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Cows' methane emissions trump gas operations in latest EPA greenhouse gas inventory

The cattle industry topped the natural gas sector as the primary methane-emitting sources in the U.S. in 2012, even as greenhouse gas emissions from the nation overall decreased by 3.3 percent.

The reductions were in part due to power plants using natural gas, which is a cleaner fuel than coal, as their fuel source. Other reasons included more fuel-efficient vehicles and a warm winter in 2012 that reduced demand for heating in homes and offices.

The results were published yesterday in the draft greenhouse gas inventory for 1990 to 2012 put together by U.S. EPA. The agency releases the inventory yearly and submits it to the U.N. Framework Convention on Climate Change.

The inventory is supposed to be a comprehensive account of emissions in the U.S., but recent studies have suggested EPA is underestimating the amount of methane being emitted across all sectors (EnergyWire, Feb. 14).

Petroleum sector's 7 largest methane emitters:

  • shallow water offshore oil platforms
  • natural-gas-powered high-bleed pneumatic devices
  • oil tanks
  • natural-gas-powered low-bleed pneumatic devices
  • gas engines
  • deepwater offshore oil platforms
  • chemical injection pumps

Source: U.S. EPA greenhouse gas inventory

The latest inventory finds that the U.S. emitted 6,501 million metric tons of carbon dioxide equivalents (CO2e) of greenhouse gases in 2012, which is 4 percent higher than in 1990. Fossil fuel combustion, especially for electricity generation and transport, contributed 94 percent of the total emissions.

The oil and gas sector was a small but significant contributor. Its major problem remained methane, which is 20 times as effective a greenhouse gas as CO2 at trapping heat in the atmosphere. The natural gas sector released 127.1 million metric tons CO2e in methane between 2011 and 2012.

That was less than the emissions from the beef cattle industry. Enteric fermentation in the rumen of cows released 141 million metric tons CO2e of methane in 2012.

EPA also revised previous years' inventories to make the cattle industry the top methane emitter since 2009. The 2011 inventory had erroneously found the natural gas sector was the largest methane emitter in the U.S. (EnergyWire, April 16, 2013).

Use of controls

Methane emissions from the natural gas sector decreased by 17 percent since 1990, even though production has significantly increased. The reductions are due to voluntary measures adopted by producers, such as the use of low-bleed pneumatic devices and the use of plunger lifts for liquids unloading, according to EPA. Natural gas pipelines are less leaky as cast-iron pipes carrying the gas into homes are being removed.

Emissions from gas wells and upstream components of field production contributed 31 percent, or 39 million metric tons CO2e of methane, in 2012. This was 40 percent lower than levels in 2006 since gas producers have been using reduced emissions completions, plunger lifts and other controls to reduce emissions, EPA found. Producers are currently capturing or flaring about half the excess methane they generate, the agency notes.

Petroleum systems

Given the low price natural gas has fetched on the market since 2008, producers have been switching en masse to oil. EPA has not tracked methane emissions from oil wells as comprehensively as it has emissions from natural gas systems.

For instance, oil wells often co-produce methane, which is emitted when a well gets fracked or worked over. EPA does not include these emissions in its inventory.

Without these emissions, oil wells generated 31 million metric tons CO2e of methane in 2012, a 10 percent increase since 2008. Meanwhile, CO2 emissions increased by 35 percent since 2008.

EPA noted that it is considering including fracking and work-over emissions in the future. Scientific studies have found wells could emit between 3.1 and 24.7 metric tons of methane per completion or work-over, it said.

Methane studies

The agency acknowledged various studies have found issues with its greenhouse gas inventory, specifically with the methane emissions from the natural gas sector. It requested comments on how best it could incorporate the results of various studies to improve its inventory.

It is taking comments until March 26.

Gayathri Vaidyanathan, EnergyWire - Tuesday, February 25, 2014

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