1: Per Cent Extraction of Weight of Cane
2: Per Cent Moisture in Bagasse
3: Per Cent in Bagasse
4: Fuel Value, B. t. u.
5: Per Cent in Bagasse
6: Fuel Value, B. t. u.
7: Per Cent in Bagasse
8: Fuel Value, B. t. u.
9: Total Heat Developed per Pound of Bagasse
10: Heat Required to Evaporate Moisture[42]
11: Heat Available for Steam Generation
12: Pounds of Bagasse Equivalent to one Pound of Coal of 14,000 B. t. u.
+----------------------------------------------------------------+
|+---+-----+----------+---------+---------+----------------+----+|
|| | | | | |B.t.u. Value per| ||
|| | | Fiber | Sugar |Molasses |Pound of Bagasse| ||
|| | +-----+----+----+----+----+----+-----+----+-----+ ||
|| | | | | | | | | | | | ||
|| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 ||
|+---+-----+-----+----+----+----+----+----+-----+----+-----+----+|
|| BASED UPON CANE OF 12 PER CENT FIBER AND JUICE CONTAINING ||
||18 PER CENT OF SOLID MATTER. REPRESENTING TROPICAL CONDITIONS ||
|+---+-----+-----+----+----+----+----+----+-----+----+-----+----+|
||75 |42.64|48.00|3996|6.24|451 |3.12|217 |4664 |525 |4139 |3.38||
||77 |39.22|52.17|4343|5.74|414 |2.87|200 |4958 |483 |4475 |3.13||
||79 |35.15|57.14|4757|5.14|371 |2.57|179 |5307 |433 |4874 |2.87||
||81 |30.21|63.16|5258|4.42|319 |2.21|154 |5731 |372 |5359 |2.61||
||83 |24.12|70.59|5877|3.53|256 |1.76|122 |6255 |297 |5958 |2.35||
||85 |16.20|80.00|6660|2.40|173 |1.20| 83 |6916 |200 |6716 |2.08||
|+---+-----+-----+----+----+----+----+----+-----+----+-----+----+|
|| BASED UPON CANE OF 10 PER CENT FIBER AND JUICE CONTAINING ||
||15 PER CENT OF SOLID MATTER. REPRESENTING LOUISIANA CONDITIONS||
|+---+-----+-----+----+----+----+----+----+-----+----+-----+----+|
||75 |51.00|40.00|3330|6.00|433 |3.00|209 |3972 |678 |3294 |4.25||
||77 |48.07|43.45|3617|5.66|409 |2.82|196 |4222 |592 |3630 |3.86||
||79 |44.52|47.62|3964|5.24|378 |2.62|182 |4524 |548 |3976 |3.52||
||81 |40.18|52.63|4381|4.73|342 |2.36|164 |4887 |495 |4392 |3.19||
||83 |35.00|58.82|4897|4.12|298 |2.06|143 |5436 |431 |5005 |2.80||
||85 |28.33|66.67|5550|3.33|241 |1.67|116 |5907 |349 |5558 |2.52||
|+---+-----+-----+----+----+----+----+----+-----+----+-----+----+|
+----------------------------------------------------------------+
Furnace Design and the Combustion of Bagasse--With the advance in sugar
manufacture there came, as described, a decrease in the amount of
bagasse available for fuel. As the general efficiency of a plant of this
description is measured by the amount of auxiliary fuel required per ton
of cane, the relative importance of the furnace design for the burning
of this fuel is apparent.
In modern practice, under certain conditions of mill operation, and with
bagasse of certain physical properties, the bagasse available from the
cane ground will meet the total steam requirements of the plant as a
whole; such conditions prevail, as described, in Java. In the United
States, Cuba, Porto Rico and like countries, however, auxiliary fuel is
almost universally a necessity. The amount will vary, depending to a
great extent upon the proportion of fiber in the cane, which varies
widely with the locality and with the age at which it is cut, and to a
lesser extent upon the degree of purity of the manufactured sugar, the
use of the maceration water and the efficiency of the mill apparatus as
a whole.
[Illustration: Fig. 27. Babcock & Wilcox Boiler Set with Green Bagasse
Furnace]
Experience has shown that this fuel may be burned with the best results
in large quantities. A given amount of bagasse burned in one furnace
between two boilers will give better results than the same quantity
burned in a number of smaller furnaces. An objection has been raised
against such practice on the grounds that the necessity of shutting down
two boiler units when it is necessary for any reason to take off a
furnace, requires a larger combined boiler capacity to insure continuity
of service. As a matter of fact, several small furnaces will cost
considerably more than one large furnace, and the saving in original
furnace cost by such an installation, taken in conjunction with the
added efficiency of the larger furnace over the small, will probably
more than offset the cost of additional boiler units for spares.
The essential features in furnace design for this class of fuel are
ample combustion space and a length of gas travel sufficient to enable
the gases to be completely burned before the boiler heating surfaces are
encountered. Experience has shown that better results are secured where
the fuel is burned on a hearth rather than on grates, the objection to
the latter method being that the air for combustion enters largely
around the edges, where the fuel pile is thinnest. When burned on a
hearth the air for combustion is introduced into the furnace through
several rows of tuyeres placed above and symmetrically around the
hearth. An arrangement of such tuyeres over a grate, and a proper
manipulation of the ashpit doors, will overcome largely the objection to
grates and at the same time enable other fuel to be burned in the
furnace when necessary. This arrangement of grates and tuyeres is
probably the better from a commercially efficient standpoint. Where the
air is admitted through tuyeres over the grate or hearth line, it
impinges on the fuel pile as a whole and causes a uniform combustion.
Such tuyeres connect with an annular space in which, where a blast is
used, the air pressure is controlled by a blower.
All experience with this class of fuel indicates that the best results
are secured with high combustion rates. With a natural draft in the
furnace of, say, three-tenths inch of water, a combustion rate of from
250 to 300 pounds per square foot of grate surface per hour may be
obtained. With a blast of, say, five-tenths inch of water, this rate can
be increased to 450 pounds per square foot of grate surface per hour.
These rates apply to bagasse as fired containing approximately 50 per
cent of moisture. It would appear that the most economical results are
secured with a combustion rate of approximately 300 pounds per square
foot per hour which, as stated, may be obtained with natural draft.
Where a natural draft is available sufficient to give such a rate, it is
in general to be preferred to a blast.
Fig. 27 shows a typical bagasse furnace with which very satisfactory
results have been obtained. The design of this furnace may be altered to
suit the boilers to which it is connected. It may be changed slightly in
its proportions and in certain instances in its position relative to the
boiler. The furnace as shown is essentially a bagasse furnace and may be
modified somewhat to accommodate auxiliary fuel.
The fuel is ignited in a pit A on a hearth which is ordinarily
elliptical in shape. Air for combustion is admitted through the tuyeres
B connected to an annular space C through which the amount of air is
controlled. Above the pit the furnace widens out to form a combustion
space D which has a cylindrical or spherical roof with its top
ordinarily from 11 to 13 feet above the floor. The gases pass from this
space horizontally to a second combustion chamber E from which they are
led through arches F to the boiler. The arrangement of such arches is
modified to suit the boiler or boilers with which the furnace is
operated. A furnace of such design embodies the essential features of
ample combustion space and long gas travel.
The fuel should be fed to the furnace through an opening in the roof
above the pit by some mechanical means which will insure a constant fuel
feed and at the same time prevent the inrush of cold air into the
furnace.
This class of fuel deposits a considerable quantity of dust, which if
not removed promptly will fuse into a hard glass-like clinker. Ample
provision should be made for the removal of such dust from the furnace,
the gas ducts and the boiler setting, and these should be thoroughly
cleaned once in 24 hours.
Table 45 gives the results of several tests on Babcock & Wilcox boilers
using fuel of this character.