Energy and Power Engineering, 2013, 5, 324-329
doi:10.4236/epe.2013.54B064 Published Online July 2013 (http://www.scirp.org/journal/epe)
Cold-state Experimental Study on Flow Characteristics of
Multi-nozzle in Natural Gas Reburning Burner
Baoming Chen, Zhongxiao Zhang, Degui Bi
College of Power Engineering, University of Shanghai for Science and Technology, Shanghai, China
Received April, 2013
ABSTRACT
Based on the prototypes of a 130 t/h boiler, constant proportional cold-state test bench is established, flow characteris-
tics of multi-nozzle in natural gas reburning burner and its influence on the covering effect for the upflow in the furnace
are researched. Numerical simulations of this process are also made with standard k


turbulence model. The results
show that air flow fullness in furnace is better in the case of the reburning zone with 8 nozzles compared to 4 nozzles
and also coverage effect of the reburning flow for the updraft gas in the furnace is better. In the condition each nozzle
airflow velocity is constant, the effect of reburning flow on coverage of up-secondary air is best when the incident angle
for four corners is 14.17˚, while Center of the furnace wall is 84.57. And while the best incident angle is invariable, the
effect of reburning flow on coverage of up-secondary air is best when the speed of reburning gas in the corners of fur-
nace is 51 m/s, the same to the center of the furnace wall’s.
Keywords: Reburning Burner; Multi-nozzle; Cold-state Flow Field; Numerical Simulation; Covering Effect
1. Introduction
Natur al gas reburning technology is a promising method
for denitrification[1], In recent years, study on mecha-
nism of reduce NOx with natural gas reburning in many
countries found that compounds containing nitrogen
HCN in reburning fuel has important influence on the
reburning process. Patry and Engel [2] think that the
product of reactions for nitrogen oxide and methane is
HCN、water and hydrogen after study, also found that
about 70% of the NOx has been transformed in a short
time . One of the key technologies is the mixing proper-
ties of reburning flow and flue gas in the furnace [3] . So,
study on reburning gas flow field is important for inves-
tigation on gas mixing characteristics in furnace and its
effect on the reduction of NOx.
In this paper, compared with only 4 reburning nozzles
in four corners, another 4 reburning nozzles is added in
the center of the furnace wall in order to improve air flow
fullness inside the furnace, achieving the purpose of ef-
fective reduction NOx. Similarity simulation principle is
applied with velocity characterization of concentration
and the two phase flow processing for the single phase
fluid.T he influence of reburning nozzle numbers, inci-
dent angle and airflow speed on the flow field inside the
furnace will be studied with the method of numerical
simulation and cold-state test in order to optimize the
natural gas reburning technology and provide reference
for engineering application.
2. Introduction of Cold-state Experiment
2.1. Experiment Device
Cold-state test bench is built in the proportion of 1:5 ac-
cording to the prototype of a 130 t/h boiler which con-
sists of three parts: boiler furnace, the reburning burner
and air distribution piping, and the bench system diagram
is shown in Figure 1. The whole furnace is made of or-
ganic glass, the burner components is placed bottom-up
in turn: lower primary air nozzle, lower secondary air
nozzle, upper primary air nozzle, upper secondary air
1. Fan; 2. Valve; 3. Burner; 4. Air Chambe; 5. Furnace.
Figure 1. Cold-state test bench system.

B. M. CHEN ET AL. 325
nozzle, reburning nozzle and OFA nozzle. Working me-
dium is air while the measuring instrument is thermal
anemometer in the experiment. Test area and measuring
point distribution in a corner of the furnace is shown in
Figure 2: take X as the nozzle jet axis with the origin at
the center of the nozzle, while Y is the horizontal axis
along the nozzle, O point is the center of the nozzle.
2.2. The Experimental Parameters
According to similarity theory, cold modeling calculation
is made based on operation parameters of the boiler, out-
let air velocity of reburning nozzle in the experiment can
be got. Experimental parameters and the actual operation
parameters is shown in Table 1.
According to the research content, 6 kinds of operating
modes are arranged in the experiment, which parameters
are shown in Table 2.
(a) Test area (b) Measuring point in a corner
Figure 2. Test area and the measuring points in corner.
Table 1. The parameters of the boiler and experiment.
Parameters The boiler
operation (m·s-1)
The experiment
(m·s-1)
Primary air velocity 22 11.4
Secondary air velocity 50 22.6
Reburning air velocity in corners 100 51
80 40.8
100 51
Reburning air velocity in the
center of the furnace wall
120 61.2
Actual velocity of OFA 64.2 29
Table 2. Reburning nozzles parameters of 1 ~ 6 operating
modes in the test.
item 1 2 3 4 5 6
incident angle
in corners α/(°) 41.17 41.17 41.17 41.17 41.17 41.17
air velocity
in corners /(m·s-1) 51 51 51 51 51 51
Incident angle in the
furnace wall/(°) / 90°84.57 74.57 84.5784.57
air velocity in the
furnace wall /(m·s-1) / 51 51 51 40.861.2
3. Theory of Experiment and Numerical Si-
mulation
3.1. The Experiment Principle
According to the theory of similarity modeling,
cold-state experiment must adhere to the following prin-
ciples [4]: (1) geometric similarity between the model
and the actual object is needed. (2) Under the corre-
sponding conditions, air movement of model and the
actual object must be in the automatic modeling area. (3)
The momentum ratio each shared airflow of model keeps
in equal with the actual objects.
Due to primary air and secondary air momentum ratio
of the model and the actual object is equal [5, 6], as fol-
lows:
11 11
22 22
11
22
(1 )
M
MOOP
MM oo
oo
oo
mmm
mm
mk
m
P









 (1)
In the formula, M-the model; O-the actual object;
1-the primary air; 2- secondary air; m-the mass flow rate;
ω - average velocity of the flowing nozzles; U -the pul-
verized coal concentration of the primary air(0.36), kg/kg;
k- 0. 8 (consider different coefficient of pulverized coal
flow rate and wind speed).
Secondary air velocity of the model can be calculated
according to the euler number is equal between the mod-
el and the actual object.
22
22
2
*
oM
Mo
m2O
P
P




 (2)
In the formula, △P2M/△P2o - flow resistance of
model and the actual object, 2; ρ – density of flow.
Take the temperature of each nozzle working medium
as equal, the fllowing formula can be obtained using
formula (1) and (2):

1
11
12 2
2212 12
1
O
OO
MM M
M
OMO M
k
O
f
f
f
f



 

 
(3)
Each nozzle parameter of the cold model is obtained
according to the operation parameters of the boiler.
3.2. Mathematical Model of Numerical
Simulation
Gas flow in the furnace is considered as the three
-dimensional turbulent flow, standard turbulence model
has good adaptability with large numbers of studies. The
gas phase turbulent flow control equations can be ob-
tained under the three-dimensional rectangular coordi-
nate system, according to the N- S equation, the general
form as follows: [7]

B. M. CHEN ET AL.
326
 UVW
xyz
S
xxyyzz


 



 

 
 


 

 



(4)
Inlet boundary condition: the primary air V1 -11.4 m/s,
the secondary air V2 - 22.6 m/s, Reburning air V3 - 51
m/s, OFA V4 - 29 m/s;
Outlet boundary conditions: the negative pressure of
export of furnace-(- 50 )Pa;
Wall boundary conditions: no velocity slip and no
quality penetration, the border of the turbulent kinetic
energy-
2
-
3
k= 2
I
u



Turbulent kinetic energy dissipation
rate-
3/2
3/4
=0.09k
l
 , Turbulence intensity is ,
-1/8
=0.16ReI
turbulence length is . =0.007lL
The SIMPLE algorithm is adopted for the numerical
simulation in the paper, using the algorithm to iterative
calculation the equations of each variable [8]. The
method of “speculation – revised” is mainly used in
SIMPLE algorithm, computing pressure field on the ba-
sis of staggered grid in order to achieve the purpose of
solving the momentum equations.
4. Analysis of Results
Under the conditions of 1~6 operating modes, air speed
of 5 set of dimensionless distance (X/b=2.5,7.5 ,5, 10, b
is 2 cm) is measured , which are distributed On the cross
section of the reburning nozzle and the upper secondary
air nozzle.The measurement results are compared with
the results of numerical simulation, flow field character-
istics of the furnace center (X/b = 7.5, 10)is mainly ana-
lysised in this paper .
4.1. The Influence of Reburning Nozzle Number
on Covering Effect in Reburn Zone
Numerous of studies have shown that There are similari-
ties between the distribution of gas concentration profile
and the distribution of velocity profile in the furnace
along the horizontal cross section of a furnace [9], so the
speed distribution will represent the concentration distri-
bution in this experiment [10].
As shown in Figure 3, in the operating mode 1 and 2,
gas flow injection into the furnace with high speed. The
direction of reburning airflow is coincide with the central
axis of up secondary air roughly due to the large wind
speed and airflow strong rigidity in the nozzle front end
(X /,2.5 b = 1, 5). Reburning air velocity is higher than
the upper secondary air in range of - 2 < y/b < 2, so up-
drafts at the bottom of the furnace can be effectively
covered by reburning airflow.
Operating mode 1 Operating mode 1
(a) X /b = 7.5
Operating mode 1 Operating mode 1
(b) X /b = 10
Figure 3. Velocity distri bution of reburning air in four cor-
ners and upper secondary air for operating mode 1,2.
As shown in Figure 3(a), rebuirning Air flow to the
boiler furnace wall side deflected under the Impact of
rotating airflow in furnace due to reburning air rate at-
tenuation in mode 1 near the furnace center (X/b = 7.5),
the maximum its speed is near the Y/b = 1. Air deflection
ratio is smaller compared to mode 2. as shown in Figure
3(b), maximum speed of reburning flow under the mode
1 is 13.1 m/s, which cannot be significantly higher than
the secondary air speed, so its coverage effect is poorer.,
under the mode 2, because 4 reburning nozzles have been
added in the center of the furnace wall, enough momen-
tum of the reburning air in the four horns is made to be
shot into the rotating airflow of furnace center through
the surrounding air entrainment effect, and coverage ef-
fect for the secondary air is higher.
4.2. The Influence of the Incident Angle in Side
Walls on Covering Effect in Reburn Zone
For coal-fired boiler, tangential diameter is an important
characteristic of air flow in the furnace [8], the ideal flow
state inside the furnace is forming right circular rotating
in the center of the furnace flame. [11, 12]
Keep the velocity of reburning air and the upper sec-
ondary air unchanged, gradually adjusted the incident
angle in the center of furnace wall, from 90˚ to 74.57˚.
As is shown in Figure 4(a), under the mode 2,3 and 4,
with the reduce of the incident angle, the maximum ve-
locity in the center of reborn gas flow occur in y/b = 0.5,
1,1.5 or so, far away from the nozzle position (x/b =
7.5).The phenomenon shows that due to defend incident

B. M. CHEN ET AL.
327
angle, the gas mixing intensity of the four corners and the
furnace wall is different, which causes the difference of
the four horns side gas flow deflection slope again, fi-
nally form the different diameter of tangentia under the
different incident angles.
Maximum speed of reburning gas flow by the center
points occur in y/b = 1.5 in operating mode 4, then gas
flow deflection slope. The largest in the furnace form
bigger tangential, because tangential diameter is too large,
the gas flow is easy to stick on the wall, which causes the
water wall slugging in the actual operation.
According to the Figure 4(b), reburning air velocity is
higher than the upprt secondary air speed in -2 < y/b < 2
range, and there is good coincidence on the center axis
between reburning air and secondary air in mode 2 , 3.
Maximum reburning air speed is smaller at the x / b =10
in mode 3compaired with mode 2, and larger deflection.
It means mode 3 form larger reactions in the furnace
tangential, and its covering effect is better.
4.3. The Influence of Flow Velocity in Side Walls
on Covering Effect in Reburn Zone
By studying the influence of different incident angles
on the gas flow in furnace, reburning air has good cover
effect on the upper secondary air when the incident angle
of the four horns s is 41.17˚ and the incident angle in the
center of side wall is 84.57˚. The results are as follows
when change the velocity of nozzle inlet with the inci-
dent, angle keep the same as above.
As is shown in Figure 5, with the high-speed and
Strong rigidity of reburning air at X /, 2.5 b = 1, 5, cal-
culated value and the simulation values coincide well
under the mode 5. In the center of the furnace (x/b = 7.5,
10), speed of reburning air can not be completely obvi-
ous higher than the second blast velocity. In actual op-
eration, NOX content is relatively high in the heat of the
furnace flame, so if the reburning air in the region cannot
be completely surrounded rotating airflow in furnace,
generation efficiency will be poor.
In the operating mode 3, the highest speed central axis
of the nozzle in corners and the nozzle in side wall
roughly Keep consistent at the nozzle outlet (x / b = 1, 5,
2.5). And in - 2 < y/b < 2 range, velocity in four corners
is greater than the upper secondary air, due to the deflec-
tion is far stronger than which in mode 5 and air en-
trainment effect of up flow in the center of the furnace,
the reburning air can be mixed with rotating airflow in
the furnace, which has a better gentrification efficiency.
In the operating mode 6, the area of high-speed reborn
gas cover on the secondary air is almost at low speed
area. In the real operation, a lot of NOx pollutants pro-
duced in the region of high-speed flue gas in the furnace,
so in mode 6, reburning air and rotating air in furnace
cannot happen strong disturbance and hybrid, which led
to a decline in gentrification efficiency
Operating mode2 Operating mode 3 Operating mode 4
(a) X /b = 7.5
Operating mode 2 Operating mode 3 Operating mode 4
(b) X /b =10
Figure 4. Velocity distribution of reburning air in four corners and upper secondary air for operating mode 2,3,4.

B. M. CHEN ET AL.
328
Operating mode5 Operating mode 3 Operating mode 6
(a) X /b = 7.5
Operating mode5 Operating mode 3 Operating mode 6
(b) X /b = 10
Figure 5. Velocity distribution of reburning air in four corners and upper secondary air for operating mode 3,5,6.
5. Conclusions
1) Effect on reduction NOx is better when eight noz-
zles are arranged in the four corners of the furnace and
the center of side wall, compared with only arranges four
nozzles in the four corners of the furnace.
The mutil-nozzles setting can decrease the NOx more
efficiently for have better effect on the coverage of the
updraft in the furnace, which is advantageous for im-
proving the generation efficiency.
2) When eight nozzles are set in the reburner, the in-
cident angle of the nozzle has a huge impact on the cov-
erage effect of the updraft in the furnace. In the condition
that the velocity of the reburning air is constant, the re-
burning flow has the best coverage quality for overfire
air with the incident angle in corners of 41.17°and the
incident angle in the furnace wall of 84.57°.
3) The speed of the reburning flow also has an effect
on the decreasing of NOx. In the condition of maintain-
ing the incident angle of four-corner nozzles with 41.17
°and the incident angle of side wall nozzles with 84.57,
the reburning air has the best coverage quality for the
upper secondary air when the speed of reburning air in
corner is 51m/s and so as to the speed of reburning air in
the center of the furnace wall .
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