Control of corrosion in coal liquefaction plant fractionation columns

Severe corrosion has been encountered in fractionation columns at the solvent refined coal (SRC) pilot plants in Fort Lewis, Washington, and Wilsonville, Alabama, as well as at the H-Coal Pilot plant in Catlettsburg, Kentucky, and the Exxon Donor Solvent Pilot Plant in Baytown, Texas. At the SRC plants, corrosion rates as high as 25 mm per year (one inch per year) on carbon steel and 6.4 mm per year (250 mils per year) on type 18-8 stainless steels have been measured in the portions of the columns operating at 220 to 260 °C (428 to 500 °F). Less severe corrosion is generally found at temperatures outside this range. The severity of this corrosion is related to the chlorine content of the coal. Studies of this corrosion problem by Oak Ridge National Laboratory (ORNL) personnel included exposure of corrosion coupons in the pilot plant columns, analyses of liquids collected at the pilot plants, and performance of laboratory experiments. As a result of this work, we can specify alloys with adequate corrosion resistance for construction of fractionation columns, identify the chlorine-bearing compounds, and propose chlorine transport and corrosion mechanisms. Identification of the corrodent and its mechanisms enables us to suggest process changes to remove the corrodent and thereby to control the corrosion.