APPLYING OZONE NOx CONTROL TO AN HRSG FOR A FOSSIL FUEL TURBINE APPLICATION

Abstract

A method for reducing NOx and recovering waste heat from a stream of exhaust gas from a fossil fuel fired turbine includes contacting the stream of exhaust gas between an economizer and an evaporator with ozone gas to convert the NO to nitrogen dioxide (NO2) thereby forming a stream of exhaust gas comprising NO2 and residual NO. The method further includes, contacting the stream of exhaust gas comprising NO2 and residual NO with water mist to create an exhaust stream comprising nitric acid (HNO3) and residual NO. The method further includes cooling the stream of exhaust gas comprising HNO3 and residual NO, collecting a first residual water film on a first condensing medium to capture the HNO3 and removing the first water film and HNO3.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application Ser. No. 61/429,627, filed on Jan. 4, 2011, which is incorporated herein by reference in its entirety.

BACKGROUND

In a combined-cycle power station, the hot exhaust from the natural gas-fired turbine is fed into a Heat Recovery Steam Generation (“HRSG”) unit to generate steam that is used to drive a steam turbine. The hot exhaust gases from the turbine are directed through the HRSG unit, which may be a combination of evaporators, economizers, superheaters, and reheaters. The amount of evaporators, economizers, superheaters and reheaters is determined by how many pressure levels are needed to drive the steam turbine. Furthermore, HRSGs can be categorized as vertical or horizontal types based on the exhaust gas stream. In the horizontal type, exhaust gas streams horizontally over vertical tubes, whereas in the vertical type, exhaust gas streams vertically over horizontal tubes. HRSGs can have single pressure applications or multi-pressure applications. Single pressure HRSGs have a single steam drum and steam is generated at a single pressure level. Multi-pressure HRSGs may employ two or three pressure levels.

Reducing NOx along with other unwanted pollutants from fossil fuel-fired turbine exhaust gas is an important process for atmospheric and environmental protection on a global scale. Natural-fired, oil-fired, diesel-fired and biofuel-fired turbines produce tons NOx per year, along with other unwanted pollutants such as SOx, HCL, mercury and particulate.

Capturing and reusing waste energy is another important factor in the operation of the fossil fuel fired turbine plant. To this end, the use of HRSG captures most of the waste heat and produces steam to increase the operational output of the plant. However, there is still approximately 10% of the waste heat that escapes to atmosphere that can be captured and reused in the plant to increase the efficiencies of the operations of the fossil-fired fuel turbine plant.

As demand for power is increasing every year with strict environmental laws for fossil fuel emissions, there remains a need for a system that may effectively remove exhaust emissions (e.g., NOx) while capturing and reusing waste heat, thus, resulting in a more efficient and clean power generation system.

SUMMARY

In general, in one aspect, the invention relates to a system for reducing NOx and recovering waste heat from a stream of exhaust gas from a fossil fuel fired turbine. The system includes a superheater configured to recover heat from the stream of exhaust gas, an evaporator configured to recover heat from the stream of exhaust gas, wherein the evaporator is located downstream from the superheater, an economizer configured to recover heat from the stream of exhaust gas, wherein the economizer is located downstream from the evaporator. The system further includes an ozone aspirator, located between the evaporator and economizer, wherein the ozone aspirator is configured to receive the stream of exhaust gas comprising nitric oxide (NO) from the evaporator and contact the stream of exhaust gas comprising NO with ozone to convert the NO to nitrogen dioxide (NO₂) thereby forming a stream of exhaust gas comprising NO₂ and residual NO. The system further includes a heat reclaim coil configured to capture heat from the stream of exhaust gas comprising NO₂ and residual NO to produce hot water, a first misting stage configured to receive the stream of exhaust gas comprising NO₂ and residual NO and contact the stream of exhaust gas comprising NO₂ and residual NO with water mist to create an exhaust stream comprising nitric acid (HNO₃) and residual NO, and a first condensing medium configured to cool the stream of exhaust gas comprising HNO₃ and residual NO and to collect a first residual water film thereon to capture the HNO₃, thereby creating an exhaust stream comprising residual HNO₃ and residual NO.

In general, in one aspect, the invention relates to a method for reducing NOx and recovering waste heat from a stream of exhaust gas from a fossil fuel fired turbine. The method includes recovering heat from the stream of exhaust gas by passing the stream of exhaust gas through a superheater, an evaporator located downstream from the superheater, and an economizer located downstream from the evaporator. The method further includes contacting the stream of exhaust gas between the economizer and the evaporator with ozone gas to convert the NO to nitrogen dioxide (NO₂) thereby forming a stream of exhaust gas comprising NO₂ and residual NO. The method further includes recovering heat from the stream of exhaust gas comprising NO₂ and residual NO to produce hot water, contacting the stream of exhaust gas comprising NO₂ and residual NO with water mist to create an exhaust stream comprising nitric acid (HNO₃) and residual NO, cooling the stream of exhaust gas comprising HNO₃ and residual NO, collecting a first residual water film on a first condensing medium to capture the HNO₃, thereby creating an exhaust stream comprising residual HNO₃ and residual NO, and removing the first water film and HNO₃.

Other aspects of the invention will be apparent from the following description and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a system in accordance with one or more embodiments.

FIG. 2 shows a system in accordance with one or more embodiments.

FIG. 3 shows an ozone generation system in accordance with one or more embodiments.

FIG. 4 shows a fogging system in accordance with one or more embodiments.

FIG. 5 shows a condensing and wastewater management system in accordance with one or more embodiments.

FIG. 6 shows a heat reclaim system in accordance with one or more embodiments.

FIG. 7 shows a flowchart in accordance with one or more embodiments.

DETAILED DESCRIPTION

Specific embodiments of applying ozone NOx control in combination with heat recovery steam generation (“HRSG”) for a fossil fuel turbine application will now be described in detail with reference to the accompanying figures. Like elements in the various figures (also referred to as FIGs.) are denoted by like reference numerals for consistency.

In the following detailed description of embodiments, numerous specific details are set forth in order to provide a more thorough understanding of the method and system. However, it will be apparent to one of ordinary skill in the art that these embodiments may be practiced without these specific details. In other instances, well-known features have not been described in detail to avoid unnecessarily complicating the description.

In general, embodiments of the invention relate to a method and system for ozone NOx control technology components employed in conjunction with an HRSG unit for reducing the NOx levels of the exhaust gases from a gas-fired, oil-fired, diesel-fire or biofuel-fired turbine. In many cogeneration and combination power plants built today, combined cycle plants utilize a Brayton cycle and a Rankine cycle. A gas or combustion turbine is used as the prime mover (Brayton Cycle) with a waste heat boiler at the exhaust of the gas turbine to recover waste heat. The HRSG unit produces steam that is directed to a steam turbine to produce more power. Due to the recovery of waste heat, the combined cycle plants are much more fuel efficient than the conventional steam plants. The HRSG unit does not use any fuels at all but only captures and utilizes the exhaust waste heat from the gas turbine.

An HRSG unit does not form nitrogen oxides (NOx), but rather, receives NOx along with other contaminants such as carbon monoxide CO and carbon dioxide CO₂ from the exhaust gases directed into the HRSG unit from the gas-fired, oil-fired, diesel-fired or biofuel-fired turbine. NOx is a combination of NO and NO₂, where NO₂ can be removed by coming in contact with water, thereby creating an acid liquid. Thus, any NO present in the exhaust stream composition needs to be oxidized before being converted to a condition that can be removed from the exhaust gases. In accordance with one or more embodiments, ozone (O₃) may used to convert NO to NO₂ which then may be removed with water as generally described below.

In accordance with one or more embodiments of the invention, NOx control may be added to an HRSG unit by adding at least one ozone (O₃) aspirator, one or more misting stages and one or more condensing mediums. To remove the NOx, O₃ may be added to the exhaust gas stream to act as a reagent. The introduction of O₃ into the exhaust gas stream causes the following reaction to occur: NO+O₃→NO₂+O₂. Thus, the nitric oxide (NO) is converted to nitrogen dioxide (NO₂). The seeded exhaust gas may then come in contact with a misting stage, located downstream of the aspirator, where the exhaust gas is fogged in a substantially uniform manner. The introduction of mist or fog (e.g., in the form of H₂O) with the exhaust gas causes the following reaction to occur: 3NO₂+H₂O→2HNO₃. Thus, the nitrogen dioxide (NO₂) is converted to nitric acid (HNO₃). The exhaust gas may then come in contact with one or more condensing media, located downstream the aspirator, where the exhaust gas is cooled to saturation, causing a water film to develop on the one or more media. The water film is used to capture the nitric acid (HNO₃) to be removed from the HRSG and directed to wastewater facility.

FIG. 1 shows an HRSG unit with NOx control in accordance with one or more embodiments. A fossil fuel fired turbine (not shown) such as natural gas-fired, oil-fired, and diesel-fired or biofuel fired turbine directs hot exhaust gases 101 through a special fabricated breech 103 that joins the fossil fuel fired turbine exhaust gas outlet (not shown) to the HRSG unit with NOx control. In accordance with one or more embodiments, the HRSG unit with NOx control includes superheater 105, high pressure evaporator 107, high pressure economizer 109, low pressure evaporator 111, aspirator 113, low pressure economizer 115, heat reclaim coil 117, first misting/fogging array 119, first condensing section 121, second misting/fogging array 123, second condensing section 125, reheat coil 127, and fan 129. The superheater 105, high pressure evaporator 107, high pressure economizer 109, low pressure evaporator 111, and low pressure economizer 115 operate as the HRSG unit, serving to reclaim heat from the exhaust gas and transfer it to the fluid (e.g., water) and/or vapor (e.g., steam) used to drive a turbine, thus, increasing the overall efficiency of the system. The aspirator 113, heat reclaim coil 117, first misting/fogging array 119, first condensing section 121, second misting/fogging array 123, second condensing section 125, reheat coil 127, and fan 129 operate as the unit for NOx control, thus, providing NOx removal from the exhaust gas stream. The individual components of the HRSG unit with NOx control are described in more detail below.

In accordance with one or more embodiments, superheater section 105 is configured to reclaim heat from the exhaust gas stream and use this heat to dry the saturated vapor (e.g., steam) and increase pressure of the vapor to be sent to, e.g., a steam turbine. The high pressure evaporator 107 is configured to reclaim heat from the exhaust gas stream and use this heat to convert the fluid (e.g., water) to vapor (e.g., steam). The high pressure economizer 109, sometimes called a pre-heater or preheat coil, configured to reclaim heat from the exhaust gas stream and to use this heat to increase the temperature and pressure of the fluid being directed from the low pressure evaporator 111 into the high pressure economizer 109. One of ordinary skill will appreciate that any known type or combination of superheaters, economizers, and evaporators may be used in the HRSG unit without departing from the scope of the present invention.

The aspirator 113 is configured to introduce and mix a gas with the exhaust gas stream in order to convert the NO in the exhaust gas to NO₂. In accordance with one or more embodiments, the gas introduced by the aspirator may be ozone (O₃). The introduction of O₃ into the exhaust gas stream causes the following reaction to occur: NO+O₃→NO₂+O₂. Downstream from the aspirator, the low pressure economizer 115 is configured to reclaim heat from the exhaust gas stream and use this heat to preheat the feed water being introduced to the system to replace the vapor (steam) being removed from the steam turbine. In addition, heat reclaim coil 117 may be used to further reclaim heat from the exhaust gas stream and use this heat to create low quality hot water for applications within the plant process.

The first misting/fogging array 119 includes a series of misting nozzles 119a placed after the heat reclaim coil 117. The mist comes into contact with the exhaust gas, thereby creating a chemical reaction in the exhaust gas. The introduction of mist or fog (e.g., in the form of H₂O mist/fog) with the exhaust gas causes the following reaction to occur: 3NO₂+H₂O→2HNO₃. The first condensing section 121, placed after the first misting/fogging array 119, is configured to condense the exhaust gases that have become saturated. The second misting/fogging array 123 and second condensing section 125 are located after the first condensing section 121 and are configured to repeat the misting and condensing in a manner similar to the first misting/fogging and condensing stages, to maximize the HNO₃ removal rates. Reheat coil 127 then reheats and dries out the saturated exhaust gases before they are directed out to the atmosphere by fan 129. Additional misting/fogging and condensing stages may be added depending on the precise requirements of the system without departing from the scope of the present invention.

FIG. 2 shows an example of system in accordance with one or more embodiments of the invention. More specifically, this system is similar to that described above with respect to FIG. 1, but is described in further detail from the perspective of the hot exhaust gas flow originating from a fossil fuel fired turbine. In accordance with one or more embodiments, a fossil fuel fired turbine 201 such as a natural gas-fired, oil-fired, diesel-fired, or biofuel fired turbine directs its hot exhaust gases 200 through a specially fabricated breech 203 that joins the fossil fuel fired turbine exhaust gas outlet to the HRSG with NOx control. In accordance with one or more embodiments, the HRSG with NOx control includes one or more elements 205-229, as described in further detail below.

The hot exhaust gas from the fossil fuel fired turbine comes into contact with a superheater 205, where the superheater is a device used to convert saturated steam or wet steam into dry steam. In accordance with one or more embodiments, the superheater may be of the water tube type, but one of ordinary skill will appreciate that any known type of superheater may be employed without departing from the scope of the present invention. The water tube type superheater takes the saturated steam supplied in the dry pipe into a superheater header mounted against the tube sheet where steam is then passed through a number of superheater elements comprised of long pipes which are placed inside special, widened tubes.

The exhaust gas then is directed to the high pressure evaporator 207. In accordance with one or more embodiments, the high pressure evaporator 201 may include carbon steel tubes known in the art. Most commonly used is an O-Frame evaporator layout. The O-Frame evaporator layout has the advantage that the upper header may be configured as the steam separation drum. The upper header can also be connected to the steam drum by risers, allowing more than one O-Frame evaporator to be connected to the same steam drum, resulting in shippable modules being able to handle very large gas flows. The evaporator section, includes all the evaporator coils making up the total evaporator for a pressure system. A pressure system includes all the components included in the various streams associated with that pressure level. The high pressure evaporator increases the saturated steam in temperature and pressure. In above example, the O-Frame evaporator is used. However, one of ordinary skill will appreciate that any known evaporator configuration may be used, including, but not limited to O-Frame, A-Frame, I-Frame, horizontal tube layout or vertical tube layout.

After passing through the high pressure evaporator 207, the exhaust gas comes in contact with a high pressure economizer 209, where hot water is heated to higher temperature to produce saturated steam. The high pressure economizer absorbs heat from the hot exhaust gases by convective heat transfer. In the convection section of the high pressure economizer 209, heat is transferred by both radiation and convection. In the larger HRSG applications it is common to have the high pressure economizer 209 formed of bare carbon steel tubes.

The exhaust gas is then directed to the low pressure evaporator 211 where low temperatures and pressures are present, and hot water is at a point of saturated steam. The low pressure evaporator 211, like the high pressure evaporator 207 may be constructed out of carbon steel or a similar alloy that allows maximum heat transfer and/or designed for lower exhaust temperature.

In accordance with one or more embodiments, the O₃ aspirator 213 may be constructed of a series of small tubes that introduce ozone into the exhaust gas that has passed through the low pressure evaporator 211. The series of small tubes may be constructed of stainless steel, wherein the lower ends of the tubes are designed on a 45 degree angle with the opening of the tube pointed in the direction co-current with the flow of the exhaust gas. The ozone is drawn into the exhaust gas and mixed with the pollutants, thereby converting the NO to NO₂. The introduction of O₃ into the exhaust gas causes the following reaction to occur:

NO+O₃→NO₂+O₂

In accordance with one or more embodiments, as the gas flow continues, it comes in contact with the low pressure economizer 215, where water is heated in a series of bare tubes to increase the temperature of the water that is transferred to produce steam. The bare tubes may be formed of carbon steel or a similar alloy designed to maximize heat transfer.

In accordance with one or more embodiments, from the low pressure economizer 215, the exhaust gas is directed to the heat reclaim coil 217 where the lower exhaust heat is captured to produce low grade hot water. The hot water produced can range in temperature from 200 degrees F. to 210 degrees F. The low exhaust heat ranges from 280 degrees F. to 350 degrees F. and is suitable to create hot water for common use in a plant such as pre-heat for combustion air (thus, creating a fuel savings), domestic hot water, building hot water heating, and absorption chilling, where the chiller cycle requires hot water.

In accordance with one or more embodiments, the exhaust gas is directed from the heat reclaim coil 217 to the first misting device 219, where the mist scrubbing uses approximately 10 micron diameter water droplets of controlled chemical solution to react with the contaminated exhaust gas stream. These small droplets have a tremendous amount of surface area. Due to the speed of the induced droplets or mist (MACH-1), the gas to liquid contact may be extremely fast. Contact is against the exhaust gas flow and the liquid solution is not re-circulated.

In accordance with one or more embodiments, by applying the mist to the exhaust gas stream, the humidity of the exhaust gas stream will come close to saturation. The first misting device 219 injects a high speed water droplet which is 10 microns in diameter against the flow of the exhaust gas stream. This high speed water using misting attacks the scrubbing problem differently by employing a high rate of boundary air induction. The water molecules hit the exhaust gas molecules causing flashing. Thus, atmospheric steam is the hydrolysis reactant with NOx which forms acid vapors. The introduction of fog (H₂O) to the exhaust gas causes the following reaction to occur:

3NO₂+H₂O→2HNO₃

In accordance with one or more embodiments, after the first misting device 219, the exhaust gas comes in contact with the first condensing medium 221, where the formation of absorptive wetted films collect on the medium's extended surface area. The condensing medium 221 is a very effective trap wherein volatile organic compounds (“VOCs”) such as chlorofluorocarbons or chlorocarbons and products of combustion (“POCs”) such as NO_x, SO_x and CO₂ collect on freely draining film surfaces.

In accordance with one or more embodiments, the exhaust gas then is directed to the second misting device 223 where the exhaust gas increases to saturation and the fine water droplet captures any of the remaining NO₃. Then, the cool exhaust gas comes in contact with the second condensing medium 225 where the same process occurs as the first condensing medium 221.

In accordance with one or more embodiments, the exhaust gas then passes through a reheat coil 227 where the saturated exhaust gas is heated to remove the moisture from the exhaust gas then directed to the exhaust stack by a variable speed controlled fan 229. The clean exhaust gas is directed to the atmosphere by the exhaust stack where the NOx and water has been removed from the process.

FIG. 3 shows an ozone generation system in accordance with one or more embodiments of the invention. The O₃ aspirator 301, situated between a low pressure evaporator 303 and a low pressure economizer 305, is used to introduce a controlled ozone into the exhaust gas 300. The ozone is produced by ozone generator 307. Ozone generator 307 generates ozone from O₂ (oxygen) that has been directed through piping 311 from O₂ tanks 309. Ozone generator 307 may employ any method known in the art for generating ozone gas, e.g., electric arc discharge. The generated ozone is then directed through piping 311 into the ozone generator 307 and an electric arc converts the O₂ into O₃.

Once the ozone has been produced it is directed through a series of tubes to a control valve 313 where the pressure and flow is regulated going into the O₃ aspirator 301. The O₃ aspirator 301 is a flow through nozzle device in which the kinetic energy is increased by way of an adiabatic process.

From the O₃ aspirator, the ozone is directed through a series of tubes were each tube includes a balancing valve 317 for equal flow of ozone being delivered to each individual tube 319. Each tube 319 may be installed within an HRSG unit (e.g., between the low pressure evaporator 303 and the low pressure economizer 305, as shown, e.g., in FIG. 1, or in any other suitable configuration based on the design requirements of the process and/or plant) where the ozone is directed into the exhaust gas to uniformly mix with the exhaust gas. The series of tubes 319 may be constructed of stainless steel and the lower ends 319a of tubes 319 are designed on a 45 degree angle with the opening of the tube pointed in the direction current with, or parallel to, the flow of the exhaust gas.

FIG. 4 shows the misting stage arrays 400a and 400b with their associated equipment in accordance with one or more embodiments. City water is directed through a pump 401 where the water is delivered to the reverse osmosis (“RO”) system 403. The RO system 403 is a water treatment process that filters undesirable materials from water by using pressure to force the water molecules through a semi-permeable reverse osmosis membrane. RO water treatment removes ionized salts, colloids, and organic molecules.

In accordance with one or more embodiments, after the RO system 403, the RO water is directed to a RO reservoir 405 and/or tank where the RO water is distributed when needed. From the RO reservoir 405 the RO water is directed to the misting/fogging high pressure pumps 407, where the water pressure is increased to 3000 psi. Each misting stage array 400a and 400b has a high pressure pump so as to deliver high pressure water to the misting or fogging nozzles 409. The high pressure RO water is directed from the high pressure pump 407 through a series of strainers, valves, meters and tubing 419 and delivered to control valves 411 so as to control the flow being directed to the misting arrays.

From the control valves 411, the high pressure RO water is directed to the misting and or fogging nozzles 409 where the high pressure RO water is dispersed into the exhaust gas against the flow of the exhaust gas.

In accordance with one or more embodiments, when there is SO₂ present in the exhaust gas caused by the type of fuel used in the turbine such as diesel or oil, a solution of H₂O₂ (hydrogen peroxide) solution is introduced into the misting/fogging system. A H₂O₂ solution tank 413 directs the H₂O₂ solution through a control valve 415 and a chemical pump 417 to the misting system where the H₂O₂ solution is mixed with the high pressure RO water. In accordance with one or more embodiments, the mixed H₂O₂ and RO water sprays against the exhaust gas flow causing the following reaction:

SO₂+H₂O→H₂SO₄

FIG. 5 shows a condensing system in accordance with one or more embodiments. The condensing mediums 501 and 503 generate large acid (e.g., H₂SO₄ and/or HNO₃) droplets on their surfaces. A wet film is produced and as more wet acid is collected, the acids are forced down by gravity to the wastewater drains. In accordance with one or more embodiments, the condensing mediums 501 and 503 can be built out of different materials such as stainless steel finned tube coils with Teflon coating when condensing just nitric acid (HNO₃), but if both nitric acid and sulfuric acid (H₂SO₄) are present, then the condensing medium materials may be critical polyvinyl chloride (CPVC) packing, which will withstand the strong acid environment while still allowing for a large surface area upon which acids may be collected.

In accordance with one or more embodiments, when the acid is collected and drained, the acid is directed through a wastewater pipe 505 to a wastewater holding tank 507, which is constructed to withstand liquid acids. From the wastewater hold tank 507, the acid liquid is directed through a wastewater pump 509 to a chemical neutralization tank 511, where the acids are neutralized. Chemical holding tank 513 stores a chemical concentration used for neutralization of nitric acid and sulfuric acid, e.g., a concentration of limestone. The chemical concentration is pumped by the chemical pump 515 to the chemical neutralization tank 511 where the chemical is mixed with a chemical mixer 517 and monitored with a chemical meter 519 to ensure all acids have been neutralized.

In accordance with one or more embodiments, the neutralized acids are directed to a wastewater press 521, where the salts and particulate are pressed out and sent to a disposal site. The wastewater that is clear of all salts and particulate is directed to another wastewater pump 523 and sent to a holding tank 525. There, the wastewater will collect and sit until a demand from the RO system requires additional water, at which time the clean wastewater 527 will be sent to the RO system. Wastewater process is designed based on an individual plant's wastewater streams to ensure the best combination of physical and chemical treatment, so as to bring a plant into compliance with federal, state, and local discharge standards. The wastewater system will recycle, neutralize acids and alkalis, precipitate metals, clarify output water and concentrate solids for disposal. Efficiency is stressed through waste minimization and process bath reclamation to reduce operating costs.

FIG. 6 shows the heat reclaim system in accordance with one or more embodiments of the invention. Exhaust gas passes through the heat reclaim coil 601, which is a standard finned tube water coil, designed using an accurate simulation model. In accordance with one or more embodiments, the cold fluid enters the heat reclaim coil 601, where the fluids are heated by energy transfer from the hot exhaust gas. The hot water fluid is directed from the heat reclaim coils 601 through a three way control valve 605 where the hot water fluid is directed to the reheat coil 603, where the hot water fluid transfers its energy from the reheat coil 603 back into the exhaust gas to dry the saturated exhaust gas before going to atmosphere. From the reheat coil 603, the hot water fluid has cooled to a warm water fluid condition and is directed to the cooling tower pump 607, where the warm water fluid is forced into the cooling tower 609 so as to cool the warm water fluid to ambient temperature and then is sent back to the heat reclaim coil 601 to restart the cycle. When the reheat coil 603 demand is satisfied the control valve 605 delivers a portion of the hot water to a heat exchanger 611 through a control valve 613. The hot water 615 is then transferred to the building process heating system (not shown), then returned to the heat exchanger 611 to be reheated. When there is no demand for heat at the heat exchanger 611, the control valve 613 is closed and the hot water is delivered to the deaerator 617, where the heat is stored until the HRSG is in demand for make-up water. The hot water then is directed through another control valve 619 to the low pressure economizer (not shown) where the hot water starts the cycle in the HRSG.

FIG. 7 shows a flow chart illustrating a method for ozone NOx control in combination with heat recovery steam generation (“HRSG”) for a fossil fuel turbine application. In ST701, initial heat from a stream of exhaust gas from a fossil fuel fired turbine is recovered by passing the stream of exhaust gas through the front end of a heat recovery steam generation system (HRSG). The front end of the HRSG may comprise three major components: superheaters, evaporators, economizers. One of ordinary skill will appreciate that the above components of the HRSG are modular and that, depending on how large the system and/or number of pressure levels needed, additional components may be added onto the system as needed.

In ST703, the stream of exhaust gas between the HRSG economizer and the evaporator is contacted with ozone thereby forming a stream of exhaust gas comprising NO₂ and residual NO. The ozone may originate from an ozone generation system similar to that described in more detail above with reference to FIG. 3.

In ST705, heat may be recovered from the stream of exhaust gas comprising

NO₂ and residual NO to produce hot water. In accordance with one or more embodiments, the heat may be recovered by passing the exhaust gas comprising NO₂ and residual NO through a heat reclaim coil as described in more detail above in reference to FIG. 6.

In ST707, the stream of exhaust gas comprising NO₂ and residual NO is contacted with a water mist/fog to create an exhaust stream comprising nitric acid (HNO₃) and residual NO. In ST709, the stream of exhaust gas comprising HNO₃ is cooled by encountering a first condensing medium. In ST711, a first residual water film is collected on a first condensing medium to capture the HNO₃, thereby creating an exhaust stream comprising residual HNO₃ and residual NO. In ST 713, the first water film and HNO₃ is removed from the first condensing medium and sent to a wastewater processing system as discussed in more detail in reference to FIG. 5.

In ST715-721, the exhaust gas stream is further cleaned/scrubbed by conducting another water mist/fog contacting step, another cooling step, and another water film collection and removal step. These and/or any number of additional cleaning steps may be accomplished by passing the exhaust stream through several consecutive misting/fogging arrays and condensing sections. In accordance with one or more embodiments, passing the gas through multiple misting/fogging arrays and condensing sections results in a saturated exhaust gas stream that has been suitably scrubbed of NOx. One of ordinary skill will appreciate that the degree of scrubbing may vary depending on many factors, e.g., initial NOx concentrations, turbine fuel types, regulatory considerations, etc.

In ST723, the saturated exhaust stream is reheated to dry the saturated exhaust stream thereby creating a clean exhaust stream wherein both NOx and water has been removed. This clean exhaust stream may be further directed to atmosphere, e.g., by way of a flue or stack.

While the embodiments have been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments may be devised which do not depart from the scope of the embodiments as disclosed herein. Accordingly, the scope of the embodiments should be limited only by the attached claims.

Claims

1. A system for reducing NOx and recovering waste heat from a stream of exhaust gas from a fossil fuel fired turbine, the system comprising:

a superheater configured to recover heat from the stream of exhaust gas;

an evaporator configured to recover heat from the stream of exhaust gas, wherein the evaporator is located downstream from the superheater;

an economizer configured to recover heat from the stream of exhaust gas, wherein the economizer is located downstream from the evaporator;

an ozone aspirator, located between the evaporator and economizer, wherein the ozone aspirator is configured to receive the stream of exhaust gas comprising nitric oxide (NO) from the evaporator and contact the stream of exhaust gas comprising NO with ozone to convert the NO to nitrogen dioxide (NO2) thereby forming a stream of exhaust gas comprising NO2 and residual NO;

a heat reclaim coil configured to capture heat from the stream of exhaust gas comprising NO2 and residual NO to produce hot water;

a first misting stage configured to receive the stream of exhaust gas comprising NO2 and residual NO and contact the stream of exhaust gas comprising NO2 and residual NO with water mist to create an exhaust stream comprising nitric acid (HNO3) and residual NO; and

a first condensing medium configured to cool the stream of exhaust gas comprising HNO3 and residual NO and to collect a first residual water film thereon to capture the HNO3, thereby creating an exhaust stream comprising residual HNO3 and residual NO.

2. The system of claim 1, further comprising:

a second misting stage configured to receive the exhaust stream comprising residual HNO3 and residual NO and contact the exhaust stream comprising residual HNO3 and residual NO with water mist to further create residual exhaust stream comprising HNO3; and

a second condensing medium configured to cool the residual exhaust stream comprising HNO3 and collect a second residual water film thereon to capture the HNO3, thereby creating a saturated exhaust stream.

3. The system of claim 2, further comprising:

a reheat coil configured to reheat the saturated exhaust stream to remove moisture from the saturated exhaust stream thereby creating a clean exhaust stream wherein the NOx and water has been removed.

4. The system of claim 3 further comprising a fan configured to direct the clean exhaust gas stream out of the system.

5. The system of claim 3 further comprising a wastewater facility configured to receive a water film comprising the first and second water films.

6. The system of claim 3 further comprising an exhaust stack configured to receive the clean exhaust gas stream.

7. The system of claim 1, wherein the first condensing medium comprises a coating of one selected from a group consisting of Teflon and critical polyvinal chloride (CPVC).

8. The system of claim 2, wherein the second condensing medium comprises a coating of one selected from a group consisting of Teflon and critical polyvinal chloride (CPVC).

9. The system of claim 1, wherein the ozone aspirator is further configured to contact the stream of exhaust gas with a mixture of ozone and hydrogen peroxide.

10. A method for reducing NOx and recovering waste heat from a stream of exhaust gas from a fossil fuel fired turbine, the method comprising:

recovering heat from the stream of exhaust gas by passing the stream of exhaust gas through a superheater, an evaporator located downstream from the superheater, and an economizer located downstream from the evaporator;

contacting the stream of exhaust gas between the economizer and the evaporator with ozone gas to convert the NO to nitrogen dioxide (NO2) thereby forming a stream of exhaust gas comprising NO2 and residual NO;

recovering heat from the stream of exhaust gas comprising NO2 and residual NO to produce hot water;

contacting the stream of exhaust gas comprising NO2 and residual NO with water mist to create an exhaust stream comprising nitric acid (HNO3) and residual NO;

cooling the stream of exhaust gas comprising HNO3 and residual NO;

collecting a first residual water film on a first condensing medium to capture the HNO3, thereby creating an exhaust stream comprising residual HNO3 and residual NO; and

removing the first water film and HNO3.

11. The method of claim 10 further comprising:

further contacting the stream of exhaust gas comprising residual HNO3 and residual NO with water mist thereby creating an exhaust stream comprising HNO3;

cooling the stream of exhaust gas comprising HNO3;

collecting a second residual water film on a second condensing medium to capture the HNO3, thereby creating a saturated exhaust stream comprising HNO3; and

removing the second water film and HNO3; and

reheating, the saturated exhaust stream to dry the saturated exhaust stream thereby creating a clean exhaust stream wherein the NOx and water has been removed.

12. The method of claim 10, further comprising directing the hot water from a reclaim coil to a reheat coil.

13. The method of claim 12, further comprising directing the hot water from the reheat coil to a cooling tower and directing the hot water from the cooling tower to the heat reclaim coil.

14. The method of claim 10, further comprising directing the hot water from the reclaim coil to a heat exchanger and directing the hot water from the heat exchanger to a building process heating system.

15. The method of claim 10, further comprising directing the hot water from a reclaim coil to a deaerator and directing the hot water from the deaerator to a low pressure economizer.

16. The method of claim 11, further comprising directing the clean exhaust gas stream out of the system.

17. The method of claim 11, further comprising providing a wastewater facility with wastewater comprising the first and second water films.

18. The method of claim 11, further comprising exhausting the clean exhaust gas stream from a stack.

19. The method of claim 10, further comprising contacting the stream of exhaust gas between the economizer and the evaporator with a mixture of ozone and hydrogen peroxide.