Coal Power Plant

Advantages of Coal

• Coal is cheap and easy for us to mine and process.
• We know enough about its properties, and have developed systems to burn it at clean as we can, and reduce emissions as much as possible.
• Most power plants use coal very efficiently, and are well built to use it.

Drawbacks of Coal

• Our coal reserves are running out. Although there's the possibility of finding new deposits, the older ones are being drained.
• Despite the fact that we've reduced emissions as much as possible, there's still a lot, whether its from the stack or not.
• Some power stations don't reuse their water, and when deposited, contains contaminants.

How it works

A typical coal fired power plant burns coal in a high efficiency boiler. Most modern applications use pulverized coal, mixed with hot intake air heated from the exhaust, sent into the furnace through jets. All the pulverized coal combusts as it enters the furnace, which usually operates around 95% efficiency.

These coal jets create very high temperatures, perfect for the amount of steam a power plant uses. The boiler is constructed in what is known as a "tube-walled" design. The walls of the furnace are covered with pipes, which connect to a drum at the very top, and sometimes one at the bottom.

The drum and tube assembly is filled with water. The immense amount of heat generated by the coal quickly heats the water into steam. Heat transfers into water quicker when there is a smaller volume of water to heat, which is why power plant boilers are good at quickly making steam.

This design is not to be confused with a steam generator. Before exiting the boiler, the produced steam, at this point known as dry, or saturated steam, is sent through a different set of piping directly above the jets, known as the superheater. The superheated, or dry steam exits the boiler, and powers the turbines.

The superheated steam condenses down to wet steam after exiting the turbines, and is condensed to water in cooling towers, and pumped back into the boiler. This feedwater passes through exhaust ducts to be reheated as it enters the furnace. The feedwater heating area is known as an economizer.

History

In 1867. Werner von Siemens devised the “self-exciting dynamo-electric generator”, an important step in power generation. Another breakthrough occurred in 1870, when a Belgian inventor, Zenobe Gramme, devised a dynamo that produced a steady direct current well-suited to powering motors.

From there, dynamos were used for lighting streets, and manufacturing. In 1879, Thomas Edison patented his light bulb, competing with other successful bulbs already in use. In 1882, Edison opened the first of his New York power stations, the Pearl Street station in lower Manhattan. Sir Charles Parsons invented the compound steam turbine in 1884.

By the early 1900s, all new power stations were built with turbines. By the 1910s, most power stations were outfitted with an economizer, feedwater heater, air preheaters, an evaporator, and a superheater, all improving efficiency to only 15%!

In 1919, the demonstration of pulverized coal steam generators at the Oneida Street Station in Milwaukee, Wisconsin vastly improved coal combustion, allowing for bigger boilers. In 1921, The Lakeside Power Plant in St. Francis, Wisconsin, began operating. It was the world’s first plant to burn pulverized coal exclusively.

Power plant steam applications are equipped with a reheater. After the steam exits the first, or high pressure turbine, it is sent strait back through the reheater, like a superheater, in the furnace. Then it goes to an intermediate pressure turbine, and finally a low pressure turbine.

The exhaust steam is sent to cooling towers where its condensed, put into either a holding pond or tank, and pumped into the boiler when necessary. High temperatures are fatal to any metal part that experiences friction or high tolerances, as the turbines can heat up far past their melting point.

The turbine housing and generator bearings are cooled by oil, and in some cases, water.

The load on a power plant always varies, it never constantly needs to make the same amount of energy. This is why a boiler is best suited for this application. The steam drum allows for a variation in pressure, if the load decreases, the drum can store the extra steam for when the load picks up again. If the load really increases, the boilers are designed to make steam very quickly.