Computer-controlled scanning electron microscopy of minerals in coal-Implications for ash deposition

The nature of mineral matter in coal determines its transformation into ash during combustion and the nature of resulting ash (e.g. chemical composition and particle size distribution), and subsequently influences the ash deposition behaviour. The behaviour of mineral matter is primarily influenced by two parameters: the mineral grain size, and whether the mineral grains are within the coal matrix or not. Computer-controlled scanning electron microscopy (CCSEM) of coal provides such information on mineral matter in coal. CCSEM data are, therefore, processed to predict the fouling and slagging characteristics of several coals. The fraction of basic oxides in each mineral grain may be considered as an indicator of stickiness of the corresponding ash particle due to formation of low melting compounds. The cumulative mass fraction of mineral grains with certain basic oxides or viscosity of resulting ash particles from included and excluded minerals are proposed as alternative indices for ash deposition.

The excluded mineral matter is in equilibrium with the combustion flue gases at the gas temperatures, whereas the included minerals are in equilibrium with the atmosphere within char at the burning char particle temperature. It is predicted from thermodynamic calculations based on this understanding that almost all the evaporation is either from the included mineral matter or from the atomically dispersed minerals in coal. This is due to the high temperature and reducing atmosphere inside the char particle. The release of the evaporated species is controlled by diffusion through the burning char particle and, therefore, may be estimated theoretically. The amount of mineral matter that is vaporized may then be related to fouling, whereas the melt phase present on the surface of large ash particles may be related to slagging. The theoretical speculations on the physical character of ash derived from these indices are compared with the experimental data obtained from combustion of coals in a drop-tube furnace.     

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.     

O_2/CO_2气氛下高岭土和石灰石控制燃煤PM_1排放的试验研究

在O2/CO2气氛和不同温度下,在沉降炉中对分别添加2种吸附剂的2种典型褐煤进行了燃烧试验,研究了高岭土和石灰石对碱金属、重金属和硫的吸附以及温度对PM1排放特性的影响.结果表明:高岭土与煤中金属发生吸附反应,而石灰石吸附了大量的硫,使金属和硫由小于1μm的颗粒向大于1μm的颗粒转移;添加吸附剂后,2种煤的PM1排放均明显减少,且均在1100℃时减排效果最好.对PM1的减少率进行比较发现,A煤添加高岭土比B煤添加高岭土的效果好,而B煤添加石灰石比A煤添加石灰石的效果好.     

Influence of operating conditions and the role of sulfur in the formation of aerosols from biomass combustion

The properties of the fine particles generated from burning biomass have been experimentally studied in a laboratory facility under a variety of combustion and postcombustion conditions; the parameters varied include combustion temperature and the concentrations of oxygen and SO₂ in the flue gases. SO₂ was added as a pure gas or generated in cofiring experiments. Fine particles are composed only of K, Cl, and S, in the form of potassium sulfate and chloride, except for the tests at 1450 °C, where phosphorus appeared also in significant amounts, although the species in which it was contained could not be determined exactly. From previous studies, K₂SO₄ is known to nucleate first when the gas cools, KCl condensing on these nuclei at lower temperatures. The chloride/sulfate ratio in fine particles is shown here to be greatly affected by the initial [SO₂] and [O₂] in the flue gases; this dependence can be adequately modeled if the conversion of SO₂ to SO₃ is assumed to be the only limiting step in the route to K₂SO₄ formation. Evidence for such a kinetic limitation is provided. Both the experimental results and theoretical considerations show that the presence of Cl in the submicron particles, associated with severe boiler corrosion, can be at least partly avoided with adequate combustion strategies (e.g., cofiring). The properties of coarse (>1 μm) particles have also been studied; both their chemical composition and size distribution are consistent with the break-up model of fly-ash formation.     

Combustion of levitated titanium particles in air

Combustion of titanium particles in air is of interest for applications to pyrotechnic countermeasures against chemical or bacteriological pollution, synthesis of nanoscale titania, and fire/explosion safety in metallurgical and SHS technologies. In this work, combustion of single titanium particles is studied using electrodynamic balance (levitation) and laser ignition. High-speed digital video recording and a photomultiplier are used for diagnostics. Based on the obtained data, empirical formulas are proposed for the dependence of the burning time on the initial particle size. Thermodynamic calculations are made to support the conclusions based on experimental data. The particle burning time is estimated using a simplified mathematical model for diffusion-controlled combustion. Comparison of the estimates with the experimental results suggests that oxygen diffusion in the gas phase plays a major role in the combustion mechanism of 100-μm or larger titanium particles in air, while kinetics become more important for smaller sizes.     

Numerical studies on the reaction of carbon particles in a vacuum residue—air flame

A computational work was carried out for the study of one‐dimensional, laminar, premixed, flat, atomized vacuum residue (VR) particle–air flames. The mathematical model includes the specified pyrolysis scheme, soot and char oxidation scheme. With some experimental works, the product composition and kinetic parameters of VR pyrolysis were determined and used for the present computational work. The computed results show that the oxidation of VR carbon char and soot occurs mainly in the reaction zone and the oxidation rate of soot is much higher than that of VR carbon char. The oxidation rates of carbon char and soot can be increased with the decrease in particle diameter, and it might be accomplished by the more effective atomization and mixing of solid particles with combustion air. Copyright © 2005 John Wiley & Sons, Ltd.     

Combustion,cofiring and emissions characteristics of torrefied biomass in a drop tube reactor

The study investigates cofiring characteristics of torrefied biomass fuels at 50% thermal shares with coals and 100% combustion cases. Experiments were carried out in a 20 kW, electrically heated, drop-tube reactor. Fuels used include a range of torrefied biomass fuels, non-thermally treated white wood pellets, a high volatile bituminous coal and a lignite coal. The reactor was maintained at 1200 °C while the overall stoichiometric ratio was kept constant at 1.15 for all combustion cases. Measurements were performed to evaluate combustion reactivity, emissions and burn-out.Torrefied biomass fuels in comparison to non-thermally treated wood contain a lower amount of volatiles. For the tests performed at a similar particle size distribution, the reduced volatile content did not impact combustion reactivity significantly. Delay in combustion was only observed for test fuel with a lower amount of fine particles. The particle size distribution of the pulverised grinds therefore impacts combustion reactivity more.Sulphur and nitrogen contents of woody biomass fuels are low. Blending woody biomass with coal lowers the emissions of SO₂ mainly as a result of dilution. NO_X emissions have a more complex dependency on the nitrogen content. Factors such as volatile content of the fuels, fuel type, furnace and burner configurations also impact the final NO_X emissions. In comparison to unstaged combustion, the nitrogen conversion to NO_X declined from 34% to 9% for air-staged co-combustion of torrefied biomass and hard coal. For the air-staged mono-combustion cases, nitrogen conversion to NO_X declined from between 42% and 48% to about 10%–14%.     

Particle combustion rates in premixed flames of polydisperse metal—air aerosols

A new approach and experimental technique are proposed to determine times of metal particle combustion in flames of polydisperse aerosols. Laminar flames are produced in air at 1 atm, using aerosol jets formed by an electrostatic particulate method. The flame radiation intensities as a function of vertical coordinate are measured and compared with the flame radiation profiles reconstructed using experimental data and simplified models. The experimental data used include particle size distributions, flame velocities, and temperatures of metal ignition and combustion. The simplified models describe the particle ignition delay, combustion time, and particle flame radiation intensity as a function of particle diameter, D. Variable parameters of the models describing particle radiation intensities and combustion times are adjusted to achieve the best fit between the reconstructed and measured flame radiation profiles. A set of parameters providing the best agreement between the reconstructed and measured profiles is selected for several aerosol flames produced by powders of different sizes of the same material. These parameters are assumed to adequately describe particle combustion times and radiation intensities for the chosen material. The experimental radiation profiles for both aluminum and magnesium aerosol flames with particles of different sizes were found to be in very good agreement with the respective reconstructed profiles. For both metals, particle radiation intensities were well described by a D³-type expression. The combustion times for magnesium aerosol particles were well described by the traditional D²-law with the evaporation constant close to those reported earlier for single particles. Aluminum aerosol particle combustion was better described by a D¹-law and combustion times of fine (<80 μm) aluminum particles in the aerosol were somewhat longer than the reported earlier combustion times for single aluminum particles.     

The simulation of the combustion of micrometer-sized aluminum particles with oxygen and carbon dioxide

The Liang/Beckstead aluminum-particle combustion model has been successfully joined with a detailed chemical-kinetic mechanism. The model has been used to investigate the effect of oxidizer concentration, pressure, and particle diameter on the combustion of CO₂/Ar and O₂/Ar with micrometer-sized aluminum particles. The simulation results when varying the oxidizer compare well with experimental data. With CO₂ as the oxidizer, the trend of each simulated diameter follows that of the experimental data, especially the simulated 7 μm particles compared to experimental data for a mass average diameter of 11 μm. For oxygen, the simulated burn times with a particle size of 11 μm has excellent agreement compared to experimental data with a mass average diameter of 11 μm. The simulation results for both CO₂/Ar and O₂/Ar show a transition from kinetically-controlled combustion to diffusion-controlled combustion as the pressure increases. The burn time of the particles decreases as the pressure increases, until the diffusion-controlled combustion regime is reached and then the pressure has no effect on burn time. The opposite is true for the CO₂ experimental data, in that the observed burn time increases with increasing pressure. The simulations indicate that the observed experimental trend could be the result of using a distribution of particle diameters. As the pressure decreases, larger particles may not ignite and the apparent burn time does not increase. The effect of particle diameter was also investigated. The effects of particle size, oxidizer, and oxidizer concentration on the calculated surface temperatures are also shown. This is the first model to show the beginning of the transition from diffusion-limited to kinetic-limited combustion control for aluminum particles.     

On the survivability and pyrosynthesis of PAH during combustion of pulverized coal and tire crumb

Results are presented on the emissions of semivolatile polycyclic aromatic hydrocarbons (PAH) from the combustion of a pulverized bituminous coal and ground waste automobile tires. Streams of fuel particles were injected at steady-state steady-flow conditions, and burned inside an isothermal drop-tube furnace, in air, at a gas temperature and gas residence time of 1150°C and 0.75 s, respectively. Combustion occurred under either very fuel-lean conditions (bulk equivalence ratio, φ < 0.5) or substantially fuel-rich conditions (φ = 1.6–1.9). Emissions from fuel pyrolysis, in the absence of oxygen, were also examined. The survivability of the fuel-PAHs during combustion/pyrolysis was assessed by examining the reactants (fuels) and the products of their oxidation/pyrolysis. The PAH species in the effluent of combustion were: 1) qualitatively compared with indigenous PAH constituents of the input fuels, and 2) quantitatively contrasted with known amounts of deuterium-labeled PAH standards, which were absorbed on the input fuels. No PAHs were detected in the effluent of combustion of either fuel under sufficiently fuel-lean conditions, e.g., φ < 0.5. This indicated that the PAH constituents of the input fuels, either indigenous or adsorbed, as well as those formed by pyrosynthesis in either the diffusion volatile flames or during the heterogeneous oxidation of the chars were destroyed. Significant amounts of PAHs were detected in the effluent of the combustion of both fuels under sufficiently fuel-rich conditions, e.g., φ > 1.6 and, especially, under pyrolytic conditions in N₂. These PAHs were mostly attributed to pyrosynthesis since none of the deuterated PAHs, adsorbed on the fuels, survived the combustion process. Small amounts of the labeled compounds, however, survived under purely pyrolytic conditions. These results were confirmed with separate experiments, where deuterium-labeled PAH standards were adsorbed on highly porous calcium/magnesium oxide or mullite particles. Again, small amounts of some PAHs survived in high-temperature pyrolytic conditions, but none in oxidative environments. These observations suggest that pyrosynthesis is the major contributing mechanism to the PAH emissions from the combustion of these fuels. Survivability of parent PAHs may be a minor mechanism at very high equivalence ratios.

Finally, both fuels were mixed with powders of calcium magnesium acetate (CMA), calcium carbonate (CaCO₃), and calcium oxide (CaO), all of which are known sulfur reduction agents, at a molar Ca/S ratio of 1. Combustion of the fuels mixed with CMA or CaCO₃ generated enhanced amounts of PAHs, while combustion with CaO had no effect on the PAH emissions.     

Studies of transformations of inorganic constituents in a Texas lignite during combustion

The mechanisms of coal ash formation were studied under closely controlled combustion conditions. Monticello lignite, from Titus County, Texas was combusted in a drop-tube furnace at 1500°C and fly ash was collected and aerodynamically size segregated into six stages. Short residence time chars were also produced in the drop-tube furnace. The coal was analyzed to determine both mineralogical and organically bound components using computer controlled scanning electron microscopy (CCSEM) and chemical fractionation techniques, respectively. Scanning electron microscopy/microprobe techniques were used to classify and determine the distribution of various ash particle types in each size fraction. Over 80% of the Monticello mineral content consisted of quartz and clay minerals and relatively large quantities of Ca, Mg, and possibly Fe were organically bound. Extensive reaction between the quartz and clay minerals and organically bound Ca resulted in amorphous and crystalline Ca-silicate and Ca-aluminosilicate phases in the fly ash. Finely subdivided discrete minerals or organically bound cations of Mg and Fe were concentrated in the finer fraction of the fly ash.     

Catalytic effect of KOH on the reaction of NO with char

Experimental results are presented on the reduction of NO by char particles prepared from a Chinese low-volatile coal. The experiment was conducted in a drop-tube furnace at 1173 to 1323 K. Kinetic parameters of the global reaction of NO with char were determined, and the effects on these kinetic parameters of adding the catalyst KOH to the char particles and also of [O₂] in the flue gas were studied. The results show that KOH can increase the frequency factor and reduce the activation energy of the reduction of NO by char. However, the benefit of increasing the KOH content in char particles decreases when there is more than 1.0 wt % of the catalyst in the char. The activation energy of the global NO-char reaction is shown to be independent of [O₂], but the frequency factor strongly depends on the equivalence ratio under oxygen-lean conditions.     

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。     

煤粉颗粒群着火和燃烧过程的数值模拟

建立了一维非稳态球形煤粉颗粒团的群燃烧模型.数值模拟煤粉颗粒团的着火和燃烧过程,获得了颗粒团燃烧火焰随时间的变迁.分析了煤粉颗粒团内部参数和外部环境参数对颗粒团着火和燃烧的影响.随着颗粒团内煤粉浓度的增加,颗粒团的均相着火延迟先减小后增加.增加煤粉颗粒尺寸和降低外部温度都会明显延迟均相着火.环境氧气含量的增加会减小着火延迟,同时增加颗粒团的燃烧速率.模拟计算和文献试验结果的变化趋势相吻合.     

Physical characterization of biomass fuels prepared for suspension firing in utility boilers for CFD modelling

A sample of 1.2 kg Danish wheat straw (Jutland, 1997) prepared for suspension firing in a PF boiler has been analyzed for the purpose of generating size and shape distribution functions applicable to numerical modelling of combustion processes involving biomass, characterised by highly anisotropic shapes. The sample is subdivided by straw type, and coherent size, type and mass distribution parameters are reported for the entire sample. This type of data is necessary in order to use CFD reliably as a design and retrofit tool for co-firing biomass with fossil fuels, as the combustion processes of the biomass particles are highly dependent on these parameters.     

Measurement of in-cylinder soot particles and their distribution in an optical HSDI diesel engine using time resolved laser induced incandescence (TR-LII)

Soot particles generated during combustion process in a single cylinder optical diesel engine were measured using time-resolved laser induced incandescence technique (TR-LII). Experimentally measured LII data were fitted with theoretically simulated LII data to determine the count median particle diameter and the geometric width of the particle size distribution. TR-LII signals were acquired simultaneously at two different wavelengths for crank angles in the range from 48.4° to 111.4° after top dead centre (aTDC) and for various engine operating conditions. Particle size was found to be dependent on the engine load but no significant change in particle sizes were observed for different fuel injection timings. The average size of the primary particles measured over different crank angles were larger at higher engine loads compared to lower engine loads. A general trend of the soot particle diameter decreasing with crank angle was observed due to oxidation, but towards the end of the expansion stroke a marginal increase in size was noted. For all operating conditions the width of the particle size distribution was found to decrease with crank angle until 61.4°aTDC, and thereafter it increases to reach a well defined size distribution for a given fuel injection quantity despite the differences in the injection timing. The soot particles are randomly transported by the complex fluid motion present within the combustion chamber leading to strong cycle to cycle fluctuations of the measured time resolved LII data. The in-cylinder soot volume fraction derived from TR-LII data revealed that for different fuel injection timings, a relatively higher amount of soot was produced for shorter ignition delay compared to larger ignition delays.     

Melting and oxidation behavior of in-flight particles in plasma spray process

The melting and oxidation behavior of in-flight particles and their effects on the splat formation are studied. An order-of-magnitude analysis and one-dimensional thermal resistance analysis are employed to investigate the heat and mass transport from the plasma flame to the particle. Two dimensionless parameters, “melting index” and “oxidation index” are derived to characterize the melting status and the oxide content of the in-flight particles, respectively. These two indice are correlated with experimentally measurable parameters, such as particle size, velocity, temperature and spray distance. Numerical simulations and experimental results are discussed to validate the analytic solutions. Effects of particle size on the melting and oxidation behavior are also investigated.     

Comparison of nitrogen oxide emissions from boilers for a wide range of coal qualities

This paper presents the results of experimental researches on nitrogen oxide emissions from coal-fired boilers. Two Chinese lean coals have been fired in two full scale boilers (1 025 t · h ⁻¹ ) and in a pilot scale test furnace (Drop-Tube Furnace) to study the influence of nitrogen content in coal on nitrogen oxide emission. The nitrogen oxide emission was found to correlate well with the fuel nitrogen content. The test results of the drop-tube furnace and the single burner furnace with three Chinese coals show that the staged combustion can greatly reduce the nitrogen oxide emission. Identical trends in nitrogen oxide emission as a function of the volatile matter of the coals have been obtained under different combustion conditions. The principle of low nitrogen oxide emissions of the wide range burner (biased combustion or internal air staging) and the cases with over fired air port (furnace air staging) is introduced in this paper. In addition, the influence of excess oxygen content on nitrogen oxide emission has been tested in the utility boilers and test furnaces.     

Experimental and numerical analyses of the combustion characteristics of Mg/PTFE/Viton fuel-rich pyrolants in the atmospheric environment

Abstract

Magnesium/polytetrafluoroethylene/Viton (MTV) fuel-rich pyrolants use the atmospheric oxygen as a complementary oxidizer to sustain and alter the performance of the combustion reaction. The flame diffusion characteristics of MTV fuel-rich pyrolants in the atmospheric environment are studied by a high-speed camera (HSC). The flame temperature and combustion components are measured by using remote sensing Fourier Transform Infrared Spectrometer (FTIR). In order to obtain the combustion component distribution more accurately, an aerobic combustion model containing the oxidation reaction of the excess reactant Mg and carbonaceous species with O₂ is established in this study. Eddy dissipation concept (EDC) model is applied to the numerical simulation of the three dimensional anaerobic and aerobic combustion field coupled with Realizable k-ε two-equation turbulence model. The research results show that flame temperature is mainly contributed by anaerobic combustion reaction. The flame structure obtained by the aerobic combustion model is closer to the experimental results, and the result of component distribution calculated by aerobic combustion is more consistent with characteristic spectrum. Therefore, the aerobic combustion model is more suitable for describing the actual MTV flame, and the combustion field can be divided into an anaerobic core zone and an aerobic diffusion zone.