Small-scale boilers significantly contribute to particle matter (PM) emissions, which adversely affect health and global warming. According to World Health Organization, particulate matter was ranked as the fifth significant parameter in premature death in 2015. Based on the Clean airpolicy 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 efficient system to clean the flue gas from a small-scale biomass boiler. For this purpose, a setup has been built at Luleå University. The setup consists of a boiler (20kW), three heat exchangers, and a generator. The flue gas from combustion, which heats the water in the boiler, flows through the absorber and generator. In the absorber (packed bed wet scrubber), the flue gas is in contact with an absorption solution, and at the same time, particulate matter is cleaned from the flue gas. The solution is passed through a filter and is purified. A part of the solution flows through the generator, absorbed water is evaporated, and concentrated solution 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 (8 months), which did not show any deterioration 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 by 18%.

The effect of different forces on particulate matter in a 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 phenomenon. On the other hand, the 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 obtained data can be used to build an efficient setup to clean the particulate matter released by small-scale boilers and would be interesting for companies that want to develop further the technology to be commercial for the market.