The prototype NET Power plant near Houston, Texas, is testing an emission-free technology designed to compete with conventional fossil power.

CHICAGO BRIDGE & IRON

Goodbye smokestacks: Startup invents zero-emission fossil fuel power

Between the energy hub of Houston, Texas, and the Gulf Coast lies a sprawling petropolis: a sea of refineries and oil storage tanks, power lines, and smokestacks, all dedicated to converting fossil fuels into dollars. They are the reason why the Houston area emits more carbon dioxide (CO2) than anyplace else in the United States.

But here, on the eastern edge of that CO2 hot spot, a new fossil fuel power plant showcases a potential remedy for Houston's outsized greenhouse gas footprint. The facility looks suspiciously like its forebears, a complex the size of two U.S. football fields, chock-a-block with snaking pipes and pumps. It has a turbine and a combustor. But there is one thing it doesn't need: smokestacks.

Zero-emission fossil fuel power sounds like an oxymoron. But when that 25-megawatt demonstration plant is fired up later this year, it will burn natural gas in pure oxygen. The result: a stream of nearly pure CO2, which can be piped away and stored underground or blasted into depleted oil reservoirs to free more oil, a process called enhanced oil recovery (EOR). Either way, the CO2 will be sequestered from the atmosphere and the climate.

That has long been the hope for carbon capture and storage (CCS), a strategy that climate experts say will be necessary if the world is to make any headway in limiting climate change. But CCS systems bolted to conventional fossil fuel plants have struggled to take off because CO2 makes up only a small fraction of their exhaust. Capturing it saps up to 30% of a power plant's energy and drives up the cost of electricity.

In contrast, NET Power, the startup backing the new plant, says it expects to produce emission-free power at about $0.06 per kilowatt-hour. That's about the same cost as power from a state-of-the-art natural gas-fired plant—and cheaper than most renewable energy. The key to its efficiency is a new thermodynamic cycle that swaps CO2 for the steam that drives turbines in conventional plants. Invented by an unlikely trio—a retired British engineer and a pair of technology geeks who had tired of their day jobs—the scheme may soon get a bigger test. If the prototype lives up to hopes, NET Power says, it will forge ahead with a full-scale, 300-megawatt power plant—enough to power more than 200,000 homes—which could open in 2021 at a cost of about $300 million. Both the company and CCS experts hope that the technology will then proliferate. "This is a game-changer if they achieve 100% of their goals," says John Thompson, a carbon capture expert at the Clean Air Task Force, an environmental nonprofit with an office in Carbondale, Illinois.

Engineer Rodney Allam conceived the carbon dioxide cycle at the heart of the new power plant.

MARC WILSON

NET Power CEO Bill Brown, 62, never set out to remake the energy market. A decade ago, as a dealmaking lawyer in New York City, he crafted financial trading strategies for Morgan Stanley. But he was restless. So he called Miles Palmer, a buddy from his undergraduate days at the Massachusetts Institute of Technology (MIT) in Cambridge. Palmer was a chemist for Science Applications International Corporation (SAIC), a defense contractor that designed everything from rail guns to drones. Brown suggested they "make something good for a change." In 2008, as the economy was collapsing, they left their jobs and started 8 Rivers, a technology incubator in Durham, North Carolina, where Brown also taught law at Duke University.

They needed something to incubate. They liked the thought of doing something in the energy sector, a famously risk-averse arena, but one in which a breakthrough technology can make a fortune. First came a brief, fruitless attempt to make biofuels from algae. Then, in 2009, the Obama administration's stimulus package offered billions of dollars in grants for "clean coal" projects—ways to reduce coal's CO2 emissions. Palmer knew that, worldwide, coal wasn't going away anytime soon, and he understood how it threatened the climate. "I wanted to solve that problem," he says.

Cleaning up coal has been tough. Not only does coal release twice as much carbon pollution as natural gas, but that CO2 also makes up just 14% of the flue gas from a conventional power plant. Still, coal is plentiful and cheap, and until recently few people cared about the CO2 it unleashes. So coal-fired power plants haven't changed much since 1882, when Thomas Edison's company built the first one in London. Most still burn coal to boil water. The steam drives a turbine to generate electricity. At the turbine's back end, cooling towers condense the steam into water, lest the high-pressure steam there drive the turbine in reverse. Those towers vent much of the energy used to boil the water in the first place. Overall, just 38% of coal's energy yields electricity. "All that energy is just wasted," Brown says.

That inefficiency helped drive utilities to natural gas. Not only is gas cleaner—and, in the United States, cheaper than coal—but because it is a gas to begin with, engineers can take advantage of an explosive expansion as it burns to drive a gas turbine. The heat of the turbine exhaust then boils water to make steam that drives additional turbines. The best natural gas "combined cycle" plants achieve nearly 60% efficiency.

Still, Palmer was focused on coal, the bigger climate problem. He built on work he had done at SAIC on a high-pressure combustor for burning coal in pure oxygen. It was more efficient and smaller, and so it would cost less to build. It also produced an exhaust of concentrated CO2, thus avoiding the separation costs. "I got it to work almost as well as a conventional coal plant, but with zero emissions," Palmer says. "But it wasn't good enough."

Palmer and Brown needed to nudge the efficiency higher. In 2009, they contacted Rodney Allam, a chemical engineer who had run European R&D operations for Air Products, an industrial giant in the United Kingdom. Later, in 2012, Allam won a share of the $600,000 Global Energy Prize, sponsored by the Russian energy industry, for his work on industrial gas production. But at the time, he was mostly retired, concentrating on his fishing, lawn bowling, and gardening.

Palmer and Brown hired Allam as a consultant. Inspired by some Russian research from the 1930s, Allam thought he saw a way to radically reinvent the staid steam cycle. Forget about boilers, he thought. He would drive everything with the CO2 itself, making an ally out of his enemy. "The only way you could proceed was to develop a totally new power system," Allam says.

Allam envisioned the CO2 circulating in a loop, cycling between a gas and what's called a supercritical fluid. At high pressure and temperature, supercritical CO2 expands to fill a container like a gas but flows like a liquid.

For decades, engineers have worked on Brayton cycles—thermodynamic loops that take advantage of the properties of supercritical fluids, which could be air or CO2. Supercritical fluids offer advantages: Because they are fluids, a pump can pressurize them, which takes far less energy than a compressor needs to pressurize a gas. And because of the fluidlike gas's extra density, it can efficiently gain or shed heat at heat exchangers.

In Allam's particular Brayton cycle, CO2 is compressed to 300 times atmospheric pressure—equivalent to a depth of 3 kilometers in the ocean. Then fuel is burned to heat the CO2 to 1150°C, which turns it supercritical. After the CO2 drives a turbine, the gas's pressure drops and it turns into a normal gas again. The CO2 is then repressurized and returned to the front end of the loop. A tiny amount of excess CO2—exactly as much as burning the fuel created—is shunted into a pipeline for disposal.

The Allam cycle, as it is now called, comes with costs. Giant cryogenic refrigerators must chill air—which is mostly nitrogen—to extract the pure oxygen needed for combustion. Compressing CO2 into a supercritical state also sucks up energy. But both steps are well-known industrial processes. Allam calculated that discarding the steam cycle would boost the 38% efficiency of a coal plant to 56%. That would put it within striking distance of the efficiency of a contemporary combined cycle plant. As a bonus, the exhaust is nearly pure CO2 that can be sold for EOR. Another perk is that the Allam cycle generates water as a byproduct of combustion, instead of consuming it voraciously as conventional steam cycles do, which could make plants easier to site in arid parts of the world.

At this point, Brown and Palmer were still planning to use coal as their fuel. But when they sent Allam's handiwork to the engineering firm Babcock & Wilcox, to see whether the system would work on an industrial scale, "they had good news and bad news," Brown says. On the downside, the Allam cycle would be tough to pull off with coal, at least initially, because the coal would first have to be converted to a synthetic gas, which adds cost. Also, sulfur and mercury in that syngas would have to be filtered out of the exhaust. But on the upside, the engineers saw no reason why the technique wouldn't work with natural gas, which is ready to burn and doesn't have the extra contaminants.

Brown and Palmer gave up on winning a clean coal grant from the government. Instead, they sought private investment for a far bigger prize: revolutionizing energy production with carbon capture. By 2014, 8 Rivers had secured $140 million in funding from Exelon and Chicago Bridge & Iron, two industrial giants that now co-own the NET Power demo plant. In March 2016, the company broke ground on its pilot plant outside Houston.

"This is the biggest thing in carbon capture," says Howard Herzog, a chemical engineer and carbon capture expert at MIT. "It's very sound on paper. We'll see soon if it works in reality. There are only a million things that can go wrong."

Smoke out A new power plant will use carbon dioxide (CO 2 ) instead of steam. Rather than venting CO 2 , it can sequester the greenhouse gas underground. And it approaches the eff - ciency of the best conventional natural gas plants. Power Compressorand pump Combuster Gas turbine Steam turbine Combuster Air separationunit CO 2 turbine CO 2 and H 2 0 CO 2 and H 2 0 CO 2 Pure O 2 Recycled CO 2 Natural gas supply Natural gassupply Low-pressuresteam High-pressure steam Water Turbine exhaust Hot gases Air supply Power Emissions Air Supercritical CO 2 turbine Conventionalsteam turbine The Allam cycle Invented in 2009, the Allam cycle can achieve a near 60% efficiency while emitting no CO 2 or other pollutants. Small packages CO 2 turbines are smaller than steam turbines—and less costly. Supercritical CO 2 At high temperatures and pressures, CO 2 becomes supercritical—a gas with the density of a fluid. That’s good for driving turbines. WaterExhaust gases are cooled,allowing water to condenseout, a bonus for arid regions. Sequestration High-pressure exhaust CO 2 can be sequestered under - ground or used to free up oil from depleted fields. Cooling towerMuch of the energy used to boil water is wasted. Natural gas combined cycle Natural gas power plants are cleaner than coal and can achieve efficiencies near 60%. But they still vent CO 2 and other pollutants to the atmosphere. Boiler Making steam is less efficient than producing supercritical CO 2 .
C. BICKEL/SCIENCE

One of those is the new turbine, which needs to work at intense temperatures and pressures. Some steam turbines reach those extremes, but "no one had ever designed a turbine to do that with CO2 as the working fluid," says NET Power spokesperson Walker Dimmig. In 2012, NET Power officials inked a deal to have the Japanese conglomerate Toshiba retool one of its high-pressure steam turbines to work with supercritical CO2, which required changing the lengths and angles of the turbine blades. Toshiba also engineered a new combustor to mix and burn small amounts of oxygen and natural gas in the midst of a gust of hot supercritical CO2—a problem not unlike trying to keep a fire going while dousing it with a fire extinguisher.

The re-engineered combustor and turbine were tested in 2013 and delivered to the demo plant in November 2016. Now, they are being integrated with the rest of the facility's components, and the plant is undergoing preliminary testing before ramping up to full power sometime this fall. "I'm 100% confident it will work," Allam says.

If it does, Brown says, NET Power will have advantages that could encourage widespread market adoption. First, the CO2 emerging from the plant is already pressurized, ready to be injected underground for EOR, unlike CO2 recovered from natural gas wells—the usual source.

Another advantage is the plant's size. Not only are the heat exchangers much smaller and cheaper to build than massive boilers, but so are many of the other components. The 25-megawatt supercritical CO2 turbine, for example, is about 10% the size of an equivalent steam turbine. Overall, NET Power plants are expected to be just one-quarter the size of an equivalent advanced coal plant with carbon capture, and about half the size of a natural gas combined cycle with carbon capture. That means less concrete and steel and lower capital costs. "For many CCS projects, the upfront costs are daunting," says Julio Friedmann, a carbon capture expert at Lawrence Livermore National Laboratory in Livermore, California. "Avoiding those costs really matters." What's more, unlike gas plants without carbon capture, NET Power will be able to sell its CO2 for EOR.

Even if NET Power's technology works as advertised, not everyone will be a fan. Lukas Ross, who directs the climate and energy campaign at Friends of the Earth in Washington, D.C., notes that the natural gas that powers the plant comes from hydraulic fracturing, or "fracking," and other potentially destructive practices. And providing a steady supply of high-pressure gas for EOR, he adds, will only perpetuate a reliance on fossil fuels. Ross argues that money would be better spent on encouraging broad deployment of renewable energy sources, such as solar and wind power.

Yet oddly enough, NET Power could help smooth the way for renewables to expand. The renewable portfolio standards in many countries and U.S. states require solar, wind, and other carbon-free sources to produce an increasing proportion of the electric power supply. But those sources are intermittent: The power comes only when the sun is shining and the wind is blowing. Nuclear and fossil fuel sources provide "base load" power that fills the gaps when renewables aren't available. Conventional natural gas power plants, in particular, are viewed as a renewable-friendly technology because they can be ramped up and down quickly depending on the supply of renewable power.

As an emission-free alternative, NET Power's plants could enable communities to deploy even more renewables without having to add dirty base-load sources. "Fossil fuel carbon-free power allows even more aggressive deployment of renewables," says George Peridas, an environmental policy analyst with the Natural Resources Defense Council in San Francisco, California.

That's a combination Allam wants to promote. "I'm not knocking renewables, but they can't meet future power demands by themselves," he says. Allam, a longtime member of the Intergovernmental Panel on Climate Change, says time for solving carbon pollution is running short—for both the world and himself. "I'm 76," he says. "I've got to do this quickly."