Facing up the biomass emissions

The recent growth in the use of biomass boilers means that particulate emissions from biomass are beginning to come under closer scrutiny. And while biomass still accounts for a tiny proportion of the UK’s particulate emissions, this contribution clearly has the potential to increase with the growing use of biomass.

In parallel with these developments, EC Directive 2008/50/EC, Ambient Air Quality and Clearer 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, retrofit able filtration of particulate emissions from biomass boilers. Hoval’s new CF ceramic filter, capable of removing up to 96% of particles of 10 and 2.5 microns in diameter, is discussed in more detail below.

Firstly, it’s worth reviewing the background to this situation. Directive 2008/50/EC came into force on 11 June 2008 and must be transposed into national legislation no later than June 2010. 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).

Size matters

Biomass boilers make a small contribution to particulate emissions, compared to traffic, and 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 202-5 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 multi-cyclone 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 both very expensive and very large – often too large for typical UK plant rooms.

Until recently there has not been a financially viable alternative but Hoval has now optimised a ceramic filter for use in biomass installations – 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 retrofitted to existing installations, so they have the potential to address many concerns (real of 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 gasses 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

There are already strong indications that the Directive will be largely enforced through local authority planning permissions and Defra has stated that “local authorities are delivery partners in relation to air quality management”

Indeed, some local authorities have already expressed concern about biomass in urban areas, so there is a very real 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. Hoval CF 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.

For further information please call 01636 672711