Natural gas is often touted as one of the cleaner, if not the cleanest, fossil fuel in our current energy mix. Compared to the average air emissions from coal-fired generation, natural gas produces half as much carbon dioxide, less than a third as much nitrogen oxides, and one percent as much sulfur oxides at the power plant.
Although natural gas releases a fraction of the carbon per BTU of energy produced as oil and coal, methane, a greenhouse gas (GHG) twenty-five times more potent than carbon dioxide is produced in virtually every stage of natural gas development, from production all the way to distribution. The principal GHG concern for the sector is carbon dioxide emissions from natural gas combustion, the sector also generates non-energy CO2 emissions and methane which are difficult to both measure and monitor.
Emissions associated with natural gas make up nearly 18 percent of all U.S. GHG emissions. Thanks to the requirements put in place to address the acid rain problem in the 1990s, the United States currently has good data on carbon dioxide emissions from the electricity generation sector. However, much less is known about the quantity or consistency of the GHGs released in the upstream development processes.
The most oft-overlooked component of natural gas usage, especially as it pertains to the buildup of GHGs in the earth's atmosphere, are the GHGs emitted into the atmosphere in the development and production stages. In fact, GHGs other than carbon dioxide could contribute as much as 10 to 40 percent of the warming induced by carbon dioxide alone, according to a recent study published in the journal Nature.
Methane can also be emitted into the air when natural gas is not burned completely. Similarly, methane can be emitted as the result of leaks and losses during transportation. Although drilling wastes have decreased exponentially thanks to increased well productivity and advancements in natural gas production, particularly in the American West, there is substantial room for improvement.
Finding a market for natural gas byproducts
Whatever the source of the natural gas (coal seams, tight-sand formations, shale, etc.), it commonly exists in mixtures with other hydrocarbons; principally ethane, propane, butane, and pentanes. In addition, raw natural gas contains water vapor, hydrogen sulfide, carbon dioxide, helium, nitrogen, and other compounds.
Natural gas processing consists of separating all of the various hydrocarbons and fluids from the pure natural gas, to produce 'pipeline quality' dry natural gas that can be marketed as a commodity of consistent quality.
While the sulfur, ethane, propane, butane, and pentanes must be removed from natural gas, this does not mean that they are all waste products. Cryogenic cooling processes can drop t hese impurities out of the nearly finished product so they can be marketed for other purposes. Another essential step in the processing of natural gas involves the removal of sulfur and carbon dioxide. The sulfur that exists in the natural gas stream can be extracted and marketed on its own. According to the USGS, U.S. sulfur production from gas processing plants accounts for about 15 percent of the total U.S. production of sulfur. But because it is not yet regulated in the U.S., carbon dioxide releases are often left unchecked.
Although many companies do very little with the secondary gases created in natural gas processing and merely flare or burn them off, some companies are finding creative ways to address the CO2 and other gases produced. Williams Energy, Colorado's largest natural gas producer, for example, has entered into an agreement with Solvay Chemicals to sell about half of the CO2 produced at Williams' gas processing plant in Parachute to be used in the production of sodium bicarbonate (baking soda). By injecting carbon into soda ash (sodium carbonate), Solvay produces sodium bicarbonate for use in a variety of applications including baking products, beverages, pharmaceuticals, chemicals, cleaning compounds, air pollution control, acid neutralization, water treatment, leather tanning, pulp & paper, plastic, oil well drilling, textile processing, swimming pools and animal feed.
Unfortunately, the economic viability of removing these secondary hydrocarbons from the produced gas varies as market demand dictates, as a result, so does the quantity and quality of attention paid to how such waste gases are handled. With substantial swings in the market price of natural gas byproducts, there has been little attempt to systematically capture or even account for the secondary gases produced in gas processing -- but this too may soon change. Language in a new rule proposed by the EPA, as well as language in the current iteration of the Waxman-Markey cap-and-trade legislation would require most industries and power plants to inventory and report their GHG emissions.
Moving forward, however, regulation of GHG emissions from the natural gas sector under a cap-and-trade program presents completely different challenges than those associated with coal or petroleum. As the summer progresses, and the details of Waxman-Markey are hashed out, keep a close eye on the issue of upstream gas development and the associated greenhouse gases because many see that particular sector as one with considerable opportunity for improvement based largely on implementing currently-available technologies.
Images: © Tim Hurst
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