Cellulose volatile products, formation

Essig M, Richards GN, Schenck E. Mechanisms of formation of the major volatile products from the pyrolysis of cellulose. In Schuerch C, editor. Cellulose and wood - chemistry and technology. Syracuse, NY Wiley Sons 1988. p. 841-62. 

Nearly all successful flame-resistant finishes depend upon the application of compounds contuining phosphorus to the cellulose. Not much is known about the combustion of cellulose, but it has been established that it breaks down into a solid carbonaceous char accompanied by the formation of volatile liquids, gases, and tarry substances. Anything which reduces the formation of volatile products of combustion will retard the rate of propagation... 

A great deal of work has been done on the effect of aqueous alkali on cellulose, from the viewpoint of the pulping industry (e.g., 16-28.48-59). The minor organic volatile products observed here at similar temperature to those used in Kraft pulping (150-180°C) eventually lead to colored product formation, which is of concern to the paper industry. The formation of acetone from cellulose has long been known. Generally, most interest has been shown in the nature of the residual cellulose after alkali treatment, not in the nature of the volatiles. From the viewpoint of determining the chemistry of oil formation from cellulose, the intermediate volatile products are all-important and the residual cellulose is of little interest. 

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). 

While there is controversy as to whether or not this Cellulose species exists, experimental evidence for the Cellulose species was obtained by Price et al.,60 who suggested that it could be a free radical in nature. At lower temperatures, oxygen plays a dominant role in cellulose degradation, and pyrolysis is faster in an oxidative atmosphere than in an inert one.61 Oxygen catalyzes the formation of both volatiles and char-promoting reactions.62 At higher temperatures, the degradation products are little affected.61... 

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... 

Gas theories. — These attribute the retardant action to modification of the behavior of the volatiles (from the pyrolysis) by gases evolved from the decomposition of the retardant. Two suggested modes of action are (a) prevention of the formation of inflammable mixtures of air and volatile compounds (derived from the cellulosic material), by dilution with noninflammable gases derived from decomposition of the retardant, and (b) inhibition of free-radical chain-reactions in the flame, by introduction of decomposition products (from the retardant) that act as chain breakers. 

Cracking/Reforming of the Volatile Matter. At somewhat higher temperatures (600°C or more) the volatile matter evolved by the pyrolysis reactions (step 1) reacts in the absence of oxygen to form a hydrocarbon rich synthesis gas. These gas phase reactions happen very rapidly (seconds or less) and can be manipulated to favor the formation of various hydrocarbons (such as ethylene). Rates and products of the cracking reactions for volatile matter derived from cellulose, lignin, and wood are now available in the literature (1, 3, 5, 6). 

Bryce and Greenwood studied the kinetics of formation of the major volatile fraction from potato starch, and its components. They limited their interest to the temperature range from 156 to 337 and to the formation of water, as well as of carbon mon- and di-oxide. The results revealed the following facts. Stability toward pyrolysis within the first 20 minutes of the process falls in the order amylose < starch < amylopectin < cellulose. Autocatalysis is absent, as shown by Puddington. Both carbon mon- and di-oxide are evolved as a consequence of each of two first-order reactions. The initial one is fast, and the second is slow. The reasons are not well understood, but they probably involve some secondary physical effects. The amount of both carbon oxides is a direct function of the quantity of water produced from any polysaccharide, which, furthermore, is independent of the temperature. The activation energy for the production of carbon mon-and di-oxide reaches 161.6 kJ/mol, and is practically independent of the polysaccharide formed. At the limiting rates, the approximate ratios of water carbon dioxide carbon monoxide were found to be 16 4 1 for amylopectin, 13 3 1 for starch, 10 3 1 for amylose, and 16 5 1 for cellulose. 

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