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Journal of Analytical and Applied Pyrolysis 97 (2012) 123–129

Contents lists available at SciVerse ScienceDirect

Journal of Analytical and Applied Pyrolysis

journa l h o me page: www.elsev ier .com/ locate / jaap

ontinuous high-temperature fluidized bed pyrolysis of coal in complextmospheres: Product distribution and pyrolysis gas

ei Zhonga,b, Zhikai Zhanga, Qi Zhoua, Junrong Yuea, Shiqiu Gaoa,∗, Guangwen Xua,∗

State Key Laboratory of Multi-Phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, P.O. Box 353, Beijing 100190, ChinaChemistry and Chemical Engineering, Xinjiang University, Xinjiang 830046, China

r t i c l e i n f o

rticle history:eceived 11 February 2012ccepted 29 April 2012vailable online 5 May 2012

a b s t r a c t

This study is devoted to investigating the continuous coal pyrolysis in a laboratory fluidized bed reactorthat fed coal and discharged char continuously at temperatures of 750–980 ◦C and in N2-base atmo-spheres containing O2, H2, CO, CH4 and CO2 at varied contents. The results showed that the designedcontinuous pyrolysis test provided a clear understanding of the coal pyrolysis behavior in various com-plex atmospheres free of and with O . The effect of adding H , CO, CH or CO into the atmosphere on the

eywords:oal pyrolysisartial gasificationluidized bedtaged conversiontmospheric-gas influence

2 2 4 2

tar yield was related to the O2 content in the atmosphere. Without O2 in the atmosphere, adding H2 andCO2 decreased the pyrolysis tar yield, but the tar yield was conversely higher with raising the CO and CH4

contents in the atmosphere. In O2-containing atmospheres, the influence from varying the atmosphericgas composition on the product distribution and pyrolysis gas composition was closely related to theoxidation or gasification reactions occurring to char, tar and the tested gas.

ar

. Introduction

Pyrolysis or carbonization represents the possible major way toonvert low rank coal into high rank fuel and chemicals that can beransported and used economically. In addition to the well-knownoal coking process, the so-called vertical rectangular carboniza-ion furnace illustrated in Fig. 1a is being widely used in China toroduce char and tar that are both for chemical applications (char isenerally used for making calcium carbide or gasified for syngas ofH3 synthesis at low capacities). In this furnace, the pyrolysis gas

s recycled into the bottom high-temperature zone to be a fuel thats combusted to provide at least partially the entailed endother-

ic heat and meanwhile suppress the combustion of solid char toealize the desired high yield of char [1]. Different from the mov-ng bed process illustrated in Fig. 1a for converting only the coalarticles with sizes above 20 mm (called lump coal), we proposed

fractionated pyrolysis process, as conceptualized in Fig. 1b, toreat the granular coal with sizes below 15 mm. A transport-bedop is integrated with a fluidized bottom to implement the pyrol-sis of fine coal (<15 mm) to produce tar and char simultaneously.he relatively large-size coal is pyrolyzed or partially gasified in

he oxygen-containing atmosphere at the fluidized bottom. Thisyrolysis generates the reducing gas containing H2 and CO at highemperatures (∼900 ◦C) to provide both the required heat and

∗ Corresponding authors.E-mail addresses: sqgao@home.ipe.ac.cn (S. Gao), gwxu@home.ipe.ac.cn (G. Xu).

165-2370/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.jaap.2012.04.009

© 2012 Elsevier B.V. All rights reserved.

reaction atmosphere of the fine coal pyrolysis in the top transport-bed section. In this process, the generated pyrolysis gas may bealso fed back into the fluidized bottom to increase the char yield,a means very similar to that adopted for Fig. 1a. Recently, we arealso working on a new two-stage gasification process which, asconceptualized in Fig. 1c, combines a fluidized bed pyrolyzer inoxygen-containing atmosphere at temperatures of about 900 ◦Cand a moving bed char gasifier downstream the pyrolyzer [2]. Thistwo-stage arrangement expects to produce low-tar fuel gas fromeither biomass or powder coal.

Obviously, all the conversion processes conceptualized inFig. 1 involve coal pyrolysis or partial gasification in the oxygen-containing gas (e.g., air) at relatively high temperatures. The coalpyrolysis refers usually to the thermal treatment of coal in atmo-spheres free of O2, even by an indirect heating of the coal in avessel without input of any external gas (e.g., coking or carboniza-tion). For the processes in Fig. 1b and c, the partial gasificationoccurs in a fluidized bed with continuous coal feed and char dis-charge. Consequently, their coal pyrolysis behavior is definitelydifferent from the commonly investigated coal pyrolysis in inertgases (N2 or He) at relatively low temperatures, for example, below800 ◦C. The understanding of such a kind of pyrolysis is criti-cal to the development of the processes highlighted in Fig. 1band c, while this is also highly needed for understanding the

coal conversion fundamentals occurring in the high-temperaturebottom section of the rectangular coal carbonization reac-tor shown in Fig. 1a where the recycled pyrolysis gas isburnt.

124 M. Zhong et al. / Journal of Analytical and Applied Pyrolysis 97 (2012) 123–129

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Fig. 1. Process schematic diagr

There are some literature studies about the atmospheric-gasnfluence on coal pyrolysis [3–12]. Braekman-Danheux et al. [4,5]tudied the influence of coke oven gas components on productields and char characteristics in a fixed bed reactor. They reportedhat the char obtained in a coke oven gas atmosphere was not fun-amentally different from those in pure hydrogen or pure inerttmosphere, while the overall conversion and oil yield were lowerhan in pure hydrogen but higher than in helium. Also in a fixeded reactor of atmospheric pressure, Ariunaa et al. [6] found thathere was slight difference in the yields of char and tar under var-ed atmospheric-gas compositions for lignite at temperatures of00 to 800 ◦C. Liao et al. [7] revealed that the higher pressure andlower heating rate increased the pyrolysis oil yield and improvedhe oil quality when testing the pyrolysis in a pressurized fixeded reactor. They also concluded that the conversion and oil yieldf coal pyrolysis in coke oven gas and synthesis gas were higherhan those from hydropyrolysis at the same H2 partial pressure.his indicated a kind of synergetic effect of the various gas compo-ents in the coke oven gas on coal pyrolysis. In a downer reactor foryrolyzing very fine coal in a N2 atmosphere, Wang et al. [8] clar-

fied that the gas and liquid yields increased with increasing theemperature from 570 to 660 ◦C, while the char yield decreased.here are limited studies on coal pyrolysis in O2-containing gases13–15]. Borrego et al. [14] reported that a small amount of O2 inhe atmosphere inhibited the volatile release to a certain extentnd the oxygen could involve in cross-linking the surfaces of thee-solidifying char particles and reduce the swelling of coal par-icles. Guo et al. [15] concluded that the char gasification rate inhe mixture of 2000 ppm O2 and 15% H2O was quicker than thatn 2000 ppm O2 or 15% H2O alone, implying a kind of synergeticffects among different atmospheric gas species. The key issue ishat oxygen is preferential to react with gas species like volatiles sohat the behavior of coal pyrolysis is different from that in O2-freetmospheres.

The present study succeeds to our previous investigation of coalyrolysis in a quartz-sand bed fluidized with composition-variedases but with an instantaneous coal sample drop (a few grams)nto the temperature-preset reactor [16,17]. The coal pyrolysis in

laboratory fluidized bed reactor with continuous coal feed andhar discharge was tested in this study. The fluidizing gas com-osition was varied to simulate particularly the conditions for therocesses conceptualized in Fig. 1a and b. Zeng et al. [2] also testedhe pyrolysis in a continuous fluidized bed reactor but their testedas atmosphere was limited to the mixture of N2 and O2 with anntention to provide the needed pyrolysis fundamental data for the

wo-stage gasification process illustrated in Fig. 1c. These previ-us studies clarified that the pyrolysis in simulated pyrolysis gastmosphere facilitated the formation of tar [16], while the inclusionf O2 into the reaction atmosphere incurred an obvious decrease

f a few pyrolysis technologies.

in the tar yield and formed more polycyclic aromatic hydrocar-bons (PAHs) [2]. There was yet not report on how the reactiongas containing O2, CO, CO2, CH4 and H2 affect the coal pyrolysisbehavior at relatively high temperatures of up to 980 ◦C in a con-tinuous fluidized bed reactor. This study is devoted to investigatingthe continuous coal pyrolysis in various N2-based complex atmo-spheres containing O2, H2, CO, CO2 and CH4 at 750–980 ◦C in alaboratory fluidized bed with continuous coal feed and char dis-charge. The results will be reported in three articles in series, andthis one is focused on characterization of the product distributionand pyrolysis gas composition.

2. Experimental

The adopted experimental apparatus was shown schematicallyin Fig. 2. It consisted mainly of the continuous coal feeding microscrew feeder, electric furnace, fluidized bed reactor, gas supplyingparts, char discharger/collector, tar recovery sector and gas cleaner.The coal feeder was equipped with a speed-variable motor to adjustthe feeding rate, and the fluidized bed reactor was a stainless steeltube of 46 mm in inner diameter. A stainless steel sintered plate wasmounted in the reactor to be the gas distributor above which a layerof inert Al2O3 ball in 3–4 mm was set to distribute and also preheatthe fluidizing gas. Below the gas distributor the reactor length was300 mm and the top section of the bed was an expanded stain-less steel freeboard of 100 mm in diameter and 150 mm in length.The major section excluding the connection part between the gasdistributor and the expanded top freeboard was 450 mm long, andchar was discharged from 125 mm above the distributor. Three 0.3-mm K-type thermocouples monitored the temperatures inside thefluidized particles and in the freeboard of the reactor. The super-ficial gas velocity in the reactor was fixed to fully fluidize the coalparticles so that the produced char can be smoothly discharged (∼3times of Umf).

The reactor was electrically heated to the preset temperaturefirst without any gas flow through the bed and in turn with therequired gas stream formulated by mixing the gas componentsfrom their cylinders under controls of mass flow meters. Whenboth the particle-bed temperature and the atmospheric-gas com-position reached their preset steady values, coal pyrolysis test wasstarted by feeding coal continuously into the reactor at a rate of7.5 g/min. A N2 flow of 1.0 l/min passed through the screw feederto facilitate the coal particles to enter the reactor. Generally, it tookabout 20 min to obtain a steady state of operation after starting thecoal feed. The produced char particles overflowed from the char

discharging tube that was inserted into a glass container. The col-lected char was weighted to calculate the char yield when it wascooled to room temperature and preserved in a desiccator for itscharacterization after the experiment.

M. Zhong et al. / Journal of Analytical and Applied Pyrolysis 97 (2012) 123–129 125

Fig. 2. Schematic diagram of the laboratory scale fluidized bed pyrolyzer. 1, screw feeder;7, condenser; 8 ice-water bath; 9, gas meter; 10, gas bag; 11, micro GC.

Table 1Data of proximate and ultimate analyses for the tested coal.

Proximate analysis (wt.%) Ultimate analysis (wt.%)

Ad Vd FCd Cdaf Hdaf Sdaf Ndaf Odaf

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become dominant and led to the decrease in the yield of C2 species.

4.14 30.40 65.46 77.95 3.98 0.74 1.05 16.28

The generated gaseous pyrolysis product was ventilated intoir without any treatment, but a part of the gas was sucked fromhe reactor exit to recover tar and analyze gas composition. Theucked gas was cooled down in, in succession, an air-cooled ironube (0.5 m long) connected to the reactor exit and two condenserslled with condensate (glycol/water = 1:1) at about −20 ◦C. The gasassed then through seven acetone-washing bottles immersed inn ice-water bath to gain the possible complete trapping of tarhich was indicated by the little changed color in the last acetone-ashing bottle. The washed gas passed further through a fabriclter and was monitored in a wet gas meter for its flow rate. Thelean gas, by dried further in a silica gel column, was finally col-ected using gas bags and analyzed in a micro GC (Aligent 3000A)quipped with both TCD and FID detectors.

The mass of tar was determined by removing its mixed acetoneith vaporization at 28 ± 2 ◦C under slightly negative pressure. Theyrolysis gas yield was calculated on the basis of the gas molaromposition and volume determined by taking a well-metered N2ow as the tracer gas. Most of the experiments were performed forwo times under the same conditions, and the reproducibility ofhe two results was well so that their average was reported hereins the experimental data. The total mass of tar, char, gas and wateras about 92–95% of the fed coal and O2 mass and the other 5–8% of

he coal mass should be the elutriated fine char with the ventilatedyrolysis gas which could not be accurately determined in the test.

The tested coal was a kind of sub-bituminous coal from Xinjiangimusaer of China, and Table 1 lists the data of the proximate andltimate analysis for the coal. For the tests of this study, the coal was

2, thermocouple; 3, electric furnace; 4, fluidized bed; 5, gas mixer; 6, char collector;

dried in an atmospheric oven at 110 ◦C for 3 h and further crashedinto the sizes of 1–2 mm for experiment.

3. Results and discussion

3.1. Variation with temperature

Fig. 3 shows the yields of pyrolysis products varying with thefluidized bed temperature in 750–980 ◦C at an excessive air ratio(ER) of 0.064. Comparing the data at 750 and 980 ◦C, one can seethat, the yields of char and tar decreased from 64.72 wt.% and3.06 wt.% to 59.92 wt.% and 2.41 wt.%, respectively. The water yieldalso decreased from about 3.23 wt.% to 1.60 wt.% because raisingthe temperature promoted the char steam gasification. The total gasyield exhibited a gradual increase with elevating the temperature,and according to Fig. 3b this was mainly due to the rapid increasesin the CO and H2 yields. In fact, high pyrolysis temperature facili-tated the cracking of aliphatic hydrocarbons and the condensationor polymerization reactions of aromatic hydrocarbons, thus result-ing in the gradually increased yield of hydrogen [18]. The increasein the CO yield with raising the temperature should be attributedto the cracking of phenolic, carbonyl and oxygenous heterocycliccompounds in tar [18], while the decrease in the CO2 yield at rela-tively high temperatures was because of the enhanced Boudouardreactions between CO2 and char. The C2 hydrocarbons in pyrolysiswere mainly from cracking some long-chain hydrocarbons, makingits yield first increase and then decrease with raising the reactiontemperature. This showed essentially a competition between theformation and decomposition of such species. When the bed tem-perature was below 910 ◦C, the formation of C2 gases was dominantin comparison with the thermal cracking of C2 hydrocarbons (toform CH4). Raising further the temperature caused the latter to

The gradually, but slightly increased CH4 content in Fig. 3b con-firmed actually this interpretation. In Fig. 3b, the lower heat value(LHV) of the generated pyrolysis gas (excluding N2) increased from

126 M. Zhong et al. / Journal of Analytical and Applied Pyrolysis 97 (2012) 123–129

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ptttdiffc3mdtot

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ig. 3. Product distribution and pyrolysis gas composition varying with pyrolysisemperature at an ER of 0.064.

3.17 to 14.15 MJ/Nm3 corresponding to the temperature rise from50 ◦C to 910 ◦C and then decreased to 13.72 MJ/Nm3, which com-lies well with the above-clarified gas composition variations.

.2. Variation with ER

The excessive air ratio ER significantly affected the pyrolyzererformance and the quality of pyrolysis products. Fig. 4a showshat the freeboard temperature decreased with raising ER whenhe temperature of particle bed was fixed at 850 ◦C. Correspondingo the increase of ER from 0 to 0.107, the freeboard temperatureecreased from 898 to 785 ◦C. This referred to a result from decreas-

ng the intensity of electric heating for the reactor when oxygen wased into the reactor to create internally a part of the heat requiredor stabilizing the particle bed temperature. Fig. 4b shows that thehar and tar yields decreased correspondingly from 68.62 wt.% and.18 wt.% to 52.58 wt.% and 1.69 wt.%, respectively. Elevating EReant surely more char as well as more tar to be combusted or

estroyed to lower thus their yields. The lower freeboard tempera-ure at higher ER (see Fig. 4a) would reduce the secondary crackingf tar to raise the tar yield, but this effect was much weaker thanhe oxygen-intensified tar oxidation at the raised ER.

Fig. 4b shows that at ER = 0.107 the gas yield reached 33.23 wt.%f the coal at dry base, which was about 50% higher than that in2 atmosphere. Especially, the CO production in Fig. 4c increased

inearly from 58.4 ml/g at ER = 0 to 166.1 ml/g at ER = 0.107. How-ver, the yields of the other combustible gases including H2, CH4

nd C2 decreased obviously with raising ER. Hence, the presencef O2 in the pyrolysis atmosphere tended to consume combustibleases, especially H2 and C2, but the gasification reaction of charby reacting with O2) caused the gradually higher CO production

Fig. 4. Freeboard temperature and pyrolysis product characterization at varied ERin N2 atmosphere at 850 ◦C.

at higher ER. The relatively stable CO2 yield in Fig. 4c corroboratedthis mechanism of oxygen effect on pyrolysis gas production. As aresult, the LHV of the pyrolysis gas was gradually lower with higherER in Fig. 4c.

3.3. Variation with other gases in O2-free atmosphere

Fig. 5 shows how the char yield (right ordinate) and tar yield (leftordinate) at 850 ◦C varied with the addition of the gas componentsH2, CO2, CO and CH4 at different amounts (6.86–31.22 vol.%) intothe N2-base reaction atmosphere. The compositions of their gen-erated pyrolysis gases were shown in Table 2. Compared with thepyrolysis in pure N2, the char yield increased with raising the CH4fraction in the atmosphere but decreased with increasing the CO2amount. Adding H2 and CO into the reaction atmosphere affected

little the char yield.

Against the pyrolysis in N2 atmosphere, Fig. 5 revealed that thepresence of CO and CH4 in the atmosphere facilitated the pro-duction of tar, whereas the inclusion of H2 and CO2 suppressed

M. Zhong et al. / Journal of Analytical and Applied Pyrolysis 97 (2012) 123–129 127

Table 2Pyrolysis gas yield in different N2-based atmospheres (“–” means no analysis for the gas component added to the reaction atmosphere).

Gas atmospthere Concentration (vol.%) Freeboard temperature (◦C) Gas production rate (ml/g)

H2 CO CH4 CO2 C2

N2 – 898 119.2 58.4 31.7 34.3 9.0

H211.56 905 – 52.4 33.7 25.1 12.421.41 912 – 49.2 36.4 23.4 17.8

CO11.56 900 131.4 – 27.2 50.9 11.531.22 889 143.6 – 22.6 57.2 9.1

CO211.56 944 132.0 297.8 31.3 – 11.2

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he tar production. The decrease of tar yield with the inclusion of2 into the atmosphere was related possibly to the conversion of

he tar-containing O into water and the H2-promoted secondaryracking of volatile vapors that led to more pyrolysis gas [16,19].able 2 shows that there was particularly more CH4 and C2 hydro-arbons when H2 was presented, while both the CO and CO2ractions slightly decreased due to probably the enhanced metha-ation reactions. The reduced tar production when including CO2as attributed to both the higher freeboard temperature of the

eactor (Table 2) at the increased CO2 fraction and its enhancedeforming reactions of tar [17].

The added H2 in the reaction atmosphere and its consequent Hadicals tended to react with the nascent char via the hydrocrackingr hydrogasification reaction to form more CH4 (Table 2) and high-uality tar. The involved reactions are [20,21]:

f + H2 → C(H2), (1)

(H2) + Cf → 2C(H), (2)

C(H) + H2 → 2C(H2) (3)

nd

(H2) + H2 + Ci → CH4 + Cf . (4)

The effect of CO2 on gas yield was resulted mainly from theO2 gasification of char, thus lowering the char yield in Fig. 5

nd causing the pyrolysis gas to have high production rates of CO58.4–350.0 ml/g) and H2 (119.2–160.8 ml/g) in Table 2. This waserified also by the increased freeboard temperature in Table 2, aesult from increasing the endothermic heat need (for more char

ig. 5. Influence of H2, CH4, CO2 and CH4 in O2-free atmosphere on pyrolysis productistribution at 850 ◦C.

160.8 350.0 30.6 – 9.1147.2 60.6 – 30.9 10.6168.3 73.6 – 28.2 12.2

CO2 gasification) which caused a rather extensive electric heat-ing for the reactor. The variation in the production rate of C2species was subject to the freeboard temperature exclusively. Atrelatively low freeboard temperatures (e.g., <950 ◦C) the decom-position of high-C species would be dominant, whereas the ratherhigher temperature might cause the decomposition of C2 speciesoverwhelming. This made a higher C2 production rate (from 9.0 to11.2 ml/g) when raising the CO2 fraction from 0 to 11.56 vol.% toelevate the freeboard temperature from 898 to 944 ◦C, and thena conversely lowered C2 production (11.2–9.1 ml/g) at the rise ofthe CO2 fraction from 11.56 to 21.41 vol.% to make a rather higherfreeboard temperature of 982 ◦C.

When coal was pyrolyzed in the N2 + CH4 atmosphere, thedeposited carbon from thermal cracking of CH4 increased the charyield (Fig. 5), which was in accordance with the results of Zhanget al. [22] and Sun et al. [23]. It was widely recognized that the cat-alytic effect of coal/char can cause CH4 to decompose over the charsurface at relatively low temperatures [9,23–25]. This increasedconsequently the H2 and CO production rates in Table 2, whilesimultaneously the CH4 reforming by CO2 lowered the CO2 pro-duction. On the other hand, methane may provide radicals, such asCH3, CH2 and H to stabilize the coal radicals generated in pyrolysisand in turn to elevate the tar yield. The increased production rate ofC2 species in Table 2 verified actually the char-catalyzed cracking ofCH4 and its resulting stabilization of the coal molecular fragmentsvia the so-called condensation reactions under the participation ofmethyl and methylene radicals [22,24,26].

3.4. Variation with other gases in O2-containing atmosphere

For the autothermal pyrolysis conceptualized in Fig. 1, the coalpyrolysis may use the gas produced by gasification as its trans-port gas and meanwhile mixed with air or O2 to provide the heatrequired by the pyrolysis. Some experiments were thus performedat 850 ◦C in atmospheres containing O2 (ER = 0.107) but with differ-ent other gas species. As presented in Fig. 6, the inclusion of H2, CH4or CO into the base gas atmosphere of N2 + O2 increased both thechar and tar yields. This is surely because that adding combustiblegas into the atmosphere caused a part of O2 to react with the com-bustible gas fed to the atmosphere, thus decreasing the amounts oftar and char combusted.

Table 3 shows the corresponding changes in production ratesof the various pyrolysis gas components. The presence of CH4induced some complicated heterogeneous and homogeneous reac-tions including combustion, methane cracking, steam and CO2reforming of methane [23,27]. These led to the rapid rise of H2production rate from 76.4 to 268.7 ml/g accompanying with theincrease of CH4 content from 0 to 34.91 vol.% in the reaction

atmosphere. Table 3 shows also that the production rates of CO2and H2 obviously increased when CO was into the atmosphere.The CO2 production reached 149.2 ml/g when the atmospheric COconcentration was 31.91 vol.%, which was about 3 times higher than

128 M. Zhong et al. / Journal of Analytical and Applied Pyrolysis 97 (2012) 123–129

Table 3Pyrolysis gas yield using N2 + O2 as the basic atmosphere at ER = 0.107.

Gas atmospthere Concentration (vol.%) ER Freeboard temperature (◦C) Gas production rate (ml/g)

H2 CO CH4 CO2 C2

O2 – 0.107 785 76.4 166.1 18.5 51.9 3.7

H211.56 0.107 807 – 135.6 22.1 46.6 7.831.91 0.107 764 – 82.5 23.8 36.6 8.3

CO11.56 0.107 779 122.9 – 19.1 91.2 7.031.91 0.107 762 137.7 – 22.2 149.2 5.7

CH411.56 0.107 808 222.5 155.6 – 38.8 6.331.91 0.107 812 268.7 167.0 – 30.1 7.1

Fp

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ig. 6. Influence of H2, CO and CH4 in O2-containing atmosphere (ER = 0.107) onyrolysis product distribution at 850 ◦C.

hat in the N2 + O2 atmosphere. The production rate of H2 reached22.9 ml/g at the CO content of 11.56 vol.%, which was 76.4 ml/g inhe atmosphere without CO. The production of C2 species exhibited

rise as well with the presence of CO. The enhanced CO combustion,hich reduced the combustion for the other gas species, should

e responsible for the result. Adding H2, which was easier to bexidized, into the N2 + O2 atmosphere decreased obviously the pro-uctions of CO and CO2. This was related to the reduced oxidationor the C-containing species including tar, char, CH4 and C2 species,

aking thus their higher production rates in Fig. 6 and Table 3.The results show also that the char yield decreased in the CO-

ontaining atmosphere in comparison with that in the atmospheresith H2 and CH4 (data not presented in Fig. 6 and Table 3). The

acilitated char gasification in the atmosphere with high CO2 con-entration should be the cause.

.5. Comparison between atmospheres free of and with O2

Fig. 7 compares the tar and char yields varying with the atmo-pheric gas composition for the atmospheres free and with O2 in

and b, respectively. The satellite table lists the composition ofhe plotted atmospheric gases G1–G10. The test was conducted bydding an individual gas component into the atmosphere gradually,nd the base atmosphere was N2 (7a) and N2 + O2 (7b). As shown inhe satellite table, the sequence of adding gas component was H2,

O, CH4 and CO2.

Fig. 7a shows that adding H2 decreased the tar yield, and thenclusion of CO and CH4 into the N2 + H2 atmosphere was likelyo increase the tar yield. The supply of CO2 into the atmosphere

Fig. 7. Char and tar yields of pyrolysis in various O2-free and O2-containing reac-tions.

caused lower tar yield. These results complied well with the obser-vation of Zhang et al. [17] in batch-wise coal pyrolysis test, and thusone can find their implied mechanics from that literature study.The continuous coal pyrolysis tests done here verified further the

results and mechanism. The char yield did not greatly vary withthe changes of the gas composition shown above, but one can stillsee that the addition of H2 and CO2 would lower the char yield,

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M. Zhong et al. / Journal of Analytica

nd the inclusion of CO and CH4 was likely to raise the char yield.hese were related to the preceding variations of tar yield with gastmosphere and revealed in fact the occurrence of the C depositionn the char with the presence of CO and CH4 in the atmospherend the enhanced volatilization (or gasification) of char by reactingith the atmospheric gas containing H2 and CO2.

Fig. 7b clarified that in the O2-containing atmosphere the addi-ion of the combustible gas including H2, CO and CH4 all increasedhe tar and char yields, while the inclusion of CO2 was likely to lowerhe yields of tar and char. This is true because the more combustibleas in the atmosphere would reduce the oxidation of tar and charo increase thus their yields. Carbon dioxide is not a combustibleas but a gasification or reforming reagent, and its presence in thetmosphere thus facilitated the tar reforming and char gasificationo lower their yields. Fig. 7b showed obviously lower yields thanig. 7a nearly for all the examined product species, affirming fur-her that it is the oxidation reactions which lowered those yields inhe O2-containing atmosphere.

. Conclusions

This paper investigated the pyrolysis characteristics of a sub-ituminous coal in a fluidized bed reactor with continuous coal feednd char discharge at temperatures of 750–980 ◦C and in N2-basetmospheres containing O2, H2, CO, CH4 and CO2. In O2-containingtmospheres (ER = 0.064 in this article), raising the reaction tem-erature (>700 ◦C) evidently increased the pyrolysis gas yield, andhis increase in the gas yield resulted mainly from the increases inhe productions of CO and H2. The yields of C2 hydrocarbons firstlightly increased and then decreased, whereas that of CO2 kept alight decrease with raising temperature. The heating value of theyrolysis gas exhibited the same variation tendency as for the yieldf C2 hydrocarbons. At the tested high pyrolysis temperatures (e.g.,ver 850 ◦C), increasing the ER until 0.107 decreased the yields ofhar and tar but increased the yields of gas and water. The CO2ield had only a slight increase with elevating ER, whereas for H2nd hydrocarbons their yields slightly decreased.

The addition of any combustible gas into the N2 + O2 atmosphereER = 0.107) all increased the tar and char yields due to the sup-ressed oxidation or gasification of tar and char by O2. Nonetheless,he yields of char and tar were both evidently lower than in pure-N2tmosphere. When increasing the H2 content in the N2 + O2 atmo-phere the production became higher for all combustible gasesncluding CH4 and C2 species but lower for CO2. Raising the atmo-pheric CH4 content lowered the CO2 yield but raised evidently theroductions of H2 and C2 species. Adding CO elevated the yields ofO2 and H2.

In O2-free atmospheres at the tested temperatures (up to80 ◦C), the tar yield increased with the addition of CH4 and CO butecreased with the presence of H2 and CO2. Adding CH4 into thetmosphere caused slightly higher char yield due to the cracking ofH4, while elevating the CO2 content in the atmosphere obviouslyecreased the char yield because of the facilitated reaction of charith CO2. The variation in the atmospheric CO and H2 concentra-

ions affected little the char yield. As for the pyrolysis gas product,he addition of H2 into the atmosphere caused gradually higherydrocarbon yield (including CH4) but lowered the production ofO and CO2. Raising the atmospheric CH4 content caused a slightlyigher production of C2 species but it was slightly lowered by the

nclusion of CO and CO2.

cknowledgments

The authors are grateful to the financial support of the Nationalasic Research Program of China (2011CB201304), the Nationalatural Science Foundation of China (21076217), the “Strategic

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pplied Pyrolysis 97 (2012) 123–129 129

Priority Research Program” Key technologies and demonstrationfor cleaning and highly efficient of low rank coal (XDA07010100)and the Young Teacher Scientific Research Cultivation Foundationof Xinjiang (XJEDU2011S03).

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