303
Ind. Eng. Chem. Res. 1991, 30,303-312 is interesting because Milne (1976) assumed the desorption of A1C13 from the surface of A1203in some conditions to be the rate-determining step of the overall process. Comparing the data and values of 12, in the case of Cl2/C0/ (NJ, the desorption rate of the reaction products alone was larger than that of the overall process; thus, it could not be the rate-limiting step. At the same time, at the maximum overall rate (at 550 "C, pkocl,= 0.8-LO), both processes are of the similar extent; Le., due to the strong acceleration of the chlorination reactions and mass-transfer processes, the desorption rate of the products could be the limiting step. The physical and mathematical model, kinetic data, and conclusions are to be used for the description and analysis of A1,03 chlorination in fluidized bed chlorinators.
Greek Symbols a = Peocl,/Pk!ocl,
Nomenclature
Subscripts CO = carbon monoxide Cl, = chlorine COClz = phosgene i = inert (N,) s = solid Registry No. CO, 630-08-0;AlZO3,1344-28-1;N P ,7727-37-9; AlCl,, 7446-70-0; phosgene, 75-44-5.
Symbols a = slope of linear isotherms A i = kiPcl,/(ki + P c I ~ ~ c Imol/(m2 ,), S) A2 = k2PCOCI / @ 2 + Pcoc12~coc1J~ mol/(m2 A:) = k,P,o/h3 + P C O ~ C O mol/(m2 ), s) bl = defined by eq 13, mol/(m2 s) 6 , = defined bv ea 14. mol/(m2 s) bj = defined by eq 16, mol)(m2 s) b4 = defined by eq 17, mol/(m2 s) k , = rate constant for chemisorption of chlorine, mol/(m2 s) k 2 = rate constant for chemisorption of phosgene, mol/(m2 -\
SI
k 3 = rate constant for binding of CO, mol/(m2 s) k 4 = overall rate constant for transformation of activated
phosgene-type complex into adsorbed A1Cl3and COPon the solid surface and their desorption, mol/(m2 s) K = equilibrium constant of the C1, + CO + COCl, reaction p = dimensionless partial pressure of gases rm = weight-loss rate of A1,03 related to a unit of mass, mol/(g s)
.
S = specific surface area, m2/g t = temperature, "C T = temperature, K u = volumetric flow rate of gas, m3/s
B = mass-transfer coefficient,. mol/(m2 s). , . Y1 = PEo/Pljdo Yz = Pel,/Pcl,, = PEocl,/PcfPel, 6 = Av/(vi uco. u$
73
+
+
coefficient in eq 10 exponent in eq 10 = coefficient of K in the case of COClz inlet & J ~= coefficient of K in the case of C1, + CO inlet
w1 = w2 =
Superscripts f = on the surface of aluminum oxide g = established in gas phase k = inlet
Literature Cited Landsberg, A. Chlorination Kinetics of Aluminum Bearing Materials. Metall. Trans. 1975,6B, 207-214. Milne, D. J. The Chlorination of Alumina and Bauxite with Chlorine and Carbon Monoxide. Proc. Australas. Znst. Min. Metall. 1976, 260, 23-31. Muller, H. P.; Baiker, A.; Richarz, W. Thermogravimetrische Untersuchung der reduzierenden Chlorierung von Tonerde. Helv. Chim. Acta 1979, 62, 76-85. SzabB, I.; Blickle, T.; Ujhidy, A.; JelinkB, R. Kinetics of Aluminum Oxide Chlorination I. The Mechanism and Mathematical Model. Ind. Eng. Chem. Res. 1991, preceding paper in this issue. Treadwell, W. D.; Terebesi, L. Zur Kenntnis der Chlorierung von Aluminiumoxyd mit Chlor und Kohlenoxyd. Helu. Chim. Acta 1932, 15, 1353-1362.
Received for review January 30, 1990 Accepted July 26, 1990
Hydrogen from Low-Rank Coals: Char Properties and Reactivity of Gasification Feedstocks Ronald C. Timpe,* Warrack G. Wilson, and Rodney E. Sears? Energy and Environmental Research Center, Box 8213, University Station, Grand Forks, North Dakota 58202
A comprehensive bench-scale study to determine and compare char reactivities and properties of three lignites, a subbituminous coal, and two bituminous coals in uncatalyzed, alkali-catalyzed, and demineralized forms was carried out. T h e kinetics of char-steam reactions were studied in a thermogravimetric analyzer (TGA) at temperatures ranging from 650 to 800 "C and ambient pressure. Gas concentrations of H2, CO, C 0 2 , and CH, were as predicted by a thermodynamic equilibrium model. The low-rank coals were demonstrated to be more reactive t h a n the two bituminous coals tested. The catalytic effect of laboratory-grade K2C03and ferrous ammonium sulfate (FAS) a n d t h e natural minerals trona, limestone, and taconite on t h e reactivities of t h e chars were studied. Trona, limestone, and taconite improved gasification rates of the chars by 2-10 times over the rates of t h e uncatalyzed reactions. Reactivities were shown to correlate reasonably well with measured active sites.
Introduction Hydrogen is the largest volume industrial chemical currently in use (Sinor, 1988). It is a key component in +Currentaddress: Engineering Research, Southern Company Services, Birmingham, AL 35202.
petroleum refining, petrochemical processing, fertilizer production, metallurgical refining, and the infant synfuels industry. Over the next 45 years the demand for H2in these industries alone is projected to increase by at least a factor of 27 (Pohani, 1984). Presently, most hydrogen is generated by steam reforming of natural gas or by partial
0888-5885/91/2630-0303$02.50/0 0 1991 American Chemical Society
304 Ind. Eng. Chem. Res., Vol. 30, No. 2, 1991
oxidation of petroleum residue. As the domestic gas supply is consumed and petroleum prices rise, coal gasification will become a more competitive source of hydrogen. Large coal gasification plants could ensure regional supplies of the gas for decades. Research at the Energy and Environmental Research Center (EERC), University of North Dakota, has used bench-scale to pilot-scale experiments and a marketing assessment and process design and cost estimate to determine the feasibility of producing low-cost hydrogen by gasification of low-rank coals (LRCs). The technical study was based on two favorable premises: (1) that the reactivity of low-rank coal (LRC) chars is orders of magnitude higher than that of chars from higher rank coals (Jenkins et al., 1973; Galegher et al., 1986) and ( 2 ) that by virtue of their inherently low mining costs, LRCs will be a cheaper source of energy than high-rank coals. The greater reactivity of low-rank coal chars is caused by higher concentrations of active sites, higher porosity, and a more uniform dispersion of alkali impurities that act as inherent catalysts (Soledade et al., 1978). In order to assess the potential of producing hydrogen efficiently from LRCs, the technical feasibility based on reaction kinetics must first be demonstrated. This paper reports the results of the laboratory-scale study of the gasification of coal when catalyzed by naturally occurring minerals. The results of the remainder of the project will be reported elsewhere. Important considerations in the design of a process for maximizing hydrogen production from coal are to select operating conditions that thermodynamically favor the production of hydrogen and to obtain reaction rates that result in sufficient gasifier throughput to be economically viable. Equilibrium thermodynamics predicts that the maximum single-pass hydrogen production from the steam gasification of carbon at atmospheric pressure, considering only the gasification reaction, (11, and the water-gas-shift reaction, (21, will occur at temperatures between 700 and C(S)+ HzO(g)
CO(g) + HzO(g)
-
CO(g) + H2(g)
++
CO,(g)
+ Hz(g)
(1) (2)
800 "C (Saha et al., 1984; Timpe et al., 1985; Galegher et al., 1986). Under these conditions the rate-limiting step is the carbon-steam reaction, i.e., the reaction between the devolatilized char and steam. Additional hydrogen will be produced during the coal devolatilization and will be dependent on the amount of the volatiles (tars, oils, and hydrocarbon gases) that are produced from a given coal and cracked, ultimately to hydrogen and CO (Feldmann, 1975). Since, in terms of global kinetics, these cracking reactions are far more rapid than steam-carbon reactions, the latter is rate-limiting and is the focus of gasification studies at EERC. The rate of gasification, reaction 1, is enhanced by the presence of catalysts of which the most widely studied are alkali-metal salts (Tomita et al., 1983). The alkali-metal carbonates, bicarbonates, oxides, and hydroxides are well-known as gasification rate enhancers, while the halides have shown little catalytic ability (Tomita et al., 1983). Of these, potassium carbonate (K2C03)has been considered one of the best and is typically used as a standard for comparison of catalytic effects (Tomita et al., 1983). However, the high costs associated with potassium compounds and the difficulty in recovering economically acceptable amounts of potassium from alkaline ashes have led to the search for less costly sodium minerals with catalytic activity (Veraa and Bell, 1978; Huttinger and Minges, 1986a,b; Sears et al., 1986). Trona was shown, in laboratory-scale experiments at EERC, to be as good or
better than pure potassium carbonate on a weight percent basis for catalyzing the char-steam reaction (Sears et al., 1986). Although the use of less expensive, naturally occurring materials with catalytic activity offers an additional economic advantage for producing hydrogen from LRCs, the catalytic response of various LRCs to these catalyst materials differs (Galegher et al., 1986). Therefore, thorough characterization of different coal chars generated a t the various temperatures of interest, and of their response to various catalysts, is important in understanding the inherent differences in the amenability of chars to catalysis and the underlying reaction mechanisms. Experimental Section Feedstock Selection a n d Treatment. Low-rank coals (LRCs) screened in this study included Velva and Indian Head (Zap), North Dakota, lignites; Martin Lake, Texas, lignite; and Wyodak, Wyoming, subbituminous coal. Asmined (unwashed) Illinois No. 6 and Indiana bituminous coals were also tested for comparison. Proximate, ultimate, and ash analyses, given in moles per 10 kg of coal to show that the relative atomic abundance of the elements can be readily seen, are presented in Table I. The coals selected for the bench-scale gasification tests were taken from large lots stored in bunkers for use in other pilot-scale programs. Each lot was sampled and ground to < 1 5 0 - ~ mparticle size. The laboratory test samples were stored in sealed scintillation vials until ready for use. The coals were not intentionally dried prior to use but did lose some moisture during preparation. Samples of four of the test coals were demineralized so that a series of gasification tests could be performed on the coal substance after it had been stripped of most of its mineral content. The coals were demineralized according to the method adapted a t EERC by Holm and Knudson from Schafer's work (Holm and Knudson, 1985). The method involves reacting the mineral material first with concentrated HCl followed by a wash with concentrated HF. This method of cleaning the coal is more rigorous than other techniques-and may possible alter some organic properties-although, for the purposes of this study, it was assumed that it caused only changes in inherent mineral content. Catalysts were selected from those known to enhance the rates of gasification reactions (Walker et al., 1968; McKee, 1974). They included naturally occurring minerals, trona (3Na20.4CO3-5H2O),and limestone (impure CaC03),pure alkali-metal carbonates (Na,CO,, K,C03), and iron in the form of ferrous ammonium sulfate (FeS0,~(NH4)~S04.6H20) and the mineral taconite (Fe304in ferruginous chert). Dry catalysts were ground to a fine powder with a mortar and pestle and then were mixed, usually at 10 wt %, with the coal. Reactivity Experiment. Studies on reaction kinetics were carried out using the thermogravimetric analysis (TGA) equipment described previously (Timpe et al., 1985, 1989a,b;Galegher et al., 1986; Sears et al., 1986). Reactions were carried out at temperatures in the range 650-800 "C in an argon-steam atmosphere with an excess of steam. The reaction sequence consisted of pretreatment of the raw coal sample and devolatilization in the TGA apparatus under argon in the absence of steam, followed immediately by the charsteam reaction. The data generated were used to determine carbon conversion. The small TGA instrument used was a DuPont Model 951 module interfaced to a DuPont 1090 thermoanalyzer and data processor. The instrument has a 100-mg capacity and a maximum heat-up rate of 100 "C/min. The com-
Ind. Eng. Chem. Res., Vol. 30, No. 2, 1991 305 Table I. Proximate, Ultimate, and Ash Analyses for Test Coals I11 No. 6" proximate analysis moisture (AR) volatile matter (mf) fixed carbon (mf) ash (mf) ultimate analysis (maf) hydrogen carbon nitrogen sulfur oxygen (diff) ash analyses by XRFA, mol of element/lO kg of coal aluminum silicon sodium potassium calcium magnesium iron titanium sulfur ratio catalysts/inhibitorsb a
Unwashed, as-mined coal sample.
Ind
Wyodak
Zap
M Lake
Velva
5.8 29.2 35.0 34.7
8.7 37.3 51.0 11.7
31.0 42.7 49.3 8.1
30.6 40.4 42.5 17.1
33.2 44.4 44.7 10.8
29.3 39.3 47.2 13.5
4.2 72.8 0.6 8.4 13.9
5.6 78.7 1.6 2.8 11.3
5.2 72.4 1.0 0.5 20.8
5.1 68.8 0.9 1.4 23.8
5.4 72.2 1.2 1.4 19.8
4.4 69.2 1.1 0.6 24.6
10.8 26.8
5.9 9.8
4.1 5.8
ND
ND
1.5 2.3 1.2 9.4 0.4 1.6 0.13
0.9 0.7 0.6 2.3 0.2 0.3 0.14
3.0 3.4 1.1 0.0 8.3 2.9 0.9 0.1 1.4 1.92
2.4 3.9 0.3 0.1 3.4 1.2 0.7 0.1 1.0 0.83
3.3 7.3 0.4 0.4 1.9 0.9 1.1 0.1 1.4 0.34
0.5 0.1 3.2 1.3 0.7 0.2 2.2 0.51
Alkali metals/(silica + alumina).
paratively low concentrations of gaseous products in the 0.1:O.g steam:argon resulted in low product gas concentrations and, consequently, reduced gas chromatography (GC) analysis accuracy. The gas analysis was useful in determining relative concentrations of H,,CO, and C 0 2 but was not used to determine conversion yields. The small size of this module enabled enclosing the small TGA instrument in a disposable glovebag before removing selected samples of prepared char from the inert (argon) atmosphere. The char was transferred to a sealed vial inside the glovebag and was made available for instrumental analysis without exposure to air. The sample-receiving chamber of the X-ray photoelectron instrument (ESCA) was enclosed in a disposable glovebag, and the sample was loaded into the instrument under flowing argon. The solid-state I3C nuclear magnetic resonance (NMR) spectrometer rotors were filled and sealed in a similar bag under flowing argon. A nominal 10-g capacity TGA instrument built at EERC using a Cahn 1000 electrobalance and Micricon 823 temperature controller and interfaced to a microcomputer, was used to carry out larger scale char-steam reactions. The larger quantity of product gas produced and collected reduced the uncertainty in the gas analyses. Char Characterization. Replicate coal char samples were characterized by tests carried out on one of the two thermogravimetric analyzers (TGA) and by other instrumental analyses. Determination run on TGA instruments included reactivity, proximate analysis, and active-site measurement. Other instruments used in characterizing the chars included those for scanning electron microscopy/energy-dispersive spectroscopy (SEM/EDS), electron spectroscopy for chemical analysis (ESCA), solid-state I3C nuclear magnetic resonance spectroscopy (NMR), X-ray powder diffraction (XRD), and X-ray fluorescence analysis (XRFA). Active-Site Determination. A gravimetric method was used to determine the number of C 0 2 active sites on the catalyzed and uncatalyzed LRC chars. The method was similar to a method proposed by Ratcliffe and Vaughn (1985). In this procedure, coal, suspended in the TGA sample chamber, was flushed with argon prior to heating to the temperature at which the char would ultimately be gasified. The sample was then cooled under argon to 300 "C and reweighed. Carbon dioxide was then introduced
and allowed to flow until CO, uptake by the sample became negligible as indicated by achieving a constant weight. The COz flow was then discontinued and argon flow was resumed until constant weight was again achieved. The weight of chemisorbed C 0 2 was taken as a measure of the active sites. A method of reactive-site determination using 13C NMR suggested by Mins and Pabst was also carried out on Velva chars containing varying amounts of added potassium (Mims and Pabst, 1981, 1983). Devolatilization Product Analysis. A knowledge of the types of liquid compounds produced by devolatization is valuable in predicting their reactivity under steam gasification conditions and in determining whether there are catalyst-specific effects. Pyrolysis GC/MS was carried out on Velva lignite without added catalyst and on Velva lignite containing 10 wt % K2C03,10 wt % trona, or 10 wt % CaC03 to determine the effect of the catalysts on the composition of pyrolysate from a lignite. The pyrolysate from 2-4 mg of sample was cryogenically trapped on the head of a fused silica capillary chromatographic column and then desorbed for analysis by GC/MS (Miller et al., 1988).
Results and Discussion Char Reactivity. TGA measured the weight loss of coal char as it reacted isothermally with steam in argon at a temperature between 650 and 800 "C at ambient pressure. The reactivity parameter (k) value was calculated according to pseudo-first-order kinetic theory previously reported, Table I1 (Galegher et al., 1986). For the more reactive chars, k was calculated from the reaction time required to reach 50% conversion using the relationship K = (In 0.5)/t, with t expressed in hours. For chars with low reactivity, Le., those not reaching 50% conversion within the allotted test period, k was calculated either by extrapolating to 50% conversion, as illustrated in Figure 1, or in the case of demineralized chars by computing k at 10% conversion. The k values calculated for different conversions in a given test will differ to the extent that the reaction kinetics deviate from the assumed first-order rate expression, causing comparisons a t similar conversion levels to be most meaningful. Total product gas samples collected during most tests were analyzed by gas chromatography (GC). The raw data
306 Ind. Eng. Chem. Res., Vol. 30, No. 2, 1991 Table 11. Reactivity Parameters for Steam Gasification of Char as a Function of Temperature and Catalysts Calculated for 50% Carbon Conversion and 10% for Demineralized Chars k , h-1 coal rank temp, "C demin uncat KXO, trona CaCOl Velva lig 650 0.30 1.35 1.92 0.48 Velva lig 700 1.06" 1.35 4.06 6.05 0.81 Velva lig 750 1.40" 2.10 8.17 16.46 6.49 Velva lig 800 1.05" 3.56 34.80 15.50 Zap lig 700 0.09" 0.70 2.40b 0.49 Zap lig 750 0.12" 1.40 2.74b 1.98 0.14" 2.50 2.95b 4.82 Zap lig 800 M Lake lig 650 0.36 0.45 1.05 M Lake lig 700 0.70 1.41 2.37 M Lake lig 750 1.62 3.55 3.50 Wyodak sub 650 0.37 1.25 Wyodak sub 700 0.07" 1.16 4.30 Wyodak sub 750 0.15" 1.31 8.24 Wyodak sub 800 0.38" 3.06 13.48 I11 No. 6 bit 700 0.06" 0.07b 5.00b I11 No. 6 bit 750 0.32" 0.08b 5.81b I11 No. 6 bit 800 2.49" O.llb 8.41b Ind bit 700 0.09b Ind bit 750 0.1l b Ind bit 800 0.24b
" Demineralization removed rank effects and slowed reaction rate to the extent that the k values reported are for 10% conversion only. extrapolated to 50% conversion because of low reaction rates. 100
1w
1
90
h
80
Actual
8
3
70
c .-m .-C a
60
E
50
OC
40
e
30
s
s
k
+
Extrapolated
20
+
10
0
0
20
40
60
500
80
Time, Minutes
Figure I. Wt 70char conversion vs time for Illinois No. 6 bituminous coal.
were normalized to exclude carrier gas and air and found to compare favorably with those predicted by equilibrium thermodynamics, Figure 2. The mol % CO generally increased with temperature, while hydrogen reached near-constant concentration at 680 "C. Theoretically, the "water-gas shift" reaction, being exothermic, partially explains this. At lower temperatures greater shift occurs, resulting in more CO production. However, there should also be an increase in hydrogen, which is not predicted by the model but was observed in practice. Only traces of hydrocarbon gases were detected, sinc 9rior to the start of the steam gasification tests, the coal had been almost completely devolatilized, thus removing hydrocarbon liquids and pyrolysates before gas collection. The reaction occurred at atmospheric pressure, discouraging hydrocarbon formation. Effect of Coal Rank. The reactivities of LRC chars were shown to be much greater than those of higher rank chars as seen in Figure 3, which shows typical conversions with time at 750 "C for the coals studied. Table I1 gives averaged reactivities for replicated tests at temperatures from 650 to 800 "C in the following order: Velva > Martin Lake > Wyodak > Indian Head (Zap) > Indian > River King, illustrative of a strong rank dependence. In order to assess these differences, data were obtained on surface structure by ESCA and SEM/EDS, on active sites by TGA
, ,w 540
580
820
660
Temperature,'C Figure 2. Equilibrium gas composition-char-steam 110
100
P
'E ._
80
m
a
: : : .
... . .
Indiana
70
E @ J
f 0
reaction.
,
-:
90
780
740
700
\ian
50
Head
40
30
p
20
k
e
10 J
0
20
40
60
Time, Minutes
Figure 3. Wt TGcarbon conversion vs time for two bituminous, one subbituminous, and three lignite coal chars.
and NMR, and on the inorganic constituents of the coal, each of which contributes to elucidate how the coals and their resulting chars differ. The results for gasification at 750 "C, presented in Figure 3, are a well-ordered set of data that can be used to illustrate some important differences between the LRC chars and bituminous chars used on statistical regression
Ind. Eng. Chem. Res., Vol. 30, No. 2, 1991 307 Table 111. Statistical Analysis of Reaction Order and Reactivity Data from Figure 3,750 "C: First-Order Statisticsn
coal
rank
Velva M Lake Zap Wyodak I11 No. 6 Ind
Ifg
lig lig sub bit bit
aPPb reaction orders 0.8 1.0 0.5 0.9 7.5 7.5
10
-
9
8 7
intercept 1.081
1.031 1.011 1.021 0.956 0.986
k,' 3.79 1.92 0.83 1.32 0.25 0.10
rZd 0.995 0.994 0.996 0.994 0.891 0.986
ORegression for data on In (1 - X ) vs time. *Order based on best fit. Rate constant. Correlation coefficient.
6 5 4
3-
2i 1
+
0 0
performed on the data, Table 111. The best-fit reaction order for the LRC chars falls between 0.5 and 1.0, and the statistical correlations for first (1)order are represented by correlation coefficients (r2)greater than 0.99 in each case. The highly reactive LRC chars are thus well represented by the first-order kinetics assumed in this study, and their reactivity k does not change appreciably with increasing conversion. The statistical, derived intercepts greater than 1.0 (less than zero conversion) reflect the evident lag in conversion for the reactive chars at near-zero reaction time as shown in Figure 3, which is probably due to a transient heat-transfer effect at the start of the test. The bituminous coal chars in Table I11 are indicated to have apparent reaction orders greater than 5, meaning that the correlation coefficients (r2)continued to increase with orders above 5. This high apparent order reflects a decrease in reactivity with increasing conversion for bituminous chars, as illustrated by the evident leveling-off of conversion with time after a short period of more rapid reaction, Figure 1. This was also borne out by changes in statistically derived values of k for the first 10-30 min compared to values for 60 min. Correlation coefficients (r2)for first-order regression over 60 min were lower than those for LRCs, but were much improved at the shorter reaction times. As opposed to LRC chars, bituminous chars suffered a dramatic loss in reactivity after relatively short reaction times, probably due to mass-transport limitations caused by swelling and agglomeration. One effect of this is to skew the rate constants, h's, to higher values upon extrapolation from 0 to 50% conversion as seen in Figure 1. Carbon- 13 nuclear magnetic resonance (I3C NMR) spectra of chars prepared at temperatures from 600 to 750 OC showed no signals below 50 ppm, indicating the absence of aliphatic groups in the char due to removal from the raw coal during the charring process. Aromatic CO groups and carboxyl, amide, and ester groups, abundant in the coal, are absent in the chars as well, indicating nearly complete carbonization. In this study, further evidence for correlations between rank, inherent mineral content, and reactivity was seen. Alkali and alkaline-earth metals are known gasification catalysts. Their presence as exchangeable cations in the LRCs contributes greatly to the higher reactivity noted in these coals. In bituminous coal, which has fewer exchange sites, these same metal cations are less intimately associated with the carboin matrix, making them less effective as catalysts. Silicon and aluminum are catalyst inhibitors due to their ability to form stable alkali-metal silicates or aluminosilicates, reducing the effective concentration of alkali metals (Radovic et al., 1984; Huttinger and Minges, 1986a,b; Yuh and Wolf, 1983). The ratio of catalysts to inhibitors, i.e., the ratio of alkali metals to silica plus alumina from Table I, shows that bituminous coals have
04
1.2
0.8
[Si]
1.6
k +
2
2.4
[Nal
Figure 4. Selected cation concentration in ash vs reactivity parameter for five coals.
low values around 0.13. Those of the low-rank coals increase from a low of 0.34 for Zap to a high of 1.92 for Velva. These values predict the observed reactivity, and its is tempting to presume these data may predict gasification reactivity. Analysis for stable alkali-metal silicates/aluminosilicates in the coal chars was accomplished by using X-ray techniques. Although amorphous materials are not detected by XRD, their presence on the char surface was detected by ESCA. Of the chars analyzed by ESCA, catalyst-inhibiting silicates were detected in both Velva and River King chars but not in Wyodak char. However, in contrast to River King char, Velva char had a higher calcium/silicon ratio, thus providing more than enough cations to electrically neutralize the silicate anions and leaving sufficient alkali-metal and alkaline-earth elements for catalysis. Wyodak subbituminous coal apparently does not lose catalytic cations to silicates or aluminosilicates, thereby retaining the intrinsic catalytic activity. From the demineralized coals, only the Velva char showed surface silicates. Correlations of reactivity with sodium and silicon content in the feed coal are shown in Figure 4 for five coals tested at 750 "C. Other elements quantitated by X-ray fluorescence analysis (XRFA) (Table I), including Al, Fe, Ti, Ca, Mg, K, and S, showed no correlation with reactivity. As expected, reactivity increased with sodium and decreased with silicon. Some interaction in the correlation between Na and Si is likely, owing to the effect of silicates in reducing alkali-metal mobility under reaction conditions. The increase in oxygen with decreasing rank also plays an important role in enhancing reactivity. The polar functionalities promote the reaction of char with steam in the presence of mobile cations occupying such sites. In addition, increased carbon-oxygen functionalities decrease the softeningt and agglomeration tendency of the coal upon heating, thereby preserving the high surface areas inherent to high reactivity. The lower levels of oxygen and corresponding lower number of active sites in bituminous coals result in a softening plastic hydrophobic matrix, which tends to agglomerate upon heating. This results in decreased surface areas, and limited access to active sites explains why gasification of bituminous coals is more subject to mass-transfer limitations than the lower rank coals. The effect of removing mineral matter is illustrated by the reactivity parameters for demineralized Velva, Martin Lake, Indian Head, Wyodak, and River King coals as a function of temperature, Table 11. The assumption is made that the basic carbon matrix remains relatively un-
308 Ind. Eng. Chem. Res., Vol. 30, No. 2, 1991 110,
I
(I)
C
m
E
U
C
f
V
z
z
0
10
20
30
40
Time, Minutes
Figure 5. Wt % carbon conversion vs time for the Velva char-steam reaction with various catalysts.
changed and, therefore, the reactivities shown reflect only the effects of demineralization. For all but the Velva lignite, removal of inherent mineral matter essentially eliminated any rank effect, so that the reactivities for Wyodak and Martin Lake were reduced to levels obtained for bituminous coal. In contrast the reactivity for Illinois No. 6 at 750 and 800 "C increased. This suggests that demineralization resulting in removal of inherent known metal catalyst caused a reduction in reactivity of the LRC, whereas, in the case of the bituminous coal, demineralization that removed mainly inhibitors, silica and alumina, increased the reactivity likely accompanied by changes in the carbon matrix. Catalyst Effects. The study of catalytic rate enhancement focused on relatively inexpensive, naturally occurring minerals, taconite, trona, and limestone, with baseline data obtained with K2C03. Since the capital cost of hydrogen production by coal gasification has been shown to be over half the product cost, lowering reaction severity and/or increasing throughput could significantly reduce the cost of hydrogen (Galegher et al., 1986; Michaels and Leonard, 1978). Plots sf conversion versus time for Velva char with the selected catalysts are shown in Figure 5 , and reactivity parameters calculated a t 50% conversion are given in Table 11. All catalysts tested had a positive effect on the rate of reaction, with trona being most effective followed by KzC03,limestone, and taconite. In comparing the reactivities of Martin Lake and Velva lignites in Table 11, the addition of KzCO3 or trona to Martin Lake char increased its reaction rate to approximately the same level as the uncatalyzed Velva lignite. A major difference in the analyses of these lignites is the higher levels of A1 and Si in the Martin Lake lignite (Table I), which contributes to its lower reactivity by combining with the alkali metals. The addition of alkali metal as KzC03or trona compensated somewhat for the high Si and Al, but the resulting Martin Lake reactivities remained well below those for Velva with addition of these catalysts. The reactivities of Wyodak subbituminous coal char were markedly increased by the addition of KzC03,being significantly higher than those for Martin Lake and only slightly below that of Velva, Table 11. This order of reactivity again follows a pattern based on Na and Si. Silicon, which decreases reactivity, is more abundant in Wyodak than in Martin Lake, as shown in Table I. Inherent sodium, which increases reactivity, is less abundant than in Velva. These factors help to explain why the reactivity of Wyodak falls between Velva and Martin Lake lignites in the presence of catalyst.
The reactivities of Indian Head (Zap) lignite responded differently to the addition of K2C03and limestone than those of the other two lignites tested. The addition of K2C03greatly increased the value of Ft a t 700 "C, but caused only moderate increase at the higher temperature, resulting in a nearly flat temperature response for the catalyzed case, Table 11. The apparent energy of activation with K2C03catalysis was E, = 4.30 kcal/mol versus Ea = 26.4 kcal/mol for uncatalyzed. When limestone was added, there was a marked response to increased temperature as indicated by an apparent activation energy of 48.3 kcal/ mol. The Illinois No. 6 bituminous coal, which had very low uncatalyzed reactivity, responded to the addition of K2C03 catalyst with an increase in rate of nearly 2 orders of magnitude for the initial 15% C conversion, but then tended to level off as in the uncatalyzed case, Figure 1. The k values for the bituminous coals in Table I1 were determined by extrapolating the initial rate to 50% conversion and, therefore, are representative of the higher initial reactivities and are an optimistic assessment of a 50 % conversion reactivity parameter. The tremendous increase in reactivity of bituminous chars with catalyst addition is due, in part, to the ability of alkali metals to stop or limit agglomeration with strongly caking coals (Yuh and Wolf, 1984). Char Characterization. The surface properties of chars prepared with different catalysts are distinct. Analyses carried out with the scanning electron microscope (SEM) on Velva char samples containing KzC03,NaZCO3, trona, and CaC03 (limestone) provided photographs, shown in Figure 6, and element maps of the surface of each sample. SEM photographs of the char/Na2C03 show uniform round nodules of uniform spacing, not unlike the char/K2C03, except those on the latter were more "moccasin-shaped" as shown in Figure 6 (Sears et al., 1987). The CaC03 additive did not seem to create the nodules on the char. Surface elemental distribution maps of the char/Na showed the Na to be more intense about the silica deposits. In the trona/Martin Lake char, element maps showed deposits of aluminum and silicon superimposed and Na localized in the same region, indicating the presence of sodium aluminosilicates. Calcium in this sample and in the char/Ca sample was quite uniform over the surface of the particle. SEM photographs also show the surface effect that results from charring the coals of different rank with and without K2C03catalyst, Figure 7 (Sears et al., 1987). The ragged, irregular surface and lack of apparent pores in the uncatalyzed char was in contrast to the rounded, nodular surface on the alkali-catalyzed char. The degree to which the surface of the coal changed on charring with the addition of catalyst differed among the coals. The Velva char surface in Figure 6 was remarkably porous and contained uniform, evenly spaced nodules of approximately 0.05 pm X 0.10 pm in size whereas the nodules on the surface of the catalyzed Wyodak and River King coal chars in Figure 7 were rounded and numerous, but not uniform in either size or distribution. The demineralized Wyodak and River King Illinois No. 6 chars had a good deal of surface relief in the form of a few nodules and concavities, whereas the surface of the demineralized Velva was relatively smooth. The differences in the surface relief appeared to parallel the differences in reactivity and were probably due to changes in available active sites. SEM mapping of the surface for inorganic element distribution showed that the inorganic matter was not distributed uniformly over the surface.
Ind. Eng. Chem. Res., Vol. 30,No. 2, 1991 309
4
Figure 6. Photomicrographs of Velva lignite chars containing (a) no catalyst, (b) K2C03, (c) Na2C03, (d) trona, and
(e) limestone;
(0
photomicrograph of demineralized char.
With the physical appearance of the chars from the SEM photos, the question arises as to the importance of increased surface area during carbonization of the coal. Increasing the surface area could be a major factor in exposing more active sites, resulting in increased reactivity.
BET surface area analyses were carried out at EERC and by a commercial laboratory on a few selected samples. Unfortunately, the inconsistency of the data made it inconclusive as to wheter a surface area effect was observed. Also, BET surface area measurements of chars containing
310 Ind. Eng. Chem. Res., Vol. 30, No. 2, 1991
k-1
t.
r
-
c
Figure 7. Photomicrographs of chars of (a) Wyodak, (b) River King, (c) Wyodak/K2C03*and (d) River King/K2C03 prepared under argon.
aklai catalyst lacked accuracy blocked pores due to solidification of molten catalyst. Reactivity vs Active Sites. Table IV presents selected active site data acquired by COP adsorption along with reactivities for Velva and Martin Lake lignite chars, with and without K2C03catalysis. The correlation coefficients r2 for plots of k vs active sites for samples a t each temperature are also given in Table IV. The active-site numbers correlated well with reactivity as indicated by correlation coefficients (r2)> 0.80. An attempt was made to determine active sites in Velva char by NMR spectroscopy. The proposed reaction mechanism for preparing the '%-enriched samples for determining active sites is reported to be -C-K
+ -C--O-K + 213CH31
-+
-C13CH3
-C013CH3
+ 2KI
The difference between natural abundance 13C and that of the enriched sample is then a measure of the number of potassium active sites. The enhancement factor of enriched over natural-abundance aliphatic 13C showed little difference between char prepared a t 750 "C from Velva lignite containing 2 wt 9% and 15 w t 9% K2CO3, which showed aliphatic 13C enhancement of 2.92 and 2.91, respectively. This results in ca.500 carbon atoms/potassium active site.
Table IV. Active Site and Reactivity for Catalyzed and Uncatalvzed Velva LiPnite Velva Velva Velva
M Lake
none K2C03 trona none
M Lake
K2CO3
M Lake
trona
650 700 7SO 650 700 750 650 700 750 650 700 750 650 700 .750 650 700 750
0.30 1.40 2.44 1.43 3.50 7.04 1.28 3.23 5.8 1 0.36 0.70 1.62 0.45 1.4 1 3.55 1.05 2.37 3.50
206 159 47 43 32 31 68 52 37 272 160 140 113 92 39 97 72 48
0.97 0.83
0.99 0.80 1.oo 1.oo
a Reactivity parameter in g/(h.g). *Correlation coefficient for C/active site vs k.
Effect of Catalyst on Volatiles Composition. The components making up the liquid devolatilization products for Velva coal varied with the catalyst. Common components in the four Velva and Velva/catalyst samples de-
Ind. Eng. Chem. Res., Vol. 30, No. 2, 1991 311 scribed above included benzene, toluene, C2-C,-benzenes, phenol, C1-C3-phenols, catechol, C1-catechols, guaiacol, C,-guaiacol, and cS-c26 aliphatic compounds. These components, however, were not found in the same quantities in all of the samples. The most notable variations occurred in the Velva/K2CO3with its higher phenol and lower guaiacol and C,-guaiacol contents, Velva/K,C03 and Velva/ trona with their lowered catechol and C,-catechol contents, and Velva/CaC03 with its slightly lowered benzene and toluene concentration as a compared with Velva without catalyst. The catalysts appear to be promoting devolatilization reaction mechanisms that differ from one another in the way that oxygen is reacted and distributed among the components in the volatiles. Conclusion The reactivities of LRC chars were much greater than high rank chars for steam gasification between 650 and 800 OC, giving the following order in terms of decreased reactivities: Velva > Martin Lake > Wyodak > Zap > Indiana > River King, illustrative of a strong rank dependence. Regression analyses showed that for LRC chars the first (1) order reaction approximation for rate dependence was excellent, giving correlation coefficients (r2)greater than 0.99 in each case. The highly reactive LRC chars are thus well represented by the first-order kinetics assumed in this study, and their reactivity k does not change appreciably with increasing conversion. Bituminous coal chars, on the other hand, were indicated to have apparent reaction orders greater than 5 , reflecting a decrease in reactivity with increasing conversion. This was illustrated by the leveling off of conversion with time after a short period of more rapid reaction. As opposed to LRC chars, bituminous chars suffered a dramatic loss in reactivity after relatively short reaction times, which when extrapolated to 50% conversion gave optimistic approximations of the bituminous coal char gasification rate constants. Correlations between rank, inherent mineral content, and reactivity were seen. The ratio of inherent catalysts, alkali metals, to inhibitors, silicon and aluminum, showed that bituminous coals had low values around 0.13, while this percentage increased significantly for LRCs from a low of 0.34 for Zap to 1.92 for Velva. These values predicted the observed reactivity and gave a clear indication of the critical role that inherent mineral matter plays in determining reactivity. Therefore, although “clean’! coals free of alkali metal are desirable for combustion processes, the presence of alkali metals is key in promoting rapid steam gasification rates. Removal of mineral matter from selected LRCs essentially eliminated any rank effect, so that the reactivities were reduced to levels obtained for bituminous chars. On the other hand, reactivity for Illinois No. 6 bituminous coal char increased, due to a larger reduction in inhibitoirs. All of the catalysts tested in this study increased the rate of char-steam reaction. Trona, an inexpensive, naturally occurring alkali-metal carbonate mineral and the base-case, laboratory-grade K,CO,, gave the greatest increases in reactivity; limestone at the same concentration by weight (lower cation concentration) produced less of an increase; and the iron-bearing catalysts showed still lower activity. Acknowledgment We acknowledge the U S . Department of Energy for the financial support of this project. Thanks are extended to Leland E. Paulson, Michael Baird, and Madhav Ghate of the Morgantown Energy Technology Center for their
guidance and support during the performance of this work. Registry No. H2,1333-74-0; CO, 630-08-0; C 0 2 ,124-38-9; CHI, 74-82-8; K,CO,, 584-08-7; FeS04-(NH4),S04.6H20,10045-89-3; 3Na20.4C0,.5H20, 15243-87-5.
Literature Cited Feldman, H. F. The Role of Chemical Reaction Engineering in Coal Gasification. In Chemical Reaction Engineering Reviews; Advances in Chemistry Series 148; Hulbert, H. M., Ed.; American Chemical Society: Washington, DC, 1975; Chapter 6. Galegher, S. J.; Timpe, R. C.; Wilson, W. G.; Farnum, S. A. “Kinetics of Catalyzed Steam Gasification of Low-Rank Coals to Produce Hydrogen”. Final Report DOE/Fe/60181-2034; Morgantown Energy Technology Center: Morgantown, WV, June 1986. Holm, P. L.; and Knudson, C. L. “Coal Mineral Matter and Its Effects in Low-Rank Coal Liquefaction, V”. Topical Report DOE/FC/10120; University of North Dakota Energy and Environmental Research Center: Grand Forks, ND, August 1985. Huttinger, K. J.; Minges, R. Influence of the Catalyst Precursor Anion in Catalysis of Wator Vapour Gasification of Carbon by Potassium: 1. Activation of the Catalyst Precursors. Fuel 1986a, 65, 1112-1121. Huttinger, K. J.; Minges, R. Influence of the Catalyst Precursor Anion in Catalysis of Wator Vapour Gasification of Carbon by Potassium: 2. Catalytic Activity as Influenced by Activation and Deactivation Reactions. Fuel 1986b, 65, 1122-1128. Jenkins, R. G.; Nandi, S. P.; Walker, P. L., Jr. Reactivity of HeatTreated Coals in Air a t 500 “C. Fuel 1973,52, 288-293. McKee, D. W. Effect of Metallic Impurities on Gasification of Graphite in Water-Vapor and Hydrogen. Carbon 1974,12,453-464. Michaels, H. J.; Leonard, H. F. Hydrogen Production via K-T Gasification. Chem. Eng. Prog. 1978, 74, (8), 85-91. Mims, C. A,; Pabst, J. K. Chemical Characterization of Active Sites in Alkali Catalyzed Gasification. In Proceedings of the International Conference on Coal Science; Verlag Gluckauf GmbH: Essen, FRG, 1981; pp 730-735. Mims, C. A.; Pabst, J. K. Role of Surface Salt Complexes in Alkali-Catalysed Carbon Gasification. Fuel 1983, 62, 176-179. Pohani, B. P. Economics of Texaco Gasifier for Gasification of Coal to Hydrogen. J . Jpn. Pet. Inst. 1984, 27 (l),1-7. Radovic, L. R.; Walker, P. L., Jr.; Jenkins, R. G. Catalytic Coal Gasification: Use of Calcium versus Potassium. Fuel 1984, 63, 1028-1030. Ratcliffe, C. T.; Vaughn, S. N. Population and Turnover Number of Active Potassium Sites on Bituminous Coals During Gasification. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1985, 30 (l), 304-310. Saha, R. K.; Gupta, B. R.; Sen, P. Production of Hydrogen in an Autothermal Fluidized Gasifier. Int. J. Hydrogen Energy 1984, 9 (6), 483-486. Sears, R. E.; Timpe, R. C.; Galegher, S. J.; Willson, W. G. Catalyzed Steam Gasification of Low-Rank Coals to Produce Hydrogen. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1986, 31 (3), 166-175. Sears, R. E.; Timpe, R. C.; Musich, M. A.; Cisney, S. J. “Production of Hydrogen from Low-Rank Coals”. Annual Technical Report DOE/MC/ 10637-2414; Morgantown Energy Technology Center: Morgantown, WV, 1987; Vol. 1. Sinor, J. E., “Production of Hydrogen from Low-Rank Coals: Industrial Market Assessment of the Hydrogen Produced From Low-Rank Coals”. Internal Report; University of North Dakota Energy and Environmental Research Center: Grand Forks, ND, July 1988. Soledade, L. E. B.; Mahajan, 0. P.; Walker, P. L., Jr. Fuel 1978,57 ( l ) ,56-57. Timpe, R. C.; Farnum, S. A.; Galegher, S. J.; Hendrikson, J. G.; Fegley, M. M. Arrhenius Activation Energies of the Reaction of Low-Rank Coal Chars and Steam. Prepr. Pap.-Am. Chem. SOC., Diu.Fuel Chem. 1985, 30 (4),481-487. Timpe, R. C.; Sears, R. E.; Malterer, T. J. Pine and Willow as Carbon Sources in the Reaction Between Carbon and Steam to Produce Hydrogen Gas. In Energy from Biomass and Wastes XII; Klass, D. L., Ed.; Institute of Gas Technology; Chicago, IL, 1989a; Chapter 39. Timpe, R. C.; Sears, R. E.; Montgomery, G. G. Characterization of a Texas and a North Dakota Lignite Char Used in the Production of Hydrogen. J . Coal Qual. 1989b, 8 (l),27-31. Tomita, A,; Ohtsuka, Y.; Tamai, Y. Low Temperature Gasification of Brown Coals Catalyzed by Nickel. Fuel 1983, 62, 150-154.
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Veraa, M. J.; Bell, A. T. Effect of Alkali Metal Catalysts on Gasification of Coal Char. Fuel 1978,57, 194-200. Walker, P. L., Jr.; Shelef, M.; Anderson, R. A. Catalysis of Carbon Gasification. In Chemistry and Physics of Carbon;Walker, P. L., J r . , Ed.; Marcel Dekker: New York, 1968; Vol. 4, pp 287-380. Yuh, S. J . ; Wolf, E. E. K2C03-Catalyzed Steam Gasification of Supercritical Extracted Char. Fuel 1983, 62, 738-741.
Yuh, S. J.; Wolf, E. E. Kinetic and FT-IRStudies of the SodiumCatalyzed Steam Gasification of Coal Chars. Fuel 1984, 63, 1604-1609.
Received for review January 29, 1990 Revised manuscript received August 14, 1990 Accepted August 28, 1990
Kinetics and Mechanisms in the Ammoxidation of Toluene over a TiO,(B)-Supported Vanadium Oxide Monolayer Catalyst. 1. Selective Reactions Mehri Sanati and Arne Andersson* Department of Chemical Technology, Chemical Center, University of Lund, P.O.Box 124, S-221 00 Lund, Sweden
A kinetic investigation of the ammoxidation of toluene was carried out over a TiO,(B)-supported vanadium oxide catalyst with a loading corresponding t o a theoretical monolayer. T h e partial pressures of reactants toluene, oxygen, and ammonia were varied, and rates were measured for the formations of benzaldehyde and benzonitrile. By analysis of the rate dependencies on partial pressures of reactants, rate expressions completely describing the data were derived. These show that benzaldehyde is formed from two routes in which the. active ensemble accommodates one and two toluene molecules, respectively. T h e latter route is most facile. For the formation of benzonitrile, there are also two routes. In the route that is major at low partial pressures of ammonia, one ammonia molecule is adsorbed. With increase in the partial pressure of ammonia, a second route involving adsorption of two ammonia molecules at the active ensemble becomes increasingly important. Oxidation of compounds having a methyl group in the a-position relative to a double bond or an aromatic ring can be carried out in presence of ammonia, so-called ammoxidation, to produce nitriles (Smiley, 1981). The commercially most important ammoxidation process is the one developed by Sohio for production of acrylonitrile from propylene (Grasselli and Burrington, 1981). However, alkylaromatics can also be converted to nitriles by use of a similar technique; e.g., phthalonitrile, isophthalonitrile, nicotinonitrile, and benzonitrile can be produced from o-xylene, m-xylene, 3-picoline, and toluene, respectively (Sze and Gelbein, 1976). In comparison with acrylonitrile, which is used for fibers and elastomers, the market for aromatic nitriles is limited. Phthalonitrile and isophthalonitrile can be used as intermediates in the manufacture of copper phthalocyanine pigments and agricultural chemicals,respectively (Sze and Gelbein, 1976). Hydrolysis can be used to convert nicotinonitrile into nicotinamide, vitamin B5, and niacin, provitamin (Beschke et al., 1977; Paustian et al., 1981). Of the aromatic nitriles, benzonitrile is the most important. It is used in the synthesis of benzoguanamine, which is a derivative of melamine and is used in protective coatings and molding resins. Other applications of benzonitrile are as a jet-fuel additive and as a drying additive for acrylic fibers (Smiley, 1981). Ti0,-supported vanadium oxide catalysts are active and selective in both oxidation and ammoxidation of alkylaromatic compounds (Bond and Konig, 1982; Wachs et al., 1985; Cavani et al., 1987; Cavalli et al., 1987a; Jonson et al., 1988). The anatase modification is generally considered to be a superior support, in comparison with the rutile phase (van Hengstum et al., 1983; Gasior et al., 1984, 1987). It has, however, been reported that, for the ammoxidation of toluene after activation in a reactant stream, the performances of anatase- and rutile-supported vanadium oxide catalysts were quite similar and so also were the distributions between various types of vanadium species
(Cavani et al., 1988). Recently, TiO,(B), which is isotypic with VO,(B) (Marchand et al., 1980), has been used as a support for vanadium oxide (Papachryssanthou et al., 1987; Sanati and Andersson, 1990a). The active phase-support interaction existing in this system was found to enhance the catalytic performance compared to bulk Vz05when used in toluene ammoxidation (Sanati and Andersson, 1990a). Even though numerous scientific studies of V-Ti-0 catalysts have been reported during the past decade, only a few have dealt with reaction kinetic aspects of hydrocarbon oxidation (Bond and Konig, 1982; Il’inich and Ivanov, 1983; Skrzypek et al., 1984; Raevskaya and Pyatnitskii, 1984). For the ammoxidation of toluene, a kinetic investigation has been reported using a coprecipitated catalyst with TiOz in the anatase form (Cavalli et ,al., 1987b). The rate expressions derived were not valid at low partial pressures of ammonia and oxygen. In a comprehensive kinetic study of the same reaction over bulk Vz05, it was demonstrated that it is necessary to evaluate in detail all dependencies on pressures; otherwise no relevant mechanistic information is obtained (Otamiri and Andersson 1988a,b). No kinetic study has hitherto been reported on hydrocarbon (amm)oxidation over TiO,(B)supported vanadium oxide catalysts. Therefore, the present investigation on the kinetics of the ammoxidation of toluene over such a catalyst was undertaken. A vanadium loading corresponding to a theoretical monolayer was used. Following procedures described by Schmid and Sapunov (1982), a strict mathematical analysis of experimental rate dependencies on reactant pressures was performed, in order to obtain expressions describing the data completely. The corresponding mechanism was then derived. Part 1 of this work concerns the formations of benzaldehyde and benzonitrile, while the kinetics and mechanism of combustion reactions are dealt with in part 2 (Sanati and Andersson, 1990b).
0888-5885/91/2630-0312$02.50/0 0 1991 American Chemical Society
