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The use of a coal-based energy structure generates a large amount of CO2 and NOx. The numerous emissions from these agents result in acid rain, photochemical smog, and haze. This environmental problem is considered one of the greatest challenges facing humankind in this century. Preheating combustion technology is considered an essential method for lowering the emissions of CO2 and NO. In this research, the char prepared from O2/CO2 and O2/H2O atmospheres was employed to reveal the effects of the addition of an oxidizing agent on the combustion characteristics of char. The structural features and combustion characteristics of preheated chars were determined by Raman, temperature-programmed desorption (TPD), and non-isothermal, thermo-gravimetric (TGA) experiments. According to the experimental results, the addition of oxidizing agents promoted the generation of smaller aromatic ring structures and oxygen-containing functional groups. The improvement in the surface physicochemical properties enhanced the reactivity of char and lowered its combustion activation energy. Furthermore, the combustion mechanisms of the char prepared from the O2/CO2 and O2/H2O atmospheres were investigated using the density functional theory (DFT). The simulation results illustrated that the combustion essence of char could be attributed to the migration of active atoms, the fracture of the benzene ring structure, and the reorganization of new systems. The addition of oxidizing agents weakened the conjugated components of the aromatic ring systems, promoting the successive decomposition of CO and NO. The results of this study can provide a theoretical basis for regulating the reaction atmosphere in the preheating process and promoting the development of clean combustion for high-rank coals. [ABSTRACT FROM AUTHOR]

This paper uses numerical simulation to investigate the effects of diluents on the flame structure and NO generation of H2/CO micromixing flames. The results show that under the same thermal power condition, the diluents reduce the flame temperature and decrease the combustion reaction rate and flame propagation velocity. In addition, the diluents downsize the flame and force it downstream. With an increase in the diluent fraction, these trends are amplified. The NO production decreases due to the diluents, and the NO is lowest when H2O is added. When the diluents are CO2 and H2O, the NO generation is dominated by the reactants' concentration. This results in the lowest temperature not corresponding to the lowest NO production. The diluents also reduce the sensitivity of the NO production to the temperature, and the CO2 diluent highly weakens the sensitivity. [ABSTRACT FROM AUTHOR]

To address the issues of ignition difficulty, incomplete burnout, and high NOx emissions during coal char combustion in industrial applications, a series of co-firing experiments involving bituminous coal and its medium-temperature pyrolysis char were conducted using a micro fluidized bed analysis system. The experiments explored co-firing behaviors and NOx emission characteristics for five different fuel blending ratios in an environment with 21 % oxygen concentration, across temperatures between 923 K and 1223 K. The study delved into the synergistic effects of coal and char co-firing, as well as the influence of combustion temperature and fuel blend ratio on the co-firing efficiency and NOx emissions. The results show a synergistic effect in the co-firing of bituminous coal and char, influenced by reaction temperature and coal-to-char blend ratio, which evolves as combustion progresses. In the 923–1073 K range, initial coal addition to the blend enhances reactivity. However, as the reaction advances or coal content exceeds 75 %, this synergy shifts to an inhibitory effect. At temperatures above 1023 K, increased proportions of coal in the blend are associated with improved burnout rates, especially when the coal content is at or above 50 %. Moreover, at a combustion temperature of 1123K, there is a marked decrease in nitrogen oxide emissions, with coal blend ratio of 25 % and 50 % showing particular efficacy in reducing these emissions. These results provide valuable insights for the efficient and clean utilization of coal char in industrial combustion processes. • The synergistic effect of co-firing depends on the temperature and the blend ratio, and changes with the reaction process. • Between 923 and 1073K, bituminous coal initially enhances fuel reactivity but later or above 75 % content, it hinders the reaction. • Temperatures over 1023 K combined with coal contents at or exceeding 50 % lead to increased burnout rates. • NOx emissions attain lowest value at 1123 K, with coal proportions of 25 % and 50 %. [ABSTRACT FROM AUTHOR]

To burn semi-coke with ultra-low volatile content effectively at a large blending proportion with high volatile bituminous coal using a pulverized fuel (PF) rich/lean burner, the ignition and combustion behaviors of blended fuel of semi-coke and bituminous coal (1:1 by weight) in reducing to oxidizing atmosphere were investigated in a 300 kW pilot-scale test furnace with an arrangement of varying pulverized fuel-rich side stream concentration (P fc,rich , kg/kg). The results showed that ignition distance of the fuel-rich stream decreased from 339 mm to 159 mm when P fc,rich increased from 0.37 kg/kg to 0.58 kg/kg, which then increased again to 0.63 kg/kg. The ignition distance for the fuel-lean stream increased monotonically from 332 mm to 383 mm with an increase in the P fc,rich. The intersection of two streams showed a positive effect on overall ignition when P fc,rich was varied from 0.49 kg/kg to 0.58 kg/kg, which was mainly attributed to the mean volatile content of blended fuel. A parameter V index was introduced to evaluate the effects of quantity of volatiles on the ignition behaviour of blended fuels, and it was found that the volatile effect was more sensitive to the ignition distance than the ignition temperature. The ignition of the fuel-rich stream transformed from the heterogeneous-homogeneous combined mode to the homogeneous mode at P fc,rich of 0.49 kg/kg. The burnout ratio of blended fuel was highest at P fc,rich = 0.52 kg/kg and 12.21% higher than that of the ordinary unbiased burner. NO emission was inhibited under rich/lean combustion conditions, and the existence of a strong reducing zone was the key factor for the low conversion ratio of fuel-N to NO. Based on the experimental results, P fc,rich ranging from 0.52 kg/kg to 0.58 kg/kg was observed to be optimum for rich/lean co-combustion at large proportion of semi-coke in the blends. • Two peaks appeared in axial temperature at higher fuel-rich stream concentration. • The ignition distance decreased with increasing volatile index of blended fuel. • The ignition mode changed for fuel-rich stream concentration was over 0.45 kg/kg. • NO formation was smallest for fuel-rich stream concentration was 0.52 kg/kg. [ABSTRACT FROM AUTHOR]

The volatile combustion and NO release characteristics of pulverized fuel (PF) flames in an optical flat-flame entrained-flow reactor were investigated by CH∗/NO∗ chemiluminescence imaging. Pyrolyzed bituminous (PB) char with an ultralow volatile content (V daf ≈10%) and a comparative coal sample of Shenhua (SH) bituminous coal with a high volatile content (V daf = 34.2%) were studied under different ambient O 2 mole fractions (10%∼30%). The initiation of the combustion region of PB char occurs later than SH bituminous coal, owing to its low volatile contents, a slow devolatilization rate, and homogeneous-heterogeneous ignition/combustion mode. By measuring probed solid and gas samples, the conversion ratios of fuel-N to NO and the releasing ratios of volatile-N/char-N were determined in ignition process. Semi-quantitative NO release integral intensity and volatile combustion intensity results were obtained by integrating the NO∗ or CH∗ chemiluminescence intensity in the corresponding reaction region. The volatile combustion and NO release show good synchronous for Shenhua bituminous coal, which means that the NO releasing originated predominantly and owned higher conversion ratio from the volatile-N in the ignition region. The NO release integral intensity and NO emission of SH bituminous coal decreased with O 2 concentration increased from 20% to 30%, which indicates that the enriched-O 2 environment is beneficial for high volatiles bituminous to decrease NO x emission in ignition stage. For PB char, NO releasing originated predominantly from the char heterogeneous reactions of char-N in its ignition and combustion late stage. • The volatile combustion region for PB char is slower compared to SH. • NO originates mainly from volatile-N for SH in the ignition stage. • PB char is homo-heterogeneous ignition, NO originated mainly from char-N during ignition and combustion late stage. • The enriched-O 2 environment is beneficial for SH to decrease NO x emission during ignition under certain conditions. [ABSTRACT FROM AUTHOR]

In an attempt to achieve the clean and efficient utilization of lignite, drying pre‐treatment was performed in this study before lignite combustion. The combustion reactivity and NO emission characteristics of the partially dried lignite samples in the char combustion stage were investigated by means of TG and isothermal combustion tests, and the reactivity could be summarized as the following order: L1>L0.5>raw>L3>LT>L5 (chars obtained from the devolatilization of the raw and partially dried coals at 378 K for 0.5, 1, 3, 5, and 120 minutes) and the NO conversion ratio of L1 was the lowest. When the moisture content in the coal particles was relatively high (19.68%‐35.84%), the drying treatment could increase the combustion reactivity and inhibit NO emission in the char combustion stage; When the moisture content was within a relatively low range (0.17%‐19.68%), the moisture removal had negative effects on the reactivity and NO emission in the char combustion stage. The surface behaviour and microstructure of the raw coal char and chars derived from the partially dried coals were clarified by temperature programmed desorption/reduction (TPD/TPR) and Raman spectroscopy. The results illustrated that L1 derived from Lc1 (19.68%) was the most reactive sample with the largest amount of C(O) on the particle surface. There were also more reactive aromatic structures in L1 than other samples. Compared with direct combustion or excessive drying treatment, lignite should be dried to a certain degree (19.68%) for optimized lignite combustion. [ABSTRACT FROM AUTHOR]

The char-O 2 /H 2 O combustion under high-temperature entrained flow conditions was investigated experimentally and numerically to study the effect of H 2 O on the char-nitrogen (char-N) conversion. A numerical model was established based on the unsteady convection–diffusion equations for mass and heat transfer. Local Random Pore Model (RPM) was adopted to consider the change of specific surface area during char combustion. The Langmuir-Hinshelwood mechanism was used in the rate equations of main heterogeneous reactions to consider the competitions on surface active sites. The numerical results of char conversion and fractional conversion of char-N to NO reproduce the experimental results well. Both of the numerical and experimental results shows that the added H 2 O promotes the fractional conversion of char-N to NO and the promotion effect becomes weaker with the increase of O 2 concentration. Based on numerical results, four crucial factors that affect char-N conversion were decoupled and discussed quantitatively with addition of H 2 O. The added H 2 O promotes nitrogen release rate at lower oxygen concentration but inhibits nitrogen release rate at higher oxygen concentration. The added H 2 O lowers the heterogeneous char-NO reduction rate constant greatly but enlarges the pore surface area slightly, resulting in a lower overall heterogeneous char-NO reduction rate. Char-NO reaction is the main NO reduction pathway, and most of HCN formed in char-N–H 2 O reaction is more likely to release out of char particle and converts to NO through HCN→NH 3 →NO pathway in the ambient flow. [ABSTRACT FROM AUTHOR]

Oxy-steam combustion, fuel combusts in O 2 /H 2 O atmosphere, is treated to be a novel and promising technology for the next generation oxy-fuel combustion. The purpose of this study is to identify the effects of steam concentration on NO emission characteristics, functional group distribution and char NO reducibility during oxy-steam combustion process. A typical bituminous coal (SH) was employed for demineralization, devolatilization, combustion in O 2 /H 2 O environment, and investigated by Temperature Programmed Desorption/Reduction (TPD/TPR) methods and Raman spectrometer. The combustion results illustrated that char N/NO conversion during the oxy-steam combustion tests initially decreased with an increasing steam concentration in low range (1.2–8.5 vol.%), while increased apparently in high steam range (8.5–20 vol.%). Decomposition of H 2 O molecules promoted the formation of C(O) and small aromatic rings on particle surface at the low steam range; the high concentration of steam could promote the condensation of aromatic ring structures in char, leading to significant generation of large aromatic ring structures with a lower reactivity and the reduction of surface active sites (C f). The TPD results illustrated that the chars oxidized at moderate steam concentration (8.5 vol.%), and the chars with moderate burnout degrees (such as S 3 , S 6 and S 11) had the largest amount of C(O). Due to the decomposition of the C(O) into new C f and massive CO molecules, a rapid acceleration of NO reduction rate occurred in the temperature range of 1000–1400 K during the TPR process of each sample obtained from identical reaction conditions. Thus, variations in char surface behavior and microstructure might be the primary reasons affecting char-N emission. • Increase of C(O) amount inhibited NO release in oxy-steam combustion of char. • Reduction of NO by char and CO occurred rapidly in the range of 1000–1400 K. • Additional steam had obvious effects on NO emission in oxy-steam combustion. • Evolution of char microstructure showed an identical trend to the C(O) amount. [ABSTRACT FROM AUTHOR]

Hailaer lignite (HLE) with a uniform particle size distribution (3–6 mm) was employed to investigate the parameters of the low‐temperature dewatering process influencing the emission of gaseous pollutants (NOx) during their combustion process. The combustion of HLE original/dried samples obtained from a COMBDry drying system were carried out in a fixed bed horizontal furnace under an air atmosphere at the reaction temperature of 1100 °C. The dewatering treatment led to the enhancement of the relative amount of the volatile and fixed carbon content in the upgraded coal sample. The deconvolution of the Raman spectra of each sample showed that the increased degree of drying led to an enhancement of the defects in aromatic structures, indicating an enhancement in the number of active sites or oxygen‐containing complexes on particle surface. The devolatilization and combustion experimental results showed the following: (1) with the increase in the degree of drying, more HCN and NH3 were released during the devolatilization process, and both HCN and NH3 molecules had positive effects on the consumption of NO under high temperature conditions; (2) due to the enhancement of the specific surface area and total amount of surface active sites (Cf) after the drying treatment, the combustion and reduction reactivity of the dried HLE samples increased significantly; and (3) the conversion ratio of fuel‐N to NO during the combustion process decreased significantly with the increase of the degree of dewatering. Therefore, it could be concluded that the removal of moisture content in lignite particles in advance had positive effects on lignite high efficiency combustion with low NO emission. [ABSTRACT FROM AUTHOR]

This experiment was conducted on fixed bed combustion in a one-dimensional bench. The effects of ash and moisture content on the combustion characteristics of corn straw were determined. The two parameters directly relate to the burning rate and affect combustion efficiency and the release of gas. The bed temperature distribution, mass loss rate and gas composition were measured in the bed. The results show that the optimum char combustion efficiency was achieved at 10% moisture content of corn combustion. A slight increasing the moisture content to 10% can obtain a higher bed temperature and accelerate the ignition rate in the char oxidation stage, while there is also a slight decrease in the conversion ratio of C to CO. The conversion rate of S to SO 2 for 10% moisture content was higher with the temperature zone above 1000 °C. With the increased ash content, there was a slight increase in the average ignition rate; the bottom bed temperature increased with a serious ash slagging. C was converted to CO and presented a slightly increasing trend for higher ash content and the conversion of N to HCN. This work provides an overall understanding of corn combustion for large boiler system. [ABSTRACT FROM AUTHOR]

In this paper, mathematical modelling is conducted on the combustion of corn straw in a one-dimensional bench combustion test rig, and the effects of the primary air flow rate are assessed over a wide range. Due to complex solid combustion mechanisms and inadequate knowledge of the process, the development of such combustion system is limited. Numerical modelling of this combustion system has some advantages over experimental analysis, although the development of a complete model for this type of combustion system remains a challenge. Due to its characteristic properties, modelling of biomass combustion has to overcome many difficulties. One such problem is displaying the process of initiating the combustion in numerical modelling. This study finds that the volatile release and combustion of char increases, thus increasing the amount of primary air up to a critical point, where the starting time of ignition becomes shorter as the primary air flow rate increases. The peak concentration of NO decreases with the increase of primary air, whereas with the increase in the amount of air, there is a reduction in the release of SO 2 as well as a reduction in CO emissions in the bed. [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]

• 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 presents the effect of the simulated sizes of Municipal Solid Waste (MSW) on the combustion process in a fixed bed experimentally and numerically. The effect of temperature, gas emissions, flame front velocity and process rate are discussed for three different sizes of MSW: 10, 30, and 50 mm. The study found that for the operating conditions of the current model, when the diameter of particles is decreased, the bulk density of the material is increased, resulting in a decrease of convective heat transfer as well as combustion speed. As the diameter size of the material particles increase, the height of the post-combustion zone is increased, while the temperature in a high temperature area is decreased, due to the decrease in the material’s bulk density and the excessive increase in porosity. Results also show that the average emission concentration of CO and CO 2 decreases gradually with an increase in the particle diameter size. [ABSTRACT FROM AUTHOR]

This paper experimentally and numerically investigates the effects of ash content on the combustion process of simulated Municipal Solid Waste (MSW). A fixed-bed experimental reactor was utilized to reveal the combustion characteristics. Temperature distributions, ignition front velocity, and the characteristics of gas species’ release were measured and simulated during the combustion process. In the present work, the two-dimensional unsteady mathematical heterogeneous model was developed to simulate the combustion process in the bed, including the process rate model as well as NOx production model. The simulation results in the bed are accordant with the experimental results. The results show that as ash content increases, the lower burning rate of fuel results in char particles leaving the grate without being fully burned, causing a loss of combustible material in the MSW in a fixed bed and therefore reducing the combustion efficiency and increasing the burning time of the MSW. [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]

A numerical model was employed to simulate the combustion process in a fixed porous bed of municipal solid waste (MSW). Mass, momentum, energy and species conservation equations of the waste bed were set up to describe the incineration process. The rate of moisture evaporation, volatile matter devolatilization, char combustion, NO x production, and reduction and dioxin formation were calculated and established according to the local thermal conditions and waste property characteristics. Changes in the bed volume during incineration were calculated according to the reaction rate of the process. The simulation results were compared with experimental data, which shows that the incineration process of waste in the fixed bed was reasonably simulated. The simulation results of weight loss and solid temperature in the bed agree with the experimental data, which shows that the waste combustion rate is nearly constant in the middle of the incineration process, and that moisture evaporation takes up most of the time for the overall incineration experiment. The emission of gas species from the bed surface is also agreeably simulated, with O 2 , CO 2 , and CO concentrations in flue gas agreeing with the experimental data. The simulation results benefit the understanding of the combustion process in the waste bed as well as the design of incinerator grates. [ABSTRACT FROM AUTHOR]

In order to reveal the features of the combustion process in the porous bed of a waste incinerator, a two-dimensional unsteady state model and experimental study were employed to investigate the combustion process in a fixed bed of municipal solid waste (MSW) on the combustion process in a fixed bed reactor. Conservation equations of the waste bed were implemented to describe the incineration 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. The rate of moisture evaporation, devolatilization rate, and char burnout was calculated according to the waste property characters. The simulation results were then compared with experimental data for different moisture content of MSW, which shows that the incineration process of waste 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. Due to the high moisture content of fuel, moisture evaporation consumes a vast amount of heat, and the evaporation takes up most of the combustion time (about 2/3 of the whole combustion process). The whole bed combustion process reduces greatly as MSW moisture content increases. The experimental and simulation results provide direction for design and optimization of the fixed bed of MSW. [ABSTRACT FROM AUTHOR]

To study the combination of fuel-rich/lean combustion with oxy-MILD (moderate and intense low-oxygen dilution) combustion for the reduction of NO x emissions by adopting carbon capture and storage, this paper numerically investigates the turbulence−chemistry interactions and NO x emission behaviours of semicoke mixtures under different secondary velocities V sec (from air-fuel combustion to air-MILD combustion) and under different CO 2 dilution levels (oxy-MILD combustion). The bias concentration ratio (BCR) hardly affects the establishment of air-MILD combustion, which is obtained at V sec = 158.6 m/s. The oxy-MILD combustion, which is composed of small flamelets diffusing in large-scale eddies (turbulent Damköhler number Da t = 0.1–1), is enhanced, and the ignition and char burnout times are prolonged by increasing CO 2 from 0 vol% to 79 vol%. The char oxidation reaction in the diffusion/kinetics-controlled regime and the char gasification reactions in the kinetics-controlled regime both shift towards the kinetics-controlled regime with increasing velocities and CO 2 dilution, thus leading to lower burnout rates. The heterogeneous MILD regime of the fuel-lean side is more easily established for the air-MILD combustion, while that of the fuel-rich side is more easily established for oxy-MILD combustion. This is because the intensive turbulence increases the O 2 dilution level in the fuel-lean jet, while a high concentration of CO 2 reduces the oxygen diffusivity in the fuel-rich jet, which also leads to a larger X NO in the fuel-lean jet compared to the fuel-lean jet. The NO x concentration at the furnace outlet decreases significantly from air-fuel combustion to air-MILD combustion and further decreases as the oxy-MILD combustion is enhanced, along with relatively lower homogeneous and heterogeneous Da. • Integrating fuel-rich/lean technology with oxy-MILD combustion is introduced for semicoke co-combustion. • CO 2 dilution is adopted to further enhance the oxy-MILD combustion. • Air-MILD combustion facilitates MILD regimeof fuel-lean jet, while oxy-MILD combustion promotes that of fuel-rich jet. • Low NO x emissions can be achieved in air-MILD combustion and is further promoted under enhanced oxy-MILD combustion. [ABSTRACT FROM AUTHOR]

Abstract: A fly ash sample was collected en-suite from an industrial circulating fluidized bed (CFB) gasifier. The fly ash is herein referred to as semi-char (partially burnt char) as it still contained a high amount of carbon. CFB gasification involves long sample residence times and the resulting residual chars are expected to be different from conventional chars from processes such as pulverized coal gasification or chars prepared under laboratory furnaces where usually, residence times are relatively much shorter. In this study, the semi-char was burned in a flat flame reactor (FFR) under oxygen conditions. The semi-char reactivity, pore development and microstructure were then used to characterize its combustion behavior. The ‘raw’ semi-char, before the FFR oxygen reactions was porous with a dominant meso–macro pore structure which is associated with its formation path in the CFB gasifier. A linear monotonic decrease with conversion in both internal surface area and pore volume was then observed while pore size increased slightly with conversion. Pore structure and reactivity data were used to determine the effectiveness factor at various stages of conversion of the semi-char. Effectiveness factor values reveal the semi-char reactions to be close to kinetic controlled condition. [Copyright &y& Elsevier]

Abstract: The purpose of this research is to determine the reactivity of a sample of high carbon fly ash obtained from a circulating fluidized bed (CFB) gasification system and benchmark it against three other chars prepared at different pyrolysis temperatures in a laboratory drop tube furnace (DTF). Isothermal and non-isothermal thermo-gravimetric (TGA) experiments were used to determine sample reactivities. Structural analysis tests were carried out to establish the influence of the pore characteristics and Scanning Electron Microscope (SEM) pictures were taken for sample morphology. In-spite of high (BET) surface area and pore volume; reactivity parameters such as activation energy, kinetic rate coefficient, half life, burnout temperature and time proved the fly ash to be the least reactive. The low reactivity is associated with its carbon thermal deactivation. Non-isothermal tests also show the fly ash to be heterogeneous in composition. Of all the samples, the char prepared in the laboratory at the lowest pyrolysis temperature was the most reactive. [Copyright &y& Elsevier]

With horizontal bias combustion burners, experiments have been carried out on a 670 t h-1, corner-fired, pulverized-coal fired boiler burning bituminous coal. At 200 MWe load, the furnace excess O2 remains stable. The different horizontal fuel biases are obtained by changing the tilt angle of all the Louvre enrichers' regulating blades. The tilt angles of the blades are 0, 15, 24, 32°; the result is that the enriching ratios of the fuel-rich primary air increase from 2.2 to 2.6 at No. 2 corner, and from 1.2 to 4.2 at No. 3 corner. The gas temperature increases in the burner region. The application of the horizontal bias combustion burners results in a reduction in NOx formation from 545.7 mg Nm-3 (O2=6%) to 287.9 mg Nm-3, and a substantial reduction in carbon in ash content from 5.24 to 2.48%. The boiler operated stably at a load of 80 MWe without auxiliary fuel oil. Copyright © 2004 John Wiley & Sons, Ltd. [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]

• 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]

The one-dimensional (1D) spatial distributions of OH absolute concentration in methane/air laminar premixed flat flame under different equivalence ratios at atmospheric pressure are investigated by using bi-directional laser-induced fluorescence (LIF) detection scheme combined with the direct absorption spectroscopy. The effective peak absorption cross section and the average temperature at a height of 2 mm above the burner are obtained by exciting absorption on the Q1(8) rotational line in the A2Σ+ (ʋ′ = 0) ← X2Π (ʋ″ = 0) at 309.240 nm. The measured values are 1.86×10−15 cm2 and 1719 K, respectively. Spatial filtering and frequency filtering methods of reducing noise are used to deal with the experimental data, and the smoothing effects are also compared using the two methods. The spatial distribution regularities of OH concentration are obtained with the equivalence ratios ranging from 0.8 to 1.3. The spatial resolution of the measured result is 84 μm. Finally, a comparison is made between the experimental result of this paper and other relevant study results. [ABSTRACT FROM AUTHOR]

Laser-induced breakdown spectroscopy (LIBS) and Raman spectroscopy were used to analyze the microstructural characteristics after coal pyrolysis. Two coals and their demineralized coals were pyrolyzed at a fixed-bed furnace. LIBS molecular spectra of demineralized coal should be first reported. Experimental results show that the decrease of Raman baseline drift revealed by Raman spectroscopy correlates with the dissipation of substituent, aliphatic, and carbonyl groups with increasing pyrolysis temperatures. Minerals in coal slightly suppress the baseline drift before pyrolysis, but it accounts for less than 10% variation of the α. The LIBS and Raman spectra data show that coal undergoes dry degassing, substituent and aliphatic decomposition, and aromatic-ring condensation as the pyrolysis temperature increases. In the combustion test, the demineralized char has an analogous combustion rate at α ≈ 0 since the char has a similar structure. The minerals in raw coal can improve the combustion rate by at least 39% relative to the demineralized coal. The catalytic effect caused by minerals is more pronounced than thermal inactivation caused by stable structural dissipation from 673 to 1273 K. Raman spectroscopy and LIBS offer fresh insight into the coal-maceral transformation, which provide excellent tools to diagnose structural change from quasi-coal to quasi-char rapidly. • Raman baseline drift may correlate with the substituent, aliphatic, and carbonyl groups. • Coal undergoes dry degassing, substituent and aliphatic decomposition, and aromatic-ring condensation upon pyrolysis. • The demineralized char has analogous combustion characteristics at α ≈ 0. [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]

Laser-induced breakdown spectroscopy (LIBS) was employed to study the spectral characteristics of the C 2 and CN emissions of three types of coal and their pyrolysis char samples. By analysing the atomic and molecular emissions, the electron temperature, electron density and rotational temperature were calculated to evaluate the characteristics of the laser-induced plasma. The correlation between the molecular emissions and coal combustion characteristics was also investigated, which was explained from the aspect of microstructural evolution obtained by Raman spectroscopy. The molecular emissions were gradually inhibited with increasing pyrolysis temperatures, however the molecules tended to distribute at a higher equilibrium temperature. The rotational temperatures increase by 1000–3300 K relative to the raw coal for chars prepared at pyrolysis temperatures of 293–1173 K. The reduction of C 2 and CN emissions, which are thought to be mainly caused by small aromatic rings and amorphous carbon structures, is consistent with the Raman spectroscopy results in that the microstructures evolve towards a more stable large aromatic-ring skeleton after pyrolysis. When the carbon conversion ratio is X c = 50% for three kinds of coal, the R 2 values of the exponential fitting between C 2 emission and carbon conversion rate are all higher than 0.99. • The presence of KBr binder has improved the rotation temperature of plasmas of the coal and char samples. • LIBS molecular emissions of char were inhibited with the increasing pyrolysis temperature. • The reduction of C 2 and CN emissions is consistent with microstructure evolution upon pyrolysis. • The C 2 emissions has a positive correlation with the carbon conversion rate during combustion. [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]

This investigation aims to understand the enhancement characteristics of plasma-assisted combustion in oxy-coal combustion. A 1064 nm Nd:YAG laser was added into a laminar flow reactor to produce laser-induced plasmas (LIPs). The high-speed visualization of the coal flame was used to reveal combustion's progress and evolution following laser-induced breakdown. To the best of our knowledge, this is the first time LIP has been applied to the optimization of pulverized coal combustion. The optical emission spectroscopy (OES) demonstrats that LIPs can generate adequate free electrons and ions during combustion. When the laser's pulse interval exceeds 60 ms, the LIP has a beneficial impact on combustion. For the heterogeneous reaction, the thermal effect generated by LIP only improves the intensity of flame radiation by 11–33 %. In contrast, the combination of chemical and thermal effects of plasma can result in significant enhancements of radiation intensity for high-volatile coals, with improvements of 76–203 %. Following laser-induced breakdown, the ignition delay time also decreases from lignite to anthracite, and the flame's peak intensity occurs earlier. [ABSTRACT FROM AUTHOR]

One of the major challenges regarding measuring chemical structures with laser-induced breakdown spectroscopy (LIBS) is that, it is difficult to track the source of organic components from atomic spectra since these structures have a similar elemental composition. However, LIBS molecular emissions (C 2 , CN, etc.) provide a possibility to investigate these similar compositions. In the present study, an experimental investigation was conducted to diagnose the structural evolution during coal combustion. The structural evolution of char particles was also evaluated as a reference using offline Raman spectroscopy. The significance of minerals on plasma emission was investigated based on plasmas of raw and demineralized coals. As combustion progresses, coal particles show the overall dissipation of active structures such as G r , V L , and V r bands, and the aromatization of char structures. The minerals in coal may increase the free–free bremsstrahlung and free–bound recombination events, resulting in a significant continuous baseline. Presence of minerals also reduces the breakdown threshold and enhances the LIBS's emissions. The char's structural transformation is regarded to be correlated with the rotational temperature (T rot) of CN molecules of LIBS's emissions. The mathematical correlations of linear fitting (R 2: 0.94) and exponential fitting (R 2: 0.99) exist between the drift coefficient and the T rot. This structural transformation might change the LIP's CN formation mechanism and lead to the T rot reduction. [ABSTRACT FROM AUTHOR]

Abstract: Hazardous waste disposal is vitally important as industrial production increases. Grate furnaces are a common means to incinerate hazardous waste. In this present work, a fixed bed assembly is used to experimentally model combustion within grate furnaces. Combustion characteristics are examined and the effects of primary air rate, moisture, bed height and particle size on burning rate, ignition-front speed and temperatures in the bed are also investigated. The results indicate that a rising temperature front descends through the bed while weight loss remains constant during the main combustion stage. Primary air rates and moisture content are shown to have significant effects on burning rates and average ignition-front speeds. Bed height has no effect on burning rates but does have an effect on average ignition-front speeds. Particle size is found to have slight effects on burning rates while having no effect on average ignition-front speeds. [Copyright &y& Elsevier]

The rapid development of measurement technology has created possibilities for the advancement of intelligent combustion diagnostics. The high-speed OH-PLIF was used to investigate homogeneous ignition and combustion characteristics of gas-solid two-phase flame in a laminar flow reactor of oxy-coal combustion. The primary focus is to investigate the physico-chemical processes of gas-solid interaction during the early stages of combustion. The image processing was employed to analyze the flame features and then obtain the ignition mechanism, OH distribution, species radiations, and fluctuation frequency. Results show that the homogeneous reaction dominates the ignition and volatile combustion at the early stages of oxy-coal flame. The ignition delay time and volatile burning time decrease with the increasing gas temperatures and oxygen concentrations. The fluctuation dominant frequency increases from 190.5 Hz to 243.2 Hz at the ambient temperature from 1200 to 1800 K. The dominant frequency stabilizes between 243.2 Hz and 243.5 Hz at the ambient temperature from 1500 to 1800 K. The dominant frequency undergoes a shift to a maximum limit of 243.5 Hz from 1200 to 1800 K. When the oxygen concentration increases from 20 % to 40 %, the dominant frequency continues to rise from 244.8 Hz to 268.7 Hz. • Homogeneous reaction dominates the early stages of oxy-coal flame. • Fluctuation dominant frequency is closely related to ignition processes. • Dominant frequency shifts to a maximum limit with the increasing temperatures. [ABSTRACT FROM AUTHOR]

O 2 /H 2 O combustion is now regarded as a novel and promising technology of coal utilization for next-generation oxy-fuel combustion. The combustion reaction mainly occurs in the mixture of O 2 and H 2 O instead of recycled flue gas (mainly CO 2). The purpose of this research is to clarify the evolution characteristics of O/N-containing complexes (C(O)/C(N)) during O 2 /H 2 O combustion. Fourier transform infrared spectroscopy (FT-IR), temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) were employed in this study. The addition of H 2 O in the atmosphere promoted the generation of NO and HCN in char combustion. There was a good linear relationship between the C(O) amount and char reactivity, as well as the NO conversion ratio, during the O 2 /H 2 O combustion process. Moreover, trace amounts of the precursors of CO 2 with low thermal stability remained on the particle surface in the O 2 /H 2 O combustion. The enhancement of the reaction temperature revealed positive effects on inhibiting NO emissions in the O 2 /H 2 O combustion. The relative amount of pyrrole (N-5), which was the precursor of HCN, increased apparently at the expense of pyridine (N-6) and quaternary nitrogen (N-Q) in the early reaction stage. When the char conversion degree exceeded 50%, N-Q and N-6 became the dominant forms of C(N). • NO is the main N containing gases released in demineralized char O 2 /H 2 O combustion. • Precursors of CO were the main structures of C(O) generated in O 2 /H 2 O reaction. • C(O) amount could be an indicator reflecting char combustion/reduction reactivity. • N-6/N-Q tended to convert to N-5 at early stage of demineralized char O 2 /H 2 O reaction. [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]

Laser-induced plasma ignition has been applied in various combustion systems, however, work on flame stabilization with repetitive laser-induced plasma (LIP) is rather limited. In this paper, stabilization of a premixed methane-air flame with a high repetition nanosecond LIP is reported. The plasma energy coupling and the temporal evolution of the flame kernels generated by the LIPs are investigated with different laser repetition rates, i.e., 1 Hz, 100 Hz and 250 Hz, respectively. The plasma energy coupling is not affected in the air flow and in the premixed methane-air flow with the applied laser repetition rates. Continuous combustion flame stabilization has been achieved with LIPs of 100 Hz and 250 Hz, in terms of catch-up and merging of the consecutive flame kernels. The flame kernel formed by the last LIP does not affect the evolution of the newly formed flame kernel by the next LIP. The catch-up distance, defined as the distance from the LIP initiation site to the flame kernel catch-up position, is estimated for different laser repetition rates based on the temporal evolution of the flame kernels. A higher laser repetition rate will lead to a shorter catch-up distance which is beneficial for flame stabilization. The up limit for the laser repetition rate to realize effective flame stabilization is determined from the critical inter-pulse delay defined from the onset of the LIP to the return of the initially contraflow propagating lower front to the LIP initiation site. The up limit is 377 Hz under the flow conditions of this work (equivalence ratio of 1, flow speed of 2 m/s, and Reynolds number of 1316). [ABSTRACT FROM AUTHOR]

• Homogeneous ignition and reaction control Zhundong coal combustion upstream. • The heterogeneous reaction occurs upstream but dominates burning processes downstream. • The release and combustion of volatiles strongly affect flame stability. • Optical kinetic data are obtained based on an Arrhenius expression. Non-intrusive measurement of dynamic and kinetic parameters during oxy-coal combustion has been carried out in an enclosed downward laminar flow reactor. High-speed OH-PLIF, CH* chemiluminescence, and pyrometer were used to determine the luminescence species, particle velocity, and particle temperature. A two-step model was established to predict the release of coal combustible based on the Arrhenius expression. The experimental data from traditional coal char analysis methods verified the optical kinetic model. Experiments showed that homogeneous ignition first occurred and dominated the upstream volatile combustion. The heterogeneous reaction was activated upstream due to volatile oxidization and became dominant downstream. The coal combustion formed an overlapping region between volatile and char oxidations. This region played a critical role in enhancing flame stability. The duration of the overlapping region was smaller in oxy-fuel combustion than in air-alike combustion due to the higher reaction rate and lower heat and mass diffusion. A temperature-dependent kinetic model was found to obey a two-step first-order Arrhenius expression with two pre-exponential factors (10 3.59 ± 0.22 and 10 5.81 ± 0.31 s−1) and two activation energies (173.1 and 278.0 kJ/mol) in air-alike combustion, and two pre-exponential factors (10 5.02 ± 0.47 and 10 6.37 ± 0.24 s−1) and two activation energies (217.0 and 291.1 kJ/mol) in oxy-fuel combustion. The kinetic predictions agree well with the experimental results of the combustible release in the corresponding tests. [ABSTRACT FROM AUTHOR]

Abstract: Experiments with five samples of corn straw were carried out on a one-dimensional bench combustion test rig. The bed temperature distribution and the mass loss of fuel and gas components such as O2, CO, CO2 and NO were measured in the bed. The combustion of corn straw occurred in two stages, ignition front propagation and char oxidation. The average burning rate increased with an increase in the primary air flow until a critical point was reached, beyond which a further increase in the primary air flow resulted in a decreased burning rate. The mean concentration of NO reached a minimum value and then increased with increased primary air flow. The time taken for the drying front to reach the bottom of the bed was 800s, 700s, and 500s; the temperatures in the high bed temperature zones were 900–935°C, 800–850°C and 700–743°C; and the maximum concentrations of NO were 725ppmv, 1287ppmv, and 2730ppmv, for whole corn stalks, hollow corn stalks and flaked corn stalks, respectively. The maximum concentrations of CO and NO were quite different between samples. There was only one peak in the distribution of NO concentration for sample B, but there were two peaks for whole corn stalks and sample A. [Copyright &y& Elsevier]

O 2 /H 2 O pre-oxidation could be regarded as a promising approach for clean and high efficiency utilization of high rank coals. A typical anthracite was employed for experimental clarification on anthracite-N evolution characteristics during O 2 /H 2 O pre-oxidation. The correlation among reaction atmosphere, nitrogen stripping efficiency and anthracite-N desorption characteristic was evaluated. The emission characteristics of nitrogen containing gaseous species and combustion reactivity of each pre-oxidized semi-coke were identified by utilizing XPS, TPD, Raman and TGA. Both O 2 and H 2 O could synergistically accelerate the crack of large aromatic structures into small and reactive ones, leading to the migration of abundant nitrogen atoms initially located inside the aromatic structures to the edge during the O 2 /H 2 O pre-oxidation process. The decomposition of condensed aromatic systems would strengthen the reactivity of semi-coke particles. Nevertheless, excessive H 2 O (> 10 vol%) would generate massive •H which could penetrate into the matrix of anthracite particles, reducing the total amount of active structures on semi-coke surface. The results in this research indicated that an optimal reaction atmosphere (10 vol% O 2 and 10 vol% H 2 O) was suggested for the anthracite O 2 /H 2 O pre-oxidation to realize the most effective regulation on anthracite-N emission pathway in the processes of both O 2 /H 2 O pre-oxidation and semi-coke combustion [Display omitted] • Targeted stripping mechanism of anthracite-N in O 2 /H 2 O pre-oxidation was studied. • Semi-coke characteristics pre-oxidized in O 2 /H 2 O differed from those in O 2 /N 2. • Correlation between pre-oxidation atmosphere and semi-coke features was clarified. • O 2 and H 2 O synergistically promoted migration of inner nitrogen atoms to the edge. [ABSTRACT FROM AUTHOR]

This research investigated the effects of preheating primary air and fuel size on combustion behaviors of pinewood and corn straw (50% ratio) in a fixed bed. Primary air temperature (T air) was ranged in 20–130 °C, and the fuel size was varied in the range of 3–8 cm in length, which affected the whole combustion efficiency, emissions of hazardous pollutants and alkali metals and chloride. Results showed that by increasing T air the ignition delay time first decreased by 40% and then increased, and both burning and ignition rates increased by 60%; however, the emissions of CO 2 , CO and CH 4 increased first, and then decreased. At the T air = 85 °C the smallest unburned carbon was left in the ash; the mostly generated N-bearing species was NO followed by HCN, and then by a small of NH 3. The higher T air increased the releases of K (by 16%) to the gas phase and promoted the transfer of Cl to HCl, while mildly effects on Ca. Also, increasing fuel size to 5 cm enhanced the combustion stability and decreased the emissions of NO. Accordingly, mildly increase the fuel size and T air were found to be well-operating condition, which provide guidance for selections in large-grate boiler during blend combustion. • Combustion behaviors and emissions of biomass co-combustion were performed. • The parameters of primary air temperature and fuel size were investigated. • A proper air temperature shortened ignition delay time and promote ignition rate. • By preheating primary air increased the released of K and transfer of Cl to HCl. • A proper fuel size can improve combustion stability and decrease emissions of NO x. [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]