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In Texas, a new power plant could redefine carbon capture

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A company called NET Power has begun testing a unique demonstration power plant in La Porte, Texas, that burns natural gas but releases no emissions into the atmosphere. How can it do this? The natural gas is burned in pure oxygen rather than ambient air, and the resulting heated carbon dioxide (CO2) is used to power a turbine instead of heating steam or gas.

Although the plant does create CO2 during the course of its operations, the waste CO2 that pumps through the system is already collected and doesn't require an additional process to scrub pollutants from flue gas like more traditional carbon capture technologies do. The mostly pure CO2 can then be sold to oil fields for use in enhanced oil recovery, or it can be used in some other way (although we haven't quite cracked that nut yet, recent research is working on creating byproducts like carbon nanotubes from waste CO2).
If all goes well with the testing phase, it could mean a more efficient and cost-effective solution than anything that could be bolted onto an existing power plant.

There are two sides to this technology: on the one hand, a solution like NET Power's doesn't do anything to clean up our existing fleet of fossil fuel plants. And while NET Power's system is zero-emissions, it's also still creating more CO2—something we decidedly don't need more of. But power companies continue to build fossil fuel plants throughout the US and especially overseas, with no signs of slowing down. With that context, NET Power's plant could be a reasonable transition technology as we move to a low-carbon future.

Another advantage of NET Power's system is that it can be designed to use very little water. Power plants can be massive water consumers. Many coal plants use water to create steam to power turbines, and fossil fuel plants in general often use water to cool the plant's machinery. But NET Power's system can be air-cooled (with the trade-off being "a small reduction in efficiency," according to the company's press release), and the CO2 turbine uses no steam. In a warming world where water is either scarce or too warm to use for cooling, low water use is a key advantage. NET Power spokesperson Walker Dimmig told Ars over the phone that the system could even be a net water producer, given that a small amount of water is created during the combustion process.
Allam cycle

What's the difference between a gas or steam turbine and a CO2 turbine? Dimmig told Ars that a CO2 turbine is significantly smaller than more traditional turbines. But in its operations, it fits somewhere in between a gas and steam turbine. It works at a higher temperature than a steam turbine and at a lower temperature than a gas turbine. It works at a higher pressure than a gas turbine but at a lower pressure than a steam turbine.

For context, a typical gas turbine operates at 30-bar, while the NET Power system operates at 300-bar. The turbine, Dimmig said, is designed by Toshiba because the company had significant experience building high-pressure turbines, with the highest-pressure turbine operating somewhere around 360-bar.

The overall process of turning the products of fossil fuel combustion into power is called an Allam cycle, named after contemporary British engineer Rodney John Allam. Since the CO2 is created in the presence of pure oxygen, there's no nitrogen oxide (NOx) that needs to be stripped out of the "working fluid" (the CO2).

This is something that more traditional carbon capture systems have tried already, a method of carbon capture called oxyfuel combustion. Both the NET Power system and oxyfuel combustion require an on-site air separator system to create the pure oxygen that's used in combustion.

The difference between an oxyfuel system and NET Power's system is that in the former, the CO2 isn't used and still needs to be captured from the plant's flue. In the latter, the whole system is designed with the intent to use and capture CO2. This makes carbon capture cheaper.

One criticism is that producing pure oxygen for the combustion process is not terribly efficient. But Dimmig claims that NET Power's CO2 turbine is extremely efficient on its own. If you factor in the electricity needed to power the O2 production process, the entire plant's efficiency drops down to somewhere around 60 percent, according to Dimmig, and 60 percent is a good efficiency for a more traditional combined cycle natural gas turbine.

According to an article in Nature, the plant is still behind schedule in finding a way to optimize combustion of natural gas and oxygen "in the presence of CO2, which normally acts as a fire extinguisher." Project officials say the delay is due to the fact that Toshiba wanted to test the combustor "on site rather than sending it to an independent test facility; that meant installing and reconfiguring equipment at the otherwise complete plant."
The economics

NET Power, a jointly owned company run by 8 Rivers Capital, Exelon, and engineering firm McDermott, is essentially "taking oxy-combustion and finding a better cycle to produce [electricity] with," Dimmig told Ars.

He added that the problem with carbon capture is that it’s usually "an afterthought, but that always ends up at net cost."

Part of the reason we haven't seen more carbon capture-enabled plants besides Kemper and Petra Nova is that energy companies are taking traditional fossil fuel plants and tacking carbon capture mechanisms on them. In the absence of a carbon tax or a cap-and-trade system, it will always be cheaper to run a fossil fuel plant without carbon capture than with carbon capture. To change those economics, captured carbon needs to actually run the plant.

The La Porte plant, of course, is just a demonstration project, but Dimmig said NET Power has essentially built a 300MW plant scaled down to 25MW electric/50MW thermal. There are some exceptions to that scaling, notably in the turbine itself, which could only be scaled down 2.5 times "or the blades would be too small for Toshiba to manufacture," Dimmig told Ars. This creates some inefficiencies in the operations of a demonstration project that wouldn't normally exist in a full-sized plant.

For now, though, NET Power just wants to be able to send electricity back to the grid in La Porte to get some data on how the whole thing works. From there, NET Power will be able to court customers for future projects.

The timing seems good—one advantage that NET Power now has in trying to build more of these systems is that Congress recently extended tax credits for carbon capture systems.

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