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 mass fractions. The results showed that higher flue gas velocities and larger particle sizes decreased the particle collection efficiency. The simulation results were validated through comparisons with established empirical models.
Next, the impact of the operational conditions on PM collection efficiency was investigated. Based on the simulations, experiments were conducted to analyse the effects of the water vapour concentration gradient, temperature gradient, and different heights of the packed-bed material in the absorber. The measurements indicated that higher water vapour concentrations increased the PM collection efficiency. To enhance the system’s efficiency, it is recommended to minimise the solution temperature and maximise the concentration of the absorption solution. Furthermore, a fully packed bed in the absorber provided higher particle collection performance than the half- and quarter-filled packed bed.
Additional measurements were conducted to evaluate the influence of several other parameters on system efficiency: gas velocity, bed material, humidity, solution flow rate, and using water as a cleaning liquid.
Higher flue gas velocities were observed to diminish the contact time between the flue gas and the absorption solution, increasing the effect of the drag force on PM, resulting in reduced collection efficiency. Moreover, an increased flue gas humidity had a positive impact on collection efficiency, primarily owing to its favourable effect on the diffusiophoresis force.
A half-filled packed bed of steel pall rings showed higher performance compared with a half-filled packed bed of ceramic Berl saddles. The analysis also revealed no significant difference in efficiency between the wet scrubber column with a half-filled packed bed of steel pall rings and one with a half-filled packed bed of ceramic Berl saddles. A quarter-filled absorber of steel pall rings showed similar results to an empty absorber, indicating an inadequate pressure drop.
Additionally, water was less effective than salt solutions, providing force in the opposite direction of the wet surface within the absorber and decreasing the particle collection efficiency.
In the subsequent phase, the system’s ability to remove PM from various pelletised fuels was assessed. Each fuel type, including stem wood pellets, mine waste pellets, municipal solid waste pellets, and poplar pellets, exhibited different levels of PM emissions. In the case of stem wood pellet combustion, fine particles with diameters of less than 1 μm were predominant, and the trend was consistent for the other tested fuels. Notably, the system demonstrated 50% efficacy in reducing PM emissions from poplar pellets, which exhibited the highest levels of released PM, and the efficiency may be increased further by increasing the absorber height.
The findings from this research may help in developing more efficient systems for cleaning the flue gas in small-scale boilers.