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Published on August 27th, 2018 | by Michael Barnard

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Capturing CO2 From Exhaust Pipes Is A Bad Idea That Won’t Die

August 27th, 2018 by  


So here’s an idea. A lot of CO2 comes from cars and light trucks, right? So why don’t we just capture it in the car itself? 

I’m sure a bunch of people have had that idea. And then, after about 10 seconds thought realized how daft it was and moved on. But there’s one company that really likes the idea and has built multiple working prototypes. Which one? Saudi Aramco. If you haven’t heard of it, it probably won’t surprise you to hear that it’s the largest oil and gas company in the world, and wholly owned by the Saudi government.

Yes, it keeps trotting out variants on its technology and credulous people keep reporting on it as if it is a serious thing. That picture is of its Toyota Camry sedan prototype with most of the trunk and rear seats taken up with an onboard carbon capture and compression unit.

Wait, are people really that gullible? Yes, of course. Here’s a brief sampling of press pieces.

I could go on, but you get the idea. Saudi Aramco is full of STEM types and I’m sure that they realize it’s actually nonsense, but they keep trotting their prototypes out to conferences and journalists, and people keep reporting on it as if it’s a real thing.

In the interest of at least publishing a stake for this vampire idea, if not actually driving the stake into its festering heart, I worked up the basics of what a system like this would entail, what it would mean and what the costs would be.

As always, I started with data:

  • EIA: About 19.6 pounds of CO2 are produced from burning a gallon of gasoline that does not contain fuel ethanol.
  • Garber: a gallon of gasoline weighs 6.2 lbs
  • FHWA: The average American drives 13,476 miles in a year, 37 miles a day or 259 miles a week
  • Motortrend: the US car and truck fleet averaged 24.7 mpg in total in 2016
  • UWaterloo: it is typical to compress CO2 to above 7.38 MPa for efficient transport; compression generally occurs in a number of steps or stages; there are four or five stages of compression; there is a considerable rise in the gas temperature during each stage; CO2 compression efficiency is about 80%. For example, the energy required for compressing CO2 to 7.38 MPa would be about 60 kWh per tonne of CO2

Interesting numbers, and the first two points make it clear where this is going, but let’s go there anyway.

The average American drives 37 miles in a day in their 24.7 mpg car. Except their car won’t be averaging 24.7 mpg because it will have another person’s weight minimum of extra CO2 compression and storage equipment in it. Let’s round it down to 24 mpg.

They burn a 37/24= 1.5 gallons of gas in a day. Their car gets 6.2*1.5=9.3 lbs lighter because of burning the gasoline, but then gets 19.6*1.5=29.4 pounds heavier for a net gain of 29.4-9.3=20.1 lbs in weight. In a week, because they aren’t going to be dropping the CO2 off daily, that turns into 140.7 lbs, another human body and another drop in gas mileage. And of course for a year, that’s 7316.4 lbs.

But let’s pretend we’re going to head to 7.38 MPA compression in four stage compression, otherwise there would be a huge balloon of CO2 floating behind the car. Over the week, they are going to be compressing 140.7/2000=0.07 tons of CO2 which will require 60*0.07=4.2 KHW of electricity. Where will that electricity come from? From the gasoline engine, acting as a generator. 746 Watts = 1 hp. So they’ll be turning a few percent of their burnt gasoline into CO2 compression instead of forward movement. We can probably add 5% to 10% to everything on this for that factor alone.

The net is that the average US driver will end up with a dead body’s worth of CO2 in their car every week that they will have to dispose of somehow.

Exactly where will they dispose of this dead body of CO2? Let’s imagine that gas stations choose to accept CO2. There really isn’t another distribution choice that any driver would consider. The gas station would have to have a CO2 compressor of their own and the average cost of a 3,000 hp CO2 compressor is US $4.47 million. Yeah, that’s going to make them happy.

And then there’s the huge honking tank or tanks they’ll need to install. Assuming that a single gas station sells 4,000 gallons of gas a day, they’ll need storage for the resulting CO2, and that means they’ll need to be able to compress and store 4,000*19.6=78,400 lbs of CO2 a day or 548,800 a week. That’s 39.2 tons per day and 274.4 tons per week. That means they are going to suck back an extra 39.2*60=2,352 KWH of electricity daily or 16,464 weekly. At an average US commercial electricity cost of 10.74 cents per kwh, that’s an extra cost of 2,352*10.74=$252.60 of cost per day or $1,768.23 per week.

Also, goodbye parking spots. And maybe the carwash too. Did you know that the average gas stations makes about $200 in profit selling gas per day? That’s why the convenience store and car washes are so important. And why they have automated pumps.

Oh, but they’ll make it up by selling the CO2, won’t they?

No. While CO2 has a market value of about $30 per ton, that’s exclusive of distribution. Tanker trucks have a maximum load of 80,000 lbs. As there are 548,800 lbs of CO2 to deal with that means seven trucks a week will be coming just for the CO2. It costs about 15.6 cents USD per ton-mile to ship by truck in the USA. Assuming the CO2 is delivered 50 miles away to a collection center, that’s another cost of $2,140.32 per week that has to be scrounged up somewhere. Except the trucks require refrigeration and compression units of their own, so it will be higher.

Oh wait, the average freight truck in the US emits 161.8 grams of CO2 per ton-mile! That means just transporting the tons of CO2 will cause emission of another couple of tons of CO2. Yup, this is making less and less sense the more we poke at it.


What possible reason could Saudi Aramco have for promoting this obviously nonsensical idea? And why would people fall for it so easily, even Phds with an energy background? (Think I’m joking? Look at that first article in the list.)

Well, the first one is that old bottom line of Aramco’s. $455 billion USD of annual revenue allows for a lot of fig leafs and greenwashing exercises. This gives the entire automotive industry and related fossil-fuel interests something to point to that suggests internal combustion vehicles have an extended shelf-life in the age of rapid electrification of transportation.

As for why people fall for it, there are the usual reasons. Some of it is venality. Some is a lack of any ability to project out costs and do analysis. Some of it is a lack of basic STEM skills. And some of it is just that Aramco’s PR department feeds press releases to media and friendly analysts that are hungry for eyeballs and they eat it up.


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About the Author

Mike works with startups, existing businesses and investors to identify opportunities for significant bottom line growth in the transforming low-carbon economy. He regularly publishes analyses of low-carbon technology and policy in sites including Newsweek, Slate, Forbes, Huffington Post, Quartz, CleanTechnica and RenewEconomy, with some of his work included in textbooks. Third-party articles on his analyses and interviews have been published in dozens of news sites globally and have reached #1 on Reddit Science. Much of his work originates on Quora.com, where Mike has been a Top Writer annually since 2012. He's available for consultation, speaking engagements and Board positions.



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