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Relation of the chemical structure of coal to its hydropyrolysis
Relation of the chemical to its hydropyrolysis Kwang
E. Chung
structure
of coal
and Ira B. Goldberg
Rockwell International Science Center, PO Box 1085, Oaks, CA 91360, USA (Received 24 August 1982; revised 24 May 1983)
1049
Camino
DOS Rios,
Thousand
The hydropyrolysis of Illinois No. 6 coal has been studied in a batch reactor, in which the reactions were initiated by explosion of Hz/O, mixtures. The ratio of H, to 0, was kept at 8, while the total pressure of the gas mixture was changed to vary the reaction temperature. The heating rate was ~50000°C s-l, and the reaction time was ~50 ms. The conversion of the feed coal increased from 19% at 620°C to 81 % at > 1500°C. At conversions <50%, the gaseous product consisted of mainly CH, and CO in almost equal proportions, and at conversions >60% the concentration of CO increased. Comparison with results from a large flow reactor revealed that comparable conversions were obtained in the present batch reactor, although product distributions were markedly different from each other. The dissimilar product distribution isattributed to different reacting media: preburning of H,and O2 in the flow reactor versusin situ burning of the mixture in the batch reactor. The H/C ratios of solid residues after the hydropyrolysis decreased linearly as the conversion increased, revealing that the portions of coal having high H/C ratios were preferentially gasified. This observation was substantiated by a high H/C ratio, 1 .74 of the first portion of coal gasified, and by a sharp decrease in H/C ratio in subsequent gasified portions. These data indicated that aliphatic side chains (or linkages) and single-ring aromatic clusters in the feed coal were gasified first, followed by larger aromatic clusters. Semi-quantitative determination of the distribution of different aromatic clusters showed good agreement with current structural information on coal. Thus, the effects of reaction variables were explained in terms of the structural features of coal, and the ratelimiting steps in the hydropyrolysis process were identified.
(Keywords:
coal; structure;
hydropyrolysis)
The hydropyrolysis of coal (pyrolysis in H, hydrogasification) has been actively pursued’-” in benchscale and pilot plants to obtain gaseous and light liquid products. Some important advantages of hydropyrolysis are: (a) a wide range of possible products, including CH,, CzHs, BTX and light oil, can be obtained; (b) up to 95% of the organic components of coal can be converted to CH,; and (c) the reactions often occur within several seconds at temperatures between 700 and 13OO”C, allowing rapid throughput of the feedstock. In spite of the potential advantages of hydropyrolysis, there is a limited understanding of the chemistry of the process. Most investigations have focused on the effects of reaction variables such as temperature, heating rate, mixing or mass transport, hydrogen pressure and reaction time, but have virtually ignored the chemical characteristics of the reactant coal. Although the nature of different coals has not been well understood or characterized, and different apparatus and analytical means have been used, experimental results by numerous investigators show common trends. The hydrolpyrolysis of bituminous coals has been examined extensively 1,2,10. The reaction rate was found to change with temperature, heating rate, hydrogen pressure, reaction time and the extent of conversion. The reaction is considered to consist of primary and secondary reaction steps. In the first step, coal decomposes on heating as a result of the breakage of weak bonds. In this stage volatile matter evolves, and aromatic fragments and free radicals are formed. In the second reaction step, 0016-2361/84/040482~6$3.00 @ 1984 Butterworth & Co. (Publishers)
482
Ltd
FUEL, 1984, Vol 63, April
hydrogen interacts with the primary products to extents which depend on the reaction conditions: temperature, pressure and reaction time. Gas-phase reactions take place between volatile matter and hydrogen, while condensed-phase reactions take place among non-volatile species or between non-volatile matter and hydrogen. According to the above picture of hydropyrolysis, thermal decomposition of coal is a rate-controlling step’. To further explain the kinetic behaviour, the concepts of ‘rapid-rate carbon’ or ‘active sites’ were introduced, and some empirical models were developed to correlate conversion data with some success’. Product distributions in hydropyrolysis have seldom been modelled, except in pyrolysis ‘* . There is little understanding of the nature and formation of ‘rapid-rate carbon’ or ‘active sites’, the overall reaction mechanism of the conversion, or the origin of the product distribution. In this Paper, the development of a simple batch reactor is reported with findings on the sequence of a rapid hydropyrolysis reaction with respect to the chemical structures in the feed coal. The findings are based on chemical structural information on the reacted portion of coal. Also the mechanism of the hydropyrolysis is discussed based on the experimental results and qualitative information of the coal structure. EXPERIMENTAL Materials
Dry Illinois
No. 6 coal (- 325 mesh, 45 pm) was used
Hydropyrolysis with a composition of C, 65.9 wt%; H, 4.5 wt%; N, 1.1 wt%; S, 3.0 wt%; 0,13.2 wt% (by difference) and ash, 12.3 wt%. Hydrogen and oxygen used in the experiments were 99.999% and 99.99% pure, respectively.
The type and design of the reactor and the related instrumentation have been selected with the consideration of (a) a wide range of reaction variables, (b) control and measurement of temperature and pressure, and (c) the requirement of detailed product analysis. A batch reactor was chosen because of its general characteristics of ease of product collection, few difficulties in instrumentation, simple and fast operation, and the use of less reactants than a flow system. Figure I shows the schematic of the new batch reactor. It was designed to withstand internal temperature up to 2000°C and pressures up to 44.8 MPa produced by the thermal explosion of HZ/O, mixtures. The reactor is made of 316 stainless steel with a liner made of thin stainless steel backed by asbestos insulating material. The volume of the reactor is 1000 ml with an insulation layer of 0.5 cm; 2 g of the coal sample can be dispersed in it. Reaction conditions can be varied by using different liners with insulating layers between the liner and wall, preheating, or reactant gas composition and pressure. The reactor assembly consists of a head and a body bolted together with a gas-tight sealing achieved with an O-ring. The head has live openings: one for gas inlet and outlet, two for ignition electrodes, one for a thermocouple and one for a piezo-electric transducer, and provision for colour ratio temperature sensor. This reactor is simpler than that used by Northam and Von Rosenberg13 in reactant loading, operation and construction, although both use HZ/O, mixtures as a reacting medium. Procedure and product analysis
Figure 1
coal
sample
Diagram
temperature
1
(x 1 g) was
of flash hydropyrolysis
placed
reactor
on
a screen
measured in the Hz-02
reaction
Thermocouple
Pressure reading
1010
1100
770 790, 8JOa
2 3
Batch reactor design
The
Table 1 Maximum
of coal: K. E. Chung and I. B. Goldberg
a Readings at two different
(“C)
850 680
positions in the reactor
supported by a tripod in the reactor, and the reaction chamber was evacuated to remove the residual gas. Hydrogen was then added at the desired pressure, followed by oxygen. Typically, the ratio of H, to 0, was 8. Reaction temperature was varied by changing the total pressure of H, and 0,. The reaction was initiated by a hot wire heated through the electrodes. The reaction temperature was measured by a thermocouple. The pressure was monitored by an electronic pressure gauge for long duration measurements such as filling the reactor, and by a piezoelectric transducer for the transient during the reaction. After the reaction, the reactor and its content were allowed to cool to room temperature, and then the final gas pressure was measured. Gaseous products were analysed by gas chromatography (g.c.) and i.r. spectrophotometry to identify gaseous species and their distribution. After analysis, the gaseous product was vented, and the solid residue was collected. This residue was analysed for elemental composition. Some samples were examined by 13C magic angle spinning crosspolarization nuclear magnetic resonance (MAS/CP n.m.r.). RESULTS
AND
DISCUSSION
Temperature measurement and reaction time Due to the explosive nature of the initiation reaction between 0, and H,, the adiabatic reaction temperature and heating rate in the reactor were extremely high. In addition, the reactor provided rapid cooling of the products due to a high heat transfer rate to the reactor wall. Measurements from the thermocouple and piezoelectric transducer were compared with each other by reacting H, and 0, in the absence of coal. The pressure measurements were converted to temperature readings by assuming ideal gas behaviour of the HZ/O2 reaction product, H,O, under the experimental conditions. Table 1 shows the maximum temperature determined by the two means. The measured values were much lower than calculated adiabatic temperatures: e.g., the adiabatic temperature was 2440°C in the case 1. The two sets of measured values differ from each other by about k 100°C. These differences are relatively small, considering the fast rate of reaction and the different characteristics of the two measurement methods. The temperature calculated from the pressure, based on ideal gas behaviour, is that of the average value in the reactor. The response time of the piezoelectric sensor is z 1 us. However, thermocouple measurements exhibit a response time of 1 to 10 ms and the temperature varies with position in the reactor. The relative closeness of the two sets of measurements indicates that the response time of the thermocouple was marginally short enough, due to a high heat transfer rate, to monitor the reaction. In actual experiments with coal, the temperature of coal particles may be closer to that of the thermocouple.
FUEL, 1984, Vol 63, April
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of coal: K. E. Chung and I. B. Goldberg
Hydropyrolysis
Figure 2 shows a temperature profile measured with a thermocouple and recorded on an oscilloscope during a reaction without any coal sample. The temperature reached its maximum within 20 ms, and dropped rapidly after 30 ms. In experiments with coal samples, the maximum temperatures were observed after a similar time, and 50°C of temperature drop from the maximum value was noticed in 50 ms. The temperature measurement is currently being improved, although the general agreement with the pressure measurement (Table 1) indicates that the thermocouple readings are not significantly different from the real temperature inside the reactor. As mentioned earlier, temperature measurement was not reported in a similar reaction system13. In the following discussion, the temperature reference is the maximum value obtained with the thermocouple. The reaction time is assumed to be ~50 ms for all experiments. The heating rate is of the order of 5oOOo”c s-l.
E:XPT
0.1
0.2
0.4
0.3
profile of hydropyrolysis
conditions Reactants
reactor
(obtained
and conversiona
(MPa)
Experiment No.
Temperature
Conversion
02
H2
(“a
(% C)
1 2 3 4 5
0.041 0.076 0.138 0.276 0.414
0.31 0.66 1.21 2.45 3.65
620 740 1140 >13OOb >15ooo
19 41 45 61 81
a Reaction time <50 ms b Thermocouple broken during the experiment
484
4
5
I
I
1
*
20
40
”
FUEL, 1984, Vol 63, April
Figure 3
gas&.; 0,
Gaseous
product
101D
60
CONVERSION, composition.
% C 0,
CO; A,
CH4; 0,
C,
CO2
amounts of H,O formed, and excess H, in the reactor increased. These changes brought about higher conversion and altered the composition of the the product gas as shown in Table 2 and Figure 3. At 19% conversion, measurable amounts of C2H, and C2H, were present, but they were absent at higher conversions. The product gas consisted of mainly CH, and CO in approximately equal proportion up to 50% conversion; beyond 60% conversion, the concentration of CO increased sharply to 85x, while that of CH, dropped to 14%. No other product species, such as C, or larger gases or benzene, were detected. The formation of CH, can be attributed to the hydropyrolysis of coal as observed by others’. However, the amounts of CH, produced are different from those obtained elsewhere. For example, 23% of the carbon of Illinois No. 6 was converted to CH, in the batch reactor at 740°C. This is two to five times larger than that in benchscale tests with Montana Rosebud subbituminous coal within 300 ms of vapour residence time at temperatures, 790 to 880°C in a flow reactor”. In other tests with a large flow reactor (1 ton h-l) by Rockwell International”, the total conversion was 57% at z 1000°C compared with 45% in Experiment 3 in Table 3.
TIME, SECOND
Table 2 Reaction
3
I
CARBON
The carbon balance was > 95% in all experiments with the batch reactor. In initial experiments, the conversion was calculated in three ways based on: (1) the amounts of solid residue; (2) the ash content of solid residue; and (3) the carbon contents of the feed sample, solid residue and gaseous product. The conversion values from the first two agreed within + 2% absolute. The conversion from solid residue was higher by 2 to 6% (absolute) than that based on the carbon contents. A similar trend has been observed by others”, and this indicates a faster gasification of some of the heteroatoms (e.g., oxygen) than that of carbon. At a constant ratio of HZ/O2 = 8, as the total pressure of H, and O2 was increased, the reaction temperature, the
Figure 2 Temperature with a thermocouple)
2
I
8,
Conversion experiments
0
1
A comparable conversion, 62x, was observed in the two reactors under similar conditions: the temperature was x 1300°C and the reaction time was < 100 ms in the batch reactor, while they were 1040°C and > 400 ms in the large flow reactor. In the batch reactor, conversion > 60% required much higher temperature and was accompanied by a large alteration in the product compostion as shown in Figure 3. In the flow reactor, the conversion increased from 55% to 62% as the reaction time extended from 100 ms to 400 ms. The general agreement on the conversion level indicates that coal reacted similarly under the two different reactor configurations which suggests that the results of the batch reactor may be applied to the flow reactor. The somewhat higher conversion with the latter may be due to better mixing and longer reaction time.
Hydropyrolysis Table 3 Carbon and hydrogen
composition
of incremental
of coal: K. E. Chung
portions of gasified coal Gasified portion
Starting material Experimenta
a Experiment
Feed
H/C
Composition
Coal
0.82
C100%2
Residue 1
0.61
C81H49
Residue 3
0.42
CSSH23
Residue 4
0.29
C39Hl1
Residue 5
0.14
C19H3
2 is not included because it is inconsistent
and I. B. Goldberg
Composition
Structure
1.74
C19H33
Single aromatic and aliphatic groups
1.oo
C26H26
0.75
C16H12
0.40
C2o”a
H/C
I
1-ring aromatic groups 2-3 fused rings >6 fused rings
I
1
ring
with the others
The product distributions in the two reactors, however, were significantly different from each other: with the flow reactor, up to 35% liquid yields were obtained at overall conversions ~60% within 100 ms residence time, while no large gaseous species and little soluble material were obtained with the batch reactor. The difference is possibly due to the reacting medium: preburning of H, and 0, in the flow reactor uersus in situ burning of the mixture in the present batch reactor. This suggests that the cracking of the volatilized material is considerably faster in the reacting medium of the batch reactor. The product distribution in the batch reactor remained almost the same up to 60% conversion, but it changed drastically at higher conversions. The sharp decrease in CH, concentration at > 60% conversion may be due to reaction of CH, with H,O to produce CO and H,, and preferential depletion of a certain fraction of coal which is more easily pyrolysed to become CH,. Carbon monoxide could have been produced by the reaction of hydrocarbon gases, e.g. CH,, with H,O or the direct reaction of coal or CH, with 02 The concurrent large changes in CO and CH, concentrations at higher conversions (and correspondingly higher temperatures) indicate that a large portion of CO was produced at the expense of CH,. This further suggests that either at low conversions (~60%) and low temperatures (< 1300’9 CH, production was faster than its conversion, if any, to CO, or CH, and CO might be produced separately since CH,/CO ratio remained almost the same up to 45% conversion. Differentiation of these possibilities will help understand the rapid formation of CH,. The incomplete conversion of residual coal at temperatures > 1300°C discloses that the reaction beyond 60% conversion is slow, and different from that up to 60% conversion. The H/C ratio of the solid residue decreased linearly with conversion or reaction temperature. This trend indicates that a fraction with a higher H/C ratio than the rest of coal or a solid residue was preferentially gasified. Further analysis of the H/C ratio data supplies additional information on the reaction that took place. Although the experiments were performed separately, one can imagine that they were carried out in succession. Thus, coal is reacted to obtain a residue in the first experiment. Then the residue is used to obtain another residue in the second experiment, and so forth. With this successive reaction scheme, it is possible to calculate the composition of the incremental portions of the coal which were gasified.
The results in Table 3 show that the first portion gasified from coal had a high H/C ratio, 1.74, and the successive portions had gradually smaller ratios. The H/C ratios of the first two gasified portions were larger than that of the feed coal, while the H/C ratios of the last two portions were much smaller than that of the feed coal. Considering various structural features of coal including aromatic clusters, aliphatic side chains, linkages and functional groups r4,r5, the H/C ratio of the first gasified portion indicates that it contained more aliphatic carbon and less aromatic carbon than the feed coal. The greater concentration of aliphatic carbon in this portion is expected because of the weaker strength of aliphatic bonds than that of aromatic bonds. The relative amounts of the first portion and the large difference in H/C ratio between the first portion and the others reveal that most aliphatic carbon in the feed coal was gasified. Further analysis ’ 6 of these data indicates that z 70% of carbon in the first portion was aliphatic, which is consistent with the ’ 3C MAS/CP n.m.r. spectra. Figure 4 shows a decrease in the intensity of the aliphatic portion of the spectrum (1360 ppm) in the first residue (Table 3) relative to that of the initial coal. Due to this preferential gasification of aliphatic carbon in the first portion, the second and latter portions contain
I
I 200
I
I 150
I
I
I
100 CHEMICAL
I 50
SHIFT
I
I
1
0
fppm)
Figure 4 13C n.m.r. spectra of coal and partially gasified Top, Illinois No. 6 coal; bottom, partially gasified coal
FUEL, 1984, Vol 63, April
coal.
485
Hydropyrolysis
of coal: K. E. Chung and I. B. Goldberg
mostly aromatic carbon. The examination of the H/C data and qualitative information on coal structure16 allows estimation of the relative sizes of aromatic clusters which were successively gasified. This analysis shows that the aromatic carbon in the first portion and most of the carbon in the second portion originated from single-ring aromatic clusters, while the third portion originated from two- to three-fused-ring structures. The fourth portion appears to be derived from aromatic structures of at least four fused rings. Although the fused aromatic clusters of different sizes were devolatilized, no large gaseous molecules or small liquid products were detected. This indicates that the devolatilized material cracked completely to the gaseous products, CH, and CO. Consequently, the cracking step is faster than the volatilization step. This is further supported by the fact that nearly uniform ratios of CH,/CO up to 50% conversion are obtained. The last portion of coal which was gasified had much larger fused aromatic clusters than others. Even at reaction temperatures near 15OO”C,the reaction was slow (i.e., not completed in 100 ms), and gaseous products were composed primarily of CO. These differences suggest that the final reaction step may be a two-phase reaction between the solid residue and water instead of a voltatilized material. The structural characteristics of the gasified portions of coal agree well with current understanding of the chemical structures of coal. Coal itself is made of different sizes of fused aromatic clusters. Most of the aromatic systems are distributed among single to four fused-ring clusters15. Recently, the distribution of different aromatic cluster groups in a Utah bituminous coal were determined from its liquefaction and solubilization products,i6,’ ’ and it was observed that single-ring clusters were linked to each other, two-fused-ring clusters connected among themselves, and so on. The findings on the Utah coal resemble those on the gasified portion, SO%,of the feed coal, Illinois No. 6, in this investigation. The different groups of aromatic clusters and their relative amounts based on whole coal show the same trend in both coals except that the last residue and the last portion gasified in Illinois NO. 6 coal have much larger aromatic cluster sizes than would be expected from the Utah coal. Also, the distinctive difference in the aromatic cluster size of successive gasified portions of the feed coal used here suggests that small clusters were connected to each other and larger ones were linked to each other as observed with the Utah coal. The above similarity between the structural information on coal in general’ ‘-I 7 and on the gasified portions of Illinois No. 6 coal indicates that the hydrogasification process is strongly dependent on the original chemical structures of the feed coal. This dependency has been recognized by others 1,3,18, but could not be developed because of the scarcity of information available on coal structure as well as on the gasified portion of a coal. Mechanism of the rapid hydropyrolysis reaction The present observation on the involvement
of different structures of the feed coal at different stages of the hydropyrolysis can be related to some important questions such as the nature of the ‘rapid-rate carbon’, the effect of rapid-heating rates, and the identification of the rate-limiting step. This interrelation provides different views and more detailed understanding of the reaction.
486
FUEL, 1984, Vol 63, April
As mentioned earlier, the ‘rapid-rate carbon’ appears when a high heating rate is applied. The structural information on the successively gasified portions, discussed in Table 3, reveals that the nature of the ‘rapid-rate carbon’ changes with the reaction temperature, The resemblance of these structures and their amounts to the structures in coal suggests that the rapid-rate carbon originates primarily from small aromatic clusters, 1 to 3 fused rings, and aliphatic species or side chains in the feed coal. Thus the maximum yield of the rapid-rate carbon is governed by the chemical characteristics of the feed coal. It is interesting to note that up to three-fused ring aromatic clusters in the feed coal can account for the rapid-rate carbon. Thus it appears, in principle, that a coal can be hydropyrolysed rapidly to 100% conversion under the present experimental conditions if it is made of aromatic clusters of less than three fused rings. Since the actual conversion will vary with reaction conditions, the limit of rapid conversion, ~60%, obtained needs to be carefully examined to find out whether the limitation was due to the experimental conditions or due to the structural features of coals. In general, low rank coals, e.g. lignite, subbituminous and some high volatile bituminous coals, are believed to be made of mainly small aromatic clusters14. It is planed to investigate well-characterized coals in the near future. The realization of ‘rapid-rate carbon’ only with high heating rates can be explained by introducing two different stages of thermal decomposition of coal. In the first stage, only very weak bonds are broken to form free radicals without appreciable changes in the molecular weight of the parent structures in the feed coal. This stage consists of the elimination of weakly-bonded functional groups and side chains on the aromatic or hydroaromatic clusters. Evolution of H,O, CH,, CO and CO2 at low temperatures, 7~40O”C, in pyrolysis and solvent dissolution studies’9-2’ supports the existence of such a stage. Free radicals generated at this stage can be quenched, for example, by hydrogen, or undergo intra- or inter-molecular recombination resulting in formation of new bonds. The quenching of free radicals will not appreciably help the devolatilization of the large molecules because of their similar molecular weight before and after this stage of decomposition. However, inefficient quenching will lead to recombination of the free radicals, which will retard the rate of the devolatilization or hydropyrolysis. The second stage of thermal decomposition involves the cleavage of linkages between aromatic or hydroaromatic clusters at higher temperatures than that of the first stage. At this stage, a parent molecule is divided into two smaller molecules having a molecular weight equivalent to, on average, 50% of the parent molecule by the cleavage of one linkage. Free radicals of the smaller molecules will have much greater chances of devolatilization even before quenching. The two stages of thermal decomposition will progress in a certain trend which depends on the nature of chemical bonds in a feed coal. According to experimental data’,“, slow heating rates cause the first stage thermal decomposition to proceed to the recombination of free radicals of large parent molecules, adding new linkages between the parent molecules. This will most likely happen during the heatup time, when the temperature is much lower than nominal reaction temperature and thus the second stage thermal decomposition occurs to a small extent. The
Hydropyrolysis
formation of new linkages are expected to reduce the conversion in two ways: (1) the number of bonds that must be broken to achieve devolatilization is increased; and (2) the new bonds are likely to be stronger than the originals because they are formed at higher temperatures. If there was no recombination of large free radicals, the reaction rate would be approximately the same or decrease logarithmically during gasification at a certain reaction temperature regardless of the previous heating rate; such a case, to the authors’ knowledge, has not been reported. The recombination of large free radicals formed in the first stage may be inhibited by high hydrogen pressure and high heating rates. The beneficial effect of hydrogen pressure on the reaction rate and conversion’*” indicates that the large free radicals are quenched by hydrogen before forming new bonds. Nevertheless, the increase of reaction rate with greater pressure is smaller with slow heating rates than with high heating rates at similar reaction temperatures. Thus, the high heating rate plays a uniquely effective role, which is different from that of H, gas. With higher heating rates, the two steps of thermal decomposition can occur within a short time, and large molecules fragment into much smaller entities made of aromatic clusters and free radical sites. These fragments or aromatic clusters will vaporize at temperatures which depend on the molecular size of the aromatic clusters. Thus, a rapid heating rate will facilitate the fragmentation and devolatilization, but within the limit of conversion set by the amounts of devolatilizable aromatic clusters originally present in the feed coal. This interpretation of the effect of heating rate appears to agree well with a limited conversion or slight dependence of conversion on rapid heating rates’.“. The dependency of conversion on reaction temperature3*‘0V’4is commonly observed, and the temperature effect has been attributed simply to thermal decomposition’. However, the present analysis of gasified portions of coal at different temperatures reveals different roles of temperature as the gasification proceeds. The structure of the first gasified portion (see Table 3) shows that most of the aliphatic carbon in the feed coal was consumed. Consequently, an extensive cleavage of aliphatic bonds or linkages should have taken place at the reaction temperature, 620°C. This is supported by the substantial decrease in aliphatic contents in other gasified portions at higher reaction temperatures. Thus, at temperatures, 620 to 74O”C, the large molecules of coal are fragmented, but only fragments of sufficiently small size will vaporize. At temperatures >74o”C, few aliphatic bonds remain, and higher temperatures contribute to the vaporization of progressively larger fragments. This hypothesis is supported by observation of a sharp increase in aromatic cluster size with increasing reaction temperature > 740°C. The ‘slowly reacting material’ comprising large aromatic clusters is not necessarily initially present in the feed coal. It was noted that early in the thermal decomposition, large free radicals which are generated with a slow heating rate will seek stabilization through recombination or intramolecular rearrangements. Even with a high heating rate, molecules with relatively large aromatic clusters might not be heated rapidly enough to sustain a considerable reduction in molecular size, and may combine with other reactive species which further increase their
of coal: K. E. Chung and I. B. Goldberg
size. The recombination of large free radicals is similar to the process often called secondary reactions’. These secondary reactions seem to include condensed phase as well as vapour-phase reactions which produce nonvolatile material with some gaseous product. These processes have not been as well defined as have other aspects of hydropyrolysis. According to the analysis here, the non-volatile material is mostly produced in the solid phase and results when combination of fragments is preferential to their further decomposition. This process is enhanced at slow heating rates. Considering the effects of heating rate and temperature, the rate-limiting step in the hydropyrolysis changes with reaction conditions and conversion. This change is due to the complex chemical nature of coal. Major structural features of a coal that play important roles in reaction rate and conversion include: 1. the nature and distribution of very weak bonds in side chains and functional groups; 2. the nature and distribution of linkages between stable structural units; and 3. the nature and size distribution of stable structural groups. ACKNOWLEDGEMENT The authors thank Joe Ratto for the n.m.r. measurements. REFERENCES 1 2
3 4 5 6 7 8 9 10.
11 12 13 14 15
16 17 18
Anthony, D. B. and Howard, J. B. AIChE J. 1976,22(4), 625 and references therein Howard, J. B. Chapter 12 in 2nd Supplementary Vol. ‘Chemistry of Coal Utilization’. (Ed. M. A. Elliott).II John Wilev & Sons. Inc. NY, 1981 ” Graff, R. A., Dobner, S. and Squires, A. N. Fuel 1976, 55, 109 Suuberg, E. M., Peters, W. A. and Howard, J. B. Fuel 1980,59,405 Green, M., Ladelfa, C. J. and Bivacca, S. J. Fuel Process. Technol. 1980,3,75 Tyler, R. J. Fuel 1979,58, 681 Fallon, P. T., Bhatt, B. and Steinberg, M. Fuel Process. ‘khnol. 1980,3, 155 Finn, M. J., Fynes, G., Ladner, W. R. and Newman, J. 0. H. Fuel 1980,59, 397 Von Fredersdorff, C. G. Ind. Eng. Chem. 1960,52(7), 595 Friedman, J. ‘Development of a Single-Stage, Entrained-Flow, Short-Residence-time Hydrogasifier’, Rockwell International Energy Systems Group, Final Report, FE-2518-24, July 1979 Stangebv, P. C. and Sears, P. L. Fuel 1981.60. 131 Jain,R. Ph.D. thesis, California Institute of T&hnology, 1980 Northam, D. B. and von Rosenberg, C. W., Jr., Fuel, 1979,58,264 Wender, I. Catal. Rev. 1976, 14(l), 97 Wiser, W. H. ‘Scientific Problems Relevant to Coal Utilization’, Proceedings of the Conference, US DOE Sym. Ser., West Virginia University, Morgantown, WV, 1977 Chung, K. E. and Goldberg, I. B. ‘Structural Information from H/C Ratio on Coal and Related Materials’, Manuscript in preparation, June 1982 Chung, K. E. Ph.D. thesis, University of Utah, 1980 Chung, K. E., Anderson, L. L. and Wiser, W. H. Am. Chem. Sot. Div. Fuel. Chem., Preprints 1979, 24(3), 243
19
Wiser, W. H., Anderson, L. L., Qader, S. A. and Hill, G. R. J. Appl. Chem. Biotechnol. 1971, 21, 82
20
21 22 23
Wen, C. Y. and Dutta, S. ‘Rates of Coal Pyrolysis and Gasilication Reactions’, in ‘Coal Conversion Technology’, (Eds. C. Y. Wen and E. S. Lee) Addison-Wesley, 1979 Solomon, P. R. ‘Relation Between Coal Structure and Thermal Decomposition Products’, in ‘Coal Structure’ (Eds. M. L. Gorbaty and K. Ouchi) Am. Chem. Sot. Chem. Ser. 1981,192,95 Hiteshue, R. W., Friedman, S. and Madden, R. ‘Hydrogasilication of Bituminous Coals, Lignite, Anthracite, and Char, RI 6125, Bureau of Mines, Dept. of&&or, Wash., DC, 1962 Anthony, D. B., Howard, J. B., Hottel, H. C. and Meissner, H. P. Fuel 1976,55, 121
FUEL, 1984, Vol 63, April
487
E. Chung
structure
of coal
and Ira B. Goldberg
Rockwell International Science Center, PO Box 1085, Oaks, CA 91360, USA (Received 24 August 1982; revised 24 May 1983)
1049
Camino
DOS Rios,
Thousand
The hydropyrolysis of Illinois No. 6 coal has been studied in a batch reactor, in which the reactions were initiated by explosion of Hz/O, mixtures. The ratio of H, to 0, was kept at 8, while the total pressure of the gas mixture was changed to vary the reaction temperature. The heating rate was ~50000°C s-l, and the reaction time was ~50 ms. The conversion of the feed coal increased from 19% at 620°C to 81 % at > 1500°C. At conversions <50%, the gaseous product consisted of mainly CH, and CO in almost equal proportions, and at conversions >60% the concentration of CO increased. Comparison with results from a large flow reactor revealed that comparable conversions were obtained in the present batch reactor, although product distributions were markedly different from each other. The dissimilar product distribution isattributed to different reacting media: preburning of H,and O2 in the flow reactor versusin situ burning of the mixture in the batch reactor. The H/C ratios of solid residues after the hydropyrolysis decreased linearly as the conversion increased, revealing that the portions of coal having high H/C ratios were preferentially gasified. This observation was substantiated by a high H/C ratio, 1 .74 of the first portion of coal gasified, and by a sharp decrease in H/C ratio in subsequent gasified portions. These data indicated that aliphatic side chains (or linkages) and single-ring aromatic clusters in the feed coal were gasified first, followed by larger aromatic clusters. Semi-quantitative determination of the distribution of different aromatic clusters showed good agreement with current structural information on coal. Thus, the effects of reaction variables were explained in terms of the structural features of coal, and the ratelimiting steps in the hydropyrolysis process were identified.
(Keywords:
coal; structure;
hydropyrolysis)
The hydropyrolysis of coal (pyrolysis in H, hydrogasification) has been actively pursued’-” in benchscale and pilot plants to obtain gaseous and light liquid products. Some important advantages of hydropyrolysis are: (a) a wide range of possible products, including CH,, CzHs, BTX and light oil, can be obtained; (b) up to 95% of the organic components of coal can be converted to CH,; and (c) the reactions often occur within several seconds at temperatures between 700 and 13OO”C, allowing rapid throughput of the feedstock. In spite of the potential advantages of hydropyrolysis, there is a limited understanding of the chemistry of the process. Most investigations have focused on the effects of reaction variables such as temperature, heating rate, mixing or mass transport, hydrogen pressure and reaction time, but have virtually ignored the chemical characteristics of the reactant coal. Although the nature of different coals has not been well understood or characterized, and different apparatus and analytical means have been used, experimental results by numerous investigators show common trends. The hydrolpyrolysis of bituminous coals has been examined extensively 1,2,10. The reaction rate was found to change with temperature, heating rate, hydrogen pressure, reaction time and the extent of conversion. The reaction is considered to consist of primary and secondary reaction steps. In the first step, coal decomposes on heating as a result of the breakage of weak bonds. In this stage volatile matter evolves, and aromatic fragments and free radicals are formed. In the second reaction step, 0016-2361/84/040482~6$3.00 @ 1984 Butterworth & Co. (Publishers)
482
Ltd
FUEL, 1984, Vol 63, April
hydrogen interacts with the primary products to extents which depend on the reaction conditions: temperature, pressure and reaction time. Gas-phase reactions take place between volatile matter and hydrogen, while condensed-phase reactions take place among non-volatile species or between non-volatile matter and hydrogen. According to the above picture of hydropyrolysis, thermal decomposition of coal is a rate-controlling step’. To further explain the kinetic behaviour, the concepts of ‘rapid-rate carbon’ or ‘active sites’ were introduced, and some empirical models were developed to correlate conversion data with some success’. Product distributions in hydropyrolysis have seldom been modelled, except in pyrolysis ‘* . There is little understanding of the nature and formation of ‘rapid-rate carbon’ or ‘active sites’, the overall reaction mechanism of the conversion, or the origin of the product distribution. In this Paper, the development of a simple batch reactor is reported with findings on the sequence of a rapid hydropyrolysis reaction with respect to the chemical structures in the feed coal. The findings are based on chemical structural information on the reacted portion of coal. Also the mechanism of the hydropyrolysis is discussed based on the experimental results and qualitative information of the coal structure. EXPERIMENTAL Materials
Dry Illinois
No. 6 coal (- 325 mesh, 45 pm) was used
Hydropyrolysis with a composition of C, 65.9 wt%; H, 4.5 wt%; N, 1.1 wt%; S, 3.0 wt%; 0,13.2 wt% (by difference) and ash, 12.3 wt%. Hydrogen and oxygen used in the experiments were 99.999% and 99.99% pure, respectively.
The type and design of the reactor and the related instrumentation have been selected with the consideration of (a) a wide range of reaction variables, (b) control and measurement of temperature and pressure, and (c) the requirement of detailed product analysis. A batch reactor was chosen because of its general characteristics of ease of product collection, few difficulties in instrumentation, simple and fast operation, and the use of less reactants than a flow system. Figure I shows the schematic of the new batch reactor. It was designed to withstand internal temperature up to 2000°C and pressures up to 44.8 MPa produced by the thermal explosion of HZ/O, mixtures. The reactor is made of 316 stainless steel with a liner made of thin stainless steel backed by asbestos insulating material. The volume of the reactor is 1000 ml with an insulation layer of 0.5 cm; 2 g of the coal sample can be dispersed in it. Reaction conditions can be varied by using different liners with insulating layers between the liner and wall, preheating, or reactant gas composition and pressure. The reactor assembly consists of a head and a body bolted together with a gas-tight sealing achieved with an O-ring. The head has live openings: one for gas inlet and outlet, two for ignition electrodes, one for a thermocouple and one for a piezo-electric transducer, and provision for colour ratio temperature sensor. This reactor is simpler than that used by Northam and Von Rosenberg13 in reactant loading, operation and construction, although both use HZ/O, mixtures as a reacting medium. Procedure and product analysis
Figure 1
coal
sample
Diagram
temperature
1
(x 1 g) was
of flash hydropyrolysis
placed
reactor
on
a screen
measured in the Hz-02
reaction
Thermocouple
Pressure reading
1010
1100
770 790, 8JOa
2 3
Batch reactor design
The
Table 1 Maximum
of coal: K. E. Chung and I. B. Goldberg
a Readings at two different
(“C)
850 680
positions in the reactor
supported by a tripod in the reactor, and the reaction chamber was evacuated to remove the residual gas. Hydrogen was then added at the desired pressure, followed by oxygen. Typically, the ratio of H, to 0, was 8. Reaction temperature was varied by changing the total pressure of H, and 0,. The reaction was initiated by a hot wire heated through the electrodes. The reaction temperature was measured by a thermocouple. The pressure was monitored by an electronic pressure gauge for long duration measurements such as filling the reactor, and by a piezoelectric transducer for the transient during the reaction. After the reaction, the reactor and its content were allowed to cool to room temperature, and then the final gas pressure was measured. Gaseous products were analysed by gas chromatography (g.c.) and i.r. spectrophotometry to identify gaseous species and their distribution. After analysis, the gaseous product was vented, and the solid residue was collected. This residue was analysed for elemental composition. Some samples were examined by 13C magic angle spinning crosspolarization nuclear magnetic resonance (MAS/CP n.m.r.). RESULTS
AND
DISCUSSION
Temperature measurement and reaction time Due to the explosive nature of the initiation reaction between 0, and H,, the adiabatic reaction temperature and heating rate in the reactor were extremely high. In addition, the reactor provided rapid cooling of the products due to a high heat transfer rate to the reactor wall. Measurements from the thermocouple and piezoelectric transducer were compared with each other by reacting H, and 0, in the absence of coal. The pressure measurements were converted to temperature readings by assuming ideal gas behaviour of the HZ/O2 reaction product, H,O, under the experimental conditions. Table 1 shows the maximum temperature determined by the two means. The measured values were much lower than calculated adiabatic temperatures: e.g., the adiabatic temperature was 2440°C in the case 1. The two sets of measured values differ from each other by about k 100°C. These differences are relatively small, considering the fast rate of reaction and the different characteristics of the two measurement methods. The temperature calculated from the pressure, based on ideal gas behaviour, is that of the average value in the reactor. The response time of the piezoelectric sensor is z 1 us. However, thermocouple measurements exhibit a response time of 1 to 10 ms and the temperature varies with position in the reactor. The relative closeness of the two sets of measurements indicates that the response time of the thermocouple was marginally short enough, due to a high heat transfer rate, to monitor the reaction. In actual experiments with coal, the temperature of coal particles may be closer to that of the thermocouple.
FUEL, 1984, Vol 63, April
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of coal: K. E. Chung and I. B. Goldberg
Hydropyrolysis
Figure 2 shows a temperature profile measured with a thermocouple and recorded on an oscilloscope during a reaction without any coal sample. The temperature reached its maximum within 20 ms, and dropped rapidly after 30 ms. In experiments with coal samples, the maximum temperatures were observed after a similar time, and 50°C of temperature drop from the maximum value was noticed in 50 ms. The temperature measurement is currently being improved, although the general agreement with the pressure measurement (Table 1) indicates that the thermocouple readings are not significantly different from the real temperature inside the reactor. As mentioned earlier, temperature measurement was not reported in a similar reaction system13. In the following discussion, the temperature reference is the maximum value obtained with the thermocouple. The reaction time is assumed to be ~50 ms for all experiments. The heating rate is of the order of 5oOOo”c s-l.
E:XPT
0.1
0.2
0.4
0.3
profile of hydropyrolysis
conditions Reactants
reactor
(obtained
and conversiona
(MPa)
Experiment No.
Temperature
Conversion
02
H2
(“a
(% C)
1 2 3 4 5
0.041 0.076 0.138 0.276 0.414
0.31 0.66 1.21 2.45 3.65
620 740 1140 >13OOb >15ooo
19 41 45 61 81
a Reaction time <50 ms b Thermocouple broken during the experiment
484
4
5
I
I
1
*
20
40
”
FUEL, 1984, Vol 63, April
Figure 3
gas&.; 0,
Gaseous
product
101D
60
CONVERSION, composition.
% C 0,
CO; A,
CH4; 0,
C,
CO2
amounts of H,O formed, and excess H, in the reactor increased. These changes brought about higher conversion and altered the composition of the the product gas as shown in Table 2 and Figure 3. At 19% conversion, measurable amounts of C2H, and C2H, were present, but they were absent at higher conversions. The product gas consisted of mainly CH, and CO in approximately equal proportion up to 50% conversion; beyond 60% conversion, the concentration of CO increased sharply to 85x, while that of CH, dropped to 14%. No other product species, such as C, or larger gases or benzene, were detected. The formation of CH, can be attributed to the hydropyrolysis of coal as observed by others’. However, the amounts of CH, produced are different from those obtained elsewhere. For example, 23% of the carbon of Illinois No. 6 was converted to CH, in the batch reactor at 740°C. This is two to five times larger than that in benchscale tests with Montana Rosebud subbituminous coal within 300 ms of vapour residence time at temperatures, 790 to 880°C in a flow reactor”. In other tests with a large flow reactor (1 ton h-l) by Rockwell International”, the total conversion was 57% at z 1000°C compared with 45% in Experiment 3 in Table 3.
TIME, SECOND
Table 2 Reaction
3
I
CARBON
The carbon balance was > 95% in all experiments with the batch reactor. In initial experiments, the conversion was calculated in three ways based on: (1) the amounts of solid residue; (2) the ash content of solid residue; and (3) the carbon contents of the feed sample, solid residue and gaseous product. The conversion values from the first two agreed within + 2% absolute. The conversion from solid residue was higher by 2 to 6% (absolute) than that based on the carbon contents. A similar trend has been observed by others”, and this indicates a faster gasification of some of the heteroatoms (e.g., oxygen) than that of carbon. At a constant ratio of HZ/O2 = 8, as the total pressure of H, and O2 was increased, the reaction temperature, the
Figure 2 Temperature with a thermocouple)
2
I
8,
Conversion experiments
0
1
A comparable conversion, 62x, was observed in the two reactors under similar conditions: the temperature was x 1300°C and the reaction time was < 100 ms in the batch reactor, while they were 1040°C and > 400 ms in the large flow reactor. In the batch reactor, conversion > 60% required much higher temperature and was accompanied by a large alteration in the product compostion as shown in Figure 3. In the flow reactor, the conversion increased from 55% to 62% as the reaction time extended from 100 ms to 400 ms. The general agreement on the conversion level indicates that coal reacted similarly under the two different reactor configurations which suggests that the results of the batch reactor may be applied to the flow reactor. The somewhat higher conversion with the latter may be due to better mixing and longer reaction time.
Hydropyrolysis Table 3 Carbon and hydrogen
composition
of incremental
of coal: K. E. Chung
portions of gasified coal Gasified portion
Starting material Experimenta
a Experiment
Feed
H/C
Composition
Coal
0.82
C100%2
Residue 1
0.61
C81H49
Residue 3
0.42
CSSH23
Residue 4
0.29
C39Hl1
Residue 5
0.14
C19H3
2 is not included because it is inconsistent
and I. B. Goldberg
Composition
Structure
1.74
C19H33
Single aromatic and aliphatic groups
1.oo
C26H26
0.75
C16H12
0.40
C2o”a
H/C
I
1-ring aromatic groups 2-3 fused rings >6 fused rings
I
1
ring
with the others
The product distributions in the two reactors, however, were significantly different from each other: with the flow reactor, up to 35% liquid yields were obtained at overall conversions ~60% within 100 ms residence time, while no large gaseous species and little soluble material were obtained with the batch reactor. The difference is possibly due to the reacting medium: preburning of H, and 0, in the flow reactor uersus in situ burning of the mixture in the present batch reactor. This suggests that the cracking of the volatilized material is considerably faster in the reacting medium of the batch reactor. The product distribution in the batch reactor remained almost the same up to 60% conversion, but it changed drastically at higher conversions. The sharp decrease in CH, concentration at > 60% conversion may be due to reaction of CH, with H,O to produce CO and H,, and preferential depletion of a certain fraction of coal which is more easily pyrolysed to become CH,. Carbon monoxide could have been produced by the reaction of hydrocarbon gases, e.g. CH,, with H,O or the direct reaction of coal or CH, with 02 The concurrent large changes in CO and CH, concentrations at higher conversions (and correspondingly higher temperatures) indicate that a large portion of CO was produced at the expense of CH,. This further suggests that either at low conversions (~60%) and low temperatures (< 1300’9 CH, production was faster than its conversion, if any, to CO, or CH, and CO might be produced separately since CH,/CO ratio remained almost the same up to 45% conversion. Differentiation of these possibilities will help understand the rapid formation of CH,. The incomplete conversion of residual coal at temperatures > 1300°C discloses that the reaction beyond 60% conversion is slow, and different from that up to 60% conversion. The H/C ratio of the solid residue decreased linearly with conversion or reaction temperature. This trend indicates that a fraction with a higher H/C ratio than the rest of coal or a solid residue was preferentially gasified. Further analysis of the H/C ratio data supplies additional information on the reaction that took place. Although the experiments were performed separately, one can imagine that they were carried out in succession. Thus, coal is reacted to obtain a residue in the first experiment. Then the residue is used to obtain another residue in the second experiment, and so forth. With this successive reaction scheme, it is possible to calculate the composition of the incremental portions of the coal which were gasified.
The results in Table 3 show that the first portion gasified from coal had a high H/C ratio, 1.74, and the successive portions had gradually smaller ratios. The H/C ratios of the first two gasified portions were larger than that of the feed coal, while the H/C ratios of the last two portions were much smaller than that of the feed coal. Considering various structural features of coal including aromatic clusters, aliphatic side chains, linkages and functional groups r4,r5, the H/C ratio of the first gasified portion indicates that it contained more aliphatic carbon and less aromatic carbon than the feed coal. The greater concentration of aliphatic carbon in this portion is expected because of the weaker strength of aliphatic bonds than that of aromatic bonds. The relative amounts of the first portion and the large difference in H/C ratio between the first portion and the others reveal that most aliphatic carbon in the feed coal was gasified. Further analysis ’ 6 of these data indicates that z 70% of carbon in the first portion was aliphatic, which is consistent with the ’ 3C MAS/CP n.m.r. spectra. Figure 4 shows a decrease in the intensity of the aliphatic portion of the spectrum (1360 ppm) in the first residue (Table 3) relative to that of the initial coal. Due to this preferential gasification of aliphatic carbon in the first portion, the second and latter portions contain
I
I 200
I
I 150
I
I
I
100 CHEMICAL
I 50
SHIFT
I
I
1
0
fppm)
Figure 4 13C n.m.r. spectra of coal and partially gasified Top, Illinois No. 6 coal; bottom, partially gasified coal
FUEL, 1984, Vol 63, April
coal.
485
Hydropyrolysis
of coal: K. E. Chung and I. B. Goldberg
mostly aromatic carbon. The examination of the H/C data and qualitative information on coal structure16 allows estimation of the relative sizes of aromatic clusters which were successively gasified. This analysis shows that the aromatic carbon in the first portion and most of the carbon in the second portion originated from single-ring aromatic clusters, while the third portion originated from two- to three-fused-ring structures. The fourth portion appears to be derived from aromatic structures of at least four fused rings. Although the fused aromatic clusters of different sizes were devolatilized, no large gaseous molecules or small liquid products were detected. This indicates that the devolatilized material cracked completely to the gaseous products, CH, and CO. Consequently, the cracking step is faster than the volatilization step. This is further supported by the fact that nearly uniform ratios of CH,/CO up to 50% conversion are obtained. The last portion of coal which was gasified had much larger fused aromatic clusters than others. Even at reaction temperatures near 15OO”C,the reaction was slow (i.e., not completed in 100 ms), and gaseous products were composed primarily of CO. These differences suggest that the final reaction step may be a two-phase reaction between the solid residue and water instead of a voltatilized material. The structural characteristics of the gasified portions of coal agree well with current understanding of the chemical structures of coal. Coal itself is made of different sizes of fused aromatic clusters. Most of the aromatic systems are distributed among single to four fused-ring clusters15. Recently, the distribution of different aromatic cluster groups in a Utah bituminous coal were determined from its liquefaction and solubilization products,i6,’ ’ and it was observed that single-ring clusters were linked to each other, two-fused-ring clusters connected among themselves, and so on. The findings on the Utah coal resemble those on the gasified portion, SO%,of the feed coal, Illinois No. 6, in this investigation. The different groups of aromatic clusters and their relative amounts based on whole coal show the same trend in both coals except that the last residue and the last portion gasified in Illinois NO. 6 coal have much larger aromatic cluster sizes than would be expected from the Utah coal. Also, the distinctive difference in the aromatic cluster size of successive gasified portions of the feed coal used here suggests that small clusters were connected to each other and larger ones were linked to each other as observed with the Utah coal. The above similarity between the structural information on coal in general’ ‘-I 7 and on the gasified portions of Illinois No. 6 coal indicates that the hydrogasification process is strongly dependent on the original chemical structures of the feed coal. This dependency has been recognized by others 1,3,18, but could not be developed because of the scarcity of information available on coal structure as well as on the gasified portion of a coal. Mechanism of the rapid hydropyrolysis reaction The present observation on the involvement
of different structures of the feed coal at different stages of the hydropyrolysis can be related to some important questions such as the nature of the ‘rapid-rate carbon’, the effect of rapid-heating rates, and the identification of the rate-limiting step. This interrelation provides different views and more detailed understanding of the reaction.
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FUEL, 1984, Vol 63, April
As mentioned earlier, the ‘rapid-rate carbon’ appears when a high heating rate is applied. The structural information on the successively gasified portions, discussed in Table 3, reveals that the nature of the ‘rapid-rate carbon’ changes with the reaction temperature, The resemblance of these structures and their amounts to the structures in coal suggests that the rapid-rate carbon originates primarily from small aromatic clusters, 1 to 3 fused rings, and aliphatic species or side chains in the feed coal. Thus the maximum yield of the rapid-rate carbon is governed by the chemical characteristics of the feed coal. It is interesting to note that up to three-fused ring aromatic clusters in the feed coal can account for the rapid-rate carbon. Thus it appears, in principle, that a coal can be hydropyrolysed rapidly to 100% conversion under the present experimental conditions if it is made of aromatic clusters of less than three fused rings. Since the actual conversion will vary with reaction conditions, the limit of rapid conversion, ~60%, obtained needs to be carefully examined to find out whether the limitation was due to the experimental conditions or due to the structural features of coals. In general, low rank coals, e.g. lignite, subbituminous and some high volatile bituminous coals, are believed to be made of mainly small aromatic clusters14. It is planed to investigate well-characterized coals in the near future. The realization of ‘rapid-rate carbon’ only with high heating rates can be explained by introducing two different stages of thermal decomposition of coal. In the first stage, only very weak bonds are broken to form free radicals without appreciable changes in the molecular weight of the parent structures in the feed coal. This stage consists of the elimination of weakly-bonded functional groups and side chains on the aromatic or hydroaromatic clusters. Evolution of H,O, CH,, CO and CO2 at low temperatures, 7~40O”C, in pyrolysis and solvent dissolution studies’9-2’ supports the existence of such a stage. Free radicals generated at this stage can be quenched, for example, by hydrogen, or undergo intra- or inter-molecular recombination resulting in formation of new bonds. The quenching of free radicals will not appreciably help the devolatilization of the large molecules because of their similar molecular weight before and after this stage of decomposition. However, inefficient quenching will lead to recombination of the free radicals, which will retard the rate of the devolatilization or hydropyrolysis. The second stage of thermal decomposition involves the cleavage of linkages between aromatic or hydroaromatic clusters at higher temperatures than that of the first stage. At this stage, a parent molecule is divided into two smaller molecules having a molecular weight equivalent to, on average, 50% of the parent molecule by the cleavage of one linkage. Free radicals of the smaller molecules will have much greater chances of devolatilization even before quenching. The two stages of thermal decomposition will progress in a certain trend which depends on the nature of chemical bonds in a feed coal. According to experimental data’,“, slow heating rates cause the first stage thermal decomposition to proceed to the recombination of free radicals of large parent molecules, adding new linkages between the parent molecules. This will most likely happen during the heatup time, when the temperature is much lower than nominal reaction temperature and thus the second stage thermal decomposition occurs to a small extent. The
Hydropyrolysis
formation of new linkages are expected to reduce the conversion in two ways: (1) the number of bonds that must be broken to achieve devolatilization is increased; and (2) the new bonds are likely to be stronger than the originals because they are formed at higher temperatures. If there was no recombination of large free radicals, the reaction rate would be approximately the same or decrease logarithmically during gasification at a certain reaction temperature regardless of the previous heating rate; such a case, to the authors’ knowledge, has not been reported. The recombination of large free radicals formed in the first stage may be inhibited by high hydrogen pressure and high heating rates. The beneficial effect of hydrogen pressure on the reaction rate and conversion’*” indicates that the large free radicals are quenched by hydrogen before forming new bonds. Nevertheless, the increase of reaction rate with greater pressure is smaller with slow heating rates than with high heating rates at similar reaction temperatures. Thus, the high heating rate plays a uniquely effective role, which is different from that of H, gas. With higher heating rates, the two steps of thermal decomposition can occur within a short time, and large molecules fragment into much smaller entities made of aromatic clusters and free radical sites. These fragments or aromatic clusters will vaporize at temperatures which depend on the molecular size of the aromatic clusters. Thus, a rapid heating rate will facilitate the fragmentation and devolatilization, but within the limit of conversion set by the amounts of devolatilizable aromatic clusters originally present in the feed coal. This interpretation of the effect of heating rate appears to agree well with a limited conversion or slight dependence of conversion on rapid heating rates’.“. The dependency of conversion on reaction temperature3*‘0V’4is commonly observed, and the temperature effect has been attributed simply to thermal decomposition’. However, the present analysis of gasified portions of coal at different temperatures reveals different roles of temperature as the gasification proceeds. The structure of the first gasified portion (see Table 3) shows that most of the aliphatic carbon in the feed coal was consumed. Consequently, an extensive cleavage of aliphatic bonds or linkages should have taken place at the reaction temperature, 620°C. This is supported by the substantial decrease in aliphatic contents in other gasified portions at higher reaction temperatures. Thus, at temperatures, 620 to 74O”C, the large molecules of coal are fragmented, but only fragments of sufficiently small size will vaporize. At temperatures >74o”C, few aliphatic bonds remain, and higher temperatures contribute to the vaporization of progressively larger fragments. This hypothesis is supported by observation of a sharp increase in aromatic cluster size with increasing reaction temperature > 740°C. The ‘slowly reacting material’ comprising large aromatic clusters is not necessarily initially present in the feed coal. It was noted that early in the thermal decomposition, large free radicals which are generated with a slow heating rate will seek stabilization through recombination or intramolecular rearrangements. Even with a high heating rate, molecules with relatively large aromatic clusters might not be heated rapidly enough to sustain a considerable reduction in molecular size, and may combine with other reactive species which further increase their
of coal: K. E. Chung and I. B. Goldberg
size. The recombination of large free radicals is similar to the process often called secondary reactions’. These secondary reactions seem to include condensed phase as well as vapour-phase reactions which produce nonvolatile material with some gaseous product. These processes have not been as well defined as have other aspects of hydropyrolysis. According to the analysis here, the non-volatile material is mostly produced in the solid phase and results when combination of fragments is preferential to their further decomposition. This process is enhanced at slow heating rates. Considering the effects of heating rate and temperature, the rate-limiting step in the hydropyrolysis changes with reaction conditions and conversion. This change is due to the complex chemical nature of coal. Major structural features of a coal that play important roles in reaction rate and conversion include: 1. the nature and distribution of very weak bonds in side chains and functional groups; 2. the nature and distribution of linkages between stable structural units; and 3. the nature and size distribution of stable structural groups. ACKNOWLEDGEMENT The authors thank Joe Ratto for the n.m.r. measurements. REFERENCES 1 2
3 4 5 6 7 8 9 10.
11 12 13 14 15
16 17 18
Anthony, D. B. and Howard, J. B. AIChE J. 1976,22(4), 625 and references therein Howard, J. B. Chapter 12 in 2nd Supplementary Vol. ‘Chemistry of Coal Utilization’. (Ed. M. A. Elliott).II John Wilev & Sons. Inc. NY, 1981 ” Graff, R. A., Dobner, S. and Squires, A. N. Fuel 1976, 55, 109 Suuberg, E. M., Peters, W. A. and Howard, J. B. Fuel 1980,59,405 Green, M., Ladelfa, C. J. and Bivacca, S. J. Fuel Process. Technol. 1980,3,75 Tyler, R. J. Fuel 1979,58, 681 Fallon, P. T., Bhatt, B. and Steinberg, M. Fuel Process. ‘khnol. 1980,3, 155 Finn, M. J., Fynes, G., Ladner, W. R. and Newman, J. 0. H. Fuel 1980,59, 397 Von Fredersdorff, C. G. Ind. Eng. Chem. 1960,52(7), 595 Friedman, J. ‘Development of a Single-Stage, Entrained-Flow, Short-Residence-time Hydrogasifier’, Rockwell International Energy Systems Group, Final Report, FE-2518-24, July 1979 Stangebv, P. C. and Sears, P. L. Fuel 1981.60. 131 Jain,R. Ph.D. thesis, California Institute of T&hnology, 1980 Northam, D. B. and von Rosenberg, C. W., Jr., Fuel, 1979,58,264 Wender, I. Catal. Rev. 1976, 14(l), 97 Wiser, W. H. ‘Scientific Problems Relevant to Coal Utilization’, Proceedings of the Conference, US DOE Sym. Ser., West Virginia University, Morgantown, WV, 1977 Chung, K. E. and Goldberg, I. B. ‘Structural Information from H/C Ratio on Coal and Related Materials’, Manuscript in preparation, June 1982 Chung, K. E. Ph.D. thesis, University of Utah, 1980 Chung, K. E., Anderson, L. L. and Wiser, W. H. Am. Chem. Sot. Div. Fuel. Chem., Preprints 1979, 24(3), 243
19
Wiser, W. H., Anderson, L. L., Qader, S. A. and Hill, G. R. J. Appl. Chem. Biotechnol. 1971, 21, 82
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Wen, C. Y. and Dutta, S. ‘Rates of Coal Pyrolysis and Gasilication Reactions’, in ‘Coal Conversion Technology’, (Eds. C. Y. Wen and E. S. Lee) Addison-Wesley, 1979 Solomon, P. R. ‘Relation Between Coal Structure and Thermal Decomposition Products’, in ‘Coal Structure’ (Eds. M. L. Gorbaty and K. Ouchi) Am. Chem. Sot. Chem. Ser. 1981,192,95 Hiteshue, R. W., Friedman, S. and Madden, R. ‘Hydrogasilication of Bituminous Coals, Lignite, Anthracite, and Char, RI 6125, Bureau of Mines, Dept. of&&or, Wash., DC, 1962 Anthony, D. B., Howard, J. B., Hottel, H. C. and Meissner, H. P. Fuel 1976,55, 121
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