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Flameless Oxidation Technology

A.Milani, J.G.WünningWS Wärmeprozesstechnik – Dornierstr 14 – 71272 Germany WS

Abstract Flameless combustion is the most significant recent advancement in hightemperature combustion technology and has been applied to industrial furnaces with wellproven very low NOx performance and high energy savings. This experience has producedspin-offs in power generating equipment, from innovative gas turbine combustors to smallreformers for decentralized H2 production, and R&TD of flameless oxidation techniques isquite promising for new advanced process design.

1. Introduction

The flameless combustion technology applied to high temperature industrial processes stemsfrom systematic investigations carried out at laboratory scale and from application to largeplants in the steel industry. Results are very satisfactory both for abatement of NOxemissions and for energy savings; spin-off and on-going R&TD in the field of powergeneration is very promising. All this started from looking again at basic principles. Aconventional flame is based upon a mechanism as old as fire discovered in nature manycenturies ago: a stable flame develops from a stationary flame front, that is a few mm thicklayer. Burner design is primarily concerned with the problem of stabilizing the flame frontby means of fluid dynamic devices. Typically a bluff body drives back hot reacting productsthat heat up the fresh fuel-air mixture thereby triggering a stable chain reaction. Highgradients of temperature and species concentration in a confined space are required to obtain astationary flame.

2. Flameless Combustion

In a burner stabilized flame mostreactions occur within the flamefront, where local temperatureapproaches adiabatic temperature.In a flameless burner, the flamefront is deliberately avoided andcombustion reactions occur asfuel and air mix together withentrained recirculated combustionproducts. For the process tooccur, combustion products mustbe above se l f - i gn i t i ontemperature (> 850 °C for safety).The reaction rate is determined bythe mixing pattern between threepartners: fuel, air and combustionproducts entrained b e f o r ecombustion. In the flameless

Figure 1 – NOx emissions from steel furnaces

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mode temperature profile is determined by the mixing pattern with the recirculatedcombustion products and cannot depart much from the temperature of these entrainedcombustion products.

In the flame mode, the temperature profile peaks in the flame front close to the burner: this isconducive to enhanced thermal NO formation [1]. To abate temperature peaks means to abatethermal NO and flameless combustion does abate NOx emissions by one order of magnitude.Figure 1 reports accumulated data relevant to many natural gas fired furnaces in the steelindustry: the advantage of the flameless technology for temperatures > 850 °C with respect tothe best low-NOx burners designs is quite clear. Figure 2 shows how flame and flamelessmode are implemented in high velocity burners, that are common in heat treatment furnacesfor steel products.

The domain of flameless combustion has beeninvestigated on a test furnace as a function ofthe recirculation ratio Kv defined as ratio of re-circulated mass flow of combustion products(before reaction) with respect to the drivingflow rate of reactants [1,2]

Kv = Mrec / (Mair + Mfuel)

Results are schematized in Figure 3: fortemperatures > ~ 850 °C, above self-ignition, adomain of stable reaction region without flamefront can be established, corresponding to largeKv values (order of Kv > ~3), that are obtainedwith high momentum of the injected fluids.

This domain has been called flamelessoxidation or with the trademark FLOX®. It isnot possible to establish a conventional flame

front for Kv values ~ > 0.3-0.5 and the intermediate region is typical of “lifted flames” and ofunstable combustion. Below ignition temperature, burner stabilized flame mode only isadmissible. Flameless oxidation does not produce a visible flame and furthermore thiscombustion mode is almost silent andabatement in combustion noise (~ 15dBA) is at least as impressive, asdisappearance of a visible flame, provingthat the turbulent flame front accounts formost of the typical combustion roar ofhigh velocity burners [3]. Flamelessoxidation has been thoroughlyinvestigated by WS [3,4]. FLOX® hasbeen shown to work for rich, nearstoichiometric and for very leancombustion conditions; it works with andwithout air or fuel preheat. It also worksfor diffusion, partial premixed andpremixed combustion. A well knownadvantage concerns low-NOx burnersoperated at very high air preheat: unlike

.a. recirculationr

Kv

~coldfluegases

self-ignitiontemperature

temperature

stable flamelesscombustionor FLOX®

no stable flame

possibleexplosion risk

unstable liftedflames

burnerstabilizedflames

v

temperature

Flue,air,gas,flamelesscombustion

F

Figure 2 – Flame and flameless combustion

Figure 3 – Domain of FLOX® vs Kv factor

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conventional flame mode, the flameless mode is insensitive to air preheat temperature as faras NOx is concerned, and this is very important for application to high temperature industrialprocesses or furnaces.

3. Energy savings in steel furnaces

The thermal efficiency of high temperature furnaces can be increased very much by means ofefficient heat recovery by means of air preheating: high efficiency is equivalent to reductionin fuel consumption and to a corresponding saving in greenhouse gas emissions. Highpreheat, like air at 800-1000 °C, is only technically feasible if special combustion techniquesare adopted in order to prevent unacceptable NOx emissions and local overheating. Flamelessoxidation fits perfectly this requirement and can be considered a prerequisite for suchapplications.

A preferred burner design for high air preheating is based on burner integrated heat recovery(Figure 4): flue gases are extracted through the burner itself and combustion air is preheatedin counter-current while cooling the flue gases. This is a convenient solution for furnacesequipped with several burners: cold combustion air is distributed to the burners while almostcold flue gases are extracted from a common manifold. This design offers effectivepreheating efficiency. Centralized heat recovery allows thermal efficiency ~ 60 % (~ 40% forno preheating at all), burner integrated recovery scores ~ 75-85%, which is a good stepforward, corresponding to a fuel saving 15-25% with respect the to state of the art (centralizedheat recovery).

Thousands of FLOX® burners have been installed in continuous industrial plants and performsatisfactorily. Also regenerative burners firing in FLOX® mode have been adopted in severallarge annealing lines for stainless steel strips and in batch furnaces. Regenerative airpreheating is certainly most efficient and allows energy savings in the order of 30-50% [5].

The radiant tube is a device used in large heat treatment furnaces for steel products: itradiates to the stock without permitting contact with the flue gases and combustion isdeveloped inside a long tubular chamber, which makes combustion control difficult.Experience has demonstrated that internal recirculation of combustion products is the key togood performance: “re-circulating geometries” allow low-NOx performance and uniformtemperature of the radiant tube thanks to flameless oxidation. Temperature uniformity hasbeneficial consequences on the strength of the radiant tube and on the average allowable heatflux, which implies a better exploitation of the radiating surface: in other words, a saving in

~cold flue gaseshot flue gases

cold airnat. gas

furnacewall

combustionchamber

Figure 4 – Burner-integrated heat recovery

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installation costs. A good example are the annealing furnaces equipped with ceramic “single-end” tubes in SiSiC: the cost of ceramic radiant tubes has been largely overridden by excellentperformance. Figure 5 shows pictures of large plants equipped with several hundreds FLOXburners.

4. Power generating equipment

The FLOX® principle is not limited to steel furnaces and can be applied to several hightemperature processes. Examples are the Stirling engines, where heat is made available athigh temperature with high efficiency, with the purpose of providing combined heat andpower in small power generating units. A very promising application of flameless combustionto combustors for gas turbines is being presently developed and successfully tested: aspecially designed FLOX® prototype burner (Figure 7) ensures very low-NOx, emissions andovercomes the nasty problem of fluctuations or “humming” that affects premix-based GTcombustors, where the flame front stabilization is a critical issue. R&TD is ongoing with theparticipation of several academic and industrial partners in Europe.

The inherent temperature uniformity obtained with flameless combustion finds an idealapplication in steam reformers for hydrogen production: reforming reactions take place inside

vertical tubes filled with acatalyst and reheated fromthe outside. The uniformtemperature distribution isessen t ia l fo r h ighproductivity, reduced stresson the reaction tubes andbetter control . Theexperience of the W Scompany in steel processfurnaces has been used tofound a daughter companyspecialised in “mini-reformers” for producingsmall amounts of hydrogen

Figure 5 – Continuous steel furnaces equipped with FLOX® burners

Figure 6 – CFD computations of the prototype GT combustor

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(order of 5-200 Nm3/h) for decentralised fuelling stations for future H2 powered vehicles.Figure 8 shows the scheme of the WS minireformer: such plants have been installed in theairports of Munich and of Madrid to provide the H2 used by local buses for passenger service.

Flameless oxidation has been investigatedwith gaseous fuels and in particular withnatural gas. However, the basic principleholds good for any fuel, at least any fuel assoon as it is made available in fluid form (likeevaporation of liquid droplets or release ofvolatile matter from pulverised solid fuel).Trials are being carried out together withGerman universities to test effects offlameless oxidation (Figure 8): flamelessmode occurs with any fuel and consistentNOx reduction has been observed.Encouraging tests have been carried outunder pressure, which might be applicable toenvisaged future processes aiming at CO2

sequestration.

5. Conclusions

Referring to the case of high temperaturefurnaces, the industrial application hasdemonstrated that flameless technology can

greatly renew and improve the design and theperformance of traditional plants / processes; advantages like “downsizing” (reduction of thefurnace length), NOx minimization, temperature uniformity, better control and improvedproduct quality make investment for revamping old plants advantageous. Similar argumentshold true for the R&TD applications to power generating devices as quoted in § 4 above. Wecan conclude that the principle of flameless oxidation has still a large potential for furtherdevelopment in many equipment where combustion plays the important role.

Figure 7 – The FLOX® basedminireformer

Figure 9 – FLOX® performance with different fuels

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The tendency is to tighten regulations concerning pollutant emissions and to limit specificemissions of greenhouse gases, which implies reducing specific fossil fuel consumption. Thisis based upon steady grounds: not the available or future fossil fuel resources put an effectivelimit to economic and abundant energy, but the available clean air. Clean air for combustionis a limited global resource that cannot be wasted or corrupted beyond a sustainable threshold.In former times California had promoted use of catalytic converters and had therebystimulated the competitive production of cleaner engines. A similar, virtuous pattern shouldbe followed in other domains related to fossil energy conversion as awareness of theworldwide “environmental challenge” proceeds.

References

[1] Wünning, J.A. Flammenlose Oxidation von Brennstoff mit hochvorgewärmter Luft,Chem.- Ing.Tech. 63, No.12, p1243-1245, 1991

[2] Wünning J.A., Wünning J.G. (1997): Flameless Oxidation to reduce thermal NOformation, Prog. Energy Combust. Sci., 23, 81-94, 1997

[3] Wünning J.G.(2005): Flameless Oxidation, 6th int Symposium HTACG – Essen, 17-19October, 2005

[4] WS Patents EP 0463218 and EP 0685683 (1990)

[5] Milani A., Wünning J.G. (2002): Design concepts for radiant tubes - Millennium Steel2002