By far the greatest user is the fossil fuel industry, and they want to multiply its use by orders of magnitude to perpetuate their business.
Supercritical CO2 is having a moment in the sun again, at least from my admittedly odd point of view. The US DOE is once again touting the use of the substance in thermal electrical generation. The carbon capture and sequestration crowd are touting supercritical CO2 pipelines as the answer to the reality that burning fossil fuels makes CO2 that’s 2-3 times the mass and 450 times or more the volume of the fuels.
And scientists also figured out that pressure had a lot to do with it. Higher pressures meant that water turned into steam only at much higher temperatures. Ditto the reverse, so anyone trying to make a hot cup of tea at the top of Mt. Everest gets a lukewarm cup instead. And there was a critical temperature above which no amount of pressure will force a gas to turn into a liquid and that was discovered as well.
There’s actually a third dimension to this chart which exists in tables. It’s the dimension of density of the supercritical CO2. As you move upward to the right, supercritical CO2 actually gets denser. That’s why the diagram has 83.8 bar , because it’s a bit higher pressure. The density can change quite radically with increases, so that at about three times the pressure and the same temperature, it can go from 161 kg per cubic meter to 941 kg per cubic meter.
That increase in heat means you have to remove some of the heat. Then you have to repeat the cycle. You have to get CO2 up to 74 atmospheres and 31° Celsius. The pressure is the equivalent of being about 6.4 meters or about 21 feet under water. It’s not extreme, but it’s not trivial. Thefrom an energy efficiency perspective turns out to be six to seven.
Increasingly, supercritical CO2 is turning up in heat pumps and refrigeration units. Sanden’s CO2 hot water heat pump is actually a supercritical CO2 heat pump, leveraging the physics for very efficient heat transfer. But let’s get back to some quantification. CO2 is a cheap commodity when it comes up from underground in large volumes, US$30 to $50 per ton. The fossil fuel industry doesn’t use electricity to power its compressors for the most part, but natural gas or oil. It still needs 90 kWh of energy, but it gets it from much less efficient fossil fuels.
But wait a minute. One of the interesting properties of supercritical CO2 is that it can vary radically in density with fairly small changes in temperature or pressure. To keep pipelines from exploding, the contents have to be maintained above 31° Celsius and 74 atmospheres worth of pressure. This is worse than natural gas or oil pipelines where extra pumps are used to keep the stuff moving along, and if one fails all that happens is that movement is slower.
At the US median length of 400 kilometers, that’s three booster stations or so, and another 30% energy requirements. Call it another $3 per ton of CO2. That’s $28 per ton of supercritical CO2 just for compressing it and transporting it. Never mind getting it or using it. Let’s give the fossil fuel network a break and assume that we take away half for redundancy of considering oil vs diesel and gasoline. And another half of the remainder for the lower density of natural gas. Then assume a reduction in fossil fuel consumption only to stationary point sources like oil and gas extraction facilities, oil and gas pipelines and coal, oil, and gas power stations. Call it 20% of demand. Then multiply it by 2.
By the way, this is all assuming dry CO2 which is non-corrosive, not wet CO2 which is very corrosive. As a result, add a lot of energy costs to remove water vapor from the CO2 before compression.Is there anything besides the absurd cost of building and operating pipelines for this nifty fluid/gas that would give us pause? Well, yes.
Concentrations above 10% can cause convulsions, comas, and death. It can cause long-lasting organ and brain damage in survivors. There are a lot of reasons to think that carbon capture and sequestration is a very dumb idea. But the basics of transporting even a fraction of it with their associated costs and dangers should make it clear that it’s even dumber than most people think.
Is it easy to do? No, no it’s not. One of the things I do when I assess technologies is see how long they’ve been around without being commercialized. The first prototype of a supercritical CO2 Brayton cycle generator was built in 1948. That’s 75 years ago. This technology is going to be 80 soon. It’s geriatric.
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