Gas plant: case study

Purpose

To provide gas processing plants with guidance on how to report to the National Pollutant Release Inventory (NPRI).

Background Information

The NPRI requires that facilities with over 20 000 employee hours consider all NPRI listed substances when determining their reporting status. Part 1 substances manufactured, processed, or otherwise used, meeting or exceeding the threshold quantity, will require a report stating the substance name and amount released, transferred, or disposed. For Part 2 substances, the threshold is based on the incidental manufacture and release of the listed substances. If the threshold quantity is met, then a report is required for Part 2 substances. Part 3 substances are reportable if the facility has one of the 17 specified activities occurring on site. If the any of the specified activities occurs on site, a report is required for the listed substance regardless of the amount released.

Part 4 substances, criteria air contaminants (CACs), are based on mass release thresholds. There are two reporting scenarios for Part 4 substances: releases from stationary combustion equipment only, and releases from all sources (which include stationary combustion equipment). Facilities with less than 20 000 employee hours only need to consider the releases from stationary combustion equipment when performing their mass release threshold calculations. For those facilities with greater than or equal to 20 000 employee hours, all sources of CACs must be considered in the mass release threshold calculations, including stationary combustion sources. Part 5 substances are speciated Volatile Organic Compounds (VOCs) and should only be considered if total VOCs were reported under Part 4.

For more information and a listing of Parts 1 - 5 substances refer to the Guide for Reporting to the National Pollutant Release Inventory.

Scenario

BTS Gas Processing Ltd. is a natural gas exploration and production company, which owns a sour gas processing plant in southern Alberta. Although, oil and gas exploration activities are exempt from reporting to the NPRI, this company will have to analyze it's production facilities individually to see if any of them meet the reporting requirements. For the simplicity of this example we will only look at the one sour gas plant in southern Alberta.

This particular facility employs 90 full time staff. The sour gas stream entering the facility is collected by a main field header and transmitted to the facility through one inlet pipeline. Prior to the main field header the gas undergoes primary water and solids removal. The water and solids removal are not part of the NPRI as long as no processing occurs to these streams and are returned to the original reservoir. The facility produces 120 million cubic feet (Mcf) per day which translates to 3,398,022 cubic metres (m3) per year.

The general process at the facility is illustrated in Figure 1 shown below:

Figure 1 : General Sour Gas Treatment Process Diagram

General Sour Gas Treatment Process Diagram

Each stage in the treatment process outlined above is a source of CAC emissions. In addition, some of the stages may release other NPRI listed substances and need to be addressed accordingly. Other CACs released at the facility result from the burning of natural gas for heating the offices and production area.

Step 1 - Identify Requirements

The first step is to identify the NPRI reporting requirements that pertain to the facility. Since 90 full time employees work at the facility, the employee threshold of 20 000 employee hours has been met, requiring that all NPRI listed substances be considered in the facility's threshold calculations. A report is required for each substance that meets the reporting criteria.

The first group of substances to consider is the Part 1A substances, which have a 10 tonne manufactured, processed, or otherwise used (MPO) threshold. Next the Part 1B substances should be considered, which are treated the same as Part 1A substances only at a lower mass and concentration threshold.

Substance reports for Part 2 substances, Polycyclic Aromatic Hydrocarbons (PAHs), are required if the threshold is met based on the incidental manufacture and release of the listed substance. Since Part 2 substances are not i ncidentally manufactured on site, these substances do not need to be considered by this facility.

Part 3 substances, Dioxins and Furans, need to be reported if the facility has any of 17 specified activities occurring onsite. For a list of these 17 activities refer to the Guide for Reporting to the National Pollutant Release Inventory. Part 3 substance reports, for this particular facility, will not be required since none of the 17 specified activities occur on site.

A gas plant with 1,800,000 employee hours (equivalent to 90 full time employees) falls under the second CAC reporting scenario discussed in the Background Section. This implies that the gas plant must consider all sources of CACs including stationary combustion sources. Some sources include, but are not limited to: internal and external combustion, painting activities, storage tank releases, and loading / unloading activities. More information on these sources can be found in the Guide for Reporting to the National Pollutant Release Inventory. Finally, if a report is filed for total volatile organic compounds (VOCs) under Part 4 then a report is required for each of the speciated VOCs listed in Part 5, which is released in a quantity of 1 tonne or greater.

Step 2 - Identify the Emission Groups at the Facility

After reading the NPRI general and supplementary guides, a survey of the facility's emission sources was performed. For simplification, the major sources were categorized into 8 groups. These groups are identified below:

  • Group 1: Utilities (includes other combustion sources not shown in Figure 1, the process flow diagram)
  • Group 2: Sulphur Flare Stack
  • Group 3: Horizontal Separator
  • Group 4: Gas Sweetening
  • Group 5: Dehydration
  • Group 6: Condensate Recovery System
  • Group 7: Gas Recompression
  • Group 8: Painting of Process Equipment

Step 3 - Calculate Emissions for each Identified Source at the Facility

Since the facility is large and encompasses many sources the analysis and release estimates will be handled on a grouping basis. Each of the eight groups listed above will be broken down into their individual sources and then a group total for each substance will be provided. After the emissions for the eight groups have been estimated, a facility total will be calculated and compared to the NPRI reporting thresholds.

Emission Group 1: Utilities

Identify the Substances of Concern

The only substances of concern released this group are Part 4 substances (i.e. CACs) substances.

Identify the Method of Calculating Emissions

If you are not familiar with the methods of estimation they are described in the Guide for Reporting to the National Pollutant Release Inventory. You may wish to review them before proceeding.

Table 1 identifies the specific emission sources included in the Utilities grouping for this facility. This table also presents the suggested emission estimation techniques and the amount of natural gas consumed by each source, if applicable.

Table 1: Possible Methods of Estimation for Utility Emissions and Consumption Data
Emission Source Suggested Estimation
Method(s)
Consumption Data
Leaks from operating
valve(s) (2 valves) 1(SCC 30600811)
Emission Factors N/A
Emergency Flares2 (SCC 10100602) Emission Factors Natural Gas: 50 m3/yr (1.930 GJ/yr)3
Space Heaters (SCC 10100602) Emission Factors, Direct Measurement Natural Gas: 100,000 m3 (3860 GJ/yr)3
Boilers (SCC 10100602) Emission Factors, Direct Measurement Natural Gas: 100,000 m3 (3,860 GJ/yr)3
Internal Combustion4 (SCC 20200254) Emission Factors Natural Gas: 160,000 m3 (6,176 GJ/yr)3

N/A -- Not applicable.

NOTES:
1 These emission sources will most likely only emit VOCs. However diligence must be exercised if it is expected that other CACs are being emitted from the source.
2 Emissions from emergency natural gas burning flares can be estimated as external combustion boilers.
3 Natural gas in cubic metres can be changed to gigajoules using the conversion factor 0.0386 GJ/m3.
4 Considering one internal combustion, four-stroke lean burn reciprocating engine, fueled by natural gas and operating at 90-105% load.

calculate Emissions

The decision was made to make all estimations using emission factors for these sources. The applicable emission factors can be found in the Table 2, shown below. In this case study, the emission factors from WebFIRE (based on the SCC identified in Table 1) were utilized with the appropriate conversions applied (i.e. imperial to metric conversions).

Table 2 : Emission Factors
Source NOx SO2 CO VOC TPM PM10 PM2.5
Leaks (kg/valve per year) N/A N/A N/A 234.417 N/A N/A N/A
Emergency Flares (kg/million m3) 1601.168 9.611 1345.564 88.102 30.435 30.435 30.435
Space Heaters (kg/million m3) 1601.168 9.611 1345.564 88.102 30.435 30.435 30.435
Boilers (kg/million m3) 1601.168 9.611 1345.564 88.102 30.435 30.435 30.435
Internal Combustion (kg/GJ) 1.754 2.528E-4 0.136 0.051 3.315E-5 3.315E-5 3.315E-5

Example Calculation

The equation for calculating releases from an emergency flare, using emission factors is:

Emission (kg/year) = Natural Gas Consumed (million m3/year) x EF (kg/million m3)

From Tables 1 and 2:

Natural gas flared = 50 m3 or 0.000050 million m3
VOC emission factor = 88.102 kg/million m3

Using the above equation and the known values:

VOC Emission = (0.000050 million m3) x (88.102 kg/million m3)

VOC Emission = 0.004405 kg

VOC Emission = 4.405 x 10-6 tonnes

The same method is utilized to calculate the emissions for the remaining sources using the fuel consumption data in Table 1, and emission factors presented in Table 2. Table 3 shows uncontrolled emissions from the sources in the Utility group.

Table 3 : Uncontrolled Annual CAC Releases for Group 1 Sources
Source NOx SO2 CO VOC TPM PM10 PM2.5
Leaks N/A N/A N/A 0.469 N/A N/A N/A
Emergency Flares 8.009E-5 4.806E-7 6.728E-5 4.405E-6 1.522E-6 1.522E-6 1.522E-6
Space Heaters 0.160 9.611E-4 0.135 8.810E-3 3.044E-3 3.044E-3 3.044E-3
Boilers 0.160 9.611E-4 0.135 8.810E-3 3.044E-3 3.044E-3 3.044E-3
Internal Combustion 8.4179 1.561E-3 1.025 0.321 0.105 0.105 0.105
Total 8.740 0.00348 1.300 0.808 0.111 0.111 0.111

N/A -- Not applicable.

Emission Group 2: Sulphur Flare Stack

Identify the Substances of Concern

The only substances of concern released this group are Part 4 substances (i.e. CACs) substances.

Identify the Method of Calculating Emissions

This particular facility does not have any stack monitoring system installed or source testing data. As a result, an appropriate estimation method to use for the flare would be either emission factors or a mass balance. Since an emission factor could not be found for this application, the recommended method would be a mass balance.

The general equation for a mass balance is:

Accumulation = Input + Generation - Output - Consumption

Where:
Input: Mass entering the process
Generation: Mass produced in the process
Output: Mass exiting in the process
Consumption: Mass consumed in the process
Accumulation: Mass that builds up within the process

calculate Emissions

For this example the parameters listed above are described as:

Input: Mass of H2S entering the stack
Generation = Consumption
Output: Mass of SO2 leaving the stack
Accumulation: No material accumulated

So the overall mass balance equation becomes:

Input = Output

When using a mass balance to determine the amount of a substance being produced by a reaction stoichiometry need to be considered. Based on the combustion reaction for flaring H2S, presented below, you will notice that the number of moles of H2S consumed equals the number of moles of SO2 produced.

2 H2S + 3 O2 2 H2O + 2 SO2

Arrow

Since we are interested in the mass of the SO2released, we need an equation that relates the number of moles of a substance to its weight. The following equation will allow us to make this relationship:

n = m/M

Where,
n = number of moles
m = mass of the substance [kg]
M = molar mass of the substance [kg / kmol]

Molar mass is equal to the sum of the molecular weights of the atoms that make up the substance. For example H2S has 2 atoms of hydrogen and one atom of sulphur, so you would add two times the molecular weight of hydrogen to the molecular weight of sulphur to get the molar mass of H2S. An example of this is shown in the next section.

Example Calculation

For the H2S flare stack in this example, complete combustion is assumed. That is, all the H2S is converted to SO2 (100% efficiency).

Amount of H2S burned = 20 kg per day or 7300 kg per year

calculate Molar Mass for H2S and SO2

molar mass H2S = (2 x 1.008 kg/kmol) + 32.06 kg/kmol
molar mass H2S = 34.076 kg/kmol
molar mass SO2 = 32.06 kg/kmol + (2 x 16.00 kg/kmol)
molar mass SO2 = 64.06 kg/kmol

calculate Mass of SO2 released per year

From before we know that the number of moles of H2S burned equals the number of moles of SO2 produced, that is:

Moles of H2S = Moles of SO2

Where:

Moles of H2S = (mass of H2S/molar mass of H2S)

Moles of SO2 = (mass of SO2/molar mass of SO2)

So,

(mass of H2S/molar mass of H2S) = (mass of SO2/molar mass of SO2)

Rearranging the above expression we get,

Mass SO2 = molar weight SO2 x (mass of H2S/molar weight of H2S)

mass SO2 = 64.06 kg/kmole x (7300 kg of H2S/34.076 kg/kmol)

mass SO2 = 13723 kg SO2/year

mass SO2 = 13.723 tonnes SO2/year

Note: Not all flare stacks are 100% efficient. If you flare stack is not 100% efficient you will need to make the appropriate adjustment to the above calculations. For example, if only 98% of the H2S is converted to SO2 then the mass of H2S used at the beginning (7300 kg per year) must be multiplied by 0.98. The remaining 2% will be released to air as H2S. As H2S is a Part 1A substance, and under these circumstances it is considered to manufactured. As a result this portion needs to be included in the Part 1A threshold calculations. A report will be required if the 10 tonne threshold is met.

Table 4 summarizes the emissions from the Sulphur Flare Stack.

Table 4 : Uncontrolled Annual CAC Releases for Group 2 Sources
Source NOx SO2 CO VOC TPM PM10 PM2.5
H2S Flare N/A 13.723 N/A N/A N/A N/A N/A

N/A -- Not applicable.

Emission Group 3: Horizontal Separator

Identify the Substances of Concern

The only substances of concern released this group are Part 4 substances (i.e. CACs) substances.

Identify the Method of Calculating Emissions

An appropriate estimation method to use would be emission factors since the facility does not have any monitoring system installed or stack testing data. Emission factors can either be published or site specific. Site specific emission factors would be more accurate than published emission factors because they are developed specifically for the facility with site specific data Table 5 identifies the specific emission sources included in the Horizontal Separator group for this facility. This table also presents the suggested emission estimation techniques and the amount of natural gas consumed by each source, if applicable.

Table 5 : Possible Methods of Estimation for the Separator Emissions and Consumption Data
Emission Source Suggested Estimation
Method(s)
Consumption Data
Leaks from operating
valve(s) (1 valve)1(SCC 30600811)
Emission Factors N/A
Process Heater2 Emission Factors Natural Gas: 150,000 m3/yr (5,790 GJ/yr)3

N/A -- Not applicable.

Notes:
1 These emission sources will most likely only emit VOCs. However diligence must be exercised if it is expected that other CACs are being emitted from the source.
2 No specific SCC code is given because the emission factors used are manufacturer specific for a fire tube boiler
3 Natural gas in cubic metres can be changed to gigajoules using the conversion factor 0.0386 GJ/m3.

calculate Emissions

For the sources in Group 3, published and site specific emission factors will be used to estimate the emissions for these sources. The applicable emission factors can be found in the Table 6, shown below. In this case study, published emission factors from WebFIRE (based on the SCC identified in Table 1) were utilized with the appropriate conversions applied (i.e. imperial to metric conversions).

Table 6 : Emission Factors
Source NOx SO2 CO VOC TPM PM10 PM2.5
Leaks (kg/leak per year) N/A N/A N/A 234.417 N/A N/A N/A
Process Heater1 (ng/J) 52.000 0.250 64.000 2.300 0.820 0.820 0.820

N/A -- Not applicable.

Notes:
1 Emission factors used are manufacturer specific for a fire tube boiler.

Example Calculation

The equation for calculating releases for the leaks using emission factors is:

Emission (kg/year) = Number of Leaks x EF (kg/lear per year)

From Tables 5 and 6:

Number of Leaks = 1
VOC emission factor = 234.417 kg/leak/year

Using the above equation and the known values:

VOC Emission = (1 lear) x (234.417 kg/leak)

VOC Emission = 234.417 kg

VOC Emission = 0.234 tonnes

The same method is utilized to calculate the emissions for the remaining sources using the fuel consumption data in Table 5 and emission factors presented in Table 6. Table 7 shows uncontrolled emissions from the sources in the Horizontal Separator group.

Table 7 : Uncontrolled Annual CAC Release for Group 3 Sources
Leaks N/A N/A N/A 0.234 N/A N/A N/A
Process Heater 0.301 1.448E-3 0.371 0.013 4.632E-3 4.632E-3 4.632E-3
Total 0.301 1.448E-3 0.371 0.248 4.632E-3 4.632E-3 4.632E-3

N/A -- Not applicable.

Emission Group 4: Gas Sweetening (Amine Treatment)

Identify the Substances of Concern

The only substances of concern released this group are Part 4 substances (i.e. CACs) substances.

Identify the Method of Calculating Emissions

Table 8 identifies the specific emission sources included in the Gas Sweetening grouping for this facility. This table also presents the suggested emission estimation techniques and the amount of natural gas / water consumed by each source, if applicable.

Table 8: Possible Methods of Estimation for the Gas Sweetening Emissions and Consumption Data
Emission Source Suggested Estimation Method(s) Consumption Data
Cooling Tower1 (SCC 30600701) Emission Factors, Mass Balance H2O: 1.3 MM m3
Process Heater2 Emission Factors Natural Gas: 150,000 m3/yr (5,790 GJ/yr)3
3 Natural Gas Burning Reciprocating Engines (SCC 20200254) Emission Factors Natural Gas: 480,000 m3/yr (18,528 GJ/yr)3
Leaks from operating
valve(s) (5 valves)1(SCC 30600811)
Emission Factors N/A

N/A -- Not applicable.

Notes:
1 These emission sources will most likely only emit VOCs. However diligence must be exercised if it is expected that other CACs are being emitted from the source.
2 No specific SCC code is given because the emission factors used are manufacturer specific for a fire tube boiler.
3 Natural gas in cubic metres can be changed to gigajoules using the conversion factor 0.0386 GJ/m3.
4 Considering internal combustion, four-stroke lean burn reciprocating engines, fueled by natural gas and operating at 90-105% load.

calculate Emissions

For the sources in Group 4, published emission factors will be used to estimate the emissions for these sources. The applicable emission factors can be found in the Table 9, shown below. In this case study, the emission factors from WebFIRE (based on the SCC identified in Table 8) were utilized with the appropriate conversions applied (i.e. imperial to metric conversions).

Table 9 : Emission Factors
  NOx SO2 CO VOC TPM PM10 PM2.5
Cooling Tower (kg/MM m3 H2O)1 N/A N/A N/A 718.953 *1 *1 *1
Process Heater2 (ng/J) 52 0.25 64 2.3 0.8 0.8 0.8
Natural Gas Burning Reciprocating Engines (kg/GJ) 1.754 2.528E-4 0.239 0.051 3.315E-5 3.315E-5 3.315E-5
Leaks (kg/leak per year) N/A N/A N/A 234.417 N/A N/A N/A

N/A -- Not applicable.

Notes:
1 For information on how to calculate particulate matter emissions from cooling towers refer to the guidance document titled "Wet Cooling Tower Guidance v1.0". This document can be found in the NPRI toolbox available on the NPRI's website.
2 Emission factors used are manufacturer specific for a fire tube boiler.

Example Calculation

The equation for calculating releases from the cooling tower using emission factors is:

Emission (kg/year) = water used for cooling (million m3/year) x EF (kg/millions m3H2O)

From Tables 8 and 9:

Water used for cooling = 1.3 million m3
VOC emission factor = 718.953 kg/MM m3H2O

Using the above equation and the known values:

VOC emission = (1.3 million m3)X(718.953 kg/million m3) = 935 kg = 0.935 tonnes

The same method is utilized to calculate the emissions for the remaining sources using the consumption data in Table 8 and emission factors presented in Table 9. Table 10 shows uncontrolled emissions from the sources in the Gas Sweetening group.

Table 10 : Uncontrolled Annual CAC Releases for Group 4
Source NOx SO2 CO VOC TPM PM10 PM2.5
Cooling Tower N/A N/A N/A 0.935 *1 *1 *1
Process Heater 0.301 1.448E-3 0.371 0.013 4.632E-3 4.632E-3 4.632E-3
3 Natural Gas Burning Reciprocating Engines 88.016 0.0127 11.993 0.945 1.663E-3 1.663E-3 1.663E-3
Leaks N/A N/A N/A 1.172 N/A N/A N/A
Total 88.317 1.41E-2 12.364 3.065 6.300E-3 6.300E-3 6.300E-3

N/A -- Not applicable.

Notes:
1 For information on how to calculate particulate matter emissions from cooling towers refer to the guidance document titled "Wet Cooling Tower Guidance v1.0". This document can be found in the NPRI toolbox available on the NPRI's website.

PART II

Report a problem or mistake on this page
Please select all that apply:

Privacy statement

Thank you for your help!

You will not receive a reply. For enquiries, contact us.

Date modified: