Page "Cogeneration" Paragraph 9

Steam turbines for cogeneration are designed for extraction of steam at lower pressures after it has passed through a number of turbine stages, or they may be designed for final exhaust at back pressure ( non-condensing ), or both.

A typical power generation turbine in a paper mill may have extraction pressures of 160 psig ( 1. 103 MPa ) and 60 psig ( 0. 41 MPa ).

A typical back pressure may be 60 psig ( 0. 41 MPa ).

In practice these pressures are custom designed for each facility.

The extracted or exhaust steam is used for process heating, such as drying paper, evaporation, heat for chemical reactions or distillation.

Steam at ordinary process heating conditions still has a considerable amount of enthalpy that could be used for power generation, so cogeneration has lost opportunity cost.

Conversely, simply generating steam at process pressure instead of high enough pressure to generate power at the top end also has lost opportunity cost.

( See: Steam turbine # Steam supply and exhaust conditions ) The capital and operating cost of high pressure boilers, turbines and generators are substantial, and this equipment is normally operated continuously, which usually limits self generated power to large scale operations.