Embodiments described herein relate to an exhaust system for a plurality of gas turbines. More particularly, embodiments described herein relate to a method and system for mitigating condensate formation, effecting efficient recovery of heat from the exhaust gases and rendering a stable structural arrangement for a tall exhaust stack.
Exhaust gases emitted from a gas turbine are typically vented or discharged to the atmosphere through an exhaust stack positioned on the gas turbine. The exhaust gases flow in a stream up the exhaust stack along the sidewall thereof and are pushed out of the exhaust stack by the pressure differential established across the gas turbine. The exhaust gases include a certain amount of moisture and other acidic pollutants such as SO2 and H2S that may condense when cooled.
The exhaust gases typically flow through the exhaust stack in a laminar pattern. Laminar flow is defined as fluid gliding through a channel (in this case the exhaust stack) in smooth layers, where the innermost layer flows at a higher rate than the outermost due to the effect of friction at the channel wall (in this case sidewall of the exhaust stack) interface. Laminar flow of the exhaust gases through the exhaust stack causes cool spots to be formed in the region along the sidewall of the exhaust stack. This results in condensation of the moisture and acidic pollutants contained in the exhaust gases along the exhaust stack sidewalls. Condensation slows down the flow of the exhaust gases through the exhaust stack. Condensate formation can also damage the exhaust system, shortening its life and increasing the frequency of maintenance.
Typically gas turbines are associated with a heat recovery/exchanger system for recovery of heat contained in the exhaust gases. The recovered heat can be converted into electrical power for powering or operating other devices. The heat contained in the exhaust gases may be recovered using systems based on Organic Rankine Cycle (ORC), heat pumps, or vane motors. Typically a heat exchanger has a plurality of heat pipes through which working fluid (coolant) flows. Heat from the exhaust gases flowing through the heat exchanger is transferred through the pipe wall to the working fluid. Applicant believes that since flow of exhaust gases through the gas turbine is laminar, flow of exhaust gases through the heat exchanger will also be laminar. Laminar flow develops an “insulating blanket” along the heat transfer region (along the pipe walls). The underlying physics of the blanket creation stems from the dynamic behaviour of molecules that participate in the heat transfer. As heat is transferred, the temperature of the gas molecules is lowered with a corresponding rise in surface (pipe wall) temperature. These cooler molecules insulate the surface from the higher temperature molecules further away from the surface, slowing convective heat transfer. This results in precipitate formation along the heat transfer region and inefficient heat transfer.
US Patent Application Publication No. 2012/0180485 to Smith et al. teaches an exhaust system that combines the exhaust gases from a plurality of gas turbines for increased heat recovery. US Patent Application Publication No. 2012/0180485 does not recognise issues related to condensate formation in the exhaust stack or in the heat exchanger nor does it provide a solution for addressing these issues.
Plume dispersion can be positively influenced by increasing the height of a conventional exhaust stack. However, height of the exhaust stack cannot be increased without compromising the structural integrity of the exhaust system.
Therefore, a need exists for an improved exhaust system that mitigates condensate formation in the exhaust stack, increases heat transfer efficiency and improves plume dispersion without compromising the structural integrity of the exhaust system.
Embodiments described herein relate to a system for mitigating condensate formation in the exhaust stack. Condensate formation is mitigated by inducing non-laminar flow such as turbulence to the exhaust gases flowing through the exhaust stack. Turbulence can be induced in a number of ways as described in the following description.
Embodiments described herein also relate to an improved and efficient heat transfer process. This is achieved through one or more of the aspects of inducing non-laminar flow and maintaining the temperature of the exhaust gases flowing through the exhaust stack above a threshold dew point. Dew point control can involve using an automated controller to continuously monitor the temperature, composition, and pressure of the flue gases (exhaust gases) to calculate the threshold dew point and using this information to control heat recovery from the exhaust gases. This kind of control introduces a layer of operation flexibility since the dew point can vary depending on the composition of the exhaust gases.
Embodiments described herein also relate to providing a tall exhaust stack for improved plume dispersion without compromising structural integrity of the exhaust system.
Accordingly in one broad aspect an exhaust system for a plurality of gas turbines is provided. The exhaust system comprises a common exhaust stack disposed in a generally vertical arrangement. An exhaust gas outlet positioned on each of the plurality of gas turbines is coupled to the common exhaust stack through a respective first flow-changing means for inducing non-laminar flow of exhaust gases through the common exhaust stack.
Accordingly in another broad aspect a method of recovering heat from exhaust gases flowing through a common exhaust stack receiving exhaust gases from a plurality of gas turbines connected thereto is provided. A heat exchanger is in the common exhaust stack. Non-laminar flow of exhaust gases is induced for flow through the common exhaust stack and the heat exchanger for minimizing formation of cool spots along a heat transfer interface. A threshold dew point is determined for exit of exhaust gases through the common exhaust stack. The exhaust gases are directed through the heat exchanger for recovery of heat from the exhaust gases along the heat transfer interface. The temperature at the heat exchanger is continuously monitored and heat recovery is reduced from the exhaust gases flowing through the heat exchanger when the temperature at the heat exchanger approaches the threshold dew point.
Accordingly in another broad aspect a method of recovering heat from exhaust gases flowing through a common exhaust stack receiving exhaust gases from a plurality of gas turbines connected thereto is provided. A heat exchanger is located in a heat exchanger conduit. The heat exchanger conduit is arranged in a parallel arrangement with the common exhaust stack. Non-laminar flow of exhaust gases is induced for flow through the common exhaust stack and the heat exchanger conduit for minimizing formation of cool spots along a heat transfer interface. A threshold dew point is determined for exit of exhaust gases through the common exhaust stack and/or the heat exchanger conduit. The exhaust gases are directed through the heat exchanger conduit for recovery of heat from the exhaust gases along the heat transfer interface. The temperature at the heat exchanger conduit is continuously monitored and flow of the exhaust gases through the common exhaust stack and the heat exchanger conduit is controlled in response to the temperature at the heat exchanger conduit. The threshold dew point can be continuously determined during an operation cycle.
Further, flow of exhaust gases through the common exhaust stack and the heat exchanger conduit is controlled by opening an access to the heat exchanger conduit when the temperature at the heat exchanger conduit is generously above the threshold dew point for passage of exhaust gases therethrough. An access to the common exhaust stack is opened and the access to the heat exchanger conduit is maintained open when the temperature at the heat exchanger conduit is above the threshold dew point. The access to the heat exchanger conduit is closed and the access to the common exhaust stack is maintained open when the temperature at the heat exchanger conduit approaches the threshold dew point.
Embodiments described herein relate to an exhaust system which mitigates condensate formation in an exhaust stack by creating turbulence in exhaust gases flowing through the exhaust stack.
Embodiments described herein also relate an exhaust system and method for effecting improved heat transfer.
In the instant disclosure, the exhaust gas outlet 3 of each of the plurality of gas turbines 3 is coupled to the common exhaust stack 4 through a respective first flow-changing means 5. The first flow-changing means 5 minimizes any predisposition of the exhaust gases to flow in a laminar pattern and induces non-laminar flow of exhaust gases through the common exhaust stack 4.
In one embodiment, each of the first flow-changing means 5 is connected at an angle to the common exhaust stack 4.
In one embodiment, the first flow-changing means 5 is implemented by connecting a first set of exhaust gas outlet connectors or interconnects 3a at an angle to the common exhaust stack 4. The exhaust gas outlets 3 are connected or coupled to the common exhaust stack through the angled connectors 3a and form an angled connection with the common exhaust stack 4. The angled connection causes the gases flowing into the common exhaust stack 4 through the exhaust gas outlets 3 to rotate thereby changing the flow pattern of the exhaust gases to a non-laminar flow pattern. The non-laminar flow of the exhaust gases through the common exhaust stack 4 reduces the formation of cool spots along the sidewall of the common exhaust stack 4. This is in turn minimizes condensate formation. Further, to leverage the natural up draught of the hot exhaust gases and to reduce backflow into any gas turbine 2 which may be inactive, preferably, the exhaust gas outlets 3 are also angled upwards between the gas turbines 2 and the connectors 3a.
In one embodiment and with reference to
In another embodiment and with reference to
The first elements 6 introduce local disturbances which further enhance mixing of the exhaust gases flowing along the first elements 6. The first elements 6 further aid in elimination of cool spots being formed in the common exhaust stack 4. Preferably, the first elements 6 are a plurality of fins located in the common exhaust stack 4. Local disturbances in the flow path can also be introduced by treating the internal surface of the common exhaust stack 4 and/or exhaust gas outlet 3. Internal surface treatment may include introducing surface corrugations or surface roughness.
Turbulence in the exhaust gases flowing through the common exhaust stack 4 can be enhanced by vertically offsetting the first flow-changing means 5 along the common exhaust stack 4, by offsetting the centerlines of the first flow-changing means 5 or by providing local disturbances in the flow path of the exhaust gases or a combination of the various arrangements illustrated in
In one embodiment, the exhaust system 1 comprises at least one header 7 and at least two exhaust gas outlets 3 are coupled to the at least one header through at least two second flow-changing means 8 for inducing non-laminar flow of exhaust gases through the at least one header 7. The at least one header 7 is coupled to the common exhaust stack 4 through at least one of the first-flow changing means 5 for inducing non-laminar flow of exhaust gases through the common exhaust stack 4. In this embodiment, at least some of the exhaust gas outlets 3 are connected to the header 7 through second flow-changing means 8. In another embodiment, at least some of the exhaust gas outlets 3 can be directly connected to the header 7. The flow of exhaust gases through the exhaust gas outlets 3 connected to the header 7 through the second flow-changing means 8 are more significantly induced to be non-laminar as compared to those directly connected to the header 7. In one embodiment, each second flow-changing means 8 is connected at an angle to the at least one header 7.
In one embodiment, as illustrated in
In greater detail, exhaust system 1 shown in
In one embodiment, inducement of non-laminar flow of exhaust gases in an exhaust system 1 comprising three or more headers 7 can be further enhanced by vertically offsetting each of the three or more headers 7 from one another along the common exhaust stack 4.
With reference to
In one embodiment, non-laminar flow comprises turbulent flow of exhaust gases. Each of the first flow-changing means 5 induces turbulent flow of exhaust gases.
In another embodiment, as shown in
Inducement of non-laminar flow of the exhaust gases through the header 7 can be further enhanced by providing second elements (not shown) disposed at about the second flow-changing means 8. The second elements may be similar in construction to the first elements 6 described in detail with reference to
Non-laminar flow through the header 7 and the common exhaust stack 4 can be enhanced by offsetting the centerlines of the exhaust gas outlets 3 feeding into the header 7, vertically offsetting the headers 7 along the common exhaust stack 4, offsetting the centerlines of the headers 7 feeding in to the common exhaust stack 4 (similar to
Combining exhaust gases from a plurality of gas turbines 2 into a common exhaust stack 4 results in increased plume dispersion characteristics. Due to the presence of pollutants in the exhaust gases, constant efforts are being made to disperse the exhaust gases at higher altitudes. Attempts in the past have included increasing the height of the individual exhaust stack on each gas turbine. However, increasing the stack height is not a feasible solution. Increasing the stack height results in subjecting the exhaust stack to greater static and dynamic stresses as wind loading typically increases with altitude. Under such conditions, it may become difficult to keep the exhaust stack stable and this may result in overturning or buckling of the exhaust stack, which in turn may damage the gas turbine.
The arrangement of the exhaust gas outlets 3 or headers 7 about the circumference of the common exhaust stack 4 also renders the common exhaust stack design of the instant disclosure structurally robust. These factors allow construction of a taller exhaust stack without compromising its stability and durability during exposure to wind loading. Three arrangements for increasing structural rigidity of the exhaust system 1 are contemplated. In a first arrangement three or more gas turbines 2 are distributed circumferentially about the common exhaust stack 4 for providing structural rigidity to the exhaust system 1, such as under wind loading. Preferably, the three or more gas turbines 2 are evenly spaced about the circumference of the common exhaust stack 4.
The headers 7 or exhaust gas outlets 3 around the common exhaust stack 4 act as reinforcing members and provide the additional strength and rigidity required for maintaining the common exhaust stack 4 stable under wind loading. Structural rigidity can optionally be further enhanced by providing individual support members 10 (
As wind speed typically increases with altitude, greater dispersion of the exhaust gases through the common exhaust stack 4 is achieved. This helps in alleviating local concentration of odours and pollutants contained in the exhaust gases thereby minimising undesirable and potentially hazardous effects.
The following equations explain the relationship between buoyancy of the exhaust gases and plume rise:
Plume rise dynamics are described by Briggs' expression (1.1):
Δh is effective height of the plume centreline above the exhaust stack tip, in metres; ū is average wind speed, in metre/second;
x is the distance downwind of the plume, in meters;
F is buoyancy flux of the plume, in metre4 second3;
The buoyancy flux F is calculated as follows (1.2)
g is the acceleration due to the gravity, in metre/sec2;
V is the volumetric flow rate of the stack gas, in kg/sec;
Tstack is the temperature of the exhaust gas, in ° C.;
Tambient is the temperature of ambient air, in ° C.;
Buoyancy is independent of the diameter of the exhaust stack and is defined by the volumetric flow of gas through the exhaust stack and the gas temperature in exhaust stack. The elevated (compared to ambient) temperature of the exhaust gases ensures that the exhaust system is buoyancy dominated and the combination of exhaust gases from the plurality of gas turbines 3 increases the volumetric flow through the common exhaust stack 4 leaving other parameters unchanged. This increased flow has a cubed root impact on the plume height meaning that, for a cluster of twenty gas turbines, the plume height is increased by a factor of approximately 2.7 times.
Thus, for a given stack height, each gas turbine inputting to the common exhaust stack 4 can achieve satisfactory dispersion performance at a markedly lower operating volume flow rate than would be required if the exhaust stack were isolated. The common exhaust stack design thus allows the gas turbines to continue to meet air dispersion requirements even if one or more gas turbines 3 in the exhaust system are inactive or producing less.
An example illustrating the effectiveness of a common exhaust stack 4 is set out below:
For a flow of 34,000 m3/day with an H2S content of 800 ppm it was found that, by increasing the exhaust stack height by 23% over that necessary to meet SO2 air quality objectives, the H2S handling capabilities of the exhaust stack were increased to over 2,000 ppm.
The common stack design system creates a simpler, more robust structure than would be achieved if each individual gas turbine was furnished with its own stack. Individual stacks tall enough to guarantee the same air dispersion performance as the common exhaust stack design would be considerably taller (assuming a fixed diameter) than the common exhaust stack and thus subject to greater static and dynamic stresses due to their increased exposure to higher winds. Since the common exhaust stack design combines multiple gas turbine exhausts into one, it is possible to design an exhaust stack that has a height-to-diameter ratio comparable to a small single gas turbine exhaust stack. The arrangement of the gas outlets/headers about the circumference of the common exhaust stack also renders the common exhaust stack design structurally robust. These factors allow construction of a taller exhaust stack without compromising its stability and durability during exposure to higher winds with high loading on the exhaust stack.
In one embodiment and with reference to
As described in the foregoing paragraphs, laminar flow of exhaust gases through a heat exchanger in a conventional exhaust system results in cool spots being formed along the heat transfer region and inefficient heat transfer.
Flow of the exhaust gases through the exhaust system 1 of the instant disclosure is non-laminar. Non-laminar flow results in uniformity of temperature in the working space. Working space includes the conduits/components through which the exhaust gases flow namely the headers 7, the common exhaust stack 4 and the heat exchanger 11. Non-laminar flow increases the velocity of the exhaust gas molecules. When the velocity increases, cooler molecules that have transferred energy to the surface are quickly replaced by higher temperature molecules, resulting in increased convective heat transfer. Further, non-laminar flow also minimizes the fluctuations in the temperature in the working space due to one or more inactive gas turbines 3 or when throughput from the gas turbines is not equal.
Applicant has identified that in order to significantly minimize condensate formation in the common exhaust stack 4, temperature of the exhaust gases flowing out of the heat exchanger 11 must be maintained above a certain threshold dew point. Selection of the threshold dew point depends on the composition of the exhaust gases and particular concentrations of the compounds therein. For exhaust gases generated from the burning of natural gas, the threshold dew point must be maintained between about 100° C. and about 200° C., preferably above about 150° C. One method for determining the threshold dew point is to couple a gas analyser/chromatographer (not shown) to the fuel gases to the gas turbines 2. The gas analyser continuously measures the moisture and/or acid gas content in the exhaust gases and determines a threshold dew point. Maintaining the temperature in the common exhaust stack 4 above the threshold dew point enables the exhaust gases to exit the common exhaust stack 4 without condensation. It will be understood that the determined threshold dew point will change depending on the composition of the exhaust gases and will vary during an operation cycle of the exhaust system 1.
Temperature of the exhaust gases flowing through the common exhaust stack 4 can be affected by a number of parameters—variable flow rate of exhaust gases from the gas turbines 3 for the reasons identified above, a large proportion of exhaust gases being diverted to the heat exchanger 11 for recovery of heat. In order to optimize the exhaust system 1, for recovering the available energy and the avoidance of dew point issues in the common exhaust stack 4, in one embodiment and with reference to
The automated controller 12 may be a microcontroller or other logic-based control system comprising sensors (not shown) for measuring temperature. Because the temperature in the common stack 4 is significantly uniform because of the non-laminar flow, it is possible to sense the temperature at the sidewall of the common exhaust stack 4. A less sophisticated sensor can, therefore, be used to sense the temperature. This results in significant cost savings.
In one embodiment and with reference to
In another embodiment and with reference to
In yet another embodiment and with reference to
In another embodiment and with reference to
Temperature regulation in the common exhaust stack 4 can be achieved either by changing the flow rate of the working fluid or by decreasing the residence time of the exhaust gases through the heat exchanger 11 or by providing a bypass passage 15 or by controlling access to a housing locating the heat exchanger or any combination of the alternatives stated above.
In one embodiment and with reference to
In one arrangement and with reference to
Heat recovery can be further enhanced by allowing a controlled amount of condensate to form in the common exhaust stack 4 or heat exchanger conduit 16. The amount is based on an evaluation of additional power production versus increased maintenance and repair cost of the exhaust system associated with the condensate formation. Calculation of the threshold dew point (discharge temperature) for formation of the controlled amount of condensate may be based on prior operating history (integrated condensate level estimate) to determine the degree of acceptable degradation in the exhaust materials and thus define a value-based optimal flue gas discharge temperature. Based on this recorded data a prediction model can be developed for real time regulation of flow of exhaust gases through the common exhaust stack 4 and the heat exchanger conduit 16. This involves adapting the automated controller 12 to receive input from a gas analyser, flow velocity sensors, temperature sensors and pressure sensors. The temperature sensors, pressure sensors, flow velocity sensors and the gas analyser are located onto the common pipeline that leads the solution gas to the gas turbine inlets. The automated controller 12 receives input from the various sensors, processes the input and generates an output for regulating flow of exhaust gases. The gas analyser provides measurements of the moisture and acid gas content in the exhaust gases, for example H2S, and time tags this data before transmission to the automated controller 12 paired with the corresponding flow velocity data. The automated controller 12 will use this data to calculate when each time packet will arrive at the common exhaust stack 4 and will be able to use the current temperature data in the common exhaust stack 4 to predict a threshold dew point and estimate whether the present heat recovery will cause the temperature to drop below the predicted threshold dew point.
Equations for predicting the threshold dew point are known and are as follows:
Dew points, in ° C., of the gasses SO3, SO2, HCl and NO2 can be calculated by means of the equations of Verhoff, Perry, and Kiang (W. M. M. Huijbregts, R. G. I. Leferink, “Latest advances in the understanding of acid dewpoint corrosion: corrosion and stress corrosion cracking in combustion gas condensates”, Anti-corrosion Methods and Materials, 51 (3):173-178, 2004):
A: Dew point equation of SO3 according to Verhoff:
B: Dew point equation of SO2 according to Kiang:
C: Dew point equation of HCl according to Kiang:
D: Dew point equation of NO2 according to Perry:
Px—is partial pressure, in atmospheres (equation A) and in mmHg (equation B, C, D), where the subscript x refers to the component of interest;
Td—is the acid dew point temperature for each particular acid, in Kelvins;
Compared with published measured data, the acid dew points predicted with equations A, B, C, D are said to be within 9° C. of the published measured data. When the temperature starts approaching the predicted threshold dew point, the system needs to reduce the heat transfer from the exhaust gases to the heat recovery fluid. This can be achieved by the arrangements illustrated in
The exhaust system 1 may comprise back-flow dampers (not shown) and isolation dampers (not shown) for preventing exhaust from an operating gas turbine from entering a non-operating gas turbine. US Patent Application Publication No. 2012/0180485 to Smith et al. teaches implementation of such dampers.
The exhaust system 1 may also comprise a drain (not shown) for draining any fluid that may be present in the exhaust gas outlets 3. The drain is typically positioned adjacent to the isolation damper.
This application claims the benefits under 35 U.S.C 119(e) of U.S. Provisional Application Ser. No. 61/658,542, filed Jun. 12, 2012, which is incorporated fully herein by reference.
Number | Date | Country | |
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61658542 | Jun 2012 | US |