FROM: Jeff Coburn, RTI International

TO: Andrew Bouchard and Brenda Shine, EPA/OAQPS

DATE: December 12, 2013

SUBJECT: Petroleum Refinery Sector Rule: Operating Limits for Flares

1. Purpose

This memorandum summarizes the data and analysis used to identify appropriate operating limits for flares.

2. Background

Flares used as a control device subject to national emissions standards for hazardous air pollutants (NESHAP) are required to meet certain de sign and operating require ments as specified in 40 CFR 63.11. The available data at the tim e suggested that flares meeting these design and operating requirements would achieve a minimu m destruction efficiency of no less than 98 percent emissions control. With the advent of activities to reduce th e amount of flaring at refineries, more refinery fl ares are commonly operated at high turndown (minimum flow conditions). Recent evidence through Passive Fourier Transform Infrared spectroscopy (PFTIR) testing suggests that steam- and air-assisted flares can have a significant degradation in destruction efficiency when ope rated at high turndown or at ot her times when steam- and airassist rates are too high (EPA, 2012). As many of th e flares operated at petroleum refineries are steam- or air-assisted, concerns of poor flare destru ction efficiency are particularly significant for petroleum refinery flares.

The U.S. Environmental Protection Agency (E PA) supported a study to better understand flare performance and to identify key operating/monito ring parameters that would ensure adequate performance of flares at all times (EPA, 2012) . The EPA is also performing a risk and technology review of the petrol eum refinery NESHAP (40 CFR part 63, subparts CC and UUU) as part of their requirements under the Clean Air Act (CAA) sections 112(d)(6) and (f)(2). Particularly, under CAA section 112(d)(6), the EPA is required to review these technology-based NESHAP and to revise them “as necessary (tak ing into account developments in practices, processes and control tec hnologies)” no less frequently than ev ery 8 years. Many of the organic hazardous air pollutant (HAP) emi ssion controls specified in the refinery NESHAP require 95 to 98 percent control efficiency (or reduce to 20 parts per million by volume (ppmv) total organic carbon) and flares meeting the general provisions requirements for flares as control devices were deemed acceptable control alternatives. However, given the growing body of data indicating that flares that are over-assisted are likely to not meet these target control levels, the EPA is considering revising the flare operating and monito ring requirements to the refinery NESHAP to ensure compliance with the CAA section 112(d)(2 ) and (d)(3) standards. This memorandumTechnical Memorandum – Petroleu m Refinery Sector Rule: Operating Limits for Flares December 12, 2013

2 summarizes the data from the flare performance studi es and the data analysis performed to select appropriate operating limits for flares.

3. Combustion Theory

Combustion can be defined as “the sequence of exothermic chemical r eactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species” (Wikipedia, 2013). Combustion generally requires three ingredients: fu el, an oxidizing agent (typically oxygen in air), and heat (or ignition source). The need for heat or an ignition source to start the combustion process is th e reason why pilot flames and maximum exit gas velocity limits have been required on flares for the past 30 years. As flares have flames that are external to a combustion chamber and surrounded by ambient ai r, they are rarely oxygen limited. For a refinery flare, the fuel is primarily light hydroc arbons that are either not recovered during normal refinery operations or gases produced during sudden shifts in operations, such as during startup, shutdown, or process upsets.

Generally, fuels must contain combustible gases and combustible gases generally have an upper and lower flammability limit. The upper flammabili ty limit is the highest concentration of a gas in air that is capable of burning. Above this fla mmability limit, the fuel is too rich to burn. The lower flammability limit (LFL) is the lowest concen tration of the gas in air that is capable of burning. Figure 1 shows a typical combustion path (often re ferred to as a “nose plot”) for a fuel as it is mixed with air, as typically occurs in a flare. The flammability region is the area inside the “nose.” As the fuel exits the flare, it may be too rich to combust, but as the fuel mixes with ambient air, it moves from Point A (fuel mixt ure concentration) to wards 100 percent air following the path of the red arrows (combusti on path). For combustion to occur, this combustion path must pass through the flammability re gion. If the fuel mixture contains lots of inerts, such as nitrogen and water/steam, it is po ssible that Point A may be further down and to the right on the fuel line so that the combustion path passes through the flammability region for only a very short period or does not pass through the flammability re gion at all. In this case, which may occur if too much steam is added to the fuel prior to combustion, little to no combustion of the flare gas would be expected to occur. Thus, the combustion efficiency of a flare is expected to be a f unction of how long the fuel and air mixture lies within the flammability region.Technical Memorandum – Petroleu m Refinery Sector Rule: Operating Limits for Flares December 12, 2013

3 

# Path of Combustion Zone Gas (Methane as an example)

Fuel Vol %

Inert (steam or Nitrogen) Vol % “B” point of insufficient air

“C” point of insufficient fuel, low NHV Vent gas out of the flare, moving toward 100% air as fuel/inert mixture diffuses.

26% Flammability Region

100% air 100% Inert 100% Fuel

“A” Exit Tip

Figure 1. Typical combustion path for a diffusion flame.

When considering the combusti on of hydrocarbon fuels, “comple te combustion” would be the conversion of all hydrocarbons co mponents to carbon dioxide (CO 2 ) and water. Complete combustion rarely occurs. Often, the combusti on process may be incomplete, producing carbon monoxide (CO) or other products of incomplete combustion, such as formaldehyde or unburned fuel components. Flare performance is commonl y characterized by the combustion efficiency. PFTIR measurements are used to determ ine the relative concentrations of CO 2 released from the flare to the concentrations of CO and hydrocarbon s. The combustion efficiency can be estimated

as:



##        

   

 

# 

 n

i i i 2 2

HC CMN CO CO CO %CE

1 ] [ ] [ ] [ % 100 ] [ (Eqn. 1)Technical Memorandum – Petroleu m Refinery Sector Rule: Operating Limits for Flares December 12, 2013

4 where:

%CE = Combustion efficiency of the flare, percent.

[CO 2 ] = Concentration of CO 2 in the offgas from the flare ( i.e. , above the flare flame), parts per million by volume (ppmv).

[CO] = Concentration of CO in the offgas from the flare ( i.e. , above the flare flame,
ppmv.
i = Individual hydrocarbon component in the offgas from the flare.

CMN i = Carbon mole number of hydrocarbon com ponent i in the offgas from the flare, mole carbon atoms per mole of compound. E.g. , CMN for ethane (C 2 H 6 ) is 2; CMN for propane (C 3 H 8 ) is 3.

[HC] i = Concentration of hydrocarbon component i in the offgas from the flare, ppmv.

While combustion efficiency looks at the simp le chemical reaction of burning hydrocarbons and transforming them into CO 2 and water, there is another common metric also used to describe the performance of flares known as destruction effi ciency. The destruction efficiency can be evaluated for a particular pollutant, i.e. , mass rate of a particular component exiting the flare flame divided by the mass rate of a component en tering the flare. However, it is difficult to measure the mass rate of a single component exiti ng the flare. Alternatively, one could consider conversion of hydrocarbons to CO as destruction, so the hydrocarbon destruction efficiency from PFTIR measurements could be determined as follows: 

##          

    

 

# 

 n

i i i 2 2

HC CMN CO CO CO CO %DE

1 ] [ ] [ ] [ % 100 ] [ ] [ (Eqn. 2)

where:

%DE = destruction efficiency of the flare, percent.

Thus, it is important to note that there is a difference between combustion efficiency and destruction efficiency and that destruction effici ency will always be greater than (or equal to) combustion efficiency.

Combustion Indicators for Fuels

In order to have good combustion, it is important that the fuel air mi xture pass through the flammability region (see Figure 1). The concept of upper and lower flammability limits was discussed, but other indicators (pro perties of the fuel) have been used historically to indicateTechnical Memorandum – Petroleu m Refinery Sector Rule: Operating Limits for Flares December 12, 2013

5 whether a fuel contains a sufficient amount of combustibles in order to provide good combustion in a flare. The key fuel (or “vent gas”) properties that have been used as indicators that the fuel is sufficiently rich to support combustion include the net heating value, the lower flammability limit, and the combustible gas concentration. Th e appropriate equations to determine the net heating value, the lower flammability limit, and the combustible gas concentration in the vent gas when multiple components are present follow. 

# 

 

###  n

i i i vg 

### NHV x NHV

1 (Eqn. 3)

where:

NHV vg = Net heating value of flare vent gas, British thermal units per standard cubic foot (Btu/scf).

i = Individual compone nt in flare vent gas.

n = Number of compone nts in flare vent gas.

x i = Concentration of component i in flare vent gas, volume fraction.

NHV i = Net heating value of co mponent i determined as the heat of combustion where the net enthalpy per mole of offgas is based on combustion at 25 degrees Celsius (°C) and 1 atmosphere (or co nstant pressure) with water in the gaseous state from values published in the literature, and then the values converted to a volumetric basis using 20 °C for “standard temperature.” Table 1 summarizes component properties including net heating values.



# 

 

###         

n

i i vg



### LFL χ LFL

i

1 

### 1 (Eqn. 4)

where:

LFL vg = Lower flammability limit of flare vent gas, volume fraction.

n = Number of components in the vent gas.

i = Individual compone nt in the vent gas.

χ

i = Concentration of component i in th e vent gas, volume percent (vol %).Technical Memorandum – Petroleu m Refinery Sector Rule: Operating Limits for Flares December 12, 2013

6 LFL i = Lower flammability limit of component i as determined using values published by the U.S. Bureau of Mines (Zabetakis, 1965), vol %. All inerts, including nitrogen, are assumed to have an infinite LFL ( e.g. , LFL N2 = ∞ , so that χ N2 / LFL N2 = 0). LFL values for common flare vent gas components are provided in Table 1.

Table 1— Individual Component Properties

Component Molecular Formula MW i (lb/lb-mol) a CMN i (mole/mole) NHV i (Btu/scf) LFL i (vol%) a
Acetylene C 2 H 2 26.04 2 1,404 2.5
Benzene C 6 H 6 78.11 6 3,591 1.3 1,2-Butadiene C 4 H 6 54.09 4 2,794 2.0 1,3-Butadiene C 4 H 6 54.09 4 2,690 2.0
iso-Butane C 4 H 10 58.12 4 2,957 1.8
n-Butane C 4 H 10 58.12 4 2,968 1.8
cis-Butene C 4 H 8 56.11 4 2,830 1.6
iso-Butene C 4 H 8 56.11 4 2,928 1.8
trans-Butene C 4 H 8 56.11 4 2,826 1.7
Carbon Dioxide CO 2 44.01 1 0 ∞ Carbon Monoxide CO 28.01 1 316 12.5 Cyclopropane C 3 H 6 42.08 3 2,185 2.4
Ethane C 2 H 6 30.07 2 1,595 3.0 Ethylene C 2 H 4 28.05 2 1,477 2.7 Hydrogen H 2 2.02 0 274 4.0
Methane CH 4 16.04 1 896 5.0 Methyl-Acetylene C 3 H 4 40.06 3 2,088 1.7 Nitrogen N 2 28.01 0 0 ∞ Oxygen O 2 32.00 0 0 ∞ Pentane+ (C5+) C 5 H 12 72.15 5 3,655 1.4 Propadiene C 3 H 4 40.06 3 2,066 2.16 Propane C 3 H 8 44.10 3 2,281 2.1 Propylene C 3 H 6 42.08 3 2,150 2.4
Water H 2 O 18.02 0 0 ∞

a MW i = molecular weight of component I; lb/lb-mol = pounds per pound-mole; LFL i = Lower flammability limit of

component i; vol% = volume percent.Technical Memorandum – Petroleu m Refinery Sector Rule: Operating Limits for Flares December 12, 2013

7 % 100

3 1      
       

 

# 

 h n

i i i

vg χ χ CMN

C (Eqn. 5)

where:

C vg = Total volumetric fraction of hydrogen (H 2 ) and combustible organic components present in the flare vent gas, fraction as propane. For the purposes of C vg , CO 2 is not considered to be a combustible organic component, but CO may be included in C vg .
n = Number of individual combustible organic components in flare vent gas.

i = Individual combustible orga nic component in flare vent gas.

χ

i = Concentration of combustible organic component i in flare vent gas, vol %.

CMN i = Carbon mole number of combustible or ganic component i in flare vent gas, mole carbon atoms per mole of compound. E.g. , CMN for ethane (C 2 H 6 ) is 2; CMN for propane (C 3 H 8 ) is 3.

χ

h = Concentration of H 2 in flare vent gas, vol %.

100% = Constant, used to convert volume percent to volume fraction.

Impact of Assist Gas on Combustion Indicators for Fuels

To improve mixing and to reduce soot production (s moking) in the flare flame, steam and/or air may be mixed with the flare vent gas. This gas addition is commonly referre d to as “assist gas” (either assist steam or assist ai r). Typically, assist air is intr oduced around the perimeter of the flare to support mixing of air and fuel in th e combustion zone. Representatives of John Zink noted that they have investigated aspirating air into the assist steam line as a means to provide both assist air and assist steam (Coburn, 2013); this method of adding assist air is fairly new and it is expected to be used in very few petroleum refinery flares, if any.

According to the combustion path theory represente d in Figure 1, the impact of adding assist air is expected to have a different impact on the co mbustion path. The additi on of steam is expected to shift Point A down and to the right on the fuel line toward 100 percent inerts. On the other hand, the addition of air is expected to move Po int A towards 100 percent air following the path of the red arrows. No combustion efficiency studies have been conducted on a flare using steamentrained assist air, but if the ai r is mixed prior to the flame zone or pilot gas ignition point, this assist air may act in much the same manner as inert gases (shifting point A so that the combustion path begins near the lower region or below the flammability region) so that combustion efficiency could be negatively imp acted. Consequently, a distinction is madeTechnical Memorandum – Petroleu m Refinery Sector Rule: Operating Limits for Flares December 12, 2013

8 between “premix assist air” and “per imeter assist air.” Premix assist air is assist air that is mixed with the flare vent gas prior to the outlet of the flare, such as assist air entrained with center steam. This premix assist air is treated in the same manner as inerts (steam) added prior to or within the flame combustion zone. Perimeter assist air includes all other assist air, such as traditional assist air introduced around the perimeter of the flare as well as assist air entrained in upper and lower steam.

Considering combustion path theory, the propertie s of the fuel mixture entering or within the combustion zone is expected to be a better indica tor of flare performance than the properties of the fuel (flare vent gas) alone. As measuremen t data are typically made for the flare vent gas properties (and because direct sampling of the com bustion zone gas is infeasible), the net heating value, the lower flammability limit, and the combustible gas concentration in the combustion zone must be calculated based on vent gas prope rties and steam/inert gas addition rates. The following equations were used to dete rmine the combustion zone properties.



#  
a,premix s vg vg vg

cz Q Q Q NHV Q NHV   

 (Eqn. 6)



#  

vg a,premix s vg vg

cz 

### Q Q Q Q LFL LFL   

 (Eqn. 7)



#  
a,premix s vg vg vg

cz Q Q Q C Q C   

 (Eqn. 8)

where:

NHV cz = Net heating value of co mbustion zone gas, Btu/scf.

NHV vg = Net heating value of flare vent gas, Btu/scf.

LFL cz = Lower flammability limit of combustion zone gas, volume fraction.

LFL vg = Lower flammability limit of flare vent gas, volume fraction.

C cz = Combustibles concentration in the combustion zone gas, volume fraction.

C vg = Combustibles concentration of flare vent gas, volume fraction.

Q vg = Cumulative volumetric flow of flare ve nt gas, standard cubic feet per minute (scfm).Technical Memorandum – Petroleu m Refinery Sector Rule: Operating Limits for Flares December 12, 2013

9 Q s = Cumulative volumetric flow of total steam, scfm.

Q a,premix = Cumulative volumetric flow of premix assist air, scfm.

Note that in all performance tests, there was no premix assist air, so Q a,premix = 0.

4. Performance Data for Flares Using Assist Steam

The vast majority of the recent flare performance data are for st eam-assisted flares (EPA, 2012). In 2011, the EPA conducted a detailed Informati on Collection Request (ICR) of the petroleum refining industry (EPA, 2011). Under Component 1 of this ICR, the EPA required petroleum refineries to provide informati on about specific process units a nd emission sources present at the facility, including information about flares. Ba sed on the ICR data, approximately 80 percent of refinery flares are steam-assisted, 10 percent ar e air-assisted, and 10 pe rcent are non-assisted. Therefore, the preponderance of data for steam-assisted flares is understandable.

Performance Indicators Flares Using Assist Steam

The flare performance studies used PFTIR methods to determine the relative concentrations of combustion products ( i.e. , CO 2 and CO) and unburned hydrocarbons in the plume above the flare flame. The combustion efficiency was subseque ntly calculated based on complete oxidation of the hydrocarbons to CO 2 ( i.e. , Equation 1 of this memorandum). Figure 2 presents the measured combustion efficiency as a function of th e combustion zone net heating value (NHV cz ). Figure 3 presents the measured combustion efficiency as a function of the combustion zone lower flammability limit (LFL cz ), and Figure 4 presents the measured combustion efficiency as a function of the combustion zone “combustibles” concentration (C cz ), which is the total hydrocarbon concentration as propane (C3) plus the H 2 concentration.Technical Memorandum – Petroleum Refinery Sector Rule: Operating Limits for FlaresDecember 12, 201310Figure 2. Flare combustion efficiency as a function of the net heating value of the gas within the combustion zone (NHVcz).0.30.40.50.60.70.80.9150 100 150 200 250 300 350 400 450 500

Combustion EfficiencyCombustionZoneNetHeatingValue(NHV_cz),Btu/scfNoH2‐OlefinInteractionH2‐OlefinInteractionTechnical Memorandum – Petroleum Refinery Sector Rule: Operating Limits for FlaresDecember 12, 201311Figure 3. Flare combustion efficiency as a function of the lower flammability of the gas within the combustion zone (LFLcz).0.30.40.50.60.70.80.910 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

Combustion EfficiencyCombustionZoneLowerFlammabilityLimit(LFL_cz),volumefractionNoH2‐OlefinInteractionH2‐OlefinInteractionTechnical Memorandum – Petroleum Refinery Sector Rule: Operating Limits for FlaresDecember 12, 201312Figure 4. Flare combustion efficiency as a function of the combustibles concentration within the combustion zone (Ccz).0.30.40.50.60.70.80.910 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

Combustion EfficiencyCombustionZoneCombustiblesConcentration(C_cz),volumefractionNoH2‐OlefinInteractionH2‐OlefinInteractionTechnical Memorandum – Petroleu m Refinery Sector Rule: Operating Limits for Flares December 12, 2013

13 The destruction (or control) efficiency of a flare is generally higher than the combustion efficiency as some hydrocarbons may be partially oxidized to CO rather than fully converted to CO 2 . In the flare performance study, it was es timated that a combustion efficiency of 96.5 percent was equivalent to a destruction e fficiency of 98 percent (EPA 2012). For this analysis, we considered primarily the individual, time-stamp specific measurement data, i.e.
, individual data measurements recorded every minut e or two during a test r un; the average values across a given run (typically a r un was about 15 minutes in duration) were also evaluated as part of the failure analysis. We expect a little mo re variability in the minute-by-minute recordings, but we also expect to see better correlation of the data as wast e gas flow rates or steam addition rates vary during a given run. Typically, only a single compositional analysis value for the waste gas is available during a run, si nce the gas chromatography (GC) cycle time is approximately 15 minutes. Therefore, change s in combustion zone compositi ons were calculated based on minute by minute changes in waste gas flow rates and steam additi on rates. Figures 2 through 4 are plots of the individual, time-stamp specific measurement data.

Hydrogen-Olefin Interaction Af fect on Flare Performance

In looking at Figures 2 through 4, there appears to be certain test conditions that showed significantly poorer combustion efficiencies than observed in other test runs. Upon investigation, these test runs appe ar to have considerable concentr ations of both olefin (alkenes) and H 2 in the waste gas. Although not well-underst ood, it is suspected that there are special reactions that occur between olefins and H 2 during the combustion process which competes with the direct oxidation reactions, making it more diffi cult to achieve high com bustion efficiencies at a given combustion zone parameter level as compar ed to waste gases that do not have a mixture of olefins and H 2 . Figures 2 through 4 indicate the data points that were expected to have H 2 -olefin interactions; these da ta points were selected us ing the following criteria:

1) The H 2 concentration in the combustion zone was greater than 1.2 mole percent. 2) The sum of the olefin concen trations in the combustion zone was greater than 2.5 mole percent. 3) The H 2 concentration in the combustion zone plus the sum of the olefin concentrations in the combustion zone was great er than 7.4 mole percent.

All three of these criteria had to be met in orde r for the data point to be considered as “having H 2 -olefin interactions.” There were numerous runs that had high H 2 concentrations but no/low olefins content and several runs that had hi gh olefin concentrations but no/very low H 2 concentrations. As seen in Figures 2 through 4, the combustion efficiency for the data points associated with conditions conducive to H 2 -olefin interactions are significantly different than for data points without H 2 -olefin interactions, par ticularly when using NHV cz or LFL cz as the indicator of good combustion. Based on these ob servations, separate operating limits were evaluated for flares with H 2 -olefin interactions an d for flares without H 2 -olefin interactions.

Failure Analysis of Assist Stea m Flare Performance Indicators

The ultimate goal of this failure analysis is to en sure a flare achieves a destruction efficiency of 98 percent (expected to be equiva lent to a flare combustion effi ciency as measured during theTechnical Memorandum – Petroleu m Refinery Sector Rule: Operating Limits for Flares December 12, 2013

14 test of approximately 96.5 percent) so that the fl are achieves an equivalent performance to other thermal combustion devices that can be used to comply with the refinery NESHAP. These other thermal destruction devices have en closed stacks so that the contro l efficiency of the device can be readily determined, and their performance is typically averaged over the period of the performance test, which is typically 3 hours ( i.e. , three 1-hour runs). Due to the variability of flow to most flares, a long aver aging period is not practical. For example, if a flaring event lasts only 1 hour, how does one determine a 3-hour averag e operating limit? Additionally, the data to assess flare performance is available only for shor t durations, so it is important to consider the time scale of the data used to derive an ope rating limit when determining an appropriate averaging time for the operating limit. In this analysis, we consider individual time-stamp data ( i.e. , minute-by-minute performance). However, we are also constrained by the availability of compositional data. Thus, while we may be evaluating minute-by-minute data, we must generally assume a fixed waste gas composition over the test run (which is typically about 15 minutes) when performing the minute-by-minute calculations. Typically, GC compositional analysis of refinery gases requi res 10 to 15 minutes to complete. Therefore, a 15-minute block average is considered the shorte st time from over which a flar e operating limit can be reasonably evaluated in practice.

Even though a 15-minute block average is consider ed the shortest time frame for which a flare operating limit can be reasonably evaluated for re gulatory purposes, the performance test data provide minute-by-minute combustion efficiency measurements and we also evaluated flare performance using this minute-by-minute data. Ther efore, it is possible that a flare that has a measured combustion efficiency less than 96.5 percent for a given minute to still meet a combustion efficiency of 96.5 percent on average over a 15-minute period. Consider an example in terms of destruction efficien cy. If a flare was achieving 99 percent destruction, for 14 minutes in a 15-minute block average, that flare woul d meet the 98 percent destruction efficiency provided that the destruc tion efficiency for the 15 th minute was at least 84 percent. However, 1 minute at 70 percent destructi on efficiency would make it impo ssible for the flare to achieve 98 percent destruction efficiency across the 15-minut e average even if the destruction efficiency for the other 14 minutes is 99.9 percent. Therefor e, while it may be acceptable to have some minutes with combustion efficiencies less than 96.5 percent (expected to be equivalent to 98 percent destruction efficiency), it is importa nt that the combustion efficiency does not fall below 85 or 90 percent in any one minute; otherwise, it become s extremely difficult, if not impossible, for the flare to achieve the desired 98 percent destruction efficiency.

With this in mind, a failure analysis was conduc ted using both the minute-by-minute data and the run average data to help identify appropriate operating limits for the three different performance indicators. A Type 1 error occurs when the operating limit is achieved, but the performance is below a specified lower combustion efficiency li mit. We conducted the Type 1 error analysis considering lower combustion efficiency values of 85 and 90 percent when using the minute-byminute analysis and 95 and 96.5 percent when using the run average data. A Type 2 error occurs when the performance indicators suggest non- compliant performance when the combustion efficiency is definitely compliant. We conducte d the Type 2 error anal ysis using 98 percent combustion efficiency as the “definitely” comp liant performance level for both the minute-byminute and run average data analyses. The flare performance dataset was di vided into two: oneTechnical Memorandum – Petroleu m Refinery Sector Rule: Operating Limits for Flares December 12, 2013

15 for minute-by-minute data that were not expected to have H 2 -olefin interactions and one for minute-by-minute data that were expected to have H 2 -olefin interactions ( i.e. , met all three H 2 -olefin concentration conditions outlined prev iously). A separate failure analysis was conducted for each of these datasets. Operating limits were adjusted by trial and error to minimize or eliminate Type 1 errors while also attempting to minimize the occurrence of Type 2

errors. Table 2 presents the recommended operating lim its and summarizes the results of the failure analysis for the time-stamp data analysis considering those operating limits; Table 3 summarizes the results of the failure analysis fo r the run average data analysis considering the recommended operating limits.

Key observations of the failure analysis results suggest:

 The target operating limits presented in Tables 1 and 2 result in very few Type 1 errors ( i.e. , instances where the operating limit wa s achieved but performance was well under 96.5 percent combustion efficiency), even when the facility can elect to comply with any of the limits at any time. At the 85 percen t combustion efficiency target, only one data point was not identified by th e performance indicators and target operating limits (only for the H 2 -olefin interactions group and this data poi nt is likely an outlier). None of the minute-by-minute data points without H 2 -olefin interactions and only 0.5 percent of the minute-by-minute data points with H 2 -olefin interactions did not have a combustion efficiency of at least 90 percent while co mpliant with any one of the performance indicators at the sel ected target operating limits. Cons idering the run average data, all test runs compliant with the selected ta rget operating limits had average combustion efficiencies exceeding 95 percent, and 97 perc ent of those had combustion efficiencies at or above 96.5 percent.

 Allowing compliance with any of the three performance indicators and target operating limits at any time helps to minimize the numbe r of Type 2 errors (non-compliance with the operating limit, while having high combustion efficiencies). Requiring compliance with a single operating limit would be more lik ely to identify a fac ility as non-compliant while they have high combustion efficiencies than allowing compliance with any one of the operating limits at any time.Technical Memorandum – Petroleu m Refinery Sector Rule: Operating Limits for Flares December 12, 2013

16 Table 2. Failure Analysis of Steam-Assisted Flare Performance Indicators Based on Minute-by-Minute Measurement Data

Data Set/Performance Indicator Target Operating Limit No. of Type 1 Errors with: No. of Type 2 Errors with CE ≥ 98% CE < 85% CE < 90%
Does not meet H2-Olefin interacti on criteria (2,918 data points)

NHV cz ≥ 270 0 0 347

LFL cz ≤ 0.15 0 0 337
C cz ≥ 0.18 0 0 305

NHV cz , LFL cz , or C cz See above 0 0 195
Meets all three H 2 -Olefin interaction crit eria (945 data points)

NHV cz ≥ 380 0 4 122

LFL cz ≤ 0.11 1 1 88
C cz ≥ 0.23 1 1 112

NHV cz , LFL cz , or C cz See above 1 5 87

Table 3. Failure Analysis of Steam-Assisted Flare Performance Indicators Based on Run Average Measurement Data

Data Set/Performance Indicator Target Operating Limit No. of Type 1 Errors with: No. of Type 2 Errors with CE ≥ 98% CE < 95% CE < 96.5% Does not meet H2-Olefin interaction criteria (180 data points)

NHV cz ≥ 270 0 2 18

LFL cz ≤ 0.15 0 3 21
C cz ≥ 0.18 0 2 23

NHV cz , LFL cz , or C cz See above 0 4 15
Meets all three H 2 -Olefin interaction criteria (56 data points)

NHV cz ≥ 380 0 2 5
LFL cz ≤ 0.11 0 3 4
C cz ≥ 0.23 0 2 5
NHV cz , LFL cz , or C cz See above 0 4 4

5. Performance Data for Flares Using Assist Air

There are very limited data on flares using perimete r assist air and no data on flares using premix assist air. As indicated in E quations 6 though 8, premix assist air is effectively treated as assist steam when calculating the combustion zone proper ties. Therefore, premix assist air is not considered further in this sect ion. There were a series of te st conducted in 1985 on a very small 1.5-inch diameter flare (EPA, 1985) and a series of tests conducte d on a 2-foot diameter flare by the Texas Commission on Environmental Quality (T CEQ) that used perimeter assist air (EPA,Technical Memorandum – Petroleu m Refinery Sector Rule: Operating Limits for Flares December 12, 2013

17 2012). A third test was conducted on a flare that used perimeter assist air in 1983, but this test did not report air assist rates and, therefore, could not be us ed to identify “over-assisted” conditions.

Performance Indicators for Flares Using Perimeter Assist Air

The TCEQ study used different mixtures of na tural gas (predominately methane), propane, propylene, and nitrogen to assess flare performa nce at the three differe nt flare vent gas net heating values: 2000 Btu/scf; 575 Btu/scf; and 350 Btu/scf. The 1985 study evaluated mixtures of nitrogen and propane covering a range of flare gas net heat ing values ranging from 440 to 2,350 Btu/scf. Figure 5 shows the measured combustion e fficiency as a function of the NHV vg . As seen in Figure 5, these tests indicate that the initial NHV vg is not a critical performance parameter (at least within the range of net heatin g values investigated). The flare performance was primarily dependent on the air-assist rates an d the combustion efficiency for a given flare gas net heating value could be high or lo w, depending on the amount of assist air.

Figure 5. Flare combustion efficiency as a function of the net heating value of the flare vent gas (NHV vg ). 30% 40% 50% 60% 70% 80% 90% 100%

0 500 1000 1500 2000 2500CombustionEfficiency(%)

Net Heating Value Vent Gas 

### Air

‐ assisted Flare: Net Heating Value in Vent Gas

TCEQ; NHVvg=2000
TCEQ; NHVvg=575 TCEQ; NHVvg=350
EPA, 1985Technical Memorandum – Petroleu m Refinery Sector Rule: Operating Limits for Flares December 12, 2013

18 In the Parameters for Properly Designed and Operated Flares report (EPA, 2012), the stoichiometric ratio was identified as a potential performance indicator for air-assisted flares. However, the correlation of combustion efficiency and stoichiometric ratio is very different for the two different studies (see Figure 6 ).

Figure 6. Flare combustion efficiency as a function of stoichiometric air assist rates.

In a diffusion flame, combustion takes plac e at the flame surface only, where the fuel (combustibles in the waste gas) meets oxygen within the flammability region. The interior of the flame contains unburned fuel. This type of combus tion is the simplest form, and one that occurs at the tip of a non-assist flare. Graphically, one can envision the path of the combustion starting initially on the zero air line (Point A), symbolizing the quality of the gases in the flare just prior to it leaving the flare tip as shown previously in Figure 1 . As the vent gases mix with air and burn, they proceed on a path from point A, through the flammability region, and eventually go to the origin (infinite dilution in air). If the flare vent gas and assist media mixture are already near the LFL when the gas mixture is released into ai r, the concentration coul d quickly fall below the 30% 40% 50% 60% 70% 80% 90% 100%

0 10 20 30 40 50 60 70 80CombustionEfficiency(%)

Stoichiometric Ratio 

### Air

‐ assisted Flare: Stoichiometric Ratio Correlation

TCEQ; NHVvg=2000
TCEQ; NHVvg=575 TCEQ; NHVvg=350
EPA, 1985Technical Memorandum – Petroleu m Refinery Sector Rule: Operating Limits for Flares December 12, 2013

19 mixture’s LFL (point C) before having a chance to ignite and burn completely; and therefore, it is possible to dilute the flammable gas mixture to a mixture with no combustible characteristics.

Consistent with a combustion theo ry and as demonstrated from the analysis of the steam-assist data, there needs to be a mini mum heat content (or other perf ormance indicator) of flare gas within the combustion zone in order for the co mbustion path to travel through the flammability region as air is introduced. All of the air assist runs were complia nt with at least one (if not all three) of the suggested operating limits based on the flare vent gas conditions. However, it is clear from Figure 5 that the ini tial flare vent gas properties alone are not sufficient to ensure good combustion.

For air-assisted flares, the combustion path can st ill be expected to follow the path indicated in Figure 1, but the rate of travel towards 100 percent air would be mu ch faster than for flare that does not use perimeter assist air. If the rate is too fast, the combustible mixture will not exist long enough to allow near complete combustion. Therefore, the “dilution rate” caused by the addition of assist air must be limited to some exte nt in order to ensure that the flare gas is not diluted so fast that combustion efficiency is compromised.

Based on the data in Figure 6, it was postulated th at the diameter of the flare is a significant parameter impacting the allowable dilution ratio of a flare with perimeter assist air. For small diameter flares, it is easier for the assist air to fully penetrate and dilute the combustion zone gas before significant combustion can occur. Theref ore, the “dilution factor” was thought to be a function of the flare diameter and act on the co mbustion zone gas. Using Equations 6 through 8, the combustion zone gas properties can be replac ed with the vent gas properties, which are actually measured. The following dilution factor, as a function of vent gas properties, was derived and subsequently evaluated as a means to predict flare performance for flares using perimeter assist air.

perimeter a premix a s vg vg Q Q Q Q Diam Q DF

, ,    

 (Eqn. 9)

where:

DF = Dilution factor, feet (ft).

Q vg = Volumetric flow rate of the flare vent gas, scfm.

Q s = Volumetric flow rate of the assist steam, scfm.

Q vg = Volumetric flow rate of the premix assist air, scfm.

Q air = Volumetric flow rate of the perimeter assist air, scfm.

Diam = Diameter of the fl are vent gas release stack, ft.Technical Memorandum – Petroleu m Refinery Sector Rule: Operating Limits for Flares December 12, 2013

20 The following air-assist diluti on parameters were defined:

 Net heating value indicator: NHV dil = NHV vg × DF

 Lower flammability limit indicator: LFL dil = LFL vg /DF

 Combustibles concentration indicator: C dil = C vg × DF

Figures 7, 8, and 9 present the correlation between combus tion efficiency and these air-assist dilution parameters. As seen in these figures, both the TCEQ and EPA data sets exhibit similar performance curves when correlated usi ng the air-assist dilution parameters.

Failure Analysis of Perimeter Assist Air Flare Performance Indicators

Similar to the failure analysis conducted for steam-assisted flares, a failure analysis was conducted to help select appropria te operating limits for the air-a ssist dilution parameters. The air-assisted flare data were not available minut e-by-minute. Individual run data for the TCEQ tests were typically 5 to 15 mi nutes long; individual run da ta for the EPA 1985 tests were 20 minutes long. As these data represent l onger averaging periods, the minimum target combustion efficiencies used in the air-assist failur e analysis were selected to be much closer to the target combustion efficiency of 96.5 percent. When looki ng at minute-by-minute data, one minute of 90 percent combustion efficiency di d not necessarily indicate that the target combustion efficiency could not be met on a 15-mi nute average basis. Since the air-assisted flare data typically represent average comparable to 15-minute averages, the air-assist dilution parameters were compared directly to the target combustion efficiency of 96.5 percent combustion efficiency. Table 4 presents the results of the air- assist dilution parameter failure analysis.

Table 4. Failure Analysis of Perimeter Assist Air Flare Performance Indicators

Data Set/Performance Indicator Target Operating Limit No. of Times Operating Limit was Achieved but CE < 96.5% (Type 1 Error) No. of Times Operating Limit was

Not Achieved but CE ≥ 96.5% (Type 2 Error)
Runs that do not meet H 2 -Olefin interaction criteria (79 runs)

NHV dil ≥ 22 0 1

LFL dil ≤ 2.2 0 1
C dil ≥ 0.012 0 2

NHV dil , LFL dil , or C dil See above 0 1
Runs that meet all three H 2 -Olefin interaction criteria (0 runs)Technical Memorandum – Petroleum Refinery Sector Rule: Operating Limits for FlaresDecember 12, 201321Figure 7. Correlation of combustion efficiency with the net heating value dilution parameter (NHVdil).30%40%50%60%70%80%90%100%0 5 10 15 20 25 30 35 40 45 50

Combustion Efficiency (%)NetHeatingValueVentGas× DilutionFactor(Btu/ft2)Air‐assistedFlare:NetHeatingValueDilutionParameterTCEQ;NHVvg=2000TCEQ;NHVvg=575TCEQ;NHVvg=350EPA,1985Technical Memorandum – Petroleum Refinery Sector Rule: Operating Limits for FlaresDecember 12, 201322Figure 8. Correlation of combustion efficiency with the lower flammability limit dilution parameter (LFLdil).30%40%50%60%70%80%90%100%0 2.5 5 7.5 10 12.5 15 17.5 20 22.5

Combustion Efficiency (%)LowerFlamabilityLimit/DilutionFactor(1/ft)Air‐assistedFlare:LowerFlamabilityLimitDilutionParameterTCEQ;NHVvg=2000TCEQ;NHVvg=575TCEQ;NHVvg=350EPA,1985Technical Memorandum – Petroleum Refinery Sector Rule: Operating Limits for FlaresDecember 12, 201323Figure 9. Correlation of combustion efficiency with the combustibles concentration dilution parameter (Cdil).30%40%50%60%70%80%90%100%0 0.005 0.01 0.015 0.02 0.025 0.03 0.035

Combustion Efficiency (%)CombustibleConcentrationofVentGas× DilutionFactor(ft)Air‐assistedFlare:CombustibleConcentrationDilutionParameterTCEQ;NHVvg=2000TCEQ;NHVvg=575TCEQ;NHVvg=350EPA,1985Technical Memorandum – Petroleu m Refinery Sector Rule: Operating Limits for Flares December 12, 2013

24 Based on the failure analysis, the air-assist dilution parameter targ et operating limits presented in Table 3 provide an excellent indication of when th e combustion efficiency will be greater than or less than 96.5 percent (assumed to be equivalent to a destruction efficiency of 98 percent) when no H 2 -olefin interaction is expected to be present. No data are available for air-assisted flares that meet H 2 -olefin interaction criteria. It is reasonable to conclude from the steam-assisted data analysis that air-assisted flares that meet the H 2 -olefin interaction criteria (in the combustion zone gas) should meet the more stringent H 2 -olefin interaction operating limits (in the combustion zone gas). It is also reasonable to expect that the perimeter assist air dilution parameter operating limits would need to be si milarly more stringent to ensure good combustion when H 2 -olefin interactions are present. Table 5 shows one method of estimating “similarly more stringent” dilution parameter operating limits for flares using peri meter assist air when H 2 -olefin interactions are presen t in the combustion zone gas.

Table 5. Projected Dilution Parameter Operating Limits for Flares with Perimeter Assist Air Meeting the H 2 -Olefin Interaction Criteria

Parameter

Family Target Vent Gas Operating Limit – No

Olefin Interaction Target Air Assist Dilution Operating Limit– No

Olefin Interaction Apparent Dilution Factor Target Vent Gas Operating Limit – Olefin Interactions Present Estimated Target Air Assist Dilution Operating Limit – Olefin Interactions Present

NHV ≥ 270 ≥ 22 22/270 = 0.0815 ≥ 380 ≥ 31

LFL ≤ 0.15 ≤ 2.2 0.15/2.2 = 0.068 ≤ 0.11 ≤ 1.6

C ≥ 0.18 ≥ 0.012 0.012/0.18 = 0.067 ≥ 0.23 ≥ 0.015

6. Performance Data for Non-Assisted Flares

There are very limited data on non-assisted flares. The EPA conducted a study in 1983 evaluating a steam-assisted flar e burning propylene or propylene/ni trogen mixtures (EPA, 1983). For some of the runs, the assist steam was turned off so the flare was effectively non-assisted. During the steam-assisted runs, the combustion efficiency of the flare was definitely best correlated with the combustion zone propertie s. For non-assisted flares, the vent gas composition is the same as the combustion zone composition. Figure 10 provides the correlation of the NHV cz to the combustion efficiency of the flar e. While it is understandable that a NHV vg limit of 200 Btu/scf was established based on the resu lts of this single stu dy, it is clear from the steam-assisted data that there is variability between flares and variability based on the composition of the flare gas (par ticularly when olefins and H 2 are both present). While the 1983 EPA study has limited data points in the tran sition area between good and poor combustion efficiencies, the data suggest that the same transition point between good and poor combustion for the non-assisted flares is essentially the sa me as for the steam-assisted flares when theTechnical Memorandum – Petroleu m Refinery Sector Rule: Operating Limits for Flares December 12, 2013

25 combustion zone properties are considered. This common transition point is also observed in LFL cz and C cz plots, and it is understandable since a steam-assisted flare will become unassisted if the assist steam is turned off. Consequently, it is reasonable to assert that the combustion zone operating limits established for steam-assisted flar es are also applicable the non-assisted flares with the understanding that, for non-assisted flares , the vent gas properties are equivalent to the combustion zone properties.

.

Figure 10. Correlation of combustion efficiency with combustion zone net heating value (NHV cz ) from the EPA 1983 study (EPA, 1983).

As noted previously, the data fo r steam assisted flares covers a much broader range of operating conditions than the data for non-assisted flares. Therefore, the operating limits established for steam-assisted flares are expected to ensure compliance over this broader range of operating conditions. Based on the data for steam-assisted flares, it is evident that a 200 Btu/scf NHV cz limit would not ensure compliance with the 98 per cent destruction efficiency requirement at all times. Using the same failure analysis as c onducted previously for steam-assisted flares, the impact of maintaining an NHV cz limit of 200 Btu/scf for non-assisted flares can be assessed. Table 6 summarizes the results of the failure anal ysis when using the run average data set. 60% 65% 70% 75% 80% 85% 90% 95% 100%

100 150 200 250 300 350 400 450 500CombustionEfficiency

Combustion Zone Net Heating Value (Btu/scf) EPA, 1983 Flare Study: Combustion Zone Net Heating Value

Steam Assist

Non

‐ AssistedTechnical Memorandum – Petroleu m Refinery Sector Rule: Operating Limits for Flares December 12, 2013

26 Allowing the use a 200 Btu/scf NHV cz limit would fail to identify poor flare performance approximately 20 percent of the time. That is, 20 percent of the runs where the average flare combustion efficiency was less than 95 percent (not including runs projected to have H 2 -olefin interactions) met the 200 Btu/scf NHV cz limit. This is in sharp c ontrast with the suggested 280 Btu/scf NHV cz limit where none of the runs where the average flare combustion efficiency was less than 95 percent met a 270 Btu/scf NHV cz limit. The 200 Btu/scf NHV cz limit is even worse when one considers runs projected to have H 2 -olefin interactions. While it is unlikely a flare that receives vent gas that is high in olefins and H 2 would be non-assisted, it is clear that a 200 Btu/scf NHV cz limit for flares (whether non-assisted or st eam-assisted) is not sufficient to ensure compliance with the target 98 pe rcent destruction efficiency.

Table 6. Comparison of Failure Analysis Results for Selected NHV cz Operating Limits Based on Run Average Measurement Data

Data Set/Performance Indicator Target Operating Limit No. of Type 1 Errors with: No. of Type 2 Errors with CE ≥ 98% CE < 95% CE < 96.5% Does not meet H2-Olefin interaction criteria (180 data points)

NHV cz ≥ 200 18 30 2

NHV cz ≥ 270 0 4 15
Meets all three H 2 -Olefin interaction criteria (56 data points)

NHV cz ≥ 200 7 13 0

NHV cz ≥ 380 0 2 5

The steam-assisted flare data also suggest that the 8 percent H 2 content provision for non-assisted flares in 40 CFR 63.11(b)(6)(i) may not ensure prope r flare destruction effici ency in all cases. There are several runs where the combustion zone H 2 concentration exceeded 8 percent, but the average run combustion efficiency was well unde r 95 percent. These poor performing runs included runs that had low olefins con centration (i.e., runs not meeting the H 2 -olefin interaction criteria outlined previously in this memorandum). Thus, the poor performan ce of the flares with high hydrogen content in the combusti on zone cannot be attributed to H 2 -olefin interactions. As these hydrogen concentrations were assessed in the combustion zone, they are expected to be analogous to vent gas properties for non-assi sted flares. As such, the 8 percent H 2 content provision for non-assisted flares in 40 CFR 63.11(b)(6)(i) is not expected to be sufficient to ensure proper flare performance.

7. References

Coburn, J. 2013. Meeting Minutes for February 13, 2013; Meeting Between U.S. EPA and Representatives from John Zink Hamworthy Combustion . Memorandum from Jeff Coburn, RTI International, to Andrew Bouchard and Brenda Shine, U.S. EPA. February 18, 2013.Technical Memorandum – Petroleu m Refinery Sector Rule: Operating Limits for Flares December 12, 2013

27 EPA (U.S. Environmental Protection Agency). 1983. Flare Efficiency Study . Prepared for U.S. Environmental Protection Agency, Industrial En vironmental Research Laboratory, Research Triangle Park, NC. July 1983. EPA-600/2-83-052

EPA (U.S. Environmental Protection Agency). 1985. Evaluation of the Effi ciency of Industrial Flares: Flare Head Design and Gas Composition. Prepared for U.S. Environmental Protection Agency, Air and Energy Research La boratory, Research Triangle Park, NC. September 1985. EPA-600/2-85-106.

EPA (U.S. Environmental Protection Agency). 2011. Petroleum Refinery Database, based on Refinery Information Collection Request (I CR) for Calendar Year 2010. September 2011. Docket Item Nos. EPA- HQ-OAR-2010-0682-0061 through 0069.

EPA (U.S. Environmental Protection Agency). 2012. 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.

Wikipedia. 2013. See http://en.wikipedia.org/wiki/Combustion . Last accessed December 12, 2013.

Zabetakis, M.G. 1965. Flammability Characteristics of Combustible Gases and Vapors . U.S. Department of the Interior, Bureau of Mines. Bulletin 627, Appendix A, pp. 114-116.