The Evolution of Safe, Environmentally Friendly Flares
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Flares have traditionally been thought of as a classic, end-of-pipe solution for waste gas disposal. In the early days of process engineering (the 1920s and '30s), most flares were little more than pipes in the sky - crude, smoky, bad-smelling devices, ignited by flaming arrows.
In the intervening years, extensive technology development has changed the picture. Refractory-lined burner tips, surrounded by continuously burning pilots (to instantly ignite flammable waste gases) became common. Large molecular seals were installed below those burners in an attempt to prevent air intrusion and thus keep combustible air/gas mixtures out of the open burners and elevated stacks. Other early attempts to improve safety and reliability included: development of dependable flamefront pilot-ignition systems; liquid seals to eliminate "burning rain" caused by liquid carry-over into elevated stacks; and improved methods for flare design, i.e., to prevent lazy flames from licking down the stacks of oversized flares. In the 1970s, a no-moving-parts kinetic seal was developed by NAO, known as the Fluidic Seal. It consists of a series of one-way baffles, built into the alloy tip of a refractory-free flare burner. A boundary-layer effect causes all intruding air to be turned back before it can penetrate into a flare burner or stack. Nearly 3,000 such burners have been installed throughout the process industries, either as part of a burner, or as a retrofit for elevated flares.
The flare as thermal oxidizer
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The latest evolution in flare technology is the ground-based, enclosed flare, which can be thought of either as a flare or as a specialized type of thermal oxidizer. The best of these designs can achieve destruction removal efficiencies of 99.9 percent or better; sound reduction to 60 dBA at 100 ft; and essentially no smoke or odor.
State-of-art enclosed flares are extremely safe, ultra-reliable, easy to service and environmentally friendly. They combine dependable, complete destruction of waste gases, offgases and watery low-energy slurries (or atomized sludges) - with no visible flame, no smoke, no odor and very low noise.
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Fig. 2 |
The flare design depicted in Fig. 2, for example, is being used in a densely populated area in Japan to handle eight different waste gases, plus a sour wastewater stream. They range from hydrogen-rich (89 percent) compounds to heavy, relatively incombustible offgas, comprising BTX compounds and cyclopentadiene. One of the streams, an H2S stripper gas, acts as the pilot fuel for the installation.
No steam or water injection Many current flare installations make use of steam or water injection to reduce smoke. Steam or water (and sometimes air injection) act to improve turbulence and mixing in the combustion zone, thereby achieving more-complete combustion and smokeless operation.
Steam and water injection has three significant disadvantages, however: cost, stability and maintainability. Providing steam at the right pressure and flowrate for a large industrial flare can add as much as $1 million/year to flare operating costs. The steam volume must be carefully matched to the volume of waste gas being handled; otherwise, the unit can become smoky (too little steam) or the flame itself can be quenched (too much steam). High-volume units, moreover, tend to pulsate with a jet-like drone. This droning noise is audible at large distances. If the steam or water condenses in the vicinity of hard refractory linings around the flare burner, it can crack the lining, creating a safety hazard and hindering performance.
To satisfy the most stringent environmental, noise and nuisance-control regulations, many small hidden burners replace a highly visible, and objectionable open burner; a low density ceramic replaces high-maintenance refractory linings; and a passive acoustic fence and gravel bed replace expensive, noisy and troublesome steam, water sprays and forced air blowers.
The soft ceramic insulation dampens (attenuates) and absorbs combustion-chamber pulsations. Additional features of the enclosed unit address safety concerns: the flare can be equipped with automatic interlocks, liquid seals, ultraviolet scanners (to ensure that a flame is present before waste gas is introduced), flame or detonation arrestors, and fire-suppression equipment.
The multi-tip, multi-jet enclosed flare is more expensive than a conventional elevated flare, but when operating costs, environmental compliance and related issues are considered, the payout period for the unit can be as short as two years. Thereafter, plant operators can take advantage of the inherent better performance of the unit through lower operating costs and better reliability.
By John F. Straitz III, P.E. President, NAO Inc.
FOR ADDITIONAL INFORMATION ABOUT ELEVATED FLARES OR LOW PROFILE ENCLOSED FLARES, contact NAO Inc., 1284 East Sedgley Ave., Philadelphia, PA 19134. Tel: 215.743.5300.