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Nitrogen conversion during rapid pyrolysis of coal and petroleum coke in a high-frequency furnace
Applied Energy 92 (2012) 854–859
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Nitrogen conversion during rapid pyrolysis of coal and petroleum coke in a high-frequency furnace Shuai Yuan, Zhi-jie Zhou, Jun Li, Fu-chen Wang ⇑ Key Laboratory of Coal Gasification of Ministry of Education, East China University of Science and Technology, Shanghai 200237, China
a r t i c l e
i n f o
Article history: Received 17 November 2010 Received in revised form 23 August 2011 Accepted 24 August 2011 Available online 17 September 2011 Keywords: Rapid pyrolysis Coal Nitrogen HCN NH3
a b s t r a c t Rapid pyrolysis of three typical Chinese coals, lignite from Inner Mongolia, bituminous from Shenfu coalfield, and anthracite from Guizhou, as well as a petroleum coke were carried out in a drop-style high-frequency furnace. The reactor was induction coil heated and had a very small high-temperature zone, which could restrain secondary conversions of nitrogen products. The effects of temperature and coal rank on conversions of fuel-N to primary nitrogen products (char-N, HCN–N, NH3–N and (tar + N2)–N) have been investigated. The results showed that, the increasing temperature reduced the yields of char-N and promoted the conversion of fuel-N to N2. Char-N yields increased, while volatile-N yields decreased as the coal rank increased. In most of the conditions, NH3–N yields were higher than HCN– N yields during rapid pyrolysis of coal. In the case of petroleum coke, NH3–N yields increased gradually with the increasing temperature, but no HCN was detected. We argue that NH3–N can be formed directly through the primary pyrolysis without secondary reactions. Although volatile-N yields of lignite were higher than those of bituminous, yields of (HCN + NH3)–N in volatile-N of lignite were lower than those of bituminous. While the (HCN + NH3)–N yields of anthracite were the lowest of the three coals. Both of the (HCN + NH3)–N yields and (HCN + NH3)–N proportions in volatile-N of petroleum coke were lower than the three coals. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Nitrogen in coal forms NOx during combustion, which results in the environmental problems such as acid rain and photochemical smog. During gasification, nitrogen in coal forms NH3 and HCN in the gasifier. NH3 in gasification system can react with H2O and CO2 to produce ammonium salts which will crystallize in the low temperature positions of the system and leading to the problems including pipeline abrasion and block. Dissolution of HCN in system water can produce cyanide wastewater which increases the difficulty and the cost of sewage treatment [1,2]. Rapid pyrolysis of coal is an important process in both combustion and gasification. Nitrogen forms released from coal during rapid pyrolysis influences the nitrogen subsequence conversions and the nitrogen pollutants finally formed. Therefore, many researchers focus on nitrogen conversions during rapid pyrolysis of coal. Rapid pyrolysis of coal has been carried out by Nelson and coworkers [3–5], and effects of coal rank and temperature on the formations of HCN and NH3, as well as nitrogen functionalities in tar were investigated. Cai and co-workers [6] have investigated the effects of heating rate and pressure on nitrogen conversion during ⇑ Corresponding author. Tel.: +86 21 64250784; fax: +86 21 64251312. E-mail address: [email protected] (F.-c. Wang). 0306-2619/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2011.08.042
rapid pyrolysis of coal by using a wire-mesh reactor. In a similar study, Xu and Kumagai investigated the effects of temperature, atmosphere and H2 pressure on nitrogen conversions by using a drop-tube furnace [7]. Rapid pyrolysis of twenty coals of different coal ranks has been carried out by Kambara et al. [8] in a pyroprobe reactor. And the correlations of nitrogen functionalities and nitrogen forms released from coal were investigate with XPS (X-ray photoelectron spectroscopy), and the effects of temperature and coal rank on nitrogen conversions during rapid pyrolysis were also investigated. Furthermore, Li and co-workers [9–14] used a reactor which had some features of both a drop-tube reactor and a fixedbed to investigate HCN and NH3 formations during rapid pyrolysis of different ranks of coals. A semi-entrained flow/semi-fixed bed reactor has been used by Liu and co-workers to investigate the effects of coal rank on HCN formations during rapid pyrolysis of coals [15]. Effects of coal rank, carrier gas flow rate and temperature on the formation of NOx precursors during rapid pyrolysis has been investigated by Feng and co-workers in a reactor similar to that of Liu and co-workers [15,16]. A review on nitrogen conversions during pyrolysis and gasification of coal and biomass has been made by Leppalahti and Koljonen [17]. Presently, nitrogen conversion mechanisms during rapid pyrolysis are not fully understood. The heating rate and residence time are greatly different due to the variations of reactor used by
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S. Yuan et al. / Applied Energy 92 (2012) 854–859 Table 1 Proximate analysis (dry basis, wt.%) and ultimate analysis (daf, wt.%) of fuel samples.
Lignite Bituminous Anthracite Petroleum coke a
A
V
FC
C
H
N
S
Oa
16.59 6.57 27.24 0.25
33.68 31.79 7.15 10.08
49.73 61.65 65.60 89.67
75.69 85.11 92.16 91.52
5.69 6.13 2.06 2.58
0.91 1.32 1.42 1.63
0.77 1.35 1.70 3.47
16.94 6.07 2.66 0.80
By difference.
different researchers. The nitrogen products released from primary reactions such as char-N, tar-N and other volatile-N are affected by the heating rate. However, the residence time of primary pyrolysis products in high-temperature zone varied with different studies due to the differences of reactor forms and gas velocities. Nelson et al. suggested that, HCN was formed by secondary cracking of volatile-N during rapid pyrolysis of coal in a fluidized-bed reactor [3]. But Xu and Kumagai proposed that HCN was converted to NH3 by reacting with H2 during rapid pyrolysis of coal in a pressurized drop-tube furnace [7]. The fluidized-bed reactor and droptube furnace which have large high temperature zones provide essential conditions for secondary reactions. During rapid pyrolysis of coal or biomass in fluidized-bed reactor and drop-tube furnace, the fuel samples are continuously fed into the reactor. Therefore there will be always pyrolysis gas in the reactor, and the later fed samples may react with the pyrolysis gas released from the previously fed samples. In the study of Tan and Li [9], HCN and NH3 were formed in the primary stage of pyrolysis during rapid pyrolysis of coal in a drop-tube/fixed-bed reactor, and they proposed the residence time affected the yields of HCN and NH3 remarkably [9]. The secondary reactions of char-N, tar-N, NH3, HCN and other nitrogen products which affected by the reactors and the gas velocities significantly, caused so many disagreements or even opposite conclusions reported in literature. The wire-mesh reactor is a powerful tool to realize rapid pyrolysis without secondary reactions. But there is no systematic study on nitrogen evolution during rapid pyrolysis of coal by using wire-mesh reactor. The low sample capacity of the wire-mesh reactor might be the reason. In this study, a drop-style high-frequency furnace which could limit the high temperature zone in a very small zone were used, and the secondary reactions of nitrogen products could be sharply reduced by shortening the residence time of primary volatilenitrogen products in high temperature zone. The purpose of this study was to investigate the conversion of coal-N to primary products such as char, HCN, NH3, N2 and tar without secondary reactions during the rapid pyrolysis of coal. In addition, nitrogen conversions during the rapid pyrolysis of petroleum coke which have been scarcely reported were also investigated in this study.
Sample
2. Experimental 2.1. Fuel samples Three typical Chinese coals lignite (Inner Mongolia), bituminous (Shenfu), and anthracite (Guizhou) were pyrolyzed, these coals are largely reserved in China, and be used widely. A petroleum coke which had a high graphitization degree was also pyrolyzed as a comparison. The proximate analysis and ultimate analysis of the fuel samples are listed in Table 1. The particle sizes of the four samples were chosen as 125–180 lm. 2.2. Pyrolysis The experimental device used in this study is shown in Fig. 1. Quartz tube reactor and the molybdenum crucible within it were placed in the center of induction coil which connected to the high-frequency power supplier. When the high-frequency power was turned on, high-frequency alternating magnetic field could be formed in the center of induction coil, and the molybdenum crucible in the high frequency alternating magnetic field can be self-heated rapidly. The temperature of molybdenum crucible was monitored by an S-thermocouple which inserted into the hole drilled in the bottom, and the final temperature was controlled by the current from power supplier. As the molybdenum crucible was self-heated, the high temperature zone was just limited in the molybdenum crucible and the small area around it. During pyrolysis, the volatile products released from the molybdenum crucible could be carried out by the carrier gas rapidly from the high temperature zone and be quenched rapidly, thus secondary reactions could be sharply suppressed. The temperature distribution above the crucible was measured by an S-thermocouple. But the corundum lined-pipe of the thermocouple, whose radiation absorption coefficient is much higher than the gas, might make the measured temperature higher than the actual temperature of the gas. Therefore the temperature distribution was also calculated by the ‘‘Fluent’’ software. The measured and calculated temperature distributions above the crucible under the condition of 1200 °C are shown in Fig. 2. The measured gas temperature is about 50–100 °C higher than the calculated temperature. The calculated temperature should be more approach to the actual temperature of the gas. It can be found that the temperature decreases sharply with the increasing distance above from the crucible. The quartz tube reactor was 200 mm long with an inner diameter of 35 mm. The carrier gas was high purified argon (>99.999%), and its flow rate was 500 ml/min. Fuel samples (500 ± 5 mg each time) were feed into the reactor from the top of quartz reactor though a sample feeding tube which was inserted into the reactor. Before experiment, fuel sample was weighed and putted into a dropper with rubber head, the dropper was connected with the
8
Vent
Mo - crucible
3 9
4 5
1
Ar
6 7
10
2 Fig. 1. High frequency furnace rapid pyrolysis system. 1 – Ar cylinder; 2 – flowmeter; 3 – high frequency current source; 4 – quartz tube reactor; 5 – induction coil; 6 – Mo – crucible; 7 – thermocouple and meter; 8 – filter (cotton); 9 – absorption bottles; 10 – bubble stones.
S. Yuan et al. / Applied Energy 92 (2012) 854–859
1200
Calculated Measured
(a)
Temperature,
1000 800 600 400 200 0 0
2
4
6
8
10
12
14
16
Nitrogen distribution in products, mol %
856
100 90 80 70 60 50 40 30 20 10 0
600
700
Distance above from the crucible, cm
900
1000
1100
1200
100 90 80 70 60 50 40 30 20 10 0
600
700
800
900
1000
1100
1200
1000
1100
1200
1000
1100
1200
Temprature,
(c)
2.3. Quantification
and CN ions were analyzed by Metrohm 861 ion chromatograph. The separation column of the cations was Metrosep C 4–100, and the eluent was a mixture solution of HNO3 (1.7
Nitrogen distribution in products, mol %
sample feeding tube by a short rubber tube. The rubber tube was clipped by a clip before experiment, and the dropper was upside down. The system was purged before the experiment, and then turned on the power supplier and adjusted the current to make the molybdenum crucible be heated to the target temperature. During purging of the system, the clip was removed and the dropper was kept upside down. And then the rubber head was gently extruded several times to drive out the air in the dropper but avoid blowing the fuel sample out. During experiment, the dropper was raised to make the fuel particles flow slowly into the sample feeding tube. The sample feeding tube was inserted about 20 mm upon the crucible bottom. Rapid pyrolysis happened at the moment the samples dropped into the molybdenum crucible, the pyrolysis gas was carried out from the reactor to the adsorption bottles by the carrier gas. Feeding time of the fuel sample was 2 min each cycle, and the power supplier was shut down after 2 min when the injection of fuel samples was finished. HCN and NH3 in pyrolysis gas were adsorbed by NaOH solution (20 mmol/L) and HNO3 solution (1.7 mmol/L) respectively, and the adsorptions of HCN and NH3 were carried out separately in parallel experiments.
(b)
Nitrogen distribution in products, mol %
Temprature,
Fig. 2. Temperature distribution above the crucible in the reactor.
NHþ 4
800
100 90 80 70 60 50 40 30 20 10 0
600
700
800
900
(d)
100 90
Char, wt %
80 70 60 Lignite Bituminous Anthracite Petroleumcoke
50 40 600
700
800
900
1000
1100
1200
Temperature, Fig. 3. Char yields under rapid pyrolysis of lignite, bituminous, anthracite, and petroleum coke under different temperatures.
Nitrogen distribution in products, mol %
Temprature, 100 90 80 70 60 50 40 30 20 10 0
600
700
800
900
Temprature, Fig. 4. Effect of temperature on nitrogen distributions in pyrolysis products from (a) lignite, (b) bituminous, (c) anthracite and (d) petroleum coke: gray: char-N; light gray: (HCN + NH3)–N; white: (tar + N2)–N.
857
(a)
3.1. Nitrogen distributions in pyrolysis products
Centre-N
Top-N
Valley-N
N N
N
9 8 7 6 5 4 3 2 1 0
600
700
800
900
1000
1100
1200
1100
1200
9
Yields of HCN-N and NH 3-N, mol %
(b)
3. Results and discussions
8 7 6 5 4 3 2 1 0
600
700
800
900
1000
Temperature,
(c)
Yields of HCN-N and NH 3-N, mol %
Char yields of lignite, bituminous, anthracite, and petroleum coke under rapid pyrolysis at different temperatures are shown in Fig. 3. Char yields decreased with the increasing coal rank. Char yields of petroleum coke were close to those of anthracite, and higher than those of lignite and bituminous. But char yields of anthracite were more sensitive to temperature than those of petroleum coke. The effect of temperature on nitrogen distributions in pyrolysis products from lignite, bituminous, anthracite, and petroleum coke are shown in Fig. 4a–d. Most of the fuel-N was retained in char during rapid pyrolysis. Char-N yields decreased with the increasing temperature. (HCN + NH3)–N yields of coal increased and surpassed a maximum, but their absolute amounts were much lower. The yields of (tar + N2)–N increased with the increasing temperature, however according to the literature [19], the yields of tar decreased with the increasing temperature above 600 °C. The yields of tar decreased from 20% to 5% when increasing the temperature from 600 °C to 1000 °C during rapid pyrolysis of Yallourn lignite [19]. In addition, although tar produced in experiments of this study was not qualified, it was qualitatively observed that less tar was formed under high temperature than low temperature. Therefore, it can be deduced that, more fuel-N converted to N2 under high temperature during rapid pyrolysis. From the comparisons of Fig. 4a–d, we found the yields of volatile-N decreased with the increase in coal rank. However, the proportions of (HCN + NH3)–N in volatile-N were low, the changes in volatile-N yields were mainly reflected by (tar + N2)–N. The yields of (HCN + NH3)–N during rapid pyrolysis of petroleum coke were much lower than those of the three coals. And promotion of the increasing temperature on the yields of volatile-N was weak. The
10
Temperature,
4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
600
700
800
900
1000
1100
1200
Temperature,
(d)
Yields of HCN-N and NH 3-N, mol %
mmol/L) and Pyridine 2,6-dicarboxylate (10 mmol/L). The separation column of the cations was Metrosep A Supp 1–250, and the eluent was NaOH solution with a concentration of 10 mmol/L. It should be noted that HNCO produced during rapid pyrolysis can convert to NHþ 4 in adsorption solutions through hydrolysis, but the yields of HNCO are much lower than that of HCN and NH3, and its contribution to total the NHþ 4 in solution were very small [18]. Solid products (char) in molybdenum crucible was collected and weighted after each experiment. Nitrogen contents in coal and char were analyzed by an Elemental Analyzer (Vario MACRO CHN/CHNS). While, the tar was hardly collected for the reasons that quantity of fuel samples injected into the reactor each time was low, tar yields during rapid pyrolysis especially high-temperature rapid pyrolysis are low [7,19], and proportions of tar were condensed in the inner wall of the upper part of the quartz reactor and the pipe line. In this paper tar-N together with N2–N were calculated by the mass conservation method.
Yields of HCN-N and NH 3-N, mol %
S. Yuan et al. / Applied Energy 92 (2012) 854–859
3.0 2.5 2.0 1.5 1.0 0.5 0.0
600
700
800
900
1000
1100
1200
Temperature,
Fig. 5. Schematic diagram of ‘‘centre-N’’, ‘‘valley-N’’ and ‘‘top-N’’ [19,20].
Fig. 6. Nitrogen conversion to HCN–N and NH3–N during pyrolysis of (a) lignite, (b) bituminous, (c) anthracite, and (d) petroleum coke under different temperatures: black: HCN–N; twill: NH3–N.
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S. Yuan et al. / Applied Energy 92 (2012) 854–859
100
reasons might be that, in the petroleum which has a high graphitization degree, most of the nitrogen atoms exist in the grids of macromolecules, and they might be similar to the ‘‘centre-N’’ and ‘‘valley-N’’ which is much more stable than other forms of nitrogen in coal (Fig. 5) [20,21].
Yields of HCN-N, NH 3-N, and char-N, mol%
90
3.2. NH3 and HCN formations The effects of temperature on the yields of NH3–N and HCN–N during the rapid pyrolysis of lignite, bituminous, anthracite, and petroleum coke are shown in Fig. 6a–d. In most of the conditions NH3–N yields were higher than HCN–N yields, especially for the petroleum coke where no HCN was detected in its pyrolysis products. It might be further proved that nitrogen in petroleum coke is mostly condensed in the grids of macromolecules as the form of ‘‘centre-N’’ and ‘‘valley-N’’. Due to the reasons that N atoms of ‘‘centre-N’’ and ‘‘valley-N’’ are shared by two or three aromatic rings, and the three bonds connected to the N atom have averaging bond strengths [22]. Therefore, these three bonds connected with the N atom might be cracked at the same time during rapid pyrolysis when a high energy impact can be provided in a very short time. And then N free radicals could be formed and leading to the formation of N2 and low yields of NH3. Under the experimental conditions of this study, the secondary reactions between the gas phase products could be sharply reduced, the yields of char were low and the secondary cracking of tar could also be restrained. From Fig. 6 it is obviously that, considerable NH3 was formed in the conditions described above. The results have some contradictions to the view that NH3 is mainly produced from the heterogeneous reactions between HCN and char [23]. From this study, it can be deduced that NH3 can be formed directly in the primary stage of rapid pyrolysis through the cracking of nitrogen functionalities. The process might be that –N, –NH, and –NH2 free radicals could be released from fuel under the effect of high energy impact, and then combined with H or H2 to form NH3 through the gas-phase reactions. Li and co-workers considered that H radicals released during pyrolysis could be adsorbed on the surface of char, attack heterocyclic nitrogen, and promote nitrogen release as the form of NH3 [9–11]. From the comparison of Fig. 6a–c, the NH3–N yields of the three coals were in the order of bituminous > lignite > anthracite, and the HCN–N yields of the three coals presented similar trend. It is necessary to compare the results derived from different devices for rapid pyrolysis in the literatures. Data under the condition
80 70 60 50 40 30 20 10 0
65
70
75
80
85
90
95
Carbon content of coal, wt % (daf) Fig. 7. Yields of HCN–N, NH3–N, and char-N under different devices at 800 °C: triangle: HCN–N; round: NH3–N; square: char-N (solid: results of this study).
of 800 °C which available in each of these literatures were chosen to be listed in Table 2. Results derived from different devices have large diversities. In the studies of Tan and Li [9–11] and Kamara et al. [8], NH3–N yields were lower than HCN–N yields. However, Xie et al. [12,13] and Xu and Kumagai [7] found that NH3–N yields were higher than HCN–N, which are similar to the results of this study. In this study, HCN–N/NH3–N ratios were found to be higher than those of lignite, which are similar to the results in the study of Nelson et al. [3,4]. Tan and Li [9–11] also found that HCN–N/NH3–N ratios increased with the increasing coal rank. However, HCN–N/ NH3–N ratios derived from a fluidized-bed/fixed-bed reactor in the study of Xie et al. [12,13] presented an decrease trend with the increasing coal rank. As listed in Table 2, yields of HCN–N, NH3–N, and char-N versus carbon content (daf) in coal were plotted in Fig. 7. Char-N yields presented a good regularity, which increased with the increasing carbon content. Yields of HCN–N and NH3–N presented not so good regularities as that of char-N yields. However, NH3–N yields presented better regularity than HCN–N yields. As the carbon content increasing, yields of NH3–N showed a decreasing trend, but the points of HCN–N yields were dispersed, especially at low carbon contents. Therefore it can be concluded that, during rapid pyrolysis of coal in different devices, char-N yields present small diversities, and HCN–N present largest diversities.
Table 2 Comparison of HCN–N, NH3–N, and char-N yields derived from different devices at 800 °C. Study
Reactor
Heating rate (K/s)
Coal (C, wt.%, daf)
HCN–N (mol%)
NH3–N (mol%)
Char–N (mol%)
This work
High frequency furnace
>104
Nelson et al. [3,4]
Fluidized-bed reactor
>104
Xu and Kumagai [7]
Free fall pyrolyzer, 3 MPa
>2 103
75.7 85.1 92.2 67.3 82.3 72.0 78.4 79.8
1.2 4.7 1.8 5 11 2 3 2
8.6 8.9 3.5 8 8 13 15 15
64.2 69.6 80.5 65 – 65 60 67
Tan and Li [9–11]
Drop-tube with fixed-bed reactor
>103
Xie et al. [12,13]
Fliudzed-bed with fixed-bed reactor
>104
68.5 82.1 91.0 68.5 84.3 90.0
23 18 8 12 7 3
18 8 2 12 14 9
– – – – – –
Kambara et al. [8]
Pyroprobe reactor
7.5 103
72.8 84.6 88.1
24 12 10
9 8 2
65 68 88
S. Yuan et al. / Applied Energy 92 (2012) 854–859
4. Conclusions Rapid pyrolysis of coal of three ranks and a kind of petroleum coke was carried out in a high-frequency furnace which had a small high temperature zone. Nitrogen conversions under the conditions of rapid pyrolysis were investigated. Under the conditions of this study, the increasing temperature could decrease the char-N yields during the rapid pyrolysis of coal and petroleum coke. The increasing coal rank increased the char-N yields and decreased the volatile-N yields, but more NH3 than HCN could be released in most of the conditions of this study. Both HCN and NH3 could be released directly from coal at the primary stage of rapid pyrolysis. However proportions of both NH3–N and HCN–N in volatile-N were low. During rapid pyrolysis of petroleum coke, the increasing temperature increased the NH3–N yields, but no HCN was found under all the conditions. By comparing the data from this study and the literatures, char-N yields derived from different devices present better regularity than the yields of HCN–N and NH3–N, and HCN–N yields vary largely with the devices. However, nitrogen in tar was not investigated in this study due to the difficulty of tar collection. Further efforts should be made to make quantifications of tar-N yields derived from the highfrequency furnace, and nitrogen forms in tar should also be probed into. Acknowledgments This study was supported by the National Basic Research Program of China (2010CB227000), and Shanghai ‘‘Technology Innovation Action Plan’’. The authors also acknowledge Prof. Yi-fan Han (State Key Laboratory of Chemical Engineering, ECUST) for his help on language. References [1] Chen Z, Yuan S, Liang QF, Wang FC, Yu ZH. Distribution of HCN, NH3, NO and N2 in an entrained flow gasifier. Chem Eng J 2009;148:312–8. [2] Zhao W, Feng J, Chang LP, Xie KC, Liu MR. Release of nitrogenous species during coal gasification. J Fuel Chem Technol 2002;6:519–22. Chinese journal. [3] Nelson PF, Kelly MD, Wornat MJ. Conversion of fuel nitrogen in coal volatiles to NOx precursors under rapid heating conditions. Fuel 1991;70:403–7. [4] Nelson PF, Buckley AN, Kelly MD. Functional forms of nitrogen in coals and the release of coal nitrogen as NOx precursors (HCN and NH3). Symp (Int) Combust 1992;24:1259–67.
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Nitrogen conversion during rapid pyrolysis of coal and petroleum coke in a high-frequency furnace Shuai Yuan, Zhi-jie Zhou, Jun Li, Fu-chen Wang ⇑ Key Laboratory of Coal Gasification of Ministry of Education, East China University of Science and Technology, Shanghai 200237, China
a r t i c l e
i n f o
Article history: Received 17 November 2010 Received in revised form 23 August 2011 Accepted 24 August 2011 Available online 17 September 2011 Keywords: Rapid pyrolysis Coal Nitrogen HCN NH3
a b s t r a c t Rapid pyrolysis of three typical Chinese coals, lignite from Inner Mongolia, bituminous from Shenfu coalfield, and anthracite from Guizhou, as well as a petroleum coke were carried out in a drop-style high-frequency furnace. The reactor was induction coil heated and had a very small high-temperature zone, which could restrain secondary conversions of nitrogen products. The effects of temperature and coal rank on conversions of fuel-N to primary nitrogen products (char-N, HCN–N, NH3–N and (tar + N2)–N) have been investigated. The results showed that, the increasing temperature reduced the yields of char-N and promoted the conversion of fuel-N to N2. Char-N yields increased, while volatile-N yields decreased as the coal rank increased. In most of the conditions, NH3–N yields were higher than HCN– N yields during rapid pyrolysis of coal. In the case of petroleum coke, NH3–N yields increased gradually with the increasing temperature, but no HCN was detected. We argue that NH3–N can be formed directly through the primary pyrolysis without secondary reactions. Although volatile-N yields of lignite were higher than those of bituminous, yields of (HCN + NH3)–N in volatile-N of lignite were lower than those of bituminous. While the (HCN + NH3)–N yields of anthracite were the lowest of the three coals. Both of the (HCN + NH3)–N yields and (HCN + NH3)–N proportions in volatile-N of petroleum coke were lower than the three coals. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Nitrogen in coal forms NOx during combustion, which results in the environmental problems such as acid rain and photochemical smog. During gasification, nitrogen in coal forms NH3 and HCN in the gasifier. NH3 in gasification system can react with H2O and CO2 to produce ammonium salts which will crystallize in the low temperature positions of the system and leading to the problems including pipeline abrasion and block. Dissolution of HCN in system water can produce cyanide wastewater which increases the difficulty and the cost of sewage treatment [1,2]. Rapid pyrolysis of coal is an important process in both combustion and gasification. Nitrogen forms released from coal during rapid pyrolysis influences the nitrogen subsequence conversions and the nitrogen pollutants finally formed. Therefore, many researchers focus on nitrogen conversions during rapid pyrolysis of coal. Rapid pyrolysis of coal has been carried out by Nelson and coworkers [3–5], and effects of coal rank and temperature on the formations of HCN and NH3, as well as nitrogen functionalities in tar were investigated. Cai and co-workers [6] have investigated the effects of heating rate and pressure on nitrogen conversion during ⇑ Corresponding author. Tel.: +86 21 64250784; fax: +86 21 64251312. E-mail address: [email protected] (F.-c. Wang). 0306-2619/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2011.08.042
rapid pyrolysis of coal by using a wire-mesh reactor. In a similar study, Xu and Kumagai investigated the effects of temperature, atmosphere and H2 pressure on nitrogen conversions by using a drop-tube furnace [7]. Rapid pyrolysis of twenty coals of different coal ranks has been carried out by Kambara et al. [8] in a pyroprobe reactor. And the correlations of nitrogen functionalities and nitrogen forms released from coal were investigate with XPS (X-ray photoelectron spectroscopy), and the effects of temperature and coal rank on nitrogen conversions during rapid pyrolysis were also investigated. Furthermore, Li and co-workers [9–14] used a reactor which had some features of both a drop-tube reactor and a fixedbed to investigate HCN and NH3 formations during rapid pyrolysis of different ranks of coals. A semi-entrained flow/semi-fixed bed reactor has been used by Liu and co-workers to investigate the effects of coal rank on HCN formations during rapid pyrolysis of coals [15]. Effects of coal rank, carrier gas flow rate and temperature on the formation of NOx precursors during rapid pyrolysis has been investigated by Feng and co-workers in a reactor similar to that of Liu and co-workers [15,16]. A review on nitrogen conversions during pyrolysis and gasification of coal and biomass has been made by Leppalahti and Koljonen [17]. Presently, nitrogen conversion mechanisms during rapid pyrolysis are not fully understood. The heating rate and residence time are greatly different due to the variations of reactor used by
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S. Yuan et al. / Applied Energy 92 (2012) 854–859 Table 1 Proximate analysis (dry basis, wt.%) and ultimate analysis (daf, wt.%) of fuel samples.
Lignite Bituminous Anthracite Petroleum coke a
A
V
FC
C
H
N
S
Oa
16.59 6.57 27.24 0.25
33.68 31.79 7.15 10.08
49.73 61.65 65.60 89.67
75.69 85.11 92.16 91.52
5.69 6.13 2.06 2.58
0.91 1.32 1.42 1.63
0.77 1.35 1.70 3.47
16.94 6.07 2.66 0.80
By difference.
different researchers. The nitrogen products released from primary reactions such as char-N, tar-N and other volatile-N are affected by the heating rate. However, the residence time of primary pyrolysis products in high-temperature zone varied with different studies due to the differences of reactor forms and gas velocities. Nelson et al. suggested that, HCN was formed by secondary cracking of volatile-N during rapid pyrolysis of coal in a fluidized-bed reactor [3]. But Xu and Kumagai proposed that HCN was converted to NH3 by reacting with H2 during rapid pyrolysis of coal in a pressurized drop-tube furnace [7]. The fluidized-bed reactor and droptube furnace which have large high temperature zones provide essential conditions for secondary reactions. During rapid pyrolysis of coal or biomass in fluidized-bed reactor and drop-tube furnace, the fuel samples are continuously fed into the reactor. Therefore there will be always pyrolysis gas in the reactor, and the later fed samples may react with the pyrolysis gas released from the previously fed samples. In the study of Tan and Li [9], HCN and NH3 were formed in the primary stage of pyrolysis during rapid pyrolysis of coal in a drop-tube/fixed-bed reactor, and they proposed the residence time affected the yields of HCN and NH3 remarkably [9]. The secondary reactions of char-N, tar-N, NH3, HCN and other nitrogen products which affected by the reactors and the gas velocities significantly, caused so many disagreements or even opposite conclusions reported in literature. The wire-mesh reactor is a powerful tool to realize rapid pyrolysis without secondary reactions. But there is no systematic study on nitrogen evolution during rapid pyrolysis of coal by using wire-mesh reactor. The low sample capacity of the wire-mesh reactor might be the reason. In this study, a drop-style high-frequency furnace which could limit the high temperature zone in a very small zone were used, and the secondary reactions of nitrogen products could be sharply reduced by shortening the residence time of primary volatilenitrogen products in high temperature zone. The purpose of this study was to investigate the conversion of coal-N to primary products such as char, HCN, NH3, N2 and tar without secondary reactions during the rapid pyrolysis of coal. In addition, nitrogen conversions during the rapid pyrolysis of petroleum coke which have been scarcely reported were also investigated in this study.
Sample
2. Experimental 2.1. Fuel samples Three typical Chinese coals lignite (Inner Mongolia), bituminous (Shenfu), and anthracite (Guizhou) were pyrolyzed, these coals are largely reserved in China, and be used widely. A petroleum coke which had a high graphitization degree was also pyrolyzed as a comparison. The proximate analysis and ultimate analysis of the fuel samples are listed in Table 1. The particle sizes of the four samples were chosen as 125–180 lm. 2.2. Pyrolysis The experimental device used in this study is shown in Fig. 1. Quartz tube reactor and the molybdenum crucible within it were placed in the center of induction coil which connected to the high-frequency power supplier. When the high-frequency power was turned on, high-frequency alternating magnetic field could be formed in the center of induction coil, and the molybdenum crucible in the high frequency alternating magnetic field can be self-heated rapidly. The temperature of molybdenum crucible was monitored by an S-thermocouple which inserted into the hole drilled in the bottom, and the final temperature was controlled by the current from power supplier. As the molybdenum crucible was self-heated, the high temperature zone was just limited in the molybdenum crucible and the small area around it. During pyrolysis, the volatile products released from the molybdenum crucible could be carried out by the carrier gas rapidly from the high temperature zone and be quenched rapidly, thus secondary reactions could be sharply suppressed. The temperature distribution above the crucible was measured by an S-thermocouple. But the corundum lined-pipe of the thermocouple, whose radiation absorption coefficient is much higher than the gas, might make the measured temperature higher than the actual temperature of the gas. Therefore the temperature distribution was also calculated by the ‘‘Fluent’’ software. The measured and calculated temperature distributions above the crucible under the condition of 1200 °C are shown in Fig. 2. The measured gas temperature is about 50–100 °C higher than the calculated temperature. The calculated temperature should be more approach to the actual temperature of the gas. It can be found that the temperature decreases sharply with the increasing distance above from the crucible. The quartz tube reactor was 200 mm long with an inner diameter of 35 mm. The carrier gas was high purified argon (>99.999%), and its flow rate was 500 ml/min. Fuel samples (500 ± 5 mg each time) were feed into the reactor from the top of quartz reactor though a sample feeding tube which was inserted into the reactor. Before experiment, fuel sample was weighed and putted into a dropper with rubber head, the dropper was connected with the
8
Vent
Mo - crucible
3 9
4 5
1
Ar
6 7
10
2 Fig. 1. High frequency furnace rapid pyrolysis system. 1 – Ar cylinder; 2 – flowmeter; 3 – high frequency current source; 4 – quartz tube reactor; 5 – induction coil; 6 – Mo – crucible; 7 – thermocouple and meter; 8 – filter (cotton); 9 – absorption bottles; 10 – bubble stones.
S. Yuan et al. / Applied Energy 92 (2012) 854–859
1200
Calculated Measured
(a)
Temperature,
1000 800 600 400 200 0 0
2
4
6
8
10
12
14
16
Nitrogen distribution in products, mol %
856
100 90 80 70 60 50 40 30 20 10 0
600
700
Distance above from the crucible, cm
900
1000
1100
1200
100 90 80 70 60 50 40 30 20 10 0
600
700
800
900
1000
1100
1200
1000
1100
1200
1000
1100
1200
Temprature,
(c)
2.3. Quantification
and CN ions were analyzed by Metrohm 861 ion chromatograph. The separation column of the cations was Metrosep C 4–100, and the eluent was a mixture solution of HNO3 (1.7
Nitrogen distribution in products, mol %
sample feeding tube by a short rubber tube. The rubber tube was clipped by a clip before experiment, and the dropper was upside down. The system was purged before the experiment, and then turned on the power supplier and adjusted the current to make the molybdenum crucible be heated to the target temperature. During purging of the system, the clip was removed and the dropper was kept upside down. And then the rubber head was gently extruded several times to drive out the air in the dropper but avoid blowing the fuel sample out. During experiment, the dropper was raised to make the fuel particles flow slowly into the sample feeding tube. The sample feeding tube was inserted about 20 mm upon the crucible bottom. Rapid pyrolysis happened at the moment the samples dropped into the molybdenum crucible, the pyrolysis gas was carried out from the reactor to the adsorption bottles by the carrier gas. Feeding time of the fuel sample was 2 min each cycle, and the power supplier was shut down after 2 min when the injection of fuel samples was finished. HCN and NH3 in pyrolysis gas were adsorbed by NaOH solution (20 mmol/L) and HNO3 solution (1.7 mmol/L) respectively, and the adsorptions of HCN and NH3 were carried out separately in parallel experiments.
(b)
Nitrogen distribution in products, mol %
Temprature,
Fig. 2. Temperature distribution above the crucible in the reactor.
NHþ 4
800
100 90 80 70 60 50 40 30 20 10 0
600
700
800
900
(d)
100 90
Char, wt %
80 70 60 Lignite Bituminous Anthracite Petroleumcoke
50 40 600
700
800
900
1000
1100
1200
Temperature, Fig. 3. Char yields under rapid pyrolysis of lignite, bituminous, anthracite, and petroleum coke under different temperatures.
Nitrogen distribution in products, mol %
Temprature, 100 90 80 70 60 50 40 30 20 10 0
600
700
800
900
Temprature, Fig. 4. Effect of temperature on nitrogen distributions in pyrolysis products from (a) lignite, (b) bituminous, (c) anthracite and (d) petroleum coke: gray: char-N; light gray: (HCN + NH3)–N; white: (tar + N2)–N.
857
(a)
3.1. Nitrogen distributions in pyrolysis products
Centre-N
Top-N
Valley-N
N N
N
9 8 7 6 5 4 3 2 1 0
600
700
800
900
1000
1100
1200
1100
1200
9
Yields of HCN-N and NH 3-N, mol %
(b)
3. Results and discussions
8 7 6 5 4 3 2 1 0
600
700
800
900
1000
Temperature,
(c)
Yields of HCN-N and NH 3-N, mol %
Char yields of lignite, bituminous, anthracite, and petroleum coke under rapid pyrolysis at different temperatures are shown in Fig. 3. Char yields decreased with the increasing coal rank. Char yields of petroleum coke were close to those of anthracite, and higher than those of lignite and bituminous. But char yields of anthracite were more sensitive to temperature than those of petroleum coke. The effect of temperature on nitrogen distributions in pyrolysis products from lignite, bituminous, anthracite, and petroleum coke are shown in Fig. 4a–d. Most of the fuel-N was retained in char during rapid pyrolysis. Char-N yields decreased with the increasing temperature. (HCN + NH3)–N yields of coal increased and surpassed a maximum, but their absolute amounts were much lower. The yields of (tar + N2)–N increased with the increasing temperature, however according to the literature [19], the yields of tar decreased with the increasing temperature above 600 °C. The yields of tar decreased from 20% to 5% when increasing the temperature from 600 °C to 1000 °C during rapid pyrolysis of Yallourn lignite [19]. In addition, although tar produced in experiments of this study was not qualified, it was qualitatively observed that less tar was formed under high temperature than low temperature. Therefore, it can be deduced that, more fuel-N converted to N2 under high temperature during rapid pyrolysis. From the comparisons of Fig. 4a–d, we found the yields of volatile-N decreased with the increase in coal rank. However, the proportions of (HCN + NH3)–N in volatile-N were low, the changes in volatile-N yields were mainly reflected by (tar + N2)–N. The yields of (HCN + NH3)–N during rapid pyrolysis of petroleum coke were much lower than those of the three coals. And promotion of the increasing temperature on the yields of volatile-N was weak. The
10
Temperature,
4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
600
700
800
900
1000
1100
1200
Temperature,
(d)
Yields of HCN-N and NH 3-N, mol %
mmol/L) and Pyridine 2,6-dicarboxylate (10 mmol/L). The separation column of the cations was Metrosep A Supp 1–250, and the eluent was NaOH solution with a concentration of 10 mmol/L. It should be noted that HNCO produced during rapid pyrolysis can convert to NHþ 4 in adsorption solutions through hydrolysis, but the yields of HNCO are much lower than that of HCN and NH3, and its contribution to total the NHþ 4 in solution were very small [18]. Solid products (char) in molybdenum crucible was collected and weighted after each experiment. Nitrogen contents in coal and char were analyzed by an Elemental Analyzer (Vario MACRO CHN/CHNS). While, the tar was hardly collected for the reasons that quantity of fuel samples injected into the reactor each time was low, tar yields during rapid pyrolysis especially high-temperature rapid pyrolysis are low [7,19], and proportions of tar were condensed in the inner wall of the upper part of the quartz reactor and the pipe line. In this paper tar-N together with N2–N were calculated by the mass conservation method.
Yields of HCN-N and NH 3-N, mol %
S. Yuan et al. / Applied Energy 92 (2012) 854–859
3.0 2.5 2.0 1.5 1.0 0.5 0.0
600
700
800
900
1000
1100
1200
Temperature,
Fig. 5. Schematic diagram of ‘‘centre-N’’, ‘‘valley-N’’ and ‘‘top-N’’ [19,20].
Fig. 6. Nitrogen conversion to HCN–N and NH3–N during pyrolysis of (a) lignite, (b) bituminous, (c) anthracite, and (d) petroleum coke under different temperatures: black: HCN–N; twill: NH3–N.
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100
reasons might be that, in the petroleum which has a high graphitization degree, most of the nitrogen atoms exist in the grids of macromolecules, and they might be similar to the ‘‘centre-N’’ and ‘‘valley-N’’ which is much more stable than other forms of nitrogen in coal (Fig. 5) [20,21].
Yields of HCN-N, NH 3-N, and char-N, mol%
90
3.2. NH3 and HCN formations The effects of temperature on the yields of NH3–N and HCN–N during the rapid pyrolysis of lignite, bituminous, anthracite, and petroleum coke are shown in Fig. 6a–d. In most of the conditions NH3–N yields were higher than HCN–N yields, especially for the petroleum coke where no HCN was detected in its pyrolysis products. It might be further proved that nitrogen in petroleum coke is mostly condensed in the grids of macromolecules as the form of ‘‘centre-N’’ and ‘‘valley-N’’. Due to the reasons that N atoms of ‘‘centre-N’’ and ‘‘valley-N’’ are shared by two or three aromatic rings, and the three bonds connected to the N atom have averaging bond strengths [22]. Therefore, these three bonds connected with the N atom might be cracked at the same time during rapid pyrolysis when a high energy impact can be provided in a very short time. And then N free radicals could be formed and leading to the formation of N2 and low yields of NH3. Under the experimental conditions of this study, the secondary reactions between the gas phase products could be sharply reduced, the yields of char were low and the secondary cracking of tar could also be restrained. From Fig. 6 it is obviously that, considerable NH3 was formed in the conditions described above. The results have some contradictions to the view that NH3 is mainly produced from the heterogeneous reactions between HCN and char [23]. From this study, it can be deduced that NH3 can be formed directly in the primary stage of rapid pyrolysis through the cracking of nitrogen functionalities. The process might be that –N, –NH, and –NH2 free radicals could be released from fuel under the effect of high energy impact, and then combined with H or H2 to form NH3 through the gas-phase reactions. Li and co-workers considered that H radicals released during pyrolysis could be adsorbed on the surface of char, attack heterocyclic nitrogen, and promote nitrogen release as the form of NH3 [9–11]. From the comparison of Fig. 6a–c, the NH3–N yields of the three coals were in the order of bituminous > lignite > anthracite, and the HCN–N yields of the three coals presented similar trend. It is necessary to compare the results derived from different devices for rapid pyrolysis in the literatures. Data under the condition
80 70 60 50 40 30 20 10 0
65
70
75
80
85
90
95
Carbon content of coal, wt % (daf) Fig. 7. Yields of HCN–N, NH3–N, and char-N under different devices at 800 °C: triangle: HCN–N; round: NH3–N; square: char-N (solid: results of this study).
of 800 °C which available in each of these literatures were chosen to be listed in Table 2. Results derived from different devices have large diversities. In the studies of Tan and Li [9–11] and Kamara et al. [8], NH3–N yields were lower than HCN–N yields. However, Xie et al. [12,13] and Xu and Kumagai [7] found that NH3–N yields were higher than HCN–N, which are similar to the results of this study. In this study, HCN–N/NH3–N ratios were found to be higher than those of lignite, which are similar to the results in the study of Nelson et al. [3,4]. Tan and Li [9–11] also found that HCN–N/NH3–N ratios increased with the increasing coal rank. However, HCN–N/ NH3–N ratios derived from a fluidized-bed/fixed-bed reactor in the study of Xie et al. [12,13] presented an decrease trend with the increasing coal rank. As listed in Table 2, yields of HCN–N, NH3–N, and char-N versus carbon content (daf) in coal were plotted in Fig. 7. Char-N yields presented a good regularity, which increased with the increasing carbon content. Yields of HCN–N and NH3–N presented not so good regularities as that of char-N yields. However, NH3–N yields presented better regularity than HCN–N yields. As the carbon content increasing, yields of NH3–N showed a decreasing trend, but the points of HCN–N yields were dispersed, especially at low carbon contents. Therefore it can be concluded that, during rapid pyrolysis of coal in different devices, char-N yields present small diversities, and HCN–N present largest diversities.
Table 2 Comparison of HCN–N, NH3–N, and char-N yields derived from different devices at 800 °C. Study
Reactor
Heating rate (K/s)
Coal (C, wt.%, daf)
HCN–N (mol%)
NH3–N (mol%)
Char–N (mol%)
This work
High frequency furnace
>104
Nelson et al. [3,4]
Fluidized-bed reactor
>104
Xu and Kumagai [7]
Free fall pyrolyzer, 3 MPa
>2 103
75.7 85.1 92.2 67.3 82.3 72.0 78.4 79.8
1.2 4.7 1.8 5 11 2 3 2
8.6 8.9 3.5 8 8 13 15 15
64.2 69.6 80.5 65 – 65 60 67
Tan and Li [9–11]
Drop-tube with fixed-bed reactor
>103
Xie et al. [12,13]
Fliudzed-bed with fixed-bed reactor
>104
68.5 82.1 91.0 68.5 84.3 90.0
23 18 8 12 7 3
18 8 2 12 14 9
– – – – – –
Kambara et al. [8]
Pyroprobe reactor
7.5 103
72.8 84.6 88.1
24 12 10
9 8 2
65 68 88
S. Yuan et al. / Applied Energy 92 (2012) 854–859
4. Conclusions Rapid pyrolysis of coal of three ranks and a kind of petroleum coke was carried out in a high-frequency furnace which had a small high temperature zone. Nitrogen conversions under the conditions of rapid pyrolysis were investigated. Under the conditions of this study, the increasing temperature could decrease the char-N yields during the rapid pyrolysis of coal and petroleum coke. The increasing coal rank increased the char-N yields and decreased the volatile-N yields, but more NH3 than HCN could be released in most of the conditions of this study. Both HCN and NH3 could be released directly from coal at the primary stage of rapid pyrolysis. However proportions of both NH3–N and HCN–N in volatile-N were low. During rapid pyrolysis of petroleum coke, the increasing temperature increased the NH3–N yields, but no HCN was found under all the conditions. By comparing the data from this study and the literatures, char-N yields derived from different devices present better regularity than the yields of HCN–N and NH3–N, and HCN–N yields vary largely with the devices. However, nitrogen in tar was not investigated in this study due to the difficulty of tar collection. Further efforts should be made to make quantifications of tar-N yields derived from the highfrequency furnace, and nitrogen forms in tar should also be probed into. Acknowledgments This study was supported by the National Basic Research Program of China (2010CB227000), and Shanghai ‘‘Technology Innovation Action Plan’’. The authors also acknowledge Prof. Yi-fan Han (State Key Laboratory of Chemical Engineering, ECUST) for his help on language. References [1] Chen Z, Yuan S, Liang QF, Wang FC, Yu ZH. Distribution of HCN, NH3, NO and N2 in an entrained flow gasifier. Chem Eng J 2009;148:312–8. [2] Zhao W, Feng J, Chang LP, Xie KC, Liu MR. Release of nitrogenous species during coal gasification. J Fuel Chem Technol 2002;6:519–22. Chinese journal. [3] Nelson PF, Kelly MD, Wornat MJ. Conversion of fuel nitrogen in coal volatiles to NOx precursors under rapid heating conditions. Fuel 1991;70:403–7. [4] Nelson PF, Buckley AN, Kelly MD. Functional forms of nitrogen in coals and the release of coal nitrogen as NOx precursors (HCN and NH3). Symp (Int) Combust 1992;24:1259–67.
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