If the MSW is used in an as-received condition as input to WtE or WtP pro­cesses, it can lead to variable (and even unstable) operating conditions due to variable properties of the feedstock. This, at least, will result in fluctuating product quality. Depending upon the technology used, convenient and stable feeding of waste to the conversion process is important. Refuse-derived fuel (RDF), which is a processed form of MSW, is often used to prepare waste for various WtE and WtP processes. This preparation or pretreatment usually consists of size reduction, screening, sorting, and in some cases, drying or pelletization to improve the handling characteristics and homogeneity of the waste materials. Therefore, for a given technology, a trade-off between the increased cost of producing RDF from MSW and potential cost reductions from system design and operations needs to be found. The main advantages of RDF are higher heating value; homogeneous physical and chemical struc­ture of the feed; easy storage, handling, and transportation; lower pollutant emissions; and reduced excess air requirement during combustion. In addi­tion to pelletization, biomass, in general, can undergo a variety of pretreat­ments. The advantages and disadvantages of these pretreatment methods are described in Table 6.2.

Because waste can contain numerous impurities, which during the con­version process mostly end up either in the gas phase or the solid phase, these impurities can also be removed as a part of waste preparation meth­ods. Once again it is a choice of economics between removing the impuri­ties up front or at the end of the conversion process. Waste can, in general, contain both inorganic and organic impurities. Depending upon the tech­nology used, some CO, HCl, HF, HBr, HI, NOx, SO2, VOCs, PCDD/F, PCBs, and heavy metal compounds (among others) can be formed or remain [7]. Solids from various conversion processes can also contain numerous impurities including ash.