Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Chemical and Biochemical Engineering

Supervisor

de Lasa, Hugo I.

Abstract

Biomass gasification yields a blend of H2, CO, CH4 and CO2, designated as syngas. Syngas can be further combusted using fluidizable oxygen carriers (OCs) during power generation via Chemical Looping Combustion (CLC). To improve syngas CLC and establish its application, a new Ni-based oxygen carrier with a Co and La modified γ-Al2O3 support was studied. This type of OC considerably limits the NiAl2O4 formation. Therefore, the oxygen carrier was engineered using a special preparation methodology to eliminate NiAl2O4 species formation. This promising Highly Performing Oxygen Carrier (HPOC) was characterized using XRD, BET, H2-TPR, NH3-TPD and pulse chemisorption.

Isothermal CLC runs were carried out in the CREC Riser Simulator which is a novel mini-batch fluidized bed reactor. A computational fluid dynamics (CFD) simulation was developed using the COMSOL Multiphysics® module to analyse gas-solid mixing patterns in the CREC Riser Simulator. This CFD model allowed to calculate the axial and circumferential gas velocities, the pressure changes and the geometrical modifications required in the reactor.

Reactivity runs using the CREC Riser Simulator, were developed as follows: a) 2-40s reaction times, b) 550-650°C, c) H2/CO ratios at 2.5 and 1.33 and d) 0.5 and 1 fuel to HPOC oxygen stoichiometric ratios. Encouraging results were obtained employing a 2.5 H2/CO ratio with a 92% CO2 yield and 90% CO, 95% H2 and 91% CH4 conversions. As well, the HPOC showed an oxygen transport capacity in the 1.84-3.0 wt% (gO2/gOC) range, with a 40-70% oxygen conversion.

A thermodynamic based model was established to predict the CLC syngas conversion limits. Additionally, a kinetic model was proposed for syngas CLC using the HPOC. This solid state kinetics considers a Nucleation and a Nuclei Growth Model (NNGM). This kinetics led to a ten intrinsic kinetic parameter model. These parameters were determined via numerical regression within a 95% confidence interval and with small cross-correlation coefficients. As a result, the kinetic rate constants showed that the reactivity of the syngas species could be expressed following the H2>CO>CH4 order.

Given the high performance and stability of the developed HPOC and the successful established kinetics, a computational particle fluid dynamics (CPFD) simulation for a 100kW CLC facility was developed. This simulation used a hybrid Barracuda VR CPFD model featuring “single particles” and “clustered particles”. According to the simulated CLC performance, it is anticipated that 90% CO2 yields can be achieved as follows: a) in a 25-m length downer unit simulated using the “single particle” model, b) in a 30-m downer unit simulated with the “clustered particles” model. Furthermore, it was observed that a L-type loop seal with air-pulse system eliminates the CO2 gas leakage, which usually occurs when the syngas moves from the fuel reactor to the air reactor.

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