CO2 Acceptors for Sorption- Enhanced Steam Methane Reforming

Abstract

Sorption-enhanced steam methane reforming (SESMR) is an emerging technology for H2 production from fossil fuels with CO2 capture. The aim of the present work has been to study SESMR with a multiscale approach and to gain insight into certain aspects of the process.

Specail attention has been given to the development of new high-temperature CO2 acceptors and their impact on the workability of SESMR. The preparation and CO2 capture properties of Li2ZrO3, potassium promoted Li2ZrO3, and Na2ZrO3 have been studied in detail. A novel soft-chemistry route has been developed with success for the synthesis of mixed oxides. This method involves intimate mixture of the precursors and requires lower temperatures than conventional solid-state routes, forming nanosized crystals with high purities. The properties of the powders such as capture rate of CO2 and regeneration conditions have been significantly improved.

Nanocrystalline tetragonal Li2ZrO3 could hold CO2 in amounts equivalent to 27 wt%, and saturation was reached in less than 5 min at 848 K and 100% of CO2. These results represent important improvements in the carbon dioxide capture rates compared to monoclinic Li2ZrO3 prepared by solid-state reactions. However, low capture rates were observed when operating at CO2 partial pressures lower than 0.2 bar. Controlling the Li:Zr ration, and especially ensuring the presence of free ZrO2, allowed improved uptake rates. More beneficial was the promotion of oxide with potassium due to the presence of molten carbonates. It is generally accepted that doping with potassium favours the diffusion of CO2 through the Li2CO3 layer that is formed at the surface of the acceptor during the capture reaction. On the other hand, nanocrystalline monoclinic Na2ZrO3 showed superior uptake kinetics and the ability to work efficiently at CO2 partial pressure as low as 0.025 bar. However, Na2ZrO3 requires higher temperatures for regeneration.