Sour gas contains both H2S and CO2. These gases are removed from the natural gas inlet stream in order to meet pipeline sales gas specifications (i.e., 2 mole percent CO2).
After H2S is removed from the inlet gas stream it may be recovered as elemental sulphur or burned to produce SO4. The removed CO2 is sent to the incinerator or flare, but since it is non-combustible, it becomes an atmospheric release of CO2.
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This CO2 released to the atmosphere may be estimated using the following formula:
Equation 11
tonnes CO2 released = [ (MF x VI) - (SMF x VS) ] x 1.8612 / 1000 where:
MF = mole or volume fraction CO2 in inlet gas VI = volume inlet gas in m3/yr @ STP conditions
SMF = sales gas spec. for mole fraction, CO2 = 0.02 typically VS = volume sales gas in m3/yr @ STP conditions
1.8612 = density of CO2 in tonnes/103 m3 at STP conditions
An alternate method to calculate CO2 vented is based on the amount of CO2 being sent to the acid gas incinerator. The following formula may be used for this method.
Equation 12
CO2 released (in tonnes/yr) = (MFS x AGV) x 1.8612 / 1000 where:
MFS = mole fraction CO2 in acid gas to incinerator inlet
AGV = acid gas volume to incinerator inlet in m3/yr @ STP conditions 1.8 Fugitive Emission Estimates Derived from the GFC Method
Although the Detailed Method gives the best results, it requires substantial effort to collect the actual number of valves and fittings from a given site/facility (either by site inspection or the review of facility PIDs). The Short Form method is quite simple to use; however, since it uses production-based fugitive emission factors, it usually results in high emissions volumes.
In order to improve the accuracy of the Short Form method for fugitive emissions without a substantial increase in effort, the new Generic Fitting Count (GFC) may be used.
By using a generic or average fitting count for specific equipment/processes, the fugitive emissions calculated will be based on the number of fittings without the need to count fittings at a specific site. The generic fitting counts presented here were taken from an API fugitive emission study of 20 different facilities in 1993.
Reference should be made to API Publication 4589, Dec. 1993, “Fugitive Hydrocarbon Emissions from Oil and Gas Production Operations.”
April 2003 Estimating Greenhouse Gas Emissions 1-28
Table 1-15 to Table 1-19 list the generic or average fitting counts determined from this API study. The fitting counts in these Tables do not distinguish how many fittings are in liquid or vapour service. Equipment with both liquid and gas fittings such as separators and dehydrators can be considered to have 50 percent of their fittings in gas service (i.e., as an approximation).
Example 6
Use the Generic Fitting Count to calculate the annual fugitive emissions from a sweet gas plant that has:
• 1 inlet separator
• 2 reciprocating compressors
• 1 dehydrator
• 1 fractionation tower
• 2 scrubbers
• 1 sales gas meter
The plant operates 360 days per year, has 1 PRV relieving to atmosphere and has 85 percent methane content (by weight) in its gas.
Solution
Using Table 1-15, the total number of gas connectors is:
(48 x 0.5) + 2(129) + (105 x 0.5) + (81 x 0.5) + 2(81 x 0.5) + 160 = 617 where 0.5 is used for the inlet separator, dehy, fractionator and scrubbers since half of the connectors are in gas service.
Using Table 1-16, the total number of gas valves is:
(17 x 0.5) + 2(26) + (25 x 0.5) + (23 x 0.5) + 2(23 x 0.5) + 41 = 149 Using Table 1-17, the total number of open-ended gas lines is:
(3 x 0.5) + 2(2) + (3 x 0.5) + (2 x 0.5) + 2(2 x 0.5) + 13 = 23 Using
Table 1-18, the number of compressor seals is:
2(3) = 6
The number of PRVs vented to the atmosphere is 1.
Therefore, using the fugitive emission factors from Table 1-11 and Equation 7, the total fugitive emissions are:
April 2003 Estimating Greenhouse Gas Emissions 1-29
= [(617 connectors x 0.00253 kg/hr/connector) + (149 valves x 0.04351
kg/hr/valve) + (23 open-ended lines x 0.00373 kg/hr/OEL) + (6 seals x 0.80488 kg/hr/seal) + (1 PRV x 0.12096 kg/hr/PRV) ] x 24 hr/day x 365 days/yr x 360/365 x 0.85
= [1.56 + 6.48 + 0.09 + 4.83 + 0.12] x 8760 x 0.986 x 0.85
= 13.08 x 8760 x 0.986 x 0.85
= 96,030 kg CH4/yr
= 96 tonnes CH4/yr
= 96 x 21 (GWP factor)
= 2,017 tonnes CO2E/yr
Table 1-15: Generic or Average Connector Counts by Equipment/Process Type Equipment/Process Light Oil
Facilities
Heavy Oil Facilities
Gas
Production
Gas Plants
Offshore Platforms
Overall Onshore Average
Well 53 44 60 195 52
Header 389 108 105 145 310 187
Heater 146 147 197 147
Separator 111 41 160 48 299 90
Filter 122 269 122
Chiller 94 94
Meter 91 55 160 383 102
Dehydrator 119 155 105 210 126
Fractionation 81 81
Sulphur 109 144 127
Compressor 163 195 129 417 162
Vapour Recovery 78 162 78
Scrubber 105 120 81 177 102
Flare 114 221 376 168
April 2003 Estimating Greenhouse Gas Emissions 1-30
Table 1-16: Generic or Average Valve Counts by Equipment/Process Type Equipment/Process Light Oil
Facilities
Heavy Oil Facilities
Gas
Production
Gas Plants
Offshore Platforms
Overall Onshore Average
Well 13 8 16 61 12
Header 109 17 26 38 82 48
Heater 28 22 45 25
Separator 24 10 30 17 81 20
Filter 19 42 19
Chiller 25 25
Meter 21 13 41 84 25
Dehydrator 26 31 25 46 27
Fractionation 23 23
Sulphur 34 42 38
Compressor 34 31 26 88 30
Vapour Recovery 10 41 10
Scrubber 22 24 23 39 23
Flare 35 71 74 53
April 2003 Estimating Greenhouse Gas Emissions 1-31
Table 1-17: Generic or Average Open-ended Line Counts by Equipment/Process Type
Equipment/Process Light Oil Facilities
Table 1-18: Generic or Average Compressor Seal Counts by Equipment/Process Type Equipment/Process Light Oil
Facilities
Vapour Recovery 2
Since the fugitive emission factor for compressor seals is quite high, it is advisable to use the actual number of seals at your specific facility. Generally, centrifugal compressors have one seal per unit, while reciprocating compressors have one seal per cylinder.
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Table 1-19: Generic or Average PRV Counts by Equipment/Process Type Equipment/Process Light Oil
Facilities
Table 1-19 should be used with caution. Although the number of PRVs shown per equipment is reasonable, it is unlikely that this number of PRVs will be vented to atmosphere at Canadian facilities. Operations staff should be consulted about how many PRVs actually vent to atmosphere at their facility. Operations staff should also review all allocated generic fitting counts prior to finalizing fugitive emission calculations.
After determining the number of equipment/processes that exist at a specific facility (i.e., through examination of Process Flow Diagrams or interviews with Operations), the number of fittings per process (in Table 1-15 to Table 1-19) may be used to determine the total number of fittings. Using this total fitting count along with the fugitive emission factors listed in Table 1-11, the total fugitive emission losses can be calculated. This GFC procedure does not take into account venting from gas operated field instrumentation and controllers. These losses must be determined separately.
April 2003 Estimating Greenhouse Gas Emissions 1-33