Part 23

Lignites--As the content of volatile matter and moisture in lignite is higher than in bituminous coal, the difficulties encountered in burning them are greater. A large combustion space is required and the best results are obtained where a furnace of the reverberatory type is used, giving the gases a long travel before meeting the tube surfaces. A fuel bed from 4 to 6 inches in depth can be maintained, and the coal should be fired in small quantities by the alternate method. Above certain rates of combustion clinker forms rapidly, and a steam jet in the ashpit for softening this clinker is often desirable. A considerable draft should be available, but it should be carefully regulated by the boiler damper to suit the condition of the fire. Smokelessness with hand firing with this class of fuel is a practical impossibility. It has a strong tendency to foul the heating surfaces rapidly and these surfaces should be cleaned frequently. Shaking grates, intelligently handled, aid in cleaning the fires, but their manipulation must be carefully watched to prevent good coal being lost in the ashpit.

Stokers--The term "automatic stoker" oftentimes conveys the erroneous impression that such an apparatus takes care of itself, and it must be thoroughly understood that any stoker requires expert attention to as high if not higher degree than do hand-fired furnaces.

Stoker-fired furnaces have many advantages over hand firing, but where a stoker installation is contemplated there are many factors to be considered. It is true that stokers feed coal to the fire automatically, but if the coal has first to be fed to the stoker hopper by hand, its automatic advantage is lost. This is as true of the removal of ash from a stoker. In a general way, it may be stated that a stoker installation is not advantageous except possibly for diminishing smoke, unless the automatic feature is carried to the handling of the coal and ash, as where coal and ash handling apparatus is not installed there is no saving in labor. In large plants, however, stokers used in conjunction with the modern methods of coal storage and coal and ash handling, make possible a large labor saving. In small plants the labor saving for stokers over hand-fired furnaces is negligible, and the expense of the installation no less proportionately than in large plants. Stokers are, therefore, advisable in small plants only where the saving in fuel will be large, or where the smoke question is important.

Interest on investment, repairs, depreciation and steam required for blast and stoker drive must all be considered. The upkeep cost will, in general, be higher than for hand-fired furnaces. Stokers, however, make possible the use of cheaper fuels with as high or higher economy than is obtainable under operating conditions in hand-fired furnaces with a better grade of fuel. The better efficiency obtainable with a good stoker is due to more even and continuous firing as against the intermittent firing of hand-fired furnaces; constant air supply as against a variation in this supply to meet varying furnace conditions in hand-fired furnaces; and the doing away to a great extent with the necessity of working the fires.

Stokers under ordinary operating conditions will give more nearly smokeless combustion than will hand-fired furnaces and for this reason must often be installed regardless of other considerations. While a constant air supply for a given power is theoretically secured by the use of a stoker, and in many instances the draft is automatically governed, the air supply should, nevertheless, be as carefully watched and checked by flue gas analyses as in the case of hand-fired furnaces.

There is a tendency in all stokers to cause the loss of some good fuel or siftings in the ashpit, but suitable arrangements may be made to reclaim this.

In respect to efficiency of combustion, other conditions being equal, there will be no appreciable difference with the different types of stokers, provided that the proper type is used for the grade of fuel to be burned and the conditions of operation to be fulfilled. No stoker will satisfactorily handle all classes of fuel, and in making a selection, care should be taken that the type is suited to the fuel and the operating conditions. A cheap stoker is a poor investment. Only the best stoker suited to the conditions which are to be met should be adopted, for if there is to be a saving, it will more than cover the cost of the best over the cheaper stoker.

Mechanical Stokers are of three general types: 1st, overfeed; 2nd, underfeed; and 3rd, traveling grate. The traveling grate stokers are sometimes classed as overfeed but properly should be classed by themselves as under certain conditions they are of the underfeed rather than the overfeed type.

Overfeed Stokers in general may be divided into two classes, the distinction being in the direction in which the coal is fed relative to the furnaces. In one class the coal is fed into hoppers at the front end of the furnace onto grates with an inclination downward toward the rear of about 45 degrees. These grates are reciprocated, being made to take alternately level and inclined positions and this motion gradually carries the fuel as it is burned toward the rear and bottom of the furnace. At the bottom of the grates flat dumping sections are supplied for completing the combustion and for cleaning. The fuel is partly burned or coked on the upper portion of the grates, the volatile gases driven off in this process for a perfect action being ignited and burned in their passage over the bed of burning carbon lower on the grates, or on becoming mixed with the hot gases in the furnace chamber. In the second class the fuel is fed from the sides of the furnace for its full depth from front to rear onto grates inclined toward the center of the furnace. It is moved by rocking bars and is gradually carried to the bottom and center of the furnace as combustion advances. Here some type of a so-called clinker breaker removes the refuse.

Underfeed Stokers are either horizontal or inclined. The fuel is fed from underneath, either continuously by a screw, or intermittently by plungers. The principle upon which these stokers base their claims for efficiency and smokelessness is that the green fuel is fed under the coked and burning coal, the volatile gases from this fresh fuel being heated and ignited in their passage through the hottest portion of the fire on the top. In the horizontal classes of underfeed stokers, the

## action of a screw carries the fuel back through a retort from which it

passes upward, as the fuel above is consumed, the ash being finally deposited on dead plates on either side of the retort, from which it can be removed. In the inclined class, the refuse is carried downward to the rear of the furnace where there are dumping plates, as in some of the overfeed types.

Underfeed stokers are ordinarily operated with a forced blast, this in some cases being operated by the same mechanism as the stoker drive, thus automatically meeting the requirements of various combustion rates.

Traveling Grates are of the class best illustrated by chain grate stokers. As implied by the name these consist of endless grates composed of short sections of bars, passing over sprockets at the front and rear of the furnace. Coal is fed by gravity onto the forward end of the grates through suitable hoppers, is ignited under ignition arches and is carried with the grate toward the rear of the furnace as its combustion progresses. When operated properly, the combustion is completed as the fire reaches the end of the grate and the refuse is carried over this rear end by the grate in making the turn over the rear sprocket. In some cases auxiliary dumping grates at the rear of the chain grates are used with success.

Chain grate stokers in general produce less smoke than either overfeed or underfeed types, due to the fact that there are no cleaning periods necessary. Such periods occur with the latter types of stokers at intervals depending upon the character of the fuel used and the rate of combustion. With chain grate stokers the cleaning is continuous and automatic, and no periods occur when smoke will necessarily be produced.

In the earlier forms, chain grates had an objectionable feature in that the admission of large amounts of excess air at the rear of the furnace through the grates was possible. This objection has been largely overcome in recent models by the use of some such device as the bridge wall water box and suitable dampers. A distinct advantage of chain grates over other types is that they can be withdrawn from the furnace for inspection or repairs without interfering in any way with the boiler setting.

This class of stoker is particularly successful in burning low grades of coal running high in ash and volatile matter which can only be burned with difficulty on the other types. The cost of up-keep in a chain grate, properly constructed and operated, is low in comparison with the same cost for other stokers.

The Babcock & Wilcox chain grate is representative of this design of stoker.

Smoke--The question of smoke and smokelessness in burning fuels has recently become a very important factor of the problem of combustion. Cities and communities throughout the country have passed ordinances relative to the quantities of smoke that may be emitted from a stack, and the failure of operators to live up to the requirements of such ordinances, resulting as it does in fines and annoyance, has brought their attention forcibly to the matter.

The whole question of smoke and smokelessness is to a large extent a comparative one. There are any number of plants burning a wide variety of fuels in ordinary hand-fired furnaces, in extension furnaces and on automatic stokers that are operating under service conditions, practically without smoke. It is safe to say, however, that no plant will operate smokelessly under any and all conditions of service, nor is there a plant in which the degree of smokelessness does not depend largely upon the intelligence of the operating force.

[Illustration: Fig. 26. Babcock & Wilcox Boiler and Superheater Equipped with Babcock & Wilcox Chain Grate Stoker. This Setting has been

## Particularly Successful in Minimizing Smoke]

When a condition arises in a boiler room requiring the fires to be brought up quickly, the operatives in handling certain types of stokers will use their slice bars freely to break up the green portion of the fire over the bed of partially burned coal. In fact, when a load is suddenly thrown on a station the steam pressure can often be maintained only in this way, and such use of the slice bar will cause smoke with the very best type of stoker. In a certain plant using a highly volatile coal and operating boilers equipped with ordinary hand-fired furnaces, extension hand-fired furnaces and stokers, in which the boilers with the different types of furnaces were on separate stacks, a difference in smoke from the different types of furnaces was apparent at light loads, but when a heavy load was thrown on the plant, all three stacks would smoke to the same extent, and it was impossible to judge which type of furnace was on one or the other of the stacks.

In hand-fired furnaces much can be accomplished by proper firing. A combination of the alternate and spreading methods should be used, the coal being fired evenly, quickly, lightly and often, and the fires worked as little as possible. Smoke can be diminished by giving the gases a long travel under the action of heated brickwork before they strike the boiler heating surfaces. Air introduced over the fires and the use of heated arches, etc., for mingling the air with the gases distilled from the coal will also diminish smoke. Extension furnaces will undoubtedly lessen smoke where hand firing is used, due to the increase in length of gas travel and the fact that this travel is

## partially under heated brickwork. Where hand-fired grates are

immediately under the boiler tubes, and a high volatile coal is used, if sufficient combustion space is not provided the volatile gases, distilled as soon as the coal is thrown on the fire, strike the tube surfaces and are cooled below the burning point before they are wholly consumed and pass through as smoke. With an extension furnace, these volatile gases are acted upon by the radiant heat from the extension furnace arch and this heat, together with the added length of travel causes their more complete combustion before striking the heating surfaces than in the former case.

Smoke may be diminished by employing a baffle arrangement which gives the gases a fairly long travel under heated brickwork and by introducing air above the fire. In many cases, however, special furnaces for smoke reduction are installed at the expense of capacity and economy.

From the standpoint of smokelessness, undoubtedly the best results are obtained with a good stoker, properly operated. As stated above, the best stoker will cause smoke under certain conditions. Intelligently handled, however, under ordinary operating conditions, stoker-fired furnaces are much more nearly smokeless than those which are hand fired, and are, to all intents and purposes, smokeless. In practically all stoker installations there enters the element of time for combustion, the volatile gases as they are distilled being acted upon by ignition or other arches before they strike the heating surfaces. In many instances too, stokers are installed with an extension beyond the boiler front, which gives an added length of travel during which, the gases are acted upon by the radiant heat from the ignition or supplementary arches, and here again we see the long travel giving time for the volatile gases to be properly consumed.

To repeat, it must be emphatically borne in mind that the question of smokelessness is largely one of degree, and dependent to an extent much greater than is ordinarily appreciated upon the handling of the fuel and the furnaces by the operators, be these furnaces hand fired or automatically fired.

[Illustration: 3520 Horse-power Installation of Babcock & Wilcox 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 ANALYSES 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 classified 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 class 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 class 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 constituents may vary greatly. Hogged wood is mill refuse and logs that have been passed 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 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.