Applied Energy 92 (2012) 854–859

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Applied Energy

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Nitrogen conversion during rapid pyrolysis of coal and petroleum cokein 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 2010Received in revised form 23 August 2011Accepted 24 August 2011Available online 17 September 2011

Keywords:Rapid pyrolysisCoalNitrogenHCNNH3

0306-2619/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.apenergy.2011.08.042

⇑ Corresponding author. Tel.: +86 21 64250784; faxE-mail address: wfch@ecust.edu.cn (F.-c. Wang).

a b s t r a c t

Rapid pyrolysis of three typical Chinese coals, lignite from Inner Mongolia, bituminous from Shenfu coal-field, and anthracite from Guizhou, as well as a petroleum coke were carried out in a drop-style high-fre-quency 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 coalrank 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 ofchar-N and promoted the conversion of fuel-N to N2. Char-N yields increased, while volatile-N yieldsdecreased 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 graduallywith the increasing temperature, but no HCN was detected. We argue that NH3–N can be formed directlythrough the primary pyrolysis without secondary reactions. Although volatile-N yields of lignite werehigher than those of bituminous, yields of (HCN + NH3)–N in volatile-N of lignite were lower than thoseof bituminous. While the (HCN + NH3)–N yields of anthracite were the lowest of the three coals. Both ofthe (HCN + NH3)–N yields and (HCN + NH3)–N proportions in volatile-N of petroleum coke were lowerthan the three coals.

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1. Introduction

Nitrogen in coal forms NOx during combustion, which results inthe environmental problems such as acid rain and photochemicalsmog. During gasification, nitrogen in coal forms NH3 and HCN inthe gasifier. NH3 in gasification system can react with H2O andCO2 to produce ammonium salts which will crystallize in the lowtemperature positions of the system and leading to the problemsincluding pipeline abrasion and block. Dissolution of HCN in sys-tem water can produce cyanide wastewater which increases thedifficulty and the cost of sewage treatment [1,2]. Rapid pyrolysisof coal is an important process in both combustion and gasifica-tion. Nitrogen forms released from coal during rapid pyrolysisinfluences the nitrogen subsequence conversions and the nitrogenpollutants finally formed. Therefore, many researchers focus onnitrogen conversions during rapid pyrolysis of coal.

Rapid pyrolysis of coal has been carried out by Nelson and co-workers [3–5], and effects of coal rank and temperature on the for-mations of HCN and NH3, as well as nitrogen functionalities in tarwere investigated. Cai and co-workers [6] have investigated the ef-fects of heating rate and pressure on nitrogen conversion during

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rapid pyrolysis of coal by using a wire-mesh reactor. In a similarstudy, Xu and Kumagai investigated the effects of temperature,atmosphere and H2 pressure on nitrogen conversions by using adrop-tube furnace [7]. Rapid pyrolysis of twenty coals of differentcoal ranks has been carried out by Kambara et al. [8] in a pyroprobereactor. And the correlations of nitrogen functionalities and nitro-gen forms released from coal were investigate with XPS (X-rayphotoelectron spectroscopy), and the effects of temperature andcoal rank on nitrogen conversions during rapid pyrolysis were alsoinvestigated. Furthermore, Li and co-workers [9–14] used a reactorwhich had some features of both a drop-tube reactor and a fixed-bed to investigate HCN and NH3 formations during rapid pyrolysisof different ranks of coals. A semi-entrained flow/semi-fixed bedreactor has been used by Liu and co-workers to investigate the ef-fects of coal rank on HCN formations during rapid pyrolysis of coals[15]. Effects of coal rank, carrier gas flow rate and temperature onthe formation of NOx precursors during rapid pyrolysis has beeninvestigated by Feng and co-workers in a reactor similar to thatof Liu and co-workers [15,16]. A review on nitrogen conversionsduring pyrolysis and gasification of coal and biomass has beenmade by Leppalahti and Koljonen [17].

Presently, nitrogen conversion mechanisms during rapid pyro-lysis are not fully understood. The heating rate and residence timeare greatly different due to the variations of reactor used by

Table 1Proximate analysis (dry basis, wt.%) and ultimate analysis (daf, wt.%) of fuel samples.

A V FC C H N S Oa

Lignite 16.59 33.68 49.73 75.69 5.69 0.91 0.77 16.94Bituminous 6.57 31.79 61.65 85.11 6.13 1.32 1.35 6.07Anthracite 27.24 7.15 65.60 92.16 2.06 1.42 1.70 2.66Petroleum

coke0.25 10.08 89.67 91.52 2.58 1.63 3.47 0.80

a By difference.

S. Yuan et al. / Applied Energy 92 (2012) 854–859 855

different researchers. The nitrogen products released from primaryreactions such as char-N, tar-N and other volatile-N are affected bythe heating rate. However, the residence time of primary pyrolysisproducts in high-temperature zone varied with different studiesdue to the differences of reactor forms and gas velocities. Nelsonet al. suggested that, HCN was formed by secondary cracking ofvolatile-N during rapid pyrolysis of coal in a fluidized-bed reactor[3]. But Xu and Kumagai proposed that HCN was converted toNH3 by reacting with H2 during rapid pyrolysis of coal in a pressur-ized drop-tube furnace [7]. The fluidized-bed reactor and drop-tube furnace which have large high temperature zones provideessential conditions for secondary reactions. During rapid pyrolysisof coal or biomass in fluidized-bed reactor and drop-tube furnace,the fuel samples are continuously fed into the reactor. Thereforethere will be always pyrolysis gas in the reactor, and the laterfed samples may react with the pyrolysis gas released from thepreviously fed samples. In the study of Tan and Li [9], HCN andNH3 were formed in the primary stage of pyrolysis during rapidpyrolysis of coal in a drop-tube/fixed-bed reactor, and they pro-posed 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 reactorsand the gas velocities significantly, caused so many disagreementsor even opposite conclusions reported in literature. The wire-meshreactor is a powerful tool to realize rapid pyrolysis without second-ary reactions. But there is no systematic study on nitrogen evolu-tion during rapid pyrolysis of coal by using wire-mesh reactor.The low sample capacity of the wire-mesh reactor might be thereason.

In this study, a drop-style high-frequency furnace which couldlimit the high temperature zone in a very small zone were used,and the secondary reactions of nitrogen products could be sharplyreduced by shortening the residence time of primary volatile-nitrogen products in high temperature zone. The purpose of thisstudy was to investigate the conversion of coal-N to primary prod-ucts such as char, HCN, NH3, N2 and tar without secondary reac-tions during the rapid pyrolysis of coal. In addition, nitrogenconversions during the rapid pyrolysis of petroleum coke whichhave been scarcely reported were also investigated in this study.

Ar

1

6

5

7

3

2

Sample

4

Fig. 1. High frequency furnace rapid pyrolysis system. 1 – Ar cylinder; 2 – flowmeter; 3 –crucible; 7 – thermocouple and meter; 8 – filter (cotton); 9 – absorption bottles; 10 – b

2. Experimental

2.1. Fuel samples

Three typical Chinese coals lignite (Inner Mongolia), bituminous(Shenfu), and anthracite (Guizhou) were pyrolyzed, these coals arelargely reserved in China, and be used widely. A petroleum cokewhich had a high graphitization degree was also pyrolyzed as acomparison. The proximate analysis and ultimate analysis of thefuel samples are listed in Table 1. The particle sizes of the four sam-ples 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 wereplaced in the center of induction coil which connected to thehigh-frequency power supplier. When the high-frequency powerwas turned on, high-frequency alternating magnetic field couldbe formed in the center of induction coil, and the molybdenumcrucible in the high frequency alternating magnetic field can beself-heated rapidly. The temperature of molybdenum cruciblewas monitored by an S-thermocouple which inserted into the holedrilled in the bottom, and the final temperature was controlled bythe current from power supplier. As the molybdenum crucible wasself-heated, the high temperature zone was just limited in themolybdenum crucible and the small area around it. During pyroly-sis, the volatile products released from the molybdenum cruciblecould be carried out by the carrier gas rapidly from the high tem-perature zone and be quenched rapidly, thus secondary reactionscould be sharply suppressed. The temperature distribution abovethe crucible was measured by an S-thermocouple. But the corun-dum lined-pipe of the thermocouple, whose radiation absorptioncoefficient is much higher than the gas, might make the measuredtemperature higher than the actual temperature of the gas. There-fore the temperature distribution was also calculated by the‘‘Fluent’’ software. The measured and calculated temperature dis-tributions above the crucible under the condition of 1200 �C areshown in Fig. 2. The measured gas temperature is about50–100 �C higher than the calculated temperature. The calculatedtemperature should be more approach to the actual temperatureof the gas. It can be found that the temperature decreases sharplywith the increasing distance above from the crucible.

The quartz tube reactor was 200 mm long with an inner diam-eter 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 eachtime) were feed into the reactor from the top of quartz reactorthough a sample feeding tube which was inserted into the reactor.Before experiment, fuel sample was weighed and putted into adropper with rubber head, the dropper was connected with the

10

9

Vent Mo -crucible8

high frequency current source; 4 – quartz tube reactor; 5 – induction coil; 6 – Mo –ubble stones.

0 2 4 6 8 10 12 14 160

200

400

600

800

1000

1200

Tem

pera

ture

,

Distance above from the crucible, cm

Calculated Measured

Fig. 2. Temperature distribution above the crucible in the reactor.

0

10

20

30

40

50

60

70

80

90

100

Nitr

ogen

dis

trib

utio

n in

pro

duct

s, m

ol %

600 700 800 900 1000 1100 1200Temprature,

(a)

0

10

20

30

40

50

60

70

80

90

100

Nitr

ogen

dis

trib

utio

n in

pro

duct

s, m

ol %

600 700 800 900 1000 1100 1200Temprature,

(b)

50

60

70

80

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100

tion

in p

rodu

cts,

mol

% (c)

856 S. Yuan et al. / Applied Energy 92 (2012) 854–859

sample feeding tube by a short rubber tube. The rubber tube wasclipped by a clip before experiment, and the dropper was upsidedown. The system was purged before the experiment, and thenturned on the power supplier and adjusted the current to makethe molybdenum crucible be heated to the target temperature.During purging of the system, the clip was removed and the drop-per was kept upside down. And then the rubber head was gentlyextruded several times to drive out the air in the dropper but avoidblowing the fuel sample out. During experiment, the dropper wasraised to make the fuel particles flow slowly into the sample feed-ing tube. The sample feeding tube was inserted about 20 mm uponthe crucible bottom. Rapid pyrolysis happened at the moment thesamples dropped into the molybdenum crucible, the pyrolysis gaswas carried out from the reactor to the adsorption bottles by thecarrier gas. Feeding time of the fuel sample was 2 min each cycle,and the power supplier was shut down after 2 min when the injec-tion of fuel samples was finished. HCN and NH3 in pyrolysis gaswere 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.

0

10

20

30

40

Nitr

ogen

dis

trib

u

600 700 800 900 1000 1100 1200

2.3. Quantification

NHþ4 and CN� ions were analyzed by Metrohm 861 ion chro-matograph. The separation column of the cations was Metrosep C4–100, and the eluent was a mixture solution of HNO3 (1.7

600 700 800 900 1000 1100 1200

40

50

60

70

80

90

100

Cha

r, w

t %

Temperature,

Lignite Bituminous Anthracite Petroleumcoke

Fig. 3. Char yields under rapid pyrolysis of lignite, bituminous, anthracite, andpetroleum coke under different temperatures.

Temprature,

0

10

20

30

40

50

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70

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90

100

Nitr

ogen

dis

trib

utio

n in

pro

duct

s, m

ol %

600 700 800 900 1000 1100 1200Temprature,

(d)

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; lightgray: (HCN + NH3)–N; white: (tar + N2)–N.

0

1

2

3

4

5

6

7

8

9

10

Temperature, 600 700 800 900 1000 1100 1200

Yie

lds

of H

CN

-N a

nd N

H3-

N, m

ol %

(a)

0

1

2

3

4

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9

Temperature, 600 700 800 900 1000 1100 1200

Yie

lds

of H

CN

-N a

nd N

H3-

N, m

ol %

(b)

0.0

0.5

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4.5

Temperature, 600 700 800 900 1000 1100 1200

Yie

lds

of H

CN

-N a

nd N

H3-

N, m

ol %

(c)

1.5

2.0

2.5

3.0

and

NH

3-N

, mol

%(d)

S. Yuan et al. / Applied Energy 92 (2012) 854–859 857

mmol/L) and Pyridine 2,6-dicarboxylate (10 mmol/L). The separa-tion column of the cations was Metrosep A Supp 1–250, and theeluent was NaOH solution with a concentration of 10 mmol/L. Itshould be noted that HNCO produced during rapid pyrolysis canconvert to NHþ4 in adsorption solutions through hydrolysis, butthe yields of HNCO are much lower than that of HCN and NH3,and its contribution to total the NHþ4 in solution were very small[18].

Solid products (char) in molybdenum crucible was collectedand weighted after each experiment. Nitrogen contents in coaland char were analyzed by an Elemental Analyzer (Vario MACROCHN/CHNS). While, the tar was hardly collected for the reasonsthat quantity of fuel samples injected into the reactor each timewas low, tar yields during rapid pyrolysis especially high-temper-ature rapid pyrolysis are low [7,19], and proportions of tar werecondensed in the inner wall of the upper part of the quartz reactorand the pipe line. In this paper tar-N together with N2–N were cal-culated by the mass conservation method.

3. Results and discussions

3.1. Nitrogen distributions in pyrolysis products

Char yields of lignite, bituminous, anthracite, and petroleumcoke under rapid pyrolysis at different temperatures are shownin Fig. 3. Char yields decreased with the increasing coal rank. Charyields of petroleum coke were close to those of anthracite, andhigher than those of lignite and bituminous. But char yields ofanthracite were more sensitive to temperature than those of petro-leum coke.

The effect of temperature on nitrogen distributions in pyrolysisproducts from lignite, bituminous, anthracite, and petroleum cokeare shown in Fig. 4a–d. Most of the fuel-N was retained in char dur-ing rapid pyrolysis. Char-N yields decreased with the increasingtemperature. (HCN + NH3)–N yields of coal increased and sur-passed a maximum, but their absolute amounts were much lower.The yields of (tar + N2)–N increased with the increasing tempera-ture, however according to the literature [19], the yields of tar de-creased with the increasing temperature above 600 �C. The yieldsof tar decreased from 20% to 5% when increasing the temperaturefrom 600 �C to 1000 �C during rapid pyrolysis of Yallourn lignite[19]. In addition, although tar produced in experiments of thisstudy was not qualified, it was qualitatively observed that lesstar was formed under high temperature than low temperature.Therefore, it can be deduced that, more fuel-N converted to N2 un-der high temperature during rapid pyrolysis.

From the comparisons of Fig. 4a–d, we found the yields of vol-atile-N decreased with the increase in coal rank. However, the pro-portions of (HCN + NH3)–N in volatile-N were low, the changes involatile-N yields were mainly reflected by (tar + N2)–N. The yieldsof (HCN + NH3)–N during rapid pyrolysis of petroleum coke weremuch lower than those of the three coals. And promotion of theincreasing temperature on the yields of volatile-N was weak. The

N

N N

Valley-NCentre-N Top-N

Fig. 5. Schematic diagram of ‘‘centre-N’’, ‘‘valley-N’’ and ‘‘top-N’’ [19,20].

0.0

0.5

1.0

Yie

lds

of H

CN

-N

Temperature, 600 700 800 900 1000 1100 1200

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.

65 70 75 80 85 90 950

10

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100

NH

3-N

, and

cha

r-N

, mol

%

Carbon content of coal, wt % (daf)

Yie

lds

of H

CN

-N,

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).

858 S. Yuan et al. / Applied Energy 92 (2012) 854–859

reasons might be that, in the petroleum which has a high graphiti-zation degree, most of the nitrogen atoms exist in the grids of mac-romolecules, and they might be similar to the ‘‘centre-N’’ and‘‘valley-N’’ which is much more stable than other forms of nitrogenin coal (Fig. 5) [20,21].

3.2. NH3 and HCN formations

The effects of temperature on the yields of NH3–N and HCN–Nduring the rapid pyrolysis of lignite, bituminous, anthracite, andpetroleum coke are shown in Fig. 6a–d. In most of the conditionsNH3–N yields were higher than HCN–N yields, especially for thepetroleum coke where no HCN was detected in its pyrolysis prod-ucts. It might be further proved that nitrogen in petroleum coke ismostly 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 aromaticrings, and the three bonds connected to the N atom have averagingbond strengths [22]. Therefore, these three bonds connected withthe N atom might be cracked at the same time during rapid pyro-lysis when a high energy impact can be provided in a very shorttime. And then N free radicals could be formed and leading tothe formation of N2 and low yields of NH3.

Under the experimental conditions of this study, the secondaryreactions between the gas phase products could be sharply re-duced, the yields of char were low and the secondary cracking oftar could also be restrained. From Fig. 6 it is obviously that, consid-erable NH3 was formed in the conditions described above. The re-sults have some contradictions to the view that NH3 is mainlyproduced from the heterogeneous reactions between HCN and char[23]. From this study, it can be deduced that NH3 can be formed di-rectly in the primary stage of rapid pyrolysis through the crackingof nitrogen functionalities. The process might be that –N, –NH, and–NH2 free radicals could be released from fuel under the effect ofhigh energy impact, and then combined with H or H2 to formNH3 through the gas-phase reactions. Li and co-workers consid-ered that H radicals released during pyrolysis could be adsorbedon the surface of char, attack heterocyclic nitrogen, and promotenitrogen release as the form of NH3 [9–11].

From the comparison of Fig. 6a–c, the NH3–N yields of the threecoals were in the order of bituminous > lignite > anthracite, and theHCN–N yields of the three coals presented similar trend.

It is necessary to compare the results derived from different de-vices for rapid pyrolysis in the literatures. Data under the condition

Table 2Comparison of HCN–N, NH3–N, and char-N yields derived from different devices at 800 �C

Study Reactor Heating rate (K/s)

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

Tan and Li [9–11] Drop-tube with fixed-bed reactor >103

Xie et al. [12,13] Fliudzed-bed with fixed-bed reactor >104

Kambara et al. [8] Pyroprobe reactor 7.5 � 103

of 800 �C which available in each of these literatures were chosento be listed in Table 2. Results derived from different devices havelarge diversities. In the studies of Tan and Li [9–11] and Kamaraet 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 yieldswere higher than HCN–N, which are similar to the results of thisstudy. In this study, HCN–N/NH3–N ratios were found to be higherthan those of lignite, which are similar to the results in the study ofNelson et al. [3,4]. Tan and Li [9–11] also found that HCN–N/NH3–Nratios increased with the increasing coal rank. However, HCN–N/NH3–N ratios derived from a fluidized-bed/fixed-bed reactor inthe study of Xie et al. [12,13] presented an decrease trend withthe increasing coal rank.

As listed in Table 2, yields of HCN–N, NH3–N, and char-N versuscarbon content (daf) in coal were plotted in Fig. 7. Char-N yieldspresented a good regularity, which increased with the increasingcarbon content. Yields of HCN–N and NH3–N presented not so goodregularities as that of char-N yields. However, NH3–N yields pre-sented better regularity than HCN–N yields. As the carbon contentincreasing, yields of NH3–N showed a decreasing trend, but thepoints of HCN–N yields were dispersed, especially at low carboncontents. Therefore it can be concluded that, during rapid pyrolysisof coal in different devices, char-N yields present small diversities,and HCN–N present largest diversities.

.

Coal (C, wt.%, daf) HCN–N (mol%) NH3–N (mol%) Char–N (mol%)

75.7 1.2 8.6 64.285.1 4.7 8.9 69.692.2 1.8 3.5 80.567.3 5 8 6582.3 11 8 –72.0 2 13 6578.4 3 15 6079.8 2 15 67

68.5 23 18 –82.1 18 8 –91.0 8 2 –68.5 12 12 –84.3 7 14 –90.0 3 9 –

72.8 24 9 6584.6 12 8 6888.1 10 2 88

S. Yuan et al. / Applied Energy 92 (2012) 854–859 859

4. Conclusions

Rapid pyrolysis of coal of three ranks and a kind of petroleumcoke was carried out in a high-frequency furnace which had asmall high temperature zone. Nitrogen conversions under the con-ditions of rapid pyrolysis were investigated. Under the conditionsof this study, the increasing temperature could decrease thechar-N yields during the rapid pyrolysis of coal and petroleumcoke. The increasing coal rank increased the char-N yields and de-creased the volatile-N yields, but more NH3 than HCN could be re-leased in most of the conditions of this study. Both HCN and NH3

could be released directly from coal at the primary stage of rapidpyrolysis. However proportions of both NH3–N and HCN–N in vol-atile-N were low. During rapid pyrolysis of petroleum coke, theincreasing temperature increased the NH3–N yields, but no HCNwas found under all the conditions. By comparing the data fromthis study and the literatures, char-N yields derived from differentdevices present better regularity than the yields of HCN–N andNH3–N, and HCN–N yields vary largely with the devices.

However, nitrogen in tar was not investigated in this study due tothe difficulty of tar collection. Further efforts should be made tomake quantifications of tar-N yields derived from the high-frequency furnace, and nitrogen forms in tar should also be probedinto.

Acknowledgments

This study was supported by the National Basic Research Pro-gram of China (2010CB227000), and Shanghai ‘‘Technology Innova-tion Action Plan’’. The authors also acknowledge Prof. Yi-fan Han(State Key Laboratory of Chemical Engineering, ECUST) for his helpon language.

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