Coal molecules

Aromaticity of coal molecules increases with coal rank. Calculations based on several models indicate that the number of aromatic carbons per cluster varies from nine for lignite to 20 for low volatile bituminous coal, and the number of attachments per cluster varies from three for lignite to five for subbituminous through medium bituminous coal. The value is four for low volatile bituminous (21). 

Electron spin resonance (esr) (6,44) has had more limited use in coal studies. A rough estimate of the free-radical concentration or unsatisfied chemical bonds in the coal stmcture has been obtained as a function of coal rank and heat treatment. For example, the concentration increases from 2 X 10 radicals/g at 80 wt % carbon to a sharp peak of about 50 x 10 radicals/g at 95 wt % carbon content and drops almost to zero at 97 wt % carbon. The concentration of these radicals is less than that of the common functional groups such as hydroxyl. However, radical existence seems to be intrinsic to the coal molecule and may affect the reactivity of the coal as well as its absorption of ultraviolet radiation. Measurements from room... 

Several methods ean be employed to eonvert eoal into liquids, with or without the addition of a solvent or vehiele. Those methods which rely on simple pyrolysis or carbonization produce some liquids, but the mam produet is eoke or char Extraction yields can be dramatically increased by heating the coal over 350°C in heavy solvents sueh as anthraeene or eoal-tar oils, sometimes with applied hydrogen pressure, or the addition of a eatalyst Solvent eomponents whieh are espeeially benefieial to the dissolution and stability of the produets eontain saturated aromatic structures, for example, as found in 1,2,3,4 tctrahydronaphthalene Ilydroaromatie eompounds are known to transfer hydrogen atoms to the coal molecules and, thus, prevent polymerization... 

The solvent s role in the first step of the process is clearly based as much on dispersing and dissolving the coal molecules resulting from thermal bond breaking as it is on stabilizing the molecules by hydrogen transfer. 

The Effect of Phenol. Three types of phenol compounds have been identified in the fractions derived from the product of the phenol ati on reaction (.1,2) alkyl phenols and alkyl-aryl ethers, both formed by combining phenol with alkyl side chains cleaved from the coal molecule, and compounds made up of aromatic fragments attached to phenol by a methylene bridge, formed by... 

The reaction may contain hydrogenation, alkylation, and depolymerization of coal molecules assisted by the liquefying solvent. Through these reactions, coal molecules can be converted to be fusible or soluble in the solvent. 

FIGURE 9.19 Hypothetical coal molecule as represented by Solomon [39]. Reprinted with permission from the American Chemical Society. 

To evaluate further the CAMD results, a number of atomic and chemical parameters from each structure (number of atoms, fractions of aromatic carbon and hydrogen, weight fraction or each atomic species, empirical formula) were compared with the original literature for each structure. This provided a useful check on the accuracy of the computer models. Results of the computer analyses for the four coal structures are given in Table I. The total numbers of atoms only appear as guides to the size and complexity of each structure, and bear no relationship to the size of a "coal molecule" or a decomposition product. 

Confirmation that conversion of coal Into liquids depends on limited pyrolytic disruption of coal "molecules" and on prompt stabilization of the resultant fragments by hydrogen Is provided by liquefaction In a hydrogen-donor viUxcU allows such reactions as... 

To account for this behaviour, it is generally assumed that the coal "molecule" breaks down much more drastically when very rapidly raised to decomposition temperatures, and that repoly-merlzatlon of the radical fragments Is Inhibited by their fast discharge from the coal. Some support for this thesis can be seen In the fact that. In a coke oven, "primary" tars will aromatize the more the longer they are In Intimate contact with the hot coke. But a necessary corollary Is, obviously, that the fragments must be supposed to stabilize themselves In the vapour-phase by Internal disproportionation. 

Leslie Reggel Could you say more about the structure of the coal molecule you showed us Is enough hydroaromatic hydrogen included ... 

Dr. Sternberg We believe that it is the solvated lithium cation that is being reduced at the cathode to solvated zero charged lithium atom. The solvated lithium then reacts with the coal. This latter reaction may be homogeneous or heterogeneous depending on whether the coal molecule is solvated or not. 

The high activation energies resulting from the chemisorption hypothesis appear to favor its validity. However, it is possible that such a high activation energy could be caused by an equilibrium between the undecomposed coal molecules and the diffusion species resulting from thermal decomposition. That is, for diffusion in the Z dimension only, a simplified mass balance would lead to a differential equation of the form... 

Coal density is a useful parameter not only for deducing the spatial structure of coal molecules, but the relationship between the density and porosity suggests that emphasis must be given to density and its determination. Porosity measurement, in turn, provides useful information on the technical behavior of coal toward its end use. Particle density is required for calculating the porosity of individual coal particles. 

All of the papers in this book deal with the chemistry and characterization of coal macerals and, as such, recognize the heterogeneous nature of coal. Coal is, in fact, a rock derived from a variety of plant materials which have undergone a variety of physical and chemical transformations. While a number of chemical studies of coal have found the concept of a single "coal molecule" useful, the papers in this book are aimed at characterizing in some way the various macromolecules that comprise the many kinds of coal macerals. 

The concept that free radical reactions are responsible for the initial fragmentation reactions of large coal molecules and for the essential hydrogen transfer reactions that occur during coal dissolution reactions is quite widely accepted (1). The in-... 

Promotion of Hydrogen Exchange The radicals derived from the coal molecules by simple thermal homolyses and by molecule-induced homolyses initiate exchange reactions between tetralin-d, and the donors in the reaction system—principally diphenyl-methane and the hydroaromatic compounds in the macerals (2. ... 

The hypothetical coal molecule and thermal decomposition products for PSOC 170 are presented in Figs. 1 and 2. The structure parameters for the suggested molecule, the corresponding parameters determined for PSOC 170 and the source for the data are summarized in Table I. Some of the details of the model are as follows 1) The structure contains only one carboxyl. The carboxyl concentration is based on the yield of CO2 which is believed to be its thermal decomposition product. No other carbonyl s are included as the carbonyl peaks... 

Figure 1. Summary of coal structure information in a hypothetical coal molecule...

Figure 2. Cracking of hypothetical coal molecule during thermal decomposition...

In thermal coal solvation, a reaction occurs between dissolved hydrogen (or hydrogenated solvent) and the fragmented coal molecules to produce a host of products. 

As well as these reactions, large coal molecules undergo pyrolysis and hydro-pyrolysis to smaller molecules. 

In order to obtain Insight Into the chemical structure of coal, methods are needed to convert coal into substances which are soluble In suitable solvents. This conversion should be conducted at mild conditions so that the coal molecules remain unchanged as much as possible. So far, the most usual method applied to make bituminous coals more soluble In simple liquids Is hydrogenation which, however, requires temperatures exceeding 350° to 400°C consequently cracking reactions also occur. 

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