Subcritical or supercritical?

Telangana Chief Minister K Chandrasekhar Rao on Sunday met Union Minister for Power Piyush Goyal in New Delhi and sought exemption for the 270x4 MW Bhadradri thermal power plant. The National Green Tribunal has de-listed the State application on the issue of use of subcritical technologies for setting up the plant at Manuguru in newly created Kothagudem district. However, the State Energy Minister exudes optimism that the plant will be commissioned by March next. He pointed out that there is no ban on subcritical technology plants till the end of the 12th Plan (2012-17).

Central Electricity Authority (CEA) in September 2015 discussed the “Replacement of old and inefficient units with supercritical units” and possibility of replacing the old & inefficient thermal generating units by installing supercritical units. The design life of 25 years is generally considered for coal fired units. As per data available in CEA as on 31.3.2015 around 32830 MW capacity in State, Central sector and 1450 MW in Private sector is more than 25 years old.

According to Dhananjay Kavathekar (www.quora.com), Supercritical units are the standard for future power plants in many countries including China. Water with pressure and heat first reach saturated temperature, then the steam is wet which further gets dry and then superheat. With higher pressure latent heat gets lesser and lesser. At about 225 kg/cm2 pressure latent heat become zero and water transform directly to superheated steam.

So below this citical pressure it is subcritical and above 225 it is supercritical. Boilers generating steam above 225 kg are supercritical boilers. Accordingly boilers are designed. Higher the steam pressure lesser the volume which would save piping, turbine volume thereby saving equipment weights and costs.

The main advantage and the reason for a higher pressure operation is the increase in the thermodynamic efficiency of the Rankine cycle. Large subcritical thermal power plants with 170 bar and 540 / 540 ° C (SH / RH) operate at an efficiency of 38 %. Supercritical units operating at 250 bar and 600/615 ° C can have efficiencies in the range of 42 %. Ultra supercritical units at 300 bar and 615 / 630 °C will still increase the efficiency up to 44 %. Increase in efficiency directly leads to reductions in unit cost of power and CO2 emissions. (brighthubengineering.com).