Cleaner Coal-fired ‘Supercritical Power plants’

High performance Coal-fired ‘Supercritical Power plants’ – Promotes cleaner environment

As name suggests, Coal-fired Supercritical power plants operate at very high temperature and pressure (580 degree centigrade temp. and at a pressure of 23 MPa) resulting much higher heat efficiencies (46%), as compare to sub-critical coal-fired plants which operates at 455 degree centigrade temp., and efficiency of within 40%. Some of the benefits of advanced supercritical power plants include:

• Reduced fuel costs due to improved plant efficiency;

• Significant improvement of environment by reduction in CO2 emissions;

• Plant costs comparable with sub-critical technology and less than other clean coal technologies;

• Much reduced NOx, SOx and particulate emissions;

• Can be fully integrated with appropriate CO2 capture technology.

In other words, supercritical power plants are highly efficient plants with best available pollution control technology, reduces existing pollution levels by burning less coal per megawatt-hour produced, capturing the vast majority of the pollutants. This increases the kWh produced per kg of coal burned, with fewer emissions.

Because of the above techno-economic benefits along with its environment-friendly cleaner technology; more and new power plants are coming-up with this state-of-the-art technology. As environment legislations are becoming more stringent, adopting this cleaner technology have benefited immensely in all respect. As LHV (lower heating value) is improved (from 40% to more than 45%); a one percent increase in efficiency reduces by two percent, specific emissions such as CO2, NOx, SOx and particulate matters.

"Supercritical" is a thermodynamic expression describing the state of a substance where there is no clear distinction between the liquid and the gaseous phase (i.e. they are a homogenous fluid). Water reaches this state at a pressure above 22.1 MPa. The efficiency of the thermodynamic process of a coal-fired power describes how much of the energy that is fed into the cycle is converted into electrical energy. The greater the output of electrical energy for a given amount of energy input, the higher the efficiency. If the energy input to the cycle is kept constant, the output can be increased by selecting elevated pressures and temperatures for the water-steam cycle.

There are various operational advantages in case of supercritical power plant:

• There are several turbine designs available for use in supercritical power plants. These designs need not fundamentally differ from designs used in sub-critical power plants. However, due to the fact that the steam pressure and temperature are more elevated in supercritical plants, the wall-thickness and the materials selected for the high-pressure turbine section need reconsideration. The supercritical plant needs once-through boiler, where as drum type boiler is required by sub-critical power plant. In fact, once-through boilers are better suited to frequent load variations than drum type boilers, since the drum is a component with a high wall thickness, requiring controlled heating.

• The performance of supercritical plant depends on steam condition. Steam conditions up to 30 MPa/600°C/620°C are achieved using steels with 12 % chromium content. Up to 31.5 MPa/620°C/620°C is achieved using Austenite, which is a proven, but expensive, material. Nickel-based alloys, would permit 35 MPa/700°C/720°C, yielding efficiencies up to 48%. Lot R&D inputs and allying with suppliers are required to achieve higher performance.

• Moreover, fuel Flexibility is not compromised in Once-Through Boilers. A wide variety of fuels have already been implemented for once-through boilers. All types of coal as well as oil and gas have been used.

• Current designs of supercritical plants have installation costs that are only 2% higher than those of sub-critical plants. Fuel costs are considerably lower due to the increased efficiency and operating costs are at the same level as sub-critical plants. Specific installation cost i.e. the cost per megawatt (MW) decreases with increased plant size.

Because of the high performance, efficiency and preservation of much cleaner environments than sub-critical coal-fired power plants, more than 400 supercritical coal-fired power plants are operating in the developed countries like US, Europe, Russia and in Japan. Most of the new power plants coming up now-a-days are of supercritical coal-fired technology.