Boiler soot blowing system
During the combustion of pulverized coal in boiler, coal will turn into slag and get discharged, but still some unburnt carbon, volatile matters, fly ash will go with flue gas. These things will cool off and adhere to heating surface. They settle down on heating surface to cause problem called soot fouling and slagging. Boiler soot blowing system could keep boiler clean. Here are 4 reasons why it is necessary.
This would increase the thermal resistance and reduce thermal efficiency. Thermal conductivity coefficient of soot is only 0.1163w/(m2℃), it is only 1/600 of steel. As thermal conductivity of heating surface is reduced, heat contained by flue gas can’t be absorbed sufficiently, boiler steam rate is reduced, coal consumption is increased, boiler can’t run at full load. Economic benefit is significantly influenced.
During combustion, the matters containing alkali metals (K, Na, Ca, Mg) are sublimated and condensate on tubes when reaching superheater, reheater area. They will react with SO3, oxides within flue gas and form composite sulfate, which has fast corrosion rate to boiler tubes. Tube leaking would happen easily. That’s why accidents happen at a high chance at superheater area. It’s hidden danger to boiler operation safety.
Moreover, fouling and slagging would increase the resistance between boiler bank for flue gas. Flue gas speed is reduced. And in the area with less fouling and slagging, flue gas speed is increased, heat transfer is improved. The abrasion of fly ash to boiler tube is in directional proportion with the cube of flue gas speed. The faster speed, the heavier abrasion. Tubes with fouling and slagging have lower heat transfer ability, heat absorption between tubes is uneven. It would cause bigger thermal deviation, tube at superheater and reheater is in overtemperature. Boiler can’t run at full load. If overtemperature is 10-20℃ for a long time, service life of tubes is only half.
Soot fouling on the heating surface of superheater, reheater, economizer, air preheater, evaporator would increase the resistance of flue gas passthrough, power consumption of FD fan and ID fan is increased. Heat exchange efficiency in air preheater is decreased, temperature of primary and secondary air is reduced, combustion inside boiler furnace is influenced. Normally speaking, if coal feeding system is the same, temperature of primary and secondary air is higher, combustion speed of pulverized coal in furnace is faster. Then volatile matters and carbon particles generated during combustion from coal powder of coarse particles would burn completely before reaching convention bank, under the same flue gas flow and residence time inside furnace. Carbon residue in fly ash and slag is much lower. On the contrary, temperature of primary and secondary air is lower, combustion speed inside furnace is slower, volatile matters and carbon particles can’t burn complete, carbon residue is higher. Pulverized coal combustion is at a low efficiency. In turn, it makes the fouling on heating surface a worse situation.
Inside furnace, it is very complex physical and chemical process. Property of coal, structure of heating surface, temperature, air condition would all have an influence on the fouling and slagging. Soot deposit can’t be avoided only by boiler design and operation. Practice has proved that boiler soot blowing system could effectively avoid fouling, slagging and corrosion of tubes. It could ensure the safety of boiler, improve availability of unit. And due to heating surface keeps clean condition and effective heat transfer, boiler efficiency is improved and auxiliary equipment power consumption is reduced.
before soot blowing
after soot blowing
boiler soot blowing system
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