| 1. |
- Andersson, Klas, 1977, et al.
(författare)
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Radiation intensity of lignite-fired oxy-fuel flames
- 2008
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Ingår i: Experimental Thermal and Fluid Science. - : Elsevier BV. - 0894-1777. ; 33:1, s. 67-76
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Tidskriftsartikel (refereegranskat)abstract
- The radiative heat transfer in oxy-fuel flames is compared to corresponding conditions in air-fuel flames during combustion of lignite in the Chalmers 100 kW oxy-fuel test facility. In the oxy-fuel cases the flue-gas recycle rate was varied, so that, in principle, the same stoichiometry was kept in all cases, whereas the oxygen fraction in the recycled flue-gas mixture ranged from 25 to 29 vol.%. Radial profiles of gas concentration, temperature and total radiation intensity were measured in the furnace. The temperature, and thereby the total radiation intensity of the oxy-fuel flames, increases with decreasing flue-gas recycle rate. The ratio of gas and total radiation intensities increases under oxy-fuel conditions compared to air-firing. However, when radiation overlap between gas and particles is considered the ratios for air-firing and oxy-fuel conditions become more similar, since the gas-particle overlap is increased in the CO2-rich atmosphere. A large fraction of the radiation in these lignite flames is emitted by particles whose radiation was not significantly influenced by oxy-fuel operation. Therefore, an increment of gas radiation due to higher CO2 concentration is not evident because of the background of particle radiation, and, the total radiation intensities are similar during oxy-fuel and air-fuel operation as long as the temperature distributions are similar.
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| 2. |
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| 3. |
- Hjärtstam, Stefan, 1978, et al.
(författare)
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Combustion characteristics of lignite-fired oxy-fuel flames
- 2009
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Ingår i: Fuel. - : Elsevier BV. - 0016-2361. ; 88:11, s. 2216-2224
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Tidskriftsartikel (refereegranskat)abstract
- This experimental work describes the combustion characteristics of lignite-fired oxy-fuel flames, in terms of temperature distribution, gas composition (O2, CO2, CO, total hydrocarbon concentration and NO) and ignition behaviour. The aim is to evaluate the flame structure of three oxy-fuel cases (obtained by changing the flue gas recycle rate) including a comparison with an air-fired reference case. Measurements were performed in Chalmers 100 kW test unit, which facilitates oxy-fuel combustion under flue gas recycling conditions. Temperature, O2 and CO concentration profiles and images of the flames indicate that earlier ignition and more intense combustion with higher peak temperatures follow from reduction of the recycle rate during oxy-fuel operation. This is mostly due to higher O2 concentration in the feed gas, reduced cooling from the recycled flue gas, and change in flow patterns between the cases. The air case and the oxy-fuel case with the highest recycle rate were most sensitive to changes in overall stoichiometry. Despite significant differences in local CO concentration between the cases, the stack concentrations of CO are comparable. Hence, limiting CO emissions from oxy-fuel combustion is not more challenging than during air-firing. The NO emission, as shown previously, was significantly reduced by flue gas recycling.
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| 4. |
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| 5. |
- Hjärtstam, Stefan, 1978, et al.
(författare)
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Computational Fluid Dynamics Modeling of Oxy-Fuel Flames: The Role of Soot and Gas Radiation
- 2012
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Ingår i: Energy & Fuels. - : American Chemical Society (ACS). - 1520-5029 .- 0887-0624. ; 26:5, s. 2786-2797
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Tidskriftsartikel (refereegranskat)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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| 6. |
- Hjärtstam, Stefan, 1978, et al.
(författare)
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Evaluation of gas radiation modeling in oxy-fired furnaces
- 2010
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Ingår i: Conference Proceedings; 2010 AIChE Annual Meeting, 10AIChE; Salt Lake City, UT; 7 November 2010 through 12 November 2010. - 9780816910656
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Konferensbidrag (refereegranskat)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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| 7. |
- Hjärtstam, Stefan, 1978, et al.
(författare)
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Oxy-Fuel Combustion Modeling: Performance of Global Reaction Mechanisms
- 2012
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Ingår i: Industrial & Engineering Chemistry Research. - : American Chemical Society (ACS). - 1520-5045 .- 0888-5885. ; 51:31, s. 10327-10337
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Tidskriftsartikel (refereegranskat)abstract
- Three global reaction mechanisms derived for oxy-fuel combustion and one global reference mechanism are investigated and compared under gaseous oxy-fuel combustion conditions. The aim is to evaluate their prediction of major in-flame species and temperature by comparison with a detailed reaction mechanism (validated for oxy-fuel conditions) and experimental data. The evaluation is performed using a 1D plug flow reactor (PFR) method and 3D CFD calculations. Through the PFR calculations, it is found that the global mechanisms all predict a too early onset of fuel oxidation compared to the detailed mechanism. Furthermore, the global reference mechanism predicts gas concentrations more in line with the detailed mechanism than the oxy-fuel mechanisms, which yield incorrect reaction sequences. In the CFD analysis, significant differences in the predicted gas concentrations and temperature fields between the global mechanisms show that the choice of reaction mechanism strongly influences the results. In summary, the global reference mechanism is a preferable alternative to represent the combustion chemistry when modeling oxy-fuel combustion using CFD, if the use of a detailed reaction mechanism is prohibited due to computational limitations.
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