THE COAL/AIR MIXTURE INSIDE A DIRECT FIRED COAL MILLING SYSTEM IGNITED EXPLOSIVELY. DAMAGE: 1) COAL MILL FAN TO BURNER DUCT RUPTURED ALONG A WELD SEAM APPROX. 24"" CIRCUMFERENTIALLY. 2) A BELLOWS EXPANSION JOINT AT THE BURNER WAS DISTENDED. 3) COAL MILL FAN IMPELLAR SHOWED CRACKS AT THE RADIAL BLADE ROOT WELD.