Detailed modelling of biomass steam gasification in a dual fluidized bed gasifier with temperature variation

Biomass gasification is a very efficient process to produce clean energy in the form of green hydrogen, synthetic natural gas (methane) and liquid chemicals. The products of biomass gasification process can be employed for energy production in a more efficient way. The modelling of biomass gasification enables the optimization of the process designs, but it is a challenge due to its high complexity. In most cases this process is treated with a black box approach where the sub-processes are neglected and only changes between the input and output are assessed. Here a model for prediction of the performance of a 100-kW dual bed fluidized biomass gasifier is derived and implemented. Detailed pyrolysis modelling is properly addressed, and this is believed to be a key factor of this approach and enables more accurate results. The proposed model and its basic assumptions were extensively validated on a range of operating temperature by conducting experiments using softwood pellets as fuel and fresh olivine sand as bed material. To achieve the objective, experimental tests have been conducted and theoretical models have been developed. The results provide understanding of the conversion processes occurring in the different parts of the gasifier enabling optimization of the system under different conditions. The main achievements are summarized in the following: • The fluid dynamics of the system, i.e. the distribution of gas and solids in different parts of the gasifier, the mixing of fuel with bed particles and the operational range at which the gasifier can be safely operated are calculated and validated against the experimental measurements. • The main fuel conversion processes (devolatilization and char gasification) were studied thoroughly. The former through the literature and the latter by the experiments conducted in a thermogravimetric analyzer (TGA). • Water gas shift reaction as the main and most important homogenous reaction in the process of biomass steam gasification has been investigated experimentally to obtain the proper kinetic parameters. • A reactor model of the DFB gasifier system of TUW was developed using the findings from the experimental studies conducted previously, supported by additional data from literature. Simulations were performed to obtain the main outputs of the biomass gasification process and the results have been validated with the experimental measurements. The overall conclusion of this work is that the proposed model for the DFB gasifier is an interesting method to simulate the thermochemical conversion of biomass in the gasifier and this technology is very suitable for electricity production from biomass and waste. Further research on tar conversion processes is necessary as well as testing the flexibility of the model against various bed materials.