The effect of ferrocene addition on particle formation and burnout in combustion processes

Abstract

THE EFFECT OF FERROCENE ADDITION ON PARTICLE FORMATION AND BURNOUT IN COMBUSTION PROCESSES M. KASPER’, K. SATTLER2, K. SIEGMANN’ and U.MATTER’ ’ ETH-Zurich, Laboratory for Combustion Aerosols and Suspended Particles, 8093 Zurich, Switzerland; 2 University of Hawaii, Department of Physics and Astronomy, Watanabe Hall / 2505 Correa Road, Honolulu Hawaii 96822, USA KEYWORDS Combustion Aerosol; Fuel Additive: Ferrocene; Aerosol Photoemission; Size Distributions The soot suppressing properties of ferrocene (Fe(CsH5)2) are well-known from different combustion systems (Howard and Kausch 1980). Various studies provide evidence that ferrocene reduces carbonaceous matter in combustion by more efficient burnout rather than by inhibition of soot formation (e.g. Zhang and Megaridis 1996). In this work we show that in a ferrocene doped diffusion flame, iron oxide indeed nucleates before soot inception and subsequently serves as soot formation nuclei. This process has been postulated earlier (Ritrievi et al. 1987) but is now directly observed. With the technique of photoelectric charging of the particles in their natural gaseous environment (Burtscher 1992), we obtain a characteristic signal of the chemical state of the particle surface as it depends on the time spent in the combustion zone. Furthermore, we show that the transition metal (iron) reappears in the form of very fine respiratory oxide nuclei in the exhaust. Fig. 1 shows particle size distributions from laminar methane diffusion flames with and without ferrocene vapor added to the fuel gas. In the flame with ferrocene vapor, the first particles appear earlier than in the unseeded flame. These particles are not carbonaceous soot particles, because their photoelectric activity is much lower than that of soot particles sampled slightly above the soot inception point. Most likely, these are iron oxide particles formed from the Fe-ions liberated when the ferrocene molecules are cracked. These iron oxide particles increase their photoelectric activity as combustion proceeds approaching the photoelectric activity of the carbonaceous soot particles after the soot inception point. This shows that the iron oxide particles act as condensation nuclei for the carbonaceous particles as their surface cannot be distinguished any more from the one of the genuine soot particles. After the soot burnout at the tip of the flame, the iron oxide particles reappear with their original low photoelectric activity. The fast drop of the photoelectric activity at the burnout of the soot particles suggests that the carbonaceous matter that condensed on the iron oxide nuclei surface was burnt. Results obtained with an acetylene diffusion flame show that iron oxide incorporated in the soot particles acts as catalyst to promote soot burnout at the tip of the flame. Above the flame the iron oxide particles reappear in the exhaust. The phenomena observed in the model experiment are compared to the more complicated and less transparent case of the real diesel engine.