Donald F. McMillen

Mechanisms of hydrogen transfer and bond scission of strongly bonded coal structures in donor-solvent systems

Abstract Five possible non-ionic mechanisms for hydrogen transfer leading to scission of strongly bonded coal structures are considered. Cleavage of diphenyl ether, diphenylmethane, and 1,2′-dinaphthylmethane was studied in a number of aromatic and hydroaromatic solvents. Relative cleavage rates and product selectivity data show that a substantial portion of the cleavage takes place as a consequence of direct H-atom transfer from solvent derived cyclohexadienyl radical intermediates to the ipso position of the substrate in a single biomolecular step. This reaction is given the name radical hydrogen transfer or RHT.

The case for induced bond scission during coal pyrolysis

Bond scissions during coal pyrolysis are generally considered to result solely from the thermolysis of weak bonds in coal structures. Evidence is presented from published data showing that a significant amount of bond scission under pyrolysis cannot be due to simple thermolysis and must be induced by some other means. The evidence includes the following observations: 1, polymeric models consisting of no weak linkages are volatilized in the 450 to 550 °C range; 2, polymeric coal models designed specifically to degrade by thermolysis of weak bibenzylic linkages show nonetheless a significant fraction of the cleavage of strong CarCal linkage; 3, under pyrolysis conditions bibenzyl type structures immobilized on silica yield benzene and ethylbenzene in addition to the expected toluene; 4, pyrolysis of O-benzylated coals shows besides the expected cleavage of the benzyl ether linkage products from the scission of the strong phenyl-C bond. By analogy with coal liquefaction, the scission of strong bonds results from β-scission of radical intermediates, and from hydrogenolysis engendered by either free H-atoms or cyclohexadienyl radicals. These reactive cyclohexadienyl radicals themselves may be formed from the scavenging of radicals (produced by weak-bond thermolysis) by dihydroaromatic structures or from reverse radical-disproportionation between aromatic and hydroaromatic structures. Finally, the importance of induced bond scissions was illustrated by showing how they help rationalize the observed selective production of oils as opposed to gas in the hydropyrolysis of coals when the pyrolysis temperature is maintained below a certain threshold.

Characteristics of CANMET coprocessing distillates at different coal concentrations

Abstract The effect of feed coal concentration on the characteristics of gross and fractionated coprocessing distillates was investigated. Elemental analysis, 1H n.m.r., high performance liquid chromatography (h.p.l.c.) and field ionization mass spectrometry (f.i.m.s.) were used in this study. The distillates (205–525 °C) were obtained by coprocessing Forestburg subbituminous coal at three concentration levels, 3.7, 23.9 and 39.5 wt% (based on a maf slurry feed) and Cold Lake vacuum bottoms from Alberta in a nominal 1 kg/h bench-scale unit. Gross distillate characteristics as determined by elemental and 1H n.m.r. analyses indicate that as coal concentration is increased, more coal-derived liquids contribute to the distillate. For detailed analysis, distillates from 3.7 and 23.9 wt% coal concentration were separated into five different fractions using a Polar Amino Cyano (PAC) column. Analysis of the fractions indicated that increasing coal concentration from 3.7 to 23.9 wt% results in an increase in the amount of polar materials and a subsequent decrease in the amount of saturated compounds. The wt% of monoaromatics and polyaromatics was not affected by the amount of coal present in the slurry feed. The number and weight average molecular weights of the distillates boiling between 205 °C–525 °C and their hydrocarbon-type fractions decreased as the coal concentration increased. Each hydrocarbon-type fraction was analysed by 1H n.m.r. and f.i.m.s. to determine the effect of coal concentration on compound-type distribution. The results show that increasing coal concentration has a significant effect on compound-type distribution and, in part, coal may enhance the upgrading of bitumen. Based on preliminary isotopic mass balance measurements that take advantage of the difference between the 13 C 12 C ratio of coal and bitumen, the amount of coal-derived carbon in the coprocessing distillate was estimated.

Laser pyrolysis of an entrained stream of coal particles

Publisher Summary This chapter discusses the laser pyrolysis of entrained stream of coal particles. The simplicity of pyrolysis as a basis for deriving liquid and gaseous fuels from coal is offset by the problem of poor yields and product quality. Efforts to improve yields have been limited by a traditional mechanistic picture of pyrolysis that is, at best, incomplete. To contribute to an improved understanding of the chemical and physical processes that control the formation of volatiles during pyrolysis, an apparatus has been developed for pyrolyzing an entrained flow of coal particles with a CW infrared laser. The advantages of this pyrolysis mode are that it provides a very rapid, in-depth heating to a steady-state temperature determined by the balance between radiative input to the particle and the sum of convective and radiative heat losses and secondary reactions are minimized by the cold-gas atmosphere, which rapidly quenches the initially evolved volatiles. The observation indicating substantial nonvapor tar transport comes from the appearance of the tar collected during pyrolysis of coals into which small amounts of hydrogenated coal tar have been loaded.

STUDIES OF RETROGRESSIVE REACTIONS IN DIRECT LIQUEFACTION

Publisher Summary This chapter discusses a study to examine the importance of key factors, such as oxygen functional group content, water content, and cation content, that are important in retrograde reactions that occur under pyrolysis conditions. Three coals from the Argonne premium sample bank were used. The results from various techniques suggested that nearly all the hydroxyls and carboxyls were methylated for each coal. The results of the X-ray analysis of the exchanged, demineralized, and methylated samples confirmed that the concentrations of exchangeable metal cations (Ca, Mg, Na, and K) were significantly reduced. The results of programmed pyrolysis in the TG-FTIR apparatus of the raw, ion-exchanged, demineralized, and methylated coals indicated significant increases in the yield of tar for the latter two modifications in the case of the two low rank coals. After the samples were characterized, experiments were done under coal liquefaction conditions in a microautoclave reactor, which allows for gas analysis. The extent of retrogressive reactions in pyrolysis and liquefaction for low rank coals (lignite, subbituminous) is significantly reduced by methylation and demineralization.