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Innovative zero-emissions power plant begins battery of tests

Net Power carbon capture plant, Houston.

A demonstration power plant run by NET Power in Houston, Texas, uses carbon dioxide to drive the turbine that generates electricity.Courtesy of NET Power and McDermott.

A team of engineers in La Porte, Texas, has spent the past several weeks running tests on a prototype power plant that uses a stream of pure carbon dioxide — not air — to drive a turbine. If the zero-emission technology developed by NET Power in Durham, North Carolina, succeeds, it could help to usher in an era of clean power from fossil fuels.

The company broke ground on the roughly 25-megawatt plant in March 2016, after raising US$140 million for the project, and completed construction last year. It is now running a battery of tests on the combustor that powers the plant, a one-of-a-kind device built by the Japanese industrial giant Toshiba. If the tests go as planned, NET Power will hook up the turbine and begin generating electricity later this year.

Officials say everything is running smoothly so far. “We’re still smiling,” says chemical engineer Rodney Allam, the facility’s lead designer. Allam is now a partner with 8 Rivers, a technology company in Durham that co-owns NET Power with Exelon, a major electricity provider in Chicago, Illinois, and McDermott International, an energy-services company in Houston, Texas.

What separates the La Porte facility from a standard power plant is the CO2 cycle at its core. A conventional power plant burns fossil fuels to generate steam that drives a turbine — and it also emits CO2 as a byproduct.

By contrast, NET Power will drive its turbine with a loop of hot, pressurized CO2. The first step is to fill the system with CO2, which must then be heated in order to drive the turbine — much like a conventional power plant heats water to create steam.

The combustor then ignites a mixture of natural gas and oxygen, which is extracted from the atmosphere in a separate facility. This heats up the CO2 in the loop that drives the turbine, but it also produces additional CO2 that must be siphoned off to keep the system in balance.

Energy economics

The result is a stream of pure CO2 that can be buried or put into a pipeline – rather than the atmosphere – at virtually no cost. That gives it an edge over existing technologies for stripping CO2 out of a conventional power plant’s exhaust; these drive up costs while sapping around 20% of the plant’s power.

Allam says that, if all goes well, NET Power’s technology will produce electricity as cheaply and efficiently as a conventional, modern gas-fired power plant — and earn additional revenue by other means. For instance, oil companies might buy the plant’s excess CO2 and pump it into their wells to boost oil production. NET Power could also sell nitrogen and argon produced by the plant’s air separator.

A coal-fired power plant in Houston that is equipped with a competing CO2-capture technology is already delivering the gas it collects to a nearby oil field. The $1-billion Petra Nova project came online in January 2017. It uses an amine-based solvent to capture about one-third of the emissions from a single power-generating unit — up to 1.6 million tonnes of CO2 annually.

But the project — a joint venture between NRG Energy in Princeton, New Jersey, and JX Nippon Oil and Gas Exploration in Tokyo — depended on both a $190 million grant from the US Department of Energy and additional oilfield revenue to turn a profit, says Daniel Cohan, an atmospheric scientist at Rice University in Houston. By contrast, he notes NET Power’s claim that its power plant will turn a profit even before it begins selling CO2. “If the plant does everything they say, it’s hard to imagine why you would want to build a traditional power plant,” Cohan says. “But there are still a lot of ifs ahead.”

Hurdles ahead

One major challenge will be ensuring proper combustion of oxygen and methane in the presence of CO2, which normally acts as a fire extinguisher. NET Power is several months behind schedule on this task, but project officials say that was the result of Toshiba’s decision to test the plant’s combustor on site rather than sending it to an independent test facility; that meant installing and reconfiguring equipment at the otherwise complete plant.

Once the project begins producing electricity, NET Power engineers must also show that the plant operates as efficiently as advertised, says Howard Herzog, who studies carbon capture and sequestration at the Massachusetts Institute of Technology in Cambridge. The challenge, he says, will be to address the inevitable problems that arise when engineers are building the first-of-a-kind facility without sacrificing energy efficiency or driving up costs.

NET Power officials say they are ready to take advantage of recently expanded US-government tax credits for carbon capture and sequestration projects — beginning with a proposed 300-megawatt plant that could be operational by 2021. But the company’s chief executive, Bill Brown, says the company isn’t reliant on subsidies, and is already seeking customers and manufacturing partners abroad. It is also looking at potential markets for CO2, which could soon become a cheap chemical feedstock.

”We don’t like to rely on policy around here,” Brown says. “We like to rely on science.”

Nature 557, 622-623 (2018)



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