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Module 6: Air Pollutants and Control Techniques - Nitrogen Oxides - Practice Problems
- Instructions:
- Answer these questions on a sheet of paper and check your responses against those provided below.
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#1
- What types of sources emit nitrogen oxides (NOx)?
- Answer: Combustion sources such as automobiles, boilers, and incinerators, as well as other high-temperature industrial operations such as metallurgical furnaces, blast furnaces, plasma furnaces, and kilns emit nitrogen oxides. Other sources include nitric acid plants and other types of industrial processes involving the use and/or generation of nitric acid.
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#2
- Describe the characteristics of the NOx compounds NO and NO2.
-
- Answer: Nitric oxide (NO) is a colorless and odorless gas, which is essentially insoluble in water. Nitrogen dioxide (NO2) usually exists as a dimer compound (N2O4) at low temperatures such as those that occur in the ambient air. This dimer compound has a distinctly reddish-brown color, which contributes to the brown haze that occurs with smog. Nitrogen dioxide is moderately soluble in aqueous liquids. Both NOx compounds are toxic.
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#3
- Why are nitrogen oxide emissions regulated if ambient concentrations of NOx usually remain below levels considered to be harmful?
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- Answer: Ambient NOx concentrations are low primarily because NO and NO2 react relatively rapidly with other compounds once they are emitted into the atmosphere. Nitrogen oxide emissions are regulated to suppress these atmospheric reactions, which create ozone and other compounds that are associated with adverse health effects.
Practice Problems
Formation Mechanisms
- Instructions:
- Answer these questions on a sheet of paper and check your responses against those provided below.
-
#1
- Which NOx compound is emitted from boilers and incinerators at the highest concentration?
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- Answer: Nitric oxide (NO)
- Approximately 90 to 95% of NOx compounds generated in combustion processes are in the form of nitric oxide (NO). Once in the atmosphere, NO reacts in a variety of photochemical and thermal reactions to form NO2.
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#2
- What are the two sources of nitrogen for the formation of NOx compounds?
-
- Answer: Atmospheric nitrogen and nitrogen compounds found in the fuel or waste material.
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#3
- Nitrogen oxides generated from atmospheric nitrogen are called ____________ and nitrogen oxides generated from fuel or waste are called ____________.
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- Answer: Thermal NOx, fuel NOx
- Nitrogen oxides generated from atmospheric nitrogen are called thermal NOx and nitrogen oxides generated from fuel or waste are called fuel NOx.
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#4
- Explain how combustion and incineration processes generate NOx compounds.
-
- Answer: During the normal process of burning fuel in the high temperature zones of combustion chambers, ambient air, which consists of approximately 79% nitrogen, enters the combustion chamber. At these high temperatures, the nitrogen gas reacts with oxygen present in the air to form nitrogen oxides, primarily in the form of nitric oxide (NO). Nitric oxide that reaches the atmosphere reacts in a variety of photochemical and thermal reactions to form nitrogen dioxide (NO2). Through similar formation mechanisms, nitrogen oxides can also be generated from nitrogen compounds contained in the fuel or waste that is burned, although a significant fraction of the fuel nitrogen remains in the bottom ash or the fly ash.
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#5
- What set of conditions in the combustion zone influence the amount of NOx produced?
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- Answer: The complex set of reactions responsible for nitrogen oxides generation are very sensitive to high oxygen concentrations and high gas temperatures in the combustion zone.
Practice Problems
Control Techniques
- Instructions:
- Answer these questions on a sheet of paper and check your responses against those provided below.
-
#1
- What are the two general approaches primarily used to control nitrogen oxide emissions?
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- Answer: The two general approaches are: (1) combustion modifications to suppress the formation of nitrogen oxides, and (2) add-on controls to reduce nitrogen oxides to molecular nitrogen.
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#2
- Which types of control techniques can be used to prevent the formation
of NOx emissions from combustion processes? Select all that
apply.
- Off-stoichiometric combustion
- Dry scrubbers (dry injection)
- SNCRs
- SNRs
- Off-stoichiometric combustion
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- Answer: a. Off-stoichiometric combustion
- Off-stoichiometric combustion is one technique for limiting the production of NOx. Other techniques discussed in the lesson are low excess air operation, flue gas recirculation, and natural gas reburning.
- SCRs and SNCRs are add-on control systems for NOx. They remove NOx after it has already been formed.
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#3
- What is the typical NOx suppression efficiency of combustion
modifications in fossil-fuel-fired boilers?
- 5 to 10%
- 10 to 30%
- 30 to 50%
- 50 to 90%
- 5 to 10%
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- Answer: c. 30 to 50%
- The typical NOx suppression efficiency of combustion modifications in fossil-fuel-fired boilers is 30 to 50%.
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#4
- Which of the following reagents is (are) typically used in NOx
add-on control systems? Select all that apply.
- Ammonia
- Calcium hydroxide
- Alkalis
- Urea
- Ammonia
-
- Answer:
- a. Ammonia
- d. Urea
- Ammonia and urea are typically used as reagents in NOx add-on control systems.
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#5
- Describe the similarities and differences between SNCR and SCR control techniques.
-
- Answer: Similarities: Both types of systems inject ammonia or urea into the gas stream to reduce nitrogen oxides to molecular nitrogen and water.
- Differences: In SNCR systems, the reducing gases (ammonia or urea) are injected into a very hot gas zone where thermal reactions leading to the reduction of nitrogen oxides can occur. SCR systems utilize a catalyst to reduce the nitrogen oxides to nitrogen and therefore can operate at a lower temperature. The catalysts are usually composed of tungsten or vanadium deposited in a substrate, which is extruded into a honeycomb arrangement. The gas stream passes through channels in the honeycomb. Two or three catalyst beds are usually arranged in series. The two systems vary in their typical NOx reduction efficiency (see questions below).
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#6
- What is the typical NOx reduction efficiency of SNCR
systems?
- 5 to 20%
- 20 to 60%
- 60 to 80%
- 80 to 90%
- 5 to 20%
-
- Answer: b. 20 to 60%
- The typical NOx reduction efficiency of SNCR systems is 20 to 60%.
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#7
- What is the typical NOx reduction efficiency of SCR
systems?
- 5 to 20%
- 20 to 50%
- 50 to 75%
- 75 to 90%
- 5 to 20%
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- Answer: d. 75 to 90%
- The typical NOx reduction efficiency of SCR systems is 75 to 90%.