Matthew Todd 

1220 L Street NW

Washington, DC 20005-4070

USA

Telephone: 202-682-8319 

Email: toddm@api.org

www.api.org

August 28, 2012

Ms. Brenda Shine

US EPA

Office of Air Quality Planning and Standards

Mail Drop E143-01

Research Triangle Park, NC 27711

shine.brenda@epa.gov

Re: Potential Delayed Coker Emission Controls

Dear Brenda:

The American Petroleum Institute (“API”) has been working with EPA on the Refinery Sector 

Rulemaking since the beginning of that effort. We appreciate the dialogue we have been able to 

have about emissions from delayed cokers and their control. In particular, we appreciate the 

recent sharing of your impressions from your June visit to three delayed coking units (DCUs). 

We believe those impressions reflect a reasonable understanding of the complex, inter-related

issues associated with managing DCU operations and their emissions, the individuality of each 

installation, and the criticality of coker throughput to meeting refined product market demand.

Introduction

Coke drum vent pressure at the time the drum vent is routed to the atmosphere is a readily 

measurable parameter that has been established as the key parameter reflective of coke drum 

vent emissions. A pressure criterion is already used in RMACT 1, NSPS Ja, state regulations, 

and facility permits as the basis for allowing release of the coke drum vent to the atmosphere. 

Therefore, establishing a revised RMACT 1 pressure criterion as a work practice for limiting 

coke drum emissions to the atmosphere represents the best, most consistent way to further 

manage coke drum emissions in the Refinery Sector Rulemaking. 

In order to help inform your decision on what the most justifiable pressure criterion would be,

refinery coker experts have evaluated the potential emission reductions and costs for achieving 

various pressure levels. They conclude that the available data shows that there is little difference 

in coke drum emissions at vent pressure thresholds between 5 psig and 2 psig and that there can 

be significant costs associated with meeting a 2 psig criterion versus meeting a 5 psig criterion. 

Thus, while the available emissions and cost data are sparse and of varying quality, they do not 
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support establishing a work practice standard for existing DCUs that is any more stringent than 

that recently adopted for modified and reconstructed DCUs in NSPS Ja (i.e., 5 psig).

Emissions and risk

The pressure at which coke drums are vented to the atmosphere varies from unit to unit and 

facility to facility based upon a number of criteria, including type of coke produced, equipment 

design, and fractionator, flare, or fuel gas system backpressure. Some drums can be vented at 

pressures as low as 2 psig and others approach 15 psig before being vented. As we discuss in 

Attachment 1, the available data do not demonstrate a significant emissions difference for an 

atmospheric release criterion below 5 psig. This is consistent with the estimates EPA developed 

in the NSPS Ja rulemaking where the Agency estimated an insignificant incremental VOC 

reduction of only 1.31 lb/event for 2 psig versus 5 psig1

.

For this rulemaking, the impact of coke drum emissions on risk are being assessed by using HAP 

emission estimates collected in Part 2 of the recent Refinery Information Collection Request 

(ICR). As we also discuss in Attachment 1, the emission factors that were required to be used 

for these HAP estimates were based on a single data set (the HOVENSA data). The additional 

data recently collected in Part 4 of the ICR indicate that those emission factors and thus the 

calculated HAP emissions and risk are significantly overstated and actual HAP emissions and 

risk from coke drums may be as much as several orders of magnitude lower than were used by 

EPA in the risk modeling for this rulemaking.

In summary, our review of the available data indicates that (1), there is likely only a small VOC 

and HAP emission reduction potential associated with setting a coke drum release to the 

atmosphere criterion below 5 psig and (2), the HAP emissions and thus risk increment attributed 

to coke drum emissions in the current rulemaking effort is likely significantly overstated because 

the HAP emission factors used in the ICR significantly overstate the actual emissions. 

Equipment considerations and costs of pressure reduction

The quenching of a coke drum full of hot coke involves adding water from the bottom of the 

drum. The water cools the coke by means of a phase change from liquid to vapor. The steam 

vapor formed in this process passes across the remaining hot coke and out the top of the coke 

drum. During the quenching operation, the coke drum vapors are routed to the DCU

“blowdown” section where the steam and most hydrocarbons are cooled, condensed, and 

separated into a water phase and a hydrocarbon phase. 

The blowdown section of the plant typically uses fin fan coolers (air cooled heat exchangers) for 

removing heat from the coke drum vapor. Under fin fan cooler conditions, a portion of the 

hydrocarbon leaving the coke drum cannot be fully condensed. These non-condensable 

hydrocarbon molecules were originally routed to a flare system. Over time, flare gas recovery 

has been added to many DCUs to compress these gases and deliver them to a refinery fuel gas 

 1 Docket Item EPA-HQ-OAR-2007-0011-0226.1, Delayed Coker Cost Effectiveness Spreadsheet. EPA calculated, based on an 

assumed 20 hour cycle time and no outages, a model coke drum would have 438 such events per year, resulting in an annual 

estimated emission of 0.29 Tons of VOC.
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system rather than to a flare. On some cokers, non-condensable hydrocarbon molecules from the 

blowdown section are routed to the coker main fractionator overhead system for recovery during 

at least part of the coke drum quench cycle rather than to a flare or flare gas recovery system.

Once the water level in the coke drum is above the level of coke in the drum, the generation of 

steam is reduced significantly. At that point, the pressure on the drum is a function of: 

1) the pressure drop between the coke drum and the blowdown system caused by the 

remaining flow of vapor; and 

2) the operating pressure of the blowdown system. 

Due to physical layout constraints, DCUs vary greatly in the configuration, length, and diameter 

of the piping between the coke drums and blowdown section. Some blowdown sections are 

closer to the drums in some DCUs than are others. In DCUs with multiple drums, the blowdown 

section is farther away from some drums than from other drums. As a result, the pressure on a 

well-quenched drum will be different from unit to unit and drum to drum.

Lowering the back-pressure on a quenched coke drum below the pressure set by the DCU design 

without increasing cycle time requires significant changes in the mechanical hardware in the 

DCU. Generally, this involves lowering the blowdown section operating pressure and/or 

reducing the pressure drop between the coke drum and the blowdown section (by upsizing long 

runs of large diameter pipe, adding eductors and/or compressors, and/or adding blowdown 

section cooling). Attachment 2 discusses the types of changes that might be required in more 

detail. Of course, the specific changes that will be needed will be different from unit to unit and 

drum to drum. In addition to the typical problems associated with installing new facilities in an 

existing, congested unit without impacting production, as the pressure is lowered in the 

blowdown section, there is increased risk of creating a negative pressure somewhere in the 

system, and safeguards must be incorporated into the design to prevent such an occurrence. 

Little data is available on the cost of reducing coke drum vent pressure from 5 psig to 2 psig 

because these changes are often done as part of larger projects and cannot easily be separated and 

because each situation is different. For some DCUs our members report that there is no 

investment associated with meeting a 2 psig limit because they already are achieving that level or 

because of their DCU’s design. Other members indicate that significant investments will be 

needed.

One of our members reports that the cost to lower the coke drum outlet release pressure from 5 

psig to 2 psig for one DCU was $3.7 million for steam eductors and additional blowdown fin fan 

coolers. For this particular DCU, no investment in compression was required. Another member 

reports an investment of approximately $1.5 million for adding eductors to achieve 2 psig. On 

the other hand, two members identified seven DCUs where an incremental investment of $5-8 

million per DCU will be needed to meet a 2 psig criterion versus meeting a 5 psig criterion. This 

is consistent with the $6 million incremental investment EPA used in evaluating the 2 psig 

criterion versus a 5 psig criterion in the NSPS Ja rulemaking.

Even with these investments, a lower vent pressure criterion may cause coke drum operating 

cycles to be longer, as it may take longer to reach the target vent pressure. Longer cycle times 
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can result in lower coker throughput and, thus, large refinery product production debits. These 

large lost opportunity costs have not been quantified in evaluating the cost effectiveness of DCU 

vent limitations.

Conclusion

In summary, the incremental investment cost for meeting a 2 psig vent criterion versus a 5 psig 

criterion likely ranges from no capital investment to at least $8 million dollars. Even assuming a 

high emission reduction potential of 3 tons of VOC2

, where investment is needed, annualized 

investment costs3 alone will easily exceed $100,000 per ton of VOC reduced and the cost per ton 

will be much higher than that for those DCUs where more than $3.2 million investment is 

needed and when annual costs and lost production debits are included. Costs per ton of HAP 

reduced will be even higher than the costs per ton of VOC.

We believe that using coke drum pressure as the compliance factor is the correct approach and 

that a release to the atmosphere criterion of no less than 5 psig is the appropriate target metric, 

based on the high costs associated with an industry-wide standard of 2 psig as well as the small 

emissions reduction benefit that would be expected with that lower threshold.

Again, we appreciate the dialogue that we continue to have with you on this and other refinery 

sector issues. Please let me know if you would like to discuss our analysis further.

Sincerely,

/s/

Matthew Todd

cc Ms. Penny Lassiter, US EPA

 2 Five times what EPA calculated for a 2 coke drum DCU in the NSPS Ja rulemaking. 3 Based on 7% interest for 20 years (CRF = 0.094392926)
Attachment 1

Coke Drum Emissions and Risk

Available stack test data do not demonstrate significant additional emissions reduction with a

reduction in the coke drum atmospheric release criterion from 5 psig to 2 psig.

For example, if one considers the VOC and benzene vent data from the recent Refinery ICR 

data4 set as a function of pressure, it appears that, within the scatter of the data, there is little 

potential reduction per drum opening.

 4 The data sets include ExxonMobil, HOVENSA, Houston Refining, Marathon and Husky. Both the Houston Refining and the 

Husky data assume a nominal vent pressure of 5 psig as presented. HOVENSA data are indicated in red.
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The data from the HOVENSA report appears to be an outlier when compared to more recent

DCU vent tests with respect to both the magnitude of reported gaseous emissions and the length 

of the vent time (>2 hours)5

. It is our understanding that the mass of PAH constituents emitted 

per coke drum venting used to estimate emissions under the ICR protocol (and which are the risk 

drivers motivating changing the coke drum depressurization work practice) were derived 

exclusively from the HOVENSA data set. For instance, as shown in the table below, examining 

2-methylnaphthalene, one can see that the 2.913 lb/drum venting reported in the HOVENSA

study contrasts significantly (by more than 2 orders of magnitude in one case on a lb/drum 

venting basis) from the other available data. Similarly, other hydrocarbons were significantly 

lower in the new Part 4 ICR data sets when compared to the HOVENSA data set. This

comparison strongly suggests that the HAP emissions and risks calculated from emission 

estimates based on the ICR Protocol are significantly overstated.

Measured Emission Rates for Four Hydrocarbons

(Note: Methane and Ethane are not VOCs or HAPs)

HOVENSA ExxonMobil Houston Refining Husky Marathon

2-methylnaphthalene 

(lb/drum venting) 2.913 0.256 0.0132167 0.727 -

Methane

(lb/drum venting) 795.6 75.3 5.3 321 263

Ethane

(lb/drum venting) 168.6 21.4 0.77 54.9 67.2

Benzene

(lb/drum venting) 11 0.341 0.012 1.9 0.76

 5 The reasons for these differences are unknown, but the HOVENSA tests were among the first using a new protocol and the 

individual test runs showed significant differences, which suggests possible operating or sampling problems.
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Attachment 2

Coke Drum Vent Pressure Reduction Considerations

Once the drum is quenched, lowering the coke drum pressure below the minimum pressure set 

by the system design requires significant changes in the mechanical hardware in the unit. The 

required changes typically fall into two categories – lowering the blowdown section operating 

pressure and reducing the pressure drop between the coke drum outlet and the blowdown section. 

The types of changes often needed are described as follows.

1. Lower the blowdown section operating pressure.

a. In most plants, the DCU blowdown section is designed to float on the relief (e.g., 

flare) header. Depending on the specific equipment configuration, installing 

larger pipe between the blowdown system and the relief system (perhaps all the 

way to the flare) may decrease pressure drop somewhat and allow the blowdown 

operating system pressure to be reduced somewhat. However, installing larger 

diameter pipe can be very expensive where space is restricted by existing 

equipment and/or the distance to the relief device (i.e., flare) is significant. Also,

piping changes can only reduce blowdown system backpressure to the operating 

pressure of the relief system or the relief system compressor suction, which may 

not be much different than currently achieved.

b. For some DCUs, additional fin fan coolers may be required in the blowdown 

section to better condense coker vent steam and hydrocarbon to reduce the flow of 

vapor out of the blowdown section to the relief system.

c. Where pressure drop is not the issue, addition of eductors and condensers in the 

relief system to forcibly move material out of the blowdown section and into the 

relief system may be required to lower the blowdown section pressure. Coke 

drums and blowdown systems are not typically designed to handle a vacuum, so 

safeguards must be included to prevent the blowdown section pressure from 

dropping too low.

d. Finally, compression may be needed to lower the blowdown section pressure 

enough to meet the release criterion or to allow routing the vapors to higher 

pressure dispositions such as fuel gas.

2. Lower the pressure drop between the coke drum and blowdown section of the plant.

If, after getting the blowdown section operating pressure as low as possible, the coke drum 

pressure is still too high, one or more of the following may be necessary to achieve a lower 

coke drum outlet pressure.
Page 8

a. Increase the diameter of some or all of the piping from the coke drum to the 

blowdown section.

b. Addition of educators and condensers at the coke drum to forcibly move material 

out of the drum and then into the blowdown section.