How Boilers Are Used in Power Generation: Water and Steam
March 21, 2014
In the first article in this series about how boilers are used for power generation we looked at their combustion system and how they extract stored energy from a fuel source. In this article we will take a closer look at their core function: boiling water into steam. Boiling water into steam yields the thermodynamic energy that is ultimately used to power the turbines that generate the power.
How Is Steam Used for Power Generation?
The stored energy released from the boiler’s fuel source in the act of combustion is transferred into heat. The heat enters the water, converting it to steam and creating thermodynamic energy. This thermodynamic steam energy then acts as the energy source needed to power the massive turbines that create the power.
Types of Boilers
Boilers are often categorized as either fire-tube boilers or water-tube boilers. This distinction is used based on whether or not it is the heat or the water that passes through the tubes. There are many subtypes of boilers in each category that are used in different industries for various purposes.
Fire-Tube Boilers - Fire-tube boilers have tubes through which the hot gases, or "fire," created from combustion pass through. The tubes are surrounded by a sealed tank of water and as the heat passes through the tubes it heats the water by the process of thermal conduction. Eventually the water becomes hot enough to boil and converts into steam.
Water-Tube Boilers - Water-tube boilers use the opposite configuration compared to fire-tube boilers. With water-tube boilers, instead of the hot gases passing through tubes contained within the water, the water instead passes through tubes contained within the hot gases. Once again this allows heat to be transferred by convection or thermal radiation and results in the water boiling into steam.
Saturated Steam, Superheated Steam, and Supercritical Steam Generation
There are several important terms describing the nature of the steam and how it is generated:
Saturated Steam - Water converts into steam at 212 °F (100°C). Steam naturally rises above the water because it is much less dense than water. In fact, steam is so much less dense than water, that a given amount of steam will occupy a space over 1,000 times the volume of the same amount of water. As the liquid water converts into steam and separates it will remain at the temperature of its boiling point, 212 °F, unless the pressure is increased. When steam is at this equilibrium point with water it is called saturated steam.
Superheated Steam - Regular, saturated steam can be expanded into even more steam by superheating it. The remaining water droplets that are suspended in saturated steam can themselves become steam. Superheated steam is important because it increases the efficiency of the boiler. In the power generation industry it serves an additional crucial function: "drying out" the steam. It is important that dry steam is used for power generation because water droplets could damage the power-generating turbines.
Supercritical Steam Generation - Another process often used in the production of electricity is subcritical steam generation. Standard boilers function at subcritical levels, defined as less than 3,200 psi. Below the critical level boiling bubbles will form as the water is converted to steam. Supercritical generation occurs at pressure levels above the critical threshold of 3,200 psi. At this level the liquid water instantly becomes steam without ever going through the bubbling and boiling phase. For this reason the term "boiler" is often avoided for supercritical steam generators since true boiling does not occur. An advantage of supercritical steam generation is that it ultimately uses less fuel and releases less greenhouse gas emissions.
Feed Water, Purification, and Preheating
For the performace of the boiler and purity of the steam it yields it's important to use feed water that is as pure as possible. If the feed water contains too many suspended solids or dissolved impurities this can cause corrosion or a buildup of sediment and sludge in the boiler.
If the feedwater is at room temperature or colder it requires the boiler to have to work harder to heat the water. This reduces efficiency and wastes fuel. For this reason it is very common for the feed water to be preheated before it enters the boiler.
Preheating is often accomplished by outfitting the boiler with an economizer. An economizer is a set of water-filled coils located at the boiler’s stack where the hot gases pass. The economizer captures the heat from these hot gases and uses it to warm the water within its coils before feeding it into the drum, thereby preheating the boiler’s feedwater and making the process more efficient.
The conversion of water into steam is obviously the most salient feature of industrial boilers. For power generation...converting steam into water is still only one phase in the process. The final step involves using the thermodynamic energy present in the steam to turn the power generating turbines.