SMOKE IN CABIN OF SAAB 340 DURING TAXI RESULTS IN EMERGENCY DECLARATION AND EVACUATION ON THE TXWY.

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SUPPLEMENTAL AIR CONDITIONING AIR COMPRESSOR OVERHEATED CAUSING THE BELT TO POUR SMOKE INTO THE CABIN. WHILE TAXI TO RWY AT ZZZZ ARPT; FLT ATTENDANT NOTIFIED FO SHE COULD SMELL SMOKE. FO THEN INFORMED MYSELF. I HAD FO CALL FLT ATTENDANT TO SEE HOW BAD WAS THE SMOKE. AFTER FO SPOKE TO FLT ATTENDANT; HE SAID SHE COULD NOW SEE SMOKE IN THE CABIN. I TOLD FO WE WERE GOING TO EVAC. I ALSO TOLD FO TO GO BACK AND HELP FLT ATTENDANT EVAC THE ACFT; I AM SHUTTING DOWN ENGS. I SHUT DOWN THE ENGS THEN INFORMED ATC WE WERE DECLARING AN EMER: SMOKE IN THE CABIN; EVACING PAX ON TXWY; COULD YOU SEND THE FIRE TRUCKS; ALSO CALL MY DISPATCH. I CALLED GND OPS; TOLD THEM WE WERE DEPLANING PAX ON TXWY. AT THAT TIME DISPATCH WAS UP ON OPS FREQ. I INFORMED HIM OF OUR SITUATION. ALL PAX WERE OFFLOADED AND LED AWAY FROM ACFT BY FO AND FLT ATTENDANT. THE FIREMEN BOARDED THE ACFT SEARCHING FOR A FIRE. SMOKE HAD LEFT THE CABIN BUT FLOOR WAS STILL VERY HOT. THE FIRE CHIEF WANTED TO USE AN AX AND THE JAWS OF LIFE TO OPEN FLOOR TO GET TO THE HEAT SOURCE. I TOLD THE FIREMEN THAT THERE WAS NO MORE SMOKE WOULD LIKE TO TALK TO A MECH BEFORE THEY OPENED THE FLOOR. A COUPLE OF MINS LATER; MECHS FROM ANOTHER ACR SHOWED UP; AGREEING NOT TO OPEN UP THE FLOOR. THE PAX WERE SHUTTLED BACK TO THE ARPT WITHOUT FURTHER INCIDENT.