FTRT.D OF THE INVENTION AND PRIOR ART

The present invention relates to a power plant comprising a combustor, m which a combustion of a fuel while producing hot flue gases is intended to take place, means for supplying com¬ bustible gases to a location in or m the intermediate proxim¬ ity of the path of said flue gases downstream of the combustor and at least a burner adapted to cause a combustion of the combustible gases at said location for mixing the gases re¬ sulting therefrom with said flue gases for increasing the temperature of the latter.

The supply of combustible gases to and the combustion thereof in or m the proximity of the flue gas path is in such a power plant taken place primarily so as to raise the temperature of the flue gases for being able to increase the efficiency of the power plant or simply increase the possible power output therefrom. It may be a question of all conceivable types of power plants m which flue gases are formed during a combus¬ tion of a fuel, such as power plants having fluidised beds of for example bubbling or circulating type for combustion of particle-like fuel, oil-fired boilers, soda pans and so on. "Power plant" is as appears above m this disclosure defined as comprising not only plants for generating electricity but also all types of plants for generating heat or heat and elec¬ tricity.

In order to illuminate the problem on which the invention is based and accordingly not in any way for restriction thereof,

a PFBC-power plant, i.e. a plant for combustion of particle- like fuel m a pressurised fluidised bed, will heremaftei be described. A power plant of this type is already known through for example SE patent publication 458 955 and the Swedish patent application 9501097-1 of the applicant, m which the combustible gases are supplied to a topping combustor, m which they are combusted and the hot gases resulting therefrom are mixed with the flue gases for increasing the temperature of the latters before they reach a gas turbine of the power plant. The chemistry of the fuel bed in said combustor makes it impossible to obtain flue gases having a temperature higher than about 950°C m such a power plant, but the power that may be generated by the gas turbine increases very strongly with a temperature of the propellant gas, i.e. the flue gases, raised thereabove, so that it is desirable to raise the temperature thereof to 1200-1500°C, so that a higher total efficiency cf the plant is obtained.

When such a power plant is operated under partial load, i.e. while taking a lower power from the power plant than it is able to generate, the pressure in the pressure vessel and the flue gas paths of the power plant is lower than during maximum load. When starting from a partial load operation and a load increase is desired such a load increase has m the power plants already known of this type so far resulted m an in¬ itial lowering of the power of the entire power plant of a short duration, since the compressor driven by the gas turbine and pressurising said pressure vessel immediately requires more power for being able to press an increased air flow into the pressure vessel. Such a power increase may initially not be achieved m the gas turbine, since the combustion process has a large time constant . The increased power need oϊ the compressor is therefore taken from that part of the gas tur¬ bine power which goes to the generator, through which the useful power is reduced. The combustible gases derives mostly, but this is not necessary, from a gasifier which gasifies a

fuel under supply of air from the compressor driven by the gas turbine, which has as a consequence that also the gas flow from the gasifier to said location is initially reduced upon a load increase and this needs a longer time than would be de- sired. Sometimes problems with the gasifier also arise in such a plant, which means that the power generation of the plant has to be rapidly reduced, maybe to a "stand by" -state i.e. a so called "trip" of the plant has to be done, and a compara¬ tively long time is needed to bring the plant back to the desired power again. Thus, the plant will in such a case dur¬ ing a considerable period of time from the financial point of view not be able to deliver the power desired.

SUMMERY OF THE INVENTION

The object of the present invention is to find a remedy to the inconveniences mentioned above of already known power plants of the type defined in the introduction by providing a power plant having an increased flexibility with respect to adapta- tion to the power output required therefrom and primarily an increased ability to rapidly be adapted to an increased power output required and in which a rapid power reduction of the plant may be avoided should said means for supplying the com¬ bustible gases, such as the gasifier, fail.

This object is according to the invention obtained by provid¬ ing such a power plant with an arrangement for supplying an additional fuel to said location and a burner adapted to cause a combustion of this fuel at said location for mixing the gases resulting also therefrom with said flue gases for in¬ creasing the temperature thereof, and said arrangement com¬ prises means for optionally regulating the supply of said additional fuel. It will in this way be possible to obtain a very rapid load increase when it is suddenly called for a load increase from a partial load operation, since said means may be regulated to supply said additional fuel to said location,

so that the gas flow to a gas turbine, if the plant has one, may directly be increased and the power increase may be accel¬ erated during the load rise Thus, by the additional combus¬ tion at said location a power addition is achieved, which m the case of a power plant having a gas turbine is given to the gas turbine, which then may more rapidly generate more air and power Thanks to the provision of said arrangement it would also be possible to ensure supply of said additional fuel to said location through a suitable regulation by means of said means when problems arises or a maintenance of said means, such as a gasifier, for supplying said combustible gases to said location is required, so that m such a case a "trip" of the plant or another fast reduction of the power thereof is avoided.

According to a preferred embodiment of the invention the ar¬ rangement is adapted to supply said additional fuel in liquid state or m gas state to said location, and this additional fuel may advantageously be oil. The two fuels "combustible gases" and "additional fuel" may in this way be complementary to each other m an advantageous way, so that it will always be possible to supply said additional fuel even if the combus¬ tible gases may not be supplied. However, it is of course withm the scope of the invention to supply an additional fuel through said arrangement m the form of combustible gases to said location completely separated from the supply of the combustible gases through said means

According to another preferred embodiment of the invention the power plant comprises a container for storing said additional fuel and this is connected to said location through a inter¬ connecting conduit which may be optionally opened By storing said additional fuel m this way m a container being a part of the very power plant it may be instantaneously supplied to said location through opening said interconnection It is then of course possible that the container is also so equipped that

said additional fuel may be filled therein as it is consumed during the operation of the plant. According to another pre¬ ferred embodiment of the invention the power plant comprises means for controlling said arrangement for supplying an addi- tional fuel to supply this to said location when an increase of the power desired to be taken from the power plant occurs It will by this be possible to ensure a very rapid such power increase for the reasons discussed above .

According to a further preferred embodiment of the invention the power plant comprises means for controlling said arrange¬ ment for supplying said additional fuel to supply it to said location when it is desired or necessary to disconnect said means for supplying the combustible gases to said location when there is a need for maintenance of these means, problems of the function thereof arise or the like. The expression "necessary to disconnect" also comprises the case in which said means switch itself completely or partially off, for instance by lowering of the power thereof drastically as a consequence of an operation fault thereof. By controlling the arrangement to supply said additional fuel exactly in such a case, a so called "trip" of the plant or another rapid reduc¬ tion of the power thereof and high costs, may be avoided.

According to another preferred embodiment of the invention said regulating means is adapted to keep the supply of said additional fuel to said location closed at normal operation of the power plant when the power output therefrom is substan¬ tially constant. The power plant will by this in an advanta- geous way obtain a desired temperature increase of the flue gases through supply and combustion of the combustible gases at said location and the arrangement for supply of said addi¬ tional fuel will only be there for an emergency case, for instance when there is a need of a fast load increase of the power plant or when there is a problem with the supply of the combustible gases to said location.

According to another preferred embodiment of the invention the power plant comprises a gas turbine intended to be driven by said flue gases. The problem according to the invention is particularly accentuated in the case in which a power plant of the type defined in the introduction has such a gas turbine, and the advantages of supplying said additional fuel in such a case are particularly great, although the problem according to the invention is not at all only applicable to power plants having a gas turbine.

Other preferred embodiments of the invention, which are espe¬ cially related to power plants having a pressurising of the combustor and the flue gas paths downstream thereof, and in particular PFBC-power plants, are the subject matter of a number of dependent claims .

According to a further preferred embodiment of the invention the power plant comprises at least a room, in which the com- bustion of said combustible gases and said additional fuel as well as mixing of the gases resulting therefrom with said flue gases for temperature increase thereof is intended to take place, and said room is arranged downstream of the combustor in the path of the flue gases . Such a room may according to another preferred embodiment of the invention be a so called topping combustor, which is arranged in the flue gas path upstream of the gas turbine, and it may according to a still further preferred embodiment of the invention be a so called reheating combustion chamber or reheating combustor, which in the case of at least two gas turbines of the power plant is arranged in the flue gas path between the two gas turbines so as to raise the temperature of the flue gases experienced a temperature reduction in the first gas turbine before they are supplied to the following gas turbine, so that the power of the gas turbine may be increased rapidly through the supply of said additional fuel thereto and it is efficiently contributed

to a fast load increase of the power plant or a maintained uniform operation of the plant when problems with the supply of the combustible gases to this room suddenly occur. It is then also possible that said arrangement for supplying said additional fuel is adapted to supply this both to a so called topping combuster and a so called reheating combustor for a possibility to a very rapid load increase and avoiding the problems described above when there is a problem of supplying the combustible gases to these combustion chambers of such a power plant.

Further advantages and advantageous features of the invention will appear from the following description and the other de¬ pendant claims .

BRIEF DESCRIPTION OF THE DRAWING

With reference to the appended drawing, below follows a de¬ scription of a preferred embodiment of the invention cited as an example.

In the drawing:

The single fig 1 illustrates schematically a PFBC-power plant having a combined gas and steam cycle (the latter is not shown) according to a preferred embodiment of the invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVEN¬ TION

The invention is as already mentioned not in any way re¬ stricted to the application thereof on a PFBC-power plant, i.e. a plant for combustion of a particle-like fuel in a pres¬ surised fluidised bed, but it will nevertheless hereinafter be described as applied to such a plant for explaining the basic idea of the invention, since the problem upon which the inven-

tion is based is particularly pronounced for this type of power plants Thus, in the sole figure a part of such a power plant is schematically illustrated, said plant comprising a combustor 1, which is enclosed m a vessel 2, which nas a volume in the order of 10 ⁴ rr and which may be pressurised to for example about 16 bars. Compressed air 3 for pressurising the combustor 1 and for fluidising a bed 4 m the combustoi or combusting chamber is supplied to the pressure vessel The compressed air is supplied to the combustor through fluidising nozzles 5 being schematically indicated and arranged m the Dottom of the combustor for fluidising tne bed enclosed m the combustor. The bed is made of bed material, granule-shaped absorbent and a particle-like fuel, preferably crushed carbon, which is combusted m the fluidising an led to the bed The combustion gases, hereinafter called flue gases, from the bed are then supplied through a purification plant 6, which m the example consists of a high temperature filter intended for high pressures, and an intercept valve 7 to a first room 8 in the form of a topping combustor A combustible gas or fuel gas is also led to the topping combustor 1 through a conduit 9 from a gasifier 10 of known type through a high temperature filter 11. The combustible gases are combusted m the topping combustor 8 m connection with supply of compressed air through the conduit 12 from a high pressure compressor 13 through the influence of a burner 42 and mixed with the flue gases from the combustor 1 for raising the temperature thereof, so that the gases leaving the topping combustor have a temperature (1200-1500°C) making them well suited as propel lant gas for propelling a first gas turbine 14 m the form of a high pressure turbine The temperature of said combustion gases has through the topping combustor been increased from about 850-950°C to 1200-1500°C

The high pressure turbine and the high pressure compressor are arranged on the same axle as a generator 15, from which usaole energy may be taken The high pressure compressor 13 delivers

also compressed air to the PFBC-combustor 1 through the con¬ duit 16 from which the conduit 12 is branched An intercept valve 17 is arranged between the high pressure compressor and the combustor 1. It delivers also air through the conduit 18 for gasifying m the gasifier 10, and m this conduit 18 is a compressor 19 arranged for increasing the gas pressure further m the gasifier, since it is desired that the gas flow deliv¬ ered therefrom has a higher pressure than the flue gas flow arriving to the topping combustor, so that the combustible gases in every given pressure situation may easily be supplied to the topping combustor A liquid or solid fuel, m this example particle-like carbon, is gasified m the gasifier and generates combustible gases through an sub-stoichiometric process m a known way The remaining or residual fuel from the gasifier 10 may be supplied to the bed 4 m the combustor 1 through a fuel conduit 20

The PFBC-power plant shown m the figure is of an advanced type since it has an additional gas turbine 21, m the form of an intermediate pressure turbine, which is arranged on the same axle 22 as the high pressure turbine 14 The gas, which has been expanded and experienced a temperature reduction m the high pressure turbine 14, is led through a conduit 23 to a second room 24, which is called reheating combustor or rehea - mg combustion chamber The reheating combustor 24 receives a flow of said first combustible gases deriving from the gasi¬ fier 10 and compressed air from the high pressure compressor 13 the same way as the topping combustor 8, which is shown in the drawing through the conduits 25 and 26, respectively, whereby these combustible gases are combusted there through a burner 43 and the hot gases so created are mixed with the flue gases from the high pressure turbine so as to raise the tern perature thereof again before they are led through the conduit 27 to the intermediate pressure turbine 21 Tne power output from the intermediate pressure turbine may this way be increased considerably

The flue gases expanded in the intermediate pressure turbine 21 are led to a low pressure turbine 28. The exhaust gases leaving the low pressure turbine still contain energy, which an economiser 29 may take care of. A low pressure compressor 31 is also arranged on the axle 30 of the low pressure turbine 28 and atmospheric air is supplied thereto through a filter 32. Thus, the low pressure compressor is driven by the low pressure turbine and provides through the outlet thereof the high pressure compressor 13 with air which has been compressed in a first step. An intercooler 33 is arranged between the low pressure compressor and the high pressure compressor for low¬ ering the temperature of the air supplied to the inlet of the high pressure compressor 13.

The power plant has a steam turbine side, which is not shown here, but it is indicated through the set of tubes 41 immersed into the fluidised bed 4, in which water is circulated and evaporated and superheated through heat exchange between the tubes and the bed material for absorbing heat generated by the combustion carried out in the bed.

The power plant described so far is in principle already known, and the new characteristics of the invention will now be described.

The power plant comprises further a container 34 for storing an additional fuel, which in the embodiment shown is oil. A pump 35 is arranged to feed said additional fuel in a conduit 36 from the container 34 through two conduits 37 and 38 branched therefrom to the topping combustor 8 and the reheat¬ ing combustor 24, respectively, in which the additional fuel may be combusted in a conventional way through burners 42, 43, for mixing the hot gases resulting therefrom with the flue gases for increasing the temperature thereof to temperatures suitable for obtaining a high power in the gas turbines 14 and

21, respectively. Furthermore, the conduits 37 and 38 are provided with means 39 and 40, respectively, for regulating the flow of the additional fuel to the respective room 8, 24. These regulating means 39, 40 are constituted by valves which may be completely closed so as to ensure that the hot gases for the intermixing in the flue gas flow in the topping com¬ bustor and the reheating combustor are only provided through the supply of combustible gases from the gasifier, which is intended to be the case under normal operation of the power plant when the power output therefrom is substantially con¬ stant. The regulating means 39 and 40 are preferably connected to any control means not shown, which control the function thereof through hand operation or better through measuring any or several of the operation parameters.

The regulating means 39 and 40 are arranged to open the flow the conduits 37 and 38, respectively, and the pump 35 to be driven when there is a need to supply the additional fuel, m this case the oil, to the combustors 8 and 24. Such a need is there when a load increase of the plant is desired, since when this is operated under partial load the vessel 2 and by that the combustor 1 and the flue gas paths are pressurised through the compressor 31 and 13 to a lower level than during maximum load operation. When there is suddenly a desire to increase the power output from the power plant during such a partial load operation, i.e. increasing the load thereof, possibly to maximum load operation, it is of vital importance for the speed of this load increase how quick the pressure in the system may be increased. The power of the gas turbine must be increased for increasing the air flow from the compressor, which in power plants already known has resulted in an initial reduction of the number of revolutions of the gas turbine and by that of the air pressure compressor, so that there will be a delay of the pressure increase in the system, so that also the pressure in the gas flow from the gasifier 10 is reduced, which makes the possible load mcrease slower. However, the

present invention takes care of this problem by arranging the regulating means 39 and 40 to open when there is a desire of a load increase, whereby said additional fuel is supplied to the topping combustor 8 and/or the reheating combustor 24. The flue gas flow is in this way increased at this location and an additional power is obtained in the respective turbine 14 and 21 and also indirectly in the turbine 28, so that the gas turbines may directly deliver more power and increase the power of the compressors 13 and 31 pressurising the system. The load increase may by this be accomplished rapidly without the slightest requirement of an initial reduction of the power of the power plant because the gas turbine compressor directly demands more power for pressing an increased air flow into the pressure vessel. When the rise of the increased load is com- pleted, the regulating means 39 and 40 are preferably closed for being opened later on when a new load rise is desired from this level or from a lower level established after a load reduction.

The other case in which there is a need of supplying said additional fuel to the topping combustor 8 and/or the reheat¬ ing combustor 24 is when suddenly a problem with the gasifier 10 arises, so that this should be switched off or it switches itself off completely or partially as a consequence of distur- bances. In such a case it has in power plants of this type known until now been necessary to rapidly reduce the power generated by the plant by taking the plant to a "stand by" - state or in any other way. However, this is avoided in the power plant according to the invention by opening one or both regulating means 39, 40 when such problems arises, so that additional fuel is supplied to the combustor 8 and/or 24 as a substitute for the combustible gases from the gasifier 10 otherwise led thereto. Thus, in this case said additional fuel is supplied during a preferably continued substantially con- stant power output from the power plant, but it is not any question about any normal operation thereof.

The invention is of course not in any way restricted to the preferred embodiment described above, but several possibili¬ ties to modifications thereof will be apparent to a man skilled in the art without departing from the basic idea of the invention.

The patent claim definition "optionally" is also intended to comprise an automatic adjustment of the regulating means in question depending upon any particular operation parameter.

In a power plant having a topping combustor and a reheating combustor it is possible to arrange the flow of the additional fuel connected to only one of these combustors .

The burners mentioned in the appended claim 1 for causing combustion of the combustible gases and the additional fuel could consist of one single common burner and "burner" is to give a broad meaning and comprises all conceivable means which may ensure a combustion of a fuel.