What Causes Slagging in Coal Fired Boilers?

Written by chandra anderson
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What Causes Slagging in Coal Fired Boilers?
Coal-fired boilers are used to generate electricity. (Boilers, ladders and pipes at a power plant, image by Andrei Merkulov from Fotolia.com)

The term "slag" was derived from the German word "slagge" and dates to at least 1552. Slag is molten rock and minerals, created when coal is burnt at high temperatures, and it is similar to lava and volcanic ash. Slagging has been an industry problem as far back as 1916 when the Babcock and Wilcox Company (a leader in boiler manufacturing and maintenance) mentioned it in their comprehensive manual on boiler operations. Proper air circulation is vital to steam generation. Too much slag can literally choke a boiler because air is prevented from moving around inside.

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The most common cause of slag in a boiler is inferior fuel sources. Low-quality coal, such as lignite, contains a higher concentration of water and minerals including pyrite, iron, calcium, sodium and potassium. When heated to the extreme temperatures needed for boiler operations, these compounds break down and interact with the boiler interior, the air, released moisture and with each other. The resulting debris then either sticks to the walls of the boiler or falls to the floor where it is covered by another boiler operation by-product, ash. Too much of this results in poor operations and equipment failures.


Sometimes poorly maintained or operated equipment is the cause of slagging. If the boiler's air-to-fuel ratio is not correct, the coal will not burn properly. Coal is pulverised before it is burnt, and if it is not done correctly, even the best coal will not burn thoroughly. Soot blowers, designed to remove ash and cut the potential for slagging, must be used according to operating procedures to be effective.


A boiler furnace, too small for the fuel type and amount, will not efficiently burn the coal fed into it. This inefficiency will lead to slagging as the unspent fuel mixes with the ash and covers the boiler's inner walls. Another design flaw is that of the inability to monitor slag build-up and address the issue before it becomes a major maintenance problem. Sightglasses are, as the name implies, external ports on the outside of boilers that let personnel see what is going on inside. If the sightglasses are damaged, dirty, improperly mounted or missing, slagging may go unnoticed. Inadequate soot blower and ash removal equipment are also contributing factors.

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