1. Air pollution
Lime is used in exhaust treatment process to significantly reduce air pollution.
The growing in demand of electricity leads to the increasing of fossil fuels burning. These fuels contain a lot of sulfur which was discharged directly into the atmosphere. The process is the main reason of acid rain, killed plants, dried up ponds and lake, and destruction of environment.
Lime and dolomite are used in exhaust treatment processes to significantly reduce air pollution.
1.1 Using lime cream
This method is widely used in treatment technology, because of high efficiency, cheap materials available everywwhere.
The reactions in treatment process as belows:
Ca(OH) + SO2 = CaSO3 + H2O
2CaSO3 + O2 = 2CaSO4
SO2 absorption performance by lime cream achieves 98%. Aerodynamic resistance of the system is not exceed 20mm water column
Lime materials are used completely. In particular, the sludge from exhaust treatment system can be used as an adhesive in construction after converting sulfide to sulfate in the kiln.
Advantages of this method:
Simple technology process, low cost, cheap and easy to find sorbent and inexpensive, emission handling without cooling and preliminary dust handling.
Absorption by suspensions CaCO3 (Component of Solid: liquid = 1: 10, CaCO3 particle size: 0,1mm)
When absorbing much air at once, the velocity absorption of each air is reduced. In membrane flow tower or buffer tower, chemical absorbed air has convection phenomena on the exposure surface of air phase and liquid phase. The essence of this phenomenon is that forced convection which promotes mass transfer process appears on the exposure surface.
1.2 Using Dolomite
Using of dolomite to remove SO2 exhaust has been known for a long time, but application in industry is performed recently by the technology scientists of the former Soviet Union
SO2 is absorbed by oxide – magnesium hydroxide, to create magnesium sulfite hydrated crystalline. The reactions occur in absorbing equipment as belows:
MgO + SO2 = MgSO3
MgO + H2O = Mg(OH)2
MgSO3 + H2O + SO2 + Mg(HSO3)2 = Mg(OH)2 + Mg(HSO3)22MgSO3 + 2H2O
Solubility of magnesium sulfite in water is limited, thus the excess of magnesium sulfite in form of MgSO3.6H2O and MgSO3.3H2O falls to residue at the botton
The rate of solid : liquid in suspensions is 1 : 10; input pH is 6.8 – 7.5; and output pH is 5.5 – 6.0; magnesium sulfate is formed by oxidation of magnesium sulfite
MgSO3 + O2 = MgSO4
The formation of MgSO4 is not of use to regeneration of MgO (MgSO4 decomposition temperature is 1200 – 1300oC). Therefore, this reaction should be limited by reducing the exposure time between the air phase and liquid phase or use oxidative reducing substance
Magnesium regeneration is performed in kiln at 900oC with coke
Released SO2 in burning is 7-15%. The released SO2 is cooled, removed dust and sulfuric acid mist and be taken to produce sulfuric acid.