The invention described herein generally pertains to coupling microwave energy into a tuned multi-section WR-975 waveguide assembly, connected to a waveguide containing a ceramic cylindrical applicator, terminated in a dummy load, for pre-heating fuels to improve vaporization, combustion efficiency, and soot reduction in combustion chamber(s). Designed primarily for processing liquid byproducts produced during biodiesel fuel manufacturing, this invention establishes a charge density in a cross-coupled applicator, according to the applicator's volume, dielectric characteristics of the materials being processed, applied frequency, and voltage. This invention may also be used for preheating or polarization of solids within a pipe-conveyed slurry, such as biosolids, bituminous coal, crude oil sludge, paper pulp or shale oil rock. Finally, this invention may serve as an integral part of a coal gasification process, employing both microwave drying and reduction methods, to produce syngas, hydrogen, and/or liquid fuels.