines to strengthen the fire safety management of coal mining, transportation, storage, processing and combustion production (Glushkov et al., 2018).

Due to the heat transfer of the hot particles to the coal dust, exothermic pyrolysis of coal dust near hot particles will occur, and volatiles will be generated at the same time. The precipitated volatiles may be ignited in the form of flaming combustion after being affected by the hot particles. Therefore, when hot particles contact with coal dust, they may ignite coal dust in two different ways, namely, volatiles ignition and coal smoldering ignition (Glushkov et al., 2018; Wang et al., 2015; Fereres et al., 2012). Vershinina et al. have found that the ignition of coal particles is affected by coal rank and volatiles, and the ignition mode of coal particles with high volatiles is mainly gas-phase combustion (Vershinina et al., 2020). The contact between hot particles and coal dust will lead to more complex fluid flow, which in turn promotes the transfer of volatiles. Compared with coal dust ignited by hot contact surface, the physicochemical process of ignition of coal dust caused by hot particle (ICDHP) is more complex.

Numerical simulation is helpful to establish a reasonable physical explanation to more deeply understand the phenomenon and characteristics of coal dust ignited by the hot particles. Further, detailed numerical simulation research can effectively understand the local heat source for coal dust ignition mechanism and key incentives, so as to establish more targeted safety protection means and safety management measures (Shi et al., 2021b; Zhuo et al., 2019; Song et al., 2021a, Song et al., 2021b). Previous studies have found that the temperature and properties of the hot particles have a great influence on the ignition of solid combustibles (Urban et al., 2017). In addition, the response characteristics of different solid combustibles to the hot particle are also inconsistent.

Previous studies have focused on hot particles igniting other types of solid materials such as cellulose, polyurethane foam and plant beds, etc (Ellis, 2015; Fang et al., 2021; Hadden et al., 2011). There is less work related to the ICDHP. Obviously, the process of ICDHP is a very complex multi–physical field coupling process of fluid flow, heat transfer, mass transfer and chemical reaction. It is necessary to establish a more detailed numerical description method of the ICDHP. The detailed mechanism of the ICDHP under various conditions also needs to be explained.

In this paper, two different chemical reactions of coal pyrolysis and volatiles combustion were established, and the detailed processes of the ICDHP were studied. The evolution characteristics of temperature, ignition delay time (IDT), heat release rate and volatiles molar concentration under different particle temperatures and contact conditions between the hot particle and coal dust were studied respectively. The main mechanism of coal dust ignition under different hot particle temperatures and contact conditions between the hot particle and coal dust was revealed. It is worth mentioning that this article has carried out an innovative survey on: 1) A multi-physics coupling model describing the ICDHP; 2) The different thermal contact conditions of hot particles and coal dust; 3) A series of new characteristics of coal dust smoldering and volatile combustion; 4) The potential mechanism of ICDHP. The goal of this paper is to understand the detailed characteristics of the ignition of coal dust caused by a hot particle and to establish reasonable industrial safety measures for process safety and risk engineering, such as high-temperature metal particles or local heat sources caused by metal welding, mechanical processing, etc. This study is of great significance to understand the characteristics and mechanism of the ignition and combustion propagation under the influence of local heat source on coal dust in industry and to establish reasonable industrial safety measures for process safety and risk engineering.

Section snippets

Model analysis

There have been many accidents of violent combustion or explosion caused by the ICDHP. Based on real scenarios, the physical and chemical processes of the ICDHP are simplified. In the numerical model, the falling process of the hot particle and the inhomogeneity of coal dust are ignored. The physical model adopted in this paper was based on the experiments of (Glushkov et al. (2018)). Before performing numerical simulations, some main idealized assumptions should be made about actual physical

Verification of mathematical model

The mathematical model of hot particles igniting coal dust is very complicated, and the physical and chemical parameters in this process are highly coupled. Therefore, it is necessary to verify the numerical model. For ICDHT, IDT directly determines the degree of danger of hot particles igniting coal dust. Accordingly, IDT is the result that needs the most attention (Glushkov et al., 2018; Wang et al., 2015). IDT is considered to be the time corresponding to the maximum temperature of coal dust

Conclusions

Based on two different chemical reactions of coal dust pyrolysis and volatiles combustion, the detailed process of the ICDHP was studied. The evolution of temperature, volatiles concentration and heat release characteristics under the different hot particle temperatures and contact conditions between the particle and coal dust were investigated. The main conclusions obtained are as follows:

  • 1)

    The IDT increases exponentially with the decrease of Tp. For Tp = 1100 K, no thermal runaway (flaming

Declaration of Competing Interest

The authors report no declarations of interest.

Acknowledgements

This study was supported by National Key R&D Program of China (2018YFC0807900), National Natural Science Foundation of China (Grant Nos. 51974235, 51774233) and Natural Science Foundation of Shaanxi Province (Grant Nos. 2018JZ5007).

References (62)

  • M. Li et al.

    Risk assessment of mine ignition sources using fuzzy Bayesian network

    Process Saf. Environ.

    (2019)
  • S. Lin et al.

    A study on the FTIR spectra of pre- and post-explosion coal dust to evaluate the effect of functional groups on dust explosion

    Process Saf. Environ.

    (2019)
  • H. Park et al.

    A means to estimate thermal and kinetic parameters of coal dust layer from hot surface ignition tests

    J. Hazard. Mater.

    (2009)
  • A.K. Sahu et al.

    Comprehensive numerical modeling of ignition of coal dust layers in different configurations

    Proc. Combust. Inst.

    (2015)
  • M. Schmidt et al.

    Self–ignition of dust at reduced volume fractions of ambient oxygen

    J. Loss Prevent. Proc.

    (2003)
  • A. Shah et al.

    The ignition of low–exothermicity solids by local heating

    Process Saf. Environ.

    (2004)
  • S.X. Song et al.

    Hybrid CH4/coal dust explosions in a 20-L spherical vessel

    Process Saf. Environ.

    (2019)
  • Z.Y. Song et al.

    Chimney effect induced by smoldering fire in a U-shaped porous channel: A governing mechanism of the persistent underground coal fires

    Process Saf. Environ.

    (2020)
  • Z.Y. Song et al.

    Numerical study on effects of air leakages from abandoned galleries on hill-side coal fires

    Fire Saf. J.

    (2014)
  • H.T. Su et al.

    Risk analysis of spontaneous coal combustion in steeply inclined longwall gobs using a scaled-down experimental set-up

    Process Saf. Environ.

    (2017)
  • J.L. Urban et al.

    Smoldering spot ignition of natural fuels by a hot metal particle

    Proc. Combust. Inst.

    (2017)
  • K.Y.U. Vershinina et al.

    Comparing the ignition parameters of promising coal fuels

    Process Saf. Environ.

    (2020)
  • S.P. Wang et al.

    Ignition of expandable polystyrene foam by a hot particle: an experimental and numerical study

    J. Hazard. Mater.

    (2015)
  • D.J. Wu et al.

    Experimental investigation on the self–ignition behaviour of coal dust accumulations in oxy–fuel combustion system

    Fuel

    (2015)
  • Q. Xu et al.

    Effect of particle size and low-temperature secondary oxidation on the active groups in coal structures

    Process Saf. Environ.

    (2021)
  • J.L. Yang et al.

    Effect of interface thermal resistance on ignition of reactive material by a hot particle

    Int. J. Heat Mass Transf.

    (2016)
  • H. Yuan et al.

    A computational model to simulate self–heating ignition across scales, configurations, and coal origins

    Fuel

    (2019)
  • Y.L. Zhang et al.

    A clean coal utilization technology based on coal pyrolysis and chemical looping with oxygen uncoupling: principle and experimental validation

    Energy

    (2016)
  • Y.T. Zhang et al.

    Risk evaluation of coal spontaneous combustion on the basis of auto–ignition temperature

    Fuel

    (2018)
  • Y.T. Zhang et al.

    Ultrasonic extraction and oxidation characteristics of functional groups during coal spontaneous combustion

    Fuel

    (2019)
  • Y.T. Zhang et al.

    Study on the characteristics of coal spontaneous combustion during the development and decaying processes

    Process Saf. Environ.

    (2020)
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