During fuel combustion, waste is formed, which is called fly ash. Special devices are installed next to the furnaces to trap these particles. They are a dispersion material having components less than 0.3 mm in size.
What is fly ash?
Fly ash is a finely dispersed material with small particle sizes. It is formed when burning solid fuel at elevated temperatures (+800 degrees). It contains up to 6% of unburned to the end substance and iron.
Fly ash is generated when burning mineral impurities that are in the fuel. For various substances, its content is not the same. For example, in wood, the ash content of fly ash is only 0.5-2%, in fuel peat 2-30%, and in brown and hard coal 1 - 45%.
Getting
Fly ash is generated during fuel combustion. The properties of the substance obtained in boilers differ from those created in the laboratory. These differences affect the physico-chemical characteristics and composition. In particular, during combustion in the furnace, the mineral substances of the fuel melt, which leads to the appearance of components of the unburnt composite. Such a process, which is called mechanical burning, is associated with an increase in the temperature of the furnace to 800 degrees and above.
To capture fly ash, special devices are needed, which can be of two types: mechanical and electrical. During the operation of the GZU a large amount of water is consumed (10-50 m 3 of water per 1 ton of ash and slag). This is a significant drawback. To get out of this situation, use a circulating system: water after cleansing from particles of ash re-enters the main mechanism.
Main characteristics
- Workability. The finer the particles, the greater the influence of fly ash. Adding ash increases the uniformity of the concrete mixture and its density, improves styling, and also reduces the consumption of mixing water with the same workability.
- Decrease in heat of hydration, which is especially important in the hot season. The ash content in the solution is proportional to the decrease in the heat of hydration.
- Capillary absorption. With the addition of 10% fly ash to cement, the capillary absorption of water increases by 10-20%. This, in turn, reduces frost resistance. To eliminate this drawback, it is necessary to slightly increase air intake due to special additives.
- Stability in aggressive water. Cements, which are 20% ash, are more resistant to immersion in aggressive water.
Pros and cons of using fly ash
Additive to the mixture in the form of fly ash entails a number of advantages:
- Clinker consumption is reduced.
- Grinding improves.
- Strength increases.
- Workability improves, which facilitates formwork.
- Shrinkage is reduced.
- Heat is reduced during hydration.
- The time until cracks appear.
- Resistance to water is improved (both clean and aggressive).
- The mass of the solution decreases.
- Fire resistance increases.
Along with the advantages, there are some disadvantages:
- The addition of ash with a high content of underburn changes the color of the cement mortar.
- Reduces initial strength at low temperatures.
- Reduces frost resistance.
- The number of mixture components that need to be controlled is increasing.
Types of fly ash
There are several classifications by which fly ash can be divided.
By the type of fuel that is burned, ash can be:
The composition of the ash are:
- Acidic (with a calcium oxide content of up to 10%).
- Basic ( calcium oxide content above 10%).
Depending on the quality and further use, 4 types of ash are distinguished - from I to IV. Moreover, ash of the latter type is used for concrete structures that are used in harsh conditions.
Fly ash processing
For industrial purposes, unprocessed fly ash is most often used (without grinding, sieving, and so on).
When fuel burns, ash is formed. Light and small particles due to the movement of flue gases are carried away from the furnace and captured by special filters in ash collectors. These particles are fly ash. The remainder is referred to as dry ash.
The ratio between these fractions depends on the type of fuel and the design features of the furnace itself:
- with solid removal, 10-20% of the ash remains in the slag;
- with liquid slag removal - 20-40%;
- in cyclone type furnaces - up to 90%.
During processing, particles of slag, soot and ash can get into the air.
Dry fly ash is always sorted into fractions under the influence of electric fields that are created in the filters. Therefore, it is most suitable for use.
To reduce the loss of substance during calcination (up to 5%), fly ash is necessarily homogenized and sorted into fractions. Ash, which is formed after the burning of low-reactive coals, contains up to 25% of the combustible mixture. Therefore, it is additionally enriched and used as energy fuel.
Where are fly ash used?
Ashes are widely used in various fields of life. It can be construction, agriculture, industry, sanitation
In the production of certain types of concrete, fly ash is used. Application depends on its type. Granular ash is used in road construction to establish parking lots, solid waste storage sites, bicycle paths, embankments.
Dry ash entrainment ash is used to strengthen soils as an independent astringent and quickly hardening substance. It can also be used for the construction of dams, dams and other hydraulic structures.
For the production of hydraulic concrete, ash is used as a substitute for cement (up to 25%). As a filler (small and large), ash is included in the process in the production of slag concrete and blocks used in the construction of walls.
It is widely used in the production of foam concrete. Adding ash to the foam concrete mixture increases its aggregate stability.
Ashes in agriculture are used as potash fertilizers. They contain potassium in the form of potash, which is easily soluble in water and available for plants. In addition, ash is rich in other useful substances: phosphorus, magnesium, sulfur, calcium, manganese, boron, micro and macro elements. The presence of calcium carbonate allows the use of ash to reduce soil acidity. Ash can be applied to various crops in the garden after plowing, fertilize near-tree circles of trees and shrubs with it, as well as meadows and pastures. The use of ash simultaneously with other organic or mineral fertilizers (especially phosphorus) is not recommended.
Ash is used for sanitation in the absence of water. It increases pH and kills microorganisms. It is used in latrines, as well as in places of sewage sludge.
From all of the above, we can conclude about the widespread use of such a substance as fly ash. The price for it varies from 500 r. per ton (with large wholesale) up to 850 rubles. It should be noted that when using pickup from distant regions, the cost can vary significantly.
GOST
The documents that control the production and processing of fly ash have been developed and are in force:
- GOST 25818-91 "Fly ash of thermal power plants for concrete."
- GOST 25592-91 "Ash and slag mixtures of thermal power plants for concrete".
Other additional standards are used to control the quality of the produced ash and mixtures with its use. Moreover, sampling and all types of measurements are also carried out in accordance with the requirements of GOSTs.