Study on compositions of FCC flue gas and pollutant precursors from FCC catalysts

Journal Pre-proof Study on Compositions of FCC flue gas and pollutant precursors from FCC catalysts

Hui Luan, Jueying Lin, Guangli Xiu, Feng Ju, Hao Ling PII:

S0045-6535(19)32768-7

DOI:

https://doi.org/10.1016/j.chemosphere.2019.125528

Reference:

CHEM 125528

To appear in:

Chemosphere

Received Date:

08 September 2019

Accepted Date:

29 November 2019

Please cite this article as: Hui Luan, Jueying Lin, Guangli Xiu, Feng Ju, Hao Ling, Study on Compositions of FCC flue gas and pollutant precursors from FCC catalysts, Chemosphere (2019), https://doi.org/10.1016/j.chemosphere.2019.125528

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Journal Pre-proof

Study on Compositions of FCC flue gas and pollutant precursors from FCC catalysts Hui Luan1, Jueying Lin2, Guangli Xiu1,3,4*, Feng Ju2*, Hao Ling2

1. School of Resources and Environmental Engineering, East China University of Science and Technology, Shanghai 200237, China 2. State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China. 3. Shanghai Environmental Protection Key Laboratory on Environmental standard and risk management of chemical pollutants, East China University of Science and Technology, Shanghai 200237, China 4. Shanghai Institute of Pollution Control and Ecological Security, Shanghai 200092, China

Abstract: Fluid catalytic cracking (FCC) unit emits a large amount of flue gas, which is a major issue of environmental protection supervision. A spent FCC catalyst from a typical FCC unit was characterized by NMR, XPS, EA TGA and TPD-MS methods. XPS results presented that the nitrogen compounds in coke were pyridine nitrogen, pyrrole and quaternary nitrogen. Sulfur compounds in coke are in the form of thiophene and thiols. TGA and TPD-MS results indicated that the soft coke in the spent catalyst may decompose to small molecular such as NOx, SO2 and HCN. NH3 and HCN are mainly emitted due to the incomplete combustion. The flue gas from the FCC unit is monitored by different on-line monitoring instruments. Results showed that the existence of ammonia greatly affected the value of SO2 during the venting process or instruments’ inlet piping system, where saturated vapor in flue gas is partially condensed. 1

Journal Pre-proof The concentration of NH3 and HCN is more than 100 ppm, which should be paid more attention to. Taken together, fourier transform infrared method was more applicable for monitoring FCC flue gas than non-dispersed infrared method and ultraviolet fluorescence method. Keywords: FCC flue gas; pollutant precursor; ammonia; HCN; FCC catalysts

1. Introduction Fluid catalytic cracking (FCC) unit is one of the most important processes used for conversion of high-boiling hydrocarbon fractions of petroleum into valuable products, particularly gasoline and light cycle oil, olefinic gases and some other products(Bai et al., 2018). In the cracking process, large amount of catalysts are used to offer acid sites to crack high-molecular weight fractions, meanwhile leading to the formation of products deposited on the catalyst surface via a series of polymerisation and dehydrogenation reactions. After the oxidation regeneration, the catalysts could be reused. Typically, regeneration process includes two stages: partial combustion and complete combustion. In the partial combustion stage, the regenerator is operated under oxygen limited conditions and relatively high levels (1% to 5%) of CO. In the complete combustion stage, the regenerator is operated with excess O2 and low levels (< 500 ppmv) of CO in the exhaust flue gas. During the regeneration process, a wide variety of air pollutants, such as SOx, NOx, CO, VOC, organic hazard air pollutant (HAP) and ammonia, are generated. Before these air pollutants are released into the atmosphere, multiple measures are used to reduce the emission of SOx and NOx. (Emissions Estimation Protocol for Petroleum Refineries,2015) Stringent environmental regulations have forced refineries to reduce emissions of SOx and NOx to the atmosphere. Lots of attention has been paid to the control of SOx and NOx emissions (Busca et al., 1998; Chen et al., 2010; Skalska et al., 2010; Jiang et al., 2011; Wang et al., 2013; Wang et al., 2015), and less study has been conducted regarding the controlling pollutant emission from FCC unit, which is worthy of 2

Journal Pre-proof studying in order to understand the formation mechanism of gas pollutant. NOx formation during FCC regeneration was investigated (Dishman et al., 1998; Skalska et al., 2010). Dishman et al. reported that NOx was not detected until the end of the burn when concentrations of carbon on catalyst and gaseous carbon monoxide are exhausted. Hence, fuel NOx is considered the main source of NOx emissions from FCC regeneration. Shi (Shi et al., 2016) has summarized the NOx formation routes during FCC regeneration. HCN and NH3 as the volatile intermediates are initially generated from the nitrogen compounds in the coke (Barth et al., 2004). Then, NOx is formed by the oxidation of these nitrogen compounds intermediates. Due to the presence of CO, NO is reduced to N2 by CO in the regenerator under reducing atmosphere (Zhao et al., 1997; Barth et al., 2004). The emission concentration of SOx in flue gas can be studied by measuring the sulfur containing deposits on catalyst coke. During the cracking process, sulfur compounds, including thiol, thioether, thiophene, benzothiophene and thiophene derivatives, could form sulfur deposits via various condensation and polymerization reactions (Corma et al., 2001). Jaimes (Jaimes et al., 2011) studied interactive effects of sulfur containing species and gasoline components over a ZSM-5 catalyst and found more than 90% of the converted thiophene ended up forming coke. Zhang (Zhang and Yani, 2011) studied sulfur transformation during pyrolysis of Australian lignite and suggested that organic sulfur would firstly decompose to form -SH radicals, and then the radicals undergo complex interactions with organic matrix of the lignite to produce various sulfur compounds such as H2S, COS, CS2, SO2. The pyrolysis process of lignite is similar to the FCC regeneration process. However, less research focuses on the SOx forming mechanism during FCC regeneration and research has been rarely reported on the precursors of SOx emission. Although the research on the mechanism of air pollutants from FCC has been widely concerned, little 3

Journal Pre-proof has reported the composition of flue gas including NH3 and HCN from FCC unit with monitoring data acquired on-sight.. Moreover, the precursors and the forming mechanism of these flue gases have not been fully elucidated. In this paper, the composition of flue gas from a typical FCC unit is monitored by different methods and the coke catalyst is collected during the monitoring for research on the precursors of flue gas. The coked catalyst is analyzed with NMR and XPS. Hence, the monitoring results of flue gas, combined with the analysis of the coke species, will provide further insight into the mechanism of FCC coke transformation and improve understanding of the composition of flue gas.

2. Methods 2.1. FCC process The monitoring data of FCC flue gas is from a typical refinery, whose design processing capacity is 1.2 million tons/year. The catalytic cracking divides into following processes.

Firstly, fresh raw oil is heated

up, and then enters the catalytic cracking lifting tube reactor under high temperature. The reaction product is separated by a cyclone separator, and the catalyst after separation enters the stripping section. Next, the catalyst enters the regenerator which is designed as two-stage regeneration. The first regenerator is for the partial combustion stage, the coke burning ratio is 80%, and the temperature is about 660 °C. The second regenerator is for the complete combustion stage, the coke ratio is 20%, and the temperature is about 730 °C. Finally, the flue gas from the regeneration process enters the desulfurization device. 2.2. Characterization of spent catalyst The non-destructive technique of X-ray photoelectron spectros copy (XPS) was conducted by a Thermo escalab 250Xi with a hemispheric detector, using Al Ka nonmonochromatic radiation (1486.6 eV). Curve fitting method was employed to illustrate the enveloped XPS spectra that were resolved into multiple 4

Journal Pre-proof superimposed peaks induced by distinctive functionalities. The Peakfit software was applied for all the deconvolution process, and the XPS spectra were deconvoluted by a number of mixed Gaussian/Lorentzian lines based on the assignment of binding energies, applying different peak parameters. The best approximations with the highest correlation coefficients were accepted (larger than 0.999). Solid-state NMR experiments were performed on 500MHZ/AVANCEIII. Data is collected byusing 14 mm pencil probes with rotation speed at 3.7-4.8 KHZ. The contact time of 1.5ms and the relaxation delay of 1.0s were used in each experiment. The TGA pyrolysis experiments in this work were carried out on the Q600 simultaneous DSC-TGA (America). About 10 mg spent catalyst samples were heated from ambient temperature to 800° C at a heating rate of 10° C/min. A constant flow of 100 mL/min high-purity Ar was blown into the furnace to ensure an inert atmosphere during the pyrolysis process. MS Hiden HPR 20 is used to analyze gas composition. The precision of the apparatus can reach 0.1 μ g of the weight loss and 0.01° C of the temperature. 2.3. Characterization of FCC regenerated flue gas The Gasmet Dx-4000 instrument is based on Fourier transform infrared spectroscopy (FT-IR) method with wide dynamic ranges. The sample cell can be heated up to 180 °C. Sample cell absorption path length is selected according to the application. Sick Maihak S810 instrument is used as continues emission monitoring system (CEMS) to monitor the on-line data of FCC flue gas. Nitrogen content is conducted by chemiluminescence method and sulfur content is conducted by ultraviolet fluorescence method. PG-350 instrument, based on the non-dispersed infrared analysis, is used to measure the concentrations of the components of the sample gas. 5

Journal Pre-proof 3. Results and discussion 3.1. Nitrogen and sulfur compounds distribution of spent FCC catalyst 3.1.1 Elemental Analysis Two samples of spent catalysts are analyzed by EA method. The average contents of carbon, hydrogen, sulfur and nitrogen are shown in Table 1. Table 1 Results of spent FCC catalyst elemental analysis Sample Quality （mg）

2.17

Elements

Contents (wt%)

C

0.82

N H S

0.32 0.71 0.76

According to the data, the content of these four elements in different samples are similar. The content of carbon and hydrogen are about 0.8% and 0.71%, respectively, leading to the emission of CO, CH4 and H2O in the flue gas. The content of nitrogen and sulfur are about 0.3% and 0.75%, respectively, contributing to the pollutant such as NH3, NOx, HCN, and SO2 monitored by FT-IR. However, the substances on the surface of spent catalyst are complex and the formation of pollutants is complicated in the regeneration stage. Elemental analysis can only obtain the content of carbon, nitrogen and sulfur of spent catalysts, while, the forms of the compounds cannot be distinguished. Thus, it needs further studies in combination with other characterization techniques to identify the formation of pollutants. 3.1.2 XPS results XPS method could provide information of functional forms of nitrogen and sulfur coke in the spent catalysts (Qian et al., 1997; Babich et al., 2005) . Figure 1a presents the XPS results of N1s spectrum of spent FCC catalyst. The XPS spectrum was divided into three subpeaks located at 398.5, 400.1 and 401.1 eV, corresponding to pyridine nitrogen (N-6), pyrrole (N-5) and quaternary nitrogen(N-Q), respectively 6

Journal Pre-proof (Kapteijn et al., 1999). Table 2 shows the peaking area ratio of nitrogen compounds on the catalyst surface. The area ratios of pyrrole nitrogen, quaternary nitrogen and pyridine nitrogen are 38.58%, 35.01% and 26.41%, respectively. This distribution is consistent with the previous publication(Liu et al., 2017). During the FCC process, due to the strong interaction between the center of the surface acid and basic compounds, the nitrogen-containing compounds such as pyridine and pyrrole in the feed or their cracked fragments are easily adsorbed on the catalyst surface. Under reaction conditions, the C-N bond of the edge pyridine or pyrrole would break, and then nitrogen could be mixed in aromatic substances, moving from edge to center to form nitrogen coke.

(a)

(b)

Figure 1. XPS spectra of N and S elements (a) N; (b) S

Table 2

Binding energy and peak area ratio of Nitrogen and Sulfur compounds N1s fitting peak position and material relative contents 7

Journal Pre-proof Substance

Binding energy (eV)

Peak area ration%

Pyridinic (N-6)

398.7±0.2

26.41%

Pyrrolic (N-5)

400.3±0.2

38.58%

Quaternary nitrogen（N-Q）

401.4±0.2

35.01%

S2p fitting peak position and material relative contents Substance

Binding energy (eV)

Peak area ration%

Thiophene

168.9±0.2

68.43%

Thioether

170.1±0.2

31.57%

The sulfur distribution on the surface of spent catalyst was also analyzed by XPS. The S2p spectrum of spent catalyst is shown in figure 1b. There are two forms of sulfur in the spent catalyst, which are thiophene and thioether. As shown in Table 2, the peak area ratios of these two sulfur species are 68.43% and 31.57%, respectively, of which thiophene accounts for a considerable proportion. There is less research on sulfur coke in spent FCC catalyst. Marinov et al.(Marinov et al., 2004) studied the changes of organic sulfur in the pyrolysis process of two kinds of coke by XPS combined with temperature programmed reduction (TPR). At the same time, it was proved that the removal of non-thiophene sulfur could be realized by adding water vapor in the pyrolysis process. Liu et al. (Liu et al., 2007) studied the uneven distribution of sulfur during pyrolysis by XPS. The results showed that sulfur gradually migrated from the inside of coke to the outside with the increase of pyrolysis temperature. Due to the stability of thiophene cyclic conjugated system and the instability of positive carbon ions under acidic catalyst conditions, thiophene will form dithiophene active intermediates, which are difficult to inducering-opening cracking desulfurization reaction under the condition of catalytic cracking. The content of thiophene sulfide accounts for about 70% of the total sulfur in straight distillate oil. It means that the content of thiophene sulfide in the catalyst is obviously higher than that of thioether, which is consistent with the XPS data. 8

Journal Pre-proof 3.1.3 TG-MS results Thermo-gravimetric analysis (TGA) was conducted in 100% Ar. TGA data (Figure 2) shows that the catalyst loses less weight during pyrolysis, which is about 6%wt. When the temperature is about 100-120°C, the small molecular substances such as H2O and NH3 would attach to the surface of the firstly-desorbed catalyst (Cerqueira et al., 2008). Normally, FCC coke can be divided into soft coke and hard coke (Behera and Ray, 2009; Behera et al., 2013). The former is primarily composed of small aliphatic molecules, and the latter is mainly of stable aromatic compounds. When the temperature ranges from 380 to 550°C, the weight loses significantly. It can be inferred that in this stage the soft coke in the spent catalyst may decompose to the small molecular such as NOx, SO2 and HCN, at the edge of the catalyst. When the temperature increases to 800° C, the soft coke has been decomposed, however, the hard carbon begin to react in the partial-combustion condition, leading to the less weight loss.

Figure 2

TGA result of spent catalyst

Temperature programmed decomposition (TPD) of FCC coked catalyst combined with MS detector are used to analyze the decomposition process of spent catalyst, which is shown in Figure 3. It can be seen that TPD results coincide well with the TGA results. H2O(m/e=18), NH3 (m/e=17), HCN (m/e=27), 9

Journal Pre-proof CO(m/e=28), NO2(m/e=46), SO2(m/e=64), and CO2 (m/e= 44) were detected simultaneously in the emitted gases (Figure 3), which were observed by Barth’s study (Barth et al., 2004). TGA data estimated that these small molecules would be desorped which is further confirmed in the TPD (in Figure 3). At 10 min (100 C), almost at the same time, the signals of H2O (m/e=18), NH3 (m/e=17) and CH4 (m/e=16) are detected. And then, at 30 min (320 °C) the signal of HCN (m/e=27) and NO2(m/e=46）appeared. This is mainly due to the pyridine and pyrrole at the edge of the catalyst (Shi et al., 2016). The finding is consistent with the monitoring data from FT-IR method. When the temperature increases to 580°C, the signal of SO2 (m/e=64) can be observed. Liu et al (Liu et al., 2010) reported that SO2 was as a result of the C-S break, causing the sulfur and the oxygen free radical union. Thus, under reduction atmosphere, SO2 emission requires high temperature. NH3 and HCN are mainly emitted during the partial combustion stage. However, NH3 and HCN are not unstable even under high reaction temperature. (Mo et al.,2013)

Figure 3

Gas species during the spent catalyst TPD

3.1.2 NMR results In order to understand the forms of carbon on the surface of spent catalysts, 10

13C

CP-MAS NMR

Journal Pre-proof (carbon-13 cross-polarisation magic-angle spinning nuclear-magnetic) was used for characterization. As shown in Figure 4, in spent FCC catalyst catalysts, aromatic carbon can be identified and the signal is concentrated around 125 ppm. The signal of aliphatic carbon is around 20ppm, but the intensity is too low to identify aliphatic carbon. As reported previously, typical FCC coke is dominated by aromatic carbon and a small fraction of aliphatic carbon (Qian et al., 1997; Behera et al., 2013), which is consistent with NMR results. In the regeneration stage, the aromatic carbon may form aromatic compounds such as benzene, which is detected by FT-IR.

Figure 4

Results of NMR method

3.2. Composition of FCC flue gas Three different monitoring methods were adopted in this field monitoring. As can be seen from Table 3, the concentrations of emission gases such as NOx, SO2, NH3, C6H6, HCN, C8H8, CH4, and CO, are given based on the FT-IR method. The concentration of these pollutants is at a high level. Due to the complicated composition of FCC flue gas and the different monitoring principles, there is a large difference among the monitoring data of different methods. The reasons need to be further discussed based on different pollutants. Table 3 Results of FT-IR on characteristic contaminants monitoring（mg/m3） 11

Journal Pre-proof contaminant

NOx

SO2

NH3

C6H6

HCN

C8H8

CH4

CO

11:00-12:00

14.4

82.9

176.4

47.1

110.29

34.07

594.44

2.99%

12:00-13:00

22.4

135.6

218.4

46.2

108.30

36.52

588.82

2.99%

13:00-14:00

53.56

208.5

252.7

55.5

107.83

35.86

572.22

2.99%

Time

3.2.1 SO2 results Three monitoring data of different methods on SO2 are illustrated in Table 4. The results monitored by different monitoring methods vary considerably. The SO2 monitoring values increased from 80 to 220mg/m3 given by FT-IR, however, the values from non-dispersed infrared kept on 210 mg/m3 steadily. For these two monitoring methods, the flue gas is pumped into monitoring instrument directly through heating pipe, without any other pretreatment methods. Moreover, the concentration of CO is more than 30000 ppm, which is too high to lead to the positive deviation of the non-dispersive infrared method, making the value keep at 210 mg/m3. It can be concluded that the non-dispersive infrared method is not suitable for high CO content condition, like FCC flue gas. In contrast, the flue gas monitored by CEMS was pretreated to remove the water vapor to avoid the condensation in the instrument. Normally, the removal of water could reduce less than 15% monitoring value of SO2, which does not have much effect on monitoring data. However, comparing the data in Table 2, it can be seen that the data of CEMS is reduced to less than 10mg/m3, signifying that the pretreatment of condensation could reduce it significantly. Based on the monitoring data of different methods, it seems that most of SO2 are absorbed during the condensation process. The monitoring results of Fourier infrared spectrum revealed that the flue gas composition has a relatively high level of ammonia, leading to the reaction process as follows: NH3 + H2O → NH3·H2O NH3·H2O + SO2 → (NH4)2SO3 + H2O

12

Journal Pre-proof This byproduct reaction is similar to the ammonia desulfurization process, and most of the pollutants will be absorbed into the condensed water, resulting in the reduction of monitoring value. In order to confirm the byproduct process, condensation water was detected by ion chromatography, and the concentration of each ion is shown in Table 5. Table 4 Results of different monitoring methods on SO2 monitoring (mg/m3) Time

11:00—12:00

12:00--13:00

13:00--14:00

Gasmet Dx-4000 (FT-IR)

82.86

135.62

208.49

PG-350 (non-dispersed infrared)

210

214

213

CEMS (ultraviolet fluorescence)

3

2.7

2.6

Methods

Table 5 Results of ions in water sample by Ion Chromatograph (mg/L) Ion

SO42-

SO32-

NO3-

NH4+

CN-

Content

11880.1

11095.2

0.8

3352.2

155.5

In Table 5, the content of SO42- , SO32- and NH4+ are 11880.1 mg/L, 11095.2 mg/L and 335.22 mg/L, respectively. It indicates that the condensation process removed more than 90% SO2 in the flue gas before entering the monitoring instrument because of the existence of ammonia. Therefore, it can be considered that the results from Fourier infrared method are relatively accurate. The problem of the removal of SO2 by ammonia in the flue gas needs to be considered. 3.2.2 NOx results NOx monitoring results of different monitoring methods and the hourly averages are shown in Table 6. From the table, the results monitored by different monitoring methods also vary considerably. The order of monitoring values can be obtained from Table 4: Fourier transform infrared spectroscopy > non-dispersed infrared > CEMS (chemiluminescence). Since the regeneration temperature is about 760 °C, the thermal 13

Journal Pre-proof NOx could not be formed a lot. Therefore, the concentrations of NOx from different methods are up to 53.6 mg/m3. The high concentration of CO would lead to the negative deviation of the non-dispersive infrared method, resulting in the value as low as 4 mg/m3. The high content of NH3 may cause the reduction of NOx before the flue gas pumping into CEMS. Thus, the existence of NH3 and HCN in flue gas needs to be taken into consideration during the monitoring. Table 6 Results of different monitoring methods on NOX monitoring (mg/m3)

Time

11:00—12:00

12:00--13:00

13:00--14:00

Gasmet Dx-4000 (FT-IR)

14.4

22.4

53.6

PG-350 (non-dispersed infrared)

4.1

4.2

4.1

CEMS (chemiluminescence)

3.1

2.9

2.8

Methods

3.2.3 NH3 &HCN results Table 7 shows the hour average value of NH3 and Table 8 shows hour average value of HCN measured by FT-IR. Table 7 Results of FT-IR on NH3 monitoring（mg/m3） Time

11:00-12:00

12:00-13:00

13:00-14:00

Content

176.38

218.35

252.73

Table 8

Results of FT-IR on HCN monitoring（mg/m3）

Time

11:00-12:00

12:00-13:00

13:00-14:00

Content

110.29

108.30

107.83

NH3 and HCN are considered as the intermediates of NOx. The nitrogen in coke may initially form HCN in regeneration conditions. HCN is a thermodynamically unstable species, which could transfer to NH3 under reduction atmosphere. Amino groups in nitrogen coke may form NH3 directly. And then, most of 14

Journal Pre-proof NH3 and HCN may react with NOx or oxygen to form N2, NOx, CO2, CO and H2O. However, lots of attention has been paid to controlling NOx, while NH3 and HCN concentration in FCC flue gas have been underestimated. Less research has been conducted on the two characteristic pollutants of NH3 and HCN in FCC regenerated flue gas. Mo (Mo, 2013) had studied HCN emissions in fluid catalytic cracking and found that HCN content in flue gas is at up to about 150 ppm. Since NH3 is a major odorous contaminant and HCN has a high toxicity, the emission of these two pollutants should be paid more attentions. In Table 7, the content of NH3 ranges from 176 to 253 mg/m3, which is relatively high compared with the emission of SOx and NOx. In Table 8, the content of HCN remains at 110 mg/m3, which is consistent with the data of Mo’s. The TPD-MS data (Figure 4) could explain the existence of NH3 and HCN in the FCC flue gas. It can be concluded that the concentration of NH3 and HCN as the air pollutants are quite high and their emissions need to be considered. 3.2.4 Benzene results Figure 5 is the results of continuous monitoring of benzene by FT-IR.

Figure 5 Results of benzene by FT-IR Some researchers have reported the catalytic cracking properties of different reactants (alkane, 15

Journal Pre-proof cycloalkane, olefine, aromatic hydrocarbon) (Zhao et al., 1997; Barth et al., 2004; Shi et al., 2016). It is found that no matter what kind of substance, the process of forming coke must go through the step of aromatic hydrocarbons. Alkanes first cracked to produce olefins, then oligomerized to form C6+ chain olefins, and then through hydrogen transfer reaction to produce diolefins, which were then converted to soluble coke by cyclization and aromatization. On the other hand, there are only hydrogen transfer reaction and condensation reaction in the coke formation of naphthene, and even fewer steps are needed in the coke formation of aromatics. Cerqueira (Cerqueira et al., 2008) characterized the coke agent by 13 C NMR. The peak with chemical shift of 130 was assigned to C in the aromatic ring, and the peak of 18 to 20 was assigned to the C on the aliphatic chain. 80% and 90% of the C was located in the aromatic ring, indicating that coke is mostly aromatic hydrocarbons with fatty side chain. This conclusion has also been verified in this study. Through the characterization of

13

C NMR, the chemical shift is 130, but because of the low

carbon content, the aliphatic peak is not found at 18 to 20. It can be seen that coke is mostly aromatic hydrocarbons. Aromatic hydrocarbons go through a complex reaction process in the process of thermal conversion, accompanied by varying degrees of hydrogen transfer, disproportion and molecular rearrangement. Holmes et al (Holmes et al., 1997) studied thestructure of insoluble coke by gas chromatography-mass spectrometry (GC-MS). The coke produced benzene and benzene series at high temperature.

4. Conclusions In this study, the spent catalysts from a typical FCC unit were characterized by NMR, XPS, EA, TGA and TPD-MS methods. NMR results showed that aromatic carbon may form aromatic compounds such as benzene, which is detected by FT-IR. XPS results presented that the nitrogen compounds in coke were pyridine nitrogen (N-6), pyrrole (N-5) and quaternary nitrogen (N-Q), and the sulfur compounds in coke 16

Journal Pre-proof were thiophene and thiols. TPD of spent FCC catalyst combined with MS detector is used to analyze the decomposition process, and found that H2O and NH3 had been attach to the surface of catalyst desorbed firstly. Soft coke in the spent catalyst may decompose to the small molecular such as NOx, SO2 and HCN. NH3 and HCN are mainly emitted during the partial combustion stage. However, NH3 and HCN are not unstable even under high reaction temperature. Based on the analysis of spent catalysts, the flue gas of the typical FCC unit is monitored by different on-line monitoring instruments. The results from FT-IR method showed that the concentration of SO2 was over 100 mg/m3, which is quite higher than the CEMS results. The main reason is that the existence of ammonia affects the value of SO2 significantly during the condensation process. FT-IR method could avoid the effect of CO and water vapor in high concentration, and is applicable for the complex compositions flue gas. The concentrations of NH3 and HCN as the air pollutants are more than 100 mg/m3 and their emissions need to be considered. Fourier transform infrared method was more applicable for monitoring FCC flue gas than non-dispersed infrared method and ultraviolet fluorescence method.

Acknowledgements The financial support from the Fundamental Research Funds for the Central Universities of China, grant from Science and Technology Commission Shanghai Municipality (19DZ1205000), grant from Shanghai Department of Ecology and Environment ( 2018-37), and is gratefully acknowledged.

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Hui Luan and Guangli Xiu finished the on-site monitoring experiments and wrote the monitoring experiments section. Hui Luan, Jueying Lin and Feng Ju carried out catalyst experiments and wrote the catalyst analysis section. Feng Ju, Guangli Xiu and Hao Ling designed the experiments. Hui Luan, Jueying Lin and Feng Ju prepared the draft. Hao Ling, Guangli Xiu and Feng Ju reviewed the manuscript.

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Dear Editor, We declare that no conflict of interest exists in the submission of this manuscript entitled “Study on Compositions of FCC flue gas and pollutant precursors from FCC catalysts”.

Best regards, Guangli Xiu Ph.D., Professor School of Resources and Environmental Engineering East China University of Science and Technology Meilong Road. 130, Shanghai 200237, China E-mail: [email protected] Tel.:+86-21-64253113

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Aromatic carbon may form aromatic compounds such as benzene in the flue gas. XPS results presented that the nitrogen compounds in coke were pyridine nitrogen (N-6), pyrrole (N-5) and quaternary nitrogen (N-Q), and the sulfur compounds in coke were thiophene and thiols. NH3 and HCN are mainly emitted during the partial combustion stage. FT-IR method is more applicable for monitoring FCC flue gas than non-dispersed infrared method and ultraviolet fluorescence method.