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HEATINC VENTILATING AIR CONDITIONING CUIDE 1944 sents the fixed carbon. The unburned residue is ash. Although deter mined separately, the sulphur content is frequently reported with a proximate analysis. Other important qualities of coals are the screen sizes, ash fusion temperature, friability, caking tendency, and the qualities of the volatile matter. In considering these factors the following points are of interest. The volatile products given off by coals when they are heated differ materially in the ratios by weight of the gases to the oils and tars. No heavy oils or tars are given off by anthracite, and very small quantities are given off by semi-anthracite. As the volatile matter in the coal increases to as much as 40 per cent of ash and moisture-free coal, in creasing amounts of oils and tars are released. For coals of higher volatile content, the relative quantity of oils and tars decreases, so it is low in the sub-bituminous coals and in lignite. The percentage of ash and its fusion temperature do not indicate the composition or distribu tion of its constituents. A classification of coals is given in Table 5, and a brief description of the kinds of fuels is given in the following paragraphs, but it should be recognized that there are no distinct lines of demarcation between the kinds, and that they graduate into each other. Anthracite is a clean, dense, hardcoal which creates little dust in handling. It is com paratively hard to ignite but it burns freely when well started. It is non-caking, it burns uniformly and smokelessly with a short flame, and it requires no attention to the fuel bed between firings. It is capable of giving a high efficiency in the common types of handfired furnaces. A tabulation of the quality of the various anthracite sizes will be found in a Bureau of Mines Report*. Semi-anthracite has a higher volatile content than anthracite. It is not so hard and ignites somewhat more easily; otherwise its properties are similar to those of anthracite. Semi-bituminous coal is soft and friable, and fines and dust are created by handling it. It ignites somewhat slowly and burns with a medium length of flame. Its caking prop erties increase as the volatile matter increases, but the coke formed is relatively weak. Having only half the volatile matter content of the more abundant bituminous coals it can be burned with less production of smoke, and it is sometimes called smokeless coal. The term bituminous coal covers a large range of coals andineludes many types having distinctly different composition, properties, and burning characteristics. The coals range from the high-grade bituminous coals of the East to the poorer coals of the West. Their caking properties range from coals which completely melt, to those from which the volatiles and tars are distilled without change of form, so that they are classed as noncaking or free-burning. Most bituminous coals are strong and non-friable enough to permit of the screened sizes being delivered free from fines. In general, they ignite easily and burn freely; the length of flame varies with different coals, but it is long. Much smoke and soot are possible, if improperly fired, 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 ashpit continue to burn. Very little smoke or soot is formed. ^Quality of Anthracite as Prepared at Breakers. 1935 (U. S. Bureau of Mines Report of Investigations, R. 1. 3283). 174 CHAPTER 8. COMBUSTION AND FUELS Table 5. Classification of Coals by Rank* Legend: F.C. = Fixed Carbon. V.M. = Volatile Matter. Btu = British thermal units. Class I. Anthracite _ II. Bituminous*- III. Sub-bituminous. IV. Lignitie Geocf Limits op Fitted Carbon or Btu MnraRAT^MATraw-FBCT BASIS Requisite Physical Properties 1. Meta-anthracite Dry F.C., 98 per cent or mare (Dry V.M., 2 per cent or less) 2. Anthracite ,, Dry F.C., 92 per cent or more and less than 98 per cent (Dry V.M., 8 per centor lessand more than 2 per cent) 3. Semi-anthracite-- Dry F.C., 86 per cent or more and less than 92 per cent (Dry V.M., 14 per centor Irasand more than 8 per cent) 1. Low volatile bituminous coal___ Dry F.C., 78 per cent or more and less than 86 per cent (Dry V.M., 22 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 78 per cent (Dry V.M., 31 per cent or less and more than 22 per cent) 3. High volatile A bituminous coaL Dry F.C., less than 69 per cent (Dry V.M., more than 31 per cent); and moist4 Btu, 14,000* or more 4. High volatile B bituminous coaL Merisi* Btu, 13,000 or more and less than 14,000* 5. High volatile C bituminous CoaL Moist Btu, 11,000 or more and less than 13,000* 1. Sub-bituminous A coal______ Moist Btu, 11,000 or more and less than 13,000* 2. Sub-bituminous B coal_ Moist Btu, 9500 or more and less than 11,000* 3. Sub-bituminous C coaL. Moist Btu, 8300 or more and less than 9500* 1. Lignite*-- Moist Btu less than 8300 Moist Btu less than 8300 Non-agglomerating* Either agglomerating* or non-weathering/ Both weathering and non-agglomerating* Consolidated Unconsolidated This classification does not include a few coals which have unusual physical and chemical properties and which come within the limits of fixed carbon or Btu of the high-volatile bituminous and sub-bituminous ranks. All of these coals either contain less than'48 per cent dry, mineral-matter-free fixed carbon or have more than 15,500 moist, mineral-matter-free Btu. *If agglomerating, 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. *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, agglomerating and non-weathering; Variety 2, agglomerating and weathering; Variety 3, non-agglomerating and non-weathering. Adapted from A.S.T.M. Standards, 1937, Supplement, p. 145, American Society for Testing Materials, Philadelphia. CLASSIFICATION OF COKES Coke is produced by the distillation of the volatile matter from coal. The type of coke depends on 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. High-temperature cokes. Coke as usually 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 175