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- Pallarès, David, 1975, et al.
(författare)
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Modeling of the heat transfer in large-scale fluidized bed furnaces
- 2012
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Ingår i: 21st International Conference on Fluidized Bed Combustion, Naples (Italy).
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Konferensbidrag (refereegranskat)abstract
- A 3-dimensional model for the heat transfer in the furnace of a fluidized bed boiler is presented. The model, which is part of a comprehensive modeling work for large-scale CFB boilers, describes separately the convective and radiative heat transfer mechanisms at different heat extraction surfaces in the furnace (waterwalls, wing walls and division walls). The focus of the paper is on the heat transfer in the furnace, but the heat balance closure at a unit level including the return leg is also treated.Modeled data are compared to measurements from CFB boilers at two different scales: in the Chalmers 12 MWth research boiler and in a large scale boiler of about 300 MWth. Modeled and measured data generally show a good agreement.Since convective heat extraction depends strongly on the local properties of the solids flow, 3D modeling of the convective heat transfer requires other expressions than those found in literature (which are typically. based on cross-sectional averaged solids concentration). In regions with low solids concentration (upper part of furnace), the radiative heat transfer is significantly influenced from regions several meters away from the heat extraction surface. Thus, in the description of the radiative heat transfer, optical factors accounting for the absorption in the gas-solids suspension are used. The model results reveal the importance of the exchange of radiative heat between the upflowing core and the downflowing wall layers. In addition, the importance of the fluid dynamics (wall layer flow properties, local solids flow properties such as the backflow effect and corner effects,) on the heat transfer is discussed with help of the model presented.
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| 4. |
- Seddighi, Sadegh, 1977, et al.
(författare)
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Assessment of Oxyfuel Circulating Fluidized Bed Boilers – Modeling and Experiments in a 5 MW Pilot Plant
- 2011
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Ingår i: 2nd International Oxyfuel Combustion Conference, International Energy Agency Greenhouse Gas R&D Programme.
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Konferensbidrag (refereegranskat)abstract
- As in air-firing, oxy-fired combustion using the Circulating Fluidized Bed (CFB) technology may offer advantages in that the CFB technology provides high fuel flexibility, in-furnace reduction of SOx emissions and a relatively smooth distribution of the extracted heat flux. Furthermore, the thermal flywheel induced by the solids flow and the possibility of heat extraction in the solids recirculation system (i.e. outside the furnace) represent a potential to achieve high oxygen concentrations while limiting the temperature level. Allowing such higher oxygen concentrations would imply significantly more compact (and thus less costly) furnaces than those used in air combustion.The present work summarizes the current status of an ongoing project which has the aim to assess the oxyfuel CFB technology by means of pilot testing and modeling. The pilot testing is carried out in a 5 MW oxyfuel CFB pilot plant (representing the largest oxy-fired CFB unit running at present date). The model is an extended version of a model for air-fired CFB combustion developed over the last six years by the authors and validated for utility-scale units.Considering that the model has previously undergone an extensive validation against data from air-fired large-scale boilers, the model is expected to represent a useful tool for the assessment, design and scale-up of oxyfuel boilers once validated with measurements from the 5 MW oxyfuel unit,. An obvious application of the model is in the dimensioning and designing heat extraction surfaces. High oxygen concentrations require more heat extraction in the solids recirculation system, which is allowed by means of increased external circulation of solids. Such increased solids circulation can be established with modifications to the furnace design and a change in operating conditions and/or solids properties.
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| 5. |
- Seddighi, Sadegh, 1977, et al.
(författare)
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Heat transfer in a 4 MWth circulating fluidized bed furnace operated under oxy-fired and air-fired conditions: modeling and measurements
- 2015
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Ingår i: International Journal of Greenhouse Gas Control. - : Elsevier BV. - 1750-5836. ; 37, s. 267-273
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Tidskriftsartikel (refereegranskat)abstract
- Heat transfer to wall panels in the furnace of a circulating fluidized bed (CFB) is investigated by means of a 1.5-dimensional mathematical model together with measurements from a 4-MWth CFB unit under air and oxy-fuel conditions. The conditions at the wall panels correspond to solids concentration between 0.35 and 18 kg/m3 and temperatures between 1054 and 1168 K. The heat transfer coefficient to the wall panels is similar in oxy-fuel and air-firing due to that the solids flow, which plays the main role in CFB furnace heat transfer, was kept similar in air and oxyfuel conditions. The modeled furnace heat extraction and in-furnace vertical profiles of temperature and solids concentration show generally good agreement with the corresponding measured values. Modeling results show that for all cases studied, the share of radiation in the total furnace heat extraction exceeds 70% and increases with the increase in furnace temperature. Modeling results also show that gas radiation has a small influence on furnace heat extraction.
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