Plastic production vs automobile fuel

A friend and cousin-in-law asked about a photo showing waste plastic bags in woodsy area. The caption indicated that a plastic bag contains enough oil to drive a car 11 meters. I wasn’t sure… This sounded a bit extreme, so I had to research.

It’s a mass thing. A gram of oil makes a gram of plastic. Any waste is used for other plastics, and the catalysts are recycled pretty efficiently in the big shops.

Assuming your car is 20MPG, this is 32187 meters per gallon. Gasoline averages 6 pounds per gallon, or 2722 grams per gallon. That’s 11 meters per gram. I checked, and my plastic shopping bags weigh SIX grams. That would be 66 meters fuel equivalent per bag.

Plastic in the US is made mostly from LPG and NLG, though some raw methane. LPG and NLG have lower energy per gallon, but the same energy per gram. (Energy is released by combining with oxygen, which is based on number of atoms of carbon and hydrogen, which is a mass thing, not a volume thing.) So, gram for gram, it would be the same. 99.45% of the input petroleum is feedstock, and the other 0.55% is fuel for heating, etc.

This seems like a lot, but compare this to paper sacks. It takes 7 times more fuel to deliver paper sacks than plastic sacks. It takes 11 times more fuel to recycle a paper sack than a plastic bag. Finally, 65% of the US reuse their plastic grocery bags, which costs no additional energy. So, each paper sack uses the energy required to push a car 462 meters.

It gets fuzzy on actual production/consumption, but the US used 191 million barrels in 2010, and 416 billion cubic feet of natural gas to product plastic. Worldwide, 280 million tonnes of plastic were produced (2011) and this grows by about 4% per year.

Also, 90% of propane consumed in the US is produced locally.
While the US produces a lot of wood and pulp locally, it’s the #1 importer in the world.

Both paper and plastic calculations here ignore the energy input to produce the feedstock (trees, million year old dead animals). Our biggest problem is not the feedstock for plastic bags (PLA is nice, and biodegradable, made from plant based lactic acid), but our fuel supplies. We desperately need a biofuel with high energy density. Ethanol is a poor choice, and is only in use now because we needed SOMETHING.

Legume oil is about the best we have now. As fuel prices go up, then it becomes more cost effective to use. Algae-Oil would completely reverse our trends. We could supply the planet, plus refill the oil wells. The problem is that it’s delicate, and crops die off too easily to be effective. Selective breeding, crossbreeding, or even direct genetic manipulation may help. Exxon is probably 10 years out from having a commercial production of algal oil. Until then, use of vegetable oil as biofuel continues to cause a huge pressure on foodcrops for impoverished nations.