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Reaction of char nitrogen during fluidized bed coal combustion—Influence of nitric oxide and oxygen on nitrous oxide
C O M B U S T I O N A N D F L A M E 97:118-124(1994)
118
Reaction of Char Nitrogen During Fluidized Bed Coal Combustion--Influence of Nitric Oxide and Oxygen on Nitrous Oxide G. F. KRAMMER and A. F. SAROFIM Department of ChemicalEngineering~MassachusettsInstitute of Technology, Cambridge,MA 02139 The conditions that favor the formation of nitrous oxide from char under fluidized bed combustion conditions are shown to be the oxidation of the char to make the nitrogen bound in the heterocyclic rings accessible so that it can react heterogeneously either with oxygen to form NO or with nitric oxide to form N20. This was demonstrated by measurement of the gas composition when a batch of coal particles was introduced into a fluidized sand bed electrically heated to 1023 K. Oxygen and nitric oxide were added to the fluidizing helium gas in varying concentrations. N20 was formed in amounts that increased with increasing NO concentration showing the importance of NO for N20 formation. The N20 concentration, however, fell to zero when the 02 supply was interrupted underlying the essential role of oxygen in freeing the organically bound nitrogen so that it can react with NO to form N20.
NOMENCLATURE b c f
fractional carbon burnout ( p p m / p p m ) molar concentration (ppm) fraction, f c o = Cco/(Cco + Cco2 + Ccn,), fNo = CNO/[(XN/XcXCco + CCO2 +
CcH4)]' F r x t
fN20 = 2"CN20/[(XN/Xc)(Cco + CCOz + CCn,)] ( p p m / p p m ) cumulative fraction ( p p m / p p m ) fractional survival Cout/Cir, ( p p m / p p m ) molar ratio ( m o l / m o l ) time (s)
INTRODUCTION NzO formation is known to be favored in the temperature range of fluidized bed coal combustion. The mechanisms by which the N 2 0 is formed from H C N and N H 3 in the volatiles are relatively well understood [1]. The conditions that favor the conversion of char nitrogen to N20, however, are incompletely understood. The N 2 0 formation from char can be equal to [2] or greater than [3] that formed from the volatiles. The most thorough study of char nitrogen conversion to N 2 0 is that by de Soete [4]. H e has identified many of the reactions governing the conversion of organically bound nitrogen to N O and NaO in a series of elegant 0010-2180/94/$7.00
experiments. He, however, showed that when N O is reduced by char little N 2 0 is formed, and concluded that the N O char reaction is not an important pathway to N20. This is correct in the absence of oxygen but, as will be shown in this article, the presence of oxygen greatly augments the formation of N 2 0 from N O and char. A small fluidized bed reactor was used in order to investigate the influence of the temperature, the oxygen concentration and the particle size of the coal particle on the formation and reduction of N O and N20, respectively [3]. The experimental setup is described elsewhere [3]. The influence of the amount of oxygen and nitric oxide on the production of N 2 0 is considered in more detail here.
E X P E R I M E N T A L PROCEDURES AND M E A S U R E M E N T S A fluidized 57-mm-diameter quartz sand bed was electrically heated to 1023 K and a batch of coal particles about 4 m m in diameter were introduced to the bed (Fig. 1). First the volatiles were released and then the char was cornbusted. Helium as a carrier gas with different concentrations of oxygen and nitric oxide was used to maintain the bed fluidized. The gas flow was 2.5 L / m i n at s.t.p, at the entrance of Copyright © 1994 by The Combustion Institute Published by Elsevier Science Inc.
N20 FORMATION IN FLUIDIZED BED COMBUSTION
119
A deconvolution method was derived by Tullin et al. [8] in order to evaluate the influence of the broadening of the gas concentration profiles due to mixing downstream from the reaction zone. The influence, however, is small on the gas concentration profiles during the char oxidation stage and it is not taken into account in the data shown here. The composition of the coal which was used in the experiments is given in Table 2. The experiments were designed to obtain the conversion of fuel nitrogen to NO and to N20 for single particles with a minimum of secondary reactions with either other particles or the bed material. The uncertainty in the gas analysis decreases with the increasing concentration encountered when several particles are injected into the bed. Tests were therefore carried out to evaluate the effects of particle interaction on the N20, NO, and CO yields by injecting one and five particles. The fractional conversion of carbon to CO is given by
tk
qt
ele, quo
de CG
clio ql
sil
9as in Fig. 1. Schematics of the fluidized bed reactor.
the reactor. The gas released was analyzed for CO, CO2, NO, N20 , CH4, NH3, and NO 2 in an FTIR. The fractional conversion of the organically bound nitrogen to NO and N20 and the fraction of the carbon released that is in the form of CO were derived from the experimental data as a function of burnout. The experimental conditions such as coal mass, number of coal particles, and concentration of the fluidizing gas are listed in Table 1. The volume gas flow was constant at 2.5 L / m i n at normal conditions (273.5 K and 1 atm).
¢co
]:co = CCO q- CCO 2 -'b ¢ C H 4
and the fractional conversion to NO and N20, assuming that the rates of consumption of nitrogen and carbon are in the ratio of the atomic ratio of nitrogen and carbon in the char, are given by
fNo
¢NO
XN(cco + CCO: + CcH) Xc
TABLE 1 Experimental Conditions Exp. No.
Coal Mass [mg]
No. of Particles
Gas Concentration
Deviation of c-balance (%)
Figure No.
1 8
200.0 202.0
5 5
3.8 --
2, 3 4
11 12 13 14 15
44.0 211.5 227.3 169.5 239.8
1 5 5 5 5
5.0 --1.1 6.0
2, 3 3,5 3, 5 3,6,7 6, 7
17
188.3
5
4% 0 2 in He 275 ppm NO, 4% 0 2 in He 4% O 2 in He 1.12% 0 2 in He 0.8% 0 2 in He 2% 0 2 in He 205 ppm NO, 2% 0 2 in He 723 ppm NO, 2% O2 in He
3.3
6, 7
120
G . F . KRAMMER AND A. F. SAROFIM TABLE 2 Proximate and Ultimate Analysis of Newlands Coal (Data Provided by Hitachi Ltd., Japan)
Proximate Analysis
Wt. %
Ultimate Analysis
Wt. %
Ash Volatile matter Fixed carbon Water
17.44 26.49 56.07 2.44
Ash Carbon Hydrogen Oxygen Nitrogen total sulfur Inflammable sulfur Nonflammable sulfur
17.44 68.83 4.38 7.71 1.20 0.54 0.44 0.10
Gross heating value: 6490 [BTU/Ib]
and 2.CN20 fN20 =
XN
- - ( C c o + Cco2 + CCH,) XC Figure 2 shows the fraction of CO, NO, and N20 plotted versus the fractional carbon burnout given by f 0 /( C c o
-F
CCO 2 -I'- CCH4)
dt
b CC,coal, t = 0 The results of a single particle experiment are compared with the results of an experiment with five particles. At total coal burnout the cumulative fractional conversions were found to be Fco = 0.074, F N O = 0.608, and FN2o = 0.096 for the five particle experiment whereas ~
1.Or
0.40
0,8
o 8
•
0.6
~ 0.4 & 0.2
0.0: 0.0
o
o. O ~. filled symbols: 1 particle • • o
0.2
0.4
0.6
fractionol corbon burnout
0.8
o
"l 0.16 -0,08
1.0
b
Fig. 2. Instantaneous fraction of CO, NO, and N20 at the combustion of 1 and 5 Newland coal particles at 1023 K. The reaction gas was constant at 4% O 2 in helium.
they were Fco = 0.149, FNO = 0.700, and FN20 = 0.08 for the single-particle experiment. It can be seen that the results with the single particle, because of the low concentrations, show much more scatter than with five particles. Interaction of Coal Particles in the Fluidized Bed Reactor
The NO profiles show larger differences at the beginning of char burnout, but this is still within the standard error range of the gas analyzer. The N20 profiles are within the error range whereas the CO profile is not, especially at the later stages of burnout. The CO burns to CO2 in the boundary layer surrounding the particle and therefore the CO burnout is inhibited when several particles are introduced because of the overlap of their layers. Figure 3 shows the fractional carbon burnout versus reaction time. An analytic function could be derived by fitting the data of each curve to a quadratic function with correlation coefficient always larger than 0.995. It is seen that the single particle burns approximately 10% faster than the batch of five particles which was due to the fact that some of the particles for the five-particle experiment were about 10% larger in mass than the single particle. However, the observed trends of the concentrations profiles are unchanged.
N20 Decomposition The ratio r of the outlet to inlet concentration of N20 in the empty reactor (no sand and coal particles) with no oxygen and 4% oxygen in
N 2 0 F O R M A T I O N IN F L U I D I Z E D B E D C O M B U S T I O N i
i
i
ance on the particle, equating the rate of heat generation to the heat transfer to the bed. The Nusselt number was calculated using a correlation in the literature [5] for particle-gas heat transfer in gas fluidized beds and found to be in the range of 8-12. The burnout rate was derived from the generation of CO and CO2, the sum of which is shown in Fig. 3. It was found that the temperature increase of the particle in the case of 4% oxygen in helium and a flow rate of 2.5 L / m i n was about 25 ° C at maximum. In the case of 0.8% and 1.12% oxygen in helium and 2.5 L / m i n , the particle temperature increased by no more than 8 ° C. Tullin et al. [3] found that at temperatures below 1073 K the fractional conversion of char nitrogen to NO increased and that to N 2 0 decreased with increasing temperature. However, the results obtained from the experiment at a higher oxygen concentration and consequently at a higher particle temperature show a lower NO and a higher N 2 0 fraction (Fig. 4), which indicates that the temperature increase due to burnout has a minor effect on the char nitrogen reaction.
i
1.0
# f
o8
° o
I
, ~
c o o o
0.4
&
0.2
~
i 500
27. 02
v
1.12g 02
symbols:
hollow
O.C
°
5
i
i
1000
1500
ti~
t
121
particles
I 2000
b]
Fig. 3. The fractional carbon burnout versus reaction time at the combustion of Newland coal particles at 1023 K.
helium was found to be 0.95 and 0.97, respectively. In the presence of the fluidized quartz sand bed r was determined to be 0.81 when no oxygen was present and 0.92 with 4% oxygen. The volume flow rate was constant at 2.5 L / m i n at normal conditions. The temperature was set to 1023 K. The values of r at oxygen concentration of 8%, 12%, and 20% were the same as for 4% oxygen. The N 2 0 decomposition is compensated for in the results presented in this paper.
RESULTS AND DISCUSSION Influence of O z o n N 2 0 Formation Figure 4 shows the gas concentrations of CO, CO 2, NO, and N 2 0 as a function of time. At the start of the reaction the gas concentration of the fluidizing gas was 4% O 2, 275 ppm NO, and the balance helium. A batch of five coal particles were introduced to the fluidized bed after 40 s. After 375 s the gas concentration
Actual Temperature of the Particle The temperature of the coal particles in the fluidized quartz sand bed was obviously higher than the bulk temperature which is the one mentioned in Table 1. The difference in temperature was estimated from an energy bal-
8
7000 6000 -
5000
8 8
~oo:°°-
{,~,,.
8
o
~-00
cN2 o
25
• CNo v Cco
0 z on
¢00, /
35O
02 off
/
300 -~ 4000
•Cco 2
8 300, /
350 -
-
3000
250 '~
&
250 200
200
2000 -
0
-
?
o
150
1000 -
2O
300
T
A
30
*5O
500
10
100
10o I 50
50
0 0
250
500 time t
1500 IS]
p
i
1750
2000
o
5
0
Fig. 4. Gas concentrations of CO, CO2, NO, and N20 at the combustion of five Newland coal particles at 1023 K. The reaction gas was changed from 4% 02, 275 ppm NO in helium to 275 ppm in helium and back again.
122 was changed to 275 ppm NO with helium as the carrier gas and after 1470 s the original gas concentration of 4% 0 2, 275 ppm NO and helium carrier was used again. The experiment was terminated after 2000 s. Figure 4 provides a critical test of the role of 02 on N20 production. In the first stage of the reaction oxygen as well as additional NO is available in the carrier gas. One can expect, therefore, that the NO measured is the difference between NO production by the oxidation of the bound nitrogen in the volatiles or char and the NO reduction by hydrocarbons in the volatiles or by char nitrogen. Soon after the particles were introduced into the reactor the reduction of NO on the coal particles was exceeded by the production of NO from the bound nitrogen compounds in the volatiles accounting for the peak in NO at 65 s. After the volatiles burned out at a time of 78 s, the NO concentration was almost constant at a value about 50 ppm below that of the NO in the carrier gas. In this regime the NO decrease as a result of the N O / c h a r reaction exceeds the NO formation from the oxidation of char nitrogen. When the oxygen was turned off CO2, CO, and N20 declined rapidly to zero. (The slower transient for CO and N20 indicates some slower desorption of these species.) The NO concentration, after an initial peak due to the volatile oxidation, dropped below the inlet value of 275 ppm as a consequence of the char NO reactions. The NO concentration then increased with time, which means that the reduction capacity of the char particles decreased with time. This transient is quite possibly a consequence of the build up of chemisorbed oxygen atoms on the surface [ ( - C O ) in the reaction mechanism of de Soete] reducing the number of carbon sites available to react with the NO. After the oxygen flow was resumed, N20 , CO, and CO 2 were formed again and the level of NO increased as the result of the contribution from the oxidation of char nitrogen. Surprisingly the level of N20 and CO 2 were lower and the ones of CO and NO were higher compared with the concentration level when oxygen was turned off. The higher CO level can be explained by assuming that some of the accumulated adsorbed ( - CO) molecules are released. (After the small drop at about
G . F . KRAMMER AND A. F. SAROFIM 1650 seconds the increase of CO concentration continued as expected.) It is uncertain whether the higher CO concentration level during this time had an influence on the NO and N20 concentration. Figure 5 shows the results of two experiments where the fluidizing gas contained 0.8% and 1.12% 02 in helium. The fraction of NO, CO, and N20 is plotted versus the normalized burnout rate. By increasing the oxygen concentration in the helium carrier gas the fractions of N20 and CO increase whereas that of NO decreases. An influence of oxygen on the formation of nitric oxide and nitrous oxide was also observed by de Soete [4], Tullin et al. [3], and Mochizuki et al. [6].
Influence of Nitric Oxide on N20 Formation
In order to evaluate the influence of the NO concentration on the N20 concentration a set of experiments were carried out which are shown in Fig. 6. It was found that with increasing NO concentration the amount of N20 increases. With increasing burnout and a NO concentration of 723 ppm in the carrier gas the N20 fraction rises at the later stage of burnout which is contrary to the results where no NO or less NO was added to the carrier gas. The cumulative fraction of N20 produced, defined
1.0
[
I
0.9 0.8 0.7
I
~ o
0 ° o
~
I
•
,~o
:,,0 fNO
• filled symbols: 08% 0z hollow symbols: I. 12%02 ~
0.2
o.1
o:.
-.,
f
1
~
"• ~ ~ ~ ' , . * ~
oo_; ' 0.0
0.1
, ,
0.2
!
0.3
- ~ ~ o7
0.4
0,5
0.6
fractional carbon burnout b
Fig. 5. Instantaneous fraction of CO, NO, and N20 at the combustion of five Newland coal particles at 1023 K at 1.12% 02 and 0.8% 02 in helium, respectively.
N20 FORMATION IN FLUIDIZED BED COMBUSTION 0.40
k
o',,0..j
0.35
e
'
'
/
u 2%02, 2 0 5 p p m NO i n H e o 2 Z O 2, 7 2 3 p p m NO in H e
0.30 o0,25
e~
g 020 &
0.15 0.10
0.05 0.00 0.0
02
0.4
0.6
fractional carbon burnout
00~ I 0 0.8
.0
b
Fig. 6. Instantaneous fraction of N20 at the combustion of five Newland coal particles at 1023 K at three different concentrations of NO added to 2% 0 2 in helium.
as
o2.C N20
dt
fractional conversion of char nitrogen to NO increases and the fractional conversion to N20 declines with increasing burnout. De Soete's [4] result as well as our own results show that both the formation as well as the reduction of both species are heterogeneous reactions. Mochizuki et al. [6] presented the results of fluidized bed combustion experiments and they showed that nitric oxide and oxygen play an important role in the formation of N20. They propose that oxygen exposes the bound nitrogen to the gas phase, where it reacts at the char surface with NO to form N20; this suggested mechanism is consistent with our resuits. The experimental data provide some guidance on the mechanism. The oxygen must first break up the ring to form some intermediate active bound nitrogen species, say - N in the absence of any information on the intermediate. This intermediate can then react with either 02 to form NO or with NO to form N20:
FN2 o =
1
Xc [Cco + Cco 2 + CCH4] dt is shown in Fig. 7. At complete burnout, the total conversion of char nitrogen to N20 was found to be FN: o = 0.080 for 2% 02 in the absence of added NO, FN2o = 0.125 for 2% 0 2 with 205 ppm NO in the inlet gas and FN2o = 0.206 for 2% 0 2 with 723 ppm NO in helium in the inlet gas. Figure 2 shows that the i
i
i
i
0.20 O 2Z 0 2 in He a 2% 02 . 2 0 3 p p m o 2% Oa, 7 2 3 p p m
, I . ~ NO i n H e NO in H e
z o 01 5
0.10 R D o
123
0.05
0.00 00
0.2
0.4
0.6
fractional carbon burnout
0.8
1.0
b
Fig. 7. Cumulative fraction of NzO at the combustion of five Newland coal particles at 1023 K at three different concentrations of NO added to 2% 0 2 in helium.
( - C N ) + 3 0 2 ---+ CO q- ( - N ) ,
(1)
2 ( - N) + O e --+ 2NO,
(21)
( - N) + NO ---, N20.
(31)
One possibility for the reactive intermediate is ( - C N O ) as suggested by de Soete, in which case one can postulate the following reactions for parallel NO and N20 formation paths: 0 2+(-CN)
+ (-C)
(-CNO) + (-CO),
(4)
NO + ( - C N O ) --+ N20 + ( - CO),
(5)
(-CNO) ~ NO+ (-C).
(6)
With increasing burnout the char particle radius decreases and the NO can rapidly diffuse out of the particle. Therefore the concentration of the NO at the site of the adsorbed ( - CNO) species will be diminished and so less N20 will be produced [7]. The ratio of NO and N20 will therefore increase with increasing carbon burnout. This reaction mechanism also provides a good explanation for the observed increase in N20 as the NO concentration is increased. An additional path to N20, that of - C N with - C N O , has been proposed by de Soete. This can account for.some conversion of
124 char nitrogen to N 2 0 but only w h e n char b o u n d nitrogen a t o m s h a p p e n to be adjacent. F o r a coal char, with a N / C ratio of a b o u t 0.01, this m e c h a n i s m can account for no m o r e than a one p e r c e n t conversion of char nitrogen to N20. T h e current experiments cannot be used to identify the nitrogen i n t e r m e d i a t e and differentiate b e t w e e n the c o m p e t i n g reactions 2 + 3 and 5 + 6. They, however, clearly establish that this intermediate nitrogen surface species was only m a d e available by the oxidation of the heterocyclic nitrogen containing rings in the char; once m a d e available the b o u n d nitrogen i n t e r m e d i a t e can u n d e r g o reactions to f o r m N O (reactions 2 or 6) or with N O to f o r m N 2 0 (reactions 3 or 5).
The authors which to acknowledge the support of this study by the Hitachi Research Laboratory, Hitachi Limited. The authors thank Prof. M. Horio for making available Ref 6 during the review process. G. Krammer is a visiting engineer
G.F.
KRAMMER
A N D A. F. S A R O F I M
from Graz, University of Technology, Austria and he is supported by the Austrian Science Foundation. REFERENCES 1. Kilpinen, P., and Hupa, M., Combust. Flame, 85:94104 (1991). 2. Hayhurst, A. N., and Lawrence, A. D., Prog. Ener. Combust. Sci., 18:529-552 (1992). 3. Tullin, C., Sarofim, A. F., and Be6r, J. M., J. Inst. Ener., (in press). 4. de Soete, G. G., Twenty-Third International Symposium on Combustion, The Combustion Institute, Pittsburgh, 1990, pp. 1257-1264. 5. Agarwal, P. K., Chem. Eng. Sci., 46:1115-1127 (1991). 6. Mochizuki, M., Koike, J., and Horio, M., Fifth International Workshop on Nitrous Oxide Emissions, Tsukuba, Japan, 5-3-1 to 5-3-8 (1992). 7. Tullin, C., Goel, S., Morihara, A., Sarofim, A. F., and Be6r, J. M., Ener. Fuels (in press). 8. Sarofim, A. F., Be6r, J, M., TuUin, C., Teare, J. D., and Goel, S., Formation and emission of NO X from low NO x combustion systems, Report to the Hitachi Research Laboratory, Hitatchi Ltd., August 1992.
Received 9 June 1993; revised 23 November 1993
118
Reaction of Char Nitrogen During Fluidized Bed Coal Combustion--Influence of Nitric Oxide and Oxygen on Nitrous Oxide G. F. KRAMMER and A. F. SAROFIM Department of ChemicalEngineering~MassachusettsInstitute of Technology, Cambridge,MA 02139 The conditions that favor the formation of nitrous oxide from char under fluidized bed combustion conditions are shown to be the oxidation of the char to make the nitrogen bound in the heterocyclic rings accessible so that it can react heterogeneously either with oxygen to form NO or with nitric oxide to form N20. This was demonstrated by measurement of the gas composition when a batch of coal particles was introduced into a fluidized sand bed electrically heated to 1023 K. Oxygen and nitric oxide were added to the fluidizing helium gas in varying concentrations. N20 was formed in amounts that increased with increasing NO concentration showing the importance of NO for N20 formation. The N20 concentration, however, fell to zero when the 02 supply was interrupted underlying the essential role of oxygen in freeing the organically bound nitrogen so that it can react with NO to form N20.
NOMENCLATURE b c f
fractional carbon burnout ( p p m / p p m ) molar concentration (ppm) fraction, f c o = Cco/(Cco + Cco2 + Ccn,), fNo = CNO/[(XN/XcXCco + CCO2 +
CcH4)]' F r x t
fN20 = 2"CN20/[(XN/Xc)(Cco + CCOz + CCn,)] ( p p m / p p m ) cumulative fraction ( p p m / p p m ) fractional survival Cout/Cir, ( p p m / p p m ) molar ratio ( m o l / m o l ) time (s)
INTRODUCTION NzO formation is known to be favored in the temperature range of fluidized bed coal combustion. The mechanisms by which the N 2 0 is formed from H C N and N H 3 in the volatiles are relatively well understood [1]. The conditions that favor the conversion of char nitrogen to N20, however, are incompletely understood. The N 2 0 formation from char can be equal to [2] or greater than [3] that formed from the volatiles. The most thorough study of char nitrogen conversion to N 2 0 is that by de Soete [4]. H e has identified many of the reactions governing the conversion of organically bound nitrogen to N O and NaO in a series of elegant 0010-2180/94/$7.00
experiments. He, however, showed that when N O is reduced by char little N 2 0 is formed, and concluded that the N O char reaction is not an important pathway to N20. This is correct in the absence of oxygen but, as will be shown in this article, the presence of oxygen greatly augments the formation of N 2 0 from N O and char. A small fluidized bed reactor was used in order to investigate the influence of the temperature, the oxygen concentration and the particle size of the coal particle on the formation and reduction of N O and N20, respectively [3]. The experimental setup is described elsewhere [3]. The influence of the amount of oxygen and nitric oxide on the production of N 2 0 is considered in more detail here.
E X P E R I M E N T A L PROCEDURES AND M E A S U R E M E N T S A fluidized 57-mm-diameter quartz sand bed was electrically heated to 1023 K and a batch of coal particles about 4 m m in diameter were introduced to the bed (Fig. 1). First the volatiles were released and then the char was cornbusted. Helium as a carrier gas with different concentrations of oxygen and nitric oxide was used to maintain the bed fluidized. The gas flow was 2.5 L / m i n at s.t.p, at the entrance of Copyright © 1994 by The Combustion Institute Published by Elsevier Science Inc.
N20 FORMATION IN FLUIDIZED BED COMBUSTION
119
A deconvolution method was derived by Tullin et al. [8] in order to evaluate the influence of the broadening of the gas concentration profiles due to mixing downstream from the reaction zone. The influence, however, is small on the gas concentration profiles during the char oxidation stage and it is not taken into account in the data shown here. The composition of the coal which was used in the experiments is given in Table 2. The experiments were designed to obtain the conversion of fuel nitrogen to NO and to N20 for single particles with a minimum of secondary reactions with either other particles or the bed material. The uncertainty in the gas analysis decreases with the increasing concentration encountered when several particles are injected into the bed. Tests were therefore carried out to evaluate the effects of particle interaction on the N20, NO, and CO yields by injecting one and five particles. The fractional conversion of carbon to CO is given by
tk
qt
ele, quo
de CG
clio ql
sil
9as in Fig. 1. Schematics of the fluidized bed reactor.
the reactor. The gas released was analyzed for CO, CO2, NO, N20 , CH4, NH3, and NO 2 in an FTIR. The fractional conversion of the organically bound nitrogen to NO and N20 and the fraction of the carbon released that is in the form of CO were derived from the experimental data as a function of burnout. The experimental conditions such as coal mass, number of coal particles, and concentration of the fluidizing gas are listed in Table 1. The volume gas flow was constant at 2.5 L / m i n at normal conditions (273.5 K and 1 atm).
¢co
]:co = CCO q- CCO 2 -'b ¢ C H 4
and the fractional conversion to NO and N20, assuming that the rates of consumption of nitrogen and carbon are in the ratio of the atomic ratio of nitrogen and carbon in the char, are given by
fNo
¢NO
XN(cco + CCO: + CcH) Xc
TABLE 1 Experimental Conditions Exp. No.
Coal Mass [mg]
No. of Particles
Gas Concentration
Deviation of c-balance (%)
Figure No.
1 8
200.0 202.0
5 5
3.8 --
2, 3 4
11 12 13 14 15
44.0 211.5 227.3 169.5 239.8
1 5 5 5 5
5.0 --1.1 6.0
2, 3 3,5 3, 5 3,6,7 6, 7
17
188.3
5
4% 0 2 in He 275 ppm NO, 4% 0 2 in He 4% O 2 in He 1.12% 0 2 in He 0.8% 0 2 in He 2% 0 2 in He 205 ppm NO, 2% 0 2 in He 723 ppm NO, 2% O2 in He
3.3
6, 7
120
G . F . KRAMMER AND A. F. SAROFIM TABLE 2 Proximate and Ultimate Analysis of Newlands Coal (Data Provided by Hitachi Ltd., Japan)
Proximate Analysis
Wt. %
Ultimate Analysis
Wt. %
Ash Volatile matter Fixed carbon Water
17.44 26.49 56.07 2.44
Ash Carbon Hydrogen Oxygen Nitrogen total sulfur Inflammable sulfur Nonflammable sulfur
17.44 68.83 4.38 7.71 1.20 0.54 0.44 0.10
Gross heating value: 6490 [BTU/Ib]
and 2.CN20 fN20 =
XN
- - ( C c o + Cco2 + CCH,) XC Figure 2 shows the fraction of CO, NO, and N20 plotted versus the fractional carbon burnout given by f 0 /( C c o
-F
CCO 2 -I'- CCH4)
dt
b CC,coal, t = 0 The results of a single particle experiment are compared with the results of an experiment with five particles. At total coal burnout the cumulative fractional conversions were found to be Fco = 0.074, F N O = 0.608, and FN2o = 0.096 for the five particle experiment whereas ~
1.Or
0.40
0,8
o 8
•
0.6
~ 0.4 & 0.2
0.0: 0.0
o
o. O ~. filled symbols: 1 particle • • o
0.2
0.4
0.6
fractionol corbon burnout
0.8
o
"l 0.16 -0,08
1.0
b
Fig. 2. Instantaneous fraction of CO, NO, and N20 at the combustion of 1 and 5 Newland coal particles at 1023 K. The reaction gas was constant at 4% O 2 in helium.
they were Fco = 0.149, FNO = 0.700, and FN20 = 0.08 for the single-particle experiment. It can be seen that the results with the single particle, because of the low concentrations, show much more scatter than with five particles. Interaction of Coal Particles in the Fluidized Bed Reactor
The NO profiles show larger differences at the beginning of char burnout, but this is still within the standard error range of the gas analyzer. The N20 profiles are within the error range whereas the CO profile is not, especially at the later stages of burnout. The CO burns to CO2 in the boundary layer surrounding the particle and therefore the CO burnout is inhibited when several particles are introduced because of the overlap of their layers. Figure 3 shows the fractional carbon burnout versus reaction time. An analytic function could be derived by fitting the data of each curve to a quadratic function with correlation coefficient always larger than 0.995. It is seen that the single particle burns approximately 10% faster than the batch of five particles which was due to the fact that some of the particles for the five-particle experiment were about 10% larger in mass than the single particle. However, the observed trends of the concentrations profiles are unchanged.
N20 Decomposition The ratio r of the outlet to inlet concentration of N20 in the empty reactor (no sand and coal particles) with no oxygen and 4% oxygen in
N 2 0 F O R M A T I O N IN F L U I D I Z E D B E D C O M B U S T I O N i
i
i
ance on the particle, equating the rate of heat generation to the heat transfer to the bed. The Nusselt number was calculated using a correlation in the literature [5] for particle-gas heat transfer in gas fluidized beds and found to be in the range of 8-12. The burnout rate was derived from the generation of CO and CO2, the sum of which is shown in Fig. 3. It was found that the temperature increase of the particle in the case of 4% oxygen in helium and a flow rate of 2.5 L / m i n was about 25 ° C at maximum. In the case of 0.8% and 1.12% oxygen in helium and 2.5 L / m i n , the particle temperature increased by no more than 8 ° C. Tullin et al. [3] found that at temperatures below 1073 K the fractional conversion of char nitrogen to NO increased and that to N 2 0 decreased with increasing temperature. However, the results obtained from the experiment at a higher oxygen concentration and consequently at a higher particle temperature show a lower NO and a higher N 2 0 fraction (Fig. 4), which indicates that the temperature increase due to burnout has a minor effect on the char nitrogen reaction.
i
1.0
# f
o8
° o
I
, ~
c o o o
0.4
&
0.2
~
i 500
27. 02
v
1.12g 02
symbols:
hollow
O.C
°
5
i
i
1000
1500
ti~
t
121
particles
I 2000
b]
Fig. 3. The fractional carbon burnout versus reaction time at the combustion of Newland coal particles at 1023 K.
helium was found to be 0.95 and 0.97, respectively. In the presence of the fluidized quartz sand bed r was determined to be 0.81 when no oxygen was present and 0.92 with 4% oxygen. The volume flow rate was constant at 2.5 L / m i n at normal conditions. The temperature was set to 1023 K. The values of r at oxygen concentration of 8%, 12%, and 20% were the same as for 4% oxygen. The N 2 0 decomposition is compensated for in the results presented in this paper.
RESULTS AND DISCUSSION Influence of O z o n N 2 0 Formation Figure 4 shows the gas concentrations of CO, CO 2, NO, and N 2 0 as a function of time. At the start of the reaction the gas concentration of the fluidizing gas was 4% O 2, 275 ppm NO, and the balance helium. A batch of five coal particles were introduced to the fluidized bed after 40 s. After 375 s the gas concentration
Actual Temperature of the Particle The temperature of the coal particles in the fluidized quartz sand bed was obviously higher than the bulk temperature which is the one mentioned in Table 1. The difference in temperature was estimated from an energy bal-
8
7000 6000 -
5000
8 8
~oo:°°-
{,~,,.
8
o
~-00
cN2 o
25
• CNo v Cco
0 z on
¢00, /
35O
02 off
/
300 -~ 4000
•Cco 2
8 300, /
350 -
-
3000
250 '~
&
250 200
200
2000 -
0
-
?
o
150
1000 -
2O
300
T
A
30
*5O
500
10
100
10o I 50
50
0 0
250
500 time t
1500 IS]
p
i
1750
2000
o
5
0
Fig. 4. Gas concentrations of CO, CO2, NO, and N20 at the combustion of five Newland coal particles at 1023 K. The reaction gas was changed from 4% 02, 275 ppm NO in helium to 275 ppm in helium and back again.
122 was changed to 275 ppm NO with helium as the carrier gas and after 1470 s the original gas concentration of 4% 0 2, 275 ppm NO and helium carrier was used again. The experiment was terminated after 2000 s. Figure 4 provides a critical test of the role of 02 on N20 production. In the first stage of the reaction oxygen as well as additional NO is available in the carrier gas. One can expect, therefore, that the NO measured is the difference between NO production by the oxidation of the bound nitrogen in the volatiles or char and the NO reduction by hydrocarbons in the volatiles or by char nitrogen. Soon after the particles were introduced into the reactor the reduction of NO on the coal particles was exceeded by the production of NO from the bound nitrogen compounds in the volatiles accounting for the peak in NO at 65 s. After the volatiles burned out at a time of 78 s, the NO concentration was almost constant at a value about 50 ppm below that of the NO in the carrier gas. In this regime the NO decrease as a result of the N O / c h a r reaction exceeds the NO formation from the oxidation of char nitrogen. When the oxygen was turned off CO2, CO, and N20 declined rapidly to zero. (The slower transient for CO and N20 indicates some slower desorption of these species.) The NO concentration, after an initial peak due to the volatile oxidation, dropped below the inlet value of 275 ppm as a consequence of the char NO reactions. The NO concentration then increased with time, which means that the reduction capacity of the char particles decreased with time. This transient is quite possibly a consequence of the build up of chemisorbed oxygen atoms on the surface [ ( - C O ) in the reaction mechanism of de Soete] reducing the number of carbon sites available to react with the NO. After the oxygen flow was resumed, N20 , CO, and CO 2 were formed again and the level of NO increased as the result of the contribution from the oxidation of char nitrogen. Surprisingly the level of N20 and CO 2 were lower and the ones of CO and NO were higher compared with the concentration level when oxygen was turned off. The higher CO level can be explained by assuming that some of the accumulated adsorbed ( - CO) molecules are released. (After the small drop at about
G . F . KRAMMER AND A. F. SAROFIM 1650 seconds the increase of CO concentration continued as expected.) It is uncertain whether the higher CO concentration level during this time had an influence on the NO and N20 concentration. Figure 5 shows the results of two experiments where the fluidizing gas contained 0.8% and 1.12% 02 in helium. The fraction of NO, CO, and N20 is plotted versus the normalized burnout rate. By increasing the oxygen concentration in the helium carrier gas the fractions of N20 and CO increase whereas that of NO decreases. An influence of oxygen on the formation of nitric oxide and nitrous oxide was also observed by de Soete [4], Tullin et al. [3], and Mochizuki et al. [6].
Influence of Nitric Oxide on N20 Formation
In order to evaluate the influence of the NO concentration on the N20 concentration a set of experiments were carried out which are shown in Fig. 6. It was found that with increasing NO concentration the amount of N20 increases. With increasing burnout and a NO concentration of 723 ppm in the carrier gas the N20 fraction rises at the later stage of burnout which is contrary to the results where no NO or less NO was added to the carrier gas. The cumulative fraction of N20 produced, defined
1.0
[
I
0.9 0.8 0.7
I
~ o
0 ° o
~
I
•
,~o
:,,0 fNO
• filled symbols: 08% 0z hollow symbols: I. 12%02 ~
0.2
o.1
o:.
-.,
f
1
~
"• ~ ~ ~ ' , . * ~
oo_; ' 0.0
0.1
, ,
0.2
!
0.3
- ~ ~ o7
0.4
0,5
0.6
fractional carbon burnout b
Fig. 5. Instantaneous fraction of CO, NO, and N20 at the combustion of five Newland coal particles at 1023 K at 1.12% 02 and 0.8% 02 in helium, respectively.
N20 FORMATION IN FLUIDIZED BED COMBUSTION 0.40
k
o',,0..j
0.35
e
'
'
/
u 2%02, 2 0 5 p p m NO i n H e o 2 Z O 2, 7 2 3 p p m NO in H e
0.30 o0,25
e~
g 020 &
0.15 0.10
0.05 0.00 0.0
02
0.4
0.6
fractional carbon burnout
00~ I 0 0.8
.0
b
Fig. 6. Instantaneous fraction of N20 at the combustion of five Newland coal particles at 1023 K at three different concentrations of NO added to 2% 0 2 in helium.
as
o2.C N20
dt
fractional conversion of char nitrogen to NO increases and the fractional conversion to N20 declines with increasing burnout. De Soete's [4] result as well as our own results show that both the formation as well as the reduction of both species are heterogeneous reactions. Mochizuki et al. [6] presented the results of fluidized bed combustion experiments and they showed that nitric oxide and oxygen play an important role in the formation of N20. They propose that oxygen exposes the bound nitrogen to the gas phase, where it reacts at the char surface with NO to form N20; this suggested mechanism is consistent with our resuits. The experimental data provide some guidance on the mechanism. The oxygen must first break up the ring to form some intermediate active bound nitrogen species, say - N in the absence of any information on the intermediate. This intermediate can then react with either 02 to form NO or with NO to form N20:
FN2 o =
1
Xc [Cco + Cco 2 + CCH4] dt is shown in Fig. 7. At complete burnout, the total conversion of char nitrogen to N20 was found to be FN: o = 0.080 for 2% 02 in the absence of added NO, FN2o = 0.125 for 2% 0 2 with 205 ppm NO in the inlet gas and FN2o = 0.206 for 2% 0 2 with 723 ppm NO in helium in the inlet gas. Figure 2 shows that the i
i
i
i
0.20 O 2Z 0 2 in He a 2% 02 . 2 0 3 p p m o 2% Oa, 7 2 3 p p m
, I . ~ NO i n H e NO in H e
z o 01 5
0.10 R D o
123
0.05
0.00 00
0.2
0.4
0.6
fractional carbon burnout
0.8
1.0
b
Fig. 7. Cumulative fraction of NzO at the combustion of five Newland coal particles at 1023 K at three different concentrations of NO added to 2% 0 2 in helium.
( - C N ) + 3 0 2 ---+ CO q- ( - N ) ,
(1)
2 ( - N) + O e --+ 2NO,
(21)
( - N) + NO ---, N20.
(31)
One possibility for the reactive intermediate is ( - C N O ) as suggested by de Soete, in which case one can postulate the following reactions for parallel NO and N20 formation paths: 0 2+(-CN)
+ (-C)
(-CNO) + (-CO),
(4)
NO + ( - C N O ) --+ N20 + ( - CO),
(5)
(-CNO) ~ NO+ (-C).
(6)
With increasing burnout the char particle radius decreases and the NO can rapidly diffuse out of the particle. Therefore the concentration of the NO at the site of the adsorbed ( - CNO) species will be diminished and so less N20 will be produced [7]. The ratio of NO and N20 will therefore increase with increasing carbon burnout. This reaction mechanism also provides a good explanation for the observed increase in N20 as the NO concentration is increased. An additional path to N20, that of - C N with - C N O , has been proposed by de Soete. This can account for.some conversion of
124 char nitrogen to N 2 0 but only w h e n char b o u n d nitrogen a t o m s h a p p e n to be adjacent. F o r a coal char, with a N / C ratio of a b o u t 0.01, this m e c h a n i s m can account for no m o r e than a one p e r c e n t conversion of char nitrogen to N20. T h e current experiments cannot be used to identify the nitrogen i n t e r m e d i a t e and differentiate b e t w e e n the c o m p e t i n g reactions 2 + 3 and 5 + 6. They, however, clearly establish that this intermediate nitrogen surface species was only m a d e available by the oxidation of the heterocyclic nitrogen containing rings in the char; once m a d e available the b o u n d nitrogen i n t e r m e d i a t e can u n d e r g o reactions to f o r m N O (reactions 2 or 6) or with N O to f o r m N 2 0 (reactions 3 or 5).
The authors which to acknowledge the support of this study by the Hitachi Research Laboratory, Hitachi Limited. The authors thank Prof. M. Horio for making available Ref 6 during the review process. G. Krammer is a visiting engineer
G.F.
KRAMMER
A N D A. F. S A R O F I M
from Graz, University of Technology, Austria and he is supported by the Austrian Science Foundation. REFERENCES 1. Kilpinen, P., and Hupa, M., Combust. Flame, 85:94104 (1991). 2. Hayhurst, A. N., and Lawrence, A. D., Prog. Ener. Combust. Sci., 18:529-552 (1992). 3. Tullin, C., Sarofim, A. F., and Be6r, J. M., J. Inst. Ener., (in press). 4. de Soete, G. G., Twenty-Third International Symposium on Combustion, The Combustion Institute, Pittsburgh, 1990, pp. 1257-1264. 5. Agarwal, P. K., Chem. Eng. Sci., 46:1115-1127 (1991). 6. Mochizuki, M., Koike, J., and Horio, M., Fifth International Workshop on Nitrous Oxide Emissions, Tsukuba, Japan, 5-3-1 to 5-3-8 (1992). 7. Tullin, C., Goel, S., Morihara, A., Sarofim, A. F., and Be6r, J. M., Ener. Fuels (in press). 8. Sarofim, A. F., Be6r, J, M., TuUin, C., Teare, J. D., and Goel, S., Formation and emission of NO X from low NO x combustion systems, Report to the Hitachi Research Laboratory, Hitatchi Ltd., August 1992.
Received 9 June 1993; revised 23 November 1993