The burning of straw and woodchips can be a good way of reducing the consumption of fossil fuels, which in turn will benefit both the environment and the climate. But the burning of straw and woodchips can be a dusty affair, since particularly the use of straw results in the emission of large quantities of very fine dust from the chimney.

This is what scientists from Aarhus University will be trying to address in partnership with the engineering company REKA A/S and electromechanical engineers Magnussen & Speiermann. In a new project granted 2.9 million Danish kroner from the Energy Technology Development and Demonstration Programme they will develop an electrostatic precipitator that can reduce dust emissions from biofuel boilers. Basically, the scientists will further develop a prototype built in a previous project.

- Our aim is to develop a cheap and reliable precipitator that can be used for smaller straw and woodchip boilers, says project manager Erik Fløjgaard Kristensen from the Department of Engineering.

Currently, there are no limits in Denmark on how much dust may be emitted from small-scale boilers in the agricultural zone, but from 1 January 2018 the common European regulations will apply also to these. This means that the threshold for automatically fired boilers will be 40 mg of dust for a normal cubic meter volume of flue gas. There are filters available that can be used to capture the particles from straw and woodchip-fired boilers, but there is a need for a cheaper and more efficient technology.

No dust, please

With straw-burning, large amounts of fine dust are normally emitted from the chimney. The dust is so fine that it cannot be separated using a cyclone. A cyclone removes dust particles from air or gas through its centrifugal powers. This means that the dust is thrown against the wall of the centrifuge and falls down and out through an outlet at the bottom. This is a cheap and effective solution for removing medium and coarse-sized dust from air or gas, but is not good enough to remove very fine particles.

- The dust from the combustion of straw is mainly composed of alkaline salts that have a very low conductivity. This means that in practice a lining of dust measuring as little as 0.10 mm is enough to drastically reduce a traditional filter's cleaning ability, says engineer Jens Kristian Kristensen from the Department of Engineering.

A conventional filter is therefore not suitable for removing particles from the burning of straw. The Aarhus University scientists and REKA A/S have in a previous project developed a prototype that scrapes the dust off rather than knocking it off. This technique results in a dust removal efficiency of 97 percent for straw smoke and 99 percent for woodchip smoke. In the new project the technique will be refined, made stable and upscaled to larger biomass boilers. The final model will be thoroughly tested before it is ready for marketing.

- With the introduction of the new regulations for flue dust emissions in three years’ time there is a strong need to further develop and commercialise the prototype to ensure there is a cheap and reliable electrostatic precipitator available on the market at the time, says Erik Fløjgaard Kristensen.

The two-year project has a total budget of 4.3 million Danish kroner and has been granted 2.9 million Danish kroner from the Energy Technology Development and Demonstration Programme.

For further information please contact:

Academic employe Erik Fløjgaard Kristensen, Department of Engineering, telephone: +45 8715 7659, mobile: +45 2266 8217, email: ErikF.Kristensen@eng.au.dk

Technician Jens Kristian Kristensen, Department of Engineering, mobile: +45 2511 6175, email: jkk@eng.au.dk