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Abstract The combustion kinetics of char-O 2 /H 2 O reactions under high-temperature entrained flow conditions involving char-H 2 O and char-O 2 reactions were investigated experimentally and numerically. A numerical model was established based on the unsteady convection-diffusion equations, and the random pore model (RPM) was adopted locally to predict the change in intraparticle pore surface area. The Langmuir-Hinshelwood form rate equation was used to consider the possible competitive relationship between char-O 2 and char-H 2 O reactions, and the most probable intrinsic kinetics parameters were found by checking the correlation coefficient between the experimental and numerical results. For H 2 O gasification, the numerical results show that the enrichment of H 2 inside char particles decreases the intrinsic char-H 2 O gasification rate, especially at the outer layer regions. For O 2 /H 2 O combustion, the experimental results show that H 2 O has a promotion effect on the total carbon conversion at low SRs (stoichiometric ratio of O 2 to fuel) but has an inhibition effect on the total carbon conversion at high SRs. Using the common active sites assumption, the numerical results match well with the experimental results. According to the numerical results, H 2 O penetrates deeper inside the char particle than does O 2 , contributing more to the local carbon conversion at the inner layer region, while H 2 O inhibits the char-O 2 combustion rate at the outer layer regions due to H 2 formation. As a result, H 2 O pushes the kinetics-diffusion-controlled reaction towards the kinetics-controlled regime. As the SR increases, the char-O 2 combustion contributes more, magnifying the inhibition effects of H 2 O on the char-O 2 combustion rate; therefore, the total O 2 /H 2 O combustion is gradually inhibited by H 2 O. [ABSTRACT FROM AUTHOR]

Shenhua (SH) demineralized char was employed to determine the relationship between the char conversion ratio and the surface behavior of particles. The amount, distribution and thermal stability of C(O) generated during the combustion process were clarified by temperature-programmed desorption (TPD) and Fourier transform infrared spectroscopy (FTIR). CO 2 was the primary gaseous product released during the TPD process of each partially oxidized sample. Based on the TPD and FTIR deconvolution results, the decomposition temperature of each functional group could be determined as follows: phenol (1000 K) < carboxyl (1150 K) < ether/anhydride (1400 K) < quinone (1600 K) < lactone (1650 K). Partial oxidative treatment promoted the generation of C(O) on the particle surface, with the maximum amount of C(O) existing on the surface of samples with an intermediate conversion ratio (0.33–0.47). Due to the increase in C(O), the reducibility of the partially oxidized sample was also enhanced. The demineralized chars collected in the intermediate stage (0.33–0.47) expressed the strongest reducibility, and the maximum NO was consumed by per unit mass of these samples. Phenol, carboxyl, anhydride and ether were the main forms of C(O) and were involved in the rapid and successive consumption of NO as major reactants, leading to an apparent reduction of the NO released during the char combustion process. [ABSTRACT FROM AUTHOR]

In actual combustion facilities, coal chars are often generated using a variety of pyrolysis processes, such as secondary pyrolysis, which is characterized by a long residence time in high temperature zone. The effects of such processes on the conversion of char N to NO during combustion have seldom been explored. In this study, the releases of NO during the combustion of coal chars obtained from different pyrolysis processes in a drop tube and in a fixed bed reactor were investigated. In addition, the extent of char N/NO conversion was studied in relation to the char reactivity, pore surface structure and carbon conversion in a horizontal tube furnace. The results show that, compared with chars generated by a single pyrolysis, chars treated by a subsequent secondary pyrolysis process exhibit larger pore surface areas but less reactivity because of the thermal annealing resulting from a longer thermal history. Chars with higher intrinsic reactivity were also found to release a lower amount of NO. However, a weak correlation was identified between the apparent reactivity and char N/NO conversion, indicating that intrinsic reactivity is more important and directly determines the NO reduction process under combustion conditions. Moreover, char N/NO conversion was significantly affected by the coal rank, and a greater extent of conversion of char N to NO was observed in the case of high-rank coal chars. At a high combustion temperature (1373 K), variations in the bulk O 2 concentration had little effect on the char N/NO conversion, and an apparent correlation was found between the extents of char N/NO conversion and the accessible pore surface area. These results indicate that at high temperatures, the char N/NO conversion is directly determined by the accessible pore surface area due to transportation limitations. The NO/(CO + CO 2 ) ratio increased with increasing burn-off in the latter stages of char conversion, which can be attributed to decreases in both the BET surface area and accessible pore surface area available for NO reduction during combustion. [ABSTRACT FROM AUTHOR]

Two typical types of bituminous coal (JN and SH) were employed for thorough devolatilization, then the samples were functionalized by O 2 at 1073 K to different conversion ratios (JN: 0.21, 0.34, 0.40, 0.53 and 0.63; SH: 0.15, 0.22, 0.32, 0.42 and 0.52) before the surface oxygen containing complexes C(O) were qualitatively and semi-quantitatively characterized by Fourier transform infrared spectroscopy (FT-IR) and temperature programmed desorption (TPD). A deconvolution method was applied to analyze the C(O) adsorption in the spectral region of 1800–1000 cm −1 in the FT-IR spectra, and the results indicated that the type and thermal stability of these functional groups were as follows: carboxylic (1250 K) < phenol (1350 K) < ether/anhydride (1550 K) < lactone/quinone (1650 K). The TPD results demonstrated that more active sites were generated on the particle surface after the oxidative treatment under O 2 atmosphere, especially the samples with low conversion degree (J2, S2, and S3), coal type and conversion ratio had small effects on the ratios of metastable stable complexes to stable complexes C wea (O)/C str (O) under the experimental conditions. The oxidized char samples were significantly more reactive than the raw char, and per unit mass of each oxidized sample could reduce more NO than raw char during the temperature programmed reduction (TPR) process, and phenol, ether and anhydride were the main reactants participated in the NO reduction reaction under high-temperature condition (1173–1600 K). [ABSTRACT FROM AUTHOR]

Elevated emissions of hydrogen chloride (HCl) from combustion of biomass in utility boilers is a major issue as it can cause corrosion and, in combination with the high alkali content often encountered in these fuels, it can also deposit molten alkali chloride salts on the boiler’s water tubes. Such deposition can impede heat transfer and cause further corrosion. This work torrefied and then burned herbaceous biomass (corn straw) as well as crop-derived biomass (olive residue and corn-based Distillers Dried Grains with Solubles, DDGS), all pulverized in the size range of 75–150 µm. It monitored the HCl emissions from torrefaction of biomass and, subsequently, the comparative HCl emissions from combustion of both raw and torrefied biomass. Results showed that during torrefaction most of the chlorine of biomass was released in the gas phase, predominately as HCl. Consequentially, combustion of torrefied biomass, which contained less chlorine than raw biomass, generated significantly lower HCl emissions than raw biomass, particularly so for biomass of low alkali content. This observation complements previous findings in this laboratory that torrefied biomass also generated lower SO 2 emissions than raw biomass, albeit by a smaller factor. Both of these findings enhance the appeal of torrefied biomass as a substitute fuel in utility boilers. [ABSTRACT FROM AUTHOR]

• The replacement of O 2 /Ar with O 2 /H 2 O is beneficial to the formation of NH 3. • C(O) facilitates the chemisorption of H 2 O on the surface of coal-N. • The conversion of HCN and the heterogeneous hydrogenation influenced NH 3 formation. • NH 3 emission followed the heterogeneous hydrogenation of coal-N in O 2 /Ar combustion. • The conversion of HCN is the major way for the emission of NH 3 in O 2 /H 2 O combustion. Understanding the formation mechanism of NH 3 during the pulverized coal combustion process is a feasible approach to reducing NOx emissions. In this study, the emission characteristics of NH 3 during the O 2 /Ar and O 2 /H 2 O combustion processes were investigated by the isothermal combustion experiments and the Density Functional Theory (DFT) method. The experimental results demonstrated that H 2 O accelerated the conversion of HCN to NH 3 , and the positive effects of H 2 O were applied to all kinds of coal-based fuels. The DFT calculations deeply revealed the formation mechanism of NH 3 through the homogeneous conversion of HCN and the heterogeneous hydrogenation of coal-N. The hydrogenation of coal-N is the primary source of NH 3 emissions during O 2 /Ar combustion. During O 2 /H 2 O combustion process, the heterogeneous generation of NH 3 was modified by the synergetic effects of O 2 and H 2 O, and the additional H 2 O strengthened the proportion of the homogeneous reaction. The homogeneous conversion of HCN was the main NH 3 formation pathway. This study is prone to coupling the experiments and quantum chemistry to deeply reveal the formation mechanisms of NH 3 during O 2 /Ar and O 2 /H 2 O combustion processes, providing a relatively complete pathway for the emissions of NH 3. [ABSTRACT FROM AUTHOR]

This work contrasted gaseous emissions of carbon, sulfur and nitrogen oxides from combustion of several types of biomass including woody, herbaceous and crop-derived wastes, pulverized in the size range of 75–150 μm. Both raw and torrefied biomass were exposed to high heating rates (10 4 –10 5 K/s) in a laboratory-scale electrically-heated drop-tube furnace, operated at 1400 K. Combustion occurred under fuel-lean conditions. Torrefied biomass has lower volatile matter content, higher fixed carbon content and higher heating value than raw biomass. Results revealed that (a) CO 2 emission factors from torrefied biomass were higher than those from raw biomass, reflecting the higher carbon content of the former, however there was no uniform trend in emission factors (kg/GJ); (b) SO 2 emission factors of torrefied biomass were lower than those from raw biomass, even if some torrefied biomass types contained higher sulfur mass fractions than their raw biomass precursors. All raw and torrefied biomass, with one exemption, generated lower SO 2 emission factors than a typical sub-bituminous coal; (c) torrefied biomass has higher fuel-nitrogen mass fractions than their raw biomass precursors; however, there was no clear trend in NO x emission factors between raw and torrefied biomass, as torrefied herbaceous and woody biomass types generated higher while torrefied crop biomass types generated lower emission factors than their raw biomass precursors. Comparing with the sub-bituminous coal, some raw and torrefied biomass types generated lower and some higher NO x emission factors. Overall, combustion of most types of biomass, either in raw or torrefied state, can result in lower SO 2 and NO x emissions factors than those of typical western US sub-bituminous coals while, in principle, generating minimal net emissions of carbon. [ABSTRACT FROM AUTHOR]

• Semichar preheated in CO 2 /Ar expressed higher NO initial reduction temperature. • Inert pretreatment was beneficial to arising the NO reduction index of semichar. • Occupation of active sites by oxygen atoms mildly inhibited the adsorption of NO. • Evolutionary pathways of C(O) in semichar affected the NO reduction reactivity. • The NO reduction pathways could be regulated by the preheating atmospheres. The extensive utilization of coal-based solid fuels in thermal power plants has led to massive emissions of greenhouse gases (GHG) and gaseous pollutants, resulting in serious environmental issues. In this study, the NO reduction reactivity of semichars preheated in Ar and Ar/CO 2 atmospheres was investigated via temperature-programmed reduction (TPR) experiments. Based on the TPR-DTPR method, the NO initial temperature, the maximum NO reduction rate, and the NO reduction indices of different semichars were determined. Compared with CO 2 /Ar pretreatment, the inert pretreatment enhanced the NO reduction index of the semichar. The CO 2 /Ar pretreatment was beneficial for decreasing the total Raman area and the ratio of I (Gr+VL+Vr) /I D due to the preferential consumption of high-reactivity structures and the condensation of aromatic ring structures. Furthermore, the detrimental effect of CO 2 /Ar treatment on semichar reactivity was relevant to the volumetric fraction, exerting an extraordinary correspondence with R r. Density functional theory (DFT) was employed further to analyse the NO reduction mechanisms by semichars derived from Ar and CO 2 /Ar atmospheres. For the inert pretreatment, the NO reduction pathways consisted of two pathways ( path 1 and path 2 ). Path 1 was supported by thermodynamics and dynamics. The energy barrier of the rate-determining step was 280.53 kJ/mol. Owing to the difference in the evolutionary pathways of C(O) on the semichar surface, the reaction between NO and the semichars derived from the CO 2 /Ar atmospheres could be summarized as two pathways ( path 3 and path 4 ). Path 3 was identified as the better method, and the energy barrier of the rate-determining step was 285.39 kJ/mol. The slightly adverse effects of CO 2 /Ar pretreatment on NO heterogeneous reduction were determined by coupling the experimental and theoretical methods, providing new insights into nitrogen migration and transformation mechanisms. [ABSTRACT FROM AUTHOR]

[Display omitted] • Bluff-body MILD technology is used to numerically investigate semicoke co-combustion. • Turbulence-chemistry interactions are characterized by homo- and heterogeneous Da. • As V sec rises, local MILD (1< Da t <10) is enhanced until a complete MILD is realized. • Weakening transitional MILD regime (10< Da t <30) is critical to create MILD regime. Secondary air jet velocity is strongly related to flue gas entrainment in semicoke-blend MILD combustion, which exerts a critical influence on the turbulence–chemistry interaction and combustion characteristics. To extend the application of moderate or intense low oxygen dilution (MILD) combustion to tangential fired boilers, this paper numerically investigates the effect of secondary air velocity (V sec) on the turbulent flow and chemical reaction interaction behaviors of large-proportion semicoke and bituminous coal co-firing to establish the bluff-body MILD combustion regime. The results show that increasing V sec abates the peak temperature by enlarging radiative heat transfer and enhances flue gas recirculation. The MILD regime first occurs at V sec = 51.8 m/s and then extends quickly from 0.4 m to 1.2 m with further increasing V sec. Increasing V sec decreases the maximum of the turbulent Damköhler number Da t and increases the minimum of Karlovitz number Ka , implying that the predominant role of turbulent mixing gradually is transferred into the comparable role. The local-MILD regime (1 < Da t < 10) is enhanced significantly and an ideal MILD combustion regime occurs at a velocity of 158.6 m/s, i.e., Da t ≈ 1 , and Ka ≫ 1 , denoting a slow-chemistry regime. Meanwhile, the maximum heterogeneous Damköhler number Da, C-O2 decreases from 1.09 to 0.86, while the maximum Da, C-CO2 and Da, C-H2O decrease from 0.013 to 0.007 and from 0.003 to 0.0018, respectively, hinting that the char-O 2 reaction is controlled by diffusion/kinetics, char-CO 2 and char-H 2 O reactions are determined by kinetics, and they all proceed toward the kinetics-controlled regime with increasing V sec. Low NO x emissions can be captured and coupled with small Da t by increasing V sec , and weakening the transitional MILD regime (10 < Da t < 30) is of great importance when establishing the MILD regime. [ABSTRACT FROM AUTHOR]

• The hydration heat release process of the cement with ultrafine CFB fly ash was obtained. • The dissolution and polymerization characteristics of Si-O and Al-O polyhedrons were studied. • The affecting mechanism of ultrafine grinding on pozzolanic activity has been reported. The pozzolanic activity of ultrafine circulating fluidized-bed (CFB) fly ash prepared by jet mill has been investigated in this research. The ultrafine powder properties and the hydration heat release process of the cement blended with ultrafine CFB fly ash have been obtained. Furthermore, the dissolution characteristics of SiO 2 and Al 2 O 3 , the Ca2+ absorption capacity in saturated Ca(OH) 2 solution, and the polymerization degree of Si-O and Al-O polyhedrons have been studied to further clarify the mechanism of ultrafine grinding on pozzolanic activity. Compared to raw CFB fly ash (RCFA), the particle morphology, the pore structure of ultrafine CFB fly ash (UCFA, D 50 ≤ 5 μm), and the pozzolanic activity of UCFA are significantly improved. The hydration rate of cement blended with UCFA is accelerated, and the total heat release is close to that of pure cement within 72 h. Ultrafine grinding increases the SiO 2 and Al 2 O 3 dissolution rate of CFB fly ash and Ca2+ absorption capacity in saturated Ca(OH) 2 solution, mainly due to the decrease of polymerization degree of Si-O and Al-O polyhedrons and particle size of CFB fly ash. [ABSTRACT FROM AUTHOR]

• The ignition and combustion of co-combustion is studied using OH-PLIF. • The synergistic effect on blends ignition is volatile content and K, Na, and Ca. • The high flame temperature and Na/ Fe promote the fixed carbon burnout of blends. • The promotion on ignition and burnout is dominant at 20% pyrolyzed char in blends. This work studies the ignition behavior and combustion characteristics of pulverized pyrolyzed bituminous (PB) char blended with pulverized bituminous coal at different mass fractions. Experimental studies were conducted with OH planar laser-induced fluorescence (OH-PLIF) diagnosis technology in a flat-flame optical entrained flow reactor for pulverized coal/char. An interaction occurs between pyrolyzed char and bituminous coal, which is indicated by the relationship observed between the pyrolyzed char mass fraction mean values (MFMVs) and the combustion properties of the blended fuels. The addition of bituminous coal enhances the competitive effects of the two fuels on ignition and burnout. The synergistic effect observed upon ignition of the blends is mainly attributed to the high volatile contents and the potential catalysis effects from the alkali and alkaline earth metal species in the blends. The important factor leading to this synergistic effect is that the alkali and alkaline earth metal species in the ash can reduce the ignition temperature and accelerate the release of volatiles, thereby shortening the ignition delay. The high flame temperature and the potential catalysis effects from the alkali metal and transition metal species promote blended fuel burnout. Considering both the ignition delay and the residual char burnout, the optimal blending ratio of pyrolyzed char is approximately 20% because the fixed carbon burnout of the blends is considerably promoted and the ignition delay is significantly shortened. In comparison with PB char, the ignition delay decreased by 28.95% and the burnout ratio increased by 39% at the 20% PB char blending ratio. [ABSTRACT FROM AUTHOR]

[Display omitted] • O 2 /H 2 O pretreatment with Na participation accelerated the formation of micropores. • Sodium addition enhanced the reactivity of semichar via the structure of C-O-Na. • Increasing the quantity of small aromatic rings could accelerate the emission of CN/C 2. • The addition of O 2 , H 2 O, and Na could synergistically increase the NO reduction ability of semichar. • Na promoted the reduction of NO by attenuating the conjugated components of the aromatic system. The surface physicochemical features of semichar play a crucial role in determining its reactivity, dominating the subsequent reaction between semichar and NO. Therefore, the synergistic effect of O 2 /H 2 O pretreatment and Na addition on the microstructural characteristics of semichar and NO reduction characteristics is revealed through experimental and density functional theory (DFT) methods. Experimental results indicate that O 2 /H 2 O pretreatments with Na addition promote the development of pores. Micropores are considered the major pore structures in semichar. Based on Raman and laser-induced breakdown spectroscopy (LIBS) results, the combined effect of O 2 /H 2 O pretreatments and Na promotes the formation of small aromatic rings, exhibiting a positive correlation with molecular emissions. Temperature-programmed reduction experiments (TPR) demonstrate that the inclusion of Na in O 2 /H 2 O pretreatments enhances the reduction ability of NO. The reduction mechanism of NO-semichar is determined using the DFT method. The simulation results indicate that the adsorption of H 2 O during O 2 /H 2 O pretreatment is chemisorption and independent of Na atom. The additional Na reduces the energy gap of the NO-semichar, facilitating the decomposition of N 2 by weakening the conjugated components of the aromatic structures. [ABSTRACT FROM AUTHOR]

• Fuel-N mainly released in the form of NO in O 2 /H 2 O combustion of demineralized char. • Increase of T r and O 2 concentration could inhibit NO emission in O 2 /H 2 O combustion. • C(O) amount reflected demineralized char reactivity and NO emission characteristics in combustion. • There was linear correlation between I (Gr+VL+Vr) /I D and demineralized char reactivity. Surface behavior and structural characteristics of char particles generally play a vital role in affecting char NO emission characteristics during combustion process. A typical bituminous Shenhua from northwest China was employed for demineralization in this research. The demineralized coal without catalytic interference of alkali metal salts was employed as the experimental sample. Moreover, by means of the utilization of isothermal combustion test, Temperature Programmed Desorption (TPD) and Raman spectrometer, the conversion ratio of nitrogen content in char to NO, as well as the evolution of char surface behavior and micro-structure in the process of O 2 /H 2 O combustion under different conditions were determined. With the increase of reaction temperature (T r) and O 2 concentration, more surface active sites (C f), oxygen containing functional groups (C(O)) and small aromatic ring structures generated on char particle surface inhibiting the emission of NO during O 2 /H 2 O combustion process. Under identical reaction conditions (Tr, O 2 and H 2 O concentration), the partially oxidized chars with moderate burnout degree (X c ≈ 0.3) had the largest amount of C f on particle surface. On the other hand, the amount of C f and small aromatic ring structures first increased and then decreased as the H 2 O concentration enhanced. When the C f amount and the value of I (Gr+VL+Vr) /I D reached the maximum at the H 2 O concentration of approximately 8.5 vol%, the conversion ratio of char-N to NO reached the minimum value. Meanwhile, the increase of H 2 O concentration initially accelerated the NO reduction rate and this positive effects gradually diminished when the H 2 O concentration exceeded the critical value (8.5 vol%). Thus, the optimal combustion condition for char combustion with the lowest NO conversion ratio in this study was: 1473 K, 30% O 2 and 8.5 vol% H 2 O. [ABSTRACT FROM AUTHOR]

• Homo-/heterogeneous Damkhöler numbers are adopted to characterize the MILD regime. • A low O 2 content and a CO 2 environment promote the homo-MILD regime while hetero-MILD regime is inhibited. • An ideal hetero-MILD combustion can be realized by increasing O 2 content. • Reduced effect of char-O 2 reaction share and promoted effect of char-CO 2 reaction share with increasing X O2 are weakened. • The surface behavior of char and NO x emission are affected by X O2 and CO 2 dilution. The present study numerically investigates the effects of the O 2 content (X O2) in the oxidizer on the MILD combustion behaviours of large-proportion semicoke mixtures, such as those involving temperature, MILD regime recognition, char surface consumption, homo/heterogeneous Damkhöler number, and NO x emission, under O 2 /N 2 and O 2 /CO 2 atmospheres. The results indicate that by increasing X O2 from 15 % to 30 %, the peak temperature increases significantly due to the increased char consumption rate, the high temperature region shrinks, and the axial length of the MILD regime region slightly increases from ∼1 to ∼1.4 m because of enhanced flue gas recirculation under O 2 /N 2 or O 2 /CO 2 atmosphere. The reduction effects of the char-O 2 reaction proportions and the promoted effects of the char-CO 2 reaction proportions due to the increased XO 2 are both obviously weakened when N 2 is substituted for CO 2. A high O 2 content and an O 2 /N 2 atmosphere reduce the char burnout time. With increasing XO 2 , the maximum value of the turbulent Damkhöler number Dat increases from 6.79 to 10.16 under the O 2 /N 2 atmosphere and from 3.99 to 10.6 under the O 2 /CO 2 atmosphere, implying that a low O 2 dilution and a CO 2 environment promote the establishment of the homo-MILD regime. The maximum values of the surface Damkhöler number (Das-CO 2 and Das-H 2 O) of the two gasification reactions (Char-CO 2 and Char-H 2 O) have the same order of magnitude and are much lower than those of the surface Damkhöler number (Das-O 2) of the Char-O 2 oxidation reaction, i.e., Das-H 2 O < Das-CO 2 ≪ Das-O 2 < 1. The maximum values of the three surface Damkhöler numbers all increase with increasing XO 2 , while they decrease with the replacement of N 2 by CO 2 , indicating that a high O 2 content and an O 2 /N 2 atmosphere could facilitate the establishment of an ideal hetero-MILD combustion regime to some extent. Low NO x emissions can be realized under moderate O 2 and CO 2 dilution conditions by regulating the oxy-MILD combustion regime of semicoke blends. [ABSTRACT FROM AUTHOR]

• Ignition distance of the same trend was obtained by multiple methods. • Ignition transformed from homogeneous-heterogeneous to homogeneous mode at particle size of R 90 = 25%. • The ignition share of semicoke decreased with increasing particle size. • Strong reducing zone was crucial to low NO emission for finer particle sized fuel stream. The characteristics of semicoke and high-volatile bituminous coal comprising different-sized particles were experimentally studied during co-firing in a 300-kW pilot-scale down-fired furnace. Ignition was investigated in detail in the near-burner region wherein ignition transitioned from reducing to an oxidizing atmosphere similar to that in pulverized coal (pc) furnaces. Combustion characteristics—including ignition distance and mode, NO emission, and char burnout—were evaluated based on chemiluminescence spectra, flame temperature, and flue gas species. The results showed that the chemiluminescence-spectra measured ignition distance decreased with increasing particle size, which was consistent with the trends determined using gas temperature and flue-gas species analyses. An attempt was made to distinguish the relative contributions of semicoke and bituminous coal to the initiation ignition. The ignition mode transformed from nonhomogeneous to homogeneous at particle size of R 90 = 25% because the semicoke ignition share was reduced in the blended fuel. A method was proposed for determining the intensive combustion zone and strong reducing zones (S C and S R , respectively). The S C and S R areas both decreased with increasing particle size. The correlation between S R and the fuel-N/NO conversion ratio showed that a larger S R is the key to low NO emission. Because bituminous coal promoted semicoke ignition, stabler combustion and lower downstream NO emission, R 90 ≤ 20% particles may be suitable for cofiring semicoke and bituminous coal. [ABSTRACT FROM AUTHOR]

• The inhibitors can significantly increase the adsorption capacity of CO 2 in coal. • The inhibitor can increase the amount of micropores and mesopores with pore size <7.5 nm in coal. • The intrusion of inhibitors can reduce the surface roughness of coal, break the macropores and generate new micropores in coal. Injecting power plant flue gas into goaf for sequestration can not only reduce the emission of CO 2 , but can also effectively inhibit the spontaneous combustion of coal left in the goaf. To analyze the effect of the chemical environment in the goaf on the sequestration of CO 2 in a coal seam, a home-made adsorption experiment device and ASAP 2020 specific surface analyzer were used to study the regularity of CO 2 adsorption in coal and the pore structure of coal before and after inhibition. The results indicate that the adsorption capacity of CO 2 in coal increased by 14.05%–21.04% after inhibition. The inhibitors can significantly increase the adsorption capacity of CO 2 in coal, enhancing the sequestration of flue gas in goaf. The pore volume and specific surface area of micropores in the coal samples after inhibition are larger compared to those in the raw coal, whereas those of mesopores and macropores are smaller, but those of the micropores are 4.3 and 205 times those of mesopores and macropores, respectively. Thus, the total pore volume and specific surface area of coal increased significantly after inhibition, which is the reason for the increase of CO 2 adsorption capacity. The inhibitor does not fundamentally change the pore size distribution of coal, most of the pores in coal are in the pore size range of 0.4–0.6, 0.8–0.9, 1.5–75, and 120–170 nm, which greatly contribute to its pore volume and specific surface area. The pores with a pore size of <7.5 nm are the main adsorption pores in coal, and the inhibitors can significantly increase the number of micropores and mesopores with a pore size of <7.5 nm. The fractal dimension of the coal samples was calculated using the FHH model, which indicated that the intrusion of inhibitors can remove the impurities in pores and reduce the surface roughness of coal. Conversely, the macropores in the coal samples after inhibition are broken and new micropores are generated. This study is of great significance for increasing the sequestration capacity of CO 2 in goaf. [ABSTRACT FROM AUTHOR]

[Display omitted] • Fuel-rich/lean technology is used for large-proportion semicoke co-combustion. • Adding bluff bodies at the fuel-rich side facilitates fuel-rich/lean technology. • Turbulence-chemistry interaction behaviour is featured by Damköhler number. • Low NO x formation is due to a local homogeneous MILD regime at the fuel-rich side. Fuel-rich/lean combustion is considered as a novel combustion technology due to its enhanced ignition performance and low NO x emissions. To further improve the poor performance of large-proportion semicoke and bituminous coal blended combustion and reduce NO x emissions, under a bias concentration ratio (BCR) of 2:1, an addition of bluff bodies at the fuel-rich side is proposed to numerically and experimentally investigate the turbulence-chemistry interaction behaviours of the fuel-rich/lean combustion regime in a 0.3 kW pilot-scale bias combustion furnace for various bluff body blockage ratios (BR = 0%, 11% and 21%). The results show that, increasing the blockage ratio evidently shortens the ignition delay time and burnout time of bituminous coal and semicoke, and the promotive effect of triangular bluff-body (BR = 21%) is greater. The turbulent Damköhler number Da t maximums decrease from 194 to 113 with increasing BR, denoting the regime is traditional combustion mode controlled by mixing. On the fuel-rich side, a local homogeneous MILD combustion zone (Da t <10) extends with increasing BR. For the heterogeneous combustion, the char-O 2 reaction determined by diffusion/kinetics and the char gasification reaction controlled by kinetics both show a kinetically controlled trend with increasing BR. Moreover, with increasing BR, a decreasing in the NO concentration in the main reaction zone can be attributed to an enhanced recirculation rate K v. There is a strong correlation between the maximum NO and the local homogeneous/heterogeneous Damkhöler number on the fuel-rich/lean side in the plane of maximum K v. Increasing BR can evidently decrease the maximum NO concentration on the fuel-rich side due to the enhanced local MILD combustion regime, and thus reducing NOx exit emissions. Considering temperature, ignition, burnout, turbulence-chemistry interactions and NO x emissions, a triangular-shaped bluff body on the fuel-rich side (BR = 21%) is recommended rather than a cubic bluff body in the fuel-rich/lean co-combustion of semicoke. [ABSTRACT FROM AUTHOR]

• CN and C 2 emissions were positively correlated to the amounts of small aromatic rings. • The synergetic effects of O 2 /Ar and sodium salt enhanced the combustibility index of semichar. • O 2 /Ar pretreatments and sodium salt addition altered the distribution of charge in semichar. • Additional Na during O 2 /Ar pretreatment decreased the combustion barrier of semichar. Using Raman spectroscopy, temperature-programmed desorption (TPD) and laser-induced breakdown spectroscopy (LIBS), the relationship between microstructural characteristics and molecular emissions was determined. It can be concluded that the enhancement of C 2 and CN emissions was related to the generation of small aromatic rings and C(O) groups. Compared with the Ar preheating treatment, the presence of additional amount of O 2 and sodium salt accelerated the dissociation of the large aromatic ring skeleton, leading to the significant enhancement of small aromatic rings, C(O) and C O Na groups. Subsequently, the combustion performance of preheated semichar was investigated by the TG-DTG method. The experimental results demonstrated that pretreatment under oxidizing atmospheres (without and with sodium salt addition) enhanced the comprehensive combustion index (S) and the combustion stability (R w) of semichars. The corresponding combustion activation energy (E) was weakened. Furthermore, the DFT method was used to analyse the combustion behaviours of semichars. This result demonstrated that preheating treatment (with or without sodium participation) altered the distribution of the Hirshfeld atomic charge (ADCH) of preheated samples, and the combustion essence of semichar was the fracture and recombination of aromatic clusters. For the semichar preheated in the O 2 /Ar atmosphere, the energy gap of the most optimized pathway was 436.26 kJ/mol. When sodium participated in the oxidizing pretreatment, the energy gap of the most optimized pathway was 413.80 kJ/mol. The positive effect of sodium on the combustion performance of semichar was identified, which was in good agreement with the experimental results. [ABSTRACT FROM AUTHOR]

• The material combined with BNNS and CeVO 4 was prepared for efficient desulfurization. • The desulfurization rate maintained over 91% after six times recycle. • The superoxide species were generated during the reaction between the catalyst and O 2. Removal of sulfur-containing compounds from fuel oil is one of the effective ways to reduce the emission of harmful substances and is of great environmental importance. In this work, a novel cerium-vanadium bimetallic catalyst supported by boron nitride nanosheets (CeVO 4 /BNNS) is reported for the efficient removal of sulfur-containing compounds from diesel fuel through catalyzing the molecular oxygen in the air. The successful preparation of the CeVO 4 /BNNS catalyst was demonstrated by SEM, XRD, and BET characterization, while its conformational relationship was also revealed. Synergistic effect of CeVO 4 bimetallic oxide together with the strong affinity of BNNS for sulfur-containing compounds achieved the efficient removal of of 4,6-DMDBT (100 %), DBT (99.62 %) and BT (71.29 %) within 3 h. The structure of CeVO 4 /BNNS catalyst remained basically unchanged after six times recycle, and the desulfurization rate maintained above 91 %, indicating a good stability. What'more, the free radical scavenging experiments and the analysis of oxidation products revealed the possible reaction mechanism, i.e., the activation of molecular oxygen by CeVO 4 /BNNS catalyst generated superoxide radicals, which oxidized DBT to DBTO 2. [ABSTRACT FROM AUTHOR]

[Display omitted] • Characterization of bottom ashes/slags (BAS) from a pulverized coal boiler was studied. • BAS mainly contains SiO 2 , CaCO 3 and Al 2 O 3 except for C element enrichment. • Effect of fine/coarse BAS on slagging behavior of coal blend was studied. • 5% fine BAS addition converted glassy slags at 1200℃ into low molten deposits. • Different groups composed of BAS and PD inhibited the formation of more melting minerals. Coal cofiring with carbon-containing solid wastes can achieve resource utilization and promote the circular economy development. This work aims to investigate the effects of C-rich bottom ashes/slags (BAS), fine/coarse BAS separated by mechanical sieving, and the mixture of untreated BAS with purified dust (PD) from CaC 2 production acting as additive on slagging behavior and mineral phase transformation of Zhundong (ZD) coal blend. Detail characterizations including chemical composition, micro-morphology, mineral phases and weight loss of BAS were performed. The ash fusion tests for coal blend with varying proportions of additive were conducted. The compositional analyses of deposits on an un-cooled probe at different sampling temperatures (600/800/1000/1200 ℃) in a drop-tube furnace were investigated. The results show that three types of untreated BAS have similar physicochemical properties. The BAS addition has a negative influence on ash fusion temperatures, but they would completely change the types of crystal minerals in the melted deposits dominated by amorphous substance at 1200 ℃. The addition of 5 wt% coarse (>1 mm) or fine (<53 μm) BAS promotes glassy slags at 1200 ℃ converting into deposits with a low melt fraction, but the influence of fine BAS is more pronounced. A group composed of untreated BAS and PD at an appropriate ratio, promotes the formation of more crystal minerals with high melting points in deposits at 1200 ℃. [ABSTRACT FROM AUTHOR]

A numerical model is developed to investigate the effects of horizontal bias combustion (HBC) and air staging combustion (over-fire air, OFA) technologies on the performance of a 200 MWe tangentially-fired pulverized-coal boiler. The devolatilization rate and volatiles amount are determined by a kinetic devolatilization model, which predicts the coal devolatilization prior to the volatile matter and char combustion. The characteristics of the devolatilization, combustion, heat transfer and NO x emission are studied and compared to achieve a comprehensive understanding of the low NO x combustion. The prediction shows a good agreement with the on-site measurement results, which confirms that the model is capable of predicting the characteristics of the investigated boiler. The predicted results have shown that the OFA has a remarkable effect on the reduction of NO x emission. The HBC makes a significant NO x reduction in the primary combustion zone (PCZ) when there is no air staging. In terms of the NO x reduction, the air staging plays a dominant role in comparison with HBC burners. The application of OFA tends to lead to slagging in the PCZ, which can be avoided using HBC due to the higher stoichiometry close to the furnace wall. The details of this study improve the understanding of combustion and NO x emissions in tangentially-fired pulverized-coal boilers with low NO x combustion technologies, especially for boilers adopting HBC burners and OFA. [ABSTRACT FROM AUTHOR]

• Oxidative torrefaction modified elemental and compositional characteristics more effectively; • Increasing torrefaction severity lowered the amount of hydrophilic group; • Flue gas was more economical and efficient to strengthen biomass stability and reactivity; • Relative amount of N-5 increased after undergoing flue gas torrefaction; • Hydrophobicity was strengthened for appropriate oxidizing agent addition. Rice husk, a typical agricultural by-product was used for experimental investigation on characterizing the variations in fuel behaviors before and after torrefaction performed under different conditions. The effects of torrefaction temperature (493, 543 and 573 K), atmosphere (argon, air and flue gas) and duration (30 and 60 min) on the compositional variation, microstructural evolution, functional group distribution and surface hydrophobicity were evaluated systemically through the utilization of elemental analysis, FTIR, Raman spectra, XPS and contact angle. Oxidative torrefaction expressed a tendency to improve the torrefaction efficiency and increase the carbon content in torrefied sample due to the simultaneous occurrence of volatile release, carbonization and surface oxidation. Oxidizing agents could accelerate the decomposition or conversion of surface hydrophilic groups during oxidative torrefaction process effectively, strengthening the hydrophobicity of torrefied sample significantly. Furthermore, partial oxidation and thermal degradation tended to promote the crack of condense aromatic structures synergistically, increasing the proportion of active structures in the torrefied rice husk. Simultaneously, some nitrogen atoms initially located inside the aromatic structures with high thermal stability would migrate to the edge rapidly during the oxidative torrefaction (N-Q → N-5 + N-6). In the process of air torrefaction, excessive oxidants would reduce the content of reactive structure and functional group with low thermal stability through severe oxidation reaction, reducing the quality of torrefied sample, especially at high temperatures. The results in this study illustrated that torrefaction performed in flue gas atmosphere at 573 K for 30 min was the optimum torrefaction condition for rice husk upgradation, and employing flue gas as the carrier gas for biomass torrefaction was more effective and economical than N 2 and air in improving the fuel quality of rice husk. [ABSTRACT FROM AUTHOR]

• In both reducing and oxidizing atmospheres, biochar exhibited better NO reduction than coal char. • O 2 made chars' structures developed and enriched with functional groups, which facilitated char-N oxidation. • Effects of CO 2 on NO reduction depended on the presence or absence of O 2. • Without O 2 , the contribution of AAEMs in biochar on NO reduction was more pronounced. Experiments were conducted on chars prepared from R- and H- form sawdust (SD) and Shenmu coal (SM) to investigate NO reduction in different atmospheres, and physicochemical structures of chars after the reaction were analyzed. The effects of atmospheres on NO reduction reactivities and structural evolution characteristics of different chars were discussed. The NO reduction reactivity of SD chars was always better than that of SM chars, and there were some differences in the specific reactivity of four chars under different atmospheres. When NO, CO 2 and O 2 were sequentially involved, the char ordering degree tended to increase, which also had a clear positive impact on the pore development. With the participation of CO 2 and O 2 in the reaction, some N-6 and N-X in R SD char were converted into N-5 and N-Q, while N-6, N-5 and N-X in the other three chars were converted into N-Q. [ABSTRACT FROM AUTHOR]

• Biomass chars have a more favorable NO reduction effect than coal chars. • For biomass chars, H-form chars are more active than R-form chars on NO reduction. • The acid-soaking treatment contributed to the development of char physical structures. • The oxidation and reduction reactions lead to different evolution paths of nitrogen-containing functional groups. Under simulated flue gas conditions, the effect of the residence time of different chars on the evolution of physiochemical structures during the NO reduction process was evaluated. The experimental data proved that the acid-soaking treatment contributed to the development of char structures. Both biomass chars and H-form chars showed better reactivity on NO reduction, which may be related to smaller average pore diameters, larger BET surface areas and larger pore volumes. The increasing of residence time increased the graphitization degree of all chars, and main reductions of structural defects were from within and between carbon graphene layers. Besides, with the residence time increasing, N-6 and N-5 of R-form Shenmu chars were transformed into N-Q and N-X, while N-X and N-Q (and some N-5) of R-form and H-form sawdust chars were transformed into N-6, which is related to the oxidation or reduction reaction for different chars. [ABSTRACT FROM AUTHOR]

• More nitrogen was stripped from condensed aromatic structures in O 2 /H 2 O pre-oxidation; • O 2 /H 2 O pre-oxidation strengthened the conversion of anthracite-N to HCN and NH 3 ; • •O and •OH accelerated the decomposition of condensed structures into small ones; • N-5 became dominant structure at expense of N-Q and N-6 after O 2 /H 2 O pre-oxidation. Pre-oxidation is considered a promising technology for improving combustion efficiency and inhibiting the emission of NO x in high rank coal use. This investigation concentrated on the evolution characteristics of anthracite-N during pre-oxidation processes under O 2 /H 2 O atmospheres. A typical high rank coal was used in this study. Isothermal pre-oxidation tests were performed under different reaction atmospheres (O 2 : 3, 6, 10 vol%; H 2 O: 0, 10 vol%), and the results illustrated that the enhancement of the O 2 volumetric fraction and the addition of H 2 O expressed positive effects on promoting the release of anthracite-N (16.7% → 31.7%) rapidly, and more structural nitrogen tended to be removed as HCN and NH 3 (0.85% → 5.48%) during the O 2 /H 2 O pre-oxidation process. The XPS results demonstrated that the increasing O 2 volumetric fraction and addition of H 2 O in the pre-oxidation atmosphere led to the enrichment of pyrrole (N-5), which was the precursor of HCN, at the expense of pyridine (N-6) and quaternary nitrogen (N-Q). Furthermore, the increasing pre-oxidation degree accelerated the attachment of oxygen atoms on the anthracite surface, generating massive amounts of reactive functional groups. The Raman results illustrated that the variations in the chemical structure of nitrogen-containing complexes was caused by the cracking of condensed aromatic structures into reactive fragments or defect structures due to carbon-O 2 /H 2 O reactions during the pre-oxidation process, leading to massive amounts of internal structural nitrogen exposed to the edge of aromatic structures (N-Q/N-6 → N-5). When the pre-oxidation occurred in an atmosphere consisting of 10 vol% O 2 and 10 vol% H 2 O, the conversion ratio of anthracite-N to NO during the whole pre-oxidation process reached the minimum value of 5.31%, and the reactivity of the semi-char was the highest. [ABSTRACT FROM AUTHOR]

The effect of preheating primary air on the co-combustion characteristics of a 50–50% blend of pinewood and corn straw in a fixed bed. The primary air temperatures were assessed from 20 to 130 °C. The co-combustion characteristics were included the co-combustion behaviors and emissions. In order to reveal the features of the combustion process in the porous bed, a two-dimensional unsteady state model was employed to investigate the combustion process in a fixed bed of blended biomass on the combustion process in a fixed bed reactor. Conservation equations of the bed were implemented to describe the combustion process. The gas phase turbulence was modeled using the k-ε turbulent model and the particle phase was modeled using the kinetic theory of granular flow. Results showed that by increasing primary air temperature the residual mass on bed decreased, while the average burning rates and ignition front propagation velocity increased At the primary air temperature of 85 °C the smallest unburned carbon was left in the ash, and the emissions of nitrogen-compounds were relatively small. In contrast, the primary air temperature of 85 °C was found to be well-operating condition, which can be suggested for industrial boiler during blend co-combustion. The simulation results were then compared with experimental data for different temperature, which shows that the combustion process in the fixed bed is reasonably simulated. The simulation results of solid temperature, gas species and process rate in the bed are accordant with experimental data. [ABSTRACT FROM AUTHOR]

Pulverized corn straw biomass was torrefied in carrier gases which contain oxygen and carbon dioxide gases with the following proportions (3–7% O 2 , 13–17% CO 2 , 80% N 2). Torrefaction took place at 275–375 °C, under atmospheric conditions. Upon such oxidative torrefaction, the mass and energy yields of this biomass decreased with increasing temperature and increasing oxygen concentration. Most of the chlorine and sulfur contents of the biomass were released to the gas phase, 52–90% and 55–98% respectively, with amounts increasing with increasing both torrefaction temperature and oxygen concentration. In the examined parameter ranges, temperature had a pronounced effect on the mass and energy yields and on the evolution of chlorine and other elements to the gas phase. Low temperatures decrease the energy input to the process and enhance the mass and energy yields of the solid product. At the examined temperatures, the mass and energy yields were also affected by the variation in the oxygen content of the input gas. [ABSTRACT FROM AUTHOR]

• High-speed multi-species optical diagnostic techniques and DMD analysis method are performed. • Continuous and whole volatile combustion process of the same particle is obtained. • The oscillation frequency characteristics of pulverized coal flame were captured. • Both oxygen concentration and particle aggregation have effects on the frequency characteristics. This paper investigated the oscillation characterization of volatile combustion of single coal particles using high-speed OH-PLIF and CH* chemiluminescence, meanwhile advanced data-driven algorithm as post-processing. The measurement region of in-situ 500 Hz OH-PLIF system was set to 53mm × 35 mm with a spatial resolution of ~23 µm per pixel, which was used to capture the continuous and whole volatile combustion process of single particle in a group of pulverized coal particles (SPGCP). A high-volatiles bituminous coal (30 wt%) burned at a gas temperature of 1700 K with oxygen mole fractions in the range of 10%–30%. The morphology of volatile flame and dynamic evolution was obtained. Dynamic mode decomposition (DMD) method can provide a new insight into combustion oscillation of pulverized coal. The oscillation frequency characteristics of pulverized coal flame were captured, and the red-shift phenomenon of dominant frequency with the increase of oxygen concentration was found. The particle aggregation might lead the low-frequency oscillation of pulverized coal combustion. On the contrary, individual or separated particle combustion could generate larger oscillation frequency. [ABSTRACT FROM AUTHOR]

• The ignition of bituminous coal and lignite is dominated by their volatiles. • The devolatilization rate of bituminous coal changes less than that of lignite. • The flame stability of the bituminous coal particle stream is much higher. • The volatile releasing ratio of bituminous coal was less than that of lignite. An optical entrained flow reactor, based on a downward flat-flame, was established to explore the devolatilization and volatile combustion behaviours of coal particle streams with surrounding temperatures from 1600 K to 1800 K and oxygen mole fractions in the range of 10%–30%. The coal particle streams with particle sizes of 53–80 μm are two kinds of high-volatile pulverized bituminous coal and lignite. Using High-speed Planar Laser-Induced Fluorescence to diagnose OH (OH-PLIF), the planar sheet-imaging of OH-radicals was obtained to visualize the coal particle flames, and then, the normalized fluorescence signal intensity along the distance downstream from the burner and along the radial distance was obtained to investigate the volatiles combustion process. From the planar sheet-imaging, the ignition of tested high-volatile bituminous coal and lignite is dominated by the combustion of their volatiles. The combustion processes of volatiles can be divided into 4 stages: ignition, accelerating burning, stable burning and feeble burning, to investigate the intrinsic combustion correlation phenomena. The relative standard deviation (RSD) changes of the OH radial profiles can provide a new insight into flame stabilization, wherein the flame stability of the bituminous coal particle stream is higher than that of lignite under the same combustion conditions. The volatiles combustion of lignite is more sensitive to the oxygen concentration and temperature than that of bituminous coal, and the volatile releasing ratio of bituminous coal (65%–81%) was substantially less than that of lignite (92%–100%) by the analysis of the char residues. [ABSTRACT FROM AUTHOR]