The combustion of natural untreated wood in firewood stoves generates ashes with high contents of minor and trace elements. Those ashes can be disposed via garbage collection. Those ashes can also be used as a fertilizer. This study investigates the elemental composition of four natural untreated wood species (beech, oak, spruce, and fir) and their ashes produced in a firewood stove. In total, 30 minor and trace elements were analyzed using inductively coupled plasma-optical emission spectroscopy. With a complete set of fuel and ash data, possible effects of ashes from untreated wood on the environment are discussed. They contain nutrients in concentrations so that ashes are comparable to commercial fertilizers, but they also contain toxic trace elements in concentrations at or above limits given by relevant German legislation. However, the trace elements are not only potentially harmful but also valuable. Therefore, ashes are also valuable in terms of an urban mining prospectus because they do contain certain elements in comparable concentrations as commercially used ores. It is therefore necessary to separate the trace elements from the ash matrix before it is put on soil, and the separated trace elements need to be processed to crude materials for demanded rare earth elements.

Particulate matter emissions are a key issue of modern biomass boilers. A novel gas cleaning method using a metal mesh filter combined with water-based cleaning was developed and tested. The filter was tested batch-wise. Flue gas of a commercial 50-kW boiler was filtered until a pressure drop of 2000 Pa was reached. Afterwards, the filter was regenerated. The initial prototype used ultrasound in order to remove the filter cake from the filter candles. Regeneration was complete and, even after boiler malfunctions producing tar, the filter cake could still be removed. Given the good results, a second cleaning mode, flushing the filter candles with water, was tested. The results were as good as with ultrasonic cleaning. Peak mass collection efficiency was very high with 98 ± 2% (burning wood pellets). However, directly after cleaning, the first layer of filter cake has to be developed. In this initial phase, collection efficiency is low. Service time until maximum pressure drop was reached depended on the gas velocity. Using pellets as fuel, at a gas velocity of 66.6 m/h, 12-h service time was reached and 4.1 g dust was collected per square meter filter surface, while at 33.3 m/h, service time increased to 55 h and collected dust to 13.9 g/m2. Using low-quality wood chips, the raw gas dust loading was much higher but also the maximum loading of the filter was higher with 13.3 to 28.9 g dust separated per square meter. Still, the service time decreased to 3.4 respective 38 h. Peak collection efficiency increased to 99.5 ± 0.8%. The overall collection efficiency including the buildup of the filter cake depends on the gas velocity and fuel. It ranges from 74 ± 4 to 91 ± 1%. The feasibility of the filter concept could be proven, and further development towards a commercial application is in progress. Metal mesh filters with countercurrent cleaning showed a high potential given their simple and robust design, as well as high collection efficiency.

Pollutants from combustion in wood‐burning stoves are influenced by many factors, including the exhaust gas system. This is responsible for the air supply due to the draught generated. Contrary to results reported in literature the experiments in this study show no effect on temperature related to increasing air supply and a total dust decreasing at higher draughts. The results also show no significant impact of the draught on gaseous emissions (CO/OGC/NOx). The most significant loss was determined at thermal efficiency, which lost up to 13 % absolute with increasing draught.

Die Studie zeigt den Einfluss der Zersetzungsprozesse bei Buchen‐ und Kiefernholz auf die Brennstoffqualität und Verbrennungschemie in einem Scheitholzofen. Insbesondere bei einer volumenbezogenen Betrachtung der aufgegebenen Holzmenge nehmen die Abbrandzeit und die Energieabgabe mit dem Zersetzungsgrad ab. Die gasförmigen Emissionen sowie die Staubemissionen ändern sich, in Abhängigkeit der Zersetzungsprozesse, nicht. Bei den hier durchgeführten Versuchen zeigten sich bei der Verbrennung von Nadelholz generell geringere gasförmige Emissionen als bei Laubholz.

Wood combustion is a major part of the current efforts to reduce CO2 emissions. However, wood combustion leads to emissions of other pollutants like fine particulate matter. A new option to reduce particulate matter emissions is a metal mesh filter with counter current flushing. An automatic prototype was tested under realistic conditions including starts and stops of the boiler. For regeneration, the filter was flushed using water in opposite flow direction. The water was recycled multiple times to limit water consumption. The results are very promising. Regeneration was successful and no signs of decay could be observed over 419.5 h of operation and 234 regenerations. The filter can be operated during all phases of boiler operation, which is a major step forward compared to alternative secondary measures. Separation efficiency was high with 80–86%, even though the filter showed internal leakage, which reduced the separation efficiency. Additionally, waste products were examined. About 1000 l wastewater can be expected to be produced every month, which could be disposed using the communal waste water system, given the low heavy metal loading. A part of the fine particulate matter is unsoluble and has to be removed from the regeneration water before reuse. The unsoluble fraction contains the majority of heavy metals and has to be disposed as fly ash or used for urban mining. Generally spoken, the metal mesh filter is a new, promising option which can overcome limitations of current secondary measures without increasing costs given its simple and robust construction.