the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 02 Jun 2025
Primary particle emissions and atmospheric secondary aerosol formation potential from a large-scale wood pellet-fired heating plant
Abstract. Solid biofuels are one option to reduce fossil fuel combustion and mitigate climate change. However, large-scale combustion of solid biofuels can have significant impacts on air quality and the emissions of short-lived climate forcers. Due to lack of detailed scientific experimental data, these atmospheric emissions and their impacts are mostly unknown. In this study, we characterized primary particle emissions before and after the flue gas cleaning as well as the potential of emissions to form secondary particulate mass in the atmosphere from the compounds emitted from a large-scale, biomass-fired modern heating plant. Experiments were made at three power plant loads, i.e., 30 MW, 60 MW, and 100 MW (full load), and at each of these loads, flue gas particles were characterized by comprehensive instrument setup both for their physical and chemical characteristics. The study highlights the importance of efficient flue gas cleaning in biofuel applications; the bag-house filters (BHFs) utilized to clean the flue gas from combustion boiler reduced the particle number emissions three orders of magnitudes and the BC emissions close to zero. After the filtration, the measured primary particle number emissions were at 30 MW, 60 MW, and 100 MW 1.7∙103 MJ-1, 5.2∙103 MJ-1, and 7.2∙103 MJ-1, respectively. By number, emitted particles existed mostly in sub-200 nm mobility particle size range. When measuring the potential of flue gas to form secondary aerosol in the atmosphere, for the first time according to authors knowledge, we observed that the secondary aerosol formation potential of the flue gas is high; the total impact of flue gases to atmospheric particulate matter concentrations can be even 100–1000 times higher than the impact of primary particle emissions. In general, the results of the study enable emission inventory updates, improved air-quality assessments, and climate modeling to support the transition toward climate-neutral societies.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Aerosol Research.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.- Preprint
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RC1: 'Comment on ar-2025-14', Anonymous Referee #1, 25 Jun 2025
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General comment
This study by Mylläri et al. investigates emissions from a large-scale (100 MW) wood pellet–fired heating plant in Helsinki, focusing on both primary particle emissions and the potential for secondary aerosol formation in the atmosphere. They conducted comprehensive measurements before and after the plant’s baghouse filter (BHF) system across different load conditions (30, 60, and 100 MW), and used an oxidation flow reactor to simulate atmospheric aging and evaluate secondary aerosol formation. They found that BHF filtration removed over 99.95% of particle number emissions, with BC reduced below detection limits, indicating very low direct particle emissions under normal operation. However, they also discovered that the potential for secondary particle formation was up to 1,000 times higher than the remaining primary particle emissions, primarily driven by nitrate formation from NOₓ. The authors state that while biomass combustion with efficient filtration can minimize primary emissions, secondary aerosol formation remains a significant concern and must be considered in future emission regulations and impact assessments.
The manuscript is written in appropriate English and the topic is within the scope of Aerosol Research. Although I do not share the stated general low level of information on power plant emissions (see for example Guevara 2024 ESSD), the study provides detailed information of physical particle properties. I am mainly criticizing the discussion of the results, i.e. the missing context, so it remains much as a presentation of results. I would suggest to compare emission factor from the 100 MW biomass power plant (1) to emissions from common coal- or gas-fired plants, also co-fired with biomass or peat, (2) to small-scale combustion appliances of similar operation mode and draw a quantitative conclusion about the apparent benefit from a central heating over residential heating in individual houses, (3) among different operation modes regarding the filtration efficiency of the BHF, and (4) the relevance of such this biomass power plants for the air quality of Helsinki. After addressing these aspects and my specific comments below, I would recommend this study for publication in Aerosol Research.Specific comments
Line 44: I think this is to general, and not valid for e.g. H2 combustion. You may constraint to solid fuel combustion or carbon-based fuels?
Line 47-48: I guess you refer to Sippula et al. (2009a).
Lien 48: The upper limit of 92 mg/MJ is principally correct, but this emission factor results from plant operation without exhaust aftertreatment. The upper limit of PM1 emission factor is then 31 mg/MJ, but different from a filtration process of this study. Scrubbing may be less efficient than filtration, but also remove gases and increase the particle diameter, which might be beneficial in terms of deposition in the human lung. A fair comparison would be the two electrostatic precipitators used in Sippula et al. (2009a) with only 4 and 6 mg/MJ of PM1. Please have a look on the particle number emission factors accordingly.
Line 56: kWh is not a unit for power.
Line 79-81: In Ortega et al. (2013) it is described that “FLAME-3 was a targeted investigation of emissions from burning of plant species that are relevant to North American local and regional air quality and often undergo prescribed and wildfire burning”. Burning experiments were done by igniting 0.1 to 1kg of biomass on a ceramic plate by heating coils, so combustion is not conducted “in a small-scale device”. For example, Czech et al (2017) presents secondary emissions from a small-scale pellet boiler in addition to Heringa et al (2011).
Line 78: What does “full-scale” mean in this context? Do you mean rather large-scale?
Line 102-103: Please provide more information about the boiler configuration and combustion technology. Based on the Valmet Ltd. Website, pellets are pulverized before burning and a low-NOx combustion technology is used.
Line 105: Please define what is “dust” in this context. To the best of my knowledge, Dusthunter P100 is not measuring gases, so please provide instrument information.
Lien 112-113: What was the raw material of the pellets? The difference in elemental composition is quite remarkably because the most different elements are determining largely the PM1 emissions (Kleinhans 2018 Progress Energy Combust Sci).
Line 128-130: If you fix the diluted emissions to 30°C, what does it refer to in terms of dilution ratio? How much is the primary emission temperature changing (with changing load for example)? What does “to mimic the atmospheric dilution of the aerosol” mean? After atmospheric dilution, i.e. atmospheric concentrations, further additional dilution for analysis is surprising.
Line 142-143: Assuming dilution air of 20°C, the undiluted flue gas has only a temperature of 140°C. Is this correct and if yes, was any intense heat recovery involved?
Lien 159: How was the TSAR operated? External or in situ generation of ozone, at which humidity and proximate OH reactivity? What about the influence of non-OH chemistry compared to OH-driven photooxidation?
Line 248: Please provide an upper limit of the EF for THC based on your LOD.
Line 273-276: In line 167 you refer to Li et al. (2015) for the determination of OH exposure by CO, but disregarding that it is recommended for OH exposures of 5e11 to 1e13 s/cm3. CO reacts slowly with OH having an atmospheric half-life of 22 days. To determine an OH exposure equivalent to 0.5 days, you have to detect a different in CO_in and CO_out of about 1.5%, to differentiate between 0.46 and 0.82 days of about 1%, to give significant digits of 0.01 days even 0.3%. Additionally, in using the concept of photochemical clock, also the rate constant has an error of typically 25-30% which needs to be considered. Therefore, I am curious how you performed the measurements with the CO12M instrument to obtain results with the precision required.
Line 290: Often, OFR aging leads to intense formation of nucleation mode particles from oxidized vapors. How significant was this fraction compared to the particle size range accessible by the AMS?
Line 303-305: How does the aging affect the composition of the PM1 emissions from fresh to aged?
Line 314: I would not call your analysis “comprehensive” if the only information about the aerosol chemical composition is derived from AMS, see also line 381.
Line 320-321: To draw such a conclusion, you need to provide emission data from fossil fuel burning. Moreover, impacts on air quality were not investigated in this study and almost only particle emissions are presented. What about the air quality impact of gaseous emission constituents?
Line 322-326: Several studies (e.g. Lepistö 2023 Environ Int) pointed out that ultrafine particles should receive more attention explaining health effects. If the particle size is shifted towards smaller ones, how does this fit to the conclusion about positive population health?
Line 337-342: This paragraph is correct but lacks of novelty. Bag-house filters became popular in the 70s and can be considered as standard technology for power plant emission mitigation since the 90s. Therefore, I encourage the authors to compare the filtration efficiency to previous similar studies in a quantitative manner or discuss filtration efficiency for different loads as the flue gas velocity has an impact.
Line 343-350: In lines 319-323, there are only positive aspects discussed with respect to particle emissions, but here suddenly negative impacts on air quality by BC are addressed despite being irrelevant from the data presented in this study.
Line 355-357: Whenever fresh particle emissions are filtered and subsequently compared to the secondary aerosol formation potential, the enhancement ratio turns out to be extraordinarily high suggesting air quality problems. However, the secondary aerosol formation has to be discussed on an EF-basis for a meaningful comparison. Following this, my conclusion would be that biomass combustion in a power plant generates orders of magnitude lower (primary and secondary) aerosol emission than small-scale (Ozgen 2014 Atmos Environ; Shen 2012 Environ Sci Tehcnol; Syc 2011 Environ Sci Technol; Aurell 2012 Environ Sci Technol), turning the enhancement ratio irrelevant.
Lien 363: Which secondary aerosol precursors may be formed during NOx removal? In biomass burning, air staging or flue gas recirculation (Steiner 2024 Renew Energy) is much more common compared to exhaust aftreatment like SCR.
Line 375-377: If there are only two orders of magnitude between the emission and the ambient concentration of Hg, I would conclude that the power plant is a well below average contributor. Dilution ratios of emissions from a point source to ambient air rather ranges from three to five orders of magnitude (Robinson 2007 Science).
Lien 380: I know small- and large-scale appliances, but what is real-scale in this context? I would expect that you design a lab scale experiment and set it to real-scale in the second setup.
LanguageLine 41-34: I suggest to remove the brackets.
Line 309: Replace “societies’” by “anthropogenic”.
Line 358: Two times “especially”.
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Citation: https://doi.org/10.5194/ar-2025-14-RC1
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