Types of Wood Fuel

Log

Logs are easy to obtain especially in rural locations or from your own woodland/wood supply. When buying logs it is best to buy them by volume and not weight because you could be buying unnecessary water. The moisture content of the timber should be below 25% (air dried), otherwise your boiler will be making steam. Generally the heavier the seasoned log, the better the calorific value of that log. Log gasification boilers combined with large storage tanks can give long times between firings depending on the size of the system and heat load placed on the system. The main drawback with logs is that they require more user input to keep the boiler running.

Pellet

Pellets are generally manufactured from co-product from sawmills and other wood processing industries. This is not waste wood or contaminated wood, just the leftover wood from various processes. The raw materials include ground woodchips, sawdust and bark. No chemical additives are needed, the natural lignin of the wood serves as a binder, although sometimes small quantities of maize starch are added as well.

Pellets have a moisture content of 6 to 10% and need less storage than log or chip as they have a higher calorific value for the same volume. Pellet systems are generally fully automated, with the pellets sucked by vacuum straight into the boiler from the store.

Chip

Wood chip is timber that has been shredded in a chipper. It is an easy fuel for automation, with a conveyor belt or screw-feed taking the chips straight from the store into the boiler. Like logs the chipped material needs to be dried to achieve a low moisture content, 25 – 35 % moisture is a good result. Full requirements are provided with each boiler for the type of fuel required. Chip systems give very low running fuel costs, however initial capital outlay is greater than the other technologies. The interest free Carbon Trust loans and The Renewable Heat Incentive do make this an attractive technology for larger premises and those with their own fuel supply.