Relation Between Coal Structure and Thermal Decomposition

7 Relation Between Coal Structure and Thermal Decomposition Products

Downloaded by TUFTS UNIV on December 2, 2015 | http://pubs.acs.org Publication Date: June 1, 1981 | doi: 10.1021/ba-1981-0192.ch007

P. R. S O L O M O N

1

United Technologies Research Center, East Hartford, CT 06108

A relation between coal organic structure and the products of thermal decomposition was developed by measuring and modeling vacuum devolatilization of 12 bituminous coals and a lignite. The distribution of devolatilization products is related to the concentration of aromatic and aliphatic carbons and hydrogens, and oxygen functional groups. Structure information on the coals, tars, and chars was provided by IR spectra obtained with a Fourier transform infrared spectrometer (FTIR). Quantitative determinations of aromatic and aliphatic hydrogens and hydroxyl oxygens can be made from the IR spectra, and concentrations of aromatic and aliphatic carbons and ether oxygens may be inferred. These quantities may be used to predict the time -dependent evolution of thermal decomposition products by using kinetic constants that depend on the functional group but are independent of coal rank. In a recent study the thermal decomposition of 13 coals was examined in over 600 vacuum devolatilization experiments (1). The results for all 13 coals were successfully simulated in a model that assumes that large molecular fragments (monomers) are released from the coal polymer, with only minor alteration, to form tar, while simultaneous cracking of the chemical structure forms the light molecules of the gas (2, 3). The evolution of each species is characterized by rate constants that do not vary with coal rank. The differences between coals are due to differ ences i n the mix of sources in the coal for the evolved species. The sources were tentatively related to the functional group concentrations in the coal. Current address: Advanced Fuel Research, Inc., 87 Church Street, East Hartford, C T 06108. 1

0065-2393/81/0192-0095$05.00/0 © 1981 American Chemical Society

In Coal Structure; Gorbaty, M., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1981.

96

COAL STRUCTURE

This paper reports a study to verify the relationship between func tional group distribution and thermal decomposition behavior. A Fourier transform infrared spectrometer (FTIR) has been employed to obtain quantitative infrared spectra of the coals, chars, and tars produced in the devolatilization experiments. The spectra have been deconvoluted by using a computerized spectral synthesis routine to obtain functional group distributions, which are compared to the model parameters.


Model The thermal decomposition model is illustrated in Figure 1. The coal is represented as an area with X and Y dimensions. The Y dimen sion is divided into fractions according to the chemical composition of the coal. Y°. represents the initial fraction of a particular component (carboxyl, aromatic hydrogen, etc.) and 2 Y°. = 1. The evolution of each component into the gas (carboxyl into C 0 , aromatic hydrogen into H , etc.) is represented by the first-order d i m i n i s h i n g of the Y. d i m e n s i o n , Y = Y° exp(—kf). The relationship between the functional groups and the thermal decomposition products is discussed by Solomon and Colket (2). More complicated reaction steps have been observed at high tem perature (4). The X dimension is divided into a potential tar-forming fraction X° and a non-tar-forming fraction 1 - X°, with the evolution of the tar being represented by the first-order diminishing of the X dimension, X = X° exp(—k t). fc. and k are rate constants given in Table I. The amount of a component in each of the pyrolysis products is given by the following equations: 2

2

(

x

W(t)

w(«) W(i)

char

tar

gas

=

(1 - X° + X)Y,

= onri-xYjkjac, =

x

(1 - X°) (Y°. -

+

k) x

Yj) + W(t)

tar

Table I. Kinetic Constants for Lignite and Bituminous Coals Functional Group Carboxyl Hydroxyl Ether loose Ether tight Aliphatic Aromatic H Nonvolatile C Tar

*i = = = = K = h = h = =

Kinetic Rates 6 exp(-4,000/r) s e c 15 exp(-4,950/T) 7,000 exp(-10,300/r) 890 exp(-12,000/r) 4,200 exp(-9,000/T) 3,600 exp(-12,700/r) 0 750 exp(-8,000/D 1


7.

SOLOMON

Coal Structure and Thermal Decomposition

CD

GAS

E3

97

CHAR

TAR

(b)

(a) CARBOXYL

•co _

HYDROXYL

-*-HoO

2

ETHER — C H | V,°J A L I P H A T I C 2

p

4

C H ETC

6


AROMATIC H

i

^NONVOLATILE CARBON' !

N O N - T A R FORMING FRACTION ( 1 - X ° )

TAR

FORMING

FRACTION X°

Figure 1. Progress of thermal decomposition according to model: (a) functional group composition of coal; (b) initial state of decomposition; (c) later stage of decomposition; (d) completion of decomposition According to this model the tar contains a mix of functional groups similar to that in the parent coal. This concept is based on the strong similarity between vacuum devolatilized tar and the parent coal observed in chemical composition (1, 2, 3), infrared spectra (1, 3, 5, 6), and N M R spectra (1, 3). The similarity of the IR spectra is illustrated in Figure 2 for four coals. The resemblance is strong for the three bituminous coals but not for the lignite. The resemblance of the tar and parent coal suggests that the tar consists of monomers released from the coal poly mer. The major difference observed in the IR spectra between tar and parent coal is the higher quantity of aliphatic C H and C H , presumably resulting because the monomers abstract hydrogen to stabilize the free radical sites produced when the monomer was freed. Similar arguments 2

3



98

COAL STRUCTURE

u -T—

4000

n

2800

-=~n

1600

WAVENUMBERS

^

400

-r~

1

4000

1

2800

1600

WAVENUMBERS

1

400

Figure 2. IR spectra of coals and tars. The absorbance is normalized to a sample size of 1 mg/cm in the KBr disk. 2

were given for pyrolysis of model compounds by Wolfs, van Krevelen, and Waterman (7). Figure 1(b) illustrates the initial stage of thermal decomposition during which the volatile components H 0 and C 0 evolve from the hydroxyl and carboxyl groups, respectively, along with aliphatics and tar. At a later stage [Figure 1(c)] C O and H are evolved from the ether and aromatic H . Some C O is also released at low temperatures, so two kinetic rates are listed for ether. To simulate the abstraction of H by the tar, the aliphatic fraction in the tar is assumed to be retained together with some additional aliphatic material, which may be added directly or may contribute its hydrogen. When hydrogen from the aliphatic material is contributed, its associated carbons remain with the nonvolatile carbon fraction, which eventually forms the char [Figure 1(d)]. The nonvolatile carbon fraction is the aromatic carbon fraction minus the fraction of carbons released as C O from ether tight. 2

2

2

Infrared Spectra Infrared spectra of coals, tars, and chars were obtained on a Nicolet FTIR. K B r pellets of coals and chars were prepared by mixing 1 mg of a


7.

SOLOMON


99

dry, finely ground (20 m i n in a W i g - L - B u g ) sample with 300 mg of K B r . Pellets 13 mm in diameter were pressed in an evacuated die under 20,000 l b of pressure for 1 min and dried at 110°C overnight to remove water. Since the heating process destroyed similarly prepared tar pel lets, the water correction was obtained by subtracting the spectrum of a blank disk prepared under the same conditions. The H 0 - c o r r e c t e d spectra matched those from samples prepared by allowing the tar pro duced in thermal decomposition to fall directly onto a water-free blank K B r disk. Spectra are obtained by using 4-wave number resolution, 100 scans of the sample, and 100 scans of the background. A globar source and cooled mercury-cadmium-telluride detector are used. The F T I R obtains spectra in digital form, and corrections for particle scattering, mineral content, and water may be easily made. A typical correction sequence is illustrated in Figure 3. The lower curve in Figure 3(a) is the uncorrected spectrum of a dried coal. It has a slope, owing to particle scattering, and peaks from the mineral components near 3600, 1000, and 450 c m . The middle spectrum in Figure 3(a) has had the mineral peaks removed, which is done by subtracting appropriate amounts of the reference spectra shown in Figure 3(b) to produce a smooth spectrum in the region of the mineral peaks. The sum of the reference spectra subtracted from the coal is shown as the top spectrum in Figure 3(b). Similar procedures for mineral analysis using FTIR have recently been reported (8). The top spectrum in Figure 3(a) represents the coal on a mineral-matter-free basis. It has had a straight-line scattering correction and has been scaled to give the absorbance for 1 mg/cm of dry, mineralmatter-free ( D M M F ) coal.


2

1

2

M u c h previous work has been done to identify the functional groups responsible for the observed peaks. Extensive references may be found in Lowry (9) and van Krevelen (10). To get a quantitative measure of the functional group concentrations, we used a curve analysis program (CAP), available in the Nicolet library, to synthesize the IR spectra. The synthesis is accomplished by adding 26 absorption peaks with Gaussian shapes and variable position, width, and height, as shown in Figure 4. The peaks are separated according to the identified functional group. A l l the coals, chars, and tars that were studied could be syn thesized by varying only the magnitudes of a set of Gaussians whose widths and positions were held constant. These samples could be ana lyzed, therefore, in terms of a fixed mix of functional groups. The set of peaks M , N , and Q simulates the absorption assumed to result from hydrogen-bonded O H . To verify this assumption, we ob tained spectra, at elevated temperatures, of a tar sample coated on a K B r pellet. Figure 5 shows a spectrum taken near room temperature, one at 250°C, and their difference. As expected for hydrogen bonding,the ab sorption for bonded O H groups (3400-2400 c m " ) decreased, while that 1


100

COAL STRUCTURE O

M

Q O O


u go 5". in m a

o

O M

8SbbO o OP kJ

3600

3200

2600

2400 2000 MRVENUHBER5

b)

o

U z

M1NERRL SPECTRUM 6.6% A-V.

CD a o

KP0L1N 1.6%

s< CJ Q

A—*

1LL1TE 5.0% .1000

36b0

32b0

2600

24bO 2000 WRVENUMBER5

1600

ebo

«*bo

Figure 3. Correction of coal spectrum for scattering and minerals: (a) correction of coal spectrum; (b) determination of mineral spectrum by addition of reference spectra. The absorbance is normalized to a sample size of 1 mg/cm in the KBr disk. 2

for free O H (3500 c m ) increased. The change in the high-temperature spectrum was not permanent; the room temperature spectrum returned after cooling. Peak O at 1600 c m has been i n c l u d e d w i t h the h y d r o x y l group. The identity of this peak has caused much speculation in the literature (9, 10). Fujii et al. (11) showed that the intensity of the - 1

- 1


SOLOMON


7.

101


WAVENUMBERS

Figure 4. Synthesis of IR spectrum 1600-cm" peak varied linearly with oxygen content in the coal. The present study narrows the correlation still further. Figure 6 shows a linear relation between the intensity of the O peak at 1600 c m " and the hydroxyl content measure by peaks L , M , N , and Q . The correlation includes chars that have a high oxygen content in ether groups but low O peaks, so that the correlation with total oxygen would no longer hold. It also includes samples that have a large aliphatic content and a small value of O and samples with a small aliphatic content and a large value of O , so that enhanced aromatic ring resonance due to aliphatic substitutions is unlikely. It therefore has been concluded that the sharp line at 1600 c m is caused mainly by enhanced aromatic ring resonance due to O H substitutions. 1

1

- 1


102

COAL STRUCTURE

• IFF id ' (j ~" z a


Relation Between Coal Structure and Thermal Decomposition

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