Pyrolysis, flash cellulose

As indicated above, to achieve control of the pyrolysis course in flash pyrolysis, it is necessary for the sample to be reproducibly heated. Ideally, the total decomposition of the sample should occur over the same temperature range. The reason for a precise temperature control is illustrated in an example shown in Figure 4.1.3. This figure gives the weight variation of a sample where the pyrolytic processes may occur following two independent reaction kinetics, both of the first order process (1) with E = 100.7 kJ/mol and A = 9.6 10 sec and process (2) with E = 65 kJ/mol and A = 5.5 10 sec (the kinetics parameters were selected from data indicated for cellulose pyrolysis). 

J. Piskorz, P. Majerski, D. Radlein, A. Vladars-Usas, and D. S. Scott, Flash pyrolysis of cellulose for production of anhydro-oligomers, J. Anal. Appl. Pyrol., 56 (2000) 145-166. 

A report on the continuous flash pyrolysis of biomass at atmospheric pressure to produce liquids indicates that pyrolysis temperatures must be optimized to maximize liquid yields (36). It has been found that a sharp maximum in the liquid yields vs temperature curves exist and that the yields drop off sharply on both sides of this maximum. Pure cellulose has been found to have an optimum temperature for liquids at 500°C, while the wheat straw and wood species tested have optimum temperatures at 600°C and 500°C, respectively. Organic liquid yields were of the order of 65 wt % of the dry biomass fed, but contained relatively large quantities of oiganic acids. 

Flash pyrolysis in FFR is a usefnl means to remove snlphnr from coal [19, 21]. As shown by Li et al. [22], it can also be ntilized to remove heteroatom molecules from biomass. Both the yield and the composition of the resnltant gas depend on the biomass composition. The gas ontpnt is richer in hydrogen in the case of cellulose and hemicel-Inlose than in the case of lignin. Smaller biomass particle sizes and higher fast pyrolysis temperatnres also boost hydrogen content. The total of carbon monoxide and hydrogen content is reported to be 65.4% for legnme straw and 55.7% for apricot stone. 

The yield and properties of the char depend also on the rate of heating. Flash pyrolysis by intense, thermal radiation leaves little char (see Section III,2b, p. 456), in contrast to heating at slowly rising temperatures, which results in carbonization of cellulose. 

The complexity arises from the degradation of lignin, cellulose, hetnicellulose and any other organics in the feed material, giving a broad spectrum of phenolic and many other classes of compounds that result from uncontrolled degradation as described below. The liquid from fast or flash pyrolysis has significantly different physical and chemical properties compared to the liquid from slow pyrolysis processes, which is more like a tar. 

Boutin O., Ferrer M. and Lede J. (1998) Radiant Flash Pyrolysis of Cellulose -Evidence for the Formation of Short Life Time Intermediate Liquid Species. J. 

Lanzetta M, Di Blasi C., Buonanno F. (1997) An Experimental Investigation of Heat Transfer Limitations in the Flash Pyrolysis of Cellulose Ind. Eng. Chem. Res., 36, 542-552. 

Boutin, O. et. at. (1998) Radiant flash pyrolysis of cellulose — evidence for the formation of short life time irtennediate liquid species. Journal of Analytical and applied pyrolysis, Vol.47, 13-31... 

From the results obtained, the studied celluloses could be arranged in the following order with the respect to the yield of levoglucosenonc obtained by catalytic flash pyrolysis ... 

With but one exception, past studies of the flash pyrolysis of cellulose using laboratory equipment have relied on radiation to achieve the high heating rates required. Lincoln (11) used pulsed carbon arcs and xenon flash tubes to achieve the complete volatilization of cellulose by pyrolysis reactions. Berkowitz-Mattuck... 

Lewellen et al. (9) at M.I.T. studied flash pyrolysis of cellulose using a bench scale, electrically heated screen. They found that for heating rates ranging between 400 and 10,000°C/sec in an inert helium atmosphere the cellulose completely vaporized by pyrolysis reactions , leaving no char residue. Only by extended heating of the cellulose at 250°C was the M.I.T. group able to produce some char (2% by weight of the initial cellulose). 

Only limited data is available on the flash pyrolysis of ligno-cellulosic materials. Rensfelt et al. have reported a significant reduction in char yields following the flash pyrolysis of various biomass materials (17). Diebolt has obtained high gas yields from Eco Fuel II by flash pyrolysis (18). These results suggest that flash pyrolysis may be the preferred thermochemical method for obtaining gaseous fuels and chemicals from all biomass materials. 

TABLE VI. Characteristic Times for Solar Flash Pyrolysis of Avicel PH102 Cellulose... 

TABLE VII. A Summary of the Results of Prior Research on the Flash Pyrolysis of Cellulose... 

The flash pyrolysis of starch has not yet been reported. In this technique, a thin film of polymer is heated rapidly (in one second, or less) to about 600", and the resultant, volatile compounds are immediately swept onto the gas-chromatographic column for analysis. The effects of thermal gradients in the sample, the diffusion of products, and secondary reactions are thus minimized. In the field of synthetic polymers, flash pyrolysis provides a convenient and rapid method of analysis, because the chromatogram produced is characteristic of the material. Chromatograms from the flash pyrolysis of cellulose have been described. ... 

Funazukuri, T. A Study of Flash Pyrolysis of Cellulose, Ph.D. Thesis, Dept, of Chemical Engineering, University of Waterloo, Waterloo, Ontario, Canada, 1983. 

Most promising is flash pyrolysis in which fuel particles are heated very rapidly (more than 1,000 °Cs ) and remain in the hot zone for a very short time (in general less than 1 s). After this very short time period, the liquid compounds produced from the solid biomass by decomposing the organic compounds (z.e. lignin, celluloses) have to be removed and cooled rapidly to avoid further decomposition into gases. To date, flash pyrolysis reactors have reaehed laboratory stage development level and the first pilot plants are available. 

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