Factors Influencing the Time-Resolved Evolution of NO, HCN, and

Factors Influencing the Time-Resolved Evolution of NO,. HCN, and N2O during Char Oxidation at Fluidized Bed. Conditions. Shakti Goel. Department of ...
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Energy & Fuels 2002, 16, 823-830

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Factors Influencing the Time-Resolved Evolution of NO, HCN, and N2O during Char Oxidation at Fluidized Bed Conditions Shakti Goel Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

Alejandro Molina and Adel F. Sarofim* Department of Chemical Engineering, University of Utah, 1495E 100S, Room 109, Salt Lake City, Utah 84112-1114 Received June 4, 2001. Revised Manuscript Received February 4, 2002

There is increasing evidence that HCN is an important intermediate to the production of N2O during the oxidation of char nitrogen at fluidized bed combustion conditions. The experimental evidence shows that N2O is formed primarily during oxidation so that the fundamental question is on which fuel N intermediate is formed during char consumption. This paper models first the conversion of char nitrogen assumed to be released as HCN, simulating the homogeneous nitrogen chemistry in the boundary layer of a single char particle. This pathway, which is based on 180 reversible gas-phase reactions and 33 chemical species, assumes that HCN is released at a rate proportional to carbon consumption. The oxidation of HCN occurs in the boundary layer of the particle. An alternative pathway assumes that during oxidation, NO is formed in addition to a cyano species by heterogeneous processes and then both NO and the cyano species react in the boundary layer. The temporal evolution of NO, N2O, and cyano compounds from both models have been compared against the observations obtained at fluidized bed conditions. The results suggest that solely HCN oxidation on the boundary layer does not predict the most important trends observed for the production of NO and N2O for char oxidation at fluidized bed combustion conditions. On the contrary, the simultaneous heterogeneous production of NO and a cyano-like species can predict to a certain extent the experimental observations but will require a more detailed understanding of the surface chemistry which controls the temporal evolution of NO and cyano species.

Introduction Formation of nitrogen oxides during combustion of coal has been a topic of widespread interest. Nitrogen oxides can react with water vapor present in the atmosphere to form acids, or can react with oxygen in the presence of sunlight to form photochemical smog.1 Stationary combustion units burning solid fossil fuels such as coal are important sources for the emissions of nitrogen oxides. The solid fuels, on combustion, split into a gaseous phase called the volatile matter, and a residual solid-phase called the char. Nitrogen is present in both of these phases.2 The principal nitrogenous compounds formed in the gas phase are ammonia, hydrogen cyanide, acetonitrile, and nitrogen.3,4 While the nitrogen combustion chemistry for the volatile phase has been well studied,5,6 the fate of char nitrogen is still * Corresponding author. Fax: 801 585 5607. E-mail: sarofim@ reaction-eng.com. (1) Goel, S. Environmental Problems: Fundamental Studies and Global Ramifications. Ph.D. Thesis, Department of Chemical Engineering, MIT, Cambridge, MA, 1996. (2) Solomon, P.; Colket, M. Fuel 1978, 57, 749. (3) Nelson, P.; Buckley, A.; Kelly, M. Twenty-Fourth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1992; p 1259.

uncertain. Several hypotheses7,8 have been presented for the formation of nitric and nitrous oxides during combustion of char. Sarofim and co-workers have extended the mechanism proposed by de Soete9 to include steps in which an oxidizing agent like oxygen splits open the aromatic rings containing nitrogen,10 and exposes the nitrogen atom by forming a surface intermediate (-I). This intermediate can be considered as an active (-I) site. The intermediate splits to form nitric oxide or reacts with nitric oxide to form nitrous oxide. The nitrogen oxides so formed can further react with char to form molecular nitrogen. This mechanism explained the major trends observed in single particle experiments performed in a fluidized bed.10-16 Refinement of the (4) Ikeda, E.; Mackie, J. Twenty-Sixth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1990; p 597. (5) Kilpinen, P.; Hupa, M. Combust. Flame 1991, 85, 94. (6) Kramlich, J.; Cole, J.; McCarthy, J.; Lanier, W.; McSorley, J. Combust. Flame 1989, 77, 375. (7) Glarborg, P.; Dam-Johansen, K.; Kristensen, P. Gas Research Institute Contract No. 5091-260-2126, Denmark, Dec. 1993. (8) Molina, A.; Eddings, E. G.; Pershing, D. W.; Sarofim, A. F. Prog. Energy Combust. Sci. 2000, 26, 507. (9) de Soete, G. Twenty-Third Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1990; p 1257.

10.1021/ef010117o CCC: $22.00 © 2002 American Chemical Society Published on Web 04/17/2002

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parameters they used have been proposed by Kilpinen et al.17 to better fit the effects of temperature on the NO evolution during char oxidation. Despite good agreement between experimental and modeling data obtained by the heterogeneous mechanism proposed by these investigators, recent experimental evidence casts doubts on the exclusive existence of a heterogeneous intermediate in the production of N2O and provides strong evidence for the existence of an alternative homogeneous pathway. Kramlich et al.6 proposed that the devolatilization or gasification of char nitrogen followed by gas-phase reaction could lead to N2O formation. They suggested that HCN might be the species that once released from the char is oxidized in the homogeneous phase to produce N2O. A similar hypothesis was proposed by Amand and Leckner18 when explaining why the conversion of char nitrogen to N2O increased during CH3CN addition to a fluidized bed reactor. Miettinen and co-workers19-21 also presented experimental data that support the homogeneous route, although they did not identify HCN as the intermediate. Winter et al.22,23 using an iodine addition technique for suppressing radical reactions (basically HCN oxidation to N2O) were able to detect HCN at fluidized bed conditions. Furthermore, they observed that the appearance of HCN was followed by the disappearance of N2O. This observation was presented as evidence supporting the homogeneous route to N2O. Further support for the homogeneous pathway is provided by Haynes and co-workers24,25 who have measured HCN and HNCO during the low-temperature oxidation of char and by the calculations of Kilpinen et al.17 that show that the kinetic parameters obtained by Goel et al.10 predict a reduction of the conversion of char nitrogen to NO when the temperature is increased, in contrast to the experimental results26 for which this conversion increases with temperature. It must be recognized that the heterogeneous pathway is complex and is subject to modifications. De Soete et al.27 have refined their seminal publication9 to include (10) Goel, S.; Morihara, A.; Tullin, C.; Sarofim, A. Twenty-Fifth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1994; p 1051. (11) Goel, S.; Zhang, B.; Sarofim, A. Combust. Flame 1996, 104, 213. (12) Tullin, C.; Goel, S.; Morihara, A.; Bee´r, J.; Sarofim, A. Energy Fuels 1993, 7, 796. (13) Tullin, C.; Bee´r, J.; Sarofim, A. J. Inst. Energy 1993, 66, 207. (14) Krammer, G.; Sarofim, A. Combust. Flame 1994, 97, 118. (15) Goel, S.; Sarofim, A.; Kilpinen, P.; Hupa, M. Twenty-Sixth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1996; p 3317. (16) Goel, S.; Bee´r, J.; Sarofim, A. J. Inst. Energy 1996, 69, 201. (17) Kilpinen, P.; Kallio, S.; Konttinen, J.; Forssen, M. Prepr. Pap.s Am. Chem. Soc., Div. Fuel Chem. 2001, 46, 167. (18) Amand, L.; Leckner, B. Energy Fuels 1993, 7, 1097. (19) Miettinen, H.; Paulsson, M.; Stro¨mberg, D. Energy Fuels 1995, 9, 10. (20) Miettinen, H.; Abul-Milh, M. Energy Fuels 1996, 10, 421. (21) Miettinen, H. Energy Fuels 1996, 10, 197. (22) Winter, F.; Christian, W.; Gerhard, L.; Herman, H. TwentySixth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1996; p 3325. (23) Winter, F.; Lo¨ffler, G.; Wartha, C.; Horbauer, H.; Preto, F.; Anthony, E. Can. J. Chem. Eng. 1999, 77, 275. (24) Ashman, P. J.; Haynes, B. S.; Buckley, A. N.; Nelson, P. F. Twenty-seventh Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1998; p 3069. (25) Ashman, P. J.; Haynes, B. S.; Nicholls, P. M.; Nelson, P. F. Twenty-eighth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 2000; p 2171. (26) Tullin, C. J. Academic Dissertation. Chalmers University of Technology, Sweden, 1995.

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the reactions involving heterogeneous complexes as intermediates for the formation of N2O. Experimental data of Ashman et al.24 and quantum chemical calculations by Montoya et al.28 show that the heterogeneous pathways involving oxygen intermediates are more complicated than proposed by the early investigators and may lead to N2 as well as N2O. And a recent experimental study by Liu et al.29 explains the conversion of char nitrogen to N2O exclusively through heterogeneous routes. From the review of the literature it is clear that char oxidation is needed for significant production of N2O since interruption of the flow of oxygen or another oxidizer to reactors results in an interruption in the N2O formation. What is in question is the intermediate (-I) that the char nitrogen forms during char oxidation. The early hypothesis that the intermediate is a heterogeneous complex such as -CNO does not support later data. This paper first examines the hypothesis proposed by Winter22 and others that the intermediate is exclusively HCN. Models of the detailed gas-phase chemistry have not accompanied experimental observations on the importance of gas-phase reactions of HCN both at single particle and system scales. Most models consider the heterogeneous chemistry associated with char combustion and simplify the homogeneous chemistry to a given boundary condition for the bulk concentration of all homogeneous species. Contrary to these exclusively heterogeneous models, Mitchell et al.30 and Goel et al.31 using a refined numerical solver were able to couple the heterogeneous chemistry of char to the homogeneous chemistry present in the boundary layer. The primary aim of this paper, therefore, is to apply this single particle boundary layer model to the complex process of N2O production where the heterogeneous and homogeneous chemistries are well coupled. This approach allows the treatment of both (heterogeneous or homogeneous) reactions in any combination. Particularly we have aimed the simulations on trying to understand under what conditions the homogeneous oxidation of nitrogen-containing species evolved from the char will produce N2O. In this way we can examine the possibilities that the intermediate species (-I) defined by Sarofim and co-workers can be solely gas-phase HCN, or complexes that can lead to a distribution of gas-phase compounds. Two different scenarios were analyzed for the conversion of char nitrogen to N2O. In the first, the interaction of oxygen with char nitrogen forms hydrogen cyanide, which diffuses out of the pores of the char particle. HCN is then oxidized in the boundary layer to form NO and N2O. In the second, char nitrogen is converted to nitric oxide and a cyano species, and the latter is oxidized in the boundary layer to form NO and N2O. It is arbitrarily (27) de Soete, G. G.; Croiset, E.; Richard, J. R. Combust. Flame 1999, 117, 140. (28) Montoya, A.; Truong, T. N.; Sarofim, A. F. J. Phys. Chem. A 2000, 104, 6899. (29) Liu, H.; Kojima, T.; Feng, B.; Liu, D.; Lu, J. Energy Fuels 2001, 15, 696-701. (30) Mitchell, R. M.; Kee, R. J.; Glarborg, P.; Coltrin, M. E. TwentyThird Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1990; p 1169. (31) Goel, S.; Lee, C.; Longwell, J.; Sarofim, A. Energy Fuels 1996, 10, 1091.

Evolution of NO, HCN, and N2O in Char Oxidation

Figure 1. Pictorial representation of the model developed to understand the fate of char nitrogen.

Energy & Fuels, Vol. 16, No. 4, 2002 825

the stiffness of the differential equations. A good description of various numerical techniques used to solve stiff differential equations involving reactive flow chemistry can be found in ref 32. Earlier work in solving boundary layer equations using detailed kinetic schemes consists of identifying the location of CO oxidation. Mitchell and co-workers30 found that CO oxidation in the boundary layer was negligible in the temperature range 1500-1700 K and 12% oxygen concentration. A simple one-film model was also found to be applicable for char combustion. Using a transient approach and a five-step surface reaction mechanism, Dryer and coworkers33 found that moisture was required for CO oxidation. Goel and co-workers31 simulated CO oxidation in the boundary layer of a char particle and compared the CO/CO2 ratio with the experimental data obtained in an electrodynamic balance.34 In the present study the CO oxidation model31 has been extended to include the nitrogen chemistry. The modified model now consists of a greater number of chemical species and homogeneous reactions. The essence of the problem, however, remains unchanged. The conservation for total mass, individual species, and energy for a spherically symmetric particle is simply:

assumed for the second scenario that the surface complexes decompose to yield a mixture of 0.4 HCN, 0.5 NO, and 0.05 N2 per char nitrogen. A single particle model is used to simulate the reaction pathways outlined above. The model couples a nitrogen reaction set, homogeneous and heterogeneous, with the diffusive processes associated with various species involved. The model assumes that char nitrogen is released as a cyano species and nitric oxide at a rate proportional to carbon consumption. As opposed to its oxidation in the pores, the cyano species is oxidized in the boundary layer of the char particle leading to the formation of NO and N2O. The kinetic scheme consists of five heterogeneous reactions for the consumption of oxygen and the formation of CO, CO2, a cyano species, and NO, and 180 gas-phase reversible reactions for 33 chemical species. Due to the complex homogeneous chemistry and the multicomponent diffusion processes in the boundary layer, the task of developing a boundary layer model is challenging. A series of numerical simulations have been carried out to predict the trends for the cumulative and differential conversion of char nitrogen to nitric and nitrous oxides at various conditions of surrounding oxygen and nitric oxide concentrations. The model trends have been compared against the experimental observations available in the literature. In the following sections, model development and results from the numerical simulation studies and their comparison to experimental data are discussed.

net accumulation + net convection in ) net diffusion out + net generation

Development of the Model

The values of Tg,∞, Yi,∞ were set equal to the gas temperature and composition of the simulated experiments.

Figure 1 presents a pictorial representation of the model developed in this work. A detailed numerical description is given below. Reactive flow chemistry problems as applicable for a boundary layer around a char particle have posed a numerical challenge to the research community due to

The terms are given in Table 1. These equations describe the diffusive and reactive phenomena occurring in the boundary layer around the char particle as opposed to those occurring within the pores of the particle. The two boundary conditions required to solve the above set of PDEs are given below. At the surface of the particle:

For the species balance:

(

)

DTi ∂T ∂Xi 1 Rs,i ) FgYi Ur - Di,m Xi ∂r FgYiT ∂r

For the energy balance: FcCp,cRp ∂Ts ∂Tg 4 λr ) -ωcHc + σ(T4s - Tg,∞ )+ ∂r 3 ∂t At infinity:

Species: Yi(t,r)|rf∞ ) Yi,∞ Energy: Tg(t,r)|rf∞ ) Tg,∞ Initial conditions at all r:

Species: Yi(0,r) ) Yi,∞ Energy: Tg(0,r) ) Tg,∞

(32) Oran, E.; Boris, J. Numerical Simulation of Reactive Flow; Elsevier: New York, 1987. (33) Lee, C.; Yetter, R.; Dryer, F. Combust. Flame 1995, 101, 387. (34) Tognotti, L.; Longwell, J.; Sarofim, A. Twenty-Third Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1990; p 1207.

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Table 1. Summary of Conservation Equations net accumulation mass

net convection in

1 ∂ 2 (r FgUr) r2 ∂r

∂Fg ∂t

species

energy

Fg

∂Yi ∂t

Fg Cp,g

Fg Cp,gUr

(

∂Tg ∂r

1 ∂ r2 ∂r

The spatial discretization was carried out using orthogonal collocation on finite elements.35 A simple finite difference approach was used for the temporal term. The resultant set of discretized equations was solved implicitly using the Newton-Raphson’s method. Further details on the solution procedure for the conservation equations using finite elements are given in ref 31. The model uses a detailed homogeneous kinetic scheme consisting of 180 reversible reactions and 33 chemical species, viz., O2, CO, CO2, O, H2, H, OH, HO2, H2O2, H2O, HCO, NO, N2O, NCN, NH, HCN, N2H2, HONO, H2NO, CN, HCNO, HOCN, HNCO, NCO, HNO, NNH, N, HNNO, NO2, NO3, NH2, N2, and an inert species such as Ar or He. The complete set of reactions used for the model can be found as supporting material and is an extension of pioneer work by Miller and Bownan.36 The reactions for the oxidation of HCN, NH3, and CO have been taken from ref 7. The kinetic schemes for the hydrocarbon oxidation and hydrocarbon-nitrogen interactions have been taken from ref 37. The thermodynamic data were taken from ref 38. The diffusive transport coefficients were calculated using the transport software package developed at Sandia National Laboratories.39 The char carbon consumption in the model is described by34

C + (1 - R/2)O2 ) RCO + (1 - R)CO2 where R is the fraction of char carbon converted to CO. The rates of consumption of char and O2, and formation of CO and CO2 are given by

Rchar )

(

CO2,∞ 1 3 + kg ηksSR

RO2 ) 1 -

)

Yi ∂Xi DTi ∂T 1 ∂ 2 r Fg Di,m + r2 2 ∂r Xi ∂r T ∂r r

∂Yi FgUr ∂r ∂Tg ∂t

net chemical generation

net diffusion out

R R 2 char

)

( ) r2λg

∂Tg ∂r

(

)

Yi ∂Xi DTi ∂T ∂Tg Cp,g Fg Di,m + Xi ∂r T ∂r ∂r i)1 Nsp

+

ωi

∑

Nsp

∑ωH i

g,i

i)1

radical are given by

RHCN ) RH )

N R C char

H R C char

We recognize that several studies19-21,24,25,40 have shown interactions between the nitrogen in NO and N2O in the gaseous phase with surface complexes. This suggests the existence of reactions different from the one described above. However, since the objective of this study is to understand under what conditions nitrogencontaining species can produce N2O, the simple heterogeneous mechanism explained above was considered adequate. The expressions used for the production of HCN neglect the fact that the formation of HCN can depend on the hydrogen availability in the char. In fact, Ren et al.41 found that chars with lower hydrogen content (as the result of higher devolatilization temperatures) produced less N2O. Nevertheless, this study does not evaluate the probability of the formation of HCN or the other nitrogen-containing species but the feasibility of the formation of N2O by homogeneous reactions of these species in the boundary layer. The mechanism for the release of HCN and similar nitrogen-containing species should be, of course, the focus of future studies. The surface area variation during the course of the reaction will depend on the char. The function used here is for spherocarb particles42 and it is represented by a seventh order polynomial.

S ) (920.9089 + 2661.0706Xc - 27273.1303Xc2 + 112965.3388Xc3 - 53322.8987Xc4 + 312434.902Xc5-199151.1856Xc6 + 51245.1435Xc7) × 104 (cm2/g of C)

RCO ) RRchar

The rate constant for spherocarb oxidation is also taken from the literature:34

RCO2 ) (1 - R)Rchar

ks ) 0.184 e-16980/Ts

In addition, the rate of emanation of HCN and hydrogen

(38) Kee, R.; Rupley, F.; Miller, J. Sandia National Laboratories Report, SAND87-8215B, 1990. (39) Kee, R.; Dixon-Lewis, G.; Warnatz, J.; Coltrin, M.; Miller, J. Sandia National Laboratories Report, SAND86-8246, 1986. (40) Aihara, T.; Matsuoka, K.; Kyotani, T.; Tomita, A. Proc. Combust. Inst. 2000, 28, 2189-2195. (41) Ren, W.; Lu, J.; Yue, G.; Bee´r, J. M.; Molina, A.; Sarofim, A. F. CFB7: 7th International Conference on Circulating Fluidized Beds, Niagara Falls, Canada 2002, in press.

(35) Carey, G.; Bruce, A. Chem. Eng. Sci. 1974, 30, 587. (36) Miller, J.; Bownan, C. Prog. Energy Combust. Sci. 1989, 15, 287. (37) Glarborg, P.; Hadvig, S. Development and test of a kinetic model for Natural Gas Combustion; Nordic Gas Technology Center Report, Denmark, March 1991.

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The rate expressions remain the same for the two pathways except that for the second pathway direct NO formation from char combustion is also possible. The nitrogenous surface intermediate can form any of the following cyano species: HCN, HNCO, HCNO, or CN. In this way char nitrogen is considered to split to form NO and a cyano species in a ratio such that their formation rates are

Rcyano ) 0.4 RNO ) 0.5

N R C char

N R C char

The thickness of the boundary layer was assumed to be 100 times that of the particle size. The model parameters such as transport parameters and concentrations were updated at each iteration. Homogeneous oxidation of HCN within the pore of the char particle was neglected as pore diffusion is controlled by mesopores,43,44 suggesting that the probability of two gas molecules colliding with each other is less than their probability of reacting with the char surface. But allowance for homogeneous reactions in the pores should be included since the gas-phase reactions still apply, even though multiple collisions with the wall occur between collisions.45 Results The model was applied at low temperatures (750900 °C) at which the char nitrogen chemistry has been extensively studied. The trends for the conversion of char nitrogen to NO and N2O were predicted at various operating conditions. The simulations were carried out on a 500 MHz Pentium III machine with 128 MB of memory. The simulations took three to six hours to complete, depending on the initial model input. The differential conversion of char nitrogen to NO and N2O obtained with HCN as the gas-phase intermediate is shown in Figure 2. The HCN is oxidized in the boundary layer to form NO and N2O. The following equations were used to calculate the differential conversion of char nitrogen to NO, N2O, and HCN.

∫r∞ CNO r2 dr cf

fNO )

∫r (∑ CN-containing species)r2 dr ∞ cf

f N 2O )

∫r∞ 2CN O r2 dr cf

2

∫r (∑ CN-containing species)r2C dr ∞ cf

fHCN )

∫r∞ CHCN r2 dr cf

∫r (∑ CN-containing species)r2 dr ∞ cf

where rcf stands for the distance from the particle center where there is no variation in the total molar flux. It is (42) Dudek, D. Single particle, high temperature, gas-solid reactions in an electrodynamic balance. Ph.D. Thesis, Department of Chemical Engineering, MIT, Cambridge MA, 1989.

Figure 2. Differential conversion of char nitrogen to NO, N2O, and HCN. Results for NO, N2O are presented for two different oxygen concentrations (Results for HCN at 8% O2 are similar to those at 4%). Solely HCN is considered as the nitrogenous species evolving from char. Gas temperature ) 1023 K.

assumed that the reaction chemistry is frozen from this point until the end of the boundary layer. Figure 2 shows that the fN2O profile follows the experimental trends. The differential conversion to nitrous oxide drops down to zero at complete burnout. However, the profile depicting differential conversion to nitric oxide shows a curvature in a direction opposite to observations. Experimentally it was observed10 that char nitrogen is entirely converted to nitric oxide at high carbon conversions whereas the model predicts that the conversion drops to zero. In the case of the HCN pathway this is because the HCN conversion to NO and N2O decreases as the particle diameter shrinks. Also, with increases in the bulk oxygen levels, more of char nitrogen is converted to NO, while less of it is converted to N2O. Experimental results of the dependence of char nitrogen conversion to N2O on increasing O2 concentrations are not consistent, showing both increases46 and decreases.47 The fHCN profile does not start from zero, but instead approaches zero when the char carbon conversion is around 0.05. There is an initial accumulation of hydrogen cyanide in the boundary layer during this induction period as the O and OH radicals build up when the char particle starts burning; during this initial period hydrogen cyanide is not completely oxidized. Once the concentration of radicals has reached a certain level, HCN ignites and the conversion of char nitrogen to HCN drops to zero. Figure 3 shows the concentration of O2, CO, CO2, O, NO, N2O, and HCN in the boundary layer at different carbon conversion levels. The concentration of O radicals is zero when the char carbon conversion is zero. At 30% conversion, the O radical concentration is higher than that at 0.15% conversion suggesting that there is a transient phase during which the radical concentration (43) Goel, S.; Sarofim, A.; Lu, J. Twenty-Sixth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1996; p 3127. (44) D’Amore, M.; Tognotti, L.; Sarofim, A. Combust. Flame 1993, 95, 374. (45) Haynes, B. S. Personal Communication, 2000. (46) Feng, B.; Liu, H.; Yuan, J.; Liu, D.; Leckner, B. Energy Fuels 1996, 10, 203. (47) Mockizuki, M.; Koike, M.; Hori, M. 5th International Workshop of Nitrous Oxide Emisions; NIRE: Tsukuba, Japan, 1992.

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Figure 3. Concentration of O2, CO, CO2, HCN, NO, N2O, and O species in the boundary layer (mol fraction). Solely HCN is considered as the nitrogenous species evolving from char. Xc represents carbon burnout. The abscissa is defined as the normalized distance from particle surface x/R. Gas temperature ) 1023 K; O2 concentration ) 4%.

in the boundary layer builds up. As char conversion proceeds, there is a net accumulation of HCN though it is partially consumed. A fraction of the char nitrogen is also converted to molecular nitrogen. As the particle size becomes small, the boundary layer O radical concentration decreases and HCN escapes through the boundary layer into the bulk and is not oxidized. The

diffusive processes are faster than the reaction kinetics when the particle nears complete burnout. As a result, NO and N2O are not formed from char nitrogen at high char carbon conversions, and all the char nitrogen appears as HCN (see Figure 3). As expected, HCN did not oxidize when water vapor or the hydrogen-containing radicals were absent. The

Evolution of NO, HCN, and N2O in Char Oxidation

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Figure 4. Differential conversion of char nitrogen to N2O as a function of carbon conversion for two different ambient NO concentrations. Solely HCN is considered as the nitrogenous species evolving from char. Gas temperature ) 1023 K; O2 concentration ) 4%.

results presented above were, therefore, obtained when emanation of hydrogen radicals from the char surface was assumed. An earlier study showed that in order to ignite CO in the boundary layer, presence of hydrogencontaining radicals was important.33 Figure 4 shows that in the presence of background NO, the fraction of char nitrogen converted to nitrous oxide increases at lower levels of char carbon conversion. The fN2O profile, however, once again drops to zero at high carbon conversion. This is in complete contrast to the experimental observations where an upward curvature is obtained when high levels of background NO are present.43 If HCN is released during char combustion then even high levels of NO are not able to contribute to its oxidation in the boundary layer of a particle nearing complete burnout. The fact that the model predictions do not match the experimental trends suggests that the complete conversion of char nitrogen to HCN is not the dominant pathway for N2O production at low-temperature conditions. It is, however, possible that nitric oxide may also be released in addition to a cyano species as the primary product during combustion of char. The cyano species can be formed for example by the release of nitrogen complexes formed during char oxidation. Such reaction, in turn, can prevent the heterogeneous formation of nitrous oxide. On the second set of calculations it was assumed that nitric oxide can be formed both heterogeneously and homogeneously but nitrous oxide is formed from the homogeneous oxidation of a cyano species. The split of char nitrogen to NO and a cyano species is arbitrarily assumed to be in the ratio 5:4 and to remain constant during carbon conversion. The remainder of char nitrogen is assumed to form molecular nitrogen. Figure 5 presents the results of numerical simulation when the bulk temperature was 1023 K and the bulk oxygen concentration was 4%. In the absence of any reactions in the boundary layer, the conversion to NO will be 0.5 and to N2O will be 0. The conversion of the char nitrogen to NO increases to 0.8 as a result of the gas-phase oxidation of the fuel nitrogen species to NO for most carbon conversions as shown in the bottom panel of Figure 5. As complete carbon conversion is approached, the boundary layer reactions decrease and the NO conversion approaches the 0.5 asymptote. A

Figure 5. Differential conversion of char nitrogen to NO and N2O. The split of char nitrogen to NO and the cyano species was 5:4. Gas temperature ) 1023 K; O2 concentration ) 4%.

small and varying fraction of fuel nitrogen species is converted to N2O as shown in the top panel. At complete char conversion the amount of N2O goes to zero for all species other than CN, which being unstable will continue to react with NO even at low concentrations. The results presented in Figure 5 do not match exactly the experimental observations from previous studies.43 Particularly, the observed increase in the char nitrogen conversion to NO as char burnout increases. However, they suggest that N2O can be formed from the homogeneous oxidation of a cyano species, if NO is formed from the direct oxidation of char nitrogen. The combined homogeneous/heterogeneous scheme in this study shows how the temporal evolution of NO and N2O during char combustion depends on both homogeneous and heterogeneous reactions. There are numerous approximations that can affect the final predictions. One of them is the rate of N and H release from the char. The present model assumes both to be proportional to the rate of char oxidation. However, a study by Ashman et al.24 shows that at these temperatures, NO accumulates in the particle at low conversions, and then it is released at high conversions. If this effect is added to the model, it may explain the differences in the prediction of NO evolution during char oxidation. Furthermore, there is clearly a need to understand the heterogeneous reactions in sufficient detail to predict the distribution of fuel nitrogen species evolved from the surface as a function of char burnout.

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Summary

Symbols

A model simulating the formation of nitrogen oxides at fluidized bed conditions was developed. The model couples heterogeneous and homogeneous kinetic pathways with the diffusional processes associated with the oxidation of a single char particle. This paper aimed to evaluate under what conditions the homogeneous oxidation of a nitrogen-containing species can produce N2O during char oxidation since new evidence collected by different groups17-25 questioned the exclusive existence of a heterogeneous intermediate as previously suggested. Further work, both experimental and numerical, is required to identify the dominant pathway, homogeneous or heterogeneous, leading to the formation of nitrous oxide. Two cases were evaluated in this study. The first, assuming that char nitrogen was exclusively released as HCN during char oxidation, failed to predict many of the experimental trends, due to the low reactivity of HCN in the boundary layer as char burn-out advances. A second, assuming distribution of nitrogenous products at the surface in which char nitrogen is converted to NO and a cyano species in a ratio 5:4, produced predictions that although not in complete agreement with experimental results suggested that homogeneous reactions may lead to the formation of N2O during the oxidation of char at fluidized bed conditions. Although the model based on homogeneous chemistry predicted the qualitative trends for nitrous oxide to some extent, the quantitative predictions were significantly lower than the experimental observations. To match the experimental trends, both quantitative and qualitative, allowance for the variation of the product ratio during char burnout will be needed to combine both homogeneous and heterogeneous reactions. The main contribution of this paper is, therefore, to show the importance of the coupling of homogeneous and heterogeneous mechanisms on the prediction of species evolution during char oxidation.

A ) cross section area Ci ) concentration of species i dz ) incremental change in height De ) effective pore diffusivity Di,m ) diffusivity of species in a mixture of gases DiT ) thermal diffusivity fNO, fN2O, fHCN ) differential conversion of char nitrogen to NO, N2O, and HCN, respectively FNO, FN2O ) cumulative conversion of char nitrogen to NO and N2O, respectively Hg,i ) heat of formation of species i ki ) rate constant for species i kg ) mass transfer coefficient ks ) rate constant for the char consumption reaction N/C ) atomic ratio of nitrogen to carbon in char Nsp ) number of species r ) radial position rcf ) radial position after which the flux is constant R ) particle radius Ri ) net rate of formation (formationEnDash-destruction) for species i Rs,i ) surface reaction rate for species i S ) surface area of the char particle T ) temperature t ) time Ur ) radial velocity X ) mole fraction Xc ) char carbon conversion Y ) mass fraction R ) fraction of char carbon converted to CO  ) emissivity σ ) Stefan-Boltzmann constant φ ) Thiele modulus F ) density λ ) thermal conductivity η ) effectiveness factor ω ) homogeneous reaction rate g ) gas phase c ) char phase p ) particle phase s ) surface ∞ ) infinity

Acknowledgment. The financial support for this research was provided in part by the Department of Energy, USA, under Contract DE-PS22-94PC94200. A.F.S. and A.M. also thank Dr. Baldur Eliasson at ABB Corporate Research LTD, Segelhof, Switzerland, for partial support of the research under a grant entitled “A Study of NO and N2O Formation and Reduction During Coal Combustion in Fluidized Beds”.

Supporting Information Available: Complete set of reactions used for the model. This material is available free of charge via the Internet at http://pubs.acs.org. EF010117O