Coal is an abundant source of energy. But burning it spews billions of tons of climate-warming CO2 into the air every year.
Much hope has been placed on a developing technology known as carbon capture and sequestration (CCS). The idea is to extract the carbon before it’s emitted from smokestacks, compress it, and store it underground.
That could allow humans to keep using coal, without further loading the atmosphere and oceans with more CO2.
The hope is misplaced, according to a new analysis by University of Michigan engineering researchers, largely because the cost of CCS is far greater than previously assumed.
The U of M's Sarang Supekar says researchers have been relying on a flawed interpretation of a single 1991 pilot study, which pegged the cost of a CCS-equipped plant at $29 million more per year than a conventional plant.
But that study assumed that the CCS-equipped plant would operate at a lower power output than a conventional plant. In reality, companies operate power plants at full capacity as much as possible, to maximize output and profitability.
Supekar says the 1991 study makes it clear that there is a very large efficiency reduction associated with a CCS-equipped plant operating at full capacity. That’s because of what he calls the “energy loop.”
"You need energy to capture CO2, and that energy's going to come from coal," says Supekar. "So you burn more coal, but that also creates more CO2, which needs to be captured."
Supekar says the actual additional cost of a CCS plant is closer to $126 million annually – four times the number that’s been assumed. He says at that cost, a coal-burning plant with CCS technology would cost as much as a current solar energy project.
The analysis explains why CCS test projects have been seeing higher than expected energy penalties, says Supekar.
“The one-line conclusion is that coal should stay in the ground,” Supekar says. "It doesn't make sense to mine it, then get heat out of it, then capture that CO2 that you mined, then put it back into the earth, it just seems illogical to do that.”
Supekar acknowledges that it’s difficult to run a reliable energy grid using only solar and wind energy (wind energy is already cheaper than a new coal-burning power plant). That’s because solar and wind are intermittently available.
He says carbon capture technology might still be useful – but on natural gas power plants, since natural gas is already a lower carbon fuel than coal.