SCR DeNOx Technology
1. Purpose and Principle
DeNOx technology mainly includes Selective non-Catalytic Reduction (SNCR), Selective Catalytic Reduction (SCR) and combination of SNCR and SCR.
SCR has high NOx reduction efficiency（70-90%），and it is a mature technology, and is the most common technology used for DeNOx.
SCR means the NOx in flue gas selectively reacts with the regent within the presence of the catalyst bed, producing N2 and H2O, and removing the NOx in the flue gas.
SCR Chemical reaction is:
4NO + 4NH3+O2 → 4N2 +6H2O
6NO2 + 8NH3 + O2 → 7N2 + 12H2O
The typicle process of SCR DeNOx system
In SCR De-NOx system, NOx contained in many kinds of exhaust gas is reduced by ammonia (NH3), urea [(NH2)2CO], etc. called "Ammonia like material" to nitrogen (N2) and water (H2O).
The most suitable reducing agent can be selected out of ammonia like materials based on economics, handling and safety.
SCR DeNOx system is mainly consisted of reactor, reductant tank/storage, reductant injection system and catalyst. After injection and completely mixing of reducing agent at the inlet of reactor, the exhaust gas is led into catalyst bed where the NOx removing reaction will take place.
NOx is converted into N2 and H2O on catalyst surface. When reducing agent is NH3, chemical equations are represented as follows:
4NO+4NH3+O2 -> 4N2+6H2O
6NO2 +8NH3 -> 7N2+12H2O
2. Performance (range of general use)
Application: (Gas, Oil, Coal fuel) Boiler, Gas turbine (HRSG), Waste to Energy plants and Diesel engines
Efficiency: More than 90% of De-NOx Ratio
Operation Temp.: 200-420ºC
NOx Concentration: 10-2,000 ppm
Pressure Drop: less than 1.2 kPa/reactor
Reactor: Catalyst bed can be installed in heat recovery boiler
Operation: Steady state can be achieved within 30 minutes from cold start
1) When De-NOx efficiency is required more than 95%, the emission of excess reducing agent will be increased distinctly.
2) Operation temperature should be determined by considering SOx concentration contained in exhaust gas to avoid formation of ABS, resulting in clogging the catalyst.
The performance of SCR De-NOx system is mainly estimated based on catalyst performance (activity, durability, etc.) and control techniques of reducing agent injection.
V/TiO2 catalyst which is obtained by VO3, ion adsorption on TiO2 crystals and calcination, is generally used in SCR system.
De-NOx reaction is very rapid. Therefore the reaction rate is mainly controlled by the diffusivity of reactants: diffusivity in laminar boundary layer and in catalyst micro pore.
Catalyst technology is progressing day by day. But now the monolithic catalyst with ceramic fiber reinforcements has highly evaluation as De-NOx catalyst. This type of catalyst has the structure as follows:
Catalyst fine powder (1- 30µm in diameter) is held and dispersed in the matrix of ceramic fiber.
As diffusivity of reactant is widely accelerated, this catalyst has high activity for De-NOx reactions and very low activity for SO2 oxidation:
SO2 oxidation to SO3 affects some down stream equipments on corrosion and dust deposition.
Because of high diffusivity, reactor has high response for condition change. Then it is possible to control the reductant injection at the most suitable range in transient state.
When reaction temperature is higher than 350ºC, V/TiO2 is mainly poisoned by SO3 and shows steady activity for long term operation.
Alkaline metals in dust are known as the catalyst poison. However, because of their low possibility to contact with active site in normal operation, generally they are not so harmful for activity. But it should be taken into account that Se, Zn, As compounds and their vapor is catalyst poison.
Monolithic type catalysts are installed in catalyst bed of reactor. Its void fraction is more than 80% so the pressure drop in catalyst bed is very low, and the dust in exhaust gas gives no clogging in bed as far as the dust is not adhesive.