Page 71 - Sustainable On-Site CHP Systems Design, Construction, and Operations

50 CHP B a s i c s

above, the heat rate for combustion turbines increases with increased inlet air tempera-
ture while, at the same time, power output capacity falls linearly. As a rule of thumb, a
10°F increase in air temperature approximately equates to about a 5 percent decrease in
power output. Combustion turbine inlet cooling, as discussed, can be effective in main-
taining consistent power output even at higher outside air temperatures.
The amount of inlet pressure loss and combustion turbine backpressure also affects
the performance of the combustion turbine generator, and the CTG inlet and outlet
pressure drops need to be kept within the turbine manufacturer’s allowable limits. An
approximate 0.5 percent decrease in power output can be expected for each inch of
water column increase in air inlet pressure drop, therefore the design of the combustion
turbine air inlet system is critical to successful, sustainable CHP operations.

Heat Rate and Electric Efficiency
Combustion turbine’s average fuel-to-electrical shaft efficiencies generally range from
about 25 to 40 percent based on the HHV. Larger combustion turbine generators are
more efficient than smaller combustion turbine generators. Heat rates vary by manufac-
turer and model, and in general range from about 8500 to almost 14,000 Btu/kWh.
The remainder of the fuel energy is discharged in the exhaust and a minor amount
through radiation or internal coolants in large turbines. A minimum stack exhaust tem-
perature of approximately 300°F is typically required to prevent condensation, unless
the exhaust system is specifically designed for exhaust gas condensation, which if not
will lead to rapid corrosion of most metal exhaust systems (stainless steal is typically
required for condensing exhaust systems). Neutralization of the any condensate before
discharging to the sewer is also often required.
Useable Exhaust Temperatures/Useable Heat
Combustion turbines typically run very hot with combustor exhaust gases sometimes
exceeding 2300°F. At the turbine exit, exhaust temperatures are reduced down to
temperatures between 850 and 1100°F, due to the expansion of the hot gas through the
turbine(s). The exhaust temperatures coupled with high exhaust flow rates lead to
opportunities for heat recovery and duct firing, which are not feasible with reciprocating
engines. In CHP plants, where combustion turbine electric generation efficiency is of
utmost importance, regenerators or recuperators can be employed in the exhaust air
stream to preheat the compressed air that enters the combustor thereby leading to
higher electrical efficiency and slightly decreased fuel consumption.
The combustion turbine exhaust contains a large percentage of excess air; therefore,
afterburners/duct burners may be installed in the exhaust to create a supplementary
boiler system providing additional steam. Duct burners can be very efficient, reaching
an estimated maximum efficiency that exceeds 90 percent.
Cooling Water Requirements
Combustion turbines do not have the same cooling requirements as IC reciprocating
engines. Turbines do not have a crankcase or reciprocating parts that require cooling
and the only internal cooling typically required is for the oil that lubricates the compres-
sor/turbine shaft bearings and possibly the electric generator.
As noted above, cooling is, however, often utilized to precool the intake air stream.
Since the power output of a turbine is decreased by approximately 1/2 percent for each
1°F rise in intake air temperature. Ambient air to the compressor intake is never the
same temperature at all times throughout the year. In most cases, there is a significant