There is another consideration that is often overlooked in microwave processing of materials. Microwaves are generated by devices requiring electrical energy. Electrical energy is generated primarily from fossil fuels. The conversion of the energy in the fuel to electrical energy is less than 40 percent efficient. In addition, microwave generators (magnetrons, etc.) are not generally better than 85 percent efficient in converting electric power to microwaves, and the microwaves are not perfectly coupled to the material (90 percent coupling would be very good), so the total energy generated is probably less than 30 percent of the energy content of the fossil fuel used in generating the electricity. This means there are real limitations to the economics of bulk heating. Direct heating with fossil fuels makes much more efficient use of energy, and microwaves can only be economically competitive when electric heating is mandated or the selective heating capability of microwaves, or some other factor, more than compensates for the inefficiency of electric heating. An example, discussed later in this report, is the removal of volatiles from soil, where it is not necessary to heat the soil as is required when heating by conventional means.
The successful use of microwaves requires the processor to have a good understanding of the strengths and limitations of microwaves. Among the strengths are penetrating radiation, controllable electric field distributions, rapid heating, selective heating, and self-limiting reactions. But, simply putting a material into the microwave oven and "zapping" it in the hopes of solving a problem is risky. The materials processor must understand and match the special capabilities of microwave processing to the material and the properties required in order to design an appropriate process. In some cases, incomplete understanding exists, requiring research to improve the knowledge base for using microwaves to process materials.
The information contained in this report is intended to ease the work of those interested in using microwaves to solve a problem or improve a current process. Examples of successful applications are given to illustrate the characteristics of a material and process that are amenable to microwave processing. An equipment section describes the alternatives available and the "setup" required to apply the microwaves to the material. Economic considerations are described, and where possible, costs are provided as an aid in determining the economic consequences of using microwaves. A theory section is provided to help both the materials processor and the equipment designer understand the fundamental limitations and advantages of microwaves in the processing of materials. In addition, the limitations in present understanding are delineated as a caution to users and as a guide for future research activities.