Coal Combustion Model Research Articles

Integrating Flow Field Dynamics and Chemical Atmosphere Predictions for Enhanced Sulfur Corrosion Risk Assessment in Power Boilers.

In this work, we attempt to explain the phenomenon of sulfur corrosion of power boiler water walls under the conditions of large fluctuations in carbon monoxide concentrations. To assess the conditions required for corrosion formation, a criterion based on the chemical and flow field parameters of the flue gas is proposed. The formulated sulfur corrosion criterion is based on the mixture fraction variance and the turbulence time scale. Numerical modeling of coal combustion in a 250 MW power boiler is performed using ANSYS. Two cases of combustion in a boiler are analyzed, with the first simulating the boiler operated using classic high-swirl burners and the second one accounting for boiler operation with modified low-swirl burners. Calculations of pulverized coal combustion are performed using the standard k-ε turbulence model and the combustion described by the mixture fraction. The simulation results reveal that the low-swirl burner is characterized by higher values of the mixture fraction variance and a higher frequency of fluctuation of the velocity field, which is strongly related to an increased corrosion rate. The study outcomes show the validity of using the criterion of the mixture fraction variance and velocity field fluctuations to determine the areas at risk of sulfur corrosion.

New exploration on the goaf inerting with hidden fire source under CO2 injection volume of adsorption compensation

To strengthen the rationality of the design of CO2 injection volume in goaf, a framework is proposed to study goaf inerting characteristics with hidden fire sources under CO2 injection volume of adsorption compensation. The zone of hidden fire source is determined by similar simulation experiment of coal spontaneous combustion on a working face of coal mine. The range of injection volume without adsorption is calculated theoretically. A three-dimensional initial flow field model of coal spontaneous combustion is established combined with field measurement. The error of oxidation zone width is 3.64 %, which verifies the reliability of model. The influence of injection volume on the inerting effect in goaf is studied, and optimal injection volume without adsorption is 1000 m3/h. Based on the relationship between adsorption capacity and temperature, a quantization model of injection volume with adsorption compensation is established. The maximum dissipation ratio is then proposed. The optimal injection volume after adsorption compensation is calculated. The results show that intermittent injection of gaseous CO2 is recommended under simulated conditions. The maximum dissipation ratio is 1/4, and the CO2 injection volume compensated by adsorption is 1333 m3/h. The research results can provide theoretical guidance for the optimization of CO2 inerting parameters in goaf.

Coal combustion modeling: A comparative study

Coal combustion modeling: A comparative study

Towards realistic structural char models: Generation of stacked graphene-like layers using constrained reactive molecular dynamics simulations

For a molecular understanding of coal combustion, realistic coal and char models are pivotal. We propose a novel top-down method for the construction of char models, consisting of stacked graphene-like layers with defects. We start on the mesoscale with corrugated stacked surfaces as a template. Then the carbon atoms are pre-positioned on these surfaces and surrounded by hydrogen atoms. In a constrained reactive MD simulation the carbon atoms are then rearranged into a chemically reasonable state with dangling bonds saturated by hydrogen atoms. Importantly, the initial shape of the template is maintained to a great extent, leading to a substantial amount of defects in the graphitic planes, especially for high curvatures. In a final step, small molecular fragments and unfavorable arrangements are removed and remaining dangling bonds are saturated with hydrogen atoms. The central advantage over other approaches is the ability to choose the shape and size of the templating planes, with the possibility to use experimental input.The method is validated on single sheets with a protrusion and application examples are shown for stacked graphitic layers with up to 100 Å size and more than 20000 atoms. It represents the first step of a longer term project. In the future, heteroatoms and functional groups will be implemented, and by combining several such stacks, the approach will allow the construction of more realistic char models using the data from HRTEM images as an experimental boundary condition. On this basis important questions like the adsorption and diffusion of volatiles under high temperatures can be studied on a molecular level.

Research Status and Development Trend of Coal Spontaneous Combustion Fire and Prevention Technology in China: A Review.

Coal seam spontaneous combustion fire is not only one of the main forms of the five major mine disasters, but also the main cause of secondary disasters such as mine gas and coal dust explosions. In recent years, with the advancement of mechanization, automation, and intelligent mine construction, spontaneous coal fires in mines have presented a series of new characteristics, and the prevention and control of spontaneous coal fires are also facing new challenges. On the basis of literature research, this paper summarizes and discusses the basic theory of coal spontaneous combustion, monitoring and early warning methods, and prevention and control technology, summarizes the development process of coal spontaneous combustion theory, reviews the research progress of coal spontaneous combustion monitoring and early warning methods and prevention and control technologies, and discusses the future development direction. In terms of the basic theory of spontaneous combustion of coal, from the initial hypothesis of spontaneous combustion of multielement coal to the unified understanding of coal-oxygen composite theory, a complete set of theoretical systems have been established, and a lot of macro and micro studies have been carried out and analyzed from multiple perspectives. In terms of coal spontaneous combustion monitoring and early warning, from the initial single indicator gas early warning to the construction of gas index system, the hierarchical early warning system is studied, and gradually tends to be perfect. With the development of automation and intelligence technology, the monitoring of coal spontaneous combustion disasters has also formed a new monitoring technology with beam tube monitoring as the traditional method, distributed optical fiber, wireless AD hoc network temperature measurement, and a coal spontaneous combustion multiparameter wireless monitoring system. In terms of fire prevention and control, the traditional "prevention" and "treatment" have changed to the "prevention-control-extinction" technical system based on hierarchical early warning, and the focus has gradually shifted to "prevention", and a large number of antifire materials have been developed, including blocking materials and fire-fighting materials. However, the precise inhibition and control of coal spontaneous combustion disasters, the evolution model of coal spontaneous combustion under the conditions of multifactor coupling in the field, the reliability and stability of intelligent monitoring system, the noncontact detection method of fire source, and the collaborative adaptation of multiple prevention and control techniques are not yet clear. In the future development, the mechanism of spontaneous coal combustion and its evolution process and other basic theories should be deeply studied. On the basis of the mechanism optimization early warning method of spontaneous coal combustion process, flame retardant and fire prevention materials should be targeted and developed. On the basis of the spatiotemporal evolution of spontaneous coal combustion, monitoring and monitoring system equipment with high speed, high precision, and high stability should be developed, which should accelerate the realization of accurate dynamic sensing and intelligent early warning of coal spontaneous combustion, and form an active hierarchical collaborative prevention and control system based on the trinity of "prevention-control-extinction" of coal spontaneous combustion. The conclusions and prospects of this paper can be used for reference in the future research direction, and have a certain role in promoting the exchange of research results of coal science and technology workers.

Prediction of Coal Spontaneous Combustion Hazard Grades Based on Fuzzy Clustered Case-Based Reasoning

Accurate prediction of the coal spontaneous combustion hazard grades is of great significance to ensure the safe production of coal mines. However, traditional coal temperature prediction models have low accuracy and do not predict the coal spontaneous combustion hazard grades. In order to accurately predict coal spontaneous combustion hazard grades, a prediction model of coal spontaneous combustion based on principal component analysis (PCA), case-based reasoning (CBR), fuzzy clustering (FM), and the snake optimization (SO) algorithm was proposed in this manuscript. Firstly, based on the change rule of the concentration of signature gases in the process of coal warming, a new method of classifying the risk of spontaneous combustion of coal was established. Secondly, MeanRadius-SMOTE was adopted to balance the data structure. The weights of the prediction indicators were calculated through PCA to enhance the prediction precision of the CBR model. Then, by employing FM in the case base, the computational cost of CBR was reduced and its computational efficiency was improved. The SO algorithm was used to determine the hyperparameters in the PCA-FM-CBR model. In addition, multiple comparative experiments were conducted to verify the superiority of the model proposed in this manuscript. The results indicated that SO-PCA-FM-CBR possesses good prediction performance and also improves computational efficiency. Finally, the authors of this manuscript adopted the Random Balance Designs—Fourier Amplitude Sensitivity Test (RBD-FAST) to explain the output of the model and analyzed the global importance of input variables. The results demonstrated that CO is the most important variable affecting the coal spontaneous combustion hazard grades.

Combustion in a chamber furnace with a tangentially swirling vortex

Relevance. The need to analyze the furnace processes in boiler units with vertical vortex when arranging pulverized coal combustion with the enhancement of environmental parameters through the installation of tertiary blast nozzles. Despite the carbon-free policy in modern power engineering coal remains one of the main sources of energy, so the reconstruction of boiler units in order to reduce harmful emissions is very relevant. Aim. To study aerodynamics and combustion processes of pulverized coal fuel in the furnace chamber of a boiler unit with a tangential arrangement of burner devices using a combustion scheme with tertiary air blowing. Methods. The Eulerian–Lagrangian approach was applied to numerical study of flow characteristics, heat transfer and combustion in a furnace with a tangential burner arrangement. Also, k-ε model of gas turbulence, particle-turbulence interaction, diffusion model of particle dispersion, P-1 radiation modeling method and pulverized coal particle combustion model based on global particle kinetics and experimental data were used for numerical calculations. Results. Based on numerical calculations and analysis, the dependences of combustion product concentrations, temperature, hydrodynamics at combustion of Kuznetsky coal grade D in the furnace chamber with tangential arrangement of burners for brown coal, as well as at installation of tertiary blast nozzles have been obtained. Insufficient redistribution of air between burners and tertiary nozzles is revealed, as for maintenance of stable twist of vertical vortex and accordingly presence of high values of velocities at the outlet of burner devices it is not possible to increase the portion of tertiary air without reconstruction of burners. It is also established that the volume of the furnace chamber below the burners is poorly involved in heat exchange.

A spatio-temporal temperature prediction model for coal spontaneous combustion based on back propagation neural network

Coal spontaneous combustion (CSC) occurs with the change of temperature and gas concentration which is useful for early warning. To explore the intrinsic correlation between temperature evolution and gas distribution, a Back Propagation Neural Network (BPNN) model of coal temperature - gas composition - spatial distance was constructed, which is based on the gas characteristics and spatial distribution of coal temperature in large-scale coal spontaneous combustion experiments. The results showed that the O2 concentration reached the lowest value (8.058%) at 45 cm on Day 52 with the time. CO and CO2 also formed concentration peaks at 45 cm, and the gas distribution continuously moved towards the inlet position. The hydrocarbon gas components showed periodic increases, with values below 100 ppm. When the supply air time increased, the temperature rise rate in the initial stage of oxidation was slow but relatively uniform. Affected by ventilation conditions and coal density, two high-temperature zones were observed at 45 cm and 85 cm in the experimental furnace. With the inclusion of spatial distance parameters in coal temperature prediction, the BPNN model exhibits accurate predictive capability, with a total error of 0.94997. This can provide a method for predicting temperature changes in coal mine.

Study on experiments and numerical simulation of coal combustion characteristics under different thermal environments

In order to study heat release rate and flue gas generation during residual coal in goaf combustion, a cone calorimeter was used to research on combustion characteristics of bituminous coal under different heat radiation flux conditions. A novel coal fire FOAM solver for coal combustion is developed, numerically simulated by varying the gas atmosphere, and compared with the above experimental results. The results showed that with the increase in heat radiation flux, the shorter the time to reach the peak heat release rate, the greater the maximum heat release rate. When the coal does not enter the ignition stage, the CO production rate is higher than that after ignition, and the CO2 production rate is lower than that after ignition. Compared with the experimental results of the cone calorimeter, the difference between the two results is very small, which verified the feasibility and accuracy of the coal combustion model. After the injection of the composite inert gas, the heat release rate and mass loss rate of coal combustion have varying degrees of lag. The numerical simulation results can provide a theoretical basis for injecting inert compounds into the goaves of coal mines to prevent the spontaneous combustion of coal.

Study on controlling factors and developing a quantitative assessment model for spontaneous combustion hazard of coal gangue

Developing a quantitative hazard assessment model for spontaneous coal gangue combustion is imperative to ensuring the safety of coal mining operations. In this study, eight types of coal gangue from different coalfields were used to determine the controlling factors for spontaneous combustion. A variety of physical and chemical characterization methods were employed to investigate the physical and chemical characteristics of the samples. Based on the experimental results, six key parameters were identified, namely the aliphatic C–H components, pyrite sulfur content, physical oxygen absorption capacity, ash content, ignition point temperature, and activation energy for thermal decomposition. The results show that the spontaneous combustion hazard of coal gangue is positively correlated with the first three of these and is negatively correlated with the rest. Using the entropy-weight method combined with the analytic hierarchy process, appropriate weighting factors for each parameter were obtained and used to establish a quantitative assessment model for the spontaneous combustion hazard of coal gangue. This model is expected to enhance the current industrial best practices for the prevention and control of spontaneous coal gangue combustion.

Long-distance migration law of radon in overburden of abandoned goaf during coal spontaneous combustion

The surface isotope radon measurement method (SIRMM) is widely used in fire source detection in abandoned mines. However, studies on the long-distance migration of radon during coal spontaneous combustion are lacking, which hinders the further popularization of this technology in coal fire prevention and control. For this reason, the migration law of radon in overlying strata in fire areas was studied through experiments and numerical simulation. The radon exhalation concentration of coal was found to increase at first and then decrease in the range of 30–350 °C through experiments. The radon concentration reaches the maximum value (557.1 Bq/m3) at 150 °C, which is 6.3 times higher than that at 30 °C. Based on the radon source term obtained by fitting the experimental data, the radon migration model of coal spontaneous combustion in abandoned goaf was constructed, and the dynamic distribution characteristics of the airflow, temperature, and radon concentration fields in the overlying strata area were analyzed. The internal relationship between surface radon and underground fire source was discussed. The simulation results revealed the sharp change in the porosity of the overlying rock causes radon concentration at the interface between the caving and fissure zones to increase continually with the process of spontaneous combustion, providing material and energy support for the long-distance radon migration. When the maximum temperature of the coal pile reaches 70 °C, the concentration of radon released from the coal pile increases rapidly to 13696 Bq/m3, and the radon from the underground space appears on the surface at this temperature. In the range of 70–150 °C, with rapid increase in radon released from coal piles, the surface concentration of radon also increased rapidly to 225 Bq/m3. At the high-temperature stage exceeding 150 °C, the concentration of radon released from coal piles exhibited a downward trend, resulting in a decrease in the rate of increase of radon concentration on the surface. A close relationship between the surface radon concentration and underground fire source temperature in the process of coal spontaneous combustion was observed. In the spatial position, the peak position of radon on the surface was highly consistent with that of the fire source longitudinally, which ensures the accuracy of the SIRMM to determine the location of the hidden fire source. This suggests that the SIRMM can accurately evaluate the fire source's temperature and fire area's development trend.

Study on the Effect of External Air Supply and Temperature Control on Coal Spontaneous Combustion Characteristics

Coal spontaneous combustion in underground mine goaf has a great impact on coal mining. The temperature-programmed experiment is a commonly used and effective method for studying the characteristics of coal spontaneous combustion. Aiming at the problem that the numerical simulation of coal spontaneous combustion characteristics under the condition of external air supply and temperature control in a temperature-programmed experiment is insufficient, a multi-physical field coupling numerical model of coal spontaneous combustion in the temperature-programmed experiment is established. The variation characteristics of coal temperature, oxygen, and oxidation products under external air supply and temperature control were studied. The results show that the numerical simulation results are consistent with the experimental results. With the increase in temperature, the volume fractions of oxygen and carbon dioxide decrease and increase, respectively. As the air supply volume increases, the oxygen volume fraction at the outlet increases, and the peak value of the oxygen volume fraction change rate exhibits a “hysteresis” feature, and the time corresponding to the peak value increases. Moreover, the temperature change rate increases. With the increase in the heating rate, the peak value of the oxygen volume fraction change rate increases and shows an “early appearance” characteristic, at the same time, the maximum coal temperature displays a linear increase trend.

Study on the evolution characteristics of coal spontaneous combustion and gas coupling disaster region in goaf

In order to study the evolution of multiple fields and the risk degree of combined disasters in the process of coal spontaneous combustion under the gas-containing environment in the goaf, a simulation platform for the thermal test of the thermal dynamic disaster in the goaf was built, and the gas–solid coupling heat transfer of multi-component gas model for coal spontaneous combustion was constructed. Through similar simulation experiments and numerical simulation, it is clear that the gas accumulation law during the coal spontaneous combustion process in the goaf. The ventilation process causes the gas to be driven along the air leakage channel to the deep part of the air-return side of the goaf, and finally a high gas concentration area is formed. Meanwhile, the change of gas density inside and outside the high-temperature area causes a small area of gas accumulation that migrates with the coal spontaneous combustion. According to the results of experiments and simulations, a risk assessment model of coal spontaneous combustion and gas combined disaster based on the dimensionless dynamic explosion limit of multi-gas was constructed. It is found that due to the massive consumption of oxygen by coal spontaneous combustion, the hazardous area of gas explosion has been continuously reduced, but the risk has been continuously increased. The risk of combined disasters is close to 1 in the corners of the working face and the back of the working face.

Simulation of co-production system based on coal partial gasification under different reactant gas atmospheres

In order to explore the comparison of coal partial gasification hydrogen-electricity cogeneration systems under different gasification agent atmospheres, based coal partial gasification, including steam cycle power generation and syngas processing for hydrogen production, a hydrogen-electric co-production system and its model of partial coal gasification and semi-coke combustion are constructed, respectively using (H2O/O2) and (CO2/O2) as gasification reactant gas. The effects of oxygen-coal ratio and carbon conversion on exergy efficiency and syngas composition under different gasification reaction gas atmospheres were studied by Aspen Plus simulation. The exergy efficiency under different gasification agent atmospheres increases first and then decreases with the increase of oxygen-coal ratio and carbon conversion rate. Using (CO2/O2) as gasification agent has higher exergy efficiency. The composition of syngas changes little under different oxygen-coal ratios, while the overall content of H2, CO and CH4in syngas will increase with the increase of carbon conversion rate, but the growth rate will continue to decline.

Study on the formation mechanism of a radon source during coal spontaneous combustion in a goaf

Surface isotope radon detection technology (SIRDT) is an effective physical exploration technology that provides a solution for detecting hidden underground fire sources. However, because of the inaccessibility of deep underground spaces, current research on the formation mechanism of radon sources during coal spontaneous combustion in a goaf is insufficient, which severely restricts the use of SIRDT to detect such fires. In this study, the quantitative relationship between the coal temperature and radon exhalation rate was revealed through radon exhalation experiments during the low-temperature oxidation of coal, which provides measurement data for the radon source term of coal spontaneous combustion in a goaf. A multi-physical, coupled numerical model of coal spontaneous combustion in a goaf was constructed, and the intrinsic dynamics of radon migration and the formation mechanism of the radon source in the process of coal oxidation were thoroughly investigated. The results show that the radon distribution in a goaf is because of the competition between air leakage and the “porous chimney effect.” When the temperature of the coal was low, air leakage dominated the radon transportation in the goaf, causing radon to accumulate in the area close to the air-return side. When the temperature of the coal increased, the thermal buoyancy and vortex phenomena appeared in the high-temperature area of the goaf, at this point, the “porous chimney effect” took the leading role. The updraft generated by the thermal buoyancy effect rapidly carried the radon to the top of the goaf. The vortex promoted airflow from the surrounding area of coal spontaneous combustion into the interior of the “porous chimney,” which accelerated the process of coal spontaneous combustion and increased the amount of radon transported upward; thus, further intensifying the accumulation of radon at the top of the goaf. This accumulation becomes a source of radon that continues to migrate toward the surface.

Development of a Coal Combustion Model Compliant with the CHEMKIN Format for Use in Co-Firing Analysis with Hydrogen and Low-Carbon Fuels

We developed a simplified coal combustion model that is used in combination with detailed gas reaction models. This model aimed to develop co-firing systems for decarbonized fuels such as coal and hydrogen. The reaction mechanism was described in the CHEMKIN format. We aimed to develop a coal combustion model that is easy to handle even for experts in gas combustion. Based on the experimental results, we modified the following reaction models. (1) Determining the rate constant of NOx reduction by char in the presence of oxygen. (2) Determining the gasification reaction rate constants, especially the rate constant of the steam gasification reaction, from staged combustion experiments. (3) Adoption of a chemical structural model of char based on elemental analysis results. Char was composed of carbon; C, nitrogen; N, and hydrocarbons; CnHm. As a result of the improvement, we achieved the NOx prediction error halved. Improving the calculation accuracy of the coal burnout was most important for improving the accuracy of the gas reactions. If there was an error in the calculated coal burnout due to an error in the gasification reaction, and so forth, the amount of C, H, O, and N species in the coal released into the gas phase would change. These changed the burning conditions, for example, stoichiometric ratio, in the gas phase. As a result, the concentration of radicals such as H, O, and OH changed, causing a decrease in NOx prediction accuracy. We obtained outline of the combustion performances when decarbonized fuel such as hydrogen and coal were co-combusted.

Numerical modeling of the co-firing process of an in situ steam-torrefied biomass with coal in a 230 MW industrial-scale boiler

This paper presents the CFD modeling results of the torrefied maize straw co-firing with sub-bituminous coal in various mass ratios in the industrial scale boiler, to recognize possible application issues of the coal substitution with upgraded biomass. The steam torrefaction of biomass took place in a pilot in a counter-flow torrefaction reactor fed with superheated steam from the OP-230 (Rafako, Poland) boiler. Using a TGA, it was possible to analyze the combustion indexes and synergy effects after burning the torrefied biomass-coal mixtures. Additionally, a kinetic model of pyrolysis devolatilization was established and used in the modeling along with Ansys Fluent kinetics of the coal and gas-phase combustion models. Due to the numerical modeling, it was possible to determine the temperature distribution in the boiler's furnace chamber, the heat flux densities, the simulated distribution of carbon monoxides and carbon dioxide concentration, and the decomposition of nitrogen oxides resulting from co-combustion. Steam torrefied biomass indicates higher combustion activity compared to coal, ignites easier, and burns more intensely with better combustion stability. A synergistic effect between the coal-torrefied blend was observed. According to numerical analysis, it was found that with the increase of the share of torrefaction in the fuel mixture, the share of unburned fuel in fly ash increases. Additionally, an increased share of the torrefied biomass in the fuel blend from 30% to 40% results in a slight increase in the molar NO concentration in the furnace chamber. The authors strongly recommend the continuation of work on further investigation of the co-firing of the coal with torrefied biomass in the pre-mixed blends injected through all burners.

Risk Evaluation Model of Coal Spontaneous Combustion Based on AEM-AHP-LSTM

Immediately and accurately assessing the risk of coal spontaneous combustion and taking targeted action are crucial steps in coal spontaneous combustion prevention and control. A new model, AEM-AHP-LSTM, was proposed to solve the weight calculation problem of multiobjective evaluation in the process of coal spontaneous combustion. Firstly, the key indicators of coal spontaneous combustion were analyzed and used as risk factors to establish an evaluation system. Next, the objective and subjective weights were calculated using AEM and AHP, respectively. The objective and subjective weights were then combined, and TOPSIS was used to calculate the score of the evaluation sample. Finally, the obtained evaluation samples were trained with the BP, RBF, and LSTM model to resolve the problem of model overdependence on historical data and achieve the auto-adapt adjustment of weight with data change. Additionally, data from 15 typical Chinese coal mines were applied to the model. The results indicate that, compared with the BP and RBF neural networks, the LSTM model has higher prediction accuracy, stronger generalization ability, and stronger practicability. The modeling and application findings show that the AEM-AHP-LSTM model was better appropriate for the risk assessment of coal spontaneous combustion. This method can potentially be further applied as reliable approach for the assessment of mine disaster risk.

Evaluation of ammonia co-firing in the CRIEPI coal jet flame using a three mixture fraction FPV-LES

Highly resolved Large Eddy Simulations (LES) are performed to investigate co-firing of coal and ammonia in a burner experiment conducted by the Central Research Institute of Electric Power Industry (CRIEPI) in Japan. The coaxial burner with a hydrogen supported pulverized coal flame is modeled using the in-house code PsiPhi. A three mixture fraction flamelet/progress variable (FPV) approach is employed to simulate coal particle conversion due to devolatilization, hydrogen combustion, and ammonia combustion. Three cases are investigated and compared to each other: 1) a coal combustion case, injecting air and coal particles, 2) an ammonia combustion case, injecting a mixture of ammonia and air, and 3) a co-firing combustion case, injecting a mixture of coal, ammonia and air in the center tube. Two mechanisms are used to build the chemistry table and are compared against each other: a reduced CRECK mechanism with 120 reaction species and 1551 elementary reactions, originally reduced for coal combustion modeling, and a newly introduced reduced CRECK mechanism with 129 reaction species and 1644 elementary reactions, including the detailed NH3 reaction paths in addition to the coal chemistry. Species are compared for the coal case and temperature fields are compared for both the coal and co-firing case. Normalized LIF signals for OH and NH are presented for all three cases. The gas composition profiles are in good agreement with the experiment and the temperature fields are consistent with previous results for the pure coal flames. For pure ammonia and for ammonia co-firing, the new mechanism shows an improved prediction of the reaction zone.

Analysis of oxidation pathways for characteristic groups in coal spontaneous combustion

This study was designed to clarify the molecular characteristics and reactivities involved in coal spontaneous combustion. In the study, the structures of the characteristic groups –OH, –CHO, –CHOHCH2-, –COOH, –CH2CH2CH3, and –CH2CHCH2 were the research objects. The B3LYP-311G method of DFT was used to construct a molecular structure model for coal spontaneous combustion. Based on quantum-chemistry theory, the oxidation characteristics and reaction pathways of characteristic groups were analysed, and the reaction channels and oxidation products involved in reactions of –CHO, –COOH and –CHOHCH2- moieties were studied. Analyses of the oxidation characteristics and reaction pathways of the characteristic groups showed that the reactions of active groups with oxygen depend on the energies of bonds between the characteristic groups and hydrogen. The smaller the bond energy is, the more exothermic the reaction and the more readily the reaction will take place. When characteristic groups in coal react with oxygen, oxygen first attacks hydrogen atoms directly connected to carbon atoms close to aromatic rings. At normal temperatures and pressures, the rate-determining step of the –CHO conversion to CO2 is decarboxylation of ▪; the rate-determining steps for conversion of –CH2CHOH to CO2 and CO were decarboxylation and decarbonylation of ▪, respectively.