Microchemical phase characterization of petroleum coke gasification slags
The inorganic chemistries of coal and petroleum differ because of their disparate geologic environments of formation, the physical state of the fuels, and the type and quantity of minerals and organic compounds in the fuels. Commercial coals typically contain 2 to 25% ash (average ~ 10%) while petroleums contain 0.003 to 0.07% ash (average ~ 0.01 %).
Globally averaged, coal ash is dominated by Si, Fe, Ca, AI, and S, whereas petroleum ash contains significant quantities of V, Ni, S, Fe, Ca, Na, K, Mg, Si and AI. This larger number of important elements causes petroleum combustion slags to have more complex phase assemblages. The high vanadium contents of petroleum-based combustion feedstocks yield numerous crystalline V -oxides with stoichiometric amounts of Ca, Fe, Mg, AI, Ni andlor Na in the resulting slags. Slightly lower nickel contents yield abundant NiFe and Ni sulfides. The dominance of metals over silicon results in the formation of crystalline silicates following metal saturation of immiscible Si-rich glasses. High gasification temperatures (1200 - 1500°C) promote the development of equilibrial assemblages.
Chemical variations between individual feedstock cokes coupled with nonuniform operational conditions result in three principal categories of petroleum coke slag; 1) sulfide dominant, 2) silica dominant, and 3) oxide dominant. Sulfide dominant slags are not necessarily derived from feedstocks with high sulfur contents, instead they appear to derive from feedstocks rich in chalcophile elements, predominantly Fe and Ni, by attracting sulfur otherwise lost by volatilization. Slagging additives can change the chemical categorization of resulting slags through phase modifications and the formation of new phases; this in tum can strongly alter the physical behavior of the slags.
Compositionally diverse spinel oxides are the most common crystalline slag phase because of their wide thermal and compositional stability ranges, refractory nature, and rapid growth kinetics. Spinel compositions are strongly influenced by the inorganic chemistry of the feedstock, the composition of host phases, and the composition of additives. Coke slag spinels are generally enriched in AI, Fe, V, Mg, and Ni, and often contain Cr that is derived from reaction with refractory material.