Carbon is by far the most abundant as is indicated in the chapters on fuels.
Hydrogen in a free state occurs in small quant.i.ties in some fuels, but is usually found in combination with carbon, in the form of hydrocarbons. The density of hydrogen is 0.0696 (Air = 1) and its weight per cubic foot, at 32 degrees Fahrenheit and under atmospheric pressure, is 0.005621 pounds.
Sulphur is found in most coals and some oils. It is usually present in combined form, either as sulphide of iron or sulphate of lime; in the latter form it has no heat value. Its presence in fuel is objectionable because of its tendency to aid in the formation of clinkers, and the gases from its combustion, when in the presence of moisture, may cause corrosion.
Nitrogen is drawn into the furnace with the air. Its density is 0.9673 (Air = 1); its weight, at 32 degrees Fahrenheit and under atmospheric pressure, is 0.07829 pounds per cubic foot; each pound of air at atmospheric pressure contains 0.7685 pounds of nitrogen, and one pound of nitrogen is contained in 1.301 pounds of air.
Nitrogen performs no useful office in combustion and pa.s.ses through the furnace without change. It dilutes the air, absorbs heat, reduces the temperature of the products of combustion, and is the chief source of heat losses in furnaces.
Calorific Value--Each combustible element of gas will combine with oxygen in certain definite proportions and will generate a definite amount of heat, measured in B. t. u. This definite amount of heat per pound liberated by perfect combustion is termed the calorific value of that substance. Table 31, gives certain data on the reactions and results of combustion for elementary combustibles and several compounds.
TABLE 31
OXYGEN AND AIR REQUIRED FOR COMBUSTION
AT 32 DEGREES AND 29.92 INCHES
Column headings:
1: Oxidizable Substance or Combustible 2: Chemical Symbol 3: Atomic or Combining Weight 4: Chemical Reaction 5: Product of Combustion 6: Oxygen per Pound of Column 1 Pounds 7: Nitrogen per Pound of Column 1. 3.32[23] O Pounds 8: Air per Pound of Column 1. 4.32[24] O Pounds 9: Gaseous Product per Pound of Column 1[25] + Column 8 Pounds 10: Heat Value per Pound of Column 1 B. t. u.
11: Volumes of Column 1 Entering Combination Volume 12: Volumes of Oxygen Combining with Column 11 Volume 13: Volumes of Product Formed Volume 14: Volume per Pound of Column 1 in Gaseous Form Cubic Feet 15: Volume of Oxygen per Pound of Column 1 Cubic Feet 16: Volume of Products of Combustion per Pound of Column 1 Cubic Feet 17: Volume of Nitrogen per Pound of Column 1 3.782[26] Column 15 Cubic Feet 18: Volume of Gas per pound of Column 1 = Column 10 Column 17 Cubic Feet
BY WEIGHT ________________________________________________________________________ | | | | | | | | 1 | 2 | 3 | 4 | 5 | 6 | |________________|_______|____|________________|_________________|_______| | | | | | | | | Carbon | C | 12 | C+2O = CO_{2} | Carbon Dioxide | 2.667 | | Carbon | C | 12 | C+O = CO | Carbon Monoxide | 1.333 | | Carbon Monoxide| CO | 28 | CO+O = CO_{2} | Carbon Dioxide | .571 | | Hydrogen | H | 1 | 2H+O = H_{2}O | Water | 8 | | | | / CH_{4}+4O = | Carbon Dioxide | | Methane | CH_{4}| 16 | | | 4 | | | | CO_{2}+2H_{2}O | and Water / | | Sulphur | S | 32 | S+2O = SO_{2} | Sulphur Dioxide | 1 | |________________|_______|____|________________|_________________|_______|
________________________________________________________ | | | | | | | | 1 | 2 | 7 | 8 | 9 | 10 | |________________|_______|_______|_______|_______|_______| | | | | | | | | Carbon | C | 8.85 | 11.52 | 12.52 | 14600 | | Carbon | C | 4.43 | 5.76 | 6.76 | 4450 | | Carbon Monoxide| CO | 1.90 | 2.47 | 3.47 | 10150 | | Hydrogen | H | 26.56 | 34.56 | 35.56 | 62000 | | | | | | | | | Methane | CH_{4}| 13.28 | 17.28 | 18.28 | 23550 | | | | | | | | | Sulphur | S | 3.32 | 4.32 | 5.32 | 4050 | |________________|_______|_______|_______|_______|_______|
BY VOLUME
________________________________________________________________ | | | | | | | | 1 | 2 | 11 | 12 | 13 | 14 | |_________________|________|______|____|________________|________| | | | | | | | | Carbon | C | 1C | 2 | 2CO_{2} | 14.95 | | Carbon | C | 1C | 1 | 2CO | 14.95 | | Carbon Monoxide | CO | 2CO | 1 | 2CO_{2} | 12.80 | | Hydrogen | H | 2H | 1 | 2H_{2}O | 179.32 | | Methane | CH_{4} | 1C4H | 4 | 1CO_{2} 2H_{2}O| 22.41 | | Sulphur | S | 1S | 2 | 1SO_{2} | 5.60 | |_________________|________|______|____|________________|________|
_____________________________________________________________ | | | | | | | | 1 | 2 | 15 | 16 | 17 | 18 | |_________________|________|_______|________|________|________| | | | | | | | | Carbon | C | 29.89 | 29.89 | 112.98 | 142.87 | | Carbon | C | 14.95 | 29.89 | 56.49 | 86.38 | | Carbon Monoxide | CO | 6.40 | 12.80 | 24.20 | 37.00 | | Hydrogen | H | 89.66 | 179.32 | 339.09 | 518.41 | | Methane | CH_{4} | 44.83 | 67.34 | 169.55 | 236.89 | | Sulphur | S | 11.21 | 11.21 | 42.39 | 53.60 | |_________________|________|_______|________|________|________|
It will be seen from this table that a pound of carbon will unite with 2-2/3 pounds of oxygen to form carbon dioxide, and will evolve 14,600 B. t. u. As an intermediate step, a pound of carbon may unite with 1-1/3 pounds of oxygen to form carbon monoxide and evolve 4450 B. t. u., but in its further conversion to CO_{2} it would unite with an additional 1-1/3 times its weight of oxygen and evolve the remaining 10,150 B. t. u. When a pound of CO burns to CO_{2}, however, only 4350 B. t. u.
are evolved since the pound of CO contains but 3/7 pound carbon.
Air Required for Combustion--It has already been shown that each combustible element in fuel will unite with a definite amount of oxygen.
With the ultimate a.n.a.lysis of the fuel known, in connection with Table 31, the theoretical amount of air required for combustion may be readily calculated.
Let the ultimate a.n.a.lysis be as follows:
_Per Cent_ Carbon 74.79 Hydrogen 4.98 Oxygen 6.42 Nitrogen 1.20 Sulphur 3.24 Water 1.55 Ash 7.82 ------ 100.00
When complete combustion takes place, as already pointed out, the carbon in the fuel unites with a definite amount of oxygen to form CO_{2}. The hydrogen, either in a free or combined state, will unite with oxygen to form water vapor, H_{2}O. Not all of the hydrogen shown in a fuel a.n.a.lysis, however, is available for the production of heat, as a portion of it is already united with the oxygen shown by the a.n.a.lysis in the form of water, H_{2}O. Since the atomic weights of H and O are respectively 1 and 16, the weight of the combined hydrogen will be 1/8 of the weight of the oxygen, and the hydrogen available for combustion will be H - 1/8 O. In complete combustion of the sulphur, sulphur dioxide SO_{2} is formed, which in solution in water forms sulphuric acid.
Expressed numerically, the theoretical amount of air for the above a.n.a.lysis is as follows:
0.7479 C 2-2/3 = 1.9944 O needed ( 0.0642 ) ( 0.0498 - -------) H 8 = 0.3262 O needed ( 8 ) 0.0324 S 1 = 0.0324 O needed ------ Total 2.3530 O needed
One pound of oxygen is contained in 4.32 pounds of air.
The total air needed per pound of coal, therefore, will be 2.353 4.32 = 10.165.
The weight of combustible per pound of fuel is .7479 + .0418[27] + .0324 + .012 = .83 pounds, and the air theoretically required per pound of combustible is 10.165 .83 = 12.2 pounds.
The above is equivalent to computing the theoretical amount of air required per pound of fuel by the formula:
( O) Weight per pound = 11.52 C + 34.56 (H - -) + 4.32 S (10) ( 8)
where C, H, O and S are proportional parts by weight of carbon, hydrogen, oxygen and sulphur by ultimate a.n.a.lysis.
In practice it is impossible to obtain perfect combustion with the theoretical amount of air, and an excess may be required, amounting to sometimes double the theoretical supply, depending upon the nature of the fuel to be burned and the method of burning it. The reason for this is that it is impossible to bring each particle of oxygen in the air into intimate contact with the particles in the fuel that are to be oxidized, due not only to the dilution of the oxygen in the air by nitrogen, but because of such factors as the irregular thickness of the fire, the varying resistance to the pa.s.sage of the air through the fire in separate parts on account of ash, clinker, etc. Where the difficulties of drawing air uniformly through a fuel bed are eliminated, as in the case of burning oil fuel or gas, the air supply may be materially less than would be required for coal. Experiment has shown that coal will usually require 50 per cent more than the theoretical net calculated amount of air, or about 18 pounds per pound of fuel either under natural or forced draft, though this amount may vary widely with the type of furnace, the nature of the coal, and the method of firing.
If less than this amount of air is supplied, the carbon burns to monoxide instead of dioxide and its full heat value is not developed.
An excess of air is also a source of waste, as the products of combustion will be diluted and carry off an excessive amount of heat in the chimney gases, or the air will so lower the temperature of the furnace gases as to delay the combustion to an extent that will cause carbon monoxide to pa.s.s off unburned from the furnace. A sufficient amount of carbon monoxide in the gases may cause the action known as secondary combustion, by igniting or mingling with air after leaving the furnace or in the flues or stack. Such secondary combustion which takes place either within the setting after leaving the furnace or in the flues or stack always leads to a loss of efficiency and, in some instances, leads to overheating of the flues and stack.
Table 32 gives the theoretical amount of air required for various fuels calculated from formula (10) a.s.suming the a.n.a.lyses of the fuels given in the table.
The process of combustion of different fuels and the effect of variation in the air supply for their combustion is treated in detail in the chapters dealing with the various fuels.
TABLE 32
CALCULATED THEORETICAL AMOUNT OF AIR REQUIRED PER POUND OF VARIOUS FUELS
____________________________________________________________ | |Weight of Const.i.tuents in One |Air Required| | Fuel |Pound Dry Fuel |per Pound | | |______________________________|of Fuel | | | Carbon | Hydrogen| Oxygen |Pounds | | | Per Cent| Per Cent| Per Cent | | |________________|_________|_________|__________|____________| |c.o.ke | 94.0 | . | . | 10.8 | |Anthracite Coal | 91.5 | 3.5 | 2.6 | 11.7 | |Bituminous Coal | 87.0 | 5.0 | 4.0 | 11.6 | |Lignite | 70.0 | 5.0 | 20.0 | 8.9 | |Wood | 50.0 | 6.0 | 43.5 | 6.0 | |Oil | 85.0 | 13.0 | 1.0 | 14.3 | |________________|_________|_________|__________|____________|
[Ill.u.s.tration: 4064 HORSE-POWER Installation of Babc.o.c.k & Wilc.o.x Boilers and Superheaters, Equipped with Babc.o.c.k & Wilc.o.x Chain Grate Stokers, at the Cosmopolitan Electric Co., Chicago, Ill.]
a.n.a.lYSIS OF FLUE GASES
The object of a flue gas a.n.a.lysis is the determination of the completeness of the combustion of the carbon in the fuel, and the amount and distribution of the heat losses due to incomplete combustion. The quant.i.ties actually determined by an a.n.a.lysis are the relative proportions by volume, of carbon dioxide (CO_{2}), oxygen (O), and carbon monoxide (CO), the determinations being made in this order.
The variations of the percentages of these gases in an a.n.a.lysis is best ill.u.s.trated in the consideration of the complete combustion of pure carbon, a pound of which requires 2.67 pounds of oxygen,[28] or 32 cubic feet at 60 degrees Fahrenheit. The gaseous product of such combustion will occupy, when cooled, the same volume as the oxygen, namely, 32 cubic feet. The air supplied for the combustion is made up of 20.91 per cent oxygen and 79.09 per cent nitrogen by volume. The carbon united with the oxygen in the form of carbon dioxide will have the same volume as the oxygen in the air originally supplied. The volume of the nitrogen when cooled will be the same as in the air supplied, as it undergoes no change. Hence for complete combustion of one pound of carbon, where no excess of air is supplied, an a.n.a.lysis of the products of combustion will show the following percentages by volume:
_Actual Volume_ _for One Pound Carbon_ _Per Cent_ _Cubic Feet_ _by Volume_ Carbon Dioxide 32 = 20.91 Oxygen 0 = 0.00 Nitrogen 121 = 79.09 --- ------ Air required for one pound Carbon 153 = 100.00
For 50 per cent excess air the volume will be as follows:
153 1 = 229.5 cubic feet of air per pound of carbon.
_Actual Volume_ _for One Pound Carbon_ _Per Cent_ _Cubic Feet_ _by Volume_ Carbon Dioxide 32 = 13.91 } Oxygen 16 = 7.00 } = 20.91 per cent Nitrogen 181.5 = 79.09 ----- ------ 229.5 = 100.00
For 100 per cent excess air the volume will be as follows: