This chapter discusses the main physical laws applied to thermal power plant modeling, control, and improving energy efficiency. Heat transfer including conduction, convection, and radiation occurs in different sections of the furnace. In the center of the furnace, heat transfers to the metal surface of water wall pipes from the flames of pulverized coal burning by radiation. Slagging occurs in the radiant section of a coal boiler with a high temperature. The aerodynamics of the flue gas in the combustion process can convey ash particles to the vicinity of the heat transfer, and the ash particles can pass to the boundary area by inertia. Maintaining an appropriate temperature in the boundary of the furnace and keeping the incoming particles from melting can decrease slagging. Based on these mechanisms of heat transfer, chemical reactions, and slagging, the research proposes a novel way to improve coal-fired power plant boiler efficiency and decrease slagging.