Weighted Sum Of Gray Gases Model Research Articles

Effects of total pressure on non-grey gas radiation transfer in oxy-fuel combustion using the LBL, SNB, SNBCK, WSGG, and FSCK methods

The effects of total pressure on gas radiation heat transfer are investigated in 1D parallel plate geometry containing isothermal and homogeneous media and an inhomogeneous and non-isothermal CO2–H2O mixture under conditions relevant to oxy-fuel combustion using the line-by-line (LBL), statistical narrow-band (SNB), statistical narrow-band correlated-k (SNBCK), weighted-sum-of-grey-gases (WSGG), and full-spectrum correlated-k (FSCK) models. The LBL calculations were conducted using the HITEMP2010 and CDSD-1000 databases and the LBL results serve as the benchmark solution to evaluate the accuracy of the other models. Calculations of the SNB, SNBCK, and FSCK were conducted using both the 1997 EM2C SNB parameters and their recently updated 2012 parameters to investigate how the SNB model parameters affect the results under oxy-fuel combustion conditions at high pressures. The WSGG model considered is the recently developed one by Bordbar et al. [19] for oxy-fuel combustion based on LBL calculations using HITEMP2010. The total pressure considered ranges from 1 up to 30atm. The total pressure significantly affects gas radiation transfer primarily through the increase in molecule number density and only slightly through spectral line broadening. Using the 1997 EM2C SNB model parameters the accuracy of SNB and SNBCK is very good and remains essentially independent of the total pressure. When using the 2012 EM2C SNB model parameters the SNB and SNBCK results are less accurate and their error increases with increasing the total pressure. The WSGG model has the lowest accuracy and the best computational efficiency among the models investigated. The errors of both WSGG and FSCK using the 2012 EM2C SNB model parameters increase when the total pressure is increased from 1 to 10atm, but remain nearly independent of the total pressure beyond 10atm. When using the 1997 EM2C SNB model parameters the accuracy of FSCK only slightly decreases with increasing the total pressure.

The usability and limits of the steady flamelet approach in oxy-fuel combustions

This paper investigates two furnaces which work under oxy-fuel condition with natural gas. One is a 0.8 MW furnace where detailed inflame measurements are available. The other furnace is an 11.5 kW lab-scale furnace with temperature measurements. The furnaces were investigated by CFD (Computational fluid dynamics) analysis. The main focus was on using combustion models that are not computationally demanding. Therefore the SFM (steady flamelet) approach was used with two detailed mechanisms. The advantage of the SFM is that the calculation time can be reduced from 4 weeks to 4 days on 8 CPU-cores. The applicability of two detailed mechanisms under oxy-fuel condition is pointed out in this paper. The investigation showed that the skeletal25 mechanism and the SFM are in very good accordance with measurements. If the strain rate between CH4 and O2 stream is too low, the SFM fails to predict the flame shape correctly. The influence of three different turbulence models was also investigated. Furthermore simulations with the eddy dissipation model and numerically expensive eddy dissipation concept model were conducted. Different WSGGM (weighted sum of grey gases model) were applied. The comparison of the WSGGMs showed that the difference between them is insignificant for small furnaces.

A full spectrum k-distribution based weighted-sum-of-gray-gases model for oxy-fuel combustion

Oxy-fuel combustion has become one of the most promising technologies for carbon capture and storage (CCS) in coal-fired plants. Radiative properties of combustion gases in oxy-fuel combustion are significantly different from those found in air-fired due to the change of the gas composition. A few modified weighted-sum-of-gray-gases (WSGG) models had been proposed to consider the wide H2O to CO2 ratio for oxy-fuel combustion in recent years. As most of the models are validated using simple homogenous benchmark, the applicability of such models to non-homogeneous media is questionable. In this paper, based on the up-to-date HITEMP 2010 database, we obtain new parameters of WSGG model for oxy-fuel combustion by combining the features of full-spectrum k-distribution (FSK) model with WSGG model, in which the weighting factors and absorption coefficients are directly obtained from the k-distribution, and the polynomial expression of absorption coefficient is improved. The new parameters of WSGG model are extensively validated by comparing the radiative source terms and radiative heat fluxes, predicted with the line-by-line (LBL) model integration of HITEMP 2010 database at a one-dimensional slab system. The predictions by different parameters of WSGG model from the literature are also included in comparison. The investigative cases include the non-isothermal and/or non-homogeneous gas mixture. The maximum relative error of heat fluxes is 11.2% for the new parameters, 33.5% for Yin model (Yin et al., 2010) and 23.0% for Johansson model (Johansson et al., 2011) in non-isothermal non-homogeneous gas mixture. The results show the new parameters can significantly improve the prediction accuracy in non-isothermal non-homogeneous CO2/H2O mixture.

Evaluation of gas radiation heat transfer in a 2D axisymmetric geometry using the line-by-line integration and WSGG models

The weighted-sum-of-gray-gases (WSGG) model is widely used in engineering computations of radiative heat transfer due to its relative simplicity, robustness and flexibility. This paper presents the computation of radiative heat transfer in a 2D axisymmetric chamber using two WSGG models to compute radiation in H2O and CO2 mixtures. The first model considers a fixed ratio between the molar concentrations of H2O and CO2, while the second allows the solution for arbitrary ratios. The correlations for both models are based on the HITEMP2010 database. The test case considers typical conditions found in turbulent methane flames, with steep variations in the temperature field as well as in the molar concentrations of the participating species. To assess the accuracy of the WSGG model, the results are compared with a solution obtained by line-by-line integration (LBL) of the spectrum.

Predicting Radiative Heat Transfer in Oxy-Methane Flame Simulations: An Examination of Its Sensitivities to Chemistry and Radiative Property Models

Measurements from confined, laminar oxy-methane flames at different O2/CO2dilution ratios in the oxidizer are first reported with measurements from methane-air flames included for comparison. Simulations of these flames employing appropriate chemistry and radiative property modeling options were performed to garner insights into the experimental trends and assess prediction sensitivities to the choice of modeling options. The chemistry was modeled employing a mixture-fraction based approach, Eddy dissipation concept (EDC), and refined global finite rate (FR) models. Radiative properties were estimated employing four weighted-sum-of-gray-gases (WSGG) models formulated from different spectroscopic/model databases. The mixture fraction and EDC models correctly predicted the trends in flame length and OH concentration variations, and the O2, CO2, and temperature measurements outside the flames. The refined FR chemistry model predictions of CO2and O2deviated from their measured values in the flame with 50% O2in the oxidizer. Flame radiant power estimates varied by less than 10% between the mixture fraction and EDC models but more than 60% between the different WSGG models. The largest variations were attributed to the postcombustion gases in the temperature range 500 K–800 K in the upper sections of the furnace which also contributed significantly to the overall radiative transfer.

Numerical investigation of oxy-coal combustion in a large-scale furnace: Non-gray effect of gas and role of particle radiation

Computational fluid dynamics (CFD) simulations of the oxy-coal combustion in a large-scale furnace were performed by applying different weighted sum of gray-gases models (WSGGMs). Before simulation of the furnace, the different WSGGMs were tested in an enclosed rectangular box containing only gases inside. In both cases of air and oxy-fuel combustion, the calculated radiative sources and wall heat fluxes obtained with gray and non-gray WSGGMs were completely different. Only with the non-gray WSGGMs (i.e., non-gray implementation), the calculated results could well agree with the benchmark data. In the simulation of furnace, three WSGGMs were adopted: Smith’s gray WSGGM (i.e., gray implementation), Johansson’s non-gray-A WSGGM with uniform distribution of radiative energy of particles in each gray band, and Johansson’s non-gray-B WSGGM, in which the radiative energy of particles was distributed using weighting factors. The weighting factors for the particles can be calculated with the same function as that used for gas but using the particle temperature instead of gas temperature, i.e., wp,i=wg,i (Tp). Being different from the rectangular box without particle radiation, in the real furnace, the gas temperature and wall heat flux calculated by gray and non-gray-A WSGGMs almost show no obvious difference. This is attributed to the fact that particle radiation plays a more significant role than gas radiation in the real furnace. However, using the non-gray-B model instead of Smith’s gray model or non-gray-A model can obviously influence the distribution of wall heat flux and total amount of heat absorbed by the walls. This indicates that in the numerical simulation of oxy-fuel combustion, the non-gray WSGGMs should be used by combining with the weighting factors of wp,i=wg,i (Tp) for particles, in order to rigorously describe both the gas and particle radiation during oxy-fuel combustion of coal in real furnace.

CFD modelling of air and oxy-coal combustion

Purpose – The main goal of this work was the CFD analysis of air and oxy-coal combustion, in order to develop a validated with experimental measurements model of the combustion chamber. Moreover, the purpose of this paper is to provide information about limitations of the sub-models implemented in commercial CFD code ANSYS Fluent version 13.0 for the oxy-coal combustion simulations. The influence of implementation of the weighted sum of gray gas model (WSGGM) with coefficients updated to oxy-coal combustion environment has been investigated. Design/methodology/approach – The sub-models validated with experimental measurements model for the air combustion has been used to predict the oxy-coal combustion case and subsequently the numerical solutions have been compared with the experimental data, which enclose the surface incident radiation (SIR) and the flue gas temperature. To improve the numerical prediction of the oxy-coal combustion process the own routine for calculating properties of the oxy-combustion product has been implemented. Findings – The results of numerical simulation of combustion in the air environment fitted within the experimental measurements accuracy. However, the air combustion sub-models implemented for the oxy-coal combustion simulations does not predict the SIR within the experimental data accuracy. The implementation of own routine, which uses the coefficients calculated for oxy-coal combustion environment shows improvement in numerical prediction of oxy-coal combustion. Originality/value – The radiative properties of gases in the combustion chamber during oxy-coal combustion calculated using the WSGGM implemented in ANSYS Fluent 13.0 do not predict the SIR within experimental measurement accuracy, however, implementation of WSGGM with updated coefficients provide a reasonable improvement in numerical prediction of SIR in the oxy-coal combustion.

A line by line based weighted sum of gray gases model for inhomogeneous CO2–H2O mixture in oxy-fired combustion

The HITEMP 2010 spectral emissivity database has been employed in line by line (LBL) calculation to produce an accurate total emissivity database for H2O–CO2 mixtures of the composition characteristics for the oxy-fired combustion. A wide range of temperatures, pressure-path length products and molar fraction ratios have been covered in the database. By using the LBL based emissivity database, an accurate set of coefficients has been obtained for the weighted sum of gray gases model (WSGGM). Compared to the standard WSGGM, the present model includes the molar ratio of H2O–CO2 mixtures in its formulation which leads to one set of coefficients required to represent the entire range of molar ratio. This simplifies the implementation of the model in the simulation of radiative heat transfer in high inhomogeneous media providing higher accuracy. The model is validated against the benchmark solutions for oxy-fired conditions. Obtained results have been compared with some of the previously reported WSGGMs.

Radiative heat transfer analysis in modern rocket combustion chambers

Radiative heat transfer is analyzed for subscale and fullscale rocket combustion chambers for H2/O2 and CH4/O2 combustion using the P1 radiation transport model in combination with various Weighted Sum of Gray Gases Models (WSGGMs). The influence of different wall emissivities, as well as the results using different WSGGMs, the size of the combustion chamber and the coupling of radiation and fluid dynamics, is investigated. Using rather simple WSGGMs for homogeneous systems yields similar results as using sophisticated models. With models for nonhomogeneous systems the radiative wall heat flux (RWHF) decreases by 25–30 % for H2/O2 combustion and by almost 50 % for CH4/O2 combustion. Enlarging the volume of the combustion chamber increases the RWHF. The influence of radiation on the flow field is found to be negligible. The local ratio of RWHF to total wall heat flux shows a maximum of 9–10 % for H2/O2 and 8 % for CH4/O2 combustion. The integrated heat load ratio is around 3 % for H2/O2 and 2.5 % for CH4/O2 combustion. With WSGGMs for nonhomogeneous systems, the local ratio decreases to 5 % (H2/O2) and 3 % (CH4/O2) while the integrated ratio is only 2 % (H2/O2) and 1.3 % (CH4/O2).

3-D radiation modeling of nongray gases–particles mixture by two different numerical methods

A new methodology and code for 3-D radiation of nongray gases mixtures containing gray anisotropically scattering particles is developed. Radiative transfer equation (RTE) is solved by finite volume method (FVM) coupled with Smith’s weighted sum of gray gases model (WSGGM) with 5 gray gases, but any other gas radiative properties model can be incorporated. Evaluation of scattering phase function (SPF) and analysis of anisotropically scattering particles were performed by Mie Theory (MT). Missing benchmark, another new 3-D model and code, which solve same problems, based on a combination of Zone Method (ZM) and Monte Carlo method (MC) is developed. In this method the same WSGGM and MT, as in previous methodology, is incorporated. Physical and mathematical concepts of both models are presented. To author’s best knowledge these methodologies were not developed before. Series of predictions for the mixtures of real gases as carbon dioxide, water vapor and nitrogen containing various types of particles for non-uniform temperature field were performed by both methods in order to be compared. Agreement between the predictions of these two different numerical methods is very good. Model developed is sufficiently accurate and convenient for engineering calculations as well as for incorporation in computational fluid dynamics codes.

Evaluation of a steady flamelet approach for use in oxy-fuel combustion

The present work investigates CFD analysis of an 11.5kW lab-scale furnace with oxy-natural-gas combustion for high temperature processes. In recent years many researches were done for oxy-fuel combustion with flue gas recycle in power plants. However, this paper considers combustion models which are applicable for oxy-fuel environment in melting and annealing furnaces. The main emphasis was to use the steady laminar flamelet model (SFM) associated with three detailed chemical mechanisms to predict the combustion process in the furnace. This approach was chosen to avoid time expensive chemical calculations in CFD while calculating the flow field, temperature and species concentrations. Just two additional transport equations for mixture fraction and mixture fraction variance have to be solved to determine the reactions in the flow field. It was found, that the skeletal mechanism with 17 species and 25 reversible reactions generated the best results with SFM. CFD calculations with SFM were compared with results from a 2-step eddy dissipation model (EDM) and a numerically expensive eddy dissipation concept (EDC) model with 17 species and 46 reversible reactions. The EDM was exposed to be unsuitable for modelling oxy-fuel combustion due to dissociation effects and forming of radicals in high temperature applications. In contrast to the EDM results, temperatures calculated by EDC and SFM showed close agreement with measured data in the furnace. Also for the species concentrations, obtained with both models, no significant differences were observed. A significant improvement was the reduction of computational time from 3weeks to 4days on 8 CPU-cores by using the SFM. The P1 and discrete ordinates (DO) model were also applied to solve the radiative transfer equation. A weighted sum of grey-gases model (WSGGM) was applied to model the radiative properties of the flue gas. Due to the special composition of the flue gas in oxy-fuel combustion, which consists mainly of H2O and CO2, the radiative properties should be treated with a non-grey gas approach. Nevertheless, the WSGGM was used in this study because of the small beam length of the furnace. Simulation with the P1 model overestimates the radiation, which leads to lower temperatures in the furnace. The DO model calculated temperatures in good agreement with the measurement with the SFM and EDC.

Refined Weighted Sum of Gray Gases Model for Air-Fuel Combustion and Its Impacts

Radiation is the principal mode of heat transfer in utility boiler furnaces. Models for radiative properties play a vital role in reliable simulations of utility boilers and simulation-based design and optimization. The weighted sum of gray gases model (WSGGM) is one of the most widely used models in computational fluid dynamics (CFD) simulation of air-fuel combustion processes. It represents a reasonable compromise between an oversimplified gray gas model and a comprehensive approach addressing high-resolution dependency of radiative properties and intensity upon wavelength. The WSGGM coefficients evaluated by Smith et al. for several partial pressures of CO2 and H2O vapor are often used for gas temperatures up to 2400 K, which is supplemented by the coefficient values presented by Coppalle and Vervisch for higher temperatures until 3000 K. This paper refines the air-fuel WSGGM in terms of accuracy, completeness, and implementation and demonstrates the use and impacts of the refined model in CFD simulati...

Radiation Effects on the Flame Structure and Extinction Limit of Counterflow Methane Partially Premixed Flames Diluted with Water Vapor in the Air Stream

The radiation effects on counterflow methane partially premixed flames (PPFs), diluted by introducing water (H2O) vapor into the air stream, were investigated numerically using the OPPDIF code and GRI-v3.0. Three radiation models were compared. Adiabatic model (ADIA) simulations were compared with optically thin model (OTM) and weighted sum of gray gases model (WSGGM) simulations. The numerical simulation results for the three models were also compared with the experimental flame structure for two different equivalence ratios, Φ = 1.5 and 2.5. The ADIA overestimated the maximum flame temperature, flame width, and extinction limit of H2O, compared with the WSGGM and OTM. However, there was only a slight difference in the WSGGM and OTM predictions for the flame structure. The difference in the estimates among the models for the flame speed had a considerable effect on the location of the rich premixed flame and flame width. The radiation effect on the flame structure was smaller for Φ = 2.5 compared with Φ = 1.5, due to the low, narrow flame temperature profile and the reabsorption effect of the radiative heat. The difference in the extinction limits predicted by the WSGGM and OTM was small, compared with the ADIA. The extinction limit predicted by each model was, from largest to smallest, ADIA > WSGGM > OTM. The maximum heat release rate for Φ = 2.5 showed a peculiar behavior, which was closely related to the complicated distribution of the heat release rate of the methane-rich PPF. Additionally, it was found that H2O played an important role in radiation heat loss, as well as reabsorption. The radiation reabsorption effect on the flame structure was small for a small addition of H2O, and this effect became more prominent as the H2O concentration.

Computational Fluid Dynamics Modeling on the Air-Firing and Oxy-fuel Combustion of Dried Victorian Brown Coal

The numerical modeling of the combustion of air-dried Victorian brown coal in O2/N2 and O2/CO2 mixtures with 21–30% O2 has been conducted via the use of computational fluid dynamics (CFD), ANSYS FLUENT 13.0, with the refined weighted-sum-of-gray-gases model (WSGGM), the single-film model with multiple surface reactions (i.e., char-O2, char-CO2, and char-H2O) for char particle, and the refined two-step mechanism for the oxy-firing of methane to mimic the volatile oxidation. The purpose of this study is to verify the experimental observations in a lab-scale drop-tube furnace (DTF) and to promote the understanding on the details underpinning the combustion characteristics of Victorian brown coal, the youngest coal in the world and the single largest source for power generation in Victoria, Australia. As confirmed, the modeling results show good agreement with the experimental measurements on the particle temperature, coal ignition delay photographed by a high-speed camera, carbon burnout rate, and particle velocity. The air-dried Victorian brown coal bears an extremely high reactivity for devolatilization and char-O2 and char-CO2 reactions. The inherent moisture in coal was released with volatile matter simultaneously rather than as that predicted by the CFD wet combustion module for pulverized coal with surface moisture. Increasing the secondary gas temperature greatly narrowed coal ignition delay caused by the substitution of 21% O2 balanced in CO2 for air. At the furnace temperatures of 1073 and 1273 K, the contribution of char-CO2 to coal burnout reached approximately 10 and 25% in the oxy-fuel mode, respectively, which, in turn, reduced the coal particle temperature by a maximum of 300 K. To achieve an identical flue gas temperature with the air-firing case, the use of 30% O2 in CO2 is essential. However, the radiation heat flux match can be achieved by the substitution of 27% O2 in CO2 for air.

Comparison of different WSGG correlations in the computation of thermal radiation in a 2D axisymmetric turbulent non-premixed methane–air flame

This study makes an analysis of the radiation heat transfer in a turbulent non-premixed methane–air cylindrical combustion chamber. The highly complex dependence of the radiative properties with the wavenumber spectrum is modeled with the weighted-sum-of-gray-gas (WSGG), making use of the classical correlations of Smith et al. (J Heat Transfer 104:602–608, 1982) and of the more recently in obtained correlations of Dorigon et al. (IJHMT 64:863–873, 2013), based on HITEMP2010. The reaction rates were considered as the minimum values between the Arrhenius and Eddy Break-Up rates. A two-step global reaction mechanism was used, and turbulence modeling was considered via standard k–e model. The source terms of the energy equation consisted of the energy involved in the reaction rates and radiation exchanges. The discrete ordinates method (DOM) was employed to solve the radiative transfer equation (RTE). The results show that the temperature, the radiative heat source, and the wall heat flux can be importantly affected by the WSGG correlations, while their influence on the species concentrations tends to be negligible. Numerical results considering the WSGG model with the new correlations were closer to experimental data presented in the literature.

WSGG correlations based on HITEMP2010 for computation of thermal radiation in non-isothermal, non-homogeneous H2O/CO2 mixtures

In this study, new coefficients for the weighted-sum-of-gray-gas (WSGG) model are proposed based on the up-to-date HITEMP2010 database. The coefficients are determined for gas mixtures of water vapor and carbon dioxide with partial pressure (or molar concentration) ratios equal to 1.0 and 2.0, representing typical products of the combustion of fuel oil and methane. The correlations are valid for pressure path-lengths ranging from 0.001atmm to 10atmm, and for temperatures varying between 400K and 2500K. The WSGG model with the new coefficients are applied to the solution of the radiation heat transfer in non-isothermal, non-homogeneous gas mixtures, which are then compared with benchmark line-by-line solutions to access the accuracy of the model for a few illustrative test cases. Results show that, in spite of its simplicity in comparison to modern gas models, the WSGG model is still a competitive alternative to provide fast but reliable solutions of radiation heat transfer in non-uniform media.

A new weighted-sum-of-gray-gases model for oxy-combustion scenarios

A new weighted-sum-of-gray gases (WSGG) model that is based on the statistical narrow band model (SNB) RADCAL is proposed for use in computational fluid dynamic (CFD) simulations of air and oxy-combustion. When employed in conjunction with the discrete ordinates (DO) method, the model predictions compare well against line-by-line benchmark data that have been made available recently that are based on the latest spectroscopic databases. Furthermore, the model compares well against the EM2C SNB model calculations that have served as benchmark data in three-dimensional geometries. Radiative transfer calculations in these prototypical problems therefore confirm recent experimental observations that SNB RADCAL and EM2C SNB serve as good model databases to develop approximate radiative property models. To achieve an optimum balance of speed and accuracy in computationally intensive CFD simulations, non-gray formulations of the WSGG model are also employed with the P1 model and solutions are compared against those generated by the DO model. While the P1 model gave favorable comparisons when cold, black walls were present, the errors in the surface incident radiative flux predictions increased in the presence of hot, reflecting walls. Finally, in fully coupled simulations of natural gas combustion under air-firing and oxy-firing modes, the predicted incident radiative flux profiles were distinctly different between the gray and non-gray calculations at regions of high temperature gradients, while the centerline temperature predictions were comparatively unaffected. The effects of turbulence radiation interactions were also accounted for through the temperature self-correlation term. However, the magnitudes of the temperature fluctuations were small and localized within this furnace and did not significantly alter our predictions. Copyright © 2012 John Wiley & Sons, Ltd.

Numerical investigation of processes in the lignite-fired furnace when simple gray gas and weighted sum of gray gases models are used

Comparison of the numerical investigation results was carried out when the simple gray gas (SGG) and weighted sum of gray gases (WSGG) models are used to model the radiative properties of the gas phase inside the lignite fired furnaces. Comprehensive mathematical model of the tangentially fired furnace by pulverized lignite was made. Gas radiative properties were modeled by the SGG and WSGG models. Radiative heat transfer was modeled by the zonal model. Gas-phase variables and absorbed wall fluxes were compared on the basis of the relative differences that were determined for all control volumes and surface zones. Average relative differences of the gas-phase temperatures were about 1.0%. Average relative differences of the absorbed wall fluxes were from 2.0% to 5.0%. Absorbed wall fluxes determined by the SGG model were bigger than those determined by the WSGG model. Differences of the heat transfer rates of the absorbed radiation through the furnace walls were expressed in percents of heat transfer rates determined by the SGG model and were similar to the average relative differences of absorbed wall fluxes. Results justify application of the SGG model in comprehensive mathematical models of lignite-fired furnaces.

Nongray-Gas Effects in Modeling of Large-Scale Oxy–Fuel Combustion Processes

Studies have been conducted to implement oxy–fuel combustion with flue gas recycle in conventional utility boilers as an effective effort of carbon capture and storage. However, combustion under oxy–fuel conditions is significantly different from conventional air–fuel firing, and radiative heat transfer under oxy–fuel conditions is one of the fundamental issues. This paper demonstrates the nongray-gas effects in modeling of large-scale oxy–fuel combustion processes. Oxy–fuel combustion of natural gas in a large-scale utility boiler is numerically investigated, in which investigation a recently refined weighted-sum-of-gray-gases model (WSGGM) applicable to oxy–fuel conditions is used to perform nongray and gray calculations, respectively, and a widely used air–fuel WSGGM is also employed for gray calculation. This makes the only difference among the different computational cases. The simulation results show that the gray and nongray calculations of the same oxy–fuel WSGGM make distinctly different predictions in the wall radiative heat transfer, incident radiative flux, radiative source, gas temperature, and species profiles. Relative to the nongray implementation, the gray calculation of the oxy–fuel WSGGM remarkably overpredicts the radiative heat transfer to the furnace walls and underpredicts the gas temperature at the furnace exit plane, which also result in a higher incomplete combustion in the gray calculation. Moreover, the gray and nongray calculations of the same WSGGM make much more pronounced difference in the results than the gray implementation of different WSGGMs does (i.e., the oxy–fuel and air–fuel WSGGMs). Even though particle radiation also has an important impact and will compromise the demonstrated nongray-gas effects to some extent in large-scale oxy–coal combustion, the nongray formulation of an oxy–fuel WSGGM is still highly recommended for a reliable oxy–fuel combustion modeling.

Effects of gas and particle emissions on wall radiative heat flux in oxy-fuel combustion

Oxy-fuel combustion exhibits combustion and heat transfer characteristics different from air-fuel combustion due to high concentrations of CO2 and H2O. This study evaluated the effect of gas and particle emissions on radiative heat transfer in oxy-fuel combustion of coal. For a hexahedral furnace, prescribed gas compositions based on combustion calculation were used to simplify the combustion reactions. The values of radiative heat fluxes (qrad) were compared for different combustion modes, flue gas recirculation (FGR) methods, particle concentrations, furnace sizes and O2 concentrations in the oxidizer. The radiation was calculated by the discrete ordinate method with gaseous emission predicted by the weighted sum of gray gases models (WSGGMs). The results showed that employing an optimized WSGGM is essential for the accurate prediction of qrad in oxy-fuel combustion for gaseous fuels. The conventional WSGGM showed large errors for larger furnace volumes or under dry FGR conditions. With higher particle concentrations such as in pulverized coal combustion, however, qrad was dominated by emission of particles. The effect of gas emissivity was not critical in the furnace with a mean beam length of 8.3m. Oxy-fuel combustion with wet FGR had higher qrad than dry FGR. The O2 concentration in the oxidizer was a key parameter for oxy-fuel combustion since increasing its value linearly increased qrad.