A Study of Catalytic Carbon Dioxide Methanation Leading to the Development of Dual Function Materials for Carbon Capture and Utilization
Creators: Duyar, Melis Seher
Publisher: name: Columbia University
Year: 2015
Subjects: Engineering

description: 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 (320°C) 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 280°C with a space velocity of 4720 h⁻¹. 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 260°C, 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 Eley-Rideal model at 230°C 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.