Short-term corrosion testing in a burner rig with oxy-fuel and conventional firing

As part of the European ENCAP project, a test rig has been modified by IVD (Institute of Process - Engineering and Power Plant Technology), Stuttgart, Germany to compare oxy-fuel firing with conventional firing. A broad spectrum of alloys from low alloyed steels to nickel alloys were exposed on a cooled probe in this rig at a metal temperature of 630 C for up to 40 hours with lignite from the Lausitz region as the fuel, and for 26 hours with bituminous coal (Kleinkopje) as fuel. For lignite firing, both exposure with oxy-fuel firing with recirculation of flue gas and conventional firing has been conducted to compare the corrosion attack and deposit composition. Only oxy-fuel tests were conducted with the bituminous coal. For the lignite fuel, the deposit composition from oxy-fuel and conventional firing was similar, and consisted of calcium sulphate (anhydrite) and iron oxide (hematite). The corrosion attack for the different alloys was also similar for both types of combustion. The corrosion attack was oxidation with some sulfidation/sulphation at the oxide-metal interface. In general, the thickness of the corrosion product decreased for the higher alloyed steels. The ferritic steels formed a two layered oxide. The high nickel containing alloy Hastelloy C-2000 showed a surprisingly high corrosion rate with internal attack and an outer nickel oxide. The Kanthal APM and Nimonic 263 alloys had the best oxide with even surface coverage. For the bituminous coal test, there was very little deposit on the specimens, and the deposit was rich in aluminium, silicon, oxygen and iron with lesser amounts of calcium and sulphur. The corrosion attack for the different alloys was similar to that observed for lignite where Kanthal APM had the most protective oxide coverage. Generally the mass gain rates are similar between the different tests for the same material. The main findings from these short term tests are that alumina forming alloys and super austenitic stainless steels show the lowest corrosion rates. Nickel based alloys show a relatively high corrosion rate and ferritic steels show the highest corrosion rate. (authors)