Supercritical Technology in Indian Power Sector | Original Article
Supercritical is a thermodynamic expression describing the state of a fluid above a certain pressure when there exists no clear distinction between the liquid and gaseous phases. The pressure at which such a transition to the supercritical state takes place is known as the critical pressure. For water this pressure is 22.064 Mpa. The corresponding saturation temperature is known as the critical temperature and for water it is 647.096 K. The term critical has been used for this thermodynamic state because it is a singularity in the fluid property states. Fluids below the supercritical pressure are termed as subcritical whereas those above the critical pressure are regarded as supercritical. Several modern plants operate at peak pressures of more than 24 Mpa and hence function as supercritical power plants. Supercritical coal fired power plants with efficiencies of around 45% have much lower emissions than subcritical plants for a given power output. Today’s state of the art in supercritical coal fired power plants permits efficiencies that exceed 45%, depending on cooling conditions. Options to increase efficiency above 50 % in ultra-supercritical power plants rely on elevated steam conditions as well as on improved process and component quality. Steam conditions up to 30 MPa/600°C/620°C are achieved using steels with 12 % chromium content. Pressures of up to 31.5 MPa/620°C/620°C have been proposed using Austenite, which is a proven, but expensive, material. Nickel-based alloys, e.g. Inconel, may permit 35 MPa/720°C/720°C, yielding efficiencies of nearly 50%.In order to improve coal-fired power plant efficiency leading to a proportional reduction in coal consumption and carbon dioxide emissions, it is widely accepted that the domestic power industry must move from sub critical to supercritical steam cycles. Medium to large capacity thermal power plants in India are now increasingly adopting the more efficient & bigger 660/800 MW supercritical units. Higher efficiency translates into reduced environmental impact. Less coal and water are used, smaller volumes of fly ash and scrubber waste is produced, and uncontrolled emissions of CO2 and mercury will be lowered.