AT APPROX. 11 AM, A CONTRACT EE & A MILLWRIGHT EE WERE IN A MAN LIFT BASKET EXTENDED INSIDE OF THE SMA HOPPER-STILLING SHED. THE CONTRACT EE WAS WORKING ON CUTTING A WEAR PLATE LOOSE IN THE HOPPER. HE WAS HOLDING A TORCH IN ONE HAND & USED A HAMMER WITH THE OTHER HAND TO KNOCK A SECTION OF THE PLATE LOOSE. THE IMPACT OF HAMMER BLOW VIBRATED. THE TORCH IGNITED THE COAL DUST.