THE


REFRACTORIES ENGINEER


Stuck in the 90’s (alumina)


21st Century Mullite Refractories for Optimum SRU Lining Stability


by Chris Windle, Technical Director, DSF Refractories & Minerals Ltd


INTRODUCTION Increasing consumption of fossil fuel based energy emits SO2


into the


atmosphere. This pollutant can harm the human respiratory system creating breathing difficulties and epidemiological studies have shown heightened sensitivity to SO2


in asthma sufferers particularly children [1] High concentrations of SO2 in the air can lead to SOx based compounds


which manifest as particulate matter. These small particles may penetrate deep into the lungs and in sufficient amounts to cause health problems.


In addition to the human interaction; SOx effects the wider environment contributing to acid deposition, affecting soil and water quality which in turn has adverse effects on aquatic ecosystems and damage to forests, crops and other vegetation.


In refining processes it is therefore vital that elemental sulphur is reduced and therefore recovered from fuels and associated acid gas streams.


The Claus process (patented 1883 Carl Friedrich Claus) has been the workhorse of the recovery of elemental sulphur from H2


S containing gas


streams for over 50 years and the reaction furnace (sulphur recovery unit) lies at the heart of the process.


In the reaction furnace acid gas (H2S bearing) is oxidised (burned) with sufficient air to convert ~1/3rd


of the H2 with the SO2 S + SO2


to as the Claus Reaction [2]:- 2H2


↔ SO2 + H2 O


The refractory lining of the reaction vessel has to endure a severe thermo- mechanical and thermo-chemical environment.


BACKGROUND


The traditional materials for SRU linings are based on 90’s alumina and whilst these operate well; there has been a step change in recovery technology and therefore there should be a step change in the refractories available.


Although DSF has supplied generic mullite bonded Corundum (Fricor) materials for the hot face lining of the SRU since 2006, the adoption of oxygen enrichment for lean acid gas streams, additional capacity and


S to SO2 ; the unburned H2 S reacts formed to yield elemental sulphur. This reaction is referred


Technical Paper


capacity redundancy has created a more demanding environment for the hot face refractory.


DSF Refractories & Minerals are acknowledged as suppliers of premium grade re-bonded Fused Mullite refractories for long campaign (7 to 9 years) E glass and speciality glass melters.


Forming sprung self- supporting crown structures; DSF Fused Mullites are subjected to temperatures up to 3000°F (1650°C) and therefore must have intrinsic thermo-mechanical stability, that is, creep deformation resistance.


In reaction furnaces with 40% O2 enrichment steady temperatures


of 2700°F(1490°C) can be achieved and it is predicted that a system running at 60% O2


to ~2750°F (1510°C).


These temperatures per se, pose no significant threat to the lining stability, however it is abnormal process temperatures (temperature excursions) which although sporadic and short term can lead to lining deformation and subsequently campaign limiting.


Following anecdotal reports of linings failing prematurely and taking into consideration the views of industry experts; DSF developed a creep resilient mullite specifically for SRU linings; DSF Frimul FX.


The remit for this product was to exhibit minimal creep deformation at 3000°F (1650°C).


Frimul FX is fired to 3100°F (1700°C) in batch intermittent kilns which ensures extensive solid state bonding throughout the brick matrix, this in turn imparts structural integrity at high temperatures and loads.


Whilst Frimul FX is a derivative of the predecessor product DSF Frimul F, the latter with proven thermo-mechanical stability over many years, it is never the less a significant step forward in materials design.


Following extensive research the binary phase diagram (Al2O3 O3 enrichment at elevated temperatures. :SiO2 ) has


been reassessed and shows that the mullite stable phase field veers toward Al2


enrichment will increase the operating temperature


Figure 1. Alumina-silica binary phase diagram showing mullite phase field veering towards alumina enrichment with increasing temperature (Klug et al 1987) [3]


Frimul FX composition mitigates for this phenomenon which in conjunction with ultra pure fused and reactive insitu bonding materials contribute to low creep values at 3000°F (1650°C).


November 2019 Issue


13


www.irengineers.co.uk www.ireng.org


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