Mechanisms of N₂O Formation from Char Combustion

Abstract

This paper describes an experimental study on N2O and NO x emission from char combustion. Experiments have been carried out in a fixed bed reactor in the temperature range of 973−1323 K. Oxygen concentrations for combustion, temperature, and char type have been found to have strong effects on both N2O and NO x emissions. With an increasing temperature, N2O reaches a peak at 1073 K and then decreases with temperature. At the same time NO x remains nearly the same. When oxygen concentration increases, the concentration of N2O increases and NO x first increases and then decreases. The char with higher nitrogen content emits more N2O, indicating that nitrogen oxides come from char-N. When NO is included in the inlet gases, much more N2O is produced from char combustion. This suggests that the reaction of NO + O2 + char is the main pathway for N2O formation. Another pathway, i.e., the homogeneous oxidation of HCN from the gasfication of char, seems to be important as well.

Simultaneous thermogravimetric–mass spectrometric study on the pyrolysis behaviour of different rank coals

“…As in this work the experiments were carried out in an inert atmosphere, the main reduction reactions will occur via homogeneous mechanisms. N 2 O and N 2 , reduced species that contribute to m/z 14, will be produced from the oxidation of HCN by NO in the gas phase [30].…”

Nitrogen oxides emission from a circulating fluidized bed combustor

“…The results of the latter investigators include not only char-N oxidation, but also char-N conversion by other reactions, for example, the reaction of NO with char-N. NO has been found to improve the conversion of char-N to N,O (Tullin et al 1993;Feng et al 1994b). So, during the combustion of char in fluidized beds, N,O formation can be split into two parts: one from the direct oxidation of char-N (de Soete, 1990;Feng et al 1994a), the other from the reaction of NO with char-N, (Tullin et al 1993;Feng et al 1994b). …”

“…A rough indication of the contribution of the above mechanisms to N,O emission is also given in Figure 1. The estimate is based on the experimental results, (Feng et al 1994b). For this coal (GL coal in Feng et al, 1994b), more than 60% of the N,O emission comes from char combustion.…”

“…As discussed by Feng et al (1994b), this is because coals with a high content of carbon more easily produce (-CNO) sites, which then react with NO and (-CN) to release N,O by mechanism 2 and 3.…”

Effects of flue gas re-circulation and nitrogen contents in coal on NOX emissions under oxy-fuel coal combustion

“…In general, however, N 2 O formation becomes larger at a lower combustion temperature in CFB. It has been reported that the reaction of NO and char-N with O 2 becomes the main pathway for N 2 O formation when NO is included in the inlet gases [13].…”