28-03-2014, 11:06 AM
COGENERATION
COGENERATION.pdf (Size: 415.99 KB / Downloads: 143)
Need for cogeneration
Thermal power plants are a major source of electricity supply in India. The conventional
method of power generation and supply to the customer is wasteful in the sense that only
about a third of the primary energy fed into the power plant is actually made available to
the user in the form of electricity (Figure 7.1). In conventional power plant, efficiency is
only 35% and remaining 65% of energy is lost. The major source of loss in the
conversion process is the heat rejected to the surrounding water or air due to the inherent
constraints of the different thermodynamic cycles employed in power generation. Also
further losses of around 10-15% are associated with the transmission and distribution of
electricity in the electrical grid.
Principle of Cogeneration
Cogeneration or Combined Heat and Power (CHP) is defined as the sequential generation
of two different forms of useful energy from a single primary energy source, typically
mechanical energy and thermal energy. Mechanical energy may be used either to drive an
alternator for producing electricity, or rotating equipment such as motor, compressor,
pump or fan for delivering various services. Thermal energy can be used either for direct
process applications or for indirectly producing steam, hot water, hot air for dryer or
chilled water for process cooling.
Cogeneration provides a wide range of technologies for application in various domains of
economic activities. The overall efficiency of energy use in cogeneration mode can be up
to 85 per cent and above in some cases.
Technical Options for Cogeneration
Cogeneration technologies that have been widely commercialized include extraction/back
pressure steam turbines, gas turbine with heat recovery boiler (with or without bottoming
steam turbine) and reciprocating engines with heat recovery boiler.
Steam turbine cogeneration systems
The two types of steam turbines most widely used are the backpressure and the
extraction-condensing types (see Figure 7.3). The choice between backpressure turbine
and extraction-condensing turbine depends mainly on the quantities of power and heat,
quality of heat, and economic factors. The extraction points of steam from the turbine
could be more than one, depending on the temperature levels of heat required by the
processes.
Gas turbine cogeneration systems
Gas turbine cogeneration systems can produce all or a part of the energy requirement of
the site, and the energy released at high temperature in the exhaust stack can be recovered
for various heating and cooling applications (see Figure 7.4). Though natural gas is most
commonly used, other fuels such as light fuel oil or diesel can also be employed. The
typical range of gas turbines varies from a fraction of a MW to around 100 MW.
Gas turbine cogeneration has probably experienced the most rapid development in the
recent years due to the greater availability of natural gas, rapid progress in the
technology, significant reduction in installation costs, and better environmental
performance. Furthermore, the gestation period for developing a project is shorter and the
equipment can be delivered in a modular manner. Gas turbine has a short start-up time
and provides the flexibility of intermittent operation. Though it has a low heat to power
conversion efficiency, more heat can be recovered at higher temperatures. If the heat
output is less than that required by the user, it is possible to have supplementary natural
gas firing by mixing additional fuel to the oxygen-rich exhaust gas to boost the thermal
output more efficiently.
Bottoming Cycle
In a bottoming cycle, the primary fuel produces high temperature thermal energy and the
heat rejected from the process is used to generate power through a recovery boiler and a
turbine generator. Bottoming cycles are suitable for manufacturing processes that require
heat at high temperature in furnaces and kilns, and reject heat at significantly high
temperatures. Typical areas of application include cement, steel, ceramic, gas and
petrochemical industries. Bottoming cycle plants are much less common than topping
cycle plants. The Figure 7.6 illustrates the bottoming cycle where fuel is burnt in a
furnace to produce synthetic rutile. The waste gases coming out of the furnace is utilized
in a boiler to generate steam, which drives the turbine to produce electricity.