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Study on the migration characteristics of nitrogen and sulfur during co-combustion of oil sludge char and microalgae residue
Fuel 238 (2019) 1–9
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Study on the migration characteristics of nitrogen and sulfur during cocombustion of oil sludge char and microalgae residue
T
⁎
Zhiqiang Gong , Zhentong Wang, Zhenbo Wang, Peiwen Fang, Fanzhi Meng State Key Laboratory of Heavy Oil, China University of Petroleum (East China), Qingdao 266580, China
A R T I C LE I N FO
A B S T R A C T
Keywords: OS char TG-MS XPS Microalgae residue Fluidized bed combustion
Integrated thermal treatment (coupled pyrolysis and combustion) has great potential to be a clean and effective method for massive oil sludge (OS) utilization. Produced OS pyrolysis char (OS char) can be effectively utilized through fluidized bed incinerator with auxiliary fuels. A study on co-combustion of OS char and microalgae extraction residue (MR) were conducted with a coupled thermogravimetric-mass spectroscopy system (TG-MS), and a fluidized bed reactor system. Co-combustion process of OS char and MR could be divided into three stages, including water evaporation, volatiles release and combustion, fixed carbon combustion and minerals decomposition. SO2 emissions decreased with the increase of MR ratio and NOx emissions initially decreased followed by an increase until the MR ratio exceeded 50%. Both NOx and SO2 emissions initially increased before a decrease with combustion temperature raised. Higher excessive air ratio (α) promoted the pollutants emission. Pyrrole (N-5) content increased with the increase of MR ratio and more N-5 and pyridine (N-6) could be converted to oxidized nitrogen (N-X) in the bottom ash under a higher temperature during OS combustion. Besides, the temperature range (900–1000 °C) was conductive to SO2 release, and S4+ formation.
1. Introduction Oil sludge (OS), which is an oil-containing solid waste produced during oil extraction, transportation, refining, and oily waste-water treatment [1,2], has been listed as a hazardous waste by law in China. Over several million tons of OS are produced per year from various sources [3]. Public and environmental health would be under significant threat unless it is treated properly [4]. Pyrolysis and incineration have gained extensive attention due to their remarkable advantages of energy recovery and significant reduction in waste volume [5]. OS pyrolysis process produces hydrocarbons with lower molecular weight in condensation (i.e., liquid) and/or noncondensable gases. It also generates a solid product, OS char [6]. OS char can be utilized as fuels for power plants. The integrated thermal treatment (coupled pyrolysis and combustion) has great potential to be a clean and effective method for OS treatment [5]. Basil et al. [7,8] investigated the combustion of char from New Zealand coals using thermogravimetric analysis (TGA) and found that the differences in reactivity between chars from different coal ranks are significant. Arenillas et al. used a thermogravimetry-mass spectrometry (TG-MS) system to study the behavior of char combustion, and they indicated that the heating rate and parent coal rank had a significant influence on the char texture and NO reduction on the char surface. In addition to ⁎
the TG analyzer or the TG-MS system, some fluidized bed reactors were used to investigate char combustion [9–11]. Kim et al. [9] used a wire heating reactor, which can use a synchronized experimental method, to obtain the intrinsic kinetics of 0.4–1.0 mm char particles, and they found that internal conduction was important in large char particles. Gong et al. [12] investigated the combustion and pollutants characteristics of char in a 2 MW CFB and found that the combustion of char has a higher dependence upon the combustion temperature, and it improves when the temperature exceeded 900 °C. It can be seen that most of these aforementioned investigations were about char combustion and pollutant emissions characteristics, which could not prove the morphologies and complex migration process of nitrogen and sulfur during the char combustion. Furthermore, researchers [11–13] have pointed out that pyrolysis char combustion technology was accompanied by some issues such as low efficiency and combustion stability. Microalgae residues (MR), a by-product after chemical extraction of astaxanthin and bio-oil in microalgae, had the advantages of high heating value and combustion stability. Related experiments [14,15] had verified MR addition could help improve the combustion performance and reduce NOx and SO2 emissions during solid fuel combustion. NOx and SO2 emissions from combustion of fossil fuels have raised much attention. Krzywanski et al. [16–18] investigated NOx and SO2 emissions from combustion of coal under air
Corresponding author. E-mail address: [email protected] (Z. Gong).
https://doi.org/10.1016/j.fuel.2018.10.087 Received 6 July 2018; Received in revised form 21 August 2018; Accepted 13 October 2018 0016-2361/ © 2018 Elsevier Ltd. All rights reserved.
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Nomenclature A C C() C(N) C(O) d50 FC H H/C HHV M MMR MOS-char MR
MR ratio N N-5 N-6 N-Q N-X O OS O/C S S2− S4+ S6+ T V α
ash content in sample wt% carbon content in sample wt% surface free sites in NOx reduction reactions surface nitrogen in NOx reduction reactions surface oxygen species in NOx reduction reactions 50% cut mean diameter μm fixed carbon content in sample wt% hydrogen content in sample wt% hydrogen-carbon molar ratio high heating value MJ·kg−1 moisture content in sample wt% mass of MR mg mass of OS char mg microalgae residue (chemical extraction)
mass mixing ratio of MR wt% nitrogen content in sample wt% pyrrole pyridine quaternary nitrogen oxidized nitrogen oxygen content in sample wt% oil sludge oxygen-carbon molar ratio sulfur content in sample wt% sulfide/pyrite sulfoxide/sulfite sulfate/sulfone combustion temperature °C volatile content in sample wt% excessive air ratio
2. Experimental
and oxygen-enriched conditions. Gungor et al. [19–21] and Jia et al. [22,23] studied NOx and SO2 emissions in both small and large scale circulated fludized bed reactor. However, the morphologies and migration of nitrogen and sulfur elements during the co-combustion of OS char and MR are quite complex, which involves a variety of physical and chemical changes. To figure out these complicated migration process, X-ray photoelectron spectroscopy (XPS) is used for detecting the morphology of nitrogen and sulfur in this study, and XPS is recognized as one of the most effective means of characterizing elements and their bonds. XPS can be used to conduct qualitative and quantitative analysis with high sensitivity while determining chemical element states [24]. The integrated thermal treatment (coupled pyrolysis and combustion) has great potential to be a clean and effective method for OS treatment, which has been successfully applied to the utilization of lowrank coal. OS char has a relatively lower heating value. Therefore, adding microalgae residue to the OS char were proposed to improve the combustion performance of the mixed fuel and reduce pollutant emissions. The presented work focused on the migration of nitrogen and sulfur during combustion of OS char blended with MR with the application of a TG-MS system, and a novel fluidized bed reactor system. NOx and SO2 emissions were detected by a gas analyzer. Besides, the XPS spectra of raw samples and bottom ash under different combustion temperature and MR ratio were investigated and presented. The results could provide basic data and theoretical support for high efficiency and low pollutants in char combustion.
2.1. Sample preparation OS samples are supplied from oil tanks in Shengli oilfield, located in Dongying, Shandong Province. OS char preparation was carried out on a horizontal tube furnace with a heating rate of 5 °C/min. The horizontal tube furnace reactor system is shown in Fig. 1. The internal atmosphere was N2 with a flowrate of 500 ml/min. Pyrolysis char, gas and oil can be obtained from OS pyrolysis. OS chars were dried in an oven at 105 °C for 24 h and stored in a desiccator. The mass of pyrolysis oil and chars can be measured by weighting and the mass of pyrolysis gas was calculated by the difference between the weight of OS sample and the weight of pyrolysis oil and char. Wang et al. [5] demonstrated that the yields of oil did not exhibit an obvious change when the pyrolysis temperature exceeded 600 °C. Hence, OS char under pyrolysis temperature of 600 °C was investigated in this work. The MR was supplied by Qingdao Xuneng Biological Engineering Co. Ltd. The sample in this work is the residue after the astaxanthin and bio-oil were obtained through a chemical process. Table 1 shows the proximate and ultimate analysis of OS char and MR. It can be seen that OS char and MR differed in their compositions obviously. OS char contained higher carbon and ash content as a solid fuel, while MR had higher volatile and heating value. Thus, MR addition could improve the combustion performance of OS char. The molar ratio of H/C as a parameter for aromaticity and carbonization degree was lower for OS char [25]. Similarly, the molar ratio of O/C ratio was lower for OS char which meant that OS char had lower hydrophilicity with less polar-
Fig. 1. Schematic diagram of horizontal tube furnace reactor system. 2
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300 ml/min was introduced into the reactor after the temperature of the furnace reached the set value with a heating rate of 5 °C/min, which was proposed to preheat the air and replace the atmosphere of the whole equipment with an insert atmosphere. Then the screw feed system was turned on with a feeding rate of 0.5 g/min. The combustion process lasted for 60 min and the power of the electric furnace was shut off, while the air gas was continuously supplied until the temperature in the reactor dropped bellowed 50 °C. The bottom ash was collected and saved in a desiccant flask for further analysis. 2.4. Experimental conditions in the fluidized bed reactor system Experimental conditions in the fluidized bed reactor system are presented in Table 2. Effects of final temperature, excessive air ratio (α) and MR ratio on the migration and emission characteristics of nitrogen and sulfur have been investigated. MR ratio is defined as the mass of MR to the mass of the sample, which is shown in Eq. (1) [14].
Fig. 2. Distribution of the particle sizes of OS and MR.
groups than MR did [26]. The inherent calcium content in OS and MR is 2.45%, and 2.41%, respectively.
MR ratio =
2.2. Analysis methods
MMR × 100% MMR + MOS − char
(1)
where MMR is the mass of MR, mg; and MOS-char is the mass of OS char, mg. To guarantee the reproducibility, experiments for combustion of the samples were repeated three times on TG system. The relative errors are < 3% of the averaged values. Besides, experiments were repeated three times with the tube furnace system with error bars marked in the figures.
Thermogravimetric experiments were carried out with a TG-MS system, including a STA449F3 NETZSCH thermogravimetric analyzer (TGA) and a QMS403C Aeolos type quadrupole mass spectrometer. Firstly, a sample of about 10 mg with particle sizes of 0.5–1.0 mm was tiled at the bottom of an Al2O3 crucible, and the internal atmosphere of the TGA was set to the air atmosphere of 60 ml/min in combustion. The initial temperature of the furnace in the STA was set as 50 °C and it would last for 60 min after the sample was fed into the furnace. The purpose was to replace the atmosphere of the whole equipment, including the furnace, and the air inside the balance of the gas chamber, and ensure that the pyrolysis and combustion process were carried out in an inert atmosphere, making the MS fully prepared for the detection of evolved gaseous products. After the initial steady state, the sample was heated with a heating rate of 20 °C/min in the temperature range of 50–1200 °C. The gaseous products were sampled by the mass spectrometer for detection and analysis simultaneously. To figure out the NOx and SO2 conversion process, an XPS analyzer (ESCALAB 250) was used to determine the nitrogen and sulfur morphology in OS char and bottom ash samples. The XPS spectrum was obtained using a fixed analyzer transmission mode with Al Kα as the anode operating at 200 W. The vacuum in the analysis chamber was 10–8 mbar, with C 1 s (284.6 eV) as the standard for calibration.
3. Results and discussion 3.1. TG-MS system analysis 3.1.1. TG and DTG profiles of OS char blended with MR TG and DTG profiles during the combustion of OS char, MR and the mixture at a heating rate of 20 °C/min are shown in Fig. 4. There were mainly two weight losses during combustion of OS char. The first weight loss (< 400 °C) was due to the removal of water and volatiles. The second weight loss (400–600 °C) was attributed to the combustion of fixed carbon and decomposition of inorganic matters [27,28]. Actually, the second weight loss was much larger with a weight loss of 20% than the first one with a weight loss of 7%, which was consistent with the results of proximate analysis. There were some heavy metal salts in OS char and these salts had a complex reaction at high temperature [28]. As for the MR combustion process, the weight losses region can be divided into three stages. The first weight loss (< 250 °C) was due to water evaporation. In the second stage (250–380 °C), there was a large weight loss peak of 23.5%/min, a maximum one, which was mainly caused by the release and combustion of volatiles. The last weight loss, occurred at 380–600 °C, was attributed to the combustion and decomposition of fixed carbon and inorganic minerals, such as aluminosilicate and calcium aluminate compounds [14]. The weight loss regions of OS char blended with MR could be divided three stages, which consisted of water evaporation, volatiles release and combustion, fixed carbon combustion and minerals decomposition. With an increase of MR ratio in the mixture, the whole TG
2.3. Fluidized bed reactor system A fluidized bed reactor system for OS char combustion was designed [15]. The schematic diagram of the system can be seen in Fig. 3. The system consists of a gas supply system, a screw feed system, an electric furnace, a quartz tube reactor, a flue gas filter, and the auxiliary system. The diameter of the tube reactor is 50 mm, with a height of 1020 mm. Air gas were supplied by Qingdao instrument & equipment center, with a purity > 99.999%. The gas flows are controlled by a float flowmeter. At the beginning of the experiments, air gas with a flow rate of Table 1 Proximate and ultimate analysis of OS char and MR. Samples
OS char MR
HHVa (MJ·kg−1)
Proximate analysis (wt%) Ma
Va
Aa
FCa,b
0 0
7.75 42.67
78.38 47.91
13.87 8.42
6.65 21.41
Ultimate analysis (wt%) C
H
Ob
N
S
H/C
O/C
17.11 33.97
0.60 7.12
0.72 2.03
0.46 7.53
2.73 0.44
0.42 2.52
0.032 0.044
M, V, A, FC and HHV refers to moisture, volatile, ash, fixed carbon and high heating value, respectively. aas air-dried basis. bO and FC, calculated by difference. The diameters of OS and MR are 0–1 mm, with a 50% cut mean diameter (d50) of 29.23 and 42.95 μm, respectively, as shown in Fig. 2. 3
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Fig. 3. Schematic diagram of the fluidized bed reactor system.
curves descended and the total weight loss increased from 27% to 45%, which was attributed to the fact that the volatiles flame supplied lots of heat for combustion and decomposition of the mixture, resulting in a lower ignited temperature with MR addition for the mixture [29]. In addition, the maximum of weight loss peak increased from 5.9%/min to 14.8%/min with the increase of MR ratio, which indicated that MR addition could help improve the combustion performance of OS char.
Table 2 Experimental conditions in the fluidized bed reactor system. Case
MR ratio
Final temperature (°C)
Excessive air ratio (α)
1 2 3 4 5 6 7 8 9 10 11 12 13 15
0% 0% 0% 0% 50% 50% 50% 25% 50% 75% 100% 50% 50% 50%
800 900 1000 1100 900 900 900 900 900 900 900 800 1000 1100
1.2 1.2 1.2 1.2 1.1 1.3 1.4 1.2 1.2 1.2 1.2 1.2 1.2 1.2
3.1.2. Combustion products identification of mixture with different MR ratio During the co-combustion process of OS char and MR, the signal change of combustion products with the lapse of time could be obtained through the online sampling capability of MID pattern of MS. Figs. 5 and 6 illustrates the evolved profiles of nitrogen and sulfur containing gaseous products during the combustion process, respectively. The spectra were recorded continuously for 60 min, with the background noise subtracted. As shown in Fig. 5, NH3, NO, NO2 exhibited the maximum peaks in the MR combustion profiles, which was attributed to the fact that large amounts of volatile-N released in MR combustion. Only small amounts of nitrogen containing compounds with high binding energy preserved in OS chars. HCN and NH3 were considered as the main precursor of NOx in solid fuel (co) combustion [30,31]. Evolved profiles for NH3 of mixtures shifted left with the increase of MR ratio, while the relative intensity increased dramatically at 50%-75% and showed no change above 75%. Compared with OS char combustion, the generation of nitrogen containing precursor significantly increased and the temperature range of gas release narrowed slightly in (co) combustion with MR addition, which was due to the formation of nitrogen containing compounds with lower binding energy and weaker stability in MR. The shapes of the NO release profiles of mixtures were similar, which were mainly appeared at 200–700 °C. A weak peak of NO profiles approximately appeared at 300 °C, which was caused by the adsorption of OS char, then NO adsorbed was released before fixed carbon combustion [32]. NO and NO2 from combustion of OS char and mixtures appeared and reached the maximum peak at the same time, which indicated that the combustion temperature was determined to the release of NOx [5]. When the MR ratio was 25%, the generation of NO and NO2 decreased obviously indicating that an appropriate MR addition ratio was conductive to the NOx reduction in OS char combustion. The maximum peak of all the COS products occurred approximately
Fig. 4. TG and DTG profiles of combustion of OS char blended with MR. 4
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Fig. 5. Nitrogen containing compounds release in (co) combustion.
Fig. 6. Sulfur containing compounds release in (co) combustion.
at the same time (Fig. 6). The mechanisms of COS are complicated and not fully understood until now [33]. With an increase in MR ratio, the COS profiles of mixtures increased and the peak intensity during combustion of the mixture was lower than that in OS char combustion when
the MR ratio was below 50%. H2S was mainly released between 150 °C and 600 °C, and the peak in MR combustion occurred earlier than that in OS char combustion. With the increase of MR ratio, the peak of H2S release profiles in mixtures
5
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reduction reaction rates increased, but the NO reduction catalyzed by ash grew faster when the temperature exceeded 900 °C, resulting in a lower NOx emission. As for SO2 emissions, the increased temperature promoted the decomposition of pyrite and sulfur oxidation reactions between 800 and 1000 °C. However, the higher combustion temperature enhanced the transmission resistance of SO2, resulting in a decreased SO2 emission [5]. As shown in Fig. 7(c), NOx and SO2 emissions exhibited a similar growth trend with the increase of α. This was because an increased oxygen concentration facilitated oxidation reactions of nitrogen-containing, and sulfurous precursors in dense phase region and suppressed
combustion was higher, and did not demonstrate an obvious increase when the MR ratio exceeded 50%. H2S mainly came from the decomposition of pyrite and aliphatic sulfur. MR addition improved the performance of mixtures and supplied heat for the decomposition of pyrite. The shapes of the SO2 release profiles in OS char, MR and mixtures were similarly. All the SO2 products were released between 250 °C and 600 °C, and the maximum peak was exhibited at the SO2 profile of OS char combustion. SO2 mainly came from the oxidation of aromatic sulfur and pyrite [31]. The ion intensity of SO2 in mixtures combustion decreased with an increase in MR ratio, which was due to the fact that MR addition promoted vulcanization reaction of gaseous sulfur and inorganic minerals. Besides, sulfur content of mixtures decreased with the increase of MR ratio. When the MR ratio exceeded 75%, the SO2 profiles of mixtures exhibited little change. This illustrated that excessive MR addition had no effects on desulfurization in OS char combustion. 3.2. Nox and SO2 emission in the fluidized bed reactor system Fig. 7(a) shows NOx and SO2 emissions during the combustion of OS char blended with MR at 900 °C with the α value of 1.2. NOx emissions initially decreased and then increased with MR ratio increased. The mechanism of NOx emissions was rather complicated [14]. NOx emissions reached to the minimum (90 mg/m3) when the MR ratio was 50%, which was mainly attributed to two reasons. The first one was due to NO reduction can be strongly catalyzed by char and ash in combustion since OS char and MR had a high ash content of 78.38% and 47.91%, respectively. The second was caused by the reduction reaction of heavy metals in OS char [14]. Chen pointed out that heavy metals could reduce NOx and SO2 emissions during biomass combustion [34]. NOx emissions slightly increased when the MR ratio exceeded 50%, which was due to NOx reduction competition was weaker than the effect on high nitrogen content in the mixtures. With the increase of MR ratio and N content in the blends, fuel-N conversion amounts increased, but the NO reduction catalyzed by ash and heavy metals still played an important role, resulting in a relatively lower NOx emission. Meanwhile, SO2 emissions decreased dramatically from 6500 mg/m3 to 500 mg/m3 with an increase in MR ratio, and changed smoothly above 50%, which was consistent with the results of SO2 profiles in Fig. 6(c). Sulfur content of OS char was 2.73%, much higher than that of MR (0.44%). Therefore, the overall sulfur content of the mixture decreased with the increase of MR ratio. Besides, as aforementioned, the SO2 reduction caused by heavy metals in OS char resulted in a lower emission. NOx and SO2 emissions under different combustion temperature and α value during the combustion of OS char blended with 50% MR are shown in Fig. 7(b) and (c), respectively. It can be seen that NOx emissions increased at 800–900 °C and then dropped to 96 mg/m3 at 1100 °C. Similarly, SO2 emissions increased between 800 and 1000 °C and then dropped to 90 mg/m3 at 1100 °C. In this work, NOx was mainly generated from the conversion of fuel nitrogen (fuel-N) below 1100 °C. NOx can be converted into N2 and H2O through a serious of complex heterogeneous and homogeneous reactions between NOx and C or CO [12].
NO + 2C () → C (N ) + C (O)
(2)
NO + 2C (N ) → N2 + C (O)
(3)
2C (N ) → N2 + C ()
(4)
2CO + 2NO → N2 + 2CO2
(5)
where C(), C(N) and C(O) were surface free sites, surface nitrogen and oxygen species, respectively. Experiments have verified that NO reduction catalyzed by char or ash became more intensified with an increase in combustion temperature than fuel-N oxidation reactions [35,36]. With the increase of combustion temperature, both the fuel-N conversion rates and NO
Fig. 7. NOx and SO2 emissions under the different combustion conditions. 6
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Table 3 Binding Energy of Different Nitrogen and Sulfur Morphologies [40]. Items
Nitrogen morphology
Symbol
Binding energy (eV)
Nitrogen
pyridine pyrrole quaternary nitrogen oxidized nitrogen sulfide/pyrite sulfoxide/sulfite sulfate/sulfone
N-6 N-5 N-Q N-X S2− S4+ S6+
395.7 399.5 401.5 403.5 163.8 168.5 170.4
Sulfur
± ± ± ± ± ± ±
0.4 0.3 0.3 0.3 0.2 0.5 0.3
NO reduction reactions in dilute phase region, resulting in a higher NOx and SO2 emission [14]. 3.3. Nitrogen and sulfur morphologies in the bottom ash. XPS has been used to study nitrogen morphology in chars for years [37–39]. Researchers have pointed out the main nitrogen species in coal/char are pyridine (N-6), pyrrole (N-5), quaternary nitrogen (N-Q), and oxidized nitrogen (N-X). The main sulfur species in char contained sulfide/pyrite (S2−), sulfoxide/sulfite (S4+), and sulfate/sulfone (S6+). Analysis of the nitrogen 1s (N 1s) peak and sulfur 2p (S 2p) peak can conclude changes occurring in nitrogen and sulfur species present on carbon surfaces during the combustion process. Binding energies given by the fitting of the N 1s and S 2p peaks are presented in Table 3[38,40]. N 1s and S 2p photoelectron spectra for OS char are presented in Fig. 8(a) and Fig. 8(b), respectively. It can be seen that nitrogen existed as N-Q and N-5, and sulfur existed as S6+, S4+, and S2+ in OS char. The N-Q content was higher of 70% than N-5 content of 30%, while the S2− content was higher than S6+ and S4+ content, which illustrated that sulfur mainly existed as sulfide or pyrite in OS char. As for the S 2p photoelectron spectra of MR, Fig. 8(c) exhibited obviously that N-5 was the main nitrogen morphology of MR. Sulfur morphologies were not detected out, which was due to a low sulfur content of 0.44% in MR. Fig. 9 shows nitrogen and sulfur morphologies of bottom ash of mixtures with different MR ratio under 900 °C. N-5 and N-Q were detected out in the bottom ash and N-5 content was the main nitrogen form in bottom ash no matter what MR ratio was. With the increase of MR ratio, N-Q content decreased slightly. When the MR ratio reached 75%, N-Q peak disappeared. This was because the more and more MR addition from 25% to 75% significantly increased N-5 content. As for the sulfur forms in bottom ash, Fig. 9(b) presented that S4+ and S6+ were the main sulfur morphologies in bottom ash of mixtures combustion. Compared with sulfur forms of OS char, S2− was not detected out in the ash, which was mainly caused by two reasons. The first one was that MR addition decreased the sulfur content. The second one high combustion temperature promoted the decomposition of pyrite [31], resulting in the S2+ conversion to S4+ and sulfur-containing gases. Besides, S4+ content was obviously higher than S6+, which was attributed that most sulfur-containing gases released in a form of SO2, and SO2 rapidly reacted with calcium oxide in high combustion temperature to form sulfite. Nitrogen and sulfur morphologies of bottom ash of OS char under different combustion temperature are shown in Fig. 10. It can be seen that N-X, N-5, and N-6 can detected out in the ash. Compared with OS char, N-X and N-6 occurred at the bottom ash and N-Q disappeared, which was because N-Q has been completely converted to other nitrogen forms in fly ash and gases. Besides, more N-5 can be converted to N-X and N-6, while N-X and N-6 was more stable than N-5 [41–43] during the thermal process. N-5 and N-6 contents decreased, while N-X content increased obviously and became the main nitrogen form, with the increase of final temperature. This was attributed that N-X were considered to be the most inactive forms of nitrogen binding [42]. N-6 was completely converted to N-X [41], and nitrogen-containing gases as
Fig. 8. Nitrogen and sulfur morphologies of OS char and MR.
shown in Fig. 10(a). As for sulfur morphologies of ash under different final temperature, when the temperature was below 900 °C, the form and content of sulfur in bottom ash did not exhibited an obvious change. With the increase of temperature from 900 °C to 1000 °C, S4+ content increased and S6+ decreased, which illustrated that this temperature range was conductive to SO2 release. However, when the final temperature reached 1100 °C, more S4+ was converted to S6+ and S2−. 7
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Fig. 9. Nitrogen and sulfur morphologies of bottom ash under different MR ratio.
Fig. 10. Nitrogen and sulfur morphologies of bottom ash under different final temperature.
This was probably attributed that high combustion temperature promoted the conversion of SO2 to SO3 and the formation of gaseous sulfur elementary substance, then the reaction with inorganic minerals caused the increase of S6+ and S2−.
was the main nitrogen-containing gas in combustion, and showed no change when the MR ratio exceeded 50%. NO was the main nitrogen oxide, when the MR ratio was 25%, the NOx profiles exhibited an obvious reduction. Similarly, MR ration was conductive to SO2 reduction. In the combustion carried on fluidized bed reactor, NOx emissions reached the minimum value 96 mg/m3 with a MR ratio of 50%. Higher temperature was conductive to the NOx and SO2 reduction. Besides, an increased α value could promote the pollutants emissions. XPS results illustrated the main nitrogen morphologies were N-Q and N-5 in OS char, and N-5 in MR, respectively. High temperature promoted the conversion of N-5 and N-6 to N-X. The temperature range (900–1000 °C) was conductive to SO2 release, and when the final temperature exceeded 1000 °C, more S4+ was converted to S6+ and S2−.
4. Conclusions The migration and emission characteristics of nitrogen and sulfur in (co) combustion of OS char and MR were studied with a TG-MS system and a fluidized bed reactor system. The weight loss regions of cocombustion of OS char and MR consisted of water evaporation, volatiles release and combustion, fixed carbon combustion and minerals decomposition. An increased MR addition could promote the volatiles combustion and improve the combustion performance of OS char·NH3 8
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Acknowledgements [21]
The research was supported by the Natural Science Foundation of Shandong Province (No. ZR2017BEE042), the Fundamental Research Funds for the Central Universities (No. 18CX02150A), the Talent Introduction Project of China University of Petroleum (East China) (No. 2017010068), the Graduate Innovation Project of China University of Petroleum (East China) (No. YCX2018029)
[24]
References
[25]
[22]
[23]
[1] Mrayyan B, Battikhi MN. Biodegradation of total organic carbons (TOC) in Jordanian petroleum sludge. J Hazard Mater 2005;120(1):127–34. [2] Xu N, Wang W, Han P, Lu X. Effects of ultrasound on oily sludge deoiling. J Hazard Mater 2009;171(1):914–7. [3] Hu G, Li J, Hou H. A combination of solvent extraction and freeze thaw for oil recovery from petroleum refinery wastewater treatment pond sludge. J Hazard Mater 2015;283:832–40. [4] Chen J, Xie C, Liu J, He Y, Xie W, Zhang X, et al. Co-combustion of sewage sludge and coffee grounds under increased O2/CO2 atmospheres: thermodynamic characteristics, kinetics and artificial neural network modeling. Bioresour Technol 2017:230–8. [5] Wang Z, Gong Z, Wang Z, Fang P, Han D. A TG-MS study on the coupled pyrolysis and combustion of oil sludge. Thermochim Acta 2018;663:137–44. [6] Hu G, Li J, Zeng G. Recent development in the treatment of oily sludge from petroleum industry: a review. J Hazard Mater 2013;261(13):470–90. [7] Shaw KJ, Beamish BB, Rodgers KA. Thermogravimetric analytical procedures for determining reactivities of chars from New Zealand coals. Thermochim Acta 1997;302(1–2):181–7. [8] Beamish BB, Shaw KJ, Rodgers KA, Newman J. Thermogravimetric determination of the carbon dioxide reactivity of char from some New Zealand coals and its association with the inorganic geochemistry of the parent coal. Fuel Process Technol 1998;53(3):243–53. [9] Kim RG, Jeon CH. Intrinsic reaction kinetics of coal char combustion by direct measurement of ignition temperature. Appl Therm Eng 2014;63(2):565–76. [10] Kelebopile L, Sun R, Wang H, Zhang X, Wu S. Pore development and combustion behavior of gasified semi-char in a drop tube furnace. Fuel Process Technol 2013;111(8):42–54. [11] Sadhukhan AK, Gupta P, Saha RK. Modeling and experimental studies on single particle coal devolatilization and residual char combustion in fluidized bed. Fuel 2011;90(6):2132–41. [12] Gong Z, Liu Z, Zhou T, Lu Q, Sun Y. Combustion and NO emission of Shenmu char in a 2 MW circulating fluidized bed. Enegy Fuels 2015;29(2):1219–26. [13] He R, Sato JI, Chen C. Modeling char combustion with fractal pore effects. Combust Sci Technol 2002;174(4):19–37. [14] Gong Z, Wang L, Wang Z, Wang Z, Xu Y, Sun F, et al. Experimental Study on Combustion and Pollutants Emissions of Oil Sludge Blended with Microalgae Residue. J Energy Inst 2017. https://doi.org/10.1016/j.joei.2017.10.001. [15] Gong Z, Wang Z, Wang Z. Study on migration characteristics of heavy metals during oil sludge incineration. Pet Sci Technol 2018;3:1–6. [16] Krzywanski J, Czakiert T, Shimizu T, Majchrzak-Kuceba I, Shimazaki Y, Zylka A, et al. NOx Emissions from Regenerator of Calcium Looping Process. Energy Fuels 2018;32(5):6355–62. [17] Krzywanski J, Nowak W. Artificial Intelligence Treatment of SO2 Emissions from CFBC in Air and Oxygen-Enriched Conditions. J Energy Eng 2016;142(1).. [18] Krzywanski J, Nowak W. Neurocomputing approach for the prediction of NOx emissions from CFBC in air-fired and oxygen-enriched atmospheres. J Power Technol 2017;97(2):75–84. [19] Gungor A. Prediction of SO2 and NOx emissions for low-grade Turkish lignites in CFB combustors. Chem Eng J 2009;146(3):388–400. [20] Gungor A, Eskin N. Effects of operational parameters on emission performance and
[26]
[27] [28]
[29]
[30] [31] [32] [33]
[34]
[35]
[36] [37]
[38] [39]
[40]
[41]
[42]
[43]
9
combustion efficiency in small-scale CFBCs. J Chin Inst Chem Eng 2008;39(6):541–56. Gungor A, Eskin N. Two-dimensional coal combustion modeling of CFB. Int J Therm Sci 2008;47(2):157–74. Jia L, Tan Y, Anthony EJ. Emissions of SO2 and NOx during Oxy−Fuel CFB Combustion Tests in a Mini-Circulating Fluidized Bed Combustion Reactor. Energy Fuels 2010;24(2):910–5. Jia Q, Che D, Liu Y, Liu Y. Effect of the cooling and reheating during coal pyrolysis on the conversion from char-N to NO/N2O. Fuel Process Technol 2009;90(1):8–15. Xu AJ, Zhaorigetu B, Lin Q, Liu LY. Study of X-ray photoelectron spectroscopy and catalytic performance in ODH of pyro-Ni2V2O7 catalysts. J Functional Mater 2007;38(9):1489–91. Yuan H, Lu T, Huang H, Noriyuki K, Chen Y. Influence of temperature on product distribution and biochar properties by municipal sludge pyrolysis. J Mater Cycles Waste Manage 2013;15(3):357–61. Tan C, Yaxin Z, Hongtao W, Wenjing L, Zeyu Z, Yuancheng Z, et al. Influence of pyrolysis temperature on characteristics and heavy metal adsorptive performance of biochar derived from municipal sewage sludge. Bioresour Technol 2014;164(7):47–54. Scott SA, Dennis JS, Davidson JF, Hayhurst AN. Thermogravimetric measurements of the kinetics of pyrolysis of dried sewage sludge. Fuel 2006;85(9):1248–53. Deng S, Wang X, Tan H, Mikulčić H, Yang F, Li Z, et al. Thermogravimetric Study on the Co-combustion Characteristics of Oily Sludge with Plant Biomass. Thermochim Acta 2016;633:69–76. Yu J, Zhang MC, Zhang J. Experimental and numerical investigations on the interactions of volatile flame and char combustion of a coal particle. Proc Combust Inst 2009;32(2):2037–42. Friebel J, Köpsel RFW. The fate of nitrogen during pyrolysis of German low rank coals — a parameter study. Fuel 1999;78(8):923–32. And ZW, Ohtsuka Y. Nitrogen Distribution in a Fixed Bed Pyrolysis of Coals with Different Ranks: Formation and Source of N2. Energy Fuels 1997;11(2):477–82. Pevida C, Arenillas A, Rubiera F, Pis JJ. Synthetic coal chars for the elucidation of NO heterogeneous reduction mechanisms. Fuel 2007;86(1):41–9. Chowanietz V, Pasel CHR, Eckardt T, Siegel A, Bathen D. Formation of Carbonyl Sulfide (COS) on Different Adsorbents in Natural Gas Treatment Plants. Oil Gas Eur Mag 2016;42(2):82–5. Chen C, Chen F, Cheng Z, Chan QN, Kook S, Guan HY. Emissions characteristics of NO x and SO 2 in the combustion of microalgae biomass using a tube furnace. J Energy Inst 2016;90(5):806–12. He J, Song W, Gao S, Dong L, Barz M, Li J, et al. Experimental study of the reduction mechanisms of NO emission in decoupling combustion of coal. Fuel Process Technol 2006;87(9):803–10. Spinti JP, Pershing DW. The fate of char-N at pulverized coal conditions. Combust Flame 2003;135(3):299–313. Gong B, Pigram PJ, Lamb RN. Identification of inorganic nitrogen in an Australian bituminous coal using x-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOFSIMS). Int J Coal Geol 1997;34(1–2):53–68. Jones JM, Zhu Q, Thomas KM. Metalloporphyrin-derived carbons: models for investigating NOx release from coal char combustion. Carbon 1999;37(7):1123–31. Pietrzak R. XPS study and physico-chemical properties of nitrogen-enriched microporous activated carbon from high volatile bituminous coal. Fuel 2009;88(10):1871–7. Kapteijn F, Moulijn JA, Matzner S, Boehm HP. The development of nitrogen functionality in model chars during gasification in CO2 and O2. Carbon 1999;37(7):1143–50. Bimer J, Sałbut PD, Berłożecki S, Boudou JP, Broniek E, Siemieniewska T. Modified active carbons from precursors enriched with nitrogen functions: Sulfur removal capabilities. Fuel 1998;77(6):519–25. Stańczyk K, Dziembaj R, Piwowarska Z, Witkowski S. Transformation of nitrogen structures in carbonization of model compounds determined by XPS. Carbon 1995;33(10):1383–92. Pels JR, Kapteijn F, Moulijn JA, Zhu Q, Thomas KM. Evolution of nitrogen functionalities in carbonaceous materials during pyrolysis. Carbon 1995;33(11):1641–53.
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Study on the migration characteristics of nitrogen and sulfur during cocombustion of oil sludge char and microalgae residue
T
⁎
Zhiqiang Gong , Zhentong Wang, Zhenbo Wang, Peiwen Fang, Fanzhi Meng State Key Laboratory of Heavy Oil, China University of Petroleum (East China), Qingdao 266580, China
A R T I C LE I N FO
A B S T R A C T
Keywords: OS char TG-MS XPS Microalgae residue Fluidized bed combustion
Integrated thermal treatment (coupled pyrolysis and combustion) has great potential to be a clean and effective method for massive oil sludge (OS) utilization. Produced OS pyrolysis char (OS char) can be effectively utilized through fluidized bed incinerator with auxiliary fuels. A study on co-combustion of OS char and microalgae extraction residue (MR) were conducted with a coupled thermogravimetric-mass spectroscopy system (TG-MS), and a fluidized bed reactor system. Co-combustion process of OS char and MR could be divided into three stages, including water evaporation, volatiles release and combustion, fixed carbon combustion and minerals decomposition. SO2 emissions decreased with the increase of MR ratio and NOx emissions initially decreased followed by an increase until the MR ratio exceeded 50%. Both NOx and SO2 emissions initially increased before a decrease with combustion temperature raised. Higher excessive air ratio (α) promoted the pollutants emission. Pyrrole (N-5) content increased with the increase of MR ratio and more N-5 and pyridine (N-6) could be converted to oxidized nitrogen (N-X) in the bottom ash under a higher temperature during OS combustion. Besides, the temperature range (900–1000 °C) was conductive to SO2 release, and S4+ formation.
1. Introduction Oil sludge (OS), which is an oil-containing solid waste produced during oil extraction, transportation, refining, and oily waste-water treatment [1,2], has been listed as a hazardous waste by law in China. Over several million tons of OS are produced per year from various sources [3]. Public and environmental health would be under significant threat unless it is treated properly [4]. Pyrolysis and incineration have gained extensive attention due to their remarkable advantages of energy recovery and significant reduction in waste volume [5]. OS pyrolysis process produces hydrocarbons with lower molecular weight in condensation (i.e., liquid) and/or noncondensable gases. It also generates a solid product, OS char [6]. OS char can be utilized as fuels for power plants. The integrated thermal treatment (coupled pyrolysis and combustion) has great potential to be a clean and effective method for OS treatment [5]. Basil et al. [7,8] investigated the combustion of char from New Zealand coals using thermogravimetric analysis (TGA) and found that the differences in reactivity between chars from different coal ranks are significant. Arenillas et al. used a thermogravimetry-mass spectrometry (TG-MS) system to study the behavior of char combustion, and they indicated that the heating rate and parent coal rank had a significant influence on the char texture and NO reduction on the char surface. In addition to ⁎
the TG analyzer or the TG-MS system, some fluidized bed reactors were used to investigate char combustion [9–11]. Kim et al. [9] used a wire heating reactor, which can use a synchronized experimental method, to obtain the intrinsic kinetics of 0.4–1.0 mm char particles, and they found that internal conduction was important in large char particles. Gong et al. [12] investigated the combustion and pollutants characteristics of char in a 2 MW CFB and found that the combustion of char has a higher dependence upon the combustion temperature, and it improves when the temperature exceeded 900 °C. It can be seen that most of these aforementioned investigations were about char combustion and pollutant emissions characteristics, which could not prove the morphologies and complex migration process of nitrogen and sulfur during the char combustion. Furthermore, researchers [11–13] have pointed out that pyrolysis char combustion technology was accompanied by some issues such as low efficiency and combustion stability. Microalgae residues (MR), a by-product after chemical extraction of astaxanthin and bio-oil in microalgae, had the advantages of high heating value and combustion stability. Related experiments [14,15] had verified MR addition could help improve the combustion performance and reduce NOx and SO2 emissions during solid fuel combustion. NOx and SO2 emissions from combustion of fossil fuels have raised much attention. Krzywanski et al. [16–18] investigated NOx and SO2 emissions from combustion of coal under air
Corresponding author. E-mail address: [email protected] (Z. Gong).
https://doi.org/10.1016/j.fuel.2018.10.087 Received 6 July 2018; Received in revised form 21 August 2018; Accepted 13 October 2018 0016-2361/ © 2018 Elsevier Ltd. All rights reserved.
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Nomenclature A C C() C(N) C(O) d50 FC H H/C HHV M MMR MOS-char MR
MR ratio N N-5 N-6 N-Q N-X O OS O/C S S2− S4+ S6+ T V α
ash content in sample wt% carbon content in sample wt% surface free sites in NOx reduction reactions surface nitrogen in NOx reduction reactions surface oxygen species in NOx reduction reactions 50% cut mean diameter μm fixed carbon content in sample wt% hydrogen content in sample wt% hydrogen-carbon molar ratio high heating value MJ·kg−1 moisture content in sample wt% mass of MR mg mass of OS char mg microalgae residue (chemical extraction)
mass mixing ratio of MR wt% nitrogen content in sample wt% pyrrole pyridine quaternary nitrogen oxidized nitrogen oxygen content in sample wt% oil sludge oxygen-carbon molar ratio sulfur content in sample wt% sulfide/pyrite sulfoxide/sulfite sulfate/sulfone combustion temperature °C volatile content in sample wt% excessive air ratio
2. Experimental
and oxygen-enriched conditions. Gungor et al. [19–21] and Jia et al. [22,23] studied NOx and SO2 emissions in both small and large scale circulated fludized bed reactor. However, the morphologies and migration of nitrogen and sulfur elements during the co-combustion of OS char and MR are quite complex, which involves a variety of physical and chemical changes. To figure out these complicated migration process, X-ray photoelectron spectroscopy (XPS) is used for detecting the morphology of nitrogen and sulfur in this study, and XPS is recognized as one of the most effective means of characterizing elements and their bonds. XPS can be used to conduct qualitative and quantitative analysis with high sensitivity while determining chemical element states [24]. The integrated thermal treatment (coupled pyrolysis and combustion) has great potential to be a clean and effective method for OS treatment, which has been successfully applied to the utilization of lowrank coal. OS char has a relatively lower heating value. Therefore, adding microalgae residue to the OS char were proposed to improve the combustion performance of the mixed fuel and reduce pollutant emissions. The presented work focused on the migration of nitrogen and sulfur during combustion of OS char blended with MR with the application of a TG-MS system, and a novel fluidized bed reactor system. NOx and SO2 emissions were detected by a gas analyzer. Besides, the XPS spectra of raw samples and bottom ash under different combustion temperature and MR ratio were investigated and presented. The results could provide basic data and theoretical support for high efficiency and low pollutants in char combustion.
2.1. Sample preparation OS samples are supplied from oil tanks in Shengli oilfield, located in Dongying, Shandong Province. OS char preparation was carried out on a horizontal tube furnace with a heating rate of 5 °C/min. The horizontal tube furnace reactor system is shown in Fig. 1. The internal atmosphere was N2 with a flowrate of 500 ml/min. Pyrolysis char, gas and oil can be obtained from OS pyrolysis. OS chars were dried in an oven at 105 °C for 24 h and stored in a desiccator. The mass of pyrolysis oil and chars can be measured by weighting and the mass of pyrolysis gas was calculated by the difference between the weight of OS sample and the weight of pyrolysis oil and char. Wang et al. [5] demonstrated that the yields of oil did not exhibit an obvious change when the pyrolysis temperature exceeded 600 °C. Hence, OS char under pyrolysis temperature of 600 °C was investigated in this work. The MR was supplied by Qingdao Xuneng Biological Engineering Co. Ltd. The sample in this work is the residue after the astaxanthin and bio-oil were obtained through a chemical process. Table 1 shows the proximate and ultimate analysis of OS char and MR. It can be seen that OS char and MR differed in their compositions obviously. OS char contained higher carbon and ash content as a solid fuel, while MR had higher volatile and heating value. Thus, MR addition could improve the combustion performance of OS char. The molar ratio of H/C as a parameter for aromaticity and carbonization degree was lower for OS char [25]. Similarly, the molar ratio of O/C ratio was lower for OS char which meant that OS char had lower hydrophilicity with less polar-
Fig. 1. Schematic diagram of horizontal tube furnace reactor system. 2
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300 ml/min was introduced into the reactor after the temperature of the furnace reached the set value with a heating rate of 5 °C/min, which was proposed to preheat the air and replace the atmosphere of the whole equipment with an insert atmosphere. Then the screw feed system was turned on with a feeding rate of 0.5 g/min. The combustion process lasted for 60 min and the power of the electric furnace was shut off, while the air gas was continuously supplied until the temperature in the reactor dropped bellowed 50 °C. The bottom ash was collected and saved in a desiccant flask for further analysis. 2.4. Experimental conditions in the fluidized bed reactor system Experimental conditions in the fluidized bed reactor system are presented in Table 2. Effects of final temperature, excessive air ratio (α) and MR ratio on the migration and emission characteristics of nitrogen and sulfur have been investigated. MR ratio is defined as the mass of MR to the mass of the sample, which is shown in Eq. (1) [14].
Fig. 2. Distribution of the particle sizes of OS and MR.
groups than MR did [26]. The inherent calcium content in OS and MR is 2.45%, and 2.41%, respectively.
MR ratio =
2.2. Analysis methods
MMR × 100% MMR + MOS − char
(1)
where MMR is the mass of MR, mg; and MOS-char is the mass of OS char, mg. To guarantee the reproducibility, experiments for combustion of the samples were repeated three times on TG system. The relative errors are < 3% of the averaged values. Besides, experiments were repeated three times with the tube furnace system with error bars marked in the figures.
Thermogravimetric experiments were carried out with a TG-MS system, including a STA449F3 NETZSCH thermogravimetric analyzer (TGA) and a QMS403C Aeolos type quadrupole mass spectrometer. Firstly, a sample of about 10 mg with particle sizes of 0.5–1.0 mm was tiled at the bottom of an Al2O3 crucible, and the internal atmosphere of the TGA was set to the air atmosphere of 60 ml/min in combustion. The initial temperature of the furnace in the STA was set as 50 °C and it would last for 60 min after the sample was fed into the furnace. The purpose was to replace the atmosphere of the whole equipment, including the furnace, and the air inside the balance of the gas chamber, and ensure that the pyrolysis and combustion process were carried out in an inert atmosphere, making the MS fully prepared for the detection of evolved gaseous products. After the initial steady state, the sample was heated with a heating rate of 20 °C/min in the temperature range of 50–1200 °C. The gaseous products were sampled by the mass spectrometer for detection and analysis simultaneously. To figure out the NOx and SO2 conversion process, an XPS analyzer (ESCALAB 250) was used to determine the nitrogen and sulfur morphology in OS char and bottom ash samples. The XPS spectrum was obtained using a fixed analyzer transmission mode with Al Kα as the anode operating at 200 W. The vacuum in the analysis chamber was 10–8 mbar, with C 1 s (284.6 eV) as the standard for calibration.
3. Results and discussion 3.1. TG-MS system analysis 3.1.1. TG and DTG profiles of OS char blended with MR TG and DTG profiles during the combustion of OS char, MR and the mixture at a heating rate of 20 °C/min are shown in Fig. 4. There were mainly two weight losses during combustion of OS char. The first weight loss (< 400 °C) was due to the removal of water and volatiles. The second weight loss (400–600 °C) was attributed to the combustion of fixed carbon and decomposition of inorganic matters [27,28]. Actually, the second weight loss was much larger with a weight loss of 20% than the first one with a weight loss of 7%, which was consistent with the results of proximate analysis. There were some heavy metal salts in OS char and these salts had a complex reaction at high temperature [28]. As for the MR combustion process, the weight losses region can be divided into three stages. The first weight loss (< 250 °C) was due to water evaporation. In the second stage (250–380 °C), there was a large weight loss peak of 23.5%/min, a maximum one, which was mainly caused by the release and combustion of volatiles. The last weight loss, occurred at 380–600 °C, was attributed to the combustion and decomposition of fixed carbon and inorganic minerals, such as aluminosilicate and calcium aluminate compounds [14]. The weight loss regions of OS char blended with MR could be divided three stages, which consisted of water evaporation, volatiles release and combustion, fixed carbon combustion and minerals decomposition. With an increase of MR ratio in the mixture, the whole TG
2.3. Fluidized bed reactor system A fluidized bed reactor system for OS char combustion was designed [15]. The schematic diagram of the system can be seen in Fig. 3. The system consists of a gas supply system, a screw feed system, an electric furnace, a quartz tube reactor, a flue gas filter, and the auxiliary system. The diameter of the tube reactor is 50 mm, with a height of 1020 mm. Air gas were supplied by Qingdao instrument & equipment center, with a purity > 99.999%. The gas flows are controlled by a float flowmeter. At the beginning of the experiments, air gas with a flow rate of Table 1 Proximate and ultimate analysis of OS char and MR. Samples
OS char MR
HHVa (MJ·kg−1)
Proximate analysis (wt%) Ma
Va
Aa
FCa,b
0 0
7.75 42.67
78.38 47.91
13.87 8.42
6.65 21.41
Ultimate analysis (wt%) C
H
Ob
N
S
H/C
O/C
17.11 33.97
0.60 7.12
0.72 2.03
0.46 7.53
2.73 0.44
0.42 2.52
0.032 0.044
M, V, A, FC and HHV refers to moisture, volatile, ash, fixed carbon and high heating value, respectively. aas air-dried basis. bO and FC, calculated by difference. The diameters of OS and MR are 0–1 mm, with a 50% cut mean diameter (d50) of 29.23 and 42.95 μm, respectively, as shown in Fig. 2. 3
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Fig. 3. Schematic diagram of the fluidized bed reactor system.
curves descended and the total weight loss increased from 27% to 45%, which was attributed to the fact that the volatiles flame supplied lots of heat for combustion and decomposition of the mixture, resulting in a lower ignited temperature with MR addition for the mixture [29]. In addition, the maximum of weight loss peak increased from 5.9%/min to 14.8%/min with the increase of MR ratio, which indicated that MR addition could help improve the combustion performance of OS char.
Table 2 Experimental conditions in the fluidized bed reactor system. Case
MR ratio
Final temperature (°C)
Excessive air ratio (α)
1 2 3 4 5 6 7 8 9 10 11 12 13 15
0% 0% 0% 0% 50% 50% 50% 25% 50% 75% 100% 50% 50% 50%
800 900 1000 1100 900 900 900 900 900 900 900 800 1000 1100
1.2 1.2 1.2 1.2 1.1 1.3 1.4 1.2 1.2 1.2 1.2 1.2 1.2 1.2
3.1.2. Combustion products identification of mixture with different MR ratio During the co-combustion process of OS char and MR, the signal change of combustion products with the lapse of time could be obtained through the online sampling capability of MID pattern of MS. Figs. 5 and 6 illustrates the evolved profiles of nitrogen and sulfur containing gaseous products during the combustion process, respectively. The spectra were recorded continuously for 60 min, with the background noise subtracted. As shown in Fig. 5, NH3, NO, NO2 exhibited the maximum peaks in the MR combustion profiles, which was attributed to the fact that large amounts of volatile-N released in MR combustion. Only small amounts of nitrogen containing compounds with high binding energy preserved in OS chars. HCN and NH3 were considered as the main precursor of NOx in solid fuel (co) combustion [30,31]. Evolved profiles for NH3 of mixtures shifted left with the increase of MR ratio, while the relative intensity increased dramatically at 50%-75% and showed no change above 75%. Compared with OS char combustion, the generation of nitrogen containing precursor significantly increased and the temperature range of gas release narrowed slightly in (co) combustion with MR addition, which was due to the formation of nitrogen containing compounds with lower binding energy and weaker stability in MR. The shapes of the NO release profiles of mixtures were similar, which were mainly appeared at 200–700 °C. A weak peak of NO profiles approximately appeared at 300 °C, which was caused by the adsorption of OS char, then NO adsorbed was released before fixed carbon combustion [32]. NO and NO2 from combustion of OS char and mixtures appeared and reached the maximum peak at the same time, which indicated that the combustion temperature was determined to the release of NOx [5]. When the MR ratio was 25%, the generation of NO and NO2 decreased obviously indicating that an appropriate MR addition ratio was conductive to the NOx reduction in OS char combustion. The maximum peak of all the COS products occurred approximately
Fig. 4. TG and DTG profiles of combustion of OS char blended with MR. 4
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Fig. 5. Nitrogen containing compounds release in (co) combustion.
Fig. 6. Sulfur containing compounds release in (co) combustion.
at the same time (Fig. 6). The mechanisms of COS are complicated and not fully understood until now [33]. With an increase in MR ratio, the COS profiles of mixtures increased and the peak intensity during combustion of the mixture was lower than that in OS char combustion when
the MR ratio was below 50%. H2S was mainly released between 150 °C and 600 °C, and the peak in MR combustion occurred earlier than that in OS char combustion. With the increase of MR ratio, the peak of H2S release profiles in mixtures
5
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reduction reaction rates increased, but the NO reduction catalyzed by ash grew faster when the temperature exceeded 900 °C, resulting in a lower NOx emission. As for SO2 emissions, the increased temperature promoted the decomposition of pyrite and sulfur oxidation reactions between 800 and 1000 °C. However, the higher combustion temperature enhanced the transmission resistance of SO2, resulting in a decreased SO2 emission [5]. As shown in Fig. 7(c), NOx and SO2 emissions exhibited a similar growth trend with the increase of α. This was because an increased oxygen concentration facilitated oxidation reactions of nitrogen-containing, and sulfurous precursors in dense phase region and suppressed
combustion was higher, and did not demonstrate an obvious increase when the MR ratio exceeded 50%. H2S mainly came from the decomposition of pyrite and aliphatic sulfur. MR addition improved the performance of mixtures and supplied heat for the decomposition of pyrite. The shapes of the SO2 release profiles in OS char, MR and mixtures were similarly. All the SO2 products were released between 250 °C and 600 °C, and the maximum peak was exhibited at the SO2 profile of OS char combustion. SO2 mainly came from the oxidation of aromatic sulfur and pyrite [31]. The ion intensity of SO2 in mixtures combustion decreased with an increase in MR ratio, which was due to the fact that MR addition promoted vulcanization reaction of gaseous sulfur and inorganic minerals. Besides, sulfur content of mixtures decreased with the increase of MR ratio. When the MR ratio exceeded 75%, the SO2 profiles of mixtures exhibited little change. This illustrated that excessive MR addition had no effects on desulfurization in OS char combustion. 3.2. Nox and SO2 emission in the fluidized bed reactor system Fig. 7(a) shows NOx and SO2 emissions during the combustion of OS char blended with MR at 900 °C with the α value of 1.2. NOx emissions initially decreased and then increased with MR ratio increased. The mechanism of NOx emissions was rather complicated [14]. NOx emissions reached to the minimum (90 mg/m3) when the MR ratio was 50%, which was mainly attributed to two reasons. The first one was due to NO reduction can be strongly catalyzed by char and ash in combustion since OS char and MR had a high ash content of 78.38% and 47.91%, respectively. The second was caused by the reduction reaction of heavy metals in OS char [14]. Chen pointed out that heavy metals could reduce NOx and SO2 emissions during biomass combustion [34]. NOx emissions slightly increased when the MR ratio exceeded 50%, which was due to NOx reduction competition was weaker than the effect on high nitrogen content in the mixtures. With the increase of MR ratio and N content in the blends, fuel-N conversion amounts increased, but the NO reduction catalyzed by ash and heavy metals still played an important role, resulting in a relatively lower NOx emission. Meanwhile, SO2 emissions decreased dramatically from 6500 mg/m3 to 500 mg/m3 with an increase in MR ratio, and changed smoothly above 50%, which was consistent with the results of SO2 profiles in Fig. 6(c). Sulfur content of OS char was 2.73%, much higher than that of MR (0.44%). Therefore, the overall sulfur content of the mixture decreased with the increase of MR ratio. Besides, as aforementioned, the SO2 reduction caused by heavy metals in OS char resulted in a lower emission. NOx and SO2 emissions under different combustion temperature and α value during the combustion of OS char blended with 50% MR are shown in Fig. 7(b) and (c), respectively. It can be seen that NOx emissions increased at 800–900 °C and then dropped to 96 mg/m3 at 1100 °C. Similarly, SO2 emissions increased between 800 and 1000 °C and then dropped to 90 mg/m3 at 1100 °C. In this work, NOx was mainly generated from the conversion of fuel nitrogen (fuel-N) below 1100 °C. NOx can be converted into N2 and H2O through a serious of complex heterogeneous and homogeneous reactions between NOx and C or CO [12].
NO + 2C () → C (N ) + C (O)
(2)
NO + 2C (N ) → N2 + C (O)
(3)
2C (N ) → N2 + C ()
(4)
2CO + 2NO → N2 + 2CO2
(5)
where C(), C(N) and C(O) were surface free sites, surface nitrogen and oxygen species, respectively. Experiments have verified that NO reduction catalyzed by char or ash became more intensified with an increase in combustion temperature than fuel-N oxidation reactions [35,36]. With the increase of combustion temperature, both the fuel-N conversion rates and NO
Fig. 7. NOx and SO2 emissions under the different combustion conditions. 6
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Table 3 Binding Energy of Different Nitrogen and Sulfur Morphologies [40]. Items
Nitrogen morphology
Symbol
Binding energy (eV)
Nitrogen
pyridine pyrrole quaternary nitrogen oxidized nitrogen sulfide/pyrite sulfoxide/sulfite sulfate/sulfone
N-6 N-5 N-Q N-X S2− S4+ S6+
395.7 399.5 401.5 403.5 163.8 168.5 170.4
Sulfur
± ± ± ± ± ± ±
0.4 0.3 0.3 0.3 0.2 0.5 0.3
NO reduction reactions in dilute phase region, resulting in a higher NOx and SO2 emission [14]. 3.3. Nitrogen and sulfur morphologies in the bottom ash. XPS has been used to study nitrogen morphology in chars for years [37–39]. Researchers have pointed out the main nitrogen species in coal/char are pyridine (N-6), pyrrole (N-5), quaternary nitrogen (N-Q), and oxidized nitrogen (N-X). The main sulfur species in char contained sulfide/pyrite (S2−), sulfoxide/sulfite (S4+), and sulfate/sulfone (S6+). Analysis of the nitrogen 1s (N 1s) peak and sulfur 2p (S 2p) peak can conclude changes occurring in nitrogen and sulfur species present on carbon surfaces during the combustion process. Binding energies given by the fitting of the N 1s and S 2p peaks are presented in Table 3[38,40]. N 1s and S 2p photoelectron spectra for OS char are presented in Fig. 8(a) and Fig. 8(b), respectively. It can be seen that nitrogen existed as N-Q and N-5, and sulfur existed as S6+, S4+, and S2+ in OS char. The N-Q content was higher of 70% than N-5 content of 30%, while the S2− content was higher than S6+ and S4+ content, which illustrated that sulfur mainly existed as sulfide or pyrite in OS char. As for the S 2p photoelectron spectra of MR, Fig. 8(c) exhibited obviously that N-5 was the main nitrogen morphology of MR. Sulfur morphologies were not detected out, which was due to a low sulfur content of 0.44% in MR. Fig. 9 shows nitrogen and sulfur morphologies of bottom ash of mixtures with different MR ratio under 900 °C. N-5 and N-Q were detected out in the bottom ash and N-5 content was the main nitrogen form in bottom ash no matter what MR ratio was. With the increase of MR ratio, N-Q content decreased slightly. When the MR ratio reached 75%, N-Q peak disappeared. This was because the more and more MR addition from 25% to 75% significantly increased N-5 content. As for the sulfur forms in bottom ash, Fig. 9(b) presented that S4+ and S6+ were the main sulfur morphologies in bottom ash of mixtures combustion. Compared with sulfur forms of OS char, S2− was not detected out in the ash, which was mainly caused by two reasons. The first one was that MR addition decreased the sulfur content. The second one high combustion temperature promoted the decomposition of pyrite [31], resulting in the S2+ conversion to S4+ and sulfur-containing gases. Besides, S4+ content was obviously higher than S6+, which was attributed that most sulfur-containing gases released in a form of SO2, and SO2 rapidly reacted with calcium oxide in high combustion temperature to form sulfite. Nitrogen and sulfur morphologies of bottom ash of OS char under different combustion temperature are shown in Fig. 10. It can be seen that N-X, N-5, and N-6 can detected out in the ash. Compared with OS char, N-X and N-6 occurred at the bottom ash and N-Q disappeared, which was because N-Q has been completely converted to other nitrogen forms in fly ash and gases. Besides, more N-5 can be converted to N-X and N-6, while N-X and N-6 was more stable than N-5 [41–43] during the thermal process. N-5 and N-6 contents decreased, while N-X content increased obviously and became the main nitrogen form, with the increase of final temperature. This was attributed that N-X were considered to be the most inactive forms of nitrogen binding [42]. N-6 was completely converted to N-X [41], and nitrogen-containing gases as
Fig. 8. Nitrogen and sulfur morphologies of OS char and MR.
shown in Fig. 10(a). As for sulfur morphologies of ash under different final temperature, when the temperature was below 900 °C, the form and content of sulfur in bottom ash did not exhibited an obvious change. With the increase of temperature from 900 °C to 1000 °C, S4+ content increased and S6+ decreased, which illustrated that this temperature range was conductive to SO2 release. However, when the final temperature reached 1100 °C, more S4+ was converted to S6+ and S2−. 7
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Fig. 9. Nitrogen and sulfur morphologies of bottom ash under different MR ratio.
Fig. 10. Nitrogen and sulfur morphologies of bottom ash under different final temperature.
This was probably attributed that high combustion temperature promoted the conversion of SO2 to SO3 and the formation of gaseous sulfur elementary substance, then the reaction with inorganic minerals caused the increase of S6+ and S2−.
was the main nitrogen-containing gas in combustion, and showed no change when the MR ratio exceeded 50%. NO was the main nitrogen oxide, when the MR ratio was 25%, the NOx profiles exhibited an obvious reduction. Similarly, MR ration was conductive to SO2 reduction. In the combustion carried on fluidized bed reactor, NOx emissions reached the minimum value 96 mg/m3 with a MR ratio of 50%. Higher temperature was conductive to the NOx and SO2 reduction. Besides, an increased α value could promote the pollutants emissions. XPS results illustrated the main nitrogen morphologies were N-Q and N-5 in OS char, and N-5 in MR, respectively. High temperature promoted the conversion of N-5 and N-6 to N-X. The temperature range (900–1000 °C) was conductive to SO2 release, and when the final temperature exceeded 1000 °C, more S4+ was converted to S6+ and S2−.
4. Conclusions The migration and emission characteristics of nitrogen and sulfur in (co) combustion of OS char and MR were studied with a TG-MS system and a fluidized bed reactor system. The weight loss regions of cocombustion of OS char and MR consisted of water evaporation, volatiles release and combustion, fixed carbon combustion and minerals decomposition. An increased MR addition could promote the volatiles combustion and improve the combustion performance of OS char·NH3 8
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Acknowledgements [21]
The research was supported by the Natural Science Foundation of Shandong Province (No. ZR2017BEE042), the Fundamental Research Funds for the Central Universities (No. 18CX02150A), the Talent Introduction Project of China University of Petroleum (East China) (No. 2017010068), the Graduate Innovation Project of China University of Petroleum (East China) (No. YCX2018029)
[24]
References
[25]
[22]
[23]
[1] Mrayyan B, Battikhi MN. Biodegradation of total organic carbons (TOC) in Jordanian petroleum sludge. J Hazard Mater 2005;120(1):127–34. [2] Xu N, Wang W, Han P, Lu X. Effects of ultrasound on oily sludge deoiling. J Hazard Mater 2009;171(1):914–7. [3] Hu G, Li J, Hou H. A combination of solvent extraction and freeze thaw for oil recovery from petroleum refinery wastewater treatment pond sludge. J Hazard Mater 2015;283:832–40. [4] Chen J, Xie C, Liu J, He Y, Xie W, Zhang X, et al. Co-combustion of sewage sludge and coffee grounds under increased O2/CO2 atmospheres: thermodynamic characteristics, kinetics and artificial neural network modeling. Bioresour Technol 2017:230–8. [5] Wang Z, Gong Z, Wang Z, Fang P, Han D. A TG-MS study on the coupled pyrolysis and combustion of oil sludge. Thermochim Acta 2018;663:137–44. [6] Hu G, Li J, Zeng G. Recent development in the treatment of oily sludge from petroleum industry: a review. J Hazard Mater 2013;261(13):470–90. [7] Shaw KJ, Beamish BB, Rodgers KA. Thermogravimetric analytical procedures for determining reactivities of chars from New Zealand coals. Thermochim Acta 1997;302(1–2):181–7. [8] Beamish BB, Shaw KJ, Rodgers KA, Newman J. Thermogravimetric determination of the carbon dioxide reactivity of char from some New Zealand coals and its association with the inorganic geochemistry of the parent coal. Fuel Process Technol 1998;53(3):243–53. [9] Kim RG, Jeon CH. Intrinsic reaction kinetics of coal char combustion by direct measurement of ignition temperature. Appl Therm Eng 2014;63(2):565–76. [10] Kelebopile L, Sun R, Wang H, Zhang X, Wu S. Pore development and combustion behavior of gasified semi-char in a drop tube furnace. Fuel Process Technol 2013;111(8):42–54. [11] Sadhukhan AK, Gupta P, Saha RK. Modeling and experimental studies on single particle coal devolatilization and residual char combustion in fluidized bed. Fuel 2011;90(6):2132–41. [12] Gong Z, Liu Z, Zhou T, Lu Q, Sun Y. Combustion and NO emission of Shenmu char in a 2 MW circulating fluidized bed. Enegy Fuels 2015;29(2):1219–26. [13] He R, Sato JI, Chen C. Modeling char combustion with fractal pore effects. Combust Sci Technol 2002;174(4):19–37. [14] Gong Z, Wang L, Wang Z, Wang Z, Xu Y, Sun F, et al. Experimental Study on Combustion and Pollutants Emissions of Oil Sludge Blended with Microalgae Residue. J Energy Inst 2017. https://doi.org/10.1016/j.joei.2017.10.001. [15] Gong Z, Wang Z, Wang Z. Study on migration characteristics of heavy metals during oil sludge incineration. Pet Sci Technol 2018;3:1–6. [16] Krzywanski J, Czakiert T, Shimizu T, Majchrzak-Kuceba I, Shimazaki Y, Zylka A, et al. NOx Emissions from Regenerator of Calcium Looping Process. Energy Fuels 2018;32(5):6355–62. [17] Krzywanski J, Nowak W. Artificial Intelligence Treatment of SO2 Emissions from CFBC in Air and Oxygen-Enriched Conditions. J Energy Eng 2016;142(1).. [18] Krzywanski J, Nowak W. Neurocomputing approach for the prediction of NOx emissions from CFBC in air-fired and oxygen-enriched atmospheres. J Power Technol 2017;97(2):75–84. [19] Gungor A. Prediction of SO2 and NOx emissions for low-grade Turkish lignites in CFB combustors. Chem Eng J 2009;146(3):388–400. [20] Gungor A, Eskin N. Effects of operational parameters on emission performance and
[26]
[27] [28]
[29]
[30] [31] [32] [33]
[34]
[35]
[36] [37]
[38] [39]
[40]
[41]
[42]
[43]
9
combustion efficiency in small-scale CFBCs. J Chin Inst Chem Eng 2008;39(6):541–56. Gungor A, Eskin N. Two-dimensional coal combustion modeling of CFB. Int J Therm Sci 2008;47(2):157–74. Jia L, Tan Y, Anthony EJ. Emissions of SO2 and NOx during Oxy−Fuel CFB Combustion Tests in a Mini-Circulating Fluidized Bed Combustion Reactor. Energy Fuels 2010;24(2):910–5. Jia Q, Che D, Liu Y, Liu Y. Effect of the cooling and reheating during coal pyrolysis on the conversion from char-N to NO/N2O. Fuel Process Technol 2009;90(1):8–15. Xu AJ, Zhaorigetu B, Lin Q, Liu LY. Study of X-ray photoelectron spectroscopy and catalytic performance in ODH of pyro-Ni2V2O7 catalysts. J Functional Mater 2007;38(9):1489–91. Yuan H, Lu T, Huang H, Noriyuki K, Chen Y. Influence of temperature on product distribution and biochar properties by municipal sludge pyrolysis. J Mater Cycles Waste Manage 2013;15(3):357–61. Tan C, Yaxin Z, Hongtao W, Wenjing L, Zeyu Z, Yuancheng Z, et al. Influence of pyrolysis temperature on characteristics and heavy metal adsorptive performance of biochar derived from municipal sewage sludge. Bioresour Technol 2014;164(7):47–54. Scott SA, Dennis JS, Davidson JF, Hayhurst AN. Thermogravimetric measurements of the kinetics of pyrolysis of dried sewage sludge. Fuel 2006;85(9):1248–53. Deng S, Wang X, Tan H, Mikulčić H, Yang F, Li Z, et al. Thermogravimetric Study on the Co-combustion Characteristics of Oily Sludge with Plant Biomass. Thermochim Acta 2016;633:69–76. Yu J, Zhang MC, Zhang J. Experimental and numerical investigations on the interactions of volatile flame and char combustion of a coal particle. Proc Combust Inst 2009;32(2):2037–42. Friebel J, Köpsel RFW. The fate of nitrogen during pyrolysis of German low rank coals — a parameter study. Fuel 1999;78(8):923–32. And ZW, Ohtsuka Y. Nitrogen Distribution in a Fixed Bed Pyrolysis of Coals with Different Ranks: Formation and Source of N2. Energy Fuels 1997;11(2):477–82. Pevida C, Arenillas A, Rubiera F, Pis JJ. Synthetic coal chars for the elucidation of NO heterogeneous reduction mechanisms. Fuel 2007;86(1):41–9. Chowanietz V, Pasel CHR, Eckardt T, Siegel A, Bathen D. Formation of Carbonyl Sulfide (COS) on Different Adsorbents in Natural Gas Treatment Plants. Oil Gas Eur Mag 2016;42(2):82–5. Chen C, Chen F, Cheng Z, Chan QN, Kook S, Guan HY. Emissions characteristics of NO x and SO 2 in the combustion of microalgae biomass using a tube furnace. J Energy Inst 2016;90(5):806–12. He J, Song W, Gao S, Dong L, Barz M, Li J, et al. Experimental study of the reduction mechanisms of NO emission in decoupling combustion of coal. Fuel Process Technol 2006;87(9):803–10. Spinti JP, Pershing DW. The fate of char-N at pulverized coal conditions. Combust Flame 2003;135(3):299–313. Gong B, Pigram PJ, Lamb RN. Identification of inorganic nitrogen in an Australian bituminous coal using x-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOFSIMS). Int J Coal Geol 1997;34(1–2):53–68. Jones JM, Zhu Q, Thomas KM. Metalloporphyrin-derived carbons: models for investigating NOx release from coal char combustion. Carbon 1999;37(7):1123–31. Pietrzak R. XPS study and physico-chemical properties of nitrogen-enriched microporous activated carbon from high volatile bituminous coal. Fuel 2009;88(10):1871–7. Kapteijn F, Moulijn JA, Matzner S, Boehm HP. The development of nitrogen functionality in model chars during gasification in CO2 and O2. Carbon 1999;37(7):1143–50. Bimer J, Sałbut PD, Berłożecki S, Boudou JP, Broniek E, Siemieniewska T. Modified active carbons from precursors enriched with nitrogen functions: Sulfur removal capabilities. Fuel 1998;77(6):519–25. Stańczyk K, Dziembaj R, Piwowarska Z, Witkowski S. Transformation of nitrogen structures in carbonization of model compounds determined by XPS. Carbon 1995;33(10):1383–92. Pels JR, Kapteijn F, Moulijn JA, Zhu Q, Thomas KM. Evolution of nitrogen functionalities in carbonaceous materials during pyrolysis. Carbon 1995;33(11):1641–53.