Cogeneration Plants - Energy Conversion and Management - page 147

Figure 9.1 represents a cogeneration plant with a gas turbine if efficiency
coefficients from Table 9.1 are used.
The maximum mechanical power available at the shaft does not gener-
ally exceed 30-32 % of the turbine input power as fuel in small and
medium-sized units (1-5 MW). The amount of recoverable heat depends
on the bottom temperature level required by the end user and in conse-
quence on stack-gas temperature.
Typical input power values, in the case of natural gas input, are
350 Sm 3 /h (3,330 kW) to produce 1,000 kW of electric power and
1,750 kW of recoverable heat. Specific consumptions, Sm 3 of natural
gas per kWh, are lower for bigger gas turbine units.
The overall efficiency of the system, that is, the ratio of total output to input
power, is roughly 70-80 %, depending on the bottom level of the thermal energy
user which determines the temperature of the stack exhausts. Higher values are not
generally possible, because of the great amount of airflow and so of the exhausts.
Gasoil instead of natural gas can also be used with appropriate burners.
Combined cycles, where exhausts with or without supplementary burners pro-
duce steam and then electric energy in a steam turbine, can also be used for sizes
higher than 20 MW of total electric output. These cycles allow for much higher
power-producing capability per unit of steam than the backpressure system or the
gas turbine by themselves.
Combined cycles with gas turbine and condensing or backpressure
turbines are widely used with the ratio of output electric power to
input power equal to 46-60 %.
The main data to take into account for selecting gas turbines are as follows:
• Unit fuel consumption/output power. Typical values range between 3 and
3.5 kW input (0.31-0.37 Sm 3 /h of natural gas) per 1 kW of output shaft
power. Gasoil can be used instead of natural gas;
• Exhaust flow and temperature values, on which the selection of the recovery
system can be based;
• The bottom level of temperature required from process end users that limits the
temperature at which the exhaust can be cooled;
• Ambient temperature on which depend the density, and so the mass of air
flowing through the compressor which is a nearly constant volume flow-rate
machine, as well as the bottom temperature of the cycle;
• Atmospheric pressure which varies according to the altitude. In consequence, the
density and so the mass of air flowing through the compressor changes;
• Pressure drops upstream the compressor and turbine system and downstream the
turbine that may influence the gas turbine useful output;