The rotary kiln is more flexible and provides increased heat transfer. The kiln can be heated indirectly, or the heat can be furnished by partially burning the pyrolysis products. The gas flow can be either parallel or countercurrent to the waste flow. The gas and liquid products do not have to escape through thick layers of char as in the batch retort; therefore, fewer complex solid-gas reactions occur. The heat cycle is much faster than in the retort, and the gas yield is higher and the liquid yield lower. The size of the indirectly fired kiln, because of the high temperatures involved and the need to transfer energy through the walls, is severely limited. The maximum capacity is in the range of 2 tn per hour for wood waste and is similar for solid waste.
If a limited amount of oxygen is used as the energy needed for pyrolysis, refractory lined kilns can be used, and large systems become feasible. If the oxygen and the feed are introduced countercurrently, the oxygen contacts the pyrolysis char first and tends to burn this char to furnish the heat for pyrolysis, which reduces the char yield. If air is introduced parallel to the feed, the oxygen reacts with the raw feed and the pyrolysis gas and gives a lower gas yield.
Rotary equipment, however, is more expensive to build, is more difficult to design with positive seals, and requires more maintenance.
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