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A comparative evaluation of gray and non-gray radiation modeling strategies in oxy-coal combustion simulations
Applied Thermal Engineering 54 (2013) 422e432
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A comparative evaluation of gray and non-gray radiation modeling strategies in oxy-coal combustion simulations Pravin Nakod a, Gautham Krishnamoorthy b, *, Muhammad Sami c, Stefano Orsino d a
ANSYS Software Pvt. Ltd., 34/2 Rajiv Gandhi Infotech Park, MIDC, Hinjewadi, Pune 411057, India Department of Chemical Engineering, PO Box 7101, Harrington Hall Room 323, 241 Centennial Drive, University of North Dakota, Grand Forks, ND 582027101, USA c ANSYS Inc., 16350 Park Ten Place, Suite #140, Houston, TX 77084, USA d ANSYS Inc., 10 Cavendish Ct., Lebanon, NH 03766, USA b
h i g h l i g h t s < Oxy-coal combustion simulations with dry and wet flue gas recycle was studied. < Temperatures were more sensitive to devolatilization models than radiation models. < Gray and non-gray modeling differences were amplified in larger geometries. < Higher flame temperatures also contributed to model differences. < Predictions from two recently proposed non-gray WSGG models were similar.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 August 2012 Accepted 27 January 2013 Available online 24 February 2013
Computational fluid dynamic simulations of oxy-coal combustion are demonstrated in a lab-scale furnace and full-scale boiler employing gray and non-gray formulations of recently proposed radiative property models for the gas-phase. The investigated scenarios included: air-firing, oxy-firing with dry and wet flue-gas recycle (FGR). The study confirms that the temperature and wall radiative flux profiles encountered during air firing can be replicated in both dry and wet FGR scenarios through an appropriate selection of (CO2 þ H2O)/O2 molar ratios in the oxidizer stream. The computed temperature profiles were in reasonable agreement with the experimental measurements. In the lab-scale furnace, lower flame temperatures and smaller path lengths minimized the differences between the gray and non-gray model predictions. Within the full-scale boiler, large volume pockets were present where the radiation was dominated by the gas-phase. This combined with higher peak flame temperatures and longer path lengths resulted in: a 10% variation between the gray and non-gray radiative fluxes and a 50 K difference in the predicted average outlet gas temperatures. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Oxy-coal combustion Radiative heat transfer CFD WSGGM
1. Introduction 1.1. Oxy-coal combustion Oxy-coal combustion is one of the promising power production technologies being actively investigated for carbon capture [1]. During oxy-coal combustion, coal is burnt in an oxygen stream to produce an exhaust gas consisting primarily of H2O and CO2. In order to regulate the operating temperature within current material limits, it is anticipated that part of the flue gas will be recycled back into the
* Corresponding author. Tel.: þ1 701 777 6699; fax: þ1 701 777 3773. E-mail address: [email protected] (G. Krishnamoorthy). 1359-4311/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.applthermaleng.2013.01.049
furnace. The flue-gas recycle (FGR) could either be “dry” containing CO2 alone after condensing out the H2O or “wet” containing a mixture of H2O and CO2. While the combustion of coal in an oxy-fired environment has been studied in detail for over two decades now on a pilot-scale, demonstration of the technology at an industrial scale will be a vital step toward confidence building within the power industry, among the public and the commercial deployment of this technology [2]. Critical issues such as the flame lengths, flame luminosities, radiant heat transfer in the furnace, the formation of gaseous pollutants (CO, SOx and NOx), ash properties along with their slagging and fouling propensities that have been studied extensively at small scales now need to be examined at commercial scales. High-fidelity computational fluid dynamic (CFD) models of oxycoal combustion can provide valuable insights into the application
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of this technology at different reactor scales and help bridge this knowledge gap. However, as a result of the high concentrations of CO2 and H2O in an oxy-fired environment, the coal reactivities, gas emissivities and gas-phase reaction rates are anticipated to be different than those encountered during combustion in air. Subsequently, CFD sub-models developed for air-firing scenarios would need to be refined to be applicable to oxy-firing. The opportunities and challenges with regards to the development of these CFD submodels in oxy-firing environments have been discussed in a recent review [3]. The focus of this manuscript is an evaluation of the gas-phase radiative property models that have been refined for oxy-fired combustion scenarios. Models for the radiative properties of gas mixtures encountered during oxy-fired combustion have been proposed by several researchers in recent years [4e7]. Most of the proposed models fall under the category of weighted-sum-of-graygases (WSGG) models where the emissivities of gas mixtures are expressed in a functional form consisting of temperature independent absorption coefficients (kj) and temperature dependent weighting fractions aj(T):
3
¼
n X
aj ðTÞ 1 exp kj ðPc þ Pw ÞL
(1)
j¼0
In Eq. (1), Pc and Pw are the partial pressures of CO2 and H2O respectively, n is the number of gray gases and L is the path length. The sum of weighting factors (aj) has the constraint that it must equal unity. The transparent regions of the spectrum are accounted for by the term j ¼ 0. The temperature dependence of aj can be expressed in a polynomial form, for instance, Krishnamoorthy et al. [4] adopted a first-order form:
aj ðTÞ ¼ C1 T þ C2
(2)
Comparisons against benchmark data for media conditions representative of oxy-fired combustion scenarios have deemed these models as sufficiently accurate [4e7]. This accuracy translates to a modest increase in computational cost in computational fluid dynamic (CFD) simulations particularly when radiative transfer calculations are carried out once in several fluid iterations. Among the other advantages of the WSGG models are: ease of implementation within the framework of CFD codes since the coefficients aj and kj are direct inputs to the standard differential form of the radiative transport equation (RTE) and the flexibility to be employed in either gray or non-gray radiative transfer calculations. However, due to the spectral line structure of CO2 and H2O and the increased concentration of these gases in oxyfired combustion scenarios, calculations in “prototypical” furnace geometries with fixed temperature and gas compositions representative of dry FGR [8] first demonstrated the importance of performing non-gray calculations. Subsequent, studies employed these models in CFD simulations of oxy-fired combustion [8e12]. Table 1 Summary of WSGG models examined in this study. Source
Model notation
Radiative property database
Hottel et al. [14], Krishnamoorthy et al. [4]
Perry (gray)
Krishnamoorthy [7,16]
Perry (5 gg)
Johansson et al. [5]
Chalmers (5 gg)
Hottel charts and SNB RADCAL (Hottel et al. [14]; Grosshandler [15]) Hottel charts and SNB RADCAL (Hottel et al. [14]; Grosshandler [15]) EM2C SNB (Soufiani and Taine [17])
423
Table 2 Absorption (Qabs) and scattering efficiencies (Qscat) used in the coal combustion simulations. Particle
Coal
Char
Ash
Qabs Qscat
1.13 1.3
0.59 1.5
0.05 1.7
Yin [9] carried out simulations of oxy-fired combustion with natural gas as fuel in a large scale utility boiler and showed distinctly different predictions of: wall radiative heat transfer, incident radiative fluxes, radiative source, gas temperature, and species profiles between the gray and non-gray calculations. Relative to the nongray predictions, the gray calculations were found to over-predict the radiative heat transfer to the furnace walls and under-predict the gas temperature at the furnace exit plane. Edge et al. [10] carried out gray and non-gray calculations of oxy-coal combustion in a laboratory scale burner employing the Reynolds Averaged Navier Stokes (RANS) and Large Eddy Simulation (LES) approach and found significant differences in the in the near-burner region. In the oxyfired combustion simulations, the RANS approach using a spectral gas radiation model was found to capture the surface incident radiation more accurately than the gray model. Therefore, the use of a spectral radiation model with the LES approach was suggested for use in oxy-coal combustion simulations. Kangwanpongpan et al. [11] compared gray and non-gray simulations of radiative heat transfer during oxy-coal combustion in a laboratory-scale furnace and attributed an improved agreement to the temperature predictions to the employing of a non-gray radiation model. More recently, Hjärtstam et al. [12] examined gas and soot-related radiation mechanisms in air-fired and oxy-fired flames operated with propane as fuel. While the non-gray modeling more accurately predicted the radiative source term in the combustion environments compared to the gray model, the work also showed that the inclusion of soot radiation was more critical in sooty air and oxyfired flames than the use of a more rigorous description of the radiative properties of the gas. However, none of the aforementioned studies rigorously investigated the effects of employing gray and non-gray calculations in simulations of oxy-coal combustion in an industrial scale reactor unit or in a wet FGR scenario. Wet FGR is also a likely candidate for retrofitting existing boilers, and therefore a comparative evaluation of gray and non-gray radiation modeling strategies need to be carried out when high concentrations of water-vapor are present in the furnace. For instance, Denison [13] carried out a comparative evaluation of gray and non-gray radiation modeling in one-dimensional prototypical geometries. The media conditions consisted of pure H2O, CO2 as well as mixtures of these participating gases present under isothermal and non-isothermal conditions. By comparing against line-by-line benchmark data, his study demonstrated the need to employ high-fidelity models for non-gray radiation in the calculations. The resulting inaccuracies from employing gray models in the Table 3 Boundary conditions for the full scale boiler simulations. Oxidizer Primary gas Secondary gas Overfire gas Coal flow composition flow rate flow rate flow rate rate (mol%) (ton/h) (ton/h) (ton/h) (ton/h) Air firing O2 ¼ 21%; N2 ¼ 79% Dry FGR O2 ¼ 29%; CO2 ¼ 71% Wet FGR O2 ¼ 24%; CO2 ¼ 50%; H2O ¼ 26%
305.2
574.8
257
110.2
305.2
574.8
257
110.2
305.2
574.8
257
110.2
424
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calculations were found to be dependent on the scale of the geometry, the temperature and concentration gradients within the domain and the thermal boundary conditions. Therefore, this paper provides for the first time a comprehensive assessment of gray and non-gray radiation modeling in oxy-coal combustion with dry and wet FGR in two reactor geometries varying widely on the geometric scale. The choice of the reactors was determined by the availability of critical experimental information such as the reactor and burner dimensions, proximate and
ultimate analysis of the fuel, thermal boundary conditions, all of which impact the accuracies of the numerical predictions. The configurations were intentionally chosen to represent a wide peak temperature range, temperature gradients and length scales for radiative heat transfer. While the intent of the study is insights and not validation, simulation results are also compared against experimental measurements of temperature where available to assess the fidelity of the calculations and the impacts of the gray and non-gray modeling strategies on the radiative fluxes and the temperature field are examined. The WSGG models examined in this study are summarized in Table 1. The WSGG models for oxy-fired combustion have been fit to emissivity data of gas-mixtures that were generated employing different radiative property models, spectroscopic databases and experimental data. Recent gas cell experiments conducted by Becher et al. [18] concluded that the statistically narrow band models (SNB) RADCAL and EM2C to be accurate within 3% of the measured transmissivity at gas concentrations representative of oxy-fired combustion scenarios. Therefore, WSGG models that are based on these two SNB model databases are employed in this study. All three WSGG models were implemented as user-definedfunctions (UDFs) and employed in conjunction with the commercial CFD code ANSYS FLUENT (version 12) [19]. The non-gray models are denoted by the number of gray gases (gg) employed in their formation. A domain based mean-beam length was employed in the calculations involving the Perry (gray) model. The test cases examined in this study are described next. 2. Lab-scale furnace (Aachen U. furnace) 2.1. Geometry and boundary conditions Oxy-coal combustion in a laboratory scale furnace experimentally investigated by Toporov et al. [20] was investigated in this study. The furnace is 4.2 m long and the length of the combustion chamber within the furnace was 2.1 m with an inner diameter of 0.4 m. In the primary stream, the volume fractions of O2 and CO2 were set at 0.19 and 0.81 respectively whereas the corresponding volume fractions were 0.21 and 0.79 respectively, in the secondary, tertiary and staging streams. The particle and oxidizer flow rates, fuel compositions and the particle size distributions along with the wall boundary conditions are specified in Toporov et al. [20]. In order to investigate the differences between oxy-firing with dry FGR and air-firing conditions, additional simulations corresponding to hypothetical “air-firing” conditions were carried out by replacing the CO2 in the oxidizer streams with N2. Additionally, a hypothetical “wet FGR” case was also investigated with the molar compositions of the primary, secondary and staging streams set to: 50% CO2, 24% O2 and 26% H2O, corresponding to a (CO2 þ H2O)/O2 molar ratio of 3.17. 2.2. Modeling approach
Fig. 1. The axial temperature profiles predicted by the various models in the Aachen U. coal furnace during: (a) air-firing; (b) oxy-firing with dry FGR; (c) oxy-firing with wet FGR.
An EulereLagrangian approach was undertaken to model the coal combustion. A stochastic technique was employed to model the dispersion of the particles in the turbulent flow field with the coupling between the particle phase and the fluid phase represented via particle source terms. The fluid turbulence was modeled employing the Shear Stress Transport (SST) keu turbulence model to account for the highly swirling nature of the flow. A constant devolatilization rate of 50 s1 was used to model the coal devolatilization and the eddy-dissipation model was used to compute the chemistry of the volatile components in the gas phase. The heterogeneous reaction of char with the oxidizer stream at the particle surface was assumed to be limited by the rate at which the
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425
Fig. 2. Radial temperature variations at different axial burner distances in the Aachen U. coal furnace during: (a, b) air-firing; (c, d) oxy-firing with dry FGR; (e, f) oxy-firing with wet FGR.
oxidant stream diffuses to the surface of the particle. A two-step mechanism was employed for the production of CO2. First, the char oxidizes to form CO which further oxidizes to form CO2. The radiation was modeled employing the discrete ordinates method employing an angular resolution of 4 4 in the theta and phi direction in every quadrant. As a coal particle undergoes devolatilization and subsequently burns out leaving behind ash, its radiative properties (absorption and scattering efficiencies) also vary. This variation was accounted for through a UDF with the values of absorption and scattering efficiencies of coal and ash obtained from Ref. [21]. The absorption and scattering efficiencies of char was assumed to be an average of that of coal and ash with an anisotropic forward scattering phase-function assumed for the coal, char and
ash particles. These properties are summarized in Table 2. The simulations were performed in 2D axisymmetric co-ordinate system to take advantage of the symmetry in the geometry employing a mesh of resolution 58,691 cells. The results were found to be invariant with any further increase in the number of cells. 3. Full-scale boiler 3.1. Geometry and boundary conditions To investigate the effect of reactor size on the radiative property models, a 300 MW front-wall-fired, pulverized-coal, utility boiler experimentally investigated by Costa and Azevedo [22] for air-
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firing was simulated in this study. The boiler and burner dimensions as well as the coal and air flow rates corresponding to air firing along with the proximate and ultimate analyses as well as the heating values of the coal mixture are all summarized in Ref. [22]. Since the coal size distributions were not explicitly mentioned, a coal particle diameter of 100 microns was employed in the
calculations. The boiler was meshed utilizing 916,934 computational cells. In addition to the experimentally investigated conventional air fired, dry FGR where the CO2/O2 molar ratio was 2.45 in the oxidizer stream and wet FGR conditions where the molar composition of the oxidizer stream was set to 50% CO2, 24% O2 and 26% H2O, resulting in a (CO2 þ H2O)/O2 molar ratio of 3.17 were also investigated. These boundary conditions are summarized in Table 3. Since the wall emissivity was not explicitly specified by Costa and Azevedo [22] a value of 0.725 was assumed in the calculations. The wall temperature was maintained at 750 K. A total of 72 angular directions were employed in the discrete ordinates model. 3.2. Modeling approach The standard ke3 model was used to model the turbulence. A constant devolatilization rate of 100 s1 was used to model the coal devolatilization and the eddy-dissipation model was used to compute the chemistry of the volatile components in the gas phase. The heterogeneous reaction of char with the oxidizer stream at the particle surface was assumed to be limited by the rate at which the oxidant stream diffuses to the surface of the particle. As with the lab-scale study, the particles were assumed to be forwardscattering and the variation in the radiative properties of coal, char and ash were varied as shown in Table 2. 4. Results and discussion 4.1. Lab-scale furnace
Fig. 3. The wall incident radiative flux profiles predicted by the various models in the Aachen U. coal furnace during: (a) air-firing; (b) oxy-firing with dry FGR; (c) oxy-firing with wet FGR.
Fig. 1 shows the axial temperature profiles along the length of the lab-scale furnace during air-firing and oxy-firing conditions. First, the simulations show that at the specified molar ratios of (CO2 þ H2O)/O2 in the recycle stream the temperature profiles encountered during air-firing can be replicated. Second, the differences in the temperature predictions between the gray and non-gray model predictions are observed to be small for all three scenarios. Finally, there are no observable variations resulting from employing different radiative property model databases in the Perry (5 gg) and the Chalmers (5 gg) model formulations. Fig. 2 shows the radial temperature variations at different axial burner distances in the Aechen U. coal furnace during air and oxy-firing. The impact of the gray and non-gray modeling strategies on the temperature predictions is not seen to be significant. Furthermore, the numerical predictions are within 100 K of the experimental measurements downstream of the flame zone (cf. Fig. 2(d)). These downstream temperature prediction accuracies compare well with previous numerical simulations of this furnace that have been carried out in 3D periodic domains employing more advanced models for coal devolatilization [11,20]. However, in the nearburner region (cf. Fig. 2(c)), the agreement with the experimental measurements is poor. Also shown in Fig. 2(c) are the corresponding predictions from Toporov et al. [20]. The simulations in Toporov et al. [20] were carried out employing the Chemical Percolation Model (CPD) model for coal devolatilization. In Fig. 2(c) we notice that the choice of the coal devolatilization model has a greater impact on the numerical predictions of temperature than the gray or non-gray modeling strategy for the gasphase radiative properties. The combustion of the volatiles occurs at the axis and clearly the choice of the devolatilization model has a significant bearing on the temperature predictions. The steep temperature gradients observed numerically but not captured experimentally at radial distances less than 0.075 m have been previously attributed to the coarse spatial resolution of the temperature measurements [20].
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427
Fig. 4. The ratio of the particle to gas-phase absorption coefficients (when employing the Perry (gray) model) during: (a) air-firing; (b) oxy-firing with dry FGR; (c) oxy-firing with wet FGR.
Fig. 5. Comparisons of numerical predictions of temperature against experimental measurements during air-firing in the full scale boiler.
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Fig. 6. Contours of temperature within the full scale boiler during air-firing, dry and wet FGR.
P. Nakod et al. / Applied Thermal Engineering 54 (2013) 422e432
Fig. 7. Ratio of particle absorption coefficient to gas absorption coefficients within the full scale boiler employing the Perry (gray) model.
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Fig. 8. Contours of surface incident radiation (in W/m2) along the walls of the full scale boiler.
P. Nakod et al. / Applied Thermal Engineering 54 (2013) 422e432 Table 4 The total incident radiation to the walls of the boiler and the pendant plates normalized with respect to the Perry (gray) model predictions. Air-fired
Perrygray Perry5 gg Chalmers5 gg
Dry FGR
Wet FGR
Pendant plates
Furnace wall
Pendant plates
Furnace wall
Pendant plates
Furnace wall
1
1
1
1
1
1
0.908
0.925
0.831
0.898
0.966
0.955
0.912
0.930
0.805
0.887
0.933
0.951
Fig. 3 shows wall incident radiative flux profiles predicted by the various models during air and oxy-firing conditions. Fig. 3 also confirms that dry and wet FGR at these (CO2 þ H2O)/O2 molar ratios in the oxidizer stream, the heat flux profiles encountered during air-firing can be replicated. However, no major differences between the gray and non-gray calculations are observed within this furnace geometry. Fig. 4 shows the ratio of the particle to gasphase absorption coefficients during air-fired and oxy-firing conditions when employing the Perry (gray) model. The particle absorption coefficient is several times larger than the gas absorption coefficient in the near burner region and decreases as we move axially along the length of the furnace. These correspond to regions of coal devolatilization and burnout occurring in the near burner regions and particle radiation mainly due to ash particles in the downstream regions of the furnace. From Figs. 1e4 we may summarize that when simulating this lab-scale furnace geometry, there is very little observable variation between gray and non-gray radiation modeling when employing WSGG parameters in the Perry model that have been optimized to cover the entire spectrum of scenarios likely to be encountered in air and oxy-fired combustion. Furthermore, differences in the radiative property model databases employed in formulating these optimized models do not significantly affect the numerical predictions. These are attributed to the smaller path lengths and the relatively low flame temperatures [cf. Figs. 1 and 2] encountered within this furnace. While these low temperatures and path-lengths encountered within the lab-scale furnace are not representative of a full-scale coal combustion scenario, the numerical simulations provide valuable insights into the radiation modeling fidelity (and therefore the computational cost) that is required during the modeling of such furnaces and help identify the combustion models that are most in need of refinement to improve agreement with the experimental data. 4.2. Full-scale boiler Fig. 5 compares the numerical predictions of temperature against the experimental measurements reported in Ref. [22]. First, it is important to remember that the accuracy of the simulations was limited by the lack of fuel specific kinetic data and the exact particle diameter size distributions at the inlets. To determine the impact of devolatilization rates on the simulations, an additional Table 5 Ratios of the total surface incident radiation to the furnace walls and the pendant plates in the absence of particle radiation to those obtained when both particle and gas radiation are present. All results were obtained employing the Perry (5 gg) model. Air-fired
Dry FGR
Wet FGR
Pendant plates
Furnace wall
Pendant plates
Furnace wall
Pendant plates
Furnace wall
0.750
0.750
0.836
0.852
0.849
0.882
431
simulation was carried out employing the two competing rates (Kobayashi) model in ANSYS FLUENT [19] with the Perry (5 gg) model. The observed agreement between the experimental measurements and simulations is within 100 K in most of the locations and may be deemed as acceptable. An agreement to within 100 K of the experimental measurements is generally the level of modeling accuracies that can be expected in simulations of such large scale furnaces even when accurate particle size distribution and coal kinetic parameters are available [23]. Fig. 5 also shows that the choice of the devolatilization model has a greater impact on the numerical predictions of temperature than the choice of the radiative property model for the gas-phase. The temperature contours within the full scale boiler during air-firing, dry and wet FGR are shown in Fig. 6. The similarities between the contours obtained from the three firing conditions confirm that the combustion of coal can be completed at these H2O/CO2 molar ratios in the dry and wet FGR scenarios to achieve a temperature profile within the boiler that is identical to that found in air-burn. These conclusions are consistent with the observations of Habermehl et al. [24] who also determined through numerical simulations of an industrial scale boiler that oxy-fired combustion could be completed at similar (CO2 þ H2O)/O2 mole fractions in the dry and wet FGR streams. The average outlet temperatures predicted by the different models are also reported in Fig. 6. Although no noticeable difference is observable from the gray and non-gray model temperature contours, 40e50 K temperature difference is seen between the average outlet temperatures predicted by the gray and non-gray models. Fig. 7 shows the ratios of the particle absorption coefficient to gas absorption coefficients within the full scale boiler employing the Perry (gray) model. In all three scenarios there are pockets within the furnace where the gas radiation is greater than or of the same magnitude of the particle radiation. Fig. 8 shows the contours of surface incident radiation along the walls of the full scale boiler. In order to closely examine the differences between the gray and non-gray modeling strategies, the total incident radiation along the walls of the furnace and the pendant plates in the convective pass of the furnace was determined. These are reported in Table 4, where the values are normalized with respect to the corresponding predictions from the Perry (gray) model. Results from Table 4 indicate that the non-gray model predictions of the total wall incident radiation are about 10% lower than that of the Perry (gray) model. The largest variations are seen during dry FGR with the smallest variations encountered during wet FGR. Again, the variations between the Perry (5 gg) and Chalmers (5 gg) models are small. In order to assess the relative importance of particle radiation to gas radiation in these scenarios, the incident radiative fluxes to the walls of the boiler were determined by turning “off” the particle radiation contribution. This was undertaken by “freezing” the temperature and specie concentration fields in the air, dry and wet FGR cases, turning off “particleeradiation interactions” and carrying out radiative transfer calculations till convergence. These calculations were carried out employing the Perry (5 gg) model. Table 5 summarizes the ratios of the total surface incident radiation to the boiler walls and the pendant plates in the absence of particle radiation to those obtained when both particle and gas radiation are present. The contribution of the gas radiation to the total radiation increases under oxy-firing conditions. This is consistent with the observations of Andersson et al. [25] who noted an increase in the gas radiative intensities in a 100 kW lignite-fired boiler. They also noted that as long as the temperature distributions are similar the total intensities are the same in both air and oxy-firing scenarios. This is consistent with our findings in Figs. 1, 3, 6 and 8 where similar temperature profiles encountered in air-firing, dry FGR and wet FGR scenarios are seen to result in similar incident radiative heat flux profiles. The results
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in Table 5 shows that the gas radiation is a significant contributor to the total surface heat fluxes in all three scenarios. Therefore, an accurate specification of non-gray particle radiative properties and scattering phase functions for instance (which was not undertaken in this study) is likely to play a smaller role in the determination of wall heat fluxes than that played by non-gray gas radiative properties. However, previous studies [8,26] have shown the relative differences in the heat flux predictions between the gray and nongray models will be determined more by the temperature and composition gradients and in homogeneities encountered within the reactor. While the computational cost of the radiation calculations when employing the Perry (5 gg) and Chalmers (5 gg) models is 5 times greater than that of the Perry (gray) model, the observed variations in the radiative flux predictions between the gray and non-gray modeling strategies (cf. Table 4) might necessitate their usage in large scale furnaces where large temperature and composition gradients are encountered. However, this increase in computational expense can be partially overcome by performing the radiation calculation only once several fluid iterations. This often is an acceptable strategy since the radiation field does not change as fast as the fluid field between iterations. 5. Conclusions Simulations of coal combustion in a lab-scale furnace and full-scale boiler at air-fired and oxy-fired combustion conditions were carried out employing two recently proposed WSGG models for the radiative property of the gas phase. The models differed in the radiative property models/databases employed in their formulation. Based on the results of this study, the following conclusions can be drawn: 1. The study first confirms that at certain (CO2 þ H2O)/O2 molar ratios in the dry and wet FGR scenarios, the temperature and wall radiative flux profiles encountered during coal combustion with air can be replicated with oxy-fired combustion in lab-scale furnaces as well as full-scale boilers. 2. The computed temperature profiles were in reasonable agreement with the experimental measurements. The accuracies of the predictions were however limited by the lack of fuel specific kinetic data. Furthermore, comparisons from employing different coal devolatilization models showed that the temperature predictions were more sensitive to the choice of devolatilization model rather than the radiative property model for the gas-phase. 3. In the lab-scale furnace, lower flame temperatures and smaller path lengths minimized the differences between the gray and non-gray radiative flux and temperature profiles. 4. Within the full-scale boiler, large volume pockets were present where the radiation was dominated by the gas-phase. This combined with higher peak flame temperatures and longer path lengths resulted in: a 10% variation between the gray and non-gray total incident radiative flux predictions and 40e50 K difference in the average outlet gas temperature. The largest variation between the gray and non-gray heat fluxes was observed during dry FGR with the smallest variation encountered during wet FGR. 5. The contribution of the gas-radiation to the wall incident radiative fluxes was determined to be more significant than that of particle radiation in all three scenarios. Therefore, an accurate specification of the gas radiation properties is likely to be more important than the particle radiative properties for wall radiative flux predictions. 6. There were no significant differences in the predictions between the Perry (5 gg) and the Chalmers (5 gg) models. Therefore, variations among the WSGG models due to differ-
ences in the experimental and spectroscopic databases employed and their formulation were minimal. References [1] B.J.P. Buhre, L.K. Elliott, C.D. Sheng, R.P. Gupta, T.F. Wall, Oxy-fuel combustion technology for coal-fired power generation, Progress in Energy and Combustion Science 31 (2005) 283e307. [2] T. Wall, R. Stanger, S. Santos, Demonstrations of coal-fired oxy-fuel technology for carbon capture and storage and issues with commercial deployment, International Journal of Greenhouse Gas Control 5 (2011) S5eS15. [3] P. Edge, M. Gharebaghi, R. Irons, R. Porter, R.T.J. Porter, M. Pourkashanian, D. Smith, P. Stephenson, A. Williams, Combustion modelling opportunities and challenges for oxy-coal carbon capture technology, Chemical Engineering Research and Design 89 (2011) 1470e1493. [4] G. Krishnamoorthy, M. Sami, S. Orsino, A. Perera, M. Shahnam, E.D. Huckaby, Radiation modeling in oxy-fuel combustion scenarios, International Journal of Computational Fluid Dynamics 24 (2010) 69e82. [5] R. Johansson, K. Andersson, B. Leckner, H. Thunman, Models for gaseous radiative heat transfer applied to oxy-fuel conditions in boilers, International Journal of Heat and Mass Transfer 53 (2010) 220e230. [6] C. Yin, L.C.R. Johansen, L.A. Rosendahl, S.K. Kær, New weighted sum of gray gases model applicable to computational fluid dynamics (CFD) modeling of oxy-fuel combustion: derivation, validation, and implementation, Energy and Fuels 24 (2010) 6275e6282. [7] G. Krishnamoorthy, A new weighted-sum-of-gray-gases model for oxycombustion scenarios, International Journal of Energy Research, (2012), http://dx.doi.org/10.1002/er.2988. [8] R. Porter, F. Liu, M. Pourkashanian, A. Williams, D. Smith, Evaluation of solution methods for radiative heat transfer in gaseous oxy-fuel combustion environments, Journal of Quantitative Spectroscopy and Radiative Transfer 111 (2010) 2084e2094. [9] C. Yin, Nongray-gas effects in modeling of large-scale oxy-fuel combustion processes, Energy and Fuels 26 (2012) 3349e3356. [10] P. Edge, S.R. Gubba, L. Ma, R. Porter, M. Pourkashanian, A. Williams, LES modelling of air and oxy-fuel pulverized coal combustiondimpact on flame properties, Proceedings of the Combustion Institute 33 (2011) 2709e2716. [11] T. Kangwanpongpan, R.C. da Silva, H.J. Krautz, Prediction of oxy-coal combustion through an optimized weighted sum of gray gases model, Energy (2011) 1e8. [12] S. Hjärtstam, R. Johansson, K. Andersson, F. Johnsson, Computational fluid dynamics modeling of oxy-fuel flames: the role of soot and gas radiation, Energy and Fuels 26 (2012) 2786e2797. [13] M.K. Denison, A spectral line-based weighted-sum-of-gray-gases model for arbitrary RTE solvers, Ph.D. Thesis, Brigham Young University, Provo, UT, 1994. [14] H.C. Hottel, J.J. Noble, A.F. Sarofim, Heat and mass transfer, in: D.W. Green, R.H. Perry (Eds.), Perry’s Chemical Engineers’ Handbook, eighth ed., McGrawHill, New York, 2007 (Chapter 5). [15] W.L. Grosshandler, Radiative heat transfer in nonhomogeneous gases: a simplified approach, International Journal of Heat and Mass Transfer 23 (1980) 1447e1459. [16] G. Krishnamoorthy, A new weighted-sum-of-gray gases model for CO2eH2O gas mixtures, International Communications in Heat and Mass Transfer 37 (2010) 1182e1186. [17] A. Soufiani, J. Taine, High temperature gas radiative property parameters of statistical narrow-band model for H2O, CO2 and CO, and correlated-K model for H2O and CO2, International Journal of Heat and Mass Transfer 40 (1997) 987e991. [18] V. Becher, S. Clausen, A. Fateev, H. Spliethoff, Validation of spectral gas radiation models under oxyfuel conditions. Part A: gas cell experiments, International Journal of Greenhouse Gas Control 5S (2011) S76eS99. [19] ANSYS FLUENT User’s Guide, Version 12, ANSYS Inc., Canonsburg, PA, 2009. [20] D. Toporov, P. Bocian, P. Heil, A. Kellermann, H. Stadler, S. Tschunko, M. Forster, R. Kneer, Detailed investigation of a pulverized fuel swirl flame in CO2/O2 atmosphere, Combustion and Flame 155 (4) (2008) 605e618. [21] K. Kuehlert, Modellbildung und Berechnung der Wärmestrahlung in Gas- und Kohlenstaubfeuerungen, Ph.D. Dissertation, RWTH Aachen, 1998. [22] M. Costa, J.L.T. Azevedo, Experimental characterization of an industrial pulverized coal-fired furnace under deep staging conditions, Combustion Science and Technology 179 (2007) 1923e1935. [23] R.V. Filkoski, Pulverised-coal combustion with staged air introduction: CFD analysis with different thermal radiation methods, The Open Thermodynamics Journal 4 (2010) 2e12. [24] M. Habermehl, J. Erfurth, D. Toporov, M. Förster, R. Kneer, Experimental and numerical investigations on a swirl oxycoal flame, Applied Thermal Engineering 49 (2012) 161e169. [25] K. Andersson, R. Johansson, S. Hjärtstam, P. Johnsson, B. Leckner, Radiation intensity of lignite-fired oxy-fuel flames, Experimental Thermal and Fluid Science 33 (2008) 67e76. [26] F. Liu, Numerical solutions of three-dimensional non-grey gas radiative transfer using the statistical narrow-band model, ASME Journal of Heat Transfer 121 (1999) 200e203.
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
A comparative evaluation of gray and non-gray radiation modeling strategies in oxy-coal combustion simulations Pravin Nakod a, Gautham Krishnamoorthy b, *, Muhammad Sami c, Stefano Orsino d a
ANSYS Software Pvt. Ltd., 34/2 Rajiv Gandhi Infotech Park, MIDC, Hinjewadi, Pune 411057, India Department of Chemical Engineering, PO Box 7101, Harrington Hall Room 323, 241 Centennial Drive, University of North Dakota, Grand Forks, ND 582027101, USA c ANSYS Inc., 16350 Park Ten Place, Suite #140, Houston, TX 77084, USA d ANSYS Inc., 10 Cavendish Ct., Lebanon, NH 03766, USA b
h i g h l i g h t s < Oxy-coal combustion simulations with dry and wet flue gas recycle was studied. < Temperatures were more sensitive to devolatilization models than radiation models. < Gray and non-gray modeling differences were amplified in larger geometries. < Higher flame temperatures also contributed to model differences. < Predictions from two recently proposed non-gray WSGG models were similar.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 August 2012 Accepted 27 January 2013 Available online 24 February 2013
Computational fluid dynamic simulations of oxy-coal combustion are demonstrated in a lab-scale furnace and full-scale boiler employing gray and non-gray formulations of recently proposed radiative property models for the gas-phase. The investigated scenarios included: air-firing, oxy-firing with dry and wet flue-gas recycle (FGR). The study confirms that the temperature and wall radiative flux profiles encountered during air firing can be replicated in both dry and wet FGR scenarios through an appropriate selection of (CO2 þ H2O)/O2 molar ratios in the oxidizer stream. The computed temperature profiles were in reasonable agreement with the experimental measurements. In the lab-scale furnace, lower flame temperatures and smaller path lengths minimized the differences between the gray and non-gray model predictions. Within the full-scale boiler, large volume pockets were present where the radiation was dominated by the gas-phase. This combined with higher peak flame temperatures and longer path lengths resulted in: a 10% variation between the gray and non-gray radiative fluxes and a 50 K difference in the predicted average outlet gas temperatures. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Oxy-coal combustion Radiative heat transfer CFD WSGGM
1. Introduction 1.1. Oxy-coal combustion Oxy-coal combustion is one of the promising power production technologies being actively investigated for carbon capture [1]. During oxy-coal combustion, coal is burnt in an oxygen stream to produce an exhaust gas consisting primarily of H2O and CO2. In order to regulate the operating temperature within current material limits, it is anticipated that part of the flue gas will be recycled back into the
* Corresponding author. Tel.: þ1 701 777 6699; fax: þ1 701 777 3773. E-mail address: [email protected] (G. Krishnamoorthy). 1359-4311/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.applthermaleng.2013.01.049
furnace. The flue-gas recycle (FGR) could either be “dry” containing CO2 alone after condensing out the H2O or “wet” containing a mixture of H2O and CO2. While the combustion of coal in an oxy-fired environment has been studied in detail for over two decades now on a pilot-scale, demonstration of the technology at an industrial scale will be a vital step toward confidence building within the power industry, among the public and the commercial deployment of this technology [2]. Critical issues such as the flame lengths, flame luminosities, radiant heat transfer in the furnace, the formation of gaseous pollutants (CO, SOx and NOx), ash properties along with their slagging and fouling propensities that have been studied extensively at small scales now need to be examined at commercial scales. High-fidelity computational fluid dynamic (CFD) models of oxycoal combustion can provide valuable insights into the application
P. Nakod et al. / Applied Thermal Engineering 54 (2013) 422e432
of this technology at different reactor scales and help bridge this knowledge gap. However, as a result of the high concentrations of CO2 and H2O in an oxy-fired environment, the coal reactivities, gas emissivities and gas-phase reaction rates are anticipated to be different than those encountered during combustion in air. Subsequently, CFD sub-models developed for air-firing scenarios would need to be refined to be applicable to oxy-firing. The opportunities and challenges with regards to the development of these CFD submodels in oxy-firing environments have been discussed in a recent review [3]. The focus of this manuscript is an evaluation of the gas-phase radiative property models that have been refined for oxy-fired combustion scenarios. Models for the radiative properties of gas mixtures encountered during oxy-fired combustion have been proposed by several researchers in recent years [4e7]. Most of the proposed models fall under the category of weighted-sum-of-graygases (WSGG) models where the emissivities of gas mixtures are expressed in a functional form consisting of temperature independent absorption coefficients (kj) and temperature dependent weighting fractions aj(T):
3
¼
n X
aj ðTÞ 1 exp kj ðPc þ Pw ÞL
(1)
j¼0
In Eq. (1), Pc and Pw are the partial pressures of CO2 and H2O respectively, n is the number of gray gases and L is the path length. The sum of weighting factors (aj) has the constraint that it must equal unity. The transparent regions of the spectrum are accounted for by the term j ¼ 0. The temperature dependence of aj can be expressed in a polynomial form, for instance, Krishnamoorthy et al. [4] adopted a first-order form:
aj ðTÞ ¼ C1 T þ C2
(2)
Comparisons against benchmark data for media conditions representative of oxy-fired combustion scenarios have deemed these models as sufficiently accurate [4e7]. This accuracy translates to a modest increase in computational cost in computational fluid dynamic (CFD) simulations particularly when radiative transfer calculations are carried out once in several fluid iterations. Among the other advantages of the WSGG models are: ease of implementation within the framework of CFD codes since the coefficients aj and kj are direct inputs to the standard differential form of the radiative transport equation (RTE) and the flexibility to be employed in either gray or non-gray radiative transfer calculations. However, due to the spectral line structure of CO2 and H2O and the increased concentration of these gases in oxyfired combustion scenarios, calculations in “prototypical” furnace geometries with fixed temperature and gas compositions representative of dry FGR [8] first demonstrated the importance of performing non-gray calculations. Subsequent, studies employed these models in CFD simulations of oxy-fired combustion [8e12]. Table 1 Summary of WSGG models examined in this study. Source
Model notation
Radiative property database
Hottel et al. [14], Krishnamoorthy et al. [4]
Perry (gray)
Krishnamoorthy [7,16]
Perry (5 gg)
Johansson et al. [5]
Chalmers (5 gg)
Hottel charts and SNB RADCAL (Hottel et al. [14]; Grosshandler [15]) Hottel charts and SNB RADCAL (Hottel et al. [14]; Grosshandler [15]) EM2C SNB (Soufiani and Taine [17])
423
Table 2 Absorption (Qabs) and scattering efficiencies (Qscat) used in the coal combustion simulations. Particle
Coal
Char
Ash
Qabs Qscat
1.13 1.3
0.59 1.5
0.05 1.7
Yin [9] carried out simulations of oxy-fired combustion with natural gas as fuel in a large scale utility boiler and showed distinctly different predictions of: wall radiative heat transfer, incident radiative fluxes, radiative source, gas temperature, and species profiles between the gray and non-gray calculations. Relative to the nongray predictions, the gray calculations were found to over-predict the radiative heat transfer to the furnace walls and under-predict the gas temperature at the furnace exit plane. Edge et al. [10] carried out gray and non-gray calculations of oxy-coal combustion in a laboratory scale burner employing the Reynolds Averaged Navier Stokes (RANS) and Large Eddy Simulation (LES) approach and found significant differences in the in the near-burner region. In the oxyfired combustion simulations, the RANS approach using a spectral gas radiation model was found to capture the surface incident radiation more accurately than the gray model. Therefore, the use of a spectral radiation model with the LES approach was suggested for use in oxy-coal combustion simulations. Kangwanpongpan et al. [11] compared gray and non-gray simulations of radiative heat transfer during oxy-coal combustion in a laboratory-scale furnace and attributed an improved agreement to the temperature predictions to the employing of a non-gray radiation model. More recently, Hjärtstam et al. [12] examined gas and soot-related radiation mechanisms in air-fired and oxy-fired flames operated with propane as fuel. While the non-gray modeling more accurately predicted the radiative source term in the combustion environments compared to the gray model, the work also showed that the inclusion of soot radiation was more critical in sooty air and oxyfired flames than the use of a more rigorous description of the radiative properties of the gas. However, none of the aforementioned studies rigorously investigated the effects of employing gray and non-gray calculations in simulations of oxy-coal combustion in an industrial scale reactor unit or in a wet FGR scenario. Wet FGR is also a likely candidate for retrofitting existing boilers, and therefore a comparative evaluation of gray and non-gray radiation modeling strategies need to be carried out when high concentrations of water-vapor are present in the furnace. For instance, Denison [13] carried out a comparative evaluation of gray and non-gray radiation modeling in one-dimensional prototypical geometries. The media conditions consisted of pure H2O, CO2 as well as mixtures of these participating gases present under isothermal and non-isothermal conditions. By comparing against line-by-line benchmark data, his study demonstrated the need to employ high-fidelity models for non-gray radiation in the calculations. The resulting inaccuracies from employing gray models in the Table 3 Boundary conditions for the full scale boiler simulations. Oxidizer Primary gas Secondary gas Overfire gas Coal flow composition flow rate flow rate flow rate rate (mol%) (ton/h) (ton/h) (ton/h) (ton/h) Air firing O2 ¼ 21%; N2 ¼ 79% Dry FGR O2 ¼ 29%; CO2 ¼ 71% Wet FGR O2 ¼ 24%; CO2 ¼ 50%; H2O ¼ 26%
305.2
574.8
257
110.2
305.2
574.8
257
110.2
305.2
574.8
257
110.2
424
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calculations were found to be dependent on the scale of the geometry, the temperature and concentration gradients within the domain and the thermal boundary conditions. Therefore, this paper provides for the first time a comprehensive assessment of gray and non-gray radiation modeling in oxy-coal combustion with dry and wet FGR in two reactor geometries varying widely on the geometric scale. The choice of the reactors was determined by the availability of critical experimental information such as the reactor and burner dimensions, proximate and
ultimate analysis of the fuel, thermal boundary conditions, all of which impact the accuracies of the numerical predictions. The configurations were intentionally chosen to represent a wide peak temperature range, temperature gradients and length scales for radiative heat transfer. While the intent of the study is insights and not validation, simulation results are also compared against experimental measurements of temperature where available to assess the fidelity of the calculations and the impacts of the gray and non-gray modeling strategies on the radiative fluxes and the temperature field are examined. The WSGG models examined in this study are summarized in Table 1. The WSGG models for oxy-fired combustion have been fit to emissivity data of gas-mixtures that were generated employing different radiative property models, spectroscopic databases and experimental data. Recent gas cell experiments conducted by Becher et al. [18] concluded that the statistically narrow band models (SNB) RADCAL and EM2C to be accurate within 3% of the measured transmissivity at gas concentrations representative of oxy-fired combustion scenarios. Therefore, WSGG models that are based on these two SNB model databases are employed in this study. All three WSGG models were implemented as user-definedfunctions (UDFs) and employed in conjunction with the commercial CFD code ANSYS FLUENT (version 12) [19]. The non-gray models are denoted by the number of gray gases (gg) employed in their formation. A domain based mean-beam length was employed in the calculations involving the Perry (gray) model. The test cases examined in this study are described next. 2. Lab-scale furnace (Aachen U. furnace) 2.1. Geometry and boundary conditions Oxy-coal combustion in a laboratory scale furnace experimentally investigated by Toporov et al. [20] was investigated in this study. The furnace is 4.2 m long and the length of the combustion chamber within the furnace was 2.1 m with an inner diameter of 0.4 m. In the primary stream, the volume fractions of O2 and CO2 were set at 0.19 and 0.81 respectively whereas the corresponding volume fractions were 0.21 and 0.79 respectively, in the secondary, tertiary and staging streams. The particle and oxidizer flow rates, fuel compositions and the particle size distributions along with the wall boundary conditions are specified in Toporov et al. [20]. In order to investigate the differences between oxy-firing with dry FGR and air-firing conditions, additional simulations corresponding to hypothetical “air-firing” conditions were carried out by replacing the CO2 in the oxidizer streams with N2. Additionally, a hypothetical “wet FGR” case was also investigated with the molar compositions of the primary, secondary and staging streams set to: 50% CO2, 24% O2 and 26% H2O, corresponding to a (CO2 þ H2O)/O2 molar ratio of 3.17. 2.2. Modeling approach
Fig. 1. The axial temperature profiles predicted by the various models in the Aachen U. coal furnace during: (a) air-firing; (b) oxy-firing with dry FGR; (c) oxy-firing with wet FGR.
An EulereLagrangian approach was undertaken to model the coal combustion. A stochastic technique was employed to model the dispersion of the particles in the turbulent flow field with the coupling between the particle phase and the fluid phase represented via particle source terms. The fluid turbulence was modeled employing the Shear Stress Transport (SST) keu turbulence model to account for the highly swirling nature of the flow. A constant devolatilization rate of 50 s1 was used to model the coal devolatilization and the eddy-dissipation model was used to compute the chemistry of the volatile components in the gas phase. The heterogeneous reaction of char with the oxidizer stream at the particle surface was assumed to be limited by the rate at which the
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425
Fig. 2. Radial temperature variations at different axial burner distances in the Aachen U. coal furnace during: (a, b) air-firing; (c, d) oxy-firing with dry FGR; (e, f) oxy-firing with wet FGR.
oxidant stream diffuses to the surface of the particle. A two-step mechanism was employed for the production of CO2. First, the char oxidizes to form CO which further oxidizes to form CO2. The radiation was modeled employing the discrete ordinates method employing an angular resolution of 4 4 in the theta and phi direction in every quadrant. As a coal particle undergoes devolatilization and subsequently burns out leaving behind ash, its radiative properties (absorption and scattering efficiencies) also vary. This variation was accounted for through a UDF with the values of absorption and scattering efficiencies of coal and ash obtained from Ref. [21]. The absorption and scattering efficiencies of char was assumed to be an average of that of coal and ash with an anisotropic forward scattering phase-function assumed for the coal, char and
ash particles. These properties are summarized in Table 2. The simulations were performed in 2D axisymmetric co-ordinate system to take advantage of the symmetry in the geometry employing a mesh of resolution 58,691 cells. The results were found to be invariant with any further increase in the number of cells. 3. Full-scale boiler 3.1. Geometry and boundary conditions To investigate the effect of reactor size on the radiative property models, a 300 MW front-wall-fired, pulverized-coal, utility boiler experimentally investigated by Costa and Azevedo [22] for air-
426
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firing was simulated in this study. The boiler and burner dimensions as well as the coal and air flow rates corresponding to air firing along with the proximate and ultimate analyses as well as the heating values of the coal mixture are all summarized in Ref. [22]. Since the coal size distributions were not explicitly mentioned, a coal particle diameter of 100 microns was employed in the
calculations. The boiler was meshed utilizing 916,934 computational cells. In addition to the experimentally investigated conventional air fired, dry FGR where the CO2/O2 molar ratio was 2.45 in the oxidizer stream and wet FGR conditions where the molar composition of the oxidizer stream was set to 50% CO2, 24% O2 and 26% H2O, resulting in a (CO2 þ H2O)/O2 molar ratio of 3.17 were also investigated. These boundary conditions are summarized in Table 3. Since the wall emissivity was not explicitly specified by Costa and Azevedo [22] a value of 0.725 was assumed in the calculations. The wall temperature was maintained at 750 K. A total of 72 angular directions were employed in the discrete ordinates model. 3.2. Modeling approach The standard ke3 model was used to model the turbulence. A constant devolatilization rate of 100 s1 was used to model the coal devolatilization and the eddy-dissipation model was used to compute the chemistry of the volatile components in the gas phase. The heterogeneous reaction of char with the oxidizer stream at the particle surface was assumed to be limited by the rate at which the oxidant stream diffuses to the surface of the particle. As with the lab-scale study, the particles were assumed to be forwardscattering and the variation in the radiative properties of coal, char and ash were varied as shown in Table 2. 4. Results and discussion 4.1. Lab-scale furnace
Fig. 3. The wall incident radiative flux profiles predicted by the various models in the Aachen U. coal furnace during: (a) air-firing; (b) oxy-firing with dry FGR; (c) oxy-firing with wet FGR.
Fig. 1 shows the axial temperature profiles along the length of the lab-scale furnace during air-firing and oxy-firing conditions. First, the simulations show that at the specified molar ratios of (CO2 þ H2O)/O2 in the recycle stream the temperature profiles encountered during air-firing can be replicated. Second, the differences in the temperature predictions between the gray and non-gray model predictions are observed to be small for all three scenarios. Finally, there are no observable variations resulting from employing different radiative property model databases in the Perry (5 gg) and the Chalmers (5 gg) model formulations. Fig. 2 shows the radial temperature variations at different axial burner distances in the Aechen U. coal furnace during air and oxy-firing. The impact of the gray and non-gray modeling strategies on the temperature predictions is not seen to be significant. Furthermore, the numerical predictions are within 100 K of the experimental measurements downstream of the flame zone (cf. Fig. 2(d)). These downstream temperature prediction accuracies compare well with previous numerical simulations of this furnace that have been carried out in 3D periodic domains employing more advanced models for coal devolatilization [11,20]. However, in the nearburner region (cf. Fig. 2(c)), the agreement with the experimental measurements is poor. Also shown in Fig. 2(c) are the corresponding predictions from Toporov et al. [20]. The simulations in Toporov et al. [20] were carried out employing the Chemical Percolation Model (CPD) model for coal devolatilization. In Fig. 2(c) we notice that the choice of the coal devolatilization model has a greater impact on the numerical predictions of temperature than the gray or non-gray modeling strategy for the gasphase radiative properties. The combustion of the volatiles occurs at the axis and clearly the choice of the devolatilization model has a significant bearing on the temperature predictions. The steep temperature gradients observed numerically but not captured experimentally at radial distances less than 0.075 m have been previously attributed to the coarse spatial resolution of the temperature measurements [20].
P. Nakod et al. / Applied Thermal Engineering 54 (2013) 422e432
427
Fig. 4. The ratio of the particle to gas-phase absorption coefficients (when employing the Perry (gray) model) during: (a) air-firing; (b) oxy-firing with dry FGR; (c) oxy-firing with wet FGR.
Fig. 5. Comparisons of numerical predictions of temperature against experimental measurements during air-firing in the full scale boiler.
428
P. Nakod et al. / Applied Thermal Engineering 54 (2013) 422e432
Fig. 6. Contours of temperature within the full scale boiler during air-firing, dry and wet FGR.
P. Nakod et al. / Applied Thermal Engineering 54 (2013) 422e432
Fig. 7. Ratio of particle absorption coefficient to gas absorption coefficients within the full scale boiler employing the Perry (gray) model.
429
430
P. Nakod et al. / Applied Thermal Engineering 54 (2013) 422e432
Fig. 8. Contours of surface incident radiation (in W/m2) along the walls of the full scale boiler.
P. Nakod et al. / Applied Thermal Engineering 54 (2013) 422e432 Table 4 The total incident radiation to the walls of the boiler and the pendant plates normalized with respect to the Perry (gray) model predictions. Air-fired
Perrygray Perry5 gg Chalmers5 gg
Dry FGR
Wet FGR
Pendant plates
Furnace wall
Pendant plates
Furnace wall
Pendant plates
Furnace wall
1
1
1
1
1
1
0.908
0.925
0.831
0.898
0.966
0.955
0.912
0.930
0.805
0.887
0.933
0.951
Fig. 3 shows wall incident radiative flux profiles predicted by the various models during air and oxy-firing conditions. Fig. 3 also confirms that dry and wet FGR at these (CO2 þ H2O)/O2 molar ratios in the oxidizer stream, the heat flux profiles encountered during air-firing can be replicated. However, no major differences between the gray and non-gray calculations are observed within this furnace geometry. Fig. 4 shows the ratio of the particle to gasphase absorption coefficients during air-fired and oxy-firing conditions when employing the Perry (gray) model. The particle absorption coefficient is several times larger than the gas absorption coefficient in the near burner region and decreases as we move axially along the length of the furnace. These correspond to regions of coal devolatilization and burnout occurring in the near burner regions and particle radiation mainly due to ash particles in the downstream regions of the furnace. From Figs. 1e4 we may summarize that when simulating this lab-scale furnace geometry, there is very little observable variation between gray and non-gray radiation modeling when employing WSGG parameters in the Perry model that have been optimized to cover the entire spectrum of scenarios likely to be encountered in air and oxy-fired combustion. Furthermore, differences in the radiative property model databases employed in formulating these optimized models do not significantly affect the numerical predictions. These are attributed to the smaller path lengths and the relatively low flame temperatures [cf. Figs. 1 and 2] encountered within this furnace. While these low temperatures and path-lengths encountered within the lab-scale furnace are not representative of a full-scale coal combustion scenario, the numerical simulations provide valuable insights into the radiation modeling fidelity (and therefore the computational cost) that is required during the modeling of such furnaces and help identify the combustion models that are most in need of refinement to improve agreement with the experimental data. 4.2. Full-scale boiler Fig. 5 compares the numerical predictions of temperature against the experimental measurements reported in Ref. [22]. First, it is important to remember that the accuracy of the simulations was limited by the lack of fuel specific kinetic data and the exact particle diameter size distributions at the inlets. To determine the impact of devolatilization rates on the simulations, an additional Table 5 Ratios of the total surface incident radiation to the furnace walls and the pendant plates in the absence of particle radiation to those obtained when both particle and gas radiation are present. All results were obtained employing the Perry (5 gg) model. Air-fired
Dry FGR
Wet FGR
Pendant plates
Furnace wall
Pendant plates
Furnace wall
Pendant plates
Furnace wall
0.750
0.750
0.836
0.852
0.849
0.882
431
simulation was carried out employing the two competing rates (Kobayashi) model in ANSYS FLUENT [19] with the Perry (5 gg) model. The observed agreement between the experimental measurements and simulations is within 100 K in most of the locations and may be deemed as acceptable. An agreement to within 100 K of the experimental measurements is generally the level of modeling accuracies that can be expected in simulations of such large scale furnaces even when accurate particle size distribution and coal kinetic parameters are available [23]. Fig. 5 also shows that the choice of the devolatilization model has a greater impact on the numerical predictions of temperature than the choice of the radiative property model for the gas-phase. The temperature contours within the full scale boiler during air-firing, dry and wet FGR are shown in Fig. 6. The similarities between the contours obtained from the three firing conditions confirm that the combustion of coal can be completed at these H2O/CO2 molar ratios in the dry and wet FGR scenarios to achieve a temperature profile within the boiler that is identical to that found in air-burn. These conclusions are consistent with the observations of Habermehl et al. [24] who also determined through numerical simulations of an industrial scale boiler that oxy-fired combustion could be completed at similar (CO2 þ H2O)/O2 mole fractions in the dry and wet FGR streams. The average outlet temperatures predicted by the different models are also reported in Fig. 6. Although no noticeable difference is observable from the gray and non-gray model temperature contours, 40e50 K temperature difference is seen between the average outlet temperatures predicted by the gray and non-gray models. Fig. 7 shows the ratios of the particle absorption coefficient to gas absorption coefficients within the full scale boiler employing the Perry (gray) model. In all three scenarios there are pockets within the furnace where the gas radiation is greater than or of the same magnitude of the particle radiation. Fig. 8 shows the contours of surface incident radiation along the walls of the full scale boiler. In order to closely examine the differences between the gray and non-gray modeling strategies, the total incident radiation along the walls of the furnace and the pendant plates in the convective pass of the furnace was determined. These are reported in Table 4, where the values are normalized with respect to the corresponding predictions from the Perry (gray) model. Results from Table 4 indicate that the non-gray model predictions of the total wall incident radiation are about 10% lower than that of the Perry (gray) model. The largest variations are seen during dry FGR with the smallest variations encountered during wet FGR. Again, the variations between the Perry (5 gg) and Chalmers (5 gg) models are small. In order to assess the relative importance of particle radiation to gas radiation in these scenarios, the incident radiative fluxes to the walls of the boiler were determined by turning “off” the particle radiation contribution. This was undertaken by “freezing” the temperature and specie concentration fields in the air, dry and wet FGR cases, turning off “particleeradiation interactions” and carrying out radiative transfer calculations till convergence. These calculations were carried out employing the Perry (5 gg) model. Table 5 summarizes the ratios of the total surface incident radiation to the boiler walls and the pendant plates in the absence of particle radiation to those obtained when both particle and gas radiation are present. The contribution of the gas radiation to the total radiation increases under oxy-firing conditions. This is consistent with the observations of Andersson et al. [25] who noted an increase in the gas radiative intensities in a 100 kW lignite-fired boiler. They also noted that as long as the temperature distributions are similar the total intensities are the same in both air and oxy-firing scenarios. This is consistent with our findings in Figs. 1, 3, 6 and 8 where similar temperature profiles encountered in air-firing, dry FGR and wet FGR scenarios are seen to result in similar incident radiative heat flux profiles. The results
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in Table 5 shows that the gas radiation is a significant contributor to the total surface heat fluxes in all three scenarios. Therefore, an accurate specification of non-gray particle radiative properties and scattering phase functions for instance (which was not undertaken in this study) is likely to play a smaller role in the determination of wall heat fluxes than that played by non-gray gas radiative properties. However, previous studies [8,26] have shown the relative differences in the heat flux predictions between the gray and nongray models will be determined more by the temperature and composition gradients and in homogeneities encountered within the reactor. While the computational cost of the radiation calculations when employing the Perry (5 gg) and Chalmers (5 gg) models is 5 times greater than that of the Perry (gray) model, the observed variations in the radiative flux predictions between the gray and non-gray modeling strategies (cf. Table 4) might necessitate their usage in large scale furnaces where large temperature and composition gradients are encountered. However, this increase in computational expense can be partially overcome by performing the radiation calculation only once several fluid iterations. This often is an acceptable strategy since the radiation field does not change as fast as the fluid field between iterations. 5. Conclusions Simulations of coal combustion in a lab-scale furnace and full-scale boiler at air-fired and oxy-fired combustion conditions were carried out employing two recently proposed WSGG models for the radiative property of the gas phase. The models differed in the radiative property models/databases employed in their formulation. Based on the results of this study, the following conclusions can be drawn: 1. The study first confirms that at certain (CO2 þ H2O)/O2 molar ratios in the dry and wet FGR scenarios, the temperature and wall radiative flux profiles encountered during coal combustion with air can be replicated with oxy-fired combustion in lab-scale furnaces as well as full-scale boilers. 2. The computed temperature profiles were in reasonable agreement with the experimental measurements. The accuracies of the predictions were however limited by the lack of fuel specific kinetic data. Furthermore, comparisons from employing different coal devolatilization models showed that the temperature predictions were more sensitive to the choice of devolatilization model rather than the radiative property model for the gas-phase. 3. In the lab-scale furnace, lower flame temperatures and smaller path lengths minimized the differences between the gray and non-gray radiative flux and temperature profiles. 4. Within the full-scale boiler, large volume pockets were present where the radiation was dominated by the gas-phase. This combined with higher peak flame temperatures and longer path lengths resulted in: a 10% variation between the gray and non-gray total incident radiative flux predictions and 40e50 K difference in the average outlet gas temperature. The largest variation between the gray and non-gray heat fluxes was observed during dry FGR with the smallest variation encountered during wet FGR. 5. The contribution of the gas-radiation to the wall incident radiative fluxes was determined to be more significant than that of particle radiation in all three scenarios. Therefore, an accurate specification of the gas radiation properties is likely to be more important than the particle radiative properties for wall radiative flux predictions. 6. There were no significant differences in the predictions between the Perry (5 gg) and the Chalmers (5 gg) models. Therefore, variations among the WSGG models due to differ-
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