Fine Particle Collection in Small-Scale Biofuel Boilers Using Packed-Bed Wet Scrubbers

Small-scale biofuel boilers are one source of particulate matter (PM) emissions, advertently discharging PM into the air and affecting both human health and the environment. According to the World Health Organization, PM was the fifth leading factor of premature death in 2015. To address this issue, the European Commission’s Clean Air Policy Package was established in 2013, aiming to reduce emissions from energy sources by half by 2030. In Europe, small-scale biofuel boilers and domestic heating systems significantly contribute to the total PM emissions. Therefore, it is imperative to find an economical method for cleaning the flue gas expelled from small-scale boilers. The primary aim of this thesis is to investigate the mechanics of PM cleaning and identify the major parameters that influence cleaning efficiency in the case of flue gas in small-scale biomass boilers. To achieve this goal, an experimental setup has been constructed at Luleå University, comprising a 20kW boiler, three heat exchangers, a generator, and a packed-bed wet scrubber. The flue gas generated during combustion heats the water in the boiler, and heats the absorption solution in the generator, then the total flue gas flow through the absorber (packed bed wet scrubber). A packed-bed wet scrubber is used to bring the flue gas into contact with the absorption solution, thereby removing PM from the gas. The solution is then passed through a filter for purification. A portion of the solution is directed to the generator, where absorbed water in the scrubber is evaporated, and the concentrated solution is returned to the absorber. During an extensive 8-month study, the stability of the solution in collecting PM was tested and showed no signs of deterioration. The system’s average efficiency in collecting PM with a size range of D50 (0.8–10 µm) was found to be 60%. Additionally, the heat recovery of the system was improved by 18%. To elucidate the forces acting on PM within the wet scrubber, CFD simulations of various operational conditions were conducted using Ansys Fluent 19.2. These simulations revealed that the concentration gradient had the most significant impact on PM collection, which is explained by the diffusiophoresis phenomenon. However, the temperature gradient (thermophoresis) did not significantly affect PM collection. The influence of diffusiophoresis and thermophoresis on different PM sizes was also examined for varying flue gas velocities, temperatures, and water vapour ma