AU2007100222A4 - Sequestering carbon dioxide - Google Patents

Description

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION INNOVATION PATENT Applicant: Earth Ocean Space Pty Ltd Invention Title: SEQUESTERING CARBON DIOXIDE The following statement is a full description of this invention, including the best method of performing it known to me/us: -2- Sequestering Carbon Dioxide Technical Field A specific method for sequestering carbon dioxide (C 2 Oz) is disclosed that makes use of a change in alkalinity of particularly sea water. The method has particular though not exclusive application in reducing the amount of CO 2 emitted with power station flue gas.

Background Art Increased levels of atmospheric carbon dioxide from the burning of fossil fuels are the cause of the so-called greenhouse effect. Many argue that this effect is the major global environmental issue. Mitigation of the greenhouse effect therefore requires that current atmospheric carbon dioxide levels and carbon dioxide emissions be reduced.

The art is replete with methods of atmospheric carbon dioxide capture and sequestration for the purpose of reducing atmospheric carbon dioxide levels. For example, since at least 1995 it has been known to sequester carbon dioxide by changing ocean alkalinity (Kheshgi, H S (1995) "Sequestering Atmospheric Carbon Dioxide by Increasing Ocean Alkalinity" Energy, 20, 915-922). A more recent patent that discusses developments in this area is US6,890,497. The method of this patent hydrates the atmospheric CO 2 and then reacts the resulting formed carbonic acid with carbonate.

A reference herein to a prior art document is not an admission that the document forms part of the common general knowledge of a skilled person in the art in Australia or elsewhere.

Summary of the Disclosure In a first aspect there is disclosed a method of reducing the amount of CO 2 emitted in flue gas resulting from a fossil fuel burning process. The process is one that employs a flow of cooling water from a body of sea water, or from water having a high carbonate content. The method comprises the steps of: directing and releasing an offtake stream of the flue gas into the cooling water flow, whereby the CO 2 in the offtake stream forms carbonic acid in the water; -3locating a source of carbonate in the cooling water flow in the vicinity of where the flue gas offtake stream is released such that the carbonic acid can react with the carbonate source to form bicarbonate.

Such a method sequesters CO 2 from the flue gas offtake and is able to be implemented simply and economically, as it can make use of existing infrastructure of a fossil fuel burning process, especially the cooling water flow, in a manner not previously contemplated.

In this regard, a simple bleed stream configuration (eg. a gas offtake pipe or conduit) can be fitted to the flue (eg. to extend from the flue interior at an intermediate point along its length). This bleed stream can then be directed into a pre-existing flow of cooling water. This flow of cooling water may be defined in a cooling water intake channel extending from the cooling water body and into proximity of burners for the fossil fuel burning process. In this regard, cooling condensers for the burners can be brought into contact with the cooling water of the intake channel. Whilst the bleed stream is typically directed into the cooling water intake channel after the cooling condensers, depending on the location of the flue with respect to the channel the bleed stream may be directed into the cooling water intake channel prior to the cooling condensers.

In either case, the gas offtake pipe or conduit can be fed into the cooling water flow (eg. intake channel) at a location towards an in-use lower region of the flow to then permeate up into the cooling water to ensure maximal reaction of the CO 2 into carbonic acid. For example, the CO 2 may be released to bubble out (eg. as a fine dispersion of bubbles for maximum reaction) of the gas offtake pipe or conduit and up into the cooling water flow. The high carbonate content in the cooling water body, when brought into contact with the CO 2 in the flue offtake gas, will promote the formation of carbonic acid, typically at an economically feasible rate when compared to lower carbonate waters.

The method then locates a source of carbonate in the cooling water flow in the vicinity of the where the flue gas offtake stream is released into the cooling water flow.

For example, a mined metal carbonate (eg. calcium carbonate) by-product can be suspended in water pervious baskets (eg. of stainless steel) in the vicinity of where the flue offiake gas bubbles into the cooling water intake channel. This location may be downstream of where the flue gas offtake stream is released into the cooling water intake channel.

G Alternatively, a finely ground metal carbonate powder (eg. calcium carbonate powder) can be added to the cooling water intake channel. This location may be upstream of where the flue gas offtake stream is released into the cooling water intake channel, to facilitate the reaction of the carbonic acid with the metal carbonate.

C1 When the terminology "high carbonate content" is used in relation to the water Sbody, it is by way of reference and comparison to typical carbonate levels in naturally occurring fresh water bodies. For example, sea water is a water body having a naturally high carbonate content (with the total concentration of carbonate and bicarbonate ions N, typically amounting to approximately 2200 ltmoles/litre). Whilst some water bodies other than sea water may, eg. due to geological factors, have carbonate levels approaching such high concentrations, this would not typically be the case.

Thus, in one form the method is applied to flue gas where the flue is located close to a body of sea water (eg. adjacent to a coast).

The method is typically applied to a flue gas having a high CO 2 content, for example around 15% by volume or greater. Such high C02 content can occur in a flue gas emitted from a power station that bums a fossil fuel (eg. coal and gas) to produce electricity. Thus, in one form the method is applied to a flue gas emitted from a power station that bums a fossil fuel to produce electricity. In applying the method in this context, the pre-existing infrastructure of such a power station can easily and readily be employed.

However, it should be appreciated that the method is not limited to being applied in power stations. In this regard, it can be easily applied wherever a fossil fuel burning process is employed close to a body of sea water that is also used for cooling (eg. at a coastal steelworks).

In a second aspect there is disclosed a method for determining the amount of

CO

2 sequestered from flue gas emitted from a fossil fuel burning process. In this method the process employs a flow of cooling water from a body of sea water, or from water having a high carbonate content. The method comprises the steps of: directing and releasing an offtake stream of the flue gas into the cooling water flow, whereby the C02 in the offtake stream forms carbonic acid in the water; locating a source of carbonate in the cooling water flow in the vicinity of where the flue gas offtake stream is released such that the carbonic acid can react with the carbonate source to form bicarbonate; measuring pH in the cooling water flow upstream of flue gas release, and downstream of the carbonate source location; from these measures, determining the amount of carbon sequestered by a detected change in water alkalinity.

The method of the second aspect may, in other respects, be practised according to those features of the first aspect as outlined above.

In a third aspect there is disclosed apparatus for reducing the amount of CO 2 emitted in flue gas resulting from a fossil fuel burning process. That process is one that employs a flow of cooling water from a body of sea water, or from water having a high carbonate content. The apparatus comprises an offtake pipe or conduit that is located to receive, direct and release the flue gas from a flue of the process and into the cooling water flow.

The apparatus of the third aspect may further comprise a carbonate release device for location in the cooling water flow in the vicinity of where the flue gas is released from the offtake pipe or conduit. The carbonate release device can comprise a mechanism for introducing the source of carbonate as defined in the first aspect.

The offtake pipe or conduit, and the mechanism, may also be as defined in the first aspect.

Further, the apparatus of the third aspect may be suitable for implementing the method as defined above in the first and second aspects.

Brief Description of the Drawing Notwithstanding any other forms that may fall within the method and apparatus as set forth in the Summary, specific embodiments of the method and apparatus will now be described, by way of example only, with reference to Examples and the accompanying drawing in which Figure 1 shows a schematic side view of a configuration and apparatus for implementing one form of the method, with a flue and an adjacent flow of cooling water being depicted.

Detailed Description of a Specific Embodiment -6- O Prior to describing specific embodiments of the method and apparatus, the N, context in which the method and apparatus arises will be described. In this regard, it is Sto be noted that rapid changes in climate are a global threat. For example, the food security of many poor coastal nations is being affected by increasing acidity of the ocean. Such increases diminish marine life and thus food (protein) supply. A major cause of increased ocean acidity is high atmospheric carbon dioxide levels resulting CI from fossil fuel burning.

SThe method and apparatus disclosed herein allow for fossil fuel to be burned (ie. whilst this is still the most cost effective fuel supply) to the benefit of mankind, whilst also allowing substantial amounts of a major negative environmental by-product, carbon dioxide, to be captured and stored as inorganic carbon. This by-product is already present in the ocean, and its capture and storage as inorganic carbon has additional benefits of enhancing marine life. The method also does not increase the ocean acidity.

In this regard, at equilibrium, the atmospheric partial pressure of carbon dioxide and of the surface ocean below are equal. A change either away from equilibrium leads to a flux of carbon dioxide either into or out of the ocean. The partial pressure of the ocean surface water depends on many factors such as temperature, total alkalinity, total carbon and pH. It has been found that alkalinity adjustment of surface waters increases the total carbon in the sea water without increasing the partial pressure of carbon dioxide in the water.

Total alkalinity of sea water is a measure of carbonate and bicarbonate ions with a typical concentration being 2200pmoles/litre (2200Leq/kg). Increasing the total alkalinity increases the total carbon in the water whilst maintaining the same partial pressure of carbon dioxide. The most straightforward way of increasing total alkalinity is to dissolve calcium carbonate or calcium oxide to form bicarbonate ions. Optimally, to enable rapid dissolution of the calcium carbonate, the pH of the water is lowered.

Description of Apparatus and Method Referring now to Figure 1, apparatus 10 is schematically depicted for implementing a method of capturing and storing a proportion of carbon dioxide (C 2 Oz) that would otherwise be released to atmosphere with the flue gas exiting a flue 11 of a power station 12. The method makes use of an existing sea water cooling channel 14 -7for the power station 12 which draws off a larger body of sea water (usually in flow communication with the ocean at a coastal location).

In the apparatus depicted the flue gas offtake stream comprising say 15 vol% carbon dioxide is drawn off a flue gas flowing through flue 11 via a flue gas offtake (bleed) pipe 16. The pipe is configured such that an in-use lower portion 18 thereof is located adjacent to a bottom region 20 of the sea water cooling channel 14.

As shown, the flue gas offtake permeates up into the cooling water W as a typically fine dispersion of bubbles B released via a series of holes in pipe lower portion 18 to ensure maximal reaction of the C02 with the water into carbonic acid. The formation of carbonic acid lowers the pH of the water. The pipe lower portion 18 extends transversely across the bottom of channel 14 to ensure maximum release/reactivity of the CO 2 The high carbonate content of the sea water promotes the formation of carbonic acid at an economically feasible rate.

In the method a source of carbonate, in the form of calcium carbonate 24, is located in the water flow in channel 14. In this regard the calcium carbonate is suspended at 26 in one or more water pervious baskets 28 of stainless steel which are then lowered into the water of channel 14 in the vicinity of where the flue offtake gas bubbles B rise into the water flow. The calcium carbonate is provided in a form that has a large surface area exposed for maximum reactivity. In the apparatus depicted the baskets 28 are lowered into the channel 22 immediately downstream of the pipe lower portion 18.

Alternatively, a finely ground metal calcium carbonate powder is added to the cooling water intake upstream of where the flue gas offtake stream is released into the cooling water, to facilitate a more complete reaction of the carbonic acid with the metal carbonate.

The carbonic acid formed in the sea water reacts with the suspended calcium carbonate to form bicarbonate in the water. The method and apparatus therefore captures and sequesters the flue gas offtake COz in a simple manner, and in a way that is easy to retrofit to existing power station infrastructure.

Examples Non-limiting Examples are described below, however, prior to describing the Examples, it was first noted that the waters of the surface ocean were supersaturated with respect to calcium carbonate, and that precipitation was impeded (apparently) by naturally occurring ions present in the water. In laboratory testing of natural sea water much higher concentrations of calcium carbonate were able to be achieved than were initially present in the water.

The pH of ocean surface water was noted to be between 8 and 8.5 attributable to the present global levels of carbon dioxide in the atmosphere. It was also noted that rising levels of carbon dioxide in the atmosphere are expected in the future and that this will lower the pH of the ocean surface water. It was further noted that, should the pH be lowered by a corresponding increase of carbonic acid, solid calcium carbonate would go into solution.

Example 1 Using the apparatus schematically depicted in Figure 1, flue gas with a concentration of carbon dioxide of order 15% was bubbled through sea water used for cooling. This was noted to lower the pH of the sea water. The water was then allowed to flow past calcium carbonate previously broken into small pieces and retained in a number of porous baskets. The pH was observed to rise as the calcium carbonate went into solution. Chemical additives were also used to impede calcium carbonate precipitation. The baskets were refilled from trucks bringing the waste carbonate from a near by quarry.

The total alkalinity of ocean surface water was of the order of 2200 jeq/kg. At a CO 2 partial pressure of 150,000 ppm the pH was observed to be low. Such a low pH was enough to rapidly dissolve the calcium carbonate. The rate of dissolution depended on the surface area of the calcium carbonate.

Example 2 Again, using the apparatus schematically depicted in Figure 1, flue gas with a concentration of carbon dioxide of order 15% was bubbled through sea water used for cooling. Finely ground (or fine as-mined) calcium carbonate with a large surface area was supplied upstream of flue gas introduction and, because of the turbulence in the cooling water channel, stayed in suspension in the water. Example 2 was otherwise as per Example 1.

Example 3 The amount of carbon dioxide (carbon) sequestered was determined from a measured changing alkalinity as per the methods of Example 1 and 2. In this regard, the channel sea water pH was measured upstream of the point of introduction of the flue gas, and was measured downstream of the calcium carbonate (basket) introduction. The pH differential was then used to calculate the amount of carbon sequestered.

Example 4 The environmental impacts of discharging high alkalinity water were assessed from an examination of, and by comparison to, data sourced regarding the discharge water from desalination plants. Here the salinity and alkalinity were noted to be about doubled over that of the adjacent ocean. Flow rates of discharge water from desalination plants of capacity 200ML per day were noted to be about the same as a 400MW power station. Thus it was determined that if the alkalinity adjustment lead to a discharge with a concentration less than 4,400pteq/kg, the desalination experience should be directly relevant as to the environmental compatibility of the present method and apparatus.

Flue gas was also noted to contain NOx and SOx gas fractions. Since NOx and SOx are soluble, the flue gas that bubbled out of the cooling water was noted to have reduced concentrations of these gases. The abundant level of calcium carbonate in the present method and apparatus was also observed to neutralise acids that were formed (potentially) by the scrubbing of these gases.

Shelf fish adjacent to the cooling water exit were expected to prosper with the higher levels of carbonates, especially bicarbonates, as such marine life use carbonate in their shell formation.

Example The methodology was noted to be most likely applicable to a coastal power station, with the CO 2 from its flue stacks pumped through cooling water and channelled into the sea. In such a situation, the project boundary was noted to be the physical boundary of the power station and cooling water channel, and extended into the sea. For example, a boundary was determined that extended by a 20km radius from the cooling water discharge point back into the sea water body. In this example, the calcium carbonate quarry was noted to be outside this boundary.

It was noted that only a small amount of specialist apparatus was required to be manufactured to retrofit the coastal power station to make it suitable for the method.

Example 6 Two possible generators of carbon dioxide that occurred outside the boundary in the alkalinity adjustment method of Example 5 were noted as: 1. The mining of calcium carbonate in the current method it was expected that waste calcium carbonate from commercial mining would be used.

2. Transport of calcium carbonate to the power station in the current method it was expected that truck transport would generate about 0.2 kg CO 2 per tonnekilometre. This would become 4.4 kg of CO 2 per tonne of CO 2 sequestered when the method assumed 10 km of transportation to the boundary.

Even with much larger distances, it was noted that these possible generators of carbon dioxide would be considered negligible (less than 1% of flue gas emissions).

Example 7 Once the carbon dioxide was converted to bicarbonate, the possibility of enhanced precipitation of calcium carbonate, either on the sea floor or in the hard shells of marine organisms was considered, in case such an action would release carbon dioxide back to the environment. While it was noted that the pH of the ocean is falling (due to increasing atmospheric concentration of C0 2 additional carbonate ions would go into solution (from coral reefs and the like) and there was no evidence to suggest that any significant fraction of the additional alkalinity provided by the present method would therefore be lost by precipitation.

Example 8 From within the project boundary of Example 5 there was: 1. Extra energy, taken from the power station, to urge the flow of cooling water against the extra resistance of the suspended calcium carbonate, and the extra pressure head on the flue gases. This energy was estimated for a pilot plant to be kWh for 400kg of CO 2 per hour. This energy drawn from the power station was converted to carbon dioxide (for gas fired power stations) at 0.6 kg CO 2 per kWhr.

Again, it was noted that these generators of carbon dioxide would be considered negligible (less than 1% of flue gas emissions).

2. Reduced emission of CO 2 to the atmosphere was calculated from the change of carbon dioxide in the cooling water (at equilibrium with the atmosphere).

Whilst specific methods and apparatus of sequestering CO 2 has been described, it should be appreciated that the method and apparatus can be embodied in many other forms.

In the claims which follow and in the preceding description, except where the context requires otherwise due to express language or necessary implication, the word S-11 "comprise" or variations such as "comprises" or "comprising" is used in an inclusive

C

sense, i.e. to specify the presence of the stated features but not to preclude the presence Sor addition of further features.

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Claims (4)

1. A method of reducing the amount of CO 2 emitted in flue gas resulting from a fossil fuel burning process, wherein the process employs a flow of cooling water from a body of sea water, or from water having a high carbonate content, the method comprising the steps of: directing and releasing an offtake stream of the flue gas into the cooling water flow, whereby the CO 2 in the offtake stream forms carbonic acid in the water; locating a source of carbonate in the cooling water flow in the vicinity of where the flue gas offiake stream is released such that the carbonic acid can react with the carbonate source to form bicarbonate.

2. A method as claimed in claim 1 wherein the flue gas offtake stream is a bleed stream that is drawn off via an offtake pipe or conduit fitted to the flue and extending from the flue interior at an intermediate point along its length, and is then directed to extend transversely in a cooling water intake channel that extends from the cooling water body and that passes around cooling condensers for fossil fuel burners, the offtake pipe or conduit being located towards an in-use lower region of the channel to enable the flue gas offtake to permeate up into the cooling water and ensure maximal reaction of the CO 2 into carbonic acid, with the C0 2 being released as a fine dispersion of bubbles from the pipe or conduit and up into the cooling water flow.

3. A method as claimed in claim 2 wherein the source of carbonate is a mined metal carbonate that is suspended in the cooling water flow in one or more water pervious baskets in the vicinity of where the flue offtake gas bubbles into the cooling water flow, or wherein the source of carbonate is a finely ground metal carbonate powder that is added to the cooling water intake channel upstream of where the flue gas offtake stream is released into the cooling water intake channel.

4. A method for determining the amount of CO 2 sequestered from flue gas emitted from a fossil fuel burning process, wherein the process employs a flow of cooling water from a body of sea water, or from water having a high carbonate content, the method comprising the steps of: directing and releasing an offtake stream of the flue gas into the cooling water flow, whereby the CO 2 in the offtake stream forms carbonic acid in the water; -13- locating a source of carbonate in the cooling water flow in the vicinity of C where the flue gas offtake stream is released such that the carbonic acid can react with it the carbonate source to form bicarbonate; measuring pH in the cooling water flow upstream of flue gas release, and downstream of the carbonate source location; from these measures, determining the amount of carbon sequestered by a CI detected change in water alkalinity. Apparatus for reducing the amount of CO 2 emitted in flue gas resulting from a fossil fuel burning process, wherein the process employs a flow of cooling water from a body of sea water, or from water having a high carbonate content, the apparatus C comprising an offtake pipe or conduit that is located to receive, direct and release the flue gas from a flue of the process and into the cooling water flow.