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 analysis 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 analysis 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 analysis.
[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 passes 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 Analysis--The Orsat apparatus, illustrated in
Fig. 22, is generally used for analyzing 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 analyzed.
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 glass 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 glass, or similar material, to clean the gas. To discharge any
air or gas in the apparatus, the cock 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 cock 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 cock 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 cock 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.
[Illustration: Fig. 22. Orsat Apparatus]
Before taking a final sample for analysis, 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 cock G is then closed and the
cock 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
cock 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.
The gas is then driven over into the pipette C and a similar operation
is carried out. The difference between the resulting reading and the
first reading gives the percentage of oxygen in the flue gases.
The next operation is to drive the gas into the pipette D, the gas being
given a final wash in E, and then passed into the pipette C to
neutralize any hydrochloric acid fumes which may have been given off by
the cuprous chloride solution, which, especially if it be old, may give
off such fumes, thus increasing the volume of the gases and making the
reading on the burette less than the true amount.
The process must be carried out in the order named, as the pyrogallol
solution will also absorb carbon dioxide, while the cuprous chloride
solution will also absorb oxygen.
As the pressure of the gases in the flue is less than the atmospheric
pressure, they will not of themselves flow through the pipe connecting
the flue to the apparatus. The gas may be drawn into the pipe in the way
already described for filling the apparatus, but this is a tedious
method. For rapid work a rubber bulb aspirator connected to the air
outlet of the cock G will enable a new supply of gas to be drawn into
the pipe, the apparatus then being filled as already described. Another
form of aspirator draws the gas from the flue in a constant stream, thus
insuring a fresh supply for each sample.
The analysis made by the Orsat apparatus is volumetric; if the analysis
by weight is required, it can be found from the volumetric analysis as
follows:
Multiply the percentages by volume by either the densities or the
molecular weight of each gas, and divide the products by the sum of all
the products; the quotients will be the percentages by weight. For most
work sufficient accuracy is secured by using the even values of the
molecular weights.
The even values of the molecular weights of the gases appearing in an
analysis by an Orsat are:
Carbon Dioxide 44
Carbon Monoxide 28
Oxygen 32
Nitrogen 28
Table 33 indicates the method of converting a volumetric flue gas
analysis into an analysis by weight.