[Tanada]

For all the engineering stuff I have read up on since I joined PO dot com in 2005 this one sort of fell through the cracks.

Over a century ago it was discovered that certain materials like CO2 and Water have an interesting and useful fourth kind of matter state. You can make these materials solid, liquid or gas by adding or removing heat and pressure to the basic material. Interestingly enough if you keep adding heat and pressure and the container is strong enough both CO2 and H2O transition into a "Supercritical fluid" state where they have the low viscosity of a gaseous material but the high mass and energy transfer of a liquid material.

Early in the 20th century it was a laboratory stunt to back a test cylinder with dry ice, seal the container and then observe what happened through a little port hole type window to the CO2 when you heated it. Dry ice in this situation would at first sublimate directly into a gas because the relatively low pressure in the container at first, but once the first critical point was reached liquid CO2 would form and the rest of the ice would transition into liquid instead of sublimating into gas. If you continued to heat the sealed container however the phase line between the liquid and gas would suddenly disappear as the contents reached the Supercritical Point.

The resulting liquidy gas or gassy liquid came to be defined as a Supercritical Fluid with extremely low viscosity but substantial mass making it an ideal substance for spinning turbines. You can think of it as steam under so much pressure it has the same density of water filling the same volume would have.

At first this Supercritical Fluid CO2 was just a lab trick that was used with physics students and sometimes engineering students as a teaching tool. Once they started playing around with larger volumes of material however a few interesting physical traits were discovered. For one, you can take filtered sea water, heat and compress it until the water becomes a Supercritical Fluid and as the liquid transitions to the higher state of matter all the dissolved solids precipitate out. Yes this means all the sea salts and dissolved calcium and magnesium and sulfur carbonates precipitate out like a tourists souvenir snow globe left to settle after a good shaking. It was also discovered sort of by accident that if you took a bag of coffee beans and flushed it with supercritical CO2 the Caffeine molecules would dissolve right out of the beans attached to the CO2, then as you lowered the Supercritical fluid close to the normal liquid pressure the caffeine would precipitate out as a white powder. This is how modern coffee is decaffeinated without greatly altering the flavor profile and also how caffeine is collected to use in stay awake pills and migraine tablets.

Turning CO2 into a supercritical fluid is not to difficult which is what made it such an attractive laboratory demonstration and the useful aspects were learned more as a side effect of its ease of experimentation than anything else.

Turning Water into a supercritical fluid takes very high pressure and temperature which means you need a very tough container to safely play around with it. However engineers are pretty clever and all the way back in 1957 the Babcock & Wilcox steam boiler company USA sold their very first "Supercritical Boiler" to a coal burning power plant.

If it is so hard and dangerous to make water into a supercritical fluid why would anyone want to do such a thing? Well it turns out that because Supercritical Water flows like steam but carries mass like liquid water that makes it a very efficient material for spinning a turbine to get useful work. It turns out that in 1957 steam boiler power plants were around 28-30% efficient at converting Coal into Electricity. However a Supercritical Boiler/Turbine system had a potential efficiency of 45% with something like 40% being achieved by that very first system.

The plain fact is in the USA coal was and remains pretty darn cheap and the Supercritical Boiler/Turbine equipment was and remains a bit pricey compared to a standard lower pressure system so in the USA the option did not become the industry standard. However in Japan where most coal burned had to be imported from the international market the price incentive was very much a factor and something like half of current generation Japanese coal burning plants operate with Supercritical Water temperatures and pressures which gets their generation up into that 45% range of efficiency. Japanese big boiler manufacturers like Mitsubishi Heavy Industries have adopted Supercritical Water boiler/turbine systems as a major export item and have built power plants using the technology all over Asia including India, Vietnam, China and South Korea. Intellectual property being what it is the Chinese have switched to building many coal fired Supercritical systems instead of the older simple steam turbine systems that remain more common in the USA and the cost savings on fuel have been substantial.

Now for the piece that makes itself irresistible. NASA has been developing this technology for use by the ISS and other crewed space environments because of another one of those accidental discoveries. It turns out that if you take raw sewage and compress+heat it to Supercritical Fluid levels you get a massively accelerated chemical interaction between the solid matter, dissolved gasses from the atmosphere and the water itself. In a process called "Hydrothermal Combustion" that breaks down the waste into CO2 and water without the hazards of having an open flame in a small enclosed space. It is expected this methodology can be used widely in the near future to remediate many types of hazardous waste liquids contaminating water by fully breaking them down without open air combustion being used. Also because the US Navy has developed techniques for using water with dissolved CO2 as feedstock for manufacturing synthetic hydrocarbon fuels the two technologies could be mated together to take contaminated water into one end of the system and produce purified water and synthetic fuel as final products.

One proposed use is for conversion of finely ground biomass, lignite or coal in water into synthetic Methane and liquid fuels. If done strictly using biomass this results in carbon neutral fuels that can drop directly into the already existing fossil fuel distribution network seamlessly.

For less wealthy nations like South Africa that are rich in coal resources converting over to Supercritical Water Boiler/Turbine technology will make their power grid 50% more efficient therefore reducing their average carbon intensity of generation. Yes Cheng Cycle and Combined Cycle systems burning Natural Gas or Petroleum Products can achieve even better efficiencies in the 55% range but coal is still by far the cheapest fossil fuel and most likely to continue being used for the foreseeable future unless we have a world wide technological collapse from a nuclear war, volcanic super-eruption, asteroid impact or climate flip from global warming.

One last thing, the proposed Generation IV nuclear prototypes include a "Supercritical Water Reactor" which is as simple as a modern Boiling Water Reactor but using even higher pressure than modern Gen III PWR designs to increase thermal efficiency from 33-35% to 45-48%. As a side benefit neutron behavior in a supercritical fluid is also different from the behavior in either liquid water or steam and in theory allows replacement level breeding to vastly improve fuel cycle lifetime possibly extending it from the current 48 month industry maximum to 124 months without greatly changing the enrichment level of the fuel.

[Newfie]

VERY interesting.

[Subjectivist]

For all the engineering stuff I have read up on since I joined PO dot com in 2005 this one sort of fell through the cracks.

Over a century ago it was discovered that certain materials like CO2 and Water have an interesting and useful fourth kind of matter state. You can make these materials solid, liquid or gas by adding or removing heat and pressure to the basic material. Interestingly enough if you keep adding heat and pressure and the container is strong enough both CO2 and H2O transition into a "Supercritical fluid" state where they have the low viscosity of a gaseous material but the high mass and energy transfer of a liquid material.

Early in the 20th century it was a laboratory stunt to back a test cylinder with dry ice, seal the container and then observe what happened through a little port hole type window to the CO2 when you heated it. Dry ice in this situation would at first sublimate directly into a gas because the relatively low pressure in the container at first, but once the first critical point was reached liquid CO2 would form and the rest of the ice would transition into liquid instead of sublimating into gas. If you continued to heat the sealed container however the phase line between the liquid and gas would suddenly disappear as the contents reached the Supercritical Point.

The resulting liquidy gas or gassy liquid came to be defined as a Supercritical Fluid with extremely low viscosity but substantial mass making it an ideal substance for spinning turbines. You can think of it as steam under so much pressure it has the same density of water filling the same volume would have.

At first this Supercritical Fluid CO2 was just a lab trick that was used with physics students and sometimes engineering students as a teaching tool. Once they started playing around with larger volumes of material however a few interesting physical traits were discovered. For one, you can take filtered sea water, heat and compress it until the water becomes a Supercritical Fluid and as the liquid transitions to the higher state of matter all the dissolved solids precipitate out. Yes this means all the sea salts and dissolved calcium and magnesium and sulfur carbonates precipitate out like a tourists souvenir snow globe left to settle after a good shaking. It was also discovered sort of by accident that if you took a bag of coffee beans and flushed it with supercritical CO2 the Caffeine molecules would dissolve right out of the beans attached to the CO2, then as you lowered the Supercritical fluid close to the normal liquid pressure the caffeine would precipitate out as a white powder. This is how modern coffee is decaffeinated without greatly altering the flavor profile and also how caffeine is collected to use in stay awake pills and migraine tablets.

Turning CO2 into a supercritical fluid is not to difficult which is what made it such an attractive laboratory demonstration and the useful aspects were learned more as a side effect of its ease of experimentation than anything else.

Turning Water into a supercritical fluid takes very high pressure and temperature which means you need a very tough container to safely play around with it. However engineers are pretty clever and all the way back in 1957 the Babcock & Wilcox steam boiler company USA sold their very first "Supercritical Boiler" to a coal burning power plant.

If it is so hard and dangerous to make water into a supercritical fluid why would anyone want to do such a thing? Well it turns out that because Supercritical Water flows like steam but carries mass like liquid water that makes it a very efficient material for spinning a turbine to get useful work. It turns out that in 1957 steam boiler power plants were around 28-30% efficient at converting Coal into Electricity. However a Supercritical Boiler/Turbine system had a potential efficiency of 45% with something like 40% being achieved by that very first system.

The plain fact is in the USA coal was and remains pretty darn cheap and the Supercritical Boiler/Turbine equipment was and remains a bit pricey compared to a standard lower pressure system so in the USA the option did not become the industry standard. However in Japan where most coal burned had to be imported from the international market the price incentive was very much a factor and something like half of current generation Japanese coal burning plants operate with Supercritical Water temperatures and pressures which gets their generation up into that 45% range of efficiency. Japanese big boiler manufacturers like Mitsubishi Heavy Industries have adopted Supercritical Water boiler/turbine systems as a major export item and have built power plants using the technology all over Asia including India, Vietnam, China and South Korea. Intellectual property being what it is the Chinese have switched to building many coal fired Supercritical systems instead of the older simple steam turbine systems that remain more common in the USA and the cost savings on fuel have been substantial.

Now for the piece that makes itself irresistible. NASA has been developing this technology for use by the ISS and other crewed space environments because of another one of those accidental discoveries. It turns out that if you take raw sewage and compress+heat it to Supercritical Fluid levels you get a massively accelerated chemical interaction between the solid matter, dissolved gasses from the atmosphere and the water itself. In a process called "Hydrothermal Combustion" that breaks down the waste into CO2 and water without the hazards of having an open flame in a small enclosed space. It is expected this methodology can be used widely in the near future to remediate many types of hazardous waste liquids contaminating water by fully breaking them down without open air combustion being used. Also because the US Navy has developed techniques for using water with dissolved CO2 as feedstock for manufacturing synthetic hydrocarbon fuels the two technologies could be mated together to take contaminated water into one end of the system and produce purified water and synthetic fuel as final products.

One proposed use is for conversion of finely ground biomass, lignite or coal in water into synthetic Methane and liquid fuels. If done strictly using biomass this results in carbon neutral fuels that can drop directly into the already existing fossil fuel distribution network seamlessly.

For less wealthy nations like South Africa that are rich in coal resources converting over to Supercritical Water Boiler/Turbine technology will make their power grid 50% more efficient therefore reducing their average carbon intensity of generation. Yes Cheng Cycle and Combined Cycle systems burning Natural Gas or Petroleum Products can achieve even better efficiencies in the 55% range but coal is still by far the cheapest fossil fuel and most likely to continue being used for the foreseeable future unless we have a world wide technological collapse from a nuclear war, volcanic super-eruption, asteroid impact or climate flip from global warming.

One last thing, the proposed Generation IV nuclear prototypes include a "Supercritical Water Reactor" which is as simple as a modern Boiling Water Reactor but using even higher pressure than modern Gen III PWR designs to increase thermal efficiency from 33-35% to 45-48%. As a side benefit neutron behavior in a supercritical fluid is also different from the behavior in either liquid water or steam and in theory allows replacement level breeding to vastly improve fuel cycle lifetime possibly extending it from the current 48 month industry maximum to 124 months without greatly changing the enrichment level of the fuel.

Sounds great, why aren't we using it?

[theluckycountry]

Sounds great, why aren't we using it?

Economics isn't your strong suit is it.

[theluckycountry]

Fascinating post, thanks Tanada.

[careinke]

Great post, thanks for sharing.

Peace