Soot abatement from biomass boilers by means of open-cell foams filters

Currently, fossil fuels such as oil, coal and natural gas represent the main energy sources in the world, but it is well known that these sources of energy will deplete within the next years. So, due to increasing environmental concerns especially related with the use of these fossil fuels, new solutions to limit the greenhouse gas effect are continuously sought. Among the available alternative energy sources, including hydro, solar, wind etc. to mitigate greenhouse emissions, biomass is the only carbon-based sustainable option. However despite an environmentally friendly use of renewable energies, incomplete combustion of biomass can lead to the emission of environmental pollutants as well as substances which are hazardous to health. In comparison to gaseous and liquid fossil fuels, the emissions of particulate matter (PM, or soot) are higher, leading to concerns about the availability of cost-effective techniques to reduce emissions in biomass combustion plants: so filtration devices are mandatory. Actually different filtration systems are available; some have reached a technological maturity and a consequent wide diffusion while others, though promising, have not yet demonstrated adequate features to be considered competitive. In the first class there are electrostatic precipitators, sleeve filters and wet scrubbers, which are standard solutions in medium to large plants. In the second class there are ceramic filters which in principle can be applied both in new equipment and in retrofit of existing boilers, and may be easily scaled according to the size of the boiler. In our previous works we studied the use of catalytic ceramic wall flow filters as soot emission control devices of biomass-fired boilers and stoves. Starting from those results, in this work we investigated the use of a different kind of filters, the open-cell ceramic foams, characterized by a different porosity if compared to wall flow filters. The prepared filters were tested in a customised sampling line at the exhaust of a 30 kW pellets boiler, and regeneration was specifically obtained by a high-temperature electrical heater. PM concentration in the flue gas was monitored by means of a real-time continuous detector and a cascade impactor. The tests evidenced that the higher average pores diameter of the foams, compared to ceramic wall-flow filters, resulted in two main consequences, (i) lower pressure drop, and (ii) a filtration efficiency higher than 50%. These results are very important, in particular because the pressure drop never reached a critical value for the normal biomass boiler functioning: in this way this solution could be a feasible device for soot emissions control. Further studies are still running to investigate the deposition of a catalyst on the foams.