WASTES
“Waste” as a gasification feedstock covers a wide range of materials, both solid and liquid. Some materials, which we include in this section, could as easily have been included under biomass. Other chemical liquid wastes described here could also have been included in Section 4.2. The selection of materials described here is therefore to some extent arbitrary, but it has mainly been guided by self-classification of specific projects and general tradition within the gasification industry.
One of the most difficult aspects of waste as a feedstock, whether for gasification or incineration, is its heterogeneous nature. In this connection the interconnected properties of heating value and moisture content play an important part for both solid and liquid feeds. For solids, particle size is another key parameter. The presence of a number of components in the waste such as sulfur, chlorides, or metals can also vary considerably. The variations can be seasonal or even daily. Even with well-defined industrial wastes, where several waste streams are fed together to the gasifier, differing proportions of the individual streams can have an influence on gasifier performance.
For these reasons feedstock preparation plays an important role in the design of any waste gasification project. Equally important, this wide variation tends to make each project unique.
The long-term supply of the feedstock is another aspect to be considered. For this and other reasons it is often important that the refuse is processed with a support feedstock such as coal or residual oil. Ideally, the economics of the plant should be robust even in the absence of the refuse feedstock.
In the European Union there are about 500 million tons per year of waste that is suitable for thermal treatment. Of this some 6% is classified as hazardous waste, which includes clinical wastes, some wastes derived from agriculture, and chemical industry wastes (Schwager and Whiting 2002).
