... 4 OEM offered Heating furnace/air furnacebrief introduction:Direct type air heater is by the way of directly ... The fuels can be:1, solid fuel, such as: coal, coke.2, liquid fuel, such as: diesel oil, ... heavy oil.3, gas fuel, such as: coal gas, natural gas, liquid gas. High temperature combustion gas ... a fuel burning device, such as: coal burner, fuel oil burner, coal gas burner and so on. The ...