Initial Design of a Dual Fluidized Bed Reactor

Steam hydro-gasification (SH) of biomass holds great potential to produce transportable and storable fuels to replace fossil fuel. There is a critical task which needs to be addressed in order to scale up the process. SH is an endothermic reaction which requires external heat to operate. The use of two highly coupled reactors: one for SH and the other for combustion of solid feedstock may provide sufficient and efficient heat management and produce an outlet product with high carbon conversion. A dual fluidized bed (DFB) gasifier has been selected for this purpose. A cold mode DFB was built with acrylic plastic to simulate the gasifier in order to develop insight for the optimization of the reactor for SH. This is the main objective of my thesis. Hydrodynamics tests were carried out to better understand the solid flow behavior in the cold mode DFB. The mixing test found that the gases from two reactors within the cold DFB mixed in the fast bed. The mixing level decreased with increase in the gas velocity in the fast bed and the BFB. Also the degree of gas mixing decreased with the increase in solid inventory. The hydrodynamics test found that increase in the gas velocity in the fast bed and the BFB leads to increase in the solid holdup in the fast bed. This same trend was observed with the three sizes of sand. Design modifications are made to improve the design of DFB for SH based on the cold model studies. Heat and mass balance of SH in the DFB was calculated using the Aspen plus simulation tool. Combustion of 13.8% of char from SH produces the required heat for SH with the net heat duty of -0.4kw, when 1 dry ton/ day of pine wood is fed into SHR. The results of these studies are presented in the thesis and will contribute to the development of the dual fluidized bed reactor optimized for SH with a potential for commercialization of the process.