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Types of Air Abatement

Absorption Abatement

Suitable for high-flow, low-concentration (e.g. 1-200 mg/m3 VOCs), low-temperature gas streams, where the pollutant is chemically reactive (or soluble in the case of VOC contaminants). A common use is the treatment of extracted air from reactor vessels with the scrubber liquor typically a caustic solution. Hypochlorite may be used for cyanide scrubbing and odour control. A two-stage system could be utilised, e.g. alkali and hypochlorite scrubbers in series. Water supply and effluent disposal facilities must be available. Monitoring provisions include: • pH, flow rate and level of scrubber liquors and scrubber pressure drop • pressure drop monitoring with alarm • exit concentrations should be periodically monitored under different operating conditions There should be a programme for the regular changing of absorbent in absorption units.

Adsorption Abatement

Carbon adsorption is commonly used as abatement for local extraction points e.g. bulking up and sampling points. It should be avoided where the air steam is moist because the polar nature of the common adsorbents will preferentially adsorb water vapour. For this reason carbon adsorption is not generally suitable for abatement of air emissions from oil processing heating vessels (see Section 2.1.7 on page 50).

Condensation Abatement

Condenser is a term describing a heat exchanger where a gas is cooled to effect a change to the liquid phase. Applications would include emissions from oil reprocessing heating vessel (see Section 2.1.7 on page 50) which would incorporate recovery of oil components. It can be used as a pre-treatment for thermal oxidation, reducing the fuel requirement and the overall size of the oxidiser. To Thermal oxidation Used for VOC control and will usually require the addition of supplementary fuel to support the combustion process. The Operator can offset the cost of the supplementary fuel when there is a requirement elsewhere on-site for the waste heat that is generated.

Biological Oxidation Abatement

Biofilter is a generic term applied to any biological oxidation process taking place in a packed system. This includes the conventional trickling filters, bioscrubbers (microbial population supported in scrubber liquor) or biobeds (packed system using soil, peat and bark). Biobeds have been installed on waste treatment sites for the abatement of odorous emissions. Operational conditions include:

• Incoming air must have a relative humidity of >90% (this may require the use of a humidifier).

• Particulate must be removed.

• Hot gases may need to be cooled closer to the optimal activity temperature for aerobic micro-organisms, generally 25 to 35oC, and the potential temperature rise across the bed of up to 20oC should be taken into account.

• The major operating parameters such as the off-gas temperature and backpressure should be checked daily. • The moisture content in the filters should be monitored regularly.

• A low-temperature alarm should be fitted to warn of freezing, which may damage the filter and may affect the growth of the microbial population. • The packing medium must be supported to allow a fast, even air flow without pressure drop. • The medium should be removed when it starts to disintegrate, affecting air flow (bark is less resistant than, for example, heather).

• Choice of medium and supporting system affects the power requirement to maintain air flow with power to overcome bed resistance. This is the largest operational cost.

• Biofiltration and bioscrubbing have lower operating costs than many other air pollution control technologies for treating low concentrations of biodegradable organic pollutants. Bioscrubbers have the higher maintenance cost of the two. Environmental benefits include low energy requirements and the avoidance of cross-media transfer of pollutants. Consideration should be given to the effect of loss of biomass due to the introduction of toxic compounds, and a stand-by procedure should be developed for such an event.

Cyclones Abatement

Relatively cheap and reliable. Not effective against particle sizes < 10 µm, which may require additional measures, for example fabric filters. Operational requirements include: • monitoring of exit concentrations under different operating conditions

Fabric Filters Abatement

The industrial fabric filter is generally constructed from a woven material or a felted fabric to provide the filtration medium through which the particle-laden gases are passed. Fabric filters are typically used as secondary or tertiary gas cleaning devices with a cyclone, dry scrubber located upstream. Filter efficiency may be enhanced by pre-coating the filter cloth prior to being brought on-line. Fabric filters are not generally suitable for use in moisture-laden streams or those with acidic, tarry or sticky characteristics due to the adverse effects of fabric "blinding" and adherence problems. Pressure drop should be monitored with alarm and measurement of inlet and exit concentrations is required. An opacity meter or particle impingement detector can be used to monitor performance. There should be a programme for the regular cleaning of filters.