Corrosion Behavior of Candidate Materials Used for Urea Hydrolysis Equipment in Coal-Fired Selective Catalytic Reduction
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JMEPEG DOI: 10.1007/s11665-017-3001-3
Corrosion Behavior of Candidate Materials Used for Urea Hydrolysis Equipment in Coal-Fired Selective Catalytic Reduction Units Jintao Lu, Zhen Yang, Bo Zhang, Jinyang Huang, and Hongjie Xu (Submitted February 15, 2017; in revised form June 23, 2017) Corrosion tests were performed in the laboratory in order to assess the corrosion resistance of candidate materials used in urea hydrolysis equipment. The materials to be evaluated were exposed at 145 °C for 1000 h. Alloys 316L, 316L Mod., HR3C, Inconel 718, and TC4 were evaluated. Additionally, aluminide and chromate coatings applied to a 316L substrate were examined. After exposure, the mass changes in the test samples were measured by a discontinuous weighing method, and the morphologies, compositions, and phases of the corrosion products were analyzed using scanning electron microscopy, energy-dispersive spectroscopy, and x-ray diffraction. Results indicated that continuous pitting and dissolution corrosion were the main failure modes for 316L stainless steel. 316L Mod. and HR3C alloy showed better corrosion resistance than 316L due to their relatively high Cr contents, but HR3C exhibited a strong tendency toward intergranular corrosion. Inconel 718, TC4, and aluminide and chromate coating samples showed similar corrosion processes: only depositions formed by hydrothermal reactions were observed. Based on these results, a possible corrosion process in the urea hydrolysis environment was discussed for these candidate materials and questions to be clarified were proposed. Keywords
alloy, coating, corrosion, selective catalytic reduction, urea hydrolysis
1. Introduction Selective catalytic reduction (SCR) has been strongly promoted by the power industry for the efficient removal of NOx from the exhaust gases of industrial boilers (Ref 1). The key process of SCR, however, is a reduction reaction in which ammonia gas reacts as a reductant reacts with the NOxcontaining flue gas in the presence of a catalyst, which leads to the formation of nitrogen and water, thereby achieving denitration. Because the reducing ammonia gas is an indispensable raw material, potential sources of this reductant have received attention. Although both ammonia solution and liquid ammonia could be used as reductants, ammonia gas generated by hydrolysis of urea is more appropriate to satisfy the demands of a coal-fired boiler: urea is much safer to transport and the urea hydrolysis process goes to completion so no by-products are generated (Ref 2). Furthermore, only low-quality steam is needed to meet the heat demand of the hydrolysis reaction (Ref 3). Urea hydrolysis as a source of reductant for SCR has therefore been used extensively; however, corrosion damage of the key components in a urea hydrolysis unit is a limiting factor in service (Ref 4) and an important factor influencing the equipment cost. Jintao Lu, Zhen Yang, Bo Zhang, Jinyang Huang, and Hongjie Xu, National Energy R&D Center of Clean and High-Efficiency FossilFired Power Generation Technology, Xi
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