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.
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