The nat Some explanations of these analyses are necessary. ural gas is that of Findlay, O. The coal gas is probably an average sample of coal gas, purified for use as an illuminant. The water gas is that of a sample of gas made for heating, and consequently not purified, hence the larger per cent. of CO2 that it contains. 2 Since calculating the tables used in this paper, I am satisfied that the sample of water gas is not an average one. The CO is too high and H is too low. Were proper corrections made in this respect, it would increase the value in heat units of a pound, but not materially change the value when volume is considered, and as that is the way in which gases are sold, the tables will not be recalculated. The producer gas is that of an average sample of the Pennsylvania Steel Works, made from anthracite, and is not of so high grade as would be that made from soft coal. The natural gas excels, as shown in Table I, because of the large per cent. of marsh gas. In no other form, in the gases mentioned, do we get so much hydrogen in a given volume of gas. It is the large per cent. of hydrogen in the coal gas that makes it so nearly equivalent to the natural gas in a given weight, but much of the hydrogen in coal gas, being free, makes it fall short of natural gas in calorific value per unit of volume. Water gas is composed, as you know, chiefly of carbonic oxide and hydrogen, both good heating gases, but nearly all the hydrogen being free, makes the actual weight of hydrogen in a given volume, far below that of natural gas, and again you will notice by the analyses given, that three-quarters of the weight of water gas is CO, and of this compound four-sevenths is oxygen, which possesses no calorific value; in other words, more than forty per cent. of the weight of uncarburetted water gas, even when free from CO2, and nitrogen fails to add anything to the value of the gas as a heat producer. The producer gas, of which an analysis is given above, does not possess as high calorific value as it would if made from soft coal, but a greater volume can be made from a ton of anthracite or hard coke than can be made from a like quantity of bituminous coal. A further comparison of the value of the several gases named may be made by showing the quantity of water that would be evaporated by 1,000 feet of each kind of gas, allowing an excess of twenty per cent. of air, and permitting the resultant gases to escape at a temperature of 500°. This sort of comparison, probably, has more practical value than either of the others that have been previously given. We will assume that the air for combustion is entering at a temperature of 60°. The theoretical temperature that may be produced by these several gases does not differ greatly as between the first three named. The producer gas falls about twenty-five per cent. below the others, giving a temperature of only 3,441° F. Water gas leads in this respect, with a temperature of 4,850°. A formula for calculating the temperature theoretically obtained may be as follows: A comparison of the resultant products of combustion also shows water gas to possess merit over either natural or coal gas, when the combustion of equal quantities-say 1,000 feet-is considered. An excess of twenty per cent. of air is calculated in the following table: Steam.... Carbonic acid.. Sulphuric acid.. Total weight after com- Pounds oxygen for combination... 94.25 69.718 25.104 6.921 119.59 68.586 61.754 36.456 00.36 664.96 427.222 170.958 126.568 879.16 565.526 257.816 169.945 167.462 107.961 43.149 19.677 While the combustion of 1,000 feet of natural gas vitiates the atmosphere when consumed under the conditions named, more than five times as much as does the combustion of 1,000 feet of producer gas, yet for the work performed the former vitiates the atmosphere less than does the latter gas. You will observe, by the following table, that, with the exception of producer gas, each kind gives off nearly one pound of waste gases for each pound of water evaporated. This quantity includes twenty per cent. excess of air. TABLE VI.-Weights of WATER EVAPORATED AND of Result ANT GASES. Natural Coal Water Producer Weight of water evaporated 893.25 591. 262. Weight of gases after combustion 879.16 565.526 257.816 169.945 The vitiation of the atmosphere per unit of value in water evaporation is practically the same in water gas as in natural gas. Coal gas shows about three per cent. better than either of the two gases named, in this respect, and about fifty per cent. better than producer gas. In the above comparisons, all of the resulting products of combustion, including excess of oxygen, are regarded as being of a deleterious character, tending to vitiate the atmosphere. However, the excess of oxygen does no harm, and the steam and nitrogen cannot be regarded as very objectionable products. The gas that robs the air permanently of the most oxygen, and produces the greatest quantity of carbonic acid per unit of work, must be classed as the most objectionable from a sanitary standpoint. TABLE VII.-OXYGEN ABSORBED AND CARBONIC ACID PRO Here, then, it is shown that if pollution by carbonic acid and impoverishment by the absorption of oxygen are equally deleterious to the atmosphere, coal gas stands at the head as being the least objectionable. It will be conceded that the gas that gives the greatest number of heat units for a given volume, with the least weight of resultant gases of combustion, must possess the greatest value for all ordinary purposes of heating. Notwithstanding this fact, there are purposes for which gas is used where this rule would not apply. For instance: Natural gas is not so desirable for gas engines as is coal gas, chiefly because the flame that lights the explosive mixture is too easily extinguished; again, the explosive mixture will not ignite so readily when natural gas is used as it will when coal gas or water gas is used. For metallurgical purposes, the producer gas has one advantage over all other gases in this—that it contains less hydrogen in any and every form than either of the others, and where metal, and especially iron or steel, is to be heated to very high temperatures by direct contact, it is questionable if anything like the full value of the hydrogen is utilized, unless it be combined with oxygen in recuperators and the heat returned in the air used for secondary combustion. While this advantage does not bring producer gas nearly up to the value of any of the other gases, yet it does bring its value nearer to the others than appears from a theoretical calculation. The producer gas, on the other hand, possesses a disadvantage in connection with metallurgical work, in this, that the resultant gases having to pass off at high temperatures, and the weight of these per unit of work being so much greater than that of the other gases, more heat units will thus be wasted. This disadvantage, however, will not be very great where recuperators are employed, nor will it be great when the gas is used for raising steam, or for culinary purposes, or for any purpose which permits the waste gas to finally escape at a low degree of temperature. In the utilization of coal gas for fuel we do not obtain, even theoretically, but little more than twenty-five per cent. of the energy of the coal from which the gas was made; but this is not a fair way to put it, because we have in the coal gas manufacture the valuable residuals, coke, tar and ammonia. It will only be convenient to compare the processes for the manufacture of two of the gases, water gas and producer gas, and ascertain in which the greater loss of energy is sustained. In making these comparisons, we must accept data obtained from practice rather than what might be deduced by theoretical calculation. It would be difficult to calculate what quantity of energy would be wasted in blowing up a heat in a water-gas apparatus. We might tell how much heat would be lost in the generation of the steam required, if we knew exactly how much of the steam was decomposed. But upon this point there is a great diversity of opinion, some claiming that ninety per cent. is decomposed, while others claim that not more than twenty-five to forty per cent. is utilized in the cupolas. The net results obtained from fuel by the many different works using water-gas apparatus vary greatly, doubtless because of the varied conditions under which they are operated. In some works the cupola may run almost constantly, and there the minimum quantity of hard fuel will be required. In smaller works the cupolas will be idle half the time, and there the maximum quantity will be used. |