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Carbon Sequestering Energy Production

Global warming is concentrating two things in the ocean, heat and carbon dioxide.

It is estimated that ocean storage has taken up over ninety percent of the heat attributed to global warming and that the amount of carbon dioxide that has been taken up since the start of the Industrial Revolution has been sufficient to raise the ocean’s acidity by about 30 percent.

Its acidity has dropped 0.1 units on the pH scale which is logarithmic, thus the 30 percent increase.

This increase in acidity is believed to be detrimental to many forms of marine life.

According to the world ocean review, once a new carbon equilibrium between the atmosphere and the world’s ocean is re-established – once we stopped adding CO2 to the atmosphere – the oceanic reservoir will have assimilated around 80 per cent of the anthropogenic CO2. This CO2 will have combined with the water to form carbonic acid, H2CO3.

If we could draw down the ocean’s CO2 concentration, the new equilibrium state would equate to less CO2 in the atmosphere as well.

According to a recent finding of scientists from Lawrence Livermore National Laboratory we can do just that, even as we generate carbon-negative hydrogen that neutralizes ocean acidity.

As I have proposed many times in this forum, the accumulation of heat in the ocean, mostly in the upper 700 meters, sets up the potential for producing energy by way of ocean thermal energy conversion or OTEC. Since most of the best locations for producing this energy are offshore, it becomes necessary to produce an energy carrier, such as hydrogen, to bring the power to market. The way this is done is by the process of electrolysis.

As the Lawrence Livermore team has demonstrated, at lab scale, electrolysis of saline water produces not only hydrogen, chlorine and oxygen gases, the resulting electrolyte solution is significantly elevated in hydroxide concentration, which are strongly absorptive and retentive of atmospheric CO2. And the carbonate and bicarbonate produced in the process could be used to mitigate ongoing ocean acidification, similar to how an Alka Seltzer tablet neutralizes excess acid in the stomach.

The following is a schematic of the chemical reactions that take place.

 

The following diagram from the Chemical Education Digital Library shows the reaction that takes place when a brine solution is electrolyzed and how the solution evolves from one of sodium chloride to sodium hydroxide, which in turn precipitates the CO2 out of the water in the preceding reaction.

 

Seawater contains only about 3.5% salt and thus when it is electrolyzed you get oxygen production as well as chlorine at the anode. You also get less Na+ which is the key to the desired end of precipitating C02 from the air and water.

One way to increase the production of Na+ would be to pass the seawater through a reverse osmosis desalinator and then electrolyze the concentrate. With most desalination process the concentrate is virtually toxic waste but in this case it would be gold, and the desalinate water would remain available.

One of the main problems with reverse osmosis is the cost of pressurizing seawater to the 55 to 85 bar that are necessary for the process to work. With OTEC the desalinator would operate at a depth of 1000 meters or 100 bar per the following diagram (the desalinators are the white cylinders at the bottom of the stack).

 

One of the significant problems with OTEC is the biofouling of the heat exchangers, particularly with the evaporator. In the deep, cold water environment of the condenser it is not a problem.

A good way to overcome biofouling of the evaporators would be to vent a portion of the chlorine gas produce by the electrolyzer into the evaporator along with the warm water.

According to a 2005 NREL it takes about 52.5 kWh of electricity to produce 1 kg of hydrogen through electrolysis. If my calculations are correct a 100 MW OTEC plant could therefore produce about 16,500,000 kilograms of hydrogen a year and since this is equivalent to 16,500,000 gallons of gasoline at current price of roughly $3.70/gallon, the value of the hydrogen generated would be about $62 million.

With every mole of hydrogen produced however you would also generate about 1 mole of Na, if you electrolyzed the desalination concentrate, and this in turn would precipitate 1 mole of CO2 out of the ocean and/or atmosphere. Since a mole of CO2 weighs 44 grams (*) you could therefore sequester about 726,000 tonnes of  CO2 per year with a 100 MW OTEC plant.

Even at a modest price of $15/tonne for carbon, this is about $11 million/year.

The question presented then is, how do you want to sequester your carbon?

Do you want to consume more energy to do it, or have it done as a natural consequence of producing energy that ameliorates virtually every problem associated with global warming and increases in value as a consequence?

I am not an economist, but I know which way I believe we should go.

*The molecular weight of CO2 and resulting calculations are corrected from origninal post.

Content Discussion

Hops Gegangen's picture
Hops Gegangen on July 10, 2014

 

 I’ve been thinking that it might be easier politically if instead of trying to have a global carbon tax or cap and trade, the major nations redirected a fraction of the military budget to some massive carbon capture facilities. I was thinking of something like this but driven by nuclear power on floating platforms. This might be better. 

Engineer- Poet's picture
Engineer- Poet on July 10, 2014

This post doesn’t distinguish between the production of hydrogen and oxygen vs. hydrogen and caustic soda.  This begs the question:  if you’ve electrolyzed saltwater to NaOH and Cl2, what do you do with the chlorine?

If you add chlorine to water, it will form HCl and oxygen.  That HCl re-acidifies the water and you are right back where you began acidically, but a lot poorer.

Roger Arnold's picture
Roger Arnold on July 10, 2014

You’re correct, as usual, EP.  Jim’s description of how carbon-negative hydrogen production works is misleading.  The process does work, but there’s more to it than what Jim describes.

It’s best to view the process in terms of separate analyte and cotholyte solutions — the electrolytes in the anode and cathode sections of the cell, respectively.  For carbon-negative hydrogen production, the two start out as common salt solutions.  But as the solutions flow through the electrolysis cell, Na+ ions migrate from the analyte to the catholyte and are replaced in the analyte by H+ ions from the anode.  At the same time, Cl- ions from the catholyte migrate into the analyte and are replaced by OH- ions from the cathode.

The result is that a comon stream of salt solution entering the cell exits as separate acid and base streams. For seawater, that would be HCl on the anode side, and NaOH on the cathode side.  The two are balanced, and if they were simply discharged into the ocean, nothing would be accomplished (beyond production of hydrogen).  The  hydrogen would only be “carbon-neutral”, not “carbon-negative”.

The key step that Jim failed to mention is neutralization of the acid analyte stream by reaction with silicate minerals. That produces silca and soluble metal chloride salts.  It mimics the natural weathering of exposed rock surfaces by carbonic acid in rainwater, but is millions of times faster.  What gets discharged into the ocean is then metal chloride salts and sodium hydroxide.  The latter counters ocean acidification and enhances its capacity to hold dissolved CO2.

Thermodynamic calculations indicate that for every net increase of ten hydroxyl ions added to the ocean, nine CO2 molecultes from the atmosphere will ultimately be absorbed from the atmosphere and retained in the ocean as carbonate and bicarbonate ions.  Over time, and with a little help from sea life, the carbonate ions will combine with calcium and magnesium and precipitate as new carbonate minerals.  

Neutralization of the acid analyte can be handled in several ways.  For the LLNL studies, crushed ultra-mafic (highly basic) rocks were simply packed around the anode.  Not very practical for a production process, since the mineral alkalinity is consumed and the minerals have to be continuously replaced.  But that was only a lab demonstration to show the concept.  For production, the acid analyte solution leaving the cell can be discharged over a leaching bed of crushed rock, replenished from the top.  The neutralized solution of metal chloride salts would be withdrawn from the bottom.  

The most practical approach, however, is probably just to pump the discharged analyte into deep injection wells drilled into porous basalt layers.  Volcanic basalt is porous and ultra-mafic, and is the basement rock for the ocean basins.

Roger Arnold's picture
Roger Arnold on July 10, 2014

You really don’t want your process to be producing significant amounts of chlorine gas.  

To have any effect in countering CO2 emissions, the production volumes for carbon-negative hydrogen must be huge — tens to  hundreds of megatonnes H2 per year, or it’s likely not worth bothering with.  If you allow the cells to produce chlorine, that would be hundreds to thousands of megatonnes of chlorine gas annually.  No way could even a tiny fraction of that amount of chlorine be discharged into the oceans without disasterous impact.

Fortunately, there’s a way to avoid production of chlorine in electrolysis of saline solutions.  It requires “hiding” the anode behind a thin layer of cation exchange resin.  The resin has a high concentration of bound anion groups.  It is permeable to water and allows for free flow of H+ ions from the anode, but isolates the anode from chloride ions in the analyte.

Greg Rau's picture
Greg Rau on July 10, 2014

Yes, I really don’t want to produce Cl2 or other chlorine compounds. I’m also not a big fan of membranes because of cost and fouling. Plan B would be to employ O2 selective anodes, e.g.: http://infosys.korea.ac.kr/PDF/JIEC/IE13/IE13-4-0545.pdf

Plan C is to precipitate solid carbonate and allow the electrolyte salt to be regenerated, meaning we could use a non-chloride electrolyte, but we’d still need a source of H2O.

I also don’t want or need either the acid neutralized anolyte or the  CO2 neutralized catholyte injected into the deep ocean. Much prefer these alkalized solution to remain in surface waters to help maintain seawater pH and carbonate saturation state.

Also, if we are using silicate as your anolyte neutralizer we’ll have to deal with SiO2 or compounds thereof.  Adding silicic acid to the ocean could act as a fertilizer in certain regions, not to mention the bio effects of the Fe released from the mineral. This could spur further bio CO2 uptake, but could also have ecosystem negatives.

So need to do more research before going full scale. Partners and funding solicited

 

Roger Arnold's picture
Roger Arnold on July 10, 2014

Aha! After re-reading what Jim wrote, I think I see where some of the confusion between us regarding the carbon-negative H2 production process is coming from.  He and I are thinking of somewhat different processes.

Jim has precipitation of sodium carbonate as the means for sequestration and storage of CO2.  I suspect he’s been reading Calera’s patents for “carbon-negative cement” production.  That approach will work, provided you have a concentrated source of CO2 available.  In that case, there’s no need to neutralize an acid analyte stream by reaction with silicate minerals.  If you allow chlorine to be produced at the anode, there’s no acid analyte stream to be neutralized in the first place.  Instead of producing hydrogen ions, the anode consumes chloride ions.  That’s essentially the chlor-alkali process. 

The problem with that approach is that sodium carbonate is highly soluble under most conditions.  It takes high concentrations at high pH to cause it to precipitate.  You can get the high pH in the catholyte solution, but you can’t get the high carbonate concentration without having a concentrated source of CO2 to bubble through the catholyte.  You could make the process work at a coastal coal-fired power plant, but it’s not suitable for an OTEC platform.

The carbon-negative H2 process that Greg Rau and the LLNL team were looking at is different.  It does not precipitate sodium carbonate, nor produce chlorine.  It produces an acid analyte stream and a basic catholyte stream.  The acid analyte stream is neutralized through reaction with basic minerals that comprise the bulk of earth’s crust.  The basic catholyte stream goes into the ocean to raise its pH.  Raising ocean pH automatically leads to enhanced uptake of CO2 from the atmosphere.

Greg Rau's picture
Greg Rau on July 10, 2014

Thanks, Jim.  I would be interesting to see if we could merge OTEC with my idea for C-negative H2. Cl2 generation is a major concern, and despite you desire to use this as a defoulant, we’d have to figure out how to avoid Cl discharge if we are going to get past the marine ecosystem police.  I posted some ideas on this elsewhere in the comments.

Other details: I would prefer not to precipitate C out of solution but to keep it as dissolved bicarbonate to help maintain ocean alkalinity. Ideally the cation balancing this bicarbonate would be the metal derived from the mineral, Ca or Mg, rather than Na.  Still working on that aspect. 1 mole of CO2 = 44 g (not 32 g) and the market value for avoiding CO2 I think is currently $5/tonne (or $0.018/kg C, not $0.15/kg C) so you might want to redo your economics. Still, OTEC needs an energy carrier, H2 via electrolysis is a prime candidate, and if we can cost effectively get additional CO2 and ocean antacid benefits that’s a lot of wins for one technology. Anyway, with more than a few TWhrs out there going to waste, and the CO2 problem worsening, now what?   

Engineer- Poet's picture
Engineer- Poet on July 10, 2014

It would be best to put the chlorine out of circulation for a much longer time.  Since liquid chlorine is much more dense than water, perhaps it could be packaged in sealed plastic sleeves and just laid gently on the ocean floor (possibly in deep trenches where the bottom is unlikely to be trawled).

It may well be simpler, easier and cheaper just to pulverize silicates like olivine and convert the CO2 into carbonates.

 

Hops Gegangen's picture
Hops Gegangen on July 11, 2014

 

Minng and crushing and moving a lot of silicate rock sounds like a carbon-intensive operation on the scale at which it would be required to move the needle, no?

I’ve wondered whether it would be feasible to sequester CO2 by combining it with hydrogen to create polymers for plastics. Most plastic just ends up sequestered in landfills. A quick look on the internet shows it taking about 450 yeras for a plastic bottle to decompose. I assume a more solid object would take a lot longer, so perhaps some forms of plastics could be used for infrastructure.

I think the navy is already looking at creating jet fuel using the power of the on-board reactors, and doing so at a reasonable overall cost. 

 

 

Roger Arnold's picture
Roger Arnold on July 11, 2014

The energy cost of mining and crushing operations wouldn’t actually be too bad. Suitable source rocks, such as serpentines, can be found in large deposits with little or no overburden.  Open pit rock quarries are energy efficient.  But the scale would be huge.  It would take two to four tons of crushed serpentine rocks to provide the alkalinity to capture and sequester one ton of CO2 in the ocean.  

The logistics cost of those operations, on top of the even larger costs for loading the crushed rock onto rail cars, rail transport to a seaport, offloading onto an ore carrier, shipping to an offshore OTEC site, and offloading from the ore carrier onto the leaching bed ,,, I don’t have specific cost estimates, but I expect they’d prove prohibitive.

That’s why I suggested the alternative of injecting the acid anolyte (sorry, Greg, about all those misspellings elsewhere) into porous basalt on the sea floor. The basalt would neutralize the acid in-situ almost as well as leaching over crushed serpentine.  At a small fraction of the cost.  

It didn’t occur to me that one might want to distribute the iron salts and silicic acid from the neutralized anolyte to fertilize ocean surface waters.  But I can see that fertilization would amplify ocean CCS and perhaps help the oceans recover from the damage we’ve done through overfishing.  It’s something that should be studied.  But it doesn’t rule out injection as an economical way neutralize the anolyte.  It just means you’d want to sink some production wells into the basalt layer around the injection wells.  That would allow some of the neutralized anolyte to be recovered and distributed to surface waters.

Engineer- Poet's picture
Engineer- Poet on July 11, 2014

Here’s why you probably want to get the iron into solution near the surface, and silicic acid too:

http://nextbigfuture.com/2014/06/120-tons-of-iron-sulphate-dumped-into.html

Roy Wagner's picture
Roy Wagner on July 13, 2014

I am not a chemist however my understanding is that one of the best solutions for the production of hydrogen from electrolisis is hot saltwater.

The choices of anode and cathode materials and the voltage applied also determine what gases are produced.

After this process the concentrated brine could be used for thermal energy storage or salt production.

Any thoughts on these options?

Roy Wagner's picture
Roy Wagner on July 13, 2014

Jim and Greg  I think you should propose your ideas on this website they are offering potential funding for new ocean energy and industries https://ouroceanschallenge.org/