If this gas is generated with dry air in con tact with fuel free from impurities, it will still contain all the nitrogen of the air and will be composed approximately of carbon monoxide (CO), 34.7 per cent; nitrogen (N), 65.3 per cent by volume, and will possess a heating value of about 118 British thermal units per cubic foot.
In actual practice, however, it usually con tains also a certain amount of carbon dioxide CO, and a little hydrogen (H), the H being de rived from the fuel and the decomposition of the moisture in the air supply when it comes in contact with the glowing carbon.
When fuels such as soft coals are used, the products of distillation of the raw fuel in the upper zone of combustion, consisting chiefly of hydrogen and the hydrocarbons — marsh gas (CH.) and olefiant gas (C,F14) will become mixed with the products of gasification in the zone below.
Air when passed over incandescent carbon maintain a given quali of gas with a minimum percentage of CO,. Practically, ractically, bowever, the heat employed must not be so great as to de stroy the producer, and the lower the exit tem perature of the gases the better.
It is important to note that wet coals re tard the development of high temperature in a gas producer, on account of the great heat ex penditure required for the vaporization of such water; and also, that for like reasons, carbon ized fuels work at a higher temperature than uncarbonized fuels.
A condition which affects favorably the per centage of combustibles in the gas is the pres ence of steam in the air supply. Blowing with air alone introduces a large amount of inert nitrogen which dilutes the gas and reduces its calorific value. On the other hand, the use of steam increases the percentage of combustibles by adding hydrogen and supplying in its disso ciation a quantity of oxygen which is not di luted with nitrogen, thus increasing the calorific value of the gas, reducing its exit temperature and preventing clinkering.
(C) gives minimum CO2 at a temperature of 2000 F., therefore, the temperature of the pro.. ducer should he kept above this point. Further more, the formation of CO is greatly affected by the character of the fuel and the depth of the fuel bed, so that the greater the depth of the fuel bed and the more finely divided the fuel, provided it does not offer too much resistance to the passage of the air and gases, the greater will be the percentage of CO formed.
Other things being equal, the higher the tem perature of the producer the greater will be the amount of fuel gasified per unit of time, but as this depends largely upon the air supply, an increased air supply signifies more rapid com bustion, greater velocity of the gases through the fuel bed, briefer contact of the gases with the fuel, and, therefore, indicates that a greater depth of contact is required if it is desired to The use of steam does not, however, result in the production of a greater amount of heat in the producer. The heat is simply transferred
from the generator to the furnace by the higher potential heat value of H, instead of the less efficient, though greater sensible, heat in the gas itself. In practice it is found that not all of the water content of the air-blast is con, verted into hydrogen and oxygen. A small per centage slips through unchanged, and in the combustion chamber of the furnace absorbs heat in its dissociation, and, to that degree, reduces the temperature of the furnace. For this rea son the amount of water supplied (as steam) must be carefully regulated according to • the amount of moisture present in the fuel. When the fuel is dry and highly carbonized the equiva lent of one pound of water may be injected with each 80 cubic feet of air at ordinary tern- • peratures. If the air-blast and steam are pre heated, say to 1000° F., one pound of water may be injected with each 40 cubic feet of air. With fuel of the composition of bituminous coal one pound of water is supplied with each 85 feet of air.
It is apparent, therefore, that the composition of producer gas is likely to be very variable, owing in the first place to the lack of uniform ity in the fuel, and secondarily to the varying adjustments of the air and steam supplies and the degree of heat at which the gas is produced. An analysis of producer gas made from Illinois bituminous coal showed, in combustible con stitutents: Carbon monoxide, 15.12 per cent; hydrogen, 9.98 per cent; methane (marsh gas), 6 per cent; and of incombustible constituents: nitrogen, 59.06 per cent; carbon dioxide, 9.72 per cent ; and oxygen, 0.12 per cent. Where the gas is not used directly as it comes from the producer, but is cooled and the moisture content condensed. a larger quantity of water may be used during gasification, with the beneficial ef fect of reducing the nitrogen content to or slightly below 50 per cent, and replacing the 9 or 10 per cent thus eliminated with hydrogen. The percentage of carbon dioxide will be larger in the case of fuels with a low melting slag, as in such cases the temperature of the producer must be kept so low that not all of the dioxide is transformed into monoxide. A higher per centage of dioxide than 4 or at most 5 per cent is objectionable and indicates that the pro ducer is not working properly, or that its fuel is not suitable for making profitable producer gas.