| 241. |
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| 242. |
- Kuehnemuth, Daniel, 1979, et al.
(författare)
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NOX reburning in oxy-fuel combustion - An experimental investigation
- 2010
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Ingår i: 27th Annual International Pittsburgh Coal Conference 2010, PCC 2010; Istanbul; Turkey; 11 October 2010. - 9781617823213 ; 1, s. 256-270
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- This work investigates the reburning reduction of nitric oxide (NO) in a 100 kW propane-fired oxyfuel flame. The conducted experiments include a comprehensive parameter study: NO was injected into the recycled flue-gas, the inlet oxygen concentration was varied between 25 and 37 vol. % and the stoichiometric ratios at the burner inlet ranged from 0.7 and 1.15. The respective influence of inlet oxygen concentration and burner stoichiometry on once-through and total reduction of NO was measured. Furthermore, concentration and temperature in the furnace were mapped to identify important differences between oxy and air-fired conditions. The furnace measurements show that the peak concentration of carbon monoxide may be more than twice as high as in air-fired conditions. The formation paths of CO and its influence on the NO x chemistry are therefore discussed. The results of the parameter study show that reburning is favored by decreased burner stoichiometry. The effect of inlet oxygen concentration on once-through NO reduction is of minor importance. Changes in stoichiometry and oxygen inlet concentration are associated with changes in recycle ratio. The influence of the recycle ratio on the NO reduction is of great importance and is investigated as separate parameter.
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| 243. |
- Kuehnemuth, Daniel, 1979, et al.
(författare)
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On the carbon monoxide formation in oxy-fuel combustion-Contribution by homogenous and heterogeneous reactions
- 2014
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Ingår i: International Journal of Greenhouse Gas Control. - : Elsevier BV. - 1750-5836. ; 25, s. 33-41
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Tidskriftsartikel (refereegranskat)abstract
- This work investigates CO formation mechanisms under oxy-fuel combustion conditions. The importance of the possible explanations for increased inflame CO concentrations in oxy-fuel flames compared to air-firing are discussed. A model based on a detailed gas-phase reaction mechanism is combined with a lignite char combustion model, including apparent surface kinetics for oxidation as well as carbon dioxide and steam gasification and implication of diffusion limitation. In agreement with other authors work, it is concluded that in gas-fired oxy-fuel flames the CO formation is promoted by a homogenous reaction between hydrogen radicals and CO2. Additionally, this work concludes that in lignite-fired oxy-fuel combustion, this gaseous reaction route is of less importance. In oxy-lignite flames, CO2 gasification is the largest contributor to the increased CO formation compared to air firing. The substitution of CO2 with steam in the oxidizer during wet oxy-fuel combustion has moderate influence on the CO2 gasification, whereas the homogenous CO formation is strongly reduced.
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| 244. |
- Kuehnemuth, Daniel, 1979, et al.
(författare)
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Reburning of Nitric Oxide in Oxy-Fuel Firing-The Influence of Combustion Conditions
- 2011
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Ingår i: Energy & Fuels. - : American Chemical Society (ACS). - 1520-5029 .- 0887-0624. ; 25:2, s. 624-631
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Tidskriftsartikel (refereegranskat)abstract
- Experiments were carried out to obtain data on the efficiency of nitric oxide (NO) reduction by reburning under oxy-fuel conditions. The work was carried out in a 100 kW test facility fired with propane. The data were analyzed by means of a combustion model, which included a detailed description of the reburning chemistry. The importance of several combustion parameters on the reburning efficiency was studied: inlet oxygen concentration, flue gas recycle ratio, temperature, and stoichiometric ratio. The inlet oxygen concentration was kept between 25 and 37 vol % and the burner stoichiometric ratio between 0.7 and 1.15. NO was injected in the oxidizer. As expected, similar to air-firing, reburning in oxy-fuel is favored by substoichiometric conditions. A decrease in combustion temperature, caused by a lowered stoichiometric ratio, is shown to be advantageous for reduction of NO under oxy-fuel conditions. The effect of inlet oxygen concentration on reburning is not significant as long as the combustion conditions are fuel-lean. However, the amount of recycled flue gas, which increases with decreasing oxygen content, significantly improves the total reduction. Instead, when the stoichiometric ratio is decreased, the recycle flow of the flue gas is reduced, which, to some extent, counteracts the otherwise positive effect of fuel-rich conditions in the flame zone in oxy-fuel conditions. Thus, during oxygen-rich combustion, the total reburning efficiency in oxy-fuel combustion is superior to once-through reburning in air-firing, but during substoichiometric conditions, the reduction in air and oxy-fuel combustion is comparable.
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| 245. |
- Köhler, Anna, 1989, et al.
(författare)
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Determination of the Apparent Viscosity of Dense Gas-Solids Emulsion by Magnetic Particle Tracking
- 2018
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- When designing fluidised bed units a key to ensure efficient conversion is proper control of the mixing of the fuel in both lateral and axial directions in the bed. In order to mechanistically describe the mixing of fuel particles in a fluidised bed, there is a need to determine the apparent viscosity of the gas-solids emulsion, which determines the drag on the fuel particles. In this work the apparent viscosity of a bed of spherical glass beads and air at minimum fluidisation was determined by means of the falling sphere method. Hereto the drag of the bed on a single immersed object was obtained by measuring the velocity of a negatively buoyant tracer with magnetic particle tracking (MPT). MPT allows for highly temporally and spatially resolved trajectories (10-3 s and 10-3 m, respectively) in all 3-dimensions. The bed consisted of glass beads with a narrow size distribution (215 to 250 μm) and tracers with a size from 5 to 20 mm and densities from 4340 to 7500 kg/m3 were used. Hence, the literature, which typically covers data for velocities lying within or just above the Stoke flow regime (0.002 < Re < 2.0) could be expanded to Re numbers (53 to 152) well within the transition flow regime. The drag and apparent viscosity was compared to different fluid models and agreed well with the Newtonian model, when taking into account possible effects of the bed walls. Comparing the drag coefficient of data of free falling spheres and data of spheres falling with controlled velocities, the latter showed a dependence on the product of tracer diameter and falling velocity, dput, while the former was constant over dput. This indicates the method with controlled falling velocities to be intrusive and influencing the result of the apparent viscosity of the bed. Using the free falling sphere method this work obtained an apparent viscosity of 0.24 Pa s, which is consistent with values found in earlier literature for an emulsion of air and sand of similar size and density.
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| 246. |
- Köhler, Anna, 1989, et al.
(författare)
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Experimental characterization of axial fuel mixing in fluidized beds by magnetic particle tracking
- 2016
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- A Magnetic Particle Tracking (MPT) system is applied to a bubbling fluidized bed to study how axial mixing and segregation of fuel are influenced by the fuel density and operational conditions (fluidization velocity, bed height and pressure drop across the gas distributor). The MPT system is used to determine the vertical distribution of the tracer particle in a fluid-dynamically down-scaled cold unit resembling a 0.74×0.74 m2 fluidized bed reactor operating at 800 °C. This work uses a tracer particle of 10 mm in diameter, corresponding to a fuel particle of 44 mm. Different tracer particles are applied with solids density representing biomass, biomass char and that of the average bulk. The MPT system yields a spatial accuracy in the order of 10-3 m and a time resolution of 10-3 s. For the operational range investigated, three fuel segregation regimes can be identified from the MPT measurements: 1) A flotsam regime which occurs at low fluidization velocities and for low density tracer particles, 2) A transition regime over which an increase in fluidization velocity results in that the presence of fuel particles at the bed surface decreases rapidly, and 3) A fully developed mixing regime in which the presence of tracer particle at the bed surface and the splash zone remains constant with fluidization velocity. The transition velocities between the regimes depend on bed height and density of the tracer particle.
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| 247. |
- Köhler, Anna, 1989, et al.
(författare)
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Experimental characterization of axial fuel mixing in fluidized beds by magnetic particle tracking
- 2017
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Ingår i: Powder Technology. - : Elsevier BV. - 1873-328X .- 0032-5910. ; 316:SI, s. 492-499
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Tidskriftsartikel (refereegranskat)abstract
- A magnetic particle tracking (MPT) system is applied to a bubbling fluidized bed to study how axial mixing and segregation of fuel are influenced by the fuel density and operational conditions (fluidization velocity, bed height and pressure drop across the gas distributor). The MPT system is used to determine the vertical distribution of the tracer particle in a fluid-dynamically down-scaled cold unit resembling a 0.74×0.74 m^2 fluidized bed reactor operating at 800°C. This work uses a tracer particle of 10 mm in diameter, corresponding to a fuel particle of 44 mm. Different tracer particles are applied with solids density representing biomass, biomass char and that of the average bulk. The MPT system yields a spatial accuracy in the order of 10^-3 m and a time resolution of 10^-3 s.For the operational range investigated, three fuel segregation regimes can be identified from the MPT measurements: 1) A flotsam regime which occurs at low fluidization velocities and for low density tracer particles, 2) A transition regime over which an increase in fluidization velocity results in the presence of fuel particles at the bed surface decreases rapidly, and 3) A fully developed mixing regime in which the presence of tracer particle at the bed surface and the splash zone remains constant with fluidization velocity. The transition velocities between the regimes depend on bed height and density of the tracer particle.
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| 248. |
- Köhler, Anna, 1989, et al.
(författare)
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Magnetic tracking of a fuel particle in a fluid-dynamically down-scaled fluidised bed
- 2017
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Ingår i: Fuel Processing Technology. - : Elsevier BV. - 0378-3820. ; 162, s. 147-156
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Tidskriftsartikel (refereegranskat)abstract
- The mixing of a fuel particle in a fluid-dynamically down-scaled bubbling fluidised bed was studied using magnetic particle tracking. Both the resulting steady-state fuel distributions and the underlying mixing dynamics (fuel velocity field) were investigated. The experimental set-up applied resembles the mixing of an anthracite coal particle in a bed with a cross-section of 0.85 × 0.85 m2 operated at 900 °C with fluidisation velocities in the range of 0.16–0.45 m/s and bed heights in the range of 0.25–0.35 m. Four different gas distributors with variable pressure drops and orifice configurations were investigated. For the cases studied, 7.5 min of sampling time at a sampling frequency of 20 Hz was found to be sufficient to resolve the spatial distribution of the tracer. However, to provide a reliable estimate of the mixing dynamics, a sampling frequency of at least 100 Hz was required, together with a sampling time of approximately 20 s. Results on axial mixing showed improved mixing with increasing fluidisation velocity and bed height. The lateral dispersion coefficients were in the order of 10− 3–10− 2 m2/s (on an up-scaled basis), increased with fluidisation velocity, and were only moderately influenced by the configuration of the gas distributor.
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| 249. |
- Köhler, Anna, 1989, et al.
(författare)
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Modeling Axial Mixing of Fuel Particles in the Dense Region of a Fluidized Bed
- 2020
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Ingår i: Energy & Fuels. - : American Chemical Society (ACS). - 1520-5029 .- 0887-0624. ; 34:3, s. 3294-3304
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Tidskriftsartikel (refereegranskat)abstract
- A semiempirical model for the axial mixing of fuel particles in the dense region of a fluidized bed is presented and validated against experimental magnetic particle tracking in a fluid-dynamically downscaled fluidized bed (Köhler et al. Powder Technol., 2017, 316, 492-499) that resembles hot, large-scale conditions. The model divides the bottom region into three mixing zones: a rising bubble wake solid zone, a zone with sinking emulsion solids, and the splash zone above the dense bed. In the emulsion zone, which is crucial for the mixing, the axial motion of the fuel particle is shown to be satisfactorily described by a force balance that applies experimental values from the literature and an apparent emulsion viscosity of Newtonian character. In contrast, the values derived from the literature for key model parameters related to the bubble wake zone (such as the upward velocity of the tracer), which are derived from measurements carried out under cold laboratory-scale conditions, are known to underestimate systematically the measurements relevant to hot large-scale conditions. When applying values measured in a fluid-dynamically downscaled fluidized bed (Köhler et al. Powder Technol., 2017, 316, 492-499), the modeled axial mixing of fuel tracers shows good agreement with the experimental data. © 2020 American Chemical Society.
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| 250. |
- Köhler, Anna, 1989, et al.
(författare)
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Modeling the motion of fuel particles in a fluidized bed
- 2021
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Ingår i: Fuel. - : Elsevier BV. - 0016-2361. ; 305
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Tidskriftsartikel (refereegranskat)abstract
- A semiempirical model for the mixing of fuel particles in a fluidized bed is presented and validated against experimental data from the literature regarding lateral fuel mixing. The model of fuel particle mixing categorizes the fluidized bed into three mixing zones: a rising bubble wake solid zone, an emulsion zone with sinking bulk solids, and a splash zone located above the dense bed. In the emulsion zone, the axial motion of the fuel particle is described by a force balance, applying a viscoplastic stress model, i.e., with a dominant yield stress and only a minor contribution of the shear stress, using an empirical expression from the literature. In the lateral direction, the model is divided into so-called ‘recirculation cells’, which are crucial for the lateral mixing. Comparisons of the modeled and measured lateral dispersion coefficients of different fuel types measured in three different large-scale fluidized bed units under both hot and cold conditions (covering a broad range of coefficients: 10−4–10−1 m2/s) reveal satisfactory agreement. The validated model was used to investigate how the lateral mixing of fuel particles depends on the excess gas velocity, the bed height, and the lateral distribution of bubbles over the bed cross-section (which is typically uneven in industrial FB furnaces), as well as the size and density of the fuel particles.
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