K. R. Doolan

Products from rapid heating of a brown coal in the temperature range 400–2300 °C

The devolatilisation behaviour of Yallourn brown coal was investigated under rapid heating conditions using two different flash pyrolysers: a fluid-bed reactor giving coal particle heating rates of 104 °Cs−1 with a gas residence time of about 0.5 s and a shock tube which generated heating rates of the order of 107 °Cs−1 and a 1 ms reaction time. Yields of products are reported covering pyrolysis temperatures in the range 400–2300 °C. Hydrocarbon gas yields reached maximum values which were remarkably similar for both reactors although occurring at different temperatures. Carbon oxide production was also similar for both reactors with CO yields reaching 30% wt/wt daf coal. These high yields of CO are very different from those reported for slow heating conditions. It appears that on flash heating, coal decomposition pathways change in a manner which increases CO yields at the expense of H20 and to a lesser extent C02, resulting in the volatilisation of additional carbon from the coal.

Coal flash pyrolysis: secondary cracking of tar vapours in the range 870–2000 K☆

Abstract Tar free from influences of the original coal or char was cracked in two reaction systems, one using tar vapour in tubular quartz reactors at 900–1400 K and reactor residence times of ≈ 0.2 and ≈ 1 s, the other using tar aerosol in a shock tube at 1100–2000 K and residence time ≈ 1 ms. In the latter system the aerosol evaporated rapidly after passage of the shock front and the tar behaved kinetically as a vapour. Yields of light hydrocarbons including C6H6 and CO from tar cracking were determined as a function of temperature. Maximum yields of individual species agreed well between the two reactors but the temperatures of maximum yield depended on residence time. Hydrocarbon yields on a daf coal basis were very similar to those obtained previously by direct flash pyrolysis of the same coal at 870–1270 K. Kinetic analyses of the formation of C2H4, C3H6 and C2H2 gave activation energies of 220–260 kJ mol−1, similar to those for formation of the same hydrocarbons from n-hexadecane. The precursors of the alkenes could be polymethylene groups in the tar. CH4, C6H6 and CO all had low activation energies of formation, reflecting the many different functional groups capable of eliminating these molecules.

Kinetics of pyrolysis of octane in argonhydrogen mixtures

Abstract The pyrolysis kinetics of n -octane dilute in both argon and 50 mol% mixtures of argon and hydrogen have been studied in the temperature range 1000–1400K in a single-pulse shock tube. In the inert gas pyrolyses principal products of reaction are ethylene, methane, and propene. Yields of methane and ethane are increased in pyrolyses in 50 mol% mixtures of argon and hydrogen and the rate of decomposition of octane is approximately an order of magnitude larger than that in inert gas pyrolyses. Methyl radical and hydrogen atom attack on the parent octane determines the octane decomposition kinetics and methyl, and ethyl radical attack on hydrogen have been found to be important in determining product distributions in pyrolyses containing hydrogen. Both overall kinetics and product distributions can be simulated by a reaction model based on a modified Kossiakoff-Rice mechanism involving coupled isomerization and unimolecular decomposition of large alkyl radicals.

Products from the rapid pyrolysis of a brown coal in inert and reducing atmospheres

Abstract Yallourn brown coal and its calcium form have been pyrolysed in three different gas mixtures: 23% methane in argon, 50% hydrogen in argon, and 9.4% toluene in argon, at heating rates of 10 7 Ks −1 and residence times of ≈2 ms. The products have been compared with those obtained in pure argon. The highly reactive molecule, ketene, was detected in the pyrolysate gases from both the ‘raw’ and calcium-form coals when pyrolysed in the above mixtures. Ketene was not detected in the pyrolysate gases, however, when the coals were pyrolysed in argon alone. When pyrolysed in 23% methane/argon the maximum yield of carbon monoxide from the ‘raw’ coal was found to be much lower than when pyrolysed in pure argon. The fully exchanged calcium-form coal when pyrolysed in pure argon, gave a maximum yield of carbon dioxide of oxygen content equal to the total oxygen content of the carboxyl groups originally present in the coal; the C0 2 yield from the ‘raw’ coal was about half that from the calcium-form in pure argon.

Heat transfer to fine coal particles in flash pyrolysis

Abstract Heat transfer to fine bituminous coal particles suspended in argon has been studied by recording infrared emission from the coal as a function of time at wavelengths 3.42 and 2.26 μm when the coal was subjected to very rapid heating by a shock wave. The rate of particle heating was found to fit a Nusselt model of conductive heat transfer. Light extinction studies on coal suspensions showed that particles of diameters ≤5 μm undergo velocity relaxation behind the shock front in less than 30 μs. Allowance for forced convective heat transfer to the coal particles in the velocity relaxation zone was made in the heat transfer analysis but forced convection was found to be unimportant in these studies. The Nusselt heat conduction model has been found to hold for heat transfer to micron-sized coal particles up to 1200K; above this temperature infrared emission from soot produced from products of devolatilization of the coal becomes important.

Kinetics of rapid pyrolysis and hydropyrolysis of a sub-bituminous coal

Coal pyrolysis in both argon and hydrogen atmospheres has been investigated in the temperature range 1100–2300K using a shock tube. Particles of size 7 Ks −1 were achieved giving particle heating times of 5 Ks −1 . Residence times ranged from 0.25–1.3 ms and total pressures from 17–35 atmospheres. C 1 –C 7 hydrocarbons and benzene, toluene and xylene yields were determined for pyrolysisin both argon and hydrogen/inert gas mixtures. Major gaseous products included CH 4 , C 2 H 4 , C 3 H 6 , 1, 3-butadiene and benzene. C 2 H 2 only became important at temperatures above 1500K and was attributed to secondary gas phase decomposition of the volatilised species. In the presence of hydrogen the major effect observed was an increase in the yields of CH 4 and C 6 H 6 . C 2 H 2 yields were markedly lower in H 2 . The results were fitted by a first order evolution and secondary decomposition model which included equations to allow for particle heatup and quench. The model enabled kinetic parameters for volatile species evolution and decomposition to be evaluated. The implications of these results to coal pyrolysis are discussed.

Kinetics of rapid pyrolysis of a calcium-exchanged brown coal and of a calcium model compound

The pyrolysis of calcium exchanged Yallourn brown coal has been carried out under rapid heating conditions in a shock tube which generated heating rates of 10 7 K s −1 . The results are compared with the pyrolysis of untreated Yallourn brown coal and with calcium acetate chosen as a model for carboxylate group decomposition. Light hydrocarbon yields from the two coals were the same within experimental error. It is suggested that these products arise from secondary decomposition of tars of similar composition. Compared to the untreated coal, increased CO 2 and decreased CO were found for the calcium-exchanged coal. The primary products of thermal decomposition of calcium acetate were found to be acetone, carbon dioxide, methane and ketene for temperatures in the range 900–1500 K. The overall rate of decomposition and primary product yields were fitted by means of a detailed model consisting of 16 chemical reactions. For the initial decomposition reactions of the calcium acetate low Arrhenius parameters were found. These parameters were similar to those found for formation of volatile products from the coals when based on a first order kinetic coal decomposition model.