Renove Biomass

About Biomass

About Biomass

Biomass is a general term for burning with wood. Biomass boilers come in three main forms:

  • Wood Pellets
  • Wood Chips
  • Logs

Wood Pellets

The most common form of Biomass heating systems are based on wood pellets. Pellets are made from sawdust usually generated by saw mills as a by-product of wood production. The sawdust is dried down to moisture levels less than 10%, and then put through a high pressure press to create a firm and even pellet. Wood pellets normally have no added chemicals and can be considered completely green. Wood pellet come in the UK mainly in 6mmm format and have a limited length. They are either supplied in bags, or can be blown into a silo from a truck or dedicated tanker. They are easy to feed into a boiler using either a augur (screw) feed or a vacuum feed (using air to transport the pellets). Wood pellets have a high calorific value which means that the boilers and the pellet store can be relatively small. Most domestic boilers are based on wood pellets.

Wood pellet boilers come in a great variety of sizes, starting with small domestic wood pellet stove boilers, and going up to 1000kW industrial systems. Most wood pellet boilers expect Grade A pellets and conform to a standard like the DIN EN+. This ensures that each and every bag will have consistent fuel quality and the boilers can achieve really high efficiencies.

Wood Chips

Wood Chips are shredded trees, and like wood pellets can be fed into a fully automatic heating system. The chip is bulkier than wood pellets taking 3-4 times the space a pellet boiler would require, and securing a good quality chip is vital to achieving reliable and consistent heat. Most problems related to wood chip boilers come from inconsistent fuel quality (either moisture content, or contaminated fuel) and we normally would not recommend installation of chip boilers on systems less than 100kW in size.

Wood chip boilers use a screw or augur feed system and tend to be bulkier than wood pellets. The fuel has an inconsistent size and so the feed system use industrial motors capable of crushing oversize fragment in the feed. Many chip boilers use three phase electrics as standard. Wood Chip Boiler tend to be more expensive than their pellet counterparts.

Chip can be very cheap to run, with a unit of heat varing between 1.5-2.5p. This makes the systems very attractive when combined with the Renewable Heat Incentive.


Log boilers are batch heating systems and involve manually loading once or twice a day. Like all the other types of Biomass, their performance depends on the quality and moisture content of the fuel you are using. Typically logs should have a moisture content of 20% and no more than 30%. Moisture content above these levels will result in poor system performance and high levels of tar and klinker forming in the boilers.

Log boilers generate a large amount of water in a short amount of time and work best when combined with a large energy store called a buffer. This stores that hot water until it is required by the heating system. A good rule of thumb with Log boilers is that a buffer should have 50Ltrs of hot water for every kw output of the system. This would mean a typical domestic 20kW boiler would require a 1000ltr buffer tank. Due to their size and nature, log boilers tend to be located in dedicated plant room or outhouses and require day to day upkeep (from the owner or a caretaker), saying this, Log boilers can be very cheap to run and attract the same level of RHI as any other boiler system.