April N. Covington

Near-Field Characterization of Methane Emission Variability from a Compressor Station Using a Model Aircraft

A model aircraft equipped with a custom laser-based, open-path methane sensor was deployed around a natural gas compressor station to quantify the methane leak rate and its variability at a compressor station in the Barnett Shale. The open-path, laser-based sensor provides fast (10 Hz) and precise (0.1 ppmv) measurements of methane in a compact package while the remote control aircraft provides nimble and safe operation around a local source. Emission rates were measured from 22 flights over a one-week period. Mean emission rates of 14 ± 8 g CH4 s(-1) (7.4 ± 4.2 g CH4 s(-1) median) from the station were observed or approximately 0.02% of the station throughput. Significant variability in emission rates (0.3-73 g CH4 s(-1) range) was observed on time scales of hours to days, and plumes showed high spatial variability in the horizontal and vertical dimensions. Given the high spatiotemporal variability of emissions, individual measurements taken over short durations and from ground-based platforms should be used with caution when examining compressor station emissions. More generally, our results demonstrate the unique advantages and challenges of platforms like small unmanned aerial vehicles for quantifying local emission sources to the atmosphere.

Methane Emissions from Leak and Loss Audits of Natural Gas Compressor Stations and Storage Facilities

As part of the Environmental Defense Funds Barnett Coordinated Campaign, researchers completed leak and loss audits for methane emissions at three natural gas compressor stations and two natural gas storage facilities. Researchers employed microdilution high-volume sampling systems in conjunction with in situ methane analyzers, bag samples, and Fourier transform infrared analyzers for emissions rate quantification. All sites had a combined total methane emissions rate of 94.2 kg/h, yet only 12% of the emissions total resulted from leaks. Methane slip from exhausts represented 44% of the total emissions. Remaining methane emissions were attributed to losses from pneumatic actuators and controls, engine crankcases, compressor packing vents, wet seal vents, and slop tanks. Measured values were compared with those reported in literature. Exhaust methane emissions were lower than emissions factor estimates for engine exhausts, but when combined with crankcase emissions, measured values were 11.4% lower than predicted by AP-42 as applicable to emissions factors for four-stroke, lean-burn engines. Average measured wet seal emissions were 3.5 times higher than GRI values but 14 times lower than those reported by Allen et al. Reciprocating compressor packing vent emissions were 39 times higher than values reported by GRI, but about half of values reported by Allen et al. Though the data set was small, researchers have suggested a method to estimate site-wide emissions factors for those powered by four-stroke, lean-burn engines based on fuel consumption and site throughput.

Pump-to-Wheels Methane Emissions from the Heavy-Duty Transportation Sector

Pump-to-wheels (PTW) methane emissions from the heavy-duty (HD) transportation sector, which have climate change implications, are poorly documented. In this study, methane emissions from HD natural gas fueled vehicles and the compressed natural gas (CNG) and liquefied natural gas (LNG) fueling stations that serve them were characterized. A novel measurement system was developed to quantify methane leaks and losses. Engine related emissions were characterized from twenty-two natural gas fueled transit buses, refuse trucks, and over-the-road (OTR) tractors. Losses from six LNG and eight CNG stations were characterized during compression, fuel delivery, storage, and from leaks. Cryogenic boil-off pressure rise and pressure control venting from LNG storage tanks were characterized using theoretical and empirical modeling. Field and laboratory observations of LNG storage tanks were used for model development and evaluation. PTW emissions were combined with a specific scenario to view emissions as a percent of throughput. Vehicle tailpipe and crankcase emissions were the highest sources of methane. Data from this research are being applied by the authors to develop models to forecast methane emissions from the future HD transportation sector.

Design and Use of a Full Flow Sampling System (FFS) for the Quantification of Methane Emissions.

The use of natural gas continues to grow with increased discovery and production of unconventional shale resources. At the same time, the natural gas industry faces continued scrutiny for methane emissions from across the supply chain, due to methanes relatively high global warming potential (25-84x that of carbon dioxide, according to the Energy Information Administration). Currently, a variety of techniques of varied uncertainties exists to measure or estimate methane emissions from components or facilities. Currently, only one commercial system is available for quantification of component level emissions and recent reports have highlighted its weaknesses. In order to improve accuracy and increase measurement flexibility, we have designed, developed, and implemented a novel full flow sampling system (FFS) for quantification of methane emissions and greenhouse gases based on transportation emissions measurement principles. The FFS is a modular system that consists of an explosive-proof blower(s), mass airflow sensor(s) (MAF), thermocouple, sample probe, constant volume sampling pump, laser based greenhouse gas sensor, data acquisition device, and analysis software. Dependent upon the blower and hose configuration employed, the current FFS is able to achieve a flow rate ranging from 40 to 1,500 standard cubic feet per minute (SCFM). Utilization of laser-based sensors mitigates interference from higher hydrocarbons (C2+). Co-measurement of water vapor allows for humidity correction. The system is portable, with multiple configurations for a variety of applications ranging from being carried by a person to being mounted in a hand drawn cart, on-road vehicle bed, or from the bed of utility terrain vehicles (UTVs). The FFS is able to quantify methane emission rates with a relative uncertainty of ± 4.4%. The FFS has proven, real world operation for the quantification of methane emissions occurring in conventional and remote facilities.

Methane Leak and Loss Audits of Natural Gas Fueled Compressor

Natural gas reserves within the United States continue to rise. According to the Energy Information Administration, dry natural gas reserves increased by ten percent from 2010 to 2011, while wet natural gas reserves increased by 38% in 2011. Natural gas consumption also increased from 24.09 trillion cubic feet (TCF) to 24.48 TCF over the same period. As the natural gas supply, demand, and industry continue to grow methane losses across the supply chain will be inevitable. Since methane is a potent greenhouse gas, many studies are currently analyzing the loss of methane from the wells to the end user. As natural gas transmission systems grow there must be an increase in natural gas compressor and storage facilities.Currently, there is not a detailed inventory describing the emissions associated with natural gas compressor system engines in terms of the emissions resulting from engine unit losses and leaks. Researchers from West Virginia University’s Center for Alternative Fuels, Engines, and Emissions (CAFEE) recently conducted methane leak and loss audits at five compressor stations with a special focus placed on the engine and compressor units. These audits focused on identifying and quantifying the leaks and losses associated with the engines and compressor units of a typically operating site. A micro dilution high volume sampling system was used in conjunction with a portable methane analyzer to quantify leaks and losses. Bag samples of exhaust gas and engine operating parameters were used to calculate the methane flow rate from the reciprocating engines and turbines used to operate compressors at these sites. Leaks are defined as unintended methane releases from components not designed to emit methane. Losses are defined as methane releases that are known to exist or exist by design.Copyright

Future methane emissions from the heavy-duty natural gas transportation sector for stasis, high, medium, and low scenarios in 2035

ABSTRACT Today’s heavy-duty natural gas–fueled fleet is estimated to represent less than 2% of the total fleet. However, over the next couple of decades, predictions are that the percentage could grow to represent as much as 50%. Although fueling switching to natural gas could provide a climate benefit relative to diesel fuel, the potential for emissions of methane (a potent greenhouse gas) from natural gas–fueled vehicles has been identified as a concern. Since today’s heavy-duty natural gas–fueled fleet penetration is low, today’s total fleet-wide emissions will be also be low regardless of per vehicle emissions. However, predicted growth could result in a significant quantity of methane emissions. To evaluate this potential and identify effective options for minimizing emissions, future growth scenarios of heavy-duty natural gas–fueled vehicles, and compressed natural gas and liquefied natural gas fueling stations that serve them, have been developed for 2035, when the populations could be significant. The scenarios rely on the most recent measurement campaign of the latest manufactured technology, equipment, and vehicles reported in a companion paper as well as projections of technology and practice advances. These “pump-to-wheels”(PTW) projections do not include methane emissions outside of the bounds of the vehicles and fuel stations themselves and should not be confused with a complete wells-to-wheels analysis. Stasis, high, medium, and low scenario PTW emissions projections for 2035 were 1.32%, 0.67%, 0.33%, and 0.15% of the fuel used. The scenarios highlight that a large emissions reductions could be realized with closed crankcase operation, improved best practices, and implementation of vent mitigation technologies. Recognition of the potential pathways for emissions reductions could further enhance the heavy-duty transportation sectors ability to reduce carbon emissions. Implications: Newly collected pump-to-wheels methane emissions data for current natural gas technologies were combined with future market growth scenarios, estimated technology advancements, and best practices to examine the climate benefit of future fuel switching. The analysis indicates the necessary targets of efficiency, methane emissions, market penetration, and best practices necessary to enable a pathway for natural gas to reduce the carbon intensity of the heavy-duty transportation sector.

Potential Reduction of Fugitive Methane Emissions at Compressor Stations and Storage Facilities Powered by Natural Gas Engines

The American Gas Association (AGA) and the United States (US) Energy Information Administration (EIA) report that natural gas reserves, production, and consumption are increasing. Current estimates show over 100 years worth of recoverable reserves. As production increases, the natural gas pipeline interstate will grow or at least experience increased throughput. With the industry expanding at rapid rates and the high global warming potential of methane (21 over a 100 year period), it is important to identify potential sources for reductions in fugitive methane emissions. This research group conducted leak and loss audits at five natural gas compressor station and storage facilities. The majority of methane losses were associated with the operation of the lean-burn, natural gas engines (open crankcases, exhaust), compressor seal vents, and open liquid storage tanks. This paper focuses on the potential reduction in fugitive methane emissions of the discovered industry weaknesses through application of various proven technologies. As engines are not perfectly sealed, blow-by of intake air, fuel, and combustion gases occurs past the piston rings. In order to prevent a build-up of pressure within the crankcase, it must be vented. Diesel engines have lower hydrocarbon emissions from their crankcases due to the short duration of fuel addition after compression of the intake charge. Lean-burn, natural gas engines, like conventional gasoline engines, compress both the fuel and intake air during the compression stroke. During the 1960s, many passenger vehicles adopted positive crankcase ventilation (PCV) or closed crankcase ventilation (CCV) systems to reduce significantly hydrocarbon emissions from engines. Currently, some heavy-duty on-road engines still have open crankcase systems and most off-road engines have crankcases simply vented to the atmosphere. In this paper, researchers will examine the potential reduction in methane emissions that could be realized with the installation of retrofitted CCV systems at these locations. In addition to the reduction of methane losses from the crankcase, it is realized that with proper plumbing, flow control, and safety parameters, all of the losses typically vented to atmosphere could be ducted into the engine intake for combustion. Preliminary results show that applications of closed crankcase systems could reduce emissions from these sites by 1–11% while modifying these systems to include the losses from compressor seal vents and storage tanks could yield potential reductions in methane emissions by 10–57%.Copyright