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Study on Emission Control of NOx and N2O in a Fly-ash Recycle Fluidized Bed Test Rig.

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Dev. Chem. Eng. Mineral Process., 8(3/4),pp.281-292, 2000.
Study on Emission Control of NO, and N 2 0
in a Fly-ash Recycle Fluidized Bed Test Rig
Z. Zhong*, J.Lan, and B. Jin
Thermal Energy Engineering Research Institute, Southeast Universio,
Nanjing 210096, P. R. CHINA
and Y.Han
T h e m 1 Tube Research Centel; Nanjing Chemical Engineering
Universio, Nanjing 21 0000, r! R. CHINA
Experiments on NO, and N20 release characteristics and emission control were
srudied in ajly-ash recyclefluidized bed test rig. The total height of thejluidized bed
is 4.4m. Thefludization section is 1.2m high with a cross-section 0.23mx0.23m. The
freeboard is 3.0m high with a cross-section 0.460mxO.345m. The coal-fired weight of
the test rig is 30 k g h , and coal is fed by a screw feedel: Fine ash is collected by a
cyclone and recycled to the bed surface through a J-valve, slag is then removedfrom
the bottom. An oil-fired start-up burner is also used. Testfuels are Xuzhou bituminite
and Henan anthracite from China with particle range of 0-8mm. When the bed
temperature rises, NO, increases and NzO decreases. At the same temperature, N 2 0
emission of bituminite is less than that of anthracite. While the secondary air ratio
reached 15% under staged combustion, NO, emission decreased to 14% and N 2 0
emission decreased to 24%. As the CdS molar ratio increases, NO, emission also
increases while NzO emission decreases. Ammonia injection decreases NO, emission
significantly, but it has no effect on the emission of N20.
Keywords: fluidized bed; fly-ash recycle; coal-fired;NO, N20; emission control.
* Authorfor correspondence.
281
Z. B o n g , J. Lan, B. Jin and Y.Han
Introduction
NO, and N20 are the main pollutants from coal-fired process. NO, is a toxic gas in
air and can become optical-chemical smoke when it co-exists with &Hy. N20 effects
of global warming and the N20 infrared ray absorbing capacity is 200 times greater
than that of C02. N20 can react with 03,thus increasing the ray absorbing capacity of
ultra-violet rays and hence increasing the incidence of skin cancers, e.g. black
pigment cancer, etc. Presently, the atmospheric concentration of N2O is 300mgkg
increasing at 0.18-0.26% per year, and the effect on the weather is closely monitored.
Fluidized bed boilers may desulfurize during the combustion process. In addition,
they have a lower combustion temperature and thus produce less NO,. Hence, the
technology of coal-fired fluidized beds will become more important. However,
fluidized bed boilers produce more N2O than that of powder-coal boilers. In recent
years, many researchers have been interested in emissions and control of NO, and
N20 in fluidized bed boilers. This paper present the test results of NO, and N20
release characteristics and emission control in a fly-ash recycle fluidized bed test rig.
Test Method
Figure 1 shows the coal-fred fly-ash recycle fluidized bed test rig. The total height of
the fluidized bed is 4.4m. The fluidization section is 1.2m high with a cross-section
0.23mx0.23m, and the freeboard is 3.Om high with a cross-section 0.460mx0.345m.
The coal-fired weight of the test rig is 30 kgh. Two layers of the secondary air
nozzles and two layers of the ammonia injection nozzles are set longitudinally. Eight
movable exchanger heat tubes adjust the bed temperature. Coal and limestone are fed
into the bed by a screw feeder. Fine ash is collected by a cyclone and then recycled to
the bed surface through a J-valve. Slag is removed from the bottom. An oil-fired
start-up burner is also used. Gas components are analyzed by a Binos-1004 multicomponent flue gas analyzer from the Rosemount Company, Germany.
This
apparatus has a sample gas pre-handling system with rapid gas cooling and moisture
removal functions. It can detect C02, CO, 0 2 , NO,, NzO and SO2 simultaneously.
282
Emission Control in a Fly-ash Recycle Fluidized Bed Test Rig
L
I
I
I
I
I
air
Figure 1. Schematic diagram of the fly-ash recyclefluidized bed test rig.
The test fuels are Xuzhou bituminite and Henan anthracite, and the average
particle sizes are 1.09mm and 1.33mm,respectively. The analytical data for the
experimental coals are given in Table 1. The sorbent is Nanjing limestone with 99.6%
CaC03, and the average particle diameter is 2.14mm.
Table 1. Analytical results of coals.
bituminite
I
Henan
anthracite
I
283
Z Zhong, J. LUG B. Jin and Y. Han
Experiments of NO, and NzO Emission and Control
Effect of different coals and bed temperature (Tb)on NO, and N 2 0 emission
As the combustion temperature is low, a fluidized bed boiler produces very little
thermodynamic NO,. The NO, gases are produced by fuel-N homogeneous and nonhomogenous oxide reactions. At the conditions of CFBC, the characteristics of NO,
and NzO release with change of bed temperature are shown in Figures 2 and 3,
respectively. The fly-ash recycle ratio (R)is 2.53, CdS molar ratio is 1.95, and O2is
6% in the flue gas. With increments of bed temperature, NO, increases and NzO
decreases. NO, emission from bituminite is greater than from anthracite, but for NzO
the opposite occurs. When the bed temperature is 880°C, NO, and NzO emission
concentrations of Xuzhou bituminite are 2.35~10% and 8 ~ 1 0 - ~ %
and, of Henan
anthracite are 1.75~10%and 4x109%, respectively.
Tanveer et al. (1993) suggest that the main intermediate product of volatile-N is
HCN in which fuel-N exists as phenyl components. The main intermediate product of
volatile-N is NH3 in which form volatile-N exists as amido components. NzO is the
main product from the HCN homogenous oxide reaction:
HCN+ 0 + NCO+ H
+H
HNCO + OH + NCO + H20
NCO + NO + NzO + CO
HCN + OH +HNCO
(1)
(2)
(3)
(4)
NO is the main product for the NH3 homogenous oxide reaction:
NH3 + 0 + N H 2 + OH
(5)
NH3+OH+NHz+HzO
N H 2 + 0 + NH + OH
(6)
NHz+ OH +NH + H20
NH + OH +NO + Hz
NH + 0 + N O + H
(7)
(8)
(9)
(10)
Either NO or NzO is produced in the char-N non-homogenous oxide reaction. Volatile
matter quality decreases from bituminite to anthracite, and the proportion of fuel-N
284
Emission Control in a Fly-ash Recycle Fluidized Bed Test Rig
s%
9-
0
3
P
8
T,es
i,es
Figwe 2. Effect of &Teerent coals
and Tb on NO, emiwion
Figure 3. Effect of &peerent coals
and Tb on N20 emission
1151
I
1101
@/
160+
0
-
.
-
,
.
,
.
,
.
.
.
,
5 1 0 1 5 2 0 2 5 3 0
Figure 4. Effect of x on NO, emission.
Figure 5. Effectof K on
N20 emission.
285
Z Zhong, J. Lun, B. Jin and Y. Han
which exists as phenyl components increases.
Therefore, at the same bed
temperature, NO, emission of bituminite is greater than that of anthracite, but N20 is
the reverse. The change of trend for NO, and NzO emission is the reverse when the
bed temperature changes. With increments of bed temperature, NO, increases. This
is because NH3 is increasingly becoming NO, and the NO, non-homogenousreducing
reactions are restrained (Kramlich et al., 1989).
NO + 2(C) + (CO) + (CN)
(11)
Nz + 2(C)
(12)
2(cN)
-+
NO + (C) + (CH)+(CO)
NO + (CO)
Char
>
+(
0
(13)
1/2N2+ C02
(14)
When the bed temperature increases, the combustion rate also increases and
percentages of CO and char decrease. Therefore, it is not favourable for the reduction
of NO. Meanwhile, at higher temperatures, thermodynamic NO, will increase. With
the increments of bed temperature, the reaction of HCN -+ NzO will weaken, and the
non-homogenous reaction of char-N
-+
N2O will also weaken, but the reaction of
NzO homogenous decomposition is very fast (Roby and Bowman, 1987).
NZO + H + Nz+ OH
(15)
NzO + OH -+ Nz+ HOz
(16)
Therefore, high temperature combustion may give significantly reduced NzO
emissions.
Eflect of secondary air ratio(%)on NO, and NzO emission
Figures 4 and 5 are the test results of the effect of yz on NO, and NzO emission under
different temperatures, respectively. The molar ratio of CdS is 1.95 and R is 2.53.
The following experiments use Xuzhou bituminite. With increments of secondary air
ratio (yz), NO, emission decreases but NzO emission decreases gradually at first then
it increases. When the secondary air ratio (yz) increases from 0 to 25% at a bed
temperature of 870°C, NO, emission decreases from 2.68~10% to 1.44x102%(46%
286
Emission Control in a Fly-ash Recycle Fluidized Bed Test Rig
reduction). When yzincreases from 5 to 25% N20 emission increases from 9.65~10'
% to 1.14~10-~%
(18.1% rise).
A reducing atmosphere in the bed is stronger under staged combustions, char and
CO increase, and reactions (11) to (14) also increase. Meanwhile, decomposition
products of fuel-N become N2. Therefore, NO, emission decreases significantly as
shown by Equations (17) and (18):
XN + XN +N2+ 2X
(17)
N+N+Nz
(18)
Test results of staged combustion for N2O emission are different to literature values.
In the test, the freeboard temperature increased by 70-801 thus reinforcing N20
homogenous decomposition reactions (15) and (16), but NzO forming reactions (1) to
(4) also are reinforced. Therefore, N20 emission increases.
Effect offly-ash recycle on NO,and N 2 0 emisswn
From Figures 6 and 7, the test conditions are 4% O2in flue gas, secondary air ratio 'y2
is 1596, CalS is 1.95, and the bed temperature is 880OC. With increments of fly-ash
recycle ratio (R),NO, emission decreases significantly and for N20 emission the
reverse occurs. When R increases from 0 to 2.5, the concentration of NO, decreases
by 36%, and the N20 emission increases by 44%. This is because non-combustion-C
increased, and char-N becomes N20 slowly in the freeboard while there is fly-ash
recycle (Dulce et al., 1991) as shown below:
02+
(C) + (W-+ (CO) + (CNO)
+ (C) + N20
CNO + (0
+ N20 + 2(C)
2(CNO)
3 (CO)
(19)
(20)
(21)
VolatileN release is very fast and forms NO in the freeboard, then NO is reduced to
N20 on char surface as represented in the following equations:
NO+(CN)
char
> N20 + (C)
(22)
N2O + (C) + N2+ CO
(23)
2N20 + (C) -+ 2N2 + C02
(24)
287
2. Zhong, J. Lan, B. Jin and Y. Han
1404
0.0
.
I
0.5
. , .
1.0
I
1.5
,
I
20
'
I
I
'
25
3.0
0.0
0.5
1.0
R
Figure 6. Effect of R on NO, emission
1.5
R
20
25
3.0
Figure 7. Effect of R on N20 emission.
'I
110
1.0
1.5
20
25
30
35
4.0
WSmlariatiO
Figure 8. Effect of W S on NO, emission.
288
1.0
1.5
20
25
80
35
Wsndarrabb
Figure 9. Effect of CdS on N 2 0 emission.
4.0
Emission Control in a Fly-ash Recycle Fluidized Bed Test Rig
Because NO and NzO non-homogenous reducing reactions increase in the char
surface, they may result in formation of NzO.
Eflect of limestone desulfurimdion on NO, and N 2 0 emisswn
Figures 8 and 9 show NO, emission with limestone desulfurization: yz=15%,
flue gas is 496, R = 2.5, Tb = 860-88OOC.
0 2
in
Under conditions of staged air and
limestone desulfurization, the reducing atmosphere in the bed increases, then CaO
and CaS co-exist. CaO and CaS are catalysts for NO and N20 reducing reaction. In
addition, CaS can directly reduce NzO (Peter et al., 1993) in reactions (25) to (28):
NO + CO
NzO + CO
CaS
G O
or
CaS
>
1/2N2+ COZ
(25)
9 N2 + COz
(26)
> Nz+ 1/2O2
3Nz0+ CaS + 3Nz + SO2 + CaO
NZO
(27)
(28)
Perhaps because limestone is injected, oxidizing of volatile-N is changed from
homogenous reaction to non-homogenous.
For the non-homogenous oxidizing
reaction of volatile-N such as HCN, NH3 selectively forms NO, but it has low
selectivity for forming N20 (Shimizu et al., 1993). Meanwhile, the quantity of NO is
reduced by the CO on the limestone surface, however it is lower than that of HCN
and NH3 non-homogenous oxidization forms significant NO. In total, NO, emissions
increase but NzO emission decreases.
Eflect of excess air comient (a)on NO, and N 2 0 emission
Effect of excess air coefficient (a)on NO, and N20 emissions is shown in Figures 10
and 11, respectively, for CdS = 1.95, R = 2.53, y2 = 0, Tb = 87O-88O0C. When cf
increases, NO, and NzO emissions increase very rapidly. When a increases from 1.05
to 1.35, NO, emission increases from 1.6~10-~%
to 2.96~10% (increased by 85%),
and NzO emission increases from 4x10”% to lxlO%h (increased by 150%). This is
because combustion intensifies, thus increasing y2. Then NO, and N20 homogenous
289
Z. Zhong, J. Lan, B. Jin and Y. Han
c
1.05 1.10 1.15 la
1.30 1.36
1.00 1.06 1.10 1.15 l
a la 1.33 1.35 1.40
1.00
1.Z
oc
Figure 10. Effect of = on NO, emission.
Figure 11. Effect of = on N20 emission.
3
40-1
0
.
I
1
. , . , . , . ,
2
. ,
6
hYM3,mdarraio
3
4
5
. , . ,
7
8
Figure 12. Effect of ammonia injection on NO, emission.
(])Ed, Tb=873”c
( 2 )yi= 15% Tb=85j0c
( 3 ) ~ = 1 5 %Tb=82j0c
,
290
Emission Control in a Fly-ash Recycle Fluidized Bed Test Rig
oxidizing reactions (1) to (10) are reinforced and their non-homogenous reducing
reactions (11) to (14) and (23) to (28) are weakened. Meanwhile, N2O homogenous
reducing reactions (15) and (16) also weaken because the concentrations of
[HIand
[OH] decrease.
Effect of ammonia injection on NO, and NzO emission
NH3has a selectivity reducing effect on NO as shown below:
4NH3 + 6NO + 5N2 + 6H20
(29)
4NH3+ 4 N 0 + O24 4N2+ 6H20
(30)
As seen in Figure 12, staged combustion and ammonia injection in the bed surface
may decrease NO, emission for CdS = 1.95, [02] = 4%, and R = 2.5. When the
NHflO, molar ratio is 5, NO, decreases by 39% in non-staged combustion. NO,
decreases by 60% when the secondary air ratio is 15% and when it has ammonia
injection. When NHflO, molar ratio is greater than 5, the effect of NO, decreasing
is restrained.
Results are different for ammonia injection and N20 emissions when compared to
the literature. With increments of N H f l O , molar ratio, N20 emission increases
slightly in our experiment. It may be related to the reactions of NHi
No
> N20.
Conclusions
NO, emission is not only related to different coals but also the combustion
conditions in a fly-ash recycle coal-fired fluidized bed.
When the bed
temperature increases, NO, emission increases and N20 emission decreases. At
the same bed temperature, NO, emission for bituminite is greater than that for
anthracite, but for N20 the opposite happens.
Fly-ash recycle and staged combustion can significantly control NO, emission.
But in the freeboard, the char-N non-homogenous oxide reaction and NO is
291
Z. Urong,J. Lan, B. Jin and Y.Han
reduced on char surfaces resulting in NO, emission increases. It has very strong
selectivity such that volatile-N non-homogenous oxide reaction on CaO surface
becomes NO. Limestone desulfurization has an adverse effect on NO, emission,
but it can decreases NzO emission.
3. Ammonia injection decreases NO, emission significantly, but has less effect on
NzO emission.
4.
Considering combustion efficiency, desulfurization and denitrification, then the
following test conditions are preferred: n = 15%, = = 1.2, R = 2.5, Ca/S = 2.0,
"0,
= 3.
Acknowledgement
This work was supported by the Trans-Century Training Programme Foundation for
the Talents of the State Education Ministry of China. The authors would like to
express their thanks for this support.
References
Dulce, H., et al. (1991) NzO Formation during fluidized bed combustion of chars. Rocecdings of the 11th
InternationalConference on FBC.
Kramlich, J.C., Cole, J.A. et al. (1989) Mechanism of nitrous oxide fonnation in coal flamcs. Combustion
and Flame,77,375-384.
Peter, F.B.. et al. (1993) Limestone catalyzed reduction of NO and N20 under fluidized bed combustion
conditions. The 12th International Conference on FBC, Rubow, LN.(Ed.).
Roby, R.J. and Bowman, C.T. (1987) Combustion and Flame, 70,119-123.
Shimizu. T.,et. al. (1993) Effect of limestone on emission of NO. and N20 from a circulating fluidized
bed combustors. The 12th Intunational Conference on FBC. Rubow, L.N.(Ed.).
Tanveer, K., Lee, Y.Y.and Brown. R.A. (1993) Homogenous phase fonnation of nitrous oxide from
hydrogen. Ibid
292
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