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Abstract: Particle clustering is an important phenomenon in dense particle–gas two-phase flow. One of the key problems worth studying is the reacting properties of particle clusters in coal particle combustion process in the dense particle region. In this paper, a two-dimensional mathematical model for the char cluster combustion in airflow field is established. This char cluster consists of several individual particles. The comprehensive model includes mass, momentum, and energy conservation equations for both gas and particle phases. Detailed results regarding velocity vector, mass component, and temperature distributions inside and around the cluster are obtained. The micro-scale mass and heat transfer occurred inside and around the char cluster are revealed. By contrastively studying the stable combustion of char particle clusters consisting of different particles, the combustion properties of char clusters in various particle concentrations are presented and discussed. [Copyright &y& Elsevier]

Abstract: An experimental study was performed on a 36 kWth down-fired furnace in order to obtain a wider and more detailed knowledge of the influence of the key parameters on the gaseous fuel reburning process when typical Chinese coals served as the primary fuel. The experimental results show that both above 50% nitric oxide reduction and low carbon loss can be obtained when reburn zone residence time is about 0.6 s–0.9 s, gaseous reburn fuel percent is about 10%–15% and average excess air coefficient of reburn zone is about 0.8–0.9. According to the optimizational reburning operating parameters, a 350 MWe coal-fired boiler was retrofitted for reburning application. The retrofitted boiler had about 0.7 s reburn zone residence time and about 2 s residence time after the burnout air was injected. The results of the demonstration on the full-scale boiler showed that the nitric oxide emission was about 235 mg/Nm3 (O2 =3%, dry) from the retrofitted boiler and above 60% nitric oxide reduction efficiency could be achieved with no adverse effects on the boiler operability when the coke-oven gas used as the reburn fuel. This successful demonstration project filled the gaps in the gaseous fuel reburning application on 300 MWe grade unit boiler in China and showed that the values of the optimizational reburning parameters obtained from experiments could be successfully applied to the designing, retrofitting and optimizing for the gaseous fuel reburning process. [Copyright &y& Elsevier]

• The addition of inorganic sodium inhibits the OH generation and ignition of coal. • High sodium coal exhibits lower PAHs and soot generation during coal early stage combustion. • Pathways of cleavage of coal matrix affected by sodium are studied by DFT. • Sodium in the coal increases the energy barrier of cleavage of coal matrix in the early stage of coal combustion. Inorganic sodium in low rank coal exerts significant influence on the soot generation in the early stage of combustion. The effect of inorganic sodium on the PAHs, soot formation and OH generation of coal particle was investigated on a concentrating light heating platform using Planar Laser Induced Fluorescence (PLIF) of OH radicals and PAHs. The generation characteristics of PAHs during coal particles pyrolysis with the temperature of 500 °C and 800 °C were also investigated. Raw coal, acid-washed (AW) coal and AW impregnated with different mass ratio of NaCl (1 %, 3 % and 5 % NaCl-loaded) were studied. Theoretical calculations using density functional theory was performed to explore the effect of sodium on the conversion of small molecule to macromolecular substances during early coal combustion. The results showed that the NaCl addition inhibited the OH radical and soot generation in the early stage of coal combustion, as demonstrated by the reduced OH intensity, and decreased soot intensity by observing the flame luminosity for the NaCl-loaded coals, compared with AW coal. Simultaneously, the PAHs generation was significantly reduced for NaCl-loaded coals, both at the early stage of coal combustion and under pyrolysis conditions, compared with AW coal. The quantum chemistry calculation revealed that the formed C-Na structure enhanced coal structural stability, making the chemical bonds less prone to cleavage, thereby inhibiting the volatile and PAHs release during the early stage of coal combustion. On the other hand, the released Na+ can catalyze the addition reactions of small ring PAHs (naphthalene, naphthalene derivatives) to larger ring PAHs (phenanthrene, pyrene), which would promote the PAHs and soot generation in homogeneous reactions, explaining the highest PAHs with 3-to-5 rings for 5 % NaCl-loaded coal among NaCl-loaded coals. The above two kings of reaction pathways were related to soot formation, but considering the results of reduced soot yield after NaCl addition, the former dominated. Furthermore, the results from the two pathways also illustrated that the Na inhibit the OH formation, which was corresponding to the decreased OH intensity for NaCl-loaded coals, as detected by OH-PLIF, and thus was detrimental to soot consumption. Therefore, the presence of Na in the early stage of combustion inhibited the release of volatiles and PAHs, which was the main reason for the decreased soot generation amount of NaCl-loaded coal. [ABSTRACT FROM AUTHOR]

Co-pyrolysis of coal slime (CS) and sewage sludge (SS) is a promising technology to achieve waste reduction and recovery of value-added products. For the first time, pyrolysis behavior, kinetics, thermodynamics, dendrite neural network modelling, and evolved char structure were examined in the co-pyrolysis of CS and SS. The results showed that blending CS and SS at a weight ratio of 6:4 yielded a higher synergy in relation to the obtained kinetic and thermodynamic parameters. At lower temperatures, the active hydroxyl radicals released from SS pyrolyzed volatiles accelerate the dehydrogenation reactions of CS. The interaction between oxygen radicals, such as ether bonds in SS volatiles, and aromatics or hydrocarbon compounds in CS char increases the content of oxygenated compounds in the pyrolytic char by significantly enhancing its reactivity. As the temperature rises, the demethylation reaction of aliphatic hydrocarbons in SS volatiles generates a considerable amount of hydrogen radicals. This intensifies the formation of small-ring aromatic compounds during co-pyrolysis, further facilitating the condensation of small-ring aromatics into larger-ring aromatics at higher temperatures. In addition, a novel dendrite neural network model was successfully applied to predict the co-pyrolysis behavior of CS and SS, whereby the transfer function of interaction was obtained. [Display omitted] • The addition of SS enhances the pyrolytic reactivity of CS. • CS and SS blend at 6:4 yielded the highest synergetic effect than other ratios. • Interaction increased small aromatic rings and O-containing functional groups in co-pyrolysis char. • The interaction raised the formation of ethers and π-type oxygenated radicals in co-pyrolysis char. • Dendrite Net is, for the first time, applied for the co-pyrolysis process. [ABSTRACT FROM AUTHOR]

Coal Spontaneous Combustion (CSC) is one of the most important natural disasters in coal mining safety production. To deeply restore the real state of CSC left in goaf during the face mining of coal mines, the experiments are carried out to simulate the oxidation characteristics of coal left in goaf under the coupling of the stresses (0 MPa, 4 MPa, 8 MPa) and the pre-oxidation temperatures (25 °C, 70 °C, 120 °C, 180 °C) employing the load-pressure-based device equipped with temperature-programmed. The oxidation characteristic parameter of CO gas generation, the O 2 consumption rate, the apparent activation energy (E a), and the critical temperature(T 1) are calculated. It is found that both the pre-oxidation temperature and the stress have a promotion range for coal spontaneous combustion, and although they inhibited each other beyond the promotion range, they still show a promotion result. The pre-oxidation temperature and stress mainly affect the content of oxygen-containing functional groups and the permeability, thus influencing the process of coal spontaneous combustion. Based on the engineering application, a cusp catastrophe model is established to predict the catastrophic temperature of coal spontaneous combustion, and it is verified that the catastrophic temperature could correctly characterize the transition of coal spontaneous combustion, and it is found that the catastrophic temperature lagged behind the critical temperature, which provides sufficient time and space for the prevention and control of coal spontaneous combustion. • The pre-oxidation and stress display highly nonlinear effect for the coal spontaneous combustion (CSC). • Stress and pre-oxidation affect the oxygen-containing functional groups and permeability. • The catastrophic temperature lags relatively behind the critical temperature. • The more energy accumulation is required for the occurrence of catastrophe in CSC. [ABSTRACT FROM AUTHOR]

Abstract: When predicting the variation of pore structure during O2/CO2 combustion of coal chars using the random pore model (RPM), it is impossible to calculate exactly the structure parameter ψ from the pore characteristics. The values of structure parameter ψ, which were calculated based on its fractal feature at various carbon conversions, should be almost constant. However, this investigation exhibited a drastic increase of ψ at the end of combustion reaction. In this work, structure parameter ψ of the RPM was modified according to the experimental analysis and a new model, fractal random pore model (FRPM), was constructed. Compared with other models such as RPM, discrete random pore model (DRPM), the Struis model (Model I) and the Liu model (Model II), it was found that fractal random pore model was more accurate to describe coal chars combustion, especially at higher conversions. Using the FRPM, O2/CO2 isotherm combustion of coal chars were analyzed at different temperatures. [ABSTRACT FROM AUTHOR]

Abstract: To estimate the unmeasured parameter from experts and running data, in this paper, a novel method named “immune genetic algorithm-based adaptive evidential classification rule (IGA-EC)” was proposed. The IGA-EC model was realized by two strategies: (1) a new parametric distance metric was applied instead of Euclidean distance to enhance the robust adaptive ability of the traditional evidence-theoretic classification rule; and (2) the powerful evolutionary algorithm immune genetic algorithm was used to parallel search the global optimal solutions of the parameters involved in the proposed model. To validate IGA-EC model, some experiments were conducted based on some popular data sets, and the experimental results show that the proposed method was powerful with respect to the accuracy. Finally, the IGA-EC model was used to estimate the unmeasured parameter level of coal powder filling in the ball mill in power plant. From the analysis of the estimating results, it suggests that the proposed method was applicable for estimating the level of coal powder, and the proposed method can also be applied for estimating other unmeasured parameters in industry. [Copyright &y& Elsevier]

Coal is naturally heterogeneous on chemical structure, which significantly affects its pyrolysis behaviours. However, due to the limit of characterization tools, quantification of the chemical structural heterogeneity and its effects on pyrolysis are still challenging. In this study, a quantification method for chemical structural heterogeneity by Raman mapping technique was firstly proposed, and the heterogeneous chemical structure of coal was investigated. The results show that the distributions of the chemical structure are highly uneven, even though the average structural characteristics are highly similar. Besides, the effects of chemical structural heterogeneity on pyrolysis behaviours were studied. It is found that except for the average structural properties, the chemical structural heterogeneity affects the pyrolysis behaviours significantly. It is attributed to the different pyrolysis mechanisms and uneven distribution of these chemical structures with different reactivity. The effects of the average chemical structures and the chemical structural heterogeneity on the pyrolysis behaviours are independent. Furthermore, new predicting models for pyrolysis behaviours by considering both the average structure and the chemical structural heterogeneity were established and validated. The final models were proved to be adequate to effectively predict the pyrolysis behaviours of raw and blended coals. [Display omitted] • Coal chemical structural heterogeneity was quantified by Raman mapping. • 3 kinds of structures varied in reactivity were identified by chemical distribution. • Effects of chemical structural heterogeneity on pyrolysis behaviours were revealed. • Predicting models of pyrolysis behaviours considering heterogeneity were built up. [ABSTRACT FROM AUTHOR]

• The blending of spent SCR catalyst with coal was proposed. • Spent catalyst blended with coal could promote coal combustion. • The V and As could well solidify in the bottom ash. • The leaching of V and As in bottom ash was very low. The spent SCR catalyst as the hazardous waste would seriously affect the ecological environment and human life. In this study, a new method of spent SCR catalyst harmless treatment was proposed by blending the spent SCR catalyst with coal for combustion. The combustion characteristics of coal blending with spent SCR catalyst, melting solidification characteristics of V/As in spent catalyst, and leaching characteristics of V/As in bottom ash were explored by TG, ICP-MS, XRD, SEM analysis. The results show that the proper amount of spent catalyst blending with coal could promote coal combustion by reducing the ignition and burnout temperature of coal particles due to the catalytic effects of TiO 2. The co-combustion of spent SCR catalyst with coal could well solidify the V/As in the bottom ash. In this process, the solidification rate of V decreased but the solidification rate of As increased with the increasing of temperature. Both of V and As solidification rate decreased with the increasing of spent catalyst blending ratios. The leaching concentration and leaching rate of V and As in the bottom ash were lower than the emission standards. The leaching concentration of As and V of the bottom ash significantly decreased due to melting of bottom ash when the temperature reached to 1300 °C. When the blending ration of catalyst blending amount increased, the leaching concentration and leaching rate of V and As increased slightly due to the decreasing of the relative amount of CaO in bottom ash, which played an important roles in the solidification of V and As. [ABSTRACT FROM AUTHOR]

Structures and combustion characteristics of 32 kinds of Chinese coals were studied by Micro-Raman spectroscopy and thermal gravimetric analyzer. Changes in coal structures with coalification were investigated by detailed curve-fitting the Raman spectrum with ten Gaussian bands. The relationships between the Raman spectral parameters and coal combustion characteristics were set up and evaluated. The results indicate that the loss of aromatic substituents or aliphatic structures can be responsible for the decrease of V daf for low rank coals (volatiles content in dry ash-free basis (V daf ) > 25%), while the rapid growth of aromatic rings along with the increase of cross-linking density of coals mainly occurs under coalification for relative high rank coal (V daf  > 25%). Besides, C   O structures in coal increase monotonously with the increase of V daf . The condensation of aromatic rings, loss of C   O structures and reduction of “impurity” structures among large aromatic rings all can increase the coal combustion characteristic temperatures. Reasonable correlations between the coal combustion characteristic temperatures: T i , T m , T b and Raman spectral parameters: A (GR+VL+VR) /A D , A GL /A Total , A S /A D , A D /A Total have been found respectively, and the relationships are all better than that between T i , T m , T b and V daf . Particularly, the Raman spectral parameter A D /A Total is a combination of above key parameters and related to the coal ignition temperature best with the R-square higher than 0.9. A D /A Total can act as a good indicator for coal combustion characteristics. This study directly demonstrates that Raman spectroscopy can play a probe not only for coal ranks/structures but also coal combustion characteristics and it can provide a new approach to rapidly predict the coal properties. [ABSTRACT FROM AUTHOR]

• Interaction between the sewage sludge and coal slime during their co-combustion was found. • SO 2 formation and release mechanism during co-combustion of sewage sludge and coal slime was revealed. • AAEMs play a key role on the evolution of S-functional groups. The production of sewage sludge (SS) and coal slime (CS) is increasing year by year, which affects the ecological environment and human daily life, and it is urgent to deal with them reasonably. This study investigated the SO 2 release characteristics and the evolution of S functional groups and minerals during the combustion of SS, CS and their blends. The results showed that the co-combustion of SS and CS had an interaction on desulfurization. When the temperature was 800 ℃ and the blend ratio of SS was 20%, it was most beneficial to suppress the emission of SO 2. The types of S functional groups in SS and CS were similar but the relative content was different. The evolution of some S functional groups showed various laws. In the early stage of co-combustion, Sulfate (S1) reached maximum value earlier and decreased at the end of co-combustion, and Sulfide (S5) were more likely to be converted to aromatic-S (S4). According to XRD and SEM-EDS results, Ca (mainly CaCO 3) in the fuels had a significant effect on SO 2 release characteristics during the co-combustion. CaO formed by thermal decomposition of CaCO 3 could react with SO 2 to form intermediate solid products (CaS and CaSO 3). These intermediates further reacted with O 2 to generate CaSO 4 , thus retaining SO 2 in the ash. Some CaSO 4 was decomposed in the later stage of combustion, leading the re-release of SO 2. Excessive combustion temperature and reductive gases would lead to the decomposition of CaSO 4 , which reduced the ability of Ca to absorb SO 2. [ABSTRACT FROM AUTHOR]

The devolatilization and volatile combustion of lignite coal (LC), sub-bituminous coal (SBC) and bituminous coal (BC) with different O 2 concentrations (21%-50%) were investigated on a concentrating photothermal experiment platform. The characteristics and mechanism of different coal particle combustion were analyzed by using multiple diagnostics, including Planar Laser Induced Fluorescence (PLIF) of OH radicals and polycyclic aromatic hydrocarbons (PAHs), high-speed cinematography and FT-IR. The results showed that, with increasing O 2 concentration, the ignition delay time (t ign) of different ranks of coal decreased, which was associated with the increase of OH concentrations during the ignition stage. The good correlation between t ign and OH concentration was observed. For the volatile combustion, the OH concentration, volatile release amount and volatile flame duration time (t vol) significantly increased with the increasing of O 2 concentration at low O 2 concentrations. In the higher O 2 concentrations, coals with different PAHs generation amount responded differently on the increasing of O 2 concentration. On one hand, for the coals with lower PAHs generation amount, the OH concentration increased with the increasing of O 2 concentration, while the t vol was not sensitive to changes of the O 2 concentration. On the other hand, for the coals with higher PAHs generation amount, the OH concentration decreased with the increasing of O 2 concentration while the t vol increased with the increasing of O 2 concentration. It can be concluded that PAHs inhibited the OH generation, and the generation characteristics of OH and PAHs affected the sustained state of volatile combustion. The higher PAHs concentration consumed higher amount of OH during the volatile combustion. Meanwhile, with the increasing of PAHs concentration, the reaction rate of H + O 2 +M → HO 2 +M in which PAHs participate was increased, inhibiting the reaction H + O 2 → OH+O , and the increasing of polymer soot of PAHs will reduce the flame temperature through thermal radiation, which further inhibited the OH formation. [ABSTRACT FROM AUTHOR]

• Inorganic sodium inhibits coal combustion in early stage under fast-heating rate. • Ignition delay time of NaCl-loaded coal increases visibly under fast-heating rate. • Evolution pathways of condensation reactions catalyzed by Na are studied by DFT. • Na in studied coal reduces energy barrier of condensation reactions in early stage. The combustion characteristics of raw (RAW), acid-washed (AW) and inorganic sodium impregnated (NaCl-loaded) Zhundong (ZD) coals were researched in a fast-heating rate combustion system and thermal gravimetric (TG) analyzer, aiming to exhibit the effect of inorganic sodium on coal combustion processes under different heating rates. The results showed that the ignition delay time decreased in the order: NaCl-loaded coal > AW coal > RAW coal in the early stage with fast-heating rate. The surface temperature of NaCl-loaded coal increased the slowest, while with the highest T is , reflecting that the ignition and combustion behavior became more difficult after adding NaCl in the early stage. However, in the later stage, the fastest increase in temperature and the shortest t b , together with the shorter extinction time of volatile flame, were observed on NaCl-loaded coal. The size of NaCl-loaded coal decreased faster than that of AW coal. Thus, it can be concluded that inorganic sodium will inhibit coal combustion in the early stage, while promotes coal combustion in the later stage under fast-heating rate. In the TG experiment, the T i-TG and T b-TG of NaCl-loaded coal were lower than that of AW coal, and the T m-TG of NaCl-loaded coal moved to the low temperature area, revealing that inorganic sodium can promote coal combustion with low-heating rate during the whole combustion process. The quantum chemistry calculation at M06-2X/6-31G(d,p)//CBS-QB3 level reveals that inorganic sodium can catalyze the addition and cyclization reactions of small chain molecules in the early stage with fast-heating rate, thereby inhibiting coal combustion in the early stage, which was different from that in the later stage. Inorganic sodium will promote the formation of OH radical, and the organic sodium obtained from sodium-containing surface compounds of the char can be beneficial for the oxidation reaction of the carbon residue, thereby promoting coal combustion in the later stage with fast-heating rate, which was consistent with the references. [ABSTRACT FROM AUTHOR]

This paper reports an experimental study to assess the effects of CO 2 and heating rate on the characteristics of the chars formed in CO 2 and N 2 atmospheres in the initial stage of oxy-fuel combustion. The chars of a Chinese bituminous coal were prepared in a fluidised-bed/fixed-bed reactor at 900 °C and their chemical structural features were further characterised with FT-Raman spectroscopy. Our results indicate that changes in CO 2 concentration influence not only the char yield but also the char chemical structural features, which become more condensed and increasingly oxygenated with increasing CO 2 concentration. The chars formed in CO 2 atmosphere at fast heating rates are less condensed but more oxygenated than those formed at a slow heating rate. The O-holding capacity of the fast heating char decreases with the aromatic condensation as the char is held in CO 2 atmosphere at 900 °C. [ABSTRACT FROM AUTHOR]

Char-bound nitrogen [char(N)] is confirmed to be the real intermediate in heterogeneous reduction during coal combustion. Aiming to have a deep insight into the interaction mechanism between NO and char(N) containing different functional forms of nitrogen, a comprehensive theoretical exploration with density functional theory at M06–2X/6–31G(d,p)//def-TZVP level is performed. The detailed electronic descriptions of char(N) surface based on spin density, Mulliken atomic charge and electrostatic potential exhibit the attack sites and electron donating capability during NO chemisorption in the following order: char(N-5) < char(N-X) < char(N-6). Oxygen surface complex, an intermediate for N 2 O formation and a catalyst for NO reduction, can weaken the connected C N bond and promote N 2 separation. The migration of oxygen atoms to adjacent active sites is thermodynamically conducive to reduction reactions, leading to the preferred pathway of N 2 release on char(N-5) and char(N-X) surfaces. Contiguous active sites remaining along char edge are unfavorable for N 2 O detaching, while beneficial to NO further chemisorption for consecutive reactions, resulting in the great contributions of char(N-5) in reducing NO. The lower energy barriers for NO reduction follow the sequence coincident with the results of electronic property analysis. The kinetically favorable temperatures and activation energies of different char(N) for reducing NO are determined, which is consistent with previous experiments. The theoretical results provide evidences to explain microscopic mechanism of NO-char(N) interaction, making contributions to effectively minimize NOx emissions in the future. [ABSTRACT FROM AUTHOR]

[Display omitted] • The furnace-side combustion process is simulated coupled with S-CO 2 heat transfer. • A new method to predict simulation results via RBF neural network is proposed. • 3D Eulerian-Lagrangian model computationally facilitated by the MP-PIC scheme. • Heat transfer characteristics of the novel S-CO 2 CFB boundary are studied. • Predicted and simulated results are compared to verify the neural network accuracy. The supercritical CO 2 (S-CO 2) power cycle has a wide application prospect in coal-fired power generation field because it's highly effective, compactly structured, and flexible of operation. To observe more accurate heat transfer and coal combustion characteristics in the circulating fluidized bed (CFB) with the distinctive S-CO 2 boundary, a 3D computational fluid dynamics (CFD) simulation of the furnace-side combustion process treated by the multiphase particle-in-cell (MP-PIC) method was conducted in a 600 MW S-CO 2 CFB boiler coupled with the heat transfer process on working fluid side based on the polynomial fitting calculation model. Furthermore, a novel method to predict simulation results via Radial Base Function (RBF) neural network was proposed to simplify the computational process, enhance the sample data fusion, and improve the prediction accuracy. Results show that staggered high-temperature fluid and high heat flux was a major concern in S-CO 2 heating surface arrangement. The temperature rise of wall heaters was less than the conventional steam CFB, and the heat flux of spiral and vertical heat transfer tubes decreased along the tube. The predicted temperature distribution of tubes and cold walls was in a good agreement with the coupling simulation results, whose accuracy can meet the engineering requirements. [ABSTRACT FROM AUTHOR]

• The co-combustion characteristics of sewage sludge (SS) and coal slime (CS) were investigated by thermogravimetric analyzer. • The interaction between SS and CS improved the ignition performance and burnout performance of the blends. • A synergy index was proposed to quantify the degree of synergy. • The reaction rate equations of SS-CS were obtain based on FWO and KAS methods combined with the master curve method. Based on the physical and chemical properties of municipal sewage sludge (SS) and coal slime (CS), the combustion behaviors of SS, CS and their blends were investigated by the thermogravimetric analyzer. The interaction between SS and CS was evaluated, and the combustion reaction rate equations were obtained. The results suggest that during the co-combustion, SS could improve the ignition performance, while CS could enhance the activity in the high temperature zone and improve the slagging characteristics. There was the most significant interaction in 550–600℃, which had a positive effect on the entire combustion process, and the synergy reached its peak when the CS ratio was 40% with synergy index SI=1.53. The combustion of the blend was a multi-step process, which conformed to the n th-order model, and the addition of CS could reduce the apparent activation energy of SS during the entire combustion process. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: The Fe2O3–SiO2 composite (FS) with high specific surface area and pore volume was successfully synthesized using sol–gel method. BET surface area analysis, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), thermal gravity analysis (TGA), temperature programmed desorption (TPD) were used to characterize the catalyst. The performance of prepared FS for elemental mercury removal was evaluated under simulated flue gas. This study identified the effective temperature range (80–450°C) for elemental mercury removal, with the optimal temperature of 350°C. A high SO2 (>1500ppm) concentration is not conducive to elemental mercury removal by FS catalyst. The removal mechanism has been discussed based on the experimental and analytical results. [Copyright &y& Elsevier]

Abstract: Recently the oxyfuel combustion of coal chars having a significant impact on reducing greenhouse emissions is gradually paid extensive attention by many researchers, but only a limited number of studies have focused on its reaction mechanism. Therefore, it is important to investigate the combustion mechanism of coal chars in oxyfuel atmosphere, while random pore model (RPM) is usually recommended as a model for the comprehensive simulation of coal chars reaction. In this context, the values of structure parameter ψ in RPM were calculated based on pore structural character at various carbon conversions, and show interesting evolution phenomena keeping constant at the preceding reaction stage before increasing remarkably at the end of stage. Consequently, a new model, two-stage random pore model (TRPM), was applied to the coal chars combustion in oxyfuel atmosphere. Compared to other models such as RPM, the Struis model (Model I), the Liu model (Model II), and fractal random pore model (FRPM), it shows that two-stage random pore model was more accurate to describe coal chars combustion under oxyfuel conditions, especially at higher carbon conversions. In addition, the oxyfuel combustion process of coal chars at 1323K and 1373K were analyzed. [Copyright &y& Elsevier]

Abstract: Mercury emission from coal combustion is an increasing environmental concern due to its high volatility and toxicity, and activated carbon (AC) adsorption has been proven an effective mercury-control method, with high-cost limit. The renewable bioresource of bamboo constitutes an important precursor for activated carbon, and the bamboo charcoal (BC) may act as low-cost sorbent used in the mercury-control. The adsorptive potential of BC and modified BC using H2O2 for elemental mercury was investigated for the first time through a parametric study conducted with a bench-scale bed. The effects of pore structure and surface chemistry were investigated based on BET, XPS. Which suggest that BC materials have excellent adsorption potential for elemental mercury, especially after modified by H2O2. The modification using H2O2 altered the physical and chemical properties of BC materials, making the sorbents more effective in mercury adsorption even at a relative higher temperature, and the enhancing-effect was more obvious with increasing H2O2. [Copyright &y& Elsevier]

• A concentrating photothermal reactor at heating rate of 4800 °C/min was conducted. • Combustion characteristics in O 2 /N 2 , O 2 /CO 2 and O 2 /H 2 O at fast heating rate were studied. • Faster consumption rates of functional groups and higher condensed char in O 2 /H 2 O were evidenced. • Second-order function relationship expressions between R and I (Gr+Vl+Vr) /I D were observed. To comprehensively investigate the combustion characteristics and structural evolutions of Zhundong coal pellet at fast heating rate in O 2 /N 2 , O 2 /CO 2 and O 2 /H 2 O atmospheres, the real-time mass loss, temperature and combustion images were directly obtained on a novel concentrating photothermal reactor with heating rate of 4800 °C/min. The structural features of the coal/char with different holding times were analyzed by FT-IR and Raman. The experimental results showed that the maximum temperature and the maximum mass loss rate of coal pellet increased with the increasing of H 2 O concentration but decreased with the increasing of CO 2 concentration. Under the experimental condition with the fast heating rate, CO 2 prolonged while H 2 O shortened the ignition delay of coal pellet. In the initial stage of coal combustion, faster consumption rates of all functional groups were evidenced in O 2 /H 2 O while slower consumption rates were evidenced in O 2 /CO 2 compared with those in O 2 /N 2. Raman band area ratio I (Gr+Vl+Vr) /I D of chars also exhibited that the less condensed char was observed in O 2 /CO 2 but more condensed char was obtained in O 2 /H 2 O. Such results indicated that the coal conversion was slowed down with the addition of CO 2 but accelerated with the addition of H 2 O in the initial stage at fast heating rate. The specific reaction rates were reasonably characterized by the Raman band area ratio of char in O 2 /N 2 , O 2 /CO 2 and O 2 /H 2 O atmospheres, and they were accorded with the second-order function relationship expressions. [ABSTRACT FROM AUTHOR]

[Display omitted] • Raman mapping technology was developed for char structure characterization. • More than 150 individual particles in micro-scale were studied for each sample. • H/C atom ratio of individual char particle in micro-scale was quantified. • Structure evolution characteristics during Zhundong coal pyrolysis was revealed. Heterogeneity of coal in micro-scale is significant and can affect coal thermal behavior. In this study, micro-Raman spectroscopy was developed for characterizing coal/char in the scale of micro- and bulk with a case study of Zhundong coal pyrolysis. More than 150 particles in micro-scale were analyzed for each sample by micro-Raman mapping technology controlled by a computer program. Individual Raman spectrum of each particles in micro-scale and average Raman spectrum for each sample were analyzed, respectively. The results indicate that the evolution characteristics of aromatic rings, C–H in methyl or methylene, and C = O groups can be revealed by combined deconvolution of first- and second-order Raman spectrum. The fluorescence interference in Raman spectrum defined as drift coefficient α can quantitate volatiles content and H/C atom ratio of individual coal/char particle in micro-scale. The results reveal that the chemical structures of the Zhundong coal in micro-scale are significantly heterogeneous. The structural evolution of char particle in micro-scale differs from each other, especially in the early stage of the Zhundong coal pyrolysis. The chemical structure of char particles in micro-scale tends to converge as the pyrolysis progresses. A more severe pyrolysis condition can generally lead to a char with higher heterogeneity. For the bulk structural evolution, the Zhundong coal pyrolysis can be mainly divided into two stages. Firstly, the C–H in methyl or methylene releases rapidly, and the relative amount of small aromatic rings and C = O groups decrease mildly. And then, the aromatization degree of char increases rapidly along with the significant release of C = O groups. [ABSTRACT FROM AUTHOR]

Revealing the pyrolysis reaction pathway related to coal structure evolution under different temperatures is important for the industry to design reactors and process operations. In this study, all the pyrolysis products obtained under different pyrolysis temperatures were comprehensively analyzed by multiple techniques. The results showed that the aromatic nucleus were inclined to react with methyl radicals, forming char with abundant methyl substitution aromatic structure at low pyrolysis temperature. At medium pyrolysis temperature, dramatically condensation reaction occurred, resulting in the formation of aromatic protonated carbon and abundant polycyclic aromatic hydrocarbons (PAHs) containing benzenoid rings joined by a carbon‑carbon single bond remaining in char. The concentration of aliphatic hydrocarbons in light tar decreased dramatically and converted into light hydrocarbon gas compounds (especially H 2 and CH 4). At high pyrolysis temperature, severe carbonation of char led to the formation of abundant PAHs with reduced aliphatic side chain in light tar. Tar precursors preferred to form larger PAHs due to occurrence of drastic secondary reaction. The contents and diversity of oxygen−/nitrogen-containing heteroatom compounds in heavy tar were significantly reduced with the increase of temperature. Finally, the devolatilization mechanism responsible for the evolution of chemical structure of Shenfu coal pyrolyzed at elevated temperatures was proposed. Unlabelled Image • Low temperature leads to char with abundant methyl substitution aromatic structure. • Medium temperature results in aromatic protonated carbon and single-bond PAHs in char. • Tar precursors prefer to form larger PAHs at high pyrolysis temperature. • Contents and diversity of heteroatom compounds in heavy tar decreased with elevated temperature. • Evolution pathways of coal chemical structure at elevated temperature was proposed. [ABSTRACT FROM AUTHOR]

Ignition of large size coal was the basis of the design and application of Fluidized Bed (FB) and Circulating Fluidized Bed (CFB) combustion technologies. The strongly coupled gas-phase and solid-phase temperature fields in existing experimental apparatus provoked obstacles for the determination of the influence of volumetric reactions on large size coal ignition. In this study, a gas-phase temperature adjustable concentrating photothermal reactor (GTA-CPR) was developed, and the gas-phase temperature could be effectively regulated by the preheated gas flow while the solid-phase temperature was determined by the irradiation voltage. 25 experimental conditions corresponded to 5 levels of solid-phase temperatures with 5 levels of gas-phase temperatures. Enhanced volumetric reactions (by higher gas-phase temperature) led to significant decrease in ignition delay (≥50%) and prolongation of flame duration (≥1.1 times). Real-time CH 4 concentration in volumetric reaction boundary was detected with Mass Spectrometry (MS), which exhibited double peaks. The first CH 4 peak height decreased dramatically while the second CH 4 peak height increased with the gas-phase temperature, indicating the advanced ignition and enhanced volumetric reactions. Ultimately, an ignition delay prediction model was proposed through Backward induction and the indispensable importance of volumetric reactions to coal ignition was evidenced. • Ignition of large size coal were experimentally investigated in the GTA-CPR. • Independent regulation of gas-phase and solid-phase temperature was realized. • Volumetric reactions significantly reduced ignition delay and enhanced flame intensity. • The 1st CH 4 peak decreased, the 2nd CH 4 peak increased with gas-phase temperature. • Ignition delay prediction model with form of τ i = a × (f(T) - b) -c was proposed. [ABSTRACT FROM AUTHOR]

Supercritical carbon dioxide (S–CO 2) coal-fired power generation technology is considered as the transformative technology in the energy sector due to its high efficiency, simple turbomachinery and high power density though the concept design of S–CO 2 boiler is still in the infancy stage. This study constructs a modified combustion and heat transfer thermodynamic calculation method of the S–CO 2 boiler adapted for multi-zone furnace. The effects of parameters and configurations on the characteristics of the S–CO 2 boiler are compared and analyzed. The results show that the increase in the separated over-fired air ratio can effectively equalize the heat duty distribution. And the increasing of the furnace wall temperature will affect the furnace radiation heat transfer performance, which increases the flue gas average temperature. The boiler configurations including the flue gas recirculation, double furnace strategy, thermal insulation strategy and boiler local expansion strategy have the positive effects on decreasing the furnace heat duty to ensure the boiler safety. Finally, the novel S–CO 2 boiler configurations are proposed and the comprehensive evaluations are conducted on the boiler combustion, heat transfer and burnout characteristics, which show that the novel S–CO 2 boiler performance is promising and can provide a basis for the design of S–CO 2 boiler. • Modified thermodynamic calculation method of S–CO 2 boiler is constructed. • Effects of parameters and configurations on boiler characteristics are analyzed. • Combined strategies of FGR, DBS, TIS, BLES are for lowering the furnace heat duty. • Novel S–CO 2 boiler configuration adapted for S–CO 2 cycle is proposed and evaluated. [ABSTRACT FROM AUTHOR]

Comparative combustion studies were performed in a concentrating photothermal platform (room temperature chamber type experimental bench with less volumetric reactions) on coal pellets (>6mm) from three different ranks: bituminous coal, sub-bituminous coal, and lignite. The combustion of single coal pellet in 21 ~ 50% oxygen concentration was observed with IR pyrometry and high-speed cinematography to obtain temperature–time–size histories. Based on the combined mass measurement and gaseous product analysis, a comprehensive understanding of the effects of oxygen concentration on single coal pellet combustion behaviours was developed. Distinctive three-stage processes were proposed with characteristical temperature and cinematography variations. No obvious flame duration was detected in 21% or 30% O 2 while transient but luminous flames were clearly observed in 40% and 50% O 2 and formed notable temperature peaks in the second stage during combustion of all the three ranks coal pellets. Volume and density both changed non-linearly during combustion and such processes were accelerated with the increase of oxygen concentration. High concentrations of CO and CH 4 were detected due to the suppression of volumetric reactions. CO accounted for about 9.3% of the gaseous products and consistently exhibited two peaks with the first peak independent from oxygen concentration. Three-stage combustion process was further validated by CO 2 /CO ratio and the effective reaction orders of oxygen concentration were 1.91 (bituminous coal), 1.81 (sub-bituminous coal) and 1.26 (lignite). [ABSTRACT FROM AUTHOR]