st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
A New Concept of low NOx Burner based on the Flame Stabilization
by Plasma assisted Flame
D. H. Lee, K.-T. Kim, H. S. Kang, Y.-H. Song
Korea Institute of Machinery and Materials
Abstract: Ultra low NOx burner is introduced. Modification of burner by introducing plasma
reformer inside a commercial industrial burner is carried. Modification of the burner does not
change any outer configuration of prototype burner. 4~10% of fuel is supplied for the reformer. Plasma reformer produces hydrogen and carbon monoxide. Insertion of the plasma burner
results in supply of hydrogen to main flame. Because of the reformate gas with high temperature, main flame is lifted. But different with typical burner flame is not blown off. Hydrogen
can hold a flame with lifted condition. The distance of flame lift ranges about 10~15cm and
the distance enhances mixing of fuel and air resulting in partially premixed flame. Also, syngas from the reformer itself can function as a reductant for a kind of selective non catalytic
reduction. All of the effect results in reduction of NOx generation. Even single digit ppm of
NOx can be obtained by the modification.
Keywords: Plasma burner, NOx, Hydrogen, lifted flame
1. Introduction
NOx is major emission in combustion process.[1] Because of its hazardous effects on the climate and human
body, diverse regulations have been tried to reduce NOx
emission. In case of Korea, the government has legislated
regulation to suppress the total amount of NOx generation
in industry.[2] Among possible source of NOx, internal
combustion engine, industrial burner occupies largest
portion among diverse sources.[3]
Approaches to reduce NOx in combustion facilities can
be classified into two different methods. One is after
treatment such as selective catalytic reduction (SCR)
process suing NH3 as reductant.[4] The other is low NOx
combustion technology such as exhaust gas recirculation
(EGR), staged combustion and so on.[5] In general, low
NOx combustion technology has merits of cost effectiveness and smaller volume of facility over after treatment
technology.
Regarding low NOx combustion technology, EGR is
most common way to reduce NOx.[6] EGR can be sub
divided into internal EGR and external EGR. Internal
EGR induces recirculation flow inside the combustor and
external EGR drives part of burnt gas into combustor with
external flow path. External EGR has more capability of
reducing NOx but requires rather larger facility and complex flow paths.
On the other hand, plasma has been applied for diverse
combustion process and chemical reactions. Plasma has
chemically active species such as high energy electron
and excited species that can function to sustain reaction
even in harsh condition. Plasma application in combustion
of very high flow velocity, ultra lean or rich mixture has
been tried.[7,8]
This paper introduces means of reducing NOx without
help of external facility, EGR or other auxiliary mechanism. In this study, to reduce NOx, industrial burner modified by combining plasma reformer inside the burner
head is introduced.
2. Plasma reformer
Plasma reformer used in this study is based on a rotating arc.[9] Schematic of basic structure of rotating arc is
shown in Fig.1
Fig.1 Schematic of rotating arc reactor
Rotating arc has been successfully applied for plasma
reformer.[10] The amount of hydrogen produced by reforming process, is almost linearly proportional to the
electric power supplied. However, only little amount of
hydrogen can alter the stabilization mechanism of the
flame. Dimension of rotating arc reactor is rather small.
Typical rotating arc reactor can covers up to hundreds of
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
liter per minimum with reactor diameter of about 50mm.
the scale of the reformer enables embedment of the reformer inside the burner.
3. Modification of the burner with plasma
Strategy of the work is to modify existing industrial
burner for LNG with minimal change. Function of plasma
is introducing reformate gas into flame where the reformate gas contains hydrogen and carbon monoxide. Reforming is obtained by introducin part of fuel supplied to
the burner into plasma reformer placed inside the burner.
The procedure for the modification is given in Fig.2 [11]
outer configuration of the prototype burner head is not
changed from the prototype burner after the modification.
Only additional fuel line and air supply line is added for
reformer.
fuel for reformer. Electric power supply for the reformer
is fixed in this study.
Parametric effect of these factors for the operation and
NOx generation of the burner are investigated and all of
the tested conditions are tabulated in Table 1. Electric
power of 50~150 W was used with AC power supply operated with 10 kHz.
Table 1. Test condition matrix
Parameter
Ranges
Burner capacity
(x1,000 kcal/hr)
100
Excess air ratio
Several points between 1.02-1.45
O2/C ratio
(1st stage)
Fuel division
(%, 1st/2nd)
160
200
225
250
0.7-2.0 with interval of 0.1
4
5
8
10
Modified burner is installed in the boiler with dimension of diameter of 600mm and length of 1m. Water jacket is placed circumferentially to maintain the exit temperature of boiler to be 500 OC. Product gas is sampled out of
the boiler exit.
5. Results
According to the operation condition of reformer the
configuration of main flame is changed. Figure 3 compares the illustration of flame in prototype burner and
modified burner.[11]
(a) Flame configuration in prototype burner
Fig.2 Illustration for the procedure of burner modification
[11]
4. Experimental condition
By modification of the burner, operating parameter for
the plasma reformer can function as main parameter of
the burner. Parameter for the plasma reformer can be
listed to be O2/C ratio and electric power. Considering
these, parameters affecting the NOx generation in the
modified burner can be listed to be 1) Heat capacity, 2)
Burner excess ratio, 3) Reformer O2/C ratio, 4) Portion of
(b) Flame configuration in modified burner
Fig.3 Comparison of flame configuration in prototype and
modified burner [11]
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
NOx generations from the burner according to the
change of aforementioned parameters are investigated.
At first, effect of heat capacity is observed.
cess controlling the amount of product hydrogen and
temperature of the product. It seems that NOx reduction
experiences two different mechanisms along with varying
O2/C ratio. In the case of lower O2/C ratio, reforming
process takes place out of the reformer and around O 2/C
ratio of 1.1 the reforming process (or rich flame) is swallowed into the burner head or inside of the reformer. And
the change is most important reason of the change of NOx
reduction trend according to the O2/C
25
NOx
Fig.4 Comparison of NOx/CO generation in reference
burner and modified burner according to the heating load
of the burner [11]
Modified burner shows about half of the NOx
generaiton compared to reference burner.
In general, about the effect of excess air ratio, larger the
ratio cause higher the NOx generation because of lean
burn condition. However, modified burner shows almost
no change across tested excess air ratio as shown in Fig 5
in contrast to the result of reference burner. The results
reflect that modified burner shows characteristics somewhat deviate from typical diffusion flame burner.
Concentraion (ppm)
20
15
10
5
0
0.6
0.8
1
1.2
1.4
1.6
1.8
2
st
O2/C ratio in 1 stage combustor
Fig.6 NOx generation according to the O2/C ratio of
plasma reformer inside the burner head
Finally, the effect of fuel load division is observed. As
shown in Fig.7
25
40
20
Concentraion (ppm)
NOx concentration (ppm)
35
30
25
NOx (Plasma)
NOx (Reference)
20
15
15
NOx
10
5
10
0
5
4
5
6
7
8
9
10
st
0
Fuel in 1 stage combustor (%)
1.1
1.15
1.2
1.25
1.3
Excess air ratio
Fig.5 NOx generation according to the excess air ratio
of burner operation
O2/C ratio is actually parameter for the reformer operation. O2/C ratio determines the degree of reforming pro-
Fig.7 NOx generation according to the fuel division in
plasma reformer.
5. Discussion and Conclusion
What is most important change in adopting plasma reformer for the modification of the burner is the change of
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21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
flame shape and structure. Figure 8 shows the change in
flame shape.
Before modification
After modification
Fig.8 Flame shape before and after modification
What is observed in this change can be listed to be 1)
detach of the flame.- hot reformate gas induces lift off of
the flame from the burner rim, however, the flame is not
blown off by stabilization of hydrogen. Detach of flame
enables the flame to have time for mixing of fuel and air
resulting in partially premixed condition that generates
lower NOx that typical diffusion flame. 2) Removal of hot
spots that formed around the nozzle exit of fuel. Removal
of hot spots is beneficial for the reduction of thermal NOx.
3) Area of flame root is widened. Widened flame area has
effect of lowering thermal density around the flame root
that is also beneficial for the reduction of NOx. All of the
above mechanism functions for the reduction of NOx in
modified burner. It is revealed that plasma can be successfully applied for burner industry.
References
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