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EARLY PRODUCTS OF PYROLYSIS OF WOOD
William F. DeGroot. Wei-Ping Pan, M. Dalilur Rahman, Geoffrey N. Richards Wood Chemistry Laboratory. University of Montana, Missoula. Montana 59812 USA
We are studying the f i r s t stages in pyrolysis of wood and other cellulosic materials as part of a study of the chemistry of smoldering or low-temperature combustion. In the 1atter processes both oxidative and non-oxidative pyrolysis occur. This paper describes an investigation of the first volatile products of pyrolysis of wood and we believe that many of the conclusions regarding the first pyrolysis reactions at low temperatures are applicable also t o higher temperature treatments of wood such as are utilized i n liquifaction processes. Our aim was primarily to determine the relative rates and modes of pyrolysis of the three major constituents of wood (viz. cellulose, hemicelluloses and lignin) from their chemically intact situation in whole wood. We believe that interactions of these substances and of t h e i r degradation products during pyrolysis make t h i s approach necessary and may invalidate some aspects of earlier studies of pyrolysis of isolated wood fractions.
Our major technique has been the coupling of a thermogravimetric analysis system (TG) with a Fourier transform infrared spectrometer. The coupling i s by a heated 1 m. teflon tube from the TG to an infrared gas cell. All of the results were obtained with cottonwood sapwood (Populus trichocarpa) which had been ground to pass an 80 mesh sieve. A l l pyrolyses were carried out i n flowing nitrogen, the time lag between the thermal balance and the gas cell was 1 min. and heating was either isothermal at 250°C or at J"/min. from 100" to 500'. In the isothermal runs for periods up to 2 hrs. the whole infrared spectrum could be accounted for by the absorption bands o f the 6 components shown i n Table 1. The system was therefore calibrated for these compounds by heating the pure liquid in a loosely covered pan on the thermal balance at an appropriate temperature and relating the rate of weight loss to the infrared spectrum. Where a wavenumber range i s shown i n Table 1. the absorbance was integrated between these values and where a single wavenumber i s given, the height of that Q branch was used. These values bore a linear relationship t o the rate of weight loss for an individual compound, and the resultant calculated response factors were used to calculate the rates of production of each product from heated wood samples. For carbon dioxide the calibration was based on air versus a pure nitrogen blank and the carbon monoxide calibration was then deduced from a standard mixture of the two oxides i n nitrogen.
The effects o f cations on the pyrolyses were investigated w i t h wood which had been washed with acid to remove all metal ions and with samples in which the indigenous cations had been replaced entirely with either potassium or calcium ions. These i o n exchange processes caused no other chemical change i n the wood. E.g. the content of L-arabinofuranose units (which comprise one of the most acid-labile groups i n wocd) was unchanged.
Total absolute glycose contents o f the wood samples were determined by acid hydrolysis. reduction, acetylation and gas chromatography using i-inositol as i n t e r n a l standard (1). Uronic acids were determined on a aliquot portion o f the hydrolyzates (2) and v a n i l l i n and syringaldehyde from l i g n i n were generated by nitrobenzene oxidation (3) and determined by gas chromatography of trimethylsilyl ethers. The glucan components of the heated wood samples were very resistant to hydrolysis with 72% sulfuric acid and it was necessary to "reactivate" with water before hydrolysis.
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