(780d) Oxide-Independent Thermodynamics of Solar Chemical-Looping Reforming for Producing Synthesis Gas and Hydrogen

The solar upgrading of natural gas to produce synthesis gas is an attractive option for producing more sustainable transportation fuels. Synthesis gas is a valuable feedstock for producing high octane gasoline, kerosene, and high cetane diesel. Synthesis gas produced from natural gas, water, and process heat from concentrated solar energy contains up to 28% more energy than the chemical feedstock, offering a corresponding 22% reduction in carbon dioxide emissions on an energetic basis. Solar chemical-looping reforming (SCLR) with a metal oxide oxygen carrier has attracted attention for the application because it offers an opportunity to produce separate product streams of pure hydrogen and synthesis gas with a two to one ratio of hydrogen to carbon monoxide. Furthermore, the solar to chemical energy conversion via SCLR has potential for high efficiency. With realistic accounting energy requirements including parasitic work, the projected efficiency for ideal chemical conversion with SCLR is 54%.

In this work, oxide-independent chemical and process thermodynamics analyses are applied to frame the requirements for attaining high efficiency with SCLR in practice. Chemical equilibrium influences on process efficiency are presented for both the syngas-producing methane partial oxidation reaction and the hydrogen-producing water splitting reaction. Key drivers of efficiency include the ratio of methane to oxygen in the methane partial oxidation reaction and the conversion of oxidizer in the splitting reaction. The equilibrium oxygen partial pressure provides a link between the performance of the two reactions. A process thermodynamics analysis quantifies the impact of methane conversion, hydrogen and carbon monoxide selectivity, and oxidizer conversion on efficiency. The parasitic work for separating synthesis gas from the products of the methane partial oxidation reaction is also considered. Methane conversion and oxidizer conversion are shown to have the largest impact on efficiency. With complete conversion of methane to hydrogen and carbon monoxide in the methane partial oxidation reaction, the projected efficiency increases from 18% to 54% as the conversion of water to hydrogen in the splitting reaction is increased from 10% to 100%.