FROM: Jeff Coburn, RTI International

TO: Brenda Shine, EPA/OAQPS/SPPD

FOR: Docket ID No. EPA-HQ-OAR-2010-0682

DATE: August 28, 2013

SUBJECT: Applicability of Emission Limits for Sulfur Recovery Plants that Use Oxygen Enrichment in Claus Burner for Refinery MACT 2

1. Purpose

The purpose of this memorandum is to determine if the correlation equa tions developed for the second set of new source performance standard s (NSPS) for petroleum refineries (40 CFR part 60, subpart Ja) for sulfur recovery plan ts that use oxygen enrichment are suitable for inclusion in Refinery MACT 2 (40 CFR part 63, subpart UUU).

2. Regulatory Background

Refinery MACT 2, promulgated in April 2002, c ontains hazardous air pollutant (HAP) standards for sulfur recovery units (SRU) that were based on the first set of NSPS for petroleum refineries (40 CFR part 60, subpart J). The NSPS subpart J limits were predicated on the use of a Claus sulfur recovery train followed by a tail gas trea tment unit (TGTU) “control system” to achieve an estimated 99.9 percent sulfur r ecovery efficiency across the sulf ur recovery plant. The NSPS subpart J includes an emission limit of 300 parts per million by volume (ppmv) reduced sulfur compounds for a reduction control system not fo llowed by an incinerator and an emission limit of 250 ppmv sulfur dioxide (SO 2 ) (dry basis, 0 percent excess air) for oxidative control systems or reductive control systems follo wed by incineration. These NSPS subpart J limits applied only to Claus sulfur recovery plants (sulfur recovery plants can contain one or more SRU trains) with a sulfur recovery capacity greater the 20 long tons per day (LTD).

Refinery MACT 2 allows owners or operators of an SRU (defined as “a process unit that recovers elemental sulfur from gases that contai n reduced sulfur compounds and other pollutants, usually by a vapor-phase catal ytic reaction of sulfur di oxide and hydrogen sulfide” and specifically excludes the LO-CAT II process) to meet the NSPS subpart J limit or to meet reduced sulfur compound limit of 300 ppmv (dry ba sis, 0 percent excess ai r), regardless of the type of control system or the presence of an in cinerator. In fact, it was anticipated that an incinerator would be used to comply with this reduced sulfur compound limit for any SRU that did not have a TGTU control system ( i.e. , units not subject to NSPS subpart J). Furthermore, unlike NSPS subpart J, Refinery MACT 2 did not have a capacity applicability limit, so this 300 ppmv reduced sulfur compound limit is appli cable to all Claus SRU, regardless of size.Technical Memorandum – Applicability of Emission Limits for Sulfur Recovery Plants that Use Oxygen Enrichment in Claus Burner for Refinery MACT 2 August 28, 2013

2 In June 2008, the U.S. Environmental Protecti on Agency (EPA) promulgated Refinery NSPS subpart Ja, which included new provisions for the sulfur recovery plant. First, for sulfur recovery plants with a capac ity of more than 20 LTD, a new correlation was introduced to provide equivalent emission limits for systems that use oxygen-enriched air in their Claus burner. Second, emission limits were added for sulfur rec overy plants with a capac ity of 20 LTD or less, to require new, small sulfur rec overy plants to achieve a target sulfur recovery efficiency of 99 percent.

3. Technical Background

The Claus reaction used in most petroleum refinery SRU is:



### O 2H S SO S 2H 2 2 2 

###   

Sour gas feed to the sulfur recovery plant contains primarily hydrogen sulfide (H 2 S), carbon dioxide (CO 2 ), ammonia (NH 3 ), and trace quantities of hydro carbons (both light “fuel gas” hydrocarbons and aromatics, which have higher solubi lity in amine solutions). Prior to the Claus reactors, one-third of th e feed gas is oxidized in the Claus burner to provide the stoichiometric amount of SO 2 for the Claus reaction. A typical Clau s SRU consists of two to four Claus reactors. Elemental sulfur is condensed after ea ch Claus reactor, driving the equilibrium of the Claus reaction to the right. Therefore, the more Claus reactors in the sulfur recovery train, the more efficient the sulfur recovery plant. A typical Claus sulfur r ecovery train has sulfur recovery efficiencies of 95 to 97 percent. The gas rema ining after the sulfur is recovered is commonly referred to as “tail gas.” It is common practice to use a thermal oxidizer or incinerator to convert any reduced sulfur compounds in the Claus tail gas to SO 2 prior to discharging the tail gas to atmosphere.

There are processes to extend the Claus reaction or to further treat (“control”) the tail gas. Processes that extend the Claus reaction include PROClaus, sub-dewpoint Claus reactor, and direct oxidation. These processes allow the sulf ur recovery plant to achieve greater than 99 percent sulfur recovery effici ency. Alternatively, a TGTU control system, such as a SCOT or Beavon unit, can be used. These TGTU common ly use a hydrogenation process to convert the sulfur compounds in the Claus tail gas to H 2 S, which can then be reprocessed through the Claus train. Typical Claus/TGTU sulfur recovery pl ants achieve greater than 99.9 percent sulfur recovery efficiencies. Even with the use of an extended Claus of TGTU, most sulfur recovery plants at petroleum refineries still use a thermal oxidizer or incinerator to convert any remaining reduced sulfur compounds to SO 2 . The NSPS subpart Ja limits fo r small sulfur recovery plants (20 LTD or less) are based on use of processe s to extend the Claus reaction. The NSPS subpart J/Ja limits for large sulfur recovery pl ants (greater than 20 LTD ) are based on use of a TGTU control system.

The NSPS subpart J emission limits were establis hed on a concentration basis for SRU achieving the target sulfur recovery efficiency assuming ambient air is used in the Claus burner. As ambient air is about 79 percent inert (78 percen t nitrogen and 1 percen t argon), a considerableTechnical Memorandum – Applicability of Emission Limits for Sulfur Recovery Plants that Use Oxygen Enrichment in Claus Burner for Refinery MACT 2 August 28, 2013

3 amount of inerts are added to the process flow when ambient air is used in the Claus burner. The proportion of inerts in the proce ss flow increases through the Claus train as sulfur is condensed. The owner or operator of a sulfur recovery plan t can use oxygen-enriched air in the Claus burner to improve operational reliability a nd increase the effective capacity of the sulfur recovery plant by reducing the amount of inerts in troduced in the Claus burner. However, inerts introduced in the Claus burner act to dilute the SO 2 or reduced sulfur compound con centration in the tail gas. Without the inerts present, it b ecomes difficult to meet the concentration-based limits, event with high sulfur recovery efficiencies. Conseque ntly, the concentration-based NSPS subpart J standards tend to discourage the use of oxygen-enriched air. The new NSPS subpart Ja introduces a correlation equation de signed to provide owners or ope rators of sulfur recovery plants that use oxygen-enriched ai r a means to determine an appr opriate (comparable) tail gas concentration emissions limit.

4. SRU Flow Model and Emission Limit Correlation

An SRU flow model was develope d to project the flow and composition of sulfur compounds in the sulfur recovery plant. The model includes inputs for the overall target sulfur recovery efficiency, the dry gas composition of the sour gas feed (considers only the concentrations of H 2 S, CO 2 , NH 3 , and total hydrocarbons), the oxygen con centration of the air or oxygen-enriched air used in the Claus burner, a nd the TGTU flow factor . The TGTU flow f actor accounts for the air and fuel added in typical TGTU processes.

The SRU flow model considers the oxygen need ed to burn one-third of the gas stream. Combustion of the gas stream includes converting NH 3 to nitric oxide (NO) (and water), hydrocarbons (as propane) to CO 2 , H 2 S to SO 2 . The dry gas molar volume change, which only occurs for hydrocarbon combustion, is determined assuming C3-hydrocarbons (because the input hydrocarbon concentration is measured as propane). The amount of nitrogen introduced to the Claus burner is determined by the input oxygen enrichment level. The model projects the loss of dry gas flow from the recovery of elemental sulfur (based on the input sulfur recovery efficiency) and the additional flow needed fo r TGTU hydrogenation (based on the input flow factor). The flow model outputs the total concentration of sulfur compounds (as H 2 S) in the tail gas prior to incineration a nd the concentration of SO 2 in the exhaust from the tail gas incinerator (based on ratio of NSPS subpart J limits, 300/250 = 1.2 flow factor for in cinerator air/fuel).

Table 1 provides a summary of the SRU flow model input parameters used. The oxygen concentration in the Claus burner was vari ed to project the reduced sulfur compound concentration from a reductive control system and the SO 2 concentration in the tail gas incinerator exhaust that corres ponds to 99.9 percent sulfur recove ry efficiency for the various oxygen enrichment levels. Table 2 summarizes the results of th e SRU flow model simulations for 99.9 percent sulfur recovery.Technical Memorandum – Applicability of Emission Limits for Sulfur Recovery Plants that Use Oxygen Enrichment in Claus Burner for Refinery MACT 2 August 28, 2013

4 Table 1. SRU Flow Model Input Parameters a

Parameters Value Units
H 2 S concentration 74% % by volume, dry
NH 3 concentration 10% % by volume, dry
CO 2 concentration 15% % by volume, dry Total hydrocarbon concentration 1% % by volume, dry
O 2 concentration in air/O 2 stream inlet to Claus burner Varied from 20.9 to 100% % by volume, dry

Flow factor for TGTU process 1.06 Assumed sulfur recovery efficiency 99.9% or 99% % inlet S recovered as elemental S

a % = percent; O 2 = oxygen; S = sulfur.

Table 2. SRU Flow Model Outlet Concentrations

Assumed O 2 Concentration Inlet to Claus Burner (% by volume, dry) Reduced Sulfur Compound Concentration (ppmv, dry, 0% O 2 )

in Reductive Control System at 99.9% Sulfur Recovery Efficiency SO 2 Concentration (ppmv, dry, 0% O 2 ) from Tail Gas Incinerator at 99.9% Sulfur Recovery Efficiency 20.9 % (ambient air) 297.0 247.5
35 % 457.9 381.6
50 % 603.2 502.6
75 % 800.7 667.3
100 % 957.5 797.9

A second order polynomial equation provided the best fit of the data in Table 2 with an R 2 of 0.9999. The parameters of the best fit polynomial equation were adjusted to meet the following conditions:

1. The equation was required to predict out put concentration of 300/250 ppmv (reduced sulfur compound or SO 2 , respectively) at a Claus burne r inlet oxygen concentration of 20.9 percent (ambient air).

2. The equation was required to output con centrations greater than the modeled concentrations for all Claus bur ner inlet oxygen concentrations.

3. The ratio of the modeled concentration and th e equation’s output concentration was to be as close as practical to th e ratio determined for a Claus burner with an inlet oxygen concentration of 20.9 pe rcent (ambient air).

Based on these criteria, the fo llowing equation was derived:Technical Memorandum – Applicability of Emission Limits for Sulfur Recovery Plants that Use Oxygen Enrichment in Claus Burner for Refinery MACT 2 August 28, 2013

5 

##     

#   6

. 25 53

. 11 038

. 0 2    

  2 2 1 %O %O k EmissLimit (Eqn. 1)

where:

EmissLimit = Emission limit for a 99.9 percen t efficient sulfur recovery plant; ppmv, dry basis, at 0 percent excess air of either SO 2 or reduced sulfur compounds.

k 1 = Constant factor for emission limit conversion: k 1 = 1 for incinerator outlet SO 2 limit and k 1 = 1.2 for tail gas reduced sulfur compounds limit.

%O 2 = Oxygen (O 2 ) concentration of the inlet air/O 2 stream to the Claus burner, percent by volume (dry basis).

A similar analysis was done at various oxygen enrichment levels assuming 99 percent sulfur recovery efficiency. As the modeled concen trations for a 99 percent efficient SRU were essentially equivalent to 10 times the concentr ation of a 99.9 percent efficient SRU, the same correlation, multiplied by a factor of 10, was selected for determining appropriate emission limits for smaller SRU that use oxygen enrichment in the new NSPS subpart Ja.

5. Appropriateness of New Subpart Ja Emi ssion Limits for Refinery MACT 2

For sulfur recovery plants with a capacity greater than 20 LTD and that use ambient air in the Claus burner, the NSPS subpart Ja limits are th e same as those in NSPS subpart J (i.e., §60.102a(f)(1)(i) and (ii) of NSPS subpart Ja contain the same lim its as §60.104(a)(2)(i) and (ii) of NSPS subpart J). As the NSPS subpart J lim its were the basis of the Refinery MACT 2 emission limits, compliance with the emission limits in §60.102a(f)(1)(i) and (ii) of NSPS subpart Ja is sufficient to demonstrate complia nce with the requirements in Refinery MACT 2 sulfur recovery plants with a capacity greater than 20 LTD.

For sulfur recovery plants with a capacity greater than 20 LTD and that use oxygen-enriched air in the Claus burner, NSPS subpart Jaallows the owner or operator of the sulfur recovery plant to use Equation 1 above to determine the appropria te emission concentration. As Equation 1 was developed based on a fixed sulfur recovery effi ciency, the mass emissions rate for the sulfur recovery plant was held constant while the level of oxygen enrich ment was varied. Thus, while Equation 1 may allow owners or operators of sulfur recovery plants to emit a higher concentration of SO 2 or reduced sulfur compounds in the tail gas when oxygen enrichment is used, it does not result inan increase in the mass emission rate. That is, Equation 1 yields a mass emission rate for an oxygen-enriched SRU that is equivalent to the mass emission limit required in Refinery MACT 2 (or NSPS subpart J/Ja) for an SRU that uses only ambient air. Consequently, it is reasonable to allow the use of the emission limits in §60.102a(f)(1)(iii) of NSPS subpart Ja (i.e., Equation 1 for sulfur recovery plants th at use oxygen enrichment) as a means to comply with Refinery MACT 2.Technical Memorandum – Applicability of Emission Limits for Sulfur Recovery Plants that Use Oxygen Enrichment in Claus Burner for Refinery MACT 2 August 28, 2013

6 For sulfur recovery plants with a capacity of 20 LTD or less, NSPS subpart Ja provides emission limits that are a factor of 10 highe r than those required for larger sulfur recovery plants. There were no “sulfur oxide” emission limits for these smaller sulfur recovery plants in NSPS subpart J. The compliance options in Refinery MA CT 2 effectively requiredthese smaller sulfur recovery plants to either meet the NSPS limits for larger sulfur recovery plants (effectively achieve 99.9 percent sulfur recovery) or use an incinerator to meet a 300 ppmv reduced sulfur compound emissions limit regardless of the sulfur recovery efficiency of the plant. The emission limits for small sulfur recovery plants in NSPS subpart Ja are not equivalent to the emission limits required in Refinery MACT 2. Therefor e, the emission limits provided in §60.102a(f)(2) of NSPS subpart Ja cannot be used to dem onstrate compliance with the Refinery MACT 2 standards. In reviewing the emission limits in NSPS subpart Ja, it appears that the emission limits in NSPS subpart Ja for a small SRU empl oying a reductive contro l systems not followed by incineration are meaningless. Refinery MACT 2 effectively requires that an incinerator be used for any sulfur recovery plant not achieving a sulfur recovery efficiency of approximately 99.9 percent. Thus, while the new emission limit s in NSPS subpart Ja e ffectively establish a minimum sulfur recovery effici ency of 99 percent for small su lfur recovery plants where no standard existed under NSPS subpart J, when c onsidered with the Refinery MACT 2 emission limits, any small sulfur recovery plant with a re ductive control system ac hieving only 99 percent sulfur recovery efficiency would have to be fo llowed by an incinerator. As such, it does not appear that the 3,000 ppmv reduced sulf ur compound emission limit provided in §60.102a(f)(2)(ii) of NSPS subpart Ja can be us ed in practice at a petroleum refinery.