Passive gas control

Figure 10a. Passive gas control using a permeable trench (U.S. EPA, 1985).

Passive gas control systems control gas movement by altering the paths of flow without the use of mechanical components. There are generally two types, high-permeability and low-permeability. 

The applications and limitations of passive gas control systems must also be understood. They can be used at virtually any site where there is the capability to trench or drill and excavate to at least the same depth as the landfill. Limiting factors could include the presence of a perched water table or rock strata. Passive vents should generally be expected to be less effective in areas of high rainfall or prolonged freezing temperatures. 

FIGURE 16.6 Passive gas control using apermeable trench. 

The cost of passive gas control systems is low. The passive concept has virtually no operating or maintenance costs. However, it is recommended that periodic inspections be made and that the surface gas be periodically monitored in the area being protected to ensure that the systems are performing their intended functions. 

Stone-filled vent trenches represent a second form of passive gas control. These trenches are often used on site boundaries where gas migration has been identified, but such systems can only be effectively used when the landfill depth does not exceed 8m (DoE, 1992a), and they are seldom used at depths greater than 5m. The limit on the effective depth of trench is controlled by the depth to which an excavator may dig. To excavate deeper than this requires specialised equipment or techniques and is not cost-effective. The effectiveness of vent trenches (Figure 8) increases when they are used in conjunction with a geomembrane, placed within the trench along the wall furthest from the gas source. The vent trenches again... 

Passive perimeter gas control systems are designed to alter the path of contaminant flow through the use of trenches or wells, and typically include synthetic flexible membrane liners (FMLs) and/or natural clays as containment materials. The membrane is held in place by a backfilled trench, the depth of which is determined by the distance to a limiting structure, such as groundwater or bedrock. A permeable trench installation functions to direct lateral migration to the surface, where the gases can be vented (if acceptable) or collected and conveyed to a treatment system (Figure 10a and 10b). 

An active perimeter gas control system can have any of the same configurations as a passive perimeter system with the addition of any combination of gas extraction wells, gas collection headers, vacuum blowers or compressors. Their ultimate purpose is to direct the gas to a treatment or utilization system. 

High-permeability passive perimeter gas control systems entail the installation of highly permeable (relative to the surrounding soil) trenches or wells between the hazardous waste site and the area to be protected (Figure 16.6). The permeable material offers conditions more conductive to gas flow than the surrounding soil, and provides paths of flow to the points of release. High-permeability systems usually take the form of trenches or wells excavated outside the site, then backfilled with a highly permeable medium such as coarse crushed stone. 

Low-permeability passive perimeter gas control systems (Figure 16.7) effectively block gas flow into the areas of concern by using barriers (such as synthetic membranes or natural clays) between the contaminated site and the area to be protected. In the low-permeability system, gases are not collected and therefore cannot be conveyed to a point of controlled release or treatment. The low-permeability system can also alter the paths of convective flow. 

High-permeability and low-permeability passive perimeter gas control systems are often combined to provide controlled venting of gases and blockage of available paths for gas migration.15... 

Particles produced in the gas phase must be trapped in condensed media, such as on solid substrates or in liquids, in order to accumulate, stock, and handle them. The surface of newly formed metallic fine particles is very active and is impossible to keep clean in an ambient condition, including gold. The surface must be stabilized by virtue of appropriate surface stabilizers or passivated with controlled surface chemical reaction or protected by inert materials. Low-temperature technique is also applied to depress surface activity. Many nanoparticles are stabilized in a solid matrix such as an inert gas at cryogenic temperature. At the laboratory scale, there are many reports on physical properties of nanometer-sized metallic particles measured at low temperature. However, we have difficulty in handling particles if they are in a solid matrix or on a solid substrate, especially at cryogenic temperature. On the other hand, a dispersion system in fluids is good for handling, characterization, and advanced treatment of particles if the particles are stabilized. 

Active gas control systems utilise energy to "puU" the gas from the landfill waste. The gas collection systems usually comprise an array of interconnected vertical, or in some cases, horizontal, perforated pipes within the waste, through which gas is abstracted. The systems described under "passive control" (above) may all be modified for active gas abstraction. The collected gas may be either flared, or if sufficient gas is present and the economics are viable, may be burned as a direct heating fuel or as a fuel in electricity generatiorL... 

The advantages of an active gas control system are that the abstraction can be controlled, the reduction of within-waste gas pressure will be more effective than with passive systems, and because in most situations, the collected gas is burned, the atmospheric pollution potential is reduced. Although gas flaring does not remove all trace components and bulk gases, the concentrations of each are significantly reduced. 

This list shows that a lot of relevant information must be gathered over a period of time before an appropriately informed decision can be made. Once this information has been collected and assessed then the most effeaive gas control systems can be designed and implemented. Gas control may be achieved both beyond th building and within the building. Gas control beyond the building may be either active or passive and will utilise the types of control described above. 

Under-floor voids are often used to aid gas control, whereby the underfloor flow of air, which may be either actively or passively undertaken, is used to dilute gas entering the void. The most effective use of this method would also require the use of an impermeable membrane barrier below the void, as described above. 

Many systems rely on "passive" air flow in which the pressure differential across an underflow void induces air flow. Semi-passive systems utilise rotating cowels, or similar devices to induce air flow as wind flows over them. Active gas control is carried out by means of gas blowers which may be linked to gas sensors and which will cut-in when a lower gas level is exceeded. If the fans are ineffective, and an upper gas level is subsequently exceeded then alarms will sound to signal building evacuation. 

Inflam gas control The cordainment has inerted anno dtere during operation passive Conuinmeni is always mirogen-filled, besides, a recombiner system prevents accumulation of oxy/hydrogen... 

Inflammable gas control Flammabilit control system passive Passive auto-calalytic recombmers... 

Inflam gas control Catalytic recombmers Passive System under R D stage... 

Inflam gas control not necessary passive Inert gas environment (N,)... 

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