In coal gasifiers, refractories and metals must withstand the coal/air or coal/oxygen combusting atmospheres. The Zr-Nb-Ti system is under consideration as alternative to the Fe and Ni alloy systems because the coal combustion degrades these alloy systems for the oxygen potentials ranging from 10-20 to 10-10 atm and sulphur potentials ranging from 10-8 to 10-3 atm, as well as the carbon. To provide a detailed understanding of the possible reactions on the metal surface or even the refractory, suggest possible reactions for the combustion chamber or the adjacent heat recovery chamber, which may use the alloy system. In your discussion, you should include the following supportive material:
A. The Ti-C-O stability diagram (i.e., log pCO – log pO2) calculated at 1800°C.
B. The Nb-C-O stability diagram (i.e., log pCO – log pO2) calculated at 1800°C.
C. An engineer also believes that the Nb-O system will result in significant vaporization and suggested the use of a Zr-Ta-Ti system instead of a Zr-Nb-Ti system. Calculate the Kellogg diagrams for the Nb-O and Ta-O systems at 1800°C to determine whether vaporization is a major concern and under what conditions.
D. Determine the stabilities of the Nb, Ta, Ti and Zr oxides via an Ellingham diagram and suggest whether the carbon oxidation will reduce the oxides and form carbides.
E. Your engineering office partner considers Zr, Ti, Nb and Ta too expensive so he wants to use either Fe or Ni alloys. Discuss your reasoning for or against using either Fe or Ni alloys for the coal gasifier.