| Front page | | Contents | | Previous | | Next |

Industrial odour control

6 Measures to reduce odour emissions

6.1 Measures relating to production and technical design

It is not possible to give a complete description of how odour nuisances can be prevented or removed. However, below are listed a number of factors, which should be considered in connection with the establishment of new plants and control of odour from existing plants.

Temperature can considerably influence odour emissions in three different ways. The generation of odorous compounds depends on the temperature. Odorous compounds are produced more quickly at high temperatures (summer/winter, not cooled/cooled). Heating of heat-sensitive substances may lead to generation of odorants. Also the release of odorants to the ambient air is influenced by temperature (e.g. processes with hot liquids result in greater release than if cold liquids are used). Very high temperatures may decompose odorants during incineration with sufficient oxygen and appropriate incineration throughput rates.

If pressure in a process plant is changed from a small positive pressure to a small negative pressure by relocating valves or ventilators, the number of possible uncontrolled leakages is reduced.

If the exchange of air surrounding an odorous process is large the emitted odorants will be diluted. But if the odour comes from evaporated material, the emission may increase. In normal circumstances it is best to reduce the amount of air which gets in contact with odorous materials. In this way odour control equipment will be required for smaller volumes of air; the equipment may be simpler and costs be reduced. Due regard must of course be had to the risk of explosion and health hazards in the plant when the air volume is reduced.

If equipment is supervised and maintained frequently, leakages in joints, pump gaskets, boilers etc. can be avoided and odour nuisances be prevented. Ordinary operational practice should include maintenance of equipment to prevent uncontrolled escape of odour. Odour emissions are often a result of plant overloading or chemicals spill. It is therefore essential that processes operate correctly and chemicals are handled carefully. Odour emitted in connection with tank filling operations can be minimised by means of floating covers or smell charcoal filters at ventilation outlets.

Evaporation of odorants from stored decaying material may often lead to odour emission (e.g. food, waste products). Good housekeeping may eliminate the problems. It is recommended to process animal products quickly, and not to store them in the open air.

If preventive measures or changed process parameters are not enough to avoid annoying emissions of odorous compounds from a plant, it must be placed in a building equipped with ventilation and, where required, air cleaning equipment. It may be necessary to have non-opening windows and automatic gate and door closing devices, and to provide a negative pressure in the building. Odour nuisances can be prevented either by cleaning of exhaust air or by dilution in outlets.

6.2 Odour removal techniques

Below is given a brief survey of the efficiency of various equipment for cleaning of air containing odorous constituents. For more detailed information, see for instance (NRC, 1979), (Hansen, et al., 1979) and (Menig, 1977).

A number of methods are used to remove odorants from gases, based on adsorption, absorption, incineration, catalytic oxidation or biological cleaning.

Adsorption

During adsorption gases are retained and concentrated on the surface of a solid substance consisting of very porous material. Activated carbon is used most frequently for adsorption of odorants and poison.

To achieve the most efficient operation of the carbon filter, substances like dust, tar, mineral oil and large quantities of steam must be removed from the gas before it passes through the filter bed, to prevent these substances from clogging up the small charcoal pores, thus reducing their adsorption capacity. Also certain metal compounds quickly reduce the char- coal adsorption capacity, often as a result of heavy oxidation of the coal and destruction of the pore structure. To improve the adsorption capacity of activated carbon for certain purposes the coal is impregnated with various agents so that the substances intended for retention react chemically with the impregnation agent. Activated carbon can often be regenerated in a process where odorants are removed with steam. The condensated steam may present wastewater problems.

Absorption

Odorants are often removed from the air by means of washing in scrubbers. Scrubbers are for instance used to remove odorant compounds with chlorine sulphur and fluoride.

A scrubber is a fairly simple device, which is able to treat large volumes of air. Gas washing in a scrubber is, therefore, often a cheap way of removing odorants from process gases.

In order to achieve the most efficient scrubber operation, an acid, alkaline or oxidising agent is added to the water, depending on the circumstances. Chemicals should be added very carefully to prevent overloading of the plant. In a well-operated scrubber the reaction products are often salts and non-smelling acids.

Heavily oxidising liquids, e.g. hypochlorites, are often used for removal of odorous compounds from fish and meat meal processing plants. Acid gases are needed for alkaline solutions and vice versa.

There are different types of scrubbers, for instance packed tower scrubbers, spray and venturi scrubbers etc. A common characteristic is the effort to make the efficient contact area between air and liquid as big as possible.

Thermal incineration

By thermal incineration odorous chemicals are oxidised into less odorous or non-smelling substances, for instance carbon dioxide, water and sulphur dioxide. The incineration temperature is normally 500-1200 ^oC.

Odour abatement by incineration in combustion plants is well suited in plants with large consumption or energy (fishmeal, meat meal, bone meal factories etc.). The air required for the boiler plant can be taken from the exhaust system. This method allows the factory to clean large volumes of air at low costs.

In plants with serious odour problems and low energy consumption the operation costs of afterburning are often so high that other methods must be used. Here the best and cheapest solution is often a combination of ad/absorption and incineration, and the heavily contaminated air emitted from regenerated plants is then cleaned in an incineration process.

The combustion temperature shall be so high and the residence time so long that the odorants are completely burnt. In order to reduce the operation costs the afterheat must be used to the greatest possible extent. The plant should for instance always be fitted with a combustion air preheater.

Catalytic oxidation

Operating temperatures in catalytic oxidation are lower (250-500 ^oC) than incineration temperatures.

The usual catalyst is a metal, for instance platinum or vanadium, or a ceramic porous material. Given the high price of these metals the catalytic afterburner is normally rather expensive to buy. On the other hand the operating costs are small because the combustion temperature is low.

If catalytic oxidation is based on a metal removing malodorous gas, care should be taken to avoid that the gas contains compounds, which may poison the catalyst. Poisons are for instance silicates, silicones, phosphorous compounds and metal compounds.

Biological cleaning

In recent years new methods for biological odour abatement have been developed. The odours are treated in a microbially active bed or particles with microbes. The filter material is humus-like products (sphagnum, compost, etc.). The filter bed is placed at soil surface, and the odorant gases are fed in through a network of pipes (GUST et al., 1979).

| Front page | | Contents | | Previous | | Next | | Top |