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Living and learning with biomass

Biomass offers a very effective heating solution for many schools but itís important to address the key practical issues that go with it. Andy Owens, biomass technical sales manager with Hoval, explains.

In striving to meet its carbon emissions targets the education sector has been quick to take advantage of the benefits of biomass. Consequently, we have much experience of school biomass projects and it is clear there is a need to take account of more than just the basic engineering design. In particular, the specifier needs to evaluate a range of day-to-day practicalities that accompany any biomass system.

Clearly the first stage is to understand the range of heat loads through the year and to design as much as possible for continual load with minimum cycling. Other considerations include the potential benefits of a suitably sized buffer vessel and whether other heat sources Ė conventional or renewable Ė should be included.

Itís also important to understand how different biomass boilers deal with variable loads. Some may go into standby mode without going out, offering a rapid response to a call for heat. Others may go out completely and use auto-ignition to re-fire when required, providing a slightly slower response time.

Additional issues include the overall suitability of the site in relation to fuel storage and deliveries; both of which also impact on the choice of wood fuel to be used. In the UK that choice usually comes down to wood chips or wood pellets, both of which have their place but offer different characteristics.

Wood chips, for instance, show greater variation in combustion efficiency and physical size and have a lower calorific value than pellets, so for a given heating capacity they will require either higher storage volumes or more frequent deliveries.

Because of the volumes involved, chips are normally delivered by tipper lorry and this will determine the dimensions for access routes and turning circles. Below-ground storage will also be required to receive fuel from the tipper, along with a mechanism (e.g. push/pull floor) for moving the chips from the store to the fuel transport system. A ceiling mounted fuel dispenser can be used to spread the chips throughout the store. If a fuel dispenser is not used multiple fuel delivery hatches will be required to enable the full capacity of the store to be useable.

The irregular shape and size of chips means they need more robust handling equipment with larger augers and motors, compared to pellets. In addition, the distance from storage to boiler should be as short as possible to minimise the number of augers required, the chances of fuel blockages and the cost.

On the plus side, chips cost less than pellets so this may enter the overall equation, though itís worth noting that the cheapest chips will usually produce higher volumes of ash, which has implications for everyday maintenance.

Pellets are usually delivered by a smaller Ďblowerí vehicle with less demanding access requirements and the best results are achieved by using a vertical storage silo as this maximises fuel recovery. The silo may be manufactured from GRP (glass reinforced polyester) for outdoor use, or a fabric silo can be assembled in-situ indoors. Because of their height, outdoor silos may require planning permission.

Fuel and emissions

All biomass fuels produce particulate emissions and the volume and nature of these will vary with fuel quality, as well as combustion efficiency. Using boilers that comply with EN 303-5 Class 3, in conjunction with high quality fuel, is the first step in minimising particulate emissions but some filtration will also be necessary.

Until recently cyclones and multi-cyclones have been the only viable option but their operation depends on particle size and they are only able to remove about 50% of the particles down to a diameter of 10 microns (PM10). However, EC Directive 2008/50/EC (Ambient Air Quality and Cleaner Air for Europe), which comes into force in June 2010, will require control of particles down to 2.5 microns (PM2.5).

Clearly this is a cause of concern for many local authorities but a new ceramic filter has recently entered the market that is capable of removing up to 96% of PM2.5 and PM10 particles. Given the sensitivity of air quality issues it seems likely that most local authorities will require specification of this higher level of filtration in the future.

Living with biomass

It is also vital to get the buy-in of the people who will use the boilers. They need to understand how they differ from more familiar gas or oil, particularly any maintenance aspects. Furthermore, many schools only have part-time janitorial services available so this may have a bearing on the choice of boiler and feed system. Understanding the precise requirements and resources of each school will be key to delivering the best solution.

In parallel, several schools are using their biomass plant as part of their environmental awareness teaching, so the design of the boiler room may need to facilitate observation through a viewing window or similar.

For all of these reasons itís vital to look beyond the engineering design when specifying biomass. But the building services engineer is still best placed to address these issues, perhaps working with suppliers that have specialist knowledge and experience of biomass.