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.
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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, [Pg 203] 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 space 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 Babcock & Wilcox boilers this fuel may be burned without extension furnaces, provided that the boilers are set at a sufficient height to provide ample combustion space 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 quantity. 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 quantity 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 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. Ample combustion space 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.
Bagasse —Bagasse 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 bagasse 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.