Chapter 25

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.

TABLE 45

TESTS OF BABCOCK & WILCOX BOILERS WITH GREEN BAGASSE ____________________________________________________________________ | Duration of Test | Hours | 12 | 10 | 10 | 10 | | Rated Capacity of Boiler |Horse Power| 319 | 319 | 319 | 319 | | Grate Surface |Square Feet| 33 | 33 | 16.5 | 16.5 | | Draft in Furnace | Inches | .30 | .28 | .29 | .27 | | Draft at Damper | Inches | .47 | .45 | .46 | .48 | | Blast under Grates | Inches | ... | ... | ... | .34 | | Temperature of Exit Gases | Degrees F.| 536 | 541 | 522 | 547 | | /CO_{2} | Per Cent | 13.8 | 12.6 | 11.7 | 12.8 | | Flue Gas Analysis { O | Per Cent | 5.9 | 7.6 | 8.2 | 6.9 | | \CO | Per Cent | 0.0 | 0.0 | 0.0 | 0.0 | | Bagasse per Hour as Fired | Pounds | 4980 | 4479 | 5040 | 5586 | | Moisture in Bagasse | Per Cent |52.39 |52.93 |51.84 |51.71 | | Dry Bagasse per Hour | Pounds | 2371 | 2108 | 2427 | 2697 | | Dry Bagasse per Square Foot| | | | | | | of Grate Surface per Hour| Pounds | 71.9 | 63.9 |147.1 |163.4 | | Water per Hour from and at | | | | | | | 212 Degrees | Pounds |10141 | 9850 |10430 |11229 | | Per Cent of Rated Capacity | | | | | | | Developed | Per Cent | 92.1 | 89.2 | 94.7 |102.0 | |____________________________|___________|______|______|______|______|

Tan Bark--Tan bark, or spent tan, is the fibrous portion of bark remaining after use in the tanning industry. It is usually very high in its moisture content, a number of samples giving an average of 65 per cent or about two-thirds of the total weight of the fuel. The weight of the spent tan is about 2.13 times as great as the weight of the bark ground. In calorific value an average of 10 samples gives 9500 B. t. u. per pound dry.[43] The available heat per pound as fired, owing to the great percentage of moisture usually found, will be approximately 2700 B. t. u. Since the weight of the spent tan as fired is 2.13 as great as the weight of the bark as ground at the mill, one pound of ground bark produces an available heat of approximately 5700 B. t. u. Relative to bituminous coal, a ton of bark is equivalent to 0.4 ton of coal. An average chemical analysis of the bark is, carbon 51.8 per cent, hydrogen 6.04, oxygen 40.74, ash 1.42.

Tan bark is burned in isolated cases and in general the remarks on burning wet wood fuel apply to its combustion. The essential features are a large combustion space, large areas of heated brickwork radiating to the fuel bed, and draft sufficient for high combustion rates. The ratings obtainable with this class of fuel will not be as high as with wet wood fuel, because of the heat value and the excessive moisture content. Mr. D. M. Meyers found in a series of experiments that an average of from 1.5 to 2.08 horse power could be developed per square foot of grate surface with horizontal return tubular boilers. This horse power would vary considerably with the method in which the spent tan was fired.

LIQUID FUELS AND THEIR COMBUSTION

Petroleum is practically the only liquid fuel sufficiently abundant and cheap to be used for the generation of steam. It possesses many advantages over coal and is extensively used in many localities.

There are three kinds of petroleum in use, namely those yielding on distillation: 1st, paraffin; 2nd, asphalt; 3rd, olefine. To the first group belong the oils of the Appalachian Range and the Middle West of the United States. These are a dark brown in color with a greenish tinge. Upon their distillation such a variety of valuable light oils are obtained that their use as fuel is prohibitive because of price.

To the second group belong the oils found in Texas and California. These vary in color from a reddish brown to a jet black and are used very largely as fuel.

The third group comprises the oils from Russia, which, like the second, are used largely for fuel purposes.

The light and easily ignited constituents of petroleum, such as naphtha, gasolene and kerosene, are oftentimes driven off by a partial distillation, these products being of greater value for other purposes than for use as fuel. This partial distillation does not decrease the value of petroleum as a fuel; in fact, the residuum known in trade as "fuel oil" has a slightly higher calorific value than petroleum and because of its higher flash point, it may be more safely handled. Statements made with reference to petroleum apply as well to fuel oil.

In general crude oil consists of carbon and hydrogen, though it also contains varying quantities of moisture, sulphur, nitrogen, arsenic, phosphorus and silt. The moisture contained may vary from less than 1 to over 30 per cent, depending upon the care taken to separate the water from the oil in pumping from the well. As in any fuel, this moisture affects the available heat of the oil, and in contracting for the purchase of fuel of this nature it is well to limit the per cent of moisture it may contain. A large portion of any contained moisture can be separated by settling and for this reason sufficient storage capacity should be supplied to provide time for such action.

A method of obtaining approximately the percentage of moisture in crude oil which may be used successfully, particularly with lighter oils, is as follows. A burette graduated into 200 divisions is filled to the 100 mark with gasolene, and the remaining 100 divisions with the oil, which should be slightly warmed before mixing. The two are then shaken together and any shrinkage below the 200 mark filled up with oil. The mixture should then be allowed to stand in a warm place for 24 hours, during which the water and silt will settle to the bottom. Their percentage by volume can then be correctly read on the burette divisions, and the percentage by weight calculated from the specific gravities. This method is exceedingly approximate and where accurate results are required it should not be used. For such work, the distillation method should be used as follows:

Gradually heat 100 cubic centimeters of the oil in a distillation flask to a temperature of 150 degrees centigrade; collect the distillate in a graduated tube and measure the resulting water. Such a method insures complete removal of water and reduces the error arising from the slight solubility of the water in gasolene. Two samples checked by the two methods for the amount of moisture present gave,

_Distillation_ _Dilution_ _Per Cent_ _Per Cent_ 8.71 6.25 8.82 6.26

TABLE 46

COMPOSITION AND CALORIFIC VALUE OF VARIOUS OILS

+-------------------------+-----+-----+----+--------+----+---+--------+-----+------------------------+ | Kind of Oil | %C | %H | %S | %O |S.G.|FP | %H2O |Btu |Authority | +-------------------------+-----+-----+----+--------+----+---+--------+-----+------------------------+ |California, Coaling | | | | |.927|134| |17117|Babcock & Wilcox Co. | |California, Bakersfield | | | | |.975| | |17600|Wade | |California, Bakersfield | | |1.30| |.992| | |18257|Wade | |California, Kern River | | | | |.950|140| |18845|Babcock & Wilcox Co. | |California, Los Angeles | | |2.56| | | | |18328|Babcock & Wilcox Co. | |California, Los Angeles | | | | |.957|196| |18855|Babcock & Wilcox Co. | |California, Los Angeles | | | | |.977| | .40 |18280|Babcock & Wilcox Co. | |California, Monte Christo| | | | |.966|205| |18878|Babcock & Wilcox Co. | |California, Whittier | | | .98| |.944| |1.06 |18507|Wade | |California, Whittier | | | .72| |.936| |1.06 |18240|Wade | |California |85.04|11.52|2.45| .99[44]| | |1.40 |17871|Babcock & Wilcox Co. | |California |81.52|11.51| .55|6.92[44]| |230| |18667|U.S.N. Liquid Fuel Board| |California | | | .87| | | | .95 |18533|Blasdale | |California | | | | |.891|257| |18655|Babcock & Wilcox Co. | |California | | |2.45| |.973| |1.50[45]|17976|O'Neill | |California | | |2.46| |.975| |1.32 |18104|Shepherd | |Texas, Beaumont |84.6 |10.9 |1.63|2.87 |.924|180| |19060|U.S.N. Liquid Fuel Board| |Texas, Beaumont |83.3 |12.4 | .50|3.83 |.926|216| |19481|U.S.N. Liquid Fuel Board| |Texas, Beaumont |85.0 |12.3 |1.75| .92[44]| | | |19060|Denton | |Texas, Beaumont |86.1 |12.3 |1.60| |.942| | |20152|Sparkes | |Texas, Beaumont | | | | |.903|222| |19349|Babcock & Wilcox Co. | |Texas, Sabine | | | | |.937|143| |18662|Babcock & Wilcox Co. | |Texas |87.15|12.33|0.32| |.908|370| |19338|U. S. N. | |Texas |87.29|12.32|0.43| |.910|375| |19659|U. S. N. | |Ohio |83.4 |14.7 |0.6 |1.3 | | | |19580| | |Pennsylvania |84.9 |13.7 | |1.4 |.886| | |19210|Booth | |West Virginia |84.3 |14.1 | |1.6 |.841| | |21240| | |Mexico | | | | |.921|162| |18840|Babcock & Wilcox Co. | |Russia, Baku |86.7 |12.9 | | |.884| | |20691|Booth | |Russia, Novorossick |84.9 |11.6 | |3.46 | | | |19452|Booth | |Russia, Caucasus |86.6 |12.3 | |1.10 |.938| | |20138| | |Java |87.1 |12.0 | | .9 |.923| | |21163| | |Austria, Galicia |82.2 |12.1 |5.7 | |.870| | |18416| | |Italy, Parma |84.0 |13.4 |1.8 | |.786| | | | | |Borneo |85.7 |11.0 | |3.31 | | | |19240|Orde | +-------------------------+-----+-----+----+--------+----+---+--------+-----+------------------------+

%C = Per Cent Carbon %H = Per Cent Hydrogen %S = Per Cent Sulphur %O = Per Cent Oxygen S.G. = Specific Gravity FP = Degrees Flash Point %H_{2}O = Per Cent Moisture Btu = B. t. u. Per Pound

Calorific Value--A pound of petroleum usually has a calorific value of from 18,000 to 22,000 B. t. u. If an ultimate analysis of an average sample be, carbon 84 per cent, hydrogen 14 per cent, oxygen 2 per cent, and assuming that the oxygen is combined with its equivalent of hydrogen as water, the analysis would become, carbon 84 per cent, hydrogen 13.75 per cent, water 2.25 per cent, and the heat value per pound including its contained water would be,

Carbon .8400 × 14,600 = 12,264 B. t. u. Hydrogen .1375 × 62,100 = 8,625 B. t. u. ------[**Should be .1375 x 62,000 = 8,525] Total 20,889 B. t. u.[**Would be Total = 20,789]

The nitrogen in petroleum varies from 0.008 to 1.0 per cent, while the sulphur varies from 0.07 to 3.0 per cent.

Table 46, compiled from various sources, gives the composition, calorific value and other data relative to oil from different localities.

The flash point of crude oil is the temperature at which it gives off inflammable gases. While information on the actual flash points of the various oils is meager, it is, nevertheless, a question of importance in determining their availability as fuels. In general it may be stated that the light oils have a low, and the heavy oils a much higher flash point. A division is sometimes made at oils having a specific gravity of 0.85, with a statement that where the specific gravity is below this point the flash point is below 60 degrees Fahrenheit, and where it is above, the flash point is above 60 degrees Fahrenheit. There are, however, many exceptions to this rule. As the flash point is lower the danger of ignition or explosion becomes greater, and the utmost care should be taken in handling the oils with a low flash point to avoid this danger. On the other hand, because the flash point is high is no justification for carelessness in handling those fuels. With proper precautions taken, in general, the use of oil as fuel is practically as safe as the use of coal.

Gravity of Oils--Oils are frequently classified according to their gravity as indicated by the Beaume hydrometer scale. Such a classification is by no means an accurate measure of their relative calorific values.

Petroleum as Compared with Coal--The advantages of the use of oil fuel over coal may be summarized as follows:

1st. The cost of handling is much lower, the oil being fed by simple mechanical means, resulting in,

2nd. A general labor saving throughout the plant in the elimination of stokers, coal passers, ash handlers, etc.

3rd. For equal heat value, oil occupies very much less space than coal. This storage space may be at a distance from the boiler without detriment.

4th. Higher efficiencies and capacities are obtainable with oil than with coal. The combustion is more perfect as the excess air is reduced to a minimum; the furnace temperature may be kept practically constant as the furnace doors need not be opened for cleaning or working fires; smoke may be eliminated with the consequent increased cleanliness of the heating surfaces.

5th. The intensity of the fire can be almost instantaneously regulated to meet load fluctuations.

6th. Oil when stored does not lose in calorific value as does coal, nor are there any difficulties arising from disintegration, such as may be found when coal is stored.

7th. Cleanliness and freedom from dust and ashes in the boiler room with a consequent saving in wear and tear on machinery; little or no damage to surrounding property due to such dust.

The disadvantages of oil are:

1st. The necessity that the oil have a reasonably high flash point to minimize the danger of explosions.

2nd. City or town ordinances may impose burdensome conditions relative to location and isolation of storage tanks, which in the case of a plant situated in a congested portion of the city, might make use of this fuel prohibitive.

3rd. Unless the boilers and furnaces are especially adapted for the use of this fuel, the boiler upkeep cost will be higher than if coal were used. This objection can be entirely obviated, however, if the installation is entrusted to those who have had experience in the work, and the operation of a properly designed plant is placed in the hands of intelligent labor.

TABLE 47

RELATIVE VALUE OF COAL AND OIL FUEL

+------+--------+-------+-----------------------------------------------+ |Gross | Net | Net | Water Evaporated from and at | |Boiler| Boiler |Evap- | 212 Degrees Fahrenheit per Pound of Coal | |Effic-|Effici- |oration+-----+-----+-----+-----+-----+-----+-----+-----+ | iency|ency[46]| from | | | | | | | | | | with | with |and at | | | | | | | | | | Oil | Oil | 212 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | | Fuel | Fuel |Degrees| | | | | | | | | | | |Fahren-| | | | | | | | | | | | heit +-----+-----+-----+-----+-----+-----+-----+-----+ | | | per | | | | | Pound | Pounds of Oil Equal to One Pound of Coal | | | |of Oil | | +------+--------+-------+-----+-----+-----+-----+-----+-----+-----+-----+ | 73 | 71 | 13.54 |.3693|.4431|.5170|.5909|.6647|.7386|.8124|.8863| | 74 | 72 | 13.73 |.3642|.4370|.5099|.5827|.6556|.7283|.8011|.8740| | 75 | 73 | 13.92 |.3592|.4310|.5029|.5747|.6466|.7184|.7903|.8621| | 76 | 74 | 14.11 |.3544|.4253|.4961|.5670|.6378|.7087|.7796|.8505| | 77 | 75 | 14.30 |.3497|.4196|.4895|.5594|.6294|.6993|.7692|.8392| | 78 | 76 | 14.49 |.3451|.4141|.4831|.5521|.6211|.6901|.7591|.8281| | 79 | 77 | 14.68 |.3406|.4087|.4768|.5450|.6131|.6812|.7493|.8174| | 80 | 78 | 14.87 |.3363|.4035|.4708|.5380|.6053|.6725|.7398|.8070| | 81 | 79 | 15.06 |.3320|.3984|.4648|.5312|.5976|.6640|.7304|.7968| | 82 | 80 | 15.25 |.3279|.3934|.4590|.5246|.5902|.6557|.7213|.7869| | 83 | 81 | 15.44 |.3238|.3886|.4534|.5181|.5829|.6447|.7125|.7772| +------+--------+-------+-----+-----+-----+-----+-----+-----+-----+-----+ | | | Net | | | | |Evap- | | | | |oration| | | | | from | | | | |and at | | | | | 212 | Barrels of Oil Equal to One Ton of Coal | | | |Degrees| | | | |Fahren-| | | | | heit | | | | | per | | | | |Barrel | | | | |of Oil | | +------+--------+-------+-----+-----+-----+-----+-----+-----+-----+-----+ | 73 | 71 | 4549 |2.198|2.638|3.077|3.516|3.955|4.395|4.835|5.275| | 74 | 72 | 4613 |2.168|2.601|3.035|3.468|3.902|4.335|4.769|5.202| | 75 | 73 | 4677 |2.138|2.565|2.993|3.420|3.848|4.275|4.703|5.131| | 76 | 74 | 4741 |2.110|2.532|2.954|3.376|3.798|4.220|4.642|5.063| | 77 | 75 | 4807 |2.082|2.498|2.914|3.330|3.746|4.162|4.578|4.994| | 78 | 76 | 4869 |2.054|2.465|2.876|3.286|3.697|4.108|4.518|4.929| | 79 | 77 | 4932 |2.027|2.433|2.838|3.243|3.649|4.054|4.460|4.865| | 80 | 78 | 4996 |2.002|2.402|2.802|3.202|3.602|4.003|4.403|4.803| | 81 | 79 | 5060 |1.976|2.371|2.767|3.162|3.557|3.952|4.348|4.743| | 82 | 80 | 5124 |1.952|2.342|2.732|3.122|3.513|3.903|4.293|4.683| | 83 | 81 | 5187 |1.927|2.313|2.699|3.085|3.470|3.856|4.241|4.627| +------+--------+-------+-----+-----+-----+-----+-----+-----+-----+-----+