Modelling coal combustion: the current position

At the present time, computer models for coal combustion are not sufficiently accurate to enable the design of combustion plant or the selection of a coal based on combustion behaviour. Most comprehensive combustion models can predict with reasonable accuracy flow fields and heat transfer, but usually with a much lesser degree of accuracy than the combustion of the coal particles through to char burnout. Many research programmes are aimed at developing a much more accurate predictive tool for assessing coals specially fired in burners or furnaces employing a range of NO_x abatement technologies. Some of the current developments in CFD coal combustion modelling are outlined here. Particular attention is paid to the first step, where the devolatilisation pre-processor code is used to compute the pyrolysis rate, the yields and the composition of volatiles and char. These parameters are used as inputs to the devolatilisation and volatile combustion sub-models, where various options can be used, and also the char burnout sub-models. The accuracy of the sub-models is examined using data from four well-studied coals, three from the UK and one from the US. The main network devolatilisation codes are compared with experimental data. Two char combustion models have also been investigated in order to compare char burnout predictions and the development of char morphology and surface area during burnout are considered. The applications of these sub-models to two combustion situations were considered. These involve reactions in a drop tube furnace and a low NO_x industrial burner and in both cases, the model predictions were compared with experimental measurements.     

Emission of suspended PM10 from laboratory-scale coal combustion and its correlation with coal mineral properties

Four pulverized coals were subjected to combustion in a laboratory-scale drop tube furnace to investigate the emission of suspended particulate matter smaller than 10 μm (PM₁₀) and to study the correlation of PM₁₀ emission with mineral properties of the coals. Combustion conditions of 1200 °C, 2.4 s and 20% atmospheric oxygen content were used and all the carbon was consumed under given conditions. The properties of PM₁₀ were studied including its concentration, particle size distribution and elemental composition. Two typical sizes were also subjected to Computer controlled scanning electron microscopy (CCSEM) analysis for determination of chemical species within them. To investigate the influence of coal mineral properties, the metallic elements in the raw coals were divided into three parts: organically bound, included inorganic particles and excluded ones. The results indicated that during coal combustion, about 0.5-2.5 wt% of inherent minerals changed into the suspended PM₁₀. With an increase in the coal ash content, the concentration of PM₁₀ increased proportionally. The resulting PM₁₀ had a bimodal size distribution with two peaks around 2.5 and 0.06 μm, respectively. SiO₂ and Al₂O₃ dominated the large mode around 2.5 μm, which is formed by the direct transformation of inherent minerals. On the other hand, SO₃ and P₂O₅ were prevalent in the small mode around 0.06 μm, which is formed by vaporization of these two elements. For other metals found in PM₁₀, the refractory metals were enriched in the large mode, with concentrations proportional to their content in the excluded minerals in the raw coal. Volatile metals were however enriched in the small mode since, they react with gaseous SO₂ and P₂O₅ to form sulfates and phosphates in the solid phase. The study showed that experimental observations agree with thermodynamic equilibrium considerations.     

Kinetics of mineral matter transformation during coal combustion

The transformation of individual minerals was investigated based on TG and DTG analysis at temperature up to 1700 K in inert and oxidizing atmospheres. The decomposition of minerals in inert atmosphere and the reaction with gaseous atmosphere was described by first order reactions for which the kinetic data were found. The evaluated kinetic parameters were then tested on a complex mineral matter of coals. It has been demonstrated on example of two different compositions that the mass loss during the transformation of coal mineral matter during combustion can be modelled as a mixture of individual minerals.     

Environmental impacts of coal combustion: A risk approach to assessment of emissions

This paper summarises some of the work performed in the Cooperative Research Centre for Coal in Sustainable Development (CCSD) on emissions from current power generation. A comprehensive approach was taken in the CCSD program to assessing environmental issues of concern for the power, and by implication the coal, industries. Here results of sampling on full scale operating plants are described, and detailed data on emission fluxes, particle size distributions, trace element concentrations as a function of particle size, and speciation of the trace elements are illustrated. The results show that particle capture in electrostatic precipitators (ESPs) is significantly less efficient than in fabric filters (FFs), particularly for submicron material, and that significant enrichment is observed in the finer particle sizes emitted from both ESPs and FFs. Results for the speciation of chromium, arsenic and selenium in coals, bottom ash and fly ash are also presented. The majority of chromium in fly ash is present in the less toxic Cr³⁺ form. Speciation of arsenic in feed coals is variable but the dominant form of As in fly ash is the less toxic As⁵⁺.     

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.     

Iron transformations during combustion of Pittsburgh no. 8 coal

The chemical speciation of iron in combustion-derived ash is an important factor in determining the likelihood of ash deposit formation and buildup. In this study, the transformation of iron were examined in ash produced by the combustion of a beneficiated Pittsburgh no. 8 bituminous coal under a range of oxygen concentration ranging from 0% to 100% O₂ in a drop tube furnace. The speciation of iron was found to be strongly dependent upon ash particle size, with the lowest fraction of glassy state iron found in the largest ash particles. Both the fraction of iron in a glassy state and the ratio of Fe⁺⁺⁺(glass)/Fe⁺⁺(glass) increased with increasing O₂ concentration in the furnace. For the submicron ash particles, about 10% of the iron is formed by direct disintegration of pyrite and pyrrhotite during combustion. Most of the iron is however, present as Fe⁺⁺⁺(glass), which results from the vaporization, recondensation, and coagulation of iron and silicates. For ash particles with size between 1 and 9 microns, most of the ash derives from mineral coalescence within the reacting char, with additional contribution from extraneous minerals. The fraction of glassy iron in those particles is high because of the high contact probability between iron melt and silicates. For the coarsest ash particles with size greater than 9 microns, extraneous pyrite is changed into hematite, and iron in the core of the char is changed into a glassy state.     

Numerical study of coal particle cluster combustion under quiescent conditions

Behavior of ignition and combustion of coal particle cluster under a quiescent condition was numerically simulated by solving balance equations of mass and enthalpy with combustion kinetic models of volatiles and char. Two-flame structure, one flame penetrating into the cluster and the other moving out of the cluster, was predicted during the combustion of coal particle cluster. Effects of radiative heat transfer, group number, ambient temperature, coal particle size, and oxygen concentration on ignition and combustion of coal particle clusters were also analyzed. Simulations indicated that the gas volume fraction of coal particle cluster increases with time after devolatilization. Gas velocity passing through the cluster surface varied significantly at volatile liberation. The ignition time delay was reduced with the increase of ambient temperature. The cluster devolatilization rate and char burning rate increased while the ignition time delay decreased with the increase of ambient oxygen concentration.     

Analysis of mercury species present during coal combustion by thermal desorption

Mercury in coal and its emissions from coal-fired boilers is a topic of primary environmental concern in the United States and Europe. The predominant forms of mercury in coal-fired flue gas are elemental (Hg⁰) and oxidized (Hg²⁺, primarily as HgCl₂). Because Hg²⁺ is more condensable and far more water soluble than Hg⁰, the wide variability in mercury speciation in coal-fired flue gases undermines the total mercury removal efficiency of most mercury emission control technologies. It is important therefore to have an understanding of the behaviour of mercury during coal combustion and the mechanisms of mercury oxidation along the flue gas path. In this study, a temperature programmed decomposition technique was applied in order to acquire an understanding of the mode of decomposition of mercury species during coal combustion. A series of mercury model compounds were used for qualitative calibration. The temperature appearance range of the main mercury species can be arranged in increasing order as HgCl₂ < HgS < HgO < HgSO₄. Different fly ashes with certified and reference values for mercury concentration were used to evaluate the method. This study has shown that the thermal decomposition test is a newly developed efficient method for identifying and quantifying mercury species from coal combustion products.     

Synthesis of zeolitic materials from natural clinker: A new alternative for recycling coal combustion by-products

In this study, we report an innovative process of zeolite synthesis through transforming of natural raw clinker (NRC) via (1) conventional hydrothermal alkaline activation and (2) alkaline fusion prior to hydrothermal reaction. Both methods were evaluated at 100 °C as a function of the alkaline agent, although in the first method other variables such as concentration of the activation solution and reaction time were selected, whereas in the second method similar ratios of alkaline activator/NRC and H₂O/alkali fused product during different reaction times were used. Experimental results indicate that the method, alkali activator, concentration and time have a strong effect on the type and degree of crystallinity of the synthesized zeolite. Low-silica sodium or potassium zeolitic materials were synthesized, which include Na-phillipsite, hydroxysodalite and hydroxycancrinite, K-chabazite, zeolite K-F, faujasite and zeolite Linde Type A. The use of a recycling process of NRC in zeolite synthesis could become an area of important interest and research, playing an active role in encouraging and promoting technological advances, research and technology transfer related to the use and disposal of mining wastes, remaining after the natural and spontaneous combustion of coal seams. NRC-based zeolites (NRCZs) synthesized under well-optimized experimental conditions could be used in applications for environmental waste treatment.     

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.     

Mercury control technologies for coal combustion and gasification systems

Development and testing of mercury control technologies have largely focused on coal-fired combustion systems, with less emphasis on advanced power systems. Mercury control is influenced by coal properties and chemistry, plant configuration, pollution control devices, flue gas conditions, and plant operations, which differ between combustion and gasification systems. Sorbents such as treated activated carbons have shown promising results in low-temperature environments; however, elevated temperature and reducing environments of many advanced systems remain challenging, requiring research and development to obtain acceptable mercury control levels. Concurrent pollutant/multipollutant control strategies that include CO₂ control are critically needed for both conventional and advanced power systems.     

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.     

Physical structure and combustion properties of super fine pulverized coal particle

Physical structures and combustion properties of super fine pulverized coal particles of eight Chinese coals, Heshan subbituminous coals and Jincheng lean coals from two areas of China, have been investigated using accelerated surface area and porosimetry, thermobalance (TGA), and Fourier transform infrared spectrometer. Results showed that the particle specific surface area and pore volume increased greatly when the coal particle size was reduced. The higher the carbon content on a dry ash-free basis is, the larger the particle specific surface area and pore volume are. When the coal particle size decreases, the combustion process can be largely improved, ignition temperature is reduced, and SO₂ emission from coal combustion is also lower.     

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.     

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.     

燃烧处理挥发性有机污染物的研究进展

随着环境问题的日益严重,治理作为PM_2.5前体的挥发性有机物（VOCs）越来越受到重视,燃烧法是目前常用的处理VOCs污染物技术之一。本文从燃烧的机理出发综述了燃烧法处理VOCs的研究进展,将燃烧法分为两大类,即非催化燃烧法和催化燃烧法。非催化燃烧法中从燃烧方式出发,总结了直接燃烧法、蓄热式热力燃烧法、多孔介质燃烧法的研究进展,并对燃烧影响因素进行了综述。在催化燃烧法中阐述了贵金属催化剂、非贵金属催化剂和复合金属氧化物催化剂的研究进展,探讨了催化剂的失活问题,分析了每种催化剂的优势与不足。贵金属催化剂活性高,但是价格昂贵、稳定性差;非贵金属催化剂价格低廉、寿命长,但是起燃温度高;复合金属氧化物催化剂活性高、抗毒性强,但是制备工艺复杂。最后基于目前的研究现状和不足,展望了未来燃烧法处理VOCs的研究方向为：结合实际应用的工艺条件和催化燃烧的机理,制备出活性高、价格低廉、抗毒性强和寿命长的催化剂用于蓄热式催化燃烧技术;将催化燃烧和多孔介质燃烧相结合,开发出高效、稳定、经济的燃烧技术处理VOCs污染物。     

挥发性有机物燃烧催化剂的研究进展

挥发性有机物（VOCs）是主要大气污染物质,也是形成PM_2.5和臭氧的重要前体。强化挥发性有机物控制,是改善大气环境的重要途径。催化燃烧（氧化）被认为是去除VOCs最有效的方式之一,本文综述了VOCs催化燃烧常用的贵金属催化剂、非贵金属催化剂。其中,贵金属催化剂主要包括基于Pd、Pt、Ru等的催化剂,非贵金属催化剂主要包括Mn、Co、Ce、Zr等的氧化物,通常贵金属催化剂具有比非贵金属更高的氧化活性和稳定性,但对于含氮VOCs非贵金属催化剂（Mn、Cu）具有更好的氮气选择性,Cr基催化剂对于含氯VOCs燃烧具有更好的效果。此外,还重点讨论了载体、分散度、催化剂制备方法对贵金属催化剂性能的影响,并对发展VOCs氧化催化剂的研究提出了展望。     

Application of percolation model to particulate matter formation in pressurized coal combustion

Coal is an important energy resource for meeting the future demand for electricity, as coal reserves are much more abundant than those of other fossil fuels. In this study, the percolation model, which can account for swelling due to devolatilization and ash agglomeration, is applied to particulate matter formation process in coal combustion, and the effects of coal properties, ambient temperature, ambient pressure and initial coal size on the characteristics of a burning coal particle are studied. The devolatilization rate of coal is given by the first-order reaction model with FLASHCHAIN^® model [Niksa, S., Combust. Flame, 100, (1995) 384-394.]. The characteristics of a burning coal particle are investigated under the atmospheric and high pressure conditions. The results show that in the atmospheric pressure condition, the characteristics of the burning coal particle obtained by the percolation model are in general agreement with the experimental data. The particle diameter of Newlands coal with higher fuel ratio and ash content is larger than that of Plateau coal in the char-combustion-dominant process. As the ambient temperature increases, the particle diameter becomes small in the early stage of the char-combustion-dominant process, but becomes large afterward. The porosity in the char-combustion-dominant process decreases with decreasing the initial coal size. It is also observed that the effect of ambient pressure is prominent in the char-combustion-dominant process. The particle diameter and porosity in the pressurized condition are greater than those in the atmospheric pressure condition. These behaviors can be explained by the interaction between char reaction and ash agglomeration.     

Characterization of unburned carbon from ash after bituminous coal and lignite combustion in CFBs

This paper deals with the characterisation of carbon (UC) from bottom ash (BA) and fly ash (FA) samples from two fluidised-bed power stations burning bituminous coal and lignite. The laboratory results for the carbon determinations and its mass balances are evaluated. Chemical and mineral analyses and textural characteristics (specific surface area and pore-size distribution) are presented. Depletion/enrichment of selected elements (S, Cl, V, Cr, Ni, Cu, Zn, As, Se, Sb, Hg, and Pb) in carbon from the bottom ash are compared with both ash compostions. The strong positive relationships between the concentrations of some trace element contents (Hg, Se, As, Cu, Ni, V and Cl) in fly ash with the content of carbon and the specific surface area of FA are presented and expressed by regression equations with very high correlation coefficients. Laser ablation-ICP-MS has been used to obtain an insight into element distributions within carbon grains from the bottom ash.     

Simulation of swirling coal combustion using a full two-fluid model and an AUSM turbulence-chemistry model

A full two-fluid model of reacting gas-particle flows with an algebraic unified second-order moment turbulence-chemistry model for the turbulent reaction rate of NO formation are used to simulate swirling coal combustion. The sub-models are the k-ε-k_p two-phase turbulence model, the EBU-Arrhenius volatile and CO combustion model, the six-flux radiation model, coal devolatilization model and char combustion model. The prediction results are in good agreement with the experimental results taken from references.