The migration and transformation mechanism of N in ammonia/coal volatile co-combustion: Experimental and quantum chemical calculation

It is very important to understand the NO formation characteristics and N conversion mechanism of ammonia/coal volatile combustion for low nitrogen combustion during ammonia-coal co-combustion. In this work, constant temperature furnace experiment and quantum chemistry calculation were used to study the migration and transformation characteristics of N for ammonia/coal volatile co-combustion. The experimental results showed that the ammonia blending ratio and temperature have significant effects on the formation of NO in volatile combustion. When the ammonia ratio was less than 10 %, volatile combustion preceded ammonia combustion, and unburned ammonia reduced part of NO at high temperature, so that the stable concentration of NO in ammonia/volatile combustion increased first and then decreased with the temperature increasing. The combustion performance of ammonia was enhanced with the further increase of ammonia blending ratio, and the stable concentration of NO in ammonia/volatile combustion gradually increased with the temperature increasing. The theoretical calculation showed that the ammonia/volatile co-combustion system firstly oxidized ammonia-N, and then oxidized C and N in the volatile. The addition of ammonia reduced the rate-limiting step barrier value of volatile-N oxidation about 53.11 kJ/mol or 99.11 kJ/mol, and promoted the formation of N-containing oxidation products in co-combustion system. The kinetic results showed that the rate-limiting step reaction rate of ammonia/volatile co-combustion system was about 2–4 orders of magnitude higher than that of pure volatile oxidation, and the formation rate of NO gradually increased with the increase of temperature. The theoretical calculation confirmed the experimental phenomenon and revealed the molecular mechanism of N conversion in ammonia/volatile combustion system. • Ammonia blending ratio and temperature have significant effects on NO formationin volatile combustion. • Volatile burnt prior to ammonia combustion when the ammonia co-firng ratio was less than 10 %. • Ammonia combustion occurred first at a high ammonia blending ratio in ammonia/volatile co-firing system. • Ammonia reduced the energy barrier value of the rate-limiting step of volatile-N oxidation by about 50 or 100 kJ/mol. • The reaction rate of the rate-limiting step in ammonia/volatile co-firing was significantly higher than that of the volatile.