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306 CHAPTER 16 1946 Guide pound of coal, in the handling and storing properties, and in the burning characteristics. A description of the relationship between the qualities of coals and these characteristics requires considerable space: a treatment applicable to heating boilers is given in a Bureau of Mines Bulletin2. Coal composition may be expressed by either an ultimate or proximate analysis. In the ultimate analysis the proportions of carbon, hydrogen, oxygen, nitrogen, sulphur, and ash are determined. This form of analysis is difficult to make and is used only for extremely, close* studies. The proximate analysis is more easily made and is satisfactory for most purposes. In this analysis, the proportions of moisture, volatile matter, fixed carbon, and ash are determined. Moisture is obtained by noting the loss of weight of a sample of coal when dried at about 220 F. To .determine volatile matter, the dried sample is heated to about 1750 F, 1 in a closed crucible, and the loss of weight is noted. The sample is then burned in an open crucible, and the accompanying loss of weight represents 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 fuel 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, hard coal 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 efficifency in the common types of hand fired furnaces. A tabulation of the quality of the various anthracite sizes will be found in a Bureau of Mines Report *. Standard anthracite sizing specifications are shown in Table 6. 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 and includes 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 non-, caking 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* . j ' Fuels and Combustion 307 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. 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 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. ^ Petroleum cokes, which.are obtained by coking the residue left from the distillation of petroleum, 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. Table 6. Standard Anthracite Sizing Specifications3 Size Test Mesh, In. Through Over Broken...... ... .... 4% Egg . ------- _ 3K to 3 2% Stove................. 2% m Nut______:_______ m % Pea,,.............. .... % Buckwheat. % Hi % %. Barley-- ___ .. % % Round Mesh Oversize Max. Per Cent .Undersize Max; Min. Per Cent Per Cent Maximum Impurities. Per Cent Slateb Bone 5 7^ 7K 10 . 10 10 10 15 15 12H 10 15 15 15 20 .m m . m 5 m m. m 10 m2 m2 23 34 45 12 Ash 13 Ash . -----------' ` . Approved and adopted by Anthracite Committee. State Street Building, Harrisburg, Pa. bWhen slate content on Broken to Pea inclusive is lessthan above standards, bone content may be cor respondingly increased, but slate content specified above shall not be exceeded in any event and the total maximum impurities shall not exceed those above specified. *' ^