Controlling dust

Written by: Fiona Miles | Published:
Spittelau waste incineration plant in Vienna

WRAP guidance for EfW plants says dust is a hazard that must be controlled, and those that don’t tackle the problem effectively will be penalised. Fiona Miles from dust suppression and conveying solutions specialist, Renby, looks at dust control strategies for each of the major dust emission areas

Energy from waste and their feeder plants generate large amounts of dust in several parts of the process. Respirable dust creates a serious health hazard and HSE exposure limits are tightly controlled. For EfW plants, the total dust emission limit of 10mg/m3 is a relatively low regulatory limitation compared with what may be required in the future. Especially as other award-winning plants exist, such as the Spittelau waste incineration plant in Vienna which can achieve total dust emissions of less than 5mg/m3.

Where is dust produced in the EfW process?

Dust is not only generated by fly-ash and gas flue dust emissions or even the handling and storage of bottom ash before it is sent on to be used in cement or other by-product processes. The fuel preparation process can be a major contributor to the total dust emission. Sorting, crushing, screening, separating, pelletising and handling of waste prior to incineration or other energy generation method creates dust.

Let’s look at some examples of these and how we can control the dust levels.

Preparation, sorting and screening

As dry waste is handled and moved, often in large open buildings, dust from the waste and movement of vehicles becomes airborne. The sheer scale of the operation means that this is often on a massive scale. Much of the dust contains respirable crystalline silica (RCS) which can cause life-limiting lung conditions. That is clearly bad for workers’ health and morale, and can lead to expensive litigation against organisations that are not doing enough to protect staff.

The size of the buildings can make it difficult for dust collectors and extractors to be effective. Ventilation and extraction cannot be used to tackle dust alone. PPE is uncomfortable and expensive, and creates its own waste problem. Also, it cannot help with the problem of poor visibility caused by dust.

An alternative or additional tool is needed, as we need to get the dust to ground level to suppress dust and protect workers from its effects. Fine fog is often the most effective tool and has proven to dramatically reduce dust emissions in pellet handling halls, SRF, sawdust, and other fuels.

How does fog control dust?

Airborne dust particles are attracted to the fog droplets. The heavier combined dust/water particle falls to the ground. A MicronFog system generates a consistent fog of 10-20 micron droplets. This is a highly effective way to suppress dust. Dust collectors and extractors can be effective to deal with airborne dust in certain situations, but in an entire building application, fog gives those systems a headstart and provides a more energy-efficient dust control system.

Fog is made using drinking water. It contains no chemicals and clears the air using far less energy than dust extraction systems.
A good system will include a water conditioner, particle filtration and, most importantly, a UV disinfection system to kill any pathogens present such as legionella and cryptosporidium. The water is then pressurised and sent through high-pressure tubing to laser-drilled ruby nozzles.

What about wetting the area?

There is a difference between fog and mist. A misting system might make an area damp. In a genuine fogging system, the droplets are so small that flash evaporation occurs, leaving a dry floor and a dry product.

MicronFog systems have been used for kiln-dried sand loading and vegetable processing where it is imperative to keep the product and work area dry.

Storage bunkers

Fog is also used in storage bunkers to keep the dust down. Alternatively, Spittelau extracts the fresh air for the incineration process from its storage bunker, thus generating a partial vacuum which also suppresses dust and transports any airborne dust into the incinerator.

Maximised burn efficiency

Of all the EfW methods, incineration carries the greatest dust emission risk, and yet it is the most popular. It may seem obvious, but maximising the amount of material burned and turned into heat or gas is the best way to reduce ash in the flue or in the bottom of the furnace. Maximum ash burnout means maximised energy production, as well as minimum ash production.

Fly and bottom ash

Dust and other pollutant emissions are controlled from the flue by techniques such as catalysts, sieving and adsorption (the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface), particle inertia, electrostatic precipitation and gas scrubbers. In areas where the flue gas exceeds 60°C, fog is ineffective as it would evaporate too quickly to be of great use.

Fly and bottom ash handling to minimise dust

The fly ash, once collected, is conveyed to a storage bunker ready to be used in other processes such as chemical manufacture and cement. The hot fly ash and bottom ash can be an aggressive material to convey, so a robust conveying method is needed to handle these materials. Pneumatic conveyors are sometimes used for this purpose, but they do carry disadvantages: Excessive wear around bends can limit the EfW plant designer in conveying routes and height of silos. The energy required to pneumatically convey ash can also be great and thus costly.

A more efficient conveyor for this task is a tubular drag conveyor. These are made up of either a cable or chain with circular discs at regular intervals along the cable or chain which push the ash along the tube. The cable versions can suffer from problems with cable wear, tensioning and stretch. This is eliminated with tube chain conveyors.

The tube chain conveyor fits the bill better than any other solution. In Germany, Schrage Conveyors are used by many EfW plants to convey ash. They are sturdy, powerful, and can convey in any plane and around bends. The tubes and discs are designed and materials selected for each specific product they are required to handle; in this case, ash.

Dust emissions are a constant source of concern for the EfW sector, but it is by no means insurmountable. Care needs to be taken to select the most effective and efficient techniques to minimise it. Fog and tube chain conveyors are highly efficient and effective tools in the challenge to control dust.

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