Destructive Distillation of Wood

the decomposition of wood by heating to a temperature of 450°–550°C in the absence of air. The products of this process are gases (carbon dioxide, carbon monoxide, methane), liquids, and a solid residue —charcoal. The gaseous and liquid products separate out as a mixture of steam and gases, and when the mixture is cooled, a distillate is obtained. The distillate, in turn, separates into pyro-ligneous acid and wood tar. The acid can be treated to yield acetic acid, methanol, and other products. Fractional distillation of the wood tar yields inhibitors (the fraction rich in phenols), used to stabilize the oils and benzines obtained by the cracking process, and flotation oil, used in ore dressing. The charcoal is used in the production of activated charcoal, carbon disulfide, and other substances. On the average, destructive distillation yields approximately 32–38 percent charcoal, 45–50 percent liquid products, and 16.5–18 percent gaseous products (see Table 1).

The size and moisture content of the pieces of wood being distilled have a bearing on the product yield, as do the temperature and rate of the process. Higher temperatures tend to increase the carbon content of the charcoal; the yield of tar and gases also increases with higher temperatures, but the yield of acetic acid and alcohols declines. Increasing the rate of pyrolysis reduces the output of charcoal and acetic acid but increases the output of tar.

Wood can be destructively distilled in a variety of retorts and furnaces, but the greatest productivity is offered by continuous-action vertical steel retorts with internally circulating coolants. Here, the coolants are usually fuel gases evolved during the combustion of noncondensing gases generated by destructive distillation. In retorts of this type, wood is charged in batches at the top and charcoal is withdrawn from the bottom. The capacity of these retorts is 60,000 m³ of compacted wood per year, although models have been developed with twice the capacity.

Approximately 3 million m³ of compacted wood are destructively distilled each year (1970’s) in the USSR.