Liquefied natural gas, or LNG -- used in cooking and in power plants and industries -- leaves behind a greenhouse gas footprint about a third more than that of coal when looked at over a 20-year period, according to a new study.
Over a 100-year period, LNG's greenhouse gas footprint was found to be the same as or exceeding that of coal.
Greenhouse gas emissions, such as carbon dioxide and methane, are released into the atmosphere, while a greenhouse gas footprint indicates the impact of these emissions on the environment.
Natural gas, an odourless gas primarily made up of methane, is cooled down to a liquid state at about -106 degrees Celsius to form liquefied natural gas, or LNG, thereby reducing the gas's original volume by 600 times. Being liquid, LNG can be shipped efficiently and safely.
Although LNG is considered a cleaner, low-carbon alternative to coal, when processing and shipping are taken into account, its greenhouse gas footprint is about a third worse than that of coal, Robert Howarth, a professor of ecology and environmental biology at Cornell University, US, said.
"LNG is made from shale gas (a type of natural gas), and to make it you must supercool it to liquid form and then transport it to market in large tankers. That takes energy. While natural gas and shale gas are all bad for the climate, LNG is worse," Howarth, author of the study published in the journal Energy Science and Engineering, said.
Shale gas is extracted from shale rock formations through a process called 'fracking'.
For the study, the researchers looked at previous studies to estimate emissions from the stage of shale rock formations to that of combustion by the final consumer.
The carbon dioxide and methane released while LNG is being extracted, processed, transported and stored make up for about half of its total greenhouse gas footprint, Howarth said.
"Almost all methane emissions occur upstream when you're extracting the shale gas and liquefying it. This is all magnified just to get the liquefied natural gas to market," the author said.
The author wrote, "Overall, the greenhouse gas footprint for LNG as a fuel source is 33 per cent greater than that for coal when analysed for 20 years global warming potential."
Further, "Even considered on the time frame of 100 years (global warming potential), which severely understates the climatic damage of methane, the LNG footprint equals or exceeds that of coal," Howarth wrote.
"So, LNG will always have a bigger climate footprint than the natural gas, no matter what the assumptions of being a bridge fuel are. It still ends up substantially worse than coal," the author said.
According to a draft policy by the Ministry of Petroleum and Natural Gas, India plans to fuel a third of its heavy-duty trucks by LNG, instead of diesel over the next five to seven years, to bring down pollution, the news agency Reuters reported on September 9.
Even as a fraction of cars and buses run on compressed natural gas -- the compressed form of natural gas -- LNG would be a better option for long-haul trucks as it gives a higher range, the report quoted the government as saying.
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