(59e) Exergetic Analysis of Chemical Looping Reforming

Chemical looping combustion (CLC) is an emerging clean combustion technology in which metal oxygen carriers are exposed to oxidation/reduction cycles with air and fuel, respectively.  The oxygen carrier is first oxidized by air in the oxidizer, forming a metal oxide, and then reduced by a fuel in the reducer, forming CO₂, water, and the reduced form of the metal.  Overall, CLC yields a flameless, NOx-free combustion and produces sequestration-ready CO₂.  CLC has long been recognized as a highly efficient energy process when compared to conventional combustion.  This increased energy efficiency is achieved via the division of the conventional gas phase combustion reaction into the two reversible gas-solid reactions in the CLC oxidizer and reducer.  The high efficiency is due to a reduction in the entropic losses, or irreversibilities, associated with the combustion process, i.e. chemical looping has high exergetic efficiency, resulting in more usable energy when compared to conventional gas phase combustion. 

Beyond CLC, several chemical looping reforming (CLR) technologies have also been identified.  For instance, the use of steam instead of air as the oxidant has been demonstrated by our group and others to yield an efficient route produce high-purity H₂ as the oxidizer effluent.  Similarly, our group has recently proposed the use of CO₂ instead of air as the oxidant, resulting in chemical looping dry reforming (CLDR) as an novel method to utilize CO₂ via reduction to CO (i.e. via “CO₂ activation”).  While the thermodynamic and kinetic feasibility was demonstrated for these chemical looping reforming processes, their exergetic efficiency has not been studied to-date. 

In the present work, we have developed a process model for CLR processes and implemented it in the Aspen modeling software.  First results from these studies to-date indicate an increased exergetic efficiency of chemical looping dry reforming (CLDR) compared to conventional dry reforming.  Extension onto H₂ production via CLR processes and comparison to conventional H₂ production via steam reforming and partial oxidation is currently under way.