EP0854970A1

EP0854970A1 - Method of using solid secondary fuel in firing the gas turbine of a combined-cycle power plant and a connection for implementing said method

Info

Publication number: EP0854970A1
Application number: EP96933459A
Authority: EP
Inventors: Martti Äijälä; Markku Raiko; Janne Tiihonen
Original assignee: Imatran Voima Oy
Current assignee: Imatran Voima Oy
Priority date: 1995-10-12
Filing date: 1996-10-10
Publication date: 1998-07-29
Also published as: FI102630B; PL326198A1; FI954847A; FI102630B1; FI954847A0; HUP9802914A2; AU7218496A; HUP9802914A3; WO1997013962A1

Abstract

The invention relates to a method and connection suited for using biomass, solid waste or other solid fuel as a secondary fuel for an oil- or gas-fired gas turbine. Such solid fuel may have an extremely high moisture content or be mixed with water into a slurry whose heating value may even be negative. According to the invention, the solid secondary fuel is gasified in a separate gasification unit (6, 14 - 18, 20, 21), and into the secondary fuel to be gasified is introduced supplementary energy by firing the main fuel of the gas turbine (2) in, e.g., said gasification unit (6, 14 - 18, 20, 21). The product gas made from the solid secondary fuel is used in conjonction with the main fuel for firing the gas turbine (2).

Description

Method of using solid secondary fuel in firing the gas turbine of a combined-cycle power plant and a connection for implementing said method

The present invention relates to a method according to the preamble of claim 1 for using biomass fuel, solid waste or other -solid fuel as the secondary fuel for an oil- or gas-fired gas turbine. The solid fuel may have an extremely high total moisture content, or when slurried with water, even have a negative caloric value.

The invention also concerns a -connection based on said method for producing energy.

Today, the use of biomass as a fuel in electric power and district heat energy production is increasingly desir¬ able, and in the disposal of refuse, a parallel goal is to recover the energy released in refuse incineration or to_ utilize at least partially the heat used in the incin- eration of refuse, unless the refuse itself lacks suffi¬ cient heat value. Optimally, biomass is used in conjunc¬ tion with paper mills, because these plants produce large amounts of biomass suited for energy production, and rather abundant quantities of combustible byproduct waste are left over from the paper and pulp process. Hence, a particularly interesting target is to use the biomass arising in paper mills maximally efficiently in energy generation, since a majority of paper mills today already have generating plants which provide the plant with both electric power and heat distributed in the form of pro¬ cess steam. Conventionally, reaction turbines are used for generating electric power in paper and pulp mills. The process steam needed in the plant is taken from the backpressure section, and even if the steam consumption would be substantial, the amount of electric power gener¬ ated remains low. In conventional biomass-fuelled steam cycle plants of the paper and pulp industry, the ratio of generated electric energy to heat energy used is rela¬ tively low, typically below 0.3. For instance, in district heat cogeneration this ratio reaches 0.5.

In addition to paper and pulp mills, byproduct biomass and other solid waste suited for energy generation are produced in agriculture, for instance, and as municipal and erection site refuse. Particularly the combustion and use of municipal refuse in energy generation is attrac- tive, because in this fashion the amount of refuse to be dumped can be reduced and partially utilized in replacing fossil fuels. Especially advantageous would be the util¬ ization of nonrecyclable paper waste and biomass result¬ ing from recycling processes. Thereby, the lifecycle of wood biomass could be extended from harvesting, through paper-making and a few recyclings, to energy generation, whereby the harvested biomass would be utilized to its maximum potential. Because the increasing efficiency of recycling increases the amount of low-quality, nonrepulp- able paper refuse and deinking process waste, improved methods are required for the disposal of these waste com¬ ponents. Such a low-quality paper refuse could be de- structed in units located in conjunction with pulp plants using recycled paper as raw material, whereby also the waste resulting from the deinking process could be incinerated.

The utilization of bio asses and incinerable waste is further hampered in that the small size of locally col- lected lots and the unprofitable transportation of bio¬ mass to incineration plants from distant locations often makes an incineration plant unprofitable. Even in paper and pulp plants, where the amounts of byproduct biomass waste are substantial, the accumulated quantities remain too small to fulfill the total energy needs, thus neces¬ sitating the use of supplementary fuels. Due to limited availability of combustible biomasses, a condensing power plant fuelled by biomasses alone is not economically viable. In CHP (Combined Heat and Power) processes, direct combustion of biomass for steam generation and the use of so-generated steam in the steam circuit of the plant results in a low electric power-to-heat ratio of the cycle. Then the total efficiency of the electricity generating plant is impaired, resulting in lower cogener- ation efficiency than is achievable by means of a com¬ bined cycle using a gas turbine alone.

While the efficiency of solid waste combustion could be improved by gasification techniques, also for such plants the minimum economically viable plant size is so large that difficulties arise in the collection of sufficient amounts of combustible biomass fuel. Because gasification techniques today are still relatively costly and only a small plant can be erected due to the small annually available volume of biomass fuel, the investment costs of a _.plant fuelled by biomass or solid waste alone will rise to excessive amounts. If the price of such fuel is further elevated by high transportation costs, erection of an economically justifiable plant becomes difficult and commitment to extensive investments is hard to reach.

One method of using solid fuel in conjunction with a gas- turbine cycle is so-called topping combustion. In this process, the gas turbine is fired with product gas made from solid fuel by means of a gasification system and the inlet gas temperature to the gas turbine is then elevated sufficiently high by supplementary firing using a liquid or gaseous topping fuel such as natural gas. Frequently, a two-stage gasification process is employed in which the volatile components of the fuel are gasified and the non- gasified char residue is combusted in either the bottom part of the gasifier or a separate combustion chamber. In these plants, the firing process of the solid fuel is crucial to the operation of the plant and extremely crucial to the total economics of the plant. In this system, the main fuel is the solid fuel taken to the gasifier and the topping fuel is used to operate the gas turbine with maximum possible efficiency. In addition to the limited durability of filter materials, the combus¬ tion of the char remaining from the solid fuel gasifica¬ tion at a sufficiently high temperature is also restrict¬ ed by other factors including the evaporation of alkali compounds which are most damaging to the blades of the gas turbine wheels.

It is an object of the present invention to provide a method suited for using waste-like collected solids such as biomass advantageously in energy generation in combined-cycle gas turbine power plants.

The goal of the invention is achieved by gasifying the solid fuel to be fired at least partially by virtue of the heat content of the gas turbine main fuel and taking the product gas into the fuel circulation of the gas turbine.

According to a preferred embodiment of the invention, the gas turbine is fired by natural gas and the solid fuel is biomass.

More specifically, the method according to the invention is characterized by what is stated in the characterizing part of claim 1.

Furthermore, the connection according to the invention is characterized by what is stated in the characterizing part of claim 12.

The invention provides significant benefits. A benefit of the invention is that the side-stream gasi¬ fication connection offers high power-to-heat ratio in plant designs partially fired by solid fuel and having a combined-cycle process as its main process. Small amounts of biomass and waste can be utilized at the efficiency rates of the large and efficient main process, partially drawing upon the existing system resources of the large main process. In side-stream gasification, the gasifica¬ tion equipment is smaller and the gasification and pro- duct gas clean-up processes proper are simpler in design and less costly than full-scale gasification systems fired by product gas alone. As- the gasification is made with the help of the main fuel, the heating values of the secondary fuel and the product gas may be selected more freely, because a portion of the main fuel may be intro¬ duced into the side-stream gasifier. The heating value of the product gas exiting the gasifier may be very low and even negative, because the proportion of the side-stream gas in the total fuel firing rate remains small. The heating value of the gas mixture can be kept sufficiently high by combusting the main fuel of the gas turbine. Another significant benefit is the improved availability of the process according to the invention over a conven¬ tional IGCC (Integrated Gasification Combined Cycle) process, because the production of the side-stream gas flow in the gasifier does not cause a similar nonavail¬ ability risk and need for keeping a reserve fuel storage as is the case with a conventional IGCC process.

By introducing a portion of the energy required for gasi¬ fication through combusting a portion of the main fuel of the gas turbine, instead of producing the entire energy consumed in gasification by combusting the fuel to be gasified, also the gasification of fuels of low caloric value becomes possible. When using fuels of extremely high moisture content and/or high ash content, gasifica¬ tion without introducing external energy is either cumbersome or uneconomic. This is due to the low chemical energy content of solid fuel, which is insufficient for attaining a good gasification result and reliable con¬ trol. The external energy may be introduced into the gasification process by substantially heating at least a portion of the gasification air flow entering the gasi¬ fier or by burning a portion of the main fuel flow in the gasifier.

The present invention offers a significant simplification of the gasification plant design. Here, it is also possi¬ ble to omit the erection of a dryer in conjunction with the gasifier of solid fuel of high moisture content. Moreover, the control of the gasifier will become easier as the inlet flow of the external energy is principally used for gasifier control. Furthermore, the product gas cooler may be replaced by an arrangement of simpler design and lower cost such as water spray cooling.

The introduction of external energy also makes it possi¬ ble to provide wider margin in shifting the gasification atmosphere to the oxidizing side in the gasification of fuels of high moisture content. This facility allows operation of the gasifier at a reduced level of autom- ation and cuts down the operating risks of the gasifica¬ tion unit. Furthermore, it becomes possible to ensure a sufficiently high gasification temperature, whereby also the tar load exiting the gasifier can be kept low, thus relaxing the need for gas clean-up.

The side-stream gasification arrangement according to the invention improves significantly the electric-to-thermal energy ratio and the power-to-heat ratio of the power plant with respect to boiler plants fired by solid biomass fuel. By virtue of the gasification technique, the use of biomass fuels of high moisture content in a gas-turbine plant achieves a more economical outcome than what is possible using a conventional steam cycle. Moreover, a high-pressure steam dryer may advantageously be connected to a pressurized gasifier. Using a proper connection, the dryer, the gasifier and the gas turbine can be economically combined together, simultaneously maximizing the electric power generation capacity. Thus, the invention makes it possible to increase the firing percentage of renewable fuels with respect to fossil fuels.

The clean-up of the product gas is easy as the gas may be cooled down to temperatures at which the gas may be cleaned free from obnoxious substances by means of metal or ceramic filters, whereby alkali compounds contained in the gas are condensed in the filters on the separated particulate matter. Cooling may be performed using water or steam spraying, whereby the generated steam acts as an auxiliary expanding medium in the gas turbine. Alterna¬ tively, clean-up of the product gas may be performed using water-spray washer equipment. The method according to the invention is well suited for incinerating materi¬ als containing extremely toxic substances, since the high temperature in the combustion chamber of the gas turbine disintegrates efficiently such toxic compounds.

In comparison of the combustion process according to the invention with conventional topping combustion, it must be noted that in topping combustion the fuel being gasified is used for elevating the temperature of the inlet gas flow to the gas turbine sufficiently high to permit economical operation of the gas turbine. By contrast, in the process according to the invention, the gas turbine may be operated at maximum efficiency also without the gasifier. In theory, also topping combustion could be performed without the product gas from the gasifier, but not without significantly compromising the total efficiency of the power plant. Moreover, topping combustion gasifies the main fuel, while the process according to the invention can use almost any available fuel in the gasifier. Furthermore, topping combustion requires a solid fuel of relatively easy gasification in order to achieve a sufficiently high temperature for the gasifier discharge gases taken to the gas turbine, while the heating value of the product gas from the gasifier in the process according to the invention is not crucial to the total energy balance of the power plant, in the pro- cess according to the invention, a natural-gas combustor placed in front of the gasifier can be used in order to ensure gasification of extremely problematic fuels, or alternatively, to aid the gasification step of reasonably easily gasifiable fuels. Without significantly degrading the efficiency of the plant, the process employed in the invention can use even such fuels that have a negative heating value. By contrast, in topping combustion, the quality of the product gas delivered from the gasifier can affect the plant total efficiency crucially, thus requiring the use of a complicated and therefore expen¬ sive system in order to achieve a good gasification effect, while the process according to the invention can be built on less expensive arrangements based on, e.g., cooling the product gas with water-spray coolers. In topping combustion, a sufficiently high gasification temperature can be obtained only by burning the fuel proper to be gasified in order to make gasification possible at all, while in the process according to the invention, a significant fraction of the gasification heat can be introduced by burning externally fed supple¬ mentary fuel in either the gasifier or the inlet air to the gasifier. While the entire heat generating process in topping combustion requires latest technology of maximum performance, that is, complicated and expensive equip- ment, the process according to the invention can be erected around a conventional power plant of established reliability using advanced but simplified novel techniques, In the following the invention will be examined in greater detail by making reference to the appended drawings in which

Figure 1 shows a connection according to the invention;

Figure 2 shows a second alternative connection according to the invention;

Figure 3 shows a third alternative connection according to the invention; and

Figure 4 shows a fourth alternative connection according to the invention;

Referring to Fig. 1, a simplest possible connection according to the invention is shown therein. As the con¬ nection has no dryer, it is best suited for solid fuel of relatively low moisture content such as recycle paper, building site refuse or relatively dry biomass such as straw. The connection incorporates a gas turbine 2 to which air is fed by a compressor 1. The exhaust gas of the gas turbine 2 is taken to a boiler 3 in which steam is generated for a steam turbine 4. From the steam turbine 4, steam is taken to a process or condenser 5 using the steam. The main fuel of the gas turbine 2 is natural gas, which is taken to a combustor 9 of the turbine over a gas line 12. A side line 13 is taken from the compressor 1 of the gas turbine 2 to the combustor 11, and therefrom further to the gasifier 6. The com¬ bustor 11 is operated on the gas turbine main fuel. The side line 13 is provided with a booster fan 10 for blowing air into the combustor 11 and the gasifier 6. The air is heated in the combustor, wherefrom the heated air is taken together with the gas turbine exhaust gases to the gasifier 6, into which the solid fuel is taken along a feed line 14. The solid fuel is gasified in the gasi- fier under air-lean conditions, thus forming combustible product gas. The product gas is taken first to a cooler 7 and subsequently cleaned in a water-spray washer 8. The purified product gas is passed into a natural gas feed line 12, mixed therein with the natural gas and the mixture is fed into the combustor 9 of the gas turbine 2.

The gasifier 6 can be operated with a plurality of fuels having different heating values, and although the connec- tion described herein is not provided with a separate dryer, the fuel may have a relatively high moisture con¬ tent, since the energy balance of the gasifier may be adjusted by elevating the inlet air temperature to the gasifier prior to its introduction into the gasifier. The gasification process itself may be enhanced by taking a fraction of the main fuel into the gasifier proper. Water spraying is well suited for cooling the product gas, because the steam generated herein can be used as injec¬ tion steam into the gas turbine 2. Cooling may also be implemented with the help of heat exchanger walls, whereby steam can be generated for the steam turbine or industrial processes. Furthermore, the heat recovered from cooling may be used for preheating the inlet air to the gasifier. In the clean-up of the cooled product gas, an alternative approach is to replace the water-spray washer with mechanical filters. However, the use of water-spray washing permits the omission of the cooler 7 in some cases provided that the gas is cooled with suffi¬ cient efficiency by the water spray in the washer.

Referring to Fig. 2, another connection is shown having the gasification step complemented with drying of the solid fuel. Therein, the fuel to be fed to the gasifier is first taken a high-pressure dryer 15, and the fuel is next separated from the mixture of the circulating steam and dried fuel in a cyclone 16, whereafter the fuel is routed via a fuel feed nozzle 14 to the gasifier 6. From the circulating steam flow is separated that portion of the steam which corresponds to the amount of water evaporated from the fuel and this portion of the steam is added along a line 18 to the product gas flow prior to the cooler 7. By means of such a connection, the energy used for drying the fuel can be recovered in the form of injection steam mixed in the fuel flow of the gas turbine 2. Simultaneously, the injected steam cools the product gas flow. While the separated steam can be taken to another point in the fuel circuit of the gas turbine, obviously the most advantageous arrangement is to introduce the steam into a pressurized section of the system prior to the gas turbine or to directly inject the steam into the turbine. Alternatively, the steam can be used in the steam circulation of the plant.

The circulating steam of the dryer 15 is taken along a line 17 blown by a fan 19 into the combustor 20 of the dryer, where the circulating steam is reheated prior to its return into the dryer. The combustor 20 may have a design of the heat-exchanger type, wherein the energy of combustion is transferred through heat-exchanger walls into the circulating steam, or alternatively, the flue gas of the combustor 20 can be mixed into circulating steam. Instead of generating the heating energy required by the dryer 15 by means of the main-fuel-fired combustor 20 of the gas turbine 2, at least a fraction of this heating energy can be generated in the exhaust heat recovery boiler 3 of the gas turbine 2 or in the cooler of the product gas. One further technique of altering the energy balance sheet of the dryer 15 is to fire main fuel directly into the dryer 15. In this connection, the gasi¬ fication of the solid fuel is supported by introducing the required supplementary energy into the solid fuel contained in the dryer 15 either in the form of heat, or alternatively, by combusting the main fuel. The connec¬ tion is suited for such fuels whose heating value may be elevated so high by drying that gasification may be carried out simply by feeding air from the compressor 1 of the gas turbine 2 into the gasifier 6. Hence, the supplementary energy introduced by combusting the main fuel is in this embodiment brought via the dryer to the gasification step, whereby the dryer and the gasifier form an integrated gasification unit.

Referring to Fig. 3, the embodiment shown therein can use fuels of extremely low caloric value. In this connection, to the inlet air line 13 of the gasifier 6 is adapted a combustor 21 fired with the main fuel of the gas turbine 2. Also the dryer circuit 15 - 17, 19 is provided with a combustor 20 in the same fashion as in the embodiment of Fig. 2. With the help of these combustors, a substantial amount of energy can be introduced into the solid second¬ ary fuel, making this connection suitable for fuels of extremely high moisture content and/or low caloric value. Hence, the connection could be used for burning of waste which is problematic to dispose of and difficult to in¬ cinerate. The connection is also most flexible as the ex¬ ternal heat added to the process by means of the supple¬ mentary combustors of the dryer and the gasifier is easy to control according to the properties of the solid fuel, in some cases even permitting operation with only one of the combustors 20, 21.

While gasification of the solid fuel is advantageously carried out in a pressurized stage, operation at atmos- pheric pressure is also possible, Then, however, the product gas must be compressed prior to the gas turbine. Fig. 4 shows a connection for atmospheric-pressure gasi¬ fication. Here, the inlet air to the gasifier 6 is fed to the combustor 11 of the gasifier 6 with the help of a fan 22, whereby the internal pressure in the gasifier 6 is approximately equal to the ambient pressure. Obviously, the product gas exiting the gasifier is at the ambient pressure, and any possible increase of internal pressure in the gasifier due to the compression heating of the inlet air is cancelled by the pressure drop that occurs when the temperature of the product gas falls in the cooler 7. The cooled product gas is taken via the washer 8 to a compressor 23. As a compressor driven by an electric motor -24 is required for compressing the product gas, the pressure elevation of the product gas consumes electric power.

Besides those described above, the present invention may have alternative embodiments. -

In the above exemplifying embodiments, the heat required by the dryer was produced by combusting the main fuel. At least a fraction of this energy could be generated in the waste heat recovery boiler 3 of the gas turbine 2. In this case, for example, the connection of Fig. 3 could be provided with a heat exchanger, which is connected to said waste heat recovery boiler 3, in parallel with or replacing the combustor 20 of the dryer 15.

BALANCE SHEET EXAMPLE

Initial values of computation

The following calculation is based on the assumption that a plant to be erected is required to supply a given heat load for which a combined-cycle power plant has to be designed. The main product of this combined-cycle plant is process steam with electric power generated as a by-product. Hence, the size of this plant is dictated by the required steam output, to which a gas turbine of optimal size is adapted dimensioned on the basis of the steam consumption. The required steam-generating capacity of the example plant at two pressure levels is: 14 bara: 1.6 kg/s (5.75 ton/h) 4.0 MJ/s 2.8 bara: 22.65 kg/s (81.5 ton/h) 54.5 MJ/s (bara = absolute pressure in bars)

A power plant of the above-outline type could be erected in conjunction with, e.g., a pulp and paper mill, whereby the solid fuel would be obtained as the sludge of a wastewater treatment plant. Such a sludge chiefly com¬ prises extremely fine fiber material collected from manu- facturing steps of fiber from wood and a certain amount of bacterial biomass escaping the treatment process plus some other components carried over along the process waters.

Comparative example

This calculation, in turn, iε based on the assumption that no essential amount of electric power is generated in a condensing cycle. Then, the size of the gas turbine in the comparative process can be made smaller as a por¬ tion of the generated steam is obtained from a separate solid-fuel-fired boiler. Thus, the present invention makes it possible to select a larger gas turbine to supply the same given heat load than is possible in con- ventional connections, since the application of the in¬ vention makes it possible to generate the entire steam output in the exhaust gas heat recovery boiler of the gas turbine. Accordingly, the gas turbine selected for the comparative example is type GE F5 (made by General Electric), whose nominal output power (26 MW_e) is approx. 2/3 of the nominal output (38.4 MW_e) of the gas turbine type GE F6B selected for the side-stream gasification connection. In the comparative example, the solid-fuel- fired boiler combusts the sludge using natural gas as supplementary fuel so that the fraction of natural gas combustion is approx. 40 % of the boiler overall output. Then, approx. 3.3 % (0.66 kg/s) of the overall steam output reaches the condenser, corresponding to a genera¬ tor output power of approx. 0.3 MW_e. By virtue of the in¬ vention, the gasification of the solid fuel can be per¬ formed using approx. 0.5 - 1.0 MJ/s of natural gas energy to heat the gasification air prior to the gasifier. With this portion of natural gas combustion, the gasification of difficult-to-gasify fuels such as masses of high moisture content can be improved. Additionally, it is possible to gasify fuels having a low caloric value but offering economical benefits. This situation may occur if the compensation paid for the incineration of the fuel is sufficiently rewarding.

Improvement by virtue of the invention in generation

In the two processes compared above, the main difference is in the gas turbine unit used for electric power gener¬ ation. Both steam turbine units have approximately iden¬ tical capacities, namely, about 11 MW_e gross electric power output and slightly below 60 MJ/s process steam, while in the gas turbine units, the difference between the gross electric power outputs is about 15 MW_e, corre¬ sponding to approx. 56 % increase of power output in the gas turbine unit operated using the side-stream gasifica- tion connection with regard to operation according to the comparative example, and a 37 % increase in the total electric power output of the entire plant. Simultaneous¬ ly, the firing rate is increased 15 %, that is, 17 MJ/s. This increase in the firing rate is covered by firing natural gas in the combustion chamber of the gas turbine. When computing the firing rates according to the lower heating values (LHV), the following table summarizes the above-mentioned differences between the compared pro¬ cesses. The table is computed for a situation in which the sludge is fed undried into the gasifier and the generated product gas is cooled in a water-spray washer to approx. 550 °C. The portion of supplementary energy input required for the gas turbine unit is covered by combusting natural gas.

Firing rate Process output Electric power [MJ/s] [MJ/s] output [M _e]

Comparative process 117.2 58.5 36.0

Side-stream 134.6 58.5 49.3 gasification process

Additional

Power-to-heat Electric power electric power ratio generating generating efficiency efficiency

Comparative process 0.62 30.7 %

Side-stream 0.84 36.6 % 76.5 % gasification process

Estimated effect of invention on plant overall economy

If the sludge from the mill is used locally as a fuel, the internal acquisition cost of such solid fuel can be assumed to be essentially zero. Then, the generated output of additional electric power should cover all extra costs incurred by the side-stream gasification connection. If the additional electric power is valued at 200 FIM/MWh_e, the annual value of the additional electric power generation according to the invention at 7000 h/a peak demand is 18.6 MFIM/a, of which the increased con¬ sumption of natural gas shaves off about 8.6 MFIM/a if the purchase cost of natural gas is estimated at a thermal energy level of 65 FIM/MWh_th. Then, the addi¬ tional electric power generation leaves about 10 MFIM per annum to be divided between the extra investment costs and higher operating margin. Taking 20 years as the effective operating life of the plant and applying a 12 % interest rate, the difference between the discounted values of electric power generation and fuel costs will be 75 MFIM. As the procurement cost difference between the gas turbine units (F6B vs. F5) can be estimated at 15 MFIM, the remaining discounted difference (60 MFIM) will cover the higher operating costs and the investment costs of the side-stream gasifier, while the rest will contribute to improved operating margin of the plant. It must further be noted that the investment into the side¬ stream gasifier can replace the solid-fuel-fired steam generator in the comparative process. Hence, the money saved from such a boiler of the fluidized-bed type, for instance, can be diverted to the investment into the side-stream gasifier.

Claims

Claims :

1. A method of generating energy, in which method

- a gas turbine (2) fired with oil or gas as its main fuel is used as the prime mover for an electric generator, and

- the exhaust gases of the gas turbine (2) are taken to a steam generator ( 3 ) in which the heat content of the exhaust gases is used for generating steam,

c h a r a c t e r i z e d in that

- the solid secondary fuel is gasified in a separate gasification unit (6, 14 - 18, 20, 21),

- supplementary energy is introduced into the secondary fuel to be gasified by firing the main fuel of the gas turbine (2) and using at least the heat thus generated in the gasifica¬ tion process, and

- the product gas made from the solid secondary fuel is used in conjunction with the main fuel as the fuel of the gas turbine (2).

2. A method as defined in claim 1, c h a r a c t e r ¬ i z e d in that the main fuel of the gas turbine (2) is natural gas .

3. A method as defined in claim 1, c h a r a c t e r - i z e d in that undried, solid secondary fuel is gasified directly.

4. A method as defined in claim 1, c h a r a c t e r ¬ i z e d in that gasification is carried out under pressurized conditions.

5. A method as defined in claim 1, c h a r a c t e r ¬ i z e d in that gasification is carried out at atmospheric pressure.

6. A method as defined in any of foregoing claims, c h a r a c t e r i z e d in that the solid fuel is dried prior to gasification and at least a fraction of the heat energy used for its drying is produced by firing the main fuel of the gas turbine (2) and that the steam separated from the solid fuel in the drying step is taken as injection steam into the fuel circulation of the gas turbine (2) .

7. A method as defined in any of claims 1 - 5, ch a r a c t e r i z e d in that the solid fuel is dried prior to gasification and at least a fraction of the heat energy used for its drying is produced in the exhaust gas heat recovery boiler (3) of the gas turbine (2) and that the steam separated from the solid fuel in the drying step is taken as injection steam into the fuel circulation of the gas turbine (2).

8. A method as defined in claim 7, c h a r a c t e r ¬ i z e d in that at least a fraction of the heat energy used for drying is produced by cooling the product gas.

9. A method as defined in claim 1, c h a r a c t e r ¬ i z e d in that energy is introduced into the gasifica¬ tion process by combusting the main fuel and taking the hot flue gases from the combustion of the main fuel into the flow of the secondary fuel.

10. A method as defined in claim 9, c h a r a c t e r ¬ i z e d in that energy is introduced into the gasifica¬ tion process by combusting the main fuel and taking the hot flue gases from the combustion of the main fuel at least into the gasifier (6) of the solid fuel.

11. A method as defined in claim 9, c h a r a c t e r ¬ i z e d in that energy is introduced into the gasifica¬ tion process by combusting the main fuel and taking the hot flue gases from the combustion of the main fuel at least into a dryer (15) of the solid fuel.

12. A connection for generating energy, said connection comprising

- a gas turbine (2) fired with oil or gas as its main fuel,

- an electric generator for generating electric energy using said gas turbine (2) as its prime mover, and

- a steam generator (3) and a line for taking the exhaust gases of the gas turbine (2) to said steam generator (3) for generating steam,

c h a r a c t e r i z e d by

- a gasification unit (6, 14 - 18, 20, 21) for gasifying a separate solid secondary fuel,

- equipment (10, 11) for combusting the main fuel of said gas turbine (2) and introducing supplementary energy into said secondary fuel to be gasified and using at least the heat thus generated in the gasification process, and - means for taking the product gas made from the solid secondary fuel to said gas turbine (2) for using said product gas therein in conjunction with the main fuel as the fuel of said gas turbine (2).

13. A connection as defined in claim 12, c h a r a c ¬ t e r i z e d in that said gasification unit (6, 14 - 18, 20, 21) is operated under pressurized conditions.

14. A connection as defined in claim 12, c h a r a c ¬ t e r i z e d in that said gasification unit (6, 14 - 18, 20, 21) is operated at atmospheric pressure.

15. A connection suited for implementing the method according to any foregoing claim, c h a r a c t e r ¬ i z e d by

- dryer equipment (14, 15, 16) for drying said solid fuel prior to its gasification,

- means (20) for combusting the main fuel of said gas turbine (2), said means being suited for producing at least a fraction of the heat energy required in the drying step of said solid fuel, and

- a line (18) for taking the steam, which is separated from the solid fuel in the dryer equipment (14, 15, 16), as injection steam into the fuel circulation of the gas turbine (2) .

16. A connection suited for implementing the method according to any of claims 12 - 15, c h a r a c t e - i z e d by - dryer equipment (14, 15, 16) for drying said solid fuel prior to its gasification,

- heat exchanger means (20) adapted into the exhaust gas recovery boiler (3) of said gas turbine (2), said means being suited for producing at least a fraction of the heat energy required in the drying step of said solid fuel, and

- a line (18) for taking the steam, which is separated from the solid fuel in the dryer equipment (14, 15, 16), as injection steam into the fuel circulation of the gas turbine (2).

17. A connection as defined in claim 16, c h a r a c ¬ t e r i z e d by means (7) for cooling said product gas and taking the heat energy recovered from the cooling step into said dryer (15).

18. A connection as defined in claim 12, c h a r a c ¬ t e r i z e d by a combustor (11) for firing said main fuel for introducing supplementary energy into said gasification process and by means for taking the hot flue gases from the combustion of the main fuel into the flow of said secondary fuel.

19. A connection as defined in claim 18, c h a r a c - t e r i z e d by means (10, 11) for taking the hot flue gases from the combustion of the main fuel into at least said gasifier (6) .

20. A connection as defined in claim 18, c h a r a c - t e r i z e d by means for taking the hot flue gases from the combustion of the main fuel into at least said dryer (15) .

21. A connection as defined in any of foregoing claims 12 - 20, c h a r a c t e r i z e d by a water-spray washer (8) for cleaning said product gas.

22. A connection as defined in any of foregoing claims 12 - 16, 18 - 21, c h a r a c t e r i z e d by water- spray equipment- (7) for cooling said product gas.