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. 

Recent pellet stoves perform excellently under type test conditions. In contrast, typical real life emissions show significantly higher values under usual operational conditions. Consequently, type testing procedures may not account for real life stove operation and, thus, do not allow to distinguish between low- and high-tech appliances. The EU-project beReal aimed at the development of a testing method for pellet stoves that reflects real life operations better and to support innovative pellet stoves that perform well under typical operational conditions. Based on an online survey and field observations, an advanced real life testing procedure for pellet stoves was established reflecting real life user behavior, e.g. regarding different load levels and the ignition phase. A field test was designed at the end of the project to demonstrate the applicability and practical relevance. The field test proved that emission values for beReal at the test stand and in the field stay within the same range. A Round Robin test proved the repeatability and reproducibility of the beReal testing procedure. The beReal method can be reproduced with the same statistical variability or performed even better than the type testing method with exception of PM between different laboratories. © 2017, ETA-Florence Renewable Energies. All rights reserved.

The Wood Stove year 2020 - a part of the living environment

This project is a collaboration between Nibe AB / Contura and SP Technical Research Institute of Sweden. Based on earlier evaluations of six different technology tracks, three tracks were selected to further evaluate: CFD modeling, active control and catalyst technology, with the long term goals of particle emission, control of maximum output and long burning time. A literature review of available sensors for O2, CO and hydrocarbons have been conducted followed by a market research, focusing on developments during the last decade and experiences from biomass combustion. Sensors were long-term tested. The lambda probe works well and is affordable, while sensors for unburnt gases still need to be further optimized for accuracy and long-term stability and also become less expensive to be of practical use. Active control has been applied to a prototype and evaluated with good results. In further work with active control, emphasis should be on safety. A CFD method for stove combustion has led to an industrial application with significant cuts in time and resources. Advanced CFD simulation has been used to examine the impact of furnace temperature on wood conversion and how wood log size can be used to counteract an accelerating power output. Simulation results show that log size has good potential to contribute to an improved combustion, but should be combined with further design of the stove. A rig for evaluation of catalyst coatings has been built and experimental results are promising. CO was in principle eliminated from the flue gas and hydrocarbon concentration were reduced down to a level equivalent to 25-70% of the original content.