153 2 = 306 cubic feet of air per pound of carbon.
_Actual Volume_ _for One Pound Carbon_ _Per Cent_ _Cubic Feet_ _by Volume_ Carbon Dioxide 32 = 10.45 } Oxygen 32 = 10.45 } = 20.91 per cent Nitrogen 242 = 79.09 --- ------ 306 = 100.00
In each case the volume of oxygen which combines with the carbon is equal to (cubic feet of air 20.91 per cent)--32 cubic feet.
It will be seen that no matter what the excess of air supplied, the actual amount of carbon dioxide per pound of carbon remains the same, while the percentage by volume decreases as the excess of air increases.
The actual volume of oxygen and the percentage by volume increases with the excess of air, and the percentage of oxygen is, therefore, an indication of the amount of excess air. In each case the sum of the percentages of CO_{2} and O is the same, 20.9. Although the volume of nitrogen increases with the excess of air, its percentage by volume remains the same as it undergoes no change while combustion takes place; its percentage for any amount of air excess, therefore, will be the same after combustion as before, if cooled to the same temperature. It must be borne in mind that the above conditions hold only for the perfect combustion of a pound of pure carbon.
Carbon monoxide (CO) produced by the imperfect combustion of carbon, will occupy twice the volume of the oxygen entering into its composition and will increase the volume of the flue gases over that of the air supplied for combustion in the proportion of
100 + the per cent CO 1 to ----------------------- 100
When pure carbon is the fuel, the sum of the percentages by volume of carbon dioxide, oxygen and one-half of the carbon monoxide, must be in the same ratio to the nitrogen in the flue gases as is the oxygen to the nitrogen in the air supplied, that is, 20.91 to 79.09. When burning coal, however, the percentage of nitrogen is obtained by subtracting the sum of the percentages by volume of the other gases from 100. Thus if an a.n.a.lysis shows 12.5 per cent CO_{2}, 6.5 per cent O, and 0.6 per cent CO, the percentage of nitrogen which ordinarily is the only other const.i.tuent of the gas which need be considered, is found as follows:
100 - (12.5 + 6.5 + 0.6) = 80.4 per cent.
The action of the hydrogen in the volatile const.i.tuents of the fuel is to increase the apparent percentage of the nitrogen in the flue gases.
This is due to the fact that the water vapor formed by the combustion of the hydrogen will condense at a temperature at which the a.n.a.lysis is made, while the nitrogen which accompanied the oxygen with which the hydrogen originally combined maintains its gaseous form and pa.s.ses into the sampling apparatus with the other gases. For this reason coals containing high percentages of volatile matter will produce a larger quant.i.ty of water vapor, and thus increase the apparent percentage of nitrogen.
Air Required and Supplied--When the ultimate a.n.a.lysis of a fuel is known, the air required for complete combustion with no excess can be found as shown in the chapter on combustion, or from the following approximate formula:
Pounds of air required per pound of fuel =
(C O S) 34.56 (- + (H - -) + -)[29] (11) (3 8 8)
where C, H and O equal the percentage by weight of carbon, hydrogen and oxygen in the fuel divided by 100.
When the flue gas a.n.a.lysis is known, the total, amount of air supplied is:
Pounds of air supplied per pound of fuel =
N 3.036 (-----------) C[30] (12) CO_{2} + CO
where N, CO_{2} and CO are the percentages by volume of nitrogen, carbon dioxide and carbon monoxide in the flue gases, and C the percentage by weight of carbon which is burned from the fuel and pa.s.ses up the stack as flue gas. This percentage of C which is burned must be distinguished from the percentage of C as found by an ultimate a.n.a.lysis of the fuel.
To find the percentage of C which is burned, deduct from the total percentage of carbon as found in the ultimate a.n.a.lysis, the percentage of unconsumed carbon found in the ash. This latter quant.i.ty is the difference between the percentage of ash found by an a.n.a.lysis and that as determined by a boiler test. It is usually a.s.sumed that the entire combustible element in the ash is carbon, which a.s.sumption is practically correct. Thus if the ash in a boiler test were 16 per cent and by an a.n.a.lysis contained 25 per cent of carbon, the percentage of unconsumed carbon would be 16 .25 = 4 per cent of the total coal burned. If the coal contained by ultimate a.n.a.lysis 80 per cent of carbon the percentage burned, and of which the products of combustion pa.s.s up the chimney would be 80 - 4 = 76 per cent, which is the correct figure to use in calculating the total amount of air supplied by formula (12).
The weight of flue gases resulting from the combustion of a pound of dry coal will be the sum of the weights of the air per pound of coal and the combustible per pound of coal, the latter being equal to one minus the percentage of ash as found in the boiler test. The weight of flue gases per pound of dry fuel may, however, be computed directly from the a.n.a.lyses, as shown later, and the direct computation is that ordinarily used.
The ratio of the air actually supplied per pound of fuel to that theoretically required to burn it is:
N 3.036(---------)C CO_{2}+CO ------------------ (13) C O 34.56(- + H - -) 3 8
in which the letters have the same significance as in formulae (11) and (12).
The ratio of the air supplied per pound of combustible to the amount theoretically required is:
N ------------------ (14) N - 3.782(O - CO)
which is derived as follows:
The N in the flue gas is the content of nitrogen in the whole amount of air supplied. The oxygen in the flue gas is that contained in the air supplied and which was not utilized in combustion. This oxygen was accompanied by 3.782 times its volume of nitrogen. The total amount of excess oxygen in the flue gases is (O - CO); hence N - 3.782(O - CO) represents the nitrogen content in the air actually required for combustion and N (N - 3.782[O - CO]) is the ratio of the air supplied to that required. This ratio minus one will be the proportion of excess air.
The heat lost in the flue gases is L = 0.24 W (T - t) (15)
Where L = B. t. u. lost per pound of fuel, W = weight of flue gases in pounds per pound of dry coal, T = temperature of flue gases, t = temperature of atmosphere, 0.24 = specific heat of the flue gases.
The weight of flue gases, W, per pound of carbon can be computed directly from the flue gas a.n.a.lysis from the formula:
11 CO_{2} + 8 O + 7 (CO + N) ---------------------------- (16) 3 (CO_{2} + CO)
where CO_{2}, O, CO, and N are the percentages by volume as determined by the flue gas a.n.a.lysis of carbon dioxide, oxygen, carbon monoxide and nitrogen.
The weight of flue gas per pound of dry coal will be the weight determined by this formula multiplied by the percentage of carbon in the coal from an ultimate a.n.a.lysis.
[Graph: Temperature of Escaping Gases--Deg. Fahr.
against Heat carried away by Chimney Gases--In B.t.u.
per pound of Carbon burned.[31]
Fig. 20. Loss Due to Heat Carried Away by Chimney Gases for Varying Percentages of Carbon Dioxide. Based on Boiler Room Temperature = 80 Degrees Fahrenheit. Nitrogen in Flue Gas = 80.5 Per Cent. Carbon Monoxide in Flue Gas = 0. Per Cent]
Fig. 20 represents graphically the loss due to heat carried away by dry chimney gases for varying percentages of CO_{2}, and different temperatures of exit gases.
The heat lost, due to the fact that the carbon in the fuel is not completely burned and carbon monoxide is present in the flue gases, in B. t. u. per pound of fuel burned is:
( CO ) L" = 10,150 (-----------) (17) (CO + CO_{2})
where, as before, CO and CO_{2} are the percentages by volume in the flue gases and C is the proportion by weight of carbon which is burned and pa.s.ses up the stack.
Fig. 21 represents graphically the loss due to such carbon in the fuel as is not completely burned but escapes up the stack in the form of carbon monoxide.
[Graph: Loss in B.T.U. per Pound of Carbon Burned[32]
against Per Cent CO_{2} in Flue Gas
Fig. 21. Loss Due to Unconsumed Carbon Contained in the CO in the Flue Gases]
Apparatus for Flue Gas a.n.a.lysis--The Orsat apparatus, ill.u.s.trated in Fig. 22, is generally used for a.n.a.lyzing flue gases. The burette A is graduated in cubic centimeters up to 100, and is surrounded by a water jacket to prevent any change in temperature from affecting the density of the gas being a.n.a.lyzed.
For accurate work it is advisable to use four pipettes, B, C, D, E, the first containing a solution of caustic potash for the absorption of carbon dioxide, the second an alkaline solution of pyrogallol for the absorption of oxygen, and the remaining two an acid solution of cuprous chloride for absorbing the carbon monoxide. Each pipette contains a number of gla.s.s tubes, to which some of the solution clings, thus facilitating the absorption of the gas. In the pipettes D and E, copper wire is placed in these tubes to re-energize the solution as it becomes weakened. The rear half of each pipette is fitted with a rubber bag, one of which is shown at K, to protect the solution from the action of the air. The solution in each pipette should be drawn up to the mark on the capillary tube.
The gas is drawn into the burette through the U-tube H, which is filled with spun gla.s.s, or similar material, to clean the gas. To discharge any air or gas in the apparatus, the c.o.c.k G is opened to the air and the bottle F is raised until the water in the burette reaches the 100 cubic centimeters mark. The c.o.c.k G is then turned so as to close the air opening and allow gas to be drawn through H, the bottle F being lowered for this purpose. The gas is drawn into the burette to a point below the zero mark, the c.o.c.k G then being opened to the air and the excess gas expelled until the level of the water in F and in A are at the zero mark. This operation is necessary in order to obtain the zero reading at atmospheric pressure.
The apparatus should be carefully tested for leakage as well as all connections leading thereto. Simple tests can be made; for example: If after the c.o.c.k G is closed, the bottle F is placed on top of the frame for a short time and again brought to the zero mark, the level of the water in A is above the zero mark, a leak is indicated.
[Ill.u.s.tration: Fig. 22. Orsat Apparatus]
Before taking a final sample for a.n.a.lysis, the burette A should be filled with gas and emptied once or twice, to make sure that all the apparatus is filled with the new gas. The c.o.c.k G is then closed and the c.o.c.k I in the pipette B is opened and the gas driven over into B by raising the bottle F. The gas is drawn back into A by lowering F and when the solution in B has reached the mark in the capillary tube, the c.o.c.k I is closed and a reading is taken on the burette, the level of the water in the bottle F being brought to the same level as the water in A.
The operation is repeated until a constant reading is obtained, the number of cubic centimeters being the percentage of CO_{2} in the flue gases.