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FUELS

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NATURAL GAS

Natural gas is a fossil fuel composed of methane, ethane and other hydrocarbons drawn from gas or crude oil wells. It is stored in compressed (CNG) or liquefied (LNG) form in high pressure cylinders and more commonly fed into spark-ignited engines. However, newer heavy-duty natural gas-powered engines without spark ignition have become available. Currently one out of every five alternatively fueled transit buses is powered by natural gas. In 2001, CNG buses accounted for one-fifth of all new bus orders[1].

Table 1 Natural Gas Emissions Compared to Conventional Diesel[2]

Natural gas engines produce substantially fewer emissions of particulate matter, carbon monoxide and nitrogen oxides than conventional diesel engines. Many natural gas engines already meet or exceed the 2004 EPA heavy-duty engine emission standards. Research is currently underway to develop emissions control devices for natural gas engines to meet engine certification standards for 2007.

In the year 2000, domestic production accounted for 84 percent of total natural gas consumption. Canadian imports account for the majority of the remaining 16% of natural gas consumed in the US.

Costs of bus fleet conversions to natural gas technologies are concentrated in the initial capital investment. The purchase cost of a CNG bus can range from $25,000 to $50,000 more than a diesel bus. This price differential is expected to grow narrower as production volumes of CNG buses increase. Also, investment in natural gas fueling infrastructure usually requires construction of a new fueling facility. These fueling facilities can be an investment in future natural gas vehicle fleets that will demand public refueling stations. The Federal Transit Administration has estimated the cost of a fueling station for a fleet of 200 buses at approximately $1.7 million. Finally, National Fire Protection Association (NFPA) codes may require modification of bus storage depots to incorporate gaseous fuel detection systems. Despite these added costs, operational and maintenance expenses tend to be lower than those for diesel-powered buses, including newer ”clean diesel” models fitted with after-treatment devices. Natural gas-powered buses save money because they do not require frequent oil changes and regular cleaning and eventual replacement of new filters. Since natural gas engines are inherently cleaner burning and experience less engine wear, normal maintenance costs tend to remain low, but these costs have varied among transit agencies.[4]

In 1993, the Los Angeles Metropolitan Transit Authority committed to purchasing only alternative fuel buses. It currently operates over 1,900 dedicated CNG buses, the nation’s largest fleet. The Metropolitan Atlanta Regional Transit Authority and New York MTA Long Island Bus Co. have also committed to alternative fuel-only purchases and operate substantial numbers of CNG buses. SunLine Transit Agency in Thousand Palms, CA became the nation’s first 100 percent CNG bus fleet in 1994 while Pierce Transit Agency in Tacoma , Washington was one of the first to use a natural gas-powered heavy-duty engine.

LNG buses are less common than CNG, although market share for LNG buses is growing rapidly. The slower market penetration is due in part to the current limitations in LNG fueling supply and distribution network. One advantage is that fuel density of LNG is higher than CNG, which means that the same BTU content (and thus, vehicle range) weighs less and may be stored in less space, typically a single cryogenic liquid tank rather than the multiple gaseous fuel storage cylinders associated with CNG units. Emissions performance is about the same as CNG because they both start as natural gas although the methane content of LNG is usually slightly higher than CNG originating from pipeline gas. Transit agencies in Orange County , CA ; Dallas, TX ; and Phoenix, AZ operate some of the nation’s largest LNG bus fleets.