NEW DELHI (AP) — U.S. oil refineries that are unable to sell a dirty fuel waste product at home are exporting vast quantities of it to India instead.

Petroleum coke, the bottom-of-the-barrel leftover from refining Canadian tar sands crude and other heavy oils, is cheaper and burns hotter than coal. But it also contains more planet-warming carbon and far more heart- and lung-damaging sulfur — a key reason few American companies use it.

Refineries instead are sending it around the world, especially to energy-hungry India, which last year got almost a fourth of all the fuel-grade "petcoke" the U....