Hot directed metal oxidation

As previously discussed in more detail, there are two general types of hot gas desulfurization systems direct injection of limestone with the gas into the gasifier, and metal oxides. The limestone systems suffer from lower sulfur removal/recovery efficiency and the generation of large amounts of reactive solid waste material. The metal oxide systems are more complex, and generate a relatively dilute SO2 stream that must be converted to sulfur or sulftiric acid. Two other potential problems with both types of hot desulfurization systems are NO, and trace components. 

To illustrate the thermodynamic complexities that arise because of the presence of a molten metal in the directed metal oxidation process, a detailed analysis is presented both with and without the Si metal present. The former analysis is applicable to more traditional ceramic processing such as sintering or hot-pressing of SiC/Si3N4 composites, whereas the latter is applicable to the directed metal oxidation process, or any other composite process where molten Si may be present. 

Chemical.—All forms of C combine with O at high temperatures, with light and heat. The product of the union is carbon dioxid if the supply of air or O be sufficient but it O be present in limited quantity, carbon monoxid is formed. The alflnity of C for O renders it a valuable reducing agent. Many metallic oxids are reduced, when heated with C, and steam is decomposed when passed over red-hot C HaO- -C=CO-i-H2. At elevated temperatures C also combines directly with S, to form carbon disulfld. With H, carbon also combines directly, under the influence of the voltaic arc. 

Other preparation methods have recently been developed. Sulfated metal oxides have been prepared by a sol-gel method [42,57,58], which involves the formation of a zirconium-sulfate co-gel by adding sulfuric acid to zirconium n-propoxide in isopropyl alcohol. This one step method appears to be simpler than the two step preparation procedures and allows a better control of the variables. It also allows the direct formation of biiunctional catalysts by the addition of chloroplatinic acid to the gel mixture. A new preparation method, named rapid thermal decomposition of precursors in solution (RTDS), which involves the use of hot pressurized water at hydrotheimal conditions to force metal ion precursors to go into phases of oxyhydroxides and oxyhydrosulfates, has been used to produce sulfated zirconia with crystallite sizes below 100 A [59]. 

The use of tubes to increase the residence time of atoms in the analytical zone, and hence improve detection limits have more recently been reported for a variety of applications. Such tubes are often fabricated from silica and employ slots, one directly above the burner slot and the other usually at 180°, to decrease the turbulence of the hot gases. The improvement in sensitivity associated with these tubes is generally confined to those elements readily dissociated to their ground state atoms in the flame. Elements with relatively high metal-oxide dissociation energies such as some of the transition metals that 

---