Lab-scale Investigation of Deposit-induced Chlorine Corrosion of Superheater Materials under Simulated Biomass-firing Conditions. Part 1: Exposure at 560 °C†

Abstract

Deposit-induced chlorine corrosion was studied under well-controlled laboratory conditions, simulating the conditions in straw-fired boilers and boilers cofiring coal and straw. This was done by exposing pieces of superheater tube (TP 347H FG) covered with synthetic deposits of known Cl content to gas mixtures simulating straw-firing and cofiring of coal and straw, at 560 °C (1040 °F), for 3 days. The corroded specimens, and the reacted deposits, were studied in detail using a scanning electron microscope to determine the corrosion rate, investigate the chemistry and morphology of the corrosion attack, and study the sulfation behavior. Besides the gas compositions, various parameters were studied systematically. Most specimens suffered some internal attack, mostly by selective corrosion and in some cases by grain boundary attack. In all experiments with KCl and KCl−SiO2 deposits, the corrosion products consisted of an oxide scale, containing oxides of Cr and Fe, and on top of that a characteristic mixed layer of iron oxide threads in a potassium sulfate matrix. However, the thickness and shape of this layer was found to be strongly dependent on the experimental conditions. An increase of the percentage of KCl in the deposit resulted in a more uniform and deeper internal corrosion attack. The presence of HCl in the flue gas did not seem to be essential for chlorine-induced corrosion to occur, when a deposit containing KCl was present, but it enhanced the corrosion rate. The degree of sulfation of KCl in the deposits after exposure was quantified by wet chemical analysis and was shown to be dependent on many parameters, including the SO2 concentration in the gas flow, the concentration of KCl in the deposit, and the SiO2 and KCl particle sizes in the deposit. No simple relation was observed between the degree of sulfation in the deposit and the depth of internal attack or the thickness of the oxide or mixed layer. Whereas SiO2 particles were found to be chemically inert with respect to the flue gas and the corrosion attack, CaO particles reacted with HCl from the flue gas, and the resulting CaCl2 played an important role in the corrosion mechanism. As a result, the corrosion rate was strongly enhanced when CaO was present in the deposit, instead of SiO2.