Dehydration products, 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. 

Mechanism. No single mechanism explains the action of all fire retardants, so they probably work through a combination of several mechanisms. The mechanisms of fire retardants in wood involve a complex series of simultaneous reactions whose products may affect subsequent reactions. Pyrolysis of cellulose involves dehydration, depolymerization, decarbonylation, decomposition of smaller compounds, condensation, and other reactions. These pyrolysis reactions occur both in the solid phase and vapor phase. Addition of fire retardants will alter the reactions however, this alteration will depend on the additives, the material, and the thermal-physical environment. The presence of oxygen adds subsequent and competitive oxidation reactions to the above series. These oxidative reactions can take place in both the solid and vapor phases. Evidence indicates that most fire retardants reduce combustible volatiles production and limit combustion to the solid phase. The best retardants also inhibit solid-phase oxidation to effectively remove the fuel from the fire. 

At higher temperatures, the intermediates, including levoglucosan and the condensation products further pyrolyze to give various products by fission of the carbohydrate units and rearrangement of the intermediate products. Table III shows the products obtained from the pyrolysis of cellulose and treated cellulose at 600° (8). The significant increase in the yields of water and char and decrease in the yield of tar in the acid treated cellulose verifies the previously mentioned promotion of dehydration and charring reactions by acidic additives. 

Carbon and graphite fibers are made by the pyrolysis of certain naturally occurring and man-made fibers, such as regenerated cellulose (rayon) fibers. A wide range of physical, mechanical and chemical properties may be obtained dependent on amount of dehydration. This product is one of the most structurally efficient reinforcements. Unlike any other reinforcement, it retains its 2,800 MPa (400,000 psi) tensile strength when tested up to a temperature of 2700 C (4800F). 

An approach to the production of ethylene from biomass that does not involve pyrolysis is ethanol dehydration. The catalytic conversion of syngas to ethanol from low-grade biomass (or fossil) feedstocks, and fermentation ethanol via advanced cellulose hydrolysis and fermentation methods, which make it possible to obtain high yields of ethanol from low-grade biomass feedstocks as well, are both expected to be commercialized in the United States (Chapter 11). Which technology becomes dominant in the market place has... 

It is known that the qualitative and quantitative composition of the thennal degradation products from polysaccharides can be altered by use of different catalysts [1]. Inorganic acids are not well selective catalysts as they affect both the degradation and condensation reactions in the pyrolysis process. A relationship between dehydration, degradation and condensation reactions is determined by the individual properties of the acid, the characteristics of the cellulose structure and by the pyrolysis conditions... 

Pyrolysis of alkali cellulose generates qualitatively the same decomposition products as cellulose, but the decomposition starts at lower temperatures. Also, the amount of each compound in the pyrolysate is modified, the amount of levoglucosan being lower than in pure cellulose (see Section 7.2) because of a higher tendency to form dehydrated cellulose as a first step in pyrolysis. 

The optimum temperature for the vacuum pyrolysis of starch and cellulose lies around 350°. At lower temperatures (—200°), dehydration, particularly in the presence of inorganic salts, and thermo-oxidation accompanied by partial depolymerization and progressive carbonization predominate at temperatures above 500°, ready carbonization and decomposition of the primary pyrolysis-products occur. Low pressures favor the formation of levoglucosan, which should be rapidly removed from the pyrolysis oven by a flow of an inert gas, preferably, superheated steam.171,172,178,179,181,182,209... 

The general pyrolysis mechanisms of polysaccharides have been determined from model studies on cellulose and involve the splitting of the polysaccharide structure by three basic chemical reaction mechanisms dehydration, retroaldolization, and decarboxylation. Using these basic pyrolysis mechanisms, it is possible to explain the pyrolysis of polysaccharides and evolved pyrolysis products. The hexose degradation pathway for cellulose results in formation of furan- and pyran-type fragments and smaller acyclic aldehyde and ketone fragments. ... 

Konkin [50] divides the chemical processes undergone in the transition of cellulose to carbon fiber into four groups heterolytic depolymerization, dehydration, homolytic depolymerization and a more thorough thermal deeomposition. Since the initial pyrolytic reactions are heterolytic, the eourse of pyrolysis and the evolved products can be substantially changed by the addition of acidie or alkaline catalysts [51]. 

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