MOISTURE AND CALORIFIC VALUE OF SAWMILL REFUSE
_____________________________________________________________________
| | | | |
| | | Per Cent | B. t. u. |
| Kind of Wood | Nature of Refuse | Moisture | per Pound |
| | | | Dry Fuel |
|_____________________|_______________________|__________|____________|
| | | | |
| Mexican White Pine | Sawdust and Hog Chips | 51.90 | 9020 |
| Yosemite Sugar Pine | Sawdust and Hog Chips | 62.85 | 9010 |
| Redwood 75%, | Sawdust, Box Mill | | |
| Douglas Fir 25% | Refuse and Hog | 42.20 | 8977[41] |
| Redwood | Sawdust and Hog Chips | 52.98 | 9040[41] |
| Redwood | Sawdust and Hog Chips | 49.11 | 9204[41] |
| Fir, Hemlock, | | | |
| Spruce and Cedar | Sawdust | 42.06 | 8949[41] |
|_____________________|_______________________|__________|____________|
It is essential in the burning of this class of fuel that a large
combustion space be supplied, and on account of the usually high
moisture content there should be much heated brickwork to radiate heat
to the fuel bed and thus evaporate the moisture. Extension furnaces of
the proper size are usually essential for good results and when this
fuel is used alone, grates dropped to the floor line with an ashpit
below give additional volume for combustion and space for maintaining a
thick fuel bed. A thick fuel bed is necessary in order to avoid
excessive quantities of air passing through the boiler. Where the fuel
consists of hogged wood and sawdust alone, it is best to feed it
automatically into the furnace through chutes on the top of the
extension. The best results are secured when the fuel is allowed to pile
up in the furnace to a height of 3 or 4 feet in the form of a cone under
each chute. The fuel burns best when not disturbed in the furnace. Each
fuel chute, when a proper distance from the grates and with the piles
maintained at their proper height, will supply about 30 or 35 square
feet of grate surface. While large quantities of air are required for
burning this fuel, excess air is as harmful as with coal, and care must
be taken that such an excess is not admitted through fire doors or fuel
chutes. A strong natural draft usually is preferable to a blast with
this fuel. The action of blast is to make the regulation of the furnace
conditions more difficult and to blow over unconsumed fuel on the
heating surfaces and into the stack. This unconsumed fuel settling in
portions of the setting out of the direct path of the gases will have a
tendency to ignite provided any air reaches it, with results harmful to
the setting and breeching connection. This action is particularly
objectionable if these particles are carried over into the base of a
stack, where they will settle below the point at which the flue enters
and if ignited may cause the stack to become overheated and buckle.
Whether natural draft or blast is used, much of the fuel is carried onto
the heating surfaces and these should be cleaned regularly to maintain a
good efficiency. Collecting chambers in various portions of the setting
should be provided for this unconsumed fuel, and these should be kept
clean.
With proper draft conditions, 150 pounds of this fuel containing about
30 to 40 per cent of moisture can be burned per square foot of grate
surface per hour, and in a properly designed furnace one square foot of
grate surface can develop from 5 to 6 boiler horse power. Where the wood
contains 50 per cent of moisture or over, it is not usually safe to
figure on obtaining more than 3 to 4 horse power per square foot of
grate surface.
Dry sawdust, chips and blocks are also used as fuel in many wood-working
industries. Here, as with the wet wood, ample combustion space should be
supplied, but as this fuel is ordinarily kiln dried, large brickwork
surfaces in the furnace are not necessary for the evaporation of
moisture in the fuel. This fuel may be burned in extension furnaces
though these are not required unless they are necessary to secure an
added furnace volume, to get in sufficient grate surface, or where such
an arrangement must be used to allow for a fuel bed of sufficient
thickness. Depth of fuel bed with the dry fuel is as important as with
the moist fuel. If extension furnaces are used with this dry wood, care
must be taken in their design that there is no excessive throttling of
the gases in the furnace, or brickwork trouble will result. In Babcock &
Wilcox boilers this fuel may be burned without extension furnaces,
provided that the boilers are set at a sufficient height to provide
ample combustion space and to allow for proper depth of fuel bed.
Sometimes this is gained by lowering the grates to the floor line and
excavating for an ashpit. Where the fuel is largely sawdust, it may be
introduced over the fire doors through inclined chutes. The old methods
of handling and collecting sawdust by means of air suction and blast
were such that the amount of air admitted through such chutes was
excessive, but with improved methods the amount of air so admitted may
be reduced to a negligible quantity. The blocks and refuse which cannot
be handled through chutes may be fired through fire doors in the front
of the boiler, which should be made sufficiently large to accommodate
the larger sizes of fuel. As with wet fuel, there will be a quantity of
unconsumed wood carried over and the heating surfaces must be kept
clean.
In a few localities cord wood is burned. With this as with other classes
of wood fuel, a large combustion space is an essential feature. The
percentage of moisture in cord wood may make it necessary to use an
extension furnace, but ordinarily this is not required. Ample combustion
space is in most cases secured by dropping the grates to the floor line,
large double-deck fire doors being supplied at the usual fire door level
through which the wood is thrown by hand. Air is admitted under the
grates through an excavated ashpit. The side, front and rear walls of
the furnace should be corbelled out to cover about one-third of the
total grate surface. This prevents cold air from laneing up the sides of
the furnace and also reduces the grate surface. Cord wood and slabs form
an open fire through which the frictional loss of the air is much less
than in the case of sawdust or hogged material. The combustion rate with
cord wood is, therefore, higher and the grate surface may be
considerably reduced. Such wood is usually cut in lengths of 4 feet or 4
feet 6 inches, and the depth of the grates should be kept approximately
5 feet to get the best results.
Bagasse--Bagasse is the refuse of sugar cane from which the juice has
been extracted by pressure between the rolls of the mill. From the start
of the sugar industry bagasse has been considered the natural fuel for
sugar plantations, and in view of the importance of the industry a word
of history relative to the use of this fuel is not out of place.
When the manufacture of sugar was in its infancy the cane was passed
through but a single mill and the defecation and concentration of the
saccharine juice took place in a series of vessels mounted one after
another over a common fire at one end and connected to a stack at the
opposite end. This primitive method was known in the English colonies as
the "Open Wall" and in the Spanish-American countries as the "Jamaica
Train".
The evaporation and concentration of the juice in the open air and over
a direct fire required such quantities of fuel, and the bagasse, in
fact, played such an important part in the manufacture of sugar, that
oftentimes the degree of extraction, which was already low, would be
sacrificed to the necessity of obtaining a bagasse that might be readily
burned.
The furnaces in use with these methods were as primitive as the rest of
the apparatus, and the bagasse could be burned in them only by first
drying it. This naturally required an enormous quantity of handling of
the fuel in spreading and collecting and frequently entailed a shutting
down of the mill, because a shower would spoil the supply which had been
dried.
The difficulties arising from the necessity of drying this fuel caused a
widespread attempt on the part of inventors to the turning out of a
furnace which would successfully burn green bagasse. Some of the designs
were more or less clever, and about the year 1880 several such green
bagasse furnaces were installed. These did not come up to expectations,
however, and almost invariably they were abandoned and recourse had to
be taken to the old method of drying in the sun.
From 1880 the new era in the sugar industry may be dated. Slavery was
almost universally abolished and it became necessary to pay for labor.
The cost of production was thus increased, while growing competition of
European beet sugar lowered the prices. The only remedy for the new
state of affairs was the cheapening of the production by the increase of
extraction and improvement in manufacture. The double mill took the
place of the single, the open wall method of extraction was replaced by
vacuum evaporative apparatus and centrifugal machines were introduced to
do the work of the great curing houses. As opposed to these
improvements, however, the steam plants remained as they started,
consisting of double flue boilers externally fired with dry bagasse.
On several of the plantations horizontal multitubular boilers externally
fired were installed and at the time were considered the acme of
perfection. Numerous attempts were made to burn the bagasse green, among
others the step grates imported from Louisiana and known as the Leon
Marie furnaces, but satisfactory results were obtained in none of the
appliances tried.
The Babcock & Wilcox Co. at this time turned their attention to the
problem with the results which ultimately led to its solution. Their New
Orleans representative, Mr. Frederick Cook, invented a hot forced blast
bagasse furnace and conveyed the patent rights to this company. This
furnace while not as efficient as the standard of to-day, and expensive
in its construction, did, nevertheless, burn the bagasse green and
enabled the boilers to develop their normal rated capacity. The first
furnace of this type was installed at the Southwood and Mt. Houmas
plantations and on a small plantation in Florida. About the year 1888
two furnaces were erected in Cuba, one on the plantation Senado and the
other at the Central Hormiguero. The results obtained with these
furnaces were so remarkable in comparison with what had previously been
accomplished that the company was overwhelmed with orders. The expense
of auxiliary fuel, usually wood, which was costly and indispensable in
rainy weather, was done away with and as the mill could be operated on
bagasse alone, the steam production and the factory work could be
regulated with natural increase in daily output.
Progress and improvement in the manufacture itself was going on at a
remarkable rate, the single grinding had been replaced by a double
grinding, this in turn by a third grinding, and finally the maceration
and dilution of the bagasse was carried to the extraction of practically
the last trace of sugar contained in it. The quantity of juice to be
treated was increased in this way 20 or 30 per cent but was accompanied
by the reduction to a minimum of the bagasse available as a fuel, and
led to demands upon the furnace beyond its capacity.
With the improvements in the manufacture, planters had been compelled to
make enormous sacrifices to change radically their systems, and the
heavy disbursement necessary for mill apparatus left few in a financial
position to make costly installations of good furnaces. The necessity of
turning to something cheap in furnace construction but which was
nevertheless better than the early method of burning the fuel dry led to
the invention of numerous furnaces by all classes of engineers
regardless of their knowledge of the subject and based upon no
experience. None of the furnaces thus produced were in any sense
inventions but were more or less barefaced infringements of the patents
of The Babcock & Wilcox Co. As the company could not protect its rights
without hurting its clients, who in many cases against their own will
were infringing upon these patents, and as on the other hand they were
anxious to do something to meet the wants of the planters, a series of
experiments were started, at their own rather than at their customers'
expense, with a view to developing a furnace which, without being as
expensive, would still fulfill all the requirements of the manufacturer.
The result was the cold blast green bagasse furnace which is now
offered, and it has been adopted as standard for this class of work
after years of study and observation in our installations in the sugar
countries of the world. Such a furnace is described later in considering
the combustion of bagasse.
Composition and Calorific Value of Bagasse--The proportion of fiber
contained in the cane and density of the juice are important factors in
the relation the bagasse fuel will have to the total fuel necessary to
generate the steam required in a mill's operation. A cane rich in wood
fiber produces more bagasse than a poor one and a thicker juice is
subject to a higher degree of dilution than one not so rich.
Besides the percentage of bagasse in the cane, its physical condition
has a bearing on its calorific value. The factors here entering are the
age at which the cane must be cut, the locality in which it is grown,
etc. From the analysis of any sample of bagasse its approximate
calorific value may be calculated from the formula,
8550F + 7119S + 6750G - 972W
B. t. u. per pound bagasse = ---------------------------- (22)
100
Where F = per cent of fiber in cane, S = per cent sucrose, G = per cent
glucose, W = per cent water.
This formula gives the total available heat per pound of bagasse, that
is, the heat generated per pound less the heat required to evaporate its
moisture and superheat the steam thus formed to the temperature of the
stack gases.
Three samples of bagasse in which the ash is assumed to be 3 per cent
give from the formula:
F = 50 S and G = 4.5 W = 42.5 B. t. u. = 4183
F = 40 S and G = 6.0 W = 51.0 B. t. u. = 3351
F = 33.3 S and G = 7.0 W = 56.7 B. t. u. = 2797
A sample of Java bagasse having F = 46.5, S = 4.50, G = 0.5, W = 47.5
gives B. t. u. 3868.
These figures show that the dryer the bagasse is crushed, the higher the
calorific value, though this is accompanied by a decrease in sucrose.
The explanation lies in the fact that the presence of sucrose in an
analysis is accompanied by a definite amount of water, and that the
residual juice contains sufficient organic substance to evaporate the
water present when a fuel is burned in a furnace. For example, assume
the residual juice (100 per cent) to contain 12 per cent organic matter.
From the constant in formula,
12×7119 (100-12)×972
------- = 854.3 and ------------ = 855.4.
100 100
That is, the moisture in a juice containing 12 per cent of sugar will be
evaporated by the heat developed by the combustion of the contained
sugar. It would, therefore, appear that a bagasse containing such juice
has a calorific value due only to its fiber content. This is, of course,
true only where the highest products of oxidization are formed during
the combustion of the organic matter. This is not strictly the case,
especially with a bagasse of a high moisture content which will not burn
properly but which smoulders and produces a large quantity of products
of destructive distillation, chiefly heavy hydrocarbons, which escape
unburnt. The reasoning, however, is sufficient to explain the steam
making properties of bagasse of a low sucrose content, such as are
secured in Java, as when the sucrose content is lower, the heat value is
increased by extracting more juice, and hence more sugar from it. The
sugar operations in Java exemplify this and show that with a high
dilution by maceration and heavy pressure the bagasse meets all of the
steam requirements of the mills without auxiliary fuel.
A high percentage of silica or salts in bagasse has sometimes been
ascribed as the reason for the tendency to smoulder in certain cases of
soft fiber bagasse. This, however, is due to the large moisture content
of the sample resulting directly from the nature of the cane. Soluble
salts in the bagasse has also been given as the explanation of such
smouldering action of the fire, but here too the explanation lies solely
in the high moisture content, this resulting in the development of only
sufficient heat to evaporate the moisture.