The LSS type methanol (alcohol based fuel) steam boiler is a vertical fast loading internal combustion three return fire tube boiler. The fuel enters the burner from an external gas source and generates high-temperature flames through automatic adjustment and ignition functions. It enters the inner side of the dense discharge water pipe inside the furnace from the top of the furnace, bends 180 ° at the bottom of the furnace to enter the interlayer between the furnace and the dense discharge water pipe, and then bends 180 ° at the lower flue opening of the furnace to enter the rib layer between the cylinder and the packaging layer. After heat transfer between the ribs, the flue gas leaves the boiler through the upper flue opening and is discharged into the atmosphere through the chimney. The boiler is equipped with a burner with excellent technical performance, and adopts advanced technologies such as automatic proportional adjustment of combustion, automatic adjustment of feedwater, program start stop, and fully automatic operation. It also has high and low water level alarms and automatic protection functions such as extremely low water level, ultra-high steam pressure, and shutdown.