Smoke and Smoke Prevention

Since 192o the total home consumption of coal in Great Britain has averaged roughly 175 million tons annually, of which about 40 million tons has gone to meet domestic requirements, most of the remainder being burned in industrial furnaces, mainly for the production of steam. Coals of relatively low volatile content are largely absorbed for steam raising purposes and when due atten tion is paid to furnace design, stoking procedure and draught con trol, they can be burned without much smoke and with remark ably high efficiencies of steam generation. On the other hand, in certain metallurgical furnaces, etc., it is claimed that a smoky atmosphere is essential.

Most of the British domestic coal supplies are drawn from highly bituminous seams, and since they are burned in a very haphazard and careless manner, with little possibility of proper air regulation, it is not surprising to learn that the domestic chimney is on the whole responsible for far more than its proportionate share of smoke and soot. In London, domestic smoke forms more than two-thirds of the total, but in centres of iron, steel or pottery manufacture, or in the immediate vicinity of large works, industrial smoke may obviously preponderate. Apart from con siderations of its quantity, however, domestic smoke is in char acter dissimilar to, and far more objectionable than, factory smoke, in so far as it contains relatively high proportions of tar, while boiler smoke consists largely of ash and dust. This point is amply illustrated by the following analyses, due to Professor Cohen, of soot from similar coal burned respectively in a sitting room grate and a boiler furnace.

Bituminous coal normally contains 1% to 2% of sulphur. Most of this is evolved in the gaseous form ; but some is found in the soot, mainly in the combined state as ammonium sulphate; and some is left behind in the residue. The sticky nature of tarry soot causes it to adhere tenaciously to objects with which it comes into contact, and since it usually contains free sulphur acids in addition to ammonium sulphate, it has a destructive action upon stone, fabrics, metals and vegetation, apart from the widespread dirt and discolouration which it causes.

A number of investigators have made direct determinations of the actual amounts of soot emitted from burning coal, usually by aspirating a measured volume of the flue gases through suitable filters. Roberts-Austen, in analyses carried out in connection with the 1884 London Smoke Abatement Exhibition, obtained from domestic grates soot equivalent to 6% by weight of the coal burned, a value in agreement with later measurements by Cohen and Hefford. Sinnatt, however, in experiments undertaken for the Manchester Corporation, found a corresponding soot loss of only 2%, but unburned gaseous products represented thermal losses of as much as 15-2o%. Scheiner-Kestner's boiler furnace

soot determinations gave values averaging under i%, but prob ably in small inefficient plant, and certainly in other types of industrial furnaces, this amount is often exceeded. Further, it is not always feasible, even in efficient boiler practice, to avoid altogether an escape of unburned gaseous products. For while it is desirable in the interests of economy to aim at complete com bustion, increase of excess air to this end is justifiable only so long as the additional heat generated is sufficient to offset the increased losses in sensible heat.

Investigation of Atmospheric Pollution.

In 1912, as the outcome of a conference held in London under the auspices of the Coal Smoke Abatement Society, a voluntary committee was ap pointed for the investigation of atmospheric pollution. Initiated as a private body, this later became an advisory committee of the Meteorologicai Office, but in 1927 responsibility for the direction of subsequent work was transferred to the Department of Scientific and Industrial Research. Annual reports of the investigations undertaken and the results obtained have been published by H. M. Stationery Office. The chief aim throughout has been the compilation and collation of systematic information respecting the nature and amount of atmospheric pollution in various localities, with a view to obtaining both seasonal varia tions and variations from place to place; but investigations of a more academic nature have also been undertaken. Standard methods of measurement have been adopted for the statistical work, which has been carried out in co-operation with various local authorities and institutions.

Town air always contains large numbers of suspended solid particles, which of course are not derived exclusively from smoke, but include also dust, vegetable matter, etc. These tend gradually to settle down under the force of gravity. The impurities so deposited can be caught in a suitable receptacle and analysed, and if collected under similar conditions in different places afford material for interesting comparisons. The organisation of an investigation on these lines has formed the chief work of the above-mentioned committee, and in 1927 seventy-nine deposit gauges, maintained by thirty-four authorities, were in operation. These return monthly records not only of the total weight of deposited matter, but also of its insoluble and soluble constituents separately. The insoluble fraction is further divided into com bustible and ash, and is analysed for sulphates, chlorine and am monia. The lowest deposits, measured in the smaller towns or in suburban districts, amount annually to some 75-100 tons per square mile; the highest deposits, measured in the heart of large industrial areas, reach ten times these figures.