Consequences of Unwanted Local Reducing Conditions in Biomass-Fired Boilers on Chemistry and Operation: A Thermodynamic Evaluation

The case studies investigated are focusing on uncontrolled local reducing conditions and their effects on alkali and trace metal chemistry in biomass-fired boilers. The chemical trends obtained by thermodynamic equilibrium were interpreted in terms of practical outcomes for thermal systems. There is a lack of knowledge concerning the fate of Na and K in reducing atmospheres. This study shows the following: (1) Corrosion is expected to be more severe at reducing than oxidizing conditions; however, the corrosive species and therewith the corrosion mechanisms are changing with the oxygen levels. The main corrosive species under pyrolytic conditions are Na2S (s), NaCl (g), and KCl (g), while the main corrosive species at mildly reducing conditions are H2S (g), NaCl (g), and KCl (g). Under oxidizing conditions, alkali chlorides are the main alkali species involved in corrosion. (2) Some chemical elements have stable chemistries; i.e., their speciation and/or phase distribution are little affected by oxygen levels, while (3) other elements are very sensible to changing oxygen availability.