Chilled ammonia carbon capture system with refrigeration system and heat pump

Abstract

A chilled ammonia carbon capture system, using a first working fluid, preferably ammonia, and a heat pump system, using a second working fluid, preferably water, the refrigeration system and the heat pump system being coupled through a vaporizer wherein the heat of the working fluid of the refrigeration system is used to evaporate the working fluid of the heat pump system, so that the waste heat from the chilled ammonia carbon capture system is used to obtain high temperature and high pressure steam. Steam extraction is configured to be utilized in the reboilers of the chilled ammonia carbon capture unit, in such a way that steam refurbishment and additional equipment for the production of high temperature and high pressure steam are not required.

Description

CHILLED AMMONIA CARBON CAPTURE SYSTEM WITH REFRIGERATION SYSTEM AND HEAT PUMPDescriptionTECHNICAL FIELD

[0001] The present disclosure concerns a chilled ammonia carbon capture system configured to exploit low temperature waste heat, by means of a cascade heat pump system, the cascade heat pump system comprising at least a first heat pump system using a first working fluid and a second heat pump system using a second working fluid, different from the first working fluid. Embodiments disclosed herein comprise a cascade heat pump system wherein the first heat pump system comprises a first evaporator configured to heat and evaporate the first working fluid, namely ammonia, by heat exchange with the refrigeration system of a chilled ammonia carbon capture unit and the second heat pump system comprises a second evaporator configured to heat and evaporate the second working fluid, namely water, by heat exchange with the first working fluid, the second heat pump system being configured to heat and compress the second working fluid, to obtain steam at a temperature and pressure suitable to be utilized in the reboilers of the chilled ammonia carbon capture unit, acting as the condenser of the second heat pump system without any additional heat transfer.BACKGROUND ART

[0002] The need for a reduction of carbon dioxide emissions has become a major concern to avoid global warming. The accelerated increase of carbon dioxide concentration in the atmosphere is attributed to the growing use of fuels, such as coal, oil and gas, which release billions of tons of carbon dioxide to the atmosphere every year.

[0003] Many technologies have been developed allowing the decreasing of the emissions, in particular carbon dioxide emissions, from industrial plants. Carbon dioxide capture implies separating the CO2 from the rest of the flue gases from an industrial plant instead of releasing the CO2 in the atmosphere. Several methods can be used to capture CO2 from coal-fired plants. Post combustion techniques separate the carbon dioxide from the flue gas after a traditional combustion process. The main advantage of such technique is that the combustion at the power plant is unaltered, so the processcan be implemented on existing power plants. A process using aqueous ammonia as solvent and operating at low temperature (2-10°C), also known as Chilled Ammonia carbon capture Process (CAP), has been developed and involves many advantages including: i) low cost and large availability of the solvent needed, ii) chemically stable solution, iii) regeneration at medium pressure and iv) high CO2 carrying capacity.

[0004] The use of chilled ammonia to capture carbon dioxide was disclosed in W02006022885. The purpose of the process is to absorb the carbon dioxide at a low temperature, in particular at a temperature range from 0 to 20°C, and preferably from 0 to 10°C. The corresponding cooling duties are typically rejected to cooling utilities, like cooling water or refrigeration systems. The actual configuration is depending on the actual availability of cooling utilities, their capacities and supply temperatures.

[0005] The flue gas to the chilled ammonia process is firstly treated in a reactor to remove contaminants, then, cooled down in a plurality of heat exchangers. The cooled flue gas enters a CO2 capture section, composed by an absorber and a desorber operating under high pressure (typically 21 bar). The flue gas enters the bottom of the absorber in countercurrent with a CO2 lean stream, mainly composed of water and ammonia, and including little amount of carbon dioxide, entering the top of the absorber and coming from the bottom of the desorber. The carbon dioxide of the flue gas is absorbed by the ammonia in the absorber. A low temperature in the absorber prevents the ammonia from evaporating and enhances the mass transfer of CO2 to the solution. According to W02006022885, more than 90% of the CO2 from the flue gas can be captured.

[0006] A cleaned gas stream leaves the absorber from its top, while a CO2 rich stream leaves the bottom of the absorber and is sent by means of a pump to a heat exchanger where it is warmed, and then sent to the desorber. Inside the desorber, CO2 separates from the solution and leaves the top of the desorber as a relatively clean and high- pressure stream. According to W02006022885, a condenser is provided at the top of the desorber to separate water vapor and ammonia contained in the CO2 stream and recirculate them to the desorber. A CO2 lean stream leaves the bottom of the desorber and is routed to an air cooler and subsequently to the top of the absorber, to absorb CO2 from the flue gas. The desorption reaction is endothermic, the energy that has to be supplied highly depending on the composition of the CO2 rich stream that entersthe desorber. The required heat for desorption is typically provided by steam at about 8 bara and 175 °C.

[0007] As previously described, the Chilled Ammonia carbon capture Process generates waste heat, which needs to be rejected to a cooling or refrigeration system. On the other hand, the CAP also requires heat to be provided to the desorption reaction. In many cases, the host plant of the CAP is not configured to supply heat and cooling for the CAP system. As a consequence, in order to allow the CAP to be as independent as possible, an electric power operated heating and cooling system need to be added to the system. However, this solution implies additional complexity, higher costs and undesirable emission of CO2.

[0008] Accordingly, an improved system and method for operating a Chilled Ammonia carbon capture Process to address the issues of complexity, costs and carbon oxide emission of the systems of the current art would be beneficial and would be welcomed in the technology.SUMMARY

[0009] In one aspect, the subject matter disclosed herein is directed to a chilled ammonia carbon capture refrigeration system combined with heat pump, wherein the waste heat of the refrigeration system is used to evaporate a fluid (namely water), which can be compressed to higher pressure / temperature levels, up to values allowing to provide heat to the desorber of the CAP. Therefore, the aim of the present disclosure is to “upgrade” part of the waste heat from the CAP to provide heating duty required in the chilled ammonia process. In particular, the solution disclosed allows to generate the steam required heat for desorption in the CAP.BRIEF DESCRIPTION OF THE DRAWINGS

[0010] A more complete appreciation of the disclosed embodiments of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:Fig.1 illustrates a process flow diagram of a combined refrigeration system of a chilled ammonia carbon capture system’s with a heat pump, according to an exemplary embodiment.DETAILED DESCRIPTION OF EMBODIMENTS

[0011] Reference now will be made in detail to one embodiments of the disclosure, which is illustrated in figure 1 by way of explanation of the disclosure, not limitation of the disclosure. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present disclosure without departing from the scope or spirit of the disclosure. Reference throughout the specification to “one embodiment” or “an embodiment” or “some embodiments” means that the particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrase “in one embodiment” or “in an embodiment” or “in some embodiments” in various places throughout the specification is not necessarily referring to the same embodiment s). Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.

[0012] When introducing elements of various embodiments, the articles “a”, “an”, “the”, and “said” are intended to mean that there are one or more of the elements. The terms “comprising”, “including”, and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements.

[0013] Referring to Fig.1, it is shown a block diagram of a combination of multistage refrigeration system 50 of a chilled ammonia carbon capture system (hereafter CAP system) 30 with a steam heat pump system 10, according to the invention.

[0014] The CAP SYSTEM 30 comprises a plurality of reboilers 31 and a plurality of coolers 32, 32’, 32”, by way of example a first cooler 32 working at a higher temperature, typically comprised between 35°C and 45°C, a second cooler 32’ at an intermediate temperature and a third cooler 32” at a lower temperature, typically between 20°C and 27°C. Heat from the coolers is recovered through the lines QP, Qp’, Qp”.

[0015] The multistage refrigeration system 50 consists of three refrigeration thermodynamic cycles 50’, 50”, 50’”, through which a stream of a working fluid, preferablyammonia, carrying low temperature waste heat Qp generated from the CAP SYSTEM 30 is compressed to a higher pressure and temperature and is finally condensed in a condenser 59 at the end of the refrigeration system 50, by exchanging heat QA with water in an evaporator 11 of a heat pump system 10, consisting of three interconnected heat cycles 10’, 10”, 10’”. The number of refrigeration thermodynamic systems and the number of cycles in Fig.1 is given by way of explanation, but it is apparent to those skilled in the art that both the number of refrigeration thermodynamic systems and the number of cycles can change, independently from each other, depending on the operative parameters of the CAP SYSTEM 30, the working fluid and possible auxiliary heat sources and / or heat loads connected to the system, as will be disclosed herein below.

[0016] More in detail, with reference to Fig.1, heat Qp from the CAP SYSTEM 30 is transferred to the refrigeration system 50 through an evaporator 51, which evaporate the working fluid of the first refrigeration thermodynamic cycle 50’ of the refrigeration system 50 from a liquid phase to a vapor phase at a first temperature and pressure. A vapor stream from the evaporator 51 is routed to a compressor 52’, which is driven by an electric motor 53’, to be compressed to a second temperature and pressure, higher than said first temperature and pressure. The compressed stream is directed to an economizer 55’, wherein the compressed stream is mixed with a stream expanded in valve 56” coming from a second refrigeration thermodynamic cycle 50”, operating at higher temperature and pressure. The liquid from the economizer 55’ is directed to an expansion valve 56’ and to the evaporator 51, while the vapor stream at said second temperature and pressure is directed to a compressor 52” of the second refrigeration thermodynamic cycle 50”, which is driven by an electric motor 53”, to be compressed to a third temperature and pressure, higher than said second temperature and pressure. The compressed stream is directed to an economizer 55” of the second refrigeration thermodynamic cycle 50”, wherein the compressed stream is mixed with a stream expanded in valve 56’” coming from a third refrigeration thermodynamic cycle 50’”, operating at higher temperature and pressure, and directed to an expansion valve 56” and to the economizer 55’ of the first refrigeration thermodynamic cycle 50’, while the vapor stream at said third temperature and pressure is directed to a compressor 52’” of the third refrigeration thermodynamic cycle 50’”, which is driven by an electric motor 53’”, to be compressed to a fourth temperature and pressure, higher than said thirdtemperature and pressure. The compressed stream is then directed to a condenser 59, as will be described herein after, to be condensed and subsequently directed to an expansion valve 56”’ of a third refrigeration thermodynamic cycle 50’” and to the economizer 55” of the second refrigeration thermodynamic cycle 50”.

[0017] Still with reference to Fig. 1, a portion of the flow coming out of the compressor 52’ of the first refrigeration thermodynamic cycle 50’ can be optionally directed to an evaporative condenser unit 57’ , which exchanges heat with a mixed stream of a flue gas condensate from the CAP and make-up water in order to remove excess heat from the first refrigeration thermodynamic cycle 50’ by condensing a part of the vapor stream. The evaporative condenser unit 57’ has two functions: to reject the remaining heat that is not required for an optional district heating unit 58 installed downstream in the refrigeration system 50, as will be described herein after, and to concentrate the flue gas condensate from the CAP to reduce the load of the wastewater treatment plant.

[0018] Furthermore, always with reference to Fig. 1, the second refrigeration cycle 50” (or another refrigeration cycle of the refrigeration system 50) can be provided with a district heating unit 58 which receives a portion of the compressed stream from the compressor 52”, in order to distribute excess heat of the second refrigeration cycle 50” to other locations / systems, namely domestic users, through a system of insulated pipes. The evaporative-condenser unit 57 and the district heating unit 58 can be omitted from the refrigeration system 50 due to space requirements, by releasing the rejected heat to the atmosphere. Therefore, depending on requirements of the system of the present disclosure, the district heating unit 58 can offer additional financial revenue.

[0019] By means of the condenser unit 59, an amount QA of heat is exchanged between the refrigeration system 50 and the heat pump system 10 through an evaporator 11 of a first heat pump 10’ the heat pump system 10. Inside the evaporator I l a working fluid of the heat pump system 10, preferably water, at a first heat pump temperature, higher than the temperature of the third refrigeration thermodynamic cycle 50’”, is evaporated and is subsequently routed to a compressor 12’, which is driven by an electric motor 13’, to be compressed to a second heat pump temperature and pressure, higher than the first heat pump temperature and pressure. The compressed vapor stream is directed to an economizer 15’, wherein the compressed stream is mixed witha stream expanded in expansion valve 16” of a second heat pump 10”, operating at higher temperature and pressure and is subsequently directed to an expansion valve 16’ and back to the evaporator 11, while the vapor stream at said second heat pump temperature and pressure is directed to a compressor 12” of the second heat pump 10”, which is driven by an electric motor 13”, to be compressed to a third heat pump temperature and pressure, higher than said second heat pump temperature and pressure. The compressed stream is directed to an economizer 15” of the second heat pump 10”, wherein the compressed stream is mixed with a stream expanded in expansion valve 16”’ of a third heat pump 10’”, operating at higher temperature and pressure and is subsequently directed to an expansion valve 16” and to the economizer 15’ of the first heat pump 10’, while the vapor stream at said third heat pump temperature and pressure is directed to a compressor 12’” of the third heat pump 10’”, which is driven by an electric motor 13’”, to be compressed to a fourth heat pump temperature and pressure, higher than said third heat pump temperature and pressure. The compressed stream is then directed to a trim condenser 17 and to the CAP reboilers 31 (in Fig. 1 the reboilers 31 being represented both as part of the heat pump system 10 and as part of the CAP SYSTEM 30, and linked by the line 31’), as will be described herein after, to be condensed and, after collection in a condensate receiver unit 19, directed to an expansion valve 16’” of the third heat pump 10’” and to the economizer 15” of the second heat pump 10”.

[0020] In particular, through the CAP reboilers 31, heat obtained through the heat pump system 10 is exchanged with the CAP system 30, thus achieving the aim of the invention to provide heat to the CAP system 30 without the need of additional electric power operated heating and cooling system and therefore reducing complexity, costs and carbon oxide emission of the system.

[0021] Optionally, the steam heat pump system 10 includes various steam import / ex- port conducts 11’, 11”, I T”, 14’, 14”, 14’”, 17’, 18’ at different temperature and pressure, depending on the host plant circumstances and requirements. In particular, steam import reduces steam generation compression power. On the other hand, steam export can offer additional financial revenue if there are low pressure steam users to which produced steam can be delivered. This can potentially save fuel consumption if low pressure steam would be produced otherwise by combustion.

[0022] With reference to Fig. 1, steam export conducts 11’, 11”, 11’”, 17’ can be provided: from the inlet of the compressor 12’ to the outlet of the expansion valve 16’ of the first heat pump 10’ and / or from the inlet of the compressor 12” to the outlet of the expansion valve 16” of the second heat pump 10” and / or from the inlet of the compressor 12’” to the outlet of the expansion valve 16’” of the of the third heat pump 10’” and / or from the outlet of compressor 12’” to the inlet of condensate receiver 19, in parallel to the trim condenser 17 and CAP reboilers 31. Superheating (SH) of the steam streams is achieved by the transfer of heat from subcooling (SC) the liquid streams from the economizers. Superheated steam is typically required to account for heat loss in the steam pipes to the respective steam consumers.

[0023] Steam import conduct 14’can be optionally provided along recirculation lines connecting the outlet of the subcooling of the economizer 15’ with the inlet of the suction drum 15 of compressor 12’in the respective heat pump 10’in parallel to the line passing through the expansion valves 16’ and the evaporators 11

[0024] Steam import conduct 14”, can be optionally provided along recirculation lines connecting the outlet of the subcooling of the economizer 15” with the inlet of the economizer 15’ of compressor 12” in the respective heat pump 10”, in parallel to the line passing through the expansion valves 16”. Finally, two additional steam import conducts 14’”, 18’ can be provided according to the present disclosure, along the line downstream the condensate receiver unit 19. In particular the steam import conduct 14’” connects the outlet of the condensate receiver 19 with the inlet of the economizer 15” of the second heat pump 10”, in parallel to the line passing through the expansion valve 16’”, while the steam import conduct 18’ is arranged along the line connecting the outlet of the condensate receiver 19 with the outlet of the compressor 12’” of the third heat pump 10’” (a pump is needed along the line).

[0025] While aspects of the invention have been described in terms of various specific embodiments, it will be apparent to those of ordinary skill in the art that many modifications, changes, and omissions are possible without departing form the spirit and scope of the claims.

Claims

CLAIMS1. A chilled ammonia carbon capture system (30) comprising a refrigeration system (50) and a heat pump system (10), the heat pump system (10) including a plurality of heat cycles (10’, 10”, 10’”), each heat pump operating at higher temperature and pressure than the upstream heat pump, wherein the refrigeration system (50) is coupled to the heat pump system (10) through an evaporator (11), wherein the heat from the refrigeration system (50) is used to evaporate a working fluid of a low temperature heat cycle (10’) of the heat pump system (10), the heat cycles (10’, 10”, 10’”) being configured to use water as working fluid, to obtain high temperature and high pressure steam to provide heat to the chilled ammonia carbon capture system (30).

2. The chilled ammonia carbon capture system (30) of claim 1, wherein the chilled ammonia carbon capture system (30) comprises a CO2 capture section, composed by an absorber and a desorber, the absorber being configured to absorb carbon dioxide within a CO2 lean solution, mainly composed of water and ammonia, the desorber being configured to separate CO2 from the solution, a condenser being provided at the top of the desorber to separate water vapor and ammonia contained in the CO2 stream and recirculate them to the desorber, the heat cycles (10’, 10”, 10’”) being configured to use water as working fluid, to obtain high temperature and high pressure steam to provide heat to the desorber of the chilled ammonia carbon capture system (30).

3. The chilled ammonia carbon capture system (30) of claim 1 or 2, wherein the refrigeration system (50) includes a plurality of refrigeration thermodynamic cycles (50’, 50”, 50’”), each refrigeration thermodynamic cycle operating at higher temperature and pressure than the upstream refrigeration thermodynamic cycle.

4. The chilled ammonia carbon capture system (30) of claim 3, wherein one or more heat exchangers are coupled with any refrigeration thermodynamic cycle of the plurality of refrigeration thermodynamic cycles (50’, 50”, 50’”), to provide heat to external services.

5. The chilled ammonia carbon capture system (30) of claim 4, wherein the external services include district heating.

6. The chilled ammonia carbon capture system (30) of any of the previous claims, wherein one or more heat exchangers are coupled with any heat cycle of the plurality of heat cycles (10’, 10”, 10’”), to provide heat to external services.

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