In this article we will discuss about the cogeneration of electricity and steam in a power plant.
Introduction to Cogeneration:
Cogeneration is the simultaneous generation of electricity and steam (or heat) in a single power plant. It has long been used by certain industries which need large quantities of steam or heating as well as electrical power. Examples are cement factories, paper mills, chemical industries, textile plants, fertilizer plants, extraction plants, edible oil industries, refineries etc. Cogeneration is used if the electricity thus produced is cheaper, than that purchased from the state electricity boards.
In a cogeneration plant if,
Types of Cogeneration:
There are two broad categories of cogeneration:
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1. The Topping Cycle:
In this type the primary heat at the higher temperature end of the Rankine cycle is used to generate high-pressure and high temperature steam and electricity in the usual manner.
The process steam at low pressure and temperature is then taken from the turbines as per the process requirements. There are two ways in which the steam is utilized.
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They are as follows:
(a) By using steam at the turbine exhaust – The steam turbine in this case is called back pressure turbine. The condenser is removed, and turbine exhaust is led to the process plant. Since condenser is absent the exhaust pressure of the turbine is more (hence the name back pressure turbine) and its efficiency is lower.
(b) By extracting steam from the turbine at an intermediate state; much as for feed water heating. The turbine of this type is known as extraction turbine or pass-out turbine. Since condenser is used the efficiency of the cycle is higher; and steam extracted has much higher pressure and temperature.
The main advantage of this cycle is its ability to use wide range of fuels including wood, coal, petroleum, coke, refuse etc. and hence is used where low quality refuse and byproduct fuels are readily available.
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This type however is seldom used because of its high initial cost and low power-to-heat ratio; especially at high process-steam pressures.
2. Bottoming Cycle:
Here the first primary heat at high temperature is directly used for process requirements. The low grade (low temperature and availability) waste heat from process plant is used to generate electricity. The efficiency of the cycle is obviously low, –10 percent for low and upto 20 percent at high temperature applications. Given that the waste heat is free, the cycle economics may be favourable.
The main applications of bottoming cycles are in the processes of the chemical and material processing industries such as cement, limestone and steel.
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Since the temperature of waste heat is low, hydrocarbon fluids are often used as working medium.
Since the bottoming cycle has very low combined efficiency; it is of little thermodynamic or economic interest and only the topping cycle therefore can provide true savings in primary energy. In addition, most process applications require a low grade (low temperature and availability) steam which can be conveniently produced in a topping cycle.
Some of the arrangements for cogeneration in topping cycle are as under:
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(a) Steam turbine power plant with a back pressure turbine.
(b) Steam turbine power plant with steam extraction from condensing turbine.
(c) Gas turbine power plant with a heat recovery boiler – using gas turbine exhaust to generate steam.
(d) Combined steam gas turbine power plant – The steam turbine is either of the back pressure type (a) or the extraction-condensing type (b) above.
The back pressure steam-turbine plant (a) is most suitable only when electricity demand is low compared with the heat demand. The combined cycle plant (d) above is most suitable only when the electric demand is high, about comparable to heat demand or higher. The gas turbine cycle (c) lies in between. Only the extraction condensing plant (b) above is suitable over a wide range of ratios.
Cogeneration with a Gas Turbine Plant:
Gas turbines are used by themselves in wide range of services.
Some of them are:
1. As a powering unit for all types of air-crafts.
2. For driving mechanical equipment such as pumps, compressors in industrial plants.
3. As a powering unit for railways and racing cars.
4. To drive electric generators in electric power plants – To produce power for peak loads.
There is also growing interest in using gas turbines for cogeneration and in combined cycle plants. The working temperatures of Gas Turbine are quite high (Max. temperature about 1200 – 1300°C) and hot exhaust gases leaving the turbine at about 500°C, still have a great deal of heat energy stored in them.
This heat energy can be used for various purposes such as:
1. Drying and direct heating – In food industry.
2. Heating of fluids in process industries – to heat air, oil, water which is a working medium in a process plant.
3. Producing steam by using a waste recovery boiler – The steam produced may be used for process application or for generation of electricity by using steam turbines. This is known as “cogeneration” for the former and “combined cycle” for the latter.
A simple gas turbine cycle consisting of an Air Compressor (AC), Combustion Chamber (CC) and Gas Turbine (GT) is used. The turbine exhaust gases are passed through a heat recovery boiler (HRB) to generate process steam which is then taken out for process plant. Supplementary fuel burners are provided to burn additional fuel to increase the mass flow rate of process steam for a short period, if required.