Analysis of flame structure using detailed chemistry and applicability of flamelet/progress variable model in the laminar counter-flow diffusion flames of pulverized coals

Pulverized coal is still found in many practical devices even though it is recognized as “dirty fuel” because of its CO₂ and pollutant emissions. To overcome this problem, advanced coal utilization technologies have been developed using numerical simulations. In this study, the structures of the laminar counter-flow diffusion flames of pulverized coals were investigated by performing simulations based on detailed chemistry. The high-temperature region became narrower as the coal/air ratio increased, because of the departure from the stoichiometric mixture and local quenching by the heat transfer between the gas and solid phases. Further, the applicability of the flamelet/progress-variable (FPV) model was investigated through a priori and a posteriori tests. The a priori test confirmed that the FPV model is capable of reproducing the numerical solutions obtained using the detailed chemistry, including the mass fractions of minor species. In the a posteriori test, there was a slight difference between the FPV model and detailed chemistry results due to overestimation of the progress of the chemical reactions. Given the sufficiently high accuracy of the FPV model in various numerical conditions, it can be concluded that the extended FPV model has potential for use in turbulent coal combustion simulations.     

Waste plastics as supplemental fuel in the blast furnace process: improving combustion efficiencies

The possibility of using waste plastics as a source of secondary fuel in a blast furnace has been of recent interest. The success of this process, however, will be critically dependent upon the optimization of operating systems. For instance, the supply of waste plastics must be reliable as well as economically attractive compared with conventional secondary fuels such as heavy oil, natural gas and pulverized coal. In this work, we put special importance on the improvement of the combustibility of waste plastics as a way to enhance energy efficiency in a blast furnace. As experimental variables to approach this target, the effects of plastic particle size, blast temperature, and the level of oxygen enrichment were investigated using a custom-made blast model designed to simulate a real furnace. Lastly, the combustion efficiency of the mixture of waste plastics and pulverized coal was tested. The observations made from these experiments led us to the conclusion that with the increase of both blast temperature and the level of oxygen enrichment, and with a decrease in particle size, the combustibility of waste polyethylene could be improved at a given distance from the tuyere. Also it was found that the efficiency of coal combustion decreased with the addition of plastics; however, the combustion efficiency of mixture could be comparable at a longer distance from the tuyere.     

Soot formation characteristics in a pulverized coal flame formed in a swirling flow

To understand the soot formation characteristics in a pulverized coal flame with a swirling flow, simultaneous imaging of Mie scattering of coal particles and laser induced incandescence (LII) of soot were performed in this study. The pulverized coal flame was stabilized by a hydrogen diffusion pilot flame. The characteristic structures of soot formation in the pulverized coal flame with a swirling flow were analyzed based on a comparison of the experimental results and two-dimensional numerical simulation in the mixing region of coal particles and the oxidizer. The interactions between coal particle clouds and soot formation are discussed in detail. The results clearly show that the averaged radial dispersions of scattering signals from coal particles and of the LII signals from soot are overlapped. The overlapping region appeared nearby the nozzle exit due to the turbulent mixing and the high temperature region formed by swirl-induced recirculation flow. This overlapping region radially expands with increasing the height from the burner. Additionally, the characteristic areas of soot formation were observed in the results of simultaneous imaging of Mie scattering and LII. These areas are 1) streaky soot formation areas around the particle clouds, 2) soot formation areas inside of the particle clouds and 3) soot formation areas around the large coal particles.     

Flamelet modeling of laminar pulverized coal combustion with different particle sizes

In this work, flamelet modeling of pulverized coal flame stabilized in a laminar counterflow is conducted. Particularly, the effects of particle size on the flamelet model performance are investigated. The a priori tabulated thermo-chemical quantities are compared with the corresponding values in the detailed chemistry solutions, which are obtained by solving the transport equations for all species mass fractions existing in the employed detailed chemical reaction mechanism. The detailed chemistry solutions show that the pulverized coal flame structure can be changed significantly by varying the particle size. While pulverized coal flame structure with small particles is similar to the pure gas flame, it becomes much more complex when the particle Stokes number is larger than unity. Large coal particles can directly cross the flame front to the opposite side and disperse over a large region, resulting a significantly wrinkled flame front and a considerably broadened reaction zone. Comparisons between the tabulated thermo-chemical quantities and the detailed chemistry solutions show that the CH₄, O₂ and H₂O species mass fractions and their RMS (root mean square) values for all cases can be well predicted by the classical flamelet model without any modification, while the peak values of OH mass fraction, gas temperature and heat release rate for the large particle case cannot be correctly captured. For all cases, the mass fractions of combustion-mode-sensitive species cannot be correctly predicted in the premixed flame reaction zone by the employed diffusion-flame-based flamelet model. Such discrepancies are even more pronounced at large particle conditions since the premixed flame reaction zone tends to be broadened.     

Influence of operating temperature on performance of electrostatic precipitator for pulverized coal combustion boiler

Electrostatic precipitator (ESP) is a major dust collection device used in pulverized coal combustion boilers in Japan. Although the collection efficiency of ESP is over 99%, the performance of ESP is affected by dust properties, especially the electric resistivity of the dust. In Japan, many different kinds of coal are used in pulverized coal combustion boilers. The property of coal ash formed in pulverized coal combustion is different for different coals. It is very important to develop high performance ESPs for different kinds of coal ash. In general, the low temperature ESP operated at about 423 K has been used for a long time, but the electric resistivity of some kinds of coal ash is higher than suitable for the ESP in this temperature range. A high temperature ESP operated at 623 K was developed about 20 years ago and an advanced low temperature ESP operated at 373 K has been used recently for control of the electric resistivity of coal ash.In this paper, we report on the influence of operating temperature from 363 K to 623 K on performance of ESP for a pulverized coal combustion boiler. The influence of coal ash properties including electric resistivity, alkali metal concentration, and operating condition on collection efficiency of an ESP is estimated.     

Plasma thermochemical preparation for combustion of pulverized coal

A plasma model of thermochemical preparation for the combustion of pulverized coal implemented through the PlasmaKinTherm program for the calculation of plasma-fuel systems has been described. Such systems are used at nonfuel-oil start-up of boilers and the stabilization of the combustion of a pulverized coal torch. The model combines kinetic and thermodynamic methods describing the process of the thermochemical preparation of fuel in the volume of the system. The numerical study of the regime parameters of the plasma-fuel system as a function of plasmatron power providing the ignition of the high-ash coal air-petrol mixture is carried out. Distributions of temperatures and velocities of gas and coal particles and concentrations of products of the thermochemical preparation over the length of the system are obtained. The main regularities of the process of the plasma ignition of fuel are revealed consisting in the displacement of the maxima of temperatures and velocities of products of thermochemical preparation upstream (in the direction of the plasmatron), and the independence of plasmatron power maximal values of temperatures and velocities. The results of calculations are compared with experimental data confirming the validity of assumptions accepted at the development of the model.     

Soluble polycyclic aromatic hydrocarbons in raw coals

Polycyclic aromatic hydrocarbons (PAHs) are considered to be a group of compounds that pose potential health hazards since some PAHs are known carcinogens. During coal utilization processes, such as coal combustion and pyrolysis, PAHs released may be divided into two categories according to their formation pathways. One category is derived from complex chemical reactions and the other is from free PAHs transferred from the original coals. PAHs released from complex chemical reactions during combustion and pyrolysis have received considerable attention in recent years. However, free PAHs contained in raw coals have not been seriously considered as a source of these materials to be released during the utilization of coal. The goal of this study was to observe the relation between the content of PAHs in different coals and the elemental composition of the coals. In this study, eight bituminous coals with dry, ash-free carbon values varying from 65% to 90% were selected. Each coal was extracted with dichloromethane in a Soxhlet extractor for 6 h. The extracts were quantitatively analyzed with a gas chromatograph/mass spectrometer (GC-MS). More than 20 kinds of PAHs were identified. The total amount of PAHs determined varied from 1.2 to 28.3 mg/kg from the various coal types. The maximum total PAHs extracted was reached when the carbon content exceeded 84% by weight.     

Recent advances in high-fidelity simulations of pulverized coal combustion

Although renewable energy has recently attracted a lot of attention, coal-fired power plants will still play an important role in electricity production in the future due to coal’s abundance in the world. However, combustion of pulverized coal is extremely complex which makes experimental measurements very challenging even impossible. As an alternative, computational fluid dynamics (CFD) is a powerful tool to investigate pulverized coal combustion (PCC), and can help to design and develop advanced coal-fired facilities. With the development of computational capacity, high-fidelity simulations of PCC have been developed and significant achievements have been obtained in the recent years. In this review, the recent advances in CFD of PCC, especially in fully-resolved direct numerical simulation (DNS), point-particle DNS and large eddy simulation (LES) of PCC are summarized. The progresses on both numerical methods and coal combustion models are highlighted. The state-of-the-art applications of these high-fidelity simulations are also reviewed, including the investigation of combustion characteristics of pulverized coals, the development of advanced coal combustion models, and the development of clean coal technologies. It is concluded that fully-resolved DNS, point-particle DNS, and LES have their merits and demerits, so the optimal use of each approach depends on specific research purpose. For coal devolatilization and char-oxidation models, the combined models in which advanced models are used to calibrate the kinetic parameters of simple models are quite attractive in terms of computational accuracy and computational burden. For chemical reactions, the flamelet model is popular, but the predictive capability and robustness need to be further improved. Besides, LES is rapidly approaching to simulate industrial PCC. However, the computational cost is still quite expensive, and more efficient algorithms and sub-grid models are required. To this end, DNS database, benchmark measurements, and international collaborations are important. Machine learning may also play an important role in promoting the development of high-fidelity simulations of PCC in the future.     

火焰燃烧区域空气声速与温度关系的实验研究

为使声学方法能对火焰温度进行精确测量,实验研究了火焰燃烧区域中空气声速与温度的关系。首先对非火焰气体环境中的声速与温度进行测量,然后在此基础上对不同燃料燃烧的火焰区域进行声速测量实验,并结合热电偶测得火焰温度,进而得到火焰中空气声速与温度的关系。结果表明:在固定距离下,与室温空气环境相比,高温烟气环境会使声波的传播时间减小,火焰环境会使声波的传播时间变长;在非火焰区域,空气声速与温度的关系符合理想气体中声速与温度的关系;在火焰燃烧区域,空气声速与温度关系偏离理想气体的声速与温度方程,与按照理想气体计算的声速结果相比,实际声速测量值偏小;对于同种燃料的火焰,随着火焰温度升高会出现空气中的声速减小的现象。     

Numerical analysis of cement calciner fuel efficiency and pollutant emissions

Efficient mixing of pulverized fuel and limestone particles inside cement calciners is important due to the reason that the calcination process directly affects the final fuel consumption. The focus of this paper is on the numerical analysis of cement calciner’s operating conditions and pollutant emissions. The paper analyzes the influence of different amounts of fuel, mass flow of the tertiary air and the adiabatic wall condition on the decomposition rate of limestone particles, burnout rate of coal particles, and pollutant emissions of a newly designed cement calciner. Numerical models of calcination process and pulverized coal combustion were developed and implemented into a commercial computational fluid dynamics code, which was then used for the analysis. This code was used to simulate turbulent flow field, interaction of particles with the gas phase, temperature field, and concentrations of the reactants and products, by solving the set of conservation equations for mass, momentum, and enthalpy that govern these processes. A three-dimensional geometry of a real industrial cement calciner was used for numerical simulations. The results gained by these numerical simulations can be used for the optimization of cement calciner’s operating conditions, and for the reducing of its pollutant emissions.     

Gasdynamic stimulation of combustion of lean fuel mixtures. 1. experimental study

We studied combustion modes experimentally for a lean fuel mixture of propane-air, including combustion of a quiescent mixture, combustion with annular gas swirling, and combustion stimulated by the jets of a burning gas that are injected into the combustion chamber (pulsed jet combustion — PJC). It is shown that, in providing a maximum combustion rate, the PJC mode has an obvious advantage in initiation of combustion of extremely lean fuel mixtures over other gasdynamic combustion modes. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 73, No. 2, pp. 302–308, March–April, 2000.     

Numerical simulation of soot formation in pulverized coal combustion with detailed chemical reaction mechanism

A two-dimensional unsteady numerical simulation with a detailed chemical reaction mechanism that considers 158 species and 1804 reactions is applied to pulverized coal combustion in a mixing layer and the soot formation behavior is investigated in detail. The computational conditions and ignition process are the same as those in our previous work (Muto et al., 2017). The results show that the peak of the mass density of the soot is distributed in the region where the gas temperature is higher than the unburned gas temperature of the mixture of volatile matter and air (1300–1400?K) and lower than the flame temperature (2000?K $～$). This is due to the fact that soot formation from the precursors (C₂H₂ and C₆H₆) is enhanced as the gas temperature increases, whereas the quantities of the precursors and the produced soot are reduced due to oxidation at the higher gas temperature condition that exists close to the flame. The peak value of the mass density of the soot is also distributed in the region between the peak values of the gas temperature and the probability density function of the number of coal particles.     

Chemical properties of superfine pulverized coal particles. Part 1. Electron paramagnetic resonance analysis of free radical characteristics

Superfine pulverized coal combustion is a new pulverized coal combustion technology with lots of advantages. A mechanochemical effect exists during the comminution process, which changes the chemical properties of coal significantly. Free radical concentrations and certain functional groups would increase with the decrease of particle sizes. In this paper, we combined electron paramagnetic resonance (EPR) and ¹³C solid-state nuclear magnetic resonance (NMR) techniques to study the free radical characteristics of superfine pulverized coal thoroughly. The final results indicate that the EPR spectra of coal are the superimpositions of several lines induced by different paramagnetic centers, which can be fitted by 1 Gaussian and 3 Lorentzian lines. The influences of coal maturities and particle sizes on EPR parameters, such as g-values, linewidths, and spin concentrations, are analyzed in detail. It is shown that with the decrease of particle sizes, more free radicals are induced through bond cleavages. Mechanical forces initiate the accumulation of free radicals in the fractures and inner pore surfaces of coal. Furthermore, the influence of particle sizes on oxygen-containing radicals (i.e., Lorentzian 1 types) is the greatest. This work provides a primary picture of the occurrence modes and spatial distributions of free radicals in superfine pulverized coal. The findings will help form the basis and provide guidance for further studies on revealing the correlations between the free radical reaction pathways and NO_x formation mechanisms.     

Explosibility boundaries for fly ash/pulverized fuel mixtures

Incomplete combustion and subsequent fuel contamination of a waste stream can pose a serious explosion hazard. An example of this type of incident is the contamination of fly ash with unburned pulverized coal. The coal, if present in sufficient quantities in the mixture, can act as a fuel source for a potential explosion. Experiments were conducted in a 20l Siwek explosibility test chamber to determine the minimum fuel contamination of fly ash required to form an explosible mixture. A sample of fly ash from Ontario Power Generation (OPG) (Ont., Canada) was artificially contaminated with Pittsburgh pulverized coal dust (the surrogate used to represent unburned fuel dust). Additionally, the influence of fly ash particle size on the amount of fuel contaminant required to form an explosible mixture was examined. Fine and coarse size fractions of fly ash were obtained by screening the original sample of OPG fly ash.The results show that at least 21% Pittsburgh pulverized coal (or 10% volatile matter) was required to form an explosible mixture of the original fly ash sample and coal dust. The results also illustrate that fly ash particle size is important when examining the explosibility of the mixture. The fine size fraction of fly ash required a minimum of 25% coal dust (12% volatile matter) in the mixture for explosibility, whereas the coarse fly ash required only 10% coal dust (7% volatile matter). Thus, the larger the particle size of the inert fly ash component in the mixture, the greater the hazard.     

ANN–GA approach for predictive modeling and optimization of NOx emission in a tangentially fired boiler

An artificial neural network (ANN) and genetic algorithm (GA) approach to predict NOx emission of a 210 MW capacity pulverized coal-fired boiler and combustion parameter optimization to reduce NOx emission in flue gas, is proposed. The effects of oxygen concentration in flue gas, coal properties, coal flow, boiler load, air distribution scheme, flue gas outlet temperature, and nozzle tilt were studied. The data collected from parametric field experiments was used to build a feed-forward back-propagation neural net. The coal combustion parameters were used as inputs and NOx emission as outputs of the model. The ANN model was developed for full load conditions and its predicted values were verified with the actual values. The algebraic equation containing weights and biases of the trained net was used as fitness function in GA. The genetic search was used to find the optimum level of input operating conditions corresponding to low NOx emission. The results proved that the proposed approach could be used for generating feasible operating conditions.     

用DSC法研究混入煤粉的铵油炸药相容性

为了研究混入煤粉的铵油炸药的相容性以及煤粉与铵油炸药之间的相互作用,采用差示扫描量热仪(DSC)研究了铵油炸药、混入煤粉的铵油炸药、硝酸铵和混入煤粉的硝酸铵的热分解特性。用Kissinger方程求解了铵油炸药和混入煤粉的铵油炸药的表观活化能(E_a),考察了煤粉与铵油炸药的化学相容性。研究结果表明:在常压氮气氛围的环境中,铵油炸药和硝酸铵的DSC曲线中的热分解均为吸热峰;当煤粉和硝酸铵混合后,煤粉与硝酸铵会在硝酸铵的热分解温度之前发生化学反应,随着煤粉含量的提高,硝酸铵的热分解逐渐转变为放热峰,说明煤粉能极大地降低硝酸铵的热稳定性;使用DSC法研究混入煤粉的铵油炸药相容性等级为4级,煤粉与铵油炸药的相容性差。     

高炉煤粉喷吹燃烧行为的数值模拟

为探究煤粉喷吹微观机理,分析煤粉喷吹行为规律及其它相态物料对其影响,采用计算流体动力学(CFD)技术对高炉内多相流动过程进行建模。以某1 750 m3炼铁高炉为研究对象,将喷入的煤粉作为粉相,计算得到典型炉况下多相态的温度、速度等物理量可视化结果,以及高煤比时炉内温度状态。实践运行与计算结果表明,适当调整布料结构、富氧量等因素,可进一步加大煤粉喷吹量,从而提高炉利用系数,降低焦比和生产成本。     

Partitioning behavior of trace elements during pilot-scale combustion of pulverized coal and coal-water slurry fuel

Release pathways for inorganic hazardous air pollutants (IHAPs) from a pilot-scale, down-fired combustor (DFC) when firing pulverized coal (PC) and coal-water slurry fuel (CWSF) were identified and quantified to demonstrate the effect of fuel form on IHAP partitioning, enrichment and emissions. The baghouse capturing efficiency for each element was calculated to determine the effectiveness of IHAP emission control. Most of the IHAPs were enriched in the fly ash and depleted in the bottom ash. Mercury was found to be enriched in the flue gas, and preferentially emitted in the vapor phase. When firing CWSF, more IHAPs were partitioned in the bottom ash than when firing PC. Significant reduction of Hg emissions during CWSF combustion was also observed.     

Simulation of turbulent mode of combustion of gas jets

The diffusion mode of combustion of fuel jets in an oxygen-containing atmosphere occurs very widely both in nature and in engineering. In so doing, a glowing flame is formed, whose geometric dimensions define the energy efficiency of combustion of fuel.The hydrodynamic structure of the flame exhibits a number of differences from the classical jet flows, in particular, from well-studied subsonic turbulent jets [1]. The flames are characterized by a high degree of hydrodynamic instability; however, at the same time, the flame height clearly follows the phenomenon such as the change of laminar mode for turbulent one; by the way, this phenomenon presents a difficult problem for investigation in flows of other types.The diffusion mode of jet combustion is of further interest because of the presence of “scaling” effect, i.e., the dependence of the relative flame height both on the Reynolds number (which is typical of all flows with laminarto-turbulent transition) and on the dimensionless nozzle diameter (d ₀/d*); in so doing, the threshold value of d* divides the entire diversity of dimensions into regions with the presence and absence of this effect.These and other features of burning fuel jets present a serious problem for mathematical simulation of burners and combustors.The paper contains analysis of a number of relations used in different models of turbulent combustion.     

微油点火燃烧器一级燃烧室点火性能试验研究

试验研究了不同一次风速和煤粉质量分数条件下微油点火燃烧器一级燃烧室的点火性能.研究结果表明,燃烧室中心温度随一次风速和煤粉质量分数的增加,呈先降低后增加的趋势;燃烧室中心线上的CO体积分数随一次风速的增加而增大,但随煤粉质量分数的增加而减小;燃烧室出口中心处的NOx体积分数随一次风速和煤粉浓度的增加则均呈现上升趋势.