Recuperators which can transfer waste heat from industrial furnaces to the combustion air can result in fuel conservation. Available alloys cannot currently satisfy the demand for higher thermodynamic efficiencies from higher temperature recuperated combustion air because of their properties at high temperatures. Therefore, selected ceramic materials are principal candidates for use in waste-heat recovery devices for flue gas temperatures of 1100 to 1650 exp 0 C. A leading configuration now under investigation is the cross-flow fixed-boundary extended-surface ceramic honeycomb heat exchanger. The need for a material that has both low thermal expansion and high thermal shock resistance and is also fabricable and relatively inexpensive has led to the development of cordierite (magnesium aluminosilicate - MAS). A cordierite honeycomb recuperator was exposed for about 2200 h to representative process temperatures from 1100 to 1550 exp 0 C in a molybdenum and tungsten reheating furnace. This recuperator performed satisfactorily for test and exhaust inlet temperatures of 1100 to 1400 exp 0 C. Electron microprobe and ceramographic examination demonstrated that molybdenum and tungsten carryover in the waste gas stream significantly penetrated the cordierite at temperatures as low as 675 exp 0 C. The leading edges of the exhaust inlet section showed significant microstructural modification. A subsequent accidental overfiring to 1560 exp 0 C resulted in slumping of this modified material. Exhaust-to-air channel leakage caused deposits in air channels and modified the microstructure in bonding lines between layers in the honeycomb. Cracking also occurred in the air outlet section. Significant microstructural modification, including coarsening within the walls and glass formation on the air-channel wall surfaces, was observed. (ERA citation 04:041171)
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