Slag (combustion residue)
As slag is in the firing technology , the ash called when it was heated above its softening point, it is not that more fine-grained or powdery state, but doughy or even (zäh-) liquid is.
Slagging
Since the ash is an amorphous, ceramic-like material mixture , it does not have a sharply defined melting point , but rather the softening takes place continuously over a wide temperature range. The viscosity decreases with increasing temperature, the ash becomes sticky, the fine ash grains agglomerate and sinter to form larger chunks, the ash begins to creep and finally even to flow.
Similar to other amorphous substances , such as the transition temperature of glass , the softening temperature of bitumen or the softening temperature of plastics , different temperature values can also be defined for ash:
- Sintering temperature
- Softening temperature
- Hemisphere / melting / flow temperature
The measurement of the different temperatures is standardized, for example for coal in DIN 51730 or ISO 540 .
Depending on the composition of the ash, the softening point is between about 900 and 1400 ° C, the sintering point about 100 to 200 K below, the pour point about 100 to 200 K above.
Theoretically, the temperatures mentioned above can only be predicted very imprecisely, since the melting behavior of the substance mixture is extremely complex. Even the smallest changes in the proportions of the substance can have a strong influence on the softening point if the mixture forms a eutectic . In general, the higher the coalification, the higher the temperature . Some substances (such as some potassium, aluminum, sodium, magnesium and silicon compounds) are known to massively lower the softening point. This is particularly evident in the case of stalk-like biomasses (straw and grass), which contain such substances in high concentrations and which are among the fuels with the lowest ash softening temperatures.
Slagging problems and countermeasures
As it cools, the slag solidifies and sintered into a solid, rock-like mass. Depending on the maximum temperature and speed of cooling, it can be porous / brittle / crumbly to glass-like hard.
The cooling and solidification of the slag takes place during the transition from the hot zone of the furnace to the cold edge areas or to any downstream heat exchanger . Here caking that the form heat transfer obstruct and degrade and increase the flue gas side pressure drop of the furnace. One speaks of “slagging” in the furnace. In extreme cases, large, dangerous cornices can form which, if they ultimately break off and fall, can cause considerable damage.
In grate firing systems , slag can block the grate's movement or the openings for the wind below, i.e. the combustion air that flows through the grate from below, can be clogged up. The units for discharging and conveying the ash can be blocked by large, hard cinder blocks, so that crushers have to be provided to crush the cinder blocks . In waste incineration plants in particular, the incineration residue contains not only the actual ash but also a high proportion of inert foreign substances such as metal scrap, glass and ceramic shards, stones, etc., which mix with the slag.
To prevent the furnace from being excessively soiled by ash and slag caking, industrial furnaces and steam boilers usually have cleaning devices ( soot blowers , steam or water blower lances, knockers / vibrators , ...) with which the caking can be removed during operation. This shows that slag is much more difficult to handle than powdery ash. If the caking is so severe and hard that it can no longer be removed with the company cleaning equipment, the furnace must be switched off in order to remove the caking manually ("using mining methods") .
Because of the problems with caking, the formation of slag in firing is normally undesirable and attempts are made to keep the temperature in the furnace below the ash softening temperature by means of suitable measures (e.g. flue gas recirculation ).
With the furnace , the temperature is deliberately very high, above the slag flow point, and the slag runs down the walls of the furnace, where the slag drips into a water bath. Due to the high temperatures, however, more and more environmentally harmful nitrogen oxides are produced , which is why this type of furnace has rarely been used in recent times.
Recovery / disposal
The type of use or disposal of the slag depends primarily on its chemical composition, in particular on the content of poisonous and harmful substances . Particularly dangerous are those constituents that are not safely integrated, but that can be outgassed (→ air pollutant ) or washed out (“eluted”) by rain or groundwater (→ water pollutant ).
Certain pollutants from the fuel or its combustion ( heavy metals , dioxins , PCB , PAHs , ...) have a tendency to attach themselves to solids such as ash / slag. While organic pollutants are mainly found in the fly ash , heavy metals increasingly remain in the furnace ash and thus in the slag.
- According to DIN EN 450, slag with a low pollutant content can be used as an aggregate (artificial aggregate ) for non-metallic-inorganic building materials such as cement, asphalt, etc. For this, the slag must also have certain mechanical properties.
- Slag with a high pollutant content must be disposed of , with the proportion of pollutants determining the landfill class (I to IV).
See also
Individual evidence
- ↑ a b Technical University of Munich - Chair of Energy Systems: Ash Melting Microscope (accessed on February 2, 2016).
- ↑ Hans Oechsner: Fuel properties of biomass (PDF; 1.2 MB), lecture at the workshop "Bioenergy Use in Baden-Württemberg - Heating with Grain" , November 9th and 22nd, 2006 (PDF; 1.2 MB).
- ↑ a b Zwahr, Heiner (Hanseatisches Schlackenkontor GmbH, Hamburg): Page no longer available , search in web archives: properties of mineral waste, state of processing technology and investigation methods for incineration waste slag