Journal of Power and Energy Engineering, 2015, 3, 178-184
Published Online April 2015 in SciRes.
How to cite this paper: Iqbal, J. and Gao, S. (2015) Computational Analysis of Gas Phase Mixing in a Co-Fired Burner with
Two Different Designs. Journal of Power and Energy Engineering, 3, 178-184.
Computational Analysis of Gas Phase Mixing
in a Co-Fired Burner with Two
Different Designs
J. Iqbal, S. Gao*
Department of Engineering, University of Leicester, Leicester, UK
Email:, * .uk
Received January 2015
The study of swirling jet combustor for biomass coal co-firing is of great interest for energy indus-
try. The biomass co-firing can serve as a NOx reduction method as well as the better use of renew-
able energy source. Large eddy simulation (LES) and RANS modelling have been performed with
two different burner designs. Usually pulverized coal-biomass mixture enters the furnace along
with primary air through primary pipe, and the secondary pipe provides necessary air and mixing
for combustion. The improved model has three passages including primary, secondary and middle
passage for swirling. The simulations on two geometries have been compared, and the aim is to
design a better and improved burner model for better pre-combustion mixing in the biomass co-
fired furnace. The results from two-way and three-way geometry have been compared with each
other as well as with the results from the furnace model used by Apte and Mahesh [8].
Swirling Jet, Co-Firing, Biomass, Turbulence, LES
1. Introduction
During combustion process the fossil fuels release carbon dioxide, carbon monoxide, SOx and NOx. Coal has
different types and its chemical and physical properties of coal change with varying amount of carbon [1].
Biomass is one of the main renewable energy sources, having reasonable cost compared with other renewable
sources. Biomass may be forestry material, agriculture waste, straw, municipal waste, sewage sludge etc. [3].
The biomass utilization can contribute in reducing NOx, SOx, CO2 and CO. The biomass material has more vola-
tility but low energy than hard coal, while a biomass-coal blend contains balanced volatility and energy. Usually
biomass coal co-fired or retrofitted burners consist of primary air pipe conveying air fuel mixture, secondary air
(swirled) pipe conveying additional air for turbulence and mixing [4]. In this project, the modified geometrical
model consists of primary air pipe, secondary pipe and tertiary flow pipe. The air from secondary pipe helps to
increase the turbulence mixing and produce wakes in the region away from the center line. The results from
Corresponding author.
J. Iqbal, S. Gao
two-way and three-way geometry have been compared with each other as well as with the results from the fur-
nace model used by Apte and Mahesh [8], which is two-way model geometry. Different turnulence models and
schemes have been used, and it is found that the three-way assembly is better than the two-way assembly for
pre-combustion mixing. The geometrical models are shown schematically in Figure 1.
2. Methodology
The characteristics of fluid flow in the burner can be described by the fundamental conservation laws. This
combines the continuity equation and momentum equation which are listed as follows:
Figure 1. Schematic of biomass co-fired models. (a) Three-way assem-
bly; (b) Two-way assembly.
J. Iqbal, S. Gao
( )
+∇⋅ =
( )()
uuu p
+∇⋅= −∇+∇⋅
( )
( )
u uIu
τη η
= −∇+∇+∇⋅
 
The burner geometry generation, meshing and calculation of the conservation of mass and momentum equa-
tions are carried out by using the commercial software package ICEMCFD and ANSYS Fluent.
Large eddy and RANS k-model simulations have been performed in this project to investigate the turbulence
effects on the flow [2] [4]. It is well known that LES is a better tool for flows such as jet shear and wall jet im-
pingement [7]. The LES scheme solves the filtered Navier-Stokes equation and can produce more useful infor-
mation, e.g. instantaneous flow fluctuations, than the RANS viscous models. The leading flow parameters for
the three-way assembly are given in Table 1. For the two-way assembly geometry, there is no tertiary entry, and
the secondary is swirling flow.
The 2nd order upwind scheme for flow and QUICK scheme for turbulence are selected with SIMPLE algo-
rithm for the simulations. The turbulence simulation results are compared with each other and also with the
simulation results from Apte and Mahesh [8].
3. Results and Discussion
The geometry from the reference [8] consists of two jets from primary and secondary annular pipe discharging
into a huge cylinder. The effect of the jet velocity ratio shows the complexity of the isothermal models [4], while
the new geometry has an additional annular pipe. It can be seen in Figure 2 that the flow in the new three-way
geometry has better mixing than the two-way assembly from the reference of Apte and Mahesh [8]. The con-
tours in Figure 2 show that the assembly with additional entry is more helpful for better pre-combustion mixing.
The additional air produces additional turbulence and wake effects, and this additional air is also helpful to re-
burn the solid particles like coal/biomass in the burner [1].
Figure 3 and Figure 4 show the mean axial velocity and RMS of mean axial velocity variation with respect to
the characteristic length for different assemblies; the swirling motion governs the flow to increase the rapid
spreading and the reattachment of shear layers [8]. It can be seen that the axial velocity decreases with the char-
acteristic length. The three-way assembly and two-way assembly have been compared with the results from the
simulations of Apte and Mahesh [8] and the experimental work of Somerfield and Qiu [9]. In Figure 3 the mean
axial velocity for the three-way assembly agrees well with the reference data. Comparison of radial and swirl
velocity has also been made with the reference data [8] [9]. The swirl velocity decreases with the characteristic
length as shown in the graphs. The radial velocity graphs show the different flow property than the swirling and
axial velocity; here the radial velocity has the increased value at X/R = 0.8 than the initial point X/R = 0.1,
slightly increase at X/R = 1.6 and starts to decrease and becomes zero at the end point. The increasing and de-
creeing of the radial velocity produce the desired mixing in the combustor. The pressure is also a property of the
moving fluid depending on the axial velocity, which decreases and increases with the axial velocity. The wall
shear problems can also better be solved by using the three-way.
It is the pressure variation created in the fluid due to the kinetic energy fluctuations that produces the wake
and mixing effects, which contribute positively to the mixing phenomena inside the furnace. Clearly, the im-
proved three-way burner model has better mixing efficiency than the simple co-fired burner, mainly due to the
addition of the new annular entry.
Table 1. Flow parameters for the three-way assembly.
Primary (m/sec) Secondary (m/sec) Tertiary (m/sec) Temperat ure Pressure (Pa)
15 10 18 Not considered Atmospheric
J. Iqbal, S. Gao
Figure 2. Axial velocity for (a) two-way geometry; (b) three-
way geometry; (c) y-velocity for three-way geometry.
J. Iqbal, S. Gao
X/R = 0.1
X/R = 0.8 X/R = 1.6 X/R = 2.6
X/R = 3.5 X/R = 4.8 X/R = 6.1
Figure 3. Axial velocity comparison between LES on two-way (), three-way (----) and experiment [9] () for
swirling flow in combustor.
4. Conclusion
The physics of flow has been analyzed to understand the pre-combustion mixing behavior of air in two different
burner geometries. The aim of the research is to design a better burner/furnace assembly for solid fuel bio-
mass-coal co-firing. The additional un-swirled pipe as secondary entry increases the performance of the biomass
coal co-fired burner because the solid fuel needs additional air for re-burning. The swirling flow produces tur-
bulence, and the secondary pipe entry maintains the swirling for most of the time, which can produce wake ef-
fects in axial and radial directions. The efficient co-fired burner results in better use of the biomass as the main
renewable energy source, and in turn it will result in the reduction of SOx, NOx, CO and CO2 from the emissions
of coal fired power plants. The swirling and dynamic pressure effects mostly depend upon the axial velocity or
J. Iqbal, S. Gao
X/R = 0.1
X/R = 0.8 X/R = 1.6 X/R = 2.6
X/R = 3.5 X/R = 4.8 X/R = 6.1
Figure 4. RMS of axial velocity comparison between LES on two-way (), three-way (----) and experimental data [9]
() for swirling flow in combustor.
kinetic energy of the flow. The radial velocity variations are necessary for maintaining the efficient mixing in
the burner.
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J. Iqbal, S. Gao
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