Chunkwood fuel feeding and combustion experiments in small-scale boilers to provide design suggestions for chunkwood friendly boiler construction

• Chunkwood fuel is suitable for small scale biomass combustion. • Two stoker boilers for chips were tested but failed to feed chunks. • The main fuel feeding problems were located to the outlet of the fuel bin. • Backfire is more problematic with chunks, which requires feeder with airlock. • Partial load combustion is more challenging for chunks than for chips. Chunkwood fuels have a particle size larger than normal chips which enables good drying and storage properties and are therefore appreciated by small-scale users. However, small-scale boilers optimized for chunkwood are not commercially available and the research question is if modern wood chip stokers, selected for having a robust fuel feeding system could feed and combust the fuel. Chunkwood fuel feeding, and combustion tests are performed in a 27-kW and a 240-kW wood chip stoker. Both boilers fulfill Ecodesign emission requirements for carbon monoxide (CO) at nominal load, but further optimization is required to fulfil requirements for dust. Partial load combustion needs to be further studied. There were problems with high stress on the fuel feeding system in both stokers, traced to when excessively large fuel pieces passed the outlet of the fuel bin and when fuel discs became trapped between the auger screw and the lid of the conveyor. Suggestions to solve the fuel feeding problems includes redesign of the fuel bin auger screw to cut oversized pieces, alternatively use of previously developed prototype conveyors that worked. Further studies are required to optimize the fuel feeding system and the combustion performance including a solution for partial load operation.