## 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. 

## BACKGROUND ART 

15 20 25 [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). 

45 50 [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 

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. 

[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. 

[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$_{3}$. 

[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. 

[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. 

[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. 

[0015] In the current state of the art, apart from the advantages that they make possible, combined- cycle plants present various energy losses. 

## DISCLOSURE OF INVENTION 

[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. 

[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: 

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; 

