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Emissions control: Ceramic solutions

25 January 2013

As more attention is paid to particulate emissions from biomass boilers, forthcoming air quality legislation is set to force a re-think on how we handle these emissions.Kevin Stones, engineering and service director with H

As more attention is paid to particulate emissions from biomass boilers, forthcoming air quality legislation is set to force a re-think on how we handle these emissions.Kevin Stones, engineering and service director with Hoval, considers the implications

While there has been considerable discussion about the impact of vehicles and other heavy polluters on air quality for some years, the recent growth in the use of biomass boilers means that particulate emissions from biomass are beginning to come under closer scrutiny.

This is because EC Directive 2008/50/EC, Ambient Air Quality and Cleaner Air for Europe, creates a requirement for tighter control of the smaller particles that are not filtered by traditional mechanisms such as multi-cyclones.

Consequently, we will soon be faced with a need for more efficient, retrofittable filtration of particulate emissions from biomass boilers. 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 we'll be 4% short of target, the full data will not be collated until early 2012.

Size matters Of particular significance in these regulations is the size of particles that will be controlled. In the past, emphasis has been on particles with a diameter of 10 microns or above (PM10).

However, the new Directive will seek to introduce a new control framework for particles down to 2.5 microns in diameter (PM2.5).

As noted above, the majority of these particles in the UK come from vehicle emissions but all biomass boilers also make a small contribution to particulate emissions. The level of emissions will vary with the quality of the fuel and combustion efficiency of the boiler, so using boilers that comply with EN 303-5 Class 3, in conjunction with high quality fuel, is the first step in minimising particulate emissions.

Of course, many biomass installations already use a cyclone or multi-cyclone to remove particles from flue gases. However, 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.

One alternative to cyclones and multicyclones is electrostatic precipitation, whereby particles are charged and removed from the flue gases in an electrostatic field to a collector. This is very effective for smaller particles but electrostatic filters tend to be expensive and often too large for typical UK plant rooms.

However, there is now a ceramic filter that has been optimised for biomass use without making 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 concerns (real or perceived) about particulate emissions.

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.

Moving forward Given that some local authorities have expressed concern about biomass in urban areas, there is a concern that failing to find a suitable solution could severely curtail the potential for biomass in the UK. This, in turn, would have a significant impact on our ability to meeting the country's commitments to reducing carbon emissions.

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.