EPA-452/F-03-019 

# Air Pollution Control Technology Fact Sheet

EPA-CICA Fact Sheet Flare 

### 1 Name of Technology : Flare This includes elevated flares, steam-assisted flares , air-assisted flares, non-assisted flares, pressureassisted flares, and enclosed ground flares. Type of Technology: Destruction by thermal oxidation. Applicable Pollutants: Volatile organic compounds (VOC), with the exception of halogenated compounds (EPA, 1995).

Achievable Emission Limits/Reductions: VOC destruction efficiency depends upon an adequate flame te mperature, sufficient residence time in the combustion zone, and turbulent mixing (EPA, 1992). A properly operated flare can achieve a destruction efficiency of 98 percent or greater when controlling emission streams with heatcontents greater than 11 megajoules per standard cubic meter (MJ/sm 3 ) (300 British thermal units per standard cubic foot (Btu/scf)) (EPA, 1995; AWMA, 1992; EPA, 1992; EPA, 1991). Applicable Source Type: Point Typical Industrial Applications: Flares can be used to control almost any VOC stream , and can typically handle large fluctuations in VOC concentration, flow rate, heating value, and inert spec ies content. Flaring is appropriate for continuous, batch, and variable flow vent stream applications, but the primary use is that of a safety device used to control a large volume of pollutant resulting from ups et conditions. Flares find their primary application in the petroleum and petrochemical industries. The majo rity of chemical plants and refineries have existing flare systems designed to relieve emergency process upset s that require release of large volumes of gas. These large diameter flares are designed to handle em ergency releases, but can also be used to control vent streams from various process operations. Ga ses flared from refineries, petroleum production, and the chemical industry are composed largely of lo w molecular weight VOC and have high heating values. Flares used to control waste gases from blast furnaces c onsist of inert species and carbon monoxide with a low heating value. Gases flared from coke ovens are intermediate in composition to the other two groups and have a moderate heating value (EPA, 1995; EPA, 1992).

Emission Stream Characteristics:

a. Air Flow: The flow rate through the flare is dependent upon the properties of the waste gas stream and the configuration of the flare. St eam-, air-, and pressure-assisted flares add flow to the waste stream in order to improve flame stab ility. In cases where the heating value of the waste gas is too low or too high, auxiliary f uel or additional air must be added to the flow, respectively. The maximum flow through comme rcially available flares is about 500 standard cubic meters per second (sm 3/sec) (1,060,000 standard cubic feetper minute (scfm)), and the minimum can approach zero flow (EPA, 1995).EPA-CICA Fact Sheet Flare 

### 2 b. Temperature: The discharge temperature is typically in the range of 500 to 1100°C (1000 to 2000°F), depending upon the composition of the waste gas flow (AWMA, 1992). c. Pollutant Loading: Depending upon the type of flare conf iguration (e.g., elevated or ground flares) and the source of the waste stream, the c apacity of flares to tr eat waste gases can vary up to about 50,000 kilograms per hour (kg/hr) (100,000 pounds per hour (lb/hr)) of hydrocarbon gases for ground flares and about 1 million kg/hr (2 million lb/hr) or more for elevated flares (EPA, 1991). Flares are not subject to the sa fety concern of incinerators regarding having a high concentration of organics in the waste gas. This is because flaring is an open combustion process and does not have an enclosed combusti on chamber that can create an explosive environment. Incinerators, however, have an enclosed combustion chamber, which requires that the concentration of the waste gas be subst antially below the lower flammable level (lower explosive limit, or LEL) of the specific co mpound being controlled to avoid the potential for explosion (as a rule, a safety factor offour (i.e., 25% of the LEL) is used).

d. Other Considerations: The waste gas stream must hav e a heating value of greater than 11 MJ/scm (300 Btu/scf). If this minimum is not met by the waste gas, auxiliary fuel must be introduced in sufficient quantity to make up the difference (EPA, 1995). Emission Stream Pretreatment Requirements: Liquids that may be in the vent stream gas or t hat may condense out in the collection header and transfer lines are removed by a knock-out drum. The knock-out or disentrainment drum is typically either a horizontal or vertical vessel located at or close to t he base of the flare, or a vertical vessel located inside the base of the flare stack. Li quid in the vent stream can exti nguish the flame or cause irregular combustion and smoking. In addition, flaring liquids can generate a spray of burning chemicals that could reach ground level and create a safety hazard (EPA, 1995).

Cost Information: Typical elevated flares are primarilysafety devices which prevent the emissions of large quantities of raw unburned hydrocarbons during plant upset conditions. The capital costs of elevated flare systems can range from $10,000 to $3,000,000, depending upon the applic ation (Gonzalez, 1999). The controlling factors in the cost of the flare are the basic support structure of the flare, the size and height, and the auxiliary equipment. Other factors influencing the co st are the degree of sophistication desired (i.e., manual vs. automatic control) and the number of appurtenances selected, such as knock-out drums, seals, controls, ladders, and platforms. The minimum fl are diameter is 2.5 centimeters (cm) (1 inch); the maximum flare diameter currently commercially av ailable is 2.3 meters (90 inches). (EPA, 1996) Operating costs for an elevated flare depend largely upon the design of the flare (e.g., a steam-assisted flare will require steam), the flow rate (this will det ermine the diameter of the flare tip), and the heating value of the gas to be controlled (this will be a fact or in determining the height of the flare and the amount of auxiliary natural gas required to achieve the desired destruction te mperature) (EPA, 1996). The following are cost ranges (expressed in 2002 dollars) for elevated steam-assisted flares of conventional design under typical operating c onditions, developed using EPA cost-estimating spreadsheets (EPA, 1996) and referenced to the volumetric flow rate of the waste stream treated. Costs were calculated for flares with tips between 2.5 cm (2 in) and 2.3 m (90 in) in diameter, burning 100 percent combustible waste gas (no air) with a heat content of approximately 4000 kcal/m 3(450 Btu/scf), and operated between 1 and 100 hours per year. Flares in the lower end of the capital, operating &EPA-CICA Fact Sheet Flare 

### 3 maintenance, and annualized cost ranges have hi gher flow capacity (approximately 90 m 3/s or 190,000 scfm), with a flare tip diameter of up to 2.3 m (90 in), and operate 100 hours per year or more. The higher end of the cost ranges have lowe r flow capacity (approximately 0.01 m 3/s, or 24 scfm), flare tip diameters as small as 2.5 cm (1 inch ), and operate fewer than ten hours per year. Because flares are primarily safe ty devices which deal with flows of short duration (generally an upset condition or an accidental release from a process) rather than a control device which treats a continuous waste stream, it is not entirely appropriate to compar e the cost effectiveness of flares to other control devices. Cost per ton of pollutant contro lled largely depends upon the annual hours of operation. Infrequent use of the flare (approximately ten hours per y ear) will result in greater cost per ton of pollutant controlled., while more frequent use (approximately 100 hours per year) is represented by the lower costs per ton of pollutant controlled in the ranges presented below. a. Capital Cost: $27,000 to $4,000,000 per sm 3/sec ($13 to $21,000 per scfm)

b. O & M Cost: $2,000 to $20,000 per sm 3/sec ($1 to $10 per scfm), annually

c. Annualized Cost: $6,000 to $650,000 per sm 3/sec ($3 to $300 per scfm), annually

d. Cost Effectiveness: $17 to $6,500 per metric ton ( $15 to $5,800 per short ton), annualized cost per ton per year of pollutant controlled

Theory of Operation: Flaring is a VOC combustion control process in wh ich the VOC are piped to a remote, usually elevated, location and burned in an open flame in the open air usi ng a specially designed burner tip, auxiliary fuel, and steam or air to promote mixing for nearly comp lete (> 98%) VOC destruction. Completeness of combustion in a flare is governed by flame temperatur e, residence time in the combustion zone, turbulent mixing of the gas stream components to complete t he oxidation reaction, and available oxygen for free radical formation. Combustion is complete if all VOC are converted to carbon dioxide and water. Incomplete combustion results in some of the VOC being unaltered or converted to other organic compounds such as aldehydes or acids. Flares are generally categorized in tw o ways: (1) by the height of the flare tip (i.e., ground or elevated), and (2) by the method of enhancing mixing at the flar e tip (i.e., steam-assisted, air-assisted, pressureassisted, or non-assisted). Elevating the flare can prevent potentially dangerous conditions at ground level where the open flame (i.e., an ignition source) is located near a process unit. Elevating the flare also allows the products of combustion to be dis persed above working areas to reduce the effects of noise, heat, smoke, and objectionable odors. In most flares, combustion occurs by means of a di ffusion flame. A diffusion flame is one in which air diffuses across the boundary of the fuel/combustion product stream toward the center of the fuel flow, forming the envelope of a combustible gas mixture around a core of fuel gas. This mixture, on ignition, establishes a stable flame zone around the gas core abov e the burner tip. This inner gas core is heated by diffusion of hot combustion products from the flame zone. Cracking can occur with the formation of small hot parti cles of carbon that give the flame its characteristic luminosity. If there is an oxygen deficiency and if t he carbon particles are cooled to below their ignition temperature, smoking occurs. In large diffusion flames, combustion product vortices can form around burning portions of the gas and shut off the supply of oxygen. This localized instability causes flameEPA-CICA Fact Sheet Flare 

### 4 flickering, which can be accompanied by soot formati on. As in all combustion processes, an adequate air supply and good mixing are required to complete com bustion and minimize smoke. The various flare designs differ primarily in their accomplishment of mixing. Steam-assisted flares are single burner tips, elevated above ground level for safety reasons, that burn the vented gas in a diffusion flame. They reportedly acc ount for the majority of the flares installed and are the predominant flare type found in refineries and c hemical plants. To ensure an adequate air supply and good mixing, this type of flare system injects steam into the combustion zone to promote turbulence for mixing and to induce air into the flame. Some flares use forced air to provide the combusti on air and the mixing required for smokeless operation. These flares are built with a spider-shaped burner (w ith many small gas orifices) located inside but near the top of a steel cylinder 0.6 meters (24 inches) or more in diameter. Combustion air is provided by a fan in the bottom of the cylinder. The amount of co mbustion air can be varied by varying the fan speed. The principal advantage of air-assisted flares is that they can be used where steam is not available. Although air assistance is not usually used on large fl ares (because it is gener ally not economical when the gas volume is large) the number of lar ge air-assisted flares being built is increasing. The non-assisted flare consists of a flare tip withoutany auxiliary provision for enhancing the mixing of air into its flame. Its use is limited to gas stream s that have a low heat cont ent and a low carbon/hydrogen ratio that burn readily without producing smoke. T hese streams require less air for complete combustion, have lower combustion temperatures that minimi ze cracking reactions, and are more resistant to cracking.

Pressure-assisted flares use the ventstream pressure to promote mixing at the burner tip. Several vendors now market proprietary, high pressure drop burnertip designs. If sufficient vent stream pressure is available, these flares can be applied to streams pr eviously requiring steam or air assist for smokeless operation. Pressure-assisted flares generally ( but not necessarily) have the burner arrangement at ground level, and consequently, must be located in a re mote area of the plant where there is plenty of space available. They have multiple burner heads that are staged to operate based on the quantity of gas being released. The size, design, number , and group arrangement of the burner heads depend on the vent gas characteristics.

An enclosed flare's burner heads are in side a shell that is internally in sulated. The shell reduces noise, luminosity, and heat radiation and provides wind pr otection. Enclosed, orground-based flares are generally used instead of elevated flar es for aesthetic or safety reas ons. A high nozzle pressure drop is usually adequate to provide the mixing necessary for smokeless operation and air or steam assistance is not required. In this context, enc losed flares can be considered a special class of pressure-assisted or non-assisted flares. The height must be adequate for creating enough draft to supply sufficient air for smokeless combustion and for dispersion of the thermal plume. These flares are always at ground level. Enclosed flares generally have less capacity than open flares and are used to combust continuous, constant flow vent streams, al though reliable and efficient operation can be attained over a wide range of design capacity. Stable combustion can be obtained with lower heat content vent gases than is possible with open flare designs (1.9 to 2.2 MJ/sm 3(50 to 60 Btu/scf)), probably due to their isolation from wind effects. Enclosed flares are typically us ed at landfills to destroy landfill gas. (EPA, 1995) Advantages: Advantages of flares over other types ofVOC oxidizers include (EPA, 1992; EPA, 1991):EPA-CICA Fact Sheet Flare 

### 5 1. Can be an economical way to dispose of sudden releases of large amounts of gas; 2. In many cases do not require auxiliary fuel to support combustion; and 3. Can be used to control intermitt ent or fluctuating waste streams. 12. Disadvantages: Disadvantages of flares include (EPA, 1995): d. Can produce undesirable noise, smoke, heat radiation, and light; e. Can be a source of SO x , NO x , and CO; f. Cannot be used to treat waste streams with halogenated compounds; and g. Released heat from combustion is lost.

Other Considerations: Flaring is considered as a control option when t he heating value of the emission stream cannot be recovered because of uncertain of intermittent flow as in process upsets of emergencies. If the waste gas has a heating value high enough to sustain combustion (i.e. greater than 11 MJ/sm 3or 300 Btu/scf), the stream may serve as a fuel gas for an inciner ator if one is employed at the site (EPA, 1991).

References: AWMA, 1992. Air & Waste Management Associ ation, Air Pollution Engineering Manual . Van Nostrand Reinhold, New York. EPA, 1991. U.S. EPA, Office of Research and Deve lopment, “Control Technologies for Hazardous Air Pollutants,” EPA/625/6-91/014, Washington, D.C., June. EPA, 1992. U.S. EPA, Office of Air Quality Pl anning and Standards, “Control Techniques for Volatile Organic Emissions from Stationary Sources,” EPA-453/R-92-018, Research Triangle Park, NC., December. EPA, 1995. U.S. EPA, Office of Air Quality Planni ng and Standards, “Survey of Control Technologies for Low Concentration Organic Vapor Gas Streams,” EPA- 456/R-95-003, Research Triangle Park, NC., May. EPA, 1996. U.S. EPA, Office of Air Quality Pl anning and Standards, “OAQPS Control Cost Manual,” Fifth Edition, EPA 453/B-96-001, Research Triangle Park, NC. February. Gonzalez, 1999. Steve Gonzalez, Kaldair, Inc., Houston, Texas, (800) 525-3247, personal communications with Eric Albright, April 15 and 16, 1999.