part 1: overview of cogeneration and its status in asia - Fire

State of art review of cogeneration 15
higher ratings and efficiencies and they are not sensitive to the gas quality. High-pressure
dual-fuel engines are available in both two-stroke and four-stroke versions.
2.3 Gas Turbines
Gas turbines used for cogeneration are usually designed for continuous duty because gas
turbines for stand-by use normally have low efficiencies and are most suitable for
applications where the operating periods are short.
Gas turbines for continuous duty are traditionally divided into two groups on the basis of
differences in design philosophy (there is now some convergence in their design).
The aero-derivative gas turbine, as its name indicates, is more or less derived from an
aircraft propulsion engine. The characteristics of aero-derivative gas turbines are low specific
weight, low fuel consumption, high reliability, etc. The major advantages of aero-derivative
gas turbines are high levels of efficiency and a compact and modular design with easy
access for maintenance. However, because skilled service personnel are required, gas
turbines of this type are often taken off the site for maintenance. Aero-derivative gas turbines
require a relatively high specific investment cost ($/kWe), high quality fuel and may
experience a lowering in output and efficiency after a long period of operation.
The industrial gas turbine, also referred to as the heavy duty or heavy frame gas turbine, is a
robust unit constructed for stationary duty and continuous operation. It has a somewhat lower
efficiency than the aero-derivative type, but usually maintains its performance over a longer
period of operation. Maintenance can be easily carried out on site, and maintenance costs
are low. The industrial gas turbine usually has a lower specific investment cost than its aeroderivative
counterpart. Furthermore, it has the ability to make use of low quality fuel.
The performance of a gas turbine depends on the pressure and temperature of ambient air
that is compressed. Since the ambient conditions vary from day-to-day and from location-tolocation,
it is convenient to consider some standard conditions for comparative purposes.
The standard conditions used by the gas turbine industry are 15°C, 1.013 bar (14.7 psia) and
60 per cent relative humidity, which are established by the International Standards
Organization (ISO). The performance of gas turbines is expressed under ISO conditions.
The actual power output of a gas turbine varies with ambient conditions. The power output of
a gas turbine decreases when the ambient temperature rises. In contrast, the power output
increases with the ambient pressure. The variations in power outputs of a typical gas turbine
with ambient conditions are shown in Figure 2.2 as a percentage of ISO power output.
The heat recovery steam generator (HRSG) is one of the major components of the gas
turbine cogeneration system. Since the energy content of the exhaust gas rejected to the
atmosphere is considerably high, HRSGs are designed to produce process steam (or hot
water) by recovering a large share of the energy contained in the exhaust stream. The
exhaust gas at 500-550°C is cooled in the HRSG to about 150°C to extract useful heat. A
temperature of 150°C is recommended at the outlet of the HRSG to avoid condensation of
exhaust gases. At lower temperature levels, gases such as SOx and NOx would form acids
along with the condensation and corrode the materials of HRSG.