US refineries are expanding operations to process oil from Canada’s tar sands just as efforts are building to limit the use of the more polluting fuels.

There have been at least two proposals for new refineries in the US and about two dozen expansions to process tar sands oil. Canada’s tar sands – the largest proven reserves outside Saudi Arabia – have become the focal point for the world’s leading oil companies, which are increasingly being blocked from new opportunities by nationalism in oil-rich countries.