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

State of art review of cogeneration 17
Recuperators are used to increase the power output of gas turbine cogeneration systems if
the heat demands are low. The recuperator is in fact only a heat exchanger that is employed
to heat the air leaving the compressor. The exhaust stream from the turbine is passed
through the recuperator before going into the HRSG so that a part of the energy contained in
turbine exhaust is utilized in the recuperator. The gas turbine cogeneration system with
recuperator is sometimes known as the heat exchange cycle.
2.4 Steam Turbines
Steam turbines are the most commonly employed prime movers for cogeneration
applications, particularly in industries and for district heating. The technology is well proven in
sugar and paper mills having demand for both electricity and large quantity of steam at high
and low pressures. Some steam turbine manufacturers are over 100 years old and have
products ranging from a few kW to 80 MW. However, turbines below two MW may be
uneconomical except where the fuel has no commercial value.
A cogeneration system using a backpressure steam turbine (see Figure 1.2) consists of
boiler, turbine, heat exchanger and pump. In the steam turbine, the incoming high pressure
steam is expanded to a lower pressure level, converting the thermal energy of high pressure
steam to kinetic energy through nozzles and then to mechanical power through rotating
blades. Thermal energy of the turbine exhaust steam is then transferred to another fluid,
water, air, etc., in a heat exchanger, providing heat to the processes. For instance, the air
heated by heat exchanger can be used to dry products in food processing industries.
Depending on the pressure (or temperature) levels at which process steam is required,
backpressure steam turbines can have different configurations. The most common types of
backpressure steam turbines are shown in Figure 2.4. In extraction and double extraction
backpressure turbines, some amount of steam is extracted from the turbine after being
expanded to a certain pressure level. The extracted steam meets the heat demands at
pressure levels higher than the exhaust pressure of the steam turbine.
The backpressure steam turbine has a higher heat to power ratio and higher overall
efficiency. Furthermore, back pressure turbine cogeneration systems need less auxiliary
equipment than condensing systems, leading to lower initial investment costs.
The extraction condensing turbines have higher power to heat ratio in comparison with
backpressure turbines. Although condensing systems need more auxiliary equipment such
as the condenser and cooling towers, better matching of electrical power and heat demand
can be obtained where electricity demand is much higher than the steam demand and the
load patterns are highly fluctuating.
In the reheat cycle, steam is extracted from the turbine and reheated in the boiler during the
expansion process. Reheat cycles improve the overall thermal efficiency and eliminate any
moisture that may form as the steam pressure and temperature are lowered in the turbine.
Steam turbines may also include a regenerative cycle where the steam is extracted from the
turbine and used to preheat the boiler feedwater.
The efficiency of a backpressure steam turbine cogeneration system is the highest. In cases
where 100 per cent backpressure exhaust steam is used, the only inefficiencies are gear
drive and electric generator losses, and the inefficiency of steam generation. Therefore, with
an efficient boiler, the overall thermal efficiency of the system could reach as much as 90 per
cent.