Modeling of Biomass Gasification with Adaptation to Biological Methanation Using Aspen Plus®

University essay from Lunds universitet/Kemiteknik (CI)

Abstract: With reduced resources of fossil fuels and increased environmental requirements, renewable substitutes have received greater attention. The production of renewable chemicals and fuels through biomass gasification presents a promising route to drive this energy transition. Synthe-sis gas (syngas), a mixture of mainly CO and H2, can be produced from biomass via gasifica-tion. Biological methanation, as a potential alternative to catalytic methanation, can then be used to produce biogas from biomass-derived syngas. The aim of this work was to model the gasification process of biomass in the Aspen Plus® simulation software with an adaptation to the subsequent biological methanation of syngas. The study was based on Meva Energy’s 5-MWth cyclone gasifier, which operates at 0.65 barg and 850–1000 °C. The generated syngas consisted mainly of N2, H2, CO, CO2, CH4, light non-aromatic hydrocarbons, and tars. As of today, most simulations in Aspen Plus® found in literature are based on equilibrium cal-culations. The main drawbacks of equilibrium-based models are the overestimation of char conversion and the neglect of tars and their reactions during gasification. In this study, equi-librium and kinetic models were developed for modeling biomass gasification in a cyclone gasifier. In addition, a third model was developed for the succeeding gas-cleaning system in the plant. All models were validated against experimental data. The lower heating value of produced gas and the cold gas efficiency of the process were calculated for all cases. Predicted temperature in the gasifier was lower than experimental data when equilibrium was assumed. A large deviation was also observed between the equilibrium model and measure-ments with respect to concentrations of H2, CH4 and light non-aromatic hydrocarbons. On the other hand, a better agreement between simulation and experimental data was found when employing the kinetic-based model. When validating the gas-cleaning model, predicted values showed some agreement with measurements, but big errors were observed for some compo-nents, such as indene and naphthalene. For the biological methanation process, the main requirement is a syngas free of N2. There is no negative effect of N2 on the biological methanation itself, however, the separation of N2 in the product can be associated with high costs. This requirement can be achieved by replacing air as the gasification medium with a mixture of O2 and recycled CO2 which has been separat-ed from the gas after the methanation. When gasification was adapted to biological methana-tion, a lower gasification temperature was observed compared to conventional air-blown gasi-fication at the same air–fuel equivalence ratio (lambda). This comes with a cost of lower cold gas efficiency and higher tar content in the syngas. Therefore, lower concentration of CO2 in the gasifying medium is desired, but particle separation will be affected as the gas volumetric flow rate decreases.

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