A Study of Catalytic Carbon Dioxide Methanation Leading to the Development of Dual Function Materials for Carbon Capture and Utilization

Abstract

A Study of Catalytic Carbon Dioxide Methanation Leading to the Development of Dual Function Materials for Carbon Capture and Utilization Melis S. Duyar The accumulation of CO2 emissions in the atmosphere due to industrialization is being held responsible for climate change with increasing certainty by the scientific community. In order to prevent its further accumulation, CO2 must be captured for storage or conversion to useful products. Current materials and processes for CO2 capture rely on the toxic and corrosive methylethanolamine (MEA) absorbents and are energy intensive due to the large amount of heat that needs to be supplied to release CO2 from these absorbents. CO2 storage technologies suffer from a lack of infrastructure for transporting CO2 from many point sources to the storage sites as well as the need to monitor CO2 against the risk of leakage in most cases. Conversion of CO2 to useful products can offer a way of recycling carbon within the industries that produce it, thus creating processes approaching carbon neutrality. This is particularly useful for mitigation of emissions if CO2 is converted to fuels, which are the major sources of emissions through combustion. This thesis aims to address the issues related to carbon capture and storage (CCS) by coupling a CO2 conversion process with a CO2 capture process to design a system that has a more favorable energy balance than existing technologies. This thesis presents a feasibility study of dual function materials (DFM), which capture CO2 from an emission source and at the same temperature (320C) in the same reactor convert it to synthetic natural gas (SNG), requiring no additional heat input. The conversion of CO2 to SNG is accomplished by supplying hydrogen, which in a real application will be supplied from excess renewable energy (solar and/or wind). The DFM consists of Ru as methanation catalyst and nano dispersed CaO as CO2 adsorbent, both supported on a porous γ-Al2O3 carrier. A spillover process drives CO2 from the sorbent to the Ru sites where methanation occurs using stored H2 from excess renewable power. This approach utilizes flue gas sensible heat and eliminates the current energy intensive and corrosive capture (amine solutions) and storage processes without having to transport captured CO2 or add external heat. The catalytic component (Ru/γ-Al2O3) has been investigated in terms of its suitability for a DFM process. Process conditions for methanation have been optimized. It has been observed that the equilibrium product distribution for CO2 methanation with a H2:CO2 ratio of 4:1 can be attained at a temperature of 280C with a space velocity of 4720 h. TGA-DSC has been employed to observe the sequential adsorption and reaction of CO2 and H2 over Ru/γ-Al2O3. It was shown that H2 only reacts with a CO2-saturated Ru/γ-Al2O3 surface but does not adsorb on the bare Ru surface at 260C, consistent with an Eley-Rideal type reaction. In this rate model CO2 adsorbs strongly on the catalyst surface and reacts with gas phase H2. Kinetic tests were employed to confirm this observation and demonstrated that the rate dependence on CO2 and H2 was also consistent with an Eley-Rideal mechanism. A rate expression according to the EleyRideal model at 230C was developed. Activation energy, pre-exponential factor and reaction orders with respect to CO2, H2, and products CH4, and H2O were determined in order to develop an empirical rate equation in a range of commercial significance. Methane was the only hydrocarbon product observed during CO2 hydrogenation. The activation energy was found to be 66.084 kJ/g-mole CH4. The empirical reaction order for H2 was 0.88 and for CO2 0.34. Product reaction orders were essentially zero.