Novel metal mesh filter using water-based regeneration for small-scale biomass boilers

Abstract

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.

Evaluation of a metal mesh filter prototype with wet regeneration

“…Given a considerable amount of flue gas bypassed the filter, the measured separation efficiency was lower. According to the previous work on a smaller scale [35], a separation of around 80-90% was expected. As the deformation of the filters progressed and the amount of bypassed gas increased, the separation efficiency decreased to less than 70%.…”

“…A proof of concept was achieved in a previous work [35]. There, it was shown that metal mesh filters can be an effective solution to limit the particulate matter emissions of wood boilers to a minimum.…”

“…The operation time has a direct influence on the separation efficiency as discussed during the earlier work [35]. Depending on the selected values for gas velocity, a higher separation efficiency can be achieved, for example if this is required to fulfil the requirements for subventions.…”

Demonstration of a Top-Lit Updraft Based Pyrolytic Burner with Low Emission Operation and Automatic Process Control

“…7). On the other hand, Baumgarten et al [35] investigated the effects of novel metal mesh filter in small-scale commercial biomass boiler with 50 kW heat output.…”

Strategies to save energy in the context of the energy crisis: a review

“…Biomass boilers are renowned for their high heating efficiency (Kütt et al 2018 ), especially when using solid biomass fuels with 14–23 MJ/kg calorific values (Gravalos et al 2016 ). Wood chip pellets powered by 50 kW boilers have peak mass collection efficiencies of around 98%, with overall collection efficiencies ranging from 70 to 90% (Baumgarten et al 2022 ; Wang et al 2017a ). The combustion efficiency of biomass wood can be further enhanced by compressing it into pellets under high pressure and temperature (Hartmann and Lenz 2019 ).…”