Coal heteroatoms

The complex nature of coal as a molecular entity (2,3,24,25,35,37,53) has resulted ia the chemical explanations of coal combustion being confined to the carbon ia the system. The hydrogen and other elements have received much less attention but the system is extremely complex and the heteroatoms, eg, nitrogen, oxygen, and sulfur, exert an influence on the combustion. It is this latter that influences environmental aspects. 

High resolution mass spectrometry (qv) has been used with extracts of a series of coals to indicate the association of different heteroatoms (27). Various types of chromatography (qv) have also been used to identify the smaller species that can be extracted from coal. 

Coal Hquefaction iavolves raising the atomic hydrogen-to-carbon ratio from approximately 0.8/1.0 for a typical bituminous coal, to 2/1 for Hquid transportation fuels or 4/1 for methane (4). In this process, molecular weight reduction and removal of mineral matter and heteroatoms such as sulfur, oxygen, and nitrogen may need to be effected. 

Tai yields of approximately 0.32 m /1 were quoted. Because the scheme used hydrogen, the pyrolysis Hquids generally exhibited lower heteroatom contents than conventional tars derived from coal pyrolysis ia an iaert atmosphere. Process development proceeded through a 270 t/d semiworks plant which was operated successfully on noncaking coals. OperabiUty for caking coals was difficult, however. 

In thermal processes the formation of asphaltols always precedes other reactions such as major heteroatom rejection and distillate formation. In fact, in the SRC process bituminous coals are actually dissolved by the time the coal slurry exits the preheater (4,11). This has recently been demonstrated at the SRC process development unit (PDU) in Wilsonville, Alabama (11) (see Figure 2). 

The changes that occur with solid residence time in the hot-rod reactor were not very pronounced because only the nonvolatile portion of the oil remaining on the coal bed would be expected to undergo secondary reactions such as aromatization and loss of heteroatoms. However, the oils from the hot-rod reactor were also compared with those obtained in a rotating autoclave with much longer solid and vapour residence times and the changes with residence time were more noticeable in this case as can be seen in Table II. 

The parent materials differ from each other in many aspects the differences are being related both to their origin (coal or pitch) and heat treatment temperature. Clearly, coal should be classified as a polymeric type precursor while the others, such as carbonaceous precursors of relatively low, except for AC, carbonization degree. Specific of pitch-derived materials is distinctly lower mineral matter and heteroatoms content. Anisotropic appearance with predominating flow type texture proves the superior extent of structural ordering in pitch-derived materials. 

The species which are unknown and have not been identified as one of the major chemical lump such as alkanes, phenols and aromatics are lumped together as unidentified. However, the species in this lump include saturated and unsaturated cycloalkanes with or without side chains, which resembles the naphthenes, a petroleum refinery product group. A number of well known species in coal liquid are not mentioned in this lumping scheme. Such as heterocyclic compounds with sulfur, nitrogen or oxygen as the heteroatom, and other heteroatora containing species. Some of these compounds appear with aromatics (e.g. thiophenes, quinolines) and with phenols (e.g. aromatic amines), and most of them are lumped with the unidentified species lump. 

Shale oils are dark-brown, viscous, waxy liquids that usually contain higher concentrations of nitrogen and oxygen heteroatoms than coal liquids or crude oil. Oils can often have high pour point temperatures and may have trace concentrations of arsenic. Nitrogen levels of 2% and oxygen concentrations of 1.5% are typical. The sulfur content averages about 1%. 

C02 laser pyrolysis of reactant gases has been used to produce a wide variety of dispersed, single crystal nanoparticles (average size 2 to 20 nm). This chapter discusses the production of nitrides (oxynitrides) and carbides (oxycarbides) of Group 6B elements (Mo and W) and Fe by this technique. The emphasis is on the characterization of the atomic order in the particle and the chemical state of the particle surface. The catalytic properties of these particles for coal liquefaction and heteroatom removal from model compounds is also addressed briefly. 

The catalytic activity of Fe carbides, molybdenum oxynitride and oxycarbide has been evaluated for coal liquefaction and heteroatom removal of model compounds related to coal. Preliminary results show that the LP nanoparticles are active catalysts for coal liquefaction. In fact, they are more active for heteroatom removal than a molybdenum promoted sulfated hematite, even though surface characterization indicates that as introduced into the reactor they exhibit surface oxidation. 

Tandem mass spectrometry is also developing into an important analytical method for application to coal-derived materials (Wood, 1987). The analysis of heteroatom ring species and hydrocarbon species in coal-derived liquids offers indications of the location of the heteroatoms in, or on, ring systems, as well as indications of the hydrocarbon systems. 

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