The gas is of low to medium heating value, 5-15 MJ/Nm3, depending on the method of pyrolysis. Even after efficient vapour condensation and liquid collection, it still has a relatively high oil content and either needs to be burned hot such as for drying feedstock, or a tertiary clean-up stage may be needed according to how it is used. Physical gas cleaning is difficult due to the complex physical and chemical characteristics of the organics. If water scrubbing is used, this gives a substantial wastewater disposal problem. The most effective utilisation method for the gas is as the fluidising medium if a fluid bed is used and use in-plant for some of the process energy requirement, although its specific energy content is rather low. The gas from high temperature processes can contain a significant proportion of non­equilibrium products such as olefins. These are of potential interest to the petrochemical industry when high temperature flash pyrolysis is employed (9). The yields of any individual or group of constituents is, however, relatively low. Ethylene yields have not exceeded 15% for example which is considered too low to be of economic interest (10). Catalytic conversion of such intermediates is possible by any established petroleum or petrochemical process, although this is not known to have been investigated.

The solid product from pyrolysis is char with any ash or inerts present in the feed.

Carbonisation is pyrolysis operated at low reaction rates and low temperatures to maximise charcoal production. This is established technology in both industrialised and developing countries. High pressure pyrolysis gives higher solid yields, but low pressure (vacuum) pyrolysis behaves analogously to flash pyrolysis in terms of liquids as the primary products are rapidly removed and do not contribute to char formation. In flash pyrolysis the char can be exported or used in — plant for process energy demands.