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    "source_title": "Encyclopaedia Britannica (1911)",
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    "chunk_id": "1911:co2:ac3efd64cf68",
    "title": "CO2",
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    "verified_text": "co2, and this is of course the maximum effect obtainable from this source. if the same gram-molecule of carbon is used for making water gas, that is, co + h2, the heat produced by the combustion of the product is 68.4 + 57.6 = 126 great calories, an apparent surplus of 29 calories, which cannot be got out of nothing. this is made evident by another consideration. in the above reaction c is not burned to co2, but to co, a reaction which produces 28.6 calories per gram-molecule. but as the oxygen is furnished from water, which must first be decomposed by the expenditure of energy, we must introduce this amount, 68.5 calories in the case of liquid water, or 57.6 calories in the case of steam, as a negative quantity, and the difference, viz. + 28.6 - 57.6 = 29 great calories, represents the amount of heat to be expended from another source in order to bring about the reaction of one gram-molecule of carbon on one gram-molecule of h2o in the shape of steam. this explains why steam directed upon incandescent coal will produce water gas only for a very short time: even a large mass of coal will quickly be cooled down so much that at first a gas of different composition is formed and soon the process will cease altogether. we can avoid this result by carrying on the process in a retort heated from without by an ordinary coal fire, and all the early water gas apparatus was constructed in this way; but such a method is very uneconomical, and was long ago replaced by a process first patented by j. and t.n. kirkham in 1854, and very much improved by successive inventors. this process consists in conducting the operation in an upright brick shaft, charged with anthracite, coke or other suitable fuel. this shaft resembles an ordinary gas producer, but it differs in being worked, not in a continuous manner, which, as shown above, would be impossible, but by alternately blowing air and steam through the coal for periods of a few minutes each. during the first phase, when carbon is burned by atmospheric oxygen, and thereby heat is produced, this heat, or rather that part of it which is not carried away by radiation and by the products of combustion on leaving the apparatus, is employed in raising the temperature of the remaining mass of fuel, and is thus available for the second phase, in which the reaction (b) c + h2o = co + h2 goes on with the abstraction of a corresponding amount of heat from the incandescent fuel, so that the latter rapidly cools down, and the process must be reversed by blowing in air and so forth. the formation of exactly equal volumes of carbon monoxide and hydrogen goes on only at temperatures over 1200 deg. c., that is, for a very few minutes. even at 1100 deg. c. a little co2 can be proved to exist in the gas, and at 900 deg. its proportion becomes too high to allow the process to go on. about 650 deg. c. the co has fallen to a minimum, and the reaction is now essentially (c) c + 2h2o = co2 + 2h2; soon after the temperature of the mass will have fallen to such a low point that the steam passes through it without any perceptible action. the gas produced by reaction (c) contains only two-thirds of combustible matter, and is on that account less valuable than proper water gas formed by reaction (b); moreover, it requires the generation of twice the amount of steam, and its presence is all the less desirable since it must soon lead to a total cessation of the process. in ordinary circumstances it is evident that the more steam is blown in during a unit of time, the sooner reaction (c) will set in; on the other hand, the more heat has been accumulated in the producer the longer can the blowing-in of steam be continued. the process of making water gas consequently comprises two alternating operations, viz. first \"blowing-up\" by means of a current of air, by which the heat of the mass of fuel is raised to about 1200 deg. c.; and, secondly \"steaming,\" by injecting a current of (preferably superheated) steam until the temperature of the fuel had fallen to about 900 deg. c., and too much carbon dioxide appears in the product. during the steaming the gas is carried off by a special conduit into a scrubber, where the dust mechanically carried away in the current is washed out, and the gas is at the same time cooled down nearly to the ordinary temperature. it is generally stored in a gas-holder, from which it is conducted away as required. it is never quite free from nitrogen, as the producer at the beginning of steaming contains much of this gas, together with co or co2. the proportion of hydrogen may exceed 50%, in consequence of reaction (c) setting in at the close of the steaming. ordinary \"blue\" water gas, if, as usual, made from coke or anthracite, contains 48-52%",
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