A discrete particle swarm optimization approach for classification of Indian coal seams with respect to their spontaneous combustion susceptibility

Mine fires due to spontaneous combustion in coal mines is a global concern. This leads to serious environmental and safety hazards and considerable economic losses. Therefore it is essential to assess and classify the coal seams with respect to their proneness to spontaneous combustion to plan the production, storage and transportation capabilities in mines. This paper presents the formulation of clustering problem into a linear assignment model and the application of a discrete particle swarm optimization approach for the classification of coal seams based on their proneness to spontaneous combustion. In this research work, twenty nine coal samples of varying ranks belonging to both high and low susceptibilities to spontaneous combustion have been collected from all the major coalfields of India. Using moisture, volatile matter, and ash content and crossing point temperature of the coal samples as the parameters, the proposed algorithm has been applied to classify the coal seams into three different categories. This classification will be useful for the planners and field engineers for taking ameliorative measures in advance for preventing the occurrence of mine fires.     

An investigation of mercury distribution and speciation during coal combustion

A multi-field electrostatic precipitator (ESP) and a two-stage condensing heat exchanger (CHX^®) have been added to the pilot scale Vertical Combustion Research Facility (VCRF) in CETC-O to further research into integrated emissions control for coal fired power plants. A series of combustion trials were conducted on the VCRF with three different coals (bituminous, sub-bituminous and lignite) to study mercury distribution and speciation at various VCRF locations. Results showed that, with the bituminous coal, as the flue gas cools down from 700 to 200 °C, 80% of total mercury in the gas phase existed in oxidized form and 20% in elemental form. For sub-bituminous and lignite coals, elemental mercury was the dominant form throughout the system. Analysis of deposited ash samples showed that oxidized mercury can be absorbed on carbon-rich ash deposits, although overall only a very small percentage of total mercury was absorbed on the ash. The potential of the CHX^® at removing mercury from the flue gas was also explored. Results indicated that, using wet scrubbing, the CHX^® was able to remove 98% of oxidized mercury. Though elemental mercury went through the system unabated, it is suggested that, with appropriate agent to oxidize elemental mercury in the CHX^®, it is conceivable to use CHX^® to remove both oxidized and elemental mercury. Finally, mercury balance was performed and good mercury balance was obtained across the VCRF, validating our sampling procedures and analysis methods.     

煤的自燃倾向性鉴定法操作过程分析

将煤样装入装样箱后彻底摇匀,并用硬物件轻轻敲下底,听无回音,测得准确的煤的自燃倾向性等级结果。     

The effect of moisture content and air-drying on spontaneous combustion characteristics of two Turkish lignites

The results of an experimental study aimed at evaluating the spontaneous combustion characteristics of two Turkish lignites moistured and air-dried at varying times are discussed. The spontaneous combustion characteristics of the coals were determined using Crossing Point Methods adapted to our laboratories conditions. The content of three predominant oxygen functional groups (carboxyl, hydroxyl, and carbonyl) of untreated, moisten and air-dried coal samples were also determined with wet chemical methods. The content of oxygen functional groups in moisten coal samples do not differ significantly that of untreated coal samples, for realized in vacuum desicator to moistured of coal samples. The liability of spontaneous combustion of the two coals was reduced when moisture content increased with increase in contacted time to water vapour. The moisten coal samples was dried in laboratory during 24 and 48 h time period. The concentration of oxygen contain functional groups of drying coal samples increased with increase of contact time with air and decrease of particle size. The liability of spontaneous combustion of the air-dried coal samples increased with increase of contacted time with air and with decrease of moisture content.     

Spontaneous combustion of carbonaceous stockpiles. Part I: the relative importance of various intrinsic coal properties and properties of the reaction system

The likelihood of a coal stockpile or carbonaceous waste dump undergoing spontaneous combustion is strongly influenced by factors that relate to both the intrinsic properties of the material itself, as well as the properties of the reaction system. An understanding of these material properties is a pre-requisite for the simulation of the large-scale reaction system with the aim of preventing or containing this phenomenon.The first part of this paper investigates a range of coal properties that were considered to be the most critical in terms of heat ‘generation’ in a bulk coal medium. Certain properties, such as the heat capacity, the heat of reaction with oxygen and the activation energy for this reaction, were found to vary to a relatively minor degree for coals of varying rank and geological origin. However, rates of reaction with oxygen were found to vary by orders of magnitude for different coals. The reaction kinetics is consistent with a diffusion-limited shrinking-core model. A study of the coal properties that most strongly influence these rates of reaction is the subject of part two of this paper.     

A novel sodium carboxymethyl ellulose/aluminium citrate gel for extinguishing spontaneous fire in coal mines

Spontaneous combustion of coal is one of the main disasters affecting safe production in mines. A sodium carboxymethyl cellulose/aluminium citrate gel for prevention and extinguishing of mine fires was prepared. Based on a temperature‐controlled experiment, the authors studied the influences of the gel on indicator gases generated in the spontaneous combustion of coal, crossing point temperature, and activation energy. The results showed that the crossing point temperature of coal samples treated by the gel increased by more than 13.9°C and the activation energy rose by 7.17 to 16.34%. Moreover, the indicator gases, such as CO and C²H⁴ generated in the oxidation, decreased significantly. After treatment with the gel, the weight loss rate of the coal samples was reduced, and less energy was released. By using the self‐developed test platform (coal capacity of 2 m³), the spontaneous combustion of coal on the roof of a roadway was simulated, and the carboxymethyl cellulose/aluminium citrate gel was used for fire extinguishing. After injecting the gel, the temperature in the furnace rapidly decreased, and the concentration of O² remained at less than 3%, without showing any after‐combustion effects.     

The fragmentation of coal particles during the coal combustion in a fluidized bed

This paper presents an experimental method for studying the fragmentation of coal particles during coal combustion in a fluidized bed and the quantitative fragmentation indexes of 10 typical Chinese coal ranks. The influences of a variety of factors such as the bed temperature, the size of coal particles, the coal rank and the fluidizing medium on the fragmentation index of coal particles are also studied. The research results show that the main reason for the fragmentation of coal particles is the primary fragmentation, and that the volatile matter can drastically influence the degree of fragmentation of coal particles.     

Spontaneous combustion of carbonaceous stockpiles. Part II. Factors affecting the rate of the low-temperature oxidation reaction

In part I of this paper, the rate of the low-temperature oxidation reaction was found to be critical in determining the relative tendency of a carbonaceous material to self heat and hence undergo spontaneous combustion. A static isothermal apparatus was designed to directly measure rates of reaction at constant oxygen concentrations and at low temperatures. The results show the relative importance of extrinsic factors such as particle size, ambient humidity, oxygen concentration and concentration of reaction product. A distributed-rate model shows that the apparent fractional order of reaction with respect to oxygen can be explained by allowing for a distribution of first-order rate constants. The results are consistent with the porous structure of coal and with microscopic examination of oxidised coals.Sufficient samples were tested and analysed to perform statistical modelling, which relates the intrinsic properties measured by proximate, ultimate and petrographic analyses to the rate of oxidation of a coal. It was shown that the two factors that had the most statistical significance in determining the propensity for self-heating were the volatile content and the inherent moisture. It is suggested that the inherent moisture is related to the total surface area of the sample, and that the volatile matter component represents the reactive component. Measurement of these two parameters gives a reasonable prediction of a sub-bituminous coal's rate of reaction with oxygen at ambient temperature, and quickly identifies ‘at risk’ samples for further testing.A separate set of experiments was conducted to examine the comparative importance of the sorption of moisture. It was shown that the rate of vapour phase adsorption and desorption of moisture is slow compared to the oxidation reaction.     

Oxidation behavior of carbon steel in simulated kerosene combustion atmosphere: A valuable tool for fire investigations

Fire investigations aim to establish the origin and cause of fires by collecting and analyzing the comprehensive fire‐related evidences. Metallic materials exposed to the fire scene environments are usually subjected to melting and/or high‐temperature oxidation, and they have been considered vital parameters for temperature determination, as recommended in NFPA 921. The oxide characteristics obtained from the conventional fire investigations primarily rely on simple visual observations such as the variations in oxide color, the so‐called “oxidation patterns.” However, such information is not sufficiently convincing due to the complex nature of oxides formed in the fire scene. The oxide color is strongly affected by the type of oxide, the oxide thickness, the concentration of contaminant, and the interactions among different oxides. In this study, Q235 structural steel samples have been exposed to high‐temperature air and simulated kerosene combustion conditions at certain temperatures and for indicated periods. The oxidation rate was examined by thermogravimetric analysis. The morphologies and microstructures of the oxide scales were investigated by scanning electron microscopy, energy dispersive spectroscopy, and X‐ray diffractions. The results show that the oxide properties are strongly dependent on the oxidation temperature and oxidation atmospheres. These oxidation behaviors are expected to provide useful information on identifying fire characteristics.     

原煤自燃特性的试验研究

介绍了电加热柱形原煤自燃试验台测试原煤煤堆自燃性能的方法,并将测试结果与已有的判别指标进行了对比分析,指出了煤的品级对煤自燃特性的影响,并得出基于基本煤质参数的简易自燃判别指标。     

A method for measuring the kinetics of organically associated inorganic contaminant vaporization during coal combustion

A novel method is presented for the direct measurement of kinetic parameters under conditions that simulate the vaporization of inorganic material during coal combustion. This method will ultimately be used to generate key physical property data necessary to model and assess the vaporization behavior of trace elements (TEs) during coal combustion. The method uses a graphite furnace atomic absorption spectrometer (GFAAS) as the experimental platform for vaporization measurements for temperatures up to 2800 K. The method was used to model vaporization during coal pyrolysis for one of the three key contaminant domains: organically associated material. Significant testing was conducted for selected TEs under varied instrumental operational conditions that could affect activation energies, such as GFAAS ashing and atomization temperatures, TE concentration, and carrier gas flow rate. The vaporization activation energies determined were found to be independent of these operating conditions despite a significant difference in the degree of analyte vaporization observed at varied ashing temperatures.     

DEM-LES simulation of coal combustion in a bubbling fluidized bed Part II: coal combustion at the particle level

The discrete element method-large eddy simulation (DEM-LES) is used to model coal combustion at the particle level in a bubbling fluidized bed. The gas phase is modelled as a continuum and the solid phase is modeled by DEM. Chemical reactions consist in the heterogeneous reactions of char with O₂, CO, CO₂, NO, and N₂O, and in the homogeneous reactions involving CO, O₂, NO, and N₂O. The colliding particle-particle heat transfer is based on the analysis of the elastic deformation of the spheres during their contact. The model predicts the effects of the particle heterogeneous flow structure on the thermal characteristics of coal particles when heating and burning, and the gaseous emissions from a fluidized sand-coal binary mixture. The heating rates are 1627 and for, respectively, 0.8 and diameter coal particles fed into the fluidized bed. The instantaneous contribution of the collision heat transfer is weak, less than 5.0% of the total power exchanges (coal combustion, radiation, convection and collision) during the heating and 1.5% during the combustion. The temperature of the coal particles exceeds the bed temperature, which is in qualitative agreement with experimental data from literature. The effects of the diameter of coal particles, of the bed temperature, and of the inlet gas velocity on the thermal characteristics are also studied.     

Numerical studies of pulverized coal combustion in a tubular coal combustor with slanted oxygen jet

A pure two-fluid model for turbulent reacting gas-particle flow of coal particles is developed using a unified Eulerian treatment of both the gas and particle phases. The particles' history caused by mass transfer due to moisture evaporation, devolatilization and char reaction is described. Both velocity and temperature of the coal particles and the gas phase are predicted by solving the momentum and energy equations of the gas and particle phases, respectively. A k-ε-k_k two-phase turbulence model, EBU-Arrhenius turbulent combustion model and four-flux radiation heat transfer model are incorporated into a comprehensive model. The above comprehensive mathematical model is used to simulate two-dimensional gas-particle flows and pulverized coal combustion in a newly designed tubular oxygen-coal combustor of blast furnace. Predicted results of isothermal gas-particle flows are in good agreement with those obtained by measurements. The results also show that the proposed tubular oxygen-coal combustor prolongs the coal particle residence time and enhances the mixing of coal and oxygen. Results indicate that smaller coal particles of 10 μm diameter are heated and devolatilized rapidly and have volatile combustion in the combustor, while larger coal particles of 40 and 70 μm in diameter are heated but not devolatilized, and combustion of such particles does not occur in the tubular combustor.     

Effect of early stages of coal oxidation on its reaction with elemental sulphur

The present research shows how mild oxidation of coal mostly affects the evolution of H₂S produced in the reaction of coal with elemental sulphur. Coal samples oxidized at 30, 50, 80 and 150°C were reacted with sulphur in a temperature-programmed reactor. The H₂S produced in the reaction is very sensitive to the initial stage of the oxidation of coal. The strongest reduction in the amount of H₂S evolved was observed in the samples oxidized at 30°C. This temperature is lower than the one found in most coal storage places. The reaction with elemental sulphur could be used to monitor the initial stages of coal oxidation, which otherwise would be difficult to follow by conventional analytical methods.     

Modeling the temperature in coal char particle during fluidized bed combustion

The temperatures of a coal char particle in hot bubbling fluidized bed (FB) were analyzed by a model of combustion. The unsteady model includes phenomena of heat and mass transfer through a porous char particle, as well as heterogeneous reaction at the interior char surface and homogeneous reaction in the pores. The parametric analysis of the model has shown that above 550 °C combustion occurs under the regime limited by diffusion. The experimental results of temperature measurements by thermocouple in the particle center during FB combustion at temperatures in the range 590-710 °C were compared with the model predictions. Two coals of different rank were used: lignite and brown coal, with particle size in the range 5-10 mm. The comparisons have shown that the model can adequately predict the histories of temperatures in char particles during combustion in FB. In the first order, the model predicts the influence of the particle size, coal rank (via porosity), and oxygen concentration in its surroundings.     

Perovskite-type oxide monolithic catalysts for combustion of chlorinated hydrocarbons

The catalytic activity of perovskite-coated and perovskite-extruded monolithic catalysts was studied in the total oxidation of several aliphatic chlorinated hydrocarbons. At low reaction temperatures a reversible catalyst deactivation takes place. The complete decomposition of the chlorinated hydrocarbons without formation of by-products depends on the reaction conditions, the kind of chlorinated hydrocarbon and the monolith preparation.     

Analysis of combustion efficiency in CFB coal combustors

Fluidized bed technology is well known for its high combustion efficiency and is widely used in coal combustion. In this study, the combustor efficiency has been defined and investigated for CFB coal combustor based on the losses using a dynamic 2D model. The model is shown to agree well with the published data. The effect of operating parameters such as excess air ratio, bed operational velocity, coal particle diameter and combustor load and the effect of design variables such as bed height and bed diameter on the mean bed temperature, the overall CO emission and the combustion efficiency are analyzed for the small-scale of CFBC in the presently developed model. As a result of this analysis, it is observed that the combustion efficiency decreases with increasing excess air value. The combustion efficiency increases with the bed operational velocity. Increasing coal particle size results in higher combustion efficiency values. The coal feed rate has negative effect on the combustion efficiency. The combustor efficiency considerably increases with increasing combustor height and diameter if other parameters are kept unchanged.     

含硫油品储罐腐蚀产物自燃性研究进展

储罐中含硫油品腐蚀产物自燃会引起储油罐发生火灾及爆炸事故。作者综述了储罐中硫腐蚀产物的主要来源及其自燃性,阐述了硫化温度、硫化时间、硫化环境中氧气浓度和氧化环境温度、氧化环境中氧气浓度、空气流速、含水量、生成方式及其它因素对硫腐蚀产物自燃性的影响。     

Effect of longwall face advance rate on spontaneous heating process in the gob area - CFD modelling

A commercial CFD software programme, FLUENT, was used to study the oxidation process of coal in the mined-out longwall (gob) area. A three-dimensional, single-phase model with a continuously advancing longwall face has been developed. For the model, the gob longwall area was designed on the basis of the actual longwall panel operating in the Ostrava-Karviná Coal Mines (OKD, Czech Republic). The behaviour of the coal to oxygen was modelled using the results arising mainly from the former laboratory-scale experiments with Czech bituminous coals. Basically, the technique of pulse flow calorimetry and measurements at a continuous airflow reactor were applied during the laboratory investigations. In the contribution, the main focus was to understand the effect of the longwall face advancing speed on the oxidation heat production as well as evolution of the gases in the gob area. Simultaneously, the effect of coal crushing in the mined-out area on the spontaneous heating process was examined.Numerical simulations confirmed the existence of a “favourable” zone for the onset and development of the spontaneous heating process in the gob area. The location and the maximal temperature reached in the “favourable” zone were found to be significantly affected by the advancing rate of the coalface. The slower the advancing rate is, the higher the maximal temperature and smaller the depth of the “favourable” zone in the gob area are. When the rate drops to a certain “critical” value, spontaneous heating turns to flammable combustion of the coal. The value of the “critical” advancing rate was confirmed to increase if the grain size of the coal left in the gob decreases. Numerical examinations of carbon monoxide concentrations then proved that small incidents of spontaneous heating could occur in the gob area that need not be detected in the airflow of the longwall tail gate.