Los mbororo y su “despertar” político

Wood is vegetable tissue that has undergone no geological change. When newly cut, wood contains from 30 to 50% of moisture. When dried in the atmosphere for approximately 1 year, the moisture content is reduced to 18 or 20%.

Wood is ordinarily classified as hardwood, including oak, maple, hickory, birch, walnut and beech, and softwood, including pine, fir, spruce, elm, chestnut, poplar, and willow. While, theoretically, equal weights of wood substance should generate the same amount of heat, regardless of species, practically the varying form of wood tissue and the presence of rosins, gums, tannin, oils, and pigments result in different heating values, and, more particularly, in a difference in the ease with which combustion can be accomplished. Rosin may increase the heating value as much as 12%. Contrary to general opinion, the heat value per pound of softwood is slightly greater than that of hardwood.

The heat values of wood fuels are ordinarily reported on a dry basis. It is to be remembered, however, that because of the high moisture content, the ratio of the amount of heat available for steam generation to that of the dry fuel is much lower than that of practically all other solid fuels. Even woods that are air dried contain approximately 20% moisture, and this moisture must be evaporated and superheated to the temperature of the escaping gases before the heat evolved, for absorption by the boiler, can be determined.

In industrial wood refuse from lumber mills and sawmills, the moisture content may run as high as 60% and the composition of the fuel may vary over a wide range during different periods of mill operation. The fuel consists of saw-dust, ‘‘hogged’’ wood, and slabs, and the proportions of these may vary widely.

Hogged wood is mill refuse and logs that have been passed through a ‘‘hog’’

machine or macerator that cuts or shreds the wood with rotating knives to a state in which it may be readily handled as fuel.

1. Furnace Design

The principal features of furnace design for the satisfactory combustion of wood fuel are ample furnace volume and the presence of a large area of heated brick-work to radiate heat to the fuel bed. The latter factor is of particular importance in the case of wet wood, and ordinarily necessitates the use of an extension fur-nace. A furnace of this form not only gives the required amount of heated brick-work for proper combustion, but enables the fuel, in the case of hogged wood and sawdust, to be most readily fed to the furnace. With wet mill refuse, the furnace should be ‘‘bottled’’ at its exit to maintain as high a temperature as possible, the extent to which the bottling effect is carried being primarily depen-dent on the moisture content of the fuel and being greater as the moisture content is higher. The bottling effect, which is ordinarily secured by a variation in the height of the extension furnace bridge wall, has, in several recent installations, been accomplished by the use of a ‘‘drop-nose’’ arch at the rear of the furnace combustion arch.

Secondary air for combustion is of assistance in securing proper results and may be admitted through the bridge wall to the furnace or, where there is a secondary combustion space behind the bridge wall, into that space.

For hogged wood and sawdust, the fuel is fed through fuel chutes in the roof of the extension furnace, ordinarily being brought from the storage supply to the chutes by some type of conveyor system. With this class of wood fuel, in-swinging fire doors are placed at the furnace front for fire-inspection purposes.

Where slabs are burned in addition to hogged wood and sawdust, large side-hinged slab firing doors are usually installed above the in-swinging doors.

Fuel chutes should be circular on the inside and square outside, such design enabling them to be installed most readily in the furnace roof. For ordinary mill refuse, the chute should be 12 in. in diameter, although for shingle mill refuse the size should be 18 in.

Each fuel chute should handle a square unit of grate surface, the dimensions of such units varying from 4⫻ 4 to 8 ⫻ 8 ft, depending on the moisture content and nature of the fuel.

Dry sawdust, chips, blocks, and veneer are frequently burned in plants of the woodworking industry. With such fuel, as with wet wood refuse, an ample furnace volume is essential, although because of the lower moisture content, the presence of heated brickwork is not as necessary as with wet wood fuel.

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. Cord wood and slabs form an open fire through which the frictional loss of the air is much less than for sawdust or hogged mate-rial. The combustion rate with cord wood is, therefore, higher, and the grate surface may be considerable reduced. Such wood is usually cut in lengths of 4

108 Chapter 4

ft or 4 ft 6 in., and the depth of the grates should be kept approximately 5 ft to obtain the best results.

D. Bagasse [31, 32]

Bagasse is the refuse of sugar cane from which the juice has been extracted, and from the beginning of the sugar industry, it has been the natural fuel for sugar plantation power plants. Physically it consists of matted cellulose fibers and fine particles, the percentage of each varying with the process. Bagasse generally contains about 50% moisture and has a heat content of 3600–4200 Btu/lb as fired. It is used chiefly as a fuel to generate steam and power for the plant. Other by-product uses are for cellulose, for paper and paperboard manufacture, and for furfural production.

In the early days of sugar manufacture, the cane was passed through a single mill and the defecation and concentration of the saccharine juice took place in a series of vessels mounted over a common flue with a fire at one end and a stack at the other. This method required an enormous amount of fuel, and it was frequently necessary to sacrifice the degree of extraction to obtain the necessary amount of bagasse and a bagasse that could be burned. In the primitive furnaces of early practice, it was necessary to dry the bagasse before it could be burned, and the amount of labor involved in spreading and collecting it was great.

With the general abolition of slavery and resulting increased labor cost of production, and with growing competition from European beet sugar, it was necessary to increase the degree of extraction, the single mill being replaced by the double mill, and the open wall or Jamaica train method of extraction as just described was replaced by vacuum-evaporating apparatus and centrifugal ma-chines. Later a third grinding was introduced, and the maceration and dilution of the bagasse were carried to a point where the last trace of sugar in the bagasse was practically eliminated. The amount of juice to be treated was increased by these improved manufacturing methods from 20 to 30%, but the amount of ba-gasse available for fuel and its calorific value as fuel were decreased to an extent that the combustion capacity of the furnaces available could not meet. In the older plants the raw cane was ground by passing it in series through sets of grooved rolls, each set comprising a mill having finer groves than the preceding one. Modern practice incorporates a shredder that cuts the cane with revolving knives before the tandem milling previously described. The end product has a higher percentage of fines and short fibers. For the steam-generation end of manu-facture to keep pace with the process end, it was necessary to develop a more efficient method of burning the bagasse commercially than that employed in the drying of the fuel.

During the transition period of manufacture may furnaces were ‘‘invented’’

for burning green bagasse, the saving in labor by this method over that necessary in spreading, drying, and collecting the fuel obviously being the primary factor

in reduction of the cost of steam generation. None of these furnaces, however, gave satisfactory results until the hot-blast bagasse furnace was introduced in 1888. Although furnaces of this design operated satisfactorily, their construction was expensive and, because of the cost to the planters in changing to improved sugar manufacture apparatus, they were difficult to introduce.

1. Composition and Calorific Value of Bagasse

The proportion of fiber contained in the cane and the density of the juice are important factors in the relation the bagasse fuel will have to the total fuel neces-sary 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 subjected 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 caloric value. The factors that enter here are the age at which the cane must be cut, the locality in which it is grown, and so on. From the analysis of any sample of bagasse its approximate caloric value may be calculated from the formula

Btu/lb bagasse⫽8550F⫹ 7119S ⫹ 6750G ⫺ 972W 100

Where F⫽ percentage of fiber in cane, S ⫽ percentage of sucrose, G ⫽ percentage of glucose, and W⫽ percentage of 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.

A sample of Java bagasse having F⫽ 46.5, S ⫽ 4.5, G ⫽ 0.5, W ⫽ 47.5 gives Btu of 3868. These figures show that the more nearly dry the bagasse is, the higher the caloric value, although this is accompanied by a decrease in su-crose. The explanation is that the presence of sucrose in an analysis is accompa-nied 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 fur-nace.

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 have also been given as the explanation of such smoldering action of the fire, but here too, the explana-tion lies solely in the high moisture content, this resulting in the development of only sufficient heat to evaporate the moisture.

2. 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

110 Chapter 4

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, Puerto Rico, and like countries, however, auxiliary fuel is almost universally a necessity. The amount will vary, largely depending on 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 on the degree of purity of the manufactured sugar, the use of the maceration water, and the efficiency of the mill apparatus as a whole.

In general, it may be stated that this class of fuel may be best burned in large quantities. Because of this fact, and to obtain the efficient combustion re-sulting from burning a bulk of this fuel, a single large furnace is frequently in-stalled between two boilers, serving both, although there is a limit to the size of boiler units that may be set in this manner. A disadvantage of this type of installa-tion results from the necessity of having two boiler units out of service when it is necessary to take the furnace down for repairs, requiring a greater boiler capac-ity than if single furnaces are installed to assure continucapac-ity of service. On the other hand, the lower cost of one large furnace as against that of two individual smaller furnaces, and the increased efficiency of combustion with the former, may more than offset this disadvantage.

As with wet wood refuse and, as a matter of fact, for all fuels containing an excessive moisture content, the essential features of furnace design for the proper combustion of green bagasse are ample combustion space, a large mass of furnace brickwork for maintaining furnace temperature, and a length of gas travel sufficient to enable combustion to be completed before the boiler-heating surfaces are encountered. The fuel is burned either on a hearth or on grates. The objection to the latter method, particularly where blast is used, is that the air for combustion enters largely around the edges of the fuel pile where the bed is thinnest. Furthermore, when the fuel is burned on grates, the tendency of the ash and refuse to stop the air spaces does not allow a constant combustion rate for a given draft, and because there is a combustion rate that represents the best efficiency with this class of fuel, such efficiency cannot be maintained throughout the entire period between cleaning intervals. If the bagasse is burned on a hearth, the ash and refuse form on the hearth, do not affect the air supply, and allow a constant combustion rate to be maintained. When burned on a hearth, the air for combustion is admitted through a series of tuyeres extending around the furnace and upward from the hearth. In some cases a combination of grates and tuyeres has been used. When air is admitted through tuyeres, it impinges on the fuel pile as a whole and gives a uniform combustion. The tuyeres are connected to an annular space in which, where blast is used, the pressure is controlled by a blower.

As stated, bagasse is best burned in large quantities, with corresponding high combustion rates. When burned on grates with a natural draft of 0.3 in. of water in the furnace, a combustion rate of from 250 to 300 lb/ft²of grate surface per hour may be obtained, whereas with a blast of 0.5 in. this rate may be in-creased to approximately 450 lbs. When burned on a hearth with a blast of 0.75 in. a combustion rate of approximately 450 lb/ft² of hearth per hour may be obtained, whereas with the blast increased to 1.6 in., this rate may be increased to approximately 650 lbs. These rates apply to bagasse containing about 50%

moisture. It would appear that when burned on grates the most efficient combus-tion rate is approximately 300 lb/ft² of grate per hour, and as stated this rate is obtainable with natural draft. When burned on a hearth, and with blast, the most efficient rate is about 450 lb/ft² of hearth per hour, which rate requires a blast of approximately 0.75 in.

The hearth on which the bagasse is burned is ordinarily elliptical. Air for combustion is admitted through a series of tuyeres above the hearth line. The supply of air is controlled by the amount and pressure of the air within the annular space to which the tuyeres are connected. Secondary air for combustion is admit-ted at the rear of the bridge wall, as indicaadmit-ted. The roof of the furnace is ordinarily spherical, with its top from 11 to 13 ft above the grate or hearth. The products of combustion pass from the primary combustion chamber under an arch to a secondary combustion chamber. A furnace of this design embodies the essential features of ample combustion space, the mass of heated brickwork necessitated by the high moisture content of the fuel, and a long travel of gases before the boiler-heating surfaces are encountered. The fuel is fed through the roof of the furnace, preferably by some mechanical method that will assure a constant fuel supply and at the same time prevent the inrush of cold air into the furnace.

This class of fuel deposits an appreciable quantity of dust and ash which, if not removed promptly, fuses into a hard, glass-like clinker. Ample provision should be made for the removal of this material from the furnace, the gas ducts, and the boiler setting and heating surfaces.

As a fuel for the production of steam, bagasse has been burned in several types of furnaces, the oldest being a Dutch oven with flat grates. Since it was difficult to distribute the bagasse evenly on the grates, the latter were subject to high maintenance costs from burning. Therefore, a new type of furnace was developed to burn the bagasse in a pile on a refractory hearth. Air was admitted to the pile around its circumference through tuyeres. The most popular of these extension furnaces was the Cooke, but it also suffered from high-cost mainte-nance because of excessive radiation and cleaning difficulties. To overcome these problems the Ward furnace was designed. The Ward furnace has been very suc-cessfully used under sugar-mill boilers. It is easy to operate and maintain. Bagasse is gravity fed through chutes to the individual cells, where it burns from the surface of the pile with approximately 85% of the air that is injected into the sides of the pile adjacent to the hearth. This causes local incomplete combustion,

112 Chapter 4

but there is sufficient heat released to partially dry the entering raw fuel. Addi-tional drying is accomplished by radiant heat reflected from the hot refractory to the cells. Combustion is completed in the secondary furnace above the arch.

Ward furnaces are now equipped with dumping hearths, which permit the ashes to be removed while the unit is in operation.

Mechanical harvesting of sugar cane increases the amount of dirt in the bagasse to as much as 5–10%. To overcome the resultant slagging tendency of the ash, watercooling is incorporated in the furnaces. In the older mills, the drives for the milling equipment were large reciprocating steam engines, which used steam at a maximum of 150 psi and with a few degrees of superheat, exhausting at 15 psi to the boilinghouse steam supply. In more modern mills the drives are

Mechanical harvesting of sugar cane increases the amount of dirt in the bagasse to as much as 5–10%. To overcome the resultant slagging tendency of the ash, watercooling is incorporated in the furnaces. In the older mills, the drives for the milling equipment were large reciprocating steam engines, which used steam at a maximum of 150 psi and with a few degrees of superheat, exhausting at 15 psi to the boilinghouse steam supply. In more modern mills the drives are