Producer gas is a combustible gas manufactured by blowing a mixture of steam and air upwards through a bed of hot coke, or coal, such that the fuel is completely gasified. The gas obtained from coke consists mainly of a mixture of carbon monoxide and hydrogen with the nitrogen from the blast of air. When coal is used the gas will contain, in addition, tar and the gases liberated during the carbonization of the coal in the fuel bed.
The process is carried out usually by charging the fuel by gravity from a hopper into a vertical, cylindrical, steel chamber, which is either lined with fire bricks, or it has an annular water-jacket from which steam required for gasification can be raised by heat transmitted from the fuel bed. The fuel bed is supported at the bottom by a grate/distributor, through which is introduced the blast, made by adding steam to the air supply such that it is saturated at a temperature of about 50°C.
A layer of ash is maintained at the bottom of the fuel bed; it serves to protect the grate and to distribute and preheat the blast. Immediately above the ash zone is a narrow combustion zone in which oxygen in the air reacts with carbon in the fuel to form carbon dioxide, thereby generating the heat to sustain the subsequent gasification reactions. The hot gases and steam move upwards into the reduction zone where endothermic reactions occur between carbon and carbon dioxide, and between carbon and steam to produce carbon monoxide and hydrogen. At the top of the bed the incoming fuel is dried and carbonized. Most of the sulfur in the coal appears as hydrogen sulfide in the product gas.
The hot gases leaving the producer are quenched and washed with water to remove dust and tar and, if required, purified to remove hydrogen sulfide. A typical producer gas obtained from coke contains 27% carbon monoxide, 12% hydrogen, 0.5% methane, 5% carbon dioxide and 55% nitrogen, by volume. It has a heating value of about 5,000 kJ/m3. When coal is used as fuel the producer gas contains about 3% methane and 0.5% higher hydrocarbons.
Himus, G. W. (1972) The Elements of Fuel Technology, Leonard Hill, London. Gas Making and Natural Gas, BP Trading Ltd.