Thermal Decomposition of CO2 with Nuclear Heat

A lot of effort must and will be put into combating climate change. We can however directly attack the root cause of it by reducing the amount of CO2 that we have released into the atmosphere.

We can enlist the help of ultra-high-temperature nuclear reactors to do this rapidly and efficiently.

Climate change is caused by greenhouse gases, principal of which is CO2. Our developing economies use plenty of power, and the principal source of it is cheap fossil fuels: oil, gas, coal. 

Beijing smog.

More CO2 means that more heat is trapped within the atmosphere, leading to increases in global average temperature. More heat also means more extreme weather and faster swings in temperature and humidity, while the CO2 molecule’s reaction with seawater causes ocean acidification and a decrease in its ability to absorb CO2. 

Our immediate response to worsening climate conditions is to burn more fuels. Air conditioning consumes a third of the peak power in US cities and the number of air conditioned homes is increasing by the tens of millions every year. Less developed countries that struggle with uncertainty in weather disasters and crop yield double down on the most secure energy source currently available.

Short-term relief, long-term pain.

Alternative energy sources, such as renewable energy or nuclear power, have their own issues. Solar panels and windmills must be paired with expensive energy storage solutions if they are to cover the needs of an electrical power grid. Nuclear reactors have been developed to the point where they are the safest, cleanest and most effective option, but only a handful of forward-thinking countries are building the new generation of reactors, while others are denied them for political reasons. How then to defeat the self-reinforcing cycle of fossil fuel consumption that leads to worsening climate conditions?

CO2 targets vs CO2 as the target

Punitive action has been put forward to reduce CO2 emissions. Carbon taxes, carbon quotas, carbon limits… they have varying results and can only realistically be burdened by the largest economies in the world. Smaller economies cannot afford to hamper their growth and development, especially seeing that they are only minor contributors to CO2 in the world. 

Tesla cars have very good air filters but come at 3-5x yearly Chinese wages in price.

Furthermore, no country, large or small, has the budget to switch over to fully carbon-free economies within the next century. Imagine every family being asked to buy a new electric car, every plane having to load up on synthetic fuels, every factory torn down to be replaced with electrical machines. 

You might be willing to and able to afford it. The richest companies in the world might have the budget for it….but half the world’s population lives on $2.50 per day. And even if all this is accomplished, it would not rid us of the damage already done to the environment.

Unless a radical change in economic structures and political motivations occur worldwide, controlling the release of CO2 cannot be relied upon. Instead, we can take CO2 out of the atmosphere.

 

This has been proposed many times, and demonstrated at multiple sites and scales.

Most solutions require a large quantity of precious metals to serve as catalysts, consume alkali metals or simply try to hide it away in underground chambers. This is evidently unable to deal with the 40+ gigatons of carbon dioxide we release each year. 

Absorbing CO2 using soda/lime.

A quick calculation shows that just as many gigatons of calcium carbonate would have to be found, or finding volumes greater than 20 trillion m^3 to store CO2 underground.

 

Furthermore, using grid power to capture CO2 is a very expensive option. We rely on cheap electricity to run our global economies and raising prices by up to +75% will only push weaker economies to burn the cheapest energy source for power (and strong economies to buy their power or produce their goods abroad with ‘dirty’ grids). Again, this is for simply capturing CO2 and not eliminating it.

 

A solar or wind-powered carbon capture plant beats the energy balance problem but just does not have enough power output to remove enough CO2 to make a difference. Each square meter of solar panels would produce 30W/m^2 on average throughout a day. This is just enough to remove 7 kilograms per day of CO2 from the air, if we take the capture cost in energy to be 370.8 kJ/kg. Not to solve the carbon problem, but just to contain it.

Solar thermal is more effective but still insufficient.

Nuclear power can come to the rescue.

 

Thermal decomposition

Construction of San Ofre Nuclear Generating Station began in 1964.

A typical nuclear reactor is over 40 years old. It has a multitude of uranium fuel rods that are packed together in a large pressure vessel and their heat is used to boil water into a steam at about 600K. A huge installation of turbines, generators and coolers must draw water from a nearby source to convert that heated steam into electricity. If the reactor fails, it fails spectacularly. 

A 4^th generation or newer reactor would use fuel held in self-contained, hermetically sealed graphite/ceramic ‘pebbles’. It reaches much higher temperatures – over 1000K – by using molten salts instead of steam. This allows it to be both more compact and more efficient than before. If the cooling is cut off or the reactor is breached, there is no chance of a meltdown or leak of radioactive material.

While the question of why we are not building as many of the new and improved nuclear reactors as possible is straightforward and easy to answer, it is not the topic of this post. Instead, we are concerning ourselves with the technology that allows for the reactor to run at higher temperatures.

Some designs, such as Very High Temperature reactors use helium as a coolant that will reach over 1500K. This document states that molten fluoride-salt coolant only fail at 1800K+. We know that the pebble fuel elements themselves can survive more than 2800K.

 

Going further, we can look at nuclear reactors meant for propulsion in space. They have the highest temperatures because it would mean propellant is ejected at the highest velocities, a key metric in determining rocket performance.

KANUTER

We find that solid-core nuclear thermal rockets had core temperatures of over 2800K, with the Project Timberwind engines maintaining 3000K for several minutes. More recently, we have research on uranium fuel held in niobium-carbides by engines that operate at 3250K.

At the extremes, we have reactors designs where the uranium is liquid. Designs such as this heat the coolant to 4,000K. Another NASA rocket design has spinning liquid uranium held at 5,250K. It is not implausible to develop a nuclear reactor core that operates at these temperatures.

 

What is the point of these extreme temperatures for helping with climate change?
 

CO2 can be broken up into carbon and oxygen. It is an energy-consuming process that is accomplished naturally by plants using enzymes or artificially in certain catalysts. Neither is a good option for our purposes, as they are slow and expensive methods.

 

We want to thermally decompose CO2. At a high enough temperature, CO2 simply turns into a plasma where carbon and oxygen ions dissociate freely. Efficiency is massively increased, as every joule added to the plasma goes into breaking up chemical bonds and any further heating just makes the reaction faster.

 

Thermal decomposition has already been studied as an option for producing hydrogen from water. Oxygen is reluctant to let go of its hydrogen atoms due to its electronegativity, meaning that a lot of electrical power is required, but the task becomes much easier to do at temperatures of over 2500K. 

Thermal decomposition of water.

You would not even need electricity to split water once you reach 3500K. That is an important fact, because producing heat is easy for a nuclear reactor, but turning it into electricity requires turbines, steam and a power cycle that costs you 50 to 70% of the reactor’s output.

We find that at 3000K, 40% of CO2 molecules break up into CO and O particles. The fraction becomes 50% at 3600K. Carbon monoxide has an even higher thermal decomposition temperature, beyond 3800K.

 

At 4000K, we can expect that from every 1 mole of CO2, we get 0.15 moles of carbon, 0.5 moles of oxygen and 0.2 moles of CO. Each mole of CO2 fully broken up requires to 530kJ. This corresponds to 12 MJ per kg of CO2 that is decomposed.

 

Thankfully, nuclear heat is in no short supply. Even small reactor cores can produce gigawatts of thermal energy… indeed, most of the cost of a nuclear reactor comes from the difficulty of containing the heat, not in producing it. 

This sodium-cooled fast reactor has a volumetric energy density of 300 MW/m^3. Rocket engine cores are on another level. The Pewee reactor released 2.34 GW per cubic meter, and the same page cites ‘advanced’ reactors having 40 GW/m^3.

The task and challenges

Hopefully, you are starting to understand how ultra-high-temperature nuclear reactors and the thermal decomposition of CO2 can go together.

 

If we manage to put CO2 in a 4000K environment, we could be converting it into simple carbon and oxygen. This directly eliminates the root cause of global warming.

 

Nuclear reactors are able to put out gigawatts of heat and therefore process several thousands of tons of CO2 every day.

 

Even better, the high temperatures and plentiful heat can be used in many alternative ways. The most obvious is to siphon off a bit of that power to produce electricity. Another is to produce split water at similarly high temperatures, creating hydrogen that can go towards synthetic fuels.

 

The ‘catch’ is that no usual material can handle these temperatures. The highest melting point ceramic we know of is tantalum-hafnium-carbide (TaHfC) at 4488K.  This is likely the only material we can build the reactor walls out of.

Investigation of TaHfC's properties.

Holding liquid uranium at 4000K inside a container that melts at 4488K is no easy task, but it is simpler to accomplish than for the liquid core nuclear rockets cited above. We are not worried about uranium flying out of a nozzle, nor do we need to transfer heat quickly to a propellant. We also get a 488K temperature margin to lean on.

 

Another challenge is preventing the radioactive products of uranium from contaminating the CO2 we are trying to break down. Necessarily, the reactor accepts air from the external environment and releases exhaust; we must make sure that we do not contaminate the environment with fission products. This means a reactor design made of multiple hermetically sealed vessels nestled inside of each other.

 

Finally, we have to manage the gases and products entering and leaving the reactor. Air is composed not just of CO2, but plenty of other things, such as water moisture and nitrogen. Only pure CO2 should be accepted into the 4000K zone, and carbon must be separated from oxygen before the gases cool down, otherwise the carbon would burn in the oxygen and we’d get back the CO2 we started with. We cannot have the exhaust be at a high temperature either, as it will react with the air and produce toxic carbon monoxide or polluting nitrous oxides.

Proposed system

 

In this section, we attempt to conceptualize a reactor design that solve the challenges mentioned above. It is only one of the potential options we have for using extreme heat to remove CO2 from the atmosphere.

 

It uses two key components: a tantalum-hafnium-carbide container that doubles as heat exchanger, and a supersonic expansion nozzle.

 

The container holds a critical volume of liquid uranium at 4000K. TaHfC survives these temperatures without the need for active cooling. The roof of the container would have channels for CO2 to pass through and be heated. Having the hot gas vent into an open space above the heat exchanger allow for further heating through blackbody radiation. A black surface at 4000K emits 14.75 MW/m^2.

Vapor core nuclear power system operating at 4000K.

For safety, the container is held within ‘hot’ pressure vessel, itself separated from the environment by a ‘cold’ pressure vessel. There are drain ports underneath the container to quickly bring the uranium liquid below the critical point inside a TaHfC drain pool.

 

The nozzle is how carbon is separated from the thermal decomposition products. Slowly cooling them would allow oxygen to react with carbon monoxide and carbon to re-form CO2. Very rapid cooling by using supersonic expansion through a nozzle could bring oxygen below the ignition temperature needed to burn with carbon (680-1200K) or carbon monoxide (880K). This implies a pressure drop of at least 6.7 times if we start at 4000K. The carbon should condense at these lower temperatures and fall out of the gas flow as a dust. For safety and efficiency, we will use a ten-fold expansion, in addition to flushing the exhaust with cold air and using an oil spray to separate the carbon dust from the oxygen.

 

Here’s how it would work in sequence:   

 

1) Extracting CO2 from the air

 

We cannot use atmospheric CO2 directly. It is mixed in with oxygen, nitrogen, water vapor and dust that would not respond to the temperatures inside the reactor well.  

The first step is therefore to filter out the dust, cool the air to condense out water vapor, and then liquefy CO2 by compressing it to over 25 bars at below room temperature. The liquid CO2 rains out of the compressed gas and can be drained away.

 

2) Multi-stage heating of CO2 to 4000K

We need to bring the high pressure liquid CO2 up to 4000K.

 

To prevent thermal stresses, we do this in multiple stages. The first stage might be to increase the temperature from 300K to 400K, so that it becomes a gas again. Then, we take it from 400K to 1000K, then 1000K to 1600K and so on. The start and finish temperatures could be made to correspond to external thermal cycles, for example to feed hot gases through a turbine. The final stage has the CO2 cross from a stable state at 3000K, to the thermal decomposition temperature of 4000K all at once.

This final stage takes place in a heat exchanger built into the uranium container as described above.

 

3) Supersonic expansion and extraction of carbon

 

The high temperature gases leaving the heat exchanger are composed of carbon dioxide, carbon monoxide, oxygen and carbon. They would be at a high pressure, perhaps 10 bars.

By passing the gases through a nozzle, they can be made to expand very quickly. This ‘freezes’ them before they have a chance to chemically react and recombine into carbon dioxide.

 

An example of this is the passage of a 4000K gas at 10 bars through a nozzle with an expansion ratio of 10. The gas exiting the nozzle would have its temperature reduced to 400K, its pressure down to 1 bar and its velocity increased to about 2.3 km/s. The transition from hot to cold would occur in less than a millisecond.

Oxygen would still react a bit with carbon and carbon monoxide. To make this even less likely, we can add a load of cold air to the exhaust stream. Adding mass to the exhaust slows it down and maintains a below-ignition-threshold temperature. Adding 10 kg/s of motionless air to 1 kg/s of supersonic exhaust can reduce the velocity by 90%.

It is best to spray oil into the exhaust as well. This helps ‘rain out’ the carbon dust, much the same way rain helps remove dust from the atmosphere. The oil would protect carbon from attack by oxygen molecules and makes it easier to extract carbon dust from the exhaust chamber as dust does not normally flow into tubes.

 

4) Exhaust treatment

 

The cold exhaust is either recycled through the condenser of the first step or released into the environment. If it is released, special precaution must be made to prevent carbon monoxide from reaching the environment.

 

The oil laden with carbon dust is filtered and dried to create a sort of powdered charcoal, or graphitized using reactor heat to form storable bricks.

Graphite bricks.

Graphite bricks can then be used in construction, steelmaking, batteries or just stacked inside the empty coal mines and oil wells we got all of our fossil fuels (and CO2 problem) from.

Funnily enough, coal miners could find new work as coal mine refillers...

A full-size diagram of this process will be created in the next post.

Efficiency and performance

The source of all power in this system is the nuclear reactor’s heat. It is consumed in three ways: thermal decomposition of CO2, running the pumps and inefficiencies in heat transport or isolation.

 

The thermal decomposition process is not 100% efficient. Heating up CO2 from room temperature to 4000K requires 4.4 MJ/kg. The thermal decomposition process itself requires 12 MJ/kg. For every 1 mole of CO2 that goes in, we get 0.15 moles of untouched CO2 and 0.2 moles of carbon monoxide. This is an efficiency of 65%, which bumps up our energy requirements to a total of 22.8 MJ/kg.

This is a lot!

 

However, it must be compared to the raw heat output of a nuclear reactor. 1 GW of nuclear heat would remove 3,790 tons of CO2 per day. This can be the output of a reactor core that fits on a truck bed.

Power in small packages.

Eliminating 40 gigatons of CO2 per year would require 912 exajoules, delivered as 29 TW of continuous power. This is a lot, but we are also talking about geo-engineering, which is the manipulation of the environment on a planetary scale… terraforming Mars is similar in scope.

 

In terms of uranium consumption, this 162,400 tons per year with 5% enriched high burnup fuel (65 GWd/t), and perhaps just 7,200 tons per year with fuel reprocessing allowing 90% burnup instead of 4%. Worldwide uranium production is at 66,500 tons.

 

If we want to compare it to our existing nuclear output, it is about 34x the thermal output if we assume 33% efficiency (the figures are given in electrical output).

Improvements and alternatives

 

There are many ways to improve on this design, or to do things differently.

-Gas recycling

 

The exhaust of the design described above is depleted in carbon but still contains oxygen, carbon monoxide and CO2. Recycling these gases into the reactor allows for a greater thermal decomposition efficiency. If the exhaust chamber can handle it, 2.3 km/s gases can be directed back into the heat exchanger. As they strike the exchanger’s surfaces, the gases compress and heat back up to a temperature close to 4000K, which allows for a recycling of most of the heat energy lost to the exhaust.

 

A reduction in energy consumption and getting a higher fraction of CO2 broken down can bring the 22.8 MJ/kg requirement down closer to the 12 MJ/kg minimum.

 

-MHD separation

A supersonic expander can be enhanced or replaced by electromagnetic separation of the carbon from the thermal decomposition products. A magnetohydrodynamic device acting on a 4000K stream of gases would even be able to generate electricity at high efficiency.
 

-Electric heating

 

Electricity produced using an MHD device or a gas turbine siphoning heat from the reactor can be used to further heat the CO2. We can both lower the material temperature requirement down to the point where a solid core reactor is usable (3000K) and increase the temperature that the CO2 reaches (4400K+).

 

The heating can take the form of an electric arc, a laser or radiofrequency heaters.

 

-Reactor laser

 

An even more speculative heating method is to use the reactor’s fission output directly to produce a laser beam that acts upon the CO2.

 

-Pebble bed reactor

 

The unsurpassed security of a pebble bed reactor could be achieved at 4000K if we solve the thermal expansion issues that might arise if we held liquid uranium inside a TaHfC shell.

The TaHfC would need great strength to handle the pressure of an expanding uranium center as it passes from solid to liquid phase. This implies a very thick shell, which hinders thermal conductivity… but this is of lesser concern in a reactor sitting on the ground when compared to a nuclear rocket.