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Modeling the Gasification Based Biomass
Co-firing in a 600MW Pulverized Coal BoilerDalong Jiang, Changqing Dong*, RuiYang, Junjiao Zhang, and Yongping Yang
Abstract--Gasification based biomass co-firing was an
attractive technology for utilizing biomass as an additional
fuel in utility boilers. Compared to directly co-firing biomass
and coal, it showed following benefits: (1) avoiding biomass
delivery into the boiler, (2) reduced boiler slagging, (3)
avoiding altered ash characteristics. In co-firing
demonstration project, higher percentage of biomass gas for
co-firing was expected. But the effect of percentage of biomass
gas on combustion efficiency, boiler efficiency and pollutant
emission was not clear. In the study, numerical simulation of
co-firing producer gas from biomass gasification and coal in a
600MWe tangential PC boiler was carried out with CFD
method. Combustion behavior and pollutant emission for the
coal fired only case and six co-firing cases were compared.
The results showed that The effect of co-firing on combustion
efficiency was slight, the reduction of NOx emission can be
achieved. The NO removal rate was between 45% and 71%.
In addition, slagging can be reduced for the temperature
decreasing. And the convection heat transfer area should be
increased or the biomass gas should be limited to a low
percentage to achieve higher boiler efficiency.
Key words--biomass gas, co-firing, nitrogen monoxide
I.NOMENCLATURE
Symbol Meaning expression
Gk
the generation of turbulence kinetic energy due to the mean
velocity gradients
Gb
the generation of turbulence kinetic energy due to buoyancy
YM
the contribution of the fluctuating dilatation in compressible
turbulence to the overall dissipation rate
This work was supported by the National Basic Research Program
(2009CB219900), and the National High Technology Research and
Development of China (2008AA05Z302).
D.L. Jiang, C.Q. Dong*, R. Yang, J.J. Zhang, and Y.P. Yang are all with
the National Engineering Laboratory of Biomass Power Generation
Equipment, Key Laboratory of Condition Monitoring and Control for
Power Plant Equipment, Ministry of Education, North China Electric
Power University, China, 102206 (*Corresponding author:
1C constant=1.44
2C constant=1.92
3C constant=0.09
k turbulent Prandtl numbers for k
turbulent Prandtl numbers for
Sk
user-defined source term
S user-defined source term
( )m tv
volatile yield up to time t
,0m
p initial particle mass at injection
1 ,
2 yield factors
ma
ash content in the particle
H the total height of the boiler
z the ratio of the actual elevation to the total height of the boiler
II.INTRODUCTION
Biomass co-firing, the practice of supplementing a base
fuel with biomass fuels which include wood waste, short
rotation woody crops, short rotation herbaceous crops (e.g.,
switchgrass), alfalfa stems, various types of manure,
landfill gas and wastewater treatment gas, began in the
1980s, and become the normal power generation
technology in Europe and the United States.
With minimum modifications to the existing boiler
systems, co-firing was generally viewed as the most cost
effective approach to biomass utilization by the electric
utility industry. It was a family of technologies [1]. These
included: (1) blending biomass with coal on the fuel pile,
then pulverizing and injecting the mixture into the boiler,
(2) preparing the biomass separately from coal, and
injecting it into the boiler without impacting the
conventional coal delivery system, (3) feeding the biomass
into the gasifier to generate producer gas, and injecting the
biomass gas into the boiler through the gas burner.
Reviews of co-firing experiences identified over 100
successful field demonstrations in 16 countries that used
essentially major type of biomass (herbaceous, woody,
animal waste, anthropomorphic wastes) combined with
essentially every rank of coal and combusted in major type
of boiler (tangential, wall, and cyclone fired) [2,3]. The
technical evaluations showed the potential project
benefits:(1) reduced fossil CO2 emissions, (2)reduced other
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airborne emissions including NOx, SO2 and trace
metals,(3)potential for reduced fuel cost,(4) supporting
economic development among wood products and
agricultural industries in a given service area. So co-firing
was a low-cost, low-risk, environment friendly technology
to biomass utilization by the electric utility industry.
However, while biomass was directly co-fired with coal,
some technical challenges appeared, including [4]:
Limited percentage of biomass for co-firing (e.g.
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B. Mathematical Models
The combustion in the furnace was a very complicated
process that included gas turbulent flow, turbulent
combustion, particle movement, volatile devolatilization,
particle combustion, radioactive heat transfer, and these
interacted. In the paper, standard k was selected for
simulating the gas turbulent flow field. The turbulence
kinetic energy kand its rate of dissipationwere obtained
from the following transport equations[6]:
( ) ( ) [( ) ]ti k b M k i j k j
kk ku G G Y S
t x x x
+ = + + + +
(1)
2
1 3 2( ) ( ) [( ) ] ( )t
i k b
i j j
u C G C G C S t x x x k k
+ = + + + +
(2)
/ /uj
G u uk i j u
i
=
Pr
ttG gb i x
t i
=
2kC
t
= (3)
22Y Mt
=2
kM
ta
= (4)
Two mixture fraction/PDF (Probability Density Function)
was selected as the gas turbulent combustion model. The
PDF modeling approach involves the solution of transport
equations for one or two conserved scalars (the mixture
fractions). Equations for individual species are not solved.
Instead, species concentrations are derived from the
predicted mixture fraction fields. The thermo-chemistry
calculations are preprocessed in prePDF and tabulated for
look-up in FLUENT. Interaction of turbulence and
chemistry is accounted for with a probability density
function.
*
sec( , , )
i i fuel f p H =
*
sec( , , )
i i fuel f p H =
/ 2 / 2 *
s ec s ec, , , ,fuel fuelf p p H
i
*
sec( , , )i i fuel f p H =
1( )
fuelp f
2 sec( )p p
Fig.3. Computational tasks between FLUENT and prePDF for a
Two-Mixture-Fraction case
The dispersion of particles due to turbulence in the fluid
phase was predicted using the stochastic tracking model.
The two competing rates Kobayashi model was selected as
the devolatilization model. The kinetic devolatilization rate
expressions of the form proposed by Kobayashi:
1( )
1 1PE RTR A e
= (5)
2( )
2 2PE RTR A e= (6)
Where1
R and2
R are competing rates that may
control the devolatilization over different temperature
ranges. The two kinetic rates are weighted to yield an
expression for the devolatilization as:
1 1 2 2 1 20 0
,0 ,0
( )( )exp( ( ) )
(1 )
t tv
w p a
m tR R R R dt dt
f m m = + +
(7)
The Kobayashi model requires input of the kinetic rate
parameters1
A ,1
E ,2
A ,2
E ,1
,2
. These parameters
recommended by the literature [7] were used.
The Kinetic/Diffusion surface reaction rate model was
selected as the surface combustion model and the P1 as the
radioactive heat transfer model. Segregated solution
method was used and the standard pressure scheme was
chosen. First-Order Upwind Scheme was used in the
discretization of governing equations and simple algorithm
for pressure-velocity coupling [8].
IV.SIMULATIONRESULTSANDDISCUSSION
Seven cases including coal fired only and six producer
gas co-firing cases were calculated with Fluent software
package. For co-firing cases, the percentage of biomass gas
was 3%, 5%, 8%, 10%, 15%, 50% separately. The velocity
vector, coal particles mass, temperature, the concentration
of CO, CO2, O2 and NO emissions in seven conditions
were compared. The excess air ratio at the outlet of furnace
was kept as 1.125. The initial parameters were showed in
table IV. The heat transfer area above the furnace arch was
not considered.
Fig 4 Velocity vectors distribution in the first layer of primary air injection
Fig.5. Velocity vectors distribution in the fourth layer of primary air
injection
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TABLE IV
THE MAIN INLET PARAMETERS OF THE SEVEN CASES
Parameters Coal combustion 3% co-firing5%
co-firing8% co-firing 10% co-firing 15% co-firing 50% co-firing
The layer of the coal
burner
123456
7
1234
567
1234
567
1234
567
1234
567
1234
567
1234
567
The layer of the biomass
gas burner/ 1 1 1 1 1 1234
Coal quantity(t/h) 419 406.43 398.05 385.48 377.1 356.15 209.5
The flow rate of
biomass(m3/s)/ 9.18 15.3 24.48 30.6 45.92 153.01
Outlet velocity of
biomass gas(m/s)/ 10.67 17.79 28.46 35.58 26.69 25.41
Outlet velocity of primary
air(m/s)23.73 23.73 23.73 23.73 23.73 23.73 23.73
Outlet velocity of
secondary air(m/s)
54.712345
6
69.6154.723
456
65.97154.723
456
60.09154.7234
56
56.16154.7234
56
67.47154.7234
56
71.1612354.74
56
Outlet velocity of
OFA(m/s)45.44 45.44 45.44 45.44 45.44 45.44 45.44
(In the option of outlet velocity of secondary air, the number in the bracket represented the layer of the secondary air)
Figure 4, 5 showed the velocity vector distribution in the
first and the fourth layer of primary air injection. Four jet
streams interacted and formed good corner tangential firing.
This suggested that the aerodynamic field was proper and
the air distribution mode meet the demand of corner
tangential firing. The diameter of the tangential circle did
not changed in all cases, which suggested that co-firing did
not impact the field in the furnace.
Fig.6. The mass history of the coal particles
Figure 6 showed the particle traces colored by particle
mass. For co-firing 3%,5%,8%,10%,15% biomass gas and
coal combustion case, some particles fell into the cold ash
hopper under gravity action, which caused solid
incomplete combustion heat loss. However, most particles
moved upward with flow gas and burned out. When
co-firing 50% biomass gas, pulverized coal was injected
from the upper layer of burners, no particle fell into the
cold ash hopper.
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
800
1000
1200
1400
1600
1800
2000
T/K
z/H
coal combustion
3% co-firing
5% co-firing
8% co-firing
10% co-firing
15% co-firing
50% co-firing
Fig 7 Mean temperature distribution along with the height of the furnace
The mean temperature distribution along the height of
furnace was similar for the seven cases as showed in Fig 7.
There were two peak temperatures which were
representing the burner region and the area in the upper
reaches of OFA injection. For the co-firing cases, the mean
temperature was lower than that of the coal combustion,
and decreased with the percentage of the biomass gas
increased. The gas volume was higher for co-firing cases
than that of the coal fired only case, thus, the temperature
thereupon decreased. When the percentage of biomass gas
increased to 50% the temperature of outlet flue gas
decreased by 300K.
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0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
0
1
2
3
4
5
6
7
CO/mo
lefraction
z/H
coal combustion
3% co-firing
5% co-firing
8% co-firing
10% co-firing
15% co-firing
50% co-firing
a
b
cd
Fig 8 Mean CO concentration distribution along with the height of
furnace
The mean CO concentration was similar for all the cases.
There are two maximum and two minimum which were
marked by a, b, c and d in figure 9. a and b
were present to near the first and the fifth layer of burner
separately. c was present to the space between the first
group burner and second group burner. With the feeding of
the over-fire air, the second minimum d appeared. For
co-firing cases, the CO level decrease with the increasing
proportion of the biomass gas. When the percentage of
biomass gas was increased to 50%, the CO level was lower
than that of coal combustion case.
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
13
14
15
16
17
18
CO2/molefraction
z/H
coal combustion
3% co-firing
5% co-firing
8% co-firing
10% co-firing
15% co-firing
50% co-firing
Figure 9 Mean CO2 concentration distribution along with the height of
furnace
Figure 9 presented the CO2 level. The peak value was
nearby the space between the first group burner and second
group burner. The CO2 level decrease for the co-firing
cases but the difference of the outlet value was not
significant. Figure 10 presented the O2 concentration. The
trends were similar and two peak values were present to
the burner area and the OFA injection area separately. The
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
O2/molefraction
z/H
coal combustion
3Data1_Co-firing
5% co-firing
8% co-firing
10% co-firing
15% co-firing
50% co-firing
Figure 10 Mean O2 concentration distribution along with the height
of furnace
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
0
00
00
00
00
z/H
coal combustion3% co-firing5% co-firing8% co-firing10% co-firing
15% co-firing50% co-firing
Figure 11 Mean NO concentration distribution along with the height
of furnace
O2 level increased with the increasing proportion of the
biomass gas. Figure 11 presented the effect of the co-firing
on NO emission. There are two peak values, one was
present to the burner area, and the other was present to the
OFA injection area. Table V showed NO emission at outlet
and NO removal rate of co-firing. The NO removal rates
were between 45% and 71% for co-firing cases. As
compared to coal, producer gas was characterized by low
nitrogen content and can reduce the NO formation, thus,
the NO level decreased dramatically when co-firing.
TABLE V
THE NO EMISSION AT OUTLET AND NO REMOVAL RATE OF
CO-FIRING
ConditionOutlet NO mole
fraction/ppm
Removal rate of
NO/%
Coal combustion 149.697 /
3% Co-firing 82.246 45%
5% Co-firing 60.707 59%
8% Co-firing 60.065 60%
10% Co-firing 51.792 65%
15% Co-firing 50.631 66%
50% Co-firing 44.138 71%
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For all the co-firing cases, the CO, CO2 and O2
concentration distributions were similar to that of the coal
fired only case. The differences of the outlet values of these
parameters were slight. Aerodynamic field was proper and
coal particles burned completely in all cases. The
temperature level was lower when co-firing, this can relief
the slagging problem. These analyses showed that the
effect of co-firing on combustion efficiency was slight. The
NO emission was reduced when co-firing.
V.CONCLUSIONS
In this study, 3%, 5%, 8%, 10% 15%, 50% by heat basis
producer gas from biomass gasification was co-fired with
coal in the 600MW tangential PC boiler. The results
showed that:
NO emission decreased with the percentage of the
biomass gas increased. The removal rate was between
45% and 71%.
The effect of co-firing on combustion efficiency was
slight. The flue gas temperature was lower and the
flue gas quantity was higher when co-firing, and
decreased with the percentage of the biomass gas
increased. The convection heat transfer area should be
increased or the biomass gas should be limited to a
low percentage to achieve higher boiler efficiency.
The slagging can be reduced for lower co-firing
temperature.
VI.REFERENCES
[1]D.A.Tillman, Biomass co-firing: the technology, the experience, the
combustion consequences, Biomass and Bioenergy , 2000,19: 365-384.
[2] Baxter L., Biomass co-firing overview, Second world conference
and exhibition on biomass for energy, industry and climate protection.
Rome, Italy, 2004.
[3] Koppejan J., Introduction and overview of technologies applied
worldwide, Second world conference and exhibition on biomass for
energy, industry and climate protection. Rome, Italy, 2004.
[4] Larry Baxter, Biomass-coal co-combustion: opportunity for
affordable renewable energy, Fuel, 2005, 84: 1295-1302.
[5] Magn Lapuerta, Juan J. Hernndez, Amparo Pazo, et al., Gasification
and co-gasification of biomass wastes: Effect of the biomass origin and
the gasifier operating conditions, Fuel processing technology. 2008,
89(9): 828-837.
[6] Fujun Wang, Computational Fluid Dynamic Analysis: Principle and
Application of CFD Software, Beijing, China, 2004.120-123.
[7] Luis I. Dez, Cristbal Corts, Javier Pallars, et al., Numerical
investigation of NOx emissions from a tangentially-fired utility boiler
under conventional and overfire air operation, 2007, 87(7):1259-1269.
[8] Ryan Zarnitz, Sarma V. Pisupati, Evaluation of the use of coal
volatiles as reburning fuel for NOx reduction, Fuel, 2007, 86(4):
554-559.
VII.BIOGRAPHIES
Dalong Jiang was majoring PH.D. program
in North China Electric Power University. He was
the chairman of Dragon Power Corp.( China). His
special fields of interest are the clean utilization of
Biomass energy.
Rui Yang was born in Shaanxi province in
china on November 30, 1983. She is studying at
North China Electric Power University. Her
special fields of interest are efficient and clean
utilization of biomass.