Deposition and high temperature corrosion in a 10 MW straw fired boiler
Introduction
In recent years, much attention has been drawn toward the burning of biomass for power production. First of all a fairly large surplus of, e.g., wheat straw exists in certain parts of Denmark and the use of this for energy production could solve a waste problem and at the same time substitute coal. Secondly, the Danish Government has decided a 20% reduction of the carbon dioxide (CO2) emissions before year 2005 with reference to 1988. Biomass is considered CO2 neutral, which means that during growth it accumulates the same amount of CO2 by photosynthesis as is released during combustion. In 1993, the Danish power producers were enjoined to burn 1,200,000 metric tons of straw and 200,000 metric tons of wood chips every year beyond the year 2000. As a result, feasible ways of burning biomass for heat and power production have been a major question, e.g., small scale furnaces and co-combustion with coal [1]. The use of straw for heat production in small scale furnaces and on individual farms has been practiced for a number of years. Generating power from biomass, however, is a fairly new task. Biomass fired boilers have generally been experiencing great problems with slagging and fouling [2], and the steam temperature is traditionally kept below 450°C in order to limit the corrosion damage of the superheater tubes. Especially, straw fired boilers have been experiencing problems due to the inorganic metal constituents such as alkali metals and chlorine present in the straw.
A demand to raise the temperature of the superheated steam to improve the electrical efficiency and to make the plants more economically feasible has been an area of discussion especially with regards to corrosion of the superheater tubes. In order to determine the effect on corrosion from raising the steam temperature Elsam (the Jutland–Funen Electricity Consortium) performed corrosion studies on an existing 10.7 MW wheat straw fired boiler (Rudkøbing KVV). Only negligible corrosion of the existing superheater tubes, having a steam temperature of 450°C, has been observed, but corrosion probe measurements showed that severe corrosion takes place at test probes with metal temperatures above 520°C [3]. The metal test elements were found to suffer from severe selective corrosion, where chlorine attack the chromium and iron in the steel leaving a nickel enriched skeleton behind. The corrosion of superheater tubes is closely related to the composition of deposited material, and especially the presence of potassium chloride (KCl) in the deposit is expected to play a major role in the mechanism of selective chlorine corrosion.
This paper presents the results of deposit measurements carried out in connection to the above-mentioned corrosion studies. The aim of the deposit measurements was to characterize and determine the appearance, structure, and chemical composition of the deposits associated with the superheater corrosion. Two metal temperatures were used for the deposit measurements: 460°C was used to simulate the metal temperature of the existing superheater tubes, and 550°C was used for simulation of the elevated steam temperatures in potential future superheater tubes. Deposition probe exposure times of 2, 4, and 14–16 h were used to determine eventual changes in deposit composition with time. Samples of deposits collected from the corrosion probes were analyzed to give information about the chemical composition of deposits with long exposure times.
Section snippets
The Rudkøbing thermal power plant
The measurements were carried out at Rudkøbing KVV, Denmark, which is a 10.7 MW stoker fired unit producing 2.3 MW electricity and 7 MW heat. The unit is 100% fired with wheat straw, and at full load this equals about 3 metric tons of straw per hour. A schematic drawing of Rudkøbing KVV is shown in Fig. 1. The first pass consists of the boiler. The primary combustion takes place on a stationary grate followed by a vibrating grate. From this grate the ash drops to the hopper and is removed to a
Bulk analysis of fuel, fly ash and bottom ash
Samples of fly ash and bottom ash were collected and analyzed in order to determine the pathways and behavior of the inorganic constituents in the boiler. The composition of the wheat straw burned in the experimental period is shown in Table 1.
The analyses of three fly ash samples and one bottom ash sample are shown in Table 2. As mentioned above, the bottom ash drops into a water-filled slag remover and the sample of the bottom ash is found as a slurry of ash in water. A part of the water
Deposition probe measurements
The deposits formed were light grey mostly on the upstream side of the probe. The deposit on the back of the test element was a white powder and easy to brush off. The thicker deposit on the front of the test element was darker and more dense. There was no direct sign of molten phases in the deposits. The hard deposit seemed to be strongly bonded to the metal oxide layer than the metal oxide layer was bonded to the metal itself.
The deposits were quite hard to remove from the metal surface, and
Corrosion measurements
At the existing superheater tubes with a steam temperature of 450°C (approximately Tmetal=480°C) the corrosion was found to be negligible. In the steam temperature range 490–520°C, the corrosion rate increases somewhat whereas at temperatures above 520°C the corrosion rate increases significantly. At steam temperatures above 490°C selective corrosion is observed at the austenitic steels. Especially, chromium and to a lesser extent iron are removed from the alloy leaving a degraded metal phase
Summary and conclusion
Large amounts of deposits are formed in the convective pass of the boiler. The corrosion of superheater tubes is closely related to the deposits. Straw has a high content of potassium and chlorine which plays a major role in the deposition process.
The fly ash has a high content of unburned carbon (15–20 wt.%). Furthermore, the fly ash is enriched in potassium and chlorine. Approximately 35% of the total chlorine leaves the plant in a condensed form (KCl) and not as a gaseous compound in the
Acknowledgements
This work was carried out as a part of the CHEC (Combustion and Harmful Emission Control) research programme. The CHEC programme is financially supported by ELSAM, ELKRAFT, The Danish Technical Research Council, the Danish Energy Research Program, and the Nordic Energy Research Programme. Henning Sørensen, Geological Survey of Denmark and Greenland, is greatly acknowledged for performing the SEM analyses.
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