Heterogeneous formation of nitrous oxide from char-bound nitrogen

Abstract

It is generally recognized [1–4] that the reaction NO + (CNO) ▪ N 2O + (CO) is the most probable heterogeneous way of forming N 2O from char-nitrogen. During the combustion of char, the main heterogeneous reactions of nitrogen appear to be: NO,out +O 2 +(−CNO) (−CN) (−CNO) NO N 2O N 2O,out (IV) (V) (VII) +(−C) (VIII) +(−C)↓ N 2 N 2,out (VI) This work aims to: (i) determine the rate constant, k 7, of the reaction NO + (CNO) ▪ N 2O + (CO) at the temperatures of fluidized bed combustors and (ii) discuss the kinetic aspects of the fractional conversion of char-nitrogen into N 2O and NO, as well as the relative importance of the two competing reaction routes leading to reduction of NO to N 2. mProper measurement of k 7 requires the concomitant determination of the rate constants of reactions (V), (VI), and (VIII). Furthermore, since reaction (V) is linked to the oxidation of carbon in reaction (IV), the rate constants for oxygen adsorption on both (C) sites and on (CN) sites and the desorption constants of CO and CO 2 from (CO) sites have to be determined as well. For this, a time-resolved study was performed in the transient state created by suddenly removing oxygen during the combustion of char. Values of k 7 have been determined in this way for two chars. For “Prosper,” burning at a total pressure of 100 kPa, k 7 = 1.1 × 10 2 exp(−9350/ T) (g Ng C −1Pa −1); for the other (“Westerholt”) char, k 7 = 1.6 × 10 2 exp(−14,300/ T) (g Ng C −1 Pa −1) at a total pressure of 200 kPa. mThe rates of production of nitrogen, normalized by the oxygen adsorption rate on (CN) (i.e., step IV), were derived for one char and show interesting features: (i) the fraction of char-nitrogen converted to N 2O is strongly dependent on the combustion temperature and increases up to 1100 K; (ii) analysis of the triple trade-off between N 2O, NO, and N 2 shows that, in the range 900–1050 K, the major part of N 2 production from NO passes indirectly via reaction (VII), rather than the direct route, reaction (VI).