Rapid bituminous coals

In contrast to sporinite, resinite from a Utah high volatile A bituminous coal reacted rapidly and more completely than the corresponding vitrinite. Table V shows the conversion levels achieved for a concentrate containing 75% resinite (mineral-free basis) reacted under relatively mild conditions. The results are curious. A fairly respectable level of conversion is achieved in 15 minutes at 350°C (under which conditions the associated vitrinite would presumably show little conversion), but longer times and a temperature of 370° have little further effect even raising the temperature to 400° does not show a major increase in conversion. 

Studies initiated by the author in CSIRO (13) seek to throw light on the role of the various macerals by studying the conversion, under catalytic hydrogenation conditions, in Tetralin as vehicle, of maceral concentrates from a high volatile bituminous coal. Some preliminary results, given in Fig. 3, show conversions as almost complete for the hand picked vitrain (>90% vitrinite) from a high volatile bituminous coal (Liddell seam N.S.W., 83.6% carbon and 43% volatile matter both expressed on a dry ash-free basis). However, it is evident that the conversion of the whole coal increases rapidly with increase in hydrogen pressure (under otherwise similar conditions - batch autoclave, 4h. 400°C). 

The coal-oil-catalyst paste and an excess of hydrogen (300 to 500 cubic meters per ton of brown coal paste, 500 to 1000 cubic meters per ton of bituminous coal paste) are pumped at 250 to 300 atmospheres pressure through a heat exchanger and a gas-fired preheater into the reactors. The reactants enter the first of three or four converters at about 430° C. (806° F.) and are rapidly heated to 470° to 490° C. (878° to 915° F.) by the exothermic heat of the hydrogenation reaction. The temperature in the reactors is kept in the range 470° to 490° C. (878° to 915° F.) by injection of about 1000 cubic meters of cold hydrogen per ton of coal paste. 

Three methods for determining mineral carbon dioxide in coal were investigated using bituminous coal. The titrimetric method is claimed to be superior to either of the then-used British standard gravimetric or manometric methods (BS 1016). The procedure involves the decomposition of carbonate minerals with hydrochloric acid and absorption of the evolved carbon dioxide in a mixture of benzylamine, ethanol, and dioxan. This mixture forms a stable salt of benzylcar-bamic acid, which is then titrated with sodium methoxide. The method was said to be suitable for all concentrations of carbon dioxide. It is especially accurate for low concentrations, and it is much more rapid than other methods tested. 

Bituminous coal tars tend to be more aromatic (and relatively more thermally stable) than the tars generated from lignites. When heated, bituminous coals soften, become plastic, and swell to varying degrees, whereas lower-rank coals generally do not become plastic. However, at a rapid heating rate (about 10°C/min) or elevated pressure, certain lower-rank coals may melt and demonstrate some plastic and swelling characteristics. 

Anthony et al. [10] studied rapid devolatilization of monolayers of lignite and bituminous coals supported on wire mesh heating elements in helium. They calculated... 

A Comparison of the Rapid Pyrolysis of a Lignite and a Bituminous Coal... 

For the reaction to occur both phenol and a solvated proton must diffuse into the coal to reach an aromatic methylene bond. Phenol is a better solvent for bituminous coals than is pyridine (28) and is expected to interact strongly with coal and probably swell it. The penetration of coal by phenol at its boiling point should be rapid and extensive. However, the proton has little affinity for a hydrocarbon environment, and its diffusion into the hydrophobic coal is not expected to be facile. The way in which the solvation of the proton will affect its diffusion in coals is unknown. As a result of limitations on proton transport, this reaction may be subject to accessibility limitations. 

Flame Volatile and Silicate Sodium in Coal. Sodium is rapidly volatilized in the flame when it occurs in a non-silicate compound form, chiefly associated with chlorine in bituminous coals and combined with organic compounds in the lignite and sub-bituminous fuels. The fraction of sodium combined with coal silicates remains largely involatile in the pulverized fuel flame. 

A large volume of work has been reported on rapid devolatilization of coal (heating rates approximating process conditions (21,22). Recently, the effects of coal minerals on the rapid pyrolysis of a bituminous coal were reported by Franklin, et al ( 23). They found that only the calcium minerals affected the pyrolysis products. Addition of CaCO3 reduced the tar, hydrocarbon gas and liquid yields by 20-30%. The calcium minerals also altered the oxygen release mechanism from the coal. Franklin, et al. attribute these effects to CaCOj reduction to CaO, which acts as a solid base catalyst for a keto-enol isomerization reaction that produces the observed CO and H2O. 

Further experimental data and further model comparisons relate to the rapid pyrolysis of different coals. In the absence of air, this experimental device heats and converts small coal particles (10-200 pm) in gas and distillates. Figure 20 shows a very satisfactory agreement between experimental data relating to a bituminous coal and model results at 1,260 K. It is noteworthy that despite the strong differences between carbon deposit and bituminous coal, the characteristic times for the dehydrogenation processes are practically the same. Further data on this subject, as well as a detailed model for the analysis of the pyrolysis and devolatilization process of coal particles, are available in a recent paper (Migliavacca et al., 2005). 

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