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Last time we learned how enthalpy is used to measure heat energy contained in the steam inside a power plant. The higher the steam pressure, the higher the enthalpy, and vice versa, and we touched upon the concept of work, or the potential for a useful outcome of a process. Today we’ll see how to get the maximum work out of a steam turbine by attaching a condenser at the point of its exhaust and making the most of the vacuum that exists within its condenser. Let’s revisit the equation introduced last time, which allows us to determine the amount of useful work output: W = h1 – h2 Applied to a power plant’s water-to-steam cycle, enthalpy h1 is solely dependent on the pressure and temperature of steam entering the turbine from the boiler and superheater, as contained within the purple dashed line in the diagram below.
As for enthalpy h2, it’s solely dependent on the pressure and temperature of steam within the condenser portion of the water-to-steam cycle, as shown by the blue dashed circle of the diagram. Next week we’ll see how the condenser, and more specifically the vacuum inside of it, sets the platform for increased energy production, a/k/a work.
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Posts Tagged ‘turbine generator’
Enthalpy and the Potential for More Work
Monday, November 18th, 2013
Coal Power Plant Fundamentals – The Steam Turbine
Sunday, February 20th, 2011|
When I was a kid I didn’t have video games or cable TV to help me occupy my time. Back then parents tended to be frugal, and the few games I had were cheap to buy and simple in operation, like the plastic toy windmill I’d play with for hours on end. All I had to do to make it spin was take a deep breath, pucker my lips together, fill my cheeks with breath, then blow hard into the windmill blades. Its spin was fascinating to watch. Little did I know that as an adult I would come to work with a much larger and complex version of it, in the form of a power plant’s steam turbine. You see, when you trap breath within bulging cheeks and then squeeze your cheek muscles together, you actually create a pressurized environment. This air pressure buildup transfers energy from your mouth muscles into the trapped breath within your mouth, so that when you open your lips to release the breath through your puckered lips, the pressurized energy is converted into kinetic energy, a/k/a the energy of movement. The breath molecules flow at high speed from your lips to the toy windmill’s blades, and as they come into contact with the blades their energy is transferred to them, causing the blades to move. A similar process takes place in the coal power plant, where steam from a boiler takes the place of pressurized breath and a steam turbine takes the place of the toy windmill. If you recall from my previous article, the heat energy released by burning coal is transferred to water in the boiler, turning it to steam. This steam leaves the boiler under great pressure, causing it to travel through pipe to the steam turbine, as shown in Figure 1.
Figure 1 – A Basic Steam Turbine and Generator In A Coal Fired Power Plant At its most basic level the inside of a steam turbine looks much like our toy windmill, of course on a much larger scale, and it is very appropriately called a “wheel.” See Figure 2.
Figure 2 – A Very Basic Steam Turbine Wheel The wheel is mounted on a shaft and has numerous blades. It makes use of the pressurized steam that has made its way to it from the boiler. This steam has ultimately passed through a nozzle in the turbine that is directed towards the blades on the wheel. This is the point at which heat energy in the steam is converted into kinetic energy. The steam shoots out of the nozzle at high speed, coming into contact with the blades and transferring energy to them, which causes the turbine shaft to spin. The turbine shaft is connected to a generator, so the generator spins as well. Finally, the spinning generator converts the mechanical energy from the turbine into electrical energy. In actuality, most coal power plant steam turbines have more than one wheel and there are many nozzles. The blades are also more numerous and complex in shape in order to maximize the energy transfer from the steam to the wheels. My Coal Power Plant Fundamentals seminar goes into far greater detail on this and other aspects of steam turbines, but what I have shared with you above will give you a basic understanding of how they operate. So to sum it all up, the steam turbine’s job is to convert the heat energy of steam into mechanical energy capable of spinning the electrical generator. Next time we’ll see how the generator works to complete the last step in the energy conversion process, that is, conversion of mechanical energy into electrical energy. _____________________________________________ |
Coal Power Plant Fundamentals – Combustion
Sunday, February 13th, 2011| Ever have a small child threaten to hold his breath until he passes out and he actually managed to do it? It’s not that unusual. And if his body were prevented from acting in self preservation, that is, taking in breaths while he was unconscious, leading to his eventual awakening, he would die. While the human body can survive about a month without eating and three days without water, under normal conditions it can survive only a matter of minutes without breathing. Power plants, too, require oxygen to function, and this process is called combustion.
Human lungs, along with the diaphragm which works to expand and release the lung cavities, enable our bodies to breathe in air, then expel the waste product, carbon dioxide. Oxygen is needed to metabolize, that is burn, our food, enabling the food cells’ energy to be absorbed by our bodies and converted into energy to live. Like us, coal power plants need to breathe in oxygen in order to convert coal’s latent energy into a usable form. Previously we learned how coal is fed to a coal mill where it is pulverized into a fine powder. This powder is then sucked out of the mill by the exhauster and blown through a serpentine path of pipes leading to the burners on the furnace. The burners will then act upon the coal, combining it with the oxygen in our atmosphere to create a chemical reaction capable of releasing coal’s energy in the form of heat. All this activity looks to a bystander like a massive, sustained fire in the furnace. See Figure l.
Figure 1 – Coal Power Plant Combustion The boiler is contained within the furnace and is situated so it is exposed to fire from the combustion process. Heat energy from the fire transfers into the water in the boiler, much like when you boil water for tea in a kettle on your stovetop. If you’ve ever boiled water, you know that once it gets hot enough it will turn into steam, and the same for our furnace boiler. The steam emitting from the boiler will cause a turbine-generator to spin, and the end result will be electricity for our use. In the simple diagram of Figure 1, waste products from the combustion process, like carbon dioxide, go up the smoke stack and are released into the atmosphere. Incidentally, this is the same type of carbon dioxide that we exhale from our bodies when we breathe. Please keep in mind that Figure 1 is a very simplified diagram. In reality waste products leaving the furnace go through various pollution control devices where most pollutants are removed before they reach the smoke stack. These details, and many more, are the type of information that would be covered during my training seminar, Coal Power Plant Fundamentals. Next time we’ll learn how the heat energy in steam is converted into mechanical energy capable of spinning a turbine generator to make electricity.
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