This set of Air Pollution Control Multiple Choice Questions & Answers (MCQs) focuses on “Gaseous Pollutants – Control of Nitrogen Oxides”.
1. How many sources of nitrogen contribute to the formation of nitrogen oxides during combustion?
a) 1
b) 2
c) 3
d) 0
View Answer
Explanation: There are two sources of nitrogen that cause the production of its oxides during the combustion process. One of these is the atmospheric nitrogen, and the other is the nitrogen present in the fuel itself.
2. Which compound acts as the catalyst in the combustion chambers to produce nitrogen oxides?
a) Water vapour
b) Platinum
c) Metal oxides
d) Palladium
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Explanation: Platinum and palladium listed are elemental, while the question asks for compounds. There are no catalysts added to the combustion reaction. Studies show that water vapour present in the fuel-air mixture acts as a catalyst in the formation of nitrogen oxides.
3. How does the residence time of the gas in the combustion chamber affect the formation of nitric oxide?
a) It has no effect
b) Longer the residence time, the lesser is the formation
c) Shorted the residence time, the lesser is the formation
d) It affects nitrogen dioxide formation
View Answer
Explanation: The relationship is directly proportional. The longer the residence time of the gases in the combustion chamber, the higher the concentration of nitric oxide produced. Actual residence time is about 2 seconds in power plant combustion units.
4. Which of the following is not one of the methods to reduce NOX formation during stationary combustion?
a) Reduce residence time
b) Reduce peak reaction temperature
c) Reduce the presence of oxygen
d) Reduce the pressure of operation
View Answer
Explanation: Reducing residence time reduces the formation of nitrogen oxides. The reduction of oxygen concentration reduces nitrogen oxide formation. The temperature of the reaction reduces the production because the oxidation occurs at high temperatures.
5. Which of the following control measures can reduce nitrogen oxide formation in stationary combustion processes?
a) High temperature, more excess oxygen
b) Low temperature, more excess oxygen
c) Low temperature, insufficient oxygen
d) High temperature, insufficient oxygen
View Answer
Explanation: Equilibrium studies show that reducing the temperature of the combustion reaction and reducing the oxygen concentration in the reaction favours lower production of nitrogen oxide. Additionally, the residence time of gases also affects the formation.
6. Which is the most effective way to suppress the formation of nitrogen oxides during combustion?
a) Use low excess-air combustion
b) Use two-stage combustion
c) Flue gas recirculation
d) Inject water into the unit
View Answer
Explanation: All the options help with the suppression of nitrogen oxide formation during the combustion process. Ideally, the most effective method is the use of a two-stage combustion process. However, a combination of two or more of the listed methods is often incorporated.
7. By how much is the formation of nitrogen oxides reduced when a two-step coal-fired combustion unit is used?
a) About 15%
b) About 25%
c) About 35%
d) About 45%
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Explanation: Using the two-stage combustion process reduces the formation of nitrogen oxides by 35% for coal- and oil-fired units. This value is 50% for units using natural gas. Using such a setup reduces the operating temperature, which reduces NOx production.
8. Which is conditionally effective when suppressing the formation of nitrogen oxides during stationary combustion?
a) Use low excess-air combustion
b) Use two-stage combustion
c) Inject water into the unit
d) Flue gas recirculation
View Answer
Explanation: Flue gas recirculation is only conditionally effective when it comes to the reduction in the production of nitrogen oxides. It only occurs when the flue gas is thoroughly mixed with the air used for combustion, before the combustion.
9. How much water should be injected into the combustion process to help with the reduction of nitrogen oxide production?
a) 10-15% of flue gases recirculated
b) 5-10% of the air used
c) 1-5% of the fuel used
d) 7-13% of the products formed
View Answer
Explanation: The quantity of air used dictates the quantity of water that needs to be injected. Additionally, the amount of water to be injected is between 1% to 5% of the air used. The injected water reduces the temperature of the operation.
10. What is the best method to control the production of nitrogen oxides?
a) Use methods in combination
b) Use two-stage combustion
c) Inject water into the unit
d) Flue gas recirculation
View Answer
Explanation: The most effective method is to use the two-stage combustion method. Using a combination of the methods listed, along with reducing operating temperatures and less excess oxygen is the best method.
11. What is a limitation of using lower temperatures for the reduction of nitrogen oxide formation in coal-fired combustion processes?
a) Higher operating costs
b) Higher installation costs
c) Increase in CO production
d) Reduction in efficiency
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Explanation: Reducing temperature and lowering the amount of excess oxygen used increases the carbon monoxide formed. These conditions show that the modification of operating conditions has many limitations, and they need to be thoroughly studied before they are incorporated.
12. Which of the following burner orientations results in reduced nitrogen oxide formation?
a) Direct firing
b) Vertical firing
c) Horizontal firing
d) Tangential firing
View Answer
Explanation: Tangential firing results in a lower formation of nitrogen oxides. This is because of the boiler configuration itself. More intense combustion with higher temperatures leads to higher concentrations; this happens with horizontal firing boilers.
13. Which of the following boiler designs results in reduced production of nitrogen oxides?
a) Fluidized beds
b) Blast furnaces
c) Horizontal firing furnaces
d) Electric arc furnaces
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Explanation: Fluidized beds result in reduced concentrations. That is because fluidized beds have the capability of operating at lower temperatures. Low temperatures favour lesser nitrogen oxide formation.
14. Which of the following is not an advantage of fluidized bed combustion units?
a) Lower temperature of operation
b) Less pressure drop
c) Reduced nitrogen oxide formation
d) Less excess oxygen required
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Explanation: Fluidized beds have the capacity of operating at lower temperatures of about 1000 degrees Celsius. Fluidized beds are also capable of providing sufficient contact to justify lesser oxygen requirement. These two conditions result in lower concentrations of the formation of oxides of nitrogen.
15. When is the control of stack gas emissions performed for the control of nitrogen oxides?
a) Always
b) When excess nitrogen oxides are formed
c) When modification of operating and design conditions is costly
d) When more than one nitrogen oxide is present in the exhaust
View Answer
Explanation: When the modification of operating and design conditions is not enough to curb the formation of nitrogen oxides, emission control is performed. Additionally, it is done when the modification is too expensive.
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