By Richard Nemec, Daily Gas Price Index.

For climate change and alternative transportation fuel advocates, renewable natural gas (RNG) may eventually be the fuel of choice, but right now even its advocates note there are challenges to making a reality out of vehicles fueled by a combination of landfill, digester and waste treatment gas supplies.

"There are substantial challenges for RNG," said Harrison Clay, president of Clean Energy Fuels Corp.'s Clean Energy Renewables unit, on a webinar last Tuesday called "Renewable Natural Gas Gets Real." The challenges are particularly telling on the production side, where the cost of production exceeds the price of conventionally produced natural gas in the national pipeline grid, Clay said.