(503d) Design of Intensified Reactor for Lean Methane Emissions Treatment

A recent study of the Permian Basin [1] reported 194 metric tones of methane emissions to be released every hour. This makes up the methane emitted in the area to be 9.4 % of the total gas production [1], which is more than 6 times of the recommended value by the EPA [2]. Given the high global warming potential (GWP) of methane, solutions are urgently needed to reduce the impact of fossil fuels extraction and conversion on climate change [3]. Besides fossil fuels, the livestock sector accounts for about 31% of methane emissions in the US [4]. These emissions are commonly lean, diluted, and fluctuating in flow rate and composition, which makes it challenging to be converted into CO₂ in an economically sustainable manner. In this presentation, the design of a fixed bed reactor will be communicated, with the specific objective of handling lean methane emissions. The reactor is loaded with a metal-oxygen carrier and operates on a chemical looping scheme of alternating reduction-oxidation (redox) stages, while handling a continuous gas stream of lean methane. During the reduction stage, the metal oxygen carrier is reduced by the incoming gas. The oxygen carrier is, subsequently, oxidized with air in the following oxidation stage. The heat source for the overall process is provided by the exothermic reaction of methane and oxygen carrier oxidation during the oxidation stage. The metal-oxygen carrier functions as a thermal buffer that maintains the bed temperature during the alternating redox process. The feasibility of this concept is explored through dynamic simulation of a conceptual reactor loaded with a Ni-based oxygen carrier, for which reduction and oxidation kinetics have been studied extensively. The reactor model accounts for the mass, energy, and momentum balance of the fluid and solid phases inside the reactor, and the detailed kinetics of the gas, gas-solid and Ni-catalyzed reactions. We demonstrate the operational benefits of the alternating redox process by elucidating the benefits of a high-temperature zone inside the reactor bed that serves as the reaction front for lean methane conversion reactions. The resulting reduced Ni is continuously regenerated with air. We benchmark this concept against the performance of a reactor loaded with NiO operating as a reducer only, where the NiO bed gets saturated with Ni and then only promotes catalytic reforming of the lean methane to syngas. At cyclic steady-state the redox reactor system converts lean methane at 99 % conversion efficiency at a mean bed temperature of 720 °C, without the need for added heat. The proposed reactor concept was optimized to minimize methane emissions, by manipulating the air feed temperature, Ni oxygen carrier loading, and the intervals of the redox process stages. At the optimal operating condition, the reactor reached complete methane conversion at bed temperature of 850 °C, with a reaction front at the first 20% of the reactor length. The presentation will present the mathematical formulations of the reactor model and the optimization problem, simulation results that illustrate the fundamental benefits of the proposed concept and a discussion of practical limitations of the proposed concept.

Acknowledgements

This study was supported by the UTC Institute for Advanced Systems Engineering (UTC-IASE) at the University of Connecticut (UConn).

References

[1] Y. Chen et al., “Quantifying Regional Methane Emissions in the New Mexico Permian Basin with a Comprehensive Aerial Survey,” Environ. Sci. Technol., 2022, doi: 10.1021/acs.est.1c06458.

[2] M. R. Harrison, T. M. Shires, J. K. Wessels, and R. M. Cowgill, “Methane Emissions from the Natural Gas Industry, Volume 1: Executive Summary,” Epa, no. June, pp. 1–24, 1996.

[3] EPA, “Overview of Greenhouse Gas.” https://www.epa.gov/ghgemissions/overview-greenhouse-gases#methane.

[4] H. Aguirre-Villegas, R. A. Larson, and M. D. Ruark, “Methane Emissons from Dairy Cattle,” 2016. Accessed: Apr. 04, 2020. [Online]. Available: http://www.sustainabledairy.org/publications/Documents/DairyCap_Methane_....