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Seeing the wood for the trees 

When considering biomass heating as part of a carbon-cutting strategy it’s important to understand the implications relating to fuel supply, storage and delivery, as well as the potential issue of particulate emissions. Paul Haddlesey reports 

Biomass is seen as a key component in the government’s carbon reduction strategy and many organisations are now including this renewable fuel source in their own carbon reduction strategies. But while biomass offers many potential benefits it’s important to be aware of the practical differences between biomass heating systems and those using gas or oil. 

“When considering biomass it’s necessary to understand the heat loads, how to address them efficiently and how different boilers handle variable heating loads,” observes Simon Matthews, Director of Energy & Sustainability with Cofely. “But it’s also important to look beyond the engineering aspects to evaluate other factors such as storage capacity for fuel, frequency of deliveries required in relation to predicted fuel consumption and access for delivery vehicles. There may also be planning issues relating to external fuel storage areas,” he warns. 

Biomass fuels are derived from recently living plants, as opposed to the fossil fuels that formed from plants and animals that died millions of years ago. Currently in the UK the most commonly used biomass fuel for heating is wood, either in the form of pellets or chips, though it is likely that bio oils derived from crops will become commercially viable in future years. 

Part of the attraction of biomass fuels is that they are considered to be nearly carbon-neutral. Although burning them emits carbon dioxide (CO2), this is CO2 that was removed from the atmosphere and fixed by the plants through photosynthesis only a few years earlier. Consequently, the carbon footprint of biomass fuels relates only to the carbon emitted during harvesting, processing and transportation. In contrast, when we burn fossil fuels we are releasing CO2 that was fixed over a period of millions of years within just a few hundred years, thus altering the carbon balance. 

Renewable Heat Incentive 

In order to encourage people to invest in renewable heating technologies the government is introducing the Renewable Heat Incentive (RHI) from April 2011. This pays a tariff to any organisation (or individual) using renewable heat technologies such as biomass. Depending on the size of the installation, the tariff paid for using biomass can range from as little as 1.6p/kWh for very large installations up to 9p/kWh for installations below 45kW. The RHI will apply to any renewable heat systems that were completed after 15th July 2009. 

The right fuel 

There is considerable variation between the properties of wood chips and wood pellets. Wood chips are produced by chipping or shredding various sources of wood, including traditionally managed woodlands and short rotation coppice (using fast-growing species such as willow). However, chips tend to vary in size and moisture content (typically 25-35%), so that sophisticated boiler controls are generally required to handle the varying fuel properties. 

Wood pellets are manufactured from sawdust, shavings and finely reduced wood waste (some of which comes from further processing of wood chips), compressed into pellets of consistent size with a consistent moisture content of around 10%. This means they have a higher calorific value than chips and will burn predictably with a consistent heat output.  

“Using carbon-neutral biomass fuels is a very quick way to reduce carbon emissions, especially in older buildings where heating may account for as much as 40% of emissions,” notes George Sands of York City Council, “However, before proceeding with any such project it’s vital to establish which type of technology will deliver the best results.  

“After a lot of research we concluded that wood pellets offered the best results. Not only do pellets give more consistent and reliable combustion compared to wood chips, they also require less storage space, they lend themselves to automatic fuel feeding mechanisms and produce less ash.” he continued. 

In other situations, wood chips may be the most viable option, such as when there is a consistent supply of wood from local forestry and/or short rotation coppice plantations. For example, Pilgrim Hospital in Lincolnshire is using wood chips for its new biomass boiler because it has been able to source a plentiful and consistent supply from local Forestry Commission operations. 

These two projects illustrate the importance of considering all of the implications of fuel choice, including cost, maintenance requirements, storage volumes and deliveries. 

Cheap as chips 

 “Because of the volumes involved, chips are normally delivered by tipper lorry and this will determine the dimensions for access routes and turning circles,” notes Andy Owens of Hoval. “Below-ground storage will also be required to receive fuel from the tipper, and the irregular shape and size of chips means they need more robust handling equipment, compared to pellets. 

 “On the plus side, chips cost less than pellets so this may enter the overall equation, though it’s 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 ‘blower’ 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,” Owens added. 

Day-to-day operation is another important consideration as there are additional demands, such as removing ash, when dealing with biomass boilers, compared to gas or oil. This requires communicating with maintenance staff and ensuring they are ‘on board’. 

“Most people have no experience of solid fuel boilers and, while modern biomass boilers are very easy to use, different procedures are involved,” explains Lawrence Dixon of specialist biomass installer Instatherm. “For that reason we explain the details to the staff before a project begins so they are fully conversant with routines, such as removing ash every fortnight or so, once the boilers are handed over,” he adds. 

Air quality legislation 

All biomass fuels produce particulate emissions and the volume and nature of these will vary with fuel quality, as well as combustion efficiency. Until recently air quality legislation focused on those particles with a diameter of 10 microns (1 micron = 1 millionth of a metre) or more, denoted as PM10. In June 2010, however, EC Directive 2008/50/EC (Ambient Air Quality and Cleaner Air for Europe) came into force and this requires control of particles down to 2.5 microns (PM2.5). 

Traditionally, biomass boilers have used cyclones or multi-cyclones to remove particles from emissions but these are only able to remove about 50% of the particles down to PM10. However, new ceramic filters are now entering the market that are capable of removing up to 96% of particles down to PM2.5. So the emissions control mechanism for any biomass installation will also need to be evaluated. 

There can be little doubt that biomass will continue to make an important contribution to the UK’s carbon reduction objectives, either on its own or integrated with other renewable heat sources. However, it needs to be selected on the basis of offering the best solution for a project, having considered all of the options and practical details, rather than simply to exploit the PR benefits of having a biomass installation.