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Fine Particle Collection in Small-Scale Biofuel Boilers Using Packed-Bed Wet Scrubbers
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.ORCID iD: 0000-0003-2646-5220
2024 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Place, publisher, year, edition, pages
Luleå: Luleå tekniska universitet, 2024.
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
Keywords [en]
Packed-bed wet scrubber, Efficiency, Particulate matter, Flue gas humidity, Diffusiophoresis
National Category
Energy Engineering
Research subject
Energy Engineering
Identifiers
URN: urn:nbn:se:ltu:diva-106161ISBN: 978-91-8048-598-2 (print)ISBN: 978-91-8048-599-9 (electronic)OAI: oai:DiVA.org:ltu-106161DiVA, id: diva2:1867357
Public defence
2024-09-12, E632, Luleå University of Technology, Luleå, 09:00 (English)
Opponent
Supervisors
Available from: 2024-06-10 Created: 2024-06-10 Last updated: 2024-06-25Bibliographically approved
List of papers
1. Efficient Cleaning and Heat Recovery of Flue Gas from a Small-Scale Boiler
Open this publication in new window or tab >>Efficient Cleaning and Heat Recovery of Flue Gas from a Small-Scale Boiler
2021 (English)In: Chemical Engineering & Technology, ISSN 0930-7516, E-ISSN 1521-4125, Vol. 44, no 11, p. 2116-2125Article in journal (Refereed) Published
Abstract [en]

Small-scale biomass boilers contribute to the emission of particulate matter (PM) to the environment. In this study, the performance of a wet scrubber purification system for flue gas was experimentally investigated. The experimental setup consisted of a boiler, a wet scrubber, a generator, and heat exchangers. The results show an average particulate collection efficiency of around 42% for a particulate matter size range of 0.08-10 µm, within a testing period of 5 months. Furthermore, the results show an improvement in the heat recovery of about18%. Focusing only on the heat losses through exhaust flue gases, the losses were shown to have decreased by 72%. During the total testing period (8.5 months), no decrease could be noticed in the absorption solution ability.  

Place, publisher, year, edition, pages
John Wiley & Sons, 2021
Keywords
Flue gas cleaning, Heat recovery, Pellet boiler, Particulate matter, Wet scrubber
National Category
Energy Engineering
Research subject
Energy Engineering
Identifiers
urn:nbn:se:ltu:diva-87050 (URN)10.1002/ceat.202100274 (DOI)000703614600001 ()2-s2.0-85116285715 (Scopus ID)
Funder
Interreg Nord
Note

Validerad;2021;Nivå 2;2021-10-26 (beamah)

Available from: 2021-09-13 Created: 2021-09-13 Last updated: 2024-06-10Bibliographically approved
2. CFD modeling of the forces in the wet scrubber acting on particulate matter released from biomass combustion
Open this publication in new window or tab >>CFD modeling of the forces in the wet scrubber acting on particulate matter released from biomass combustion
2021 (English)In: Thermal Science and Engineering Progress, ISSN 2451-9049, Vol. 25, article id 100997Article in journal (Refereed) Published
Abstract [en]

The flue gas from biomass combustion contains particulate matter, which is a considerable precarious constituent cause of serious health issues. The wet flue gas cleaning method is one of the most efficient solutions for small-scale boilers (≈100 kW) and small particles. In this study, the forces on particulate matter in a wet flue gas cleaning process using an absorption solution have been studied with the implementation of compiled user-defined function code in Ansys Fluent 19.2®. The forces governing the wet cleaning process as well as the corresponding conditions of the system have also been studied. Drag, buoyancy, diffusiophoresis, and thermophoresis forces have been analyzed as the most important forces acting on particulate matters. Simulations have been conducted for velocities, particle sizes, temperatures, and water vapor mass fractions within the same range as experiments in order to acquire trends for particle collection for these variables.

Moreover, the influence of diffusiophoresis force was compared to that of the thermophoresis force under different conditions. It was unveiled that the diffusiophoresis force had a significant effect on nanoparticle collection. The impact of diffusiophoresis is increased by the ascending gradient of temperature as well as water vapor mass fraction. Simulations declare that the thermophoresis force effect is small compared to the diffusiophoresis force effect in the particle collection process. Thereby, one could conclude that the diffusiophoresis force governs the collection of particulate matter in the wet scrubber method. The model validation is confirmed by comparing the results with previous empirical models.

Place, publisher, year, edition, pages
Elsevier, 2021
Keywords
Biomass combustion, Flue gas cleaning, Diffusiophoresis, Thermophoresis, Wet scrubber, Particulate matter
National Category
Energy Engineering
Research subject
Energy Engineering
Identifiers
urn:nbn:se:ltu:diva-85935 (URN)10.1016/j.tsep.2021.100997 (DOI)000704170800011 ()2-s2.0-85111344856 (Scopus ID)
Funder
Interreg Nord
Note

Validerad;2021;Nivå 2;2021-08-13 (alebob)

Available from: 2021-06-23 Created: 2021-06-23 Last updated: 2024-06-10Bibliographically approved
3. Effect of operation conditions on particulate matter removal by a packed-bed wet scrubber for a small-scale biofuel boiler
Open this publication in new window or tab >>Effect of operation conditions on particulate matter removal by a packed-bed wet scrubber for a small-scale biofuel boiler
2024 (English)In: Thermal Science and Engineering Progress, ISSN 2451-9049, Vol. 47, article id 102290Article in journal (Refereed) Published
Abstract [en]

In 2013 the EU’s Clean Air Policy Package was established, aiming to reduce air pollution to half by 2030 compared to the level in 2005. Small-scale (<500 kW)biofuel boilers play a key role in particulate matter emission, and exposure to particulate matter even in the short term can cause different diseases. With the aim of reducing particulate matter emission in Europe, this study presents an approach to improve the removal of particulate matter emitted by small-scale boilers. A biofuel combustion boiler was equipped with a packed-bed wet scrubber, and the flue gas emitted through combustion was cleaned through the wet scrubber using a saltwater mixture. The performance of a packed-bed wet scrubber was investigated under different operating conditions. The effect of the salt concentration of the absorption solution, the temperature of the absorption solution fed to the absorber, and the height of the packed-bed material on the particle collection efficiency were measured. The operating conditions were selected based on the results obtained in a previous computational fluid dynamic simulation study. The results obtained in the present study show that an absorption solution temperature of 30 °C and an absorption solution concentration of 75 % with a full height of the packed-bed material lead to the best performance in the system. Totally keeping the absorption solution temperature as low as possible, increasing the absorption solution concentration, and raising the packed-bed material height could improve the particle collection efficiency by enhancing the effect of the diffusiophoresis and thermophoresis forces and the contact time between the flue gas and solution.

Place, publisher, year, edition, pages
Elsevier, 2024
Keywords
Wet scrubber, Diffusiophoresis, Thermophoresis, Particulate matter, Small-scale boiler
National Category
Energy Engineering
Research subject
Energy Engineering
Identifiers
urn:nbn:se:ltu:diva-93513 (URN)10.1016/j.tsep.2023.102290 (DOI)001123659300001 ()2-s2.0-85177495572 (Scopus ID)
Note

Validerad;2023;Nivå 2;2023-11-22 (hanlid);

Full text license: CC BY

This article has previously appeared as a manuscript in a thesis

Available from: 2022-10-09 Created: 2022-10-09 Last updated: 2024-06-27Bibliographically approved
4. Enhancing Particle Segregation in Stem Wood Combustion Flue Gas Wet Scrubbers: Experimental Investigation of Operational Conditions
Open this publication in new window or tab >>Enhancing Particle Segregation in Stem Wood Combustion Flue Gas Wet Scrubbers: Experimental Investigation of Operational Conditions
(English)Manuscript (preprint) (Other academic)
Keywords
Packed-bed wet scrubber, Efficiency, Particulate matter, Flue gas humidity, Diffusiophoresis
National Category
Energy Engineering
Research subject
Energy Engineering
Identifiers
urn:nbn:se:ltu:diva-106159 (URN)
Available from: 2024-06-10 Created: 2024-06-10 Last updated: 2024-06-10
5. An Analysis of Packed Bed Wet Scrubber Efficiency for Particulate Matter Collection with Different Fuels
Open this publication in new window or tab >>An Analysis of Packed Bed Wet Scrubber Efficiency for Particulate Matter Collection with Different Fuels
(English)Manuscript (preprint) (Other academic)
Keywords
Wet scrubber, combustion, particulate matter, municipality waste, mine waste
National Category
Energy Engineering
Research subject
Energy Engineering
Identifiers
urn:nbn:se:ltu:diva-106160 (URN)
Available from: 2024-06-10 Created: 2024-06-10 Last updated: 2024-06-25

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The full text will be freely available from 2024-08-22 09:00
Available from 2024-08-22 09:00

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Darbandi, Tayebeh

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