Journal of Thermal
Science and
Technology
Vol. 8, No. 1, 2013
Formation of Lean Premixed Surface Flame
Using Porous Baffle Plate and Flame Holder*
Pil Hyong LEE** and Sang Soon HWANG**
**Department of Mechanical Engineering, University of Incheon
12-1 Songdo-dong, Yuensu-Gu, Incheon, Korea
E-mail: [email protected]
Abstract
A lean premixed surface flame has many advantages including low CO(Carbon
Monoxide) and NOx(Nitrogen Oxide) emission and applicability of a small
combustion volume leading to compact design. These advantages make it
applicable to burner for condensing boilers with high thermal efficiency. Moreover
recent severe regulation of global warming gas favored a lean premixed surface
flame in development of a condensing boiler burner.
This study focused on emission characteristics of lean premixed flame and the
effect of flow distribution on flame stability of a surface flame in a cylindrical
porous metal plate burner. For conceptual design of surface flame burner, the
numerical calculation of a flow pattern inside the burner was performed and the
calculated data were used for design of the burner system including the baffle plate
and flame holder.
The results show that the surface and stable premixed flame can be generated by
implementing the proper baffle plate and flame holder. The surface cylindrical
flame mode is changed into green flame, yellow radiation flame, blue flame and
blow off with decreasing equivalence ratio. The blue flame has a wide stability
region in the stability curve and showed the lowest CO and NOx emission at low
equivalence ratio. And CO decreased as the mixture ratio became leaner but NOx
showed almost the same emission level. For stability of a surface cylindrical flame,
it was found to be very important to select the proper distribution of holes in a
baffle plate and install the flame holder to prevent blow off at the rim of the
cylindrical burner. NOx was measured below 6 ppm (0% oxygen base) from
equivalence ratios 0.706 to 0.769 through the proper design of baffle plate and
flame holder. CO which is a very important emission index in residential gas boiler
was observed below 49.1ppm under the same equivalence ratio range.
Key words: Lean Premixed Surface Flame, Condensing Boiler, Cylindrical Porous
Metal Plate Burner, Flame Stability, CO, NOx
1. Introduction
*Received 27 Dec., 2012 (No. 12-0521)
[DOI: 10.1299/jtst.8.178]
Copyright © 2013 by JSME
Rapid depletion of fossil fuel resources and the severe environmental pollution are the
major issues caused by the widespread use of fossil fuels. Since energy saved is more
precious than the energy generated, the demand for various efficient and eco-friendly
energy management schemes to be implemented in the industrial, commercial and domestic
(1)
sectors has become increasingly.
As conventional burner for residential boiler, Bunsen burner which can form separate
and multiple flames at rich mixture condition has been widely used. Since multiple flames
in a Bunsen burner are long and separate from each other, the temperature distribution at the
flame tip region is not uniform. Thus the combustion chamber should be very long to have
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enough mixing time to satisfy the uniform temperature distribution for efficient heat transfer
to the heat exchanger. This causes to make a gas boiler size large. In addition, flames in
Bunsen burner can not be stable in the downward and sideward positions because of the
effect of buoyancy forces on long flames. Condensing boiler needs extra latent heat transfer
area for condensation of the water vapor into liquid water in order to make use of the latent
heat of water vapor generated from combustion. This condensation process makes the
Bunsen flame very unstable due to the falling down of water droplets formed at upper side
heat exchange. But it is very difficult for Bunsen burner to be placed at downward or
sideward direction to avoid the falling down of water droplets. Therefore flat flame burner
which is very stable independent of orientation is needed for condensing boiler. Moreover it
is not easy to realize the NOx reduction by lean combustion because Bunsen burner is
(2-4)
usually operated at a rich mixture ratio.
Recently, a porous premixed flat flame burner has been proposed for perfect
(5-8)
combustion and uniform heating in a condensing gas boiler.
Porous burners such as
metal fiber and perforated ceramic tile and multi-hole plate have been used with different
(9)
porous materials. Metal fiber burners have been widely used to form a lean premixed flat
(10)
flame in the main burner of condensing boiler.
But metal fiber burner which consists of
very thin metal fibers has thermal durability problems due to frequent contacts with high
temperature flat flame front during operation. And its combustion noise is sometimes very
high and it is also very expensive. Perforated ceramic tile has high thermal resistance but it
is very weak with respect to small external shock. On the other hand, a multi hole plate
made from thin steel material is very thermally and mechanically very durable and
moreover cost competitive but should be carefully designed for formation of stable lean
premixes flame.
In this study, a premixed surface flame from a multi hole cylindrical burner was
examined using a porous baffle plate and multi-flame holes plate. The effect of baffle plate
shape and flame holder was examined to achieve the wider stability limit. And a premixed
surface flame using a cylindrical multi-hole burner was characterized using various
equivalence ratios and heating capacity and its CO and NOx emission were measured and
analyzed as well.
2. Numerical Analysis and Discussions
2.1 Numerical analysis
Figure 1 Schematic of reference cylindrical premixed burner Assembly
It is very important to distribute flow velocity on the burner surface uniform in order to
form the surface flame. In our study, the cylindrical burner type is adopted because the
cylindrical condensing boiler has recently become widespread. Baffle plate with different
porosity is used for controlling the mixture flow distribution on the cylindrical surface of
burner. To find out the effect of different porosity on distribution of flow velocity on the
burner surface, numerical simulation was carried out.
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Figure 2 Computational grid for cylindrical premixed burner
The numerical simulation model for calculation of in and outside flow in cylindrical
burner is based on SIMPLE (semi-implicit method for pressure linked equations consistent)
algorithm using segregated solver of Ansys Fluent®. Typical schematic of cylindrical
premixed burner is shown in Fig. 1. Dimension of cylindrical burner with multiple holes is
35mm(radius) × 130mm(length) × 1mm(thickness). The dimension of baffle plate is set
to 30mm(radius) × 120mm(length) × 1mm(thickness). The baffle plate with small holes
and burner surface with many flame holes are simulated as porous surface with equivalent
porosity in numerical simulation. Figure 2 shows the computational mesh structure for
cylindrical premixed burner assembly. The radial and hexagonal grids for computational
domain are employed. At the inlet, the fluid is supposed to flow into the cylindrical
premixed burner at known velocity and the atmospheric pressure is applied to the outlet.
2.2 Numerical Discussions
To analyze internal flow characteristics in a cylindrical burner with a metal baffle plate
with different porosity and thickness, a numerical analysis was conducted by applying
three-dimensional mesh as shown in Fig. 2. At the inlets, the fluid is supposed to flow into
the cylindrical burner at known velocity and the atmospheric pressure is applied at the
outlets. And the numerical conditions are listed in Table 1.
Table 1 Numerical Conditions
Air flow rate
Fuel flow rate
Equivalence ratio
Temperature(℃)
31.8 l/min
273.9 l/min
0.83
25
Figure 3 shows the pressure and contour of flow velocity when applying a metal baffle
plate of 1 mm thick with porosity of 0.5 which is usually used in conventional multi holes
burner. Figure 3-(a) shows that high flow velocity due to the stagnation flow around the end
plate was observed leading to non uniform velocity distribution on the burner surface. This
kind of non uniform velocity distribution may cause formation of surface flame very
difficult and produce the lift–off flame at the end region of burner. The pressure drop of 3
Pascal was observed at the very small level as shown in Fig. 3-(b).
Low porosity baffle plate was applied in order to induce uniform velocity distribution
on the burner surface by applying higher pressure drop across the baffle plate. Low porous
baffle plate has porosity of 0.3 and thickness of 3 mm as shown in Fig. 4.
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Figure 5 shows the velocity distribution and pressure drop with baffle plate with low
porosity. Figure 5-(a) shows that distribution of flow velocity becomes more uniform across
the burner surface compared to Fig. 4-(a). Figure 5-(b) shows that a pressure drop around 7
Pascal was higher than with baffle with porosity of 0.5 but the difference is very small.
(a) Flow velocity
(b) Pressure drop
Figure 3 Contour of pressure drop and velocity at baffle plate with porosity 0.5.
Figure 4 Schematic of cylindrical premixed burner assembly using baffle plate with porosity
0.3.
(a) Flow velocity
(b) Pressure drop
Figure 5 Contour of pressure drop and velocity at permeability baffle plate
Figure 6 compares the flow velocity on the burner surface which is very important to
make surface flame. Flow velocity of 0.1-1.7 m/s was observed and flow velocity becomes
faster approaching the end zone of burner for the case of high porosity and 0.13-0.17 m/s
was maintained uniformly for low porosity case as expected. It was confirmed that porosity
level for surface flame was estimated through the numerical simulation and can be used for
design of the baffle plate for experimental use.
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Figure 6 Comparison of reference baffle plate and permeability baffle plate velocity profile.
3. Experimental Apparatus and Method
3.1 Experimental methods
1) Fuel/air Mixer
The design of mixer is very prerequisite to determine the proper mixing of reactant and
low pressure drop. Figure 7 shows schematics of cylindrical multi holes burner. AGM(Air
Gas Mixer) was used for fuel gas and air mixing. AGM shown in Fig. 8 makes very rapid
and complete mixing by the entrainment of air/fuel gas induced by fast rotation of fan. This
kind of mixer is very compact and has ability of fine control of mixture ratio and low
pressure drop and reduces combustion noise as well.
Figure 7 Schematic diagram of premixing fan and air gas
Figure 8 Schematic diagram of Air Gas Mixer(AGM)
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Direct introduction of fuel gas into rotating fan in this kind of mixer makes mixing of
fuel and air more efficiently. So the mixture of fuel and air becomes more homogeneous
resulting to low flame front fluctuation and then reduce combustion noise.
2) Flame hole and Baffle Plate
Figure 9 Cylindrical lean premixed burner assembly
Figure 9 represents the configuration of flame hole and baffle plate. The mixture gases
formed in AGM was introduced to baffle plate through manifold. Mixture gases became to
have uniform velocity through baffle plate which forms the uniform pressure drop
throughout the baffle plate surface.
The reference baffle plate which is available commercially and the baffle plate
proposed in this study are compared in Fig. 10. A reference cylindrical baffle plate is
120mm long and has 60mm diameter with 2.4mm or 3.0mm holes, which has equivalent
porosity of 0.5. The bronze sintering baffle plate has same dimension as a reference baffle
plate but composed of a fine porous sintered bronze whose permeability is 100 micro
meters, which is equivalent to porosity 0.3 given in numerical simulation. More uniform
pressure distribution can be expected by using a porous sintered bronze material through the
numerical simulation. Surface flames were investigated under range of the heating
capacities of 8,000 - 32,000 kcal / hr , which is the average heat capacity in residential
boiler.
(a) Multi hole metal baffle plate
Figure 10 Comparison of baffle plates
(b)Bronze sintering baffle plate
3) Flame Holder
Figure 11 Configuration of Flame holder
Blow off at the edge of flame hole was usually observed in cylindrical multi-hole
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burner due to difference between velocity at the entrance and the end zone. This blow-off
was observed in numerical simulation for high porosity case. In order to prevent blow off at
the edge, a flame holder shown in Fig. 11 was used in a cylindrical burner. Flame holder
was made of stainless steel mesh and fabricated in 10mm wide and 0.7mm thickness, which
makes the flow resistance higher and reduce the flow velocity. The diameter of mesh wire is
0.2mm and 40 mesh per inch.
4) Schematics of experimental apparatus
LNG(Liquid Natural Gas) used as fuel gas in this study was composed of methane,
propane, ethane and its heating capacity is 10,500 kcal / m3 . The LNG for this experiment
consists of 90% of methane and ethane of about 8-9%, propane, about 2%.
Figure 12 shows the schematics of experimental apparatus. Combustion chamber is
cylindrical for cylindrical premixed burner. Pressure of LNG is set to 250 mmH 2 O through
pressure regulator and flow rate of fuel gas was measured by wet gas
meter(Sinagawa-W-NK Type). The flow rate and air fuel mixing rate were regulated by a
proportional air ratio valve(SIT) accurately. Equivalence ratio was adjusted by varying the
fan rotational speed regulated by DC power supply. Emission data was analyzed by
emission gas analyzer(TESTO-330) which can measure NOX , NO , CO , CO2 , O2
simultaneously. And DSLR(Digital Single Lens Reflex) camera(D-80, Nikon) was used in
order to observe the flame pattern with equivalence ratio and heating capacities. Other
specifications necessary for experimental condition is shown in Table 2.
Figure 12 Schematics of combustion experimental apparatus
Table 2 Experimental Conditions
Air Temperature
Heating rate
Equivalence ratio
Humidity
25℃
8,000 - 32,000 kcal / hr
0.50 - 0.95
70%
3.2 Experimental Results and Discussions
1) Flame patterns of burner flame with multi hole baffle plate
The flame patterns of a burner used with multi hole baffle plate is shown in Fig. 13. The
flame patterns were visualized using a DSLR camera. Flame Figs were taken by decreasing
the equivalence ratio at a fixed heating capacity. Figure 14 shows the flame pattern when
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the heating capacity was set to 20,000 kcal / hr . As the equivalence ratio was decreased, the
flame patterns were changed as follows.
Green flame → Yellow radiation flame → Blue flame → Blow off
It is postulated that the green flame is colored by activated molecules and has a surface
(6)
flame pattern. It was found that yellow radiation flame formed near to the stoichiometric
ratio has the typical radiation mode which has most intense intensity at yellow wave
frequency. Yellow radiation flame was submerged inside the burner surface and its main
heat transfer mode seems to be radiation. As the mixture ratio became leaner, a blue colored
flame just above surface of burner was observed. As the mixture ratio became much leaner,
blow off at the edge was detected and the flame surface was fluctuated unstably. As more
air was supplied, typical blow off on the entire flame surface was observed.
(a)Green Flame( φ = 0.94 )
(b)Red Flame ( φ = 0.90 )
(c)Blue Flame ( φ = 0.83 )
(d)Lift off flame ( φ = 0.69 )
Figure 13 Comparison of flame patterns at different equivalence ratio at heating capacity
20,000 kcal / hr
(a) Metal baffle plate( φ = 0.83 )
(b) Bronze sintering baffle plate( φ = 0.83 )
Figure 14 Flame phenomena by using types of baffle plate material at heating capacity
20,000 kcal / h
2) Flame pattern of burner with sintered bronze baffle plate
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Figure 14 shows the flame pattern using a sintered bronze baffle plate(b) compared to
the flame pattern of the multi hole baffle plate burner(a). As shown in Figure 14-(a) and (b),
both flames exhibit almost same blow off phenomena. However, the flame with the sintered
bronze baffle plate showed clear blue color and fluctuation of the flame front was greatly
reduced. The Figs also show that a more stable flame can be established by using a
permeability baffle plate. This means the bronze sintered baffle plate can produce more
uniform and stable flow on the burner surface.
3) Flame stabilization with flame holder
Figure 15 represents flame patterns of a multi hole baffle plate burner without flame
holder and with flame holder to see the effect of flame holder. As shown in Fig. 15-(a)
flame pattern is very unstable and also exhibit blow off phenomena at the several regions on
the flame surface. But in Fig. 15-(b), flame patterns is very stable and blow off on the front
and end regions was almost suppressed. This is because the flow velocity at the front and
end zone of flame surface were decreased due to flow resistance by flame holder. It is very
important to know the flame characteristics inside boiler combustion chamber in real boiler
system because combustion chamber shape and heat exchanger arrangement could affects
the mixture flow inside and outside burner.
(a)Non flame holder( φ = 0.80 )
(b)Flame holder( φ = 0.80 )
Figure 15 Effect of flame holder of flame pattern at heating capacity 32,000 kcal / h
(a) Flames with metal baffle plate
(b) Flames with bronze sintering baffle plate
and flame holder
Figure 16 Comparison of flame pattern inside combustion chamber at heating capacity
32,000 kcal / h
Figure 16 shows the flame pattern using a sintered bronze baffle plate and flame
holder(b) compared to the flame pattern of the multi hole baffle plate burner(a) inside
combustion chamber of the real boiler. In Fig. 16-(b), cylindrical flame is very stable and
clear blue color inside combustion chamber of boiler system. But Fig. 16-(a) shows
fluctuation of the combustion flame in front of burner and flame patterns is very unstable.
And no flame zone in the front edge region was observed. The formation of no flame zone
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makes the combustion efficiency worse and increases exhaust emission. The stable flat
flame can be destabilized when applied to the real boiler combustion chamber.
4) Emission characteristics of burner with flame holder and sintered bronze baffle plate
It is well known that maximum NOx emission occurs at around 1 of equivalence ratio
and that CO emission increases as equivalence ratio becomes rich. The CO and NOx of
stable green flame and stable yellow flame was very higher than that of blue flame because
the blue flame was maintained at very low equivalence ratio(around 0.7 equivalence ratio).
Figure 17 shows CO and NOx emission pattern with increasing equivalence ratio measured
by a TESTO-330 for the blue flame. Generally, CO and NOx decrease with decreasing
equivalence ratio for an equivalence ratio of less than 1. NOx was measured below 6
ppm(0% oxygen)from equivalence ratios between 0.706 to 0.769. CO which is a very
important emission index in a gas boiler was observed below 49.1ppm under the same
equivalence range. This CO emission level is blow 100 ppm of CO regulation value for
residential boiler class.
Figure 17 CO and NOx emission with equivalence ratio
5) Stability curve of burner with flame holder and sintered bronze baffle plate
Figure 18 shows a flame stability diagram with equivalence ratio and heating capacity.
Stability curve is very important factor of boiler burner, because turn down ratio is
significant parameter during boiler operation. The surface flames were divided into three
flame pattern by equivalence ratio. Each flame pattern has appeared within almost same
equivalence ratio range under a heating capacity of 8,000 to 32,000 kcal / hr . Yellow
radiation surface flame was formed between Φ=0.89 and Φ= 0.94 at a heating capacity of
8,000 to 16,000 kcal / hr . The yellow radiation flame was located on the burner surface and
its main heat transfer mode was radiation. Radiation surface flame has been widely used in
industrial steel heating and drying where radiation heat transfer is very useful. As the
equivalence ratio decreased, blue flame was detected at around Φ = 0.57 to 0.93 at all
heating capacities. This blue flame mode was formed at the wider equivalence ratio. As the
equivalence ratio became much leaner and the velocity of the air-fuel mixture gas increases
more, the surface flame detached from the burner surface and blow off flame appeared at
Φ= 0.5 to 0.6 for all heating capacities. Blue flame which is typical lean premixed flame
and has low emission characteristics occupies widest area in stability diagram. It can be
found that the turn down ratio of this burner(maximum heating value/minimum heating
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value) is approximately 4 at between 0.65 and 0.75equivalence ratio compared to turn down
ratio of Bunsen burner of 2-3. The operating range of this burner for stable blue flame lies
between equivalence ratio of 0.65 to 0.75 for turn down ratio of 4.
Combustion efficiency calculated from experimental data of CO2 and O2 for the
blue flame range was 97.3-98.0%, which means the almost perfect combustion under lean
mixture ratio.
Figure 18 Stability curve with heating capacity
4. Conclusion
The combustion characteristics of a multi-hole cylindrical burner with a sintered bronze
baffle plate and a flame holder can be summarized as follows,
1) Numerical simulation show that the porosity of baffle plate is very important to make
premixed flat flame and blow off of flow velocity at end zone of cylindrical burner can be
produced due to stagnation flow of end plate.
2) As the equivalence ratio decreased, the flame pattern of a cylindrical multi hole
burner was generally changed into green flame → yellow radiation flame → blue flame
→ blow off.
3) A significant reduction in flame fluctuation was attributed to uniform pressure
distribution by the application of a porous sintered bronze baffle plate. Blow off at the edge
of a cylindrical burner with a sintered bronze baffle plate can be suppressed by applying a
mesh flame holder.
4) NOx was measured below 6ppm (0% oxygen base) from an equivalence ratio of
0.706 to 0.769. CO which is a very important emission index in a gas boiler was observed
below 49.1ppm(0% oxygen base) under the same equivalence ratio region. This low level of
emission can be achieved due to stable flame formation at very lean equivalence.
5) Turn down ratio for lean blue flame burner is found to be approximately 4 at between
equivalence ratio of 0.65 to 0.75 compared to turn down ratio of Bunsen burner of 2-3.
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Acknowledgment
This work was supported by the University of Incheon Research Grant in 2010.
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