•First time to evaluate the louver concentrator performance in an arch-fired boiler•Ignition and combustion problems were experimentally and numerically assessed.•Effect of louver concentrator performance on ignition was clarified.•Suggestions for the retrofit of the combustion system were provided.A set of comprehensive on-site experiments was conducted to assess the performance of the louver-type primary air (PA) concentrators on anthracite ignition and combustion in a 600 MW supercritical arch-fired boiler (AFB) equipped with slot burners. The temperature profiles along the PA stream from fuel-rich and fuel-lean streams were measured under various conditions. The separation efficiency of the PA concentrators of two selected burners was also measured and its effect on the ignition of the PA streams was studied. Experimental results revealed that separation performance of some louver concentrators was even opposite to design setting. The ignitability of PA stream, however, was more profoundly depended on the configuration of burner, the position of the nozzle and the SA distribution than the separation performance of the louver concentrators. On the other hand, confirmed by numerical simulation, the unsatisfied separation performance of the louver-type concentrators could result in overall poor ignition of PA, causing over-shooting of the flame penetration length, and high loss of ignition and difficulties for combustion organization. Some suggestions are given to the retrofit and new design of combustion system.Download high-res image (199KB)Download full-size image

•Experimentally studied the flow characteristics of two parallel rectangular jets through exits with sudden contraction.•The sudden contraction remarkably enhances the mixing between the parallel jets.•The sudden contraction makes the parallel jets combined earlier.•The sudden contraction influences the turbulence characteristics of the jet flows.The flow characteristics of two parallel jets ejecting from the nozzles with sudden contraction exits were studied using particle imaging velocimetry (PIV) technique. The measured mean velocity components, velocity vector contours, turbulence characteristics and Reynolds shear stresses showed that the flow fields adjacent to the exits are significantly different from those of the dual flows ejecting from the nozzles with constant cross section area. The sudden contraction at the exits strongly promotes the merging of the two parallel jets and enhances the turbulence intensity in the merging region. The location of the combined point locates much closer to the exits when the contraction ratio in area varies from 1 to 1.5 and thereafter changes slowly when the contraction ratio further increases. The measured results also showed that the parallel jets ejecting from the nozzles with sudden contraction exits creates much stronger turbulence in the inner and outer shear layers of the two parallel jets than the jets ejecting from the nozzles with constant cross-section area.Download high-res image (331KB)Download full-size image

•Extinction limits of lean premixed H2-CO-air flames were measured and computed.•H2 addition restrains the flames from being extinguished.•Chemical kinetics is crucial in determining the H2-CO-air flame extinction.•An extinction exponent was proposed based on the chain theory.•The critical extinction exponent characterises the near-limit kinetics.The extinction limit of laminar lean premixed stretched H2-CO-air flames was investigated with particular attention to the chemical kinetic characteristics under near-limit conditions. The extinction stretch rates were determined by the digital particle image velocimetry measurement using the Counterflow Twin-Flame method. Corresponding numerical simulations were conducted using CHEMKIN II with detailed chemical kinetic mechanisms. The simulation results generally agreed well with the experimental data. It was found that increasing H2 ratio restrained the flame from being extinguished, and this restraining effect was more profound low H2 ratios. Further analyses of the flame speed, temperature and elementary reaction sensitivities revealed that H2 addition played a crucial role in flame extinction by changing the competition between chain branching and termination reactions. To quantitatively measure the kinetic effect of H2 addition, a dimensionless extinction exponent (β) defined as the ratio between the relative sensitivities of termination (ωT) and branching (ωB) reaction rates, i.e., β = ∂lnωT/∂lnωB, was proposed. For a given syngas, β rapidly increased to a certain constant, defined as the critical extinction exponent, when the flame extinction was approached, despite variations in the stretch rate and changes in the loss mechanism. The chemical kinetic characteristics and loss mechanism jointly determined the extinction limit of lean premixed H2-CO-air flames.

•Effects of extraction condition on AAEM determination in were experimentally studied.•An improved sequential extraction method was proposed with clearly defined conditions.•The new method has higher accuracy and is less time consuming.•AAEM occurrence modes and contents in three Zhundong coals were determined by the new method.•Mineral patterns in raw coal and extraction residues were analyzed by XRD.Zhundong coal is one of the most important coals in China due to its huge reserve, but it causes severe fouling and slagging problems when it is burnt in boilers for its high AAEMs (alkali and alkaline earth metal species) contents. Accurately determination of the occurrence modes and contents of AAEMs in Zhundong coals is a necessary step to overcome the ash-related problems. Nowadays, different researchers used different methods or procedures to determine AAEMs in Zhundong coals, and the data were scattering. Based on a series of experiments on the effects of extraction conditions, an improved extraction method with clearly defined conditions for each extraction step was proposed. 0.1 mol/L NH4Cl (pH = 8.5) instead of 1 mol/L NH4OAc (pH = 7) was used to extract the exchangeable AAEMs in Zhundong coals to avoid the high solubility of carbonates in the extraction solution. The improved method was more accurate and took much less time than the conventional one. With the proposed extraction method, the occurrence modes and contents of AAEMs in three Zhundong coals were measured. The results revealed that Na is mainly water soluble (Naw/Natotal = 50–85%) which is halite or in the form of surface bound Na+, while Ca and Mg are mainly acid soluble (Caac/Catotal = 60–90%, Mgac/Mgtotal = 45–90%) which are carbonates together with a little amount of sulfates. Exchangeable AAEMs organically bonded to the coal matrix are not the main occurrence mode of AAEMs. Acid insoluble Ca-species could be rankinite. The amounts of AAEMs in the same occurrence mode could be remarkably different among coals from different Zhundong district. The improved sequential extraction method can be extended to determine AAEMs in high carbonate-containing coals.

Ignition behaviors of pulverized coal particle clouds in a jet with different turbulent intensity, O2 concentration and coal concentration were experimentally studied using an entrained flow reactor. Mie scattering technique and high-speed cameras were employed to record the particle motions and flame behavior. Results revealed that a cloud flame consisted of a number of parcel flames and stripe flames, which were located in the center and around boundary of the cloud flame, and resulted by the burning of the evolved volatile matter or the clusters of fine fuel particles, and the burning of the single particles respectively. As Reynolds number of the primary flow increased, the cloud flame changed from narrow and structured to wide and turbulent. At the same time, the particle dispersion became more intensive, leading to a lower flame incandescence. The increase of O2 concentration in the primary or secondary flow promoted ignition of cloud flames. For laminar cloud flame, ignition distance was more sensitive the O2 concentration in the primary flow, and a minimum value was found at certain coal concentration, but for turbulent cloud flame, ignition distance was shorter, and no obvious non-monotonic trend was found over the tested coal concentration range.

An experimental study on ignition and combustion of single coal particles under different O 2 concentrations was conducted at both normal (1-g) and microgravity (μ-g) in the first time. The surface and centre temperatures of the bituminous coal particle with initial diameter of ∼ 2.0mm were measured by the monochromatic imaging technique using a short wavelength infrared (SWIR) camera and an embedded fine thermocouple respectively. Results revealed that at μ-g, ignition of the tested coal particles was homogeneous. O 2 concentration significantly affects the shape, ignition temperature and ignition delay time of the volatile flames. A mathematical model considering thermal conduction inside the coal particle was developed to describe the ignition process of single particle, adopting the volatile matter flammability limit as the homogeneous ignition criterion. The predicted ignition temperatures were slightly lower but closer to μ-g data. And the predicted variation trends of ignition temperature and delay time under different O 2 concentrations agreed well with the μ-g experimental results.

•New experimental extinction data for the diluted lean premixed H2/CO/air flames.•Detailed simulation on the propagation and extinction of syngas flames.•Assessment of dilution mechanisms on the propagation of syngas flames.•Assessment of dilution mechanisms on the extinction of syngas flames.The dilution effects of inert components N2 and CO2 on the propagation and extinction of lean premixed H2/CO/air syngas flames were experimentally and numerically investigated. Extinction stretch rates were measured using the counterflow technique while laminar flame speed data were obtained from literatures. Numerical simulations were conducted at 1-D freely propagating configuration and opposed-jet configuration with detailed chemistry and molecular transport description. The numerical results well predicted the experimental measurements. Both results revealed that CO2 dilution had more profound effect on flame propagation and extinction than N2 dilution. In addition, numerical simulation assessed the preferential importance in a rather quantitative manner among the three effects, i.e., the thermal effect, the diffusivity change effect and the chemical effect with artificial manipulation of mass diffusivities and chemical reactions of CO2 and N2. The results showed that the thermal effect dominated the reduction of laminar flame speed and extinction strain rate. The chemical effect caused by CO2 dilution was slightly stronger to the reduction of extinction limit than to that of laminar flame speed. The diffusivity change effect is negligible for both CO2 and N2 dilutions. N2 only acts as a thermal inert in the propagation and extinction of the H2/CO/air flames.

•Experimental study on convection effect on ∼1.5 mm single coal particle ignition.•Effect of O2 concentration under convection was also studied.•Influences on coal ignition mechanisms, delay time and temperature were accessed.•Typical ignition regimes of bituminous coal particles under convection were given.•The influencing mechanisms of convection on coal particle ignition were discussed.An experimental study on the convection effect on the ignition behavior of single coal particles with size of ∼1.5 mm was carried out in an electrically heated vertical tube furnace under various O2 concentrations and temperatures ranging from 873 K to 1123 K. Under all test conditions, anthracite particles ignited heterogeneously. However, the ignition mechanism of the bituminous coal particles was conditionally changed by the convention. For both coals, the ignition delay time decreased and then increased with the increasing convection intensity, and the variation trends were insignificant under strong convections or high furnace temperatures. Under O2-enriched conditions, convection is in favor to coal ignition at low furnace temperatures and even cause asymmetrical hetero-homogeneous ignition. The ignition temperature of bituminous coal decreased and then increased with increasing convection intensity. It slightly decreased with the increasing O2 concentration at a weak convection and remained nearly constant when convection was rather strong. The influencing mechanisms of convection on coal particle ignition were discussed.Graphic abstractTypical ignition regimes for single particles of a bituminous coal under different convection intensities and furnace temperatures in air condition (coal size: ∼1.5 mm).

•Extinction limits of near-limit syngas/air flames were measured at micro-gravity.•Branching and terminating determine the premixed syngas/air flame extinction.•Diffusivity is more important than chemistry in syngas/air flame extinction.•Suppressing H2 diffusivity makes the syngas/air flame easier to extinguish.•Suppressing H diffusivity makes the syngas/air flame slightly stronger.Extinction studies of weakly-stretched near-limit lean premixed syngas/air flames were conducted in a twin-flame counterflow configuration. Experiments showed that buoyancy-induced natural convection at normal gravity strongly disturbed these flames. In order to validate the simulation, accurate extinction data was obtained at micro-gravity. Experimental data obtained from the 3.6 s micro-gravity drop tower showed that the extinction equivalence ratio increased with the increasing global stretch rate and decreased with the increasing H2 mole fraction in the fuel. Numerical simulation was conducted with CHEMKIN software using GRI 3.0 and USC-Mech II mechanisms. The predicted extinction limit trend was in agreement with the micro-gravity experimental data. Sensitivity analyses showed that the competition between the main branching reaction H + O2 ⇔ O + OH and the main termination reaction H + O2 + M ⇔ HO2 + M in the H2/O2 chemistry determined the extinction limits of the flames. The dominant species for syngas/air flame extinction was the H radical. The key exothermal reaction changed from OH + CO ⇔ H + CO2 to OH + H2 ⇔ H + H2O with the increasing H2 mole fraction in the fuel. Also, the mass diffusion played a more important role than chemical kinetics in the flame extinction. When the H2 mass diffusion was suppressed, the reaction zone was pushed to the stagnation plane and the flame became weaker; while H mass diffusion is suppressed, the reaction zone slightly shifted towards the upstream and the flame was slightly strengthened.

The eddy dissipation concept (EDC) model with the consideration of the intermediate reactions for the volatile matter (VM) combustion was applied to simulate turbulent pulverized coal/air jet combustion adjacent to a bluff-boy type model burner. The VM of the coal was assumed to consist of CO, H2, and CH4, and its chemistry was described by a 16-speices and 41-step skeletal mechanism for CH4/air combustion. The model was compared with some other conventionally used ones, including the eddy dissipation (ED) model, eddy dissipation or finite rate (ED-FR) model, mixture fraction probability density function (MF-PDF) model, and the EDC model with a global reaction mechanism for gas phase combustion (EDC_G), in predicting temperature profiles, maximum flame temperature, flame shape, and ignition position and in CPU cost. The predicted temperature profiles and flame positions were further compared with reported experimental data. It was found that the EDC model with the consideration of the intermediate reactions for VM combustion improved prediction in the temperature field and thus ignition position. With the adoption of the full in situ adaptive tabulation (ISAT) method, two-third of the overall CPU time could be saved, making the EDC model more acceptable in comparison with the ED-type models and MF-PDF model. On the basis of the simulation results, it is suggested that intermediate reactions of the VM should be considered when high accuracy of flame temperature and ignition position prediction is desired in simulation of pulverized coal combustion.

In this paper, the attrition characteristics of ashes of 25 different coals used in circulating fluidized bed boilers were experimentally studied. The attrition characteristics were described by the attrition rate constant Kaf, which was obtained by the static combustion and cold sieving method. The effects of 10 chemical components and 6 mineral components on Kaf were evaluated by the gray relational analysis method. The main phases of the mineral components in the coal ashes were determined by X-ray diffraction, and anhydrite (CaSO4), lime (CaO), quartz (SiO2), hematite (Fe2O3), metakaolinite (Al2Si2O7), and metaillite (KAl2AlSi3O11) were identified. The results showed that, for a given mass fraction, the significance of the influence of the ash chemical components on Kaf follows the sequence: CaO > MgO > SO3 > P2O5 > Na2O > Fe2O3 > K2O > TiO2 > Al2O3 > SiO2. The significance of the influence of the ash mineral components on Kaf follows the sequence: lime (CaO) > anhydrite (CaSO4) > hematite (Fe2O3) > metaillite (KAl2AlSi3O11) > metakaolinite (Al2Si2O7) > quartz (SiO2). In addition, the effect of the hardness of individual mineral components on Kaf was also discussed.

The NOx emission from a circulating fluidized bed (CFB) boiler with 50 MW rated thermal output (50 MWth) during the cofiring of anthracite coal and pelletized corn stalk biomass was investigated. Cofiring could improve combustion efficiency and, thus, the thermal efficiency of the boiler. However, contrary to most results found in bubbling fluidized bed (BFB) combustors and the one found in a CFB boiler burning rice husk, NOx concentration in flue gas and fuel nitrogen conversion ratio increased with the biomass mass fraction. Several causes for such a contradiction were discussed and the configuration in combination with feedstock location of biomass was regarded as the most important one. In a CFB boiler, fuel is mostly burnt above the dense bed, opposite to that in a BFB boiler, and, thus, the average oxygen concentration above the dense bed, where biomass was fed, was higher. High bed temperature and high primary air ratio used for burning the anthracite coal increased NOx emission and enhanced the variation trend. In addition, the catalyst-contained ash also could promote NOx formation. The results indicated that biomass is not necessarily an effective cofiring fuel to reduce NOx emission in a CFB boiler and in order to reduce NOx emission, low primary air ratio, and locating the feedstock ports of biomass below the secondary air should be considered.

The phase transformation during pyrite concentrate oxidation under the conditions of the circulating fluidized bed (CFB) roasting process was experimentally studied at temperatures of 600–900 °C, O2 concentrations of 1%–21%, and particle residence times of ∼5.0 s in a drop tube furnace (DTF). The X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to analyze the phase composition and microstructure of the product samples. For the roasting process with a short residence time, the products of pyrite concentrate oxidation are in a nonequilibrium phase. At higher O2 concentration and temperature, Fe3O4 is the dominant product phase. When O2 concentration increases, the micromorphology of sample products tends to be perfectly spherical particles with smooth surface, difficult for the formed Fe3O4 to be oxidized to Fe2O3. Experimental results indicated that, in the CFB roasting, relatively high temperature and low O2 concentration are needed to control the phase transformation into Fe2O3.

In this paper, the first Chinese microgravity (μ-g) experimental study on coal combustion was introduced. An experimental system used to study the ignition process of single coal particles was built up, complying with the requirements of the 3.5 s drop tower in the National Microgravity Laboratory of China (NMLC). High volatile bituminous and lignite coal particles with diameter of 1.5 and 2.0 mm were tested. The ignition and combustion process was recorded by a color CCD and the particle surface temperature before and at the ignition was determined by the RGB colorimetric method. Comparative experiments were conducted at normal gravity (1-g). The experiments revealed that at different gravity levels, the ignition of all tested coal particles commenced in homogeneous phase, while the shape, structure, brightness and development of the flames, as well as the volatile matter release during the ignition process are different. At μ-g, the part of volatile was released as a jet, while such a phenomenon was barely observed at 1-g. Also, after ignition, flames were more spherical, thicker, laminated and dimmer at μ-g. It was confirmed that ignition temperature decreased as the particle size or volatile content increased. However, contradicted to existing experimental results, provided other experimental conditions except gravity level were the same, ignition temperature of coal particles was about 50–80 K lower at μ-g than that at 1-g.

In the cyclone of a circulating fluidized bed (CFB) boiler, a noticeable increment of flue gas temperature, caused by combustion of combustible gas and unburnt carbon content, is often found. Such phenomenon is defined as postcombustion, and it could introduce overheating of reheated and superheated steam and extra heat loss of exhaust flue gas. In this paper, mathematical modeling and field measurements on postcombustion in 135MWe commercial CFB boilers were conducted. A novel one-dimensional combustion model taking postcombustion into account was developed. With this model, the overall combustion performance, including size distribution of various ashes, temperature profile, and carbon content profiles along the furnace height, heat release fraction in the cyclone and furnace were predicted. Field measurements were conducted by sampling gas and solid at different positions in the boiler under different loads. The measured data and corresponding model-calculated results were compared. Both prediction and field measurements showed postcombustion introduced a temperature increment of flue gas in the cyclone of the 135MWe CFB boiler in the range of 20−50 °C when a low-volatile bituminous coal was fired. Although it had little influence on ash size distribution, postcombustion had a remarkable influence on the carbon content profile and temperature profile in the furnace. Moreover, it introduced about 4−7% heat release in the cyclone over the total heat release in the boiler. This fraction slightly increased with total air flow rate and boiler load. Model calculations were also conducted on other two 135MWe CFB boilers burning lignite and anthracite coal, respectively. The results confirmed that postcombustion was sensitive to coal type and became more severe as the volatile content of the coal decreased.

The effect of bed pressure drop and bed inventory on the performances of a circulating fluidized bed (CFB) boiler was studied. By using the state specification design theory, the fluidization state of the gas−solids flow in the furnace of conventional CFB boilers was reconstructed to operate at a much lower bed pressure drop by reducing bed inventory and control bed quality. Through theoretical analysis, it was suggested that there would exist a theoretical optimal value of bed pressure drop, around which the boiler operation can achieve the maximal combustion efficiency and with significant reduction of the wear of the heating surface and fan energy consumption. The analysis was validated by field tests carried out in a 75 t/h CFB boiler. At full boiler load, when bed pressure drop was reduced from 7.3 to 3.2 kPa, the height of the dense zone in the lower furnace decreased, but the solid suspension density profile in the upper furnace and solid flow rate were barely influenced. Consequently, the average heat transfer coefficient in the furnace was kept nearly the same and the furnace temperature increment was less than 17 °C. It was also found that the carbon content in the fly ash decreased first with decreasing bed pressure drop and then increased with further increasing bed pressure drop. The turning point with minimal carbon content was referred to as the point with optimal bed pressure drop. For this boiler, at the optimum point the bed pressure was around 5.7 kPa with the overall excess air ratio of 1.06. When the boiler was operated around this optimal point, not only the combustion efficiency was improved, but also fan energy consumption and wear of heating surface were reduced.

A sodium–zinc sorbent based flue gas desulfurization technology (Na–Zn-FGD) was proposed based on the experiments and analyses of the thermal decomposition characteristics of CaSO3 and ZnSO3·2.5H2O, the waste products of calcium-based semi-dry and zinc-based flue gas desulfurization (Ca–SD-FGD and Zn–SD-FGD) technologies, respectively. It was found that ZnSO3·2.5H2O first lost crystal H2O at 100 °C and then decomposed into SO2 and solid ZnO at 260 °C in the air, while CaSO3 is oxidized at 450 °C before it decomposed in the air. The experimental results confirm that Zn–SD-FGD technology is good for SO2 removal and recycling, but with problem in clogging and high operational cost. The proposed Na–Zn-FGD is clogging proof, and more cost-effective. In the new process, Na2CO3 is used to generate Na2SO3 for SO2 absorption, and the intermediate product NaHSO3 reacts with ZnO powders, producing ZnSO3·2.5H2O precipitate and Na2SO3 solution. The Na2SO3 solution is clogging proof, which is re-used for SO2 absorption. By thermal decomposition of ZnSO3·2.5H2O, ZnO is re-generated and SO2 with high purity is co-produced as well. The cycle consumes some amount of raw material Na2CO3 and a small amount of ZnO only. The newly proposed FGD technology could be a substitute of the traditional semi-dry FGD technologies.Download full-size image

The characteristics of oxy-coal combustion for a swirl burner with a specially designed preheating chamber are studied numerically. In order to increase the accuracy in the prediction of flame temperature and ignition position, eddy dissipation concept (EDC) model with a skeletal chemical reaction mechanism was adopted to describe the combustion of volatile matter. Simulation was conducted under six oxidant stream conditions with different O2/N2/CO2 molar ratios: 21/79/0, 30/70/0, 50/50/0, 21/0/79, 30/0/70 and 50/0/50. Results showed that O2 enrichment in the primary oxidant stream is in favor of combustion stabilization, acceleration of ignition and increase of maximum flame temperature, while the full substitution of N2 by CO2 in the oxidant stream delays ignition and decreases the maximum flame temperature. However, the overall flow field and flame shapes in these cases are very similar at the same flow rate of the primary oxidant stream. Combustion characteristics of the air-coal is similar to that of the oxy-coal with 30% O2 and 70% CO2 in the oxidant stream, indicating that the rear condition is suitable for retrofitting an air-coal fired boiler to an oxy-coal one. The swirl burner with a specially designed preheating chamber can increase flame temperature, accelerate ignition and enhance burning intensity of pulverized coal under oxy-coal combustion. Also, qualitative experimental validation indicated the burner can reduce the overall NOx emission under certain O2 enrichment and oxy-coal combustion conditions against the air-coal combustion.

A transient mathematical model was developed to describe soot formation during the combustion of single coal particles based on the static semi-empirical model presented by Fletcher and coworkers. Sensitivity analyses of the model parameters show that soot emissivity and mass diffusivity of tar play an important role in predicting soot volume fraction (fv) and flame temperature (Tf). The model was applied to simulate the combustion of single bituminous coal particles with initial diameter (2r0) of 83 µm in a drop tube furnace and air atmosphere. It was found that soot is only formed within the first ∼ 5 ms after the appearance of the volatile flame. Although most of the soot is oxidized during the volatile flame phase, a small portion of soot still remains during the char combustion. Due to the soot presence, the volatile flame duration is extended by 2.6 ms. Compared with the soot-free flame, the sooting flame has remarkable lower Tf and its peak Tf value is ∼ 410 K lower. As a consequence, char combustion starts at a temperature that is ∼ 125 K lower than that of the soot-free case. Spatially, the peak fv at 16.6 ms appears at 4.5 r0 and soot oxidation zone spans to ∼ 10 r0. The model was validated by comparing the predicted Tf and fv under different O2 mole fractions (xO2) with recent experimental results reported by Khatami and coworkers. The predicted trends are consistent with those of the experimental results. With increasing xO2, Tf increases, but the increase rate becomes more gradual at a large xO2. While for fv, a non-monotonic variation is observed, where soot first increases and then decreases with a peak value occurring at xO2 ≈ 40%.

An experimental study on ignition and combustion of single particles was conducted at normal gravity (1-g) and microgravity (μ-g) for three high volatile coals with initial diameter of 1.5 and 2.0 mm, respectively. The non-intrusive twin-color pyrometry method was used to retrieve the surface temperature of the coal particle through processing the images taken by a color CCD camera. At the same time, a mathematical model considering thermal conduction inside the coal particle was developed to simulate the ignition process.Both experiments and modeling found that ignition occurred homogeneously at the beginning and then heterogeneously for the testing coal particles burning at μ-g. Experimental results confirmed that ignition temperature decreased with increasing volatile content and increasing particle size. However, contradicted to previous studies, this study found that for a given coal with certain particle size, ignition temperature was about 50–80 K lower at μ-g than that at 1-g.The model predictions agreed well with the μ-g experimental data on ignition temperature. The criterion that the temperature gradient in the space away from the particle surface equaled to zero was validated to determine the commence of homogeneous ignition. Thermal conduction inside the particle could have a noticeable effect for determining the ignition temperature. With the consideration of thermal conduction, the critical size for the phase transient from homogeneous to heterogeneous is about 700 μm at ambient temperature 1500 K and oxygen concentration 0.23.

The distributions of soot volume fraction, soot number density, and soot particle size in a coal-air turbulent jet flame was numerically investigated using a soot formation model developed in our previous study. In addition, the effect of radiation from different radiative media on the flame temperature was assessed and discussed. Validated by the reported experimental measurements, the distribution of soot volume fraction in the turbulent air-coal jet flame was well predicted. It was also found that soot particles were formed and accumulated in the high temperature zone with remarkable oxygen deficient. In the same time, the temperature of the coal-air jet flame was reduced remarkably by soot radiation but barely by soot formation. For the simulated flame, at a downstream location of 200 mm from the jet exit, the heat loss caused by the thermal radiation of coal particles, soot particles and gas species resulted in a temperature decrease of 262 K, 238 K and 102 K respectively. Different radiative media induced remarkably different distributions of absorption coefficient, and the emitting heat loss from the media was determined by the local absorption coefficient and temperature.