Cogeneration is defined as the simultaneous generation of heat and power. Cogeneration plants became popular due to their high thermal efficiency (e.g., 84 percent). They can be arranged as topping or bottoming cycles. In a topping cycle, power is generated before delivering heat to the process. The following are typical examples of topping cycles:
Noncondensing steam turbines (commonly used in the pulp and paper industry).
Gas turbine heat recovery and combined cycles. In these applications, the exhaust heat from gas turbines is recovered and used as an efficient source of heat.
Power is generated in a bottoming cycle by recovering heat from kilns, process heaters, and furnaces. This design normally has low thermal efficiency. This is mainly due to the low temperature of the steam used for power generation.
The thermal efficiency of a modern coal-fired power plant is around 35 percent. Most of the remaining energy (65 percent) is discharged to the environment (e.g., lake, ambient air). Thus, for every megawatt (MW) electric generated in a coal-fired power plant, almost 2 MW of heat is discharged to the ambient. Cogeneration plants use most of the heat input in a useful manner. Only a small amount of energy is lost (e.g., 16 percent). This is due to their high thermal efficiency (e.g., 84 percent).
The thermal efficiency of a cogeneration plant decreases significantly as the process steam is diverted to power generation. For example, the thermal efficiency of a cogeneration plant will decrease from 84 to 35...
TABLE OF CONTENTS 
