PROCESS FOR THE SEPARATION OF PRESSURISED CARBON DIOXIDE

FROM STEAM

This invention relates to the separation of carbon dioxide gas from a pressurised stream of steam and carbon dioxide. Such a stream is typically produced by the combustion of a fuel gas with oxygen at elevated pressures and is used to generate shaft power, which may be used for the generation of electric power. Such pressurised stream of steam and carbon dioxide can contain particulate matter depending on the method of combustion. Such particulate matter is preferentially substantially removed using well known means such as filtration, by cyclone separation or, preferentially, liquid washing.

Often this carbon dioxide is separated from the steam. In the present invention the separation step is carried out at elevated pressure. By this means the separated carbon dioxide remains at elevated pressure and the sensible heat in the stream is recovered and used in the raising of steam for the further generation of shaft power which may be used for the generation of further electric power. This invention has particular utility in the case where the combustion gases are fed to a hot expander to generate shaft power.

To obviate confusion, there follow definitions of the terms "direct cooling" and "indirect cooling" as used in this document .

Indirect cooling means the transfer of heat from a stream at one temperature to a lower temperature stream without any mixing of the stream, such as through the medium of a metal wall separating the two streams in a heat exchanger .

Direct cooling means the transfer of heat from a stream at one temperature to a lower temperature stream through direct contact of the two streams in a vessel.

Traditional power generation from fossil feedstocks, such as coal, consists of the release of the fossil feedstock chemical energy in the form of heat through its oxidation or combustion with oxygen. The released heat is used to raise steam which may be expanded in an expander or steam turbine to generate shaft power. Because carbon dioxide is the principal non-condensable by-product of the oxidation of a fossil feedstock, this results in a major contribution of carbon dioxide emissions normally released to the atmosphere. Traditional power generation also uses atmospheric air as the normal source of oxygen for the oxidation or combustion process. This means that the very high concentration of nitrogen associated with oxygen in atmospheric air will be mixed with the byproduct carbon dioxide resulting from the oxidation or combustion process.

It may be required to remove or "capture" the CO ₂ by- product of such a power generation process in order to place it in a suitable storage system to obviate any undesirable effects of its release into the atmosphere.

In such a case, the use of atmospheric air for combustion and the resulting mixture of by-product gases makes the selective capture of carbon dioxide cumbersome .

It is well known that if high purity oxygen, that is oxygen produced by a cryogenic air separation unit such as that disclosed in US Patent No. 3,134,228, were substituted for atmospheric air, then the products of combustion will consist almost entirely of water vapour and carbon dioxide. This creates the possibility to condense out the water leaving the non-condensable carbon dioxide available for compression and transport, preferably by pipeline, to a storage site. Air separation units typically provide oxygen of from 95% up to 99.5% by volume purity, the contaminants consisting mainly of nitrogen together with a small proportion of argon. Here the higher the purity of the oxygen, the lower the proportion of these contaminants in the captured carbon dioxide .

Such a known process may be carried out either at atmospheric pressure or at elevated pressure.

For the atmospheric case, the heats of combustion and water condensation are recovered by indirect heat exchange to raise steam to be used to generate shaft power and to substantially condense by indirectly cooling with typically water or air all the steam, thus leaving the residual carbon dioxide to be compressed from sub- atmospheric pressure .

The elevated pressure alternative conventionally recovers the heats of combustion and water condensation by a combination of hot expansion of the products of combustion to generate shaft power plus indirect heat exchange to raise steam to be used to generate shaft power and to substantially condense by indirectly cooling with typically water or air all the steam, thus leaving the residual carbon dioxide to be compressed from sub- atmospheric pressure.

The hot expander device used to expand the products of combustion to generate power may require cooling for the mechanical parts of the expander. Hitherto such cooling has been provided either by steam or else by recycled carbon dioxide. In both cases the cooling medium must be sufficiently dry to avoid any condensation of water vapour in the expander, particularly if any acid gases are present .

In order to maximise heat recovery in both the atmospheric and elevated pressure cases, it is standard practice for the final condensation pressure for the steam and carbon dioxide to be taken as low as possible, and in any case at least below 1 bar absolute, through indirect heat exchange which means that the compression of the carbon dioxide from this low pressure is expensive in both energy and capital cost . The standard steam condenser is two phase as the steam is mixed with a high proportion of non-condensable carbon dioxide. Such two phase condensers are used in the recovery of heat from geologically sourced natural steam and are known as "geothermal condensers" . They are characterised by being larger and therefore more expensive than conventional

steam condensers of the same heat load because of the higher volume required to permit effective separation of the steam condensate from the non-condensable gas content .

Furthermore, any other water soluble non-condensing contaminants contained in the carbon dioxide, resulting from impurities in the fossil fuel, and particularly any oxides of sulphur, will be partially dissolved in the steam condensate.

When recycled carbon dioxide is used as a cooling medium for the hot expander, this is generally taken off after condensation which means that although the gas is dry it has to be compressed up to expander pressure from a relatively low pressure starting point.

In this invention, the condensation of the steam content of the steam/carbon dioxide mixture is effected at an elevated pressure above 1 bar absolute, and preferably between 4 and 15 bars absolute, by heat exchange with a stream of water from which the heat content of the steam is recovered for use in the raising of steam. By using this pressurised condensation system a side stream of the steam/carbon dioxide mixture can be taken off - still at an elevated pressure and before condensation - to act as a cooling medium for the hot expander without the risk of, particularly, acid gas condensation and with a much reduced requirement for compression.

Preferably direct cooling using a counter-current stream of circulating water is used. This cooling is effected sufficiently to condense out essentially all, i.e. as

much as is practically possible, viz. usually at least 90% by volume, of the steam present in the stream. This leaves a separated carbon dioxide stream still at the condensing pressure of at least 1 bar absolute. Any non- water soluble non-condensing contaminants such as nitrogen can be left in the carbon dioxide stream as a diluent or separated out if desired.

The advantages of this invention are:

1. The carbon dioxide and other non-condensing gases are available at a pressure of at least 1 bar absolute and preferably between 4 and 15 bar absolute.

This means that the compression energy required to compress the carbon dioxide and other non-condensing gases to a pressure sufficient for their transportation to a suitable storage site is significantly reduced below that required if the carbon dioxide and other non-condensing gases were made available for compression only at pressures at or below 1 bar absolute.

2. Some of the steam/carbon dioxide mixture can be extracted before cooling down to the water condensation point, compressed, and recycled for use as a cooling medium for the hot expander used to generate power, as an alternative to steam or recycled carbon dioxide .

3. The carbon dioxide is separated from the steam/carbon dioxide stream in a simple and non- expensive system.

4. The heat of condensation of the steam is recovered at a higher temperature and at a relatively low cost compared to the case when two phase condensers operating at pressures at or below 1 bar absolute are used.

5. Any superheat content, that is, the heat content above condensation of the steam and carbon dioxide stream can be recovered by indirect heat exchange down to the saturation/condensation point.

6. Condensation of the stream above 1 bar absolute means that the condensation and separation occurs at a higher temperature than it would be at or below 1 bar absolute. This means that a much higher proportion of sulphur oxides in the steam/carbon dioxide stream resulting from the combustion of sulphur impurities in the fossil feedstock will be present in the non-condensing part of the stream compressed for transportation to storage. If desired, this can be removed from the compressed non-condensing stream using means of scrubbing with a suitable solvent .

The present invention will now be described by way of examples with reference to the accompanying drawing of a schematic flow sheet. This flow sheet shows the carbon dioxide/steam separation section of a power generation process in which a fossil feedstock is oxidised to form a pressurised stream of steam and carbon dioxide which is used to produce electric power via shaft power.

In the specific embodiment of the invention shown in Figure 1 103978 kg.mols of a mixture (1) coming from a hot expander (not shown) and containing more than 80% by volume of steam and 20% by volume of non-condensable gases, which gases are more than 90% by volume CO ₂ plus nitrogen, sulphur dioxide, argon and oxygen, at a pressure of 11 bar absolute and a temperature of 289°C is cooled to its water saturation point of 160 ⁰ C in heat exchanger (10) and then enters packed vessel (11) below the packing. The steam/gas mixture passes up the packing and is cooled by a 122195 kg.mols counter-flow stream of water (5) which enters vessel (11) at 45°C above the packing. 17302 kg.mols of separated and cooled CO ₂ (2) leave vessel (11) at 11 bar absolute pressure at a temperature of 47 ⁰ C.

Stream (5) is taken from the bottom of vessel (11) at 154°C, fed to pump (12) , and then cooled in heat exchanger (13) to 45°C and circulated to the top of vessel (11) to enter above the packing. 79411 kg.mols of surplus water (6) are taken from the water circuit for recycle to the front end of the overall process.

74299 kg.mols of boiler feedwater (4) at 20°C is heated in heat exchangers (13) and (10) to produce saturated steam (3) at 5 bar absolute and 152°C. This steam is then taken off to be used for power generation.

The CO ₂ stream (2) is scrubbed by water to remove oxides of sulphur in packed vessel (14) leaving the resulting cleaned CO2 stream still at a pressure of greater than 1 bar absolute. Pump (15) circulates the water stream (7) from the bottom of vessel (14) to the top with a

concentrate of dissolved acid gases being taken off as stream (8) . The water balance of the circuit is maintained with water stream (9) taken from water stream (6) at 45°C.

Because the mixture (1) is at pressure a side stream can be taken off before heat exchanger (10) and recycled after compression to the hot expander (not shown) from which the stream (1) came to act as a cooling medium for the mechanical parts thereof .