(439d) Methane to Syngas By Chemical Looping Using FE-NI Oxygen Carriers: Reactor Design and Process Modeling

Chemical looping reaction scheme involves separation of a reaction into two separate steps, either spatially or temporally. One of the reactants, usually oxygen, is circled between the two steps using a metal-based oxygen carrier. Applications of chemical looping include total or partial oxidation of fuels. Typical oxygen carriers use transition metals such as Fe, Cu and Ni. Partial oxidation of methane (POM) to syngas (mixture of CO and H₂) using chemical looping has shown several advantages over the conventional process such as steam methane reforming (SMR) or catalytic partial oxidation (POX). The Fe-based oxygen carriers have the advantage of high selectivity to CO and H₂, while having low reactivity. On the other hand, Ni-based oxygen carriers show high selectivity towards total oxidation (production of CO₂), but have high reactivity. In previous work done by our group, novel oxygen carriers (Ni_xFe_(1-x)-CeO₂) were developed which combined the high selectivity of Fe for partial oxidation and high reactivity of Ni to produce syngas by methane reduction. Ni_0.12Fe_0.88-CeO₂ was identified as the optimal choice that maximizes the production of syngas while minimizing the formation of CO₂ and coking.

In this paper, we focus on a systems-level analysis of a methane-to-syngas process utilizing a chemical-looping fixed bed reactor with mixed Fe-Ni oxygen carrier material mentioned above. The overall system is designed to produce a desired flow rate and composition of syngas (as measured by the amount of CO and H₂ and the ratio of CO/H₂) from natural gas, while maximizing the yield and minimizing the overall energy consumption and CO₂ emissions. The system involves heating of natural gas and air (for reduction and oxidation steps, respectively), chemical looping reactor with alternating redox cycles, downstream cooling of gases with heat recovery, steam generation from waste heat and optionally, using steam for electricity generation. The model involves a detailed chemical looping reactor model, incorporated into the systems-level model. Reactor-level mass and energy balance calculations will be performed to calculate the effect of operating conditions such as flow rates, solids conversion and temperature on the final product composition, selectivity and yields of different components and solids inventory. The systems-level model includes thermodynamic calculations of individual process components such as blowers, compressors and heat recovery equipment, as well as steam turbines. The overall system efficiency and CO₂ emissions will be calculated for a design syngas flow rate and composition. Using sensitivity analysis on the process parameters, we will identify operating conditions required to improve process performance. The results will be compared to the conventional alternatives such as SMR and POX processes.