Tiny particles (< 107 #m) of seven coals have been added continuously to flat, premixed flames of H 2 + 02 + Ar with temperatures at or above 2000 K. Every flame contained excess 02, so that, e.g., carbon ends up totally as CO 2. The first volatiles burned on a particle's exterior, but subsequent ones did so in a diffusion flame surrounding each particle. The resulting concentrations of NO x and CO 2 were measured along the axis of each flame. Some coals produced volatiles with an N/C ratio that increased only slightly with time. Other coals gave volatiles with an unchanging ratio of N/C. Most coals yielded more CO 2 after devolatilization under these conditions than expected. However, quite strikingly, 57% + 5% of the nitrogen contained originally in the volatiles always appeared as NO or NO 2; the rest of the nitrogen ended up as N 2. This constancy of the yields of NO x and N 2 (from N originally bound in the volatiles) held despite variations of: coal type, particle size, 02 concentration in the flame gases, the extent of devolatilization, flame temperature, and the nitrogen content of the volatiles. By way of explanation, it seems that N 2 might appear to be a primary product of pyrolysis along with HCN and NHi species. When the diluent Ar was replaced by N2, -20% of the NO x observed during devolatilization originated from fixation of this N 2. Such an extra contribution to the observed NO x increased with the volatile content of the coal and was also larger for smaller particles. These observations can be explained in terms of the temperatures of the flame front around each devolatilizing particle.
NOMENCLATURE
Bi Blot number (= hd/6kc) d effective diameter of a coal particle (m) h heat transfer coefficient for heat transfer between a devolatilizing coal particle and its surrounding gases (Wm -z K-l ) kc thermal conductivity of a coal particle (W m-lK-l) kf thermal conductivity of the gases around a devolatilizing coal particle (W m-lK -l)