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The combustion and gasification of coke and coal chars
Twenty-first Symposium (International) on Combustion/The Combustion Institute, 1986/pp. 221-230
THE COMBUSTION
A N D G A S I F I C A T I O N OF COKE A N D C O A L C H A R S
G. HARGRAVE, M. POURKASHANIAN AYD A. WILLIAMS Department of Fuel and Ener~, The University of Leeds
The oxidative reactivities of two cokes, two laboratory prepared chars and one char produced in situ in a methane/air flame were measured in various gaseous environments in the temperature range of 770-2000 K. The methods used consisted of methane/air flame for the highest temperatures, a falling particle reactor for intermediate temperatures and an isothermal thermogravimetric method for the lowest temperature range. The rate of mass loss was inferred from the particle surface temperatures in the first two techniques and was measured directly in the third. Fragmentation was observed when cold coke or char particles were injected into the high temperature environments but not with the char formed in situ from the coal. The chemical reaction rates for these samples depend on the physical nature and morphology of the chars. Low temperature measurements showed that the reactivity of the cokes and chars varied by about three orders of magnitude. At high temperatures the difference in reactivity is reduced to one order of magnitude, The intrinsic reactivity of the petroleum and metallurgical coke was higher than the other chars, especially at high temperature. In general the intrinsic reactivity is reduced as the surface area of the sample particle increased. The activation energies obtained using two very different techniques were in agreement with results of other workers. In addition, the functional variation of burning rate with temperature were used to investigate the effect of different reaction zones on the activation energies.
1. Introduction T h e r e is considerable interest in the m o d elling o f the c o m b u s t i o n or gasification behaviour o f coals, chars a n d cokes. T h e p r o b l e m is c o m p l i c a t e d because o f the wide r a n g e o f coal types and the fact that the reactivity o f a coal c h a r or coke is very m u c h d e t e r m i n e d by the reaction conditions d u r i n g its f o r m a t i o n 1. T h e s e conditions, particularly the h e a t i n g rate, determ i n e the surface area, porosity and crystallog r a p h i c structure o f the c h a r o r coke. O f these the porosity is very i m p o r t a n t since it can control the rates o f diffusion o f chemical species into a n d o u t o f the c h a r s t r u c t u r e a n d the extent o f the reactive area. T h e effect o f t e m p e r a t u r e , p r e s s u r e and gaseous e n v i r o n m e n t s on the rate o f c o m b u s t i o n o r gasification on coal c h a r or coke has b e e n extensively studied by various investigators 1-4. N e v e r t h e l e s s significant problems remain, particularly in relation to the factors that d e t e r m i n e the chemical reactivity of the char. T h e c h a r b u r n - o u t reaction is relatively slow a n d d e p e n d s on the reaction of gaseous oxygen, as well as with c a r b o n d i o x i d e and water v a p o u r , with a p p r o p r i a t e l y o r i e n t a t e d carbon atoms in the char, this in t u r n b e i n g d e t e r m i n e d by the way the c h a r is f o r m e d . T h e s e reactions are not well u n d e r s t o o d a n d f u r t h e r i n f o r m a -
tion is r e q u i r e d on the factors controlling these processes. T h e p r e s e n t study r e p o r t s data on the oxidation and gasification o f d i f f e r e n t types o f coke and c h a r in a t e m p e r a t u r e r a n g e o f 7 7 0 - 2 0 0 0 K using two e x p e r i m e n t a l techniques. T h e s e involved firstly the i n t r o d u c t i o n o f sample particles into an electrically h e a t e d 'falling particle' reactor, and secondly the use o f a m e t h a n e - a i r flame as the source o f the gaseous e n v i r o n m e n t . T h e two-colour p y r o m e t e r t e c h n i q u e was used to m e a s u r e the surface t e m p e r a t u r e and velocity o f the particles, and the physical changes in the samples were e x a m i n e d using electron microscopy. A t h e r m o g r a v i m e t r i c t e c h n i q u e was used to obtain reaction rates in air atmospheres. T h e c o m b i n a t i o n o f these d i f f e r e n t techniques enable data to be o b t a i n e d o v e r a wide range of conditions for overall b u r n i n g rates, reaction o r d e r and activation energies. In addition the relative reactivities o f c h a r a n d coke in air were c o m p a r e d and the effect o f t e m p e r a t u r e , char surface and porosity w e r e e x a m i n e d .
2. Experimental Method E x p e r i m e n t s w e r e u n d e r t a k e n on a UK b i t u m i n o u s coal f r o m M a r k h a m Main (NCB
221
222
COAL COMBUSTION
sample particles by means of a vibrated fluidized rank 702), two bituminous coal chars derived bed. Individual particles were injected into the from the coal, and two types of coke, a petroleum coke (Conoco) and a hard metallurgical flame through a 5 m m diameter orifice in the centre of the b u r n e r . Particles were collected coke. These samples were crushed, g r o u n d and sized by dry sieving a n d the fraction used stored after various residence times in the flame on 0.2 u n d e r an argon atmosphere. The n u m b e r arith~m polycarbonate Nuclepore filters, subsequent metic mean diameter of this fraction was deteranalysis being made by electron microscopy. mined by a Quantimet 720 Analyser and was In situ measurements of particle velocity and found to be 85 I~m with 95% of the fraction temperature were made by the two-colour between 78 and 92 t~m. The coal chars were pyrometer a r r a n g e m e n t 5'6. The optical fibre prepared from the bituminous coal (sample A) collected radiation from particles present in a by pyrolysing the coal u n d e r an argon atmo- control volume defined by a 0.3 mm by 1.0 mm sphere at different heating rates. Char B was slit. The emitted light was viewed in two narrow prepared at a low heating rate (10 K/rain) in a wavelength bands by means of interference quartz tube heated in a furnace, and char C was filters centred on 550 nm (8 n m band pass) and prepared by the flash pyrolysis of the coal A on 649 nm (10 n m band pass), and the intensity an electrically heated grid at a high heating rate was measured by two calibrated photomuhi(1000 K/s). The final pyrolysis temperature in plier tubes using a data acquisition system. both cases was 1223 K and samples were cooled (b) Combustion experiments on char partito room temperature u n d e r argon. The proxi- cles in air were conducted in an electrically mate and ultimate analyses for all samples are heated vertical reactor consisting of a I00 m m given in Table I, together with particle sizes, diameter silica tube which is 2 m long with surface areas, and water densities. Maceral viewing ports at 20 cm intervals 7. The sample analyses showed that the coal contained 74% particles were introduced at a rate of 5 mg/s Vitrinite, 10% Exinite and 16% Inertinite. Surinto a hot zone through a water-cooled probe. face area measurements were carried out using a Once in the reactor the particles are heated Quantisorb continuous flow surface analyser rapidly by radiation from the furnace wall and using N J H e mixtures at 77 K; N2/He was used conduction from the hot gases. The gas and because it largely excludes the area contained in wall temperature of the reactor were adjusted the micropores and is consistent with the apto be equal to each other and were controlled in proach of other investigatorsk Before use the the range of 773-1223 K. The surface temcoal samples were conditioned to remove mo- perature and velocity of the sample particles lecular oxygen u n d e r a continuous nitrogen were measured at several stations along the flow at 105~ tor 7 hours. The water density was reactor using the two colour pyrometer techused as a measure of true density because it niques described above. corresponds closely to fielimn density for cokes (c) The reactivity of the char and coke and chars, and porosity was also calculated from samples in air at 1 atmosphere pressure at apparent and true densities. lower temperatures (770-1200 K) were meaThree experimental techniques were used to sured using an isothermal thermogravimetric examine the combustion and gasification of technique 7'~. Typically about 5 - 8 mg of coke or individual coke or char particles, these being char particles were spread uniformly in a monolayer in a stainless steel mesh basket (1 • (a) a methane/air flame into which char parti1 cm) which was suspended from an arm of the cles were injected, was used to simulate gasifica- microbalance by a quartz fibre. At these loadtion in the temperature range of 1300-2000 K, ings the size of the sample used is not importhe 02 and CO2 concentrations were in the tant. A thermocouple was h u n g parallel to the ranges of 1-18 and 3-11 mol % respectively. quartz fibre to give the temperature of the char The b u r n e r consisted of a 70 m m diameter sample, and the change in the weight of the water-cooled sintered brass disk, and methane/ sample was recorded by a microprocessor data air mixtures were b u r n e d as a flat flame 2 mm recording system. T h e thermocouple was situabove the bed surface, and in some experiments ated 1 to 3 m m from the char particles. the flame was protected by a housing containing windows. The flame gas composition was deter3. Experimental Results mined by sampling using a silica probe, and a gas chromatograph. Flame temperature profiles were measured using a Pt-Pt 13% Rh thermo- Experiments at High Temperature couple of 50 t~m diameter and corrected for The high temperature experiments were radiation. Part of the premixed methane/air undertaken using a methane~air~oxygen flame supplied to the b u r n e r was seeded with the and the vertical electrically heated falling parti-
COMBUSTION AND GASIFICATION OF COKE AND CHAR cle furnace. T h e samples used r e p r e s e n t a wide r a n g e o f carbon samples with d i f f e r i n g crystall o g r a p h i c structure a n d surface activity, surface area, density and ash c o n t e n t (Table 1). E x p e r i m e n t a l data w e r e obtained o v e r a r a n g e o f reaction conditions o n particle surface t e m p e r a t u r e s a n d these w e r e c o n v e r t e d to give overall b u r n i n g rates. E x p e r i m e n t s in the falling particle reactor c o v e r e d a gas t e m p e r a t u r e r a n g e o f 773 to 1223 K with particles being in o n e a t m o s p h e r e p r e s s u r e o f air and with particle surface t e m p e r a t u r e s o f 1 5 0 0 - 1 8 0 0 K. Studies using the m e t h a n e / a i r flame were und e r t a k e n with gas t e m p e r a t u r e s in the r a n g e o f 1400 to 2000 K, a n d with particle surface t e m p e r a t u r e s o f 1520 to 2440 K. T h e equations used to d e t e r m i n e the chemical (P~) and intrinsic reactivity (Pi) of the samples to o x y g e n were essentially those described by 9 Field et al 2 and S m i9t h ~0111z ' ' ' . T h e che mical reaction rate coefficient, R , is obtained f r o m the relationship b e t w e e n d i f f u s i o n reaction rate coefficient RD and R~: X R~ -=RDPG~('-l) (1 - X )- ~
(1)
w h e r e Po~ is the o x y g e n partial pressure in the s u r r o u n d i n g gas, R is the gas constant, Tp is the particle t e m p e r a t u r e , n is the reaction o r d e r and X is the ratio o f actual b u r n i n g rate to m a x i m u m b u r n i n g rate. T h e rate coefficient for o x y g e n diffusion to the particle, RD, is e x p r e s s e d as
223
w h e r e t~ is the m e c h a n i s m factor 2, D is the diffusion coefficient o f o x y g e n in the gas, d~ is the particle d i a m e t e r and Tg is the s u r r o u n d i n g gas t e m p e r a t u r e . Since most o f the reaction occurs within the porous structure o f the particles it is more m e a n i n g f u l a'9 to express the char reactivity in terms o f reaction between gases and porous solids, ie
Ri = R~/Ag~,crp'q
(3)
w h e r e Ri is the intrinsic reaction rate coefficient, Ag is the specific surface area, ~ is the characteristic d i m e n s i o n o f the particle arid "q is the effectiveness factor, calculated using T h i e l e ' s m o d u l u s in a u n i m o d a l p o r e systemL In o r d e r to m a k e c o m p a r i s o n of the reactivities of the coke a n d c h a r samples, on a same o x y g e n partial p r e s s u r e the chemical reaction rate, pc, and the intrinsic reaction rate, Pi, were calculated by m e a n s o f the relations
p~ = R~(Po~)"
(4)
o, = R,(P~)~)"
(5)
and
T h e reaction o r d e r at low f u r n a c e t e m p e r a tures were calculated f r o m the relationship b e t w e e n PE and Cx e x p r e s s e d in equation (6):
(6)
pE-BAgC~ exp R _
RD = 24 O D/dp R ((Tp + Tg)/2)
n
_
_
(2)
TABLE 1. Elemental and proximate analysis of cokes, char and parent coal and Particle Parameters
Fuel Source % moisture % volatile matter % ash % fixed carbon Swelling no. C% H% N% Diameter (p.m) Surface area (mZ/g) Water density (g/cm ~) Porosity (%)
Coal (A)
Coal Char (B)
3.61 35.90 2.98 57.51 1.5 76.40 4.88 1.67 85.6 12.50
4.22 5.80 4.97 85.01 -87.37 1.76 1.73 87.4 29.4
3.25 4.14 5.42 87.24 -90.64 0.97 t.91 87.5 74.5
1.14 0.89 0.07 97.91 -98.7 0.37 0.79 87.4 2.10
1.39
1.61
1.44
1.93
34
46
Coal Char (C)
51
Petroleum Coke (D)
14
Met
Coke (E) 2.40 1.10 7.9 88.6 89.43 0.72 1.15 85.5 3.25 1.87
19
224
COAL COMBUSTION
where Pe is the observed reaction rate of sample per unit external surface area, B is the preexponential factor, Cg is the concentration of oxygen, Et is the true activation energy. T h e true activation energy can be calculated from the measurements of pe at different temperatures, The reaction order at high temperature (zone II) was obtained following the method described by SmithS). The overall b u r n i n g rates of particles per unit external surface area, 9o, can be calculated from the experimental data obtained from the twocolour pyrometer measurements. It is assumed that particles are in thermal equilibrium with their surroundings 13. Therefore we obtain: Q,-..... + Q~aa = QE
(7)
where Q~..... and Qra~ are the rate of heat loss by convection and radiation respectively, and QE represents the rate of heat generation per unit area. Equation 7 can be rewritten as
%,
LOGIO 0 -rr,
....
j ......
I,rL
.........
t .........
,
........
1
-3 ~ '
Zone I
-zl
-~ "~ -5 ~
V 9 § Zone Ii
9
4-
-6
-7 , 1 I I
-8
I
Po = (h(Tp - T~) + ~cr(Tp 4 - T,.4))l~rt
(8)
--~
. . . . . . . .
, ......
15
where h is the heat transfer coefficient, ~ is emissivity of the particle, ~r is the Stefan-Boltzmann constant, and Tm is the medium gas temperature. T h e value of the heat released at the surface per unit mass of carbon burnt, M-/, was assumed 2 to be 9.781 MJ/kg. In this way values of X and therefore the reaction rate pc were obtained for all the samples and these are given in Fig. 1. The reaction rates given for coal A represent the combustion of the char immediately following devolatilisation. Intrinsic reactivities Pi, obtained by means of equations 3 and 5 are given for all these samples in Fig. 2 and the intrinsic reactivities tot all samples when Po2 is at I atmosphere pressure are summarised in Fig. 3. Activation energies relating to the chemical reaction rates, Pc, derived from Fig. 1 for the high temperature region are given in Table 2. Since kinetic data based on mass loss can be invalidated by fragmentation of the particles the method we used and described earlier was only dependent on the surface temperature and not on particle size. However, since the occurrence of particle fragmentation is of great practical interest then samples were examined by scanning electron microscopy (max x 60,000). It was found that in the case of petroleum coke with<2% volatile matter, particularly in high temperature flames, cracks appeared in the surface of the particles. With residence times equivalent to 30% b u r n o u t cracks appeared, with 60% mass loss fragmentation was observed to take place for 70% of samples. At lower
J,,i
. . . . . . . . .
8
i . . . . . . . . .
i .....
t0
t2
,,,I
t4
104 x l/Tp (K-l)
FIG. 1. Variation of chemical reaction rate, Pc, with particle temperature: char from Coal A, A; char sample B, V; char sample C, + ; petroleum coke, • ; metallurgical coke,m temperatures (ca 1000 K) and lower heating rates, cracks only appeared when 40% of the mass had been lost. With chars, with higher volatile matter (<6%) cracks were observed but not fragmentation. However, at above 80% carbon conversion disintegration o'f the char particle was observed. With the coal, no fragmentation was observed in the char particles produced in the flame u n d e r any of the conditions studied here. Fig. 4 shows SEM photographs of (a) the formation of cracks on the surface of petroleum particles, and (b) where fragmentation has taken place. Experiments at L o w Temperature
At lower temperatures data on mass loss was determined by the thermogravimetric method over the temperature range of 770 to 1200 K in air for the two chars and the two cokes, and for the coal which was decomposed in situ prior to the reactivity measurements. Data from the thermogravimetric measurements were analysed by the method described by Smith s. T h e observed rate of coke or char b u r n i n g (PE) were determined directly from the initial weight of
COMBUSTION AND GASIFICATION OF COKE AND CHAR LOG~O O__
225
LOGIO
-I
Ky•
o i = 52 exp( T-R~6 .5 )
-2
-3 -4
-4
c,,
X
++V
x
x
A
I I t .
4
6
8
iO
~2
t4
.
a
T0 4 1/T (K -1 )
.
.
.
.
.
6
.
.
.
.
.
.
.
.
.
.
8 10 4 I / T
.
.
.
.
.
.
.
.
.
I0
.
.
.
.
.
12
t4
(K -1 )
FIG. 2. Relation between intrinsic reactivity, p, and particle temperature: Symbols as in Fig. 1.
FIG. 3. Comparison of intrinsic char and coke reactivity in oxygen (at an oxygen pressure of 1 bar): Symbols as in Fig. 1;----correlation of Smith 3.
the particles and the w e i g h t o f sample r e m a i n ing at any time. T h e relationship b e t w e e n OE, the observed b u r n i n g rate o f c a r b o n p e r u n i t external surface a r e a and the c h e m i c a l reactivity is expressed by e q u a t i o n (9)
with the particle surface the particle t e m p e r a t u r e was calculated by the heat balance m e t h o d 2. T h e r e was in fact little d i f f e r e n c e b e t w e e n the m e a s u r e d a n d calculated particle t e m p e r a t u r e s but the latter was c o n s i d e r e d to be the f u n d a m e n t a l l y m o r e accurate m e t h o d a n d was used in the calculations. T h e results o b t a i n e d are plotted in Fig. 1 for the chemical reaction rate a n d in Fig. 2 for the intrinsic reaction rates as before. Low t e m p e r a t u r e activation e n e r g i e s o b t a i n e d f r o m Fig. 1 are given in T a b l e 2. In Fig. 3 the data are for 1 a t m o s p h e r e p r e s s u r e o x y g e n to p e r m i t comparison with the h i g h t e m p e r a t u r e data.
Rc = [RD R E / ( R e -
Re)]"(pe) 1-n
(9)
in place o f e q u a t i o n (1) used for the high t e m p e r a t u r e data. E q u a t i o n (9) is not very d e p e n d e n t on the accuracy o f the particle t e m p e r a t u r e and only RD uses it directly. Howe v e r since the t h e r m o c o u p l e was not in contact
TABLE 2. Activation Energies and Pre-exponential Factors for High and Low Temperatures Zone II
Zone I Samples
Coal char (A) Char (B) Char (C) Metallurgical coke (D) Petroleum coke (E)
6.1 7.1 6.8 5.7 3.0
x x x x x
l0 s 103 l0 s 103 103
Etrue
Eapparent
kJ/tool.
kJ/tool.
143.9 153.1 137.4 168.3 174.1
41.4 45.3 31.3 34.2 25.6
76.8 83.4 69.2 90.1 95.5
226
COAL COMBUSTION
FIG. 4. Electron micrograph of petroleum coke particle in CH4/air flame, (a) formation of cracks on the surface followed by (b) fragmentation of particle.
4. D i s c u s s i o n and Conclusion
Reactivity and Reaction Kinetics T h e variation o f chemical reaction rate as function of particle t e m p e r a t u r e for all samples are shown in Fig. 1. T h e dashed line separates the two reaction zones observed. At low temperature diffusion resistance does not affect the chemical reaction rate which is taking place at the surface o f the char or coke particle (zone I) and therefore the chemical reaction is the rate determining step. T h e activation energy obtained from e x p e r i m e n t a l data in this stage (zone I) is the true activation energy. In zone II the reaction rate d e p e n d s on the rate of both diffusion into the pore and the chemical reaction behaviour at the pore walls, and the activation energy obtained experimentally is approximately one-half of the true value. Values obtained in zone II represent the influence of both pore structure and chemical reaction rate and because o f this are more significant in many practical applications. At higher particle temperatures, the reaction rate becomes very
rapid and it is limited by the transportation o f oxygen t h r o u g h the b o u n d a r y layer a r o u n d the particle. In this zone (zone III) the experimentally d e t e r m i n e d activation energy is small. During this investigation zones I, II and the transition region from zone II to zone I I I were observed d u r i n g combustion of coke or char particles. T h e activation energies obtained from experimental data for both these zones are shown in Fig. 3. T h e variation in the chemical reactivity o f the samples indicates that in both zones the char C has the highest reactivity and petroleum coke was the least reactive; the char from coal A produced directly in the methane/air flame had a reactivity lower than char C. T h e activation energies have values shown in Table 2 in the range of 173-137 kJ/mol in zone I and reduces to 9 5 - 6 9 kJ/mol in zone II. T h e experimental values obtained for zones I and II fall within the range expected for the true and a p p a r e n t activation energies for i m p u r e carbons. (Etude = 168 kJ/mol). Differences in chemical reactivity of the samples can be related to the original physical properties of the samples. Increase in the volatile matter present in the coke or char samples from 1-5.5% increased the observed burning rate, which in turn affected the chemical reaction rate. T h e chars p r o d u c e d by a high heating rate and m e d i u m final pyrolysis temperature o f the p a r e n t coal had a higher chemical reactivity than chars p r o d u c e d with a low heating rate or high final pyrolysis temperature and high heating rate. No correlation could be f o u n d between the densities and moisture content o f samples with chemical reactivity. However experimental results indicated that the observed reaction rate could be influenced by the mineral matter present in the char. T h e difference in chemical reactivity of samples becomes relatively small at high particle temperature, as at high t e m p e r a t u r e the chemical reaction rate becomes fast so the effect of the internal surface area becomes negligible. Fig. 2 shows the relationship between intrinsic reactivity and t e m p e r a t u r e of the particles for the samples investigated. This is a more realistic treatment o f the coke or char reactivity because it eliminates the effect of pore size and pore surface area a n d therefore provides information on the reactivity of the chars on the basis of f u n d a m e n t a l chemical reaction rates. The active surface area in intrinsic reaction rate becomes significant, especially at low particle temperatures. It was found that the intrinsic reactivity of the cokes differ by a r o u n d two to three orders o f magnitude; the petroleum coke had the highest intrinsic reactivity followed by metallurgical coke, and the char C is less
COMBUSTION AND GASIFICATION OF COKE AND CHAR reactive. Smith 11 suggested that the presence o f the inhibiting impurities and the effect of atomic structure of the carbon influences the intrinsic reactivity o f the coal. Fig. 3 compares the intrinsic reaction rates of all the samples investigated at low and high temperatures. T h e activation energy obtained from the least square regression over all the data points shows a value o f 161 -+ 6 kJ/mol. Smith 3 r e p o r t e d the value o f 179.4 kJ/mol for an activation energy for a wide range o f chars and carbon samples and his correlation is shown in Fig. 3. Because of his inclusion of carbons we consider o u r data to be more accurate for chars and cokes. T h e true and a p p a r e n t activation energies obtained from chemical reactivities are in a good agreement with values d e t e r m i n e d by other investigators a'3'4'13 for similar samples.
Fragmentation of coke or char particle In recent years theoretical and experimental work has been carried out to investigate the effect of porosity, chemical and physical composition and combustion environment on char fragmentation 14-x6. During this investigation electron microscope studies of coke and char particle at high gas t e m p e r a t u r e s indicated that cracks a p p e a r at the surface of the particles followed by fragmentation as shown in Fig. 4. In most cases fragmentation of coke particles takes place at a r o u n d 6 0 - 7 0 % conversion in comparison with chars which disintegrate at a higher conversion stage. T h e result showed that cracks and f r a g m e n t a t i o n depends on the physical properties o f the char or coke. At high t e m p e r a t u r e the b u r n i n g rate o f the petroleum coke is controlled mainly by oxygen diffusion and external mass transfer is a main rate limiting factor. T h e r e f o r e there is a steady decrease in the d i a m e t e r o f the particles and the density remains approximately constant d u r i n g combustion process, and internal surface reaction is less significant. T h e electron microscope studies showed that when the diameter of the particle is r e d u c e d by 3 0 - 5 0 % of the initial size cracks were visible at the surface of the particle followed by fragmentation of almost 2 0 - 3 0 % when the carbon conversion exceeds 60%. Clearly stress effects occur which result in fragmentation. However, previous investigations "''is suggest that morphology of chars depends on the coal rank and maceral type of individual coal particles. T h e chars p r o d u c e d from coal A were mainly thick walled ballon type chars which is generated from the vitrinite. SEM studies showed that disintegra-
227
tion of the char occurred at a higher conversion stage than petroleum coke (>80% carbon conversion). However, fragmentation in the char did not follow a similar r o u t e and in some case separation of 50% of the particles were observed. T h e fragmentation of the particles during combustion process could have an important impact on weight loss. I n d e e d our measurements indicated that fragmentation can reduce the initial weight o f carbon by a r o u n d 20-30% and therefore introducing serious e r r o r in the weight loss calculation obtained from the size variation of carbon particles in CH4/air flame when d e t e r m i n e d by optical techniques 19. In addition the size distribution of flyash produced d u r i n g the char combustion stages can be influenced by fragmentation of the particles. Reduction in size distribution of the flyash due to fragmentation can affect the process of removal of the flyash in reactors. T h e r e f o r e the carbon fragmentation must be included in calculations of b u r n i n g rates and design of the reactors.
Concluding Remarks A considerable a m o u n t o f information is now available (for example refs. 17, 20-22) on the combustion and gasification of Markham Main coal. W h e n it u n d e r g o e s decomposition: coal
~char + tar + gases
the initial reaction can be described 23 by the reaction rate k = 10 13 e x p ( - 2 4 , 6 5 0 / T ) s- 1 . T h e tar consists of high boiling components which remain in the liquid phase together with volatile components and gases. Generally sufficient kinetic data are available to describe the rate of oxidation of the hydrocarbons and gaseous components. T h e rate o f b u r n i n g of the tar droplets can be described 24 by a modified dZ-burning law, the value being 3 x 10 .3 g c m -2 s- 1 a t l 0 0 0 K a n d 4 x 10- 3 g c m - 2 s- 1 a t l l 5 0 K . T h e reactivity o f the char that is p r o d u c e d is d e p e n d e n t on its heating rate, the maceral content of the coal from which it is f o r m e d and on its ash constituents. T h e char formation condition plays an important role on the chemical reactivity especially in zone I. T h e char p r o d u c e d at high heating rate and m e d i u m final pyrolysis temperature had a h i g h e r reactivity than the char p r o d u c e d at high heating rate and high final pyrolysis temperature. Such differences become less significant in zone II, because the
228
COAL COMBUSTION
c h a r m o r p h o l o g y (porosity a n d p o r e size) is n o w t h e i m p o r t a n t p a r a m e t e r in d e t e r m i n i n g reactivity. I n a d d i t i o n t h e c h a r s p r o d u c e d in i n e r t gas t e n d s to b e m o r e o p e n t h a n c h a r s p r o d u c e d in air o r CO2, a n d t h e final pyrolysis temperature and heating rate of char formation i n f l u e n c e s t h e d e v e l o p m e n t o f i n t e r n a l s t r u c t u r e a n d r e a c t i v e s u r f a c e area. It also d e t e r m i n e s t h e c o n c e n t r a t i o n o f r e s i d u a l volatile m a t t e r w h i c h in t u r n i n f l u e n c e t h e n u m b e r o f reactive sites. T h e c h a r s h a v e a h i g h e r c h e m i c a l reactivity t h a n the p e t r o l e u m a n d m e t a l l u r g i c a l coke, b u t t h e i n t r i n s i c reactivity f o r t h e cokes a r e h i g h e r t h a n the c h a r s . T h i s reflects t h e i n f l u e n c e o f t h e reactive s u r f a c e a r e a .
Acknowledgements We wish to thank British Gas for financial support under the Fellowship Scheme and Mr. M. Chan for assistance with thermogravimetric experiments.
REFERENCES 1. WELLS, W.F., KRAMER, S.T., SMOOT, L.D., BLACKHAM, A.V.: Twentieth Symposium (International) on Combustion, p. 15-39, T h e Combustion Institute, 1984. 2. FIELD, M.A., GILL, D.W., MORGAN, B.B. AND HAWKSLEY, P.G.W.: Combustion of Pulverized Coal, p. 89, BCURA, Leatherhead, 1967, 3. SMITH, I.W.: Nineteenth Symposium (International) on Combustion, p. 1045, The Combustion Institute, 1982. 4. MULCAHY, M.F.R. AND SMITH, I.W.: Rev Appl. Chem. 19, 81, 1969. 5. TIMOTHY, L.D., SAROFIM, A.F. AND BEER, J.W.: Nineteenth Symposium (International) on Combustion, p. 1123, T h e Combustion Institute, 1982. 6. MURDOCH, P., POURKASHANIAN, M., AND WILLIAMS, A.: Second European Conference on Coal Liquid Mixtures, p. 177, I.Chem.E. Symposium Series No. 95, 1985.
7. MURDOCH, P., POURKASHANIAN, M. AND WILLIAMS, A.: Twentieth Symposium (International) on Combustion, p. 1409, The Combustion Institute, 1984. 8. JORDAN, J.B. ANn WILLIAMS, A.: Alternative Hydrocarbon Fuels, Combustion and Chemical Kinetics, Progress in Astronautics and Aeronautics, Vol. 62, p. 180, 1978. 9. LEWIS, P.F. ANn SIMONS, G.A.: Combustion Science and Technology 20, 117, 1979. 10. LAURENDEAU,N.M.: Prog. Eng. Comb. Science 4, 221, 1978. 11. SMITH, I.W.: Fuel 57, 409, 1978. 12. HAMOR, R.J., SMrrH, I.W. AND TYLER, R.J.: Combustion and Flame 21, 153, 1973. 13. MITCHELL, R.E. AND McLEAN, W.J.: Nineteenth Symposium (International) on Combustion, p. 1113, T h e Combustion Institute, 1982. 14. GIVINOT, J., AUDIER, M., COULON, M., AND BONNETAIN, E.: Carbon 19, 95, 1981. 15. KERSTEIN, A.R. AND NIKSA, S.: Twentieth Symposium (International) on Combustion, p. 941, The Combustion Institute, 1984. 16. SUNDBACK, C.A., BEER, J.M. AND SAROFIM, A.R.: Twentieth Symposium (International) on Combustion, p. 1495, T h e Combustion Institute, 1984. 17. STREET, P.J., WEIGHT, R.P. AND LIGHTMAN, P.: Fuel 48, 343, 1969. 18. OKA, N,, MURAYAMA, T., MATSUOKA, H., YAMADA, S., YAMADA, T., SHINOZAKI, S., SHIBAOKA, M. AND THOMAS, C.G.: Proceedings of International Symposium on Fundamentals of Catalytic Coal and Carbon Gasification, May 1986. 19. TICHENOR, D.A., MITCHELL, R.E., HENCKEN, K.R. AND NIKSA, S.: Twentieth Symposium (International) on Combustion, p. 1213, The Combustion Institute, 1984. 20. CAUSTON, P. AND MCENANEY: B. Fuel 64, 1447, 1985. 21. DEW, B.A. AND ROBINSON, S.C.F.: Fuel 64, 861, 1985. 22. DESYPRIS, J., MURDOCH, P. AND WILLIAMS, A.: Fuel 61, 807, 1982. 23. JOHNSON, G., MURDOCH, P. AND WILLIAMS, A.: Submitted for publication. 24. POURKASHANIAN, M. AND WILLIAMS, A.: Submitted for publication.
COMMENTS I. W. Smith, CSIRO Division of Fossil Fuels, Australia. It is unusual to find in practice that in zone IIEA -" 2 x Ezo,,, i due to problems of evolving pore structure, transitions between regimes, etc. Do you plan to take better pore structure measurement and use better
pore structure models to rationalize your Zone I and Zone II models? Author's Reply. We agree with Mr. Smith's comments and during this investigation we were of course con-
COMBUSTION AND GASIFICATION OF COKE AND CHAR cerned with the effect of the pore structure model employed and the separation of Zone I and II regimes. We found that the activation energies in Zone II were roughly one half of the true value in Zone I with a variation of 5-8%. Our experimental data indicates that the transitional stage, where both Zone I and Zone II behaviour occurs, makes it difficult to separate the theoretical activation energies with precision. We also agree that knowledge of internal surface area, the assignment of accurate diffusion coefficients, and development of the pore structure are important factors in determining heterogeneous char combustion. In this investigation we have used both uni- and bimodal models of pore structure and obtained similar results, thus giving some evidence of the accuracy of our results. However, it is necessary to improve models of pore system development, taking into account particle fragmentation, blind pore and pore coalescence.
S. Srinivasachar, MIT, USA. Is the BET N2 surface area the correct one to use for deriving reaction rates of char oxidation (from experiiments)? How do you account for the varying surface area as a function of reaction extent, and that not all the surface area available is put to use in the oxidation reaction? Author's Reply. We believe that the BET N2 surface area is the best one to use because the measurement relates to pores with radii greater than 5 A, which we calculate to be more representative of the surface area available for reaction with molecular oxygen. The development of char surface area during the combustion process was predicted at high temperatures by means of a theoretical model (as described in the paper) or measured directly during burn-off in the thermogravimetric measurements.
R. E. Mitchell, Sandia National Laboratories, USA. In order to derive reaction order from experimental measurements, one must vary Oz levels over a considerable range. Over this range, the effects of Stefan flow become appreciable. Yet, you neglect Stefan flow in your analysis. I assert that your results for reaction order are influenced by this neglect. Author's Reply. It is certainly possible that under certain conditions convective flow of the gaseous medium may take place normal to the carbon surface (Stephan flow). 1 However this only amounts to a second-order correction in most chemically reacting systems.2 In this investigation the effect of Stephan flow was negligible for an oxygen concentration up to 12%. Using expressions given by Frank-Kamenetskii2 the effect is found to be less than 5%. At higher oxygen
229
concentrations and at higher temperatures neglect of the Stephan flow can influence the values of apparent activation energy and apparent reaction order in the diffusion limit. However, in these experiment the higher oxygen concentrations were used tor the thermogravimetric experiment at lower initial temperatures and the consequential effect is small.
REFERENCES 1. STEPHAN,J., Ann. Phys., 17, 550(1882) 41,725, (1890). 2. FRANK-KAMENETSKII, D.A., Diffusion and Heat Exchange in Chemical Kinetics, Plenum Press, London, 1969.
R. H. Essenhigh, Ohio State University, USA. 1) Did reactivity increase with decreasing density, or with increasing density? 2) Could cracks he responsible for fast access of O2 to the particle interiors and be responsible for higher "intrinsic" reactivity? In flame studies with coals, chars, and petroleum cokes we have been successful in modelling the flames using the same kinetic constants, but flame differences were found to be due to the mode of reaction: shrinking core or shrinking density (see the 15th Symposium). The macropore volume appeared to be critical in determining the mode of reaction. Cracks may do the same thing. What is your evaluation?
Author's Reply. We attempted to find correlation between intrinsic chemical reactivity and ash, volatile matter, H content and true density, We could only obtain a correlation with true density and the reactivity increased with increasing density, suggesting that the level of graphitisation is the important factor. The formation of cracks can increase the active surface area but in our experiments our kinetic data was primarily derived from char particles in circumstances in which cracking was small. In general we believe that the influence of cracks is small, however the occurrence of fragmentation can have a significant effect on its mode of reaction.
S. Niksa, Stanford University, USA. Considering the broad temperature range represented in your TGA and flow reactor data, and the body of data which suggests substantial production of CO2 at moderate temperatures, can you assess the influence of simultaneous production of CO and COz on your analysis? Specifically, is the magnitude of the discrepancy between the measured and calculated temperatures in the TGA experiments indicative of the larger reaction exotherm for CO2 production?
230
COAL COMBUSTION
Author's Reply. In this investigation it was assumed that carbon particle reacts with oxygen to p r o d u c e CO or CO2 and their a m o u n t d e p e n d i n g on experimental conditions. At p r e s e n t there is no reliable method to obtain exactly the a m o u n t o f CO or CO2 f o r m e d at the surface o f small (<100p~m) carbon particle d u r i n g combustion process. T h e r e f o r e the relationship r e c o m m e n d e d by Smith (Ref. 3) a n d Field et al (Ref. 2) are used in this investigation. In CH4/Air flame reactor w h e r e the particle temperature was >1500 K, it is assumed that CO is the primary product, and we have no evidence to the contrary. H o w e v e r in the thermogravimetric measurements it was a s s u m e d that CO2 as well as CO is the primary p r o d u c t w h e r e ratio o f CO2: CO varied with temperature (Ref. 2). T h u s for the reaction: OC + O2---~ 2 (O - 1) CO + (2 - tO) CO2 where tO = 2 ( 1 - to) + to, to = fraction o f carbon oxides f o r m e d as CO2 We also assume that AH = 33.02 (2/tO - 1) + 9.78 (2 - 2/tO) MJ/Kg. A comparison o f calculcated and measured particle temperature indicated that CO2: CO ratio decreased from 2.8 to 0.20 as the t e m p e r a t u r e increased. T h e r e f o r e we agree that to make the assumption that carbon burns to p r o d u c e only CO2 or CO can cause an error in reactivity calculation, especially below 1200 K, and more i n f o r m a t i o n is required on the nature o f the oxidation p r o d u c t for small particles.
many practical applications, one can expect to find a rather high a m o u n t o f mineral matter at least in a fraction o f the pulverized coal. How do you expect this will affect the concept o f an intrinsic reaction rate and the global reaction rate o f the particles as a whole?
Author's Reply. T h e role o f ash is significant in practical applications. Whilst we used relatively low ash fuels in o u r paper, many industrial fuels initially contain 2 0 - 3 0 % ash and in the latter stages o f combustion may contain 80% ash before disintegration o f the char particle. Although the kinetic role played by ash in such circumstances is not well defined, in general its effect on combustion is twofold, namely: (a) a catalytic effect o f ash on the h e t e r o g e n e o u s carbon oxidation, and (b) an increase in diffusion resistance, these two effects opposing one a n o t h e r as the ash content increases. T h e char reaction rate will increase with an increasing a m o u n t o f metal catalyst but will saturate when the active surface area is completely covered, and the catalyst effect o f impurities decreases with increasing t e m p e r a t u r e m. T h e r e f o r e probably ash is unlikely to have any m a r k e d affect on reactivity at the pulverised fuel combustion temperatures. T h e formation o f the ash shell a r o u n d the char particle will increase the diffusion resistance which in turn will reduce the reactivity. It is t h e r e f o r e important to include correction terms for both these effects, otherwise predicted char lifetimes are shorter than that realised in practice. REFERENCE
W. Zinser, University of Stuttgart, Germany. I f you are investigating coal with an ash content as f o u n d in
HAWTIN, P., AND GmsoN,J.A., Carbon, 4, 501, (1966).
THE COMBUSTION
A N D G A S I F I C A T I O N OF COKE A N D C O A L C H A R S
G. HARGRAVE, M. POURKASHANIAN AYD A. WILLIAMS Department of Fuel and Ener~, The University of Leeds
The oxidative reactivities of two cokes, two laboratory prepared chars and one char produced in situ in a methane/air flame were measured in various gaseous environments in the temperature range of 770-2000 K. The methods used consisted of methane/air flame for the highest temperatures, a falling particle reactor for intermediate temperatures and an isothermal thermogravimetric method for the lowest temperature range. The rate of mass loss was inferred from the particle surface temperatures in the first two techniques and was measured directly in the third. Fragmentation was observed when cold coke or char particles were injected into the high temperature environments but not with the char formed in situ from the coal. The chemical reaction rates for these samples depend on the physical nature and morphology of the chars. Low temperature measurements showed that the reactivity of the cokes and chars varied by about three orders of magnitude. At high temperatures the difference in reactivity is reduced to one order of magnitude, The intrinsic reactivity of the petroleum and metallurgical coke was higher than the other chars, especially at high temperature. In general the intrinsic reactivity is reduced as the surface area of the sample particle increased. The activation energies obtained using two very different techniques were in agreement with results of other workers. In addition, the functional variation of burning rate with temperature were used to investigate the effect of different reaction zones on the activation energies.
1. Introduction T h e r e is considerable interest in the m o d elling o f the c o m b u s t i o n or gasification behaviour o f coals, chars a n d cokes. T h e p r o b l e m is c o m p l i c a t e d because o f the wide r a n g e o f coal types and the fact that the reactivity o f a coal c h a r or coke is very m u c h d e t e r m i n e d by the reaction conditions d u r i n g its f o r m a t i o n 1. T h e s e conditions, particularly the h e a t i n g rate, determ i n e the surface area, porosity and crystallog r a p h i c structure o f the c h a r o r coke. O f these the porosity is very i m p o r t a n t since it can control the rates o f diffusion o f chemical species into a n d o u t o f the c h a r s t r u c t u r e a n d the extent o f the reactive area. T h e effect o f t e m p e r a t u r e , p r e s s u r e and gaseous e n v i r o n m e n t s on the rate o f c o m b u s t i o n o r gasification on coal c h a r or coke has b e e n extensively studied by various investigators 1-4. N e v e r t h e l e s s significant problems remain, particularly in relation to the factors that d e t e r m i n e the chemical reactivity of the char. T h e c h a r b u r n - o u t reaction is relatively slow a n d d e p e n d s on the reaction of gaseous oxygen, as well as with c a r b o n d i o x i d e and water v a p o u r , with a p p r o p r i a t e l y o r i e n t a t e d carbon atoms in the char, this in t u r n b e i n g d e t e r m i n e d by the way the c h a r is f o r m e d . T h e s e reactions are not well u n d e r s t o o d a n d f u r t h e r i n f o r m a -
tion is r e q u i r e d on the factors controlling these processes. T h e p r e s e n t study r e p o r t s data on the oxidation and gasification o f d i f f e r e n t types o f coke and c h a r in a t e m p e r a t u r e r a n g e o f 7 7 0 - 2 0 0 0 K using two e x p e r i m e n t a l techniques. T h e s e involved firstly the i n t r o d u c t i o n o f sample particles into an electrically h e a t e d 'falling particle' reactor, and secondly the use o f a m e t h a n e - a i r flame as the source o f the gaseous e n v i r o n m e n t . T h e two-colour p y r o m e t e r t e c h n i q u e was used to m e a s u r e the surface t e m p e r a t u r e and velocity o f the particles, and the physical changes in the samples were e x a m i n e d using electron microscopy. A t h e r m o g r a v i m e t r i c t e c h n i q u e was used to obtain reaction rates in air atmospheres. T h e c o m b i n a t i o n o f these d i f f e r e n t techniques enable data to be o b t a i n e d o v e r a wide range of conditions for overall b u r n i n g rates, reaction o r d e r and activation energies. In addition the relative reactivities o f c h a r a n d coke in air were c o m p a r e d and the effect o f t e m p e r a t u r e , char surface and porosity w e r e e x a m i n e d .
2. Experimental Method E x p e r i m e n t s w e r e u n d e r t a k e n on a UK b i t u m i n o u s coal f r o m M a r k h a m Main (NCB
221
222
COAL COMBUSTION
sample particles by means of a vibrated fluidized rank 702), two bituminous coal chars derived bed. Individual particles were injected into the from the coal, and two types of coke, a petroleum coke (Conoco) and a hard metallurgical flame through a 5 m m diameter orifice in the centre of the b u r n e r . Particles were collected coke. These samples were crushed, g r o u n d and sized by dry sieving a n d the fraction used stored after various residence times in the flame on 0.2 u n d e r an argon atmosphere. The n u m b e r arith~m polycarbonate Nuclepore filters, subsequent metic mean diameter of this fraction was deteranalysis being made by electron microscopy. mined by a Quantimet 720 Analyser and was In situ measurements of particle velocity and found to be 85 I~m with 95% of the fraction temperature were made by the two-colour between 78 and 92 t~m. The coal chars were pyrometer a r r a n g e m e n t 5'6. The optical fibre prepared from the bituminous coal (sample A) collected radiation from particles present in a by pyrolysing the coal u n d e r an argon atmo- control volume defined by a 0.3 mm by 1.0 mm sphere at different heating rates. Char B was slit. The emitted light was viewed in two narrow prepared at a low heating rate (10 K/rain) in a wavelength bands by means of interference quartz tube heated in a furnace, and char C was filters centred on 550 nm (8 n m band pass) and prepared by the flash pyrolysis of the coal A on 649 nm (10 n m band pass), and the intensity an electrically heated grid at a high heating rate was measured by two calibrated photomuhi(1000 K/s). The final pyrolysis temperature in plier tubes using a data acquisition system. both cases was 1223 K and samples were cooled (b) Combustion experiments on char partito room temperature u n d e r argon. The proxi- cles in air were conducted in an electrically mate and ultimate analyses for all samples are heated vertical reactor consisting of a I00 m m given in Table I, together with particle sizes, diameter silica tube which is 2 m long with surface areas, and water densities. Maceral viewing ports at 20 cm intervals 7. The sample analyses showed that the coal contained 74% particles were introduced at a rate of 5 mg/s Vitrinite, 10% Exinite and 16% Inertinite. Surinto a hot zone through a water-cooled probe. face area measurements were carried out using a Once in the reactor the particles are heated Quantisorb continuous flow surface analyser rapidly by radiation from the furnace wall and using N J H e mixtures at 77 K; N2/He was used conduction from the hot gases. The gas and because it largely excludes the area contained in wall temperature of the reactor were adjusted the micropores and is consistent with the apto be equal to each other and were controlled in proach of other investigatorsk Before use the the range of 773-1223 K. The surface temcoal samples were conditioned to remove mo- perature and velocity of the sample particles lecular oxygen u n d e r a continuous nitrogen were measured at several stations along the flow at 105~ tor 7 hours. The water density was reactor using the two colour pyrometer techused as a measure of true density because it niques described above. corresponds closely to fielimn density for cokes (c) The reactivity of the char and coke and chars, and porosity was also calculated from samples in air at 1 atmosphere pressure at apparent and true densities. lower temperatures (770-1200 K) were meaThree experimental techniques were used to sured using an isothermal thermogravimetric examine the combustion and gasification of technique 7'~. Typically about 5 - 8 mg of coke or individual coke or char particles, these being char particles were spread uniformly in a monolayer in a stainless steel mesh basket (1 • (a) a methane/air flame into which char parti1 cm) which was suspended from an arm of the cles were injected, was used to simulate gasifica- microbalance by a quartz fibre. At these loadtion in the temperature range of 1300-2000 K, ings the size of the sample used is not importhe 02 and CO2 concentrations were in the tant. A thermocouple was h u n g parallel to the ranges of 1-18 and 3-11 mol % respectively. quartz fibre to give the temperature of the char The b u r n e r consisted of a 70 m m diameter sample, and the change in the weight of the water-cooled sintered brass disk, and methane/ sample was recorded by a microprocessor data air mixtures were b u r n e d as a flat flame 2 mm recording system. T h e thermocouple was situabove the bed surface, and in some experiments ated 1 to 3 m m from the char particles. the flame was protected by a housing containing windows. The flame gas composition was deter3. Experimental Results mined by sampling using a silica probe, and a gas chromatograph. Flame temperature profiles were measured using a Pt-Pt 13% Rh thermo- Experiments at High Temperature couple of 50 t~m diameter and corrected for The high temperature experiments were radiation. Part of the premixed methane/air undertaken using a methane~air~oxygen flame supplied to the b u r n e r was seeded with the and the vertical electrically heated falling parti-
COMBUSTION AND GASIFICATION OF COKE AND CHAR cle furnace. T h e samples used r e p r e s e n t a wide r a n g e o f carbon samples with d i f f e r i n g crystall o g r a p h i c structure a n d surface activity, surface area, density and ash c o n t e n t (Table 1). E x p e r i m e n t a l data w e r e obtained o v e r a r a n g e o f reaction conditions o n particle surface t e m p e r a t u r e s a n d these w e r e c o n v e r t e d to give overall b u r n i n g rates. E x p e r i m e n t s in the falling particle reactor c o v e r e d a gas t e m p e r a t u r e r a n g e o f 773 to 1223 K with particles being in o n e a t m o s p h e r e p r e s s u r e o f air and with particle surface t e m p e r a t u r e s o f 1 5 0 0 - 1 8 0 0 K. Studies using the m e t h a n e / a i r flame were und e r t a k e n with gas t e m p e r a t u r e s in the r a n g e o f 1400 to 2000 K, a n d with particle surface t e m p e r a t u r e s o f 1520 to 2440 K. T h e equations used to d e t e r m i n e the chemical (P~) and intrinsic reactivity (Pi) of the samples to o x y g e n were essentially those described by 9 Field et al 2 and S m i9t h ~0111z ' ' ' . T h e che mical reaction rate coefficient, R , is obtained f r o m the relationship b e t w e e n d i f f u s i o n reaction rate coefficient RD and R~: X R~ -=RDPG~('-l) (1 - X )- ~
(1)
w h e r e Po~ is the o x y g e n partial pressure in the s u r r o u n d i n g gas, R is the gas constant, Tp is the particle t e m p e r a t u r e , n is the reaction o r d e r and X is the ratio o f actual b u r n i n g rate to m a x i m u m b u r n i n g rate. T h e rate coefficient for o x y g e n diffusion to the particle, RD, is e x p r e s s e d as
223
w h e r e t~ is the m e c h a n i s m factor 2, D is the diffusion coefficient o f o x y g e n in the gas, d~ is the particle d i a m e t e r and Tg is the s u r r o u n d i n g gas t e m p e r a t u r e . Since most o f the reaction occurs within the porous structure o f the particles it is more m e a n i n g f u l a'9 to express the char reactivity in terms o f reaction between gases and porous solids, ie
Ri = R~/Ag~,crp'q
(3)
w h e r e Ri is the intrinsic reaction rate coefficient, Ag is the specific surface area, ~ is the characteristic d i m e n s i o n o f the particle arid "q is the effectiveness factor, calculated using T h i e l e ' s m o d u l u s in a u n i m o d a l p o r e systemL In o r d e r to m a k e c o m p a r i s o n of the reactivities of the coke a n d c h a r samples, on a same o x y g e n partial p r e s s u r e the chemical reaction rate, pc, and the intrinsic reaction rate, Pi, were calculated by m e a n s o f the relations
p~ = R~(Po~)"
(4)
o, = R,(P~)~)"
(5)
and
T h e reaction o r d e r at low f u r n a c e t e m p e r a tures were calculated f r o m the relationship b e t w e e n PE and Cx e x p r e s s e d in equation (6):
(6)
pE-BAgC~ exp R _
RD = 24 O D/dp R ((Tp + Tg)/2)
n
_
_
(2)
TABLE 1. Elemental and proximate analysis of cokes, char and parent coal and Particle Parameters
Fuel Source % moisture % volatile matter % ash % fixed carbon Swelling no. C% H% N% Diameter (p.m) Surface area (mZ/g) Water density (g/cm ~) Porosity (%)
Coal (A)
Coal Char (B)
3.61 35.90 2.98 57.51 1.5 76.40 4.88 1.67 85.6 12.50
4.22 5.80 4.97 85.01 -87.37 1.76 1.73 87.4 29.4
3.25 4.14 5.42 87.24 -90.64 0.97 t.91 87.5 74.5
1.14 0.89 0.07 97.91 -98.7 0.37 0.79 87.4 2.10
1.39
1.61
1.44
1.93
34
46
Coal Char (C)
51
Petroleum Coke (D)
14
Met
Coke (E) 2.40 1.10 7.9 88.6 89.43 0.72 1.15 85.5 3.25 1.87
19
224
COAL COMBUSTION
where Pe is the observed reaction rate of sample per unit external surface area, B is the preexponential factor, Cg is the concentration of oxygen, Et is the true activation energy. T h e true activation energy can be calculated from the measurements of pe at different temperatures, The reaction order at high temperature (zone II) was obtained following the method described by SmithS). The overall b u r n i n g rates of particles per unit external surface area, 9o, can be calculated from the experimental data obtained from the twocolour pyrometer measurements. It is assumed that particles are in thermal equilibrium with their surroundings 13. Therefore we obtain: Q,-..... + Q~aa = QE
(7)
where Q~..... and Qra~ are the rate of heat loss by convection and radiation respectively, and QE represents the rate of heat generation per unit area. Equation 7 can be rewritten as
%,
LOGIO 0 -rr,
....
j ......
I,rL
.........
t .........
,
........
1
-3 ~ '
Zone I
-zl
-~ "~ -5 ~
V 9 § Zone Ii
9
4-
-6
-7 , 1 I I
-8
I
Po = (h(Tp - T~) + ~cr(Tp 4 - T,.4))l~rt
(8)
--~
. . . . . . . .
, ......
15
where h is the heat transfer coefficient, ~ is emissivity of the particle, ~r is the Stefan-Boltzmann constant, and Tm is the medium gas temperature. T h e value of the heat released at the surface per unit mass of carbon burnt, M-/, was assumed 2 to be 9.781 MJ/kg. In this way values of X and therefore the reaction rate pc were obtained for all the samples and these are given in Fig. 1. The reaction rates given for coal A represent the combustion of the char immediately following devolatilisation. Intrinsic reactivities Pi, obtained by means of equations 3 and 5 are given for all these samples in Fig. 2 and the intrinsic reactivities tot all samples when Po2 is at I atmosphere pressure are summarised in Fig. 3. Activation energies relating to the chemical reaction rates, Pc, derived from Fig. 1 for the high temperature region are given in Table 2. Since kinetic data based on mass loss can be invalidated by fragmentation of the particles the method we used and described earlier was only dependent on the surface temperature and not on particle size. However, since the occurrence of particle fragmentation is of great practical interest then samples were examined by scanning electron microscopy (max x 60,000). It was found that in the case of petroleum coke with<2% volatile matter, particularly in high temperature flames, cracks appeared in the surface of the particles. With residence times equivalent to 30% b u r n o u t cracks appeared, with 60% mass loss fragmentation was observed to take place for 70% of samples. At lower
J,,i
. . . . . . . . .
8
i . . . . . . . . .
i .....
t0
t2
,,,I
t4
104 x l/Tp (K-l)
FIG. 1. Variation of chemical reaction rate, Pc, with particle temperature: char from Coal A, A; char sample B, V; char sample C, + ; petroleum coke, • ; metallurgical coke,m temperatures (ca 1000 K) and lower heating rates, cracks only appeared when 40% of the mass had been lost. With chars, with higher volatile matter (<6%) cracks were observed but not fragmentation. However, at above 80% carbon conversion disintegration o'f the char particle was observed. With the coal, no fragmentation was observed in the char particles produced in the flame u n d e r any of the conditions studied here. Fig. 4 shows SEM photographs of (a) the formation of cracks on the surface of petroleum particles, and (b) where fragmentation has taken place. Experiments at L o w Temperature
At lower temperatures data on mass loss was determined by the thermogravimetric method over the temperature range of 770 to 1200 K in air for the two chars and the two cokes, and for the coal which was decomposed in situ prior to the reactivity measurements. Data from the thermogravimetric measurements were analysed by the method described by Smith s. T h e observed rate of coke or char b u r n i n g (PE) were determined directly from the initial weight of
COMBUSTION AND GASIFICATION OF COKE AND CHAR LOG~O O__
225
LOGIO
-I
Ky•
o i = 52 exp( T-R~6 .5 )
-2
-3 -4
-4
c,,
X
++V
x
x
A
I I t .
4
6
8
iO
~2
t4
.
a
T0 4 1/T (K -1 )
.
.
.
.
.
6
.
.
.
.
.
.
.
.
.
.
8 10 4 I / T
.
.
.
.
.
.
.
.
.
I0
.
.
.
.
.
12
t4
(K -1 )
FIG. 2. Relation between intrinsic reactivity, p, and particle temperature: Symbols as in Fig. 1.
FIG. 3. Comparison of intrinsic char and coke reactivity in oxygen (at an oxygen pressure of 1 bar): Symbols as in Fig. 1;----correlation of Smith 3.
the particles and the w e i g h t o f sample r e m a i n ing at any time. T h e relationship b e t w e e n OE, the observed b u r n i n g rate o f c a r b o n p e r u n i t external surface a r e a and the c h e m i c a l reactivity is expressed by e q u a t i o n (9)
with the particle surface the particle t e m p e r a t u r e was calculated by the heat balance m e t h o d 2. T h e r e was in fact little d i f f e r e n c e b e t w e e n the m e a s u r e d a n d calculated particle t e m p e r a t u r e s but the latter was c o n s i d e r e d to be the f u n d a m e n t a l l y m o r e accurate m e t h o d a n d was used in the calculations. T h e results o b t a i n e d are plotted in Fig. 1 for the chemical reaction rate a n d in Fig. 2 for the intrinsic reaction rates as before. Low t e m p e r a t u r e activation e n e r g i e s o b t a i n e d f r o m Fig. 1 are given in T a b l e 2. In Fig. 3 the data are for 1 a t m o s p h e r e p r e s s u r e o x y g e n to p e r m i t comparison with the h i g h t e m p e r a t u r e data.
Rc = [RD R E / ( R e -
Re)]"(pe) 1-n
(9)
in place o f e q u a t i o n (1) used for the high t e m p e r a t u r e data. E q u a t i o n (9) is not very d e p e n d e n t on the accuracy o f the particle t e m p e r a t u r e and only RD uses it directly. Howe v e r since the t h e r m o c o u p l e was not in contact
TABLE 2. Activation Energies and Pre-exponential Factors for High and Low Temperatures Zone II
Zone I Samples
Coal char (A) Char (B) Char (C) Metallurgical coke (D) Petroleum coke (E)
6.1 7.1 6.8 5.7 3.0
x x x x x
l0 s 103 l0 s 103 103
Etrue
Eapparent
kJ/tool.
kJ/tool.
143.9 153.1 137.4 168.3 174.1
41.4 45.3 31.3 34.2 25.6
76.8 83.4 69.2 90.1 95.5
226
COAL COMBUSTION
FIG. 4. Electron micrograph of petroleum coke particle in CH4/air flame, (a) formation of cracks on the surface followed by (b) fragmentation of particle.
4. D i s c u s s i o n and Conclusion
Reactivity and Reaction Kinetics T h e variation o f chemical reaction rate as function of particle t e m p e r a t u r e for all samples are shown in Fig. 1. T h e dashed line separates the two reaction zones observed. At low temperature diffusion resistance does not affect the chemical reaction rate which is taking place at the surface o f the char or coke particle (zone I) and therefore the chemical reaction is the rate determining step. T h e activation energy obtained from e x p e r i m e n t a l data in this stage (zone I) is the true activation energy. In zone II the reaction rate d e p e n d s on the rate of both diffusion into the pore and the chemical reaction behaviour at the pore walls, and the activation energy obtained experimentally is approximately one-half of the true value. Values obtained in zone II represent the influence of both pore structure and chemical reaction rate and because o f this are more significant in many practical applications. At higher particle temperatures, the reaction rate becomes very
rapid and it is limited by the transportation o f oxygen t h r o u g h the b o u n d a r y layer a r o u n d the particle. In this zone (zone III) the experimentally d e t e r m i n e d activation energy is small. During this investigation zones I, II and the transition region from zone II to zone I I I were observed d u r i n g combustion of coke or char particles. T h e activation energies obtained from experimental data for both these zones are shown in Fig. 3. T h e variation in the chemical reactivity o f the samples indicates that in both zones the char C has the highest reactivity and petroleum coke was the least reactive; the char from coal A produced directly in the methane/air flame had a reactivity lower than char C. T h e activation energies have values shown in Table 2 in the range of 173-137 kJ/mol in zone I and reduces to 9 5 - 6 9 kJ/mol in zone II. T h e experimental values obtained for zones I and II fall within the range expected for the true and a p p a r e n t activation energies for i m p u r e carbons. (Etude = 168 kJ/mol). Differences in chemical reactivity of the samples can be related to the original physical properties of the samples. Increase in the volatile matter present in the coke or char samples from 1-5.5% increased the observed burning rate, which in turn affected the chemical reaction rate. T h e chars p r o d u c e d by a high heating rate and m e d i u m final pyrolysis temperature o f the p a r e n t coal had a higher chemical reactivity than chars p r o d u c e d with a low heating rate or high final pyrolysis temperature and high heating rate. No correlation could be f o u n d between the densities and moisture content o f samples with chemical reactivity. However experimental results indicated that the observed reaction rate could be influenced by the mineral matter present in the char. T h e difference in chemical reactivity of samples becomes relatively small at high particle temperature, as at high t e m p e r a t u r e the chemical reaction rate becomes fast so the effect of the internal surface area becomes negligible. Fig. 2 shows the relationship between intrinsic reactivity and t e m p e r a t u r e of the particles for the samples investigated. This is a more realistic treatment o f the coke or char reactivity because it eliminates the effect of pore size and pore surface area a n d therefore provides information on the reactivity of the chars on the basis of f u n d a m e n t a l chemical reaction rates. The active surface area in intrinsic reaction rate becomes significant, especially at low particle temperatures. It was found that the intrinsic reactivity of the cokes differ by a r o u n d two to three orders o f magnitude; the petroleum coke had the highest intrinsic reactivity followed by metallurgical coke, and the char C is less
COMBUSTION AND GASIFICATION OF COKE AND CHAR reactive. Smith 11 suggested that the presence o f the inhibiting impurities and the effect of atomic structure of the carbon influences the intrinsic reactivity o f the coal. Fig. 3 compares the intrinsic reaction rates of all the samples investigated at low and high temperatures. T h e activation energy obtained from the least square regression over all the data points shows a value o f 161 -+ 6 kJ/mol. Smith 3 r e p o r t e d the value o f 179.4 kJ/mol for an activation energy for a wide range o f chars and carbon samples and his correlation is shown in Fig. 3. Because of his inclusion of carbons we consider o u r data to be more accurate for chars and cokes. T h e true and a p p a r e n t activation energies obtained from chemical reactivities are in a good agreement with values d e t e r m i n e d by other investigators a'3'4'13 for similar samples.
Fragmentation of coke or char particle In recent years theoretical and experimental work has been carried out to investigate the effect of porosity, chemical and physical composition and combustion environment on char fragmentation 14-x6. During this investigation electron microscope studies of coke and char particle at high gas t e m p e r a t u r e s indicated that cracks a p p e a r at the surface of the particles followed by fragmentation as shown in Fig. 4. In most cases fragmentation of coke particles takes place at a r o u n d 6 0 - 7 0 % conversion in comparison with chars which disintegrate at a higher conversion stage. T h e result showed that cracks and f r a g m e n t a t i o n depends on the physical properties o f the char or coke. At high t e m p e r a t u r e the b u r n i n g rate o f the petroleum coke is controlled mainly by oxygen diffusion and external mass transfer is a main rate limiting factor. T h e r e f o r e there is a steady decrease in the d i a m e t e r o f the particles and the density remains approximately constant d u r i n g combustion process, and internal surface reaction is less significant. T h e electron microscope studies showed that when the diameter of the particle is r e d u c e d by 3 0 - 5 0 % of the initial size cracks were visible at the surface of the particle followed by fragmentation of almost 2 0 - 3 0 % when the carbon conversion exceeds 60%. Clearly stress effects occur which result in fragmentation. However, previous investigations "''is suggest that morphology of chars depends on the coal rank and maceral type of individual coal particles. T h e chars p r o d u c e d from coal A were mainly thick walled ballon type chars which is generated from the vitrinite. SEM studies showed that disintegra-
227
tion of the char occurred at a higher conversion stage than petroleum coke (>80% carbon conversion). However, fragmentation in the char did not follow a similar r o u t e and in some case separation of 50% of the particles were observed. T h e fragmentation of the particles during combustion process could have an important impact on weight loss. I n d e e d our measurements indicated that fragmentation can reduce the initial weight o f carbon by a r o u n d 20-30% and therefore introducing serious e r r o r in the weight loss calculation obtained from the size variation of carbon particles in CH4/air flame when d e t e r m i n e d by optical techniques 19. In addition the size distribution of flyash produced d u r i n g the char combustion stages can be influenced by fragmentation of the particles. Reduction in size distribution of the flyash due to fragmentation can affect the process of removal of the flyash in reactors. T h e r e f o r e the carbon fragmentation must be included in calculations of b u r n i n g rates and design of the reactors.
Concluding Remarks A considerable a m o u n t o f information is now available (for example refs. 17, 20-22) on the combustion and gasification of Markham Main coal. W h e n it u n d e r g o e s decomposition: coal
~char + tar + gases
the initial reaction can be described 23 by the reaction rate k = 10 13 e x p ( - 2 4 , 6 5 0 / T ) s- 1 . T h e tar consists of high boiling components which remain in the liquid phase together with volatile components and gases. Generally sufficient kinetic data are available to describe the rate of oxidation of the hydrocarbons and gaseous components. T h e rate o f b u r n i n g of the tar droplets can be described 24 by a modified dZ-burning law, the value being 3 x 10 .3 g c m -2 s- 1 a t l 0 0 0 K a n d 4 x 10- 3 g c m - 2 s- 1 a t l l 5 0 K . T h e reactivity o f the char that is p r o d u c e d is d e p e n d e n t on its heating rate, the maceral content of the coal from which it is f o r m e d and on its ash constituents. T h e char formation condition plays an important role on the chemical reactivity especially in zone I. T h e char p r o d u c e d at high heating rate and m e d i u m final pyrolysis temperature had a h i g h e r reactivity than the char p r o d u c e d at high heating rate and high final pyrolysis temperature. Such differences become less significant in zone II, because the
228
COAL COMBUSTION
c h a r m o r p h o l o g y (porosity a n d p o r e size) is n o w t h e i m p o r t a n t p a r a m e t e r in d e t e r m i n i n g reactivity. I n a d d i t i o n t h e c h a r s p r o d u c e d in i n e r t gas t e n d s to b e m o r e o p e n t h a n c h a r s p r o d u c e d in air o r CO2, a n d t h e final pyrolysis temperature and heating rate of char formation i n f l u e n c e s t h e d e v e l o p m e n t o f i n t e r n a l s t r u c t u r e a n d r e a c t i v e s u r f a c e area. It also d e t e r m i n e s t h e c o n c e n t r a t i o n o f r e s i d u a l volatile m a t t e r w h i c h in t u r n i n f l u e n c e t h e n u m b e r o f reactive sites. T h e c h a r s h a v e a h i g h e r c h e m i c a l reactivity t h a n the p e t r o l e u m a n d m e t a l l u r g i c a l coke, b u t t h e i n t r i n s i c reactivity f o r t h e cokes a r e h i g h e r t h a n the c h a r s . T h i s reflects t h e i n f l u e n c e o f t h e reactive s u r f a c e a r e a .
Acknowledgements We wish to thank British Gas for financial support under the Fellowship Scheme and Mr. M. Chan for assistance with thermogravimetric experiments.
REFERENCES 1. WELLS, W.F., KRAMER, S.T., SMOOT, L.D., BLACKHAM, A.V.: Twentieth Symposium (International) on Combustion, p. 15-39, T h e Combustion Institute, 1984. 2. FIELD, M.A., GILL, D.W., MORGAN, B.B. AND HAWKSLEY, P.G.W.: Combustion of Pulverized Coal, p. 89, BCURA, Leatherhead, 1967, 3. SMITH, I.W.: Nineteenth Symposium (International) on Combustion, p. 1045, The Combustion Institute, 1982. 4. MULCAHY, M.F.R. AND SMITH, I.W.: Rev Appl. Chem. 19, 81, 1969. 5. TIMOTHY, L.D., SAROFIM, A.F. AND BEER, J.W.: Nineteenth Symposium (International) on Combustion, p. 1123, T h e Combustion Institute, 1982. 6. MURDOCH, P., POURKASHANIAN, M., AND WILLIAMS, A.: Second European Conference on Coal Liquid Mixtures, p. 177, I.Chem.E. Symposium Series No. 95, 1985.
7. MURDOCH, P., POURKASHANIAN, M. AND WILLIAMS, A.: Twentieth Symposium (International) on Combustion, p. 1409, The Combustion Institute, 1984. 8. JORDAN, J.B. ANn WILLIAMS, A.: Alternative Hydrocarbon Fuels, Combustion and Chemical Kinetics, Progress in Astronautics and Aeronautics, Vol. 62, p. 180, 1978. 9. LEWIS, P.F. ANn SIMONS, G.A.: Combustion Science and Technology 20, 117, 1979. 10. LAURENDEAU,N.M.: Prog. Eng. Comb. Science 4, 221, 1978. 11. SMITH, I.W.: Fuel 57, 409, 1978. 12. HAMOR, R.J., SMrrH, I.W. AND TYLER, R.J.: Combustion and Flame 21, 153, 1973. 13. MITCHELL, R.E. AND McLEAN, W.J.: Nineteenth Symposium (International) on Combustion, p. 1113, T h e Combustion Institute, 1982. 14. GIVINOT, J., AUDIER, M., COULON, M., AND BONNETAIN, E.: Carbon 19, 95, 1981. 15. KERSTEIN, A.R. AND NIKSA, S.: Twentieth Symposium (International) on Combustion, p. 941, The Combustion Institute, 1984. 16. SUNDBACK, C.A., BEER, J.M. AND SAROFIM, A.R.: Twentieth Symposium (International) on Combustion, p. 1495, T h e Combustion Institute, 1984. 17. STREET, P.J., WEIGHT, R.P. AND LIGHTMAN, P.: Fuel 48, 343, 1969. 18. OKA, N,, MURAYAMA, T., MATSUOKA, H., YAMADA, S., YAMADA, T., SHINOZAKI, S., SHIBAOKA, M. AND THOMAS, C.G.: Proceedings of International Symposium on Fundamentals of Catalytic Coal and Carbon Gasification, May 1986. 19. TICHENOR, D.A., MITCHELL, R.E., HENCKEN, K.R. AND NIKSA, S.: Twentieth Symposium (International) on Combustion, p. 1213, The Combustion Institute, 1984. 20. CAUSTON, P. AND MCENANEY: B. Fuel 64, 1447, 1985. 21. DEW, B.A. AND ROBINSON, S.C.F.: Fuel 64, 861, 1985. 22. DESYPRIS, J., MURDOCH, P. AND WILLIAMS, A.: Fuel 61, 807, 1982. 23. JOHNSON, G., MURDOCH, P. AND WILLIAMS, A.: Submitted for publication. 24. POURKASHANIAN, M. AND WILLIAMS, A.: Submitted for publication.
COMMENTS I. W. Smith, CSIRO Division of Fossil Fuels, Australia. It is unusual to find in practice that in zone IIEA -" 2 x Ezo,,, i due to problems of evolving pore structure, transitions between regimes, etc. Do you plan to take better pore structure measurement and use better
pore structure models to rationalize your Zone I and Zone II models? Author's Reply. We agree with Mr. Smith's comments and during this investigation we were of course con-
COMBUSTION AND GASIFICATION OF COKE AND CHAR cerned with the effect of the pore structure model employed and the separation of Zone I and II regimes. We found that the activation energies in Zone II were roughly one half of the true value in Zone I with a variation of 5-8%. Our experimental data indicates that the transitional stage, where both Zone I and Zone II behaviour occurs, makes it difficult to separate the theoretical activation energies with precision. We also agree that knowledge of internal surface area, the assignment of accurate diffusion coefficients, and development of the pore structure are important factors in determining heterogeneous char combustion. In this investigation we have used both uni- and bimodal models of pore structure and obtained similar results, thus giving some evidence of the accuracy of our results. However, it is necessary to improve models of pore system development, taking into account particle fragmentation, blind pore and pore coalescence.
S. Srinivasachar, MIT, USA. Is the BET N2 surface area the correct one to use for deriving reaction rates of char oxidation (from experiiments)? How do you account for the varying surface area as a function of reaction extent, and that not all the surface area available is put to use in the oxidation reaction? Author's Reply. We believe that the BET N2 surface area is the best one to use because the measurement relates to pores with radii greater than 5 A, which we calculate to be more representative of the surface area available for reaction with molecular oxygen. The development of char surface area during the combustion process was predicted at high temperatures by means of a theoretical model (as described in the paper) or measured directly during burn-off in the thermogravimetric measurements.
R. E. Mitchell, Sandia National Laboratories, USA. In order to derive reaction order from experimental measurements, one must vary Oz levels over a considerable range. Over this range, the effects of Stefan flow become appreciable. Yet, you neglect Stefan flow in your analysis. I assert that your results for reaction order are influenced by this neglect. Author's Reply. It is certainly possible that under certain conditions convective flow of the gaseous medium may take place normal to the carbon surface (Stephan flow). 1 However this only amounts to a second-order correction in most chemically reacting systems.2 In this investigation the effect of Stephan flow was negligible for an oxygen concentration up to 12%. Using expressions given by Frank-Kamenetskii2 the effect is found to be less than 5%. At higher oxygen
229
concentrations and at higher temperatures neglect of the Stephan flow can influence the values of apparent activation energy and apparent reaction order in the diffusion limit. However, in these experiment the higher oxygen concentrations were used tor the thermogravimetric experiment at lower initial temperatures and the consequential effect is small.
REFERENCES 1. STEPHAN,J., Ann. Phys., 17, 550(1882) 41,725, (1890). 2. FRANK-KAMENETSKII, D.A., Diffusion and Heat Exchange in Chemical Kinetics, Plenum Press, London, 1969.
R. H. Essenhigh, Ohio State University, USA. 1) Did reactivity increase with decreasing density, or with increasing density? 2) Could cracks he responsible for fast access of O2 to the particle interiors and be responsible for higher "intrinsic" reactivity? In flame studies with coals, chars, and petroleum cokes we have been successful in modelling the flames using the same kinetic constants, but flame differences were found to be due to the mode of reaction: shrinking core or shrinking density (see the 15th Symposium). The macropore volume appeared to be critical in determining the mode of reaction. Cracks may do the same thing. What is your evaluation?
Author's Reply. We attempted to find correlation between intrinsic chemical reactivity and ash, volatile matter, H content and true density, We could only obtain a correlation with true density and the reactivity increased with increasing density, suggesting that the level of graphitisation is the important factor. The formation of cracks can increase the active surface area but in our experiments our kinetic data was primarily derived from char particles in circumstances in which cracking was small. In general we believe that the influence of cracks is small, however the occurrence of fragmentation can have a significant effect on its mode of reaction.
S. Niksa, Stanford University, USA. Considering the broad temperature range represented in your TGA and flow reactor data, and the body of data which suggests substantial production of CO2 at moderate temperatures, can you assess the influence of simultaneous production of CO and COz on your analysis? Specifically, is the magnitude of the discrepancy between the measured and calculated temperatures in the TGA experiments indicative of the larger reaction exotherm for CO2 production?
230
COAL COMBUSTION
Author's Reply. In this investigation it was assumed that carbon particle reacts with oxygen to p r o d u c e CO or CO2 and their a m o u n t d e p e n d i n g on experimental conditions. At p r e s e n t there is no reliable method to obtain exactly the a m o u n t o f CO or CO2 f o r m e d at the surface o f small (<100p~m) carbon particle d u r i n g combustion process. T h e r e f o r e the relationship r e c o m m e n d e d by Smith (Ref. 3) a n d Field et al (Ref. 2) are used in this investigation. In CH4/Air flame reactor w h e r e the particle temperature was >1500 K, it is assumed that CO is the primary product, and we have no evidence to the contrary. H o w e v e r in the thermogravimetric measurements it was a s s u m e d that CO2 as well as CO is the primary p r o d u c t w h e r e ratio o f CO2: CO varied with temperature (Ref. 2). T h u s for the reaction: OC + O2---~ 2 (O - 1) CO + (2 - tO) CO2 where tO = 2 ( 1 - to) + to, to = fraction o f carbon oxides f o r m e d as CO2 We also assume that AH = 33.02 (2/tO - 1) + 9.78 (2 - 2/tO) MJ/Kg. A comparison o f calculcated and measured particle temperature indicated that CO2: CO ratio decreased from 2.8 to 0.20 as the t e m p e r a t u r e increased. T h e r e f o r e we agree that to make the assumption that carbon burns to p r o d u c e only CO2 or CO can cause an error in reactivity calculation, especially below 1200 K, and more i n f o r m a t i o n is required on the nature o f the oxidation p r o d u c t for small particles.
many practical applications, one can expect to find a rather high a m o u n t o f mineral matter at least in a fraction o f the pulverized coal. How do you expect this will affect the concept o f an intrinsic reaction rate and the global reaction rate o f the particles as a whole?
Author's Reply. T h e role o f ash is significant in practical applications. Whilst we used relatively low ash fuels in o u r paper, many industrial fuels initially contain 2 0 - 3 0 % ash and in the latter stages o f combustion may contain 80% ash before disintegration o f the char particle. Although the kinetic role played by ash in such circumstances is not well defined, in general its effect on combustion is twofold, namely: (a) a catalytic effect o f ash on the h e t e r o g e n e o u s carbon oxidation, and (b) an increase in diffusion resistance, these two effects opposing one a n o t h e r as the ash content increases. T h e char reaction rate will increase with an increasing a m o u n t o f metal catalyst but will saturate when the active surface area is completely covered, and the catalyst effect o f impurities decreases with increasing t e m p e r a t u r e m. T h e r e f o r e probably ash is unlikely to have any m a r k e d affect on reactivity at the pulverised fuel combustion temperatures. T h e formation o f the ash shell a r o u n d the char particle will increase the diffusion resistance which in turn will reduce the reactivity. It is t h e r e f o r e important to include correction terms for both these effects, otherwise predicted char lifetimes are shorter than that realised in practice. REFERENCE
W. Zinser, University of Stuttgart, Germany. I f you are investigating coal with an ash content as f o u n d in
HAWTIN, P., AND GmsoN,J.A., Carbon, 4, 501, (1966).