A B727-200 IN CLB AT FL240 DECLARED AN EMER AND DIVERTED DUE TO LOWER BODY OVERHEAT WARNING AND BURNING FIBERGLAS ODOR CAUSED BY A #2 ENG BLEED DUCT FAILURE.

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ON FIRST FLT OF DAY; ACFT EXPERIENCED A 'LOWER AFT BODY OVERHEAT.' THE (DIFFERENT) CAPT RETARDED #2 THROTTLE AND WENT THROUGH ABNORMAL CHKLIST. ON LNDG AT DFW; PROB WAS MISDIAGNOSED AS A FAULTY #1 BLEED VALVE. THAT ITEM WAS MEL'ED. AFTER TKOF; FROM DFW-DEN; WE GOT ANOTHER 'LOWER AFT BODY HEAT.' WE WENT THROUGH THE ABNORMAL PROCS; WHICH CALL FOR RETARDING EACH THROTTLE IN TURN; REDUCING OUR RATE OF CLB TO APPROX 400-500 FPM. OUR PROCS LED US TO THE CONCLUSION THAT THE #2 BLEED SYS WAS FAULTY AND WE CONTINUED CLBOUT WITH #2 THROTTLE RETARDED. WE CONTACTED COMPANY MAINT; WHO REQUESTED WE RETURN TO DFW. WE ATTEMPTED SEVERAL COMMERCIAL RADIO CONTACTS WITH OUR DISPATCH IN IND WITHOUT SUCCESS. CLBING THROUGH APPROX 15000-16000 FT; OUR CABIN HAD REACHED 8000 FT WITH NEARLY 1000 FPM CLB. THE FE REQUESTED PERMISSION TO OPEN THE MEL'ED #1 BLEED; IN ORDER TO STABILIZE THE CABIN PRESSURE BY OPERATING 2 PACKS. OPERATING INOP'ED ITEMS IS NORMALLY AGAINST OUR COMPANY POLICY; BUT BECAUSE WE NOW KNEW THE MAINT SIGNOFF TO BE FAULTY; WE ELECTED TO OPEN THE BLEED. THIS ALSO PREVENTED US FROM DEVELOPING A COMPOUND PROB: RETARDED PWR ON #2 AND CABIN AT 10000 FT PRESSURE. DURING THIS PERIOD; I HAD BEEN GRADUALLY INCREASING PWR ON #2; TO SEE IF THE OVERHEAT LIGHT WOULD RECUR. AT APPROX 80-83% N1 ON #2; WE BEGAN TO SMELL A BURNING 'HOT FIBERGLAS' SMELL IN COCKPIT. I THEN RETARDED #2 TO IDLE; DECLARED AN EMER; AND REQUESTED A RETURN TO DFW. COMPANY MAINT FOUND THE #2 ENG'S 6TH STAGE BLEED DUCT HAD DISINTEGRATED; CAUSING AN APPROX 12 INCH BY 19 INCH HOLE IN THE ENG'S FIREWALL.