Contexto histórico-político y legado colonial

Woodworking plants, such as furniture factories, box factories, planing mills, and other similar industries are the principal sources of dry wood for steam-generation purposes. Although the refuse from these plants may contain as high as 25% moisture, the average will generally be in the neighborhood of 20%. The wood to be burned consists of large percentages of sawdust and shaving, with considerable lesser amounts of edgings, blocks, slabs, and sticks. Because dry wood burns readily, it is necessary to apply different principles in the design of furnaces for this fuel. Furthermore, the problem of providing suitable furnace cooling is of great importance, as high flame temperatures are developed when burning wood with low excess-air quantities. Under these conditions, the silica

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and alkaline constituents of the wood ash are combined to form a low-fusion–

temperature slag, which fluxes with the silica in the refractories. As a result, there is considerable wall erosion. Even though air-cooled refractory walls have served to reduce somewhat the extent and penetration of this erosion, the use of substan-tial amounts of watercooled surface over the furnace walls minimizes mainte-nance and considerably lengthens the time between outages for necessary repair.

The location and area of these furnace watercooling surfaces must be carefully studied, so that sufficiently high furnace temperatures, as required for smokeless combustion, may be maintained and, at the same time, fusion of any exposed refractories may be avoided.

Dry wood furnaces may be divided into two general types: one for burning the wood partly in suspension and partly on flat or sloping grates, and the other for burning sawdust, shavings, and other hogged wood in suspension.

1. Flat Grate Furnace

The flat grate-type furnace was formerly commonly used in many furniture facto-ries and planing mills. Cyclone collectors supplied the dry wood, simultaneously with large quantities of excess air, through chutes to the furnace. The wood was burned as produced, and the quantity was therefore irregular. Some of it burned in suspension, while the remainder smoldered in uneven piles that spotted the grate because distribution and supply lacked uniformity. The furnace volume provided was exceedingly small, and this lack of sufficient combustion space resulted in incomplete combustion of the large amount of volatile matter in the wood, notwithstanding the presence of large quantities of excess air, and caused the production of dense smoke at practically all burning rates. Furthermore, flame impingement on the boiler surfaces, because of the short distance between them and the fuel bed, resulted in chilling of the burning gas to produce additional smoke, along with deposits of soot, in large quantities, throughout the boiler passes. Many of these older installations are now being replaced with properly designed furnaces in which modern feeding and burning equipment are used.

2. Fuel Feeding Equipment

An important requirement for these newer installations is the use of suitable equipment to feed properly sized fuel to the furnace. The regulation of dry wood supply is important, because the steam demands for plant operation may bear only a small relation to the simultaneous fuel production cycle. Therefore, it is necessary to control the wood fed to the furnace to avoid flooding with fuel at times of low steam demand, or starving at times of high demand. The use of a fuel storage bin, equipped with some form of feeding device, provides the means for control of fuel flow. These bins are usually of the flat-bottom type, with slightly tapering sides. In the bottom are several helicoid screws used to agitate the fuel, to overcome any tendency to arching, and at the same time slip it forward to a horizontal screw conveyor. Operating in synchronism with this screw

con-veyor is a star wheel feeder to control fuel supply and provide sealing against any sparks or backfiring into the storage bin, or against needless infiltration of air into the furnace through the feed openings.

3. Inclined Grate Furnace

An inclined grate, similar to that for wet wood, may be used in burning hogged dry wood. The slope of the fuel supporting surface, however, is decreased to approximately 30 degrees. The upper section is composed of stationary elements, with horizontal air spaces formed by a series of ledges, which also act as retarders to fuel slippage. This construction provides a nonsifting feature to prevent wood particles from falling into the windbox. Alternate longitudinal sections of the lower grate are equipped with pushers to move the fuel gradually, as it burns, down the grate. Retarders are located at the end of the grate, so that accumulated refuse can be dumped without danger of the entire fuel bed slipping into the ashpit. These inclined grates are applicable to both large and small boiler units.

4. Furnaces for Suspension Burning

Furnaces for burning sawdust, shavings, and hogged dry wood in suspension find their application primarily in those industries where the steam demands are such that large units are required. In most of these it is also necessary to provide for an auxiliary fuel that is used when the wood supply is low. Pulverized coal, oil, and gas are well adapted to these applications because design requirements and disposition of furnace volume are practically the same as for the wood.

Because of the ease with which dry wood is kindled, temperature in a re-fractory furnace is sufficiently high to maintain ignition at all loads, and arches are not required. The wood is supplied to the furnace through openings in the upper part of the frontwall. As it falls, the major portion is burned in suspension, while the larger particles drop to the hearth and are burned in the same manner as on a flat grate. Air for combustion is supplied through a series of tuyeres located in the lower portion of the furnace walls. The air streams from these are directed to sweep the pile of accumulated wood, and also to set up a zone of turbulence that breaks up any stratification and produces uniform mixture of the gas leaving the furnace. The earlier design used air-cooled refractory walls, the lanes of which were connected to the wood-burning tuyeres or the auxiliary fuel burners. For reasonable maintenance, the heat liberation rate in these refractory furnaces is limited to approximately 15,000 Btu/ft³hr^⫺1.

5. Watercooled Wall Construction

The application of watercooled wall construction resulted from the necessity for overcoming excessive outage, caused by fluxing of refractories by the wood ash, when furnace volume is otherwise insufficient to develop the required rate of steam output. At first there was a feeling that the cooling effect of bare metallic walls, capable of high rates of radiant heat absorption, would chill the furnace

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to a point where ignition would be impaired. Nevertheless, a number of units were installed in several plants. In these a large portion if the furnace sidewalls were watercooled, whereas the front and rearwalls were of refractory construc-tion. Operation was successful, and availability increased to the extent that out-age, owing to furnacewall failure, became practically nonexistent. In addition, it was possible to maintain combustion rates of 20,000 Btu/ft³hr^⫺1for long periods.

The final step came with the use of fully watercooled furnaces, in which the liberation rates were 25,000 Btu/ft³hr^⫺1, and even higher in some instances.

In some industries refuse wood supply is small and erratic; therefore, it does not warrant the use of special furnace designs. A satisfactory solution for disposing of this refuse is, then, to burn it on stoker fuel beds. When this is done, however, provision must be made for supplying controlled amounts of overfire air to burn the wood quickly and thus prevent it from eventually blanketing the stoker fuel bed.