During the combustion of fuel, waste products are produced, which are called fly ash. Special devices are installed next to the furnaces to trap these particles. They are a dispersion material having components smaller than 0.3 mm.
What is fly ash?
Fly ash is a fine material with small particle sizes. It is formed during the combustion of solid fuels at elevated temperatures (+800 degrees). It contains up to 6% of the unburned substance and iron.
Fly ash is formed from the combustion of mineral impurities that are in the fuel. For different substances, its content is not the same. For example, in firewood, the content of fly ash is only 0.5-2%, in fuel peat 2-30%, and in brown and hard coal 1 - 45%.
Receive
Fly ash is formed during the combustion of fuel. The properties of the substance obtained in boilers differ from those created in the laboratory. These differences affect the physico-chemical characteristicsand composition. In particular, when burning in a furnace, the mineral substances of the fuel melt, which leads to the appearance of components of an unburned composite. Such a process, which is called mechanical underburning, is associated with an increase in the temperature in the furnace up to 800 degrees and above.
To capture fly ash, special devices are needed, which can be of two types: mechanical and electrical. A large amount of water is consumed during the operation of the gas storage unit (10-50 m3 of water per 1 ton of ash and slag). This is a significant disadvantage. To get out of this situation, a reverse system is used: water, after being purified from ash particles, re-enters the main mechanism.
Key Features
- Workability. The finer the particles, the greater the effect of fly ash. The addition of ash increases the homogeneity of the concrete mixture and its density, improves placement, and also reduces the consumption of mixing water with the same workability.
- Reducing the heat of hydration, which is especially important in the hot season. The ash content of the solution is proportional to the decrease in the heat of hydration.
- Capillary absorption. Adding 10% fly ash to cement increases the capillary absorption of water by 10-20%. This, in turn, reduces frost resistance. To eliminate this shortcoming, it is necessary to slightly increase the air entrainment due to special additives.
- Stability in aggressive water. Cements, which are 20% ash, are more resistant to immersion in aggressivewater.
Pros and cons of using fly ash
Adding fly ash to the mix has a number of benefits:
- Clinker consumption is reduced.
- Grinding improves.
- Increases durability.
- Improves workability for easier stripping.
- Shrinkage is reduced.
- Reduces heat generation during hydration.
- Increases time to crack.
- Improves resistance to water (both clean and aggressive).
- The mass of the solution decreases.
- Increases fire resistance.
Along with the advantages, there are also some disadvantages:
- Addition of ash with a high content of underburning changes the color of the cement mortar.
- Decreases initial strength at low temperatures.
- Reduces frost resistance.
- Increases the number of ingredients in the mix that need to be controlled.
Types of fly ash
There are several classifications by which fly ash can be classified.
According to the type of fuel that is burned, ash can be:
- Anthracite.
- Coal.
- Brown coal.
According to their composition, ash is:
- Acidic (with up to 10% calcium oxide).
- Basic (calcium oxide above 10%).
Depending on the quality and further use, 4 types of ash are distinguished - from I to IV. And the last kind of ashused for concrete structures that are used in harsh environments.
Fly ash processing
For industrial purposes, untreated fly ash is most often used (without grinding, sieving, etc.).
When fuel is burned, ash is produced. Light and small particles are carried away from the furnace due to the movement of flue gases and are captured by special filters in the ash collectors. These particles are fly ash. The rest is called dry selection ash.
The ratio between the indicated fractions depends on the type of fuel and the design features of the furnace itself:
- with solid removal, 10-20% 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 enter the air.
Fly ash from dry selection is always sorted into fractions under the influence of electric fields that are created in the filters. Therefore, it is the most suitable for application.
To reduce the loss of matter during calcination (up to 5%), fly ash is necessarily homogenized and sorted into fractions. The ash that is formed after the combustion of low-reactive coals contains up to 25% of the combustible mixture. Therefore, it is additionally enriched and used as energy fuel.
Where is fly ash used?
Ashes are widely used in various areas of life. It can be construction, agriculture, industry, sanitation
Fly ash is used in the production of certain types of concrete. Application depends on its type. Granulated ash is used in road construction for the foundation of parking lots, solid waste storage sites, bicycle paths, embankments.
Dry fly ash is used to strengthen soils as an independent binder and quickly hardening substance. It can also be used to build dams, dams and other hydraulic structures.
For the production of hydraulic concrete, ash is used as a substitute for cement (up to 25%). As an aggregate (fine and coarse), ash is included in the process in the production of cinder concrete and blocks used in the construction of walls.
Widely used in the production of foam concrete. The addition of ash to the foam concrete mixture increases its aggregative 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 to plants. In addition, the 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 for various crops in the garden after plowing, it can be used to fertilize tree and shrub circles around the trunks, as well as sprinkle meadows and pastures. It is not recommended to use ash simultaneously with other organic or mineral fertilizers (especially phosphorus).
Ash is used forsanitation in the absence of water. It increases the pH level and kills microorganisms. It is used in latrines, as well as in places of sewage sludge.
From all of the above, we can conclude that a substance such as fly ash is widely used. The price for it varies from 500 r. per ton (with large wholesale) up to 850 rubles. It should be noted that when using self-delivery from distant regions, the cost may vary significantly.
GOSTs
Documents that control the production and processing of fly ash have been developed and are in effect:
- GOST 25818-91 "Fly ash from thermal power plants for concrete".
- GOST 25592-91 "Ash and slag mixtures at TPPs for concrete".
To control the quality of the produced ash and mixtures with its use, other additional standards are used. At the same time, sampling and all types of measurements are also carried out in accordance with the requirements of GOSTs.
