WITNESSES SAW THE ACFT APCH THE ACDNT SITE AT LOW ALT TRAILING A HVY VOLUME OF SPRAY OR SMOKE. THE ACFT SUDDENLY ALTERED COURSE, AND THE PLT SEEMED TO BE TURNING TO LND ON A ROAD. SOME WITNESSES SAID THEY SAW FLAMES COMING FM THE ENG COMPARTMENT AREA; OTRS SAID THE ENG 'BURST INTO FLAMES' WHILE THE ACFT WAS AIRBORNE. AS THE ACFT TURNED TWD THE ROAD, IT COLLIDED WITH PWR LINES & CRASHED. A POST-CRASH FIRE CONSUMED THE ACFT. AN EXAM OF THE ENG, WHICH SEPD FM THE FUSELAGE DRG IMPACT, REVEALED NO EVIDENCE OF A FIRE. ALL HOSES WERE NOTED TO BE INTACT. THE WOODEN HOPPER DOOR HAD SEPD FM THE FUSELAGE & IT WAS FND ALONG THE FLT PATH. IT WAS FND HEAVILY CHARRED ON THE INTERIOR SIDE WITH NO EVIDENCE OF FIRE ON THE EXTERIOR SIDE. RECORDS SHOWED THE ACFT HAD BEEN LOADED WITH POTASSIUM PERMANGANATE AFTER SULFUR HAD BEEN HAULED ON A PREVIOUS FLT. CHEMICAL MANUFACTURER RPRTD THAT POTASSIUM PERMANGANATE WAS AN OXIDIZER, WHICH WAS REACTIVE WITH EVEN SMALL AMOUNTS OF SULFUR.