CALCINER PRODUCT ACCUMULATED IN DUCT LEADING TO BAGHOUSE & PLUGGED IT. A 20' LONG AIR LANCE HAD BEEN FABRICATED TO BLOW BLOCKAGE FREE. HE PLANNED TO INSERT AIR LANCE INTO DUCT & SIGNAL FOR AIR TO BE TURNED ON. OTHER EE THOUGHT HE SIGNALED FOR HIM TO TURN VALVE BUT BEFORE HE COULD MOVE, WAS SPRAYED WITH HOT GAS & PRODUCT FROM CONVEYING DUCT. BURNED ON BOTH FOREARMS & LEFT FOOT.