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A high voltage electrode electrically charges the particulate matters in the flue gas. The charged particles can then be electrostatically separated.
© CCA – Carola Clean Air GmbH
Electrostatically separating particulate matter
Projektinfo 06/2016

Dust content in the flue gas with recurring chimney sweep measurements made on 94 wood chip boilers
© HDG Bavaria

Structure of the flue gas cleaning system
© CCA – Carola Clean Air GmbH
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Clean heating with wood

Despite considerable advances in firing technology, harmful particulate matter is produced when wood is combusted. Electrostatic precipitators, however, filter up to 90 per cent of particulate emissions from biomass boilers. These therefore enable wood burners to use a wider range of fuel and still meet the tightened requirements of Germany‘s 1st Ordinance on the Implementation of the Federal Immission Control Act. The major advantage: Both new and old heating plants can benefit from the new system.

Heating with wooden logs, briquettes, chips or pellets is becoming increasingly popular. In addition to the fuel costs, users are particularly motivated by the environmental aspects. If the fuel stems from sustainable forestry and is available without long transport routes, it causes on balance only a fraction of the carbon dioxide emissions produced by comparable gas- or oil-fired combustion systems. However, wood burners also emit harmful particulate matter. In the winter time, their particulate emissions often exceed those produced by vehicular traffic.

Therefore since 2015, the 1st Ordinance on the Implementation of the Federal Immission Control Act (1. BImSchV) has stipulated more stringent limit values (second limit level) for emissions of particulate matter and carbon monoxide from biomass-fired boiler systems. In a research project jointly conducted with HDG Bavaria GmbH and CCA-Carola Clean Air, researchers at Karlsruhe Institute of Technology (KIT) have therefore developed an electrostatic filter that ensures that dust emissions are reduced below the new limits.

„We previously evaluated chimney sweep measurement reports for a sample of 94 wood chip burners,“ says Dr Hanns-Rudolf Paur, director of the research project, in explaining the initial situation (Fig. 1). „Although almost all the systems were within the limits permitted at that time, only 10 per cent of the systems would have complied with the currently applicable requirements.“ In accordance with the year of manufacture, the second limit level of the 1st Ordinance on the Implementation of the Federal Immission Control Act stipulates different transition periods for biomass-fired boilers with thermal outputs between 4 and 1,000 kW. There is already a considerable need for flue gas purification. According to the Fachagentur Nachwachsende Rohstoffe (Agency for Renewable Resources), about 850,000 woodfired boiler systems are currently operated in Germany. In principle, particulate emissions can be minimised during the wood combustion by optimising the combustion and fuel quality and through the post combustion of carbon-containing dusts. In practice, however, fluctuating fuel compositions limit the success of these measures. For many wood chip and wood log boilers, it will therefore be necessary to install particle precipitators that have separation efficiencies ranging between 70 and 90 per cent. Modern filter technology makes it possible to use a wider range of fuel and will play a significant role when the utilisation of thermal biomass takes on increasing importance as part of the energy transition.


Electrically charging and separating particles

The precipitator is installed directly in the flue gas path between the boiler and chimney. It consists of ionisation and separation chambers, through which the flue gas successively flows. In the ionisation chamber, a corona discharge on a high voltage electrode ensures that the particles in the flue gas are electrically charged. The flue gas then enters the tubular separation chamber and flows along a spiral, grounded steel brush. The particles are discharged to the bristles and then precipitate. The flue gas cleaned in this manner then passes into the chimney.

The steel brush rotates at periodic intervals over a scraping edge and thus cleans the walls of the collector chamber and its bristles. The dust falls into an ash pan. Almost all that needs to be done in maintenance terms is to empty the ash pan. Paur sees particular advantages in the dry cleaning: „This therefore makes it easier to install and only dry dust is produced. No official approval is required in order to dispose of soot-containing cleaning water. „Thanks to the regular cleaning of the electrodes, the efficiency is maintained even under difficult combustion conditions.

With a corresponding chimney design, the filter can be operated without blowers. The structural principle enables the precipitator to be integrated downstream of the boiler or directly in it. Since no additional openings in the precipitator are required, it is structurally impossible for toxic fumes to escape into the building.

Depending on the system size, the corona discharge and the motor for the automatic brush cleaning system consume between 40 and 100 watts.

On the test rig

The electrostatic fine particle precipitator was operated on test rigs and in field trials for over 30,000 hours. For the service life investigations and quality control, the researchers at the KIT site are operating a test rig with a 100-kW wood chip boiler. It produces at full load a flue gas volume of 350 m³ per hour. This is fed at a temperature of between 150 and 200 °C into the measurement test section designed in accordance with the German VDI Guideline 2066. Here up to three precipitators can be tested in parallel. Particle and gas gauges as well as temperature and pressure sensors provide readings from the flue gas stream upstream and downstream of the precipitator.

„Combining the optimum combustion technology used in our boilers with the precipitator ensures that the limits of the 1st Ordinance on the Implementation of the Federal Immission Control Act are securely met even with varying fuel qualities,“ emphasises Martin Ecker, Managing Director of HDG Bavaria GmbH.

Projektinfo 06/2016:
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Biomass boiler manufacturer – adaptation of the precipitator for HDG wood chip boilers
HDG Bavaria GmbH

Development of the Carola particle precipitator


BINE-Projektinfo 06/2016
(PDF, 4 pages, 771 kB)

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Deutsches Biomasseforschungszentrum gemeinnützige GmbH (DBFZ)