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.

The CFD modeling based on the two-fluid model (TFM) was used on studying the novel indirectly heated bubbling fluidized bed steam reformer (IHBFB-SR). This is a collaborative project between Petrogas and TU Delft for testing the advance’s reactor configuration, which indirectly supplies the heat via radiant burner on the center toward the surrounding bed, thereby improving heat transfer efficiency and reduced the losses. The present work aimed to observe the hydrodynamic and heat transfer of the reactor by first employing air as the fluidizing gas and corundum (Geldart B size) as the bed material. The minimum fluidization condition, bubbles development, and voidage profile are the main objectives for the hydrodynamic simulation. In the heat transfer study, the effect of radiative heat transfer, bubbles and voidage profiles, and different radiative models (P1 and DO) on the heat transfer mechanism were examined. The 2D and 3D models were built, and three drag models: Gidaspow, adjusted Syamlal, and EMMS/Bubbling was employed. The simulation results were then compared to the experimental data obtained, such as minimum fluidization flowrate, pressure drop, bed expansion, and temperature profile on a specific flow rate.

Initially, a grid independency test was conducted using five different grid sizes. It is concluded that the appropriate grid size for simulating the IHBFB-SR with a bed material particle size of 0.496 mm should be at least 7.5 mm or 15 times the corundum particle size. The present research used 2.5 mm or five times dp. The minimum fluidization obtained was in the range of 14-16 kg/h based on both 2D and 3D simulation. Nonetheless, if primarily refers to the 3D model results, the minimum fluidization condition should lie around 14 kg/h. Three drag models have also been compared. It was found that the adjusted Syamlal gives the closest result compared to the experimental data. Nevertheless, the drag modification, based only on minimum fluidization conditions like modified Syamlal, tends to overpredict the drag coefficient on the entire range of solid volume fraction. There are also no significant differences in the bubbles or voidage profiles among those three drag models. Adjusted Syamlal has a slightly larger bubble while Gidaspow and EMMS bubbling has a bit smaller one. The expected better result of using the EMMS bubbling drag model does not appear to have a considerable impact on the case of Geldart B or larger particles. There was also an underprediction of pressure drop and bed expansion on the simulation. Two major factors were the absence of proper particle shape representation through a sphericity factor and the lack of precise simulation of particle size distribution. Only a perfect rounded sphere of corundum with a sphericity factor of one and one uniform size of the particle was assumed.

In the case of heat transfer simulation, bubbles and voidage effect firstly studied. It was found that the increase of the bubbles frequency and size, as represented on the voidage profile, would improve the heat transfer process indicated by the increase of the heat flux. The bubbles’ occurrence on the bed plays a critical role in the mass transfer’s improvement from and to the vicinity of the radiant burner wall, thus increasing heat transfer. In contrast with the voidage, the increase of superficial gas velocity does not directly influence the heat flux. It was also proved that the radiative heat transfer improved the overall heat flux in this bubbling fluidized bed reactor by about 16.11 %. Though it appears small, this contribution fits the range of other proven works, with a similar operating environment and particle size used. There are two different radiative models performed in the simulation: first-order spherical harmonics method (P1) and discrete ordinate method (DO). Both models presented a similar trend, with P1, has a slightly higher magnitude. However, the P1 model shows a peculiar result of having a strange lower temperature lower on the bottom part of the bed. On the contrary, that is not the case for the DO, which is well known for its accuracy but a higher computational cost demand. It is then concluded that for the present setup, the DO model could perform better. Lastly, the overall heat transfer process was investigated. Since no specific experimental data available for validating the heat transfer properties, only the final steady temperature values of five different thermocouples were used. Comparing these two, it was found that the present model did not give satisfactory results due to overprediction of air temperature above the bed and underprediction of bed temperature at the same time. Some improvements are required, as will be presented in the recommendations section.

The Process and Energy department of the 3mE faculty of TU Delft and the Dutch company Petrogas Gas-Systems B.V. are working together on the commissioning of a small 50 kWth Indirectly Heated Bubbling Fluidized Bed Steam Reformer (IHBFB-SR) heated by two radiant tube burners placed vertically inside the reactor. This is a new approach on indirectly heating as the heat is released from the inside to the outside, compared to existing indirectly heated gasifiers, where the heat is released from the outside to the inside. The main objective of this thesis has been to analyze the hydrodynamics and heat transfer occurring within the reactor, by applying "Computational Fluid Dynamics" (CFD) techniques. The analysis has been carried out using the commercial CFD software ANSYS® FLUENT. First the physical phenomena occurring within the reactor have been identified and then research was done on the models and parameters developed to describe the physical phenomena. First the hydrodynamic behaviour was evaluated and then it was looked into how the heat transfer can be coupled to the hydrodynamics in the reactor. In regards to the hydrodynamics, the Euler-Euler Two-Fluid Model (TFM) has been found to be appropriate, and the inter-phase drag coefficient was chosen as a parameter of interest. The Gidaspow drag model and the Syamlal-O-Brien modelwere compared to one another, and the solid volume fraction, pressure, axial velocity and granular temperature within the reactor were evaluated. The results were compared to the numerical solutions obtained in previous work, in order to understand which model gives a better prediction. The Gidaspow drag model was found to provide a better prediction of the core annular flow in the bed zone, and was therefore implemented in the heat transfer evaluation. In regards to the heat transfer, models were already developed for the conductive and convective heat transfer in multiphase flows. For radiation there is still a lack of rigorous coupling between radiative heat transfer and hydrodynamics in simulations of non-dilute multiphase flows. The heat transfer simulation was performed in two steps, first with only the conductive and convective heat transfer, and then with radiation added to the system. The thermal properties such as the thermal conductivity, absorption and scattering coefficient have been made dependent of the volume fraction. For the case without radiation, a small temperature increase was observed with high temperature gradients near the wall. For the case with radiation, the Discrete-Ordinates (DO) Radiation model was evaluated. From the results it can be concluded that the DO Radiation model overpredicts the radiation that is emitted and scattered from the bed, causing the bed to heat up continuously to temperatures much higher than the radiant tube. More research and experimental validation is required in order to improve the coupling of the radiative heat transfer to the hydrodynamics.