Figure 5.21.5 shows an example of an enclosed flare, as used in landfill gas disposal. A multiple-head, gas burner is mounted at ground level inside a refractory-lined, combustion chamber that is open at the top. Enclosed flares have better combustion efficiencies than elevated flares, and emission testing is more readily performed. Typical minimum performance parameters include 0.3 sec residence time at 1000°F and CO < 100 ppm. Most refinery flares are elevated and steam assisted (see Figure 5.21.6). The steam promotes turbulence, and the induction of air into the flare improves combustion. The amount of steam required depends partly on the C/H ratio of the VOCs to be destroyed; a high ratio requires more steam to prevent a smoking flare.
U.S. EPA studies (Joseph et al. 1983; McCrillis 1988) identify several parameters important to flare design: flare head design, flare exit velocity, VOC heating value, and whether the flame is assisted by steam or air. They acquired the data from a specially constructed flare test facility. For a given flare head design, they found that the flame stability (the limit of flame stability is reached when the flame propagation speed is exceeded by the gas velocity) is a function of the fuel (VOC), gas velocity at the flare tip, and the lower heating value (LHV) of the fuel. For a given velocity, a minimum LHV is required for stable combustion. A combustion efficiency of >98% is maintained as long as the ratio of the LHV to the minimum LHV required for stable combustion is >1.2. Below that value, the DRE drops rapidly.
Gas (VOC) composition has a major influence on the stability limit. Figure 5.21.7 illustrates this influence with the results from the EPA flare test facility. Particularly interesting is the difference between methyl chloride (MeCl), propane (Prop), and butadiene (But) at a given exit velocity. The figure shows that with most gases, an increase in the flare exit velocity must be accompanied by an in-
Enclosed flare combustion chamber
Landfill gas from collection wells and header system
Exhaust Gas (1500° F) 63,179 acfm
Landfill Gas 5 Thermocouple 'nlet
Was this article helpful?