Appendix 5: Odour Control Systems
1 Incineration of gaseous contaminants
Thermal incinerators
Thermal incinerators, referred to as 'afterburners' in some industrial applications, combust odorous materials to form mainly water and carbon dioxide. A straight thermal incinerator has a combustion chamber and does not include any heat recovery of exhaust air. The destruction efficiency of the contaminants depends on design criteria, including:
- chamber temperature
- residence time
- inlet contaminant concentration
- compound type
- degree of mixing.
Typical thermal incinerator design efficiencies range from 98-99.99% and above. Typical design conditions needed to meet more than 98% control are an 870°C combustion temperature, 0.75 second residence time, and flow velocities of at least 6-12 metres/second to ensure proper mixing (Buonicore and Davis, 1992).
Resource consent conditions typically require temperatures ranging from 750 to 850oC with a 0.5 second residence time, depending on the application. Sometimes a minimum oxygen concentration is specified. The lower range of temperatures is suitable for easily combustible odorous compounds, while the higher temperatures are necessary for less reactive pollutants and when very high destruction efficiencies are required.
For vent streams with low contaminant concentrations (typically below about 2000 ppmv for VOCs), reaction rates decrease and the maximum destruction efficiency achievable decreases. High destruction efficiencies may also be difficult to measure with low inlet concentrations because of the detection limits of measurement instruments. In these cases performance criteria may be better expressed as a maximum emission concentration (e.g. less than 20 ppmv for VOCs).
Recuperative incinerators
Recuperative incinerator systems use heat exchangers to preheat the waste-gas stream prior to combustion, and may recover heat to generate steam or hot water or to provide process heating. Shell-and-tube and plate-heat exchangers may be used. Shell-and-tube units are more common and have advantages when temperatures exceed 540oC.
Recuperative incinerators have similar destruction efficiencies to thermal incinerators, but they can be limited by the need to operate the heat exchanger at lower temperatures to prevent damage. These incinerators are usually more economical to operate than straight thermal incinerators because they can recover 40-70% of the waste heat from the exhaust gases, but they do have higher maintenance costs.
Suitable design and performance criteria for recuperative incinerators are similar to those for simple thermal incinerators (see above).
Catalytic incinerators
With catalytic incinerators gas passes through the flame area and then to a catalyst bed. The catalyst increases the oxidation reaction rate and enables conversion at lower reaction temperatures than in thermal incinerator units. Catalysts typically used for VOC incineration include platinum and palladium. Other formulations include metal oxides, which are used for gas streams containing chlorinated compounds (USEPA, 1998).
Several different types of catalytic incinerators are available, which are largely distinguished by the method of contacting the contaminated gas stream with the catalyst. Both fixed-bed and fluid-bed systems are used.
Contaminant destruction efficiency is dependent on the composition of the gas, operating temperature, oxygen concentration, catalyst type and space velocity. Temperature and space velocities are particularly important. High temperatures and low space velocities produce increasing destruction efficiencies. Performance criteria of 95-99% destruction could be required for inlet gases with high contaminant concentrations, or a minimum outlet concentration specified for treatment of low-concentration waste streams.
Regenerative incinerators
Regenerative thermal incinerators use direct contact with a high-density medium such as a ceramic-packed bed for heat recovery and to preheat the waste gas. Preheated and partially oxidised gases enter the combustion chamber, where final destruction takes place. Cleaned gases are then directed to one or more packed beds to heat the bed, and the gas flow is periodically reversed.
Regenerative incinerators can use a catalyst rather than ceramic material in the packed bed, which allows for destruction at a lower temperatures. Contaminant destruction efficiencies of thermal regenerative incinerators typically range from 95 to 99%, while catalytic units range from 90 to 99%. Catalytic units have the advantage of being able to remove carbon monoxide from VOC-laden air.
Regenerative incinerators are expensive and difficult to install, are large and heavy, and have a high maintenance demand for moving parts. Advantages include their low fuel requirements, an ability to operate at higher temperatures than recuperative incinerators, and their are suitability for high-flow, low-concentration waste streams.
Flares
Flares are primarily safety devices, which deal with flows of short duration such as an upset condition or an accidental release from a process, rather than a control device that treats a continuous waste stream.
Flares are generally categorised by the:
- height of the flare tip - ground or elevated
- method of enhancing mixing at the flare tip - steam-assisted, air-assisted, pressure-assisted or non-assisted
- candle type or enclosed flare.
Elevating the flare can prevent potentially dangerous conditions at ground level, and also allows the products of combustion to be dispersed. Flares can be used to control almost any VOC stream, and can typically handle large fluctuations in concentration, flow rate, and other characteristics. The primary application of flares is in the petroleum and petrochemical industries, but flares are also common for landfill gas treatment, and biogas from anaerobic digestion of sludge at wastewater treatment plants. Pilot flames can run continuously or by auto-ignition. It is important to monitor the flare to ensure that the flame does not go out in strong winds. Monitoring may be by regular inspection or automatic monitoring and an alarm.
2 Scrubbing and adsorption systems
Scrubbing systems
Scrubbing systems can vary from a simple spray tower to multiple counter-flow packed towers. Packed scrubbers are generally in the form of a tower, with the gas inlet at the base and outlet at the top. The scrubbing liquid flows counter-current to the gas stream. The tower is filled with packing material, which increases the surface area for absorption. Packing materials may be symmetrical in shape (e.g. saddles or rings), or random (e.g. coke, plastic scrap and scoria).
Plate scrubbers operate in a similar way to the packed tower. The scrubbing liquid contacts the gas stream in a series of stages. The liquid enters the top stage, flows across the plate and discharges through holes to the next plate. The gas stream rises through the same holes or openings, creating bubbles or froth where removal of the contaminant takes place.
The scrubbing liquid may be water or a chemical solution. Other solvents may be used to remove substances with a low solubility in water. The scrubbing liquid should have high gas solubility (or reaction), low volatility, be chemically stable and non-corrosive, and preferably have a low toxicity. Scrubbing liquor could include acid solutions, alkaline solutions, hypochlorite, or catalysed systems. Multi-stage systems with different scrubbing solutions are sometimes needed.
Scrubbing systems can be bought 'off the shelf' and can often be trialled for particular applications at particular sites.
Purpose-built scrubbing towers designed for a specific duty may reach efficiencies of 99.99% for certain contaminants. Common efficiencies are in the 90-99% range. The effectiveness depends on inlet concentrations, and whether equilibrium is approached between the gas and the liquid. A disadvantage of scrubbing systems is the production of a liquid waste that requires treatment for reuse or disposal.
Adsorption systems
With adsorption, contaminants attach or condense onto the surface of a porous solid (adsorbent). Carbon, zeolite and polymer adsorbents have been used to adsorb VOCs and other pollutants from relatively low-concentration gas streams. Other adsorbents used industrially include alumina, activated clay, silica gel and molecular sieves. A large surface area is key because this increases the amount of adsorption that can be achieved per unit of adsorbent.
Adsorbents eventually become exhausted when all the surface area is taken up by the contaminant and 'breakthrough' is reached. Monitoring for breakthrough is important. Adsorbents can be regenerated by incineration, or desorption with another gas or liquid, and the contaminant may be either recovered or destroyed.
The most common adsorption systems used in New Zealand use activated carbon. Systems range in size and complexity from small systems designed to remove odours from cooking operations, to complex solvent-recovery systems for the surface-coating and pharmaceutical industries. They have also been used successfully to control odours from asphalt manufacture.
Well-designed adsorption equipment can achieve control efficiencies of 95-98% for VOC inlet concentrations in the range 500-2000 ppm, independent of the recovery or disposal process. If incineration at, for example, 98% efficiency is used for regeneration, total removal efficiencies may be 93-96% (USEPA CATC, 1998). Lower efficiencies are achieved where regeneration is less effective.
3 Biofilters and bioreactors
Biofiltration is where vapour-phase organic contaminants are passed through a bed of material and adsorb to the substrate surface, where they are degraded by micro-organisms. The bed material may be soil, bark, compost or any mixture of these components. Synthetic bed materials are also used. Bed material is either contained in a structure or in a depression in the ground, and the gas stream is distributed through pipes placed under the bed. More information on biofiltration can be found in the appendices of the Manual for Wastewater Odour Management (New Zealand Water and Waste Association, 1999).
Bioreactors are a development of the biofilter and operate in a similar way, but use an inert support medium such as plastic rings, scoria or pumice. The support medium used can vary widely depending on the application. The micro-organisms are cultured as a biofilm on the surface of the support medium, which is supported by recirculating water.
Biofiltration is dependent on the biodegradability of the contaminants. Under proper conditions, biofilters can remove virtually all selected contaminants. Biofiltration is used primarily to treat hydrogen sulphide, organosulphides, organonitrogen compounds and non-halogenated hydrocarbons. Halogenated hydrocarbons can also be treated, but the process may be less effective because the compounds can inhibit biological activity.
Inlet concentrations of contaminants in the gas stream may range from fractions of a part per million (ppm) up to 1000 ppm, or higher. The efficiency of removal is dependent on the system and contaminant. General odour removal (measured by olfactometry) from wastewater treatment plants is expected to be at least 90%. Removal efficiencies for hydrogen sulphide and methyl mercaptan are greater than 99% and 95% respectively (Brenman et al, 1996). Biofiltration efficiency is limited by the inlet odour concentration, because it is difficult to achieve high efficiencies with a low-concentration effluent gas due to residual odour in the outlet from the filter medium itself.
Biofilter design is based on the required gas residence time in the bed. Typical gas-volume to bed-area ratios to ensure adequate residence time range from 50 to 100 m3/m2/hr, with bed depths typically 0.8-1.2 m. The principal disadvantage of biofilters is the large space required. This can be overcome by using stacked systems with synthetic media, or bioreactors, which have less demanding requirements on residence time.
To maintain maximum efficiency, moisture levels must be maintained at higher than 60% and temperature in the 20-35°C range. Control of pH is less critical but should be within the range 4-8. Bed moisture content is very important and humidity of the gas stream should be maintained at near to 100% to prevent drying of the underside of the bed. Overhead watering systems are also common. The filter bed should be maintained in an aerobic condition. A humidifier may be necessary before the effluent gas is passed to the biofilter to ensure that the bed moisture is maintained.
Biofilters have advantages over conventional adsorbers: bio-regeneration keeps the maximum adsorption capacity available constantly, and contaminants are destroyed - not just separated, as with adsorption systems. In biofilters the bed material will need replacing from time to time depending on the media used. Experience shows that bark and compost filters start to break down over time, increasing back pressure, which can cause problems in the process. In any case the bed media should be completely replaced on about a five-yearly basis, but this will depend on the conditions under which the biofilter is required to operate. Monitoring back pressure is one indicator of when the filter will require turning or replacement. Biofilters may be designed in two cells so that one can be isolated while maintenance is carried out on the other.
Bioreactors using an inert bed material normally require the biofilm to be seeded with the most appropriate bacteria and a liquor circulated to provide nutrients for microbial activity.
Biofilters and bioreactors are suitable for many applications, and the variety of processes using them is growing. In New Zealand biofilters are used in wastewater treatment, composting, and the food and animal products industries. They may be applicable for the treatment of VOCs and other contaminants from the surface coating, printing and petrochemical industries, but their success has not been well proven in these areas.
