Be alert to biomass fuel needs

from Mike Clare

There has been a bit of a rush to install biomass boilers – but in the flurry of activity, crucial flue requirements are being overlooked. People have got so used to oil and gas over the years they don’t realise a number of other critical factors must be dealt with when using the different types of biomass fuels in order to maintain the safety and integrity of their system.
The first decision is which fuel will be used. This may sound obvious, but you really do have to make a choice as confusion will lead to real problems later on. For example, wood pellets have a higher calorific value, burn hotter and produce less ash than woodchip, which in turn has a higher sulphur content because there is more bark present.
Sizing must be carefully considered and can vary depending on the fuel chosen. Some appliances are sold on the basis that they can burn either fuel, but the fuel supply mechanism must still be altered to accommodate the different fuel sizes. A lesser calorific value will not achieve the required output so, for example, if pellet supplies run out and woodchip is substituted, failure to correctly alter the feed mechanism will cause the appliance output to drop by around 66%. Additional factors include different fuel storage system requirements.
Aesthetics also suffer in this type of scenario. To work properly, the chimney height needs to be at a minimum of five metres on all biomass appliances in order to achieve the right efflux properties. The architect may make a decision not to have the chimney rising higher than say 3.6m, but for the system to work properly this would have to be revised.
Designers, manufacturers and installers are going to have to learn to communicate with each other far more effectively. We had an example recently where natural ventilation terminals had been selected for location adjacent to the the flue arrangement for the biomass boiler – not a situation anyone would want as the natural ventilation would simply draw the waste products back into the building. Fortunately, because we were supplying both elements of the contract, we were able to correct what would have potentially been a considerable error and provide the right advice before the installation had taken place.
We can absolutely understand why, in the planning process, the two types of system were selected but in our efforts to create greener buildings we have to ensure that the systems we choose don’t end up clashing with each other, causing long term problems and ultimately greater expense.
Collaboration, communication and a sense of partnership among those who have the expertise to share and those designing the buildings is essential if we are really going to make a difference in the future.
In some instances, planning for cleaning access is not being done properly. Access is required because particle build up occurs in the horizontal sections and although the incorporation of flue gas cyclones is said to ease the problem of Pm10 particles -the worst kind for air quality purposes – build up will still occur.
To date we have found that maintenance is not being properly undertaken, which is extremely dangerous. Our anecdotal evidence suggests that most people under a certain age do not appreciate the reality of wood burning, a reality that means when you burn wood you produce ash and other particles that must be dealt with – or ignored at your peril.
Add to this the Clean Air Act requirement that stipulates during full load conditions the efflux velocity for appliances up to 2500kW should not be less than 6m/sec, and you have a challenge on your hands. Achieving those figures is virtually impossible on condensing appliances and difficult on woodchip because of the low flue gas temperatures. With pellets it is possible but only by oversizing the flue, then using the excess draught to reduce the terminal diameter. This builds efflux velocity and improves the potential for good dispersion.
People need to think carefully about the outcomes they are looking for and turn to those of us who have been dealing with a wide range of fuels and flue requirements over the years so they can get some practical help in finding the best solution. Time and again we stress the need for customers to involve us at the design stage. With the correct design input we can make sure that the flues are correctly sized, that they achieve the right efflux velocity and they suit the system.
Advice should be sought at the earliest possible stage of the planning process so that headaches, costs and, most significantly, dangers, are minimised from the outset – so come and talk to us, we really want to help!