Modeling circulating fluidized bed biomass gasifiers. Results from a pseudo-rigorous 1-dimensional model for stationary state

doi:10.1016/j.fuproc.2005.08.003

Fuel Processing Technology

Volume 87, Issue 3, February 2006, Pages 247-258

https://doi.org/10.1016/j.fuproc.2005.08.003 Get rights and content

Abstract

Results from a 1-dimensional and semirigorous model for atmospheric and circulating fluidized bed biomass gasifiers (CFBBGs), presented in the (previous) paper by Corella and Sanz [J. Corella, A. Sanz, Modeling circulating fluidized bed biomass gasifiers. A pseudo-rigorous model for stationary state. Fuel Process. Technol. 86 (2005) 1021–1053], are shown here. Process variables predicted by the model are gas composition (H₂, CO, CO₂, CH₄, C₂H_n, H₂O and O₂ contents), gas yield, tar content in the flue gas and char concentration in the solids. Both axial profiles in the riser and values at the gasifier exit are calculated from the model and are shown here for some selected sets of process variables. Variables analyzed in depth are: total air flow (used as equivalence ratio, ER), percentage of secondary air flow, height (location) of the secondary air flow, biomass moisture and biomass flow rate, expressed as the biomass weight hourly space velocity in the gasifier. All the results from the model agree both with known published data and with some tests made to check the model.

Introduction

It is very well known how thermochemical gasification produces a valuable gas, a mixture of H₂, CO, CO₂, CH₄, C₂H_n, etc., with some tar and other impurities, by using a gasifying agent and a organic feedstock (biomass, coal, residues, etc.). Atmospheric circulating fluidized bed (CFBs) reactors are promising gasifiers because of their very high throughputs, which in the case of biomass range between 1500 and 4000 kg biomass fed/h m² of cross-sectional area of the gasifier. Nevertheless, CFB gasifiers face some technical and economical problems which difficult their use. A good model for CFB gasifiers could improve both their design and operation, reduce any associated problems and facilitate the implantation of this technology.

Corella et al. at the Universities of Zaragoza and Madrid (Spain) started to study the modeling of fluidized bed biomass gasifiers in the mid-1980s (i.e. Ref. 2). More recently [3], they discussed the reaction network existing in a CFB biomass gasifier and the problems associated with the accuracy of the kinetic equations needed for the existing complex reaction network. Further, Corella et al. [4], presented a model for bubbling fluidized bed (BFB) biomass gasifiers, gasifying with pure steam. That model identified the four main, for modelling purposes, chemical reactions among the reaction network existing in the gasification process. With only four kinetic parameters, the model predicted quite well the BFB gasifier. Finally, Corella and Sanz [1] have presented a whole model for CFBBGs. Such model is 1-dimensional and for steady state. The model has a semirigorous character because of the assumptions that had to be introduced by lack of accurate knowledge in some parts of the modeling. A deep literature review on the field was also made in that previous paper, reason why no much literature will be cited in this paper now. This paper will now show some significative results from the model developed by Corella and Sanz [1].

Among the number of variables which can be calculated for many different experimental conditions, the results presented here will always point out the tar content in the produced gas which is the key index of its quality. Even more, to use the gasification gas in gas turbines or in gas engines the tar content must be below 50 mg/N m³. This very low tar content can be obtained by using nickel-based catalysts, rings as well as monoliths, downstream from the gasifier. To avoid their deactivation by coke, they require a tar content in the fuel gas at the inlet of the catalyst reactor below 2 g/N m³ [5]. This catalytic hot gas cleaning method require a quite clean fuel gas which, as it will be shown in this paper, may be obtained in a fluidized bed biomass gasifier only under very special operating conditions. Results presented here will show these conditions and serve to improve both design and operation of existing and future CFBBGs.

Section snippets

Outputs from the model

Using the model shown in detail in the previous paper [1], the longitudinal profiles in the riser of a CFBBG have been calculated for the following variables:

–
Gas composition (H₂, CO, CO₂, CH₄, C₂H_n, O₂, in vol.%, dry basis, and H₂O, vol.%).
–
LHV (MJ/N m³, dry gas).
–
Tar content (g/N m³, dry gas).
–
Char concentration [g/kg (sand + dolomite)].

Once the axial profiles of these variables are known, their values at the gasifier exit are further calculated. The model also allows the calculation of the gas

Limitations or constrains of the results here presented

Trends and magnitude of the results presented here have a general character, usefulness and application. Nevertheless, some details concerning these results may change from one gasifier to another one because these results were obtained for only some specific conditions. They have to be taken into consideration when the results presented here are applied to another different situation. These conditions that may change from one gasifier to another one are as follows.

Operating conditions

Results will be presented for only some sets of process parameters. The sets of operation conditions for each of the cases studied here (ER, biomass moisture, etc.) are shown in Table 1. Sets shown in Table 1 correspond to selected and realistic situations in commercial CFBBGs.

It has to be pointed out that the process operating parameters in the above indicated sets are not independent of each other, but they are related through heat, mass and hydrodynamic balances and by the stoichiometry of

Results

We now give details of the effects of the process variables considered.

Perspectives and checking of the results

It is well known that all models need their experimental results checked, and also that all authors provide experimental validation in such a way that it is easy to find in literature contradictory models with some experimental data provided to validate each model. So, experimental checking or validation of the results presented here is very important for these authors. Nevertheless, no comparison between theory and experimental will be provided here because a full or deep comparison cannot be

Conclusion

The 1-dimensional model presented in the previous paper [1] has been used to predict the gas composition, quality (tar content, mainly) and yield from a CFBBG. The model predicts the axial profiles of these variables and their values at the exit of the gasifier. Their variation by the ER value, the percentage of secondary air flow, secondary air inlet height, biomass flow rate (as WHSV, h⁻ ¹) and biomass moisture have been shown here for a set of selected operating conditions.

The parameters with

Acknowledgement

The present work was made under the EC-financed project No. JOR3-CT99-0053. The authors thank Dr. Wennan Zhang, at Mid-Sweden University, for his effort and encouraging work as Project Coordinator.

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Modeling circulating fluidized bed biomass gasifiers. Results from a pseudo-rigorous 1-dimensional model for stationary state

Abstract

Introduction

Section snippets

Outputs from the model

Limitations or constrains of the results here presented

Operating conditions

Results

Perspectives and checking of the results

Conclusion

Acknowledgement

Fuel Process. Technol.

Chem. Eng. Sci.

Chem. Eng. Sci.

Chem. Eng. Sci.

Chem. Eng. Sci.

Fuel Process. Technol.

A model for the non-stationary states of a commercial fluidized bed air gasifier of biomass

Modeling a CFB biomass gasifier: Part I. Model formulation