Current technology involves improvement in capturing fugitive process emissions (FPEs) and prevention of open source fugitive emissions from industries.
One of the major difficulties in controlling FPEs is that they are often dispersed as low-concentration/high-volume emissions. If the particulate were more concentrated, it could be collected by a high efficiency hood and ducted to a conventional control device.
Similarly, building evacuation would be practical if the particulate were highly concentrated in a small building volume. As an alternative, EPA is actively pursuing the use of chemically and electrostatically treated water spray droplets in cases where the FPEs cannot be hooded or evacuated to a control device.
Fugitive particles entrained in a gas stream may be collected with charged or uncharged water sprays by mechanisms such as diffusion, inertial impaction, interception and electrophoresis. Larger water droplets would enable separation of the dust/water droplets from the gas stream by such methods as gravitational settling or entrainment separation.
Figure 13.1 is a functional diagram of the process anticipated for controlling fugitive emissions. The process steps represented in this diagram could occur concurrently, sequentially or separately, depending on the type of equipment. Two basic approaches include the spray charging and trapping (SCAT) scrubber system and the charged fogger.
In contrast to typical FPE collection systems that use secondary hooding or total building enclosure and evacuation, the SCAT system controls fugitive emissions by diverting the FPEs into a charged spray scrubber located near the source. This is a relatively simple and inexpensive method for controlling FPEs in that it minimises the apparatus required to contain, convey and control the FPEs.
Figure 13.2 shows one of the many possible designs. Air curtains consist of one or more high- velocity air streams, flowing as a sheet, that are produced by one or more air jets. These high-velocity air streams push and entrain FPEs plus some additional air and carry them away from the source. Downstream, water is sprayed co-currently into the gas stream to remove the entrained dust.
After a sufficient contacting distance to effect capture of the gas stream particles, a low-pressure drop entrainment separator is used to separate the water spray drops from the gas stream. A final water treatment step would consist of filtering or separating the collected dust particles from the effluent stream enabling both water recycle and disposal of the dust in a way to minimise its redispersion.
The basic SCAT features, outlined briefly, are:
1. Minimum use of solid enclosure (hooding).
2. Air curtain(s) and/or air jet(s) applied to divert, contain and convey the FPE.
3. Charged sprays of water or aqueous solutions to collect FPE’s and to aid in moving and containing the air being cleaned.
4. Trapping of collected dust and disposal so as to prevent redispersion.
5. Ability to divert crosswinds and contain hot buoyant plumes.
6. Minimum size of scrubber and entrainment separator section.
7. Minimum consumption of water.
Since the design of the SCAT system is basically simple, capital investment should be low. It is expected that the use of air curtains and/or barriers would be cost effective. Air curtains have the potential for deflecting wind, thereby minimising total air volume to be treated. Some potential SCAT applications include metal pouring and foundry operations; raw material charging into furnaces, roasters and converters; dump sites; raw material storage, loading and unloading; sand, gravel and asphalt batching; transfer points on conveyor belts; and coke oven pushing operations.
Experiments have been performed on a 224 m3/min (8000 acfm) bench-scale scrubber to verify the theory and demonstrate the feasibility of collecting fugitive particles with charged sprays. The effects of charge level, nozzle type, droplet size, and gas velocity and liquid-to-gas ratio were determined experimentally.
A prototype SCAT system has been built and tested in crosswind conditions and on a hot, buoyant smoke plume. It has been shown that the system is effective with crosswinds up to 24 km/hr. (15 mph) and with buoyant plumes of temperatures up to 470°C (880°F). It has been found that at aerodynamic particle diameters of 5 μm and below, the use of charged water droplets can significantly increase SCAT particle collection efficiency as illustrated in Fig. 13.3.
The present SCAT prototype uses only a single spray bank and depending on nozzle diameter, a liquid-to-gas ratio as low as 0.41/m3 (3 gal/1000 scf). Higher efficiencies can be achieved by using more water and additional spray banks. Both capital and operating costs for the SCAT system are expected to be lower than for hooded systems.
Various types of wet scrubbers rely on water droplets to scour dust from industrial gas streams and water sprays have historically been used to reduce ambient dust levels from mining and materials handling operations. In contrast to conventional water sprays, charged fog sprays consist of droplets that carry a charge of static electricity and are generally of finer size.
It has been found that many respirable materials are negatively charged and most of the finer (<1 μm) particles are always negatively charged. By exposing these particles to electrostatically charged water fog, the electrostatic effect augments fog/dust contact and the wetted particulates would be expected to agglomerate and settle out.
Initial studies were carried out under EPA sponsorship at the University of Arizona to determine technical feasibility, probable applications and expected performance of the charged fogger.
Potential advantages of this system are:
1. The quantity of water involved would be very low, thereby conserving water resources in the arid southwest. Limited water use would permit the application of fog to water-sensitive materials; for example, flour, cement.
2. A system of this type would be suitable for control of moving dust sources (e.g., trucks, sweepers and front-end loaders) where conventional methods cannot be applied.
3. Charged water fog might be used for preagglomeration of dusts before final collection by other means. One example might be cyclones, which are generally not efficient in collecting dust below 10 μm in diameter. If the respirable dust were agglomerated before it entered the cyclone, it might be removed by the centrifugal action.
In laboratory experiments conducted at the University of Arizona, the charged fogger was used to enhance particulate and gaseous collection primarily due to preagglomeration effects. In a series of tests to improve cyclone performance, greatest reduction of dust (viz. 71 per cent) occurred with a positively charged fogger with low water requirements. Preagglomeration using the charged fogger accounted for up to 52 per cent particulate collection in the 100-150°C (212-302°F) temperature range. Work is also under way involving the use of additives to reduce surface tension and to improve dust/water contact.
In a study performed recently on the applicability of the charged fog spray for primary lead and copper smelters, several important conclusions were reached. Using uncharged sprays, a minimum collection efficiency was noted for 2 μm diameter particles, whereas no such minimum appears to exist for charged sprays. Therefore, some improvement in collection of respirable dust can be expected from charging. Charged sprays appear to be best suited to localised sources of dust, suspended in a low velocity or stationary gas stream.
The charged fog sprays are not expected to be satisfactory for high temperatures or very turbulent, open areas encountered in many of the major sources of primary smelter fugitive emissions such as converter leakage or furnace tap-hole emissions. Finally, at practical water application rates, the nature of charged fog sprays limits them to a maximum of 50-60 per cent collection efficiency compared to 95 per cent for building evacuation.
Table 13.1 summarises the utility requirements and control efficiencies determined in the recent study. While both capital investment and energy consumption are lower compared to building evacuation, the reduction of total particulate and elemental lead emissions is also lower. This is due to the larger number of sources covered by a building evacuation system. For equivalent degrees of control, it appears that charged fog sprays require approximately 10 per cent of the capital and utility requirement for building evacuation.
The problem of re-entrained dust from unpaved roads can be solved if the surface of the dust can be stabilised. Such measures will include application of water, oil or chemical surface agents. With respect to unpaved roads, application of these materials has been shown to be less cost effective than paving. Advances in fabric development now provide an alternative to the use of stabilisation for unpaved roads.
In this approach a civil engineering fabric is laid over the soil to help support and contain the overburden aggregate. It helps to spread the concentrated stress resulting from heavy-wheeled traffic over a wide area, siphons away ground water and contains fine soil particles in the roadbed that can otherwise contaminate the road ballast.
The fundamental concept behind the use of a civil engineering fabric or ‘road carpet’ for the control of fine particle emissions from unpaved roads is prevention of vortex re-entrainment by isolating vehicular traffic from entrainable particles. As a vehicle passes over a poor load-bearing road surface, air compression and expansion of the road results in a draft that ‘pumps’ dust into the air. In addition, commination (caused by slippage between the road and vehicle tyres) contributes a major portion of the re-entrained emissions from unpaved roads.
With a road carpet, large aggregate is prevented from settling, while the newly deposited fines (<70 μm) are filtered by gravitation and hydraulic action down through the fabric away from vortex re-entrainment. The important characteristic of being able to transport water along the plane of the fabric aids in subsurface stabilisation.
Ambient concentrations downwind of the fabric/aggregate road were found to be lower by an average of 44 per cent, with a range of 30-70 per cent, compared to a control road cross section without the road carpet. Wind speeds in November were noted to be lower than during April, which corresponded to greater emission reductions during November than during April.
Data for the control of particles specifically in the inhalable size range (diameters less than 15 μm) obtained from the Fort Carson testing apparatus are shown in Table 13.2. Concentrations of respirable dust generated from vehicles passing over the fabric/aggregate road are shown to be reduced from 26 to 53 per cent, with an average of 43 per cent, compared to the control road. Again, the reduction was slightly greater at lower wind speeds.
A number of studies have been conducted to evaluate street cleaning programmes and sweeper effectiveness. However, these studies have almost exclusively centered around existing street cleaning methods and practices. Results have shown primarily that such existing practices are relatively ineffective. At present, street sweepers are not designed to collect inhalable particles that are dispersed during street cleaning. EPA has identified the research needs in this area as focusing on improvements to existing equipment with emphasis on greater removal efficiencies.
Thus, a special-purpose street sweeper programme has been developed in USA, Europe and other countries. The objective of this study is to retrofit existing vacuum sweepers with an air pollution control system designed to collect 90 per cent of the inhalable particles dispersed by the sweeper. The control system uses air jets, hooding and spray scrubbing to capture and retain particles for subsequent disposal. Analysis of conventional street sweeper operation indicates that the gutter broom and leakage from the ‘pickup hood’ are the major sources of inhalable particulate emissions from the regenerative type vacuum sweepers. Therefore, the particulate control approach consists of containing emissions from these sources.
The recirculating air sweeper design is used so that only a portion of the total air flow requires scrubbing. This minimises the size and water requirements of the scrubber and creates a positive influx of air to the pickup hood, thereby eliminating leakage. The gutterbroom emissions are controlled by interactive hoods that are specifically designed to collect the dust dispersed by the gutterbroom and convey it to the hopper, while not restricting the gutterbroom’s ability to sweep the street surface and gutter.
Preliminary test results are presented in Fig. 13.5, showing street dust density before and after sweeping as a function of distance from the curb. It is observed that street dust is generally concentrated in the gutter area. Removal efficiencies calculated from the measured data shown in Fig. 13.5 range from 75 to 97 per cent, which is a substantial improvement over the performance capability of the best conventional vacuum sweepers currently available.
The major breakthrough has been the development of an effective gutterbroom head. Additional power requirements are minimal since the sweeper provides sufficient blower power, pumps and auxiliary power for the required service. The improved street sweeper could also be used for special problems such as sweeping roads at heavy industrial sites and after sanding and salting operations used for general snow and ice control on any paved road.
Screen fencing has historically been used to reduce glare along highway medians and to reduce snow drifting on roads. Screening systems can also be designed to reduce wind velocity, creating a significant lee zone and helping to control the migration of fugitive dust. In Sweden windscreens are used to confine and reduce fugitive dust emissions from iron ore, coal, coke and gypsum storage piles.
Windscreens have also been installed for glare and snow control in the United States. Where this system can be sited in the prevailing upwind direction from open storage areas, it provides continuous protection without the potential undesirable side effects attendant with wet suppression using water and/or chemicals or the impracticably in certain cases of confinement by covering or enclosure.
The design and siting of a wind fence for optimum wind reduction for a given application must include the following factors:
1. Windscreen positioning.
2. Windscreen height and width.
3. Windscreen curvature.
4. Windscreen permeability.
5. Size of the windscreen elements or openings.
6. Terrain roughness.
7. Turbulent intensity in the incident wind field and in the air flow downwind of the screen.
The heart of the system is an extremely tough and durable knitted polyester screen secured by weather- resistant fasteners that enable the screen to be adapted to a variety of support structures. Generally, a 50 per cent screen porosity has been found to be an optimum balance between wind speed reduction and created turbulence. Typical field data, reported for a fence height of 0.5 m (1.64 ft), indicate that an effective windscreen should give a mean velocity reduction of 50 per cent up to 10 fence heights downstream and a maximum reduction of 70 to 80 per cent 1 to 5 fence heights downstream.
Tests conducted with the 50 per cent permeable fence for an incident wind of 4.2 m/sec (13.8 ft/sec) showed a reduction in wind velocity of 60 per cent. This result is comparable to the efficiency of the 65 per cent permeable fence tested at an incident wind speed of 3.0 m/sec (9.84 ft/sec). In this case, however, there is no apparent zone of stagnation separating the ‘bleed flow’ through the windscreen from the ‘displacement flow’ that passes over the fence.