Coal volatile aromatics

Aliphatic structures are still of major importance in the second group of resinites, those of the bituminous coals, but aromatic structures are present in significant amounts. The spectra of these resinites display the type of absorption pattern that has come to be associated with other coal macerals, particularly the sporinites and to a large extent the vitrinites. This pattern is established in the resinites of the high volatile bituminous coals. Furthermore, resinites of this group are reactive during carbonization and oxidation processes in which their behavior parallels that of similarly affected vitrinites of equivalent rank. 

Application of supercritical gas extraction (Schneider et al., 1980 Bright and McNally, 1992 Kiran and Brennecke, 1993) has lately received considerable attention when applied to coal. For example, studies have been reported relating to the chemical nature of extracts of coal (volatile matter content in excess of 36%) obtained using toluene (under pressure) at 350°C (660°F). The extracts contained aromatics (benzene derivatives, naphthalene derivatives, and phenanthrene derivatives) as well as n-paraffins, sterane, and materials such as phytane, pristane, and farnesane (Table 10.2) (Bartle et al., 1975 Smith and Smoot, 1990). There was a predominance of n-alkanes. 

On the monomeric LCSP called 4,4 -biphenylene-bis (4-n-butyloxy-benzoate) placed in a capillary column, 12 polynuclear aromatic hydrocarbon (PAHs) were separated in the mixture derived from coal tar (Fig. 4). The chromatogram of selected PAHs on metallomesogen stationary phase is presented in Fig. 5. Chromatograms of selected volatile aromatics compounds on monomeric stationary phases are presented in Figs. 6 and 7. 

Production (Source, Use, Shipment) and Emissions Volatile Aromatics from Coal and Lignite... 

In the USSR and in Japan processes were developed to keep the working atmosphere - for instance of the paint and printing industry - free from volatile aromatics. The processes are based on the catalytic oxidation or decomposition of toluene and other offensive and malodorous components [79-81]. The polluted air is oxidised catalytically at 80 °-100 °C at with calgon activated coal coated with metals [79] or at platinum catalysts on alumina oxide [80]. The components may also be decomposed at 225 °-300 °C quantitatively by cobalt oxides on a carrier, such as aluminum oxide [81]. Replacing the volatile aromatics or preventing them from evaporating into the air, are better solutions of the problem in this case. 

Coals (the plural is deliberately used because coal has no defined, uniform nature or structure) are fossil sources with low hydrogen content. The structure of coals means only the structural models depicting major bonding types and components relating changes with coal rank. Coal is classified, or ranked, as lignite, subbituminous, bituminous, and anthracite. This is also the order of increased aromaticity and decreased volatile matter. The H C ratio of bituminous coal is about 0.8, whereas anthracite has H C ratios as low as 0.2. 

Many valuable chemicals can be recovered from the volatile fractions produced in coke ovens. Eor many years coal tar was the primary source for chemicals such as naphthalene [91-20-3] anthracene [120-12-7] and other aromatic and heterocycHc hydrocarbons. The routes to production of important coal-tar derivatives are shown in Eigure 1. Much of the production of these chemicals, especially tar bases such as the pyridines and picolines, is based on synthesis from petroleum feedstocks. Nevertheless, a number of important materials continue to be derived from coal tar. 

Aromatic Hydrocarbons. These are the most toxic of the hydrocarbons and inhalation of the vapor can cause acute intoxication. Benzene is particularly toxic and long-term exposure can cause anemia and leukopenia, even with concentrations too low for detection by odor or simple instmments. The currendy acceptable average vapor concentration for benzene is no more than 1 ppm. PolycycHc aromatics are not sufftcientiy volatile to present a threat by inhalation (except from pyrolysis of tobacco), but it is known that certain industrial products, such as coal tar, are rich in polycycHc aromatics and continued exposure of human skin to these products results in cancer. 

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). 

Particulate polycyclic aromatic hydrocarbons (PPAH), see Coal tar pitch volatiles... 

Coal tar pitch volatiles, see Particulate polycyclic aromatic hydrocarbons (PPAH), as benzene solubles Cobalt metal, dust and fume (as Co)... 

Benzene An aromatic hydrocarbon which is a colorless, volatile, flammable liquid. Benzene is obtained chiefly from coal tar and is used as a solvent for resins and fats in dye manufacture. 

Simple aromatic hydrocarbons come from two main sources coal and petroleum. Coal is an enormously complex mixture made up primarily of large arrays of benzene-like rings joined together. Thermal breakdown of coal occurs when it is heated to 1000 °C in the absence of air, and a mixture of volatile products called coal for boils off. Fractional distillation of coal tar yields benzene, toluene, xylene (dimethylbenzene), naphthalene, and a host of other aromatic compounds (Figure 15.1). 

The high-volatile Liddell bituminous coal (Figure 2 (E)) shows little indication of thermally-activated molecular mobility below 500 K. There is some fusion between 500 and 600 K followed by a major fusion transition above 600 K which appears very similar to the high temperature transition of the Amberley coal. This Liddell coal, however, has only 6% liptinite, has a crucible swelling number of 6.5 and exhibits considerable Gieseler fluidity. We therefore attribute this high temperature fusion event to the aromatic-rich macerals of the coal and associate it with the thermoplastic phenomenon. This implies that a stage has been reached in the coalification processes at which aromatic-rich material becomes fusible. 

The low-volatile bituminous Bulli coal which contains no liptinite and has significant thermoplastic properties has a M2J pyrogram (Figure 2 (F)) showing only one fusion transition which is lesser in extent and shifted to higher temperatures than that of the Liddell coal. This transition is, of course, attributed to aromatic-rich macerals. 

---