Man has mastered so many arts, from intricately carving artefacts to building ballistic machines! Making power by burning fuels such as coal, petroleum or natural gas was one such achievement that lighted up our lives. Thermal generators or specially designed furnaces produce this wonderful thermal electricity. Thermal power plant burn fuels to boil water and make steam. The steam is then used to spin a turbine which is connected to a generator that weaves electricity. Turning on a light or a fan seems so effortless, but do you know how electricity is made? It is time that we peered into the working of a thermal power station.
What goes on within a thermal power station?
Among several systems required in a thermal power plant the key ones are a boiler, a turbine and a generator. What are they? Well, read on to find out (Fig 1)!
Getting to know the boiler!
A boiler, as the name suggests, is a place where water is boiled to make steam. It is made up of about 100 miles of pipes, all welded together for better heat flow from the fire to the water. A big furnace, 14 storeys high provides the required heat to the water. The reservoir at the top collects steam for delivery to a high pressure turbine.
Well, after all the energy from steam is used up by the turbine, it is sent to a condenser. The condenser turns the leftover steam to water and a pump sends it back to the boiler. This completes the boiler cycle.
The condenser is a big heat exchanger that works like a radiator in your car. The engine in the car gets hot when it is running and a radiator is used to cool it. The same thing happens in a thermal power plant. The leftover steam is hot and has to be liquefied before the pump can send it back to the boiler. Generally, a natural water body comes in handy to cool a thermal power plant.
Now the Turbine!
The steam made in the boiler feeds the turbine arriving first at a nozzle block, which is made of thick steel. Remember that it has to hold the pressure and heat that comes from the steam. The steam exits the nozzle block from about 100 small holes, approximately one inch high and one-quarter inch wide.
The high pressure steam generated in the boiler is fed by pipe to a turbine. The turbine uses the kinetic and/or reactionary energy from the steam to spin a shaft. The figure 1 below will give you a clear view. See the cup-shape of the blades.
They quickly capture the steam and move rapidly. The turbine has many stages. Can you count them all? Yes, there are three stages to this turbine, a high pressure on the left, an intermediate pressure in the middle, and a low pressure on the right. The turbine has tremendous torque and is connected to a generator by a shaft. The shaft can be 18 to 24 inches thick. This brings us to the next part which is a generator.
Face to face with the generator
What is a generator? It is simply a big electric motor. The generator pins at 3600 rpm and provides the electricity that comes to your home. There are four main parts to a generator, a stator, rotor, brushes, and a shaft. The generator makes electricity at a specific voltage and phase so that an electrical grid may use it. What is a grid? You must be indeed so familiar with the World Wide Web. Well, the grid is somewhat like that! How? In the World Wide Web many computers are all connected together and they can talk to each other. In the electrical grid, the electrical generators are all connected by wire. They all have a physical connection to each other. But whether they can converse with each other is still unknown! Figure 2 below shows a pictorial representation of how a generator works.
A generator is online when it is connected to the grid and is providing electricity to the customers. It is said to be offline when it is not providing electrical power to the grid. A generator may be taken offline for repairs or because your city is not using a lot of power. For example, during the night, a generator may be taken offline because your city is using less power than it does during the day.
What is co-generation?
Co-generation is the concept of producing two forms of energy from one fuel. One of the forms of energy must always be heat and the other may be electricity or mechanical energy. In a conventional power plant, fuel is burnt in a boiler to generate high-pressure steam. This steam is used to drive a turbine, which in turn drives an alternator to produce electric power. The exhaust steam is generally condensed to water which goes back to the boiler. Since the low-pressure steam has a large quantum of heat which is lost in the process of condensing, the efficiency of conventional power plants is only around 35 per cent.
In a cogeneration plant, a very high efficiency level, in the range of 75 per cent – 90 per cent, can be reached. Why? Because the low-pressure exhaust steam coming out of the turbine is not condensed but used for heating purposes in factories or houses! Co-generation has other advantages as well, especially in the form of significant cost savings for the plant and reduction in emissions of pollutants due to reduced fuel consumption.
Even at conservative estimates, the potential of power generation from co-generation in India is more than 20,000 MW. Since India produces an enormous amount of sugar, bagasse-based cogeneration is being promoted. The potential for cogeneration is primarily in sugar and rice mills, distilleries, petrochemical sector and industries such as fertilizers, steel, chemical, cement, pulp and paper, and aluminium.
What fuels a thermal power station?
Coal, black or brown, consists mainly of carbon, formed by the compressed vegetative remains of past ages. Coal is extracted from the depths of the earth, in layers sandwiched by shale, sandstone, limestone and other rocks. Mining can be of two types: open cast and underground mining. The largest part of coal production in India comes from the coalfields of West Bengal, Bihar, Jharkhand and Odisha. Take a look at the geological layout of the Talcher coal fields (Fig 3) you will get a feel of how complicated the mining business is!
What about petroleum and natural gas?
Petroleum, an inflammable liquid, composed primarily of 90 to 98 per cent of hydrocarbons, the rest comprising chiefly of organic compounds. In India, as you know, crude oil is produced both onshore and offshore. The offshore production contributes nearly two-thirds of the total crude oil production in India, while the onshore production is restricted to the states of Gujarat, Assam, Nagaland, Tamil Nadu, Andhra Pradesh and Arunachal Pradesh. However, the demand for oil in India cannot be met by domestic production. As a result, the country has to import, both in the form of crude oil and oil products.
Use of oil for power generation is indeed limited in our country. It is used mainly in the thermal stations close to the oil wells and refineries and maybe used in diesel and gas turbine stations.
Natural gas is mainly available both alone and in association with crude oil. In India, most of the natural gas output comes from associated gas fields. Did you know that around 70 per cent of our current gas production comes from offshore Bombay High field and onshore fields in Gujarat? The estimated gas reserve in India is about 685 billion cubic meters (BCM), with the current production of 74 million standard cubic meter per day (MMSCMD). The production from established fields is expected to reach a higher level as the demand for natural gas for power generation increases.
How much thermal power is generated in India?
As on August 31, 2016, the total installed capacity of utilities in India stood at 212,568.90 MW. In the earlier stages of power development, the thermal power stations consisted of several small widely scattered units. The first step in development of thermal power on a large scale was the establishment of power station in Bokaro. This station later became the forerunner for the chain of thermal power stations that have developed subsequently, including large thermal stations like Neyveli, Dhuvaran, Korba, Trombay, etc. Large findings of coal have also made it possible to set up new projects in Singrauli, Korba, Talcher and several other places.
Take a look at the Graph below. Which states have the maximum generating capacity? Yes, it is Maharashtra generating 29922.19 MW of thermal energy, followed by Gujarat, which generates 23556.71 MW. Other major contributors to thermal energy are Chhattisgarh (15752.54 MW), Uttar Pradesh (15728.92 MW), Tamil Nadu (13313.94 MW), Madhya Pradesh (12216.23 MW), Andhra Pradesh (10875.69 MW) and Rajasthan (10225.75).
Coal Reserves in India
As a result of exploration carried out up to the maximum depth of 1200m by the Geological Survey of India (GSI), Central Mine Planning and Design Institute (CMPDI), The Singareni Collieries Company Limited (SCCL) and Mineral Exploration Corporation Limited (MECL), etc, a cumulative total of 301.56 billion tonnes of Geological Resources of Coal have so far been estimated in the country as on 1.4.2014. The details of state-wise geological resources of coal are given as under:
Data collected from Ministry of Coal (http://coal.nic.in/content/coal-reserves)