Cold gas efficiency enhancement in a chemical looping combustion system using staged H2 separation approach

Significance Statement

Hydrogen energy is as an important replacement to fossil fuels and hydrocarbon-based fuels. A hydrogen economy is expected to reduce the dependence on fossil fuels and ensure energy security for developing countries. Therefore, there is an increasing need for efficient H2 production with minimal carbon emissions. Steam methane reforming process entails the reaction of natural gas with steam over a nickel-based catalyst. Although the process is cost effective, there is still the concern in the significant carbon dioxide emissions associated with it.

Chemical looping technology allows for carbon dioxide capture coupled with significant enhancement in exergy efficiency of the process. Therefore, chemical looping combustion for hydrogen production from methane has been focused on as a substitute as well as an enhancement of the typical steam methane reforming method. However, there is a thermodynamic limit to the amount of hydrogen gas that can be generated in the oxidizer of the chemical looping combustion systems, therefore, restricting the steam conversion.

Fortunately, it has been found a suitable method for surpassing this limit for enhancing cold gas efficiency of the chemical looping combustion process by raising the hydrogen gas yield. Researchers led by Professor Liang-Shih Fan from The Ohio State University investigated the application of the staged hydrogen separation for the oxidizer reactor implementing the ASPEN Plus software as an approach for  increasing the hydrogen gas yield from the chemical looping process. Their work is published in International Journal of Hydrogen Energy.

The major limitation for the production of hydrogen gas in the chemical looping combustion system is observed as the steam oxidation of the reduced iron oxide in the oxidizer reactor. The oxidizer reactor functions partially in oxidizing the reduced iron oxide particles, and hydrogen gas is produced from the steam. This is an energy intensive process that entails the use of large amount of high temperature steam as the oxidizing agent. It was realized that the production of the hydrogen gas was dependent on the degree of steam conversion.

One possible approach for increasing the hydrogen gas production was to shift the reaction equilibrium of the steam oxidation reaction towards generating more hydrogen gas. To achieve this, the authors had to conform to the Le Chatelier’s principle where hydrogen gas had to be continually removed from the system which was simulated through the use of hydrogen separation modules in ASPEN Plus simulation software. The use of hydrogen separation modules was found to have suitable effects on the hydrogen yield from the reactor by increasing the conversion of steam beyond the equilibrium limit. The effect of these modules on the operating lines of the oxidizer reactor was a reduced difference between gas and solids conversions against their equilibrium concentrations compared to the original reactor. Chemical looping combustion integrated with hydrogen separation modules was found to further increase the cold gas efficiency as compared to the typical steam methane reforming process for the production of hydrogen gas.

Cold gas efficiency enhancement in a chemical looping combustion system using staged H2 separation approach

 Reference

Sourabh G. Nadgouda, Mandar V. Kathe, Liang-Shih Fan. Cold gas efficiency enhancement in a chemical looping combustion system using staged H2 separation approach. International Journal of Hydrogen Energy 42 (2017), pages 4751-4763.

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