Performance prediction and optimization of a waste-to-energy cogeneration plant with combined wet and dry cooling system

The present study is focused on a waste-to-energy plant located in Northern Italy, that produces electric power and thermal energy from the non-recyclable fraction of municipal and industrial solid waste (8,00,000 tons/year). In cold months heat is provided to a district heating system. Another peculiarity of this plant is that the condenser system is organized with an air condenser and a water cooled condenser, coupled with a wet cooling tower. This work shows how the net power output can be maximized by properly regulating the combined wet and dry units of the combined cooling system. A detailed model of the steam cycle was performed by means of a commercial code (Thermoflex®). Off design performance were carefully predicted to simulate accurately the real thermal cycle behaviour. Once the power cycle performance has been predicted over an extensive range of operating conditions, an optimal search method was implemented to find the set of variables allowing the wet and dry cooling system to be regulated so that the maximum net power is achieved. In general, the best strategy resulted in loading as much as possible the wet cooling system to reduce the operational cost of the dry air condenser. Conversely, the whole exhaust steam flow rate has to be sent only to the air condenser when the district heating water request is very large, i.e. in coldest months.