IFRF Combustion Journal
Article Number 200107, September 2001
ISSN 1562-479X
MODELING NITROGEN CHEMISTRY IN THE FREEBOARD
OF BIOMASS-FBC
A. Brink, S. Boström, P. Kilpinen and M. Hupa
Åbo Akademi University
Lemminkäsenkatu 14-18 B
FIN-20520 Åbo
FINLAND
Corresponding Author:
A. Brink
Åbo Akademi University
Lemminkäsenkatu 14-18 B
FIN-20520 Åbo
FINLAND
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Fax:
+358 2 215 4931
+358 2 215 4780
E-mail:[email protected]
 IFRF - Combustion Journal – 1999 - 2001
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ABSTRACT
A recently developed two-step mechanism for modeling the fate of fuel-N from
biomass fired combustion devices has been applied in the modeling of the freeboard
of a forest residue fired fluidized bubbling bed. With the model the influence of two
different air-staging strategies on the NO emissions were studied. Also the
description of the release of the fuel-N was addressed by modeling the char-N
release in two ways. The predicted NO emissions agreed well with measured ones.
Changing the air staging strategy did not change the NO emissions significantly.
The reason for this was that most of the nitrogen chemistry occurred in the lower
part of the freeboard where the conditions remained almost unchanged.
Key Words:
fuel-nitrogen, fluidised bed combustion, biomass combustion, modelling
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1. INTRODUCTION
There is an urge to increase the use of biomass in order not to increase the CO2
emissions from energy production The combustion temperature when firing
biomass is typically fairly low and most of the NOx emissions stem from the fuel
bound nitrogen. However, the NOx emissions can still be reduced by primary
means, such as air staging. Support in minimizing NOx emission can be obtained
using computational fluid dynamics (CFD). However, using commercial CFD
packages it is difficult to use a comprehensive description of the combustion
chemistry. In practice, global reaction schemes have to be used. In the literature no
simplified reaction models describing the fate of the volatile-N in biomass
combustion has been reported. However, a number of global schemes for the
selective non-catalytic reduction (SNCR) process can be found [1-2]. The
temperature range this process is working is similar to those found in biomass
combustion, but the SNCR process work in flue gas conditions where the free
radical level is much lower than in the flame region. Mitchell and Tarbell [3] have
proposed a scheme including nitrogen chemistry for pulverized coal combustion,
but also they used experiments at SNCR conditions to establish their reaction rate
parameters. De Soete [4] has proposed another set of reaction that describes the
correct reactions for biomass application, but his rate coefficients were determined
from experiments with much higher temperatures.
The fluidized bubbling bed combustor (FBC) is one option when firing biomass. In
this paper a new model [5] recently developed at the Åbo Akademi University is
used to access the possibilities of diminishing the NOx emissions from a FBC by
changing the air distribution. This mechanism was especially developed for the
conditions found in biomass combustion. In the CFD modeling the turbulencechemistry interaction was modeled with the Eddy Dissipation Combustion Model,
originally put forth for a single step fast reaction. Also two ways of describing the
release of the fuel bound nitrogen are evaluated.
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2. CASE DESCRIPTION
Figure 1 shows the geometry of a fluidized bubbling bed that was studied. The
geometry only describes the freeboard part of the boiler. The height of the modeled
volume is 15.83 m, the width (half width of the boiler) is 4.76 m, and the depth is
11.03 m. The boiler is fired with forest residue and has a thermal output of 105 MW.
The primary air is fed through the bed. The rest of the air is introduced through air ports
at three different levels: 2.3 m, 4.5 m and 8.7 m above the bed surface. The distribution
of the secondary air is given in Table 1. Two different air distributions have been tested
in this study, here refereed to as “normal” and “modified”.
outlet
air ports, level 4
air ports, level 3
air ports, level 2
bed surface
Figure 1. The boiler geometry (Only half of the boiler is shown).
Table 1. Secondary air feed at three different air distributions (m3N/s).
“Normal”
“Modified”
Level 2
6.49
6.49
Level 3
11.48
6.96
Level 4
6.96
11.48
Sum
24.92
24.92
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3. GAS PHASE MODELS
3.1. Main chemistry
In this study only gas phase reactions were modeled. The CFD modeling was done
using the commercial software FLUENT 5.4. An unstructured grid consisting of
260 000 cells were used for the freeboard. Only one half of the freeboard was
modeled, hence, the mean size of the cells was 3 dm3. The heterogeneous chemistry
was not taken into account. The boiler is fired with forest residue, which is assumed
to consist of 50% wood and 50% bark. Table 2 shows the assumed compositions.
In the calculations it is assumed that the wood residue particles reach the bed
surface where they dry. The dried fuel is pyrolysed in the bed and partly oxidized.
It is assumed that the gas mixture leaving the bed is that obtained assuming
chemical equilibrium at the conditions the bed is operate at, i.e., at a equivalence
ratio of 0.7 and a temperature of 850 °C. The assumed composition of the gas
emerging from the bed is given in Table 3.
Table 2. Typical composition (wt.-% of dry matter) and moisture of wood and bark.
Calculated values for wood residue (50% wood and 50% bark).
Wood
Bark
Forest residue
C
50.4%
54.5%
52.5%
H
6.2%
5.9%
6.1%
S
0.0%
0.0%
0.0%
O
42.5%
37.7%
40.1%
N
0.5%
0.3%
0.4%
Ash
0.4%
1.7%
0.9%
100.0%
100.0%
100.0%
55.0%
60.0%
58.0%
Moisture content
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Table 3. Assumed compositions of the primary gas flowing from the bed to the
freeboard (vol.-%).
Component
Content
CO
6.2%
H2
4.0%
CO2
9.9%
H2O
35.2%
N2 (rest)
44.8%
The reaction kinetics of the main species released as volatiles were modeled with
the mechanism given by Jones and Lindstedt [6]:
CH4 + ½O2 → CO + 2H2
CH4 + H2O → CO + 3H2
H2 + ½O2 ↔ H2O
CO + H2O ↔ CO2 + H2
The mechanism consists of four reactions. In this work, only the two last reactions
were used. The mechanism was slightly modified in that the reaction describing the
oxidation of H2 was modeled as an irreversible reaction. The reason for this lays in
shortcoming of the turbulence–chemistry interaction model [7]. The turbulence
chemistry interaction was modeled with the ”Eddy Dissipation Combustion Model”
[8] as implemented in FLUENT 5.4.
3.2. Nitrogen chemistry
It is assumed that the forest residue contains 0.4 wt.-% nitrogen. As the fuel is
pyrolyzed 80% of the fuel-N is assumed to release, 50% as NH3 and 50% as N2.
The remaining 20% is assumed to be bound in the char residue. Here, the release of
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the char-N was modeled in two alternative ways. In the first the char-N was
released as N2, and in the second the char-N was assumed to be oxidized to NO
when released. The content of nitrogen containing species in the gas emerging from
the bed is shown in Table 4.
Table 4. Assumptions regarding the chemical form of fuel nitrogen in the primary
gas.
Volatile-N
Char-N
Fuel-N 1
Fuel-N 2
419 ppm NH3
419 ppm NH3
209 ppm N2
209 ppm N2
0 ppm NO
209 ppm NO
105 ppm N2
0 ppm N2
The gas phase nitrogen chemistry was described with a simplified two-step model
[5] recently developed at Åbo Akademi University. The model describes the fate of
the volatile-N, here assumed to be released as NH3. The model consists of two
reactions:
NH3 + O2 → NO + H2O + ½H2
(1)
NH3 + NO → N2 + H2O + ½H2
(2)
where the first describes the oxidation of the volatile-N to NO. The other describes
the reduction of NO by NH3. The expressions describing the kinetics of these
reactions are:
r1 = 1.21 ⋅ 10 8 T 2 e −8000 / T [NH 3 ][O 2 ]
[H 2 ]0.5
r2 = 8.73 ⋅ 1017 T −1 e −8000 / T [NH 3 ][NO]
0.5
where the temperature is expressed in K, concentrations in mole cm-3 and the rate in
mole cm–3s-1. The rate expressions have been extracted from perfectly stirred
reactor calculations with a comprehensive mechanism. Figure 2 shows a
comparison between the two-step mechanism and the comprehensive mechanism
KILPINEN97 [9,10]. The effect of the turbulence on the nitrogen chemistry was
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modeled with the Eddy Dissipation Combustion Model as found in FLUENT 5.4.
However, the nitrogen-chemistry was calculated using the post processing
approach, i.e., at this stage the flow field, the temperature field, and the
concentration fields of the main components were not modified.
4. RESULTS & DISCUSSION
4.1. Main chemistry
Table 5 gives concentrations at the outlet of the boiler. The measurements were
carried out while firing forest residue. However, no other information regarding the
conditions during the measurements was available. From the table it can be seen
that the air staging strategy does not influence the composition of the flue gas.
Principally, some minor change in the CO and H2 concentrations could have been
anticipated. In both cases the temperature of the gas leaving the boiler was
approximately 800°C, the difference between the two cases was only 2°C.
However, it can be noticed that the calculated H2O content of the flue gas is much
higher than the measured one. Also the O2 level is quite different. This points to
that the fuel has been much drier than assumed, and that the stoichiometric air to
fuel ratio has not been the same.
Table 5. Calculated results compared to measurement data.
“Normal”
“Modified”
Measured (mean)
Measured (σ2)
H2O
29.9%
29.9%
19.6%
0.5%
O2 (dry)
2.7%
2.7%
4.9%
0.3%
CO2
12.2%
12.2%
12.4%
0.2%
CO
1 ppm
1 ppm
2 ppm
24 ppm
CH4
-
-
17 ppm
3 ppm
1 ppm
1 ppm
-
-
H2
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NH3=1000 ppm, NO= 0 ppm, λ=0.7
0.00125
Mole fraction
Comprehensive mechanism
Comprehensive mechanism
0.001
0.001
0.00075
0.00075
NO
NO
0.0005
0.0005
NH3
0.00025
0
0.000001
0.00125
0.0001
0.00025
0.01
1
NH3
0
0.000001
0.001
0.00075
0.00075
1
0.01
1
NO
NO
0.0005
0
0.000001
0.01
New 2-step
0.001
0.00025
0.0001
0.00125
New 2-step
Mole fraction
NH3=1000 ppm, NO= 0 ppm, λ=1.3
0.0005
NH3
0.00025
NH3
0
0.0001
0.01
Residence time (s)
1
0.000001
0.0001
Residence time (s)
Figure 2. Comparison of results obtained with the comprehensive mechanism and the
new simplified 2-step model. Mole fraction of NH3 (––) and NO (- - -) as a function of
the residence time in an ideal perfectly stirred reactor at λ=0.7 and λ=1.3
Figure 3 shows the calculated H2 fields in the two cases. Here the effect of the
differences in the air distribution can be seen. In the “normal” setup the λ-value is
higher in the lower part of the boiler. In this case H2 is oxidized quicker than in the
“modified” setup, where much of the air is supplied at the fourth air port level.
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Figure 3. H2 concentration profiles in the freeboard. To the left the “normal” case, to
the right the “modified” case. The concentrations are in mole fraction in wet flue gas.
4.2. Nitrogen chemistry
Figure 4 shows that the NH3 in the primary gas emerging from the bed surface is
rapidly converted to NO and N2 at the first level of secondary air. In this area the
NH3 is almost completely consumed. The volume of the area where most of the
NH3 conversion takes place is very small compared to the volume of the boiler.
According to the model also the conversion of NH3 is to a large extent controlled by
the turbulent mixing. Only in the vicinity of the air jets the conversion rate is
kinetically controlled. In the standard case 61% of the NH3 is converted to NO and
39% is consumed in the reaction between NH3 and NO. It should be remembered
that no NO is emerging from the bed: most of the NO formed from the NH3 reacts
with additional NH3 to N2. The calculated NO level in the dry flue gas expressed
for a 3% O2 level is 103 ppm, which is in very close agreement with the
measurements. The results of the nitrogen chemistry calculations are compiled in
Table 6.
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Table 6. NO concentration in the boiler outlet normalized to dry flue gas conditions
with an O2 level of 3%.
Case
NO
NO
(outlet, mean)
(σ2)
Measurements
110 ppm
7 ppm
Standard, Fuel-N 1
103 ppm
-
Modified, Fuel-N1
103 ppm
-
Standard, Fuel-N 2
133 ppm
-
Modified, Fuel-N 2
135 ppm
-
Kinetics, Fuel-N 1
27 ppm
-
Figure 4. NO (left) and NH3 (right) concentration profiles in the “normal” case
assuming the volatile-N is released according to assumption “Fuel-N 1”.
INFLUENCE OF AIR STAGING
In the calculations no influence of the air staging strategy on the NOx emissions
could be seen. Also the NH3 and NO concentrations in the freeboard are almost
identical. This is a consequence of that in the lower part where much of the nitrogen
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chemistry occurs the conditions are very similar. Only higher up in the freeboard a
difference in the conditions could be seen. However, in this region the NH3 level
has already dropped to practically zero.
INFLUENCE OF NITROGEN RELEASE MODELLING
As described in Table 4, two different assumptions for the release of the fuel-N
from the bed are used. These differ in that in the standard case the char-N is
released as N2 whereas in the second case the char-N is released as NO. In this case
there is 209 ppm NO in the gas emerging from the bed surface. Now, the
consumption of NH3 by NO starts already in the lowest part of the freeboard. The
importance of the reaction between NH3 and NO is larger as well. In this case only
39% the NH3 is consumed in the reaction forming NO, whereas 61% is consumed in
the reaction between NH3 and NO. Despite the increased importance of the second
reaction the NO emission is larger. With both air staging strategies the NO emission
is around 135 ppm expressed for a dry flue gas with an O2 content of 3%.
INFLUENCE OF TURBULENCE
In the Eddy Dissipation Combustion Model the reactions are assumed to be either
mixing limited or kinetically limited. In an earlier section it was stated that in most
parts of the boiler the reactions were limited by the mixing. To test the influence of
the turbulence-chemistry interaction model, a case was calculated where the
limiting mixing rate was not taken into account. In this case the reaction proceeded
with a much higher speed, almost all NH3 reacted at the level of the lowest air
ports. Also the predicted NO emission was strongly influenced, only 27 ppm NO
expressed for a dry flue gas with an O2 content of 3% was found at the outlet.
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5. CONCLUSIONS
The freeboard of a biomass-fired fluidizing bed has been modeled with simple
descriptions of the chemistry and the turbulence-chemistry interaction model. The
agreement between measured and calculated NO emissions is in this particular case
excellent.
According to the calculations also the nitrogen conversion is to a large extent
mixing controlled, this suggests that the result is sensitive to a correct description
of the turbulence. Calculations without any turbulence-chemistry interaction model
predicted far too low NO emissions. Based on the present calculation it is not
possible to judge how the release of char-N should be modeled. Both the results
obtained assuming that the char-N is released as N2 or as NO are realistic.
No influence on the NO emissions could be obtained using a different air-staging
strategy. The reason was that most of the volatile-N reacted in the lower part of the
freeboard between the second and third level of air ports. Here the conditions
remained similar regardless of the air-staging strategy.
The results suggest that influencing the NO emissions would require modifications
to the air distributions in the lower part of the boiler. The results also points to the
importance of an adequate bed model since much of the fuel-N conversion occurs in
the bed, and in the free board part below the first level of air ports.
ACKNOWLEDGEMENT
This work is part of the activities at the Åbo Akademi Process Chemistry Group
within the Finnish Centre of Excellence Programme (2000-2005) by the Academy
of Finland. It has been supported financially by the Finnish National Technology
Agency, Andritz-Ahlstrom, Fortum Power and Heat Oy, and Kvaerner Pulping Oy.
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We also like to acknowledge Dr.-Ing. Christian Mueller and Staffan Nickull for
their contributions.
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