Pyrolysis of a lignite in an entrained flow reactor

Pyrolysis of a lignite reactor

in an entrained

2. Effect of metal cations on decarboxylation Mark

E. Morgan*

and Robert

flow and tar yield

G. Jenkins

Fuel Science Section, Department of Materials Science and Engineering, Pennsylvania State University, University Park, PA 76802, USA (Received 14 June 1985; revised 2 1 October 1985)

The

The role of exchangeable cations in the release of carboxylic functional groups and tars from a Montana lignite has been studied. Pyrolysis was carried out in an entrained flow reactor using a dilute stream of coal particles, and high heating and cooling rates. Direct determination of the carboxylic functional groups on the coal and char samples by ion exchange with barium acetate was employed to study both the amount and kinetics of decarboxylation. These results indicate that the presence of metal cations decreases the extent of decarboxylation at temperatures of 973, 1073 and 1173 K when compared with the behaviour of an acid washed lignite. It was also possible to determine first order activation energy values using this direct measure of decarboxylation. The activation energies determined for decarboxylation were 100 and 97 kJ mol - ’ for the raw and acid washed samples, respectively. Furthermore, it is reported that the presence of metal cations on the coal resulted in a decrease of tar release of 70-94x. Analysis of the tars by Fourier transform infrared spectroscopy indicated that the tars released during pyrolysis were more aliphatic when metal cations were present. (Keywords: lignite; pyrolysis

decarboxylation)

In a previous paper’, it was found that, in several respects, the presence of ion-exchangeable metal cations can be detrimental to rapid lignite pyrolysis in entrained flow

reactors. The presence of metal cations results in a number of effects: a reduction in the yield of volatile matter in an entrained flow reactor, and a reduction in the toal possible weight loss. This paper is concerned with the effect of ion-exchangeable metal cations on the decomposition of two specific components of a lignitic coal, namely; carboxylic functional groups and tarforming materials. There are a number of reasons for considering these two components. First, other studies with different types of reactors have shown the decomposition of both the carboxyl functional groups and the tar producing fraction can be strongly affected by the presence of metal cations. Previous researcherszm4 found that the presence of metal cations greatly decreased the extent of decarboxylation when low rank coals were pyrolysed in fixed bed reactors. Other researchers studied pyrolysis of low-rank coals in a fluidized bed5 and on a wire grid apparatus6 and found a very large decrease in total volatile matter and tar yield when metal cations were present. The second reason for interest in decarboxylation and tar release is that a significant portion of the pyrolysis products released by low-rank coals is accounted for by these species. Carboxyl functional groups can be shown to account for 10-20x of the original coal weight2-4, while tar species have been shown to account for up to 40% of the volatiles released5v6. A number of indicators have been employed to determine the effect of metal cations on the pyrolysis * International Flame Research Foundation, PO Box 10.000,1970CA IJmuiden, The Netherlands 0016-2361/86/060764-05$3.00 0 1986 Butterworth & Co. (Publishers)

764

Ltd

FUEL, 1986, Vol 65, June

behaviour of carboxylic functional groups and tars. In the case of carboxyl group decomposition, it was possible to determine the amount and kinetics of decarboxylation by determination of the quantity of carboxyl groups on the char samples. Direct determination of the concentration of carboxyl groups on the char gives information that studies utilizing overall weight loss, or measurement of pyrolysis gas product composition, cannot yield. It regard to tar release, it was also possible to obtain information about the amount and kinetics of release. Additionally, FT-i.r. was employed to obtain some information about the chemical nature of the tars released and the effect of cations.

EXPERIMENTAL The entrained flow reactor used in this study is described elsewhere’. The samples studied were: a raw Montana lignite (PSOC 833); the acid washed lignite; and the acid washed lignite back exchanged with calcium. For the sake of comparison, it should be noted that; the raw lignite contains the original cations and minerals, and the acid washed sample contained hydrogen ions in place of the original cations and the acid insoluble minerals from the raw coal. The back exchanged lignite (Ca 1) was prepared so that it contained approximately the same quantity of calcium ions as the raw coal and the acid insoluble portion of the original minerals. Analyses of the samples and pertinent experimental techniques are described in detail elsewhere’. All pyrolysis experiments were performed in an inert atmosphere (He-primary, entraining gas; N,-preheated, secondary gas).

Pyrolysis

of a lignite in an entrained

Decarboxylation

Decarboxylation during pyrolysis was monitored by direct determination of the concentration of carboxyl groups remaining on the char samples. The chars analysed were the same chars for which total weight loss data were previously reported’. The analytical technique used is outlined in detail in an earlier study’ which was based on the work of Schafer*,9. There are three steps involved, namely: acid washing; exchange with barium acetate; and determination of the extent of exchange. By this technique, acid form carboxyl groups were exchanged with barium and the extent of exchange was determined by titrating the acid released. The only alteration of the original technique is in the acid washing step. In the original determination, the coal was demineralized using the technique of Bishop and Ward”. In this work the coal and chars were washed with 1 N HCl at 355 to 365 K for three 24 h periods. As with the demineralization step, this treatment was followed by stirring the coal in boiling deionized water for l-l.5 h to remove adsorbed HCl. This new treatment does not require as much attention as the demineralization procedure, and it removes the hazard of working with hydrofluoric acid. Acid washing in the manner described was found to be sufficient to remove all the exchangeable cations on the coal and the chars. The extent of removal was determined by analysing the extracts of acid washing by atomic absorption spectroscopy.

2: M. E. Morgan

and R. G. Jenkins

2.5-

6 u a D >

2.0-

-3 IS-

B

The amount of tars released was determined by collecting the condensible pyrolysis products on a tar filter located in the exit stream of the reactor. It should be noted that the definition of ‘tars’ used in this paper is an operational definition. As will be seen, despite the qualitative nature of the data, it is felt that valuable information about the effect of metal cations on tar yield has been found. Furthermore, the tars collected were examined by FT-i.r. spectroscopy to determine differences in their chemical composition. For the i.r. spectroscopy, samples were prepared using KBr pellets, and detailed procedures can be found elsewhere”.

RESULTS

reactor.

within the full residence time of the reactor. In Figure 2, the results for the acid washed coal are presented. Again, pyrolysis behaviour of the carboxyl groups was found to be qualitatively the same as the total weight loss data. At 1173 K, there is a rapid loss of carboxyl groups up to ~0.05 s after which little change is observed. Figure 3 presents results of the pyrolysis of the Cal sample at 1173 K. As would be predicted from the total weight loss data, decarboxylation of Ca 1 at 1173 K is very similar to decarboxylation of the raw coal at 1173 K. Decarboxylation at maximum residence time (0.25 s) at 1173 K was virtually the same for the raw and Cal samples (2.6 meq g - ’ dicf) while the acid washed sample lost essentially all of the carboxyl groups present (3.1 meq g-l dice). With regard to the total weight loss at 1173 K and maximum residence time, the losses were 30.8x, 28.7x, 49.4% (dice) for the raw, Cal and acid washed samples, respectively. Furthermore, the trend of lower decarboxylation with a corresponding decrease in reactor temperature found in this study is very similar to the total

4

Tar yield

flow

s -. j

l.O-

s

0.5-

0.0 0.00

0.05

I 0.10

I 0.15

1 0.20

I 0.25

C 0

ResidenceTime (I)

Figure 1 53x38pm

Loss of carboxyl

groups

for the raw lignite

(PSOC

833);

1173K

AND DISCUSSION

h/--O

3.0

0

O-

Decarboxylation

Data on the decomposition of carboxyl groups enables one to study the behaviour of a single species during pyrolysis, thus yielding information that cannot be extracted from overall weight loss data. Results from this study are plotted in Figures I,2 and 3. It will be recalled that carboxyl group concentration was determined by the ion exchange technique. Figure I displays the quantity of carboxyl groups lost from the raw lignite sample (on a dicf coal basis) as a function of time reactor temperature. residence and The decarboxylation bahaviour of the raw lignite is qualitatively very similar to the total weight loss behaviour of the same sample pyrolysed in this reactor’. That is, the data in Figure I display a region of rapid loss of carboxyl groups up to ~0.15 s, followed by a region of slow decomposition. As expected, decreases in temperatures result in decreases in the rate of carboxyl group decomposition. At 1073 and 973 K, the raw coal decarboxylation data show no trend of being completed

2.5 5 0 D

1

I

0:oo

/

/ OO"/Y

0:05

/

O.‘lO

973K /

0.15

o.io

0:25

0

RasidcnceT,m. (I)

Figure 2 Loss of carboxyl 833); 53 x 38 pm

groups

for the acid washed

lignite (PSOC

FUEL, 1986, Vol 65, June

765

Pyrolysis of a lignite in an entrained

flow reactor. 2: M. E. Morgan

JoI

1173K

2.5 -

F b ? D

2.0-

z j

c,

Is-

9 s ,. 2

l.O-

J

0.5 -

0.01

0.00

’

0, 0.05

I

0.10

I

I

I

0.15

0.20

0.25

Residence

Figure 3 Loss of carboxyl 833); 53 x 38 pm

0.30

Time (I)

groups for the Cal form of the lignite (PSOC

weight loss data. Decarboxylation at maximum residence time dropped from 2.6 to 0.9 meq g-’ dicf for the raw coal and from 3.1 to 2.3 meq g-i dicf for the acid washed sample over the range 1173-973 K. In the case of total weight loss, the corresponding values were a decrease from 30.8 to 22.9% for the raw sample and 49.4 to 27.4% for the acid washed sample. The decomposition of carboxyl groups in lignites can account for a relatively large amount of volatile material. In the case of the three samples studied, the fraction of total weight loss which can be attributed to carboxyl group decomposition (1173 K and maximum residence time) is 30% for the acid washed coal, 35% for the raw coal and 35% for Cal form coal. The striking similarity in total weight loss and decarboxylation data strongly. suggests that the mechanisms controlling both of these phenomena are similar. Thus, as with total weight loss data, it is believed that loss of carboxyl groups occurs via a number of mechanisms, ranging from the direct scission of the carboxyl groups yielding carbon dioxide to the loss of large tar molecules which contain carboxyl groups. The presence of metal cations influences the degree of secondary reactions that the primary products undergo. As was suggested for the role of metal cations on total weight loss, it is believed that the main involvement of metal cations is to promote secondary char-forming reactions. The most likely mechanism for these secondary char-forming reactions. The most likely mechanism for these secondary reactions are cracking of tar molecules to carbon rich deposits and light gases, and polymerization reactions leading to non-volatile species. Although previous researchers have studied the effects of cations on the pyrolysis of carboxyl groups, most of the work has been concerned with determination of the total extent of decarboxylation as a function of temperature. There has been little concern about the kinetics of decarboxylation. In this study, it was found that decarboxylation can be described well by a first order kinetic approach. To execute this analysis, (ln( 1 -A W/ A W,) was plotted versus residence time, the slope of this line is k (rate constant). The residence times were the

766

FUEL, 1986,

Vol 65, June

and R. G. Jenkins

results of the particle velocity model described previously and the temperature was assumed to be that of the reactor. The activation energy for decarboxylation, and the preexpontial factor were estimated from Arrhenius plots. In this analysis, the value for AW,, the ultimate possible weight loss due to decarboxylation, was the carboxyl group content of the raw coal (3.14 meq g-’ dice). The ability to determine quantitatively AW, is an advantage of the kinetic analysis of decarboxylation when compared with the kinetic analysis of total weight loss. In the case of total weight loss, there is no independent measure of A W,. It will be recalled’ that the value used for AW, in the case of total weight loss was somewhat arbitrarily selected as the maximum weight loss in the entrained flow reactor at 1173 K. Tables 1 and 2 contain the results of the kinetic analysis. Correlation coefficients of the first-order plots were always greater than 0.96. The data presented in Table I displays the rate constants for decarboxylation. It can be seen that the rates of decarboxylation of the raw coal were less than those for the acid washed coal over this temperature range. This trend is in agreement with the trend displayed previously for the kinetics of total weight loss data generated during the same pyrolysis runs’. Again, it is evident that the presence of the metal cations significantly inhibits the rate of loss of the carboxyl groups from the coal at any temperature. The correlation coefficients gained in the Arrhenius plots were very high with values >0.99. A list of the activation energies and pre-exponential factors parameters are presented in Table 2. The values found in these two tables are similar to rate constants, activation energies and preexponential values found in other similar studies’ 2. The activation energies are low when compared with chemical bond breaking processes, and a number of explanations for the low apparent energies can be invoked. First, previous research has indicated that heat and mass transfer processes play an important role in rapid pyrolysis1&16. However, it is felt that the experimental conditions selected in this study (entrained flow reactor and small particle sizes) will, to some degree, minimize the influences of heat and mass transport. The second explanation of low activation energies relates to the assertion of Anthony and Howard’ 2 that the use of single activation energy models to analyse overall pyrolysis kinetics can yield low activation energies even if the actual activation energy is in the range of chemical bond breaking. However, the kinetic calculations reported in this study were made for the loss of a specific fraction of the coal, carboxyl groups, and are not for overall weight loss. The third explanation is that loss of Table 1

Rate constants

for decarboxylation

(s-l)

Sample

913 K

1073 K

1173K

Raw Acid washed

0.91 3.9

3.2 10.0

9.1 31.5

Table 2

Kinetic

parameters

Sample

Activation (kJ mol-‘)

Raw Acid washed

110 97

for decarboxylation energy

Pre-exponential (s-l) 7 X 105 6x lo5

factor

Pyrolysis of a lignite in an entrained flow reactor. 2: M. E. Morgan and R. G. Jenkins carboxyl groups is dominated by the release of large molecules which are weakly held in the coal structure. That is, it will be shown shortly that a significant portion of the carboxyl groups are released as a part of larger tar molecules. In low-rank coals some 1040% of the coal material can be released as tars when secondary reactions are minimized’ 4. These tars have been postulated to be held within the coal structure by weak bonds with low activation energies 13*14. Thus, the loss of a significant quantity of carboxyl groups because of their association with the tars will result in the observed low activation energies. If one compares the kinetic parameters of the raw and acid washed coals, the activation energy and preexponential values for these two samples are virtually identical with the activation energy for the raw coal being about 10% greater than that calculated for the acid washed coal. This similarity in activation energy values is quite unexpected. The influence of metal cations on decarboxylation is in sharp contrast to that displayed for total weight loss kinetics. In the case of total weight loss, the activation energy of the acid washed coal was 250% greater than the activation energy for the raw coal’. It must be recalled, however, that it was possible to directly determine AW, for decarboxylation while an estimate was utilized for total weight loss. Because of this factor, it is felt that the kinetic data for decarboxylation are most representative of actual values.

considered. The reduction in tar release caused by the presence of metal cations ranged from between 70 to 94%. These numbers display the fact that the tar fraction was more sensitive to the presence of metal cations than total weight loss (40% reduction at 1173 K and 0.25 s) or decarboxylation (30% reduction at 1173 K and 0.25 s). It can also be shown that the chemical nature of the tars released is affected by the presence of cations. First, there was a significant difference in the physical appearance of the tars collected. The tars released from the raw coal were black and gummy while the tars from the acid washed coal were light to dark brown and powdery. Second, two tar samples were analysed by FT-i.r. spectroscopy. In Figure 4, spectra from the raw and acid washed coal tars are presented as well as the ‘difference spectrum’ which is simply the raw coal spectrum subtracted from the acid washed coal spectrum l1 These tars were collected at a reactor temperature of 1173 K and a residence time of 0.078 s. The tar fractions from pyrolysis of the raw and acid washed coals are similar in many ways. The same general major features are present in both of the spectra, such as methyl, phenol and carboxyl groups. However, the two spectra differ significantly in the intensity of the carbon-hydrogen aliphatic bond stretching absorbance at about 2800-3000 cm - ‘. Table 3

Tars released

Residence

Tar release Table 3 lists the quantity of tars collected when the raw and acid washed samples were pyrolysed at 1173 K at three collection positions. The presence of metal cations resulted in a reduction of tars collected at every position

in entrained flow reactor Tars collected (mg g

’ coal feed)

time

(s)

Raw coal

Acid washed

0.042 0.078 0.112

3 9 3

10 30 48

coal

Aromatic c-c

c=o

I

Corboxyl ,I

Acid Woshed

I’

I

3800

Figure 4

‘I

’

3400

Infrared

spectra

I

’

I

3000

’

1

’

I

2600

’

I

’

I

’

I

2200 ., cnl

of lignite tars. A, Raw coal tar; B, tar from acid washed

-

1.1'1' 1800

I 1400

lignite; A-B, difference

-

I’ 1000

I

‘I’

1 600

4llo

spectrum

FUEL, 1986, Vol 65, June

767

Pyrolysis

of a lignite in an entrained

flow reactor. 2: M. E. Morgan

It is evident from the difference spectrum that there is a large discrepancy in the number of these bonds. Integration of the peak for the two samples leads to the conclusion that tars from the raw coal have about three times as many aliphatic hydrogen atoms as tars from the acid washed coal. The other large peak in the difference spectra is located between 100 and 1200cm-’ and it is indicative of oxygen functionalities. It is very difficult to assign this peak to any particular oxygen group since a number of oxygen groups have minor peaks in this region. However, because no other bands indicative of a significant concentration of any particular oxygen functionality can be seen in the difference spectra, it can be inferred that the peak observed is the result of a combination of a number of minor peaks. The fact that the tar fraction is very sensitive to the presence of metal cations in terms of weight loss and chemical structure supports the contention that the main role of metal cations in lignite pyrolysis is the promotion of secondary char forming reactions. The tar fraction which is high in molecular weight would be expected to be the most susceptible to the proposed cracking and polymerization reactions that the primary pyrolysis products undergo. The fact that the amount of tars collected decreases when metal cations are present is an obvious result of this hypothesis; however, the fact that the tars from the metal cation form coal are more aliphatic is more difficult to explain. Either these highly aliphatic tars are the only species which are able to escape without undergoing char-forming reactions, or these more aliphatic tars are the products of cracking reactions occurring to the primary tar species which are more aromatic in nature. It should also be noted that evidence of the existence of carboxyl groups in the tar fractions of both raw and acid washed samples can be seen in Figure 4. The presence of carboxylic acids on these tars supports the mechanisms proposed for decarboxylation and the role of metal cations. That is, it was postulated that decarboxylation occurs via a number of mechanisms ranging from the direct scission of the carboxyl groups yielding carbon dioxide to the release of large tar molecules which contain carboxyl groups. The presence of carboxyl groups on the tars collected from both the raw and acid washed samples supports the latter mechanism. Furthermore, this evidence suggests that a major role of metal cations in decarboxylation is the promotion of the redeposition of tars which contain carboxyl groups.

CONCLUSIONS It has been found that the presence of metal cations dramatically affects the release of carboxylic functional groups and tars. In the case of the decarboxylation, it was

768

FUEL, 1986, Vol 65, June

and R. G. Jenkins

shown that the presence of metal cations decreases the amount and rate of loss. The activation energies found for decarboxylation of the raw and acid washed coals were similar and somewhat lower than that predicted for chemical bond breakage. In the case of tar release, it was shown that when metal cations are present there is a dramatic reduction in the amount of tars released and collected. It was also found that the tars which were collected from the metal cation containing lignite were more aliphatic in nature than the tars collected from the coal with carboxyl groups were in the acid form. It is felt that the similarity in the way which the presence of metal cations influences the total weight loss, decarboxylation and tar evolution points to an obvious similarity in the machanism responsible. Thus, in parallel to the conclusions about total weight loss’, it is believed that metal cations influence decarboxylation and tar release by promoting the re-deposition of primary volatile material. The two most likely re-deposition reactions are cracking and polymerization. Furthermore, the results of the kinetic study on decarboxylation led to the conclusion that a significant portion of the pyrolysis products were generated by the release of large molecules which are weakly held within the coal structure. The release of these weakly held species can account for the low activation energies observed in the kinetic analysis. ACKNOWLEDGEMENTS This study was supported by USDOE contract No. DEACOl-79ET14882 and, in part, by The Pennsylvania State University Cooperative Program in Coal Research. The authors wish to thank W. Spackman, Director of the Coal Research Section (PSU) for supplying the lignite. REFERENCES

I 8 9 10 11 12 13 14 15 16

Morgan, M. E. and Jenkins, R. G. Fuel 1986,65, 757 Schafer, H. N. S. Fuel 1979,58,667 Schafer, H. N. S. Fuel 1979,58,673 Murray, J. B. Fuel 1973, 52, 105 Tyler, R. J. and Schafer, H. N. S. Fuel 1980, 59, 487 Franklin, H. D., Cosway, R. G., Peters, W. A. and Howard, J. B. Ind. Eng. Chem. Process Des. Dev. 1983, 22, 39 Morgan, M. E., Jenkins, R. G. and Walker, P. L., Jr. Fuel 198 1,60, 189 Schafer, H. N. S. Fuel 1970,49, 197 Schafer, H. N. S. Fuel 1970,49,271 Bishop, M. and Ward, D. L. Fuel 1958,37, 191 Painter, P. C., Kuehn, D. W., Snyder, R. W. and Davis, A. Fuel 1982,61, 691 Anthony, D. B. and Howard, J. B. AIChE J. 1976,22,625 Given, P. H. Prog. Energy Cornbust. Sci. 1984, 10, 149 Freihaut, J. D. and Seery, D. J. Nineteenth Symp. (Int.) on Comb., The Combustion Institute, Pittsburgh PA, 1982 Simons, G. A. Prog. Energy Cornbust. Sri. 1983, 9, 269 Maloney, D. J. and Jenkins, R. G. Twentieth Symp. (Int.) on Comb., The Combustion Institute, Pittsburgh PA, 1985, p. 1435