Natural gas (NG), which is 90+% methane, heats half of America’s households and fuels many important industrial processes including production of electricity, fertilizer, and polymers. NG is collected from various sources including petroleum wells, landfills, sewage treatment plants, etc.
Our society runs on NG (methane), refined petroleum, and coal. Coal has its problems with pollution from mercury and CO2, but methane is also problematic, partly because it is ubiquitous. However, if one looks at the details, one can usually differentiate between the various sources. For example, methane from wells is too old to have significant 14C radioactivity. Methane from surface sources contains 14C, which can be measured by accelerator mass spectrometry. NG with a high percentage of ethane indicates that the gas came as a byproduct from petroleum production. Some formations produce methane only.
Methane in our atmosphere arises from natural and industrial sources. Globally, methane concentration in the earth’s atmosphere is about 1.6 ppm. Annual addition each year is 900 Tg methane. Of this, 400 Tg comes from anthropogenic sources and the remaining majority from methanogenesis in wetlands, etc., where microbes decompose organic matter. Plus there are natural seeps from various geologic formations. Methane hydrates in the deep ocean are another potential source.
Methane is a greenhouse gas with a warming potency about 20‒35 times that of CO2. Recently, reports of methane or NG leaks have fueled the charges of those against hydraulic fracturing to produce petroleum hydrocarbons, particularly methane from tight formations. They argue that if leaks release as little as 3‒4% of a natural gas stream, the impact on global warming would be as bad for the environment as the 96‒97% methane combusted to CO2. They extend the concern to replacing coal with NG as an energy source. Their goal appears to be stopping hydraulic fracturing as a technology.
How bad are the leaks? Brandt et al. summarizes nine different reports of NG leakage from anthropogenic sources in the U.S.A. The highest estimate of leakage is at 100 Tg/yr methane. Other estimates range downward to the single-digit Tg range.
A critical review of these reports led by Prof. Brandt in Science concludes: “We find (i) measurements at all scales show that official inventories consistently underestimate actual CH4 emissions, with the NG and oil sectors as important contributors; (ii) many independent experiments suggest that a small number of “superemitters” could be responsible for a large fraction of leakage; (iii) recent regional atmospheric studies with very high emissions rates are unlikely to be representative of typical NG system leakage rates; and (iv) assessments using 100-year impact indicators show system-wide leakage is unlikely to be large enough to negate climate benefits of coal-to-NG substitution.”
Brandt’s data appear to be most consistent with large leaks in the distribution system.1 While most leaks are small, none should be neglected. NG mixed with air is a powerful explosive, as seen in New York City on March 12, 2014, when a NG explosion destroyed two apartment buildings and killed eight.
In 2011, NG was gathered from 460,000 wellheads through 19,662 miles of gathering pipelines. Methane is distributed through a network of nearly 2 million miles of pipes.2 Usually the older pipes are iron or steel. Most are underground, which means wet. Wet iron and steel rust, especially if the pipes are not protected with anodes. Rusted pipes fail. Plus, some pipes are not even on maps. Worse, existing maps are sometimes inaccurate.
For example, PGE (formerly Pacific Gas & Electric) distributes NG and electricity to Central and Northern California. PGE is a regulated utility, but for decades, the company has benefited from oversight from the California Public Utilities Commission and federal regulators. The most graphic consequence was the San Bruno pipe failure in September 2010. Several years prior to the explosion, PGE had requested and received additional funds to study and remediate NG pipeline deterioration. It appears they pocketed most of the funds, improving their profitability.
On April 1, 2014, PGE was indicted on 12 criminal counts of violating the Federal Pipeline Safety Act between 2003 and 2010.3 Should the indictments be proven, the potential liability is expected to be more than $4 billion. Plus, if the injured individuals are successful in seeking civil restitution for the eight deaths, 66 injured and loss of 38 homes, the total could be even larger.
These examples show that NG leaks need to be aggressively searched for and remediated. If NG pipelines and distribution firms fail to act, they will face a continuing series of disasters and associated legal expense. The public will certainly deal harshly with incidents caused by negligence.
Leak detection technology
This leads to the question: What analytical technology is available to detect methane leaks? The technology has advanced in the last 60 years. When I was a child, some 60 years ago, I recall that a gas main on the way to elementary school was trenched by the local utility in Long Beach, CA. I recall that the trench revealed NG leak points by the olive drab color of the soil, plus the aroma of the odorant. Sections of the pipeline had to be replaced.
A decade later, I worked as a roustabout in the oil fields. Our leak detector was a bucket of dilute soap solution, which we slopped on potential leaks such as gaskets and fittings. If it bubbled, the order was “Fix it!”
Decades later, in Lafayette, CA, before starting to resurface our street, the gas distribution network for our neighborhood was surveyed for leaks with a simple sniffer probe connected to a flame ionization detector (FID). The FID responds to hydrocarbons, including methane. The probe was waved over or poked into the soil about every 25 ft. Leaking hardware was located and replaced. The distribution network was about 50 years old.
More recently, methane detectors based in absorbance in the near infrared (NIR) have been reported. Sampling is via optical fibers, which can be placed near potential leak points. Detection sensitivity is sufficient to detect methane below the lower explosive limit (5% by volume) in air.4 Optical fibers seem to be suitable for locating high-volume leaks in NG processing operations described in the supplemental information online.1
For field surveys of buried pipes, one would like better detection sensitivity. A recent report by Suresh Pisharody of Picarro, Inc. (Santa Clara, CA) describes the use of the company’s Cavity Ring Down Spectrometry (CRDS) for rapid methane detection with low-ppb detection sensitivity.2 CRDS uses a three-mirror cavity and a tunable, single-frequency diode laser to create a continuous laser wave. By measuring the time it takes for the light intensity generated by this wave to decay, or “ring down,” the technology can automatically and continuously detect gases in the illuminated path volume. Picarro’s specific implementation of a three-mirror cavity increases the pathlength of the continuous wave to several kilometers. Additionally, by tuning the laser to different wavelengths where a target gas absorbs light and mapping the various points, the technology is able to determine the precise concentration of gases. More importantly, by accounting for the range of stable isotope values for a given gas, CRDS is able to distinguish between different forms of the same gas, such as petrochemical methane and microbial methane, based upon stable isotope ratios.
A report by the membership of the Pipeline Research Council International (PRCI) and led by PG&E, Picarro’s Surveyor was field tested in four regions in CA and NV including Diablo, Livermore, Sacramento, and Southern Nevada. In side-by-side studies with legacy industry methods, the car-mounted Surveyor identified significantly more leaks in considerably less time, including several that required immediate remediation.
The point is that we have the technology to locate most leaks in the production and distribution system. While the focus on global warming is probably misdirected, there is no doubt that leaks can be located and should be fixed for public safety. Apparently, the major problem is the lack of will from the organizations involved.
While humans can and should mitigate emission of methane from anthropogenic sources, controlling natural sources may be an increasing problem. Two data points are:
- A recent article reports that thawing of arctic permafrost in peat deposits leads to humidification with an increasing emission of methane and CO2. The team warns that permafrost thawing triggers a positive feedback process accelerating emission of greenhouse gases.5
- Another article describes how geese and shore bird activists are paying rice farmers in the Central Valley of California to keep the rice fields wet during the winter, rather than the past practice of drying the fields after harvest and burning the stubble.6 Thus, one replaces CO2 with a comparable amount of methane, since the decaying straw is digested anaerobically.
These sources may be small compared to the potential of release of methane from deep-water hydrates in the oceans. The methane concentration in the atmosphere is 1.6 ppm. Even if it is 30 times more effective than CO2, this would be comparable to about 50 ppm of CO2. Currently, CO2 is about 400 ppm. Thus, methane is a significant potential contributor, but CO2 appears to be a much larger problem. Even so, can anything be done to mitigate its effect?
Atmospheric methane has a lifetime of 8‒12 years. The degradation is a complex series of photochemical gas phase oxidations according to Eqs. (1‒7) below.7 The degradation process is described in Ref. 7.
The methane degradation scheme is three complex parts: Oxidation to formaldehyde, followed by oxidation to carbon monoxide, which is then oxidized to CO2. In more detail, the oxidation of atmospheric methane starts with reactions with OH· according to the reaction sequence 1‒4.
Most of the steps involve reaction with other chemicals. However, one seems particularly significant. Eq. (2) involves oxidation at a surface of a particle, probably an aerosol. This surface dissipates excess energy released in this reaction. Atmospheric aerosols are important in atmospheric heat balance. After the eruption of Mount Pinatubo, airborne debris increased reflection of solar radiance as the cause of global cooling for four years posteruption. Recent air pollution efforts have also reduced atmospheric aerosols. This may extend the half-life of methane. Is aerosol generation a defense mechanism for global warming? Is it practical?
Since natural sources of methane are diffuse and dominate, mitigation strategies designed to eliminate emissions are probably not practical. Similarly, reducing emission of carbon dioxide is probably only practical for larger point sources. Perhaps this is an opportunity for novel R&D programs to scrub these pollutants from the atmosphere. Plus, America has several powerful national laboratories run by the Department of Energy. It should be easy to connect the dots between need, charter, and funding. The scale seems daunting, but so is the impact of doing nothing.
- Brandt, A.R. et al. Methane leaks from North American natural gas systems. Science 14 Feb 2014, 343(6172) 733‒5 and www.sciencemag.org/content/343/6172/733/suppl/DC1
- Pisharody, S. Spectroscopy method detects and measures methane leaks; http://www.novuslight.com/spectroscopy-method-detects-and-measures-methane-leaks_N1359.html#sthash.GhYaKlzH.dpuf
- Avalos, G. PG&E hit with federal charges. Contra Costa Times Apr 2, 2014, p 1A.
- Culshaw, B.; Stewart, G. et al. Fibre optic techniques for remote spectroscopic methane detection—from concept to system realization. Sensors and Actuators B 31 Aug 1998; 51(1‒3), 25‒37; http://www.sciencedirect.com/science/article/pii/S0925400598001841
- Hodgkins, S.B.; Tfaily, M.M. et al. Changes in peat chemistry associated with permafrost thaw increase greenhouse gas production; online before print, Apr 7, 2014, doi: 10.1073/pnas.1314641111; http://www.pnas.org/content/early/2014/04/02/1314641111.short; PNAS Apr 7, 2014.
- Robins, J. Paying farmers to welcome birds. NY Times 4/15/14, p D1.
- Anonymous. http://environmentofearth.wordpress.com/2009/09/13/methane-cycle-in-atmosphere/
Robert L. Stevenson, Ph.D., is Editor, American Laboratory/Labcompare; e-mail: email@example.com .