April 15, 1909] 



NA TURE 



monoxide instead of carbon dioxide, for when there is an 

 excess of highly heated carbon the dioxide formed in the 

 lower part of the fire is reduced to the monoxide. Carbon 

 monoxide may also be formed by the direct combustion of 

 the carbon with oxygen, and actually both these reactions 

 may, and probably do, occur. Theoretically, if we were 

 dealing only with carbon and air, about 30 per cent, of 

 the heat of combustion would be liberated in the producer, 

 and about 70 per cent, would be liberated when the carbon 

 monoxide is afterwards burnt to carbon dioxide in a 

 furnace or engine, &c. ; the practical result, however, is 

 still less favourable, and pr:m& facie the conversion of 

 solid fuel into gas does not seem a promising performance. 



It is true that not all the heat set free in the producer 

 need be lost if the gas can be used while it is hot (as in 

 furnace work) ; but for gas engines it must be cold. Apart 

 from this, the liberation of so much sensible heat in the 

 producer overheats it, and indirectly it promotes the forma- 

 tion of clinker, which is a practical drawback. To avoid 

 these and other difliculties, the almost invariable practice 

 is to add a certain proportion of steam or aqueous vapour 

 to the air sent into the producer. 



It should, however, be clearly understood that from the 

 point of view of the heat quantities involved, the use of 

 steam in a gas producer is simply a means for absorbing 

 the sensible heat developed by the partial combustion of 

 the fuel, and storing it for future use. Obviously there 

 can be no actual increase of the total amount of heat 

 which can be obtained from a given quantity of fuel. 

 Besides avoiding excessive heat in the producer, the use of 

 steam has the further practical advantage that a gas of 

 considerably greater calorific power per unit volume can 

 be obtained than is possible when air alone is used. The 

 use of air necessarily involves the presence of the diluent 

 nitrogen, and when steam is decomposed the resulting 

 hydrogen and carbon monoxide displace some of the 

 nitrogen. 



With the exception of coke and charcoal, all ordinary 

 fuels give off volatile substances when subjected to heat ; 

 and in a gas producer, working in the ordinary way with 

 an upward draught, each fresh charge of fuel is heated, 

 and is then subjected in some degree to a process of dis- 

 tillation before it descends into the zone where partial 

 combustion takes place. The gas actually obtained may 



Fig. 1. — Steavi-jctp 



flatit. (z, superheating steam boiler ; b, steam jet and air injecto 

 scrubber with water seal \/^ water sprayer ; g^ gas-hold 



, .., gas producer ; d^ waste cock ; 



and tank ; h gas outlet. 



We are therefore led to consider how steam reacts with 

 the carbon with which it comes in contact. If the carbon 

 is at a sufficiently high temperature, the steam (H,0) is 

 decomposed, and an equal volume of hydrogen is produced ; 

 the oxygen of the steam combines with the carbon to form 

 either carbon monoxide or carbon dioxide, according to the 

 conditions under which the reaction takes place. When 

 hydrogen combines with oxygen to form water vapour 

 heat is liberated, and when this water vapour is decom- 

 posed by the reaction of highly heated carbon (or by any 

 other means) an equal amount of heat is absorbed. The 

 combination of the oxygen of the steam with the carbon , 

 is accompanied by the evolution of heat, but the quantity 

 of heat thns evolved is much less than the quantity of 

 heat absorbed by the decomposition of the steam, and 

 this is why the addition of small quantities of steam to 

 the air going into the producer reduces the working 

 temperatui;e. Part of the sensible heat is absorbed bv the 

 reactions which take place between the steam and the 

 incandescent carbon, so that the gas leaves the producer 

 at a low'er temperature than is the case when air alone is 

 used ; the heat so absorbed is stored up in the gas, and is 

 again set free when the gas is burnt. 



NO. 2059, VOL. 80] 



therefore be regarded as producer gas obtained from 

 carbon, mixed with the volatile substances given off by the 

 distillation. The actual composition of the gas depends a 

 good deal on the nature and amount of these volatile sub- 

 stances, and they vary considerably, the fuels used being 

 chiefly anthracite, coke, and bituminous or semi-bituminous 

 coal. Both these coals give off a considerable quantity of 

 tarry matter, which may represent as much as 8 per cent, 

 or 9 per cent, of the total heat value of the fuel. When 

 the gas is cooled and scrubbed before use, the tar which 

 is removed has little value ; it is therefore desirable that 

 producers should be designed to burn the tar in the pro- 

 ducer itself, or to decompose it and convert it into com- 

 bustible gases which will not condense at ordinary tempera- 

 tures. Even anthracite when heated yields both hydrogen 

 and methane, and this is why it makes a better gas than 

 coke. 



In Fig. I we give a typical example of a gas plant in 

 which the producer is worked with a jet of superheated 

 steam which injects the air required. 



In some plants the steam required is produced by the 

 sensible heat of the gas after it has left the producer, and 

 this effects a certain saving ; but even then the gas must 



