Flare Control under the Refinery Sector Rule

Paper from the AFRC 2018 conference titled Flare Control under the Refinery Sector Rule

Petroleum refineries throughout the United States are less than a year away from having to comply with the new flare operating parameters contained in amendments to the Refinery Sector Rule. With these new operating parameters come new monitoring and control requirements. This presentation will examine the inherent complexities of flare control and the implementation challenges facing refiners as they race towards the January 2019 compliance date. The presentation will also review the design documentation required to be included in each refinery's flare management plan and the questions which may arise when refiners attempt to incorporate these values into flare control systems.

Stuck 1 Flare Control under the Refinery Sector Rule Derek Stuck, Spectrum Environmental Solutions American Flame Research Committee - Industrial Combustion Symposium Salt Lake City, Utah September 17-19, 2018 In 2015, the United States Environmental Protection Agency (USEPA) finalized regulations related to refinery flares, which included standards for improving combustion efficiency, along with requirements for parameters previously covered under other regulations. In order to comply with the new requirements, refineries throughout the United States have installed new instrumentation to allow flares affected by the new requirements to demonstrate compliance by January 30, 2019. Regulatory Background Amendments to the Refinery Sector Rule (RSR) were published by the USPEA on December 1, 2015. The changes to the RSR include amendments to both existing refinery Maximum Achievable Control Technology (MACT) rules, which are found in the National Emission Standards for Hazardous Air Pollutants (NESHAPs) Part 63 Subpart CC and Subpart UUU (referred to here as MACT CC and MACT UUU, respectively). Flares used to control emissions from sources covered by either MACT CC or MACT UUU are subject to the flare requirements found in §63.670 of MACT CC, which include the following: • • • • • Pilot flare monitoring and presence, Flare tip exit velocity, Visible emissions, Net heating value in the combustion zone (NHVcz), and Net heating value dilution parameter (NHVdil) for flares which receive perimeter assist air. Several of these requirements (specifically those related to tip velocity, pilot monitoring and presence, and visible emissions) were previously included in the General Provisions (Subpart A) of the both the MACT and New Source Performance Standard (NSPS) rules. In order to avoid duplicative requirements, MACT CC now includes the provision that if a refinery flare is in compliance with the amended requirements in MACT CC, that flare is no longer required to comply with the requirements of §63.11 of MACT Subpart A or §60.18 of NSPS Subpart A. In order to ensure sufficient combustion efficiency at the flare tip, MACT CC requires refiners to maintain the NHVcz at or above 270 British thermal units per standard cubic foot (Btu/scf)1. Flares receiving perimeter assist air must maintain the NHVdil at or above 22 British thermal units per square 1 40 CFR §63.670(e) Stuck 2 foot (Btu/ft2)2. As shown below in Equations 1 through 4, MACT CC provides two methods (direct and feed-forward) which may be used to demonstrate compliance with each of these operating parameters. Direct Compliance Calculation 𝑁𝐻𝑉𝑐𝑧 = 𝑁𝐻𝑉𝑑𝑖𝑙 = 𝑄𝑣𝑔 × 𝑁𝐻𝑉𝑣𝑔 𝑄𝑣𝑔 + 𝑄𝑠 + 𝑄𝑎,𝑝𝑟𝑒𝑚𝑖𝑥 𝑄𝑣𝑔 × 𝑁𝐻𝑉𝑣𝑔 × 𝐷𝑖𝑎𝑚 𝑄𝑣𝑔 + 𝑄𝑠 + 𝑄𝑎,𝑝𝑟𝑒𝑚𝑖𝑥 + 𝑄𝑎,𝑝𝑒𝑟𝑖𝑚𝑒𝑡𝑒𝑟 𝑬𝒒𝒖𝒂𝒕𝒊𝒐𝒏 𝟏 𝑬𝒒𝒖𝒂𝒕𝒊𝒐𝒏 𝟐 Feed Forward Compliance Calculation 𝑁𝐻𝑉𝑐𝑧 = 𝑁𝐻𝑉𝑑𝑖𝑙 = (𝑄𝑣𝑔 − 𝑄𝑁𝐺2 + 𝑄𝑁𝐺1 ) × 𝑁𝐻𝑉𝑣𝑔 + (𝑄𝑁𝐺2 − 𝑄𝑁𝐺1 ) × 𝑁𝐻𝑉𝑁𝐺 𝑄𝑣𝑔 + 𝑄𝑠 + 𝑄𝑎,𝑝𝑟𝑒𝑚𝑖𝑥 [(𝑄𝑣𝑔 − 𝑄𝑁𝐺2 + 𝑄𝑁𝐺1 ) × 𝑁𝐻𝑉𝑣𝑔 + (𝑄𝑁𝐺2 − 𝑄𝑁𝐺1 ) × 𝑁𝐻𝑉𝑁𝐺 ] × 𝐷𝑖𝑎𝑚 𝑄𝑣𝑔 + 𝑄𝑠 + 𝑄𝑎,𝑝𝑟𝑒𝑚𝑖𝑥 + 𝑄𝑎,𝑝𝑒𝑟𝑖𝑚𝑒𝑡𝑒𝑟 𝑬𝒒𝒖𝒂𝒕𝒊𝒐𝒏 𝟑 𝑬𝒒𝒖𝒂𝒕𝒊𝒐𝒏 𝟒 Where: NHVdil = Net heating value dilution parameter, Btu/ft2. NHVvg = Net heating value of flare vent gas determined for the 15-minute block period, Btu/scf. Qvg = Cumulative volumetric flow of flare vent gas during the 15-minute block period, scf. QNG2 = Cumulative volumetric flow of supplemental natural gas to the flare during the 15-minute block period, scf. QNG1 = Cumulative volumetric flow of supplemental natural gas to the flare during the previous 15minute block period, scf. NHVNG = Net heating value of supplemental natural gas to the flare for the 15-minute block period, Btu/scf. Diam = Effective diameter of the unobstructed area of the flare tip for flare vent gas flow, ft. Qs = Cumulative volumetric flow of total steam during the 15-minute block period, scf. Qa,premix = Cumulative volumetric flow of premix assist air during the 15-minute block period, scf. Qa,perimeter = Cumulative volumetric flow of perimeter assist air during the 15-minute block period, scf. MACT CC requires the installation of new monitoring instrumentation to measure the various parameters required by the compliance calculations. Although many refinery flares are already equipped with vent gas flow meters (typically as a result of either previous flare regulations or permitting requirements), fewer are equipped with assist gas or NHV/composition monitoring. This new monitoring forms the basis for not only the compliance calculations but also control scheme necessary for keeping the flare in compliance. 2 40 CFR §63.670(f) Stuck 3 Compliance calculations are performed for each 15-minute block period demonstrate that the NHVcz was at least 270 Btu/scf and the NHVdil (where applicable) was at least 22 Btu/ft2. Although the Rule specifies how refiners are to demonstrate compliance, specific control requirements are not included. Therefore, calculations to adjust the flow of supplemental gas in efforts to raise the NHVcz or NHVdil (if necessary) may occur on a more frequent basis. Additional control measures (such as ratio control of assist gas) may also be used to maintain compliance. Selecting the Right Instrumentation for Flare Control Unlike boilers, heaters, and many other process units, flares are not expected to operate at constant conditions. Instead, flares typically handle a small fraction of their total capacity, while maintaining the ability handle a maximum relief scenario with little to no warning. These dramatic swings in operation can present challenges for control due to the required flexibility of the instruments monitoring the flare. Flow meters must be able to measure as low as 0.1 feet per second and as high as hundreds of feet per second across a wide range of potential compositions. Composition monitors must measure a wide range of compounds and quickly report results to enable facilities to make adjustments to maintain compliance with the operating limits. Since few flares are designed with the level of instrumentation now required by MACT CC in mind, each flare typically presents a unique set of challenges related to its physical design. Straight runs of pipe for flow meters and good sampling points for composition monitoring are often difficult to identify because the flare was not designed to accommodate them. With that in mind, every flare must be approached as a unique challenge, as no two flares are identical. MACT CC requires flow monitoring for vent gas, assist gas, and, potentially, supplemental gas. • Vent Gas Flow Monitoring As mentioned above, the flow of vent gas to a flare is subject to wide variations in both magnitude and composition. These variations impact which technologies can be used. The changes in magnitude necessitate flow meters with a wide turndown ratio, while the composition changes can affect the accuracy and signal strength of the flow meter. Although assist gas flow measurement is not subject to the same changes in composition as the vent gas, assist gas (such as steam or air) can also see wide ranges of flow, although not as severe as those seen in the vent gas. Accuracy requirements dictated by regulations can also impact which flow monitoring technologies may be used for a given system. For example, MACT CC requires steam flow meters to be accurate to within +/-5% over the normal range of flows. Special consideration must be given to what the "normal range" of flow is and what technology can accurately measure across that range. • Vent Gas Composition When considering instrumentation to determine the net heating value in the vent gas (NHV vg), the first decision comes down to how much information is required. If only heat content is required, a calorimeter may be sufficient. However, in many cases, knowing the composition of the vent Stuck 4 gas may offer additional operational benefits to the facility. For instance, monitoring specific compounds may allow operators to identify sources contributing to the flare and facilitate corrective actions to minimize flaring. When knowing the composition would be beneficial, technologies such as gas chromatographs (GC) and mass spectrometers rise to the forefront of the decision-making process. The desire to know what is actually in the gas being flared must also be balanced with how quickly that knowledge must be available to implement control of the flare. MACT CC requires flares to demonstrate compliance every 15 minutes, which may be problematic if an instrument (such as a GC) only reports back a value one or two times during each 15-minute block. Since compliance is determined in 15-minute blocks, control must happen at a faster interval. As with flow monitoring technologies, the sample point location for composition monitors must also be carefully considered. When choosing the sample point, facilities should consider whether the instrument will see gas that is representative of the gas reaching the flare tip. If gas (such as supplemental gas or purge gas) is added downstream of the sampling location, additional monitoring may be required to account for this additional gas in the combustion zone. Flare Control Challenges under the Refinery Sector Rule Although MACT CC will dictate how refinery flares demonstrate compliance, no specific requirements are established for how that compliance is maintained. Refiners have therefore chosen to follow a number of different paths to building their flare control systems. Many have chosen to directly program the controls into their existing distributed control system (DCS). Third-party systems have been developed to handle the control calculations and provide the DCS with an input to establish the setpoint for any required supplemental gas. Neither of these approaches is inherently better than the other, and the decision is typically driven by site-specific factors such as staffing and time constraints. One of the principle concerns for any flare control system is the speed with which the system is able to respond to changes in flow of composition of the flare vent gas. Instrumentation will play a significant role in determining how fast a facility can respond to a low Btu flaring event; however, instrument lag created by slow response times can be exacerbated by other factors, including location of supplemental gas injection. Depending on where supplemental gas is added, slower instruments may require two analytical cycles to reflect the supplemental gas addition, at which point conditions in the flare likely no longer reflect those at the time the supplemental gas was added. Instrument reliability can also potentially play havoc with a flare control system. For instance, ultrasonic flow meters rely on a number of independent sensors (flow velocity, temperature, and pressure) to measure the flow rate of gas. Should any one of those sensors, the validity of the data provided by the is called into question. Composition monitors must undergo routine quality assurance checks to insure they are operating properly. MACT CC has specific thresholds (two times the accuracy requirements specified in the rule) at which an instrument becomes out of control. From a compliance standpoint, if an instrument is "out-of-control" cannot be used to demonstrate compliance. However, just because the data may not be valid from a compliance standpoint, doesn't Stuck 5 mean it should be ignored from a control standpoint. Data, even out-of-control data, is better than no data when it comes to flare control. Future Flare Control Schemes In recent years, several different approaches to directly monitoring the combustion efficiency of a flare have been introduced. Some, such as passive Fourier transform infrared (PFTIR) spectroscopy and direct combustion zone sampling, have been used repeatedly by both the USEPA and the petrochemical industry to study flare combustion efficiency. Several additional approaches have sought to bridge the gap between direct monitoring and regulatory compliance. There are several issues, however, with directly monitoring the combustion efficiency and complying with the requirements of MACT CC. First, it is unclear how a facility would know how to respond to a low measured combustion efficiency without the instrumentation already required by MACT CC. Since a decrease in combustion efficiency may be the result of any number of factors (low NHVvg, low vent gas flow, high assist gas flow, etc.), a facility would still need to know these parameters in order to make an informed decision on how best to control the flare. Additionally, any approach which would seek to bypass the monitoring requirements of MACT CC would be required to submit an Alternative Means of Emissions Limitation (AMEL) to the USEPA. Any AMEL would likely require significant side-by-side comparisons of the direct combustion zone monitoring and the required flare instrumentation. Due to the unique nature of many flare systems, this approach may be required for every flare.