Fluid dynamic analysis of dual fluidized bed gasifier for solar applications

A hydrodynamic model of a dual fluidized bed gasifier (DFBG) is developed and its predictions are compared with measurements of solids flux and pressure profiles from a cold flow model (CFM). Then, the performance of a DFBG gasifier is theoretically analyzed in terms of solids circulation and solids distribution under changes in riser and loop seal aeration, solids inventory and particle size, and a sensitivity analysis is made to delimit the model prediction capability. Furthermore, the model is applied to analyze the effects of key design aspects of DFBG, such as the relative size of riser and gasifier, the connection between both units, the circulation rate of solids and their distribution around the system. The model is further used to extend the DFBG operation with external solar energy carried by heated solid particles, i.e. to design solar DFBG (SDFBG). The analysis is focused on the performance with high solids inventory in the gasifier to increase the char conversion (operation with a large solar share) and the control of solids circulation to meet the heat demand of the gasifier with the availability of solar energy. The operation with large solids inventory in the gasifier requires the size of the gasifier to increase considerably compared to that of the conventional DFBG. The substitution of the connection pipe between the riser and the bubbling bed (current design in commercial DFBG) by a lower loop seal enables better control of the solids circulation, thus, benefiting the solar design

Modeling the transient response of a fluidized-bed biomass gasifier

The dynamic response of a bubbling fluidized-bed biomass gasifier (FBG) is examined. A transient model is developed by extending a previous steady-state model to account for key processes occurring during the ramp-up and/or changes in loading of fuel and gasification agent. The model is validated against measurements from transient tests in a laboratory-scale FBG. The model results are also compared with steady-state measurements and previous FBG models from the literature. A sensitivity analysis is performed to identify the most influencing parameters. The model is then used to study the transient response of industrial FBG under different operating conditions. It is shown that for given operational conditions (biomass flowrate, equivalence ratio, initial temperature, and initial char inventory in the bed), there is always an optimal start-up procedure (rate of change in feeding the gasifying agent and/or the fuel) leading to the shortest start-up time and lowest peak temperature. The transient period can be reduced by up to 75% compared to the reference value, in which the transient response can extend for more than an hour, due to the slow change in the inventory of char in the reactor. The model can be used to optimize the operation of hybridized power plants with biomass gasification and thermal energy storage