Why do coal-fired furnaces need to be larger than gas or oil-fired furnaces?

Coal-fired furnaces need to be larger than gas or oil-fired furnaces primarily due to the slower combustion process associated with solid fuel. When burning coal, the combustion reaction occurs more slowly compared to the more efficient burning of gas or oil, which are vaporized and mixed with air in a more uniform manner. This slower burning requires additional space within the furnace to accommodate the longer residence time necessary for complete combustion of the solid fuel.

Furthermore, solid fuels like coal have a lower surface area compared to gases or liquid fuels, leading to a need for increased furnace volume to ensure that the coal particles are adequately heated and combusted uniformly. This design consideration ensures that the furnace operates efficiently and effectively, preventing incomplete combustion and excessive buildup of unburned fuel.

While other factors like ash content and ventilation might influence design to some extent, the primary reason for the larger furnace size is fundamentally tied to the characteristics of solid fuel combustion.