Posts Tagged ‘boiler’

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.

Electric Utility Power Plant

      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.

electric utility power plant boiler and steam turbine

      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.

coal power plant expert

      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.

boiler

      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.

utility boiler expert

     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 – Latent

Monday, July 15th, 2013

      If you took high school chemistry, you learned that water is created when two gases, hydrogen and oxygen are combined.   You may have even been lucky enough to have a teacher who was able to perform this magical transformation live during class.

      Depending primarily on the amount of heat energy absorbed, water exists in one of the three states of matter, gas, liquid, or solid.   Its states also depend on surrounding atmospheric pressure, but more about that later.    For our discussion, the water will reside at the atmospheric pressure present at sea level, which is around 14.7 pounds per square inch.

      Last time we learned that the heat energy absorbed by water before it begins to boil inside our example tea kettle is known as sensible heat within the field of thermodynamics.   The more sensible heat that’s applied, the more the water temperature rises, but only up to a point.

      The boiling point of water is 212°F.    In fact this is the maximum temperature it will achieve, no matter how much heat energy is applied to it.   That’s because once this temperature is reached water begins to change its state of matter so that it becomes steam.   At this point the energy absorbed by the water is said to become the latent heat of vaporization, that is, the energy absorbed by the water becomes latent, or masked to the naked eye, because it is working behind the scenes to transform the water into steam.

      As the water in a tea kettle is transformed into steam, it expands and escapes through the spout, producing that familiar shrill whistle.   But what if we prevented the steam from dispersing into the environment and continued to add heat energy?   Ironically enough, under these conditions temperature would continue to rise, upwards of 1500°F, if the stove’s burner were powerful enough.   This process is known as superheating.   Now hold your hats on, because even more ironically, the heat added to this superheated steam is also said to be sensible heat.

      Confused?    Let’s take a look at the graph below to clear things up.

power plant engineering

      Sensible heat is heat energy that’s added to water, H2O, in its liquid state.   It’s also the term used to describe the heat energy added to steam that’s held within a captive environment, such as takes place during superheating.    On the other hand, the latent heat of vaporization, that is the heat energy that’s applied to water once it’s reached boiling point, does not lead to a further rise in temperature, as least as measured by a thermometer.

      Next time we’ll see how surrounding air pressure affects water’s transition from liquid to steam.

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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?

sensible heat power plant boiler

      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 – 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.

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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. 

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Coal Power Plant Fundamentals

Sunday, January 23rd, 2011

     Several years ago I was asked by power producers within the electric utility industry to write and then present a training course on the subject of coal power plant fundamentals.  The finished product was a two day introductory course on the energy transformation process within a coal fired plant.

     Since that time my seminar, entitled Coal Power Plant Fundamentals, has been presented to a variety of audiences, including Mirant Corporation, Platte River Power Authority, and Integrys Energy Group, Inc.  Audience makeup has been diverse and has included equipment manufacturers, mining companies, power industry consultants, and regulatory agencies.

     This seminar, which I continue to present today in meeting rooms across the country, covers all major systems in a typical power plant, from coal handling when the coal first enters the plant, to its eventual end destination, the electrical switch yard which facilitates power transmission to customers.  My Power Point presentation is embellished with ample illustrations, including photographs that I have taken during the course of my career and diagrams which I created using CAD, or Computer Aided Drawing software, one of which is featured below.  In addition to the overhead slides, I provide a 150-page bound book which is distributed to seminar attendees.  They use it to both follow along with my lecture and have a source of refresher material to take home with them.  I’ve been told that having my illustrations in front of them makes a world of difference towards their understanding of the subject matter.

     The unique thing about my course is that it focuses on the simplified presentation of complex engineering concepts, much like my blogs do.  Of course it always helps to have an engineering background or scientific background of sorts, but I wrote the course to accommodate understanding of the subject matter by individuals without any technical background.  Accountants, salespersons, administrative staff, plant operating and maintenance workers, and journalists have all found the course to be easy to follow, interesting, and informative.

     So how do you get electricity from coal?  To answer this question and give you a sampling of my seminar material let’s take a look at Figure 1. 

Figure 1 – The Coal Power Plant Energy Transformation Process

     Following along from left to right, the coal is first burned in order to transform the chemical energy which it contains into heat energy.  That heat energy is then absorbed by water inside a nearby boiler, where it is converted into steam.  The heat energy in the steam flows through a pipe into a steam turbine where it is again transformed, this time into mechanical energy that enables the turbine shaft to spin.  The mechanical energy in the turbine is then transmitted by its shaft, enabling it to turn an electrical generator.  And, finally, the mechanical energy is transformed by the generator into electrical energy for our usage.

     Simple process, right?  Well, maybe, maybe not.  My illustration certainly helped to simplify things, but there are a lot of details that were purposely omitted so as not to “muddy the waters.”  It’s those details which have the potential to make things a lot more complicated, and next week we’ll begin to take a closer look at some of them.  

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Pressurized Containers – Industrial Overpressure Devices

Sunday, October 17th, 2010

     Perhaps you went out on a drive to enjoy a nice summer day.  As you ventured into uncharted territory, you might have ended up in an industrial area.  There, you noticed factories, chemical plants, and oil refinery complexes, each surrounded by a huge system of pipes and tanks.  You might have considered it to be an eyesore, but if you’re an artist and engineer like I am, you might look at it as a form of art, composed of interesting shapes, colors, and patterns.  No matter how you look at it, you can bet that there are at least a few pressurized containers in there.

     Last time we saw how something as seemingly harmless as a home water heater could become a dangerous missile if the pressure inside builds to the point where the tank ruptures.  You can imagine what kind of explosive forces, steam, and chemicals would be unleashed into the surroundings if an industrial sized pressurized container failed due to overpressure.  Let’s explore some other types of overpressure devices that are commonly used in industrial settings.

     One type of overpressure device is a safety valve.  They are similar to a water heater relief valve, but they are generally used to relieve overpressure of gases and steam.  How do they work?  Basically, a safety valve is attached to the top of a pressurized container as shown in the cut away view in Figure 1 below.

Figure 1 – A Basic Safety Valve In The Closed Position

A powerful spring in the valve body is designed to force down on the valve and keep it closed if there is normal pressure inside the container.  Once the pressure begins to rise to an unsafe level, it pushes up against the valve and overcomes the force of the spring.  The valve opens, as shown in Figure 2 below, and the contents of the pressurized container are safely vented out to an area that is normally unoccupied by people.  In case you’re wondering, safety valves are commonly used on pressurized storage tanks and boilers.

Figure 2 – A Basic Safety Valve In The Open Position

     Another way to address the overpressure scenario is to employ a rupture disc.  This is in fact a purposely constructed weak spot.  It is intentionally built into a pressurized container and is designed so that it will fail when pressure starts to rise.  In fact, this disc is designed to fail at a pressure point just below the pressure at which the container itself would fail.  The disc is usually located within a vent pipe, which is in turn connected to the container.  Should the disc rupture in an overpressure situation, the contents of the pressurized container will safely flow out of the vent pipe to a place normally unoccupied by people.  The advantage of using a rupture disc is that they are made to safely release huge quantities of pressurized substances very quickly.  The disadvantage in their usage is that they’re a one-time fix.  That is, unlike relief or safety valves which may perform their function a multitude of times, a rupture disc is destroyed once it does its job.  They are generally used in industrial settings where potential hazards are greater than at home, so once the rupture disc blows, the complete system generally undergoes a shut down so that the disc an be replaced before the pressurized container can be used again.

     Another option to pressure containment is the use of a fusible plug, usually constructed of a metal that will melt if the temperature within a pressurized container rises above a certain level. The metal plug melts, and excess pressure is vented through the aperture formed into a safe location. These are often used on locomotive boilers and compressed gas cylinders. Like rupture discs, fusible plugs are a one-time fix and must be replaced once they have done their job.

     Yet another option to pressure containment is to use a temperature limiting control.  This category includes devices that monitor temperature and pressure within a pressurized container.  If a dangerous situation should develop, the control system reacts, effectively reducing the pressure to prevent failure of the vessel.  Automatic combustion control systems for boilers in electric utility power plants use temperature and pressure sensors to keep pressures within safe limits by regulating fuel and air input to the boiler.

     Next time we’ll cover the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (BPVC), which establishes rules governing the design, fabrication, testing, inspection, and repair of boilers and other pressurized containers.

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