What was found
FLT 820 SDQ-JFK, CREW DECLARED EMERGENCY DUE PASSING 14,000FT ON DESCENT SMELLED IN COCKPIT ONLY INITIALLY THE AROMA SIMILAR TO SULFUR OF A LIT MATCH. SMELL DISSIPATED FOR 5 MIN AND THEN RETURNED UNTIL BLOWER & EXTRACT FAN WAS SELECTED TO OVERRIDE PER QRH PROCEDURE INITIAL STEPS. ON POST FLIGHT NOTICED RECIRC FAN X18 TRIPPED ON BACK PANEL. T/S PER TSM 24-00-00-810-823-A, REMOVED AND REPLACED RECIRCULATION FAN LEFT SIDE (15HG) IAW AMM 21-21-51. AS PRECAUTION REMOVED AND REPLACED RECIRC FAN FILTERS LEFT AND RIGHT SIDE IAW AMM 21-21-41 OPS CHECK GOOD. C/B X18 (L RECIRC) "IN" CONTINUITY GOOD. ALSO RAN PACKS OPS CHECK GOOD. NO BURNING SMELL FOUND PER AMM 21-60-00.