key: cord-0872954-lp54ycff authors: Dandajeh, Hamisu Adamu; Ladommatos, Nicos; Hellier, Paul title: Influence of unsaturation of hydrocarbons on the characteristics and carcinogenicity of soot particles date: 2020-08-12 journal: J Anal Appl Pyrolysis DOI: 10.1016/j.jaap.2020.104900 sha: e068a22d2c3d66b1eb7624c11b10b28d5eb33e35 doc_id: 872954 cord_uid: lp54ycff Abstract This paper concerns the effect of unsaturation of hydrocarbons (single, double, and triple bonds) on soot particle characteristics (mass, number, and size) and on the carcinogenicity of soot particles. The soot particles were produced from oxygen-free pyrolysis of five hydrocarbons, namely: propane, propylene, ethane, ethylene, and acetylene. The characteristics of soot particles were measured with the aid of a differential mobility spectrometer (Cambustion-DMS-500) and measurement of soot mass concentration was confirmed using gravimetric filter measurements. The soot particle carcinogenicity was estimated from the emission quantities of total polyaromatic hydrocarbons (PAHs) and the toxicity equivalent factor (TEF) of each PAH. Oxygen-free pyrolysis of the hydrocarbon fuels was conducted in a laminar tube reactor within the temperature range of 1050 –1350oC at a constant nitrogen flow rate of 20 L/min and constant fuel flow rate of 1% (vol) on carbon-1 basis. The experimental results showed that increasing unsaturation of fuels from single to double and to triple bonds increased the mass concentration, particle size, number concentration, and carcinogenicity of soot particle notably at the initial temperature of 1050 oC. Increase in the pyrolysis temperature of the tube reactor from 1050 – 1350oC, increased the mass concentration and sizes of the soot particle while the number concentration and carcinogenicity of the soot particle decreased. There was a positive correlation between the soot particle number and the corresponding soot particle carcinogenicity, while a negative correlation was observed between the soot particle mass and size with soot particle carcinogenicity regardless of the pyrolysis temperature examined. The potential implication of these observations is that, low-temperature combustion (LTC) applications, aimed at reducing emissions of soot and NOx, could produce higher soot particle number concentration of higher carcinogenicity. Inhalation of atmospheric air contaminated with soot particles has led, and is still leading to the rising global human morbidities and mortalities [1] . The correlation between human exposure to atmospheric air contaminated by soot emissions and recent fatalities due to COVID-19 in the United States has been established [2] . While Poly-cyclic aromatic hydrocarbons (PAHs) are known as the primary precursors of soot particles; they also constitute some sizable noxious components deposited onto soot particles [3] . PAHs are carcinogenic and mutagenic and can be found both attached onto the surfaces of particulates and as inhalable gaseous substances available in the atmosphere. Large atmospheric soot particles of diameter lower than 10µm emitted from various combustion sources could be accumulated into the superior airways of respiration [4] , [5] . Inhalable soot particles of diameter lower than 1µm and ultrafine particles of diameter less than 100nm, carrying toxic PAHs, are possibly deposited into the lung of a human being, leading to health complications such as lung cancer, cardiovascular diseases and ultimately premature death [6] . Incidences of human mortality related to air-bone soot particles carrying PAHs have been revealed in many global cities [7] , [8] [9] , [10] . There are presently several hundreds of PAHs in nature, but only 16 Carcinogenicity of particulates bearing adsorbed PAHs is affected by the molecular configuration of the fuel [10] , pyrolysis/combustion temperature [11] and particle characteristics of the exhaust soot formed. For example, Sánchez et al., [11] investigated acetylene pyrolysis in a tube-reactor. They reported that the concentration and toxicity of both PAHs and soot depend on the pyrolysis temperature. Dandajeh et al. [12] researched the effect of the unsaturation of hydrocarbons having carbon numbers of 2 and 3 on the PAHs formed by those hydrocarbons. They concluded that increasing the unsaturation of fuels increased the type and concentration of PAHs formed depending on the pyrolysis temperature. In a related study, Dandajeh et al. [13] experimentally investigated the effect of increasing J o u r n a l P r e -p r o o f carbon number of fuels from 1 to 7 in forming PAHs in a pyrolysis tube reactor. They reported that increasing the carbon number of fuels from 1 to 7 increased the propensity of fuel molecules to soot depending on the pyrolysis temperature. Mei et al., [14] recently studied the experimental effects of adding CO2 in the pyrolysis of ethylene. Their results revealed that adding CO2 in small quantities (up to 10%) promoted the quantity of soot formed while also increasing the soot particle volume, soot particle number concentrations and, decrease in soot induction time. However, increasing CO2 proportion beyond 10%, Mei et al reported a decrease in the rates of soot nucleation and soot mass growth. Kashiwa et al. [15] investigated the influence of oxygen on PAH and particulate formation in benzene pyrolysis at low temperatures. They reported forming small fraction of particulates at a low temperature of 900 o C and zero oxygen and particulate formation increased with increasing oxygen level. Despite these remarkable contributions by numerous researchers on hydrocarbon pyrolysis, investigating the influence of unsaturation of hydrocarbon fuels on soot particle characteristics (mass, number, size, and surface area) and soot particle carcinogenicity has been rarely studied. The interdependence of soot particle mass, size, number, and surface area on the carcinogenicity of soot particles was, to the best of our knowledge, also not investigated. The paper herein, therefore, describes an experimental analysis on the influence of unsaturation of hydrocarbons on soot particle carcinogenicity and the interdependence of soot particle characteristics on soot carcinogenicity. The study was carried out in a pyrolysis furnace at a temperature range of 1050 -1350 o C. PAHs in the gas phase and those attached to soot particles were trapped on XAD-2 resin and a glass microfibre filter, respectively. These PAHs were extracted using an accelerated solvent extraction system. The extracts of the PAHs were later analysed using Gas Chromatography (GC) coupled to Mass Spectrometry (MS). The carcinogenic equivalent sum of the soot particles produced by each fuel was calculated from the total PAH concentration and the toxicity equivalent factors (TEFs) shown in Table 1 . Soot J o u r n a l P r e -p r o o f particle characteristics (particle mean diameter, number concentrations, surface area and mass concentrations) were measured and analysed using Cambustion-DMS 500 instrument. Five gaseous hydrocarbon molecules are propane, propylene, ethane, ethylene, and acetylene. Each of the hydrocarbon fuel had a purity of over 99 %. Table 2 shows the properties of the hydrocarbons. The objective of this study is to analyse the influence of unsaturation of fuels listed in Table 2 as follows: Determine the effect of unsaturation of fuels on the gravimetric filter measurements of mass concentration Characterise the soot particles based on size, mass and number concentration III. Confirm the soot mass concentration measurements from the gravimetric filter values and those obtained from DMS 500. Investigate the interdependence of soot particle characteristics on the soot particle carcinogenicity V. Investigate temperature influence on the characteristics and carcinogenicity of soot particles Samples of soot particles were generated by pyrolysis within the temperature range of 1050 to 1350 °C. The pyrolysis was conducted at temperature intervals of 100 o C using a tube reactor J o u r n a l P r e -p r o o f 6 shown in Figure 1 . The carrier gas (nitrogen) was measured at a fixed flow rate of 20 L/min. All the five fuel molecules had a fixed flow rate of carbon of 1% on C1 basis. Example, the flow rate of ethylene, with 2 carbon number, was about 1.5 times higher than that for propylene, a fuel with a carbon number of 3. To check daily repeatability and to detect any potential change in the experimentation, ethane pyrolysis was used as a baseline data. Gas-phase PAHs were also sampled under the same conditions. probe [12] . The Alumina tube was put in a vertical form in an electrically controlled furnace and 60 cm of the tube central length was heated between 1050 -1350 °C. The axial temperature distribution profile in the tube reactor over this 60 cm length axis was uniform as reported in earlier published works [16] . The gas average residence time (tr) was evaluated using equation 1. Where Vr (L) is the volume (in litres) of the 60 cm heated portion of the tube. Q is the flow rate (litres/second) at the test temperatures. The average residence time was temperature (T) dependent. Soot particles and gaseous PAHs were captured from the reactor's outlet using a stainless-steel probe. The probe was attached to a vacuum-pump and soot samples were drawn at a temperature of 100 o C intervals. The sampling probe leading to where the microfibre filter is housed was heated and sustained at a temperature of 120 °C. This was done primarily, to avoid gas phase PAHs condensing within the sampling line. The sampling probe was installed at the outlet of the reactor and was also connected to the filter by a 12.5 mm stainless-steel tube. A control valve was used to maintain the vacuum pump's flow rate always at less than 18 L/min. A filter and XAD-2 resin systems were placed in tandem, to collect soot particles and gas phase PAHs, respectively. Before sampling, the glass micro-fiber filter was baked to 120 o C in an electrical oven for a period of 8 hours in order to reduce moisture on the filter. The baked filter was used immediately after removal from the oven. The filter is 70 mm in diameter, 700 nm pore size, and has 310 seconds filtration speed. The filter was selected since it can withstand high temperatures and was also found suitable by many other studies [11, 17] . Soot samples were measured on a mass balance having 0.001 mg resolution. Figure 2a and b show samples of soot and gas-phase PAHs collected during ethane pyrolysis at a temperature of 1150 o C. The XAD-2 resin, onto which the gas phase PAHs were collected, had a surface area of ~300 m 2 /g, 90 Å average pore size and 1.02 g/mL density at a temperature of 25 °C. XAD-2 resin was chosen due to its superior retention and trapping of gaseous PAHs compared with other materials [17] . To capture the gas phase PAHs effectively, a 5g of XAD-2 resin, embedded within two pieces of glass wool, was initially put into a glass cartridge. The cartridge was loaded into a customized housing made from stainless steel. The XAD-2 resin housing system was coupled in tandem with the glass microfiber filter system. Glass wool Adsorbed PAHs on XAD-2 resin Soot sample The volume of gas (Vg), passing through the microfiber filter and the XAD-2 resin was metered using a volumetric diaphragm gas meter. The gas meter has a maximum volumetric flow (Qmax) rate of 6 m 3 /h. The duration of sampling the soot particles and gaseous PAHs were kept at 15 min for all the test temperatures. The sampling duration was optimised to collect a satisfactory mass of soot that can yield visible PAH peaks on the GC-MS. The filter gravimetric soot mass (Ms) and the cumulative volume of gas (Vg) were measured for each test duration. PAH extraction from the samples of the soot and XAD-2 resin, shown in Figure Table 3 . The DCM in the extracted PAH sample was evaporated by bubbling nitrogen gas into the PAH-DCM solution; thereby leaving concentrated PAH extracts. The concentrated PAH extracts were identified using Gas chromatograph (GC) and quantified using the Mass Spectrometer (MS). The GC was first calibrated using a standard QTM PAH Mix Standard prior to the PAH quantification. Details of the calibration procedure can be found in the supplementary material. The PAH standard contained the 16 EPA PAHs (see Table 1 ). The optimised GC-MS operating conditions are shown in Table 4 and detailed PAH extraction and analysis procedures can be found in [12] , [13] . Table 5 . Figure 3 , therefore, shows the calculated gravimetric filter mass concentration (Ms/Vg) emanating from Table 5 . The error bars on the ethane data in Figure 3 denotes standard deviation derived from the repeat tests with ethane. It can be deduced from Table 5 that, for all the hydrocarbons pyrolysed, increasing temperature of the reactor from 1050 -1350 o C resulted in an increase in the mass of soot collected (Ms) and a decrease in the corresponding volume of gas (Vg). The temperature increase in the reactor also led to the increase in the calculated soot mass concentration as shown in Figure 4 regardless of the unsaturation of the hydrocarbon. This trend of increasing soot mass concentration and a decrease in gas volume with temperature rise is in line with those reported previously in the literature [18, 19] . TIt can also be seen from Table 5 and Figure Table 2 ). However, at a temperature of 1250 o C, ethane (single- [12] that propylene is a C3 hydrocarbon that could generate propargyl radicals (C3H3) during its pyrolysis [21] . These radicals are key in accelerating PAH growth and subsequent production of incipient soot [3] . Significant soot formation for propylene over propane was also described by Wang and Chung [22] in diffusion flames. From Table 5 and J o u r n a l P r e -p r o o f Figure 3 , it can be finally deduced that the propensity of a hydrocarbon to soot increases when the degree of unsaturation and carbon to hydrogen ratio increases. irrespective of the degree of unsaturation of the fuels. This trend can be attributed to the primary particles agglomerating into larger, fewer, particles of several hundred nm in size [24] , [25] as the reactor's temperature was raised. Therefore, from Figure 5 , it could be summarised that as the temperature was increased, the particles agglomerated into fewer bigger particles, but with the bigger particles still growing, with the target that the total mass of the particles increases. So, although agglomeration yielded fewer bigger particles, the fewer particles continued to grow through agglomeration but also new soot deposition on them. to 1150 o C, beyond which soot particle agglomeration may be less significant within the temperature range of 1150 to 1350 o C. Nevertheless, Figure 5a shows that in the temperature range of 1150 to 1350 o C, agglomeration may have ceased, but particle growth continues through new soot deposits on the particles already formed. Figures 5a, b and c showed that the total soot mass concentration measured by the DMS-500 is influenced by the larger soot particle sizes of a smaller number, than the much smaller particles of smaller sizes. It is also important to note that changing the bond of the hydrocarbons shown in Table 1 from single to double and to triple bonds, increases the propensity of the hydrocarbons to produce soot particles of bigger sizes. Fig .6 shows the particle-phase PAH concentrations found by summing-up the total 16 PAHs (see Table 1 ) extracted from the surfaces of the soot particles of the hydrocarbons examined. Complete characterisation of these PAHs have been presented in the supplementary material and was discussed more extensively in [12] . Fig.6a and b show particle-phase PAH concentration normalised with gas volume and soot mass, respectively. It is obvious that at the lowest temperature of 1050 o C, both figure 6a and b show a high concentration of PAHs for all the hydrocarbons and this concentration, decreased markedly when the pyrolysis tempetature was raised to 1350 o C. This trend is consistent with those shown in Fig.5c of soot particle number concentration with rising pyrolysis temperature to 1350 o C. The trend was based on the increased agglomeration of soot particles, leading to fewer but larger particles of reduced surface area [12] ; so lower PAH concentration was found on those larger soot particles. Table 1 and were those proposed by Nisbet and Lagoy [26] . These factors are widely used by investigators worldwide to assess the toxicity of PAHs. The CES-PAHs has a unit of concentration, since TEF is a dimensionless relative factor. CES -PAHs adsorbed on soot particles = ∑ ( * ) 16 =1 2) Figure 7a shows that the CES decreased with temperature increase, the soot particle mean diameter on the primary axis also increased with a rise in temperature. In other words, soot particles of smaller mean diameters (29 -77nm) bearing adsorbed PAHs have significant carcinogenic potential and these ultrafine particles are likely inhalable by humans [5] , passing through the bloodstream. Fig.7a also shows that increasing the unsaturation of the hydrocarbons, especially from ethane to ethylene and to acetylene at a temperature of 1050 o C also increased the CES. Figure 7b shows that while soot particle mass concentration increased due to increasing pyrolysis temperature of the reactor, there was also corresponding decrease in the CES of PAHs. That is, the carcinogenic potential of PAHs is higher on soot particles of lower mass concentrations. However, Figure 7c shows a decreasing trend of both CES of PAHs and soot particle number concentrations with increasing pyrolysis temperature. That is, while the particle size increase (see Figure 7a ) and particle number decrease (see 7c) due to agglomeration, the carcinogenic potential of soot particles carrying PAHs also decreased. These results further confirm that a high number of ultrafine particles (soot particles of high SPNC) could potentially pose allergic inflamations of the lungs [27] and cardiovascular disease [28] than fewer numbers of large coarse particles (soot particles of high SPMD). J o u r n a l P r e -p r o o f  Increasing unsaturation of fuels from single to double and to triple bonds increased the soot mass concentration, soot particle size, soot number concentration, particle phase PAHs and soot particle carcinogenicity particularly at the initial temperature of 1050 o C.  As the degree of unsaturation of the hydrocarbons was increased, it was also observed that the ranges of soot particle mean diameter become broader and bigger regardless of the pyrolysis temperature.  Increasing the pyrolysis temperature of the reactor from 1050 o C -1350 o C, increased the soot mass concentration and soot particle size while the soot particle number concentration, soot surface area concentration, particle-phase PAHs and associated soot carcinogenicity decreased substantially.  There was a positive correlation between the soot particle number concentration and the corresponding soot particle carcinogenicity, while a negative correlation was observed between the soot particle mass and size with soot particle carcinogenicity regardless of the pyrolysis temperature examined  The decreasing trend of PAH concentration and associated soot carcinogenicity with rising temperature was believed to be due to the increased agglomeration of soot particles, leading to fewer but larger particles of reduced surface area J o u r n a l P r e -p r o o f J o u r n a l P r e -p r o o f Table 4 : Optimised conditions for GCMS analysis [13] Gas Com-ruelle, I. 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