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BAILEY, ELY—COAL-ASH FUSING TEM PERATU RE IN LIG H T OF R ECEN T FURNACE STUDIES 473
constituents of the ash, such as oxide of iron, are reduced to
oxides with lower melting points, and (6) the A.S.T.M. method
of sampling and analysis represents the temperature at which a
thorough mixture of the whole ash of the coal fuses, the mixing
having taken place before the analysis is made. In actual practice,
neither the entire furnace is in a reducing atmosphere nor is
the entire fuel supply thoroughly mixed upon entering the furnace.
The investigation of Gould and Brunjes10 had for its purpose
the determination of softening temperatures of the ash in physically
separable portions of coal and the determination of the relative
quantities of ash contributed by each portion. To this end
samples of five different coals were pulverized to pass a 200-mesh
screen and then separated into gravity fractions from 1.3 to 1.9.
It is known that reducing coal frees the impurities, this being a
method employed in coal preparation. These separations indicated
that, in the main, the ash in these coals is composed of
highly refractory material (2800 F) and easily fusible material
(under 2300 F). It further showed that these fractions are
not only physically separable, but are separated in the reduction
process. The proportions of easily fusible material varied from
12.6 to 31.4 per cent of the total ash in the coal.
Applying the work of Gould and Brunjes10 to the authors’
studies of slag characteristics, as given in Figs. 1 and 2 of the
paper, one can visualize that the easily fusible material quickly
melts upon entering the furnace, and upon striking a tube or the
furnace floor, in a sticky condition, adheres to it. The refractory
ash, in union with buming-coal particles or alone, passes through
the furnace to deposit itself upon generating or superheater tubes
or other heating surfaces in varying forms of sticky, spongy, or
granular ash, depending upon whether or not it combines with
ash of different fusing temperature. The greater concentration
of iron in the furnaces of Figs. 1 and 2 of the paper may in part
be accounted for by the presence of a reducing atmosphere
therein and also11 to the greater density of the low-fusion-ash
particles, causing them, in accordance with Stokes’ law, to be
more quickly precipitated to the floor. The original form in
which the iron in the ash existed such as FeSj, Fe2S04 is likely
reduced to Fe by combustion of the S2 and in its passage through
the furnace picks up 0 2 to form FeO, Fe2Os, and Fe30 4.
The work of Gould and Brunjes,10 therefore, complements the
authors’ studies and helps to explain the varying forms and constituents
of ash, clinker, and slag found in pulverized-coal-fired
furnaces. In this and previous similar investigations, the authors
have given a clear picture of the results of coal ash in the furnaces
studied, whereas, Messrs. Gould and Brunjes devoted
their studies to causes creating these results. They merit the
thanks of those interested in coal utilization and steam generation.
P. N i c h o l l s 12 a n d W. T. R e i d . 13 This paper14 is timely and
helpful because it again draws attention to our neglect of opportunities
in that we have made little attempt to use the initial and
fluid cone-fusion temperatures, but have optimistically hoped
that all ash behaviors and all troubles could be explained as be-
10 ‘‘Proportions of Free Fusible Material in Coal Ash as an Index
to Clinker and Slag Formation,” by G. B. Gould and H. L. Brunjes,
American Institute of Mining and Metallurgical Engineers, Tech.
Pub. No. 1175, 1940.
11 "Fusion Characteristics of Fractionated Coal Ashes,” by A. H.
Moody and D. D. Langan, Jr., Combustion, vol. 5, Oct., 1933, pp. 13-
17- 12 Supervising Fuel Engineer, Bureau of Mines, Central Experiment
Station, Pittsburgh, Pa. Mem. A.S.M.E.
13 Associate Fuel Engineer, Bureau of Mines, Central Experiment
Station, Pittsburgh, Pa.
14 Discussion of this paper is published by permission of the Director,
Bureau of Mines, United States Department of the Interior.
ing proportional to the softening temperature. That such
optimism was not justified has been proved by many failures of
coordination between the softening temperatures and relative
clinkering or slagging troubles.
In our attempts at coordination there has been equal fault in
failing to analyze the dependence of ash troubles on the conditions
to which that ash has been subjected, or, as we have expressed
it, a consideration of the life history of the ash in terms
of path of travel, temperatures, and time. For a number of
years our reports have urged that correct interpretation of clinkering
and slagging requires much more analysis of conditions than
has been customary.
The latter part of the paper suggests uses that could be made
of the three cone-fusion temperatures in connection with different
types of furnaces or methods of burning; such extended
use of the cone fusions would undoubtedly be an advance, and
engineers should make a practice of asking for the three fusion
temperatures when cone tests are made.
However, even with the values for the three cone-fusion temperatures,
there will always be a limitation to the interpretation
and analysis of observations of clinkering and slagging, and also
of the ability to predict the suitability of a coal, unless and until
we have data on the viscosity of slags. The ease with which a
molten slag will run or the rate at which it will flow are evidently
very important in the slagging of tubes or walls in all types of
furnaces, in fuel beds as affecting the density of the clinkers and
the clogging of grate bars, and in slag-tap furnaces as affecting
the ability to tap and the time required.
The authors bring out a number of interesting points that invite
discussion. First it is suggested that the paper does not
place enough importance on the lime content of ashes and slags;
its effects in many respects equal those of iron oxides. Lime becomes
of more importance as the iron content is low, or as the
ferric percentage is high, i.e., under oxidizing conditions. Plots,
such as those of Figs. 5 and 6 of the paper, will give scattering
of the points if the lime contents vary.
The segregation of iron in the slag bed or primary furnace,
shown in Fig. 4, averages higher than in the eighteen furnaces
we reported on in 1934 (Table 2, Bibliography (4) of the authors).
For the slag-tap furnaces of Fig. 4, the increase in iron, expressed
as percentage of iron in the coal ash, averages 70, with a maximum
of 90, whereas, ours averaged 11 per cent with a maximum of 32.
Presumably this is due to higher rates of burning in the primary
furnace in later designs.
Tables 1 and 2 of the same paper (4) show that there was
segregation not only of the iron but also of the lime; in addition,
there was increase of the silica-alumina ratio. The lime was increased
in twelve and the silica-alumina ratio in sixteen out of
the eighteen furnaces.
The authors show that the ferric percentage of the slags and
ashes increased along the path of travel of the gases and imply
that what occurs is that the iron in the ash particles is highly reduced
in the primary furnace and is then reoxidized along the-
path of travel. We did not examine wall or tube deposits from'
the eighteen boilers, but eleven stations furnished samples of
fly ash, which were so taken as to match the slag samples; theaverage
ferric percentage of ten samples was 77, and the maximum
84. Our deduction was that the iron in most particles does not
have time or opportunity to be reduced in their passage through
the furnace. However, particles deposited on a wall will haveample
time to reach equilibrium, and there will be more reduction
to FeO as the particle is maintained at a higher temperature,
or is in contact with carbon.
Whether reduction and then reoxidation occur could be proved
by collecting fly ash in water-cooled samplers at different positions
along the path of travel. However, studies we have- m&da