(628f) Process Analysis for Co-Removal of Methane and Carbon-Dioxide from Air

The Sixth Assessment Report of the IPCC Report [1] has clearly highlighted the contribution and the effects of non-CO₂ greenhouse gases on global warming. CH₄ is the second largest contributor to global warming after CO₂, with a concentration in the atmosphere that has increased by 63 ppb from 2011. In any scenario where the global average temperature rise is limited to 1-1.9 °C by the end of 2100, these emissions must be significantly reduced (>50%). However, mitigating CH₄ emissions in challenging since majority of the emissions are from distributed artificial and natural sources, and the concentration of it is very low (<2 ppmv) in the atmosphere. Agricultural sector is the major emitter of methane accounting for ~40% of the emissions in the world [2]. CH₄ emissions from the agricultural are more concentrated that the concentration in atmosphere. For example, the concentration of CH₄ in the air from ventilation stables is 10-300 ppmv, and the is 140-25000 ppmv in the headspace of enclosed manure storage [3]. These concentrations are very similar to the concentration of CO₂ in atmosphere. Therefore, synergies can be mapped between direct air capture (DAC) of CO₂ and methane removal from air. This study presents analysis of processes to remove CH₄ and CO₂ from air.

Two routes have been proposed for CH4 removal: (i) separation, capture and regeneration (ii) catalytic (thermal- or photo-) conversion. This work focuses on the second route, where CH₄ is first catalytically converted to CO₂ and H₂O. In the next step, the CO2 formed from catalytic conversion of CH₄ and the existing CO₂ in air is captured in a DAC unit. A first of its kind integrated process to co-remove CH₄ and CO₂ was presented by Sirigina, Goel [4]. In this process, air from ventilation stable having 300 ppmv of CH₄ is blown into a de-humidifier. The de-humidified air is pre-heated to 330 °C and is sent to the catalytic reactor where CH₄ is converted to CO₂ and H₂O. Heat from the air leaving the reactor is recovered before the air enters the DAC unit for CO₂ capture. For an air stream containing 300 ppmv CH₄ at the inlet of de-humidifier, the total energy to remove 1-ton CO₂-equivalent is 9.09 GJ. The energy consumption can be significantly reduced is photo-catalytic reactor designed at room temperature is used to convert CH₄. In this work, we present the comparison of thermal- and phot-catalytic routes to convert CH₄ followed by removing CO₂ in a DAC system.

Methods and assumptions

Air containing 300 ppmv is assumed in the analysis. 6.5 wt% Pd/Al₂O₃ has been considered as the thermal catalyst in this study and the kinetics has been obtained from Alyani and Smith [5]. The de-humidifier is important to the process since the amount of thermal catalyst is sensitive to the water vapor concentration in the feeds stream. The reactor is assumed to be isothermal. The 0D reactor model is developed in Python. 100% conversion of CH₄ is assumed in the reactor. 2% pressure is assumed in the heat-exchangers and de-humidifier. 5% pressure drop is assumed in the reactor and DAC system respectively. A vacuum temperature swing adsorption (VTSA) process is assumed for DAC, with energy requirements obtained from [6]. In the photocatalytic route, the reaction occurs at room temperature (25 °C). The pressure drops in the process are similar to the thermal-catalytic route. The key performance indicator in this study is the energy (GJ) per ton CO₂-equivalent removed from the process. Global warming potential of CH₄ is assumed to be 25.

Results

The thermal-catalytic route for CH₄ conversion followed by DAC for CO₂ utilizes 9.09 GJ per ton of CO₂-equivalent removed from air. This is very similar to the energy consumed in the DAC processes designed for only CO₂ removal [6]. However, removal of CH₄ has added climate benefits, since CH₄ is a more potent greenhouse gas than CO₂. The energy consumed for co-removal of CH₄ and CO₂ can be significantly lowered through the photo-catalytic route that does not require pre-heating of the feed stream to the reactor. In this study, we compare the performance in terms of energy per ton CO₂-equivalent removed of process routes integrated with thermal- and photo-catalytic reactors. In addition, sensitivity studies for different concentration of CH₄ in feed stream and pressure drop is presented. Results will be summarized in the article.

Acknowledgements

The work is part of the project “Energieffektiv negativa utsläpp frånjordbrukssektorn” (project number 50340-1) (https://www.mgm-negaf.proj.kth.se/) funded by Energimyndigheten (Swedish Energy Agency). This work is also financially supported by the Swedish governmental initiative StandUp for Energy.

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