Heavier crudes, heavier footprints

Transition to viscous feedstocks is upping greenhouse-gas emissions associated with liquid fuels

Relying on heavy oils and tar sands as the feedstock for liquid fuels will exaggerate the greenhouse-gas emissions associated with fossil-fuel use, a new study finds.

FUEL INTENSIVE FUEL-MAKING Refining heavier crudes at this California facility and others takes more energy — and spews more greenhouse gases. iStockPhoto/© Daniel Stein
CRUDE CHANGES Graphs chart federal data on increasing viscosity, aka falling gravity (top) and rising sulfur content (bottom) of crude oil entering U.S. refineries since 1985. DOE/ Energy Info. Admin.

Light crudes are the easiest to work with. But as their biggest and most accessible reservoirs have been tapped — and often tapped out — the oil industry has increasingly been turning to what has been termed “unconventional” stocks. These are viscous, if not tarry, forms of petroleum.  And as the upper graph below shows, the average “gravity” — viscosity of crude — has fallen into the heavy range (below an average of about 31 degrees on the American Petroleum Institute scale) beginning in 2000. At least for oil processed by U.S. refineries.

Not surprisingly, it takes extra work to convert viscous gunk into the gasoline, diesel and other high-value fuels that power engines the world over. And the extra fuel that powers those upgrades releases bonus greenhouse-gas emissions, thereby upping the carbon footprint of each gallon of refined product created.

It also takes extra energy to process “sour” crude —  petroleum naturally bearing high concentrations of sulfur. That sulfur can poison catalysts used in refining. And as the lower graph below (courtesy of the Department of Energy) indicates, the sulfur content of crude oil entering U.S. refineries has been climbing steadily since at least 1985.

The higher energy intensity inherent in processing sour, heavy crudes has been known for a long time, observes Greg Karras of the Oakland, Calif.-based Communities for a Better Environment. Indeed, the pollution-prevention engineer suspects, refiners probably know to the dime what it costs to convert each barrel of raw crude that they purchase into liquid fuels — and can calculate how that translates to greenhouse-gas emissions.

But owing to trade secrets, companies haven’t shared this information, Karras says. So it’s been virtually impossible for the public and policymakers to quantify the climate implications of moves to rely on some of the really heavy crudes — especially those tar sands bitumens that may have a API gravity of around 8.

Karras undertook an analysis of the publicly available data reported between 1999 and 2008 by refineries accounting for 97 percent of U.S. capacity: what types of crude oil were entering refineries and in what amounts, what products they generated and in what amounts. Then he used what was known about the energy intensity of processing steps used to refine and upgrade various crudes into particular products.

The evolution toward more sour and viscous petroleum sources “drove a 39 percent increase in emissions across regions and years,” Karras reports in a paper posted early online in Environmental Science & Technology. Fossil-fuel use increased 61 megajoules for each kilogram of sulfur in a cubic meter of oil and increased 44 megajoules for each kilogram per cubic meter of oil density.

“We currently import roughly 1 million barrels of Alberta [Canada] tar sands,” for refining in the United States, notes Ryan Salmon, an energy policy advisor to the National Wildlife Federation in Washington, D.C. And with two pipelines in place to move that low-quality petroleum to the Midwest for refining — and a third to southern Texas in the planning — the United States is committing itself to feedstocks that will require huge quantities of energy to process.

Keep in mind, he notes, in addition to the extra processing costs associated with tarry crude, there can be huge extraction costs. And the direct, non-climate fallout from that extraction can also prove costly.

Right now, a transition to heavier crudes is well underway. If these more viscous petroleum sources substituted entirely for the light, sweet (low-sulfur) crude that had been the mainstay of U.S. refining, emissions from this processing could “double or triple,” Karras says, depending on whether heavy crudes were used versus tar sands, “and add 1.6 to 3.7 gigatons of carbon dioxide to the atmosphere annually from fuel combustion to process the oil.”

That’s not what negotiators meeting right now at the United Nations Framework Convention on Climate Change in Cancun, Mexico, want to hear. They’re trying to find every easy and not-so-easy reasonable means for nations around the world to cut back on greenhouse-gas emissions.

At last year’s UNFCCC meeting in Copenhagen, the Obama administration pledged to work toward cutting U.S. greenhouse-gas emissions 17 percent — relative to 2005 values — by 2020, and to cut them 80 percent by 2050. “It wasn’t a binding agreement,” Salmon acknowledges — nothing at that meeting was. But it signaled the intent of the administration. Challenging as those targets are, they will become stickier still if the contribution of liquid fuels escalates owing to relying on super-viscous crude, his group explained in an analysis it issued December 2.

Karras agrees. “While we talk about the need to transition to more sustainable forms of energy, actual dollar investments are increasingly skewed towards investing in more pollution intensive — more energy intensive — primary sources of petroleum.” If industrial nations continue down this path, he asks, “will it foreclose the ability to achieve the total emissions reductions that we need?”

The National Petrochemical & Refiners Association and the American Petroleum Institute were each reached a day ago (Dec. 2) and invited to weigh in on the Karras report. They have not done so, other than for an NPRA spokesman to say he thought Karras’ emissions numbers for unconventional oils seemed higher than estimates that he’d heard attributed to Cambridge Energy Research Associates, earlier this year. I attempted to confirm this, but CERA did not return my phone call.

Janet Raloff

Janet Raloff is the editor of Science News for Students, a daily online magazine for middle school students. She started at Science News in 1977 as the environment and policy writer.

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