Desmond Radlein

On the mechanism of the rapid pyrolysis of cellulose

Abstract Experimental yields and liquid product analyses obtained from the fast pyrolysis of biomass and cellulose in a fluidized bed reactor have been used to demonstrate that the decomposition of cellulose to liquid products proceeds to a major degree by a single mechanism. At temperatures over about 450°C and at vapor residence times of one second or less, it is proposed that the monomer unit of cellulose decomposes preferentially to a two-carbon and a four-carbon fragment, with the two-carbon fragment rearranging to give a yield of hydroxyacetaldehyde (glycolaldehyde) which is 75% or more of that theoretically probable. The most likely route for decomposition or rearrangement of the four-carbon moiety is suggested. The formation of carbonyl or hydroxycarbonyl compounds with two to four carbon atoms is favored. Experimental results from the fast pyrolysis of poplar wood indicate that the cellulose decomposition in wood follows the same path as that of pure cellulose in the production of liquid products.

Pretreatment of wood and cellulose for production of sugars by fast pyrolysis

Abstract The liquid products (tars or sirups) obtained from the fast pyrolysis of cellulosics show a wide variation in composition depending on the cellulosic feedstock used. Native cellulose in wood gives significant yields of hydroxyacetaldehyde and other low molecular weight oxygenated compounds but low yields of anhydrosugars, while highly altered microcrystalline cellulose gives the reverse. Experimental results from fluidized bed fast pyrolysis are given for poplar wood and for a number of types of cellulose produced by different processes. It is shown that high anhydrosugar yields can be obtained by fast pyrolysis at atmospheric pressure. The effect on product nature and yields as a result of different pretreatments of the wood or cellulose before pyrolysis is also reported. From these observations, as well as from the variation of product yields with temperature for one cellulose product, possible mechanisms for the primary decomposition of cellulose are proposed. Two major parallel pathways appear to account for the yields of major products. The content and nature of inorganic salts and the degree of polymerization of the cellulose play an important role in determining the relative importance of these two decomposition pathways, at least in the medium temperature range, 400 to 550°C.

Fast pyrolysis of natural polysaccharides as a potential industrial process

Abstract Existing technology for fast pyrolysis processes will be reviewed briefly. It will be shown that in most cases, provided certain criteria of heating rate and residence time are met, similar products will be obtained. The emphasis will be on pyrolysis in the regime of moderate temperatures and relatively short residence times which leads to high liquid yields. Effects due to the source and pretreatment of feedstocks will be outlined. Comparisons will be made with results from the scientific literature. The extent to which it is possible to alter and increase the specificity of product yields will be examined. Pyrolytic mechanisms which have been proposed and which might account for the reported experimental product spectra will be presented and analysed. Finally, based on the presently known chemistry and technology, future alternatives and possibilities will be suggested.

Flash pyrolysis of cellulose for production of anhydro-oligomers

Evidence is presented showing that it should be possible to depolymerize a purified cellulose to produce anhydro-oligosaccharides containing small numbers of glucosidic units if suitable reaction conditions are chosen. Experiments were carried out in a stream of nitrogen in a downflow cocurrent tubular reactor with a high temperature wall. Two sizes of particles of Avicel pH102 were used and two sizes of reactor tube with wall temperatures from 850 to 1200°C and residence times from 35 to 75 ms. From the solid residue of the reaction, a water soluble fraction was recovered which contained anhydrosugars. Yields of soluble solids of up to 44% of the cellulose fed were obtained in a small diameter tube, and up to 34% in a larger diameter reactor. Only a narrow window of reaction conditions exists which is optimal for maximum yields. Carbohydrate analyses showed significant yields of anhydro-oligosaccharides from G2 to G7, the limit of resolution for the HPLC column used. Normally, these compounds made up ∼50% of the soluble solids fraction. Substantial amounts of material >G7 were also produced but could not be identified. Tests were also done with pretreated Avicel, but only one method gave improved results (slow preheating) Given the probable high unit value of anhydro-ologosaccharides, it would appear that this method of flash pyrolysis may be worth further exploitation.

Liquid Products from the Fast Pyrolysis of Wood and Cellulose

The liquid products (tars or sirups) obtained from the fast pyrolysis of cellulosics show a wide variation in composition depending on the cellulosic feedstock used. Native cellulose in wood gives significant yields of hydroxyacetaldehyde and other low molecular weight oxygenated compounds but low yields of anhydrosugars, while highly altered micro-crystalline cellulose gives the reverse. Experimental results from fluidzed bed fast pyrolysis are given for poplar wood and for a number of types of cellulose produced by different processes. The effect on product nature and yields as a result of different pretreatments of the wood or cellulose before pyrolysis is also reported. From these observations, as well as from the variation of product yields with temperature for one cellulose product, possible mechanisms for the primary decomposition of cellulose are proposed. Two major parallel pathways appear to account for the yields of major products. The content and nature of inorganic salts and the degree of polymerization of the cellulose play an important role in determining the relative importance of these two decomposition pathways.

Thermal Conversion of Cellulose and Hemicellulose in Wood to Sugars

It has been shown in earlier work that good conversions of the cellulose in wood to anhydrosugars can be obtained in fast pyrolysis by suitable pretreatments of the wood. Further results are now given to show the effects of operating conditions in a fluidized bed reactor on sugar yields from poplar wood which had been subjected to a dilute acid prehydrolysis to remove hemicelluloses.

New Catalysts for the Fluid Bed Catalytic Gasification of Biomass

Various kinds of catalysts are being developed and tested for the pyrolytic gasification of biomass using the Waterloo Fast Pyrolysis Process (WFPP) technology.