Method and apparatus for drying bagasse

An outer cylindrical drum has an inner drum concentrically mounted therein in spaced relationship, to form coextensive outer and inner drying chambers that are connected to a common source of hot exhaust gases from a sugar factory boiler. Raw bagasse is first passed through the outer annular chamber to effect primary drying thereof, and then a portion of the primary dried bagasse is divided from the remainder and is passed through the inner chamber to produce secondary dried bagasse. The conveyance path between the point where the primary bagasse is divided and the inlet of the inner chamber is short in both time and distance to preserve heat in the divided portion, and an entrainment unit is utilized to preheat the divided portion before it enters the inner chamber.

TECHNICAL FIELD OF THE INVENTION 
This invention relates generally to a method and apparatus for drying 
bagasse, which is the plant material remaining after the sugar has been 
pressed from sugar cane. More particularly, it relates to a novel method 
and apparatus for efficiently utilizing hot exhaust gases to produce 
primary and secondary dried bagasse, with the latter being suitable for 
pelletizing into a fuel. 
BACKGROUND OF THE INVENTION 
After sugar cane has been cut and pressed at the mill to remove sugar, the 
crushed plant stalk material that remains is called bagasse. Typically, 
the raw bagasse consists of about 48% fiber, 50% moisture and 2% of 
soluble matter such as residual sugar and other organic and inorganic 
substances. Once regarded as a waste material, bagasse is now being looked 
to as a source of energy to operate plant boilers and the like. However, 
in its raw form the bagasse is a low-grade fuel that requires large 
furnaces because of the large volume of flue gases produced, and typically 
it will have a heat content of 2,005-2,340 kilocalories per kilogram. 
While useful, the raw bagasse is thus not an especially desirable fuel. 
It has been found, however, that if the moisture content of the raw bagasse 
is lowered, its desirability as a fuel can be greatly increased. Thus, 
efforts have been made to dry the raw bagasse, and commonly the hot 
exhaust or flue gases from the sugar factory boilers or similar sources 
are utilized for this purpose. In the usual arrangement, the raw bagasse 
is placed in a large dryer to which the hot exhaust gases are admitted, 
with the raw bagasse being agitated as the gases pass therethrough. This 
process can lower the moisture content of the raw bagasse by several 
percentage points, making it a more desirable fuel. 
With the high cost of conventional energy, attention has recently been 
directed toward producing a more efficient fuel from bagasse. It has been 
found that if the moisture content of the raw bagasse is lowered 
sufficiently and if the particle size of the material is proper, the 
bagasse can then be pelletized to form a fuel with relatively high BTU 
value, compared to the raw bagasse. To achieve this, the raw bagasse first 
undergoes a primary or initial drying cycle done much as in the past, to 
produce primary dried bagasse. A portion of the primary dried bagasse is 
thereafter passed through a secondary drying cycle, to further lower its 
moisture content and produce secondary dried bagasse suitable for 
pelletizing. 
In processing the bagasse it is still desired to utilize the hot exhaust 
gases to produce both primary and secondary dried bagasse for overall 
efficiency in the energy cycle. However, difficulties have been 
encountered in doing this in an efficient manner. 
Part of the problem flows from the fact that the hot exhaust gases actually 
are relatively low in temperature, usually falling into the range of 
between about 375.degree. F. and 425.degree. F. This has required drying 
drums that are large in dimension, to lower the moisture content of the 
raw bagasse. More particularly, in the past two separate drying operations 
with separate equipment have been required to produce both primary dried 
bagasse and secondary dried bagasse, with the processing being done in a 
sequential manner. The equipment required occupies considerable space, is 
expensive, and most importantly does not utilize the hot exhaust gases in 
the most efficient manner. 
The need thus exists for a new method and apparatus for producing primary 
and secondary dried bagasse, the latter being suitable for pelletizing, 
designed to make maximum efficient use of the energy provided by the 
relatively low temperature exhaust gases of a sugar factory or the like so 
as to maximize the overall energy cycle efficiency and minimize the need 
to utilize oil and other conventional fuels. The present invention is 
intended to satisfy that need. 
BRIEF SUMMARY OF THE INVENTION 
In the method of the invention, the whole batch of raw bagasse is exposed 
within an annular chamber formed in a first rotating cylindrical drum to a 
first portion of the hot exhaust gas flow from a boiler flue or the like, 
to produce primary dried bagasse. The primary dried bagasse is separated 
in the apparatus, and a portion thereof is then passed through a rotating 
cylindrical drum placed concentrically within the first drum and exposed 
to a second portion of the hot exhaust gases. The apparatus includes 
transfer means for transmitting the separated portion of primary dried 
bagasse to the second drum over a short path and in minimum time, to 
minimize heat loss therein. The separated portion of bagasse is normally 
subjected to shredding during transport, to produce particles of the 
correct dimensions for pelletizing. 
By dividing the hot exhaust gas flow into two portions passing through 
concentrically arranged first and second drying drums, maximum efficiency 
in using the gas flow is obtained. The second drying drum is surrounded by 
the first, which minimizes heat loss from the more secondary drying 
operation. Further, the whole apparatus is relatively compact and 
mechanically simple. The present apparatus and method constitute a 
significant advance in the art of utilizing bagasse as fuel and, in 
particular, assure that a factory's exhaust or flue gases are utilized to 
the maximum in producing the primary and secondary dried bagasse. 
The principal object of the present invention is to provide a method and 
apparatus for producing both primary and secondary dried bagasse, wherein 
drying is achieved by utilizing hot exhaust or flue gases in a manner to 
assure maximum energy efficiency. 
Another object is to provide a method and apparatus for producing both 
primary and secondary dried bagasse, wherein the apparatus is of minimum 
length and diameter, and of relatively simple mechanical construction. 
A further object is to provide a dryer apparatus wherein the secondary 
chamber is concentrically disposed with the primary chamber, and part of 
the product produced from the primary chamber is then passed through the 
secondary chamber for further drying. 
Other objects and many of the attendant advantages of the present invention 
will become readily apparent from the following detailed description of 
the preferred embodiment, when taken in conjunction with the accompaning 
drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings, an outer cylindrical drying drum is 
indicated at 2, the inlet end 4 thereof having thereon an annular ring 6 
that rests on bearing units 8 mounted on a first support 10. The outlet 
end 12 of the drum 2 carries a similar annular ring 14, which rests on 
bearing units 16 mounted on a second support 18. Mounted concentrically 
within the outer drum 2 and generally coextensive therewith is an inner 
cylindrical drum 20, positioned to lie concentrically about the 
longitudinal axis of the outer drum 2 and supported from the walls thereof 
by radial spacers 22. The inner surface of the first or outer drum 2 and 
the outer surface of the second or inner drum 20 define therebetween an 
outer annular chamber 24, and carry thereon a plurality of fins 26 of 
suitable design and arrangement to help agitate raw bagasse as it passes 
through the chamber 24. The inner surface of the inner drum 20 defines an 
inner chamber 28, and carries thereon fins 30 that are similar in 
arrangement and purpose to the fins 26. 
The inlet end 4 of the outer drying drum 2 has an inwardly directed sealing 
flange 32 thereon, and the inlet end of the inner drying drum 20 has a 
similar sealing flange 34. The outlet ends of the drums 2 and 20 are 
provided with similar sealing flanges 36 and 38, respectively. A motor 40 
is connected by a chain or belt 42 to one of the bearing units 8 upon 
which the annular ring 6 rests, and functions to rotate the two 
concentrically arranged drying drums about the longitudinal axis thereof. 
Positioned at the inlet ends of the outer and inner drying drums 2 and 20 
is a vertical frame 44, which supports an outer inlet housing 46 having a 
shorter, smaller diameter inner inlet housing 48 mounted concentrically 
therein. The outlet ends of the housings 46 and 48 carry external sealing 
flanges 50 and 52 thereon, respectively, which are positioned to abut the 
flanges 32 and 34 on the rotating drying drums 2 and 20. Suitable annular 
sealing rings 54 and 56 are engaged between the flanges 50 and 32 and the 
flanges 52 and 34, respectively, whereby a rotating sealed joint results. 
A gas supply conduit 58 is connected with the outer inlet housing 46, and 
leads from the flue or exhaust of sugar factory boiler units, or the like 
(not shown). The inlet end of the inner inlet housing 48 has adjustable 
dampers 60 mounted therein, which can be adjusted to control the flow of 
heated gases into the housing 48 from the larger outer inlet housing 46. 
Turning to the outlet end of the drying drums 2 and 20, a collector unit 62 
is mounted at said end and includes a vertical collection chamber 64 
having an outlet 66 at its upper end which leads to cyclone and fan 
apparatus (not shown) for handling the primary dried bagasse as it leaves 
the annular outer drying chamber 24. The fan apparatus also serves to help 
draw the exhaust gases through the dryer. The collection chamber 64 has an 
inlet portion 68 provided with an external flange 70 that is positioned to 
confront the flange 36 on the drum 2, and a sealing ring 72 is positioned 
therebetween. 
A secondary dried bagasse collection conduit 74 is carried by the 
collection chamber 64 and passes completely through the outer wall 
thereof. The inner end of the conduit 74 carries an external flange 76 
that confronts the flange 38, and a sealing ring 78 is positioned 
therebetween. The collection conduit 74 collects the secondary bagasse 
from the inner chamber 28 of the drying unit, and prevents it from being 
mixed with the primary dried bagasse. 
The collection chamber 64 contains a set of vanes 80 therein, arranged to 
direct the flow of the primary bagasse exiting from the horizontally 
disposed annular drying chamber 24 in an upward direction, toward the 
outlet 66. As this occurs, some of the primary bagasse will drop into a 
hopper 82 at the bottom of the chamber 64, from which it is withdrawn 
through an air lock 84 and deposited on suitable screening or sorting 
equipment 86 that separates the material into fines and larger particles. 
A fines conveyor 88 is positioned to receive the fines as they are sorted, 
and similarly a coarse particle conveyor 90 collects the coarser material 
flowing over the sorting equipment 86. The two conveyors 88 and 90 are 
fitted with control gates 92 and 94, respectively, which are positioned 
over the collection hopper 96 of an inclined feed conveyor 98 that leads 
from the sorting equipment 86 upwardly to an airlock 100 mounted atop an 
entrainment unit 102. 
The entrainment unit 102 is mounted on the inlet end of a shredder 104 that 
is carried on the vertical frame 44 above the outer inlet housing 46, and 
which is operated by a motor 106. The entrainment unit 102 includes a 
slotted portion 108 enclosed by an annular manifold 110, which is 
connected with the inlet housing 46 by a bleed conduit 112 that serves to 
bleed hot gas flow from the inlet housing 46 and supply it to the 
entrainment unit. The outlet of the shredder 104 is connected to one end 
of a supply conduit 114 that passes through the outer inlet housing 46 and 
connects with the inner inlet housing 48, at a point between the control 
dampers 60 and the inner chamber 28. 
Raw bagasse is provided to the outer inlet housing 46 by a raw bagasse 
conveyor 116, which dumps the material into an airlock 118 mounted on top 
of a main supply conduit 120 that connects to the outer inlet housing 46 
close to the annular chamber 24. 
In operation, the conduit 58 is connected with a source of hot gas flow 
which, as has been noted, will normally be the exhaust gases taken from 
the flue of the sugar factory's boilers. The flow of gas into the outer 
inlet housing 46 is divided into three parts. The first and major part 
flows into the annular chamber 24, to effect primary drying of the raw 
bagasse. A second portion enters the inner inlet housing 48 through the 
dampers 60, the dampers being set according to the temperature and 
velocity of the gas flow to obtain the desired drying temperature within 
the inner chamber 28. A small third portion of the gas flow is bled off 
through the bleed conduit 112, and entrains the primary dried bagasse 
portion that is being supplied to the shredder 104 to help it move through 
the shredder and to help maintain its temperature. 
With the gas flow established and the drying drums 2 and 20 rotating, raw 
bagasse is then fed unto the operating conveyor 116 and drops into the 
rotating air lock 118, which allows entry of the raw bagasse into the 
outer inlet housing 46 without incurring loss of heated gas therefrom. 
Referring to FIG. 2, it will be noted that the conduit 120 is tangential 
to the cylindrical outer inlet housing 46, which facilitates feeding the 
raw bagasse into the annular chamber 24. As the raw bagasse enters the 
outer inlet housing 46 from the supply conduit 120, it is entrained by the 
gas flow and carried into and through the rotating outer chamber 24, the 
fins 26 serving to agitate the bagasse and help keep it moving. Passage of 
the raw bagasse through the annular chamber 24 effects primary drying 
thereof, and the primary dried bagasse is collected by the collector unit 
62. As has been noted, a portion of the primary bagasse is drawn off into 
the hopper 82, while the main body thereof is moved out of the collector 
unit 62 and transferred away from the dryer apparatus. The primary dried 
bagasse can then be utilized for fuel directly, or for other suitable 
purposes. 
As the screening or sorting unit 86 is operated to sort the primary bagasse 
material collected in the hopper 82 and discharged through the airlock 84, 
the control gates 92 and 94 are operated to admit a correctly blended flow 
of fine and coarse material into the hopper 96. The separated primary 
bagasse portion is then elevated to the air lock 100, which admits it to 
the entrainment unit 102, where it is mixed with hot gas flow from the 
conduit 58 and then enters the shredder 104. The shredder 104 is chosen to 
assure that the secondary dried bagasse will have the correct particle 
size necessary for pelletizing. 
From the shredder 104, the primary bagasse enters the inner chamber 28, and 
is moved therethrough by gas flow entering through the dampers 60. The 
dampers 60 are set to effect the desired degree of drying in the chamber 
28, to produce secondary dried bagasse with the desired characteristics. 
The conduit 74 takes the secondary dried bagasse from the outlet end of 
the inner chamber 28, and it is then transported to a processing location. 
Among the features of the apparatus just described are several that 
contribute to maximum usage of the drying energy found in the hot exhaust 
gas flow which, as has been mentioned, will normally have a temperature of 
between about 375.degree. and 425.degree. F. A first feature is that the 
portion of the primary bagasse subjected to secondary drying is taken 
immediately from the hopper 82 while it is still hot, and is quickly 
conveyed over a short path to the entrainment unit 102, where it is 
immediately subjected to heated gas flow. Thus, the temperature of the 
primary bagasse portion selected for secondary treatment is maintained 
high, which contributes to the effectiveness of the secondary drying 
operation. 
A second important feature is that the temperature within the annular 
chamber 24 is maintained relatively high because of the direct gas flow 
from the conduit 58, through the inlet housing 46, and into the annular 
chamber. Also, because the inner drying drum 20 constitutes the inner wall 
of the annular chamber 24 and is itself heated by the flow of hot exhaust 
gases therethrough, an efficient drying environment is established in the 
annular chamber. 
Further, because the inner drying drum 20 is surrounded and insulated by 
the outer drying drum 2, the loss of heat from the inner chamber 28 is 
minimized. The flow of hot exhaust gases enters the inner chamber 28 with 
essentially the same ease as it enters the outer chamber 24, with no 
significant loss of heat because of lengthy transit. At the same time, the 
dampers 60 allow the rate of gas flow in the chamber 28 to be adjusted to 
achieve a desired degree of drying. Thus, the concentric chambers 24 and 
28 are mutually reinforcing, and the two chambers cooperate to assure 
maximum energy efficiency in utilizing the hot flue gases. 
Another feature derived from the apparatus is its compactness and 
mechanical simplicity, as compared to the separate dryer arrangements 
utilized for bagasse in the past. Obviously, the invention also includes a 
number of other operational advantages. 
Turning now to the present method, it is seen that such includes the 
following steps: 
1. Effecting primary drying of raw bagasse by passing it through an annular 
chamber connected directly with a source of hot exhaust gases, the hot 
gases being effective to move the raw bagasse through the chamber and to 
remove moisture therefrom, and the walls defining the annular chamber 
being rotated about a horizontal axis to agitate the raw bagasse; 
2. Collecting the primary dried bagasse at the outlet of the annular 
chamber; 
3. Dividing a portion of the primary dried bagasse from the rest as it is 
collected; 
4. Immediately conveying the divided portion of primary dried bagasse into 
a cylindrical inner chamber disposed concentrically within said annular 
chamber, the inner chamber being directly connected with the same source 
of hot exhaust gases and the portion of bagasse being moved through said 
cylindrical chamber and having moisture removed therefrom by the flow of 
said gases, the wall defining said inner chamber also being rotated about 
said horizontal axis; and 
5. Collecting the secondary dried bagasse from the cylindrical chamber, the 
flow of hot gases into said cylindrical chamber being regulated to effect 
the desired degree of moisture removal from the secondary bagasse. 
In the invention, the primary drying step is effective to reduce moisture 
in the raw bagasse as it comes from the mill from a level of about 50% to 
a more acceptable level of from 35%-40%. The secondary drying of a portion 
of the primary dried bagasse according to the present method can lower the 
moisture content in the secondary dried bagasse to about 12%. 
For a typical installation, the inner diameter of the outer drying drum 2 
might be about 14 feet, and that of the inner drum 20 about 5-6 feet. 
Overall length of the chambers 24 and 28 would be from 18-20 feet. 
In a typical example of the method, a sugar mill would provide about 60 
tons of bagasse per hour, and the factory boiler would exhaust 
approximately 185,000 CFM of exhaust gas at about 400.degree. F. All of 
the raw bagasse, having a moisture content of about 50%, would be passed 
through the outer annular chamber 24 of the dryer for primary drying, 
utilizing about 150,000 CFM of the hot exhaust gas. Segregation of a 
portion of the primary dried bagasse would then occur, and the balance of 
the primary bagasse would typically be transported to the boiler for use 
directly as fuel. 
The segregated portion would then be passed through the inner chamber 28 to 
effect secondary drying thereof, utilizing about 35,000 CFM of the exhaust 
gases, which would produce secondary dried bagasse with a moisture content 
of about 12%. The secondary dried bagasse would then be taken to the 
pellet mill for pelletizing. 
Segregation of a portion of the primary bagasse can be done pneumatically, 
by screening, or by any other conventional operation. The fines are 
primarily wanted for secondary drying, while the larger and coarser 
material is simply burned as fuel as in the past. It should also be noted 
that while a shredder will normally be utilized on the segregated bagasse 
before it enters the inner chamber 28 for secondary drying, in some 
instances it may not actually be needed. The condition of sugar cane stalk 
can vary from time to time and field to field, and the process is adjusted 
to take these variations into account. 
It is to be understood that the mechanical details of the apparatus can be 
varied without departing from the invention. For example, the shape and 
placement of the agitating fins can be changed from what is shown herein, 
and the rotating seals can be constructed differently, without departing 
from the invention. 
Obviously, many modifications and variations of the invention are possible.