Principles of Nitrogen Oxides (NOx)
1. What are the hazards of nitrogen oxides (NOx)?
Answer:
- NO can paralyze the central nervous system and cause death, while NO₂ leads to asthma, emphysema, and damage to vital organs such as the heart, lungs, liver, kidneys, and hematopoietic system. Its toxicity is higher than NO. The maximum permissible concentration of NO, NO₂, or N₂O in air is 5 mg/m³ (calculated as NO₂).
- Like SO₂, NOx deposits onto the ground through dry and wet deposition, ultimately forming nitrate or nitric acid. Nitric acid rain is more harmful than sulfuric acid rain, as it causes water acidification, soil nutrient leaching, crop and forest damage, and corrosion of buildings and cultural artifacts. Although nitrate rain provides some beneficial nitrogen to the soil, its negative impacts outweigh the benefits, causing surface water eutrophication and ecological damage.
- A portion of atmospheric NOx enters the stratosphere, damaging the ozone layer, which results in ozone depletion or holes that adversely affect human life. Additionally, N₂O in NOx is a greenhouse gas contributing to global warming, with a greenhouse effect 200–300 times stronger than CO₂, despite its small quantity.
2. What factors influence NOx formation?
Answer:
NOx in boiler flue gas primarily originates from nitrogen in the fuel. Higher fuel nitrogen content generally leads to increased NOx emissions. Other influencing factors include:
- Fuel characteristics: The O/N ratio of coal influences NOx formation; a higher O/N ratio leads to more NOx. Similarly, the S/N ratio affects SO₂ and NOx emissions due to competition during oxidation.
- Excess air coefficient: Lowering the coefficient can limit oxygen concentration in the reaction zone, reducing NOx formation by 15–20%, although it may increase CO emissions and reduce combustion efficiency.
- Combustion temperature: Higher combustion temperatures increase NOx emissions.
- Boiler load rate: Higher load rates increase combustion temperature and NOx formation from volatile nitrogen compounds.
3. How can NOx emissions be controlled?
Answer:
Control methods address the fuel lifecycle in three stages: pre-combustion, during combustion, and post-combustion. Current research focuses on the latter two, termed primary (low-NOx combustion) and secondary measures (flue gas denitrification).
- Primary measures include low-NOx burners, air staging, and fuel staging combustion technologies.
- Secondary measures involve Selective Catalytic Reduction (SCR) and Selective Non-Catalytic Reduction (SNCR), or hybrid SCR/SNCR technologies.
4. What is low-NOx combustion technology?
Answer:
Low-NOx combustion reduces NOx formation by controlling combustion zone temperature and oxygen levels. Common technologies include:
- Combustion optimization: Adjusting air distribution and maintaining air-fuel balance to minimize NOx.
- Air staging: Dividing the combustion process into phases by using primary and secondary air supplies to create oxygen-deficient zones for lower NOx formation.
- Low-NOx burners: Specially designed to reduce oxygen concentration and temperature in ignition zones, cutting NOx emissions by 30–60%.
5. Features of SCR Denitrification Technology
Answer:
Definition: Selective Catalytic Reduction (SCR) involves injecting ammonia or other reductants into the flue gas upstream of a catalyst to convert NOx into nitrogen and water. This occurs at 200–450°C and is selective, reacting only with NOx.
Advantages: High NOx removal efficiency (>85%), compact equipment, and low operating temperatures.
Disadvantages: Catalyst poisoning by contaminants, high costs, and formation of ammonium sulfate and bisulfate, which can clog equipment.
6. Catalyst Selection in SCR Technology
Answer:
Key requirements for catalysts include high activity, long life, cost-effectiveness, and resistance to secondary pollution. Common catalysts include alkali metal-based types with titanium dioxide as the base, which are effective at 300–400°C and resistant to fouling by flue gas impurities.
7. Ensuring SCR Efficiency
Answer:
Maintaining efficiency requires proper NH₃/NOx molar ratios, regular catalyst replacement, and controlling ammonia slip to minimize unreacted ammonia and secondary pollutants.