Steam, Its Generation and Use Part 20

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 pa.s.sed 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 c.o.c.k 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 a.n.a.lysis made by the Orsat apparatus is volumetric; if the a.n.a.lysis by weight is required, it can be found from the volumetric a.n.a.lysis 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 a.n.a.lysis 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 a.n.a.lysis into an a.n.a.lysis by weight.

TABLE 33

CONVERSION OF A FLUE GAS a.n.a.lYSIS BY VOLUME TO ONE BY WEIGHT

Column Headings:

A: a.n.a.lysis by Volume Per Cent B: Molecular Weight C: Volume times Molecular Weight D: a.n.a.lysis by Weight Per Cent _____________________________________________________________________ | | | | | | | Gas | A | B | C | D | |________________________|_______|___________|________|_______________| | | | | | | | | | | | | | | | | | 536.8 | | Carbon Dioxide CO_{2} | 12.2 | 12+(216) | 536.8 | ------ = 17.7 | | | | | | 3022.8 | | | | | | | | | | | | 11.2 | | Carbon Monoxide CO | .4 | 12+16 | 11.2 | ------ = .4 | | | | | | 3022.8 | | | | | | | | | | | | 220.8 | | Oxygen O | 6.9 | 216 | 220.8 | ------ = 7.3 | | | | | | 3022.8 | | | | | | | | | | | | 2254.0 | | Nitrogen N | 80.5 | 214 | 2254.0 | ------ = 74.6 | | | | | | 3022.8 | |________________________|_______|___________|________|_______________| | | | | | | | Total | 100.0 | | 3022.8 | 100.0 | |________________________|_______|___________|________|_______________|

Application of Formulae and Rules--Pocahontas coal is burned in the furnace, a partial ultimate a.n.a.lysis being:

_Per Cent_ Carbon 82.1 Hydrogen 4.25 Oxygen 2.6 Sulphur 1.6 Ash 6.0 B. t. u., per pound dry 14500

The flue gas a.n.a.lysis shows:

_Per Cent_

CO_{2} 10.7 O 9.0 CO 0.0 N (by difference) 80.3

Determine: The flue gas a.n.a.lysis by weight (see Table 33), the amount of air required for perfect combustion, the actual weight of air per pound of fuel, the weight of flue gas per pound of coal, the heat lost in the chimney gases if the temperature of these is 500 degrees Fahrenheit, and the ratio of the air supplied to that theoretically required.

Solution: The theoretical weight of air required for perfect combustion, per pound of fuel, from formula (11) will be,

(.821 .026 .016) W = 34.56 (---- + (.0425 - ----) + ----) = 10.88 pounds.

( 3 8 8 )

If the amount of carbon which is burned and pa.s.ses away as flue gas is 80 per cent, which would allow for 2.1 per cent of unburned carbon in terms of the total weight of dry fuel burned, the weight of dry gas per pound of carbon burned will be from formula (16):

11 10.7 + 8 9.0 + 7(0 + 80.3) W = --------------------------------- = 23.42 pounds 3(10.7 + 0)

and the weight of flue gas per pound of coal burned will be .80 23.42 = 18.74 pounds.

The heat lost in the flue gases per pound of coal burned will be from formula (15) and the value 18.74 just determined.

Loss = .24 18.74 (500 - 60) = 1979 B. t. u.

The percentage of heat lost in the flue gases will be 1979 14500 = 13.6 per cent.

The ratio of air supplied per pound of coal to that theoretically required will be 18.74 10.88 = 1.72 per cent.

The ratio of air supplied per pound of combustible to that required will be from formula (14):

.803 ------------------------- = 1.73 .803 - 3.782(.09 - 0)

The ratio based on combustible will be greater than the ratio based on fuel if there is unconsumed carbon in the ash.

Unreliability of CO_{2} Readings Taken Alone--It is generally a.s.sumed that high CO_{2} readings are indicative of good combustion and hence of high efficiency. This is true only in the sense that such high readings do indicate the small amount of excess air that usually accompanies good combustion, and for this reason high CO_{2} readings alone are not considered entirely reliable. Wherever an automatic CO_{2} recorder is used, it should be checked from time to time and the a.n.a.lysis carried further with a view to ascertaining whether there is CO present. As the percentage of CO_{2} in these gases increases, there is a tendency toward the presence of CO, which, of course, cannot be shown by a CO_{2} recorder, and which is often difficult to detect with an Orsat apparatus. The greatest care should be taken in preparing the cuprous chloride solution in making a.n.a.lyses and it must be known to be fresh and capable of absorbing CO. In one instance that came to our attention, in using an Orsat apparatus where the cuprous chloride solution was believed to be fresh, no CO was indicated in the flue gases but on pa.s.sing the same sample into a Hempel apparatus, a considerable percentage was found. It is not safe, therefore, to a.s.sume without question from a high CO_{2} reading that the combustion is correspondingly good, and the question of excess air alone should be distinguished from that of good combustion. The effect of a small quant.i.ty of CO, say one per cent, present in the flue gases will have a negligible influence on the quant.i.ty of excess air, but the presence of such an amount would mean a loss due to the incomplete combustion of the carbon in the fuel of possibly 4.5 per cent of the total heat in the fuel burned. When this is considered, the importance of a complete flue gas a.n.a.lysis is apparent.

Table 34 gives the densities of various gases together with other data that will be of service in gas a.n.a.lysis work.

TABLE 34

DENSITY OF GASES AT 32 DEGREES FAHRENHEIT AND ATMOSPHERIC PRESSURE ADAPTED FROM SMITHSONIAN TABLES

+----------+----------+--------+---------+----------+---------------+ | | | | | | Relative | | | | | Weight | | Density, | | | | | of | Volume | Hydrogen = 1 | | | |Specific|One Cubic| of +-------+-------+ | Gas | Chemical |Gravity | Foot |One Pound | |Approx-| | | Symbol | Air=1 | Pounds |Cubic Feet| Exact | imate | +----------+----------+--------+---------+----------+-------+-------+ |Oxygen | O | 1.053 | .08922 | 11.208 | 15.87 | 16 | |Nitrogen | N | 0.9673 | .07829 | 12.773 | 13.92 | 14 | |Hydrogen | H | 0.0696 | .005621 | 177.90 | 1.00 | 1 | |Carbon | | | | | | | | Dioxide | CO_{2} | 1.5291 | .12269 | 8.151 | 21.83 | 22 | |Carbon | | | | | | | | Monoxide | CO | 0.9672 | .07807 | 12.809 | 13.89 | 14 | |Methane | CH_{4} | 0.5576 | .04470 | 22.371 | 7.95 | 8 | |Ethane |C_{2}H_{6}| 1.075 | .08379 | 11.935 | 14.91 | 15 | |Acetylene |C_{2}H_{2}| 0.920 | .07254 | 13.785 | 12.91 | 13 | |Sulphur | | | | | | | | Dioxide | SO_{2} | 2.2639 | .17862 | 5.598 | 31.96 | 32 | |Air | ... | 1.0000 | .08071 | 12.390 | ... | ... | +----------+----------+--------+---------+----------+-------+-------+

[Ill.u.s.tration: 1942 Horse-power Installation of Babc.o.c.k & Wilc.o.x Boilers and Superheaters in the Singer Building, New York City]

CLa.s.sIFICATION OF FUELS

(WITH PARTICULAR REFERENCE TO COAL)

Fuels for steam boilers may be cla.s.sified as solid, liquid or gaseous.

Of the solid fuels, anthracite and bituminous coals are the most common, but in this cla.s.s must also be included lignite, peat, wood, baga.s.se and the refuse from certain industrial processes such as sawdust, shavings, tan bark and the like. Straw, corn and coffee husks are utilized in isolated cases.

The cla.s.s of liquid fuels is represented chiefly by petroleum, though coal tar and water-gas tar are used to a limited extent.

Gaseous fuels are limited to natural gas, blast furnace gas and c.o.ke oven gas, the first being a natural product and the two latter by-products from industrial processes. Though waste gases from certain processes may be considered as gaseous fuels, inasmuch as the question of combustion does not enter, the methods of utilizing them differ from that for combustible gaseous fuel, and the question will be dealt with separately.

Since coal is by far the most generally used of all fuels, this chapter will be devoted entirely to the formation, composition and distribution of the various grades, from anthracite to peat. The other fuels will be discussed in succeeding chapters and their combustion dealt with in connection with their composition.

Formation of Coal--All coals are of vegetable origin and are the remains of prehistoric forests. Destructive distillation due to great pressures and temperatures, has resolved the organic matter into its invariable ultimate const.i.tuents, carbon, hydrogen, oxygen and other substances, in varying proportions. The factors of time, depth of beds, disturbance of beds and the intrusion of mineral matter resulting from such disturbances have produced the variation in the degree of evolution from vegetable fiber to hard coal. This variation is shown chiefly in the content of carbon, and Table 35 shows the steps of such variation.

TABLE 35

APPROXIMATE CHEMICAL CHANGES FROM WOOD FIBER TO ANTHRACITE COAL