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The effects of ion-exchanged cobalt catalysts on the gasification of wood chars in carbon dioxide
The effects of ion-exchanged catalysts on the gasification in carbon dioxide William
F. DeGroot
cobalt of wood
chars
and G. N. Richards
Wood Chemistry laboratory, University (Received 13 November 1986; revised
of Montana, Missoula, I June 1987)
Montana
59812,
USA
The high catalytic activity of cobalt in gasification of wood chars by carbon dioxide has been studied by determining the effect of heat treatment temperature (HTT) on reactivity of the char and the chemical state of the catalyst. The pyrolytic transformations occurring as low-temperature chars (HTT 400°C) are heated to higher gasification temperatures have also been studied in order to determined first, the nature of the interaction of the catalyst with the pyrolytic carbon; and second, the type of gases produced by pyrolytic reactions prior to or during gasification by carbon dioxide. Maximum catalytic activity was not observed until the HTT exceeded 600°C. The maximum rate ofgasification was not strongly dependent on HTT, butas the HTT was increased the duration of the maximum level of reactivity was reduced, and an increasingly smaller fraction of the carbon maintained contact with the catalyst. The most reactive chars were those of HTT 600°C; these chars gasified completely in approximately 5 min at 600°C. Elemental cobalt was detected in chars of HTT 1OCKF’C by X-ray diffractions, and this is believed to be the active catalytic species at lower temperatures as well. The activity of the catalyst is believed to depend upon reduction to the elemental state, and is further related to the rate of agglomeration of the catalytic species and the efficiency of the reduced catalyst in dissociating carbon dioxide. (Keywords: wood; char; gasification)
The effects of several ion-exchanged catalysts on gasification of wood chars in carbon dioxide are described in the accompanying paper’. Cobalt was found to be a very effective catalyst of gasification of wood chars, and in this paper the effects of cobalt catalysts are discussed in detail with emphasis on the effect of heat treatment temperature (HTT) on the rate of gasification. This paper also includes discussion of the nature of the gases formed by pyrolysis of low-temperature chars, as determined by mass spectrometry. This analysis serves two purposes. First, identification of the gases formed
when low-temperature chars (HTT 400°C) are heated to a higher gasification temperature is indicative of structural features of the char and helps to elucidate the chemical transformations in biomass at high temperatures. Secondly, this investigation showed the quality of the gaseous mixture obtainable by combined pyrolysis and CO, gasification of the car. The tar-forming reactions of biomass are essentially complete at 400°C and as the char is heated to higher temperatures small molecules are eliminated and aliphatic structures are transformed into aromatic structures. Some of the expected components of the pyrolysis gases, e.g. H, and CO, are also products of gasification by steam or carbon dioxide and the heat content of these gases could be recovered without substantially complicating the final gas mixture. This analysis therefore provides an assessment of the opportunities afforded by combined pyrolysis/gasilication of low-HIT chars for enhancing the yield of gases, the overall rate of gasification, and possibly the quality of the product gas.
EXPERIMENTAL The details of sample preparation and gasification rate determination are given in the accompanying paper’. The gasification system was modified to include a unit resolution mass spectrometer (Hewlett-Packard model 5970B Mass Selective Detector) for qualitative analysis of gas mixtures formed during pyrolysis of the sample prior to gasification. When the mass spectrometer was in use helium was used as the reactor purge gas. A splitter (Scientific Glass Engineering, Austin, TX, USA) was placed in the gas line between the reactor outlet and the combustion air inlet. One metre of uncoated vitreous silica capillary tubing (0.20 mm i.d.) connected the splitter to the mass spectrometer. The capillary tubing was contained in a transfer line heated to 100°C. This arrangement diverted approximately 0.1 ml min - ’ (0.3 “/,,) of the reactor gas flow to the mass spectrometer. On the basis of preliminary runs which revealed no high molecular weight pyrolysis products, the mass range of 10 to 110 amu was scanned and approximately fifteen mass spectra were accumulated per minute. X-ray diffraction patterns were obtained using a Phillips diffractometer (CuKcr, 35 kV, 20mA) at a scan rate of 1 degreemin-‘. The sample was mounted on a glass slide using a Vaseline smear. RESULTS Determination
of gasljkation
rate
The gasification by CO, of chars (HTT 800°C) prepared from wood treated at and beyond the ion
001662361/88/03034547$3.00
0
1988 Butterworth
& Co. (Publishers)
Ltd.
FUEL, 1988, Vol 67, March
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Ion-exchanged
Co catalyst effects on wood char gasification in CO,: W. F. DeGroot and G. N. Richards
TaMe 1 Effectof HTT on extent of CO, gasification of chars from ion-exchanged wood. Gasification was for 30 min in 90.9 kPa of CO,
Char yield* (%, dmmf)
Gasification temperature (“C)
Treatment
;;
Co-exchanged
1000 800
1.4 9.2
800 800
600 400
12.9 21.3
K-exchanged
1000 800 600
Ca-exchanged
1000 800 600
Gasification (%)“ P
G
Total
z 400
9 9 6 9 2
>69 73 46 96 0
>78 82 52 105’ 2
9.9 13.4 17.5
800 800 600
1 2 2
6 13 0
7 15 2
8.1 9.3 12.7
800 800 600
14 5 2
36 102 0
50 107’ 2
‘Measured weight loss was 73 %, indicating that most of the combustible gases were detected bChar yields are reported on a dry, ash-free basis assuming that the weight of the ash in the original wood remains in the char ’ Sample gasified completely. Percentage conversion determined by integration of combustible gas detector signal was consistently lOtILl10 % of weight of samples which gasified completely ’ P and G denote the extents of gasification due to pyrolysis and gasification due to reaction with CO,, respectively Table 2 Effect of heat treatment temperature (HTT) on the yield and reactivity of chars prepared from cobalt-exchanged wood and wood treated beyond the ion exchange capacity with cobalt acetate. Gasification was carried out for 30 min at 500°C in 90.9 kPa of CO, Extent of gasification (%) H’lT
Sample
Char yield (%, dmmf)
P
G
Total
Co-exchanged wood 800 (0.23 Co, 0.34% ash) 700 600 5QO
9.2 10.9 12.9 17.2
3 5 5 4
16 66 IO 3
19 71 75 7
Co(Ac),-treated wood (0.40%Co,0.89%ash)
10.6 14.4
11 8
59 82
70 90
800 600
exchange capacity with cobalt is described in the accompanying paper’. In Table 1 the yields and reactivities of chars prepared at different HTT values from cobalt-exchanged wood are compared with those of chars prepared using calcium as a representative alkaline earth metal catalyst and potassium as a representative alkali metal catalyst. As indicated in Table 1, the chars prepared from cobalt-exchanged wood were so reactive that the rate of gasification often exceeded the detection limits of the detector, i.e. all of the oxygen in the combustion air supply was consumed by combustion of product gases. The sample size was reduced to counteract this, but other considerations placed a lower limit on the sample size. In these cases, therefore, the maximum rate of gasification was not observable and the extents of gasification obtained by integrating the detector signal indicate a minimum extent of gasification. The extent of gasification is therefore indicated as being ‘greater than’ the value of the integrated detector response when the detector limit was exceeded and the change in sample weight is shown to indicate the total extent of reaction. The reactivities of the chars containing cobalt catalyst are clearly less dependent on HTT than are those of the chars containing calcium and potassium. The reactivities of the latter chars toward gasification at 800°C increase by at least a factor of two as the HTT is reduced from 1000 to 800°C. By contrast, chars prepared from cobaltexchanged wood at 800 and 1000°C are gasified to a
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FUEL, 1988, Vol 67, March
similar extent at 800°C. When the HTT and gasification temperature are reduced to 6OO”C,complete gasification results, whereas the chars prepared from calcium- and potassium-exchanged wood are completely unreactive at this temperature. When cobalt-exchanged wood was charred and gasified at 4OO“C,no gasification occurred. In order to assess the dependence of reactivity on HTT better, chars prepared from cobalt-exchanged wood at 100°C increments were gasified at a common temperature of 500°C. The results shown in Table 2 indicate that the reactivity peaks at HTT 600°C. The reactivity decreases slightly when the HTT is raised to 700°C and more dramatically when the HTT is raised to 800°C. There is very little reaction for the sample of HTT 500°C. This suggests that there is a threshold temperature for activation of the catalyst between 500 and 600°C. Such a threshold temperature implies the occurrence of a ncessary chemical transformation of the catalyst such as decarboxylation of carbonate salts, or a direct reaction of the catalyst with the carbon. To obtain a more detailed description of the nature of catalysis by cobalt it is helpful to consider the changes in reaction rate with extent of gasification or ‘gasification rate profiles’ shown in Figure 1. The gasification rate profile for the HTT 1000°C char prepared from Coexchanged wood undergoes two phases of reaction when gasified at 800°C. The first stage is very rapid but decreases as the reaction proceeds. The second phase is characterized by a lower, nearly constant rate of reaction, characteristic of a reaction which is of zero order with respect to the quantity of char. The gasification of char prepared from the same wood at lower temperatures (HTT 800°C) shows the same trend, but the transition between the two phases is less distinct. An interesting feature of these gasification rate profiles is that the maximum rate of reaction of the higher temperature char is much higher than the maximum rate for the lower temperature char. This is in contrast to the trends shown in Table 1 for total extents of gasification and also in contrast to the reports of several investigators who have studied the effects of HTT on the catalysis of gasification of lignite chars by a variety of catalysts2-4. The gasification rate profiles for gasification at 600°C of chars (HTT 800 and 600°C) prepared from Coexchanged wood are shown in Figure 2. At this
Ion-exchanged
Co catalyst effects on wood char gasification
Carbon dloxlde I
,
Nitrogen
T
1000
1
I
I1 I I I
I 20
30 Time (mm)
40
50
0
Figure 1
Gasification rate profiles for thegasification at 800°C ofchars 1OOOT;. ., 800°C prepared from cobalt-exchanged wood at: -,
Figure 2 Gasification rate profdes for the gasification at 600°C of char 6OOT; .., 800°C prepared from cobalt-exchanged wood at: -,
temperature the gasification of the HTT 800°C char is much less rapid and gasification is no doubt only due to the more reactive fraction of the char, since the less reactive fraction would not be expected to gasify at this temperature. Comparison of the overall extents of gasilicaton at 600 and 8OO”C,shown in Table 1, confirms this, since the extent of conversion at 600°C is only slightly more than half of that obtained at 800°C. Considering the lower gasification temperature, the overall extent of gasification of this char (HTT 800°C) at 600°C is very similar to that obtained at 800°C although the time required for reaction is longer. For comparison, it should be noted that the char obtained from calciumexchanged wood (HTT 8OO”C),which gasified completely at 800°C was completely unreactive at 600°C. Figure 2 also shows the gasification rate profile of a low temperature char (HTT 600°C) gasified at 600°C. This char was much more reactive than the char prepared at 800°C. As shown by the extents of gasification in Table I, the low-temperature char was completely gasified and there was no evidence of the second, slower stage of gasification, which was observed in gasification of higher temperature chars (HTT 800 and 1000°C) at 800°C. The rates of gasification of chars prepared from wood treated beyond the exchange capacity with cobalt acetate have also been determined and the char yields and extents of reaction of these chars are shown in Table 2. The char
in CO+ W. F. DeGroot
and G. IV. Richards
yields from wood treated beyond the ion exchange capacity are slightly higher than those obtained from wood treated by ion exchange. Chars prepared from wood treated with excess cobalt acetate were very reactive, with the more reactive char (HTT 600°C) being nearly completely gasified in 30 min at only 500°C. In this case, the effects of HTT on reactivity appear to be smaller; for example, chars prepared from cobalt acetate-treated wood at HTT values of 800 and 600°C gave extents of gasification of 59 and 82 %, respectively, when gasified at 500°C. By contrast the corresponding chars prepared from cobalt-exchanged wood gave extents of gasification of 16 and 70%, respectively. Comparison of the maximum rates of gasification shown in Figures 2 and 3 leads to the same conclusion. As shown in Figure 2, the maximum rate ofgasilication of the low-temperature char (HTT 600°C) prepared from cobalt-exchanged wood is approximately eight times higher than that of the high temperature char (HTT 800°C); Figure 3 shows that the maximum reactivities of the corresponding chars prepared from cobalt acetate-treated wood differ by a factor of approximately 2.5. These results are consistent with the results reported in the accompanying paper’ for chars of HTT 8OO”C,in which the char prepared from the ion-exchanged sample was apparently deactivated by pyrolysis to a greater extent than the sample treated beyond the ion exchange capacity with cobalt acetate. These results suggest that the ion-exchanged catalysts are deactivated by high temperatures (>6OO”C) to a greater extent than are added cobalt salts. This could be due to higher mobility of the cobalt species derived from the exchanged cations during char formation, which would result in greater agglomeration of the originally ion-exchanged catalyst. Alternatively, it is possible that the two forms of the catalyst decompose to different products at temperatures >6OO”C and these products could have different catalytic activities, although it is not clear at this time what these forms might be. Pyrolytic gasijkation of low-temperature chars
The chars discussed previously were prepared at or above the gasification temperature in order to minimize the contribution of pyrolysis to the gasification process. We now discuss the pyrolytic transformations that occur in chars prepared at 400°C as they are heated to a
IO-
Nitrogen
I
Carbon dloxlde
- 1000
I
Nitrogen
I 1 I I I I I I I I I
g F
o
4b
Time (mln) Fire 3 Gasification rate profdes for the gasification at 500°C ofchars 600°C and ., 8OOT, from wood treated beyond prepared at -, the exchange capacity with cobalt acetate
FUEL, 1988, Vol 67, March
2
-500;
_
‘. 30
20
_
347
Ion-exchanged
r50°C
is0 7
Co catalyst effects on wood
100
I
I
0’
Temperature 300 500
I
5
I
(“Cl 700
I
Time
,
t=-8oo”C
r
I
char gasification
IS0 -
IO
(mm)
Figure 4 Gasification rate profile and total ion profile for pyrolytic gasification of char (HIT 4OOT) prepared from untreated wood
4 6.5 min 400 oc 2
0
8.5 mm
e
g
600°C
6
:: c 4 s E 0, cc
4
2
in CO,:
W. F. DeGroot
and G. N. Richards
distinctly different from that of the total ion profile. The former peaks at 10.5 min, corresponding to the end of the temperature ramp, while the total ion profile peaks at 8.5min (600°C). The difference in the two profiles is apparently due to evolution of molecular hydrogen. Hydrogen is not detected by the mass spectrometer, but the combustible gas detector is very sensitive to the evolution of hydrogen, since its combustion requires three times more oxygen per unit weight than does combustion of carbon. Comparison of these curves therefore suggests that evolution of molecular hydrogen predominates at high temperatures (T > 65O”C), while carbonaceous compounds are evolved at lower temperatures. The mass spectra of the pyrolysis gases evolved at different times in the pyrolysis are shown in Figure 5. The mass spectra of the pyrolysis gas mixtures indicate that the predominant carbon species evolved during pyrolysis are CO (m/z 28) and CO, (m/z 44), with CO, evolution decreasing at higher temperatures. Surprisingly, there is also a smaller, but significant quantity of water (m/z 18) evolved throughout the pyrolysis process. Mass peaks indicative of methanol, low molecular weight aldehydes and hydrocarbons are also present in very low abundances. The gasification rate profile shown in Figure 4 and the relative level of pyrolysis gases shown in Figure 5 for untreated wood have been demonstrated to be representative of untreated wood as well as wood treated with alkali and alkaline earth metals’. However, as shown in Figure 6, the gasification rate profile of lowtemperature chars prepared from cobalt-exchanged wood differs from this pattern. The profile for the char containing cobalt peaks much earlier than that of the char from untreated wood and this appears to be due to a peak (8.8mi1-1, 630°C) superimposed on the profile of the untreated wood. Analysis of the total ion profile and mass spectra corresponding to this peak suggest that this peak involves evolution of carbonaceous species but does not indicate that it corresponds to the enhanced evolution of any single product which would be indicative of a specific solid phase reaction between carbon and the cobalt catalyst. The nature of this transition is therefore
0 4
50°C
10.5 min 800 ‘C
is0-y
100
I
I
300
I
Temperoture 500
I
I
PC)
1
700
+-8OoYI
1
I
so-
2
0
Llll
a .
20
u I
40
I
I
1
60
I.,
-,
80
! i
I-
,
100
m/z
Figure 5
Mass spectra of combined product gases obtained during pyrolytic gasification of char (HIT 4CKK) prepared from untreated wood
gasification temperature of 800°C. Figure 4 shows the gasification rate profile and the total ion profile for a char from untreated cottonwood (HTT 400°C) heated in flowing helium to 800°C; both curves are from the same experiment. The shape of the gasification rate profile is
348
FUEL, 1988, Vol 67, March
L
Time
(mm)
_
Figure 6 Gasification rate profiles obtained during pyrolytic gasification of chars (HTT 400°C) prepared from: -, untreated wood; and ---, cobalt-exchanged wood
Ion-exchanged
Co catalyst effects on wood char gasification
unknown; however, we believe that it is due to reduction of the catalyst to the active elemental state, since it corresponds approximately to the HTT at which the catalyst became active (see Tables 1 and 2).
X-ray diffraction (XRD) patterns for chars prepared at 800 and 1000°C from cobalt-exchanged wood are shown in Figure 7. The diffraction patterns show evidence of different crystalline phases in the two chars. Two of the peaks in the diffraction pattern of the higher temperature char, correspond to d-spacings characteristic of elemental cobalt (28 = 44.2” and 51.5°)6. The large, broad peak in the XRD pattern for this char is not identified and probably corresponds to ordered regions in the carbon lattice. The peaks in the diffraction pattern of the char of HTT 800°C could not be correlated with a crystalline cobalt compound or with compounds of the small quantity of calcium remaining in this sample. No XRD peaks were observed in chars prepared at 800°C (10 min) from potassium- or calcium-exchanged samples. DISCUSSION The results obtained in this study demonstrate the exceptionally high activity of cobalt in catalysing the gasification of wood chars in carbon dioxide. A pyrolysis temperature of 600°C appears to be required to activate
300
lso~100
Temperature 500
PC) 700
Time (mln)
~800°Ciso--cl
IO
Figure 7 Gasification rate profile and total ion profile for pyrolytic gasification of char (HIT 4OO”kC) prepared from cobalt-exchanged wood
I
I
I
I
XRD patterns
obtained
from chars
prepared
from cobalt-
I
I
50
40 Degrees
Figure 8
I
I
30
20
and G. N. Richards
the catalyst. At higher pyrolysis temperatures the maximum rate of gasification remains fairly constant, but the duration of maximum catalytic activity, and consequently the total extent ofgasification, declines. The combined data from gasification rate determinations and XRD measurements suggest a picture of catalysis by cobalt involving (1) reduction to an active catalytic state, probably elemental cobalt, near 600°C followed by (2) agglomeration of the catalyst particles to form crystalline cobalt. The latter process, which is much more important at temperatures > 8OO”C,gives rise to two distinct phases of gasification. The first, which corresponds to gasification of carbon in contact with cobalt crystallites is fast, while the second, slower rate corresponds to gasification of char which is not in contact with catalyst. The higher the temperature of char formation, the more distinct these two phases of reaction become (see Figure 1). On the basis of the XRD evidence for crystalline elemental cobalt in chars of HTT lOOO”C,we believe that elemental cobalt is the active phase of the catalyst throughout the reaction. The XRD patterns are detectable only when crystallite growth is at a relatively advanced stage and it is assumed that dispersed elemental cobalt is present in the chars prepared at 600 and 800°C. A similar description of the effects of HTT on catalysis of coal gasification in air, involving CaO as the catalytic species, has been suggested by Radovic el ~1’. Diffraction peaks characteristic of CaO were observed in coal chars after 5 min pyrolysis at 1275 K. These conditions are similar to those required for the production of detectable quantities of cobalt crystals in wood chars. The same authors also demonstrated that loss of catalyst dispersion (formation of crystallites) was very rapid in coals containing ion-exchanged copper, and attributed this to the lower Tammann temperature, T,, of copper’ (& =43O”C for Cu versus 1210°C for CaO). The Tammann temperature, which is expressed as a fraction of the absolute melting point of a solid*, corresponds to the temperature at which the mobility of the elements of a crystal lattice becomes ‘appreciable’ allowing, for example, crystal growth. As shown in Tuble 3, the reduction of the metal oxide is important in terms of the Tammann temperature, since the melting point of the solid decreases in going from the metal oxide to the metal. The metal oxides of cobalt and nickel would therefore not be expected to agglomerate significantly at temperatures <8OO”C, while agglomeration in the reduced metal would be expected to be relatively rapid. These considerations suggest that the low activity found for
X-ray diflaction
-50%
in CO,: W. F. DeGroot
I
60
28
exchanged
wood at HTT values of: A, 800°C; and B, 1000°C
FUEL, 1988, Vol 67, March
349
Ion-exchanged
Co catalyst effects on wood char gasification
copper catalysts in the related study’ might be due in part to rapid agglomeration of copper metal during char formation. On the basis of Tammann temperatures nickel and cobalt would be expected to crystallize at similar, slower rates and therefore the rate of gasification of wood chars containing nickel would be expected to be enhanced by reducing the HTT, as was found with the cobalt catalyst. The presence of reduced cobalt in wood chars also suggests that the relative activities of transition metal catalysts in gasification by carbon dioxide might be associated with the rate of dissociation of the reactant gas on the metal surface. Grabke’ has reported rate constants for dissociation of CO, on cobalt, nickel and copper at lOOo”C, which are pertinent to the non-equilibrium conditions used in our studies. The rate constants decrease in the order: kc,> kNi+ kc,, which is the same order of relative activities reported for transition metal Table 3 Calculated Tammann temperatures of transition metals and their oxides Melting points (“C)
Tammann temperatures (“C?
Element
Oxide”
Metal
Oxide”
Metal
A
co
1935 1990 1326
1495 1453 1083
880 900 560
650 620 430
- 230 - 280 - 130
Ni CU
“COO, NiO and CuO *Calculated as 0.52 x (m.p., K) (Ref. 8). Final values are rounded to tens
Table 4
in CO,: W. F. DeGroot
catalysts in the accompanying paper’. The results of these studies are therefore consistent with an oxygen transfer mechanism, as described by Walker et al. in the catalysis by iron of gasification of graphite by carbon dioxidelo. In this case, however, deactivation of the catalyst due to oxidation is not as important as has been found with iron catalysts in gasification of graphite” and wood chars” under COz. Since gasification was complete in chars of HTT 6OO”C,it is believed that the two distinct phases of reaction in chars of HTT 8OO”C,which were also reported by Figueiredo et al. in chars (HTT 85O’C) prepared from cobalt-treated wood’*, are due to agglomeration of the catalyst rather than conversion of the catalytically active reduced cobalt to the inactive oxide. The relative activities of transition metal catalysts in the gasification of wood chars at a given temperature are therefore expected to be related to the relative effectiveness of the metal in dissociating the reactant gas as well as the relative mobilities of the catalysts, as discussed above. Table 4 compares the maximum reaction rates, R,,,, of the chars prepared in this study with those of other chars which have been reported to undergo low-temperature gasification in CO, or H,O. In every case the gasification was catalysed by cobalt or nickel. The maximum rate of 36% min-’ achieved in this study is more than three times higher than the next highest rate at a comparable temperature. Tomita et al. achieved appreciable rates of gasification of coal chars in CO2 and H,O at comparable temperatures, but higher catalyst loadings were used and ~20% of the sample was unreactive13. The maximum rate obtained by Figueiredo et al.” at 805°C is only slightly less than that obtained in this study under similar conditions, but their rate data extrapolate to a rate of less
Comparison of published data on maximum rates of C-CO2 and C-H,0 Catalyst
reactions catalysed by cobalt and nickel
Reactant gas T
Carbod
Type
Concentration (%) Type 0.1’ 0.4
and G. N. Richards
R maxa
(% min- ‘)
Referen&
Pressure (atm)
(“C)
CO, CO2
1.0 1.0
620 620
0.17 0.22
14, Figure 1 14, Figure 1
PFA carbon
850 850
Ni Ni
Yallourn coal
raw raw
Ni Ni
lC+ 10
H,O H,O
0.7 0.7
5ocl 450
2.0 1.3
15, Figure 1 15, Figure 5
Yalloum coal
raw 500 raw
Ni Ni Ni
lad 10 10
H,O %Y
0.7 0.7 1.0
650 650 600
10 0.4 5-i
13, Figure 2 13, Figure 2 13, p. 153
Pine wood char
850 850
co co
CO2 CO,
1.0 1.0
805 149
18 15
12, Table 1 12, Table I
1.6 1.6
Activated carbon
900
co
H,O
0.2
800
2
16, Figure 5
Douglas Fir char
1000
Ni
6@
CO*
0.3
650
9
11, Figure 5
2.5 1.8 2.9 1.8
CO* CO* CO, CO,
1.0 1.0 1.0
800 600 500
22 36 10 4
Cottonwood char
800
co
600
co
Z
co
16
This study, Figure 1 This study, Figure2 Unpublished Thisstudy,Figure3
‘Maximum rate or average rate over initial period of high reactivity. (After period of pyrolytic weight loss in raw coals.) Where rate data are not given they have been estimated from slopes of bum-off curves %ince reactivity data within a given paper are sometimes contradictory, specific sources of reactivity data are given within cited reference ‘Added to furfuryl alcohol as Ni(NOJ ‘Added to raw coal by grinding coal in ammoniacal Ni(NH,),C03 solution ‘Added to acid-washed pine wood from 0.1 M CoNO, solution ‘Added to demineralized activated carbon from aqueous CoCl, solution ‘Added to untreated wood from Ni(CH3C02)2 solution “Added to demineralized wood by ion exchange ‘Added to demineralized wood from 0.01 M Co(CH,CO,), solution ‘Includes pyrolytic gasification. Rate of gasification due to reaction with CO, would be significantly lower
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FUEL, 1988, Vol 67, March
Ion-exchanged
Co catalyst effects on wood char gasification in CO,: W. F. DeGroot and G. A!. Richards
than 10 % min - 1 at 600°C. Their data also indicate that x40% of the carbon was resistant to gasification. The lower reactivity of their chars is no doubt due to their higher HTT and longer soak time (1 h). 6
ACKNOWLEDGEMENTS This work was supported by the Gas Research Institute under Grant No. 5082-260-0683. We are grateful to Dr John Wehrenberg of the University of Montana Department of Geology for providing the XRD data and to Dr R. K. Osterheld of the University of Montana Department of Chemistry for helpful discussions.
7 8 9 10
11 12
REFERENCES 1 2
DeGroot, W. F. and Richards, G. N. Fuel 1988, 67, 352 Radovic, L. R., Walker, P. L., Jr. and Jenkins, R. G. Fuel 1983, 62,209
13 14 15 16
Jenkins, R. G., Nandi, S. P. and Walker, P. L., Jr. Fuel 1973,52, 288 Hippo, E. J., Jenkins, R. G. and Walker, P. L., Jr. Fuel 1979,58, 388 DeGroot, W. F. and Richards, G. N. Final Report to Gas Research Institute for July 1982 to December 1985 (GRI86/0049), 1986,99 pp. ASTM Publication No. PDlS-17i, ‘Index (Inorganic) to the Powder Diffraction File’, Amer. Sot. For Testing and Materials, Philadelphia, 1967 Radovic, L. R., Walker, P. L., Jr. and Jenkins, R. G. Fuet 1983, 82,382 Baker, R. T. K. J. Catal. 1982,78,473 Grabke, H. J. Ber. Bunsenges Phys. Chem. 1967,71, 1067 Walker. P. L.. Jr.. Shelef. M. and Anderson. R. A. ‘Chemistrv and Phisics oiCa;bon’, (Ed. P. L. Walker, J;), Marcel Dekke;, New York, 1968, Vol. 4, p. 287 DeGroot, W. F. and Shalizadeh, F. Fuel 1984,63,210 Figueiredo, J. L., Orfao, J. J. M. and Ferraz, M. C. A. Fuel 1984, 63, 1059 Tomita, A., Ohtsuka, Y. and Tamai, Y. Fuel 1983,62, 150 Marsh, H. and Adair, R. R. Carbon 1975, 13, 327 Ohtsuka,Y ., Tom&a, A. and Tamai, Y. Appl. Catal. 1986,28,105 Kasaoka, S., Sakata, Y., Yamashita, H. and Nishino, T. ht. Chem. Eng. 1981,21,419
FUEL, 1988, Vol 67, March
351
F. DeGroot
cobalt of wood
chars
and G. N. Richards
Wood Chemistry laboratory, University (Received 13 November 1986; revised
of Montana, Missoula, I June 1987)
Montana
59812,
USA
The high catalytic activity of cobalt in gasification of wood chars by carbon dioxide has been studied by determining the effect of heat treatment temperature (HTT) on reactivity of the char and the chemical state of the catalyst. The pyrolytic transformations occurring as low-temperature chars (HTT 400°C) are heated to higher gasification temperatures have also been studied in order to determined first, the nature of the interaction of the catalyst with the pyrolytic carbon; and second, the type of gases produced by pyrolytic reactions prior to or during gasification by carbon dioxide. Maximum catalytic activity was not observed until the HTT exceeded 600°C. The maximum rate ofgasification was not strongly dependent on HTT, butas the HTT was increased the duration of the maximum level of reactivity was reduced, and an increasingly smaller fraction of the carbon maintained contact with the catalyst. The most reactive chars were those of HTT 600°C; these chars gasified completely in approximately 5 min at 600°C. Elemental cobalt was detected in chars of HTT 1OCKF’C by X-ray diffractions, and this is believed to be the active catalytic species at lower temperatures as well. The activity of the catalyst is believed to depend upon reduction to the elemental state, and is further related to the rate of agglomeration of the catalytic species and the efficiency of the reduced catalyst in dissociating carbon dioxide. (Keywords: wood; char; gasification)
The effects of several ion-exchanged catalysts on gasification of wood chars in carbon dioxide are described in the accompanying paper’. Cobalt was found to be a very effective catalyst of gasification of wood chars, and in this paper the effects of cobalt catalysts are discussed in detail with emphasis on the effect of heat treatment temperature (HTT) on the rate of gasification. This paper also includes discussion of the nature of the gases formed by pyrolysis of low-temperature chars, as determined by mass spectrometry. This analysis serves two purposes. First, identification of the gases formed
when low-temperature chars (HTT 400°C) are heated to a higher gasification temperature is indicative of structural features of the char and helps to elucidate the chemical transformations in biomass at high temperatures. Secondly, this investigation showed the quality of the gaseous mixture obtainable by combined pyrolysis and CO, gasification of the car. The tar-forming reactions of biomass are essentially complete at 400°C and as the char is heated to higher temperatures small molecules are eliminated and aliphatic structures are transformed into aromatic structures. Some of the expected components of the pyrolysis gases, e.g. H, and CO, are also products of gasification by steam or carbon dioxide and the heat content of these gases could be recovered without substantially complicating the final gas mixture. This analysis therefore provides an assessment of the opportunities afforded by combined pyrolysis/gasilication of low-HIT chars for enhancing the yield of gases, the overall rate of gasification, and possibly the quality of the product gas.
EXPERIMENTAL The details of sample preparation and gasification rate determination are given in the accompanying paper’. The gasification system was modified to include a unit resolution mass spectrometer (Hewlett-Packard model 5970B Mass Selective Detector) for qualitative analysis of gas mixtures formed during pyrolysis of the sample prior to gasification. When the mass spectrometer was in use helium was used as the reactor purge gas. A splitter (Scientific Glass Engineering, Austin, TX, USA) was placed in the gas line between the reactor outlet and the combustion air inlet. One metre of uncoated vitreous silica capillary tubing (0.20 mm i.d.) connected the splitter to the mass spectrometer. The capillary tubing was contained in a transfer line heated to 100°C. This arrangement diverted approximately 0.1 ml min - ’ (0.3 “/,,) of the reactor gas flow to the mass spectrometer. On the basis of preliminary runs which revealed no high molecular weight pyrolysis products, the mass range of 10 to 110 amu was scanned and approximately fifteen mass spectra were accumulated per minute. X-ray diffraction patterns were obtained using a Phillips diffractometer (CuKcr, 35 kV, 20mA) at a scan rate of 1 degreemin-‘. The sample was mounted on a glass slide using a Vaseline smear. RESULTS Determination
of gasljkation
rate
The gasification by CO, of chars (HTT 800°C) prepared from wood treated at and beyond the ion
001662361/88/03034547$3.00
0
1988 Butterworth
& Co. (Publishers)
Ltd.
FUEL, 1988, Vol 67, March
345
Ion-exchanged
Co catalyst effects on wood char gasification in CO,: W. F. DeGroot and G. N. Richards
TaMe 1 Effectof HTT on extent of CO, gasification of chars from ion-exchanged wood. Gasification was for 30 min in 90.9 kPa of CO,
Char yield* (%, dmmf)
Gasification temperature (“C)
Treatment
;;
Co-exchanged
1000 800
1.4 9.2
800 800
600 400
12.9 21.3
K-exchanged
1000 800 600
Ca-exchanged
1000 800 600
Gasification (%)“ P
G
Total
z 400
9 9 6 9 2
>69 73 46 96 0
>78 82 52 105’ 2
9.9 13.4 17.5
800 800 600
1 2 2
6 13 0
7 15 2
8.1 9.3 12.7
800 800 600
14 5 2
36 102 0
50 107’ 2
‘Measured weight loss was 73 %, indicating that most of the combustible gases were detected bChar yields are reported on a dry, ash-free basis assuming that the weight of the ash in the original wood remains in the char ’ Sample gasified completely. Percentage conversion determined by integration of combustible gas detector signal was consistently lOtILl10 % of weight of samples which gasified completely ’ P and G denote the extents of gasification due to pyrolysis and gasification due to reaction with CO,, respectively Table 2 Effect of heat treatment temperature (HTT) on the yield and reactivity of chars prepared from cobalt-exchanged wood and wood treated beyond the ion exchange capacity with cobalt acetate. Gasification was carried out for 30 min at 500°C in 90.9 kPa of CO, Extent of gasification (%) H’lT
Sample
Char yield (%, dmmf)
P
G
Total
Co-exchanged wood 800 (0.23 Co, 0.34% ash) 700 600 5QO
9.2 10.9 12.9 17.2
3 5 5 4
16 66 IO 3
19 71 75 7
Co(Ac),-treated wood (0.40%Co,0.89%ash)
10.6 14.4
11 8
59 82
70 90
800 600
exchange capacity with cobalt is described in the accompanying paper’. In Table 1 the yields and reactivities of chars prepared at different HTT values from cobalt-exchanged wood are compared with those of chars prepared using calcium as a representative alkaline earth metal catalyst and potassium as a representative alkali metal catalyst. As indicated in Table 1, the chars prepared from cobalt-exchanged wood were so reactive that the rate of gasification often exceeded the detection limits of the detector, i.e. all of the oxygen in the combustion air supply was consumed by combustion of product gases. The sample size was reduced to counteract this, but other considerations placed a lower limit on the sample size. In these cases, therefore, the maximum rate of gasification was not observable and the extents of gasification obtained by integrating the detector signal indicate a minimum extent of gasification. The extent of gasification is therefore indicated as being ‘greater than’ the value of the integrated detector response when the detector limit was exceeded and the change in sample weight is shown to indicate the total extent of reaction. The reactivities of the chars containing cobalt catalyst are clearly less dependent on HTT than are those of the chars containing calcium and potassium. The reactivities of the latter chars toward gasification at 800°C increase by at least a factor of two as the HTT is reduced from 1000 to 800°C. By contrast, chars prepared from cobaltexchanged wood at 800 and 1000°C are gasified to a
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FUEL, 1988, Vol 67, March
similar extent at 800°C. When the HTT and gasification temperature are reduced to 6OO”C,complete gasification results, whereas the chars prepared from calcium- and potassium-exchanged wood are completely unreactive at this temperature. When cobalt-exchanged wood was charred and gasified at 4OO“C,no gasification occurred. In order to assess the dependence of reactivity on HTT better, chars prepared from cobalt-exchanged wood at 100°C increments were gasified at a common temperature of 500°C. The results shown in Table 2 indicate that the reactivity peaks at HTT 600°C. The reactivity decreases slightly when the HTT is raised to 700°C and more dramatically when the HTT is raised to 800°C. There is very little reaction for the sample of HTT 500°C. This suggests that there is a threshold temperature for activation of the catalyst between 500 and 600°C. Such a threshold temperature implies the occurrence of a ncessary chemical transformation of the catalyst such as decarboxylation of carbonate salts, or a direct reaction of the catalyst with the carbon. To obtain a more detailed description of the nature of catalysis by cobalt it is helpful to consider the changes in reaction rate with extent of gasification or ‘gasification rate profiles’ shown in Figure 1. The gasification rate profile for the HTT 1000°C char prepared from Coexchanged wood undergoes two phases of reaction when gasified at 800°C. The first stage is very rapid but decreases as the reaction proceeds. The second phase is characterized by a lower, nearly constant rate of reaction, characteristic of a reaction which is of zero order with respect to the quantity of char. The gasification of char prepared from the same wood at lower temperatures (HTT 800°C) shows the same trend, but the transition between the two phases is less distinct. An interesting feature of these gasification rate profiles is that the maximum rate of reaction of the higher temperature char is much higher than the maximum rate for the lower temperature char. This is in contrast to the trends shown in Table 1 for total extents of gasification and also in contrast to the reports of several investigators who have studied the effects of HTT on the catalysis of gasification of lignite chars by a variety of catalysts2-4. The gasification rate profiles for gasification at 600°C of chars (HTT 800 and 600°C) prepared from Coexchanged wood are shown in Figure 2. At this
Ion-exchanged
Co catalyst effects on wood char gasification
Carbon dloxlde I
,
Nitrogen
T
1000
1
I
I1 I I I
I 20
30 Time (mm)
40
50
0
Figure 1
Gasification rate profiles for thegasification at 800°C ofchars 1OOOT;. ., 800°C prepared from cobalt-exchanged wood at: -,
Figure 2 Gasification rate profdes for the gasification at 600°C of char 6OOT; .., 800°C prepared from cobalt-exchanged wood at: -,
temperature the gasification of the HTT 800°C char is much less rapid and gasification is no doubt only due to the more reactive fraction of the char, since the less reactive fraction would not be expected to gasify at this temperature. Comparison of the overall extents of gasilicaton at 600 and 8OO”C,shown in Table 1, confirms this, since the extent of conversion at 600°C is only slightly more than half of that obtained at 800°C. Considering the lower gasification temperature, the overall extent of gasification of this char (HTT 800°C) at 600°C is very similar to that obtained at 800°C although the time required for reaction is longer. For comparison, it should be noted that the char obtained from calciumexchanged wood (HTT 8OO”C),which gasified completely at 800°C was completely unreactive at 600°C. Figure 2 also shows the gasification rate profile of a low temperature char (HTT 600°C) gasified at 600°C. This char was much more reactive than the char prepared at 800°C. As shown by the extents of gasification in Table I, the low-temperature char was completely gasified and there was no evidence of the second, slower stage of gasification, which was observed in gasification of higher temperature chars (HTT 800 and 1000°C) at 800°C. The rates of gasification of chars prepared from wood treated beyond the exchange capacity with cobalt acetate have also been determined and the char yields and extents of reaction of these chars are shown in Table 2. The char
in CO+ W. F. DeGroot
and G. IV. Richards
yields from wood treated beyond the ion exchange capacity are slightly higher than those obtained from wood treated by ion exchange. Chars prepared from wood treated with excess cobalt acetate were very reactive, with the more reactive char (HTT 600°C) being nearly completely gasified in 30 min at only 500°C. In this case, the effects of HTT on reactivity appear to be smaller; for example, chars prepared from cobalt acetate-treated wood at HTT values of 800 and 600°C gave extents of gasification of 59 and 82 %, respectively, when gasified at 500°C. By contrast the corresponding chars prepared from cobalt-exchanged wood gave extents of gasification of 16 and 70%, respectively. Comparison of the maximum rates of gasification shown in Figures 2 and 3 leads to the same conclusion. As shown in Figure 2, the maximum rate ofgasilication of the low-temperature char (HTT 600°C) prepared from cobalt-exchanged wood is approximately eight times higher than that of the high temperature char (HTT 800°C); Figure 3 shows that the maximum reactivities of the corresponding chars prepared from cobalt acetate-treated wood differ by a factor of approximately 2.5. These results are consistent with the results reported in the accompanying paper’ for chars of HTT 8OO”C,in which the char prepared from the ion-exchanged sample was apparently deactivated by pyrolysis to a greater extent than the sample treated beyond the ion exchange capacity with cobalt acetate. These results suggest that the ion-exchanged catalysts are deactivated by high temperatures (>6OO”C) to a greater extent than are added cobalt salts. This could be due to higher mobility of the cobalt species derived from the exchanged cations during char formation, which would result in greater agglomeration of the originally ion-exchanged catalyst. Alternatively, it is possible that the two forms of the catalyst decompose to different products at temperatures >6OO”C and these products could have different catalytic activities, although it is not clear at this time what these forms might be. Pyrolytic gasijkation of low-temperature chars
The chars discussed previously were prepared at or above the gasification temperature in order to minimize the contribution of pyrolysis to the gasification process. We now discuss the pyrolytic transformations that occur in chars prepared at 400°C as they are heated to a
IO-
Nitrogen
I
Carbon dloxlde
- 1000
I
Nitrogen
I 1 I I I I I I I I I
g F
o
4b
Time (mln) Fire 3 Gasification rate profdes for the gasification at 500°C ofchars 600°C and ., 8OOT, from wood treated beyond prepared at -, the exchange capacity with cobalt acetate
FUEL, 1988, Vol 67, March
2
-500;
_
‘. 30
20
_
347
Ion-exchanged
r50°C
is0 7
Co catalyst effects on wood
100
I
I
0’
Temperature 300 500
I
5
I
(“Cl 700
I
Time
,
t=-8oo”C
r
I
char gasification
IS0 -
IO
(mm)
Figure 4 Gasification rate profile and total ion profile for pyrolytic gasification of char (HIT 4OOT) prepared from untreated wood
4 6.5 min 400 oc 2
0
8.5 mm
e
g
600°C
6
:: c 4 s E 0, cc
4
2
in CO,:
W. F. DeGroot
and G. N. Richards
distinctly different from that of the total ion profile. The former peaks at 10.5 min, corresponding to the end of the temperature ramp, while the total ion profile peaks at 8.5min (600°C). The difference in the two profiles is apparently due to evolution of molecular hydrogen. Hydrogen is not detected by the mass spectrometer, but the combustible gas detector is very sensitive to the evolution of hydrogen, since its combustion requires three times more oxygen per unit weight than does combustion of carbon. Comparison of these curves therefore suggests that evolution of molecular hydrogen predominates at high temperatures (T > 65O”C), while carbonaceous compounds are evolved at lower temperatures. The mass spectra of the pyrolysis gases evolved at different times in the pyrolysis are shown in Figure 5. The mass spectra of the pyrolysis gas mixtures indicate that the predominant carbon species evolved during pyrolysis are CO (m/z 28) and CO, (m/z 44), with CO, evolution decreasing at higher temperatures. Surprisingly, there is also a smaller, but significant quantity of water (m/z 18) evolved throughout the pyrolysis process. Mass peaks indicative of methanol, low molecular weight aldehydes and hydrocarbons are also present in very low abundances. The gasification rate profile shown in Figure 4 and the relative level of pyrolysis gases shown in Figure 5 for untreated wood have been demonstrated to be representative of untreated wood as well as wood treated with alkali and alkaline earth metals’. However, as shown in Figure 6, the gasification rate profile of lowtemperature chars prepared from cobalt-exchanged wood differs from this pattern. The profile for the char containing cobalt peaks much earlier than that of the char from untreated wood and this appears to be due to a peak (8.8mi1-1, 630°C) superimposed on the profile of the untreated wood. Analysis of the total ion profile and mass spectra corresponding to this peak suggest that this peak involves evolution of carbonaceous species but does not indicate that it corresponds to the enhanced evolution of any single product which would be indicative of a specific solid phase reaction between carbon and the cobalt catalyst. The nature of this transition is therefore
0 4
50°C
10.5 min 800 ‘C
is0-y
100
I
I
300
I
Temperoture 500
I
I
PC)
1
700
+-8OoYI
1
I
so-
2
0
Llll
a .
20
u I
40
I
I
1
60
I.,
-,
80
! i
I-
,
100
m/z
Figure 5
Mass spectra of combined product gases obtained during pyrolytic gasification of char (HIT 4CKK) prepared from untreated wood
gasification temperature of 800°C. Figure 4 shows the gasification rate profile and the total ion profile for a char from untreated cottonwood (HTT 400°C) heated in flowing helium to 800°C; both curves are from the same experiment. The shape of the gasification rate profile is
348
FUEL, 1988, Vol 67, March
L
Time
(mm)
_
Figure 6 Gasification rate profiles obtained during pyrolytic gasification of chars (HTT 400°C) prepared from: -, untreated wood; and ---, cobalt-exchanged wood
Ion-exchanged
Co catalyst effects on wood char gasification
unknown; however, we believe that it is due to reduction of the catalyst to the active elemental state, since it corresponds approximately to the HTT at which the catalyst became active (see Tables 1 and 2).
X-ray diffraction (XRD) patterns for chars prepared at 800 and 1000°C from cobalt-exchanged wood are shown in Figure 7. The diffraction patterns show evidence of different crystalline phases in the two chars. Two of the peaks in the diffraction pattern of the higher temperature char, correspond to d-spacings characteristic of elemental cobalt (28 = 44.2” and 51.5°)6. The large, broad peak in the XRD pattern for this char is not identified and probably corresponds to ordered regions in the carbon lattice. The peaks in the diffraction pattern of the char of HTT 800°C could not be correlated with a crystalline cobalt compound or with compounds of the small quantity of calcium remaining in this sample. No XRD peaks were observed in chars prepared at 800°C (10 min) from potassium- or calcium-exchanged samples. DISCUSSION The results obtained in this study demonstrate the exceptionally high activity of cobalt in catalysing the gasification of wood chars in carbon dioxide. A pyrolysis temperature of 600°C appears to be required to activate
300
lso~100
Temperature 500
PC) 700
Time (mln)
~800°Ciso--cl
IO
Figure 7 Gasification rate profile and total ion profile for pyrolytic gasification of char (HIT 4OO”kC) prepared from cobalt-exchanged wood
I
I
I
I
XRD patterns
obtained
from chars
prepared
from cobalt-
I
I
50
40 Degrees
Figure 8
I
I
30
20
and G. N. Richards
the catalyst. At higher pyrolysis temperatures the maximum rate of gasification remains fairly constant, but the duration of maximum catalytic activity, and consequently the total extent ofgasification, declines. The combined data from gasification rate determinations and XRD measurements suggest a picture of catalysis by cobalt involving (1) reduction to an active catalytic state, probably elemental cobalt, near 600°C followed by (2) agglomeration of the catalyst particles to form crystalline cobalt. The latter process, which is much more important at temperatures > 8OO”C,gives rise to two distinct phases of gasification. The first, which corresponds to gasification of carbon in contact with cobalt crystallites is fast, while the second, slower rate corresponds to gasification of char which is not in contact with catalyst. The higher the temperature of char formation, the more distinct these two phases of reaction become (see Figure 1). On the basis of the XRD evidence for crystalline elemental cobalt in chars of HTT lOOO”C,we believe that elemental cobalt is the active phase of the catalyst throughout the reaction. The XRD patterns are detectable only when crystallite growth is at a relatively advanced stage and it is assumed that dispersed elemental cobalt is present in the chars prepared at 600 and 800°C. A similar description of the effects of HTT on catalysis of coal gasification in air, involving CaO as the catalytic species, has been suggested by Radovic el ~1’. Diffraction peaks characteristic of CaO were observed in coal chars after 5 min pyrolysis at 1275 K. These conditions are similar to those required for the production of detectable quantities of cobalt crystals in wood chars. The same authors also demonstrated that loss of catalyst dispersion (formation of crystallites) was very rapid in coals containing ion-exchanged copper, and attributed this to the lower Tammann temperature, T,, of copper’ (& =43O”C for Cu versus 1210°C for CaO). The Tammann temperature, which is expressed as a fraction of the absolute melting point of a solid*, corresponds to the temperature at which the mobility of the elements of a crystal lattice becomes ‘appreciable’ allowing, for example, crystal growth. As shown in Tuble 3, the reduction of the metal oxide is important in terms of the Tammann temperature, since the melting point of the solid decreases in going from the metal oxide to the metal. The metal oxides of cobalt and nickel would therefore not be expected to agglomerate significantly at temperatures <8OO”C, while agglomeration in the reduced metal would be expected to be relatively rapid. These considerations suggest that the low activity found for
X-ray diflaction
-50%
in CO,: W. F. DeGroot
I
60
28
exchanged
wood at HTT values of: A, 800°C; and B, 1000°C
FUEL, 1988, Vol 67, March
349
Ion-exchanged
Co catalyst effects on wood char gasification
copper catalysts in the related study’ might be due in part to rapid agglomeration of copper metal during char formation. On the basis of Tammann temperatures nickel and cobalt would be expected to crystallize at similar, slower rates and therefore the rate of gasification of wood chars containing nickel would be expected to be enhanced by reducing the HTT, as was found with the cobalt catalyst. The presence of reduced cobalt in wood chars also suggests that the relative activities of transition metal catalysts in gasification by carbon dioxide might be associated with the rate of dissociation of the reactant gas on the metal surface. Grabke’ has reported rate constants for dissociation of CO, on cobalt, nickel and copper at lOOo”C, which are pertinent to the non-equilibrium conditions used in our studies. The rate constants decrease in the order: kc,> kNi+ kc,, which is the same order of relative activities reported for transition metal Table 3 Calculated Tammann temperatures of transition metals and their oxides Melting points (“C)
Tammann temperatures (“C?
Element
Oxide”
Metal
Oxide”
Metal
A
co
1935 1990 1326
1495 1453 1083
880 900 560
650 620 430
- 230 - 280 - 130
Ni CU
“COO, NiO and CuO *Calculated as 0.52 x (m.p., K) (Ref. 8). Final values are rounded to tens
Table 4
in CO,: W. F. DeGroot
catalysts in the accompanying paper’. The results of these studies are therefore consistent with an oxygen transfer mechanism, as described by Walker et al. in the catalysis by iron of gasification of graphite by carbon dioxidelo. In this case, however, deactivation of the catalyst due to oxidation is not as important as has been found with iron catalysts in gasification of graphite” and wood chars” under COz. Since gasification was complete in chars of HTT 6OO”C,it is believed that the two distinct phases of reaction in chars of HTT 8OO”C,which were also reported by Figueiredo et al. in chars (HTT 85O’C) prepared from cobalt-treated wood’*, are due to agglomeration of the catalyst rather than conversion of the catalytically active reduced cobalt to the inactive oxide. The relative activities of transition metal catalysts in the gasification of wood chars at a given temperature are therefore expected to be related to the relative effectiveness of the metal in dissociating the reactant gas as well as the relative mobilities of the catalysts, as discussed above. Table 4 compares the maximum reaction rates, R,,,, of the chars prepared in this study with those of other chars which have been reported to undergo low-temperature gasification in CO, or H,O. In every case the gasification was catalysed by cobalt or nickel. The maximum rate of 36% min-’ achieved in this study is more than three times higher than the next highest rate at a comparable temperature. Tomita et al. achieved appreciable rates of gasification of coal chars in CO2 and H,O at comparable temperatures, but higher catalyst loadings were used and ~20% of the sample was unreactive13. The maximum rate obtained by Figueiredo et al.” at 805°C is only slightly less than that obtained in this study under similar conditions, but their rate data extrapolate to a rate of less
Comparison of published data on maximum rates of C-CO2 and C-H,0 Catalyst
reactions catalysed by cobalt and nickel
Reactant gas T
Carbod
Type
Concentration (%) Type 0.1’ 0.4
and G. N. Richards
R maxa
(% min- ‘)
Referen&
Pressure (atm)
(“C)
CO, CO2
1.0 1.0
620 620
0.17 0.22
14, Figure 1 14, Figure 1
PFA carbon
850 850
Ni Ni
Yallourn coal
raw raw
Ni Ni
lC+ 10
H,O H,O
0.7 0.7
5ocl 450
2.0 1.3
15, Figure 1 15, Figure 5
Yalloum coal
raw 500 raw
Ni Ni Ni
lad 10 10
H,O %Y
0.7 0.7 1.0
650 650 600
10 0.4 5-i
13, Figure 2 13, Figure 2 13, p. 153
Pine wood char
850 850
co co
CO2 CO,
1.0 1.0
805 149
18 15
12, Table 1 12, Table I
1.6 1.6
Activated carbon
900
co
H,O
0.2
800
2
16, Figure 5
Douglas Fir char
1000
Ni
6@
CO*
0.3
650
9
11, Figure 5
2.5 1.8 2.9 1.8
CO* CO* CO, CO,
1.0 1.0 1.0
800 600 500
22 36 10 4
Cottonwood char
800
co
600
co
Z
co
16
This study, Figure 1 This study, Figure2 Unpublished Thisstudy,Figure3
‘Maximum rate or average rate over initial period of high reactivity. (After period of pyrolytic weight loss in raw coals.) Where rate data are not given they have been estimated from slopes of bum-off curves %ince reactivity data within a given paper are sometimes contradictory, specific sources of reactivity data are given within cited reference ‘Added to furfuryl alcohol as Ni(NOJ ‘Added to raw coal by grinding coal in ammoniacal Ni(NH,),C03 solution ‘Added to acid-washed pine wood from 0.1 M CoNO, solution ‘Added to demineralized activated carbon from aqueous CoCl, solution ‘Added to untreated wood from Ni(CH3C02)2 solution “Added to demineralized wood by ion exchange ‘Added to demineralized wood from 0.01 M Co(CH,CO,), solution ‘Includes pyrolytic gasification. Rate of gasification due to reaction with CO, would be significantly lower
350
FUEL, 1988, Vol 67, March
Ion-exchanged
Co catalyst effects on wood char gasification in CO,: W. F. DeGroot and G. A!. Richards
than 10 % min - 1 at 600°C. Their data also indicate that x40% of the carbon was resistant to gasification. The lower reactivity of their chars is no doubt due to their higher HTT and longer soak time (1 h). 6
ACKNOWLEDGEMENTS This work was supported by the Gas Research Institute under Grant No. 5082-260-0683. We are grateful to Dr John Wehrenberg of the University of Montana Department of Geology for providing the XRD data and to Dr R. K. Osterheld of the University of Montana Department of Chemistry for helpful discussions.
7 8 9 10
11 12
REFERENCES 1 2
DeGroot, W. F. and Richards, G. N. Fuel 1988, 67, 352 Radovic, L. R., Walker, P. L., Jr. and Jenkins, R. G. Fuel 1983, 62,209
13 14 15 16
Jenkins, R. G., Nandi, S. P. and Walker, P. L., Jr. Fuel 1973,52, 288 Hippo, E. J., Jenkins, R. G. and Walker, P. L., Jr. Fuel 1979,58, 388 DeGroot, W. F. and Richards, G. N. Final Report to Gas Research Institute for July 1982 to December 1985 (GRI86/0049), 1986,99 pp. ASTM Publication No. PDlS-17i, ‘Index (Inorganic) to the Powder Diffraction File’, Amer. Sot. For Testing and Materials, Philadelphia, 1967 Radovic, L. R., Walker, P. L., Jr. and Jenkins, R. G. Fuet 1983, 82,382 Baker, R. T. K. J. Catal. 1982,78,473 Grabke, H. J. Ber. Bunsenges Phys. Chem. 1967,71, 1067 Walker. P. L.. Jr.. Shelef. M. and Anderson. R. A. ‘Chemistrv and Phisics oiCa;bon’, (Ed. P. L. Walker, J;), Marcel Dekke;, New York, 1968, Vol. 4, p. 287 DeGroot, W. F. and Shalizadeh, F. Fuel 1984,63,210 Figueiredo, J. L., Orfao, J. J. M. and Ferraz, M. C. A. Fuel 1984, 63, 1059 Tomita, A., Ohtsuka, Y. and Tamai, Y. Fuel 1983,62, 150 Marsh, H. and Adair, R. R. Carbon 1975, 13, 327 Ohtsuka,Y ., Tom&a, A. and Tamai, Y. Appl. Catal. 1986,28,105 Kasaoka, S., Sakata, Y., Yamashita, H. and Nishino, T. ht. Chem. Eng. 1981,21,419
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