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Assessment on the impact of including hydrodynamics on the performance of kinetic based models for bubbling fluidized bed gasifiers
Mälardalen University, School of Business, Society and Engineering, Future Energy Center.ORCID iD: 0000-0002-0895-8286
(English)Manuscript (preprint) (Other academic)
Abstract [en]

One of the approaches to model fluidized bed gasifiers is to combine the reaction kinetics and bed hydrodynamics. The kinetic part of the model deals with the chemical reactions occurring in different steps of gasification while hydrodynamics of the bed gives more information about the physical phenomena inside the bed. In this paper two major fluidization models; two phase theory (TPT) and counter current back mixing (CCBM) and one kinetic rate model have been developed and compared in terms of accuracy in predicting product gas concentration and the generality of the model to different ranges of input parameters. The results show that including hydrodynamics of the bed as in TPT improves the accuracy of the kinetic rate model. TPT model can be used for air gasification in bubbling fluidized bed gasifiers while for steam gasification it needs further modification.

Keyword [en]
Kinetic, hydrodynamics, biomass gasification, fluidized bed gasifiers, modeling
National Category
Chemical Process Engineering
Identifiers
URN: urn:nbn:se:mdh:diva-33425OAI: oai:DiVA.org:mdh-33425DiVA: diva2:1037735
Available from: 2016-10-17 Created: 2016-10-17 Last updated: 2017-02-02Bibliographically approved
In thesis
1. Biomass gasification in fluidized bed gasifiers: Modeling and simulation
Open this publication in new window or tab >>Biomass gasification in fluidized bed gasifiers: Modeling and simulation
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Using woody biomass as a resource for production of biofuel, heat and power through gasification has been studied for years. In order to reduce the cost of operating and to design the full-scale gasification plant developing a general model to be applicable for different ranges of input data with acceptable level of accuracy, is needed. In order to develop such model for the gasifier, as the main component in the process, three major models have been studied in this thesis; theoretical model (Equilibrium model), semi-empirical model (modified equilibrium model, kinetic combined with hydrodynamic model) and empirical model (statistical model).

Equilibrium model (EM), shows low accuracy in predicting the content ofmajor components in product gas especially CH4 and CO. Therefore to improve the accuracy of prediction modification of EM is needed. Analyzing the semi-empirical approaches show that although the accuracy of EM can be improved, the generality of the modified models are still low. Therefore two new modified models have been developed. The first model is based on including data from wider range of operating condition to develop the empirical equation. The second model is based on combining QET and reaction kinetics for char gasification approaches. The first model decreases the overall error from 44% to 31% while the overall error of second model is decreased from 36% to 8%. Other semi-empirical model for fluidized bed gasifiers which is not equilibrium-based is developed by combining reaction kinetics with hydrodynamic equations. Investigating different hydrodynamic models show that combining two-phase-structure model with reaction kinetics for bubbling fluidized bed gasifiers improves the accuracy of the kinetic-only model.

The third type of approaches, investigated in this thesis, towards developing a general model is the empirical model. This model has been developed based on Partial least square (PLS) approach. The PLS-R model show high level of accuracy within the specific range of empirical data used for developing the model. Further analysis on the experimental dataset by PLS-R model show that equivalence ratio (ER) is the operating parameter with the most significant impact on the performance of fluidized bed gasifiers. Optimizing the operation of fluidized bed gasifiers based on this model shows that high gas quality (high volume fraction of H2, CO and CH4 and low volume fraction of CO2), high carbon conversion and low tar yield is achieved when ER≈0.3, Steam to Biomass ratio≈0.7, moisture content≈9% and particle size≈3mm and olivine is the bed material. 

Place, publisher, year, edition, pages
Västerås: Mälardalen University, 2016
Series
Mälardalen University Press Dissertations, ISSN 1651-4238 ; 216
National Category
Chemical Process Engineering Energy Systems
Research subject
Energy- and Environmental Engineering
Identifiers
urn:nbn:se:mdh:diva-33426 (URN)978-91-7485-296-7 (ISBN)
Public defence
2016-12-02, Pi, Mälardalens högskola, Västerås, 09:15 (English)
Opponent
Supervisors
Available from: 2016-10-18 Created: 2016-10-18 Last updated: 2016-11-11Bibliographically approved

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