High Temperature Corrosion on Biodust Firing

The high content of alkali metals and chlorine in biomass gives rise to fouling/slagging and corrosion of heat exchange components, such as superheaters, in biomass fired power plants. Increasing the lifetime of these components, and in addition, preventing unwarranted plant shutdowns due to their failure, requires understanding of the complex corrosion mechanisms, as well as development of materials that are resistant to corrosion under biomass firing conditions, thereby motivating the current work. To understand the mechanisms of corrosion attack, comprehensive analysis of corrosion products was necessary. In the present work, two complementary methodologies based on analysis of cross sections and plan views were applied to achieve comprehensive characterization of corrosion products. The suitability of these methods for both laboratory scale and full scale corrosion investigations was demonstrated by the combined use of complementary information from microscopy, energy dispersive X-ray spectroscopy and various X-ray diffraction characterization techniques. In light of the wide variation in operating conditions in biomass fired power plants, systematic and wellcontrolled, but realistic laboratory scale exposures were carried out to understand the effect of process parameters such as gas phase chemistry, time and temperature variations, on the corrosion process. By conducting corrosion exposures under: oxidizing, oxidizing-chlorinating, oxidizing-sulphidizing and oxidizing-chlorinating-sulphidizing gas mixtures, it was possible to observe the contributions of chlorination, sulphation and sulphidation on the complex corrosion mechanisms. Comparative exposure of KCl-coated and KCl-free samples under each of the gas atmospheres revealed that the corrosion attack due to the presence of a deposit overrules the effect of the gas phase chemistry. Further, using the oxidizing-chlorinating-sulphidizing gas mixture corresponding to straw-firing conditions, the evolution of corrosion with time and temperature variation was studied. Corrosion attack does not decrease with time after longer exposures under isothermal conditions and is accelerated once the material encounters a higher temperature. For the investigated austenitic stainless steels, FeCrAl alloys and Ni-based superalloy, the formation of a protective oxide layer that suppressed corrosion attack was not observed. However, it was established that among the alloying elements present in these alloys, Ni exhibits a relatively greater resistance to corrosion attack. Surface modification approaches with the aim to form resistant oxides or coatings on superheater surfaces were evaluated. This included preoxidation to obtain Al2O3 and Ti-Cr-rich oxides, as well as formation of SiOx coating, Ni electroplating and NiAl coatings on commercial alloys. For most -Al2O3 obtained by preoxidation, poor mechanical integrity of the oxide layer nullifies its otherwise excellent chemical integrity towards corrosion attack. Nonetheless, corrosion attack on most of the modified surfaces was substantially lower than attack on the virgin alloys. Thus, with further optimization, these approaches may provide alternative corrosion resistant materials for application in biomass fired power plants.