PSI - Issue 77

Alexander Backa et al. / Procedia Structural Integrity 77 (2026) 143–151 A. Backa et al. / Structural Integrity Procedia 00 (2026) 000 – 000

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1. Introduction Combustion of solid fuels, whether in industrial facilities or in small household burners, ranks as a major source of particulate matter (PM). When combustion is incomplete, it can emit PM together with harmful gases such as carbon monoxide (CO), sulfur oxides (SO x ), and nitrogen oxides (NO x ), all of which negatively impact air quality and pose serious risks to both human health and the environment (Kantová et al., 2021; Tamang and Park, 2023). From an environmental viewpoint, PM significantly impacts climate and visibility. Biomass combustion, commonly used as a renewable alternative to fossil fuels, is one of the leading sources of PM. Though biomass is considered carbon-neutral, it can still release considerable PM quantities, especially in small-scale burners used in home heating (Jiang et al., 2024). The combustion efficiency, determined by variables such as fuel characteristics, air supply (stoichiometric ratio), and achieved temperature, is profoundly affected by the design and operational parameters of burners (Drobniak et al., 2022). Inadequate burner configurations or improper system adjustments can promote incomplete combustion, thereby significantly elevating particulate matter emissions. The harmful effects of particulate matter released during biomass combustion emphasize the importance of investigating how these particles form and how to effectively reduce their generation. This is especially important for compact combustion systems, where fluctuating operating conditions make emission control more difficult. Studying PM formation factors, such as fuel composition, air supply, and combustion efficiency, is crucial for devising strategies to limit emissions. While most studies measure particulate matter (PM) concentrations at the flue gas outlet and relate them to diverse combustion parameters (Garcia-Maraver et al., 2014; Kantová et al., 2024; Siegmund et al., 2024), the direct generation of PM inside the combustion chamber remains less thoroughly investigated. Many studies rely on simplified lab-scale setups examining single-particle burning rather than realistic in-chamber dynamics (Sładek et al., 2020; Yilmaz et al., 2023) . In the context of ongoing research into the performance of this pellet burner under diverse operational parameters, including air supply and fuel variations (Backa et al., 2025a) and spatial distribution of emissions in combustion chamber (Backa et al., 2025b), this specific study investigates PM concentration above the burner in three central cross-sections. This study aims to quantify the link between PM concentration and temperature, using a calculated temperature-to-PM ratio and linear model. The derived linear model offers a practical tool for estimating PM levels based on in-situ thermal conditions and provides a foundation for validating and improving numerical models such as CFD simulations. 2. Material and Methods 2.1. Used fuel In this study, commercially available pellets produced entirely from spruce sawdust with a diameter of 6 mm were used. The fuel met the ENplus A1 certification standard in accordance with ISO 17225-2:2021. Table 1 presents the elemental and proximate composition of the pellets, along with their net calorific value.

Table 1. Wood pellets Analysis. MC- Moisture content, VM- volatile matter, FC- fixed carbon, LHV- Lower heating value. (O 2 content was determined by subtracting the other components.) Proximate analysis (%) Ultimate analysis (%), (dry basis) LHV (kJ/kg) MC VM FC Ash C H O N S 17 780 5.56 77.13 16.95 0.36 50.43 6.61 42.47 - 0.12

2.2. Operating conditions of combustion The combustion experiments were carried out in a compact heating system featuring a bottom-feed retort burner. Although the boiler was originally rated at 18 kW, this value reflects its nominal heat output. Wood pellets were automatically delivered from a fuel hopper to the combustion chamber via a screw-type conveyor, operating in 18 second feed cycles separated by 25-second pauses. This corresponded to an average fuel flow rate of approximately 5.8 kg/h. Combustion air was introduced using a fan, supplying around 126 kg/h of air. During all measurements,