Transactions

482 TRANSACTIONS OF TH E A.S.M.E. AUGUST, 1941
torts at points inside the envelope of semicoke. It contains
small concentrations of carbon dioxide, corresponding to that
at levels of V* to 1 in. above the tuyeres, and increasing concentrations
of hydrogen and methane as the coking zone is approached.
At yet higher levels in the retort, the carbon dioxide
and monoxide also increase, while the oxygen decreases, until
the skin of coke at the top of the retort is penetrated, when, in
many cases the oxygen concentration again increases while the
combustible constituents tend to disappear. This indicates that
there may be a layer of comparatively cold stagnant air lying on
top of the retorts, which is only slowly aspirated into the main
gas stream rising from the burning lanes. Additional evidence
of the existence of this phenomenon will be apparent later.
Some of the air, carrying products of combustion from the
retort, may pass into the burning lane at levels below the top of
the retort through shrinkage cracks in the coke and semicoke
walls. Such shrinkage cracks are likely to be much deeper and
more open in these walls than they are in coke formed in a byproduct-oven,
because of the higher temperatures to which the
walls are exposed, and the agitating action of the feeding mechanism.
Thus, there may be quite free passage, at some levels,
from the retorts out to the open space in the top of the burning
lane. The entrance of this gas mixture into the burning lane is
marked by the appearance of an increase in the concentration of
the constituents hydrogen and methane in the gas, and a simultaneous
increase in the concentration of oxygen. The combustible
gases, both those brought by the air from the retort, and
the carbon monoxide from the gasification that took place in the
lower parts of the burning lane where a continuous fuel bed
existed, combine rapidly with the available oxygen. The resulting
gas flame, in the protection of the burning-lane walls, results
in the highest temperatures observed in the bed, observations of
3000 F and even higher being not uncommon. In general, even
with the air entering from the retort, there is insufficient oxygen
completely to burn out the combustible gases issuing from the
top of the burning lane. But, as the gases leave the lane, the
oxygen concentration again increases, as shown in Fig. 7 for
levels 9 to 11 in. above the tuyeres. This also supports the conclusion
that there is a rather stagnant layer of relatively cool
unconsumed air above the retorts which is slowly aspirated into the
jets rising from the burning lanes.
Visual evidence of the existence of this stagnant zone above
the retorts appeared during tests of high-volatile coal in the
work (14) referred to. During some of the preliminary runs with
this coal, the boiler was brought off bank to a comparatively
high load rather rapidly, so that about 50 per cent of the maximum
rating (75 per cent of the test rating) was obtained before
the furnace walls reached their normal temperature. At this
time, clouds of smoke came out of the top of the retorts, but did
not ignite as they did later on when the walls were hot. These
clouds did not flow rapidly; they appeared to ooze out, and to
roll slowly downward along the top of the retort. The ends
gradually approached the stream rising from the burning lane,
into which they were sucked and dispersed. Thus, the flow above
the retorts must have been very slow; it had a negligible upward
component, but was mainly along the top of the retort, lengthwise
of the stoker. Evidently, the gas velocity distribution in
the vertical direction shows marked discontinuities in the furnace
just above the fuel bed; high values exist in the jets rising
from the burning lanes, with relatively small upward velocities
in the spaces between. A vector plot of gas velocities at the level
of the top of the bed would look like a comb, with teeth pointing
upward above each burning lane. The jets spread, as has been
shown for other types of jet, with increasing distance from the
level of the bed, and gradually entrain the gases from the space
above the retorts. If cold secondary air is admitted to the furnace
at low velocity, it will fall down on the retorts because of its
high density and blanket them, only slowly being drawn into the
main stream of combustion gases where it is needed. Thus, the
need for very high pressures in the secondary-air jets is demonstrated,
for only if sufficient velocity is supplied will the jets carry
through the furnace and produce adequate mixing of the highvelocity
streams from the burning lanes and the relatively stagnant
gas above the retorts.
In its passage through the fuel bed at the bottom of the burning
lane, the air and combustion gas obey laws similar to those observed
for flow of cold gas through beds of broken solids (15, 16).
The pressure drop of the gas is proportional to the height of the
bed traversed, i.e., the pressure gradient is practically constant
over a considerable portion of the height of the bed. This is the
normal behavior of a stream of fluid passing through a uniformly
packed passage of constant cross section. Moreover, the pressure
gradient is proportional to a power of the rate of gas flow slightly
greater than 2. While the exponent observed in the Hell Gate
tests 2.1 is somewhat greater than has been observed in closely
controlled experiments at normal temperatures, this discrepancy
may be accounted for by the relatively few data from which it
was determined, by probable inaccuracies in the data inseparable
from plant-scale testing, by a possible temperature effect on turbulent
flow through such beds not now recognized because of the
lack of data for such high-temperature zones, or, by an effect due
to the expansion and contraction of the gas caused by the violent
changes in temperature during its passage through the burning
zone.
Since the gas flow appears to obey the laws previously found by
investigations on cool fluids not undergoing reaction, it is probably
permissible to turn to these investigations for other information
concerning flow phenomena in fuel beds. These experiments
show that for small flow velocities, a bed of broken solids acts
like a pipe, tube, or other resistance to flow. For very small
flows, the streaming is viscous, and the resistance to flow increases
as the first power of the mass velocity. Beyond a rather
well-defined critical point, flow enters the turbulent regime, when
the resistance increases with a power of the velocity (15) somewhat
less than 2., This continues up to the point when the
pressure gradient through the bed approaches in value the bulk
density of the bed reduced to the same element of volume. That
is, for a bed of coke, whose bulk density may be 36 lb per cu ft,
the limiting pressure gradient will be ——
36
= 0.0209 psi per in. of
1728
36
bed depth, or expressed differently X 12 = 7 in. of water
column per ft of bed depth.
When this critical flow is reached, the bed is, on the average,
supported by a force equal to its own weight, so that it becomes
loose, almost fluid, and the interlocking of neighboring pieces,
which gave it its character as a pure resistance, is reduced (16).
If the air-flow rate is increased slightly beyond this point, the
bed maintains its general form, but becomes capable of great extension
in the direction of flow; any slight disturbance, however,
or a slight further increase in flow is sufficient to disrupt it entirely
so that the bed is completely blown away. The last phenomenon
is, of course, known to every fuel engineer; it has not,
however, been generally recognized that the point at which it
occurs is so definitely fixed by the fundamental properties of the
bed as an interlocking aggregate of particles.
Carman (15) has shown that the resistance to flow through a
bed of broken solids can be estimated if the void volume and the
specific surface of the bed are known. These properties can be
measured (17, 18) or estimated from the size analysis (19).
Fig. 8 shows the resistance to air flow in the turbulent regime for