Experimental investigation on combustion and emission characteristics for heavy fuel and used motor oils blends in a free-flames burner

•Heavy fuel and used motor (UMO) oils blends were used in a free-flames burner.•Fuel blends were preheated, and air injection regulated for minimum CO emissions.•Temperatures and chemical products were measured and analyzed.•Flames radiation heat transfer for blends were higher than for pure heavy fuel oil.•Combustion efficiencies increased from 48.8% for UMO25 to 63.5% for UMO100.