Thermal degradation product cellulosics

Early interest in the thermal degradation of cellulose was stimulated by lively academic discussions about the origin of bituminous coal — whether it is derived from the lignin or carbohydrate constituents of wood. This subject was also investigated in an attempt to determine the significance of cellulose in commercial destructive distillation of wood and its contribution to the mixture of degradation products. 

In the presence of acid additives thermal degradation of cellulose is intensified at lower temperatures, due to the occunence of the dehydration reactions [3], Condensation reactions result in the decrease of volatile products at 450"C combined... 

Koll and Metzger (1978) report on the use of supercritical acetone as the reaction medium for the thermal degradation of cellulose and chitin. Since the pyrolysis of these polysaccharides occurs at such high temperatures, it is necessary to remove the primary products from the reaction zone as soon as they are formed to avoid degradation of the products into coke. The high operating temperature also adversely affects both yield and product distribution. It is possible to reduce the carbon formation by carrying out the pyrolysis under vacuum but the reaction rate is also reduced because of the poor heat transfer to the reactants. 

However, few details of the chemical nature of the pyrolytic oils produced from wood or other biomass have been reported. Some recent studies of the composition of pyrolysis oils obtained from poplar wood were carried out by workers of the Pacific Northwest Laboratories of Battelle Institute [7] and the Universite de Sherbrooke [8]. Methods of quantitative determination of functional groups in the pyrolytic oils from wood were tested in our laboratory by Nicolaides [9]. However, more detailed characterisation exists in the literature for the products of the thermal degradation of cellulose [10,11,12,13,14]. 

Table 1. Some products of the thermal degradation of cellulose acetate analysed by TGA/MS/MS...

The self-ignition temperature of PVF film is 390°C. The limiting oxygen index (LOI) for PVF is 22.6% (120), which can be raised to 30% in antimony oxide-modified film (121). HF and a mixture of aromatic and aliphatic hydrocarbons (122) are generated from the thermal degradation of PVF. Toxicity studies, ie survival and time to incapacitation, of polymers, cellulosics (123,124), and airplane interior materials (125) expose mice to pyrolysis products and show PVF thermal degradation products to have relatively low toxicity. 

The list of pyrolysis products of cottonwood shown in Table VII (llj reflects the summation of the pyrolysis products of its three major components. The higher yields of acetone, propenal, methanol, acetic acid, CO, water and char from cottonwood, as compared to those obtained from cellulose and xylan, are likely attributed to lignin pyrolysis. Other results are similar to those obtained from the pyrolysis of cell-wall polysaccharides. This further verifies that there is no significant interaction among the three major components during the thermal degradation of wood. 

One important thermal degradation mechanism of cellulose fibres (cotton, rayon, linen, etc.) is the formation of the small depolymerisation product levoglucosan (Fig. 8.7). Levoglucosan and its volatile pyrolysis products are extremely flammable materials and are the main contributors to cellulose combustion. Compounds that are able to hinder levoglucosan formation are expected to function as flame retardants for cellulose. The crosslinking and the single type of esterification of... 

In wood pyrolysis, it is known that several parameters influence the yield of pyrolytic oil and its composition. Among these parameters, wood composition, heating rate, pressure, moisture content, presence of catalyst, particle size and combined effects of these variables are known to be important. The thermal degradation of wood starts with free water evaporation. This endothermic process takes place at 120 to 150 C, followed by several exothermic reactions at 200 to 250°C, 280 to 320 C, and around 400 C, corresponding to the thermal degradation of hemicelluloses, cellulose, and lignin respectively. In addition to the extractives, the biomass pyrolytic liquid product represents a proportional combination of pyrolysates from cellulose, hemicelluloses. 

Pyrolysis of cellulose at temperatures below 300 0 results mainly in char formation. Any lignin present in the MSW (Kraft paper, cardboard, and wood waste contain significant proportions) tends to char, even at higher temperatures. On the other hand, the cellulose and hemicelluloses readily decompose to volatile products at temperatures above 300 0. Most of the plastics present thermally degrade at a significantly higher temperature (400-450OC) (2). 

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