The coal entering the gas generator must be completely retorted in the retort layer, otherwise there will be more tar in the gas in the lower section. In the heat transfer process in the low-temperature retort layer, three heat transfer modes: radiation, convection and conduction all exist, but the heat transfer is mainly convection and thermal conduction. According to the heat balance, the heat required for complete low-temperature carbonization of coal entering the furnace can be calculated. Depending on the fuel and the moisture content of the fuel, the gas entering the carbonization layer accounts for about 30 to 45%. The higher the moisture content of the fuel, the more heat is required for drying, the longer the carbonization time is, and the greater the amount of gas entering the carbonization layer. Therefore, measures should be taken to control the moisture content of coal entering the furnace.

       The retort layer of a traditional two-stage furnace has both a peripheral refractory brick wall, a vertical ventilation channel in the brick wall, and a longitudinal internal partition wall to divide the retort layer into several retort chambers.φThe 3.2m two-stage furnace has no longitudinal internal partition wall and is equipped with a central tube made of stainless steel. There is a central tube regulator on the upper part to adjust the rising flow rate of the lower section gas. There are six air flow regulators evenly distributed along the circumference below the lower gas outlet to adjust the rising amount of gas at the bottom, which is beneficial to the even distribution of gas and furnace temperature across the entire section. The gas outlet of the lower section is located at the top of the carbonization layer.