Transterrestrial Musings

Comments

Welcome back, Sam!

Posted by John Breen III at March 21, 2007 07:51 AM

Oxyfuel combustion of coal doesn't burn the coal with pure oxygen, liquid or otherwise. Instead, the coal reacted with a combination of gaseous oxygen and recycled CO2. The CO2 prevents the temperature from getting too high for the materials of the system to withstand.

The gas coming out of this is mostly CO2 and water, with some minor amounts of nitrogen (from the N contained in the coal). After the water is condensed some of the CO2 gets sent around again, and the remainder is available for sequestration.

It's not clear this is the best way to get a pure CO2 stream out of a coal burning powerplant. Other techniques, such as scrubbing the CO2 from the flue gas with chilled ammonia, immobilized carbonic anhydrase enzymes, or removal from gasified coal before combustion, are all competing possibilities.

The gasification system is probably the one you are garbledly refering to. CGCC allows the CO2 to be scrubbed from shifted syngas before it gets mixed with atmospheric nitrogen. The hydrogen can then be burned in a combustion turbine, which is more efficient (when used with a steam bottoming cycle) than driving a steam cycle alone. It's also much cleaner than ordinary powdered coal plants, and the sulfur removed from the syngas comes off in elemental form, which is valuable as a precursor for making sulfuric acid.

Posted by Paul Dietz at March 21, 2007 07:58 AM

If the output from the plant is 'clean' as far as radioactivity, particulates, and other pollutants (as opposed to greenhouse gases), what is preventing a greenhouse-based farm from being part of the 'exhaust processing'?

High CO2, high H2O, what's not to like?

Posted by Al at March 21, 2007 08:25 AM

what is preventing a greenhouse-based farm from being part of the 'exhaust processing'?

There are a number of companies looking at exploiting the CO2 from coal plants in order to grow algae. Some varieties of algae are high in valuable oils and are much more productive than conventional crop plants if given enough CO2.

The capital cost of closed photobioreactors and contamination problems in open ponds have been problems preventing adoption of this approach. I imagine the cost of greenhouses, as well as consumption of water for evaporative cooling, could also be problematic for growing conventional plants.

In both these cases the CO2 in the fuel being produced will be released into the atmosophere if it is used for transportation fuel (for stationary applications, the coal itself would be hard to beat economically), but one could argue it would displace CO2 released by burning oil-derived fuels. You get two bites of the apple before the CO2 finally escapes to the air.

Posted by Paul Dietz at March 21, 2007 08:32 AM

Two points. Sam doesn't mention the cost of injecting the CO2 into the ground(compression, monitor wells, pipeline all are not cheap). Paul, Last I looked, sulfer prices were in the toilet so this sulfer would be a further drug on the market. You can try to minimize the cost of CO2 and sulfer extraction but they are still an expense we will all have to pay for unfortunately.

Posted by Bill Maron at March 21, 2007 08:37 AM

Paul, Last I looked, [sulfur] prices were in the toilet so this [sulfur] would be a further drug on the market.

Hey, it beats the alternative byproduct (spent lime from post-combustion scrubbing).

There's a cellulosic ethanol technology that seems to me to be closest to practical use that uses concentrated sulfuric acid to break the cellulose and hemicellulose down into sugars. Although most of the acid is recycled by chromatography, about 1 kg of acid is consumed per 10 kg of ethanol produced. This could be a very large market for the acid.

Posted by Paul Dietz at March 21, 2007 08:59 AM

I wonder how the ethanol subsidy program will fare in that environment. One more thing, coal prices delivered are more like $50/ton I believe.

Posted by Bill Maron at March 21, 2007 09:23 AM

I wonder how the ethanol subsidy program will fare in that environment.

The estimated cost of production for that ethanol is slightly over $1/gallon, which, if accurate, means it could survive without subsidy at current gasoline prices. That technology can also use highly varied feedstocks, including paper, cardboard and wood from partially sorted municipal waste.

Posted by Paul Dietz at March 21, 2007 10:01 AM

One more thing, coal prices delivered are more like $50/ton I believe.

Wyoming coal (Powder River Basin) is $15/ton, delivered to an Iowa power plant.

The reason that O2 is proposed as an oxidizer instead of air, is because if you use air you get 4 parts of N2 in your exhaust stream for every 1 part CO2. And N2 is a permanent gas, i.e. it won't liquefy at ~800psi like CO2 does. Presumably storage of compressed 4*N2 + CO2 would be far more uneconomical than pure CO2.

Still though, the notion of using O2 as an oxidizer in a low-value process (viz: burn 1.7 cents worth of coal in a $1000-$2000 plant, to produce 5 cents worth of electricity) strikes me as problematic. This is a serious fight against two formidable opponents, economics and thermodynamics...

BTW LOX isn't the only way to produce the O2; look up a few different IGCC "Integrated Gasification Combined Cycle", and see the ASU "Air Separation Unit" component - there are several approaches to producing tonnage oxygen. In some ways it's impressive how "cheap" tonnage oxygen is.

Don't forget though that you are burning a $2/mmBTU fuel. Each mmBTU (about 70 pounds carbon) needs about 185 pounds of O2. Don't pay too much for the oxidizer for this two bucks worth of fuel...

Posted by Daniel Chisholm at March 21, 2007 10:53 AM

I suspect what was causing the confusion was that the cheapest way to separate air on a large scale is cryogenic -- but LOX is only produced inside the ASU. It isn't an output of the ASU (unless you want it) since it's thermodynamically more efficient to use the outgoing cryogens to precool the incoming air. The output is gas at close to am-bient temperature (pressurized if you like). Refrigeration is needed, but only to make up losses in the system.

Interestingly, if you need pure nitrogen gas, in some ranges of production volume it's most economical to do this with a cryogenic ASU in which the refrigeration is provided by additional LN2 trucked in from outside.

Posted by Paul Dietz at March 21, 2007 11:27 AM

Dear Rand: I am under attack by a real-live communist. What should I do?

Posted by Louise at March 21, 2007 02:19 PM

"Wyoming coal (Powder River Basin) is $15/ton, delivered to an Iowa power plant"

It is $15 per ton for a reason, it is lignite and low BTU.

Posted by Mike Puckett at March 21, 2007 05:45 PM

LOX is pretty energy intensive to make, then you have to deliver it in tanker trucks, or send it via pipelines. However, LOX is a byproduct of LN2 production that is frequently discarded, so maybe it can be obtained cheaper than expected.....

BTW, Air Liquide has a big O2 pipeline along the Gulf coast.

Posted by ech at March 22, 2007 06:48 AM

However, LOX is a byproduct of LN2 production that is frequently discarded, so maybe it can be obtained cheaper than expected...

LOX wouldn't be discarded. Gaseous oxygen, maybe, but not LOX. If you just want LN2, you design the air separation unit to cool the incoming air with the outgoing waste oxygen. Discarding liquid oxygen as a liquid would be throwing away its cooling capacity for no good reason.

Posted by Paul Dietz at March 22, 2007 07:25 AM

John: Glad to be back. I'll post something over at Decisive Win and cross post here later this week.
Paul: thanks for ungarbling this for the team. Carting CO2 to the ground will likely be as expensive as carting CH4 from the ground so power plants may have to be sited near dead wells. The good news is that the wells don't have to be dug for injection. I think it is clear that sequestering 4xN2 is a lot cheaper to sequester than getting power plant quantities of oxygen, but that's just a guess. In any case coal with oxygen will be uncompetitive if the oxygen costs more than 3-5 cents per kwh which is enough to burn .6 cents of coal or about 3 kg of carbon.

Louise: Tell the Commissar.

Posted by Sam Dinkin at March 22, 2007 09:25 AM

I think it is clear that sequestering 4xN2 is a lot cheaper to sequester than getting power plant quantities of oxygen,

I think precisely the opposite is clear. Otherwise, why would companies be investing in this? Your notion requires that they be idiots.

Compression costs on nitrogen-diluted flue gases would be a killer.

Posted by Paul Dietz at March 22, 2007 10:28 AM

Or instead of this sequestering, they could build nuclear plants and do research on breeder reactors so uranium wouldn't be needed in such vast quantities.

It is also possible to avoid transuranics if you use a thorium molten salt reactor with constant reprocessing. The waste would thus be dangerous only for 300 years, not the tens of thousands like current nuclear waste.
A 1 GW reactor could use as little as 1000 kg of thorium per year - running 30 years on a single truckload!

How long do you have to watch over this CO2? It doesn't decompose into anything... What if hundreds of years from now, we have massive CO2 leaks from sequestered places? It seems like a bad bandaid solution... A quick and dirty hack.

Posted by mz at March 22, 2007 12:24 PM

Ok, one last comment.

This slide presentation on oxy-fuel combustion has some interesting facts that should inform some of the statements made previously:

Oxyfuel combustion is already extensively used in the glass and pyrometallurgy industries
The cost of oxygen gas in high volume is less than $20/ton, far far cheaper than LOX at $1/gallon.
Even at that low price, the energy cost of cryogenic separation (around 200 kWh/ton) is still far above the thermodynamic minimum (around 30 kWh/ton) for producing oxygen at 1 bar from air. So there's still room for improvement.

The slide show goes on about one possibility, using oxygen-permeable ceramic membranes (like those proposed for producing O2 and CO from martian CO2) to produce low pressure oxygen from air, then burning fuel in that dilute oxygen. Chemical looping combustion is also mentioned (using chemical reactions with solid carriers to separate oxygen from air.)

Posted by Paul Dietz at March 22, 2007 01:34 PM

Paul: Now we are talking: $20/ton, we would only need 2.5 tons of O to burn 1 ton of C so fuel and oxidizer costs would only go up from 0.6 cents per kwh to 2.1 cents per kwh keeping coal ccompetitive. Nevertheless, $50/ton of CO2 is not currently competitive to be a technology to use to sequester carbon even if the capture and storage were free.

Tell me why sequestering nitrogen into a salt dome is so expensive. Can't you just take the exhaust gas and pump it into the salt dome at high speed?

Posted by Sam Dinkin at March 22, 2007 03:01 PM

Sam: Per the 2 March newsletter on www.uic.com.au "In 2006 the 103 US nuclear plants in 31 states produced 787.6 billion kWh at a record low production cost of 1.66 c/kWh (excluding capital component)."

If you gan generate electricity without CO2 at $.0166/KWh, why would you pay $.02 just for one component of a competing technology? Granted, the capital equipment cost(about $2500/KW) to build a nuke is higher than for a coal burner, particularly in the US where the permitting process takes several years. But the capital cost is a small fraction of the total life cycle cost, especially if the long-term environmental cost is included. Decomisssioning and waste storage costs are inclued for nukes in the price of electricity, but typically not for fossil burners.

Posted by Dan DeLong at March 22, 2007 03:53 PM

If you gan generate electricity without CO2 at $.0166/KWh, why would you pay $.02 just for one component of a competing technology?

As usual, just how you look at it depends on what problem you're trying to solve.

Your question would be the relevant question if the issue was 'do we shut down existing, operating nuclear plants and replace them with this technology?' But that's not relevant to the US. The questions here are 'what will we build for the new baseload capacity that has to be added' and, possibly, 'how do we reduce CO2 emissions from existing plants'? For that, capital cost of the added equipment is important. Understand that a significant part of the cost of the air separation is amortized capital cost of the ASU, so counting that, but not capital cost of a new nuclear plant, is not a fair comparison.

Having said that, oxycombustion looks like it needs oxygen production costs to drop further before it's really competitive. So I would not be surprised if some other technology were adopted instead. It does have the advantage of allowing existing powdered coal plants to be retrofitted, something IGCC with precombustion removal of CO2 does not.

Posted by Paul Dietz at March 23, 2007 06:42 AM

It's all well and good to analyze whether sequestration or ethanol could in principle lead to practical ways to produce energy without trashing the Earth's atmosphere. But as such they are both unproven. Some forms of ethanol are in production, but not in ways that really prove anything. Corn ethanol in particular is a crime of false environmental accounting. And for all of Paul Dietz's intelligent comments about sequestration, it is also a pretext to do the stupidest thing possible, namely to keep burning the coal with no sequestration.

But there are two ways to produce pollution-free electricity that actually work: Wind power and nuclear power. They are both in production and they are not cheats like ethanol. They are also both missed opportunities for America. I respect research into new solutions, but we need to take action with the solutions we have.

Posted by at March 23, 2007 07:38 AM

One could also seriously look at energy conservation. Stuff like better house insulation reducing heating / air conditioning needs.

Posted by mz at March 23, 2007 07:45 AM

Tell me why sequestering nitrogen into a salt dome is so expensive. Can't you just take the exhaust gas and pump it into the salt dome at high speed?

Well, salt domes are limited in volume, so you wouldn't use them.

If you compress to 800 psia the density of N2 is less than .07 g/cm^3, vs. around 1 g/cm^3 for liquid CO2 (depending on temperature). So not only is the mass you're manipulating much greater, so is the volume. This will make the pipeline more expensive. You could increase the pressure, but this also increases the cost of the pipeline. And, your underground reservoir now has more than an order of magnitude lower CO2 capacity.

I'll add that the cost of compressing N2 from 14.7 psia to 800 psia will be at least ~100 kWh/ton, and that's if you do it perfectly and isothermally. In practice it will be even higher. So you're already losing relative to the energy cost of O2 separation by a factor of nearly 2, perhaps more.

It might make the most sense to compress the flue gas, remove the liquefied CO2, then reexpand the residual nitrogen (with an external heat source) to recover some of the energy of compression. The nitrogen would then be vented. Even this isn't being considered, telling me it doesn't cut it for some reason (economics or too much CO2 in the residual gas).

Posted by Paul Dietz at March 23, 2007 10:35 AM

"But there are two ways to produce pollution-free electricity that actually work: Wind power and nuclear power."

Nuclear, sure. And it can also be made intrinsically safe (pebble-bed I believe) which negates the need for hugely expensive triply-redundant safety systems and therefore makes it cheaper.

Wind power? Nonsense. There is some question whether the energy yield from a wind turbine during its life even pays for the energy used to make it, never mind any more than that. In addition to this, wind power is unreliable, which means you need a backup plant to run when the wind dies down. Also, there are serious noise pollution problems, and if you are not very careful about siting the turbines kill flying wildlife.

Two renewables that have been very little studied are wave power (also unreliable but a lot less so) and ocean thermal. The latter we know works; the pilot plant (about 100kW) was built in 1932!

Of course, the real answer is SPS, but I don't expect a wind-power advocate to agree with that!

And as something worth a small bet ($200 million or so, a drop in the bucket of corporate finance) how about electrostatic-confinement fusion?

Lastly, how about renewable fuel? And I don't mean ethanol brewed from corn, either - another dish of Congress pork.

Of course, the oil-industry lobby will resist any research on anything that might actually work with every last drop of American soldiers' blood, until every last drop of oil has been pumped and every last dollar extracted from it. With America's current president (small p intended) as an accomplice.

"Only after the last tree has been cut down; Only after the last fish has been caught; Only after the last river has been poisoned; Only then will you realize that money cannot be eaten."

Posted by Fletcher Christian at March 24, 2007 05:25 AM

"I like Air Liquide."

Booo. Praxair all the way!

Posted by Josh Reiter at March 25, 2007 10:09 PM

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