by Monte Davis
12+16+16 = 44. We can all agree on that.
44/12 = 3 2/3. Good so far?
That’s all the math needed to ask some pointed questions about CCS. That stands for carbon capture and storage (or “sequestration”), a technology discussed and explored at pilot-project scale for decades. Its goal is to separate and collect the carbon dioxide from combustion, before it is released into the atmosphere, then put it in very long term storage so it doesn’t contribute to further climate change. So CCS is really CO2 capture and storage – and the difference between carbon and carbon dioxide is where the arithmetic above comes in.
Carbon’s atomic mass is 12; oxygen’s is 16. (Never mind isotopes.) So combining a carbon atom and two oxygen atoms – combustion — yields a molecular mass for CO2 of 44… or 3 2/3 times that of the “unburnt” carbon atom. These numbers are ratios, so the math is the same for any units: completely burn 12 grams of pure carbon, and get 44 grams of CO2; burn 12 million tons, get 44 million tons of CO2. Fossil fuels aren’t pure carbon, of course, and combustion rarely burns every bit of what there is, so the emission ratio varies. Coal typically yields 2.1 times its mass in CO2; firewood, 1.6 to 1.8 times; gasoline, 2.3 times; natural gas, about 2.8 times.
Pause here, because this is deeply counterintuitive – so deeply that we don’t realize it. We’ve had chemistry for a few centuries, arithmetic for millennia. But we’ve been using fire deliberately for a million or two years, seeing the aftermath of wildfire for much longer. All that experience taught us in our bones that the ashes always weigh less than the fuel did. As for the smoke – why, just look at it! Any hominid can see that it weighs nothing at all!
Because CO2 swirls invisibly away with that smoke, and soon mixes with the air and dissipates, we’ve learned only in the latest eyeblink of time to account for all the combustion products. It takes an effort to grasp that CO2 is as much “ash” as the gray powder in the fireplace – ash that weighs more than the logs we burned. The 15 gallons of gasoline in your car’s tank weigh about 100 pounds. When it’s gone, you’ve made a present to the world of about 230 pounds of carbon dioxide, along with much smaller quantities of carbon monoxide, benzene, 1,3-butadiene, etc.
Should we call CO2 emissions “pollution,” as we do those noxious traces? In the broadest sense – something our activities put into the environment that is doing harm – it is, but there are good reasons to keep it in a category of its own.
First, nearly all the pollutants that come to mind from fifty years of modern environmentalism have been “side effects” of useful processes or activities. We could find ways to reduce or eliminate them while still achieving the desired ends. But the chemical reaction of carbon and oxygen, with its release of energy as heat and light, has been the whole point of combustion ever since firewood was all we had.
Second, the quantities of “side effect” pollutants were typically very small fractions of the total throughput of a process, or of the whole volume of water, air, or other resource concerned. But CO2 is literally massive: fully reckoned, the “ash” of a coal-fired power plant is not only greater than the hills of coal ash and slag, but greater than the coal that went into the boiler.
Both of these differences cast an interesting light on the evolution of the phrase “clean coal.” For decades before CO2 and climate change took center stage, clean coal meant mitigating the health and environmental impacts of coal power: pre-treating the coal to remove incombustible components, filtering and “scrubbing” the stack exhaust to remove mercury and nitrogen and sulfur compounds, and more secure storage of coal ash. Those were widely adopted, and genuinely mitigated acid rain and other forms of pollution.
Increasingly in the last twenty years, though, advocacy of clean coal has been stretched to cover CCS — with an unmistakable subtext of we tackled those other pollutants, and we’ll find a solution for CO2 as well! Several technologies under investigation modify the chemistry of the combustion process to make CO2 capture from the flue gas more efficient. So far, the record of CCS projects, many with government agency support, has been dismal. Tellingly, 30 of the 42 operational commercial projects use the captured CO2 as a pressurant, injected into oil fields to extract more petroleum. Some might consider this an infraction of “If you find yourself in a hole, stop digging”… or at any rate a plan B that provides some return on an otherwise dubious investment.
What is most remarkable about public perception and discussion of CCS is that attention is directed primarily to the competing technologies for CO2 capture… secondarily to where and how to store what’s captured… and hardly at all to the enormous scale of infrastructure and investment that would be required to make a significant difference.
Let’s try to reverse that order. To explain and compare CO2 capture approaches quickly gets into the chemical-engineering weeds, so let’s stipulate that someone will come up with a cost-effective design that can be adapted and retrofitted to existing fossil-fueled power plants, cement works, steel mills, and other large CO2 emitters. Because that’s not what matters.
Where and how to store it? The answer is almost always “underground,” whether on land or under the ocean floor, in deep strata that have pore space to hold pressurized gas and impervious “cap” strata above to prevent the gas from finding a route back to the surface. A lot of geological study will be needed, but there’s no question that such strata exist in many places. The question is how many of those places are conveniently close to large CO2 emitters. If they aren’t, that means storing and transporting the captured CO2 to the deep wells where it will be injected into the storage strata.
Storing and transporting… and now we get to the arithmetic that matters.
Picture Georgia Power’s 3.5-gigawatt Plant Scherer, near Macon, the largest coal-fired power plant in the US. It came on line in the 1980s, and at peak output required up to five trains a day, 100 coal cars or more long, from the Powder River basin in Wyoming. In 2018 it burned on the order of 9 million tons of coal, with CO2 emissions of 20 million tons.
Now, picture that 20 million tons of CO2 not drifting invisibly away from the smokestacks, but being dealt with: Stored in a giant tank farm. Pumped into pipelines or long trains of tank cars. At the end of its journey, forced down large numbers of dedicated shafts by powerful pumps. And that is what’s going to happen to nearly all of it, because while there is a market for CO2 in industrial processes, it’s trivial compared to supply on this scale. So nearly all that new investment and operating cost is economic dead weight: 20 million tons of former “externality” turned into the utility’s red ink. Even if coal-fired generation weren’t already under a lot of pressure from cheaper oil and gas, its economics can’t possibly survive that burden.
The allure of the Breakthrough Innovation is powerful, but scaling up CCS to national (let alone global) significance requires engagement with boring, off-the shelf infrastructure and engineering: it’s a materials handling problem. We can reliably project its costs, because we already have mature technology all over the world handling megatons of oil, natural gas, and other fluids. Whatever the cost of the CO2 capture tech itself, no dazzling breakthrough is going to make pumps, tanks, pipelines, tank cars, and deep boreholes orders of magnitude cheaper.
In 2011, the respected energy analyst and historian of technology Vaclav Smil wrote: “In order to sequester just a fifth of current CO2 emissions we would have to create an entirely new worldwide absorption – gathering – compression – transportation – storage industry whose annual throughput [of CO2] would have to be about 70 percent larger than the annual volume now handled by the global crude oil industry — whose immense infrastructure of wells, pipelines, compressor stations and storage took generations to build.” And, of course, there’d be a short useful time for that vast investment not in selling energy, but in simply taking out the ashes.
The greatest accomplishment of CCS so far has been to buy more time for fossil fuels. There may be niches for it where truly crucial activities simply cannot use zero-carbon power, or where profitable uses for CO2 and ideal storage conditions combine near existing large emitters. But as a large-scale solution to CO2-driven climate change, it is Not. Gonna. Happen.