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 equations developed for the 

second set of new source performance standards (NSPS) for petroleum refineries (40 CFR 

part 60, subpart Ja) for sulfur recovery plants 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, contains 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 treatment unit (TGTU) “control system” to achieve an 

estimated 99.9 percent sulfur recovery efficiency across the sulfur 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 followed by an incinerator and an emission limit 

of 250 ppmv sulfur dioxide (SO2) (dry basis, 0 percent excess air) for oxidative control systems 

or reductive control systems followed 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 contain reduced sulfur compounds and other pollutants, 

usually by a vapor-phase catalytic reaction of sulfur dioxide 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 basis, 0 percent excess air), regardless of the 

type of control system or the presence of an incinerator. 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 applicable 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 Protection Agency (EPA) promulgated Refinery NSPS 

subpart Ja, which included new provisions for the sulfur recovery plant. First, for sulfur 

recovery plants with a capacity 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 recovery plants with a capacity of 20 LTD or less, 

to require new, small sulfur recovery plants to achieve a target sulfur recovery efficiency of 

99 percent. 

3. Technical Background 

The Claus reaction used in most petroleum refinery SRU is: 

2H2S  SO2  S  2H2O

Sour gas feed to the sulfur recovery plant contains primarily hydrogen sulfide (H2S), carbon 

dioxide (CO2), ammonia (NH3), and trace quantities of hydrocarbons (both light “fuel gas” 

hydrocarbons and aromatics, which have higher solubility in amine solutions). Prior to the Claus 

reactors, one-third of the feed gas is oxidized in the Claus burner to provide the stoichiometric 

amount of SO2 for the Claus reaction. A typical Claus SRU consists of two to four Claus 

reactors. Elemental sulfur is condensed after each 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 recovery train has sulfur recovery 

efficiencies of 95 to 97 percent. The gas remaining 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 SO2 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 sulfur recovery plant to achieve greater than 

99 percent sulfur recovery efficiency. Alternatively, a TGTU control system, such as a SCOT or 

Beavon unit, can be used. These TGTU commonly use a hydrogenation process to convert the 

sulfur compounds in the Claus tail gas to H2S, which can then be reprocessed through the Claus 

train. Typical Claus/TGTU sulfur recovery plants 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 SO2. The NSPS subpart Ja limits for small sulfur recovery plants 

(20 LTD or less) are based on use of processes to extend the Claus reaction. The NSPS 

subpart J/Ja limits for large sulfur recovery plants (greater than 20 LTD) are based on use of a 

TGTU control system. 

The NSPS subpart J emission limits were established 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 percent nitrogen and 1 percent argon), a considerable 
Technical 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 process flow increases through the Claus train as sulfur is condensed. 

The owner or operator of a sulfur recovery plant can use oxygen-enriched air in the Claus burner 

to improve operational reliability and increase the effective capacity of the sulfur recovery plant 

by reducing the amount of inerts introduced in the Claus burner. However, inerts introduced in 

the Claus burner act to dilute the SO2 or reduced sulfur compound concentration in the tail gas. 

Without the inerts present, it becomes difficult to meet the concentration-based limits, event with 

high sulfur recovery efficiencies. Consequently, 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 designed to provide owners or operators of sulfur recovery 

plants that use oxygen-enriched air a means to determine an appropriate (comparable) tail gas 

concentration emissions limit. 

4. SRU Flow Model and Emission Limit Correlation 

An SRU flow model was developed 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 

H2S, CO2, NH3, and total hydrocarbons), the oxygen concentration of the air or oxygen-enriched 

air used in the Claus burner, and the TGTU flow factor. The TGTU flow factor accounts for the 

air and fuel added in typical TGTU processes. 

The SRU flow model considers the oxygen needed to burn one-third of the gas stream. 

Combustion of the gas stream includes converting NH3 to nitric oxide (NO) (and water), 

hydrocarbons (as propane) to CO2, H2S to SO2. 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 for TGTU hydrogenation (based on the input flow 

factor). The flow model outputs the total concentration of sulfur compounds (as H2S) in the tail 

gas prior to incineration and the concentration of SO2 in the exhaust from the tail gas incinerator 

(based on ratio of NSPS subpart J limits, 300/250 = 1.2 flow factor for incinerator air/fuel). 

Table 1 provides a summary of the SRU flow model input parameters used. The oxygen 

concentration in the Claus burner was varied to project the reduced sulfur compound 

concentration from a reductive control system and the SO2 concentration in the tail gas 

incinerator exhaust that corresponds to 99.9 percent sulfur recovery efficiency for the various 

oxygen enrichment levels. Table 2 summarizes the results of the 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 Parametersa

Parameters Value Units 

H2S concentration 74% % by volume, dry 

NH3 concentration 10% % by volume, dry 

CO2 concentration 15% % by volume, dry 

Total hydrocarbon concentration 1% % by volume, dry 

O2 concentration in air/O2 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; O2 = oxygen; S = sulfur. 

Table 2. SRU Flow Model Outlet Concentrations 

Assumed O2 Concentration 

Inlet to Claus Burner (% by 

volume, dry) 

Reduced Sulfur Compound 

Concentration (ppmv, dry, 0% O2) 

in Reductive Control System at 

99.9% Sulfur Recovery Efficiency 

SO2 Concentration (ppmv, 

dry, 0% O2) 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 R2

 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 output concentration of 300/250 ppmv (reduced 

sulfur compound or SO2, respectively) at a Claus burner inlet oxygen concentration of 

20.9 percent (ambient air). 

2. The equation was required to output concentrations greater than the modeled 

concentrations for all Claus burner inlet oxygen concentrations. 

3. The ratio of the modeled concentration and the equation’s output concentration was to be 

as close as practical to the ratio determined for a Claus burner with an inlet oxygen 

concentration of 20.9 percent (ambient air). 

Based on these criteria, the following 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 

 0.038   11.53   25.6 2 EmissLimit  k1   %O2   %O2  (Eqn. 1) 

where: 

 EmissLimit = Emission limit for a 99.9 percent efficient sulfur recovery plant; ppmv, dry 

basis, at 0 percent excess air of either SO2 or reduced sulfur compounds. 

 k1 = Constant factor for emission limit conversion: k1 = 1 for incinerator outlet SO2

limit and k1 = 1.2 for tail gas reduced sulfur compounds limit. 

 %O2 = Oxygen (O2) concentration of the inlet air/O2 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 concentrations for a 99 percent efficient SRU were 

essentially equivalent to 10 times the concentration 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 Emission 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 the same as those in NSPS subpart J (i.e., 

§60.102a(f)(1)(i) and (ii) of NSPS subpart Ja contain the same limits as §60.104(a)(2)(i) and (ii) 

of NSPS subpart J). As the NSPS subpart J limits 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 compliance 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 Ja allows the owner or operator of the sulfur recovery plant to 

use Equation 1 above to determine the appropriate emission concentration. As Equation 1 was 

developed based on a fixed sulfur recovery efficiency, the mass emissions rate for the sulfur 

recovery plant was held constant while the level of oxygen enrichment was varied. Thus, while 

Equation 1 may allow owners or operators of sulfur recovery plants to emit a higher 

concentration of SO2 or reduced sulfur compounds in the tail gas when oxygen enrichment is 

used, it does not result in an 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 that 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 higher 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 MACT 2 effectively required these 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. Therefore, the emission limits provided in §60.102a(f)(2) 

of NSPS subpart Ja cannot be used to demonstrate 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 employing a reductive control 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 limits in NSPS subpart Ja effectively establish a 

minimum sulfur recovery efficiency of 99 percent for small sulfur recovery plants where no 

standard existed under NSPS subpart J, when considered with the Refinery MACT 2 emission 

limits, any small sulfur recovery plant with a reductive control system achieving only 99 percent 

sulfur recovery efficiency would have to be followed by an incinerator. As such, it does not 

appear that the 3,000 ppmv reduced sulfur compound emission limit provided in 

§60.102a(f)(2)(ii) of NSPS subpart Ja can be used in practice at a petroleum refinery.