Chemical looping gasification of biomass

he steam gasification of biomass is endothermic. The energy can be supplied either by an external source or by combusting a part of biomass feed with oxygen. Air dilutes the produced syngas with nitrogen; while, pure oxygen increases the total production cost. Besides air and pure oxygen, the chemical looping process is an alternative to provide the required oxygen. Using the chemical looping system, the produced syngas has a higher calorific value compared to a conventional biomass gasification process with air. The oxygen is separated from air at high temperature using metallic oxides. The reduced metal is oxidized with air and transferred to the reducer where it releases oxygen. The reduced metal oxide is recycled to the oxidizer for regeneration, and the released oxygen in the reducer is available in the gaseous form to combust or gasify the biomass. To increase the thermal and mechanical stability of the metal oxide, it is supported with a high strength material, and in combination is called an oxygen carrier.;Oxygen carriers for biomass gasification are capable of absorbing oxygen from air and desorbing it in the gasifier. Based on thermodynamic equilibrium, copper, manganese and cobalt oxides have the highest oxygen release capacities among the different oxygen carriers. These oxygen carriers were deposited on alumina via incipient wetness impregnation. The weight loss of the CuO-Cu2O carrier, as measured in a thermo-gravimetric analyzer, was 10 %, while it was 7 %, for the Co3O4-CoO couple, and only 3 % for the Mn2O3-Mn3O4 couple. The optimum operating temperature for the CuO oxygen carrier was 100 °C higher compared to the other two at 950 °C. A modified nuclei growth model (MNG) characterizes the weight loss/gain during the reductionoxidation cycles. The reduction rate is 3 times higher at 875 °C compared to 825 °C, while, the oxidation rate decreases more than 10 times. The CuO carrier surface area decreased by 70 %, while it was 30 % and 60 % in the Co3O 4 and Mn2O3 carriers, respectively. Cobalt has a lower tendency to sinter at high temperature compared to either copper or manganese and has a higher oxygen transport capacity and oxidation-reduction rates. Therefore, despite its higher cost and toxicity it might be considered as a potential oxygen carrier especially for solid fuel gasification.;As the second part of the thesis, biomass was gasified in a bubbling fluidized bed with steam and oxygen. The oxygen was supplied by reducing Co 3O4 to CoO. The reduced cobalt oxide (CoO) was regenerated by switching the fluidizing gas to air. This technology known as chemical looping, is to combust or gasify fuels. The effluent gas is free of nitrogen and has a higher calorific value compared to gasification with air. From 825 to 875 °C, H2 yield increased up to 60 %. From 0 to 18 % of steam, H2 yield increased 4 times. Substituting 50 % of the sand as bed material with Co3O4, increased the CO yield up to 45 %, while lowering the H2:CO ratio. A two phase model for the dense bed and a plug flow model for the freeboard region were used for reactor modeling. The solids were assumed to be well mixed. From ambient to 850 °C, the axial dispersion coefficient increased from 0.09 to 0.12 m 2/s. Furthermore, the difference between the gas composition in the radial direction was negligible. The presented hydrodynamic model together with the kinetic expression from the literature characterized the transient gas compositions at the reactor outlet very well.;In the final stage of this thesis, a conventional gasification (CG) system and a chemical looping gasification (CLG) system to treat 86 t/d biomass were simulated with Aspen Plus and the operating and capital cost of them were compared. Conventional gasification (CG) systems use air as an oxygen source. Besides air, the chemical looping process is an alternative method to provide the system with the required oxygen. The syngas produced using the chemical looping has a higher calorific value than that produced using a conventional process with air. For comparison purposes, a conventional gasification unit with pure oxygen (CGPO) and a chemical looping gasification (CLG) system were simulated using Aspen Plus to treat 86 t/d biomass, and an economic analysis comparing the operating and capital costs of the two systems was performed. The two systems were identical except for the reactor configuration. The "CGPO" reactor consisted of a bubbling fluidized bed (ID=1.8 m and H=6.6 m) as gasifier and sand as bed material with oxygen supplied via a pressure swing adsorption unit. The CLG consisted of a bubbling fluidized bed gasifier (ID=1.8 m and H=6.6 m) working in parallel with a fast fluidized bed oxidizer (ID=1 m and H=10 m). Co3O4(8 %)/Al2O3i with a circulation rate of 44.6 kg/s between gasifier and oxidizer supplied the oxygen for the CLG system. The total capital investment (TCI) of the CGPO and CLG units were...