Pyrolysis cellulose

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. 

In order to fireproof wood and cotton products and to thermally convert biomass into chemicals, researchers must understand cellulose pyrolysis. Extensive research has been conducted in this area and several reviews are available (1-7). 

The objective of this research was to examine the effect of crystallinity, additives and data analysis technique on isothermally pyrolyzed cellulose. The Ea, activation enthalpy (AH+) and activation entropy (AS+) were determined from the mass loss rates. This data was used to develop an understanding of how cellulose pyrolysis is affected by crystallinity and additives and how the results obtained are dependent on the data analysis technique. 

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. 

Table I. Comparison of Cellulose Pyrolysis Rates (Rates of Weight Loss) Rnalyzed by Differential Methods for Control...

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... 

Several compounds were also found to have a seasonal distribution. Kubatova et al. (2002) found that concentrations of lignin and cellulose pyrolysis products from wood burning were higher in aerosol samples collected during low-temperature conditions. On the other hand, concentrations of dicarboxylic acids and related products that are believed to be the oxidation products of hydrocarbons and fatty acids were highest in summer aerosols. PAHs, which are susceptible to atmospheric oxidation, were also more prevalent in winter than in summer. These results suggest that atmospheric oxidation of VOCs into secondary OAs and related oxidative degradation products are key factors in any OA mass closure, source identification, and source apportionment study. However, additional work is much desirable to assess the extent and seasonal variation of these processes. 

Figure 4. Chromatogram of cellulose pyrolysis tar after reduction with NaBHa, unknown b, 1,6-anhydro-p-n-glucopyranose c, 1,6-an-hydro-p-v-glucofuranose d, 3-deoxyhexitols e, d-glucitol f, oligosaccharide derivatives...

The pyrolysis reactions involved in hemicellulose, i.e., xylan, are similar to those involved in cellulose pyrolysis. 

The addition of a Lewis acid, i.e., ZnC significantly decreases the production of tar and enhances the production of char due to the enhanced dehydration reactions. At higher temperatures the glycosyl units and the random condensation products are further degraded to a variety of volatile products, as shown in Table V (9). Comparison of this table with the high temperature pyrolysis products listed for cellulose in Table III shows that the products of both fractions are basically similar. The significant increase in the yields of 2-furaldehyde, water and char and decrease in the yield of tar by the addition of ZnCl verifies the enhanced dehydration and is similar to observed effects in cellulose pyrolysis. 

Besides the cellulose pyrolysis producing levoglucosenone (-)-l, syntheses of both enantiomers of 1 have been reported.1,3 However, these methods use natural carbohydrate precursors and the results are practically less than satisfactory. We, therefore, explored first the synthesis of levoglucosenone 1 without using a naturally occurring starting material. 

Kandola, B., Horrocks, A. R., Price, D., and Coleman, G., Flame retardant treatment of cellulose and their influence on the mechanism of cellulose pyrolysis, Revs. Macromol. Chem. Phys., 1996, C36, 721-794. 

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. 

S. Li et al. Real-time evolved gas analysis by FTIR method an experimental study of cellulose pyrolysis. Fuel, 80, 1809-1817 (2001). 

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

TABLE 13.3 Apparent Activation Energies of Product Gases from Short-Residence-Time Cellulose Pyrolysis at 500 to 675°C ... 

Biomass is a complex polymeric material and its thermal deconqiosition is a multistage complicated process. Many pathways and mechanisms have been proposed to illustrate/cxplain the fundamental steps in pyrolysis (e.g. 5, 6, 7, 8, 9, 10, 11, 12, 13). Broido-Shafizadeh type kinetic models are perhaps the most widely used for cellulose pyrolysis but they can be also applied, at least qualitatively, to the whole biomass (see Figure 1). 

The main purpose of the present paper is to report several new (qualitative and quantitative) results obtained in an original set up, in order to better understand the mechanisms of cellulose pyrolysis. 

The large hydrocarbons and H2 contents of the gases, as well as the absence of CO2, bring another proof of the secondary nature of the gases (formed by thermal cracking reactions). All these results provide evidence for the validity of the Broido Shafizadeh type model for representing the elementary processes of cellulose pyrolysis (Fig. 3). 

Varhegyi G, Jakab M., Antal M.J. (1994) Is the Broido-Shafizadeh Model for Cellulose Pyrolysis True Energy and Fuels, 8, 1345-1352. 

Antal M.J. and Vdrhegyi G. (1995) Cellulose pyrolysis kinetics The current state of knowledge. Ittd. ng. Chem. Res., 34,703-17. 

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