FIELD OF THE INVENTION

The invention concerns a method of operating a gas turbine combined heat and power plant including a compressor assembly and a turbine assembly, useful work being extracted by at least one device being included in the plant, wherein combustion gases are produced by a combustion device, upstream of the turbine assembly, and wherein water and/or steam is heated through heat exchanging with a hot gas stream

downstream of the turbine assembly and/or in a compressor flow path and water and/or steam is injected into said gas flow stream upstream of and/or in the combustion device in such amounts that at least 80% of oxygen contents in the air in the gas flow stream is consumed through combustion in the

combustion device and wherein a flue gas condenser is

operative in a flue gas stream downstream of the turbine assembly in order to extract heat therefrom for heating heat carrying fluid used in a heating device. The invention also concerns a gas turbine combined heat and power plant .

BACKGROUND OF THE INVENTION

WO 2004/106718 (Euroturbine AB) describes a method of operating a gas turbine power plant with two gas turbine groups. The main purpose of the invention described in that document is to provide high efficiency by introducing water and/or steam into the process flow, thereby allowing a close to stoichiometric combustion process. Water is recovered downstream by a flue gas condenser and is re-introduced at various places upstream in the process flow. AIM AND MOST IMPORTANT FEATURES OF THE INVENTION

The invention according to the background art works well and it is an aim of the present invention to provide a further development of means for performing a method of operating a combined heat and power plant, wherein the emphasis is to enhance properties for supplying heat used in a heating

device, preferably of a district heating net.

These aims are achieved according to the invention in a method as indicated above through the characterizing features of the independent claims .

It is achievable to reach as high temperature levels as at least close to 80°C in the heat supply conduit for the heat carrying fluid through the use of a flue gas condenser/heat recovery device. This is achieved in that a high dew point is reached because of great amounts of water being present in the flue gases, whereby condensation is performed at high

temperature levels. Thus, condensation is performed from 80 °C down to about 50 °C, which is a typical return temperature in a district heating net, a major part of the waste heat from the power plant will in this way become available for district heating purposes.

Hereby it is thus possible to reach such high temperature levels in the heat carrying fluid that it thereby can be advantageously used for external heating purposes such as district heating of buildings in operating conditions that prevail during a great part of the year in most parts of the world. For colder periods, when extra heat requirements prevail, the invention provides additional solutions for supply temperature topping.

It is important to realise that the invention concerns a method in connection with a close to stoichiometric process wherein water contents in the process gas is as great as is practically possible. Air access is held at the lowest level necessary in order to achieve efficient combustion in the combustor. Hereby unnecessary work for compressing air

containing oxygen not used for combustion can be held to a minimum. A part of the combustion heat may also be used for transforming water into steam.

In previously known heating plants for district heating, up to now, it has been necessary to fire great amounts of fuels in order to reach heat demands and sufficient

temperature levels, leading to high energy consumption and high energy costs. This drawback is not necessary with the method according to the invention. By this method, also, heat inherent in steam used in the process including water vapour produced by the combustion of the fuel itself will be

recovered.

According to a first inventive aspect of the invention the flue gas stream downstream of the turbine assembly and upstream of the flue gas condenser is supplemented with flue gases from an additional combustor in order to arrange for enhancement of the temperature level for the heat carrying fluid, wherein combustion oxygen for the additional combustor comes from ambient air oxygen and/or oxygen from an inlet air humidifier and/or residue oxygen in the flue gases downstream of the turbine assembly. Advantageously, this additional combustor also enhance the conditions for producing both combustor steam and top up temperature levels for heat

production. Conditions close to stoichiometry can be

maintained resulting in efficient power production and

maintained high flue gas dew temperature.

Further, according to the first inventive aspect of the invention, an additional condenser is arranged for extracting water and heat from the flue gas stream downstream of the flue gas condenser and the inlet air humidifier is arranged for injecting extracted heat and water into intake air to the compressor assembly. Hereby, heat having such a temperature level that it is below return temperature of a district net can also preferably be extracted from the flue gas stream downstream of the flue gas condenser and used for heating and humidifying intake air to the compressor assembly. Thus heat is extracted by further condensation of the gas and recycled into the process by means of an intake air humidifier.

According to a second inventive aspect of the invention, heat is extracted by a heat exchange device being placed in a compressor gas stream upstream of the combustion device and used for heating said fluid which makes it possible to reach higher supply temperature levels in the heat carrying fluid in an advantageous way. It is also advantageous to be able to use this principle for adapting cooling of the compressor gas stream for the purpose of adjusting to compressor and

combustor characteristics.

Through this aspect, enhanced temperature performance and possibility of reaching higher supply temperatures are

obtained. Heat exchange, for this purpose, with the compressor gas stream upstream of the combustion device is particularly advantageous in that the temperature in that stream is high, typically as high as 500°C, which makes is possible in an effective way for getting access to such high temperatures that are desired in respect of various conditions.

This is achieved, according to this aspect, without declined plant performance and does not reduce the total efficiency .

When, in particular according to the second inventive aspect, the heat is extracted by said heat exchange device having at least one heat exchange unit being arranged in the compressor gas stream respectively between at least two compressor units the advantage of an intercooler solution is obtained in that the gas temperature in the second (and possibly third etc.) compressor unit (or compressor step) is reduced leading to reduced compressor work requirement. It is thus possible to have plural heat exchange units, each one being arranged between two compressor units. One further advantage is that, according to the invention, cooling is effected by means of water whereby cooling is possible to lower levels than would have been possible with otherwise preferably used steam generator.

Preferably combustion gas being fed to the turbine assembly has a pressure of 50 - 300 bar.

It is also preferred that the water and/or steam being injected into said gas flow stream upstream of and/or in the combustion device is water and/or steam produced by being heated through heat exchanging with said hot gas stream

downstream of the turbine assembly and/or in said compressor flow path since this is most efficient for the process.

It is preferred that available heat extracted by a further heat exchange device being placed in said flue gas stream between the turbine assembly and the flue gas condenser and used for heating said fluid since this makes it possible to reach even higher supply temperature levels in the heat carrying fluid.

Said hot gas stream is advantageously the flue gas stream downstream of the turbine assembly.

Said hot gas stream is as an alternative the gas flow stream upstream of the combustion device.

To further enhance the temperature level the heat carrying fluid, steam or hot water can be extracted from a heat exchanger being placed downstream of the turbine assembly being originally adapted for production of steam for

introduction into the combustor can according to this aspect be transferred to the heat exchanger for further heating of the heat carrying fluid. This aspect reduces the efficiency for the production of electricity but can be advantageous for reaching high supply temperatures in the heat carrying fluid.

However, instead of possibly lacking steam for

introduction into the combustor because of heat allocation for heating heat carrying fluid, more water can be injected therein for production of steam inside the combustor, thus still maintaining stoichiometric conditions in the process.

The most important function of the condensers is to utilize steam vaporization heat of the steam content in the flue gases.

It is preferred that the proportional amounts of water and steam relative to each other injected into said gas flow stream upstream of and/or in the combustion device are

controlled through control means with maintained close to stoichiometric conditions in order to regulate the amount of heat being available for being extracted from said flue gas stream downstream of the turbine assembly for heating said heat carrying fluid and subsequently the impact on supply temperature.

Increased extraction of heat and subsequent higher supply temperature is obtained by routing an increased cooling flow through the further heat exchange device being placed in the flue gas stream between the turbine assembly and the flue gas condenser and transfer extracted heat for heating the heat carrying fluid and vice versa. This allows an incremental switch between steam production and hot water production in said further heat exchange device. Allocating heat for heating heat carrying fluid generally speaking leads to reduced steam production which in turn is remedied by the injection of more water (which can be for example water from the flue gas condenser) into the combustor so as to maintain the

stoichiometric conditions that are crucial for the invention. Hereby fast response to occurring requirements is made

possible maintaining excellent efficiency and control. This aspect does not result in any loss of energy, basically only in advantages in respect of regulation.

In particular it is preferred that oxygen contents in said flue gas stream downstream of the turbine assembly is detected through detection means and the amount of water and/or steam injected into said gas flow stream upstream of and/or in the combustion device is controlled through said control means as a response to detected oxygen contents value. A typical practical target oxygen contents value in said flue gases is between about 0,4 - 4% by weight of flue gases, preferably between about 0,6 - 2%, more preferred 0,6 - 1,5% using 1,0% as most preferred value. Detection can be

accomplished in various ways, for example by using a so called Lambda sond interfering in the flue gas stream, and water and/or steam injection can be regulated by means of a CPU which controls the associated water/steam flows.

All measures described above, and in particular when they are combined, result in that higher degree of process

efficiency can be obtained.

All measures and features described above can be combined with the first as well as with the second inventive aspect of the invention.

Corresponding advantages are obtained in respect of the claims directed to a gas turbine combined heat and power plant .

BRIEF DESCRIPTION OF DRAWINGS

The invention will now be described in greater detail at the background of embodiments and with reference to the annexed drawings, wherein: Fig. 1 diagrammatical ly illustrates a combined heat and power plant according to the invention,

Fig. 2 illustrates typical heat demand in a district heating net,

Fig. 3 illustrates temperature requirements in a heat carrying fluid in a district heating net,

Fig. 4 diagrammatically illustrates a combined heat and power plant according to a second embodiment of the invention, and

Fig. 5 diagrammatically illustrates a combined heat and power plant according to a third embodiment of the invention.

DESCRIPTION OF THE EMBODIMENTS

In Fig. 1 there is shown diagrammatically a combined heat and power plant, wherein a first gas turbine group includes, on the one hand a turbine assembly 1 and on the other hand a compressor assembly 2 which are mechanically interconnected over a shaft 3. The shaft 3 is also used for driving a device 4 for extracting useful work from the plant, such as an electric generator.

A second gas turbine group includes on the one hand a turbine assembly 1 ^' and on the other hand a compressor

assembly 2 ^' which are mechanically interconnected over a shaft 3 ^' . In this embodiment there is thus provided twin gas turbine groups corresponding essentially to what is illustrated in WO 2004/106718 Al .

In a gas flow stream 13 there is arranged downstream of the compressor assembly 2 ^' and upstream the turbine assembly 1 ^" , a combustion device 5 for producing combustion gases/flue gases for driving the turbine assembly 1" and the turbine assembly 1. In a flue gas stream 7 downstream of the turbine assembly 1, a heat exchanger 6 is arranged for heating steam which is introduced upstream of or in the combustion device 5 over a conduit 8. The arrow 22 indicates fuel supply.

A further heat exchange device 14 for the production of hot water is placed in said flue gas stream 7 between the turbine assembly 1 and the flue gas condenser 9 and is also used for heating said heat carrying fluid by being connected to said heat exchanger 21 by use of surplus heat available in the flue gas stream upstream of the condenser. The heat

exchange device 14 can preferably be integrated with the heat exchanger 6 which i.a. makes it more easily capable of making incremental changes between steam and hot water production.

Under maintained stoichiometric conditions, output of heat from the integrated heat exchanger unit 6, 14 to the heat exchanger 21 can be controlled by varying the production of steam and/or hot water to be injected into said gas flow stream upstream of and/or in the combustion device.

8' indicates a conduit for leading hot water from the heat exchange device 14 to the combustor 5.

Further downstream in the flue gas stream 7, there is arranged a flue gas condenser 9, which extracts heat from the flue gases, while water content in the flue gases is extracted by condensation, for heating, in a heat exchanger 21, heat carrying fluid inside a conduit 10 leading to a heating device 11 for example for a district heating net.

Water condensed by the flue gas condenser 9 is heated and re-entered into the cycle in or upstream of the combustion device 5.

A heat exchange device 12 is arranged in a compressor gas stream 13 between the compressor assemblies 2 and 2' for cooling said stream after being compressed by the first

compressor assembly 2. Heat extracted by the heat exchange device 12 is entered into the heat exchanger 21 for further heating of said heat carrying fluid for topping of supply temperature .

The heat exchanger 12 can preferably be arranged such that production of steam and hot water can be easily switched in respect of each other. When requirement for heat in a district heating net or the like so allows, typically is not high, more heat extracted by the heat exchanger 12 can be used for heating water and/or steam for injection into the gas flow stream upstream of or in the combustion device.

An additional combustor 16 is positioned for producing hot flue gases to be introduced into the flue gas stream 7 downstream of the turbine assembly 1 and upstream of the flue gas condenser 9. Combustion oxygen for the additional

combustor can be supplied from ambient air oxygen and/or from a humidifier 18 and/or residue oxygen in the flue gases downstream of the turbine assembly (see below) .

This additional combustor 16 is used for topping of temperature in the flue gas stream 7, in particular in

conditions were extra high heat carrying fluid temperatures are necessary such as, for example, in respect of district heating nets during very cold weather conditions.

Downstream of the flue gas condenser 9 there is arranged an additional condenser 17 for further condensation of the flue gases which are thus further dehumidified, before they are being emitted from the plant. Water and heat extracted by the additional condenser 17 is preferably introduced into an inlet airstream 23 through an inlet air humidifier 18.

Figs 2 and 3 illustrate, as an example, diagrams related to a district heating net. In Fig. 2 there is illustrated a typical one year heat demand distribution structure, whereby can be seen that for a relatively very short time period there is a high heat demand, whereas for the major part of the year the demands are considerably lower. In Fig. 3 there is shown a diagram illustrating supply- line temperature requirements for heat carrying fluid in the conduit 10 as a function of number of hours during a year. As the diagram illustrates, for most part of the year, a supply line temperature of around 90°C (or slightly lower) is

required whereas extremely high supply line temperatures up to about 130°C are required only during a short period each year.

Typically, the major parts of the required heat for a district net, such as the one having the requirements in Fig. 3, are covered by recovering heat from a flue gas condenser which typically, because of high water contents in the flue gases reaches as high as about 80°C corresponding to a first level LI.

Typically also, a second level L2 temperature of 90°C is relatively easily reached by the heat exchange device 12 positioned between the first and the second compressor units in the compressor assembly.

A third level L3 , up to about 100°C, is reached by further heat exchange upstream of the flue gas condenser. This level L3 is also preferably supported by extracting heat from the steam generator/heat exchanger 6 and from the heat

exchange device 14 which can be achieved by introducing steam produced and leading heated excess water to the heat exchanger 21. With the term "heated excess water" is here intended water in excess of what is necessary for the production of hot water and steam to be supplied upstream or in the combustion device 5. Supply of heated excess water thus means reduced steam production for supply upstream of or in the combustion device, which then is compensated by extra water injection in the combustion device for maintaining close to stoichiometric conditions .

At a fourth level L4 , the topping heat is typically reached by use of the additional combustor, which is positioned downstream of the turbine assembly 1. It should be noted that also at various operational cases associated with lower temperature levels, heat production by the additional combustor can be beneficial for the process .

In Fig. 4 there is shown a second embodiment of the invention having a turbine assembly 1 being connected to a compressor assembly 2 with two compressor units 2' and 2"' as well as to a device 4 for extracting work, such as an electric generator. The basic features of the elements in respect of external heat delivery are, however, the same as is

illustrated in Fig. 1 and discussed above. The remaining reference numerals generally correspond to the ones appearing in Fig 1.

Further, the flue gases from the combustion device are in this embodiment lead to the turbine assembly 1.

In Fig. 5 there is shown a third embodiment of the invention having a turbine unit 1'' being connected to a compressor assembly 2 with two compressor units 2" and 2'". A second turbine unit 1' is connected to a device 4 for

extracting work, such as an electric generator. The basic features of the elements in respect of external heat delivery are much the same as is discussed above except for the

coupling of the heat exchanger 12 being arranged in the compressor gas stream 13 between the compressor units 2' and 2" for cooling said stream after being compressed by the first compressor unit 2'. Heat extracted by the heat exchange device 12 is entered into the heat exchanger 21 for further heating of said heat carrying fluid.

As is indicated above, when requirement for heat in a district heating net or the like so allows, typically is not high, heat extracted from the heat exchanger 12 can be used for heating water and/or steam for injection over the conduit 8'" into the combustion device 5. Further, the flue gases from the combustion device 5 are in this embodiment lead to a turbine unit 1" being a first part of the turbine assembly 1. A turbine unit 1' being a second part of the turbine assembly 1 is coaxial therewith and is rotationally free from the turbine unit 1''. The turbine unit 1' is over a shaft 3'" connected to a device 4 for extracting work, such as an electric generator. The remaining reference numerals generally correspond to the ones appearing in Fig 4.

As an explanation of the advantages of the present invention, it can be noted that background art Combined Heat and Power plants (CHP:s) have a total efficiency (including production of electricity and heat) of up to about 90% in the best case calculated on the fuel's low heating value.

The invention, however, provides for the possibility of far better total efficiency in that it gives the possibility to utilize the energy in the flue gases being in the form of the contained steam's vaporisation heat. For that reason a plant having the combined measures described above exhibits a total efficiency of up to about 115% calculated on the fuel's low heating value which can be compared to a typical total efficiency of 80 - 90% for a method according to the

background art. Because of its principle, the efficiency of a plant according to the invention is also not as sensitive to high water contents fuels, such as bio fuels, as the

previously known CHP, since the energy to vaporize the water in the fuel can be recovered in the condenser or condensers.

The invention can be modified within the scope of the patent claims in that i.a. the flue gas condenser can be supplemented with the various heat exchange devices discussed above according to need and conditions. In another

modification, heat exchanger 6 being arranged for heating water and/or steam and the heat exchange device 14 are integrated functional parts of one single device. As a complementary measure to heating water and/or steam through heat exchanging with the hot gas stream downstream of the turbine assembly 1, heated water and/or steam for introduction into the process gas stream is advantageously produced in the compressor flow path 13.