Introduction of flue gas SCR denitration system

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    Flue gas denitration system, i.e. gas generating set exhaust denitration device (SCR selective catalystic reduction system) is the device which has the principle of converting nitrogen oxide (NO_X) into harmless nitrogen (N₂) and water (H₂O) with catalyst. Requirement of NO_X≤250mg/Nm³ in flue gas after denitration.

1. Harm of nitrogen oxide (NO_X)

In NO_X discharged from generating unit, NO accounts for about 90%, and NO₂ only accounts for a small part of NO_X. Co is a colorless, odorless, and not very poisonous gas, however, Co of high concentration can cause spasticity and paralysis to nerve center, and it can gradually be oxidized to NO₂ after being emitted into the atmosphere. NO₂ is a reddish brown gas with pungent smell and highly toxicity (toxicity is about five times of that of NO), which can cause cause damage to respiratory tract and lungs, and can cause emphysema in severe case. It can become life-threatening at any time when its concentration reaches a certain value. NO_X and HC can generate photochemical smog in the presence of sunlight, and NO_X can also increase the concentration of surrounding ozone, whereas which can undermine the growth of plants. In addition, NO_X has adverse effects on all kinds of fibers, rubbers, plastics, electronic materials. etc. For all the above reasons, the serious pollution on environment by NO_X in emissions has aroused general concerns in the world scope, therefore, regulations of all countries limiting its emission also becomes more and more strict.

 

2，Working principle

Working principle of the flue gas denitration system is that the urea solution is resolved into ammonia (NH3) and water (H2O) in high temperature in the mixing zone of exhaust pipe, and then enters catalyst module after being fully mixed with flue gas, ammonia (NH3) and water (H2O) react to form harmless nitrogen (N2) and water (H2O) in the catalytic reaction zone, and finally is discharged into the atmosphere via exhaust pipe.

SCR Denitration Reactive Principle Drawing

The urea solution is resolved as follows:

 

CO(NH₂)2 + H₂O = 2NH₃ + CO₂

The urea solution generates NH3 through pyrolysis and hydrolysis, which provides material for the reaction.

 NO_X reactions occurred in SCR denitration system are shown as follows:

3) Technological process

The flue gas denitration system is mainly made up of: SCR reactor, control system, urea tank and urea injection system, flue gas detection and reducing agent feed system, etc. The technological process is as follows:

Under the control of electronic control unit, the urea solution is drawn from the urea tank with urea pump, and is delivered to the two fluid atomizing spray gun after being measured and pressurized, the compressed air is also delivered to the metering spray gun after going through pressure regulating by control unit, the urea ejects under the injecting force of compressed air after the injection valve is opened, and injects into the exhaust pipe after atomization by nozzle after being mixed with the compressed air. The electronic control unit calculates accurately the amount of injection of urea solution according to exhaust flow rate, temperature of catalyst and NO_X concentration,and sends corresponding pulse width signalling to metering pump, which carries out metering for the urea solution based on signal so as to ensure that accurate urea solution is injected into the exhaust pipe at any time. The flue gas will start selective catalytic reduction reaction when going through the catalyst layer of SCR reaction chamber.