coal selection criteria for industrial pfbc firing project 3.2 - CCSD

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“Coal Selection Criteria for Industrial PFBC Firing” less free SiO2. Also, a maximum of 10% ash contained in the coal is needed to obtain an excellent result. Karita requires the soot and dust concentration at the gas turbine inlet to be 840 mg/Nm 3 or less. 3.6.2 Gas Turbine Blade Corrosion Corrosion is due to fine ash particles and corrosive compounds of sulfur, alkali and alkaline earth elements contained in the coals (Li, Chuming et al. 1991). As mentioned before, cyclones are not 100% effective and hence some particulates and corrosive compounds managed to escape and enter the gas turbine especially when the cyclones were clogged. Metal corrosion occurs as a result of complex chemical reactions at high temperature. Sulfur (SO2 and SO3) and alkali (Na2O and K2O) react to form alkali sulfates with low melting points (Li, Chuming et al. 1991). Deposition of such sulfates in their molten state act as an adhesive to stick the micro particles on the blades, promoting complex chemical reactions forming low melting point complexes impairing the oxide protection on the blade thereby exfoliating the metal surface by gas and particles flow (Li, Chuming et al. 1991). To minimize the total alkali release from dolomites, an extremely pure metamorphic dolomite (e.g. Kaiser Dolowhite) may be used. Alkali removal sorbents, such as emathlite, have been tested for PFBC application. Up to a temperature of 1200 o C, either a packed bed of emathlite was placed after the cyclone or small emathlite particles were injected directly into the combustion products prior to the cyclone entrance to control the alkali to acceptable level (Newby, Keairns et al. 1989). To reduce this type of corrosion, it is recommended to operate the plant with coal that is low in sulfur and alkali, such as Australian black coals. 3.6.3 In-bed Heat Exchanger Tubes Erosion This type of erosion was experienced by Wakamatsu plant. The erosion mechanism is complicated due to the high operating temperature and the following interaction of Page 16
“Coal Selection Criteria for Industrial PFBC Firing” oxidation and erosion (Tsumita, Namba et al. 1997). In preventing such a phenomenon, at low temperature it is important to apply thermal spray coatings. For high temperature, the tube materials should have sufficient erosion and corrosion resistance due to the formation of hard oxide scale on the surfaces (Tsumita, Namba et al. 1997). Abrasion and leakage was found in Osaki’s boiler tube. The cause of this problem was believed to be solidified fluid deposits which remained on tubes in the furnace. When high velocity fluid flowed, abrasion progressed ten times faster. Modifications in the shutdown procedure were proved to prevent such occurrence (Matsumoto and Kawahara ). 3.6.4 In-bed Heat Exchanger Tubes Corrosion Some researchers found that the presence of ash in the bed accommodated the competition between gaseous halide carrier and solid alumino-silicate for corrosion. The kinetics of this competition were controlled by the location of chlorine and alkali release from the coal, re-trapping of alkali may occur if the bed is relatively high (~ 10 ft) (Keairns, Alvin et al. 1977). The same researchers discovered that more alkali will be released at lower pressure (Keairns, Alvin et al. 1977). Decreasing the bed temperature will reduce the alkali emission, but at the expense of reduced efficiency. When alumino-silicate was present, in sufficient concentration, it would reduce the alkali emissions from the bed by forming feldspars (Keairns, Alvin et al. 1977). Aluminosilicate, as a getter, is cheap and effective over a wide range of temperature. It is an attractive option in reducing alkali emissions, however neither its capacity nor concentrations were known in order to design an effective alkali suppression stage (Keairns, Alvin et al. 1977). In addition, its presence could cause plant deterioration through erosion. 3.7 Environmental Performances Most PFBC plants do not encounter any environmental problems since PFBC is already an environmentalal friendly technology. Nevertheless, the local Environmental Protection Agency (EPA) sets the pollutant emission regulation to achieve sustainable environment control. Page 17
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Page 5 and 6: LIST OF TABLES “Coal Selection Cr
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Page 23 and 24: 5. CONCLUSIONS “Coal Selection Cr
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coal selection criteria for industrial pfbc firing project 3.2 - CCSD

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