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Moisture and ash are always present in wood to some extent, but their affect on its chemical behavior is not fully known. The influence of moisture and ash on the thermal degradation of wood was investigated by pyrolyzing samples of ground wood waste in a batch fluid-bed reactor at between 320 and 470°C in helium at 101-104 kPa. The wood samples were heated at about 300°C/min. so that drying and pyrolysis were simultaneous, Woodex® pellets were used in this study because their density was suitable for fluid-bed tests.

In ash-free samples moisture suppressed the formation of pyrolysis tar at temperatures above 390ºC and increased the yield of char, relative to dry samples. A model for the behavior of free radicals during pyrolysis is proposed which gives qualitative agreement with the observed effect of temperature. The ion-exchange capacity of wood was used to disperse calcium atoms in the polymer matrix, which increased the formation of aqueous product during pyrolysis at the expense of tar by enhancing dehydration and fragmentation reactions. The native mineral components in the wood waste gave effects equivalent to calcium.

The effect of moisture on char yield was independent of the ash components, but the yield of tar from Woodex containing moisture and ash exhibited a minimum at 390°C. Below 390°C the water hydrated catalyst sites to reduce reactivity. A kinetic model for hydration gave qualitative agreement with the observed effects of temperature. Above 390°C the degradation of tar became independent of the availability of catalyst sites, and was suppressed by the effect of water on the concentration of tar within the particles, and on the equilibrium of dehydration reactions.

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