Researchers assess power plants that convert all of their CO2 emissions into carbon nanotubes

June 20, 2016 by Lisa Zyga report
CO2, captured either directly from the atmosphere or from a combined cycle (CC) natural gas power plant, can be electrolyzed to produce oxygen gas and carbon nanofibers, which can then be used to make a variety of valuable products. Credit: Lau et al. ©2016 Elsevier Ltd

(Phys.org)—Last year, researchers at George Washington University proposed a method for transforming CO2 emissions into carbon nanotubes (CNTs). When applied to power plants, the technology could completely eliminate the power plants' CO2 emissions while simultaneously producing a valuable product that is used for a variety of applications, including batteries, consumer electronics, airplanes, and athletic equipment.

The technology can work with almost any kind of power plant, but the researchers specifically investigated its application for combined cycle (CC) natural , which are the most efficient kind of electrical power plant yet still emit massive amounts of CO2.

The idea is to add a molten lithium carbonate electrolyzer to a conventional CC plant, creating a CC nanofiber (CC CNF) plant. Using electrolysis—the same technology that splits water to produce hydrogen—the system applies a voltage to split CO2 into oxygen gas and solid . Adding in small quantities of nickel causes the carbon nanofibers to be hollow, forming CNTs.

To make sure that the idea isn't too good to be true, in a new study the same researchers have performed a thermodynamic assessment of the proposed CC CNF plant. They found that the concept is economically feasible and even improves the power plants' energy efficiency.

"The technology incentivizes carbon dioxide removal by transforming this greenhouse gas into a valuable product to ameliorate the impact of climate change," Stuart Licht, a chemistry professor at George Washington University and leader of the study, told Phys.org. "The production of CNTs will actually be more profitable for fossils fuel than making power, and this should incentivize the transition to a renewable, sustainable society. CNTs have over twenty times the strength of steel or aluminum and are lower weight, and we hope that the CNTs will provide a complete replacement for the trillion-dollar steel and aluminum market. CNTs are also useful in nanoelectronics and new medicine delivery systems, and are already being used in textiles [such as bullet-proof clothing]."

The researchers' assessment shows that, for every metric ton of methane fuel consumed, a conventional CC power plant produces $909 of electricity and emits 2.74 tons of CO2. In contrast, the proposed CC CNF plant would produce about $835 of electricity, which is about 8% less than the CC plant. But the CC CNF plant would also produce about 0.75 tons of CNTs, which is worth an estimated $225,000, and emits no CO2.

(Left) A conventional CC power plant uses a gas turbine to convert heat to mechanical energy, followed by a steam turbine to convert the residual heat to mechanical energy. (Right) The proposed CC CNF power plant adds an electrolyzer that converts carbon dioxide into carbon nanotubes, eliminating carbon dioxide emissions. Credit: Lau et al. ©2016 Elsevier Ltd

In other words, the small decrease in power output is more than compensated for by the highly valuable carbon nanofibers and nanotubes that could be produced. This is mainly because industrial-grade carbon nanotubes are such an expensive commodity, which currently cost about $300,000/ton ($130/pound) to produce using methods available today. Using the new method, the researchers estimate that it would cost just $2,000/ton to produce CNTs—less than 1% of current production costs.

The researchers hope that this large profit potential will make the technology seem like an obvious choice. Since CNTs are about 10,000 times more valuable than carbon tax credits (which are roughly $30/ton), the researchers predict that CNT production will offer a greater incentive for the energy industry to reduce carbon emissions than carbon tax credits offer.

Even though the value of CNTs would likely decrease in the future since they can be produced much more easily and cheaply using this new method, that would simply spread part of the economic benefit to other industries. Lower CNT prices would spur CNT market growth and positively impact the many industries that use them, including the automobile, airline, and wind turbine industries.

The researchers' assessment also shows that the CC CNF plants make sense from a thermodynamic perspective when compared to conventional CC plants with and without carbon sequestration, as well as conventional coal plants.

Even though the CC CNF plants would produce somewhat less electricity than the other types of plants, they would do so at a higher efficiency. The improved efficiency is due to heat energy gained in several areas that could be recycled back into the steam turbines. The heat energy comes, for instance, from the energy produced from chemical reactions with lithium oxide; the energy gained from cooling the carbon and oxygen products; the energy gained from burning the natural gas with a mix of pure oxygen and air (splitting the CO2 releases pure oxygen as well CNTs); the energy saved from preventing CO2 emissions; and the energy gained by capturing CO2 at a much higher temperature than the temperatures at which CC plants with carbon sequestration operate. And unlike carbon capture technologies, energy is not need to store the CO2 as a waste material, since instead it is converted to a valuable product.

Currently, the researchers are working to build and implement the technology as quickly as possible.

"We are quickly scaling up the process, which is the challenge to rapid deployment and substantial CO2 reduction," Licht said.

Explore further: ExxonMobil launches venture for low-cost carbon capture

More information: Jason Lau, Gangotri Dey, and Stuart Licht. "Thermodynamic assessment of CO2 to carbon nanofiber transformation for carbon sequestration in a combined cycle gas or a coal power plant." Energy Conversion and Management. DOI: 10.1016/j.enconman.2016.06.007

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57 comments

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MR166
2.7 / 5 (6) Jun 20, 2016
If this plan works as stated it is the first CO2 sequestering proposal that actually makes any sense or cents as the case may be.
MR166
4.1 / 5 (9) Jun 20, 2016
On a separate note, can carbon nano tubes produce cancer if ingested or inhaled? I have never seen any research on this.
shavera
5 / 5 (6) Jun 20, 2016
can carbon nano tubes produce cancer if ingested or inhaled?


To the best of my knowledge this is a field of very active research at the moment.
tekram
5 / 5 (8) Jun 20, 2016
A recent investigation indicated that multiwalled carbon nanotube (MWCNT) act as a strong promoter of lung tumors (Sargent et al. 2014).

http://www.ncbi.n...4706753/
winthrom
3.2 / 5 (6) Jun 20, 2016
Once the carbon nano-tubes are created, they are fuel (if nothing else) that burns. In the worst case, this technique could double the power output of a natural gas power plant by adding a "carbon burning" power cycle. Effectively we get twice the power from the same amount of raw fuel. (Then we do it again??)

The gain comes from:adding " ... a molten lithium carbonate electrolyzer to a conventional CC plant, creating a CC carbon nanofiber (CC CNF) plant. Using electrolysis—the same technology that splits water to produce hydrogen—the system applies a voltage to split CO2 into oxygen gas and solid carbon nanofibers. Adding in small quantities of nickel causes the carbon nanofibers to be hollow, forming CNTs."

The lithium/nickel catalysts seem to make a nearly perpetual motion process out of the conventional CC power plant. Something is unexplained here. Electrolysis uses up lots of energy.
winthrom
3.4 / 5 (5) Jun 20, 2016
The following statement is suspicious:

"The researchers' assessment shows that, for every metric ton of methane fuel consumed, a conventional CC power plant produces $909 of electricity and emits 2.74 tons of CO2. In contrast, the proposed CC CNF plant would produce about $835 of electricity, which is about 8% less than the CC plant. But the CC CNF plant would also produce about 0.75 tons of CNTs, which is worth an estimated $225,000, and emits no CO2."

This says that ALL the CH4 carbon is captured as CNF at no significant energy cost. A true miracle. If real, one could imagine making the CNFs (before turning them into CNTs) using solar energy and then burying the result in abandoned coal pits/mines.

"The required energy balance for a carbon nanotube production from an analogous coal power plant consumes a larger fraction of the coal energy, and encourages co-generation with renewable electric energy." Read link to DOE abstract in article above.
MR166
1 / 5 (4) Jun 20, 2016
Deleted
freeiam
1.3 / 5 (21) Jun 20, 2016
Oh no, IPCC's non existent self created problem solved.
That means all climate whining must go elsewhere; I'm not worried though, they are very inventive and always find a fear elsewhere so the funding never stops, maybe warn against meteors and catastrophes from space.
Danielshead
3 / 5 (6) Jun 20, 2016
Looks like they are feeding additional O2 into the intake of the gas turbine. I'm betting their thermodynamics is sound, but that they increased the fuel to keep the combustion ratio the same. That will increase the temperature of the combustion gasses and increase the efficiency of the system. Unfortunately existing turbines already run the blades as close to the edge of melting as possible. Neat idea though.
antigoracle
1.4 / 5 (15) Jun 20, 2016
Wow, talk about win WIN. They can convert the electricity they produce to 3000 times its worth while saving the emission of the waste CO2. So, this begs the question, why bother selling the electricity when they can afford to just burn methane to feed this process. What is it they say about something that's too good to be true?
Eikka
3.5 / 5 (6) Jun 20, 2016
In the worst case, this technique could double the power output of a natural gas power plant by adding a "carbon burning" power cycle.


There already is such a cycle in the original gas turbine. It takes energy to split CO2 back to carbon to make the nanotubes, and it's that energy you get out by burning them.

For a coal burning powerplant, the system cannot possibly work because it would take all the energy produced by burning the coal to turn the resulting CO2 back to carbon. The whole process means un-burning the fuel.

That's why the numbers are highly suspicious - it can't possibly work like that.
carbon_unit
1.4 / 5 (22) Jun 20, 2016
I agree that this whole thing is thermodynamically fishy. The nanotubes would be uneconomical to produce (as fuel) because the electrolyzer can't use less energy to separate the CO2 (unburn it) than one would get when one later burns this "hairy coal." There would be a deficit in electrical power, not a surplus. And of course the burning of the natural gas in the first place can't release more energy than the later unburning in the electrolyzer would require.

Unless I'm missing something, this whole thing is scientifically bogus and I'm wondering why phys.org keeps publishing perpetual energy scheme BS like this??
Osiris1
1 / 5 (12) Jun 20, 2016
Now the manufacture of unlimited amounts of carbon nanotubes in any length becomes feasible economically. You ALL know what that means
Sigh
4 / 5 (8) Jun 20, 2016
I agree that this whole thing is thermodynamically fishy. The nanotubes would be uneconomical to produce (as fuel) because the electrolyzer can't use less energy to separate the CO2 (unburn it) than one would get when one later burns this "hairy coal." There would be a deficit in electrical power, not a surplus.

I agree so far.
And of course the burning of the natural gas in the first place can't release more energy than the later unburning in the electrolyzer would require.

The hydrogen in CH4 is still burned to produce water. I expect that's where the electricity comes from.
retrosurf
5 / 5 (9) Jun 20, 2016
From the abstract, the process costs 219 Kj to convert a mole of CO2 to nanotubes and oxygen.
Also from the abstract:
Converting to power and ton units, per metric ton of methane fuel consumed the CC CNF plant is thermodynamically assessed to produce 8350 kW h of electricity and 0.75 ton of CNT and emits no CO2, while the CC plant produces 9090 kW h of electricity and emits 2.74 ton CO2.


Let us calculate. 2.74 tons of CO2 is 56543 moles, times 219 kiloJoules is 1.238291 x 10**10 joules, or 3440 kWh to electrolyze that CO2 to nanotubes and oxygen. The difference in the power output of the two plants is 740 kWh.

So, that's a 2700 kWh difference. Of course, boosting the fuel charge with oxygen from the electrolyzer is an improvement in two different ways: reducing the proportion of inert N2 in the input, and improving the expansion of the combustion products.

Seems believeable, like the boost in distillation efficiency when you capture the heat of condensation.

Da Schneib
4.5 / 5 (16) Jun 20, 2016
The lithium/nickel catalysts seem to make a nearly perpetual motion process out of the conventional CC power plant. Something is unexplained here. Electrolysis uses up lots of energy.
That's what catalysts do: make a reaction require less energy input. Note that the plant still puts out less energy per amount of fuel than it would without the electrolysis. There's no free energy here.
Da Schneib
4.5 / 5 (15) Jun 20, 2016
In the worst case, this technique could double the power output of a natural gas power plant by adding a "carbon burning" power cycle.


There already is such a cycle in the original gas turbine. It takes energy to split CO2 back to carbon to make the nanotubes, and it's that energy you get out by burning them.

For a coal burning powerplant, the system cannot possibly work because it would take all the energy produced by burning the coal to turn the resulting CO2 back to carbon. The whole process means un-burning the fuel.

That's why the numbers are highly suspicious - it can't possibly work like that.
You, too, forgot the catalyst. You'd be right except for that.
Da Schneib
4.3 / 5 (12) Jun 20, 2016
And two others: @carbon_unit and @Sigh, just to be fair.
LifeBasedLogic
Jun 20, 2016
This comment has been removed by a moderator.
Osiris1
2.4 / 5 (11) Jun 20, 2016
This makes the generation of 60,000 mile CNT cables for the space elevator economically feasible!
RealScience
5 / 5 (11) Jun 20, 2016
Note that this does not start with coal, it starts with methane.

After netting out the intermediate steps, they are basically splitting the hydrogen off from the methane and burning that for fuel, leaving carbon behind, which does work thermodynamically.

However instead of producing coal dust an burying it, they are turning the carbon into carbon nanotubes and will turn a profit (if their cost figures are correct).

One potential issue is that while carbon nanotubes are now ~$300,000 per tonne, the market at that price is small so if they start dumping millions of tonnes on the market the price will crash. Still, if their $2000/ton cost pans out, they can make money at a 100x lower price.

Another issue is health - CNTs may be the new asbestos. However really long CNTs are less likely to be troublesome, so let's get really long CNTs by the ton and get that space elevator going!
Tenstats
1.9 / 5 (8) Jun 20, 2016
Let's begin by saying that perpetual motion is impossible. The first concern is that all of the energy released by burning the C in CH4 to CO2 cannot then be used to convert the CO2 back into CO2 since there will be thermodynamic losses, so a power generation cycle that burns C to CO2 and then back to C is an impossibility without an additional source of energy. In the process as discussed in this article, the only useful energy is the energy that is released by burning the H4 in the methane into 2 H2O. The energy from this source must be sufficient to cover the fact that the overall thermodynamic is such that one unit of energy input doesn't result in one unit of energy to the grid, or otherwise be available for electrolysis.
Da Schneib
4.2 / 5 (15) Jun 20, 2016
You can't compare the energy required in a catalyzed reaction with the energy in an uncatalyzed reaction. The entire point of catalysis is that it allows reactions to proceed that would otherwise not be able to because it would take too much energy. Once this is factored in, we find that the power output per unit methane burned is slightly less than the energy if we merely allowed the CO2 to float away. So you pay to make the nanotubes, but not as much as bare (i.e. without catalyst) electrolysis.

And that's five.
Shootist
1.5 / 5 (16) Jun 21, 2016
Dyson was right again. Carbon isn't a problem and any downside will be solved by technology.

In the mean time if youse is worried about AGW, paint your roof and roadways white. That is the most effective thing your insignificant self can do to solve "anthropogenic global" yadayada.
Eikka
4.5 / 5 (8) Jun 21, 2016
That's what catalysts do: make a reaction require less energy input.


That's not what catalysts do. They lower the energy threshold of reactions, but do not lower the total amount of energy needed to complete it.

It's the difference of having to lift a weight up on a wall, or up a staircase. The latter can be done gradually with small inputs while the former has to be done all at once in one big grunt. Both result in the same total energy spent.

You can't compare the energy required in a catalyzed reaction with the energy in an uncatalyzed reaction.


Yes you can. Catalysts are not magical over-unity reagents.
EnricM
5 / 5 (4) Jun 21, 2016
Oh no, IPCC's non existent self created problem solved.



Oh yes, the evil minds of the Illuminati Pokemon Collectors Club... quick, fasten your thin foil hat, I heard black helicopters flying your way!!!
antialias_physorg
4.7 / 5 (12) Jun 21, 2016
yson was right again. Carbon isn't a problem and any downside will be solved by technology.

Who are you going to quote next? Ronald McDonald?
And FYI: Dyson said no such thing.

The only thing Dyson says regarding climate is:
a) that he admits he has no clue on how climate works
and
b) that he's not sure that climate models give correct answers

..and that's it.

So if you're going to quote someone who isn't even supporting your position then do it right.
Da Schneib
4.3 / 5 (11) Jun 21, 2016
That's what catalysts do: make a reaction require less energy input.
That's not what catalysts do. They lower the energy threshold of reactions, but do not lower the total amount of energy needed to complete it.
You're correct; I was inexact. But while this is true, it's also true that the power output of the plant is less per ton of gas consumed.

You can't compare the energy required in a catalyzed reaction with the energy in an uncatalyzed reaction.


Yes you can. Catalysts are not magical over-unity reagents.
Right, but that's not what's being claimed here. Again, the power output per unit fuel consumed is less.

I'll also point out that the thermodynamic benefits of running the plant at higher temperatures partly offset the extra energy needed to electrolyze the CO2. And that it's the valuable CNTs that make the economic difference.
BurnsTrollsAlive
Jun 21, 2016
This comment has been removed by a moderator.
Eikka
4.2 / 5 (5) Jun 21, 2016
I'll also point out that the thermodynamic benefits of running the plant at higher temperatures partly offset the extra energy needed to electrolyze the CO2.


The energy content of CH4 is roughly 891 kJ/mol while the carbon content contributes in at 388.8kJ/mol which means that you have to spend 43% of the energy present in the fuel to un-burn the CO2.

Now, a CCGT powerplant isn't working at 100% efficiency to electricity in the first place, but more like 60% so by burning 100 units energy worth of methane, you get 60 units worth of electricity and out of that you have to spend at least 43 units to turn the CO2 back to carbon.

The powerplant has to spend at least 2/3rds of its output to split the CO2 back to carbon, and even if the kickback increased the efficiency of the plant all the way to 100% - which is impossible - the output would still be reduced by nearly half instead of 8%

The numbers just don't add up without external energy input.
Eikka
4 / 5 (4) Jun 21, 2016
The only way it sort-of works is if you use the ~40% of waste heat energy from the turbines instead of electricity to split the CO2 molecule, but the problem there is that the waste heat from the gas turbine is required to run the combined cycle steam turbine.

If you tap into that stream of heat and use all of it to make CNTs, you obviously lose temperature at the steam turbine inlet and the steam turbine stops running, and if the steam turbine doesn't run then you don't get 60% efficiency but more like 30%.

And the waste heat from the steam turbine outlet is too low in temperature to run a CO2 splitting reaction anymore because it comes out at close to room temperature to condense the steam, so that obviously won't work either.

So again, I can see no plausible way the plant could operate and lose only 8% output energy.

carbon_unit
1.6 / 5 (21) Jun 21, 2016
@sigh: Of course, the Hydrogen... Thanks!

Osiris1:
This makes the generation of 60,000 mile CNT cables for the space elevator economically feasible!
Maybe. https://www.newsc...-ground/
But now it seems a single out-of-place atom is enough to cut their strength by more than half. That means one of the more outlandish applications for CNT fibres – a sci-fi space elevator – might never happen.
RealScience
4.6 / 5 (10) Jun 21, 2016
I am skeptical of losing only 8% in a real implementation, but let's explore the numbers.
A ~60% efficient combined cycle gas turbine has a turbine efficiency of ~40% and then uses a team cycle to convert ~1/3 of the 60% exhaust heat for another 20%, bringing the total efficiency to the ~60%.
So if the waste heat is used to separate the carbon it costs less than 43% because the steam cycle is inefficient. But it still costs more than 1/3 of the 43% because the temperature drops and so the steam cycle becomes even less efficient so at best we can get ~5% electricity out of the remaining 17% thermal energy, for a total of 45%.

However the diagram shows that at least some of the 43% carbon separation energy is electricity, so let's guess that it is 30% heat and 13% electricity. Now our steam cycle adds only 10% to the gas turbine's 40%, but we consume 13% for separating the carbon, leaving 37%.

So is there anywhere that extra efficiency can come from?

- continued -
RealScience
4.6 / 5 (10) Jun 21, 2016

- continued -

Separating the carbon releases enough oxygen to double the oxygen content of the air entering the gas turbine, which will significantly increase the maximum combustion temperature, and thus the theoretical maximum efficiency, by eliminating half of the nitrogen. So IF one compares the highest-efficiency at high temperature operation of the CNT-producing turbine to the actual efficiency of a real CCGT power plant, then an 8% efficiency drop is plausible.

But that ignores the challenges of extreme-temperature turbines, so a comparison to a syngas-in-enhanced oxygen Graz cycle would be more realistic. These are approaching 70% efficiency (~50% turbine plus ~40% of the waste heat), which, at the 30%/13% guess used before, would net ~45% total efficiency. So a 'guestimate' for a real-world efficiency is 45%, or 3/4 of CCGT's 60% efficiency, with the cost of the hotter turbine and the separation unit offset by producing CNTs rather than CO2).
Da Schneib
4.6 / 5 (9) Jun 21, 2016
Nice analysis, @RealScience.
RealScience
4.4 / 5 (7) Jun 21, 2016
Nice analysis, @RealScience.

Thanks. The pieces were in other comments - lots of people commented on the numbers being optimistic, Winthrom and Eikka commented on the energy used in electrolysis, Danielshead commented on the challenges of higher temperature turbines, and he and you and Retrosurf all commented on the O2-> higher temperature -> higher efficiency turbine operation, Sigh commented on the hydrogen, and Eikka hunted down some hard numbers for the thermodynamics, so it was mainly summarizing what others had already commented on.
Eikka
1 / 5 (2) Jun 22, 2016
So if the waste heat is used to separate the carbon it costs less than 43%


You're confusing ratios. The 43% or 43 units out of 100 is the absolute amount of energy needed to separate the carbon - it does not depend on turbine efficiency ratios. It's what the energy contents of the amount of carbon is, and you can't change that with different configurations - it's always going to be 43% of the energy contents of the methane you put in.

The gas turbine and steam turbine are actually both approximately 37% efficient - that will give a combined cycle efficiency of 60.31%

If you take the energy to separate the carbon out of the gas turbine exhaust, you will be left with 20 units of waste heat to the steam boiler, and 7.4 units out as electricity. Now you have a total efficiency of 44.4% instead of 60.31% which is a 26.4% loss.

Eikka
1 / 5 (2) Jun 22, 2016
But it still costs more than 1/3 of the 43% because the temperature drops


You don't need to run the steam boiler in series with the system. You can divert a portion of the hot exhaust to the steam turbine and a portion to the CNT reactor. That way the steam turbine efficency remains a constant. It just gets less energy.

Any energy you cycle back in the form of electricity is going to cost you more than using the heat directly, which is why my estimate says the lower bound for energy loss is in the 25% range.

Now let's say the gas turbine efficiency improves with the recycling of oxygen. Suppose it goes up to 45% instead. Great, but that means there's only 12% left over for the steam boiler so the total efficiency improves to 49.4% and that's still an 18% loss.

Eikka
1 / 5 (2) Jun 22, 2016
The reason why both gas and steam turbines are approximately 37% efficient is because of technical limitations in the steam turbines. The gas turbines burn at very high temperatures and the exhaust is hot enough to exceed the thermodynamic limits of steam boilers because the water turns corrosive at high enough temp and pressure. The practical (economic) upper limit for cheap bog standard steam turbines is about 37-38% without the use of exotic materials.

And if the steam turbine is 37% then the math turns out correct if the gas turbine is also 37% efficient. The combined efficiency is 0.37 + (1 - 0.37) * 0.37
RealScience
4.5 / 5 (8) Jun 22, 2016
So if the waste heat is used to separate the carbon it costs less than 43%


You're confusing ratios. The 43% or 43 units out of 100 is the absolute amount of energy needed to separate the carbon ...


As the calculations that follow the phrase 'costs less' show, 'costs less' refers to the less loss of ELECTRICAL output when using the heat rather than the electricity.

Here is a longer version, with additions in '[]':
So if the waste heat is used to separate the carbon it costs less than 43% [less than 43 MJ of electrical output for each 100 MJ of methane input] because the steam cycle is inefficient [and only converts anything close to 100% of its input thermal energy to heat].

I should have been clearer in the original, but since the rough calculations came out to 45% and your more precise calculations came out to 44.4%, it should have been clear to you that there was a miscommunication rather than a science misunderstanding.
RealScience
4.5 / 5 (8) Jun 22, 2016


You don't need to run the steam boiler in series with the system. You can divert a portion of the hot exhaust to the steam turbine and a portion to the CNT reactor. That way the steam turbine efficency remains a constant. It just gets less energy.


Even even a lithium carbonate eutectic with sodium and potassium melts at ~400C, so fully parallel would waste its exhaust heat. I therefore simply put it fully in series between the gas turbine and the steam turbine.

However you are CORRECT that it probably wouldn't lower the efficiency of the steam cycle in a practical system - since the turbine exhaust is hotter than a low-cost turbine can handle, running the lithium carbonate 'hot' will leave its leftover heat at a nice temperature for the steam cycle and thus not impact the steam cycle's efficiency (but just reduce the input energy to the steam cycle).
RealScience
4.5 / 5 (8) Jun 22, 2016

The gas turbine and steam turbine are actually both approximately 37% efficient - that will give a combined cycle efficiency of 60.31%

I used 40% and 1/3 (rather than 37% and 37%) because a good gas turbine is 40% and a nice round 1/3 makes the total a nice round 60% (I like round numbers because I try to keep in practice by doing the math in my head, hence my addiction to the '~').

But I just checked the most efficient CCGT that I know of - the new Siemens systems: http://www.power-...r-plant/
These use the Siemens SGT5-8000H gas turbine, whose technical data sheet lists an efficiency of 40%.
Yes, it is unfair to attribute all losses from combining two cycles to the steam cycle, but that's the way Siemens breaks out the numbers.
And the difference is small - the Dusseldorf plant just hit ~61.5%, which gives it a steam cycle efficiency of ~36%, which is not far from your 37%-38% figure...
RealScience
4.5 / 5 (8) Jun 22, 2016
(The Dusseldorf plant also uses the 'waste' heat from the steam cycle for district heating and process heat as well, bringing its total efficiency to ~85%. With the bulk of being electricity generated with record-setting efficiency, the process heat is a nice extra.

The Dusseldorf plant can also ramp up by up to 20 MW per minute to compensate for variable renewable energy.)

Returning to the CNT-producing electricity generation:

Any energy you cycle back in the form of electricity is going to cost you more than using the heat directly, which is why my estimate says the lower bound for energy loss is in the 25% range.


"... the 30%/13% guess used before, would net ~45% total efficiency. So a 'guestimate' for a real-world efficiency is 45%" is clearly based on a GUESS for the electricity-to-heat used in carbon separation. If it is mostly electricity, then my 'guestimate' comes into agreement with your estimate.
Shakescene21
3.7 / 5 (6) Jun 22, 2016
Although I'm not a scientist, I have learned a lot from this article and especially from the comments which show Physorg at its best. For the most part your comments have been expert and on-topic, and much more polite than usual for Physorg.

Thank you.
Tenstats
1 / 5 (2) Jun 22, 2016
There is one inescapable problem: the process requires the electrolysis of the CO2 back into C + O2. Now, the conversion of the C in the methane (CH4) to CO2 emits heat as the heat of formation and this same amount of heat (energy) is the minimum required to convert the CO2 into C. Therefore, any thermodynamic process must neglect any energy derived from the combustion process and electricity generation.
SNAFU
Tenstats
2.3 / 5 (3) Jun 22, 2016
Eikka, I was just restating one of your earlier comments, although from a heat of formation approach. I can't see from the actual paper that the authors negate this issue. There are still no perpetual motion machines. BTW I am a chem engineer (B.S. and M.S.)
RealScience
4.6 / 5 (9) Jun 23, 2016
Huh - GE just beat Siemens' record, hitting 62.2% overall efficiency:
http://www.powerm...a5347531
Their specs say >41% gas turbine, >61% combined cycle:http://www.powerm...elivery/
BiteMe
Jun 25, 2016
This comment has been removed by a moderator.
tear88
4 / 5 (4) Jun 25, 2016
The energy content of CH4 is roughly 891 kJ/mol while the carbon content contributes in at 388.8kJ/mol which means that you have to spend 43% of the energy present in the fuel to un-burn the CO2.


The numbers may not add up, but I think you've answered my question as to whether converting CnHn + O2 (or, CH4 specificallly + O2) into C + H2O can be an exothermic reaction. Apparently, it can. IMO, while hydrocarbon extraction is still a bad idea, eliminating the continuing pollution of the environment with massive amounts of CO2 would be well worth the reduced output.
Caliban
1.5 / 5 (13) Jun 25, 2016
A neat trick, to be sure.

Unfortunately, it is a trick, just the same.

Big Carbon gets a nice greenwashing while continuing to emit gigatonnes of carbon waste.

Carbon nanotubes are just that --waste-- until or unless a market that uses them at nearly 100 percent of production is found or developed --even whileentirely ignoring environmental/health consequences, both known and unknown. Meanwhile, Big Carbon may have backed its ass into a new profit center(YAY for Big Carbon & "stakeholders")

So we are now being sold on a system that still requires fossil fuel production in order to operate and emits pollution in a form that may or may not be useful --much less benign-- simply in the name of CO2 emissions reduction.

Essentially, another ploy to continue the disruptive, polluting, and OH-SO-LUCRATIVE exploitation of goddam fossil fuel. Big Carbon is the only winner here.

This isn't "Gee Whiz" --it's: WTF?!?!

antigoracle
1 / 5 (5) Jun 26, 2016
Love is blind.

Yes, but significantly much less than what your mental condition has left you to your mental condition. Take your meds AGreatWanker.
Da Schneib
4.6 / 5 (10) Jun 26, 2016
I don't think you've thought this all the way through, @Caliban. It's a lot easier to sequester carbon nanotubes that just sit there than CO2 that floats around, for one example.
RealScience
4.6 / 5 (11) Jun 26, 2016
I don't think you've thought this all the way through, @Caliban. It's a lot easier to sequester carbon nanotubes that just sit there than CO2 that floats around, for one example.


Exactly - carbon is a pretty stable solid (but that applies to coal or graphite (or diamond) as well as to carbon nanotubes). We know how to store gigatonnes of solid carbon - bury it in old coal mines (and, in the Appalachians, put the mountain tops back).

What if we could do this with the methane clathrates? The hydrogen released would power industry for many CO2-free decades while we build up solar and storage, and this would also defuse the ticking time bomb of warming oceans destabilizing the clathrates and releasing the methane in a run-away feedback.

xponen
1 / 5 (2) Jun 26, 2016
The nanofibril property will make it as popular & useful as asbestos were... and carcinogenic.
Da Schneib
4.4 / 5 (7) Jun 26, 2016
The nanofibril property will make it as popular & useful as asbestos were... and carcinogenic.
Yeah, good thing they don't get released into the air.

Just sayin'.
snerdguy
1 / 5 (2) Jun 27, 2016
It has tremendous commercial potential as long as they develop a process that assures that none of the nanotube material will be released into the environment either intentionally or accidentally. There are some pretty good reasons to believe that they are potentially carcinogenic because they are small enough to damage cells. It would be stupid to develop something that turns one hazard into a different hazard. We can't afford the mistakes like the nuclear power industry made and end up with a bunch of toxic material we can't get rid of.
Tenstats
1 / 5 (2) Jun 28, 2016
Has anyone evaluated the energy requirements for the electrolysis process meaning where is the electricity sourced? Next, balance of plant requirements to separate the CO2 from the rest of the gas, including water vapor which could react violently with the the molten lithium carbonate.

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