De-Nitrogen Oxides (De-NOx) Technologies: Overview and Applications Nitrogen oxides (NOx), including nitric oxide (NO) and nitrogen dioxide (NO₂), are harmful pollutants produced primarily during high-temperature combustion processes in industries, power plants, and vehicles. NOx emissions contribute to smog, acid rain, respiratory diseases, and ozone layer depletion. To mitigate these environmental and health impacts, various De-NOx technologies have been developed to reduce or eliminate NOx emissions. 1. Selective Catalytic Reduction (SCR) SCR is one of the most effective and widely used De-NOx methods. It involves injecting a reducing agent, typically ammonia (NH₃) or urea (CO(NH₂)₂), into exhaust gases. The gases then pass over a catalyst (e.g., vanadium-tungsten-titanium oxides), where NOx reacts with the reducing agent to form harmless nitrogen (N₂) and water (H₂O). SCR systems achieve NOx reduction efficiencies of up to 90% and are commonly applied in power plants, industrial boilers, and marine engines. 2. Selective Non-Catalytic Reduction (SNCR) SNCR is a simpler, lower-cost alternative to SCR, suitable for medium-temperature applications (900–1100°C). In this process, ammonia or urea is injected directly into the combustion chamber or flue gas stream, where it reacts with NOx to form N₂ and H₂O. However, SNCR has lower efficiency (30–70%) and requires precise temperature control to avoid ammonia slip (unreacted NH₃ emissions). It is often used in smaller industrial units or as a supplementary measure. 3. Low-NOx Burners (LNB) and Combustion Modification Preventing NOx formation at the source is another key strategy. Low-NOx burners optimize air-fuel mixing and combustion conditions to reduce peak flame temperatures, minimizing thermal NOx production. Techniques like flue gas recirculation (FGR) dilute oxygen concentrations and lower combustion temperatures. These methods are cost-effective but typically achieve moderate NOx reductions (30–50%). 4. Adsorption and Absorption Methods Some systems use adsorbents (e.g., activated carbon, zeolites) or liquid absorbents (e.g., alkaline solutions) to capture NOx from exhaust streams. While less common for large-scale applications, these methods are useful in niche scenarios where other technologies are impractical. 5. Emerging Technologies Research continues into advanced De-NOx solutions, such as plasma-assisted catalysis, photocatalytic oxidation, and biofiltration. These innovations aim to improve efficiency, reduce energy consumption, and address challenges like low-temperature NOx removal. Conclusion De-NOx technologies play a critical role in reducing air pollution and meeting regulatory standards. SCR and SNCR dominate industrial applications, while combustion modifications are favored for their simplicity. The choice of technology depends on factors like cost, efficiency, and operational conditions. Continued advancements will further enhance NOx control, supporting global efforts toward cleaner air and sustainable industrial practices.