MOVING-BED PROCESSES
Historically, moving-bed processes are the oldest processes, and two in particular, the producer gas process and the water gas process, played an important role in the early production of synthesis gas from coal and coke. The preferred feedstocks were in general coke or anthracite, as otherwise the gas needs extensive cleaning to remove tars from the gas. Both processes operate at atmospheric pressure.
Producer Gas
In the producer gas process, humidified air is blown upward through a deep bed of coal or coke. The coal is fed from the top and moves slowly downwards as it is consumed. Ash is drawn off at the bottom of the reactor from a rotating grate. The air reacts with the coal, thereby producing a gas with a lower heating value of 6500 kJ/m3. The presence of about 50% nitrogen in the product gas is the main cause for this low value. The advantage of the process was that it was continuous. The main drawback is that because of its low heating value, producer gas cannot be transported over long distances. When using low-rank feedstocks such as wood, the heating value of the gas can be as low as 3500 kJ/m3. Gas producers continue to be built in small numbers today where gas is required locally for firing industrial furnaces, particularly in locations where coal is the only available energy form as in parts of China (Wellman undated).
Water Gas
The water gas process is a discontinuous process in which steam reacts with red-hot coke to form hydrogen and carbon monoxide. First, the coal or coke bed is heated by blowing air upward through the bed to temperatures of about 1300°C. Then the air flow is stopped and steam is passed through the coal or coke bed, first upward and then downward, thereby producing synthesis gas. The reason for the two different directions of steam flow was to make optimum use of the heat in the bed, which is required do drive the endothermic water gas reaction. When the temperature has dropped to about 900°C, the steam “run” is stopped and the cycle is repeated. After purification the syngas can be used for the synthesis of ammonia or methanol. Before the advent of large-scale air separation plants, the water gas process was the only means for making high-grade synthesis gas for chemical purposes. In order to obtain a continuous gas flow, at least three water gas reactors are required.
