Supercritical technology in thermal power plants
It employs higher temperature and pressure resulting in better efficiency, reduced fuel and less green house gas (GHG) emissions, says A Pattabhi Raman, even as he gives worldwide trends and advantages offered by the technology.
Power generation by coal as a fuel is around 55 - 65 per cent of the total power generation in India. In India, around 200 - 250 billion tonne of coal have been identified as fuel resources. The cost of coal production is high while coal extraction is also low compared to demand. Indian coals are generally low grade with ash content as high as say 35 to 45 per cent. The fired coal cost will be high due to long distance transportation i.e., from the mine to the power station. Further, there are costs in preparing coal and its handling or disposal of ash. Pithead power stations would overcome some of these disadvantages, but there are other limitations like availability of water, land, etc., in the proximity of these pit head locations. So it is always not possible to establish a thermal power plant at coal mines or at the pithead.
The other alternative location to establish a power plant is on the coast where sea water is available in plenty and coal can be imported and unloaded through barges or jetties and transported to the plant. Imported coal in general has higher heat value and lower ash content as compared to Indian coal but it is expensive.
It is evident that availability of coal (fuel), water and land are very important and precious to set up a power station and hence need to be conserved. Land has been optimised by stipulating 100 per cent utilisation of ash. Water use has been minimised by higher cycle of concentration, minimum blow-down etc. The main item which requires concern in economics and ecology is fuel i.e., coal, the consumption has been reduced by higher efficiency technology, which is currently super-critical technology.
Due to the advent of better metallurgy, the boiler and turbine components are manufactured to withstand higher operating temperatures and pressures. This results in achieving higher efficiencies, in other words better heat rate which means less heat per kilowatt hour. Lower heat rate will result in less fuel consumption. In coal-fired power plants, less coal consumption means less ash, less green house gas emission, less wear and tear, etc.
The term 'super-critical' is used for power plants with operating pressures above critical pressure. Thermodynamic cycles which operate at parameters above critical point (at 225.56 kg/cm2 and 374.15 °C ) are called 'supercritical cycles'. At critical point, density of water and steam are same. Further, the latent heat (the heat required for change of phase from water to steam) at this point is zero which means that there is no steam-water mixed phase, which consequently reduces the fuel heat input. The cycle efficiency is improved by adopting super critical steam parameters.
Another advantage in super-critical boilers is that the load variation can be faster due to the absence of the drum. Higher temperature and pressure translate into better efficiency, i.e., more electricity per ton of coal fired. This increased efficiencies result in reduced fuel and hence less green house gas (GHG) emissions.
Super-critical technology is proven and is widely used since the 1950s. Over 600 super critical units from 300 to 1,200 MW size are in operation around the world.
Coal continues to be a major energy source for power generation worldwide. However, as carbon or carbon dioxide emission awareness is becoming an important aspect, technologies to reduce emissions from coal-fired plants become more pertinent. Supercritical and ultra-supercritical technologies are becoming popular in power generation. Upgradation of metallurgy for boiler and turbine components has given way to adopt higher steam parameters (pressure and temperature).
The units operating with super-critical steam parameters have better efficiency than conventional sub-critical units. The improvement in thermal cycle efficiency with increased steam parameters is indicated below:
Due to higher thermal efficiency, supercritical plants have the following benefits:
Further, super-critical units have other advantages like:
- Lower heat rate,
- Lower specific fuel consumption and hence reduced ash generation
- Reduced GHG emissions (CO2, SOX and NOX).
The advantages of 660 MW super-critical units as compared to 500 MW sub-critical units are many and these are listed below:
- Fast start-up times,
- Faster load-changes
- High part load efficiency
- Higher adaptability for sliding pressure operation and
- Fuel quality variation.
Central Electricity Authority (CEA) and Electric Power Development Corporation (EPDC) Japan have evaluated supercritical technology from technical and economical point of view for the Indian perspective and recommended supercritical technology for Indian plants.
The author is CEO of East Coast Energy and can be reached at Pattabhi@eastcoastenergy.in. Views are personal.