Anomalous carbon dioxide gasification behaviour of high temperature coal chars

Anomalous carbon dioxide gasification behaviour of high temperature coal chars

Fuel Processing Technology, 36 (1993) 243-250 243 Elsevier Science Publishers B.V., Amsterdam ANOMALOUS CARBON DIOXIDE GASIFICATION BEHAVIOUR OF HI...

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Fuel Processing Technology, 36 (1993) 243-250

243

Elsevier Science Publishers B.V., Amsterdam

ANOMALOUS CARBON DIOXIDE GASIFICATION BEHAVIOUR OF HIGH TEMPERATURE COAL CHARS M. W e e d a a, F. Ermers a, B. v.d. Linden a, F. Kapteijn b, J.A. Moulijn b a Department of Chemical Engineering, University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands. b Faculty of Chemical Technology and Material Science, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands. Abstract The effect of HTF on the CO2-gasification reactivity of five different coal chars was investigated. The reactivity of three of the five chars was enhanced by HTT up to temperatures of 1673 K to 1800 K. The TSA and ASA of a hvB bituminous coal char decreased considerably by heat treatment. HTF activation was absent for a demineralized char, clearly indicating a correlation between HTI" activation and catalysis by inorganics, presumably iron.

INTRODUCTION The reactivity of coal chars in gasification is usually considered to depend on three main factors; i) the accessibility of the char to the reactant gas, ii) the concentration of carbon active sites, and iii) the presence of catalytic material. It is generally claimed that increasing the severity of heat treatment, i.e. increasing the temperature or time of heating, decreases the reactivity of a char due to the process of thermal annealing, which is thought to involve a combination of micropore collapse, structural ordering of the carbon on a molecular level, and catalyst deactivation [1-3]. In a kinetic study into the CO2-gasification of a bituminous coal char [4], the effect of thermal annealing on the char reactivity was investigated in order to mark out reaction conditions to obtain "clean" kinetics. For this purpose char samples were subjected to different initial heat treatment temperatures (HTT) in an inert atmosphere, and then checked for reactivity and burn-off behaviour under "standard" conditions. In contrast to what was expected on the basis of the general trend, the sample that had been subjected to the highest H T T showed the highest reactivity. Similar results have been reported by R o h d e and Arendt [5] for the CO2-reactivity of heat treated metallurgical coke, but no explanation of the p h e n o m e n o n was given. To elucidate the origin of this anomalous behaviour the effect of the heat treatment on the three factors previously mentioned was investigated through measurement of the total surface area (TSA) and oxygen chemisorption capacity of the "original" and heat treated char, and by checking the effect for a demineralized char. Furthermore, different chars were subjected to the same heat treatment procedure to determine whether the observed effect is truly anomalous.

0378-3820/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.

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EXPERIMENTAL Most chars used in this study originate from a 38Table I Parent coals used. 53/~m size fraction of the parent coal (table 1), SBN-codec coal rank which was pyrolysed in a laminar entrained flow reactor [6] in N2, at 1313 K for 0.6 s. (semi-anthr.), at 134GB04 anthracite 1373 K for 1.4 s. (lignite, hvA bit., hvB bit.-A), or at 108DE10 semi-anthracite 1673 K for 0.5 s. (hvB bit.-B). The anthracite char 516BE30 hvA bituminous was prepared by outgassing of a 106-150 #m size 513DE38 hvB bituminous fraction in a tube oven, under a N z flow at 1133 K 114DE53 lignite for 35 hours. The analysis of the coals can be found c Code of European Centre for elsewhere [4,6]. Apart from the char preparation Coal Specimens step, all experiments were performed in a Setaram TG16/18 thermobalance. Heat treatment was performed as follows. Depending on the char used, about 15 to 30 mg of char was placed in a small ceramic sample pan, dried at 423 K for 15 min. in a 100 ml rain q argon flow, and then subjected to TPD at a heating rate of 10 K min -1 to temperatures of up to 1800 K. The gases (H2, CO and CO2) evolved during TPD were analysed with a Balzers Q M G 112 quadrupole mass spectrometer. The reactivity of the chars was determined by isothermal gasification of about 1.5 mg of sample in 15% CO2/85% Ar gas mixture (285 ml rain "1) at atmospheric pressure. The samples were dried at 423 K for 15 min. and then heated at a rate of 50 K min -1 to the gasification temperature. Due to differences in reactivity of the "original" chars the experiments were performed at temperatures ranging from 1073 K to 1423 K. The TSA of the chars from the hvB bituminous coal was determined from the CO 2 adsorption isotherm at 293 K and pressures of up to 0.1 MPa, using the Dubinin-Polanyi equation. To compare the active surface area (ASA) of these chars, the O2-chemisorption capacity was determined in a 5% O2/95% Ar gas mixture (100 ml rain "1) at 423 K. Prior to chemisorption the char was outgassed in argon at 1273 K for 1 hour. Samples of about 30 mg were used for both the CO2-adsorption and the Oz-chemisorption measurements. Removal of the inorganic constituents of the hvB bituminous coal char was accomplished by washing with HC1 and HF as described by Radovi6 et al. [7]. Through acid washing the ash content of the char decreased from 20 wt% to 2.7 wt%.

RESULTS

Figure 1 shows the results for TPD up to 1800 K of the hvB bit. coal char-A. The results are expressed per unit weight of carbon initially present. Otherwise, the mass spectrometer signals are given as measured. By taking into account a correction for the molar mass of the molecules, the H 2 mass signal would be much smaller indicating that the observed weight loss during heating is almost entirely due to CO. The reactivity profile of the char before and after TPD are presented in figure 2. The results show that TPD brought about an increase in initial reactivity by a factor of 4. In contrast to the reactivity profile of the "original" char, which displays a slight maximum in the reaction rate, the profile after TPD shows an almost linear decrease in reaction rate until it reaches the profiles of the "original" char at about 75% burn-off.

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T P D up to 1800 K resulted in a collapse of the T S A of the char. Measurement of the T S A before and after T P D produced values of 280 m 2 g-Z and < 10 m 2 g-Z, respectively. A similar result was obtained for the capacity of O2-chemisorption. During 1 hour at 423 K, after only outgassing the char at 1273 K for 1 hour, the weight increase per unit of char due to O2-chemisorption was a factor of 10 larger than after T P D up to 1800 K. In fact, with the HT]" char no more weight increase was observed 10 min. after the O 2 was introduced into the gas flow, whereas after outgassing of the char at 1273 K, the oxygen uptake continued for at least 3 hours. No attempt was made to optimize the conditions for chemisorption. Therefore no absolute figures for the A S A of the chars can be given. 30

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The effect of the removal of the inorganic constituents on the reactivity profile of the "original" and HTI" hvB bit. coal char-B (char-A and -B differ only in conditions applied during pyrolysis), are summarized infigure 3. T P D up to 1673 K for this char resulted in an increase in reactivity similar to that for char-A (figure 2). T P D up to 1773 K for the demineralized char-B resulted in a marked decrease in char reactivity, apart from a small initial effect, while in comparison with the "original" char demineralization was observed to have a small positive effect on the reactivity. Results with respect to the chars of the other coals are presented in figure 4A to D. T P D up to 1773 K resulted in a considerable increase in reactivity for the semi-anthracite

246 char. An increase in reactivity was also observed for the hvA bit. coal char. It did not affect the reactivity of the anthracite, while it had a destructive effect on the reactivity of the lignite char. 30 0 D

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Figure 4 Reactivity profiles for gasification of different "original" (m) and HTT 1773 K (e) chars at different temperatures: A anthracite at 1423 K, B semi-anthracite at 1373 K, C hvA bituminous coal at 1373 K, D lignite at 1073 K. DISCUSSION As far as TSA and O2-chemisorption capacity are concerned, the results show the typical symptoms of thermal annealing of the carbon structure. The T P D data show that the decrease in TSA and O2-chemisorption capacity is accompanied by the evolution of CO and H2, consistent w_ith the viewpoint that structural ordering of char is related to the process of graphitization. As this process is known to result in a decrease in char reactivity, it is clear that the changes in TSA and Oz-chemisorption capacity do not correlate with the change in reactivity. The present results are, therefore, further evidence that the concept of O2-chemisorption capacity [7,9] does not generally correlate with the reactivity of coal chars [8]. The strong decrease in reactivity upon heat treatment of the demineralized char, together with the results for the TSA and the O2-chemisorption capacity, leaves no alternative than to attribute the anomalous increase in reactivity to catalysis of inorganic constituents. Without treatment, demineralization resulted in a small increase in reactivity, which is probably due to creation of additional porosity upon removal of the mineral matter [101. With the hvB bit. coal char-A it was observed that the increase in reactivity was realised particularly upon heating of the char from 1673 K to 1800 K. As the T P D data

247 show a significant weight loss, accompanied by the evolution of CO in this temperature region, it seems likely that the catalytically active species are formed upon reduction of metal oxides by carbon. Iron is considered the most likely catalyst for several reasons. Elemental Fe is known to be a good catalyst for CO2-gasification Table 2 Mineral matter analysis of the hvB bit. coal. of carbon, but the oxides are not [11]. According to the mineral Quartz (SIO2) 7.3% matter analysis (table 2) Fe is Kaolinite (A14Si4010(OH)8) 8.1% present in a significant amount. Illite (KI.I.5AI4Si7_6AI1_1.5020(OH)4) 12.1% Furthermore, it is expected that Pyrite (FeS2) 19.5% Calcite (CaCO3) 14.1% reduction to elemental Fe can Siderite (FeCO3) 11.3% easily take place under the TPD Ankerite (Ca(FeMg)(CO3)2) 6.6% conditions applied in this study. Mixed silicates 9.1% This view is supported by results Not determined 12.0% of Kyotani et al. [12], who observed that the reduction of Fe30 4 by carbon already starts around 1050 K in vacuum, while atmospheric pressure will shift this about 100 K to higher temperatures. Further support follows from an investigation into the chemical state of Fe during COz-gasification by Ohtsuka et al. [13]. Their results show that the higher the gasification temperature and the larger the ratio of CO/CO2, the lower is the Fe oxidation state. For instance, metallic ~,-Fe was observed in a 80% CO/20% CO 2 gas flow starting at 1073 K. Results of Hurley [14] show that even during pulverized coal combustion metallic Fe can exists up to high levels of burn-off. The linear part of the reactivity profiles of the H T T chars (figure 2 and 3) means that the reaction rate per amount of carbon actually present is approximately constant during gasification. This indicates that catalytic sites are deactivated as gasification proceeds, as in catalysed gasification the reaction rate is usually controlled by the total number of catalytic species available. It is expected that Fe is oxidized by CO 2 during gasification. Nevertheless, the high activity was observed to persist up to a high level of burn-off. This is probably connected to the poor accessibility of the char, which prevents the Fe inside the particles from oxidation directly at the onset of gasification, and is assumed to result in shrinking sphere like gasification. It has been shown previously by 22 73 K Walker Jr. and coworkers [15,16] that E : '.CO, 15~ pretreatment of different carbons in H 2, IH, 55% at 1123 K for several hours, resulted in an r 30~ 15 increased reactivity during subsequent gasification in CO 2 and HzO due to CO. 15% Ar H, 6 5 % 1 \ g Ar 85% Ar 35% reduction of iron present in the carbon to ¢0 the elemental state. A similar test was performed for the hvB bit. coal char-A. 0 1 2 3 4 For this purpose the char was subjected Time I hr to a H2-treatment in a 65% H2/35% Ar gas flow at 1173 K for more than an hour, Figure 5 Reactivity of the hvB bit. char-A in after partial gasification in a gas flow of a C02/Ar mixture and a CO2/H2/Ar mixture. 15% CO2/85% Ar. Subsequently, the char reactivity was tested by adding a similar

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amount of C O 2 to the H2/Ar as was used for partial gasification in CO2/Ar. The results, which are summarized in figure 5, clearly show that a much higher reaction rate was obtained after H2-treatment, despite the fact that H 2 is generally known to be a strong inhibitor for the uncatalysed gasification reaction [2]. The results thus further support the assumption that catalysis by Fe is responsible for the increase in reactivity as a result of T P D up to high temperature. The results for the other coal chars indicate that the HTI" effect is not truly an anomaly, but is a more general phenomenon which may be related to the a m o u n t ' o f iron present in the coal minerals. The only exception is the marked decrease in reactivity of the lignite char which is most likely due to deactivation of catalytically active calcium. A detailed description of the results of the other coal chars is given elsewhere [4]. CONCLUSIONS

It can be concluded that a high temperature treatment of coal char does not generally lead to a decrease in the gasification reactivity of the char. Three out of five coal chars investigated showed an increase in reactivity towards gasification, as a result of heat treatment up to temperatures ranging from 1673 K to 1800 K in an inert atmosphere. This effect is most likely due to catalysis by iron. H e a t treatment of the hvB bituminous coal char resulted in a collapse of the pore structure of the char, as indicated by a decrease in T S A and O2-chemisorption capacity of the char. The fact that at the same time the gasification reactivity increased clearly shows that the ASA-method does not generally correlate with the reactivity of coal chars. REFERENCES

1 R.H. Essenhigh, in Chemistry of Coal Utilization 2 no Suppl. Vol. (M.A. Elliot, ed.), John Wiley & Sons, New York, 1981, p. 1153. 2 N.M. Laurendeau, N.M., Prog. Energy Combust. Sci., 4 (1978) 221. 3 E.M. Suuberg, in Fundamental Issues in Control of Carbon Gasification Reactivity (J. Lahaye and P. Ehrburger, eds.), Kluwer Academic Publishers, Dordrecht, 1991, p. 269. 4 M. Weeda, Ph.D. Thesis, University of Amsterdam, 1993. 5 W. Rohde and P. Arendt, in Proc. 1991 Int. Conf. on Coal Sci., Butterworth-Heinemann Ltd, Oxford, 1991, p. 432. 6 P.J.J. Tromp, F. Kapteijn and J.A. Moulijn, in Clean Utilization of Coal (Y. Yiiriim, ed.), Kluwer Academic Publishers, Dordrecht, 1992, p. 75. 7 L.R. Radovir, P.L Walker Jr. and R.G. Jenkins, Fuel 67 (1983) 1691. 8 H.J. Miihlen, in Coal Characterisation for Conversion Processes 1989 (W. Prim, K.A. Nater, H.A.G. Chermin and J.A. Moulijn, eds.), Elsevier, Amsterdam, 1990, p. 285. 9 B. McEnaney, in Fundamental Issues in Control of Carbon Gasification Reactivity (J. Lahaye and P. Ehrburger, eds.), Kluwer Academic Publishers, Dordrecht, 1991, p. 175. 10 E. Hippo and P.L Walker Jr., Fuel, 54 (1975) 245. 11 P.L Walker Jr., M. Shelef and R.A. Anderson, in Chemistry and Physics of Carbon Vol.4 (P.L Walker Jr., ed.), Marcel Dekker, New York, 1968, p. 287. 12 T. Kyotani, S. Karasawa and A. Tomita, Fuel, 65 (1986) 1466. 13 Y. Othsuka, Y. Kuroda, Y. Tamai and A. Tomita, Fuel 65 (1476) 1476. 14 J.P. Hurley, Ph.D. Thesis, Pennsylvania State University, 1990. 15 S. Matsumoto and P.L. Walker Jr., Carbon, 24 (1986) 277. 16 T.E. Easier, R.C. Bradt and P.L Walker Jr., Carbon, 29 (1991) 1125.

249 Discussion

Anomalous carbon dioxide gasification behaviour of high temperature coal chars M. Weeda, F. Ermers, B.v.d. Linden, F. Kapteijn and J.A. Moulijn Question: J.-H. Miihlen Why did you use a mixture of Ar, CO2 (15%) and I-I2 (55%) to test the reaction behaviour of a char sample in which iron has been reduced by H 2 treatment? Is it to keep the iron reduced and active? What would have happened, if the reactivity test were carried out in CO2 only? Would the iron be oxidized and deactivated? Answer

The reason why this mixture was used to test the reaction behaviour is that it appeared more convenient to just add the COz instead of first removing the H 2 before adding the CO2. If hydrogen would have been absent in this test, we would still have obtained a high reactivity, but I believe that the iron would be oxidized more quickly then. The reason why it is possible to have an enhanced reactivity over almost the entire burn-off range after HTI' is that the chars become badly accessible as indicated by specific surface area measurements. I suppose that there is a sort of shrinking sphere behaviour. The catalyst stayed active until the gasification front has reached the catalytic species; then it will be deactivated. Question: H.E. van Dam (Fig. 5) At the conditions used, the watergas shift is instantaneous. Is the final rapid gasification in Fig. 5 possibly a fast steam gasification process? AllSWelg

I agree that there will be a watergas shift reaction. On the other hand when all the COz would be 'converted' to H20, there would still be 40% H: left. Hydrogen is known to strongly inhibit the gasification reaction. Based on the results I presented earlier this day I would have expected a much lower reactivity in the absence of a catalyst. Question" A. Tomita This is a comment. It is hard to believe that the oxidation of metallic iron proceeds so slowly under your gasification conditions. The active species should be an oxide. Although the catalytic activity of iron oxides is much smaller than that of metallic iron, they are certainly active to some extent. The large enhancing effects you observed after heat treatment may be not due to the catalytic effect of metallic iron, but perhaps because you got a different form of iron species, for example, a highly dispersed one.

250 Answer Thank you for your comment. I indeed think that oxidation of iron proceeds very rapidly at the gasification conditions applied. However, I do not think that the iron can be reached by C02 directly at the beginning of gasification. The reason for this is the bad accessibility of the char indicated by the low CO2 specific surface area. As we do not have any direct proof of metallic iron being present, the possibility remains that a different form of iron species is responsible for catalysis.