Practical flare gas recovery for minimising flaring
Flares play a critical role as a safety device in refineries, and in most oil and gas producing or processing facilities.
Brian Blackwell, Trevor Leagas and Greg Seefeldt
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Flares provide a safe, effective means for the destruction of Volatile Organic Compounds (VOCs) and other gases during plant emergencies, upsets, and normal operating conditions. Historically, the flare has served as the vapour “sewer” for the facility, collecting and burning all of the unwanted gases and system leakage.
In recent years some refinery owners and operators decided to recover the potentially high heat-value gasses existing in their flare systems due to normal operations in lieu of flaring. Flare gas recovery offers some real and tangible benefits:
• Recovered flare gas can be re-used in plant process heater burners and boiler burners
• Reduces the amount of natural gas purchased by the facility
• Less purchased gas means a fairly short return on investment for the cost of the flare gas recovery system
• If the flare tip is an assist-type, employing flare gas recovery reduces the consumption of steam or air while increasing the life of the flare tip
Flare gas recovery provides some intangible benefits, as well. Reducing the amount of flaring overall reduces the visibility of the flare, improving public perceptions of the facility.
Flaring in the United States is addressed by the Environmental Protection Agency (EPA) in Section 111 of the Clean Air Act of 1970, which authorised the EPA to develop technology- based standards that apply to specific categories of stationary sources. These standards are referred to as New Source Performance Standards (NSPS) and are found in 40 CFR Part 60. Subpart J and Subpart Ja of the NSPS deal specifically with petroleum refineries. While NSPS Subpart Ja (40 CFR 60; §60.100a-§60.109a) has a much broader application than just the reduction or elimination of routine, or non-emergency, flaring, the standard does directly address reducing routine flaring overall.
Three applicability triggers in NSPS Subpart Ja are directly related to flare systems: construction, modification, and reconstruction: Construction is the erection of a new flare on or after June 24, 2008. Construction is, of course, any new flare built in a refinery.
Reconstruction is defined in the NSPS General Practices §60.15(b) as having occurred when the current-day capital cost of all flare-related capital projects over any two-year period is greater than 50% of the current-day capital cost to totally replace the flare with a comparable new flare.
Modification of a flare, for the purposes of meeting Subpart Ja, commences (1) when a project that includes any new piping from a refinery process unit, including ancillary equipment, or a fuel gas system is physically connected to the flare (e.g., for direct emergency relief or some form of continuous or intermittent venting); or (2) when a flare is physically altered to increase the flow capacity of the flare. However, only a few specific exceptions exist to the “new connections” passage to the flare rule, and refinery operators should familiarise themselves with the ruling or seek expert assistance.
Basically, if a refinery has constructed a new flare, reconstructed a flare, or modified its existing flare system since June 2008, NSPS Subpart Ja will apply to that flare.
• New or reconstructed flares must comply with all portions of Subpart Ja upon startup of the flare.
• Modified flares must comply as follows:
- 162 ppmv H2S (three-hour rolling average) limit at startup of the flare with exceptions allowed as follows: (1) modified flares not previously subject to the H2S limit in 40 CFR 60 Subpart J; (2) modified flares with monitoring alternative as defined in subpart Ja; or (3) flares complying with Subpart J as specified in a consent decree. In these cases the flare will need to comply with the 162 ppmv requirement by November 13, 2015.
- Compliance with all other portions of Subpart Ja by November 13, 2015.
- The 162 ppmv H2S limit does not apply to the combustion in the flare of process upset gases.
Subpart Ja also contains specific design, equipment, work practice, or operation standards that apply to any owner or operator of an affected flare. §60.103a states that each owner or operator who operates a flare subject to this subpart shall develop and implement a written flare management plan no later than November 11, 2015 or upon startup of the modified flare, whichever is later. Requirements for continuous flow monitoring, H2S concentration monitoring and Total Reduced Sulphur monitoring for any affected flare, whether constructed, reconstructed or modified are also a part of the rule. The continuous monitoring requirements in Subpart Ja present both a cost to refineries as well as potential cost savings over time. Once refinery operators can quantify the amount of gas being burned in the flare on a regular basis, they can identify cost-effective flare gas minimisation or recovery projects to improve operational profitability by capturing high energy-value gas. The H2S and Total Reduced Sulphur continuous monitoring is included for environmental reasons to reduce the occurrences of exceeding the 162 ppm H2S in a three-hour rolling average, or 500 lbs of S02 emitted in a 24-hour period.
While enforcement lies at the discretion of the regulator, (and penalties for failing to comply with the provisions of NSPS Subpart Ja are still not clear), the EPA’s “Enforcement Alert, EPA 325-F-012-002” was sent out in August 2012 announcing that penalties under the Clean Air Act for violations of US federal requirements can result in fines of as much as $37,500 per violation, per day.
Likely Outcomes of Flaring Regulations
While flaring and emissions control regulations vary worldwide, they are in general becoming more and more stringent. Even operators in areas not yet required to have flare mitigation plans or flare gas recovery options in place can benefit from the economic, environmental, and public perception benefits of reducing routine flaring through a flare gas recovery system. In the United States, the potential for costly fines or even plant shutdowns makes investigating this option a far better short-term time investment with the November 2015 compliance date looming. Other economic and intangible benefits for installing flare gas recovery include reduction in purchased gas for the refinery, reduced utility consumption by the flare, extension of the life of the flare tip, reduced noise produced by the flare during normal operations, reduced visibility of the flare by neighbours, and improved perception of the facility within the community.
Step by Step System Design Feasibility Study
In most situations, the initial step to introduce flare gas recovery is the completion of a feasibility study. This study should include an extended period of flare gas flow metering to help identify the true flow rates of gas to be recovered. Zeeco recommends a monitoring minimum time of three weeks, although engineers prefer to evaluate months’ or even years’ worth of flow data, if available.
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