selective catalytic (SCR) v/s non -catalytic reduction(SNCR)
SCR
1. An SCR (Selective Catalytic Reduction) unit is an effective means of conditioning the exhaust gas after the combustion process for reducing NOx already formed in the combustion process.
2. SCR is the method for NOx reduction on diesel engines today that can give the largest reductions.
3. The process essentially involves injecting ammonia in the exhaust stream and in the presence of a catalyst the NOx reacts with the ammonia and forms water vapour and nitrogen.Due to the hazardous properties of ammonia, urea solution is generally used to provide the required ammonia.
4. With the SCR technique, the exhaust gas is mixed with ammonia NH3 or urea (as NH3 carrier) before passing through a layer of a special catalyst at a temperature between 300 and 400°C, whereby NOx is reduced to N2 and H2O.
6. NOx reduction by means of SCR can only be carried out in this specific tem-perature window:
If the temperature is too high, NH3 will burn rather than react with the NO/NO2.
If the temperature is too low, the reaction rate will also be too low, and con-densation of ammonium sulphates will destroy the catalyst.
7. When engine exhaust gas is released from the exhaust gas receiver, urea or ammonia is supplied to the pipeline via double-wall piping into a mixer. The engine exhaust gas is mixed with the agent and led into the turbocharger in the turbine side.
8. To compensate for the pressure loss across the SCR system, high-efficiency turbochargers and high performing auxiliary blowers are mandatory. Due to the ammonia/urea heat release in the SCR process, the exhaust gas temperature from the turbocharger is slightly higher than the exhaust gas temperature in engines without SCR.
It consists of the following main components:A reducing agent storage tank, a reducing agent feeding and dosing unit, the reducing agent injection and mixing element, a reactor with catalyst elements, a soot blowing system for keeping the catalyst elements clean and the control system. A pump unit transfers urea from the storage tank to the dosing unit, which regulates the flow of urea to the injection system based on the operation of the engine. The dosing unit also controls the compressed air flow to the injector. The urea injector sprays reducing agent into the exhaust gas duct. After the injection of reducing agent, the exhaust gas flows through the mixing duct to the reactor, where the catalytic reduction takes place.
SNCR
Selective non-catalytic reduction (SNCR) is a chemical process that changes nitrogen oxides (NOx) into molecular nitrogen (N2), carbon dioxide (CO2) (if urea is used), and water vapor.
A reducing agent, typically anhydrous gaseous ammonia or liquid urea, is injected into the combustion/process gases. At suitably high temperatures (870 to 1200 deg c) , the desired chemical reactions occur. Conceptually, the SNCR process is quite simple.
A gaseous or aqueous reagent of a selected nitrogenous compound is injected into, and mixed with, the hot flue gas in the proper temperature range. The reagent then, without a catalyst, reacts with the NOx in the gas stream, converting it to harmless nitrogen gas, carbon dioxide gas (if urea is injected), and water vapor. SNCR is “selective” in that the reagent reacts primarily with NOx,.
No solid or liquid wastes are created in the SNCR process. While either urea or ammonia can be used as the reagent, for most commercial SNCR systems, urea has become the prevalent reagent used. Urea is injected as an aqueous solution while ammonia is typically injected in either its gaseous or anhydrous form using carrier air as a dilutive and support medium.
The principal components of the SNCR system are the reagent storage and injection system, which includes tanks, pumps, injectors, distribution modules, and associated controls. Given the simplicity of these components, installation of SNCR is easy relative to the installation of other NOx control technologies. While SNCR performance is specific to each unique application, NOx reduction levels ranging from 30 % to more than 75 % have been reported.
The NOx reduction efficiency of both SNCR processes depends on the following factors:
• Flue gas temperature in reaction zone
• Uniformity of flue gas temperature in the reaction zone
• Normal flue gas temperature variation with load
• Residence time
• Distribution and mixing of ammonia/urea into the flue gases
• Initial NOx concentration
• Ammonia/urea injection rate
• Heater configuration, which affects location and design of injection nozzles
