1 EP 2 100 011 B1 2  Description TECHNICAL FIELD [0001] The present invention relates to a combined- cycle plant for the production of electrical energy. [0002] In particular, the present invention regards a combined-cycle plant for the production of electrical en- ergy that can be selectively supplied with gases with low calorific powerand ata low pressure,in the case in point siderurgical gases generated by a steel plant. [0003] Generally, a combined-cycle plant of the type identified above comprises a gas- turbine unit and  a steam-turbine unit. The gas-turbine unit comprises: an air compressor; a combustion chamber; and a gas tur- bine. The combustion chamber can be supplied wit h siderurgical gases, i.e., mixtures of gases produced in steel plants during the working cycles for the production of steel. The steam-turbine unit comprises, in succes- sion,along theductforsupply ofthe steam:acondenser; a degasser; and a boiler, which is supplied with the ex- haustgases,produced bythe combustion thathastaken place in the combustion chamber of the gas- turbine unit to carry out recovery of heat from said exhaust gases. A combined-cycle having the above identified structure is disclosed in US patent 4,932,204. [0004] US patent2,613,495 teaches to evaporate part of the condensed steam by the heat generated in an air compressor and supplying said steam directly to an ex- pansion stage of a steam turbine. [0005] Patent application JP 2007-023976 teac hes compressing the fuel, which is burnt partly in the com- bustion chamberofa gasturbine unit,and partlyexpand- ed in a turbine for recovery of part of the energy. [0006] Steel plants comprise: blast furnaces, which generate a gas referred to as BFG (Blast Furnace Gas); electrical converters, which generate a gas referred to as BOFG(BasicOxygen Furnace Gas);and cokeovens, which generate a gas referred to as COG (Coke Oven Gas). [0007] Siderurgical gases are produced at a pressure slightly higher than atmospheric pressure, whilst the combustion chamber requires that the combustible gas be ata higher pressure.Consequently, siderurgical gas- es must be compressed by a compression unit, which is actuated bythe combined- cycle plantand absorbs a non- negligible part of the power produced by the plant (typi- cally 15-30% of the power generated). [0008] Siderurgical gases have a calorific power that isrelativelylow as compared totheso- called naturalgas, and it is therefore necessary to burn a high amount of siderurgicalgases.Inaddition,since the siderurgicalgas- es are not continuously available, it is necessary for the combustion chamber to be able to be supplied also with natural gas to prevent arrest of the plant and stoppage a gas-turbine unitprovided witha combustion cham- ber; a steam-turbine unitprovided witha degasser,a boil- er, and a circuit for supply of the condensed steam to the degasser and, subsequently, to the boiler; [0017] According to the present invention there is pro- vided a combined-cycle plant for the production of elec- trical energy, the combined-cycle plant comprising: [0016] The aim of the present invention is to provide a combined-cycle plant that is able to overcome the draw- backs of the known art so as to increase the power and improve the efficiency thereof. [0015] In the current state of the art, apart from the advantages that they make possible, combined- cycle plants present various energy losses. [0014] In combined cycles, this fact leads to a loss of electrical power that can be produced and a consequent loss of efficiency of the combined-cycle plant, since, in order to prevent problems of corrosion in the first banks of the low-pressure economizer, there typically needs to be introduced the steam that has undergone condensa- tion directlyin the degasser,with consequentabsorption of steam coming from the outside to guarantee proper degassingofthe condensed steam.Obviously,the steam consumed by the degasser would have contributed to producing electrical energy. [0013] With the known solutions, in order to prevent formation of sulphuric acid, an effort is made to avoid cooling of the fumes below the threshold temperature referred to above and/orto avoid havingareas within the boiler below said threshold temperature. [0012] When a flow of gas containing SO $_{3}$ is cooled below a threshold temperature equal to the Acid Dew Point Temperature increased by 10° C, SO$_{3}$ combines with the water vapour present in the fumes, thus giving rise to sulphuric acid H$_{2}$SO$_{4}$, which is highly corrosive. [0010] In addition, siderurgical gases typically contain an amount of sulphur products (H$_{2}$S, COS, etc.) that is sufficiently high as to create problems of corrosion in the end part of the boiler. [0011] In fact, when the aforesaid components of the siderurgicalgases are burntinthecombustion chambers ofgas turbines and/orinthe post- combustions processes thatoccur in the boilers, they give rise, to a major extent, to S0$_{2}$ and, to a minor extent, to SO$_{3}$. [0009] Compression of siderurgical gases generates, in addition to the increase in pressure, a considerable rise in the temperature of the siderurgical gases, which in effect worsens the efficiency of the compression unit. of production of electrical energy. Supply ofthecombus- tion chamber with natural gas (with high calorific power) is used also in the step of start-up of the plant when the plant is not able to function properly burning siderurgical gases. BACKGROUND ART DISCLOSURE OF INVENTION 2