Chris Franklin MD at Ranheat Engineering Ltd. a leading UK manufacturer of wood combustion equipment, continues his series of articles exclusively for Panel and Joinery Production. This month he writes about boiler design for burning industrial wood-waste that Ranheat manufacture.
Ranheat has nearly always made its systems with a sperate boiler/heater foundation. The boiler or heater is made using a classic “Horseshoe” design or put in another way an open bottom design.
Ranheat and Danstoker A/S have been designing Biomass Boilers for over 50 years, this is long before Biomass Boilers became Fashionable as a source of renewable energy.
Why? Well in simple terms Denmark had no oil, coal or gas as conventional fuels but they did have large amounts of wood and straw. Other Scandinavian countries like Sweden also had vast amounts of Forestry.
Add to this that Scandinavia has long cold winters and it’s easy to see why the Danes got good at designing boilers to run on wood.
Ranheat 3 pass multitube boiler tube nest.
So, there are some essentials that the design of a successful wood fired boiler must follow:
- Large water content to cope with power loss (not be reliant on separate accumulators).
- Open bottom design (horseshoe) boilers to enable many different combustion systems to be used with the same boiler.
- Multi-tube heat exchangers.
- Genuine 3 or 5 pass design for low flue temperatures and high efficiency.
- Wet back boiler for increased efficiency (as opposed to dry back boilers)
- Automated tube cleaning, woodburning inevitably produces soot deposits as burning a solid fuel.
- Separate considerations when using wood-waste as a fuel.
- Wood has a higher ash content particularly man-made boards.
- Long thin boilers with long tube path lengths are hard to clean.
- Ranheat boilers are wide with short tube lengths and are easier to clean.
- Good practice is for a true 3 pass boiler with a larger number of shorter tubes as opposed to a smaller number of longer tubes.
Ranheat 4 pass high efficiency boiler
Equally there are further considerations when using wood-waste as a fuel to the design of a boiler.
Some of these are conditions placed on a system by legislation as the boiler is burning a waste product even if Virgin timber if it is a by-product of a wood manufacturing process.
- Wood-waste burning boilers are not allowed to idle.
- This makes automatic ignition a No-No as large amounts of NOx and CO are likely to be emitted during start-up and shut down as stoichiometric combustion is not possible during these periods.
- Wood-waste often contains fine dusts particularly if burning MDF (flour)
- Glues and resins in man-made boards MDF, MFC, OSB, plywood and hardboard.
As well as the boiler design considerations there are also the permitting requirements:
- All wood-waste burning operations require a permit to burn waste that is a by-product of a wood machining process even if burning Virgin Timber.
- The systems are divided into 3 ranges, up to 50kgs/hr. up to 90 kgs/hr and over 90kgs/hr. There are 3 different permits or exemptions one for each size.
- All installations must have planning permission for the chimney (or written permitted development)
- All installations need chimney height approval from environmental health.
All of these permits and permissions must be in place before the installation goes ahead.
All installations must meet the guidance as laid down in the MCPD (Medium Combustion Plants Directive)
Unlike previous legislation the MCPD is retrospective, and will apply to all installations by 2030, Plants over 5MW had to comply by 1st January 2025 and plants under 5MW have until 2030.
For further information:
Tel: 01604 750005
Email: [email protected]
