Energy, in general, and electricity in particular, plays a vital role in improving the standard of life everywhere. World has abundant proven reserves of coal and thus coal-based thermal power plants dominate almost everywhere. The development of coal fired supercritical power plant technology can be described as an evolutionary advancement towards greater power output per unit and higher efficiency. Energy conversion efficiency of steam turbine cycle can be improved by increasing the main steam pressure and temperature.
(a) Reduced fuel costs due to improved plant efficiency;
(b) Significant improvement of environment by reduction in CO2 emissions;
(c) Plant costs comparable with sub-critical technology and less than other clean coal technologies;
(d) Much reduced NOx, SOx and particulate emissions;
(e) Can be fully integrated with appropriate CO2 capture technology.
Supercritical technology and its advantages:
Since the energy crisis in 70’s a lot of effort has been made to reduce the energy consumption worldwide and increase the use of indigenously available fuels and renewables. In recent years also the need of reduced C02 emission from power generation has been a prime driver in developed nation’s energy policy. Thus, most of coal- fired power plants commissioned after 1980’s have been designed for supercritical operation with high efficiency ratings. Before the introduction of the super critical power plant concept, the generally applied concept was a drum-type boiler, which was replaced by the once-through type boiler to increase efficiency by at least 3% (relative) for practically the same investment.
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.
Moreover, 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.
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.
Conclusion:
Thus, new pulverized coal combustion systems – utilizing supercritical and ultra-supercritical technology – operate at increasingly higher temperatures and pressures and therefore achieve higher efficiencies than conventional sub-critical units with significant CO2 reductions. The objective of power plants within toady’s market boundaries is more than ever to ensure high efficiency (to reduce the environmental impact as much as possible) while at the same time to increase their economics in competition to existing alternatives. The development of an economical and efficient concept needs to look at the steam turbine all other main components like boiler, flue gas cleaning equipment and the optimization of the water-steam-cycle as main parts for the optimization.
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. This plant concept fulfils the requirement to balance reliable power supply, sustainable use of existing resources and economic operation.
L&T Venture into Super Critical Power:
L&T has identified power as major growth area and recently entered into the two JVs with MHI, one for Boilers and other for Turbine Generators with the objective to bring world-class technology to India. Both the JVs have been signed for design and manufacturing of power equipment for use in supercritical power plants (which are superior to conventional sub-critical plants in terms of fuel efficiency and environment-friendliness).
L&T is also setting up manufacturing capabilities for other critical equipment, including pressure piping, coal pulverizers, ESPs, and heavy forging, which will streamline the availability of components, and increase overall plant reliability and availability.
L&T secured one of the single largest EPC orders in India’s power sector by winning a contract valued at Rs 6897 crores from Maharashtra State Power Generation Co. Ltd. (Mahagenco) for 3X 660 MW super-critical Boiler Turbine Generator (BTG) units.
Contributed by
P Nagaraja
(Based on an article by Mr Partha Das Sharma)
