Document 93wgY9qn66xDwnZzp1rx0mw23

HEATING VENTILATINC AIR CONDITIONING GUIDE 1942 having the same chemical analysis may have distinctly different burning characteristics. Users are mainly interested in the available heat per 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 Bureau of Mines Bulletin No. 276. 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 analysisis 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 Fig. 4. Correct and Incorrect Methods of Draft Regulation in a Hand-Fired Furnace determine volatile matter, the dried sample is heated to about 1750 F 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 repre 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 how the ash is distributed or how much of it is less fusible lumps of slate or shale. 166 CHAPTER 8. COMBUSTION AND FUELS 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, 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 efficiency in the common types of handfired furnaces. A tabulation of the quality of the various anthracite sizes will be found in Bureau of Mines Report of Investigations No. 3283. 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 pro perties increase as the volatile matter increases, but the coke formed is relatively weak. Table 5. Classification of Coals by Rank Legend: F.C. = Fixed Carbon. V.M. = Volatile Matter. Btu = British thermal units. Class Gbodp Limits or Fixed Carbon ob Btu Mineral-Matteh-Free Basis Requisite Physical Properties I. Anthracite II. Bituminous4___ Dry F.C., 98 per cent 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 less and more than 2 per cent) Non-agglomerating6 Dry F.C., 86 per cent or more and less than 92 per cent (Dry V.M., 14 per cent or less and more than 8 per cent) 1. Low volatile bituminous coal___ Dry F.C., 78 per cent or more and lea 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 eent or lees and more than 22 per cent) 3. High volatile A bituminous ooaL Dty F.C., lees than 69 per cent (Dry V.M., more than 31 per eent); and moist* Btu, 14,000* or more 4. High volatile B bituminous ccaL Moist* 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* Either agglomerating6 or non-weathering/ IIL Sub-bituminousIV. Iignitic._______ \ than 13,000* Both weathering .and than 11,000* non-agglomerating6 Moist Btu, 8300 or more and less than 9500* Moist Btu less than 8300 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 haye more than 15,500 moist, mineral-matter-free Btu. 6If agglomerating, classify in low-volatile group of the bituminous das9. 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. 167