Size reduction of solid waste and its components is an energy- and maintenance-intensive operation. Energy requirements for size reducing some solid waste fractions in one type of hammermill shredder are indicated in Figure VI-5. In the figure, specific energy (i.e., kWh/Mg processed) is plotted as a function of degree of size reduction. Degree of size reduction (zo) is defined as unity minus the ratio of characteristic product size (xo) to the feedstock characteristic size (fo) [3]. The characteristic particle size is the screen size on a size distribution curve corresponding to 63.2% cumulative weight passing. (Values of zo of zero and unity refer to no size reduction and infinite (though not achievable in practice) size reduction, respectively.) The curves in the figure clearly demonstrate that composition of the material influences the energy requirements for size reduction. For example, screened light fraction (primarily paper and plastic) requires more energy (on a unit basis) to achieve a given degree of size reduction than does mixed waste.
Energy requirements for the size reduction of mixed waste vary depending on the type and design of the size reduction equipment. The requirements for size reducing mixed waste to various product sizes using different types of horizontal hammermills are illustrated in Figure VI-6. The curves shown in the figure are the results of the field testing of a variety of size-reduction devices. Evident from the figure is the fact that the energy requirement increases dramatically if product sizes less than 1 to 2 cm are required. In terms of average gross power requirements for shredding mixed wastes, the requirement is calculated as the product of the specific energy requirement and the average design throughput, with an appropriate assumption to accommodate for the power needed to freewheel the machine. Average gross power requirements are shown as a function of nominal size of shredded product in Figure VI-7.