This paper presents results from a series of experiments involving the intermittent exposure of several steels to a CO-based gas mixture containing 0.1?AHTS for periods of up to 1000 hours. The flow of the gas was maintained at a rate sufficiently high to prevent equilibration of the mixture occurring at the reaction temperature of 550C, thereby simulating conditions found in commercial coal gasification plants.
Five materials were selected for this study, i.e. three commercial alloys containing 9, 12 and 20?% Cr and two specially cast 12%Cr model alloys with additions of 1% and 2?% Si. Several specimens of each alloy were exposed at the same time; some samples were also coated with a Cl-containing ash mixture prior to exposure in order to study the influence of fly-ash deposits upon degradation. In addition, after intermittently cooling the samples to room temperature, half were desiccated dry whilst the remainder were placed in moisture-saturated air at 30°C in order to simulate conditions arising during periods of plant down-time, i. e. down-time corrosion (DTC).
Corrosion kinetics are presented in terms of weight change and metal loss measurements
indicating the significant improvement accompanying the addition of 1-2% Si to the basic 12% Cr
composition. The contributions of ash deposits and periods of moist down-time are also discussed.
High efficiency coupled with environmentally acceptable levels of emissions have established coal gasification power plants as attractive alternatives for producing electricity from coal. Of particular appeal are entrained slagging gasifiers featuring rapid gasification rates and non-hazardous liquid and solid waste products. Large heat exchangers are used to recover sensible heat from the raw syngas and thus the selection of appropriate corrosion-resistant alloys is essential in guaranteeing long-term service. However, sulfidation in these low-oxygen process atmospheres is of concern as this is a particularly aggressive form of attack which may result in unacceptably high wastage rates for metallic components.
Most research studies focusing on materials behavior in mixed oxidant environments typical of the process atmospheres generated during the gasification of coal have been carried out in gases containing high levels of H2S, (0.2 - 2.0 % H2S). The reason for this was that in present coal gasification processes, all sulfur present in the coal is released as H2S and only removed from the gas after cooling-down in heat exchangers followed by a water quench. The heat exchangers are thus in contact with highly reducing gases consisting mainly of CO and H2, with lesser quantities of C02, H20 and H2S present.
