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Article

Emission Characteristics and Environmental Impact of VOCs from Bagasse-Fired Biomass Boilers

by
Xia Yang
1,2,
Xuan Xu
1,
Jianguo Ni
3,
Qun Zhang
4,
Gexiang Chen
1,2,
Ying Liu
1,2,
Wei Hong
1,
Qiming Liao
1,2 and
Xiongbo Chen
1,2,*
1
Guangdong Province Engineering Laboratory for Air Pollution Control, South China Institute of Environmental Sciences, The Ministry of Ecology and Environment of PRC, No. 7 West Street Yuancun, Guangzhou 510655, China
2
Guangdong Provincial Key Laboratory of Water and Air Pollution Control, South China Institute of Environmental Sciences, The Ministry of Ecology and Environment of PRC, No. 7 West Street Yuancun, Guangzhou 510655, China
3
Xiaoshan Branch of Hangzhou Ecological Environment Bureau, Hangzhou 311200, China
4
Yuhang Branch of Hangzhou Ecological Environment Bureau, Hangzhou 311121, China
*
Author to whom correspondence should be addressed.
Sustainability 2025, 17(14), 6343; https://doi.org/10.3390/su17146343
Submission received: 25 April 2025 / Revised: 5 July 2025 / Accepted: 7 July 2025 / Published: 10 July 2025
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Abstract

This study investigates the emission characteristics and environmental impacts of pollutants from bagasse-fired biomass boilers through the integrated field monitoring of two sugarcane processing plants in Guangxi, China. Comprehensive analyses of flue gas components, including PM2.5, NOx, CO, heavy metals, VOCs, HCl, and HF, revealed distinct physicochemical and emission profiles. Bagasse exhibited lower C, H, and S content but higher moisture (47~53%) and O (24~30%) levels compared to coal, reducing the calorific values (8.93~11.89 MJ/kg). Particulate matter removal efficiency exceeded 98% (water film dust collector) and 95% (bag filter), while NOx removal varied (10~56%) due to water solubility differences. Heavy metals (Cu, Cr, Ni, Pb) in fuel migrated to fly ash and flue gas, with Hg and Mn showing notable volatility. VOC speciation identified oxygenated compounds (OVOCs, 87%) as dominant in small boilers, while aromatics (60%) and alkenes (34%) prevailed in larger systems. Ozone formation potential (OFP: 3.34~4.39 mg/m3) and secondary organic aerosol formation potential (SOAFP: 0.33~1.9 mg/m3) highlighted aromatic hydrocarbons (e.g., benzene, xylene) as critical contributors to secondary pollution. Despite compliance with current emission standards (e.g., PM < 20 mg/m3), elevated CO (>1000 mg/m3) in large boilers indicated incomplete combustion. This work underscores the necessity of tailored control strategies for OVOCs, aromatics, and heavy metals, advocating for stricter fuel quality and clear emission standards to align biomass energy utilization with environmental sustainability goals.

1. Introduction

Biomass energy, as a crucial renewable resource, plays a strategic role in achieving carbon neutrality goals [1,2]. Biomass boilers, as critical carriers of renewable energy technology, demonstrate significant advantages in replacing fossil fuels and reducing net carbon dioxide (CO2) emissions [3]. However, the complex flue gas components released during biomass combustion—including particulate matter (PM), nitrogen oxides (NOx), volatile organic compounds (VOCs), and heavy metals—may trigger regional atmospheric pollution [4,5,6,7,8,9].
Biomass fuels are mainly agricultural and forestry waste, which can be divided into woody biomass fuel and straw biomass fuel. The chemical composition and physical properties of biomass fuel determine its combustion and flue gas emission characteristics [10,11,12]. Monitored CO and VOC concentrations were lower in the flue gas of wheat straw pellet combustion when compared to those in biomass boilers fueled with wood and rape straws under the same conditions [13]. In addition, referring to various biomass fuels, including pine, mixed wood, and corn straw, the study found that NOx and SO2 emissions were closely related to the nitrogen and sulfur content of fuel, and SO2 emissions of three fuels were nearly undetectable under high temperature and high air flow [14]. For particulate matter (PM) emissions, fine particles (i.e., PM1) constituted the primary component across all tested fuels, including sunflower stalks, rice straw, buckwheat husks, corn stalks, wheat husks, wood, and sewage sludge particles [15]. Studies have also performed kinetic analyses on pellet fuels composed of varying blend ratios of wood shavings and peanut shells, examining NOx and CO emissions from five pellet samples. Significant improvement in the integrated combustion performance of blended fuels was observed, with a 41–56% reduction in CO emissions and a moderate 14–28% increase in NOx emissions [16].
Sugarcane bagasse, as a by-product of the sugar industry, exhibits distinct emission characteristics compared to woody biomass due to its high moisture content, chlorine concentration, and heterogeneous combustion behavior [17,18]. Although existing studies have focused on the emission factors of PM and NOx from biomass boilers [19,20,21], systematic analyses of VOC speciation and their secondary pollution potential remain scarce. This gap is particularly pronounced for bagasse-fired systems, hindering the accurate construction of emission inventories and the targeted development of pollution control technologies.
As a primary byproduct of tropical sugar mills, bagasse combustion flue gas contains elevated Cl− and K+ ratios that may enhance the hygroscopic growth of PM2.5, while its high volatile content facilitates incomplete combustion, leading to increased CO and VOC emissions [22,23,24,25]. The current research predominantly focuses on the monitoring of conventional pollutants, e.g., NOx and PM2.5, neglecting the speciation and photochemical reactivity of trace VOCs [26]. For instance, Primo et al. (2006) studied the formation and control mechanism of NOx emission from biomass boilers during bagasse combustion and evaluated the influence of NOx emission from sugar factories on the surrounding air quality through the atmospheric diffusion model [27]. As critical precursors of O3 and secondary organic aerosol (SOA), variations in VOC source profiles can lead to significant divergence in atmospheric oxidation pathways [28,29]. Bagasse-derived VOCs may contain high proportions of oxygenated VOCs, whose photolysis yield should be more deeply quantified [30,31,32,33]. Additionally, quantitative assessments of heavy metal enrichment in fly ash or flue gas are lacking, which is a potential risk for human health when in the air or the soil.
The goals of this study were to (1) investigate two typical bagasse-fired biomass boilers and conduct integrated field monitoring of flue gas components (PM2.5, SO2, NOx, CO, VOCs, heavy metals), (2) establish a comprehensive emission profile database for bagasse combustion and estimate the ozone formation potential (OFP) and secondary organic aerosol formation potential (SOAFP) of atmospheric VOCs using maximum incremental reactivity (MIR) and aerosol generation coefficient (FAC) methods, and (3) evaluate the impact of bagasse-fired biomass combustion on the environmental atmosphere.
The structure of the remainder of this paper is as follows: Section 2 introduces the information on biomass, the monitoring analysis method, and the ozone and aerosol formation potential calculation methods. Detailed results and discussions are described in Section 3, including the properties of fuel and the emission characteristics of conventional and unconventional pollutants. Section 4 summarizes the conclusions, and Section 5 shows the limitations.

2. Materials and Methods

2.1. Overview of Biomass Boilers

The biomass boiler flue gas pollutant emissions of two sugar enterprises in Guangxi were monitored. Both biomass boilers were based on the bagasse briquette fuel. The scale of each biomass boiler and the pollution control measures are shown in Table 1. The biomass boiler flue gas of Enterprise 2 was divided into two flues at the outlet of the boiler for waste gas treatment and then gathered in a chimney for emission. The flue gas pollutants produced by biomass boiler combustion were generally not effectively treated, and only simple dust removal facilities were installed.

2.2. Sampling Monitoring Analysis Method

Regarding the sampling point location, there were a total of two sampling points in Enterprise 1 and four sampling points in Enterprise 2 (equipped with two flues), with monitoring sites established both upstream and downstream of the dust removal facilities at each enterprise.
For the flue gas sample collection and analysis, all species were monitored at a single time point. O2, CO, NO, NO2, NOx, and SO2 concentrations were measured using a flue gas analyzer (Testo350) for sampling and analysis, and the average value of five to fifteen minutes after stabilization was recorded as one data point. After obtaining three data records, the average value of these three data points was used as the concentration of the above species; particulate matter was sampled using a filter cartridge and automatic soot tester (Laoying 3012H, made by Qingdao Laoying Environmental Technology Co., Ltd, Qingdao, China). VOCs were collected by suma tank and flowmeter, and 105 components were analyzed with GC–MS. Heavy metals were collected according to the air sampling tank and absorption solution specified in the “Ambient air and waste gas from stationary sources emission—Determination of metal elements in ambient particle matter-Inductively coupled plasma optical emission spectrometry” (HJ 777-2015 [34]), and the analysis was repeated three times to take the average to eliminate errors; non-methane total hydrocarbons were sampled and analyzed using PID gas detector. HCl and HF were sampled according to the fixed pollution source exhaust gas sample collection device specified in “Stationary source emission—Determination of hydrogen fluoride—Ion chromatography” (HJ 688-2019 [35]) and “Ambient air and stationary source emissions—Determination of Hydrogen Chloride-Ion Chromatography” (HJ 549-2016 [36]). The monitoring methods and detection limits for the above substances are attached in Table S1.
For the solid sample collection and analysis, the calorific values of fuel, fly ash, and bottom slag samples from biomass boilers of enterprises were analyzed using oxygen bomb calorimeter. Proximate analysis (moisture, ash, volatile matter, and fixed carbon content) was conducted in accordance with the standard “Proximate analysis of coal” (GB/T 212-2008 [37]). Elemental composition (C, H, N, and other elements) was analyzed with organic element analyzer (German elementar, UNICUBE). Heavy metal content in the samples was determined with inductively coupled plasma atomic emission spectrometer (ThermolCPOES 7200, made by ThermoFisher, Waltham, MA, USA) and inductively coupled plasma mass spectrometer (PerkinElmer NexION 300D ICP-MS, made by PerkinElmer, Waltham, MA, USA).

2.3. Ozone and SOA Formation Potential

To quantify the environmental impacts of volatile organic compounds (VOCs) emitted from bagasse combustion, the ozone formation potential (OFP) and secondary organic aerosol formation potential (SOAFP) were calculated based on VOC speciation data. The methodology integrated reactivity-based metrics and thermodynamic models to evaluate the photochemical contributions of individual VOC species.
The OFP for each VOC species was determined using the maximum incremental reactivity (MIR) approach proposed by Carter (1994) [30]. The MIR value represented the incremental ozone produced per unit mass of VOCs reacted under optimal NOx conditions. The OFP was calculated as follows:
OFPi = [VOC]i × MIRi
where [VOC]i is the measured concentration (μg/m3) of the i-th species, and MIRi is its corresponding MIR coefficient (g O3/g VOC), obtained from Zhang (2021) (Table S2) [31].
The secondary organic aerosol formation potential (SOAFP) was estimated using the fractional aerosol coefficient (FAC), which quantifies the mass yield of secondary organic aerosol (SOA) per unit mass of VOC oxidized [32]. The FAC values were sourced from chamber experiments and volatility basis set (VBS) model predictions (Table S3) [33]:
SOAFPi = [VOC]i × FACi
where [VOC]i is the measured concentration (μg/m3) of the i-th species, and FACi (g SOA/g VOC) corresponds to the i-th species.
For VOC species below the detection limit, a conservation approach by assigning half of the monitoring detection limit values was applied when calculating OFP and SOAFP, which was widely adopted in studies to minimize bias in reactivity assessments.

3. Results and Discussion

3.1. Physicochemical Properties of Biomass Fuel (Bagasse), Bottom Slag, and Fly Ash

The results of the elemental analysis, industrial analysis, and calorific value of biomass fuel, bottom slag, and fly ash from two sugar enterprises in Guangxi are shown in Figure 1 and Table 2. The results of the elemental analysis show that compared with traditional energy coal [38], the content of C, H, N, and S in bagasse fuel is generally lower—especially, N is 50% or less—while the content of O is more than three times higher, which may cause significant differences in the emission concentrations of SO2 and NOx [5,39]. Additionally, S in the biomass fuel of both enterprises is too low to be detected. The content of C, H, and N in the fuel used by Enterprise 1 is higher than that of Enterprise 2, and the content of O is lower than that of Enterprise 2. High C and H contents tend to increase the calorific value (Table 2), and high O content tends to reduce the calorific value, where the calorific value of Enterprise 2 fuel is lower [40]. The reason for this phenomenon is that the high oxygen content in the recycled flue gas replaces a portion of the oxygen naturally present in the air, thereby necessitating an increased amount of fuel to fully combine with this supplementary oxygen. This leads to a further reduction in the overall thermal efficiency [41].
The results of industrial analysis show that, compared with traditional energy coal [38], the moisture content (Mar) of biomass fuel is higher by about ten times (47.71~53.34%), and the evaporation of this moisture requires the consumption of heat [41], once again verifying that high moisture content will lead to a decrease in calorific value. The ash content (Ad) and fixed carbon (FCddiff) content are much lower, and the volatile content (Vd) is slightly higher, which indicates that biomass is easy to burn, but if the combustion is not sufficient, it is easy to produce VOCs [43].
The main metal element contents of biomass fuel, bottom slag, and fly ash produced by combustion in two sugar enterprises are shown in Figure 1. The contents are of great difference among metal elements, while basically consistent for the two enterprises. For the fuel of both enterprises, the four elements with the highest content are Cu, Cr, Ni, and Pb, which is about ten times higher than other metal elements. Mn, Ti, Na, and Hg have the lowest contents among them, which is very different from wood, in which Mn has a high proportion [6,42]. Nevertheless, for bottom slag and fly ash, the contents of Mn and Na exceed most metal elements and are only lower than Ca and K, which is due to their lower volatility. This phenomenon could be due to the fact that these metal elements are not easily volatile and remain in solid pollutants. Hg has the lowest content (0.074–0.236 mg/kg) because of its high volatility.

3.2. Emission Characteristics of Conventional Pollutants Before and After the Treatment Process

3.2.1. Particulate Matter

The biomass boiler with bagasse as fuel will produce a certain amount of particulate matter, which mainly comes from the incomplete combustion of fuel and the secondary formation of combustion products [7,44]. Figure 2 shows the generation and emission concentration of particulate matter in the flue gas of biomass boilers in two enterprises.
In order to control the concentration of particulate matter emissions, Enterprise 1 adopts a Venturi water film dust collector, and Enterprise 2 adopts a bag filter to remove particulate matter in flue gas. According to Figure 2, the content of particulate matter in the flue gas produced by the biomass boiler of Enterprise 1 is much higher than that of Enterprise 2. In addition, it can be seen that the particulate matter emission concentration of the two enterprises is less than 20 mg/m3, which meets the requirements of the “Emission standard of air pollutants for boiler” (GB 13271-2014) [45], where the emission standard is 80 mg/m3 and 50 mg/m3 for Enterprises 1 and 2, respectively, based on the boiler construction year. The removal rate of particulate matter in Enterprise 1 exceeds 98%, and the removal rate of particulate matter in Enterprise 2 exceeds 63%. The reason for the low removal rate of particulate matter in Enterprise 2 is mainly in line with the low content of particulate matter in the flue gas generated by the boiler. According to the conclusion of Price-Allison et al. (2021) that the concentration of particulate matter produced in the combustion process of high humidity woody biomass fuel is higher [46], and the result of higher moisture content of fuel in Enterprise 1 shown in Table 2, it can be speculated that the higher the moisture content of bagasse fuel, the greater the concentration of particulate matter produced.

3.2.2. NOx

There are three ways to generate NOx during fuel combustion, namely fast NOx, thermal Nox, and fuel NOx [8]. It is difficult for the biomass combustion temperature to reach 1300 °C, so thermal NOx is basically not produced. With the proportion of fast NOx also being small, NOx in the flue gas is mainly composed of fuel NOx. Therefore, the production of nitrogen oxides in flue gas is closely related to the nitrogen content in fuel. According to Table 2, it can be seen that the nitrogen content in the fuel of Enterprise 1 is higher, which is consistent with the fact that the NOx concentration in the flue gas produced by Enterprise 1 in Figure 3 is higher. It is found that the NOx concentration changes little before and after flue gas treatment. The NOx removal rate of Enterprise 1 is 10~56% after flue gas treatment, and the NOx removal rate of Enterprise 2 is between 0 and 34%. The main reason for the higher removal rate of NOx in the flue gas of Enterprise 1 is that the water film dust collector can absorb part of the water-soluble NOx in the flue gas and achieve the effect of reducing NOx concentration, while the bag filter employed in Enterprise 2 is a dry dust collector. Even though the concentration of NOx emissions in the flue gas of the two enterprises meets current standards [45], the NOx emission concentration is still high. Given the current level of technology, achieving lower NOx emission concentrations is feasible and necessary to comply with environmental sustainability requirements [47].

3.3. Unconventional Pollutant Emission Characteristic for Biomass Boiler

3.3.1. CO

In the process of bagasse combustion, CO is a product of insufficient fuel combustion, which can be used as an indicator gas of combustion efficiency. The CO emission concentration of biomass boiler in Enterprise 1 is low (<6 mg/m3), and the CO production and emission concentration of two flues in Enterprise 2 are high (>1000 mg/m3). When evaluated against the CO emission concentration limit of 200 mg/m3 for biomass boilers specified in Guangdong Province’s “Emission standard of air pollutants for boilers” (DB 44/765-2019 [47]), the CO concentration in the flue gas emitted by Enterprise 1 complies with the standard requirements, while both flue gas streams from Enterprise 2 significantly exceed the regulatory limit [48]. The reason is that the large furnace may contribute to incomplete combustion conditions, resulting in substantially elevated CO emissions. This is a common issue, as most biomass boilers are retrofitted from coal-fired boilers, which typically have oversized furnaces and lack internal mixing mechanisms. Although larger boilers can provide longer residence time, which is conducive to the complete combustion of fuel, if the boiler design is not reasonable, resulting in the uneven mixing of fuel and air, it will reduce combustion efficiency and increase pollutant emissions [49].

3.3.2. Heavy Metal

The emission of heavy metals in the flue gas of biomass boilers is closely related to the content of heavy metal elements in the burned biomass (bagasse) and the volatility of different heavy metals. As a global pollutant, Hg has strong genotoxicity and neurotoxicity and has a wide impact on the health of humans and ecosystems [50]. In addition, Hg exhibits high volatility and low water solubility, and among heavy metals, Hg is the only heavy metal with long-range atmospheric transport and significant bioaccumulation in ecosystems [51,52,53]. Therefore, Hg is specified as the only heavy metal that is limited in the “Emission standard of air pollutants for boiler” (GB 13271-2014) [45].
According to the mean concentration of heavy metal monitoring in flue gas (Figure 4), the top five heavy metals in flue gas produced by bagasse enterprises are Cr, Ni, Mn, Hg, and Cu. Compared with the heavy metal concentrations in the biomass fuels in Figure 1, the concentrations of Cr, Ni, and Cu in biomass fuels and flue gas are ranked higher. The higher concentrations of Mn and Hg in the flue gas are very low in the biomass fuel, which further confirms that Hg and its compounds are highly volatile [54]. After the exhaust gas treatment facility, the concentrations of Hg, Mn, Cr, and Ni elements are still high compared to other heavy metals. Some elements, including As, Pb, Ti, etc., are not detected.
Hg emissions from both enterprises meet the standard (<0.05 mg/m3) for flue gas emissions [48]. Additional heavy metal emissions are below their respective thresholds specified in reference to the “Integrated emission standard of air pollutants” (GB 16297-1996) [55], where Pb < 0.7 mg/m3, Be < 0.012 mg/m3, Sn < 8.5 mg/m3, Cd < 0.85 mg/m3, and Ni < 4.3 mg/m3. However, the test results of flue gas emissions from biomass boilers show that Cr and Mn concentrations remain high, suggesting that they should be paid special attention in addition to the heavy metal Hg. Especially, there are studies that have indicated that the emissions of Cr, Mn, and other heavy metals into the atmosphere tend to form aerosols. These aerosols subsequently enter the soil through wet deposition, where they accumulate and are further concentrated in plants, ultimately impacting human health. Additionally, inhaled atmospheric Cr may be carcinogenic to humans, while Mn exposure has been found to interfere with cardiovascular autonomic nervous function [56,57].

3.3.3. HF and HCl

The bagasse biomass fuel used contains a certain amount of chlorine and fluorine. Biomass combustion converts chlorine and fluorine into HCl and HF. These acidic gases accelerate the corrosion of flue gas pipelines under high temperature conditions, and their emissions also promote the formation of dioxins and VOCs in the atmospheric environment [58].
The actual monitoring of HCl and HF is shown in Figure 5. Referring to the emission limits of HCl (<100 mg/m3) and HF (<9 mg/m3) in the “Integrated emission standard of air pollutants” (GB 16297-1996) [55], the HCl and HF concentrations in the flue gas produced and discharged by the two enterprises reached the standard without treatment. This is mainly due to the low content of Cl and F in the biomass fuels used by the two companies.

3.3.4. Volatile Organic Compounds (VOCs) and Potential of Ozone and Aerosol Generation

In this study, 105 components of VOCs in the flue gas after flue gas treatment facilities were quantitatively detected. The results show that 57 kinds of VOCs reached the detection limit, and the monitoring results are shown in Table S4. According to the classification of chemical functional groups, 57 kinds of VOCs can be divided into six categories: alkanes, alkenes, halocarbons, aromatics, OVOCs, and others. The composition of VOCs is shown in Figure 6.
In Figure 6, the proportion of VOCs components in Enterprise 1 and Enterprise 2 is quite different. The highest VOC content in the flue gas emitted by Enterprise 1 is OVOCs, accounting for 87%. The species with the highest VOC content in two flues of Enterprise 2 are also different. The highest VOC content in the flue gas of flue 1 is aromatics, accounting for 60%, while that from flue 2 is OVOCs, accounting for 34%. The above results do not reflect the characteristic species of VOCs in the flue gas when bagasse is used as a biomass boiler fuel. The total VOC emissions in the flue gas of two companies are 0.95–3.94 mg/m3. The top five substances of VOC content in Enterprise 1 are ethyl acetate > carbon disulfide > n-octane > vinyl acetate > isopropanol. The top five substances of VOC content in Enterprise 2 flue 1 are benzene > ethylene > carbon disulfide > trans-2-butene > 1-butene. The top five substances of VOC content in flue 2 of Enterprise 2 are 2-hexanone > ethylene > carbon disulfide > p/m-xylene > trans-2-butene. In Enterprise 1, VOCs of oxygen-containing groups are the main product of complete combustion. In Enterprise 2, VOCs of ketones and alkenes are the main products, which are usually characteristics of incomplete fuel combustion. This may be caused by the lack of an air–fuel mixture device [59]. The above analysis is consistent with the above conclusions.
Based on the types of VOC-rich substances in the flue gas emitted by two enterprises, the concentration of CS2 is always high in the bagasse boiler. It is speculated that CS2 may be an indicative VOC compound using bagasse as a biomass boiler fuel. Although chloromethane is generally regarded as an indicative species of biomass combustion [60], in our study focused on bagasse boiler combustion scenarios, the measured chloromethane concentrations were relatively low. Specifically, Enterprise 1 emitted flue gas containing chloromethane at a concentration of 0.0262 mg/m3, accounting for 0.67% of the total VOC mass. In Enterprise 2, the chloromethane concentrations in the flue gas from stack 1 and stack 2 were 0.027 mg/m3 (1.90% of total VOCs) and 0.00155 mg/m3 (0.17% of total VOCs), respectively. Compared to the chloromethane emissions from biomass combustion in household boilers, such as wood (0.008 mg/m3) and cornstalks (0.56 mg/m3), as reported by Shi et al. [60], the measured chloromethane concentrations in our study were notably lower. This suggests that the applicability of chloromethane as an indicative species for bagasse boiler combustion may be limited.
(1) 
Ozone formation potential (OFP)
This study employed the maximum incremental reaction activity method (MIR method) to systematically evaluate the ozone generation potential (OFP) of volatile organic compounds (VOCs) emitted by two biomass boiler enterprises. As depicted in Figure 7, the total OFP values ranged from 3.42 to 4.45 mg/m3. In comparison to OFP values reported by Geng et al. (2019) for coal-fired boiler retrofits utilizing wood fuel (81 mg/m3) and dedicated biomass boilers (6 mg/m3) [61], our investigation demonstrates significantly lower emissions for bagasse-fueled systems. Conversely, our results are higher than the OFP value (0.27 mg/m3) reported by Yang et al. (2020) for conventional coal-fired boilers [62].
For Enterprise 1, the OFP contribution exhibited pronounced species: of the specific variations, 76% were oxygenated volatile organic compounds (OVOCs), and 10% were aromatic hydrocarbons and alkanes. Among these, ethyl acetate, vinyl acetate, methyl acrylate, p/m-xylene, and 2,4-dimethylpentane were the top five contributors, collectively accounting for 81.5% of the OFP.
For Enterprise 2, flue 1 comprised olefins (60%), aromatics (25%), and OVOCs (9%), with trans-2-butene, benzene, ethene, 1-butene, and acrolein being the top five contributors (cumulative contribution rate: 67.7%). Flue 2 consisted of olefins (56%), aromatics (15%), and OVOCs (24%), with trans-2-butene, 2-hexanone, ethylene, 1-butene, and p/m-xylene as the top five contributors (cumulative contribution rate: 67.3%). Despite the differing emission profiles, the contribution structure of VOC species to OFP was comparable, with olefins, OVOCs, and aromatic hydrocarbons being the primary control species. This similarity likely stems from analogous combustion organization modes, leading to comparable formation patterns of homologous compounds.
The OFP analysis reveals that it is necessary to consider the characteristics of the boiler when we identify high-OFP species. For small-sized boilers, particular attention should be directed towards the high-OFP risks associated with OVOCs, such as ethyl acetate, vinyl acetate, and methyl methacrylate. In contrast, for large-sized boilers, the combined effects of aromatic hydrocarbons, olefins, and OVOCs, including trans-2-butene, ethylene, p/m-xylene, benzene, and 2-hexanone, require special attention.
(2) 
Secondary organic aerosol formation potential (SOAFP)
There were 20 secondary organic aerosol precursors among the 57 detected VOCs, comprising 12 alkanes and eight aromatic hydrocarbons, with significant differences in SOAFP contributions (Figure 8). When bagasse was used as fuel, the SOAFP value ranged from 0.33 to 1.9 mg/m3. For both reference systems—wood-fired boilers (61.56–211.67 mg/m3) [61] and coal-fired boilers (4.6 mg/m3) [62]—our results were significantly lower. Enterprise 1 had an SOAFP value of 0.37 mg/m3, where alkanes contributed 20% and aromatic hydrocarbons 80%. The top five dominant species were toluene, p/m-xylene, o-xylene, 3-methylheptane, and ethylbenzene. Similarly, the double flue system of Enterprise 2 showed aromatic hydrocarbons as the absolute dominant contributor. Flue 1 had a high SOAFP value of 1.95 mg/m3, with aromatic hydrocarbons contributing 99%, led by benzene (benzene > p/m-xylene > toluene > o-xylene > ethylbenzene). Flue 2 had a SOAFP value of 0.52 mg/m3, with aromatic hydrocarbons still contributing 91%, and p/m-xylene being the most dominant (p/m-xylene > toluene > o-xylene > ethylbenzene > dodecane).
Compared to the wood/straw boiler study of Geng et al. (2019) (where the SOAFP-dominant species were toluene, ethylbenzene, and p/o-xylene) [61], this study’s bagasse boiler exhibited a more concentrated aromatic hydrocarbon contribution and a significantly increased proportion of benzene, toluene, ethylbenzene, and xylene (BTEX, a class of volatile toxic aromatic compounds). As a typical volatile organic pollutant, the increase in BTEX concentration may enhance the potential formation of secondary organic aerosol. The SOAFP analysis underscores the dominant role of aromatic hydrocarbons, highlighting the need to strengthen control of species like toluene, p/m-xylene, and o-xylene, especially when local PM2.5 levels exceed standards.

4. Conclusions

This study comprehensively investigated the flue gas pollutant characteristics of bagasse-fired biomass boilers. Through field monitoring at two Guangxi sugar mills, we analyzed the emission profiles of various pollutants, including particulate matter (PM), NOx, SO2, HCl, HF, CO, heavy metals, and VOCs. The key findings include:
Bagasse exhibits higher oxygen and lower carbon, hydrogen, and sulfur content compared to coal, resulting in lower heat values. High moisture content further contributes to reduced combustion efficiency.
Water film dust collectors demonstrated superior PM removal efficiency (>98%) compared to bag filters (>63%). NOx removal efficiency was slightly higher for water film collectors (~25%) due to partial water solubility.
Insufficient combustion in large-scale biomass boilers led to significantly elevated CO emissions, exceeding acceptable limits and highlighting the need for optimization.
Cu, Cr, Ni, and Pb were the predominant heavy metals in bagasse fuel. Ca, K, Mn, and Na were enriched in bottom slag and fly ash due to their lower volatility. In addition to Hg, special attention is also required for Cr and Mn due to their high emission concentration in flue gas in this study.
VOC emission profiles varied between boiler sizes, although two boiler fuels are bagasse. Notably, the carbon disulfide concentration ranks high and can be an indicator for VOC compounds when bagasse is used for biomass boiler fuel. The total values in the range of 3.34~4.39 mg/m3 for OFP and 0.37~1.95 mg/m3 for SOAFP have been found, indicating that VOC emission control cannot be ignored for a bagasse-fired biomass boiler. For small-sized biomass boilers, oxygenated volatile organic compounds (OVOCs) pose a high OFP risk (76%). For large-sized ones, aromatic hydrocarbons (56~60%), olefins, and OVOCs jointly affect the OFP. The SOAFP analysis shows that aromatic hydrocarbons play a leading role. Therefore, it is essential to strengthen the control of aromatic hydrocarbons from bagasse boilers for PM2.5 and O3 control.
While biomass boilers offer clean energy advantages, this study underscores the importance of establishing stringent biomass fuel quality standards and flue gas emission regulations. Effective control measures, particularly targeting aromatic hydrocarbons and Hg, are crucial for mitigating secondary organic aerosol and O3 impacts.
This research provides valuable data and insights for optimizing biomass boiler operations and developing targeted pollution control strategies to promote sustainable biomass energy utilization.

5. Limitations

While this study provides a systematic analysis of flue gas emissions from bagasse-fired biomass boilers in Guangxi, China, several limitations should be noted. First, the dataset is currently limited to two boiler capacities (75 and 180 t/h) due to field-access constraints and the geographic concentration of bagasse utilization in this region. Although these units represent typical operational scales for sugarcane-processing industries, future studies should include a wider range of boiler sizes (e.g., <65 t/h or >200 t/h) to assess capacity-dependent emission trends. Second, the temporal resolution of our measurements may not fully capture emission variability during unstable boiler operations. Extending the monitoring duration or employing online monitoring instruments (if economically feasible) could provide more robust, time-resolved data and help distinguish more true emission factors from operational transients. These limitations will guide our future work on industrial-scale biomass combustion systems.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/su17146343/s1, Table S1: List of detection and analysis methods, instruments, and detection limits; Table S2: MIR value for VOC species; Table S3: FAC value for VOC species; Table S4: The emission concentrations of VOCs from biomass boilers flue gas.

Author Contributions

Conceptualization, X.Y. and X.C.; methodology, X.Y. and X.X.; formal analysis, X.X. and X.Y.; investigation, J.N., Q.Z., G.C. and Y.L.; data curation, G.C. and Y.L.; writing—original draft preparation, X.Y. and X.X.; writing—review and editing, Q.L., W.H. and X.C.; visualization, X.Y., X.X. and Q.L.; supervision, X.C.; project administration, X.Y. and X.C.; funding acquisition, X.Y., J.N. and Q.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Key Research and Development Plan (Grant No. 2022YFC3701600).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Dataset available on request from the authors.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Figure 1. Main metal elements in (a) biomass fuel, (b) bottom slag, (c) fly ash [6,42].
Figure 1. Main metal elements in (a) biomass fuel, (b) bottom slag, (c) fly ash [6,42].
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Figure 2. The production and emission concentration of particulates in biomass boiler flue gas.
Figure 2. The production and emission concentration of particulates in biomass boiler flue gas.
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Figure 3. The production and emission concentrations of NOx from biomass boiler flue gas.
Figure 3. The production and emission concentrations of NOx from biomass boiler flue gas.
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Figure 4. The production and emission concentrations of heavy metals from biomass boiler flue gas.
Figure 4. The production and emission concentrations of heavy metals from biomass boiler flue gas.
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Figure 5. The production and emission concentrations of HCl and HF from biomass boiler flue gas.
Figure 5. The production and emission concentrations of HCl and HF from biomass boiler flue gas.
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Figure 6. Composition characteristics of VOCs from biomass boiler flue gas.
Figure 6. Composition characteristics of VOCs from biomass boiler flue gas.
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Figure 7. VOC compositions by mass concentration and contributions of VOCs to OFP: (a) flue of Enterprise 1, (b) flue 1 of Enterprise 2, (c) flue 2 of Enterprise 2.
Figure 7. VOC compositions by mass concentration and contributions of VOCs to OFP: (a) flue of Enterprise 1, (b) flue 1 of Enterprise 2, (c) flue 2 of Enterprise 2.
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Figure 8. VOC compositions by mass concentration and contributions of VOCs to SOAFP: (a) flue of Enterprise 1, (b) flue 1 of Enterprise 2, (c) flue 2 of Enterprise 2.
Figure 8. VOC compositions by mass concentration and contributions of VOCs to SOAFP: (a) flue of Enterprise 1, (b) flue 1 of Enterprise 2, (c) flue 2 of Enterprise 2.
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Table 1. Overview of biomass boilers.
Table 1. Overview of biomass boilers.
Enterprise 1Enterprise 2
Boiler size/t·h−175180
Fuel typeBagasseBagasse
Fuel consumption/t·d−11550–16001500
Pollution treatment facilitiesWater film removal dusterBag filter
Chimney height/m8080
Boiler temperature/°C627~668796~966.6
Air–fuel ratio8.98.4
Table 2. Ultimate and proximate analyses of fuel for two enterprises.
Table 2. Ultimate and proximate analyses of fuel for two enterprises.
SampleUltimate Analyses (wt.%)Proximate Analyses (wt.%)Heat Value (MJ/Kg)
NdafCdafHdafSdafOdafdiffMarAdVdFCddiffLHVHHV
Enterprise 10.1830.803.72024.0453.341.3439.716.6011.8912.67
Enterprise 20.1424.983.20029.9347.710.6344.207.468.939.63
Peanut shell [16]1.53/////7.0365.3/15.4~16.9
Wood [16]0.38/////1.7173.87/17.6~19.2
Corn [17]0.83/////8.0271.26/16.7
Bituminous coal [38]1.265.75.60.57.74.914.732.348.118.24~24.93 [39]
diff: calculated by difference.
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MDPI and ACS Style

Yang, X.; Xu, X.; Ni, J.; Zhang, Q.; Chen, G.; Liu, Y.; Hong, W.; Liao, Q.; Chen, X. Emission Characteristics and Environmental Impact of VOCs from Bagasse-Fired Biomass Boilers. Sustainability 2025, 17, 6343. https://doi.org/10.3390/su17146343

AMA Style

Yang X, Xu X, Ni J, Zhang Q, Chen G, Liu Y, Hong W, Liao Q, Chen X. Emission Characteristics and Environmental Impact of VOCs from Bagasse-Fired Biomass Boilers. Sustainability. 2025; 17(14):6343. https://doi.org/10.3390/su17146343

Chicago/Turabian Style

Yang, Xia, Xuan Xu, Jianguo Ni, Qun Zhang, Gexiang Chen, Ying Liu, Wei Hong, Qiming Liao, and Xiongbo Chen. 2025. "Emission Characteristics and Environmental Impact of VOCs from Bagasse-Fired Biomass Boilers" Sustainability 17, no. 14: 6343. https://doi.org/10.3390/su17146343

APA Style

Yang, X., Xu, X., Ni, J., Zhang, Q., Chen, G., Liu, Y., Hong, W., Liao, Q., & Chen, X. (2025). Emission Characteristics and Environmental Impact of VOCs from Bagasse-Fired Biomass Boilers. Sustainability, 17(14), 6343. https://doi.org/10.3390/su17146343

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