Engineers and chemists are now closely co-operating to solve smoke and waste gas problems arising from the growth of power supply on a large scale. The main object is to lessen the damage caused by the emission of grit, dust, tar and harmful acids
DESIGNED TO BURN ANTHRACITE—the Tir John power station at Swansea, South Wales. Anthracite, which consists almost entirely of carbon, burns with a smokeless flame. Designers of anthracite-burning power stations do not, therefore, have to allow for the treatment of waste gases.
AN enormous amount of damage is done by smoke and atmospheric pollution. It has been estimated by one competent authority that in Great Britain the damage may be as much as £500,000,000 in a year.
With the growth of power supply on a large scale, and the building of giant power stations throughout the country, atmospheric pollution has become a problem of the first magnitude. It is not surprising, therefore, that the problem has received much study from engineers and chemists. The various means adopted for its ultimate solution are fine examples of collaboration between these two professions.
The huge boilers used in modern power stations emit flue gases which contain oxides of sulphur and of nitrogen. In addition, hydrochloric acid, which eats away stonework and corrodes metals rapidly, is given off in large quantities. For example, the combustion of 1,000 tons of coal will not only cause the emission of grit, dust and tar, but will also cause the formation of enormous quantities of these harmful acids — forty-five tons of sulphuric acid from three to seven tons of nitric acid and half a ton of hydrochloric acid. All these substances are harmful, and will be carried by the grit and tar and deposited on buildings, unless preventive means are adopted before the gases leave the chimneys.
When Parliamentary sanction was sought for the building of Battersea Power Station, it was granted only on condition that the best possible means were adopted for removing the sulphurous compounds and grit from the flue gases before these were emitted to the atmosphere. This condition was prompted by the extensive damage caused by atmospheric pollution to the stonework of the Houses of Parliament and of many other buildings in London.
In the past attention was confined mainly to the elimination of grit and dust from the discharged gases, but technical experts are now fully alive to the fact that it. is the acid gases which do the greatest damage. This side of the problem, in itself one of great difficulty, has been solved only after some years of careful research and experiment. In spite of these difficulties, an official embargo upon “acid” emissions from new power stations was enforced with the companies operating in London, even before a satisfactory solution of the problem had been achieved.
It is only within recent years that acid pollution has been closely studied. In large towns stonework, masonry, metal, paintwork, and fabrics all suffer from this scourge, which silently destroys beauty and substitutes grime.
The effects of smoke pollution on health are of considerable importance. It is disturbing to reflect that even in moderately clear winter weather the air in London contains between 4,000 and 5,000 particles of soot per cubic centimetre. In time of fog, when the smoke becomes more concentrated and stabilized by the acid contamination in the atmosphere, this figure may increase to more than 100,000 particles in the same volume of air.
Again, it has been calculated that an adult human being consumes each day, on an average, 30 lb. of air, 2.7 lb. of food and 4.5 lb. of water. It is remarkable that so much thought is directed towards the improvement of food quality, whereas the cleansing of the atmosphere has only recently received the attention it deserves.
There are also many indirect effects of polluted air on health, due to the lessening of sunlight and the loss of daylight combined with the lack of sufficient ultra-violet radiation. In the great industrial cities and towns it is unfortunate that the hours during which the atmosphere is comparatively clean lie between 1 a.m. and 6 a.m.
The greatest pollution occurs when it is liable to do the most harm, and it has been definitely proved that the suspended matter is responsible for bronchial and other troubles. It is also well known that harmful bacilli are carried by this matter from infected persons, thus helping to spread disease. Pulmonary tuberculosis may be transmitted far and wide by individuate inhaling dust containing the deadly tubercle bacilli. It is almost impossible to arrive at an accurate estimate of the cost of damage from smoke pollution, but it has been calculated that in London the cost between 1921 and 1936 amounted to about twenty-four shillings a year per head of population. The unfortunate thing is that the smoke producers do not suffer appreciably from the nuisance which they create. It is the general public which has to bear the burden and foot the bill. This bill includes such items as painting, cleaning, artificial lighting, repairs to buildings and remedying the corrosion of metals.
Apart from the material damage caused, the acid pollution also has a. pernicious effect in the production of fog, which is in its turn so damaging to the health of the people. The acid contamination causes the fog to become “stabilized”, because the atmosphere is not sufficiently scavenged.
The large power stations which have been recently built have shown that we must expect the stations of the future to burn more than 1,000 tons of coal a day. It has been estimated that domestic, or household coal consumption accounts for as much as 23 per cent of the total coal consumption in Great. Britain, and power station consumption for only 6 per cent. A domestic consumption of 1,000 tons a day is equivalent to that of 70,000 houses each consuming some two hundredweight of coal a week.
When this domestic pollution is spread over a wide area, the concentration of the acids is small. With a large power station, however, the acid concentration is much higher, as the smoke is emitted from a small source. It has been found that this affects atmospheric conditions for some considerable distance on the lee side of the chimney, irrespective of its height.
The modern power plant is so designed that it is possible to burn cheap coal in the boilers and yet to -obtain a high efficiency. This means, however, that the coal generally contains a relatively large amount of sulphur, about 3 per cent. To remove all this sulphur from the coal before it reaches the boiler is not practicable, so that the necessity for efficient cleaning of the flue gases is paramount.
LIQUOR PUMPS for liquid sulphur dioxide at a copper smelting plant at Imatra, Finland. Sulhur dioxide is a serious cause of atmospheric pollution, and this plant extracts sulphur dioxide from the waste gases of the smelter before converting it into liquid sulphur dioxide.
Some people might argue that power stations should be built well away from large towns and cities; thus the proprietors would obtain cheap supplies of fuel and avoid the necessity for providing special plant for flue-gas cleaning. It is, however, far more economical to locate the power station alongside the load than to face the extra cost of transmitting current over long distances.
Another important consideration which must be borne in mind is the provision of a large volume of cooling water for condensing the steam leaving the turbines. River water is ideal for this purpose.
General atmospheric pollution has been carefully studied for a number of years by the Department of Scientific and Industrial Research, which has published a vast amount of information on the subject. By means of special gauges, scientists of the Department have collected samples of the soluble and the insoluble matter floating in the atmosphere. Their work has revealed a deplorable state of affairs, but it is not yet possible to differentiate between the domestic and industrial pollution. Sufficient has been learnt, however, to show that it is possible and desirable to enlist all the forces of science in fighting pollution from whatever source it may originate.
The acid contamination is measured by the chemist with a special standard known as the “pH value”, which is a measure of small concentrations of acidity or alkalinity. For example, pure water has a pH value of between 5.5 and 6.0; tap water of 7.5 and a solution of chalk in pure water of 8.5. A neutral solution, neither acid nor alkaline, has a pH of between 6.5 and 7.0. A solution of 1/20 per cent of sulphuric acid in water has a pH of 2 and a solution of 0.04 per cent of caustic soda has a pH of 12.0. The pH value serves as a measuring rod for gauging the acidity of rainwater collected in receivers, and thus obtaining a direct measurement of acid contamination.
The pH value of collected rainwater in Burnley (Lancs), for example, has been found to lie between 3 and 4 in the course of a normal year. In London, on the other hand, measurements of the pH value have shown that it lies between 4 in January and 8 in July. A pH value below 5.5 will attack steel and stonework, so that the atmosphere of Burnley is much more corrosive than that of London. This is only to be expected, for London is not as highly industrialized as Burnley. For the greater part of the year the atmosphere of London is not extremely acidic.
Before this question of acid pollution had received the close attention which is directed towards it to-day, manufacturers of flue-gas cleaning plant concentrated their efforts on removing the grit and dust only. The removal of sulphur compounds was looked upon as desirable, but not absolutely essential. It was in 1933 that Imperial Chemical Industries built a special pilot plant at their Billingham works (see the chapter “Petrol from Coal”) for the purpose of studying a system of lime washing which they had designed for this purpose. The plant was so designed that it could deal with the gas from one large boiler, so that the results obtained could be applied to ordinary commercial practice. The gases are constrained to pass upwards through a scrubber which is fitted with timber baffles, over which there flows a continuous stream of lime water. This lime has the effect of neutralizing the acids in the gas and forming sulphurous compounds, which can be easily removed from the plant by a special filtering arrangement.
PLANT FOR DRYING SULPHUR DIOXIDE before it is liquified. Smelters which produce metals from sulphide ores liberate large volumes of sulphur dioxide in the process. Until recently the normal method of disposing of the waste gas was to discharge it into the atmosphere.
The sludge collected in a conical settler was removed from the system and passed through a special filter, which removed the solid sulphurous compounds and returned the water to the system. In a plant of this type an important point is to control the pH value to a close degree of accuracy. A remarkable instrument has been designed for this purpose and it was thoroughly tested in the pilot plant at Billingham, where it was used to provide the means for controlling the amount of lime that had to be added to the plant.
This amazingly simple and efficient instrument measures the electrical conductivity of the lime solution. The outstanding feature of this particular type of instrument is that it obtains a continuous record of the pH value, and marks it on a chart. The instrument consists of two electrodes or conductors which are immersed in the liquid of which the pH value is required. The recording apparatus can be placed at any convenient distance from the electrodes, and it can also be arranged to control the pH value to within certain small limits. Any deviation from this set value causes a switch to come into operation and to increase or reduce the supply of lime to the flue gas washing system. It is by this means that the whole system is accurately and simply controlled.
After the tests had been carried out on the pilot plant at Billingham, two large flue-gas washing plants, operating on the I.C.I. system, were installed at the large power stations at Swansea (South Wales) and Fulham (London).
Wealth from Waste
A remarkable new process has recently been perfected by Imperial Chemical Industries, Ltd., and the Swedish concern of Bolidens Gruvaktie-bolag, for the conversion of sulphurous products into that valuable commodity, sulphur. By this means the chemical engineer is extracting wealth from waste, and it is by the collaboration of these two firms that Sulphur Patents, Ltd., has become the sole exploiting agent for the patented I.C.I. and Bolidens process.
In the process of smelting ores to obtain metals there are liberated large volumes of sulphur dioxide. In the past the normal method of disposing of this gas was to discharge it from a tall stack.
Apart from the fact that this was a wasteful method of getting rid of a valuable raw material, the smelting companies concerned were involved in considerable financial loss because of the necessity for paying compensation on account of atmospheric pollution. Considerable damage was done to human beings, crops and forests by this pollution.
Another method of disposing of this gas has been to make sulphuric acid from it. Here again there are various difficulties. Many smelters are situated in remote country districts where the consumption of sulphuric acid is small and the freight charges to the consuming centres are so high as to be almost prohibitive.
The problem which these two firms therefore set out to solve concerned the elimination of atmospheric pollution, and the production of sulphur in a marketable form.
FILLING A TANK WAGON with liquid sulphur dioxide at Imatra, Finland. The plant installed at the large copper smelter at Imatra recovers 50 tons of liquid sulphur dioxide a day from the smelter gases. The gas that would otherwise cause serious pollution of the atmosphere is converted into sulphur at a competitive price.
For a considerable number of years this important subject has been intensively studied by many patient research workers, until at last victory has been achieved. The difficulties at times seemed almost unsurmountable, particularly because the sulphur had to be produced at a price which would compete with that of the cheap native product of Texas, Sicily and other places.
In some localities there is an outlet for the sulphur dioxide, which is used for the manufacture of sulphite, or paper pulp. In general, however, the sulphur dioxide is reduced to sulphur and the most suitable reducing agent has been found to be coke. The Bolidens process is one in which the reduction of the gas to sulphur is done directly and it operates with remarkable efficiency.
The Swedish company uses this process in its smelter at Ronnskar, where between 20,000 tons and 25,000 tons of high quality sulphur are produced annually.
During the spring of 1936 the first large plant using the I.C.I. process was put into operation in a copper smelter at Imatra, Finland.
This huge plant produces 52 tons of sulphur dioxide a day. The process of reducing sulphur dioxide to sulphur is accomplished in three stages. First a reducing gas is manufactured and used for reducing the sulphur with what is known as a catalyst, the process being analogous to that described on page 727 of this work.
After this catalytic action, the gases are cooled and the sulphur is extracted by special electrical methods. The liquid sulphur then runs from the cooler and the electrical plant into steam-heated containers, from which it is poured into small trucks. It undergoes a further process of purification in large tanks, after which it is pumped into huge wooden moulds, where it solidifies. These moulds contain several thousand tons of sulphur, which is excavated by mechanical shovels and loaded into railway wagons.
The whole process is extremely simple, and is yet another indication of how science can turn what was formerly a nuisance into a valuable commercial product.
Extraction of Sulphur
An outstanding feature of the Bolidens plant is the use made of accurate automatic instruments for the control of gas flow and of the composition of the finished product. All the instruments and controls are grouped in one room, and the whole plant is under the control of one man. It is remarkable that this man has not received any special chemical training, but he is able to make the necessary changes according to the readings of the various instruments. A complete record is kept of these readings, so that the plant manager will have no difficulty in following the progress of any change. For several years an experimental plant has been in operation at Billingham, and now gives a continuous production of twenty tons of sulphur dioxide a day. Part of this gas is treated in a plant having a daily output of between five and six tons of sulphur, which is of a high quality.
It is possible that this process may yet be applied to the extraction of sulphur from flue gases of power stations burning coals of high sulphur content. It is estimated that, if only a quarter of the sulphur from coal burning were saved, a quantity exceeding the world’s output of native sulphur would become available.
Sulphur is a valuable raw material and is used for many different purposes. It can well be imagined what an enormous advantage it would be to a country to produce sulphur from waste flue gases. It would be extremely valuable for Great Britain, which has no sulphur in its native form. It would be yet another means of making the fullest possible use of the country’s vast coal resources. As time goes on there is no doubt that processes of this nature will exert considerable influence in attaining the ideal of a smokeless uncontaminated atmosphere in cities and towns.
STORAGE VESSELS and liquor cooler at a sulphur dioxide recovery plant. Acia gases are the most serious cause of atmospheric pollution, and their elimination is more important than the removal of grit and dust from flue gases. Stonework, metal, paint and fabrics are all damaged by acid emissions from smelting or coal-burning plants.