Flameless Oxidation Technology - Italian Section of the Combustion

Flameless Oxidation Technology
A.Milani, J.G.Wünning
WS Wärmeprozesstechnik – Dornierstr 14 – 71272 Germany WS
Abstract Flameless combustion is the most significant recent advancement in high
temperature combustion technology and has been applied to industrial furnaces with well
proven very low NOx performance and high energy savings. This experience has produced
spin-offs in power generating equipment, from innovative gas turbine combustors to small
reformers for decentralized H2 production, and R&TD of flameless oxidation techniques is
quite promising for new advanced process design.
1. Introduction
The flameless combustion technology applied to high temperature industrial processes stems
from systematic investigations carried out at laboratory scale and from application to large
plants in the steel industry. Results are very satisfactory both for abatement of NOx
emissions and for energy savings; spin-off and on-going R&TD in the field of power
generation is very promising. All this started from looking again at basic principles. A
conventional flame is based upon a mechanism as old as fire discovered in nature many
centuries ago: a stable flame develops from a stationary flame front, that is a few mm thick
layer. Burner design is primarily concerned with the problem of stabilizing the flame front
by means of fluid dynamic devices. Typically a bluff body drives back hot reacting products
that heat up the fresh fuel-air mixture thereby triggering a stable chain reaction. High
gradients of temperature and species concentration in a confined space are required to obtain a
stationary flame.
2. Flameless Combustion
Figure 1 – NOx emissions from steel furnaces
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In a burner stabilized flame most
reactions occur within the flame
front, where local temperature
approaches adiabatic temperature.
In a flameless burner, the flame
front is deliberately avoided and
combustion reactions occur as
fuel and air mix together with
entrained recirculated combustion
products. For the process to
occur, combustion products must
be
above
self-ignition
temperature (> 850 °C for safety).
The reaction rate is determined by
the mixing pattern between three
partners: fuel, air and combustion
products entrained b e f o r e
combustion. In the flameless
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mode temperature profile is determined by the mixing pattern with the recirculated
combustion products and cannot depart much from the temperature of these entrained
combustion products.
In the flame mode, the temperature profile peaks in the flame front close to the burner: this is
conducive to enhanced thermal NO formation [1]. To abate temperature peaks means to abate
thermal 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 steel
industry: the advantage of the flameless technology for temperatures > 850 °C with respect to
the best low-NOx burners designs is quite clear. Figure 2 shows how flame and flameless
mode are implemented in high velocity burners, that are common in heat treatment furnaces
for steel products.
The domain of flameless combustion has been
investigated on a test furnace as a function of
the recirculation ratio Kv defined as ratio of recirculated mass flow of combustion products
(before reaction) with respect to the driving
flow rate of reactants [1,2]
Kv = Mrec / (Mair + Mfuel)
F
Results are schematized in Figure 3: for
temperatures > ~ 850 °C, above self-ignition, a
domain of stable reaction region without flame
front can be established, corresponding to large
Kv values (order of Kv > ~3), that are obtained
with high momentum of the injected fluids.
Flue,air,gas,flameless
combustion
This domain has been called flameless
Figure 2 – Flame and flameless combustion
oxidation or with the trademark FLOX®. It is
not possible to establish a conventional flame
front for Kv values ~ > 0.3-0.5 and the intermediate region is typical of “lifted flames” and of
unstable combustion. Below ignition temperature, burner stabilized flame mode only is
admissible. Flameless oxidation does not produce a visible flame and furthermore this
combustion mode is almost silent and
abatement in combustion noise (~ 15
dBA) is at least as impressive, as
stable flameless
burner
combustion
stabilized
disappearance of a visible flame, proving
or FLOX®
flames
unstable lifted
that the turbulent flame front accounts for
flames
~
most of the typical combustion roar of
ctemperature
high velocity burners [3]. Flameless
ol
oxidation has been thoroughly
d
self-ignition
investigated by WS [3,4]. FLOX® has
temperature
fl
been shown to work for rich, near
temperature
u
stoichiometric and for very lean
no stable flame
e
combustion conditions; it works with and
explosion risk
possible
g
without air or fuel preheat. It also works
as
for diffusion, partial premixed and
e
premixed combustion. A well known
Kv
s
advantage concerns low-NOx burners
.a.
recirculation
operated at very high air preheat: unlike
r
Figure 3 – Domain
of FLOX® vs Kv factor
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v
Italian Section of the Combustion Institute
conventional flame mode, the flameless mode is insensitive to air preheat temperature as far
as NOx is concerned, and this is very important for application to high temperature industrial
processes or furnaces.
3. Energy savings in steel furnaces
The thermal efficiency of high temperature furnaces can be increased very much by means of
efficient heat recovery by means of air preheating: high efficiency is equivalent to reduction
in fuel consumption and to a corresponding saving in greenhouse gas emissions. High
preheat, like air at 800-1000 °C, is only technically feasible if special combustion techniques
are adopted in order to prevent unacceptable NOx emissions and local overheating. Flameless
oxidation fits perfectly this requirement and can be considered a prerequisite for such
applications.
~cold flue gases
hot flue gases
nat. gas
cold air
furnace
wall
combustion
chamber
Figure 4 – Burner-integrated heat recovery
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 preheated
in counter-current while cooling the flue gases. This is a convenient solution for furnaces
equipped with several burners: cold combustion air is distributed to the burners while almost
cold flue gases are extracted from a common manifold. This design offers effective
preheating efficiency. Centralized heat recovery allows thermal efficiency ~ 60 % (~ 40% for
no preheating at all), burner integrated recovery scores ~ 75-85%, which is a good step
forward, corresponding to a fuel saving 15-25% with respect the to state of the art (centralized
heat recovery).
Thousands of FLOX® burners have been installed in continuous industrial plants and perform
satisfactorily. Also regenerative burners firing in FLOX® mode have been adopted in several
large annealing lines for stainless steel strips and in batch furnaces. Regenerative air
preheating 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: it
radiates to the stock without permitting contact with the flue gases and combustion is
developed inside a long tubular chamber, which makes combustion control difficult.
Experience has demonstrated that internal recirculation of combustion products is the key to
good performance: “re-circulating geometries” allow low-NOx performance and uniform
temperature of the radiant tube thanks to flameless oxidation. Temperature uniformity has
beneficial consequences on the strength of the radiant tube and on the average allowable heat
flux, which implies a better exploitation of the radiating surface: in other words, a saving in
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installation costs. A good example are the annealing furnaces equipped with ceramic “singleend” tubes in SiSiC: the cost of ceramic radiant tubes has been largely overridden by excellent
performance. Figure 5 shows pictures of large plants equipped with several hundreds FLOX
burners.
Figure 5 – Continuous steel furnaces equipped with FLOX® burners
4. Power generating equipment
The FLOX® principle is not limited to steel furnaces and can be applied to several high
temperature processes. Examples are the Stirling engines, where heat is made available at
high temperature with high efficiency, with the purpose of providing combined heat and
power in small power generating units. A very promising application of flameless combustion
to combustors for gas turbines is being presently developed and successfully tested: a
specially designed FLOX® prototype burner (Figure 7) ensures very low-NOx, emissions and
overcomes the nasty problem of fluctuations or “humming” that affects premix-based GT
combustors, where the flame front stabilization is a critical issue. R&TD is ongoing with the
participation of several academic and industrial partners in Europe.
The inherent temperature uniformity obtained with flameless combustion finds an ideal
application in steam reformers for hydrogen production: reforming reactions take place inside
vertical tubes filled with a
catalyst and reheated from
the outside. The uniform
temperature distribution is
essential
for
high
productivity, reduced stress
on the reaction tubes and
better control. The
experience of the W S
company in steel process
furnaces has been used to
found a daughter company
specialised in “minireformers” for producing
Figure 6 – CFD computations of the prototype GT combustor
small amounts of hydrogen
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Italian Section of the Combustion Institute
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(order of 5-200 Nm /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 the
airports of Munich and of Madrid to provide the H2 used by local buses for passenger service.
Flameless oxidation has been investigated
with gaseous fuels and in particular with
natural gas. However, the basic principle
holds good for any fuel, at least any fuel as
soon as it is made available in fluid form (like
evaporation of liquid droplets or release of
volatile matter from pulverised solid fuel).
Trials are being carried out together with
German universities to test effects of
flameless oxidation (Figure 8): flameless
mode occurs with any fuel and consistent
NOx reduction has been observed.
Encouraging tests have been carried out
under pressure, which might be applicable to
envisaged future processes aiming at CO2
sequestration.
5. Conclusions
Referring to the case of high temperature
furnaces, the industrial application has
Figure 7 – The FLOX® based
demonstrated that flameless technology can
minireformer
greatly renew and improve the design and the
performance of traditional plants / processes; advantages like “downsizing” (reduction of the
furnace length), NOx minimization, temperature uniformity, better control and improved
product quality make investment for revamping old plants advantageous. Similar arguments
hold true for the R&TD applications to power generating devices as quoted in § 4 above. We
can conclude that the principle of flameless oxidation has still a large potential for further
development in many equipment where combustion plays the important role.
Figure 9 – FLOX® performance with different fuels
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The tendency is to tighten regulations concerning pollutant emissions and to limit specific
emissions of greenhouse gases, which implies reducing specific fossil fuel consumption. This
is based upon steady grounds: not the available or future fossil fuel resources put an effective
limit to economic and abundant energy, but the available clean air. Clean air for combustion
is 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 thereby
stimulated the competitive production of cleaner engines. A similar, virtuous pattern should
be followed in other domains related to fossil energy conversion as awareness of the
worldwide “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 NO
formation, Prog. Energy Combust. Sci., 23, 81-94, 1997
[3] Wünning J.G.(2005): Flameless Oxidation, 6th int Symposium HTACG – Essen, 17-19
October, 2005
[4] WS Patents EP 0463218 and EP 0685683 (1990)
[5] Milani A., Wünning J.G. (2002): Design concepts for radiant tubes - Millennium Steel
2002
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