Investigation of hydrogen generation in a three reactor chemical looping reforming process

Abstract

Chemical looping reforming (CLR) is a relatively new method to produce hydrogen (H2) and is also used as an energy conversion method for solid, liquid or gaseous fuels. There are various advantages of this method such as inherent carbon dioxide (CO2) capture, minimal NOx emissions and the H2 production. In this process, there is no direct contact between the fuel and oxidizer. This method utilizes oxygen from an oxygen carrier which may be a transition metal. The idea is to split the combustion process into three separate sub-processes by employing three separate reactors: air reactor where the oxygen carrier is oxidized by air, fuel reactor where natural gas is oxidized to produce a stream of CO2 and H2O and steam reactor where the steam is reduced to produce H2.In this study, a thermodynamic model with iron oxides as oxygen carrier has been developed using Aspen Plus by employing conservation of mass and energy for all the components of the CLR system. The developed model was employed to investigate the effect of various operating parameters such as mass flow rates of air, fuel, steam and oxygen carrier and fraction of inert material on H2 and CO2 production and key reactor temperatures. The results show that the H2 production increases with the increase in air, fuel and steam flow rates up to a certain limit and stays constant for higher flow rates. The CO2 production follows a similar trend. Similarly, the H2 production also increases with the increase in oxide flow rate and fraction of inert material up to a particular value, but then decrease for higher oxide flow rates and inert fractions. Reactor temperatures were also observed in this study. The temperatures increase with the increase of steam and air flow rates till the stoichiometric values and decrease thereafter. Increase in the fuel flow rate shows a decreasing trend in all the reactor temperatures. The reactor temperatures increase with the increase in oxide flow rates and the decrease of the inert mass fraction. The data obtained from the simulations is also used for exergetic analysis of the system. The overall exergetic efficiency was found to decrease with steam, air and oxide flow rates and increase with the increase in the fuel flow rate and the fraction of inert material.