| 1. |
- Ge, Yaxin, 1992, et al.
(author)
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Effect of fresh bed materials on alkali release and thermogravimetric behavior during straw gasification
- 2023
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In: Fuel. - : Elsevier BV. - 0016-2361 .- 1873-7153. ; 336
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Journal article (peer-reviewed)abstract
- Alkali-associated problems are key issues for the efficient use of straw that is available as a major renewable energy resource worldwide. The effects of six bed materials commonly used in fluidized bed reactors on straw pyrolysis and char gasification were evaluated using online monitoring of alkali release and thermogravimetric analysis. Scanning electron microscopy with energy dispersive spectroscopy was used to determine the elemental composition of the char surface. In the straw pyrolysis stage, alkali release is reduced by the addition of dolomite and silica due to alkali adsorption on the bed materials, and enhanced by the addition of alumina because of its high sodium content. In the char gasification stage, silica, sea sand, olivine, and ilmenite reduce the char reactivity and alkali release, which is attributed to transfer of Si and Ti from the bed materials to the char and reaction with alkali to form stable and catalytically inactive compounds. Alumina also reduces the char conversion rate by transfer of Al to the char and formation of K-Al-Si and Ca-Al-Si compounds, while alkali release from the straw and alumina blend remains high due to the high Na content in alumina. Dolomite initially appears to increase the char gasification reactivity, but the results are affected by conversion of volatile matter that deposited on the dolomite in the straw pyrolysis stage. Dolomite also significantly increases the alkali release, which is attributed to Ca reactions with aluminosilicate compounds that allow potassium to remain in volatile form. Fresh bed materials are concluded to have significant effects on straw conversion depending on their chemical composition, and the results can contribute to the understanding required for efficient use of straw in commercial applications of biomass thermochemical conversion.
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| 2. |
- Ge, Yaxin, 1992, et al.
(author)
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Impacts of fresh bed materials on alkali release and fuel conversion rate during wood pyrolysis and char gasification
- 2023
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In: Fuel. - : Elsevier BV. - 0016-2361 .- 1873-7153. ; 353
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Journal article (peer-reviewed)abstract
- Bed materials provide efficient heat transfer and catalytic function in the thermochemical conversion of biomass, but their interactions with the fuel remain incompletely understood. In this study, the effects of bed materials on alkali release and fuel conversion during wood pyrolysis and CO2 gasification are investigated by online alkali detection combined with thermogravimetric analysis. The investigated bed materials include silica, sea sand, alumina and the natural ores olivine, ilmenite and dolomite. Only dolomite has a significant effect on fuel mass loss and alkali release during wood pyrolysis, while all bed materials influence char reactivity and alkali release during gasification. Sea sand, alumina and dolomite enhance the char gasification during the whole or most of the gasification process, which is related to alkali migration from the bed materials. All bed materials affect char reactivity and alkali release when the conversion approaches completion, and small amounts of some bed materials reduce the alkali release by an order of magnitude. The findings can be understood based on the chemical composition of the different materials. Silicon-rich materials reduce the levels of catalytically active alkali by formation of stable alkali silicates, and a similar explanation applies for ilmenite that captures alkali efficiently. Magnesium and calcium in contrast promote alkali release through their influence on alkali silicate chemistry. Analysis of char surfaces using scanning electron microscopy with energy dispersive spectroscopy indicates that low amounts of several elements are transferred from the bed material to the char where they may be directly involved in the char conversion process. The transferred elements are specific for each bed material and relates to their chemical composition. Mechanisms for material exchange between bed material and char are discussed.
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| 3. |
- Johnsson, Filip, 1960, et al.
(author)
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Characteristics of the formation of particle wall-layers in CFB boilers
- 1995
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In: Proc of the 2nd International Conference on Multiphase Flow. ; 3, s. FB1-25-FB1-32-
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Conference paper (peer-reviewed)abstract
- The character of the vertical two-phase flow in circulating fluidized bed (CFB)boiler furnaces is compared with results from small CFB risers presented in literature. The particle flow in furnaces as well as in the smaller risers shows a core/wall-layer structure. In the CFB boilers the particle flow pattern develops up through the furnace with the major back-flow caused by separation to the furnace walls. Both the particle wall-layer thickness and the downward flux within this layer increase downwards along the furnace walls. In the smaller units this downward increase is either absent or much smaller, a consequence of a more developed particle flow profile, which, typically, is parabolic in shape.
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| 4. |
- Johnsson, Filip, 1960, et al.
(author)
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Optical and momentum probe measurements in a CFB boiler
- 1996
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In: Proc of the 5th International Conference on Circulating Fluidized Bed. ; , s. 652-657
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Conference paper (peer-reviewed)abstract
- Methods for the characterization of the two-phase flow in the transport zone of circulating fluidized bed (CFB) boilers are discussed based on measurements of fluctuations in local solids concentration and momentum flux using an optimum probe and a momentum probe. Both probes were designed to withstand the environment inside a furnace during combustion. The dynamic information on the flow obtained by the two probes was in agreement. The time-resolved measurements in the core region as well as in the wall layer by continuous increases and decreases in solids concentration and momentum flow from a base level at low solids concentration.
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| 5. |
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| 6. |
- Zhang, Wennan, 1962, et al.
(author)
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Characteristics of the Lateral Particle Distribution in Circulating Fluidized Red Boilers
- 1993
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In: Proc of the 4th International Conference on Circulating Fluidized Beds. ; , s. 266-273
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Conference paper (peer-reviewed)abstract
- The local time-averaged solids volume fraction was measured in the 12MW~1 Circulating Ruidized Bed (CFR) boiler at Chalmers University of Technology. It was found that the corelwall-layer structure of the flow pattern observed in small-scale beds also characterizes large-scale CFR boilers. The local solids volume fractions close to the wall and in the core region of the furnace were found to be directly proportional to the cross-sectional average solids volume fraetjon, c, as ohtained from pressure rneasurements. If the shape of the membrane-tube wall is taken into account, the relationship for the solids fraction close to the wall (related to a plane wall) is c~=3.6c, which is similar to that reported in literature for plain walls.
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| 7. |
- Zhang, Wennan, 1962, et al.
(author)
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Fluid-dynamic boundary layers in CFB boilers
- 1995
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In: Chemical Engineering Science. - 0009-2509. ; 50:2, s. 201-210
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Journal article (peer-reviewed)abstract
- The characteristics of the fluid-dynamic boundary layer (FBL) were studied in the 12 MW circulating fluidized bed boiler at Chalmers University of Technology. For a comparison, some additional measurements were made in a 165 MW CFB boiler. The FBL consists mainly of the falling film of particles at the furnace wall. The distance from the wall to the zero net particle flux position is used as a measure of the thickness of the layer. Horizontal profiles of particle flux, velocity and concentration were measured in the transport zone of the furnace. It was found that the FBL thickness varies only slightly or not at all with particle concentration, gas velocity and height in the furnace. An estimation of the thickness is given as a function of the horizontal bed dimension. The particle falling velocity inside the FBL is about 0.9–1.6 m/s depending on location. There is a sharp increase in particle concentration in the FBL all the way towards the wall. The disparity between the particle flux profile normal to a plain refractory-lined wall and the corresponding profile normal to a membrane-tube wall can be compensated for by taking the fin-tube configuration into account. From this point of view, the particle redistribution mechanism at the membrane-tube wall is discussed
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| 8. |
- Zhang, Wennan, 1962
(author)
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Fluid Dynamics of the Transport Zone of Circulating Fluidized Beds - with Application to Boilers
- 1995
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Doctoral thesis (other academic/artistic)abstract
- The particle suspension flow structure in circulating fluidized beds (CFB) was studied with emphasis on the layer of downflowing particles at the wall. The main experiments were carried out in the furnace of the 12 MWth CFB boiler of 1.7x1.7 m cross-section, and in a two-dimensional CFB cold bed of 0.1x0.7 m cross-section at Chalmers University of Technology. Optical-fibre probes were used in determining the local, time-averaged, particle concentration in CFB cold beds and in studying the time-dependent characteristics of the wall-layer in the boiler furnace. In view of the rough environment in the furnace, a particle flux probe and a suspension flow momentum probe were developed to determine the local particle velocity and concentration. A detailed theoretical analysis on the momentum probe is given. The average particle concentration, based on an integration of the concentration profile measured by the probes, is consistent with that obtained by pressure-drop measurements. The flux probe works through non-isokinetic sampling. It is shown to give reliable results by a good mass flow balance in the cold bed. It is found that the core/annulus structure of the flow pattern found in many small-scale beds with a circular cross-section is also characteristic of large-scale CFB boilers, but is more pronounced. There is a strong particle downflow in the region close to the furnace walls with the falling velocity about 0.9-1.6 m/s. The distribution of particle velocity and concentration is flat over the core region. The particle flow pattern is developed in small-scale risers but is developing both down along the walls and up through the core in the large-scale CFBs. This situation is described by a simple mathematical model which is focused on a coefficient of mass transfer from the core to the wall. The expressions of the particle downflow at the wall, the local and average concentrations, as well as the wall-layer thickness as a function of the bed height, were obtained from this model. The local particle concentration in the transport zone, i.e. the main extension of a CFB, is found to be directly proportional to bed bulk density as obtained by pressure drop measurements. Empirical correlations describing this self-similarity relation are obtained. The particle flux measurements show that the downflow is unevenly distributed around the bounding walls. In the rectangular cold bed, most particles tend to go down and accumulate along the two wide walls. In the furnace of the boiler, the strongest downflow was detected in the corners. It was found that the formation and development of the particle wall-layer are closely related to the exit configuration. The wall-layer at lower levels of the furnace is strongly affected by the secondary air penetration. The configuration of the boiler membrane-tube wall enhances the particle accumulation in the region close to the fins between the tubes of the membrane-tube wall. A diagnostic measurement on the fluctuation characteristics in the boiler furnace was carried out by simultaneously sampling the signals of both the local fluctuation of particle downflow and the global fluctuation of pressure drops in the upper zone, in the bottom zone and from the air plenum to atmosphere. Spectrum and probability analysis on the signals were performed, which give a detailed insight into the time-dependent particle flow pattern and fluid-dynamics in the furnace of CFB boilers.
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| 9. |
- Zhang, Wennan, 1962, et al.
(author)
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Momentum probe and sampling probe for measurement of particle flow properties in CFB boilers
- 1997
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In: Chemical Engineering Science. - 0009-2509. ; 52:4, s. 497-509
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Journal article (peer-reviewed)abstract
- With the aim of making local particle flow structure studies in commercial scalecirculating fluidized-bed (CFB) boilers, a momentum probe and a non-isokinetic particlesampling probe have been designed to measure local momentum flux of gas-solids flow andparticle flux. The time-averaged, local particle velocity and concentration are deduced from themeasurement results of these two probes. The two probes were calibrated and validated in situin a CFB boiler furnace. In order to better understand the principle of the momentum probeand to ensure reliable measurements, a theoretical model is established by focusing on theparticle deceleration movement inside the probe tip. This model is able to predict the relationshipbetween the gas-solids momentum flux and the impact pressure as obtained from thecalibration.
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