Clinker Formation In Thermal Power Plant Boilers.docx

Avoiding Clinker Formation in Thermal Power Plant Boilers Introduction Clinker formation is a serious problem in the boilers of thermal power plants which results in forced outage for very long duration and generation loss. 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. The removal of clinkers from the water wall and hopper of boilers is very difficult because clinkers are very hard lumps due to presence of metals in them. Removal of clinkers from boiler is very difficult procedure which takes a long time and the cost involved is also very high. Pneumatic hammering is done manually to dislodge clinkers from boilers, and clinker grinders can be used in grind large pieces of clinkers into small ones.

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. 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.

Causes of Clinker Formation The causes of clinker formation are low quality coal having low gross calorific value, more ash content, high mineral content, low fusion temperature of ash below 1500 C, and overfiring of the molten slag. In India the coal used for thermal power plants is often of low quality with low gross calorific value, high ash and minerals content, so the problems of clinker formation and deposition are evident in many plants. The best solution is to avoid the deposition of clinker is precaution. Blaming poor quality coal may prove harmful for the boiler. Instead of very poor quality coal having high ash and mineral content, clinker formation may be avoided by a sincere attitude of power plant engineers.

Contributing Factors 1. Bituminous coal that's rich in sulfur, iron compounds and clay minerals tends to produce the most clinkers. 2. Boiler operating practices that reduce emissions of oxides of nitrogen tend to increase formation of clinkers. 3. 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. 4. A firebox that's too small for the type of coal being burned coupled with infrequent ash removal also encourages clinker formation. 5. The problem is compounded if there's no way to observe clinker buildup in the boiler.

Typical Clinker

Dealing with Clinker Removal 1. If great care is not taken for poking, then deposition may start, and its sudden collapse may cause injury and burns to workers. 2. A sufficient quantity of water must be provided in the hoppers to flush molten slag and ash so they will not deposit. 3. For flushing of molten ash, water is sprayed into the hopper by the flush pump. 4. Soot blowing must be regularly done to avoid deposition of soot in the water wall by injecting steam at sufficient pressure and temperature to dislodge soot from the water walls tubes. Clinker grinders are provided in boilers to grind large pieces of clinkers into small ones so that choking of the exhaust hole may be avoided, but it is observed in many thermal power plants that clinker grinders are out of service due to negligence or being considered a redundant feature. Due to this negligence large pieces of clinkers can choke the exhaust hole of the hopper and accumulation of small pieces of clinkers and water starts above it. Clinker grinders must be made operational.

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.

Summary The major cause of clinker formation in a boiler is over-firing ( High temperature) due to which ash fuses, hence firing must be properly adjusted by using the right quantity of primary and secondary air ratio in the boiler. Good quality coal having more gross calorific value, less ash content, less mineral content in the coal, and ash fusion temperature above 1500 C must be used to avoid clinker formation. Regular watch must be done on poking. Precautions may avoid clinker formation and its deposition in boilers. Precaution is the best solution.