Structure and composition of coal tars: An attempt to correlate molecular structure with increasing molecular mass

Abstract

Near-burner-zone combustion rates in pulverized-fuel (PF) combustors depend primarily on the amount and the structure=composition of volatiles released from the fuel. Mathematical simulations of PF flames often assume that volatiles combustion takes place at rates similar to those of light hydrocarbons. However, tars constitute 60-70% of volatiles released by most power station coals; tar molecules are far larger and oxidize more slowly. The problem is usually ignored. This article describes the characterization of structures and compositions of molecules in a low-temperature coal tar, typical of material combusted in the near-burner zone. Several techniques have been used in combination. The procedure relies on the estimation of molecular mass distributions by size exclusion chromatography coupled with bulk structural characterization by several established techniques (e.g., 13 C-NMR, UV-fluorescence spectrometry). Sample fractionation by polarity and molecular mass have been found necessary for enhancing the resolution of the analytical tools, as well as for allowing meaningful correlations to be established between changing molecular mass ranges of successive fractions of the tar and the structural features of each fraction. The approach, described in some detail, extends the range of molecular masses amenable to examination to levels far above ceilings imposed by limitations of conventional GC, GC-MS, and probe-MS. The distribution of structures and molecular masses found in the tar suggests reaction pathways for the formation of soot during PF combustion.

Pyrolysis‐gas chromatography/mass spectrometry of pitch fractions recovered from preparative size exclusion chromatography: structural differences with increasing molecular size

“…Broadly similar results were obtained when a coal extract18 and a low‐temperature tar19,, 20 were characterised by the same methods.…”

“…Application of these methods to a low‐temperature tar19,, 20 led to similar observations. 13 C‐NMR data indicated the heaviest fraction (soluble only in NMP during column chromatography) to contain less than 50% aromatics.…”

“…In recent work, we probed relationships between the distribution of structural features and changes in molecular mass in samples of coal tar pitch,12,, 17 a coal extract18 and a low‐temperature tar 19,. 20 The method involved fractionation of the samples by planar chromatography (PC) or column chromatography.…”

Analysis of Recovered Solvents from Coal Liquefaction in a Flowing-Solvent Reactor by SEC and UV-Fluorescence

“…However, the use of THF in SEC has been shown to promote adsorption of coal derived materials on the column packing, giving results not related to size exclusion, as well as being a poor solvent compared to NMP (Begon, 2002;Islas et al, 2002Islas et al, , 2003. In addition, the use of only one wavelength, 254 nm, tended to discriminate against the largest aromatic clusters observed in the THF insoluble materials (Gibbins, 1991;Herod et al, 1996;Zhang et al, 1997).…”

Identification, Quantification and Distribution of Pac-Metabolites, Heterocyclic Pac and Substituted Pac in Groundwater Samples of Tar-Contaminated Sites From Germany

“…Furthermore, in several European countries tar was used, for example, as corrosion protection for water pipes (6,7). Original tar contains a higher percentage of non-polar PAH compared to PAC-metabolites, substituted PAC and heterocyclic PAC (PASH, PANH, PAOH) as degradation and transformation products (8)(9)(10)(11)(12)(13)(14)(15). Due to higher water solubility and corresponding lower K OWvalues, polar PAH derivatives show an increasing tendency for distribution in groundwater flow direction and bioavailability.…”