AU2010101085A4 - Improved method of capturing and increasing solubility of carbon dioxide and conversion to bicarbonate anions and sequestering as calcium bicarbonate in aqueous solution - Google Patents

Description

AUSTRALIA Patents Act 1990 COMPLETE SPECIFICATION INNOVATION PATENT IMPROVED METHOD OF CAPTURING AND INCREASING SOLUBILITY OF CARBON DIOXIDE AND CONVERSION TO BICARBONATE ANIONS AND SEQUESTERING AS CALCIUM 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 AND INCREASING SOLUBILITY OF CARBON DIOXIDE AND CONVERSION TO BICARBONATE ANIONS AND SEQUESTERING AS CALCIUM BICARBONATE IN AQUEOUS SOLUTION 5 Introduction This Innovation Patent is an extension on Innovation Patent 2009100001 10 IMPROVED METHOD OF CAPTURING CARBON DIOXIDE AND CONVERTING TO BICARBONATE ANIONS AND THEN SEQUESTERING AS CALCIUM BICARBONATE IN AQUEOUS SOLUTION 15 Description The purpose of this Innovation Patent is to specify the method of how the solubility of carbon dioxide is increased in the alkaline water capture system of 20 carbon dioxide from the emissions of a power station or other fossil fuelled heat raising plant. Further clarifications are made on the speed of reaction between carbon dioxide/carbonic acid and calcium carbonate in nature and the enhanced speed of the designed process. 25 Fig 1 shows a simple process of absorbing C02 into water and reacting with an excess of calcium carbonate in a single pass. This is how the natural ocean system works in the conversion of ocean absorbed C02 in seawater to 30 bicarbonate where there is a huge quantity of water involved and where the concentration of C02 is very low at around 1 00ppm. The industrial process must be designed to use much less water. 35 The state of the art in understanding how carbon dioxide is captured in water is that the maximum solubility of carbon dioxide is about 0.2% (2000ppm) in ambient temperature water. The solubility ranges from 0.15% (1500ppm) at about 24Deg C to 0.25% (2500ppm) at about 10deg C. Fig A. This low solubility would make it necessary to use large volumes of water to capture 40 the carbon dioxide emissions from the power station or other fossil fuel burning plant. About 500 tonnes of water would be needed to absorb 1 (one) tonne of carbon dioxide at ambient 15deg C. The process developed increases the amount of carbon dioxide which may be 45 absorbed by a factor of many times which reduces the amount of process water that is needed to achieve significant removal of carbon dioxide emissions. This will be described step by step.

2 4 3,5 3 -C02 2,5 2 15 . to engineeringtoolbox-corn o I 0 10 20 30 40 50 60 Water Temperature (deg C) Figure A: Solubility of CO 2 in water with temperature pH/CO2 equilibra 1.0 0.9 - -- 0.8 0.7

-[HCOM

0.6 -- [--C03]2 ] 0.5 [HCO3]=[CO2]I 0.4 0.2 - 0.2 0.1 I 0.0, 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 pH Figure B: pH and CO 2 species The form of carbon dioxide species in water at various pH values is shown in the Figure B. This shows the relative equilibrium concentrations of the species carbon dioxide, C02, bicarbonate anion, HC03- and carbonate anion C03= through the pH range 0 - 14 5 * Carbon dioxide, C02, is the predominant species under acidic conditions pH<5. * Bicarbonate, HC03-, is the predominant species under intermediate conditions pH 7-9. 10 * Carbonate, C03=, is the predominant species under alkaline conditions pHlO.5+ Step 1 15 Carbon dioxide C02 gas dissolves in water to give dissolved carbon dioxide/ carbonic acid. The main form is carbon dioxide in solution. A small amount of the dissolved carbon dioxide (about 1 %) converts to carbonic acid. An acidic solution of carbonic acid forms nominally at about pH5. The solubility of the C02 will be limited under these acidic conditions since the 20 predominant species is C02. Low pH (acid) tends to evolve C02 gas hence limiting the solubility. C02 + H20 ' C02. H20 / H2CO3 Carbon dioxide water -> dissolved carbon dioxide / carbonic acid 25 Step 2 If the acidic dissolved C02/carbonic acid solution is added to a large excess 30 of calcium carbonate then neutralisation occurs and bicarbonate anions are formed. This is the well known formation of calcium bicarbonate and the temporary hardness of water. The pH will increase since bicarbonate anions have been formed. Calcium carbonate (CaCO3) provides a large reservoir of alkalinity in the form 35 of carbonate anions C03= which react with carbonic acid (H2CO3) or dissolved (C02.H20) to form two bicarbonate anions 2(HCO3)-. C03= + H2CO3 - o 2(HC03) carbonate carbonic acid bicarbonate 40 Note that the pH at maximum concentration of bicarbonate anions is pH 8.3 which is the ideal pH for seawater. 45 Step 3 It will now be possible to dissolve more C02 gas in the same water since the C02 species has been removed by converting it to bicarbonate anions and also the solution is no longer as acidic.

4 This extra dissolved C02/carbonic acid can be further neutralised with more excess of calcium carbonate converting to more bicarbonate anions. This will enable further C02 to dissolve and the pH of the solution will increase further 5 The Continuous Process A continuous process can be created by repeating Step 3 over and over. The solubility of calcium bicarbonate in water is over 16%. The stochiometric 10 equivalent of C02 in the Ca(HCO3)2 is about 50% by weight so the equivalent solubility of C02 as the bicarbonate species is about 8%, forty times more that the initial C02 solubility. The amount of water necessary for the scrubbing of the C02 from the emissions is thus reduced dramatically. As the concentration of calcium bicarbonate increases the pH will also 15 increase so that the process will probably run at about pH 8 to pH8.5 The process is driven by a large excess of carbonate anions, C03=, (on the right hand side of the Fig B and at high pH) providing a large reservoir of alkalinity to force the molecular C02, carbon dioxide, from the left of the Fig B 20 (and at low pH) to be converted to bicarbonate at an intermediate pH. The Fig B thus gives a pictorial illustration of this transient conversion situation. Newly delivered C02 is continuously being converted to bicarbonate due to the ever present large excess of alkaline carbonate reservoir. The C02 is in a rock world. 25 The effluent flow may be arranged to remove bicarbonate solution to the ocean and maintain a suitable bicarbonate level to ensure efficient absorption of the carbon dioxide emissions. 30 The mass of the calcium carbonate can be made large enough and the particle size made small enough to give sufficient surface area and contact time to allow the conversion of the carbon dioxide to bicarbonate to be achieved at an appropriate rate. 35 SOx and NOx emissions (Power stations and other heat raising plant) See Fig 3 Power stations and other fossil fuelled heat raising plant produce SOx and 40 NOx gases. Coal contains 1 -3% of sulphur S. This oxidizes to oxides of sulphur (SOx) in the combustion process. SOx reacts with water to form sulphurous and sulphuric acids. NOx gases form in the high temperature combustion process and form nitrogen oxide acids with water. 45 These SOx.H20 and NOx.H20 mineral acids are fully ionized and are very strong acids. They are responsible for acid rain and the detrimental effects on the environment such as the devastating effects on the Black Forest in Germany. They will also have a significant effect on the acidification of the oceans in addition to the effect of carbonic acid.

0 They must be removed before carbon dioxide can be captured, Fig 2, since the predominant form under acidic conditions is the C02 gas. The aqueous capture system for carbon dioxide needs to be alkaline at a nominal pH of 8 to pH 8.3 which is the maximum in the curve for bicarbonate equilibrium. 5 There is a process for the neutralisation of SOx and NOx gases in fossil fuelled heat raising plant in industry with limestone to try and reduce the effects of the sulphur and nitrogen acids on the environment but there is great 10 reluctance to do this. One factor will be cost. The neutralisation process will add costs to the power station costs of generating electricity which may be regarded as avoidable and it will be done reluctantly. A second reason which has emerged with the increase in anthropogenic atmospheric C02 is that the product of the neutralisation 15 process will be more anthropogenic carbon dioxide from the neutralisation process. Only enough CaCO3 will be used to neutralise so the conditions will not be alkaline and C02 will be introduced under the neutral /slightly acid conditions. 20 Calcium carbonate + sulphur acids 4 calcium suphates/ites + carbon dioxide CaCO3 SOx.H20 Ca SOx C02 The process could receive more costs from the additional cost of a carbon tax or other penalties associated with emissions of carbon dioxide. 25 This should be contrasted to the proposed sequestration neutralisation reaction between calcium carbonate and carbon dioxide/carbonic acid where the neutralisation is carried out in a large excess of calcium carbonate and where the product of neutralisation is the calcium bicarbonate. 30 The sulphur and nitrogen oxides captured in aqueous solution may be neutralized in a LARGE excess of calcium carbonate to generate the bicarbonate. The reaction between a strong acid and a large excess of calcium carbonate thus produces a net alkaline solution. 35 calcium + sulphur oxide 4 calcium + calcium carbonate acids bicarbonate sulphates (excess) Ca C03 SOx.H20 Ca(HC03)2 Ca S04 40 calcium + nitrogen oxide 4 calcium + calcium carbonate acids bicarbonate nitrite/ates (excess) Ca C03 NOx.H20 Ca(HCO3)2 Ca NOx 45 This quick removal of the highly reactive SOx and NOx gases can be seen to benefit the C02 capture by quickly increasing the pH of the water system by speeding up the establishment of the alkaline bicarbonate medium. 50 0 The emission gases may then be cooled and the C02 absorbed into the water system with the increased pH, The cooled emissions may then be treated with a combination of or individually with the alkaline materials smoke, ash, limestone and other 5 alkaline minerals to form bicarbonate. The use of the smoke and ash which is essentially a free incidental material which is alkaline may be able to reduce the amount of limestone needed for the conversion of dissolved carbon dioxide/ carbonic acid to bicarbonate. 10 SOx and NOx emissions - Absorption into the ocean Sulphur oxides from the combustion of fossil fuels would have been absorbed from the atmosphere into the oceans. The amount of sulphur in coal is 1-3% 15 so a significant amount of SOx gases dissolved in ocean water would be responsible for some of the acidification and drop in pH. SOx.H20 acids are fully ionised so a 1% by stochiometric weight could have a similar effect on pH drop compared carbonic acid which is only slightly dissociated at about 1 %. 20 The oceans are becoming more acidic with a pH drop of ocean water from C02 absorption mainly with some contribution from SOx acidic gases and to a lesser degree NOx acidic gases. 25 A lower pH of ocean water has been shown to cause a change in behaviour of fish fry who become more at risk from predators. This represents an enormous risk on the future of fish survival and fish stocks for human consumption in the future. 30 As the pH of ocean water is reduced there also becomes a greater tendency for the vast store of bicarbonate to be released as carbon dioxide. I ppm of bicarbonate in the oceans will cause a huge increase in atmospheric C02 which could more than double the level to 800 - 1000ppm with unknown consequenses to the environment. It is essential that this problem be 35 addressed. The neutralisation of these acids with calcium carbonate and other alkaline materials provides a solution to the problem. There are those who believe that the saturated calcium carbonate in the oceans will be sufficient to neutralize the dissolved carbon dioxide/carbonic 40 acid. The reaction rate may not be fast enough to keep up with the generation of anthropogenic carbon dioxide (and SOx) so an enhanced process is needed. Reaction rate of dissolved carbon dioxide with "saturated" calcium carbonate in the oceans is very slow due to the relative low concentrations of about 45 1 00ppm (nominally) of the carbon dioxide and 1 00ppm (nominally) of the "saturated" calcium carbonate. 1 00ppm is one ten thousandth. There is a low probability of dissolved carbon dioxide at 100ppm coming into contact with a particle of saturated calcium carbonate at 100ppm. This could be expressed as one ten thousandth times 50 one ten thousandth or one in a hundred millionth as a concept of probability.

f If sea water with dissolved carbon dioxide at 100ppm is brought into contact with a large formation mass of limestone in the designed process way then the probability of contact of the dissolved carbon dioxide with the calcium carbonate is 100%. The reaction rate with the designed process is thus likely 5 to be of the order many millions of times faster than the natural process. This process will also take out the sulphurous and sulphuric acids as shown. When ocean water is passed through the large limestone bed to convert the carbon dioxide to bicarbonate the SOx acids will be neutralised and readily 10 converted to sulphates and bicarbonate. This will increase the pH of the ocean water. Evidence for this type of reaction in the past is in the oceans where it may be seen that the concentration of sulphate anions is over 2700ppm. 15 Preferred embodiments 20 There will be a first stage in the industrial process where SOx and NOx gases which form strong acids will be removed. (Fig 2) They must be removed since C02 absorption will not be possible under acidic conditions. The SOx and NOx gases will be captured in the first stream since they are very readily soluble in hot water. The sulphur oxide acids and nitrogen oxide 25 acids will be taken to large beds of limestone to convert to the bicarbonate solution which may be then sent to the carbon dioxide absorber scrubber stage to to assist in the capture of the C02 from the cooled emissions (Fig 3). 30 The absorber/scrubber of the carbon dioxide emissions will utilise a water recirculation system. Calcium carbonate/limestone will be included in the reactor or the process water will be sent to a vessel containing calcium carbonate limestone before being returned to the scrubber (Fig 3). 35 The dissolved carbon dioxide will be converted to and sequestered as bicarbonate. This reaction increases the pH of the water system which stabilises the bicarbonate anion. It also removes the C02 species encouraging more C02 emissions gas to be dissolved. 40 The concentration of calcium bicarbonate effluent may be chosen to control the removal rate of absorbed and converted C02 whilst maintaining the bicarbonate level and pH conditions in the recirculation water system to promote the absorption of further C02 from the cooled emissions stage. The amount of water need to absorb the C02 emissions in the process will be 45 reduced dramatically from about 500 tonnes per tonne of C02 (for the normal C02 solubility) to about 15 to 20 tonnes of water per tonne of C02 for the recirculation process. 50

Claims (5)

1. The reaction of neutralising the acidic carbon dioxide/carbonic acid 5 with a large excess of alkaline calcium carbonate/ limestone converts the carbon dioxide to bicarbonate and results in a higher pH solution which stabilises the bicarbonate anion . 10

2. Carbon dioxide solubility is increased since the original carbon dioxide has been removed and more carbon dioxide is able to further dissolve in the recirculated water and react with the excess of limestone to produce further bicarbonate which further increases the pH of the solution. The recirculation process continues and is thus self catalysing 15 as the product of the neutralisation reaction, bicarbonate, increasingly promotes the reaction due to the increasing solubility of carbon dioxide and the increasing pH which stabilises the bicarbonate product of reaction. 20

3. The rate at which the carbon dioxide converts to bicarbonate is proportional to the surface area of calcium carbonate which is increased by a larger mass and smaller particle size of the calcium carbonate bed or formation 25

4. The amount of water needed to capture the carbon dioxide emissions is dramatically reduced due to the much higher effective solubility of the carbon dioxide in the water capture system, 30

5. The SOx and NOx emissions of the plant are reacted with limestone quickly to form bicarbonate to neutralise the dangerous acidic gases and to produce bicarbonate quickly which gives a rapid increase in pH which kick starts the process. 35 40 KENNETH GREEN 1 OCTOBER 2010 45 GRAHAM EDWARD THOMS 50