CFD Investigation on the Effect of Angled Secondary

Air to the Air Flow and Coal Combustion of a 660

Mw Tangential-Fired Utility Boiler
ISSN 2319-9725
Mr. Ashish Fande
PG Scholar,
Department of Mechanical Engineering
S.S.C.E.T., Bhilai, Chhatisgarh, India
Mr. P. V. Joshi
Professor, Department of Mechanical Engineering
S.S.C.E.T.,Bhilai, Chhatisgarh, India
Mr. Sachine Gabhane
Assist. Engineer AtAdani Power Plant.
Tirora. India

Abstract: A CFD investigation has been carried out about the performance of a 660 MW
tangentially coal-red boiler, focusing on the reduction of NOx attainable by changing angle of
secondary air fire with the direction from horizontal to an angle of inclination. Due to changing
angle of secondary air fire air The NOx value of calculation at the outlet of furnace is 15% less than
without changing angle of secondary air fire air .The aerodynamic field in the furnace with various
secondary air fire parameters are arranged to analyze the effect of secondary air fire air added to
the boiler furnace. The optimized secondary air fire parameters and arrangements are gained and
simulated in reacting flow. The result shows that the NOx emission of the furnace with changing
angle of secondary air fire is reduced and the NOx concentration of outlet decreases from 166 to
143 ppm (O2=6%). A reasonable agreement has been attained in most cases. For experimental set
up and validating the result, the 660 MW super critical boiler at Adani Power Maharashtra Limited,
Tirora (M.S.) in India is used.
Keywords: tangential fired boiler,cfd,temperature distribution,species concentration,NOx emission.

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1. Introduction:
Tangentially fired pulverized-coal boilers are the mostly used type of boiler in the power
generation. However, there are still some problems like NOx formation, unburned carbon in
ash, heat imbalance and gas temperature deviation in super-heaters and repeaters, and
slagging in the furnace [3]. The combustion of blends of coals is now often conventional
practice in power stations either for economic reasons or to improve the combustion
properties of the blended coal [1, 2].
Formation of nitrogen oxides in mega combustion systems is a relevant problem due to their
harmful impact upon the environment. Their control and reduction is a worldwide concern,
thus governments and institutions facing environmental protection have established
restrictive legislations that limit NOx emissions to the atmosphere[2].
In pulverised-coal-red utility boilers, the so-called primary techniques for reduction of
nitrogen oxides mainly consist in the modication of combustion conditions to restrict their
formation. Depending on fuel and also on the design of furnace and combustion system,
deviation of gas, diverse technologies and measures are available like low NOx burners,
fuel/air staging, overre air, reburning, ue gas recirculation. In many cases these primary
techniques are not sufficient to meet the new, very stringent regulations, so that secondary
techniques, i.e., NOx selective chemical reduction, are unavoidable. However, the cost of
some primary measures can be quite low, so that an optimization problem arises, related to
the possibility that a change of combustion conditions may lead to a substantial reduction of
NOx and thus to a reduction of size and operation requirements of costly NOx abatement
installations[3].
However, a previous and precise estimation of the change in NOx levels as a function of
operating conditions is always needed. Obtaining it in situation, by empirically characterizing
the behavior of the installation, is normally an task in such a large industrial plant as a coalred utility boiler. Since generation schedules are now a day very constrained, and size and
complexity of equipment are important, only specic and limited tests can be planned and
executed, and the gathered data sets are usually incomplete and subjected to a large measure
of uncertainty [4]. To predict the response of the boiler, the use of comprehensive
mathematical models and numerical methods seems presently to be more suitable, since a
great amount of complete and detailed data can be generated in a feasible way.

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The present work describes the experimental and numerical investigations of with or without
changing secondary air fire in a 660 MW boiler NOx reduction. CFD simulation and gas
temperature measurements, together with the evaluation of the relevant plant data have been
carried out to find out the principal cause of this reduction. . In this boiler opposed concentric
tangential & counter-tangential tangential firing system is adopted for firing equipment,
tangential firing mainly depends on the 4.5 ~ 15 deflections of some secondary air nozzles.
Because some secondary air nozzles deflect in the clockwise direction, driving the flue gas to
rotate in the clockwise direction. This makes up the tangential firing, and this part of
secondary air is called rotation-starting secondary air. On the contrary, part of secondary air
nozzles adopts the counterclockwise deflection of 5~25. Through the deflection in the
counterclockwise deflection, the rotating strength of the air stream entering the upper
combustion zone is alleviated or even eliminated. So it is called rotation-eliminating
secondary air. In the meantime, the upper over-fire air nozzles can be adjusted horizontally
for 15. The rotation eliminating action can also be realized through the deflection in the
counterclockwise direction. With the opposed concentric tangential & counter-tangential
tangential firing mode, part of the secondary air stream is staged in the horizontal direction.
The mixing of air and pulverized coal is delayed on the initial firing stage, and the quantity of
NOx generated is low[17]. This firing system is shown in figure 2(b) [17].

2. Case-Study Boiler:
The boiler furnace geometry of the simulated boiler can be seen in Fig. 1. The threedimensional geometry was created using Ansys workbench. The boiler geometry is to be
constructed in such a way that depending upon its dimension as height, length and depth from
ground .The width of the furnace is 19824mmwhilethe depth is 17640mm. Elevation of the
water wall lower header is 9000mm; elevation of centerline of roof is 77800mm, and
elevation of the main girder bottom is 85700mm. Thirty two burners are arranged in the
corner of tangential fired furnace. Total 8 mills are located for coal with primary air firing
each mill having four burners and the mill are spaced at the certain distance as 912.4 mm.
with each other. In between mill there is some provision for firing secondary air .Boiler
combustion system is design according to medium speed pulverizers and cold primary air
direct-fired system. 32 direct air burners are arranged at the lower four corners of the furnace
in 8 layers, and power coal and air are loaded through the four corners to burn in the furnace
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tangentially. Nozzle center elevation of the top row of burner is 39780mm and is 20020mm
far from the platen bottom. Nozzle center elevation of the bottom row of burner is 26150mm
and is 5131mm far from the furnace hopper corner. Technology, quick ignition pulverized
coal nozzle etc. burners in the corners. The velocity of coal+air is shown in table 2. On upper
part of the furnace platen superheater and high temperature superheater are arranged. Platen
superheater, high temperature superheater and reheater are fixed by fluid cooled spacer tube
along the furnace width direction. The fluid cooled spacer tube is led out from the backpass
extending side wall lower header, converging with the main pipe before entering the
secondary desuperheater. Superheater steam temperature is controlled by coal-water ratio
adjustment and two-stage spray water. The primary desuperheater is located at the low
temperature superheater outlet piping, while the secondary one is at the platen Geometry and
distribution of inlet sections along the height of each corner of the furnace Superheating and
two reheating sections are located. Economizers are located in the rear pass of combustion
gases. The coal used in this boiler based on the proximate and ultimate analysis shown in
table 3.
Fig. 2(a) .shows in detail the arrangement of air and coal nozzles. Dierent inlet sections are
represented, corresponding to the mixture of primary air and pulverized-coal (PA + PC), the
secondary air (SA) and also the fuel-oil used during start-up (FO). Surrounding the primary
air and pulverized-coal entrance, a narrow square ring also introduces a small ow of
peripheral secondary air. Dierent orientations can be imposed for the entering streams,
separately for primary air + pulverized-coal and for secondary air. Tilting angle can be also
varied independently, within the range 4.5 ~ 15 .In below table summarizes the Main boiler
operating conditions of boiler.

3. Mathematical Modelling:
A commercial CFD code [2] has been used to simulate combustion, uid and particle ow,
heat and mass transfer, under the assumptions explained below, inside the furnace.
The mesh adopted is unstructured structured and consists of 1,72,436 tetrahedral elements.
Mesh density is rened in the neighbourhood of coal + air inlet sections, being progressively
decreased inside the furnace. To calculate a numerical approximation to the elds of Table 1
Main boiler operating conditions in boiler[17].
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Case description

1st Mill (In

Each
Burner)
24.6m/sec
&343k
37m/sec
& 598k
19 m/sec
&598k

T.P.A.velocity(m/s)
& temperature (K)
S.A velocity(m/s)
& temperature (k)
P.S.A velocity(m/s)
& temperature(K)
Total coal feed rate
3.47
(kg/s)
Calorific value
1.64e+07

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2nd Mill(In
Each
Burner)
24.6m/sec
&343k
37m/sec
& 598k
19 m/sec
&598k

3rd Mill(In
Each
Burner)
24.6m/sec
&343k
37m/sec
& 598k
19 m/sec
&598k

4rth Mill In
Each
Burner)
24.6m/sec
&343k
37m/sec
& 598k
19 m/sec
&598k

3.47

3.47

3.47

3.47

1.64e+07
Table 1

1.64e+07

1.64e+07

1.64e+07

5th
Mill(In
Each Burner)
24.6m/sec
&343k
37m/sec
& 598k
19 m/sec
&598k

Where,
P.A.For Primary Air Velocity.
S.A. For Secondary Air Velocity.
P.S.A.For Peripheral Secondary Air Velocity.
Table 2 Proximate and ultimate analysis of the coal-red during the tests[17].
Proximate analysis

(% db)

Ultimate analysis

(% daf)

Moisture
Ash content
Volatile matter
Fixed carbon
Total sulphure

17
4
37
43
0.83

Carbon
Hydrogen
Nitrogen
Sulfur
Oxygen

74.42
4.9
1.5
1.0
18.13

Table 2
The general layout and line diagram of simulated boiler geometry is shown below with
burner details

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(a)

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(b)

Figure 1: (a) Three dimensional arrangement of boiler (b) side view of tangential fired boiler

(a)

(b)
Figure 2: (a) arrangement of air and coal nozzles (b) firing system inside the boiler.
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temperature, concentration of species and NOx distribution , the discretized conservation

equations for mass, momentum and energy aresolved. Only mass-weighted averaged values
are sought, assumed stationary, so that the Reynolds-averaged versions are adopted.
Turbulence closure is eected by means of the standard ke model [4]. As for boundary
conditions, axial velocities and temperatures are xed on the inlet sections and standard wall
functions are used for solid contours. Representative, constant values of temperature and
emissivity are also imposed on the walls of the enclosure: 673 K and 0.85, respectively.
3.1. Mathematical Modeling Of NOx Formation:
A realistic mathematical model of NOx formation in reactive systems can be accomplished
from three dierent perspectives [3]:
i. A detailed chemistry schemes are implemented (including hundreds of elementary
reactions), coupled with a estimated elds of velocity and temperature, these being,
necessarily, very simplified.
ii. A simplied chemical kinetics based on a less number of overall reactions is coupled
with detailed explanation of the velocity, temperature and species concentration elds
throughout the system.
iii. Advanced engineering correlations are inferred from a large variety of operating
conditions in the real system, which are used to predict the emissions under different
scenarios.
The rst alternative has been implemented with gaseous reactions i.e. homogeneous reactions
of gaseous fuels and focuses mainly on chemical kinetics. Comprehensive modeling of NOx
formation and depletion in pulverized fuel combustion additionally demands the simulation
of complex turbulent two-phase ow, heterogeneous reactions, and heat and mass transfer.
Since all these phenomena are coupled, the use of detailed models of chemical mechanisms is
neither feasible not warranted, so that a simplied approach has to be assumed. The third
alternative requires the execution of a large amount of well-controlled experiments, which is
in fact unfeasible for a utility boiler.
Consequently, global reaction models incorporated within computational tools is the
preferred approach for predicting NOx in large coal-red utility boilers.
Formation and depletion of nitrogen oxides are highly complicated processes, involving a
largenumber of intermediate species, in most case. Generally, the most relevant are NO,
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NO2, N2O, NH3 and HCN [8]. Their concentration and signicance mainly depend on
gastemperature and fuel/oxygen ratio in the neighbourhood of the burning particle. For
typical temperature and air excess ranges in large pulverized-coal ames, NO amounts to
more than 90% of the total NOx produced, which results from three main mechanisms:
thermal NOx, fuel NOx and prompt NOx. The contribution of prompt NOx is usually
negligible in coal-red furnaces [9], being only noticeable in hydrocarbon ames under rich
fuel conditions [3].
3.1.1. Thermal NOx:
Thermal NOx is generated by oxidation of the atmospheric nitrogen present in the
combustion air. The formation of thermal NOx is evaluated by a set of strongly temperaturedependent chemical reactions, known as the Zeldovich mechanism. The reactions governing
the process are the following:
O+N2N+NO

(1)

N+O2O+NO

(2)

N+OHH+NO

(3)

Then net formation of nitric oxide is given by the following expression:

[

[ ][

[ ][

[ ][

][ ]

][ ]

][ ]

(4)

Where k1, k2, k3 are the kinetic rate constants for each of the forward/reverse reactions
(1)(3) of the extended Zeldovich mechanism. Dierent authors [7,8] have critically
evaluated data available and correlation of these constants. Assuming steady-state production
and consumption of atomic nitrogen, which is reasonable in this case, the formation rate of
thermal NOx given by Eq. (4) can be simplied as follows:
[

[ ]

[
[
[

]
[

]
]

(5)
]

In order to use this equation, some hypotheses to estimate local concentrations of O and OH
radicals are required, since these are normally not considered by common combustion
models, which only handle major species. Several works [3,5,9] describe dierent approaches
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to the question, mostly involving assumptions of equilibrium or partial equilibrium depending

on ame region and fuel nature. The questionis still the subject of some controversy. In this
work, the partial equilibrium approach, both for atomic oxygen and hydroxyl radical, has
been adopted.
3.1.2. Fuel NOx:
This is the main contribution in large pulverised-coal ames. Fuel NOx is produced by
oxidation of the nitrogen bound in the coal, both in the volatile matter and in the char. The
pathway leading to formation and depletion of fuel NOx is not yet completely understood.
Usually, it is assumed that HCN and NH3 are the main species formed as nitrogen-bearing
intermediates from the volatiles, under rates depending both on local combustion conditions
and characteristics of nitrogen in the specic fuel. HCN and NH3 are competitively oxidized
and reduced by means of the following generic scheme involving four reactions:
HCN+O2 NO+..

(6)

HCN+NO N2+

(7)

NH3+O2 NO+..(8)
NH3+NO N2+.(9)
As for the solidgas reactions, a fraction of the nitrogen in the char is directly converted to
NO, in(rough) proportion to the carbon consumption rate. Additionally, heterogeneous
reduction to N2 by interaction with the char particles can contribute to slightly decrease the
total amount of fuel NOx
Char+ NO

N2+.(10)

In pulverized-coal ames, 2040% of the nitrogen bound in the fuel results in NOx formation
and remaining produces molecular N2 [9]. Several have deduced experimental values for the
kinetic constants of either the four homogeneous reactions (6)(9) or just the NH3 scheme
(8), (9), whereas the work of Levy et al. [15] is of widespread use to model the heterogeneous
reduction. The net formation of nitrogen oxides accordingly to the kinetic scheme (6)(10) is
given by the following expression:

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[

][

][

][

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]

][

(11)

Where is the oxygen-reaction order, correlated by desoete [16] as only dependent on the O2
concentration in the gas stream, and char is the char surface density per volume unit.
3.1.3 Prompt NOx:
This third source is contribute to the reaction of atmospheric nitrogen, N2, with radicals such
as C, CH and CH2 fragments obtained from fuel from fuel, where this cannot be explained by
either the thermal or fuel processes, this results in the formation of fixed species of nitrogen
such as nitrogen monohydride(NH), hydrogen cyanide (HCN), dihydrogen cyanide( H2CN)
and cyano radical (CN-) which can oxidize to NO. The fuels that contain nitrogen, the
incidence of prompt NOx is especially minimal and it is generally only of interest for the
most exacting emission targets.[13]
Formation of N2Ofrom NO
At high pressures NO formation via N2O becomes important:
N2 + O + N2O

(12)

N2O + O 2 NO (activation energy 97 kj/mol)

(13)

N2O + O N2 + O2(14)
Competing Reactions :
N2O + O NO + N (thermal NO)
N2O + O + N2O

(15)
(16)

All these processes can be modelled by means of kinetic rate expressions, detailed enough to
(hopefully) characterize the process, but consisting of only a few global steps, to facilitate
their interaction with the computation of temperature and concentration elds. Within the
realm of CFD modeling of large pulverized-coal furnaces, NOx simulation is usually
decoupled from the main calculation of the ame structure [7]. The calculation sequence is
therefore made up of three stages. Calculations of flow and turbulence is first solved after
convergence then in second stage start energy, pdf temperature and concentration of major
species are rstly solved to convergence, and NOx prediction is executed thereafter as a postprocessing task. This expedient is reasonable, since the presence and evolution of trace
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species hardly aect the overall ame evolution, controlled by comparatively faster fuel
combustion mechanisms. Coupled resolution of coal combustion and NOx chemistry would
result in any case in an unreasonable computational demand.

4. Results: Validation And Discussion:

At first, only the gas flow equations are solved in order to achieve for stable calculations and
fast convergences for two-phase flow, combustion, heat-transfer and chemical reactions.
After the flow field converges, the trajectories of the coal particles interacting with the flue
gas are calculated. Next, the chemical reaction and enthalpy equations for coal combustion
are taken into account. Numerical calculations are repeated until the flow and temperature
converge. Finally, the NOx transport equations are solved based on the predicted flow and
temperature. A convergence criterion is that the normalized residuals for all the variables
need to be less than 10-3. Flow and temperature field convergence is obtained after more than
1465 iterations. The results obtained from the calculation without changing SAFA operation
are presented first and discussed in Sections 4.14.3. Then, the results with the changing
SAFA operation are presented and discussed along with a comparison of the two results in
Section 4.4. The results predicted from the numerical calculations are then compared with the
measured values at the furnace exit .The comparison between the measured values and
predicted results shows good agreement.
The predicted temperatures at the furnace exit are 1545K, while the measured temperatures
are 1600K.The predicted concentrations of O2, CO2 and NOx at the boiler exit are 0.78%,
23% and 166 ppm, respectively.
The differences between the measured and predicted value of temperature and NOxare given
in Table 3.
Section name
Temperature[K]
NOx [ppm]

Furnace exit
Furnace exit

Plant
measurement
1600
90
Table 3

Predicted (without SAFA) Predicted

with (SAFA)
1545
1540
166
143

Note: The value means the mass weighted average in each section
4.1. Temperature Distributions:
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The temperature distributions in the cross sections are shown in Fig. 3.

Figure 3: a) depicts contours of mass weighted average of temperature along central vertical
intersection of the furnace. b) mass weighted average of temperature along the furnace
elevation in horizontal cross section .c) Surface indicating high temperature zones
The temperature of the flue gas is relatively high in the central region of the furnace where
coal combustion actively takes place. The change pattern of the flue gas temperature is
clearly shown along the furnace height in Fig. 3b. The temperature of the air injected from
the corner burners is 343 K, and it increases up to a maximum temperature of 1800 K in the
central region of the furnace. A notable temperature deviation is seen in the lower level of the
furnace, but in the higher levels, the temperature deviation decreases due to stronger swirling
and increased mixing. With the increased height, the average temperatures in each cross
sections also increase because of high combustion intensity. However, as the flue gas goes
upward in the furnace , the temperature of the flue gas decreases due to the heat transfer
between the flue gas and furnace walls via convection and radiation. Finally, the flue gas
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leaves the boiler furnace at an average temperature of 1,545 K. The iso-surfaces at

temperatures of 1700K and 1800 K indicating high temperature zones in the furnace are
depicted in Fig. 3c. These high temperature regions are closely related to NOx formation
which is dependent on local temperature
4.2. Species Distributions:
4.2.1. O2 Distributions:
The O2 distributions in the cross sections are shown in Fig. 5.
The O2 concentration in the furnace is relatively higher near the burners. The O2 contained in
the air injected into the furnace, where the temperature is relatively higher, is quickly
consumed during the combustion processes. As a result, the O2 mass fraction rapidly
decreases. In particular, the O2 mass fraction remarkably decreases near the burners, where
combustion is more active, and the fuel volatile species are more rapidly consumed. As
depicted in Fig. 4, the high temperature regions in the furnace roughly correspond to the
regions of the lower O2 mass fraction.
4.2.2. CO2 Distributions:
The CO2 distributions in the cross sections are shown in Fig. 6.
In contrast to the O2 mass fraction, the CO2 mass fraction significantly increases as the air
moves from the burners due to the active combustion processes. high peaks of the O2 mass
fraction are shown in the middle zone of the furnace when the CO2 mass fraction is low,
which suggests that the sudden variation in their values is due to the supply of combustion air
through the burners. In Fig. 5b, as the temperature of the flue gas increases, the O2 mass
fraction decreases steeply while the CO2 mass fraction increases because volatile
combustible contents of the coal burn near the burners.

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Figure 4: depicts contours of mass weighted average of O2mass fractionalong central

vertical intersection of the furnace (XY plane=8m).b) mass weighted average of O2 in mass
fraction along the furnace elevation.

Figure 5: a)depicts contours of mass weighted average of CO2 mass fraction along central
vertical intersection of the furnace (XY plane=8m).b) mass weighted average of CO2 in mass
fraction along the furnace elevation in horizontal section
4.3. NOx Emissions:
The NOx distributions in the cross sections are shown in Fig. 7.

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The NOx concentration contour and the iso-surface showing a NOx concentration zone of
180 ppm are depicted in Fig. 7. The predicted maximum NOx concentration is 166 ppm and
the relatively high NOx concentration zones are found in the furnace center where the
temperature is higher and the combustion processes are more active. The predicted total NOx
emission is 166 ppm with CO2 levels at 23 % (mass fraction) at the furnace exit. These
predictions agree well with the measured values listed in Table 3. The results show that the
NOx emission is fairly low due to the combination of lower temperature peaks, use of low
NOx burners and staging combustion technology Fig. 7c shows the zones of the total NOx
formation. The NOx formation rates are low since the oxygen concentration is very low in the
central zones, although the temperature is relatively high.

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Figure 6: depicts a)contours of mass weighted average of NOx along central vertical
intersection of the furnace.b) mass weighted average of NOx along the furnace elevation.c)
Surface indicating high NOx zones
4.4. NOx Emissions With Changing SAFA Operation:
The calculation under changing SAFA operation, where the angle of secondary air is changed
through the SAFA ports, is carried out to investigate NOx reduction. The predicted
temperature distribution in the vertical section and the iso-surface at 1800 K indicating a
relatively high temperature zone in the furnace, are depicted in Fig. 8.
Compared with the temperature distribution without changing SAFA operation discussed in
Section 4.1, a relatively high temperature region is moved upward and slightly enlarged in the
upper furnace due to the occurrence of combustion consuming the air supplied through the
changing SAFA ports.
The predicted temperatures at the furnace exit is 1,540 K. The predicted concentrations of
species O2, CO2 and NOx at the boiler exit are 0.73%, 23.00% and 143 ppm, respectively.
Based on the data with changing SAFA operation,the total NOx emissions in the boiler are
reduced by 15% as noted in Table 3. The reduction in NOx might be the result of decreased
contact of nitrogen from the fuel with oxygen in the combustion air, which makes a fuel-rich
zone where NO could be reduced to N2 and the reduction due to the decreased temperature in
the furnace. These features are consistent with the concept of air staging, in which the
reaction temperature is decreased and thus the formation of NOx decreases. The variances in
temperatures, species mass fractions and NOx formation rates along the furnace height with
and without changing SAFA operation are presented in Fig. 8,9,10.and the values correspond
to the average in each horizontal cross section. In particular, the NOx concentration
significantly decreases in the case with changing SAFA operation. while the significant
decrease in the case with changing SAFA operation is due to dilution caused by air injected
through the changing SAFA ports.
Numerical predictions vary signicantly with the model used to reproduce devolatilisation.
Dierences are as high as 15%, which optimistically may indicate a substantial improvement
of two-step over single-step schemes. However, the only rm conclusion can be that
predictions are very sensitive to the devolatilisation model itself, which is indeed a relevant
fact.
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Figure 7: a)depicts contours of mass weighted average of temperature along central vertical
intersection of the furnace (XY plane=8m).b) mass weighted average of CO2 in mass fraction
along the furnace elevation in horizontal section c) Surface indicating high temperature
zones

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Figure 8: concentration of species along the furnace elevation

Figure 9: a)depicts contours of mass weighted average of NOx in ppm along central vertical
intersection of the furnace (XY plane=8m). b) mass weighted average of NOx in ppm along
the furnace elevation in horizontal section

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5. Conclusion:
The characteristics of the flow, combustion, temperature and NOx emissions in the 660MW
tangentially fired pulverized-coal boiler have been numerically investigated using
comprehensive models for the combustion processes and NOx formation. In order to generate
accurate predictions, additional attentions have been paid in selecting the calculation domain,
generating mesh, and choosing numerical models, since NOx formation is affected by fluid
flow, temperature and oxygen concentration distributions. The flow fields, flue gas and coal
particle motion, temperature distributions, species distributions and NOx emissions in the
boiler have been obtained and compared with the measured values. The comparison between
the predicted results and measured values have shown a good agreement, which implies that
the adopted combustion and NOx formation models are suitable for predicting the
characteristics of the flow, combustion, temperature and NOx emissions in the boiler. The
relation among the temperature, O2 mass fraction and CO2 mass fraction has been clearly
demonstrated based on the calculated distributions. The predicted results have shown that the
NOx formation in the boiler highly depended on the combustion processes as well as the
temperature and species concentrations. The results obtained from this study have shown that
changing SAFA operation is a good way to reduce the NOx emissions from the pulverizedcoal fired boiler. The decrease in the NOx formation is due to the decreased contact of
nitrogen from the fuel with oxygen in the combustion air and the decrease in temperature. It
is generally accepted that at temperatures below 1800 K, NOx is not significantly formed by
the Zeldovich mechanism and is also not a major source of NOx in local fuel-rich zones [1].
Therefore, for more accurate control of NOx formation, it is important to control
temperatures above 1,800 K and encourage fuel-rich conditions.The detailed results presented
in this paper may enhance the understanding of complex flow patterns, combustion processes
and NOx emissions in tangentially fired pulverized-coal boilers. This paper may also provide
a useful basis for NOx reduction and control. Further research focused on the reduction and
control of NOx emissions may be needed under various operating conditions such as
implementing overfireair ,different flow rates of over fire air and operating loads.

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