The Anglesea Power Station of Alcoa of Australia Limited uses 1.2 million tons of brown coal a year to produce 150 Mw of electric power for the company's alumina smelter and semi-fabricating facilities at Point Henry, Victoria. Joy's Western Precipitation Division designed and built the elec- trostatic precipitators which effectively clean the station's
JOY PROVIDED IT.
flue gases and keep the sky blue over Anglesea.
Western Precipitation equip- ment has been helping to hold back the health threat of air pollution for 60 years and Joy has the design and manufac- turing facilities to produce air cleaning equipment for any in- dustry—including electrostatic precipitators, mechanical col- lectors, filter type collectors
and wet scrubbers. Western Precipitation Division, Joy Manufacturing Company Pty.
Ltd., 78-86 Kent Road, Mascot, N.S.W. 2020. Tel.: 67 5161.
Cover ii Clean Air / May, 1971
Ceiicote scrubbers settle pollution problems
With pollution control regulations becoming more and more stringent, it will pay you to investigate the overall long-term advantages of Ceiicote wet packed scrubbers.
Ceiicote has a scrubber to handle your noxious gases, corrosive mists, and solid particulates.
Ceiicote manufactures corrosion-resistant air pollution control equipment from fibreglass reinforced plastic or lined steel. Wet scrubbers are our specialty. We design, engineer, and build them. For example: • Cross-flow Packed Scrubbers — Capacity 2,100 — 50,000 cfm. • Countercurrent-flow Scrubbers — Capacity 330 — 47,500 cfm. • Wet Cyclone Scrubbers — Capacity 3000 — 50,000 cfm. • Vertical Air Washers — Capacity 2900 — 67,000 cfm.
And if these types are not suitable for your needs, we can build a scrubber to your specification. Unique Tellerette packing — exclusive to Ceiicote — is used in many of our scrubbers. The advantages are tremendous. Please write for fully descriptive literature.
Ceiicote
Designers, Manufacturers and Installers of Corrosion Resistant Materials, Process Equipment, and Air Pollution Control Systems Ceiicote Pty. Ltd.', P. O. Box 563, Devonport, Tasmania 7310. Telephone: 27 8461 (Std. prefix: 004) Telex: 58S10
Sydney 439 7760. Melbourne 813047. Adelaide 84 173. Perth 24 6044. Brisbane 5 3331.
CEILCOTE SCRUBBER AT GEELONG, VIC.
A typical installation of a Ceiicote Duracor FRP scrubber at the Phosphate Co-operative at Geelong, Vic. This Tellerette filled, cross-flow scrubber weighs 12,800 lbs. and was designed by Ceiicote to treat 32,000 cfm. of Dens gas tormed during
superphosphate produc- tion at 130° F.
S p -
Bailey lets you know exactly how much is leaving your stack before someone else does
The Bailey Smoke/Dust Recorder lets you prove com- pliance with air pollution control regulations. It can be calibrated over its entire range
—0 to 5 Ringlemann numbers for smoke, 0.0005 grains/cu. ft.
to full saturation for dust.
Accuracy (± 2% of maximum range) is unaffected by effluent colour, and is main- tained without frequent recali- bration (once every two or three years is enough).
The sensor is a highly accurate, patented bolometer. It goes inside your stack, where smoke and dust are present. It works on draught or pressurized stacks
up to 20 feet in diameter, at ambient temperatures to 250° F and stack gas temper- atures to 900° F (it includes a temperature compensating filament).
The recorder goes inside your control room, up to 500 feet away. It can include a running time recorder to record periods
of compliance and excessive emission on the same chart with smoke or dust density, electrical contacts for alarm- ing, and/or an electric or pneumatic retransmitter for control.
Price of the system is $1100.
You can't buy more versatility or higher accuracy regardless of price (and we have over 8000 installations to prove it).
For more information, ask for Product Specification E66-1.
Contact your Bailey represent- ative, or write Bailey Meter Australia Pty. Ltd., 26 Auburn Rd., Regents Park,
N.S.W. 2143.
TECHNICAL PAPERS Air Resource Management:
Planning on Acceptable Urban Environment, Werner Strauss
Air Pollution f r o m Wood Waste Incineration
R. P. Murphy and J. F. Pottinger Industrial Combustion of Oil Fuels H.W. Baddams
FEATURES
1972 Clean Air Conference Book Review
Journal Review
Page
19
26 31
25 25 37
Clean Air is the Journal of the Clean Air Society of Australia and New Zealand President: Federal Secretary:
J. G. Schroder Dr. J. Harry, . P.O. Box 130, Lidcombe, N.S.W., 2141
A5
EDITOR W. Strauss ASSISTANT EDITOR J. R. Alonso EDITORIAL BOARD W. H. Cock H. Hartmann J. Maher N. Hawthorn EDITORIAL OFFICE
Department of Industrial Science, University of Melbourne, Parkville, V i c , 3052, Australia.
The journal is published quarterly, in February, May, August
and November.
Manuscripts of original papers in the area of Air Pollution, its measurement, effects and control should be sent to the Editor.
Annual Subscription rates (inc. postage) for non-members and libraries:
Australia $A2.50 U.S.A. $US3.50 U.K. ES1.10.0 Elsewhere $A3.50 Single copies 0.75 cents.
Subscriptions and Subscription enquiries should be directed to the
Circulation Manager, Mr. W. H. Cock, 151 Northern Road, Heidelberg, V i c , 3 0 8 1 , Australia.
ADVERTISING H. E. Pett 8. Co.,
29 Crossley Street, Melbourne, 3000.
V o l . 5 / No. 2 / MAY, 1971
Werner Strauss AIR RESOURCE MANAGEMENT:
PLANNING AN ACCEPTABLE URBAN AIR ENVIRONMENT
From detailed knowledge of levels of air pollution and acceptable air quality standards, it should be possible to plan industrial areas and other urban features to maintain this air quality.
The author is head of the Department of Industrial Science, University of Melbourne. This paper was presented at the Tewksbury Symposium on the Analysis of Urban Development, held at the University of Melbourne, July 1970.
Introduction: It is significant that the reports presented to the Minister for Local Government in the State of Victoria on the future planning of Melbourne — a region which in just 30 years, in the year 2000, will contain 5 million persons — do not mention
"air resource management" or plan- ning and zoning to minimise air pollution.
The Melbourne and Metropolitan Board of Works, the principal plan- ning authority for this region, has not yet considered this aspect of plan- ning, but it is not unique, as very few cities have so far examined their air resources. A notable exception is St.
Louis, where the U.S. Public Health Service has carried out an elaborate investigation(i) which has led to a comprehensive programme for air resource management.
The location and growth of cities has rarely been an orderly and plan- ned process. For the most part com- mercial considerations — access to ports and trade routes or the ready availability of industrial raw mate- rials — have played a more import- ant role than aesthetic aspects of landscape or environmental aspects such as climate and topography.
Secondary factors which influence detailed planning are the availability of suitable land for roads and build- ings and an adequate water supply.
The air resources of a city and its micrometeorology have, until recently, been completely ignored in all urban planning, although in some cases aesthetic appearances have been considered as well as the need for essential services.
Two cities have been traditionally troubled by serious air pollution;
London and Los Angeles. In London, following the 1952 smog disaster, the U.K. government appointed a "Com- mittee on Air Pollution" under the chairmanship of Sir Hugh Beaver(3) which recognised the serious health effects as well as economic cost of air pollution. The Beaver Committee did not have detailed quantitative air quality data available for the different pollutants, but recognised that smoke from the combustion of bituminous coal in domestic chimneys was the major cause of the "smogs" in the
major cities of the United Kingdom.
The Beaver Committee recommended the setting up of "smokeless zones"
and smoke control areas in these cities in which only smokeless fuels could be burned in conventional grates and appliances, while special appliances had to be installed for the smokeless combustion of bituminous coals.
This recommendation was included in the subsequent "Clean Air Act" of 1956(4), and as a result a number of major cities including London, Man- chester and Sheffield have set up sub- stantial smokeless zones. Visitors to these cities some ten years later have been surprised at the cleaner air and the reduction in smogs that has been achieved with this comparatively crude form of air resources manage- ment.
The problem in Los Angeles is more complex, and it is a typical example of the problem in air resource management which is likely to be faced by major cities elsewhere in the future. Before 1940 Los Angeles was known for its blue skies and in- frequent rain, and for this reason had been chosen by film makers as suit- able for outdoor stages. But Los Angeles, which is situated between the Pacific Ocean and a 5000 ft.
mountain range, the San Gabriel mountains, has frequent stable tem- perature inversions and wind speeds averaging less than 4 m.p.h. (i.e.
2 m/s). Los Angeles is a centre for heavy industry, power generation and oil refining, and has a high motor car density. In 1942 when the first
"smogs" were experienced, the popul- ation was 3.15 million, which owned 1.1 million automobiles. In 1965 this had increased to 6.82 million persons with 3.45 million automobiles. Smog conditions were now present on 240 days each year. Table 1, which lists air contaminants, shows that on smog days the concentrations of some of these — hydrocarbons, nitrogen oxides, oxidants and ozone — have increased significantly, while others, notably carbon monoxide, aldehydes and sulphur dioxide have not changed. Eye irritation, the most marked "smog" symptom, appears to be associated with high concentra- 19
Table 1. Ranges of air Contaminants Measured in Los Angeles on Smog and non-Smog Days(5)
Contaminant
Aldehydes Carbon monoxide Hydrocarbons Oxides of Nitrogen Oxidant
Ozone
Sulphur dioxide Particulate matter
(arbitrary units)
Typical Contaminant Range ppm ( V / V ) Smoggy day(*)
0.05 — 0.60 8 — 60 0.20 — 2.00 0.25 — 2.00 0.20 — 0.65 0.20 — 0.65 0.15 — 0.70 5 — 5 0
non-Smoggy day(b) 0.05 — 0.60
5 — 15 0.10 — 2.00 0 . 0 5 — 1.30 0.10 — 0.35 0.05 — 0.30 0.15 — 0.70 4 — 1 4
Maximum Value ppm ( V / V )
1.87 72.0
4.66 2.65 0.75 0.90 2.49 24.8 ( a ) Defined as a day with severe eye irritation.
( b ) Defined as a day with no eye irritation.
tions of ozone and the oxides of nitro- gen.
In the period 1942-1965 very strin- gent restrictions were placed on the production of air contaminants by industry in Los Angeles, and the in- creases in smog must be largely ascribed to the increase in the quantity of automobile exhaust gases.
Thus, while these two examples show how the domestic combustion of soft coal leads to one type of smog, and the automobile is a major con- tributor to the other type, industrial sources are also major sources of air contaminants, which have to be con- trolled. As industrial process units become larger, and production in- creases, the quantities of waste gases will also increase. As a result, it may be expected that stricter controls will have to be enforced.
The different atmospheric pollu- tants produced by an urban environ- ment depend on a number of factors.
In a cold climate, domestic heating will be a major contributor, while in a city with no public transport, private automobiles will contribute relatively more to pollution than in a closely settled European city with greater use of public transport. The type of industry, its controls, and its location within a city will also play a part, as will the siting of refuse- incineration plants and of major electric power generating stations, burning high sulphur fuels. Some typical emissions which can be ex- pected in an urban location are listed in Table 2. This table is not an ex- haustive list, but it illustrates that some contaminants such as hydro- carbons and sulphur dioxide can arise from a number of sources.
What is a polluted environment?
The Establishment of Air Quality Standards.
"Pure Air" is a mixture of oxygen and nitrogen, with small quantities of water vapour, carbon dioxide, and traces of the inert gases and a number of other gases (Table 3). The air in our cities will contain somewhat greater quantities of carbon dioxide and sulphur dioxide, with lesser quantities of water vapour than in the surrounding country, and if these vary beyond certain somewhat arbi- trary standards, then we call the air
"polluted."
However, excessive quantities of some contaminants can come from natural sources. Thus in the thermal regions of New Zealand, appreciable quantities of hydrogen sulphide and greater than normal concentrations of carbon dioxide are found. These atmospheres are so corrosive that special precautions have to be taken to protect metals which are usually not attacked, such as copper and silver. In these areas, for example,
telephone exchanges, have to be pro- vided with protective atmosphere in critical areas.
Cities have a profound influence on the microclimatic conditions, for example by forming "heat islands", which in turn, influence the reten- tion and accumulation of air pollu- tants in their atmospheres. These effects and possible plans for their modification are discussed in Sec- tion 4. But there are no Australian data on the effects of Australian cities on their microclimate, nor are there Australian data on "typical"
urban and rural concentrations of polluting gases such as sulphur diox- ide, nitrogen dioxide or carbon dioxide (in high concentrations), which would enable us to determine desirable and realistic air quality standards.
However, it is possible to assess the order of magnitude of the differ-
ences in concentrations of polluting gases and in climatic effects which may be expected from recently pub- lished German studies based on extensive measurements during 1967 and 1968(6). These measurements were made at four rural locations, all of which were a considerable distance from cities, and two urban stations.
The first of the urban sites (GE) was on a three storey building in the steel manufacturing town of Gelsen- kirchen (population 40,000) while the second urban site was on a meadow north of Mannheim (MA) (population 300,000), where different industries, including chemical plants and en- gineering works as well as residential and farming districts, contributed to the sample.
Fig. la compares the average sul- phur content in the air at the four country stations, with the average for the two urban stations. There are
Clean Air / May, 1971 20
Table 2.
Domestic & Commercial Heating Carbon Dioxide Carbon Monoxide Sulphur Dioxide Nitrogen Oxides Fly Ash Soot
Metallurgical Industry Metallurgical Fume (Sub-Micron Size) Dust
Carbon Dioxide Sulphur Dioxide Odours
Fluorides
Emissions to the Atmosphere Automobile
Exhausts Carbon Dioxide Carbon Monoxide Sulphur Dioxide Aldehydes Nitrogen Oxides Organic Acids Ammonia Solids Dust Lead Oxide Chemical
Industry Hydrocarbons Sulphur Oxides Nitrogen Oxides Carbon Monoxide Mercaptans Aldehydes Acid Mists Dusts Odours
Expected in Urban Area:
Electricity Generation Carbon Dioxide Oxides of Nitrogen Fly Ash
Sulphur Dioxide
Other Industry Dark Smoke Cement Dust Grain Dust Nitrogen Oxides from Electroplating Odours
significant peaks in winter and sum- mer in the urban locations, compared with the uniform concentrations in the country. Fig. lb shows similar trends for sulphur dioxide. Nitrogen dioxide (Fig. lc) shows that city temperatures are always higher than in the country, and wind velocities consistently lower. Diurnal variations of temperature and wind velocity are also greater in the city than country.
(Fig. 3). More detailed measurements of sulphur dioxide and carbon dioxide concentrations are shown in Fig. 4 and Fig. 5 respectively, demonstrat- ing the lesser values found at the Mannheim site (MA) compared with Gelsenkirchen (GE), and using the measurements at Westerland as a
"unpolluted base line" comparison.
It would be unrealistic to attempt to achieve a completely unpolluted country environment in a city where there is industry, domestic heating, transport, power generation and the numerous polluting activities of modern man. Pollution from these sources can however be controlled to a greater or lesser extent, depending on the type of process and the efflu- ent gas. Furthermore, the degree of control which is required is also a function of location of the process.
This decision as to degree of contro- and siting must be made as a com- bination of the desired air quality and the best possible and feasible means of control. It must be recog- nised that the latter may not give the desired air quality if the process is placed on a site where its contribution will add to the total emissions and
exceed the air quality standards which have been laid down, unless other emitters are reduced.
The World Health Organization in its Inter-Regional Symposium on Criteria for Air Quality and Methods of Measurement (1963) agreed on the definition of four "levels of pollution", expressed in terms of pollutant con- centration and exposure times, on which air quality standards can be based (7). These levels were subse- quently endorsed by the W.H.O.
Expert Committee on Atmospheric Pollutants. They are:
Level l. Concentrations and ex- posure times below which, according to present knowledge, neither direct or indirect effects (including alter- ation of reflexes or of adaptive or protective reactions) have been obser- ved. This is the level of "detectable"
effect,
Level 2. Concentrations and expo- sure times at, and above which, there is likely to be irritation of the sen- sory organs, harmful effects on vege- tation, visibility reduction and other adverse effects on the environment.
Level 3. Concentrations and expo- sure times at and above which there is likely to be impairment of vital physiological functions or changes that may lead to chronic diseases or shortening of life.
Level 4. Concentrations and ex- posure times at and above which there is likely to be acute illness or death in susceptible groups of the population.
Air quality standards correspond- ing to Level 1 are probably a reason-
Table 3.
Gaseous Components Present in Normal Air(5) Constituent
Nitrogen Oxygen Carbon dioxide Argon
Neon Helium Krypton Xenon Hydrogen Methane Nitrous Oxide
Concentration 78.084 ± 0.004 20.946 ± 0.002 0.033 ± 0.001 0.934 ± 0.001 18.18 ± 0.04
5.24 ± 0.004 1.14 ± 0.01 0.087 ± 0.001 0.05
2.0
0.5 ± 0.1
Range vol. % vol. % vol. % vol. % ppm (vol.) ppm (vol.) ppm (vol.) ppm (vol.) ppm (vol.) ppm (vol.) ppm (vol.)
able basis on which to get the goals of an urban environment. However, the United States has set these only for sulphur dioxide and particulates (0.08 mg/m:i) although an extensive list of "hygienic standards" which correspond to Level 1 has been pub- lished in the U.S.S.R. Some typical values for the latter are given in Table 4, and in the case of sulphur dioxide agree closely with the new U.S. standards for air quality.
A further factor which has so far not been considered as an integral part of proposals for air quality standards, is the interaction factor.
Two or more pollutant gases may interact, giving rise to more toxic or corrosive matter t h a n the individual gas. Thus if a process introduces a gas not previously present which reacts with existing air pollutants, then new restrictions will have to be introduced. This may limit the new process or add further restrictions to those in force on existing processes.
Much more relaxed standards of air quality than those in the U.S.S.R., or the new U.S. standards were recommended for St. LouisW. (Table 5). This was a very heavily polluted area, and the more relaxed standards may have been attainable within a short period. It may be a realistic approach in such cases to set an in- terim standard of air quality, which can be achieved within 5 to 7 years, followed by higher air quality stand- ards which are made effective over a subsequent period.
Heavily polluted areas elsewhere
Clean Air / May, 1971
are also resorting to special emer- gency measures. An example is the Ruhr valley in Germany, where under adverse meteorological conditions, in- dustries are required to switch to low sulphur fuels for processing and elec- tricity generation, while other indus- trial operations are closed down till the emergency has passed. It is un- likely that such measures will have to be used in Australia, because similar concentrations of industry do not exist.
Air Pollution Levels in Australia.
The most comprehensive published data of pollution levels in Australia were contained in an appendix to the Senate Select Committee's Report on Air Pollution (8). New South Wales, Queensland, South Australia and Vic- toria report dustfall, smoke density (COH units) and sulphur dioxide concentrations. Some more detailed studies of a range of gases (hydro- carbons, aldehydes, oxides of nitrogen, carbon monoxide, lead and inert particulates) have been reported for the air close to traffic lanes in Sydney.
It is of interest that in New South Wales, South Australia and Victoria, recorded values of dust fall, smoke density and sulphur dioxide con- centrations at some locations are in excess of the rather generous air quality goals suggested for St. Louis.
However, the data on pollution levels currently are very sparse, and have been collected using only the most simple apparatus, and these do not permit an accurate and compre- hensive assessment of the present air quality over our cities. For example, there are no measurements of sulphur dioxide concentrations over brief "3 minute" or "5 minute" periods, or of gases from automobile exhausts or from the chemical industry. There is a most urgent need for these meas- urements, for which there is an established technology, so that accu- rate and comprehensive values can be used for the correlation of data for setting air quality standards.
There is a further need for assess- ing the amount of pollution produced by an area — for example, a metro- politan area — based on fuel con- sumption and industrial processes.
Ventilation and diffusion rates for these areas also have to be established so that maximum emission limits can be set for the area based on present and future emissions. My colleagues and I, in the Department of Indus- trial Science are currently engaged on a preliminary study based on fuel consumption. This will establish total quantities for Australia for gases emitted by boilers, furnaces and transportation. A second stage will in- clude waste gases from industrial processes. A further development will
be a detailed estimate of all pollu- tants emitted in a pilot area (Mel- bourne). It may then be possible to apply the method to other state capitals and industrial areas.
It is hoped that these data will
enable governments to establish air quality standards and air quality goals. This will not mean that exist- ing and new industries will be permit- ted to emit pollutants as long as t h e y are in an area in which their efflu-
22
Table 4. Some Maximum Permissible Standards for Air Quality in the U.S.S.R. (From ref. 7) Maximum 24 Hour Average
Material at any one time Maximum mg/m3 ppm mg/m3 ppm
Ammonia 0.20 0.265 0.20 0.265 Benzene 1.5 0.43 0.80 0.23 Hydrogen Sulphide 0.008 0.0053 0.008 0.0053 Nitrogen Dioxide 0.085 0.042 0.085 0.042 Sulphur Dioxide 0.5 0.167 0.15 0.050 Dust (inert) 0.5 — 0.15 — Soot 0.15 — 0.05 —
Table 5. Suggested Air Quality Goals for the Interstate (St. Louis) Air Pollution Study Area(2>
Sulphur Dioxide: Maximum annual average 0.02 ppm.
24 hour average 0.1 ppm, not to exceed 1% of the days in any 100 day period.
1 hour average not to exceed 0.2 ppm more than once in any 4 consecutive days.
5 min. average not to exceed 0.5 ppm more than once in any 8 hour period.
Sulphuric Acid: Maximum annual average not to exceed 4 u g / m ^ . Not to exceed 12 / i g / m3 over 1% of time.
Not to exceed 30 /ag/m3 hourly average over 1 % of time.
Hydrogen Sulphide: 0.05 ppm Vs hour average, not to exceed 2 times/year.
0.03 ppm Vi hour average, not to exceed 2 times in any 5 consecutive day period.
Oxidant: 0.15 ppm for 1 hour (not to be exceeded).
Carbon Monoxide: 30 ppm for 8 hours.
120 ppm for 1 hour.
Dustfall: 10 tons/( mile2) (month), 3 month average (non-heavy industrial).
25 t o n s / ( m i l e2) ( m o n t h ) , 3 month average (heavy industrial).
(Values above a 5 tons/(mile2) (month) background).
Suspended Particulate Matter: 7 5 / g / m2 (annual mean).
200 g / m2, annual 99th percentile.
Soiling Index: 0.4 COH Units/1000 lineal feet.
(COH units = co-efficient of haze, based en the darkening of a filter through which the air is drawn).
ents would be within the air quality standards, the best practicable means of control must still be employed. On the other hand, if a new industry wishes to set up in an area where there are existing industries, or an existing industry wishes to expand, the new industry or expansion will have to adhere to the new overall limits. This may require modification of the original processes so that they comply with stricter requirements than at the original establishment time.
Planning Urban Form for Better Air Resource Management.
A programme of air resource manage- ment must form part of a general plan for urban development. It is therefore necessary to consider, in some detail the air resources of a city, and of areas within the city.
The effect of the presence of natural sinks, such as large bodies of water, and of barriers to air movement, such as mountain ranges, must be exam- ined, together with direction and strength of prevailing winds. Further factors are the presence of the city,
and its effect on the microclimate.
Cities consist of large masses of stonework, concrete, bricks, sand gravel, and asphalt which warm up and cool down more slowly than moist soil, open country and large bodies of water. Evaporation from water sur- face and from transpired moisture on leaves keeps these surfaces cooler than stone or dry sand. On a sunny day this can be a difference of the order of 10°F.
Our central city areas consist of rows of tall buildings lining compar- atively narrow streets. When winds blow along the streets, a funnel effect will give high wind velocities, but if winds enter the street at an angle, velocities can be much higher on the windward side of the street than on the sheltered side. In general, though, wind speeds are less in a city than in the environs (Table 6)0>.
Buildings surrounded by sealed parking areas tend to be in a dry, desert like climate, while those sur- rounded by moist soil and planted areas tend to be cooler and more humid. The walls of tall buildings in a city tend to reflect radiation down- ward to the street, and streets and
lower levels of buildings tend to become warmer than upper floors.
The upper floors also cool off more rapidly in the evening, and these temperature gradients create upward air currents which move pollutants upwards to roof level.
However, at roof level, if this is fairly uniform, heat accumulated during the day will radiate to lower levels, giving an upward rise of air, and permitting a cool layer to come in at roof level. This can blanket an area with narrow streets, trapping warm air and pollutants at lower levels.
In winter in a cold climate, houses are heated and very large amounts of heat are produced in a large city.
However, throughout the whole year, during each working week factories produce heat in boilers and furnaces, while automobiles, buses and trucks produce both heat and fumes, allow- ing people to come to work and transport goods. Thus, cities become
"heat islands". It can be shown that even non-industrial cities such as Washington D.C.(10) show "heat island" effects of about 5°F.
This effect gives rise to an upward current of warm air over the city centre, which entrains dust, smoke and fumes. This settles over the cooler environs, and is the cause of the "dust dome" which can, perio- dically be observed over several of our large cities. In the absence of a strong wind or heavy rain which clears the dust dome, the haze becomes denser.
Less sunshine penetrates this dust dome, and can, in winter, lead to some increase in fuel consumption offsetting the "heat island" effect. It has been shown in a 20 year study in Toronto that this amounts to about 3% Less solar radiation on week days than on Sundays.
A large body of relatively deep water, such as a lake, estuary or ocean shore can act as a natural sink. These bodies of water retain an almost con- stant temperature, while the land mass goes through a diurnal temper- ature cycle. During the day, as the land mass warms up, an up-current of air from it, dragging in a cooler breeze off the water. In the evening, as the land mass cools down, the air above the land, containing the pollu- tants gets swept over the water and is dispersed. The construction of buildings, railway, freeway and road embankments can act as local traps for air pollutants, and some care should be taken to provide adequate ventilation. In the same way, building freeways and railways in cuttings can provide traps for pollutants.
Location of industrial sites down- wind from residential areas is a simple strategy, but not one that is often feasible either in large urban centres, or for other reasons of plan- ning. Thus effective pollution con- 23
trol, either by process modification or by installation of control devices, is of primary importance.
Chimneys, which are needed for the dispersal of the residual effluent gases must be sufficiently high so that ground level concentrations are in- significant. Furthermore, allowances have to be made so t h a t aerodynamic downwash is minimised. Vortex eddies in the lee of buildings, which can cause a looping plume which would bring the plume to ground level with little dilution, must also be avoided, by shaping and position- ing the buildings as well as increas- ing the stack height.
Open spaces can help to reduce certain types of air pollution from an industrial area. Russian workers have shown that a 1,500 ft. wide belt of open land reduces concentrations of pollutants (which were not specified) to about half, while this was even more effective (to more t h a n 80%) when planted with trees, shrubs and bushes. A German worker found that a 600 ft. strip of green space gave a 75% reduction in dust particle count downwind from an industrial area in Leipzig(12).
Studies of sulphur dioxide con- centrations made across Hyde Park, London (13), showed that with wind- speeds greater t h a n 3 knots, a de- crease of more than 90% could be obtained in 3,500 ft. downwind under certain circumstances, while at lower wind speeds an initial decrease (up to 70% in 2,000 ft.) would be followed by an increase. The general behaviour could be closely related to the atmos- pheric temperature lapse rate.
Thus wide strips of park land (say 1,500 ft. or greater) are certainly effective in reducing air pollution downwind from an industrial area, and would have further benefits such as noise barriers. Narrower strips, which are occasionally used as buffer zones between residential areas, although they help the aesthetic appearance of the area and "defend"
land values in the residential area, are not so effective in dilution of pollutants. Other "buffer sandwiches,"
such as commercial zones can also be used, but because of their nature, will not be as effective. A broader band would have to be used to give similar
"sink" properties as a band of park land. Aesthetic factors should also encourage the growth of belts of trees to hide the uglier aspects of indus- try such as open air drum storages, some of the untidier facets of a chemical plant, or mounds of raw materials.
This suggestion would broaden the spread of the urban cornucopia, which is not entirely desirable from other points of view such as cost of services.
It must therefore be weighed against arguments favouring the decentraliz- ation of industry.
Clean Air / May, 1971
Table 6.
Element Contaminants
Dust p a r t i c l e s S u l p h u r d i o x i d e C a r b o n d i o x i d e C a r b o n m o n o x i d e Radiation
T o t a l o n h o r i z o n t a l surface U l t r a v i o l e t , w i n t e r U l t r a v i o l e t , s u m m e r
Cloudiness C l o u d s Fog, w i n t e r Fog, s u m m e r
Prescription A m o u n t s
Days w i t h 0.2 i n .
T e m p e r a t u r e A n n u a l m e a n W i n t e r m i n i m u m
Relative H u m i d i t y A n n u a l mean W i n t e r S u m m e r W i n d Speed
A n n u a l m e a n E x t r e m e gusts C a l m s
United States sources have shown that in 1964 more than half the total air pollution, on a weight basis, was due to automobiles. In the absence of Australian data, it is probably a not unreasonable assumption that here too, the automobile is a major con- tributor to urban pollution*. There are many suggestions, such as electric or electric battery/petrol engine hybrid mini cars, which would reduce this source of pollution (14). While these are possible long term solu- tions, modifications to conventional petrol engines, which are currently being enforced in the United States, as well as "cleaner" fuels, will prob- ably keep pollution from automobiles at approximately current levels if these are also introduced in Australia.
In the long term some reduction can be envisaged. As an additional factor, more effective and popular public transport would also help in reducing the number of automobiles.
In the case of serious morning inversions, such as occur in Los Angeles, a delay in the morning
"rush hour" by about one hour could be used to give a 24% improvement in air pollution as measured by oxid- ant concentration (is). A staggering of business and office hours could there- fore assist both traffic problems as
* A current project In the Department of Industrial Science will investigate this statement.
Climate Changes Produced by C i t i e s C ' i Comparison with Rural Environs
10 times more 5 times more 10 times more 25 times more
1 5 - 2 0 % less 3 0 % less 5% less
5 - 1 0 % more 1 0 0 % more 3 0 % more
5 - 1 0 % more 1 0 % more
1 deg. to 1.5 deg. F. m o r e 2 deg. to 3 deg. F. m o r e
6% less 2 % less 8% less
2 0 - 3 0 % less 1 0 - 2 0 % less 5 - 2 0 % more
well as reducing air pollution result- ing from this source.
Topographical factors must also be considered in the siting of industrial, commercial and residential areas.
While, as mentioned above, it is g e n - erally unwise to place industry up- wind from commercial and residential areas, it is even worse to have dwell- ings level with, or higher than t h e stack emitting the waste gases.
Conclusions and Recommendations The setting of acceptable air quality standards for Australian cities a n d industrial areas is an urgent need. It may be possible that their attainment will be a two stage process in a i r resources management, but before t h e levels for the two stages are defined, far more data on present and future pollution emissions will be required.
These can be based on a combination of detailed measurement of present pollution, assessment of present in- dustrial processes, power generation, and other emissions, prediction of their expansion, and micrometeoro- logical studies of critical areas.
There is a further requirement to co-ordinate these investigations in planning the development of u r b a n forms as a realistic air management programme. This must form an in- tegral part of our urban development if we wish to preserve what is w o r t h in our cities and build better ones.
9 4
References
1. Williams, J. D. a n d o t h e r s . I n t e r s t a t e Air P o l l u t i o n S t u d y . P h a s e I I Project Report.
U.S. Dept. of H e a l t h , E d u c a t i o n a n d Wel- fare, P u b l i c H e a l t h Service, W a s h i n g t o n . Vol. I, I n t r o d u c t i o n (May 1966).
Vol. I I , Air P o l l u t i o n Emission I n v e n t o r y (December 1966).
Vol. I l l , Air Q u a l i t y M e a s u r e m e n t (De- c e m b e r 1966).
Vol. IV, O d o u r s — R e s u l t s of Survey ( J u n e 1966).
Vol. V, Meteorology a n d T o p o g r a p h y (April 1967).
Vol. VI, Effects of Air P o l l u t i o n (Decem- ber 1966).
Vol. VII, Opinion Surveys a n d Air Quality S t a t i s t i c a l Survey (May 1966).
M a n a g e m e n t P r o g r a m m e (May 1967).
Vol. VIII, A Proposal for an Air Resource M a n a g e m e n t P r o g r a m m e (May 1967), P u b l i c Awareness a n d Concern w i t h Air P o l l u t i o n i n t h e St. Louis M e t r o p o l i t a n Area (May 1965).
2. Williams, J.D.; Ozolius, G.; Sadler, J.W.;
a n d Farmer, J.R.; Vol. VIII, Ref. 1.
3. Report by t h e C o m m i t t e e on Air Pollu- tion; Presented to Parliament, November 1954. London, H.M.S.O. R e p r i n t e d (1960).
4. An Act to make provision for a b a t i n g t h e p o l l u t i o n of t h e air (Clean Air Act, 1956).
Section 11 — Smoke Control Areas. See also "A descriptive s u m m a r y of t h e Clean Air Act 1956", National Society for Clean Air, London, S.W.I.
o. Dickinson, J a n e t E.; "Air Quality of Los Angeles County", Technical Progress Report Vol. II. C o u n t y of Los Angeles, F e b r u a r y 1961.
6. Kohler, A., a n d Fleck, W.; " K o n z e n t r a t l o n gasformiger L u f t v e r u n r e i n i g u n g e n in b e l - a s t e t e n u n d " r e i n e n " Gebieten". S t a u b - R e i n h a l t . Luft 29 499-503 (1969).
7. S t r a u s s , W.; " I n d u s t r i a l Gas C l e a n i n g "
(2nd Edition) P e r g a m o n Press, Oxford (in press).
8. R e p o r t from t h e Senate Select C o m m i t t e e on Air Pollution, C o m m o n w e a l t h Govern- m e n t P r i n t i n g Office, Canberra, 1969, p . p . 108.
9. Landsberg, H.E.; "City Air — B e t t e r or Worse" in "Air Over Cities" — Sympo-
s i u m s p o n s o r e d by t h e L a b o r a t o r y of E n g i n e e r i n g a n d P h y s i c a l Sciences, Division of Air P o l l u t i o n , U.S. Dept. of H.E.W., T a f t S a n i t a r y E n g i n e e r i n g Centre, C i n c l n - n a t t i , Ohio (1961).
10. Lowry, W.P.; " T h e C l i m a t e of Cities"
Scientific A m e r i c a n , 217 (2) 15 (August 1967).
11. Mateer, C.L.; Note on t h e effect of t h e of t h e weekly cycle of air p o l l u t i o n on solar r a d i a t i o n a t T o r o n t o . I n t . J . Air W a t e r P o l l u t i o n 4 52 (1961).
12. K a l y u z h n i , D.N.; et al, a n d K u h n , q u o t e d by Rydell, C.P. a n d S c h w a r z , G r e t c h e n ;
"Air P o l l u t i o n a n d U r b a n F o r m " . J . Amer.
I n s t . P l a n n e r s 34 (11) M a r c h 1968 ( R e p r i n t ) . 13. W a i n w r i g h t , C.W.K., a n d Wilson, M.J.G.;
" A t m o s p h e r i c P o l l u t i o n in a L o n d o n P a r k " , I n t . J. Air W a t e r P o l l u t i o n 6 337
(1962).
14. S t e i n h a g e n , W.K.; C l e a n Air, 4 (i) 3 (1969).
15. K a u p e r . E.K., a n d Hopper, C.J.; " T h e U t i l i z a t i o n of O p t i m u m Meteorological C o n d i t i o n s for t h e R e d u c t i o n of Los Angeles A u t o m o t i v e P o l l u t i o n " , Air Poll- t i o n C o n t r o l Assn. J. 15 210 (1965).
1972 CLEAN AIR CONFERENCE Dr. John T. Middleton, Director of the Air Pollution Control Office of the U.S. Environmental Protection Agency, has accepted the Society's invitation to be the keynote speaker at the 1972 Clean Air Conference in Melbourne.
The Conference, as announced earlier, will be from 15th to 17th May at the University of Melbourne. A preliminary pamphlet was distributed in Washington during the 2nd Inter- national Conference of the Air Pollu- tion Prevention Associations, as well as with the previous issue of Clean Air. As a result, about 50 persons from the U.S.A., Canada, France, Mexico, U.K., Japan and Germany have indic- ated their intention to attend. Further- more, technical tours are being arranged by travel agents in Germany and Japan which will feature the Conference.
Conference Organizers are calling for papers for presentation at the Conference, so If you would like to present a paper, it is suggested that you write, enclosing an abstract of not more than 250 words, to —
The Organising Secretary, 1972 Clean Air Conference, 191 Royal Parade,
Parkville, Vic, 3052, Australia before 18th June.
The papers presented will cover all aspects of air pollution and its control: meteorology and atmospheric effects, medical, biological and socio- logical factors, measurement of air pollution and air pollution control.
BOOK REVIEW
POLLUTION: THE WORLD CRISIS Lynette Hamblyn, Published by T.
Stacey Limited, London, and dis- tributed in Australia by Thomas C.
Lothian Pty. Ltd., Melbourne, Aust- ralia. Price $5.40, pp. 168, 1970.
This brief account of pollution prob-
lems has been written by a young and articulate member of the "Prophets of Doom" school. The whole sorry lineup of pollution problems, from the London 1952 smog disaster to Cleveland's inflammable Cuyahoga River is presented. The writer, how- ever, is not without some hope for civilization. She feels that our prob- lems are not insuperable; that we have the knowledge to control and
"Clean up the mess already made."
The authors source materials, as listed, is limited to popular scientific journals, the New York Times and a few books such as Rachel Carson's
"Silent Spring". The book could, therefore, be worthwhile supplement- ary reading for those doing projects at the junior High School level, but one would expect students in senior years to go to the source material, which is both readily accessible and very readable.
The book is well written, but the production — paper and typeface — are surprisingly old fashioned, while the dust cover is reminiscent of a nineteen twenties romance of a Welsh mining town.
W. STRAUSS
Melbourne University Open Day The University of Melbourne held its Annual "Open Day" on 1st May, featuring work on environmental problems which is going on in many Departments within the University.
The Department of Industrial Science had a special display of air pollution sampling and control. This included a model sampling train operating on a smoky flame system, environmental air pollution monitoring and models of typical scrubbers and bag filters supplied by the industry.
AIR POLLUTION CONTROL CONSULTANTS
W. HOWARD COCK & ASSOCIATES PTY. LTD.
Consulting Chemists & Engineers Air Pollution Control & Environmental Studies, Design of Dust & Fume Collection Systems etc.
Emmision and Efficiency Testing
Dust & Toxic Vapour Determinations in Air etc.
Research Projects undertaken for I.R. & D. (A.R.O. No. 83)
151 Northern Road, Heidelberg West, Vic. 3081 Phone: 4 5 4 5 0 6 , 4 5 9 3 0 9 5
25 Clean Air / May, 1971
R. P. Murphy and J. F. Porringer
AIR POLLUTION FROM WOOD WASTE INCINERATION
Introduction: The production of sawn timber in Australia amounts to approximately 4,000,000 tons per year, of which some 30% is seasoned and dressed and re-processed before final use. The total annual wastage result- ing from these operations is of the order of 4,800,000 tons, comprising 3,600,000 tons of slabs, trimmings and dockings and 1,200,000 tons of saw- dust, shavings, etc. Timber processing in New South Wales accounts for about one-third of these quantities.
The disposal of this waste, in a way which will not create environmental problems, is becoming an increasingly difficult task for the timber industry.
Apart from minor agricultural and industrial utilisation of wood waste, the three principal methods employed for disposal are as follows:
1. Dumping. It is estimated that about 20% of the total waste is trans- ported to dump sites. This method results in a minimum of air pollution, and is encouraged in many instances but it is limited by the availability of suitable sites within a reasonable dis- tance of the works. Local Councils are often reluctant to accept wood waste at municipal refuse dumps.
2. As a fuel for heat recovery.
Approximately 10% of the total is used as a fuel in boilers, kilns and similar plants. Disposal in this way depends on the location of suitable consumers in the vicinity of the works; even then, conflictions are likely to arise between the fuel re- quirements and the quantities of waste available, and dumping or in- cineration is often necessary as a sup- plementary method of disposal.
3. Incineration. The majority of wood waste, something of the order of 70% of the total, is incinerated either by open burning or in en- closed incinerators. This method offers certain advantages in as much as it can be carried out on site with neg- ligible transportation costs and with few limitations on the quantities which may be handled. However, it can be responsible for emissions to atmosphere which may cause severe local problems in residential areas.
The combustion of wood waste can give rise to excessive emissions of smoke, odours, ash and charred par-
tides unless carried out in a properly designed incinerator equipped with adequate pollution controls. For this reason, open burning and the use of poorly constructed incinerators and furnaces cannot be considered accept- able for wood waste disposal.
Characteristics of Wood Waste The combustion characteristics of waste vary considerably due to major differences in density and moisture content and the physical size and shape of the material. Wood waste originates from many sources ranging from timber harvesting to finished manufacture and may take the form of slabs, trimmings, dock- ings, off-cuts, veneers, chips, shavings or dust. Frequently, disposal problems are complicated by the variety of waste produced at a single site.
The gross calorific value of wood substance (including resins) on a dry-ash-free basis lies in the region of 8,750 B.Th.U/lb. for most timbers.
The ash content is normally low and has little effect on calorific value but moisture in the wood may reduce the net value considerably.
The green moisture content varies widely between wood species, e.g.
sassafras and coach-wood may con- tain as much as 60% moisture when freshly sawn whilst cypress pine heartwood may contain no more than 25%. In general, the denser the tim- ber, the less moisture it contains when green. Air dry wood stacked in the open is likely to have between 10% and 20% moisture.
Densities of N.S.W. timbers vary between approximately 45 and 80 lb.
per cu. ft. in the green state, whilst air dried weights lie in the range 25-70 lb. per cu. ft.
Like most other fuels, wood is composed largely of carbon, hydrogen and oxygen. Typical analyses for hard and soft woods are shown in Table 1.
Emissions from Wood Waste Combustion
Since wood is composed of moisture, volatile matter, fixed carbon and ash, combination proceeds in distinct phases associated with each com- ponent.
Clean Air / May, 1971 26
Mr. Murphy is the principal Engineer, Air Pollution Control Branch, New South Wales Department of Health, and Mr. Pottinger is t h e Director of Environmental C o n t r o l , Tasmania.
This paper was one of those presented at the 1971 Australian Waste Disposal Conference which w a s arranged by the Department of Fuel Technology, University of N e w South Wales.
The moisture must first be evap- orated. This is an endothermic process and sufficient heat must be available from the burning wood or from sup- plementary fuel to complete the evap- oration of moisture as well as sustain proper combustion. When the total moisture content exceeds 50% com- bustion is likely to be difficult in any kind of furnace or incinerator unless auxiliary burners are employed.
The volatile matter in wood, com- prising a variety of hydrocarbon compounds, is responsible for the majority of emissions of smoke and odours from wood waste burning.
Distillation of these volatiles is en- dothermic and takes place at temper- atures in the region of 300-800° F. If the volatiles reach ignition temper- atures of the order of 1000° P, com- bustion will take place and provided ignition temperature is maintained for a sufficient period of time and adequate air is available, the volatile compounds will burn to carbon dioxide and water. In practice, ideal combustion conditions are rarely achieved and combustion of the volatiles is often incomplete due to low furnace temperatures, insufficient residence time in the furnace or in- adequate admixture with air. Under adverse conditions dense smoke may be emitted to atmosphere from burn- ing wood waste. The colour of the smoke may range from white to black depending on the particular furnace conditions, and statutory regulations which are related to the Ringelmann scale of smoke density sometimes cannot be applied directly to smoke emissions from burning wood waste.
Even when combustion is considered to be good, a light grey smoke and some odour may be evident indicating the escape of unburnt organic mat- erials.
Ignition of the fixed carbon com- mences at approximately 650° F. but much higher temperatures are neces- sary if a satisfactory rate of com- bustion is to be achieved. As with other combustible materials, the car- bon must remain at ignition temper- ature for a sufficient length of time to complete combustion and there must be adequate air in contact with the burning surface. Frequently small wood waste particles in a furnace are entrained in the waste gases and pass out of the flame zone before the car-
bon has burnt completely. Deposits of charred particles escaping from in- cinerators in this way are a common cause of complaint. The soiling nature of these deposits aggravates the problem considerably.
The amount of residual ash pro- duced from burning wood is small and it is not normally responsible for excessive emissions. However, furnaces which burn sawdust entirely in sus- pension are likely to require special attention in regard to fly ash control.
Emissions from wood waste burn- ing can cause severe local air pollution problems. Excessive smoke, whether white, grey or black, is unacceptable, especially in residential areas, and the acrid odour commonly associated with wood burning, is considered objec- tionable by many people. Deposits of charred particles and ash in the vicinity of wood waste burners cause annoyance as well as soiling of pro- perty. Deposits of unburnt sawdust, originating from cyclones, stockpiles, etc., add to air pollution problems associated with wood waste disposal.
Refractory Incinerators
Certain fundamental requirements must be met in order to achieve satis- factory combustion of wood waste and control of emissions. First and fore- most the furnace must be designed to maintain ignition throughout the combustion zone. This means operat- ing at the maximum temperatures possible without damage to the furn- ace lining, and the minimum excess air rate consistent with achieving complete combustion. The furnace configuration should be arranged so as to minimise heat loss whilst pro- viding adequate volume for combus- tion. Lining of the combustion cham- ber with refractory materials reduces heat losses and improves radiant heat reflection.
Refractory incinerators can be designed to burn most forms of wood waste successfully with a minimum of smoke emission. It is recommended that, where the waste is burnt on a static fuel bed under natural draught conditions, a multiple chamber unit comprising primary, mixing and secondary chambers should be em- ployed. The waste should preferably be fed by mechanical means at a steady rate as close as possible to the
burning rate for which the inciner- ator is designed. Large sections are best docked or hogged and also fed mechanically to the burner. Any major variations in feed rate or dis- turbance of the fuel bed are likely to give rise to smoke emissions. Overflre air ports need to be positioned to give maximum efficiency of air distribution and mixing in the combustion zone.
Emissions of solid particles from refractory incinerators fed at an irregular rate frequently exceeds the prescribed concentration under the N.S.W. Clean Air Act (0.2 grains/
standard cubic foot at 12% Co2).
Emissions from incinerators operat- ing under natural draught normally lie in the range 0.1-0.5 gr./cu.ft. de- pending on the design, method of operation and type of waste. Table 2 shows typical values measured on two and three chamber units.
Provided combustion is good, satis- factory control of particular emissions from a static bed incinerator can be achieved by low velocity settlement in a specially built chamber incorporated in the unit. Gas velocities in the settling zone should not exceed 5 ft./sec. and the length of horizontal gas travel should be at least equal to the square root of the cross sectional area of the passage.
Where combustion of the waste takes place in suspension, as for example in the McCashney design, or where forced draugh overflre air jets are employed, high rates of combus- tion are possible. Air flow normally occurs in an intense swirl or vortex motion in the combustion zone, and the resultant high local velocities are likely to cause entrainment of fly ash in the discharge gases. With such in- cinerator designs the installation of efficient fly ash collectors, e.g.
cyclones or spray type scrubbers, is normally required for satisfactory control of paticulate emissions. A controlled fuel feed rate should be maintained and correct proportioning of air to feed rate is essential. Fine sander dust, if fed in significant quantities pneumatically, should be injected at a steady rate with a throat velocity in excess of the flame pro- pogation velocity. The high temper- atures and heat release rates obtain- able with forced draught combustion necessitate careful sizing of combus-
TABLE 1
Typical Composition and Calorific Values of Hard and Soft Woods
Type
Hardwood Softwood
Air Dry Density Ib./cu. ft.
40-70 25-40
Carbon
%
50 51
Hydrogen
%
6 6
Oxygen
%
44 43
Volatiles
%
78 81
Ash
%
0.1-2.0 0.5-1.0
Gross Calorific Value O.A.F.,
B.Th.U/lb.
8,500 9,000
TABLE 2
Emmissions f r o m Refractory Incinerators
Type of Incinerator
Type of Woodwaste
Feed rate I b / h r
Gas flow rate c.f.m. at s.t.p.
Gas temp. °F
Fly ash concentration g r . / c u . ft a t s.t.p. and 1 2 % C O2 ....
Emission rate I b . / h r
Three Chamber Incinerator
Shavings, sawdust
7 9 0 5 , 0 0 0 6 0 0 3.0
0 . 2 2 2.4
Three Chamber Incinerator
Shavings, sawdust off cuts
1,520 4 , 0 0 0 1,270 7.1
0 . 0 8 0.9
Two Chamber Incinerator
Shavings, Sawdust
2 4 0 3,590 6 5 0 1.3
0 . 3 2 1.1
Two Chamber incinerator
Sawdust, off cuts
—
3,495 380 2.4
0.42 2.6
tion chamber and choice of refractory materials.
Tepee Burners
Wood waste burners of the conical or tepee design are used widely by the timber industry. Their principal advantage to the industry are their low capital cost for a given capacity and their ability to handle slabs and trimmings without the necessity for docking. There can be little doubt that tepee burners are very effective in handling the quantities and types of waste generated by sawmills, but unfortunately they can be responsible for major problems as regards emis- sions to atmosphere.
The shell of the tepee burner is not refractory lined and heat losses are high. In addition, large quantities of excess air are used in the burner to keep the shell cool. As a result, combustion zone temperatures are relatively low outside the immediate flame area and emissions of smoke, unburnt gases and solid particles frequently occur. The strong up- draught in the core of the burner also leads to entrainment of excessive quantities of ash and chared particles, particles.
Tests have been conducted on two tepee incinerators in N.S.W. and a resume of these results is shown in Table 3.
The first incinerator tested was a 19 ft. base diameter unit burning hardwood off-cuts and sawdust. This incinerator was not equipped with any air pollution control device apart from a wire mesh spark arrester. The second burner was a 35 ft. base dia- meter unit burning hardwood chips.
This unit was equipped with a cone and water spray arrester on the top;
the water being recirculated through a sludge tank at ground level.
The tests on both of these incin- erators indicate that emissions of solid particles are above the statutory
limit. There was evidence, however, that, when the spray arrester was operated on the second unit, emissions of coarse particles were significantly reduced. Smoke emissions, although sometimes very dense, usually com- plied with the statutory regulations.
As a result of these tests and the accumulations of evidence of problems associated with tepee burners, it has been concluded that incinerators of this design are, in their present form, incapable of meeting the requirements of the New South Wales Clean Air Act.
Equipment for improved combus- tion and fly ash control has been designed and proposed by a number of manufacturers, but no evidence is yet available that such equipment is capable of reducing emissions to a level acceptable in urban areas.
Devices for temperature control, secondary air injection, fly ash collec- tion, etc., can, however, achieve a significant reduction in emissions;
and their installation on existing tepee burners, where problems are known to occur, is to be encouraged.
Clearly further development is necessary if tepee burners are to meet all requirements for air pollution con- trol, and it is the responsibility of the timber industry and tepee manufact- urers to demonstrate that incinerators of this type can be designed to oper- ate within the law. The following measures are considered to be pre- requisite to satisfactory control of emissions from tepees.
1. Operation of incinerator as close as possible to maximum design burn- ing rate.
2. Minimum excess air introduc- tion consistent with permissible shell temperature. This necessitates an air- tight shell and elimination of large openings for wood waste charging.
3. Forced draught underfire and overfire air systems.
4. Control of wood waste charact-
eristics such as size and total moist- ure.
5. Controlled rate of feed of waste to burner.
6. Installation of efficient fly ash arresting equipment.
7. Adequate maintenance of burner and auxiliaries.
Boilers
The low calorific value, bulk density, and ash content of sawdust and shavings presents some difficulties in the use of conventional stoker equip- ment to fire boilers consuming this fuel. Spreader stokers have been used in a number of instances but gener- ally in regard to smoke emission, the most successful installations have been boilers which were equipped with a Dutch oven and step grate.
The wood fuel was charged at a regular rate onto the step grate with screw or flight conveyors. In other instances, a Dutch oven with a flat grate was used and the wood waste was fired at intervals through the crown of the oven to form a conical pile on the grate. Hand firing either alone or in conjunction with mech- anical feeding methods was also used, particularly where off-cuts were to be disposed of.
With the above methods of firing, heavy smoke emission frequently occurred. In an attempt to prevent this, overfire jets were installed on a number of plants in New South Wales.
Overfire jets have been used on coal fired boilers for many years as a means of improving combustion effec- iency and to control smoke emission.
The design criteria for these jets has been described by Engdahl and with minor variation the methods proposed were found to be successful in con- trolling smoke emission from wood waste fired boilers.
Jets were most effective when the boilers were fed at a controlled rate.
However, for the most part, for boilers
Clean Air / May, 1971 28
