LOCATION carbon Ash Sulphur Connellsville, Pa 89% 10% 1% Pocahontas, Va 93% 6% 1% Combustion takes place when the elements or the constituents of a fuel, which are mainly carbon and hydrogen, form a chemical combination with oxygen anti produce oxides. Heat accompanies this chemi cal action and it is this heat which is partially utilized in generating steam.
Air Required for Combustion.—Air is used to supply the necessary oxygen for maintaining and supporting the combustion. The composi tion of air is approximately 23 per cent oxygen (02) and 77 per cent nitrogen (N2) by weight, or 21 per cent 0, and 79 per cent N2 by vol ume. To secure one pound of oxygen in the furnace it will therefore be necessary to sup 1.
ply or 4.35 pounds of air. Ordinarily in practice from 25 to 100 per cent more air is provided than is required for complete combus tion. Naturally the less this excess, still main taining complete combustion, the better the efficiency as this excess air leaves the boiler at a much higher temperature than when intro duced into the furnace, thus carrying away heat.
Let C, H and 0 denote respectively the parts by weight of carbon, hydrogen and oxy gen in one pound of fuel. Carbon dioxide (CO2) is formed by the complete combustion of carbon and by the proportion of molecular weights 12 pounds of C combine with 32 pounds of oxygen to form 44 pounds of CO,. In other words one pound of carbon requires 2.67 pounds of 02 or 11.6 pounds of air. By a similar method one pound of H. requires 8 pounds of 0, or 34.8 pounds of air.
There is often a small quantity of oxygen in the fuel itself and we consider that this will be united with the hydrogen as far as it is pos sible. Then the minimum weight of air re quired for complete combustion may be ob tained by the following equation.
0 11.6C+34.8(H— )Pounds of •air.
The volume of air at 62° F. and atmospheric pressure theoretically required for complete combustion is arrived at by multiplying by the corresponding volume per pound of air which gives 0 147C+441 (H— )=Cu. ft. of air.
Example: Determine the pounds of air and cubic feet of air theoretically required for the complete combustion of the wood with the composition as given above.
.4135 11.6X.4975+34.8(.0605— — ):15.077 lbs. of 8 air.
351 147X.4975+441(.0605— .4g cu. ft. of air.
One pound of fuel oil requires considerably more air than wood and more than any of the fuels mentioned heretofore.
lbs. of air. =178.19 cu. ft. of air.
The above figures indicate the importance of giving due consideration to the fuel used as space must be provided for admitting the proper quantity of air. At the same time the resistance offered to the passage of air through the fuel bed should be considered.
Heating As mentioned before a chemical action is accompanied by the gener ation of heat. The heat generated when one pound of combustible is completely burned is called the heating value. Heating values are determined as a rule by various calorimeters. The heating value of a fuel depends upon the quantities of carbon and hydrogen which it contains. Some fuels contain sulphur which generates some heat and requires a definite amount of oxygen for combustion. It is gen erally disregarded however in making computa tions.
The heating value of a fuel may be approxi mated by computing the quantities of heat which the combustible constituents contained in the fuel would produce if burned separately and taking the sum.
The heat liberated in burning one pound of carbon completely is 14,650 B. T. U. One pound of hydrogen completely burned liberates 62,100 B. T. U.
Using the same symbols for the constituents as were used in arriving at the theoretical quantity of air necessary for combustion we can write.
British thermal units (B. T. C+62,100 (H— 0 —).
8 The above formula is known as Dulong's equation and is used extensively in estimating the heating value of fuels when the ultimate analysis is known. To determine the heating value of wood from the composition hereto fore given, we write 1 14,650X .4975+62,100 )835 B. T. U. per lb.