Last time we learned that our power plant boiler as presently designed doesn’t do a good job of producing ample amounts of superheated steam, the kind of steam that turbines need to spin and power generators. During the process of superheating the more heat energy that’s added to the steam in our boiler, the higher its temperature becomes. However, our boiler can only produce a limited amount of superheated steam as it stands now. So how do we get more heat energy into the superheated steam? Take a look at the illustration below for the solution to the problem. You’ll note a prominent new addition to our illustration. It’s called a superheater. The superheater performs the function of raising the temperature of the steam produced in our boiler to the incredibly high temperatures required to run steam turbines and electrical generators down the line, as explained in my blog on steam turbines. The superheater adds more heat energy to the steam than the boiler can alone. In fact, the amount of heat energy in the superheated steam produced with our new design is proportional to the amount of electrical energy that power plant generators produce. Its addition to our setup will result in more energy supplied to the turbine, which in turn spins the generator. The result is more electricity for consumers to use and a more efficiently operating power plant. But inefficiency isn’t the only problem addressed by the superheater. We’ll see what else it can do next week. ________________________________________ |
Posts Tagged ‘boiler’
Superheating, Part I
Monday, August 19th, 2013Heat Energy Within the Power Plant— Water and Steam Cycle, Part 2
Wednesday, August 14th, 2013
Last time we learned that electric utility power plants must have water treatment systems in place to remove contaminants from incoming feed water before it can be used. This clarified water is then fed to a boiler by the boiler feed pump as shown below. As it stands this setup will work to provide electricity, however in this state it’s both inefficient and wasteful. We’ll see why in a minute. Boilers, as their name implies, do a great job of heating water to boiling point to produce steam. They do this by adding the heat energy produced by burning fuel, such as coal, to water, then steam. We learned in earlier blogs in this series that the energy used to heat water to boiling point temperature is known as sensible heat, whereas the heat energy used to produce steam is known as latent heat. The key distinction between these two phases is that during sensible heating there is a rise in temperature, during latent heating there is not. For a review on this, see this blog article. When water starts to heat inside the boiler, sensible heat energy is said to be added. This is represented by phase A of the graph below. During A, heat energy will raise the temperature of the water to boiling point. As the water continues to boil in phase B, water is transforming into steam. During this phase latent heat energy is said to be added, and the temperature will remain at boiling point. In phase C something new takes place. The temperature rises beyond boiling point and only steam is present. This is known as superheated steam. For example, if the boiler pressure is at 1,500 pounds per square inch, steam becomes superheated at temperatures greater than 600°F. Unfortunately, boilers alone do a poor job of superheating steam, that is, continuing to raise the temperature of the steam present in phase C. This is evident by the fact that phase C is quite small in comparison to phases A and B before it. This inefficiency in producing ample amounts of superheated steam results in a small amount of useful energy being provided to the turbine down the line, which is bad, because steam turbines require exclusively superheated steam to run the generator. Next time we’ll see how to provide our steam turbine with more of what it needs to run the generator, more superheated steam. ___________________________________________
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Heat Energy Within the Power Plant – Water and Steam Cycle, Part 1
Monday, August 5th, 2013
Last time we learned that electric utility power plant boilers are vessels that are reinforced with thick steel and are closed off from the surrounding atmosphere so as to facilitate the building up of highly pressurized steam. This steam is laden with sensible heat energy, meaning it’s a useful energy, and it’s used to run steam turbines, which in turn drive electrical generators. The end result is power to consumers. Let’s now revisit our basic electric utility boiler diagram to see how water and steam flow. Water is fed into the boiler, heat is applied externally, and steam exits through a pipe leading to the steam turbine. You’ll notice that after the steam passes through the turbine, some of it is expelled into the surrounding atmosphere. Since water is being continuously boiled off to produce steam, the boiler must be continuously replenished with a fresh supply. This is typically supplied by a nearby body of water, hence one reason that power plants are often situated on a lake or river. Since water contains both minerals and organic matter, including algae, a treatment system to remove these contaminants must be added to the water’s inlet area before it can be used. This will keep operating parts such as the boiler and turbine free of damaging deposits. The treatment system operates much like the water softener in your home, but on a larger scale. Lake water is drawn into the system by a make-up pump, so named because it makes up, or replenishes spent water with a fresh supply. The result is clean, mineral-free water that’s delivered to the boiler by a boiler feed pump, so named because its specific function is to feed water to the boiler. Feeding water to the boiler on a continuous basis is no easy task because of the steam straining to break free, and boiler feed pumps are massively powerful devices built to accomplish this. They effectively force water into the boiler even as high internal pressures try to force the water out. This pressure is often greater than 1,500 pounds per square inch (PSI) in modern power plants. So at this point we’ve discussed the fact that the boiler requires a continuous supply of fresh water, which is converted into high pressure steam, which is then sent on to spin a steam turbine. The turbine powers an electrical generator, resulting in usable energy. If you’ve been reading along closely, you will have identified that as things stand now it’s a rather inefficient and wasteful system, a point which we’ll address in next week’s blog. ___________________________________________
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Heat Energy Within the Power Plant—The Power Behind the Turbines
Monday, July 29th, 2013
Last time we discovered that the boiling point of water varies. It’s dependent upon the amount of pressure exerted on its surface, which varies due to a variety of reasons, including where it is in relation to sea level. Before we see what happens under higher than atmospheric pressures, such as exist in an electric utility power plant boiler, let’s cover some basics. In the power plant, water is heated in a boiler specifically to produce steam, unlike our tea kettle where the primary purpose is to produce hot water. The steam produced is used to spin turbine generators, which in turn generate electricity, as I explained in a previous blog on steam turbines. Unlike a tea kettle, which is open to the atmosphere on your kitchen stove, the boiler in a power plant is an enclosed, reinforced steel vessel. See illustration below. The reinforced steel boiler vessel is designed to withstand great internal pressure as temperatures rise within. In addition to providing a safety feature, the enclosed space provides a sheltered environment for collecting steam so it can later be put to use spinning power generating turbines down the line. In other words, surface water inside the boiler is closed off from the surrounding atmosphere, allowing its internal pressure to build for our specific purposes. As heat energy is added to water within the boiler, the water boils and steam bubbles break out from its surface, filling the empty space above the surface with pressurized steam. This steam will try to expand here, but it can’t, because it’s being constrained by the reinforced steel vessel within which it is enclosed. Instead, steam pressure builds up on the surface of the water inside the boiler until it is high enough to be released through an attached pipe which is connected to a nearby turbine. We’ll talk more about this pent-up energy and how it is put to use within the power plant in next week’s blog. ___________________________________________
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Forms of Heat Energy – Boiling Water and Atmospheric Pressure
Sunday, July 21st, 2013
If you’ve ever baked from a pre-packaged cake or cookie mix, you’ve probably noticed the warning that baking times will vary. That’s because the elevation of the area in which you’re doing the baking makes a difference in the baking time required. Living in New Orleans? Then you’re at or below sea level. In Colorado? Then you’re above sea level. Your cake will be in the oven more or less time at the prescribed temp, depending on your location. Last time we learned how the heat energy absorbed by water determines whether it exists in one of the three states of matter, gas, liquid, or solid. We also learned that at the atmospheric pressure present at sea level, which is about 14.7 pounds per square inch (PSI), the boiling point of water is 212°F. At sea level there are 14.7 pounds of air pressure bearing down on every square inch of water surface. Again, I said sea level for a reason. The boiling point of water, just like cake batter baking times, is dependent upon the amount of pressure that’s being exerted on its surface from the surrounding atmosphere. When heat energy is absorbed, it causes the water or cake batter molecules to move around. In fact, the temperature measured is a reflection of this molecular movement. As more heat energy is absorbed, the molecules move more and more rapidly, causing temperature to increase. When the water temperature in our tea kettle reaches its boiling point of 212°F at sea level, the steam molecules in the bubbles that form have enough energy to overcome the atmospheric pressure on the surface of the water. They become airborne and escape in the form of steam. If we’re up in the Rockies at say an altitude of 7000 feet above sea level, the atmospheric pressure is only about 10.8 PSI. There’s just less air up there. That means there’s less air pressure resting upon the surface of the water, so it’s far easier for steam molecules to form into bubbles and leave the surface. As a result the boiling point is much lower in the Rockies than it is at sea level, 196°F versus 212°F. So what if the water was boiling in an environment that had even higher pressures exerted upon it than just atmospheric? We’ll see how to put this pent-up energy to good use next week, when we begin our discussion on how steam is used within electric utility power plants.
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Forms of Heat Energy – Sensible
Sunday, July 7th, 2013 In our house the whistle of a tea kettle is heard throughout the day, no matter the temp outside. So what produces that familiar high pitched sound?
When a tea kettle filled with room temperature water, say about 70°F, is heating on the stove top, the heat energy from the burner flame will transfer to the water in the kettle and its temperature will steadily rise. This heat energy that is absorbed by the water before it begins to boil is known as sensible heat in thermodynamics. To read more about thermodynamics, click on this hyperlink to one of my previous blog articles on the topic. So, why is it called sensible heat? It’s so named because it seems to make sense. The term was first used in the early 19th Century by some of the first engineers who were working on the development of boilers and steam engines to power factories and railways. Simply stated, it’s sensible to assume that the more heat you add to the water in the kettle, the more its temperature will rise. So how high will the temperature rise? Is there a point when it will cease to rise? Good questions. We’ll answer them next week, along with a discussion on another form of heat energy known as the latent heat of vaporization. ___________________________________________
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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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Coal Power Plant Fundamentals – Feeding The Furnace
Sunday, February 6th, 2011 Today we’ll continue our discussion of coal’s journey through a power plant. Keep in mind that the material presented in this series of blogs is meant to be a primer. It is a simplification of what actually goes on. My training seminars go into much more depth.
Now imagine a five course meal spread out on the table before you. You load up your plate and pack a forkful of food into your mouth. You instinctively chew, getting the digestive process underway and making it easier to swallow. Power plants approach their consumption of coal in much the same way. Last time we talked about handling the coal and filling up silos for short term storage within the power plant building. The coal silo is analogous to a dinner plate, and the furnace, which heats up the boiler water to make steam for the turbine, acts very much like a diner’s stomach. As for the fork and your teeth, there are a couple of machines within power plants which mimic their behavior. They’re called the coal feeder and coal mill. The coal feeder does as its name implies, it systematically feeds a measured amount of coal to the coal mill. The coal mill, also known as a pulverizer, then grinds the coal to make it easier for the furnace to burn it. Let’s take a look at Figure 1 below. At the top of the configuration is the coal silo, which is fully open at the bottom. Gravity draws the coal within the silo downward, facilitating the coal’s dropping through the opening into a chute, on its way to the coal feeder. The coal from the silo spills into little buckets on a wheel within the feeder, and as the wheel turns, the coal spills out and falls down into another chute leading to the mill. Figure 1 – Feeding Coal To A Power Plant Furnace Now you could have the coal spill down a chute directly from the silo into the mill, bypassing the coal feeder entirely, but that’s really not a good idea. Just think how difficult it would be to chew if you tried to stuff an entire plate of food into your mouth at once. Just as your mouth requires to be fed in mouth-sized amounts, the coal mill must be fed coal in a size that it can handle. It’s the job of the spinning wheel inside the coal feeder to keep coal flowing in measured amounts to the mill. You see, the wheel is attached to a variable speed motor, and depending on how quickly the furnace needs to be fed, the wheel will either turn faster or slower. Once inside the mill, the coal is ground up before moving on to the furnace. The coal mill contains massive steel parts capable of pulverizing chunks of coal into a fine black powder. This pulverized coal is then propelled by means of an exhauster towards the burners. The exhauster sits next to the coal mill and both are often driven by the same electric motor. The exhauster is connected to the top of the mill by a pipe, and another pipe connects the exhauster to burners on the furnace. The exhauster acts like a big vacuum cleaner, sucking coal powder out of the mill, then blowing it through pipes leading to the burners. Finally, the powder ignites within the furnace, heating the water inside the boiler. Next time we’ll learn about the combustion process in the power plant furnace. _____________________________________________ |