Coal Ignition Research Articles

Modeling and experiments on plasma ignition of Ekibastuz coal in the form of dust

Due to the low cost of coal and the fact that it is readily available in most parts of the world, coal is a convenient fuel source. Despite the inefficiency of the systems when it comes to converting heat energy into electricity, new technologies are necessary in order to improve their efficiency. In contrast to traditional methods of starting-up boilers and stabilizing combustion, plasma ignition and combustion stabilization (PICS) of pulverized coal flames offers an effective and sustainable alternative to the use of fuel oil or gas. This technology involves heating the air-coal mixture with electric arc plasma until the coal devolatilizes and the coke residue partially gasifies. Consequently, low-rank coal is converted into a highly reactive two-component fuel (HRTF) consisting of combustible gas and coke residue. For these processes in a plasma-coal burner (PCB) using Ekibastuz coal in the form of dust, a kinetic analysis was conducted using the PlasmaKinTherm program. Modeling the kinetics of PICS of pulverized fuel allowed changes in temperature, velocity, and concentration to be determined along the length of a PCB. The composition, degree of carbon gasification, and temperature of a stable coal-dust flame were determined using plasma ignition of solid fuel. Based on the comparison between experimental and calculated data, it was found that the results were satisfactory.

Design method and experimental study on a novel self-sustaining internal combustion burner

Peak shaving represents a particular challenge with regard to coal-fired power plants, as methods to achieve stable, high-efficiency, and low-nitrogen-oxide combustion at ultralow loads remain unavailable. In this study, based on analysis of the ignition mechanism, heating mode, and theory of preheating method, we proposed a novel pulverized coal full-time self-sustaining internal combustion burner by combining the processes of plasma ignition, reverse jetting, and multi-stage ignition. A flame stability model for the burner's recirculation zone was established using the temperature change rate of the recirculation zone as a criterion. The ignition position of the stable self-sustained combustion of the pulverized coal gas stream was determined under various working conditions, and the length of the ignition core stage, a component of the burner inside which the pulverized coal gas stream achieves stable self-sustained combustion, was determined based on the calculated maximum ignition position; a self-sustaining internal combustion burner was then designed based on the ascertained length. Subsequently, we established an experimental system for the self-sustaining internal combustion burner and conducted tests in the load range of 25 %–100 %. Notably, the results of the pulverized coal self-sustained ignition position experiment were consistent with the computational results of the model. The self-sustaining internal combustion burner offers a quick start–stop feature, with the pulverized coal able to achieve stable self-sustained combustion after the plasma igniter shuts off. The resultant gas-phase products exiting the burner met the boiler's concentration requirements for enhanced nitrogen reduction, and the burner exhibited good anti-interference properties. The influence of the pulverized coal gas stream velocity and concentration on the self-sustained ignition was expounded. Under the experimental conditions, an optimal concentration range was ascertained for stable ignition of the pulverized coal gas stream, with the ignition position gradually advancing backward with increasing gas stream velocity. Together, our findings highlight the potential of the proposed burner design to support clean and efficient pulverized coal combustion in coal-fired power plants.

Oxy-fuel co-combustion properties and N-containing pollutants release characteristics of biomass/coal blends

Thermal properties and NO release characteristics during oxy-fuel co-combustion of pine sawdust (PS) and Shenmu coal (SM) were investigated based on the effects of various parameters, including atmosphere, temperature, blending ratio, and oxygen concentration in this research. The results indicate that the release of NO decreased with the increasing temperature during decomposition process of PS and SM. The content of N-Q and N-X in semicoke (SC) increased at higher torrefaction temperature. The N-Q and N-X content for SC at the temperature of 1000 °C was 36.9 % and 23.2 %, respectively. When 40 % PS was added to the SM, the NO release amount was significantly reduced by 16.3–23.1 % in the oxy-fuel atmosphere compared to the O2/N2 atmosphere, and the ignition time was reduced from 4.0s to 0.66s. The NO emission initially increased and then decreased as the combustion temperature increased from 800 to 1000 °C. Furthermore, increasing the oxygen concentration from 10 % to 40 % shortens the combustion time and increases the emissions of NO. Conversely, increasing PS blending ratios from 10 % to 40 % decreased NO emission and the conversion ratio. These findings emphasize that adding biomass can effectively improve coal ignition, increase combustion rates, reduce NO emissions, and address the air pollution problems associated with NOx emissions.

Experimental analyses of CO2 sequestration potential in mineralized fly ash and its efficacy in mitigating coal spontaneous ignition risks in underground mine goafs

To effectively utilize fly ash solid waste and realize fly ash resource utilization, carbon sequestration and emission reduction, as well as mine fire prevention and suppression, this paper analyses the components of fly ash by X-ray fluorescence spectrometer (XRF), rheological experiments, the CO2 sequestration experiments by the mineralization of fly ash, thermogravimetric analysis experiments, and flame-retardant property experiments. Additionally, the changes in the physical structure and morphology of fly ash before and after mineralization were analyzed by X-ray diffraction (XRD) and scanning electron microscope (SEM). The carbon sequestration capacity and fluidity of fly ash slurry, as well as the ability of mineralized fly ash slurry to inhibit spontaneous coal combustion are comprehensively investigated. The research results indicate that CO2 is sequestered as calcite on the fly ash surface after mineralization. The optimal CO2 sequestration capacity of fly ash is 65.12 kg/t when 0.5 mol/L NaHCO3 is used as an additive. The fluidity of mineralized fly ash slurry is greater than that of fly ash slurry, and the lower the water-cement ratio is, the worse the fluidity. Mineralized fly ash slurry with a water-cement ratio of 3:1 has a strong carbon sequestration capacity and high fluidity, making it suitable for preventing coal spontaneous ignition in mine goafs. Mineralized fly ash slurry exhibits a regular trend where the better the mineralization effect, the lower the CO release during the low-temperature oxidation stage of coal, and the higher the cross-point temperature. Mineralized fly ash slurry has a stronger ability to inhibit spontaneous coal combustion than does ordinary fly ash slurry.

Effect of the Initial Moisture Content of Cedar Needles on the Time Characteristics of the Ignition of Coal–Water Suspensions

Effect of the Initial Moisture Content of Cedar Needles on the Time Characteristics of the Ignition of Coal–Water Suspensions

Features of ignition and combustion of low-grade coal mixed with pyrolysis liquid

THE RELEVANCE of the study lies in the need to use pyrolysis liquid from wood processing for combustion in boilers of medium and low power as part of mixtures with coal. The unpredictability of the ignition and combustion properties of such mixtures, as well as the lack of calculation methods, predetermined a predominantly experimental approach to the study of these processes. At the same time, for the widespread implementation of such technological solutions in practice, it is necessary to have relatively simple methods for assessing the ignition and combustion characteristics of these components. THE PURPOSE. Synthesize samples of pyrolysis liquid from two different sorts of wood and perform their characterization. Develop an experimental method for studying ignition and combustion characteristics of pyrolysis liquid mixed with low-reactivity coal. Perform research on ignition delay times and the composition of released gas-phase products. Determine the main mechanisms of action of the components in the mixture of low-grade coal with pyrolysis liquid they’re during ignition and combustion. METHODS. The study was carried out using an experimental technique based on a high-temperature muffle furnace equipped with a high-speed video camera and a once-through gas analyzer. RESULTS. The article presents the dependences of the ignition delay times and the concentration curves of gas-phase products released during combustion of mixtures of low-grade coal and two types of pyrolysis liquid in the temperature range of the heating medium 600-800 °C and additive concentrations from 0 to 20 wt.%. Linear approximation dependences of ignition delay times on additive concentrations were obtained, and the mechanism of action of the additive during combustion was formulated. CONCLUSION. It was found that an increase in the concentration of the additive in the mixture leads to a stable decrease in the ignition delay times, while the amount of gas-phase products formed varied additively with respect to the composition of the mixture. The shape of the concentration curves for the release of CO and NOx indicates the independent action of the components of the mixture. The mechanism of action of additives was proposed. It was based on significant contribution of their evaporation process followed by combustion in the gas phase, which, in turn, initiats the ignition of coal.

Numerical simulation study on chemical ignition process of underground coal gasification

Underground coal gasification (UCG) is a process that produces combustible gas by the in-situ gasification of coal seams. For a steady and efficient gasification project, the ignition of coal seam is a prerequisite. The heating method and reacting process of coal seam play an essential role to the ignition. A coal seam ignition model coupled with heptane combustion was created to simulate the deep UCG ignition stage employing the chemical ignition method. A 1700 K heptane jet flame and a U-shaped pyrolysis gas flame formed the heat source. The flame temperature of pyrolysis gas was around 1300 K. The effective heating length of the flame increased with the heating time, which reached 600 mm at 400 s. The coal seam reaction process was divided into four stages: (1) heptane flame stabilization, (2) coal seam heating, (3) pyrolysis gas release, and (4) pyrolysis gas flame stabilization. It was the third stage when the coal seam ignited. The coal seam would ignite faster with a lower excess air coefficient, and the ignition time ranged from 225 to 275 s when the excess air coefficient was 1.39. The outlet carbon dioxide concentration increased by about 5 %–17 % when the coal seam ignited.

Экспериментальные исследования воспламенения и горения каменного угля с применением плазменной активации

This work presents experimental studies of a promising method of ignition and combustion of coal fuel using plasma activation. Experimental studies were carried out on a bench with a thermal power of up to 5 MW. As an experimental sample, hard coal of the GD grade was chosen selected after a ball drum mill with a sieve residue of R90 = 15%. The conditions were close to industrial ones used when lighting coal boilers. The operating range of pulverized coal flow velocities in the electric arc unit has been determined. It has been established that a stable process of ignition and further combustion of coal is possible at flow rates on the electric arc unit from 6 to 13 m/s. The experimental stand was kindled from a cold state to a temperature of 1000 °C at excess air coefficients a from 0.5 to 0.7, the kindling time did not exceed 200 seconds. Based on the experiments, CFD modeling determined whether the process conditions were met.

Simulation and experiment of plasma ignition of low-grade coal

A plasma-coal burner is studied utilizing a model of plasma thermochemical preparation of coal for combustion, implemented in the PlasmaKinTherm program. For boiler start-up and coal combustion stabilization, plasma-coal burners do not require fuel oil or gas. The PlasmaKinTherm program combines thermodynamics and kinetics to describe the thermochemical preparation of fuel in the plasma-coal burner volume. The purpose of the simulation was to determine the conditions for plasma ignition of low-grade coal. A numerical study was carried out of the influence of the plasmatron power on the ignition of the air mixture (coal + air). High-ash Ekibastuz coal was used in the calculations. The distributions of temperature and velocity of gas and coal particles and concentrations of products of plasma thermochemical preparation of coal for combustion along the length of the burner were calculated. As a result of the analysis of the processes of plasma ignition of coal, their main patterns were revealed, including the shift of the maximum temperatures and velocities of the products of thermochemical preparation of coal for combustion upstream (towards the plasma torch), as well as the fact that the maximum values of temperatures and velocities of the products do not depend on power plasmatron. At the plasmatron power determined by kinetic modeling, experiments were conducted to test and validate the ignition and combustion conditions for a highly reactive two-component fuel torch. The assumptions made during the development of the mathematical model were confirmed by comparing the calculations with experimental data.

Effect of turbulent fluctuation on the ignition of millimeter particle: Experimental studies and numerical modelling with a new correlation of nusselt number

Understanding the influencing mechanism of turbulent fluctuation on the ignition characteristics of millimeter coal particles is essential. In this work, to study the effect of turbulent fluctuation on ignition time, millimeter coal particles are subjected to a specific flow field, generated in a furnace with symmetric fans. A one-dimensional model with the new proposed correlation and the Ranz-Marshall (R-M) correlation for Nu (Nusselt number) is established to simulate the coal ignition process. In addition, the effects of fan speed, temperature, particle diameter, particle distance and coal type on the ignition time are investigated. It is found that an increase in fan speed from 0 to 3000 rpm leads to a particle Reynolds number Rep increase from 0 to 22.5, and a turbulent particle Reynolds number Ret∗ increase from 0 to 71.5. With a consideration of the fluctuation effect, the new correlation of Nu gives a better prediction of ignition time compared to the R-M correlation. Moreover, the ignition time is revealed to decrease with an increasing fan speed and an elevating temperature. While the ignition time shows merely an initial boost with enlarging particle distance, it exhibits a linearity with the term of particle diameter dp1.3–1.7 and Reynolds numbers (Nu∗/Nu)–0.6 (Nu∗ is turbulent Nusselt number). Based on this relationship, the difference of predicted ignition time is calculated at different Rep and Ret∗. It is shown that at low Rep or high Ret∗ values, the new correlation should substitute for the R-M correlation.

Scientific bases and peculiarities of conversion of CHPP anthracite boilers to sub-bituminous coal combustion

Scientific bases and peculiarities of conversion of CHPP anthracite boilers to sub-bituminous coal combustion Print EmailUser Rating: / 0 PoorBest Category: Content №1 2024 Last Updated on 29 February 2024 Published on 30 November -0001 Hits: 1 SocButtons v1.4 Authors: M.V.Chernyavskyy*, orcid.org/0000-0003-4225-4984, Thermal Energy Technology Institute of National Academy of Sciences of Ukraine, Kyiv, Ukraine, e-mail: mchernyavski@gmail.com O.Yu.Provalov, orcid.org/0000-0002-5191-2259, Thermal Energy Technology Institute of National Academy of Sciences of Ukraine, Kyiv, Ukraine, e-mail: eproval@ukr.net Ye.S.Miroshnychenko, orcid.org/0000-0003-2487-6886, Thermal Energy Technology Institute of National Academy of Sciences of Ukraine, Kyiv, Ukraine, e-mail: yevhenmi@gmail.com O.V.Kosyachkov, orcid.org/0000-0002-9445-8738, Thermal Energy Technology Institute of National Academy of Sciences of Ukraine, Kyiv, Ukraine, e-mail: alexkosoy@ukr.net * Corresponding author e-mail: mchernyavski@gmail.com повний текст / full article Naukovyi Visnyk Natsionalnoho Hirnychoho Universytetu. 2024, (1): 041 - 049 https://doi.org/10.33271/nvngu/2024-1/041 Abstract: Purpose. Development of scientific foundations and generalization of experience in development and implementation of technical solutions for conversion of CHPP anthracite boilers with steam productivity up to 250 t/h for combusting sub-bituminous coal with maximum use of existing equipment. Methodology. Consumption and heat calculations of pulverizing systems, aerodynamic calculations of pulverized coal pipes and burners, thermal calculations of boilers and combustion chambers. Calculation justification of technical solutions to eliminate the risk of coal ignition in pulverizing systems and in the burners. Industrial tests on CHPP boiler units. Findings. Different types of pulverizing systems of anthracite CHPP boilers with ball-drum mills, an intermediate pulverized coal hopper and with hot air as a drying agent were considered, and a classification of pulverizing systems by the method of pulverized coal transport to the burners (with drying agent or hot air), and boilers – by the type and location of the burners and the geometry of the furnace, was performed. The problems were analyzed, the calculations of pulverizing systems, pipes, burners, and boilers were performed according to the applied technical solutions, and the experience was summarized of conversion from anthracite to sub-bituminous coal in the boilers of Myronivska, Darnytska CHPP and of the first line of Slovianska TPP. Recommendations are given on methods of conversion of anthracite boilers with a steam capacity of up to 250 t/h with different types of pulverizing systems for burning sub-bituminous coal with maximum use of existing equipment. Originality. For pulverizing systems with ball-drum mills and an intermediate hopper for pulverized coal with hot air as a drying agent classification was made for the first time by the type of transporting agent, and it was shown that when converting from anthracite to sub-bituminous coal with the air transport of pulverized coal to the burners, it is necessary to use the selection of slightly heated air from the first stage of the air heater. It is substantiated that maintaining the temperature conditions of molten slag removal while reducing the share of hot air consumption to the furnace requires the preferential operation of pulverizing systems in a single-mill mode, which is possible due to the greater grindability of sub-bituminous coal. Practical value. Based on the experience of approving technical solutions at Myronivska, Darnytska CHPPs, and at the first line of Slovianska TPP, recommendations are given on how to transfer anthracite boilers with a steam capacity of up to 250 t/h with different types of pulverizing systems for burning sub-bituminous coal with maximum use of existing equipment.

Thermodynamic and kinetic modeling and experiment on plasma ignition of pulverized high-ash coal

Solid fuels are relatively inexpensive and widely available, making them an attractive option for energy producers. However, they are currently inefficient in converting heat energy into electricity, and new technological advances are needed to make them more efficient. Plasma ignition and stabilization of pulverized coal flame provides a cost-effective and sustainable approach to boiler start-up and combustion stabilization, avoiding the traditionally used fuel oil or gas. This technology consists of heating the air-coal mixture with electric arc plasma to the temperature of coal devolatilization and partial gasification of the coke residue. Thus, a highly reactive two-component fuel, consisting of combustible gas and coke residue, is obtained from the original coal. In this paper, a comprehensive thermodynamic and kinetic analysis was carried out. A thermodynamic analysis was carried out to study the optimal parameters of plasma ignition and stabilization of combustion of a pulverized fuel flame. Kinetic modeling of the process of plasma ignition and stabilization of combustion of pulverized fuel made it possible to obtain changes in temperatures, velocities and concentrations of high-temperature two-component fuel components along the length of the plasma-fuel system. In experiments on plasma ignition of thermal coal, a stable coal-dust flame was obtained, and its temperature, composition and degree of carbon gasification were determined. Comparison of experimental and calculated data showed satisfactory agreement.

Pulverized coal combustion in boundary layer turbulence using direct numerical simulation

The present study investigates pulverized coal combustion under oxy-fuel conditions interacting with turbulent boundary layer flows over a flat plate using direct numerical simulation (DNS). The coal particles are tracked in the Lagrangian framework while the flow is solved in an Eulerian way. Three cases with reacting particles, inert particles and single-phase flows were performed. The interplay between pulverized coal combustion, turbulent boundary layer flows and the wall was examined. It was shown that the coal particles first ignite in the outer layer of the boundary layer turbulence, where individual coal particle ignition occurs. The wall-normal velocity in the reacting case is larger compared with that of the non-reacting cases due to flow acceleration induced by combustion. The volatile matter mass fraction is high in the inner layer in the downstream region. However, the heat release rate is low due to the cold wall effect. The strong correlation between the wall heat flux and streak structures in the near-wall region is observed in all cases. The values of radiative heat transfer are similar in the upstream region of both the cases with reacting and inert particles. However, as combustion occurs and particle temperature increases in the reacting case, the radiative heat transfer from particles to the wall increases significantly in the downstream region. Particle accumulation due to turbophoresis in the near-wall region is analyzed. It was shown that the magnitude of streamwise vorticity is attenuated by coal combustion. Therefore, particle wall-accumulation is less prominent in the reacting case compared with the inert case.

Ignition and thermal decomposition of solid organic fuel: The influence of activation, deactivation, and composite formation

The article discusses research on the thermal decomposition and ignition of coal, pine sawdust, and composite fuels derived from them. Through simultaneous thermal analysis, it was observed that the combustion process occurs in two primary stages. In the first stage, gaseous products are released and burned, while in the second stage, carbon produced in the first stage undergoes oxidation. The study also revealed the impact of mechanical activation and deactivation of ground fuel. Interestingly, the properties of coal and sawdust in the composite fuel formation are not simply additive. When the composite contains more carbon, there is a shift in the maximum decomposition rates towards higher temperatures in the first stage. Additionally, there is a decrease in decomposition magnitude during the first stage and an increase during the second stage. Moreover, it was found that the combustion temperatures of the composite fuel and mixture are higher than those of sawdust alone. Furthermore, the composite fuel exhibits superior burning characteristics as compared to the mixture. It ignites earlier, and the combustion zone shifts towards the center, resulting in enhanced fuel combustion efficiency.

Ignition characteristics of the high-velocity pulverized coal jet in MILD combustion mode: Experiments and prediction improvements

The influence of high jet velocity on the ignition characteristics of pulverized coal is very important for the Moderate or Intense Low-oxygen Dilution (MILD) combustion technology of pulverized coal. This paper conducts experimental and numerical simulation research on the ignition characteristics of the pulverized coal jet in a high-temperature environment based on the Hencken-type flat flame burner. The results show that when the ambient temperature is 1873 K and the ambient oxygen mole fraction is 5 %, the ignition delay distance of the pulverized coal jet decreases as the jet velocity increases from 15 m/s to 100 m/s for the formation of MILD combustion. The adoption of the turbulent particle heat transfer model and the non-spherical particle motion model effectively improves the prediction of particle dispersion, ignition delay, and reaction zone in the high-velocity coal jet flame. The enhancement of gas–solid heat transfer by turbulent fluctuation plays a dominant role in advancing the coal ignition in the high-velocity jet, while the non-spherical shape of coal particles mainly improves the uniformity of reaction distribution. The influence of ignition characteristics in the high-velocity jet on the MILD combustion formation of pulverized coal has been also analyzed in detail.

Characteristics of OH formation during single coal particle ignition and volatile combustion in O2/N2 and O2/CO2 atmospheres

The behaviors of volatile combustion and ignition of lignite coal were assessed on a concentrating light heating platform using hydroxyl-Planar laser-induced fluorescence (OH-PLIF) over a range of 21–60% O2 in both N2 and CO2 diluent gases. The results indicated that when the O2 concentration was below 40%, substituting CO2 for N2 delayed the ignition delay time (tign). This was evident as no ignition occurred in the 21% O2/CO2 mixture, and a nearly 1.0 s difference in tign emerged when substituting CO2 for N2 in 30% and 40% air atmospheres. Additionally, the weaker flame luminosity and the shorter volatile flame duration time (tvol) were also observed in O2/CO2 compared with that in O2/N2. Correspondingly, a significantly lower OH signal intensity was observed in O2/CO2 compared to O2/N2, which was primarily due to physical and chemical effects of CO2. Firstly, the higher heat capacity and lower diffusion rate of O2 in O2/CO2 leads to a lower volatile release rate and decreasing flame temperature, as evidenced by the closer proximity of the flame front to the coal particle surface. The lower flame temperature and volatile release rate would reduce the number of hydrocarbon molecule (RH) and the reaction rate of RH→hydrocarbon radical (R) + H, leading to a decrease in the concentration of OH radical from the reaction: H + O2→OH + O. Secondly, CO2 participates directly in H + OH + three-body (M)→H2O + M reaction, competing with the primary OH formation pathway, H + O2→OH + O, for H radicals, thus suppressing the OH generation. However, when O2 concentration exceeded 40%, the tign was shorter in O2/CO2 by about 1.2–1.4 s than in O2/N2 with the same O2 concentration. Simultaneously, compared to the O2/N2, similar flame luminosity and tvol were observed in the O2/CO2. An obvious increase in OH intensity was also observed in O2/CO2. With higher O2 concentrations in the O2/CO2, the absorption and emission radiation characteristics of CO2, increased O2 diffusivity and the decreased specific heat capacity in the O2/CO2 gas mixture, synergistically facilitate temperature rise and volatile release, which promotes the generation of RH, and then increases the H concentration due to the reaction RH→R + H. This would lead to a substantial increase in concentration of OH produced by the reaction H + O2→OH + O, which was detected by OH-PLIF.

Prediction model of goaf coal temperature based on PSO-GRU deep neural network

The accurate determination of coal temperature in hidden space such as goaf has always been a worldwide problem that needs to be solved. The research of coal temperature prediction model has important practical significance for the accurate detection of loose coal temperature. Based on the coal natural ignition experiment and coal spontaneous combustion gas characterization index as the initial data set GRU neurons are used to mine the nonlinear relationship between the index gas and temperature, and the GRU model parameters are optimized by PSO to obtain the predicted value of coal body temperature. The results show that the predicted MAE value of the PSO-GRU model is 1.37 °C, 6.51 °C, 11.40 °C, 15.90 °C, 20.20 °C lower than that of the PSO-SVM, PSO-BP, BP, RF and SVM prediction models respectively. The RMSE value decreased by 0.45 °C, 4.44 °C, 10.33 °C, 15.71 °C and 24.24 °C respectively. The judgment coefficient R2 on the test set and the training set of the PSO-GRU model is 0.99, and the generalization, prediction accuracy and robustness are all good. The experimental results show that the inversion temperature error of the model is within the range of 3.87 %, the maximum temperature difference is 5.69 °C, and the average temperature difference is 2.83 °C, which can meet the accuracy requirements of the field temperature measurement.

Simulation of underground coal gasification ignition in deep coal seam based on transitional diffusion mechanism: Influence of inlet temperature and O2

Underground coal gasification (UCG) in deep coal seam was attracting more and more attentions all over the world due to its characteristics of safety, clean and low carbon. In the study, the ignition of UCG in deep coal seam was simulated by employing computational fluid dynamics (CFD). A new method setting gas diffusion in coal seam as transitional diffusion (TD) was put forward, and the TD and Fickian diffusion (FD) mechanism were compared. It was elucidated that TD mechanism was more accurate for UCG model. The default FD mechanism in coal seam overestimated the reaction rate of coal seam. With TD mechanism, coal seam in turn went through external heating controlled (EHC) region and hybrid heating controlled (HHC) region during the ignition of UCG. The demarcation temperature between EHC and HHC regions was 720 K. Then, various inlet temperatures and mass fractions of O2 were attempted during the simulation of UCG ignition. The time spent by the ignition was shortest with 1100 K inlet temperature and it was ∼180 s. The less the inlet mass fraction of O2 was used, the slower combustion rate was. CO2 was the most potential indicator to get access to the underground condition.

Analysis of the possibility of solid-phase ignition of coal fuel

The article presents the results of theoretical and experimental studies of the ignition process of coal fuel particles in a pelletized state during high-temperature radiation-convective heating in an oxidizing environment. A new, different from the known, “two-temperature” mathematical model of the ignition process of a cylindrical coal pellet under intense heating conditions is presented. During the modeling, a detailed kinetic scheme of the reaction of gaseous products of thermal decomposition of coal with atmospheric oxygen was considered. The mathematical model was tested through a comparative analysis of the theoretical and experimental values of the ignition delay times of coal pellet fuel with experimental data. A wide range of heating conditions that could potentially lead to solid-phase ignition were analyzed. The research results showed that even under conditions of high oxygen content in the original coal, solid-phase ignition of coal pellets does not occur during intense heating. Based on the results of numerical modeling, it was established that the ignition process occurs in the gas phase. The oxygen released during the thermal decomposition of coal is not enough for stable gas-phase or heterogeneous ignition of pyrolysis products in the intrapore space of the fuel pellet. Based on the results of the numerical modeling, it was established that the oxygen released during the thermal decomposition of coal is significantly deficient in the intrapore space. The O2 concentration is not enough for a significant (in terms of heat flow) thermochemical intrapore reaction with gaseous and solid products of thermal decomposition.

Co-hydrothermally carbonized sewage sludge and lignocellulosic biomass: An efficiently renewable solid fuel

The objectives of this investigation were to elucidate the potential use of metal oxide-rich sewage sludge obtained from the treated brewery wastewater into a value-added solid fuel via co-hydrothermal carbonization process (co-HTC). Two residue biomass including spent coffee grounds and bagasse were supplied as co-combustion. The effects of sewage sludge and biomass addition on fuel properties were evaluated to optimize the best condition for biocoal-liked production. The chemical composition and mineral phase of solid product were further analyzed. Combustion kinetics analysis including activation energy (E) and pre-exponential factor (A) were derived from thermogravimetric analysis. It was found that the addition of coffee grounds and bagasse enhanced the fuel properties of the solid products, remarkably increasing high heating value (HHV) along with a low ash content, providing an increased fuel ratio of 0.34 – 1.01 and higher HHV as 14.29 – 22.19 MJ/kg. The highest rate of energy recovery was achieved when combining 75 wt% sewage sludge with 25 wt% spent coffee grounds. A substantial decrease of H/C and O/C atomic ratios was distinguished after bagasse addition compared to commercial lignite coal. It was also noticed that the relationship between the sewage sludge and biomass feedstocks during co-HTC is synergistic by increasing the amount of oxidative carbon during the char combustion stage and enhancing the degree of thermal stability. Moreover, it was also emphasized that during co-HTC some carbon and inorganic contents of sludge and lignocellulosic biomass were partially transferred into a liquid phase confirmed by TOC and ICP-OES analyses. A heavy metal leaching toxicity in a liquid product was also determined according to the USA-EPA standard. The combustion reactivity was improved, especially combustion reactivity of the biomass-sewage sludge-derived hydrochar. Interestingly, the hydrochar product was anticipated to possess enhanced safety and stability. Moreover, the co-combustion of hydrochar and coal improved the devolatilization properties and ignition of coal. This strategy revealed that co-hydrothermal process with biomass is a prospective approach to increase the value-added of sewage sludge feedstock as bicoal-like solid fuel.