| 1. |
- Djerf, Tove, 1989, et al.
(författare)
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Bottom-bed fluid dynamics – Influence on solids entrainment
- 2018
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Ingår i: Fuel Processing Technology. - : Elsevier BV. - 0378-3820. ; 173, s. 112-118
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Tidskriftsartikel (refereegranskat)abstract
- In CFB boilers, the solids concentration along the riser and the external solids circulation are important design parameters. This work provides an experimental investigation on how the solids entrainment from the bottom region of a CFB riser is influenced by the fluidization conditions, in particular if there is a bottom bed or not. Measurements are carried out in a CFB riser with a height of 3 m and 0.45 m 2 in cross section. The solids inventory consists of glass spheres with a mean size of 112 μm, employing fluidization velocities up to 1.4 m/s and riser pressure drops in the range 0.15–1.5 kPa. The vertical distribution of solids concentration is determined through pressure drop measurements along the riser height. The external solids circulation is measured with a valve system in the return leg. The results show that the bottom region conditions govern how operational parameters influence the characteristics of the solids entrainment from the bottom. The vertical extension of the splash zone above the dense bed depends strongly on the dense bed height. In the absence of a dense bed, a bottom region with strong solids back-mixing is established which has similarities with the splash region.
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| 2. |
- Djerf, Tove, 1989, et al.
(författare)
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Bottom-bed fluid dynamics - Influence on solids entrainment
- 2017
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Ingår i: 12th International Conference on Fluidized Bed Technology, CFB 2017. ; 2017, s. 183-190
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Konferensbidrag (refereegranskat)abstract
- In CFB boilers, the solids concentration along the riser and the external solids circulation are important design parameters, mainly in terms of the heat balance but also influencing the risk of wear on heat transfer surfaces. This work investigates experimentally how the amount of solids entrained from the bottom region of a CFB riser is influenced by the fluidization conditions, including the presence or absence of a dense bottom bed. The paper presents first measurements in a new cold lab-scale unit (3 m tall, 0.45 m2in cross section), which is a scale model of a large utility boiler. The solids inventory consists of glass spheres with a mean size of 112 µm. The operational range covers fluidization velocities between 0.1 and 1.4 m/s and riser pressure drops between 0.2 and 1.5 kPa. The vertical distribution of solids concentration is determined through pressure drop measurements between densely spaced pressure taps (15 in total) along the riser height. The external solids circulation is measured with an automatic valve system in the return leg. The results show that the presence or absence of a dense bed govern how operational parameters influence the characteristics of the solids entrainment from the bottom region. The vertical extension of the splash zone above the dense bed depends strongly on the dense bed height. In the absence of a dense bed, a bottom region with strong solids back-mixing is established which has similarities with the splash region.
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| 3. |
- Djerf, Tove, 1989, et al.
(författare)
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Solids back-mixing in the transport zone of circulating fluidized bed boilers
- 2022
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Ingår i: Chemical Engineering Journal. - : Elsevier BV. - 1385-8947. ; 428
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Tidskriftsartikel (refereegranskat)abstract
- This work investigates the back-mixing of solids in the transport zone of large-scale circulating fluidized bed (CFB) boilers, with the aims of identifying and evaluating the governing mechanisms and providing a mathematical description based on a solid theoretical background rather than on purely empirical correlations. In addition, transient Direct Numerical Simulation (DNS) modeling is used to identify the mechanism that drives migration of the solids from the dilute up-flow in the core region to the down-flow at the furnace walls. Previously published concentration and pressure profiles are collated and analyzed through modeling of the steady-state mass balance of the dispersed solids in the transport zone. The study shows that solids back-mixing at the furnace wall layers is limited (hence governed) by the core-to-wall layer mass transfer transport mechanism rather than by the lateral movement of solids within the core region. The latter is shown by the 3-dimensional (3D) mass balance model, and the transient DNS modeling indicates that this is due to a turbophoresis mechanism. We also show that the use of Pe-numbers to describe the lateral solids dispersion is not straightforward but rather depends on the unit scale, and that Pe-numbers < 26 are needed to yield the solids back-mixing rates measured in large-scale CFB boilers. Finally, we propose a mathematical expression for the core-to-wall layer mass transfer coefficient derived from a Sherwood number (Sh)-correlation fitted to measured values of the characteristic decay constant that result from the solids back-mixing. This expression shows better agreement with the large-scale measurements than do the expressions given in the literature.
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| 4. |
- Djerf, Tove, 1989, et al.
(författare)
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Solids backmixing and entrainment in the splash zone of large-scale fluidized bed boilers
- 2022
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Ingår i: Powder Technology. - : Elsevier BV. - 1873-328X .- 0032-5910. ; 404
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Tidskriftsartikel (refereegranskat)abstract
- This work studies the fluid dynamics of the solids in the splash zone of fluidized bed furnaces, with focus set on solids back-mixing and solids entrainment in order to enhance the understanding and prediction of the solids flow in the bottom region of the furnace. Experimental results show the establishment of a splash zone also for runs in absence of a dense bottom bed. A simple model assuming ballistic trajectories of the ejected solids is shown to satisfactorily estimate the solids back-mixing rate. The flux of non-backmixed solids, which are entrained from the bottom region, is found to be unaffected by the bottom wall configuration (tapered/vertical) for a given gas flow. Finally, an empirical expression is proposed for the solids entrainment from the bottom region which covers wide operational and unit size ranges.
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| 5. |
- Djerf, Tove, 1989, et al.
(författare)
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Solids circulation in circulating fluidized beds with low riser aspect ratio and varying total solids inventory
- 2016
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Ingår i: Fluidization XV (2016), Quebec, Canada.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- This paper presents an experimental study with the aim tounderstand the relation between the flow conditions - the riser pressure drop and fluidization velocity - in a CFB riser and the net (external)solids flux (Gs [kg/m2s]), applying a riser geometry and overall flow conditions similar to CFB boilers.The experiments are carried out in a CFB unit operated under ambient conditions. The riser has a cross section of 0.7 m x 0.12 m and a height of 8.5 m, yielding a riser height-to-width aspect ratio of 10.6 (in the wide dimension), similar to that of CFB boilers. The unit is equipped with densely spaced pressure taps providing a fine resolution of the measured vertical pressure profile along the riser and an automatic system to accurately measure Gs. The experiments cover fluidization velocities of 0.3-7 m/s, riser pressure drops of 1.7-10.5 kPa and loopseal fluidization velocities of 0.12-0.54 m/s (secondary air flows are not considered). These ranges correspond to conditions both with and without a dense bottom region.The results show that Gs is determined by the solids concentration at the riser top, which depends riser pressure drop and fluidization velocity, and the backflow effect, which depends on the configuration and flow conditions of the loop seal and the exit region. For operating conditions with a dense bottom bed present, Gs is independent of riser pressure drop, whereas when operating without a dense bed an increase in riser pressure drop yields an increase in Gs.
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| 6. |
- Djerf, Tove, 1989, et al.
(författare)
-
Solids circulation in circulating fluidized beds with low riser aspect ratio and varying total solids inventory
- 2017
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Ingår i: Powder Technology. - : Elsevier BV. - 1873-328X .- 0032-5910. ; 316, s. 670-676
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents an experimental study with the aim to understand the relation between the flow conditions - the riser pressure drop and fluidization velocity - in a CFB riser and the net (external) solids flux (Gs [kg/m(2) s]), applying a riser geometry and overall flow conditions similar to CFB boilers. The experiments are carried out in a CFB unit operated under ambient conditions. The riser has a cross section of 0.7 m x 0.12 m and a height of 8.5 m, yielding a riser height-to-width aspect ratio of 10.6 (in the wide dimension), similar to that of CFB boilers. The unit is equipped with densely spaced pressure taps providing a fine resolution of the measured vertical pressure profile along the riser and an automatic system to accurately measure G5. The experiments cover fluidization velocities of 03-7 m/s, riser pressure drops of 1.7-10.5 kPa and loop-seal fluidization velocities of 0.12-0.54 m/s (secondary air flow is not included). These ranges correspond to conditions both with and without a dense bottom region. The results show that G(s) is determined by the solids concentration at the riser top, which in turn depends on riser pressure drop and fluidization velocity, and the backflow effect, which depends on the configuration and flow conditions of the loop seal and the exit region. For operating conditions with a dense bottom bed present, G(s) is independent of riser pressure drop at any fluidization velocity, whereas when operating without a dense bed an increase in riser pressure drop increases G(s).
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| 7. |
- Djerf, Tove, 1989
(författare)
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Solids Flow in Large-Scale Circulating Fluidized Bed Furnaces
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The flow pattern of solids established in large-scale circulating fluidized bed (CFB) furnaces is of great importance for the performance of commercial CFB boilers, as it governs the heat transfer and mixing of the fuel and any other reactive solids. The solids flow pattern in the riser is crucial for the design and scaling up of large-scale CFB technologies for the thermochemical conversion of solids. The aim of this work is to acquire new knowledge and understanding of the solids flow patterns in CFBs that are representative of large-scale furnaces. The goal is to improve the reliability of predictive modeling tools and, thereby expand the development of new and existing CFB technologies within the energy field. The solids flow of a CFB furnace is characterized by a bottom region with a high concentration of solids, a splash zone with strong solids back-mixing, and a transport zone that covers the major height of the furnace and has a lower level of solids back-mixing, from the bottom and upwards. This thesis uses experimental campaigns and various modeling tools to elucidate the CFB solids flow. The experimental work is carried out in two cold units: a pseudo-2-dimensional unit that allows visual observation of the flow; and a 3-dimensional unit that can be operated under fluid-dynamical scaling, which has been shown to reflect accurately the solids flow in an existing reference >200-MWth CFB boiler. Furthermore, the data derived from the different sizes and operational ranges of these experimental units are linked to previous measurements of large-scale CFB combustion. Examinations of the solids back-mixing phenomena are supported by different modeling tools, including Direct Number Simulations, semi-empirical modeling through the Finite Volume Method, and Monte Carlo modeling. The results of this work show that: (i) the presence/absence of a dense bottom bed affects the extent of solids entrainment from the bottom region; (ii) a fluid-dynamical region similar to the splash zone is established even in the absence of a dense bottom bed; (iii) the rate of solids back-mixing in the splash zone can be predicted from modeling of the gravity-driven ballistic trajectories; (iv) the solids back-mixing in the transport zone is governed by the transfer of solids through the core-wall layer boundary, which is driven by turbophoresis (i.e., the migration of particles in the direction of increasing particle concentration), and for which a Sherwood number-based expression is proposed that improves on the former empirical expressions; and (v) the solids back-flow effect at the riser exit cannot generally be neglected when predicting the in-furnace back-flow, and is substantial at gas velocities that are typical for commercial CFB boilers. Validated expressions are proposed for the decay coefficients of the splash and transport zones and the solids entrainment from the bottom region. Taking together this collected knowledge, this thesis improves the reliability of semi-empirical modeling tools for the prediction of the solids flow patterns in large-scale CFB furnaces for a wide range of operational conditions.
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| 8. |
- Djerf, Tove, 1989, et al.
(författare)
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Solids flow patterns in large-scale circulating fluidised bed boilers: Experimental evaluation under fluid-dynamically down-scaled conditions
- 2021
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Ingår i: Chemical Engineering Science. - : Elsevier BV. - 0009-2509. ; 231
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Tidskriftsartikel (refereegranskat)abstract
- This work aims at gaining novel knowledge of the mechanisms governing the solids flow pattern in the furnace of large-scale Circulating Fluidised Bed (CFB) boilers. A fluid-dynamically down-scaled unit resembling an existing 200-MWth CFB boiler was built and validated against full-scale data. The extensive experimental campaign showed, among others, that the presence or absence of a dense bed governs the entrainment of solids from the bottom region of the furnace, and that the back-flow of solids at the exit region is negligible at low gas velocities although it quickly becomes significant with an increase in gas velocity. Thus, it is shown that the estimation of the external solids flux by the top flux in the furnace is not generally valid.
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| 9. |
- Djerf, Tove, 1989
(författare)
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Solids flows in circulating fluidized beds: explorations of phenomena with applications to boilers
- 2020
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The circulating fluidized bed (CFB), which is a technology that is commonly used in the heat and power sector, efficiently converts renewable and/or and low-grade solid fuels, such as biomass and biogenic waste fractions. CFB units serve also as a technological platform for carbon capture processes (combustion by oxy-firing or chemical looping), which are foreseen to play a key role in the transition of the energy system towards decreased atmospheric CO2 emissions. However, the development of commercial CFB boilers is limited by key gaps in the knowledge, one of which is the solids flow pattern, which is not sufficiently understood even though it is an important phenomenon that governs both mass and heat transfers in the system. This thesis aims to elucidate the solids flow patterns in CFB units that are representative of large-scale boilers. More specifically, the objectives are to identify and characterize the interlinked underlying phenomena that govern the solids flow pattern in the riser and the external circulation of solids. The specific phenomena studied here are: depletion of the dense bed; solids entrainment from the bottom region; the back-mixing in the splash and transport zones; and the riser exit backflow. Also examined is how these factors are affected by the unit size. For these purposes we conducted experimental analyses in two units: a pseudo-2-dimensional unit; and a fluid-dynamically down-scaled unit (in which studies were carried out with and without scaled bed material). Validation of the cold-flow scale model with scaled material shows very good similarity between the concentration profiles obtained from the cold-flow scale model and from the large-scale (>200-MWth) CFB reference boiler. The results show that the presence/absence of a dense bed affects the entrainment of particles from the bottom region into the freeboard. The expansion of the splash zone immediately above the dense bed is affected by the dense bed height due to the modified bubble growth. The solids back-mixing from the core region to the wall layers is mostly affected by the gas velocity and the cross-sectional geometry of the riser. Finally, the external circulation of solids is shown to be non-equal the upwards solids flux at the top of the riser, with the exceptions of cases with very low gas velocities, revealing significant local back-mixing of solids at the furnace exit at nominal loads.
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| 10. |
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