Emission concentrations and source profiles of VOCs
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.
Emission factors of VOCs
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.
Table 1
Comparison of VOC EFs in this study with previous studies
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) |
Factors influencing VOCs emissions
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
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.
Fuel type
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.
Combustion phase
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.
Assessment of ozone formation potential
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.