1、 Pollutants and sources
The kiln tail gas contains a variety of gaseous substances regarded as pollutants: carbon dioxide, CO2, carbon monoxide, Co, sulfur dioxide, SO2, hydrogen sulfide, H2S, nitric oxide and No.
Carbon monoxide can be produced in the kiln if there is not enough air to completely burn the fuel, or if the fuel does not take enough time to complete the combustion reaction at the combustion temperature.
Carbon dioxide comes from the decomposition of raw carbonate and the combustion of carbonaceous fuel.
Sulfur dioxide comes from the combustion of sulfur and sulfide in fuel and raw materials, as well as the reduction and decomposition of sulfate.
Nitric oxide is produced by two mechanisms: fuel nitrogen oxidation, which usually occurs in the flame of the kiln main burner, and the thermal reaction between air nitrogen and oxygen in the hot combustion zone. Nitric oxide is not only the producer of "acid rain" oxidized to nitric acid, but also a cause of photochemical smog.
Pollutants SO2, particulate matter, NOx, fluoride, chloride and heavy metals (Pb, CD) in kiln flue gas. SO2 :
1) The hot blast stove of spray drying tower uses pulverized coal or coal water slurry to cause sulfur entrainment in the powder;
2) The kiln uses fuel without purification and desulfurization (such as water, coal, etc.);
3) Sulfur containing minerals in green body raw materials. Particulate matter: green body dust, fuel entrainment, suction under working conditions; NOx: the main source is thermal NOx, that is, during fuel combustion, nitrogen (N2) in the air reacts with oxygen (O2) at high temperature to generate NOx; Fluoride: fluorine-containing minerals in billets and glazes, which decompose when fired at high temperature and overflow in the form of SiF4 and HF; Heavy metals: heavy metals such as Pb and CD contained in body glaze;
2、 Excess air coefficient and oxygen content
Oxygen content in boiler flue gas and excess air coefficient are two different concepts, but they are closely related. The oxygen content of boiler flue gas is the percentage content of oxygen in boiler flue gas, which is a ratio greater than 0 and less than 1. The excess air coefficient is the ratio of actual air input to theoretical air demand in the actual boiler combustion process, which is generally 1.2-1.3 times of theoretical air. In order to ensure the complete combustion of boiler fuel, this is a ratio greater than 1.
However, there is a direct connection between the two. If the excess air coefficient is large, the oxygen content of boiler flue gas is high. Therefore, generally, the excess air coefficient can be determined by measuring the oxygen content of boiler flue gas.
2、 Introduction of chemical reduction treatment technology for tail gas with low nitrogen
PLC control system
compressed air supply system
urea supply system
jet mixing section
Selective catalytic reduction,
The following tests (SCR) and reducing agent urea solution (or ammonia, NH3 · H2O) are the core purification components.
The filter unit can selectively reduce nitrogen oxides (NOx) in the tail gas to nitrogen.
SCR reaction is a complex and multivariate catalytic reduction reaction. The main reactions are as follows:
NH3 · H2O → NH3 ↑ + H2O (urea hydrolysis reaction)
No + NO2 + 2nh3 → 2n2 ↑ + 3H2O (NOx reduction reaction)
4NO + O2 + 4nh3 → 4n2 ↑ + 6H2O (nitric oxide reduction reaction)
2no2 + O2 + 4nh3 → 3N2 ↑ + 6H2O (nitrogen dioxide reduction reaction)