Melting chamber firing

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The slag tap , also briefly slagging furnaces called, is a furnace for solid fuels can be achieved in the constructive measures on the combustion chamber temperatures so high that the slag remains liquid and can be removed so the deslaggers.

description

A major problem when operating solid fuel-fired steam boilers is the contamination of the heating surfaces . Residues on the heating surfaces ( slag ) have an insulating effect and impair the heat transfer to the water to be evaporated . The more dirty the boiler is, the lower its steam output. The flue gas temperatures rise and the firing efficiency is reduced . Furthermore, downstream heating surfaces can be damaged if the flue gas on superheaters and feed water preheaters is hotter than in the thermal design of the steam boiler.

To prevent the slag from sticking to the heating surfaces, the furnace temperature is increased to at least 1550 ° C. At this temperature the slag remains liquid and can flow out of the furnace. The concept was technically implemented by means of cyclone melting , with which slag temperatures of up to approx. 1800 ° C are reached. In order to achieve such high temperatures, the combustion chamber is separated from the rest of the boiler by a narrowing.

Each smelting furnace consists of the melting chamber, in which the slag is melted, and the boiler. A distinction must be made between open and closed melting chambers.

The open melting chamber is usually located directly below the boiler, while the closed melting chamber is separated from the main boiler by a constriction. It is located to the side of the main boiler ( horizontal cyclone , U and SU firing , VKW melting chamber, etc.). The melting chamber usually has a slightly sloping floor so that the molten slag can easily drain out of the chamber. At the lowest point of the chamber, it flows into a shaft that opens into a water bath (wet slag remover). There the slag is quenched and solidifies as a versatile granulate (e.g. road construction).

The closed melting chamber and the actual boiler are always separated by a grate made of water-cooled pipes, the so-called catch grate . This grate is supposed to catch the last liquid slag particles from the flue gas flow before they can reach the heating surfaces in the main boiler. The catch grate also forms a so-called temperature threshold in the boiler. The cremation should be completed behind him. Due to the high temperatures of well over 1500 ° C, uncooled boiler steels cannot be used because they would melt or soon scale. The catch grate therefore consists of water-cooled pipes.

At the high temperatures in the melting chamber, unprotected water-cooled pipe walls cannot be used. On the one hand, the heat flux density would be too high so that film boiling could start and, on the other hand, the slag on the water-cooled pipes would solidify immediately. The melting chamber is therefore lined with a temperature-resistant and insulating protective film. For this purpose, pins are welded onto the pipe walls, which are usually made of the temperature-resistant material Sicromal 12 (a high-temperature, stainless steel). Ramming compound is applied to the surface prepared in this way, which clings to the pins. The rammed earth consists of a silicon compound ( silicon carbide and chrome ores ) and is temperature resistant up to over 1700 ° C. Since a high heat transfer still occurs in the melting chamber despite the insulating rammed earth, the pipe walls belong to the evaporator part of the steam boiler.

Different construction methods

A wide range of designs can be distinguished. The first two types of the following examples are open melt furnaces, the other types are closed. The mother of all melting furnaces is the melting funnel. There is also the ignition table, the "U" and "SU" melting chambers, the melting chamber according to VKW , the vortex melting chamber and, as a result, the cyclone melting furnace . The cyclone furnace was built vertically (e.g. KSG, Dürr ) and horizontally ( Babcock ).

The temperature difference between the slag melting point and the furnace temperature (furnace temperature> melting temperature) is called the length of the slag . If the temperature difference is large, one speaks of a long slag , if it is small, one speaks of a short slag .

Due to the high temperatures in the melting chamber, damage to the pressure-bearing walls occurs here frequently.

Emissions protection

The decisive disadvantage, which is why melting chamber furnaces are no longer installed, is the emission behavior. Due to the very high temperatures , the nitrogen molecules introduced with the combustion air dissociate . The free nitrogen atoms thus formed can react with free oxygen atoms to form nitrogen oxides. So-called thermal nitrogen oxides are formed, the rate of which is correspondingly high at the very high temperatures and the residence times of the combustion gases in the melting chamber.

With modern flue gas cleaning systems , such as B. denitrification by selective catalytic reduction , you manage the high nitrogen oxide - emissions to master. Of course, a considerable amount of sulfur dioxide is also produced during combustion , but this always depends on the sulfur content of the coal. Most of the sulfur dioxide is removed from the exhaust gas using flue gas desulphurisation (FGD).

particularities

The first melting chamber furnace in Germany was put into operation in 1932 in the Cuno power plant in Herdecke .