Denitration

2024-04-25 14:35

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The Introduction of Denitrification


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1. Properties and sources of nitrogen oxide (NOx)
Nitrogen oxide (NOx) includes: N2O、NO、N2O3、NO2、N2O4、N2O5. In the atmosphere, Nox mainly exists in the form of NO (90%) and NO2 (10%).
Nitrogen oxide (NOx) sources: (1). Natural sources: lightning, fire, NH3,N2O oxidation in the atmosphere.
(2). Artificial sources: coal-fired power plants, coking plants, automobile exhaust, and so on.
2. Hazard of nitrogen oxides
(1) . Form acid rain, corrode crops, build (construct) structures, equipment.
(2) . Damage to the ozone layer, enhance ultraviolet radiation.
(3) . Produce photochemical smoke, causing animal and plant discomfort and even death.
(4) . Greenhouse Effect caused climate change and sea level rise.
(5) . The harm of NOx pollution to human health: mainly destroy respiratory system, can cause bronchitis and emphysema.

The Technology Introduction of Denitrification

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The Introduction of SCR(Selective Catalytic Reduction)Technology

SCR refers to that under the action of catalyst, ammonia or urea is used as reducing agent, and the flue gas temperature at the outlet of boiler (320 °C~400°C) is selectively redox with nitrogen oxide in flue gas to produce non-toxic and pollution-free nitrogen gas and water, achieving the purpose of degradation of nitrogen oxides.
SCR Performance Parameter Selection and Operation Control Requirements:
1, The removal rate of NOX is generally 60%~90%
The denitrification rate is moderate, the ammonia escape is suitable, and the air preheater is less polluted.
2, The escape rate of NH3 is generally ≤3ppm, to prevent secondary pollution.
3, The conversion rate of SO2/SO3 is generally ≤ 1%, to prevent adverse effects on downstream equipment.
4, Operating temperature: is required for the operation of the catalyst
5, The flue gas pressure drop:1000pa
6, The catalytic agent chemical lifetime:24,000hours
Advantages and Disadvantages of SCR Technology:
Advantages:
(1) , After the SCR unit is installed in the boiler economizer, it has no effect on the boiler performance and structure.
(2) , The removal rate of denitrification is high, which can reach more than 90%.
(3) , The desulphurization unit is reliable and stable, and the availability of the equipment is 98%.
(4) , The escape rate ≤3ppm
Disadvantages:
(1), The temperature of catalytic reaction is limited, and the typical temperature is 300°C~400°C.
(2), Active maintenance of Catalysts-removal of sulfur and Ash from flue Gas.
(3), Catalyst short life, high cost: generally 4 years, and difficult to regenerate.
(4), The system is complex: economizer and reactor should be designed with bypass; SCR reactor is huge, affecting air preheater and boiler operation.

The Introduction of SNCR(Selective Non-Catalytic Reduction)Technology

SNCR principle: Injecting ammonia, urea and other chemicals into the furnace to react with NOX in flue gas and convert it to nitrogen (N2) and water (H2O).
The effective reaction temperature ranges from 900℃ to 1090℃. Because of the narrow temperature range of the SNCR reaction, the denitrification efficiency of the boiler decreases when the load is changed.
Denitrification efficiency: 25- 40% for large coal-fired units
Response Equation:
4 NO + 4 NH3 + O2 -> 4 N2 + 6 H2O
4 NH3 + 5 O2 -> 4 NO + 6 H2O
4 NH3 + 3 O2 -> 2 N2 + 6 H2O
At temperatures above 1093 °C ammonia decomposes:
4 NH3 + 5 O2 -> 4 NO + 6 H2O
In that case NO is created instead of removed.
Application Features:
1, No catalyst, the cost of the project is greatly reduced
2, Complete layout on boiler body, no area required
3, It is especially suitable for the retrofit of old units and denitrification of small units.
4, Can be used in conjunction with SCR to achieve better results