Game changer for propellant generation on the Moon.

 Copyright 2023 Robert Clark

ABSTRACT

 Proponents of getting propellant from the Moon for spacecraft already in Earth orbit, as for orbital propellant depots, base this on using high power to separate hydrogen and oxygen by electrolysis applied to the water ice in the Moon's south polar region. However, quite surprisingly there may already be free hydrogen at the Moon's south pole or available in far more energy efficient way then by electrolysis. This hydrogen may be so easily obtainable then it may be it can be profitably shipped to Earth for a clean energy economy.

Free Propellant on Mars and the Moon.

 In the blog post "Potential Game-Changer for Generating Propellant on Mars," I suggested that getting free oxygen and carbon monoxide from the Martian atmosphere would provide a free means of producing propellant on Mars. This is important since rather than requiring several football fields of solar panels or a nuclear power plant to generate the megawatts of power needed to separate the CO2 or H2O into carbon, oxygen, and hydrogen by electrolysis, we could actually generate megawatts of power after collecting the free CO and O2 in the Martian air then combusting them together:

2CO+O2​→2CO2​;ΔH=−569kJ/mol

 This may may seem odd that we can generate power by collecting the natural resources already there but note this is exactly what we're doing when we collect the coal or petroleum or uranium from the ground on Earth. You get far more potential energy out than what you put in to collect it.

 And in fact we could also generate hydrogen in a way that at the same time generates power as well. This comes from the reaction:

1.)CO + H2O → CO2+ H2, ΔH = -41 kJ/mol 

 The H2O could come from the abundant ice in the Martian soil or in fact from the Martian air as well. The hydrogen(H2) would then come from what is called the water-gas shift reaction: 

The water–gas shift reaction (WGSR) describes the reaction of carbon monoxide and water vapor to form carbon dioxide and hydrogen:

CO + H₂O ⇌ CO₂ + H₂

The water gas shift reaction was discovered by Italian physicist Felice Fontana in 1780. It was not until much later that the industrial value of this reaction was realized. Before the early 20th century, hydrogen was obtained by reacting steam under high pressure with iron to produce iron oxide and hydrogen. With the development of industrial processes that required hydrogen, such as the Haber–Bosch ammonia synthesis, a less expensive and more efficient method of hydrogen production was needed. As a resolution to this problem, the WGSR was combined with the gasification of coal to produce hydrogen. As the idea of hydrogen economy gains popularity, the focus on hydrogen as an energy storage medium when an alternative replacement energy source for hydrocarbons is used.

https://en.wikipedia.org/wiki/Water%E2%80%93gas_shift_reaction

 

 However, it may be this process can also work on the Moon. The LCROSS mission to my mind may have been one of the most significant planetary science missions ever conducted by NASA, as measured in relation to its cost. It was designed as a low cost "Discovery" class mission but what it returned was profoundly important. It had an ingenious design that sent a rocket stage to impact the Moon at the south polar region and observe spectroscopically the material sent up in the impact plume by a second spacecraft..

 The mission confirmed what was suspected that there was great amounts of water as ice in the Moon's south polar region. In fact LCROSS found an extensive list of volatiles concentrated at the south pole of the Moon:

Moon Blast Reveals Lunar Surface Rich With Compounds.
Science Oct 21, 2010 2:05 PM EDT.
There is water on the moon … along with a long list of other compounds, including, mercury, gold and silver. That’s according to a more detailed analysis of the chilled lunar soil near the moon’s South Pole, released as six papers by a large team of scientists in the journal, Science Thursday.
https://www.pbs.org/newshour/science/its-confirmed-there-is-water

 The amounts of volatiles found by LCROSS was extraordinary:

Lunar Impact Uncovered More Than Just Moon Water.

Oct. 21, 2010:  Nearly a year after announcing the discovery of water molecules on the moon, scientists have revealed new data uncovered by NASA's Lunar CRater Observation and Sensing Satellite, or LCROSS, and Lunar Reconnaissance Orbiter, or LRO—and it's more than just water.

In addition to water, the plume contained "volatiles." These are compounds that freeze in the cold lunar craters and vaporize easily when warmed by the sun. The suite of LCROSS and LRO instruments determined as much as 20 percent of the material kicked up by the LCROSS impact was volatiles, including methane, ammonia, hydrogen gas, carbon dioxide and carbon monoxide.

https://science.nasa.gov/science-news/science-at-nasa/2010/21oct_lcross2 

 These last three are quite important in regards to the lunar propellant issue. If the hydrogen is indeed free then that gives us a ready made source for the clean-energy hydrogen. However, I suspect the hydrogen seen from orbit after the rocket stage impact, was derived from the water-gas shift reaction. This reaction shown in eq. 1.) reacts carbon monoxide and water to produce hydrogen and carbon dioxide and is helped by high temperatures, as would have obtained after the rocket stage impact. So the carbon monoxide could have reacted with the water to generate hydrogen.

 Once we have hydrogen then we can send it back to Earth for clean energy. The gravity and required launch speed from the Moon is so low we could use continual launch methods such as railguns or even the space elevator, which is possible with current materials under the Moons low gravity.

 As in the case of the getting hydrogen or other propellants on Mars discussed in the prior blog post, we would need low energy separation methods.  A new method sounds quite promising:

Mechanochemical breakthrough unlocks cheap, safe, powdered hydrogen

By Loz Blain July 18, 2022

Australian scientists say they've made a "eureka moment" breakthrough in gas separation and storage that could radically reduce energy use in the petrochemical industry, while making hydrogen much easier and safer to store and transport in a powder.

https://newatlas.com/energy/mechanochemical-breakthrough-unlocks-cheap-safe-powdered-hydrogen/

 

 This proposed solution for low energy gas separation though still needs to be confirmed by other researchers. 

 Hydrogen on the Moon Cheaper and Cleaner than on Earth?

 The Department of Energy(DOE) set a goal in 2021 to reduce the cost of producing hydrogen in a clean fashion by 80% to $1 per kg in 1 decade. The chemical methods for producing hydrogen can already do it about a $1 per kg cost, but results in additional CO2 being added to the atmosphere. Using solar power for a clean energy hydrogen generation approach is more expensive. This described on this DOE page:

7.1. WHY HYDROGEN?

Clean hydrogen is of the greatest interest in addressing the climate crisis, with the recognition that hydrogen could play a crucial role in economy-wide decarbonization, particularly in the transportation and industrial sectors including sustainable fuels, iron & steel, ammonia and chemicals, and more.1 Hydrogen is an extremely clean energy carrier, as its consumption produces only water; it also has a high energy density by mass. Hydrogen can be used to power fuel cells or combusted in a hydrogen turbine to generate electricity, and could also serve as clean transportation fuel.

The first Energy Earthshot, the Hydrogen Shot launched in 2021, seeks to reduce the cost of clean hydrogen by 80% to $1 per kilogram in 1 decade. This is an ambitious initiative, since current costs of clean hydrogen are much higher. For example, electrolytic generation of hydrogen using renewable electricity costs at least $6 per kilogram.

Worldwide, about half of all hydrogen production is from reforming of natural gas (mainly steam methane reforming or SMR), with the remainder deriving from gasification of liquid and solid feedstocks such as coal, petcoke, and petroleum residuals, from oil as a byproduct, and with a few percent from electrolysis. Syngas from gasification already contains a significant amount of hydrogen, which can be increased through water gas shift (WGS) and separated into a pure hydrogen product meeting industry product quality standards. There are several conventional hydrogen separation processes, with the well-proven and moderate cost pressure swing adsorption (PSA) methods commonly chosen. PSA has the ability to produce high purity (99.9%) hydrogen at near feed pressure; however, relatively high hydrogen concentration in feed gases is required for its economics to remain favorable.

For either natural gas reforming or gasification routes, hydrogen production costs are lower than the electrolytic route, but the challenge is that the carbon footprint of conventional hydrogen production is large, as shown in the following table:

	Hydrogen product (kg/h)	CO2 product (kg/h)	CO2 emitted in stack gas (kg/h)	Carbon intensity kg CO2/kg H2
SMR w/ 90% capture	25,700	227,600	25,300	1.0
Coal gasification w/ 90% capture	25,700	456,700	49,300	1.9

Carbon Intensities of SMR and Coal Gasification-based Hydrogen Production (with 90% capture)

If all CO2 produced in these processes is emitted, and CO2 footprint ranges from about 10 to 20 times the mass of the hydrogen produced. Only 0.7% of fossil fuel-based hydrogen production is currently performed in conjunction with carbon capture and storage.2

The gasification route to hydrogen presents an opportunity to use low-cost and liability feedstocks including biomass, solid wastes, and waste coal, which could help reduce the environmental costs and liabilities of solid waste disposal and legacy waste impoundments. Carbon-neutral biomass feedstocks significant reduce the carbon footprint of gasification, and combined with high levels of carbon capture facilitated by high syngas concentrations of CO2 and hydrogen characteristic of efficient gasification processes, production of clean hydrogen from gasification could be a compelling option in the emerging decarbonized economy.

__________________________________________________________________

https://www.netl.doe.gov/research/Coal/energy-systems/gasification/gasifipedia/hydrogen

  Note then on the Moon, either the hydrogen is already free, or it can be obtained by the water-gas shift reaction that obtains for the low cost chemical production methods used on Earth. But now this can be considered clean production, at least in regards to Earth, in that the CO2 released would be on the Moon.

 As for energy generation actually solar or nuclear generation would not be needed. The materials for energy generation would be available in the Moon's regolith, according to the LCROSS results. See here:.

 You see there are abundances listed for H2 and H2O and CO and CO2 but nothing for O2 for combustion. Oxygen could be obtained from high temperature burning of lunar rocks, but this is energy intensive. We want an energy production method that does not require high energy input in the first place.

 One possibility might be to use the free magnesium Mg seen in the LCROSS data. Magnesium can burn in CO2 without requiring oxygen:

The General Chemistry Demo Lab

Reaction Of Magnesium Metal With Carbon Dioxide.

http://www.ilpi.com/genchem/demo/co2mg/index.html

  Another possibility would be to use the free sodium Na seen by LCROSS. Sodium reacts explosively with water releasing heat and also hydrogen, so this might provide an additional method of producing hydrogen.

 Robert Clark

Potential Game-Changer for Generating Propellant on Mars.

 Copyright 2023 Robert Clark

  As an educator it’s quite exciting that an important unsolved problem may be one university or high school students, or even amateur scientists can address.

  A possibly limiting technology for a manned Mars mission is the high energy requirements to produce the return propellant on Mars. Robert Zubrin estimated for a Starship-sized vehicle as it requiring possibly 10 football fields worth of solar panels, or possibly even a nuclear reactor placed on Mars:

Elon Musk’s Plan to Settle Mars Robert Zubrin.
https://www.yahoo.com/news/elon-musk-plan-settle-mars-093002155.html

 Marcus House in his video series also discusses the problem:

 SpaceX Starship can return from Mars without surface refilling.

Slide from the video showing proposed size of Mars solar panels for ISRU propellant production:

 

 Beginning at about the 12 minute point, House runs the numbers and considers it so daunting, at least for initial missions, that he suggests it might be better instead to transport the propellant from Earth with Starship tanker flights all the way to Mars. This of course discounts the entire advantage of using Mars generated propellant to achieve a smaller mission size.

 The problem is the method envisioned would be by either splitting CO2 into carbon and oxygen and/or H2O into hydrogen and oxygen by electrolysis to be used for the propellant. However, these are energy intensive operations, which explains why in the reverse direction you get so much energy out when you combine them for combustion.

 What the researchers for this new idea are proposing instead is getting the oxygen for Mars atmosphere by filtering out the free O2 already there:

Feb 19, 2022
Breathing Mars Air: Stationary and Portable O2 Generation.
Ivan Ermanoski
Arizona State University
The basis for TSSD is a two-step thermally-driven cycle operating below ~260 C. thermal swing sorption/desorption (TSSD)--to generate oxygen from the Mars atmosphere with 10x less energy than the state of the art. Our approach is motivated by thermodynamics: the minimum theoretical work to separate oxygen from the Mars atmosphere is ~30-50 times lower than to obtain it by splitting carbon dioxide.
Efficiency: TSSD is expected to be ~10x more efficient than MOXIE. For MOXIE, the target power requirement for oxygen propellant production is 30 kW. The TSSD estimate is only 4 kW; i.e., 90% less than MOXIE. Applying TSSD in rovers, the estimated power for oxygen pro-duction is only ~50 W/person.
https://www.nasa.gov/directorates/spacetech/niac/2022/Breathing_Mars_Air/

 Here’s the breakdown of the Martian atmosphere:

Atmosphere of Mars. General information[2]
Average surface pressure 610 Pa (0.088 psi; 4.6 mmHg; 0.0060 atm)
Mass 2.5x1016 kg[1]
Composition[3][4]
Carbon dioxide 95%
Nitrogen 2.8%
Argon 2%
Oxygen 0.174%
Carbon monoxide 0.0747%
Water vapor 0.03% (variable)
https://en.wikipedia.org/wiki/Atmosphere_of_Mars

 The authors so far are proposing just getting O2 from the Martian air. But note there is also free carbon monoxide(CO) in its atmosphere. Then it may also be possible to filter out CO from Mars air. This is quite important because CO can be made to combust with O2:

1.)2CO+O2​→2CO2​;ΔH=−569kJ/mol

 However, when CO can be obtained from low energy filtration from the Martian atmosphere then free hydrogen for propulsion can be obtained by the reaction:

2.)CO + H2O → CO2+ H2, ΔH = -41 kJ/mol 

 This uses water that both orbiters and landers have shown to be wide spread on Mars even in low latitude locations in ice form. 

You can then get methane, if that is the preferred fuel over hydrogen, by reacting the free hydrogen with CO2 by the famous Sabatier reaction:

3.) ∆H = −165.0 kJ/mol

 

 The nice thing about this proposal is that any decent high school or university lab or even well-appointed amateur lab can test it out themselves. Key question: what is the method requiring least energy to concentrate and filter out the O2 and CO when in the concentrations known in the Martian atmosphere?

 About how to filter out the O2 and CO, three possibilities come to mind:

1.)Gas centrifuge separation.

2.)Fractional distillation.

3.)Membrane separation.

 As discussed here: https://en.m.wikipedia.org/wiki/Air_separation

 Of game-changing importance is that rather than football fields of solar cells or even a nuclear power plant being needed to generate the propellant, it may be the process of producing the propellant actually generates power.

 But if this is so easy, why has this not been done before? Mars atmosphere is almost all CO2, so the obvious thing to investigate is getting O2 by splitting CO2 into carbon and O2 by electrolysis.

 On the other hand, O2 is only a little more than 1/1000th of Mars atmosphere, which itself is only 1/100ths Earth’s air density. So the oxygen there would be in the range of only 1/100,000th the density we normally see on Earth. So this would require an extreme level of densification to be usable.

 However, in this regard, it may be sufficient to just cool the oxygen to liquid form which is needed for the propellant application anyway. This might not require too high energy input since at night the temperatures drop to well below freezing on Mars. 

 The most optimal generation and separation/filtration methods have to take into account the conditions on Mars. For instance in eq. (2.) the energy released of -41 kJ/mol is when the water is in gaseous form. It's much less if it is liquid or frozen water form. But in this regard it is notable temperatures on Mars surface do reach above the melting point of water on large swaths of Mars during the the mid afternoon hours on Mars.

 It had been frequently said that liquid water can not exist on Mars because Mars landers had shown the air temperatures were always below freezing. But what was missed was the actual surface temperatures were often well above the air temperatures. This is a rather obvious point that's easily understood when you compare for example on Earth the temperature of the air on a hot Summer day with what the temperature on the ground might be, especially if the ground is a dark surface absorbing sunlight rather than reflecting it. See the graphs from the Spirit and Opportunity MER rovers:

 But because of the low atmospheric pressure the time at which the water will remain liquid is relatively short, even if the ground temperature will be above the melting point for hours. This is good for our propellant generation purposes though since we will get the water in gaseous form without additionally adding energy.

 

 Hydrogen fuel from Mars to Earth?

 The propellant generation from taking atmospheric oxygen and carbon monoxide might actually generate energy. But this means we could actually produce hydrogen and methane at low cost, unlike the electrolysis methods that require high energy input. This shows the importance of this in general not just for Mars. It could be a means of producing clean energy in the form of hydrogen being transported from Mars to Earth. Again, it is remarkable that this is something high school and university teams can contribute to establishing its feasibility.

 But for it to be feasible, the energy requirements for producing the O2 and CO will have to be improved. The NASA release on the new TSSD method touts its reduced energy requirements over the usual electrolysis method, MOXIE, giving its power requirements as 4 kW. But the news release doesn't say how much oxygen is produced by the process for that power. This article allows us to estimate it:

MARCH 3, 2022 BY ANDY TOMASWICK

Mars Explorers are Going to Need air, and Lots of it. Here’s a Technology That Might Help Them Breath Easy.

...

All that power consumption makes for an expensive system.  A MOXIE machine that can create 2 kg O2 per hour (enough to support two explorers) would require around 25 kW of power, or slightly less than the average American house uses per day.    While that may not seem like a lot, utilizing solar energy is a much more difficult prospect on Mars. Any early solar farm built to run the MOXIE system would dwarf the habitat it could supply with oxygen for just two astronauts.

Enter the TSSD, which nicely eliminates all three major problems with MOXIE.  The system itself relies on a thermochemical pumping system, which relies on heat differentials to move the atmosphere to the appropriate place, eliminating the need for a mechanical pump.  It also doesn’t suffer from carbon fouling as it doesn’t break apart CO2. Lastly, it doesn’t require too much energy, with Dr. Ivan Ermanoski, the PI on the NIAC funded project and a research professor at Arizona State University, expecting that it will be 90% more efficient than MOXIE.

https://www.universetoday.com/154809/mars-explorers-are-going-to-need-air-and-lots-of-it-heres-a-technology-that-might-help-them-breath-easy/

 TSSD needing 1/10th the power of MOXIE would suggest 2 kg per hour of O2 for 2,500 watts, or 1 kg per hour for 1,250 watts. The process operating at an hour would require 1,250w*3,600s = 4,500,000 joules, 4.5 MJ, of energy per kg of O2 generated.

 Then we also need to separate out the CO. The CO content in the Mars atmosphere is about half that of the O2 so as a first order estimate we can estimate it as taking 9,000,000 joules per kg generated, though to get the actual value would take closer examination of the whole process used to generate the O2 and how it would have to be adapted to generate the CO.

 To get an idea of how much total energy this is in practical terms for both O2 and CO production, equation 1.) gives the energy generated by combusting CO in O2 as 569,000 joules per mol of O2. Since O2 is 32 grams per mol, this is 569,000J/32gm = 17,800 joules/gm or 17.8 MJ per kg of O2. But the CO needed for the reaction is 2 moles for each mole of O2, and since they are close to the same molecular weight you need about 2 kg of CO for each kg of oxygen.

 So you need 4.5 MJ of energy to generate a kg of O2 and then 2*9 MJ = 18 MJ to generate the needed 2 kg of CO. This is a total of 22.5 MJ required to put in to get out the 17.8 MJ you would get by combusting the CO with the O2.

 For only producing the H2 by equation 2.), you really need only the CO but the molecular weight of the CO is 15 times that of the H2 so you would need 15 times as much CO and its high energy cost by the TSSD process for each kg of H2 produced.

 Then the major impediment for making this feasible is finding low energy methods of separating out O2 and CO at approx. 1 part per thousand concentrations from a CO2 atmosphere only 1/100th as dense as Earths.

 TSSD is about 1/10th as power intensive as the usual electrolysis method. To put this in perspective, instead of 10 footballs fields of solar cells to run the propellant generating plant for the Starship on Mars, it would only take 1 football field worth of solar cells. The energy requirements probably need to be reduced again by at least another factor of 10.

 Some recent advances in gas separation by absorption of surfaces might improve on the energy requirements of TSSD:

Weird Crystal Can Absorb All The Oxygen In A Room — And Then Release It Later

This could potentially make fuel cells, space travel, and scuba diving a lot more efficient.

BY SARAH FECHT | PUBLISHED OCT 2, 2014 1:15 AM EDT

...

Researchers from the University of Southern Denmark say they’ve invented a crystal that pulls oxygen out of the air and even water. Apparently, just a spoonful of the stuff can suck up all the oxygen in a room.

The crystal is a salt made from cobalt*, and it appears to be capable of holding oxygen at a concentration that is 160 times higher than the air we breathe. The paper notes that “an excess” of the substance would bind up to 99 percent of the oxygen in a room.

But what’s more remarkable is that the crystal can later release the oxygen when exposed to heat or low-oxygen conditions. In a press release, study author Christine McKenzie likens it to the hemoglobin in our blood, which uses iron to bind and release oxygen in the human body.

https://www.popsci.com/article/science/weird-crystal-can-absorb-all-oxygen-room-and-then-release-it-later/

 And:

Nanowindows Poke Holes in Graphene

By Shinshu University | June 4, 2018

Unlike the windows of your house, nanoscale holes in graphene (named as “nanowindows”) can selectively choose which type of air molecules can pass through.

Scientists from Shinshu University and PSL University, France, theoretically proved concerted motion of the nanowindow-rim to selectively allow molecules to pass, in an energy-efficiently and fast way (Nature Communications, “Air separation with graphene mediated by nanowindow-rim concerted motion”). This brings up new possibilities to create an advanced molecular separation membrane technology.

The mechanism of separation by nanowindows is that the atomic vibration of the nanowindow-rim changes the effective nanowidow size. When the rim of one side is deviated and the other is deviated to the opposite direction, the effective nanowindow size becomes larger than when the rim does not move. This effect is very predominant for molecules of oxygen, nitrogen, and argon, inducing an efficient separation of oxygen from air.

https://www.rdworldonline.com/nanowindows-poke-holes-in-graphene/

Transportation of Cargo from Earth to Mars.  

 As for the prospect of sending the hydrogen thus produced backed to Earth, that's actually the easy  part. The delta-v to reach Earth from the surface of Mars is 6.4 km/s, if you use aerobraking on arrival at Earth. This can easily be supplied by a reusable single-stage-to-orbit from the surface of Mars. And if the CO and O2 can be obtained in an airbreathing manner for propulsion then it can be done even more simply, as discussed in the blog post, "Low cost commercial Mars Sample Return". The propellant could also be transported by a Mars cycler.

 Alternatively, this is low enough that a space elevator can do it using currently available materials. Still another possibility is by using a railgun, i.e., electromagnetic accelerator.

  

   Robert Clark