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that is also found in coke discharged from by-product coke ovens,
as shown by line 2 in Fig. 10. When, however, the same coal is
coked at about the same temperature in continuous vertical retorts,
in which the coke is severely abraded during formation, the
size distribution of the coke produced is that given as line 3. It
is evident that a larger proportion of fine sizes is produced than
corresponds to impact breakage. A fuel bed formed of the coke
represented by line 3 has a much lower limit of instability than
one formed of that represented by line 2 and would produce a
much larger proportion of fly coke. This is shown in Fig. 8,
where line 1 represents coke 2 and line 2 represents coke 3.
It can be readily seen that the superposition of a large longitudinal
component on the flow of coal in the retort may produce
abrasive action resulting in a yield of coke whose size distribution
is more like that represented by line 3 than that represented by
line 2, thus limiting the rate of air flow through the bed and,
hence, the rating attainable. Therefore, it may be desirable to
separate the function of longitudinal distribution from that of
carbonization and delivery of coke to the burning line, if by so
doing, it is possible to produce coke having characteristics more
like that of line 2. Separation of these functions would, in addition,
permit improved control of the fuel feed to different portions
of the bed.
MAYERS—FLOW PROCESSES IN U N D ERFEED STOKERS 485
F l o w o f H e a t
The flow of heat in a stoker is very complex and has so far defied
exact analysis. We may, however, by drawing on the results
of several other simpler processes, gain some insight into what
must take place in the stoker. The principal portion of the heat
released in the burning lanes remains in the gaseous products of
the combustion and flows with them out of the burning lanes into
the furnace and so to the boiler. This portion obviously will be
distributed at its release from the bed in the same way as the gas
flow itself is distributed, and will be transferred from the gases to
cold surfaces according to well-understood laws of radiation- and
convection-heat transfer. Another but very much smaller portion
of the heat released will be transferred by radiation from the
top of the fuel bed directly to cold surfaces. This portion is
probably smaller than is usually estimated, since only a very
limited fraction of the top surface of the bed is at extremely high
temperatures, e.g., those portions represented by the burning
lanes. The other 60 to 85 per cent of the fuel bed, i.e., the area
over the retorts and walls of the burning lane, is at a very much
lower temperature, probably not above 2200 to 2300 F and so
radiates at a very much lower rate. It is well known that this
portion of the fuel bed looks black when observed through a fire
glass.
A third portion of the heat released is used to ignite the incoming
fuel and thus recirculates within the fuel bed itself in the same
way as the heat in preheated air recirculates in the steam-generatiug
unit. Most of this heat is conducted through the coke and
semicoke walls of the burning lane transversely into the green
coal in the retorts. Thus, its direction of travel is directly
opposed to the direction of flow of the fuel. In any steadily burning
fire, this results in setting up a quasi-steady state, in which
the temperatures at any point either do not change or fluctuate
within limits about a constant mean value. Under these conditions
just enough heat flows across the walls of the burning lane
at any point to heat up the fuel flowing out from the retort
toward the burning lane at that same point to a constant temperature.
Taking the retort as a whole, it is evident that at any distance
from the front wall the conditions of heat flow are similar to those
in a by-product coke oven at some stage during the process of
coking the charge. At the head end of the stoker, this stage is
similar to that immediately after the oven is charged. At the tail
end of the stoker, the condition should be, if the stoker is being
properly operated, similar to that just before the oven is pushed.
That is, at the head end of the retort, almost all of the width of
the retort is filled with green coal, with only a very thin skin of
coke and semicoke set up at the walls of the burning lane. As
we progress further from the front wall of the stoker, the coke
wall increases in thickness, just as in the coke oven it does at
later times during the coking period, until finally, at the tail
end, the plastic layers produced in the coking process have met at
the center of the retort, just as they do at the end of the coking
period in a by-product coke oven.
The high rates of ignition, by comparison with those found
in pure underfeed burning, observed in multiple-retort stokers
can be understood in the light of this picture. In the first place,
the ignition surface is not a plane parallel to the plane of the
stoker, but the sum of all the nearly vertical surfaces which
separate the semicoke walls of the burning lanes from green
coal. Thus, the ignition surface may be greater than was previously
thought. In the second place, there is no flow of air
through the ignition zone, which was previously shown to return
conducted heat to the high-temperature region in pure underfeed
burning, so that all of the heat conducted into coke and green
coal is available for coking and igniting it.
The similarity between the stoker retort and the by-product
coke oven provides a means for calculating the rate of coking in a
retort and so of controlling it, for a great deal of research has
been done on this problem in connection with by-product coke
ovens and the results of such research are immediately applicable
here. It has long been known that the carbonizing time in coke
ovens, other factors being held constant, varies as a power of the
oven width greater than 1. Since the volume of coke produced
is directly proportional to the oven width, it follows that an increase
in output can be obtained by the use of narrower ovens, a
fact which is made use of in modern construction. If simple heat
conduction were responsible for the coking process, the coking
time would vary as the square of the oven width, but the correlation
by H. H. Lowry, director of the Coal Research Laboratory,
of data on carbonization in experimental retorts (22), indicates
that the exponent 1.6 more nearly represents the dependence of
coking time on oven width. The same exponent appears to apply,
as well, to many different types of commercial ovens.
Thus, we are justified in saying that the average rate of coking
in an oven, hence in a retort, varies inversely as the 1.6 power of
the retort width, so that the rate of coking per retort varies as the
inverse 0.6 power of the width. It is evident that, in order to
secure higher rates of coking with other conditions constant, it
is necessary to decrease the width of the oven or retort. By the
use of this principle, a formula can be derived expressing the
correct proportioning of retort and tuyere widths for any desired
rates of burning.
A H e a v y - D u t y S t o k e r
On the basis of the descriptions of flow processes in the stoker
which have been presented, we may state the features that would
be possessed by a stoker designed for much higher duty than any
now in existence. In the first place, such a stoker must have
narrower retorts than appear in existing stokers. This will make
possible the preparation and coking of coal at a much higher
rate than existing stokers can now perform. If the retort width
were reduced to 25 per cent of that in conventional stokers, coal
could be coked in each retort at nearly 2.3 times the rate it is now
done. This would permit the maintenance of deeper fuel beds
over tuyeres of the same width as now are used, thus raising the
ratio of active surface to the total area of the stoker and permitting
the average burning rate to approach more closely the
burning rate referred to the air-admission surface.