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Size matters (written for Water Energy & Environment)
Imminent air quality legislation is set to have an impact on the way particulate emissions from biomass boilers are managed. Kevin Stones, Engineering and Service Director with Hoval, explains
The levels of small particles in the air, primarily from vehicles but with a very small contribution from solid fuel boilers, have the potential to cause respiratory problems and there are currently regulations designed to reduce particles down to a diameter of 10 microns (PM10). However, EC Directive 2008/50/EC, Ambient Air Quality and Cleaner Air for Europe, seeks to introduce a new control framework for particles down to 2.5 microns in diameter (PM2.5).
Directive 2008/50/EC came into force on 11 June 2008 and must be transposed into national legislation no later than June 2010. At this point, the Air Quality Standards Regulations 2007 will be revoked. 2010 will also be the reference year for targeted reductions by 2020 and while modelling predicts this will fall 4% short of target, the full data will not be collated until early 2012.
This situation has led to some concern about the particulate emissions from biomass boilers, not least amongst local authorities that have been described by Defra in the past as “delivery partners in relation to air quality management”.
Although the majority of such particulate emissions are from internal combustion engines, it is certainly true that all biomass boilers make a small contribution to particulate emissions. And the level of these emissions will vary with the quality of the fuel and the combustion efficiency of the boiler. Consequently, using boilers that comply with EN 303-5 Class 3, in conjunction with high quality fuel, is the first step in minimising particulate emissions.
In addition, there is a strong case for introducing filtration that will improve filtration of PM10 and PM2.5 particles – something that is not achieved by the cyclone and multi-cyclone filters currently used with biomass boilers. This is because cyclones are totally dependent on the mass of the particles for removal, so while they will remove around 50% of the coarser particles they do not remove particles below PM10. This is why the new Directive and its emphasis on PM2.5 has such significance for biomass installations.
An alternative to cyclones and multi-cyclones is electrostatic precipitation, whereby particles are charged and removed from the flue gases in an electrostatic field and transferred to a collector. This is very effective for smaller particles but electrostatic filters tend to be both very expensive and very large – often too large for typical UK plant rooms.
Another option is to make use of ceramic filters that have been tried and tested in industrial applications but in the past these too have been an expensive option. Recently, though, ceramic filtration technologies have been developed that are optimised for biomass applications and do not make the overall cost of a biomass installation prohibitive.
Capable of removing up to 96% of PM2.5 and PM10 particles, ceramic filters can be used with any type of biomass boiler and can be retrofitted to existing installations, so they have the potential to address many of concerns (real or perceived) about particulate emissions from biomass.
Ceramic filters are connected to the back of the boiler, in the same way as a cyclone. Each unit contains a matrix of porous ceramic tubes which are closed at the lower end. The number of tubes in each matrix is aligned with flue gas volumes for each boiler.
As flue gases are drawn through the filter by an inline fan, the gases are able to pass through the walls of the ceramic tubes, while particles are trapped. At regular intervals (timed and/or in response to a pressure drop across the filter) a pulse of air is used to dislodge the particles, which fall into a collection bin.
Maximising biomass opportunities
Biomass has been targeted to play a major role in assisting the UK to meet its emissions targets so it is clearly important to maximise the opportunities to make use of this renewable heat source in the most appropriate ways. Clearly, then, anything that could limit the use of biomass as a renewable fuel for heating could also impact on the UK’s ability to meet our environmental commitments.
Ceramic filters have the potential to meet the challenge of particulate emissions and ensure that biomass remains as a viable element in the UK’s renewable heating strategy without compromising on air quality.
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