concroller) prevencs air leaks from oucside. This increases ehe efficiency and furchermore lowers ehe oxidizacion of ehe molcen mecal. The emergence of new. chermomechanical ceramics lecs chink ofa possible heac recovery co preheac ehe combuscion air, even in furnaces using fluxes, which is not ehe case wich mecallic recuperacors. This kinci offurnaces ranges from 1 . 5 ro 5 c/h in melcing capacicy, and may score up ro 30 mecric cons in holding capacicy. They may be scanding or cilcing, wich or wichouc an aucomacic loading device. *Anocher way co improve ehe operacion resulcs of an aluminium melcing furnace is co use regeneracive burners: wich a 1100 °C cemperacureofche flue gases at cheouclec, checombuscion air can be preheaced up co 880 °C, chus decreasing ehe specific consumpcion down ro 540 kWh (NCV)/c for a mecal pouring cemperacure of740°C. 4. COGENERATION Cogeneracion, a decencralized power generacion cechnique, includes differenc cechnologies wich a simulcaneous produccion ofehermal and mechanical energy. In mosc cases, ehe mechanical energy is couplecl wich ageneracor co produce eleccrici ey. In rhis process, heac and mechanical energy are available wich an overnll efficiency largely higher chan when boch energies are separacely (ehe chermal energy, usually wasced when producing energy, being recovered). Cogeneracion is oneofehe mosc efficienc namrnl gas cechnologies and offers a wide range of possible applicacions elue ro eheprogresses made in gas fireci engines and curbines. 4-1. Advamages Cogeneracion or combined heac and power produccion, presencs a wide array of cechniques wich an oucpuc ranging from some hunclred kW ro several MW. Ic is an efficienc, safe and environmenc respeccfull solucion co meec energy requiremecs and co concrol power generacion. Efficienc, cogeneracion helps opcimize ehe energecical balance by seleccing ehe leasc expansive power source, using ehe price as a basic cricerium. Safe Power Supply, cogeneracion garancees a concinous supply, which is imporranc when supply securicy is required. Clean, Cogeneracion wich namral gas combines low C02 emissions (duc co ics high efficiency) co ehe environmenc proceccion (due co ehe inherenc qualicies of nacural gas). DOGAL GAZ DERGİSİ SAYI. 29 102 4-2. Operation Principles Whacever ehe cechnology, cogeneracion requires a "prime mover", wbich can be a sceam or a gas curbine or an incernal combuscion engine, depending on ehe eleccrical power outpuc and chermal needs (sceıtm, hor wacer,. . .) In a sceam curbine cogeneracion process, dry high or medium presssure sceam is provicled by a convencional boiler fireci by any fuel available. This sceam is expanded in a curbine driving a generacor, eben may be used for ocher purposes, improving ehe overall efficiency. Sceam nırbine power plancs usually have a minimum oucpuc around 1,500 MW for eleccric produccion and 5,000 MW for rhermal produccion, sceam being available at clifferenc pressures. Tbe overall efficiency reaches 80 %, che eleccrical produccion efficiency being around 1O co 18 % . Increasing ehe sbare of eleccric produccion can impair ehe overall efficiency, wbich can drop down co some 35 %. Gas mrbine cogeneracion syscems presenc a far beccer share for eleccric produccion, ranging from 20 co 40 %. In a gas engine driven cogeneracion, chermal energy is recovered from ehe cooling wacer, ehe lubricacing oil ancl ebe exhausc gases. In mosc cases, ebe mecanical energy is cransformed inco eleccricicy, generally low volcage, by a generacor, wicb a power oucpm ranging from some one hundred kw up co a few MW. When run ac racios becween 50 and 100 % of ics eleccrical racing, chis cecbnique is highly efficienc: irs overa.. efficiency is around 80-90 %, as ics eleccrical efficiency varies from 30 co 40 %. This device is mainly usecl co generace eleccricicy, beside hor wacer, hor ancl cold air, ehe laccer wich a compression or absorpcion syscem. 4-3. Environmemal Impacc Cogeneracion proceccs ehe environmenc. Wich a highly efficienc use of ehe primary energy, ic decreases ehe heac losses and, as far as natura! gas is concerned, it diminishes pollmion, as illuscraced below: *The METZ cogeneracion power srncion: To generace 370 GWh of elececricicy and 250 GWh ofheac, it consumes 1 ,230 GWh ofcoal, wasce and gas. Wich convencional separnce plancs, che consumpcion would be 1 ,540 GWh. Tbac means primary energy savings of more chan 20 %, a 54 % reduccion in dusc emissions, 26 % in SO,, 25 % in NOX and HCI and 26 % in co2. *Wicb a gas curbine cogenecarion associaced wich a sceam produccion by heac recovery, a French papermill saved 9 % in primary energy, while reducing polucing emissions by 20 % .. , .
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