4 Coal Structure and Thermal Decomposition P E T E R R.
SOLOMON
Downloaded by GEORGE MASON UNIV on December 23, 2014 | http://pubs.acs.org Publication Date: October 26, 1981 | doi: 10.1021/bk-1981-0169.ch004
Advanced Fuel Research, Inc., 87 Church Street, East Hartford, CT 06108
Although coal has been in use for over six hundred years, there is s t i l l no agreed-upon model for the chemical structure of coal or how it comes apart during a chemical reaction. The increased use of complicated processing of coal to produce alternative fuels or reduce pollution has made it imperative to obtain an understanding of the fundamental chemistry. The most widely held view of coal structure pictures coal as groups of fused aromatic and hydroaromatic ring clusters, possibly linked by relatively weak aliphatic bridges. The ring clusters contain heteroatoms (oxygen, sulfur and nitrogen) and have a variety of attached functional groups. Several investigators have suggested coal structures based on constraints provided by available data. Much of this work was recently reviewed by Davidson (1). In the early I960's Given (2,3) suggested a structure based on x-ray data for the cluster size and infrared measurement for the concentrations of aliphatic and aromatic hydrogens. To fit Brown's values for aromatic hydrogen, Har ( 4 ) , Given made extensive use of dihydroanthracene or dihydrophenanthrene linkages. These s t r u c t u r e s make the maximum use of a l i p h a t i c carbons as subs t i t u t e s f o r aromatic hydrogens r e s u l t i n g i n a low value f o r H . Brown's values f o r H , however, appear to be too low when compared to other methods of determining H including recent FTIR measurements (_5). Wiser s model ( 6 ) uses a higher value of H and r e f l e c t s liquefaction and thermal decomposition data by i n c l u d i n g more e a s i l y broken weak a l i p h a t i c bonds such as ethylene bridges between r i n g c l u s t e r s . A recent s t r u c t u r e c o n t r i b u t e d by Heredy and Wender ( 7 ) i s s i m i l a r to W i s e r s. It i s based on a combination of a n a l y t i c a l data and used a mathematical a n a l y s i s to determine H . Experiments on c o a l which change the hydrogen c o n c e n t r a t i o n by r e d u c t i o n or c a t a l y t i c dehydrogenation agree with what would be expected for t h i s model. Suggested s t r u c t u r e s by Whitehurst et a l ( 8 ) have been based on the a n a l y s i s of short contact time c o a l l i q u e f a c t i o n products which are believed to r e f l e c t s t r u c t u r e i n the a r
a
r
a
r
1
a
r
1
a
r
0097-6156/81/0169-0061$05.00/0 ©
1981
A m e r i c a n Chemical
Society
In New Approaches in Coal Chemistry; Blaustein, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
Downloaded by GEORGE MASON UNIV on December 23, 2014 | http://pubs.acs.org Publication Date: October 26, 1981 | doi: 10.1021/bk-1981-0169.ch004
62
NEW
APPROACHES IN
COAL
CHEMISTRY
parent c o a l . Other models which propose d i f f e r e n t views of the s t r u c t u r e have been presented by P i t t , ( 9 ) Chakrabarty and Berkowitz (10) and H i l l and Lyon (_U_). This paper c o n s i d e r s the i m p l i c a t i o n s of a number of ana l y t i c a l techniques and extensive thermal decomposition data (5,12-15) i n d e f i n i n g a s t r u c t u r e f o r a P i t t s b u r g h seam c o a l , PSOC 170. The thermal decomposition experiments y i e l d informat i o n on the c o a l fragments which are s i m i l a r i n composition to the parent c o a l and on the c o n c e n t r a t i o n of chemical species and t h e i r r e l a t i v e bond s t r e n g t h s . The a n a l y t i c a l techniques a p p l i e d to the coals and thermal decompositon products i n c l u d e : 1) F o u r i e r Transform I n f r a r e d (FTIR) Spectroscopy f o r the quantitative determination of hydroxyl, aliphatic (or hydroaromatic) and aromatic hydrogen and a l i p h a t i c and aromatic carbon and qualitative determination of oxygen f u n c t i o n a l groups and the d i s t r i b u t i o n of aromatic hydrogen (whether 1,2 or more adjacent hydrogens on a r i n g ) , (5,15). 2) Carbon and proton NMR f o r determination of the f r a c t i o n s of aromatic c a r bons and hydrogens (16). 3) Elemental a n a l y s i s . 4) G e l permeation chromatography (GPC) and vapor phase osmometry (VPO) f o r determining the molecular weight of c o a l fragments (17). The above r e s u l t s are summarized i n a proposed molecular s t r u c t u r e and proposed d e s c r i p t i o n f o r the o r i g i n of thermal decomposition products. The purpose of presenting molecular s t r u c t u r e s i s to summarize research r e s u l t s i n a c o n c i s e form which w i l l allow a convenient comparison to other work. For example, the proposed s t r u c t u r e i s s i m i l a r to those of Wiser (6) and of Heredy and Wender Ç7). But these s t r u c t u r e s only represent aspects of the s t r u c t u r e which are c o n s t r a i n e d by a v a i l a b l e data, and are not unique. They may be wrong i n det a i l s f o r which data i s m i s s i n g . Care should t h e r e f o r e be exe r c i s e d i n the way such models are i n t e r p r e t e d . An important factor influencing the d i s t r i b u t i o n of thermal decomposition products i s the a v a i l a b i l i t y of a l i p h a t i c or hydroaromatic hydrogen. T h i s f a c t o r i s considered f o r s e v e r a l examples i n which the d i s t r i b u t i o n of products may be varied substantially by varying the thermal decomposition conditions. Coal S t r u c t u r e and Thermal
Decomposition
The h y p o t h e t i c a l c o a l molecule and thermal decomposition products f o r PSOC 170 are presented i n F i g s . 1 and 2. The s t r u c t u r e parameters f o r the suggested molecule, the c o r responding parameters determined f o r PSOC 170 and the source f o r the data are summarized i n Table I. Some of the d e t a i l s of the model are as f o l l o w s : 1) The s t r u c t u r e contains only one c a r b o x y l . The c a r b o x y l c o n c e n t r a t i o n i s based on the y i e l d of CO2 which i s believed to be i t s thermal decomposition product. No other carbonyl's are included as the carbonyl peaks
In New Approaches in Coal Chemistry; Blaustein, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
Downloaded by GEORGE MASON UNIV on December 23, 2014 | http://pubs.acs.org Publication Date: October 26, 1981 | doi: 10.1021/bk-1981-0169.ch004
4.
SOLOMON
Figure 1.
Figure 2.
Coal
Structure
and Thermal
Decomposition
Summary of coal structure information in a hypothetical coal molecule
Cracking of hypothetical coal molecule during thermal decomposition
In New Approaches in Coal Chemistry; Blaustein, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
63
64
NEW
APPROACHES IN C O A L
CHEMISTRY
TABLE I
COAL STRUCTURE PARAMETERS (weight fraction DMMF)
Downloaded by GEORGE MASON UNIV on December 23, 2014 | http://pubs.acs.org Publication Date: October 26, 1981 | doi: 10.1021/bk-1981-0169.ch004
PARAMETER C H 0 Ν S(0)
ar
C ^ y l
al ar H
0 H
[Jmethyl [jar-1 adj ar-2 adj H!ar-3or4 adj
J
co
ring «ring mercaptan 1 2 H
c
al/ al
MODEL
SOURCE
COAL
.81 .055 .098 .011 .026
.82 .054 .094 .014 .019
CHN Analyzer 1| CHN Analyzer Difference CHN Analyzer Li SEMPil
.60 .20 .030 .74
.60 .22 .03 .73
NMR
