Assessment of Online Corrosion Measurements in Combination with Fuel Analyses and Aerosol and Deposit Measurements in a Biomass Combined Heat and Power Plant

Abstract

To systematically investigate high-temperature corrosion of superheaters in biomass combined heat and power (CHP) plants, a long-term test run (5 months) with online corrosion probes was performed in an Austrian CHP plant (28 MWNCV; steam parameters: 32 t/h at 480 °C and 63 bar) firing chemically untreated wood chips. Two corrosion probes were applied in parallel in the radiative section of the boiler at average flue gas temperatures of 880 and 780 °C using the steel 13CrMo4-5 for the measurements. Corrosion rates were determined for surface temperatures between 400 and 560 °C. The results show generally moderate corrosion rates and a clear dependence upon the flue gas temperatures and the surface temperatures of the corrosion probes, but no influence of the flue gas velocity has been observed. The data are to be used to create corrosion diagrams to determine maximum steam temperatures for superheaters in future plants, which are justifiable regarding the corrosion rate. Dedicated measurements were performed at the plant during the long-term corrosion probe test run to gain insight into the chemical environment of the corrosion probes. From fuel analyses, the molar 2S/Cl ratio was calculated with an average of 6.0, which indicates a low risk for high-temperature corrosion. Chemical analyses of aerosols sampled at the positions of the corrosion probes showed that no chlorine is present in condensed form at the positions investigated. Deposit probe measurements performed at the same positions and analyses of the deposits also showed only small amounts of chlorine in the deposits, mainly found at the leeward position of the probes. Subsequent to the test run, the corrosion probes have been investigated by means of scanning electron microscopy/energy-dispersive X-ray spectroscopy analyses. The results confirmed the deposit probe measurements and showed only minor Cl concentrations in the deposits and no Cl at the corrosion front. Because, in the case of Cl-catalyzed active oxidation, a layer of Cl is known to be found at the corrosion front, this mechanism is assumed to be not of relevance in the case at hand. Instead, elevated S concentrations were detected at the corrosion front, but the corrosion mechanism has not yet been clarified.

Application of an empirical model in CFD simulations to predict the local high temperature corrosion potential in biomass fired boilers

“…4 and 5). This result is in good agreement with long-term corrosion probe measurements performed at a CHP plant fired with forest wood chips [12]. Furthermore, a weaker, linear dependence could be found on the flue gas velocity (see Fig.…”

“…The model is validated for the parameter range investigated within the test runs. Although the model developed is not able to predict actual corrosion rates due to the fact that the measurement time of the test run has been kept rather short (~300 h), the general dependence on the parameters is comparable to those found during long-term measurements in a real-scale, forest wood chips fired biomass steam boiler [12].…”

“…A detailed description of the probe can be found in Refs. [11] and [12]. The corrosion probe used has already been successfully applied in waste incinerators [11,13] as well as biomass fired boilers [12].…”

Porosity-Based Corrosion Model for Alkali Halide Ash Deposits during Biomass Co-firing

“…The maximum temperature reached in the fluidised bed plants, relevant to this work is 920 C, thus the amount of CaSO 4 in the fly ash remains high. 23 , it is considered important to investigate its impact on the corrosion process. It is noted that bromine is present in the synthetic salt and is expected to contribute qualitatively to the corrosion process in a manner similar to chlorine.…”

“…The thermodynamic phase stability diagram for the Fe-OCl system at 540 C can be calculated using chemical reaction and equilibrium software and is shown in Previous work has included the development of custom experimental rigs 20 which attempt to replicate fluidized bed combustors as well tests in which samples are exposed to gas flows containing HCl or similar gases 21,22 . Work has also been conducted in which corrosion probes were placed in a biomass CHP plant 23 . A review of recent research on ash-related issues in fluidized bed combustion of biomass has been carried out by Hupa 1 while a review paper focusing specifically on the chlorine-associated corrosion in biomass boilers was carried out by Nielsen et al 6 .…”