Due to growing awareness and rising concern over the climate change impact, the demand for renewable energy has been increasing. In the coming decades, biomass is expected to play a crucial role as it is one of the most plentiful and well-utilized renewable resources in the world. Biomass can be sustainably converted to solid/liquid/gaseous biofuels which in turn can be used to produce both, power and heat. Among the many thermochemical conversion technologies, conventional gasification technology is one of the widely used conversion routes. However, the use of conventional gasifiers for the conversion of biomass feedstocks with more than 70% moisture content is not suitable without their pre-treatment. Having the advantage of avoiding energy- and cost-intensive drying process, Supercritical Water Gasification (SCWG), offers a promising approach in converting these biogenic residues into valuable biofuels.
SCWG is an alternate thermochemical conversion route and is suitable for the conversion of wet biomass feedstocks having very high moisture content. The thermochemical conversion takes place in Supercritical Water (SCW) having temperatures and pressures higher than 374.29 °C and 221 bar, respectively. At such conditions, the thermo-physical properties of water change in a way that causes water to act as a solvent and catalyst at the same time. With the use of SCWG, large amounts of wet biomass wastes such as cattle manure, fruit/vegetable waste, and cheese whey residual streams which get disposed from farming and food processing industries globally, can be sustainably treated. Since an in-depth investigation of SCWG of the noted real wet biomass wastes is still at an early stage, in this study, we have therefore concentrated on the SCWG of these specific classes of waste. To this end, different modelling scenarios, including global, constrained, and quasi-thermal thermodynamic equilibria models have been pursued so as to effectively predict system behavior. We used Factsage and MATLAB modelling tools to develop and analyze these models. We observed reasonable agreements between experimental results and predictions from constrained and quasi-thermal equilibrium models, effectively emanating from conceptual improvements due to experimental data.
The results showed that the superimposition of carbon conversion efficiency together with the use of a constant molar amount of specific compounds can improve the accuracy of the global equilibrium model. For example, deviation of CO2 yield from experimental data significantly improved from 55% to 0.3% for fruit/vegetable residue gasification using a constrained equilibrium model. Furthermore, comparisons revealed the advantage of using a quasi-thermal equilibrium model which uses the ‘’approach temperature” concept over the constrained equilibrium model. Results for fruit/vegetable waste showed an approach temperature between 60 and 80 °C for H2 yield. Overall, the quasi-thermal equilibrium approach has its advantages of lumping all the additional constraints to be used in constrained equilibrium model into an effective approach temperature, offering a much better prediction of the compositions with an error margin of maximum 0.001%.
Furthermore, the results of this effort assisted us in designing a conceptual bio-refinery model based on the SCWG process. Using the ASPEN modelling tool, we were able to optimize and analyze the entire process for its chemical and thermal behavior. Using the results, the SCWG process was found to be thermally self-sustaining for the assessed reactor conditions. However, with the reactor conditions; temperature (600 and 650 °C), pressure (240 bar), and fruit/vegetable waste feed concentration (11wt%), the process was assessed to be practically infeasible as larger part of the produced gas stream (i.e. more than 70%) was getting recycled back to the system. Finally, we compared the process modelling results based on global and constrained modelling scenarios and the use of GTE modelling for process designing was found to have its limitations. Overall the result of this thesis shows the great potential of using SCWG for thermochemically upgrading wet biomass feedstocks. Comparing the results from different modelling scenarios gave an insight into the process and the reactions taking place inside an SCW gasifier, thereby assisting in better reactor designing.