The continued use of fossil fuels has significant adverse climate effects. To reduce carbon emissions and develop a more sustainable energy sector, widespread development and use of cleaner, affordable and dependable alternatives have been promoted globally. In particular, biomass has drawn significant interest as its application is generally considered a promising renewable energy source even though its production is accompanied by invariant release of CO2. This can be attributed to its abundant availability, its synthesis of different chemicals and fuels and simple transportation and storage requirements.
Biomass production involves a complicated gasification process. Moreover, the product distribution is highly sensitive to the residence time and rate of heating in biomass gasifiers. This has directly contributed to the development of different mathematical models to aid in characterizing and predicting syngas production. These models not only provide a reliable representation of the physical and chemical phenomena in the gasifier but also provide guidance on the design and optimization of the gasifier.
Several approaches have been adopted to model biomass gasification systems. Among them, equilibrium models have proved effective in describing gasification processes, especially downdraft biomass gasifiers. While thermodynamic equilibrium cannot be attained in a gasifier, it allows prediction of the final composition of syngas and is often used as a simulation tool in processes with longer durations and high gasification temperatures. It can be categorized into stoichiometric and non- stoichiometric models. Although numerous studies have been conducted to understand better the parameters affecting the gasification kinetics, most fail to account for the production of char and tar despite being important outputs. Additionally, the existing models do not allow simultaneous prediction of gasification parameters or explore their effects.
On this account, PhD graduate Ahmad Ibrahim, Dr. Sergii Veremieiev and Professor Philip H. Gaskell from Durham University in England proposed a stoichiometric model to predict syngas yield in the reduction zone of a downdraft biomass gasifier. It incorporated a thermochemical equilibrium model of global gasification reaction. The governing model equations were solved in a fully coupled fashion. The char output was predicted via a boudouard reaction, ammonia production was predicted via ammonia synthesis reaction, while a new empirical experimental correlation derived from existing pertinent experimental data was used to calculate tar yield. Their work is currently published in the journal, Renewable Energy.
The research team showed that while the model did not require the use of correction factors, it satisfactorily predicted the concentration of methane in the producer gas, an output that was challenging to predict using existing equilibrium models. Char and tar yields, lower heating value, syngas composition, cold gas efficiency and gasification temperature were obtained for different biomass feedstock using specific ultimate analysis for varying equivalence ratio and moisture content. Using the solution of a fully coupled equation was not only beneficial in determining the gasification temperature but also contributed to improving the convergence properties.
The authors discussed the effects of different parameters. For example, the concentrations of CO2 and hydrogen (H2) increased with moisture content, while the concentration of CO decreased. Although CH4 concentration gradually increased with increased moisture content, N2 concentration remained constant. The gasification temperature decreased with moisture content but increased with equivalence ratio, while the tar yield decreased with increasing moisture content and equivalence ratio due to improved syngas quality. The concentration of NH3 decreased with increased moisture content and equivalence ratios, while the concentration of H2S remained nearly constant. Finally, cold gas efficiency and lower heating value decreased with increased equivalence ratio and gasification temperature.
In summary, the authors developed a comprehensive thermochemical model for understanding the gasification process in downdraft biomass gasifier consisting of a global reaction of all gaseous species. Whenever possible, they successfully compared their model predictions with experimental data, and a good agreement was established. The results demonstrated the capability of equilibrium models in providing reliable predictions of syngas composition. In a statement to Advances in Engineering, Professor Gaskell stated that their findings would contribute to advancement in process design, evaluation and optimization of downdraft gasification systems.
Ibrahim, A., Veremieiev, S., & Gaskell, P. H. (2022). An advanced, comprehensive thermochemical equilibrium model of a downdraft biomass gasifier. Renewable Energy, 194, 912-925.