Steam, Its Generation and Use Part 26

[Ill.u.s.tration: 3520 Horse-power Installation of Babc.o.c.k & Wilc.o.x Boilers at the Portland Railway, Light and Power Co., Portland, Ore. These Boilers are Equipped with Wood Refuse Extension Furnaces at the Front and Oil Burning Furnaces at the Mud Drum End]

SOLID FUELS OTHER THAN COAL AND THEIR COMBUSTION

Wood--Wood is vegetable tissue which has undergone no geological change.

Usually the term is used to designate those compact substances familiarly known as tree trunks and limbs. When newly cut, wood contains moisture varying from 30 per cent to 50 per cent. When dried for a period of about a year in the atmosphere, the moisture content will be reduced to 18 or 20 per cent.

TABLE 41

ULTIMATE a.n.a.lYSES AND CALORIFIC VALUES OF DRY WOOD (GOTTLIEB)

_______________________________________________________ | | | | | | | | | Kind | | | | | | B. t. u.| | of | C | H | N | O | Ash | per | | Wood | | | | | | Pound | |________|_______|______|______|_______|______|_________| | | | | | | | | | Oak | 50.16 | 6.02 | 0.09 | 43.36 | 0.37 | 8316 | | Ash | 49.18 | 6.27 | 0.07 | 43.91 | 0.57 | 8480 | | Elm | 48.99 | 6.20 | 0.06 | 44.25 | 0.50 | 8510 | | Beech | 49.06 | 6.11 | 0.09 | 44.17 | 0.57 | 8391 | | Birch | 48.88 | 6.06 | 0.10 | 44.67 | 0.29 | 8586 | | Fir | 50.36 | 5.92 | 0.05 | 43.39 | 0.28 | 9063 | | Pine | 50.31 | 6.20 | 0.04 | 43.08 | 0.37 | 9153 | | Poplar | 49.37 | 6.21 | 0.96 | 41.60 | 1.86 | 7834[40]| | Willow | 49.96 | 5.96 | 0.96 | 39.56 | 3.37 | 7926[40]| |________|_______|______|______|_______|______|_________|

Wood is usually cla.s.sified as hard wood, including oak, maple, hickory, birch, walnut and beech; and soft wood, including pine, fir, spruce, elm, chestnut, poplar and willow. Contrary to general opinion, the heat value per pound of soft wood is slightly greater than the same value per pound of hard wood. Table 41 gives the chemical composition and the heat values of the common woods. Ordinarily the heating value of wood is considered equivalent to 0.4 that of bituminous coal. In considering the calorific value of wood as given in this table, it is to be remembered that while this value is based on air-dried wood, the moisture content is still about 20 per cent of the whole, and the heat produced in burning it will be diminished by this amount and by the heat required to evaporate the moisture and superheat it to the temperature of the gases.

The heat so absorbed may be calculated by the formula giving the loss due to moisture in the fuel, and the net calorific value determined.

In designing furnaces for burning wood, the question resolves itself into: 1st, the essential elements to give maximum capacity and efficiency with this cla.s.s of fuel; and 2nd, the construction which will entail the least labor in handling and feeding the fuel and removing the refuse after combustion.

Wood, as used commercially for steam generating purposes, is usually a waste product from some industrial process. At the present time refuse from lumber and sawmills forms by far the greater part of this cla.s.s of fuel. In such refuse the moisture may run as high as 60 per cent and the composition of the fuel may vary over wide ranges during different portions of the mill operation. The fuel consists of sawdust, "hogged"

wood and slabs, and the percentage of each of these const.i.tuents may vary greatly. Hogged wood is mill refuse and logs that have been pa.s.sed through a "hogging machine" or macerator. This machine, through the action of revolving knives, cuts or shreds the wood into a state in which it may readily be handled as fuel.

Table 42 gives the moisture content and heat value of typical sawmill refuse from various woods.

TABLE 42

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 cla.s.s of fuel that a large combustion s.p.a.ce 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 s.p.a.ce for maintaining a thick fuel bed. A thick fuel bed is necessary in order to avoid excessive quant.i.ties of air pa.s.sing 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 quant.i.ties 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 s.p.a.ce 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 Babc.o.c.k & Wilc.o.x boilers this fuel may be burned without extension furnaces, provided that the boilers are set at a sufficient height to provide ample combustion s.p.a.ce 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 quant.i.ty. 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 quant.i.ty 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 cla.s.ses of wood fuel, a large combustion s.p.a.ce 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 s.p.a.ce 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.

Baga.s.se--Baga.s.se 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 baga.s.se 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 pa.s.sed 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 quant.i.ties of fuel, and the baga.s.se, 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 baga.s.se that might be readily burned.

The furnaces in use with these methods were as primitive as the rest of the apparatus, and the baga.s.se could be burned in them only by first drying it. This naturally required an enormous quant.i.ty 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 baga.s.se. Some of the designs were more or less clever, and about the year 1880 several such green baga.s.se 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 compet.i.tion 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 baga.s.se.

On several of the plantations horizontal mult.i.tubular boilers externally fired were installed and at the time were considered the acme of perfection. Numerous attempts were made to burn the baga.s.se 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 Babc.o.c.k & Wilc.o.x 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 baga.s.se 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 baga.s.se 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 baga.s.se 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 baga.s.se was carried to the extraction of practically the last trace of sugar contained in it. The quant.i.ty 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 baga.s.se 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 disburs.e.m.e.nt 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 cla.s.ses 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 Babc.o.c.k & Wilc.o.x 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 baga.s.se furnace which is now offered, and it has been adopted as standard for this cla.s.s 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 baga.s.se.

Composition and Calorific Value of Baga.s.se--The proportion of fiber contained in the cane and density of the juice are important factors in the relation the baga.s.se 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 baga.s.se 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 baga.s.se 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 a.n.a.lysis of any sample of baga.s.se its approximate calorific value may be calculated from the formula,

8550F + 7119S + 6750G - 972W B. t. u. per pound baga.s.se = ---------------------------- (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 baga.s.se, 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 baga.s.se in which the ash is a.s.sumed 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 baga.s.se 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 baga.s.se 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 a.n.a.lysis 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, a.s.sume the residual juice (100 per cent) to contain 12 per cent organic matter.

From the constant in formula,

127119 (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 baga.s.se 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 baga.s.se of a high moisture content which will not burn properly but which smoulders and produces a large quant.i.ty of products of destructive distillation, chiefly heavy hydrocarbons, which escape unburnt. The reasoning, however, is sufficient to explain the steam making properties of baga.s.se 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 baga.s.se meets all of the steam requirements of the mills without auxiliary fuel.

A high percentage of silica or salts in baga.s.se has sometimes been ascribed as the reason for the tendency to smoulder in certain cases of soft fiber baga.s.se. This, however, is due to the large moisture content of the sample resulting directly from the nature of the cane. Soluble salts in the baga.s.se 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

a.n.a.lYSES AND CALORIFIC VALUES OF BAGa.s.sE +---------------------------------------------------------------------+ |+----------+--------+-------+-------+-------+-------+-------+-------+| || | | | | | | |B.t.u. || || | | | | | | | per || || Source |Moisture| C | H | O | N | Ash | Pound || || | | | | | | | Dry || || | | | | | | |Baga.s.se|| |+----------+--------+-------+-------+-------+-------+-------+-------+| ||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 a.n.a.lyses and heat values of baga.s.se from various localities. Table 44 gives the value of mill baga.s.se 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