Wood combustion is a way to reduce fossil CO2 emissions from the heating sector. However, it results in the local emission of other air pollutants such as particulate matter. A metal mesh filter with wet regeneration was tested as a novel option to reduce particulate matter emissions. Two different options were considered: Counter-current flushing of the mesh with water, and ultrasound-assisted cleaning.
First, fundamental research was performed using a manually operated prototype. The main focus of the investigations were the filtering processes and the general behavior of the metal mesh. Secondary phenomena which could influence the behavior of the metal mesh and the filter cake, like condensation and tar generation, were minimized, and data regarding regeneration, potential collection efficiency, and operation time were collected.
Based on this research, an automatic prototype for a 180 kW furnace was built, to demonstrate long-time feasibility and to evaluate the entire system under realistic conditions, including condensation processes. The system's furnace was operated both at stable conditions and using a modulating load profile including ignitions, stops, and changes of load, causing condensation and tar emissions.
During the fundamental research, it could be shown that filtration underwent two stages: First surface separation on the mesh and second depth-filtration in the filter cake. The collection efficiency was below 50\% during surface separation, but increased to more than 95% during depth- filtration after a filter cake was formed. The pressure drop during the filtration follows an exponential function, depending on the gas velocity, the dust per m² filter surface, and two coefficients. These coefficients depend on the furnace, the quality of the burnout, and the quality of the fuel.
Both regeneration modes were successful, and it could be shown that collection efficiency and operation time (before a regeneration was required) depend on fuel choice and gas velocity, without any signs of decay due to residues or tarring. At a gas velocity of 33.3 m/h, up to 91 ± 1 % collection efficiency and 55 h (pellets) or 38 h (wood chips) operation time was measured. At 66.6 m/h, the collection efficiency decreased to 74 ± 4% and 12 h (pellets) / 3.4 h (wood chips) operation time.
Long-time operation of the automatic prototype was successful. The filter was operated for 419.5 h, and 234 regenerations were performed without signs of insufficient cleaning as long as particle enrichment in the regeneration water was avoided. No differences between the different regeneration methods could be observed. Ultrasound cleaning requires an expensive ultrasound generator, thus, counter-current flushing is favorable.
Over an entire filtration (including both phases), a collection efficiency between 80 and 86% could be reached.
Additionally, the waste products, insoluble solid residue and regeneration water loaded with the soluble part of particulate matter, were evaluated. Heavy metals found in wood formed mostly insoluble compounds during combustion, which remained in the solid fraction along with other valuable metals in high concentrations. If the filter would be used on a large scale, the use of the solid fraction for urban mining should be considered. The liquid fraction can be disposed of using the communal wastewater system.
In conclusion, metal mesh filters with wet regeneration are a viable option. The main advantage over current technologies is the ability to filter the emissions during all times of furnace operation, including ignition and while modulating load, enabling a 100% filter availability.