Effect of ammonia on the soot properties in a laminar ethylene flame

Abstract

Ammonia, as a non-carbon fuel, has been regarded as an alternative fuel for combustion. In this investigation, the effect of ammonia on the soot properties such as morphology, nanostructure, graphitization degree, surface functional groups, and oxidation energy in a laminar ethylene flame has been studied using different detection techniques at 15 mm and 30 mm height above burner (HAB). The results show that the soot basically appears as a chain-like aggregate, which is composed of several tens of primary particles, and it also finds that the use of ammonia decreases diameters of primary particles, the diameter of A60 (ethylene with 60 ml/min ammonia) decreases by 28.0 % compared with that of A0 (ethylene without ammonia) at 30 mm HAB. Moreover, the addition of ammonia promotes the regularity degree of soot, slightly increases the mean separation distance at 30 mm HAB, and decreases the mean fringe tortuosity both of 15 mm and 30 mm HAB based on the results from high-resolution transmission electron microscope (HRTEM). Meanwhile, the ratio of ID1/IG, ID3/IG, and ID4/IG has been used to evaluate the degree of graphitization, it indicates that the use of ammonia increases the degree of graphitization, but both of the hybridization degree of sp3 and sp2 in amorphous carbon and carbon atoms decrease. Furthermore, the proportion of oxygenated functional groups decreases with the increase of ammonia. The highest content of C-OH functional group is about 9 % to 15 %, followed by C=O and O-C=O functional groups, which is about 3 % to 6 % and 2.5 % to 3.5 %, respectively. Eventually, the oxidation process of the particles has been analyzed, it is indicated that the use of ammonia decreases the oxidation activity, which is more difficult to oxidize.