Selective catalytic reduction (SCR) is a means of converting nitrogen oxides, also referred to as nitrogen oxides (NOx). When burning coal, petroleum products, natural gas as well as domestic and hazardous waste for industrial production pollutions are created, that are emitted with off gas. Thus, significant amounts of NOx, one of the main pollutants, forms despite optimized combustion. Through the reaction of nitrogen oxides with other atmospheric constituents, ozone is created and leads to the so-called summer smog. Other reaction products such as nitric acid, reach the water and soil with the precipitation in the form of acid rain. The resulting nitrate accumulation has a negative impact on human, plants and animals. For these reasons almost all industrialized countries have adopted laws to strict nitrogen oxide emissions. GEA is a leader in DeNOx plants worldwide and develops and manufactures highly-efficient denitrification systems, individually tailored for different customer needs. Aqueous ammonia solution or urea is injected into the ductwork upstream of the SCR and evaporates immediately. Two-fluid nozzles (ammonia and compressed air) are used for the injection of the ammonia solution. A system of static mixers installed in the ductwork downstream of the ammonia injection points ensure adequate mixing of ammonia. The gas and ammonia mixture enters the unit from the top and exits horizontally at the bottom. A gas distribution system (swirl breaker) ensures adequate gas distribution over the entire cross-section of the unit. The reaction between ammonia and NOX is enhanced by the presence of the catalyst bed located at the bottom of the unit.
GEA - Leader in DeNOx
The reduction occurs in the presence of the catalyst at a temperature range between 250°C to 380°C (570°F to 716°F).
|The main chemical reactions are:|
|4 NO + 4 NH3 + O2 -> 4 N2 + 6 H2O (1)|
|6 NO2 + 8 NH3 -> 7 N2 + 12 H2O (2)|
The catalyst bed is cleaned in an intermittent fashion (once or more per 24 hours) with compressed air and steam. A raking arm travels slowly over the catalyst, ensuring efficient cleaning of the top surface and inside the channels.
The ammonia feed rate is controlled by the NOX outlet concentration of the gas. The resulting products from chemical reactions are nitrogen and water vapor, which constitute the natural components of ambient air, and can be discharged to atmosphere.
There are sealing profiles in order to secure low ammonia slip.
SCR Plant for Cement Industry
For the reduction of the nitrogen oxide emissions from cement plants the selective non catalytic reduction process, SNCR, is usually applied by dosing of ammonia or urea at high gas temperatures. However this process is limited in terms of NOx removal efficiency and produces a fairly high ammonia emission (slip). Therefore GEA is using the advanced SCR technology with catalysts, which reduces the reaction temperature from approximately 1000°C to range of 230°C to 400°C. High reduction rates can be realized with a minimum of ammonia slip. GEA prefers the low dust arrangement.
Advantages and Disadvantages of SCR-Systems
|Low Ddust||Semi Low||High Dust|
|Risk of Poisoning||++||-||-|
|Risk of Inkrustations||+||+||-|
|Frequency of Surface Cleaning||++||+||--|
|Necessary Catalyzer Volume||+||+||-|
|Lifetime of Catalyzer||++||-||-|
|Spec. Investment cost||-||-||+|
|Typical low dust SCR parameters:|
|Temperature||250 - 275 °C|
|Dust load||5 - 20 mg/Nm³|
|Catalyst openings||3.5 - 5mm|
|Catalyst Layers / no cleaning device||1 - 2|
Such SCR type is in general over the life cycle (CAPEX & OPEX) favorable in comparison to other arrangements.
SCR for Glass Industry
In the glass production sector, GEA provides tailor-made SCR-DeNOx plants, characterized by:
For selenium glass production specific procedures have to be considered.
Fluid Catalytic Cracking (FCC) Units in Refineries
Refineries vary by complexity; more complex refineries have more secondary conversion capability, meaning they can produce different types of petroleum products. Fluid catalytic cracking, a type of secondary unit operation, is mainly producing additional lighter oil fractions out of the crude oil. Here, GEA provides customized gas cleaning systems including specific effluent treatment.
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