Running a power plant on carbon dioxide instead of steam

Running a power plant on carbon dioxide instead of steam
The critical point of CO2 is near room temperature and allows efficient input of compressive work; steam's high critical point prohibits this and actually acts as a heat sink, decreasing cycle efficiency. Credit: (c) C. Bickel/Science, DOI: 10.1126/science.aam8281

(Tech Xplore)—A team with NET Power is currently in the process of building a power plant in Texas that will use a form of carbon dioxide to turn turbines instead of using steam to make electricity. The plant will be the first in the world to attempt to utilize the new technology. Levi Irwin and Yann Le Moullec with SETA, ManTech SRS Technologies, Inc., and Électricité de France, China Holding R&D respectively offer a Perspective piece on the work being done by the company in the journal Science.

To make large quantities of electricity, huge turbines are powered by the steam produced from boiling water. Despite being inefficient, this process is the worldwide standard—the only variable is the means by which the water is boiled—by burning coal or natural gas, or splitting atoms. But now the standard may change as the team at NET Power is building a plant that replaces steam with that has been heated and pressurized to induce a in which it has the density of a liquid but still expands to fill a space like a gas. Because the gas is much denser, the CO2 approach can use much smaller turbines, increasing efficiency.

But as Irwin and Moullec note, it is not easy to make such a , which is why no one has tried it before, despite the technique's promise. Pressurizing CO2 to the degree needed presents challenges and requires a lot of energy. Also, temperature and pressure have to be controlled precisely, or the CO2 will condense into droplets, causing problems. Also, a turbine had to be modified to work with the supercritical CO2.

In the new plant, natural gas is burned inside an environment of pure oxygen to prevent the release of anything but pure CO2—any excess can be buried or sold, which means that other than water, the plant will be completely emission free. The new facility, which will start operating later this year, is a test site, but the company already has plans to build a much larger plant that will power 200,000 homes. They also believe they can produce electricity at a cost very nearly equal to a plant.


Explore further

May carbon dioxide turbine help address clean power generation?

More information: Levi Irwin et al. Turbines can use COto cut CO, Science (2017). DOI: 10.1126/science.aam8281

Summary
Two-thirds of the electricity in the United States is generated from fossil fuel via combustion-powered steam turbines. To get to the high temperatures needed for high efficiency, steam must first be vaporized from liquid water. The steam is further heated, expanded through the turbine, and condensed to water on the other side. In this process, called the Rankine cycle, the vaporization step is a phase change that requires a large heat input but delivers no increase in temperature (or efficiency). Advanced steam turbines try to avoid the phase change by going to supercritical conditions, but attempts to exhaust heat at low temperatures push parts of this cycle to operate just above water's critical point (374°C and 218 atm). Near this point, steam's heat capacity increases sharply, so up to 36% of total heat input still goes to a low-temperature, vaporizer-like process (see the figure). By switching from steam to supercritical CO2 (scCO2) and running a Brayton cycle (the same cycle run by natural gas turbines), the "vaporizer" step can be avoided, providing an opportunity to replace subcritical steam plants with a cycle that could be up to 30% more efficient. These gains are expected to persist in the smaller turbine sizes suited for harvesting solar thermal energy.

Journal information: Science

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May 26, 2017
This article:
"Despite being inefficient, this [steam turbine electricity generation] process is the worldwide standard"

http://www.turbin...iciency/
"The electrical generating efficiency of standard steam turbine power plants varies from a high of 37% HHV4 for large, electric utility plants designed for the highest practical annual capacity factor, to under 10% HHV for small, simple plants which make electricity as a byproduct of delivering steam to processes or district heating systems."

The fuel->electricity energy efficiency is a fraction of the efficiency of the heat generation phase to boil the water, burning the coal. Other inefficiences - in mining, processing, distributing and storing the coal and its burnt products - are not included in those efficiency rates.

May 26, 2017
"In the new plant, natural gas is burned inside an environment of pure oxygen to prevent the release of anything but pure CO2—any excess can be buried or sold, which means that other than water, the plant will be completely emission free."

"any excess can be buried or sold": That is a stupid lie. Burying the CO2 leaks CO2 into groundwater and eventually the atmosphere (the Greenhouse). And any excess "sold" is going into the Greenhouse too.

May 26, 2017
The most exciting aspect of using supercritical CO2 as the working fluid for heat engines is that it offers economics for actively storing and monitoring possibly large quantities of CO2 instead of discarding it into the Greenhouse and acid seas or just pumping it out of sight into the ground where it will leak into groundwater and the atmosphere, and so eventually into the Greenhouse.

Geothermal plants could provide very large reservoirs in which to sequester CO2. Both cutting almost all emissions from the combustion plants they replace, and providing infrastructure and incentive to monitor their reservoir for leaks and mitigate them. Even solar thermal plants might use large CO2 stores for such benefits.

May 28, 2017
"That is a stupid lie. Burying the CO2 leaks CO2 into groundwater and eventually the atmosphere"


It's not necessary to pump it into aquifiers. An icelandic study pointed out that the CO2 can become mineralized into certain types of rock as well.

It's not an infinitely sustainable solution, but a stopgap solution.

May 28, 2017
Eikka:
It's not


Some CO2 in some locations can become mineralized. Those sequestrations are legit, but the exception rather than the rule. The rule is leaky burials that don't protect the Greenhouse from thickening. And from acidifying groundwater - not just aquifers (which collapse when carbonic acid water dissolves aquifer limestone structure). Acidified groundwater flows to outgas CO2 into the atmosphere.

So burying the CO2 is not worth it. It's not a stopgap solution, it's a distraction to keep polluters in business. Polluters who would bury their waste only to kick the can down the road as they always do, externalize the cost to others. That's as essential to their business model in faked sequestration as it's always been.

Instead the effort must be spent reducing the CO2e output, and changing the processes so carbon waste can be sunk as solids and/or consumed in some other process - to eliminate the previous other process' own polluting carbon feeds.

May 29, 2017
EmceeSquared - as to your first point, the method of heat production is completely separate from the use of that heat in power generation. There is certainly nothing that in principle that says for example, that solar concentrators or waste heat from industrial processes cannot be used to power the generators, in which case the efficiency considerations for coal that you mentioned do not apply. Actually most locations have available the sorts of minerals that can trap CO2 using a chemical reaction to mineralize it. And lastly, while its absolutely true that CO2 leakage would be a major consideration, there is nothing in principle that states leakage cannot be kept very small with careful design. But it certainly is a major concern.

May 29, 2017
Parsec:
EmceeSquared - as to your first point


I posted the 37% max efficiency of steam turbines to quantify the article's vague mention of their inefficiency.

Yes, coal fired turbines can be fired by other heat sources, and are being converted (typically to natgas). The inefficiency of the heat generation phase is an additional factor in the plant's overall inefficiency, as I stated for coal.

I don't know what your point is here.

May 29, 2017
Parsec:
Actually most locations


I've seen only a few early demos of mineralizing CO2 emissions in any practical rate/quantity. Please post stats, preferably a map, showing "most locations" have the required conditions to mineralize power plant emissions.

May 29, 2017
Parsec:
And lastly

What about the principle that the old mines where many polluters propose to store CO2, like old coal mines, are very leaky? That principle states leakage cannot be kept very small. We're talking about pumping a tiny molecule into the ground, not a sealed container. It's only in the past year or so that engineers are even claiming that the leakage could possibly be accurately measured, using new techniques not yet proven, certainly not in the widely varying geology the polluters assured us would be no problem (until after the polluters are dead).

"Principles" are nice idealized goals. Geology is complex and dynamic, diverging mightily from the principles used to describe them.

Jun 10, 2017
I hope the people that plan to run that place understand they are going to have to wear o2 tanks and masks or at least have them readily available and easy to get to. I've worked around and in enough steam plants to know that there are going to be leaks coming from valves, pumps etc.... Unless that plant has a whole lot of exhaust and supply air fans that place is not going to be safe to work in.

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