AN EXPLOSION OCCURRED IN THE LINE 1 PREHEAT TOWER ALTERNATE FUEL ELEVATOR UPON RESTART FROM MAINT. NO ONE WAS INJURED IN INITIAL BLAST. EXPLOSION VENTING OPERATED AS DESIGNED. ELEVATOR WAS SHU T DOWN & WATER APPLIED FROM FIXED SPRAY SYSTEM. RESIDUAL FIRE WAS NOTED IN SEVERAL BUCKES & ELEVATOR WAS RESTARTED TO CLEAR MATERIAL & EXTINGUISH AT BASE OF ELEVATOR. EE WAS TOO CLOSE TO OPEN