Production of Inherently Separated Syngas Streams Via Chemical Looping

 Production of Inherently Separated Syngas Streams via Chemical Looping

Amey More and Götz Veser

 Department of Chemical Engineering, University of Pittsburgh, Pittsburgh, PA

Abstract:

‘Chemical Looping Combustion' (CLC) is an emerging clean combustion technology which offers an efficient and elegant route for fossil fuel combustion with inherent CO₂ capture based on the cyclic oxidation and reduction of an “oxygen carrier” (typically a metal) with air and a fuel, respectively. However, chemical looping can be used for fuel conversion well beyond combustion. We have previously shown that chemical looping allows for highly efficient production of synthesis gas from methane via partial oxidation. In the present work, we propose the formation of separated streams of CO and high-purity H₂ via non-oxidative methane cracking and oxidative removal of the produced carbon using CO₂ in a “looping configuration”, i.e. at periodic process conditions.  The proposed process utilizes the well-known reactivity of Ni towards C-H bond activation (which also makes Ni/NiO a popular oxygen carrier for conventional chemical looping combustion). CH₄ is cracked catalytically over Ni, producing gaseous H₂ and solid carbon which deactivates the carrier. The carrier is then regenerated by burning off the carbon using CO₂ as oxidizer_, enabling the reduction of CO₂ to CO.

Supported Ni carriers were synthesized using a simple wet-impregnation procedure and characterized using X-Ray Diffraction (XRD) and Electron Microscopy (SEM, TEM, EDX). Solid state and gas phase conversions and selectivities were determined via thermo-gravimetric analysis (TGA) combined with fixed bed reactor studies. We found that the nature of the support strongly influences particle size and distribution of the active metal species (Ni) and thus impacts the overall activity of the carrier towards methane cracking. Furthermore, these differences also impact the morphology of the carbon formed during the cracking reaction, which further impact overall carrier reactivity. Similarly, the oxidative removal of the carbon by CO₂ depends strongly on the type of carbon formed and hence on support properties. Based on these observations, suitable process conditions were identified and a simple, yet promising carrier system for an efficient chemical looping process for the production of separated syngas streams (i.e. separate H₂ and CO effluent streams) is proposed.