A B757-200 CREW EXPERIENCED A STRONG SMELL; FOLLOWED BY HIGH ENG EGT; AND RPM ROLLBACK. EMER DECLARED AND CREW RETURN LAND.

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FLT XXXX TOOK OFF FROM ZZZ AT APPROX XA30L AND BEGAN A CLB WITH WX DEVS DUE TO MULTIPLE TSTM CELLS THROUGHOUT THE S OF THE STATE. CLB THROUGH FL180 WAS UNEVENTFUL EXCEPT FOR CONTINUOUS LIGHT TO OCCASIONAL MODERATE CHOP (FLT ATTENDANTS WERE TOLD TO REMAIN SEATED THROUGHOUT). ENG ANTI-ICE WAS TURNED ON AT APPROX FL180. FLT XXXX WAS HDG ABOUT 340 DEGS IN THE CLB. FLT XXXX LEVELED AT FL230 PER ATC INSTRUCTIONS; AND THEN RECEIVED A CLB CLRNC TO FL320. AT THE TIME CLRNC WAS RECEIVED; FO NOTICED AN ACRID SMELL AND THE L ENG VIBRATION INDICATOR IN THE AMBER/CAUTION RANGE. HE CALLED ON INTERPHONE TO FLT ATTENDANTS TO CONFIRM THEY SMELLED THE ACRID ODOR AS WELL; AND THEY DID. FO THEN SUGGESTED AND CAPT APPROVED CLOSING THE L ENG BLEED AND BOTH RECONFIRMED THAT THE ENG ANTI-ICE WAS TURNED ON. INCREASING HIGH FREQ RUMBLE WAS NOTED; ENG N1 BEGAN TO ROLL BACK; AND EGT BEGAN TO CLB TO 900 DEGS C. DURING DEBRIEF; CABIN CREW MENTIONED THAT THERE WERE LOUD BANGS HEARD DURING THIS TIME. CAPT THEN DECLARED AN EMER WITH ARTCC AND REQUESTED A L TURN TO HDG 180 DEGS AND DSCNT TO RETURN TO ZZZ. ENG PWR WAS REDUCED AND THE CAPT PERFORMED THE ENG FAILURE/FIRE/SEVERE DAMAGE QRH PROC AS THE FO DSNDED THE ACFT IN HVY PRECIP AND MODERATE TURB. THE WX CONDITIONS PERSISTED FOR ABOUT THE FIRST 40 MI OF THE 80+ MI RETURN TO ZZZ. CAPT THEN PERFORMED THE SINGLE ENG LNDG AND NORMAL DSCNT CHKLIST; WHILE ALSO ADVISING FLT ATTENDANTS THAT A NORMAL LNDG WITHOUT GND EVAC WAS PLANNED. CAPT ALSO CALLED OPS TO COORD FOR A TUG TO TOW ACFT IN FROM THE RWY AFTER LNDG. CAPT MADE 2 PA'S DURING THE DSCNT TO REASSURE THE PAX THAT THE SIT WAS WELL IN HAND. FO PERFORMED AN ILS TO RWY 26L APCH IN VMC AND STRONG GUSTY XWINDS WITH TOUCHDOWN AT APPROX XB08L. OVERWT LNDG WAS FIRM BUT NOT HARD AT 229000 LBS GROSS WT. FLT XXXX CLRED THE RWY; CONFIRMED WITH CFR THAT THERE WAS NO SIGN OF FIRE OR SMOKE FROM THE DAMAGED L ENG; THEN SHUT DOWN THE R ENG AND AWAITED A TOW-IN TO THE GATE.