Part 41

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Progress and improvement in the manufacture itself was going on at a remarkable rate, the single grinding had been replaced by a double grinding, this in turn by a third grinding, and finally the maceration and dilution of the bagasse was carried to the extraction of practically the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.

TABLE 43

ANALYSES AND CALORIFIC VALUES OF BAGASSE +---------------------------------------------------------------------+ |+----------+--------+-------+-------+-------+-------+-------+-------+| || | | | | | | |B.t.u. || || | | | | | | | per || || Source |Moisture| C | H | O | N | Ash | Pound || || | | | | | | | Dry || || | | | | | | |Bagasse|| |+----------+--------+-------+-------+-------+-------+-------+-------+| ||Cuba | 51.50 | 43.15 | 6.00 | 47.95 | | 2.90 | 7985 || ||Cuba | 49.10 | 43.74 | 6.08 | 48.61 | | 1.57 | 8300 || ||Cuba | 42.50 | 43.61 | 6.06 | 48.45 | | 1.88 | 8240 || ||Cuba | 51.61 | 46.80 | 5.34 | 46.35 | | 1.51 | || ||Cuba | 52.80 | 46.78 | 5.74 | 45.38 | | 2.10 | || ||Porto Rico| 41.60 | 44.28 | 6.66 | 47.10 | 0.41 | 1.35 | 8359 || ||Porto Rico| 43.50 | 44.21 | 6.31 | 47.72 | 0.41 | 1.35 | 8386 || ||Porto Rico| 44.20 | 44.92 | 6.27 | 46.50 | 0.41 | 1.90 | 8380 || ||Louisiana | 52.10 | | | | | 2.27 | 8230 || ||Louisiana | 54.00 | | | | | | 8370 || ||Louisiana | 51.80 | | | | | | 8371 || ||Java | | 46.03 | 6.56 | 45.55 | 0.18 | 1.68 | 8681 || |+----------+--------+-------+-------+-------+-------+-------+-------+| +---------------------------------------------------------------------+

Table 43 gives the analyses and heat values of bagasse from various localities. Table 44 gives the value of mill bagasse at different extractions, which data may be of service in making approximations as to its fuel value as compared with that of other fuels.

TABLE 44

VALUE OF ONE POUND OF MILL BAGASSE AT DIFFERENT EXTRACTIONS

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 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.

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.