Inside a Coal-Fired Power Plant Quiz

Answer: 4 million tons of coal

The coal can be transported to the plant in various ways -- train, river barge and truck are popular solutions, depending on the plant's location -- but the sheer scale of the operation requires streamlined loading, unloading and storage procedures. Coal is by far the most expensive component of day-to-day running, and a lot of effort goes into working out where best to get it from, how much to transport at a time, and what exactly to do with it once it gets there.

Answer: If the air contains too much oxygen, a spark will make the coal powder explode.

In general, powders are extremely combustible: their large surface area ensures that powder grains burn fast and hot, and when they're tightly packed it's very easy for the fire to spread in an explosive way. Explosions are also a major safety hazard in grain elevators, for the same reason.

Of course, all the factors that make coal powder dangerous in the pulverizer make it a great fuel when the burning can be controlled. Let's move on to the boiler!

Answer: At the boiler's corners, so that the streams of air and coal produce vortices and turbulence for better oxygen circulation.

Like anything else, coal needs oxygen to burn. Improving the efficiency of the fire boils down to one thing: exposing as much of the coal as possible to oxygen. This is why powdered coal burns more efficiently than the large coal chunks that used to power boilers (see, for example, the boilers depicted in the movie "Titanic"). It also means that it's important to have a lot of movement in the air. If much of your coal is sitting in a relatively stagnant section, all the oxygen in that volume of air will be consumed and the coal will cease to burn. Blasting in coal powder and air from the corners of the boilers creates whirlwinds within the boiler -- no stagnant air here!

At many plants, the fire in the boiler is started with fuel oil, and coal is not added until the fire is already hot.

Answer: The heat of the boiler makes the steel walls expand by several feet from their installation size.

Metals, like most substances, expand when they get hot. This is why bridges usually contain metal expansion joints; if there weren't a way for the bridge to safely expand during a hot summer day, it might crack.

Hot bridge sections expand by a few inches. The steel walls of a boiler - which reaches over a thousand degrees Fahrenheit - expand quite a bit more. When the plant is running at full power, the boiler is three or four feet taller than it is when the plant has been shut down. The architecture of the plant has to be able to adapt not only to that height change, but also to the necessary changes in all of the tubes and nozzles connected to the boiler - hence the solution of suspending the whole assembly on springs. One imagines the engineering discussions preceding this inspiration: "Oh, hang it all!" snaps a frustrated engineer - "Wait a minute - we CAN hang it all!"

Answer: In pipes near the edges of the boiler.

As in any power generation process, efficiency is the key. Coal releases tremendous amounts of energy as heat when it burns, but only some of that energy can be converted into electrical energy at the end of the process: there is loss (wasted energy) at every stage. Minimizing that loss maximizes efficiency and profit.

Pipes are better containers for boiler water than tanks because more of the water has direct contact with a heated surface, so the water is heated more efficiently. Keeping the pipes near the edges of the boiler simplifies the connections for the plumbing and the pumps. When the water first boils, the steam flows into a second set of pipes that hangs from the ceiling, making contact with the hottest part of the gases in the boiler. The resulting steam -- superheated to over 1000 degrees Fahrenheit -- is now directed to the turbines.

Answer: Constant closed-circuit television monitoring of all joints in the piping.

The high pressure inside the pipes makes leaks quite dangerous, which is why the problem is taken so seriously. Plants go offline for routine maintenance once every one or two years, and one of the engineers' most important maintenance tasks is to inspect and X-ray every weld. There is very little margin for error here!

If the plant suddenly goes offline -- for example, if its outgoing power lines are felled in a storm -- the valves close and the steam stops flowing to the turbines. But there's a lot of kinetic energy in so much steam moving so quickly, so the pipes kick. Shock absorbers prevent them from tearing themselves loose.

Answer: The turbines get larger as the steam moves down the line, so that lower-energy steam meets longer blades.

The steam begins its turbine journey highly pressurized and confined to a relatively small volume. As it loses energy turning the narrow turbine blades, its pressure decreases and it expands into a larger volume. The turbine blades thus have to be larger and larger at each stage, in order to extract the maximum energy from a dispersing cloud of steam.

A plant will generally have several turbine systems, all rotating on the same shaft: a high-pressure turbine, one or two intermediate-pressure turbines, and a low-pressure turbine. After the high-pressure turbine, most plants return the steam to the boiler to be reheated (note that it is reheated in hanging pipes, NOT in the turbine itself!) before running it through the lower-pressure turbines.

Answer: It's a safety issue: the turbine blades turn so fast that the impact of a single water droplet could tear them apart.

This is an area where a great deal of energy is wasted: there's lots of heat even in steam that's quite cool by power-plant standards. Damage to the turbine blades, however, is a risk that isn't worth running, so engineers bite their lips and work in a safety margin.

Turbines are surprisingly delicate for such massive pieces of machinery. The rotating parts of the turbine assembly may add up to over 200 tons. Because it is so heavy, it must be kept turning constantly, lest it deform itself under the force of gravity. A plant's emergency batteries thus have four functions: emergency lights, the alarm system, communications between parts of the plant and with the outside world, and keeping the turbines spinning at a low rate of speed.

Answer: It is condensed into water, treated, and reintroduced to the boiler to begin the cycle all over again.

The water used for steam is heavily purified and thus very expensive to waste; it's therefore important to save it for re-use. The condenser uses un-purified water -- perhaps from a nearby river -- to cool the steam so that it condenses; the cooling water is kept in separate pipes, so that it never touches the steam and can be released downstream. (While the cooling water is not polluted, it does emerge several degrees warmer than it entered; this often makes the plant's output pipe an extremely popular spot for fish.) The condenser serves another purpose as well: it creates a low-pressure region by cooling the nearby steam, thereby dragging the hotter steam all the way through the turbine assembly! This step dramatically increases turbine efficiency.

Condensed steam (called "feedwater") is heated in stages before being reintroduced to the boiler; usually, it's heated by small amounts of steam siphoned off from various stages of the turbine assembly. It's also re-purified, going through a device called a deaerator to remove dissolved air.

Answer: The rotating coils are located inside a stationary electromagnet.

This effect -- whose importance to modern civilization cannot be overstated -- was discovered by Michael Faraday, a brilliant 19th-century English physicist. When a conducting coil is placed inside a working electromagnet, it experiences a certain magnetic flux -- that is, a certain amount of magnetic field strength density in a given area. When the coil rotates, the magnetic field changes its direction, and this starts an electrical current in the coil. (Note that the electrical charges are already present in the conductor -- that's what makes it a conductor! The current represents a net flow of charge.)

The field strengths required to generate power-plant levels of current are massive -- so massive that permanent magnets cannot be used. This is why the stator (the stationary part of the generator, as opposed to the rotor) is an electromagnet. When it starts up, the plant must draw some power from the grid to run the electromagnet; when the plant is running, some of the power it generates is used to keep the electromagnet going.

Answer: An electrostatic precipitator uses electrical charge to filter ash particles out of the emitted gases.

The most basic electrostatic precipitator consists of a large number of thin wires followed by metal plates in a row. A large voltage is applied to the wires, so that particles traveling through the system pick up an electric charge. As they move further down the line, they are electrically attracted to the plates (which are grounded) and collect on the surface. The gases flow past, unimpeded.

The plates are vertical to ease their cleaning, which is computer-controlled: periodically, an automatic hammer will strike the top of each plate, shaking the collected ash loose into a hopper below. It can then be disposed of responsibly; often, the power plant can sell it! Fly ash is used as fill material in large construction projects, and as a replacement for Portland cement in concrete; in fact, using fly ash from coal power plants *reduces* the carbon footprint of making concrete.

Answer: Sulfur dioxide

Carbon and nitrogen oxides are standard byproducts of combustion, but of all fossil fuels, only coal contains significant amounts of sulfur. Thus, it's only coal power plants (and car engines, which run on petroleum) that must take measures to reduce the amount of sulfur dioxide they emit; as a precursor to acid rain, SO2 is a vital environmental concern.

The best available technology for reducing SO2 emissions is a scrubber, generally located inside the stack. An alkaline material, like lime or limestone, is added to the flue gases. Because SO2 is acidic, the resulting chemical reaction produces a solid precipitate. (Depending on exactly what reaction is occurring, it will also produce CO2 or water.) Many plants then oxidize the precipitate to produce gypsum, which can then be sold as a component of plaster, drywall, and even tofu (it's an excellent source of calcium).

Scrubbers reduce SO2 emissions by more than 95%. Plants without scrubbers can somewhat reduce their SO2 emissions by choosing types of coal with relatively low sulfur levels. (For example, coal from West Virginia and western Pennsylvania tends to be relatively high in sulfur, while coal from Montana contains much less.) Unfortunately, low sulfur content makes the electrostatic precipitator (Question 11) less efficient at removing ash, so a scrubber (which can be used after the ash-removal stage) is generally a much better choice.

Natural gas initially contains some sulfur, but this is removed in processing before the fuel ever makes it to a power plant.

Answer: NO and NO2 are major greenhouse gases and are regulated by the Kyoto Protocol.

The Kyoto Protocol regulates N2O (nitrous oxide), which is not a byproduct of coal burning. NO and NO2 are not greenhouse gases, but they are dangerous: they aggravate respiratory conditions like asthma. They are key components in chemical reactions that create ozone, which harms respiratory systems at ground level (even though it's a helpful barrier to ultraviolet light in the upper atmosphere). And they dissolve in water to form nitric acid, leading inexorably to acid rain.

Removal of NO and NO2 from a power plant's emissions operates on a similar principle to the scrubber (Question 12) that removes sulfur dioxide. In a selective catalytic converter, ammonia or urea is added to the flue gases; in the presence of a metallic catalyst (like vanadium or tungsten), it reacts with NOx to produce water vapor and nitrogen gas.

NOx emissions can also be reduced by changing the combustion process, whether by controlling the amount of air added to the boiler or by adding water vapor (outside the steam pipes, that is).

Answer: Carbon monoxide

Carbon monoxide (CO) forms when the fuel has access to too little oxygen to burn completely. An incomplete burn means that some of the fuel's energy is not released in the form of heat, so the plant's incentives are clear. More efficient burning means both lower CO emissions and more money! The distribution of fuel nozzles described in Question 3 -- with nozzles located around the corners of the boiler -- is designed to improve the flow of oxygen, maximize the efficiency of the burn and minimize carbon monoxide emissions.

After combustion, carbon monoxide can be reduced by running the gas through a catalytic converter, which oxidizes it into the less harmful carbon dioxide (CO2). Most modern cars are equipped with this technology.

Answer: Coal naturally contains trace amounts of these elements, which are released when it burns.

Although only trace amounts of these elements are present in coal, a sizable power plant will go through millions of tons in a year -- so the trace amounts add up. Depending on the type of coal used, a 1000 MegaWatt plant might release 5 tons of uranium and 12 tons of thorium every year. Plants don't create these pollutants, but they do release them.

Other types of power plants pollute in different ways, of course, and no type of power generation is the sole answer to all our needs.