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How Stuff Works: How coal-fired power plants work

By Marshall Brain

HowStuffWorks.com

Published: Saturday, Nov. 13 2010 1:06 p.m. MST

In the United States there are several different technologies that we use to generate electricity. We have hydroelectric dams, nuclear power plants, geothermal, wind turbines and solar panels. But by far the most common way, making up about 56 percent of the electricity generated in the United States, is the coal-fired power plant. The idea of generating power from coal has been around for centuries, starting with the first steam engines invented in the 1700s. Large-scale coal mining took off about the same time. Steam locomotives appeared in the early 1800s, and by the end of the 1800s people were getting electricity from coal. The fact that we are still using that same basic technology more than 200 years later shows how effective these ideas are. Let's take a look at how coal and water get the job done today.

The modern coal-fired power plant is a massive thing. That's because it takes a lot of coal to keep the lights on. If you want to keep a 100 watt light bulb burning for a year, it will take roughly 700 pounds of coal to do it. When you figure that a typical city might have hundreds of thousands of homes and businesses, and they all have several light bulbs (along with computers, TVs, refrigerators, etc.) lit up at any given moment, we are talking about a lot of coal the needs to get processed. A coal-fired plant is consuming thousands of tons of coal every day. Sitting outside the typical plant is a million-ton stockpile of coal that might keep the plant supplied for a month or two. Trains and barges are arriving every day delivering the coal the plant needs.

The first step in turning the coal into electricity is a pulverizer. The coal turns into a powder that will make it easier to burn cleanly. The coal powder enters a boiler on a stream of air. In the boiler the coal burns and in the process heats a set of tubes filled with water. The water flashes into steam at 1,000 degrees F and 3,500 PSI. All that steam pressure is used to spin a steam turbine. The basic idea behind a steam turbine is similar to a fan. If you blow on a fan, it will spin. In a steam turbine, the steam is blowing on a fan with hundreds of carefully designed blades that try to extract every bit of power available. Those blades spin a shaft that turns a gigantic electrical generator. The generator might produce up to a billion watts of electricity in the largest plants.

Now the power plant has four things it needs to deal with. The first is the electricity coming out of the generator. The electricity flows onto the power grid, which takes it to all of the customers who need it. The second is the spent steam leaving the turbines. It needs to be condensed back into water so it can be reused. The cooling towers that you see outside of most power plants takes care of that process. The third thing is the ash left behind by the coal. This often gets sold for things like asphalt and cinder blocks.

And then there is the exhaust gas that will eventually exit out the smoke stacks. If coal were pure carbon, then the only thing we would have to worry about at coal-fired power plants is the carbon dioxide formed when coal burns. Unfortunately, coal often contains other chemicals. Sulfur is one common element found in coal. When burned, the sulfur in coal turns into sulfur dioxide. When sulfur dioxide mixes with moisture in clouds, it creates acid rain. And acid rain can create lots of problems in the environment.

So a coal-fired power plant will use filters, scrubbers and catalytic converters to clean up the exhaust stream as best it can. The smoke from the burning coal first goes through a particulate filter to remove soot and ash. Then it enters a scrubber. In the scrubber, there is a water spray that produces a cloud of fine water droplets. The water is mixed with crushed limestone, which reacts with the sulfur and pulls it out of the exhaust. The unexpected side effect is that the calcium in the limestone plus the sulfur form gypsum (CaSO4-2H2O). Gypsum is best known as the material inside gypsum wallboard, so the gypsum gets sold to wallboard companies. Then catalytic converters remove nitrogen oxides from the exhaust, using techniques similar to the catalytic converters found in a car. A ton of coal turns into roughly 2,500 kilowatt-hours of electricity in a typical power plant. It might be worth $250 when purchased by retail customers, not counting the amount lost to heat in the distribution process. When you think about all the work, equipment and resources needed to generate it, electricity seems like an amazing bargain.

Looking for more? For extra info on this or the scoop on other fascinating topics, go to HowStuffWorks.com. Contact Marshall Brain, founder of HowStuffWorks, at marshall.brain@howstuffworks.com.

Distributed by McClatchy-Tribune Information Services.

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