| 1. |
- Yang, Changwon, et al.
(författare)
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One-dimensional numerical simulation of CFB combustion: Improvement of hydrodynamic models for accurate prediction of pressure and particle size distributions
- 2023
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Ingår i: Fuel. - 0016-2361. ; 346
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Tidskriftsartikel (refereegranskat)abstract
- This study aims to improve the accuracy of one-dimensional numerical simulations of circulating fluidized bed (CFB) boilers by evaluating models related to solid properties, such as attrition, fragmentation, cyclone efficiency, and bed material discharge. The study validates models for estimating solids holdup and particle size distribution (PSD) using experimental results from a 12 MWth CFB boiler. The study found that accurate prediction of solid axial holdup and local PSD requires proper consideration of attrition and fragmentation, and the simulation results show that attrition has a more significant effect on numerical analysis performance than fragmentation. By using appropriate values for sand attrition constant (5E-7), char attrition constant (6E-6), and coal:1.5, sand:1.2, and char(ash):1.2 fragmentation constants, the pressure distribution measured from experiments was best predicted. The simulation successfully predicted the particle PSD distribution at a specific location in the CFB boiler and pressure change by height, similar to the experimental results In addition, this study investigated the accuracy of 3D computational fluid dynamics (CFD) simulations using solid properties obtained through 1D simulation as initial conditions compared to those based on experimental conditions. The results revealed that 3D CFD simulations using the solid properties from 1D numerical analysis are more accurate, implying that 1D-3D co-simulation approach is a good strategy to increase the simulation performance.
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| 2. |
- Gómez Barea, Alberto, et al.
(författare)
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Modeling of biomass gasification in fluidized bed
- 2010
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Ingår i: Progress in Energy and Combustion Science. - : Elsevier BV. - 0360-1285. ; 36:4, s. 444-509
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Tidskriftsartikel (refereegranskat)abstract
- Modeling of biomass gasification in bubbling and circulating fluidized bed (FB) gasifiers is reviewed. Approaches applied for reactor modeling, from black-box models to computational fluid-dynamic models, are described. Special attention is paid to comprehensive fluidization models, where semi-empirical correlations are used to simplify the fluid-dynamics. The conversion of single fuel particles, char, and gas is examined in detail. The most relevant phenomena to be considered in modeling of FB biomass gasifiers are outlined, and the need for further investigation is identified. An updated survey of published mathematical reactor models for biomass and waste gasification in FB is presented. The overall conclusion is that most of the FB biomass gasification models fit reasonably well experiments selected for validation, despite the various formulations and input data. However, there are few measurements available for comparison with detailed model results. Also, validation of models with data from full-scale FB biomass gasification units remains to be done.
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| 3. |
- Leckner, Bo G, 1936, et al.
(författare)
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Emissions from co-combustion of coal, wood and sludge in CFB
- 2003
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Ingår i: Fluidization 2003, Proceedings of the 8th China-Japan Symposium on Fluidization, Gifu Japan, December 3-5, 2003. ; 8, s. 28-35
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Konferensbidrag (refereegranskat)abstract
- This paper reports on gaseous emission from co-combustion of sewage sludge with coal or wood in circulating fluidised bed (CFB). Dried, or in some cases "wet" (mechanically dewatered), municipal sewage sludge was investigated. The fuels coal and wood were used in contrast to each other. The emissions of CO, NO, N2O, SO2 and HCl were measured from various sludge fractions and air supply methods in two CFB combustion units: the 12 MWth boiler at Chalmers University of Technology and the laboratory scale unit at the Technical University Hamburg-Harburg. The two units were scaled according to approximate scaling rules and give comparable emissions. It was shown that the emissions were moderate despite the considerable quantities of fuel nitrogen and sulphur in the sludge. For commercially interesting sludge fractions (
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| 4. |
- Lyngfelt, Anders, 1955, et al.
(författare)
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Dependence of Sulphur Capture Performance on Air Staging in a 12 MW Circulating Fluidised Bed Boiler
- 1993
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Ingår i: Gas Cleaning at High Temperatures, Eds. R. Clift and J.P.K. Seville, Blackie Academic & Professional, Glasgow, ISBN 0 7514 0178 1.. - 0751401781 ; , s. 470-491
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- Three cases of air staging were examined in a 12 MW circulating fluidised bed boiler: i) no staging, ii} normal staging and iii) intensified staging. The conditions inside the combustion chamber were investigated by zirconia cell measurements of the oxygen partial pressure, 0.35, 0.65 and 8 m above the bottom air distributor plate. A significant effect of the degree of staging was seen in the two lower locations: At 0.65 m height the fraction of time under substoichiometric conditions was low in the no-staging case (2-35%), at normal staging it was 70-90%, whereas at intensified staging it was 100Y.. At 0.35 m height, i.e. in the dense bed, a similar effect was seen, although the fraction of time under reducing conditions was lower. The fraction of time under reducing conditions was low in the top of the combustion chamber in all three cases . The increase in the fraction of time under reducing conditions with a higher degree of staging is associated with a decrease in sulphur capture. It is assumed that a release of SO2 from CaSO4 takes place during the transitions between oxidising and reducing conditions. Thus, the rapid alternations between oxidising and reducing conditions, as seen with the zirconia cell, offer an explanation of the reductive decomposition and, accordingly, of the dependence of sulphur capture on temperature and on the extent of staging.
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| 5. |
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| 6. |
- Leckner, Bo G, 1936
(författare)
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Fluid dynamic regimes in circulating fluidized bed boilers—A mini-review
- 2022
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Ingår i: Chemical Engineering Science. - : Elsevier BV. - 0009-2509. ; 247
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Tidskriftsartikel (refereegranskat)abstract
- The fluid dynamics in the furnaces of large-scale circulating fluidized bed (CFB) boilers are surprisingly little known in contrast to the many laboratory studies made on conditions related to chemical reactors. Two areas are surveyed in the present work: the bottom bed and the upper dilute zone of a furnace. The bottom bed is considered bubbling, but the general opinion is that either it does not exist, or it is turbulent. The flow in the upper furnace is dilute phase transport, judging from regime maps, showing that the state of the flow is outside of the range of fast fluidization. However, this is also not generally accepted. Usually, the regime of fluidization in CFB boilers is said to be fast fluidization. In one work it is considered fast fluidization even though the authors agree that it is different from the general definition. In another investigation it is called entrained flow. Here, the conclusion is that the diversity of opinions should be resolved by further investigations with the aim of defining the conditions for the fluidized flow in furnaces, including the influence of particle size and density, fluidization velocity, gas properties, and effects from the furnace dimensions, if any.
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| 7. |
- Leckner, Bo G, 1936
(författare)
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Fluidization Characteristics of Circulating Fluidized Bed Boilers
- 2023
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Ingår i: Chemie-Ingenieur-Technik. - : Wiley. - 0009-286X .- 1522-2640. ; 95:1-2, s. 32-39
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Forskningsöversikt (refereegranskat)abstract
- The fluidization in the furnace of circulating fluidized bed (CFB) boilers is described. Data have been obtained from a 12 MWth CFB boiler and from literature. The bottom bed is bubbling rather than fast or turbulently fluidized as is often assumed. There is an extended splash zone above this bed, in which particles are thrown up by the bubbles and fall back onto the bottom bed. Other particles are carried further into the transport zone, initially as a saturated gas flow, but some particles move into the wall layers and the density decays towards the exit. The density of the transport zone is low, and the circulation rate is relatively seen small. Therefore, this flow is in a state of dilute-phase transport and not in the fast fluidization regime, such as is often claimed.
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| 8. |
- Suárez-Almeida, M., et al.
(författare)
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Fluid dynamic analysis of dual fluidized bed gasifier for solar applications
- 2021
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Ingår i: Powder Technology. - : Elsevier BV. - 1873-328X .- 0032-5910. ; 390, s. 482-495
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Tidskriftsartikel (refereegranskat)abstract
- A hydrodynamic model of a dual fluidized bed gasifier (DFBG) is developed and its predictions are compared with measurements of solids flux and pressure profiles from a cold flow model (CFM). Then, the performance of a DFBG gasifier is theoretically analyzed in terms of solids circulation and solids distribution under changes in riser and loop seal aeration, solids inventory and particle size, and a sensitivity analysis is made to delimit the model prediction capability. Furthermore, the model is applied to analyze the effects of key design aspects of DFBG, such as the relative size of riser and gasifier, the connection between both units, the circulation rate of solids and their distribution around the system. The model is further used to extend the DFBG operation with external solar energy carried by heated solid particles, i.e. to design solar DFBG (SDFBG). The analysis is focused on the performance with high solids inventory in the gasifier to increase the char conversion (operation with a large solar share) and the control of solids circulation to meet the heat demand of the gasifier with the availability of solar energy. The operation with large solids inventory in the gasifier requires the size of the gasifier to increase considerably compared to that of the conventional DFBG. The substitution of the connection pipe between the riser and the bubbling bed (current design in commercial DFBG) by a lower loop seal enables better control of the solids circulation, thus, benefiting the solar design.
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| 9. |
- Wu, Wanqiang, 1992, et al.
(författare)
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Solids separation efficiency at the outlet of a circulating fluidized bed riser
- 2023
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Ingår i: Powder Technology. - 1873-328X .- 0032-5910. ; 428
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Tidskriftsartikel (refereegranskat)abstract
- In a circulating fluidized bed (CFB) fuel converter the outlet geometry plays a decisive role for the solids flow in the circulating loop, thus affecting the mass and heat balances and the conversion process. A Dense Discrete Particle Model (DDPM) framework is used to investigate the solids flow, with model validation against experimental data from a cold model, scaled to represent the fluid dynamics of a commercial CFB boiler operated under hot conditions. Riser outlets with different configurations (L-shape and T-shape) are studied. The results show that the solids separation efficiency of the outlet (and, hence, of the riser) can be related to the Stokes number (Stk, which varied within 0.034–1.24). Decreased outlet area at a given riser cross-section and/or increased distance of the exit window from the top of the riser yield a higher solids separation efficiency. The smaller the Stk, the lower the solids separation in general, and the less it will be affected by the geometrical configuration. The solids flux value did not show any major impact on the solids separation efficiency attained. Finally, an expression for the solids separation efficiency of the riser outlet is derived, covering a range of efficiencies within 0.3–0.9 and yielding an average error of <20% compared to experimental data from literature.
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| 10. |
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