Heat Recovery From Gas Turbine Exhaust
Another significant area of waste heat recovery is the Combustion Gas Turbine (CGT), used either as:
• A direct mechanical drive for process equipment. In some instances, the hot turbine exhaust is used as combustion air for a fire heater in the plant where the mechanical drive machine is located.
• Part of a “Cogeneration” System. In these plants, the gas turbine shaft produces electricity, and steam to operate the plant is produced from the hot CGT exhaust.
For both situations, the CGT produces hot exhaust gases, typically in the 950°F to 1000°F range. This exhaust is used as:
• Preheated combustion gas for fired heaters, or
• Hot gases to a Steam Generator. The exhaust is almost always supplementary fired, up to 1600°F and 1700°F, to normally produce both dry (saturated) and superheated steam at different pressures for the operating center, or in the case of producing, a 60% to 80% quality steam for downhole injection.
Production of electricity and steam from one fuel source is called Cogeneration, which cuts the amount of energy necessary to make electric power to nearly onehalf. This is based on an economic amount of steam and electrical power being required at the plant.
Public Utilities typically furnish electric power to our operating centers at an efficiency of 32% to 35%.
Steam is typically generated in most of our plants at efficiencies of 70% to 75% without waste heat recovery, and 85% to 90% with it (all on a HHV basis).
The overall weighted efficiency for power purchased from a public utility power plant and steam produced at a Company facility is in the 60% to 70% range. This results from averaging the electric power at its low efficiency (from the power plant) with the higher efficiency for steam produced (in the Company facility).
By using an on-site gas turbine to simultaneously generate both electric power and steam, the overall “weighted” efficiency can be improved to the 75% to 85% range.
Figure 3200-6 compares the efficiency of Cogeneration with the conventional way of making steam at the plant and purchasing electric power from the utility.