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February 10, 2003
The road to zero emissions
DART advances leading-edge clean fuels program
Increasing transit use is one of the easiest ways to reduce air pollution, and Dallas Area Rapid Transit -- using clean fuels to power its fleet of high-tech buses and trains -- is making transit the cleanest commuting choice around.
"While DART vehicles are responsible for less than one-half of one percent of the area's air pollution, using transit can make a big impact in air quality," said DART President/Executive Director Gary Thomas. "Our buses and vans must be seen as a clean alternative to cars, just as light rail is. Using the latest technologies in practical ways, we are reducing our emissions every year, with a target of operating a zero emissions bus by 2010."
In August 2002, the North Texas region suffered its first-ever "level purple" air quality alert, where the air was so bad that everyone was cautioned to stay inside. After smog build-ups violated EPA ozone standards 22 times that summer, federal authorities warned that highway funds could be withheld unless improvements are made.
The cleanest choice is transit
An American Public Transportation Association study reported that, on average, a mile traveled on transit uses only half the fuel needed for a single-occupant vehicle and results in only a fraction of the pollution. The study said increased transit use is the most effective strategy available for reducing energy consumption and improving the environment without new taxes, government mandates or regulations on the economy or consumers.
DART meets or exceeds all current federal and state air quality standards. However, regulations for transit vehicles are becoming stricter, aiming at a 90% reduction in emissions by 2010, notes Mike Hubbell, DART's vice president of Maintenance.
"The 2007 federal standard for NOx cannot be met by any currently available transit engine technology," he said. "But both diesel and natural gas technologies are improving, and we're involved with both of them. We'll get there." Oxides of nitrogen emissions, or NOx, is one of the major contributors to the formation of ozone.
Natural Gas Experience
DART began its push for cleaner fuels in 1996 with a commitment to bring ultra-low emission (ULEV) buses into the fleet that use liquefied natural gas (LNG), a cryogenic fuel that must be kept at 260 degrees below zero.
"When we started our LNG program, we had to overcome a lot of unanticipated problems, including engine failures, fuel loading difficulties and even fuel availability that made it difficult to keep our bus routes on schedule," said Hubbell.
With the addition of 45 LNG buses in 2002, DART's LNG fleet now totals 184 buses. Additionally, DART uses compressed natural gas (CNG) to fuel 20 ULEV Trolley-Buses and 200 ULEV non-revenue vehicles.
DART's LNG experience also includes better fuel purchasing techniques. "We went through a period when we didn't know in the morning if enough LNG would arrive to fuel all our buses that day," said Hubbell. "Now we have a long-term contract for lower-cost LNG through 2005."
While natural gas-fueled vehicles have shown promise, a study done for DART by Battelle, a national engineering and technology firm, showed that natural gas wasn't the best or cleanest fuel for every bus in the fleet. "On suburban bus routes with more periods of steady driving, natural gas engines do well, but for stop-and-go operation on city streets, a hybrid engine may be better," said Hubbell.
Clean diesel advances
DART has moved to the forefront of ultra-low-sulfur diesel utilization with a $7.5 million grant awarded by the State of Texas under the Texas Emissions Reduction Plan (TERP) in July 2002. The grant, presented by Texas Gov. Rick Perry, allow DART to rework 360 engines in the agency's 1999-2000 model year diesel buses.
The engines will be fitted with a low-pressure exhaust gas recirculation (EGR) system and improved catalytic converter that can only operate with diesel fuel that has substantially less sulfur. The grant also allows DART to contract with a Texas refinery for an assured supply of the ultra-low-sulfur diesel. The fuel contract is with the Valero Three Rivers refinery. The retrofit program is scheduled for completion by March 2004.
"Besides making a huge improvement in emissions from the reworked buses, our access to ultra-low-sulfur diesel also accelerates DART's compliance with state regulations for diesel emissions scheduled for early 2005," said Hubbell. "Our addition of 45 new clean diesel buses last year also helps."
The push to fuel cells
Despite the advances with clean diesel and natural gas, the fact that both rely on internal combustion means they can never provide zero-emission power. Ultimately, Hubbell believes achieving a zero-emissions bus fleet will probably require some form of electric propulsion system in conjunction with hydrogen-powered fuel cells.
"In learning how to handle LNG fuels at 260 degrees below zero, we've become better prepared for when we use hydrogen, which is also best handled as a cryogenic fuel," said Hubbell. "Right now, fuel cell buses cost a million dollars, which makes them economically unfeasible," said Hubbell. "Additionally, fuel cell propulsion has to be supported with the infrastructure and a source of fuel-quality hydrogen."
DART buses log 38 million miles a year serving 150,000 passengers daily. Besides the LNG and CNG vehicles, the fleet includes 95 zero-emission electric light rail vehicles, 170 low-emission Paratransit vehicles fueled by diesel or gasoline, 91 low-emission diesel buses and 75 low-emission non-revenue vehicles fueled by diesel or gasoline.
"We're in a strong position to continue to meet or exceed emissions requirements for the next few years," said Hubbell. "For years further out, we will watch new developments and stay ready to implement the next level of technology."
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