DOI: https://doi.org/10.21203/rs.3.rs-1688150/v1
Industrial biomass boilers (IBBs) are widely promoted in China as a type of clean energy. However, they emit large amount of volatile organic compounds (VOCs) and the emission characteristics and the underlying factors are largely unknown due to the sampling difficulties. In this study, three wood pellet-fueled and two wood residue-fueled IBBs were selected to investigate the characteristics of VOCs emissions and to discover their underlying impacting factors. The emission factor of VOCs varied from 21.6 ± 2.8 mg/kg to 286.2 ± 10.8 mg/kg for the IBBs. Oxygenated VOCs (OVOCs) were the largest group, contributing to 30.3%-73.6% of the VOCs emissions. Significant differences were revealed in the VOCs source profiles between wood pellet-fueled and wood residue-fueled IBBs. Operating load, excess air, furnace temperature and fuel type were identified as the primary factors influencing VOCs emissions. The excess air coefficient should be limited below 3.5, roughly corresponding to the operating load of 62% and furnace temperature of 630 °C, to effectively reduce VOCs emissions. VOCs emissions also showed great differences in different combustion phases, with the ignition phase having much greater VOCs emissions than the stable combustion and the ember phases. The ozone formation potential (OFP) ranged from 4.3 mg/m3 to 31.2 mg/m3 for the IBBs, and the wood residue-fueled IBBs yielded higher OFP than the wood pellet-fueled ones. This study underscored the importance of OVOCs in IBB emissions, and reducing OVOCs emissions should be prioritized in formulating control measures to mitigate their impacts on the atmospheric environment and human health.
Biomass is considered as a type of clean and renewable energy source due to its low emissions of sulfur dioxide (SO2) and nitrogen oxide (NOx) during combustion (Vassilev et al., 2015). However, biomass burning is second only to biogenic sources in terms of volatile organic compounds (VOCs) emissions globally (Yokelson et al., 2013; Brilli et al., 2014; Vassilev et al., 2015). VOCs not only pose adverse impact on human health by containing a great number of toxic and harmful constituents (Boeglin et al., 2006; Chen et al., 2017; Ballesteros-González et al., 2020), but also act as important precursors in the formation of ozone and secondary organic aerosol (SOA) in the atmosphere (Crutzen and Andreae, 1990; Johnson et al., 2006; Yuan et al., 2013; Alvarado et al., 2015; Zhang et al., 2017). In order to formulate effective control strategies to negate the environmental and human health impacts of VOCs, characterization of VOCs from biomass burning is urgently needed.
Biomass burning refers to a large class of activities involving combustion of biomass. Over the past few decades, extensive researches have been conducted to investigate VOCs emissions from open biomass burning of specific fuels (e.g., Christian, 2003; Yang et al., 2020), open biomass burning of crop residues, forest, shrub land and grassland (e.g., Karl et al., 2007; Kudo et al., 2014; Sirithian et al., 2018; Mor et al., 2021), biomass fuels in residential boilers and stoves (e.g., Zhang and Smith, 1999; Huy et al., 2021; Zhang et al., 2021), and biomass fuels in domestic fireplaces (e.g., McDonald et al., 2000; Evtyugina et al., 2014). However, studies on VOCs emissions from biomass fuels in industrial biomass boilers (IBBs) are relatively scarce (Yan et al., 2016; Geng et al., 2019; Zheng et al., 2020).
Biomass fuels in IBBs are mostly in the forms of molded pellets, including wood pellets and straw pellets, as well as raw materials, including wood residues such as eucalyptus wood chips, bark, sawdust, and wood shavings (Santana et al., 2021). Geng et al. (2019) characterized 107 VOC species from two IBBs fueled by wood pellets and straw pellets, respectively. They found that oxygenated VOCs (OVOCs) and aromatics accounted for the major part of VOCs, and the emission factors (EFs) of straw pellets were higher than that of wood pellets. Zhang et al. (2020) measured 86 VOC species from five bagasse-fueled IBBs and found alkenes and OVOCs contributed the most to VOCs. Still in the initial stage, the studies of IBB emissions have two major limitations. One is the lack of VOCs characterization in different burning phases. Burning phase-specific VOCs compositions have been measured for residential biomass combustion and small-scale pellet boiler (Bhattu et al., 2019; Czech et al., 2017), but not for IBBs. In addition, as VOCs emissions from IBBs are strongly dependent upon operating conditions, the operational factors influencing VOCs emissions from IBBs needed to be well investigated to provide scientific support in formulating of control measures. Influencing factors of VOCs emissions have been studied in detail for residential stoves (Fachinger et al., 2017; Bhattu et al., 2019; Sun et al., 2019; Thompson et al., 2019), but not for IBBs either.
It is noted that the number of samples collected in the published IBB source characterization studies are relatively fewer, mostly no more than five (Yan et al., 2016; Geng et al., 2019; Zheng et al., 2020). This is because IBB sampling is technically challenging. Compared with other types of biomass burning such as residential stove and crop residual, the IBB sampling platform is narrow and the sampling port is high from the ground, as illustrated in Fig. S1 in the supporting information (SI), thus posing great challenges to the safety of sampling personnel and the placement of sampling system. In addition, IBB sampling is largely restricted by the operational time of the factory, significantly reducing the flexibility of the sample collection. These technical difficulties make IBB probably the least studied category in the biomass burning emission characterization.
Considering the scientific limitations of the current studies and the challenges in IBB sampling, we conducted measurement of VOCs emissions from five IBBs in this study, determined their EFs and source profiles, evaluated the chemical reactivity of VOCs emissions, and estimated the impacts of a set of parameters including operating load, excess air coefficient, furnace temperature, fuel type and different combustion phases on the VOCs emissions from IBBs. The results of this study would deepen understanding on VOCs emissions from IBBs in China, and provide a useful reference for optimizing the operating conditions of IBBs so as to reduce VOCs emissions and alleviate their contributions to the ambient ozone and particulate matter pollution.
In this study, three wood pellet-fueled IBBs (BB1, BB2 and BB3) and two wood residue-fueled IBBs (BB4 and BB5) in Zhaoqing City, Guangdong Province, China were selected for VOCs sampling and analysis. Detailed information of the selected IBBs is listed in Table S1 in the SI, and the sampling photos are presented in Fig. S1 in the SI. The average operating load of BB1−BB5 were 52%, 59%, 77%, 35% and 71%, respectively. All IBBs were equipped with dust removal devices, while no desulfurization and denitrification unit was installed in these boilers. Flue gas treatment systems were operating normally during sampling period. The oxygen concentrations of flue gas were monitored by a portable infrared flue gas analyzer (Photon, Madur, Austria). Detailed information on the fuels of wood pellets and wood residues, including elemental composition and heating value, is provided in Table S2 in the SI, and the photos of biomass fuels used in this study are shown in Fig. S2 in the SI. For both kinds of fuels, the percentages of volatile matter contents by weight were greater than 74.6%, and the moisture contents of fuels on an air-dried basis ranged from 6.2–10.7%.
The VOCs sampling system for IBBs is illustrated in Fig. 1. The sampling outlet was located between the flue gas treatment device and the chimney vent. The flue gas was pumped for several minutes to assure the air in the pipeline was drained away before sampling. The sampling system consisted of three modules. The first module was a portable flowmeter (Series 2440, Kurz Instruments Inc., USA) which detected the flue gas flow in the chimney, and the sampling probe was inserted at the end of the flow tube. The second module was a portable infrared flue gas analyzer (Photon, Madur., Austria) that measured the oxygen content of the flue gas. The third module was the VOCs sampling channel composed of the heated pipelines and filters to remove water and particulate matter within the flue gas, respectively. The heating tube was maintained at a temperature about 120 °C to prevent condensation of the VOCs, and the Teflon pipe was used behind the stainless steel tube to reduce adsorption. The flue gas was then separated into two branches. The first branch connected a SUMMA canister (3.2 L, Entech Instruments., USA) with a flow-limiting valve, and the sampling time was 20 min. The SUMMA canister was rinsed by Nutech 2101DS cleaner for eight cycles using high-purity nitrogen before sampling to ensure no residual contamination. The second branch connected in serial two silica cartridges coated with 2,4-dinitrophenylhydrazine (DNPH) (200 mg/6 mL, Bonna-Agela., China) to prevent penetration. The OVOCs concentration was obtained by adding the concentrations in the two tubes. The flow rate was approximately 200 mL/min, and the sampling time was 20 min. A KI oxidant scrubber (Bonna-Agela., China) was used before the DNPH cartridges to eliminate the interference from ozone. After sampling, DNPH cartridges were packed tightly with tin papers or aluminum foils, stored at temperature lower than 4 °C, and analyzed within 7 days after sampling. To increase the representativeness, we collected three VOCs samples consecutively for each IBB.
All five IBBs were measured under the stable combustion phase. To examine the differences of VOCs emissions in different combustion phases, we also collected VOCs samples during the ignition phase, the stable combustion phase, and the ember phase separately for the BB1 boiler. Due to the short duration of the ignition phase, only one sample was collected. An 8 L Tedlar sampling bag was used to collected VOCs sample for 2 min, and the sample was immediately transferred to the SUMMA canister and the DNPH cartridge for storage at the same flow rate as the stable combustion.
C2−C12 VOCs sampled by the SUMMA canister were analyzed according to the TO-14 and TO-15 methods recommended by the United States Environmental Protection Agency (USEPA). The samples were concentrated using a three-stage channel cryogenic pre-concentrator (Model 7200, Entech Instruments Inc., USA) and the moisture and CO2 were removed by a water management trap and a soda asbestos tube, respectively. VOCs species were characterized by a gas chromatography (7890B, Agilent Technologies Inc., USA) coupled with a quadrupole mass spectrometry/flame ionization detection system (5977B, Agilent Technologies Inc., USA). The OVOCs analysis was conducted according to the TO-11A method recommended by the USEPA. OVOCs collected in the DNPH cartridges were eluted with 5 mL of acetonitrile in a pollution-free environment, and the eluent was extracted to a 3 mL volumetric flask for testing and finally analyzed by the high performance liquid chromatography (1260, Agilent Technologies Inc., USA). Detailed analysis methods were described in the previous studies (Wang et al., 2013; Yan et al., 2016). In brief, the VOC samples were cryogenically concentrated and removed H2O and CO2 through a liquid nitrogen–cooled cryogenic trap (0.32 cm × 20 cm) at − 160 °C, a trap (0.32 cm × 20 cm) at − 40 °C with TenaxTA as adsorbents and cryofocus trap (0.08 cm × 5 cm) at − 170 °C in sequence. After that, the temperature of the trap will rise rapidly and the VOCs were transferred to the GC-MSD/FID system. The GC equipped with a DB-1 capillary column (60 m × 0.32 mm × 1.0 µm, Agilent Technologies, USA), helium was used as the carrier gas. The initial GC oven temperature was 10 °C for 3 min, and the temperature was raised to 120 °C at 5 °C/min, then the temperature was raised to 250 °C at 10 °C/min, and finally held at 250 °C for 10 min. The MSD was performed under the selective ion mode, and the ion source of mass spectrometry was electron ionization (EI) with electron impacting of 70 eV.
The EFs of IBBs were calculated on a mass basis by Eq. (1) (Yan et al., 2016).
where EFs is the emission factor (mg/kg fuel); \({C}_{i}\) is the converted mass concentration at a reference oxygen level of the total VOCs (mg/m3); Q is the volume flow rate of flue gas in the chimney (m³/hr); M is fuel consumption per hour (kg/hr).
In order to avoid the impact of different levels of excess air in different IBBs, the measured concentrations of each VOCs species were converted by Eq. (2)
where \({C}_{i}\) is the converted mass concentration at a reference oxygen; \({C}_{i,M}\) is the measured concentration in the flue gas; \({\phi \left({O}_{2}\right)}_{A}\) is the oxygen content in the ambient air, which is 21%; \({\phi \left({O}_{2}\right)}_{M}\) is the measured oxygen content in the flue gas; \({\phi \left({O}_{2}\right)}_{R}\) is the reference oxygen content. A value of 9% was used for \({\phi \left({O}_{2}\right)}_{R}\) in this calculation, as suggested by the Chinese standard GB/T 13271 − 2014 for the coal-fired and biomass-fired boilers (PRC MEE, 2014).
In general, combustion processes occur with an amount of excess air. The excess air coefficient, α, can be estimated based on the measured oxygen content by Eq. (3).
The ozone formation potential (OFP) of a VOC species was calculated using the maximum incremental reactivity (MIR) value by Eq. (4).
where OFP is the ozone formation potential (mg/m3); \({C}_{i}\) is mass concentration of species i (mg/m3); \({MIR}_{i}\) is the MIR value of species i extracted from Carter. (2010).
In this study, 109 VOCs species were characterized from the five IBBs, including 28 alkanes, 11 alkenes, 1 alkyne, 16 aromatics, 35 halocarbons and 18 OVOCs (Table S3 in the SI). The measured total VOCs (TVOCs) was converted according to Eq. (1). The concentrations of TVOCs emitted from BB1-BB5 were 1.1 ± 0.1 mg/m3, 3.2 ± 2.3 mg/m3, 1.3 ± 0.2 mg/m3, 7.7 ± 1.7 mg/m3, 2.6 ± 0.1 mg/m3, respectively.
By normalizing concentrations of VOCs species with respect to the total VOCs mass concentration, we obtained the VOCs source profiles, as shown in Fig. 2. The top 10 species in abundance from the five IBBs are marked blue. They accounted for 71.4 ± 0.9%, 80.9 ± 6.3%, 75.7 ± 1.5%, 74.2 ± 1.1% and 74.8 ± 2.5% of the total VOCs in BB1−BB5, respectively. Although the total proportions of the dominant species were similar among the five IBBs, large differences existed in the VOCs source profiles. In general, wood pellet-fueled IBBs emitted higher amounts of formaldehyde, acetaldehyde, benzene and acetylene, whereas wood residue-fueled IBBs emitted much greater proportions of methyl ethyl ketone (MEK), ethane and acrolein but significantly lower benzene and acetylene. Chloromethane, a tracer for VOCs emissions from biomass burning (Wang et al., 2009; Sun et al., 2019), was detected from all five IBBs.
As OVOCs was the dominant group, we further analyzed the relative contributions of 18 OVOCs species in the five IBBs, as shown in Fig. 3. Formaldehyde and acetaldehyde were the most abundant species, accounting for 42.9%−61.0% of the total OVOCs. This finding was consistent with those from previous biomass combustion studies. For example, Hedberg et al. (2002) found that formaldehyde and acetaldehyde accounted for 49.8% of the total OVOCs emitted from the residential stoves. Mo et al. (2016) reported their total proportions of 68.3%, 43.7% and 52.4% from the burning of rice stalks, corn stalks and wheat straw, respectively. We also found that acrolein and acetone accounted for 5.9%−33.8% of the total OVOCs. The predominance of OVOCs species with relatively lower molecular weights may be due to the incomplete oxidation combustion of alkanes and alkenes in the furnace with a high temperature. Apart from the low-molecule OVOCs, some high-molecule OVOCs, including aldehydes, alcohols and lipids, were also observed in the IBB emissions. For example, benzaldehyde accounted for 7.6% and 3.8% in BB1 and BB5, and vinyl acetate accounted for 11.2% and 7.5% in BB2 and BB5, respectively.
The total and group-specific EFs are shown in Fig. 4a. Among the five IBBs, the total EFs ranged from 21.6 ± 2.8 mg/kg to 286.2 ± 10.8 mg/kg of fuel. Significant differences in EFs were revealed among IBBs, which cannot be solely explained by the difference in fuel types. Studies have shown that a series of factors, including fuel type, combustion mode, boiler type, and smoke treatment, can impact VOCs emissions (Burling et al., 2010; Achaw and Afriyie, 2015; Sefidari et al., 2014; Szyszlak-Bargłowicz et al., 2015; Pognant et al., 2018; Geng et al., 2019). Influences of the factors on VOC emissions were analyzed in detail in “Factors influencing VOCs emissions” section.
We further examined EFs in different VOCs groups. OVOCs were consistently the largest contributor to the total VOCs emissions. Their EFs ranged from 6.5 ± 3.3 mg/kg to 120.1 ± 11.5 mg/kg, accounting for 30.3%−73.6% of the total VOCs, as shown in Fig. 4b. Zhang et al. (2020) found that OVOCs was the second largest group from IBBs, accounting for 13.4%−42.2% of the total VOCs. Their relatively lower OVOCs contribution was mainly because they did not measure formaldehyde. The largest contribution of OVOCs highlighted the importance of including OVOCs in the emission characterization of the IBBs.
The second largest group was aromatics, with the EFs ranging from 5.9 ± 0.2 mg/kg to 64.1 ± 8.5 mg/kg and accounting for 16.1%−27.3% of the total VOCs. This portion was close to 27.9%−29.2% reported by Geng et al. (2019). Benzene and some other aromatics are mainly produced by the degradation of lignin, so the high content of lignin in the biomass fuel may be responsible for the high emissions of aromatics (Greenberg et al., 2006; Evtyugina et al., 2014). Emissions of alkanes and alkenes were relatively lower from all five IBBs, accounting for 3.6%−14.6% and 0.9%−9.2% of the total VOCs, respectively. Alkanes and alkenes are mainly produced by the thermal degradation of cellulose and hemicellulose in 240−350 °C and 200−260 °C, respectively (Greenberg et al., 2006). The low emissions of alkanes and alkenes from the IBBs may be due to the much higher furnace temperature (> 500 °C) which further oxidized alkanes and alkenes into OVOCs.
Table 1 compares the VOCs EFs in this study with those reported in previous studies and assessed the differences between them. VOCs EFs from IBBs in this study were generally within the range of reported values (Geng et al., 2019; Zhang et al., 2020), but much lower than the official recommended value for IBBs in China (1130 mg/kg) (PRC MEE, 2014). VOCs emission inventories using the recommended EFs may markedly overestimate the actual emission from IBBs. EFs of VOCs from IBBs in this study were about three times those from power plants boilers (Yan et al., 2016). This is mainly due to the fact that OVOCs, accounting for up to 30.3%-72.1% in this study, had not been detected in power plant boilers. VOCs EFs from IBBs in this study were significantly lower than that of residential crop residue briquettes stove (Wang et al., 2013). Apart from the difference in fuel, the equipment of a special combustion chamber with an air supply system and a bag filter as the flue gas treatment device are the primary factors for the low EFs from IBBs. In comparison, the residential biomass stove usually only has a simple combustion chamber without an air supply system, leading to lower combustion efficiency and higher emissions from incomplete fuel combustion (Evtyugina et al., 2014; Thompson et al., 2019). Flue gas is often emitted directly through the chimney without flue gas treatment device or even without the chimney (Wang et al., 2013; Weyant et al., 2019). Therefore, lower combustion efficiency and the absence of flue gas treatment devices collectively contribute to the higher VOCs EFs from residential stoves than those of IBBs.
| Appliance | Fuel | VOC EFs (mg/kg) |
|---|---|---|
| Industrial boiler | Wood pellet | 21.6–163.9 a |
| Industrial boiler | Wood residue | 41.5–286.2 a |
| Industrial boiler | Sugarcane bagasse | 71–161 b |
| Industrial boiler | Wood pellet | 41.40–147.44 c |
| Industrial boiler | Straw pellet | 86.07–131.8 c |
| Industrial boiler | Biomass | 1130 d |
| Biomass-fired power plants | Maize straw | 57 e |
| Residential stove | Crop residue briquettes | 6160 ± 1140 f |
| Source: a This study; b Zhang et al. (2020); c Geng et al. (2019); d PRC MEE (2014); e Yan et al. (2016); f Wang et al. (2013) | ||
Previous studies have demonstrated that fuel properties (e.g., heating value, fixed carbon, element carbon, moisture) and operating conditions (e.g., operating load, furnace temperature) can impact VOCs emissions from biomass burning (Evtyugina et al., 2014; Sefidari et al., 2014; Aurell et al., 2017; Geng et al., 2019; Křůmal et al., 2019; Weyant et al., 2019). To examine the influence of fuel properties and operating conditions on VOCs emissions from the IBBs, correlations between EFs and a set of parameters were calculated, as shown in Table S4 in the SI. A first glance revealed that furnace temperature, operating load, heating value, fixed carbon, and element carbon all showed negative correlations with the EFs. We shall analyze their impacts on VOCs emissions in detail in the following sections.
Operating load, excess air coefficient and furnace temperature are three parameters that are interrelated with each other. Operating load refers to the percentage of the rated load of the boiler during operation, and it is one of the key factors to govern the completeness of fuel combustion (Pognant et al., 2018). Excess air coefficient refers to the ratio of the actual air supply to the theoretical air supply for fuel combustion. Since excess air brings the heat from the combustion chamber to outside, high excess air coefficient would not only reduce the temperature of the furnace, but decrease the combustion efficiency and release more pollutants from incomplete combustion (Szyszlak-Bargłowicz et al., 2015; Geng et al., 2019). As shown in Table S5 in the SI, the oxygen contents in the flue gases of all five IBBs were greater than 13%. The excess air coefficient of IBBs ranged from 3.0 to 4.5.
Figure 5 shows the relationship between operating load, excess air coefficient and furnace temperature with the EFs of the five IBBs. As expected, the operating load and furnace temperature were negatively correlated with the EFs, while the excess air coefficient was positively correlated with the EFs. BB4 with the lowest operating load (35%) and furnace temperature (510 °C) and the highest excess air coefficient (4.5) had the highest EFs of 293.1 mg/kg, whereas BB3 with the highest operating load (78%) and furnace temperature (850 °C) and the lowest excess air coefficient (3.0) showed the lowest EFs of 19.6 mg/kg.
Linear fitting was used between operating load, excess air coefficient and EFs, whereas exponential fitting was adopted between furnace temperature and EFs (Zhang et al., 2020). When the operating load increased by 10%, the excess air coefficient decreased by 1.0, the EFs decreased by 64.0 mg/kg, 200.4 mg/kg, respectively. Therefore, in order to reduce VOCs emissions from the IBBs, it is essential to control the appropriate operating load and excess air coefficient to increase the combustion efficiency. In the National Standard GB 13271 − 2001 of China, the recommended value of excess air coefficient for the coal-fired and biomass-fired boilers is 1.8 (PRC MEE, 2001). Considering the air leakage from the flue and auxiliary boilers, the measured oxygen content at the end of flue may rise to some extent. According to the results from this study, it is suggested that the excess air coefficient should be limited below 3.5, roughly corresponding to operating load of greater than 62% and furnace temperature higher than 630 °C.
We have demonstrated that operating load, excess air and furnace temperature are the leading factors governing VOCs emissions in “Operating load, excess air coefficient and furnace temperature” section. In this section, in order to examine the impact of fuel type on VOCs emissions, we selected the wood pellet-fueled BB1 and the wood residue-fueled BB4 for analysis. These two IBBs shared similar operating load, excess air and furnace temperature, and had the same rated evaporation capacity of 10 t/hr.
The EFs and relative contributions of the VOCs groups from BB1 and BB4 are shown in Fig. 6. The EF for BB1 and BB4 was 163.9 ± 18.4 mg/kg and 286.2 ± 10.8 mg/kg, respectively. The higher EFs of wood residue-fueled BB4 is due to its lower heating value, fixed carbon, element carbon content (Table S2 in the SI). Similar results have been reported in the literatures. Sun et al. (2019) found that heating value, fixed carbon, element carbon showed negative correlations with VOC EFs for domestic heating appliances. Yao et al. (2015) also found that the EFs of wood pellets were smaller than those of the direct firewood combustion.
Apart from measuring VOCs emissions under normal operating conditions, we also carried out VOCs sampling for BB1 in the ignition phase, the stable combustion phase, and the ember phase separately. In this section, we analyzed VOCs emissions during different phases, and discussed the impacts of the combustion phase on VOCs emissions.
Figure 7 shows the VOCs EFs during the ignition, stable combustion and ember phases. Obviously, VOCs emissions were mostly concentrated in the ignition phase. The total EFs in the ignition phase was 2,272.0 mg/kg, about 14 times that of the stable combustion phase (163.9 mg/kg) and 5 times that of the ember phase (458.1 mg/kg). The dominant percentage of 75.8% in the ignition phase was similar to 60%−75% of VOCs from the start phase in the log wood stove combustion (Bhattu et al., 2019). EFs of six groups (alkanes, alkenes, alkyne, aromatics, halocarbons, and OVOCs) all showed the same trend as the total EFs in different phases. The higher emissions during the ignition is mainly due to the low temperature in the furnace that is insufficient to ignite the volatiles released during combustion, leading to more incomplete combustion of the biomass.
In order to assess the OFP of VOCs emissions, we firstly calculate VOCs concentrations released from the five IBBs. Figure 8a shows the VOCs concentrations from the five IBBs and their percentages from different VOCs groups. The total VOCs concentration ranged from 1.1 mg/m3 to 7.7 mg/m3 for the five IBBs. VOCs concentration seemed to be more fuel type-dependent that the wood residue-fueled IBBs had higher concentration than the wood pellet-fueled ones. The structure of wood pellets is relatively dense so that the volatiles on the surface are firstly released and those inside are released slowly. The slow release of the volatiles increases the completeness of combustion, leading to relatively lower VOCs concentration (Li et al., 2011).
Figure 8b shows the OFPs of VOCs emissions from the five IBBs and their percentages from different VOCs groups. The OFP ranged from 4.3 mg/m3 to 31.2 mg/m3 for the five IBBs. The order of the five IBBs in terms of OFP (the shape of blue curve in Fig. 8a) was similar to that in terms of VOCs concentration (the shape of red curve in Fig. 8b), indicating that VOCs concentration was the leading factor for OFP and the impact of chemical composition was rather trivial. The relatively lower OFP for the wood pellet-fueled IBBs indicated that wood pellet is a relatively cleaner type of biomass fuel in comparison with wood residues. Wherever possible, replacement of the wood residues by the cleaner wood pellet is recommended to reduce VOCs emissions and their potential to form ozone and other secondary pollutants.
OVOCs accounted for 56.3%−80.7% of the total OFP, even greater than its mass contribution of 30.3%−73.6%, further underscoring their importance in the IBB emissions. Low-molecule OVOCs, especially formaldehyde, acetaldehyde and acrolein, were consistently within the top 10 VOCs species contributing to OFP for all IBBs, as shown in Fig. S3 in the SI. With a contribution of 50.4%−69.7% to the VOCs emissions, the top 10 species contributed to 83.3%−89.9% of the OFP for the five IBBs. Therefore, reducing OVOCs emissions should be prioritized in formulation of future control measures to mitigate emissions from the IBBs and their impacts on the atmospheric environment and human health.
In this study, five representative IBBs with two fuel types were selected to characterized VOCs emissions and their impacting factors. The total VOCs EFs ranged from 21.6 ± 2.8 mg/kg to 286.2 ± 10.8 mg/kg. OVOCs were the largest contributor to the total VOCs emissions, accounting for 30.3%−73.6% of the total VOCs. Although large differences existed in the VOCs source profiles among the five IBBs, formaldehyde and acetaldehyde were consistently the most abundant species, accounting for 42.9%−61.0% of the total OVOCs.
Operating load, excess air and furnace temperature were the leading factors affecting VOCs emissions. Operating load and furnace temperature were negatively, whereas excess air was positively correlated with the EFs. We suggested that the excess air coefficient should be limited below 3.5, roughly corresponding to the operating load of 62% and furnace temperature of 630 °C, to effectively reduce VOCs emissions from the IBBs. Apart from the operating conditions, fuel type also had a significant impact on VOCs emissions. The EF in the ignition phase was 14 times that in the stable combustion and 5 times that in the ember phase. The total OFP of five IBBs ranged from 4.3 mg/m3 to 31.2 mg/m3, and OVOCs contributed 56.3%−80.7% to the total OFP, much greater than their contributions in VOCs emissions. Wood residue-fueled IBBs yielded higher OFP than wood pellet-fueled ones, indicating wood pellet is a relatively cleaner types of biomass fuel in comparison with wood residues.
This study highlighted the importance of OVOCs emissions from IBBs. Reducing OVOCs emissions should be prioritized in formulating control measures to mitigate their impacts on the atmospheric environment and human health. Although with great challenges in sample collection, it is suggested to carry out a more detailed study to give a more comprehensive description on VOCs emission characteristics and a more accurate identification on the underlying factors.
Author Contributions Conceptualization: Daojian Huang, Zibing Yuan; Methodology: Ruidan Shi, Daojian Huang; Formal analysis and investigation: Ruidan Shi, Leifeng Yang; Writing - original draft preparation: Ruidan Shi; Writing - review and editing: Daojian Huang, Zibing Yuan; Funding acquisition: Daojian Huang, Zibing Yuan; Resources: Hui Ma, Zibing Yuan; Supervision: Daojian Huang
Funding This study was supported by the Key-Area Research and Development Program of Guangdong Province (2020B1111360003) and Central Public-interest Scientific Institution Basal Research Fund of South China Institute of Environmental Sciences (PM-zx703-202201-005).
Data availability The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.
Ethics approval Not applicable.
Consent to participate Not applicable.
Consent for publication Not applicable.
Competing interests The authors declare no competing interests.