In the constant search for ways and means of improving the quality of coke and thus making its use at the blast-furnace more satisfactory and economical, it has become increasingly evident that the present laboratory tests do not show the full picture of a coke’s value as a blast-furnace fuel.
That this fact has been recognized recently is evidenced by the amount of attention currently being given to the investigation of coke quality in this Country and abroad. Probably the most spectacular investigation undertaken in this Country has been the Coke Evaluation Project sponsored jointly by the Coke Oven Technical Committee of the American Iron and Steel Institute and the Coke Research Committee of the American Coke and Coal Chemicals Institute.
The fact that there exists a pronounced variation in the rate of combustion of different cokes was brought to the attention of the writer some years ago by observation of the behavior of cokes from different sources in shaft-type lime kilns. When using one coke, satisfactory kiln operation was indicated by the temperature of the burnt lime leaving the kiln and by the analysis and the temperature of the top gases. After changing to the second coke, both the temperature and analysis of the two products showed unsatisfactory changes which made this second coke unsuitable for this use. As a result of this observation, substantiated by reports from a number of other coke consuming operations, it became evident that this “neglected characteristic” was worthy of intensive study.
Factors Affecting Rate of Combustion. The Rate of Combustion of a coke is probably the factor which will largely determine its suitability as a fuel. But Rate of Combustion is dependent upon a number of other factors which may be listed in three general groups. These, in turn, are each divided into a number of sub-groups and some knowledge of the effects of these factors is essential if our purpose is to correct undesirable conditions of coke combustibility and not merely to identify them.
(A) Furnace Factors:
(a) Conductivity of furnace walls.
(b) Diameter of furnace.
(B) Air Factors:
(a) Velocity or Quantity of Air.
(b) Temperature of Air.
(c) Amount of Water Vapor in Air,
(C) Coke (or Carbon) Factors:
(a) Reactivity to Oxygen and to Carbon Dioxide.
(c) Size of Pieces.
(d) Ash Content.
The above considerations as to the effect of coke reactivity, size of pieces and porosity have been based on the behavior of pure carbon, or at least, the pure carbon portion of the coke. However, it is necessary to take into account the ash content of the coke, and there are several ways in which ash can affect the combustibility of the coke. Assuming that the ash in the coke is homogenously distributed thruout the entire coke mass, it will follow that the surface exposed to the action of oxygen (and later of carbon dioxide) is not pure carbon but carbon intermingled with ash.
The Double-zone Theory of Combustion. Since this theory has been mentioned in the above section of this paper, it might be well to enlarge upon it and explain it at this time. In our discussion we have mentioned that the effect of improving any of the factors contributing to the combustibility of coke is evidenced by a reduction in the height and volume of the combustion zone. This in turn results in the attaining of higher temperatures in this reduced zone. Therefore it would appear that with all factors of carbon, air and furnace held constant, the combustibility index of the coke under test could be determined by either a measurement of the volume of the combustion zone or of the temperature reached at the hottest part of that zone. But an alternative method is presented, involving the determination of the composition of the gases leaving the top of the furnace.
Varying Combustibility with Constant Furnace Size. In the above discussion we have assumed that when changing from a coke of high to on of low combustibility, we can increase the height of the furnace to match the increased height of the combustion zone. Now let us consider what takes place when different qualities of coke are used in furnaces of fixed dimensions. Here we show four test furnaces of exactly the same dimensions A B C D. Each of these is filled with coke to the brim and is blown with air of constant temperature and velocity. The first of these is filled with carbon of very high combustibility properties, whether due to high reactivity, small size, high porosity or low ash. Here, because of the high combustibility, all 02 in the air is converted to CO2 and all CO2 converted to CO before the upper limit of the furnace is reached.
Measuring Coke Combustibility. Since it has been shown above that the combustibility of coke is dependent on a number of factors and that some of these factors will in turn influence other factors, it appears that the determination of combustion rate should give us the most valuable index of comparison.
With air and coke size constant, altering the height of the furnace and, since it is kept completely filled with coke, simultaneously changing the height, of the combustion zone until maximum CO is attained in the top gases. This condition can be determined by observing the height of the flame on the top of the coke bed, or by analyzing the exit gases. Then the lesser the height of the combustion zone needed to give maximum CO, the greater the combustion rate of the coke.