28/02/2017 by ESI Boiler Rentals 0 Comments
How Does a Power Plant Boiler Work?
A power boiler is the primary source of steam in most utility and industrial facilities. Its purpose is to create steam by combusting fuels to generate heat and transferring that heat energy to water. Steam is the primary form of power generation in most of the world through the use of turbine generators. Steam turns the turbine which spins the generator in a magnetic field generating a current (electricity).
To better understand the subtleties of how a power plant boiler works, we took a closer look at its design, operation, and key components. Read more and find out the basics you ought to know about power plant boilers: what they are, how they work, and what are their main advantages.
A boiler is a closed, pressure vessel in which water is being converted to steam (and possibly superheated) by the transfer of energy from a fuel source – typically oil, gas, or coal. The steam exits the boiler for use in various processes or heating applications such as manufacturing equipment, central heating, steam-based power generation, cleaning, cooking, and so on.
Made from thick steel plates, boilers can generate extremely high pressures. Steam pressure and temperature are both key factors for power generation. Generally, the higher the pressure and temperature of a power boiler – the better the overall efficiency.
What is a power plant boiler and how is it different from regular heating boilers?
For starters, power plant boilers are much bigger and the system involved in the combustion process is much more complex. Unlike heating boilers, the steam pressure of a power boiler exceeds 150 psig. They are used to generate power and are intended for operations at temperatures exceeding 500°F.
Power boilers are typically used at power plants as well as other large industrial applications where extremely high-steam pressure is required.
Power plant boiler operation is highly complex and involves several stages. First, the fuel – coal, natural gas, fuel oil– is being burned to generate heat. The water is usually heated to just below its boiling point in feedwater heaters and economizers before entering the boiler. Once in the boiler the water undergoes a phase change to become steam. The steam is then superheated to reach a higher energy level which increases the power plant’s output and efficiency. We will explain the process in detail later in the article.
An essential system, designed to effectively transfer heat and energy from the fuel to water, a boiler can account for between 20% to 30% of the total cost of a power plant.
In order to understand their importance, let’s take a closer at how a power plant boiler works:
The Basics: How a boiler works
Boilers are comprised of two key components: the burner or combustor and the heat exchanger.
The burner, grate, or bed mixes fuel and air together with an ignition source, providing a method for combustion which takes place in the combustion chamber. Together they can be considered the combustor or “gas” side of the boiler since the fuel/air products of combustion are gaseous in nature. The heat that is generated by combustion is transferred to the water in the heat exchanger.The heat exchanger has hot gases from combustion on one side and water on the other which becomes steam when heated enough.
The boiler evaporates water, which is then transfered through pipes and delivered to the process whether that be heating (radiator), drive (locomotion for equipment), or power generation. Water is introduced into the boiler through pumps which must overcome the pressure of the boiler to force water in while steam is “pushed” out of the boiler by the pressure generated inside the generating section. How does this work? Steam takes up approximately 1600 times more space than the same mass of water at atmospheric pressure. Since the boiler is made of very strong steel (and does not expand) the steam is generated in a fixed space which causes the pressure to increase as more steam is generated.
How does a power plant boiler work?
Some boilers produce steam to be used at saturation temperatures which is the temperature corresponding to the pressure inside the boiler (for any given pressure there is a corresponding temperature at which water phase changes to steam and this is known as the saturation temperature). Power plant boilers, on the other hand, use an additional stage of heat exchanger to transfer more energy to the steam after it has become vapor. This is known as “superheating” the steam and takes place in a section simply called the superheater.Boilers go through two to three different stages of energy addition:
Sensible heat addition – which refers to the process of heating the water;
Latent heat addition – which refers to the energy it takes to phase change water to steam;
Superheat addition – which refers to the additional energy added above saturation energy to steam.
Boiler operation: feed water system
For the boiler to work for more than a few minutes generating steam we must continuously supply it with new water. That water is called “feedwater”. Boilers use a very high quality water since all impurities in the water will be left in the boiler when water is changed to steam. Over time those impurities would build up in the boiler and cause failures. Therefore boiler systems use a number of different types of feedwater treatment to remove impurities and produce ultra-pure water.Once the water has been treated it needs to be fed into the boiler. Remember thought that the boiler is operating at very high pressure, so it cannot simply be poured in. As the name suggests, a boiler feed water pump is a system used to pump feed water into the heat exchanger part of the boiler by increasing the pressure of the water above that of the boiler allowing it to flow in.In power generating facilities, the water is typically pre-heated before entering the boiler by using waste heat. Waster heat comes from water condensed from steam, steam that has already been used in the process, or by the exhaust gases exiting the boiler (economizer). Heat exchangers which use waste water or waste steam on one side and clean feedwater on the other side are called feedwater heaters. This use of waste energy to pre-heat the feedwater saves fuel and significantly increases the overall plant efficiency. The next section is about the heat exhcagner which uses the exhaust gases from the boiler to pre-heat feedwater.
Water often enters the boiler through a section known as “the economizer” where the warm exhaust gases exiting the boiler can be used to heat the cooler feedwater. As we discussed above, the steam generated in power boilers can be more than 500°F. Therefore the exhaust gases from the boiler are often between 450-600°F since cooler gases are no longer useful to heat the steam in the boiler. However, there is still plenty of energy left in these gases and a great way to recover some of that heat is by heating the water going into the boiler. From the economizer the water is pumped into the heat exchanger side of the boiler itself.
Boiler Heat Exchanger
A typical boiler consists of a generating section and steam drum. The generating section is made up of tubes which are exposed to the hot combustion gases and the drum is where the water/steam interface is located. Feedwater is pumped into the drum and the cold water circulates down the tubes of the generating section. As the water in the generating section is heated steam bubbles form and flow back up to the drum where they break the surface of the water and exit the boiler. This is saturated steam which can then be sent to process or can be directed through the superheater section to gain additional energy.
For power boilers, steam from the drum flows into a superheater which is located in the area of the furnace with the hottest combustion gases. The steam in the superheater picks up additional temperature above its saturation point which is valuable since it can then give up heat in the power generating turbine before it begins to condense. This greatly improves the efficiency of the overall system. Finally, from here, the steam is sent to the steam turbine generator. This is where steam transfers its thermal energy into mechanical energy via the turbine which spins a generator to produce electricity.
Advantages & disadvantages of power plant boilers
Boilers are the primary source of heat and energy in most industrial facilities. Now we will discuss the main advantages and disadvantages of power boilers.
- Power boilers are generally economic and efficient as compared to alternative sources of power production. Steam is capable of carrying a large amount of energy which makes it so popular.Boilers are low maintenance compared to many other types of power generation technology;
- Boilers have a long life expectancy (in excess of 20 years and as high as 100);
- Power boilers are reliable and have an extremely high availability (above 90%).Power boilers can be built almost anywhere and are plentiful.