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 leftover from refining Canadian tar sands and other heavy crude, 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 are sending it around the world instead, especially to energy-hungry India, which last year got almost a fourth of the fuel grade "petcoke" the U.S. ships, an Associated Press...