Baking Soda Carbon Sequestration?

The Skyonic SkyMine process is a new process which claims to be able to store carbon dioxide from power stations in the form of sodium hydrogen carbonate (baking soda). It has a great appeal - baking soda is a solid that can be easily stashed in large underground places such as salt caverns without the risk of leaks. If only continuing the fossil-fueled oblivion were so easy...

From the start, I questioned the thermodynamics of the process. Carbon sequestration is always tricky in this regard because the whole driving force behind fossil energy (especially coal) is the reaction of carbon with oxygen which yields carbon dioxide and energy. Carbon dioxide is a direct result of energy production itself and toying with that reaction or trying to alter the production of carbon dioxide will have consequences on the amount of energy that can be produced.

Apparently Skyonic is testing the process at a coal-fired power station in Texas, and has attracted large electric companies like TXU (electric generation is usually a rather conservative industry which is very attached to old ways). A lot of money has been apparently been invested in the process, and the people behind the operation also have experience in the chemical engineering field. I have taken basic chemistry courses in high school and college and other than independent research that''s about it, nothing compared to the millions of dollars of money invested in this. So what right do I have to question them? Simply the fact that it didn''t make thermodynamic sense, like just about every other carbon sequestration scheme that has been devised. It is possible that this entire argument is worthless, that I made an error, a false assumption, or that a piece of data (such as that value for delta H_f on salt) is wrong. But it seems that I have left enough leeway (such as assuming that 100% of the energy in coal can be converted to electricity) for at least some error.

What is NOT wrong is that for each carbon dioxide molecule sequestered, one sodium atom is needed (as this is a requirement for the chemical structure of baking soda). That sodium atom must come from somewhere, some foreign chemical (such as salt - the most plentiful source of sodium on Earth) must be mined at a 1:1 molar ratio with coal. If we burn a billion moles of carbon in coal, we will need to mine a billion moles of sodium from a sodium-containing mineral to make the baking soda. No one can deny that, it is the basic law of conservation of matter.

There is no cut-and-dried explanation of the chemical reactions (Skyonic is an "Intellectual Property" corporation, so they are not going to release the stuff for sake of keeping someone else from taking their ideas), possible catalysts, etc. that might be involved, but they talk of three chemicals:

• Chlorine (by-product)
• Hydrogen (by-product)
• Sodium Hydroxide

The production of chlorine suggests to me that salt (sodium chloride) is used as the sodium source for the production of sodium hydroxide, which is produced industrially using electrolytic processes described below.

The more mysterious reaction is the use of sodium hydroxide to sequester CO2 to produce baking soda (NaHCO₃ ). There may be catalysts or other reagents involved, but I have simplified it to a basic reaction (reaction 3). Either way, the conservation of matter and thermodynamics are the same.

1. 2NaCl -> 2Na + Cl₂
2. 2Na + 2H₂O -> 2NaOH + H₂
3. NaOH + CO₂ -> NaHCO₃

Using salt as the sodium source means that electrical energy must be put in to break the(quite strong) sodium-chlorine bond. The heat of formation of sodium chloride is -411.12 kJ/mol.

According to these reactions, to sequester one mole of carbon dioxide (about 44 grams) as sodium hydrogen carbonate, one mole of salt would have to be split into its constituent elements to obtain the required amount of sodium or sodium hydroxide. That process is done via electrolysis, in a device such as a Downs Cell or via the Chloralkali Process. Whatever the method, thermodynamics state that to break the sodium-chlorine bond, 411.12 kJ of energy per mole of salt is required.

The combustion of one tonne of coal will produce around 2.5-3 tonnes of carbon dioxide. That is equal to 56,818 mol of CO2 (assuming 2.5 tonnes CO2 - 2.5 megagrams). Therefore 56,818 mol NaCl needs to be electrolyzed for each ton of coal burnt (assuming 100% sequestration and process efficiency). If the NaCl formation emits 411.12 kJ/mol during formation, then it must require that same amount of energy to split it back into its elements. So 411.12 * 56818 = 23,359,016 kJ of electrical energy is needed to make the sodium, assuming that electrolysis is 100% efficient.

The amount of salt needed per tonne of coal would be about 980 kg, assuming perfect conditions and 100% capture. The amount of salt needed to sequester all of the CO2 from all COAL power plants in the U.S. each year would then be about 980 million tonnes (we burn about 1 billion tonnes to make 50% of our electric power). World production of sodium chloride in 2006 was 240 million tonnes.

The hydrogen produced contains energy, but I am going to leave it out of this calculation, since Skyonic suggests selling it to industry rather than using it for energy production.

The energy yield of the combustion of one tonne of coal averages about 29,307,600 kJ, according to Online Conversion.

The net energy production by the power plant, under perfect conditions and 100% heat-to-electricity conversion (both impossible), would therefore be a dismal 5,948,584 kJ/tonne of coal. That is about 1600 kWh. Damn, we should have built wind turbines instead!

The chemical reactions never will have 100% yield.
Steam turbines on the most advanced supercritical boilers are at the very best 45% efficient.
Some of the sodium hydroxide will react with sulfur dioxide and other gases as well as CO₂ (this is actually good to reduce pollution).
Electrolysis is not 100% efficient.
Extra electricity will be required for all of the ancillary fans, pumps, reactors, etc.

Even if the process is thermodynamically sound, can we produce as much salt/"sodium-containing feedstock" as we produce coal and stash away many billion tons more of baking soda into the earth? Do we have a use for the millions (maybe even billions) of tonnes of chlorine that will be produced? I can''t see those things going without an environmental impact.

But I could be missing something.

CREATED/WRITTEN: 2008-10-08 00:00