Sorption Enhanced High Temperature Water Gas Shift Reaction: Materials and Catalysis

Abstract

The work presented in this thesis focus on the hydrogen production by high temperature water gas shift reaction in conjunction to reforming process, as a significant industrial method for hydrogen generation. This thesis is a summary of three scientific papers. The work is directed towards synthesis of catalyst, evaluation for kinetic parameters and reaction mechanism, and sorption enhanced study to produce fuel cell grade hydrogen focusing high temperature water gas shift reaction. The main objective has been to achieve hydrogen rich product stream for fuel cell purpose and to screen a better catalyst for this purpose.

The first paper describes a much simplified route for the preparation for Ni-Co hydrotalcite derived catalysts as potential candidates for various hydrogen production processes. In this aspect, we designed catalysts by one- step spray drying technique with advantages of time economical, no separation steps, controlled morphology and promising yields with reasonable production cost as compare to conventional coprecipitation method. The results show crystalline structured, regular micro-spherical shaped catalysts with narrow particle size distribution, reasonable metallic dispersions and high surface area properties.

In the second paper, we report kinetic study over Ni-Co bimetallic system for high temperature WGS reaction, which is relevant for the pre-reforming of natural gas and the sorption enhanced reforming processes. Dependence of the reaction rate on temperature and partial pressures of CO, H2, CO2 and H2O has been determined. The reaction order was found to be negative first order in hydrogen, zero order for carbon dioxide, and first order with respect to both carbon monoxide and water partial pressures. Whereas at relatively low partial pressure condition of hydrogen, the reaction order was observed to be +0.5 with respect to hydrogen and water order goes to zero. The WGS reaction mechanism on Ni-Co catalysts by a detailed kinetic modeling in a wide range of operating conditions was elucidated. In this regard, we demonstrated two mechanisms namely redox mechanism and mechanisms with COH formation to explain experimental data. Furthermore, the catalyst performance was accessed for methanation reaction occurring as side reaction during water gas shift reaction. Particular attention is paid to develop a catalyst with high WGS activity but low methanation activity.

The third paper focus on sorption enhanced water gas shift (SEWGS) process. Here we established both experimentally and thermodynamically that integration of in-situ CO2 capture to high temperature water gas shift reaction as one pot process can effectively produce fuel cell grade hydrogen at optimum temperature of 500oC. Below this temperature the reaction is kinetically limited, whereas at higher temperatures it is thermodynamically constrained. The process optimization factors, such as temperature, steam/CO and CO partial pressure were studied. Catalysts play a significant role in CO conversion via WGS, methanation and methane steam reforming reactions. Pd promoted Ni-Co catalyst (1%Pd-20%Ni-20%Co) derived from hydrotalcite-like material (HT) showed a high activity of WGS and methane steam reforming. The methanation activity was further reduced on 30%Ni-10%Co. We demonstrated also a challenge for hydrogen production at high CO pressures where CO pressures show a negative influence on the WGS activity and an induction period exists. The CaO-based mixed oxide CO2 sorbent showed good CO2 capture capacity and stability in the cyclic operation of SEWGS reaction.