image credit: DOE

How can we make fossil fuel power plants cleaner?

We at MOBProd love talking about the energy industry. Ever since our PBS NOVA show about nuclear power, The Nuclear Option, we have been interested the way the energy industry is tied to environmental concerns.

In our recent NewsHour pieces, we have focused on clean coal: myth or reality? There were two aspects of coal we chose to focus on, on the mining and on the burning.

For mining, Miles went to West Virginia and saw coal surface mining operations and their impacts. Environmental and health professionals showed him the detrimental effects to water and air quality caused by the mining, while industry voices denied there was a link and showcased the economic benefit. A murky situation where the coal doesn’t come out too clean–but judge for yourself.

For burning, Miles went to a new coal power plant, called Petra Nova, that opened this year in Texas. This new plant has an interesting add-on: a scrubber that captures 90% of the CO2 coming off of one of its four smokestacks. Though expensive, the system works. If we really wanted to make burning coal cleaner, we have the technology… but will we use it

Here’s the thing: of course we want renewables and no emissions. But the fact of the matter is that fossil fuels aren’t going anywhere. And, even more than coal, cheap and plentiful natural gas continues to take over the US energy market.

So, though some may think “clean coal” or “clean natural gas” are mythical, misguided ideals, the reality is that we can have a large impact on the amount of greenhouse gases we are pumping into the atmosphere by cleaning up these sources of energy.

How do we do that?

When we interviewed Nathan Myhrvold for The Nuclear Option, he had a nifty way of describing how the vast majority of our energy works: a firebox heats up water to steam, which then spins turbines that generate electricity. Any fuel-based plant basically works this way, from nuclear to coal to natural gas.

That leaves two avenues to making cleaner fossil fuel energy production: clean up the firebox or make a more efficient turbine. Here’s a few interesting projects in both categories.

Firebox Innovations

Just using a cleaner fossil fuel is one way to clean up the firebox. Good thing methane, the chief component of cheap and plentiful natural gas, is the cleanest-burning fossil fuel possible. The problem with natural gas is just that: that it is a gas and not a liquid. Leaks are more common for natural gas than for oil, and harder for the production and distribution players to find and plug.

CO2 pumped into the ground can also be used to extract hard-to-recover oil. Credit: Hovorka lab.
CO2 pumped into the ground can also be used to extract hard-to-recover oil. Credit: Hovorka lab.

Carbon capture and storage technology, like the Petra Nova facilityMiles visited at the W. A. Parish coal-fired power plant, could help drastically cut down dirty emissions from various fireboxes. These systems work by selectively binding with CO2 in the flue gas, distilling the CO2 in a different container, and compressing it for other storage or other uses. We have the technology right now, it’s just very expensive to add these systems to new plants and even more expensive to retrofit existing fossil fuel plants. Read an expert breakdown here.

What can you do with captured CO2, besides burying it to extract oil? Well, there are a few out-there ideas. For example, there were several companies a few years ago that were trying to mix produced CO2 into concrete. The companies aren’t doing so well now, but we need to keep testing these kinds of innovative ideas if we want to make an impact on greenhouse gas emissions.

Turbine Innovations

Perhaps more exciting than the cleaning up of the firebox are initiatives that aim to improve the turbine part of the fossil fuel energy generation equation.

In a recent article in Science, Levi Irwin and Yann Le Moullec argue that shifting from water steam to supercritical CO2 would greatly improve efficiencies of any turbine-based plant.

Water is very good at holding heat–which is a blessing and a curse for energy generation. The more heat you can move, the more energy you can convert to electricity, so the fact that water has a high heat capacity is a blessing. But the curse is that, because of the high heat capacity of water, it takes a lot of energy to vaporize the water to steam, energy that cannot be used for production of electricity.

A supercritical CO2 turbine would fix this issue, Irwin and Le Moullec write.


Phase diagram of CO2. Credit: Wikimedia.
Phase diagram of CO2. Credit: Wikimedia.

Supercritical CO2 isn’t some kind of superhero version of carbon dioxide–it’s just a different phase like solid, liquid, or gas. Every compound has what’s called a phase diagram, which shows what phase it’s in at a specific temperature and pressure.

Usually at high temperatures and pressures, compounds will become supercritical fluids–a weird phase that has both liquid and gas properties. For example, supercritical fluids can flow through holes in surfaces, like a gas, but also can dissolve other compounds, like a liquid.

Using a supercritical fluid to drive a turbine frees engineers from worrying about a vaporizing step, since there is no phase change–no water is turning to steam and wasting energy, in other words.

There are some supercritical water turbines out there now, but they have to operate at dangerously high temperatures (above 374°C) and pressures (over 218 atmospheres) to keep the water supercritical. On the other hand, carbon dioxide goes supercritical at relatively low temperatures and pressures (over 31°C and 73 atmospheres). That way, engineers can build safer, more compact turbines that run on CO2.

Supercritical CO2 turbines could improve power plant efficiencies upwards of 30%. Credit: Science.
Supercritical CO2 turbines could improve power plant efficiencies upwards of 30%. Credit: Science.

Supercritical CO2 turbines running what’s called the Brayton cycle, Irwin and Le Moullec estimate, are 30% more efficient than the average steam turbine that run something called a Rankine cycle. And they’re smaller and have fewer moving parts, meaning they are easier to build and manage.

If every power plant could switch over to supercritical CO2 technology, the potential benefits for the consumer and the environment would be sizeable. This could actually happen soon: the Department of Energy last year pledged to invest $30 million for research over the next few years… though, who knows if those grants will continue to be doled out.

Combining Innovations

A promising way to clean up fossil fuel plants on both the firebox and the turbine sides of the equation is with Allam cycle technologies.

Basically, an Allam cycle is combines a supercritical CO2 turbine with a carbon capture system. The captured CO2 from a natural gas or gasified coal firebox is piped directly into a supercritical CO2 turbine system. In essence, there is no smokestack–the exhaust itself is what drives the turbine, with extra CO2 being compressed further and shipped off, likely for enhanced oil recovery.

The Allam cycle is named after its inventor, Rodney Allam. A British tinkerer inspired by the airplanes of World War II, Allam came up with this clean firebox and turbine combo when he was younger but could not find a funder interested in prototyping this tech.

Now in his 70s, Allam will finally have his trial by fire, so to speak. He is advising a North Carolina-based company, Net Power, which is currently building the first natural gas fired power plant off of his design near Houston.

Of all clean fossil fuel tech, this I think is the most exciting. If it works, the Allam cycle will achieve the one-two punch of cleaning up the firebox and turbine, potentially delivering a zero emission fossil fuel power plant.

Either way, we’ll know soon enough: Net Power’s Allam cycle plant is on track to open later this year.

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