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Study Comparison Reveals Methane-Emissions Reduction Opportunities in Gas Processing

A version of this article appeared in the June 13, 2005 issue of Oil & Gas Journal on global warming.

Roger Fernandez
U.S. Environmental Protection Agency
Washington, DC

Donald Robinson and Vineet Aggarwal
ICF Consulting
Fairfax, VA

Introduction

U.S. demand for natural gas continues to grow dramatically, in large part because methane is the cleanest burning fossil fuel. As natural gas production has increased over the last decade, the number of gas processing plants has decreased, with the capacity of the remaining plants increasing. Faced with growing demand, natural gas processors have an incentive to reduce methane emissions because natural gas emissions waste valuable product and revenue.

U.S. natural gas processing plants are estimated to emit about 37 billion standard cubic feet (Bscf) of methane annually1. Consequently, identifying economic means of reducing methane emissions not only contributes to a cleaner environment but improves the economics of the gas processing sector. Two studies have identified the major sources of methane emissions, one by the U.S. Environmental Protection Agency (EPA) and the Gas Research Institute (GRI, now the Gas Technology Institute, GTI), published in 1996,2 and the second by Clearstone Engineering Inc (Clearstone) and EPA,3 published in 2002. Concurrently, several economic methane emission reduction options have been developed by processing plant operators and voluntarily reported to the U.S. Natural Gas STAR Program. Most of the Natural Gas STAR technologies and practices also reduce operating costs and increase processing revenues. This paper identifies significant methane emissions reduction opportunities, how these opportunities are evident from the two studies and what these studies say about methane emissions from processing plants.

Emissions Reduction Opportunities

EPA and an industry advisory committee consisting of ten gas processing companies and the Gas Processors Association (GPA) studied methane emission reduction opportunities in the gas processing sector in the year 2000. The purpose of this study was to identify suitable Best Management Practice (BMP) candidates that would form the basis for this sector to reduce emissions through the Natural Gas STAR Program. The basis for methane emissions from gas processing was the EPA/GRI 1996 study2. The BMP analysis involved the following steps:

The following seven BMP candidates were evaluated:

  1. Economic rod packing replacement in reciprocating compressors;
  2. Replacing wet seals with dry seals in centrifugal compressors;
  3. Installing flash tank separators on glycol dehydrators;
  4. Replacing gas pneumatic devices with instrument air;
  5. Directed inspection and maintenance (DI&M) 5 of fugitive leaks in gas processing plants;
  6. Installing vapor recovery units on natural gas liquids transfer from storage tanks to rail and truck transportation vehicles;
  7. Acid gas removal unit vent controls on the CO2 stream vented from amine regenerators.

In considering the definition of gas processing sector facilities, the industry advisory committee recommended that gas gathering and booster stations be included along with gas processing plants. This was because gas processors either own, or at a minimum operate the feed facilities to their processing plants, and thereby have the opportunity to reduce emissions from those facilities. The industry advisory committee believed that there are more opportunities for cost-effective methane emission reductions in gathering/booster systems than in the processing plants themselves.

The study concluded that three emissions reduction opportunities met the standards for BMP:
BMP 1: Replacement of High-Bleed Pneumatic Devices (in gathering/booster stations);
BMP 2: Installation of Flash Tank Separators in Glycol Dehydrators (primarily in gathering/booster stations);
BMP 3: Directed Inspection and Maintenance at Gas Processing Plants and Booster Stations.

Many gas processing sector companies have since joined the Natural Gas STAR Program and reported undertaking such steps to reduce their methane emissions. However, as this paper describes, more work remains to be done. In particular, reciprocating compressor seals appear to be a large opportunity for methane emission reductions that has not been reported to the Natural Gas STAR Program, and one that can be economically reduced in processing plants and probably gathering/booster stations.

This paper continues with a discussion of where the methane emissions were found in gas processing facilities and then discusses what is learned by comparison of the Clearstone study with the EPA/GRI study, relevant to opportunities for further reducing methane emissions in the gas processing sector.

Emission Estimates from Gas Processing Plants

EPA's National Methane Emissions Inventory1, based on the GRI study, estimates that in the year 2000, gas processing plants emitted 37 Bscf of methane, and Natural Gas STAR partners reported saving over one billion cubic feet of methane from the gathering and processing sector. Fugitive emissions from equipment leaks were the largest emissions category contributing nearly 70% of total methane emissions, followed by venting and combustion emissions from normal operations at 25%, and the remainder from routine operating and maintenance activities such as equipment blowdowns. The GRI study used EPA Method 214 (sniffing components with an organic vapor analyzer) to screen for leaks and employed correlation equations to estimate methane emissions factors. The Clearstone study used a Hi-Flow Sampler™ that gives a direct reading of mass emissions to quantify emission factors. Table 1 shows average per-plant methane emissions from gas processing, in thousand standard cubic feet per year (Mscf/yr) from the GRI and Clearstone studies. This section provides an overview of these emissions estimates from the two studies.

The Clearstone study defined fugitive emissions screened above 10,000 parts per million by volume (ppmv) as leakers and those below that threshold were labeled as non-leakers. The study screened a total of 101,193 components across four processing plants, and approximately 2,630 (2.6%) of the components were found to be leaking. The total hydrocarbon emissions rate from leakers was measured using the Hi-Flow Sampler™. A default-zero methane emission factor from the EPA correlation equations4 was used for non-leakers.

The GRI study, which was published nine years earlier than the Clearstone, used data from surveys of 7 processing plants combined with results of three prior studies by EPA and API that covered 14 gas plants. The EPA/GRI/API studies screened 87,923 components, finding 5,872 (6.68%) were leaking above 10,000 ppmv. GRI used EPA correlation equations to estimate fugitive methane emissions from these 21 processing plants.

The EPA/GRI/API studies also quantified individual vented and combusted emissions from compressors, processes and pneumatic devices. Clearstone made measurements or engineering calculations of numerous vent stacks in the four plants studies but found that most of these vents were manifolded to numerous emission sources, making it impractical to reliably relate emissions to any single source. Therefore, Clearstone grouped all vented and flared emissions into one category.

Table 2 gives a summary of estimated methane emissions from components associated with the different equipment types in a gas processing plants based on the Clearstone study data. The "Gas Plant/ Non-compressor" equipment category includes equipment in the natural gas section of the plant excluding the compressors, i.e., dehydration units, nitrogen rejection unit, mercury removal unit, gas sweetening, and the demethanizer. The "NGL" category represents the natural gas liquids section of the plant after the demethanizer. This includes the deethanizer, depropanizer, debutanizers, stabilizer, propane refrigeration, liquefied petroleum gas (LPG) and natural gas liquids (NGL) storage tanks and pumps. The compressor-related equipment is sub-categorized by type of compressor – reciprocating or centrifugal. The "Cryogenics" section includes the refrigeration unit using turbo-expanders. The "Vented and Flared" category covers vented emissions from components that are manifolded either directly into vent stacks or the plant flare header. The "Utilities" includes the cogeneration equipment, the Claus sulfur plant, heat pumps and other utilities. The non-compressor related equipment, reciprocating and centrifugal compressor related components, cryogenic equipment and vents and flares are considered to have methane as the primary hydrocarbon fraction and contribute about 90% of the total methane emissions from gas plants.

Comparison of Results

The level of detail in the Clearstone study provides an opportunity to organize the data for direct comparison with the GRI study. The main fugitive emission source categories from Table 2 are shown in Table 3, broken down to sub-categories of emissions source types. The categories of NGL and Utilities have been left out of the comparison since they are minor sources of methane emissions [Note: The value for methane emissions from NGL in Table 2 is dominated by a single large emission from a NGL/water separator tank thief hatch]. The average fugitive emissions per plant based on the Clearstone study are 41 million standard cubic feet per year (MMscf/yr) per plant, which is 22% higher than the GRI estimate. The Clearstone study has identified cryogenic equipment as emission sources that were not identified in the GRI study. Table 3 shows the comparison between the fugitive emissions estimates of the two studies by equipment and component categories, and the following section gives an overview of the results.

Non-Compressor Related Emissions

For emissions from non-compressor related sources, the estimates from Clearstone are about 155% higher than the GRI study values. Although the activity factors in the Clearstone study are slightly lower, the emissions factors are much higher (between 20% and 480% higher) resulting in larger emissions. The Clearstone data do not differentiate between regular Open-Ended Lines (OEL) and Blowdown OEL (BOEL) and hence there are no separate estimates for the two. Valves and connectors are the two largest contributors of emissions in this equipment category due to their large numbers, however OELs have a much higher emission factor and are fewer in number than connectors. OELs form just 1.7% of total components but contribute nearly 21% of all emissions in this equipment category. Thus, OELs may be a prime candidate to focus the DI&M practice for emissions reduction. Clearstone has also identified "Meters" as a small methane emissions source, a source not identified in the GRI study.

Reciprocating Compressors

Reciprocating compressors are the dominant type of compressors used in the gas processing industry. The GRI report estimates that reciprocating compressors formed 85% of the total compressor population in the gas processing industry. Components associated with reciprocating compressors are subject to high thermal and vibrational stresses that make them prone to leaks and are therefore among the largest source of emissions in the gas plant. The Clearstone study estimate for emissions from this category is about 35% higher than the GRI study. However, there are significant differences in estimates at the individual component level within this category. According to GRI, BOELs are the largest emissions source, contributing about 50% of all emissions from reciprocating compressors. The Clearstone study has only two data points related to emissions from blowdown and their average is small. However, the Clearstone data estimate much higher emissions from compressor seals than does the GRI study. Clearstone has also identified valves and connectors as big sources of emissions that are not explicitly listed in the GRI study. The total emissions from GRI's miscellaneous category are far too small to include valves and connectors as identified by Clearstone. Thus, according to Clearstone, compressor seals, valves and connectors are the top three emission sources in that order and together constitute 90% of the emissions from reciprocating compressors.

Centrifugal Compressors

Centrifugal compressors are represented in these studies as a smaller source of emissions compared to reciprocating compressors. GRI shows an individual centrifugal compressor emitting more than an individual reciprocating compressor. However, due to the low numbers of centrifugal compressors used in the gas processing sector they do not contribute as much as reciprocating compressors to the total emission volume. The GRI emissions estimate is heavily weighted by one particular component – the BOEL that is estimated to contribute 6,447 Mscf/year or nearly 79% of all emissions from centrifugal compressors, plus another 15% that comes from starter OELs. The Clearstone study does not have any data points clearly relating to blowdown or starter OELs, and emissions from other components are much smaller, resulting in much lower total emissions from centrifugal compressors. The Clearstone data show compressor seals as the largest emissions source. However, Clearstone does not identify the type of seal – wet or dry – and it is difficult to discern whether the estimate for compressor seals or other OEL account for vents from wet seal oil degassing. The Clearstone study also identifies a few sources of emissions, valves, connectors, pressure relief valves, and meters, that are not shown explicitly in the GRI estimates. According to the GRI survey in 1992, centrifugal compressors accounted for 15% of all compressors in gas plants. However, anecdotal data suggests that the proportion of centrifugal compressors has been increasing steadily since the GRI data was collected.

Cryogenic Equipment

The cryogenic equipment category only contains turbo-expanders and the equipment associated with them. This category of equipment was not identified in the GRI study and so is a 'new' source of methane emissions. The total emissions from turbo-expanders are small and contribute less than 3% of total methane emissions in a gas plant. The individual components contributing to emissions are valves, connectors, pressure-relief valves, compressor seals and open ended lines.

Results from The Two Processing Studies

This section examines the evidence in the two studies that supports emission reduction opportunities described earlier in the paper. While the GRI study concluded that fugitive emissions contributed 70% of all methane emissions at gas plants, the Clearstone study estimated this value to be 56%. Thus, both studies point to fugitive emissions representing a significant opportunity to reduce methane loss, and the Clearstone study further reinforces the conclusion that DI&M is an important practice for methane emissions reductions. Clearstone fugitive emissions in non-compressor related facilities are more than double those in the GRI study. This is not due to a larger number of component count but rather due to higher emission factors primarily in valves, connects and OELs.

In the compressor related sections, there is a potential discrepancy between the two studies in emissions estimates from compressor blowdown and start-up vents. None of the Clearstone emissions factors suggest that large volumes of gas are vented during these routine venting operations, which suggests that the manifolded vents evaluated by Clearstone, included in the 'vented and flared' category, likely included some significant contributions from compressor sources. While GRI's total plant emissions from blowdown and start-up vents are 30 times higher than Clearstone, even this magnitude is probably not from actual venting, but rather from leaking shut-off valves. Clearstone's blowdown and start-up vent lines may also be included in their general category of "open ended lines" (OEL) along with the numerous small diameter vent and drain connections commonly regarded as OELs (as opposed to elevated vent stacks). GRI apparently grouped such small diameter OELs under the category of "miscellaneous/other" fugitive sources, along with valves and connectors. In any event, DI&M at Gas Processing Plants and Booster Stations5 is an even more important practice for fugitive methane emissions control in compressor related areas. A growing number of operators are also taking advantage of a new technology, optical leak imaging, to make leak detection much more cost effective. This technology allows the operator to scan a large number of components in a process area quickly and see leaks normally invisible to the human eye through an infrared camera viewfinder.

The much larger emissions from blowdown and start-up vents in GRI are offset in the Clearstone study by much larger reciprocating compressor seal emissions. This suggests that economic rod packing replacement in reciprocating compressors is an important practice in addition to those identified in the EPA Natural Gas STAR BMP study. The GRI emission factor in processing plants was found to be approximately equivalent to rod packing emissions in an economically maintained seal system. Therefore, it was concluded in the gathering and processing sector BMP study that there is little opportunity for technical or economic emission reductions with this practice. However, the Clearstone emission factor of 250 standard cubic feet per hour (scf/h), five times higher than GRI, would suggest that the reciprocating compressor seals in the four plants tested may not be maintained to an economic standard. The Natural Gas STAR Lessons Learned study on Reducing Methane Emissions from Compressor Rod Packing Systems6 shows that seals can be economically replaced when emissions increase by 55 scf/h, assuming a $3/Mscf gas price. If the Clearstone study plants are representative of the worst 20 percent of processing plants in methane emissions, the sector may be able to economically save about 1.5 Bscf/year of methane emissions by practicing economic rod packing replacement.

One other significant difference between Clearstone and GRI studies is the cryogenic equipment category. Whether the GRI plants did not include cryogenic equipment, or that equipment was simply included in normal plant fugitives is not known. All four of the Clearstone processing plants had cryogenic operations.

Outside of this one source, the fact that Clearstone and GRI total fugitive emissions are nearly in parity appears to be coincidental rather than systematic. DI&M is clearly an excellent BMP for gas processing plants, as determined by the EPA BMP study for the gas processing sector and approved by the industry advisory committee. Moreover, it appears that economic rod packing replacement is a valuable practice in addition to the gas processing sector BMPs, and could be even more valuable applied to gathering/booster station compressors where virtually all compressors are reciprocating.

Conclusions and Recommendations

Overall, the Clearstone study performed a decade later than that of GRI, reinforces the GRI/EPA study in suggesting that there are several opportunities for economic methane emissions reduction in gas processing plants. Although the individual releases of methane may appear to be minimal, they are often continuous in nature and cumulatively very significant. Following are some specific conclusions and recommendations derived from these two processing plant studies.

While gas processing plant operators have already reduced methane emissions in their operations, GRI and Clearstone study provide ample evidence that reciprocating compressor seals and fugitive leaks in other areas present opportunities for significantly more methane emission reductions and therefore additional revenue generation for gas processing plants.

References

  1. U.S. EPA. April 15, 2004. Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2002. EPA 430-R-04-003.
  2. U.S. EPA and GRI. 1996. Methane Emissions from Natural Gas Industry Volume 8: Equipment Leaks Available through NTIA. Springfield VA. Publication No. EPA-600/R- 96-080h.
  3. U.S. EPA and Clearstone Engineering Ltd. 2002. Identification and Evaluation of Opportunities to Reduce Methane Losses at Four Gas Processing Plants. GRI-02/0119.
  4. U.S. Environmental Protection Agency. Protocol for Equipment leak Emission Estimation, EPA-453/R-93-026, NTIS PB93-229219, Emission Standards Division, June 1993.
  5. U.S. EPA. October 2003. Lessons Learned From Natural Gas STAR Partners: Directed Inspection and Maintenance at Gas Processing Plants and Booster Stations. EPA430-B-03-018 (available on www.epa.gov/gasstar).
  6. U.S. EPA. July 2003. Lessons Learned From Natural Gas STAR Partners: Reducing Methane Emissions from Compressor Rod Packing Systems. EPA430-B-03-011 (available on www.epa.gov/gasstar).

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