3.6.1 General properties

The crude pyrolysis liquid is dark brown and with low viscosity which approximates to biomass in elemental composition, and is composed of a very complex mixture of oxygenated hydrocarbons with an appreciable proportion of water from both the original moisture and reaction product. Solid char (39, 40, 41124, 125, 126) and dissolved alkali metals from ash (124) may also be present.

The complexity arises from the degradation of lignin, cellulose, hemicellulose and any other organics in the feed material, giving a broad spectrum of phenolic and many other classes of compounds that result from uncontrolled degradation as described above. The liquid from fast or flash pyrolysis has significantly different physical and chemical properties compared to the liquid from slow pyrolysis processes which is more like a tar.

The primary liquid product is readily combustible and can be used directly, for example in boilers and kilns, or it may be subjected to further chemical processing to give a higher quality fuel or chemical product. The characteristics of the liquid and descriptions of the technologies for upgrading are summarised in Table 3.4 below.

Changes occur in storage due to moisture content changes which affects viscosity, oxygen absorption or reaction, polymerisation and other innate chemical inter­reactions. The effect manifests as changes, usually increases, in viscosity, and the extent and rate of change depends on the mode of production, particularly residence or contact time, feedstock, and conditions of storage. These effects have not yet been fully evaluated, but are being investigated.