(347d) Mathematical Modeling, Validation, and Optimization of a Bubbling Fluidized Bed Reactor for Biomass Gasification.

Fuel resources that are renewable, and sustainable are highly desired for reducing GHG emission.¹ Recent studies have shown biomass, a renewable energy source, as an emerging alternative to fossil fuels due to its abundance in many parts of the world. Biomass can not only used for generating electricity, or heat, but also for producing syngas and H₂ through gasification of biomass.^2,3 Compared to the large volume of research literature on coal gasification ^4,5 , literature on biomass gasification is still sparse.^6,7 Biomass has several fundamental differences compared to coal. Biomass has considerably higher oxygen content (typically >40) compared to coal (typically <10%). Biomass has typically much lower carbon content (typically <50%) compared to coal (<60%). Furthermore, biomass has negligible ash content. In addition, reactivity of various components in biomass is much different than that in coal. As a result, while coal gasification is often done in entrained flow bed, moving bed, and circulating fluidized bed and transport reactors. Bubbling fluidized beds (BFBs) are generally not preferred for coal gasification as they lead to low char conversion. On the other hand, biomass gasification can be efficiently performed in BFBs. Furthermore, catalytic gasification of coal is challenging due to the type of gasifier used in coal gasification and due to presence of large amount of ash from which separation of catalyst is challenging. Due to negligible ash content in biomass, catalytic gasification of biomass is feasible if the gasification is conducted in BFBs. Therefore, this work focuses on developing a non-isothermal mathematical model of a BFB for catalytic biomass gasification.

An isothermal model of a FBB for biomass gasification has been developed by Radmansh et al. ⁶. Agu et al., ⁸ also developed a detailed one-dimensional model for steam biomass gasification in a BFB. It can be observed in the existing literature that detailed kinetic models and hydrodynamic models are missing for the biomass-fed BFBs.⁷ Various assumptions have been made in the existing literature for the hydrodynamic models. For example, the bubble fraction and bed voidage have been fixed. However, bubble dynamics changes significantly through the bed and they play a key role in mass and heat transfer characteristics of BFBs. Temperature-dependent modeling of the pyrolysis reactions are also missing. Further, kinetic models for most heterogeneous reactions are the same as the coal gasification. Furthermore, kinetic modeling of catalytic gasification processes is practically missing in the literature. This works addresses these limitations in the existing literature.

In this work a 1-D non-isothermal model for a catalytic BFB is developed by considering mass, energy, momentum conservation for the bubble and emulsion. The countercurrent back mixing model (CCBM)⁶ is adopted. A shrinking particle model is developed for modeling the heterogeneous biomass char gasification reactions. A data reconciliation approach is developed and applied to the pyrolysis data. Reconciled data are used for optimal estimation of the kinetic parameters for the pyrolysis model. A cold flow model is developed to validate the hydrodynamic model. The cold flow model is validated by using the experimental data⁹ for two cases -biomass only and biomass with catalyst. The in-house data on pine wood gasification in a BFB are used to optimally estimate the kinetic parameters for the homogeneous and heterogeneous reactions. The developed model is used for optimization of the operating conditions and bed design for maximizing H₂ yield.

References cited

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