That Natural Gas Power Plant with No
by David Roberts
Back in April of 2016, I wrote about an exciting new technology for which construction was just getting underway: the Net Power natural gas power plant. It promised to capture its own carbon dioxide emissions, not in a separate, expensive, power-intensive process like conventional carbon-capture facilities, but as part of the combustion cycle. The company claimed that the technology will ultimately enable it to produce power at prices cheaper than conventional fossil fuel power plants -- with carbon capture built in.
Net Power had just started work on a small, 50 MW power plant in La Porte, Texas, meant to demonstrate that the technology can work.
As of last year, the plant completed construction. And as of this week, it has achieved “first fire” and is running a battery of tests meant to ensure that everything is working up to snuff. If all goes well -- lead designer and chemical engineer Rodney Allam recently told Nature, “we’re still smiling” -- the plant will begin generating electricity in earnest later this year. The company plans to build another 300 MW plant for sometime in 2020.
That will be a very big deal. Zero-carbon natural gas, with no air pollution (did I mention no air pollution?), would be an excellent complement to renewable energy and a cleaner path for countries now planning for more fossil fuel use. It would change carbon capture from something expensive, burdensome, and inessential to something integral to power plants. But it would still have to find a place to bury all that CO2, or find someone to sell it to.
For more on how the power plant works and its (considerable) implications, I give you my explanation from 2016, which holds up:
I have always been skeptical about carbon capture and sequestration at fossil-fueled power plants. It's not so much the technological barriers -- they are serious, though not insurmountable -- but the cost.
Fossil fuel power plants have steadily gotten more efficient, but the problem is, no matter how efficient your plant is, capturing the carbon dioxide emissions involves bolting on a second facility to process and separate the waste gases. That second facility requires power (it's a "parasitic load," cutting into efficiency), and it adds to capital costs.
Coal and natural gas are already losing out to wind in many areas, without sequestration. Once you add sequestration, even as wind and solar are getting cheaper and cheaper, how can fossil fuels with CCS possibly compete?
But Net Power claims it can capture the carbon without a separate facility, as part of the combustion process itself, at no extra cost.
In fact, it says it can generate power more efficiently than conventional power plants, in a smaller physical footprint, with zero air pollution, and capture the carbon -- all at a capital cost below traditional power plants.
That is a heady set of claims. If they prove out in practice, it could be a huge deal. Let's take a closer look.
Natural gas electricity without the emissions
Net Power is working in collaboration with some big names on its power plant in Texas. Exelon Generation will operate the plant. CB&I, an infrastructure firm, will provide "engineering, procurement, and construction services." Net Power's parent company, 8 Rivers Capital, will provide ongoing technology development. And Toshiba will develop the key components (mainly the turbine). The 50 MW demonstration plant is meant to reassure investors that the technology works.
So what's the technology? It's called the Allam Cycle, named after lead inventor Rodney Allam. It is -- brace yourself -- "a high-pressure, highly recuperative, oxyfuel, supercritical CO2 cycle."
Let's see if we can get our heads around that, at least enough to see why it matters. Here's the diagram:
There's a technical explanation here and a more digestible article by Allam here; we'll just hit the basics.
In typical power plants, fuel is mixed with air and burned. "Oxyfuel" means that Net Power's plant mixes fuel (in this case, natural gas) with pure oxygen, produced by an air separation unit (ASU).
Using pure oxygen rather than air virtually eliminates NOx, one of natural gas's worst air pollutants.
In the combustor, a mix of about 5 percent oxyfuel and 95 percent carbon dioxide is combusted at "supercritical" temperatures and pressure, to drive a turbine.
Unlike most electric turbines, which run on steam, the turbine in Net Power's plant is specially built to run on pressurized carbon dioxide. It is a fluid turbine rather than a steam turbine, with carbon dioxide as the working fluid. (You can read much more about the turbine in this excellent Gas Turbine World piece.)
After the turbine is spun and the power is generated, the waste fluid (carbon dioxide and water) is put through a heat exchanger, the water condensed out and separated.
What's left is pure (90-plus percent) carbon dioxide, which is then repressurized in a compressor and made ready for pipeline. It can then be used for enhanced oil recovery (EOR) or sequestered. (More on that in a minute.)
But the bulk of the carbon dioxide is reheated in the heat exchanger and used again in the combustor. That's what "highly recuperative" means -- most of the CO2 is recycled through again and again.
So there we are: "a high-pressure, highly recuperative, oxyfuel, supercritical CO2 cycle."
Why does it matter?
More efficient power in a smaller footprint, for cheaper
Burning fuel with pure oxygen at high temperatures is extremely efficient, avoiding most airborne emissions and extracting more energy from the same amount of fuel. But the biggest leap here is using carbon dioxide rather than steam as the working fluid.
When water is boiled to make steam, it expands and drives the turbine. But the steam must be taken outside the system to be cooled, lest all pressure be lost. That means waste heat, vented from cooling towers, losing 30 to 40 percent of the energy created by combustion.
In Net Power's plant, the heat from the waste fluid is recycled into the heat exchanger, preserving that energy and substantially raising efficiency.
Currently, the most efficient natural gas plants -- advanced combined-cycle plants -- are projected to hit about 62 percent efficiency. Net Power and Toshiba are expecting 59 percent efficiency in this first iteration.
And remember: That 62 percent is without carbon capture. Capturing carbon would mean bolting on another unit, siphoning off power, increasing costs, and reducing natural gas plant efficiency to 48 percent. Net Power expects 59 percent efficiency with 100 percent capture of pipeline-ready carbon dioxide.
Combined-cycle gas turbines (CCGTs) are efficient because they combine two cycles, thus the name. There's enough heat left over after the gas turbine cycle to run a (lower-energy) steam turbine cycle:
Net Power's plant eliminates that second cycle. And because of the high pressures involved, its components are smaller than conventional turbines. Both those advantages shrink the plant's footprint:
(IGCC stands for integrated gasification combined cycle, the most efficient way to burn coal. NGCC stands for natural gas combined cycle.)
Because the plants are simpler (only one cycle) and smaller, Net Power believes they will have lower capital costs than conventional plants.
As a final bonus, the only water used is a bit for cooling. The plant can switch to air cooling for a small (2.5 percent) efficiency penalty and thus become a net water producer. That matters in an era when water threatens to be the Achilles' heel of US thermal power plants.
So: more efficient power, with zero air pollution, virtually no water consumption, and pipeline-ready carbon dioxide capture built in ... for cheaper than today's best fossil fuel power plants. Quite a bold promise.
Net Power's other applications
Note that the basic technology -- oxyfuel + a carbon dioxide turbine -- is not confined to natural gas.
8 Rivers is also working on a plant for coal, which will involve the addition of a coal gasification unit and additional cleanup at the water separation stage.
The best IGCC coal plants now target about 44 percent efficiency; the Allam Cycle plant is expected to hit 51 percent (again, with carbon capture).
The Allam Cycle can theoretically clean up any fossil fuel power production if it proves as cost-effective as Net Power expects. That is, of course, a big if.
And if the technology performs, there are still challenges.
Net Power's place in the bigger climate picture
The biggest challenge is what to do with all the carbon dioxide. If Net Power plants can sell their carbon dioxide to EOR operations, it will improve their competitiveness.
But that's a limited outlet; the scale of fossil fuel power generation far exceeds the ability of EOR to soak up carbon dioxide. (And it's somewhat perverse to use avoided carbon emissions to dig up more carbon anyway.)
What happens if this technology scales up and we start having tons and tons of carbon dioxide to deal with? The sequestration part of CCS is still looming out there. Someone's got to pay for it.
If the plants have to pay for it themselves, in the absence of a price on carbon, it could damage their competitiveness. If the public pays for it, it's hard not to see it as a giant subsidy to fossil fuels.
Climate campaigners are eager to see a transition to renewable energy. They’re unlikely to get too excited about better ways to burn fossil fuels, which would, after all, only keep coal mines and fracking wells in business for longer, to say nothing of keeping fossil fuel companies themselves afloat. Combustion is only one part of the damage done by fossil fuels.
But it's best not to be shortsighted here. Even under the most optimistic scenarios, there are going to be hundreds of fossil fuel power plants built across the world in coming years. This is especially true of natural gas plants, which play an important role in "firming" the fluctuations in variable renewable energy (and could potentially be run in the future on renewable biogas).
If we could start right now making all those new coal and natural gas plants air-pollution-free, it would be a public health win of historic proportions, to say nothing of the regulatory and civic battles that could be avoided.
And capturing all that carbon rather than throwing it into the atmosphere might be enough to give the fight against climate change some much-needed breathing room.
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