Emission controll

Question 1

How can NOx be reduced with primary measures?

Answer 1

During combustion.

• Air staging
• Fuel staging
• Flue gas recirculation: exhaust gases are recirculated to decrease combustion temperatures and to utilize reburning
• Low excess air ratio: limit available oxygen that can react with nitrogen to NOx

Question 2

How can NOx be reduced with secondary measures?

Answer 2

• SCR

- SNCR

Question 3

What is air staging?

Answer 3

Performed in the furnance and/or in a burner

1. Oxygen lean zone:
   - Reduce amount of available oxygen in areas that are critical to NO formation
   - Reduce peak temperature
2. Oxygen rich zone:
   - Ensure complete burnout of the fuel

Question 4

What is fuel staging? (or Reburning)

Answer 4

1. Primary zone (fuel and oxidizer)
2. Reburning zone (more fuel)
3. Burnout zone (oxidizer to ensure sufficient burnout)

Question 5

How does SCR work?

Answer 5

• Flue gas passes through catalyst chamber
• Uses ammonia together with catalyst to reduce NO
• High investment cost
• High NOx removal efficiency, independent of fuel

Question 6

How does SNCR work?

Answer 6

Injection of reducing agent, ammonia or urea into firebox of a boiler at a location where the flue gas is between 760 and 1090 C. This reacts with the NOx that is created in the combustion process.

It requires certain temperatures and residence times.