Combustion Kinetics Research Articles

Effect of air and exhaust gas dilutions on ultra-fine particulate emissions in different combustion modes

The effect of air and exhaust dilution ratios on the characteristics of ultra-fine particulate matter (PM) and combustion process in different combustion modes at low loads were investigated based on experimental and numerical simulation in this study. The combustion modes included partially premixed compression ignition (PPCI), diffusion combustion (DC), and modulated kinetics combustion (MKC). The air and exhaust gas dilution ratio varied from 0 % to 40 % and 0 % to 55 % respectively. Additionally, the effect of dilutions and combustion modes on PM characteristics is precisely analyzed by computational fluid dynamics. The NOx and Soot emission are reduced in PPCI and DC mode of combustion with the increase of air and exhaust gas dilution rate. However, NOx emission is decreases while Soot is increases in MKC mode. The concentration of particulate number increases in case of both PPCI and DC mode and achieved highest value as 3.3 × 107 and 1.4 × 107 N/cc respectively. Although, concentration of particulate number (PN) in MKC mode first starts climbing and after attaining the highest level of 1.3 × 107 N/cc it falls down. PN concentration in PPCI, MKC, and DC modes decreases as the exhaust gas dilution rate increases. The study of nuclear and accumulate mode of PM was performed separately where the diameter of particles is <1000 nm. Under variable air and exhaust gas dilution ratios, the value of PN is always lies between 40 and 90 % in the nuclear mode PM domain for PPCI and DC modes while for MKC mode, the proportion of PN fall under 13–57 % at exhaust gas ratio above 30 %. The proportion of nuclear mode particulate mass in PPCI mode is >10 % while its values is <1 % for DC and MKC modes.

Microplastics in dyeing sludge: Whether do they affect sludge incineration?

Microplastics (MPs), as emerging contaminant detected in dyeing sludge (DS), inevitably affected the subsequent treatment and disposal of DS. However, the effect of MPs on the predominant disposal path (incineration) of DS remains far from explicit. This study used thermogravimetry-mass spectrometry (TG-MS) method to explore the effect of representative MPs, polyethylene terephthalate (PET) and polyvinyl chloride (PVC), on combustion characteristics, gas evolution and kinetics on DS combustion. Results showed that PET inhibited the whole combustion of DS by physical barrier. Relatively, PVC delayed the combustion of light volatile but promoted heavy volatile and char reaction due to HCl catalyst. Generally, MPs deteriorated the combustibility, burnout performance and combustion stability of DS. MPs aggravated HCl and gaseous N emissions. Noticeably, the interactions between DS and PVC accelerated the emissions of gaseous pollutants, especially under high dose condition. DAEM and FWO models could well describe the combustion kinetic of DS containing MPs. MPs led to an increase in activation energy of DS, namely, it deteriorated the combustion efficiency of DS. The combustion mechanisms could be divided into two stages: (1) diffusion (D3) stage: melted MPs blocked the gas channels, (2) chemical reaction (F3): the residual chars were thermally stable.

Self-propagating high-temperature synthesis of MoAlB boride ceramics based on MAB-phase

This study focuses on the combustion kinetics and mechanisms of reaction mixtures in the Mo–Al–B ternary system taken so that the MoAlB MAB phase was formed. The effect of the initial temperature on the key combustion parameters was demonstrated. Reaction mixture preheating was found to weakly affect the maximum combustion temperature. The effective activation energy of self-propagating high-temperature synthesis (SHS) was calculated. Phase diagrams in the Mo–Al–B system were built using the AFLOW and Materials Project databases. The phase composition and structure of the synthesized ceramics with MoAlB lamellar grains 0.4 μm thick and ~2–10 μm long as a main component were studied. The DXRD lines of MoB and Mo2B5 intermediate borides with their total content of ≤3 % were also identified. Scanning electron microscopy and energy dispersive spectroscopy studies revealed that the Al2O3 phase was present in the intergranular pores. A sequence of chemical transformations in the combustion wave was studied, and a hypothesis about the structure formation mechanism was put forward. MoO2 and Al2O3 can be the primary phases during SHS, and the MoAlB phase is formed from the boron-containing aluminum–molybdenum melt. Submicron-sized MoB precipitates are formed in the post-combustion zone due to the partial oxidation of aluminum by the dispersion strengthening mechanism.

Co-Combustion of Food Solid Wastes and Pulverized Coal for Blast Furnace Injection: Characteristics, Kinetics, and Superiority

The combustion characteristics and kinetics of food solid wastes (FSW), pulverized coal (PC), and their mixtures were studied by a non-isothermal thermogravimetric method. In the co-combustion of FSW and PC, with the increase in FSW content in the mixture, the initial decomposition temperature, burnout temperature, and ignition temperature of the mixture decreased, and the flammability index and comprehensive combustion characteristic index gradually increased. The co-combustion of FSW and PC showed an inhibitory effect in the devolatilization stage but exhibited a combustion-promoting effect in the fixed carbon combustion stage. The interaction between FSW and PC while co-combusting them appeared to be dominated by thermal effects. On one hand, FSW combusted first and released heat that was partially absorbed by the PC, which hence suppressed the devolatilization stage of the co-combustion process. On the other hand, the PC absorbed the heat released by the combustion of the FSW, which increased the combustion rate of the PC in the fixed carbon combustion stage of the co-combustion process. The activation energy of the devolatilization stage and the fixed carbon combustion stage of the co-combustion process was calculated to be 34.16–74.52 kJ/mol and 15.04–36.15 kJ/mol, respectively. In general, the combustion performance of FSW is better than that of PC. The mixed injection of FSW and PC can improve the overall combustion efficiency and reduce CO2 emissions in the iron-making process.

Combustion kinetics of [formula omitted]-propylamine: Theoretical calculations and ignition delay time measurements

To better understand the combustion chemistry and improve the model performance of aliphatic amines, the kinetics of H-abstraction reactions from n-propylamine (NPA) by H, CH3, OH, and NH2 radicals and the ignition characteristics of NPA are investigated. The CCSD(T)-F12/cc-pVTZ-F12 method is selected as the benchmark, and the performances of several density functional theory methods are evaluated by comparing the calculated reaction energies and barrier heights against the benchmark values. The M06-2X/jun-cc-pVTZ method is identified as the best one with a mean unsigned deviation of 0.59 kcalmol−1. With the selected method, the rate constants are calculated using the variational transition-state theory combined with the small-curvature tunneling and multi-structural torsional anharmonicity (MS-CVT/SCT) options at 298–2000 K. The H-abstraction reactions at the α-C site are the dominant channels for NPA + H/CH3/NH2, but this channel is less important for NPA + OH due to the significant variational effect. The ignition characteristics of NPA is studied in a shock tube at 1105–1441 K, 4–20 atm, and different equivalence ratios (0.5, 1.0, and 2.0). The ignition delay time of NPA decreases noticeably with increasing temperature and pressure, while it is insensitive to equivalence ratio. Based on our calculations, a literature model is updated and then assessed against the measured ignition delay times. The updated model shows a better agreement with the measurements but fails to predict the ignition behavior of NPA under certain conditions. The reaction pathway and sensitivity analyses demonstrate that the unsaturated aliphatic amines and imines (such as CH3CHCHNH2 and CH2NH) are the important nitrogen-containing intermediates, and further investigation of the subset of these species is needed to improve the combustion model of aliphatic amines.

Effect of CaO and hydrothermal carbonization conditions on the fuel characteristics of rice husk hydrochars.

The effects of hydrothermal temperature and catalyst concentration on the basic elements and combustion characteristics and kinetic parameters of hydrochars were investigated using ultimate analyzer and thermogravimetric method with rice husk as the research object and CaO as the additives. The results showed that: (1) the fixed carbon content of hydrochars gradually increased with the increase of hydrothermal temperature, whereas the volatile content gradually decreased. When CaO was added, the changes of fixed carbon and volatile fraction gradually decreased with the increase of hydrothermal temperature, and H/C atomic ratio increased to different degrees, which had a certain inhibitory effect on the degree of hydrothermal carbonization of rice husk. (2) The peak of the volatile fraction combustion section of hydrochars combustion derivative thermogravimetric curve was higher than that of the fixed carbon combustion section. CaO concentration has less effect on the volatile combustion section, and the combustion peak of the fixed carbon section is significantly reduced. (3) When the heating rate of the combustion test is accelerated, the ignition and burnout temperatures of the sample increase and the overall combustion curve shifts to the high temperature region. (4) The comprehensive combustion index SN decreases with the increase in hydrothermal temperature. When the hydrothermal temperature is certain, the CaO concentration causes the SN to increase and then decrease, which finally reduces the combustion performance of hydrochars. (5) The activation energy of the fixed carbon combustion section of hydrochars is lower than that of the volatile combustion section, and the activation energy of both volatile and fixed carbon combustion sections gradually decreases after adding CaO. The primary reaction kinetic model was used to describe the combustion kinetics of hydrochars, and the correlation coefficients (R2) were all above 0.92, and the results were reliable.

Development and validation of a dynamic flowsheet model for a 350 MWe supercritical circulating fluidized bed boiler

The circulating fluidized bed (CFB) boiler is highly recommended in peak shaving for its wide load regulating range compared with the pulverized coal (PC) boiler. However, the research on dynamic flowsheet model of the CFB boiler is quite limited due to its complex flow regime. In this paper, both steady state and dynamic flowsheet model of a supercritical 350 MWe CFB boiler was developed based on Aspen Plus and Aspen Dynamics software coupled with comprehensively user defined combustion kinetics and heat transfer coefficient with FORTRAN subroutines. This model was then validated by the operating data of the 350 MWe supercritical CFB boiler. Based on this model, the dynamic characteristics of three typical step signal disturbances, coal flow rate, primary flow rate and secondary air flow rate, were further analyzed to provide theoretical references to dynamic operation. Also, the quantitative relation between the stabilization time and step signal amplitude was calculated. The calculated dynamic characteristics demonstrates that the heat distribution does not vary much after adding step signal disturbances of coal flow rate, primary air flow rate and secondary air flow rate. The step signal disturbance of secondary air flow rate has less influence to the CFB boiler system compared with that of primary air flow rate. The superheated steam temperature is more sensitive to step signal disturbances compared with reheated steam temperature. The higher step signal amplitude results in longer stabilization time. Above all, this dynamic flowsheet model can not only be used to conduct dynamic behavior prediction and analysis but also provide transfer functions to the further development of CFB automatic control system.

Theoretical Research on the Combustion Mechanism and Kinetics for Aluminum and Carbon Dioxide

AbstractThe reaction mechanism and kinetics of ground state aluminum (Al) atom and carbon dioxide (CO2) molecule were studied by quantum chemical calculations. The transition states and reaction paths were investigated at the B3LYP/6‐311+G(d,p) (Becke type three‐parameter Lee‐Yang‐Parr functional model) basis set. The energy of each stagnation point was calculated at the high‐precision CBS‐QB3 (limiting complete basis set) level of theory. The reaction of CO2 with Al is a complex process. There are four basic reaction processes and two main reaction paths (Reaction pathway 1: Al+CO2→IM1→TS1→IM2→TS2→AlO (2Π)+CO and Reaction pathway 2: Al+CO2→IM3→TS3→AlCO2linear→TS4→AlO (2Σ)+CO). The temperature‐ and pressure‐dependent rate constants of the reaction of Al and CO2 was estimated by CBS‐QB3//B3LYP/6‐311+G(d,p) level at 300∼3000 K, which is combined with Transition State Theory (TST), Variational Transition State Theory (VTST), and Rice‐Ramsperger‐Kassel‐Marcus (RRKM) theory. The total rate constant of Reaction pathway 1 (ktotal,1) is much higher than that of Reaction pathway 2 (ktotal,2), indicating that the reaction of Al with CO2 is easier to occur through Reaction pathway 1. Compared with ktotal,2, ktotal,1 shows a strong pressure dependence at high temperature, but it is almost independent with the pressure at low temperature.

SGD-based optimization in modeling combustion kinetics: Case studies in tuning mechanistic and hybrid kinetic models

Chemical kinetic modeling is an integral part of combustion simulation, and extensive efforts have been devoted to developing high-fidelity yet computationally affordable models. Despite these efforts, modeling combustion kinetics is still challenging due to the demand for expert knowledge and high dimensional optimization against experiments. Therefore, data-driven approaches that enable efficient discovery and calibration of kinetic models have received much attention in recent years, the core of which is the high-dimensional optimization based on big data. Evolutionary algorithms are usually adopted for optimizing chemical kinetic models, although they usually suffer from high computational costs and are limited to a small number of parameters. Meanwhile, gradient-based optimizations, especially the stochastic gradient descent (SGD) methods, have shown success in developing complex models by training large-scale deep learning models. Therefore, this work explores the applications of SGD-based optimizations in tuning mechanistic kinetic models and learning hybrid kinetic models. We first showed that SGD-based optimizations could substantially save computational cost compared to evolutionary algorithms when the number of kinetic parameters in mechanistic models reached about one hundred. We then demonstrated that the SGD-based optimization enabled us to use a neural network model to represent the pyrolysis of the Hybrid Chemistry and optimize the associated hundreds of weights in the neural network. These proof-of-concept studies showed that the SGD-based optimization is more efficient than evolutionary algorithms, is a promising approach for developing chemical kinetic models with high dimensional parameters, and is capable of developing hybrid mechanistic-machine learning kinetic models.

Comparison of combustion characteristics and kinetics of Jurassic and Carboniferous-Permian coals in China

The study of combustion characteristics and kinetic parameters plays an important role in the determination of the coal fire combustion regime. Four Chinese coals from the Jurassic and Carboniferous-Permian periods were selected to study the influence of the geological time on the coal combustion characteristics. The thermogravimetry and differential scanning calorimetry with different heating rates were employed to identify the combustion characteristics and oxidation reaction kinetics of coal samples. It was found that a high heating rate could boost the characteristic temperatures of coal, and the curves characteristics of thermogravimetry and differential scanning calorimetry of all coal samples were the same. The ignition temperatures of coals from the Carboniferous-Permian period were lower than that of coals from the Jurassic period. During the coal combustion stage, the heat release of the Carboniferous-Permian coal samples was 166.86–2423.84 J/g, higher than that of the Jurassic coal samples. By Flynn-Wall-Ozawa, Kissinger-Akahira-Sunose, and Starink methods, it was found that the activation energy required for combustion reaction of Carboniferous-Permian coals was 37.18–88.95 kJ/mol, lower than that of Jurassic coals, revealing that the combustion performance of Carboniferous-Permian coals was superior to that of Jurassic coals.

Experimental study of torrefaction of camellia seed shell for solid fuel production in the context of a carbon neutrality roadmap

Camellia seed shell (CSS) was torrefied at 200, 230, 260, and 290 °C for 30 minutes in a tubular reactor under a nitrogen atmosphere. The effect of temperature on the fuel quality and combustion characteristics of the torrefaction samples was investigated. The pyrolysis characteristics and kinetics of the raw materials at different heating rates were studied. Compared to the raw material, the roasted samples had an increase in the higher heating value (HHV) range of 0.33 to 4.18 MJ/kg, which reached 24.21 MJ/kg at 290 °C. The mass and energy yield ranged from 93.8% to 65.2%. The decrease in energy yield was much higher than the increase in HHV. The enhancement factor and energy yield correlated well (R2&gt; 0.99). The combustion behavior and kinetics of the torrefaction samples were studied in detail using the First Order Pseudo Bi-component Separate-stage Model (PBSM-O1). From an economic point of view, the best torrefaction temperature for CSS was 260 °C.

Application of machine learning to optimize the combustion characteristics of RCCI engine over wide load range

The objective of this study is to investigate, optimize, and compare the combustion features and intake variables affecting the kinetics of gasoline/diesel RCCI engine by applying the genetic algorithm. Six decision variables include the fuel flow rate (FFR), premixed fuel rate (PFR), exhaust gas recirculation (EGR) rate, first injection mass ratio (IMR1), and the first and the second start of injection (SOI1 and SOI2). The objective of the optimization was to simultaneously maximize the engine gross indicated efficiency (GIE), the second law efficiency (SLE), and minimize exhaust emissions and the maximum pressure rise rate. The optimal space filling design of experiment method was applied to evaluate the effect of the decision variables on the objective functions with a minimum number of numerical experiments. The relationships between the decision variables and objective functions were approximated by the genetic aggregation response surface model. Finally, the non-dominated sorting genetic algorithm II was employed to find the optimum values. In the optimal case GIE, SLE, and Fmerit increased by 1.56%, 1.25%, and 15.14% while NOx concentration and engine noise reduced by 16.67% and 33.45%, respectively. The best range of SOI2 for GIE is between –22 and −14 degrees ATDC, while for improving SLE, SOI2 should be advance. Sensitivity analysis shows that PFR and FFR are the most influential variables to moderate the combustion kinetics and increasing IMR1 has the proper effect on combustion by creating more opportunities for a homogeneous charge and reducing hot spots.

Mechanistic understanding of methane combustion over H-SSZ-13 zeolite encapsulated palladium nanocluster catalysts

Catalytic methane (CH4) combustion to CO2 and H2O is of great practical significance and an important prototype catalytic reaction. Extensive experimental studies have suggested that zeolite supported palladium (Pd) catalysts are very active for catalytic CH4 oxidation, while the structure-performance relationship is still not clear. Herein, using H-SSZ-13 zeolite encapsulated Pd nanoclusters as a demonstration case, reaction mechanisms and kinetics of complete CH4 combustion were systematically investigated using first-principles density functional theory (DFT) calculations combined with atomistic thermodynamic analysis and the energetic span model (ESM). Four H-SSZ-13 zeolite encapsulated PdⅡxOmHn model catalysts, i.e., PdⅡ/H-SSZ-13, PdⅡO/H-SSZ-13, PdⅡ2O/H-SSZ-13, and PdⅡ3O3H/H-SSZ-13 were studied. The encapsulated [PdⅡ2O]2+ and [PdⅡ3O3H]+ nanoclusters were identified as the most stable binuclear and trinuclear Pd structures under experimental oxidative conditions. DFT calculations indicated that the most kinetically relevant step in the complete CH4 oxidation reaction over four Pd model catalysts is different. The first, third, third, and fourth C–H bond cleavage were identified as the most kinetically relevant steps over PdⅡ/H-SSZ-13, PdⅡO/H-SSZ-13, PdⅡ2O/H-SSZ-13, and PdⅡ3O3H/H-SSZ-13, respectively. Using the energetic span model, the relative turnover frequencies of CH4 oxidation over four PdⅡxOmHn/H-SSZ-13 model catalysts were calculated. Consistent with recent experimental observation, the encapsulated PdⅡ3O3H/H-SSZ-13 nanocluster was found to be the most active catalyst for complete CH4 oxidation. On the basis of DFT results and the ESM model analysis, it is noted that the reaction rate of complete CH4 oxidation over the PdⅡxOmHn/H-SSZ-13 catalysts is not completely dependent on one “highest” activation barrier, i.e., the barrier for the most kinetically relevant step. Neither one specific transition state nor one reaction step carried all the kinetics information that determines the catalytic performance of the PdⅡxOmHn/H-SSZ-13 catalysts. The present work sheds light on the structure-performance relationship for CH4 combustion over the zeolite supported noble metal catalysts.

Investigation into the combustion kinetics and spontaneous ignition of sweet sorghum as energy resource

This study investigated the combustion kinetics and spontaneous ignition of sweet sorghum using thermogravimetric analysis and the Frank-Kamenetskii theory. The aim was to determine the proper operating conditions for a direct combustion reactor and predict the safe ambient temperature limits for given silo designs. Oxidative heating rates of 2, 5, and 10 °C/min were set up. Graphical observation shows that combustion was composed of two different stages representing the overlapping processes of pyrolysis and char oxidation, at 131–336 °C and 336–475 °C, respectively. Samples were found to ignite at 215 °C and were extinguished at 433 °C. Different heating rates shifted combustion characteristics to higher temperatures and increased reactivity for ignition and combustion indices up to 12 and 10 times higher. The Friedman method determined the apparent activation energies representing the combustion reaction by 132.91 kJ/mol. Regarding spontaneous ignition, the temperature safe limits were predicted to be 83–84 °C and 84–87 °C for cylindrical and box silos with diameter and height of 15 and 10 m, respectively. Calculations of silos were designed within the limits of certain dimension ratios.Graphical

Effects of CaO-Fe2O3-Fe3(PO4)2 in sewage sludge on combustion characteristics and kinetics of coal slime

The co-combustion of sewage sludge (SS) and coal slime (CS) is a promising approach to realize the harmless disposal of these two solid wastes. Aiming at the effects of the minerals in SS on CS combustion during the co-combustion, the combustion characteristics and kinetics of CS under the action of CaO, Fe2O3, and Fe3(PO4)2 were investigated by a thermogravimetric analyzer. The effects of the single and the composite minerals on CS combustion were quantified, and the synergistic or inhibitory effects between the minerals were also analyzed. Furthermore, Flynn-Wall-Ozawa (FWO) method and the master plots method were used to calculate the combustion kinetics. The results revealed that the catalysis of the minerals in SS was an important factor for the interaction between SS and CS during the co-combustion. For the single minerals, Fe2O3 exhibited the best catalytic effect, which reduced the ignition temperature (Ti) and burnout temperature (Tb) by 8.2 °C and 42.2 °C, respectively, and increased the comprehensive combustion characteristic index (S) by 10.7%. For the composite minerals, Fe2O3 and CaO synergistically catalyzed CS combustion through a cascade chain catalytic mechanism, resulting in a 15.9% increase in S compared with CS combustion alone. However, the interaction between Fe2O3 and Fe3(PO4)2 at high temperatures produced dense products that inhibit the burnout process. The kinetic results showed that the minerals in SS led to a decrease in the apparent activation energy of CS during the combustion stage of the fixed carbon, and the order of decreasing degree was Fe2O3 + CaO > Fe2O3 > Fe2O3 + CaO + Fe3(PO4)2. In addition, the combustion of CS was a multi-step kinetic process, and the mechanism function changed from f(α)= (1-α)4 to f(α)= (1-α) as the combustion progressed. The composite mineral Fe2O3 + CaO participated in the combustion reaction of fixed carbon in CS, changing the reaction mechanism from F1 to F1.3.

A comparative study of thermal and combustion kinetics for raw and bio-chars of eucalyptus wood and bark

ABSTRACT This study investigates thermal and combustion kinetics of raw and bio-char samples of eucalyptus wood and bark. Raw samples of eucalyptus wood and bark were subjected to pyrolysis at temperatures of 300, 350, 400, and 450°C. The maximum mass yield was obtained at 350°C for both bio-char samples of wood and bark. Gross calorific value, proximate, ultimate, and chemical analyses of the raw and their derived bio-chars showed high energy yield of bio-chars as compared to raw samples. Thermal analysis of raw and their respective bio-chars was also studied using thermogravimetric analysis in air at heating rates of 25, 35, and 45°C min−1. Raw wood and bark in comparison to their bio-chars showed delayed ignition that resulted in increase of ignition, peak, and burn-out temperatures. Mean reactivity of raw and bio-char samples was obtained using Ghetti model that showed higher reactivity of raw samples than bio-chars. Kinetic parameters including activation energy and order of reaction were found using Arrhenius model. For different heating rates, activation energies lied in the range of 61–108 kJ mol−1 (wood) and 42–79 kJ mol−1 (bark). Higher carbon contents in the bio-char samples resulted in higher activation energies than raw samples. This increasing carbon contents trend also furnished increase activation energies of bio-chars.

Thermogravimetric Analysis of the Combustion Characteristics and Combustion Kinetics of Coals Subjected to Different Chemical Demineralization Processes.

In order to explore the influence of different chemical demineralizations on coal combustion characteristics and combustion kinetics, five coals subjected to different chemical demineralization processes were investigated via thermogravimetric analysis. The ash contents of clean coal was reduced to 0.1–1.55% after different chemical demineralizations. The ignition temperature of coal decreased by 12–69 °C, and the peak temperature decreased by 7–62 °C. The burnout temperature of clean coal increased by 63 °C after demineralization by NaOH. The adsorption of noncombustible NaOH into the porous structure of TaiXi-3 caused an increase in burnout temperature. Alkali-soluble minerals were proven to have a negative effect on the combustion performance of coal, while acid-soluble minerals had the opposite effect. The combustion kinetics of five kinds of coals at a heating rate of 10 °C/min was investigated. The activation energy of coal obviously changes before and after demineralization (58.39–91.39 kJ mol–1). The activation energy of clean coal is obviously lower than that of raw coal.

Combustion kinetics of hydrochar from cow-manure digestate via thermogravimetric analysis and peak deconvolution

Hydrothermal carbonization (HTC) can convert wet biomass into hydrochar (HC), a solid carbonaceous material exploitable as fuel. In this study, HTC was applied to anaerobic digestate from cow manure. HCs obtained at three HTC temperatures (180, 220, 250 °C) were characterized in detail and their combustion behavior was investigated by thermogravimetric analysis (TGA) coupled with peak deconvolution. Increasing HTC temperatures increased the fixed carbon content (17.9–20.7%), the ash content (27.2–32.5%) and the calorific value (14.3–18.2 MJ/kg), while decreased the hydrogen (5.01–4.54%) and oxygen content (24.09–12.35%) of HCs. DTG profiles peak deconvolution unveils the presence of five major components in the HCs. HCs combustion kinetics were studied applying the KAS method. Average apparent activation energy values of 100, 88, 67 kJ mol−1 were obtained for HC180, HC220, HC250, respectively. HTC at 250 °C produced the HC with the best fuel characteristics.

Gas-heat characteristics and oxidation kinetics of coal spontaneous combustion in heating and decaying processes

To reduce the hazardous effect of coal fires, gas-heat dynamic characteristics were investigated in the closed fire zone. A temperature-programmed experiment and a C80 microcalorimeter were employed to evaluate the heating and decaying processes of coal spontaneous combustion, CO production and thermal oxidation were compared to analyze the gas-heat dynamic relationship and nonlinear dynamics. The results revealed that CO production and the heat flux exhibited obvious hysteresis in the decaying process, the oxygen concentration linearly increased with the total CO production and cumulative heat release. Moreover, the heat release in the heating process was lower than that in the decaying process under identical oxygen-poor conditions, this mechanism increasingly dominated with increasing oxygen concentration. Additionally, the total CO production exhibited a good linear relationship with the cumulative heat release during the heating process, the decaying process exhibited a nonlinear trend due to the hysteresis effect. Compared with that of the heating process, the apparent activation energy of the decaying process was lower, the coal spontaneous combustion risk was higher and reignition was more likely to occur. These research results are great practical significance for inhibiting coal fire hazards and guiding fire area unsealing in the closed fire zone.

Capturing the flame structure and the transition process of the fire whirl using two combustion kinetic considerations

PurposeThe purpose of this paper is to investigate the development process of the fire whirl in the fixed-frame facility and focus on the impacts of the fire whirl’s vortex core on the formation and flame structure of the fire whirl.Design/methodology/approachThe complex turbulent reacting flame surface is captured by the large eddy simulation turbulence closure coupled with two sub-grid scale (SGS) kinetic schemes (i.e. the chemistry equilibrium and steady diffusion flamelet). Numerical predictions are validated thoroughly against the measurements by Lei et al. (2015) with excellent agreements. A double maximum tangential velocity refinement approach is proposed to quantify the vortex cores’ instantaneous location and region, addressing the missing definition in other studies.FindingsThe numerical results show that the transition process of the fire whirl is dominated by the vortex core movement, which is related to the centripetal force. The unsteadiness of the fully developed fire whirl was found depending on the instantaneous fluctuation of heat release rate. The steady diffusion flamelet scheme is essential to capture the instantaneous fluctuation. Furthermore, the axial velocity inside the vortex core is the key to determining the state of fire whirl.Practical implicationsDue to intensive interactions between buoyant fires and ambient rotating flow, the on-set and formation of fire whirl still remain largely elusive. This paper focused on the transition process of fire whirl between different development stages. This paper provides insights into the transition process from the inclined flame to the fire whirls based on the centripetal force.Originality/valueThis paper presented and compared two SGS kinetic schemes to resolve the fire whirl development process and the unsteadiness of its vortical structures. The modelling framework addresses the shortcoming of previous numerical studies where RANS turbulence closure and simplified combustion kinetics was adopted. Numerical results also revealed the fire whirl transition process and its relationship to centripetal force.