Document 0Jwqj32GEv8rnxvxZJaEQO23O

of arid 1936American Society Heating Ventilating Engineers Guide, easily and burn freely; the length of flame varies with different coals, but it is long. Much smoke and soot are possible especially at low rates of burning. Sub-bituminous coals occur in the western states; they are high in moisture when mined and tend to break up as they dry or when exposed to the weather; they are liable to ignite spontaneously when piled or stored. They ignite easily and quickly and have a medium length flame, are non-caking and free-burning; the lumps tend to break into small pieces if poked; very little smoke and soot are formed. Lignite is of woody structure, very high in moisture as mined, and of low heating value; it is clean to handle. It has a.greater tendency than the sub-bituminous coals to disintegrate as it dries, and it also is more liable to spontaneous ignition. Freshly mined lignite, because of its high moisture, ignites slowly. It is non-caking. The char left after the moisture and volatile matter are driven off burns very easily, like charcoal. The lumps tend to break up in the fuel bed and pieces of char falling into the ash pit continue to burn. Very little smoke or soot is formed. Coke is produced by the distillation of the volatile matter from coal. The type of coke depends oh the coal or mixture of coals used, the temperatures and time of distil-.lation and, to some extent, on the type of retort or oven; coke is also produced as a residue from the destructive distillation of oil. Table 1. Classification of Coals by Rank/ Legend: F.C. = Fixed Carbon. V.M. = Volatile Matter. Btu = British thermal units. Class Group Limits op Fixed Carbon or Btu Mineral-Matter-Frbb Basis .Requisite Physical Properties . I. Anthracite_____ 2. Anthracite............. .......... ........ 3. Semi-anthracite_______________ Dry F.C., 98 per eent or more (Dry V.M., 2 per cent or less) Dry F.C., 92 per cent or more and less than 98 per cent (Dry V.M., 8 per cent or Iras and more than 2 per cent) Dry F.C., 86 per cent or more and less Non-agglutinating than 92 per cent (Dry V.M., 14 per cent or less and more than 8 per cent) II: Bituminous6__ 1. Low volatile bituminous coal__1. Dry F.C., 77 per cent or more and less than 86 per cent (Dry V.M., 23 per cent or less and more than 14 per cent) 2. Medium volatile bituminous coal Dry F.C., 69 per cent or more and less than 77 peti cent (Dry V.M., 31 per cent or less and more than 23 per - cent) 3. High volatile A bituminous coal. Dry F.C., Iras than 69 per cent (Dry V.M., more than 31 per cent); and moist6 Btu, 14,000^ or more 4. High volatile B bituminous coaL Moist6 Btu, 13,000 or more and less than 14/000* 5. High volatile C bituminous coaL Moist Btu, 11,000 or more and less Either agglutinating than 13,000* - or non-weathering* IIL Sub-bituminous. Both weathering and than 13,000* non-agglutinating - Moist Btu\9500 or more and less than 11,000* . than 9500* IV. Lignitic--- --------- 1 Moist Btu less' than 8300 Consolidated Unconsolidated aIf agglutinating, classify in low-volatile group of the bituminous class. *Moist Btu refers to coal containing its natural bed moisture but not including visible water on the surface of the coal. t. Pending, the report of the Subcommittee on Origin and Composition and Methods of Analysis,-it is recognized that there may be non-caking varieties in each group of the bituminous class. *Coals having 69 per cent or more fixed carbon on the dry, mineral-matter-free basis shall be classified according to fixed carbon, regardless of Btu. There are three varieties of coal in the High-volatile C bituminous coal group, namely. Variety 1, agglutinating and non-weathering; Variety 2, agglutinating ami weathering; Variety 3, uou-agglutinating and non-weathering. /Adapted from A.S.T.M. Standards on Coal and Coke, p. 68, American Society for Testing Materials, Philadelphia, 1934C 472 27--Chapter Fuels and Combustion High-temperature cokes. Coke as usuafly available is of the high-temperature type, and contains between .1 and 2 per cent volatile matter. High-temperature cokes are sub divided into beehive coke of which comparatively little is now sold for domestic use; by product coke, which covers the greater part of the coke sold, and gas-house coke. The differences among these three cokes are relatively small; their denseness and hardness decrease and friability increases in the order named. In general, the lighter and more friable cokes ignite and burn the more easily. Low-temperature cokes are produced at low coking temperatures, and only a portion of the volatile matter is distilled off. Cokes as made by various processes under develop ment have contained from 10 to 15 per cent volatile matter. In general, these cokes ignite and burn more readily than high-temperature cokes. The properties of various low-temperature cokes may differ more than those of the various high-temperature cokes because of the differences in the quantities of volatile matter and because some may be light and others briquetted. 1 The sale of petroleum cokes for domestic furnaces has been small and is generally confined to the Middle West. They vary in the amount of volatile matter they contain, but all have the common property of a very low ash content, which necessitates the use of refractory pieces to protect the grates from being burned. In order to obtain perfect Combustion a definite amount of air is re quired,for each pound of fuel fired. A deficiency of air supply will result in combustible product's passing to the stack unburned. An excess of air absorbs heat from the products of combustion and results in a greater loss of sensible heat to the stack. Total Air Required. The theoretical amount of air required per pound of fuel for perfect combustion is dependent upon the analysis of the fuel; Table 2. Pounds of Air per Pound of Fuel as Fired Anthracite Cose Semi-Bituminous Bituminous Lignite 9.6 11.2 11.2 10.3 6.2 however, for estimating purposes the theoretical air required for different grades of fuel may roughly be taken from Table 2. An excess of about 50 per cent over the theoretical amount is considered good practice under usual operating conditions. The amount of excess air, based upon the laws of combustion, can be determined by its relation to the percentage of COi (carbon dioxide) in the products of combustion. This relationship is shown by the curves (Fig. 1) for high and low volatile coals and for coke. In hand-fired fur naces with long periods between firings the combustion goes through a cycle in each period and the quantity of excess air present varies. Secondary Air-. .The division of the total into primary and secondary air necessary to produce the same rate of burning and the same excess air depends on a number of factors which include size of fuel, depth of. fuel bed, and diameter of fire pot. The ratio of the secondary to the primary air increases with decrease in the size of the fuel pieces, with increase in the depth of the fuel bed, and with increase ini the area of the fire pot; the ratio also increases with increase in rate of burning. Size of the fuel is a very important factor in fixing the quantity of secondary air required for non-caking coals. With caking coals it is. not