Compression condensate conditioning in the flue gas condenser

The invention relates to a method of conditioning a condensate generated in the compression section of a gas purification unit. The invention also relates to system for this method.

FIELD OF THE INVENTION

The present invention relates to a method of conditioning compression condensate in a flue gas condenser. The invention relates also to the system for the conditioning of the compression condensate.

BACKGROUND

The invention relates to degassing of condensate obtained in the course of compressing a CO2rich gas stream and subsequent cooling in one or multiple stages. In previous known systems the condensate is returned directly to the waste water treatment system. Eventually necessary conditioning of the condensate has been performed directly in the pipework connecting the source and the treatment system.

For the current size of plants in operation the before described solution is adequate. However, for commercial size units an approach like the before described would impose a potential risk of asphyxiation to the operators due to accumulation of heavy gases like CO2in the sewer system and/or uncontrolled release at the waste water treatment plant. Therefore there is a need for a safer system for the conditioning and degassing of the condensate.

SUMMARY

An object of the present invention is to provide a method of removing gases, especially CO2gases from the condensate generated in a system of interstage coolers.

In one embodiment of the invention a method of conditioning the condensate generated in the compression section of a gas purification unit is provided. The method comprises the following steps:a) compressing the carbon dioxide rich flue gas from a gas cooling, condensing and/or cleaning device;b) cooling of the gas below the water dew point ;c) recirculating the condensate formed during the cooling in b) comprising carbon dioxide to the inlet of the lower end of the gas cooling, condensing and/or cleaning device;d) introducing the condensate of step c) to the gas cooling, condensing and/or cleaning device;e) degassing of the condensate whereby the carbon dioxide rich vapor is released into the vapor phase in the lower end of the gas cooling, condensing and/or cleaning device.

By the method of the invention it is possible to avoid that clouds and accumulation of CO2in the water treatment system is formed. Also other undesirable components, such as asphyxiating or toxic components in the sewer and water treatment system may be avoided by removing the CO2rich vapor phase from the condensate stream.

Further advantages achieved by the method is that a higher CO2 recovery of up to 0.3% can be achieved, than with conventional methods. Another operational advantage achieved is that smaller piping in the equipment is required due to that only a single phase is flowing in the system after the degassing.

In another embodiment of the invention a method of conditioning the condensate generated in the compression section of a gas purification unit as above is provided where the method also comprises post combustion CO2 capture purification. Examples of applicable post combustion CO2 capture purification systems are for example an amine based absorption, a chilled ammonia absorption or any other process requiring the cooling and partly water condensation of the flue gas prior to CO2 removal and compression.

In an embodiment the method also comprises a step d) introducing the condensate of step c) into a separate compartment arranged within the gas cooling, condensing and/or cleaning device, for vapor disengagement/release and optional conditioning (e.g. neutralization) of the condensate.

The water condensed, the compressed waste water can contain impurities in higher amounts, due to the increase of the partial pressure during compression or because of acid formation which promotes heavy metal leaching out of the CO2 rich flue gas. By this embodiment it is possible to treat the compressed waste water, that contains higher amounts of impurities of components like acids or heavy metals, separately, while an existing equipment may be used. Thus by the invention it is provided a method to save plot space for installation and lower investment costs for a new alternative equipment.

A further embodiment is a method wherein step d) comprisesd) introducing the condensate of step c) into the lower part of the gas cooling, condensing and/or cleaning device.
This embodiment is suitable in situations wherein the condensate is less polluted and it is possible to mix with the water/liquid of the lower part of the flue gas condenser.

Another embodiment of the invention is the method as above including the following steps: step c) recirculating the condensate of step b) into a vessel for releasing the carbon dioxide rich vapor and conditioning the condensate;d) forwarding the carbon dioxide rich vapor to the gas cooling, condensing and/or cleaning device.

An advantage of this embodiment is that it offers an easy way to upgrade the solution including the compressed waste water that contains high amounts of impurities like acids or heavy metals,

In one embodiment of the invention a system for conditioning the condensate generated in the compression section of a gas purification unit is provided. The gas cleaning system comprises a gas cooling, condensing and/or cleaning device, the so called flue gas condenser,being operative for receiving at least a portion of partly cleaned carbon dioxide rich flue gas;being operative for removing at least a portion of the water content of the partly cleaned carbon dioxide rich flue gas to condense water therefrom;means for compressing the carbon dioxide rich flue gas;means for condensing water vapor from the carbon dioxide rich flue gas;means for releasing parts of the carbon dioxide included in the water and returning it back to the flue gas.

In one embodiment of this invention the system also comprises a gas purification unit comprising a post combustion CO2 capture purification unit.

The post combustion CO2 capture purification unit may be an unit for amine based absorption. Another option would be that the post combustion CO2 capture purification is performed in a chilled ammonia system.

Another embodiment of the invention is a system for cleaning and/or conditioning a condensate as described above wherein the water comprising the carbon dioxide rich vapor phase is introduced into a separate compartment of the bottom of the flue gas condenser. The system may also comprise means for conditioning the remaining, degassed condensate. The conditioning means for example that the condensate is neutralized before further treatment.

The neutralization may be performed by conventional methods, like adding basic agents, for example caustic soda etc.

The advantage achieved by the embodiment above is that it is possible to use the existing system and by that also less piping is needed, and also less safety devices.

In one embodiment of the system for cleaning and/or conditioning a condensate as described above, the water comprising the carbon dioxide rich vapor phase is introduced into the bottom level of the flue gas condenser.

In a further embodiment the system for cleaning and/or conditioning the condensate, as described above, the system comprises:

means for forwarding the water comprising the carbon dioxide rich vapor phase to a vessel for separation of water and carbon dioxide rich vapor phase. Also means for conditioning the remaining, degassed condensate, for example by neutralization, may be included in the system. Further, the system may also include means for introducing the released carbon dioxide into the flue gas condenser.

An advantage with this embodiment is that the degassing of the condensate and flue gas condenser do not have to be located physically near to each other. Also the dimensions and lengths of the piping returning the vapor may be smaller and of cheaper material than piping equipment for the condensate.

DETAILED DESCRIPTION

FIG. 1is a schematic representation of a boiler system1, as seen from the side thereof. The boiler system1comprises, as main components, a boiler2, being in this embodiment an oxy-fuel boiler, a steam turbine, schematically indicated as4, a particulate removal device in the form of an electrostatic precipitator6, and a gas cleaning system8. The gas cleaning system8comprises, as its main components, a first gas cleaning device in the form of a wet scrubber10, and a second gas cleaning device in the form of a flue gas condenser12.

A fuel, such as coal or oil, is contained in a fuel storage14, and can be supplied to the boiler2via a supply pipe16. An oxygen gas source18is operative for providing oxygen gas in a manner which is known per se. The oxygen gas source18may be an air separation plant operative for separating oxygen gas from air, an oxygen separating membrane, a storage tank, or any other source for providing oxygen to the system1. A supply duct20is operative for forwarding the produced oxygen gas, comprising typically 90-99.9 vol. % oxygen, O2, to the boiler2. A duct22is operative for forwarding recirculated flue gas, which contains carbon dioxide, to the boiler2. As indicated inFIG. 1the supply duct20joins the duct22upstream of the boiler2, such that oxygen gas and recirculated flue gas, which contains carbon dioxide, may become mixed with each other to form a gas mixture containing typically about 20-50% by volume of oxygen gas, the balance being mainly carbon dioxide and water vapour, upstream of the boiler2. Since almost no air enters the boiler2there is almost no nitrogen gas supplied to the boiler2. In practical operation, less than 3 vol. % of the gas volume supplied to the boiler2is air, which mainly enters the boiler2as a leakage of air. The boiler2is operative for combusting the fuel, that is to be supplied via the supply pipe16, in the presence of the oxygen gas, mixed with the recirculated flue gas, which contains carbon dioxide, that is to be supplied via the duct22. A steam pipe24is operative for forwarding steam, that will be produced in the boiler2as a result of the combustion, to the steam turbine4, which is operative for generating power in the form of electric power. A duct26is operative for forwarding carbon dioxide rich flue gas generated in the boiler2to the electrostatic precipitator6. By “carbon dioxide rich flue gas” is meant that the flue gas leaving the boiler2via the duct26will contain at least 40% by volume of carbon dioxide, CO2. Often more than 50% by volume of the flue gas leaving the boiler2will be carbon dioxide. The balance of the “carbon dioxide rich flue gas” will be about 20-50% by volume of water vapour (H2O), 2-7% by volume of oxygen (O2), since a slight oxygen excess is often preferred in the boiler 2, and totally about 0-10% by volume of other gases, including mainly nitrogen (N2) and argon (Ar), since some leakage of air can seldom be completely avoided.

The electrostatic precipitator6, removes most of the dust particles from the carbon dioxide rich flue gas. As alternative to an electrostatic precipitator a fabric filter, may be utilized for removing the dust particles. A duct28is operative for forwarding the carbon dioxide rich flue gas from the electrostatic precipitator6to the wet scrubber10of the gas cleaning system8.

The wet scrubber10comprises a circulation pump30a slurry circulation pipe32, and a set of slurry nozzles34arranged in the wet scrubber10. The slurry nozzles34are operative for finely distributing slurry in the wet scrubber10and to achieve good contact between slurry and the flue gas being forwarded to the wet scrubber10.

An at least partly cleaned carbon dioxide rich flue gas leaves the wet scrubber10via a duct44which forwards the flue gas to a gas distribution point46. At the gas distribution point46, being located between the wet scrubber10and the condenser12, as seen with respect to the direction of the flow of the partly cleaned carbon dioxide rich flue gas, the partly cleaned carbon dioxide rich flue gas is divided into two portions, namely a first flow, which via the duct22is recirculated back to the boiler2, and a second flow, which via a duct48is forwarded to the condenser12. The condenser12is provided with a circulation pump50which is operative for circulating a cooling liquid, via a circulation pipe52, in the condenser12in a manner which will be described in more detail hereinafter.

The flue gas condenser12where the flue gas is cooled below its water dew point and the heat released by the resulting condensation is recovered as low temperature heat. The water content of the flue gas may for example be reduced from about 40% by volume in the flue gas fed to the flue gas condenser to about 5% by volume in the flue gas leaving the flue gas condenser. Depending on pH and temperature in the flue gas condenser, the flue gas condensation may also lead to a reduction of sulfur oxides, SOX, in the flue gas. The sulfur oxides are captured in the formed condensate and separated from the flue gas. Furthermore, wash liquid or slurry, e.g. lime slurry, entrained in the flue gas from the preceding sulfur dioxide removal step is removed during the condensation.

The cooling liquid being circulated in the condenser12cools the partly cleaned carbon dioxide rich flue gas to a temperature which is below its saturation temperature, with respect to water vapour, and, hence, causes a condensation of at least a portion of the water vapour content of the partly cleaned carbon dioxide rich flue gas being forwarded from the wet scrubber10. The condensed water leaves the condenser12via a disposal pipe54. A portion of the condensed water leaving the condenser12via the pipe54is forwarded to the wet scrubber10via a pipe56as make up water. A further portion of the condensed water is forwarded, via a pipe58, to a water treatment unit60, in which the condensed water is treated prior to being disposed. The cleaned carbon dioxide rich flue gas leaves the condenser12via a duct62and is forwarded to a gas processing unit (GPU)64in which the cleaned carbon dioxide rich flue gas is compressed followed by cryogenic CO2 separation.

In the CO2 separation system, CO2 is at least partially separated from the light gases (e.g. N2, Ar, O2) of the flue gas by compression and condensation. Compressed carbon dioxide hence leaves the CO2 separation system via a duct43and is transported away for further use or storage, which is sometimes referred to as “CO2 sequestration”.

The CO2 separation in the GPU is achieved by means of compression of the flue gas and condensation. The CO2 separation system for condensation of carbon dioxide (CO2) in a flue gas stream be implemented as shown in the boiler system ofFIG. 1.

The CO2 separation system64may optionally comprise at least one compressor44having at least one, and typically two to ten compression stages for compressing the carbon dioxide rich flue gas. The flue gas compressor is operative for compressing the flue gas to a pressure at which gaseous CO2 is converted to liquid form when the temperature of the flue gas is reduced to a temperature below −20°, preferably to a temperature of −51°, in the CO2 separation section of the GPU (not shown in detail). The carbon dioxide rich flue gas is generally compressed to a pressure of about 20 bar or higher, such as about 33 bar, in the multistage compressor. Each compression stage could be arranged as a separate unit. As an alternative several compression stages could be operated by a common drive shaft. The compressor44,44′,44″,44″ may also comprise a gas cooling unit70,70′,70″, respectively80and82, downstream of one or more of the compression stages. The gas cooling unit may further be configured to collect and dispose of any liquid condensate formed during compression and/or cooling of the carbon dioxide rich flue gas.

FIG. 2illustrates further the flue gas condenser12and the compressor44,44′,44″,44″′ may also comprise a gas cooling unit70,70′,70″. From each gas cooling unit is the liquid condensate is forwarded via a duct72,73,74, and collected in the duct78, to the compartment67placed in the lower part66of the gas cooling, condensing and/or cleaning device12. The inlet of78is placed below the inlet of duct48. An internal roof is installed above the separate compartment to prevent entrainment of cooling liquid from the flue gas condenser.

It is also possible to install level controllers (not shown) within the separate compartment67, and minimum and maximum levels of liquid to be determined. The liquid in the separate compartment67may be kept at different level than the liquid in compartment66.

The internal roof is preferably sloping to let the cooling liquid fall down into compartment66.

In the preferred embodiment the carbon dioxide rich flue gas obtained in the multistage compressor is forwarded via duct75to a first gas cooler.

The carbon dioxide rich flue gas is cooled optionally to a temperature of about 60 degrees by heat exchanger80, before the gas enters optionally an mercury absorber81. The temperature of the gas obtained after the mercury absorber81is kept 10 to 15° C. above the dew point temperature of the flue gas.

The carbon dioxide rich flue gas is then forwarded to a second gas cooler82where the temperature is lowered below the water dew point and liquid water is separated in a vessel83prior to further processing of the flue gas in the CO2 drying and separation system (not shown).

From the vessel83the waste water may be forwarded via the duct76to be combined in78and recirculated to the compartment67.

The condensate collected in the separate compartment67of the flue gas condenser for gas cooling, condensing and/or cleaning the condensate. The condensate is degassed whereby the carbon dioxide rich vapor is released into the vapor phase in the lower end (66) of the gas cooling, condensing and/or cleaning device (12). Then, optionally the conditioning of the condensate is performed, thus the condensate is neutralized, for example by treatment with caustic soda (sodium hydroxide (NaOH)).

The degassed and conditioned condensate obtained in the separate compartment67may be forwarded via duct54, and pump53, to waste water treatment according to conventional methods. The water may also be recirculated to the scrubber via duct56FIG. 1to the wet scrubber10, if the amount of pollutants acceptable for further use.

Optionally, the flue gas condenser12may be fed with flue gas from an conventional boiler system using air. The plant may also include an unit for post combustion CO2 capture purification94. From the flue gas condenser12the gas is forwarded to the unit94. The post combustion CO2 capture purification may be an amine based absorption system, or a chilled ammonia CO2 capture system. Also other conventional systems and processes may be applied for the post combustion CO2 capture purification.

Out of the then CO2 lean flue gas CO2 is separated and the concentrated CO2 sent to the compression unit44. The residual flue gas out of the post combustion unit94may be routed to stack.

FIG. 3illustrates an embodiment where the condensate is forwarded to the flue gas condenser via the duct78. This embodiment is suitable when the amount of contaminants and pollutants of the condensate is limited. The condensate is fed into the lower end of the flue gas condenser66. Also in this embodiment, an optional unit for post combustion CO2 capture purification94may be included as described above.

FIG. 4illustrates an embodiment where the condensate is recirculated to a vessel85wherein the condensate is degassed, i.e. carbon dioxide rich vapor is released, and forwarded to the flue gas condenser via duct78. Conditioning of the condensate is performed, for example by neutralization as described above before it is forwarded to further treatment via the pump53, and duct54, for example for waste water treatment.