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Effects of drying upon lignite macro-pore structure
Powder Technology, 47 (1986) 9 - 15
Effects of Drying upon Lignite Macro-Pore Structure G. P. ANDROUTSOPOULOS
and TH. J. LINARDOS
Department of Chemical Engineering, National Technical University of Athens, 42, 28th October Street, GR 106 82 Athens (Greece) (Received April 2,1985)
SUMMARY
The effect of drying upon the macro- and partly meso-pore structure of Greek lignite was investigated. Drying of lignite lumps (md,: +3 - 15 mm) placed on a fixed bed arrangement, was carried out under vacuum (- 0.2 Torr) in a constant-temperature (100 250 “C) environment, for varying drying times (5 - 180 min). Drying caused a considerable particle contmction (by -l/3 of its original size), a minor decrease in the macro- and partly meso-pore volume and a marked increase in the relevant surface area, particularly in the mnge of high weight losses (35 40%). Residual moisture contents ranged from N 0% to 42%, specific pore volume fell in the range 0.20 - 0.25 cm”/g (dry sample), and specific surface area in the range 9 - 17 m’/g (dry sample). Pore volume and surface area frequency distributions, determined from mercury porosime try (MP) measurements done on raw or partially dried samples, clearly indicate the formation of pores in the size range D,: +75 - 1500 A at the expense of pores in the range D,: + 1500 - 1 O4 A This observation might be the result of pore shrinkage and pore emptying due to the counteraction of particle contraction and moisture removal respectively. Mercury penetration - retraction hysteresis was evident in every MP run. Mercury entrapment drop (from 85% to 38%) varied with residual moisture content, an indication that drying favours the formation of a more regular and less complex physical pore structure.
INTRODUCTION
Lignites, before physical or chemical processing, contain appreciable amounts of water 0032-5910/86/$3.50
appearing in various physical or chemical states. Moisture removal is an energydemanding physical processing step, preceding any mechanical treatment or chemical conversion of lignite. It is estimated that moisture removal from the unit mass of Greek lignite requires an amount of thermal energy equal to that needed for the complete gasification of the daf coal contained in the sample, the marked difference being that the two processes take place at quite different temperature levels. Among the major factors (nature and concentration of liquid, system temperature and pressure) that decide the mode of liquid movement through a porous solid, pore structure plays a key role, because it is implicitly present in most of the proposed mechanisms of liquid movement in a solid. A summary of these mechanisms as quoted by Peck and Wasan [l] is - liquid diffusion due to differences in moisture concentrations - liquid movement due to capillary forces - vapour diffusion in partly air-filled pores, due to differences in partial pressure - liquid or vapour flow due to differences in total pressure, generated by external pressure, capillarity, shrinkage, or high temperature inside the moist material - liquid movement due to gravity. Knowledge of the pore structure of lignite is equally significant for the elucidation of the principles dominating its subsequent combustion or chemical conversion processes. It is widely known that direct combustion of predried coal particles under carefully controlled conditions is necessary to maintain the desired conditions in the combustion chamber. Similarly, investigation of the behaviour of lignite particles in a chemically reactive environment is related to changes @ Elsevier
Sequoia/Printed
in The Netherlands
10
effected on the pore structure as described either by the overall pore volume and surface area or their distributions with respect to pore size. Distributed pore structure parameters provide a unique picture of the intraparticle pore space, wherein transport of reacting species and chemical reaction occur. Coals in general appear to possess a pore structure extending over an extremely wide range of pore sizes, namely I&:+30 - lo5 A. This observation points to the fact that the investigation of the coal pore structure over an extensive pore size range entails the parallel use of various experimental techniques. Removal of water from coal matrix during drying does not necessarily lead to a pore volume evolution as one could expect, since water in lignites occupies more than 50% of the original particle volume. A marked particle contraction even at moderate heating conditions (- 100 “C, under vacuum) is inevitable, the natural consequence being a pore deformation (coalescence, shrinkage and collapse). A net increase or decrease in the pore volume and surface area are dependent upon the interaction of two competing effects, i.e., pore emptying due to moisture removal and pore shrinkage due to particle contraction. Major particle contractions should also be taken into consideration when residence time distributions within industrial lignite-processing units (drying or, chemical conversion) are studied. The choice of particle sizes is of particular importance, because drying times are strongly dependent upon particle size. Particle sizes in the range dp :+ 3 - 15 mm are thought suitable to carry out the present investigation. Information correlating drying times and pore structure variations of Greek lignite of the aforementioned particle size range may be used in the design of industrial coal driers. The design of industrial gasifiers which also operate partially as driers should take into account experimental data on drying of lignite lumps. This statement assumes a greater significance in the case of lignite gasification by means of its own natural moisture, which is driven out of the pore structure, in situ. Obviously, lignite drying kinetic equations should be developed. Studies of lignite drying rates, apart from the main kinetic equation,
should also include correlations describing the dependence of drying rates upon temperature, pressure, flow conditions and particle size.
LITERATURE
SURVEY
A review of the ‘coal pore structure research’ is presented in the introductory section of Ref. [2]. Results of macro-pore structure studies on samples of a low-rank coal and its froth flotation fractions are also reported in this publication. Specific pore volume and surface area distributions were computed from mercury porosimetry (MP) measurements. Estimated cumulative values ranged from 0.06 to 0.14 cm3/g and surface area from 5 to 12 m2/g (on a dry basis). Karsner and Perlmutter [3] reported ,MP results and the subsequently calculated pore volume and surface area changes occurring during drying of a number of coal samples (nominally, four samples of bituminous coal, one lignite sample and one anthracite sample). The conclusion was that both properties undergo a little reduction during drying and also that these changes depend upon the severity of drying and the initial water content. It was also found that oxidation at moderate temperatures produced only small additional changes. Pore volumes assumed values between 0.028 and 0.114 cm3/g, and surface areas between 12 and 53 m2/g (Dp > 35 A), being systematically lower than those calculated from CO2 adsorption experimental results (at 25 “C) and higher in comparison with those derived from N2 adsorption at -196 “C. Mercury entrapment when the pressure was reduced to atmospheric varied from 25% to 49% of the total amount which has intruded at pmax = 3450 bar. Studies of pore structure changes in Alberta hv Cb coals on partial solubilization were reported by Parkash and Moschopedis [4]. Experimental results showed that coal solubilization with various solvents in an inert atmosphere or in the presence of hydrogen (with or without the use of catalysts) at the reported reaction conditions resulted in a specific surface area increase. Total, micro-, meso- and macro-pore volume, each considered individually, also increased. The analysis of MP experimental results is based on the Washburn equation (P =
11
-47 cos e/D). The following values for surface tension and the contact angle are assumed: 7 = 480 dyn cm-l and 8 = 140” Pore surface areas are calculated by use of the Rootare and Prenzlow formula [ 51 and MP penetration data. Pore structure parameters are expressed per gram of dry solid to allow sensible comparisons.
EXPERIMENTAL
Raw material A set of drying experiments were performed on samples of Greek lignite (Megalopolis area). Megalopolis lignite is classified as a low heating value solid fuel with a Net Calorific Value of -900 kcal/kg. This is obviously due to the high moisture and ash content. Typical proximate analysis data valid for Megalopolis lignite are Moisture 60% Ash 17% Volatile matter 12% Fixed carbon 11% (calculated) Lignite samples were kept in the laboratory in contact with the atmospheric air for several weeks before their mechanical treatment and the preparation of smaller samples. Big lignite lumps were crushed into smaller pieces and samples were formed by selecting particles having a sphere equivalent diameter d,:+ 3 15 run. An ultimate moisture content (determined by heating up to 250 “C for 1 h) was found to be equal to 42% of the original weight of the sample. It seems that a moisture loss of about 18% occurred during the storage period under laboratory atmospheric conditions. Description of the drying experiments Lignite particles of the desired size were placed in a cylindrical glass burette (30 cm long and 15 mm wide). The burette, partially filled with coal particles (samples of mass 14 16 g were used) could be connected with a vacuum pump capable of generating a vacuum below 0.01 Torr. The lower part of the burette (10 cm long), filled with lignite, was surrounded by an electrically heated furnace supplied with a thermostatic control system. The maximum working temperature was
300 “C. The temperature of the sample could be periodically monitored with the use of a thermocouple. Table 1 presents the conditions of nine typical drying experiments. Temperature, drying time and weight loss values for individual samples are noted. Residual contents, defined on a wet basis, vary between -0 and 42%. Figure 1 is a schematic depiction of the change in residual moisture content (% of ultimate moisture content - weight loss %) uersus drying time (wet drying curves). Individual experimental points on each curve of Fig. 1 stand for a particular sample, heated at the indicated temperature (0 100 “C and 0 250 “C), and the corresponding drying times. Each curve, like typical drying curves, comprises a constant-rate section (for drying times TABLE 1 Greek lignite drying conditions Run No.
Vacuum drying conditions m
Weight loss (wt.%)
Residual moisture content (wet basis)
,
I
r
(“C)
(min)
25 100 250 100 100 250 100 100 250
0
5 5 10 30 10 60 180 60
25
Percentage of mercury entrapment (wt.%)
(wt.%) 42.0 34.7 28.3 28.0 12.2 11.4 5.5 4.1 0.1
0
1.3 13.7 14.0 29.8 30.6 36.5 37.9 41.9
50
75
Drying time
85.8 89.4 85.9 78.6 80.2 77.1 59.0 60.2 38.3
100
I25
150
t (min)
Fig. 1. Moisture removal from beds of lignite lumps as a function of drying time. Ultimate moisture content 42% (wet basis). Bed temperature: @ 100 “C and l 250 “C. Vacuum conditions (-0.2 Torr).
12
less than 10 min, at 250 “C, and less than 25 min, at 100 “C), and a falling-rate section for higher drying times [ 11. Drying at 250 “C caused the evolution of yellow oily liquids, which could be seen on the wall of the glass burette, an indication that drying is followed by pyrolysis. Higher drying temperatures and oil formation are expected to affect the state of the lignite physical structure in a different way than milder heating-up conditions.
Pore structure measurements Mercury porosimetric (MP) studies on lignite samples were carried out on a mercury porosimeter generating and applying a maximum pressure of 2000 bar (Carlo Erba, Porosimeter Mod. 200). It was possible to perform a penetration and a retraction run on a single sample and produce the characteristic hysteresis loop. Two typical and widely differing hysteresis loops, one for a virtuaIIy dry sample (moisture content mO.l%, run No. 9, Table 1) and the second for a sample with a considerably higher moisture content of 34.7% (run No. 2, Table l), are shown in Fig. 2. The MP experiments detected approximately equal total pore volumes for the two samples under consideration (namely, V,, = 0.217 - 0.219 cm3/g (dry sample)). The MP hysteresis loops of Fig. 2 show major deviations between the relevant penetration and retraction lines as well as the amounts of mercury entrapped within the lignite internal pore structure. The MP characteristics associated with runs 1,
3 - 8, and 10 are intermediate in comparison with those of runs 2 to 9. Total pore volumes and surface areas as well as percentage of mercury entrapment are given in Table 2. In separate columns of Table 2, meso- and macro-pore volumes and surface areas are also noted. The variation of these overall structural properties is shown as a function of weight loss %, in Figs. 3, 4 and 5. The extent of entrapment decreases monotonically with an ever-increasing slope, which becomes appreciable at levels exeeding the limit of 20% in weight loss. The maximum weight loss of 42% gave rise to a decrease of
c
0.8
log P
I.6 (P
2.4
3.2
absolute pressure,
bar)
Fig. 2. Mercury penetration and retraction on samples of Greek (Megalopolis)‘.lignite (0, 1 Moisture content 34.7%, VP, = 0.219 cm3/g; 0, moisture content O.l%, VPt = 0.217 cm3/g). Numbers on curves refer to Run No. Table 2.
TABLE 2 Lignite specific pore volume and surface area changes due to drying Run No.
Specific pore surface area (dry sample) (m*/g)
Specific pore volume (dry sample) (cm3/g) Total
0.245 0.219 0.227 0.240 0.213 0.219 0.206 0.241 0.217
Meso-pore Dp < 300 A
Meso-pore/ macro-pore ratio
Total
D, > 300 A
0.028 0.025 0.028 0.018 0.024 0.032 0.041 0.033 0.040
0.217 0.194 0.199 0.222 0.189 0.187 0.165 0.208 0.177
0.13 0.13 0.14 0.08 0.13 0.17 0.25 0.16 0.23
11.7 10.4 12.2 9.7 11.0 13.0 17.2 13.8 15.0
Macro-pore
Macro-pore
Meso-pore Dp < 300 A
Dp > 300 A
Meso-pore/ macro-pore ratio
9.1 7.6 9.1 6.3 7.1 9.1 13.2 9.5 13.0
2.6 2.8 3.1 3.4 3.9 3.9 4.0 4.3 2.0
3.5 2.7 2.9 1.9 1.8 2.3 3.3 2.2 6.5
13
solid surface do not provide the conditions that favour mercury entrapment. The reduction in the degree of pore intersection might also substantially reduce the probability of mercury entrapment in wide pore spaces surrounded by narrower entrances. Similar observations have been reported in Ref. [ 31. Theoretical predictions of mercury mechanical entrapment within a network pore structure model have been published elsewhere 20
30
Weight Loss
%
IO
40
50
Fig. 3. Variation of mercury entrapment expressed as a percentage of the total pore volume filled with mercury at the maximum applied pressure (2000 bars) us. weight loss due to drying. 0, Heating at 250 “C; 0, heating at 100 “C.
&:/ ; a”
2:.
B
ho
IO Weight Loss
(WL) %
Fig. 4. Specific macro- meso-pore (DP > 75 A) volume reduction during drying under vacuum. Drying temperature: 0,100 “C; 0, 250 “C. Straight line: VP = 0.2367 - 4.6 X 10d4 (WL %), cm3/g.
18A
[61-
It is evident from Fig. 4 that the specific pore volume shows a trend to lower values as the sample weight loss increases. The wide spread of the experimental points does not allow the best straight line drawn through these points to be regarded as a reliable representation of the observed phenomenon. Nevertheless, the indication is clear that drying caused a slight lowering of the total specific pore volume. A similar graph, shown in Fig. 5, depicts total specific surface variations versus weight loss. A weight loss between 30% and 40% is sufficient for the development of a net rise in the specific pore surface area. As discussed in following sections in this work, the increase in pore surface area from -10 m2/g to 17 m2/g, in contrast to the minor decrease in the pore volume, is due to the generation of macro- and meso-pores in the range D, :+ 75 - 1500 A at the expense of macro-pores in the range D, :+ 1500 - lo4 A. A thorough picture of lignite pore structure modification due to drying is provided in Figs. 6 - 9, in the form of pore volume and area distributions with respect to pore size.
16..
Weight
LOSS
(n)
$
Fig. 5. Specific pore surface area change us. weight loss. Drying temperature: 0, 100 “C; 0, 250 “C.
about 45% in the percentage of mercury being entrapped. The systematic reduction in mercury entrapment is perhaps due to the formation of a more regular and less complex physical pore structure. Individual pores with a rather uniform cross-section and a smoother
Pore volume and surface area frequency distribu tions Pore volume differential distributions are shown in Figs. 6 and 7. The curves in Fig. 6 have been computed by differentiating MP penetration experimental lines for the indicated drying run number, performed at 100 “C. The distribution marked 0, valid for lignite before drying, forms a sharp peak located between D, lo3 and lo5 A (macropore range), a deep section between D, 150 lo3 A (partially macro- and meso-pore range) and a rising section (a half-formed peak) in the region D, : 75 - 150 A (entirely within the meso-pore range). MP could not detect pore sizes below D, = 75 A. It is readily seen in Fig. 6 that the gradual moisture removal from
7
log D
(D : Pore Diameter,
A)
-
Fig. 6. Dfcerential pore volume distributions obtained from MP measurements on samples of Greek lignite dried under vacuum at 100 “C for varying periods of time. Numbers on the curves refer to Run No., Table 2.
0
OO"OO 0
0
0
1%
D
0 0
(D: pore Diameter, A )
Fig. 7. Differential pore volume distributions derived from MP experiments on samples of Greek lignite dried under vacuum at 250 “C for varying times. Numbers on the curves refer to Run No., Table 2.
the lignite samples causes the flattening of the peak in the higher macro-pore region, while the deep section, located partly in the macropore and partly in the meso-pore area, becomes gradually shallower. The net effect is that the contribution of pore sixes 0,:+75 lo3 A to the total pore volume becomes greater at the expense of macropores (II,: +103 - 10’ A). As stated in previous paragraphs of this work, pore volume changes were not appreciable. A similar situation appears in Fig. 7, wherein distributions were calculated from MP data
on lignite samples dried at 250 “C. The formation of oils and the effect of a higher temperature does not seem to favour a practically different mode of pore structure alteration. Similar comments can be made for the differential pore surface area distributions of Figs. 8 and 9 associated with drying runs performed at 100 “C and 250 “C respectively. Moisture removed resulted in a reduction of macro-pore area while the development of meso-pores and lower macro-pores gave rise to a net increase in the overall pore surface area.
15
effects, resulting in a virtually constant specific pore volume and a moderate increase in the total specific pore surface area by clearly shifting a fraction of the pore volume from the macro-pore towards the meso-pore region.
i
I
l\
J\
B 0 %“,
I
CONCLUSIONS
0
log
D
(II : pore diameter. A
)
Fig. 8. Differential pore surface area distributions derived from MP data obtained from MP experiments on Greek lignite dried under vacuum at 100 “C for varying times. Numbers on the curves refer to Run No., Table 2.
Heating of Greek (Megalopolis) lignite under vacuum in a constant-temperature environment causes considerable particle shrinkage and the resulting moisture removal follows the pattern of a typical drying curve. Sample contraction and moisture removal play a counterbalancing role, since the former is .responsible for pore shrinkage and the latter for pore emptying, the net effect being an increase in the surface area, while pore volume is slightly decreased. The formation of meso- and macro-pores in the pore size range II,:+ 75 - 1500 A, at the expense of macropores in the pore range D,:+ 1500 - lo4 A, during drying, evidenced from comparisons of the relevant pore volume and surface distributions provides a plausible explanation of the net effect of drying upon pore structure. The temperature level at which drying takes place does not seem to practically influence the behaviour of lignite lumps, despite the formation of oily products at higher temperatures.
ACKNOWLEDGEMENT
The authors wish to acknowledge with thanks the Public Power Corporation of Greece for supplying the lignite samples.
REFERENCES log
D
(D:
pore diameter
, A)
Fig. 9. Differential pore surface area distributions derived from MP data obtained from MP experiments on Greek lignite dried at 250 “C under vacuum, for varying drying times. Numbers on the curves refer to Run No., Table 2.
Thermal drying of the lignite samples under vacuum causes a considerable volume contraction of the lignite particles (by -l/3 of their original volume). Particle contraction and moisture withdrawal seem to be counteracting
R. E. Peck and D. T. Wasan, Advances in Chem. Eng., 9 (1974) 247. G. P. Androutsopoulos and E. T. Woodburn, Powder Technol., 33 (1982) 175. G. G. Karsner and D. D. Perlmutter, Znd. Eng. Chem. Process Des. Dev., 21 (1982) 348. S. Parkash and S. E. Moschopedis, Fuel, 62 (1983) 1231. H. M. Rootare and C. F. Prenzlow, J. Phys. Chem., 71 (1967) 2733. G. P. Androutsopoulos and R. Mann, Chem. Eng. Sci., 34 (1979) 1203.
Effects of Drying upon Lignite Macro-Pore Structure G. P. ANDROUTSOPOULOS
and TH. J. LINARDOS
Department of Chemical Engineering, National Technical University of Athens, 42, 28th October Street, GR 106 82 Athens (Greece) (Received April 2,1985)
SUMMARY
The effect of drying upon the macro- and partly meso-pore structure of Greek lignite was investigated. Drying of lignite lumps (md,: +3 - 15 mm) placed on a fixed bed arrangement, was carried out under vacuum (- 0.2 Torr) in a constant-temperature (100 250 “C) environment, for varying drying times (5 - 180 min). Drying caused a considerable particle contmction (by -l/3 of its original size), a minor decrease in the macro- and partly meso-pore volume and a marked increase in the relevant surface area, particularly in the mnge of high weight losses (35 40%). Residual moisture contents ranged from N 0% to 42%, specific pore volume fell in the range 0.20 - 0.25 cm”/g (dry sample), and specific surface area in the range 9 - 17 m’/g (dry sample). Pore volume and surface area frequency distributions, determined from mercury porosime try (MP) measurements done on raw or partially dried samples, clearly indicate the formation of pores in the size range D,: +75 - 1500 A at the expense of pores in the range D,: + 1500 - 1 O4 A This observation might be the result of pore shrinkage and pore emptying due to the counteraction of particle contraction and moisture removal respectively. Mercury penetration - retraction hysteresis was evident in every MP run. Mercury entrapment drop (from 85% to 38%) varied with residual moisture content, an indication that drying favours the formation of a more regular and less complex physical pore structure.
INTRODUCTION
Lignites, before physical or chemical processing, contain appreciable amounts of water 0032-5910/86/$3.50
appearing in various physical or chemical states. Moisture removal is an energydemanding physical processing step, preceding any mechanical treatment or chemical conversion of lignite. It is estimated that moisture removal from the unit mass of Greek lignite requires an amount of thermal energy equal to that needed for the complete gasification of the daf coal contained in the sample, the marked difference being that the two processes take place at quite different temperature levels. Among the major factors (nature and concentration of liquid, system temperature and pressure) that decide the mode of liquid movement through a porous solid, pore structure plays a key role, because it is implicitly present in most of the proposed mechanisms of liquid movement in a solid. A summary of these mechanisms as quoted by Peck and Wasan [l] is - liquid diffusion due to differences in moisture concentrations - liquid movement due to capillary forces - vapour diffusion in partly air-filled pores, due to differences in partial pressure - liquid or vapour flow due to differences in total pressure, generated by external pressure, capillarity, shrinkage, or high temperature inside the moist material - liquid movement due to gravity. Knowledge of the pore structure of lignite is equally significant for the elucidation of the principles dominating its subsequent combustion or chemical conversion processes. It is widely known that direct combustion of predried coal particles under carefully controlled conditions is necessary to maintain the desired conditions in the combustion chamber. Similarly, investigation of the behaviour of lignite particles in a chemically reactive environment is related to changes @ Elsevier
Sequoia/Printed
in The Netherlands
10
effected on the pore structure as described either by the overall pore volume and surface area or their distributions with respect to pore size. Distributed pore structure parameters provide a unique picture of the intraparticle pore space, wherein transport of reacting species and chemical reaction occur. Coals in general appear to possess a pore structure extending over an extremely wide range of pore sizes, namely I&:+30 - lo5 A. This observation points to the fact that the investigation of the coal pore structure over an extensive pore size range entails the parallel use of various experimental techniques. Removal of water from coal matrix during drying does not necessarily lead to a pore volume evolution as one could expect, since water in lignites occupies more than 50% of the original particle volume. A marked particle contraction even at moderate heating conditions (- 100 “C, under vacuum) is inevitable, the natural consequence being a pore deformation (coalescence, shrinkage and collapse). A net increase or decrease in the pore volume and surface area are dependent upon the interaction of two competing effects, i.e., pore emptying due to moisture removal and pore shrinkage due to particle contraction. Major particle contractions should also be taken into consideration when residence time distributions within industrial lignite-processing units (drying or, chemical conversion) are studied. The choice of particle sizes is of particular importance, because drying times are strongly dependent upon particle size. Particle sizes in the range dp :+ 3 - 15 mm are thought suitable to carry out the present investigation. Information correlating drying times and pore structure variations of Greek lignite of the aforementioned particle size range may be used in the design of industrial coal driers. The design of industrial gasifiers which also operate partially as driers should take into account experimental data on drying of lignite lumps. This statement assumes a greater significance in the case of lignite gasification by means of its own natural moisture, which is driven out of the pore structure, in situ. Obviously, lignite drying kinetic equations should be developed. Studies of lignite drying rates, apart from the main kinetic equation,
should also include correlations describing the dependence of drying rates upon temperature, pressure, flow conditions and particle size.
LITERATURE
SURVEY
A review of the ‘coal pore structure research’ is presented in the introductory section of Ref. [2]. Results of macro-pore structure studies on samples of a low-rank coal and its froth flotation fractions are also reported in this publication. Specific pore volume and surface area distributions were computed from mercury porosimetry (MP) measurements. Estimated cumulative values ranged from 0.06 to 0.14 cm3/g and surface area from 5 to 12 m2/g (on a dry basis). Karsner and Perlmutter [3] reported ,MP results and the subsequently calculated pore volume and surface area changes occurring during drying of a number of coal samples (nominally, four samples of bituminous coal, one lignite sample and one anthracite sample). The conclusion was that both properties undergo a little reduction during drying and also that these changes depend upon the severity of drying and the initial water content. It was also found that oxidation at moderate temperatures produced only small additional changes. Pore volumes assumed values between 0.028 and 0.114 cm3/g, and surface areas between 12 and 53 m2/g (Dp > 35 A), being systematically lower than those calculated from CO2 adsorption experimental results (at 25 “C) and higher in comparison with those derived from N2 adsorption at -196 “C. Mercury entrapment when the pressure was reduced to atmospheric varied from 25% to 49% of the total amount which has intruded at pmax = 3450 bar. Studies of pore structure changes in Alberta hv Cb coals on partial solubilization were reported by Parkash and Moschopedis [4]. Experimental results showed that coal solubilization with various solvents in an inert atmosphere or in the presence of hydrogen (with or without the use of catalysts) at the reported reaction conditions resulted in a specific surface area increase. Total, micro-, meso- and macro-pore volume, each considered individually, also increased. The analysis of MP experimental results is based on the Washburn equation (P =
11
-47 cos e/D). The following values for surface tension and the contact angle are assumed: 7 = 480 dyn cm-l and 8 = 140” Pore surface areas are calculated by use of the Rootare and Prenzlow formula [ 51 and MP penetration data. Pore structure parameters are expressed per gram of dry solid to allow sensible comparisons.
EXPERIMENTAL
Raw material A set of drying experiments were performed on samples of Greek lignite (Megalopolis area). Megalopolis lignite is classified as a low heating value solid fuel with a Net Calorific Value of -900 kcal/kg. This is obviously due to the high moisture and ash content. Typical proximate analysis data valid for Megalopolis lignite are Moisture 60% Ash 17% Volatile matter 12% Fixed carbon 11% (calculated) Lignite samples were kept in the laboratory in contact with the atmospheric air for several weeks before their mechanical treatment and the preparation of smaller samples. Big lignite lumps were crushed into smaller pieces and samples were formed by selecting particles having a sphere equivalent diameter d,:+ 3 15 run. An ultimate moisture content (determined by heating up to 250 “C for 1 h) was found to be equal to 42% of the original weight of the sample. It seems that a moisture loss of about 18% occurred during the storage period under laboratory atmospheric conditions. Description of the drying experiments Lignite particles of the desired size were placed in a cylindrical glass burette (30 cm long and 15 mm wide). The burette, partially filled with coal particles (samples of mass 14 16 g were used) could be connected with a vacuum pump capable of generating a vacuum below 0.01 Torr. The lower part of the burette (10 cm long), filled with lignite, was surrounded by an electrically heated furnace supplied with a thermostatic control system. The maximum working temperature was
300 “C. The temperature of the sample could be periodically monitored with the use of a thermocouple. Table 1 presents the conditions of nine typical drying experiments. Temperature, drying time and weight loss values for individual samples are noted. Residual contents, defined on a wet basis, vary between -0 and 42%. Figure 1 is a schematic depiction of the change in residual moisture content (% of ultimate moisture content - weight loss %) uersus drying time (wet drying curves). Individual experimental points on each curve of Fig. 1 stand for a particular sample, heated at the indicated temperature (0 100 “C and 0 250 “C), and the corresponding drying times. Each curve, like typical drying curves, comprises a constant-rate section (for drying times TABLE 1 Greek lignite drying conditions Run No.
Vacuum drying conditions m
Weight loss (wt.%)
Residual moisture content (wet basis)
,
I
r
(“C)
(min)
25 100 250 100 100 250 100 100 250
0
5 5 10 30 10 60 180 60
25
Percentage of mercury entrapment (wt.%)
(wt.%) 42.0 34.7 28.3 28.0 12.2 11.4 5.5 4.1 0.1
0
1.3 13.7 14.0 29.8 30.6 36.5 37.9 41.9
50
75
Drying time
85.8 89.4 85.9 78.6 80.2 77.1 59.0 60.2 38.3
100
I25
150
t (min)
Fig. 1. Moisture removal from beds of lignite lumps as a function of drying time. Ultimate moisture content 42% (wet basis). Bed temperature: @ 100 “C and l 250 “C. Vacuum conditions (-0.2 Torr).
12
less than 10 min, at 250 “C, and less than 25 min, at 100 “C), and a falling-rate section for higher drying times [ 11. Drying at 250 “C caused the evolution of yellow oily liquids, which could be seen on the wall of the glass burette, an indication that drying is followed by pyrolysis. Higher drying temperatures and oil formation are expected to affect the state of the lignite physical structure in a different way than milder heating-up conditions.
Pore structure measurements Mercury porosimetric (MP) studies on lignite samples were carried out on a mercury porosimeter generating and applying a maximum pressure of 2000 bar (Carlo Erba, Porosimeter Mod. 200). It was possible to perform a penetration and a retraction run on a single sample and produce the characteristic hysteresis loop. Two typical and widely differing hysteresis loops, one for a virtuaIIy dry sample (moisture content mO.l%, run No. 9, Table 1) and the second for a sample with a considerably higher moisture content of 34.7% (run No. 2, Table l), are shown in Fig. 2. The MP experiments detected approximately equal total pore volumes for the two samples under consideration (namely, V,, = 0.217 - 0.219 cm3/g (dry sample)). The MP hysteresis loops of Fig. 2 show major deviations between the relevant penetration and retraction lines as well as the amounts of mercury entrapped within the lignite internal pore structure. The MP characteristics associated with runs 1,
3 - 8, and 10 are intermediate in comparison with those of runs 2 to 9. Total pore volumes and surface areas as well as percentage of mercury entrapment are given in Table 2. In separate columns of Table 2, meso- and macro-pore volumes and surface areas are also noted. The variation of these overall structural properties is shown as a function of weight loss %, in Figs. 3, 4 and 5. The extent of entrapment decreases monotonically with an ever-increasing slope, which becomes appreciable at levels exeeding the limit of 20% in weight loss. The maximum weight loss of 42% gave rise to a decrease of
c
0.8
log P
I.6 (P
2.4
3.2
absolute pressure,
bar)
Fig. 2. Mercury penetration and retraction on samples of Greek (Megalopolis)‘.lignite (0, 1 Moisture content 34.7%, VP, = 0.219 cm3/g; 0, moisture content O.l%, VPt = 0.217 cm3/g). Numbers on curves refer to Run No. Table 2.
TABLE 2 Lignite specific pore volume and surface area changes due to drying Run No.
Specific pore surface area (dry sample) (m*/g)
Specific pore volume (dry sample) (cm3/g) Total
0.245 0.219 0.227 0.240 0.213 0.219 0.206 0.241 0.217
Meso-pore Dp < 300 A
Meso-pore/ macro-pore ratio
Total
D, > 300 A
0.028 0.025 0.028 0.018 0.024 0.032 0.041 0.033 0.040
0.217 0.194 0.199 0.222 0.189 0.187 0.165 0.208 0.177
0.13 0.13 0.14 0.08 0.13 0.17 0.25 0.16 0.23
11.7 10.4 12.2 9.7 11.0 13.0 17.2 13.8 15.0
Macro-pore
Macro-pore
Meso-pore Dp < 300 A
Dp > 300 A
Meso-pore/ macro-pore ratio
9.1 7.6 9.1 6.3 7.1 9.1 13.2 9.5 13.0
2.6 2.8 3.1 3.4 3.9 3.9 4.0 4.3 2.0
3.5 2.7 2.9 1.9 1.8 2.3 3.3 2.2 6.5
13
solid surface do not provide the conditions that favour mercury entrapment. The reduction in the degree of pore intersection might also substantially reduce the probability of mercury entrapment in wide pore spaces surrounded by narrower entrances. Similar observations have been reported in Ref. [ 31. Theoretical predictions of mercury mechanical entrapment within a network pore structure model have been published elsewhere 20
30
Weight Loss
%
IO
40
50
Fig. 3. Variation of mercury entrapment expressed as a percentage of the total pore volume filled with mercury at the maximum applied pressure (2000 bars) us. weight loss due to drying. 0, Heating at 250 “C; 0, heating at 100 “C.
&:/ ; a”
2:.
B
ho
IO Weight Loss
(WL) %
Fig. 4. Specific macro- meso-pore (DP > 75 A) volume reduction during drying under vacuum. Drying temperature: 0,100 “C; 0, 250 “C. Straight line: VP = 0.2367 - 4.6 X 10d4 (WL %), cm3/g.
18A
[61-
It is evident from Fig. 4 that the specific pore volume shows a trend to lower values as the sample weight loss increases. The wide spread of the experimental points does not allow the best straight line drawn through these points to be regarded as a reliable representation of the observed phenomenon. Nevertheless, the indication is clear that drying caused a slight lowering of the total specific pore volume. A similar graph, shown in Fig. 5, depicts total specific surface variations versus weight loss. A weight loss between 30% and 40% is sufficient for the development of a net rise in the specific pore surface area. As discussed in following sections in this work, the increase in pore surface area from -10 m2/g to 17 m2/g, in contrast to the minor decrease in the pore volume, is due to the generation of macro- and meso-pores in the range D, :+ 75 - 1500 A at the expense of macro-pores in the range D, :+ 1500 - lo4 A. A thorough picture of lignite pore structure modification due to drying is provided in Figs. 6 - 9, in the form of pore volume and area distributions with respect to pore size.
16..
Weight
LOSS
(n)
$
Fig. 5. Specific pore surface area change us. weight loss. Drying temperature: 0, 100 “C; 0, 250 “C.
about 45% in the percentage of mercury being entrapped. The systematic reduction in mercury entrapment is perhaps due to the formation of a more regular and less complex physical pore structure. Individual pores with a rather uniform cross-section and a smoother
Pore volume and surface area frequency distribu tions Pore volume differential distributions are shown in Figs. 6 and 7. The curves in Fig. 6 have been computed by differentiating MP penetration experimental lines for the indicated drying run number, performed at 100 “C. The distribution marked 0, valid for lignite before drying, forms a sharp peak located between D, lo3 and lo5 A (macropore range), a deep section between D, 150 lo3 A (partially macro- and meso-pore range) and a rising section (a half-formed peak) in the region D, : 75 - 150 A (entirely within the meso-pore range). MP could not detect pore sizes below D, = 75 A. It is readily seen in Fig. 6 that the gradual moisture removal from
7
log D
(D : Pore Diameter,
A)
-
Fig. 6. Dfcerential pore volume distributions obtained from MP measurements on samples of Greek lignite dried under vacuum at 100 “C for varying periods of time. Numbers on the curves refer to Run No., Table 2.
0
OO"OO 0
0
0
1%
D
0 0
(D: pore Diameter, A )
Fig. 7. Differential pore volume distributions derived from MP experiments on samples of Greek lignite dried under vacuum at 250 “C for varying times. Numbers on the curves refer to Run No., Table 2.
the lignite samples causes the flattening of the peak in the higher macro-pore region, while the deep section, located partly in the macropore and partly in the meso-pore area, becomes gradually shallower. The net effect is that the contribution of pore sixes 0,:+75 lo3 A to the total pore volume becomes greater at the expense of macropores (II,: +103 - 10’ A). As stated in previous paragraphs of this work, pore volume changes were not appreciable. A similar situation appears in Fig. 7, wherein distributions were calculated from MP data
on lignite samples dried at 250 “C. The formation of oils and the effect of a higher temperature does not seem to favour a practically different mode of pore structure alteration. Similar comments can be made for the differential pore surface area distributions of Figs. 8 and 9 associated with drying runs performed at 100 “C and 250 “C respectively. Moisture removed resulted in a reduction of macro-pore area while the development of meso-pores and lower macro-pores gave rise to a net increase in the overall pore surface area.
15
effects, resulting in a virtually constant specific pore volume and a moderate increase in the total specific pore surface area by clearly shifting a fraction of the pore volume from the macro-pore towards the meso-pore region.
i
I
l\
J\
B 0 %“,
I
CONCLUSIONS
0
log
D
(II : pore diameter. A
)
Fig. 8. Differential pore surface area distributions derived from MP data obtained from MP experiments on Greek lignite dried under vacuum at 100 “C for varying times. Numbers on the curves refer to Run No., Table 2.
Heating of Greek (Megalopolis) lignite under vacuum in a constant-temperature environment causes considerable particle shrinkage and the resulting moisture removal follows the pattern of a typical drying curve. Sample contraction and moisture removal play a counterbalancing role, since the former is .responsible for pore shrinkage and the latter for pore emptying, the net effect being an increase in the surface area, while pore volume is slightly decreased. The formation of meso- and macro-pores in the pore size range II,:+ 75 - 1500 A, at the expense of macropores in the pore range D,:+ 1500 - lo4 A, during drying, evidenced from comparisons of the relevant pore volume and surface distributions provides a plausible explanation of the net effect of drying upon pore structure. The temperature level at which drying takes place does not seem to practically influence the behaviour of lignite lumps, despite the formation of oily products at higher temperatures.
ACKNOWLEDGEMENT
The authors wish to acknowledge with thanks the Public Power Corporation of Greece for supplying the lignite samples.
REFERENCES log
D
(D:
pore diameter
, A)
Fig. 9. Differential pore surface area distributions derived from MP data obtained from MP experiments on Greek lignite dried at 250 “C under vacuum, for varying drying times. Numbers on the curves refer to Run No., Table 2.
Thermal drying of the lignite samples under vacuum causes a considerable volume contraction of the lignite particles (by -l/3 of their original volume). Particle contraction and moisture withdrawal seem to be counteracting
R. E. Peck and D. T. Wasan, Advances in Chem. Eng., 9 (1974) 247. G. P. Androutsopoulos and E. T. Woodburn, Powder Technol., 33 (1982) 175. G. G. Karsner and D. D. Perlmutter, Znd. Eng. Chem. Process Des. Dev., 21 (1982) 348. S. Parkash and S. E. Moschopedis, Fuel, 62 (1983) 1231. H. M. Rootare and C. F. Prenzlow, J. Phys. Chem., 71 (1967) 2733. G. P. Androutsopoulos and R. Mann, Chem. Eng. Sci., 34 (1979) 1203.