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Technical Paper


the position inside the combustion chamber. The current assessment of corrosion resistance of SiC refractory materials at high temperature follow well known standardized methods (steam oxidation).


Methods and results Four SiC based shaped refractories where investigated in this study:


• Oxide-bonded SiC quality denoted (O-SiC 1) which is widely used in French and Italian WTE plants, among other location


• Nitride- bounded SiC quality 1 (Refrax® PRO, standard N-SiC from the Saint-Gobain’s portfolio)


• Nitride-bounded SiC quality 2 (Refrax® PLUS, enhanced N-SiC from the Saint-Gobain’s portfolio)


• Nitride- bounded SiC quality 3 (Refrax® TOP, enhanced N-SiC, double firing from the Saint-Gobain’s portfolio)


ASTM C863-00 Standard Test Method for Evaluating Oxidation Resistance of Silicon Carbide Refractories at Elevated Temperatures was used to compare existing and new compositions for oxidation resistance. The ASTM C863-00 standard is an accelerated oxidation test conducted on test samples within a steam environment (steam rate of 32 kg/h/m3


) at


temperatures in the range of interest (1000°C) for up to 500 hours. Test samples are withdrawn from the furnace at 100-hour intervals, 250 h and 500 h for testing. Measurements are made for changes in weight and volume.


Graph 4 depicts the evolution of measured volume expansion evolutions during the steam oxidation test. All the materials exhibit the well-known parabolic oxidation behaviour of SiC- based materials [8]


. The kinetic of this


behavior is considerably lowered in the case of silicon nitride bounded SiC quality (PRO, PLUS, TOP) due to the formation of a protective scale at the surface of the grains and over the Si3


N4 matrix. Further material oxidation is


slowed down by the diffusion of oxygen across this scale as described by a plateau observed after 200 hours.


Refrax® TOP and Refrax® PLUS exhibit the lowest mass gain and volume expansion with volume change less than 0.4% and 0.2% respectively for PLUS and TOP. In comparison, O-SiC exhibits a volume variation up to 8%, suggesting a significant dimensional instability for the tiles inside the boilers. The better performance of the Refrax®


Graph 5: Pore size distribution measured by Hg intrusion for fresh materials- Comparison N-SiC (Refrax®) and O-SiC


that way, exhibits a very low permeability to the gas and slag penetration, a low oxygen diffusion and high intrinsic temperature stability and a high inertness to corrosive media.


Monolithic range


Indeed, Saint-Gobain not only have their own tile design system, T-Clip PRO, and the specialist material family we have discussed today (Refrax® PRO, PLUS & TOP) but also offer a range of complimentary monolithic materials again designed and aimed specifically at addressing the problems and issues encountered specifically in WtE facilities.


Concentrating on high quality raw materials, correct particle size, size distribution & optimum physical factors like applied density and porosity, we achieve the best results in real life site tests time after time. We develop products to achieve a desired change, e.g. longer life / higher conductivity/ resistance to oxidation conditions, not to copy existing market trends.


Conclusions products cannot only be attributed


to their chemical composition but also to the engineered microstructure of the nitride-bounded matrix. Indeed, the reactive nitration of the silicon during the firing process leads to the formation of a relative fine and hence low permeable matrix as shown in Graph 5 (pores distribution: N-SiC d50: <1 µm vs. O-SiC 1 d50 ~ 8 µm in the case).


This effect is further increased in the case of the Refrax® TOP in which a strong physical barrier is intentionally generated thanks to a second firing. The formation of this low permeable glazed structure is fully and carefully controlled during the process and achieved thanks to a specific treatment above the application temperature. The microstructure obtained


Due to the high thermal and complex operating conditions in incinerator boilers, high performance refractory material and systems are indispensable to protect the metal membrane from corrosive media. The most critical parameter in the choice of these materials is their resistance to oxidation.


N-SiC Refrax® range of products developed by Saint-Gobain has demonstrated performances superior to the ones of oxide bounded SiC refractories with outstanding stability in the steam oxidation test (volume change <0.4% after 500 hrs @ 1000°C). Consequently they are able to provide longer and more effective protection and limiting failure. This performance is achieved thanks to a special design of the binding phase, combining a low permeability, a finer porosity a high resistance to oxidation, and a specific morphology of the Si2


ON2 and β-Si3 N4 crystals. The real challenge where we succeed is to design, manufacture and supply Castable


= 60% SiC = 70% SiC = 80% SiC


Figure 11: Saint-Gobain monolithic range 22 ENGINEER THE REFRACTORIES


Refrax®ProCast 60 Refrax®ProCast 70 Refrax®ProCast 80


Gunning


Refrax®ProGun 60 Refrax®ProGun 70 Refrax®ProGun 80


Plastic


Refrax®ProPlast 60 Refrax®ProPlast 70 Refrax®ProPlast 80


Ramming


Refrax®ProRam 60 Refrax®ProRam 70 Refrax®ProRam 80


www.ireng.org


January 2019 Issue


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