AN ATR72 ENG FIRE DEVELOPED DURING START AFTER THE PROP BRAKE APPARENTLY DID NOT RELEASE. THE EXTINGUISHING AGENT PUT OUT THE FIRE AS PAX WERE EVACUATING.

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ACFT WAS CLRED FOR PUSHBACK TAIL N. AFTER PUSH; PARKING BRAKE WAS SET. AUX PUMP WAS DEPRESSED BEFORE PUSH WAS COMPLETED TO INCREASE HYD PRESSURE. RECEIVED PROP BRAKE READY LIGHT. GAVE SIGNAL TO GND CREW TO CLR TO SPIN #2 PROP. AFTER RECEIVING CLRNC TO SPIN PROP; REQUESTED FO TO CLR R. AT THIS TIME; PROP BRAKE SWITCH WAS SET TO UNLOCK. PROP STABILIZED AT 13% NP. AFTER WHICH; R CONDITION LEVER WAS ADVANCED TO MAX RPM. PROP WAS SLOW TO SPIN UP. REQUESTED GND CREW TO STAND BY. APPROX 20 SECONDS LATER; RECEIVED PROP BRAKE WARNING. BROUGHT CONDITION LEVER TO FEATHER AND THEN TO SHUT OFF. DURING THIS TIME; THE CAPT AND FO SMELLED SMOKE. CONTACTED FLT ATTENDANT TO INVESTIGATE POSSIBLE SMOKE FROM #2 ENG; AND THEY RPTED SMELLING SMOKE. APPROX 2 SECONDS AFTER ENG SHUTDOWN; RECEIVED #2 ENG FIRE WARNING. COMMENCED IMMEDIATE ACTION ITEMS AND REFED QRH. FIRE WAS EXTINGUISHED AFTER AGENT #1 WAS DEPRESSED. WHEN READY TO MAKE PA; NOTICED THAT PAX WERE IN THE PROCESS OF EVACING IN AN ORDERLY MANNER. REQUESTED FO TO ENSURE THAT ALL PAX HAVE DEPLANED AND TAKE FIRE EXTINGUISHER. CONTACTED RAMP; OPS; AND FLT CTL OF SITUATION. FO RETURNED TO FLT DECK TO ASSIST IN SECURING ACFT. NO INJURIES WERE RPTED. I BELIEVE THAT THE PROP BRAKE SOMEHOW FAILED TO FULLY DISENGAGE. THIS CAUSED THE ALLEGED SPARKS FROM THE ENG FOLLOWED WITH ENG FIRE. A SLOW TO RISE NP IS NOT NECESSARILY UNIQUE; SO A CHANGE IN RESPONSE FROM THE FLT CREW IS NOT WARRANTED; IN MY OPINION. THE POTENTIAL PROB WITH THE PROP BRAKE MAY HAVE BEEN CAUSED BY ONE OF SEVERAL WAYS. ONE POSSIBILITY IS A MALFUNCTION WITH THE MFC'S (MULTI-FUNCTION COMPUTER). ANOTHER IS WITHIN THE PROP BRAKE.