Kinetics of cellulose pyrolysis

The kinetics of cellulose pyrolysis was extensively studied, mainly for practical purposes. As shown in Sections 3.2 and 3.3, cellulose pyrolysis can be approximated either by a reaction of first order (pseudo first order) or by more elaborate models. Assuming a first order reaction for example, the weight variation of a cellulose sample during pyrolysis is given by the relation (see rel. 15, Section 3.2)  

Several analytical applications of cellulose pyrolysis related to plant materials will be discussed in Part 3 of this book. 

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Another model for the kinetics of cellulose pyrolysis assumes the formation of an active cellulose intermediate. Using this assumption, the kinetics of the pyrolysis process can be described similarly to the previous model as follows [9] ... 

Pyrolysis of biomass materials occurs under normal conditions at relatively low temperatures (300 to 500 C), producing volatile matter and char. Very rapid heating causes pyrolytic weight loss to occur at somewhat higher temperatures. In general, the volatile matter content of cellulosic materials approximates 90% of the dry weight of the initial feedstock. Woody materials contain between 70% and 80% volatile matter, and manures contain 60% volatile matter. However, it is known (3) that cellulosic materials can be completely volatilized when subject to very rapid heating (>10,000 C/sec). Several relatively complete reviews of the mechanisms and kinetics of cellulose pyrolysis are available in the literature (4-7). 

Researchers in previous studies generally used lst-order kinetics to describe cellulose pyrolysis, but rarely have they examined 2nd-order kinetics. Thus, discussion of our results for untreated samples will concentrate on lst-order rate constants so that our results can be directly compared with results from prior studies. A true reaction order of cellulose pyrolysis based on TGA data is essentially meaningless, however, since mass loss involves complex competing multiple reactions (2,4,8). In addition, reaction order was calculated on a dimensionless mass value rather than on the correct but uncalculable molar concentration term. 

Brown, A. L., Dayton, D. C., and Daily, J. W., A study of cellulose pyrolysis chemistry and global kinetics at high heating rates. Energy Fuels 2001, 15 (5), 1286-1294. 

A. E. Lipska and W. J. Parker, Kinetics of the Pyrolysis of Cellulose over the Temperature Range 250—300°C., USNRDL-TR-928, U. S. Naval Radiological Defense Laboratory, San Francisco, Calif., Nov. 1965. 

Considerable experimental work has been done with cellulose to clarify the kinetics of biomass pyrolysis. Most kinetic studies on cellulose pyrolysis have... 

Reynolds J.G. and Burnham A.K. (1997) Pyrolysis decomposition kinetics of cellulose-based materials by constant heating rate micropyrolysis. Energy Fuels, 11,88-97. 

Antal M.J. and Vdriiegyi G. (1997) Impact of systematic errors on the determination of cellulose pyrolysis kinetics. Energy Fuels, 11, 1309-10. 

VOlker S., Rieckmann Th. and Klose W. (2000) Thermokinetic investigation of cellulose pyrolysis - impact of fmal mass on kinetic results. Pyrolysis 2000,... 

GrPnli M., Antal M.J and Vdrhcgyi G. (1999) A round-robin study of cellulose pyrolysis kinetics by thermogravimetry. Ind. Eng. Chem. Res., 38,2238-2244... 

On the bases of the present work the heats of vaporization of the low volatility fractions fall somewhere between 110 and 140 kJ/mol. It should he enphasized here that Milosavljevic and Suuberg [1995] suggested diat primary tar forming reactions were nearly thermo-neutral in the case of cellulose pyrolysis. The results of our coworkers from the Combustion Research Laboratory i icate that the pyrolysis of other biomasses is less endothermic than cellulose. Therefore, incoipoiation of heats of vaporization into the global activation energy should be relevant for an accurate assessment of pyrolysis kinetics. 

Antal, M. J. Friedman, H. L. Rogers, F. E. Kinetic Rates of Cellulose Pyrolysis in Nitrogen and Steam , to appear in Combustion Science and Technology, 1979. 

Brown, A., D. Dayton, and J. Daily. 2001. A Study of Cellulose Pyrolysis Chemistry and Global Kinetics at High Heating Rates. ENERGY AND FUELS. 15(5) 1286-1294. 

Thermogravimetric analysis (TGA) measures cellulose pyrolytic mass loss rates and activation parameters. The technique is relatively simple, straightforward and fast, but it does have disadvantages. One disadvantage is that determination of the kinetic rate constants from TGA data is dependent on the interpretation/analysis technique used. Another disadvantage of TGA is that the rate of mass loss is probably not equivalent to the cellulose pyrolysis rate. 

Thermogravimetric analysis (TGA) has often been used to determine pyrolysis rates and activation energies (Ea). The technique is relatively fast, simple and convenient, and many experimental variables can be quickly examined. However for cellulose, as with most polymers, the kinetics of mass loss can be extremely complex (8 ) and isothermal experiments are often needed to separate and identify temperature effects (9. Also, the rate of mass loss should not be assumed to be related to the pyrolysis kinetic rate ( 6 ) since multiple competing reactions which result in different mass losses occur. Finally, kinetic rate values obtained from TGA can be dependent on the technique used to analyze the data. 

Cellulose pyrolysis kinetics, as measured by isothermal TGA mass loss, were statistically best fit using 1st- or 2nd-order for the untreated (control) samples and 2nd-order for the cellulose samples treated with three additives. Activation parameters obtained from the TGA data of the untreated samples suggest that the reaction mechanism proceeded through an ordered transition state. Sample crystallinity affected the rate constants, activation parameters, and char yields of the untreated cellulose samples. Various additives had different effects on the mass loss. For example, phosphoric acid and aluminum chloride probably increased the rate of dehydration, while boric acid may have inhibited levoglucosan... 

Simmons, G.M., Lee, W.H. Kinetics of gas formation from cellulose and wood pyrolysis, in Fundamentals of Thermochemical Biomass Conversion, Overend, R.P. et al. (Eds.), Elsevier Applied Science, London, pp. 385—395, (1985). 

Several kinetic studies of the pyrolysis of cellulose have been reported [491 95] and, possibly due to the complications arising from impurities, have not led to any agreement on the kinetic order or the mechanism. However, Chatterjee found experimentally [496] and... 

The rate and kinetics of the thermal degradation of cellulosic materials have been investigated under a variety of conditions. However, these studies often relate to one of the physical effects produced by the overall process of heating or pyrolysis, instead of the kinetics of the individual chemical reactions involved. Consequently, the results are controversial and confusing. The variation of the results obtained under different conditions provides a vivid indication of the complexity of the reactions involved and the limited value of the overall kinetic data. 

Golova and Krylova pyrolyzed cotton cellulose and measured the decrease in the D-glucose residues, instead of the weight-loss, as a function of time, and found that the reaction follows a zero order. In contrast, Tang and Neill, on the basis of thermogravimetric data to be discussed later (see p. 446), suggested that the initial state of pyrolysis of cellulose is controlled by pseudo-zero-order kinetics and the final state is of pseudo-first order. 

Application of difiFerential thermal analysis and thermogravimetric analysis techniques to the pyrolysis of cellulose is obviously complicated by the complexity of the reactions involved, and the corrections and simplifying assumptions that are required in calculating the kinetic parameters. Consequently, these methods provide general information, instead of accurate identification and definition of the individual reactions (and their kinetics), which are traditionally conducted under isothermal conditions. The data obtained by dynamic methods are, however, useful for comparing the efiFects of various conditions or treatments on the pyrolysis of cellulose. In this respect, the application of thermal analysis for investigating the effect of salts (and flame retardants in general) on the combustion of cellulosic materials is of special interest and will be discussed later (see p. 467). 

The former variables affect the deposition of heat in the solid fuel and its transient temperature-profile, as well as the diffusion of the volatile pyrolysis products and their distribution and mixing with the surrounding atmosphere. The latter factors influence the nature and sequence of the primary and secondary reactions involved, the composition of the flammable volatiles, and, ultimately, the kinetics of the combustion. Consequently, basic study of the combustion of cellulosic materials or fire research has been channeled in these two directions. 

This mechanism implies that a portion of the charred residue from the pyrolysis of cellulose could be derived from the secondary reactions involved in the decomposition of levoglucosan. In the production of levulinic acid, about 25 % of the initial D-glucose residues is converted into the humin that separates as an insoluble, charred residue. This humin could be formed from 5-(hydroxymethyl)-2-furaldehyde, which is readily polymerized and decomposed on heating under acid conditions. The kinetic investigations indicate that the humin could also be formed from the reaction of 5-(hydroxy-methyl)-2-furaldehyde with one of its precursors. ... 

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