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Modeling NOx release from a single coal particle I. Formation of NO from volatile nitrogen
Modeling NO, Release from a Single Coal Particle I. Formation of NO from Volatile Nitrogen S. P. VISONA
and B. R. STANMORE*
Department of Chemical Engineering, University of Queensland, Brisbane Qid 4072, Australia
For a single spherical coal particle, modeling has been undertaken of the processes of release of volatile fuel-nitrogen, and its subsequent conversion to nitric oxide during the devolatilization. The combustion conditions used were similar to those found in utility boilers. A finite volume numerical scheme was used to solve the equations of mass, species conservation, momentum and heat transfer about the devolatilizing particle. Particles in the size range lo-100 pm were modeled with gas temperatures from 1250 to 1900 K. Devolatilization was described by two competing reactions; results are compared with other single particle models and experimental results reported in the literature. The influence of coal rank was examined by using a lignite, a sub-bituminous and a high volatile bituminous coal. The conversion efficiencies of volatile fuel-nitrogen to nitric oxide are presented. It was found that there can be a significant conversion of the volatile fuel-nitrogen to nitric oxide before the liberated volatiles have reached the oxygen supply in the bulk gas. Conversion efficiencies of volatile nitrogen to nitric oxide as high as 54% are predicted for conditions of high gas temperature and high oxygen concentrations. For small particles (= 10 pm), most of the fuel-nitrogen is released as hydrogen cyanide (up to 70%) prior to mixing with the bulk gas.
NOMENCLATURE A
surface area of a calculation shell Cm’) diffusion coefficient Cm2 s- ’1 mixture fraction, Yfue, - Yo,/s
f”
(kg kg-l>
mass fraction of nitrogen in the volatiles (kg/kg) enthalpy (J kg-‘) heat of combustion (J kg-‘) thermal conductivity (W m-l K- ‘) Darcy constant (m2) mass of particle (kg) m. - m (kg) molecular weight (kg kmol-’ ) Nusselt number pressure (Pa> heat flow (J SC’) pulverized fuel volumetric heat flux (J me3 s- ‘> radius of the particle Cm), universal gas constant (J kmol-’ K-‘) universal gas constant (J kg- ’ K- ‘1 radial position Cm) stoichiometric constant
fN h AH, k
K m Am
MW Nu P
Q p.f.
: 3 r s
ckgO,
s HCN,
NO
kgLLiks)
source term (kg m-3 s-l>
SE T t
At V
Vol x, Y
Greek Symbols
transport coefficient change in porosity during combustion emissivity and porosity nitrogen conversion efficiency dynamic viscosity (Pa s) density (kg rnp3> Stefan Boltzmann constant -2 K-4 (Wm > process variable and fuel oxygen equivalence ratio
r
AE & VN
CL P
CT dJ
C
f fuel
I
combustion convection flame volatiles COMBUSTIONAND
OOlO-2180/96/$15.00 SSDI OOlO-2180(95)00169-7
’
Subscripts
conv * Corresponding author.
specific energy or lower calorific value (MJ kg-‘) temperature (K) time 6) solution time step for numerical integration (s) velocity Cm s- ’1 volume (m3) ratio flame radius/particle radius mass fraction (kg, kg,$ture)
I
4 FLAME
105: 92-103 (1996)
Copyright 0 1996 by The Combustion Institute Published by Elsevier Science Inc.
NO FROM COAL VOLATILES g
gas
max
maximum particle outer radius of particle radiation solid stoichiometric mixture shell wall initial far field
P R
rad s stoic : co
INTRODUCTION The combustion of pulverized coal supplies NO, into the atmosphere. The formation of NO, during this process involves a complex interaction between mass, momentum and energy transfer and chemical reaction [l]. A number of approaches have been taken to understanding the phenomenon, including laboratory studies [2], burner [3] and pilot scale furnace studies [4], and mathematical modeling [5]. In typical cases only 30%-50% of the nitrogen in the coal finally appears as NO, mainly as the result of “micro-scale diffusion phenomena around individual coal particles during devolatilization” [6]. In spite of progress, the basic mechanisms are still being clarified. Each particle will heat up, begin to devolatilize, and then ignite. The stream of volatiles is generally considered sufficient to prevent oxygen reaching the exterior surface, and a flame front (with volatiles as fuel) forms some distance from the particle. The location of the flame front will be determined by mass transport rates and the rate of evolution of the volatiles. For pulverized coal particles, molecular diffusion can safely be regarded as the transport mechanism. When the volatiles are exhausted, the residual char particle ignites ,and is consumed. Some early work [2] showed that some of the fuel-nitrogen is evolved with the volatiles, while the remainder stays in the char. The latter is released slowly as the char burns off. Most experimental studies of pulverized coal pyrolysis have found that the nitrogen partitions evenly between the volatile matter and the char [7]. It is believed that the bulk of the volatile nitrogen is released as hydrogen cyanide, together with some ammonia [8]. The
93 HCN and NH, will experience a range of chemical reactions as they traverse the pores of the particle, escape from the boundary layer and mix into the bulk gas. Within a particle, the most likely reaction is reduction as oxygen will not be present. For instance, a decrease in the stoichiometric air/fuel ratio leads to an increase in the NHJHCN ratio. This suggests that reduction of HCN to NH, probably occurs on contact with aliphatic or similar radicals present in the volatiles [9]. In the boundary layer, nitric oxide can react with the HCN as the former diffuses inwards from the flame front. Once the HCN diffuses to the flame and contacts the bulk gas, it is rapidly attacked. A survey [lo] of the reactions indicates the complexity of the gas-phase situation. The interactions of the volatile and char forms of nitrogen with the combustion gases and with the char have been summarized by Wendt [6]. Since NO is formed in the flame, it can diffuse either outwards or inwards. Within the flame envelope it can be reduced either by HCN or by the carbon of the char. The likely concentration profiles of the various species are depicted in Fig. 1. An experimental investigation around a single particle is difficult. The closest approximations to single-particle combustion are flat-flame and laminar flow furnaces where conditions ensure that particles are sufficiently separated to produce no interactions. Once example is that of Song et al. Ill], who investigated the effect of temperature and fuel/ oxygen ratio on the conversion of coal-nitrogen to NO. This was for a Montana lignite and a Montana subbituminous coal in a laminar flow furnace with a residence time of 1 s. Conversion decreased monotonically with fuel/ oxygen ratio and decreased slightly with increasing temperature. As for modeling, little attention has been paid to the events taking place within the individual particle and its environs with respect to the release of nitrogenous compounds. The present approach is focused on individual particles and examines the processes which occur at various stages in their burnout. From the dynamic model developed it is possible to calculate the fraction of volatile nitrogen which escapes from a particle into the bulk gas in the
S. P. VISONA AND B. R. STANMORE
94
..
.’
‘..
__I. ‘._.
__
‘..._
‘.
. ..*.-_ _ __.._ i.___--...-
form of NO. This paper deals with NO formation in the devolatilization stage only; the release of the residual nitrogen in the char will be dealt with in a second paper. The processes modeled include pyrolysis, nitric oxide formation and destruction in the gas phase, and appropriate flow, mass and heat transfer mechanisms. The computed results for NO conversion for two coals were compared to experimental data [ill. Some additional results for the pyrolysis process, which strongly influences the NO generation steps, were compared with data generated by Jost et al. [121. THE MODEL
For modeling purposes the particle was considered in isolation, with the surrounding gas at constant temperature and composition. Following the usual assumption for modeling the combustion of pulverized fuel (p.f.1 mass and heat transfer within the particle’s boundary layer occurs by molecular processes. The only exceptions were convection brought about by the outward flow of volatiles, and radiation. The history of the particle is tracked from heatup to the total elimination of volatile matter. The exchange of thermal radiation between the particle and its surroundings plays an important part in determining the particle’s temperature. To simulate the flame in a p.f.
Fig. 1. Concentration profiles around a devolatilizing coal particle.
burner, the particle was enclosed by a remote radiation shell or “wall” (see Fig. l), which was given an emissivity of unity. The temperature of the shell wall, T,, was set to simulate the high temperature environment of a flame. The intervening gas between the flame front and the wall was assumed to be transparent to radiation. The assumptions are: The coal particle is spherical and nonswelling; radial symmetry is assumed for all processes. Heat transfer takes place uniformly on the 1 surface by conductive and radiative exchange with the surrounding gas, which is maintained at constant temperature. A radial temperature profile is generated inside the particle. Devolatilization kinetics are described by the two step models of Ubhayakar et al. [13] or Gat et al. [141. Following Field et al.’ [15], the volatiles are regarded as homogeneous vapor with a molecular weight of 100. The local porosity varies proportionately with the amount of volatiles released. The interior pressure in the particle rises due to volatiles’ release and initiates Darcy ’ flow so that a velocity profile forms. Mass transfer in the external gas phase is driven by molecular diffusion and convection.
NO FROM COAL VOLATILES TABLE 1
5. Ignition
6.
7.
8. 9.
10.
of the volatiles occurs when the Semenov criterion [16] is satisfied, i.e., when the potential rate of heat generation from the combustion of the volatiles exceeds the rate of heat loss by convection from the particle’s surface. The position of the flame front around the particle is determined by the location of the stoichiometric mixture of fuel (volatiles) and oxygen. Fast chemistry is assumed for oxidation and the flame width is governed by the size of the computational cell. The nitrogen in the fuel partitions evenly between the char and the volatiles, i.e., has the same concentration in each [71. All gaseous nitrogen appears as hydrogen cyanide. The rate constants for all reactions and their sources are listed in Table 1. The specific energy of the volatiles is modelled by a relationship given by Wall [17]. The heat of reaction is transferred away from the flame front by conduction and convection. Thermal radiation is exchanged between the particle’s surface, the flame front and an outside “shell” of unit emissivity, which is remote from the particle. The outer shell is set at 100 particle-radii from the particle’s center. The overall nitrogen conversion is calculated at that shell.
95
The following set of equations was invoked to describe the processes.
Summary of Kinetic Rate Parameters
s-1
ki j = Ai jBi, jCi, j exp( -Ei, j/RT)
Devolatkatioh
models: (1 - VM,,)char
+ VM,, volatiles
coal (k.2
\
I
(1 - VM,,)char
+ VM,, volatiles
NO Model: i=3
4 Reaction i = 1 2 3 4
HCN + NO
+ (k,,,)
N, + ...
HCN + 0,
+ (k,,)
NO + ...
Ubhayakar, devolatilization Gat, devolatilization De Soete, NO reduction De Soete, NO production
Bi,j (mol mol-I)
Ai,j Reaction
(s-l)
L1
1.34 e5 1 1.46 e13 1 2.24 e4 1 2.85 e5 1 3.0 e12 X,,, 1.0 ell X,,,
192 271 222 331 431
Ei,j
ci,j
(mol mol-‘)
W mol-1)
VM,
1 1 1 1
VM,’ 2VM, 0.4 1.0
xNO
-
Xozb
-
Ref.
74.0 251.0 117.2 139.8 251.2 280.5
13 14 17 17
“VM, is the ASTM proximate volatile fraction on a daf basis. ‘Oxygen reaction order is zero for oxygen mole fractions greater than 18,000 ppm and unity for less than 2500 ppm
[181. For the quantities in Table 2, Qrad = /
Description of Process Equations
qrad dVo1 = 0 T=Tf
= -[A$,+;
The general transport
equation in symmetric, spherical coordinates (r term only) is used via:
before ignition
- TsqR)
+A~E~u ( T’ - T:)]
after ignition
TABLE 2 Summary of Terms Used in the Transport Equations
At any point in time six transport equations are solved (Table 2): conservation of mass, solids’ energy, energy of the gas, oxygen concentration or mixture fraction, depending if ignition has occurred, hydrogen cyanide concentration, and nitric oxide concentration.
1 T,
VP 0
% yo,
vg vg
0 k/Q, k&p, p&k
YHCN
5
P~DHCN
Y NO
%
P~DNO
f
5
PPf
dm,/(AVol dt) 0 (&I + %hem)r=rl 0 dm,/(AVol dt) SHCN
-
S NO
SNO -
-
SN2
SN~
7 8 9 10a lob 11
12
96
Q them
S. P. VISONA AND B. R. STANMORE
/
=
qchem
d%
dVo1 = -
dt
and the Darcy permeability according to
AH,
Hydrogen cyanide is assumed to form within the particle according to the release of the volatiles via
[21, 221 is varied
KO
Kc_
(1 - E) Boundary Conditions
HCN =
AVol
MWN,
At the center of the particle the temperature gradient is zero,
As devolatilization proceeds within the particle, the gaseous volatile products are transported by diffusion and convection along pores to the surface of the particle; see Eq. 7, Table 2. Outside the particle, the accumulation term ap,/at in Eq. 7 is ignored. After ignition, the flame moves away from the particle as devolatilization proceeds and the flame eventually collapses when the volatiles are exhausted. HCN is formed within the particle and is converted to nitric oxide on reacting with oxygen [18]; see Eqs. 11 and 12, Table 2. Darcy flow within the particle is modeled by
PgVg sr+ = dP
(2)
0,
K
which is solved with Eq. 1 for gas to give 1 ap ----RgTg dl
ld r2 dr
=p
The particle is heated by convection and radiation,
Q, = Qrad + Qmw Qrad = A,,R~pa(Ti
AVol dt ’
(4)
where e. is the initial porosity based on data found in Karr [19] and E,,, is based on the porosity of char [20]. The surface area is updated using
Q cow = (Nu W,M,
before ignition after ignition
Dg, R - T,, R)
For p.f. particles the Nusselt Number is taken as 2.0. Within the particle the gas temperature is assumed to equal the solid’s temperature, T,=T,
forr
(5)
for r x==R.
The mixture fraction f at r, is fixed at - Yo , ,/s (corresponding to the oxygen-massfractibn boundary condition, and the stoichiometric oxygen requirement). Far field boundary conditions (subscript m) are modelled at a sufficient radial distance from the particle so that the flame formation is not adversely influenced by the confines of this boundary. The concentrations of hydrogen cyanide and nitric oxide are assumed to be zero at r,. Prior to ignition, Eq. 1Oa (Table 2) applies, and air is allowed to diffuse into the particle. After ignition, the f Eq. lob applies and is used to locate the flame front, as f = fstoic 9
with combustion products now flowing out of the particle. Inside the particle at r = 0,
‘f
-=
A = do
- T:,)
= A,, R?? p c (T; - T;tR)
Tg = TP
dms
Equation 3 gives the pressure distribution within the particle (assuming p/p is a constant for each At). The pressure distribution may then be used to obtain the velocity distribution within the particle. During devolatilization the porosity of the particle is updated using Am max-, m.
dr
and as r bedomes large (far field condition),
(3)
E=E~+AE
aT,_ -0
dr
dYo, dr
=-=
dYHCN
dr
ayNO -
dr
=
0
NO FROM COAL VOLATILES Equations 1-12 were solved by an implicit finite volume technique. An exponential upwind differencing scheme was used to solve the combined convection/diffusion problem. The particle was discretized into a number of shells, generally, 50. The properties in each shell, such as volatile matter content and porosity in the solid phase, composition, pressure and velocity in the gas phase were updated with time. The modeling procedure will be referred to as the finite volume flame model (FVFM). The computer program was run on a 50-MHz 486DX IBM clone with 64 MB of RAM. A typical run time for simulating the combustion of a lOO-pm-diameter particle at 1900 K with 50 particle shells and 400 gas shells was 50 minutes. Combustion
Conditions
Three coals were examined in the modeling program: a lignite, subbituminous and high volatile bituminous coal. The coals studied by Song et al. [ll] were a Montana lignite-A (labeled A> and a Montana sub-bituminous (B) which were used to find NO conversions. Jost et al. [12] also studied the Montana subbituminous coal. The swelling indices of these low rank coals would be small as required by the model. On the other hand Timothy et al. 1231 burned five coals with a view of observing the release of volatiles. The devolatilization times are recorded for an Illinois No. 6 high volatile bituminous coal (labeled C) and also a Montana lignite-A. The analyses of the three coals are given in Table 3. All are similarly high in volatile matter content on a d.a.f. basis. The variables considered in the modelling program included particle size, bulk gas oxygen concentration and gas and shell wall temperatures. Particles in the size range lo-100 pm were modeled, with gas and shell wall (radiation) temperatures ranging from 1250 to 1900 K. The initial particle temperature was always 363 K. The dynamic outputs include gas and particle temperatures, position of the flame front, gas concentration profiles (volatiles, O,, HCN, NO) and nitrogen conversion to NO as functions of time. The conversion of nitrogen to either nitric oxide or hydrogen cyanide (HCN)
97 TABLE 3 Summary of Coal Properties Coal A
Coal B
Coal C
lignite A
Subbitum.
HVB bitum.
13.6 36.2 7.8 46.1
21.3 35.2 9.3 50.7
7.4 38.1 13.76 48.3
Ult. Analysis (% dry) C H N S 0 (difference)
63.1 5.7O 1.0 1.0 20.2
67.7 4.3” 1.1 1.0 14.2
61.3 5.1 1.1
SE (MJ kg-‘, wet)
16.3
20.7
28.5
Descriptor
Type Prox. Analysis (as ret) Moisture VM Ash VM (% daf)
15.4
“These two results do not appear to be consistent and may have been interchanged in Song et al.‘s [ll] tabulated values. bInconsistent with the ultimate analysis.
was defined by 77~= (mass of NO or HCN leaving the model boundary)/(mass
of NO or HCN if all
fuel nitrogen converted to NO or HCN) The HCN which escapes through the flame into the bulk gas will experience a number of fates, including thermal destruction [ll], reaction with NO to form molecular nitrogen and oxidisation of NO. The latter is the most likely as oxygen concentrations will rise rapidly outside the flame shell. This effect makes it difficult to compare the model output with experimental data. RESULTS
AND DISCUSSION
Studies Using Montana
Subbituminous
The particle conduction model was checked against standard solutions for a number of simplified cases [24] with good agreement. Initial checks on the devolatilization model was carried out by comparing the modelling results with those of Gururajan et al., 1251who used what they referred to as a diffusion-limited, volatile-combustion (DLVC) model. The coal used for this comparison was the Montana
98
S. P. VISONA AND B. R. STANMORE 8e+6 Flame ExtInction
Q s
6e+6
!
6e+6
-14
1
-
Ubhayakar Kinetics
2
Ignition
El2
-
P g10-
L $ P I I S e0
,
16 -
7e+6
B : a
4e+6
,
8-
,
,
C .-.
.
. I .
,
g6_ ” u.
3e+6
P
.
4-
2e+6
‘I
2-
0.00
.05
.lO
.lS
.20
.25
30
35
0,
.40
0
Time (milliseconds)
Fig. 2. Particle heating rate (K/s) versus time for a lo-pm subbituminous particle burning at 1900 K and 15.8% oxygen.
subbituminous coal (Coal B) of Jost et al. [12], who passed 50 pm particles through a droptube reactor to ascertain devolatilization rates. Figure 2 shows the particle’s heating rate (K/s) as a function of time for lo-pm particles. The form of the heatup curve is similar for 50- and 100~pm particles. As the particle is plunged into the hot gas, the particle’s heating rate is initially very high and decreases until ignition takes place. Following ignition, the rate rapidly increases due to the surrounding volatile flame, and then decreases as the flame moves away from the particle. As the flame’s radius decreases, due to exhaustion of the volatiles within the particle, there is a further rapid heating of the particle. The flame then becomes extinct and the remaining volatiles burn at the surface of the particle. For the lo-pm particle, the time to ignition is about 0.25 ms (and is about 2.5 and 6 ms, respectively, for 50- and 100~pm particles). The results for devolatilization are shown in Figs. 3 to 5 are for a gas/shell temperature of 1900 K and an oxygen concentration of 15.8 wt%. Curve B is for Ubhayakar kinetics [13] and curve D for Gat [141 kinetics. Also seen in this figure are curves A and C, the DLVC results presented by Gururajan et al. [25]. Using Gat kinetics for the conditions outlined, the ignition delay is about 6 ms and the maxi-
, 2
/
, 4
)
,
,
,
,
(
,
8
8
10
12
14
16
18
20
Time (milliseconds)
Fig. 3. Comparison of dimensionless flame radius versus time for a devolatilizing 50-pm subbituminous particle at 1900 K and 15.8% oxygen. A and B use Ubhayakar kinetics for the DLVC and FVFM models. C and D use Gat kinetics same models.
mum ratio
2000
yz f I!500
H
Ubhayakar Kinetics (A and B)
0
i
2
‘Ooo 600
0
j
I
I
I
I
I
0
2
4
6
6
10
-I 12
Time (mllltseconds)
Fig. 4. Comparison of particle temperature versus time for 4 a devolatilizing SO-pm subbituminous particle at 1900 K and 15.8% oxygen. A and B use Ubhayakar kinetics for the DLVC and FVFM models. C and D use Gat kinetics same models.
99
NO FROM COAL VOLATILES in X, to about 16 (see Fig. 3) and a significant reduction in both the ignition delay and the devolatilization time. A greater ignition delay is prediced by the current model than was found experimentally, or by Gururajan et al. 1251. However, particle heating rates obtained by using the DLVC model are very similar to those found by the FVFM approach described in this paper. Particle temperatures calculated with both models using the same devolatilization kinetics give very similar values, as shown in Fig. 4. As discussed in Ref. 25 the slower Gat kinetics give higher particle temperatures due to the flame being closer to the particle during devolatilization. The progress of devolatilization is depicted in Fig. 5 for both kinetics expressions. As expected, devolatilization is more rapid with Ubhayakar kinetics. The experimental rates measured by Jost et al. 1121are fitted better by Gat kinetics, which predict a much gentler asymptotic approach to completion. Nitric oxide formation within the particle has been modeled using the kinetic information given by De Soete [181. Fast chemistry has been assumed and the volatiles are also assumed to be burnt in a finite volume (computa-
tional shell). This means that there is no oxygen between the spherical flame and the particle, and only HCN, volatiles and combustion products exist in this region. Figure 6 shows the effect of gas temperature on a 50-pm particle for the conditions given in Fig. 3. Both Gat and Ubhayakar kinetics have been employed to test if there was any significant difference in the conversions of nitric oxide or HCN. Increasing the surrounding gas temperature caused an increase in the conversion to nitric oxide, and at 1900 K very little of the hydrogen cyanide has reached the exit of the model. At 1500 K, over 90% of the volatilenitrogen reaches the exit of the model, while only about 10% of it has been converted to nitric oxide. At lower gas temperatures, the reaction rate between HCN and oxygen has been reduced to such a low value that HCN is the dominant fuel nitrogen product. As seen in Fig. 6, both devolatilization models produce very similar conversions for HCN and nitric oxide. It would appear that higher particle temperatures achieved using Gat kinetics tend to offset the slower devolatilization kinetics. In terms of modeling NO, from a single particle there would appear to be no advantage in using Gat kinetics over Ubhayakar kinetics, so the latter were adopted here.
NO 15.9 % 02 (Ub) NO 1.58% 02 (Ub) NO 15.9% 02 (Gat) HCN 15.8% 02 (Ub) HCN 1.58% 02 (Ub)
. .
HCN 15.9% 02 (Gat)
0
0
5
10
15
20
Experimental
25
30
35
Time (milliseconds)
Fig. 5. Comparison of particle burnout versus time for a devolatilizing 50-pm subbituminous particle at 1900 K and 15.8% oxygen. A and B use Ubhayakar kinetics for the DLVC and FVFM models. C and D use Gat kinetics same models.
I 1400
I
I
I
I
I
1500
1600
1700
1900
1900
i 2000
Temperature (K)
Fig. 6. Effect of gas temperature, devolatilization kinetics and oxygen concentration on nitric oxide and HCN conversion efficiency for a SO-pm subbituminous particle (UbUbhayakar kinetics).
S. P. VISONA AND B. R. STANMORE
100 The effect of oxygen concentration is given in Fig. 7, and, as expected, the higher oxygen concentration gives more nitric oxide. The abscissa in that figure is expressed as the reciprocal of the oxygen concentration, so that the trend may be compared with the results of Song et al. [ll] at 1750 K. They measured a similar trend, but found a greater reduction in nitric oxide than predicted by the FVFM, but an exact comparison is not possible as Song et al. [ll] expressed the abscissa as equivalence ratio (4). Experimental conversions of volatile nitrogen were also determined by song et al. 1111at 1250 K using the sub-bituminous coal. Their results (obtained as a difference between coal and char suggest very high conversions at low 4. Using the FVFM, the conversion of the volatile nitrogen to nitric oxide is very low-about 2%; it is mostly HCN which leaves the far boundary of the model. Song et al. [ll] explain this high NO conversion as due to low rates of volatile yield at lower temperatures, causing less fuel-rich regions for the particles to penetrate. This leads to higher, conversions of nitric oxide in the drop tube furnace. One possible explanation for the difference between the FVFM and the experimental results at lower temperatures is subsequent oxidation of HCN outside the boundary layer. Miller and
Bowman [26] suggest that in fuel-rich conditions, OH is the primary radical attacking HCN. The rate of that reaction given by them [261 at 1250 K is much higher than those for the oxidation of typical hydrocarbon radicals, such as CH and CH,. Thus the HCN may be preferentially oxidised by such oxidising species as are present. There has been debate over the effect of particle size on the production of nitric oxide [27, 28, 29, 301. Figure 8 looks at the effect of particle size over the size range 10 to 50 pm. Ten pm particles gave the lowest conversions to nitric oxide but the highest HCN conversions. At a gas temperature of 1900 K, the smaller particles, such as 10 pm, have very high heating rates. This produces high devolatilization rates and hence high rates of production of HCN (kg,cN mm3 s- ‘). The conversion rises with increasing particle size, but falls again at about 30 pm. The short ignition and devolatilization time found for the 10 pm particle may be used to explain the high HCN emissions from small particles to the bulk gas. For small particles, it is difficult for the production/reduction kinetics of NO to have a significant effect within the boundaries of the model. Also plotted on Fig. 8 is the sum of the concentrations of HCN and nitric oxide I / at the exit of the model. / I I 100
“a
,
90
42
80
i
??
NitricOxide
.
Hydrogen Cyanide
A
NO+HCN
El0 E iiE 60 E w 50 5 t 40 e 8 30
322 0.00
.05
.lO
.15
.20
.25
30
Oxygen) Fig. 7. Nitric oxide conversion efficiency versus reciprocal of % oxygen for a 50-pm subbituminous particle. 1 I (X
IO
15
20
26
30
35
40
46
Ml
Particle Size (microns)
Fig. 8. Nitric oxide and HCN conversion efficiency versus particle size devolatilizing at 1900 K and 23.3% oxygen.
101
NO FROM COAL VOLATILES The overall NO, emissions will be determined both by processes within the particle and by those in the bulk. For example, if the conditions in an industrial furnace were conducive to the oxidation of the HCN that leaves the particles, then small particles have the potential to produce more nitric oxide than larger particles. On the other hand, if the HCN is released into the central recirculation zone (CRZ) or a similar area where oxygen concentrations are low, the HCN could be reduced to N,. In the 0.5 MW, swirl burner studied by Maier et al. [30], increasing fineness in the pulverized coal led to an overall diminution of NO, emissions. However, the combustion air was staged and this effect, combined with the complex flow structures associated with the CRZ, make it impossible to compare with the case of a single particle.
burning a 100~pm particle of Montana lignite. At 20 vol.% oxygen, a devolatilization time of about 22 ms is predicted and may be compared with the experimental measurement of about 19 ms. Timothy et al. [23] used higher oxygen concentrations (up to 100%) which led to devolatilization times being reduced to around 2 ms. For a 50-pm lignite particle, the yield of nitric oxide varied from about 21% to 31% for a change in oxygen concentration from 2% to 20% (see Fig. 10). Also shown are results for a 100 pm particle. There is a small increase in nitric oxide (37%-43%) when the oxygen is increased from 2 to 20%. As found previously for the subbituminous coal, the hydrogen cyanide conversion is higher for smaller particles. Modeling suggests that, for similar particles of high volatile coals, such as lignite and subbituminous, the conversions (both HCN and NO) are similar.
Studies Using Montana Lignite
Studies Using High Volatile Bituminous Coal
Timothy et al. [23] measured, using two-color optical pyrometry, the duration of volatile flames for single coal particles. Devolatilization times are presented for particles in the size range 90 to 105 pm, burning at 1700 K for a range of oxygen concentrations. Figure 9 shows the effect of oxygen concentration on devolatilization times and flame radius when
Using the data in Table 3 on a high volatile bituminous (HVB) coal, modeling was carried out for the case given by Timothy et al. [23] on a 100 pm particle in 20% oxygen at a gas temperature of 1700 K. The experimental time for devolatilization was about 10 ms. Initial results for the predicted devolatilization time of this particle gave about the same time as
3000
26.5 - 26.0 ?? Flame Radius ?? Devol. Time
2500 3 S B g 9 S a a
- 25.5 - 25.0 - 24.5
2000
- 24.0 - 23.5
1500
S II.
- 23.0 - 22.5
1000
- 22.0 500
r 0
2
4
6
6
10
12
14
Oxygen Concentration (X)
16
10
20
22
Fig. 9. Dimensionless flame radii and devolatilization times for a 100-wrn particle burning at 1700 K in a range of oxygen concentrations.
102
S. P. VISONA AND B. R. STANMORE
70 20.5 -
60
20.0 z 01 50 3 5 ‘3 46 E
19.6 19.0 -
.z f
3o
s u
20
Cl
la.5 19.0 17.5 -
10
17.0 16.5
0
I
0
2
f
4
I
6
I
a
I
IO
I
12
1
14
I
16
)
,
,
,
,
,
,
,
,
,
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
I
ia
20
Oxygen Concentration (X)
Fig. 10. Effect of oxygen concentration and particle size on nitric oxide and HCN conversion efficiency for a lignite particle.
obtained with the subbituminous coal, i.e., 20.5 ms. As swelling would seem to be one possible mechanism for a reduction in devolatilization time, tests were carried out to ascertain the sensitivity of the FVFM to swelling of the particle. The swelling factor (based on the particle diameter) was found by Essenhigh [31] to be about 1.5 for rapid heating of all coals, except anthracite. Others, such as Morris and Keairns [32], found that for small particles (defined as smaller than 1.5 mm) the swelling factor could be as high as 2. A swelling factor was incorporated such that the mass of the particle remained constant while the diameter increased. Internal mass transport was not changed, as the process is heat-transfer-limited. An increase in swelling factor caused a decrease in the devolatilization time (see Fig. ll), e.g., with a swelling factor of 2 the predicted devolatilization time was reduced from 20.5 to 16.5 ms. Swelling of the particle caused greater radiation heating due to the increased external surface area for the same mass of coal. The incorporation of the swelling factor did not have a significant effect on the yield of nitric oxide. For example, there was only a 13% increase (from 47 to 54%) in the conversion to nitric oxide for the lOO-pm
2.0
Swelling Factor
Fig. 11. Effect of swelling factor on the devolatilization time of a lOO+m high volatile bituminous particle burning at 1700 K and 20% oxygen.
HVB coal particle, if it swelled to double its original diameter. CONCLUSIONS The following conclusions have been reached on the burning of volatile-nitrogen and the formation of nitric oxide around a single coal particle, at combustion gas temperatures and oxygen partial pressures similar to those found 1 in industrial p.f. coal flames. 1. There can be a significant conversion of the i volatile-nitrogen to nitric oxide before the liberated volatiles have reached the oxygen in the bulk gas. Conversion efficiencies as high as 54 percent have been calculated for conditions of high gas temperature and high oxygen concentrations. Small p.f. particles, i.e., those below 30 pm, tend to have lower conversions of volatilenitrogen to nitric oxide. Most of the fuelnitrogen is released as hydrogen cyanide (up to 70%) prior to mixing with the bulk gas. This implies that small particles (10 pm, say> could have a greater overall conversion to nitric oxide than the larger particles. At low temperatures ( = 1500 K) when most of the volatile-nitrogen escapes from the
NO FROM COAL VOLATILES particle as HCN, the final conversion to NO will then be strongly influenced by chemical processes occurring in the bulk gas phase. Raising the gas temperature causes a greater proportion of the volatile-nitrogen to be converted to nitric oxide in the vicinity of the particle. The amount of hydrogen cyanide entering the bulk gas is greatly reduced and the NO yield is thus determined predominantly by processes within the particle. Increasing the bulk partial pressure of oxygen causes a higher conversion of the volatile nitrogen to nitric oxide in the gas before it leaves the particle. For higher volatile, lower rank coals, the primary NO emissions are reasonably independent of rank. REFERENCES 1. Beer, J. M., Twenty-Second Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1988, pp. 1-16. 2. Solomon, P. R., and Colket, M. B., Fuel 57~749-755 (1978). 3. Abbas, T., Costen, P., and Lockwood, F. C., Combust. Flame 91:346-363
103 12. Jost, M., Leslie, I., and Kruger, C., Twentieth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1984, pp. 1531-1537. 13. Ubhayakar, S. K., Stickler, D. B., von Rosenbum, C. W. Jr., and Gannon, R. E., Sixteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1976, pp. 427-436. 14. Gat, N., Cohen, L. M., Denison, M. R., and Witte, A. B., Final Report DOE Contract No. DE-AC2281PC40273, 1983. 15. Field, M. A., Gill, D. W., Morgan, B. B., and Hawkesley, P. G. W., Combustion of Pulverised Coal, BCURA, Leatherhead, 1966, p. 178. 16. Semenov, N. N. 2. Phys. 48:571 (1928). 17. Wall, T. F., in Principles of Combustion Engineering (C. J. Lawn, Ed.), Academic, New York, 1987, p. 219. 18. De Soete, G. G., Fifteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1975, pp. 1093-1102. 19. Karr, C. Analytical Methods for Coal and Coal Products, Academic, New York, 1978, p. 156. 20. Johnson, J. L., in Chemistry of Coal Utilization, 2nd Supplementary Volume, (M. A., Elliott, Ed.), Wiley, New York, 1981, pp. 1517-1524. 21. Scheidegger, A. E., The Physics of Flow Through Porous Media. University of Toronto Press, 1960. 22. Yang, J. T., and Wang, G. G., Trans. ASME .I. Heat Transf. 112:192-200
Nineteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1982, pp.
(1991).
Chen, S. L., Heap, M. P., Pershing, D. W., and Martin, G. B. Fuel 61:1218-1224 (1982). 5. Fiveland, W. A., and Wessel, R. A., .I. Inst. Ener. 64:41-54 (1991). 6. Wendt, J. O., Second International Conference on
4.
Combustion
Technologies for a Clean Environment,
Calouste Gulbenkian Foundation, Lisbon, Portugal, July 1993, pp. 1-12. 7. Wang, W., Brown, S. D., Hindmarsh, C. J., and Thomas, K. M. Fuel 73:1381-1388 (1994). 8. Nelson, P. F., Buckley, A. N., and Kelly, M. D., Twenty-Fourth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1992,
pp. 1259-1267. 9. Winstone, S. W., Phong-anant, D. Baker, J. W., Gupta, R. P., and Wall, T. F., NERDDP Project No. 991 End of Grant Report, February 1989. 10. Glarborg, P., Miller, J. A., and Knee, R. J., Combust. Flame 651177-202 (1986).
11. Song, Y. H., Pohl, J. H., Beer, J. M., and Sarofim, A. F., Combust. Sci. Technol. 28:31-39 (1982).
(1990).
23. Timothy, L. D., Sarofim, A. F., and Beer, J. M.,
24. 25.
1123-1130. McAdams, W. H., Heat Transmission, McGraw-Hill Book Company, 1954, pp. 31-54. Gururajan, V. S., Wall, T. F., and Truelove, J. S., Combust. Flame 72:1-12
(1988).
Miller, J. A., and Bowman, C. T., Prog. Energ. Combust. Sci. 15:287-338 (1989). 27. Duxbury, J., and Welford, G. B., J. Inst. Ener., 26.
62147-151
(1989).
28. Abbas, T., Costen, P., Lockwood, F. C., and RomoMillares, C. A., Cornbust. Flame 93:316-326 (1993). 29. Afonso, R., Dusatko, G. C., and Pohl, J. H., Combust. Sci. Technol. 93:45-51
(1993).
Maier, H., Spliethoff, H., Kicherer, A., Fingerle, A., and Hein, K. R. G., Fuel 73:1447-1452 (1994). 31. Essenhigh, R. H., J. Eng. Power 85:183-190 (1963). 32. Morris, J. P., and Keairns, D. L., Fuel 58, 465-471 30.
(1979). Received 17 October 1994; revised 7 July 1995
and B. R. STANMORE*
Department of Chemical Engineering, University of Queensland, Brisbane Qid 4072, Australia
For a single spherical coal particle, modeling has been undertaken of the processes of release of volatile fuel-nitrogen, and its subsequent conversion to nitric oxide during the devolatilization. The combustion conditions used were similar to those found in utility boilers. A finite volume numerical scheme was used to solve the equations of mass, species conservation, momentum and heat transfer about the devolatilizing particle. Particles in the size range lo-100 pm were modeled with gas temperatures from 1250 to 1900 K. Devolatilization was described by two competing reactions; results are compared with other single particle models and experimental results reported in the literature. The influence of coal rank was examined by using a lignite, a sub-bituminous and a high volatile bituminous coal. The conversion efficiencies of volatile fuel-nitrogen to nitric oxide are presented. It was found that there can be a significant conversion of the volatile fuel-nitrogen to nitric oxide before the liberated volatiles have reached the oxygen supply in the bulk gas. Conversion efficiencies of volatile nitrogen to nitric oxide as high as 54% are predicted for conditions of high gas temperature and high oxygen concentrations. For small particles (= 10 pm), most of the fuel-nitrogen is released as hydrogen cyanide (up to 70%) prior to mixing with the bulk gas.
NOMENCLATURE A
surface area of a calculation shell Cm’) diffusion coefficient Cm2 s- ’1 mixture fraction, Yfue, - Yo,/s
f”
(kg kg-l>
mass fraction of nitrogen in the volatiles (kg/kg) enthalpy (J kg-‘) heat of combustion (J kg-‘) thermal conductivity (W m-l K- ‘) Darcy constant (m2) mass of particle (kg) m. - m (kg) molecular weight (kg kmol-’ ) Nusselt number pressure (Pa> heat flow (J SC’) pulverized fuel volumetric heat flux (J me3 s- ‘> radius of the particle Cm), universal gas constant (J kmol-’ K-‘) universal gas constant (J kg- ’ K- ‘1 radial position Cm) stoichiometric constant
fN h AH, k
K m Am
MW Nu P
Q p.f.
: 3 r s
ckgO,
s HCN,
NO
kgLLiks)
source term (kg m-3 s-l>
SE T t
At V
Vol x, Y
Greek Symbols
transport coefficient change in porosity during combustion emissivity and porosity nitrogen conversion efficiency dynamic viscosity (Pa s) density (kg rnp3> Stefan Boltzmann constant -2 K-4 (Wm > process variable and fuel oxygen equivalence ratio
r
AE & VN
CL P
CT dJ
C
f fuel
I
combustion convection flame volatiles COMBUSTIONAND
OOlO-2180/96/$15.00 SSDI OOlO-2180(95)00169-7
’
Subscripts
conv * Corresponding author.
specific energy or lower calorific value (MJ kg-‘) temperature (K) time 6) solution time step for numerical integration (s) velocity Cm s- ’1 volume (m3) ratio flame radius/particle radius mass fraction (kg, kg,$ture)
I
4 FLAME
105: 92-103 (1996)
Copyright 0 1996 by The Combustion Institute Published by Elsevier Science Inc.
NO FROM COAL VOLATILES g
gas
max
maximum particle outer radius of particle radiation solid stoichiometric mixture shell wall initial far field
P R
rad s stoic : co
INTRODUCTION The combustion of pulverized coal supplies NO, into the atmosphere. The formation of NO, during this process involves a complex interaction between mass, momentum and energy transfer and chemical reaction [l]. A number of approaches have been taken to understanding the phenomenon, including laboratory studies [2], burner [3] and pilot scale furnace studies [4], and mathematical modeling [5]. In typical cases only 30%-50% of the nitrogen in the coal finally appears as NO, mainly as the result of “micro-scale diffusion phenomena around individual coal particles during devolatilization” [6]. In spite of progress, the basic mechanisms are still being clarified. Each particle will heat up, begin to devolatilize, and then ignite. The stream of volatiles is generally considered sufficient to prevent oxygen reaching the exterior surface, and a flame front (with volatiles as fuel) forms some distance from the particle. The location of the flame front will be determined by mass transport rates and the rate of evolution of the volatiles. For pulverized coal particles, molecular diffusion can safely be regarded as the transport mechanism. When the volatiles are exhausted, the residual char particle ignites ,and is consumed. Some early work [2] showed that some of the fuel-nitrogen is evolved with the volatiles, while the remainder stays in the char. The latter is released slowly as the char burns off. Most experimental studies of pulverized coal pyrolysis have found that the nitrogen partitions evenly between the volatile matter and the char [7]. It is believed that the bulk of the volatile nitrogen is released as hydrogen cyanide, together with some ammonia [8]. The
93 HCN and NH, will experience a range of chemical reactions as they traverse the pores of the particle, escape from the boundary layer and mix into the bulk gas. Within a particle, the most likely reaction is reduction as oxygen will not be present. For instance, a decrease in the stoichiometric air/fuel ratio leads to an increase in the NHJHCN ratio. This suggests that reduction of HCN to NH, probably occurs on contact with aliphatic or similar radicals present in the volatiles [9]. In the boundary layer, nitric oxide can react with the HCN as the former diffuses inwards from the flame front. Once the HCN diffuses to the flame and contacts the bulk gas, it is rapidly attacked. A survey [lo] of the reactions indicates the complexity of the gas-phase situation. The interactions of the volatile and char forms of nitrogen with the combustion gases and with the char have been summarized by Wendt [6]. Since NO is formed in the flame, it can diffuse either outwards or inwards. Within the flame envelope it can be reduced either by HCN or by the carbon of the char. The likely concentration profiles of the various species are depicted in Fig. 1. An experimental investigation around a single particle is difficult. The closest approximations to single-particle combustion are flat-flame and laminar flow furnaces where conditions ensure that particles are sufficiently separated to produce no interactions. Once example is that of Song et al. Ill], who investigated the effect of temperature and fuel/ oxygen ratio on the conversion of coal-nitrogen to NO. This was for a Montana lignite and a Montana subbituminous coal in a laminar flow furnace with a residence time of 1 s. Conversion decreased monotonically with fuel/ oxygen ratio and decreased slightly with increasing temperature. As for modeling, little attention has been paid to the events taking place within the individual particle and its environs with respect to the release of nitrogenous compounds. The present approach is focused on individual particles and examines the processes which occur at various stages in their burnout. From the dynamic model developed it is possible to calculate the fraction of volatile nitrogen which escapes from a particle into the bulk gas in the
S. P. VISONA AND B. R. STANMORE
94
..
.’
‘..
__I. ‘._.
__
‘..._
‘.
. ..*.-_ _ __.._ i.___--...-
form of NO. This paper deals with NO formation in the devolatilization stage only; the release of the residual nitrogen in the char will be dealt with in a second paper. The processes modeled include pyrolysis, nitric oxide formation and destruction in the gas phase, and appropriate flow, mass and heat transfer mechanisms. The computed results for NO conversion for two coals were compared to experimental data [ill. Some additional results for the pyrolysis process, which strongly influences the NO generation steps, were compared with data generated by Jost et al. [121. THE MODEL
For modeling purposes the particle was considered in isolation, with the surrounding gas at constant temperature and composition. Following the usual assumption for modeling the combustion of pulverized fuel (p.f.1 mass and heat transfer within the particle’s boundary layer occurs by molecular processes. The only exceptions were convection brought about by the outward flow of volatiles, and radiation. The history of the particle is tracked from heatup to the total elimination of volatile matter. The exchange of thermal radiation between the particle and its surroundings plays an important part in determining the particle’s temperature. To simulate the flame in a p.f.
Fig. 1. Concentration profiles around a devolatilizing coal particle.
burner, the particle was enclosed by a remote radiation shell or “wall” (see Fig. l), which was given an emissivity of unity. The temperature of the shell wall, T,, was set to simulate the high temperature environment of a flame. The intervening gas between the flame front and the wall was assumed to be transparent to radiation. The assumptions are: The coal particle is spherical and nonswelling; radial symmetry is assumed for all processes. Heat transfer takes place uniformly on the 1 surface by conductive and radiative exchange with the surrounding gas, which is maintained at constant temperature. A radial temperature profile is generated inside the particle. Devolatilization kinetics are described by the two step models of Ubhayakar et al. [13] or Gat et al. [141. Following Field et al.’ [15], the volatiles are regarded as homogeneous vapor with a molecular weight of 100. The local porosity varies proportionately with the amount of volatiles released. The interior pressure in the particle rises due to volatiles’ release and initiates Darcy ’ flow so that a velocity profile forms. Mass transfer in the external gas phase is driven by molecular diffusion and convection.
NO FROM COAL VOLATILES TABLE 1
5. Ignition
6.
7.
8. 9.
10.
of the volatiles occurs when the Semenov criterion [16] is satisfied, i.e., when the potential rate of heat generation from the combustion of the volatiles exceeds the rate of heat loss by convection from the particle’s surface. The position of the flame front around the particle is determined by the location of the stoichiometric mixture of fuel (volatiles) and oxygen. Fast chemistry is assumed for oxidation and the flame width is governed by the size of the computational cell. The nitrogen in the fuel partitions evenly between the char and the volatiles, i.e., has the same concentration in each [71. All gaseous nitrogen appears as hydrogen cyanide. The rate constants for all reactions and their sources are listed in Table 1. The specific energy of the volatiles is modelled by a relationship given by Wall [17]. The heat of reaction is transferred away from the flame front by conduction and convection. Thermal radiation is exchanged between the particle’s surface, the flame front and an outside “shell” of unit emissivity, which is remote from the particle. The outer shell is set at 100 particle-radii from the particle’s center. The overall nitrogen conversion is calculated at that shell.
95
The following set of equations was invoked to describe the processes.
Summary of Kinetic Rate Parameters
s-1
ki j = Ai jBi, jCi, j exp( -Ei, j/RT)
Devolatkatioh
models: (1 - VM,,)char
+ VM,, volatiles
coal (k.2
\
I
(1 - VM,,)char
+ VM,, volatiles
NO Model: i=3
4 Reaction i = 1 2 3 4
HCN + NO
+ (k,,,)
N, + ...
HCN + 0,
+ (k,,)
NO + ...
Ubhayakar, devolatilization Gat, devolatilization De Soete, NO reduction De Soete, NO production
Bi,j (mol mol-I)
Ai,j Reaction
(s-l)
L1
1.34 e5 1 1.46 e13 1 2.24 e4 1 2.85 e5 1 3.0 e12 X,,, 1.0 ell X,,,
192 271 222 331 431
Ei,j
ci,j
(mol mol-‘)
W mol-1)
VM,
1 1 1 1
VM,’ 2VM, 0.4 1.0
xNO
-
Xozb
-
Ref.
74.0 251.0 117.2 139.8 251.2 280.5
13 14 17 17
“VM, is the ASTM proximate volatile fraction on a daf basis. ‘Oxygen reaction order is zero for oxygen mole fractions greater than 18,000 ppm and unity for less than 2500 ppm
[181. For the quantities in Table 2, Qrad = /
Description of Process Equations
qrad dVo1 = 0 T=Tf
= -[A$,+;
The general transport
equation in symmetric, spherical coordinates (r term only) is used via:
before ignition
- TsqR)
+A~E~u ( T’ - T:)]
after ignition
TABLE 2 Summary of Terms Used in the Transport Equations
At any point in time six transport equations are solved (Table 2): conservation of mass, solids’ energy, energy of the gas, oxygen concentration or mixture fraction, depending if ignition has occurred, hydrogen cyanide concentration, and nitric oxide concentration.
1 T,
VP 0
% yo,
vg vg
0 k/Q, k&p, p&k
YHCN
5
P~DHCN
Y NO
%
P~DNO
f
5
PPf
dm,/(AVol dt) 0 (&I + %hem)r=rl 0 dm,/(AVol dt) SHCN
-
S NO
SNO -
-
SN2
SN~
7 8 9 10a lob 11
12
96
Q them
S. P. VISONA AND B. R. STANMORE
/
=
qchem
d%
dVo1 = -
dt
and the Darcy permeability according to
AH,
Hydrogen cyanide is assumed to form within the particle according to the release of the volatiles via
[21, 221 is varied
KO
Kc_
(1 - E) Boundary Conditions
HCN =
AVol
MWN,
At the center of the particle the temperature gradient is zero,
As devolatilization proceeds within the particle, the gaseous volatile products are transported by diffusion and convection along pores to the surface of the particle; see Eq. 7, Table 2. Outside the particle, the accumulation term ap,/at in Eq. 7 is ignored. After ignition, the flame moves away from the particle as devolatilization proceeds and the flame eventually collapses when the volatiles are exhausted. HCN is formed within the particle and is converted to nitric oxide on reacting with oxygen [18]; see Eqs. 11 and 12, Table 2. Darcy flow within the particle is modeled by
PgVg sr+ = dP
(2)
0,
K
which is solved with Eq. 1 for gas to give 1 ap ----RgTg dl
ld r2 dr
=p
The particle is heated by convection and radiation,
Q, = Qrad + Qmw Qrad = A,,R~pa(Ti
AVol dt ’
(4)
where e. is the initial porosity based on data found in Karr [19] and E,,, is based on the porosity of char [20]. The surface area is updated using
Q cow = (Nu W,M,
before ignition after ignition
Dg, R - T,, R)
For p.f. particles the Nusselt Number is taken as 2.0. Within the particle the gas temperature is assumed to equal the solid’s temperature, T,=T,
forr
(5)
for r x==R.
The mixture fraction f at r, is fixed at - Yo , ,/s (corresponding to the oxygen-massfractibn boundary condition, and the stoichiometric oxygen requirement). Far field boundary conditions (subscript m) are modelled at a sufficient radial distance from the particle so that the flame formation is not adversely influenced by the confines of this boundary. The concentrations of hydrogen cyanide and nitric oxide are assumed to be zero at r,. Prior to ignition, Eq. 1Oa (Table 2) applies, and air is allowed to diffuse into the particle. After ignition, the f Eq. lob applies and is used to locate the flame front, as f = fstoic 9
with combustion products now flowing out of the particle. Inside the particle at r = 0,
‘f
-=
A = do
- T:,)
= A,, R?? p c (T; - T;tR)
Tg = TP
dms
Equation 3 gives the pressure distribution within the particle (assuming p/p is a constant for each At). The pressure distribution may then be used to obtain the velocity distribution within the particle. During devolatilization the porosity of the particle is updated using Am max-, m.
dr
and as r bedomes large (far field condition),
(3)
E=E~+AE
aT,_ -0
dr
dYo, dr
=-=
dYHCN
dr
ayNO -
dr
=
0
NO FROM COAL VOLATILES Equations 1-12 were solved by an implicit finite volume technique. An exponential upwind differencing scheme was used to solve the combined convection/diffusion problem. The particle was discretized into a number of shells, generally, 50. The properties in each shell, such as volatile matter content and porosity in the solid phase, composition, pressure and velocity in the gas phase were updated with time. The modeling procedure will be referred to as the finite volume flame model (FVFM). The computer program was run on a 50-MHz 486DX IBM clone with 64 MB of RAM. A typical run time for simulating the combustion of a lOO-pm-diameter particle at 1900 K with 50 particle shells and 400 gas shells was 50 minutes. Combustion
Conditions
Three coals were examined in the modeling program: a lignite, subbituminous and high volatile bituminous coal. The coals studied by Song et al. [ll] were a Montana lignite-A (labeled A> and a Montana sub-bituminous (B) which were used to find NO conversions. Jost et al. [12] also studied the Montana subbituminous coal. The swelling indices of these low rank coals would be small as required by the model. On the other hand Timothy et al. 1231 burned five coals with a view of observing the release of volatiles. The devolatilization times are recorded for an Illinois No. 6 high volatile bituminous coal (labeled C) and also a Montana lignite-A. The analyses of the three coals are given in Table 3. All are similarly high in volatile matter content on a d.a.f. basis. The variables considered in the modelling program included particle size, bulk gas oxygen concentration and gas and shell wall temperatures. Particles in the size range lo-100 pm were modeled, with gas and shell wall (radiation) temperatures ranging from 1250 to 1900 K. The initial particle temperature was always 363 K. The dynamic outputs include gas and particle temperatures, position of the flame front, gas concentration profiles (volatiles, O,, HCN, NO) and nitrogen conversion to NO as functions of time. The conversion of nitrogen to either nitric oxide or hydrogen cyanide (HCN)
97 TABLE 3 Summary of Coal Properties Coal A
Coal B
Coal C
lignite A
Subbitum.
HVB bitum.
13.6 36.2 7.8 46.1
21.3 35.2 9.3 50.7
7.4 38.1 13.76 48.3
Ult. Analysis (% dry) C H N S 0 (difference)
63.1 5.7O 1.0 1.0 20.2
67.7 4.3” 1.1 1.0 14.2
61.3 5.1 1.1
SE (MJ kg-‘, wet)
16.3
20.7
28.5
Descriptor
Type Prox. Analysis (as ret) Moisture VM Ash VM (% daf)
15.4
“These two results do not appear to be consistent and may have been interchanged in Song et al.‘s [ll] tabulated values. bInconsistent with the ultimate analysis.
was defined by 77~= (mass of NO or HCN leaving the model boundary)/(mass
of NO or HCN if all
fuel nitrogen converted to NO or HCN) The HCN which escapes through the flame into the bulk gas will experience a number of fates, including thermal destruction [ll], reaction with NO to form molecular nitrogen and oxidisation of NO. The latter is the most likely as oxygen concentrations will rise rapidly outside the flame shell. This effect makes it difficult to compare the model output with experimental data. RESULTS
AND DISCUSSION
Studies Using Montana
Subbituminous
The particle conduction model was checked against standard solutions for a number of simplified cases [24] with good agreement. Initial checks on the devolatilization model was carried out by comparing the modelling results with those of Gururajan et al., 1251who used what they referred to as a diffusion-limited, volatile-combustion (DLVC) model. The coal used for this comparison was the Montana
98
S. P. VISONA AND B. R. STANMORE 8e+6 Flame ExtInction
Q s
6e+6
!
6e+6
-14
1
-
Ubhayakar Kinetics
2
Ignition
El2
-
P g10-
L $ P I I S e0
,
16 -
7e+6
B : a
4e+6
,
8-
,
,
C .-.
.
. I .
,
g6_ ” u.
3e+6
P
.
4-
2e+6
‘I
2-
0.00
.05
.lO
.lS
.20
.25
30
35
0,
.40
0
Time (milliseconds)
Fig. 2. Particle heating rate (K/s) versus time for a lo-pm subbituminous particle burning at 1900 K and 15.8% oxygen.
subbituminous coal (Coal B) of Jost et al. [12], who passed 50 pm particles through a droptube reactor to ascertain devolatilization rates. Figure 2 shows the particle’s heating rate (K/s) as a function of time for lo-pm particles. The form of the heatup curve is similar for 50- and 100~pm particles. As the particle is plunged into the hot gas, the particle’s heating rate is initially very high and decreases until ignition takes place. Following ignition, the rate rapidly increases due to the surrounding volatile flame, and then decreases as the flame moves away from the particle. As the flame’s radius decreases, due to exhaustion of the volatiles within the particle, there is a further rapid heating of the particle. The flame then becomes extinct and the remaining volatiles burn at the surface of the particle. For the lo-pm particle, the time to ignition is about 0.25 ms (and is about 2.5 and 6 ms, respectively, for 50- and 100~pm particles). The results for devolatilization are shown in Figs. 3 to 5 are for a gas/shell temperature of 1900 K and an oxygen concentration of 15.8 wt%. Curve B is for Ubhayakar kinetics [13] and curve D for Gat [141 kinetics. Also seen in this figure are curves A and C, the DLVC results presented by Gururajan et al. [25]. Using Gat kinetics for the conditions outlined, the ignition delay is about 6 ms and the maxi-
, 2
/
, 4
)
,
,
,
,
(
,
8
8
10
12
14
16
18
20
Time (milliseconds)
Fig. 3. Comparison of dimensionless flame radius versus time for a devolatilizing 50-pm subbituminous particle at 1900 K and 15.8% oxygen. A and B use Ubhayakar kinetics for the DLVC and FVFM models. C and D use Gat kinetics same models.
mum ratio
2000
yz f I!500
H
Ubhayakar Kinetics (A and B)
0
i
2
‘Ooo 600
0
j
I
I
I
I
I
0
2
4
6
6
10
-I 12
Time (mllltseconds)
Fig. 4. Comparison of particle temperature versus time for 4 a devolatilizing SO-pm subbituminous particle at 1900 K and 15.8% oxygen. A and B use Ubhayakar kinetics for the DLVC and FVFM models. C and D use Gat kinetics same models.
99
NO FROM COAL VOLATILES in X, to about 16 (see Fig. 3) and a significant reduction in both the ignition delay and the devolatilization time. A greater ignition delay is prediced by the current model than was found experimentally, or by Gururajan et al. 1251. However, particle heating rates obtained by using the DLVC model are very similar to those found by the FVFM approach described in this paper. Particle temperatures calculated with both models using the same devolatilization kinetics give very similar values, as shown in Fig. 4. As discussed in Ref. 25 the slower Gat kinetics give higher particle temperatures due to the flame being closer to the particle during devolatilization. The progress of devolatilization is depicted in Fig. 5 for both kinetics expressions. As expected, devolatilization is more rapid with Ubhayakar kinetics. The experimental rates measured by Jost et al. 1121are fitted better by Gat kinetics, which predict a much gentler asymptotic approach to completion. Nitric oxide formation within the particle has been modeled using the kinetic information given by De Soete [181. Fast chemistry has been assumed and the volatiles are also assumed to be burnt in a finite volume (computa-
tional shell). This means that there is no oxygen between the spherical flame and the particle, and only HCN, volatiles and combustion products exist in this region. Figure 6 shows the effect of gas temperature on a 50-pm particle for the conditions given in Fig. 3. Both Gat and Ubhayakar kinetics have been employed to test if there was any significant difference in the conversions of nitric oxide or HCN. Increasing the surrounding gas temperature caused an increase in the conversion to nitric oxide, and at 1900 K very little of the hydrogen cyanide has reached the exit of the model. At 1500 K, over 90% of the volatilenitrogen reaches the exit of the model, while only about 10% of it has been converted to nitric oxide. At lower gas temperatures, the reaction rate between HCN and oxygen has been reduced to such a low value that HCN is the dominant fuel nitrogen product. As seen in Fig. 6, both devolatilization models produce very similar conversions for HCN and nitric oxide. It would appear that higher particle temperatures achieved using Gat kinetics tend to offset the slower devolatilization kinetics. In terms of modeling NO, from a single particle there would appear to be no advantage in using Gat kinetics over Ubhayakar kinetics, so the latter were adopted here.
NO 15.9 % 02 (Ub) NO 1.58% 02 (Ub) NO 15.9% 02 (Gat) HCN 15.8% 02 (Ub) HCN 1.58% 02 (Ub)
. .
HCN 15.9% 02 (Gat)
0
0
5
10
15
20
Experimental
25
30
35
Time (milliseconds)
Fig. 5. Comparison of particle burnout versus time for a devolatilizing 50-pm subbituminous particle at 1900 K and 15.8% oxygen. A and B use Ubhayakar kinetics for the DLVC and FVFM models. C and D use Gat kinetics same models.
I 1400
I
I
I
I
I
1500
1600
1700
1900
1900
i 2000
Temperature (K)
Fig. 6. Effect of gas temperature, devolatilization kinetics and oxygen concentration on nitric oxide and HCN conversion efficiency for a SO-pm subbituminous particle (UbUbhayakar kinetics).
S. P. VISONA AND B. R. STANMORE
100 The effect of oxygen concentration is given in Fig. 7, and, as expected, the higher oxygen concentration gives more nitric oxide. The abscissa in that figure is expressed as the reciprocal of the oxygen concentration, so that the trend may be compared with the results of Song et al. [ll] at 1750 K. They measured a similar trend, but found a greater reduction in nitric oxide than predicted by the FVFM, but an exact comparison is not possible as Song et al. [ll] expressed the abscissa as equivalence ratio (4). Experimental conversions of volatile nitrogen were also determined by song et al. 1111at 1250 K using the sub-bituminous coal. Their results (obtained as a difference between coal and char suggest very high conversions at low 4. Using the FVFM, the conversion of the volatile nitrogen to nitric oxide is very low-about 2%; it is mostly HCN which leaves the far boundary of the model. Song et al. [ll] explain this high NO conversion as due to low rates of volatile yield at lower temperatures, causing less fuel-rich regions for the particles to penetrate. This leads to higher, conversions of nitric oxide in the drop tube furnace. One possible explanation for the difference between the FVFM and the experimental results at lower temperatures is subsequent oxidation of HCN outside the boundary layer. Miller and
Bowman [26] suggest that in fuel-rich conditions, OH is the primary radical attacking HCN. The rate of that reaction given by them [261 at 1250 K is much higher than those for the oxidation of typical hydrocarbon radicals, such as CH and CH,. Thus the HCN may be preferentially oxidised by such oxidising species as are present. There has been debate over the effect of particle size on the production of nitric oxide [27, 28, 29, 301. Figure 8 looks at the effect of particle size over the size range 10 to 50 pm. Ten pm particles gave the lowest conversions to nitric oxide but the highest HCN conversions. At a gas temperature of 1900 K, the smaller particles, such as 10 pm, have very high heating rates. This produces high devolatilization rates and hence high rates of production of HCN (kg,cN mm3 s- ‘). The conversion rises with increasing particle size, but falls again at about 30 pm. The short ignition and devolatilization time found for the 10 pm particle may be used to explain the high HCN emissions from small particles to the bulk gas. For small particles, it is difficult for the production/reduction kinetics of NO to have a significant effect within the boundaries of the model. Also plotted on Fig. 8 is the sum of the concentrations of HCN and nitric oxide I / at the exit of the model. / I I 100
“a
,
90
42
80
i
??
NitricOxide
.
Hydrogen Cyanide
A
NO+HCN
El0 E iiE 60 E w 50 5 t 40 e 8 30
322 0.00
.05
.lO
.15
.20
.25
30
Oxygen) Fig. 7. Nitric oxide conversion efficiency versus reciprocal of % oxygen for a 50-pm subbituminous particle. 1 I (X
IO
15
20
26
30
35
40
46
Ml
Particle Size (microns)
Fig. 8. Nitric oxide and HCN conversion efficiency versus particle size devolatilizing at 1900 K and 23.3% oxygen.
101
NO FROM COAL VOLATILES The overall NO, emissions will be determined both by processes within the particle and by those in the bulk. For example, if the conditions in an industrial furnace were conducive to the oxidation of the HCN that leaves the particles, then small particles have the potential to produce more nitric oxide than larger particles. On the other hand, if the HCN is released into the central recirculation zone (CRZ) or a similar area where oxygen concentrations are low, the HCN could be reduced to N,. In the 0.5 MW, swirl burner studied by Maier et al. [30], increasing fineness in the pulverized coal led to an overall diminution of NO, emissions. However, the combustion air was staged and this effect, combined with the complex flow structures associated with the CRZ, make it impossible to compare with the case of a single particle.
burning a 100~pm particle of Montana lignite. At 20 vol.% oxygen, a devolatilization time of about 22 ms is predicted and may be compared with the experimental measurement of about 19 ms. Timothy et al. [23] used higher oxygen concentrations (up to 100%) which led to devolatilization times being reduced to around 2 ms. For a 50-pm lignite particle, the yield of nitric oxide varied from about 21% to 31% for a change in oxygen concentration from 2% to 20% (see Fig. 10). Also shown are results for a 100 pm particle. There is a small increase in nitric oxide (37%-43%) when the oxygen is increased from 2 to 20%. As found previously for the subbituminous coal, the hydrogen cyanide conversion is higher for smaller particles. Modeling suggests that, for similar particles of high volatile coals, such as lignite and subbituminous, the conversions (both HCN and NO) are similar.
Studies Using Montana Lignite
Studies Using High Volatile Bituminous Coal
Timothy et al. [23] measured, using two-color optical pyrometry, the duration of volatile flames for single coal particles. Devolatilization times are presented for particles in the size range 90 to 105 pm, burning at 1700 K for a range of oxygen concentrations. Figure 9 shows the effect of oxygen concentration on devolatilization times and flame radius when
Using the data in Table 3 on a high volatile bituminous (HVB) coal, modeling was carried out for the case given by Timothy et al. [23] on a 100 pm particle in 20% oxygen at a gas temperature of 1700 K. The experimental time for devolatilization was about 10 ms. Initial results for the predicted devolatilization time of this particle gave about the same time as
3000
26.5 - 26.0 ?? Flame Radius ?? Devol. Time
2500 3 S B g 9 S a a
- 25.5 - 25.0 - 24.5
2000
- 24.0 - 23.5
1500
S II.
- 23.0 - 22.5
1000
- 22.0 500
r 0
2
4
6
6
10
12
14
Oxygen Concentration (X)
16
10
20
22
Fig. 9. Dimensionless flame radii and devolatilization times for a 100-wrn particle burning at 1700 K in a range of oxygen concentrations.
102
S. P. VISONA AND B. R. STANMORE
70 20.5 -
60
20.0 z 01 50 3 5 ‘3 46 E
19.6 19.0 -
.z f
3o
s u
20
Cl
la.5 19.0 17.5 -
10
17.0 16.5
0
I
0
2
f
4
I
6
I
a
I
IO
I
12
1
14
I
16
)
,
,
,
,
,
,
,
,
,
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
I
ia
20
Oxygen Concentration (X)
Fig. 10. Effect of oxygen concentration and particle size on nitric oxide and HCN conversion efficiency for a lignite particle.
obtained with the subbituminous coal, i.e., 20.5 ms. As swelling would seem to be one possible mechanism for a reduction in devolatilization time, tests were carried out to ascertain the sensitivity of the FVFM to swelling of the particle. The swelling factor (based on the particle diameter) was found by Essenhigh [31] to be about 1.5 for rapid heating of all coals, except anthracite. Others, such as Morris and Keairns [32], found that for small particles (defined as smaller than 1.5 mm) the swelling factor could be as high as 2. A swelling factor was incorporated such that the mass of the particle remained constant while the diameter increased. Internal mass transport was not changed, as the process is heat-transfer-limited. An increase in swelling factor caused a decrease in the devolatilization time (see Fig. ll), e.g., with a swelling factor of 2 the predicted devolatilization time was reduced from 20.5 to 16.5 ms. Swelling of the particle caused greater radiation heating due to the increased external surface area for the same mass of coal. The incorporation of the swelling factor did not have a significant effect on the yield of nitric oxide. For example, there was only a 13% increase (from 47 to 54%) in the conversion to nitric oxide for the lOO-pm
2.0
Swelling Factor
Fig. 11. Effect of swelling factor on the devolatilization time of a lOO+m high volatile bituminous particle burning at 1700 K and 20% oxygen.
HVB coal particle, if it swelled to double its original diameter. CONCLUSIONS The following conclusions have been reached on the burning of volatile-nitrogen and the formation of nitric oxide around a single coal particle, at combustion gas temperatures and oxygen partial pressures similar to those found 1 in industrial p.f. coal flames. 1. There can be a significant conversion of the i volatile-nitrogen to nitric oxide before the liberated volatiles have reached the oxygen in the bulk gas. Conversion efficiencies as high as 54 percent have been calculated for conditions of high gas temperature and high oxygen concentrations. Small p.f. particles, i.e., those below 30 pm, tend to have lower conversions of volatilenitrogen to nitric oxide. Most of the fuelnitrogen is released as hydrogen cyanide (up to 70%) prior to mixing with the bulk gas. This implies that small particles (10 pm, say> could have a greater overall conversion to nitric oxide than the larger particles. At low temperatures ( = 1500 K) when most of the volatile-nitrogen escapes from the
NO FROM COAL VOLATILES particle as HCN, the final conversion to NO will then be strongly influenced by chemical processes occurring in the bulk gas phase. Raising the gas temperature causes a greater proportion of the volatile-nitrogen to be converted to nitric oxide in the vicinity of the particle. The amount of hydrogen cyanide entering the bulk gas is greatly reduced and the NO yield is thus determined predominantly by processes within the particle. Increasing the bulk partial pressure of oxygen causes a higher conversion of the volatile nitrogen to nitric oxide in the gas before it leaves the particle. For higher volatile, lower rank coals, the primary NO emissions are reasonably independent of rank. REFERENCES 1. Beer, J. M., Twenty-Second Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1988, pp. 1-16. 2. Solomon, P. R., and Colket, M. B., Fuel 57~749-755 (1978). 3. Abbas, T., Costen, P., and Lockwood, F. C., Combust. Flame 91:346-363
103 12. Jost, M., Leslie, I., and Kruger, C., Twentieth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1984, pp. 1531-1537. 13. Ubhayakar, S. K., Stickler, D. B., von Rosenbum, C. W. Jr., and Gannon, R. E., Sixteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1976, pp. 427-436. 14. Gat, N., Cohen, L. M., Denison, M. R., and Witte, A. B., Final Report DOE Contract No. DE-AC2281PC40273, 1983. 15. Field, M. A., Gill, D. W., Morgan, B. B., and Hawkesley, P. G. W., Combustion of Pulverised Coal, BCURA, Leatherhead, 1966, p. 178. 16. Semenov, N. N. 2. Phys. 48:571 (1928). 17. Wall, T. F., in Principles of Combustion Engineering (C. J. Lawn, Ed.), Academic, New York, 1987, p. 219. 18. De Soete, G. G., Fifteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1975, pp. 1093-1102. 19. Karr, C. Analytical Methods for Coal and Coal Products, Academic, New York, 1978, p. 156. 20. Johnson, J. L., in Chemistry of Coal Utilization, 2nd Supplementary Volume, (M. A., Elliott, Ed.), Wiley, New York, 1981, pp. 1517-1524. 21. Scheidegger, A. E., The Physics of Flow Through Porous Media. University of Toronto Press, 1960. 22. Yang, J. T., and Wang, G. G., Trans. ASME .I. Heat Transf. 112:192-200
Nineteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1982, pp.
(1991).
Chen, S. L., Heap, M. P., Pershing, D. W., and Martin, G. B. Fuel 61:1218-1224 (1982). 5. Fiveland, W. A., and Wessel, R. A., .I. Inst. Ener. 64:41-54 (1991). 6. Wendt, J. O., Second International Conference on
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Combustion
Technologies for a Clean Environment,
Calouste Gulbenkian Foundation, Lisbon, Portugal, July 1993, pp. 1-12. 7. Wang, W., Brown, S. D., Hindmarsh, C. J., and Thomas, K. M. Fuel 73:1381-1388 (1994). 8. Nelson, P. F., Buckley, A. N., and Kelly, M. D., Twenty-Fourth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1992,
pp. 1259-1267. 9. Winstone, S. W., Phong-anant, D. Baker, J. W., Gupta, R. P., and Wall, T. F., NERDDP Project No. 991 End of Grant Report, February 1989. 10. Glarborg, P., Miller, J. A., and Knee, R. J., Combust. Flame 651177-202 (1986).
11. Song, Y. H., Pohl, J. H., Beer, J. M., and Sarofim, A. F., Combust. Sci. Technol. 28:31-39 (1982).
(1990).
23. Timothy, L. D., Sarofim, A. F., and Beer, J. M.,
24. 25.
1123-1130. McAdams, W. H., Heat Transmission, McGraw-Hill Book Company, 1954, pp. 31-54. Gururajan, V. S., Wall, T. F., and Truelove, J. S., Combust. Flame 72:1-12
(1988).
Miller, J. A., and Bowman, C. T., Prog. Energ. Combust. Sci. 15:287-338 (1989). 27. Duxbury, J., and Welford, G. B., J. Inst. Ener., 26.
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28. Abbas, T., Costen, P., Lockwood, F. C., and RomoMillares, C. A., Cornbust. Flame 93:316-326 (1993). 29. Afonso, R., Dusatko, G. C., and Pohl, J. H., Combust. Sci. Technol. 93:45-51
(1993).
Maier, H., Spliethoff, H., Kicherer, A., Fingerle, A., and Hein, K. R. G., Fuel 73:1447-1452 (1994). 31. Essenhigh, R. H., J. Eng. Power 85:183-190 (1963). 32. Morris, J. P., and Keairns, D. L., Fuel 58, 465-471 30.
(1979). Received 17 October 1994; revised 7 July 1995