A COMPREHENSIVE KINETIC MODEL FOR THE ... - Penn Engineering
Twenty-first Symposium (International) on Combustion/The Combustion Institute. 1986/pp. 1207-1216
A C O M P R E H E N S I V E K I N E T I C M O D E L F O R T H E F O R M A T I O N OF C H A R - N O D U R I N G T H E C O M B U S T I O N OF A S I N G L E P A R T I C L E OF C O A L C H A R NORIO ARAI, MASANOBU HASATANI AND YOSHIHIKO NINOMIYA
Department of Chemical Engineering Nagoya University Nagoya 464, JAPAN STUART W. CHURCHILL
Department of Chemical Engineering University of Pennsylvania Philadelphia PA. 19104, U.S.A. NOAM LIOR
Department of Mechanical Engineering and Apphed Mechanics University of Pennsylvania Philadelphia PA. 19104, U.S.A. A comprehensive kinetic mechanism is proposed for the release of char-bound nitrogen in coal (char-N) into the gas phase during the combustion of coal char particles. The salient feature of the proposed mechanism is that char-N is converted competitively to NH~ as well as to NO at the surface of the particles. Based on the proposed kinetic mechanism, a mathematical model is developed for the transient formation of char-NO originated from char-N during the combus tion of a single particle of coal char in an Oz/Ar stream. The theoretical predictions of the model are compared with the experimental data for three kinds of chars, of which the parent coals are Taisei (anthracite, China), Taiheiyo (non-caking bituminous, Japan) and an activated sludge (a pseudo low-rank coal from waste-water treatment): their fuel ratios range from 0.12 to 4.16. Quantitative agreement is found between the predictions of the model and the measured values for the transient formation of char-NO and for the overall fractional conversion of char-N into char-NO. This proposed mathematical model, based on a comprehensive kinetic mechanism, is recommended for modeling the char-NO formation step in practical coal combustors.
Introduction C o n s i d e r a b l e e f f o r t has b e e n d e v o t e d to the kinetic m e c h a n i s m of c h a r - N O f o r m a t i o n f r o m c h a r - b o u n d n i t r o g e n in coal (char-N). T h e p i o n e e r i n g works by Pohl a n d Sarofim 1, B e e r and M a r t i n 2 a n d P e r s h i n g a n d W e n d t 3 resulted in the e x p e r i m e n t a l distinction o f c h a r - N O f r o m volatile-NO f o r m e d f r o m volatile nitrogen c o m p o u n d s such as NH3 a n d H C N , T h e o r e t i c a l a p p r o a c h e s to the p r e d i c t i o n o f the fate o f char-N d u r i n g c o m b u s t i o n have b e e n based on m a n y e x p e r i m e n t a l studies. W e n d t a n d Schulze ~, p r i o r to the abovem e n t i o n e d e x p e r i m e n t a l work, p r o p o s e d a m o d e l in which char-N is c o n v e r t e d to N O at the particle surface, a n d f u r t h e r reaction to N2 occurs h o m o g e n e o u s l y e i t h e r in the pores or in
the gas film s u r r o u n d i n g the particle. T h e predictions o f their m o d e l c o m p a r e qualitatively with trends o b s e r v e d in later experiments. H o w e v e r , little attention has been directed to the ability of the m o d e l to account fnr the m u t u a l effects o f the m a i n combustion reaction, i.e. C + 1/9 02 = CO, and the c o n c u r r e n t h e t e r o g e n e o u s and h o m o g e n e o u s reactions related to c h a r - N O formation. In addition, t h e r e have b e e n no actual concrete c o m p a r i s o n s b e t w e e n the predictions o f the m o d e l a n d the e x p e r i m e n t a l data. In general, the rate o f c o m b u s t i o n o f coal c h a r has b e e n o b s e r v e d to be strongly d e p e n d e n t on the p a r e n t coal. T h e r e f o r e , a m o d e l applicable to the m a t h e m a t i c a l p r e d i c t i o n o f N O f o r m a t i o n in practical coal c o m b u s t o r s m u s t account for the characteristic combustibility o f the chars in
1207
1208
COMBUSTION GENERATED POLLUTANTS
char-NO formation. Such a model has not yet been perfected. Indeed, our recent data ~ for char-NO formation from a particle of coal char cannot be described by any existing model. T h e objective of the present study is to develop a mathematical model for char-NO formation during the combustion of a single particle of coal char. First, a comprehensive kinetic mechanism including char carbon (charC) and char h y d r o g e n (char-H) is proposed for the release of char-N into the gas phase. T h e primary feature which distinguishes the proposed kinetic mechanism from prior ones is that char-N is converted competitively to NHi, as well as to NO at the surface of the particle d u r i n g the entire combustion period. Next, a mathematical model for the p r o p o s e d kinetic mechanism involving 4 heterogeneous gas-solid reactions and 21 homogeneous gas-phase reactions is developed to describe the transient formation of char-NO during the combustion of a single particle of coal char. T h e theoretical predictions of this model are c o m p a r e d with the experimental data for three kinds of coal char whose parent coals are Taisei (anthracite, China), Taiheiyo (non-caking bituminous, Japan), and an activated sludge (a pseudo low-rank coal from waste-water treatment). T h e combustion characteristics of these chars differ significantly from one another, as will be shown. Finally, the validity of the p r o p o s e d model is examined by comparing the e x p e r i m e n t a l and the calculated results for the transient tormation of char-NO and for the fractional conversion o f c h a r - N into char-NO d u r i n g combustion, for chars of different rank parent coals. To avoid the formation of thermal-NO, a mixture of 02 and Ar was employed as the oxidizer. In addition, all of the chars were p r e p a r e d by pyrolysis at a temperature about 100 K higher than that for combustion, in order to minimize the effect of volatile matter on char-NO formation.
1. Theoretical Analysis 1.1 Kinetic mechanism Twenty-five heterogeneous and homogeneous reaction steps have been introduced in the present theoretical analysis of the overall formation of char-NO d u r i n g the combustion of a single particle of coal char in an O2/Ar stream. These mechanisms are summarized in Table I together with the rate constants and references. (1) Heterogeneous gas-solid reactions: T h e proposed reaction mechanisms for the heterogeneous combustion o f char are shown schematically in Fig. 1, Reactions R1 and R4 have
already been used in existing kinetic models. 4'6 T h e key mechanism in the present model is reaction R3, which has not previously been considered. The salient hypothesis is that a mass fraction y* of the total char-N is released into the gas phase in the form of NHI. T h e hydrogen in NHI is compensated for by the fraction (I-[3") of the total char-N. [NHi is represented by N H to simplify the theoretical approach.] T h e residual fraction of char-N, (I-y*), and of char-H, [3", are oxidized to NO and H20, respectively. T h e values of y* and [3* are characteristic parameters related to the combustibility of the chars. At the present stage, both must be determined experimentally, but fortunately the value of [3* can be obtained from the mass balance: [3* = 1 - (y*/14) (W.v/WH), provided y* can be determined independently. (2) Homogeneous gas-phase reactions: Reactions R5 through R25 are postulated to occur outside of the char particle. T h e rate expressions and the rate constants for reactions R5 to R7 in Table 1 have been taken from Howard, et al. r, Baulch, et al. 8 and Chan, et al. g, respectively. Reactions R8 to R25 have been derived from the full kinetic model 1~ for the gas-phase reaction in an NH3-H2-NO-O2-Ar mixture, but the elementary reactions involving either NH~ or NH2 have been excluded.
1.2 A mathematical modelfor the overallformation of char-NO A mathematical model was developed to describe the formation of char-NO during the combustion of a single particle of coal char. T h e following assumptions are made: 1). T h e reacting system (Fig. 2) is isothermal. T h e number of coal-char particles on the porous plate is small so that the effect of the neighboring particles on char-NO formation can be ignored. 2). Since the chars were p r e p a r e d by pyrolysis at a t e m p e r a t u r e about 100 K higher than the combustion t e m p e r a t u r e , no volatile NO is formed and since N2-free oxidizing gas is used, no thermal-NO is formed. 3). Spherical symmetry and a pseudo steadystate are assumed t h r o u g h o u t the combustion period. This postulate is applicable because the maximum t e m p e r a t u r e d r o p ratio across the particle does not exceed 0.03 for the reaction conditions 11. 4). No gas-phase formation and reduction of char-NO occurs within intraparticle pores. 5). T h e gaseous products (NO, NH, H~O and CO) evolved from the particle mix instantaneously and perfectly with the O~/Ar stream at z -= 0 in Fig. 2.
K I N E T I C MODEL OF C H A R NO FORMATION
1209
TABLE I H e t e r o g e n e o u s and h o m o g e n e o u s mechanisms o f reaction employed in this work heterogeneous gas-solid reaction R1 R2 R3 R4
Reference
rate expression
C + 1 / 2 0 z = CO H + 1/402 = 1/2H20 NH(s) = NH(g) N + 1/20~ = NO
Wendt & Schulze(1976) this work this work W e n d t & Schulze(t976)
Eq.(2) Eq.(8) Eq.(10) Eq.(9)
h o m o g e n e o u s gas-phase reaction
rate expression and rate constant
R5 CO + 1/202 = C 0 2 R6CO + OH = CO2+ H R7 CO + NO = 1/2 N2 + CO2
- r c o = 1.3x 1014(H20)l/2(CO)(O2)l!2exp( - 125.5/RT) t t o w a r d et al. (1973) i n k6 = 10.847 + 3.995x10-~T Baulch et ai.(t976) kiiiPgo(kiiPco + ki) -rx~ kiiiP.xo + kliPco + kl Chan et a1.(1983) ki = 1.5• 10 3exp(- 167.2/RT) [mol/cm2/s] kii=7.33x 10 7exp(-79.4/RT) [mol/cm2/s/kPa] kiii=2.08x 10-4exp( - 108.7/RT) [mol/cm2/s/kPa]
k* = A T ~ exp(-E/RT)
R8 N H + O2 = NO + O H R9 N H + 02 = H N O + O R10 N H + NO = N2 + O H R I I N H + O H = N O + H9 R12 H N O + O H = N O + H 2 0 RI3 H N O + M = H + N O R14 H + 0 2 = O H + O RI50 + H2 = OH + H R16 H2 + OH = H20 + H RI7 O H + O H = H20 + O R18 H2 + M = H + H + M R19 O H + H + M - H 2 0 + M R20 N + OH = NO + H R21 N + O2 = N O + O R22 N + NO = N~ + O R23 N + O + M = N O + M R24 N + N + M = N~ + M R25
O + O + M = O2 + M
A [cm s, moles, s]
a
1.0xl01~ 1.0x 1012 1.0xl013 1.6x1012 3.6x 10 I3 3.0• 2.24x 1014 1.8• 10 ~~ 1.17x109 6.3 x 1012 2.23 x 1019 2 . 2 x 1022 4.1 x 10 a3 6.4x109 1.55x1013 6.44x 1016 1.55x1017 1.9x 1013
0 0 0 0.56 0 0 0 1 1.3 0 0.5 -2 0 1 0 -0.5 -0.837 0
14 g a s e o u s c h e m i c a l s p e c i e s : CO2, CO, H2, N O , H g O , H N O , N H , O H , H , O, N, a n d A r , a r e a s s u m e d to b e i d e a l gases. 7). T h e fluid h a s a u n i f o r m v e l o c i t y d i s t r i b u t i o n ( p l u g flow). T h e basic d i f f e r e n t i a l e q u a t i o n s d e s c r i b i n g mass transfer and chemical reaction can be d e r i v e d f r o m t h e a b o v e a s s u m p t i o n s as follows: 6). T h e
char
combustion
For the pseudo steady-state hypothesis, the o v e r a l l r a t e o f o x i d a t i o n o f c h a r - C in a s p h e r i c a l p a r t i c l e o f c h a r c a n b e e x p r e s s e d as
0 13.59 0 6.28 0 203.5 70.3 68.6 15.26 4.56 387.4 0 0 26.14 0 0 0 -7.49
/02Co~
N2, 02,
Heterogeneous
E[kJ/mol]
Arai et al. (1986)
2
OCo~"~
s
"[,-E~S + 7--~7r J = M~
(1)
where
R e = - dmc" d~ = (rlAg+Ae)kl Po~
(2)
H e r e Rc is t h e o v e r a l l r a t e o f r e a c t i o n o f c h a r - C o n a m a s s basis a n d kl is t h e i n t r i n s i c r e a c t i o n r a t e c o n s t a n t o n a n a r e a basis [g-C/cm2/s/kPa]. T h e initial a n d b o u n d a r y c o n d i t i o n s are:
1210
COMBUSTION GENERATED POLLUTANTS
~,C
+ 89 ----CO
(Rz~ /It2)
, N HI -*NH(s) - - - ~ N H ( g )
' l"l*~N + 89
The effective diffusivity, D,, can be estimated using the followin~ relation derived from Bosanquet's e q u a t i o n " :
1 (R3)
---~ NO
FIG. 1. Reaction mechanism of heterogeneous char combustion s~mpli ng
De=e,P(1/DK_o~)+(1/Do~_A~)
(6)
where DK-O~ and Do~-ar are the K n u d s e n and molecular diffusivities, respectively, of 0 2. The values of DK-O2 and Do2-Ar can be estimated from the literature ~3. I n the present work, the value of p is assumed to be 2.0. The effectiveness factor, "q, in Eq. (2) can he also calculated from the existing theory a4. The mass transfer coefficient at the particle surface, hD can be obtained from Sh = 2 + 0.6 Re 1/2 Sc 1/3.
(7)
Based on the kinetic mechanism shown in Fig. 1, the overall rates of release of char-N, char-H and char-NH into the gas phase can be expressed as follows:
:z=Z gas-p~ Reactions RS~R25
R n ( = - dWH/dO) = (f3*Wn/Wc) Rc
(8)
R x ( = - dWx/dO) = ((1-7*)WN/Wc) Rc
(9)
R.vH ( = - dW.~.n/dO) = ("y*WN/Wc) Re.
(I0)
Homogeneous gas-phase reaction
-x--- 0
The target chemical species and other reactants and products are labelled Yk, Ri and P,, respectively. T h e mechanism of the elementary reactions of N are related to the species Yk as follows: R, , ~+ Ri, 2 + Ri, :3k'r k~ Pi,, + P~,2+ Y~
%(o:ol
( i = 1,2 ..... N)
where k+ and kT are the forward and reverse rate constants, respectively, for the i-th reaction. The equation for the conservation of species
FIG. 2. Geometric configuration of the isothermal reaction system Co~ = 0 0 Co~ Or = 0
at 0 = 0 for all r for0>0 andr=0
(11)
(3) (4)
Yk is
OCy~ OCyk O0 + % Oz N
= ~, (},§ t=l
-ki-CP,.,C~,.2C~) ( k = 1 , 2 . . . . . m)
(12)
and
D,L--~)=hD(Co:,~-Co:)
at r = r o
(5)
where Cy, is the concentration of Yk [mol/cm3]. The equations of conservation for the species to be analyzed (Yl, Y2. . . . . Ym) can be introduced similarly.
KINETIC MODEL OF CHAR NO FORMATION T h e initial conditions for 0 = 0 and all values of z are Co2 = Co2,0,
CAr = CAr,O
(13)
Crk -- 0 (except for 02 a n d Ar) Using assumption (5), the b o u n d a r y conditions for CO, NO, NH, H20, 02 a n d A r are: For0>0andz=
0;
Cco = 1/RTc(I+B/Rc)
(14)
CNo = 1/RTc[I+(B/RN)(MH/Mc)]
(15)
CNH = 1/RTc[I +(B/RNH)(MNH/Mc)]
(16)
CH2o = 1/2RTc[I+(B/RIq)(MH/Mc)]
(17)
Co2 = Co2,0 -- (Cco q- CNO "b CH20)/2
(18)
and CAr = PT/RTc - ( C c o + CNO + CNH + CH2o + Cos)
(19)
where
B = ugpgAT('qag+ae)(mc/ma~) / 4'n~n
(20)
and
May = (RTc/PT)(MArCAr,O+Mo2Co2,0).
(21)
PT = 101.3 kPa and n is the total n u m b e r of the char particles on the porous plate. T h e b o u n d a r y conditions for the other species are Crk=0for0>0andz=0
(22)
By solving these simultaneous differential equations numerically the time-change in the concentration of NO at z = Z can be predicted. In addition, the overall fractional conversion of char-N into char-NO, "qNo, can be obtained by integrating the concentration o f NO with respect to time over the entire combustion period.
1211
Taiheiyo (non-caking bituminous coal from J a p a n , FC = 40.0%, VM = 45.0%, M = 4.1%, ash = 10.9%); and activated sludge (a pseudo low-rank coal from waste-water treatment, FC = 11.7%, VM = 48.7%, M = 1.7%, ash = 37.9%). T h e i r fuel ratios (VM/FC) vary between 0.12 and 4.16. T h e coal particles were devolatilized before the experiments in a stream of N2 at the temperatures o f 1273, 1373 and 1473 K (Tpy) to produce samples of coal-char particles. T h e ultimate analyses of p a r e n t coals and their chars are summarized in Table II together with the fuel ratios of the p a r e n t coals. Figure 3 shows the experimentally d e t e r m i n e d relationship between the complete combustion time, 0D, o f a single char particle with an initial diameter, do, measured in an O2-Ar stream. It is evident from this figure that the chars employed in this work have very different rates o f combustion.
2.2 Experimental apparatus and procedure Figure 4 shows the experimental apparatus and Fig. 5 gives the details o f the main reaction vessel. T h e quartz inner tube (41 m m in diameter) is attached with bolts and sealed with silicon-rubber packing in the outer tube (60 m m in diameter), which has a sintered porous quartz plate on its bottom. T h e apparatus itself is essentially a radiative electric furnace with a SiC heater. T h e main reaction vessel was placed at the center of this furnace and the wall t e m p e r a t u r e of the reaction tube was maintained steady and u n i f o r m t h r o u g h o u t the whole combustion period. T h e O2/Ar mixture used as an oxidizer was p r e h e a t e d to a predet e r m i n e d combustion t e m p e r a t u r e (To) by an
electric heater. Prior to the combustion experiments, weighed particles o f char (0.02 - 5 g) were placed on the porous plate in an Ar stream (Oz-free). After confirming that isothermal, steady-state conditions existed over the entire section of the reaction vessel, the Ar gas was rapidly replaced by an O2/Ar mixture. The time-change of the concentration o f NO at z = Z was measured continuously by a high-order derivative spectro-photometer (Yanagimoto Co. Ltd., UO-1 type). Concentrations of NO2, HCN and NH~ were also m e a s u r e d but were found to be negligibly small as c o m p a r e d to the NO concentration.
2. Expe~mental 3. Results and discussion
2.1 Coal-char Samples In this study, three ranks o f parent coals were employed: Taisei (anthracite coal from China; fixed carbon = 79.3%; volatile matter -~ 9.2%; moisture = 1.1%; and ash = 10.4%),
3.1 Determination of intrinsic gas-solid reactivities and physical properties T h e intrinsic rate constants, kl, of the char particles (except for the activated-sludge char) were first measured using a high-temperature
1212
COMBUSTION GENERATED POLLUTANTS
TABLE II Ultimate analyses of coal chars and their parent coals and the fuel ratio (=VM/FC) of the parent coals C (wt %)
H (wt %)
N (wt %)
Fuel Ratio
Activated Sludge char (1273K) " (1373K) " (1473K)
22.17 16.62 14.52 14.31
4.28 0.16 0.12 0.09
4.60 0.90 0.49 0.40
4.16
Taiheiyo coal char (1273K) " (1373K) " (1473K)
67.86 63.40 62.39 61.60
5.71 0.41 0.29 0.18
1,35 0,83 0.59 0.43
1.13
Taisei coal char (1273K) " (1373K) " (1473K)
79.11 75.60 74.99 74.09
2.61 0.43 0.35 0.34
0.90 0.65 0.63 0.59
0.12
Tc=1173K 12
T~1273 K Ug = I0.I cmts
02=5% exp. calc. 0 Taisei-coal char 9 Taiheiyo-coQ[char
Taisei coal
parent coal
Taiheiyo Activated coal sludge
bulk density of
particle porosity specific internal surface area
/
/
PB [g/cm3] 1.25-1.,t 0.68-0.8 0.65-0.75 0.15-0.23 0.35-0.55 0.62-0.7 es
Ag [m~/g]
5-10
15-20
60-110
3.2 Complete combustion time of a single particle of char __~.
-~-
800 do[Pm]
I~00
FIG. 3. Complete combustion-time of a single particle of coal char T G A unit(Rigaku Electric W o r k Ltd., M o d e l A-10377). Details o f the m e t h o d o f m e a s u r e m e n t and of the d e t e r m i n a t i o n of the rate constant of an activated-sludge char are given in a previous p a p e r ] 5 T h e m e a s u r e d values o f kl for the Taisei a n d T a i h e i y o coal chars are, k 1 = 3 . 2 1 x 1 0 -3 e x p (-53.9/RT) a n d 3 . 0 4 x 1 0 -5 exp (-52.6/RT), respectively, for the following ranges o f experimental conditions: T = 950 - 1280 K, do = 180 - l1251xm, 02 = 3 - 1 0 vol%. O t h e r p r o p e r t i e s u s e d in the calculation were d e t e r m i n e d e x p e r i m e n t a l l y or estimated f r o m the literature. S o m e i m p o r t a n t p r o p e r t i e s are:
T h e calculated and the e x p e r i m e n t a l results for the c o m p l e t e combustiott time, 0D, o f a single particle o f c h a r c o m p a r e d c o n f i r m the validity of the theoretical model. T h e calculated values o f 0/3 based o n Eqs, (I) and (2) were in good a g r e e m e n t with the m e a s u r e d c o m p l e t e combustion time for the t h r e e kinds o f coalchar particles (Fig. 3).
3.3 Determination of ~* B e f o r e d e t e r m i n i n g 7* as the most i m p o r t a n t characteristic p a r a m e t e r o f the p r e s e n t kinetic model, the data for a single particle o f c h a r m u s t be extracted f r o m the data for a cluster o f particles. F i g u r e 6 shows the e x p e r i m e n t a l relationship b e t w e e n ~lyo, the overall fractional conversion o f c h a r - N to c h a r - N O , a n d Ap/At, the a p p a r e n t n u m b e r o f layers o f packed particles. H e r e Ap = (~r/4)nd2 a n d AT is the cross-sectional area of the r e a c t i o n tube. ~lyo increases with a decrease in the value o f Ap/AT a n d finally becomes constant for At/AT < 0.08, as seen in Fig. 6. T h e f o r m a t i o n data o f the N O for AlJAT
A C O M P R E H E N S I V E K I N E T I C M O D E L F O R T H E F O R M A T I O N OF C H A R - N O D U R I N G T H E C O M B U S T I O N OF A S I N G L E P A R T I C L E OF C O A L C H A R NORIO ARAI, MASANOBU HASATANI AND YOSHIHIKO NINOMIYA
Department of Chemical Engineering Nagoya University Nagoya 464, JAPAN STUART W. CHURCHILL
Department of Chemical Engineering University of Pennsylvania Philadelphia PA. 19104, U.S.A. NOAM LIOR
Department of Mechanical Engineering and Apphed Mechanics University of Pennsylvania Philadelphia PA. 19104, U.S.A. A comprehensive kinetic mechanism is proposed for the release of char-bound nitrogen in coal (char-N) into the gas phase during the combustion of coal char particles. The salient feature of the proposed mechanism is that char-N is converted competitively to NH~ as well as to NO at the surface of the particles. Based on the proposed kinetic mechanism, a mathematical model is developed for the transient formation of char-NO originated from char-N during the combus tion of a single particle of coal char in an Oz/Ar stream. The theoretical predictions of the model are compared with the experimental data for three kinds of chars, of which the parent coals are Taisei (anthracite, China), Taiheiyo (non-caking bituminous, Japan) and an activated sludge (a pseudo low-rank coal from waste-water treatment): their fuel ratios range from 0.12 to 4.16. Quantitative agreement is found between the predictions of the model and the measured values for the transient formation of char-NO and for the overall fractional conversion of char-N into char-NO. This proposed mathematical model, based on a comprehensive kinetic mechanism, is recommended for modeling the char-NO formation step in practical coal combustors.
Introduction C o n s i d e r a b l e e f f o r t has b e e n d e v o t e d to the kinetic m e c h a n i s m of c h a r - N O f o r m a t i o n f r o m c h a r - b o u n d n i t r o g e n in coal (char-N). T h e p i o n e e r i n g works by Pohl a n d Sarofim 1, B e e r and M a r t i n 2 a n d P e r s h i n g a n d W e n d t 3 resulted in the e x p e r i m e n t a l distinction o f c h a r - N O f r o m volatile-NO f o r m e d f r o m volatile nitrogen c o m p o u n d s such as NH3 a n d H C N , T h e o r e t i c a l a p p r o a c h e s to the p r e d i c t i o n o f the fate o f char-N d u r i n g c o m b u s t i o n have b e e n based on m a n y e x p e r i m e n t a l studies. W e n d t a n d Schulze ~, p r i o r to the abovem e n t i o n e d e x p e r i m e n t a l work, p r o p o s e d a m o d e l in which char-N is c o n v e r t e d to N O at the particle surface, a n d f u r t h e r reaction to N2 occurs h o m o g e n e o u s l y e i t h e r in the pores or in
the gas film s u r r o u n d i n g the particle. T h e predictions o f their m o d e l c o m p a r e qualitatively with trends o b s e r v e d in later experiments. H o w e v e r , little attention has been directed to the ability of the m o d e l to account fnr the m u t u a l effects o f the m a i n combustion reaction, i.e. C + 1/9 02 = CO, and the c o n c u r r e n t h e t e r o g e n e o u s and h o m o g e n e o u s reactions related to c h a r - N O formation. In addition, t h e r e have b e e n no actual concrete c o m p a r i s o n s b e t w e e n the predictions o f the m o d e l a n d the e x p e r i m e n t a l data. In general, the rate o f c o m b u s t i o n o f coal c h a r has b e e n o b s e r v e d to be strongly d e p e n d e n t on the p a r e n t coal. T h e r e f o r e , a m o d e l applicable to the m a t h e m a t i c a l p r e d i c t i o n o f N O f o r m a t i o n in practical coal c o m b u s t o r s m u s t account for the characteristic combustibility o f the chars in
1207
1208
COMBUSTION GENERATED POLLUTANTS
char-NO formation. Such a model has not yet been perfected. Indeed, our recent data ~ for char-NO formation from a particle of coal char cannot be described by any existing model. T h e objective of the present study is to develop a mathematical model for char-NO formation during the combustion of a single particle of coal char. First, a comprehensive kinetic mechanism including char carbon (charC) and char h y d r o g e n (char-H) is proposed for the release of char-N into the gas phase. T h e primary feature which distinguishes the proposed kinetic mechanism from prior ones is that char-N is converted competitively to NHi, as well as to NO at the surface of the particle d u r i n g the entire combustion period. Next, a mathematical model for the p r o p o s e d kinetic mechanism involving 4 heterogeneous gas-solid reactions and 21 homogeneous gas-phase reactions is developed to describe the transient formation of char-NO during the combustion of a single particle of coal char. T h e theoretical predictions of this model are c o m p a r e d with the experimental data for three kinds of coal char whose parent coals are Taisei (anthracite, China), Taiheiyo (non-caking bituminous, Japan), and an activated sludge (a pseudo low-rank coal from waste-water treatment). T h e combustion characteristics of these chars differ significantly from one another, as will be shown. Finally, the validity of the p r o p o s e d model is examined by comparing the e x p e r i m e n t a l and the calculated results for the transient tormation of char-NO and for the fractional conversion o f c h a r - N into char-NO d u r i n g combustion, for chars of different rank parent coals. To avoid the formation of thermal-NO, a mixture of 02 and Ar was employed as the oxidizer. In addition, all of the chars were p r e p a r e d by pyrolysis at a temperature about 100 K higher than that for combustion, in order to minimize the effect of volatile matter on char-NO formation.
1. Theoretical Analysis 1.1 Kinetic mechanism Twenty-five heterogeneous and homogeneous reaction steps have been introduced in the present theoretical analysis of the overall formation of char-NO d u r i n g the combustion of a single particle of coal char in an O2/Ar stream. These mechanisms are summarized in Table I together with the rate constants and references. (1) Heterogeneous gas-solid reactions: T h e proposed reaction mechanisms for the heterogeneous combustion o f char are shown schematically in Fig. 1, Reactions R1 and R4 have
already been used in existing kinetic models. 4'6 T h e key mechanism in the present model is reaction R3, which has not previously been considered. The salient hypothesis is that a mass fraction y* of the total char-N is released into the gas phase in the form of NHI. T h e hydrogen in NHI is compensated for by the fraction (I-[3") of the total char-N. [NHi is represented by N H to simplify the theoretical approach.] T h e residual fraction of char-N, (I-y*), and of char-H, [3", are oxidized to NO and H20, respectively. T h e values of y* and [3* are characteristic parameters related to the combustibility of the chars. At the present stage, both must be determined experimentally, but fortunately the value of [3* can be obtained from the mass balance: [3* = 1 - (y*/14) (W.v/WH), provided y* can be determined independently. (2) Homogeneous gas-phase reactions: Reactions R5 through R25 are postulated to occur outside of the char particle. T h e rate expressions and the rate constants for reactions R5 to R7 in Table 1 have been taken from Howard, et al. r, Baulch, et al. 8 and Chan, et al. g, respectively. Reactions R8 to R25 have been derived from the full kinetic model 1~ for the gas-phase reaction in an NH3-H2-NO-O2-Ar mixture, but the elementary reactions involving either NH~ or NH2 have been excluded.
1.2 A mathematical modelfor the overallformation of char-NO A mathematical model was developed to describe the formation of char-NO during the combustion of a single particle of coal char. T h e following assumptions are made: 1). T h e reacting system (Fig. 2) is isothermal. T h e number of coal-char particles on the porous plate is small so that the effect of the neighboring particles on char-NO formation can be ignored. 2). Since the chars were p r e p a r e d by pyrolysis at a t e m p e r a t u r e about 100 K higher than the combustion t e m p e r a t u r e , no volatile NO is formed and since N2-free oxidizing gas is used, no thermal-NO is formed. 3). Spherical symmetry and a pseudo steadystate are assumed t h r o u g h o u t the combustion period. This postulate is applicable because the maximum t e m p e r a t u r e d r o p ratio across the particle does not exceed 0.03 for the reaction conditions 11. 4). No gas-phase formation and reduction of char-NO occurs within intraparticle pores. 5). T h e gaseous products (NO, NH, H~O and CO) evolved from the particle mix instantaneously and perfectly with the O~/Ar stream at z -= 0 in Fig. 2.
K I N E T I C MODEL OF C H A R NO FORMATION
1209
TABLE I H e t e r o g e n e o u s and h o m o g e n e o u s mechanisms o f reaction employed in this work heterogeneous gas-solid reaction R1 R2 R3 R4
Reference
rate expression
C + 1 / 2 0 z = CO H + 1/402 = 1/2H20 NH(s) = NH(g) N + 1/20~ = NO
Wendt & Schulze(1976) this work this work W e n d t & Schulze(t976)
Eq.(2) Eq.(8) Eq.(10) Eq.(9)
h o m o g e n e o u s gas-phase reaction
rate expression and rate constant
R5 CO + 1/202 = C 0 2 R6CO + OH = CO2+ H R7 CO + NO = 1/2 N2 + CO2
- r c o = 1.3x 1014(H20)l/2(CO)(O2)l!2exp( - 125.5/RT) t t o w a r d et al. (1973) i n k6 = 10.847 + 3.995x10-~T Baulch et ai.(t976) kiiiPgo(kiiPco + ki) -rx~ kiiiP.xo + kliPco + kl Chan et a1.(1983) ki = 1.5• 10 3exp(- 167.2/RT) [mol/cm2/s] kii=7.33x 10 7exp(-79.4/RT) [mol/cm2/s/kPa] kiii=2.08x 10-4exp( - 108.7/RT) [mol/cm2/s/kPa]
k* = A T ~ exp(-E/RT)
R8 N H + O2 = NO + O H R9 N H + 02 = H N O + O R10 N H + NO = N2 + O H R I I N H + O H = N O + H9 R12 H N O + O H = N O + H 2 0 RI3 H N O + M = H + N O R14 H + 0 2 = O H + O RI50 + H2 = OH + H R16 H2 + OH = H20 + H RI7 O H + O H = H20 + O R18 H2 + M = H + H + M R19 O H + H + M - H 2 0 + M R20 N + OH = NO + H R21 N + O2 = N O + O R22 N + NO = N~ + O R23 N + O + M = N O + M R24 N + N + M = N~ + M R25
O + O + M = O2 + M
A [cm s, moles, s]
a
1.0xl01~ 1.0x 1012 1.0xl013 1.6x1012 3.6x 10 I3 3.0• 2.24x 1014 1.8• 10 ~~ 1.17x109 6.3 x 1012 2.23 x 1019 2 . 2 x 1022 4.1 x 10 a3 6.4x109 1.55x1013 6.44x 1016 1.55x1017 1.9x 1013
0 0 0 0.56 0 0 0 1 1.3 0 0.5 -2 0 1 0 -0.5 -0.837 0
14 g a s e o u s c h e m i c a l s p e c i e s : CO2, CO, H2, N O , H g O , H N O , N H , O H , H , O, N, a n d A r , a r e a s s u m e d to b e i d e a l gases. 7). T h e fluid h a s a u n i f o r m v e l o c i t y d i s t r i b u t i o n ( p l u g flow). T h e basic d i f f e r e n t i a l e q u a t i o n s d e s c r i b i n g mass transfer and chemical reaction can be d e r i v e d f r o m t h e a b o v e a s s u m p t i o n s as follows: 6). T h e
char
combustion
For the pseudo steady-state hypothesis, the o v e r a l l r a t e o f o x i d a t i o n o f c h a r - C in a s p h e r i c a l p a r t i c l e o f c h a r c a n b e e x p r e s s e d as
0 13.59 0 6.28 0 203.5 70.3 68.6 15.26 4.56 387.4 0 0 26.14 0 0 0 -7.49
/02Co~
N2, 02,
Heterogeneous
E[kJ/mol]
Arai et al. (1986)
2
OCo~"~
s
"[,-E~S + 7--~7r J = M~
(1)
where
R e = - dmc" d~ = (rlAg+Ae)kl Po~
(2)
H e r e Rc is t h e o v e r a l l r a t e o f r e a c t i o n o f c h a r - C o n a m a s s basis a n d kl is t h e i n t r i n s i c r e a c t i o n r a t e c o n s t a n t o n a n a r e a basis [g-C/cm2/s/kPa]. T h e initial a n d b o u n d a r y c o n d i t i o n s are:
1210
COMBUSTION GENERATED POLLUTANTS
~,C
+ 89 ----CO
(Rz~ /It2)
, N HI -*NH(s) - - - ~ N H ( g )
' l"l*~N + 89
The effective diffusivity, D,, can be estimated using the followin~ relation derived from Bosanquet's e q u a t i o n " :
1 (R3)
---~ NO
FIG. 1. Reaction mechanism of heterogeneous char combustion s~mpli ng
De=e,P(1/DK_o~)+(1/Do~_A~)
(6)
where DK-O~ and Do~-ar are the K n u d s e n and molecular diffusivities, respectively, of 0 2. The values of DK-O2 and Do2-Ar can be estimated from the literature ~3. I n the present work, the value of p is assumed to be 2.0. The effectiveness factor, "q, in Eq. (2) can he also calculated from the existing theory a4. The mass transfer coefficient at the particle surface, hD can be obtained from Sh = 2 + 0.6 Re 1/2 Sc 1/3.
(7)
Based on the kinetic mechanism shown in Fig. 1, the overall rates of release of char-N, char-H and char-NH into the gas phase can be expressed as follows:
:z=Z gas-p~ Reactions RS~R25
R n ( = - dWH/dO) = (f3*Wn/Wc) Rc
(8)
R x ( = - dWx/dO) = ((1-7*)WN/Wc) Rc
(9)
R.vH ( = - dW.~.n/dO) = ("y*WN/Wc) Re.
(I0)
Homogeneous gas-phase reaction
-x--- 0
The target chemical species and other reactants and products are labelled Yk, Ri and P,, respectively. T h e mechanism of the elementary reactions of N are related to the species Yk as follows: R, , ~+ Ri, 2 + Ri, :3k'r k~ Pi,, + P~,2+ Y~
%(o:ol
( i = 1,2 ..... N)
where k+ and kT are the forward and reverse rate constants, respectively, for the i-th reaction. The equation for the conservation of species
FIG. 2. Geometric configuration of the isothermal reaction system Co~ = 0 0 Co~ Or = 0
at 0 = 0 for all r for0>0 andr=0
(11)
(3) (4)
Yk is
OCy~ OCyk O0 + % Oz N
= ~, (},§ t=l
-ki-CP,.,C~,.2C~) ( k = 1 , 2 . . . . . m)
(12)
and
D,L--~)=hD(Co:,~-Co:)
at r = r o
(5)
where Cy, is the concentration of Yk [mol/cm3]. The equations of conservation for the species to be analyzed (Yl, Y2. . . . . Ym) can be introduced similarly.
KINETIC MODEL OF CHAR NO FORMATION T h e initial conditions for 0 = 0 and all values of z are Co2 = Co2,0,
CAr = CAr,O
(13)
Crk -- 0 (except for 02 a n d Ar) Using assumption (5), the b o u n d a r y conditions for CO, NO, NH, H20, 02 a n d A r are: For0>0andz=
0;
Cco = 1/RTc(I+B/Rc)
(14)
CNo = 1/RTc[I+(B/RN)(MH/Mc)]
(15)
CNH = 1/RTc[I +(B/RNH)(MNH/Mc)]
(16)
CH2o = 1/2RTc[I+(B/RIq)(MH/Mc)]
(17)
Co2 = Co2,0 -- (Cco q- CNO "b CH20)/2
(18)
and CAr = PT/RTc - ( C c o + CNO + CNH + CH2o + Cos)
(19)
where
B = ugpgAT('qag+ae)(mc/ma~) / 4'n~n
(20)
and
May = (RTc/PT)(MArCAr,O+Mo2Co2,0).
(21)
PT = 101.3 kPa and n is the total n u m b e r of the char particles on the porous plate. T h e b o u n d a r y conditions for the other species are Crk=0for0>0andz=0
(22)
By solving these simultaneous differential equations numerically the time-change in the concentration of NO at z = Z can be predicted. In addition, the overall fractional conversion of char-N into char-NO, "qNo, can be obtained by integrating the concentration o f NO with respect to time over the entire combustion period.
1211
Taiheiyo (non-caking bituminous coal from J a p a n , FC = 40.0%, VM = 45.0%, M = 4.1%, ash = 10.9%); and activated sludge (a pseudo low-rank coal from waste-water treatment, FC = 11.7%, VM = 48.7%, M = 1.7%, ash = 37.9%). T h e i r fuel ratios (VM/FC) vary between 0.12 and 4.16. T h e coal particles were devolatilized before the experiments in a stream of N2 at the temperatures o f 1273, 1373 and 1473 K (Tpy) to produce samples of coal-char particles. T h e ultimate analyses of p a r e n t coals and their chars are summarized in Table II together with the fuel ratios of the p a r e n t coals. Figure 3 shows the experimentally d e t e r m i n e d relationship between the complete combustion time, 0D, o f a single char particle with an initial diameter, do, measured in an O2-Ar stream. It is evident from this figure that the chars employed in this work have very different rates o f combustion.
2.2 Experimental apparatus and procedure Figure 4 shows the experimental apparatus and Fig. 5 gives the details o f the main reaction vessel. T h e quartz inner tube (41 m m in diameter) is attached with bolts and sealed with silicon-rubber packing in the outer tube (60 m m in diameter), which has a sintered porous quartz plate on its bottom. T h e apparatus itself is essentially a radiative electric furnace with a SiC heater. T h e main reaction vessel was placed at the center of this furnace and the wall t e m p e r a t u r e of the reaction tube was maintained steady and u n i f o r m t h r o u g h o u t the whole combustion period. T h e O2/Ar mixture used as an oxidizer was p r e h e a t e d to a predet e r m i n e d combustion t e m p e r a t u r e (To) by an
electric heater. Prior to the combustion experiments, weighed particles o f char (0.02 - 5 g) were placed on the porous plate in an Ar stream (Oz-free). After confirming that isothermal, steady-state conditions existed over the entire section of the reaction vessel, the Ar gas was rapidly replaced by an O2/Ar mixture. The time-change of the concentration o f NO at z = Z was measured continuously by a high-order derivative spectro-photometer (Yanagimoto Co. Ltd., UO-1 type). Concentrations of NO2, HCN and NH~ were also m e a s u r e d but were found to be negligibly small as c o m p a r e d to the NO concentration.
2. Expe~mental 3. Results and discussion
2.1 Coal-char Samples In this study, three ranks o f parent coals were employed: Taisei (anthracite coal from China; fixed carbon = 79.3%; volatile matter -~ 9.2%; moisture = 1.1%; and ash = 10.4%),
3.1 Determination of intrinsic gas-solid reactivities and physical properties T h e intrinsic rate constants, kl, of the char particles (except for the activated-sludge char) were first measured using a high-temperature
1212
COMBUSTION GENERATED POLLUTANTS
TABLE II Ultimate analyses of coal chars and their parent coals and the fuel ratio (=VM/FC) of the parent coals C (wt %)
H (wt %)
N (wt %)
Fuel Ratio
Activated Sludge char (1273K) " (1373K) " (1473K)
22.17 16.62 14.52 14.31
4.28 0.16 0.12 0.09
4.60 0.90 0.49 0.40
4.16
Taiheiyo coal char (1273K) " (1373K) " (1473K)
67.86 63.40 62.39 61.60
5.71 0.41 0.29 0.18
1,35 0,83 0.59 0.43
1.13
Taisei coal char (1273K) " (1373K) " (1473K)
79.11 75.60 74.99 74.09
2.61 0.43 0.35 0.34
0.90 0.65 0.63 0.59
0.12
Tc=1173K 12
T~1273 K Ug = I0.I cmts
02=5% exp. calc. 0 Taisei-coal char 9 Taiheiyo-coQ[char
Taisei coal
parent coal
Taiheiyo Activated coal sludge
bulk density of
particle porosity specific internal surface area
/
/
PB [g/cm3] 1.25-1.,t 0.68-0.8 0.65-0.75 0.15-0.23 0.35-0.55 0.62-0.7 es
Ag [m~/g]
5-10
15-20
60-110
3.2 Complete combustion time of a single particle of char __~.
-~-
800 do[Pm]
I~00
FIG. 3. Complete combustion-time of a single particle of coal char T G A unit(Rigaku Electric W o r k Ltd., M o d e l A-10377). Details o f the m e t h o d o f m e a s u r e m e n t and of the d e t e r m i n a t i o n of the rate constant of an activated-sludge char are given in a previous p a p e r ] 5 T h e m e a s u r e d values o f kl for the Taisei a n d T a i h e i y o coal chars are, k 1 = 3 . 2 1 x 1 0 -3 e x p (-53.9/RT) a n d 3 . 0 4 x 1 0 -5 exp (-52.6/RT), respectively, for the following ranges o f experimental conditions: T = 950 - 1280 K, do = 180 - l1251xm, 02 = 3 - 1 0 vol%. O t h e r p r o p e r t i e s u s e d in the calculation were d e t e r m i n e d e x p e r i m e n t a l l y or estimated f r o m the literature. S o m e i m p o r t a n t p r o p e r t i e s are:
T h e calculated and the e x p e r i m e n t a l results for the c o m p l e t e combustiott time, 0D, o f a single particle o f c h a r c o m p a r e d c o n f i r m the validity of the theoretical model. T h e calculated values o f 0/3 based o n Eqs, (I) and (2) were in good a g r e e m e n t with the m e a s u r e d c o m p l e t e combustion time for the t h r e e kinds o f coalchar particles (Fig. 3).
3.3 Determination of ~* B e f o r e d e t e r m i n i n g 7* as the most i m p o r t a n t characteristic p a r a m e t e r o f the p r e s e n t kinetic model, the data for a single particle o f c h a r m u s t be extracted f r o m the data for a cluster o f particles. F i g u r e 6 shows the e x p e r i m e n t a l relationship b e t w e e n ~lyo, the overall fractional conversion o f c h a r - N to c h a r - N O , a n d Ap/At, the a p p a r e n t n u m b e r o f layers o f packed particles. H e r e Ap = (~r/4)nd2 a n d AT is the cross-sectional area of the r e a c t i o n tube. ~lyo increases with a decrease in the value o f Ap/AT a n d finally becomes constant for At/AT < 0.08, as seen in Fig. 6. T h e f o r m a t i o n data o f the N O for AlJAT
