Energy & Fuels 1992,6, 242-249
242
Response of Argonne Premium Coals to Heat L. L. Isaacs,* R. Abhari, R. Ledesma, and E. Tsafantakis Department of Chemical Engineering, The School of Engineering of The City College and The Graduate School of The City University of New York, New York, New York 10031 Received July 23, 1991. Revised Manuscript Received October 17, 1991
The thermal responses of the Argonne Premium Coals and of the inorganic materials separated from the coals were obtained by differential scanning calorimetry. Thermogravimetry was used to quantify the mass loss induced due to heating. The energetics of several prepyrolytic processes in the coals were calculated.
The response of coals to heating has been the subject of numerous studies dating back to the 1890s.' Many techniques?* such as differential scanning calorimetry (DSC), differential thermal analysis (DTA), thermogravimetry (TGA), and spectral analyses (ESR,NMR,IR), have been used in the course of these studies to investigate the thermal effects. With advances in instrumental sophistication, a number of these techniques have been coupled, TGA-FTIR, to study the kinetics and mechanism of coal conversion processes, for example, pyrolysis or liquification. The 1991 Storch award lecture by Solomon9 s u m m a r b s and reviews a vast amount of literature on the subject. We have studied the response of the coals which comprise the Argonne Premium Coal (APC) bankloto heating. The upper limit of temperature, 700 K, was chosen to prevent the onset of extensive pyrolytic breakdown of the coals. The studies were conducted on the coals and on the mineral matter extracted from the coals. The thermal and fusion behavior of ashes prepared by the combustion of APC samples was also investigated and has been reported." In this paper we present the results of DSC and TGA studies. The emphasis here is on establishing the energetics of thermal processes that occur at low temperaturea. The operational definition of low temperature is that it is the temperature range below the onset of inert atmosphere py-rolysis, 725 K.
Experimental Procedure The APC sample bank consists of a suite of eight coals. Differential scanning calorimetry (DSC) was the technique employed to determine the response of the experimental samples to heating. The calorimeter was a DuPont Model 910, used in conjunction with W o n t 1090and 2100 Thermal Analyst system. The heating rates for the experiments were nominally 10 K/min. The temperature range for the measurement was between 100 and 700 K. All the experiments were done while keeping the (1) Anderson, W. C.; Roberta, J. J. SOC.Chem. Znd. London 1898, XVII, 1013. (2) Glass, H. D. Fuel 1955,34,253. (3) Berkowitz, N. Fuel 1957,36,355. (4) Kirov, N. Y.; Stevens, J. N. Physical Aspects of Coal Carbonization; University of N. S. Wales: Sidney, Australia, 1967. (5) OGorman, J. V.; Walker, P. L., Jr. Fuel 1973,52, 71. (6) Mahajan, 0.; Tomita, A,; Walker, P. L., Jr. Fuel 1976, 55, 63. (7) Marinov, V. N. Fuel 1977, 56, 153. (8) Seehra, M. S.; Ghosh, B. Anal. J. Appl. Pyrol. 1988, 13, 209. (9) Solomon, P. R.; Hamblen, D. G.; Serio, M. A.; Yu,2.;Charpenay, S . F'repr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1991,36, 268. (10) Vorres, K. S. Energy Fuels 1990, 4 , 420. (11) Ledesma, R.; Isaaca, L.L.Mater. Res. SOC.Symp. R o c . 1990,178, 35.
0887-0624/92/2506-0242$03.00/0
samples covered by dry nitrogen gas flow, 1 L (STP)/min, and the exposure of the samples to the ambient environment was limited to lese than 2 min. Based on the volume of the calorimeter's experimental chamber, gas residence in the chamber ie 0.006 min. The samples were loaded into the calorimeter sample holdere and weighed in a nitmgen gas atmosphere. Actual sample weights were eatabliahed before and after each experimental run. To allow quantitative comparison of the heat flow curves, each experiment on a sample was accompanied by an experimental run using a sapphire specimen as a standard. The experimental uncertainty is estimated at about 2%. To determine the response to heating of the mineral constituents of the coals, the coals were separated into organic and inorganic fractions by the sink-float technique. A cesium d o r i d e solution of specific gravity 1.60 was used as the floating agent. Since there was insufficient material for both experiments and compositional analysis, portions of the coals used for the separations were ashed at 973 K. The ashes were analyzed by the inductively coupled plasma technique for elemental composition. DSC experiments on the separated mineral components were performed in the manner described above. Thermogravimetry experiments were done on the coals using two modes of operation. Isothermal experiments at 305 K were used to study the kinetics of dehydration. Experimenta at a constant rate of temperature rise of 10 K/min between 300 and 700 K were used to measure volatile evolution. Detaile of the experimental procedure and the results of the isothermal experiments have been published.'*
Results and Discussion Coal is a complex mixture of organic and inorganic phases together with quantities of physically and/or chemically bound water. The inorganic phase may amount to 15-20 wt % of the coal. It is a mixture of quartz, pyrite, calcite, and silicates. Trace quantities of many metal compounds are also present. The quantity of water associated with a coal ranges from less than 1to over 30 wt %. In Table I the relevant compositional information of the APC coals is summarized. The elemental compoeitions of the inorganic phases of the coals, as determined from the ash analyses, were reported elsewhere.'l A DSC thermogram of a material reflects the occurrence of physical and chemical changes. The specific features in the thermogram indicate whether the event is athermal, endothermic, or exothermic. Figure 1is representative of the thermograms which were obtained. For the coals,there are indications of endothermic behavior in several tamperature regions and of exothermic behavior above 400 K. Thermograms for the mineral matter extracted from coal show, in general, only endothermic behavior. The TGA (12) Abhari, R.; Imacs, L.
L.Energy Fuels 1990, 4 , 448.
0 1992 American Chemical Society
Response of Argonne Premium Coals to Heat
coal Beulah-Zap lignite Wvodak-Anderaon subbit. Illinois No. 6, HVB Blind Canyon, HBV Pittsburgh No. 8, HVB Lewiston-Stockton, HVB Upper Freeport, MVB Pochahontas No. 3, LVB
Energy & Fuels, Vol. 6, No. 3, 1992 243
Table I. T h e Argonne Premium Coalsa dmmf water, miner. matter, desig- d-f C, O/C, kg/kg of kg/kg of nation w t fractn kg/kg dry coal dry coal 0.7405 0.258 0.476 0.087 ND WY 0.7604 0.222 0.391 0.087 0.8073 IL 0.125 0.087 0.181 UT 0.8132 0.134 0.049 0.053 PIT 0.8495 0.081 0.017 0.109 WV 0.8547 0.087 0.025 0.216 UF 0.8808 0.054 0.011 0.153 POC 0.9181 0.018 0.007 0.055
geologic age Paleocene Paleocene Pennsylvanian Cretaceoua Pennsylvanian Pennsylvanian Pennsylvanian Pennsylvanian
minerals quartz
fraction of miner. matter pyrite calcite clay
BB
0.07 0.23 0.19 0.15 0.16 0.12 0.10 0.05
0.03 0.01 0.30 0.09 0.22 0.01 0.22 0.02
0.20 0.05 0.10 0.24 0.05 0.01 0.07 0.31
0.70 0.71 0.41 0.52 0.57 0.86 0.61 0.62
"For a compilation of Argonne Premium Coal data refer to User's Handbook ANL/PCSP-89/1. Prepared by K. S. Vorres. 20
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Figure 1. DSC output for Beulah-Zap lignite (ND) which is typical for all the DSC data.
data shown in Figure 2 are for the coal whose thermogram is shown in Figure 1. It indicates the regimes of weight losses. Consider the data presented in Figure 1. The ordinate 0 represents the heat flow to a given quantity m, of sample. The sample weight may change with temperature if the heating process leads to volatile evolution, in which case m refers to the residual weight at temperature T. Thus Q(T) = O ( T ) / m ( T ) (1)
is the energy absorption rate per unit quantity of material remaining at temperature T. At a given temperature T, Q(T) = f(T) W " / d t ) [kW/kgl (2) C ( T ) is the sample heat capacity, dT/dt is the rate of temperature rise, and f(T) is a dimensionless but temperature-dependent factor which accounts for the thermal resistance between the sample and the heat source. Nominally dT/dt is set to some constant value, and the basis of the DSC technique is that the thermal power to the sample is varied so as to maintain a constant rate of temperature rise. If the material under investigation responds to heating in a reversible manner, i.e., no chemical reactions or volatile loss or fint-order phase transition, one can convert Q(T ) to the heat capacity C ( T ) of the material by also running the experiment on a material of known heat capacity ck(T ) under identical conditions. One measures Qk(T) and uses the heat capacity Ck(T) to calculate a conversion constant k ( T ) = Qk(T)/Ck(T) = f(T)(dT/dt) (3)
Substituting k(T) into eq 2, we obtain C(T). The use of the above equation to convert energy absorption rates into heat capacities was validated by experiments on materials with well-known heat capacity values. However, if the heat absorption by the sample under investigation leads to irreversible changes in the sample
, 350
400
.
450 500 550 Temperature ( K )
0 600
650
700
Figure 2. TGA/DTG output of constant heating rate devolatilization data for Beulah-Zap lignite (ND) which is typical of all coals studied.
and/or the thermal response of the sample to heating is exothermic, then the calculated heat capacity has no physical meaning. So, to compare data for a set of materials or to deduce general trends, the relative heat absorption with respect to a reference material is calculated. First, one needs to normalize all the data to a reference state so that they correspond to numbers that would be obtained if the experiments were conducted under identical conditions. These conditions are established by setting the value of k ( T ) to 10 K/min (1/6 K/s) for the reference state. k ( T ) for the actual experiment is calculated using the data for that experiment's sapphire run. Then, for any experimental data point Q(T)/k(T) = Q(T)scadk(T)ref = 6Q(T)wded S(T) = 1/6k(T) = c,(T)/6Qs(T) (4) is the scaling factor. Q,(T) and C,(T) are the measured energy absorption rate and the known heat capacity for sapphire. Hence, the relative heat absorption QAT) = S(T) Q(T)
QdT)
(5)
Q,(T), the reference heat absorption, is calculated from ck(T), the heat capacity of the selected reference material with the scaling factor set equal to 10 K/min. The use of
Q, allows us to deduce general trends and to compare seta of data on classes of materials. For the coals the reference material selected is POCO graphite,13J4and for the inorganic extracts sapphire15 is used as the reference. Thermogravimetric Studies of Coals. TGA heating rates for the coals were set to obtain a temperature ramp of 10 K/min. The experiments were performed in dry nitrogen atmosphere between 300 and 700 K. A typical (13) Isaacs, L. L.; Wang, W. Y. Thermal Conductivity 17; Plenum Press: New York, 1982; p 55. (14) Deshpande, M. S.; Bogaard, R. H. Thermal Conductivity 17; Plenum Press: New York, 1982; p 45. (15) Ginnings, D. C.; Furakawa, G. T. J.Am. Chem. SOC. 1953,75,522.
Isaacs et al.
244 Energy & Fuels, Vol. 6,No. 3, 1992
total loss fractn init. coal mass
ND WY IL UT PIT
wv UF
POC
0.37 0.28 0.16 0.19
0.06 0.07 0.04 0.03
Table 11. Distribution of Volatilization Mass Loss labile water bound water from miner. matter (400 550 K) (550 700 K) (280 400 K) fractn init. fractn total fractn init. fractn total fractn init. fractn total coal mass mass loss coal mass mass loss coal mass mass loss 0.26 0.69 0.05 0.17 0.01 0.02
-
-
0.17 0.04 0.04 0.01 0.02 0.01 0.005
0.62 0.27 0.23 0.22 0.25 0.24 0.15
0.002 0.001 0.002
0.11 0.03 0.01 0.03 0.03
0.02 0.02 0.003 0.025
0.07 0.15 0.01 0.11 0.34
0.0
0.0
0.01
0.18
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0.01
0.004
0.13
0.03
0.01
experimental run is shown in Figure 2. Combination of the TGA data with DSC data on the coals and the inorganics allows us to identify the causes of mass losses due to heating. The derivative of the weight loss curve in Figure 2 divides the TGA data into three regimes. There is major weight loss between room temperature and 400 K due to heating. In the DSC data (Figure l),correspondingly there is a major endotherm. We infer that the removal of weakly held, labile, water from the coal is what is occurring. The origins of the labile water are surface adsorbed, interlayer, and macropore water. There is additional small weight loss due to the removal of more strongly held water between 400 and 550 K. This loss occurs at a nearly constant rate as indicated by the derivative of the weight loss curve in Figure 2. It is likely that this bound water originates from the macropore structure of the coal and from chemically bonded water associated with the coal. Mass losses, due to dehydroxylation and decomposition of the inorganics (mineral volatiles) and to the removal of small organic molecules (COzand C 1 4 5 hydrocarbons, Le., organic volatiles), occur between 550 and 700 K. Since we had insufficient quantities of the sink-float separated coal inorganic fractions to perform both TGA and DSC experiments, only DSC experiments were done on these fractions. However, the mass loss date obtained by weighing the DSC specimens at 300 K, before heating, after heating (and recooling) to 400 K, and after reheating to 770 K and above. The mass loss data from the DSC experiments on the inorganic fractions combined with the TGA data for the coals enable the separation of the high-temperature (above 500 K)mass loss into due from the loss of mineral volatiles and due from loss of organic volatiles. In Table II the weight loss distribution attributed to the events identified above are listed for all the APC coals. It is customary to correlate mass losses with coal rank. While there are many ways to establish coal ranking, the oxygen to carbon ratio was found to be the best visual indicator for the correlation of mass loss due to volatilization. In Figure 3a the organic mass loss versus O/C ratio is presented. There is an indication that either the geographic locale and/or the geological age of the coal seam may be a factor which affects the volatility of given coal. On the other hand, the total volatile loss including water removal, as shown in Figure 3b, is more or less a function of the coal rank. The Thermal Response of the Inorganic Components of Coals. Up to 20 w t ?& of coals consists of chemical species which convert to ashes when the coals are combusted. This mixture consists of silica, pyrite, calcite, silicates, gypsum, and trace quantities of other metal compounds. Kiss and King16 consider that this mixture (16)Kiss, L.T.;King, N.T.Fuel 1977, 56, 340; 1979, 58, 547.
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Figure 3. (a) Quantity of organics volatilized versus coal rank. (b) Total quantity of volatiles versus coal rank. I
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Figure 4. Specific energy absorption rate for the inorganic/ mineral component of Illinois No. 6 and West Virginia coals.
should be classified into two groups: inorganics and minerals. The inorganics are relatively evenly distributed through the coal, while the minerals occur as discrete particles throughout the coal seam. Water of hydration and adsorbed water are associated with the minerals. The responses to heating of the separated inorganic components were obtained by differential scanning calorimetry. In the absence of phase transitions and/or chemical reactions the response curve is expected to be a smoothly varying function of the temperature and of the
Energy & Fuels, Vol. 6, No. 3, 1992 245
Response of Argonne Premium Coals to Heat I
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heating rate. Transformations and reactions introduce discontinuities, valleys and peaks into the response curve. In Figure 4 the specific energy absorption rate Q is shown for the inorganic/mineral components separated from the Illinois No. 6 (IL) and West Virginia (WV) coals. Qs were calculated using eq 1. O'Gorman and Walker5 used thermogravimetry (TGA) to investigate the thermal behavior of Ymineral" fractions separated from coals by low-temperature ashing, the individual inorganic species usually found in coals, and synthetic mixtures approximating the coal =mineral" phase. The TGA experiments were done in both inert and reactive atmospheres. The features indicated in Figure 4 were identified on the basis of their work. To quantify the data, for the extracted inorganic components of the coals, we used sapphire as the reference and calculated Q, using eqs 1-5. In Figure 5 the relative heat absorption rates versus temperature during the initial heating runs are shown. The valleys (endotherms) in the 300-550 K regime are due to labile and interlayer water removal. Up to 800 K, silica and calcite have only sensible heat contributions. The onset of a second set of endotherms begins between 620 and 720 K. The actual onset temperature depends on the clay (silicates)to pyrite ratio. There are two processes that need to be considered, pyrite decomposition and clay dehydroxylation. FeSpdecomposes to give FeS, pyrrhotite (Fel-,S), and sulfur. The reaction is endothermic. An inspection of the Fe-S phase diagram"J8 and TGA data5 indicates that on heating FeSz to 600 K sulfur will start to vaporize. The decomposition will be complete by 975 K. This is a two-stage decomposition, first forming Fel,S and then FeS and S. If there is oxygen present, the exothermic formation of SO2is also a possibility. There is a vast amount of literaturelg on the dehydroxylation of clays. (17) Burgmann, W.; Urbain, G.; Frohberg, M. G. Mem. Sci. Reu. Met. 1968,65,425. (18) Rau, H.Phys. Chem. Solids 1976,37, 425.
0
200
400
600
800
Temperature ( K )
Figure 7. Calculated heat absorption rate for pyrite decomposition. The dehydroxylation onset temperature for kaolinite is 720 K, for montmorillonite about 750 K, and for illite about 680 K. Mineral analyses'O of the coals indicate that the clay component present is a variable mix of these silicates and hence the onset temperature for dehydroxylation may vary quite widely. The experimental data is further complicated by the overlap of the pyrite decomposition and the clay dehydroxylation endotherms. In the WV material the weight to weight ratio of clay to pyrite is 86. The relative heat absorption curve Q,(WV) can be corrected for the quartz and calcite contributions using known heat capacities for these materials. In a similar manner we also calculated the response curves for the clays from the POC (weight ratio 31) and WY (weight ratio 71)coals. The calculated response curves were similar with only some variation in the amplitudes and the dehydroxylation onset temperatures. Inasmuch as the composition of the clays associated with a given coal is not available, only a simple averaging of the three calculated clay response curves was performed and this curve is shown in Figure 6. The averaged dehydroxylation onset temperature is approximately 680 K. The next step in analyzing the experimental data is to calculate a response curve for the pyrite decomposition. To accomplish this we used the experimental data for the coals with the lowest clay to pyrite ratios. These are the IL and PIT coals with clay to pyrite ratios of 1.4 and 2.6. (19) Wendland, W. W. Thermal Analysis; J. Wiley and Sone: New York, 1986.
Isaacs et al.
246 Energy & Fuels, Vol. 6,No. 3, 1992
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Figure 9. Relative heat absorption rate for coals.
The calculated response curve for pyrite is shown in Figure 7. In the region of 620 to 760 K, FeSz goes to Fel,S and FeS is formed above 760 K. In the ND inorganic there is a substantial amount of gypsum. Gypsum dehydrates between 375 and 475 K. It is evident from Figure 5 that the start of the endotherm which we associate with water removal has indeed shifted u)nigner temperature ror m e IYU materia. w e estimate from the response curve that the total "clay" in the ND inorganic contains approximately 25 w t % of gypsum. The response curve for the UT inorganic is very different in character from the other response curves. Forty percent of the mass is composed of quartz and calcite, but they contribute 80% of the response curve between 200 and 400 K. Between 400 and 500 K their apparent contribution to the response c w e is 90%. We conclude from this that the components in the sample analysis, identified as pyrite and clay, do not respond as such but must act as some combined substance. The heat capacity of this material is about 20% of the heat capacity of sapphire. Starting at 500 K the material undergoes an exothermic reaction with a peak in the response curve at 680 K. This isotherm might be caused by the reaction of calcite with pyrite to form CaS04. The same kind of reaction must also occur in the UF inorganic material but to a much smaller extent than in the UT material. The Thermal Response of Coals. In Figure 8 the response curve for the ND lignite is shown for three consecutive heatings of a coal sample. Inspection of the plota indicates the following features: (a) a low-temperature endotherm (190-280 K); (b) an endotherm between 300 and 420 K; (c) exothermic behavior above 420 K; (d) a decrease in the rate of exothermicity starting between 520 and 570 K. After the first heating, the low-temperature endotherm vanishes and the endotherm between 300 and 420 K is greatly diminished. Since there is no further mass loss in this region on reheating, we conclude that the residual endotherm may be the signature of a reversible process.
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Figure 10. Relative heat absorption rate for coals.
In the exothermic regime, on reheating there are additional mass losses, and hence the exotherm may be related to some irreversible process. The process is either quite far from completion by 700 K or is kinetically extremely slow and the rate control is diffusion in the condensed phase. In Figures 9 and 10 the response curves for the initial heating of all the coals are shown as plots of 8:s versus temperature. Q', differs from Q, defied by eq 5 because the measured heat absorption is corrected for the heat absorption of the mineral components as follows
Q"l=
((m,Q,(coal) - mi,Q,(io)/(m, - mi,)) - Q,(POCO) (6)
Response of Argonne Premium Coals t o Heat Energy & Fuels, Vol. 6, No. 3, 1992 247 Table 111. Energetics of the Thermally Induced Processes labile water removal pore water activitation onset energy, temp, k J / k K' of cod
ND WY
IL UT PIT
wv UF
POC
215 230 240
10 8 3
residual endotherm
x,kg of
(300-420 K)
HlO/kg ofdry coal-
energy, kJ/keof d& coal
energy, kJ/kgof d&&al
0.476 0.391 0.087 0.049 0.017 0.025 0.011 0.007
980 832 214 138 66 77 33 24
34 57 34
Calculated from data of Burham et al?l
11
22 11
29 6
* Bakr et al?3
exotherm (42C-680 K) mval, kg/kg of dmmf coal
energy, kJ/kgof dmmf coal
0.192 0.227 0.176 0.072 0.023 0.034 0.024
-2870 -5400 -4320 -2780 -1650 -1320 -1920 -1240
0
I,
mw,
L o ,
kg/kg of dmmf coal
kg/kgof dmmf coal
0.0225 0.0130 0.0031 0.0018 0.0014 0.0011 0.0018 O.OOO8
0.170 0.214 0.173 0.070 0.022 0.033 0.022
kJ/kg of dmmf coal -2670 -5280 -4290 -2760 -1640 -1310 -1900 -1230
0
~
kJ/kg of hydrocarbon
endotherm (500K +) thermal ESRb onset onset temp,K temp,K
-8350 -18830 -17570 -21570 -16960 -1820 -27080 (-32000)'
520 570 585 (565)c (540)c 520 570 (580)E
570 550 520 580 500/650 580 520/700
Estimated.
m,and Q8(coal)are the weight and scaled energy absorption rate for the coal sample, Q,(io) is the scaled heat absorption rate for the mineral matter in that coal, and mi, is the quantity of mineral matter in the coal. Presentation of the response data in this manner gives a better perspective of the relative energetics of the processes. An endotherm at low temperature is observed in the high-moisture coals which follow a mixed kinetics model12 for drying. Mraw and Naas-0Rourkem have investigated in detail the thermal response of the Wyodak Anderson (WY) coal at low temperature and have shown that the endotherm observed at low temperature is associated with the thermal excitation of water (referred to as nonfreezable) clusters resident in the pores of the coal. Data summary, Table 111, shows that the onset of the "pore" water endotherm and the amount of energy are both related to the total amount of water associated with the coal. The energy absorbed was calculated by integration of the pore water endotherm. Assuming direct proportinality of the absorbed energy to the quantity of total moisture associated with the coal, we obtain 30 kJ/kg total water as a measure of the thermal excitation energy of pore water. Inspedion of Figure 8 shows a major endotherm between 300 and 420 K. In the initial heating of the coal, this endotherm is accompanied by volatile evolution. The quantity of mass loss closely corresponds to the moisture content of the coal. On reheating the sample a residue of the endotherm is observed but there is no further volatile evolution in this temperature region. Thus, we conclude that there are two processes associated with this endotherm: (a) labile water removal and (b) an activation process for low-temperaturecoal reaction and/or structural rearrangement process. Integration of the endotherm between 300 and 420 K yields the total energy associated with the labile water removal and the low-temperature process activation. The numbers obtained are tabulated in Table 111. A least-squares fit of the energy absorbed against water content X yields
E = 22 + 2040X [kJ/kg of dry coal]
(7)
X is the amount of water associated with the coal. Unita of X are kg of water/kg of dry coal. The number 2040 (kJ/kg of water) is the energy required to remove a kilogram of water from a coal. Thus, it is a combination of the heat of desorption and the heat of vaporization. The enthalpy of evaporation of bulk water rangea from 2440 to 2260 kJ/kg water between 300 and 373 K. Inspection of Figure 8 further indicates that between 300 and 420 K an endothermic response is still present during (20) Mraw, S. C.; Naaa-O'Rourke, D. F. Science 1979,201,901.
t
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0 IL 0
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Y
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Fractional Weight Loss ( k g / k g DMMF COAL)
Figure 11. (a) Energy release versus volatile release. (b)Energy release versu hydrocarbon release.
the second and third heating of the coal sample. Similar behavior is observed for all the coals. In Table I11 we tabulate the integral value of the endotherm. It should be noted that on reheating of the dried coals no volatile evolution was observed until above 500 K. This is consistent with the TGA observations. A simple arithmetic average of the integrated values is 25 kJ/kg of dry coal. This is to be comparedtothe 22 kJ/kgof dry coal obtained as the leasbsquares intercept for the labile water removal. From the magnitude of this number it is inferred that it is moet likely that this endotherm is associated with weak dispersion forces and/or hydrogen bonding acting between segments of the coal "molecule". Thermal studies were performed by Mahajan et al.6 on a series of coals ranging in rank from lignite to anthracite. They reported that the thermal effects accompanying inert atmosphere pyrolysis between 373 and 853 K were endothermic for high-rank coals (HVCbituminous to antharacite) and only exotherms were observed for subbituminous and lignitic coals. In their discussion they point out that endo- or exothermicity is a relative concept and depends on the choice of a reference response curve. The thermal response of the coal samples becomes exothermic relative to POCO graphite above 420 K. In Table III the integrated values of Q: between 400 and 680 K, for the first heating of the coals, are tabulated. In Figure l l a , the integral of Q', is plotted as a function of mvol,the amount of volatile release between 400 and 680 K. The energy release may be due to several sources. Among these are formation of COz,formation, and release of low molecular weight (C,-C,) hydrocarbons, and structural rearrangement of the coal.
Isaacs et al.
248 Energy & Fuels, Vol. 6,No. 3, 1992
y WY 0
%T $
t
POC 0
9
-O 0 U F
PIT 0
IL
c
0
P I
A
wv
ND
d
0.80 DMMF Carbon [ w t fraction]
A
A UF
Figure 12. Energy release versus coal rank. The energy release for the Virginia coal (POC)is an estimate.
Burnham et aLZ1have used the Rock-Eval method to measure the amount of COz evolved below 663 K during the pyrolysis of the APC coals. The quantity of COz evolved normalized to DMMF coal, mco2,is tabulated in Table III. COzmay be a product due to internal oxidation even if the pyrolysis is carried out in an inert atmosphere.' Using the standard heat of formation of COz and the data of Burnham et aLZ1the Q', integrals are adjusted to account for COzformation. In Figure l l b the adjusted values I,,,, are plotted as a function of mev. mev = mvol- mc02
=
(Qem
+ 1250)/mev
.
0
d
0
0
~
I I B r
0
-I 0-
-I 5 0 IL
(8)
is the adjusted mass evolution between 400 and 680 K. It is evident from this plot that I,,, is not solely due to the formation and release of hydrocarbons. There is still a contribution to I, which is independent of volatile release. One possibility is that there is an internal recombination of reactive sites in the coals, possibly free-radical type polymerization and or cross-linking by bonding between fused aromatic ring clusters. Another possibility is lowtemperature o x i d a t i ~ n , which ' ~ ~ ~ also proceeds via a freeradical mechanism. A least-squares straight line fit to the points in Figure l l b yields an intercept of -1250 kJ/kg of dmmf coal. We take this number to be a measure of the energetics of the internal recombination or oxidation reaction. In Figure 12, the net heat effect for hydrocarbon production Ihcis plotted as a function of dmmf carbon content in the coals. Ihc
,
0.90
(9)
is the energy release per kilogram of "hydrocarbon" volatiles produced. The data are scattered but indicate that the exothermicity for hydrocarbon evolution increases with coal rank. It is interesting to note that the West Virginia sample which appears to be anomalous is the one which has the highest sporinite and inertinite type maceral content among the coals investigated. Inspection of Figures 9 and 10 indicates that above 500 K the slopes of the response curves for the initial heating decrease. The decrease in slope is clearly seen for all but the WY and IL coals. The change in slope for these two samples is obscured by the steepness of the response curves. However, the fact that there is a decrease in slope for all the samples is demonstrated in Figure 13. A possible reason for the change in the slope is the occurrence of two processes, one of them endothermic. During the second heating of the coal samples the decrease in the slopes is not evident. In Figure 13 the differencesbetween the heat absorption rates for the initial and second heating (21) Burnham, A. K.; Oh, M. S.;Samoun, A. M. Energy Fuels 1989, 3, 901. (22) Khan Rashid, M. Energy Fuels 1987, I , 366.
1
-2 0
400
500
600
Temperature
700 (
KI
Figure 13. High-temperature endotherms for coals.
are plotted as a function of temperature. Inasmuch as the absolute numerical values of the differences are unimportant, the maximum differences were set equal to zero and all other differences were scaled relative to the adjustment. The difference graphs allow us to conclude that indeed there is an endotherm superimposed on the exotherm. The onset temperatures for the endotherms, difference equal to zero, are tabulated in Table 111. The endotherm onset temperatures for the UT, PIT, and POC coals were obtained by estimation from the first heating curve (initial deviation of the slopes). Bakr,Yokono, and SanadaZ3used in situ ESR spectroscopy to investigate the free-radical chemistry of the Argonne Coals as a function of temperature and oxidative weathering. Their results indicate that for the ND, WY, and IL coals initially the free-radical concentration decreases starting at about 300 K. An increase in the freeradical concentration begins between 520 and 570 K. The onset points for the endotherms in these coals are in the same temperature regime but without a direct correspondence to the ESR onset temperatures. The WV coal ESR data shows a very weak increase starting about 350 K and a stronger increase starting at 580 K. The onset point for WV endotherm is about 520 K. The UF coal ESR shows two temperature regimes of free-radical concentration increase, one between 500 and 700 K the other starting at 700 K. We see one endotherm starting at 570 K. It is rather suggestive to conclude that the endotherm which is seen superimposed on the isotherm in the 520 K plus temperature region reflecta the free-radical production process indicated by the ESR studies. It is also to be noted that it is in this same temperature regime where the pyrite (23) Bakr, M.; Yokono, T.;Sanada, Y. BOC. 1989Int. Conf. Coal Sci., Tokyo 1987,1, 217. (24) Iaaacs, L. L.; Taafantakis, E. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1987,32, 243.
Energy & Fuels 1992,6 , 249-254 decomposition reaction will start. However, since the authors23of the ESR work do not report the apparent g factors for the free radicals, it is highly speculative to consider that the increase in free-radical concentration is due to the formation of sulfur radicals. Conclusions In the temperature regime of 100-700 K the magnitude of the heat absorption of the inorganic/mineral component for coals is an order of magnitude smaller then the heat absorption of the organic component. The prepyrolysis thermal response of coals is dominated by three major events: (a) The endothermic desorption and volatilization of water in the 300-400 K region.
249
(b) A combination of exothermic events between 400 and 680 K due to hydrocarbon formation, Co2formation, and an internal reaction possibly free-radical polymerization. (c) An endotherm between 500 and 700 K due to formation of free radicals and/or to hydrocarbon volatilization. In addition there is a small endothermic event between 300 and 400 K. The exact cause of this endotherm is unclear. Acknowledgment. We thank K. S. Vorres for supplying the APC coal samples. R.L.was the recipient of the W. R.Grace Fellowship. Incomplete work on the coals has been presented previously in 1987 (ref 24), and work on the inorganic coal fractions was presented at the Pacifichem 89 conference.
A Novel Liquid Fluidized Bed Microreactor for Coal Liquefaction Studies. 2. Hydrotreating Results P.D.Jacobs, T.Zhang, R.R.Borgialli, and H. W. Haynes, Jr.* Chemical Engineering Department, University of Wyoming, P.O. Box 3295, University Station, Laramie, Wyoming 82071 Received July 30, 1991. Revised Manuscript Received February 24, 1992
The novel liquid fluidized bed reactor subject of earlier cold model studies has now been utilized as a laboratory hydrotreater. In the present contribution we demonstrate how reliable hydrotreating kinetics can be obtained despite a small 5-g catalyst charge and the presence of a large exothermic heat effect. The reactor may be treated as perfectly mixed. With our benchmark commercial catalyst (Amocat 1A) no evidence of catalyst attrition has appeared after 30 days of continuous operation.
Introduction The evaluation of hydrotreating kinetics at the laboratory scale is a difficult task. Normally the experimentation is conducted in trickle bed reactors in which liquid feed and hydrogen are passed over a fixed bed of catalyst in cocurrent downflow. However, laboratory-scale trickle bed reactors suffer from poor contacting efficiencies due to the low liquid mass velocities characteristic of such systems. Koros, for example, has shown that full catalyst utilization can be expected only if the superficial liquid mass velocity is above about 3 kg/m2.s.' It is not difficult to achieve this value in a commercial unit, but laboratory reactors operate a t substantially lower values. (At a given space velocity, the liquid mass velocity is proportional to reactor height.) Some improvement in catalyst utilization has been realized at the bench scale ( 1 1 m height) by diluting the bed with inert however, the success of this approach has not been demonstrated a t the laboratory scale (
