New chemical mechanism explaining the breakdown of protective oxides on high temperature steels in biomass combustion and gasification plants

Abstract

Biomass is considered a replacement fuel over fossil fuels to mitigate climate change. The switch to biomass in the combustors changes the inorganic chemistry of the flue gases and leads to more severe corrosion of the construction materials of the combustors. The integrity of most high temperature steels relies on the formation of a protective Cr 2 O 3 layer on the steel surface at a high temperature environment. The ash compound found on the heavily corroded steel in biomass combustion and gasification plants is KCl, but the mechanism, which triggers the breakdown of the protective Cr 2 O 3 layer under the KCl salt is not known. We studied the chemical reactions involved with furnace exposure of KCl and KOH with Cr 2 O 3 and identified the formed reaction products with XRD analysis. The amount of reaction products was analyzed from the leachates of the salt-oxide mixtures by UV/VIS spectroscopy. We also used thermodynamic Gibbs energy minimization calculations to evaluate the evolution of reactions as a function of temperature. The results suggests that the reaction of KCl with Cr 2 O 3 involves a KOH reaction intermediate that forms before K 2 CrO 4 is formed. The amount of reacted potassium as a function of temperature follows the trend of KCl decomposition to KOH and HCl(g) as predicted by thermodynamics calculations. Therefore, we argue that the suggested overall reaction of KCl with Cr 2 O 3 as found in the corrosion literature: 4KCl þ Cr 2 O 3 þ 1 1 2 O 2 ðgÞ þ 2H 2 OðgÞ 0 2K 2 CrO 4 þ 4HClðgÞ, starting with the initiation step: KCl + H 2 O(g) 0 KOH + HCl(g) and then the formed KOH reacts with Cr 2 O 3 to form K 2 CrO 4 . This explains the initial breakdown of the protective Cr 2 O 3 under KCl salt in water containing high temperature atmospheres. The result is essential for the development of new alloys for biomass fired combustors.Fig. 7 Comparison of the experimentally determined amount of [Fe 3+ ] in the filtrates with the thermodynamically predicted amount of the reaction products in the KOH-Fe 2 O 3 system with pure water leaching and leaching in 1 M HCl (aq) solution.This journal is

Interaction of Iron Oxygen Carriers and Alkaline Salts Present in Biomass-Derived Ash

“…During reduction, the K 2 FeO 4 phase was observed like the KNO 3 system as steam was present during reduction, which may react with KOH. The formation of K- and Fe-based new compounds over the interaction of Fe oxides and KOH (Reactions and ) was also observed by a literature study …”

“…Even though no agglomeration formed during reduction, Fe 2 O 3 reduced to FeO. It is known that KCl exists in biomass in different forms depending on other existing elements. , KCl may create volatile species (Reaction ) depending on the system conditions such as the heating rate, other available species, or temperature . Especially, the presence of steam may give rise to highly reactive gaseous species, which may result in the complete reduction of the oxygen carriers. , …”

“…The formation of K-and Fe-based new compounds over the interaction of Fe oxides and KOH (Reactions 6 and 7) was also observed by a literature study. 46…”

Torrefaction as a Pretreatment Technology for Chlorine Elimination from Biomass: A Case Study Using Eucalyptus globulus Labill

“…Chlorine is usually present in biomass in low concentrations [84,85]. However, during combustion, hydrochloric acid is formed, which can lead to corrosion in the furnace [86][87][88]. Therefore, it is crucial to determine the torrefaction temperature that will eliminate that component.…”