DILUTED COMBUSTION TECHNOLOGIES · IFRF Combustion Journal - 3 - Milani and Saponaro Article No 200101 February 2001 3 1. INTRODUCTION The topic of Diluted Combustion Technologies

IFRF Combustion Journal Article Number 200101, February 2001 ISSN 1562-479X

IFRF - Combustion Journal – 1999 - 2001 Email: [email protected]

DILUTED COMBUSTION TECHNOLOGIES

A. Milani and A. Saponaro

Corresponding Authors:

Dr Ing Ambrogio Milani Senior Scientist, formerly with CSM – Experimental Station on Combustion Sal. Inf. S. Barnaba 10 I 16136 – Genova ITALY Telephone: +39 010 215068 E-mail: [email protected]

Dr Ing Alessandro Saponaro Technical Manager General, Centro Combustione Ambiente Termosud Via Milano, km 1,600 I 70023 - Gioia del Colle ITALY

Telephone: +39 080 9980246 Email: [email protected]

IFRF Combustion Journal - 2 - Milani and Saponaro Article No 200101 February 2001

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ABSTRACT

The topic of Diluted Combustion Technologies is reviewed from early developments

some 10 ten years ago up to date, for the sake of “design and process engineers”.

Recent efforts of the academic community to supply interpretation and quantitative

modelling of the occurring phenomena are briefly described. Diluted combustion with

high temperature reactants, in particular with gas firing, has been developed since a few

years for industrial use exhibiting ultra-low NOx emissions and suitable properties for

extremely high air preheating, and therefore substantial energy savings and process

advantages. Successful applications to high temperature furnaces, mainly in the steel

industry, are illustrated to show several advantages obtained from applying the new

combustion technologies. This story is a good lesson based on conscious, scientific

approach, backed by permanent R&TD efforts. These examples should be pursued

vigorously if concrete measures to cope with the Kyoto engagements to combat air

pollution are to be brought about in reality.

Key Words:

Diluted firing, flameless combustion, high temperature air, honeycomb

regenerative burners, kinetic control, extremely low NOx and pollutant emissions.


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1. INTRODUCTION

The topic of Diluted Combustion Technologies often referred to as flameless or FLOX®

or MILD or HiTAC or DFI etc, is increasingly important and has now been debated in

IFRF and international meetings. Not only in Japan, which has pioneered large

industrial applications, but in Europe and USA as well. Just to quote some specialised

meetings in the past couple of years: a Seminar in Stockholm [1], two international

symposia in Taiwan [2] and in Yokohama [3], and a comprehensive session within the

Italian Flame Days in Rome [4]. Quite a few consolidated industrial applications show

impressive and consistent advantages of energy savings and abatement of pollutants, so

that some specialists speak of a revolution, at least in the conceptual design of furnaces

at high temperature [5].

However, the basic chemical-physical phenomena are far from being fully understood

and are much less known to process design engineers. There is even no general

agreement on the nominal definition among specialised investigators, both from R&TD

companies and academicians, as proven by the abundance of acronyms quoted above.

The acronyms were intended to stress some features for patent purposes that often cover

particular embodiments.

Recent developments of basic understanding and progress of pilot demonstration work

[6] promise very attractive breakthroughs in new, clean and energy effective processes.

However, technical design solutions of large plants in heavy process industries still lag

behind and do not take into due consideration the above quoted very encouraging steps

forward and success of demonstration projects.

Therefore, the present article is mainly devoted to reviewing the subject for the sake of

design engineers in order to popularise the new combustion technologies. It emphasises

basic facts and summarises some theoretical background first. Industrially proven

examples are then described, aiming at showing the big potential advantages for


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existing high temperature processes. The potential advantages for high temperature

furnaces are far from being exhausted: therefore, a few suggestions are discussed

concerning processes considered to be in the pre-competitive stage and mature for

prototype development.

2. DILUTED COMBUSTION

2.1 History

Flame dilution techniques have been investigated first to abate NOx emissions [7].

Dilution means that fuel and oxidiser are mixed “locally” with a ballast of inert gases

before they react so that the oxygen concentration in the reactants is substantially

reduced with respect to the 21% of the standard oxidising air. Recirculation of flue

gases or products of combustion from inside the combustion chamber carry out the most

common dilution mechanism.

Fig. 1: Diluted or flameless combustion.

An increasing amount of recirculation of hot flue gases at 1200 °C to vitiate the

combustion air has been assumed to compute the diagram of Figure 1. While the flame

is sharply enhanced with an even moderate O2 enrichment (the opposite of dilution), it


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is impossible to stabilise a flame front attached to the burner as soon as the O2

concentration is depleted below ≈ 17% (corresponding to ≈ 20-30% of flue gas

recirculation).

21% O2 concentration, which is an accident of nature in the earth atmosphere, is close

to the flammability limit and this affects burner design very much, because of the basic

requirement of a “steady flame front” for safety reasons. However, if the combustion

chamber is above self-ignition temperature, it is possible to depart from the strict safety

constraints due to explosion risks. For continuous furnaces, steadily above ≈ 850 °C, the

present safety rules already dispense with the use of flame detectors because anyway

there is no danger of explosion any more.

Therefore, above self-ignition threshold, it is possible to enhance dilution much more

than usual and a new combustion regime or mode has been experimentally identified:

combustion is complete without appreciable unburned traces and is characterised by the

absence of a steady flame front, either attached to or lifted from the burner. This

combustion mode is more and more popularly identified with the name flameless

combustion: it is possible at low oxygen concentration and at temperatures in excess of

self-ignition, i.e. in the region labelled “FLOX® mode” in Figure 1. FLOX® is a

trademark of WS GmbH and stands for “FLameless OXidation” [8]. While a flame front

is always associated with strong gradients, almost a discontinuity of temperature and

chemical species that separates reactants from products, the flameless combustion

pattern is distributed on a much larger volume within the combustion chamber, so that

one can speak of volumetric combustion regime instead of flame front combustion

regime.

The practical difference between conventional flame and diluted or flameless firing

modes may be spectacular: compare the two pictures in Figure 2, referred to 1500 kW

natural gas fired flames, and the two pictures in Figure 3 obtained on a pilot plant fired

with heavy fuel oil [9]. By flameless mode, the furnace is almost transparent, typical

combustion roar disappears, as there is no flame front any more, and reactions are


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brought about according to chemistry paths quite different from the conventional,

burner-stabilised flame [10,11,12]. The air vitiation with an inert gas component implies

a limit to the maximum adiabatic temperature and therefore the upper temperature in the

FLOX® region is inherently limited (see Figure 1). There is no wonder that the

pollutant formation and the heat flux distribution are quite different for the conventional

and the diluted firing mode and this may be exploited for practical purposes.

Fig. 2: Comparison flame/flameless firing natural gas - CSM test furnace


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Fig. 3.a: Conventional and flameless combustion of heavy fuel oil. Prototype swirl burner for heavy fuel oil - power 100 kWth. Oxidizer: standard atmospheric air (21% O2) preheated at 250°C.

Fig. 3.b: Conventional and flameless combustion of heavy fuel oil. Same prototype swirl burner for heavy fuel oil - power 100 kWth. Oxidizer: vitiated air with 12 % O2 preheated at 500°C (flameless combustion).

The existing know-how derives basically from pioneering pilot trials and industrial

works carried out in Japan, first with Tokyo Gas and then by the late Mr Tanaka [13],


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and in Germany, in particular by Dr Wünning [14]. This work has opened up far

reaching perspectives for new firing technologies not only for high temperature

furnaces, where many industrial plants have now been successfully retrofitted or built

from greenfield, but also for power generation, gasification and several other

combustion processes [15].

2.2 High velocity burners

Figure 4 reproduces the scheme of a “high velocity” burner typical of many furnace and

boiler applications. Combustion air is usually preheated and entrainment of internal re-

circulation of hot flue gases at process temperature from the combustion chamber is

carried out by the kinetic energy of the jet (or jets, depending on details of burner

design).

Fig. 4: High velocity burners.


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High velocity is not intended to be an absolute value, but certainly implies forced air

draught, high momentum of fuel injection, and high linear velocity of hot air. The

emphasis is on fluid dynamic entrainment and mixing of flue gases (exhausted product

of combustion containing N2+CO2+H2O+residual O2) from the combustion chamber. If

a flame retention device is provided, a conventional flame front, attached to the burner

or stabilised into the burner tile itself can be set up (Figure 4, conventional flame).

If fuel and air are injected directly into the combustion chamber, according to several

possible schemes [16] and avoiding possible formation of a stable flame front (see e.g.

Figure 4, flameless firing), the jets of the reactants are diluted with surrounding flue

gases before they mix and burn. Diluted or flameless firing is then occurring, provided

process temperature is above self-ignition. This general scheme applies as well for the

case of vitiated air inlet, as in the example reported in Figure 3 (see bottom: O2=12%)

up to pure oxygen firing [17].

2.3 Modelling

In conventional burner firing, the bulk of the chemistry is already over just downstream

of the flame front and kinetics proceeds much faster than mixing; diluted firing, instead,

is controlled both by chemical kinetics and by mixing. Self-ignition temperature must

be attained locally in order to ignite reactions and furthermore there is competition

between oxidation and pyrolitic reactions due to very diluted conditions. A theoretical

analysis of methane oxidation in very diluted O2 conditions (O2≈ 5%), based on detailed

kinetic schemes applied to a well-stirred reactor (WSR) supplies an interesting

suggestion [9].

Figure 5 shows potential reaction regimes depending on the ratio C/O (0.25 being

stoichiometric and 1 being fuel rich as a reformer). It displays TWSR vs To (inlet

temperature of reactants). Self-ignition and the effect of stoichiometry are clearly

visible. The original paper [9] shows that the assumption of well stirring, and hence of

quite uniform temperature and concentration can only apply if flameless or diluted


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combustion is considered as a sort of staged combustion. In the first stage, fuel

conversion to CO and H2 occurs in diluted, rich conditions. Dilution not only reduces

the maximum attainable temperature, but also depresses the formation of species

responsible of luminous emission from flames. A further air entrainment in the reaction

zone along the burner axis leads to complete conversion of CO and H2 to CO2 and H2O.

Therefore the expression volume distributed combustion instead of flame front

combustion seems appropriate both to experimental evidence and to theoretical

prediction.

Fig. 5: Well Stirred Reactor temperature vs inlet temperature (after de Joannon [9])

Kinetic computations have been carried out in flameless conditions applied to gas

turbines, i.e. at temperatures lower than many furnace processes and at large

equivalence ratios (i.e. at overall excess air factors λ around 2-3). Results of kinetic

codes have put in evidence a large potential of reduction of NOx emission [18] and

details of NO formation via a N2O pathway, as the “traditional” NO formation


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mechanisms are almost suppressed (see § 2.4 below). Predicted NO is not very sensitive

to pressure, but is very much dependent on temperature.

This focuses attention back again onto reliable predictions of computational fluid

dynamics (“CFD” codes): experimental data measured in flame/flameless conditions on

the same test furnace are being obtained at ENEA Laboratories [19] and have been

modelled with FLUENT® [20]. Results show reasonably good agreement with the

experimental data only when the combustion model used (EDC) accounts for finite-rate

chemistry and very poor agreement when the combustion model is based on local

equilibrium (i.e., a single conserved scalar and a “pdf” modelled by a “β-function”).

With the EDC model the predicted outlet NOx is low, in agreement with measurements.

These results confirm the importance of chemistry and mixing in this flame regime;

simulated detailed chemistry with CHEMKIN (using the measured flame

temperatures and compositions) confirms the NO formation pattern via the N2O

pathway, with very little contributions from thermal and prompt -NO.

Further evidence of the importance of kinetics in predicting diluted firing is confirmed

by measurements at IFRF [21].

2.4 NOx emissions

Very high air preheating is the main energy saving measure in furnaces, but produces an

intolerably fast increase of NOx emissions in conventional burners, which puts

contradictory requirements to the designer. The basic discovery of flameless techniques

was the result of research developments aimed at overcoming this constraint [13,14].

Now, the essential difference in the two schemes of Figure 4 is temperature uniformity,

typical of flameless firing that affects NOx emissions quite drastically. Figure 6 reports

several accumulated NOx data in a log scale as a function of process temperature,

assuming very efficient preheating of the combustion air (60-80 % of the process

temperature).


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Fig. 6: NOx emissions vs process temperature.

It may be seen from Figure 6 that diluted or flameless combustion (region labelled

FLOX) may abate NOx emissions by one order of magnitude even with respect to the

best staging techniques for low-NOx, envisaged for natural gas firing. The main reason

for this excellent result stems from the well known circumstance, that thermal NO

formation is extremely sensitive to flame temperature peaks or spikes and these are now

cut away in flameless firing. But also the other known NO formation mechanisms are

positively modified, as prompt NO depends on radicals (that are abundant in a flame

front, but much reduced in flameless mode [11]) and also fuel NO may undergo

reburning effects capable of reconverting NO into N2 species [21,22].

3. HEAT RECOVERY

3.1 Burner integrated recuperator

Figure 7 reports a simple calculation of the thermal efficiency ηth (% of LHV

introduced with the fuel: lambda = λ air factor; risc = ξpre air preheat) of a well-stirred

furnace at temperature Tproc , assuming air preheated at temperature Tair = ξpre x Tproc.


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Conventional reheating or heat treating furnaces typically preheat at no more than ξpre ≈

40%, leaving a margin of 20 to 30 % of thermal efficiency with respect to an ideal air

preheater. Similarly, very effective preheating allows very high excess air firing without

negatively affecting the energy balance (compare the two red curves referring to λ = 2.5

in Figure 7). Therefore, high air preheat is desirable, but efficiency above ξpre ≈ 60%

implies combustion air temperatures about 800 – 1200 °C at the burner, so hot that it

could not be handled in heat exchangers or manifolds external to the furnace.

0 ,0 0

0 ,1 0

0 ,2 0

0 ,3 0

0 ,4 0

0 ,5 0

0 ,6 0

0 ,7 0

0 ,8 0

0 ,9 0

1 ,0 0

0 2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0 1 2 0 0 1 4 0 0 1 6 0 0

P ro c ess tem p e ra tu re °C

Ther

mal

effi

cien

cy

la m b d a = 1 .1 ; r is c .= 1 0 0 %la m b d a = 1 .1 ; r is c .= 4 0 %la m b d a = 1 .1 ;risc .= 0 %la m b d a = 2 .5 ;risc .= 1 0 0 %la m b d a = 2 .5 ;risc .= 0 %

Fig. 7: Thermal efficiency vs process temperature.

The solution has been found by decentralising heat recovery and integrating the heat

exchanger into the single burner, by extracting hot flue gases in counter-current to the

air, thereby suppressing external hot air manifolds and piping. This concept of the

burner integrated heat recovery has been followed by several manufacturers, but has

been hindered for a long time by the excessive flame peak temperature conducive to

intolerable NO formation and thermal stress of construction materials [23]. Flameless or


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diluted combustion techniques provide a sound solution to this problem and therefore

may be considered as a necessary pre-requisite for high air preheat. Suppression of the

flame front attached to the burner is also advantageous to reduce local stress of ceramic

materials and components for high temperature duty, such as SiC nozzles, heat

exchangers, honeycombs etc.

3.2 Regenerative firing

A quite effective embodiment of the counter-current flue gas/air heat exchanger rests

upon the regenerative principle, based on a couple of twin ceramic thermal capacities or

solid beds.

Fig. 8: Regenerative Burners.

With reference to Figure 8, flue gases flow through the RHS body heating the bed while

air flows into the LHS burner (firing) thereby cooling the bed; the situation is reversed

as in the lower figure with cyclic flow inversion. The principle has been in use for a

long time for large hot processes (e.g. glass melting), but the new idea is to integrate the


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regenerative bed (ceramic beads, honeycomb etc) into the burner body. By reducing the

inversion time to a few tens of seconds (typically 10-40 seconds with respect to 20

minutes in large centralised plants) it is possible to reduce the size of the heat exchanger

dramatically, while maintaining very high air preheat efficiencies in the order of ξrisc ≈

80-90%, at least for clean flue gases [24]. Ceramic regenerators are now available on

the market at an affordable price and in compact shapes.

4. APPLICATIONS TO HIGH TEMPERATURE FURNACES

4.1 Steel Industry

Small natural gas burners (≈ 20-200 kW) with integrated heat recovery have been

developed in particular in the domain of ferrous metallurgy [16]. They are designed for

both flame and flameless operation in order to be used for heating the furnace up to the

self-ignition temperature (≈ 850 °C); above the safety, self-ignition threshold, it is

allowed to switch to FLOX® mode simply with electro-valves (Figure 9).

furnace wall exhaust

air gas

recuperator eductor air

two gas valvesfor flame and FLOX mode

common valve for combustion and eductor air

Fig. 9: Auto-recuperative” burner for flame/flox firing.


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A well tested and widely applied solution concerns the radiant tubes used in many heat

treatment furnaces: combustion occurs inside the tube that radiates to the stock avoiding

any contact or contamination of flue gases with the stock surface. Best results are

obtained with geometry’s that allow internal recirculation of flue gases (Figure 10);

these more complicated arrangements allow very good temperature uniformity together

with energy savings and very low NOx emissions.

Fig. 10: Radiant tubes: recirculating and non recirculating geometries.

Burners for large reheating furnaces (e.g. ladle pre-heating, car-bottom forge furnaces,

walking beam furnaces etc) must be robust and integrated into the required harsh plant

environment. Their average size spans from ≈ 500 kW up to ≈ 4000 kW. Most recent

burners use honeycomb beds and are by far more compact and efficient than the old

ceramic bead packages used for quite a few years. An example of such a regenerative

couple of recent design is reported in the scheme of Figure 11.


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Fig. 11: Scheme of a couple of regenerative HRS burners.

4.2 Down-sizing of continuous furnaces

A commonly adopted lay-out for continuous processes of heat treating metals or of

sintering ceramic pieces is reported in Figure 12: the incoming stock of material goes

through a tunnel furnace equipped with several burners in counter-current with the flue

gases. It is common practice, especially in ferrous and non-ferrous metallurgy, to

preheat the combustion air in a central heat recuperator (scheme at the LHS in Figure

12). This is typically a metallic tubular heat exchanger that should not be overheated

above ≈ 800 °C. For this reason, the initial furnace zone is “black” or passive, i.e.

without burners. This provides energy recovery by transferring heat to the in-flowing

stock and reduces the temperature of the flue gases within limits acceptable by the

central recuperator at the chimney. In the schematic on the RHS of Figure 12, the

furnace is equipped with burner integrated heat recovery. Unlike the conventional case,

cold air is supplied to the burners and each burner extracts locally its own flue gases,

which are sucked at low temperature (order of ≈ 100-200°C) by an eductor.


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With this arrangement, heat transfer by convection is greatly enhanced (i.e. the heat

exchanger is made more compact) and combustion air can be preheated, up to

temperatures very close to process temperatures, at the right moment (i.e. at the burner

nozzle). The initial black zone can be avoided and the furnace “set-point profile” can

start with the hottest temperatures at the entrance: a simple calculation, as shown in

Figure 12, estimates this productivity increase typically around 25% or more (or an

equivalent length reduction for the same production).

FLUE GASES

NAT. GAS

AIR

INLETOUTLET

°C

SET POINT TEMPERATURES

meters from inlet

Fig. 12.a: Centralised (LHS) and burner-integrated heat recovery.


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≈15% of FLUE GASES FLUE GASES

FLUE GASES

NAT.GAS

AIR

INLET

OUTLET

°C

meters from inlet

SET POINT TEMPERATURES

Fig. 12.b: Centralised (LHS) and burner-integrated heat recovery.

This productivity increase or down-sizing has positive consequences on the investment

costs and may be a decisive factor in revamping projects aimed at concentrating

production into fewer, more productive lines. In fact, this has been an important driving


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force behind the three demonstration projects described below that refer to ferrous

metallurgy.

4.2.1 Radiant tubes

The first example concerns single ended ceramic radiant tubes (see Figure 10) operated

in FLOX® mode, installed on a continuous annealing furnace for electric steel strip

under hydrogen atmosphere in the Works Bochum (Germany) of Thyssen-Krupp. The

SiC radiant tube equipped with an auto-recuperative gas burner [25], is a

technologically advanced piece of equipment, expensive and fragile. However it allows

higher performance as far as maximum temperature and good uniformity are concerned

and also it requires less maintenance thanks to better stability of ceramics with respect

to metal. The excellent NOx emissions performance due to FLOX® operation and the

advantages of burner decentralised heat recovery make the rest, so that these proven

advantages have outweighed higher costs. After successful field experience with

revamping a first annealing furnace, further units have been built and put into operation

with economic benefits.

4.2.2 Annealing furnaces

The second example concerns continuous annealing furnaces operated with direct flame

firing. In the first case the stock consists of a stainless steel strip, to be processed for

subsequent cold rolling. The furnace is part of an annealing and pickling line in the

Works of Acciai Speciali Terni (Terni – Italy), which has been rebuilt anew four years

ago with a project initially co-sponsored by the EC (THERMIE program) with CSM and

WS as partners [26]. The furnace consists of two tunnel shaped combustion chambers,

with a central chimney (Figure 13), while the strip is supported by external rollers and

the combustion equipment includes more than 60 auto-regenerative “REGEMAT®”

burners (200 kW each – see details in [27]) installed all along the side walls.


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Fig. 13: Annealing furnace for stainless strip – Terni works.

The REGEMAT® is a compact unit that includes the couple of regenerating beds into a

single unit (Figure 14) and looks therefore much more complicated than usual burners.

The burners can be operated in flame or in FLOX® mode and are all connected in

parallel to the supply lines for combustion air and for flue gas extraction. They are

controlled by means of on-off control and sequential firing routines, which allow

distributing firing on several flames in parallel. Zone control is carried out by

alternating “on” and “off” time intervals, computed by the control loop; a much better

accuracy and response speed are obtained with respect to the traditional modulated

control, whereby air and gas flow-rates are reduced in proportion to the thermal

requirement.

This advantage of the sequential firing at full burner load, together with the favourable

characteristics of the flameless combustion, make possible a superior thermal

uniformity, which is reflected in better quality of the stock. This feature is a main item

for many applications and may also have important outcomes on the final evaluation of

running costs of the final product. The Terni furnace has been operating satisfactorily


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for many years, although it must be kept in mind that the novel burners and the related

combustion equipment are more complex and maintenance demanding than

conventional equipment.

Fig. 14: Two REGEMAT® burners on the side walls.

Following this successful demonstration project, a similar continuous heat-treating and

reheating furnace has been designed and built from Greenfield in the tube

manufacturing Works Pietra (Brescia - Italy). This is a walking beam furnace

commissioned in summer 2000, and the thermal uniformity obtained with the new

burners and the computer based sequential firing is excellent, while the low NOx and

energy savings performance are very good as expected [28].

4.2.3 Reheating furnaces

The third example concerns the revamping of a walking beam furnace for slabs of

capacity 230 t/h in the steelworks NKK in Fukuyama [29]. It has been equipped with

large regenerative “HiTAC” NFK burners based on a very efficient honeycomb bed,

which is far superior to traditional ceramic beads. As a result, the new furnace has an

increased capacity and has been able to replace two adjacent old furnaces with the same

layout, i.e. the same length between inlet and outlet roller ways. Thermal uniformity as


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well as accuracy and speed of control are claimed to be much superior to traditional

firing and control systems. Also the energy savings and the NOx performance are very

similar to the heat treatment plants quoted above. If the costs of investment are referred

to the enhanced productivity, the fixed costs become competitive with the current

technology of central air recuperator. Therefore the new technology becomes attractive

not only for increasing productivity of existing plants (revamping) but also for green

field construction and the system does not require any pay-back time based on future

fuel savings.

4.3 Conclusions

The following Table summarises the comparison between the new regenerative,

flameless technology and the conventional central air preheater and traditional burners

in continuous counter-current furnaces. These data are based on the results of the quoted

plants and consolidated by years of satisfactory operation and refer to a thermal power

per unit spanning more than two orders of magnitude, i.e. between ≈ 25 kW for radiant

tubes up to ≈5000 kW for slab reheating burners.

Table – Application of regenerative burners to continuous counter-current furnaces: comparison with a conventional central preheater solution (state of the art)

NOx emissions Abatement down to ≈ 10 –25 %

Fuel savings From 15 to 30 % or more

Productivity increase (same lay-out) ≈ 25 % or more

Specific investment cost per unit production Approximately 10% saving

Thermal uniformity (cross profile) Very much improved


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5. POWER GENERATION

5.1 Steam generators

The diluted or flameless combustion process as described above can be taken into

consideration also for steam generators fired at high temperature, keeping in mind

potential advantages like:

1. Abatement of thermal NOx formation of particulate and of organic micro-

pollutants;

2. Reburning reactions in the bulk of the flame for further NOx reduction;

3. Stable combustion without a flame front even with lean fuels;

4. Limited vibrations and noise due to absence of the flame front;

5. Efficiency increase of combustion and of power generation cycle;

6. Compact design solutions.

From the point of view of industrial application, the NOx reduction in steam generators,

is not sufficient itself to justify alone the required development of new combustion

technologies. Because of the available consolidated techniques like “OFA”, “BOOS”

and “reburning”, that allows operating safely below present limits with relatively

moderate investment costs.

However, the absence of a flame front opens new perspectives for firing those lean fuels

considered difficult for their low calorific value, the low content in volatile matters or

the variable composition. Difficult fuels should also be considered those coming from

renewable sources and recovery fuels, which should contribute in a significant measure

in the next future to the mitigation of the greenhouse gas emission. The adoption of

diluted combustion techniques could also be justified if it could exhibit substantial

advantages in the clean firing of heavy liquid fuels or residues containing organic

components difficult to burn. If the technique could be used to solve the environmental

problem, it might be quite competitive thanks to the very low price of such fuels.


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The advantage mentioned in point (5) above may be justified with a better fuel

utilisation by means of a reduction of the excess air required to avoid CO formation and

of an improved thermal uniformity, which could allow an increase in the maximum

steam temperature. An improved temperature uniformity could provide better heat

transfer distribution and control with a consequent reduction of the total heat transfer

surface; also the abatement of particulate could have the ability to reduce fouling and

therefore the required bank surfaces (point (6) above).

This new combustion regime may be implemented with different techniques, such as:

1. Regenerative air preheaters (ceramic honeycombs to be operated with clean

fuels);

2. Pre-combustors fired with gaseous fuels to preheat the combustion air;

3. Recirculation of combustion products;

4. Use of process outflows suitable as oxidisers or additives in the combustion.

As a result of technical - economic studies and of experimental results obtained with

prototype trials on the pilot facilities of Termosud, the two latter routes seem to be most

promising. Technique 3) above has been tested on a large scale with the 50 MWth test

boiler facility in Gioia del Colle (Italy – [9]), showing the feasibility of embodying

diluted or flameless combustion by acting on the fluid dynamic design of the burner

nozzle only, as it may be observed in Figure 15, comparing flameless and conventional

firing in the test furnace at 30 MWth (i.e. at full power!). The immediate observed

advantages are basically due to the abatement of the unburnt residues in the flue gases,

which allows a lower excess air and therefore a potential energy saving, plus a more

uniform heat transfer rate to the boiler surfaces. The limitation with the present burner

design is due to the turn down available for flameless firing, which requires a dual mode

working facility of the burner (conventional at low power, flameless at full load).


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Fig. 15: Two REGEMAT® burners on the side walls.


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5.2 Repowering

The route labelled as 4) above, can be realised in particular by exploiting the turbine

exhaust gas (TEG) of typical gas turbines, as the chemical and physical conditions of

the exhaust gas are already close to the thermo-chemical conditions required by diluted

or flameless combustion: oxygen dilution and high temperature. The TEG or Turbine

Exhaust Gas is typically at 450- 550°C and contains ≈ 12-15 % oxygen, which is

suitable for use as oxidiser for flameless firing.

The TEG to be used as the oxidiser is available in all repowering projects for old steam

generators fired with natural gas or a gas fuel suitable for a gas turbine. Then the TEG

can be used to burn low-grade fuels instead of recovering just the sensible heat only in

HRSG (Heat Recovery Steam Generators). Part of the TEG can be diverted to a by-pass

branch, where a combustion chamber at temperatures above self-ignition is provided to

burn the low-grade fuel. An industrial example, the co-generation power station ILVA

in Taranto (Italy), producing some 1000 MWel from recovery process gases (plus

natural gas) by-produced by the steelworks [30]. Excess lean blast furnace gas is fired in

flameless mode in the by-pass branch upstream of the recovery steam generator. The

equipment required for this application has been developed for a specific purpose. It has

been observed on pilot scale that above a threshold of about 750-800 °C, the

combustion of blast furnace gas with TEG is complete in spite of the very low calorific

value and that it takes place with a diffuse bluish luminescence without a flame front.

The three industrial HRSGs in Taranto have been working satisfactorily for the last 5

years.

To summarise, the potential advantages of flameless firing techniques in power

generation systems should be examined systematically in order to couple requirements

stemming both from the combustion process and the heat transfer process. Problems to

be investigated include:

• Thermo fluid dynamics of boiler burners with the new techniques;

• Radiative and convective heat transfer processes;


28

• Operating performance at nominal and at reduced load and transient response;

• Thermodynamic cycle optimisation in different working conditions;

• Operating stability and mastering of critical parameters;

• Diagnostics and process control;

• Evaluation of operational safety.

6. CONCLUSIONS IN PERSPECTIVE

A large-scale introduction of new combustion technologies implies large and expensive

plants (in particular in capital intensive, heavy process industries) or a great number of

small distributed utilities (like domestic heaters and micro power generators) and

therefore has to take into account severe economic and market boundary conditions.

The quoted examples concerning continuous furnaces show that the new techniques can

bring about not only savings in fuel and in air pollution burden, but also considerable

productivity advantages in the short-medium term, particularly for revamping projects.

This may be a good incentive to disseminate the technology to similar plants even

outside the ferrous metallurgy.

Public authorities in Japan support this R&D effort aimed at complying with the Kyoto

protocol engagements by means of a massive incentive program for industrial

revamping projects [31]. In the USA, energy is cheap, so that the economic incentive in

fuel savings is limited: however, the most recent DOE documents [32] support

substantial improvements for the combustion technology by the year 2020 and

American equipment manufacturers see a good business opportunity in development of

clean combustion technologies.

Diluted combustion techniques, coupled with efficient heat recovery, show a great

potential for complying with the commitments of European countries within the

framework of the “Kyoto protocol” in order to reduce the release to atmosphere of

greenhouse gas [33]. The new technologies make considerable fuel savings possible, in

the order of more than 25 % for high temperature furnaces, with a corresponding

reduction of CO2 release, and a drastic abatement of NOx emissions with consequent


29

mitigation of acid rain and of greenhouse gas pollutants. These performances have been

demonstrated on a large-scale industry, like steelmaking, which alone amounts to ≈ 20%

of the primary energy consumption in industry.

Apart from consolidated applications, the flameless combustion shows a great potential

in particular for high temperature furnaces and processes in the glass, ceramic,

petrochemical industry, small natural gas users etc. In the sector of power generation, of

combustion and gasification of low grade fuels and of waste incineration, considerable

advantages concerning air quality and plant compactness could be realised, although all

these potential applications require modifications or innovations of the conceptual study

of the equipment/process with respect to conventional design, so drastic that they may

exceed the industrial risk margins of a single manufacturer.

Therefore, further effort in Research (in particular fundamental work, as the basic

mechanisms are not yet fully understood) and in Technical Development (also in mature

sectors neglected for substantial innovation) are required in order to establish

applications that combine environmental advantages with improved economics.

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Documents

DILUTED COMBUSTION TECHNOLOGIES · IFRF Combustion Journal - 3 - Milani and Saponaro Article No 200101 February 2001 3 1. INTRODUCTION The topic of Diluted Combustion Technologies