Abstract
The depletion of fossil fuels and the increasing environmental concerns related to their use, lead to the research of new solutions to limit the greenhouse gas effects. Among the available renewable energy sources for the mitigation of greenhouse gas emissions, biomass is the only carbon-based sustainable option; however its incomplete combustion can lead to the emission of hazardous and environmental pollutants, such as particulate matter (PM, or soot). Filtration devices for PM emissions are required in biomass combustion plants, if primary measures are not sufficient to respect the stricter emissions limits set by European Directives. Actually different filtration systems are available, and some have reached a technological maturity and wide diffusion, but they are quite expensive, e.g. electrostatic precipitators, sleeve filters and wet scrubbers. Therefore, a lot of research has focused on cheaper filtration solutions that can be applied both in new equipment and in retrofit of existing boilers, and may be easily scaled according to the boiler size, such as ceramic filters. 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 filters were tested in a customised sampling line at the exhaust of a 30 kW pellets boiler, and regeneration was realised by means of a high-temperature electrical heater. During the tests, 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) lower filtration efficiency. In particular, the pressure drop never reached critical values for the actual operation of biomass boilers, and the filtration efficiency was higher than 50%. Therefore, open cells foam filters are an interesting solution for soot emissions control in biomass boilers and appliances. Further studies are currently carried out in order to investigate the effects of the deposition of a catalyst on the foams.
