Buses fueled by natural gas a better option now, study finds

The local bus system could reduce its costs and emit significantly fewer pollutants by converting its fleet to one powered by natural gas, a cleaner fuel now in greater supply and more affordable, Purdue University energy economist Wally Tyner reports in a study.

While the study was specific to the Greater Lafayette Public Transportation Corp., also known as CityBus, the approach of fueling buses with , or CNG, could apply to similar municipal bus systems nationwide, said Tyner, an economist in the Department of Agricultural Economics.

"Because of the lower fuel price and pollution reduction, the CNG bus is considered to have good potential as an alternative vehicle used in the public fleet in the United States," Tyner writes in the report "Evaluation of the Economics of Conversion to Compressed Natural Gas for a Municipal Bus Fleet."

CityBus serves the adjacent cities of West Lafayette - home to Purdue University - and Lafayette. It has 72 buses and about 30,000 riders daily.

The objective of the study was to help CityBus find the most effective way to reduce operating costs and make the fleet "greener." Tyner, graduate student Lin Yang and postdoctoral researcher Kemal Sarica performed economic and environmental analyses of the transit system using scenarios of buses fueled by compressed , diesel and a hybrid of diesel and electricity.

CityBus already runs some diesel-electric buses, which have a higher than a standard diesel bus but considerably higher capital expense in the form of higher bus costs. So it is considering natural gas among its options. Most buses in the United States use .

The report concludes that buses fueled by natural gas would be the lowest-cost option for CityBus in a 15-year project that examined expenses of buying buses, maintaining them and keeping them fueled. The diesel-electric hybrid would be the costliest.

Even with the $2 million expense of building a natural-gas , the natural-gas system would cost $48 million over the span of the project, compared with $54 million for the diesel-electric and $48.5 million for the diesel-only, according to the report.

The analysis was conducted taking into account likely future diesel and natural-gas prices. Over 20 years, the analysis concludes that the natural-gas option has a 65 percent to 100 percent chance of being lower cost than the diesel option, depending on how price uncertainty is characterized.

"Moreover, from the environmental perspective, the implementation of CNG buses in the fleet would also produce less emission and provide benefit to the environment of the local society," the report says.

Emissions of carbon dioxide and particulate matter would continually decrease over the project period if the CityBus fleet were gradually converted for natural gas. Carbon dioxide emissions from a diesel-electric hybrid system also would decrease but to a lesser extent, while those emissions from diesel-only buses would increase. Particulate matter from those two systems would remain about what they are now.

The report notes that production of natural gas in the U.S. has increased dramatically in recent years, particularly from the rapid growth in shale gas, and is expected to increase until 2035. It also says the price difference between crude oil and natural gas has grown larger since 2009.

But it also says transit companies are likely to make the switch to natural gas only when the additional capital costs, such as buying new buses and building natural-gas refueling stations, are covered by the savings in fuel costs over the life spans of the .

Officials of the transit system will consider the results of the report, said Martin Sennett, general manager of CityBus.

"Dr. Tyner was extremely helpful to CityBus in helping to formulate a decision on the pros and cons of each fuel choice," Sennett said.

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More information: onlinelibrary.wiley.com/doi/10.1002/ese3.14/pdf
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Sep 25, 2013
Transit systems in New York City, Baltimore, Chicago, Pittsburgh and around the country are adding clean diesel and diesel-hybrid to their fleets.


The Clean Air Task Force conducted an analysis comparing 2012 CNG and 2012 diesel buses. According to the analysis: "Both new diesel and new CNG buses have significantly lower emissions of NOx, PM, and HC than the older diesel buses that they replace. According to EPA's MOVES emissions model a 2012 model year diesel bus emits 94% less NOx per mile, 98% less PM, and 89% less HC than a model year 2000 (12-year old) diesel bus. A model year 2012 CNG bus emits 80% less NOx, 99% less PM, and 100% less HC than a model year 2000 diesel bus."

"Replacing 10 older diesel buses with new diesel buses will reduce annual NOx, PM, and HC emissions by 4,953 kg, 275 kg, and 421 kg respectively. Replacing 10 older diesel buses with new CNG buses will reduce annual NOx, PM, and HC emissions 4,197 kg, 279 kg, and 471 kg respectively."

Sep 25, 2013
I wonder who funded this study. Why is it limited to the environmental impacts to the local society? What about impacts on the people living in the gas fields where the fuel is extracted? What about the folks in Sublette County, Wyoming, or Weld County, Colorado, or the Uintah Basin in Utah? They are experiencing smog at Los Angeles levels, thanks to gas drilling operations . Large volumes of diesel exhaust from thousands of truck trips per well pad, compressor station engines, drilling engines, dehydration units, combine with fugitive emissions from wells, pipelines and compressors along with gasses that have been vented or flared into the air.

And how long will the low prices last when the various export terminals currently under construction begin to send natural gas out of the country? How long will the low prices last when electric generating plants, home heating, and other bus fleets switch to natural gas -as the gas industry proposes?

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