AU2008101140A4 - Improved method of capturing carbon dioxide and converting to bicarbonate anions and then sequestering as sodium bicarbonate in aqueous solution - Google Patents

Description

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION INNOVATION PATENT IMPROVED METHOD OF CAPTURING CARBON DIOXIDE AND CONVERTING TO BICARBONATE ANIONS AND THEN SEQUESTERING AS SODIUM BICARBONATE IN AQUEOUS

SOLUTION

The following statement is a full description of this invention, including the best methods of performing it known to us: IMPROVED METHOD OF CAPTURING CARBON DIOXIDE AND 0 CONVERTING TO BICARBONATE ANIONS AND THEN SEQUESTERING OAS SODIUM BICARBONATE IN AQUEOUS SOLUTION 5 This patent application is to be read in conjunction with patent application O 20071011174 "Improved method of capturing carbon dioxide and converting Z to carbonate anions and then combining with calcium cations to form calcium carbonate." Introduction Current anthropogenic carbon dioxide (CO2) emissions from fossil burning fuels are 70 million tonnes per day and 25 million tonnes of these emissions are absorbed into ocean seawater daily. This absorption is causing acidification or lowering of alkalinity of seawater particularly near the surface 15 where the concentration of absorbed CO2 is the greatest. Carbon dioxide 00 mixes with seawater to produce dissolved carbon dioxide in the first stage.

This maybe represented as CO2. H20. The CO2 absorbs directly into seawater or firstly absorbs into atmospheric moisture or rain droplets and then is carried into the seawater in rain.

The dissolved CO2 in the rain or seawater converts to carbonic acid by forming covalent bonds which may be represented by H2CO3.

The carbonic acid ionises to a hydrogen ion H+ and a bicarbonate anion HCO3- with a single negative charge. There is a small amount of carbonate anions CO3-- with a double negative charge.

The absorption of carbon dioxide into the surface seawater to form carbonic acid increases the acidity of the seawater or more accurately lowers the alkalinity of the seawater since the seawater is still slightly alkaline at a pH range of 7.5 to 8.5 over the planet. There has been a reduction in pH on average of about 0.075 over the last few decades. The pH reduction of surface water will be greater due to the higher local concentrations of the absorbed carbon dioxide and hence the carbonic acid. There have been thoughts of how to reduce this effect in the past by adding alkaline materials to counter the carbonic acid increase. These have involved highly alkaline materials such as calcium oxide, calcium hydroxide and sodium hydroxide to mention only a few. The problem with adding such strongly alkaline materials which create very high pH of the water system is that they need to be added to seawater very carefully to avoid any deleterious effects on marine life. They were also relatively expensive and use on a vast scale such as in the oceans to counter acidity would be very expensive.

The present day problem is that the rate of emission of anthropogenic carbon dioxide is so great and the rate is continually increasing with no signs of abatement that there is an unrelenting increase in atmospheric carbon dioxide levels which is being related to global warming due to the greenhouse gas effect of the carbon dioxide. This is also having a continued effect in the oceans as absorption of carbon dioxide continues unrelentingly making the seawater acidity/alkalinity situation growingly worse.

This is predicted to have serious physical, ecological and economic effects for the nations of the world and marine life.

Discussion 0 Patent application 20071011174 dealt with the electrolysis of seawater (aqueous sodium chloride solution) to create alkaline conditions to convert c dissolved carbon dioxide and bicarbonate anions to carbonate anions and 5 then combining with calcium cations to precipitate calcium carbonate. The O carbon dioxide was sequestered as the carbonate anion in the stable calcium Z carbonate.

In c An improvement on this process which is more energy efficient and which has twice the capacity (see comparison of sequestering as bicarbonate versus Scarbonate) is also more practical since large quantities (approximately 10,800ppm) of sodium cations are present in seawater to assist in forming sodium bicarbonate and it eliminates the need to produce suitable large quantities of calcium cations to enable the production of calcium carbonate is as follows: 00 1 Conversion to bicarbonate anions of seawater captured carbon dioxide.

Step 1 Seawater, brine or reconstituted salt water prepared brine) is electrolysed in the same way as in 20071011174 and initial quantities of calcium carbonate are formed as the alkaline conditions produced by the electrolysis (in situ sodium hydroxide NaOH) convert dissolved carbon dioxide and bicarbonate anions present in seawater to carbonate anions which precipitate calcium carbonate due to the presence of calcium cations in the seawater (411 ppm) Step 2 The electrolysis of the seawater, brine or salt water is continued to produce further sodium hydroxide. The seawater alkalinity or pH increases to give a very high pH. Hydrogen and chlorine are liberated at the cathode and anode respectively as valuable by-products of the process. This is equivalent to losing acid components from the seawater as hydrochloric acid (HCI) so that a net alkaline solution of sodium hydroxide is produced on the seawater.

Step3 The very high pH seawater caused by the sodium hydroxide with a pH range nominally of pH 11 -14 may then be blended into untreated seawater to enable the dissolved carbon dioxide C02 (or carbonic acid H2CO3) in the untreated seawater to be converted to bicarbonate HCO3- anions at a suitable nominal pH of say pH 9-10. The bicarbonate anion is present with the sodium cation ie a net quantity of sodium bicarbonate in solution. The bicarbonate anions will always be present in a large excess of water and hence will remain in solution in a stabilised form which is essentially sequestered as the bicarbonate anion.

Step 4 The seawater with the bicarbonate conversion may then be returned to the ocean. The bicarbonate anion is stable under the normal pH range of seawater which is pH 7.5 For the purpose of understanding we have used values for pH that may not be 0 exact. They are guidelines only for the level of pH which would allow rapid 0absorption of carbon dioxide and conversion to and stabilisation as the CN bicarbonate anion. Further research and process development may result in 5 modification of these pH figures.

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Z 2 Capture of carbon dioxide from the emissions of a powerstation or other In industrial plant The carbon dioxide containing emissions from a powerstation may be directly Sblended with the very high pH seawater at pH 11-14 and the flow rates controlled to produce bicarbonate anions at a suitable nominal pH range of Ssay pH 9-10. Some carbonate anions may be produced which may precipitate some calcium carbonate due to the slight (411ppm) concentration of calcium 15 cations in seawater. The predominant and preferred conversion will be 00 bicarbonate anions. The seawater with the slightly increased sodium bicarbonate levels may then be returned to the ocean where the bicarbonate anion will be stable under the pH range of the ocean waters of pH 7.5 to The level of bicarbonate anions HCO3- in the oceans is 145ppm. The increase in bicarbonate anion levels in ocean water that would be caused if the 100ppm increase in atmospheric carbon dioxide from 280ppm to 380ppm was absorbed into ocean water would be negligible at 0.36ppm.

Mass of carbon dioxide in the atmosphere at 380ppm is 1.95 X 10A15kg Mass of the atmosphere is 5.1X10^18 kg Mass of the increase in anthropogenic carbon dioxide is 0.5 X 10A15kg Mass of oceans is 1.4X1021 kg Bicarbonate levels increase in the ocean absorbing the 100ppm carbon dioxide increase due to anthropogenic activity is 0.36ppm Comparison of efficiency of sequestering as bicarbonate anions versus carbonate anions.

The sequence of conversion of carbon dioxide to carbonate under aqueous conditions at increasing alkalinity is as follows: C02 H20 H20.C02 H+ HC03- H+ C03pH 5 pH14 carbon dioxide carbonic acid bicarbonate carbonate The bicarbonate anion HC03- has a single negative charge. The carbonate anion C03-- has a double negative charge.

Sodium bicarbonate is Na HC03. Sodium carbonate is Na2 C03 There would be two equivalents of sodium cations required to sequester as the carbonate but only one equivalent of sodium cations required to Ssequester as the bicarbonate. Sequestering as the bicarbonate thus requires N only half the amount of sodium cations compared to the carbonate as so is 5 twice as efficient in the amount of sodium cations required for the O sequestration process.

CComparison to present sequestration methods The present methods of carbon dioxide capture involve absorbing powerstation emissions with liquid ammonia forming an addition compound with the carbon dioxide, stripping the carbon dioxide from the ammonia/carbon dioxide compound with steam, compressing the carbon dioxide to liquefy and piping and storing in underground rock formations. All of 00 these processes are energy intensive and are grid dependent for energy supply and will add carbon dioxide to the atmosphere as a footprint.

The innovation is designed to work with passive electricity sources as power for the electrolytic process to avoid adding carbon dioxide to the atmosphere as a consequence of the sequestration process.

This power needs to be generated by passive systems which are known technologies. Such systems would be solar, wind, wave, tidal or hydro.

Nuclear powered electricity generation would also be suitable.

It is vital to have a passive/renewable electrical power generating system to generate power for the sequestration process since power from the grid produces anthropogenic carbon dioxide which is counterproductive to achieve the desired reduction in carbon dioxide emissions. The efficiency of grid electrical power generation (at about 30% to 40%) would produce relatively more carbon dioxide than the amount sequestered rendering the whole process impractical.