Scientists in the U.S. are working on a scheme to remove carbon dioxide from powerplant emissions and sequester it with no net cost to the powerplant. It’s not a cure-all for the climate crisis, but it offers the opportunity to take a big slice out of overall emissions, worldwide.
“Around a quarter of current carbon dioxide emissions are coming from the power sector,” says Yuan Jiang, a chemical engineer at America’s Pacific Northwest National Laboratory (PNLL) in Richland, Washington. Another 10 percent, she says, are from the industrial sector—steel plants, pulp and paper plants, cement plants, etc., all of which could benefit from the same technology. “So, there are a number of opportunities,” she says.
Accomplishing this is a two-step process, says David Heldebrant, a green chemist also at PNLL.
The first step is to remove carbon dioxide from the gases going up the stack. In theory, that’s simple. Navies have long known that for submarines it’s necessary to prevent the build-up of carbon dioxide in order to keep their crews from smothering.
Carbon dioxide is a very weak acid gas, Heldebrant says, so the easiest way to remove it is by scrubbing the air through a filter containing a base (the opposite of an acid). The two then react to produce a combined molecule called a salt. Submarines do this with soda lime (a mix of sodium hydroxide and calcium hydroxide), but that’s not useful at the levels needed to fight climate change. For that, what’s needed are molecules that can not only filter carbon dioxide out of the air passing through them, but which can later be induced to release it for subsequent compression and storage (with the carbon-capture substrate available for reuse).
In theory, this is simple. But it generally means heating up the scrubbing agent in order to get it to release its captured gas. That’s a problem because most agents work best when dissolved in water. Heating them up to release their captured carbon dioxide means also heating up the water… and if you’ve ever waited for a pot to boil, you know that that can take a lot of energy.
“The beauty of our solvent,” Jiang says, “is that we only have two to five per cent water.” The rest “is functional chemicals doing carbon capture.” The result, her team calculates, is a significant reduction in carbon-capture costs, bringing them down from well over $US40 per tonne of CO2 to about $US38.
That’s step one. Step two is figuring out what to do with that carbon dioxide.
Conventional wisdom is to dispose of it by injecting it deep underground or finding some other way to put it where it won’t easily get back into the air.
That’s where Heldebrant’s work comes into play. In a paper in Advanced Energy Materials he and six colleagues (including Jiang) described a way in which carbon dioxide recovered from powerplant emissions could be converted to useful chemicals – valuable enough, in fact, to potentially offset the entire cost of both carbon capture and conversion.
“Basically, we are taking a technology to capture carbon dioxide from exhaust fumes and then repurpose that carbon dioxide into materials that would be usable, such that you can pay for the capture,” he says. Chemically, he adds, the process is similar to that which our own bodies use: taking waste carbon dioxide from our exertions and using it to build essential fatty acids.
“If your body [does this] why aren’t we doing the same thing?” he asks.
The simplest starting point, he says, is to use the captured carbon dioxide to make methanol (CH3OH)—something that can be done by reacting it with hydrogen.
Methanol can be used for a wide variety of purposes. As far back as 2006, a Nobel-laureate chemist at the University of Southern California named George Olah advocated its use as a green-energy fuel that, unlike hydrogen, could be used in existing pipelines, service stations, and automobiles.
But that’s just the beginning. Methanol is also what Heldebrant calls a “platform chemical” that can be used for many other purposes, including the manufacture of many other economically valuable chemical feedstocks. “So, rather than make one compound and be limited by market forces, we’re trying to make multiple things from the same CO2-loaded material,” he says.
The goal, he adds, is to make the cost of doing this with recycled carbon dioxide “close to parity” with current methodologies based on the use of oil and natural gas.
Another goal, Heldebrant says, is to incorporate captured CO2 into composite building materials, such as planks you might use for the decking on your patio.
If it works, your new patio might someday be a CO2 sink. And, Heldebrant adds, these planks won’t contain plastics that could eventually degrade into microparticles that could invade the food chain. Instead, they would contain chemicals like lignin, a natural part of wood.
The key to making this work, Heldebrant and Jiang say, is coupling the carbon capture technology and the carbon reuse/recycling technology as closely as possible to the carbon source.
Historically, Heldebrant says, the model has been to capture carbon dioxide at its source, then ship it off to a distant chemical plant. But that’s inefficient. His and Jiang’s vision is to combine the two processes. Flue gases from the powerplant (or steel mill, cement plant, etc.) would not only be collected on site, but waste heat from the powerplant or industrial facility would then be used to release the carbon dioxide from the sorbent. The hydrogen needed to convert that carbon dioxide into methanol would then (hopefully) be produced from renewable sources. The result would be that much of the energy needed to drive this process comes from a combination of waste heat and renewable sources.
That, Heldebrant says, could substantially reduce what engineers call the “parasitic load” on the powerplant—the fraction of energy drained away for carbon capture and climate control. If that captured carbon could then be converted to products valuable enough to pay for the carbon-capture process, that is pretty much the definition of a climate win/win.
That said, this isn’t the only such project designed to recycle carbon dioxide into something not only climate-saving, but economically useful. The US military and scientists in Europe have been researching processes for generating what are called solar electric fuels in which solar power is used to create jet fuel from a mix of water and carbon dioxide. And, Heldebrant says, the U.S. Navy is designing a system in which aircraft carriers can capture carbon dioxide dissolved in seawater and (powered by waste heat from their onboard nuclear reactors) convert it into synthetic aviation fuel.
Bottom line: the climate crises is real, and while shifting to green energy might be the dream way of attacking it, there is also a need for solutions that can be implemented now, without waiting decades for expensive infrastructure to be retired and replaced. If some of those solutions, such as those proposed by Jiang and Heldebrant are economic win-wins, so much the better, because if such economically feasible, relatively easy to implement, solutions add up to enough slices of the overall pie, maybe it’s not too late.