Coal the primary energy source consists mainly of Carbon. During the combustion process the Carbon in the coal combines with Oxygen in the air to produce Carbondioxide producing heat. The high heating value, the energy available in the coal, is in the range of 10,500 kJ/kg to 27,000 kJ/kg.
For example, consider a coal with a high heating value of 20,000 kJ/kg. Theoretically this is equivalent to 5.56 kwhr of electrical energy. Can we get all of this as electric power? No. In practice the effective conversion is only around one third of the theoretically possible value.
Why is it so?
The first process of energy conversion is the combustion where the potential energy in coal is converted to heat energy. The efficiency of this conversion is around 90 %. Why?
- Due to practical limitations in heat transfer, all the heat produced by combustion is not transferred to the water; some is lost to the atmosphere as hot gases.
- The coal contains moisture. Also coal contains a small percent of Hydrogen, which also gets converted to moisture during combustion. In the furnace, moisture vaporises taking Latent heat from the combustion heat and exits the boiler along with the hot gases.
- Improper combustion of coal, hot ash discharged from the boiler and radiation are some of the other losses.
The second stage of conversion is the thermodynamic stage. The heat from combustion is transferred to the water to produce steam. The energy of the steam is converted to mechanical rotation of the turbine. The steam is then condensed to water and pumped back into the boiler for re-use. This stage works on the principle of the Rankine cycle. For plants operating with steam at subcritical pressures (less than 221 bar) and steam temperatures of 570 °C, the Rankine cycle efficiency is around 43 %. For the state of the art plants running at greater than supercritical pressure and steam temperatures near to 600 °C, the efficiency is around 47 %. Why is it so low?
- The steam is condensed for re-use. During this process the latent heat of condensation is lost to the cooling water. This is the major loss and is almost 40 % of the energy input.
- Losses in the turbine blades and exit losses at turbine end are some of the other losses.
- The Rankine cycle efficiency is dictated by the maximum temperature of steam that can be admitted into the turbine. Due to metallurgical constraints steam temperatures are at present limited to slightly more than 600 °C.
The third stage converts the mechanical rotation to Electricity in a generator. Copper, magnetic and mechanical losses account for 5 % loss in the Generator. Another 3 % is lost in the step-up transformer which makes the power ready for transmission to the consumer.
To operate the power plant it is required to run various auxiliary equipment like pulverisers, fans, pumps and precipitators. The power to operate these auxiliaries has to come from the power plant itself. For large power plants around 6 % of the generator output is used for internal consumption.
This brings the overall efficiency of the power plant to around 33.5 %. This means we get only 1.9 kwhr of electrical energy from one kg of coal instead of the 5.56 kwhr that is theoretically available in the coal.
The efficiency or inefficiency of power plants is something that we have to live with for the present till technology finds away out.