Lab-Scale Studies on the Activation Energy Regarding the Surface Alteration of Super-Heater Materials in Contact with KCl at Elevated Temperatures

Biomass has gained popularity as an alternative fuel due to its CO₂ neutrality and regulatory benefits. Finland is among the leading countries in the use of biomass in energy production: the share of bioenergy is 28% of all primary energy consumption in 2012. In Finland, biomass is widely used as a fuel in electricity production, CHP plants and district heating. During biomass combustion process, among other harmful species, certain alkali salts (e.g. KCl) are released or formed in the gas phase and may form corrosive deposits on the heat exchangers. Interactions between the deposits and the heat exchangers may reduce the efficiency of the super-heater and subsequently decrease their life span. Although high-temperature corrosion has been extensively studied, details of the initial process have not been addressed comprehensively.

The aim of this work is to study the activation energy of chromium oxide alteration on the surface of genuine heat exchanger materials (high-alloy stainless steel Sanicro 28 and low-alloy ferritic steel 10CrMo) in the presence of potassium chloride. The activation energy is defined as the sum of electronic and thermal free energies, and can be calculated from data obtained using chronoamperometric techniques, varying temperature and set potential. These results have been combined with surface specific information obtained by using X-ray photoelectron spectroscopy (XPS) and Scanning Electron Microscopy with Energy Dispersive X-Ray (SEM-EDX). With these techniques, the reaction current under various conditions (E_app, T, t) has been measured and the depth profile together with the chemical composition of the formed oxides has been determined.

At the very early stage of the oxidation reaction (t≈1 min) substantial amounts of both Cr⁶⁺ and Cr³⁺ could be detected by both XPS and CA (as a continuing oxidation current). An example of a set of CA-measurements is presented in Figure 1. Therefore, it seems apparent that oxidation of trivalent chromium to hexavalent chromium takes place despite temperatures well below the melting point of the used salt. For this solid-solid interaction, assuming a linearly changing reaction quotient (Q) under each condition, the activation energy can be calculated. However, due to overlapping reactions, the calculated activation energy reflects the sum reaction rather than one individual reaction, albeit the reaction with the highest activation energy will dictate the calculated value. Based on these results, it is apparent that the changes in the Cr⁶⁺/Cr³⁺-ratio and oxide layer thickness can be determined and activation energy for the sum reaction can be calculated in selected points at the very early stage of the reaction. This information can potentially increase the understanding of the overall reaction mechanism of KCl-induced corrosion at elevated temperatures.

Figure 1. Corrosion currents as functions of time applying 200mV, 400mV and 600mV potential differences at a) 450°C  b) 500°C c) 550°C.  At t = 1 minute the salt piece was contacted with the sample.