Gas Turbine Handbook : Principles and Practices

Case Study #2 253
The Motor Oil Refinery decided to install its own in-plant cogeneration
plant in early 1980’s with the following objectives:
1. Energy conservation by utilizing the refinery flare gas (11,000
ton/year);
2. Increase refinery productivity by avoiding the costly process
shut-downs due to interruption of electrical power supply;
3. Increase refinery process steam availability and reliability by
utilizing the gas turbine exhaust energy;
4. Reduce refinery operating costs;
5. Reduce refinery pollution.
In 1985, the refinery cogeneration plant, consisting of two ABB
STAL GT35 Gas Turbine-Generator units, was placed in commercial
operation. The two gas turbines exhaust into a single two pressure
Heat Recovery Steam Generator (HRSG) with supplementary firing
capability. The combined electrical output of the two units is 27 MW
and a total 52 tons/hr of high pressure steam for power generation
and 16 tons/hr of low pressure process steam for refinery purposes.
The primary combustion fuel is refinery flare gas originating
from different refinery process streams as waste by-products. It consists
essentially of propane and butane in varying proportions ranging
from 60% to 100% propane, and 40% to 0% butane by volume.
This by-product flare gas also contains a varying concentration of
H 2
S, up to a maximum of 10,000 ppm. 20-25 ppm of H 2
S concentration
is considered a highly corrosive environment for gas turbine
application. Because the GT35 operates at a low hot blade path
temperature profile (exhaust temperature of 710°F) which is below
the melting point of sulfur, it is not affected by the high temperature
corrosion phenomenon; thus making it an ideal machine for refinery
flare gas application.
The flare gas varies in qualities, pressures and temperatures.
Its heating value approximates that of natural gas, i.e. 1,145 Btu/scf.
The cogen plant back-up fuel is gasified LPG (liquefied petroleum
gas) with a heating value of 2,500 Btu/scf.
To use this flare gas for gas turbine combustion, it is initially
de-slugged by a liquid trap which separate the liquid phase from the
gas stream. It is then processed through a low pressure compressor.
The condensates from the compressed fuel gas is further removed by