Energy 6 Fuels 1994,8, 984-989
984
Catalysis of Char Oxidation by Calcium Minerals: Effects of Calcium Compound Chemistry on Intrinsic Reactivity of Doped Spherocarb and Zap Chars R. Gopalakrishnan, M. J. Fullwood, and C . H. Bartholomew' BYU Catalysis Laboratory, Department of Chemical Engineering and Advanced Combustion Engineering Research Center, Srigham Young University, Provo, Utah 84602 Received February 24, 1994. Revised Manuscript Received April 5, 1994"
Catalysis by CaO, CaC03, and Cas04 of the oxidation of a well-defined, high-purity synthetic char, Spherocarb, was investigated at low reaction temperatures using thermogravimetric analysis. The results indicate significant catalytic effects-up to a 160-fold increase for CaC03 catalysis, 290-fold increase for CaSO4, and up to 2700 times for CaO. Oxidation rates were likewise measured for fresh, demineralized, and Ca-loaded chars prepared from Beulah-Zap lignite coal in a flat flame burner at 1473 K. The oxidation rates of CaO-catalyzed Spherocarb and Zap char are the same within experimental error, suggesting that the high reactivity of the Zap char is due in large part to catalysis by CaO. It was also found that chlorine added to Ca-loaded char had a negligible effect on its low-temperature reactivity. Introduction
catalyst particles the instantaneous pore structure can be greatly modified due to the migration of catalyst parChar oxidation is a complex heterogeneous process which t i c l e ~ Pore . ~ ~ structure ~ evolution is a critical phenomenon often governs the overall rate of coal combustion. At low in deciding the overall char combustion rate at high reaction temperatures the observed intrinsic char oxidation temperatures.' Therefore, catalysts such as CaO can rate is generally an additive combination of the rates of largely influence char combustion rate and morphology of two processes:l-l6 (1) mineral-catalyzed oxidation of the reacting char, the latter largely governing the highcarbon1-6+10J3-16 and (2) noncatalytic carbon oxidatemperature combustion kinetics. t i ~ n . ~ J l The J ~ relative contribution of the catalyzed The study of mineral effects in char oxidation generally oxidation to the overall rate depends on the chemical involves (1) removal of mineral matter (demineralization), composition, concentration, and dispersion (fraction of (2) calcium reloading into the demineralized sample, and surface exposed) of mineral phases in the char. For (3) measurement of the physical, chemical, and reactivity example, well-dispersed CaO can enhance the oxidation properties of the demineralized and Ca-loaded samples. rate by 3000-foldover the rate of the noncatalytic carbon The first two steps can be carried out either on coal (before oxidation.8 converting coal to char) or on char. Kyotani et al.,13 Structure and activity of catalytic mineral matter can Radovic et al.,%13Levendis et a1.,14and other investigators's dictate the pore structure evolution of char particles during prepared char from coal that had been demineralized and combustion as the reaction progresses at the catalystremineralized by ion exchange with Ca(0H)Z. A drawback carbon interface14 followed by mobility of the catalyst of this approach is that the char prepared from the Caparticle^.^^^ Depending on size and dispersion of the loaded coal is not representative of the char prepared from the original coal, because the effects of the mineral matter Abstract published in Advance ACS Abstracts, May 15,1994. (1) Zhang, Z.; Kyotani, T.; Tomita, A. Energy Fuels 1988,2,679. on the evolution of pore structure and other char properties (2) Zhang, 2.; Kyotani, T.; Tomita, A. Energy Fuels 1989,3,566. during the coal to char conversion process are different. (3) Kyotani, T.; Hayashi, S.; Tomita, A. Energy Fuels 1991,5, 683. The alternate and more valid approach in our view is to (4) Lizzio, A. A.; Radovic, L. R. Znd. Eng. Chem. Res. 1991,30, 1735. (5) McKee, D. W. The chemical reactiuity of carbon-An old subject first prepare a representative char from the original coal with new releuance; in Materially Speaking; Genisio, M., Ed.; Southern followed by demineralization and remineralization with Illinois University: Carbondale, IL, 1987, Vol. 4(4). appropriate Ca compounds. To our knowledge there is (6)Matsukata, M.; Fujikawa, T.; Kikuchi, E.; Morita, Y. Energy Fuels 1988, 2, 750. only one previouslyreported study in the literature, where (7) Waters,B. J.;Squires,R.G.;Laurendeau,N.M. Comb. Sci. Technol. demineralization and Ca loading were conducted after 1988, 62, 187. (8)Bartholomew, C. H.; Gopalakrishnan, R.; Fullwood, M. Prepr. preparing the coal char.16 Ca loading on a demineralized Pap.-Am. Chem. Soc. Diu. Fuel Chem. 1991,34,982. char is difficult since organic functional groups such as (9) Radovic, L. R.; Steczko, K.; Walker, P. L.; Jenkins, R. G. Fuel carboxylic groups are destroyed during coal devolatilizaProcess. Technol. 1985, 10, 311. (10) Radovic, L. R.; Walker, P. L.; Jenkins, R. G. Fuel 1983,62, 209. tion. However, active sites for Ca exchange can be created (11) Radovic, L. R.; Walker, P. L.; Jenkins, R. G. Fuel 1983,62,849. by treating the char with a mild oxidizing agent such as (12) Radovic, L. R.; Jiang, H.; Lizzio, A. A. Energy Fuels 1991,5,68. (13) Radovic, L. R.; Walker, P. L.; Jenkins, R. G. J. Catal. 1983,82, oxygen at low temperature or a dilute nitric acid solution.l6 382. Both inorganic and organic forms of calcium in coal can (14) Levendis,Y.A.;Nam,S. W.;Lowenberg,M.;Flagan,R.C.;Gavalas, be transformed during combustion to various calcium G. R. Energy Fuels 1989,3, 28. (15) Chang, K. K.; Flagan, R. C.; Gavalas, G. R.; Sharma, P. K. Fuel compounds such as hydroxide, carbonate, oxide, sulfate, 1986,65, 75. chloride, etc. in the combustion chamber depending on (16) Marcilio, N. R.; Charcosset, H.; Joly, J. P.; Tournayar, L. Catal. mineral composition, temperature, and gas atmosphere. Today 1990, 7, 229.
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0887-062419412508-0984$04.50/0 0 1994 American Chemical Society
Catalysis of Char Oxidation by Calcium Minerals
For example, hydroxide and carbonate decompose to oxide above 400 and 800 "C,respectively. Sulfate is formed by the reaction of CaO with SO2 and SO3 in the presence of O2above 700 OC.17 Each chemical form of Ca is expected to have a unique catalytic activity for char oxidation due to its unique electronic structure and adsorption properties. Although CaO is known to catalyze char oxidation,'-2*s1s the function of other calcium compoundswhich are formed during the coal combustion is not known. The objective of this study was to investigate the role of Ca compound chemistry and dispersion by (1)studying specific rates of CaO, CaCOs, and CaSOr catalysis of the oxidation of representative synthetic and demineralized coal chars and (2) investigating the effects of preparation and catalyst dispersion on oxidation rate. This paper reports TGA measurements showing the effects of CaO, CaCOs, and Cas04 on low-temperatureoxidation rates of Spherocarb,a well-defined, high-purity synthetic char and of a Beulah Zap lignite char. The coal char was prepared at high temperature in a flat-flame burner, and further studies were made starting from this char. The Zap char was selected for this study because its physical properties have been extensively studied, and it contains significant quantities of calcium.'* Experimental Section Materials. Spherocarb (Analabs Inc. Norwalk,CT) isa highly microporousand well-characterizedsynthetic char. The average particle size and the BET surface area of Spherocarb were found to be 120 pm and 793 m2/g, respectively, which are in close agreement with the values of Waters et al.' of 140 pm and 864 m2/g. A detailed study of charactarization of Spherocarb can be found elsewhere.' Spherocarb was burned off to 50% of ita initial weight in a muffle furnace at 748 K in 10 % oxygen and 90% N2 flow (200 cms/min) for 2 h to drive off the volatiles, improve access to micropores, and prepare the surface for ion exchange. The low-temperature burn had negligible effect on the lowtemperature oxidation reactivity, although the BET surface area increased to 939 m2/g. The Spherocarb after 50% burn-off was used for further experimente, and hereafter the preburned samples are referred to simply as Spherocarb. Beulah-Zap coal was obtained from the Knife River Mine, North Dakota. Char was prepared from the 200 X 230 mesh (63-75 pm) size fraction in a methane/air flame burner with 0% poetflameoxygen accordingto procedures found e l s e ~ h e r e . ~ ~ T h e flame peak temperature and the particle residence time were 1473 K and 130 ms, respectively. Sample Preparation. About 10 g of the char was demineralized using HCl(37 wt % ) followed by HF (49 wt % ) treatment as described by Bartholomew et alS8 Calcium compounds were added to the char using aqueous impregnationor ion-exchangetachniques to investigatethe effecta of these two preparation methods on calcium dispersion. About 10 g of Spherocarb or acid washed Zap char was mixed with 250 mL of aqueous calcium acetate solution at 300 K for 8 h and washed with distilled water and dried in a vacuum oven for overnight. In the ion-exchange method, the pH of the solution was maintained between 8 and 9 using a 0.01 N solution of Ca( 0 H ) ~ a n the d sample was washed thoroughlywith distilled water and dried in a vacuum oven. The extent of calcium loading wae estimated by ashingabout 1mg of a sample in a thermogravimetric analyzer (Perkin Elmer TGA-7) using 10% oxygen (200 cms/ min) at 1023 K for 30 min; for example, CaO loadings were found to be 1.2 and 2.875, respectively, in the aqueous impregnated and ion-exchanged samples. These samples are hereafter (17)G~palaLriShnan,R.;Seehra, M. S. Energy Fuels 1990,4, 226. (18)Cope,R.F.Ph.D. Diaaertation, Brigham Young University, 1994. (19)Hyde, W.D. M. S. Theais, Brigham Young University, USA, 1990.
Energy & Fuekr, Vol. 8, No. 4, 1994 986 designated as 1.2%CaO/Spcb(aq) and 2.8%CaO/Spcb(ion-ex). The calcium aalt was converted in situ to either CaCOs or CaO in a TGA by heating the Ca-loaded Spherocarb at 823 K for 30 min or at 973 K for 1 h, respectively. A 5.0% CaCOJSpcb was prepared by heating the fresh sample of Ca-loaded (e.g., the original sample of 2.8% CaO/Spcb(ion-ex)) at 823 K in N2 for 30 min. Quantitative TGA experiments were run to determine regions in which either CaCOs or CaO are stable (see resulte below). Calcium sulfate was loaded using a colloidal solution of C&04 in acetone/water. About 1 g of Spherocarb was mixed with 15 mL of acetone at 25 OC and about 20 mL of saturated aqueous solution of CaS04(cas04is sparingly soluble in water) was added dropwiee until a colloidal suspension of C&04 was formed. The sample was then slowly evaporated and dried overnight in a vacuum oven. Kinetic Measurements. Reactivity measurements were performed by TGA. About 1 mg of sample was placed in a thin layer in a platinum pan and the system was purged for 45 min in pure nitrogen. With nitrogen flowing the temperature was increased at 40 OC/min to either 823 K or 973 K and held 30 min to decompose the calcium salt to CaCOS or CaO, respectively. The sample was then cooled to the reaction temperature. After ensuring that the reaction cell was isothermal, oxygen was introduced. All runs were performed using 10% oxygen in nitrogen flowingat 200 cms/min. Most of the runa were repeated 2-3 times at each temperature; the reproducibility was excellent and the experimental error was within A2 % For example, note the error bare shown in Figure 1for one set of data reflects the typical uncertainty of our measuremente. The reaction rates were based on available carbon. Sample Characterization. BET surface area, helium (skeletal) density, and porosity were determined for all samples; sampleawere further analyzedusingscanning electron microscopy (SEM) and X-ray diffraction. CaO surface area was measured using a selective COSchemisorption technique as developed by Radovic et al.18 and Linares-Solano et al." About 1 mgof sample was heated in TGA at 973 K for 1 h in nitrogen (180 cms/min) and cooled to 573 K. Chemisorption was followed by flowing 10%COP(200 cms/min) at 573 K for 30 min, purging with N2 for 20 min to remove physicallyadsorbed CO1molecules, and cooling to room temperature. From the amount of C02 chemisorbed the active surface area was calculated from the equation,
.
where SC,O is the surface area of CaO in m2/g, WCO,is the weight percent of Cog chemisorbed, W,is the weight percent of char, and W ~ isOthe weight percent of CaO loading. The average diameter and dispersion of CaO particles were estimated by substituting SGO in the following equations:
D = (1.241I538)Sca0
(3)
where d is the average particle diameter in nanometers and D is the percentage dispersion (percentage exposed). To study the effecte of HC1 on the oxidation of the carbon matrix, about 2 g of unburned Spherocarb was treated with 200 mL of 37 wt % HC1 at 360 K for 4 h followed by washing with water. Chlorine content was estimated following the ASTM standard procedure using the Eschka titration method.21 To study the effects of residual chlorine on the char, chlorine was loaded on char by treating with 200 mL of 0.01 M NH&l solution at 363 K for 2 h, filtered, and dried overnight in avacuum (20)Linares-Solano, A.; hela-Alarcon, M.; Salinaa-Martinez de h a , C. J. Catal. 1990, 125,401. (21)1979 A n n u l Book of ASTM Stondorde; Ammerican Society for Teating and Materiale: Philadelphia, PA, 1979;Part 26,p D2361.
Gopalakrishnan et al.
986 Energy & Fuels, Vol. 8,No. 4, 1994
Table 1. Weight Changes Indicating Stoichiometry of Decomposition of Calcium Acetate by TGA* experimental(TGA) calculated after water correction (wt % ) raw data (wt % ) Ca(Ac)z 100.00 Ca(Ac)z water 5.40 first decomposition 60.17 (724 K) 63.60 Ca(Ac)z CaCOs second decomposition 33.66 (1048 K) 35.58 Ca(Ac)z CaO
--
0
wt%
100.00 63.33 35.48
Values in the parentheses are the temperatures of maximum decomposition rate.
Table 2. Characterization of Unloaded and Ca-Loaded Spherocarb Samples BET helium surface area density sample (m2/g) (g/cmS) porosityb fresh Spherocarb 793 1.93 50%burned-off Spherocarb 939 1.97 0.24 2.8%CaO/Spcb(ion-ex)o 780 1.39 2.7 % CaSO&pcbo 805 1.57 a
Ca loadingswere carried out on the 50%burned-off Spherocarb.
* Porosity = 1 - (particle density/solid density).
,
0 . i 12
0.001 1
oven. The amounh of chlorine in the demineralized char and 1.17% CaO/Zap char were found to be 0.3 and 2.3 % ,respectively.
Results and Discussion Spherocarb and Ca/Spherocarb. A TGA experiment involving complete burning of fresh Ca-exchanged Spherocarb (containing the equivalent of 2.8% CaO) performed in 10% 02/90% N2 by ramping slowly from room temperature to 1173 K shows, based on weight changes, that the Ca-Spcb sample decomposes in two stages with maximum decomposition rates at 724 and 1048 K, respectively. During thermal decomposition under the same conditions pure calcium acetate quantitatively decomposes to CaC03 and CaO, respectively at 724 and 1048 K (Table 11, indicating that calcium in Spherocarb exists as CaC03 at temperatures below 823 K and CaO above this pretreatment temperature. Physical properties of unloaded and Ca-loaded Spherocarb samples are summarized in Table 2. The BET surface areas measured by nitrogen adsorption for fresh and 50% burned-off Spherocarbare 793 and 939 m2/g. These values compare favorablywith values of 864 and 965 m2/greported by Waters et ala7and Dudek et aLZ2 The BET surface area increased by about 18% after burning-off 50 wt % of the fresh Spherocarb (Table 2). Similar results have been reported for Sphero~arb,79~~ i.e., that the surface area increases with increasing carbon conversion up to 10-15 % and then monotonically decreases reaching 650-660 m2/g at 65-80% conversion at temperatures 780-1300 K. The decrease in surface area at high carbon conversion suggests that densification and/or graphitization processesZ3are important at higher oxidation temperatures (above 750 K). However, calcium loading in the form of either CaO or Cas04 decreases the surface area and solid density of the Spherocarb about 14-17 and 20-29%, respectively, indicating that the density of Spherocarb is greater than pure CaO and CaS04. The reactivities of fresh and 50 7% burned-off Spherocarb samples in 10% 0 2 are shown in Figure 1. The rates (2.1 X lo4 and 2.3 X lo4 g/(g-s), respectively, at 723 K) and the activation energies (36.5 and 35.8 kcal/mol, respec(22)Dudek, D. R.;Longwell, J. P.; Sarafim, A. F. Energy Fuels 1989, 3, 24. (23) Hurt, R. H.; Dudek, D. R.; Longwell, 3.P.; Sardim, A. F. Carbon 1988, 26, 433.
0.0613
0.0614
1/T ( K 1 )
Figure 1. Arrheniusplots of oxidation of uncatalyzedSpherocarb samples. TGA rates were measured in 10% oxygen. -2.0-
-
-3.0-
v!
M
3 3 -4.0-
e
M
-
-5.0- o
-6.0 , 0.0010
Unloaded 5.0% CaC 2.7% CaSOdSpcb I
I
I
0.0012
0.0014
0.0016
0.0018
l / T (K-1)
Figure 2. Arrheniumplots of oxidation rates for unloaded and Ca-loaded Spherocarb samples. TGA rates were measured in 10% oxygen.
tively) of both the samples are the same within experimental error indicating that the low-temperature pretreatment to burn-off 50 wt % of the fresh Spherocarb does not significantly affect the oxidation kinetics. The rates and activation energies are in excellent agreement with the results of Hurt et aLZ3measured on Spherocarb using 21% 0 2 . That is, the estimated oxidation rate of Spherocarb at 667 K, corrected to 21 % oxygen (assuming an oxygen order of 0.6) of 2.8 mg/(g.s) and the activation energy of 35.8-36.5 kcal/mol are in very good agreement with the values,2.3 mg/(g.s)and 36.5 kcal/mol,respectively, reported by Hurt et aLZ3 However, these values are somewhat lower than 44 kcal/mol reported by Waters et al.' for Spherocarb and 40-41 kcal/mol reported for oxidation of various synthetic mineral-free carbon^.^^^^^ Rate data plotted in Arrhenius form are shown in Figure 2 for unloaded Spherocarb, 1.2 5% CaO/Spcb(aq),and 2.8 5% CaO/Spcb(ion-ex) after pretreatment at 973 K and for 5.0% CaCO$Spcb(ion-ex) prepared in-situ by pretreating Ca-loaded Spherocarb (ion-ex)at 823 K. Rates calculated at 600 K are given in Table 3. Comparison of mass-based rates indicatesthat the 1.2 % CaO/Spcb(aq) sample is more reactive than the unloaded sample by a factor of about 12, (24) Felder, W.; Madronich,S.;Olson,D. B. Energy Fuels 1988,2,743.
Energy & Fuels, Vol. 8, No.4, 1994 987
Catalysis of Char Oxidation by Calcium Minerals
Table 3. Rater of Oxidation of Calcium-Loadedand Unloaded Spherocarb Sampler at 600 K in 10% Oxygen sample unloaded S p h e w b 5.0%CaCO$Spcb(ion ex) 1.2% CaO/Spcb(aq) 2.8% CaO/Spcb(ion ex) 2.7% CaSOJSpcb
Sc.o(m2/gc.o)
-
134 180
-
-
D(%) -
11
11
9
15
d(=)
-
Table 4. Activation Energiecl and Preexponential Factors of Oxidation of Calcium-Loaded and Unloaded Spherocarb Sampler in 10% Oxygen E. preexp factor sample (kdmol) (B/&nPb.S) 36 9.9 x l@ unloaded Spherocarb 33 3.0 X 108 5.0%CaCOa/Spcb(ionex) 1.2% CaO/Spcb(aq) 29 3.6 X 10' 23 2.0 x 108 2.8% CaO/Spcb(ionex) 31 4.4 x 106 2.7% CaSOJSpcb
while 5.0% CaCOs/Spcb(ion-ex)and 2.8% CaO/Spcb(ionex) are more reactive by factors of 160 and 2700, respectively. This enhancement in rate is consistent with that of up to 2 orders of magnitude reported by Levendis et al.14 for CaO catalysis at low temperatures. The 2.7 % CaSOdSpcb has an intermediate reactivity with a rate enhancement of about 290 times. Thus carbon oxidation is catalyzed by CaCOs, CaO, and the differences in their catalytic performance can be attributed to their different electronic and adsorption properties. The substantially higher reactivity of the ion-exchanged sample (200-fold on a maas basis) is explained in part by the fact that ion-exchange leads to better dispersion and a higher loading of Ca species on the carbon matrix.*J4@ Indeed, dispersions of CaO measured using selective C02 chemisorption technique are 15% for 2.8% CaO/Spcb(ion ex) and 11%for 1.2% CaO/Spcb(aq) (Table 3). Apparently, however, the CaO deposited by ion exchange is intrinsically more active in view of ita 150-fold higher activity on CaO surface area basis (Table 3). This suggeata that the catalysis of carbon oxidation may be structure sensitive. SEM studies of unloaded and CaO-loaded Spherocarb samples were also conducted to determine the distribution of CaO in the samples. The SEM pictures provided strong visual evidence that the aqueous impregnation deposita copious quantities of CaO on the exterior of the carbon spheres, while the ion-exchange technique provides a uniform dispersion and greater penetration of pores, especially into the partially burned-off Spherocarb. This is consistent with the higher loading, better dispersion, and increased surface area of CaO for samples prepared by ion-exchange method than those prepared by an aqueous impregnationtechnique (Table 3). Thus, smaller size crystallitesof CaO are formed during the ion-exchange process leading to better dispersion and higher surface area. Calcium loading by ion exchange probably allows calcium ions to enter into micro- and mesopores of Spherocarb, possibly accounting for ita higher intrinsic reactivity per surface area of CaO. Preexponential factors and activation energies were obtained from the Arrhenius plot (Figure 2) and are presented in Table 4. Activation energies vary between 23 kcal/mol for 2.8% CaO/Spcb(ion ex) and 36 kcaVmol for unloaded Spherocarb. The lower activation energies of calcium loaded samples are consistent with a catalytic effect of Ca species in the low temperature char oxidation process.
-
rate (B/&.mp**) 8.2 X 1 V 1.3 X 1od 1.1 x lW 2.2 x 10.4 2.4 X 1od
rate (dgws)
-
6.5 X 1.3 X 1.1 x 3.0 X
rate (g/m2c,o.s)
-
10.4 10.4 10-2 10-8
8.2 X 10-8 1.2 x 10-8
-
Table 5. Physical Properties of Beulah-Zap Lignite Chars. BET helium surface area density sample (m2/g) (g/cms) porosity Beulah-Zapcoal 0.35 1.43 0.025 Beulah-ZapChar 256 1.91 0.72 demineralized Zap 335 1.74 0.94 265 1.74 1.1% CaO/Zap 283 1.74 1.1% CaO/Zap with 2.3% Clz 322 1.9% CaO/Zap 1.64 0.76 315 1.76 0.91 2.6% CaO/Zap 1.73 197 1.9% CaSOJZap
-
0 Note: 1.1 76 CaO/Zap was made from HCl acid washed char. All otherZap samples were made from char demineralizedwith HCl and
HF.
0
1000
2000
3000
Pore radius
40oO
(A)
5000
6
Figure 3. Pore size distribution of Spherocarb and Zap chars measured using mercury intrusion technique.
Beulah-ZapCoal Char. The BET surface area of the Zap coal (0.35 m2/g)is enormously increased (to 256 m2/g for the char) due todevolatilizationin the flat flame burner. BET surface areas for nitrogen adsorption on Zap char and demineralized Zap char were determined to be 256 and 335 m2/g, respectively (Table 5). The value of 256 m2/g for the Zap char prepared with a residence time of 130me agrees within experimental error (*lo%) with the value of 229 m2/g reported by McDonald et aleafor a residence time of 104 ms. The larger surface area and porosity (Figure 3) for the demineralized char are consistent with removal of mineral matter from micro- or mesopores or pore entrances, since it is the micro- and mesopores that account for most of the surface area of chars.28 Addition of CaO significantly decreases the surface area from 335 to 265 m2/g, which is close to the value for the original char; however, addition of C&04 decreases the surface area from 335 to 197 m2/g,while the density is not affected. The surface areas of 1.1% CaO/ Zap chars with and without chlorine are somewhat higher than for the original Zap char but lower than the demineralizedchar because these were prepared from the HC1 acid washed char and not from the demineralized Char. Comparison of data from Tables 2 and 5 indicates that the density of Spherocarb is significantly higher than for ~~
(25)McDolnald,K.M.;Hyde, W. D.; Hecker, W. C. Fuel 1992,71,319. (26)White, W.E.; Bartholomew, C. H.; Hecker, W. C.; Smith,D. M. Aakorpt. SCLTechnol. 1991,7, 180.
Gopalakrishnan et al.
988 Energy & Fuels, Vol. 8, No. 4, 1994 -1.5 0
withZ.3'kQz withoutaz
,
0.0013
O.Ob14
0.OblS
Oh16
O.Ob17
l/T(K-1)
Figure 4. Effect of doped chlorine on the oxidation rates of 1.1%CaO/Zap chars. Rates were measured in TGA using 10% oxygen.
A
0.0012
Fresh Zapchar
0.0014
0.0016
0.0018
1IT ( K l )
Figure 5. Arrhenius plots of oxidation of fresh, demineralized, and Ca-loaded Zap char samples. TGA rates were measured in 10% oxygen.
the demineralized Zap char, Le., 1.97 versus 1.74 g/cma. The higher density for Spherocarbindicates that it is more graphitic than Zap char. In the process of demineralizing chars using concentrated acids (HC1 and HF), retention of chlorine is a concern, since it might affect the char reactivities. To determine the effects of chlorine on the physical properties and oxidation activity of the chars, the oxidation activity of a sample of 1.1%CaO/Zap char loaded with 2.3% chlorine was compared with the original 1.1% CaO/Zap char (Figure 4). Activation energies (35and 34 kcal/mol, respectively) and rates (1.37 X 10-4and 1.31X 10-4g/(g-s), respectively) of the chars with and without chlorine are the same within experimental error, indicating that the added chlorine has negligible effect on the char reactivity. Hengel and Walker:' however, attributed the lower gasification reactivity of calcium- and magnesium-loaded lignite chars prepared by demineralizing the coal using HC1 and HF rather than ammonium acetate to chlorine retention. Nevertheless, the results of this work indicate that chlorine does not lower char reactivity. Arrhenius plots of rate data are shown in Figure 5 for oxidation of Zap, 1.9, 2.6, and 5.0% CaO/Zap (prepared by ion exchange), 1.9% CaSOJZap, and demineralized Zap. The reactivities of all calcium-loaded Zap samples are similar and are very close to that of the original Zap char, while the reactivity of the demineralized Zap is much lower, suggesting that calcium catalysis accounts for the high reactivity of Zap char during low-temperature oxidation. These results (summarized in Table 6) are qualitatively consistent with data reported by Radovic et a l . l O (27) Hengel, T. D.; Walker, P. L. Fuel 1984, 63,1214.
for Zap, demineralized Zap, and CaO-catalyzed Zap chars prepared at a relatively high residence time (0.3 s) at 1275 K; in that work the demineralization and CaO exchange were done on the coal. Indeed these workers observed similar reactivities for Zap and CaO-catalyzed Zap chars, while that for the demineralized char was significantly lower. It is interesting that 5.0% CaO/Zap char prepared by ion exchange has a factor of 10 higher CaO surface area and dispersion relative to 1.9 and 2.6% CaO/Zap chars (prepared also by ion exchange), while its reactivity on a CaO surface area basis is a factor of 10 lower. Again, this result suggests that the intrinsic reactivity due to the catalytic reaction is structure sensitive. An explanation of this behavior requires at least some speculation, since little definitive information is available regarding (1) the relationship of CaO structure to its specificactivity and/or (2) CaO morphology and/or surface structure as a function of loading on carbon substrates. Only a few recent studies by Linares-Solano et almm0 address this subject. Two principles, emanatingfrom their work, are appropriate to mention here: (1)well-dispersed CaO in contact with carbon is responsible for catalytic activity and (2) the calcium oxide phase of samples ionexchanged at calcium contents lower than or equal to the saturation of the available carboxylic groups of the carbon surface and then heat-treated up to 950 OC is highly dispersed and largely in contact with the carbon, while that prepared a t high Ca contents in similar manner is typically less well-dispersedwith a smaller fraction of CaO in direct contact with the carbon interface. In the case of the CaO-Zap char samples prepared by ion exchange in this study, it is likely that the samples of lower Ca content (1.9 and 2.6%) have a greater fraction of the CaO phase in direct contact with carbon, while in the 5% CaO/Zap char only a small fraction of CaO contacts carbon directly, thus explaining the lower specific activity on a CaO surface area basis of 5 % CaO/Zap. The apparent higher dispersion of the 5% CaO sample relative to the 1.9 and 2.6% samples based on C02 adsorption is unexpected. It is our hypothesis that CaO in the samples of lower CaO content may adsorb less C02 per CaO unit because of its intimate contact with the carbon phase, while the larger and possibly somewhat crystalline CaO phase in the 5% sample adsorbs more COz per CaO units because its lower fractional contact with the carbon phase. It is hoped that future TPD, TEM, and EXAFS studies will shed additional light on this issue. Activation energies for the five Zap samples are summarized in Table 7. The value of 27 kcal/mol for untreated Zap char is the same as the value reported by McDonald et aLZ5for a Zap char prepared under similar conditions in a flat-flame burner. The value of 36 kcal/mol for demineralized char is the same within experimental error as the value determined for pure Spherocarb (Table 4). Again the lower activation energies for Ca/chars and Zap char relative to that for Zap demineralized is consistent with a catalytic effect. (28) Linares-Solano,A.; Salinas-Mateinezde L, C.; Cazorla-Amom, D.; Joly, J. P.; Charcoseet, H. Energy Fuels 1990,4, 467-474. (29) Cazorla-Amom,D.; Linarea-Solano,A.; Salinas-MartinezdeL, C; Yamashita, H.; Kyotani, T.; Tomita, A. Prepr. Pap.-Am. Chem. SOC., Diu.Fuel Chem. 1991,36(3), 998-1006. (30) Cazorla-Amom,D.; Lmam-Solano,A.; S a l i n a s - W i e zde Lecea, C.; Nomura, M.;Yamashita, H.; Tomita, A. Energy Fuels 1993,7,626631.
Catalysis of Char Oxidation by Calcium Minerals
Energy & Fuels, Vol. 8, No. 4,1994 989
Table 6. Rater of Oxidation of Demineralized and Calcium-Loaded Beulah-Zap Char Sampler at 600 K in 10% Oxygen Sample S m (mVg CaO) d (nm) I3 (%I rate (g/&.mp*.s) rate (B/gwa) rate (g/m2ho.s) 1.8 x 10-8 4.3 x 10-2 Zap demin. char 9.0 x 3.3 x 10-8 Zap char 1.7 4.5 x lo-' 2.4 x 10-0 2.1 x 1od 1.9% CaO/Zap char 21 73 1.6 3.0 X '-ol 1.2 x 10-2 1.7 X 1od 2.6% CaO/Zap char 18 85 20.5 6.0X lo-' 1.2 x 10-2 2.3 X 10-8 5.0% CaO/Zap char 254 6 3.4 x lo-' 1.9% CaSOJZap char
1v
Table 7. Activation Energies and Preerponential Facton of Oxidation of Deminerplieed and Calcium-Loaded Beulah-Zap Char Sampler in 10% Oxygen sample E. (kcaVmo1) preexp factor @/&.mows) Zap demin char 36 3.3 x 108 27 1.4 X 106 Zap char 1.9% CaO/Zap char 28 6.9 X 106 2.6% CaO/Zap char 34 9.9 x 108 5.0% CaOlZap char 28 1.1 x 10' 1.9% CaSOJZap char 32 1.6 X 108
"."0.0011 ,
0.413
O.obl5
Oh17
Oh19
IlT ( K l )
Figure 6. Arrheniw plots of oxidation for catalyzed and uncatalyzedSpherocarb and for Zap, demineralized Zap and Ca/ Zap chars in 10%oxygen.
Figure 6 compares rate data in Arrhenius form for oxidation of catalyzed and uncatdyzed Spherocarb with those for Zap, demineralized, and Ca-loaded Zap chars. The data for Zap, 2.6% CaO/Zap, and 2.8% CaO/ Spherocarb fall nearly along the same line, while rates for demineralized Zap are lower than for Zap, but higher than those for uncatalyzed Spherocarb. To compare the oxidation activity of Spherocarbwith that of demineralized Zap char, a sample of unloaded Spherocarb was prepared by treating it with HCl(37 wt %) at 323 K for 2 h, washed with water, and dried overnight (note that the Zap char was demineralized using HCl), and its activity and BET surface area were measured. Hydrochloric acid treatment was found to have negligible effect on the BET surface area and the oxidation rates of the Spherocarb. The oxidation activity of the demineralized Zap char is about
22-fold greater than the unloaded Spherocarb,which could be attributed to the presence of residual minerals in Zap char after demineralization and/or a difference in the nature of the carbon matrix. In fact, the solid density of the unloaded Spherocarb is significantly higher (1.97 g/cm3) than that of the demineralized Zap char (1.74 g/cm3),indicating that Spherocarb is more graphitic; also, SEM micrographs show that Spherocarb consists of ordered spherical particles while demineralized Zap char particles have rough and disordered surfaces due to devolatilization and demineralization treatments.
Conclusions 1. The intrinsic reactivity of Spherocarb, a relatively mineral-free synthetic char, is not affected by 50 wt % oxidation at 748 K in 10% oxygen. In other words, there is no evidence of further graphitization or densification under these conditions. However, there is a significant increase in BET surface area (18%) as aresult of this mild oxidation treatment. 2. Kinetic parameters obtained in this study indicate a significant catalytic effect of CaO involving at least a 2800-fold increase in oxidation rate of Spherocarb compared to a 100-fold increase in rate for CaC03 and 300fold increase in rate for CaSO,. 3. The ion-exchange method resultsin better dispersion and penetration of calcium into Spherocarb than the impregnationmethod in agreementwith previous studies. 4. The higher reactivity of Zap char relative to demineralized Zap char is due in large part to the presence of calcium minerals in the form of CaO. 5. The higher reactivity of demineralized Zap char relative to Spherocarb is due in large part to the more graphitic nature of the Spherocarb. 6. Chlorine has negligible effect on char reactivity.
Acknowledgment. The authors gratefully acknowledge financial support from the Advanced Combustion Engineering Research Center which is supported by the National Science Foundation, 28 companies and national laboratories, Brigham Young University, and the University of Utah.
