General
Nitrogen oxides (NOx) emitted from burining fossils fuels(Coal, natural gas or oil) has been a main pollutant to contruibute to acid rain, urban smog, visibility impairment and global climate changes. The NOx reducing technology has been developed in Germany and Japan, Mid 1970`s. In recent years, many other countries have increased attention to NOx emission prevention. The technology called 'Selective Catalyst Reduction' or SCR is one of the most efficient way to remove NOx.
Soures
Mobil Soures (40%)
Car, Truck, Air plane etc
Stationary Sources (60%)
Fuel Combustion Power Plant Incinerator, etc.
Applications
Gas turbines Nitric Acid Plants Waste Incineration Refinery Heater Ethylene Crackers etc.
Control of DeNOx
SCR : At low operating temperature (200-450��, the reducing agent such as Ammonia(NH3) or Urea mix with NOx Gas and forms harmless nitrogen(N2) and water(H2O). During passing through catalyst layer SNCR : At high operating temperature (800-850��, the reducing agent such as Ammonia(NH3) or urea mixes with NOx Gas forms harmless nitrogen (N2) and water(H2O) without catalytic reaction. SNCR is more economical way to remove NOx in the range of 40~60 percent removal rate.The performance of SNCR can be reduced by operating in non complete mixing of NOx and NH3 or urea at improper temperature
Comparison SCR and SNCR
Description
|
SCR
|
SNCR
|
Initial Costs
|
High
|
Low
|
Temperature
|
200~450��/div>
|
850~900��/div>
|
Efficiency
|
Up to 70%
|
30~60%
|
Catalyst
|
Required
|
Not Required
|
Operating Window
|
Wide
|
Narrow
|
Operation Principal of SCR
Entering the NOx reductuin chamber the incoming air is mixed with required amount of Ammonia or Urea, homogeneously
As the incoming air flows pass through the catalyst layer, NOx react with Ammonia or Urea on the surface of the catalysts and form harmless Nitrogen(N2) and water(H2O)
4NO + 4NH3 + O2 → 4N2 + 6H2O
2NO2 + 4NH3 + O2 → 3N2 + 6H2O
Pre-heater is installed to heat the air steam up to optimum operating temperature (200~400��
To reduce the operating costs the abator unit has a recuperative heat exchanger bullt in, by which outgoing air is to heat the incoming air steam
Constituents and Feature of Catalyst
|
Active
material |
Substrate
|
Feature
|
Temperature
(�� |
|
V2O5
|
SiO2
|
Reduction ability is lower than that of TiO2 substrate.
|
250~300
|
|
V2O5
|
Al2O3
|
The oxidation reaction is more pre-ominant than reduction.
|
250~300
|
|
V2O5
|
TiO2
|
High sulfur resistance , Most common use
|
300~350
|
|
Pd
|
α Al2O3
|
High performance in activity of low Temperature.
|
250~300
|
|
Pt
|
α Al2O3
|
Easily poisoned by sulfur components.
|
250~300
|
|
Pt + Ru
|
α Al2O3
|
High performance in activity of low Temperature.
|
250~300
|
|
Zeolite
system. |
Wide operation window.
|
300~450
|
|
|
Zeolite
system. |
Cr, Cu
|
High performance And wide operation window.
|
300~450
|
Consideration in theDesign SCR
No influence on operation of the existing processes due to building SCR system Proper reaction temperature : 200~450��/li> Enough mixing reducing agent and NOx : Homeogeneously mixing Minimization of a pressure loss at catalyst layer Proper concentration of oxygen in the reaction chamber NH3 : ratio of NH3/ NOx increase till 1, and slow reaction after 1. Make ratio of NH3 and NOx should be 1. Excessive injections of NH3 cause the rate of NH3 slip Concentration of SO2 : The SOx material easily react with NH3 or one of other catalyst material and form anti-activated componets Dust Low dust : No consider durability of catalyst against SO2 High dust : Required high durabiliable catalyst against SO2 and dust
Required Design Factor
Gas volume Gas Composition Operating Temperature Maximum required level of NOx Rateof NH slip H2O Content
Advantage of the SCR System designed by TYMAGIC and LESNI A/S
Usage of catalyst, which has the most efficent performance, produced by BASF. Design capability of SCR required operating temperature and low Dust/High Dust etc. Design technology of the SCR package system containing heat exchanger and steam heater, etc. Ability of supplying the catalyst for Dioxin control. Experience in contruction of SCR system in power plant, Incineration plant, and chemical plant
