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Metal Casting Technologies : September 2006
75 SPECIFIC EVALUATION AND TESTING CRITERIA OF REFRACTORY MATERIALS FOR BURNER TILES Burner tile temperature Essential for the material choice and design of the burner tile is the knowledge of the temperature profile in operation. Reliable data can be obtained by experiments. Experiments however are time consuming and dependent on the availability of appropriate equipment. In addition thermal boundary conditions used for the finite element model are estimated and based on the flame temperature. However, adiabatic flame temperatures are not useful data to estimate the resulting tile temperature. For example in the hydrocarbon and petrochemical industry most burners are working with excess air. Additional cooling of the tiles occurs in low NOx burners because introduction of inert flue gases into the combustion zone of the burner reduces the peak flame temperature and NOx formation. Therefore the burner type, the composition of fuel and air inlets and a possible flue gas recirculation must be considered when estimating thermal boundary conditions of the tile. A good indication about the flame temperature and pattern can be obtained from burner modeling by computational fluid dynamics (CFD). Material testing should also reflect the different dynamic temperature stages to determine suitable materials. A burner tile is exposed to one or more temperature cycles during the lifetime. A temperature cycle can be divided in different stages which are important for the material requirements. These are: a. Heat up b. Steady heat flow at operating temperature c. Thermal cycling caused by burner control and operation d. Cool-down. Hence the temperature dependent stress -- strain behavior of the refractory material must be defined. Thermochemical Considerations External chemical attack on burner tiles occurs predominantly on heavy oil fired burners. The reason for the attack is the reaction of impurities from oil like vanadium and sodium with the refractory to low melting silicates. Therefore beside the alkalis, the silica content determines the chemical stability of the material. The ISO standard 13705 specifies the required chemical composition of burner tiles for certain applications: Normal burner tile: > 40% alumina High intensity combustor: > 85% alumina Oil firing tile, <50mg/kg V, Na: > 60% alumina Oil firing tile, >50mg/kg V, Na: > 90% alumina Table 1 shows as an example the chemical analysis of a sample taken from a burner tile of an oil fired ship boiler burner. The tile failed after 6 months in operation because of surface spalling after cool down. The table shows the quantity of impurities picked-up during operation. V205 +2,7% NiO +2,0% Na20 +1,3% The mineral composition also plays an important role in the internal thermo-chemical and mineralogical reactions. A high mullite content is favorable for good thermal shock properties due to the low reversible expansion. The alkali and alkali earth metal oxide content has an influence on the modulus of elasticity, the resulting thermal shock properties and phase transitions that lead to stresses due to expansion or shrinkage. Furthermore a long term attack occurs from particles of combustion air. Silica and alkali containing particles melt in the flame, and due to recirculation of combustion products, attack the tile. Due to this, burner tiles can show severe surface cracking after years in service. Thermomechanical Considerations During high temperature service, burner tiles are required to conduct heat and they may be subjected to rapid temperature variations. This takes place during adjustments of the burner, fuel changes and shut down conditions when cooling down and heating up the burner again. The non-linear temperature gradient during rapid heat up and cool down develops thermal stresses or thermal expansion forces that may fracture the refractory. The refractory structure should resist tensile and compressive strains through a wide temperature range from a rigid solid state at low temperatures to a more viscous state at high temperatures. Cracking from these stresses can have an impact on the flame pattern and in a worst case scenario cause the flame to be in contact with the tile causing problems like hot spots and total destruction of the tile and burner casing.(2) One approach to determine the resistance of a refractory material against cracking in service is the evaluation of different stress parameters. The expressions obtained from the calculation of the stress parameter R1 -- R3 can be transformed in terms of the maximum temperature difference, heat flux or temperature gradient to which the refractory can be subjected without exceeding the thermal stress failure. The "Hasselmann thermal stress resistance parameters R1-R3" are a tool to predict thermal stresses in materials of brittle - elastic behavior for the purpose of basic materials ranking (3). R1=σ(1-μ)/(αE) A rapid thermal shock leads to thermal gradient on the surface of a material. R1 is the maximum allowable temperature difference in body under conditions of steady heat flow. (ΔT in °K) Table 1. WHO'S WHO OF METALS -- ANNUAL 2006/7