Clinkers have been a recurring problem in all coal-fired boilers for as long as humans have operated coal boilers. Clinkers occur in small stoves used for heating a home and in giant commercial/industrial boilers. Clinkers, also known as slag, consist of the noncombustible elements and minerals found in coal that melt and fuse together as lumpy ashes from coal combustion. Boiler operators consider clinkers to be miserable stuff.

Glowing hot coals
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Poor quality coal produces clinker problems along with heat.

Clinker Problem

Clinkers resemble volcanic ash and lava. They occur in the high-temperature furnace area of the boiler. Clinkers form as a result of interaction among coal quality, boiler design and boiler operation. Coal boilers draw their combustion air from the bottom, up through the combustion grate. Lots of large clinkers on the combustion grate can hinder air passage, increase operation and maintenance costs and even force shutdowns of the boiler for cleaning out the clinkers.

How They Form

Coal clinkers form when noncombustible elements contained in coal such as iron, calcium, silicon, aluminum and sodium interact with sulfur and carbon at high temperatures. These elements melt together with clays and alkalis to form a glassy slag that eventually hardens into clinkers. Clinker formation is a complex series of mineralogical transformations and phase changes that occur as coal burns. Small clinkers can fuse together to form large ones that can physically damage boilers.

Contributing Factors

Bituminous coal that's rich in sulfur, iron compounds and clay minerals tends to produce the most clinkers. Boiler operating practices that reduce emissions of oxides of nitrogen tend to increase formation of clinkers. Other contributors to clinker formation include failure to remove soot and other deposits, poorly pulverized coal, and improper burner adjustments that result in inefficient mixtures of air and fuel. A firebox that's too small for the type of coal being burned coupled with infrequent ash removal also encourages clinker formation. The problem is compounded if there's no way to observe clinker buildup in the boiler.

Chemical Cure

Some companies are offering chemical combustion enhancers to be added to coal that they claim will virtually eliminate clinker formation from burning lower-quality coal. These compounds act as a chemical catalyst that uses the moisture inherent in solid fuels to increase the temperature of combustion and thereby greatly reduce the amount of unburnt matter that otherwise would just melt to form clinkers.