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American Society of Heating and Ventilating Engineers Guide, 1930 Table 1. Approximate Chemical Changes from Wood Fiber to Anthracite Coal Substance Carbon Htdrogbn OxTGBN Wood Fiber............................................ Peat............................................................ Lignite.......................... ............................ Bituminous Coal..................................... Semi-Bituminous Coal.................. Anthracite Coal..........;..................,........ 52.65 59.57 66.04 75.06 89.29 91.58 5.25 5.96 5.27 5.84 5.05 3.96 42.10 34.47 28.69 19.10 5.66 4.46 less density, hardness, and metallic luster than anthracite, and the average specific gravity is about 1.4. . Semi-Bituminous Coal Semi-bituminous is a softer coal than anthracite or semi-anthracite, contains more volatile hydrocarbon, and will kindle more easily and burn more rapidly. It is usually free burning and owing to its high calorific, value, is very desirable for steam generation purposes. Bituminous Coal Bituminous coals are still softer than those already described and contain still more of the volatile hydrocarbons. The difference between semi-bituminous and bituminous coals is an important one, economically. The former have an average heating value per pound of combustible about 6 per cent higher than the latter, and they burn with much less smoke in ordinary furnaces. The distinctive characteristic of the bitu minous coals is the emission of yellow flame and smoke when burning. In color they range from pitch black to dark brown, having a resinous, luster in the most compact specimens, and a silky luster in such specimens as show traces of vegetable fiber. The specific gravity is ordinarily about 1.3. Bituminous coals are either of the caking or non-caking variety. The former, when heated, fuse and swell in size; the latter burn freely, do not fuse, and are commonly known as free-burning coals. Caking coals are rich in volatile hydrocarbons, and are valuable in gas manufacture. Table 2. Typical Analyses of Various Types of Fuel Kind or Coal Fixed Carbon Per Cent Volatile Matter Per Cent Moisture Per Cent Ash Per Cent Heat Value B.t.u. per Pound as Fired Anthracite......................... Semi-Anthracite.--........: Coke................................... Semi-Bituminous............ Bituminous...... . ,,........... Lignite.....1--..................... 82.6 83.3 81.7 73.2 48.4 28.6 3.7 9.4 2.8 18.7 .36.3 31.2 3.2 1.3 5.7 1.2 6.7 31.8 10.5 6.0 9.8 6.9 8.6 8.4 12,840 13,920 12,050 14,370 12,200 7,730 Chapter 10--Heating by Coal Li&nite Organic matter in the earlier stages of its conversion into coal is known as lignite and includes all varieties which are intermediate between peat and coal of the older formation. The specific gravity of lignite is low, being 1.2 to 1.23, and when freshly mined it may contain as high as 50 per cent of moisture. Its appearance varies from light brown, showing a distinctly woody structure in the poorer varieties, to black, with a pitchy luster resembling hard coal, in the best varieties. It is non-caking and burns with a bright but slightly smoky flame with moderate heat. It is easily broken, will not withstand much handling in transportation, and if exposed to the weather will disintegrate rapidly, which will increase the difficulty of burning it. Its composition varies over wide limits. The ash may run as low as 1 per cent and as high as 50 per cent. Its high content of moisture and the large quantity of air necessary for its combustion cause large stack losses. Lignite is distinctly a low-grade fuel, and it is used almost entirely in the districts where it is mined, because of its cheapness. Coke Coke is a porous product, consisting almost entirely of carbon remain ing after certain manufacturing processes have distilled off the hydro carbon gases of the fuel used. Coke is produced (1) from gas coal distilled in gas retorts; (2) from gas or ordinary bituminous coals burned in special furnaces called coke ovens; and (3) from petroleum, by carrying the dis tillation of the residuum to a red heat. Coke is a smokeless fuel. It absorbs moisture readily from the atmos phere and if not kept under cover its moisture content may be as much as 20 per cent of its own weight. Gas-house coke is generally softer and more porous than oven coke, ignites more readily, and requires less draft for its combustion. The heal values of coke range from 12,500 B.t.u. per pound to 13,500 B.t.u., depending on the ash content, which may vary from 5 to 10 per cent. COMBUSTION OF COAL The combustion of coal is the process of combining oxygen of the air with the carbon of the coal. In order to obtain perfect combustion, a definite amount of air is required for each pound of coal burned. The prime requirements for complete combustion are the combination of these elements in correct proportions and the thorough mixing of them in a zone of proper temperature. A deficiency of oxygen or of air supply causes the formation of CO (carbon monoxide), which is incompletely burned carbon and which is evident by the soot and smoke. An excess of air absorbs heat from the products of combustion and carries it out of the chimney as waste. Air Required The theoretical amount of air required per pound of fuel for perfect combustion is dependent upon the analysis of the fuel; however, for 205