Chapter 6

I am the more doubtful about this point, as in the course of our investigations I have found means to produce ammonia at small cost and in great abundance from the immense store of combined nitrogen which we possess in our coal fields.

Among the processes for obtaining ammonia from the nitrogen of the air which we investigated, was one apparently of great simplicity, patented by Messrs. Rickman and Thompson. These gentlemen state that by passing air and steam through a deep coal fire, the nitrogen so passed through is to a certain extent converted into ammonia. In investigating this statement we found that the process described certainly yields a considerable quantity of ammonia, but when we burned the same coal at a moderate temperature by means of steam alone in a tube heated from the outside, we obtained twice as much ammonia as we had done by burning it with a mixture of air and steam, proving in this case, as in all others, the source of the ammonia to have been the nitrogen contained in the coal. The quantity of ammonia obtained was, however, so large that I determined to follow up this experience, and at once commenced experiments on a semi-manufacturing scale to ascertain whether they would lead to practical and economic results.

I came to the conclusion that burning coal by steam alone at a temperature at which the ammonia formed should not be dissociated, although it yielded more ammonia, would not lead to an economic process, because it would require apparatus heated from the outside, of great complication, bulk, and costliness, on account of the immense quantity of raw material to be treated for a small amount of ammonia obtainable.

On the other hand, if the coal could be burned in gas producers by a mixture of air and steam, the plant and working of it would be simple and inexpensive, the gas obtained could be utilized in the same way as ordinary producer gas, and would pay to a large extent for the coal used in the operation, so that although only one-half of the ammonia would be obtained, it seemed probable that the result would be economical.

I consequently constructed gas producers and absorbing plant of various designs and carried on experiments for a number of years. These experiments were superintended by Mr. G. H. Beckett, Dr. Carl Markel, and, during the last four years, by Dr. Adolf Staub, to whose zeal and energy I am much indebted for the success that has been achieved. The object of these experiments was to determine the most favorable conditions for the economic working of the process with respect to both the cost of manufacture as well as the first cost and simplicity of plant. The cost of manufacture depends mainly upon the yield of ammonia, as the expenses remain almost the same whether a large or a small amount of ammonia is obtained; the only other item of importance is the quantity of steam used in the process. We found the yield of ammonia to vary with the temperature at which the producer was working, and to be highest when the producer was worked as cool as was compatible with a good combustion of the fuel. The temperature again depended upon the amount of steam introduced into the producer, and of course decreased the more steam increased. We obtained the best practical results by introducing about two tons of steam for every ton of fuel consumed. We experimented upon numerous kinds of fuel, common slack and burgy of the Lancashire, Staffordshire, and Nottinghamshire districts. We found not much difference in the amount of nitrogen contained in these fuels, which varied between 1.2 and 1.6 per cent., nor did we find much difference in the ammonia obtained from these fuels if worked under similar conditions. Employing the quantity of steam just named we recovered about half the nitrogen in the form of ammonia, yielding on an average 0.8 per cent. of ammonia, equal to 32 kilos, of sulphate per ton of fuel. In order to obtain regular results we found it necessary to work with a great depth of fuel in the producers, so that slight irregularities in the working would not affect results. Open burning kinds of slack do of course work with the greater ease, but there is no difficulty in using a caking fuel, as the low temperature at which the producers work prevents clinkering and diminishes the tendency of such fuels to cake together.

The quantity of steam thus required to obtain a good yield of ammonia is rather considerable, and threatened to become a serious item of expense. Only one-third of this steam is decomposed, in its passage through the producer, and two-thirds remain mixed with the gases which leave the producer. My endeavors were consequently directed toward finding means to recover this steam, and to return it to the producers, and also to utilize the heat of the gases which leave the producers with a temperature of 450° to 500° C., for raising steam for the same purpose. The difficulties in the way of attaining this end and at the same time of recovering, in a simple manner, the small amount of ammonia contained in the immense volume of gas we have to deal with, were very great. We obtain from one ton of coal 160,000 cubic feet of dry gas at 0° C. and atmospheric pressure. The steam mixed with this gas as it leaves the producer adds another 80,000 cubic feet to this, and the large amount of latent heat in this quantity of steam makes the problem still more difficult. The application of cooling arrangements, such as have been successfully applied to blast furnace gases, in which there is no steam present, and which depend upon the cooling through the metallic sides of the apparatus, is here practically out of the question. After trying a number of different kinds of apparatus, I have succeeded in solving the problem in the following way:

The gases issuing from the producers are led through a rectangular chamber partly filled with water, which is thrown up in a fine spray by revolving beaters so as to fill the whole area of the chamber. This water, of course, becomes hot; a certain quantity of it evaporates, the spray produced washes all dust and soot out of the gases, and also condenses the fixed ammonia. The water thus becomes, to a certain degree, saturated with ammonia salts, and a certain portion of it is regularly removed from the chamber and distilled with lime to recover the ammonia.

This chamber is provided with water lutes, through which the tar condensed in it is from time to time removed. From this chamber the gases, which are now cooled down to about 100° C., and are loaded with a large amount of water vapor, are passed through a scrubber filled with perforated bricks, in which the ammonia contained in the gases is absorbed by sulphuric acid. In this scrubber a fairly concentrated solution of sulphate of ammonia containing 36 to 38 per cent. is used, to which a small quantity of sulphuric acid is added, so that the liquid leaving the scrubber contains only 2.5 per cent. of free acid. This is necessary, as a liquid containing more acid would act upon the tarry matter and produce a very dark-colored solution. The liquid running from the scrubber is passed through a separator in which the solution of sulphate of ammonia separates from the tar. The greater portion of the clear liquid is, after adding a fresh quantity of acid to it, pumped back through the scrubber. A certain portion of it is, after treatment with a small quantity of heavy tar oils, which take the tarry matter dissolved in it out, evaporated in conical lead-lined pans furnished with lead steam coils, and which are kept constantly filled by the addition of fresh liquor until the whole mass is thick. This is then run out on a strainer and yields, after draining and washing with a little water, a sulphate of ammonia of very fair quality, which finds a ready sale. The mother liquor, which contains all the free acid, is pumped back to the scrubber.

The gas on entering this scrubber contains only 0.13 volume per cent. of ammonia, and on leaving the scrubber it contains not more than one-tenth of this quantity. Its temperature has been reduced to 80° C., and is fully saturated with moisture, so that practically no condensation of water takes place in the scrubber. The gas is next passed through a second scrubber filled with perforated wood blocks. In this it meets with a current of cold water which condenses the steam, the water being thereby heated to about 78° C. In this scrubber the gas is cooled down to about 40°-50° C., and passes from it to the gas main leading to the various places where it is to be consumed. The hot water obtained in this second scrubber is passed through a vessel suitably constructed for separating the tar which is mixed with it, and is then pumped through a third scrubber, through which, in an opposite direction to the hot water, cold air is passed. This is forced by means of a Roots blower through the scrubber into the producer.

The air gets heated to about 76° C. and saturated with moisture at that temperature by its contact with the hot water, and the water leaves this third scrubber cold enough to be pumped back through the second scrubber. The same quantity of water is thus constantly used for condensing the water vapor in one scrubber and giving it up to the air in the other. In this way we recover and return to the producer fully two-thirds of the steam which has been originally introduced, so that we have to add to the air, which has thus been loaded with moisture, an additional quantity of steam equal to only one-third of the total quantity required before it enters the producer. This additional quantity of steam, which amounts to 0.6 ton of steam for every ton of fuel burnt, we obtain as exhaust steam from the engines driving the blowers and pumps required for working the plant.

The gas producers which I prefer to use are of rectangular shape, so that a number of them can be put into a row. They are six feet wide and 12 feet long inside. The air is introduced and the ashes removed at the two small sides of the producer which taper toward the middle and are closed at the bottom by a water lute of sufficient depth for the pressure under which the air is forced in, equal to about 4 inches of water. The ashes are taken out from underneath the water, the producers having no grate or fire bars at all. The air enters just above the level of the water through a pipe connected with the blower. These small sides of the producer rest upon cast iron plates lined to a certain height with brickwork, and this brickwork is carried by horizontal cast iron plates above the air entrance. In this way a chamber is formed of triangular shape, one side of which is closed by the ashes, and thus the air is distributed over the whole width of the producer.

The gas is taken out in the middle of the top of the producer by an iron pipe, and fuel charged in by hoppers on both sides of this pipe. Between the pipe and the hoppers two hanging arches are put into the producers a certain distance down, and the fuel is kept above the bottom level of these hanging arches. This compels the products of distillation, produced when fresh fuel is charged in, to pass through the incandescent fuel between the two hanging arches, whereby the tarry products are to a considerable extent converted into permanent gas, and the coal dust arising from the charging is kept back in the producer.

The details of construction of this plant will be easily understood by reference to the diagrams before you.

The fuel we use is a common kind of slack, and contains, on an average, 33.5 per cent. of volatile matter, including water, and 11.5 per cent. of ashes, leaving 55 per cent. of non-volatile carbon.

The cinders which we take out of the producer contain, on an average, 33 per cent. of carbon. Of this we recover about one-half by riddling or picking, which we return to the producer. The amount of unburnt carbon lost in the cinders is thus not more than 3 per cent. to 4 per cent. on the weight of fuel used.

The gas we obtain contains, in a dry state, on an average, 15 per cent. of carbonic acid, 10 per cent. of carbonic oxide, 23 per cent. of hydrogen, 3 per cent. of hydrocarbons, and 49 per cent. of nitrogen.

The caloric value of this gas is very nearly equal to 73 per cent. of the caloric value of the fuel used, but in using this gas for heating purposes, such as raising steam or making salt, we utilize the heat it can give very much better than in burning fuel, as we can completely burn it with almost the theoretical quantity of air, so that the products of combustion resulting do not contain more than 1 to 2 per cent. of free oxygen. Consequently the heat escaping into the chimney is very much less than when fuel is burnt direct, and we arrive at evaporating, by means of the gas, 85 per cent. of the water that we would evaporate by burning the fuel direct, in ordinary fireplaces.

We have, however, to use a certain quantity of steam in the producers and in evaporating the sulphate of ammonia liquors, which has to be deducted from the steam that can be raised by the gas in order to get at the quantity of available steam therefrom obtainable. The former amounts, as already stated, to 0.6 ton, the latter to 0.1 ton of steam per ton of fuel burnt, making a total of 0.7 ton. The gas obtained from one ton of fuel evaporates 5.8 tons of water in good steam boilers, working at a rate of evaporation of 50 to 55 tons per 24 hours under 90 lb. pressure. Deducting from this the 0.7 ton necessary for working the plant leaves an available amount of steam raised by the gas from one ton of fuel of 5.1 tons, equal to 75 per cent. of the steam that we can obtain from the same fuel by hand firing.

In addition to the gas, we obtain about 3 per cent. of tar from the fuel. This tar is very thick, and of little commercial value. It contains only 4 per cent. of oils volatile below 200° C., and 38 per cent. of oils of a higher boiling point, consisting mostly of creosote oils very similar to those obtained from blast furnaces; and only small quantities of anthracene and paraffin wax.

I have made no attempts to utilize this tar except as fuel. It evaporates nearly twice as much water as its weight of coal, and we have thus to add its evaporative efficiency to that of the gas given above, leading to a total of about 80 per cent. of the evaporative efficiency of the fuel used in the producers. The loss involved in gasifying the fuel to recover the ammonia therefrom amounts thus to 20 per cent. of the fuel used. This means that, where we have now to burn 100 tons of fuel, we shall have to burn 125 tons in the producers in order to obtain ammonia equal to about half the nitrogen contained therein. Our actual yield of ammonia on a large scale amounting on an average to 32 kilos., equal to 70.6 lb. per ton of fuel, 125 tons of fuel will turn out 4 tons of sulphate of ammonia. We thus consume 6.25 tons of fuel for every ton of sulphate obtained, or nearly the same quantity as is used in producing a ton of caustic soda by the Le Blanc process--a product not more than half the value of ammonium sulphate. At present prices in Northwich this fuel represents a value of 35s. If we add to this the extra cost of labor over and above the cost of burning fuel in ordinary fireplaces, the cost of sulphuric acid, bags, etc., we come to a total of 4l. 10s. to 5l. per ton of sulphate of ammonia, which at the present selling price of this article, say 12l. per ton, leaves, after a liberal allowance for wear and tear of plant, an ample margin of profit. With a rise in the price of fuel, this margin, however, rapidly decreases, and the working of the process will, of course, be much more expensive on a small scale, as will also be the cost of the plant, which under all circumstances is very considerable. The great advantages incidental to this process over and above the profit arising from the manufacture of sulphate of ammonia, viz., the absolute impossibility of producing smoke and the great regularity of the heating resulting from the use of gas, are, therefore, as far as I can see for the present, only available for large consumers of cheap fuel.

We have tried many experiments to produce hydrochloric acid in the producers, with the hope of thereby increasing the yield of ammonia, as it is well known that ammonium chloride vapor, although it consists of a mixture of ammonia gas and hydrochloric acid gas, is not at all dissociated at temperatures at which the dissociation of ammonia alone has already taken place to a considerable extent.

I had also hoped that I might in this way produce the acid necessary to combine with the ammonia at very small cost. For this purpose we moistened the fuel used with concentrated brine, and also with the waste liquors from the ammonia soda manufacture, consisting mainly of chloride of calcium; and we also introduced with the fuel balls made by mixing very concentrated chloride of calcium solution with clay, which allowed us to produce a larger quantity of hydrochloric acid in the producer than by the other methods.

We did in this way succeed in producing hydrochloric acid sometimes less and sometimes more than was necessary to combine with the ammonia, but we did not succeed in producing with regularity the exact amount of acid necessary to neutralize the ammonia. When the ammonia was in excess, we had therefore to use sulphuric acid as before to absorb this excess, and we were never certain that sometimes the hydrochloric acid might not be in excess, which would have necessitated to construct the whole plant so that it could have resisted the action of weak hydrochloric acid--a difficulty which I have not ventured to attack. The yield of ammonia was not in any case increased by the presence of the hydrochloric acid. This explains itself if we consider that there is only a very small amount of ammonia and hydrochloric acid diffused through a very large volume of other gases, so that the very peculiar protective action which the hydrochloric acid does exercise in retarding the dissociation of ammonia in ammonium chloride vapor, where an atom of ammonia is always in contact with an atom of hydrochloric acid, will be diminished almost to zero in such a dilute gas where the atoms of hydrochloric acid and ammonia will only rarely come into immediate contact with each other.

When we burnt coke by a mixture of air and steam in presence of a large excess of hydrochloric acid, the yield of ammonia certainly was thereby considerably increased, but such a large excess cannot be used on an industrial scale. I have therefore for the present to rest satisfied with obtaining only half the nitrogen contained in the fuel in the form of ammonia.

The enormous consumption of fuel in this country--amounting to no less than 150 million tons per annum--would at this rate yield as much as five million tons of sulphate of ammonia a year, so that if only one-tenth of this fuel would be treated by the process, England alone could supply the whole of the nitrogenous compounds, sulphate of ammonia, and nitrate of soda at present consumed by the Old World. As the process is especially profitable for large consumers of fuel situated in districts where fuel is cheap, it seems to me particularly suitable to be adopted in this country. It promises to give England the privilege of supplying the Old World with this all-important fertilizer, and while yielding a fair profit to the invested capital and finding employment for a considerable number of men, to make us, last not least, independent of the New World for our supply of so indispensable a commodity.

Before leaving my subject, I will, if you will allow me, give you in a few words a description of two other inventions which have been the outcome of this research. While looking one day at the beautiful, almost colorless, flame of the producer gas burning under one of our boilers, it occurred to me that a gas so rich in hydrogen might be turned to better use, and that it might be possible to convert it direct into electricity by means of a gas battery.

You all know that Lord Justice Grove showed, now fifty years ago, that two strips of platinum partly immersed in dilute sulphuric acid, one of which is in contact with hydrogen and the other with oxygen, produce electricity. I will not detain you with the many and varied forms of gas batteries which Dr. Carl Langer (to whom I intrusted this investigation) has made and tried during the last four years, in order to arrive at the construction of a gas battery which would give a practical result, but I will call your attention to the battery before me on the table, which is the last result of our extended labors in this direction, and which we hope will mark a great step in advance in the economic production of electricity.

The distinguishing feature of this battery is that the electrolyte is not employed as a mobile liquid, but in a quasi-solid form, and it is, therefore, named dry gas battery. It consists of a number of elements, which are formed of a porous diaphragm of a non-conducting material (in this instance plaster of Paris), which is impregnated with dilute sulphuric acid. Both sides of this diaphragm are covered with very fine platinum leaf perforated with very numerous small holes, and over this a thin film of platinum black. Both these coatings are in contact with frameworks of lead and antimony, insulated one from the other, which conduct the electricity to the poles of the battery.

A number of these elements are placed side by side, with non-conducting frames intervening, so as to form chambers through which the hydrogen gas is passed along one side of the element and air along the other.

This peculiar construction allows us to get a very large amount of duty from a very small amount of platinum. One of the batteries before you, consisting of seven elements, with a total effective surface of half a square meter, contains 2½ grammes of platinum leaf and 7 grammes of platinum black, a total of 9½ grammes of platinum, and produces a current of 2 amperes and 5 volts, or 10 watts, when the outer resistance is properly adjusted. This current is equal to nearly 50 per cent. of the total energy obtainable from the hydrogen absorbed in the battery.

In order to maintain a constant current, we have from time to time (say once an hour) to interchange the gases, so as to counteract the disturbing influence produced by the transport of the sulphuric acid gas from one side of the diaphragm to the other. This operation can easily be performed automatically by a commutator worked by a clock.

The water produced in the battery by the oxidation of the hydrogen is carried off by the inert gas mixed with the hydrogen, and by the air, of which we use a certain excess for this purpose. This is important, as if the platinum black becomes wet, it loses its absorbing power for the gases almost completely and stops the work of the battery. To avoid this was in fact the great difficulty in designing a powerful gas battery, and all previous constructions which employed the electrolyte as a mobile liquid failed in consequence.

The results obtained by our battery are practically the same whether pure oxygen and hydrogen or air and gases containing 25 per cent. of hydrogen are used; but we found that the latter gases must be practically free from carbonic oxide and hydrocarbons, which both interfere very much with the absorbing power of the platinum black.