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Technical Paper


Nevertheless, the incinerator environment cannot be characterized as simply oxidizing. In the presence of chemically aggressive compounds as molten slag and alkali-rich compounds in the process, other reactions may occur.


In contact with alkali-rich environment, studies have shown possible mechanisms of degradation of the protective silica layer (formed from the passive oxidation); leading to the formation of liquid silicate film or the formation of new phases. These liquid silicate films can move away and the protection of SiC or Si3


N4


[f] MxCO3 [g] Mx


+ x.SiO2(s) SO4(s) → Mx + x.SiO2(s)


phases against oxidation is not insured anymore. O.xSiO2(l)


+ CO2(g) → Mx O.xSiO2(l) + SO3(g)


Another possible reaction concerns the modification of the protective layer oxidation into SiO3


)[4] . -2 followed by the reaction with calcium cation from the


dissolution of calcium sulphate at high temperature to form wollastonite (CaSiO3


[h] CaSO4→SO4 [i] SO4 [j] SiO2 [k]SiO3


-2→ SO2 + O-2


-2 + Ca+2 -2 + Ca+2 + O-2 → SiO3 -2 → CaSiO3


The third type of reactions is linked to the condensation of the calcium sulphate or alkali compounds species into the porosity of the refractory leading to the formation of Cristobalite phase at lower temperature ( below 700°C) in the matrix and around the SiC grains [9]


.


The temperature boundaries between all these mechanisms are only approximated and are dependent on the specific system. In practical cases, several mechanisms operate simultaneously. However, as general rule the kinetics of these reactions is strongly impacted by temperature, oxygen pressure, material microstructure and compositions and lead in the majority of the case to an undesirable volume expansion. Hence, the lower the volume expansion is, the better the material performance will be.


Therefore, it will be recommended to prefer refractory lining systems (i.e. material composition, microstructure and design) offering low permeability to corrosive gas, high resistance to oxidation and capable of generating a suitable thermal profile to limit the penetration of the corrosive gases and kinetics of damaging reactions at the surface of the refractory.


Advanced refractory materials for enhanced oxidation resistance


Typical SiC- based shaped refractories are composed of 3 main phases: SiC grains, with a well-defined and controlled particle size, a binding matrix and porosity. The choice of silicon carbide grogs is driven by their excellent resistance to oxidation, chemical stability, outstanding mechanical


SiC content [%] Binding phase Density [g/cc] Porosity [%]


MOR [MPa]


O-SiC 1 91


Cristobalite, Mullite 2.64 <16


48 Table 1: Basic Properties of SiC based shapes refractories 20 ENGINEER THE REFRACTORIES


www.ireng.org and thermo-mechanical properties and high thermal conductivity [9], [11] . On


the other hand, the role of the binding phase in the final product is often underestimated. Additionally, it plays an important role in the corrosion resistance of the product, since corrosive slag, fumes and ashes penetrate into the tile through it. Hence, good corrosion resistance of the binding phase itself and low permeability are key properties when selecting a lining configuration to extend the lifetime not only of the refractory lining, but also of the metallic parts protected by it.


Figure 8: Type structure of SiC materials employed in waste-to-energy plants (Refrax®)


We can distinguish 2 main families of SiC carbide refractories for WtE applications.


• Oxide Bonded SiC denoted [O-SiC)] • Silicon nitride bonded SiC denoted [N- SiC]


Saint-Gobain Performance Ceramics & Refractories develops, manufactures and sells high quality refractory products. Considering the well-known concern about oxidation of silicon carbide, our research on non-oxide ceramics composite matrix with enhanced resistance to oxidation was the main focus of the R&D team and evolved into the development of the Refrax® brand.


The table below outlines the average material properties of the different products supplied


proposed on the market.


Among the bonding phases known (silica, oxides…), the use of silicon nitride (Si3


N4 or oxy-nitride) leads to good mechanical properties and


chemical resistance in several corrosive/oxidative environments, in addition to high thermal conductivity, low thermal expansion and good abrasion resistance [10]


Refrax® PRO 75


Si3N4 , Si2


2.70 <14


62 ON2


Refrax® PLUS 77


Si3 N4, Si2 ON2


2.74 <13


60


Refrax® TOP 78


Si3 N4 , Si2


2.70 <13


52 ON2 . The specialities of the Refrax® product range lie in the special by Saint-Gobain in comparison to conventional


January 2019 Issue


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