Meeting the ever-increasing energy demands in our planet where conventional sources of energy are finite and fast depleting, controlling adverse phenomena like climate change, soil erosion, greenhouse gas emission, ozone layer thinning and pollution will remain profound challenges until widespread adoption of sustainable sources of energy is achieved. The shift to a sustainable energy system cannot be complete without the contribution of biomass energy. Within the ecosystem of biomass energy, gasification is a thermochemical conversion process undertaken to obtain a high-quality product gas by increasing the low energy density of solid biomass. The product gas can be used in power applications or be converted to liquid fuels for transportation. Gasification is carried out at high temperatures (700-1500°C) by using a gaseous agent under sub stoichiometric conditions (R.A. Kersten & De Jong, 2015). The main non-condensable permanent gases obtained from gasification are CO, H2, CO2 and CH4. Use of allothermal gasifiers, whose working is based on the separation of combustion and gasification chambers within the gasifier, with heat supply for carrying out endothermic reactions, established via heat carrier or heat exchanger, have shown many advantages compared to the conventional gasifiers (Hofbauer & Materazzi, 2019). The mix-up of product gas and flue gas is avoided in this type of gasification. Secondly, while using air for biomass combustion, the nitrogen present in air reduces the quality of the end product by not participating in gasification, thus resulting in a diluted product gas. A high-quality product gas can be obtained from allothermal gasifiers without the need for establishing an expensive air separation unit, as required in case of conventional gasifiers. The 50 kWth Indirectly Heated Bubbling Fluidized Bed Steam Reformer (IHBFBSR) at Delft University of Technology (TU Delft) represents a new concept of allothermal gasification technology where heat required for the endothermic gasification reactions is provided by two radiant tube burners placed vertically inside the reactor, one at the top and one at the bottom. The aim of this research is to optimize and validate the kinetic model developed for the IHBFBSR by a former student, Maarten Kwakkenbos in Aspen Plus®. The optimization is carried out with the help of the several steps, described in the report. The optimized model is then validated by using the results of the gasification tests performed under various operational conditions by PhD Candidates Mara del Grosso and Christos Tsekos. The optimized model predicts the gas composition obtained from the IHBFBSR quite well. In addition to the yield of permanent gases, N2, H2O and tars concentration in the product gas, along with various gas ratios (CO/H2, CH4/H2, CO/CO2) from the model are compared with the experimental values and found to be in reasonable agreement. Moreover, key performance indicators such as carbon conversion (CC), cold gas efficiency (CGE) and overall efficiency (OE) are also evaluated from both model and experiments and compared. The error ranges for most of the parameters lie within the reported deviations observed in various gasifier models in literature. After the validation of the model, the possibility of recycling a fraction of the product gas to feed the burners in order to make the set up more sustainable is evaluated. Sensitivity analyses is performed by varying steam to biomass ratios (SB*), primary and secondary air flowrate to evaluate the best process conditions under which product gas obtained from the gasifier, after subsequent cleaning, can be used for methanol production based on the H2/CO ratio. The results indicate that a higher SB* and a higher secondary air flowrate can result in a H2/CO ratio closer to the desired value required for optimum methanol production. From the heat analysis performed for the model, a possibility of increasing the overall efficiency of the process by adding a bypass line in the gasifier setup is suggested to be explored. The master thesis concludes by answering the research questions and recommendations to improve the detailing and accuracy of the model.

The effort to reduce global anthropogenic carbon dioxide emissions has caused a growing interest in the use of sustainable energy sources. One of the potentially sustainable energy sources considered as an alternative for the currently used fossil sources is biomass. Biomass is organic matter that originates from plants or animals which can be used as chemical energy carrier for fuels or for the production of chemical feed stock[Basu, 2010]. Gasification is a thermochemical process that converts materials like biomass into useful convenient gaseous fuels or chemical feedstock[Basu, 2010]. Gasification takes place at high temperature in the presence of a gaseous agent. Gasification is aimed at increasing the overall energy density of biomass by producing gaseous energy carriers with a higher energy density than the original biomass feed. Conventional gasifiers combust a fraction of the biomass or char in order to provide heat to the reduction reactions. This has two disadvantages. First, the flue gasses are mixed with the product. Secondly, the air that is used for partial biomass combustion mainly consists out of nitrogen. The nitrogen dilutes the gasification product significantly. The Indirectly Heated Bubbling Fluidized Bed Steam Reformer [IHBFBSR] set-up at the TU Delft is a novel indirectly heated gasifier or allothermal gasifier. The combustion of natural gas takes place in a separate combustion chamber inside the reactor chamber. Steam gasification can therefore be performed in the absolute absence of oxygen. A radiant tube facilitates heat transfer from the combustion to the reaction chamber. The radiant tubes are installed at the top and bottom of the IHBFBSR and work according to the heat from inside to outside principle. The aim of this study is to assess the performance of the IHBFBSR set-up at the TU Delft and compare its performance with other existing allothermal gasifiers. The main research question of this study is stated as: "How does the Indirectly Heated Fluidized Bed Reactor perform in terms of product yield, product quality and energy efficiency and how does its performance compare to other indirectly heated gasifiers?". In order to answer this question, both an equilibrium model and a kinetic model have been designed. Sub-optimal conditions for the IHBFBSR take place at a reactor temperature of 850 degrees C, which is the maximal controlable temperature of the reactor. At this temperature, the carbon limit is reached for an equivalent ratio of 0.23. The CGE at the carbon limit equals 69.7%. Optimal process conditions are where the particle size of the bed material is maximized without losing the fluidization behaviour of the bed. The maximum particle size equals 600 micron. The overall efficiency of the system can be increased by heat transfer between the inlet and outlet gasses of the burners and by the separation and combustion of tars and char to add heat to the process. This study shows that the pyrolysis step in the IHBFBSR is rate limited by internal heat transfer. In addition, the char oxidation reaction is limited by mass transfer of oxygen to the char particle.

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.

The Process and Energy department of the Mechanical, Maritime and Materials faculty of TU Delft and the Dutch company Petrogas Gas-Systems B.V. are working together on the commissioning of a small 50 kW (th) Indirectly Heated Bubbling Fluidised Bed Steam Reformer (IHBFBSR) which will be used to gasify the energy crop Miscanthus. However, the new feature is that this fluidised bed will be heated indirectly using radiant tube heaters installed in the reactor. These heaters are made by assembling radiant tubes with self-recuperative burners fired using Dutch Natural Gas. All tubes and burners have been manufactured by the German company WS Warmeprozesstechnik GmbH. This thesis consists of the study of two such distinct burner-tube assemblies, both of different heating capacity. The smaller capacity is of the C80 burner assembled with C100 tube while the larger is of the C100 burner assembled with C150 tube. The heat transfer and fluid dynamics inside both the tubes have been numerically modelled using CFD techniques. The models of the burners have been done using the commercially available software ANSYS Fluent version 18.2.
The main objective of this thesis has been to analyse the heat transfer from both the assemblies and calculation of their respective efficiency. To this end, the temperature, velocity, species and turbulence fields have been calculated for inside both the tubes. Also, the total heat output from the radiant tubes has been calculated. However, the mechanism of heat transfer from the radiant tube to the fluidised bed was not known at the time of these calculations. Hence, appropriate boundary conditions have been used to simulate the outer environment. The total heat transfer from the combusting flow to the inner surface of the radiant tube has been calculated. This total heat input has been equated to the heat transfer from the outer surface of the tube to the fluidised bed, assuming steady state condition. The variation of all these parameters has been studied with air factor and preheat temperature. The air factor has been varied from 1.0 to 1.5 for both tubes, by reducing the fuel inlet keeping the air inlet constant. The air preheat temperature has been varied from 300 C to 700 C for C80-C100 assembly and from 300 C to 800 C for C100-C150 assembly. This has been done to calculate an optimum operating condition for both the burner-tube assemblies in terms of maximising heat transfer and radiant tube efficiency and minimising fuel wastage. It has been found that operating at lean condition of 20% excess air by mass at highest air preheat temperature would be the optimum operating condition.
The last part of this thesis is the analysis of the robustness of calculations. Three grid sizes have been chosen other than the main grid and all the parameters have been checked for variations, if any. The variation is found to be within scientifically acceptable limits. Hence, it is concluded that the calculations are robust at this level.

The transition from a fossil based economy to a greener, bio-based economy remains challenged by the gap in technological maturity between bio-based processes and conventional fossil energy. Decreasing the price difference between fossil fuels and bio-fuels remains a major constraint. With 98% of the transportation sector now depending on fossil fuels (oil) as the primary fuel, methanol produced from CO2 and other greener sources is now growing in interest as a more sustainable fuel. This thesis focused on investigating the process biomass gasification, its challenges and its integration with methanol and power production. A number of gasification technologies were studied and the Indirect gasification technology was identified to be technologically advantageous to conventional gasification technologies. The Fast Internally Circulating Fluidised Bed (FICFB) gasification technology of the University of Technology, Vienna was modeled in Aspen Plus®. Different sets of kinetics were studied to improve the accuracy of the developed model in comparison with the data from the 100kW pilot plant at Gussing. The composition of the product gas from the developed model was then validated with the experimental data from the pilot plant if the FICFB gasifier, and found to be accurate up to 3.5%. The principle of Absorption Enhanced Reforming (AER) was then simulated in the developed gasifier by changing the bed material used to dolomite. With dolomite-CO2 sorption kinetics validated with literature, the AER principle in the gasifier predicted a 4% increase in hydrogen composition in the product gas obtained in addition to a slight increase in the cold gas efficiency of the system.
Different processing steps required to convert biomass to methanol were then identified and modeled in the same Aspen Plus® model. The identified blocks, Gasifier, Gas Cleaning Unit, Methanol Synthesis and the Energy network were optimised by performing a number of sensitivity studies. The now optimised model was then used as a common tool to study the effect of choosing different technologies and parameters within the blocks on the overall process behaviour. Four different case studies were defined, each varying from each other by a difference in technology of one of the blocks. Sankey plots for each of these cases were drawn to visualize the energy losses in such complicated systems. Results of incorporating AER on the end methanol yield and the overall efficiency of the process were studied as one of the cases. Dolomite (AER) although very encouraging as a bed material during gasification, was shown to be detrimental to the methanol synthesis process when used as a common catalyst/bed material for the water gas shift reactor. Two different cases of IGCC (Integrated Gasification Combined Cycle) systems were modeled and studied by varying the gas turbine technology employed. A new technology Inverted Brayton Cycle (IBC) gas turbine was simulated against a standard IGCC system. The Inverted Brayton Cycle system was shown to closely efficient to a standard gas turbine system working on product gas from the gasifier. The scale of biomass gasification was then identified as a significant parameter, which would determine the suitable choice of gas turbine technology to be employed. The thesis concluded by discussing some of the more influential parameters observed during the course of this study and recommending further optimization and mitigation steps corresponding to each of these identified losses. The thesis served its purpose by developing a quantitative tool to compare and validate different technological solutions to improve the process of producing methanol and power from biomass.