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Particle radiation has a spectral dependence and is closely related to the chemical composition of the material. Iron oxide, one of the main components of fly ash, observably affects the complex index of refraction of the particles. In this study, following the theory of the spectrum k -distribution based weighted sum of gray particles model (Guo et al. [4,13]), a non-gray fly ash radiative property model involving the chemical composition was developed. First, four typical fly ash particles with different iron oxide contents were selected, and the corresponding particle radiative parameters were obtained using the Mie theory. Then, the absorption efficiency and weighting factors of the non-gray model were directly obtained from the Gaussian integral points of the k -distribution. The scattering efficiency of the particles was obtained from the Planck mean. The accuracy of the newly developed model was evaluated in a one-dimensional plane-parallel slab system through comparison with the line-by-line (LBL) model and two commonly used gray radiative property models. The results show that the new non-gray model agrees well with the LBL solution and becomes more accurate as the iron oxide content increases. When the iron oxide content of the fly ash increased from 5.47% to 30.50%, the maximum relative error of the radiative heat flux and the radiative source term decreased from 12.50% to 5.68% and from 20.97% to 12.62%, respectively. The new model can improve the prediction accuracy of radiative heat transfer in pulverized coal-fired furnaces. [ABSTRACT FROM AUTHOR]

Abstract By using the concept of weighted sum of four gray particles and spectrum k -distribution (WSGP-SK), a non-gray radiative property model for unburnt char particles is developed. Based on the carbon burnout kinetic model for structure during oxidation, and the linear mixed approximation theory for complex index of refraction, spectral radiative properties of unburnt char particles are first calculated as function of the burnout ratio by Mie theory. Referring to the full spectrum k -distribution model, k -distribution is applied to reorder absorption and scattering efficiencies of particles. Then, weighting factors and efficiency factors of the non-gray radiative property model are directly obtained from Gaussian integral points of k -distribution. The model is validated against the benchmark solutions of line-by-line (LBL) model. Maximum relative errors of this model are 3% and 15% for radiative heat fluxes and source terms in non-isothermal inhomogeneous particulate media, respectively. The assumption of linearly varying radiative properties with burnout ratio (Lockwood et al. 1986) will result in a predicted deviation of 53% for radiative source terms. Results also show that this non-gray model is remarkably better than the Planck mean method. Moreover, a satisfactory comparison with LBL solutions is achieved in the gas and particle mixture by combining the non-gray WSGG-SK model (Guo et al. 2015). As a radiation sub-model, this non-gray radiative property model can significantly improve prediction accuracy of radiative heat transfer in oxy-fuel combustion. [ABSTRACT FROM AUTHOR]

Oxy-fuel combustion has different combustion and heat transfer characteristics from those of air-combustion, due to the high concentration of CO 2 and H 2 O in the flue gas. This study investigated the heat transfer characteristics in a 35 MW oxy-combustion large pilot plant. The experimental and numerical investigations are carried out with a sub-bituminous coal. The gas temperature, exhaust emission, and heat transfer are measured in conventional air-combustion and oxy-fuel conditions with wet and dry flue gas recycle. Detailed numerical studies of combustion and heat transfer are performed with a modified radiative property model and global reaction mechanism. The results are in good agreement with the experimental data. The effects of char and ash on radiative heat transfer are investigated. The experimental and numerical results show that although the peak temperature decreases, the heat transfer in membrane-wall and superheater in oxy-fuel combustion is slightly greater than that in air-combustion. The absorption coefficient ratios of particle to gas are approximately 2.5 and 2.4 in air-combustion and oxy-fuel combustion. The particle radiation dominates the radiative heat transfer in the pulverized coal combustion, which has a significant influence on the temperature level and distribution. [ABSTRACT FROM AUTHOR]

This paper reports on the recent experimental and numerical results from a 35 MW front wall-fired large pilot boiler. The boiler features a first-of-its-kind compatible design of oxy-fuel combustion and conventional air combustion (air-combustion). A sub-bituminous coal is used as fuel in the experiment. Wet and dry flue gas recycle oxy-fuel conditions are investigated, while air-combustion is also examined for reference. Measured in-furnace temperature, species concentration, exhaust emission, and transferred heat to membrane-wall and superheater are compared between oxy-fuel and air-combustion. Detailed numerical studies of combustion and heat transfer are performed using in-house developed sub-models for oxy-fuel condition, including a radiative property model and a global reaction mechanism. The predicted results agree well with the experimental data. The experimental and numerical results show that stable oxy-fuel combustion is achieved in the 35 MW large pilot boiler. Importantly, the oxy-fuel combustion and air-combustion can operate well with the especially designed burner system at similar heat loads in the experiment. Moreover, similar average temperatures and heat flux distributions are achieved under wet and dry flue gas recycle oxy-fuel conditions with an initial O 2 concentration of 28% in the oxidant. Transferred heat to membrane-wall and superheater in oxy-fuel combustion is slightly greater than in air-combustion. Furthermore, the flue gas recycle ratio significantly influences the heat transfer in oxy-fuel combustion. Increasing the recycle ratio from 0.71 to 0.73 reduces the transferred heat to membrane-wall by approximately 6%, while that of superheaters increases by approximately 4%. The selection of the optimized recycle ratio is a trade-off between boiler performance and flame stability of the burner system. [ABSTRACT FROM AUTHOR]

In recent years, oxy-fuel combustion in coal-fired plants has become one of the most promising technologies for carbon capture and storage (CCS). Compared with air-fired combustion, high concentrations of CO 2 and H 2 O in the flue gas cause a remarkable change in the pulverized coal combustion and heat transfer characteristics in furnace. In this study, improved models for the gas radiative properties and chemical reaction mechanisms were incorporated into the computational fluid dynamics (CFD) code to simulate a 200 MW e tangentially fired utility boiler, which is expected to operate at full load under both conventional air-fired and oxy-fuel combustion. Different flue gas recycle patterns: dry recycle and wet recycle, were also investigated. The results indicate that, stable combustion is achieved by a compatible burner design in both air-fired and oxy-fuel combustion. In the oxy-fuel combustion, the flue gas peak temperature and wall heat flux decrease and high CO concentration appears in the burner region, resulted from higher heat capacity of CO 2 and chemical effect of CO 2 (Liu et al., 2003, Glarborg and Bentzen, 2008). Based on the comparison of the wet recycle and dry recycle, it shows the wet recycle has more advantages than dry recycle. This study indicates a slight increase in total heat transfer, when the oxygen concentration in oxidant increases from 23% to 29%, consistent with the results of Vattenfall and Callide pilot scale oxy-combustion plant. Thus, compared with air-fired combustion, the full load of a boiler may decrease by 5–10% under oxy-fuel combustion. [ABSTRACT FROM AUTHOR]

Particle radiation characteristics have a strong wavelength-dependence. However, the gray particle assumption is widely used for coal combustion simulations, which cannot reflect the non-gray radiative property of the particles. In this study, based on the measured complex index of refraction from literatures (Gupta and Wall, 1985, Goodwin and Mitchner, 1989, and Lohi et al., 1992), a new non-gray particle radiative property model for fly ash is proposed by combining the features of full-spectrum k -distribution (FSK) model with the weighted sum of gray gases (WSGG) model. Four gray particles with different absorption and scattering efficiencies are used to replace the non-gray particles, for which absorption efficiency, scattering efficiency and weighting factor are directly obtained from the k -distribution, with model parameters obtained based on rational polynomials. Simultaneously, a gray particle model based on the Planck’s law is also obtained for comparison. The new model is systematically validated by comparing the radiative source terms and radiative heat fluxes, with those predicted by the line-by-line (LBL) integration of Mie-data in a one-dimensional plane-parallel slab system. The maximum relative error of radiative source term is 8% for the new non-gray radiative property model, 13% for Planck mean coefficients, 14% for modified Johansson model (Johansson, 2017), and 18% for empirical-constant model in non-isothermal inhomogeneous particle media, respectively. Combining the new non-gray radiative property model with the non-gray formulation of WSGG-SK model (Guo et al., 2015), the prediction accuracy is further validated by LBL solutions in the gas and particle mixture. Moreover, the contribution of gases and particles to radiative heat transfer is discussed at different path lengths, which shows the accuracy of the particle radiative property model determines the prediction accuracy of the radiative heat transfer in large-scale furnace. [ABSTRACT FROM AUTHOR]

Predicting thermal radiation for oxy-coal combustion highlights the importance of the radiation models for the spectral properties of gases and particles. This study numerically investigates radiation behaviours in small and large scale furnaces through refined radiative property models, using the full-spectrum correlated k (FSCK) model and Mie theory based data, compared with the conventional use of the weighted sum of grey gases (WSGG) model and the constant values of the particle radiation properties. Both oxy-coal combustion and air-fired combustion have been investigated numerically and compared with combustion plant experimental data. Reasonable agreements are obtained between the predicted results and the measured data. Employing the refined radiative property models achieves closer predicted heat transfer properties to the measured data from both furnaces. The gas-phase component of the radiation energy source term obtained from the FSCK property model is higher within the flame region than the values obtained by using the conventional methods. The impact of using non-grey radiation behaviour of gases through the FSCK is enhanced in the large scale furnace as the predicted gas radiation source term is approximately 2–3 times that obtained when using the WSGG, while the same term is in much closer agreement between the FSCK and the WSGG for the pilot-scale furnace. The predicted total radiation source term (from both gases and particles) is lower in the flame region after using the refined models, which results in a hotter flame (approximately 50–150 K higher in this study) compared with results obtained from conventional methods. In addition, the predicted surface incident radiation reduces by using the refined radiative property models for both furnaces, in which the difference is relevant with the difference in the predicted radiation properties between the two modelling techniques. Numerical uncertainties resulting from the influences of combustion model, turbulent particle dispersion and turbulence modelling on the radiation behaviours are discussed. [ABSTRACT FROM AUTHOR]

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 H 2 O to CO 2 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 CO 2 /H 2 O mixture. [ABSTRACT FROM AUTHOR]

• CFD results of a swirl oxy-coal flame are compared with detailed experiment. • Nonlinear variation particle radiative property is first used in oxy-coal modeling. • Contributions of gas and particle radiation are illustrated in oxy-coal flame. • Constant model ten times over-predicts the particle absorption after burnout. This paper reports a comprehensive computational fluid dynamics simulation of the IFRF's swirling oxy-fuel flame with pulverized coal using wet flue gas recycle. Special attention is given to the influence of particle radiative property model. Based on the burnout rate of pulverized coal, a recent proposed nonlinear conversion-dependent particle radiative property model (Guo et al., 2018) is used in the simulations. The influences of the high CO 2 concentration on the sub-models are also considered in detail. The gas-phase global reaction mechanism, kinetic parameters of char oxidation and gasification, and gas radiative property model are modified for oxy-fuel condition. The results show the prediction results agree well with the measurements, in terms of the temperature and composition of the flue gas. The particle radiation has an important effect on the flame temperature. Even in the small furnace studied here (optical length is approximately 1.6 m), the radiation in the flame zone is still dominated by particle radiation. Linear variation assumption (Yin et al., 2015) under-predicts the particle absorption in combustion zone. Ignoring the effect of burnout rate on particle radiative property will 10 times over-predict the particle absorption after burnout. [ABSTRACT FROM AUTHOR]