Flares Smokeless

Air-assisted flare test (a) blower off, (b) blower starting up, (c) blower speed increasing, and (d) blower at full speed and flare smokeless. (From Schwartz, R., White, J., and Bussman, W., The John Zink Combustion Handbook, Boca Raton, FL CRC Press, 2001.)... 

For environmental reasons, burning should be smokeless. Long-chain and unsaturated hydrocarbons crack in the flame producing soot. Steam injection helps to produce clean burning by eliminating carbon through the water gas reaction. The quantity of steam required can be as high as 0.05—0.3 kg steam per kg of gas burned. A multijet flare can also be used in which the gas bums from a number of small nozzles parallel to radiant refractory rods which provide a hot surface catalytic effect to aid combustion. 

Vapor recovery systems floating roof tanks pressure tanks vapor balance painting tanks white Cyclones-precipitator-CO boiler cyclones-water scrubber multiple cyclones Vapor recovery vapor incineration Smokeless flares-gas recovery... 

Flares ideally bum waste gas completely and smokelessly. Two types of flares are normally employed. The first is called the open flare, whereas the second is called the enclosed flare. The major components of a flare consist of the burner, stack. 

Elevated Flares See Flares for a general definition. The elevated flare, by the use of steam injection and effective tip design, operates as a smokeless combustion device. Flaring generally is of low luminosity up to about 20 % of maximum flaring load. Steam injection tends to introduce a source of noise to the operation, and a compromise between smoke elimination and noise is usually necessary. When adequately elevated (by means of a stack) this type of flare displays the best dispersion characteristics for malodorous and toxic combustion products. Visual and noise pollution often creates nuisance problems. Capital and operating costs tend to be high, and an appreciable plant area can be rendered unavailable for plant operations and equipment because of excessive radiant heat. 

Smokeless operation can generally be achieved, with essentially no noise or luminosity problems, provided that the design gas rate to the flare is not exceeded. However, since the flame is near ground level, dispersion of stack releases is poor and this may result in severe air pollution or hazard if the combustion products are toxic or in the event of flame-out. Capital and operating cost and maintenance requirements are high. 

Smokeless Center Steam Cheapest steam-injection flare tip. Steam jet emerges at high velocity and penetrates to the exit plane of the flare without mixing completely with flare gas. Results are intense steam noise (much greater than with steam ring for the same steam rate) and higher steam consumption than the steam ring. 

Non- Smokeless Utility or Field Flare Cheapest flare tip. Produces trailing smoke even with namral gas. 

Air Flow - The capacity of a multijet flare to induce air flow must be calculated, to make sure that it is adequate to meet the maximum air flow requirement for smokeless combustion. (W, of Equation 4 below must be > W, of Equation 5). The term air flow capacity refers to the primary air flow rate which will be induced around each jet, and may be estimated from the following equation ... 

The primary air flow rate per jet necessary for smokeless combustion depends on the molecular weight and degree of unsaturation of the flare gas. Experience indicates that it varies linearly with percent unsaturates, from a minimum of 20 % excess air for a flare gas containing 0 % unsaturates to 35 % excess air for a gas containing 67 mol % unsaturates. Based on this relationship and a gas flow rate of 72.2 mVh per jet, the required primary air flow rate can be computed directly from the gas composition, or approximated conservatively from the following equation ... 

Steam Injection - While the multijet flare will achieve a significant reduction in the smoke produced, it does not provide true smokeless combustion over its full operating range. This is particularly true with the heavier (C4+) and unsamrated gases. Steam injection at a rate of about 0.5 kg steam per kg of gas will provide an additional reduction in smoke for most gases. Steam should be... 

Figure 11. Continued - Other typical smokeless elevated flare tip designs.

When winterizing is required, the steam tracing and insulation should include the first 7.5 m of the flare stack above the vapor inlet and also in the case of a drum seal, the vapor line from the seal drum to the flare. Where steam injection to one of the smokeless tips, as shown in Figures 11 B and C is used, then the steam ring should remain outside the top of the flare tip (i.e., not internal). Where severe ambient conditions are encountered then it is recommended that the entire seal drum and flare be insulated in addition to steam tracing and open steam injection at base of flare. 

The flare shall provide smokeless combustion of gases up to the relief rates shown on the data sheet. 

A peak velocity through the flare end (tip) of as much as 0.5 mach is generally considered a peak, short term. A more normal steady state velocity of 0.2 mach is for normal conditions and prevents flare/lift off [58]. Smokeless (with steam injection) flare should be sized for conditions of operating smokelessly, which means vapor flow plus steam flow [33c]. Pressure drops across the tip of the flare have been used satisfactorily up to 2 psi. It is important not to be too low and get flashback (without a molecular seal) or blowoff where the flame blow s off the tip (see Ref. 57), Figure 7-71. 

For non-smokeless flares (no steam injection) about 30% higher capacity can be allowed [59]. Therefore, the diameter of a non-smokeless flare stack is approximately (0.85) (diameter of the smokeless flare stack). 

Figure 7-71. Flare stack arrangement for smokeless burning and backflash protection with Fluidic Seal molecular seal. Steam can be injected into the flare to introduce air to the fuel by use of jets inside the stream and around the periphery. By permission, Straitz, J. F. Ill, Make the Flare Protect the Environment," Hydrocarbon Processing, Oct. 1977, p. 131.

Gibson, R. O. and Vinson, D. J., Design and Installation of Smokeless Flare Systems for Gasoline Plants, ASME, Paper No. 72-Pet-I2, 1972. 

Eppig, The Development of a Smokeless Illu-minant Composition For The T-24 Flare ,, PATR 1527 (1945) 11) W.F. Ehert, Smith s... 

Smokeless flaring is required by law in the United States (40 CFR 60.18, Chap. 1) for normal process flares (continuous flaring). However, smokeless flaring is not required by the EPA for emergency flaring, but local conditions and regulations may require smokeless flaring. 

Smokeless operation not needed for emergency flares (only required for continuous flares). 

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