# Patent

## Title
Ammonia capturing by CO2 product liquid in water wash liquid

## Abstract

A method for capturing ammonia present in combustion flue gas subjected to carbon dioxide removal using a water wash unit included in a chilled ammonia process. The method includes combining a CO2 loaded liquid and a wash water liquid to form a CO2 enriched wash water liquid that is then brought into contact with the combustion flue gas.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims priority to and is a divisional application of U.S. application Ser. No. 13/357,963 filed Jan. 25, 2012 the contents of which are hereby incorporated in its entirety.


TECHNICAL FIELD
The present invention relates to a method for treating a combustion flue gas. More specifically it relates to capturing ammonia in a chilled ammonia process (CAP).
BACKGROUND
Liquid solutions comprising amine compounds or aqueous ammonia solutions are commonly used as solvents in processes used for industrial separation of acidic components such as H2S, CO2, COS and/or mercaptans from gas streams such as flue gas, natural gas, synthetic gas or other gas streams mainly containing nitrogen, oxygen, hydrogen, carbon monoxide and/or methane. The acidic components are often absorbed in the solvent in an absorption process or scrubbing process. After “scrubbing” of said acidic components by said solutions, contaminants, such as traces of ammonia, have to be removed from the gas stream in a separate process step.
The most commonly used process for this purpose is a wash or scrubbing step of the contaminants. In such a wash water step, the gas stream is scrubbed with water in a suitable contacting device. Typically, the water used to scrub the gas stream is either fresh water or very low NH3 content water obtained from a stripping process related to the treatment of the gas stream. After the gas stream is scrubbed with water, the water is 1) sent back to the stripping unit from which it was obtained or 2) simply mixed with the solution used in the main scrubbing process.
There are methods known wherein the efficiency of the system and methods are improved. In WO 2009/138363 it is disclosed a method for removal of contaminants from a gas stream by contacting the gas stream with CO2 containing liquid. The methods are said to be applicable for contaminants like ammonia, where the emission of the contaminants is reduced. Also in U.S. Pat. No. 5,378,442 there is described a method to contact CO2 containing liquid for recovering of ammonia present in the combustion exhaust gas.
Regeneration of used wash liquids in the scrubbing process, for example in a stripping unit, is generally energy intensive and by that an expensive process. Therefore, there is a constant need for processes that improve wash efficiency and/or reduce wash liquid consumption. Regeneration of used wash liquids may be accomplished via stripping where a particular component is stripped from a wash liquid to regenerate the wash liquid.
SUMMARY
It is an object of the present invention to improve the efficiency of a wash/scrubbing step in a gas purification process, more specifically, to improve the capture and recovery of ammonia from a treated combustion gas in an absorber system.
The improved method and system for capturing ammonia in a chilled ammonia process (CAP) according to various aspects described herein, ultimately allows a reduction in the concentration of ammonia exiting the wash/scrubbing step and thus increases the quantity of recycle ammonia back the absorber system. This helps to retain the concentration of ammonia in solution in the absorber system and also to prevent excessive ammonia losses.
Reducing ammonia emission in the treated flue gas flowing from the water wash unit supports retention of the ammonia in the chilled ammonia process. It will also reduce the amount of sulfuric acid needed to neutralize ammonia when reheating the treated flue gas in a downstream process.
According to aspects illustrated herein, there is provided a method for capturing ammonia present in combustion flue gas having been subjected to carbon dioxide removal in a water wash unit included in a chilled ammonia process, comprising the steps of:

    
    
        providing CO2 loaded liquid comprising CO2 dissolved in the liquid;
        providing wash water liquid;
        combining the CO2 loaded liquid with the wash water liquid to form CO2 enriched wash water liquid before the liquid is added to the water wash unit to suppress the equilibrium vapor pressure of NH3 present over the surface of the CO2 enriched wash water liquid; and
        bringing said combustion flue gas into contact with said CO2 enriched wash water liquid by adding the CO2 enriched wash water liquid to said water wash unit.
    
    


The CO2 loaded liquid from, for example, a CO2 cooler is continuously added to the wash water to maintain a low ammonia partial pressure The amount of said liquid can be adjusted, reduced or increased, based on ammonia emissions from the water wash system, and the required ammonia partial pressure in the solution, in order to meet washing requirements.
According to some embodiments of the method, the concentration of ammonia in the wash water may be in the range of 0.0005-3 mol/liter. In a water wash unit with a top stage and a lower stage, the concentration of ammonia may, for example, be about 0.005 to 0.2 mol/l in the top stage, and about 0.5 to 3 mol/l in the lower stage. This concentration covers the range for both lean wash water and wash water mixed with the CO2 loaded liquid. By operating with this concentration of ammonia in the wash water, the vapor pressure of the ammonia can be kept at low level, e.g., low enough to wash ammonia in the gas phase down to less than 200 ppm. In general, ammonia capture can be improved (and the partial pressure of NH3 kept low) by lowering the concentration of NH3 in the wash water solution, by lowering the operating temperature of the wash liquid and/or chemically depressing the partial pressure of ammonia via the mixing of CO2 loaded liquid streams. As long as the partial pressure of CO2 over the said liquid is high, and solids are not formed, the concentration of NH3 is of less importance.
According to some embodiments of the method, the ratio of moles of ammonia (NH3) to moles of carbon dioxide (CO2) (the R value) for the CO2 enriched wash water liquid is kept at about 0.05 to 10, preferably at about 0.1 to 5, more preferably at about 1.
According to some embodiments of the method, the concentration of ammonia in the wash water is in the range of 0.0005-3 mol/liter, preferably in the range of 0.05-2 mol/liter, and a partial pressure of CO2 in the liquid phase between 1 and 20 bar.
According to some embodiments of the method, the wash water liquid used for ammonia removal comprises about 0.0005 mol/liter to 0.2 mol/liter ammonia (NH3) before it is combined with the CO2 loaded liquid.
According to some embodiments of the method, the operating temperature of the wash water unit is about 1° C. to about 10° C.; preferably about 5° C.
By performing the method for recapturing ammonia in these specified temperature ranges the vapor pressure of ammonia may be kept low. Any refrigerant can be considered as working medium as long as these operating temperatures can be achieved. Suitable refrigerants may be propane, propylene as well as ammonia.
According to some embodiments of the method, the ratio of moles of ammonia (NH3) to moles of carbon dioxide (CO2), also denoted as the R value, is kept at about 0.05 to 10 for the CO2 enriched wash water liquid, preferably at about 0.1 to 5, more preferably about 1 to 4. The lower R value of the water wash liquid the better results of the ammonia capture.
According to aspects illustrated herein, there is provided a gas purification system for capturing ammonia (NH3) from combustion flue gas by bringing said gas into contact with CO2 enriched wash water liquid containing dissolved carbon dioxide CO2 in liquid form wherein the system comprises:

    
    
        a water wash unit for capturing ammonia NH3,
        one or more wash water liquid ducts for recirculating wash water liquid;
        one or more units generating CO2 loaded liquid;
        a CO2 loaded liquid duct transporting the CO2 loaded liquid to the wash water liquid duct from the one or more units for generating CO2 loaded liquid to suppress the equilibrium vapor pressure of NH3 over the wash water liquid; and
        one or more CO2 enriched wash water liquid ducts transporting the CO2 enriched wash water liquid resulting after integrating the CO2 loaded liquid and the wash water liquid to the water wash unit for bringing the CO2 enriched wash water liquid into contact with the combustion flue gas.
    
    


According to some embodiments of the gas purification system, the units for generating CO2 loaded liquid is a CO2 product cooler and/or a CO2 compressor system, working separately or together to generate CO2 loaded liquid.
According to aspects illustrated herein, there is provided a gas purification system for capturing ammonia (NH3) from combustion exhaust gas by a wash water unit comprising at least one packed bed section, preferably two or more packed bed sections.
The water wash unit may be a suitable container, like a column. The packed bed may be selected to provide a sufficient mass transfer of the components present in the water wash unit, thus to absorb the NH3 from the combustion exhaust gas. The water wash unit may comprise one or more packed beds, being the same or different, and arranged in different ways.
According to some embodiments of the gas purification system the CO2 enriched wash water liquid is introduced to the bottom section of the wash water unit by the CO2 enriched wash water liquid duct.
The integration of CO2 loaded liquid from the CO2 product cooler and/or the CO2 product compressor can be introduced to either water wash top section or water wash bottom section or in some cases in both sections of the water wash unit. Preferably it should be introduced in the top section to achieve better performance.
According to some embodiments of the gas purification system, the water liquid being subjected to ammonia capturing comprises less than 0.2 mol/l ammonia (NH3).
According to some embodiments of the gas purification system described above, water wash unit is operated at a temperature of about 1° C. to about 10° C.; preferably about 5° C. The operating temperature of the system is dependent on the particular refrigerant used in the system. Suitable refrigerants may be propane, propylene, as well as ammonia.
According to some embodiments of the gas purification system, carbon dioxide CO2 in liquid form is reintroduced into the wash water liquid after separation and liquefaction in a CO2 product cooler unit.
According to some embodiments of the gas purification system, the carbon dioxide CO2 in liquid form is reintroduced into the wash water stream after separation and liquefaction in a CO2 product cooler unit forming a CO2 cooler CO2 loaded liquid.
According to some embodiments of the gas purification system, the carbon dioxide CO2 in liquid form is reintroduced into the wash water stream after separation and liquefaction in a CO2 compressor system forming an interstage cooler CO2 rich condensate.
According to some embodiments of the gas purification system, the carbon dioxide CO2 in liquid form is reintroduced into the water wash unit after separation and liquefaction in a CO2 product cooler unit in combination with a CO2 compressor system.
The term “wash water”, as used herein, refers generally to an aqueous medium used for removal of contaminants from a gas stream by bringing said gas stream into contact with said wash water, resulting in the absorption of contaminants from said gas stream into said wash water. The wash water containing the absorbed contaminants is generally recycled, e.g., in a stripping unit, where the contaminants may be concentrated for incineration or purification and reuse. In other words, the economics of the water wash step are dictated by the amount of wash water needed to reach the required removal levels of trace contaminants. The amount of wash water needed to properly scrub the gas stream is dictated by the absorption capacity of the water for the respective trace contaminants, i.e. the vapor/liquid equilibrium between the contaminant in the gas phase and in the water phase.
Alternatively, the improved absorption capacity of the wash water may be used to further reduce the amount of contaminants present in the gas stream leaving the water wash step, without increasing wash water consumption. In other words, emissions can be reduced without a corresponding increase in costs due to increased water and energy consumption.
The use of liquid CO2 to improve the absorption capacity of wash water is further advantageous because, e.g., i) CO2 is odorless and relatively non-toxic, ii) any CO2 remaining in the wash water after use may easily be removed during the regeneration of the wash water, and iii) CO2 may, in at least some embodiments of the present invention, be readily available as a product from another process step.
Alkaline compounds are often used in absorption processes for removal of acidic gases, such as CO2, H2S from gas streams. Ammonia is one example of such alkaline compound, and the chilled ammonia process (CAP) is a method for this. The gas purification method of the present invention is efficient for the removal of ammonia contaminating the gas stream from use in the chilled ammonia process. By the invention, a gas purification system for the improved method is provided.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a flow diagram generally depicting an embodiment of an ammonia based gas purification system according to the present invention.
FIG. 2 is a flow diagram generally depicting a known ammonia based gas purification system (prior art).



DETAILED DESCRIPTION
Specific embodiments of gas purification systems of the prior art and of the present invention are described in detail hereinbelow with reference to the drawings.
FIG. 1 is a schematic representation of an embodiment of an ammonia based gas purification system 101 according to the present invention. The gas purification system 101 comprises a water wash unit 102 arranged to allow contact between a gas stream to be purified and one or more wash liquids.
In accordance with one embodiment, the water wash unit 102 is arranged for cleaning a flue gas that has passed through a CO2 absorber 140 of a chilled ammonia process. The chilled ammonia process is, as such, described in, for example, WO 2006/022885 (Eli GAL). Hence, the CO2 absorber 140 may, for example, be arranged for capturing CO2 from a flue gas of, for example, a power plant, an industrial plant, a waste incineration plant or a metallurgical plant, in accordance with the chilled ammonia process. In the chilled ammonia process CO2 is captured in an ammoniated solution in the absorber 140, and the ammoniated solution is regenerated in a regenerator unit 142. Such regeneration involves heating the ammoniated solution to cause a release of CO2. For reasons of maintaining clarity of illustration FIG. 1 does not illustrate the flows of ammoniated solution between the CO2 absorber 140 and the regenerator unit 142, or the flow of flue gas through the absorber 140.
Flue gas that has passed through the CO2 absorber 140 for carbon dioxide capture contains ammonia and is forwarded to water wash unit 102 via a duct 107a for washing, as will be described in more detail hereinafter.
CO2 product that is released as an effect of the heating of the ammoniated solution in the regenerator unit 142 is forwarded via a fluidly connected duct 142a from regenerator unit 142 to a CO2 product cooler unit 120. The CO2 product cooler unit 120 purifies the CO2 product forwarded from regenerator unit 142 by capturing ammonia and condensing water vapor from the CO2 product. A liquid that contains water is circulated, via fluidly connected loop duct 121, in the CO2 product cooler unit 120. The liquid circulated in loop duct 121 is cooled in heat exchanger 121a to cause condensation of water vapor from the CO2 product. The liquid circulating in loop duct 121 of CO2 product cooler unit 120 will capture ammonia and also some CO2 from the CO2 product of the regenerator unit 142. Hence, the liquid circulating in loop duct 121 will contain some dissolved ammonia, and some dissolved CO2.
As will be described in more detail hereinafter, regenerated wash water, having a reduced content of ammonia, is forwarded to CO2 product cooler unit 120 via duct 111, and a portion of the liquid circulated in CO2 product cooler unit 120 is forwarded from the unit 120 via duct 122 fluidly connected to loop duct 121.
The water wash unit 102 is a mass transfer unit, which may comprise mass transfer enhancing arrangements, for example the water wash unit 102 may comprise a column with a packed bed wherein the packing material is selected to optimize the mass transfer in the unit 102. The packing material may be selected from many different suitable and commercially available packing materials. Also, the water wash unit 102 may be arranged to comprise one, two or more stages of washing, wherein the material forming the packed bed in each stage may be the same or different, and the arrangements, such as, for example, random or structured packaging, may be the same or different to optimize parameters such as surface area, flow pattern, mass flow, etc. The liquid flow through the unit 102 may also be arranged differently between the different stages, to optimize the system and/or mass transfer. For example, the liquid flow may be in counter current mode, with the liquid flowing in the opposite direction of the gas, with the gas flowing vertically upwards and the liquid flowing vertically downwards, or in co-current mode, with both the liquid and the gas flowing vertically down-wards. Furthermore, the liquid could either be arranged, for each of the stages, in a circulation mode, with the liquid being recirculated several times in the stage before being removed therefrom, or in a once through arrangement, in which the liquid passes once through the stage and is then removed therefrom.
In the specific embodiment of FIG. 1, the water wash unit is a water wash unit 102 that comprises a two stage wash system having sections with different packing. The bottom section 103, i.e., the lower part of the water wash unit 102, comprises structured packing and is operated in counter current mode and with circulation mode with respect to the liquid solution, and with once through mode with respect to the flue gas. The top section 104, i.e., the second section of the water wash unit 102, comprises random packing, and is operated in counter current mode with once through water flow and once through flue gas flow. Flue gas to be cleaned enters the water wash unit 102 via duct 107a. Cleaned flue gas leaves the water wash unit 102 via duct 107b. 
The used wash liquid leaving the water wash unit 102 contains absorbed ammonia and leaves the water wash unit 102 via fluidly connected duct 108. The used wash liquid may be at least partly recirculated and reintroduced to the water wash unit 102 and its lower part 103 via fluidly connected duct 105.
An option of the invention is that a portion of CO2 may be introduced to the wash liquid in duct 105, via fluidly connected duct 125, and CO2 containing wash liquid is thus introduced to the water wash unit 102 at the bottom (first) section 103 of the unit 102. In combination with, or as alternative to, introducing a portion of CO2 to the wash liquid in duct 105, and as will also be described in more detail hereinafter, a portion of CO2 may be introduced to the wash liquid in duct 106, via fluidly connected duct 122, and CO2 containing wash liquid is thus introduced to the water wash unit 102 at the upper (second) section 104 of the unit 102.
The liquid introduced to the water wash unit 102, via duct 105 and/or duct 106, is denoted ‘CO2 enriched wash water liquid’, which is the wash water resulting after the mixing of wash water liquid with the portion of CO2. The portion of CO2 may, as illustrated in FIG. 1, be CO2 that has been captured in the liquid of the CO2 product cooler unit 120 from the CO2 product forwarded from the regenerator 142. Such liquid containing a portion of CO2 dissolved therein is forwarded from CO2 product cooler unit 120 to water wash unit 102 via fluidly connected duct 122, and, optionally, via fluidly connected duct 125. The dissolved CO2 forwarded to the water wash unit 102 via duct 122, and optionally duct 125, serves to improve the capture of ammonia in the water wash unit 102 by reducing the vapor pressure of ammonia, as will be described in more detail hereinafter.
The content of ammonia in the flue gas entering the water wash unit 102 via duct 107a may be about 5000-16000 ppm.
Flue gas with a reduced content of ammonia leaves the water wash unit 102 via fluidly connected duct 107b and is, for example, forwarded to a direct contact cooler (DCC) unit, not shown for reasons of maintaining clarity of illustration. The amount of ammonia in the flue gas leaving the water wash unit 102 via duct 107b may be about 0-500 ppm, preferably less than 200 ppm.
A portion, which may be referred to as “spent wash water”, of the wash water liquid leaving the water wash unit 102 via duct 108 may be fed to a heat exchanger 110 via fluidly connected duct 112. In the heat exchanger 110 the spent wash water coming from water wash unit 102 via ducts 108, 112 exchanges heat with a flow of regenerated wash water coming from a stripper unit 130 via a fluidly connected duct 132. The spent wash water coming from water wash unit 102 is, hence, forwarded to heat exchanger 110 via duct 112 and leaves heat exchanger 110 via fluidly connected duct 131. Fluidly connected duct 131 forwards the spent wash water to the stripper unit 130. Typically, the spent wash water forwarded to stripper unit 130 via fluidly connected duct 131 may comprise ammonia in a concentration in the range of 0.5-3 mol/liter. In stripper unit 130 at least a portion of the content of ammonia of the spent wash water is removed, thereby generating, as will be described in more detail hereinafter, a regenerated wash water, that leaves stripper unit 130 via the fluidly connected duct 132. Typically, the regenerated wash water leaving stripper unit 130 via fluidly connected duct 132 may comprise ammonia in a concentration in the range of 0.005-0.2 mol/liter.
The regenerated wash water is forwarded via duct 132 to the heat exchanger 110 in which the regenerated wash water is heat exchanged with the spent wash water transported in ducts 112, 131. The regenerated wash water forwarded via duct 132 has a higher temperature than the spent wash water forwarded via duct 112. Hence, in heat exchanger 110 the spent wash water is heated before being forwarded, via fluidly connected duct 131, to the stripper unit 130. Such reduces the amount of heat that must be supplied to stripper unit 130 to achieve the stripping of ammonia from the spent wash water. The regenerated wash water forwarded from stripper unit 130 via fluidly connected duct 132 is cooled in the heat exchanger 110 before being forwarded, via fluidly connected duct 138a, to fluidly connected duct 138 and further, optionally via heat exchanger 124, to the upper section 104 of the water wash unit 102, and via fluidly connected duct 111 to the CO2 product cooler unit 120.
Regenerated wash water is, hence, forwarded from the heat exchanger 110 to the CO2 product cooler unit 120 via fluidly connected ducts 138a, 111. The flow rate of the water flow to the CO2 product cooler unit 120 is typically about 5 l/min to 300 l/min, for example about 5 l/min to 200 l/min. In the CO2 product cooler unit 120, CO2 containing water is recirculated into the CO2 cooler unit 120 by fluidly connected loop duct 121. From the duct 121, a part of the CO2 containing water is split and water is transported to the water wash unit 102 via fluidly connected duct 122, with a flow rate of about 5 l/min to 300 l/min. The liquid forwarded in duct 122 may also be denoted ‘CO2 loaded liquid’, i.e., liquid comprising the dissolved CO2 and forwarded from the CO2 cooler unit 120.
In one embodiment, the duct 122 is connected to the recycling loop, duct 108 of the bottom section 103, via fluidly connected duct 125, wherein the CO2 containing water from the CO2 product cooler unit 120 is mixed with the water reintroduced via duct 105 after passing the heat exchanger 123, into the bottom, first section 103 of the water wash unit 102.
In one embodiment of the invention, the duct 122 is fluidly connected to the duct 138, wherein the CO2 containing water is mixed with the regenerated wash water forwarded from the heat exchanger 110, and further forwarded via duct 106, to the water wash unit 102 and its top section 104.
From the CO2 product cooler unit 120 cooled CO2 product is forwarded via a duct 126 and an optional heat exchanger 127, to a CO2 compressor system 150 generating a compressed CO2 rich gas transported via fluidly connected duct 151 for further processing. The condensate, comprising water and CO2, obtained in the CO2 compressor system 150 as an effect of intercooling between compression stages may be recycled to the gas purification system 101 via fluidly connected duct 152. The liquid is herein denoted ‘CO2 compressor interstage cooler CO2 rich condensate’. The duct 152 is fluidly connected to the duct 122 and the ‘CO2 compressor interstage cooler CO2 rich condensate’ is forwarded to the water wash unit 102 as described above.
Optionally, in the gas purification system 101 the carbon dioxide CO2 in liquid form is reintroduced into the water wash unit 102 via fluidly connected ducts 154 and 152 after separation and liquefaction in a CO2 product cooler unit 155, which may be a cryogenic unit for separating carbon dioxide from non-condensable gases, such as oxygen and nitrogen, such unit 155 being included in a high pressure CO2 compressor system 153.
In one embodiment, the CO2 containing liquid is generated by combining the CO2 cooler loaded wash water solution forwarded via duct 121 to duct 122 and the CO2 compressor interstage cooler CO2 rich condensate forwarded via duct 152.
Optionally, the CO2 containing water passes through heat exchanger units 124a, 124b before entering the water wash unit 102 at a temperature of about 3 to about 7° C.
The heat exchanger unit 110 is fluidly connected to the stripper unit 130, via fluidly connected ducts 131 and 132, wherein heat is transferred from the stripper bottom stream to the feed stream to minimize energy consumption in the stripper unit 130, as well as to provide low temperature liquid to the water wash unit 102 to reduce chiller load. For example, the stripper unit 130 may operate at a temperature of more than 120° C. and with a pressure of more than 20 bar. The stripper unit 130 is heated by steam via fluidly connected ducts 136 and 137. In the stripper unit 130 ammonia is removed from the spent wash water coming from the water wash unit 102 via duct 131 and the ammonia is, via fluidly connected duct 135, transferred to the CO2 absorber 140 for further treatment, such as capturing CO2. The gas containing ammonia and leaving the stripper unit 130 via a duct 133 passes a condenser 134 on its way to the regenerator or absorber system depending on stripper operating pressure. A cooling liquid is forwarded to condenser 134 via a fluidly connected duct 134a, and leaves the condenser 134 via fluidly connected duct 134b. The cooling liquid forwarded through condenser 134 via ducts 134a, 134b could be of various origins. For example, the cooling liquid could be ammoniated solution forwarded from absorber 140 to regenerator unit 142 for being regenerated therein. The cooling liquid of condenser 134 could also, for example, be feed water for a boiler, or another cooling water available in the plant. Vapor and liquid formed in the condenser 134 as an effect of the cooling of the gas leaving stripper unit 130 via duct 133 leave condenser 134 via fluidly connected duct 133a and are forwarded to a vapor-liquid separator 135a. In vapor-liquid separator 135a gas and liquid are separated from each other. The liquid collected at the bottom of the vapor-liquid separator 135a is returned, via fluidly connected duct 135b, to the stripper unit 130. In low-pressure stripper operation, the overhead vapor stream is then transferred to the absorber 140 via duct 135.
The systems described in detail above operate at a pressure of 20 bar. However, it shall be considered obvious that the systems are also applicable for operation at a lower pressure, in an arrangement where the available parameters have been adjusted for achieving the NH3 capturing effect as is intended.
The gas entering the water wash unit 102 via the duct 107a comprises typically CO2 in a concentration of 1.5-2.5% by volume.
The water wash unit 102 is typically operating at relatively high gas velocities, such as in the range of 2-8 m/s, for example about 2.5 m/s.
By introducing a portion of CO2, via a CO2 containing liquid, into the water wash unit 102, the mole ratio between the moles of ammonia to the moles of CO2 may be lowered. Such lowering of the mole ratio between the moles of ammonia to the moles of CO2 suppresses the equilibrium vapor pressure of NH3 present over the surface of the CO2 enriched wash water liquid utilized in the water wash unit 102. In the top section 104 of the water wash unit 102, the concentration of ammonia of the CO2 enriched wash water liquid, forwarded via duct 106, may typically be 0.005 to 0.2 mol/liter of NH3. The ratio of moles of ammonia (NH3) to moles of carbon dioxide (CO2) for the CO2 enriched wash water liquid forwarded via duct 106 may typically be kept at about 0.05 to 10, and more typically at about 0.05 to 2. In the bottom section 103 of the water wash unit 102, the concentration of ammonia of the CO2 enriched wash water liquid, forwarded via duct 105, may be 0.5 to 3 mol/liter of NH3. The ratio of moles of ammonia (NH3) to moles of carbon dioxide (CO2) for the CO2 enriched wash water liquid forwarded via duct 105 may typically be kept at about 0.05 to 10, and more typically at about 0.5 to 10.
The CO2 product cooler unit 120 is also connected to the regenerator unit 142, the regenerator unit 142 being arranged for regenerating absorption liquid that has been utilized in the absorber 140 for absorbing CO2 from, for example, flue gas in accordance with the chilled ammonia process. Hence, the CO2 product cooler unit 120 cools CO2 that has been released from the ammoniated solution in the regenerator unit 142.
FIG. 2 is a schematic representation of a previously used gas purification system 201 (prior art). The system comprises a water wash unit 202 arranged to allow contact between a gas stream to be purified and one or more wash liquids.
The water wash unit 202 is represented in FIG. 2 and comprises a two stage wash system having sections with different packing. The bottom section 203 in the lower part of the water wash unit 202 comprises a structured packed bed and is operated in circulation mode for the solution and with once through mode for the flue gas. The top section 204 in the top part of the water wash unit 202 comprises a random packed bed operating in counter current mode with once through water flow and once through flue gas flow.
The used wash water liquid leaving the water wash unit 202 and containing absorbed ammonia leave the water wash unit via fluidly connected duct 208. The used wash water liquid may be recycled and reintroduced to the water wash unit 202 and its lower part via duct 205.
Flue gas having a reduced concentration of ammonia leaves the water wash unit 202 via duct 207 and may be forwarded to a Direct Contact Cooler (DCC) unit, not illustrated for reasons of maintaining clarity of illustration.
The wash water is fed to the heat exchanger unit 210 via duct 212. Water is forwarded from the heat exchanger unit 210 to the CO2 product cooler unit 220 via the duct 211.
Advantages of embodiments described hereinabove in connection with FIG. 1 include:
Low concentration of NH3 in the treated flue gas discharged from the water wash unit 102;
Low consumption of acidifying components, like sulfuric acid, following treatment, such as in the direct contact cooling system (DCC) and direct contact heating (DCH) system;
Maintainability of the desired solution molarity in the systems for absorption and regeneration;
Lower energy consumption of the stripper process;
Minimizing of amount of liquid required in the water wash unit 102 to capture ammonia.
EXAMPLES
Example 1 (Verification of Computer Model)
A computer model with a simulated water wash unit (A) in accordance with the above described prior art system (FIG. 2) was compared with test results (B) for a similar prior art system.
The simulation results showed 2.3% lower ammonia emission compared to the test results, as shown in Table 1. Hence, the computer model was considered a reasonable representation of a physical process and system.







TABLE 1







Comparison: Computer model to test result (prior art system)














Inlet gas to







water wash
After bottom
After top


Case

unit 202
stage 203
stage 204
Unit





A (model)
NH3 in
8897
2404
312 (in duct
ppm



gas


207)


B (test)
NH3 in
8897
Not available
319 (in duct
ppm



gas


207)









Example 2 (Effect of Adding CO2 Containing Liquid)
An introduction of CO2 containing liquid from the CO2 product cooler 120 via duct 105 was made in a simulated water wash unit 102 (FIG. 1), thus CO2 containing liquid was introduced to the bottom section 103 of the water wash unit 102, and was compared to introducing CO2 containing liquid from the CO2 product cooler 120 via duct 106, thus to the top section 104 of the water wash unit 102. The effect of the CO2 containing liquid introduced in the water wash unit 102 is presented in table 2. The CO2 containing liquid had a content of ammonia (mole/liter) of 0.54, and the mole ratio R was 1.05 (mole NH3/mole CO2), and the flow rate of CO2 containing liquid was measured to about 59 l/min at a flue gas flow in duct 107b of about 40 800 kg/hour.







TABLE 2







Comparison: introduction of CO2 containing liquid via duct 105,


compared to introduction of CO2 containing liquid via duct 106














Inlet gas to







water wash
After bottom
After top


Case

unit 102
stage 103
stage 104
Unit





CO2 via
NH3 in
8897 (in
1695
294 (in duct
ppm


duct 105
gas
duct 107a)

107b)


CO2 via
NH3 in
8897 (in
1736
171 (in duct
ppm


duct 106
gas
duct 107a)

107b)









The results presented in Table 2 show that supply of CO2 containing liquid via duct 106 to the top stage 104 of the water wash unit 102 reduces the emission of ammonia by about 42% compared to introduction of CO2 containing liquid via duct 105 to bottom stage 103.
When comparing to the prior art results of Table 1, it is clear that introducing CO2 containing liquid via duct 105 results in a reduction of the ammonia emission of about 6% (reduction from 312 to 294 ppm of NH3), and that introducing CO2 containing liquid via duct 106 results in a reduction of the ammonia emission of about 45% (reduction from 312 to 171 ppm of NH3).
Example 3 (High Inlet Ammonia Concentration)
Simulations were made to test the ammonia emission at high ammonia concentration in the flue gas forwarded to the water wash unit, in the example inlet ammonia is 16 000 ppm.
Comparative Example: Table 3 Illustrates the Simulated Result with the Prior Art Water Wash Unit 202 of FIG. 2







TABLE 3







Comparative example: Ammonia capture


of prior art water wash system 202.











Inlet gas to water
After bottom
After top



wash unit 202
stage 203
stage 204
















NH3, (ppm)
15948
10157
2263










Simulation of High Ammonia Concentration in Gas and Introduction of CO2 Containing Liquid Via Duct 105 or Via Duct 106:

The gas flow rate was kept at the same level as in Comparative example. CO2 containing liquid from the CO2 product cooler unit 120 was, in a first simulation, added via the duct 105, to the bottom section 103 of the water wash unit 102. In a second simulation CO2 containing liquid from the CO2 product cooler unit 120 was added via the duct 106 to the top section 104 of the water wash unit 102. The CO2 containing liquid was, in each simulation, added with a flow rate of 227 l/min at a flue gas flow in duct 107b of about 40 800 kg/hour, concentration of ammonia was kept at 1 mole/liter, and the mole ratio (mole NH3/mole CO2) was 1.05.
The results achieved are shown in Table 4:







TABLE 4







Ammonia capture of water wash system 102,


CO2 introduced via duct 105, or via duct 106.













Inlet gas to






water wash unit
After bottom
After top


Case

102
stage 103
stage 104





CO2 via duct
NH3, (ppm)
15948
4969
710


105


CO2 via duct
NH3, ppm
15948
5605
159


106









As indicated above the emission of ammonia is reduced from about 2300 ppm (table 3) as obtained for the prior art water wash system 202, to about 710 ppm (table 4) with the water wash unit 102 with supply of CO2 to bottom section 103 via duct 105, and is reduced to about 160 ppm (table 4) by introducing the CO2 containing liquid to the wash water unit 102 at the top section 104 via the duct 106.
To summarize, a method for capturing ammonia present in combustion flue gas subjected to carbon dioxide removal, using a water wash unit (102) included in a chilled ammonia process, comprises:

    
    
        providing CO2 loaded liquid (122) comprising CO2 dissolved in the liquid;
        providing wash water liquid (108, 138);
        combining the CO2 loaded liquid with the wash water liquid to form CO2 enriched wash water liquid (105, 106) before the liquid is added said water wash unit (102); and
        bringing said combustion flue gas into contact with said CO2 enriched wash water liquid by adding the CO2 enriched wash water liquid to said water wash unit (102).
    
    


While the invention has been described with reference to various exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.

## Claims

1. A method for capturing ammonia present in combustion flue gas subjected to CO2 removal, using a water wash unit that comprises at least first and second stages included in a chilled ammonia process, comprising the steps of:
providing CO2 loaded liquid comprising CO2 dissolved in the liquid;
providing wash water liquid;
combining the CO2 loaded liquid with the wash water liquid to form a CO2 enriched wash water liquid
providing the CO2 enriched wash water liquid at each of the first and second stages of the water wash unit;
bringing the combustion flue gas into contact with the CO2 enriched wash water liquid by adding the CO2 enriched wash water liquid to said water wash; and
forming a reduced ammonia flue gas stream and a used wash water stream.

2. The method according to claim 1, wherein the concentration of ammonia in the CO2 enriched wash water liquid added to the first stage is 0.5 to 3 mol/liter.

3. The method according to claim 1, wherein the wash water liquid comprises 0.0005 mol/l to 0.2 mol/l ammonia (NH3) before it is combined with the CO2 loaded liquid.

4. The method according to claim 1, wherein the wash water unit operates at 1° C. to 10° C.

5. The method according to claim 1, wherein the ratio of moles of ammonia (NH3) to moles of carbon dioxide (CO2) for the CO2 enriched wash water liquid is kept at about 0.05 to 10.

6. A method for capturing ammonia present in combustion flue gas subjected to CO2 removal, using a water wash unit that comprises at least first and second stages included in a chilled ammonia process, comprising:
providing CO2 loaded liquid comprising CO2 dissolved in the liquid;
providing wash water liquid;
combining the CO2 loaded liquid with the wash water liquid to form a CO2 enriched wash water liquid;
providing the CO2 enriched wash water liquid to the water wash unit; and
bringing said combustion flue gas into contact with said CO2 enriched wash water liquid to form a cleaned flue gas stream and a used wash liquid.

7. The method of claim 6, further comprising:
providing a portion of the CO2 enriched wash water liquid to the water wash unit to each of the first and second stages.

8. The method according to claim 5, wherein the ratio of moles of ammonia (NH3) to moles of carbon dioxide (CO2) for the CO2 enriched wash water liquid is kept at about 0.1 to 5.

9. The method according to claim 5 wherein the ratio of moles of ammonia (NH3) to moles of carbon dioxide (CO2) for the CO2 enriched wash water liquid is kept at about 1 to 4.

10. A method for capturing ammonia present in combustion flue gas subjected to CO2 removal, using a water wash unit that comprises at least first and second stages, the method comprising:
providing a CO2 loaded liquid comprising CO2 dissolved in the liquid;
providing wash water liquid;
combining the CO2 loaded liquid with the wash water liquid to form CO2 enriched wash water liquid;
providing the CO2 enriched wash water liquid to the water wash unit to each of the at least first and second stages;
bringing said combustion flue gas into contact with said CO2 enriched wash water liquid to form a reduced ammonia flue gas stream and a used wash water stream;
wherein the concentration of ammonia in the CO2 enriched wash water liquid added to the first stage is 0.5 to 3 mol/liter; and
wherein the concentration of ammonia in the CO2 enriched wash water liquid added to the second stage is 0.005 to 0.2 mol/liter.

11. The method according to claim 10, wherein the wash water liquid comprises 0.0005 mol/l to 0.2 mol/l ammonia (NH3) before it is combined with the CO2 loaded liquid.

12. The method according to claim 10, wherein the wash water unit operates at 1° C. to 10° C.

13. The method according to claim 10, wherein the ratio of moles of ammonia to moles of carbon dioxide for the CO2 enriched wash water liquid is kept at about 0.05 to 10.

14. The method according to claim 1, wherein the concentration of ammonia in the CO2 enriched wash water liquid added to the second stage is 0.005 to 0.2 mol/liter.

15. The method according to claim 3, wherein the concentration of ammonia in the wash water entering the second stage is lower than the concentration of ammonia entering the first stage.