� Energy savings from utilisation of exhaust gas heat

� New applications for proven technology

� Decentralised hydrogen productionfrom natural gas

� Coal dust combustion low in harmful substances

� New possibilities for utilisation of lean gas

peaks do not occur, and a constant furnace temperature of up to around

1,400 °C can be maintained with low nitrogen emissions.

Thus, it is no surprise that now, 15 years after the first commercial

FLOX® burner was commissioned, there are many varieties avail-

able, for a broad range of applications. This technology is used es-

pecially in the steel industry and in burners for heat treatment.

The potential of FLOX® technology is, however, far from exhaust-

ed. Various projects, some of which sponsored by the German Fed-

eral Ministry of Economics and Technology (BMWi) and the Euro-

pean Union, are exploring the utilisation of lean gases and biofuels,

power plant technology, and combined heat and power generation.

Devices using FLOX® technology have also been developed for the

reformation of natural gas to hydrogen. Compared to burners with

no preheating, it is thus possible to reduce the fuel consumption by

up to 50%.

W hat at first sounds like a paradox – flameless combus-

tion – is being implemented in an increasing number of

industrial high temperature burners. With a sophisti-

cated mixture of combustion gas, combustion air, and recirculating

exhaust gas, it is possible, with so-called FLOX® burners, to main-

tain combustion without flame. The decisive advantage is that even

with high furnace temperatures, almost no nitrogen oxide is formed.

The process also enables efficient utilisation of fuels, as the exhaust

gases can be used for preheating the combustion air.

The nitrogen oxide emissions alone represent a significant challenge

in high temperature processes. Often, the threshold values defined in

the German Technical Instructions on Air Quality Control (TA Luft)

can only be achieved with costly downstream purification of the exhaust

gases. Atmospheric nitrogen oxidises to a notable extent in the hot

zones of the flame front. In flameless combustion, these temperature

Ceramic radiant tube in test laboratory

Flameless combustion

Fig. 1

� How do FLOX® burners work?

� A beginning in the industry

With FLOX® burners, combustion gas andcombustion air flow into the combustionchamber at a high flow rate and unmixed(fig. 2b). The main difference to convention-al burners is the very intensive internal recir-culation of the exhaust gases in the combus-tion chamber, and the mixing of these gaseswith the combustion air. This, and the de-layed mixing of air and combustion gas, pre-

vents a flame front from forming. With suffi-ciently high temperatures of at least around800°C, the fuel oxidises throughout the entirevolume of the combustion chamber. Thiscauses very homogenous temperatures. Theformation of thermal nitrogen oxide, whichprimarily takes place at the flame edge withits high peak temperatures, is prevented.With the more uniform distribution of tem-perature, not only do the nitrogen oxideemissions decrease, but it is also possible tomaintain a higher average combustion cham-ber temperature. With conventional burners, the combustionprocesses are usually monitored with UVinstruments. In flameless operation, this isnot possible. Instead, the temperature of theusable space is measured. If the thresholdtemperature is exceeded, ignition and com-plete combustion are ensured.

Fig. 3: Burner in flame operation (left)and in FLOX®® operation (right)

The early 1990s saw the development ofrecuperative FLOX® burners, with whichthe hot exhaust gases preheat the combus-tion air in the burner via a heat exchanger,and of regenerative burners, with which thewaste heat is used via intermediate storage.Now, the user can choose from a broadrange of mass-produced products for variousapplications.

Steel industryThe steel industry is a pioneer in the imple-mentation of FLOX® burners. Most burnersare implemented here, in the metal industry,and in heat treatment furnaces. A decisivebreakthrough came with the developmentof ceramic recuperative burners, which areconsiderably more temperature-resistantthan burners made of chrome-nickel steels,and which have a long service live. Withthese burners, high temperature processeswhich previously required electricity can alsobe fuelled economically with natural gas.The ceramic burners not only have advant-ages at high temperatures, but also when it comes to using as much waste heat as

possible at lower temperatures. To date,several thousand ceramic burners have beeninstalled.

Glass and ceramics industryIn glass production and processing, signifi-cant nitrogen oxide emissions arise, whichentail high costs for secondary exhaust gaspurification. Therefore, there is consider-able interest in low-emission productionprocesses. Yet at the same time, there are reservations with regard to deviationfrom proven production methods. Manyproduction details are based on empiricalexperience, which upon alterations must bereacquired, entailing considerable effort. Similar hurdles must be crossed in theceramics industry. Here too, there is consid-erable potential for implementation, butsimultaneously there are reservations when itcomes to altering highly integrated processes.In both cases, a great deal of effort must beinvested in research and persuasion in orderto depart from the beaten track.

Fig. 5: Hundreds of ceramic FLOX®® burners fire an annealing furnace

Combustion air blastFurnace wall

Exhaust gas

Recuperator

Fuel

Combustion air

Exhaust gasFLOX® operation ϑ > 800°CFlame operation ϑ > 800°C

Fig. 2: Starting the burner in flame operation (a) and FLOX®® operation (b)

Fire without flame

In 1989, experiments with a recuperativeburner lead to a surprising discovery: atfurnace temperatures of 1,000 °C, and withair preheating of 650 °C, the monitoringdevice for the burner flame ceased to indi-cate a signal, and no flame could be heard.Nevertheless, the fuel burnt completely. Thecarbon monoxide content of the exhaust gaswas below 1ppm, and the NOx emissionswere so low, that it was initially assumedthat the measuring device was malfunction-ing. However, the combustion was stableand uniform, without a flame havingformed. The phenomenon was named"flameless oxidation", or FLOX® for short. Infurther experiments in the research projectsponsored by BMWi, the researchers wereable to determine the conditions underwhich flameless combustion is possible (fig. 4). It soon became evident that theprocess enabled considerable energy sav-ings. Low emission values were alsoachieved when using exhaust gas heat forintensive combustion air preheating. Thus,BMWi approved a multitude of follow-onprojects. The discovery ultimately led to acombustion process, patented worldwide,with numerous possibilities for implementa-tion.

1,600

1,200

800

400

0

Temperature in °C

Recirculation0 2 4 6 8

FLOX®

Stable flameUnstable flameNo reaction

Fig. 4: The decisive parameters:temperature and exhaustgas recirculation

2 BINE projektinfo 07/06

� And new fields of application...

BINE projektinfo 07/06 3

Lean gases can be utilisedVarious research projects are examininghow landfill gas, coalmine gas, biogas,sewage gas, product gas, or wood gas, canbe used in an energy-efficient manner withlow emissions. This work often involveslean gases with a low energy content com-pared to natural gas. Furthermore, in manycases the fuel quality often fluctuates in theshort term, e.g. with gases from productionprocesses, or over a longer term, e.g. withlandfill gases. Today, lean gases are often stillburnt off, so that climate-damaging methanedoes not escape into the environment. FLOX® burners deal with low-energy gasesbetter than conventional burners do, andwith flameless oxidation, fluctuating fuelquality does not immediately cause prob-lems with flame stability. In the European research project BIO-PRO,burners are being developed for biorefineries.These are systems which convert biomassinto fuels, chemicals, and even foodstuffs.In these processes, solid, liquid, and gaseousresidues arise. To enable combustion of solidfuels as well, researchers at the University ofStuttgart are investigating the use of FLOX®

burners with various pregasifiers.For clean combustion of lean gases, the con-version of the nitrogen compounds con-tained in many lean gases must be keptwithin tight limits. In the laboratory, it hasbeen shown that this is possible. Now itneeds to be confirmed on an industrialscale. To this end, alongside constructiveapproaches, the development of a new typeof burner controller is being promoted. If this project succeeds in producing the re-quired confirmation, this technology can beoffered on a commercial basis in the future.Together with the University of Bochum,the Gaswärmeinstitut (Gas-Fired ThermalEnergy Institute) in Essen, Germany, isdeveloping new burners and combustionchamber concepts for micro gas turbines.Here, it is possible to use lean gases whichonly have a third of the heating value of nat-ural gas. In so doing, the levels of nitrogenoxide and carbon monoxide fall well shortof the TA Luft threshold values for the com-

bustion of landfill gas, biogas, and woodgas in gas turbines.

On-site hydrogen productionTo date, hydrogen is mainly produced in-dustrially. Consumers satisfy their require-ments by means of pressure cylinders or hy-drogen pipelines. Now, natural gas can beused to cover these requirements on site.This is made possible by modular reform-ers, which are approximately as efficient aslarge-system reformers are. The reformersare based on conventional steam reformingfrom natural gas. The temperatures ofaround 800 °C which this requires are pro-vided by FLOX® burners. Due to a new typeof heat management, the reformers achieveenergy conversion efficiencies of over 80%.In the future, this technology could enablebroad implementation of PEM fuel cells.Two product series have been developed todate. One version for industrial implemen-tation is already marketable, with hourlycapacities of 50 to 400 Nm3. The reformerhas a modular structure. The capacity canbe adapted to the application in an econom-ical manner via the number of reformertubes. Several reformers have already beencommissioned and, for example, are sup-plying a hydrogen filling station at MunichAirport. A second series, which is compact, shouldenable the use of fuel cell stacks for domes-tic power supply. Here too, the prototypesare already achieving conversion efficiencyof over 80%. Before a market launch, work

is still being done towards maximisation ofefficiency, operational safety, and optimisa-tion of gas purification.

Fig. 6: Mini CHP plant with Stirling motorMini combined heat

and power plant

For decentralised combined heat and powergeneration, SOLO Kleinmotoren GmbH hasdeveloped a marketable Stirling motor. Thisengine, with an electrical capacity of 9kW, issuitable for hotels, swimming pools, schools,hospitals, residential buildings, or businesses.It can be operated using different heat sources.For natural gas operation, a FLOX® burnerwith air preheating is provided.

Fig. 7: Fuel cell system with FLOX®®

reformer for domestic energy supply

� Clean coal-fired power plants with coal dust combustion In the European research project FLOX®-COAL, the University of Stuttgart is devel-oping a pilot system for flameless combus-tion of pulverised coal.Experiments at the University of Aachenshow that this technology can also be devel-oped for pressurised pulverised coal firingin coal-fired power plants. Here, the coaldust burns at pressures of up to 20 bar, andat temperatures of up to 1,450 °C. Mea-surements from a test system show consid-erable reduction of NOx concentrations in

comparison to a conventional referenceburner. For Rhenish lignite, the NOx valuesdrop by around 20%, and for Polish blackcoal by around 65%. Pressurised pulverisedcoal firing is a key technology for the devel-opment of highly efficient gas and steampower plants, also for operation with blackcoal. This could allow the efficiency of ablack-coal-fired power plant to be increasedfrom today's level of around 45%, to muchmore than 50%.

Rhenishlignite

Polish black coal800

400NO

x

mg/m3H

Refe

renc

e bu

rner

Flox

Flox

Refe

renc

e bu

rner

0

Fig. 8: Reduction of nitrogen oxideemissions with FLOX®® burners

PROJECT ADDRESSES▼

• WS Wärmeprozesstechnik GmbHDornierstraße 14D-71272 Renningen

▼ ADDITIONAL INFORMATION

Literature• Wünning, J.A.; Wünning J.G.: Brenner

für die flammlose Oxidation mit geringer NO-Bildung auch bei höchster Luftvor-wärmung. In: GasWärme International.Jg. 41 (1992), H. 10, S. 438-444

• Wünning J.G.: Flammlose Oxidationvon Brennstoff. Aachen : Mainz, 1996. ISBN 3-89653-053-4. Zugleich Disserta-tion, RWTH Aachen, 1995

• Wünning J.G.: Flameless Combustionand its Applications. In: Gas TechnologyInstitute, (USA) (Hrsg.): Natural GasTechnologies. Orlando (USA), 30.Jan. - 02. Febr. 2005. Proceedings. 2005

Images• Figs. 1, 3, 5, 7: WS Wärmeprozesstechnik

GmbH• Figs. 2, 4: Nach Erdgas.report 1/03 VNG

– Verbundnetz Gas Aktiengesellschaft• Fig. 6: SOLO Kleinmotoren GmbH• Fig. 7: Zentrum für Sonnenenergie-

und Wasserstoff-Forschung (ZSW)

Service• Additional information such as

literature and internet links are available online from BINE atwww.bine.info (Service/InfoPlus) in German.

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4 BINE projektinfo 07/06

� Future prospects

In industry, FLOX® burners are primarily used because they entail low nitrogen oxideemissions, even with intensive preheating of combustion air. Thus, exhaust gas heatcan be utilised, even with high process temperatures, which reduces the fuel require-ment by up to 50%. This technology offers further advantages: the combustion cham-ber has a more uniform temperature distribution, the burner's thermal load and noiseemissions are lower, the burners are often more reliable, and they have lower qualityrequirements of the combustion gas. Climbing energy prices and strict emissions re-quirements will serve to promote the implementation of FLOX® burners further. Newdevelopments, for instance in combustion processes for glass-melting tanks, broadenthe fields of application for this technology.Globally, interest in flameless combustion is growing outside the traditional fields ofapplication. In research, there is a particular focus on electricity generation, using con-ventional energy sources and also using energy sources which have as yet not seenmuch use, e.g. lean gases.With a clean and reliable technology, lean gases can be used for generation of heatand electricity much more often than they have been to date. If used consistently, it ispossible to increase the utilisation of biomass in Europe by 50%. At the same time,the nitrogen oxide emissions from biomass combustion could be decreased by 76,000tonnes per year. Flameless combustion can assume an important role in power plant technology. WS-Wärmeprozesstechnik GmbH is researching the implementation of FLOX® in gas tur-bines in cooperation with DLR, the Deutsche Zentrum für Luft- und Raumfahrt e.V.(German Aerospace Centre). By far the greatest potential for CO2 reduction is in efficient coal combustion. In theinternational research project FLOX®-COAL, a pilot system has been realised for thecombustion of pulverised coal. Here, with the flameless burner, extremely low NOxemissions are achieved. Also of significance is that flameless combustion can be animportant component of a CO2-free fossil power plant. In the new joint project OXY-COAL-AC, researchers from various tertiary institutions and companies are nowworking on the implementation of this idea. The individual projects are sponsored bythe German Federal Ministry of Economics and Technology (BMWi), the North Rhine-Westphalia Ministry of Science and Research (MWF), and industrial partners. Initialprojects were approved as part of COORETEC, the "research and development conceptfor low-emission fossil-fuelled power plants" initiated by BMWi at the end of 2003.

PROJECT ORGANISATION

■ Project FundingFederal Ministry of Economicsand Technology (BMWi)D-11019 Berlin

Project Management Organisation Jülich (PTJ)Research Centre JülichDr. Claus BörnerD-52425 Jülich

■ Project Number0328857A,B,C,D,E0326939A,B,C0326940A,B,C,D0327274A0327341A,B,C

IMPRINT

■ ISSN0937 – 8367

■ PublisherFIZ KarlsruheD-76344 Eggenstein-Leopoldshafen

■ ReprintReproduction of this text is permitted providedthe source is quoted and a complimentary copyis provided to the publisher; reproduction ofthe images contained in this newsletter requires the prior approval of the copyright owner.

■ EditorDr. Franz Meyer

FIZ Karlsruhe GmbH, Büro BonnKaiserstraße 185 – 197D-53113 Bonn

Tel.: +49 228 92379-0Fax: +49 228 92379-29

bine@fiz-karlsruhe.dewww.bine.info

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