WO2009117763A1

WO2009117763A1 - Capture of carbon dioxide from flue gases in large-scale algae cultivation

Info

Publication number: WO2009117763A1
Application number: PCT/AU2009/000327
Authority: WO
Inventors: Robert Walter Vowles
Original assignee: BIOSEQ Pty Ltd
Current assignee: BIOSEQ Pty Ltd
Priority date: 2008-03-27
Filing date: 2009-03-19
Publication date: 2009-10-01
Anticipated expiration: 2010-09-27
Also published as: AU2009227974A1

Abstract

A unit for the selective absorption into water of carbon dioxide from large volumes of flue gases or the direct absorption into water of large volumes of more or less pure gaseous carbon dioxide; said unit comprising one or more casings within which a large plurality of spray heads generate ultra-thin, fan-shaped sprays of water which are efficient absorbers of carbon dioxide; said carbon dioxide or flue gases being delivered to said casings via suitable trunks; said spray heads being supplied with pressurised water via suitable conduits; carbonated water draining away through suitable drainage lines, decompression wells and buffering units; said flue gases, stripped of carbon dioxide, discharging to exhaust stacks via pressure relief valves which maintain optimal carbonation pressure within said casings and optional water capture units; said carbon dioxide, flue gases and sorbent water being cooled to a suitable temperature for carbonation.

Description

CAPTURE OF CARBON DIOXIDE FROM FLUE GASES IN LARGE-SCALE ALGAE CULTIVATION

This invention relates generally to methods employed in the intensive cultivation of algae for the purpose of extracting products yielded by various algal species. More specifically, it relates to the capture of flue gas carbon dioxide for use in the intensive cultivation of algae in photobioreactors, water from which is brought into direct contact with flue gases and used as a sorbent.

Carbon dioxide (CO₂) is generally acknowledged as the so-called greenhouse gas most susceptible to management in efforts to control anthropogenic effects upon atmospheric temperatures. While carbon dioxide is produced by a very broad range of biological and industrial processes, the burning of carbonaceous fuels for electrical power generation represents the largest source. More efficient burning of a carbonaceous fuel reduces the amount of CO₂ produced per unit of power output and much research is directed towards this end. Advanced technologies, such as supercritical boilers, integrated gasification combined cycle (IGCC) and pressurised fluidised bed combustion (PFBC) are expected to carry electrical power generation thermal efficiency to approximately 50 per cent. The ability to produce ultra-clean coal has led to proposals for gas turbines to be fuelled with pulverised coal with combined cycle operation producing high thermal efficiency. Regardless of the level of thermal efficiency achieved, however, a substantial stream of CO₂ still remains. Ultra-clean coal from new processing technologies has been effective in reducing ash production to very low levels.

Similarly, technologies have been developed which substantially eliminate sulphur dioxide and oxides of nitrogen from flue gases. A number of processes have been developed for the post-combustion capture of CO₂ from flue gases. Amongst these are the use of a solid sorbent, such as hot calcium or potassium carbonate; the use of liquid sorbents, such as monoethanolamine or refrigerated ammonia; the use of CO₂ clathrate formation; the use of pressure swing absorption processes; the use of zeolite molecular sieves; and membrane separation processes. It is commonly proposed that captured

CO₂ be liquefied and sequestered in abyssal ocean depths, in coal seams or in suitable permeable geological structures. The processes of capture, liquefaction, transport and sequestration of CO₂ impose substantial costs which add to the delivered cost of electrical power, leading to a number of lines of research directed at performing them in a more cost-effective way. An adjunct point is the fact that the burning of one tonne of carbon generates approximately 3.7 tonnes of CO₂- Sequestration of a million tonnes of CO₂ thus involves the loss to biological processes of nearly 730,000 tonnes of oxygen. Thus, the sequestration of billions of tonnes of CO₂, as is proposed, may have secondary biological effects which, to date, have not been properly explored. Biological sequestration processes, which capture only carbon while liberating oxygen are clearly more desirable.

Where coal is burned in air, assuming it is of high purity, the reaction is:

C + O₂ + (N₂) → CO₂ + (N₂) + heat Nitrogen constitutes approximately 78 per cent of air. While nitrogen plays no part in the combustion process (signified by bracketing in the equations), at high combustion temperatures, burning a fuel in air may result in the generation of undesirable oxides of nitrogen. Additionally, a considerable proportion of the heat of combustion may be lost in the form of sensible heat of the flue gases, of which nitrogen is the greater part. Where they are burnt in air (similarly assuming high purity), typical reactions natural gas and oil, respectively, are:

CH₄ + 2O₂ + (N₂) → 2H₂O + CO₂ + (N₂) + heat C₁₈H₃₈ + 27.5O₂ + (N2) → 19H₂O + 18CO₂+ (N2) + heat.

Where a fuel is burnt in oxygen (oxy-fuel combustion), the nitrogen component of the above equations is eliminated, together with heat losses normally occurring via sensible heat of the nitrogen component of the flue gases. Typical CO₂ components of wet flue gases (by volume) are:

Coal 13.7%

Natural gas 8.8% Fuel oil 12.4%.

Even where excess air is applied to the combustion process, some 2% to 3% of oxygen will normally remain in the flue gases. In the so-called Clean Coal process, Integrated Gasification Combined Cycle, coal is pyrolised from 400° C up to produce a carbon-rich char and hydrogen-rich volatiles. The char is then gasified from 700° C up by exposure to oxygen and steam. The reactions are:

C + O₂ → CO C + H₂O → CO + H₂ CO + H₂O → CO₂ + H₂.

The products are thus concentrated CO₂ (which is captured for sequestration) and hydrogen for use as a gas turbine fuel in combined-cycle operation. In combined-cycle operation, the exhaust gases from a gas turbine are passed through a boiler to raise steam which is, in turn, employed to drive a steam turbine. Typically, the thermal efficiency for oxygen-blown coal gasification, including CO₂ capture and sequestration, is in excess of 70%, giving the possibility of an electricity generation process with an efficiency possibly as high as 60%. In a further Clean Coal process, pulverised coal is gasified with super-heated steam at approximately 800° C in a fiuidised bed of lime (CaO). The reaction is:

C + 2H₂O + CaO → 2H₂ + CaCO₃

The hot calcium carbonate product is cycled through a regeneration kiln to reduce it to CaO and release the CO₂ for capture. Where possible, the regeneration kiln is a commercial lime kiln. Where flue gases from a power station are clean and comprise, essentially, carbon dioxide, oxygen, nitrogen and water vapour, the carbon dioxide component lends itself to be captured using water as a sorbent. Carbon dioxide is readily soluble in water (900 cu cm per litre at 20° C), the rate of solubility being inversely proportional to temperature and salinity and directly proportional to pressure. The fact that CO₂ is more soluble in water than nitrogen and oxygen by a wide margin renders water suitable as a sorbent, flue gas water vapour also being captured during the absorption process by condensation. While water is not an efficient sorbent from which to separate captured CO₂, where carbon is to be sequestered by biological methods (algal cultivation), no separation is necessary. Biological carbon sequestration takes a number of forms. In general, a variety of plant crops, including trees, are cultivated and all or part of a mature plant is used to manufacture a useful product. Typical of these are sugar cane and corn, used to manufacture ethanol fuel; soya bean and oil palm, used to manufacture biodiesel fuel; and trees, to manufacture timber, charcoal and paper pulp. While all may ultimately be burnt as fuel, they replace petroleum-based fuels and the biological process through which they are produced may be repeated. The disadvantage of such crops is the fact that they displace more desirable food crops, generate only a small carbon credit, or require the clearing of land with significantly adverse environmental effects. The advantages of the cultivation of algal species for carbon sequestration purposes are the very large output per unit area compared with other methods, the fact that it can be conducted on a continuous basis, the ability to use desert or semi-desert land not required for other purposes, the ability to use of saline water unsuitable for other purposes and the release of large amounts of oxygen generated by photosynthesis. At the present time, although much experimental work is undertaken, no commercial project exists for the cultivation of algae on a large scale. For successful and rapid growth, algae require water at a suitable temperature, strong sunlight, some nutrients and carbon dioxide. It is the last requirement that the present invention addresses.

The dissolution of carbon dioxide (CO₂) into water is well known in the manufacturing of carbonated drinks. However, the methods and apparatus employed for this purpose do not lend themselves to be scaled up to process large water volumes on a continuous basis. Most of these employ a pressurised vessel in which CO₂ is mixed with and dissolved in water at controlled temperature and pressure. Examples are those taught by Aschberger et al in US 4,656,933, Hancock et al in US 4,850,269, Shannon in US 4,882,097 and Notar et al in US 5,443, 763. Of particular interest are the method and apparatus taught by Demyanovich in US 4,749,580 in which a carbonated liquid is prepared on a continuous basis by exposing the liquid in the form of ultra-thin sheets to a carbon dioxide atmosphere. The extremely high surface areas exposed coupled with turbulence within the liquid sheets result in the dissolution of high levels of CO₂ in a time scale of milliseconds.

The principal object of the present invention is to provide a method and apparatus for the continuous absorption of large volumes of carbon dioxide from an admixture of flue gases (or more or less pure carbon dioxide) into pondage water used in the large- scale cultivation of algae. Secondary objects of the present invention are to minimise outgasing of dissolved CO₂ and to adjust the pH of carbonated water.

According to the present invention, CO₂ in the form of an admixture with other flue gases or more or less pure gaseous CO₂ is passed through a CO₂ absorption unit comprising one or more casings, pressure relief valves, water feed conduits, water spray heads and fan protectors, water drainage lines, decompression wells, buffering units and exhaust stacks. Where liquid CO₂ is required to be gasified, heat is preferably taken up from the incoming water, the lower water temperature so produced improving solubility of the CO₂. Where insufficient heat is available from the incoming water flow for CO₂ gasification, additional heat is supplied, as appropriate, from power station flue gas heat, solar ponds or solar absorbers. Gas compression means are employed to raise the 5 pressure of gaseous CO₂ or flue gases to that required for efficient operation of the absorption process. Where flue gases are taken directly downstream of a combustion source, heat exchange means are interposed between the gas compression means and combustion source to reduce the temperature of the flue gases to that required for efficient operation of the absorption process. Additionally, where flue gases are taken 0 directly downstream of a combustion source, a rapid-acting diversion unit is optionally interposed to immediately divert flow to atmosphere should take-up by the gas compression means fail, the diversion avoiding the imposition of an unacceptable back pressure upon the combustion source. Water recovery units to capture water vapour are optionally positioned immediately downstream of the pressure relief valves at the bases5 of the exhaust stacks. Pure carbon dioxide or carbon dioxide in an admixture of flue gases is pressurised by the gas compression means, cooled as required and passed through the casings which are inclined at a shallow angle. A suitable pressure relief valve situated at the end of each casing maintains the gas pressure within the casings at the desired pressure. Water as a sorbent for CO₂ is pressurised to a suitable pressure by O suitable pump means, cooled as required and supplied through the water feed conduits passing coaxially more or less along the full length of the casings. A plurality of water spray heads fixed to the outer surface of the water feed conduits discharge water flows in the form of ultra-thin, fan-shaped sheets. Where a sustained flow of flue gas occurs along the casings, fan protectors are optionally provided along the upstream edge of each5 water discharge sheet to prevent their disruption by impingement of flue gas flow. Carbonated water runs down the casings and drains away via water drainage and delivery lines which pass to the bottom of the decompression wells. The depth of the decompression wells and the length of the water delivery lines is such that the pressure at the point at which the water delivery lines debouch into the wells is equal to the gas O pressure inside the conduits. The diameter of the decompression wells is such that the collected flow of carbonated water rises slowly to the surface with minimal turbulence or disturbance, thereby minimising any tendency towards outgasing of the CO₂. In passing along the full length of the conduits the flue gases have more or less all CO₂ stripped out by contact with the water discharge sheets. Flue gases reaching the upper end of the conduits pass out through the pressure relief valves and exit to atmosphere via the exhaust stacks. Where pure carbon dioxide is fed to the carbonators, a CO₂ atmosphere at optimal pressure is maintained within the casings by suitable pressure control means, the casings being made with closed ends and with the pressure relief valves and exhaust stacks deleted. Carbonated water discharging at the bottoms of the decompression wells is optionally made to pass upwardly through beds of calcium carbonate chips where acidification caused by dissolution of CO₂ is buffered, the Cθ₂-loaded water then passing to the pondage via suitable conduits or channel means.

The various aspects of the present invention will be more readily understood by reference to the following description of preferred embodiments given in relation to the accompanying drawings in which:

Figure 1 is a longitudinal cross-sectional view of a carbon dioxide absorption unit of the present invention;

Figure 2 is a longitudinal cross-sectional view of a decompression well of the present invention;

Figure 3 is a side view of a spray head of the present invention; Figure 4 is a view from above of the spray configuration of the spray head of Figure 3;

Figure 5 is a typical transverse cross-sectional view of the main part of the carbon dioxide absorption unit of Figure 1.

With reference to Figure 1, CO₂ in the form of an admixture with other flue gases or more or less pure gasified CO₂ is delivered via a distribution trunk 2 to one or more carbon dioxide absorption units 1. Each said carbon dioxide absorption unit comprises casing 3, pressure relief valve 4, water feed conduit 5, exhaust stack 6 and, optionally, condensate collection unit 7. A flow of water is delivered to said water feed conduits via supply lines 10. In an alternative embodiment (not shown), a number of said carbon dioxide absorption unit exhaust stacks discharge into a single larger condensate collection unit. With additional reference to Figure 4, a large plurality of spray heads 8 is fixed to the exterior of said water feed conduit such that pressurised water supplied through said conduit is emitted into said casing. The wall thickness of said water feed conduit is made such that threaded attachment bosses 9 of said spray heads may be screwably attached in a secure way. hi an alternative embodiment (not shown), localised reinforcements are provided to said water feed conduit in order to provided more secure attachment of said spray heads. Said localised reinforcements take the form of discrete bosses of suitable thickness or circumferential bands of suitable thickness bonded or welded to the external surface of said water feed conduit, both being provided with appropriately placed, threaded bores for attachment of said spray heads, hi an alternative embodiment (not shown), said spray heads are secured hi place in suitable sockets provided on said water feed conduit by threaded retaining collars screwably engaging complementary threads of said sockets, indexing means of said spray heads engaging complementary indexing means of said sockets to ensure correct orientation of the water sheet generated by said spray heads, hi another alternative embodiment (not shown), said spray heads are secured to complementary sockets provided on said water feed conduit, said spray heads and said sockets being provided with complementary bayonet connection means which ensure correct orientation of the water sheets generated by said spray heads. Sealingly closeable access hatches (not shown) are provided in said casings to permit access to the interior of said casings for the purposes of disconnecting and removing said water feed conduit. With a said water feed conduit removed, its said spray heads may be cleaned, replaced or otherwise serviced. Said casings are inclined at a shallow angle to permit drainage of water along them. A plurality of collection lines 15 collect drainage from said casings and conduct it to delivery lines 16 which pass down into one or more decompression wells (depicted as 17 in Figure 2).

With reference to Figures 3 and 4, in the preferred embodiment of the present invention, spray heads 8 are of the deflected fan-type, hi this type of spray head, a water stream 11 discharged from an axially arranged jet orifice (not shown), impinges upon an angled surface 12 and is deflected into a fan-shaped sheet 13 of angular width α (indicated asl4). hi an alternative embodiment (not shown), said angled surface is replaced by a conical deflector surface upon which said water stream impinges to produce a water sheet extending through 360°. This embodiment, known as a jet- impingement atomiser, is not as reliable as the deflected fan-type as the deflector surface and jet orifice are not physically connected, with the possibility of misalignment and consequent loss of sheet formation. If the jet of water does not impinge upon said deflector surface properly, droplets may form without first forming a water sheet. While they are capable of providing high rates of CO₂ absorption in extremely short time periods, jet-impingement atomisers do not meet the requirements of ease of maintenance and trouble-free operation generally required for industrial applications. Further, because fan-spray atomisers produce a water sheet encompassing only part of a circle, they can be placed in more compact arrays than jet-impingement atomisers.

In both types of spray head, because the flow rate within a water sheet is constant, as the sheet expands radially, it must thin. At a certain distance beyond the discharge point, the sheet disrupts into droplets. A typical water sheet, at flow rates of up to 11 litres per minute, thins from approximately 500 to 1,000 microns at formation to 10 to 20 microns immediately prior to sheet disruption. If the flow rate is substantially smaller, the thickness of a liquid sheet may fall to as little as 1.0 to 2.0 microns at the point of sheet disruption. Because a water sheet thins in a geometric manner with respect to its length, most sheets are less than 60 microns in thickness over two thirds of the sheet length for water flow rates of 11 litres per minute or less. Disruption into droplets occurs because surface tension forces exceed inertial forces as a sheet thins. At some point within a water sheet, the surface tension force become dominant, causing the sheet to decompose into droplets. The droplets so generated have radii typically 50 to 100 times larger than the thickness of the sheet immediately prior to disruption. The residence time of a water sheet having an approximate viscosity of 1.0 centipoise is usually less than 10 milliseconds. If a water sheet is operated in turbulent flow, the residence time decreases.

Little can be done to increase the residence time of a water sheet, making it imperative for absorption of CO₂ to occur prior to disruption of the sheet in a time frame typically of 5.0 to 10 milliseconds or less. Because a water sheet is very thin, diffusion of CO₂ into it can occur in an extremely short time. Demyanovich offers the equation t = V²ZD as estimating the time required for diffusion through a thin film, where t is the time for CO₂ to diffuse a distance y, and D is the diffusivity of CO₂ in water. As an example, the time required for CO₂ to diffuse 10 microns into a water sheet flowing in laminar flow is approximately 50 milliseconds. This is based upon a diffusion coefficient in water of

0.000020 square centimetres per second at 20°C and a water viscosity of 1.0 centipoise. However, since the flow in a water sheet is in turbulent flow, the molecular diffusivity is effectively increased by a factor of 20 or more. The increase is a result of mass convection of water in the direction of sheet thickness, permitting substantial diffusion to occur in a few milliseconds — in the same order of time as the residence time of the sheet. Although turbulent flow decreases the residence time of the water sheet, it increases the effective diffusivity of CO₂ many times more, leading to enhanced mass transfer. Turbulent flow is achieved in a water sheet by increasing sheet velocity, normally by increasing the pressure drop across a spray head. The transition from laminar to turbulent flow is indicated in a water sheet by the surface changing in appearance from glassy to wavy. In summary, by employing a large plurality of deflected fan-type spray heads to generate water sheets flowing with turbulent flow, large volumes of CO₂ are able to be dissolved in water in very short time periods. As such time periods for dissolution of CO₂ are substantially shorter than the residence time of said flue gases in said carbon dioxide absorption units, a high proportion of total CO₂ is able to be recovered from said flue gases. Such spray heads are reliable and easy to maintain in operation and, in suitable numbers, are able to pass the large volumes of water necessary for practice of the present invention. As a further benefit, the distribution of CO₂ throughout the water as produced by this method is extremely uniform, resulting in a low tendency for CO₂ to outgas and obviating the need for the subsequent mixing and stabilisation required by normal commercial carbonation methods. In the preferred embodiment, said spray heads are supplied with a flow of water through said water feed conduits at a pressure between 80 and 300 kPa; said spray heads are made with water discharge jets of a diameter falling in the range 1.5 to 5 millimetres; the angular width of said water sheets generated by said spray heads falls in the range 40° to 120°; and the length of said water sheets generated by said spray heads immediately prior to disruption into droplets fall in the range 50 to 100 millimetres. In an alternative embodiment (not shown), to prevent premature disruption of said water sheets by impingement of flue gases passing through said casings, a narrow, elongated shield of a suitable material is optionally fixed to each said spray head and extends fully along the upstream edge (upstream in the sense of flue gas flow) of the water sheet. Said shield is preferably made V-shaped or semi-circular in transverse cross-section with the edge of said water spray extending into the concave surface.

With additional reference to Figure 5, water feed conduits 5 are made with an internal diameter sufficient to carry the volume of water required to be passed through spray heads 18. Said spray heads are provided in sufficient number to pass the required volume of water. The internal diameter of casings 3 is such that water sheets 13 generated by said spray heads do not impact the internal surfaces of said casings before they disrupt into droplets. Said casings are made sufficiently long to accommodate the required number of said spray heads. Said spray heads are positioned on said water feed conduits in circumferentially arranged arrays, spray heads in adjacent arrays being staggered such that water sheets generated by spray heads in one array do not impact those generated by spray heads in another array. To improve solubility of CO₂ in the water or to compensate for the reduction in solubility induced by saline water, said water and said flue gases are preferably cooled to a temperature in the range 0° C to 20° C and said flue gases are preferably pressurised to a pressure in the range 1.0 to 5 bar.

In operation, a flow of water at suitable pressure and temperature is supplied to spray heads 8 via water feed conduits 5 thereby generating a large plurality of water sheets 13 extending substantially throughout the volumes of casings 3. Flue gases at suitable pressure and temperature are supplied to said casings via distribution trunk 2 and pass along said casings with residual gases exiting to atmosphere via pressure relief valve 4, condensate collection units 7 (where used) and exhaust stacks 6. Said pressure relief valves are positioned by suitable actuator means 18 with their circumferential edges in proximity to their seats 19 such that a suitable pressure is maintained within said casings against the supply pressure of remote gas compressors (not shown). In the preferred embodiment, pressure sensors (not shown) in said casings generate a signal which is transmitted to a programmable logic controller or other suitable microprocessor-based control unit (not shown) which controls said actuator to modulate the position of said pressure relief valves in relation to their said seats and thereby to regulate the gas pressure within said casings within a predetermined range. In an alternative embodiment, said control unit regulates the gas pressure in said casings according to the temperature of the water passing through said water feed conduits. Water sheets 13 generated by said spray heads extend radially into the annular spaces between the external surfaces said water feed conduits and the inner surface of said ducts and are aligned more or less parallel to the longitudinal axes of said ducts. CO₂ in said flue gases passing along said casings and coming into contact with said water sheets is rapidly absorbed or dissolved into the water which falls to the lower part of said casings and drains away via collection lines 15 and delivery lines 16 into one or more decompression wells (depicted as 17 in Figure 2). A typical analysis of flue gases from a natural gas-fired turbine is 8.5% carbon dioxide, 2.0% oxygen, 18% water vapour and 71% nitrogen. The considerably greater solubility of carbon dioxide in water at 20° C (1,800 mg/1) compared to that of nitrogen (90 mg/1) and oxygen (44 mg/1) results in a substantially selective take-up of the CO₂. Water vapour in the flue gases condenses within said casings and drains away together with the carbon dioxide-loaded feed water via said collection lines and said delivery lines. A considerable release of the latent heat occurs within said carbon dioxide absorption units as a result of said condensation.

With reference to Figure 2, decompression well 17 is made sufficiently deep such that the static pressure at the lower end 20 of delivery line 16 is equal to the pressure maintained within said casings. Said decompression well is preferably set below ground level and comprises vertical walls 21, lower floor 22, upper closure 23, gas venting line 24, exit conduit 25 and, optionally, bed of calcium carbonate chips 26. Where said bed of calcium carbonate chips is employed, said chips take the form of crushed limestone with a Sauter mean diameter of 5 to 15 millimetres and a depth in the range 0.5 to 3 metres. Water delivered to the lower part of said decompression well via delivery line 16 flows upwardly through said bed, the lowered pH caused by dissolution of CO₂ being buffered. Because of the elevated pressure in that zone of said well, outgasing as a result of turbulent flow through said bed is minimised. Above said bed or where said bed is not employed, water flows slowly upwards through said well with progressively reducing pressure and minimal turbulence, thereby reducing any tendency towards outgasing of CO₂. Any CO₂ coming out of solution is conducted via said gas venting line to the inlet side of a said remote gas compressor. Water rising to the top of said well is conducted to the pondage via said exit conduit. In an alternative embodiment, said calcium carbonate chips are replaced with another suitable buffering compound in granular form. A suitable hatch (not shown) is preferably provided in said upper closure to permit replenishment of said chips of calcium carbonate or other buffering compound. In the preferred embodiment, said decompression wells are made with a diameter in the range 4 to 15 times that of said delivery lines.

In an alternative embodiment (not shown), said carbon dioxide absorption units are supplied with more or less pure CO₂ in gaseous or liquid form. In this embodiment, said exhaust stacks, said condensate collection units and said pressure relief valves are deleted, the distal ends of said casings are made closed and a CO₂ atmosphere is maintained within said casing by suitable pressure control means. Where liquid CO₂ is 5 required to be gasified, heat is preferably taken up from the incoming water, the lower water temperature so produced acting to improve solubility of the CO₂. Where insufficient heat is available from the incoming water flow for CO₂ gasification, additional heat is optionally supplied from power station flue gas heat, solar ponds or solar absorbers. 0 Where flue gases are taken directly downstream of a combustion source, heat exchange means are interposed between the gas compression means and combustion source to reduce the temperature of the flue gases to that required for efficient operation of the CO₂ absorption process. Additionally, where flue gases are taken directly downstream of a combustion source, a rapid-acting diversion unit is optionally interposed5 to immediately divert flow to atmosphere should take-up by the gas compression means fail, the diversion avoiding the imposition of an unacceptable back pressure upon the combustion source.

In the preferred embodiment, said carbon dioxide absorption units are arranged in groups with valves which permit them to be conveniently isolated as required for O maintenance and repair or during times of low flue gas output. Said units are preferably supported upon raised structures to ensure sufficient head of pressure is generated in the water flow via said delivery lines.

The flow of depleted flue gases from said carbon dioxide absorption units will normally be close to saturation with water vapour and may carry entrained water mist.5 Where the supply of water is a problem, provision is optionally made to capture said water vapour and mist. Said water mist is readily captured using coalescer means which are well known in the art. Where said water vapour is to be captured, said condensate collection units preferably take the form of chiller units, chemical absorption units, solid desiccant wheel units or the like, all of which may be made sufficiently large to O accommodate large flue gas volumes and are well known in the art.

Claims

1. Apparatus for selectively absorbing into water carbon dioxide contained in large volumes of flue gases, said apparatus comprising one or more long casings within which a large plurality of spray heads generate ultra-thin, fan-shaped sprays of water which are rapid and efficient absorbers of carbon dioxide; trunks which deliver said flue gases to said casings from compression means via optional cooling heat-exchange and rapid-acting diversion means; water feed conduits and supply lines which deliver sorbent water to said spray heads from suitable pressurising means via optional cooling heat-exchange means; exhaust stacks through which flue gases stripped of carbon dioxide are discharged; pressure relief valves which control the discharge of said stripped flue gases to said exhaust stacks, thereby maintaining gas pressure within said casings in a predetermined range; collection and delivery lines that collect carbonated water and deliver it to decompression wells; decompression wells that create tranquil flow and permit a progressive reduction in pressure of said carbonated water, thereby minimising outgasing of carbon dioxide, and which optionally buffer the pH change resulting from carbonation; and optional water capture units which accept a flow of said stripped flue gases from said exhaust stacks and capture water therefrom.

2. Apparatus according to Claim 1 in which the length of said casings is such as to provide a residence time of said flue gases within said casings sufficient to ensure the absorption of a high proportion of their carbon dioxide content.

3. Apparatus according to Claim 1 in which the pressure of said flue gases within said casings is regulated to a pressure falling in the range 1.0 to 5.0 bar.

4. Apparatus according to Claim 1 in which the internal diameter of said casings is sufficient to ensure that said water sheets generated by said spray heads do not impact the internal surfaces of said casings before they disrupt into droplets.

5. Apparatus according to Claim 1 in which said water feed conduits are made removable and positioned coaxially within said casings.

6. Apparatus according to Claim 1 in which said flow of water supplied to said water feed conduits is preferably cooled to a temperature falling in the range 0° C to 20°

C.

7. Apparatus according to Claim 1 in which the volume of water flowing through said spray heads of a said casing is sufficient to absorb the carbon dioxide content of said flue gases passing through said casing at the prevailing gas and water temperature and pressure.

8. Apparatus according to Claim 1 in which said spray heads are of the deflected fan type in which a stream of water discharged from an axially arranged jet orifice impinges on an angled surface and is deflected into a fan-shaped sheet.

9. Apparatus according to Claim 1 in which said spray heads are of the jet impingement atomiser type in which a stream of water discharged from an axially arranged jet orifice impinges upon a conical deflector surface to produce a sheet of water extending through 360°.

10. Apparatus according to Claim 1 in which the pressure of water fed to said spray heads falls in the range 80 to 300 kPa.

11. Apparatus according to Claim 1 in which the pressure drop across said spray heads is maintained at a level sufficient to cause the flow within said water sheets generated by said spray heads to be turbulent.

12. Apparatus according to Claim 1 in which said spray heads have discharge jets with a diameter falling in the range 1.5 to 5.0 millimetres.

13. Apparatus according to Claim 1 in which the water sheets generated by said spray heads have an angular width falling in the range 40° to 120°.

14. Apparatus according to Claim 1 in which the water sheets generated by said spray heads, immediately prior to their disruption into droplets have a length falling in the range 50 to 100 millimetres.

15. Apparatus according to Claim 1 in which said spray heads are fixed to said water feed conduits by the screwing and tightening of threaded attachment bosses of said spray heads into threaded bores provided in the walls of said water feed conduits.

16. Apparatus according to Claim 1 in which said spray heads are fixed to said water feed conduits by the screwing and tightening of threaded attachment bosses of said spray heads into threaded bores provided in localised reinforcements in the form of discrete bosses of suitable thickness bonded or welded to the external surfaces of said water feed conduits.

17. Apparatus according to Claim 1 in which said spray heads are fixed to said water feed conduits by the screwing and tightening of threaded attachment bosses of said spray heads into threaded bores provided in localised reinforcements in the form of circumferential bands of suitable thickness bonded or welded to the external surfaces of said water feed conduits.

18. Apparatus according to Claim 1 in which said spray heads are fixed to complementary sockets provided on said water feed conduits by the screwing and tightening of threaded retaining collars of said spray heads onto complementary threaded parts of said sockets, indexing means of said spray heads engaging complementary indexing means of said sockets to ensure correct orientation of said water sheets generated by said spray heads.

19. Apparatus according to Claim 1 in which said spray heads are fixed to complementary sockets provided on said water feed conduits by bayonet connection means of said spray heads engaging complementary bayonet connection means of said sockets, said bayonet connection means ensuring correct orientation of said water sheets generated by said spray heads.

20. Apparatus according to Claim 1 in which said water sheets generated by said spray heads are protected from premature disruption as a result of impingement of flue gases passing through said casings by a narrow, elongated shield fixed to each said spray head and passing along the upstream edge (upstream edge in terms of flue gas flow) of the water sheet.

21. Apparatus according to Claim 20 in which said shields are made V-shaped or semi-circular in transverse cross-sectional shape with the edge of each said water sheet extending into the concave surface.

22. Apparatus according to Claim 1 in which said spray heads are fixed to said water feed conduits in circumferentially arranged arrays, spray heads in adjacent arrays being staggered such that the water sheets generated by spray heads in one array do not impact those generated by spray heads in another array.

23. Apparatus according to Claim 1 in which said water sheets generated by said spray heads are aligned more or less parallel with the longitudinal axes of said casings.

24. Apparatus according to Claim 1 in which said casings are provided with sealingly closeable access hatches providing access into their interiors.

25. Apparatus according to Claim 1 in which said casings are inclined at a shallow angle to permit drainage of said carbonated water along them.

26. Apparatus according to Claim 1 in which a plurality of collection lines collect carbonated water draining from said casings and conduct it to delivery lines which pass down into one or more decompression wells to a point just above their lower floors.

27. Apparatus according to Claim 1 in which the diameters of said collection lines and said delivery lines are sufficiently large to easily accommodate the flow of water draining from said casings.

28. Apparatus according to Claim 1 in which the depth of said decompression wells is such that, when full of water, the pressure just above their lower floors is at least equal to the gas pressure maintained within said casings.

29. Apparatus according to Claim 1 in which said decompression wells are set below ground level.

30. Apparatus according to Claim 1 in which said decompression wells are made with a diameter falling in the range 4 to 15 times that of said delivery lines.

31. Apparatus according to Claim 1 in which carbonated water delivered by said delivery lines to said decompression wells rises up slowly through said decompression wells, the tranquil flow and progressively decreasing pressure minimising the outgasing of carbon dioxide.

32. Apparatus according to Claim 1 in which the lower part of each said decompression well optionally accommodates a bed of buffering compound which acts to buffer acidity caused by dissolution of said carbon dioxide in said water.

33. Apparatus according to Claim 1 in which said bed of buffering compound has a depth falling in the range 0.5 to 3.0 metres.

34. Apparatus according to Claim 1 in which said buffering compound takes the form of calcium carbonate chips with a Sauter mean diameter falling in the range 5 to

15 millimetres.

35. Apparatus according to Claim 1 in which said buffering compound is another suitable buffering compound in suitable form.

36. Apparatus according to Claim 1 in which said decompression wells are each provided with an upper closure incorporating gas venting line, hatch for the replenishment of said buffering compound and exit conduit conducting a flow of said carbonated water to a biological absorption unit.

37. Apparatus according to Claim 36 in which said gas venting line returns a flow of outgased carbon dioxide to the inlet side of said flue gas compression means.

38. Apparatus according to Claim 1 in which said pressure relief valves have sufficient flow capacity to accommodate the outflow of said stripped flue gases from said casings.

39. Apparatus according to Claim 1 in which the positions of said pressure relief valves are modulated in relation to their seats by suitable actuator means.

40. Apparatus according to Claim 1 in which said actuators modulating the positions of said pressure relief valves in relation to their seats and thereby the gas pressure within said casings are controlled by one or more programmable logic controllers or other microprocessor-based control units in response to signals received from pressure sensors within said casings.

41. Apparatus according to Claim 1 in which said actuators modulating the positions of said pressure relief valves in relation to their seats are controlled by a programmable logic controller or other microprocessor-based control unit in response to signals received from temperature sensors in said water feed conduits.

42. Apparatus according to Claim 1 in which said water capture units incorporate coalescers to intercept water mist and desiccator units to collect gaseous water entrained in said stripped flue gases.

43. Apparatus according to Claim 42 in which said desiccator units take the form of chiller units, chemical absorption units, solid desiccant wheel units or the like.

44. Apparatus according to Claim 1 in which said water capture units have sufficient flow capacity to accommodate the flow of stripped flue gases through said exhaust stacks.

45. Apparatus according to Claim 1 in which said rapid-acting diversion means are triggered by failure of said compression means and act to open and dump said flue gases to atmosphere, thereby preventing the imposition of backpressure on the flue gas source.

46. Apparatus according to Claim 1 in which, where said flow of flue gases is taken off immediately downstream of a combustion source, cooling heat-exchange means are provided in said trunks to reduce the temperature of said flue gases to that required for efficient carbon dioxide absorption into water.

47. Apparatus according to Claim 1 in which said casings are supported upon raised structures, thereby creating a greater head of water within said delivery lines.

48. Apparatus according to Claim 1 in which said casings are arranged in groups with valve means permitting their isolation for maintenance or repair.

49. Apparatus according to Claim 1 in which said casings are supplied with a flow of more or less pure gaseous carbon dioxide.

50. Apparatus according to Claim 49 in which a carbon dioxide atmosphere at optimal pressure is maintained within said casings by suitable pressure regulation means.

51. Apparatus according to Claim 49 in which said casings are made with closed ends and with said pressure relief valves and said exhaust stacks deleted.

52. Apparatus according to Claim 49 in which liquid carbon dioxide required to be gasified before being supplied to said casings, acquires vaporisation heat in heat- exchange relationship with said incoming sorbent water, the lower water temperature so produced improving the solubility of said carbon dioxide.

53. Apparatus according to Claim 49 in which liquid carbon dioxide required to be gasified before being supplied to said casings acquires vaporisation heat in heat- exchange relationship with hot flue gases or heated water from solar ponds or solar absorbers.

54. A method of selectively absorbing into water carbon dioxide contained in large volumes of flue gases, said method comprising passing said flue gases in pressurised form through one or more long casings within which a large plurality of spray heads generate ultra-thin, fan-shaped sprays of water which rapidly and efficiently absorb carbon dioxide; trunks being provided to deliver said flue gases to said casings from compression means via optional cooling heat-exchange and rapid-acting diversion means; water feed conduits and supply lines being provided to deliver sorbent water to said spray heads from suitable pressurising means via optional cooling heat-exchange means; exhaust stacks being provided through which flue gases stripped of carbon dioxide are discharged; pressure relief valves being provided to control the discharge of said stripped flue gases to said exhaust stacks, thereby maintaining said gas pressure within said casings in a predetermined range; collection and delivery lines being provided to collect carbonated water from said long casings and deliver it to decompression wells; decompression wells being provided to create tranquil flow and permit a progressive reduction in pressure of said carbonated water, thereby minimising outgasing of carbon dioxide, and optionally to buffer the pH change resulting from carbonation; and water capture units optionally being provided to accept a flow of said stripped flue gases from said exhaust stacks and capture water therefrom.

55. The method of Claim 54 in which the length of said casings is such as to provide a residence time of said flue gases within said casings sufficient to ensure the absorption of a high proportion of their carbon dioxide content.

56. The method of Claim 54 in which the pressure of said flue gases within said casings is regulated to a pressure falling in the range 1.0 to 5.0 bar.

57. The method of Claim 54 in which the internal diameter of said casings is sufficient to ensure that said water sheets generated by said spray heads do not impact the internal surfaces of said casings before they disrupt into droplets.

58. The method of Claim 54 in which said flow of water supplied to said water feed conduits is preferably cooled to a temperature falling in the range 0° C to 20° C.

59. The method of Claim 54 in which the volume of water flowing through said spray heads of a said casing is sufficient to absorb the carbon dioxide content of said flue gases passing through said casing at the prevailing gas and water temperature and pressure.

60. The method of Claim 54 in which said spray heads are of the deflected fan type in which a stream of water discharged from an axially arranged jet orifice impinges on an angled surface and is deflected into a fan-shaped sheet.

61. The method of Claim 54 in which said spray heads are of the jet impingement atomiser type in which a stream of water discharged from an axially arranged jet orifice impinges upon a conical deflector surface to produce a sheet of water extending through 360°.

62. The method of Claim 54 in which the pressure of water fed to said spray heads falls in the range 80 to 300 kPa.

63. The method of Claim 54 in which the pressure drop across said spray heads is maintained at a level sufficient to cause the flow within said water sheets generated by said spray heads to be turbulent.

64. The method of Claim 54 in which said spray heads have discharge jets with a diameter falling in the range 1.5 to 5.0 millimetres.

65. The method of Claim 54 in which the water sheets generated by said spray heads have an angular width falling in the range 40° to 120°.

66. The method of Claim 54 in which the water sheets generated by said spray heads, immediately prior to their disruption into droplets have a length falling in the range 50 to 100 millimetres.

67. The method of Claim 54 in which said spray heads are fixed to said water feed conduits in circumferentially arranged arrays, spray heads in adjacent arrays being staggered such that the water sheets generated by spray heads in one array do not impact those generated by spray heads in another array.

68. The method of Claim 54 in which said water sheets generated by said spray heads are aligned more or less parallel with the longitudinal axes of said casings.

69. The method of Claim 54 in which said casings are inclined at a shallow angle to permit drainage of said carbonated water along them.

70. The method of Claim 54 in which a plurality of collection lines collect carbonated water draining from said casings and conduct it to delivery lines which pass down into one or more decompression wells to a point just above their lower floors.

71. The method of Claim 54 in which the diameters of said collection lines and said delivery lines are sufficiently large to easily accommodate the flow of water draining from said casings.

72. The method of Claim 54 in which the depth of said decompression wells is such that, when full of water, the pressure just above their lower floors is at least equal to the gas pressure maintained within said casings.

73. The method of Claim 54 in which said decompression wells are made with a diameter falling in the range 4 to 15 times that of said delivery lines.

74. The method of Claim 54 in which carbonated water delivered by said delivery lines to said decompression wells rises up slowly through said decompression wells, the tranquil flow and progressively decreasing pressure minimising the outgasing of carbon dioxide.

75. The method of Claim 54 in which the lower part of each said decompression well optionally accommodates a bed of buffering compound which acts to buffer acidity caused by dissolution of said carbon dioxide in said water.

76. The method of Claim 54 in which said bed of buffering compound has a depth falling in the range 0.5 to 3.0 metres.

77. The method of Claim 54 in which said buffering compound takes the form of calcium carbonate chips with a Sauter mean diameter falling in the range 5 to 15 millimetres.

78. The method of Claim 54 in which said buffering compound is another suitable buffering compound in suitable form.

79. The method of Claim 54 in which said pressure relief valves have sufficient flow capacity to accommodate the outflow of said stripped flue gases from said casings.

80. The method of Claim 54 in which the positions of said pressure relief valves are modulated in relation to their seats by suitable actuator means.

81. The method of Claim 54 in which said actuators modulating the positions of said pressure relief valves and thereby the gas pressure within said casings are controlled by one or more programmable logic controllers or other microprocessor-based control units in response to signals received from pressure sensors within said casings.

82. The method of Claim 54 in which said actuators modulating the positions of said pressure relief valves are controlled by a programmable logic controller or other microprocessor-based control unit in response to signals received from temperature sensors in said water feed conduits.

83. The method of Claim 54 in which said water capture units incorporate coalescers to intercept water mist and desiccator units to collect gaseous water entrained in said stripped flue gases.

84. The method of Claim 83 in which said desiccator units take the form of chiller units, chemical absorption units, solid desiccant wheel units or the like.

85. The method of Claim 54 in which said water capture units have sufficient flow capacity to accommodate the flow of stripped flue gases through said exhaust stacks.

86. The method of Claim 54 in which, where said flow of flue gases is taken off immediately downstream of a combustion source, cooling heat-exchange means are provided in said trunks to reduce the temperature of said flue gases to that required for efficient carbon dioxide absorption into water.

87. The method of Claim 54 in which said casings are supplied with a flow of more or less pure gaseous carbon dioxide.

88. The method of Claim 87 in which a carbon dioxide atmosphere at optimal pressure is maintained within said casings by suitable pressure regulation means.

89. The method of Claim 87 in which said casings are made with closed ends and with said pressure relief valves and said exhaust stacks deleted.

90. The method of Claim 87 in which liquid carbon dioxide required to be gasified before being supplied to said casings, acquires vaporisation heat in heat-exchange relationship with said incoming sorbent water, the lower water temperature so produced improving the solubility of said carbon dioxide.

91. The method of Claim 87 in which liquid carbon dioxide required to be gasified before being supplied to said casings acquires vaporisation heat in heat-exchange relationship with hot flue gases or hot water from solar ponds or solar absorbers.