Materials and Thermal Efficiency of a Power Plant

What are the limitations imposed by materials on thermal power plants achieving the highest efficiency and output at the lowest cost? How to get over these limitations? Read on...

Laws of Thermodynamics

1. You cannot win, you can only break even.
2. You can only break even at absolute zero.
3. You cannot reach absolute zero.

The thermodynamic cycle used in a Thermal Power plant utilises steam at high temperatures and pressures. Increasing the upper temperature and pressure limits of the thermodynamic cycle increases the efficiency of the cycle. Power Plants operating with steam parameters of 540 °C and 170 bar pressure have an efficiency of 38 % while Ultra super critical power plants with steam parameters of 300 bar and 620 °C can have efficiencies of 48 %. This increase in efficiency is a direct emissions reduction apart from the cost savings.

Why are the older power plants operating at a lower temperature and pressure? Why are power plants with higher temperatures greater than 615 °C not made? This is because of the limitations imposed by the materials used for making the tubes, drums, and pipes which contain and transport the steam.

Limitations

The limitations in material are due to

• Reduction in strength.
  
  
• The mechanical strength properties of steel drastically reduce with increased temperatures. This means to withstand the higher pressure, the thickness of the tubes and pipes have to be increased.
• Added to this the continued operation at high pressure and temperatures leads to creep or slow degradation in the mechanical strength properties.
• Increased thickness means higher thermal stresses which imposes severe limitations on the design engineers. Also increased thickness means higher weight, meaning more structures and foundations, all leading to design limitations and higher cost.
• Oxidation.
  
  
• At higher temperatures, due to oxidation, scales form on the tube material. This in the continuous operation effect the life of the plant. Oxidation limit for Carbon steels is around 425 °C.As the steam and gas temperatures increase above the this limit, special alloy steels have to be used to prevent oxidation.

To overcome these limitations newer and newer materials are developed. Two decades ago the tube material for carrying the Superheated steam was grade T22 or P22 which had an allowable stress value of 50 N/mm² at 570 °C. At 600 °C it reduces drastically to 34 N /mm². Today we use grade T91 or P91 that has strength of 78 N/mm² at 570 °C and 60 N/mm² at 600 °C. This relates to a 40 % reduction in thickness at 570 °C.

The difference is the addition of alloying elements to the basic Carbon Steel. Grade P22 has 2.0 % Chromium and 1 % Molybdenum whereas Grade P9 has 9 % Chromium and 1 % Molybdenum, Nickel, and Vanadium.

For the high temperature application these special steels are called creep resistant steels. These are derived from the normal Carbon steels by adding alloy elements that increase the mechanical strength and heat resistant properties.

The five criteria that the industry is looking for in developing new materials are:

• Mechanical strength of the material should be available at higher temperatures.
• Mechanical strength properties should be consistent throughout the life of the plant at these elevated operating conditions or it should be creep resistant.
• The materials should be easily produced and available.
• The materials should be easy for fabrication and construction.
• All this at a reasonable cost for investment.

But the most important thing is that this allows the power plants to operate at higher temperatures and pressure which means higher efficiency and lower emissions.