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Oxygen and natural gas preheating for

oxy-float glass

Heat recovery in air-firingconditions is a standard elementof glass furnace design.

Regenerators are complex devices withhigh investment costs related to whetherthey are common or multiple, from thechoice of brick shapes to the height ofthe regenerator. Recuperators are theother main type of system used torecover heat from flue gases. The airtemperature reaches from 700°C(recuperators) to 1250°C (regenerators),which amounts to up to one third of thepower injected into the furnace.

Other methods investigated forincreasing the efficiency of glassfurnaces include oxygen combustion,batch preheating, submergedcombustion, electricity production,boiler applications and syngasgeneration [1]. A well-known techniqueinvolves using full oxygen forcombustion, thus avoiding the nitrogenresponsible for NOx emissions.

Thus, combining heat recovery andfull oxy-combustion is seen as the idealsolution for Air Liquide and AGC GlassEurope. This technique yields thebenefit of oxy-combustion effectiveness,further increases energy efficiency,improves CO2 and NOx reduction andhas an attractive payback time.

Fuel consumption gainTo understand the effects of oxygentemperature and natural gas temperatureon burner efficiency, consider a burner continued »

� Fig 1. Measured fuel consumption gain vsnatural gas temperature for oxygen at 550°C.

Air Liquide and AGC Glass Europe have collaborated todevelop a new technology based on oxy-combustion,combining a heat recovery equipment that provides a furnacewith preheated oxygen and preheated natural gas by recoveringenergy from the flue gases, with a new generation of stagedoxygen burner named Alglass Sun. Youssef Joumani*, BertrandLeroux**, Antonella Contino***, Olivier Douxchamps**** andJohan Behen***** report for Glass International.

providing 1MW byusing 100Nm3/h ofnatural gaswith a lowcalorific valueof 10kWh/Nm3. Whenoxygen is preheated to550°C and natural gasto 450°C, extraenergy is introducedinto combustion. Replacing fossilfuel power directly with this energyreduces the combustible requirementby 6%, to94 Nm3/h.

However, this simplified fuelreduction calculation does not take intoaccount the combustion mechanisms.Because oxygen is preheated,combustion starts earlier and flameefficiency is better. In addition,separated jet technology allowsincreased flame size and thus lower fumetemperature than with cold reactants.

The difference in fume enthalpy is suchthat it induces a further decrease in fuelconsumption, by 4%, thus leading to afuel consumption gain of 10% asdemonstrated by pilot trials whenoxygen is at 550°C and natural gas at450°C (fig 1).

Separated jet burnersThe Air Liquide separated jet burnercalled Alglass Sun (fig 2), which hasbeen tested in several oxy furnaces [2],has been adapted to work with high-temperature reactants. Because of thehigh temperatures to which the naturalgas and oxygen are preheated, theycannot be premixed: CH4 self-ignites at550°C in pure oxygen, while species likeC2H6 and C3H8 burn at 470°C in as alittle as 2.3% oxygen. Front flame speed

� Fig 2. The Alglass Sun burner.

Natural Gas Temperature (°C)

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does not play an important role, astraditional velocities of reactants areapproximately 20 - 70m/s. The burnerhas the following main characteristics:

� Two fuel lances, allowing an easyswitch from natural gas and fuel oildepending on the costs of thecombustibles. Such switching is verycommon in glass factories.� High oxygen staging for better heattransfer and adjustability of flameshapes and NOx level. The typical levelof NOx measured in our pilot furnace is100ppm when oxygen is equallydistributed between the four oxygeninjectors. It goes down to 60ppm when75% oxygen is distributed to the twoextreme injectors. There is obviously alimit because when NOx decreases, theCO levels in the fumes increase. At 75%,the CO generated in the rich zones ofthe flame is burnt by the oxygen in thelean zones. At higher staging than 75%,CO increases dramatically and mayreach 500 - 1000ppm because of aninadequate distribution of O2 in thefuel-rich zones.� The burner is made in four blocks butcan be manufactured in one or twoblocks. The multi-block concept allowsadaptation of the burner position as afunction of furnace constraints.

Oxygen preheating technologyAt standard temperature, most standardmetallic materials combust very easily inan oxygen-enriched atmosphere. Toavoid this, EIGA has set rules on oxygenuse [3]: For example, the maximumvelocity allowed in pipeline at pressurebelow 10 bars is 60m/s to minimise theenergy available for ignition in case oflocal overheating by viscous heating orparticle impact.

However, ASME and EIGA expertisemostly focuses on temperatures below200°C. To analyse oxygen hazards, AGCGlass Europe and Air Liquide havecarried out several studies with topinstitutes and experts to obtainoptimum know-how.

Collaborative studiesFirstly, a set of 10 assorted metallic andnon-metallic materials was exposed tocyclic oxygen flow in order to accelerateoxidation and corrosion. Each samplewas subjected to 1000 one-hour cycles.

Through these experiments, weidentified a range of five to eightmaterials that underwent ignition testsin a hot oxygen chamber to shed lighton ignition conditions at 2 bars. Theprotocol of these lab-scale experimentswas the one defined in ASME guidelinesfor measuring the exemption pressure ofa material in contact with a pure oxygenflow. Some materials were excludedbecause their flammability propertiesagainst preheated oxygen were too lowwith threshold pressure close to 3 bars.

The materials that properly withstoodcyclic oxidation and promotedcombustion were exposed to a flow ofhot oxygen for more than one year.From this sequence of tests, monitoredby an independent expert, theconditions for heat exchanger designand safe use of hot oxygen wereclearly defined.

Air/O2 exchangersOnce suitable materials were chosen, alab-scale pilot exchanger was designedand tested at an Air Liquide researchcentre over several months. In theabsence of fumes, an electric preheaterwas used to heat 66 Nm3/h of air to700°C. Calibrated orifices were installedinside the pilot exchanger to test theeffect of oxygen velocity on oxidationand corrosion.

Several step changes were carried outon oxygen flow, air flow and airtemperature to examine how oxygentemperature varied as a function of these

parameters. For example, it was shownthat reducing air flow by 25% mightdecrease oxygen temperature by only15°C. At 50% of the nominal oxygenflow, oxygen temperature varied byabout 60°C for a given air flow.

These tests enabled us to determinehow to adapt air flow for a given O2 flow.Particular focus was given to insulation,i.e. type, thickness and mountingprocedure.

Natural gas preheatingThe use of hot natural gas is known inthe chemical industry. Natural gas isheated up to 400°C in an exchanger,using process exhaust gas or by directexchange with flue gases. However,there is large difference between this andthe characteristics of our technology:The natural gas is at several bars insteadof 2 bars, and the natural gas is heatedby flue gases at around 1200°C ratherthan by air.

A dedicated air/NG exchanger wasdesigned using materials compatiblewith preheated methane. Metal dustingand the compatibility of materials in ahot reduced atmosphere (carbon atomsare high reducing agents) were takeninto account in the choice of materials.

Before finalisation of the design, riskanalysis concerning the use of preheatednatural gas was conducted and a numberof recommendations were acted on.

Pilot testsAfter validation of the Alglass Sunburner at Air Liquide’s Claude DelormeResearch Centre, the first validation

continued »� Fig 3. Alglass Sun flame in the AGC furnace.

� Fig 4. Alglass Sun burner installation.

� Fig 5. Air/fumes recuperators.

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campaign at industrial scale was carriedout. Port #1 of an AGC Glass Europefurnace was converted to oxygen byinstalling two Alglass Sun 4MW burnersface-to-face (fig 3). Air flow was stoppedand oxygen controlled by an extra skid.

This demonstration, using preheatednatural gas, was a success. Thetemperature increase of the furnacewalls due to oxy-combustion was lessthan 30°C.

A second validation campaign wasorganized for the air/O2 exchanger andthe Alglass Sun burners with preheatedoxygen. Several parameters of theburners were tracked: Effect of oxygendistribution (50% or 75%); fuel type(natural gas or heavy fuel oil); andnozzle tip (single injection or multipleinjections).

During this campaign, the two naturalgas lines were preheated electrically. Afuel consumption gain of approximately10% was measured when using preheatedreactants (fig 4). The typical flame sizewas 4 x 2 metres. Staging the oxygen to75% induced a two-metre increase inflame length. The coverage of the batchwas fully controlled by this means.

The operators were able to use fireadjustment to manage foam, glassquality and batch line perfectly. Specialattention was paid to soot formation tocheck whether natural gas crackingoccurred in the pipes. After three weeksof operation, no soot was observed atthe gas lance tips.

Industrial resultsIn 2008, a newly rebuilt AGC GlassEurope furnace was equipped with AirLiquide Alglass Sun burners and with thecomplete package of heat recoveryequipment (fig 5). Oxygen and fuelskids (fig 6) were commissioned fromAir Liquide’s ALTEC technical centre inKrefeld, Germany.

After the furnace was heated up, theburners were successfully started one byone. A short period was dedicated tocold O2 operations, with particularattention to the behaviour of thefurnace and the interaction betweenoxy-combustion and glass bath.Operators used to working with air firinglearned to operate their furnace in oxy-firing conditions. To completely masterthe process they had to understand oxy-flames and make the links betweenburner power, flues gas temperaturesand O2 concentration in fumes. Thanksto good preparation, the transitionwas efficient.

During this time, the flames wereobserved continuously. Although theburner was designed for hot reactants, itwas shown that the flames remainedstable and straight enough in coldreactant configuration (as illustrated byfig 7). After this short period, the air/O2

heat exchangers were started. Withpreheated reactants, the flames werestraight and luminous, covered thebatch well and did not overheat thecrown. Flame lengths were as predicted:Four to six metres long, depending onthe burner’s location and power and thenature of the fuel.

As Alglass Sun burners provide wideflames, no CO was observed in the fluegases. With good management of thefurnace settings, it was possible to switchfrom cold O2 to hot O2 withoutchanging the operating mode. Furnaceoperators were able to deal perfectlywith potential extra foam, batch linechange and crown overheating.

After this step, natural gas wassuccessfully preheated to 400 - 450°C.NOx emissions have been measuredseveral times by the relevantgovernment agencies and a minimum of75% reduction was observed incomparison with air conditions. Thedecrease in CO2 emissions, even takinginto account CO2 emitted by O2

production, is near the target of 15%compared with air. The fuelconsumption target using preheated

oxygen and natural gas is a 25% gainover air. The results are already close tothis value and the objective is to reach itthis year.

ConclusionThis article has presented the results ofthe industrial full-oxy conversion of afloat glass furnace belonging to the AGCGlass Europe group. It has described thetechnology of oxygen and natural gaspreheating and staged oxy-combustion.

The joint project succeeded in solvingall the technical challenges posed.Alglass Sun burners were installed andstarted successfully, operating todaywith preheated natural gas andpreheated oxygen. The team is close toreaching the target fuel consumptiongain (vs air firing) of 25%. Achievementof such a gain would make it feasible toenvisage converting float glass furnacesworldwide to full oxy.

In addition, thanks to oxygen stagingof the Alglass Sun burners, low levels ofNOx have been reached, with a 75%reduction in NOx emissions observed(vs air). �

*Youssef Joumani, R&D ProjectManager, Air Liquide, France.**Bertrand Leroux, Co-ordinator ofCombustion ww network for ALTEC(Air Liquide Technology Centre).***Antonella Contino, R&D ProjectLeader, AGC Glass Europe, Belgium.****Olivier Douxchamps, R&DProcess Department Manager, AGCGlass Europe, Belgium.*****Johan Behen, Float LineManager, AGC Glass Europe, France.

The authors thank the EuropeanCommission, which has partly financed thiswork through the Life+ programme.

References[1] GMIC Workshop on EnergyEfficiency, Columbus (OH), 15th

October, 2009.[2] B. Leroux, J-F. Simon, A. Poirier, G.Constantin, Y. Joumani, R. Tsiava, Firstindustrial results of ALGLASS SUNoperations, XX Glass ProblemsConference, 2006.[3] EIGA, Oxygen pipeline Systems, IGCDoc 13/02/E.

� Fig 6. Air Liquide oxygen skid.

� Fig 7. Cold oxygen and natural gas.

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