What is the Difference Between Subcritical and Supercritical Boiler?

The key difference between subcritical and supercritical boilers lies in the pressure at which their steam is generated, as well as their operating efficiency and reliance on water circulation within the boiler. Here are the main differences:

1. Pressure and Temperature: Subcritical boilers operate below the critical pressure and temperature of water, while supercritical boilers operate above the critical pressure and temperature of water (220.64 bars and 374°C).
2. Boiler Efficiency: Supercritical boilers generally have higher efficiencies compared to subcritical boilers. For example, large subcritical thermal power plants with 170 bar and 540/540°C (SH/RH) operate at an efficiency of around 38%, while supercritical boiler units at 250 bar and 600/615°C can have efficiencies in the range of 40-42%. Ultra-supercritical boiler units at 300 bar and 615/630°C further increase the efficiency.
3. Water Circulation: Subcritical boilers rely on water circulation within the boiler, having different types of circulation such as forced circulation (ratio equal to 3) and natural circulation (ratio equal to 8). In contrast, supercritical boilers do not have a steam drum that separates the steam and the water, meaning water circulation is not as critical.
4. Steam and Water Phases: In supercritical boilers, there is no distinction between the steam and water phases due to the high pressure and temperature, so water acts as a fluid.
5. Materials and Superheat: Supercritical boilers require higher grade materials like alloy steels in the furnace due to the higher percentage of superheat compared to subcritical boilers.
6. Heat Transfer Area: Supercritical boilers have higher steam temperatures, resulting in a lesser differential temperature. This requires a higher heat transfer area compared to subcritical boilers.
7. Water Chemistry: The water entering the supercritical boiler must be of extremely high purity levels to avoid carryover of impurities that can result in turbine blade deposits.

Comparative Table: Subcritical vs Supercritical Boiler

The key difference between subcritical and supercritical boilers lies in their operating conditions, specifically temperature and pressure. Here is a table comparing the two types of boilers:

Subcritical boilers work at temperatures up to 374°C and pressures below 221 bar, while supercritical boilers operate at pressures above 221 bar and temperatures above 374°C. Supercritical boilers are generally more efficient than subcritical boilers due to their higher operating temperatures and pressures. They do not have a steam drum, which means they require higher water quality and different materials compared to subcritical boilers. Supercritical boilers are typically more expensive to build and maintain than subcritical boilers. The choice between the two types of boilers depends on factors such as efficiency requirements, budget constraints, and environmental regulations.