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

Abstract

The importance of H2 as a clean source of energy is growing and so is the need to produce it efficiently with low carbon emissions. Chemical looping combustion (CLC) presents an attractive technology to produce H2 and electricity from natural gas with CO2 capture. The potential for this technology to produce clean energy by reducing carbon emissions with minimal cost increment has been a focal point of research in this field. The CLC process developed at The Ohio State University uses an iron based oxygen carrier and a unique counter-current moving bed reactor configuration for gas–solid contact. There exists a thermodynamic limit to the amount of H2 that can be produced in the oxidizer of the CLC system which restricts the steam conversion. There is an approach for surpassing this limit for improving the cold gas efficiency (CGE) of the CLC process by increasing the H2 yield per mole of steam in the oxidizer. In this paper, the use of staged H2 separation as the approach has been presented to increase the H2 production beyond the thermodynamic limits existing for a counter-current moving bed oxidizer in a FeHO system. The ASPEN Plus simulation software has been used to assess the feasibility of using this approach for increasing H2 yield. The effect of using these H2 separation modules on the operating line of the oxidizer has been assessed for all the configurations and compared against the original oxidizer reactor. The H2 separation module configuration giving the highest H2 yield has been analyzed under auto-thermal conditions for calculating the CGE of the entire CLC system. A sensitivity analysis on separation efficiency of the H2 separation module has also been performed to demonstrate the flexibility in the H2 separation technologies that can be applied for staged H2 separation. A maximum CGE of about 79%, which is 7% points higher than the DOE baseline for steam methane reforming (SMR) case and 1.5% points over the original CLC system, was calculated. The results conclude that utilizing a staged H2 separation configuration in the oxidizer can further improve the CGE efficiency of CLC process which makes it even more competitive to the conventional SMR process for H2 production.