Innovative filtering technology for fine dust emissions of wood-burning furnaces
The initial situation
A fundamental drawback to the wood heating systems on the market compared with oil and gas heating systems is their emission of fine dust. From approx. 1,000 kW, involved filters are connected downstream that eliminate the fine dust for the most part. Power station technology used (e-filters & bag filters) is disproportionately involved, expensive and large for output ranges up to 1,000 kW.
A metal mesh filter is to be used for the output range of 100-540 kW, which we cover with our "PYROT" design series. This filter should guarantee more cost-effective and yet efficient filtering of the fine dust.
A fine dust filtering system should be an appropriate and thus financially acceptable investment also for the 100-540 kW output range.
Contents and objectives
When wood is burned, basically dust is created. Depending on the boiler and fuel, the total amount of dust breaks down to approx. 1/3 dust > 10 µm and 2/3 fine dust < 10 µm (PM10). The fine dust is considered as respirable and so especially hazardous to health.
Cyclone dust separators are usually used for wood heating systems in the output range < 1,000 kW. This especially makes sense when dusty fuels (e.g. carpentry waste) are incinerated, resulting in a high proportion of coarse dust. The coarse dust can be separated in a cyclone, but cyclones are not suited to separate fine dust.
In summary, there is presently no technology available which enables filtering of fine dust in our most important output range of 100-500 kW at costs well below the boiler costs.
Metal mesh filters, which display good separation of fine dust, are also in use in the stone industry to separate dry dusts. The use of these filters on wood-powered boilers has been thwarted in the past primarily due to the moist flue gas, which at low flue gas temperatures condenses on the filter, resulting in corrosion and forming deposits that cannot be cleaned off. The loss of pressure thus caused hinders the air volume through the boiler to such an extent that a standstill of the boiler comes about in a short period of time. It is necessary to prevent this condensation on the filter.
The procedure will be as follows:
- Integration of a metal mesh filter on a 540-kW wood chip boiler
- Optimisation of the hindrance of condensation through cartridge-type heater
- Measurement and documentation of the amount of flue gas, loss of pressure and effect of cleaning at several time points over the course of one year.
- Continuous recording and visualisation of pressure loss and/or fan speed
- Dismantlement of the metal mesh filter and examination for wear and corrosion
- Integration of the filter control system in the control system for the overall facility
Wood heating systems are substantial polluters in terms of fine dust emission in the form of ash. Over 50% of fine dust emissions caused by small consumers come from their heating system. A fine dust filtering system should be an appropriate and thus financially acceptable investment also for the 100-540 kW output range in order to noticeably reduce the discharge of fine dust by small and medium-size facilities.
The goal is a reduction to below 10 mg/m³ of emissions gas (the present legal limit for facilities < 1000 kW is 150 mg/m³). This would mean substantial easing on the environment and health.
DI Martin Buxbaum