A simplified simulation of the reaction mechanism of NO_x formation and non-catalytic reduction

ABSTRACT

During fossil fuel combustion, pollutants, such as NO_x, SO₂, CO, CO₂, organic compounds and fly ash are produced. Taking into consideration that emission limits have been becoming stricter it is crucial to apply technologies that reduce pollutant formation. This work focuses on NO_x formation and its consequent emission reduction via SNCR technology. A mathematical model based on the kinetic description of NO_x production and its non-catalytic reduction for a boiler operating under specified conditions was developed. A large number of chemical reactions take place during NO_x formation and reduction inside the boiler reduction zone. In this paper various important reactions that have significant influence on the SNCR process were selected. Based on the selected reactions a simplified SNCR reaction mechanism was assembled and converted into a numerical model. The model was applied for a denitrification process taking place in the temperature range 850–1050°C. Urea was used as reducing agent. Input gas contained NO in the order of 10^–5 molar fraction. Other components of input gas were 6.6 mole% water vapour, 13 mole% CO₂, 4 mole% O₂, 0.3 mole% CO, 0.05 mole% H₂ and the balance being N₂. Residence time was 0–2 s. The developed model makes possible to define the reducing zone in different types of boiler while using various reducing agents as well as to predict the degree of denitrification. As a result it is possible to optimize SNCR for any given boiler. The results obtained from model calculations demonstrated that the developed reaction mechanism of NO_x formation and non-catalytic reduction can be applied.

KEY WORDS:

• NO_x Emission
• Combustion
• Denitrification
• SNCR
• Reaction Mechanism