Compressor Station Case Study

Purpose

To provide compressor stations with guidance on how to report to the National Pollutant Release Inventory (NPRI).

Background Information

The NPRI defines a pipeline installation as a collection of equipment situated at a single site, used in the operation of a natural gas distribution or transmission pipeline. This definition includes pipeline compressor and storage stations along pipelines used to transport both raw and processed natural gas. Generally these types of facilities are referred to as compressor stations.

The NPRI requires that pipeline installations only report for the criteria air contaminants (CACs) released from stationary combustion sources. Stationary combustion sources can be broken down into two categories: external combustion equipment and internal combustion equipment. External combustion sources would include, but not be limited to: boilers, heaters, flares, etc. These are sources where the combustion occurs under atmospheric conditions with excess air. Internal combustion sources would include, but not be limited to, equipment such as: reciprocating engines, turbines, and generators. Internal combustion units are identified as ones in which the combustion of fuel takes place in a confined space above atmospheric pressure. The expanding gases produced by the combustion are used to provide mechanical power.

During all combustion processes (internal and external) CACs are released to the air. These are substances that contribute to the formation of ground level ozone and smog. Some of these contaminants are also eye and respiratory irritants to humans and animals. The NPRI is interested in seven CACs: oxides of nitrogen, sulphur dioxide, carbon monoxide, volatile organic compounds, total particulate matter, particulate matter less than or equal to 10 microns, and particulate matter less than or equal to 2.5 microns. For complete definitions of these substances and potential sources refer to the Guide for Reporting to the National Pollutant Release Inventory.

Scenario

A sales gas compressor station has a 160 horsepower (hp) reciprocating engine running the compressor, as well as a flare stack used during emergency situations. The reciprocating engine burned 45 000 GJ of natural gas during the reporting year and the flare burned 2400 m3 of processed natural gas. All process equipment onsite, including three hydrocarbon storage tanks, were painted during the reporting year in question.

Note: Both painting and hydrocarbon storage tanks are sources of volatile organic compounds (VOCs).

 

Process

Step 1- Identify Requirements

The first step is to identify the NPRI reporting requirements that pertain to the facility. Since this is a compressor station it would have to comply with the reporting requirements for a pipeline installation, meaning that only CACs released from stationary combustion equipment should be considered when performing the threshold calculations. A report is required for any CAC which meets or exceeds the release threshold.

The individual release thresholds for the seven CACs are shown in Table 1.

Table 1: Release Thresholds for Reporting CACs to the NPRI
NPRI SubstanceRelease Threshold (tonnes)
Oxides of Nitrogen, expressed as NO2 (NOx)20
Sulphur Dioxide (SO2)20
Carbon Monoxide (CO)20
Volatile Organic Compounds (VOCs)10
Total Particulate Matter (TPM)20
Particulate Matter less than or equal to 10 microns in diameter (PM10)0.5
Particulate Matter less than or equal to 2.5 microns in diameter (PM2.5)0.3

Step 2 - Identify the Emission Sources at the Facility

After reviewing the Guide for Reporting to the National Pollutant Release Inventory a survey of the facility's emission sources was performed. Based on the survey, the following sources were identified:

Source 1: A 4-cycle, lean burn natural gas reciprocating engine (SCC 20200254) vented through a 12m stack. This engine is operated 24 hours a day, 365 days a year, and burned 45 000 GJ of natural gas during the year in question. This engine was operated with no emission control devices and at 100% of rated load. Reciprocating engines are internal combustion sources and therefore the CAC releases from this source need to be included in the facility's threshold calculations.

Source 2: Hydrocarbon storage tanks. Although this is a source of VOCs, it is not a combustion source and therefore is not included in the threshold or release calculations.

Source 3: Equipment and storage tank painting. Again, although this is a source of VOCs it is not a combustion source and therefore is not included in the threshold or release calculations.

Source 4: A Pipeline flare stack used for emergency venting. The flare stack burned 2400 m3 of processed natural gas during the calendar year in question.

Step 3 - Threshold Calculations

Emission Source 1 - Reciprocating Engine Burning Natural Gas

Identify the Method of Calculating Emissions
The methods of estimation are described in the Guide for Reporting to the National Pollutant Release Inventory. You may wish to review them before proceeding.

This facility does not have an emission monitoring system installed or stack test data. As a result, an appropriate estimation method would be the use of emission factors. The following list includes possible sources of emission factors (in order of preference):

  • Site specific. Site specific emission factors are more accurate than published emission factors since they are developed specifically for the facility with site specific data.
  • Obtained from equipment dealers. Some equipment dealers can provide emission factors for the equipment they supply.
  • The US EPA. Obtained from using the US EPA's AP-42 document or the latest version of the WebFIRE database, currently version December 2005.

calculate Emissions
In this case study, the emission factors from WebFIRE (version December 2005) were utilized with the appropriate conversions applied (i.e. imperial to metric conversions). When selecting emission factors detailed information of the sources is required. For the compressor engine in this case study the following is required to select the appropriate emission factors:

  • The compressor engine is a 4-cycle, lean burn, natural gas reciprocating engine (SCC 20200254)
  • The engine was operated with no emissions control devices
  • The engine was operating at 100% of rated load

Based on the above criteria Table 2 provides the emission factors for this source.

Table 2: Emission Factors for the Combustion of Natural Gas in a Reciprocating Engine (SCC 20200254)
NPRI SubstanceEmission Factor (kg/GJ of Natural Gas)
NOx *1.754
SO2 *0.0002528
CO *0.1363
VOCs0.05073
TPM **0.00003315
PM100.00003315
PM2.50.00003315

Notes:
* Multiple emission factors appear for these substances due to different control methodologies. Since this particular engine has no emission control devices the "Uncontrolled" emission factor is used.
** WebFIRE (version December 2005) does not list a TPM emission factor. However, the PM10 factor may be used for this estimate.

The general equation for estimating uncontrolled air releases using an emission factor is:

Emission (kg/year) = Fuel Burned (gigajoules/year) x EF (kg/gigajoule)

Example calculation:
VOC emissions = (45 000 gigajoules/year) x (0.05073 kg/gigajoule)
VOC emissions = 2 282.9 kg
VOC emissions = 2.283 tonnes

The same method is used to calculate the remaining six CAC releases using the emission factors provided in Table 2. The results of these calculations are provided below in Table 3 and represent the uncontrolled CAC releases from the reciprocating engine as a result of natural gas combustion.

Table 3: Uncontrolled Annual CAC Releases from the Natural Gas Reciprocating Engine
NPRI SubstanceAnnual Uncontrolled Emissions (tonnes) 1
NOx78.930
SO20.011
CO6.133
VOC2.283
TPM0.001
PM100.001
PM2.50.001

Note:
1 All emissions have been rounded to three decimal places; however the full calculated values will be used to determine if facility wide release thresholds are met.

Emission Sources 2 and 3

As noted, pipeline installations are only required to report the emissions from stationary combustion equipment. Therefore, you are not required to report the CAC releases from the hydrocarbon storage tanks or painting to the NPRI.

Emission Source 4 - Emergency Flare

Identify the Method of Calculating Emissions

Again, this particular the facility does not have any monitoring systems installed or stack testing data. As a result, emission factors are an appropriate estimation method. The emission factors provided in Table 4 were taken from the WebFIRE (version December 2005) database and are for the combustion of commercial grade natural gas in a boiler with a capacity of less than 100 million BTU/h.

Note: A natural gas emergency flare is assumed to function similar to a natural gas boiler rated at 100 million BTU/h or less.

calculate Emissions

The assumption was made that since the compressor station was on a sale gas pipeline any gas being flared would be of commercial grade or processed natural gas. If the compressor station was on a raw gas pipeline where the methane content was less than 80% the appropriate emission factor to use would be one for the flaring of biogas/landfill gas, which can be found in WebFIRE version December 2005 under the SCCs 50100410 or 50300601.

Table 4: Emission Factors for the Flaring of Processed Natural Gas (SCC 10200602)
NPRI SubstanceEmission Factor (kg/million cubic metres of Natural Gas)
NOx*1600
SO2*9.6
CO *1344
VOC88
TPM **30.4
PM10**30.4
PM2.5**30.4

Notes:
* - Multiple emission factors appear for these substances due to different control methodologies. Since a flare has no emission control devices, the "Uncontrolled" emission factor is used.
** - Particulate matter emission from natural gas combustion is <1.0 micron in size. Therefore, the same emission factor (EF) is used for all three particulate fractions for boilers <100million BTU/h.

The general equation for estimating uncontrolled air releases using an emission factor is:

Emissions (kg/year) = Fuel Burned (million cubic metres) x EF (kg/million cubic metres)

Example Calculation:
2 400 cubic metres = 0.002400 million cubic metres
VOC emissions = (0.0024 million cubic metres) x (88.10 kg/million cubic metres)
VOC emissions = 0.2114 kg
VOC emissions = 0.0002114 tonnes

The same method is used to calculate the remaining six CAC releases using the emission factors provided in Table 4. The results of these calculations are provided in Table 5 and represent the uncontrolled CAC releases from the emergency flare.

Table 5: Uncontrolled Annual CAC releases from the Flaring of Natural Gas
NPRI SubstanceAnnual Uncontrolled Emissions (tonnes) 1
NOx0.004
SO20.000
CO0.003
VOC0.000
TPM0.000
PM100.000
PM2.50.000

Note:
1 All emissions have been rounded to three decimal places; however the full calculated values will be used to determine if facility wide release thresholds are met.

Step 4 - Add Emissions from All Sources and Compare to Thresholds

After calculating the emissions from each source individually, the next step is to add the emissions from each source to determine the facility wide total for each substance, shown in Table 6. This total is then used to determine if the NPRI release thresholds are met, as shown in Table 7.

Table 6: Total Facility Emissions
NPRI SubstanceAnnual Combustion Engine Emissions (tonnes)Annual Flare Stack Emissions (tonnes)Total Facility Release (tonnes)
NOx78.930.003225678.934
SO20.011380.000023040.011
CO6.1340.0032266.137
VOC2.2830.00021122.283
TPM0.0014920.000072960.002
PM100.0014920.000072960.002
PM2.50.0014920.000072960.002

NPRI Reporting Determination

Table 7: Comparison of Annual Emissions to the NPRI Reporting Thresholds
NPRI SubstanceTotal Emission (tonnes/year)NPRI Facility Threshold (tonnes/year)NPRI Threshold Exceeded?Rationale
NOx78.93420YesFacility emission is > than NPRI threshold
SO20.01120NoFacility emission is < than NPRI threshold
CO6.13720NoFacility emission is < than NPRI threshold
VOC2.28310NoFacility emission is > than NPRI threshold
TPM0.00220NoFacility emission is < than NPRI threshold
PM100.0020.5NoFacility emission is < than NPRI threshold
PM2.50.0020.3NoFacility emission is < than NPRI threshold

Conclusion:

Based on the calculations for this facility, a report is only required for NOx releases.

Step 5 - Reporting Facility Emissions

The NPRI Reporting Form is available via the on-line reporting system (Owners), available here. You will also find more information on Navigating in Owners and on completing a report.

Substance information is entered in the Substances tab of the reporting form. Since the NPRI requires that pipeline installations only report for the CACs released from stationary combustion sources, all releases meeting or exceeding the thresholds will be reported as - Stacks / Point Releases. Report all air releases of the NPRI substance to be reported and the basis of estimate. Air releases for each substance is entered in the "On-site Releases of the Substance to the Environment" section in the "Releases" tab.

For the scenario presented, the following table illustrates the information that would be entered.

Table 8: Stacks / Point Releases – Total Releases from Stack or Point Sources
NPRI SubstanceTotal Point Source Emissions (tonnes)Enter as Stack/Point ReleaseBasis of Estimate
NOx78.934Stack or Point ReleasesE2

The reporting form has a pick-list for choosing the "Basis of Estimate" codes. Select the letter code identifying the method that applies to the largest portion of the estimated releases (you can choose only one code). In this case study, the basis of estimates is E2 (Published Emission Factors). Note that if the substance was released through a stack greater than or equal to a height of 50m, a stack profile must be created and the substance assigned to the stack. More information on stack reporting can be found in the Guide for Reporting to the National Pollutant Release Inventory (NPRI).

Step 6 - Speciated VOC Calculations

Since the Part 4 VOC emissions did not exceed the NPRI threshold, speciated VOC calculations are not required.

Step 7 - Calculate the Monthly Breakdown of Annual Releases to Air by Percentage

The detailed emission estimates shown above indicated that the facility is required to report to the NPRI (i.e. for NOx emissions). Additional information such as monthly breakdown of emissions by percentage for each pollutant must be reported. The general equation and methodologies for estimating monthly emissions is the same as the annual emissions for each source.

Example:
Usage of natural gas for each month is obtained from facility records. You can calculate the emissions for each month and add the total to determine the annual emissions. Using the annual emission total, calculate the monthly emission percentage using the following equation:

January emission percentage = (January emission / total annual emissions) X 100%

For example, add the NOx emission from each source (for this scenario the sources are the compressor engine and the emergency flare) for the month of January. If the January emission of NOx from the emission sources was 12.1 tonnes, the January percentage NOx emissions is:

January emission percentage = (12.1 tonnes / 78.934 tonnes) X 100%

January emission percentage = 15.33%

Enter the information (calculated percentage emission) for each substance in the "Monthly Breakdown of Annual Releases to Air by Percentage" section in the "Releases" tab. The percentage for months must total 100%.

Step 8 - Reasons for Change in Quantities Released from Previous Year

In the "Reasons for Changes in Quantities Released from Previous Year" section in the "Releases" tab you are required to select all applicable reasons for changes in substance releases from the previous reporting year (check the applicable boxes).

Step 9 - Anticipated Releases to Air

Users are required to estimate anticipated releases for each substance released to the air from the facility for the three upcoming years. Reporting anticipated releases for two following years are optional. Enter the information for each substance in the "Anticipated Releases (tonnes)" section in the "Releases" tab.

Step 10 - Pollution Prevention (P2) Activities

If the facility has employed P2 activities, it should be noted in the "Pollution Prevention" tab (check the applicable boxes).

References

US EPA AP-42: compilation of Air Pollution Emission Factors, AP-42, Fifth Edition, Volume 1: Stationary Point Area Sources, 2004

US EPA WebFIRE (version December 2005)

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