Biomass boilers are well recognised, but what do they burn?
In the UK the introduction of the Renewable Heat Incentive has meant that the definition of biomass fuel has become more difficult to understand. The main driver for this are the emissions that the boilers have in addition to CO2 being of acceptable levels.
These limits are 30 grams per gigajoule (g/GJ) net heat input for particulate matter and 150g/GJ for NOx.
Each boiler range has to be tested with the fuel that is going to be used within it. Each fuel has a definition.
Pellets are the most straightforward for a homeowner as there is an ENPlus quality standard market on bagged pellets or on the website of the pellet producer.
All accredited wood pellet boilers have been tested to a European Standard EN303-5, met the standards of the UK Microgeneration Certification Scheme, and have a valid emissions certificate. Key to this is that the boilers were tested on ENplus pellets. So these are the only pellets that you can buy and use. To claim your RHI you will have to keep records and be able to show that you are burning ENplus pellets.
Wood Pellet Boiler Product Example
Pelleburn 15kW wood pellet boiler will burn ENplus pellets only. When it does this it has been tested to show 9g/GJ for Particulate Matter and 81g/MJ for Nox. You can find a copy of the emissions certificate here. http://rhieclist.org.uk/wp-content/uploads/2014/04/NES_PLB_RHI.pdf
Logs are measured by moisture and by whether they are “virgin” or used. This potentially creates 4 categories.
Virgin timber – Very dry or kiln dried logs or logs with less than 25% moisture
Used timber – waste wood with no contaminants e.g. Pallets, or waste wood with contaminants e.g. MDF, Plywood, Chipboard
Wood Logs product Example
Pyroburn 30kW gasification boiler can burn wood with up to 25% as it was tested on wood with a moisture content of 12.01 percent. Had the wood in testing been much drier, e.g. 6% then there would be a limit of 12 percent dry wood in the boiler. http://rhieclist.org.uk/wp-content/uploads/2014/05/NES_PB_RHI.pdf
Wood burns much better and cleaner when the wood is dry. The testing process and regulations ensures that if you buy and use a log boiler you take care of what you put in it. For instance the following are attractive, but not allowable
- Newly cut timber in excess of 25% moisture. Fuel that you “find” in woods, or offcuts from timber merchants, or branches from a tree surgeon. This fuel will not burn well. You do not get the heat but your neighbours get the smoke.
- MDF, Chip board or painted wood may burn, but it gives off lots of fumes. Unless there are very specific cleaning processes within the boiler to deal with paints and glues then the fumes produced will be toxic. This category of fuel (B2) cannot be burnt in the majority of boilers, and certainly no domestic or small boilers.
Logs can transformed into wood chip and sawdust and is subject to the same categories as above. Wood chip is generally quite bulky, so produced in local areas rather than pellets at one or two places. It varies in quality and next year (2015) suppliers will need to state that their fuel meets both a moisture standard and has no contaminants. We are excited about our new boiler The CB 160 which will shortly be released into the UK market.
Sawdust does not burn well and can cause back burning (fire can go back through the fuel delivery mechanism) boilers will specifically state whether they can burn sawdust or not. For instance the Catfire range of chip boilers can burn sawdust and has a mechanism to prevent backburning of sawdust.
Sawdust or shavings can also be made into briquettes easily (and pellets, but not so easily) that can burn very well and burn in most boilers.
As part of understanding the moisture content of the wood you would need to have an adequate storage and inspection process. For instance with pellets you need to check that there is not too much sawdust. For wood you will need to store so that the wood can mature. For chip you will need to keep it for a short period to ensure that it does not compost. Other than pellets, you will need to get a moisture meter to make sure that your wood is less than 25%.
If you want to install a biomass boiler you can only burn the fuel or fuels that it has been tested with. You will have to buy these fuels from accredited suppliers of biomass products from http://biomass-suppliers-list.service.gov.uk/find-a-fuel .
If you want to use your own stock of fuel you will have to self-certify and keep more detailed records. Again the fuel must relate exactly to the fuel tested on the boiler.
If you have questions about any of the above then please get in touch as we would be happy to help.