Solar Power Technology

The role of the burner is to provide a thorough mixing of the fuel and air so that the fuel is completely burned. Ignition is accomplished by a spark-ignited light oil jet until the flame is self-sustaining. In the combustion chamber a pulverized coal particle or atomized oil droplet burns in a fraction of a second.

The coal particle or oil droplet burns from the outside to the core, leaving behind incombustible mineral matter. The mineral matter is called ash. In modern pulverized coal and atomized oil fired power plants, more than 90% of the mineral matter forms the so-called fly ash, which is blown out of the boiler by forced or natural draft and is later captured in particle collectors.

About 10% of the mineral matter falls to the bottom of the boiler as bottom ash. When the bottom of the boiler is filled with water, the bottom ash forms a wet sludge, which is sluiced away into an impoundment. Some of the fly ash, however, is deposited on the water pipes lining the boiler. This forms a slag which hinders heat transfer. The slag needs to be removed from time to time by blowing steam jets against it or by mechanical scraping.

Coal burns relatively slowly, oil burns faster, and gas burns the fastest. For complete combustion (carbon burn-out), excess air is delivered—that is, more air than is required by a stoichiometric balance of fuel and the oxygen content of air. Pulverized coal requires 15–20% excess air; oil and gas 5–10%.

The central coal impeller carries the pulverized coal from the silo in a stream of primary air. Tangential doors (registers) built into the wind box allow secondary air to be admixed, generating a fast burning turbulent flame. The impeller is prone to corrosion and degradation and has to be replaced once a year or so.

The burners are usually arranged to point nearly tangentially along the boiler walls. In such a fashion a single turbulent flame ensues from all four burners in a row, facilitating the rapid and complete burn-out of the fuel. Depending on the power output of the boiler, as many as six rows of burners are employed, totaling 24 burners.

Some power plants employ cyclone furnaces, especially for poorer grades of coal with a high ash content. In a cyclone furnace the combustion of the pulverized coal is accomplished in a watercooled horizontal cylinder located outside the main boiler wall. The hot combustion gases are conveyed from the cyclone furnace to the main boiler.

The advantage of a cyclone burner is that the majority of the mineral matter forms a molten ash, called slag, which is drained into the bottom of the boiler, and only a smaller portion exits the boiler as fly ash. Thus, smaller and less expensive particle collectors are required. The disadvantage is that at the high temperatures experienced inside the cyclone furnace, copious quantities ofNOxare formed. Nowadays, plants equipped with cyclone furnaces require the installation of flue gas denitrification devices for reducing NOx concentrations in the flue gas, largely vitiating the cost savings of cyclone furnaces in terms of coal quality and ash content.

Some older power plants and smaller industrial boilers employ stoker firing. In stoker-fired boilers, the crushed coal is introduced into the boiler on an inclined, traveling grate. Primary air is blown from beneath the grate, and secondary overfire air is blown above the grate. By the time the grate traverses the boiler, the coal particles are burnt out, and the ash left behind falls into a hopper.

The carbon burn-out efficiency is lower in stoker than in pulverized coal burners because of poorer mixing of coal and air that is achievable in stoker-fired boilers. Therefore, stoker-type boilers have a lower thermal efficiency compared to pulverized coal boilers.