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Reactivities of carbon to steam and hydrogen and applications to technical gasification processes—A review
OCW6223/81/030167~7$02.00/0 Copyrighl 0 1981 Pergamon Press Ltd.
Carbon Vol. 19. pp. 167-173. 1981 Printed in Great Britain. All rights reserved
REACTIVITIES OF CARBON TO STEAM AND HYDROGEN AND APPLICATIONS TO TECHNICAL GASIFICATION PROCESSES-A REVIEW? H. J~~NTGEN Bergbau-Forschung GmbH, Franz-Fischer-Weg 61,4300, Essen 13, Germany (Receioed 29 My 1980) Abstract-Experimental results of reactivity measurements on coal and chars to steam, hydrogen and steamhydrogen mixtures are compared and their application to technical processes is discussed. The change of surface area as a function of burn-off has a minor significance for chars made from coal with high initial accessible surface areas. In this case the reaction rate under constant steam pressure is first order with respect to the amount of carbon being gasified. The reaction rate of hydrogasification markedly decreases with burn-off, since the activation energy increases with burn-off due to the consumption of reactive carbon during the reaction. The reaction of carbon with steam hydrogen mixtures can be described as a superposition of carbon steam reaction and hydrogasification. The overall rate relative to that with hydrogen or steam alone can increase if a high amount of reactive carbon is present and immediately reacts with hydrogen. It can decrease if the inhibition of the steam carbon reaction by hydrogen predominates. Some consequences for the technicalperformanceof coal gasification are: The overall reaction rate in a fluid bed decreases with increasing bed height, since hydrogen-steam ratios and therefore the inhibition by hydrogen increase. The overall reaction rate in a moving bed with countercurrent flow of carbon and reacting gases increases with increasing overall pressure since highly reactive carbon, formed in the pyrolysis zone, can immediately react with high partial pressures of hydrogen. 1.INTRODUCTION
system contains three complex consecutive reactions:
When coal or coke are gasified a stoichiometric equation
of overall reaction can be derived from the material balance. For further considerations of mechanism and kinetics it is practical to express this equation as a super-position of some selected basic reactions being kinetically independent. As an example gasification of carbon with steam under a pressure of 40 bar and temperatures between 800 and 850°C can give a product gas with the following composition: 18.5% COz, 14.8% CR, 37% Hz and 29.7% CO. A stoichiometric balance shows that this overall reaction takes place as a sequence of consecutive or parallel reactions: 0.76 x (C + Hz0 = CO t Hz) 0.24 x (C t 2H, = CH,) 0.29 x (CO-t Hz0 = CO, t HJ
(1) (2) (3)
giving the overall reaction C t l.OSH20 = 0.5782 + 0.47CO t 0.29C0, t 0.24CH,,. (4)
coal 5
I
CAR Vol. 19. No. 3-B
(5)
k,,kd k,
kz
gasification products. Gasification can take place in the temperature range 800-1800°C. Therefore we have to consider the relative importance of chemical reactions and transport phenomena like pore diffusion and film diffusion which become rate determining at higher temperatures. This paper only deals with the lower range of temperature where there is chemical control of rates of reaction. If we consider the overall reaction (4) and the three reactions (l)-(3) assuming that reaction (3) is in thermodynamic equilibrium at all times, then the rate of carbon conversion is given as the sum of the rates of the reactions (1) and (2): dXc _ dXc,HZo+ dXc,H2 dXe rate of.C-conversion , dt = ;;;I;:: to the overall dtdt dt (6)
Reaction (1) is the endothermic heterogeneous steamcarbon reaction, (2) is the exothermic hydrogasification reaction and (3) the slightly exothermic shift reaction. Sustained gasification can also be described by these three reactions if additionally there is taken into account that carbon can be gasified in oxygen producing CO, in a strongly exothermic reaction. When coal is gasified the pyrolysis of coal has now to be considered such that the tHerrn Prof. Dr. Klaus Schafer, Direktor des Instituts fur Physikalische Chimie der Universitat Heidelberg, zum 70. Geburtstag am 23. 8. 1980gewidmet. Invited paper presented at the “Carbon 80” Conference in Baden-Baden, on 3 July 1980.
carbon t other pyrolysis products
dXc,mo _
dt
dXe,rr, -= dt
rate of C-conversion according to carbon-steam reaction rate of C-conversion according to hydrogasification.
Therefore the reactivities of carbon with steam and hydrogen have great importance in the modelling and description of such gasification reactions which occur in technical systems. In general two different definitions for 67
168
H. MNTGEN
the reactivity of carbon are used: the reaction rate according to eqn (6)
following gas atmospheres, temperatures and pressures: Gas
Temperature, “C
Partial pressure
d-G
1&=-g-
and the reaction rate related on the proportion of unconverted carbon ,,_dXc
rC-dt
1-x;
t k3 . pH2’
In a differential reactor to determine gasification rates the hydrogen partial pressure is low, and we can neglect k, * pH2 in eqn (8). At low pressures the reaction rate is first order with respect to steam becoming at high pressures a zero order reaction. Figure 1, in which char reactivity r,JJis plotted vs the steam pressure, shows that this law is also valid in the pressure range between 1 and 70 bar using a char prepared from a bituminous coal at low temperatures [3]. As to the dependence of the reaction rate of the steam-carbon reaction on the carbon conversion Xc, Mahajan et a/.[41 studied chars, prepared from American coals, at temperatures of 1OOO“C. They performed isothermal experiments using a thermal balance under the
Schwelkoks 5
(9) In this way they showed that under their conditions the reactivities of the chars to the different gases were in the order:
A later mathematical analysis of these results[5] gave the following expression of reaction rate as a function of carbon conversion:
k, . PHlO
HZ0 =1+ kz *p&O
They found that the variation of Xc with time can be described with the same formula if the function Xc is taken to depend on a dimensionless time T, where T is defined as: T = & with to.s= time in which Xc becomes 0.5.
OF CARBONTO STEAM
The rate of reaction for steam gasification has been shown by many workers to obey Langmuir-Hinshelwood kinetics, noting the reaction steps of adsorption of the steam at the carbon surface, the surface reaction of adsorbed steam with carbon and the desorption of hydrogen[l, 21. In this way the rate constant of reaction (l), as dependent on the partial pressures of steam and hydrogen, can be written as: k
1 1 0.022 27.2
.- 1
The following is a survey of how such reactivity measurements are being applied in technical gasification systems. 2. REACTIVITY
405 900 910 980
air co, Hz0 H*
= k’ *Xc”.‘(1- X,-) with k’ = 1.76.
2
(10)
In Fig. 2 the expressions (dXc/dr) and (l/l -Xc) (dX,/dr) are plotted vs Xc when (dXc/dr) first increases, reaches a maximum and then decreases, whereas (l/l - Xo) (dXo/dr) increases strongly, later less strongly-over the whole range of Xc. This behaviour can be explained by the changes in accessible internal surface during the burn-off of the char. Wicke and Hedden[6] came to the same conclusion in studies of the gasification of pure carbon with CO, at 1000°C. Ad-
Leopold
0,2-0.1mm I
5
10
P,,obd
I 100
2
Fig. 1. Influence of partial pressure of steam on the carbon reactivity r” (according to eqn 8) in the range between 1 and 70 bar according to Feistel et a1.[3].
0
01
05
IO
XC
Fig. 2. Plots of dXC/dr and dXC/dT.l/l -Xc vs Xc according to the experimental data of Mabajan et nL[4]. (Equation 10) on chars, prepared at l.OWC.
169
Reactivities of carbon to steam and hydrogen ditionally they showed that the effective internal surface first increases with carbon conversion, reaches a maximum value in the same range of burn-off as that for the maximum value of reaction rate, and then decreases at higher carbon conversions (Fig. 3). Different results have been obtained from our isothermal and non-isothermal studies using coals and chars prepared at temperatures below the temperature of gasification[7]. A differential reactor was used and the reaction rate was determined from amounts and analyses of product gases. As an example Fig. 4 shows the results of an isothermal run at 10bar and 800°C from which it can be seen that(l/l -Xc x dXJdt) is nearly constant during the range of Xc greater than 0.1 and (dX,-/dt) decreases with increasing X,. Therefore we can write:
dXc r&=,,=k.(l-Xc)or
rS=tl_xcj
1
.-=dXc dt
k.
(11) In this way the reaction rate is first order with respect to the amount of carbon being gasified. For chars, Xc is defined as
m mc,o
Xc=mc.o-mc=l
mc.<,
0
01
05
(12)
For coal Xc is defined as: x,=1-
mc ” mcoal 1- jfjj i 1
(13)
with m, = mass of carbon, mc., = mass of carbon at the beginning of the reaction, mcoal = mass of coal (daf) at the beginning of the reaction, V = volatile in the coal. This behaviour of coal and low temperature chars can be explained because these products have a larger reactive surface area [8] at the beginning of the steam-carbon reaction compared with chars made at high temperature. Hence, for coals, changes of surface area during burn-off are of only minor significance to reaction rate. Therefore we can use non-isothermal experiments to give the same results as for isothermal experiments to determine k in eqn (11) and for the evaluation of apparent activation energies and frequency factors according to: (14) with E = activation energy, k, = frequency factor. As an example, Fig. 5 is an Arrhenius-diagram of a non-isothermal experiment from which, at temperatures greater than 8OO”C,the change from chemical control to pore diffusion control of rate can be seen[9]. Further, Table 1 shows some values of apparent activation energies and frequency factors (according to eqns I I and 14) for the reaction of lignites and hard coals with steam at a pressure of 40 bar[lO]. The lignite (Rheinische Braunkohle) and other iignites, have a low apparent activation energy and therefore high reactivites to steam. This
IO
Fig. 3. Dependence of the effective surface on carbon conversion Xc for the gasification of graphite with CO2 at I bar and 1.OOO”C according to Wickeand Hedden[6].
Fig. 4. Plots of dX/dr and l/IX, dX/dt vs X, for the gasification of low temperature-chars and coals at 10bar and 810°Caccording
to Wanzl et n/.[7].
Fig. 5. Temperature dependence of the reactivity r; for the non-isothermal gasification of an anthracite with steam at 40 bar according to Jiintgen et al. (91.
170
H. J~~NTGEN Table 1. Apparent kinetic parameters for the gasification of lignite and hard coal according to eqns (11)and (14) according to van Heek et al. [ lo] 'ol.Matter '% daf)
I
lignite
BK F 12
I
Lztivationenergy (kcal/mol)
hard coal
Pittsburgh Seam No.8
.requencyfactor (l/min)
29,5
6
.
lo5
46,0
38,9
2
2 Pittsburgh Seam
38,3
32,l
2
3 Hagen
36,Z
31,8
5
4 Zollverein
33,0
34,8
3
. 106 . 105 . 104 . lo5
7.8
36,0
3
lo5
1
5 Geitling
behaviour is mainly caused by these materials containing high contents of Ca-based catalysts which are finely divided in the coal substance. Values of apparent kinetic parameters for hard coals are in a narrow range and the reactivities at temperatures between 800 and 900°C are distinctly below those of Iignites. In general the catalytic compensation effect of increasing activation energies with increasing frequency factor is observed [Ill. 3. REACTIVITY OF CARBON TO HYDROGEN
Johson[l2] and Zaradnik and Glenn[ 131show that the reaction with hydrogen is more complex than with steam because the reactivity of coals or low temperature chars can change extensively with carbon conversion. Several gasification reactions are given as a simplified mode1 in Table 2, where the reactions of volatiles from coal and of so-called reactive carbon formed during pyrolysis are fast and are in parallel with the slow reaction of the usually non-reactive carbon from char[l4]. There are several ways of describing these reactions as a function of the carbon conversion, e.g. by assuming two parallel first order reactions with different rate constants according to
which we can use the following expression: r& =
kH,(l-X,)(1
tb ePxc).
(16)
Mahajan et a1.[4] observed that activation energies decreased with extent of carbon conversion (burn-off) even when reacting a high temperature char with hydrogen. Figure 6 results from a model calculation in which eqns (15) and (16) are compared, and indicates that for both equations r: first decreases with increasing carbon conversion. When all the reactive carbon is consumed it remains constant. For comparison the reactivity t-z of coal with hydrogen at 900°C[13] at pressures of 30-70 bar is shown in Fig. 7 as a function of time. During the first 18min when the coal is heated under hydrogen at a constant heating rate of SO”C/min, the reactivity increases. Afterwards we observe the decrease of reactivity expected from eqn (16) and Fig. 6 at constant temperature. Figure 7 shows that the reactivity increases with increasing hydrogen pressure, this behaviour being described by a Langmuir-Hinschelwood expression[l5]:
(17)
dX, = k2(1 - Xc) for XC > f r&= dt
(15)
with k, > kl, f = proportion of reactive carbon; or by assuming an activation energy decreasing with Xc for Table 2. Simplified model of the hydrogasification of coal according to Feistel et a[.[141
I
II
(15)with K,=IO,K,=I,f=O.25
i
(J
COOI
b
Cv+2HZ+CHL
I I/ I
Hiittinger and Schleicher [16] investigated the catalytically promoted hydrogenation reaction of coke and coal with Fe, Ni and Co catalysts and showed that (dX,/dt) remains constant in a carbon conversion range below 0.5. This indicates that it is the concentration of catalytic sites which is responsible for the reactivity, rather than
-
CH, C, C' ICI
Pyrolysis Hydrogosiflcatlon of Volotlle Matter
I
C*:ReactiveCarbon 17=lnreoctive Carbon Cv ~Carbon in Volatile Matter
-I
Fig. 6. Model calculation for the dependence of the reactivity of carbon against hydrogen during carbon gasification Xc.
171
Reactivities of carbon to steam and hydrogen Coal Pyrolysis (50 “c / minf Hydrogasificotun (900 @C:)
111 0
IO
---*-
30 bar
----
50 bar
-
70bar
1
I
I
I
I
I
I
20
x)
40
50
60
70
80.
Time,
mm
Fig. 7. Reactivity rE to hydrogen of a coal, dependent on time and hydrogen pressure according to Jiintgen et a1.[9].
the amount of reacting carbon. They observed that: wri, rd =
- I&. e)
(1 t k&&)
higher carbon conversions all of the reactive carbon is consumed in the hydrogasification and therefore the second term of eqn (19) is low compared with the first term. Additionally, the inhibition of the steam-carbon reaction by hydrogen adsorbed on reaction sites-as expressed in the Langmuir-Hinshelwood eqn (7)becomes effective. This explains why we find, from the 1.h.s. of Fig. 8, that at higher temperatures the reactivity of steam is higher than that of hydrogen and that in mixtures of steam with hydrogen the reactivity decreases with increasing hydrogen content. When the char is gasified (r.h.s. of Fig. 8) there is little reactive carbon present, and we find that inhibition of the steam-carbon reaction by hydrogen is more effective than the enhancement of overall reaction rate by the parallel hydrogasification reaction whose rate is relatively low. Table 3 shows that the mean reactivity of coals and chars in hydrogen-steam mixtures is lower than in pure steam[9], the effect increasing with decreasing volatile content in the coal or char. The highest inhibition of reaction rate by hydrogen is found when we used a residual char from a semitechnical hydrogasification
where PH2,e is the hydrogen partial pressure at equilibrium. 5. APPLICATIONOF RESULTSTO TECHNICALPROCESSES 4. REAC~VI~
OF CARBON IN STEM-HYDR~E~
MATURE
Using eqns (6), (11) and (16), based upon reactions (1) and (2) we have that the reaction rate of the overall reaction is:
dXf_ kH20(1d
t
-
Xc) + kH2(1 - b e-“XC)
(19)
in which kHzo is as given in eqn (8) and kH2 is as given in eqns (17) or (18). This equation is valid only for noncatalytic gasification reactions and does not consider changes in internal surface during gasification. We expect that this equation is applicable to the gasification of coal with steam and with mixtures of steam and hydrogen. Non-isothermal gasifications of coal and char in hydrogen and steam, and in hydrogen-steam mixtures, are reported in Fig. 8[9]. The left graph of Fig. 8 shows that at low gasification temperatures, corresponding to low carbon conversions, the reactivity of the char with hydrogen is higher than that with steam and that those with mixtures of hydrogen and steam are between these values. This is caused by a large proportion of reactive carbon from the coal which results in the r.h.s. expression of eqn (19) being greater than the 1.h.s. expression. At
These results indicate that in technical ~asi~cation processes the profiles of temperature and gas concentration in the gasifier, the mode of contact between gas and solid and the rate of heating of coal in the reactor, all influence the kinetics of the reaction of steam and hydrogasification. High rates of heating of coal cause a decrease in the proportion of reactive carbon because of conversion of reactive carbon into unreactive carbon (reaction Ifb in Table 2) is faster than the hydrogasification of the reactive carbon (reaction Ha). Efficient interaction between a freshly formed char and hydrogen is more favourable in moving bed reactors with countercurrent stream of gas and solid than in fluidized beds with high back-mixing of solids. Processes in which significant gasification takes place in the temperature range of chemical control involve gasi~cation under pressure in a moving bed with a counRaw pas --HZ CO
COzCHq
WI % x3 22 23 I I
800-I
T
-c
Fig. 8. Reactivity r$ of a coal (1.h.s.)and a low temperature char (r.h.s.) against mixtures of hydrogen and steam according to non-isothermal measurements of Jiintgen et a/.[9].
Fig. 9. Model of moving bed
gasification
of coal.
H. J~NTGEN
172
Table 3. Comparison of reactivities rc of different fuels against steam and mixtures of steam and hydrogen at 40 bar and 800°Caccording to Juntgen et al. [9] Initial material
Anthracite Niederberg
I
Coal
Hagen
I
Fluidized bed char
from coal
Hagen
100/o 86114 100/Q go/10
Residue char from the hydrogasification of Anthracite Niederberg
100/o 87113
Coal Leopold preoxidized with air
100/o
89/11
tercurrent stream of gas and coal, and the isothermal gasification of coal in a fluidized bed using nuclear heat.
In the first process (Fig. 9) there is a pyrolysis zone on the top of the reactor in which coal is slowly heated. In the upper part of the gasification zone the hydrogen concentration is relatively high so that the freshly generated char, containing a high proportion of reactive carbon, is in an atmosphere of high hydrogen partial pressure. Therefore a high rate of hydrogasification can be expected in this part of the gasifier and, according to the 1.h.s. of Fig. 8, the overall reaction rate will increase with increasing hydrogen partial pressure in the gas mixture. If the total pressure of the system is increased a further increase of overall reaction rate is possible (eqn 17 and Fig. 7). Relatively the rate of the carbon-steam reaction does not rise according to Fig. 1 and eqn (7). Experiments with increasing pressure show gasification rates in the Lurgi-gasifier actually increase with total pressure [ 171. On the contrary, in the fluidized bed process (Fig. IO) the heating rate is fast and therefore the proportion of reactive carbon is relatively small. Due to backmixing of solids in the gasifier no preferential reaction of freshly formed char with hydrogen can be expected. On the other hand there is a concentration profile in the gas phase over the height of the fluidized bed. In the lower regions, there are high steam concentrations and with increasing height the hydrogen content of the gas mixture increases according to the extent of the steamcarbon reaction (1). According to eqn (8) a higher inhibition of the steam gasification reaction can be expected in the upper parts of the fluidized bed. In Fig. 11 measurements of r6 of an anthracite in a semi-technical fluidized bed (area 800x 900 mm, maximum height 4 m) dependent on temperature are shown for bed heights of 2 and 4m[18]. The reactivity at 4 m is distinctly lower than that at 2 m, resulting
raw gas wnpositlons
TypIcal
Condltlom
:
vol %
PH,O
% Temp.:-800-900~ !=mssufe:-40 bar
HZ
53
CO
15
cop
17
CH.
15
4
Chemiculrwctlons
:
CH,O,.( I-y-$+yCCh; C+H20=CO*Hz CO+H$J-COz+H, Ct2Hz*CHq mixing
x
HP +;CHs
Steam Ash
Fig. 10. Model of isothermal fluidized bed gasification of coal.
Height 2m E= 55.66 kcd ,+“sno( \ k,=4
. 01 790
?Z~lO-~rn~n-‘,~
/
.
I
I
800
810
I
I
820 830 WB-Temp, ‘C
I 840
Fig. 11. Temperature dependence of the reactivity rz of an anthracite at two different heights of a semitechnical Auidized bed according to Kirchoff el a1.[18].
Reactivities of carbon to steam and hydrogen in a decrease in apparent values of activation energy and frequency factor. This decrease of reactivity is confirmed in a comparison using a bench scale fluidized
bed of smaller bed height. Furthermore, a correlation could be found between the reactivity as a function of bed height and mean hydrogen content of the gas atmosDhere in the fluidized bed. To understand and explain how reactivity in fluidized beds depends on bed height then possible changes in void fraction of the bed, leading to a change of mc per volume unit (see eqn 12), must also be taken into account.
173
4. 0. P. Mahajan, R. Yarzab and P. L. Walker, Jr., Fuel 57,643 (1978). 5. E. Chornet, J. M. Baldasano and H. T. Tarki, Fuel 58, 395
(1979). 6. E. Wickeand K. Hedden,2. Elekfrochem.57,636 (1953). 7. W. Wanzl, K. H. van Heek and H. Jiintgen, Compendium 78179,DGMK-Haupttagung vom 4.-6.10.78in Berlin. 8. _ P. Chiche. C. Durif and S. Pregermain, Fuel 44,5 (1965). Y. H. Jiintgen, K. H. van Heek, p. Leonhardt and A. Sulimma Nicht iotherme /en und Koksen chungen, paper
Messung der Reaktionsfiihigkeit von Kohgegen!ber Wasserdampfl Wasserstof-Mis-
presented at Berg- und Hutten-mlnnischer Tag, Friedberg (DDR) June 1979,to be published in Freiberger Forschungschefte. IO. K. H. van Heek. H. Jiintgen and W. Peters, J. Inst. Fuel 46, 249 (1973).
CONCLUSION
These two examples on the one hand stress the significance of systematical laboratory measurement of carbon reactivity to hydrogen and steam under typical conditions of technical gasification processes. On the other hand it seems to be necessary to analyse carefully the gas solid contact conditions, the temperature and concentration profiles in the reactor, and the timeltemperature history of coal particles moving through the gasification reactor. A modelling of gasification reactors is only possible by a close cooperation of physical chemists and chemical reaction engineers.
REFERENCES E. Wicke and M. Rossberg, 2. Elekrrochem. 57, 641 (1953). J. D. Balckwood and F. McGrory, Aust. J. Chem. 11, 16 (1958). P. P. Feistel, K. H. van Heek and H. Jiintgen, Chem. -IQ. -Techn. 50, 241 (1978): Ger. C/rem. Engng 1,294 (1978).
Il. P. P. Feistel, K. H. van Heek and H. Jiintgen, 12.
363 (1976). J. L. Johnson, Institute of Gas Technology No. 39, pp. 92-117 (1972).
Carbon
Research
14,
Bulletin
13. R. L. Zahradnik and R. A. Glenn, Div. of Fuel Chemistry, National Meeting of America1 Chemical Society. New York, (Sept. 8-12, 1969). 14. P. P. Feistel, K. H. van Heek and H. Jiintgen, High Temperatures-High
Pressures
9, 145 (1977).
15. P. P. Feistel, K. H. van Heek, H. Jiintgen and A. H. Pulsifer Gasification of a German bituminous Coal with HzO, H2 and HzO/H2-Mixtures. Paper presented at the 193rd National Meeting of the American Chemical Society at New Orleans (March 1977). 16. K. J. Hiittinger and P. Schleicher, Preprints Carbon 1980,Int. Conf. Carbon, p. 267. Baden-Baden (1980). 17. C. Lohmann and G. Riibke, gwf-gaslerdgas 121, 359 (1980), and H. Peyrer, private communication. 18. R. Kirchhoff, K. H. van Heek and H. Jiintgen Eetrieb einer Halbtechnischen Versuchsanlage zur vergasung von Kohle und Wasserdampf.
allothermen
Druck-
Paper presented at the 5th OGEW/DGKK-Gemeinschaftstagung, Miinchen, Oct. 22-24 (1980).
Carbon Vol. 19. pp. 167-173. 1981 Printed in Great Britain. All rights reserved
REACTIVITIES OF CARBON TO STEAM AND HYDROGEN AND APPLICATIONS TO TECHNICAL GASIFICATION PROCESSES-A REVIEW? H. J~~NTGEN Bergbau-Forschung GmbH, Franz-Fischer-Weg 61,4300, Essen 13, Germany (Receioed 29 My 1980) Abstract-Experimental results of reactivity measurements on coal and chars to steam, hydrogen and steamhydrogen mixtures are compared and their application to technical processes is discussed. The change of surface area as a function of burn-off has a minor significance for chars made from coal with high initial accessible surface areas. In this case the reaction rate under constant steam pressure is first order with respect to the amount of carbon being gasified. The reaction rate of hydrogasification markedly decreases with burn-off, since the activation energy increases with burn-off due to the consumption of reactive carbon during the reaction. The reaction of carbon with steam hydrogen mixtures can be described as a superposition of carbon steam reaction and hydrogasification. The overall rate relative to that with hydrogen or steam alone can increase if a high amount of reactive carbon is present and immediately reacts with hydrogen. It can decrease if the inhibition of the steam carbon reaction by hydrogen predominates. Some consequences for the technicalperformanceof coal gasification are: The overall reaction rate in a fluid bed decreases with increasing bed height, since hydrogen-steam ratios and therefore the inhibition by hydrogen increase. The overall reaction rate in a moving bed with countercurrent flow of carbon and reacting gases increases with increasing overall pressure since highly reactive carbon, formed in the pyrolysis zone, can immediately react with high partial pressures of hydrogen. 1.INTRODUCTION
system contains three complex consecutive reactions:
When coal or coke are gasified a stoichiometric equation
of overall reaction can be derived from the material balance. For further considerations of mechanism and kinetics it is practical to express this equation as a super-position of some selected basic reactions being kinetically independent. As an example gasification of carbon with steam under a pressure of 40 bar and temperatures between 800 and 850°C can give a product gas with the following composition: 18.5% COz, 14.8% CR, 37% Hz and 29.7% CO. A stoichiometric balance shows that this overall reaction takes place as a sequence of consecutive or parallel reactions: 0.76 x (C + Hz0 = CO t Hz) 0.24 x (C t 2H, = CH,) 0.29 x (CO-t Hz0 = CO, t HJ
(1) (2) (3)
giving the overall reaction C t l.OSH20 = 0.5782 + 0.47CO t 0.29C0, t 0.24CH,,. (4)
coal 5
I
CAR Vol. 19. No. 3-B
(5)
k,,kd k,
kz
gasification products. Gasification can take place in the temperature range 800-1800°C. Therefore we have to consider the relative importance of chemical reactions and transport phenomena like pore diffusion and film diffusion which become rate determining at higher temperatures. This paper only deals with the lower range of temperature where there is chemical control of rates of reaction. If we consider the overall reaction (4) and the three reactions (l)-(3) assuming that reaction (3) is in thermodynamic equilibrium at all times, then the rate of carbon conversion is given as the sum of the rates of the reactions (1) and (2): dXc _ dXc,HZo+ dXc,H2 dXe rate of.C-conversion , dt = ;;;I;:: to the overall dtdt dt (6)
Reaction (1) is the endothermic heterogeneous steamcarbon reaction, (2) is the exothermic hydrogasification reaction and (3) the slightly exothermic shift reaction. Sustained gasification can also be described by these three reactions if additionally there is taken into account that carbon can be gasified in oxygen producing CO, in a strongly exothermic reaction. When coal is gasified the pyrolysis of coal has now to be considered such that the tHerrn Prof. Dr. Klaus Schafer, Direktor des Instituts fur Physikalische Chimie der Universitat Heidelberg, zum 70. Geburtstag am 23. 8. 1980gewidmet. Invited paper presented at the “Carbon 80” Conference in Baden-Baden, on 3 July 1980.
carbon t other pyrolysis products
dXc,mo _
dt
dXe,rr, -= dt
rate of C-conversion according to carbon-steam reaction rate of C-conversion according to hydrogasification.
Therefore the reactivities of carbon with steam and hydrogen have great importance in the modelling and description of such gasification reactions which occur in technical systems. In general two different definitions for 67
168
H. MNTGEN
the reactivity of carbon are used: the reaction rate according to eqn (6)
following gas atmospheres, temperatures and pressures: Gas
Temperature, “C
Partial pressure
d-G
1&=-g-
and the reaction rate related on the proportion of unconverted carbon ,,_dXc
rC-dt
1-x;
t k3 . pH2’
In a differential reactor to determine gasification rates the hydrogen partial pressure is low, and we can neglect k, * pH2 in eqn (8). At low pressures the reaction rate is first order with respect to steam becoming at high pressures a zero order reaction. Figure 1, in which char reactivity r,JJis plotted vs the steam pressure, shows that this law is also valid in the pressure range between 1 and 70 bar using a char prepared from a bituminous coal at low temperatures [3]. As to the dependence of the reaction rate of the steam-carbon reaction on the carbon conversion Xc, Mahajan et a/.[41 studied chars, prepared from American coals, at temperatures of 1OOO“C. They performed isothermal experiments using a thermal balance under the
Schwelkoks 5
(9) In this way they showed that under their conditions the reactivities of the chars to the different gases were in the order:
A later mathematical analysis of these results[5] gave the following expression of reaction rate as a function of carbon conversion:
k, . PHlO
HZ0 =1+ kz *p&O
They found that the variation of Xc with time can be described with the same formula if the function Xc is taken to depend on a dimensionless time T, where T is defined as: T = & with to.s= time in which Xc becomes 0.5.
OF CARBONTO STEAM
The rate of reaction for steam gasification has been shown by many workers to obey Langmuir-Hinshelwood kinetics, noting the reaction steps of adsorption of the steam at the carbon surface, the surface reaction of adsorbed steam with carbon and the desorption of hydrogen[l, 21. In this way the rate constant of reaction (l), as dependent on the partial pressures of steam and hydrogen, can be written as: k
1 1 0.022 27.2
.- 1
The following is a survey of how such reactivity measurements are being applied in technical gasification systems. 2. REACTIVITY
405 900 910 980
air co, Hz0 H*
= k’ *Xc”.‘(1- X,-) with k’ = 1.76.
2
(10)
In Fig. 2 the expressions (dXc/dr) and (l/l -Xc) (dX,/dr) are plotted vs Xc when (dXc/dr) first increases, reaches a maximum and then decreases, whereas (l/l - Xo) (dXo/dr) increases strongly, later less strongly-over the whole range of Xc. This behaviour can be explained by the changes in accessible internal surface during the burn-off of the char. Wicke and Hedden[6] came to the same conclusion in studies of the gasification of pure carbon with CO, at 1000°C. Ad-
Leopold
0,2-0.1mm I
5
10
P,,obd
I 100
2
Fig. 1. Influence of partial pressure of steam on the carbon reactivity r” (according to eqn 8) in the range between 1 and 70 bar according to Feistel et a1.[3].
0
01
05
IO
XC
Fig. 2. Plots of dXC/dr and dXC/dT.l/l -Xc vs Xc according to the experimental data of Mabajan et nL[4]. (Equation 10) on chars, prepared at l.OWC.
169
Reactivities of carbon to steam and hydrogen ditionally they showed that the effective internal surface first increases with carbon conversion, reaches a maximum value in the same range of burn-off as that for the maximum value of reaction rate, and then decreases at higher carbon conversions (Fig. 3). Different results have been obtained from our isothermal and non-isothermal studies using coals and chars prepared at temperatures below the temperature of gasification[7]. A differential reactor was used and the reaction rate was determined from amounts and analyses of product gases. As an example Fig. 4 shows the results of an isothermal run at 10bar and 800°C from which it can be seen that(l/l -Xc x dXJdt) is nearly constant during the range of Xc greater than 0.1 and (dX,-/dt) decreases with increasing X,. Therefore we can write:
dXc r&=,,=k.(l-Xc)or
rS=tl_xcj
1
.-=dXc dt
k.
(11) In this way the reaction rate is first order with respect to the amount of carbon being gasified. For chars, Xc is defined as
m mc,o
Xc=mc.o-mc=l
mc.<,
0
01
05
(12)
For coal Xc is defined as: x,=1-
mc ” mcoal 1- jfjj i 1
(13)
with m, = mass of carbon, mc., = mass of carbon at the beginning of the reaction, mcoal = mass of coal (daf) at the beginning of the reaction, V = volatile in the coal. This behaviour of coal and low temperature chars can be explained because these products have a larger reactive surface area [8] at the beginning of the steam-carbon reaction compared with chars made at high temperature. Hence, for coals, changes of surface area during burn-off are of only minor significance to reaction rate. Therefore we can use non-isothermal experiments to give the same results as for isothermal experiments to determine k in eqn (11) and for the evaluation of apparent activation energies and frequency factors according to: (14) with E = activation energy, k, = frequency factor. As an example, Fig. 5 is an Arrhenius-diagram of a non-isothermal experiment from which, at temperatures greater than 8OO”C,the change from chemical control to pore diffusion control of rate can be seen[9]. Further, Table 1 shows some values of apparent activation energies and frequency factors (according to eqns I I and 14) for the reaction of lignites and hard coals with steam at a pressure of 40 bar[lO]. The lignite (Rheinische Braunkohle) and other iignites, have a low apparent activation energy and therefore high reactivites to steam. This
IO
Fig. 3. Dependence of the effective surface on carbon conversion Xc for the gasification of graphite with CO2 at I bar and 1.OOO”C according to Wickeand Hedden[6].
Fig. 4. Plots of dX/dr and l/IX, dX/dt vs X, for the gasification of low temperature-chars and coals at 10bar and 810°Caccording
to Wanzl et n/.[7].
Fig. 5. Temperature dependence of the reactivity r; for the non-isothermal gasification of an anthracite with steam at 40 bar according to Jiintgen et al. (91.
170
H. J~~NTGEN Table 1. Apparent kinetic parameters for the gasification of lignite and hard coal according to eqns (11)and (14) according to van Heek et al. [ lo] 'ol.Matter '% daf)
I
lignite
BK F 12
I
Lztivationenergy (kcal/mol)
hard coal
Pittsburgh Seam No.8
.requencyfactor (l/min)
29,5
6
.
lo5
46,0
38,9
2
2 Pittsburgh Seam
38,3
32,l
2
3 Hagen
36,Z
31,8
5
4 Zollverein
33,0
34,8
3
. 106 . 105 . 104 . lo5
7.8
36,0
3
lo5
1
5 Geitling
behaviour is mainly caused by these materials containing high contents of Ca-based catalysts which are finely divided in the coal substance. Values of apparent kinetic parameters for hard coals are in a narrow range and the reactivities at temperatures between 800 and 900°C are distinctly below those of Iignites. In general the catalytic compensation effect of increasing activation energies with increasing frequency factor is observed [Ill. 3. REACTIVITY OF CARBON TO HYDROGEN
Johson[l2] and Zaradnik and Glenn[ 131show that the reaction with hydrogen is more complex than with steam because the reactivity of coals or low temperature chars can change extensively with carbon conversion. Several gasification reactions are given as a simplified mode1 in Table 2, where the reactions of volatiles from coal and of so-called reactive carbon formed during pyrolysis are fast and are in parallel with the slow reaction of the usually non-reactive carbon from char[l4]. There are several ways of describing these reactions as a function of the carbon conversion, e.g. by assuming two parallel first order reactions with different rate constants according to
which we can use the following expression: r& =
kH,(l-X,)(1
tb ePxc).
(16)
Mahajan et a1.[4] observed that activation energies decreased with extent of carbon conversion (burn-off) even when reacting a high temperature char with hydrogen. Figure 6 results from a model calculation in which eqns (15) and (16) are compared, and indicates that for both equations r: first decreases with increasing carbon conversion. When all the reactive carbon is consumed it remains constant. For comparison the reactivity t-z of coal with hydrogen at 900°C[13] at pressures of 30-70 bar is shown in Fig. 7 as a function of time. During the first 18min when the coal is heated under hydrogen at a constant heating rate of SO”C/min, the reactivity increases. Afterwards we observe the decrease of reactivity expected from eqn (16) and Fig. 6 at constant temperature. Figure 7 shows that the reactivity increases with increasing hydrogen pressure, this behaviour being described by a Langmuir-Hinschelwood expression[l5]:
(17)
dX, = k2(1 - Xc) for XC > f r&= dt
(15)
with k, > kl, f = proportion of reactive carbon; or by assuming an activation energy decreasing with Xc for Table 2. Simplified model of the hydrogasification of coal according to Feistel et a[.[141
I
II
(15)with K,=IO,K,=I,f=O.25
i
(J
COOI
b
Cv+2HZ+CHL
I I/ I
Hiittinger and Schleicher [16] investigated the catalytically promoted hydrogenation reaction of coke and coal with Fe, Ni and Co catalysts and showed that (dX,/dt) remains constant in a carbon conversion range below 0.5. This indicates that it is the concentration of catalytic sites which is responsible for the reactivity, rather than
-
CH, C, C' ICI
Pyrolysis Hydrogosiflcatlon of Volotlle Matter
I
C*:ReactiveCarbon 17=lnreoctive Carbon Cv ~Carbon in Volatile Matter
-I
Fig. 6. Model calculation for the dependence of the reactivity of carbon against hydrogen during carbon gasification Xc.
171
Reactivities of carbon to steam and hydrogen Coal Pyrolysis (50 “c / minf Hydrogasificotun (900 @C:)
111 0
IO
---*-
30 bar
----
50 bar
-
70bar
1
I
I
I
I
I
I
20
x)
40
50
60
70
80.
Time,
mm
Fig. 7. Reactivity rE to hydrogen of a coal, dependent on time and hydrogen pressure according to Jiintgen et a1.[9].
the amount of reacting carbon. They observed that: wri, rd =
- I&. e)
(1 t k&&)
higher carbon conversions all of the reactive carbon is consumed in the hydrogasification and therefore the second term of eqn (19) is low compared with the first term. Additionally, the inhibition of the steam-carbon reaction by hydrogen adsorbed on reaction sites-as expressed in the Langmuir-Hinshelwood eqn (7)becomes effective. This explains why we find, from the 1.h.s. of Fig. 8, that at higher temperatures the reactivity of steam is higher than that of hydrogen and that in mixtures of steam with hydrogen the reactivity decreases with increasing hydrogen content. When the char is gasified (r.h.s. of Fig. 8) there is little reactive carbon present, and we find that inhibition of the steam-carbon reaction by hydrogen is more effective than the enhancement of overall reaction rate by the parallel hydrogasification reaction whose rate is relatively low. Table 3 shows that the mean reactivity of coals and chars in hydrogen-steam mixtures is lower than in pure steam[9], the effect increasing with decreasing volatile content in the coal or char. The highest inhibition of reaction rate by hydrogen is found when we used a residual char from a semitechnical hydrogasification
where PH2,e is the hydrogen partial pressure at equilibrium. 5. APPLICATIONOF RESULTSTO TECHNICALPROCESSES 4. REAC~VI~
OF CARBON IN STEM-HYDR~E~
MATURE
Using eqns (6), (11) and (16), based upon reactions (1) and (2) we have that the reaction rate of the overall reaction is:
dXf_ kH20(1d
t
-
Xc) + kH2(1 - b e-“XC)
(19)
in which kHzo is as given in eqn (8) and kH2 is as given in eqns (17) or (18). This equation is valid only for noncatalytic gasification reactions and does not consider changes in internal surface during gasification. We expect that this equation is applicable to the gasification of coal with steam and with mixtures of steam and hydrogen. Non-isothermal gasifications of coal and char in hydrogen and steam, and in hydrogen-steam mixtures, are reported in Fig. 8[9]. The left graph of Fig. 8 shows that at low gasification temperatures, corresponding to low carbon conversions, the reactivity of the char with hydrogen is higher than that with steam and that those with mixtures of hydrogen and steam are between these values. This is caused by a large proportion of reactive carbon from the coal which results in the r.h.s. expression of eqn (19) being greater than the 1.h.s. expression. At
These results indicate that in technical ~asi~cation processes the profiles of temperature and gas concentration in the gasifier, the mode of contact between gas and solid and the rate of heating of coal in the reactor, all influence the kinetics of the reaction of steam and hydrogasification. High rates of heating of coal cause a decrease in the proportion of reactive carbon because of conversion of reactive carbon into unreactive carbon (reaction Ifb in Table 2) is faster than the hydrogasification of the reactive carbon (reaction Ha). Efficient interaction between a freshly formed char and hydrogen is more favourable in moving bed reactors with countercurrent stream of gas and solid than in fluidized beds with high back-mixing of solids. Processes in which significant gasification takes place in the temperature range of chemical control involve gasi~cation under pressure in a moving bed with a counRaw pas --HZ CO
COzCHq
WI % x3 22 23 I I
800-I
T
-c
Fig. 8. Reactivity r$ of a coal (1.h.s.)and a low temperature char (r.h.s.) against mixtures of hydrogen and steam according to non-isothermal measurements of Jiintgen et a/.[9].
Fig. 9. Model of moving bed
gasification
of coal.
H. J~NTGEN
172
Table 3. Comparison of reactivities rc of different fuels against steam and mixtures of steam and hydrogen at 40 bar and 800°Caccording to Juntgen et al. [9] Initial material
Anthracite Niederberg
I
Coal
Hagen
I
Fluidized bed char
from coal
Hagen
100/o 86114 100/Q go/10
Residue char from the hydrogasification of Anthracite Niederberg
100/o 87113
Coal Leopold preoxidized with air
100/o
89/11
tercurrent stream of gas and coal, and the isothermal gasification of coal in a fluidized bed using nuclear heat.
In the first process (Fig. 9) there is a pyrolysis zone on the top of the reactor in which coal is slowly heated. In the upper part of the gasification zone the hydrogen concentration is relatively high so that the freshly generated char, containing a high proportion of reactive carbon, is in an atmosphere of high hydrogen partial pressure. Therefore a high rate of hydrogasification can be expected in this part of the gasifier and, according to the 1.h.s. of Fig. 8, the overall reaction rate will increase with increasing hydrogen partial pressure in the gas mixture. If the total pressure of the system is increased a further increase of overall reaction rate is possible (eqn 17 and Fig. 7). Relatively the rate of the carbon-steam reaction does not rise according to Fig. 1 and eqn (7). Experiments with increasing pressure show gasification rates in the Lurgi-gasifier actually increase with total pressure [ 171. On the contrary, in the fluidized bed process (Fig. IO) the heating rate is fast and therefore the proportion of reactive carbon is relatively small. Due to backmixing of solids in the gasifier no preferential reaction of freshly formed char with hydrogen can be expected. On the other hand there is a concentration profile in the gas phase over the height of the fluidized bed. In the lower regions, there are high steam concentrations and with increasing height the hydrogen content of the gas mixture increases according to the extent of the steamcarbon reaction (1). According to eqn (8) a higher inhibition of the steam gasification reaction can be expected in the upper parts of the fluidized bed. In Fig. 11 measurements of r6 of an anthracite in a semi-technical fluidized bed (area 800x 900 mm, maximum height 4 m) dependent on temperature are shown for bed heights of 2 and 4m[18]. The reactivity at 4 m is distinctly lower than that at 2 m, resulting
raw gas wnpositlons
TypIcal
Condltlom
:
vol %
PH,O
% Temp.:-800-900~ !=mssufe:-40 bar
HZ
53
CO
15
cop
17
CH.
15
4
Chemiculrwctlons
:
CH,O,.( I-y-$+yCCh; C+H20=CO*Hz CO+H$J-COz+H, Ct2Hz*CHq mixing
x
HP +;CHs
Steam Ash
Fig. 10. Model of isothermal fluidized bed gasification of coal.
Height 2m E= 55.66 kcd ,+“sno( \ k,=4
. 01 790
?Z~lO-~rn~n-‘,~
/
.
I
I
800
810
I
I
820 830 WB-Temp, ‘C
I 840
Fig. 11. Temperature dependence of the reactivity rz of an anthracite at two different heights of a semitechnical Auidized bed according to Kirchoff el a1.[18].
Reactivities of carbon to steam and hydrogen in a decrease in apparent values of activation energy and frequency factor. This decrease of reactivity is confirmed in a comparison using a bench scale fluidized
bed of smaller bed height. Furthermore, a correlation could be found between the reactivity as a function of bed height and mean hydrogen content of the gas atmosDhere in the fluidized bed. To understand and explain how reactivity in fluidized beds depends on bed height then possible changes in void fraction of the bed, leading to a change of mc per volume unit (see eqn 12), must also be taken into account.
173
4. 0. P. Mahajan, R. Yarzab and P. L. Walker, Jr., Fuel 57,643 (1978). 5. E. Chornet, J. M. Baldasano and H. T. Tarki, Fuel 58, 395
(1979). 6. E. Wickeand K. Hedden,2. Elekfrochem.57,636 (1953). 7. W. Wanzl, K. H. van Heek and H. Jiintgen, Compendium 78179,DGMK-Haupttagung vom 4.-6.10.78in Berlin. 8. _ P. Chiche. C. Durif and S. Pregermain, Fuel 44,5 (1965). Y. H. Jiintgen, K. H. van Heek, p. Leonhardt and A. Sulimma Nicht iotherme /en und Koksen chungen, paper
Messung der Reaktionsfiihigkeit von Kohgegen!ber Wasserdampfl Wasserstof-Mis-
presented at Berg- und Hutten-mlnnischer Tag, Friedberg (DDR) June 1979,to be published in Freiberger Forschungschefte. IO. K. H. van Heek. H. Jiintgen and W. Peters, J. Inst. Fuel 46, 249 (1973).
CONCLUSION
These two examples on the one hand stress the significance of systematical laboratory measurement of carbon reactivity to hydrogen and steam under typical conditions of technical gasification processes. On the other hand it seems to be necessary to analyse carefully the gas solid contact conditions, the temperature and concentration profiles in the reactor, and the timeltemperature history of coal particles moving through the gasification reactor. A modelling of gasification reactors is only possible by a close cooperation of physical chemists and chemical reaction engineers.
REFERENCES E. Wicke and M. Rossberg, 2. Elekrrochem. 57, 641 (1953). J. D. Balckwood and F. McGrory, Aust. J. Chem. 11, 16 (1958). P. P. Feistel, K. H. van Heek and H. Jiintgen, Chem. -IQ. -Techn. 50, 241 (1978): Ger. C/rem. Engng 1,294 (1978).
Il. P. P. Feistel, K. H. van Heek and H. Jiintgen, 12.
363 (1976). J. L. Johnson, Institute of Gas Technology No. 39, pp. 92-117 (1972).
Carbon
Research
14,
Bulletin
13. R. L. Zahradnik and R. A. Glenn, Div. of Fuel Chemistry, National Meeting of America1 Chemical Society. New York, (Sept. 8-12, 1969). 14. P. P. Feistel, K. H. van Heek and H. Jiintgen, High Temperatures-High
Pressures
9, 145 (1977).
15. P. P. Feistel, K. H. van Heek, H. Jiintgen and A. H. Pulsifer Gasification of a German bituminous Coal with HzO, H2 and HzO/H2-Mixtures. Paper presented at the 193rd National Meeting of the American Chemical Society at New Orleans (March 1977). 16. K. J. Hiittinger and P. Schleicher, Preprints Carbon 1980,Int. Conf. Carbon, p. 267. Baden-Baden (1980). 17. C. Lohmann and G. Riibke, gwf-gaslerdgas 121, 359 (1980), and H. Peyrer, private communication. 18. R. Kirchhoff, K. H. van Heek and H. Jiintgen Eetrieb einer Halbtechnischen Versuchsanlage zur vergasung von Kohle und Wasserdampf.
allothermen
Druck-
Paper presented at the 5th OGEW/DGKK-Gemeinschaftstagung, Miinchen, Oct. 22-24 (1980).