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Praxair, in collaboration withHeye Glas, has successfullyimplemented an advanced oxy-fuel glass melting technology

called “Tall Crown Furnace” technology,to reduce silica refractory corrosion,NOx emissions, SOx emissions andparticulate emissions while maintainingthe high heat transfer and energyefficiency of oxy-fuel glass melting. Thetechnology has been successfullyimplemented by HEYE International inthe engineering design of threecommercial oxy-fuel fired containerglass furnaces. The first furnace was litoff in early 1996 and currently hasreached a campaign life of about nineyears (more than 9000 tonnes of glasspulled per square metre of the melterarea) without major silica corrosionproblems. The furnace is expected tooperate for a total campaign of 10 to 11years with the original silica crown, withovercoat repair near the end of thisperiod. The furnace has alsodemonstrated a high heat transfer rateand an excellent energy efficiency with aproductivity as high as 3.5 tonnes/d/m2

(2.8 ft2/tpd) without electric boost and aspecific energy consumption of about815 kcal/kg (3.3 MMBtu/ton) for flintglass with 60% cullet. The second andthe third furnaces have operated aboutseven and five years respectively withgood refractory conditions and furnaceperformance. A fourth TCF container

rate. Other special design featuresinclude partial furnace atmospherestratification to create more oxidisingand lower H2O vapour conditions nearthe glass melt using low momentumultra low NOx burners and a single flueport to optimise energy efficiency.

Reduction of emissionsSince over 80% of particulates emissionsare caused by volatilisation of alkalispecies, the TCF design also reduces theparticulate emissions. In Fig 1,measured particulate emissions fromregenerative air fired furnaces,conventional oxy-fuel fired furnaces andadvanced TCF oxy-fuel fired furnaces areshown as a function of the specific glasspull rate. It is well known thatparticulate emissions increase sharplywith the specific glass pull rate as aresult of increased glass melt surfacetemperatures and higher gas velocityunder higher pull rates. A comparisonof the data from air fired furnaces andthose from conventional oxygen firedfurnaces shows about 20 to 30 %reduction of overall particulateemissions under oxygen firing. With theTCF design, shown as “Oxy-FurnaceNew Design” in Fig 2, particulateemissions were reduced by a factor oftwo compared to the earlier oxy-fuelfurnace design. At a specific pull rate of3 tonne/day/m2 particulate emissions of0.25 kg/tonne (0.5 lb/ton) were achieved.

The Praxair JL Burner provides aunique design for very low NOxemissions and partial furnaceatmosphere stratification. The burnerdesign utilises the patented concept of“deep oxygen staging” and a separate

glass furnace (150 tones/d) has beendesigned recently to rebuild an existingoxy-fuel furnace and is scheduled tostart in late 2005.

Silica crown corrosionAccelerated silica crown refractorycorrosion for oxy-fuel firing is caused bya three-fold increase in theconcentration of alkali vapour species,especially NaOH and KOH. In someearly oxy-fuel furnace conversions,major crown repairs were requiredwithin two years. The highest corrosionrates were often observed in the coldestarea of the furnace. In order to reducethe volatilisation and the concentrationof alkali species, an innovative furnacedesign characterised by tall crown heightand high burner elevation wasdeveloped.1 By raising the burner heightabove the glass melt the “hot spots” onthe glass melt surface created by theoxy-fuel flames and the gas velocity nearthe glass melt surface were reduced andthe overall alkali volatilisation rate wasreduced by about 50%. The low flamemomentum of the oxy-fuel burner andthe tall crown reduce the circulation ofNaOH and other volatile species fromthe glass melt surface to the crown andthe NaOH concentration and the gasvelocities near the crown are reduced.Both of these factors contributed to thereduction of the mass transfer rate ofNaOH to silica crown and the corrosion

Tall crown furnace technology for oxy-fuel firingAn advanced oxy-fuel fired furnace design has been proven to extend the life of silica crowns while achieving high-fuel efficiency and low NOx and particulates emissions. H Kobayashi*, K T Wu*, G B Tuson*, F Dumoulin* and H PKiewall** show us how.

injection of secondary oxygen underthe low temperature rich primaryflame. The rich primary zonestoichiometry produces a lowmomentum high luminosity flame forhigh heat transfer efficiency and theseparate secondary oxygen injectionproduces a long flame with a higheroxygen concentration and a lowerwater vapour concentration near theglass melt surface. NOx emissions fromJL Burners are shown to beapproximately 1/3 to 1/10 of those fromthe conventional oxy-fuel burners.

Heat transfer and fuel consumptionAlthough the larger distance betweenthe flame and glass melt surfacereduces convective heat transfer anddirect radiative heat transfer fromflame to glass melt, a good heattransfer efficiency is maintained withthe TCF design. The bulk gas to glassmelt radiative heat transfer increasesbecause of increased gas emissivity dueto the longer “radiation beam length”afforded by the tall crown height andthe longer gas residence time causedby the larger furnace volume. In thecharge end of the furnace near the fluegas exit port, the heat transferefficiency by bulk gas to the coldcharge is increased due to the highergas emissivity. With proper flue portplacement, the net effect is a reductionin flue gas temperature despite highercharge end crown temperature. Thedecrease in flue gas temperaturecorresponds to an increase in availableheat, which offsets a small increase inthe furnace wall losses caused by the

COMBUSTION EFFICIENCY

� Fig 2. Oxy-Furnace New Design.

� Fig 1. Measured Particulate Emissions from Container Glass Furnaces.

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air takes place in a specially designed mixing pipe. This ensures excellent mixingof gas and air quantities for minimum throughput.� The adjusted air quantity flows through a differential pressure orifice gauge.The differential pressure which results and which is influenced by the throughputrate is fed into a special ratio controller, which adjusts the corresponding gasquantity proportionally to the respective air quantity.� Adjustment of the required air quantity is made by a directly driven motorcontrol valve with a control cone that has linear KV value.

� In case of anunacceptable mixpressure, a gas safety stopvalve is integrated into theratio controller to enablesafety gas cutoff of thecontrol section.� Admission pressure ofcombustion air can beconstantly controlled andadjusted using a thyristor-controlled combustion airfan.

Contact HORNGlasanlagen GmbH,Plössberg, Germany. Tel: +49 9636 92040. Fax: +49 9636 920410.Email: info@hornglas.de;Website: www.hornglas.de

HORN has had many years of experience with heating feeders and distributorchannels, using a mixture of gas and air produced by Venturi mixers and

combined using Venturi and injection mixers. However following customerrequests, in 1988 HORN started to develop a new mixture heating system.

The resultant new Cora (constant ratio) mixture heating system has been usedsince 1999 in various customers’ plants. It keeps up an completely stable gas/air ratioover the entire range of adjustment and under very different operating conditions.

The Cora mixture heating system meets all the heating system requirements fordistributors, forehearths andburners with air casing, such aswide and constant control range,low maintenance and optimumcombustion. The system isdesigned to be integrated withoutdifficulty into almost any mixtureheating system.

Forehearth and mixing systemsthat do not work constantly canhave pressure fluctuations, whichlead to poor combustion andtherefore to reduced glass quality.The essential features of the Corasystem compared to conventionalsystems are:� The forced mixing of gas and

Distributor and forehearth mixture heating system

increase height of the breast wallswhich are generally very wellinsulated. Table 1 shows a comparisonof the furnace energy balances of a 330tpd (300 tonnes/d) conventional oxy-fuel fired and Tall Crown ContainerGlass Furnace.

Glass experience The TCF technology has beensuccessfully implemented by HEYE

COMBUSTION EFFICIENCY

APRIL 2005 80 GLASS

a campaign of 10 to 11 years with theoriginal silica crown of 375mmthickness, with overcoat repair nearthe end of this period. HEYEInternational commissioned a secondtall crown oxy-fuel furnace for greenglass in 1998 in the Netherlands. Athird tall crown oxy-fuel furnace formelting amber glass wascommissioned in April 2000 inObernkirchen.

The first furnace melting flint glassfeeds two IS 20-section double gobforming machines producing 1l and0.33l mineral-water bottles. From thestart-up on, the furnace has melted thedesired amount of flint glass withexcellent glass quality. Seed counts areon average below 20/oz. The twoforming machines pull 330 tonnes per24 hours on average. The furnace isoperated with approximately 60%cullet powder of external cullet. Energyconsumption figures are better thanexpected. The average consumption isaround 3.35 mJ/kg of glass (800kcal/kg) which results in melting costslower than those all other conventionalfurnaces in operation. This capacityand consumption is achieved at a pullrate of more than 3 tonne/m2 andwithout any electric boosting.

Concerning emissions, allexpectations have been more thanfulfilled. Measurements show emission

International into the engineeringdesign of three commercial oxy-fuelfired container glass furnaces. The first350 tonne/day furnace was built atObernkirchen, Germany and lit off inearly 1996 and currently has reached acampaign life of about nine years(over 9000 tons of glass pulled persquare metre of the melter area)without major silica corrosionproblems. It is expected to operate for

figures between 0.5 kg NOx/ton and0.35 kg NOx/ton of glass using naturalgas containing about 11% N2, wellbelow the German regulation of 500mg NOx/m3 of flue gas (converted to0.7 kg NOx/ton of glass based on samemass-flows of NOx).

SummaryTall Crown Furnace design has beendemonstrated to reduce corrosion ofsilica crown by and reducedvolatilisation of alkali vapours fromglass melt and batch. The life of aTCF oxy-fuel fired container glassfurnace is now expected to be over 10years. The TCF design with PraxairJL Burners reduced NOx andparticulate emissions substantially,while providing excellent heattransfer characteristics.

References1. Kobayashi, H, KT Wu, GB Tuson, FDumoulin, and HP Kiewall, CeramicBulletin, February 2005, pp. 14-19;March 2005, pp. 24-27.

* H Kobayashi, KT Wu, GB Tuson, F Dumoulin, Praxair,Inc, Danbury CT, USA. Tel +1 716 879 4077. Fax +1 716 879 2040. Email form on website; Website www.praxair.com. ** H P Kiewall, HEYE International GmbH, Obernkirchen,Germany. Tel +49 5724 26452. Fax +49 5724 1288. Email management@heye-international.de; Website www.heye-international.de.

Table I. Energy balance comparison.

Conventional Tall CrownOxy-fuel Oxy-Fuel

Glass Pull Rate (tpd) 330 330

Cullet ratio (weight % of charge) 30 30

Flue gas temperature (F) 2620 2600

Fuel Input (MMbtuh) 53.14 53.28

Energy Outputs (MMbtuh)Energy to batch reactions and glass 24.64 24.64

Tank & Furnace wall heat loss 6.60 6.84

Flue gas sensible and latent heat 21.90 21.80

Total 53.14 53.28

Specific Energy (MMbtu/ton) 3.86 3.87

� HORN’s mixture heatingsystem.

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