Fig. 27. Babcock & Wilcox Boiler Set with Green Bagasse Furnace
Experience has shown that this fuel may be burned with the best results in large quantities. A given amount of bagasse burned in one furnace between two boilers will give better results than the same quantity burned in a number of smaller furnaces. [Pg 209] An objection has been raised against such practice on the grounds that the necessity of shutting down two boiler units when it is necessary for any reason to take off a furnace, requires a larger combined boiler capacity to insure continuity of service. As a matter of fact, several small furnaces will cost considerably more than one large furnace, and the saving in original furnace cost by such an installation, taken in conjunction with the added efficiency of the larger furnace over the small, will probably more than offset the cost of additional boiler units for spares.
The essential features in furnace design for this class of fuel are ample combustion space and a length of gas travel sufficient to enable the gases to be completely burned before the boiler heating surfaces are encountered. Experience has shown that better results are secured where the fuel is burned on a hearth rather than on grates, the objection to the latter method being that the air for combustion enters largely around the edges, where the fuel pile is thinnest. When burned on a hearth the air for combustion is introduced into the furnace through several rows of tuyeres placed above and symmetrically around the hearth. An arrangement of such tuyeres over a grate, and a proper manipulation of the ashpit doors, will overcome largely the objection to grates and at the same time enable other fuel to be burned in the furnace when necessary. This arrangement of grates and tuyeres is probably the better from a commercially efficient standpoint. Where the air is admitted through tuyeres over the grate or hearth line, it impinges on the fuel pile as a whole and causes a uniform combustion. Such tuyeres connect with an annular space in which, where a blast is used, the air pressure is controlled by a blower.
All experience with this class of fuel indicates that the best results are secured with high combustion rates. With a natural draft in the furnace of, say, three-tenths inch of water, a combustion rate of from 250 to 300 pounds per square foot of grate surface per hour may be obtained. With a blast of, say, five-tenths inch of water, this rate can be increased to 450 pounds per square foot of grate surface per hour. These rates apply to bagasse as fired containing approximately 50 per cent of moisture. It would appear that the most economical results are secured with a combustion rate of approximately 300 pounds per square foot per hour which, as stated, may be obtained with natural draft. Where a natural draft is available sufficient to give such a rate, it is in general to be preferred to a blast.
Fig. 27 shows a typical bagasse furnace with which very satisfactory results have been obtained. The design of this furnace may be altered to suit the boilers to which it is connected. It may be changed slightly in its proportions and in certain instances in its position relative to the boiler. The furnace as shown is essentially a bagasse furnace and may be modified somewhat to accommodate auxiliary fuel.
The fuel is ignited in a pit A on a hearth which is ordinarily elliptical in shape. Air for combustion is admitted through the tuyeres B connected to an annular space C through which the amount of air is controlled. Above the pit the furnace widens out to form a combustion space D which has a cylindrical or spherical roof with its top ordinarily from 11 to 13 feet above the floor. The gases pass from this space horizontally to a second combustion chamber E from which they are led through arches F to the boiler. The arrangement of such arches is modified to suit the boiler or boilers with which the furnace is operated. A furnace of such design embodies the essential features of ample combustion space and long gas travel.
The fuel should be fed to the furnace through an opening in the roof above the pit by some mechanical means which will insure a constant fuel feed and at the same time prevent the inrush of cold air into the furnace.
This class of fuel deposits a considerable quantity of dust, which if not removed promptly will fuse into a hard glass-like clinker. Ample provision should be made for the removal of such dust from the furnace, the gas ducts and the boiler setting, and these should be thoroughly cleaned once in 24 hours.
[Table 45] gives the results of several tests on Babcock & Wilcox boilers using fuel of this character.