using a combined cycle

The present invention relates to method and system for generating power using a combined cycle, in

particular a combined cycle in which an organic Rankine cycle is used as second power system.

Power plants, such as gas turbines, produce power by combusting fuel. The power is usually produced in the form of electricity. This is usually referred to as the first (power) system.

In order to increase the efficiency of power plants, it is known to add a waste heat recovery system (the second (power) system) to generate additional power from the hot flue gasses produced by the first system. The combination of the first and second system is usually referred to as a combined cycle.

Often the first system is a gas turbine operated by a Brayton cycle and the second system is a Rankine cycle, such as an organic Rankine cycle (ORC) .

It is known to use water/steam as working fluid for the Rankine cycle. However, the use of water introduces corrosion risks and necessitates anti-corrosion measures.

The flue gasses produced by a gas turbine may typically have a temperature greater than 450°C, e.g. in the range of 450°C - 650°C.

Commercial available organic Rankine cycles are typically made for situations with the source heat temperatures in the range of 250°C - 300°C. At higher temperatures, the stability and operability of available organic Rankine cycle working fluids become an issue. Commercial available organic Rankine cycles therefore require an intermediate hot oil loop in order to avoid direct heat exchange between the working fluid and the turbine flue gasses. This reduces their efficiency, increases cost, and ultimately reduces their returns on investment .

US2013/0133868 describes a system for power

generation using an organic Rankine cycle. A number of possible ORC fluids are mentioned, comprising pentane, propane, cyclohexane, cyclopentane, butane,

fluorohydrocarbon, a ketone such as aceton or an aromatic such as toluene or thiophene .

US2005188697 describes the use of organic working fluids in Rankine cycles including polyfluorinated ethers and polyfluorinated ketones and mixtures thereof.

EP1764487 disclose the use of organic working fluids for use in an organic Rankine cycle for energy recovery, especially for utilization of heat sources having a temperature up to approx. 200°C, preferably up to approx. 180°C.

US2011/0100009 describes a system and method

including heat exchangers using Organic Rankine Cycle (ORC) fluids in power generation systems. The system includes a heat exchanger configured to be mounted inside an exhaust stack that guides hot flue gases . The heat exchanger is configured to receive a liquid stream of a first fluid and to generate a vapor stream of the first fluid. The heat exchanger is configured to include a double walled pipe, where the first fluid is disposed within an inner wall of the double walled pipe and a second fluid is disposed between the inner wall and an outer wall of the double walled pipe. The double walled pipe is used to shield the working fluid from direct exposure to the high temperature of the flue gasses and suggests to keep the temperature of the working fluid below 300°C.

Other examples of waste heat recovery are provided by US2013/0152576, WO2013/103447 and EP2532845.

It is an object to overcome at least one of the disadvantages associated with the prior art.

Therefore there is provided a method of generating power using a combined cycle, the method comprising:

- operating a first power system in which fuel is burned to generate primary power and a flue gas stream at a flue gas temperature greater than 450°C,

- operating a second power system to generate secondary power from the heat comprised by the flue gas stream, the second power system comprising a waste heat recovery heat exchanger,

the method further comprising:

- passing the flue gas stream through the waste heat recovery heat exchanger,

- passing a pressurized waste heat recovery fluid through the waste heat recovery heat exchanger to receive heat from the flue gas stream thereby obtaining a pressurized vaporous waste heat recovery fluid having a temperature in the range of 350°C - 500°C,

wherein the waste heat recovery fluid consists of fluorinated ketones.

Further provided is a system for generating power, the system comprises:

- a first power system comprising a fuel burning stage arranged to burn fuel to generate primary power and a flue gas stream at a flue gas temperature greater than 450°C,

- a second power system arranged to generate secondary power from the heat comprised by the flue gas stream, the second power system comprising a waste heat recovery heat exchanger and a waste heat recovery fluid, wherein the waste heat recovery heat exchanger comprises a first fluid path arranged to receive and convey at least part of the flue gas stream, and a second fluid path arranged to receive and convey the waste heat recovery fluid,

the first and second fluid paths being separated by a heat exchange wall,

the heat exchange wall being suitable to to be exposed to the flue gas stream at a flue gas temperature in the range of 450°C - 650°C, and the heat exchange wall being suitable to to be exposed to the waste heat recovery fluid at a temperature in the range of 350°C - 500°C,

wherein the working fluid comprised by the second power system consists of fluorinated ketones.

The waste heat recovery fluid is temperature stable up to a temperature of 500°C. The term temperature stable is used to indicate that the molecules don't decompose under the influence of the temperature.

The waste heat recovery fluid substantially consists of fluorinated ketones, preferably consists of

fluorinated ketones with 4 - 6 carbon atoms of which 4 - 6 are fluorinated carbon atoms.

Most preferably the waste heat recovery fluid substantially consists of dodecafluoro-2-methylpentan-3- one . Preferably the waste heat recovery fluid comprises more than 90 mol% dodecafluoro-2-methylpentan-3-one , preferably more than 95 mol% dodecafluoro-2-methylpentan-

3-one, more preferably more than 98 mol% dodecafluoro-2- methylpentan-3-one and most preferably 100 mol%

dodecafluoro-2-methylpentan-3-one . The waste heat recovery fluid may be essentially pure dodecafluoro-2-methylpentan-3-one , where the skilled person will understand that the term pure is used to indicate a level of purity that is practically

achievable, e.g. a purity of more than 99 mol%. For instance, the waste heat recovery fluid essentially consisting of pure dodecafluoro-2-methylpentan-3-one may be obtained from 3M at a purity of more than 99 mol%.

Fluorinated ketones, in particular dodecafluoro-2- methylpentan-3-one , can advantageous be used as waste heat recovery fluid, for instance in a Rankine cycle, as it can be exposed to temperatures above 450°C.

This way, an intermediate working fluid, such as an intermediate hot oil loop, could be omitted and direct heat exchange between the flue gasses and the working fluid is made possible. This reduces cost and increases the efficiency of the cycle.

Alternatively, fluorinated ketones, in particular dodecafluoro-2-methylpentan-3-one , may also be used in an intermediate loop.

The term direct heat exchange is used in this text to indicate that the exchange of heat takes place without intermediate fluid or cycles . The term direct heat exchange is not used to indicate that the fluids

exchanging heat are mixed or brought into contact as is done in a direct heat exchanger (in which the fluids to exchange heat are mixed) .

Heat exchange between the waste heat recovery fluid and the flue gas stream is typically done by an indirect heat exchanger in which the fluids are kept separated by a heat exchange wall through which the heat is

transmitted. It is discovered that a waste heat recovery fluid as defined above, in particular consisting of dodecafluoro- 2-methylpentan-3-one, is stable at relatively high temperatures, i.e. in the range of 350-500°C. This avoids degradation of circulating fluids.

Also it is discovered that the above identified waste heat exchange fluid produces power (mechanical work) in a relatively efficient way, i.e. at a 9-11% efficiency from a flue gas stream in the indicated temperature range (compared to 6 - 9% when using water) .

Furthermore, the suggested waste heat recovery fluid is non-corrosive to all metals and hard polymers.

The Global Warming Potential (GWP) of the waste heat recovery fluid is low, compared to known waste heat recovery fluids, such as chlorofluorocarbon (CFC, also known as Freon) , due to the ozone depletion potential.

Further advantages and details of the present invention will become apparent with the benefit of the following detailed description of embodiments and upon reference to the accompanying drawings, in which:

Figure 1 schematically shows a system according to an embodiment, and

Figure 2 schematically shows an system according to an alternative embodiment.

While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the figures and will herein be described in detail. It should be

understood that the figures and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the scope as defined by the appended claims .

Further, although the invention will be described in terms of specific embodiments, it will be understood that various elements of the specific embodiments of the invention will be applicable to all embodiments disclosed herein .

According to the embodiments a method and system is provided in which a first and second power system are operated, wherein the second power system is powered by the heat of the flue gas stream of the first power system. The second power system comprises a waste heat recovery heat exchanger through which a pressurized waste heat recovery fluid is circulated, wherein the waste heat recovery fluid comprises fluorinated ketones, in

particular dodecafluoro-2-methylpentan-3-one .

According to an embodiment, the first power system comprises a gas turbine operated by a Brayton cycle. The flue gass stream produced by such a first power system typically have a temperature greater than 450°C,

typically in the range of 450°C - 650°C

According to an embodiment operating the second power system comprises circulating a working fluid through a heat engine cycle, in particular a Rankine cycle. Rankine cycles are an efficient way to transform heat into power.

The waste heat recovery fluid may be circulated through the waste heat recovery heat exchanger as part of an intermediate heat transfer cycle. This embodiment will be described in more detail below with reference to Fig.

2.

According to an embodiment the working fluid circulated through the heat engine cycle is the waste heat recovery fluid. In such an embodiment, the waste heat recovery heat exchanger is part of the heat engine cycle. The above identified waste heat recovery fluid is suitable for being cycled through a waste heat recovery heat exchanger which is exposed to a flue gas stream at a flue gas temperature greater than 450°C.

According to an embodiment, the pressurized vaporous waste heat recovery fluid as obtained from the waste heat recovery heat exchanger has a temperature in the range of 350°C - 500°C, preferably in the range of 450°C - 500°C.

The waste heat recovery fluid is stable up to

temperatures in the range of 400°C - 500°C and can therefor advantageous be used in an organic Rankine cycle .

According to an embodiment the heat engine cycle comprises a condenser in which the waste heat recovery fluid is condensed against an ambient cooling stream, the ambient cooling stream being an ambient air stream or an ambient (sea) water stream. The working fluid may be cooled to a temperature in the range 15°C - 80°C in the condenser .

The waste heat recovery fluid can be used in a cycle in which it experiences a temperature difference of more than 320°C, even more than 400°C or even more than 450°C. This allows cooling the waste heat recovery fluid against the ambient and heating the waste heat recovery fluid against a flue gas stream having a temperature greater than 450°C.

According to an embodiment operating the second power system comprises circulating the waste heat recovery fluid as working fluid through a heat engine, such as a Rankine cycle. The Rankine cycle comprises the following steps, which are performed simultaneously: 1) Passing the pressurized waste heat recovery fluid through the waste heat recovery heat exchanger to receive heat from the flue gas stream thereby obtaining a pressurized vaporous waste heat recovery fluid. The pressurized vaporous waste heat recovery fluid may have a temperature in the range of 350°C - 500°C and pressure of more than 40 bar, e.g. 50 bar.

2) Expanding the pressurized vaporous waste heat recovery fluid over a (turbo-) expander, thereby

obtaining the secondary power and an expanded lower pressure vaporous waste heat recovery fluid. The expanded lower pressure vaporous waste heat recovery fluid may have a pressure of less than 3 bar, e.g. 1 bar and the temperature between 50°C - 150°C, e.g. 100°C.

3) Passing the expanded lower pressure vaporous waste heat recovery fluid through a condenser to obtain a liquid waste heat recovery fluid. The liquid waste heat recovery fluid may have a pressure of less than 3 bar, e.g. 1 bar and a temperature between 15°C - 100°C, e.g. 50°C.

4) Passing the liquid waste heat recovery fluid through a pump to obtain the pressurized liquid waste heat recovery fluid. The pressurized liquid waste heat recovery fluid may have a pressure of more than 40 bar, e.g. 50 bar and temperature in the range of 15°C - 100°C.

Fig. 1 schematically shows a system for generating power. The system comprises a first power system 1 and a second power system 2.

The first power system 1 comprises a fuel burning stage, here schematically depicted as a gas turbine. The gas turbine comprises a compressor 11, a fuel chamber 12 and an turbine 13. The turbine 13 drives the compressor 11 and excess power is used to drive shaft 14 which is coupled to a generator 15, such as an electric generator, to generate primary power.

A flue gas stream 16 leaves the turbine 13 via an exhaust 17 at a flue gas temperature greater than 450°C.

It will be understood that Fig. 1 shows a schematic view of an exemplary primary power system and that many variations are known to the skilled person.

Fig. 1 further schematically shows a second power system 2. The second power system 2 is arranged to generate secondary power from the heat of the flue gas stream 16. In order to do so, the second power system 2 comprises a waste heat recovery heat exchanger 21. In the embodiment shown in Fig. 1, the waste heat recovery heat exchanger 21 is positioned in the exhaust 17.

The waste heat recovery heat exchanger 17 comprises a first fluid path arranged to receive and convey at least part of the flue gas stream 16. The waste heat recovery heat exchanger 17 comprises a second fluid path arranged to receive and convey the waste heat recovery fluid. The waste heat recovery heat exchanger 17 may be any suitable type, including a plate heat exchanger.

According to the example shown in Fig. 1, the waste heat recovery heat exchanger 17 is a shell and tube heat exchanger, wherein the first fluid path is at the shell side and the second fluid path is at the tube side.

The first and second fluid paths are separated by a heat exchange wall, e.g. the walls forming the tubes of the shell and tube heat exchanger.

Fig. 1 shows a single tube but it will be understood that more than one tube may be present, each tube wall forming a heat exchange wall.

Preferably, for any type of waste heat recovery heat exchanger 21, the heat exchange wall is a single layer wall. The heat exchange does not comprise internal cooling facilities, intermediate isolation layers, double walls and the like.

The system as described here and shown in Fig. 1 comprises a working fluid in a cycle (21, 22, 23, 24, 25,

26, 27, 28) comprised by the second power system 2, the working fluid consisting of fluorinated ketones, in particular dodecafluoro-2-methylpentan-3-one .

The second power system comprises a heat engine comprising waste heat recovery heat exchanger 21, (turbo-

) expander 23, condenser 25 and pump 27, being in fluid communication with each other by conduits 22, 24, 26, 28. Such a cycle is known as a Rankine cycle.

An outlet of the heat recovery heat exchanger 21 is in fluid communication with an inlet of expander 23 via first conduit 22; an outlet of the expander 23 is in fluid communication with an inlet of condenser 25 via second conduit 24; an outlet of the condenser 25 is in fluid communication with an inlet of pump 27 via third conduit 26; an outlet of the pump is in fluid

communication with an inlet of the waste heat recovery heat exchanger 21 via fourth conduit 28.

The condenser 25 comprises an ambient inlet to receive an ambient cooling stream 61 and an ambient outlet to discharge a warmed ambient cooling stream 62.

In use, the first power system 1 generates primary power and flue gas stream 16, while the second power system 2 cycles the waste heat recovery fluid as working fluid through the above described Rankine cycle. The expander 23 drives drive shaft 29 which is coupled to a secondary generator 30, such as an electric generator, to generate secondary power. Fig. 2 schematically shows an alternative embodiment wherein the waste heat recovery fluid is not used as working fluid in a heat engine, but is used in an intermediate loop 3 to transfer heat from the waste heat recovery heat exchanger 21 to a heat engine wherein a different fluid is circulated as working fluid, such as water/steam. The second power system 2 comprises the heat engine and the intermediate loop 3.

According to this embodiment operating the second power system 2 comprises circulating the waste heat recovery fluid (consisting of fluorinated ketones, in particular consisting of dodecafluoro-2-methylpentan-3- one) through the intermediate loop 3 and circulating a working fluid through a heat engine, such as a Rankine cycle, to generate the secondary power, the heat engine comprising a heat source heat exchanger 42 and a heat sink heat exchanger 25, wherein the method comprises

- passing the flue gas stream through the waste heat recovery heat exchanger 21,

- passing a pressurized waste heat recovery fluid through the waste heat recovery heat exchanger 21 to receive heat from the flue gas stream thereby obtaining a pressurized vaporous waste heat recovery fluid having a temperature in the range of 350°C - 500°C,

- passing the waste heat recovery fluid through the heat source heat exchanger 42,

- passing the working fluid through the heat source heat exchanger 42 to obtain a heated working fluid by receiving heat from the waste heat recovery fluid.

Same reference numbers in Fig. 1 and 2 are used to refer to similar components.

Fig. 2 shows an intermediate loop 3 in which the waste heat recovery fluid is circulated. The intermediate loop 3 comprises the waste heat recovery heat exchanger 21, a condenser 42 (being the heat source heat exchanger of the heat engine) and a pump 44, being connected by intermediate loop conduits 41, 43 and 45.

An outlet of the heat recovery heat exchanger 21 is in fluid communication with an inlet of condenser 42 via first intermediate loop conduit 41; an outlet of the condenser 25 is in fluid communication with an inlet of pump 44 via second intermediate loop conduit 26; an outlet of the pump 44 is in fluid communication with an inlet of the waste heat recovery heat exchanger 21 via intermediate loop third conduit 45.

In use, the first power system 1 generates primary power and flue gas stream 16, while the second power system 2 cycles the waste heat recovery fluid through the above described intermediate loop 3 transferring heat from the flue gas stream 16 to the heat engine via the condenser (being the heat source heat exchanger 42) of the heat engine. In the heat engine, a working fluid is circulated, driving expander 23, which drives drive shaft 29 coupled to a secondary generator 30, such as an electric generator, to generate secondary power.

Simulation results

Simulation experiments have been performed using UniSim Design software. In the simulation the embodiment as shown in Fig. 1 has been simulated with a waste heat recovery fluid comprising 100 mol% dodecafluoro-2- methylpentan-3-one and has been compared to a similar embodiment in which the waste heat recovery fluid comprises 100 mol% water. The following parameters were used

Ambient temperature [K] 298.15 Ambient pressure [kPa] 101.325

Turboexpander efficiency [%] 85

Pump efficiency [%] 85

Heat Source Temperature [K] 686.15

Turboexpander Inlet Pressure 25 bar

Pressure ratio 25

Efficiency of the second power system is computed as the ratio of net power generated to the total amount of heat available with the exhaust gas:

H.WHR ^— ambient j ( wherein m _f is the mass flow of the waste heat recovery fluid as working fluid,

W _TE is the work done by turbo-expander 23,

Wpunp is work done by pump 27,

n haust is the mass flow of the flue gas stream 16, C _P ^exhaust is heat capacity of flue gas stream 16,

T _in ^exhaust ^ _{s tem} p _era - ₍ - _ure of - ₍ -h _g fi _ue g _a s stream 16, and

Tambient is the ambient temperature.

Above parameters were either taken from the table above or resulted from the simulations.

The simulations showed that the efficiency of 100 mol% water was found to be 7.50%, while the efficiency of 100 mol% dodecafluoro-2-methylpentan-3-one was found to be 10.68%. Using dodecafluoro-2-methylpentan-3-one thus resulted in an increase in efficiency of 42%. The person skilled in the art will readily understand that many modifications may be made without departing from the scope of the invention.