Resumen de Modelo y simulación de un proceso de gasificación en tres etapas para residuos y biomasas (modelling and simulation of a three stage gasification technology for wastes and biomass)
Abstract Gasification allows the calorific value contained in a solid fuel to be transferred to a gas that can be employed for energy production in a more efficient way. Gasification in fluidized bed (FB) presents some advantages compared to other gasification tech- nologies. However, this technology presents two main limitations: the high tar content in the gas, which limits its application, and the low char conversion, which reduces the process efficiency. Existing measures to overcome these problems in standalone FB gasifiers are not effective enough or too expensive for small-to-medium scale units. Therefore novel designs enabling conversion of tar and char inside the gasifier are necessary. For this purpose a new three-stage gasification concept based on FB design is under development by the Bioenergy Group of the University of Seville.
The objective of this thesis is to model and optimize the proposed gasifier, outlining the advantages of the new system compared to existing designs. 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: ¿ Experiments were conducted in a cold-model to characterize the fluid dynamics of the system, i.e. the distribution of gas and solids in different parts of the gasifi- er, the mixing of fuel with bed particles and the operational range at which the gasifier can be safely operated.
¿ The main fuel conversion processes (devolatilization, and char conversion) were studied by measurements in a lab-scale FB. The product distribution and rates during devolatilization and the rate of gasification in mixtures containing carbon dioxide and steam were determined. Furthermore, the effects of the composition of fluidization agent on product distribution during devolatilization and the kinet- ics of secondary conversion of volatiles were studied. ¿ A reactor model of the three-stage system was developed using the findings from the experimental studies carried out previously, supported by additional kinetics from literature. Simulations were carried out to compare the three-stage FB ga- sifier (FBG) to conventional a one-stage FBG. It was found that the three-stage system significantly improves the performance of a one-stage system. Analysis allowed understanding the main factors affecting the conversion of tar and char in different parts of the gasifier and, therefore, to identify improvements. Simula- tions were made to optimize the system, finding that adjustment of the operating conditions allows complete conversion of tar and char with high process efficien- cy. The overall conclusion of this work is that the proposed three stage gasifier is an interesting technology for electricity production from biomass and waste. Further research on tar conversion processes is necessary as well demonstration of the new prototype at pilot scale.
