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SuperSCAF: Carbon negative power plants
Karl Schroeder, 20 Jun 08

I thought I'd try my hand at envisioning what a 200 megawatt carbon-negative power plant might look like. It would have to be low-tech, simple, and based on current technologies. Now, I can't claim that this design is very efficient or that it would make a dent in the carbon that's already in the air, but it wasn't hard to come up with. A little more thought and we could probably do a lot better.

First of all: SCAF

The starting point for this design is SCAF, the Solar City Air Filtre. SCAF units would be house- to stadium-sized, and based on the technology of solar updraft towers. Basically you have a tall open tower standing atop a glass greenhouse-like structure. In the case of an urban SCAF, air flows in through air filters in the sides of the greenhouse, is warmed by the greenhouse effect, spins a vertical wind turbine to create power that you can sell, and exits (clean and green) out the stack. The SCAF site suggests planting these units in the middle of traffic circles, as a way of mitigating urban pollution.

The technology behind SCAF, the solar updraft tower, is well understood. A 200 meter high test unit was built in Spain in 1982 and produced power for a number of years. Although they're not very efficient, solar towers have the advantage of being simple. Air comes in at the edges of a vast gently sloping greenhouse; is heated; and turns fixed wind turbines before exiting up the stack. Such towers can produce power quite reliably, and with the addition of heat buffers (plastic-sheet covered ponds under the glass roof, for instance) they can even produce power 24 hours a day. The largest towers envisioned would stand a full kilometer in height, and would produce 200 megawatts of power essentially forever (assuming you replace parts that wear out).

Interesting as they are, solar towers aren't as compelling as some other renewable designs. Even ordinary windmills can probably out-compete them in most jurisdictions. But, with the addition of one low-tech feature, they could be carbon-negative. I call this design SuperSCAF.

SuperSCAF

A key ingredient in mortar and cement is a substance called quicklime. Quicklime is made very easily, by heating substances like limestone. It has an interesting quality: it absorbs CO2, and as it does it turns back into limestone.

So here's how SuperSCAF works: we dump powdered quicklime into the air stream after it's passed through the fixed wind turbines. The cloud of dust rises in the vortex within the central stack of the solar tower, and as it does it scrubs the air of CO2. Limestone dust precipitates out along the sides of the stack and is carried by conveyer belt to a solar oven that resides under the greenhouse roof. There the CO2 is released and piped away (to the sequestration method of your choice) and the product is... quicklime. What we then get (minus efficiency losses from the dust slowing the airflow) is a multi-megawatt power plant that actively sucks CO2 out of a rising column of air up to 130 meters in diameter, and does it 24 hours a day.

Would this work? Well I have absolutely no idea. The scheme depends on finely ground quicklime being able to absorb the CO2 at a respectable rate; and even if the quicklime removed 100% of the CO2 present in the air column, it would be removing it at only a tiny percentage of the rate that one modest-sized coal plant can pump it into the air. This is because a coal stack is a concentrated CO2 source, where SuperSCAF has only the highly diffuse (380 ppm or so) atmospheric concentration to work with. So you'd have to process truly gargantuan amounts of air this way to make a difference; the precondition to even bothering to try it would be a pre-existing global moratorium on coal-fired power.

Solar towers can process truly gargantuan amounts of air; hence this design. And what would happen if an international consortium subsidized the building of at least four such units in every country on Earth? What would 1000+ SuperSCAFs accomplish after running flat-out for ten years?

I don't have the math to figure it out. But maybe one of our readers does--and I'd love to see the result. I will be neither surprised nor disturbed if the answer is that such a massive effort wouldn't even scratch the surface of the gigatons of excess CO2 in the atmosphere.

Even so: it's possible to conceive of carbon-negative power. That in itself is something.

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Comments

Using quicklime like this will at best cause a net zero change in atmospheric CO₂ and a negative change in the budget (e.g. cost a lot of money). Converting limestone to quicklime releases the CO₂ into the atmosphere. The quicklime will then re-absorb at most the amount of CO₂ than was driven off in the kiln.

It's very likely, however, that the process will cause a net increase in CO₂ due to the energy used to heat the kiln to to 825°C. Unless the heating energy comes from a clean source (nukes, solar, wind, hydro, PV), then the quicklime manufacturing process actually increases the atmospheric CO₂. Cement production is another example of an industrial process driving CO₂ out of limestone and into the atmosphere by burning fossil fuels which also generates CO₂.


Posted by: Mark on 20 Jun 08

I can't speak to the energy production side of this idea, but the rate of atmospheric CO2 capture is easy to calculate.

Let's assume:


  • the cross-sectional area of the tower is about 10,000 square meters

  • the air is flowing upwards at 10 meters per second (22 miles per hour)

  • CO2 extraction is 100% efficient

  • atmospheric CO2 concentration is 350ppm

Then there's 100,000 cubic meters of air being processed each second, which will contain about 20kg of carbon (not carbon dioxide). If we want to talk about this in the context of "stabilization wedges (each wedge being equal to 1 gigaton of carbon emitted per year), then we should look at the annual capture capability of the plant, which would be about 600,000 tons of carbon, meaning you'd need nearly 2000 such plants to make up a "wedge".

In general it's going to be very, very difficult to get CO2 out of the atmosphere once it's there, because it's so diffuse. Entropy is not our friend.


Posted by: Zane Selvans on 20 Jun 08

It's an interesting idea. I don't know enough about the quicklime process to comment on whether it would be carbon-effective, but if you're using sustainable sources to power the kiln and the kiln is in a place where the CO2 being released is being released into some kind of carbon sequestration system, then sure, why not.

However, if you have a huge greenhouse with CO2 flowing through it, then you could also plant vegetation in the bottom as extra carbon-sink. I guess the trade-off then would be that you want all the available ground surface area as water for heat sinks.

But you don't need only water as heat sinks, there's a wide variety of things you could use. Also, the heat sinks could be anywhere in the structure.
It'd be relatively simple to support a globule of plastic-encased water 2 metres off the ground.

It's just an engineering issue, and in the long run, they're easier to solve than energy issues.


Posted by: Fraser on 20 Jun 08

Not feasible. The total world production of quicklime is only 130 million metric tons per year which is good for removing maybe 50 million tons of CO2

Only problem is we produce a billion (and growing) metric tonnes of CO2 each year which means even if we redirect every ounce of quicklime for this, we'll only push back CO2 levels about 6 months.


Posted by: Pierre on 20 Jun 08

I thought worldchanging had already run articles on the one power-plant technology that could absorb ALL of our Co2 emissions....

Biochar!


Posted by: Dave Lankshear on 21 Jun 08

@Mark, the article suggests using a solar oven to drive the C02 out of the limestone, in a closed environment that would allow the CO2 to be sequestered. So that aspect of the process would be carbon-negative, as long as the sequestration technique used was sensible and reliable.

I don't know a great deal about sequestration technologies, but I've always thought that a good use of captured CO2 would be to increase the CO2 concentration in greenhouses that are used to grow unseasonal or non-native crops. The advantage of using greenhouses for this is that it would permit, for instance, growing strawberries in the UK in the winter instead of airfreighting them in from thousands of miles away. (Of course, there would need to be restrictions on the number of greenhouses: no-one would want their country's land given over to them excessively, and for a number of good reasons. But might a moderate quantity be sufficient to soak up the CO2 collected from SuperSCAFs?)


Posted by: Sam Kuper on 21 Jun 08

Yes, as Dave Lankshear says above, conceiving of carbon-negative power is not news. The most widely-discussed option is a biomass power plant surrounded by a fast-growing forest to provide the fuel, with biomass sequestration.

In our work with David Wasdell and the Transition Towns movement we have been looking to three types of energy sources,

1) Carbon positive (like most of our current sources)
2) Carbon neutral (like many renewables at least strive to be)
3) Carbon negative

The question we are moving towards is how we can ensure that type 3 capacity outweighs type 1 capacity in the future. Or at least that total energy production is sufficient to allow for some energy-sink carbon removal technologies to make up for any net shortfall.

Type 2s may seem a little irrelevant, but in light of the 'peak oil' type problems of adapting to a lower energy future, they are of course critical.


Posted by: Shaun Chamberlin on 21 Jun 08

Thanks for the numbers, Zane. Actually, 2000 plants to make a wedge is not bad. In fact, let's put this in perspective: how many coal-fired power plants are there in the world? How 'bout 50,000 (and growing)? As I stated in the article, a precondition to making SuperSCAF (or any truly carbon-negative scheme) work is a moratorium on coal; we need to actually eliminate it, which is no more Utopian an idea than SuperSCAF itself. Consider this: replacing 4% of the world's existing coal plants with SuperSCAFs gets you a climate wedge. What if we replaced 20% of them?

Pierre, your criticism is predicated on the quicklime only being used once. In fact, it's recycled, and its initial production and recycling capture CO2 and can be done using solar ovens or something similar. It's the total amount "in play" in each plant that matters, and a given plant will only be using hundreds to (maybe) thousands of tonnes of it. So 130 million tonnes is completely possible.

Anyway, don't get hung up on the particulars of this one scheme. I've written about agrichar on this very site and there are many other potential solutions; the general point here is that the existing industrial infrastructure is as capable of pulling CO2 out of the air at a profit, as it is of putting it in. What matters is commitment and the simple application of imagination to come up with the solutions.

And, most importantly, it can be done without "geoengineering."


Posted by: Karl Schroeder on 22 Jun 08

If you want to 'recycle' the quicklime you need to heat it up (lots of energy again) and you have to work out how to capture the CO2 this process releases. Currently according to the new frontpage article 3.8% of CO2 emissions comes from cement manufacture already - therefore it must be more worthwhile trying to capture the CO2 in those processes instead.

Otherwise you're just adding extra steps in the process. Clearly heating and re-cycling quicklime is then what you're talking about here, and I think it's kind of silly to talk about towers without even knowing what this quicklime CO2 capture and recycle process will cost per ton of CO2 removed from the atmosphere.


Posted by: Pierre on 24 Jun 08

It's silly if you focus on that one mechanism and, if it fails, decide that the whole idea is bunk. I chose quicklime for this example because it's cheap and plentiful, and the technique for making it is ancient and hence not patented. However, there are many other CO2 sorbents; some are more efficient than others and some--including new, proprietary formulas--are very energy-efficient. All will require energy, of course--but the plant produces electricity, and if there's a price on carbon, that could offset the the additional cost. If the carbon price is high enough, CCS could even be a greater source of revenue for the plant than power production itself.

Incidentally, does that "3.8% CO2 from cement manufacturing" take into account the fact that most if not all of the CO2 directly released (as opposed to that released by the mining, heating and transportation) gets reabsorbed as the cement cures? You're right that capturing the carbon at that point would be a great idea--and in fact you could thereby make cement carbon-negative because the CO2 would be removed twice: once at manufacturing time, and once when the cement cures at the building site.

Anyway, I'll keep saying this as often as I have to: don't get hung up on specific details of this implementation. If you see a problem with an idea, don't just carp about it, suggest a solution. For instance, producing all the glass for the greenhouse area will create carbon, unless you use some of the new conversion mechanisms that sequester CO2 as plastic--and make the greenhouse itself out of carbon you've pulled from the air!

Solutions, solutions. There's always solutions.


Posted by: Karl Schroeder on 26 Jun 08

Not to be rude but you don't seem to know much about this topic - even a quick look at Wikipedia would have taught you that no, cement does NOT sequester all the released CO2 again and obviously the percentage quoted is for the NETT effect so yes we did think of that.

I just find it frustrating that we HAVE the solutions but yet armchair scientists like the author of this article thinks we need more harebrained 'ideas' removing focus from proven solutions.


Posted by: Pierre on 26 Jun 08

I certainly don't claim to be a scientist. Three people who do are David W. Keith of the University of Calgary, Minh Ha-Duong at CNRS and Joshuah K. Stolaroff of Carnegie Mellon University. In their paper, "Climate Strategy with CO2 Capture from the Air" (http://www.ucalgary.ca/~keith/papers/51.Keith.2005.ClimateStratWithAirCapture.e.pdf) they examine the feasibility of air capture and find that, using today's technology, it is quite possible. In fact, they describe two systems, one a biochar/biofuel plant and the second a system remarkably like SuperSCAF:

"...Carbon dioxide is captured in an NaOH solution
sprayed through the air in a cooling-tower-like structure, where it absorbs CO2 from
air and forms a solution of sodium carbonate (Na2CO3). The Na2CO3 is regenerated
to NaOH by addition of lime (CaO), forming calcium carbonate (CaCO3).
The CaCO3 in turn is regenerated to CaO by addition of heat in a process called
calcining."

They calculate the cost of capturing ambient CO2 from the air using such crude technology as being comparable to carbon-capture "scrubbing" from power plant stacks; however, air capture has several distinct advantages:

"Air capture
differs from conventional mitigation in three key aspects. First, it removes emissions
from any part of the economy with equal ease or difficulty. Consequently, its price
caps the cost of mitigation with a scope unmatched by any other kind of abatement
technology. Second, because air capture allows the removal of CO2 after emission
it permits reduction in concentrations more quickly than can be achieved by the
natural carbon cycle. Third, because it is weakly coupled to the energy system,
air capture may offer stronger returns-to-scale and lower adjustment costs than
conventional mitigation options."

So air capture has the potential to significantly change climate policy even if the technology is crude. The greatest risk the authors identify is that since it can actually drive CO2 levels to pre-industrial levels--unlike processes that merely cap emissions--governments and industry might use its existence as an excuse to delay implementing mitigation methods. This is a serious risk.

What's interesting about that aspect of the study (speaking here to Pierre) is that it validates your opinion that discussing air capture now may not be a good idea. I'm happy with that conclusion; at the same time, air capture appears to be a feasible way to take us, in the longer term, from a merely stabilized high-CO2 environment, back to pre-industrial levels. One significant suggestion in this paper is that other CCS techniques, which propose sequestering CO2 underground, might face leakage issues, and that by 2100 there could be as much as 1 gigatonne of CO2 leaking out of long-term storage every year. Air capture could be used to put it back, where no other mechanism exists to do so.


Posted by: Karl Schroeder on 30 Jun 08



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