What is supercritical boiler?

Answer 1

The following extract taken from an article by Ingo Paul in Energy Issues No 19 April 1999 published by the World Bank [external link] briefly describes the difference between a subcritical and a supercritical boiler. "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 megapascals (MPa) The "efficiency" of the thermodynamic process of a coal fired power (sic) describes how much of the energy 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. Up to an operating pressure of around 19 MPa in the evaporator part of the boiler, the cycle is sub-critical. This means, that there is a non-homogeneous mixture of water and steam in the evaporator part of the boiler. In this case a drum-type boiler is used because the steam needs to be separated from water in the drum of the boiler before it is superheated and led into the turbine. Above an operating pressure of 22.1 MPa in the evaporator part of the boiler, the cycle is supercritical. The cycle medium is a single phase fluid with homogeneous properties and there is no need to separate steam from water in a drum. Once-through boilers are therefore used in supercritical cycles.

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Answer 2

Subcritical and supercritical boilers are terms used in the context of steam power generation, specifically in coal-fired power plants. These terms refer to different operating conditions and technologies used in the boilers that generate steam to drive turbines and produce electricity. Here's the difference between subcritical and supercritical boilers:

Subcritical Boiler:

A subcritical boiler operates at a pressure and temperature below the critical point of water. The critical point is the temperature and pressure above which water and steam exist in the same phase (i.e., they become indistinguishable from each other). Subcritical boilers are the traditional type of boilers used in many older power plants.

Supercritical Boiler:

A supercritical boiler operates at a pressure and temperature above the critical point of water. In this state, there is no clear distinction between liquid water and steam; they form a continuous fluid. Supercritical boilers operate at higher temperatures and pressures, which can lead to increased efficiency and reduced greenhouse gas emissions compared to subcritical boilers.

Key Differences:

1. **Operating Conditions:** Subcritical boilers operate at pressures and temperatures below the critical point of water. Supercritical boilers operate above the critical point, where water and steam exist as a single fluid phase.

2. **Efficiency:** Supercritical boilers generally offer higher thermal efficiency compared to subcritical boilers. The higher temperatures and pressures in supercritical boilers lead to improved heat transfer efficiency and better conversion of thermal energy into electricity.

3. **Steam Generation:** Supercritical boilers produce steam that has higher energy content than steam produced by subcritical boilers. This higher energy content contributes to higher efficiency and power output.

4. **Emissions:** Supercritical boilers can have lower emissions of carbon dioxide (CO2) and other pollutants compared to subcritical boilers, as they can achieve better combustion efficiency.

5. **Plant Size and Cost:** Supercritical boilers can be more compact due to their higher efficiency, which can lead to reduced construction and operating costs. However, the initial capital cost of building a supercritical power plant might be higher due to the advanced technology required.

6. **Start-up Time:** Subcritical boilers generally have shorter start-up times compared to supercritical boilers, as the latter require careful temperature and pressure management during start-up.

7. **Materials and Design:** Supercritical boilers require specialized materials that can withstand the high temperatures and pressures involved. The design of supercritical boilers is more complex to ensure safe and efficient operation.

8. **Global Trends:** Newer power plants are increasingly adopting supercritical and even ultra-supercritical boiler technology due to their higher efficiency and lower emissions.

Both subcritical and supercritical boiler technologies play roles in power generation, and the choice between them depends on factors such as efficiency goals, environmental regulations, and economic considerations. Advances in technology are also leading to the development of even more advanced ultra-supercritical boilers with even higher efficiency and lower emissions.