Energy & Fuels 1993, 7, 796-802
796
NO and N20 Formation for Coal Combustion in a Fluidized Bed: Effect of Carbon Conversion and Bed Temperature Claes J. Tullin,? Shakti Goel, Atsushi Morihara,z Adel F. Sarofim,' and J h o s M. Be& Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received May 21, 1993. Revised Manuscript Received September 15,199P
The conversion of fuel nitrogen to NO and NzO have been determined in a small-scale fluidized bed. Small batches of coal particles were used, permitting the separation of the formation reactions within individual particles from subsequent destruction or re-formation on other particles. From time-resolved measurements of the concentrations of NO, NzO, COz, CO, and CH4, the instantaneous fractional conversions of coal nitrogen to NO and NzO as a function of fractional char burnout were obtained for bed temperatures between 975 and 1148 K. The conversion to N2O decreases with increasing temperature, whereas that to NO exhibits a maximum between 1023 and 1095 K. As a particle burns out, the instantaneous conversion to NzO decreases, whereas the reversed trend is seen for NO. The cumulative fuel nitrogen conversion to NO is in the range of 0.18-0.46, whereas the conversion to NzO is in the range of 0.04-0.18. The NO and Nz0 emissions can be explained by a model in which the nitrogen bound in the char is converted to NO and Nz0 on oxidation within pores. The split between the NO and Nz0 depend on the local NO concentration and the temperature. The NO and NzO formed are subsequently reduced as they diffuse out of the pores. This model explains the increase in fractional conversionto NO and decrease in fractional conversionto NzO with increasing carbon conversion. The temperature dependence is a function of the activation energies of the governing reactions.
Introduction The conversion of char nitrogen to NO and Nz0 during fluidized bed combustion is incompletely understood. The present paper provides experimental and modeling results on the effects of carbon conversion and temperature on the fractional conversions of char nitrogen to NO and NzO in single particle combustion. In fluidized bed combustors, the emission of N2O decreases with increasing temperature, in contrast to the trend for the NO emission which increases' and passes through a slight maximum or reaches an asymptote with increasing temperature.2 Although it is recognized that temperature is perhaps the most importance parameter in controlling NzO emission^,^^ the rate-controlling mechanism is not clear. The temperature dependence may be explained by homogeneous gas-phase reactions as well as heterogeneous gas-solid reactions. Experimental t Department of Inorganic Chemistry, Chalmers University of Technology and University of Gateborg, 5-412 96 Gateborg, Sweden. t Hitachi Research Laboratory, Japan. e Abstract published in Advance ACS Abstracts, October 15, 1993. (1)Amand, L.-E.; Lecher, B. Influence of fuel on the emission of nitrogen oxides (NO and N20) from an 8-MWfluidized bed boiler. Combust. Flame 1991a, 84 (1-2), 181-196. (2) Braun, A.; Renz, U. Vergleichendeuntersuchung der emission von stickoxiden aus stationben wirbelschichtfeuerungen. Verbrennung und Feuerungen. 15. Deutscher Flammentag, Bochum, September 17-18, 1991 (in German). (3) Hayhurst, A. N.; Lawrence, A. D. Emissions of nitrous oxide from combustion sources. Prog. Energy Combust. Sci. 1992, 18, 529-552. (4) Lecher, B.; Amand, L.-E. NzO emissionsfrom combustion of solid fuels in fluidized bed. Joint Meeting of the French, Italian and Swedish Sections of the Combustion Institute, Sept. 1992. (5) Mann, M. D.; Collings, M. E.; Botros, P. E. Nitrous oxide emissions in fluidized-bed combustion: Fundamental chemistry and combustion testing. Prog. Energy Combust. Sci. 1992, 18, 447-461.
0887-0624/93/2501-0796$04.00/0
data6as well as modeline7of the homogeneous gas-phase chemistry show that the volatile nitrogen, which is primarily emitted as HCN,8f9will undergo oxidation to NO rather than NzO as the temperature increases. Modeling7suggests that the most important N2O destruction reactions involve the attack of H and OH radicals. In addition, the decrease in N2O with increasing temperature may also be an effect of homogeneous thermal decomposition.1° Heterogeneous interactions with bed material" and char12must also be considered when discussing the temperature dependence in fluidized beds. The effect of temperature on NO emission is complicated by the major reduction of NO formed early during combustion by char in the upper part of the bed or in the freeboard. In laboratory-scale experiments, in which the bed carbon concentration is maintained low in order to minimize the secondary reduction of NO by carbon, the (6) Hulgaard, T.. Nitrous Oxide from Combustion. Ph.D. thesis in Chemical Engineering, Technical University of Denmark, 1991. (7) Kilpinen, P.; Hupa, M. Homogeneous Nz0 chemistry at fluidized bed combustion conditions: A kinetic modelling study. Combust.Flame 1991,85, 94-104. (8) Baumann, H.; Mbller, P. Pyrolyeia of hard coals under fluidised bed combustor conditions. Erdbl Kohle-Erdgas 1991,M (l),2+33. (9) Freihaut, J. D.; Zabieleki, M. F.; Seery, D. J. A parametric
investigation of tar release in coal devolatilization. Nineteenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1982; pp 1159-1167. (10) Johnsson, J. E.; Glarborg, P.; Dam-Johanaen, K. Thermal dissociation of nitrous oxide at medium temperatures. Twenty-Fourth Symposium (International) on Combustion; The Combustion Institute: PitGburgh, 1992; pp 917-923. (11) Miettinen, H.; StrBmberg,D.; Lindquiet, 0.The influence of some oxide and sulphatesurfacea on N10 decommition. 11th Int. Conf.Fluid. Bed Combust. 1991,999-1003. (12) de Soete, G. G. HeterogeneousNgO and NO formation frombound nitrogen atoms during coal char combuetion. Twenty-Third Sympaaium (Internotional)Combustion:Thecombustion Inetitute: Pittsbmh. 1990:, pp 1257-1264.
0 1993 American Chemical Society
NO and N& Formation for Coal Combustion
Energy & Fuels, Vol. 7, No. 6,1993 797
Table I. Proximate and Ultimate Analysis of Newlands
100 a
Coal.
90
ultimate analysis (%)
proximate analysis ( % ) ash volatile matter fixed carbon gross heating value (2.44% water content) calk a
17.44 26.49 56.07 6490
ash carbon hydrogen oxygen nitrogen totalsulfur
17.44 68.83 4.38 7.71 1.20 0.54
The analyses were provided by Hitachi.
NO emission displays a maximum between 750 and 850 0C.13-16 At lower temperatures, the NO removal will be governed by the char-catalyzed reaction between NO and CO, whereas the char consumingNO char reaction becomes increasingly important with increasing temperatures.1618 For practical combustors, the carbon loading is high enough that secondary reactions between the NO and char and NzO and the bed material (including char) have a major impact on the final emissions. It is therefore difficult to unravel the relative importance of the different parameter-carbon loading, formation kinetics, destruction kinetics-on the changes in emissions of NO and N2O with changes in temperature. In this investigation, the temperature effect on the NzO and NO formation/ destruction is studied in single particle experiments. A second objective of the paper is to test models of N2O formation in single particle combustion.
80
70 60
50 40
30 20
10
n 0
120
240
360
480
Time
(s)
600
720
Figure 1. Concentration us time profiles for NzO and NO; 1023 K, 8% 02 in He.
Experimental Section Batch combustion experiments were performed in a smallscale quartz glass bubbling fluidized bed reactor (i.d. 57 mm). A bed of silica sanc (particle size 150-212 pm), with a bed height of approximately 50 mm, was fluidized by a mixture of 8 % oxygen in helium. The composition and flow rate were controlled using mass flow controllers. The flow rate was set to 2.5 L/min NTP (273K, 1 atm). In each experiment a batch of four coal particles with a diameter of 4 mm was burned. The analysis of the coal is given in Table I. The combustion products were analyzed for NzO, NO, NOz, COz, CO, CHI, and SO2 using a FTIR spectrometer equipped with a MCT detector, and a low-volume (223cma) gas cell with variable path length, which was set to 7.25 m in the experiments reported here. The NO, concentration was for some cases also quantified by means of a chemiluminescent NO, analyzer, which was employed downstream of the FTIR spectrometer. The gas analyses were carried out a t room temperature, and excessive water was removed in a cooler operating a t a few degrees Celsius. The water concentrations were, however, low and no significant quantities of water condensed. A detailed description of the reactor and experimental setup can be found elsewhere.16 (13)Pereira, F. J.; BeBr,J. M. A mathemetical model of NO formation and destruction in fluidized combustion of coal. Eng. Found. Conf. Fluidization 1978. (14)Yue,G.X.;Pereira,F.J.;Sarofim,A.F.;BeBr,J. M.Charnitrogen conversion to NO. in a fluidized bed. Combwt. Sci. Technol. 1992,83, 245-256. (15)Tullin, C.;Sarofim, A. F.; Be&,J. M. Formation of NO and N10 in coal combustion: The relative importanceof volatile and char nitrogen, 12th Int. Conf.Fluidized Bed Combust., in press. (16)Furusawa, T.; Tsunoda, M.; Kunii, D.Nitric oxide reduction by hydrogen and carbon monoxide over char surface. ACS Symp. Ser. 1981, 196,347-357. (17)Chan, L.K.; Sarofim,A. F.; Be&,J. M. Kinetics of the NO-carbon reaction at fluidized bed combustor conditions. Combust. Flame 1983, 52,37-45. (18)Furusawa, T.; Tsunoda, M.; Tsujimura, M.; Adschiri, T. Nitric oxide reduction by char and carbon monoxide. Fundamental kinetics of nitric oxide reduction in fluidized bed combustion of coal. Fuel 1985.64 (Sept.),1306-1309.
Time (s)
Figure 2. Concentration us time profiles for COz, CO, and CH4; 1023 K, 8% 0 2 in He. Procedure. The characteristics of a char depend on temperature history, particle size, pressure, and gas atmosphere employed in the devolatilization.ls The chemical characteristics of chars are similar, implying that the differences in reactivity between different chars is an effect of differences in the physical structure.zo Chars prepared in separate pyrolysis experiments are therefore likely to be different from the chars obtained during combustion in situ. Since the evolution and combustion of volatiles and char occur in series, it is possible to separate the two stages in time-resolved batch combustion experiments.16The volatiles burn in a diffusion flame surrounding the particle and very little oxygen will penetrate to the surface of the particle until the devolatilization is almost complete.21 In Figures 1and 2,the time-resolved evolution of N20, NO, CO2, CO, and CH4 is shown. The curves display a peak resulting from the evolution and combustion of the volatiles, followed by the combustion of the char. The CO and CH4 concentrations peak during the devolatilization, whereas that of COz dominates during char burnout. The CO concentration increases slightly during the char burnout. The volatiles were assumed to be consumed when the CHI and CO concentration approached zero, which as seen from Figure 2 is at about 80 s. This time is greater than the actual devolatilization times estimated to be 15 s from the correlation of Stubington et aLZ2because of a time lag and axial dispersion in the freeboard and the sampling lines. The volatile (19)Howard,J. B. Fundamentalsof CoalPyrolysiaandHydropyrolyais. Chemistry of Coal Utilization, 2nd Supplementary Volume;Elliot, M. A., Ed.; Wiley: New York, 1981;Chapter 12. (20)Fletcher,T.H.;Solum,M.S.;Grant,D.M.;Pugmire,R.J.Chemical structure of char in the transition from devolatilization to combustion. Energy Fuels 1992, 6, 643-650. (21)Andrei, M. A.; Sarofim, A. F.; Be&,J. M. Time Resolved Burnout of Coal Particles in a Fluidized Bed. Combust. Flame 1985,61, 17-27.
Tullin et al.
798 Energy & Fuels, Vol. 7, No. 6, 1993
yields of carbon are estimatedfrom the transientgas composition to be 0.11 at 973 and 0.15-0.19 in the temperature range 10231148 K. The fraction of volatile carbon can be estimated from the ASTM analysis (Table I) to be 0.185. The deviation from closure of the carbon balance was less then 10%. The homogeneous decomposition in the bed and freeboard was investigated by injecting a flow of N20 just above the distributor plate in the reactor. The decomposition was found to be 39% at 1148 K, 25% at 1123 K,8% at 1073 K,and 2% at 1023 K,effectively unchanged if the NzO was injected at the bottom or the top of the bed. This suggeststhat the decomposition is caused by mainly homogeneous reactions in the freeboard, and the magnitude of decomposition is consistent with the data reported by Johnsson et al."J The measured N20 during char burnout was corrected for this decomposition. A step response experiment was performed in order to estimate the axialdispersiondownatream of the reador. It was established that the mixing could be described by a plugflow reactor in series with three CSTRa (continuously stirred tank reactors). The difference between the deconvoluted and the measured data was found to be of minor importance for the char burnout and the results reported in this paper are of the data as measured.
Results The instantaneous conversions to NO and N20 were calculated, assuming that the carbon and nitrogen were oxidized in proportion to their relative concentrations in the ~ 0 a l . ~It3is~ then ~ possible to calculate the fractional conversion to NO and N2O for every increment of carbon consumed. The instantaneous formation rate of a species (in this case NO or N20) is the molar gas flow rate times ita mole fraction in the gas. The instantaneous carbon consumption is the molar flow rate times the s u m of the mole fractions of C02, CO and CH4. The instantaneous rate of fuel nitrogen consumption equals the atomic nitrogen to carbon ratio in the char times the rate of carbon consumption. Consequently, the instantaneous conversions to N2O and NO can be written:
0.5 7
1
Vol..
0.4
s 0.1
0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1.0
Carbon Converalon (mollmol)
Figure 3. Instantaneous fuel nitrogen conversion to N20;T = 975-1148 K,8% 02 in He.
s 0
e
A
c 0
n
0.1
j Carbon Conversion (mollmol)
Figure 4. Instantaneous fuel nitrogen conversion to NO;T =
975-1148 K,8% 02 in He.
I
The carbon conversion was calculated from the molar gas flow rate u and the molar fractions of CO, C02, and CH4. carbon conversion =
l u ( [ C O l + [C021+ [CH41jdt
%WMC
(3)
X c is the carbon content, Mc is the molecular weight of carbon, and m,is the mass of the batch fed to the reactor. The results are shown in Figures 3 and 4. The range of carbon conversion where the volatile5 are consumed and where the char ignites is indicated in the figures, as determined from the peaks in CO, CO2, and CH4 evolution (cf. Figure 2). The rate of release of nitrogen during ~~~
~
(22)Stubington, J.F.;Cui, T. Y.5.; Saisithidej, S. Experimental factors affecting coal devolatilization time in fluidized bed combustion. Fuel Sci. Technol. Int. 1992,lO (3),397-419. (23)Song, Y. H.;Bdr, J. M.;Sarofun, A. F. Oxidation and devolatilization of nitrogen in coal char. Combust. Sci. Technol. 1982,28,177183. (24)Smoot, L. D. Pulverized coal diffusion flames: A perspective through modeling. Eighteenth Symposium (Internotional) on Combustion; The Combustion Institute. Pittsburgh, 1981;pp 1185-1202.
devolatilization is not uniform.26126 The instantaneous values of NO and N2O during devolatilization are therefore biased by the assumption of a constant N/Cratio and are artificially shown to be zero at early times when the N/C ratio would be lower than average and higher later in the devolatilization process when the N/C ratios would be higher than average. The early fluctuation in the apparent NO and N2O concentrations provides another measure of the duration of volatile release. From the areas under the curves, in Figures 1, 3, and 4 it can be noted that the contribution to NO and N20 formation is greater for the char burnout than for the volatile combustion. The instantaneous fractional conversion of char nitrogen to N20 is seen to decrease continuously during the char particle burnout (Figure 31, whereas that to NO increases continuously approaching one in the limit of complete carbon conversion (Figure 4). The rate of NzO formation is highest a t 975 K and decreases with increasing temperature. It should be noted that these results have been corrected for the thermal destruction of N2O. The net (25)Pohl, J. H.;Sarofm,A. F. Devolatilization and Oxidation of Coal Nitrogen. Sixteenth Symposium (International) on Combrurtion; The Combustion Institute: Pittsburgh, 1977;pp 491-501. (26)Blair,D. W.; Wendt, J. 0. L.; Bartok, W. Evolution of nitrogen and other species during controlled pyrolysis of coal. Sixteenth Symposium (Internotional) on Combustion; The Combustion Institute: Pittsburgh, 1977;pp 475-489.
NO and N20 Formation for Coal Combustion NO production rate shows a maximum at 1078 K. The conversion to NO is seen to increase when the temperature is increased from 975 to 1078 K,but decreases when the temperature is further increased.
Energy &Fuels, Vol. 7, No. 6,1993 799
(1 - 2a). As indicated by eq 7, CY is a function of the temperature and the local NO concentration in the pores. The stoichiometric equations for the reactions are as follows:
Discussion The upward curvature of the conversion to NO in Figure
(5)
4 can be e ~ p l a i n e d ' ~ in?terms ~ ~ of a model in which NO
is formed during char oxidation and is partially destroyed as it diffuses out of the pores. As the particle shrinks, less NO will be destroyed in the pores and as a result the conversion to NO increases with burnout. The instantaneous conversion to NO approaches a value of unity at complete carbon conversion because in the final stage of combustion the NO produced has little opportunity to react with char. Since the NzO is reduced on char similarly to NO, the curvature for N2O might be expected to show the same trend as the NO profile. The decrease in N2O yield with carbon conversion is therefore believed to be due to a decrease in the N2O formation rate as the char burns out. The opposing trends for N2O and NO can be explained in that N2O formation depends upon the availability of NO at the point at which the char nitrogen is oxidized. Mechanism. It is generally accepted that NO is formed as the nitrogen in the char is oxidized simultaneously with the carbon. The mechanism for N2O formation from char bound nitrogen is less clear. I t has been suggested12that N2O forms from two adjacent nitrogen sites during oxidation. However, this mechanism cannot explain the observedl5increase in N20 formation with increasing bulk NO concentration, or the trends in curvature and instantaneous conversion to N20 and NO as discussed above. In a circulating fluidized bed boiler, the N2O concentration has been found to increase with bed height in contrast to the NO concentration which decreases.28 In addition, the same investigators28 performed a test where the fly ash and char recirculation was interrupted, resulting in a decrease inN2O and an increase in NO. The results suggest that an alternative mechanism may involve the destruction of NO on the char and a reaction between NO and the char nitrogen to form N2O. de SoetelZhave shown that N2O is not formed from the reaction between NO and char in the absence of oxygen. In the presence of oxygen, however,the N2O formation increases with increasing bulk NO c~ncentration,'~ thus indicating the existence of a mechanism involving NO and char. Consequently, the mechanism of N2O formation appears to be tightly linked to the carbon oxidation in the presence of NO. Based on the experimental observations and the discussion above, the following reaction mechanism is proposed I t is assumed that the nitrogen and the carbon is oxidized at relative rates that are in proportion to the nitrogedcarbon atomic ratio in the char. As the coal is oxidized, the ring structure is disrupted and in the process a nitrogen in such a disrupted ring can react with either oxygen to form NO, or it can react with NO present in the pore (either formed at another char nitrogen site or diffusing into the pore from a high local NO concentration) to form N2O. Let the moles of NzO per char nitrogen be a, and consequently the moles of NO per char nitrogen will be (27) Wendt, J. 0. L.; Schulze, 0.E. On the fate of fuel nitrogen during coal char combustion. MChE J. 1976,22 (I),102-110. (28)Amend, L.-E.; Lecher, B. Formation of N20 in circulating fluidized bed boilers. Energy Fuels 1991, 5, 815-823.
Model Calculations. The experimental findings are compared with model predictions using a numerical model. With the assumption of pseudo-steady-state conditions, i.e., zero accumulation and first-order kinetics, the governing equations can be written: for 0,:
D,~~v~[O,I - kO2[O21= o
where
boundary conditions:
Deaisthe effectivediffusivity in the char, the rate constanta ki are per unit volume of the char, R, is the particle radius, and k~ is the mass-transfer coefficient. Calculated Results. The results can be generalized by using the pertinent dimensionless groups, including the Thiele moduli,
[2]''2 (15)
and the Biot number for mass transfer, Bi = kMRp/Dea. In order to apply the generalized solution to our data, the heat-transfer resistance was assumed to be negligible, and the particles were assumed to be isothermal with a temperature equal to the bed temperature. The carbon oxidation kinetics was obtained from Smith et al.29 The (29)Smith, I. W. The combustion rates of coal chars: A review. Nineteenth Symposium (Internutionul)on Combustion;Thecombustion Institute: Pittsburgh, 1982;pp 1045-1065.
Tullin et al.
800 Energy & Fuels, Vol. 7,No. 6,1993
_l,oy I
100Ol
I
100 h
v
4
8
10
-a,
0
LL
850
N20
Id00
Id50 IlbO Temperature (K)
Ilk0
1200
1
Figure 5. Estimation of k, by fitting the instantaneous char nitrogen conversion to NO and NzO at 50% carbon conversion for a Biot number of 100 (8% 0 2 in He). activation temperatures for the NO (16 900(K)/T) and NzO (13 900(K)/T) reduction reported by de Soete12was used in the calculations. The BET surface area of the char was assumed to be 500 m2/g. The values Of k N o , kNzo, and k, were fitted to f N 0 and fNzO. The mass-transfer coefficient was estimated from the Sherwood number which was calculated using a correlation given by La
A major uncertaintyexists in the estimation of the value of the effective diffusivityD,ff. The value has traditionally been calculated from the Knudsen diffusivity using a mean pore radius calculated from the BET surface area and total porosity.31 Although the values calculated by this method may differ from those measured for coal chars using a chromatographic method,32 a greater source of uncertainty is provided by the hypothesis that the reaction in char occurs in the mesopores.33 As the effective diffusivity is proportional to the pore radius, the uncertainty is large, since the micropores have radii typically of the order of 1-5 nm and the mesopores of the order 50-250 nm. The Biot numbers for these two conditions are in the range of 5000 and 100. The data will be fitted first for the two extreme values of Biot number and then it will be shown how the data can be further used to narrow down the probable value of Biot number. The values of kNO, k ~ ~and 0 , k, were estimated by fitting the instantaneous char nitrogen conversion to NO and N2O at 50% carbon conversion. The result is shown for a Biot number of 100in Figure 5. The corresponding plots ~ shown 0 for fN0 and fNzO as a function of NO and 4 ~ are in Figures 6 and 7. The points from Figure 5 used to generate kNO, kNzO, and kr, and the corresponding values , shown in the figures. The same procedure was of 4 ~ 0are used for a Biot number of 5000, and the corresponding plots for f N 0 and fNzO are given in Figures 8 and 9. Again, the data points used to obtain the values of k N o , kNzo, k,, and @NO are shown in the figures. (30) La Nauze, R. D. Fundamentale of coal combustion in fluidiaed beds. Chem. Eng. Res. Des. 1986,63 (Jan.), 3-33. (31) Sattefield, C. N.Moss Transfer in Heterogeneous Catalysis;MIT Press: Cambridge, MA,1970; p 41-42. (32) Valix,M. G.; Harris, D. J.; Smith, I. W.;Trimm, D. L. Theintrinsic combustion reactivity of pulverized coal chars: The use of experimental pore diffusion coefficients. Twenty-Fourth Symposium (Znternational) on Combustion;The Combustion Institute: Pittsburgh, 1992; pp 12171223. (33) DAmore, M.; Tognotti, L.; Sarofim, A. F. Oxidation rates of a single particle in an electrodynamic balance. Combust. Flame, in press.
10
1
100
1000
002 ( - )
Figure 6. Model calculations of the instantaneous factional conversion to NO as a function of $NO and 4% for a Biot number of 100. 1000
100
I
Biot Number
loo
I
h
v
oz
8 10
1 1
10
100
30
(Do2 ( - ) Figure 7. Model calculations of the instantaneous factional conversion to NzO as a function of $NO and 4%for a Biot number of 100.
The results in Figures 6 and 7 or Figures 8 and 9, with the corresponding correlations for the kinetic parameters, give internally consistent seta of results which fit the data on NO and N2O formation at 50 7% carbon conversion for Biot numbers of 100 and 5000, respectively. In order to decide which value of Biot number is more reasonable, one can examine the dependence of fN0 and fNzO on carbon conversions. Figure 10 show the effect of Biot number on the fN0 and fNzO for a temperature of 1095 K. At high Biot numbers (e.g., 5000) the fNo and fN# curves are fairly constant up to values of high carbon conversion at which point fN0 approaches a value of one and fNzO a value of zero. At lower Biot numbers (e.g., 100) the changes in fN0 and fNIO occur much more gradually. This difference in curvatures therefore provides a basis for selecting Biot number. This procedure has been used to fit the experimental results as shown in Figure 11. It should be cautioned, however, that in this interpretation of the data it has been assumed that the values of the effective diffusivity and reactivity do not change with carbon conversion. These assumptions can be relaxed as more information is developed on how the pore structure
NO and NzO Formation for Coal Combustion
Energy & Fuels, Vol. 7, No. 6,1993 801
1
0.6 A
I
v
5
4
n r
e
'0
0.2
1
100
10
1000
Q02 ( - )
0.8
0.6
0.4
Carbon Conversion
1.0
(mol/mol)
Figure 11. Comparison between model predictions and experimental data for the instantaneous conversions to NO and N2O for a Biot number of 100 (8% 0 2 in He).
Figure 8. Model calculations of the instantaneous factional conversion to NO as a function of NO and 4% for a Biot number
of 5000.
I
Biot Number 5000
1 Y
c 0.15
.-c L c 8
0.1
0" 1
10
100
1000 "0
@02 ( - )
Figure 9. Model calculations of the instantaneous factional conversion to N2O as a function of NO and $02for a Biot number of 5000. c
5- 0.8
'
Temperature=]095K Biot Number
E
0.6
PB .$ 0.4
z $-
0.2
ID
12 0 '
0.2
0.4
0.6
0.8
1.0
Carbon Conversion (mol/mol)
Figure 10. Model calculationsof the effect of Biot number on the instantaneous fractional conversion to NO and N2O as a function of carbon conversion at 1095 K (8% 0 2 in He).
and reactivity evolve during reaction.
0.5
1.o
1.5
2.0
Radius (mm)
Temp: 1023K
Figure 12. Dimensionless radial concentration profiles of NO and N2O during combustion; Initial particle radius = 2 mm, 1073 K, Bi = 100.
The fit Of f N 0 and fN20 versus carbon conversion shown in Figure 11 correctly shows that the conversion to NO increases with increasing carbon conversion, whereas that to NzO decreases. The temperature dependence of the data were matched using Smith's kineticsm for the OzIC reaction, whereas the activation energy for the reduction of NzO and NO by char was taken from de Soete.12 The activation energy for the NO char reaction is 33.8 kcall mol which is within the range reported by Chan et al.,17 Furusawa et al.,18and Suuberg et a1.% These investigators showed that the activation energy changes with temperature, but no attempt was made here to vary the activation energy to better fit the temperature dependence. The profiles of NO and N20 in the particles are shown in Figure 12 for a Biot number of 100, assuming that the carbon (34) Suuberg, E. M.; Teng, H.; Calo, J. M. Studies on the kinetics and mechanism of the reaction of NO with carbon. Twenty-Third Symposium (International) on Combustion;The Combustion Institute: Pittsburgh, 1990;pp 1199-1205.
Tullin et al.
802 Energy &Fuels, Val. 7, No. 6,1993
oxidation is restricted by diffusion and can be modeled as a shrinking unreacted core. At a conversion of 87.5% conversion, the particle radius has decreased to half the original value. The increase in NO is a consequence of the decreased reduction of NO as the particle shrinks. The conversion to N20 is determined by the local NO concentration at the point where the carbon is oxidized, and remains fairly constant until the last stages of burnout, when it falls to zero. The model provides a useful framework for interpreting and extrapolating the experimental data. I t can also be used to guide the design of experimenh to test critical hypothesis. For example, the value of the effective diffusivity D,ff to use might be tested by changing from helium to nitrogen, thereby changing the Biot number, in the regime where the conversion is sensitive to Biot numbers (high 40~).The role of CO on the NO reduction can be altered by adding CO to the gases supplied to the reactor to enhance the CO levels or adding H2O to decrease the levels of CO by accelerating the CO burnout.
Conclusions For the combustion of single particles, this investigation shows that the emission of NzO decreases with increasing temperature, whereas the NO emission passes through a maximum. The instantaneous emission of NO increases with carbon conversion, which is attributed to the decreasing importance of the NO destruction in the pores as the particle shrinks. The fractional instantaneous conversion to N20 decreases a t high carbon conversions. This is consistent with model predictions based on the assumption that N2O is formed by the oxidation of char in the presence of NO. The fractional conversions of char nitrogen to NO and NzO have been fitted by a model based on the above assumptions.
Acknowledgment. The authors are grateful for the support provided for this research by the Hitachi Research Laboratory, Hitachi Limited. C.J.T.acknowledges additional support from the Swedish National Board for Industrial and Technical Development.
