An investigation into NASA Mars Project Technology and its application to the carbon dioxide removal on planet Earth
As we all know, our home is under a crisis. Global warming is one of the most urgent issues to solve in today’s society. Human activities have increased the atmospheric concentration of carbon dioxide along with other greenhouse gases, and its impact reaches beyond just rising temperature — sea level rise, ocean acidification, more extreme weather events, biotas disruptions, etc. Fortunately, there are already some solutions in progress aiming to remove carbon dioxide in the atmosphere at a meaningful amount. However, in this paper, we are going to look for the solution somewhere that has not been looked into — Mars. NASA Mars project has a plan to turn CO2 (carbon dioxide) in the Martian atmosphere into use. In this article, we will investigate NASA’s plan and see if we can adapt this technology for Earth to capture and convert CO2, in order to fight against climate change.
Part I. On Earth: The Path to Take to Limit Warming to 1.5°C
We need to limit our warming to 1.5°C to reduce climate-related risks, according to the IPCC’s Global Warming of 1.5°C Special Report in 2018. To achieve this, net anthropogenic CO2 emissions need to decrease by about 45% from 2010 levels by 2030 and reach net zero around 2050. While reducing carbon emissions is important, the current level of our effort is not nearly sufficient to limit the warming to 2°C (and remember, we want to limit it to 1.5°C). Therefore, removing atmospheric carbon dioxide is also crucial.
Generally, carbon dioxide removal (CDR) can be put into three categories: biological approach such as reforestation, engineered approach, namely capture at point source or Direct Air Capture (DAC), and lastly hybrid approach such as BECCS (Bioenergy with Carbon Capture and Storage).
Part II. On Mars: NASA’s Plan
Mars is an environment quite different from the Earth and yet contains elements that are familiar to us. Its atmosphere is very thin, with a pressure of only 0.6 percent of that on Earth. The atmospheric composition (by volume) according to the Mars Fact Sheet by NASA(last updated 11/25/2020) is as followed:
- Carbon Dioxide (CO2) 95.1% — by far the largest constituent of Martian atmosphere
- Nitrogen (N2) 2.59%
- Argon (Ar) 1.94%
- Oxygen (O2) 0.16%
- Carbon Monoxide (CO) 0.06%
- Minor Constituents (ppm): Water (H2O) 210, Nitrogen Oxide (NO) 100, Neon (Ne), Hydrogen-Deuterium-Oxygen (HDO), Krypton (Kr), Xenon (Xe)
In NASA’s Human Exploration of Mars Design Reference Architecture 5.0 paper (and its addendum), they laid out a detailed plan on how to get humans to Mars. By sending humans to Mars and back and allowing extended surface stay, there is an extra challenge: the large amount of life support consumables and propellant required, resulting in a much heavier mass that must be launched from Earth. Therefore, an alternative — an In-Situ Resource Utilization (ISRU) plant, is designed, as its name suggests, to be used on Mars, with the resources available on site, to produce these critical mission consumables. This reduces the total amount of mass for launch or allows more payload or science equipment to be carried in place of the consumables, as well as reduces IMLEO (initial mass in low-Earth orbit), which means lower cost and risk. For Mars ascent vehicle propellant, liquid oxygen (LO2)/CH4, LO2/H2, and hypergolic propellants are all examined, and LO2/CH4 was chosen. In a word, ISRU is crucial to Mars human mission and has been studied extensively.
Carbon dioxide (CO2), which is the most abundant in Martian atmosphere, is a great source of carbon and oxygen to produce oxygen (O2), methane (CH4), and other hydrocarbons. However, it needs to be collected, separated, and pressurized before it can be used or processed. (Note: water is another resource of interest for ISRU but not the focus of this article, so we will not consider the usage of Martian water.)
The NASA plan evaluated three primary approaches to collect and pressurize CO2: mechanical pumps, micro-channel adsorption, and cryogenic separation (CO2 freezing).
- Mechanical pumps compress air. As of right now, a scroll compressor developed by Air Squared is in use in the 2020 Perseverance Rover on Mars.
- Micro-channel adsorption uses small beds to rapidly adsorb and desorb CO2 in cycles.
- Cryogenic separation (CO2 freezing) takes advantage of the closeness of the Mars nighttime temperature and the temperature difference between solid and gaseous CO2 (at Mars’ pressure). At Martian nighttime temperature (150K/-123°C), a CO2 freezer (solidification pump) solidifies CO2 by further lowering atmospheric gas temperature with active cooling (cryocooler). Then, frozen CO2 can be re-heated to be released in a gaseous state at a controlled rate.
Following, there are three conversion methods of CO2 into O2 for propellent fuel and life support. Their chemical formulas and information are shown in the following table. ISRU processes can either convert CO2 directly into O2 (1st process: solid oxide CO2 electrolysis) or produce water first and then use water electrolysis to produce O2. The latter option is seen in the second and third processes, where H2 is required to be brought from Earth. The second process, Sabatier reaction with water electrolysis also produces methane, CH4, as fuel.
In terms of mass, power, and volume, NASA showed that the best combination for propellant production is microchannel adsorption (or rapid cycle adsorption pump/RCAP) with the solid oxide CO2 electrolysis (SOCE). The volume of H2 needed to be brought from Earth to produce methane is too enormous to be desirable. The SOCE only needs CO2 as the reactant and will produce O2 from Mars’ atmospheric CO2 for propellants and life support (produced CO is vented). The CH4 fuel for ascent will be brought from Earth. By utilizing ISRU for oxygen production, the mass is substantially lower — 54,062 kg, compared to 79,428 kg without oxygen production.
The solid oxide CO2 electrolysis method is used in MOXIE (Mars Oxygen In-Situ Resources Utilization Experiment) in the 2020 Perseverance rover, which is currently running on Mars. It is paired with the mechanical pump (scroll compressor from Air Squared) capture method mentioned earlier. This is an experiment, so MOXIE has a similar size as a car battery, and future full-size ISRU will be 100 times larger.
Part III. Applicability of NASA Technology to the CO2 Challenge on Earth
That was the ISRU plan for Mars, but can we use their technology for our Earth CDR challenge? Yes. From capture methods offered above, microchannel adsorption has the potential to be used for Direct Air Capture (DAC) and will be the focus of this applicability analysis. If we also choose to convert the capture CO2, all three conversions methods are feasible and had been studied in the context of combating global warming and reducing reliance on fossil fuels.
Capture — Microchannel adsorption
First of all, we ruled out the other two options as they do not match with Earth’s conditions. Mechanical pump does not offer separation of CO2 from the air. As CO2 takes up 96% of the Martian air, it has shown to work well for the MOXIE on Mars, producing 99.6% pure oxygen. However, it does not serve the purpose of capturing CO2 on Earth, where it is only 0.04%, a drastically smaller constituent — it will only serve to compress air. Cryogenic separation takes advantage of the low Martian nighttime temperature, but on Earth at its pressure, the freezing point for carbon dioxide is -78°C, which is far below the average temperature. It would thus use a lot of energy (compared to micro-channel adsorption method).
Therefore, micro-channel adsorption is the winner. In fact, it has the same principle as some existing Direct Air Capture (DAC) methods, such as ones from Climeworks and Global Thermostat. They all utilize cycles of adsorption and desorption: adsorption happens at a lower temperature — CO2 bonds to the surface of the solid sorbent; desorption (regeneration) happens at a moderately higher temperature (80°C-120°C)— CO2 is freed and can be collected. This has a much lower temperature and heat requirements than aqueous solution DAC (what Carbon Engineering is using) and can utilize low-grade waste heat from industrial plants.
However, what differs micro-channel adsorption from these existing companies is that it uses small adsorption beds rather than larger ones, which reduces the needed mass and cycle time and increases the performance of CO2 collection substantially. This specific method and design come from the Pacific Northwest National Laboratory (PNNL). Climeworks’ cycle takes 4–6 hours, Global Thermostat’s cycle takes under 30 minutes, while micro-channel adsorption from PNNL takes less than 2 minutes. PNNL tested a stainless-steel unit with 1.2g of Zeolite 13X (the solid sorbent) in a temperate cycle from 12°C to 77°C — it was able to desorb 0.084g of CO2 under 2 minutes. This translates to 1.59 mmol of CO2 captured per gram of adsorbent. However, this test is run under pure CO2 stream, so we should test this system at the Earth’s atmospheric conditions. The other atmospheric content and low concentration of CO2 (0.04%) are what make DAC challenging.
We also need to find a suitable adsorbent. Zeolite 13X is the tested adsorbent in the PNNL research, as it has a high CO2 adsorption capacity, but it might not be the best choice for our atmosphere. In addition, water is highly adsorbed by zeolites so the air has to be dried by a desiccant before the system. There are many adsorbents to be considered, such as other zeolites, porous metal-organic materials (MOMs), amines, and modified ones. They have been tested but in different studies under different conditions and systems. We should test them specifically in the microchannel adsorption system with its small adsorption beds and fast cycles, consider factors such as CO2 selectivity, adsorption capacity, stability, energy requirements, and cost, and find the overall winner.
Additionally, as micro-channel adsorption goes through rapid cycles of high and low temperature, PNNL also developed a system to recuperate heat by running the heat exchange flow loop in the opposite direction of the microchannel cells. It has the potential to reach thermal recuperation efficiency of 80 to 90%, which could provide massive energy savings.
This capture technology offers some modification from what we have seen. It could be adapted or at least worth looking into for its potential. We need further testing of the system with different adsorbents and temperature ranges to find ones most efficient in energy and cost.
Then, what can we do after collecting CO2? There are many options
- sell to soda companies, greenhouses like some companies are doing
- store underground directly but there has to be a demand from the people and corporations or some government actions. This would be carbon sequestration, a carbon-negative process.
- convert it into useful products.
Sabatier with water electrolysis produces oxygen and methane. Oxygen can be sold for medical use and industrial use. If we use the produced methane as fuel, it could be carbon-neutral as it is recycled from the air but not newly released from the ground.
Solid Oxide CO2 Electrolysis and reverse water gas shift with water electrolysis both can produce oxygen and carbon monoxide. Carbon monoxide, an important industrial gas, can then be turned into useful products such as methanol and acetic acid.
There are many carbon dioxide removal methods. A big obstacle for all of them is funding and incentives. They need a viable business model and/or government regulations. Another challenge associated with Direct Air Capture (DAC) is the cost of technology development. Some debated that money should go to cheaper alternatives like reforestation. However, combating climate change requires us to take all the opportunities we can get. If we never invest in DAC technology, they would always be unobtainable. NASA’s technology for Mars could offer improvements to existing DAC methods or even be an applicable solution on its own. By drawing the connection between carbon dioxide utilization on Mars and carbon dioxide removal on Earth, it could benefit the progress for both challenges: human exploration of Mars and the protection of our planet.