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Inside a Coal-Fired Power Plant
"The electricity you're using to play this quiz may come from one of the hundreds of coal-fired power plants that together generate more than 40% of the world's electricity. Let's explore what makes it possible to turn on the lights."
15 Points Per Correct Answer - No time limit
The first ingredient in electricity from a coal-fired power plant is, naturally enough, coal. About how many tons of coal are required to fuel a 1000 MegaWatt (1 GigaWatt) plant for a year?
4 million tons of coal
400,000 tons of coal
40 million tons of coal
40,000 tons of coal
The smaller the piece of coal, the more efficiently it will burn -- so it's no surprise that a lot of work has to be done to get the coal ready for the boiler. Chunks of coal are first run through a crusher and then through a pulverizer, where they are ground to a powder the consistency of flour. The pulverizer is a dangerous place, and the exact chemical composition of the air inside is carefully controlled. Why?
If the air contains too much oxygen, a spark will make the coal powder explode.
If the air contains too much nitrogen, the plant's emissions will violate environmental protection laws.
If the air contains too little oxygen, maintenance will be difficult and dangerous.
If the air contains too much water vapor, the coal powder will burn less efficiently.
The next step is to take the fuel to the fire. Coal powder is blasted into the boiler from fuel nozzles distributed around the edges. Where are the nozzles generally located, and why?
At various points in the boiler floor, to provide easier access when maintenance is needed.
At the boiler's roof, so that less energy is required to transport the coal to the center of the fireball. Gravity does all the work!
At the boiler's corners, so that the streams of air and coal produce vortices and turbulence for better oxygen circulation.
In the center of the boiler walls, since early boiler engineers found the symmetry pleasing.
The boiler itself is an impressive feat of engineering. Several stories tall and built to tolerate immense forces, it must be suspended from the ceiling on springs. Why?
The springs absorb the force of the boiler if the plant must shut down suddenly.
This architecture makes the boiler tubes (for steam) easier to install.
Pollutants might leach into groundwater if the boiler were built on a standard foundation.
The heat of the boiler makes the steel walls expand by several feet from their installation size.
The coal fire exists for the purpose of boiling water into steam, which turns the turbines, which generates electricity. Where is the water kept in the boiler?
In pipes parallel to the streams of injected coal powder.
In a tank that sits atop the boiler.
In pipes near the edges of the boiler.
In a tank suspended in the center of the boiler.
Steam travels from the boiler to the turbines in large pipes (often more than a foot in diameter). There's a lot of energy in those pipes; controlling it safely is paramount. Which of the following is NOT a safety measure taken with these steam pipes leading to the turbines?
The water used for steam is purified intensively so as to reduce the possibility of corrosion.
The welds at the piping joints are inspected and even X-rayed at regular intervals.
Heavy-duty shock absorbers are attached to the pipe to absorb the sudden force if the plant shuts down suddenly and the valves close.
Constant closed-circuit television monitoring of all joints in the piping.
The energy in the steam is used to turn a cascade of turbine blades that rotate on a common shaft; at the far end of the shaft is the generator, an electromagnet whose rotation generates electricity. As the steam passes from turbine to turbine, it steadily loses pressure and temperature as its energy is transferred to the rotating blades. How does the design of the turbine system take this into account?
The turbines get larger as the steam moves down the line, so that lower-energy steam meets longer blades.
Additional steam is injected at a regular spacing to raise the overall pressure and temperature.
The turbines get smaller as the steam moves down the line, so that lower-energy steam meets shorter blades.
The turbines are located over the boiler, so that the steam is reheated as it passes down the line.
The steam can be made to keep turning turbine blades as long as it remains steam, even as it loses temperature and pressure -- but all plants remove the steam from the turbine well before it condenses into liquid. Why?
It's a safety issue: the turbine blades turn so fast that the impact of a single water droplet could tear them apart.
It's a legal issue: regulations for handling steam are much more lax than those for handling water.
It's an efficiency issue: after a certain point, the steam just doesn't contain enough energy to be worth the expense of the extra turbines.
It's an engineering issue: the H2O has to be removed from the turbine somehow, and steam is easier to drain than liquid water.
After it passes through the final stage of the final turbine, what happens to the remaining steam?
It is released into the atmosphere through the plant's smokestack.
It is allowed to cool and then drained into the sewers.
It is pumped into a nearby river or reservoir.
It is condensed into water, treated, and reintroduced to the boiler to begin the cycle all over again.
A lot of work has been done to reach this point. Burning coal boils water, producing steam, which travels through the turbine, which in turn rotate a large shaft. The rotating shaft also turns massive coils of copper conductor. Why does this induce an electrical current?
The rotating coils are heated to a carefully calculated temperature.
The rotating coils are carefully and precisely lit so as to encourage the photoelectric effect.
The rotation of the coils knocks electrons loose: voila, a current!
The rotating coils are located inside a stationary electromagnet.
Making electricity is only part of the battle; it's also necessary to control the plant's emissions and minimize harmful pollution. The place to start is with ash, the gritty byproduct of combustion. Depending on where the ash is created, some will fall to the bottom of the boiler and some -- "fly ash" -- will rise up with the combustion gases. How do power plants cut down on the emission of fly ash?
A centrifuge filters the heaviest ash particles out of the plant's emissions.
A fine mesh sieve is placed over the stack; it must be cleaned and replaced every six hours.
Emissions are routed through a convoluted system of pipes, so that ash tends to collect on the bottom.
An electrostatic precipitator uses electrical charge to filter ash particles out of the emitted gases.
For the most part, coal plants must deal with the same emissions problems that arise from any fuel combustion process -- but there are also problems that arise solely from using coal. Which of these pollutants is not a concern at a natural-gas power plant?
"NOx" refers to a whole class of pollutants: nitrogen oxides (NO and NO2). Which of these is NOT a reason that NOx emissions should be minimized?
NO and NO2 tend to aggravate asthma in sufferers.
NO and NO2 can form ozone in the atmosphere.
NO and NO2 can form nitric acid in the atmosphere, leading to acid rain.
NO and NO2 are major greenhouse gases and are regulated by the Kyoto Protocol.
Another major gaseous pollutant is dangerous wherever it's found. In the atmosphere, it may be a greenhouse gas; in enclosed spaces like buildings and cars, it's deadly. Luckily, since it's a product of incomplete combustion, and since incomplete combustion also means lower power-production efficiency, power plants also have a strong financial incentive to control their emissions of this gas. What is this colorless, odorless, and tasteless pollutant?
Unfortunately, coal combustion also releases a few pollutants that can't be removed from emissions with modern technology -- such as uranium, thorium, and mercury. How do these elements end up in the plant's emissions?
They're produced in the chemical reactions that take place in the boiler.
They're produced in chemical reactions between ash, steam and the metal lining the pipes and flues.
Ironically, they are products of the chemical reactions used to clean the emissions gases of other pollutants.
Coal naturally contains trace amounts of these elements, which are released when it burns.
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Compiled May 20 13