A numerical simulation of pulverized coal combustion employing a tabulated-devolatilization-process model (TDP model)

A new coal devolatilization model employing a tabulated-devolatilization-process model (TDP model) is developed, and its validity is investigated by performing a numerical simulation of a pulverized coal combustion field formed by an industrial low-NO_x burner in a 100 kg-coal/h test furnace. The predicted characteristics of the pulverized coal combustion field obtained from the simulation employing the TDP model are compared with those employing the conventional devolatilization model, those employing the two competing reaction rate model, and the experiments. The results show that drastic differences in the gas flow patterns and coal particle behavior appear between simulations. In particular, the recirculation flow behavior is strongly affected by the difference in the coal devolatilization model because of the difference in the volatile matter evolution rate. The TDP model captures the observed behavior of the coal particles in the experiment better than the other models. Although it is considered that by adjusting the devolatilization parameters the prediction similar to the TDP model is also possible by the other models, appropriate devolatilization parameters are automatically set to particles depending on the particle heating rate without trial–error method by employing the TDP model.     

A new framework for modeling coal devolatilization and combustion in boiler furnaces

This paper presents a new framework for the modeling of coal‐fired boiler furnaces. The input required for the model is the ultimate analysis of a coal sample. The model accounts for devolatilization followed by gas‐phase combustion. The devolatilization model used in this work is taken from published literature with slight modifications to match the numerical predictions with experimental measurements. This work also develops a reactor network model for simulating the performance of boiler furnaces. For the seamless integration of kinetic models of coal devolatilization and combustion with furnace numerical model, the thermochemistry data of several hypothetical and intermediate species involved in devolatilization chemistry are evaluated in the form of 14 coefficient National Aeronautics and Space Administration polynomials. The capability of the model for predicting the furnace temperature and product composition is demonstrated by simulating a single‐zone model. Copyright © 2017 John Wiley & Sons, Ltd.     

燃尽风对炉内流动和燃烧过程影响的数值模拟

燃尽风作为降低锅炉NOx排放浓度的一个措施已在我国得到逐步推广应用。应用数值模拟方法，对1台600MW对冲燃烧煤粉锅炉，在满负荷下燃尽风对炉内流动、燃烧和传热过程的影响开展了研究工作。应用混合分数／概率密度函数法模拟湍流燃烧，用P-1辐射模型开展辐射传热模拟，利用拉格朗日／欧拉法处理气固两相间的动量、质量和能量交换，对挥发份的析出采用单步反应模型，采用动力／扩散反应速率模型模拟煤粉颗粒的表面燃烧。研究发现：一方面，燃尽风的应用改善了炉内气流的充满情况，延迟了煤粉燃烧过程氧气的供应，加强了炉内的还原性气氛，降低了炉内最高火焰温度，有利于降低NOx排放浓度；但另一方面。燃尽风的应用将导致煤粉燃烧效率下降。     

马蹄形火焰玻璃熔窑燃烧空间流动与传热的数值模拟

对马蹄形火焰玻璃窑炉燃烧空间内的流动、燃烧及辐射传热等过程进行了数值模拟研究,得到了炉内燃烧空间的速度场、温度场、组分浓度分布及燃烧空间向玻璃液面传递的热流分布。探讨了燃烧空间入口的进气角度对炉内温度场和向玻璃面传递的热流的影响,模拟结果表明,当入口的进气角度在5°～10°之间时,传热效果较好。     

稻壳粉燃烧过程中颗粒尺寸和形态的演变

以颗粒尺寸为250 ~ 300 μm的稻壳粉为研究对象,通过高温沉降炉中的热解和燃烧实验结合颗粒样品的扫描电子显微镜图像分析方法,研究了稻壳粉燃烧过程中颗粒尺寸和形态的变化及热解、燃烧条件的影响。结果表明,热解时颗粒宽度等尺寸参数均缩小,温度的影响较小;焦燃烧时颗粒尺寸因破碎明显减小,温度、气氛等燃烧条件通过影响破碎进而影响尺寸变化。对于形态参数,热解和燃烧后横纵比的变化及实验条件对其变化的影响与尺寸参数相似;热解和燃烧后圆形度几乎无变化;圆度在热解后变化也较小,而燃烧后明显减小;实验条件对圆形度、圆度的变化几乎无影响。     

Flow and convective heat transfer in cylindrical reversed flow combustion chambers

This paper presents a computational study of the flow and convective heat transfer in cylindrical reversed flow combustion chambers. The computations are performed using an elliptic solver incorporates the k− turbulence model. Heat production by combustion is simulated by adding heat generation source terms in the energy equation. And it is assumed that heat generation occurs only a section of the furnace. A number of different inlet conditions with different geometries are considered, and the changes of flow structure, temperature distribution, convective heat flux rate are presented and compared. The results show that, in general, heat transfer in the reversed flow combustion chamber can be improved by properly chosen geometry for the required output.     

横火焰玻璃窑炉燃烧空间流动与传热的数值模拟

对横火焰玻璃窑炉燃烧空间内的流动、燃烧及辐射传热等过程进行了数值模拟研究,建立了玻璃窑炉燃烧空间内的综合数学模型,给出了诸控制方程的统一的数值解法,得到了炉内燃烧空间的速度场、温度场、组分浓度分布及燃烧空间向玻璃液面传递的热流分布。     

Prediction method of unburnt carbon for coal fired utility boilerusing image processing technique of combustion flame

A method for predicting unburnt carbon in a coal-fired utility boiler has been developed using an image processing technique. The method consists of an image processing unit and a furnace model unit. The temperature distribution of combustion flames can be obtained through the former unit. The latter calculates dynamics of the carbon reduction from the burner stages to the furnace outlet using coal feed rate, air flow rate, and the chemical and ash content of the coal. An experimental study in the Sendai Power Station of Tohoku Electric Company Inc. shows that the prediction error of the unburnt carbon can be reduced to 10%. The results show that the unburnt carbon prediction algorithm could be widely used for evaluation, optimization and diagnosis of combustion systems for various boiler and coal types     

Analysis of entropy generation in non-premixed hydrogen versus heated air counter-flow combustion

In this paper, entropy generation in non-premixed hydrogen versus heated air counter-flow combustion confined by planar opposing jets is investigated for the first time. The effects of the volume percentage of hydrogen in fuel mixture and the inlet Reynolds number (corresponding to the global stretch rate) on entropy generation are studied by numerical evaluating the entropy generation equation. The lattice Boltzmann model proposed in our previous work, instead of traditional numerical methods, is used to solve the governing equations for combustion process. Through the present study, three interesting features of this kind of combustion, which are quite different from that reported in previous literature on entropy generation analysis for diffusion hydrogen-air flames, are revealed. Moreover, it is observed that the whole investigated domain can be divided into two parts according to the predominant irreversibilities. The total entropy generation number can be approximated as a linear increasing function of the volume percentage of hydrogen in fuel mixture and the inlet Reynolds number for all the cases under the present study. It is very interesting that most characteristics in this kind of diffusion combustion also can be found in its premixed counterpart investigated in our previous work. Especially, the critical Reynolds numbers determining the order of the predominant irreversibilities in this type of diffusion flame are same as its premixed counterpart, although the flow and scalar fields between them are quite different.     

The devolatilization of coal and a comparison of chars produced in oxidizing and inert atmospheres in fluidized beds

This is a study of the devolatilization of coal in a laboratory-scale bed of silica sand, fluidized with either air or N₂ and electrically heated to 750 or 900°C. Coal particles (diameter 1.4–1.7 or 2.0–2.36 mm) were fed in batches to the surface of the bed and allowed to devolatilize in either an oxidizing atmosphere of air or inert N₂. In the first case, combustion of the volatiles occurred, but there was only thermal decomposition (pyrolysis) in the second situation. The resulting chars were recovered and analyzed for composition and structure, so that comparisons could be made between the effects of devolatilization with combustion and of pyrolysis in an inert atmosphere. It was found that the fractions of C and H in the char were only slightly sensitive to the type of fluidizing gas used. The amount of nitrogen in the recovered char and also the devolatilization time showed no dependence on the type of fluidizing gas, whereas BET areas were slightly larger after combustion in air. It is concluded that these effects are small relative to other errors, inherent in experiments on coal combustion, so that chars prepared in a bed fluidized by hot N₂ are very similar to those formed during coal combustion at nominally the same temperature. Equally, the overall composition of the volatile matter released during combustion in a fluidized bed is the same as in pyrolysis in nitrogen. The effects of other parameters, such as the temperature of the bed, the flow rate of the fluidizing gas and the size of the coal particles are also discussed in detail. It is concluded that most of the volatiles burn in a fluidized bed (at or less than 900°C) far away from the original coal particle. Also, NO_x is in effect a primary product of devolatilization, being produced in appreciable amounts when coal is heated in inert N₂. The ratio of C/N in the volatiles is found to be a constant during the latter stages of devolatilization, but beforehand at lower temperatures, carbon species are preferentially released. Overall, devolatilization of small particles (< 2.4 mm) in a fluidized bed at 900°C is kinetically controlled. The activation energy is small, being 15 ± 6 kJ/mol.     

Stoichiometry and coal-type effects on homogeneous vs. Heterogeneous combustion in pulverized-coal flames

Results of experiments to determine the extent of heterogeneous combustion in the rapid-devolatilization regime of fuel-rich bituminous and subbituminous coal-dust flames are reported. Major gas concentrations, gas and particle temperatures, and solids proximate and elemental compositions were measured as functions of residence time in pulverized-coal/O₂/Ar flames stabilized on a flat-flame burner. Proximate fixed carbon and volatile matter measurements are corrected to account for devolatilization exceeding that predicted by ASTM proximate analysis. The corrected proximate data are used to determine the fraction of volatile matter consumed heterogeneously in situ and the fraction released to the gas phase by pyrolysis. Seven to thirty-five percent of the initial corrected volatile matter is removed heterogeneously. A mass transfer analysis is applied to predict a time dependent critical particle size such that the volatiles flux emerging from larger particles is sufficient to prevent surface oxidation. Critical particle radii as large as 39 μm are calculated. Results of the volatile matter partitioning and critical radius calculations indicate that heterogeneous combustion is more important in the leaner flames and for the subbituminous coal.     

Kinetics of devolatilization and oxidation of a pulverized biomass in an entrained flow reactor under realistic combustion conditions

In this paper the results of a complete set of devolatilization and combustion experiments performed with pulverized (∼500 μm) biomass in an entrained flow reactor under realistic combustion conditions are presented. The data obtained are used to derive the kinetic parameters that best fit the observed behaviors, according to a simple model of particle combustion (one-step devolatilization, apparent oxidation kinetics, thermally thin particles). The model is found to adequately reproduce the experimental trends regarding both volatile release and char oxidation rates for the range of particle sizes and combustion conditions explored. The experimental and numerical procedures, similar to those recently proposed for the combustion of pulverized coal [J. Ballester, S. Jiménez, Combust. Flame 142 (2005) 210-222], have been designed to derive the parameters required for the analysis of biomass combustion in practical pulverized fuel configurations and allow a reliable characterization of any finely pulverized biomass. Additionally, the results of a limited study on the release rate of nitrogen from the biomass particle along combustion are shown.     

Analysis of the co-combustion of sewage sludge and coal by TG-MS

Combustion of sewage sludge may be a viable solution for its management in some cases and so is its co-combustion with coal. The aim of the present paper is to study the behavior of three different sludges during combustion and the modifications of combustion parameters that take place when sludges are mixed with coal, all measurements being done by thermogravimetry. The combustion of pyrolyzed sewage sludges has also been studied, related to the possibility of combining combustion with pyrolysis when trying to valorize sludges by producing active carbon. From the burning profile data it has been possible to observe that a rapid devolatilization occurs during sludge combustion compared to coal and previously pyrolyzed sludge. The exact temperatures of devolatilization and of maximum speed of weight loss have also been ascertained together with differential thermal analysis and measurements of gas emissions during combustion. The behavior of the sludges tested is qualitatively similar but quantitatively different, so tests have to be carried out before they are put in the furnace.     

Modeling of laminar pulverized coal flames with speciated devolatilization and comparisons with experiments

In an earlier mathematical model of laminar pulverized coal–air combustion, supported by added CH₄, it was assumed that the volatiles from the coal consisted solely of CH₄ and HCN. A revised model is introduced with speciated devolatilization rate constants for tar, CH₄, CO, CO₂, H₂O, H₂, and HCN. It is assumed that these rate constants can also be applied to the devolatilization of the tar. In addition, it is assumed that the soot is predominantly carbon and is oxidized by the attack of O, H, OH, and O₂, in the same way as the coal char. Because the devolatilization rate is strongly dependent on particle temperature, the latter has to be determined accurately from the momentum and energy equations of the particle. The model is one-dimensional, with axial radiative transfer. The introduction of soot formation and speciation of the volatiles results in much improved accuracy in the prediction of species and temperature profiles in subatmospheric combustion on a flat flame matrix burner. It is possible to derive an overall global devolatilization rate constant that agrees reasonably with the measurements. These computations suggest that the effective area of the assumed spherical coal char particles is four times greater than that of the assumed sphere. Modeling of atmospheric pressure flames suggests that in this case, the value of 4 should be reduced, probably because, as pressure increases, the diffusion flux of reactant is reduced. Subatmospheric pressure laminar burning velocities are predicted with satisfactory accuracy over the full range of overall equivalence ratios. Previous measurements of laminar burning velocity at atmospheric pressure are reviewed. However, the various means of supporting a stable coal flame and the associated uncertain geometries make it impossible to apply the present model to the different conditions. It is suggested that burning velocities measured on a flat flame burner, with a controlled amount of methane to support the combustion of a pulverized coal/air mixture, would provide a good test of the reactivities of different coals.     

Flame–vortex interaction driven combustion dynamics in a backward-facing step combustor

The combustion dynamics of propane–hydrogen mixtures are investigated in an atmospheric pressure, lean, premixed backward-facing step combustor. We systematically vary the equivalence ratio, inlet temperature and fuel composition to determine the stability map of the combustor. Simultaneous pressure, velocity, heat release rate and equivalence ratio measurements and high-speed video from the experiments are used to identify and characterize several distinct operating modes. When fuel is injected far upstream from the step, the equivalence ratio entering the flame is temporally and spatially uniform, and the combustion dynamics are governed only by flame–vortex interactions. Four distinct dynamic regimes are observed depending on the operating parameters. At high but lean equivalence ratios, the flame is unstable and oscillates strongly as it is wrapped around the large unsteady wake vortex. At intermediate equivalence ratios, weakly oscillating quasi-stable flames are observed. Near the lean blowout limit, long stable flames extending from the corner of the step are formed. At atmospheric inlet temperature, the unstable mode resonates at the 1/4 wavemode of the combustor. As the inlet temperature is increased, the 5/4 wavemode of the combustor is excited at high but lean equivalence ratios, forming the high-frequency unstable flames. Higher hydrogen concentration in the fuel and higher inlet temperatures reduce the equivalence ratios at which the transitions between regimes are observed. We plot combustion dynamics maps or the response curves, that is the overall sound pressure level as a function of the equivalence ratio, for different operating conditions. We demonstrate that numerical results of strained premixed flames can be used to collapse the response curves describing the transitions among the dynamic modes onto a function of the heat release rate parameter alone, rather than a function dependent on the equivalence ratio, inlet temperature and fuel composition separately. We formulate a theory for predicting the critical values of the heat release parameter at which quasi-stable to unstable and unstable to high-frequency unstable modes take place.     

A review on devolatilization of coal in fluidized bed

Devolatilization is an important step in fluidized bed combustion and gasification of coal. ‘Devolatilization’ is a general term that signifies the removal of volatile matters from the coal matrix. It is an extremely important step because the combustion of volatile matter can account for 50% of the specific energy of fluidized bed combustion of a high‐volatile coal. Significant insights into the complex physicochemical phenomena that occur during devolatilization have been obtained in the recent years. This review focuses on the devolatilization of coal in an inert gas, air, and oxygen‐enriched air, with emphasis on the effects of the operating parameters (e.g. temperature, heating rate, pressure, and gas velocity) on the yield of volatile matter. Particle size, oxygen content of the fluidizing gas, volatile content of coal and specific heat are some of the other important parameters for the devolatilization of coal. This review also explains the development and application of structural and empirical models. The structural models (e.g. FG‐DVC and CPD models) are fairly complex. However, they can accurately predict the yields of gas and tar. It is observed from the review of the literature that the mechanism of coal devolatilization needs further study. Although the shrinking‐core model can describe the devolatilization in the beginning and toward the end of the process, major deviations are often observed. The economic studies reveal that the capital cost of fluidized bed combustion reduces upon doubling the capacity. Some problems associated with bubbling fluidized bed combustion (e.g. the increase in freeboard temperature) have been explained with the present knowledge of devolatilization. Copyright © 2011 John Wiley & Sons, Ltd.     

Mathematical modeling of MILD combustion of pulverized coal

MILD (flameless) combustion is a new rapidly developing technology. The IFRF trials have demonstrated high potential of this technology also for N-containing fuels. In this work the IFRF experiments are analyzed using the CFD-based mathematical model. Both the Chemical Percolation Devolatilization (CPD) model and the char combustion intrinsic reactivity model have been adapted to Guasare coal combusted. The flow-field as well as the temperature and the oxygen fields have been accurately predicted by the CFD-based model. The predicted temperature and gas composition fields have been uniform demonstrating that slow combustion occurs in the entire furnace volume. The CFD-based predictions have highlighted the NO_x reduction potential of MILD combustion through the following mechanism. Before the coal devolatilization proceeds, the coal jet entrains a substantial amount of flue gas so that its oxygen content is typically not higher than 3-5%. The volatiles are given off in a highly sub-stoichiometric environment and their N-containing species are preferentially converted to molecular nitrogen rather than to NO. Furthermore, there exists a strong NO-reburning mechanism within the fuel jet and in the air jet downstream of the position where these two jets merge. In other words, less NO is formed from combustion of volatiles and stronger NO-reburning mechanisms exist in the MILD combustion if compared to conventional coal combustion technology.     

Flame structures and behaviors of opposed flow non-premixed flames in mesoscale channels

An opposed flow non-premixed flame (OFNPF) in a narrow channel was chosen as a model of a non-premixed flame in a mesoscale combustion space or micro-combustor. The stabilization limits and behaviors of methane-air flames and propane-air flames were compared for various experimental parameters such as flow velocity, nozzle distance, nozzle width, channel gap, and fuel dilution. Flames could be stabilized in a wide range of strain rates (0.9–150 s⁻¹) and dilution ratios (∼80% nitrogen at the fuel side). The flame extinction limits were classified into three types and their mechanisms were investigated: higher-strain-rate (HSR) extinction limit determined by the flame stretch, lower-strain-rate (LSR) extinction limit determined by the conductive or convective heat loss from the flame, and fuel-dilution-ratio (FDR) extinction limit determined by the decrease in the heat release rate from the flames. The HSR extinction limits in mesoscale channels could be explained with a modified strain rate, and the LSR extinction limits could be explained by employing a premixed quenching theory in which the heat loss through the dead space near the wall was considered as a major extinction mechanism. Finally, the variation of the extinction limits with the FDR in both the HSR and the LSR conditions could be explained with a modified global reaction rate in which the variations in flame temperature and species concentrations were reflected. This study provides an essential model for the stabilization and extinction of non-premixed flames in mesoscale combustion spaces.     

甲烷预混多喷嘴阵列燃烧器热声振荡模态实验研究

为研究燃气轮机模型燃烧室的非预混燃烧流场,采用大涡模拟方法分别结合火焰面生成流形模型（FGM）和部分预混稳态火焰面模型（PSFM）对甲烷/空气同轴射流非预混燃烧室开展了数值模拟研究,并与试验结果进行对比。结果表明：FGM所预测的速度分布、混合分数分布、燃烧产物及CO分布与试验结果更符合;两种模型均能捕捉到燃烧室中的火焰抬举现象;燃烧过程中的火焰结构较为复杂,同时存在预混燃烧区域和扩散燃烧区域,扩散燃烧主要分布在化学恰当比等值线附近,预混燃烧区域主要分布在贫油区。