Natural gas, one of Earth's cleanest, most abundant resources, accounts for 22% of total US energy use but only 2.2% of the energy used in the US for transportation, although the technology for natural gas vehicles (NGVs) has been available since World War II. The US Department of Energy expects alternative-fueled vehicles to account for 10% of the US automobile market by 2020 when more than 286 million vehicles will be on US roads. NGVs cost more due to the need for special fuel tanks and engine modifications. Heavy duty NGVs, such as trucks and buses, typically use liquefied natural gas rather than compressed natural gas for their bigger engines. Gas-to-liquids (GTL) is the newest of the gas fuel alternatives. With GTL processes, refineries can convert some gaseous waste products into valuable fuel oil virtually free of sulfur, nitrogen, and aromatics.
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