Everything you need to learn about controlling air pollution from fugitive emissions.
Introduction to Fugitive Emissions:
Fugitive particulate emissions are divided into two basic categories depending on their origin. They are fugitive process emissions (FPEs) and fugitive dust emissions (referred to as open or area source emissions). To avoid confusion regarding terminology, FPEs are generally assumed to include particulates arising from industry-related operations.
ADVERTISEMENTS:
In contrast to ducted (controlled point source) emissions, FPEs escape to the atmosphere through windows, doors or vents. FPEs may result from incompletely controlled point sources, such as furnace charging and tapping; from the handling, transfer and storage of materials; and from poor equipment maintenance and environmentally careless process operation (e.g., leaking furnaces or oven doors). Outdoor industrial processes such as rock crushing may also constitute significant sources of FPEs.
In contrast, area fugitive dust emissions are generally related to particulate matter arising from open (or area) sources as the result of re-entrainment and resuspension by wind, vehicular traffic, construction or agricultural activities.
Fugitive dust emissions include particulate matter re-suspended from road surfaces, exposed surfaces at construction sites and tilled cropland, as well as windblown particulate matter. Strictly speaking, process-related area sources may also be considered to fall within the category of area fugitive dust emissions.
In order to appreciate the rationale for the selection and application of control measures currently in use or being developed for fugitive particulate emissions, it is important to recognise the magnitude of the problem.
Magnitude of the Fugitive Emission Problem:
Traditional control strategies that focus on the reduction of particulate emissions from conventional point sources have not been successful in bringing about attainment of the National Ambient Air Quality Standards (NAAQS) for total suspended particulates (TSP)[The NAAQS for TSP are 75 μg/m3 (annual geometric mean) and 260 μg/m3 for a 24-hour period]. Recent investigations has shown, for example, that more than 60 per cent of the nation’s Air Quality Control Regions (AQCRs) were not meeting TSP standards.
Estimated fugitive particulate (dust) emissions were found to significantly exceed particulate emissions from point sources in over 90 per cent of the 150 AQCRs that were out of compliance. In nearly 40 per cent of the 150 AQCRs, fugitive dust emissions exceeded point source emissions by a factor of 10.
ADVERTISEMENTS:
Open sources account, by far, for the largest share of fugitive particulate materials. Although numerous open sources have been identified, four broad categories may be responsible for as much as 98 per cent of open source emissions. These include unpaved roads, agricultural activities (tilling and wind erosion of cropland), construction activities and paved roads.
In AQCRs where concentrated industrial activity is absent, fugitive dusts are probably responsible for most of the observed TSP standard violations. However, fugitive dusts also contribute significantly to ambient TSP levels in industrialised urban areas. An analysis of 300 filters from high-volume samplers, collected in 14 major cities in the United States, concluded that soil-like mineral matter predominated over combustion products and other types of particulate matter. The annual open source contribution to TSP levels in industrialised urban areas may be greater than 25 μg/m3.
Industrial fugitive particulate emissions arise from poorly controlled point sources as well as from plant site open sources such as haul roads. A number of industrial categories have been identified as significant fugitive emission sources, including pyro-metallurgical operations and processes involving the crushing and grinding of mineral products. Typically, there are many fugitive emission sources within a given industry (e.g., 20 separate sources have been identified for iron foundries).
Even though their total is much less than open source emissions, fugitive emissions from industrial sources may still exert an effect on ambient TSP based on the extremely high values measured in predominately industrial areas. In some industries, fugitive emissions may actually exceed those from point sources, further demonstrating their importance.
ADVERTISEMENTS:
Because of the relatively recent interest in fugitive emissions from open and industrial sources, the data base describing size properties and emission rates has not yet been adequately defined. Nevertheless, information currently available suggests that both categories contribute significant amounts of inhalable particulate matter (particles less than 15 μm diameter) to ambient TSP.
Depending on the source, from 45 to 100 per cent of emissions may consist of inhalable particulate matter (IPM). However, since open source emissions tend to be from ground level sources, the IPM fraction of emissions reaching a receptor (either a sampling instrument or a person) is likely to be increased because of the preferential settling of large particles.
Size characteristics for process fugitive emissions have been assessed by various research institutes for several industries. In the absence of direct measurement, additional data may be obtained by assuming that process fugitive emissions will be similar to previously measured stack emissions, although caution in using this approach is suggested.
A recent review of such data for the pyrometallurgical industries demonstrates that for non-ferrous sources, nearly all of the fugitive emission mass consists of IPM. For ferrous metal sources, the size range is broader but most emissions still fall within the IPM category.
ADVERTISEMENTS:
Emission factors are required for the development of fugitive emission control strategies and the determination of control technology needs. The general approach to this problem is to develop source specific emission factors that can be used to estimate the magnitude of emissions.
For an open source such as an unpaved road, the fugitive dust emission rate can be related to vehicle speed and kilometers traveled, silt content of the road surface and local precipitation history. With appropriate modifications to the last two parameters, this emission factor can be applied to different geographical areas.
For industrial processes, emission factors relate the mass of emissions to the amount of process input or output. Engineering judgment and extrapolation from similar processes have played a large part in fugitive emission factor development and few of the emission factors are supported by extensive test data.
Open and process source fugitive emissions have been identified as significant contributors to ambient TSP levels. Although data have been collected to support this observation, fugitive emission rates and chemical and physical characteristics need to be further defined. This task is especially important for the determination of the fugitive particulate influence on IPM levels, should such a requirement be implemented.
Current Control Measures for Process Fugitive Emission Sources:
The status of industrial process fugitive particulate emission (FPE) controls has been characterised by the terms ’emerging’ and ‘site-specific.’ Control of FPE sources has received attention only within the last five or six years. Sources of FPEs are numerous and quite varied, not only from industry to industry, but also within the same plant. Consequently, control equipment design parameters must be developed almost on a case-by-case basis.
For example, in the iron and steel industry, FPEs from small blast furnace cast houses have been controlled by exhausting all building air to a bag-house, while larger cast houses rely on local hooding to attain control at a more reasonable cost. In the future, it is unlikely that any single method of FPE control will predominate and it can be expected that process systems, in general, will continue to require individual control strategies.
The primary problem in FPE control is collection (capture) of the emissions. Once collected, these diffuse FPEs are transferred via a ventilation system to a gas cleaning device where particulates can be removed. Collection systems for FPEs typically consist of secondary hooding at the local source of emissions, large canopy-type hoods suspended above the source or complete building evacuation. Each method has some drawbacks, especially when retrofitted to an existing process operation.
Large air flows are often necessary, requiring large-diameter, expensive ductwork and fans that draw significant power. Often, the building structure itself must be reinforced because it is not strong enough to handle the additional load of hooding and ductwork. Ductwork routing and ventilation hood placement are often difficult due to space restrictions in a retrofit application. Also, secondary hooding or enclosures may restrict personnel and/or equipment access.
An important control strategy for FPEs consists of minor process modifications such as the substitution of clean scrap for charging in an electric arc furnace (EAF). Construction of wind screens around a bucket-loader transfer operation is another process modification that helps to reduce process-related dust emissions.
Some degree of FPE control can sometimes be achieved with little or no investment in additional equipment by adherence to good housekeeping, operation and maintenance practices. Prompt repair of exhaust hood leaks, maintenance of coke oven door seals, proper handling of bag-house dusts, quick cleanup of spills and other elements of good manufacturing practice all serve to reduce FPEs.
FPEs and process-related area sources account for about 80 per cent of current particulate emissions from steel mills. Retrofitting of FPE controls to existing steel mill sources is often difficult due to space limitations, a lack of process technology, the need for large air flows and required worker access to the process operation itself.
In a recent study of steel mill emissions, it was found that although substantial progress has been made in developing fugitive particulate emission controls for steel mill sources, several major control problems exist. Retrofitting proposed control systems to existing operations has often proved difficult and there is a serious deficiency with respect to data describing uncontrolled emission rates and composition, control device efficiency and control costs.
Available controls for paved and unpaved in-plant roads were found to be at least a factor of 20 times more cost effective than, for example, use of canopy hoods for control of electric arc furnaces. These two sources were found to constitute the largest amounts of both total particulate and fine particulate emissions from steel mills.
The coke battery represents one of the most difficult-to-control FPE sources in the steel industry due to the large number of potential sources, their diffuse nature and the harsh coke plant environment which frequently causes equipment malfunctions.
Although door leaks are caused by a number of factors, they are primarily attributable to warped door jambs, poor sealing of spring-loaded knife edges and poor cleaning practices. Older door designs have not proved very successful in eliminating door leaks.
Consequently, the American Iron and Steel Institute (AISI) funded a Battelle research programme that determined factors causing door leaks through field research and then developed an improved retrofit door seal designed to minimise door jamb warpage.
Control of basic oxygen furnace (BOF) secondary particulate emissions is often hindered by a lack of demonstrated control technology that can be retrofitted at reasonable cost.
For secondary emission control systems, fabric filters were preferred over scrubbers and electrostatic precipitators (ESPs). While partial building evacuation systems for furnace emissions were not found in the majority of cases, there are situations where this may be advantageous over local hooding. The use of a roof-mounted ESP with no fans attached (viz., no pressure drop due to air movement) may be a cost-effective alternative to complete reliance on local hooding connected to fabric filters.
In another survey of two iron and steel plants in Japan, it was found that natural draft, roof-mounted ESPs with special light-weight, widely spaced plates to control FPEs from roof monitors in steel making shops, appear to have promising applications in the U.S. industry.
Rock and Mineral Processing Operations:
Fugitive emissions from the nonmetallic minerals industry (i.e., stone quarrying, crushed limestone; quartz, sandstone, quartzite, granite and crushed stone) have been assessed in detail. Additionally, significant quantities of fugitive particulate emissions were also identified with drilling, blasting, haulage of material and beneficiation. Beneficiation operations may include crushing, screening, grinding and transport of material at a processing plant located either indoor or outdoor.
The controls used by the nonmetallic industries vary according to the process and its location. The only viable control options currently available for drilling are based on wet suppression. Water or water with surfactants is injected into a drilling hole so that the dust created will adhere to the water droplets. Foam injection systems may be used in place of water alone to increase the suppression capability while using the same amount of water.
Beneficiation operations employ wet suppression as well as hood capture and ventilation of emissions to a central control device. Wet suppression is used in virtually all nonmetallic industries although blinding of screens can occur in certain applications such as lime and limestone products preparation.
Waste dumps for asbestos cement manufacturing plants are often located in high population density areas. Asbestos fibers display carcinogenic properties when inhaled and lodged in human lungs.
An engineering study conducted by research institute found that an 87 per cent reduction in air emissions from this fugitive source could be achieved by:
(i) Bagging of fine waste;
(ii) Application of a soil vegetative cover on the inactive dump site; and
(iii) Temporary chemical stabilisation for active piles.
The taconite industry, which provides 75 per cent of the ore charged to blast furnaces for iron making, is rapidly expanding as older high grade ore deposits become depleted. Control methods currently employed include wet suppression and chemical stabilisation, which are applied to such open sources as mining areas and tailing piles, while hooding is used to collect FPEs from crushers.
Coal storage piles at industrial plants represent common sources of windblown particulate matter that may contain a number of trace elements, gaseous hydrocarbons and carbon monoxide. Most piles are controlled by wet suppression or a coating of tar derivatives. However, these practices may generate additional pollutants, such as those released upon combustion of the tar coating in the boiler.
Storage of coal in a pit or silo, where possible, appears to be a more economical and effective means to reduce particulate emissions. The investigations of fugitive emissions from urea and carbon black manufacturing operations showed the prilling tower to be the largest source of emissions from a urea plant. Due to the design of the prilling tower and the particle size of the urea, the scrubber typically used is only about 50 per cent efficient. As with urea, the fine particle size of carbon black makes it hard to contain, thus requiring an extensive housekeeping system to reduce fugitive emissions.
Fugitive dusts are also produced during the transport of fine materials such as coal and construction aggregate, due to the movement of vehicles as well as wind erosion. Use of wetting agents, tarpaulins and chemical stabilisation helps prevent fugitive emissions from trucks and rail cars. Tarpaulin covers are preferred over wetting because they do not affect the material being transported. Chemical stabilisation is used in the transportation of materials such as coal.
Current Control Measures for Area Fugitive Dust Sources:
The data on both emission quantities and their impacts on TSP levels support a strong relationship between fugitive dust emissions and non-attainment in many AQCRs. Fugitive dust sources may account for well over half of the ambient TSP in many AQCRs.
Wet suppression with water with or without wetting agents can provide temporary control for some agricultural and construction sources, unpaved roads, materials handling operations and stockpiles. Since water alone is a poor suppressant due to its high surface tension, chemical wetting agents (surfactants) are added to increase wetting effectiveness.
Use of wet suppression, however, is often not feasible due to water shortage, source size, and temporary nature of control and in many cases, the need to keep the dust-generating material dry. Foam systems have recently been shown effective for rock drilling activity and conveyor transfer systems.
Physical stabilisation is simply the covering of a surface with a material that prevents wind disturbance of that surface. Such materials include rock, soil, crushed or granulated slag, bark, wood chips and straw. Paving of dirt roads is a commonly used approach. Less widespread methods involve covering with elastomeric films, wax, tar and oil.
Chemical stabilisation, sometimes used on agricultural fields, unpaved roads and waste heaps, relies on binding materials to form a protective crust that shields surface dusts from the wind. The effectiveness of chemical stabilisation for a given application depends on the level of activity at that source. For example, required application (rate and quantity) to unpaved roads would be a function of the amount of traffic. Also the effectiveness of continuous spraying of aggregate piles will depend on such factors as the fraction of fines in the mix, the type of stone and the activity of the pile.
Vegetative stabilisation is restricted to inactive sources that will support growth after a layer of soil is applied. Coal piles are commonly stabilised by vegetation after acid neutralisation and mining overburden and gangue represent no particular problem. However, because some tailings often have a deficiency in plant nutrients, a hardy plant species must be considered. In general, selection of plant species is site and source specific and will depend on soil nutrients, pH and pile slope.
The composition of urban road dust is extremely variable due to both natural processes and human activities.
The primary contributors to deposition include:
1. Motor vehicles.
2. Sanding and salting.
3. Pavement wear.
4. Litter.
5. Biological debris.
6. Wind and water erosion from adjacent areas.
7. Atmospheric pollution fallout.
Once materials have been deposited on paved surfaces, they are removed by one or more of the following mechanisms:
1. Mechanical redispersion to the atmosphere.
2. Aerodynamic entrainment (wind erosion) to the atmosphere.
3. Displacement of adjacent surfaces.
4. Rainfall runoff to a catch basin.
5. Street cleaning.
Rainfall and wind erosion, of course, are natural phenomena, but due to their sporadic nature, they cannot be considered as reliable dust control methods. However, it has been determined that a rainfall of 1.27 cm (0.5 inch) can remove up to 50 per cent of road particulate, while heavier rainfalls can wash away as much as 90 per cent.
Re-entrainment and displacement are related to atmospheric conditions, vehicle speed and size and traffic mix and volume. Street cleaning methods include sweeping, flushing, resurfacing and coating. Street sweepers of the broom, vacuum and regenerative air types have been used with generally unsatisfactory reductions in emissions. Less than 20 per cent removal efficiency for particles below 140 μm was noted in one study.
Broom sweepers can actually increase emissions by stirring up dust, moving it to the center of the road and breaking up coarse particles into sizes that can be more readily re-entrained. Flushing, by way of contrast, can suppress fine particulate matter as well as control the larger particles. Its disadvantage is that it must be carried out on a continual basis because its effect diminishes as the pavement and dust dry.
Another approach to control fugitive dust from paved roads is to prevent truck spills and the carryout by motor vehicles of materials from unpaved areas such as construction sites and industrial plants.
Several programmes conducted by Harvard University have investigated conventional control methods for unpaved roads. These include both physical and chemical stabilisation, wetting and oiling. The most cost-effective control as reported by Cooper at 86 per cent control efficiency.
Cost and life expectancy estimates will depend on surface type to be used, conditions of existing surfaces, traffic density, climate and the costs of aggregate and labour. The efficiency of paving depends directly on the actual surface to be treated. Materials may range from semi-permanent bituminous single-chip seal to a permanent asphalt concrete layer several centimetres deep.
Wet suppression with water, plain or mixed with a wetting agent (surfactant), can provide temporary control for unpaved roads. Since water alone is a poor suppressant due to its high surface tension, the addition of the surfactant is necessary to increase its effectiveness.
Dust can also be controlled by application of oil to the road surface as infrequently as once per month. While this method is generally effective, the side effects often outweigh the benefits derived. From a safety standpoint, trucks rolling over an oily road have a greater tendency to slide off the road. Oiling also may be environmentally unattractive due to heavy runoff that is estimated to be 70-75 per cent of the oil applied. Road maintenance may also increase due to the development of potholes associated with oil application treatments.
Chemical stabilisers serve to protect road surfaces from wind and vehicle entrainment of dust by forming a protective crust with the road dirt. In order to ensure the effectiveness of control, the application programme developed must be conscientiously followed. The stabiliser is frequently mixed with water in a ratio of 1:4 to 1:7, then applied to the road surface where it agglomerates dust particles to form a binder. Well-maintained road surfaces become ‘as hard as concrete’ and dust free.
Fugitive dust emissions from agricultural activity often exert strong local effects as the emissions may contain pesticide residues. Wet suppression has a low control efficiency since the soil is continually turned over. Additional problems may include a short supply of water in some regions and the inability of cultivating machinery to carry enough water. In general, control methods are related to good soil conservation practice and efficient farming techniques. Vegetative stabilisation is the most obvious control technique supplemented by windbreaks and wet suppression.
Construction activity has the potential to generate large amounts of dust which, in some cases, may contain hazardous emissions from certain rock and soil types. Wind erosion of stripped land is also a potential source of fugitive dust. The status of control technology for construction activity is similar to that for agricultural activity. Wetting has been applied to excavating activity but continual working of the soil precludes effective control. Stabilising with a binder is an effective control method that is applicable to short-term heaping of excavated material. The control of fugitive dust from vehicle travel is approached with the same techniques employed for unpaved roads.
Summary of Effectiveness of Controls:
Effectiveness of controls for area source emissions, which depends on a number of factors, is somewhat variable. Generally, reductions of emissions by 50 per cent have been reported for unpaved roads, 40 per cent for agricultural tilling (chemical stabilisation effectiveness) and 30 per cent for construction activities (wetting effectiveness). For unpaved roads, only physical stabilisation (i.e., paving) has proved to provide adequate control. Vegetative stabilisation of road shoulders has also been used. Control of agricultural emissions is often achieved by wetting, although generally this source lacks effective controls.
Other technologies for this source are either untried or have been judged poor. Physical and vegetative stabilisation as applied to construction activities has proved acceptable; wetting is not acceptable; and chemical stabilisation has not been evaluated.
With regard to other sources, wetting can be fairly effective for stabilisation of dust from materials handling operations. Physical and vegetative stabilisation has been used with relative success on stockpiles, whereas conventional windscreens are rated very poor. Exhaust ventilation techniques have been beneficial in collecting mining operation emissions, for example, while wetting was generally found to be unacceptable. Stabilisation technologies have not been rated for this source.
Numerous other control methods are available for various sources of fugitive dust emissions. Some of the more important techniques include speed reduction on unpaved roads, street cleaning of paved roads, reduction of fall distances for materials handling and enclosure, hooding and ducting. For example, reducing the speed of vehicles from 64 to 40 km/hr., (40 to 25 mph) traveling over unpaved roads has proved successful in reducing re-entrainment of road dust by over 70 per cent.