| 1. |
- Andersson, Klas, 1977, et al.
(author)
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Thermal radiation in oxy-fuel flames
- 2011
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In: International Journal of Greenhouse Gas Control. - : Elsevier BV. - 1750-5836. ; 5:suppl. 1, s. S58-S65
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Journal article (peer-reviewed)abstract
- This work investigates thermal radiation in oxy-fuel flames, based on experiments and modelling. Experiments were conducted in a 100 kW test facility in air and oxy-fuel combustion atmospheres, using two different types of fuels, lignite and propane. In-flame measurements of gas composition, temperature and total radiation intensity, were performed and used as input to radiation modelling to examine the influence of oxy-fuel conditions on gas and particle radiation characteristics. In the modelling, the spectral properties of CO(2) and H(2)O are treated by means of a statistical narrow band model and particle radiation is modelled for both scattering and non-scattering particles. Experiments on the propane flame show that the flame radiation conditions are drastically influenced by the recycling conditions. With OF 27 conditions (27% oxygen in the feed gas) and dry recycling, the temperature is slightly lower compared to air-fired conditions, but the emitted intensity is significantly increased. Modelling shows that this is mainly caused by a significantly increased soot radiation. Propane flame images show that the presence of soot in oxy-fuel conditions varies strongly with recycling conditions. The contribution due to an increased emission by CO(2) is of minor importance. In the lignite experiments similar flame temperatures were kept during air and oxy-fuel combustion (OF 25 conditions with dry recycling). The measurements show that the intensity levels in both flames are similar which is due to a strong particle radiation in both environments. The modelling reveals that the dominance by particle radiation contra gas radiation is closely related to whether the particles are scattering or non-scattering.
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| 2. |
- Bäckström, Daniel, 1985, et al.
(author)
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Gas temperature and radiative heat transfer in oxy-fuel flames
- 2012
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In: The 37th International Technical Conference on Clean Coal & Fuel Systems, Clearwater USA, 3-7/6-2012. - 9780932066374
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Conference paper (other academic/artistic)abstract
- This work presents measurements of the gas temperature, including fluctuations, and its influence on the radiative heat transfer in oxy-fuel flames. The measurements were carried out in the Chalmers 100 kW oxy-fuel test unit. The in-furnace gas temperature was measured by a suction pyrometer and by an optical system based on FTIR-spectroscopy. The radiation intensity was measured by a Narrow Angle Radiometer and the gas radiation was calculated with a Statistical Narrow Band model. The overall agreement between the two temperature measurement techniques was good. The optical system showed a lower temperature than the suction pyrometer in the low velocity regions of the furnace, a difference which is likely to be an effect of the purge gas added in the optical probe. The measured temperature fluctuations were evaluated by modeling of the gas radiation. The influence from the measured fluctuations on the radiative heat transfer shows no effect of turbulence-radiation interaction. However, by comparing with temperature fluctuations in other flames it can be seen that the fluctuations measured here are relatively small. Further research is needed to clarify to which extent the applied methods can account for the turbulence-radiation interaction in the investigated flame.
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| 3. |
- Bäckström, Daniel, 1985, et al.
(author)
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Measurement and Modeling of Particle Radiation in Coal Flames
- 2014
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In: Energy & Fuels. - : American Chemical Society (ACS). - 1520-5029 .- 0887-0624. ; 28:3, s. 2199-2210
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Journal article (peer-reviewed)abstract
- This work aims at developing a methodology that can provide information of in-flame particle radiation in industrial-scale flames. The method is based on a combination of experimental and modeling work. The experiments have been performed in the high-temperature zone of a 77 kWth swirling lignite flame. Spectral radiation, total radiative intensity, gas temperature, and gas composition were measured, and the radiative intensity in the furnace was modeled with an axisymmetric cylindrical radiation model using Mie theory for the particle properties and a statistical narrow-band model for the gas properties. The in-flame particle radiation was measured with a Fourier transform infrared (FTIR) spectrometer connected to a water-cooled probe via fiber optics. In the cross-section of the flame investigated, the particles were found to be the dominating source of radiation. Apart from giving information about particle radiation and temperature, the methodology can also provide estimates of the amount of soot radiation and the maximum contribution from soot radiation compared to the total particle radiation. In the center position in the flame, the maximum contribution from soot radiation was estimated to be less than 40% of the particle radiation. As a validation of the methodology, the modeled total radiative intensity was compared to the total intensity measured with a narrow angle radiometer and the agreement in the results was good, supporting the validity of the used approach.
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| 4. |
- Hjärtstam, Stefan, 1978, et al.
(author)
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Computational Fluid Dynamics Modeling of Oxy-Fuel Flames: The Role of Soot and Gas Radiation
- 2012
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In: Energy & Fuels. - : American Chemical Society (ACS). - 1520-5029 .- 0887-0624. ; 26:5, s. 2786-2797
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Journal article (peer-reviewed)abstract
- This work applies a computational fluid dynamics (CFD) approach to examine gas and soot-related radiation mechanisms in air and oxy-fuel flames operated with propane as fuel. In oxy-fuel combustion, CO2 and H2O replace the N-2 in air combustion. As a result, the radiative heat transfer characteristics differ between the combustion atmospheres. Moreover, changes in soot formation have been observed in oxy-fuel compared to air-fired flames. Both gas- and soot-related radiation can be essential for the design of oxy-fuel furnaces and need to be accounted for when temperature and heat transfer conditions are modeled. The aim of the work is to determine the respective impact of combustion gases and soot in heat transfer modeling of the flames. Both gray and nongray approaches are used to account for the gas radiation and the results are compared to measured data from a 100 kW oxy-fuel unit to investigate if a gray model is sufficient to generate a reliable solution when applied in CFD simulations of oxy-fuel combustion. In addition, calculations of the radiative source term are performed for a domain between two infinite plates, with temperature and concentration profiles from the CFD simulations of the present work. It is shown that the nongray approach accurately predicts the source term in both combustion environments, whereas the gray model fails in predicting the source term. The source term has a direct influence on the temperature field in CFD calculations. However, this work also shows that the inclusion of soot radiation is more critical in sooty air and oxy-fuel flames than the use of a more rigorous description of the radiative properties of the gas.
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| 5. |
- Hjärtstam, Stefan, 1978, et al.
(author)
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Evaluation of gas radiation modeling in oxy-fired furnaces
- 2010
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In: Conference Proceedings; 2010 AIChE Annual Meeting, 10AIChE; Salt Lake City, UT; 7 November 2010 through 12 November 2010. - 9780816910656
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Conference paper (peer-reviewed)abstract
- Oxy-fuel fired furnaces will exhibit combustion conditions different from air-firing and this will have important effects on the radiative properties of the gas. In contrast to nitrogen (N2), carbon dioxide (CO 2) and water vapor (H2O) are strong infrared emitters and the radiative activity of the gas in oxy-fuel combustion is increased compared to air-firing. In an oxy-fired furnace the pressure path-lengths are several times larger than in an air-fired furnace and the ratio of H2O to CO2 can be significantly different. These differences between air and oxy-fuel combustion make the role of radiative heat transfer by gases in oxy-fired combustion different to air-firing, even in cases where the temperature distribution is similar to that of airfired combustion. These aspects are essential for design of oxy-fired furnaces and need to be accounted for when temperature and heat transfer conditions are determined by modeling. In comprehensive combustion models, for example CFD models, it is common to neglect the spectral variations of the gases and to treat the spectrum by a single average, i.e. a gray approximation. Approximate models frequently applied in combustion modeling, such as the Weighted-Sum-of-Gray-Gases (WSGG) model, are not suitable for oxy-fired boilers since their parameters are fitted to pressure path-lengths, and ratios of H2O to CO2, typical for airfired conditions. Different approximations used to account for gaseous radiation in CFD-simulations are investigated: temperature predictions by a gray model and a non-gray formulation of a WSGG model are compared in air- and oxy-fired conditions. The WSGG model used in this work is suitable for oxy-fuel conditions since it accounts for various ratios of H2O to CO 2 and the parameters are fitted to a broad range of pressure path-lengths. The modeled case is a propane flame in Chalmers 100 kW oxy-fuel rig. Two flames with similar temperature distribution are modeled and compared: an oxy-fuel case with 27 vol.% oxygen in the feed gas and an airfired reference case. To support the CFD-results, calculations of the radiative source term are carried out for a domain between two infinite plates with similar temperature and concentration profiles as in the CFD-simulations. In these calculations, the gray model and the non-gray formulation of the WSGG model are compared with a Statistical-Narrow-Band model. In addition, the role of radiative heat transfer is investigated by including soot radiation in the CFD-simulations to give an understanding of the relative importance of gaseous radiation when soot particles are present. It is shown that a use of a non-gray approach is motivated when modeling oxy-fuel combustion since the gray model fails in predicting the source term, which affects the predicted temperature field in CFD calculations. Furthermore, the inclusion of soot radiation is more crucial for the modeled flames than the use of a more rigorous description of the radiative properties of the gaseous components.
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| 6. |
- Johansson, Robert, 1977, et al.
(author)
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Account for variations in the H2O to CO2 molar ratio when modelling gaseous radiatve heat transfer with the weighted-sum-of-grey-gases model
- 2011
-
In: Combustion and Flame. - : Elsevier BV. - 1556-2921 .- 0010-2180. ; 158:5, s. 893-901
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Journal article (peer-reviewed)abstract
- This work focuses on models suitable for taking into account the spectral properties of combustion gasesin computationally demanding applications, such as computational fluid dynamics. One such model,which is often applied in combustion modelling, is the weighted-sum-of-grey-gases (WSGG) model.The standard formulation of this model uses parameters fitted to a wide range of temperatures, but onlyfor specific ratios of H2O to CO2. Then, the model is limited to gases from fuels with a given compositionof hydrogen and carbon, unless several sets of fitted parameters are used. Here, the WSGG model is modifiedto account for various ratios of H2O to CO2 concentrations. The range of molar ratios covers both oxyfuelcombustion of coal, with dry- or wet flue gas recycling, as well as combustion of natural gas. The nongreyformulation of the modified WSGG model is tested by comparing predictions of the radiative sourceterm and wall fluxes in a gaseous domain between two infinite plates with predictions by a statisticalnarrow-band model. Two grey approximations are also included in the comparison, since such modelsare frequently used for calculation of gas radiation in comprehensive combustion computations. It isshown that the modified WSGG model significantly improves the estimation of the radiative source termcompared to the grey models, while the accuracy of wall fluxes is similar to that of the grey models orbetter.
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| 7. |
- Johansson, Robert, 1977, et al.
(author)
-
Influence of ash particles on radiative heat transfer in air- and oxy-fired conditions
- 2012
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In: 37th Clean Coal Conference, Clearwater Florida.
-
Conference paper (other academic/artistic)abstract
- This work focuses on thermal radiation in oxy-fired conditions. Both gas and particle radiation is modeled in an axi-symmetric cross section of a cylindrical furnace and the influence of ash and its properties is investigated. Coal and ash particles are treated separately and their properties are calculated according to the Mie-theory. For the gas radiation, a Statistical-Narrow-Band (SNB) model is applied. The intensity field is calculated according to the discrete transfer method and scattering is assumed to be isotropic. Calculations are done on spectral basis, solving one intensity equation for each narrow band, to account for the spectral nature of the gases and the ash particles. The investigated cases cover both air-and oxy-fired conditions and the properties of the combustion gas are based on measured data from a lignite flame in Chalmers 100 kW rig. Temperatures and particle profiles are chosen to represent different sections of a boiler and gas concentrations corresponding to both dry and wet flue gas recycling are examined.
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| 8. |
- Johansson, Robert, 1977, et al.
(author)
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Influence of particle and gas radiation in oxy-fuel combustion
- 2013
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In: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310. ; 65, s. 143-152
-
Journal article (peer-reviewed)abstract
- This work investigates thermal radiation in oxy-fired conditions. Both gas and particle radiation is modelled in an axi-symmetric cross section of a cylindrical furnace and differences in the radiative transfer between air- and oxy-firing are investigated. The particle radiative properties are calculated according to the Mie-theory, accounting for the spectral properties. The scattering by the particles is assumed to be isotropic. For the gas radiation, a Statistical-Narrow-Band (SNB) model is applied as reference. The properties of the combustion gas and the particle load are derived from measurements in a lignite flame in Chalmers University's 100 kW test rig. The wall flux and the radiative source term along the cylinder's diameter are compared to evaluate the difference in radiation between air- and oxy-fuel combustion. Special emphasis is put on the influence of load and distribution of particles, both in the flame and at the furnace exit. The results show that the presence of particles suppresses the influence of gas composition and small differences are seen between the different gas mixtures. It is also concluded that variation of temperature and particle load can have a significant impact on the radiative heat transfer. (C) 2013 Elsevier Ltd. All rights reserved.
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| 9. |
- Johansson, Robert, 1977, et al.
(author)
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Models for gaseous radiative heat transfer applied to oxy-fuel conditions in boilers
- 2010
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In: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310. ; 53:1-3, s. 220-230
-
Journal article (peer-reviewed)abstract
- Models of gas radiation properties have been evaluated for conditions relevant to oxy-fired boilers, characterized by larger pressure path-lengths and possibly different ratios of H2O/CO2 compared to air-fired boilers. Statistical narrow band (SNB) models serve as reference. The other radiation models tested are the weighted-sum-of-grey-gases model, the spectral line-based weighted-sum-of-grey-gases model and two grey-gas approximations. The range of validity of the existing coefficients of the weighted-sum-of-grey-gases model is limited, and new coefficients have therefore been determined to cover the conditions of interest. Several assumed test cases, involving both uniform and non-uniform paths, have been studied to evaluate the accuracy of the models. Comparisons with experimental data are also included. The results show that a grey approximation can give accurate wall fluxes, but at the expense of errors in the radiative source term. The weighted-sum-of-grey-gases model with the new coefficients yields predictions within 20% of those of the reference model in most cases, while the spectral line-based weighted-sum-of-grey-gases model usually gives results within 10%. There are, however, discrepancies between the SNB models at high temperatures. The weighted-sum-of-grey-gases model with its low computational cost is recommended for computationally demanding applications where predictions of both wall fluxes and the radiative source term are important.
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| 10. |
- Johansson, Robert, 1977, et al.
(author)
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THE INFLUENCE OF PARTICLE AND GASEOUS RADIATION IN OXY-FUEL COMBUSTION
- 2011
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In: The 36th International Technical Conference on Clean Coal & Fuel Systems, Clearwater USA, 5-9/6 2011.
-
Conference paper (other academic/artistic)abstract
- This work focuses on radiation in oxy-fired conditions. Both gas and particle radiation is modeled in an axi-symmetric cross section of a cylinder and differences between air- and oxy-firing are investigated. The particle radiation is modeled by empirical correlations accounting for spectral properties of coal particles and scattering of the particles is assumed to be isotropic. For the gas radiation, a Statistical-Narrow-Band (SNB) model is applied as reference. Included in the analysis is also a non-gray or banded formulation of the Weighted-Sum-of-Gray-Gases (WSGG) model and grey approximations. The investigated cases cover both air-and oxy-fired conditions and the properties of the combustion gas are based on measured data from a lignite flame in Chalmers 100 kW rig. Wall fluxes and the radiative source term along the cylinder diameter are compared to evaluate differences in the radiative heat transfer between air- and oxy-fuel combustion. Special emphasis is put on the load and distribution of particles, with both in-flame conditions as well as furnace exit conditions being examined. The different approximations for gaseous radiation are evaluated to quantify the errors they result in when they are applied in conditions with a high particle load.
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