New Approaches in Coal Chemistry - American Chemical Society

6 A Study of Coal Oxidation by Charged-Particle

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Activation Analysis D. J. SCHLYER and A. P. WOLF Department of Chemistry, Brookhaven National Laboratory, Upton, N Y 11973

It has been recognized for many years that exposure of coal to air can significantly alter the processing characteristics of the coal. This effect is more pronounced in lower rank coals and in fact some brown coals will undergo spontaneous combustion at or near ambient temperatures. Both the initial oxidation and the effect this oxidation has on the processing characteristics of the coal have been an area of interest for many years (1). In the last few years there has been a renewed interest in the oxidation of coal and several new analytical techniques have been applied to this problem (2-6). Several salient features of the process have been revealed including the chemical forms the oxygen takes when bound to the coal. It has been noted that the oxidation is not uniform throughout the particle as evidenced by the formation of a discoloration of the coal near the surface (7). This "oxidation rim" is directly related to the behavior of the coal. The purpose of t h i s study i s to apply the unique technique of Charged P a r t i c l e A c t i v a t i o n A n a l y s i s (CPAA) to d i s t i n g u i s h the c h a r a c t e r i s t i c s of the o x i d a t i o n process from a d i f f e r e n t point of view and define parameters which can be determined by t h i s technique. The parameters to be determined are (1) the k i n e t i c s of the a d s o r p t i o n of oxygen c o n t a i n i n g s p e c i e s , (2) the elemental composition of the oxide l a y e r and how deeply i t extends i n t o the p a r t i c l e , and (3) the number of r e a c t i v e s i t e s on or near the surface. Experimental K i n e t i c Experiments. The technique of Charged P a r t i c l e A c t i v a t i o n A n a l y s i s (CPAA) f o r a n a l y s i s of oxygen i n c o a l has been described p r e v i o u s l y (8,). In the experiments to study the a d s o r p t i o n of oxygen onto the c o a l , a s l i g h t m o d i f i c a t i o n of t h i s procedure has been used. Small lumps of c o a l were t r a n s f e r r e d i n

0097-6156/81/0169-0087$05.00/0 © 1981 American Chemical Society

Blaustein et al.; New Approaches in Coal Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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an oxygen-free atmosphere to a spex mixer and ground so that about 80% of the p a r t i c l e s had diameters from 45 microns (325 mesh) to 74 microns (200 mesh) with the remainder of the p a r t i c l e s having diameters o u t s i d e t h i s range. The samples were then d r i e d at 100°C under vacuum f o r 18 hours to remove moisture. Again a l l handling of the samples was done under an i n e r t atmosphere. The samples were removed from the vacuum oven at room temperature and exposed to the a i r i n f l a t shallow pans f o r a given period of time. A f t e r t h i s exposure the samples were e i t h e r sealed d i r e c t l y i n t o a one m i l (.025 mm) aluminum packet or returned to the vacuum oven, evacuated at 65°C f o r 3 hours and then sealed i n t o the packets. The packets were then i r r a d i a t e d with He-3 at an energy of 10 MeV. The samples were counted and analyzed as described p r e v i o u s l y (8) to determine the oxygen to carbon r a t i o . A propagation of e r r o r s on t h i s technique gives a p r e c i s i o n of about 0.3% a b s o l u t e . In a c t u a l experiments the p r e c i s i o n has been about 1% a b s o l u t e . Oxygen Depth P r o f i l e . In order to measure the oxygen depth p r o f i l e a range of p a r t i c l e s i z e s was needed. The c o a l sample was ground as u s u a l . The p a r t i c l e s were then sorted according to s i z e by u s i n g standard s i e v e s . In most cases t h i s separation procedure was done i n a i r . An exception to t h i s was the procedure used to determine i f the oxygen to carbon r a t i o v a r i e d i n d i f f e r e n t s i z e c o a l p a r t i c l e s before exposure to a i r . In that case the s i e v i n g was done under an i n e r t atmosphere and the samples immediately i r r a d i a t e d . A f t e r s i e v i n g the c o a l p a r t i c l e s were exposed to the a i r f o r 60 days i n a c o n t r o l l e d humidity room ( r e l a t i v e humidity 45%) and then sealed i n t o the aluminum packets f o r i r r a d i a t i o n . The nuclear r e a c t i o n c r o s s - s e c t i o n f o r production of C - l l and f o r F-18 change as a f u n c t i o n of the p a r t i c l e energy. Since the change may be d i f f e r e n t f o r production of C - l l than i t i s f o r F-18, the n u c l e a r r e a c t i o n c r o s s - s e c t i o n r a t i o must be determined as a f u n c t i o n of energy. Standards to determine the nuclear r e a c t i o n c r o s s - s e c t i o n r a t i o f o r the oxygen depth p r o f i l e experiments were "sandwiches" made from v a r y i n g thicknesses of p o l y e s t e r and polyethylene f i l m s layered on top of one another. The standards c o n s i s t e d of a f i l m of (1/4 m i l to 2 m i l ) p o l y e s t e r of known composition followed by a polyethylene (1 m i l to 5 m i l ) f i l m followed by another p o l y e s t e r f i l m which together represent the oxide l a y e r , the c e n t r a l unoxidized c o a l , and another oxide l a y e r . Layers of these "sandwiches" were sealed i n the aluminum packets and i r r a d i a t e d and counted i n a manner i d e n t i c a l to that used f o r the c o a l samples. Comparison of the a c t u a l 0/C r a t i o f o r a g i v e n "sandwich" to the experimentally determined F-18/C-11 gave the c r o s s - s e c t i o n r a t i o as a f u n c t i o n of p a r t i c l e s i z e . Experiments on the s o r p t i o n of oxygen r a d i o t r a c e r s were performed i n the f o l l o w i n g way. A column was prepared by f i l l i n g a 1/4" s t a i n l e s s s t e e l tube one foot long with c o a l of 100-220


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mesh. T h i s column was placed i n a gas chromatograph equipped with both a thermal c o n d u c t i v i t y d e t e c t o r and a r a d i o d e t e c t o r so that both a mass trace and an a c t i v i t y trace could be obtained f o r each experiment. The column temperature was lowered to -40°C using a l i q u i d n i t r o g e n c o o l i n g system while f l u s h i n g with the helium c a r r i e r gas. A f t e r the gas sample was i n j e c t e d onto the column, the temperature was held at -40°C f o r two minutes at which point any unreacted oxygen had e l u t e d . The temperature of the column was then r a i s e d l i n e a r l y to a f i n a l temperature of 250°C. This temperature was held f o r 5 minutes. Two s e r i e s of experiments were c a r r i e d out using r a d i o a c t i v e oxygen-containing molecules. A column o f Pennsylvania A n t h r a c i t e never exposed to a i r a f t e r g r i n d i n g was prepared and exposed to molecular oxygen l a b e l l e d with a trace o f oxygen-15 prepared from the *6θ(ρ,ρη)Ήθ i reaction. A column of North Dakota L i g n i t e was prepared and i n j e c t e d with a mixture of l a b e l l e d O 2 , C O 2 and HoO. The oxygen-15 i n these molecules was prepared from the 1*Ν(ρ,η)*5ο n u c l e a r r e a c t i o n and thus the mixture contained only a trace of molecular oxygen. The t r a c e r , with or without added c a r r i e r , was placed i n a 5 cc gas s y r i n g e and i n j e c t e d onto the c o a l column. A f t e r the heating c y c l e was completed the column temperature was g r a d u a l l y lowered while the column was continuously flushed with the helium c a r r i e r gas. Each of these columns were used f o r s e v e r a l runs. The r e p e a t a b i l i t y of the r e s u l t s was very good. n

u

c

e

a

r

Results K i n e t i c experiments. The experimental curves f o r the uptake of oxygen-containing molecules versus time are shown i n F i g u r e 1 f o r three types of c o a l . The three types a l l f o l l o w the same general behavior i n that they show a r a p i d uptake of oxygen followed by a more gradual uptake. In the case of the lower rank c o a l , t h i s uptake l a s t s f o r a c o n s i d e r a b l e amount of time. The lower curve i n each case i s the uptake of oxygen c o n t a i n i n g species a f t e r the sample had been placed i n the vacuum oven at 65°C f o r 3 hours. In a l l cases a c o n s i d e r a b l e amount of the oxygen can be pumped o f f . The a n t h r a c i t e shows e s s e n t i a l l y no increase i n o x i d a t i o n or O 2 uptake a f t e r about 15 minutes. In the l i g n i t e the i r r e v e r s i b l e uptake continues f o r a very long period o f time. T h i s i s probably due to chemical o x i d a t i o n of the c o a l . Oxygen Depth P r o f i l e . When c o a l samples of d i f f e r e n t p a r t i c l e s i z e (400 mesh to 35 mesh) which had not been exposed to a i r a f t e r g r i n d i n g were analyzed f o r the oxygen to carbon r a t i o , i t was e s s e n t i a l l y the same f o r a l l s i z e s . T h i s Implies that the oxygen to carbon r a t i o i s constant throughout the p a r t i c l e . When the same a n a l y s i s was c a r r i e d out f o r c o a l exposed to a i r f o r two months, there was a d e f i n i t e i n c r e a s e i n the oxygen to carbon


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r a t i o as the p a r t i c l e s i z e decreased. This i n d i c a t e s that an oxide l a y e r has been produced on the o u t s i d e of the c o a l p a r t i c l e upon exposure to a i r . In order to estimate the depth of the l a y e r a model must be used. Three models have been explored i n the attempt to estimate the depth of p e n e t r a t i o n . The f i r s t of these i s a l a y e r of constant composition which f a l l s to the unoxidized value a t a point w i t h i n the sphere. The second i s a l i n e a r decrease i n the 0/C from the outside of the p a r t i c l e towards the center and the t h i r d i s an exponential decrease i n the r a t i o . The p r e d i c t e d depth of the oxide l a y e r i s f a i r l y i n s e n s i t i v e to the model chosen but the oxygen to carbon r a t i o on the s u r f a c e o f the c o a l p a r t i c l e i s extremely s e n s i t i v e to the model. As a r e s u l t i t i s p o s s i b l e to give meaningful numbers f o r the depth of the oxide l a y e r but not f o r the oxygen content o f the s u r f a c e . The curves derived from the f i r s t model are given i n F i g u r e 2. The volume of the oxide l a y e r f o r a given oxide l a y e r thickness ( V ) d i v i d e d by the t o t a l volume of the p a r t i c l e (Vp) i s p l o t t e d versus the p a r t i c l e diameter to o b t a i n the curve shown. These curves have been c o r r e c t e d f o r the experimentally determined nuclear c r o s s - s e c t i o n r a t i o . Figure 3 i s the data from the a c t i v a t i o n of New Mexico Sub-bituminous compared to the model curves. The values obtained f o r the oxide l a y e r depths f o r the three types of c o a l o x i d i z e d f o r two months at 21°C are given i n Table I. Q

Table I Oxide l a y e r depths (2 months exposure a t 2l°C) Coal type Anthracite Sub-bituminous Lignite

Depth i n microns 4 7 10

L a b e l l e d Oxygen Adsorption. When f i v e m i l l i l i t e r s o f oxygen l a b e l l e d with a trace of 0-15 were i n j e c t e d onto the Pennsylvania a n t h r a c i t e , about 5% was r e t a i n e d on the column a t low temperature. A l l o f t h i s oxygen was desorbed during the heating c y c l e . When a mixture of l a b e l l e d O 2 , C O 2 and H 2 O l a b e l l e d with 0-15 i n p r o p o r t i o n s of 80:15:5 i n 5 cc of n i t r o g e n was i n j e c t e d onto the column with no added c a r r i e r (approximately 1 0 ^ molecules o f O 2 due to ppm i m p u r i t i e s o f 0-16, O 2 ) , a l l of the a c t i v i t y was r e t a i n e d on the column. When 1 cc o f O 2 was added to the mixture only a c t i v i t i e s which corresponded to C O 2 and H 2 O were r e t a i n e d on the column. When 1 cc of a 50:50 mixture of C O 2 and O 2 was added only about 5% o f the a c t i v i t y was r e t a i n e d on the column. This a c t i v i t y probably corresponds to the water activity.


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NORTH

DAKOTA LIGNITE

NEW MEXICO

SUB-BIT. *

-A T Ô

KM5

15^5

2O0

PENN.

25l)

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v

. . \ n

ANTHRACITE A 300

2 WEEKS

TIME (MINUTES) Figure 1.

Uptake of oxygen by selected coals vs. time: no heat or pumping (%); heated at 65°C under vacuum ( Ά λ

Figure 2.

Volume of oxide layer divided by the total volume of the particle di ameter for model


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Comparison of experimental data for New Mexico subbituminous with model


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Discussion Several s a l i e n t features have emerged from work done i n other l a b o r a t o r i e s on the low temperature o x i d a t i o n of c o a l . These are l i s t e d below: (1) The adsorption of oxygen onto most types of c o a l f o l l o w E l o v i c h k i n e t i c s i . e . a r a p i d uptake followed by a much more gradual uptake (9) (2) The oxygen adsorbed by low rank c o a l s can form phenolic-OH or carbonyl groups (10-12). The phenolic groups can s p l i t out to form ether linkages. (3) Water s o r p t i o n isotherms on low rank c o a l s have a standard type I I sigmoid shape t y p i c a l of porous adsorbents (4^ . (4) The heat of d e s o r p t i o n f o r macroscopic amounts o f water o f f most c o a l s i s very near that of the heat of v a p o r i z a t i o n of water ( 4 ) . (5) The apparent a c t i v a t i o n energies f o r o x i d a t i o n are very low. The current s t u d i e s using radiochemical techniques are i n agreement with these f i n d i n g s . The a d s o r p t i o n of oxygen onto the c o a l f o l l o w s E l o v i c h - t y p e k i n e t i c s . Recent modelling studies have demonstrated the equivalence of E l o v i c h k i n e t i c s with d i f f u s i o n and adsorption i n microporous s o l i d s (13>ΙΑ)· These s t u d i e s tend to support the idea of s o l i d s t a t e d i f f u s i o n with the f a i r l y low a c t i v a t i o n a s s o c i a t e d with these processes as a r a t e l i m i t i n g step i n the c o a l o x i d a t i o n . The depth of p e n e t r a t i o n of the oxygen seems be d i r e c t l y r e l a t e d to the rank of the c o a l . In the cases studied here, the a n t h r a c i t e showed the l e a s t p e n e t r a t i o n while the l i g n i t e showed the deepest. T h i s f u n c t i o n of rank i s probably a r e s u l t of p o r o s i t y of the c o a l . I t has been p o s s i b l e to demonstrate that f o r the lower rank c o a l s that there are very few (more than l O ^ V g but l e s s than 10^^/g) r e a c t i v e s i t e s f o r strong oxygen a d s o r p t i o n . From the data c o l l e c t e d we wish to propose the f o l l o w i n g mechanism f o r low temperature o x i d a t i o n . Oxygen i s adsorbed weakly onto the surface of the c o a l . This adsorbed oxygen can then migrate across the surface or i n t o the i n t e r i o r of the c o a l p a r t i c l e to a r e a c t i v e s i t e at which point i t becomes chemically bound. The form of t h i s chemical bond i s phenolic-OH, carbonyl or peroxide type m o i e t i e s . As the surface l a y e r s become saturated, the oxygen w i l l d i f f u s e deeper i n t o the p a r t i c l e s through pores and c r e v i c e s to react with s i t e s w i t h i n the c o a l p a r t i c l e . T h i s i s the second, slower part of the a d s o r p t i o n process. At some point the chemical p o t e n t i a l against d i f f u s i o n i n t o the p a r t i c l e becomes great enough that the o x i d a t i o n from the e x t e r i o r of the p a r t i c l e ceases. This point w i l l be determined by the p o r o s i t y of the c o a l and the temperature at which the o x i d a t i o n i s c a r r i e d out. As


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temperature i s increased the oxide l a y e r w i l l extend deeper the c o a l p a r t i c l e .

into

Acknowledgment Research c a r r i e d out at Brookhaven N a t i o n a l Laboratory under c o n t r a c t with the U . S. Department of Energy and supported by i t s O f f i c e o f Basic Energy Sciences.

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

See for example Dryden, I. G. C. in Chemistry of Coal Utilization, Supplementary Volume (Ed. H. H. Lowry) Wiley, New York, 1963, Ch. 6, p. 272. Painter, P. C., Snyder, R. W., Pearson, D. E . , Kwong, J . FUEL, 59, 282 (1980). Bouwman, R. and Freriks, I. L. C. FUEL, 59, 315 (1980). Bhattacharyya, Κ. K. FUEL, 50, 214 (1971). Avison, N. L . , Winters, R. Μ., and Perlmutter, D. D. AIChE Journal 25, 773 (1979). Swann, P. D., Allardice, D. J . and Evans, D. G. FUEL, 53, 85 (1974). Dugan, P. and Moran, V. J . FUEL, 49, 415 (1970). Schlyer, D. J., Ruth, T. J. and Wolf, A. P. FUEL, 58, 208 (1979). Newman, J. O. Η., Stanley, L . , Evans, P. L . , Coldrick, A. J. T. and Kempton, T. J . Nature 214, 280 (1967). Ignasiak, B. S., Clugston, D. M. and Montgomery, D. S. FUEL, 51, 76 (1972). Swann, P. D. and Evans, D. G. FUEL, 58, 276 (1979). Ignasiak, B. S., Szaldow, A. J. and Berkowitz, N. FUEL, 53, 229 (1974). Harris, J . A. and Evans, D. G. FUEL, 54, 277 (1975). Yang, R. T. and Wong, C. J. Phys. Chem. 84, 678 (1980).

RECEIVED

March 9, 1981.


New Approaches in Coal Chemistry - American Chemical Society

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