air blown into the producer, and by utilizing the sensible heat of the gas in the combustion-furnace. It ought to be possible to oxidize one out of every four lbs. of carbon with oxygen derived from water-vapor. The thermic reactions in this operation are as follows: Heat-units. 4 lbs. C burned to CO (3 lbs. gasified with air and 1 lb. with water) develop.. 17,600 10,333 1.5 lbs. of water (which furnish 1.33 lbs. of oxygen to combine with 1 lb. of carbon) absorb by dissociation.. The gas, consisting of 9.333 lbs. CO, 0.167 lb. H, and 13.39 lbs. N, heated 600°, absorbs 3,748 3,519 17,600 The steam which is blown into a producer with the air is almost all condensed into finely-divided water before entering the fuel, and consequently is considered as water in these calculations. The 1.5 lbs. of water liberates .167 lb. of hydrogen, which is delivered to the gas, and yields in combustion the same heat that it absorbs in the producer by dissociation. According to this calculation, therefore, 60% of the heat of primary combustion is theoretically recovered by the dissociation of steam, and, even if all the sensible heat of the gas be counted, with radiation and other minor items, as loss, yet the gas must carry 4 X 14,500 (37483519) = 50,733 heat-units, or 87% of the calorific energy of the carbon. This estimate shows a loss in conversion of 13%, without crediting the gas with its sensible heat, or charging it with the heat required for generating the necessary steam, or taking into account the loss due to oxidizing some of the carbon to CO2. In good producer-practice the proportion of CO, in the gas represents from 4% to 7% of the C burned to CO2, but the extra heat of this combustion should be largely recovered in the dissociation of more water-vapor, and therefore does not represent as much loss as it would indicate. As a conveyer of energy, this gas has the advantage of carrying 4.46 lbs. less nitrogen than would be present if the fourth pound of coal had been gasified with air; and in practical working the use of steam reduces the amount of clinkering in the producer. Anthracite Gas.-In anthracite coal there is a volatile combustible varying in quantity from 1.5% to over 7%. The amount of energy derived from the coal is shown in the following theoretical gasification made with coal of assumed composition: Carbon, 85%; vol. HC, 5%; ash, 10%; 80 lbs. carbon assumed to be burned to CO; 5 lbs. carbon burned to CO2; three fourths of the necessary oxygen derived from air, and one fourth from water. Energy in the above gas obtained from 100 lbs. anthracite: Total energy in gas per lb. 66 807,304 heat-units. 2,248 66 .86%. 100 lbs. of coal..1,349,500 Efficiency of the conversion The sum of CO and H exceeds the results obtained in practice. The sensible heat of the gas will probably account for this discrepancy, and, therefore, it is safe to assume the possibility of delivering at least 82% of the energy of the anthracite. Bituminous Gas.-A theoretical gasification of 100 lbs. of coal, containing 55% of carbon and 32% of volatile combustible (which is above the average of Pittsburgh coal), is made in the following table. It is assumed that 50 lbs. of C are burned to CO and 5 lbs. to CO2; one fourth of the O is derived from steam and three fourths from air; the heat value of the volatile combustible is taken at 20,000 heat-units to the pound. In computing volumetric proportions all the volatile hydrocarbons, fixed as well as condensing, are classed as marsb-gas, since it is only by some such tentative assumption that even an approximate idea of the volumetric composition can be formed. The energy, however, is calculated from weight: Water-gas.-Water-gas is made in an intermittent process, by blowing up the fuel-bed of the producer to a high state of incandescence (and in some cases utilizing the resulting gas, which is a lean producer-gas), then shutting off the air and forcing steam through the fuel, which dissociates the water into its elements of oxygen and hydrogen, the former combining with the carbon of the coal, and the latter being liberated. This gas can never play a very important part in the industrial field, owing to the large loss of energy entailed in its production, yet there are places and special purposes where it is desirable, even at a great excess in cost per unit of heat over producer-gas; for instance, in small high-temperature furnaces, where much regeneration is impracticable, or where the "blow-up" gas can be used for other purposes instead of being wasted. The reactions and energy required in the production of 1000 feet of watergas, composed, theoretically, of equal volumes of CO and H, are as follows: 500 cubic feet of H weigh... 500 cubic feet of CO weigh.. Total weight of 1000 cubic feet........ 2.635 lbs. 36.89 66 39.525 lbs. Now, as CO is composed of 12 parts C to 16 of O, the weight of C in 36.89 lbs. is 15.81 lbs. and of O 21.08 lbs. When this oxygen is derived from water it liberates, as above, 2.635 lbs. of hydrogen. The heat developed and absorbed in these reactions (roughly, as we will not take into account the energy required to elevate the coal from the temperature of the atmosphere to say 1800°) is as follows: = = Heat-units. 163,370 69,564 = 93,806 2.635 lbs. H absorb in dissociation from water 2.635 × 62,000.. 15.81 lbs. C burned to CO develops 15.81 X 4400... Excess of heat-absorption over heat-development If this excess could be made up from C burnt to CO2 without loss by radi ation, we would only have to burn an additional 4.83 lbs. C to supply this heat, and we could then make 1000 feet of water-gas from 20.64 lbs. of carbon (equal 24 lbs. of 85% coal). This would be the perfection of gas-making. as the gas would contain really the same energy as the coal; but instead, we require in practice more than double this amount of coal, and do not deliver more than 50% of the energy of the fuel in the gas, because the supporting heat is obtained in an indirect way and with imperfect combustion. Besides this, it is not often that the sum of the CO and H exceed 90%, the balance being CO2 and N. But water-gas should be made with much less loss of energy by burning the "blow-up" (producer) gas in brick regenerators, the stored-up heat of which can be returned to the producer by the air used in blowing-up. The following table shows what may be considered average volumetric analyses, and the weight and energy of 1000 cubic feet, of the four types of gases used for heating and illuminating purposes: ton of coal. Approximately 30,000 cubic feet of gas have the heating power of one Calculated upon this basis, the 131,280 ft. of gas from the ton of coal contained 20,811,162 B.T.U., or 155 B.T. U. per cubic ft., or 2270 B.T.U. per lb. The composition of the coal from which this gas was made was as follows: Water. 1.26%; volatile matter, 36.22%; fixed carbon, 57.98%; sulphur, 0.70%; ash, 3.78% One ton contains 1159.6 lbs. carbon and 724.4 lbs. volatile combustible, the energy of which is 31,302,200 B.T.U. Hence, in the processes of gasification and purification there was a loss of 35.2% of the energy of the coal. The composition of the hydrocarbons in a soft coal is uncertain and quite complex; but the ultimate analysis of the average coal shows that it approaches quite nearly to the composition of CH, (marsh-gas). Mr. Blauvelt emphasizes the following points as highly important in soft coal producer-practice: First. That a large percentage of the energy of the coal is lost when the gas is made in the ordinary low producer and cooled to the temperature of the air before being used. To prevent these sources of loss, the producer should be placed so as to lose as little as possible of the sensible heat of the gas, and prevent condensation of the hydrocarbon vapors. A high fuel-bed should be carried, keeping the producer cool on top, thereby preventing the breaking-down of the hydrocarbons and the deposit of soot, as well as keeping the carbonic acid low. Second. That a producer should be blown with as much steam mixed with the air as will maintain incandescence. This reduces the percentage of nitrogen and increases the hydrogen, thereby greatly enriching the gas. The temperature of the producer is kept down, diminishing the loss of heat by radiation through the walls, and in a large measure preventing clinkers. The Combustion of Producer-gas. (H. H. Campbell, Trans. A. I M. E., xix. 128.)-The combustion of the components of ordinary pro ducer-gas may be represented by the following formula: C2H4 +60 = 2CO2 + 2H2O: 2H+0=H,0; AVERAGE COMPOSITION BY VOLUME OF PRODUCER-GAS: A, MADE WITH OPEN The coal used contained carbon 82%, hydrogen 4.7%. CO2. O. 0.2 17.2 (R. Use of Steam in Producers and in Boiler-furnaces. W. Raymond, Trans. A. I. M. E., xx. 635.)-No possible use of steam can cause a gain of heat. If steam be introduced into a bed of incandescent carbon it is decomposed into hydrogen and oxygen. The heat absorbed by the reduction of one pound of steam to hydrogen is much greater in amount than the heat generated by the union of the oxygen thus set free with carbon, forming either carbonic oxide or carbonic acid. Consequently, the effect of steam alone upon a bed of incandescent fuel is to chill it. In every water-gas apparatus, designed to produce by means of the decomposition of steam a fuel gas relatively free from nitrogen, the loss of heat in the producer must be compensated by some reheating device. This loss may be recovered if the hydrogen of the steam is subsequently burned, to form steam again. Such a combustion of the hydrogen is contemplated, in the case of fuel-gas, as secured in the subsequent use of that gas. Assuming the oxidation of H to be complete, the use of steam will cause neither gain nor loss of heat, but a simple transference, the heat absorbed by steam decomposition being restored by hydrogen combustion. In practice, it may be doubted whether this restoration is ever complete. But it is certain that an excess of steam would defeat the reaction altogether, and that there must be a certain proportion of steam, which permits the realization of important advantages, without too great a net loss in heat. The advantage to be secured (in boiler furnaces using small sizes of anthracite) consists principally in the transfer of heat from the lower side of the fire, where it is not wanted, to the upper side, where it is wanted. The decomposition of the steam below cools the fuel and the grate-bars, whereas a blast of air alone would produce, at that point, intense combustion (forming at first CO2), to the injury of the grate, the fusion of part of the fuel, etc. The proportion of steam most economical is not easily determined. The temperature of the steam itself, the nature of the fuel mixture, and the use or non-use of auxiliary air-supply, introduced into the gases above or beyond the fire-bed, are factors affecting the problem. (See Trans. A. I. M. E., xx. 625 ) Gas Analyses by Volume and by Weight.-To convert an an alysis of a mixed gas by volume into analysis by weight: Multiply the percentage of each constituent gas by the density of that gas (see p. 166). Divide each product by the sum of the products to obtain the percentages by weight. Gas-fuel for Small Furnaces.-E. P. Reichhelm (Am. Mach., Jan. 10, 1895) discusses the use of gaseous fuel for forge fires, for dropforging, in annealing-ovens and furnaces for melting brass and copper, for case-hardening, muffle-furnaces, and kilns. Under ordinary conditions, in such furnaces he estimates that the loss by draught, radiation, and the heating of space not occupied by work is, with coal, 80%, with petroleum 70%, and with gas above the grade of producer-gas 25%. He gives the following table of comparative cost of fuels, as used in these furnaces: .73 .73 Water-gas and producer-gas mixed.. Producer-gas Coal, $4 per ton, per 1,000,000 heat-units utilized Mr. Reichhelm gives the following figures from practice in melting brass with coal and with naphtha converted into gas: 1800 lbs. of metal require 1080 lbs. of coal, at $4.65 per ton, equal to $2 51, or, say, 15 cents per 100 lbs. Mr. T.'s report: 2500 lbs. of metal require 47 gals. of naphtha, at 6 cents per gal., equal to $2.82, or, say, 1114 cents per 100 lbs. ILLUMINATING-GAS. Coal-gas is made by distilling bituminous coal in retorts. The retort is usually a long horizontal semi-cylindrical or shaped chamber, holding from 160 to 300 lbs. of coal. The retorts are set in "benches" of from 3 to 9, heated by one fire, which is generally of coke The vapors distilled from the coal are converted into a fixed gas by passing through the retort, which is heated almost to whiteness. The gas passes out of the retort through an "ascension-pipe" into a long horizontal pipe called the hydraulic main, where it deposits a portion of the tar it contains; thence it goes into a condenser, a series of iron tubes surrounded by cold water, where it is freed from condensable vapors, as ammonia-water, then into a washer, where it is exposed to jets of water, and into a scrubber, a large chamber partially filled with trays made of wood or iron, containing coke, fragments of brick or paving-stones, which are wet with a spray of water. By the washer and scrubber the gas is freed from the last portion of tar and ammonia and from some of the sulphur compounds. The gas is then finally purified from sulphur compounds by passing it through lime or oxide of iron. The gas is drawn from the hy draulic main and forced through the washer, scrubber, etc., by an exhauster or gas pump. The kind of coal used is generally caking bituminous, but as usually this coal is deficient in gases of high illuminating power, there is added to it a portion of cannel coal or other enricher. The following table, abridged from one in Johnson's Cyclopedia, shows the analysis, candle power, etc., of some gas-coals and enrichers: |