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.
Char obtained from biomass pyrolysis is an eco-friendly porous carbon, which has potential use as a material for electrodes in supercapacitors. For that application, a high microporous specific surface area (SSA) is desired, as it relates to the accessible surface for an applied electrolyte. Currently, the incomplete understanding of the relation between porosity development and production parameters hinders the production of tailor-made, bio-based pyrochars for use as electrodes. Additionally, there is a problem with the low reliability in assessing textual properties for bio-based pyrochars by gas adsorption. To address the aforementioned problems, beech wood cylinders of two different lengths, with and without pre-treatment with citric acid were pyrolysed at temperatures of 300-900°C and analysed by gas adsorption. The pyrolyzed chars were characterised with adsorption with N 2 and CO 2 to assess the influence of production parameters on the textual properties. The new approach in processing the gas adsorption data used in this study demonstrated the required consistency in assessing the micro-and mesoporosity. The SSA of the chars rose monotonically in the investigated range of pyrolysis temperatures. The pre-treatment with citric acid led to an enhanced SSA, and the length of the cylinders correlated with a reduced SSA. With pyrolysis at 900°C, the micro-SSAs of samples with 10 mm increased by on average 717 ± 32 m 2 /g. The trends among the investigated parameters and the textual properties were rationalized and provide a sound basis for further studies of tailor-made bio-based pyrochars as electrode materials in supercapacitors.
Low-temperature corrosion often causes failures of cold-end parts (economizers, air pre-heaters, and fire tubes of hot water boilers) in biomass boilers firing chemically untreated wood chips and bark. The most relevant mechanisms causing low-temperature corrosion are condensation of acids and hygroscopic salts in the deposits on heat-exchanger surfaces. This article offers a short review on acid condensation and presents a detailed study on the formation of hygroscopic salts, which may absorb moisture from the flue gas to such an extent that they are dissolved and form highly concentrated salt solutions. Typical fuel compositions of wood chips and bark in combination with typical operating conditions of biomass grate furnaces have been used to estimate the compositions of ash deposits on heat-exchangers. Based on these compositions, thermodynamic equilibrium calculations have been conducted in the range of 70−400°C, which have revealed that the hygroscopic salts K 2 ZnCl 4 , KCaCl 3 , and CaCl 2 can be formed at these conditions. Finally, recommendations are given on how to minimize the risk for lowtemperature corrosion.
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