Chapter 7

Many forms of generator have been used in the United States for the production of water gas, which, after or during manufacture, is mixed with the vapors and permanent gases obtained by cracking various grades of paraffin oil, and "fixing" them by subjecting them to a high temperature; and in considering the subject of enrichment of coal gas by carbureted water gas, I shall be forced, by the limited time at my disposal, to confine myself to the most successful of these processes, or those which are already undergoing trial in this country.

In considering these methods, we find they can be divided into two classes:

1. Continuous processes, in which the heat necessary to bring about the interaction of the carbon and steam is obtained by performing the operation in retorts externally heated in a furnace; and

2. Intermittent processes, in which carbon is first heated to incandescence by an air blast, and then, the air blast being cut off, superheated steam is blown in until the temperature is reduced to a point at which the carbon begins to fail in its action, when the air is again admitted to bring the fuel up to the required temperature, the process consisting of alternate formation of producer gas with rise of temperature, and of water gas with lowering of the temperature.

Of the first class of generator, none, as far as I know, have as yet been practically successful, the nearest approach to this system being the "Meeze," in which fire clay retorts in an ordinary setting are employed. In the center of each retort is a pipe leading nearly to the rear end of the retort, and containing baffle plates. Through this a jet of superheated steam and hydrocarbon vapor is injected, and the mixture passes the length of the inner tube, and then back through the retort itself--which is also fitted with baffle plates--to the front of the retort, whence the fixed gases escape by the stand pipe to the hydraulic main, and the rich gas thus formed is used either to enrich coal gas or is mixed with water gas made in a separate generator. In some forms the water gas is passed with the oil through the retort. In such a process, the complete breaking down of some of the heavy hydrocarbons takes place, and the superheated steam, acting on the carbon so liberated, forms water gas which bears the lower hydrocarbons formed with it; but inasmuch as oil is not an economical source of carbon for the production of water gas, this would probably make the cost of production higher than necessary. This system has been extensively tried, and indeed used to a certain extent, but the results have not been altogether satisfactory, one of the troubles which have had to be contended with being choking of the retorts.

Of the intermittent processes, the one most in use in America is the "Lowe," in which the coke or anthracite is heated to incandescence in a generator lined with firebrick, by an air blast, the heated products of combustion as they leave the generator and enter the superheaters being supplied with more air, which causes the combustion of the carbon monoxide present in the producer gas, and heats up the firebrick "baffles" with which the superheater is filled. When the necessary temperature of fuel and superheater has been reached, the air blasts are cut off, and steam is blown through the generator, forming water gas, which meets the enriching oil at the top of the first superheater, called the 'carbureter,' and carries the vapors with it through the main superheater, where the "fixing" of the hydrocarbons takes place.

The chief advantage of this apparatus is that the enormous superheating space enables a lower temperature to be used for the "fixing." This does away, to a certain extent, with the too great breaking down of the hydrocarbons, and consequent deposition of carbon. This form of apparatus has just found its way to this country, and I describe it as being the one most used in the States, and the type upon which, practically, all water gas plant with superheaters has been founded.

The Springer apparatus, which is under trial by one of the large gas companies, differs from the Lowe merely in construction. In this apparatus the superheater is directly above the generator; and there is only one superheating chamber instead of two. The air blast is admitted at the bottom, and the producer gases heat the superheater in the usual way, and when the required temperature is reached, the steam is blown in at the top of the generator, and is made to pass through the incandescent fuel, the water gas being led from the bottom of the apparatus to the top, where it enters at the summit of the superheater, meets the oil, and passes down with it through the chamber, the finished gas escaping at the middle of the apparatus.

This same idea of making the air blast pass up through the fuel, while in the subsequent operation the steam passes down, is also to be found in the Loomis plant, and is a distinct advantage, as the fuel is at its hottest where the blast has entered, and, in order to keep down the percentage of carbon dioxide, it is important that the fuel through which the water gas last passes should be as hot as possible, to insure its reduction to carbon monoxide.

The Flannery apparatus is again but a slight modification of the Lowe plant, the chief difference being that, as the gas leaves the generator, the oil is fed into it, and, with the gas, passes through a D-shaped retort tube, which is arranged round three sides of the top of the generator; and in this the oil is volatilized, and passes, with the gas, to the bottom of the superheater, in which the vapors are converted into permanent gases.

The Van Steenbergh plant, with which I have been experimenting for some time, stands apart from all other forms of carbureted water gas plant, in that the upper layer of the fuel itself forms the superheater, and that no second part of any kind is needed for the fixation of the hydrocarbons, an arrangement which reduces the apparatus to the simplest form, and leaves no part which can choke or get out of order, an advantage which will not be underrated by any one who has had experience of these plants. While, however, this enormous advantage is gained, there is also the drawback that the apparatus is not fitted for use with crude oils of heavy specific gravity, such as can be dealt with in the big external superheaters of the Lowe class of water gas plant, but the lighter grades of oil must be used in it for carbureting purposes.

I am not sure in my own mind that this, which appears at first a disadvantage, is altogether one, as, in the first place, the lighter grades of oil, if judged by the amount of carbureting power which they have, are cheaper per candle power, added to the gas, than the crude oils, while their use entirely does away with the formation of pitch and carbon in the pipes and purifying apparatus--a factor of the greatest importance to the gas manufacturer.

The fact that light oils give a higher carburation per gallon than heavy crude oil is due to the fact that the latter have to be heated to a higher temperature to convert them into permanent gas, and this causes an over-cracking of the most valuable illuminating constituents; and this trouble cannot be avoided, as, if a lower temperature is employed, easily condensible vapors are the result, which, by their condensation in the pipes, give rise to much trouble.

The simplicity of the apparatus is a factor which causes a great saving of time and expense, as it reduces to a minimum the risk of stoppages for repairs, while the initial cost of the apparatus is, of course, low, and the expense of keeping in order practically _nil_.

When I first made the acquaintance of this form of plant, a few years ago, the promoters were confident that nothing could be used in it but American anthracite, of the kind they had been in the habit of using in America, and a light naphtha of about 0.689 specific gravity, known commercially as 76 deg Baume.

A few weeks' work with the apparatus, however, quickly showed that, with a slightly increased blow, and a rather higher column of fuel, gas coke could be used just as well as anthracite, and that by increasing the column of fuel, a lower grade of oil could be employed; so that during a considerable portion of the experimental work nothing but gas coke from the Horseferry Road Works and a petroleum of a specific gravity of about 0.709 were employed.

Having had control of the apparatus for several months, and, with the aid of a reliable assistant, having checked everything that went in and came out of the generator, I am in a position to state authoritatively that, using ordinary gas coke and a petroleum of specific gravity ranging from 0.689 to 0.709, 1,000 cubic feet of gas, having an illuminating power of twenty-two candles, can be made with an expenditure of 28 to 32 lb. of coke and 21/2 gallons of petroleum. The most important factors, i.e., the quantity of petroleum and the illuminating value of the gas, have also been checked and corroborated by Mr. Heisch and Mr. Leicester Greville.

Total gas made = 8,700 cubic feet.

Time taken: Blowing. 1 hour. Time taken: Making. 50 minutes.

Fuel used: Gas coke. 270 lb. = 31 lb. per 1,000 c.f. Fuel used: Naphtha, sp. gr. 0.709. 34 gals. = 2.7 gals. per 1,000 c.f.

Illuminating power of gas = 21.9 candles.

I must admit that these results far exceeded my expectations, although they only confirmed the figures claimed by the patentee; and there are not wanting indications that, when worked on a large scale and continuously, they might be even still further lowered, as it is impossible to obtain the most economical results when making less than 10,000 cubic feet of the gas, as the proper temperature of the walls of the generator are not obtained until after several makes; and it is only after about 8,000 cubic feet of gas has been made that the best conditions are fulfilled.

It will enable a sounder judgment to be formed of the working of the process if the complete experimental figures for a make of gas be taken.

COMPOSITION OF THE GAS.

Hydrogen. 46.75 Olefines. 7.59 Ethane. 6.82 Methane. 11.27 Carbon monoxide. 11.65 Carbon dioxide. 0.50 Oxygen. 0.17 Nitrogen. 8.25 ------- 100.00

UNPURIFIED GAS CONTAINED

Carbon dioxide. 2.32 per cent. Sulphureted hydrogen. 2.84 " Total sulphur per 100 cu. ft. = 6.67 Ammonia. nil Bisulphide of carbon. nil

Gas produced Naphtha used Gals. Pts. 1st. Make. 3,600 cu. ft. 10 7 2d. " 2,800 " 7 6 3d. " 2,300 " 5 3 ------ --- -- 8,700 24 0

The last portion of the table shows the economy which arises as the whole apparatus gets properly heated. Thus the first make used 3 gallons naphtha per 1,000 cubic feet, the second 2 gallons 6 pints per 1,000 cubic feet, and the third 2 gallons 4 pints per 1,000 cubic feet, and it is, therefore, not unreasonable to suppose that in a continuous make these figures could be kept up, if not actually reduced still lower.

In introducing the oil it is not injected, but is simply allowed to flow in by gravity, at a point about half way up the column of fuel, the taps for its admission being placed at intervals around the circumference of the generator, and oil at first begins to flow down the inside wall of the generator, but being vaporized by the heat, the vapor is borne up by the rush of steam and water gas, and is cracked to a permanent gas in the upper layer of fuel. This I think is the secret of not being able to use heavier grades of oil, these being sufficiently non-volatile to trickle down the side into the fire box at the bottom, and so to escape volatilization. I have tried to steam-inject the oil, but have not found that it yields any better results.

One of the first things that strikes any one on seeing a make of gas by this system is the enormous rapidity of generation. Mr. Leicester Greville, who is chemist to the Commercial Gas Company, in reporting on the process, says, "The make of gas was at the rate of about 86,000 cubic feet in 24 hours. A remarkable result, taking into consideration the size of the apparatus." It is quite possible, with the small apparatus, to make 100,000 cubic feet in 24 hours; indeed the run for which the figures are given are over this estimate; and it must be borne in mind that this rapidity of make gives the gas manager complete control over any such sudden strains as result from fog or other unexpected demands on the gas-producing power of his works; while a still more important point is that it does away with the necessity of keeping an enormous bulk of gas ready to meet any such emergency, and so renders unnecessary the enormous gasholders, which add so much to the expense of a works, and take up so much room.

Perhaps the greatest objection to water gas in the public mind is the dread of its poisonous properties, due to the carbon monoxide which it contains; but if we come to consider the evidence before us on the increase of accidents due to this cause, we are struck by the poor case which the opponents of water gas are able to make out. No one can for a moment doubt the fact that carbon monoxide is one of the deadliest of poisons. It acts by diffusing through the air cells of the lungs, and forming, with the coloring matter of the blood corpuscles, a definite compound, which prevents them carrying on their normal function of taking up oxygen and distributing it throughout the body, to carry on that marvelous process of slow combustion which not only gives warmth to the body, but also removes the waste tissue used up by every action, be it voluntary or involuntary, and by hindering this, it at once stops life.

All researches on this subject point to the fact that something under one per cent. only of carbon monoxide in air renders it fatal to animal life, and this at first seems an insuperable objection to the use of water gas, and has, indeed, influenced the authorities in several towns, notably Paris, to forbid its introduction for domestic consumption. Let us, however, carefully examine the subject, and see, by the aid of actual figures, what the risk amounts to compared with the risks of ordinary coal gas.

Many experiments have been made with the view of determining the percentage of carbon monoxide in air which is fatal to human or, rather, animal life, and the most reliable as well as the latest results are those obtained by Dr. Stevenson, of Guy's Hospital, in consequence of the two deaths which took place at the Leeds forge from inhaling uncarbureted water gas containing 40 per cent. of carbon monoxide. He found that one per cent. visibly affected a mouse in one and a half minutes, and in one hour and three quarters killed it, while one-tenth of a per cent. was highly injurious. Let us, for the sake of argument, take this last figure 0.1 per cent. as being a fatal quantity, so as to be well within the mark.

In ordinary carbureted water gas as supplied by the superheater processes, such as the Lowe, Springer, etc., the usual percentage of carbon monoxide is 26 per cent., but in the Van Steenbergh gas--for certain chemical reasons to be discussed later on--it is generally about 18 per cent., and rarely rises to 20 per cent. An ordinary bedroom will be say 12 ft. X 15 ft. X 10 ft., and will therefore contain 1,800 cubic feet of air, and such a room would be lighted by a single bats-wing burner consuming not more than four cubic feet of gas per hour. Suppose now the inmate of that room retires to bed in such a condition of mental aberration that he prefers to blow out the gas rather than take the ordinary course of turning it off--a process, by the way, of putting out gas which is decidedly easier in theory than in practice, especially in his presumed mental condition--you would have in one hour the 1,800 cubic feet of gas in the room mixed with four fifths of a cubic foot of carbon monoxide--the carbureted water gas being supposed to contain 20 per cent.--or 0.04 per cent. In such a room, however, if the doors and windows were absolutely air tight, and there was no fireplace, diffusion through the walls would change the entire air once an hour, so that the percentage would not rise above 0.04; while in any ordinary room imperfect workmanship and an open chimney would change it four times in the hour, reducing the percentage to 0.01, a quantity which the most inveterate enemy of water gas could not claim would do more than produce a bad headache, an ailment quite as likely to have been caused by the same factor that brought about the blowing out of the gas.

Moreover, we are now talking about the use of carbureted water gas as an enricher of coal gas, and not as an illuminant to be consumed _per se._ and we may calculate that it would be probably used to enrich a 16-candle coal gas up to 17.5 candle power. To do this 25 per cent. of 22 candle power carbureted water gas would have to be mixed with it, and taking the percentage of carbon monoxide in London gas at 5 per cent.--a very fair average figure--and 18 per cent. as the amount present in the Van Steenbergh gas, we have 8.25 per cent. of carbon monoxide in the gas as sent out--a percentage hardly exceeding that which is found in the rich cannel gas supplied to such towns as Glasgow, where I am not aware of an unusual number of deaths occurring from carbon monoxide poisoning.

The carbureted water gas has a smell every bit as strong as coal gas, and a leak would be detected with equal facility by the nose; and I think you will agree with me that the cry raised against the use of carbureted water gas, for this reason, is one of the same character that hampered the introduction of coal gas itself at the commencement of this century.

We must now turn to the chemical actions which are taking place in the generator of the water gas plant, and these are more complex in the case of the Van Steenbergh plant than in those of the Lowe type, and, for that reason, yield a gas of more satisfactory composition.

Taking gas as made by the Lowe or Springer process, and contrasting it with the Van Steenbergh gas, we are at once struck by several marked differences.

In the first place the hydrogen is far higher and the marsh gas or methane lower in the Van Steenbergh than in the Lowe process, this being due to the sharper cracking that takes place in the short column of cherry red coke, as compared with the lower temperature employed for a longer space of time in the Lowe superheater. Next we notice a difference of 10 per cent. in the carbon monoxide, which is greatly reduced in the Steenbergh generator by the carbon monoxide and marsh gas reacting on each other as they pass over the red hot surface of coke with formation of acetylene, which adds to the illuminants, this action also reducing the quantity of marsh gas present.

Lowe Van Steenbergh gas. gas.

Hydrogen..................... 27.14 46.75 Marsh gas.................... 25.35 11.27 Carbon monoxide.............. 26.84 18.65 Illuminants.................. 14.63 7.59 Ethane....................... ---- 6.82 Carbon dioxide............... 3.02 0.50 Oxygen....................... 0.15 0.17 Nitrogen..................... 2.87 8.25 ------ ------ 100.00 100.00

In the illuminants, if we add the higher members of the methane series present to the olefines, we see they are about equal in each gas, while the low percentage of nitrogen in the Lowe gas is due to more careful working, and could easily be attained with the Van Steenbergh plant by allowing the first portion of water gas to wash out the producer gas before the hopper on top is closed.

The cracking of the naphtha by the red hot coke is undoubtedly a great advantage, for, as I have pointed out, the cracking of rushing petroleum is an exothermic reaction, so that the coke at the top of the generator gets hotter and hotter, and it is no unusual thing to see the coke at the beginning of the make cherry red at the bottom and dull red at the top, while at the end of the make it is almost black at the bottom and cherry red at the top, in this way attaining the same advantage in working that the Springer and Loomis do by their down blast, that is, having the fuel at its hottest where the gas finally leaves it, so as to reduce the quantity of carbon dioxide, and so lessen the expense of purification.

It will be well now to turn for a few moments to the gas obtained by cracking the light petroleum oils by themselves. The Russian and American petroleum differ so widely in composition that it was necessary to see in what way the gases obtained from them differed; and to do this, equal quantities of American naphtha and a Russian naphtha were cracked, by passing through an iron tube filled with coke, and in each case heated to a cherry red heat, the gases being measured, and then analyzed, with the following results:

American. Russian. No. of cubic feet per gallon... 72 104 ---- ---- Hydrogen....................... 26.0 45.3 Methane........................ 41.6 22.3 Ethane......................... 12.5 13.9 Olefines....................... 14.1 11.6 Carbon monoxide................ 3.3 3.5 Carbon dioxide................. 1.7 2.3 Oxygen......................... 0.8 1.1 Nitrogen....................... Nil. Nil. ----- ----- 100.0 100.0

Showing that, if the Russian oil is a little lower in illuminants, it quite makes up by extra volume, but it seemed to me to deposit a much larger proportion of carbon.

Taking 21/2 gallons of American naphtha, it would give roughly 180 cubic feet of gas of the above composition, while the remaining gas would be the ordinary water gas. Taking the analysis of this as given, and calculating from it what would be the composition of a mixture of it with the naphtha gas, we obtain:

Calculated. Actual. Hydrogen...................... 47.09 42.09 Methane....................... 5.48 11.27 Olefines...................... 2.53 7.59 Ethane........................ 2.17 6.32 Carbon monoxide............... 30.07 18.65 Carbon dioxide................ 3.78 2.32 Oxygen........................ 0.56 0.17 Nitrogen...................... 7.17 8.25 Sulphureted hydrogen.......... 1.15 2.84 ------ ------ 100.00 100.00

Showing how great the effect is of the diluents in the water gas in preventing the overcracking of the hydrocarbons, as shown by the increase in the percentage of them present in the finished gas; while the enormous reduction in the amount of carbon monoxide present is due to the interaction between it and the paraffin hydrocarbons in the presence of red-hot carbon, a point which makes the Van Steenbergh apparatus enormously superior to any of the superheater forms of plant.