In considering these methods, we find they can be divided into two
classes:
1. Continuous processes, in which the heat necessary to bring about
the interaction of the carbon and steam is obtained by performing the
operation in retorts externally heated in a furnace; and
2. Intermittent processes, in which carbon is first heated to
incandescence by an air blast, and then, the air blast being cut off,
superheated steam is blown in until the temperature is reduced to a
point at which the carbon begins to fail in its action, when the air
is again admitted to bring the fuel up to the required temperature,
the process consisting of alternate formation of producer gas with
rise of temperature, and of water gas with lowering of the
temperature.
Of the first class of generator, none, as far as I know, have as yet
been practically successful, the nearest approach to this system being
the "Meeze," in which fire clay retorts in an ordinary setting are
employed. In the center of each retort is a pipe leading nearly to the
rear end of the retort, and containing baffle plates. Through this a
jet of superheated steam and hydrocarbon vapor is injected, and the
mixture passes the length of the inner tube, and then back through the
retort itself--which is also fitted with baffle plates--to the front
of the retort, whence the fixed gases escape by the stand pipe to the
hydraulic main, and the rich gas thus formed is used either to enrich
coal gas or is mixed with water gas made in a separate generator.
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