There are three main types of fuel feed systems that are used with biomass boilers. These are rotary extractors, free flowing fuel (hopper) and walking floors. All fuel feed systems will have an auger of some type installed. Each type of feed system will have been chosen to be suitable for the site where it is located and each system has potential issues that it is worth being aware of so that can preventative action can be taken.
A rotary extractor can be used on either wood pellet or wood chip boilers. If you use a rotary extractor on a wood pellet boiler there is a risk of damaging the fuel due to the method of extraction, with the smaller pellets getting caught between the extractor edge and the side of the feed tube. There are also some issues that can occur with rotary extraction on wood chip boilers. The fill level needs to be carried out in line with the extractor, if it is overfilled there will be too much weight on the extractor and this will not be able to feed the boiler. If fuel is left stagnant for a long period of time there is a chance it will start to solidify, this is more likely with high moisture content fuels.
A free flowing (hopper) fuel feed system would generally be used in wood pellet installations. This is where the pellets are gravity fed into the boiler. In order for this system to remain fully functional the hopper needs to have smooth sides and sufficient pitch to ensure that fuel will always naturally feed down to the bottom of the hopper. Most gravity-fed problems come when the surfaces of the hopper get too cold, suffer from condensation or frost or allow moisture ingress. These occurrences will lead to the expansion of the wood pellets, with corresponding detrimental impact on the efficiency of the system. In the worst scenario bridging of the pellets can occur when there is any moisture ingress or the storage has sides that are too shallow. This prevents normal feed into the boiler.
A walking floor system is generally used for wood chip boilers as they require much larger volumes of fuel. Due to the volumes of fuel required the fuel quality is likely to be poorer and will potentially have oversized fuel and foreign objects in it. There is often a higher moisture content in the fuel which can lead to it sticking to the side of the silos, therefore this needs to be regularly rotated and moved to ensure all of the fuel is used and not wasted. Care must be taken when delivering fuel into stores with walking floors. Vehicles that are accidentally driven onto them have the potential to cause damage that could go unnoticed when the silos are full of fuel.
Augers are present on nearly all types of fuel filling systems and are a part of the system that can often cause problems. There can be blockages within the auger system which can be caused by oversized fuel or foreign objects. There is also the potential for moist fuel which has been left in the auger for a long period of time to become less pliable.
In order to maintain all fuel feed systems it is important that all motors, bearings and moving parts are well maintained, checked on a regular basis and greased accordingly. Our range of biomass boiler maintenance contracts ensures that there is a level of cover to suit you and your system.