Bob Lucas/RTP/ 

USEPA/US@EPA 

02/06/2009 09:06 AM 

To: Vicente.Martinez@bp.com 

cc 

Subject: SO2 Emissions to 0% 

Vicente, 

I've attached a response to your email that addresses your questions on calculations.

Bob Lucas 

USEPA/OAQPS 

Sector Policies and Programs Division 

(See attached file: SO2 Emissions to 0% O2 Compensation_jbc comments.doc) 

"Martinez, Vicente" 

<Vicente.Martinez 

@bp.com> 

02/02/2009 09:00 AM 

To: Bob Lucas/RTP/USEPA/US@EPA 

cc 

Subject: SO2 Emissions to 0% O2 Compensation.doc 

Bob, 

Attached are our assumptions for the an accurate representation of the SO2 concentration from our CAT 

units. 

Let me know if you concur or recommend another approved EPA method. 

Best Regards, 

Vicente Martinez 

219-473-2175 

(See attached file: SO2 Emissions to 0% O2 Compensation.doc) 
SO2 Emissions to 0% O2 Compensation.doc 

January 29, 2009 

Methodology for Calculating SO2 Emissions to 0% O2 Compensation During Periods of 

Pure Oxygen Injection 

To: Vicente Martinez 

Keith Hammer 

In the Code of Federal Regulations 266.105, there is a calculation for calculating particulate 

matter emissions. This calculation includes a variable for the concentration of oxygen. For 

devices that feed oxygen-enriched air (such as pure O2 from Praxair), the value is the 

concentration of oxygen in the enriched air. This equation was utilized for calculating the SO2 

emissions during periods of pure oxygen usage. 

This equation is stated as follows: 

(O2 conc in combustion air measured O2 in stack )

20.9 Pollutant = Pollutant *

dry basis drybasis

corrected measured −

In this scenario, the pollutant in the equation would be SO2. The measured pollutant is the raw 

values for SO2 from the CEMS analyzer. The measured O2 in the stack on a dry basis is the 

value for O2 from the CEMS analyzer. The only variable that needs to be calculated is the O2 

concentration in combustion air on a dry basis. The following information describes the 

methodology used for calculating the O2 concentration in combustion air to account for the use 

of pure oxygen. 

Calculating Combustion Air on a Dry Basis 

The first step is to calculate the total air on a wet basis based upon the flowmeters in our system 

and then convert that flow to a dry basis. To calculate the wet basis flow, the flows from all 

sources of combustion air should be summed. 

Using the absolute humidity, the total air on a wet basis was converted to a total air on a dry 

basis. 

Total Air Total Air (Total Air * Absolute Humidity) drybasis = wetbasis − wetbasis

In order to calculate absolute humidity, there has to be a reference to ambient temperature, 

relative humidity, and barometric pressure. Recently, these weather variables have been added 

to the PI system. For the period in question however, this data is not available via PI. Rather, 

historical data for these variables were sought from http://www.weatherunderground.com. 

To calculate the absolute humidity, the following equation was utilized: 

100

Re lative Humidity Absolute Humidity = Saturated Humidity*
SO2 Emissions to 0% O2 Compensation.doc 

Absolute humidity is expressed as pounds of water per pound of air. 

To calculate the saturated humidity, the following equation was utilized: 

( ) S amb S

air

water * P / P P

MW

MW Saturated Humidity = −

Saturated humidity is expressed as pounds of water per pound of air. Values and assumptions 

for each variable in the above equation are as follows. 

• Molecular Weight of Water (MWwater) – 18.02 • Molecular Weight of Air (MWair) – 28.85 • Vapor Pressure of Water at Ambient Temperature (PS) – determined from steam tables 

based upon the ambient temperature for each day (expressed in psia). • Ambient Pressure (Pamb) – daily barometric pressure (expressed in psia).

Calculating Purchase O2 Usage 

Utilizing a PI, there is a tag for the amount of pure O2 (tons/day) that is used at FCU600. 

Calculating O2 Concentration in Combustion Air While Using Pure O2 

Utilizing the calculations above, the calculation for O2 concentration in combustion air while 

using pure O2 is stated as: 

*100

Total Air Total Purchased O2

Total Air *0.209 Total Purchased O2*1

O2 Conc in Combustion Air

drybasis

drybasis

drybasis +

+ =

This equation assumes 0.209 as the concentration of O2 in air and 1 as the concentration of O2 in 

the O2 from Praxair. 
SO2 Emissions to 0% O2 Compensation_jbc comments.doc 

January 29, 2009 

Methodology for Calculating SO2 Emissions to 0% O2 Compensation During Periods of 

Pure Oxygen Injection 

To: Vicente Martinez 

Keith Hammer 

In the Code of Federal Regulations 266.105, there is a calculation for calculating particulate 

matter emissions. This calculation includes a variable for the concentration of oxygen. For 

devices that feed oxygen-enriched air (such as pure O2 from Praxair), the value is the 

concentration of oxygen in the enriched air. This equation was utilized for calculating the SO2 

emissions during periods of pure oxygen usage. 

This equation is stated as follows: 

(O2 conc in combustion air measured O2 in stack )

20.9 Pollutant = Pollutant *

dry basis drybasis

corrected measured −

In this scenario, the pollutant in the equation would be SO2. The measured pollutant is the raw 

values for SO2 from the CEMS analyzer. The measured O2 in the stack on a dry basis is the 

value for O2 from the CEMS analyzer. The only variable that needs to be calculated is the O2 

concentration in combustion air on a dry basis. The following information describes the 

methodology used for calculating the O2 concentration in combustion air to account for the use 

of pure oxygen. 

JBC: The use of this equation is appropriate provided that the emission limit was derived 

primarily (or only) from systems that did not use oxygen enrichment. While some of the fluid 

catalytic cracking unit (FCCU) or fluid coking unit (FCU) data might have been from oxygen 

enriched systems, we do not know how many or to what extent oxygen enrichment was used. It 

is very unlikely that we have data for units that have very high oxygen enrichment. As such, the 

equation above can be used to effectively reduce the measured concentration in the oxygen 

enriched system to the concentration that would have occurred had air been used. The emission 

limit assumes a certain amount of N2 will dilute the measured pollutant concentration. Without 

the extra nitrogen, you will have less total gas flow and but higher concentrations even though 

you have the same mass emissions as a system that burns only air. The equation suggested for 

FCCU corrects this higher measured concentration downward to be comparable to concentration 

that would be present had the unit been air-fed (all other things being equal). All O2 

concentrations used in this equation should be in units of % by volume, on a dry basis. [Note: 

the above equation is not appropriate for oxygen enriched sulfur recovery units (SRU), because 

the emission limits for that affected facility were derived specific for oxygen enriched systems.] 

[end JBC]
SO2 Emissions to 0% O2 Compensation_jbc comments.doc 

Calculating Combustion Air on a Dry Basis 

The first step is to calculate the total air on a wet basis based upon the flowmeters in our system 

and then convert that flow to a dry basis. To calculate the wet basis flow, the flows from all 

sources of combustion air should be summed. 

Using the absolute humidity, the total air on a wet basis was converted to a total air on a dry 

basis. 

Total Air Total Air (Total Air * Absolute Humidity) drybasis = wetbasis − wetbasis

In order to calculate absolute humidity, there has to be a reference to ambient temperature, 

relative humidity, and barometric pressure. Recently, these weather variables have been added 

to the PI system. For the period in question however, this data is not available via PI. Rather, 

historical data for these variables were sought from http://www.weatherunderground.com. 

To calculate the absolute humidity, the following equation was utilized: 

100

Re lative Humidity Absolute Humidity = Saturated Humidity *

Absolute humidity is expressed as pounds of water per pound of air. 

To calculate the saturated humidity, the following equation was utilized: 

( ) S amb S

air

water * P / P P

MW

MW Saturated Humidity = −

Saturated humidity is expressed as pounds of water per pound of air. Values and assumptions 

for each variable in the above equation are as follows. 

• Molecular Weight of Water (MWwater) – 18.02 • Molecular Weight of Air (MWair) – 28.85 • Vapor Pressure of Water at Ambient Temperature (PS) – determined from steam tables 

based upon the ambient temperature for each day (expressed in psia). • Ambient Pressure (Pamb) – daily barometric pressure (expressed in psia).

JBC: I agree with the last equation. However, note the (Percent) Relative Humidity is actually 

defined as RH = 100 PWV/PS (where PWV is the partial pressure of water vapor). Absolute 

Humidity is defined as: 

( ) WV amb WV

air

water P P P

MW

MW Absolute Humidity = * / −

The middle equation would only change the numerator, but not the denominator. The correct 

equation for absolute humidity should be: 
SO2 Emissions to 0% O2 Compensation_jbc comments.doc 

*( )( ) P * RH /100 /( ) P P * RH /100

MW

MW Absolute Humidity S amb S

air

water = −

Finally, absolute humidity has units pounds of water per pounds of DRY air. As such, the first 

equation in this section is not correct. 

[Note: There may also be issues with the flow meter: does the flow meter measure volumetric 

flow (e.g., velocity, which is then translated to volumetric flow) or is it really a mass flow meter? 

For current purposes, I will assume the flow meter provides (or is used to estimate) the mass 

flow. However, to properly convert a volumetric flow rate to mass flow rate, you would have to 

know the average molecular with of the gas stream, and given confusion on the water content, I 

am concerned that this flow rate may have errors...] 

I believe the correct equation for mass flow of dry air is: 

(1 Absolute Humidity)

Total Mass Air Total Mass Air wetbasis

drybasis + =

[end JBC]

Calculating Purchase O2 Usage 

Utilizing a PI, there is a tag for the amount of pure O2 (tons/day) that is used at FCU600. 

Calculating O2 Concentration in Combustion Air While Using Pure O2 

Utilizing the calculations above, the calculation for O2 concentration in combustion air while 

using pure O2 is stated as: 

*100

Total Air Total Purchased O2

Total Air *0.209 Total Purchased O2*1

O2 Conc in Combustion Air

drybasis

drybasis

drybasis +

+ =

This equation assumes 0.209 as the concentration of O2 in air and 1 as the concentration of O2 in 

the O2 from Praxair. 

JBC: Again, I have issues with units in this equation. The 0.209 factor is a volumetric 

concentration of oxygen in air. Except for this term, all other factors on the RHS of the equation 

appear to be provided on a mass basis. The oxygen concentration of air on a mass basis is 

approximately 0.232. However, the “O2 Conc in Combustion Airdry basis” needs to be determined 

on volumetric basis (for use in the initial equation of this section). As such, all of the flow rates 

need to be calculated on a volumetric basis for use in this equation. [end JBC]