About two weeks ago, the City Beat got a press release from UND about this new research project to improve so-called "clean coal" technology. We’re just running the story today because it took me that long to figure out how this technology could work.
It’s hard to talk to engineers because, it seems, they either think you’re not smart enough to understand, which means they won’t explain exactly what they’re researching. Or, if you really press them, they explain half of the research because they now assume you know more than you do. You can’t win!
In short, UND wants to find a membrane that will do a really good job filtering CO2 from the exhaust of coal-fired power plants. Brian Tande, a native of western North Dakota, is the lead on this project.
I’ll assume you have no idea how clean coal technology works so the rest of this post will put that membrane technology into context. By the way, some environmentalists like to point out that clean coal isn’t totally clean because there’s no way to scrub out all the pollutants.
Basically, there are three broad classes of clean-coal technology:
- Pre-combustion: The process of extracting CO2 begins before actual combustion. Coal is heated but not actually combusted so that it forms a gas. When the gas is burned, it creates a high concentration of CO2, higher than when coal is burned regularly. This high concentration makes it easier, and therefore cheaper, to remove the CO2.
A gassification plant produces electricity at a rate of 7.8 cents per kilowatt-hour, which is more than a regular coal-fired plant. Scrubbing the exhaust of CO2 would bring the cost up to 10.2 cents per kilowatt-hour.
- Post-combustion: Here, the process of extracting CO2 happens after coal is burned in a regular power plant. Because the concentration of CO2 is lower, it has to be increased in some way to reduce the cost of extracting it. The membranes UND is researching is part of this class of clean-coal technology.
A regular coal-fired plant produces electricity at 5 cents per kilowatt-hour. Scrubbing the exhaust would bring the cost up to 8.25 cents per kilowatt-hour. The membrane technology, if it works as well as hoped, could bring that down to 7.76 cents per kilowatt-hour.
- Oxy-combustion: In this process, coal is burned with lots of oxygen, producing CO2 diluted in water. It’s apparently easier to remove CO2 from water than from membranes and solvents used in post-combustion. But separating the oxygen is expensive.
I don’t have any data on costs.
Let me zero-in on post-combustion technology now. There are three main approaches:
- Solvents: The power plant’s exhaust goes through a solvent that has an affinity for grabbing onto CO2 molecules. This is a process is one with a long track record in other industries, such as the semiconductor industry where it’s used to remove dissolved gases from water. The problem is you have to bring the temperature of the exhaust down for the amine to work well and then you have to bring the temperature of the solvent up for it to release its hold on CO2, so you can reuse the solvent. All those temperature changes take energy, which makes the process more expensive.
By the way, when you read "amine" in the literature, just translate it to solvent because that’s what the solvent’s made of.
- Membrane-based separation: The power plant’s exhaust goes through a membrane where the CO2 is filtered out. The problem is for this to work well there has to be a big difference in pressure between the one side of the membrane and the other. Either you’ve got high pressure pushing the CO2 through the filter like a spoon mashing lumpy gravy through a sieve or you’ve a a vacuum pulling the CO2 though the filter. Either way, this requires lots of energy, which makes it expensive.
- Membrane absorption: This is the hybrid process that UND is looking into. The solvent goes through the membrane where most of the CO2 is stripped from it. This means less heat is needed to remove the remaining CO2 from the solvent.
The energy savings apparently works like this: Membranes use less energy and extract less CO2 in a given amount of time. Solvents use more energy and extract more CO2 in the same amount of time. But you combine the two and you make the solvents more efficient, removing the same amount of CO2 but using less energy.
It took me about several days to figure this out, piecing together what I got from my correspondence with Brian Tande and from research online. Engineering is subtle that way. I sent this matrix over to Tande and he said it’s "basically correct":
|Energy used||CO2 extracted|
|Membrane and solvent||Medium||More|
The exact mechanism that the UND team is looking at was a bit hard to piece together because I’d assumed that the polymeric, or plastic, membrane the team wants to use would be too fragile for the heat.
There are two kinds of membranes in wide use:
- Polymeric: The advantage of these is the polymers can come in long thin tubes that offer a lot of surface area for the CO2 to bond to, which means the membranes are more efficient. The disadvantage is polymers can’t handle that much heat.
- Ceramic: The advantage is these can take the heat, but they don’t offer as much surface area. Basically, a ceramic membrane is a sieve with millions of tiny holes that block certain molecules, CO2, say, from passing through. They’re also heavier and more expensive.
Tande said the greater efficiency of the polymeric membrane might lower the amount of heat required such that the polymers won’t get destroyed. If it turns out the polymembric membrane just isn’t viable, he said the team would turn to ceramics.
The interesting thing to me is the trade-offs involved in this project as is true with so many engineering challenges. You can have clean coal, but how much would you be willing to pay? The cool thing about engineering is it can bring down those costs and actually make an unviable policy viable.
* Actually, it’s called "partial pressure," which is the pressure of one of the gases in a mixture of gas. So the partial pressure of the CO2 is the one that has to be different from the pressure on the other side of the membrane. I’m sure the chem nerds will know why this is important, but I was a biology nerd in high school and have no idea.