Kamilla
Arnesen
a,
Vegar
Andersen
a,
Katarina
Jakovljevic
a,
Ellen Katrin
Enge
b,
Heiko
Gaertner
c,
Thor Anders
Aarhaug
c,
Kristian Etienne
Einarsrud
a and
Gabriella
Tranell
*a
aDepartment of Materials Science and Engineering, Norwegian University of Science and Technology (NTNU), 7034 Trondheim, Norway. E-mail: kamilla.arnesen@ntnu.no; Gabriella.tranell@ntnu.no
bNorwegian Institute of Air Research (NILU), 2007 Kjeller, Norway
cSINTEF Industry, 7034 Trondheim, Norway
First published on 18th December 2023
Silicon alloys are produced by carbothermic reduction of quartz in a submerged arc furnace. This high-temperature pyrolytic process is a source of polycyclic aromatic hydrocarbons (PAHs), which are a group of aromatic organic molecules with known mutagenic and carcinogenic properties. In this study, the emission of oxy- and nitro-PAHs from a pilot-scale Si furnace, with varying process conditions such as oxygen level, flue gas recirculation (FGR), and off-gas flow, was investigated. Analysis shows the presence of both oxy- and nitro-PAH species in all experiments, believed to be formed from radical-induced substitution reactions initiated by SiO combustion and NOx formation. During Si production without FGR, the levels of oxy- and nitro-PAHs range between 1.1 and 4.4 μg Nm−3, independent of the flue gas flow rate. With increasing FGR (0–82.5%) and decreasing oxygen level (20.7–13.3%), the concentrations of both oxy- and nitro-PAHs increase to 36.6 and 65.9 μg Nm−3, respectively. When the levels of substituted PAHs increase, species such as 4-nitropyrene and 1,2-benzanthraquinone are in abundance compared to their parent PAHs. Experiments at lower flue gas flow (500 Nm3 h−1versus 1000 Nm3 h−1) generally produce less substituted PAHs, as well as SiO2 particulate matter and NOx, where the latter two parameters have a 99% correlation in this study.
Environmental significancePolycyclic aromatic hydrocarbons (PAHs) are a group of organic molecules consisting of aromatic rings which are considered to be hazardous to human health. Some PAH molecules are proven to be carcinogenic and reduced exposure is recommended. In the silicon process, PAHs originate from pyrolysis and combustion of the carbon materials and due to the ability of PAHs to accumulate in the environment, emissions should be reduced. Substituted PAH species are not well investigated as products of industrial processes, and some argue that they have been neglected as a monitored environmental pollutant. The result of this work showed the presence of both oxy- and nitro-PAH emissions from a pilot-scale silicon furnace and levels increasing with flue gas recirculation and poorer conditions of combustion. |
Metallurgical grade silicon (MG-Si) is typically produced through carbothermal reduction of quartz, using a mix of coal, coke, charcoal and woodchips in a semi-closed submerged arc furnace (SAF). The carbon materials reduce the quartz to a silicon alloy by the overall reaction shown in eqn (1), where σ is the Si-yield, which gives the distribution between tapped Si and gaseous silicon monoxide (SiO) leaving the furnace.3
SiO2 + (1 + σ)C = σSi + (1 − σ)SiO(g) + (1 + σ)CO(g) | (1) |
Silicon monoxide and carbon monoxide (CO) that reach the top of the furnace react with air and form silica fumes (SiO2) and carbon dioxide (CO2), respectively, in the furnace hood.3
The combustion affects the temperature at the charge surface in the furnace which can reach 700 to 1300 °C. Because the SAF is semi-closed, the under-pressure in the furnace drives the surrounding air into the furnace hood diluting the furnace off-gas, which consists mainly of air. The excess of air combined with the high temperature produces thermal NOx through combustion, the so called Zeldovich mechanism, of nitrogen and oxygen in the air by radical formation. As described by Kamfjord,4 the SiO and CO combustion produces high temperature hot spots at the top of the furnace charge, where also VOCs and moisture evaporate, creating conditions for fuel, prompt and thermal NOx formation.
PAHs are emitted from the furnace, either from the top of the gas-permeable furnace charge material or through gas released during the tapping process. Typically these PAHs originate from pyrolysis, combustion of the carbon raw materials and baking of the electrodes.5,6 Thomas and Wornat7 described the importance of oxygen and temperature in the formation and destruction of PAHs in a high temperature reactor using catechol as a model fuel. By investigating the effect of varying oxygen levels and temperatures up to 1000 °C, oxygen is recognized to have an important role in creating free radicals that lead to increased PAH formation at T < 850 °C and also increased PAH destruction at T > 850 °C at high levels of oxygen.
In the work by Hrdina et al.,8 parent PAH transformation through atmospheric oxidation is described, recognizing the key atmospheric oxidants of PAHs to include ozone (O3), hydroxyl radicals (OH˙), nitrogen dioxide (NO2), and nitrate radicals . Nitrated PAHs (nitro-PAH) and oxygenated PAHs (oxy-PAH) are two groups of substituted PAHs containing at least one nitro group (NO2) or carbonylic oxygen attached to an aromatic ring.9 As a result, some of the physiochemical properties change, compared to the parent PAHs, which in turn alter their environmental and toxicological effects. The International Agency for Research on Cancer (IARC) classifies 1-nitropyrene as “probably carcinogenic to humans”, with PAH derivatives recognized as mutagens.10 These electrophilic aromatic substitution reactions can occur both in gas and solid phases and produce various substituted PAHs, such as nitro- and oxy-PAH species.11–13 Atkinson et al.14 investigated the reactivity of naphthalene in a N2O5–NO3–NO2–air mixture under troposphere conditions and found naphthalene to form nitro-arenes by initial formation of the NO3-PAH adduct, followed by a reaction with NO2. The initial reaction to create a PAH adduct is seen to form both oxy and nitro-PAH species, based on the continued reaction of the adduct with available reactants, independent on the initiation of the reaction (by nitrate or hydroxyl radicals).8,15 Oxy- and nitro-PAH compounds are also detected in research on emissions from diesel engines, where NOx is known to be a considerable part of the exhaust. Heeb et al.16 investigated how diesel engine filters influence PAH and nitro-PAH profiles in diesel exhaust and found the filter to promote the formation of some nitro-PAHs, enhancing the degradation of others depending on the operating conditions of the engine. Hayakawa17 investigated the formation of nitro-PAHs compared to their parent PAHs with increasing combustion temperatures, representing diesel-engines, coal and wood burning stoves. The nitro-PAH/PAH ratio was observed to increase with temperature for compounds such as 1-nitropyrene, 6-nitrochrysene and 7-nitrobenz[a]anthracene. Oxy-PAHs have been found in emissions related to diesel exhaust, charcoal and coal production.18 Drotikova et al.19 measured PAHs in ambient air in Longyearbyen, Svalbard, and found coal-fired power plants to be one of the main sources of PAHs, with 9-fluorenone and 9,10-anthraquinone to be the dominating oxy-PAH species.
Flue gas recirculation (FGR), or exhaust gas recirculation, is a known method used for emission control by recycling parts of the process off-gas, e.g. in diesel engines to reduce oxygen levels, temperature and NOx formation, and as a way to concentrate off-gas species, such as CO2, for further processing for CO2 capture.20–22 The method has also been found to increase the level of hydrocarbon species (including PAHs), when temperature and oxygen concentrations reach a certain level due to reduced oxidation efficiency.23–25 Wittgens et al.26 investigated the possibility of including a post-combustion chamber for increased energy recovery with reduced PAH and NOx emissions in the ferroalloy industry with positive results. Together with the PhD thesis of Andersen (2023),27 studying flue gas recirculation (FGR) for the silicon process, and Kamfjord (2012),4 studying NOx formation in the silicon process, these represent a body of work forming the basis for understanding the off-gas systems and the current status for emissions of PAHs from the SAF-based Si production process.
With increased EU and global focus on industrial emissions (PM, PFAS, GHG, etc.), it will become essential for industries to better understand complex and less understood pollutants outside the regularly reported, such as in this case of PAH-16.
While the authors in previous work went in depth on an extensive list of native PAHs, to the authors knowledge this is the first study trying to determine nitro- and oxy-substituted PAH emissions from ferroalloy production as an area still to be explored. The focus for the current work was therefore to determine if, and potentially how much substituted PAHs was formed, evaluate and compare these concentration levels in the off-gas from a pilot scale silicon furnace with and without flue gas recirculation.
A mix of coal, coke, charcoal, woodchips and quartz made up the raw material, which were supplied by an industrial partner. The materials were used as received, except for the woodchips, which were dried at 100 °C for 24 h. Carbon raw material specifications are found in Table S3.†
Compound | MWa [g mol−1] | # of rings | IARC classificationb |
---|---|---|---|
a PAH MW.31 b PAH IARC status.10 | |||
9-Nitroanthracene | 223.2 | 3 | Group 3 |
2 + 3-Nitrofluoranthene | 247.3 | 4 | Group 3 |
1-Nitropyrene | 247.3 | 4 | Group 2A |
4-Nitropyrene | 247.3 | 4 | Group 2B |
3-Nitrobenzanthrone | 275.3 | 4 | Group 2B |
7-Nitrobenz[a]anthracene | 273.3 | 4 | Group 3 |
1,3-Dinitropyrene | 292.2 | 4 | Group 2B |
1,6-Dinitropyrene | 292.2 | 4 | Group 2B |
9-Fluorenone | 180.2 | 3 | — |
9,10-Anthraquinone | 208.2 | 3 | — |
2-Methyl-9,10-anthraquinone | 222.2 | 3 | — |
6H-Benz[de]anthracen-6-one | 230.3 | 4 | — |
1,2-Benzanthraquinone | 258.3 | 4 | — |
6H-Benzo[cd]pyren-6-one | 254.3 | 5 | — |
On average, 0.30 Nm3 off-gas was sampled per hour of silicon production. Results from 9 different experiments, out of 27 runs, will be presented in this work, which include a total of 27 samples where the FGR levels vary from 0 to 75.0% for experiments at a 500 Nm3 h−1 off-gas flow rate and from 0 to 82.5% for experiments at 1000 Nm3 h−1. An overview of the total concentration of PAH-42, oxy- and nitro-PAHs from experiments at varying FGR for 1000 and 500 Nm3 h−1 is presented in Fig. 1, and the results for all oxy- and nitro-compounds can be found in Table S1 in the ESI.†
Fig. 1 Graphs showing the total PAH-42, oxy- and nitro-PAH concentrations in the off-gas at varying FGR levels for experiments at (a) 1000 Nm3 h−1 and (b) 500 Nm3 h−1. |
The concentrations are a sum of 8 nitro-species with 3 and 4 rings, 6 oxy-species with 3 to 5 rings, and PAH-42, which represent an extended EPA-16 list that also includes heterocyclic and alkylated PAHs.
Overall, an increase in PAH concentration was observed when comparing the Si production with and without FGR for both the 1000 and 500 Nm3 h−1 flow rates. Higher PAH concentrations were generally observed for the 1000 Nm3 h−1 experiments as further elaborated by Arnesen et al.,30 where the increased temperature and residence time at low flow are believed to result in better conditions for combustion. An important note is the number of species being analyzed, which represent the different categories, and analyzing 42 species would expect to result in a higher concentration than the 14 substituted PAHs.
Fig. 2 Graph showing the variation in particulate matter and NOx produced during Si production, for experiments at both 1000 Nm3 h−1 and 500 Nm3 h−1. |
Sample nr | Flow [Nm3 h−1] | FGR [%] | Oxygen [vol%] | Flue gas temperature [°C] | Nitro-PAH [μg Nm−3] |
---|---|---|---|---|---|
1 | 965 | 0.6 | 20.7 | 208 | 1.353 |
8 | 914 | 0.9 | 20.7 | 249 | 1.869 |
5 | 935 | 60.1 | 17.6 | 238 | 18.468 |
4 | 865 | 81.3 | 15.5 | 232 | 65.867 |
13 | 919 | 82.5 | 13.3 | 237 | 9.807 |
9 | 566 | 0.3 | 20.4 | 268 | 1.678 |
3 | 440 | 56.6 | 17.9 | 275 | 18.219 |
7 | 497 | 59.6 | 17.3 | 293 | 24.010 |
6 | 425 | 75.0 | 16.4 | 275 | 14.001 |
With some variation a general trend is observed. When combustion conditions change, the level of nitro-PAHs increases up to 65.9 μg Nm−3, with lower available oxygen, which indicates that higher flue gas recirculation rates that lead to lower oxygen levels in the furnace atmosphere can be a substantial driving force for the generation of nitro-PAHs. Some variation is present between the two flow rates (500 and 1000 Nm3 h−1), where the concentration reaches higher levels for the increased flow rate. On average, 96.8% of the measured nitro-PAH species were found in the filter samples. As expected, these species are present as a condensed phase based on molecular weight and volatility, combined with the average flue gas temperature.
The compound 4-nitropyrene is the most abundant nitro-PAH species of those quantified in every sample, accounting for 39.5 to 91.5% of the total measured nitro-PAH concentration. The second most abundant species is the combined 2 and 3-nitrofluoranthene, with levels up to 34.0% of the total concentration in the samples. Fig. 3 shows comparisons between the concentration of nitro-PAH species with their parent PAHs at different FGR levels (concentrations of the parent PAHs can be found in the ESI† of Arnesen et al.30). The anthracene species are shown in Fig. 3(a) and (b), where the parent species is in abundance for most of the experiments, with the exception of experiments at 59.6 and 60.1% FGR, where the distribution is approximately equal. For fluoranthene, in Fig. 3(c) and (d), more 2 + 3-nitrofluoranthene (separation of 2- and 3-nitrofluoranthene was not achieved with the current analysis technique) than its parent is detected in three of the nine experiments. The opposite is the case for pyrene species in Fig. 3(e) and (f), where regular pyrene is only dominating in one experiment at 82.5% FGR. Overall, the dominating pyrene species are 4-nitropyrene, followed by 1,3-dinitropyrene, which in some cases exhibit the same concentration levels as pyrene.
Even though the level of NOx and the concentration of nitro-PAHs did not show a direct correlation with the level of flue gas recirculation in this study (5% correlation), it becomes clear that the formation of PAH species in a pyrolytic system such as the Si furnace is a balance between oxidative combustion, depending on temperature and oxygen, and other competing reactions.
Nitrogen oxides must be present for nitro-PAHs to form and NOx forming reactions are a source of radicals and atmospheric oxidative reactions. Hrdina et al.8 described the necessity of radicals and radical-initiated reactions to create oxidants such as and OH˙, which function as activators of the substitution reaction. Katritzky et al.11 described the nature of an aromatic substitution reaction, where the initial reaction step produces a temporary positively charged arenium ion. The initial reaction to form the arenium ion is typically the rate-limiting step, which will react further with electron rich molecules. In the case of this study, oxygen or NO2 is expected to react and form the final oxy- or nitro-PAH molecule. An example of how nitration of pyrene could occur is presented in Fig. 4.
The mechanism of the substitution reaction and preferential isomeric formation is often used to characterize PAH emissions. From studies examining nitro-PAH emissions from diesel vehicles (Hu et al.)32 and substituted PAHs in pollution at rural and urban locations in France (Albinet et al.),12 the isomer of 1-nitropyrene is observed and viewed as an favored isomeric pyrene specie. Atkinson et al.14 investigated the kinetics of nitro-PAH formation and found 1- and 2-nitropyrene to be favored over 4-nitropyrene in OH radical-initiated reactions with pyrene under ambient conditions. They also determined the rate constants for gas-phase reactions in N2O5–NO3–NO2–air, and found both 2- and 4-nitropyrene to be a product of the NO3 radical-initiated reaction. 2-Nitropyrene was found to be dependent on the NO2 concentration, but no such connection was discovered for 4-nitropyrene. In this current study, 4-nitropyrene was the abundant pyrene species in all samples, and 4-nitropyrene is thus most likely a product of the gas-phase NO3 radical-initiated reaction directly in the furnace and a result of a primary emission source.
Sample nr | Flow [Nm3 h−1] | FGR [%] | Oxygen [vol%] | Flue gas temperature [°C] | Oxy-PAH [μg Nm−3] |
---|---|---|---|---|---|
1 | 965 | 0.6 | 20.7 | 208 | 4.389 |
8 | 914 | 0.9 | 20.7 | 249 | 1.131 |
5 | 935 | 60.1 | 17.6 | 238 | 8.026 |
4 | 865 | 81.3 | 15.5 | 232 | 35.928 |
13 | 919 | 82.5 | 13.3 | 237 | 36.647 |
9 | 566 | 0.3 | 20.4 | 268 | 1.228 |
3 | 440 | 56.6 | 17.9 | 275 | 15.280 |
7 | 497 | 59.6 | 17.3 | 293 | 9.297 |
6 | 425 | 75.0 | 16.4 | 275 | 5.620 |
1,2-Benzanthraquinone is the most abundant of the measured oxy-PAH species in 7 out of 9 samples, making up between 41.2 and 69.1% of the total concentration of the total measured oxy-PAH level, followed by 9,10-anthraquinone, accounting for up to 43.5% of the total amount in some samples. A comparison between the oxy-PAH concentration and their parent PAH is presented in Fig. 5. Fluorene species are shown in Fig. 5(a) and (b), and fluorene is in abundance for all samples but one (the experiment at 59.6% FGR is the exception where the levels are approximately equal). 9,10-Anthraquinone is the dominating species in the anthracene comparison in Fig. 5(c) and (d), but 2-methyl-9,10-anthraquinone is also equal to or exceeds the concentration level of anthracene. Compared to the nitro-PAH species with anthracene as its parent molecule, their oxy-PAH equivalents are in abundance. In Fig. 5(e) and (f), oxy- and nitro-PAH species are compared with their parent PAH, benz[a]anthracene. 7-Nitrobenz[a]anthracene is not produced at significant levels for the experiments at 0% FGR, but increases with increasing FGR, where it exceeds its parent molecule. As with the other quinone molecules already mentioned, 1,2-benz[a]anthraquinone is found in high concentration in all experiments, compared to benz[a]anthracene, especially with increased FGR.
Even though only a small range of substituted PAHs were analyzed and some PAHs are not investigated, such as bicyclic PAHs, a trend is observed. In general, oxy-PAHs follow the same trend as nitro-PAHs and increase at higher levels of FGR. With increasing FGR, the substituted oxy species are equal to or exceed their parent PAH in concentration. These findings are in agreement with the study by Drotikova et al.19 9-Fluorenone and 9,10-anthraquinone were found to be the dominating oxy-PAH emissions from a coal power plant in Svalbard. Albinet et al.33 also found 9,10-anthraquinone to correlate with emission from diesel engines and described the observation as a result of gas-phase formation by ozonation. This coincides with quinones being a result of high temperature combustion processes, as the formation mechanisms that create nitro-PAHs are also applicable to oxy-PAHs. The observation where some diesel particle filters are seen to facilitate the formation of substituted PAHs16 is interesting in terms of possible parallels to the Si process, where SiO and SiO2 PM can have a catalytic effect on radical formation or on the formation of PAH precursors and substituted PAHs.
Even though process and temperature variations were not taken into account, an indication towards the PAH emissions, both PAH-42 and substituted PAHs, not being significantly increasing with added FGR is shown, as more of the increased PAHs produced would be recirculated back to the furnace. The exception is the case at 1000 Nm3 h−1 and 82.5% FGR in both Fig. 6(a) and (b), where the filter efficiency is lower due increasing the production of PAHs of low molecular weight, which can pass through the filter.30 This experiment measured on average 13.3 vol% oxygen in the furnace atmosphere and together with the general decreased off-gas temperature and residence time for the PAHs at the 1000 Nm3 h−1 flow rate, and these furnace conditions could provide poor overall combustion conditions for PAH degradation.
Footnote |
† Electronic supplementary information (ESI) available: A full overview of the oxy- and nitro-PAH analysis results, the pilot-scale set-up, and details of the raw materials and the PAH standards. See DOI: https://doi.org/10.1039/d3va00187c |
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