Char Combustion Processes Research Articles

Char combustion of ellipsoidal particles in a hot gas with fluctuating temperature

AbstractChar combustion of biomass particles is a significant thermo‐chemical conversion process. The char combustion process of biomass particles with gas temperature fluctuation is numerically investigated. Considering the non‐spherical shape of biomass particles, a fully algebraic expression of burning enhancement factor is theoretically derived to modify the char combustion rate of ellipsoidal particles. Four solid biofuel samples (cynara, pinewood, willow, and cardoon) are chosen for the calculations. The instantaneous mass variations and char combustion rates of biomass particles are calculated under different particle sphericities. Different kinetic parameters of biomass samples lead to various effects of gas temperature fluctuation on char combustion. The gas temperature fluctuation generally enhances the char combustion rate. Increasing the fluctuation frequency of gas temperature has no effect on char combustion. Under the same particle surface area, the char combustion rate is enhanced with decreasing particle sphericity.

A CCSEM study on the transformation of included and excluded minerals during coal devolatilization and char combustion

This study reports the transformation pathways of both included and excluded minerals during coal devolatilization and char combustion. Three Chinese coals of various ranks, namely YQ anthracite, DT bituminous coal and CF lignite, were size narrowed to 45−100μm, devolatilized in nitrogen and combusted in air at 1300°C, using a laboratory-scale drop tube furnace, to prepare chars and ashes. The mineral properties of the coals as well as their derived chars and ashes were analyzed using a computer-controlled scanning electron microscopy (CCSEM). The results demonstrate that, during coal devolatilization, whereas quartz and kaolinite are generally stable for all the three coals, included and excluded Fe-rich minerals (i.e., pyrite and siderite) as well as excluded Ca-rich minerals (i.e., calcite and ankerite) are decomposed and some of the intermediate products are incorporated with aluminosilicates. During char combustion, quartz participates in combustion reactions and forms aluminosilicates. Whereas the mullite-like phase in the YQ and DT chars changes little, that in the CF char absorbs Fe/Ca-rich fine particles and is incorporated into Fe–Ca-aluminosilicate. Most of the Ca/Fe-rich intermediate products react with aluminosilicates to form Fe-, Ca-, and Fe–Ca-aluminosilicates. The interactions between Fe/Ca-rich minerals and aluminosilicates are also observed. Calcium in Ca-rich minerals and iron in Fe-rich minerals of the YQ anthracite and the CF lignite interact with aluminosilicates during both coal devolatilization and char combustion processes. However, for the DT bituminous coal, such interactions between Ca-rich minerals and aluminosilicates only take place during devolatilization. The interactions between excluded Fe/Ca-rich minerals and included aluminosilicates during coal devolatilization are also observed.

Application of Different Combustion Models for Simulating the Co-combustion of Sludge with Coal in a 100 MW Tangentially Coal-Fired Utility Boiler

Co-combustion of sludge with coal is a promising sludge disposal approach. In this study, three different combustion models, including different turbulent gas-phase combustion and char surface combustion sub-models, were employed to simulate the combustion and emission characteristics of co-combusting sludge with coal in a 100 MW tangentially coal-fired utility boiler. By comparison of the numerical results from different combustion models to a set of full-scale coal–sludge co-combustion field experiments, a precise and suitable computational fluid dynamic (CFD) model was finally developed. The comparison result demonstrated that the finite-rate/eddy-dissipation model together with the multiple-surface-reaction model, which have carefully considered the effect of high moisture content on the fuel ignition and char combustion processes, were competent in modeling the characteristics of coal–sludge co-combustion. On the basis of the developed numerical model, the ignition, char burnout, and NOx emission characteristics of co-firing different mass contents of sludge were further investigated. The result demonstrated that co-firing sludge with coal would cause a slight delay on the fuel ignition and a decline on the char burnout. The high moisture and high nitrogen in sludge had a dual promotion effect on NO formation, leading to an increase of NOx emission after co-firing sludge.

Investigation of thermal degradation of different wood-based biofuels of the northwest region of the Russian Federation

The thermal degradation of different wood biofuels collected in the Arkhangelsk region (northwest of the Russian Federation) was investigated in order to extract their kinetic parameters. Thermogravimetric analysis was carried out for small samples under inert atmosphere or air. These samples were heated from 20 to 1300 � C under three heating rates of 5, 10 or 20 � C min -1 . The mass loss rates were measured during the drying, devolatilization and char combustion processes. Average values of the activation energies were determined using Friedman's and Ozawa- Flynn-Wall's methods under inert atmosphere and air. The morphological analysis of raw samples and of the corre- sponding chars was also performed. Pore development presents an effective impact on char reactivity. These experimental results and the kinetic data obtained in this work should be considered for the simulation of the pyrolysis and combustion processes of different biofuels in industrial devices.

Variation of Apparent Reaction Order in Char Combustion and Its Effect on a Fractal Char Combustion Model

The variation of the apparent reaction order, , during char combustion is studied by simulating the combustion process inside the char particle pores. An expression for is obtained as a function of the particle temperature, the O2 partial pressure, and the pore structural parameters. A modulus, , is shown to represent the pore diffusion resistance, with a sufficient condition for given based on . becomes lower as the particle temperature and the O2 partial pressure increase and the pore structure has higher diffusion resistance. The expression for is used to improve the accuracy of a previous fractal char combustion model. The effect of on the char combustion rate and the regularity of how changes in the char combustion process are also studied.

Instantaneous NO release from ellipsoidal particles during char combustion in a hot gas with fluctuating temperature

We have established the instantaneous governing equations for NO release during char combustion of nonspherical particles theoretically. The instantaneous released NO mass variations, the instantaneous char oxidation reaction rates, and the instantaneous NO reduction reaction rates of char particles were computed numerically under different fluctuation amplitudes of gas temperature, kinetic parameters of char oxidation reaction and NO reduction reaction, and particle aspect ratios. The gas temperature fluctuation results in a faster NO release rate, a faster char oxidation reaction rate, and a faster NO reduction reaction rate during the whole char combustion processes. The activation energy of char oxidation reaction has obvious influence on the NO release and char combustion processes. The kinetic parameters of NO reduction reaction have no contribution to the conversion time. Under the same particle surface area, the conversion rate of char nitrogen to NO increases with the increase in the particle aspect ratio.

Lattice Monte Carlo simulation of single coal char particle combustion under oxy–fuel conditions

A descriptive model for isolated char particle combustion under conventional and oxy–fuel conditions was developed. Suggested model is based on the percolation theory and Monte Carlo simulation technique. Char particle was modeled as a square lattice consisting of a large number of small sites. Sites correspond either to fixed carbon, ash, or pore, and they were distributed randomly inside char lattice using percolation concept, at the start of simulation. Random walk model was used to represent gaseous species diffusion through particle pores. Char combustion was modeled using power law Arrhenius model which assumes that reaction rate depends of particle temperature and oxygen partial pressure on particle surface. The main aim of the proposed model was to take into account influence of heterogeneous char particle structure to randomness of the char combustion process. The suggested model’s behavior was validated by qualitative comparison with experimental data obtained in single particle reactor. It was found that simulated combustion time, char burnout and particle temperature values are in good agreement with experimentally determined data. Special emphasis was given to the CO2 gasification reaction influence on char conversion and particle temperature values. Further development of the proposed model with appropriate simplifications would enable its inclusion in comprehensive CFD codes.

Numerical research on the homogeneous/heterogeneous ignition process of pulverized coal in oxy-fuel combustion

Oxy-fuel combustion technology has been deemed a promising technology to reduce CO2 emissions from coal-fired power stations. The ignition processes of pulverized coal in oxy-fuel combustion were detailedly simulated using a CFD model with a new devolatilization sub-model. The profiles of the rates of devolatilization, volatile content, char and coal combustion during the homogeneous and heterogeneous ignition process were obtained by the numerical simulation. The present CFD model was validated by the comparison of the predicted ignition positions of a coal particle with a diameter of 85μm and Jovanovic’s experimental results for four different temperatures of the secondary oxidant: 1073K,1273K, 1473K and 1623K, and oxygen mole fractions of 0.21 and 0.35 in nitrogen or carbon-dioxide. The ignition mechanisms were determined at the position of particle ignition based on three criteria conditions of the homogenous, heterogeneous, and heterogeneous–homogeneous ignition. The three conditions were implemented in the FLUENT code by user-defined function (UDF). The mechanisms of the coal ignition in O2/CO2 atmospheres are that for an O2 mole fraction of 0.21 in CO2, a coal particle with a diameter of 85μm ignites homogeneously when the secondary air temperature ranges from 1073K to 1473K. The particle ignites heterogeneously when the secondary air temperature exceeds 1473K. When the mole fraction of O2 in CO2 increases to 0.35, homogeneous ignition occurs at secondary air temperatures of 1073K and 1273K, while heterogeneous ignition occurs at temperature higher than 1273K. The rates of devolatilization, char and coal combustion at the ignition position changed slightly when the secondary air temperature increased. However, the rate of volatile combustion at the ignition position drastically decreased as the secondary air temperature was increased. The drastic variance of the rate of volatile combustion at the ignition position caused the change in the ignition mechanism of coal in oxy-fuel combustion. During homogeneous ignition in O2/CO2 atmospheres, the char generally ignites at the maximum mole fraction of volatiles near the surface of the coal particle. The devolatilization and volatile combustion processes are not affected by the char combustion process. During heterogeneous ignition in O2/CO2 atmospheres, volatiles generally ignite at the maximum mole fraction of volatiles near the surface of coal particle. The interaction between the combustion of volatiles and char is significant. The curve of the rate of char combustion features a v-notch due to the combustion process of volatiles, and the curves of the rate of volatiles combustion and mole fraction of volatiles are skewed due to the char combustion process.

Numerical Study of the Relationships Between Pore Structures and Reaction Parameters for Coal Char Particles

Fractal pore models, gas molecular movement models, and reaction models are combined to simulate char oxidation in char pores. The numerical results show the relationship between the pore structure and the apparent char reaction parameters. A modulus number φ is introduced to characterize the diffusion properties in fractal porous media with a relationship given for the apparent reaction parameter in terms of φ. The results show that the char combustion process is strongly impacted by the pore structure, and that the fractal dimension is an important parameter that should be considered in char combustion. Comparison with experimental data shows that this model is more accurate than the classical intrinsic combustion model.

Thermogravimetric Analysis of Different Biomass Materials and the Primary Biomass Components

Thermogravimetric analysis (TGA) was employed to study the pyrolysis and combustion characteristics of three primary biomass components (cellulose, xylan and lignin) and seven different biomass materials under both of nitrogen and air atmosphere. Based on the experimental results, the pyrolysis and combustion kinetics were calculated. The results indicated that the three primary biomass components exhibited different decomposition characteristics and pyrolysis kinetics. Xylan was the least thermal stable component, while the lignin would form much more solid residues than the cellulose and xylan under the nitrogen atmosphere. Moreover, the pyrolytic devolatilization process of the cellulose had high activation energy, so was the char combustion process of the lignin. The seven biomass materials showed similar pyrolysis characteristics, and poplar wood exhibited high activation energy values in both of pyrolysis and combustion processes.

Production, characterization and reactivity studies of chars produced by the isothermal pyrolysis of flax straw

Abstract The influence of pyrolysis temperature and residence time on the char yields and resultant char characteristics were investigated in the isothermal pyrolysis of flax straw. The pyrolysis temperature was varied in the range between 300 and 500 °C and reaction residence time was varied from 15 to 60 min. The char yield was found to decrease with both increasing pyrolysis temperature and residence time. The char structure and physical characteristics were thoroughly investigated by means of X-ray diffraction (XRD), temperature-programmed oxidation (TPO) and N 2 physisorption techniques. The results show that the degree of porosity and graphitization increased with increasing pyrolysis temperature and time. TPO studies on the char samples corroborate well with the XRD findings and showed the presence of two types of carbon; namely, amorphous filamentous carbon and crystalline graphitic carbon. Thermogravimetric analysis (TGA) of the char was performed to understand the combustion kinetics and reactivity. Chars formed at lower pyrolysis temperatures were found to be more reactive than the chars produced at higher pyrolysis temperatures, and these findings are well supported by the TPO, TGA, N 2 physisorption and XRD characterization data. Furthermore, an empirical global kinetic model was devised based on power law and used to estimate the activation energy and other kinetic parameters of both flax straw pyrolysis and char combustion processes.

Heat transfer in the rotary ash cooler with residual char combustion considered

In circulating fluidized-bed (CFB) boilers, some residual char is left in the ash when the ash is drained into the rotary ash cooler (RAC). The residue char will combust in the RAC given oxygen is present. It is important to understand and predict the influence of residual char combustion on the heat transfer process in the RAC in order to avoid slagging and fouling inside the RAC. In this paper, an improved heat transfer model of the RAC was developed in which the residual char combustion process was considered. Based on the analysis, the shrinking sphere model was selected to describe the residual char combustion. The improved model was validated by the final ash temperature and cooling water temperature data measured from a few RACs equipped with the coal fired CFB boilers with different steam output. The predictions of the improved model agree well with the field measurement data, within an error range of ± 10%. The simulation results also show that the heat released from residual char combustion counts for about 60–80% of the total heat released by ash in the inlet section of the RAC, and the influence of residual char combustion decreases rapidly along the ash flow. The final ash temperature increases accordingly with the increase of inlet carbon content, and decreases with the increases of residual char size. As particle size is larger than 3 mm, the final ash temperature changes little at a given carbon content due to a low mass burning rate. It is suggested that combustion of residual char should be considered when the inlet carbon content is over 2.5%; otherwise, the combustion of residual char in the RAC could be neglected.

A Mathematical Model of Cigarette Smoldering Process

Abstract A mathematical model for a smoldering cigarette has been proposed. In the analysis of the cigarette combustion and pyrolysis processes, a receding burning front is defined, which has a constant temperature (~450 °C) and divides the cigarette into two zones, the burning zone and the pyrolysis zone. The char combustion processes in the burning zone and the pyrolysis of virgin tobacco and evaporation of water in the pyrolysis zone are included in the model. The hot gases flow from the burning zone, are assumed to go out as sidestream smoke during smoldering. The internal heat transport is characterized by effective thermal conductivities in each zone. Thermal conduction of cigarette paper and convective and radiative heat transfer at the outer surface were also considered. The governing partial differential equations were solved using an integral method. Model predictions of smoldering speed as well as temperature and density profiles in the pyrolysis zone for different kinds of cigarettes were found to agree with the experimental data. The model also predicts the coal length and the maximum coal temperatures during smoldering conditions. The model provides a relatively fast and efficient way to simulate the cigarette burning processes. It offers a practical tool for exploring important parameters for cigarette smoldering processes, such as tobacco components, properties of cigarette paper, and heat generation in the burning zone and its dependence on the mass burn rate.

Firespread through fuel beds: Modeling of wind-aided fires and induced hydrodynamics

The propagation of wind-aided line fires through fuel beds is simulated by using a multiphase approach. In this approach, a gas phase flows through N solid phases which constitute an idealized reproduction of the heterogeneous combustible medium. A set of time-dependent equations is obtained for each phase and the coupling between the gas phase and the solid phases is rendered through exchange terms of mass, momentum, and energy. Turbulence is approached by using a RNG k−ε statistical model constructed from the Favre averaging method. The radiative transfer equation extended to multiphase media is solved using the discrete ordinates method (DOM). Soot formation is taken into account for the evaluation of the absorption coefficient of the soot/fuel particles/combustion products mixture using the gray gas assumption. First-order kinetics is incorporated to describe water vaporization, pyrolysis, and char combustion processes. The solution is performed numerically by a finite-volume method including a high-order upwind convective scheme and a flux limiter strategy along with a projection method for the pressure–velocity coupling. This model has been applied to describe the unsteady behavior of wind-aided fires spreading through a litter of dead pine needles and the induced hydrodynamics. The numerical results obtained from our model are presented and compared to measurements and predictions from other laboratory-based models.

A methodology for analysis of 002 lattice fringe images and its application to combustion-derived carbons

A technique is presented for semi-quantitative digital analysis of 002 lattice fringe images of carbon materials. The automated technique yields statistics on fringe length and tortuosity, as well as new quantitative parameters describing the mode, degree, and length scale of orientational order among graphene layers. The technique is applied to combustion-derived carbons with special emphasis on the nanostructural characterization of a variety of solid fuel chars as a function of residence time. During the combustion of pulverized coal and biomass (120 ms at 1700–1900 K), the main features of char nanostructure are established early, near the end of the rapid, in-flame pyrolysis. Only for the high-volatile bituminous coals is the nanostructural order in the young chars significantly altered by the subsequent char combustion process. A noteworthy general observation is the presence of two distinct length scales for orientational order in most chars. At short length scales (<30 Å) a high degree of orientational order is observed reflecting the presence of small molecular orientation domains. At larger length scale (>30 Å), a lesser but significant degree of orientational order is still present among the distinct domains. Quantitative analysis of this low-grade long-range order using Maier–Saupe theory indicates that it cannot arise by a liquid crystal mechanism. Long-range orientational order in these samples can thus be classified as ‘mesophasic’, exhibiting a high degree of order arising through liquid crystal formation in the fluid phase of pyrolysis (or retention of anisotropy in very high rank coals), and ‘statistical’, perhaps arising from elongational strain during carbonization.

An extended coal combustion model

Current coal combustion models are a useful tool in research but they use simplified coal particle devolatilisation and combustion steps in order to meet computational limitations. The availability of more advanced computers enables the use of more detailed steps for devolatilisation and the use of more realistic char combustion processes. In the present work the devolatilisation rates were calculated using the coal devolatilisation model FG-DVC. In this way devolatilisation rates and the yields of char and volatile were obtained. A drop tube reactor was operated at 1623 K to collect char samples, from Thoresby coal, at different sampling positions or residence times, and proximate and ultimate analysis were conducted on these char samples to confirm the results. The detailed char combustion sub-models being developed for CFD codes require char properties such as densities, surface areas and pore size distributions but a simplified model was used here. In this case the use of a simple global char oxidation model together with an empirical derived ‘volatile’ and FG-DVC predicted devolatilisation rate data seem to give good agreement with the experimental results available for the char burnout. However, there still remains considerable uncertainty in the use of char burnout models including the one used here is not sufficiently accurate in predicting carbon burnout in all conditions.

Evolution of pore fractal dimensions for burning porous chars

The evolution of pore fractal dimensions for three Chinese chars was investigated. The pores of porous chars can be classified into macro-, micro- and transition-pores based on their fractal dimensions. The radius ranges of the macro-, micro- and transition-pores may be different for various chars. For different conversions in the char combustion process, the fractal dimensions of the micro-pores for different chars are almost the same and the fractal dimensions of macro-pores for the same kind of chars are roughly unchanged. The effects of fractal structure on combustion rate are also discussed.

98/00565 Mathematical model for heat transfer in char combustion processes

Environmental legislation has had significant impact on coal utilization, especially coal combustion for power generation, in limiting emissions of potentially hazardous materials to the environment. For the most part, such emissions derive from the inorganic constituents in coal. However, as such legislation becomes ever more encompassing, it has increased the need to understand better the behavior of the inorganic species in coal processing to ensure, in part, that such legislation is not unduly burdensome. Consequently, it has led to significant development of new models for the behavior of inorganics in coal combustion and a complementary enhancement of many analytical methods for determining inorganics in coal.In this paper, analytical methods for inorganics in coal are reviewed on three fronts: (i) methods for determining elemental concentrations; (ii) methods for determining the mineralogy of coals; and (iii) methods for determining modes of occurrence (speciation) of trace elements in coal. The concept of association with respect to mineral–maceral and mineral–mineral occurrences is also discussed. Where possible, comparison of different analysis methods has been made by reference to data on well-characterized suites of coals, such as the Argonne premium coal samples.Incremental enhancements will continue to be made in analytical methods for elemental concentrations; however, major improvements are needed in the other two areas. There is a great need to verify and corroborate by direct speciation methods, the many inferences made by indirect methods regarding trace element speciation. Also, improvements in the measurement of mineral association and its integration into the coal mineral analysis by means of the computer-controlled scanning electron microscopy (CCSEM) would be a significant advance. Accurate determinations of both association and coal mineralogy would lead to significant and much-needed refinements of models for the behavior of inorganics in both coal cleaning processes and coal combustion.

97/02365 Novel designs of fluidized bed combustors for low pollutant emissions

It is known that NH{sub 3}, released during the devolatilization of fuel, is an important precursor for NO formation in fluidized bed combustors. On the other hand, NH{sub 3} may be used as a reducing agent in the thermal DeNO{sub x} process to reduce NO{sub x} emission levels. In this paper, a new concept of fluidized bed combustors is proposed based on the idea of in situ reduction of NO{sub x} by self-produced NH{sub 3} from fuel without lowering the sulfur capture level. This design is intended to separate the NH{sub 3} release process under reducing conditions from the char combustion process under oxidizing conditions; this self-released NH{sub 3}, together with some combustibles, is mixed with gaseous combustion products in the upper part of the combustor for a further reduction of the NO{sub x} formed during combustion. Furthermore, the combustion of the combustibles may cause the temperature to rise in this upper zone and thereby reduce the emission of N{sub 2}O. The applications of this design to bubbling and circulating fluidized bed combustors are described and the mechanisms of the main reactions involved discussed.

Combustion characteristics of wet sludge in a fluidized bed: Release and combustion of the volatiles

Combustion characteristics of wet (i.e. mechanically dewatered) sewage sludge were investigated in both laboratory and semi-pilot-scale fluidized bed combustors. The aims were to determine the influence of the high water content of sludge on its combustion process, the distribution of sludge carbon between the volatiles and char, the location of the release and combustion of the volatiles and the concentrations of char carbon in the bed. The results showed that there is a significant overlap of the drying, devolatilization and char combustion processes, so that the three processes may be considered to run in parallel. Both drying and devolatilization take place at very low particle temperature. Up to 80% of the sludge carbon is volatile carbon and sludge combustion is characterized by very low char carbon concentrations in the bed. Therefore sludge combustion is dominated by the gas-phase combustion of the volatiles. The release of the volatiles takes place mainly in the bed. Freeboard combustion is dominated by CO oxidation and takes place mainly within 2.5 m above the bed.