4/15 Miscellaneous Sources 13.5

- 1

13.5 Industrial Flares

13.5.1 General

Flaring is a high

- temperature oxidation process used to burn combustible components, mostly hydrocarbons, of waste gases from indus trial operations. Natural gas, propane, ethylene, propylene, butadiene and butane constitute over 95 percent of the waste gases flared. In combustion, gaseous hydrocarbons react with atmospheric oxygen to form carbon dioxide (CO 2 ) and water. In some was te gases, carbon monoxide (CO) is the major combustible component. Presented below, as an example, is the combustion reaction of propane.

C 3 H 8 + 5 O 2 > 3 CO 2 + 4 H 2 O

During a combustion reaction, several intermediate products are formed, and eventu ally, most are

converted to CO 2 and water. Some quantities of stable intermediate products such as carbon monoxide, hydrogen, and hydrocarbons will escape as emissions.

Flares are used extensively to dispose of (1) purged and wasted products from refi neries, (2) unrecoverable gases emerging with oil from oil wells, (3) vented gases from blast furnaces, (4) unused gases from coke ovens, and (5) gaseous wastes from chemical industries. Gases flared from refineries, petroleum production, chemical industr ies, and to some extent, from coke ovens, are composed largely of low molecular weight hydrocarbons with high heating value. Blast furnace flare gases are largely of inert species and CO, with low heating value. Flares are also used for burning waste gas es generated by sewage digesters, coal gasification, rocket engine testing, nuclear power plants with sodium/water heat exchangers, heavy water plants, and ammonia fertilizer plants.

There are two types of flares, elevated and ground flares. Elevated fl ares, the more common type, have larger capacities than ground flares. In elevated flares, a waste gas stream is fed through a stack anywhere from 10 to over 100 meters tall and is combusted at the tip of the stack. The flame is exposed to atmospheric di sturbances such as wind and precipitation. In ground flares, combustion takes place at ground level. Ground flares vary in complexity, and they may consist either of conventional flare burners with no enclosures or of multiple burners in refractory

- lined steel enclosures.

The typical flare system consists of (1) a gas collection header and piping for collecting gases from processing units, (2) a knockout drum (disentrainment drum) to remove and store condensables and entrained liquids, (3) a proprietary seal, water seal, or purge gas supply to prevent flash

- back, (4) a single

or multiple

- burner unit and a flare stack, (5) gas pilots and an ignitor to ignite the mixture of waste gas and air, and, if required, (6) a provision for external momentum force ( steam injection or forced air) for smokeless flaring. Natural gas, fuel gas, inert gas, or nitrogen can be used as purge gas. Figure 13.5

- 1 is a diagram of a typical steam

- assisted elevated smokeless flare system.

Combustion requires three ingredients: fuel, an oxidizing agent (typically oxygen in air), and heat (or ignition source). Flares typically operate with pilot flames to provide the ignition source, and they use ambient air as the oxidizing agent. The waste gases to be flared typically provide t he fuel necessary for

combustion. C ombustible gases generally have an upper and lower flammability limit. The upper flammability limit (UFL) is the highest concentration of a gas in air that is capable of burning. Above this flammability limit, the fuel is too rich to burn. The lower flammability limit (LFL) is the lowest concentration of the gas in air that is capable of burning

. Below the LFL, the fuel is too lean to burn. Between the upper and lower flammability limits, combustion can occur. Flare waste gases with13.5

- 2 EMISSION S FACTORS 4/15

concentrations above the UFL will become more dilute as the waste gas mixes with ambient air above the flare tip. As this dilution occurs, the air

- waste gas mixture will pass through the flammability region

, and

combustion will occur. Ho wever, if flare waste gas concentrations are near the LFL prior to mixing with air, the air

- waste gas mixture can fall below the flammability region

, and reduced combustion efficiencies

can occur. If steam is added to the flare waste gas at or prior to th e flare tip (i.e., prior to the “combustion zone” where the mixing with air occurs), the steam will act to dilute the waste gas. Thus, even if there are adequate concentrations of combustibles in the waste gas

, if too much steam is added to the waste gas so that the combustibles concentration becomes diluted to near the LFL as the steam

- waste gas mixture

enters the combustion zone

, reduced combustion efficiencies will result. Consequently, critical

consideration s of flare combustion include the net heatin g value and the combustibles concentration in the flare gas and in the combustion zone (e.g.

, accounting for the amount of dilution by steam or other assist

gas that occurs to the waste gas prior to the combustion zone )

.

Figure 13.5

- 1. Diagram of a t ypical steam

- assisted smokeless elevated flare.

Combustion efficiency is the percentage of hydro carbon in the flare vent gas that is completely

converted to CO 2 and water vapor . Destruction efficiency is the percentage of a specific pollutant in the

fla re vent gas that is converted to a different compound (such as CO 2 , CO or other hydrocarbon intermediate). The destruction efficiency of a flare will always be greater than the combustion efficiency of a flare. It is generally estimated that a combustion efficiency of 96.5 percent is equivalent to a destruction efficiency of 98 percent. 10

Smoking may result from combustion, depending upon waste gas components and the quantity and distribution of combustion air. Waste gases containing methane, hydrogen, CO, and ammonia4/15 Miscellaneous Sources 13.5

- 3

usually burn without smoke. Waste gases containing heavy hydrocarbons such as paraffins above methane, olefins, and aromatics, have a higher tendency to smoke. An external momentum force, such as steam injection or blowing air, is used fo r efficient air/waste gas mixing and turbulence, which promotes smokeless flaring of heavy hydrocarbon waste gas. Other external forces may be used for this purpose, including water spray, high velocity vortex action, or natural gas. External momentum fo rce is rarely required in ground flares.

Steam injection is accomplished either by nozzles on an external ring around the top of the flare tip or by a single nozzle located concentrically within the tip. At installations where waste gas flow varies, bot h are used. The internal nozzle provides steam at low waste gas flow rates, and the external jets are used with large waste gas flow rates. Several other special

- purpose flare tips are commercially available, one of which is for injecting both steam and air.

F lares are generally designed to handle large quantities of waste gases that may be intermittently generated during plant emergencies

, although they may also be used routinely to dispose of low

- volume

continuous or intermittent emissions from variou s sources at the plant . Flare gas volumes can vary from a few cubic meters per hour during regular operations up to several thousand cubic meters per hour during major upsets. Flow rates at a refinery could be 45 to 90 kilograms per hour (kg/hr) (100

- 2 00 pounds per hour [lb/hr]) during regular operation but could reach a full plant emergency rate of 700 megagrams per hour (Mg/hr) (750 tons/hr). Normal process blowdowns may release 450 to 900 kg/hr (1000

- 2000 lb/hr), and unit maintenance or minor fail ures may release 25 to 35 Mg/hr (27

- 39 tons/hr). Th us, th e required flare turndown ratio can be over 15,000 to 1.

Many plants have 2 or more flares, in parallel or in series. In the former, 1 flare can be shut down

for maintenance while the other ser ves the system. In systems of flares in series, 1 flare is intended to handle regular gas volumes and the other flare is generally intended to handle excess gas flows from emergencies.

13.5.2 Emissions

Noise

, heat , and visible flame and/or smoke are t he most apparent undesirable effects of flare operation. Flares are usually located away from populated areas or are sufficiently isolated, thus minimizing their effects on populations.

Emissions from flaring may include carbon particles (soot), unburne d hydrocarbons, CO, and partially burned and altered hydrocarbons. Also emitted are nitrogen oxides ( NO

x ) and, if sulfur

containing material such as hydrogen sulfide or mercaptans is flared, sulfur dioxide (SO 2 ). The quantities of hydrocarbon emissions g enerated relate to the degree of combustion. The degree of combustion depends largely on the rate and extent of fuel

- air mixing and on the flame temperatures achieved and maintained. Properly operated flares achieve at least 98 percent destruction effici ency in the flare plume, meaning that hydrocarbon emissions amount to less than 2 percent of the hydrocarbons in the gas stream.

The tendency of a fuel to smoke or make soot is influenced by fuel characteristics and by the amount and distribution of oxyg en in the combustion zone. For complete combustion, at least the stoichiometric amount of oxygen must be provided in the combustion zone. The theoretical amount of oxygen required increases with the molecular weight of the gas burned. The oxygen supplie d as air ranges from 9.6 units of air per unit of methane to 38.3 units of air per unit of pentane, by volume. Air is supplied to the flame as primary air and secondary air. Primary air is mixed with the gas before combustion, whereas secondary air is dr awn into the flame. For smokeless combustion, sufficient primary air must be supplied, this varying from about 20 percent of stoichiometric air for a paraffin to about 30 percent for an olefin. If the amount of primary air is insufficient, the gases ente ring the base of the flame are preheated by the combustion zone, and larger hydrocarbon molecules crack to form13.5

- 4 EMISSION S FACTORS 4/15

hydrogen, unsaturated hydrocarbons, and carbon. The carbon particles may escape further combustion

and cool down to form soot or smoke. Olefin s and other unsaturated hydrocarbons may polymerize to form larger molecules which crack, in turn forming more carbon.

The fuel characteristics influencing soot formation include the carbon

- to

- hydrogen (C

- to

- H) ratio and the molecular structure of the ga ses to be burned. All hydrocarbons above methane, i. e., those with a

C

- to

- H ratio of greater than 0.33, tend to soot. Branched chain paraffins smoke more readily than corresponding normal isomers. The more highly branched the paraffin, the greater the tendency to smoke. Unsaturated hydrocarbons tend more toward soot formation than do saturated ones. Soot is eliminated by adding steam or air; hence, most industrial flares are steam

- assisted and some are air

assisted. Flare gas composition is a critica l factor in determining the amount of steam necessary.

Since flares do not lend themselves to conventional emission testing techniques, until recently only a few attempts have been made to characterize flare emissions. Early EPA tests using propylene as flare gas indicated that efficiencies of 98 percent can be achieved when burning an offgas with at least 11,200 kJ/m 3 (300 Btu/ft 3 ). 1 However, recent studies on flare performance using passive Fourier Transform Infrared (pFTIR) spectroscopy have been per formed on a number of different flares

. 4

- 8 The studies cover a number of flares at different industrial sources with different waste gas compositions. The pFTIR studies support the conclusion that the combustion zone properties of the steam

- waste gas mi xture are predictive of proper flare combustion

. 10 There have also been recent studies on sources, including flares, using differential infrared absorption LIDAR [light detection and ranging] (DIAL). To date, many of these studies do not provide the data necessary to isolate the emissions from a particular flare. But enough data existed in one study that the emissions measured by DIAL could be attributed to the flare. 9

Table 13.5

- 1 presents flare emission s factors from the EPA tests 1 ; Table 13.5

- 2 pres ents flare

emission s factors from pFTIR and DIAL studies

. 4

- 9 Crude propylene was used as flare gas during the early EPA tests. Methane was a major fraction of hydrocarbons in the flare emissions, and acetylene was the dominant intermediate hydrocarbon sp ecies. Many other reports on flares indicate that acetylene is always formed as a stable intermediate product. The acetylene formed in the combustion reactions may react further with hydrocarbon radicals to form polyacetylenes followed by polycyclic hydr ocarbons. 2 Typical refinery waste gas feeds were used as flare gas during the pFTIR and DIAL studies.

In flaring waste gases containing no nitrogen compounds, NO is formed either by the fixation of atmospheric nitrogen (N) with oxygen (O) or by the rea ction between the hydrocarbon radicals present in the combustion products and atmospheric nitrogen, by way of the intermediate stages, HCN, CN, and

OCN. 2 Sulfur compounds contained in a flare gas stream are converted to SO 2 when burned. The amount

of SO 2 emitted depends directly on the quantity of sulfur in the flared gases.4/15 Miscellaneous Sources 13.5

- 5

Table 13.5

- 1 (English Units). THC AND SOOT EMISSION S FACTORS FOR FLARE OPERATIONS a

EMISSION S FACTOR RATING: B

Pollutant SCC Emissions Factor

Value Emissions Factor

Units

Tot al hydrocarbons b 30190099 0.14 lb/10 6 Btu

Nitrogen oxides c 30190099 0.068 lb/10 6 Btu

Soot c 30190099 0

- 274 μ g/L

a Reference 1. Based on tests using crude propylene containing 80% propylene and 20% propane.

b Measured as methane equivalent.

c Soot in concentration values: nonsmoking flares, 0 micrograms per liter ( μ g/L); lightly smoking flares,

40 μ g/L; avera ge smoking flares, 177 μ g/L; and heavily smoking flares, 274 μ g/L.

Table 13.5

- 2 (English Units). VOC and CO EMISSION S FACTORS FOR FLARE OPERATIONS a

Pollutant SCC Emissions Factor (lb/10 6 Btu) Representativeness

Volatile organic compounds b 30190099;

30 600904 0.5 7 Poorly

Carbon monoxide c 30190099;

30600904 0.3 1 Poorly

a These factors apply to well operated flares achieving at least 98% destruction efficiency and operating in compliance with the current General Provisions requirements of 40 CFR Part 60, i.e. >300 btu/scf net heating value in the vent gas and less than the specified maximum flare tip velocity. These factors are

based on steam

- assisted and air

- assisted flares burning a variety of vent gases.

b Reference s 4

- 9 and 11

.

c References 1

, 4

- 8 and 11

.13.5

- 6 EMISSION S FACTORS 4/15

References f

or Section 13.5

1. Flare Efficiency Study , EPA

- 600/2

- 83

- 052, U. S. Environmental Protection Agency, Cincinnati, OH, July 198 3.

2. K. D. Siegel, Degree Of Conversion Of Flare Gas In Refinery High Flares , Dissertation, University of Karlsruhe, Karlsruhe, Germany, February 1980.

3. Manual On Disposal Of Refinery Wastes, Volume On Atmospheric Emissions , API Publication 931, American Petroleum Institute, Washington, DC, June 1977.

4. TCE Q 2010 Flare Study Final Report. Texas Commission on Environmental Quality Tracking

No. 2008

- 81. Austin, Texas. The University of Texas at Austin Center for Energy and Environmental Resources. August 2011.

5. Performance Test of a Steam

- Assisted Elevated Flare with Passive FTIR

- Detroit. Marathon Petroleum Company, LP. Detroit, Michigan. Clean Air Engineering, Inc. November 2010.

6

. PFTIR Test of Steam

- Assisted Elevated Flares

- Port Arthur. Flint Hills Resources Port Arthur, LLC. Port Arthur, Texas. Clean Air Engineering, Inc. June 2011.

7

. Deer Park Refinery East Property Flare Performance Test Report. Shell Deer Parking Refining LP. Deer Park, Texas. Shell Global Solutions (US) Inc. April 2011.

8

. Passive Fourier Transform Infrared Technology (FTIR) Evaluation of P001 Process Control Device. INEOS ABS (USA) Corporation. Addyston, Ohio. INEOS ABS (USA) Corporation. July

2010.

9

. Differential Absorption Lidar Study

– Final Report . Texas Commission o n Environmental Quality. Austin , Texas. March 2010

.

10

. Parameters for Properly Designed and Operated Flares

. Prepared for U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, Research Triangle Park, NC. April 2012

.

11. Pe rformance Test of a Steam

- Assisted Elevated Flare with Passive FTIR. Marathon Petroleum Company, LLC. Texas City, Texas. Clean Air Engineering, Inc. May 2010.