Fuel composition effect on flare flame inefficiency

The effects of fuel composition on the performance of flare flames have long been known. Possibly the first performance indicator was the tendency of a fuel to smoke. Various fuel characteristics such as average molecular weight and carbon-to-hydrogen ratio were advanced to guide the design of early smokeless flares (Baukal, 2001). The US EPA controls the emission of flare flames based on 40CFD60.18. This regulation is based on the landmark studies performed by Pohl and co-workers (1983-1986). This work measured the experimental flame stability as functions of heating value and flare exit velocity. The emissions from flare flames were then simply correlated with the flame stability limit (see Fig. 1). Above the flame stability the combustion efficiency of flames of all compounds tested was greater than 98%, near the flame stability limit the combustion efficiency decreased rapidly to 85 -90%. This program did not thoroughly investigate the effect of wind and wind structure, chemical composition. In particular, Walsh et al. (2002) showed mixtures of hydrogen were much more stable than mixtures of propane-nitrogen. Gogolek and Hayden (2002) showed that increased ratio of the wind energy to the flare jet energy also resulted in combustion inefficiency. Nobel, et al. (1984) made an attempt to correlate flame stability with a flame temperature and the ratio of the lower and upper flammability limits. Pohl, et al. (1984) used his data to confirm that this relationship also held for combustion efficiency (see Fig. 2). However, the proposed relationship is based on limited data, has considerable scatter, and small differences in the temperature ratio of flammability limits predict large changes in flame stability. A brief, up-to-date review of the flare efficiency literature is found in Seebold et al., (2003). Combustion inefficiency of a flare flame results from extinguished portions of the flame. These portions become extinguished when the pocket is diluted by mixing of air or steam through molecular diffusion, flame stretch, and turbulent mixing. When a pocket is diluted sufficiently it does not generate heat faster than it loses it, the flame pocket extinguishes itself. The flame can also be extinguished through dilution to the point where the flame speed is slower than the imposed velocity and the flame blows off. Finally, the flame can be extinguished by dilution beyond the lower flammability limit