YGL Type of Vertical Spiral Lower-feed Coal-fired Thermal oil Heater. YGL Type of Vertical Spiral Lower-feed Coal-fired Thermal oil Heater• Adoption of upward mechanically feed coal from the bottom is made, which malees coal divided into four layers (i. e. pre-heating layer, oxidation layer, combustion layer and burnout layer) for combustion. The volatile composition may fully be combustible when ...