In the SNCR flue gas denitrification process, ammonia or urea solution is usually used as the denitrification agent, which is directly sprayed into the high-temperature flue gas and reacts with NOx in the flue gas to generate N2, achieving the purpose of denitrification. Urea does not have a corrosive effect on boiler tubes, but the hydrolysates and thermal decomposition products of urea solution have a strong corrosive effect on furnace tubes.
Hydrolysis reaction of urea:
CO (NH2) 2 → NH4CNO → NH3+HCNO
HCNO → H++CNO-
HCNO is a weak acid, while CNO - is a chelating agent with strong coordination ability under acidic conditions, which can form complexes with iron ions and accelerate the destruction of protective membranes.
Thermal decomposition reaction of urea:
CO (NH2) 2+H2O → CO2+2NH3
Thermal boiler tubes are usually made of 20G carbon steel, and protective oxide films such as FeO, Fe2O3, and Fe3O4 are formed on their surfaces under high temperature conditions. Under normal circumstances, the urea solution sprayed into the furnace flue gas is heated and decomposed into ammonia and carbon dioxide, which will not cause serious corrosion to the furnace tubes. If the urea injection tube is not designed properly, the urea solution will directly spray onto the surface of the furnace tubes. Under the influence of temperature, the acidic HCNO generated by urea directly reacts with the oxide on the surface of the furnace tube, and the strong coordination ability of CNO - accelerates the process of chemical corrosion. Meanwhile, the NH3 produced by the thermal decomposition of urea has multiple corrosive effects. On the one hand, NH3 is a strong ligand that can react with chromium in 20G carbon steel to accelerate corrosion; On the other hand, NH3 can disrupt the dual charge balance structure between the protective film and the substrate metal, reducing the electrode potential and making corrosion more likely to occur.
Coordination reaction of ammonia:
Cr3++6NH3 → Cr (NH3) 63+
After the coordination reaction occurs, it leads to inconsistent chromium content between crystal structures, which further triggers chromium poor intergranular corrosion.
Electrochemical corrosion process involving aerobic participation:
Anode, metal dissolves into metal ions and enters the solution;
Fe → Fe2++2e
Cathode, where electrons are bound by H+and O2 in the solution;
2H++2e → H2
O2+2H2O+4e → 4OH - (neutral or alkaline system)
O2+4H++4e → 2H2O (acidic system)
The NH3 generated by the decomposition of the furnace tube surface reacts with SO2 and SO3 in the flue gas to generate ammonium sulfite and ammonium sulfate, and the dust in the flue gas forms dirt covering the surface of the furnace tube, which disrupts the medium balance between the furnace tube and the flue gas and causes corrosion under the scale.
CO2 can also cause corrosion on furnace tubes under low-temperature acidic conditions. The aqueous solution on the surface of the furnace tube absorbs CO2 from the flue gas to form carbonic acid, which can directly corrode the furnace tube.
Fe+2CO2+H2O → Fe (HCO3) 2+H2
Fe (HCO3) 2 → FeCO3+CO2+H2O
Fe+H2CO3 → FeCO3+H2
In addition to the above-mentioned corrosion mechanisms, there are also various electrochemical corrosion phenomena on the surface of the furnace tube, such as water vapor corrosion, wet H2S corrosion, and chloride ion corrosion
In order to solve the corrosion of urea solution on the furnace tubes in SNCR flue gas denitrification process, Hongze adopts exclusive patented technology to replace urea, and uses pure physical methods to reduce nitrogen oxides in the flue gas without any damage to the boiler, which can greatly extend the service life of the boiler.
