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Flare management solutions

Hydrocarbon Engineering,

The new Maximum Achievable Control Technology (MACT) rule scheduled to be passed by the Environmental Protection Agency (EPA) in 2015 extends and strengthens the rules governing stationary emission sources defined in the NSPS JA. The new rules will likely allow refiners until November of 2018 to be in full compliance. Unlike the NSPS JA ruling, which only applied to new sources, either modified, installed or upgraded since 2012, and was primarily focused on Sulfur emissions, the MACT rule applies to all sources regardless of age. The key provisions of the new rule as it applies to flow measurement and flare burn efficiency include the following:

  • All flares must have a flow measurement sensitivity of +-10 ft3/min or 5% of the actual volumetric flow rate, whichever is greater (as in J and JA released in March 2012).
  • Flare tip velocities may not exceed 400 ft/sec., and flare tip velocity is subject limitations based on British Thermal Unit (BTU) content.
  • Flares must maintain a minimum combustion zone Net Heating Value (NHV) or a maximum combustion zone lower flammability limit or a combustibles concentration and report these values every 15 minutes.
  • Flares will need to control, maintain and demonstrate a 96.5% combustion efficiency or a 98% destruction efficiency.

Flare control

The basis for controlling the burn efficiency in flare began to take hold in the 1980s as the EPA concluded that flares that are properly designed and operated can result in combustion efficiencies in excess of 96.5%. Through extensive empirical field work, the EPA determined optimum levels of steam, air and combustible material content. Most flares in refineries today use some form of steam assistance to reduce smoking and promote mixing and cooling at the flare tip.

In many cases traditional flares were controlled by adding more steam to reduce smoking. Smoking was extinguished, however, the flare would be emitting dangerous volatile organic compounds (VOCs) into the environment, since combustion was not fully realized.In essence the new rule by the EPA is being adopted to define adequate controls for flares to assure that the flare is operating within the combustion zone and that combustion efficiencies approach the 98% target. Most of the field data gathered by the EPA to help define the rule, came from actual oil refineries in the US. These refineries volunteered to help define the new requirements that make up a properly designed and operated flare, by providing critical data on the operating parameters of the flare.

Controlling the combustion zone

Depending on the material being flared, a minimum BTU/ft3 must be maintained to assure that the flare maintains an adequate net heating value ( NHVcz) or a minimum lower flammability limit (LFLccz) or a minimum Hydrocarbon limit (Ccz)in the combustion zone.

When the following three conditions coexist; hydrogen concentrations exceed 1.2%, olefin concentrations exceed 2.5%, and the combined olefin/hydrogen concentration exceeds 7.4%, then the minimum NHZcz must be controlled to 380 BTU/ft3, or the LFLcz must be less than .11, or the Ccz must be greater than .23. If the flare also uses perimeter air assist, then additional rules apply within the combustion zone in order to maintain proper dilution levels for adequate combustion.

All other flaring conditions require the NHVcz to be greater than 270 BTU/ft3, or LFLcz less than .15, or a Ccz less than .18. If the flare also uses perimeter air assist, then NHVdil must be greater than 22 BTU/ft2 or LFLdil must be less than 2.2 or Cdil must be greater than .012.

If at any time all three of these thresholds is breached then make up gas needs to be added to boost the combustibles within the combustion zone or steam levels need to drop or both situations may apply in order to correctly maintain adequate combustion zone efficiency.

Flare tip velocity measurement and control

Flare tip velocity is another flare parameter that will fall under this new MACT rule. Flare tip velocities are being monitored and limited to assure that the flame does not ‘lift off’ the flare which could result in flame instability and improper combustion. Flare tip velocity is monitored and controlled relative to the overall NHV content in the flare. The lower the NHV content the lower the maximum allowable tip velocity.

GE Measurement and Controls offers a complete integrated solution for controlling a flare. The flow diagram below is a general layout of how a flare would need to be controlled in order to comply with the proposed MACT rule.

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