Document mp0q40RqOoMJJ8E6KpzdByQEJ

s American Society of Heating and Ventilating Engineers Guide, 1934 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 Calorific Values and Per Cent of Moisture and Ash 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 easily, burn freely; the length of flame varies with different coals, but it is long. Much smoke and soot are possible and are difficult to reduce to reasonable amounts at low rates of burning. Sub-bituminous coals occur in the western states; they are high in moisture when 366 Chapter 27--Fuels and Combustion mined and tend to break up as they dry or when exposed to the weather; they are liable to ienite 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. Lienite 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 also is more liable to spontaneous ignition. Freshly mined lienite, because of its high moisture, ignites slowly. It is non-caking. The, char left after dhe 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 bum. Very little smoke or soot is formed. 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, 6ybroduct coke, which covers the greater part of the coke sold, and gas-house coke. Ihe 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 lower 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 volatde matter In general, these cokes ienite and burn more readily than high-temperature cokes. The properties of various low-temperature cokes may differ more than various high-temperature cokes because of the differences in the quantities of volatile matter and because some may be light and others are briquetted. ... The sale of petroleum coke 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 required for each pound of fuel fired. A deficiency of air supply will result in combustible products 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 1. Pounds of Air per Pound of Fuel as Fired Anthracite Coke Semi-Bituminocs 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 1. 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 COt (carbon dioxide) in the products of combustion. This relationship is shown by the curves (Fig. 2) 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 vanes. 367