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Journal of Analytical and Applied Pyrolysis 92 (2011) 463–469

Contents lists available at SciVerse ScienceDirect

Journal of Analytical and Applied Pyrolysis

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

CN and NH3 (NOx precursors) released under rapid pyrolysis of biomass/coallends

huai Yuan, Zhi-jie Zhou, Jun Li, Xue-li Chen ∗, Fu-chen Wangey Laboratory of Coal Gasification of Ministry of Education, East China University of Science and Technology, Shanghai 200237, China

r t i c l e i n f o

rticle history:eceived 1 July 2011ccepted 26 August 2011vailable online 2 September 2011

eywords:iomass/coal blendapid pyrolysisitrogenCNH3

a b s t r a c t

Rapid pyrolysis of 6 biomass/coal blends (1:4, wt) including rice straw + bituminous (RS + B), ricestraw + anthracite (RS + A), chinar leaves + bituminous (CL + B), chinar leaves + anthracite (CL + A), pinesawdust + bituminous (PS + B), and pine sawdust + anthracite (PS + A) was carried out in a high-frequencymagnetic field based furnace at 600–1200 ◦C. The reactor could not only achieve high heating rates offuel samples but also make biomass and coal particles contact well; secondary reactions of primaryproducts during rapid pyrolysis can also be efficiently reduced. By comparing nitrogen distributions inproducts of blends (experimental values) with those of the sums of individual biomass and coal (weightedvalues), nitrogen conversion characteristics under rapid pyrolysis of biomass/coal blends were investi-gated. Results show that, biomass particles in blends lead to higher experimental char-N yields thanthe weighted values during rapid pyrolysis of biomass/anthracite blends. The decreased heating ratesof both biomass and coal particles caused by the low packing densities of biomass may be the reason.For blends of CL + B in which packing density of chinar leaves is high, and for PS + B during pyrolysis ofwhich melting and shrinkage happen to pine sawdust, both biomass and coal particles can obtain highheating rates, synergies can be found to promote nitrogen release from fuel samples and decrease char-N

yields under all the conditions. But the low fluidity and not easily collapsed carbon skeletons of rice strawmake the heating rates of rice straw and bituminous particles in RS + B lower than those of CL + B andPS + B, and weaker synergies can be found from char-N yields of RS + B. The synergies can obviously befound to decrease the (NH3 + HCN)-N yields and make more nitrogen convert to N2 except for those ofseveral low-temperature conditions (600–700 ◦C). Under the low-temperature (600–700 ◦C) condition,synergies make molar ratios of HCN-N/NH3-N higher than those of the weighted values.

. Introduction

With the increasing consumption of fossil fuels, biomass as aenewable energy has been widely concerned as a future energyource. Thermo-chemical conversions (pyrolysis, gasification, andombustion) are the main methods of biomass utilizations atresent. However, productions of biomass vary largely with theeasons, and the instability of fuel supply will be a problem ifiomass-only based plant were built. Therefore, taking biomass asn adjustable part of the fuel to the coal combustion or gasificationrocesses might be a good method for biomass utilizations.

NOx can be produced during combustion of both coal andiomass, which leads to the environmental problems. During gasi-

cation of coal, NH3 and HCN can be formed and affect the safe and

ong period run of gasification systems [1,2]. Nitrogen pollutantsuch as NH3, HCN, and HNCO can also be formed during pyrolysis

∗ Corresponding author. Tel.: +86 21 64250784; fax: +86 21 64251312.E-mail address: cxl@ecust.edu.cn (X.-l. Chen).

165-2370/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.jaap.2011.08.010

© 2011 Elsevier B.V. All rights reserved.

and gasification of biomass [3–9]. For either gasification or com-bustion, pyrolysis is an important process, thus studies on nitrogenconversions during pyrolysis of coal and biomass have importantsignificances on searching for the nitrogen pollutants control meth-ods.

During pyrolysis of biomass/coal blends, interactions (syner-gies) between biomass and coal have been found, such as thedecrease of char and tar yields and the increase of gas phase prod-ucts [10–17]. Cordero et al. found that, during pyrolysis of blendsof biomass and high-sulfur coal, desulfurization was enhanced[18]. However, Haykiri-Acma and Yaman found that hazelnut shellblended with coal made sulfur retain in char as the forms of CaSand CaSO4 [11]. It can be deduced that, nitrogen conversions dur-ing pyrolysis of biomass/coal blends may be different to nitrogenconversions during pyrolysis of biomass and coal individually.

Fixed-bed reactors (mainly TGA) are mostly used for pyrolysis

of biomass/coal blends [10–18]. Fluidized-bed reactors [19,20] andentrained-bed reactors (also called free fall reactor or drop tube fur-nace) [21,22] have also been used. During pyrolysis of biomass/coalblends in fluidized bed and entrained bed reactors, high heating

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64 S. Yuan et al. / Journal of Analytical

ates of the particles could be obtained. But for the dispersionsf particles, interactions between biomass and coal were found toe slight [22], or even no interactions were found [19–21]. Dur-

ng pyrolysis of biomass/coal blends in fixed bed, the particles ofiomass and coal can contact well, but the heating rates are low,nd it is difficult to investigate interactions between biomass andoal under rapid pyrolysis conditions. Furthermore, tube furnacessed for pyrolysis usually have long high-temperature zones, pyrol-sis products must go through the high-temperature zones beforescaping from the reactors and secondary reactions of the pyrolysisroducts cannot be avoided. The residence times depending on gaselocities and reactor types make the secondary reaction extents,roducts measured, and the final conclusions have large diversities3].

Wire-mesh reactor and heated foil/plate reactor are powerfulools to carry out rapid pyrolysis without secondary reactions. Asuel samples are fixed on the meshes or foils during rapid pyrol-sis in these reactors, both high heating rates and good contactf the fuel particles might also be obtained if co-pyrolysis wasoncerned to carry out in these reactors [23–25]. In this study, arop style high-frequency magnetic field based furnace which canlso result in both good contact between biomass and coal parti-les, and high heating rates of particles, was used for pyrolysis ofiomass/coal blends. It was suitable to investigate the interactionsetween biomass and coal under the rapid pyrolysis conditions.oreover, secondary reactions of pyrolysis primary products could

e reduced, and nitrogen distributions in primary products withcarce secondary reactions could be investigated. Impact factors onitrogen conversions during rapid pyrolysis of biomass/coal blendsere discussed by comparing the experimental values with theeighted values.

. Materials and methods

.1. Fuel samples

Three typical agricultural and forestry biomass, rice straw (RS),hinar leaves (CL), and pine sawdust (PS) were blended with twooals (bituminous (B) and anthracite (A)) respectively. The ricetraw was obtained from the farmland of Yangtze River delta, thehinar leaves were obtained from the campus of ECUST in Shang-ai, and the pine sawdust was obtained from a furniture factory ofhanghai. The bituminous and anthracite were from Shenfu coaleld of Shanxi and Zunyi coal field of Guizhou in China. Proximatenalysis and ultimate analysis of the raw biomass and coals areisted in Table 1 [9,26]. The raw biomass and coal were dried under10 ◦C for 1 h and 4 h respectively before being blended. To avoideing dispersed by the carrier gas, particle sizes of the biomassnd coal were chosen as 250–420 �m and 125–180 �m respec-ively. The 6 kinds of biomass/coal blends were as follows: ricetraw + bituminous (RS + B), rice straw + anthracite (RS + A), chinareaves + bituminous (CL + B), chinar leaves + anthracite (CL + A), pineawdust + bituminous (PS + B), pine sawdust + anthracite (PS + A).s fluidity of the RS + B blends (characterized by Hausner ratio,

nternal friction angle, wall friction angle, and natural packingepose angle) can be obviously decreased when the mass ratio ofice straw is too high [27], mass ratios of biomass to coal were cho-en as 1:4 in this study. More details about the biomass and coalamples have been reported elsewhere [9,26].

.2. Pyrolysis setup

In the high-frequency magnetic field based furnace used in thistudy, molybdenum crucible in which fuel samples were pyrolyzedas heated through the high-frequency magnetic field generated

plied Pyrolysis 92 (2011) 463–469

by an induction coil. Thus the molybdenum crucible was self-heated and the high-temperature zone was limited just in themolybdenum crucible and small zone around. The depth of the cru-cible was 15 mm, and inner diameters of the bottom and the topwere 20 mm and 22 mm respectively. Experiments were carriedour under the temperature ranged from 600 ◦C to 1200 ◦C. Rapidpyrolysis happened at the moment of the fuel samples were con-tacted with the molybdenum crucible, and the volatile productswere carried out rapidly from the high-temperature zone by car-rier gas (argon, 500 ml/min) and quenched rapidly. Therefore thesecondary reactions could be efficiently reduced. 0.4 ± 0.005 g sam-ple was pyrolyzed each time. Fuel samples were put in a dropperbefore each experiment, and then the dropper was connected tothe feeding tube by a rubber tube. During an experiment, the fuelsample was gently fed into the molybdenum crucible through thesample feeding tube. It cost approximate 4 min each time to feedthe fuel sample. The feeding tube was inserted near to the crucibleof the reactor (20 mm above from the bottom of the crucible) toavoid dispersion of the particles and make both of the biomass par-ticles and coal particles be fallen into the crucible at the same time.Scheme of the pyrolysis system and the operating conditions havebeen described in detail elsewhere [9,26].

2.3. Quantifications

HCN and NH3 in pyrolysis gas were absorbed by NaOH and HNO3solutions respectively. Two adsorption bottles were used to dis-solve NH3 or HCN each time, and volume of the adsorption solutionin each bottle was 200 ml. CN− and NH4

+ ions in the absorptionsolution were analyzed by Metrohm-861 ion chromatography withseparation columns of A Supp 1-250 for anion and C 4-100 forcation. Detection limits of the CN− and NH4

+ ions are 0.02 mg/Land 0.03 mg/L respectively. No water was found to condense inthe inner wall of the reactor or the pipeline for the low yields ofwater and low partial pressure of H2O under the effect of carrier gas,thus no error could be introduced to the detection of NH3 and HCN.Proximate analysis of the fuel samples was determined in an Auto-matic Industrial Analyzer (5E-MAG6700), during which moisturewas determined by drying the fuel samples at 105–110 ◦C under N2(99.9%) atmosphere until the weight was constant. Char in molyb-denum crucible was collected and weighted after each experiment,and ultimate analysis was carried out in an Elemental Analyzer(Vario MACRO CHN/CHNS) to determine nitrogen retained in char.The detailed adsorption methods, ion chromatography conditions,and ultimate analysis methods have also been described elsewhere[9].

3. Results and discussion

Yields of HCN-N, NH3-N, and char-N during rapid pyrolysisof the three biomass and the two coals individually have beenreported elsewhere [9,26]. This study focuses on comparisons ofchar-N yields, NH3-N yields, and HCN-N yields (experimental val-ues) during rapid pyrolysis of biomass/coal blends with the linearcombination (according to proportions) of char-N yields, NH3-Nyields, and HCN-N yields (weighted values) during rapid pyrolysisof biomass and coal individually.

3.1. Nitrogen distributions in products

The experimental values of nitrogen distributions in prod-ucts during rapid pyrolysis of biomass/coal blends are compared

with the weighted values in Fig. 1. Where char-N yields arethe proportions of nitrogen retained in char, (NH3 + HCN)-Nyields are the sums of NH3-N and HCN-N yields. For the rea-son that proportions of tar were condensed in the inner wall

S. Yuan et al. / Journal of Analytical and Applied Pyrolysis 92 (2011) 463–469 465

Table 1Proximate analysis and ultimate analysis of fuel samples [9,26].

Samples Proximate analysis (wt%) Ultimate analysis (wt%, daf)

Moisture Ash Volatiles FC C H N S Oa

Rice straw 10.00 12.52 61.53 15.95 50.26 7.12 1.51 0.84 40.27Chinar leaves 7.91 7.16 69.39 15.54 55.19 4.51 1.23 1.27 37.80Pine sawdust 9.00 4.63 74.08 12.29 53.88 6.06 1.52 0.70 37.85Bituminous 7.26 6.09 29.48 57.17 85.11 6.13 1.32 1.35 6.07Anthracite 4.38 26.05 6.84 62.73 92.16 2.06 1.42 1.70 2.66

daf, dry ash-free basis.a By difference.

600 700 800 900 1000 1100 12000

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0

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Temperature,

Fig. 1. Comparisons of experimental nitrogen distributions in products of biomass/coal blends and the weighted values (black: char-N (experimental); gray: (HCN+NH3)-N(experimental); light gray: (N2+tar)-N (experimental); grad: char-N (weighted); twill: (HCN+NH3)-N (weighted); white: (N2+tar)-N (weighted)).

466 S. Yuan et al. / Journal of Analytical and Applied Pyrolysis 92 (2011) 463–469

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ring rapid pyrolysis of biomass/coal blends (gray: experimental; white: weighted).

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Table 2Packing densities (�, kg m−3) of biomass, coal and biomass/coal blends (biomass20 wt%, dry).

Sample � Sample � (� expected)

RS 177.6 RS + B 411.0 (374.7)CL 215.8 RS + A 405.3 (396.1)PS 121.0 CL + B 512.0 (431.1)B 574.3 CL + A 504.8 (459.7)

Temperature,

Fig. 2. Comparisons of experimental and weighted values of NH3-N yields du

f the upper part of reactor, it’s difficult to collect the tarroduced and quantify tar-N yields. N2 in air may also have

ntroduced large errors in detections of N2 in pyrolysis gas,herefore the yields of N2-N together with tar-N yields werealculated by the function: (N2 + tar)-N (mol%) = 100 mol% − NH3-

(mol%) − HCN-N (mol%) − char-N (mol%).It can be found from Fig. 1 that, experimental char-N yields of the

blends of biomass and anthracite (RS + A, CL + A, and PS + A) are alligher than those of the weighted values. During pyrolysis of coal or

iomass, the increased heating rates can increase the volatile yieldsnd decrease the char yields. In other words, the decreased heatingates can decrease the volatile yields and increase the char yields.lthough the proportions of biomass in blends were just 20 wt%

A 640.6 PS + B 409.5 (328.3)– – PS + A 452.4 (344.5)

S. Yuan et al. / Journal of Analytical and Applied Pyrolysis 92 (2011) 463–469 467

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ring r

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Fig. 3. Comparisons of experimental and weighted values of HCN-N yields du

n this study, packing densities of the blends were greatly affectedTable 2), and heat transfer rates as well as the heating rate of fuelarticles can also be affected. In addition, Haykiri-Acma and Yamanound that, synergy between biomass and coal of high rank was

uch weaker than synergy between biomass and coal of low rank.or the reason of that, coal of low rank has a higher similarity withiomass, therefore volatile release stages of biomass and coal cane more coincident, and leads to the more significant interactions10]. Therefore, during rapid pyrolysis of blends of biomass and

nthracite, for the reason that anthracite has a high condensationegree and low volatile, no synergies happened between biomassnd anthracite, but the decreased heating rates lead to the increasef char-N yields.

Temperatu re,

apid pyrolysis of biomass/coal blends (black: experimental; twill: weighted).

During rapid pyrolysis of biomass/bituminous blends (RS + B,CL + B, and PS + B), it can be found that experimental char-N yieldsof CL + B and PS + B are lower than the weighted values underall the conditions. During rapid pyrolysis of CL + B, in which chi-nar leaves have high density and good fluidity, both particlesof chinar leaves and bituminous could obtain high heating ratesat the same time, and the synergies for nitrogen release can befound in all the conditions (Fig. 1). Although packing density ofPS + B is also low, melting and shrinking of particles were found

during rapid pyrolysis of pine sawdust [28], therefore high heat-ing rates can also be obtained by both of the biomass and coalparticles, and the synergies can also be found in all the condi-tions.

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68 S. Yuan et al. / Journal of Analytical

While, synergies during rapid pyrolysis of RS + B are foundeaker than those of CL + B and PS + B, and experimental char-N

ields of RS + B are even higher than the weighted values whenhe temperature is higher than 1000 ◦C. During rapid pyrolysis ofoal individually, coal particles could be spread at the bottom ofhe molybdenum crucible and the heating rates were high. Duringapid pyrolysis of biomass individually, fluidity of rice straw wasound much lower than that of chinar leaves and pine sawdust.

oreover, carbon skeletons of rice straw were found not easily toe destructed during pyrolysis [28]. During rapid pyrolysis of RS + B,

proportion of coal particles could not be spread to a thin layer onhe bottom of molybdenum crucible because of the carbon skeletontacking of rice straw. Therefore, the coal and rice straw particlesould not obtain as high heating rates as those of during pyrolysisf CL + B and PS + B, leading to the weaker synergies between ricetraw and coal. Compare the result of RS + A with those of CL + And PS + A, the increase of char-N yields caused by the decrease ofeating rates are also higher than those of CL + A and PS + A.

For RS + A, CL + A, and PS + A, whose experimental char-N yieldsre higher than the weighted values, yields of (NH3 + HCN)-N andN2 + tar)-N were lower than the weighted values. For CL + B, PS + B,nd RS + B (at low temperature range) during pyrolysis of whichynergies were found, (NH3 + HCN)-N yields are obviously lowerhan the weighted values, while (N2 + tar)-N yields are higher thanhe weighted values except for those of the low temperature con-itions (600–700 ◦C). Nelson et al. found that, as the temperature

ncreased from 600 ◦C to 1000 ◦C during pyrolysis of lignite, tarields decreased sharply from 20% to lower than 5% [29]. Zanzi et al.ound that tar yields decreased from 0.9–1.1 wt% to 0.1–0.2 wt% ashe temperature increased from 800 ◦C to 1000 ◦C during pyroly-is of biomass [30]. Although tar yields were not quantified in thistudy, it could be qualitatively observed that less tar was formednder the high temperature condition. It can be deduced that N2-

is the main component of (N2 + tar)-N under high temperatureonditions. In other words, although chinar leaves and pine saw-ust blended with coal can increase the nitrogen release from theuel samples, the released nitrogen tends to be converted to thearmless N2.

.2. NH3 yields

Experimental NH3-N yields during rapid pyrolysis ofiomass/coal blends with the weighted values are compara-ively shown in Fig. 2. It can be found that the experimental NH3-Nields are all lower than the weighted values. For RS + A, CL + A,nd PS + A, during pyrolysis of which synergies were scarcelyappened, NH3-N yields are much lower than the weighted values

or the low yields of volatile-N. But for RS + B, CL + B and PS + B,lthough NH3-N yields are also lower than the weighted values,isparities between the experimental values and the weightedalues are lower.

.3. HCN yields

Experimental HCN-N yields during rapid pyrolysis ofiomass/coal blends with the weighted values are compara-ively shown in Fig. 3. It can be found that, for RS + A, CL + A,nd PS + A, during pyrolysis of which synergies were scarcelyappened, although experimental (NH3 + HCN)-N yields are lowerhan the weighted value (Fig. 1), experimental HCN-N yields underhe conditions of 600–700 ◦C are higher than the weighted values.ame phenomenon can also be found from HCN-N yields of RS + B

nd CL + B. Although experimental (NH3 + HCN)-N yields of PS + Bre a little higher than the weighted values, experimental HCN-Nields are also found to be much higher than the weighted values at00–700 ◦C. In other words, during rapid pyrolysis of biomass/coal

[

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plied Pyrolysis 92 (2011) 463–469

blends at 600–700 ◦C, no matter whether experimental volatile-Nyields are higher or lower than the weighted values, HCN-N/NH3-Nmolar ratios are all higher than those of the weighted values.

It has been found that, HCN yields are high during rapid pyroly-sis of rice straw, chinar leaves, and pine sawdust at 600–700 ◦C [9].During rapid pyrolysis of coal at 600–700 ◦C, HCN yields are low forthe difficulty of heterocyclic nitrogen cracking in coal [26]. Duringrapid pyrolysis of biomass/coal blends at the low temperature con-dition, it takes longer time for tar to release from fuel particles [12].Therefore polymerization reactions between amino acids, proteins,cellulose, hemicellulose, and lignin may be enhanced to form het-erocyclic nitrogen which leads to the formation of HCN [9,31,32].Cracking of heterocyclic nitrogen in coal can be enhanced by theabundant H and OH radicals released from biomass. As release timeof volatiles in coal is also increased, promotions of H and OH rad-icals on the cracking of heterocyclic nitrogen in volatiles are alsoenhanced [17,19,33,34].

4. Conclusions

(1) Under the condition of this study, packing densities ofbiomass/coal blends and destruction behaviors of the biomassskeletons are important impact factors on particle heating ratesand nitrogen distributions in products during rapid pyrolysis ofbiomass/coal blends.

(2) Obvious synergies on char-N yields were found duringrapid pyrolysis of biomass/bituminous blends. The syner-gies increased volatile-N yields, but decreased (NH3 + HCN)-Nyields. During rapid pyrolysis of biomass/coal blends at600–700 ◦C, formations of HCN were obviously promoted.

(3) Further efforts should be made on quantification of tar-N yieldsand nitrogen forms in tar derived from rapid pyrolysis ofbiomass/coal blends.

Acknowledgments

This study was supported by the National Basic Research Pro-gram of China (2010CB227000), Shanghai “Technology InnovationAction Plan” and the Fundamental Research Funds for the CentralUniversities.

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