Decomposition of coal

Coal can be converted to gas by several routes (2,6—11), but often a particular process is a combination of options chosen on the basis of the product desired, ie, low, medium, or high heat-value gas. In a very general sense, coal gas is the term appHed to the mixture of gaseous constituents that are produced during the thermal decomposition of coal at temperatures in excess of 500°C (>930°F), often in the absence of oxygen (air) (see Coal CONVERSION PROCESSES, gasification) (3). A soHd residue (coke, char), tars, and other Hquids are also produced in the process ... 

Chemistry. Coal gasification iavolves the thermal decomposition of coal and the reaction of the carbon ia the coal, and other pyrolysis products with oxygen, water, and hydrogen to produce fuel gases such as methane by internal hydrogen shifts... 

A residuum, often shortened to resid, is the residue obtained from petroleum after nondestmctive distillation has removed all the volatile materials. The temperature of the distillation is usually below 345°C because the rate of thermal decomposition of petroleum constituents is substantial above 350°C. Temperatures as high as 425°C can be employed in vacuum distillation. When such temperatures are employed and thermal decomposition occurs, the residuum is usually referred to as pitch. By inference, the name is used in the same manner as when it refers to the nonvolatile residue from the thermal decomposition of coal tar (3). 

The important elementary reactions of coal liquefaction are the decomposition of coal structure with low bond dissociation energy, the stabilization of fragments by the solvent and the dissolution of coal units into the solution. 

Distillation of Coal.—A few remarks may now bo made on the changes which toko place during Hie manufacture of gas by the distillation of coal in red-hot retorts. The nitrogen in gas Is entirely derived from atmospheric air, admitted into the retorts during the oharges, and by leakage in the apparatus, and is not product of the decomposition of coal at all it need not, therefore, he further dwelt upon. 

The term volatile matter content (of coal) is actually a misnomer, insofar as the majority of the volatile matter is the volatile product of the thermal decomposition of coal through the application of high temperatures. The extent to which the more volatile smaller molecules of coal (Vahrman, 1970) add to this is dependent on the coal and should be determined by nondestructive methods such as extraction by solvent(s). Relative yields and boiling-point profiles provide the extent to which natural molecules contribute to the volatile matter without any influence from high-temperature cracking. 

The chief differences in the methods for the determination of volatile matter emanating from the thermal decomposition of coal are (1) variations in the size, weight, and materials of the crucibles used (2) the rate of temperature rise (3) the final temperature (4) the duration of heating and (5) any modifications that are required for coals which are known to decrepitate or which may lose particles as a result of the sudden release of moisture or other volatile materials. In essence, all of these variables are capable of markedly affecting the result of the tests, and it is, therefore, very necessary that the standard procedures be followed closely. 

Curie point pyrolysis mass spectrometry has also been valuable in providing information about the chemical types that are evolved during the thermal decomposition of coal (Tromp et al., 1988) and, by inference, about the nature of the potential chemical types in coal. However, absolute quantification of the product mixtures is not possible, due to the small sample size, but the composition of the pyrolysis, product mix can give valuable information about the metamorphosis of the coal precursors and on the development of the molecular structure of coal during maturation. However, as with any pyrolysis, it is very important to recognize the nature and effect that any secondary reactions have on the nature of the volatile fragments, not only individually but also collectively. 

Volatile matter hydrogen, carbon monoxide, methane, tar, other hydrocarbons, carbon dioxide, and water obtained on thermal decomposition of coal under prescribed conditions (ASTM D-3175). 

Yellow, P. C. Kinetics of the thermal decomposition of coal. BCURA Monthly Bull. 29, 285/308 (1965). 

Coal hydropyrolysis is defined as pyrolysis under hydrogen pressure and involves the thermal decomposition of coal macerals followed by evolution and cracking of volatiles in the hydrogen. It is generally agreed that the presence of hydrogen during the pyrolysis increases overall coal conversion. (, )... 

Kinetic parameters for n (reaction order) and E (activation energy) in the literature show significant variation for different techniques and coal (Table III). By considering the complexity of coal thermal degradation, many authors have contended that a simple, first-order reaction is inadequate. Wiser et al. (10) found that n=2 gave the best fit to their data, while Skylar et al. (11) observed that values of n above 2 were required to fit nonisothermal devolatilization data for different coals. The kinetic parameters obtained in this study also show a non-integral reaction order. The thermal decomposition of coals are complex because of the numerous components or species which are simultaneously decomposed and recondensed. 

Edinger et al. [4] studied rapid decomposition of coal in a transport-type reactor, with residence times 8-40 ms (COED-FMC). They found that pyrolysis atmosphere affects the products. Coal particles never reached the reactor temperature, even at the lowest particle transfer rate 59% of the coal volatilized when the reactor temperature was 1300°C. This is far above the 41% indicated by the ASTM volatile-matter determination. 

S. Badzioch, Rapid and controlled decomposition of coal. The British Coal Util. Res. Ass., Monthly Bull., 25(8), 285 (1961). 

TIjffective use of the vast coal reserves of the United States requires quantitative information on their conversion behavior in commercially important reactions. Studies using small-scale equipment and aimed at determining the effects of temperature, pressure, particle size, and heating rate on the rapid thermal decomposition of coal in atmospheres of both helium (pyrolysis) and hydrogen (hydropyrolysis) have been in... 

The proximate analysis of coal samples has also been described by Fyans (122), Sadek and Harrell (123), Earnest and Fyans (124), Hassel (125), and others. Using TG, Serageldin and Pan (126) described the reaction kinetics of the thermal decomposition of coal. 

Diagenesis Initial biological decomposition of coal-forming plant material. 

Coal devolatilization Thermal decomposition of coal to release volatiles and tars. 

In addition, an aspect of coal science that is often carried in the minds of those whose goal is structural elucidation is the thermal decomposition of coal to coke (Chapters 13 and 16). 

The thermal decomposition of coal (Chapter 13) emphasizes the need for access of reagents such as hydrogen to regions of the coal undergoing conversion. Thus, a key consideration in coal conversion is the mass transfer of reagents and products which occurs by means of coal s extensive pore network. 

Thus, the solvent extraction of coal, far from being a theoretical study, is, in fact, very pertinent to the behavior of coal in a variety of utilization operations. For example, the liquefaction of coal (Chapters 18 and 19) often relies upon the use of solvent and, in addition, the thermal decomposition of coal can also be considered to be an aspect of the solvent extraction of coal. The generation of liquid products during thermal decomposition can be considered to result in the exposure of coal to (albeit coal-derived) solvents with the result that the solvent materials being able to influence the outcome of the process. And there are many more such examples. 

At temperatures below those normally required for the thermal decomposition of coal, the yields of extract vary directly with extraction temperature. This effect is usually most pronounced with the nonspecific solvents but it has been noted that a solvent such as ethylenediamine will produce from bituminous coal almost three times as much extract at its boiling point (115°C [240°F]) as at room tanperature and also enhances the effect of extraction with other solvents such as A-methyl-2-pynoUdone (Pande and Sharma, 2002). In fact, the yields of extracts obtained with a series of primary aliphatic amines have been found to vary with the extraction temperature rather than with any other solvent property. 

Before progressing any further into the realm of the thermal decomposition of coal, it is noteworthy (and it should be of little surprise to most readers) that, as a result of the myriad of investigations, several forms of terminology have come into common usage. 

The terms thermal decomposition, pyrolysis, and carbonization often may be used interchangeably. However, it is more usual to apply the term pyrolysis (a thermochemical decomposition of coal or organic material at elevated temperatures in the absence of oxygen, which typically occurs under pressure and at operating temperatures above 430°C [800°F]) to a process that involves widespread thermal decomposition of coal (with the ensuing production of a char—carbonized residue). 

With respect to the volatile matter produced by the thermal decomposition of coal, rapid thermal decomposition enables the production of lower-molecular-weight hydrocarbons along with the char residue. For bituminous coals, the decomposition increases markedly above 400°C (750°F) and reaches a maximum in the range 700 C-900°C (1290°F-1650°F). 

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