(645f) Modelling Super-Equilibrium in Biomass Gasification With the Constrained Gibbs Energy Method

Background

Biomass gasification can be seen as a viable option for generating electricity from biomass with high efficiency or for the production of liquid biofuels and chemicals. Gasification is a thermo-chemical conversion process that takes place at high temperature and reductive environment.  For such conditions thermodynamic equilibrium is often assumed, but not reached in practice: during biomass gasification light hydrocarbons (e.g.: CH₄, C₂H₂, C₂H₄, C2H6, C₃H₈, C₆H₆), ammonia, tars and char are also formed. These phenomena cannot usually be modelled with equilibrium assumptions.

In order to increase the accuracy of gasification models, both empirical and mechanistic models are often  applied alone or combined with thermodynamic equilibrium calculations [1]. This leads to a ‘dual’ approach where different parts of the process and reactions are modelled independently and results are merged in post-processing.

This study presents a unified solution for simultaneous calculation of the super-equilibrium reactions and related reaction enthalpies in the gasification process. The solution is based on the Constrained Free Energy (CFE) method where equilibrium computation is extended with additional immaterial constraints for solving the local or partial constrained equilibrium instead of the global thermodynamic equilibrium.

Methodology

First applications of the constrained thermodynamic equilibrium method in Gibbs energy minimization were the conservation of immaterial properties [2] and  inclusion of reaction kinetics into the Gibbs’ian multicomponent models [3,4]. In the course of time the method has found new applications, ranging from modelling sorption in pulp suspensions to determining surface tension of liquid mixtures [5]. Quite recently CFE has been used to estimate the super-equilibria of alkali metals and sulphur in black liquor combustion [6]. Today constrained free energy method is seen as a versatile tool for various fields of application.

In this study the CFE methodology is applied for describing the super-equilibrium occurring in biomass gasification. Light hydrocarbons, ammonia, tars and char tend to decompose if thermodynamic equilibrium calculation is performed for high temperatures, and thus constraints are needed for the modelling of their presence in the super-equilibrium conditions.

This a computational study with all experimental data obtained from literature. The empirical model of super-equilibrium applied is from [1]. Thermodynamic data is from [7]. ChemSheet [8] is used as the modelling tool as it allows extending the thermodynamic system and thus enables the calculations of constrained free energy models.

Results

 In the present work different approaches for the modelling of global or local equilibrium of biomass gasification have been assessed. First (i) thermodynamic equilibrium without any constraints is computed and used as reference. Later additional constraints are introduced to system, such as amount of (ii) char, (iii) tars, (iv) ammonia. The light hydrocarbons are defined by (v) the amount of carbon and by (vi) amount of hydrogen in hydrocarbons. In addition constraints such as (vii) amount of double bonds in hydrocarbons or (viii) methane amount are evaluated. Finally a fully constrained system, where all species are defined according to [1] is reported.

The super-equilibrium of major syngas components (CO, CO₂, H₂, H₂O and CH₄) can be predicted with the proposed constrained free energy method. Accuracy of the model increases when an additional constraint for hydrogen in hydrocarbons is defined. At the same time the partial equilibrium moves further from the global equilibrium.

While the adapted constraints were successful in evaluating the super-equilibrium of major species, the respective predictions for minor hydrocarbon components (C₂H₂, C₂H₄, C₂H₆, C₃H₈, C₆H₆) were not as satisfying. The selection of modelled light hydrocarbon components seems to be vital, if more accuracy is required. In addition by defining all species with separate constraints, it is possible to increase the accuracy of results, but with the cost of decreasing the degrees of freedom of the thermodynamic system.

Summary

Constrained free energy method was successfully applied for the calculation of the super-equilibrium of light hydrocarbons, ammonia, tars and char in biomass gasification. Gasification can be solved as restricted partial equilibrium with a single calculation step. Thus, there is no need for post-processing corrections for e.g. volume or enthalpy flows.  For a reasonable accuracy of the simulation model, it is not necessary to define constraints for all reactions, but a set of more general constraints (e.g. carbon in hydrocarbons) remain sufficient.

Distinct benefit of presented approach is that chemical reactions, gasification enthalpy and states of the system (such as volume) are estimated concurrently. This method is also resilient: the number of details describing constraints can vary from simpler experimental results and models to complicated mechanistic models. A promising application area for the presented methodology could be as part of large scale process simulation where detailed thermodynamic models can be implemented and effects in gasification chemistry could conduct changes in the entire process.

References

[1]         Hannula, I. and Kurkela, E., "A parametric modelling study for pressurised steam/O2-blown fluidised-bed gasification of wood with catalytic reforming", Biomass and Bioenergy, 38:58–67 (2012).

[2]         Alberty, R.A., "Thermodynamics of the formation of benzene series polycyclic aromatic hydrocarbons in a benzene flame", The Journal of Physical Chemistry, 93(8):3299–3304 (1989).

[3]         Keck, J.C., "Rate-controlled constrained-equilibrium theory of chemical reactions in complex systems", Progress in Energy and Combustion Science, 16(2):125–154 (1990).

[4]         Koukkari, P., "A physico-chemical method to calculate time-dependent reaction mixtures", Computers & Chemical Engineering, 17(12):1157–1165 (1993).

[5]         Koukkari, P. and Pajarre, R., "Calculation of constrained equilibria by Gibbs energy minimization", Calphad, 30(1):18–26 (2006).

[6]         Kangas, P., Koukkari, P., Lindberg, D. and Hupa, M., "Modelling black liquor combustion with the constrained Gibbs energy method", Proceedings, 8th International Black Liquor Colloquium, Belo Horizonte, Brazil (2013).

[7]         Roine, A., Lamberg, P., Mansikka-aho, J., Björklund, P., Kentala, J., Talonen, T., et al., HSC Chemistry 6.12., Outotec Research Oy, (2007).

[8]         Koukkari, P., Penttilä, K., Hack, K. and Petersen, S., "CHEMSHEET – An Efficient Worksheet Tool for Thermodynamic Process Simulation", In: Microstructures, Mechanical Properties and Processes, Y. Bréchet (Ed.), Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim (2005).