Many proposed clean coal technologies for power generation couple a gasification process with a gas turbine combined cycle unit. In the gasifier, the coal is converted into a syngas which is then cleaned and fired before entering the turbine. A problem is that coal-derived syngases may contain alkali metal impurities that combine with the sulfur and chlorine from the coal to form salts that deposit on the turbine blades, causing corrosion. This paper describes a new model, applicable to most types of coal, for predicting the dewpoint temperatures and deposition rates of these sodium and potassium salts. When chlorine is present the main alkali species in the mainstream gas flow are the chlorides; but when chlorine is absent, the superoxides dominate. However, because the high-pressure turbine blades are film-cooled, they are at much lower temperatures than the mainstream gas flow and analysis then shows that the deposit is composed almost entirely of the sulfates in either liquid or solid form. This is true whether or not chlorine is present. Detailed calculations using the new model to predict the alkali salt deposition rates on three stages of an example utility turbine are presented. The calculations show how the dewpoint temperatures and deposition rates vary with the gas-phase chlorine and sulfur levels as well as with the concentrations of sodium and potassium. It is shown that the locations where corrosion is to be expected vary considerably with the type of coal and the levels of impurities present.

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