An important subject for investigation, which has not yet been satisfactorily determined, is the temperature at which it is most beneficial to distill coals of various qualities. The practice of allowing the charge to remain in the retort for some time after most of the gas has been driven off, to enable (it is said) the retort to recover heat for the next charge, often leads to misconception as to the true temperature of carbonization. The effect of this is to equalize the temperatures inside and outside the retort. This inside temperature is not maintained, the temperature outside not being high enough to transmit the heat with sufficient rapidity; and so, in an apparently hot retort, the coal may be carbonized at a comparatively low temperature. A truer test of temperature is that of the outside of the retort, which should be not less than 400° to 500° Fahr. above the temperature necessary for proper carbonization. In all experiments relating to temperature pretending to any degree of accuracy, a pyrometer of some kind should be used. Judging of the temperature by the color is often misleading. Not only may the eye be deceived, but different clays do not present the same appearance at the same temperature.
A good, reliable pyrometer to estimate temperatures to (say) 2500° Fahr. is much wanted.
Experience during the last few years with the high temperatures obtained by the use of regenerative furnaces has led me to the conclusion that higher heats than are usual may be employed with advantage, as regards both the quantity and the quality of gas, provided the retorts are heated uniformly throughout their length, and the weight and duration of the charge are so adjusted that the coal does not remain longer in the retort than is just sufficient to drive off the gas; and that the more rapidly the coal is carbonized, the better are the results. In two retorts of the same size, one making 5,000 and the other 10,000 cubic feet per day, the gas will be twice as long in contact with the surface of the retort in the former as in the latter - to the probable detriment of its quality, and increased tendency to stoppage in the ascension pipes.
A subject closely allied to that just alluded to is the temperature of the gas as it leaves the retort. Until within the last few years, it was generally assumed that this was not higher than from 200° to 300° Fahr.; and a very plausible theory was given to account for such a comparatively low temperature. A discussion which took place a few years ago in the Journal of Gas Lighting showed that at that time opinions on this subject were not unanimous. But the conclusion arrived at seemed to be that the gas was not higher in temperature than that before stated; and if higher temperatures were observed, they were due to the tarry matter in the gas, and were not those of the gas itself. A little reflection is sufficient to show that the existence of gas intimately mixed with tarry matter at a high temperature, without being itself raised to that temperature, is a physical impossibility.
In a paper read to a Continental gas association about a year ago, the writer stated, as the result of many experiments, that unless the temperature in the ascension pipe rises above 480° Fahr., thickening of the tar in the hydraulic main and choking of the ascension pipe will certainly occur. This led me to make a series of experiments, extending over many months, on the temperature of the gas in the ascension pipes at different points and at various times during the charge. The results of these experiments may be of some interest, and may lead to further investigation. The temperatures were taken by mercurial thermometers registering 600° Fahr., except those near the mouthpiece, which were taken by a Siemens water pyrometer. Every care was exercised to insure accuracy; and the instruments were carefully adjusted. At a distance of 18 inches from the mouthpiece, the temperatures varied from an average of 890°, shortly after the retort was charged, to 518° at the end of the charge; at 12 feet distant from the mouthpiece, the corresponding temperature was 444°, falling to 167° at the end of the charge; and at 22 feet, the average temperature varied from 246° at the commencement to 144° at the end of the charge. These are the averages of a number of experiments.
In some instances they were considerably above these averages - temperatures over 900° being frequently obtained. This is about the temperature of a low red heat, and is much higher than any I have seen recorded. When the gas was allowed to issue from a hole in the ascension pipe, 1¼ inches in diameter, 18 inches above the mouthpiece, a strip of lead held about an inch from the orifice was freely melted.
In the settings on which these experiments were made, the middle ascension pipe takes the gas from the two central retorts; and it is of interest to note that in this pipe the temperature of the gas 18 inches from the upper retort was found to be 1014° Fahr., and at the point where it entered the hydraulic main it was 440° Fahr. Zinc was freely melted by the gas issuing from a hole 18 inches from the mouthpiece. The temperatures always fall toward the end of the charge; the fall of temperature in the ascension pipe being a good indication that the charge is worked off. They increase with the heat of the retort and with the weight of the charge.
Experiments were also made to ascertain the temperature of the gas in the retort; and for this purpose one of Murrie's pyrometers was used, the action of which depends on the pressure produced by the vaporization of mercury in a malleable iron tube. The end of this tube was first rested on the top of the coal, but not in contact with the retort. It reached about 18 inches into the retort, and therefore was not in the hottest part. In this position the temperature indicated shortly after charging the retort was 1110° Fahr., gradually rising to 1640° Fahr. The end of the tube was then embedded in the coal, when the pyrometer indicated a temperature of 1260° Fahr. within 30 minutes after the retort was charged; gradually rising toward the end of the charge as before. At the time these temperatures were taken, the retorts were each producing 10,000 cubic feet of gas per day. I had no opportunity of testing the accuracy of the statement that, with lower temperatures, there is a tendency to stoppage of the ascension pipes; but with these high temperatures (contrary to what might be expected) there is no trouble from stoppages.
These experiments, so far as they have gone, lead to the conclusion that the temperature of the gas as it is evolved from the coal is not less than 1200° Fahr., and that cooling commences immediately on the gas leaving the retort. The temperatures being far above that of liquefaction, the gases are cooled very rapidly. The temperature of the gas in the ascension pipe depends on the rapidity with which the gas is evolved - that is to say, the greater the quantity produced in a given time, the less effective is the cooling action of the mouthpiece and the ascension pipe; and although I had no opportunity of testing it, I should expect to find that with retorts making from 5,000 to 6,000 cubic feet of gas per day, the maximum temperature in the ascension pipe 18 inches from the mouthpiece will not exceed 400° to 500° Fahr., while with lower heats and lighter charges the temperatures will be still lower. That these temperatures have some effect in causing or preventing stoppage in the ascension pipes there can be no doubt; and it is important that this subject should be thoroughly investigated.
It is of interest to consider what must be the physical condition of the gas at these high temperatures. All the hydrocarbons which are afterward condensed must then be in the condition of gases having various degrees of condensability, mixed with and rendered visible by a cloud of carbon particles or soot. If this soot could be removed from the gas at this stage without reducing the temperature, we should probably have no thick tar or pitch, but only comparatively light-colored oils; and it might possibly lead to an entirely different mode of conducting the process of condensation.
These are a few of the subjects on which it is extremely desirable that we should possess that complete information which can only be obtained by well-directed investigations with different materials and under varying conditions. There are many others in connection with carbonization and purification which might be mentioned; but I think I have said sufficient to show the necessity that exists for more minute investigation and research. Investigations such as are here indicated do not involve any large expenditure of money; but they do require care and intelligence to prevent errors being made. Experiments should not be condemned as defective because the results differ from old-established theories; yet when this does happen, it is in all cases better to suspect the new experiment rather than the old theory, until the results have been fully established. - Wm. Foulis, Journal of Gas Lighting.