QUTGP P
628.530994 1
VOLUME 2. No. 1
MARCH, 1968
The aim of the Society is to conserve clean air in Australia and New Zealand.
EDITOR:
Graham J. Cleary
ASSOCIATE EDITORS:
E. E. Finsten R. S. Williams H. W. Voss The Journal of the Clean Air Society of Australia
& New Zealand Published Quarterly
SUBSCRIPTION:
75c single copy
$2.50 per year
The Clean Air Society of Australia and New Zealand
PRESIDENT:
Mr. J. G. Schroder
SECRETARY:
Dr. G. J. Cleary P.O. Box 31, George St. North Post Office Sydney
2001 Australia
Telephone: 27.8541
Wholly set up and printed by The Sydney & Melbourne Publishing Co. Ply. Ltd., 29 Alberta Street,
Sydney, 2OO0. Tel. 61-4362
Vol. 2, No. 1 MARCH, 1968
TECHNICAL PAPERS
D i s p o s a l of M u n i c i p a l R e f u s e by I n c i n e r a t i o n . . . . 5 By N. Y. Kirov
The Contribution of Different Sources to Pollution by
Polycyclic Aromatic Hydrocarbons 13
By Graham J. Cleary
Incinerator Instrumentation Systems
By K. S. Basden
18
FEATURES
Electrostatic Precipitator Analyser . . Proceedings of M.E.C.A.R. Published People
New Projects
International Union Newsletter . . News f r o m the NSW Branch
Victorian Branch News
4 17 22 23 24 24 24
Mr. O. J. Tassicker, Senior Lecturer, Department of Electrical Engineering.
ELECTROSTATIC PRECIPITATOR
ANALYSER
In collaboration with the Electricity Commission of New South Wales, the staff of the Electrical Department, Wollongong University College, has developed a now instrument known as
"Electrostatic Precipitator Analyser".
This device is aimed at providing much information assisting in the early design of an electrostatic precipitator.
It also provides important information during the operation of a normal pre- cipitator in order to obtain, the best performance from it, especially during changes in combustion or process conditions.
The equipment was tested in co- operation with the E.C.N.S.W. during tests at Pyrmont Power Station in December, 1966. During these tests, coal from the Liddell seams was being fired in the Pyrmont boilers. The
"Electrostatic Precipitator Analyser"
measured the electrical resistivity of the collected fly-ash over a range of temperatures. It also measured the complete corona characteristics of the flue gas.
The equipment, which is light and portable, is expected to find uses where electrostatic precipitators are proposed or where they are already operating. The patents and world licence has been acquired by E.E.L.
International of Mountain View Road, Bayswater, Victoria. This company, which is experienced in the manufac- ture of scientific instruments of many kinds, will manufacture the apparatus and all the associated electronic measuring equipment and market it as a complete package.
Today the control of odours from known sources of industrial pollution is an established part of air pollution control technology and a number of methods are available to enable this to be done. These are process variation, thermal and catalytic incineration, surface condensation, absorption, adsorp- tion, oxidation by liquids or gases and odour masking and counteraction.
However the identification of intermittent odours in the field still con- stitutes a largely unresolved problem. In the absence of scientific means of analysis, odours are notoriously difficult to define unless they correspond closely to one of the few established smells such as "rotten egg gas". Because of the difficulties of description, attempts have been made to systematise odour sensations, none of them with conspicuous success. These range from Zwaardemaker's 9-fold classification and Henning's smell prism to the more recent "odour clusters" based on Hsu's factor analysis.
There are numbers of complicating factors. In common with other sensory systems in the body, the olfactory senses display the phenomenon of adapta- tion. There is a gain in sensitivity in the absence of odour stimulation and conversely there is a decline in sensitivity with continuous odour stimulation.
Another effect which is known but which has not been explored systematically is the selective nature of adaptation. The response to many odours is relatively specific when adaptation takes place and adaptation to one odour has been shown to affect severely the threshold of a number of other odours.
Compensation or "counteraction", the result of simultaneous interaction of two antagonistic olfactory processes which may interfere with each other and tend to obliterate sensation, has been known for some time in an empirical sense and this is the basis of one method which is used to control odour emissions, but little is known about the mechanisms involved.
Most of us recognise that large individual differences occur in the response to a given odour stimulus at a given time. The influence of factors such as motivation, attitudes and emotion on sensory processes and perception is recognised by psychologists, but little interdisciplinary work has been done.
The persistence of odours is also of importance. It has been shown in a number of instances that an odour which is regarded as pleasant when sampled intermittently can become objectionable when it persists for long periods of time.
Improved methods must be found also for the sampling and storage of trace quantities of odours as in most instances a time delay occurs between sampling and analysis. The compound causing the odour is often adsorbed on the container and is difficult to desorb without changing its chemical structure. In solution the compound may interact with the solvent or be affected by light, by dissolved oxygen, by pH or by a number of other factors.
The increased use of portable laboratories to analyse odours where they occur in the field may provide part of the answer.
It does appear that much more work needs to be done on the classifica- tion, sampling and identification of odours. Researchers from a number of fields, notably physiology, organic chemistry, chemical engineering and psychology could contribute to this interdisciplinary study.
March, 1968—CLEAN AIR 4
DISPOSAL OF MUNICIPAL REFUSE BY INCINERATION
By N. Y. Kirov, Associate Professor and Head, Department of Fuel Technology, The University of New South Wales.
Abstract: The problem of hygienic refuse disposal, which is of major and growing concern to Municipal authorities, is a direct consequence of popu- lation growth and technical advances characterising our present day indus- trialised civilisation. For reasons given in the paper, incineration of municipal refuse by modern high temperature techniques has become during the past fifteen years the preferred method of refuse disposal in most of the large cities of the world. An outline of the objectives of high temperature incineration
is followed by considsrations of refuse as a fuel and of the basic requirements for its efficient combustion. The main part of the paper, however, is con- cerned with the advances and principal design features of modern incinerator plant and of the various firing systems which have led to the successful de- velopments in municipal refuse incineration practice throughout the world.
1. INTRODUCTION—THE PROBLEM
In the past, man disposed of his refuse in dumps which usually burned and polluted the surroundings. As population density in urban areas in- creased, the sanitary disposal of muni- cipal refuse became a matter of vital importance to the health and welfare of the community.
We have heard from previous con- tributors to this symposium that bacterial decomposition of refuse by composting provides neither a practical nor an economic solution to the prob- lem. Sanitary landfill or controlled tipping has helped much to avoid the health hazard of dump disposal.
However—
(a) the decreasing availability of suit- able disposal sites near the centres of collection;
(b) the increased cost and difficulties of transportation of refuse to more distant sites;
(c) the increase in population in the large cities; and
(d) the growing rate of refuse col- lected per person, point to in- cineration as the most practical and hygienic solution to the refuse problem.
Disposal of refuse in properly operated sanitary landfills is generally cheaper than disposal by incineration (e.g., the Los Angeles area) and will continue to be practised in areas which are fortunate to have suitable disposal sites. However, for the reasons given above, it is obvious, and current world trends confirm this, that incineration will become more and more the pre- dominant method of refuse disposal.
For example, although the practice
of refuse incineration is of very recent origin in Japan, currently the Depart- ment of Public Cleansing in Tokyo is trying to construct as a matter of great urgency 10 new incineration plants and to improve the existing five so that by 1970 all collected refuse in metro- politan Tokyo will be incinerated.
In the post-war years, there have been considerable new developments of refuse incineration in modern plant at high temperatures. The trend over- seas has been towards large, con- tinuous and efficient operation, serving whole cities on a regional basis and located close to population centres to reduce operating and handling costs.
Such incinerators can now be built and operated in a town residential area, without disfiguring the land- scape, or polluting the air with un- pleasant odours, smoke and dust.
These developments have been con- siderably stimulated by the introduc- tion of legislation against air and water pollution and as a consequence of the growing public awareness of the dangers inherent in some methods of refuse collection and disposal.
Much attention is also being given to utilising the heat generated by the combustion process by coupling in- cinerators to steam and electricity generating plants. While such schemes have often helped to reduce overall costs, it should be recognised that a fuel of low and variable heating value such as refuse is not an economic fuel. Its. disposal is a community service which is costly and must be paid for. The combustion of refuse, therefore, must be regarded from the point of view of public health and convenience, and not from that of economics.
2. OBJECTIVES OF HIGH TEMPERATURE INCINERATION
The main aims of incineration are briefly as follows:
(i) To sterilize waste material and to eliminate the health hazards of disease, and to ensure that both residue and effluent gases are free of un- pleasant odours and vapours.
(ii) To reduce the volume of muni- cipal refuse. This reduction may be of the order of 10 to 20 fold by volume and up to five times or more by weight, e.g. in Prahran (Victoria) the refuse reduction by weight is approxi- mately in the ratio of eight to one.
(iii) To p r o d u c e a readily- disposable germ-free residue, which is odourless, not offensive and non- combustible (say to contain less than 5 per cent, of combustibles) and is almost free of putrescibles which would ultimately decompose and create nuisance.
(iv) To reduce haulage cost by pro- viding efficient means of waste dis- posal close to residential areas.
(v) To achieve the above aims in a building of aesthetically pleasing design, without creating nuisance by excessive noise or smoke, fly-ash and odour emission and without inter- fering with the amenities of the sur- rounding area.
(vi) It is further required that the above objectives be achieved at a minimum initial capital cost and with low operating and maintenance costs.
(vii) Its aims are not to salvage material or to produce cheap heat and power, and the heat by combustion may or may not be recovered for use- ful purposes. For reasons of economy,
high combustion efficiency has not been the object of many of the earlier plant designs and often the above aims may have not been adequately achieved.
As a result, the public image of a refuse incinerator plant is that it is some- thing undesirable, and something to be objected to, particularly by the resi- dents in whose areas incinerators are to be erected.
3. REFUSE AS A FUEL
Domestic and industrial refuse has widely v a r y i n g and fluctuating characteristics. It covers a group of materials with a wide range of com- positions and particle size. Heating value and moisture content change from day to day and sometimes from hour to hour.
Municipal refuse may contain up to 50 per cent, moisture; it consists ap- proximately by weight of—
(a) one-half to two-thirds of domestic
and commercial normal waste such as paper, rags and wood, containing up to 25 per cent, moisture, and
( b ) one-third to one-half of vegetable and partly animal waste, which is also combustible but may con- tain up to 85 per cent, moisture.
Its composition varies seasonally and with changes in locality and local habits, e.g. bottles v. canned beer;
introduction of no-deposit bottles, P.V.C. packing, etc. It is thus not uniform in heating value; the calorific value of refuse in Australia may- average between 2,500 and 4,800 B.T.U./lb which is of the same order as that of the Victorian brown coal which is mined and burned for the generation of power in that State. It is equivalent approximately to one- third to one-quarter of the heating value of bituminous coal. A fuel such as this is capable of generating be- tween two to three times its own weight of steam.
It should be noted, however, that reliable information on trends and relative refuse production of Aus- tralian cities and its moisture content, calorific value and other physical and chemical characteristics is not avail- able since systematic surveys and
records of the type regularly carried out in overseas countries, have not as yet been undertaken here. It is there- fore not possible to assess the likely variability of refuse in different areas and during different seasons. Since its calorific value and composition are changing continuously, incinerators must be both flexible and versatile.
4. REQUIREMENTS FOR EFFICIENT INCINERATION
The fundamental principles involved in burning refuse efficiently are similar to those involved in the burning of any other solid fuel. However, because of its relatively high moisture content provisions must be made to ensure early ignition. The low bulk density, the relatively h i g h and variable moisture content, the uneven and large range of particle size and the vari- ability of the charge present additional handling, feeding and combustion air- distribution problems.
It is a well-known fact that given sufficient time, temperature and turbu- lent mixing in a combustion chamber, with the appropriate quanity of air, all organic matter irrespective of its origin, is capable of being converted into the harmless and odourless gas carbon dioxide and water vapour,
leaving behind an ash residue of in- organic composition,
(i) Because of the high moisture content of the fuel, the operation must be designed to give suf- ficient time for the refuse to dry, ignite and burn out com- pletely, i.e. this means the pro- vision of adequate furnace volume.
(ii) To ensure completeness of com- bustion, furnace temperatures should not be less than 1,400 deg Fahr. and should preferably be within the range of 1,600 to
1,800 deg. Fahr. Preheating of the combustion air and the use of an auxiliary fuel such as fuel oil or gas may sometimes be necessary to achieve and maintain such conditions,
(iii) For efficient combustion, excess air should be kept to a minimum (correct proportioning of air to fuel), and overfire air, pre- heated as necessary must be supplied and so controlled as to ensure thorough mixing of the gases (turbulence).
Finally, the flue gases must be thoroughly cleaned to comply with the requirements of the Clean Air Act.
It is therefore important to design the plant with an ample furnace volume so as to ensure that the minimum of particulate matter is entrained in the products of combustion. In fact, it would be true to say that the control of air pollution is the key to success- ful incinerator planning.
5. COMPONENT PARTS OF A MODERN INCINERATOR PLANT
Many different incinerator designs have been, evolved, satisfying to a lesser or greater degree the require- ments listed under Sections (2) and
( 4 ) . Individual plants may contain all or most of the following features (see also Figures 1 and 2 ) :
(i) Refuse Handling and Storage
After the weight of refuse is re- corded on a weighbridge, the collec- tion trucks proceed to a dumping bay or a storage pit. The refuse is gener- ally lifted from the pit by a travelling overhead grab crane to the incinerator charging bunkers. To reduce odour and dust nuisances, the storage pit and the charging floor enclosure arc usually designed to operate under a slight negative pressure.
Preliminary sorting out of refuse prior to dumping is not undertaken and most modern incinerators gener- ally burn mixed, unsegregated refuse.
March, 1968—CLEAN AIR
6
(ii) Drying and Combustion
From the charging bunkers the refuse is fed to the high temperature combustion chamber, which may take one of a number of designs. Here it is dried and ignited. This may take place in one stage, or in several stages by allowing the refuse to move for- ward by using agitated, travelling, in- clined or rotating, rolling and tumbling types of grates. The process may be assisted by one or more of the follow- ing: preheated air, recirculation of hot combustion gases, radiation from sur- rounding brickwork or appropriately placed combustion arches.
Under normal operating conditions auxiliary fuel is not needed but it may sometimes be required to start the in- cinerator units from cold or to assist in the early drying and ignition of very wet refuse which by itself may not be
able to generate the high combustion temperatures required for the process.
(iii) Secondary-stage Combustion Chamber
In some systems the gases and vola- tilised combustion products pass into a second chamber along with some additional air, if required, where the organic combustible matter is com- pletely oxidised in the gas phase at high temperature to yield smokeless and odourless carbon dioxide and water vapour.
(iv) Gas Cooling
When combustion is completed the hot flue gases have to be cooled from temperature in excess of 1,600 deg.
Fahr. to about 600 deg. Fahr. If the heat is not absorbed by water-cooled surfaces and/or waste heat boilers for the purpose of raising steam, the gases
are generally cooled by using water spray systems with or without dilution with cooling air.
(v) Entrained Dust and Ash Separators
The cooled flue gases are then cleaned by use of various conventional techniques:
(a) settlement in a large chamber, (b) impingement on screens, (c) dry multiclone grit arrestors, (d) fabric bag filters,
(e) wet scrubbing with water sprays, (f) electrostatic precipitation.
(vi) Ash Quenching and Disposal
The ash and solid residue are quenched in a water-filled trough and then removed by a scraper conveyor
Fig. 3
into a storage hopper from which they are either further processed or loaded by gravity into trucks for final dis- posal. This material, after magnetic separation of the iron and appropriate screening, may be sold for road fill and other similar purposes.
(vii) Auxiliary Plant
Stack, fans, preheaters, instruments, weighbridge, storage bins and office and staff amenities have also to be provided.
6. FIRING SYSTEMS
Combustion systems for municipal incinerators fall mainly into two types, the major distinction being that some are batch fed and others use a con- tinuous feed with moving grate stokers.
6.1 Shaft Oven and Cell-Type Furnaces with Batch Feeding
These may be either with circular grate or rectangular grate furnaces.
The refuse is usually dropped on to the grate in batches through a charg- ing gate and a vertical chute. In some designs the fuel bed is agitated by a rotating cone with extended rabble arms, and in others by hydraulically actuated rocking grates or by recipro- cating grates. A longitudinal section through a batch fed cylindrical furnace is shown in Figure 3.
The dropping of large batches of refuse on to the burning fuel bed tends to quench the fire and to stir up the solids thus increasing the amount of particulate matter entrained in the gas stream. Frequent manual stoking and attention to the furnace conditions
are generally required and these, to- gether with the inrush of air during charging leads to fluctuating gas volumes and temperatures and to high amounts of excess combustion air.
Moreover, the temperatures to which the refractory furnace walls may be subjected are usually limited to a maximum of about 1,800 deg. Fahr.
which is controlled by allowing ad- ditional amounts of air to be inter- mixed with the combustion gases.
The intermittent operation of such plant and its relatively small capacity tend to increase maintenance and
labour costs and make compliance with the Clean Air Act requirements difficult and costly.
The modern trend is away from the batch-fed intermittently-operated cell- type furnaces; these could only be justified for relatively small units, i.e.
below six tons/hour.
The following are some examples of incinerators of this type recently visited by the writer:
(i) The Greenbay Avenue Incine- rator (commissioned in 1953) in the city of Milwaukee, U.S.A., consisting of two monohearth furnaces each of
150 tons/24 hour day capacity;
(ii) The Commissioner Street In- cinerator, Toronto, Canada, consisting of 6 x 150 tons/24 hours batch fed furnaces (built in 1955);
(iii) The Village of Forest Hill In- cinerator, n e a r Toronto, Canada, featuring 2 x 180 tons/24 hour furnaces;
(iv) The North York Incinerator, near Toronto, built in 1952 and con- sisting of 4 x 120 tons/24 hour furnaces;
(v) The Fulham (London), Lifford Lane, Kings Norton, Birmingham, and the Castle Bromwieh, Birmingham (officially o p e n e d in September, 1966) incinerators in the United Kingdom; and
(vi) The St. Kilda incinerator in Melbourne which has been operating since 1948.
6.2 Mechanical Stokers with Continuous Feeding
These furnaces are rapidly becom- ing the preferred method of incinerat- ing municipal refuse in the large cities of the world. They arc generally de- signed for continuous operation with completely mechanical charging, con- tinuous feeding and continuous ash extraction and are equipped either with multiple grates or inclined-type step grates.
Manual stoking and handling of the refuse is thus normally not required.
Recent trends have been towards the use of water-cooled furnace walls and utilisation of the heat generated for steam raising and electric power gene- ration, hot water supply and domestic heating, process steam, abattoirs and water purification. Such plants in Western Europe often incorporate a dual firing system using pulverised coal or fuel oil as supplementary fuels.
Their capacities may range up to 1,500 tons/day or more, and they in- corporate efficient flue gas cleaning equipment and very high stacks rang- ing from 300 to 600 ft above ground level. Examples of three such plants are illustrated in Figures 4, 5 and 6.
Excess air is kept to a minimum consistent with good operating con- ditions, more constant and high operating temperatures are possible and complete combustion is easier to ensure. With water-cooled walls, too, problems associated with slagging of the residue or spotting of the refractory walls are eliminated and labour and maintenance costs reduced. As a result, where a ready market for the generated steam is available, and credit is given to residue and metal sales, the total costs of incineration may be considerably reduced.
In contrast to modern European trends, U.S.A. current practice lags well behind; water-cooled walls are not generally used and manual raking is frequently applied. The aim in the U.S.A., is often not complete incinera- tion but volume reduction of refuse at a low cost, hence the minimum of auxiliary plant is used, large volumes of excess air are introduced and lax and rarely enforced codes of air pollution control lead to high quanti-
8 March, 1968—CLEAN AIR
ties of solids emission from relatively low stacks.
Some of the more interesting and advanced types of continuous combus- tion grates successfully used for the incineration of refuse in various parts of the world will now be discussed.
Examples of such installations are listed in Table 1 (see p. 10).
7. COMBUSTION GRATES FOR INCINERATORS
In contrast to industrial furnaces, incinerators arc required to perform satisfactorily over a wide range of operating conditions with an extremely heterogeneous fuel. It is therefore not practicable to burn efficiently and completely a low grade fuel such as refuse on conventional combustion grates. Hence it has been necessary to pioneer new combustion systems designed to meet the special require- ments of this difficult fuel and in par- ticular to provide for continuous mild mixing and agitation, of the burning refuse. A number of grate systems have now been successfully developed for the continuous feeding and burn- ing of refuse in large incinerators.
Of these, the five types which are briefly discussed below are amongst the most outstanding.
7.1 Multiple Travelling Grates
To overcome the disadvantages of the single travelling grate with which agitation, loosening and mixing of the refuse is not possible, several (up to five) travelling grates are combined in a tandem arrangement. The refuse, in passing from one grate on to the next, is subjected to a drop and a tumbling action which assist in reduc- ing to a minimum the discharge of unburned combustible matter. The
Fig.
5travelling grates are often compart- mented and have provision for inde- pendent combustion air control.
Such systems have been successfully built by Combustion Engineering Inc., in the U.S.A., K.S.G., in Germany, and International Combustion in the U.K. The system has also been used in Japan, slightly modified to enable it to cope with the feeding and ignition of a very wet refuse containing as much as 62 per cent, of moisture.
7.2 The Dusseldorf (V.K.W.) Barrel Grate
Specially developed for the burning of refuse by municipal engineers of the City of Dusseldorf, this grate is
Fig. 4
now manufactured by the V.K.W.
boiler company in Germany. The grate consists of six to eight cylinders or barrels placed in series on a down- ward slope of 30 degrees and slowly
rotating in the discharge direction.
Each barrel is about five feet in diameter, may vary in length from five to 17 feet and is composed of many single, serrated grate bars in the form of arched segments. These are keyed to a structural steel spider.
The barrels rest over individual steel compartments for purposes of zoning the input of underfire air and the dis- charge of riddlings.
Each barrel has its own drive so that its peripheral speed is adjustable and independently controlled. For example, the speeds of the Dusseldorf municipal plant vary from a maximum of 50 ft per hour for the first cylinder to a minimum of 16 ft per hour in the end cylinder. This relative movement of the drums assists in loosening up compacted material within the bed.
The rotating grate is in fact a multiple travelling grate, each barrel representing a minimum length of a single travelling grate with a resulting maximum number of tumbling zones and a reduced height of refuse drop.
This is claimed by the manufacturers to subject the burning material to a mild, continuous tumbling and agita- tion, giving it improved aeration with lei's disturbance. As a result, combus- tion is completed with a minimum of excess air and low dust burden. The maintenance cost of this grate is said
to be low. Examples of such installa- tion may be seen in Dusseldorf and Stuttgart.
The Dusseldorf incinerator plant, built at a total cost of about $7 million, consists of four furnaces of 1,000 tons/day total throughput and in- cludes waste heat boilers, equipment for shredding oversize burnable wastes and a complex residue processing system.
The Stuttgart plant of 20 tons/hour capacity is equipped for combined oil/refuse firing and incorporates a K.S.G., radiant boiler. The steam out- put from the boiler is 275,000 lb/hour at 1,100 p.s.i. and 980 deg. Fahr. and about one-quarter of this steam is produced from the burning of refuse.
The iron is separated by means of magnetic rollers and then the residue is crushed in a hammer mill. The crushed slag is size-graded and sold as aggregate.
7.3 The Volund System
This system was originally de- veloped by the Volund Company of Copenhagen, Denmark, for the incine- ration of refuse of high moisture con- tent and low calorific value. It in- corporates two mechanical recipro- cating grates—a drying grate and a combustion grate, followed by a rotary kiln in which the refuse is subjected to a tumbling action and where the final burning out takes place.
Movement of the fuel bed is achieved by the reciprocating action of the grates. Agitation and loosening of the refuse on the grates however is limited.
The kiln is a slowly revolving refractory-lined drum, slightly inclined to the horizontal, allowing a long travel time for the slower burning particles, thus ensuring their complete combustion.
Examples of Volund installations recently visited are the Southwest Incinerator Plant in Chicago, the St.
Ouen plant in Paris and the Kennedy Town incinerator in Hong Kong;
some details on these incinerators are summarised in Table I.
10 March, 1968—CLEAN A IR
7.4 The Von Roll Step-Down Reciprocating Grate
The Von Roll grate has been specially developed in Switzerland to burn difficult materials such as refuse.
It is of the reciprocating type and consists of several inclined step-down sections. Three such sections with zoned underfire air are incorporated in the latest designs. The first section is the drying stoker, from which the refuse, after ignition, is dropped vertically about 5 ft on to the second or combustion stoker and thence it is dropped another 5 ft on to the third (burnout) section, where combustion is completed.
Good mixing and effective agitation of the refuse is achieved by a movable set of blades which are raised periodic- ally across the burning section of the second and third grates, by the inter- mittent reciprocating action of the grate bars and by the tumbling during the transfer of the burning material from one section of the grate to the next. The hot residue is discharged into a water trough from which it is removed by a conveyor or scraper.
Frankfurt's latest refuse burning plant built by the Von Roll Company has furnaces with a capacity of 15 tons/hour of raw refuse and is com- bined with a corner-tube boiler of 71,500 lb/hour, steam capacity at 960 p.s.i. and 930 deg. Fahr. Other examples of installations of this type are given in Table 1.
7.5 The Martin Reverse-acting Grate
This interesting grate, designed by Messrs. Joseph Martin and Co., of Munich, was originally developed for the burning of low grade fuels, par- ticularly high moisture brown coals, ignites and high-ash middlings. It too is a stepped-down grate, inclined somewhat more steeply than other types, i.e. 30 to 45 degrees to the horizontal.
The grate surface consists of many heavy bars anchored by keys into a structural frame. These keys push uphill in a reciprocating action against the downward flow of refuse, provid- ing a counter-flow of a certain amount of burning fuel under the still un- ignited incoming refuse. This move- ment ensures rapid ignition of the freshly-charged raw materials and pro- vides continuous agitation, and loosen- ing of the burning refuse. As a result burning rates are high (up to 100 lb/sq. ft G.A./hour) and a very good burn-out is achieved in a compara-
Fig. 6
lively short grate length with a mini- mum of refuse bed disturbance.
A short relative motion also takes place between adjoining bars on the grate and helps to free any wires, clinkers and the like that might have lodged in the air spaces. The grate is compartmented into five to seven zones and is cooled by high air veloci- ties and is designed to give a high pressure drop in order to ensure uniform air distribution within the bed. Machining to close limits and special heat resisting materials (e.g., high quality chrome alloys) are neces- sary for the construction of these grates which makes them relatively more expensive both in initial and in maintenance costs than some of the more conventional types.
The fuel bed thickness is controlled by a reciprocating fan-type feeding gate at the front end and by variable- speed clinker rolls at the discharge end. A special feature is the ash ex- tractor system associated with this grate design. The residue is com- pletely burnt and quite cool on reach- ing the clinker rolls and only a small amount of water is required for quenching the hot ash. The residue is thus comparatively dry and easy to handle and is delivered on to a normal rubber belt conveyor.
Single units can be built with
capacities within the range of 25 to 1,200 tons of refuse per unit per day.
Particulars of municipal incinera- tors with M a r t i n grates, recently visited by the writer in Rotterdam, Paris (Issy-Jes Moulineaux), Munich and Stuttgart are summarised in Table
1.
8. CONCLUDING REMARKS
The efficient incineration of refuse presents a challenging problem to the combustion engineer. With the reali- sation that well established design principles for conventional fuel- burning plant cannot be readily applied to the burning of refuse of heterogeneous a n d ever - changing characteristics, much technological progress has been made and it is now possible to achieve the ultimate aim of reducing refuse efficiently to an inert, and even useful, residue with- out creating the nuisances which plagued early incinerator designs.
Modern trends overseas arc towards the construction of large units, de- signed for continuous operation, pre- ferably with water-cooled walls with some useful heat recovery where practicable, featuring continuous feed- ing, efficient combustion with con- trolled excess air, effective gas clean- ing and tall stacks.
AIR POLLUTION CONTROL EQUIPMENT
THE MIKRO-PULSAIRE
a unit for every dust recovery or dust control application . . . Reverse jet action • No moving parts
There is no dust recovery j o b t o o b i g f o r the M o d u l a r Mikro-Pulsaire because t h e r e is no l i m i t to its f i l t e r i n g capacity. Units are d e s i g n e d in precision sections w h i c h arc c o m b i n e d as r e q u i r e d to meet any CFM specifica- t i o n . K n o c k e d - d o w n , t h e y are economical t o s h i p , a n d all parts are m a n u f a c t u r e d f o r s i m p l e a l i g n m e n t a n d speedy assembly at the site. A p p l i c a b l e t h r o u g h o u t t h e processing i n d u s t r i e s , the M o d u l a r Mikro-Pulsaire is d e s i g n e d to v e n t alt types of p a r t i c l e r e d u c t i o n e q u i p - m e n t , spray d r y e r s , separators, calciners, m i x e r s , packag- i n g m a c h i n e r y , mechanical c o n v e y o r s , c a r l o a d i n g opera- tions, and many other dust g e n e r a t i n g sources. The M o d u l a r Mikro-Pulsaire is a v a i l a b l e w i t h 6 or 8 ft. f i l t e r tubes and can b e e q u i p p e d w i t h screw c o n v e y o r f o r c o n t i n u o u s discharge at one p o i n t t h r o u g h a M i k r o - A i r l o c k .
Supplied and manufactured under exclusive licence from Pulverizing Machinery, Division of Slick Industrial Company,
Summit, New Jersey, U.S.A., by
CHEMICAL PLANT & ENGINEERING
A D I V I S I O N OF INDUSTRIAL ENGINEERING LIMITED New South Wales Office:
283 PARRAMATTA ROAD, AUBURN, N.S.W.
Postal Address: Box 85, P.O., Auburn, N.S.W.
Telephone: 648-2144 — Telex: 1EL AA 20818
Also represented in WEST AUSTRALIA • QUEENSLAND TASMANIA e SOUTH AUSTRALIA
• NEW ZEALAND
THE MODULAR MIKRO-PULSAIRE
ALSO 8 in. MIKRO ROTARY AIRLOCK, AND 14 i n . ROTARY AIRLOCK, BIN VENT COLLECTORS, ARCO WET SCRUBBERS, DUSTEX MULT1CL0NE, DUST COLLECTORS,
CHEM PLANT DOUBLE FLAP VALVES Head Office:
48-50 A'BECKETT STREET, MELBOURNE, C.l.
Pos'al Address: G.P.O. Box 1700P, Melbourne, C.l.
Telephone: 34-9311 — Telex: IEL AA 30756
12 March, 1 9 6 8 — C L E A N AIR
SELF-CONTAINED DUST & FUME EXTRACTOR
Hansson's Self-Contained Dust and Fume Extractor Units contain a gravity settling compartment, filter chamber with flame-proof fabric sleeves or steel wool packet quality sewn filter envelope.
Air movement is derived from a backward curved blade centrifugal exhauster located on the clean side of the air circuit. Drive is from direct-coupled totally enclosed 2,850 r.p.m. electric motor.
Coarser particles are gravity deposi ed at inlet into a removable tray or hopper, at the underside of the unit into which the finer dust is also trapped, by working the handle at the right side.
Hansson Dust Extractors are made in six different standard sizes from 200 c.f.m to 2,500 c.f.m. but special units having a stronger velocity can be manufactured to suit individual requirements. We favour the installation of these smaller units to minimise the effects of the breakdown of a central plant.
Each machine has its own Suction Hoods so there is no drop in the required velocity due to suction having to be brought long distances to the source from the main plant line.
Our Company has been designing and building Dust Collecting Machines for a considerable number of years and we have 34 years' experience overseas. Our Machines are completely designed and built in Australia to suit the Australian market, and we claim they are equivalent in all respects to the finest imported equipment.
THE CONTRIBUTION OF DIFFERENT SOURCES TO POLLUTION BY POLYCYCLIC
AROMATIC HYDROCARBONS
By Graham J. G e a r y , Ph.D., Secretary, Clean A i r Society of A u s t r a l i a and New Z e a l a n d .
Abstract: The contribution from automobile exhausts, coal combustion sources, products from the combustion of gaseous and liquid fuels, tyre rubber, and incinerator effluents to atmospheric pollution by polycyclic hydrocarbons is examined briefly. Concentration ratios for the compounds 3,4 benzopyrene/1,12 benzoperylene and for 3,4 benzopyrene/coronene are used to examine the mode of pollution in Sydney and to compare this pattern with other cities in Great Britain and the United States of America.
LABORATORY tests have shown that gross extracts of air pollu- tants,1,2 ozonised gasoline,3 extracts from automobile exhausts conden- sates4,5,6 and coal soot7 produce cancers in test animals. A number of carcinogenic polycyclic hydrocarbons have been identified in urban atmos- pheres8,9,10,11,12 and certain members of the series such as 3,4 benzopyrene and 1,12 benzoperylene have been reported in every city in which they have been sought. As the carcinogenic polynuclear hydrocarbons reported in atmospheric extracts have been identi- fied in many samples of soot and tar the established biological activity of the gross extracts might be attributed largely to the carcinogenic potency of the polycyclic hydrocarbons present.
The relative contribution from various sources varies widely and the present paper will briefly review this contribution and also examine the mode of pollution by polycyclic hydro- carbons in Sydney.
Automobile exhaust products
Several investigators have considered aspects of petrol and diesel vehicle emissions. Kotin et al sampled the particulate phase of stationary diesel13 and petrol14 engine exhausts and re- ported the differential formation of the polycyclic hydrocarbons pyrene, 3,4 benzopyrene, 1,12 benzoperylene and anthanthrene with respect to engine speed and load. These workers ob- served that engine speed alone had little or no effect on the concentrations of the compounds found in diesel en- gine exhausts but with gasoline engines the quantities of these polycyclic hydro- carbons decreased rapidly with in- creasing engine speed. For example at zero load the quantities of 3,4 benzopyrene emitted in a one minute
sample of gasoline exhaust were 120 microgrammes at 500 r.p.m., 61 microgrammes at 1,000 r.p.m., 40 microgrammes at 2,000 r.p.m. and
13 microgrammes at 3,000 r.p.m.
The behaviour of the engines to load was in marked contrast. A rapid decrease in hydrocarbon emissions resulted from increasing the engine load for petrol engines. At a fixed speed of 1,000 r.p.m. the yield of 3,4 benzopyrene in a one minute sample decreased from 61 microgrammes at no load to 0 microgrammes at i- load.
At the same time the yield of 1,12 benzoperylene decreased from 177 microgrammes at no load to 45 micro- grammes at ¼ load, 5 microgrammes at ½ load and 2 microgrammes at full load. Conversely, increasing the load at fixed speed resulted in a marked increase in the diesel emissions. Under conditions of inefficient operation at a speed of 1,200 r.p.m., 9 micro- grammes of 3,4 benzopyrene was ob- tained at zero load, but this increased to 47 microgrammes at ¼ load, 437 microgrammes at ½ load and 1,706 microgrammes at full load.
For both types of engine, accelera- tion and deceleration resulted in a larger production of polycyclic hydro- carbons, particularly during accelera- tion. Kotin et al found that for a petrol engine,14 750 microgrammes of 3,4 benzopyrene and 1,875 micro- grammes of 1,12 benzoperylene were formed per minute during acceleration from 1,000 to 1,500 r.p.m. Also, up to 2,500 microgrammes of 3,4 benzo- pyrene and 2,000 microgrammes of 1,12 benzoperylene were recovered per minute from a diesel engine exhaust13
during acceleration from 1,200 r.p.m.
to 1,400 r.p.m.
From the above studies, Kotin et
Or. G. J. Cleary.
al13 concluded that the diesel engine can be a much greater source of pollution by polycyclic hydrocarbons.
This is mitigated, however, by the fact that a diesel engine is capable of main- tenance in a manner that can preclude the emission of hydrocarbons into the atmosphere. As a corollary, optimum operation of a diesel engine is con- sistent with almost complete freedom from hydrocarbon emissions to the atmosphere.
Because of the effects of main- tenance and operating conditions on the emissions from diesel vehicles, studies of their contribution to atmos- pheric pollution have tended to show inconsistent results. S t u d i e s by Commins et al15 inside a London omnibus garage and by Moore and Katz16 in railway tunnels in Canada revealed that concentrations of 3,4 benzopyrene in the working atmos- phere in which actual diesel vehicles were operating were of an order similar to that of the ambient atmos- phere in each case. However, later studies by Waller et al17 in London road tunnels showed that concentra- tions of polycyclic hydrocarbons tended to be higher than average atmospheric values and that pyrene, fluoranthene, 1,2 benzopyrene and 3,4 benzopyrene concentrations as well as smoke density tended to be more closely related to the density of diesel
rather than petrol powered vehicles.
Sullivan and Cleary18 compared the emissions of polycyclic hydrocarbons at a specific traffic situation in Sydney
where diesel-powered vehicles were partially segregated in one of 10 road lanes during the peak evening period.
Because of this, it was found possible to sample simultaneously in two of the road lanes, about 40 ft apart, only one of which was carrying diesel vehicles. An association was estab- lished between traffic containing diesel vehicles and the concentrations of smoke, 1,2 benzopyrene, 3,4 benzo- pyrene, 1,12 benzoperylene and coro- nene.
Coal combustion sources
Analysis of coal tars and pitches for polycyclic hydrocarbon con- stituents has been carried out by a number of workers mainly using tech- niques based on high temperature gas- liquid chromatography and invariably, significant amounts of a wide range of polycyclic hydrocarbons have been reported.19,20,21,22
Comparative work on the relative concentrations of polycyclic hydro- caibons in primary and secondary condensates has been reported by the present author.23 First the total yield of the eight hydrocarbons pyrene, fluoranthene, 1,2 benzanthracene, chrysene, 1,2 benzopyrene, 3,4 benzo- pyrene 1,12 benzoperylene and coro- nene in a series of experiments was from 32 to 110 times larger in the thermal cracked tars. In the latter, maximum yields of 3,4 benzopyrene and 1,12 benzoperylene of 142 micro- grammes per gramme of original coal and 116 microgrammes per gramme respectively were obtained.
Secondly, c e r t a i n concentration ratios were shown to be indicative of a primary or of a secondary tar. Thus chrysene/3,4 benzopyrene ratios of greater than 12:1 were found in a primary tar whereas these same ratios were less than 0.7:1 for secondary tars. Ratios of 1,2 benzanthracene/
3,4 benzopyrene greater than 1.5:1 were found in the primary distillates but these ratios were less than 1.0:1 in the secondary tars. By contrast the fluoranthene yields relative to 3,4 benzopyrene were much greater in the thermally cracked tars. In the primary distillates these ratios were less than 0.5:1 but for secondary tars the same ratios were greater than 2,1:1.
These same ratios were used to follow the sequence of combustion in a handfired intermittent brick kiln.24 A graduation was observed in the nature of the tar adsorbed by the soot, that collected early in the drying stage being primary in character but as the temperature rose there was a notice-
able transition to secondary tar. The highest concentration of polycyclic hydrocarbons, which coincided with the highest soot fall from the kiln, occurred towards the end of the pre- heating period. Concentrations of 54.0 ppm 1,2 benzanthracene, 19.0 ppm 1,2 benzopyrene, 40.3 ppm 3,4 benzopyrene 19.2 ppm 1,12 benzo- perylene and 11.6 ppm coronene were obtained in this sample of soot. A very marked reduction in the concen- tration of polycyclic compounds oc- curred during the full fire period and only trace quantities of the higher molecular weight hydrocarbons 3,4 benzopyrene, 1,12 benzoperylene and coronene were obtained in an equiva- lent sample taken at this stage.
Products from the combustion of gaseous and liquid fuels
It has b e e n demonstrated by Tebbens et al25 that even the simplest gaseous fuels can be burned in such a way as to produce polycyclic hydro- carbons. In an experimental furnace which was designed to facilitate burn- ing u n d e r controlled conditions, natural gas, butane, butene-1, acety- lene and butadiene were burned.
These fuels were chosen to span a wide range in the arbitrary scale re- lating smoking tendency to fuel com- position. In all cases polycyclic hydrocarbons were generated under all conditions short of complete combus- tion in excess of air.
In a later study, Tebbens et al26
showed that incomplete combustion of a wide range of liquid fuels (saturated and unsaturated aliphatic, alicyclic and aromatic hydrocarbons) similarly results in the formation of polycyclic hydrocarbons. It was also found by these authors that when burned under similar air to fuel ratios the relative by-product yield of extractable organic material to soot is similar within a wide range of fuels, both gaseous and liquid.
From these studies the hypothesis was advanced26 that the mechanism of formation of high molecular weight polycyclic hydrocarbons was presum- ably a series of cracking and reform- ing processes (rather than a distilla- tion of polynuclear hydrocarbons already present in a fuel) which takes place in the thermally energised re- action zone of the flame. It was also considered that the difference between liquid and gaseous fuels in the ease with which the particular organic by- products of combustion are generated is that the former require a relatively larger amount of thermal energy to activate the reaction mechanisms.
In another study, Howe27 obtained the yield of 3,4 benzopyrene formed under different conditions of oil com- bustion in domestic boilers and in- dustrial fire—and water—tube boilers.
In all cases of normal operation the yield was less than 0.006 parts per million of fuel burnt.
Other sources
The presence of carcinogenic poly- cyclic hydrocarbons and related com- pounds in processed rubber, including tyre rubber, was demonstrated by Falk et al.28,29 These compounds were also found in some furnace carbon blacks used in automobile tyre rubber but not in channel carbon blacks.30,31,32
A range of polycyclic compounds present in tyre rubber which had been removed by grinding tyres about to be retreaded was reported by the present author.33 This was character- ised particularly by the relatively high yield of chrysene but yields of the carcinogenic homologues 3,4 benzo- pyrene, 1,12 benzoperylene and 1,2 benzopyrene were 5.3 ppm, 8.7 ppm and 11.9 ppm respectively.
According to Sawicki34 the per- centage of 3,4 benzopyrene in par- ticulates from the stack of well-run incinerators u s u a l l y is less than 0.00001. As pointed out by Sawicki et al8 this is a lower concentration than that found in non-urban air- borne particulates. A comparison was also made of the composition of poly- cyclic hydrocarbons in air-borne par- ticulates collected from an incinerator before and after scrubbing with water through a series of bubblers and traps maintained at reduced temperatures and then through a glass-fibre filter.35 A decrease of 95 to 98 per cent, was found in the weight of polycyclic hydrocarbons present but this was related directly to the high efficiency of the scrubbing train (and particularly the fibre glass filter) for collecting the particulate fraction.
Urban air polluted with coal-tar pitch fumes from sidewalk - tarring operations has also been examined close to the site of these operations.
Yields of 3,4 benzopyrene as high as 78 mg./ 1,000m3 of air and of 1,12 benzoperylene as high as 40 mg./
1,000m3 of air have been reported.34
Ratios of hydrocarbons as indicators of pollution type
It has now been established that two sources arc mainly responsible for the pollution by polycyclic hydrocarbons in urban areas, namely soot derived from coal combustion products and
14 March, 1968—CLEAN AIR
from automobile exhausts. Fortunately these two sources exhibit characteristic concentration ratios for the com- pounds 3,4 benzopyrene/1,12 benzo- perylene and for 3,4 benzopyrene/
coronene.
Sawicki et al8 in a study of the atmosphere of 14 American cities and also of areas where the pollution was considered to be derived almost ex- clusively from automobile exhausts or coal combustion products concluded that concentration ratios of 3,4 benzo- pyrene to 1,12 benzoperylene greater than 1.0 and 3,4 benzoperylene to coionene greater than 1.2 are indica- tive of coal-fired combustion whilst ratios of 3,4 benzopyrene of 1,12 benzoperylene less than 0.6 and ratios of 3,4 benzopyrene to coronene less than 1.0 are indicative of auto- mobile exhaust products as the pre- dominant factor. Intermediate values would be indicative of mixtures from the two types of source.
In Great Britain these ratios are mostly greater than 1.0 while in Los Angeles, where motor vehicle pollu- tion is the dominant factor, the ratios are usually within the range 0.2 to 0.5. In all communities studied the 3,4 benzopyrene/1,12 benzoperylene and 3,4 benzopyrene/coronene ratios decreased in summer months.
Mode of pollution by polycyclic hydrocarbons in Sydney
Methods used by the present author in the analysis of air-borne particu- lates for polycyclic hydrocarbons have been detailed previously.12,36
The mean monthly concentrations of these hydrocarbons in the atmos- phere of Sydney for the year from February, 1962, to January, 1963, expressed in the units microgrammes per 1,000 cubic metres of air sampled are shown in Table l.33 The marked winter rise in the concentration of all compounds is similar to that reported by Sullivan37 for smoke density. Also, samples for the polycyclic hydro- carbon analysis were weighed before and after extraction in order to de- termine the total concentration of particulate matter in suspension in the atmosphere. The results of these de- terminations are shown in Table 2 which also lists the concentrations of the same polycyclic hydrocarbons in the total particulate matter expressed in microgrammes per gramme (parts per million).
The seasonal fluctuations in the amount of particulate matter and the associated polycyclic hydrocarbons is attributable predominantly to meteoro- logical influences, the most important
TABLE 1
Results of a 12-Month Survey of Polycyclic Hydrocarbon Concentrations in Suspended Particulate Matter
Hydrocarbon Concentrations*
Month February, 1962 M a r c h , 1962 A p r i l , 1962 M a y , 1962 June, 1962 J u l y , 1962 A u g u s t , 1962 S e p t e m b e r , 1962 . O c t o b e r , 1962 N o v e m b e r , 1962 ..
December, 1962 .. . J a n u a r y , 1963 . . .
Pyrene 0.70 0.91 1.01 1.28 3.87 3.98 2.33 2.19 1.20 0.84 0.078 0.39
Flouran- thene
0.44 0.39 0.56 0.63 2.57 1.84 1.71 1.41 0.95 0.47 0.056 0.15
1,2 Benzan- thracene 1.09 1.91 2.02 2.46 5.54 6.25 4.16 2.92 2.96 0.84 0.36 0.77
Chrysene 0.72 1.61 1.68 1.79 6.65 3.63 1.94 2.12 2.12 1.13 0.20 0.77
1,2 Benzo- pyrene
1.31 1.79 2.16 2.46 6.91 7.10 3.85 2.72 1.90 1.70 0.76 0.82
3,4 Benzo- pyrene 1.35 1.77 2.50 3.37 7.40 7.65 8.22 3.81 2.42 1.33 0.57 0.57
1,12 Benzo- perylene
2.54 3.00 4.88 5.72 9.65 7.55 8.70 4.02 3.26 2.36 0.88 1.09
Coro- nene 1.23 1.72 2.38 2.75 4.47 4.90 5.02 3.14 2.29 1.19 0.64 0.82
Results of a 12-Month Survey of Polycyclic Hydrocarbon Concentrations in Suspended Particulate Matter
Hydrocarbon Concentrations*
Particu-
Month February, 1962 M a r c h , 1962 A p r i l , 1962 M a y , 1962 ....
June, 1962 J u l y , 1962 ....
A u g u s t , 1962 . S e p t e m b e r , 1962 O o o b e r , 1962 N o v e m b e r , 1962 December, 1962 J a n u a r y , 1963
late Density ( m g . / m3) Pyrene
0.072 0.087 0.119 0.147 0 . 2 1 4 0.180 0.178 0.126 0.091 0.071 .... 0.037
0.059 9.7 10.4 8.5 8.7 18.1 22.1 13.2 17.4 13.2 11.8 2.1 6.6
Fluoran thene
6.1 4.5 4.7 4.3 12.0 10.2 9.6 11.2 10.4 6.6 1.5 2.5
1,2 - Benzan-
thracene 15.2 22.0 17.0 16.7 25.9 34.7 23.4 23.2 32.5 11.8 9.6 13.0
Chrysene 10.0 18.5 14.1 12.2 31.1 20.2 10.9 16.8 23.3 15.9 5.4 8.0
1,2 Benzo- pyrene 18.2 20.6 18.2 16.7 32.3 39.5 21.6 21.6 20.9 24.0 20.6 13.9
3,4 Benzo- pyrene 18.8 20.4 21.0 22.9 34.6 42.5 46.1 30.2 26.6 18.7 15.4 9.6
1,12 Benzo- perylene
3 5 . 4 3 4 . 4 4 1 . 0 3 8 . 8 45.1 41.9 4 8 . 8 31.9 3 5 . 8 3 3 . 3 2 3 . 7 18.4
Coro- nene 17.1 19.8 19.8 18.7 20.9 27.2 28.2 24.9 25.2 16.7 17.3 13.9
Correlation Coefficients b e t w e e n Concentrations of Various Polycyclic Hydrocarbons and Smoke
Density
Correlation
Hydrocarbon Coefficient Pyrene
F l u o r a n t h e n e 1,2 Benzanthracene Chrysene
1,2 Benzopyrene 3,4 Benzopyrene 1,12 Benzoperylene Coronene
+ 0.87 + 0.89 + 0.89 + 0.85 + 0.88 + 0.93 + 0.93 + 0.81
Monthly Concentration Ratios of 3,4 Benzopyrene to 1,12 Benzoperylene and 3,4 Benzopyrene to
Coronene.
Concentration Ratios 3,4 Benzopyrene:
1,12-Benzo-
perylene Coronene
of which is the frequency of tempera- ture inversions which markedly favour the winter months. Seasonal variations in fuel usage have a comparatively small influence on smoke density levels as the major demands for fuel utilisa- tion are relatively constant.
Other meteorological factors which contribute to the winter accumulation of polycyclic compounds in Sydney are the later dispersion of ground-level inversions in the colder months, the
Month
February, 1962 . M a r c h , 1962 .. ..
A p r i l , 1962 M a y , 1962 June, 1962 J u l y , 1962 A u g u s t , 1962 September, 1962 O c t o b e r , 1962 N o v e m b e r , 1962 December, 1962 . January, 1963
0.53 0.59 0.51 0.59 0.77 1.01 0.95 0.95 0.74 0.56 0.65 0.52
1.10 1.03 1.06 1.22 1.65 1.57 1.63 1.23 1.07 1.12 0.89 0.69
less rapid degradation of the hydro- carbons at lower temperatures and the prevailing wind patterns. With regard to the latter, a marked increase in smoke density has been shown to accompany winds from all westerly vectors. Conversely, wind flows from
* Concentra ions are expressed in microgrammes per 1,000 cubic metres of air s a m p l e d .
TABLE 2
* C o n c e n t r a t i o n s e x p r e s s e d in m i c r o g r a m m e s per g r a m m e (parts per m i l l i o n ) of s u s p e n d e d m a t t e r .
TABLE 3 TABLE 4
