Development of flue gas treatment for small-scale boilers with a focus on particulate matters purification

Sammanfattning: Small-scale boilers have a considerable contribution to particle matter (PM) emissions, which adversely affect health and global warming. According to World Health Organization, particulate matter is ranked as the fifth significant parameter in premature death. Based on the Clean air policy package, which was established in 2013 by the European Commission, it is aimed to mitigate the emission from the combustion of energy sources to half by 2030. In Europe, small-scale biofuel boilers and domestic heating systems release 25% of total particulate matter annually. Thus, finding an economical method for small-scale cleaning flue gas is necessary.This research aims to obtain an economical and efficient system to clean the flue gas from the small-scale biomass boiler. For this purpose, a setup has been built at Luleå University. The setup consists of a boiler (20kW), two heat exchangers, and a generator. The system was constructed from stainless steel. All parts of the system are insulated to decrease heat loss. The flue gas from combustion, which boils the water in the boiler, flows through the absorber and generator. In the absorber (packed bed wet scrubber), the flue gas is contacted with the absorber solution, and at the same time, particulate matter is cleaned. Next, the solution is passed from a filter and purified. Then the solution is pumped, flows through the generator, is condensed, and returns to the absorber. During this study, the stability of the solution in particulate matter collection was tested in the long-term running of the system (6 months), which did not show any deuteriation in the solution ability for particle collection. The system efficiency in particulate matter size D50 (0.8-10 µm) collection efficiency was 42%. Also, the heat recovery of the system was improved to 17.8%.            The effect of different forces on particulate matter in wet scrubber was simulated by Ansys Fluent 19. 2® under different operation conditions. The governing forces on the particulate matter were studied, and the results showed that the concentration gradient has the highest effect on the collection of particulate matter. The effect of concentration gradient is explained as diffusiophoresis phenomena. On the other hand, temperature gradient (thermophoresis) did not strongly affect particulate matter collection. The influence of diffusiophoresis and thermophoresis on different particulate matter (PM) sizes under different flue gas velocities, temperatures, and water vapor mass fractions were simulated. Results demonstrated that increasing the flue gas velocity and particle size reduces the particle collection efficiency. The simulation result was validated against previous empirical models.            In the next step, the effect of operation conditions on the PM collection efficiency was investigated. Based on the simulation results, the effect of water vapor concentration gradient, temperature gradient, and various heights of packed bed material in the absorber was studied experimentally. The measurements demonstrated that the water vapor concentration gradient greatly affects system PM collection efficiency. To improve the system's efficiency, it is suggested to keep the temperature of the solution as low as possible and the absorption solution concentration at the highest applicable concentration. The obtained results showed that the absorber with a fully packed bed material has a better performance in particle collection. The further steps of this study would be to test the system for alternative fuels from the municipality and min waste.