Rapid heating, devolatilization

The advantage of suspension processes over mass processes is the excellent temperature control that can be obtained through the suspending medium, water. This allows for rapid heat removal and shorter polymerization times. It reduces or eliminates hot spots or heat-kicks characteristic of mass reactors. It also allows the polymerization to be driven very close to completion so that no devolatilization step is normally required. 

Figure 1. Devolatilization of bituminous coals by rapid heating...

As a result of the very rapid heating of the coal, a significant devolatilization takes place in milliseconds (9). This devolatilization step produces various gases including hydrogen and methane. The remainder of the gasification process may be characterized by the carbon-hydrogen... 

Cold solids flow continuously into a fluidized bed where they disperse rapidly enough so that they can be taken as well mixed. They then heat up, they devolatilize slowly, and they leave. Devolatilization releases gaseous A which then decomposes by first-order kinetics as it passes through the bed. When the gas leaves the bed decomposition of gaseous A stops. From the following information determine the fraction of gaseous A which has decomposed. 

Models of wood pyrolysis and combustion have been developed to aid in fire safety and have treated various physical and chemical phenomena (3-9). Several studies have determined volatiles composition from rapid pyrolysis of small particles (10-13) However, few studies have combined modeling of heat transfer effects and detailed experimental resultsOJ). To our knowledge, no study has measured volatiles composition as a function of time from devolatilizing large particles of wood. 

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

Ubhayakar et al. [14] studied rapid devolatilization of pulverized coal in hot combustion gases, varying the input gas temperature between 1525 and 1975°C. They used three particle size distributions for the same type of coal as received, the fraction which remained on a 200 mesh screen and that which passed through the screen. The residence time in the gasifier was 7-70 x 10 s. The tests were conducted at a pressure of 1 atm, heating rates up to 10 °C/s, and volatile product yield up to 68% of the original dry-ash-free coal. 

Chen, J.C., Castagnoli, C., and Niksa, S. Coal devolatilization during rapid transient heating. 2. Secondary pyrolysis. Energy Fuels, 1992, 6, 264. 

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. 

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