Experimental and numerical study of coal dust ignition by a hot particle

Highlights

• Hot small-sized metal particle ignites a layer of coal dust.

• The ignition characteristics were determined from the experiments.

• The mathematical model was developed to forecast the ignition characteristics.

• The model describes the experimental results well.

• Limitations were analyzed for the practical use of the predictive mathematical model.

Abstract

This work studies the ignition of a layer of brown coal dust by hot metal particles experimentally and numerically. The experiments establish the limits of the flaming ignition of gas and ignition delay times when the parameters of solid fuel and metal particles varied in a wide range. The particle size of coal ranged from 0.1 to 1 mm; the shapes of the metal particles were sphere, disk, and cube; their initial temperature varied between 1000 and 1400 K. A mathematical model was developed for describing the processes involved in heat and mass transfer as well as chemical reactions around the local heat source. The results of the numerical simulation are in good agreement with the experimental data: boundaries of the flaming ignition of coal; coal ignition delay times; three modes of flaming ignition of coal with the ignition zone of volatiles located in the vicinity of the hot particle. The mathematical model is good at predicting the ignition conditions during interaction between a hot metal particle and a layer of coal dust. The model can also be used for developing the promising technology of steam boiler start-up by highly reactive coal instead of flammable liquid combustion. Another application is the development of fire prevention guidelines for tightening fire safety management at productions deal with coal mining, transportation, storage, processing, and combustion. Finally, the paper includes the analysis of limitations for the practical use of the predictive mathematical model in thermal power engineering and fire safety management.

Introduction

Coal is widely used in thermal power engineering as solid fuel [1], [2], [3] or a component of coal water slurry [4], [5] to produce electricity and heat. In chemical industry, coals serve as raw materials to produce synthetic liquid fuel [6], gas [7], porous carbons [8], sorbents [9], etc.

All the stages of the technological process are explosion and fire hazardous, starting from coal mining and ending with fuel preparation for combustion at thermal power plants or processing raw materials at chemical plants [10], [11], [12]. Fine dust deposits are the most fire hazardous. A fire may result from a layer of coal dust interacting with heated walls of process equipment [13], [14] or hot metal particles [15]. Local heat sources may emerge due to welding of metalworks, their mechanical treatment, friction of unlubricated moving metal parts, and power line short circuits. Over the recent years, coal dust catching fire (including due to hot metal particles) has become the reason for several major technological disasters at thermal power plants [16]. Therefore, research into the ignition patterns and properties of finely divided solid fuel heated by limited-capacity heat sources is a relevant task with a view to prevent fire outbreaks in mines, at thermal power plants, and chemical plants. The new knowledge may help engage technical experts in tackling this urgent problem, develop fire prevention guidelines, and make fire safety management more rigorous during maintenance and repair at process facilities.

Another relevant problem in thermal power engineering is a decrease in the consumption of the liquid fuel, which is burned in large quantities during boiler start-up [17]. Therefore, the results of research into the characteristics of coal dust ignition under the short-term interaction with local heat sources are a basis for developing the advanced technology of boiler start-up without using flammable liquids.

When hot metal particles interact with a layer of coal dust, this induces a number of interrelated physical and chemical processes. Solid fuel heats up, the local source cools down, coal starts decomposing, volatiles are released, a gas mixture is produced, and then it heats and starts burning. As a rule, the ignition of highly reactive brown coals containing about 50% of volatiles leads to the fire spreading over the surface of solid fuel. The energy released during the oxidation of the gas mixture heats up the near-surface layer of coal. Its thermal decomposition accelerates. More fire gases are released. It can be seen that the combustion of brown coal is a self-maintained process induced by the ignition of the decomposed gases.

There has been experimental research into the spontaneous ignition of solid natural fuel [18], [19] or its ignition when exposed to conductive heating by a large heat source (e.g., heated walls of process equipment, whose temperature usually remains constant for a long time and amounts to 400–600 K) [13], [14]. Under such conditions, the induction period may range from several minutes to several days [13], [14] depending on the properties of the heat source. The processes involved in the ignition of a layer of coal dust are more rapid during local conductive heating by hot small-sized particles [15]. A metal particle cools due to heat removal to fuel or gaseous medium [20], [21], [22], [23], which reduces the induction period within a certain range of initial temperatures down to less than 5 s, if the heat flux to the ignition zone is enough to initiate combustion.

Research [15] aimed at confirming the hypothesis of sustainable ignition induced by the interaction between a hot metal particle and a layer of coal dust. The authors [15] established the dependencies of coal ignition delay time on the initial temperature of a disk-shaped steel particle. The development of fire prevention guidelines requires more information on the patterns and attributes of the process under study as well as its description in a predictive mathematical model. Numerical simulation will help create a database of how significant factors affect the ignition characteristics without spending too much time on it.

Traditional mathematical models [24], [25] simulate the solid-phase ignition of condensed substances by dispersed gas-particle flows or isolated hot particles. These models are developed to describe the processes involved in gas-free combustion, when a fuel mixture contains a combustible component and an oxidizer. Such models, however, cannot be used to study the ignition of gaseous products of coal decomposition, when a flammable gas mixture is formed during the local heating of a solid fuel bed. Unlike solid-phase ignition of composite fuel, the characteristics of gas-phase ignition of locally heated coal dust depend on a set of interrelated processes: thermal decomposition of coal, emission of volatiles into the oxidizer medium, formation of a gas mixture through diffusion, and heating of the said mixture. Therefore, in order to forecast the ignition characteristics, it is necessary to develop a mathematical model based on the results of detailed experimental studies.

This work experimentally investigated the patterns and characteristics of brown coal dust ignition by metal particles of various shapes. Their initial temperatures varied within a wide range. Furthermore, a predictive mathematical model was developed to describe the interrelated physical and chemical processes involved in the gas-phase coal ignition. In terms of the practical application, the results can be used to enhance the ignition process of highly reactive coal in the development steam boiler start-up without flammable liquid combustion. In addition, they can help prevent local heat sources from interacting with coal dust and in case fire safety management standards are tightened.