Steam gasification of coal has been carried out in a 0.3 m-ID × 2.0 m-high circulating fluidized bed with a 0.1 m-ID × 0.9 m-height draft tube. Two reaction zones, coal combustion and gasification, can be made separately in the fluidized bed reactor with a draft tube by introducing air into the draft tube and steam into the annulus. By fluidizing the solid particles in the draft tube zone at the gas velocity of 6 - 12 $U_{mf}$ and that in the annulus zone at 0.9 - 1.1 $U_{mf}$, a gross circulation of bed material as a heat arrier from the combustion zone (draft tube region) into the endothermic gasification zone (annulus region) can be operated. The effects of operating temperature, coal feed rate (5 - 8 kg/hr), air flow rate (6 - 12 $U_{mf}$), and steam flow rate (3.5 - 5.0 kg/hr) on the product gas composition, calorific value of the product gas, gas yield, the carbon efficiency and cold gas efficiency have been determined.
The hydrogen and carbon monoxide, calorific value of the product gas, gas yield, the carbon efficiency and cold gas efficiency decrease with an increase in coal feed rate or steam flow rate. The calorific value of the product gas decrease with an increase in air flow rate due to the production of $CO_2$, whereas gas yield, carbon efficiency and total entrainment rate increase with an increase in air flow rate. With increasing bed temperature, the calorific value of product gas, carbon and cold gas efficiency increase.
By removing the product gas of coal combustion zone and the product gas of coal gasification zone separately, medium BTU gas can be obtained from the present fluidized bed coal gasifier.