Flash Pyrolysis and Hydropyrolysis

Pyrolysis is the first step of aU coal conversion processes and its understanding is essential for effective use of coal. The pyrolysis of coal consists of two series of reactions first is the primary decomposition that consists of the formation of radical fragments and their stabilization and the other is the secondary gas phase reaction of gaseous components produced by primary decomposition. The product yield during the pyrolysis depends on coal type and operating conditions of pyrolysis (Howard, 1981a,b Gavalas, 1982 Kawamura et al., 1993). 

The aim of the flash pyrolysis of coal is the production of smaller molecules from it in a shortest possible particle residence time. Therefore, the objective of studying the process chemistry of coal pyrolysis is to investigate the experimental parameters that permit this aim to be achieved and to establish the optimum conditions that produce a favorable product slate. The basic process parameters that influence the prodnct yields dnring flash pyrolysis of coal are (1) reaction temperature, (2) gas pressure, and (3) residence times of coal particles and ensuing tar vapors. In addition to these major process parameters, product yields can be influenced by other factors such as the nature of the pyrolysis gas and its partial pressure and the gas-to-coal ratio. 

Flash pyrolysis in methane, toluene, or methanol atmosphere was proposed but failed to increase the yield of liquid products (Calkins and Bonifaz, 1984 Doolan and Makie, 1985 Hayashi et al., 1996). It is evident that contact at molecular level between hydrogen donor solvents and the coal is essential for increasing the BTX liquid yields (Morgan and Jenkins, 1986 Graff and Brandes, 1987 Kahn, 1989b Kahn et al., 1989 Miura et al., 1991 Miura, 2000). 

Rapid pyrolysis has been briefly mentioned, but in view of the increased importance in this method of the thermal decomposition of coal, more detailed considerations are warranted (Takeuchi and Berkowitz, 1989). The rapid pyrolysis (also often called flash pyrolysis) concept is based on the observation that most coals will release gases, liquids, and tarry materials when heated at certain temperatures, but when the heating is extremely rapid (102°C/s to 106°C/s) even higher proportions of volatile matter can be released (Weller et al., 1950 Kimber and Gray, 1967 Gibbins and Kandiyoti, 1989 Gibbins et al., 1991). 

FIGURE 13.4 Relationship of volatile matter yield to heating rate. (From Eddinger, R.T. et al., Proceedings of the Seventh International Coal Science Conference, Prague, Czech Republic, 1968.) 

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