Research Output
Slagging of Coal Blends: Comparison between Pre-Blended and in Furnace Blend
Electrochemical Monitoring of Oxygen Potential on Fe-Cr Alloy Surface During High Temperature Oxidation in Dry and Humid Conditions
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Publication
Coal blending technique in electric generating power plant has been widely used to reduce coal consumption cost. However, due to various coals from different places blended together, it had caused problems. Slagging formed during the operations can reduced boiler efficiency, resulting high costs for maintenance caused by blockage, corrosion and erosion. Many empirical indices and predictive method have been developed to minimize these problems but most are unreliable for coals from different places and many predictive methods are carried out in different environment. In this research, the method starts from fabricating the pre-blended and in furnace blend techniques at a local coal pulverized power plant. After feeding the coal blends for coal trial burn, the resulting ash deposition are collected at the burner where slagging usually occurs. The raw coals in used and the slagging samples are then characterized and determine their crystalline phase and compositions by using X-Ray Diffraction (XRD) and Sacnning Electron Microscope Energy Dispersive Xray (SEM/EDX). From XRD analysis, compounds such as quartz, cristobalite low and manganosite were found as main species in ash deposition at all location of collected slagging. However, there are also some compound exist at specific location with specific technique being used. In the burner area for pre blended technique, compound Al2SiO5, Mg2Al4Si5, KNO2 and FeO was found, whereas in furnace blend K2Mn2(SO4)3 was found. At bottom part of the furnace, pre-blended graphite was presence and in furnace yield MnO2 and Al2SiO5. At top part where superheater reside, the existence of mullite, Al2(Al2.5Si1.5)9.75 was found for pre-blended technique and microcline, KAlSiO3O8 is observed from in furnace blend technique. From the EDX analysis, in furnace technique shows more complete combustion and feasible for less slagging inside the furnace compared to pre-blended technique.
Publication
The formation of an external Cr2O3 scale is important to obtain the oxidation resistance of Fe–Cr alloys at high temperatures. It is well known that the critical concentration of Cr to form a protective external scale of Cr2O3 in humid conditions is higher than that in dry conditions, and the criterion is expressed as Wagner’s equation (Rapp in Acta Metall 9:730, 1961) [1]. A lot of mass gain data and metallographic surveys are required to determine the above criterion experimentally. A method of continuous monitoring of surface oxygen potential by oxygen concentration cell using stabilized zirconia has been applied to the oxidation of Fe-Cr alloy in order to check the protectiveness of the scale. The objective of this study is to measure surface oxygen potentials on Fe-0-17 wt% Cr alloys in Ar-21% O2 gas in dry condition and Fe-10-22 wt% Cr alloys in Ar-20% O2-20% H2O gas in humid condition at 1073 K up to 20 ks. In dry condition, the surface oxygen potentials of Fe with more than 10 wt% Cr alloys were close to the oxygen potential of the atmosphere immediately after the heating period. It indicates that a protective Cr2O3 scale formed on these alloys at the early stage of oxidation. However, the surface oxygen potentials of these same composition alloys were lower in humid conditions than that in dry, which suggests a higher oxidation rate. This paper demonstrated that the protectiveness of scales formed on Fe–Cr alloys can be evaluated in situ in a few hours, which is beneficial to assess the high temperature oxidation of metals.