Combustion Of Coal Particles Research Articles

Catalytic pyrolysis of coal particles in a fluidized bed: Experiments and modeling

ABSTRACTCoal as a form of fossil fuel may be utilized in two thermochemical techniques: directly by combustion of coal particles and indirectly by converting it into valuable products. In this research, a kinetic model of coal devolatilization has been developed based on the Arrhenius expression. To validate the model, an experimental study also has been performed in a drop-tube fluidized bed. There was reasonably good agreement between modeling data and experimental data obtained from single-tube fluidized bed. The effects of reaction temperature on the pyrolysis products and fuel conversion were studied. As a result, the maximum tar yield obtained was 28.3 (%) at 400° C. An increase in activation energy was led to a relative decrease in the fuel conversion maybe due to a decrease of heating rate.

3D imaging of individual burning char and volatile plume in a pulverized coal flame with digital inline holography

The burning behaviors of an individual coal particle in the combustion of a cloud of coal particles is of great importance because it represents the realistic situation of pulverized coal particle utilization. Digital inline holography is proposed as a tool for 3D quantitative imaging of the burning coal particle and its associated volatile flame, and demonstrated by its application to a laboratory scale pulverized coal flame. The holographic fringes of burning coal particles with volatile plumes are characterized by concentric rings as well as parallel lines. Besides the conventional 3D positions and morphologies of irregular coal particles, the volatile flames are clearly reconstructed. The instantaneous behaviors of the early stage combustion of individual coal particle in a pulverized coal flame have been observed. Typical modes of the volatile flame with respect to the coal particle, including envelope volatile flame, attached wake flame, detached wake flame, side volatile flame, are observed in the experiments. The burning behaviors of one coal particle influenced by another particle in the combustion of a cloud of particles has also been evidenced. This work demonstrates that digital inline holography has the powerful capacity of simultaneously measuring reacting multiphase objects in hostile environments.

Experimental and modeling study of oxy-fuel combustion of coal particles for heat and power generation

ABSTRACTCoal combustion has attracted a lot of interest by generating combined heat and power (CHP) due to low costs and high efficiency. However, one of the most important challenges in coal combustion is the environmental issues related to the process. In this article, the influence of some critical parameters, i.e., oxygen ratio, system pressure, residence time, and coal particle size, on output temperature and greenhouse gas emissions was evaluated using a kinetic model and an experimental study. Results showed that an increase in oxygen ratio leads to a significant increase in output temperature. It is also found that the particle size plays a negligible role in the process; smaller particles favor the production of gas products and higher combustion efficiencies. Modeling data validated against experimental measurements and were found to be in a good agreement.

Mass spectra features of biomass burning boiler and coal burning boiler emitted particles by single particle aerosol mass spectrometer

In this study, single particle mass spectra signatures of both coal burning boiler and biomass burning boiler emitted particles were studied. Particle samples were suspended in clean Resuspension Chamber, and analyzed by ELPI and SPAMS simultaneously. The size distribution of BBB (biomass burning boiler sample) and CBB (coal burning boiler sample) are different, as BBB peaks at smaller size, and CBB peaks at larger size. Mass spectra signatures of two samples were studied by analyzing the average mass spectrum of each particle cluster extracted by ART-2a in different size ranges. In conclusion, BBB sample mostly consists of OC and EC containing particles, and a small fraction of K-rich particles in the size range of 0.2–0.5μm. In 0.5–1.0μm, BBB sample consists of EC, OC, K-rich and Al_Silicate containing particles; CBB sample consists of EC, ECOC containing particles, while Al_Silicate (including Al_Ca_Ti_Silicate, Al_Ti_Silicate, Al_Silicate) containing particles got higher fractions as size increase. The similarity of single particle mass spectrum signatures between two samples were studied by analyzing the dot product, results indicated that part of the single particle mass spectra of two samples in the same size range are similar, which bring challenge to the future source apportionment activity by using single particle aerosol mass spectrometer. Results of this study will provide physicochemical information of important sources which contribute to particle pollution, and will support source apportionment activities.

Simultaneous particle size and 3D position measurements of pulverized coal flame with digital inline holography

3D measurements of burning coal particles in the flame are challenging and of great interest. Digital inline holography is applied to perform 3D measurement of coal particle size and 3D position in a laboratory-scale pulverized coal flame. Pulsed laser beam illumination is used to record the coal particle hologram with clear holographic fringes in the presence of high refractive index gradient and strong turbulence. Over 30,000 coal particles have been reconstructed and analyzed. Results show that the particle size, 3D positions and particle number density distribution can be determined simultaneously. Digital inline holography presents to be a powerful tool in the 3D measurement of burning particles in a turbulent and reacting medium.

Heat transfer and discrete phase modelling of coal combustion in a pusher type reheating furnace

Mathematical as well as numerical modelling of a reheating furnace is a difficult task because of the complexity in geometry as well as the involved heat transfer and combustion phenomena. The complexity is further aggravated by the presence of discrete coal particle combustion and the periodic transient movement of the steel billets. The aim of this work is to perform a three-dimensional simulation for a pusher type reheating furnace of steel billets with coal burners. The reacting fluid flow and heat transfer inside the furnace is simulated using a commercial software ANSYS Fluent 15.0 and an in-house 3-D transient code is developed to simulate the thermal diffusion in the periodically moving billets. The actual process is then modelled by coupled simulations of the furnace and the billets with an iterative approach. This kind of coupling greatly reduces the computational effort without much affecting the accuracy. On comparison of the results with the data obtained from a real facility, a good agreement is found leading to the conclusion that the proposed methodology is adequate for simulating the system.

Analysis of role of erosion process of coal particles in generation of loss of incomplete combustion in fluidized bed boilers

In the herein paper the results of research carried out in the Institute of Thermal Machinery of Technical University of Czestochowa have been presented. They concern the process of combustion of coal particles in a fluidized bed combustor. The tests were carried out on a test stand which allows modelling of conditions around the fuel particle in a real object. On the basis of the registered mass loss of the particles of Polish hard and brown coal chosen, the impact of the mass flow rate of inert material with different particle size distribution and different collision velocity with burning coal particle on the rate and level of its burning out was evaluated. The obtained results enables the estimation of the influence of both parameters on the intensity of erosion and determination of its role in the generation of fine particles elutriated out of the combustion chamber. The results also facilitated the verification of the methodology [1] of calculation described in literature relating to the total mass loss of coal particles as a result of combustion and erosion. According to this methodology, the slip velocity between the char particle and the fine bed solid is the main parameter of the erosion process alongside the attrition rate constant k a .

Combustion Behavior of Relatively Large Pulverized Biomass Particles at Rapid Heating Rates

A pulverized solid fuel particle in a hot gas stream appears to have different characteristic behaviors at several stages, including heat-up, release of volatile matter, gas phase, and solid combustion. The characteristics of these stages may vary distinctly depending on devolatilization rate, the particle temperature history, and its chemical and physical properties. Biomass particles manifest different combustion behavior from that of burning coal particles under the same combustion conditions because they contain more volatiles (less fixed carbon), and they have a relatively lower particle density due to their fibrous structure. This paper presents an experimental study of burning behavior of different types of biomass particles (torrefied wood, coffee waste, and sewage sludge). The main experimental parameters—gas temperatures of 1090 and 1340 K, and O2 concentrations ranging from 10% to 40%—were employed to investigate the burning of biomass through a direct-observation approach using a high-speed ph...

Loading identical contents of sodium and quartz into different ash-removed coals to elaborately investigate the real effects of coal particle combustion on the emission behavior of PM10

This contribution reports the real effects of coal particle combustion on the emission behavior of particulate matter (PM) with aerodynamic diameters of <10µm (PM10). To do this, two coals, i.e., the ZD subbituminous coal and the PDS bituminous coal, were acid-washed to remove most inorganic compositions. The acid-washed coals were then loaded with identical contents of organically-bound Na and excluded quartz to produce the Na+Si loaded coals, which were then combusted via a high temperature drop tube furnace (DTF) at 1500°C in air to collect the inorganic PM10 for subsequent analyses. Burning the Na+Si loaded coals with simplified and identical mineral compositions enabled us to purposely study the real effects of coal particle combustion on PM10 emission. The results demonstrate the trimodal distributions of PM10 produced from the Na+Si loaded coals containing such simple inorganic compositions. The combustion of PDS Na+Si loaded coal seems to form more PM than that of ZD Na+Si loaded coal. In contrast to the combustion of the ZD subbituminous coal, more Na and SiO are vaporized to form more PM0.5 due to the higher peak burning temperature of the PDS bituminous coal during the stage of volatile matter combustion, meanwhile, more coalescence of quartz occurs due to its melting point being decreased by the heterogeneous condensation of Na, causing more PM2.5–10 being generated from the combustion of the PDS bituminous coal. When the PDS Si-only loaded coal is burned, the coalescence of quartz particles should be suppressed due to the lack of condensed Na. In addition, its lower char burning temperature can also suppress the coalescence of fine quartz particles, forming less PM2.5–10 compared with the combustion of the ZD Si-only loaded coal. The results suggest that both the stages of volatile matter combustion and char combustion of coal particles can affect the emission of PM10.

Investigation of ignition and volatile combustion of single coal particles within oxygen-enriched atmospheres using high-speed OH-PLIF

To study devolatilization, ignition, and volatile combustion processes of single coal particles at a fundamental level, a flat flame burner was designed to provide oxygen-enriched exhaust gas environments at elevated temperatures and varying gas compositions. In one set of experiments coal particles were carried by a central jet using a gas mixture of the same composition as the flat flame. By this means a homogenous and connected flame was generated. At the instant when single coal particles transited this stationary flame, start of particle heat-up was well defined. This allows studying start of ignition more accurately compared to the other set of experiments where an inert carrier gas jet was used for transportation of coal particles. Heat-up and ignition were retarded temporally due to precedent mixing between inert gas and ambient flue gases. Coal particles with a high percentage of volatiles (36wt%) and a particle distribution ranging from 90 to 125µm were used. Coherent anti-Stokes Raman spectroscopy (CARS) measurements were performed to characterize ambient gas phase temperatures. By high-speed laser-induced fluorescence, imaging of OH-radicals ignition and volatile combustion was visualized for individual coal particles. Reaction zones surrounding each coal particle and the corresponding distances to coal particles were identified. This distance depended strongly on oxygen concentration of the ambient flue gas atmosphere. In addition to an improved phenomenological understanding the data may serve for model validation.

Kinetic modeling of the formation and growth of inorganic nano-particles during pulverized coal char combustion in O2/N2 and O2/CO2 atmospheres

In the formation of nano-particles during coal char combustion, the vaporization of inorganic components in char and the subsequent homogeneous particle nucleation, heterogeneous condensation, coagulation, and coalescence play decisive roles. However, conventional measurements cannot provide detailed information on the dynamics of nano-particle formation and evolution. In this work, a sophisticated intrinsic char kinetics model that considers ash effects (including ash film formation, ash dilution, and ash vaporization acting in tandem), both oxidation and gasification by CO2 and H2O, homogeneous particle nucleation, heterogeneous vapor condensation, coagulation, and coalescence mechanisms is developed and used to compare the temporal evolution of the number and size of nano-particles during coal char particle combustion as a function of char particle size, ash content, and oxygen content in O2/N2 and O2/CO2 atmospheres. Based on comparisons with measurements of char particle temperature, carbon conversion, mineral vaporization, and mean size of nano-particles at various residence times, the model can accurately predict the transient combustion of pulverized coal char particles and nano-particle formation and growth. Model results show that in either O2/N2 or O2/CO2 atmospheres, the char combustion temperature has a dominant effect on the formation and growth of nano-particles. High char burning temperatures result in a high mineral vaporization rate within the char particle, and subsequent high nucleation and condensation rate, and consequently more and larger nano-particles. As a result, high oxygen content, low ash content, and small sized char particles, all of which promotes high local char burning temperatures, yield more nano-particles and shift the nano-particle size distribution to larger sizes. In comparison to combustion in O2/N2, both the number density and size of the nano-particles formed in O2/CO2 are lower. Unlike condensation, which contributes to particle growth until the vapor molecules are fully consumed, nucleation ceases during the last stage of char combustion.

Initiation of combustion of coal particles coated with a water film in a high-temperature air flow

Results of an experimental study of ignition of a coal–water fuel in a high-temperature air flow are presented. The fuel is obtained on the basis of the mark D coal or filter cake of the Severnaya dressing plant. Based on the experimental results, physical and mathematical models of the processes of thermal preparation and ignition of coal–water fuel particles are formulated, which take into account the joint proceeding of the most relevant physical and chemical transformations (which exert an essential effect on the ignition condition) and the processes of heat and mass transfer during the induction period. A comparison of the basic ignition characteristics (ignition delay time and limiting values of the gas medium temperature), which are obtained experimentally and theoretically (based on results of numerical simulations), leads to a conclusion that the water film affects the dynamics of the ignition process.

Prediction of unburned carbon and NO formation from low-rank coal during pulverized coal combustion: Experiments and numerical simulation

Coal in pulverized form has been utilized to generate heat for boiler operation, most of which is used to run power generation plants. Owing to the issues of the pollutant gas production within the pulverized-coal-fired boiler, the CO, NO, and unburned carbon (UBC)concentrations need to be reduced despite the formation of SO, which is also a potential pollutant. In order to study the pollutant generation mechanism of pulverized coal and predict its contents, the drop-tube furnace (DTF) experiment has been established and used extensively by many researchers worldwide. The DTF experiment itself, including the sample preparation processes, takes a long time to be performed. Therefore, in the present study, DTF modeling using one-dimensional numerical simulation is established for the rapid, good-accuracy prediction of the output gas and UBC concentrations. The DTF model is successfully established and found to provide results with trends similar to those of experimental results. Coal with high reaction rates is observed to have higher NO emissions than that with low reaction rates; this is attributed to a higher peak temperature in the combustion cloud around the coal particles, which leads to thermal NO formation. Suppression in the NO formation rate is observed in low-stoichiometric-ratio combustion, even under high-temperature conditions. The existence of a high CO gas concentration would assist in suppressing the NO formation during the combustion of coal char particles.

Effect of coal fragmentation on PCI combustion zone in blast furnace

Pulverized Coal Injection (PCI) is one of the most effective technologies for reducing coke consumption in blast furnaces. Its main role is as the heating source and for the generation of CO gas. Generally, PCI coal is a high-rank coal, but steel mill companies are attempting to use low-rank coal as an energy source in PCI combustion blast furnaces because of the increasing cost of high-rank coal and decreasing reserves. The purpose of this investigation was to determine how the coal rank is correlated to the pulverized coal particle temperature and CO emission. This study was performed using a Laminar Flow Reactor (LFR), which represents the combustion environment of pulverized coal inside a blast furnace. Since the PCI system was developed for iron making, the combustion characteristics of pulverized coal are important in the iron and steel industries. According to the different types of coals, the coal flame structure, coal particle temperature, and exhaust gases (CO and CO2) were investigated. This study demonstrated that the combustion characteristics of coal were influenced by several properties of individual coals. Primarily, the coal particle temperature and CO emission were found to have the strongest influence on the combustion characteristics of these coals. Moreover, the fragmentation phenomenon occurring in some coals was also an important factor influencing the PCI combustion characteristics. Therefore, this study significantly considered the coal properties, focusing on images of the burning coal particle flames, the particle temperature, and the CO/CO2 emissions.

Transient model for soot formation during the combustion of single coal particles

A transient mathematical model was developed to describe soot formation during the combustion of single coal particles based on the static semi-empirical model presented by Fletcher and coworkers. Sensitivity analyses of the model parameters show that soot emissivity and mass diffusivity of tar play an important role in predicting soot volume fraction (fv) and flame temperature (Tf). The model was applied to simulate the combustion of single bituminous coal particles with initial diameter (2r0) of 83µm in a drop tube furnace and air atmosphere. It was found that soot is only formed within the first ∼ 5ms after the appearance of the volatile flame. Although most of the soot is oxidized during the volatile flame phase, a small portion of soot still remains during the char combustion. Due to the soot presence, the volatile flame duration is extended by 2.6ms. Compared with the soot-free flame, the sooting flame has remarkable lower Tf and its peak Tf value is ∼ 410K lower. As a consequence, char combustion starts at a temperature that is ∼ 125K lower than that of the soot-free case. Spatially, the peak fv at 16.6ms appears at 4.5 r0 and soot oxidation zone spans to ∼ 10 r0. The model was validated by comparing the predicted Tf and fv under different O2 mole fractions (xO2) with recent experimental results reported by Khatami and coworkers. The predicted trends are consistent with those of the experimental results. With increasing xO2, Tf increases, but the increase rate becomes more gradual at a large xO2. While for fv, a non-monotonic variation is observed, where soot first increases and then decreases with a peak value occurring at xO2 ≈ 40%.

Ignition of a coal particle on a heated surface

With the use of high-speed (to 6 × 105 frames per second) video recording facilities, the following three physicochemical transformation regimes were recognized on the induction heating of the single particles (4–6 mm in size) of D coal on a steel plate (at a temperature of no higher than 1000 K): thermal decomposition without ignition, smoldering, and ignition. The limiting (minimum) values of heat source temperatures necessary and sufficient for the initiation of combustion of coal particles were established.The effect of particle sizes on the integral characteristics of the test process was found. The particle-size dependence of the time of heating of a natural solid fuel particle to the intense development of an exothermic reaction was obtained. It was found that the characteristic times of the initiation of the ignition and smoldering regimes were no longer than 10 and 20 s, respectively, with variations in coal particle size.

Devolatilization and volatiles reaction of individual coal particles in the context of FGM tabulated chemistry

The method of Flamelet Generated Manifolds (FGM) is coupled with a coal devolatilization model to perform transient simulations of a well-defined single coal particle combustion experiment for the first time. The gas phase chemistry is mapped onto a three-dimensional manifold controlled by the mixture fraction, a reaction progress variable and the enthalpy. A simulation of an electrically heated inert pyrolysis reactor is performed in order to evaluate transferability and applicability of experimentally obtained devolatilization kinetic parameters to Large Eddy Simulation (LES) codes for combustion configurations. Finally, the FGM modeling approach is applied to a premixed flat flame configuration, in which the coal particles cross a laminar flame front and are exposed to the hot gases. Numerical results regarding the volatiles reaction are compared to experimental findings. Particle and gas phase states are studied. Overall, good agreement between numerical results and experimental findings regarding the volatiles ignition range could be observed.

Pyrolysis, Combustion, and Fragmentation Model of Coal Particles: Preliminary Results

ABSTRACTA mathematical model has been developed to predict fragmentation of particles under a wide range of pyrolysis and combustion conditions. The model is an upgrade of a previous one that took into account only fragmentation during the heat up and devolatilization stage. The model calculates the temperature and oxygen profiles within the particle, the evolution of internal porosity as a consequence of both devolatilization and carbon combustion, the mechanical stress caused by temperature gradients, and by volatiles-generated overpressure inside the particles. Eventually the model calculates the probability of rupture of the particle based on the Weibull (1939) theory. The model has been used to simulate heating of coal particles under inert conditions at different heating rates and temperatures showing good agreement with previous work. The model has been further used to simulate heating under oxidative conditions in order to highlight the role of combustion on fragmentation phenomena.

Inner Relationship between CO, NO, and Hg in a 6 kWth Circulating Fluidized Bed Combustor under an O2/CO2 Atmosphere

Combustion experiments of semianthracite coal were conducted on a 6 kWth oxy-fuel circulating fluidized bed combustor. The inner relationship between CO, NO, and Hg in the combustor under an O2/CO2 atmosphere was explored. The results demonstrate that all temperatures are lower under a 21% O2/79% CO2 atmosphere than those under an air atmosphere. In oxy-coal combustion, the increasing oxygen concentration can increase CO emission. NO emission in oxy-coal is much lower than that in air combustion as a result of coal particle combustion generating less oxygen status under a 21% O2/79% CO2 atmosphere. The percentage of mercury oxidation is higher under air–coal combustion than that under 21% O2/79% CO2–coal combustion, because the specific heat of CO2 is higher in 21% O2/79% CO2 than that in air. The increase of the oxygen concentration under oxy-coal combustion leads to the total reducing atmosphere decrease, which is suitable for the mercury oxidation. These results are very significant for selection of op...

The effect of fuel pyrolysis on the coal particle combustion: An analytical investigation

The aim of this work is to analytically investigate the symmetrical combustion of an isolated coal particle with the fuel pyrolysis effect. The modelling concept of coal particles is similar to that of the liquid droplet combustion but in the case of coal devolatilization, the particles do not shrink like droplet does due to evaporation of liquid fuel. The rate of devolatilization of volatiles can be calculated using the equation that is similar to Arrhenius equation. This model is based on an assumption of combined quasi-steady and transient behaviour of the process and especially focuses on predicting the variations of temperature profile, radius of pyrolysis and transfer number. It is revealed that the entrance of pyrolysis effect into the governing equations leads to the reduction in the film radius and consequently a reduction in the stand-off ratio and transfer number.