Bituminous coal, rapid pyrolysis

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

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). 

Coals were devolatilized at rates comparable with those encountered in combustion and gasification processes. Rapid pyrolysis was attained with pulse-heating equipment developed for this purpose. This technique permitted control of the heating time and the final temperature of the coal samples. Subbituminous A to low volatile bituminous coals were studied. All bituminous coals exhibited devolatilization curves which were characteristically similar, but devolatilization curves of subbituminous A coal differed markedly. The products of devolatilization were gases, condensable material or tar, and residual char. Mass spectrometric analysis showed the gas to consist principally of H2, CHh, and CO. Higher hydrocarbons, up to C6, were present in small quantities. 

The yield of volatile matter in this process is a function of the coal type and ranges from approximately 20% w/w of the coal for a low-volatile bituminous coal to somewhat more than 55% w/w of the coal for a high-volatile C bituminous coal subbituminous coals may not show a volatile matter peak with increasing temperature. In addition to tarry products, the rapid pyrolysis of coal produces gases such as hydrogen, methane, and carbon monoxide as well as lesser amounts of hydrocarbons. Pyrolysis of coal is generally defined as the thermal decomposition of coal in the absence of air or other added substances. 

Because the coal is brought rapidly to reaction temperature and the pyrolysis products are exposed to the full temperature, the Winkler reactor produces no tars or phenols, and very little methane. A typical gas composition from bituminous coal is (dry and sulfur-free) ... 

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