BIOFLAM PROJECT: APPLICATION OF LIQUID BIOFUELS IN NEW
HEATING TECHNOLOGIES FOR DOMESTIC APPLIANCES BASED
ON COOL FLAME VAPORIZATION AND POROUS MEDIUM
COMBUSTION
T. Brehmer1, F. Heger1, K. Lucka2, J. von Schloss2, Y. Abu-Sharekh3, D. Trimis3, A. Heeb4, G. Köb4,
T. Hayashi5, J.C.F. Pereira5, M. Founti6, D. Kolaitis6, M. Molinari7, A. Ortona7, J.-B. Michel8, P.
Theurillat8
1
OMV AG, Vienna, Austria, 2 EST, RWTH-Aachen, Aachen, Germany, 3 LSTM, University of Erlangen-Nuremberg,
Erlangen, Germany, 4 Hovalwerk AG, Vaduz, Liechtenstein, 5 LASEF, Instituto Superior Tecnico (IST), Lisbon, Portugal, 6
Thermal Section, National Technical University of Athens (NTUA), Athens, Greece, 7 PTC SA, Novazzano, Switzerland, 8
CSEM SA, Neuchâtel, Switzerland.
Abstract
The BIOFLAM project intends to provide solutions to the problem of environmental impact
of combustion of liquid fuels for heating purposes. Oil burners show up to date a very poor
power modulation especially at low power outputs, as usually needed in domestic appliances.
Liquid biofuels like FAME are also not compatible with conventional oil burner technologies.
Furthermore, the integration of oil burners in wall-hung systems requires further burner size
reductions. Within the EC-funded BIOFLAM project new liquid fuel fired condensing boilers
are being developed showing such major features as a power modulation of at least 10:1, ultra
low CO and NOx emissions over the entire power modulation range, significantly more
compactness than conventional liquid fuel fired boilers and compatibility with renewable
liquid fuels like FAME. In order to reach these goals new ceramic premixed liquid fuel
burners were developed based on the combination of the innovative cool flame vaporization
process and the novel porous medium burner concept. High temperature ceramics, condensing
boiler technology with condense water neutralization and innovative burner controls with a
power modulation were utilized, in order to realize all the principal advantages of the
employed technologies.
Keywords Cool flame vaporizer, Porous media combustion, Oil burners, Condensing boilers
1. Introduction
The BIOFLAM project intends to provide solutions to the problem of environmental impact
of combustion of liquid fuels for heating purposes. Oil burners show up to date a very poor
power modulation especially at low power outputs, as usually needed in domestic appliances.
Liquid biofuels like FAME are also not compatible with conventional oil burner technologies.
Furthermore, the integration of oil burners in wall-hung systems requires further burner size
reductions. Within the EC funded BIOFLAM project new liquid fuel fired condensing boilers
are being developed showing the following major features:
• power modulation of at least 10:1
• ultra low CO and NOx emissions over the entire power modulation range
• significantly more compact than conventional liquid fuel fired boilers
• compatible with renewable liquid fuels like FAME (based on rape-seed RME, sunflower
SFME and used fried oil UFO) [13-14].
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In order to reach these goals new ceramic premixed liquid fuel burners were developed based
on the combination of the innovative cool flame vaporization process and the novel porous
medium burner concept. High temperature ceramics, condensing boiler technology with
condense water neutralization and innovative burner controls with a power modulation of at
least 10:1 were utilized, in order to realize all the principal advantages of the employed
technologies.
Combustion in porous, inert media offers exceptional advantages compared to techniques of
free flame burners. The relatively new technology of porous medium burners is characterized
by higher burning rates, increased flame stability with low noise emissions and lower
combustion zone temperatures which lead to a reduction in NOx formation. Porous medium
burners also show low emissions of CO and very small scale sizes. Additionally, complex
combustion chamber geometries, which are not feasible with conventional state of the art
combustion techniques, are possible. Research and development activities on stationary
combustion in inert porous media started in the first decades of the 20th century [1, 2] and are
still continuing up to the present time (see for example [3, 4, 5]). A novel combustion
technique based on the combustion in porous media has been developed in the last years at the
Department of Fluid Mechanics at the university of Erlangen [6, 7, 8]. The major novelty of
this work is the combustion stabilization principle, which allows an extremely stable
operation of the premixed combustion process in the porous matrix. The flame stabilization
layer is placed inside of the porous matrix and is well defined by the matrix design. The most
important criterion, which determines whether or not a combustion process can take place
inside a porous structure, is its critical pore size. If the size of the pores is smaller than this
critical dimension, flame propagation is prohibited; the flame is always quenched. On the
other hand, if the pore size exceeds the critical dimension, flame propagation inside the
porous structure is possible.
Waste gas
Heat transfer in the
axial direction by
radiation, conduction, dispersion
and convection
Heat transport for the stabilization
of the combustion process
Heat removal by radiation and conduction
of the solid, conductive heat transfer of
the gas phase and dispersion
Large pores region
(region C)
Small pores region
(region A)
Region C
Combustion zone
Ignition temperature
Region A
Preheating zone
Gas mixture (fuel and oxidant)
Figure 1: Schematic setup of a porous burner
Figure 1 shows the schematic setup of a porous burner with the preheating region A and the
actual combustion region C. The critical pore size may be determined by a modified Péclet
number. In the porous burner, the combustion process is stabilized with a sudden change of
the pore size, corresponding to a change of the Péclet number inside the combustion reactor.
In region A of the porous burner, the porous body properties are chosen in such a way that
flame propagation is not possible. In region C, the pores are large enough so that flame
propagation is possible.
Combustion of liquid fuels like Industrial Gas Oil (IGO) is taking place after the evaporation
procedure of the liquid fuel. In most cases the liquid fuel is atomized with a spray nozzle and
mixed with the combustion air. In the resulting two-phase flow the evaporation/gasification of
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the atomized liquid fuel is the first step before combustion takes place. The combustion heat
release and the heat transport properties influence significantly the evaporation process.
Because of these reasons the evaporation and the combustion process cannot be separated
clearly. Because of insufficient spray quality and flame stabilization, commercial liquid fuel
burners normally show a power modulation less than 1,4:1. The vaporizing and combustion
zone are not clearly separated, so the system is sensitive to changes in temperature and flow
field, which are arising with power rate modulation.
A porous burner for liquid fuels like IGO should be operated with a clear separation between
the combustion and the evaporation process. The concept of the cool flame vaporizer, which
was developed at the Institute for Heat and Mass Transfer of the University of Aachen (ESTAachen), offers the possibility for a clear separation between the evaporation and the
combustion process [9, 10, 11].
g a s te m p e ra tu re a fte r fu e l a d d itio n
The main problem in producing a homogeneous mixture of air and liquid fuel is the tendency
of autoignition during the vaporization due to the heat flux necessary to provide the
vaporizing enthalpy. The cool flame vaporizer uses the internal heat production to vaporize
the fuel. The slightly exothermic cool flame reactions cause an energy conversion of about
5% to 10%, so that it can be described as autothermal.
800
tR > 1 s
0 .6 s 0 .2 s
700
600
no
re a c tio n
c o o l fla m e
a u to ig n itio n
off gas
500
fuel
400
gas analysis
300
200
200
oxidizer
300
400
500
600
700
800
g a s te m p e ra tu re b e fo re fu e l a d d itio n in ° C
Figure 2: Temperature evolution of IGO in a cool flame (p = 1bar)
Figure 2 shows the temperatures before and after the fuel addition. Cool flames start at a
temperature of 300° C and stabilize, virtually independently of the air ratio, at a temperature
of 480 °C at 1 bar conditions. The final temperature is also independent of the initial
temperature, which is the temperature at the start of fuel injection. The activation energy to
reach autoignition is too high at 1 bar. This is due to reaction inhibition which leads to a
constant temperature.
49
550
gas temperature ϑ in °C
500
450
final temperature
n-heptane
IGO
n-oktane
FAME
400
350
300
initial temperature
250
0
0,00 0,6
0,8
1,0
1,2
1,4
1,6
1,8
2,0
air ratio λ
Figure 3: Start and end temperatures of cool flames for different fuel types
The mechanism works for most hydrocarbons and their mixtures as demonstrated in figure 3,
so that also renewable fuels like FAME can be applied. Because no heat exchanger is in direct
contact with the liquid fuel there are no problems with crack processes. Soot will also not be
formed.
A combination of the cool flame vaporizer with the porous medium burner concept has
already been realized in the past [12]. However the proposed concept in [12] proved to be
very sensitive on the adjustment of the heat balance between cool flame vaporizer and porous
burner. In contrast to the proposed concept of [12] in the BIOFLAM project a complete
decoupling of the porous burner from the cool flame vaporizer was realized, resulting in a
very robust operational behavior over the entire power modulation range.
In the following paragraphs the concept of the BIOFLAM boiler and operational
characteristics are presented. The research and development efforts are still in progress in the
framework of a combined research and demonstration project financed by the European
Commission. The heterogeneous consortium of the project partners includes all different
necessary expertise for such a system development like fuel properties (OMV), boiler design
(Hovalwerk), electronic controls (CSEM), ceramics (PTC), CFD (IST and NTUA), cool flame
process (EST) and porous burners (LSTM).
2. Concept of the bioflam boiler unit
Porous burner
In the case of the combination of a porous burner with a cool flame vaporizer one has to face
the increased temperatures of the incoming fuel/air mixture which should be maintained even
in the flame stabilization region in order to prevent condensation and coking of the oil vapor.
Thus, the stabilization principle and the applied materials of the porous medium burner had to
be adapted and improved in order to fulfill the requirements of the cool flame vaporizer
products.
The design of the porous burner flame trap (Region A) was adapted corresponding to the
expected conditions of the cool flame products. Materials with extremely low conductivity
(ceramic matrix composites based on alumina fibers, < 0,2 W/mK) were applied. The flame
trap faces temperature gradients up to 1000 °C/cm and has to show a very low effective
50
conductivity in the upstream direction, in order to prevent overheating and flashback. The
flame trap was realized as a hole plate with holes of 1mm diameter.
Flame detection sensor
Zone A
Zone C
Ignition electrode
Gas/gas heat exchanger
Figure 4: Porous burner design and burner in operation
The combustion region consisted of a silicon carbide (SiC) foam, developed by PTC, able to
withstand temperatures up to 1650 °C and extreme heating and cooling rates. A special
production process starting from a PU-foam precursor, coatings and with a final silicon
infiltration and conversion of carbon particles to SiC in vacuum was applied.
Cool flame vaporizer
Although the cool flame process is an exothermal one, due to heat losses and due to the
starting temperature level of the cool flame process at 300 °C, it was not possible to realize a
cool flame vaporizer operating with the complete combustion air, without external preheating.
In the development of the cool flame vaporizer mainly two aspects were significant. First the
heat production of the cool flame reaction had to be maximized in order to achieve an
autothermal operation mode. This can be accomplished, if the recirculation ratio increases by
taking a higher pressure drop into account. On the other hand a marketable fan should be used
for the BIOFLAM unit (pressure drop < 2500 Pa).
Use marketable fan
Maximize vaporizer heat production
Reduce pressure loss
Maximize recirculation ratio
Figure 5: Contradiction between minimal pressure loss and autothermal operation mode
Since one main objective for the vaporizer design was an autothermal operation, the air ratio
at the vaporizer had to be sub–stoichiometric due to the recirculation ratio limits imposed by
the available marketable fans. Thus a staged process was applied with a sub–stoichiometric
operation of the vaporizer and subsequently mixing of the secondary air before entering the
porous burner. The primary air ratio should not be too low, otherwise mixing with secondary
air is difficult and possibly inhomogeneous. To avoid condensation in the mixing process the
mixing temperature had to be higher than 250°C. For this purpose the cool flame heat
production had to be be maximized. Figure 6 shows the heat balance for the cool flame
51
vaporizer. It can be seen that for a burner inlet temperature of 250°C a fuel conversion of
minimal 11.8% for maximum burner power is needed.
Heat losses
app. 50 W
Cool Flame
product 250°C
Oil 35°C
Vaporizer
Burner
Flue gas
Air 20°C
Cool Flame Conversion: 2.3 kW burner power -> 13.8%
23 kW burner power -> 11.8%
Figure 6: Heat balance of the cool flame vaporizer
The maximal fuel conversion of the cool flame vaporizer was determined experimentally.
Therefore temperatures at- in and outlet of the vaporizer were measured for the maximal
possible air ratio with a stable cool flame.
fuel conversion Cool Flame
16%
14%
12%
10%
8%
6%
4%
conversion needed
2%
conversion staging 2
0%
0
5
10
15
20
25
burner power in kW
Figure 7: Fuel conversion of Cool Flame vaporizer
As indicated in Figure 7 the fuel conversion was not high enough, in order to reach the
desired temperature of 250 °C after mixing with the secondary air. To avoid condensation of
the cool flame products the missing heat amount had to be introduced with the secondary air.
For this purpose a heat exchanger for the secondary air was combined with the porous burner
construction (see Figure 5). The overall vaporization and premixing process is shown
schematically in Figure 8.
λ prim = 0,4
primary air, 20°C
cold flame products
400°C
vaporizer
Mix
burner
secondary air, 20°C
λ sek = 0,8
preheater
270°C
200°C
Figure 8: Staging concept of the cool flame vaporizer and premixing unit
52
3. Design of the BIOFLAM boiler unit
In figure 9 the schematic diagram of the BIOFLAM unit is shown. The liquid fuel is entering
the vaporizer through a spray nozzle. A fan is used for pressurizing the combustion air, which
is split between primary and secondary air. The primary air enters the vaporizer through an
annular gap around the injection nozzle, which was designed in order to maximize the internal
recirculation rate in the cool flame vaporizer section. Recirculation is realized through the
internal cylinder in the vaporizer section. The secondary air enters the burner section, is
preheated and subsequently enters the mixing chamber and is mixed with the cool flame
products. The complete mixture enters the porous burner at a temperature level of ca. 270 °C.
The exhaust gases enter after the porous burner the condensing boiler, which utilizes the
Hovalwerk multi-jet technology.
Automotive high pressure injection nozzles were used in the BIOFLAM unit, in order to
realize the high power modulation range with a good spray quality. The fuel oil flow was
controlled by changing the opening time and frequency of the nozzle needle without a change
of the oil pressure. A minimal opening time of 2 ms or a maximal frequency of 150 Hz was
possible. Such nozzles are developed for operation with gasoline at a pressure of 10 MPa. For
the BIOFLAM unit they were used with IGO and a pressure of 1 MPa. Due to that reason no
data about the operation regime for this nozzle under these conditions was available and an
experimental characterization of different nozzles was performed.
Oil nozzle Bosch HDEV 67 °
Ignition electrode
Flame detection
Primary air
vaporizer
porous
burner
Cool flame Vaporizer
condensing
boiler
Cool Flame
Product
Mixing chamber
Secondary air
Preheated secondary air
Zone A
Zone C
Porous burner
Gas/gas heat exchanger
Exhaust
Figure 9: Design of the BIOFLAM boiler unit
Special attention was given on the starting procedure and the control aspects. During the
burner start the vaporizer is used as a burner, until the unit reaches the proper operating
conditions. New control strategies were necessary, in order to control the vaporizer and the
53
burner operation, especially during startup. Ignition electrodes were used in the vaporizer and
in the burner section. Based on a control of the temperatures in the different sections of the
unit, the conditions for switching from the starting mode to the stationary porous burner
combustion mode were evaluated.
In Figure 10 a schematic diagram indicates the major sensors and controls applied in the
BIOFLAM unit. At the current development stage the control is realized with PC based
modules. A robust microprocessor based control is under preparation for the next evolution
stage of the unit.
Figure 10: Schematic of the major BIOFLAM unit controls
4. Preliminary results and discussion
The previously described concept was realized in 5 identical prototypes and first tests were
performed with the BIOFLAM boiler prototypes. Photographs of the realized units can de
seen in Figure 11.
An important aspect during the preliminary tests was the temperature distribution inside the
vaporizer over the entire power modulation range. The burner power was changed from
2.6 kW to 23.5 kW. The secondary air preheating was approx. 200°C.
54
Figure 11: Photographs of the BIOFLAM boiler prototype in operation
In Figure 12 the temperature measuring positions in the vaporizer and the variation of the
different temperatures over the power modulation range are shown. From high to low powers
the temperature at T(rec2) increases while T(rec1) decreases. So it can be assumed that cool
flame reaction in case of low power is finished locally earlier inside the recirculation gap or
even before entering the recirculation gap. Nevertheless the cool flame reaction is stable for
the whole modulation range. Consequently, the mixing temperature was above 250°C at any
time. Thus, re-condensation of cool flame products before entering the porous burner section
can be certainly excluded.
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temperature in °C
500
450
400
350
300
250
200
150
100
50
0
0
2
4
6
8
10
12
14
16
18
20
22
24
burner power in kW
T (air1)
T (rec2)
T (air2,heater)
T (premix)
T (air2)
T (mix)
T (rec1)
T (wall)
Figure 12: Temperature distribution over the power modulation of staged vaporizer
In Figure 13 the emission characteristics of the first prototypes over the entire power
modulation range are shown. In the diagram also emissions with a non-staged operation of the
cool flame vaporizer with preheated air at 350 °C are shown (version 1) for comparison
purposes. The non staged operation of the cool flame vaporizer needs a substantial air
preheating, in order to avoid extinction of the cool flame. It is obvious, that the staged
vaporizer concept (version 2) requires much less air preheating (only the secondary air is
preheated at a much lower level of 200 °C) and results in significantly less nitrogen oxide
emissions, due to the resulting reduced combustion temperatures, although a lower excess air
ratio was adjusted.
56
Emissions [mg/kWh]
140
1st version:
CO 2nd version
NOx 2nd Version
120
preheating of the entire air
T air,total = 350 °C
Pvaporizer = 3.5 - 18 kW ; λ = 1,35
100
2nd version:
CO 1st version
NOx 1st Version
preheating of the secondary air
T air,sec = 200 °C
Pvaporizer = 2.6 - 23 kW ; λ = 1,25
80
60
40
20
0
0
5
10
15
20
25
Power [kW]
Figure 13: Emission characteristics of the BIOFLAM unit over the power modulation range
The excellent emissions characteristic of the BIOFLAM boiler unit operating with a staged
cool flame vaporizer is comparable to the emission levels of low emission gas burners, which
represents a breakthrough for oil burners.
5. Conclusions and future work
The developed burner and boiler unit for liquid fuels in the framework of the BIOFLAM
project allows a high power modulation combined with a condensing operation mode at low
emission levels. The good operational characteristics result from the combination of the cool
flame vaporizer with the porous burner. Besides the low emission levels, the developed boiler
also shows a higher overall efficiency, in comparison to conventional oil burners, because of
the high power modulation and the condensing heat exchanger.
The obtained results are very promising and further optimization work is undergoing,
basically towards long term stability issues of the applied components, control strategy and
safety issues. Extensive laboratory tests especially with different fuel qualities and mixtures
with renewable fuels of the FAME type are already running and results are expected in the
near future. A CE certification and a field test of 2 units under real conditions in selected
households and of additional 5 units under laboratory conditions is under preparation.
57
Acknowledgments
The contribution and financial support of the European Commission and the Swiss Federal Office of
Education and Science under the contract No. ENK6-CT-2000-00317 are gratefully acknowledged.
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