Cogeneration Italy

Case Study

An industrial site uses 130,000 MWh of electricity annually and 300,000 tonnes of steam.  The cost of electricity is €63/MWh and the cost of gas is €18.50/MWh.  The site boilers raise steam at 10 barg at an efficiency of 80% with a condensate return rate of 50%.  Therefore, it takes 0.9 MWh of gas to raise a tonne of steam, giving an annual gas consumption of 270,000 MWh.  So the annual cost of electricity to the site is approximately €8,190 k and the cost of gas is €4,995 k.  In addition the labour and maintenance cost of the boilers is €550 k per annum giving a total annual energy cost of €13,775 k.

The average loads of the site are approximately 15 MW of electricity and 35 tph of steam.  The cogeneration plant most likely to be proposed by a developer for this site would be an RB211 gas turbine with twin pressure Heat Recovery Steam Generator (HRSG) and extraction/condensing steam turbine.  In addition the plant would, if necessary, include new standby package boilers.  The GT will produce approximately 29 MWe of electricity and the HRSG approximately 39 tph of steam at high pressure and 6 tph at LP of 10 barg.  The HP steam will be fed to a steam turbine and extracted at 10 barg to meet the balance of the site’s steam demand with the surplus being fed to an air cooled condenser.  At the site steam demand the steam turbine would be expected to generate an additional 6 MWe giving a total electrical output from the cogeneration plant of approximately 35 MWe.

In this example the steam take or pay at would be approximately 100,000 tonnes of steam per annum, or just over a third of the existing steam consumption.

On the basis of a neutral steam price the cost of electricity offered by the developer would currently be of the order of €47-€50/MWh.  Using the top end of the indicative price range of €50/MWh the annual cost of electricity to the site would be €6,500 k.  The site will save the entire cost of operating and maintaining the existing boilers but will still pay the same amount for steam; €4,995 k giving a new annual cost of energy from the cogeneration plant of €11,495 k.  Compared to the existing site energy cost of €13,775 k, co-generation would give a predicted annual saving of €2,280 k which in percentage term is 16.5%.

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