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(396e) Low Temperature “Super-Equilibrium” Reforming of Methane through Chemical Looping

Authors: 
Neal, L. - Presenter, North Carolina State University
Li, F., North Carolina State University
Ruan, C., NC State University
The complexity and cost of reforming reactors is a major barrier to distributed, natural gas-to-liquids technology. Such schemes rely upon partial oxidation/autothermal reforming (POx), where a pure oxygen feed is reacted with methane to form a ~2:1 H2:CO syngas. One of the more obvious limitations of the process is the cost and safety concerns of producing and using purified oxygen. We have previously shown that chemical looping reforming (CLR), in which the oxygen for the methane POx reaction is provided from a mixed-metal oxide, can greatly alleviate these issues. However, both traditional POx and CLR have a more subtle limitation for distributed scales: at lower temperatures (<750 ˚C), the equilibrium reaction favors the coexistence of significant amounts of CO2, H2O and CH4. At 650 ˚C, for example, yield of syngas is limited to 57%. While increasing the oxygen stoichiometry will increase methane conversion, syngas yield is reduced, and CO­­2­ and H2O yields increase. The inability to operate below 750 ˚C greatly reduced the types of materials reactors and valves can be made from, leading to high costs.

In this work, we present a “super-equilibrium” mode for CLR operation, which allows for avoidance of the thermodynamic limitations on reforming. In this configuration, two types of chemical looping catalyst are employed, a POx catalyst and a Splitting catalyst. A CLR POx catalyst allows for high conversion of methane at low temperatures (e.g. 650 ˚C), resulting in a mixture of syngas, CO2, and H2O. High methane conversion is achieved by using a stoichiometric excess of oxygen. To improve syngas yield, the CO2 and H2O are split to CO and H2 through the oxidation of a splitting catalyst. Integration of the splitting and POx catalysts allows for pseudo continuous operation with “super-equilibrium” syngas yields. Here we describe the scheme in detail, including thermodynamic analysis of the overall reaction and ideal mixed oxide systems. Experimental work confirming “super-equilibrium” yields is also shown.