Products and bioenergy from the pyrolysis of rice straw via radio frequency plasma and its kinetics

Products and bioenergy from the pyrolysis of rice straw

via radio frequency plasma and its kinetics

Wen-Kai Tu

, Je-Lung Shie

, Ching-Yuan Chang

, Chiung-Fen Chang

,

Cheng-Fang Lin

_{, Sen-Yeu Yang}

_{, Jing T. Kuo}

_{, Dai-Gee Shaw}

_{, Yii-Der You}

_{, Duu-Jong Lee}

Graduate Institute of Environmental Engineering, National Taiwan University, 71, Choushan Road, Taipei 10617, Taiwan

b

Department of Environmental Engineering, National I-Lan University, No. 1, Sec. 1, Shen-Lung Road, Ilan 26041, Taiwan

c

Department of Environmental Science and Engineering, Tung-Hai University, No. 181, Sec. 3, Taichung Harbor Road, Taichung 40704, Taiwan

d

Department of Mechanical Engineering, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan

e_{Chung-Hua Institution for Economic Research, No. 75, Changsing Street, Taipei 10672, Taiwan}

f_{Department of Chemical Engineering, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan}

a r t i c l e

i n f o

Article history: Received 21 June 2008

Received in revised form 25 September 2008

Accepted 26 September 2008 Available online 30 November 2008 Keywords: Biomass waste Radio frequency Rice straw Bioenergy Pyrolysis

a b s t r a c t

The radio frequency plasma pyrolysis technology, which can overcome the disadvantages of common pyrolysis methods such as less gas products while significant tar formation, was used for pyrolyzing the biomass waste of rice straw. The experiments were performed at various plateau temperatures of 740, 813, 843 and 880 K with corresponding loading powers of 357, 482, 574 and 664 W, respectively. The corresponding yields of gas products (excluding nitrogen) from rice straw are 30.7, 56.6, 62.5 and 66.5 wt.% with respect to the original dried sample and the corresponding specific heating values gained from gas products are about 4548, 4284, 4469 and 4438 kcal kg1_{, respectively, for the said cases. The}

corresponding combustible portions remained in the solid residues are about 64.7, 35, 28.2 and 23.5 wt.% with specific heating values of 4106, 4438, 4328 and 4251 kcal kg1_{with respective to solid}

residues, while that in the original dried sample is 87.2 wt.% with specific heating value of 4042 kcal kg1_.

The results indicated that the amount of combustibles converted into gas products increases with increasing plateau temperature. The kinetic model employed to describe the pyrolytic conversion of rice straw at constant temperatures agrees well with the experimental data. The best curve fittings render the frequency factor of 5759.5 s1_{, activation energy of 74.29 kJ mol}1_{and reaction order of 0.5. Data and}

information obtained are useful for the future design and operation of pyrolysis of rice straw via radio frequency plasma.

Ó 2008 Published by Elsevier Ltd.

1. Introduction

The bioenergy from biomass, which is estimated to contribute about 10–14% of the primary energy supply of the world or about 38% of that of the developing countries (Bhattacharya et al., 2000; Mckendry, 2002), has a potential to provide a significant portion of the projected renewable energy provisions for the shortage of the oil. In general, the feasible characteristics of the biomass are: (1) high yield, (2) low energy input for plantation, (3) low cost, (4) least contaminants and (5) low nutrient requirements (Mckendry, 2002). Therefore, using the biomass wastes becomes a significant way to produce the bioenergy. Among the available biomass wastes, rice straw is one of the favorable waste sources of bioener-gy, because it is the residue from the end use of the biomass

prod-ucts. The reutilization of rice straw not only saves the cost of disposal but also produces valuable bioenergy, achieving the goal of resources recovery and reuse. Taiwan locates in the subtropics and has excellent farming technology, thus producing abundant biomasses. However, this also results in a significant amount of agriculture wastes to be treated, with the rice straw contributing the most. In Taiwan, rice is one of the principal foods and the total annual generation of rice straw is about 1.4 million tons (Tu et al., 2008). The rice straw is difficult for burning in most existing com-bustion systems. The reasons are: (1) the fouling of deposits, (2) slag formation in furnaces and (3) accelerated corrosion (Bakker and Jenkins, 2003). Therefore, its common treatment is in-site burning for producing manure. However, the open burning is harmful to air quality and environment (Pütün et al., 2004).

Transform of the biomass wastes into bioenergy can be effi-ciently achieved applying thermochemical methods such as com-bustion, pyrolysis and gasification (Shie et al., 2001,2002a; Chen et al., 2003b; Wu et al., 2003; Pütün et al., 2004). There are two

0960-8524/$ - see front matter Ó 2008 Published by Elsevier Ltd. doi:10.1016/j.biortech.2008.09.052

*Corresponding author. Tel./fax: +886 2 2363 8994. E-mail address:cychang3@ntu.edu.tw(C.-Y. Chang).

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Bioresource Technology

main disadvantages of the pyrolysis and gasification of biomass wastes for producing gases of medium calorific value via the tradi-tional thermolysis technology. These are: (1) the low gas yield, reducing the total energy value of gas and (2) the high content of tar in gas, causing the corroding problem of the gas collection equipment and increasing the need for the further treatment of the gas produced (Caldeira et al., 2002; Bridgwater, 2003; Chen et al., 2003a). The biomass treated via thermolysis yields the tar in the inferior temperatures and can undergo the cracking and re-polymerization in the superior temperatures above 673 K via secondary reactions (Ferdous et al., 2001; Chen et al., 2003b; Pütün et al., 2004). In order to provide the suitable fuel for spark ignition gas engines, the bio-fuel should be either gaseous or high quality liquid form (Mckendry, 2002). The common methods to overcome the above two disadvantages of the traditional thermol-ysis of biomass are adding catalysts and steamShie et al., 2002b,c; Gullu, 2003; Atutxa et al., 2005; Demirbas, 2005; Waldner and Vo-gel, 2005).Worasuwannarak et al. (2007)applied the thermal gra-vimetry-mass spectrometry technique to study the pyrolysis behaviors of rice straw, yielding the pyrolysis products from rice straw at 873 K with compositions of char, H2O, CO, CO2, tar and

sum of H2and CH4of 31, 25, 15, 15, 10 and 4 wt.%, respectively.

Thus, other than H2O, the CO and CO2are the dominant gas

prod-ucts, while H2and CH4are minor.Chen et al. (2003b)employed the

catalysts to pyrolyze the rice straw, improving the gas yields at 1023 K with respect to the original un-dried sample containing about 10 wt.% H2O from 36 wt.% without catalyst to 41, 42.2 and

46 wt.% with CaO, Na2CO3and Cr2O3, respectively. However, even

with the use of the catalyst of Cr2O3, the yield of tar is still as high

as 10 wt.% at least.Pütün et al. (2004)reported that the feasible parameters for the pyrolysis of rice straw are with particle diame-ter of 20–40 mesh, gas flow of 200 mL min1_{, final temperature of}

823 K and the steam velocity of 2.7 cm s1_{.Huang et al. (2008)}

em-ployed the microwave-induced technology to pyrolyze the rice straw and yielded the H2-rich fuel gas of nitrogen-free with H2,

CO2, CO and CH4of 55, 17, 13 and 10 vol.%, respectively.

Application of a novel heating method via the radio frequency (RF) plasma is one of the feasible choices for overcoming the disad-vantages of thermolysis using traditional heating methods (Tu et al., 2008). The RF plasma heating method, which is a capacitive dielectric heating method, employs the alternating current with high frequency and voltage to build up electro-magnetic field pro-ducing plasma to induce the target material resulting in the vigor-ous colliding, rubbing and thus self-heating. As the material is heated, pyrolysis occurs. The heating method using RF plasma has many advantages such as high heating rate, short heating time to reach setting temperature, low heat loss and low residual tar. Hence, this novel method can overcome the problems encountered in the traditional pyrolysis of biomass (Zhao et al., 2001; Bridg-water, 2003; Chen et al., 2003a; Merida et al., 2004; Yaman, 2004; Shie et al., 2008).

The plasma technologies for the thermolysis of biomass not only give high concentration of syngas, but also result in low con-centration of tar in gas phase mostly below 10 mg Nm3_{as noted}

byHlina et al. (2006). Low tar content in product obtained from RF plasma thermolysis also can be achieved because high energy species, such as electron, ion, atom and free radical produced from RF plasma can enhance the decomposition of tar (Tang and Huang, 2005a,b; Cheng et al., 2007).Tang and Huang (2005a,b)employed RF plasma pyrolysis with N2as carrier gas at 0.8 L min1to treat

the biomass of sawdust. The gas yield on average with respect to the original dried sample can reach 66 wt.% consisting of H2, CO,

CH4, CO2and C2at an input power of 1800 W and an operating

pressure of 5000 Pa (= 0.05 atm). The corresponding compositions of H2, CO, CH4, CO2and C2in the gas including N2are 3.88–11.06,

5.21–14.82, 1.38–2.48, 1.51–5.05 and 1.5–3.92 vol.%, respectively.

The total content of CO and H2in the gas products is 76 vol.% on

a nitrogen-free basis, which can be used as syngas components.

2. Experimental section

The original RF plasma pyrolysis system (Tu et al., 2008) was modified adding the gas mixers, electromagnetic pulses stabiliza-tion device and effluent flow meter for the use in this study. The electromagnetic pulses stabilization device can stabilize the poten-tial energy of the reactor when the RF plasma is produced and lead the electromagnetic pulses to the ground wire. The modified RF plasma pyrolysis system denoted as RFPT-N system is shown in

Fig. 1. The RF plasma reactor consists of a quartz tube with outer diameter of 50 mm, wall thickness of 2 mm and length of 500 mm. The electrodes are two pieces of copper arcs with length of 320 mm, which are fixed around the outside of the quartz tube with a gap between two electrodes. The length of the RF plasma producing zone is about 320 mm. The electromagnetic pulses sta-bilization device stabilizes the potential energies of the upper and bottom stainless steel parts, hence preventing the interference of electromagnetic pulses on the digital monitors. Nitrogen with a purity of 99.99% was used as the working gas. Its flow rate through the drying tube was controlled via a mass flow controller of model 5850E from Brooks, Hatfield, PA, USA and checked by the effluent flow meter. In all experiments, nitrogen flow rate was kept at 200 mL min1_{. A pressure control valve is adopted to transfer the}

RFPT-N system from low-vacuum to atmosphere condition smoothly. A cold trap container with a volume of 3 L for controlling the outlet gas at a temperature of 298 ± 5 K is installed after the RF plasma pyrolysis reactor to collect the liquid products. In order to maintain a suitable degree of vacuum in the RFPT-N system, switch valves and vacuum meters of model CVG191GA from InstruTECH Inc., Bellmore, NY, USA, which are recorded by a vacuum monitor of Terranova model 906A from Duniway Stockroom Corp., Moun-tain View, CA, USA, are installed. Three digital thermometers of model TFC 305A, type K from Macro Fortunate Co., Taipei County, Taiwan were used to measure the temperatures at the inlet and in-side of the RF plasma reactor, and the outlet of the cold trap. A RF plasma power supply of model QINTO3013 from Huettinger, Frei-burg, Germany and an auto-matching box of model PFM3000A from Huettinger, Freiburg, Germany provide a maximum power of 2000 W and RF frequency of 13.56 MHz. The temperatures were controlled at various constant values from 740 ± 5 to 880 ± 5 K with the input powers at various constant values from 357 ± 2 to 664 ± 2 W at the pressure of 0.9 ± 0.04 torr. In this study, the RF plasma pyrolysis reactor was used for pyrolyzing the biomass waste of rice straw. In the batch experiments, the sample of rice straw was put in the crucible. After the completion of an experi-ment, the solid remained in the crucible was weighted. Three rep-licates were performed.

The effluent gas was vented to a fume hood for starting up and switched to the sampling port for collection using a sample bag as the experimental conditions of RFPT-N system were stable. When the run was finished, the nitrogen gas was kept flowing till the temperature of system was below 373 K.

The Brunauer–Emmett–Teller (BET) surface areas of samples were measured via the specific surface area analyzer of model ASAP2010 from Micromeritics, Norcross, GA, USA adopting gas adsorption method. Before analyzing, the sample was degassed at 378 K and 103_{mmHg. It followed the nitrogen adsorption at}

77 ± 0.5 K. The elements of C, H and N were analyzed via Heraeus CHN-O-Rapid analyzer from Heraeus Ltd., Hanau, Germany, and S and Cl via Tacussel Coulomax 78 automatic coulometric titrator from Tacussel, Lyons, France. The actual heating values of samples were measured using the adiabatic bomb calorimeter of model 150

Vacuum Flask Oxygen Bomb Calorimeter form Osaka Sanso Kogyo Ltd., Osaka, Japan, while the theoretic heating values of samples were calculated by Scheurer-Kestner formula (Srinivasa Reddy et al., 2005). The actual heating values can be estimated from the change of temperature in the adiabatic bomb calorimeter when the sample was burning and the detailed procedures can be re-ferred to the standard test method for gross calorific value of coal and coke by the adiabatic bomb calorimeter (ASTM D2015, 2000; ASTM D3174-04, 2006).

The gaseous products of H2, CO and CO2were analyzed using

gas chromatography-TCD of model 8900 from China Chromatogra-phy Co. Ltd., Taipei, Taiwan adopting column of #1-2390-U with dimensions of 15 ft  1/8 inch. The temperatures of injector, oven and detector were at 343, 368 and 343 K, respectively. The gaseous product of CH4was analyzed via gas chromatography-FID of model

HP 6890 from Hewlett Packard Inc., California, USA using column of #115432 GS-Q with dimensions of 30 m  0.530 mm. The tem-peratures of injector, oven and detector were 373 K, 323 K for 10 min then raised to 523 K with heating rate of 10 K min1_and

523 K, respectively. The gaseous products of total hydrocarbons were analyzed using gas chromatography-FID of model HP 5890 from Hewlett Packard Inc., California, USA with column of fused silica capillary tube with dimensions of 10 m  0.530 mm. The temperatures of injector, oven and detector were 423, 423 and 473 K, respectively. The detection limits of H2, CO, CO2, CH4 and

THCs are 2.2, 25.9, 21.9, 0.085 and 0.095 mg L1_.

3. Results and discussion

3.1. Effects of loading power on the efficiency of pyrolysis of rice straw via RF plasma

The rice straw sample was exposed under the sunlight for 10 d, broken via spiral breaker, sieved into 30–40 mesh (0.6–0.425 mm) and dried in a recycle ventilation drier for 24 h at 378 K before use. The results of proximate and elemental analyses, heating value and BET surface area of rice straw with respect to the original dried sample are listed inTable 1. On the dry basis, the heating value

is as high as 4042 kcal kg1 _{contributed by the high content of}

the combustibles of rice straw of 87.2 wt.%. The combustibles of rice straw are composed of 84 ± 5 wt.% of volatile matter and 16 ± 5 wt.% of fixed carbon (Chen et al., 2003b; Iranzo et al., 2004; Pütün et al., 2004; Worasuwannarak et al., 2007; Huang et al., 2008). Thus, the contents of volatile matter and fixed carbon of sample used in this study are 73.2 and 14 wt.%, respectively. The contents of C, H and O of rice straw are 41.8, 5.9 and 51.6 wt.% on dry basis, respectively, while those of nitrogen, sulfur and chloride are rare and can be neglected in the utilization of rice straw. The BET surface area is small with a value of 1.21 m2_g1_{. The}

utiliza-tion of biomass for producing energy containing products also as-sists the reduction of CO2and SO2emissions for preventing the

greenhouse effect and acid rain (Liang and Kozinski, 2000).

Fig. 1. The schematic diagram of the modified RF plasma pyrolysis (RFPT-N) system. 1. Mass flow rate controller. 2. Gas mixers. 3,6,11. Thermocouples. 4,12. Vacuum meters. 5. Pressure value. 7. Crucible and its support. 8. Copper electrodes. 9. Electromagnetic pulses stabilization device. 10. Condenser and liquid product collector. 13. Effluent flow meter. 14. Sampling port. 15. Vent to hood. 16. Circulating thermostat. 17. Auto-matching box. 18. RF plasma power supply. 19. Digital monitors.

Table 1

Some properties of original rice strawa

used in this studyb . Item Value Proximate analysis (wt.%) Moisture 0 Combustibles 87.2 (0.90)c Ash 12.8 (1.15)

Heating value (kcal kg1

) 4042 Elemental analyses (wt.%) C 41.8 (0.01) H 5.9 (0.06) N 0.4 (0.01) O 51.6d S 0.2 (0.02) Cl 0.1

BET surface area (m2

g1

) 1.212

a

Dry basis.

b

Note that the samples used in the and present previous studies (Tu et al., 2008) were collected from different farmlands, showing slight difference in properties.

c_{Numbers in parentheses are standard deviations (r} n-1). d_Balance.

The residual mass fraction (M) of rice straw with respect to the initial mass during pyrolysis is expressed on a normalized basis as

M ¼ W=W0 ð1Þ

where W and W0 are the present and initial masses of sample,

respectively.

The pyrolytic reaction of rice straw is significant in 550–650 K with two major reaction steps (Tu et al., 2008). Therefore, the pla-teau temperature for the pyrolysis of rice straw pyrolysis using RF plasma conducted in this study was set at the middle range of the second reaction step (740 ± 5–880 ± 5 K) to ensure an acceptable reaction rate of rice straw.

The effects of major system parameters on the performance of RF plasma pyrolysis of rice straw were examined. These parame-ters include loading power, reaction time and reaction tempera-ture. The loading power can be measured online and recorded via the RF plasma power supply device. Maintaining the stable va-lue of net pressure in the reactor can ensure the gas flowing through the plasma pyrolysis system with no leakage problem in experiments (Tu et al., 2008). The net pressure is the difference be-tween the final pressure and the initial pressure under vacuum of 89 mtorr. For the followed pyrolysis experiments using RF plasma, the gas flow rate and net pressure of N2were set at 200 mL min1

and 0.90 ± 0.04 torr, respectively.

Time variations of residual mass fraction and reaction tempera-ture at various loading powers and plateau temperatempera-tures are shown inFig. 2. The loading power used for the generation of RF plasma is equal to the input power minus reflected power. The in-put and reflected powers stand for the power supplied from the RF plasma power supply and the useless power producing the wasted electromagnetic wave from reflection, respectively. The results indicated that as the loading power increases from 357 to 664 W, the final value of residual mass fraction denoted as Mfat 45 min

decreases from 69.3 to 33.5 wt.%, while the plateau temperature of plasma increases from 740 to 880 K. The rate of variation of residual mass fraction decreases while that of reaction tempera-ture increases with the increase of loading power. The correspond-ing plateau temperatures at various loadcorrespond-ing powers are 740 K at 357 W, 813 K at 482 W, 843 K at 574 W and 880 K at 664 W, respectively. At loading powers of 357, 482, 574 and 664 W, the values of heating times tST to reach their corresponding setting

temperatures TST, which are estimated as initial temperature T0

plus 95% of the change of T0to the corresponding plateau

temper-atures, at 718, 787, 816 and 851 K are about 4.5, 4.17, 4 and 3.83 min with heating rates of 98, 124, 136 and 152 K min1_,

respectively. Thus, a higher loading power gives a higher plateau temperature with a shorter tST, resulting in a high heating rate

which can be express as the following equations:

TPðKÞ ¼ 0:4488PWLþ 585:97 R2¼ 0:984 ð2Þ

HR ðK min1Þ ¼ 0:1722PWLþ 38:064 R2¼ 0:9942 ð3Þ

where TP, PWLand HR are the plateau temperature, loading power

and heating rate, respectively. The values of TPand HR increase

lin-early with PWL. Hence, HR can reach as high as about 152 K min1at

PWLof 664 W.

The average reaction rate ravg before steady state can be

esti-mated by further examination of the results ofFig. 2a. At various values of loading power, the values of residual mass fraction and time at steady state symboled as MSS and tSS are about

69 ± 1 wt.% and 40 min at 357 W, 44 ± 1 wt.% and 30 min at 482 W, 37 ± 1 wt.% and 20 min at 574 W, and 33 ± 1 wt.% and 15 min at 664 W. The values of MSSat tSSare about equal to their

corresponding final values of residual mass fraction Mfat 45 min,

which are about 69.3, 43.4, 37.5 and 33.5 wt.%, respectively. As reaction temperature increases from T0to the setting temperature,

the residual mass fraction only slightly reduces about 3.5, 2.9, 2.8 and 2.6 wt.% while retains 96.5, 97.1, 97.2 and 97.4 wt.% denoted as MST for the cases with loading power of 357, 482, 574 and

664 W, respectively, for further pyrolysis at various plateau tem-peratures. The corresponding decomposition efficiency of rice straw for further pyrolysis of MSTto MSSsymboled as

g

as:

g

The values of

g

respec-tively. The values of ravgof RFPT-N system are estimated to be about

0.78, 2.06, 3.76 and 5.77 wt.% min1_{at 357, 482, 574 and 664 W,}

respectively, using the following equation:

ravg¼

g

Thus, a higher loading power gives higher decomposition efficiency and reaction rate.

Fig. 2. Time variations of residual mass fraction (M) and reaction temperature (Tr)

for the pyrolysis of rice straw via RF plasma method at various loading powers (PWL)

of 357 ± 2 (s), 482 ± 2 (4), 574 ± 2 (}) and 664 ± 2 (h) W with plateau temperature (TP) of 740 ± 5 (d), 813 ± 5 (N), 843 ± 5 () and 880 ± 5 (j) K, respectively. PWL= PWI

- PWR. PWI, PWR: Powers supplied and reflected. W0: 300 ± 10 mg. dP: 0.425–0.6 mm.

Carrier gas: N2. Q: 200 mL min1. P–P0: 0.9 ± 0.04 torr. T0: 298 ± 5 K. M = W/W0. W,

W0: Present and initial masses of sample. dP: Sample size. Q: Flow rate of inlet

carrier gas. P: Final pressure. P0: Initial pressure (89 m torr). T0: Initial or room

3.2. Fractions of products of solid, liquid and gas

Table 2lists the fractions of the products of solid residue M, li-quid and gas from the pyrolysis of rice straw via RFPT-N system at various loading powers PWLand plateau temperatures TP. The

val-ues are expressed with respect to the initial mass of dried sample. According to the studies ofLin and Lin (2006)andTu et al. (2008), the major mineral constituents of rice straw are Si, Al, Na, Mg and Ca and the total content of oxide compounds of mineral matter is about 12.5 wt.% or 125,074.8 pp mw, which is close to that of ash of 12.8 wt.%.

FromTable 2, the final residual mass fraction Mfat 45 min

de-creases as TP and PWL increase with Mf of 69.3, 43.4, 37.5 and

33.5 wt.% at TPof 740, 813, 843 and 880 K, and PWLof 357, 482,

574 and 664 W, respectively. No liquid products were collected from the RFPT-N system in this study. This may be due to the causes that only a small amount of rice straw less than 3.5 wt.% was decomposed in the heating stage with temperature rise and that the energy species generated via RF plasma can enhance the decomposition of tar and complex compounds if they were pro-duced. Therefore, one can derive that the pyrolyzed part of rice straw is transformed to the gas products. Thus, the corresponding fractions of gas products are 30.7, 56.6, 62.5 and 66.5 wt.%, respec-tively. A higher loading power gives a higher plateau temperature and a larger amount of gas products.

3.3. Characteristics of gas products and solid residues

In order to examine the variation of concentration of gas prod-ucts during the pyrolysis at various plateau temperatures TPand

loading powers PWL, the gas was sampled at different time

inter-vals with 4 min per interval such as 1–5 min, 6–10 min and 11– 15 min till 41–45 min. Items of gas products analyzed included H2, CO, CH4, non-methane hydrocarbons (NMHCs) and the total

hydrocarbons (THCs). The concentrations of NMHCs and THCs are expressed as CH4equivalence. The value of NMHCs is the

differ-ence between the values of THCs and CH4.Fig. 3shows the

instan-taneous concentrations of gas products. The results indicated that the gas production is negligible in the heating period with temper-ature rise before reaching the setting tempertemper-ature TSTat time tST.

After tST, gases are produced significantly as the pyrolysis proceeds

vigorously at TPwith a higher TPyielding more gases production as

expected. The production of gases then reduces as the residual mass fraction M decreases approaching its final value Mfas shown

inFig. 2a. The maximum concentrations of various gas products appear earlier for higher TPor PWL. The characteristics of gas

prod-ucts measured via GC–FID indicated that the major constituents of NMHCs are hydrocarbons of C2–C5. The accumulated amounts of

gas products WG,Accin weight including H2, CO, CH4and NMHCs

at various TPor PWLare shown inFig. 4. At 45 min, the values of

WG,Accsymboled as WG,Acc,f at various PWLof 357, 482, 574 and

664 W are 90.11, 147.2, 184.39 and 192.08 mg, respectively.

More-over,Fig. 4shows that the linear gas producing rates of the middle portions are 6.01, 7.36, 7.38 and 7.68 mg min1_{at P}

WLof 357, 482,

574 and 664 W, respectively. A higher PWLcan shorten the heating

time for the production of gas products and give a higher TP

favour-able to the pyrolytic reaction, thus resulting in a larger production of gas products with a higher producing rate. The yields and vol-ume fraction contents of different component i denoted as YW,i

and Viare listed in Table 3. As TP increases from 740 to 880 K,

sum of the yields of CO and H2of syngas increases greatly from

29.08 to 62.42 wt.%. Further, on nitrogen free basis, the volume fractions of syngas consisting of CO and H2are all above 97 vol.%

for the four cases examined.Table 3also gives the sums of yields of all gas products collected with values of 30.04, 49.07, 61.47 and 64.03 wt.% at TPof 740, 813, 843 and 880 K, respectively. These

values are close to the predicted values of 30.7, 56.6, 62.5 and 66.5 wt.% as listed inTable 2. Thus, the recoveries of gas products RCGare about 98, 87, 98 and 96% for the cases with TPof 740, 813,

843 and 880 K, respectively. Note that RCGcan also be estimated

using the following equation:

RCG¼ WG;Acc;f=½W0 ð1  MfÞ ð6Þ

Table 4lists the BET surface area and element constituents of the solid residues at various TPvia RFPT-N system. The BET surface area

increases from that of the original rice straw of 1.21 m2_g1_{to those}

of solid residues 2.11, 5.60, 5.86 and 6.50 m2g1_{at T}

Pof 740, 813,

843 and 880 K, respectively. The feature of high heating rate HR of RFPT-N system is beneficial for reducing the formation of tar, while enhancing the production of gases of low molecular weights such as C2–C5 of hydrocarbons. However, the high HR of RFPT-N system

may not provide enough time for the formation and development of pore structure of carbonaceous solid residues which although still contain about 42–50 wt.% of carbon. The common HR for the formation of pore structure of carbonaceous solid residues is about 5–20 K min1_{(Tsai et al., 2001). Thus, the BET surface area of the}

so-lid residues via RFPT-N system is inferior.

3.4. The kinetic model of combustible solid residues at constant temperature

Fig. 5shows the time variation of residual mass fraction sym-boled as MSTFafter the heating time tSTbased on the total amount

of mass pyrolyzed, which is denoted as WSTF, at various loading

powers PWL. The MSTFand WSTFare defined as

MSTF¼ ðW  WfÞ=WSTF ð7Þ

WSTF¼ WST Wf ð8Þ

In Eqs.(7) and (8), W and Wfare the present and final masses of

samples defined in the proceeding sections, WSTis the mass of

sam-ple at tSTor at the setting temperature TST. The values of MSTFare

nearly constant after 35.5, 15.83, 11 and 6.17 min for the cases with PWL of 357, 482, 574 and 664 W, respectively. The reaction rate

equation is proposed for the MSTFas

dMSTF=dðt  tSTÞ ¼ kMnSTF ð9Þ

which gives

MSTF¼ ½1  ðn þ 1Þkðt  tSTÞ½1=ðnþ1Þ ð10Þ

where n is the reaction order and k is the reaction rate constant described by Arrhenius equation as k = A exp(-Ea/RGT) with A, Ea

and RG representing the frequency factor, activation energy and

universal gas constant (Huang et al., 2004). The best curve fittings render A of 5759.5 s1_{, Ea of 74.29 kJ mol}1_{and n of 0.5.Fig. 5}

com-pares the experimental data and the predicted results. The coeffi-cients of determination R2 _{are 0.973, 0.995, 0.997 and 0.963 for}

the cases with PWLof 357, 482, 574 and 664 W, respectively. The Table 2

The fractions of products of solid remained, liquid and gas from the pyrolysis of rice straw via RFPT-N system at reaction time of 45 min and various plateau temperatures TPapplying various loading powers PWL.

TP(or PWL) Solid remained (Mf)

(wt.%) Liquid products (wt.%) Gas products (wt.%) 740 ± 5 K (or 357 ± 2 W) 69.3 0 30.7 813 ± 5 K (or 482 ± 2 W) 43.4 0 56.6 843 ± 5 K (or 574 ± 2 W) 37.5 0 62.5 880 ± 5 K (or 664 ± 2 W) 33.5 0 66.5

Initial mass of sample W0= 300 ± 10 mg, diameter of sample dP= 0.425–0.6 mm,

flow rate of carrier gas Q = 200 mL min1_{of N} 2.

good agreement between the two indicates that this model can be employed to describe the pyrolytic conversion of rice straw at con-stant temperatures.

3.5. The energy consumption and recovery and the heating values in the original sample and all products

The pyrolysis of rice straw via RF plasma can produce usable gas products not only as resources but also as energy with their corre-sponding specific heating values HV.Table 5lists the values of HVof

gas and solid products obtained at various plateau temperatures TP

applying various loading powers PWL. The specific heating values

gained from the gas products denoted as HV,G are estimated via

dividing the sum of all heating values of the corresponding com-bustible components by the total amount of gas products. The re-sults are 4548, 4284, 4469 and 4438 kcal kg1_{for the cases with T}

P

of 740, 813, 843 and 880 K, respectively.

The HVof each residual solid symboled as HV,Swas calculated

employing the Scheurer–Kestner formula, noted as S–K formula, using the element constituents in each residual solid listed in

Fig. 3. Instantaneous concentrations of gas products for the pyrolysis of rice straw via RF plasma method at various PWL. s, 4, h, h and other conditions are as those specified

Table 4. The actual HV,Sof original rice straw was also measured

using the adiabatic bomb calorimeter, giving the value of 4042 kcal kg1_{close to the theoretical H}

V,Sof 4154 kcal kg1calculated with

S–K formula. The less than 3% difference between these two values supports the validity of using the S–K formula to estimate the HV,S

of solid samples. The S–K formula is as follows. S–K formula: HV;S¼ 81 C  3 4O   þ 342:5H þ 22:5S þ 57  3 4O    6 9H þ WH2O   ð11Þ in which, the C, O, H and S represent the percent contents of ele-ments of sample in wt.% and WH2Ois the moisture content of sample

in wt.%. The numbers of 81, 342.5 22.5 and 57 are the heats of com-bustion of C to CO2, H to H2O, S to SO2and C to CO, respectively, in

kcal kg1_{. The number of 6 is the heat of evaporation of water}

trans-fer to gaseous H2O in kcal kg1. The numbers of 3/4 and 9 are the

ratios of molecular weights of C to O and H2O to H2, respectively.

The masses of S and moisture content of the samples are low and can be neglected. Thus, also for the said cases, the corresponding combustible portions remained in the solid residues are 64.7, 35, 28.2 and 23.5 wt.% with HV,S of 4106, 4438, 4328 and 4251 kcal

kg1 _{with respect to the solid residues, respectively. The original}

dried sample contains 87.2 wt.% combustibles with HV,S of

4042 kcal kg1_{as already noted. The values of H}

V,Sand HV,Gof solid

residues and gas products show small variation. However, the val-ues of HV,G (W0 Wf) of gas products, representing the total

heat-ing values gained in gas products, are 418.9, 727.4, 838 and 885.3 cal for the corresponding cases, respectively, which increase

as TPincreases. The results indicated that the amount of

combusti-bles converted into gas products increases with increasing TPas

expected.

The specific total heating value of solid and gas products de-noted as HV,Tincreases from 4042 of original dried rice straw to

about 4242, 4351, 4416 and 4375 kcal kg1_{at T}

Pof 740, 813, 843

and 880 K with corresponding PWLof 357, 482, 574 and 664 W,

respectively, as shown inTable 5. The total heating values noted as HV,T W0are then calculated to be 1.21 kcal of original dried

rice straw and 1.27, 1.31, 1.33 and 1.31 kcal for the pyrolytic prod-ucts of rice straw respectively, as listed inTable 6. The energies consumed in the temperature-rising heating period of tSTdenoted

as ESTare about 0.08, 0.1, 0.12 and 0.13 kcal, and the corresponding

energies consumed at the constant-temperature heating period at TPto reach steady state symboled as ETPare about 0.64, 0.63, 0.46

and 0.37 kcal. Note that the amount of rice straw sample W0

charged in the RFPT-N system for the pyrolysis experiments was about 0.3 g, while the allowable loading capacity Waof the reactor

is 85.12 g. Therefore, the values of ESTand ETPconsumed by W0are

computed as a fraction W0/Waof the energies input in the

corre-sponding heating periods. The bulk density of rice straw in this study of 0.2 ± 0.01 g cm3_{was determined according to the}

proce-dures ofPendyal et al. (1999). The allowable batch amount of rice straw that can be treated in RFPT-N system is estimated consider-ing the inner diameter of plasma reactor of 46 mm, length of plas-ma zone of 320 mm, bulk density of rice straw of 0.2 ± 0.01 g cm3

and percent of usage of capacity of plasma reactor of 80 vol.%. This then gives the allowable loading capacity of rice straw of about 85.12 g. In obtaining HV,T, EST and ETP, the applicable equations

are as follows.

HV;T¼ HV;S Mfþ HV;G ð1  MfÞ; in kcal kg1 ð12Þ

EST¼ ½ðPWL tST 60Þ=4:1868  ðW0=WaÞ; in cal ð13Þ

ETP¼ ½PWL ðtSS tSTÞ  60=4:1868  ðW0=WaÞ; in cal ð14Þ Table 6lists the energy recoveries RE,allof the entire process

includ-ing temperature-risinclud-ing and constant-temperature heatinclud-ing periods for the pyrolysis of rice straw via RFPT-N system, giving 0.66, 0.68, 0.74 and 0.77 at PWLof 357, 482, 574 and 664 W, respectively.

The corresponding energy recoveries RE,TPof the process

consider-ing constant-temperature heatconsider-ing at the TPare also presented in Table 6with values of 0.69, 0.71, 0.8 and 0.83, respectively. The re-sults indicated that the energy recovery increases as PWLincreases,

suggesting that the pyrolysis via RF plasma is favorable applying a higher PWL. In estimating RE,all and RE,TP, the following equations

are employed.

RE;all¼ ðHV;T W0Þ=½1:21 þ ðESTþ ETPÞ ð15Þ

RE;TP¼ ðHV;T W0Þ=ð1:21 þ ETPÞ ð16Þ Fig. 4. Accumulated amounts of gas products (WG,Acc) of H2, CO, CH4and NMHCs

collected for the pyrolysis of rice straw via RF plasma method at various PWL. (s),

(4), (h), (}) and other conditions are as those specified inFig. 2.

Table 3

The yields (YW,i) and volume fractions of different components i (Vi) in the accumulated gas products from the pyrolysis of rice straw via RFPT-N system at reaction time of 45 min

and various TPapplying various PWL.

TP(or PWL) YW,CO(wt.%) YW,H2(wt.%) YW,CH4(wt.%) YW,NMHCs(wt.%) YW,Ga(wt.%) VCO(vol.%) VH2O(vol.%) VCH4(vol.%) VNMHCs(vol.%) 740 ± 5 K (or 357 ± 2 W) 27.75b 2.03b 0.21b 0.05b 30.04b 92.39c (1.01)d 6.75c (0.07)d 0.71c (0.008)d 0.16c (0.002)d 813 ± 5 K (or 482 ± 2 W) 44.9 3.78 0.3 0.09 49.07 91.51(1.63) 7.70 (0.14) 0.61 (0.011) 0.18 (0.003) 843 ± 5 K (or 574 ± 2 W) 56.68 3.79 0.8 0.2 61.47 92.21 (2.06) 6.16 (0.14) 1.29 (0.029) 0.33 (0.007) 880 ± 5 K (or 664 ± 2 W) 58.54 3.88 0.65 0.96 64.03 91.43 (2.13) 6.06 (0.14) 1.01 (0.024) 1.51 (0.035) a_Y W,G= YW,CO+ YW,H2+ YW,CH4+ YW,NMHCs. b _{With respect to the original dried sample.} c

On nitrogen free basis.

d

4. Conclusions

The RFPT-N system overcomes the problem of tar formation during the pyrolysis of rice straw. In this system, a higher loading power gives a higher plateau temperature with a shorter heating time to reach setting temperature. The high heating rate of RF plas-ma can efficiently decompose the combustible solid to gas prod-ucts of H2, CO, CH4and low carbon hydrocarbons such as C2–C5.

The production of gases increases as the pyrolysis is proceeded vig-orously at a higher plateau temperature. Also, an increase in load-ing power increases the total heatload-ing value of the products of gas and solid residues.

The kinetic model employed to describe the pyrolytic conver-sion of rice straw at constant temperature agrees well with the experimental data. The frequency factor, activation energy and reaction order are 5759.5 s1_{, 74.29 kJ mol}1_{and 0.5, respectively.}

The obtained data and information are useful for the rational oper-ation and design of pyrolysis of rice straw via RF plasma.

The energy recovery from the pyrolysis of rice straw via RFPT-N system increases with increasing loading power. In practical, the rice straw can be pre-treated via pelletization to prepare the refuse derived fuel before being charged into the reactor. This can in-crease the density of rice straw pellet and the capacity of loading, enhancing the energy utilization efficiency.

Acknowledgement

We express our sincere thanks to the National Science Council of Taiwan for the financial support, under the contract No. NSC95-2218-E-002-035.

Appendix Notations. and abbreviations

Table 4

The BET surface area denoted as ABETand element analyses of solid residues from the

pyrolysis of rice straw via RFPT-N system at reaction time of 45 min and various TP

applying various PWL. TP(or PWL) ABET (m2_g1₎ C (wt.%) H (wt.%) N (wt.%) Oa (wt.%) S (wt.%) Cl (wt.%) Original rice straw 1.21 41.8 5.9 0.4 51.6 0.2 0.1 740 ± 5 K (or 357 ± 2 W) 2.11 42.6 5.5 0.5 51.4 – – 813 ± 5 K (or 482 ± 2 W) 5.60 49.8 4.3 0.8 45.1 – – 843 ± 5 K (or 574 ± 2 W) 5.86 48.9 4.2 0.8 46.1 – – 880 ± 5 K (or 664 ± 2 W) 6.50 49.4 3.8 0.7 46.1 – –

–: Neglected and not measured.

a

Balance.

Fig. 5. Time variations of MSTFafter tSTfor the pyrolysis of rice straw via RF plasma

method at various PWL. Mf= 69.3, 43.4, 37.5 and 33.5, tST= 4.5, 4.17, 4 and 3.83 min,

and R2

= 0.973, 0.995, 0.997 and 0.963 for PWL= 357, 482, 574 and 664 W,

respectively. n = 0.5. Symbols and lines: Experimental data and prediction. MSTF:

Residual mass fraction based on total amount of mass pyrolyzed, = (W  Wf)/(WST

Wf). W, Wf: Present and final masses of samples. WST: Mass of sample at heating

time tSTreaching setting temperature. R2: Coefficient of determination. n: Reaction

order. (s), (4), (h), (}) and other conditions are as those specified inFig. 2.

Table 5

The specific heating values (HV) of original sample and all products from the pyrolysis

of rice straw via RFPT-N system at reaction time of 45 min and various TPapplying

various PWL. TP(or PWL) HV,S (kcal kg1 ) HV,G (kcal kg1 ) HV,T (kcal kg1 )

Original rice straw (actual) 4042 NA 4042

Original rice straw (theoretical) 4154 NA 4154

740 ± 5 K (or 357 ± 2 W) 4106 4548 4242

813 ± 5 K (or 482 ± 2 W) 4438 4284 4351

843 ± 5 K (or 574 ± 2 W) 4328 4469 4416

880 ± 5 K (or 664 ± 2 W) 4251 4438 4375

HV,S and HV,G: Specific heating values of solid residues and gas products,

respectively.

HV,T: Specific total heating value of solid and gas products from pyrolysis, = HV,S

Mf+ HV,G (1- Mf).

HV,Tof original rice straw: same as HV,S.

HV,CO, HV,H2, HV,CH4and HV,NMHCs= 282.99, 285.84, 890.35 and 890.35 kJ mol1=

2413, 34136, 13291 and 13291 kcal kg1_.

NA: Not applicable.

Table 6

The energy consumption and recovery and heating values of all products from the pyrolysis of rice straw via RFPT-N system at reaction time of 45 min applying various PWL.

PWL HV,T W0(kcal) EST(kcal) ETP(kcal) RE,all RE,TP

357 W 1.27 0.08 0.64 0.66 0.69

482 W 1.31 0.1 0.63 0.68 0.71

574 W 1.33 0.12 0.46 0.74 0.8

664 W 1.31 0.13 0.37 0.77 0.83

HT,V: Specific heating value with unit in kcal kg1.

W0: Mass of sample, = 0.3  103kg.

HV,T W0of original rice straw: 1.21 kcal.

EST: Energy consumed in temperature-rising heating period of tST.

ETP: Energy consumed at constant-temperature heating at TPto reach steady state

of solid residues.

RE,all: Energy recovery ratio of the entire process including temperature-rising and

constant-temperature heating periods as calculated using Eq.(15).

RE,TP: Energy recovery ratio of the process considering constant-temperature

heating at TPas computed using Eq.(16).

A Frequency factor, s1

BET Brunauer–Emmett–Teller dP Diameter of sample, mm

Ea Activation energy, kJ mol1

EST Energy consumed in temperature-rising heating period of

tST, kcal

ETP Energy consumed at constant-temperature heating at TPto

reach steady state of solid residues, kcal HV Specific heating value, kcal kg1

HV,G HVof gas products, kcal kg1

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HV,T Specific total heating value of solid and gas products, =

HV,S Mf+ HV,Gx (1- Mf), kcal kg1

HR Heating rate, K min1

k Reaction rate constant, = A exp(Ea/RGT), s1

M Residual mass fraction defined in Eq.(1), = W/W0, –

Mf Final value of M at 45 min, –

MSS Value of M at tSS, –

MST Value of M at TSTor tST, –

MSTF Residual mass fraction after tSTbased on WSTF, = (W 

Wf)/WSTF, –

NMHCs Non-methane hydrocarbons n Reaction order, –

P Final pressure, Torr P0 Initial pressure, Torr

PWI Input power, W

PWL Loading power, = PWI- PWR, W

PWR Reflected power, W

Q Flow rate of carrier gas, mL min1

R2 _{Coefficient of determination, = 1  [}P_(y

e yp)2/P(ye

ye,Average)2], –

RCG Recovery of gas products estimated using Eq.(6), wt.%

RE,all Energy recovery ratio of the entire process including

temperature-rising and constant-temperature heating periods as calculated using Eq.(15),–

RE,TP Energy recovery ratio of the process considering

constant-temperature heating at TPas computed using

Eq.(16), –

RG Universal gas constant, 0.008314 kJ K1mol1

RF Radio frequency

RFPT RF Plasma pyrolysis (thermolysis) RFPT-N Modified RFPT

ravg Average reaction rate during tSTto tSSas calculated using

Eq.(5), wt.% min1

TP Plateau temperature, K

Tr Reaction temperature, K

TST Setting temperature, = T0+ 0.95 (TP T0), K

T0 Initial or room temperature, K

THCs Total hydrocarbons tH Heating time, min

tSS Heating time to reach steady state, min

tST Heating time to reach TST, min

Vi Volume fraction of component i in the accumulated gas

products, vol.%

W Present mass of sample, mg W0 Initial mass of sample, mg

Wa Allowable loading capacity of the reactor, g

Wf Final mass of sample after 45 min reaction, mg

WG,Acc Accumulated amount of gas products, mg

WG,Acc,f Values of WG,Accat 45 min, mg

WH2O Moisture content of sample, wt.%

WST Mass of sample at TSTor tST, mg

WSTF Total amount of mass pyrolyzed after tST, = WST Wf, mg

YW,i Yield of component i in the accumulated gas products

with respect to the original dry solid sample, wt.% ye Value of experimental data

yp Predicted value

g

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