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- Johansson, Robert, 1977, et al.
(author)
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Influence of intra-particle gradients in modelling of fixed bed combustion
- 2006
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In: Thirty-first international symposium on combustion.
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Conference paper (other academic/artistic)abstract
- The porous media approximation, neglecting intra-particle gradients, is extensively used in modelling of fixed bed combustion. The impact of this approximation has been investigated for a batch of biofuels ignited at the top, while combustion air is fed from below. The ignition is followed by a propagation of a reaction front downwards in the bed against the air flow. As the front passes, the fuel is heated, resulting in drying and devolatilisation. Volatile gases are ignited and together with the char formed they burn as long as there is oxygen available, providing heat for propagation of the front. In a bed, where an ignition front propagates counter-current to the air flow, a very steep temperature profile is created, in which the temperature rises from ambient to more than 1200 K. This means that there will be a significant difference in the surface temperature across the height of a single particle as the width of the reaction front in most cases is just a couple of particle diameters. The internal heating of a symmetric particle leads to a two-dimensional problem, since the temperature depends both on the distance from the surface of the particle and on the height in the bed. A bed model using the porous media approximation is implemented in this work. The fuel bed is assumed to consist of spherical fuel particles of the same size and the model treats the gas and solid phases with separate energy equations. Both drying and devolatilisation are modelled with Arrhenius expressions and char conversion is modelled with an effective reaction rate accounting for both diffusion and kinetics. The concentrations of gas species are described with their respective transport equation and the homogeneous reactions are taken as the minimum of a kinetic and a mixing rate. To close the system of equations the continuity equation for the gas phase is used. The result from the porous bed model has been compared with result from the same model, where the intra-particle gradients have been taken into account by a two-dimensional particle model, below referred to as the linked model. The particle model provides information on the internal heating of the solid particles and the internal rate of drying and devolatilisation. The surface temperature of the particles, given by the porous bed model, is the boundary condition for the particle model. The bed and particle model are run in series until a converged solution has been obtained. Fuel particles of sizes between 5-40 mm, consisting of wood, and inlet gas velocities ranging from 0.05 to 0.2 m/s are used. The results show that for a fuel bed consisting of large fuel particles (>2 cm) the porous media approximation clearly influences the release of volatiles and moisture in the bed, but the temperature profile is also affected. The reaction front is wider when internal particle gradients are considered, and this leads to a less steep temperature profile and a release of volatiles in a larger section of the bed height. If gradients in the particles are neglected, volatiles and moisture are released with a high rate in a narrow part of the bed and the temperature profile is steeper. For small particles,
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| 5. |
- Johansson, Robert, 1977, et al.
(author)
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Influence of intraparticle gradients in modeling of fixed bed combustion
- 2007
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In: Combustion and Flame. ; 149, s. 49-62
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Journal article (peer-reviewed)abstract
- The impact of using a porous media approximation, neglecting intraparticle gradients, for modeling fixed bed combustion has been investigated. This has been done by comparing the results from a bed model using the porous media approximation with the results from the same model, where the intraparticle gradients have been taken into account by a two-dimensional particle model. The particle model provides information on the internal heating of the solid particles and the internal rate of drying and devolatilization. The surface temperature of the particles, given by the porous bed model, is the boundary condition for the particle model. Fuel particles of sizes between 5 and 40 mm, consisting of wood, and inlet gas velocities ranging from 0.05 to 0.2 m/s are used. For large bed particles there are considerable differences between the two modeling approaches in the rates of release of volatiles and moisture. The temperature profile in the bed is also affected. For the smaller bed particles in the size range investigated, the differences are small. However, the difference in ignition rate and maximum temperature are small even for the largest particles, compared to uncertainties in the models and scattering of experimental data.
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| 6. |
- Johansson, Robert, 1977, et al.
(author)
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Limitations of the porous medium approximation in modelling of fixed bed combustion
- 2005
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In: Swedish - Finnish flamedays 2005. - 9171781854
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Conference paper (other academic/artistic)abstract
- Most models for combustion of solid fuels in fixed beds treat the bed as a porous medium and neglect gradients within the fuel particles. There is no information about the conditions for which the description of the bed as porous medium is valid. To investigate this, a one-dimensional porous bed model is used together with a two-dimensional particle model. The two models are connected to each other through the surface temperature of the particle which is given by bed simulations. The main parameter is particle size, but the influence of moisture content and inlet gas velocity is also investigated. The results show that only particles smaller than around 5mm at low gas velocities can be treated as a porous medium, without considering internal gradients.
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| 7. |
- Johansson, Robert, 1977, et al.
(author)
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Models for gaseous radiative heat transfer applied to oxy-fuel conditions in boilers
- 2010
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In: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310. ; 53:1-3, s. 220-230
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Journal article (peer-reviewed)abstract
- Models of gas radiation properties have been evaluated for conditions relevant to oxy-fired boilers, characterized by larger pressure path-lengths and possibly different ratios of H2O/CO2 compared to air-fired boilers. Statistical narrow band (SNB) models serve as reference. The other radiation models tested are the weighted-sum-of-grey-gases model, the spectral line-based weighted-sum-of-grey-gases model and two grey-gas approximations. The range of validity of the existing coefficients of the weighted-sum-of-grey-gases model is limited, and new coefficients have therefore been determined to cover the conditions of interest. Several assumed test cases, involving both uniform and non-uniform paths, have been studied to evaluate the accuracy of the models. Comparisons with experimental data are also included. The results show that a grey approximation can give accurate wall fluxes, but at the expense of errors in the radiative source term. The weighted-sum-of-grey-gases model with the new coefficients yields predictions within 20% of those of the reference model in most cases, while the spectral line-based weighted-sum-of-grey-gases model usually gives results within 10%. There are, however, discrepancies between the SNB models at high temperatures. The weighted-sum-of-grey-gases model with its low computational cost is recommended for computationally demanding applications where predictions of both wall fluxes and the radiative source term are important.
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| 8. |
- Johansson, Robert, 1977, et al.
(author)
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Sensitivity Analysis of a Fixed Bed Combustion Model
- 2007
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In: Energy & Fuels. ; 21:3, s. 1493-1503
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Journal article (peer-reviewed)abstract
- The ability to model the combustion of biofuels in a fixed bed is evaluated by a sensitivity analysis. Theanalysis treats the uncertainty of model parameters related to heat transport, reaction rates, and composition ofvolatiles. The scatter of the parameters is estimated from the differences between several published correlations.The results are compared with measurement data and possible model simplifications are discussed. It is shownthat the bed model is able to reproduce the ignition rate and the maximum temperature. Prediction of theseproperties is relatively insensitive to the uncertainty of most of the parameters. Gas concentrations within thebed are more difficult to predict. They are greatly influenced by the composition of the volatile gas releasedduring devolatilization. Also the composition of the volatile gas has a significant influence on the ignition ofthe gas in the model, affecting the ignition rate, particularly at low airflows. Moreover, the investigation showsthat treatment of radiation can be simplified, and the number of gas species included in the model can berestricted without significant losses of model generality.
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| 9. |
- Lundberg, Louise, 1987, et al.
(author)
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A 1-dimensional model of indirect biomass gasification in a dual fluidised bed system
- 2014
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In: 11th International Conference on Fluidized Bed Technology, CFB 2014; Beijing; China; 14 May 2014 through 17 May 2014. ; , s. 607-612
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Conference paper (peer-reviewed)abstract
- A 1-dimensional model is used to analyse how solids material circulation, biomass reactivity and gas mixing affect the char conversion in the Chalmers 2-4 MW indirect biomass gasifier. For the geometry and operational conditions particular to this unit, the model predicts a peak in char conversion for a solids circulation rate of around 3 kg/s. Char conversion is found to increase substantially with biomass reactivity and the level of gas mixing. At the experimental rate of solids circulation (6 kg/s), modelled char conversion values lie between 9% and 39% and are thus quite far from the experimental value of 2% (although potentially ranging between 0 and 10% due to experimental uncertainty). An explanation of the higher char conversion provided by the model could be the uncertainty in the reactivity of the biomass used. A further possible explanation, which has been studied by means of modelling in this work, is the gas mixing. The model uses expressions for the gas mixing which have been derived from measurements in smaller lab units with a high-pressure drop gas distributor, i.e. which induce a better gas mixing than the limited one existing in the large-scale unit studied here, caused by the presence of large bubbles and regions with weak fluidisation.
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| 10. |
- Lundberg, Louise, 1987, et al.
(author)
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A conversion-class model for describing fuel conversion in large-scale fluidized bed units
- 2017
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In: Fuel. - : Elsevier BV. - 0016-2361. ; 197, s. 42-50
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Journal article (peer-reviewed)abstract
- Solid fuel conversion in fluidized beds is often modelled with the use of population balances, where the fuel conversion process is divided into a number of classes based on for example fuel particle size. The present work investigates and evaluates different methods for the discretisation of the fuel conversion process into classes, as well as the number of classes necessary to yield a satisfactory accuracy. A discretisation method, which defines classes based on the conversion degree (rather than size or density) and that is valid for all conversion regimes, is proposed. The results show that application of the proposed class division method for modelling biomass gasification in a fluidized bed gives an accuracy that is up to ten times higher than that given by a distribution with equally large classes. For all three conversion processes of biomass gasification (drying, pyrolysis, and char gasification), discretisation into 6 classes is sufficient to yield errors of around 1%, when compared to the continuous conversion curves given as input to the conversion class discretisation model (generated by a particle model in the present work). In line with this, when the conversion class model is used in a semi-empirical 1D model of indirect biomass gasification, the resulting char conversion in the gasifier does not change significantly when more than 3–6 char conversion classes are used.
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