Energy and exergy analysis of rice husk high-temperature pyrolysis

Energy and exergy analysis of rice husk high-temperature pyrolysis

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 0 Available online at www.sciencedirect.com ScienceDi...

781KB Sizes 2 Downloads 34 Views

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 0

Available online at www.sciencedirect.com

ScienceDirect journal homepage: www.elsevier.com/locate/he

Energy and exergy analysis of rice husk high-temperature pyrolysis Xinyu Wang a,b, Wei Lv a,*, Li Guo b, Ming Zhai b,**, Peng Dong b,***, Guoli Qi c a

School of Mechanical Engineering, Harbin University of Science and Technology, Harbin 150080, Heilongjiang, China b School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, Heilongjiang, China c China Special Equipment Inspection and Research Institute, Beijing 100029, China

article info

abstract

Article history:

Based on a two-stage fixed bed high-temperature pyrolysis system, the influence of tem-

Received 6 June 2016

perature (800e1200  C) on total energy and exergy, as well as exergy and energy efficiency

Received in revised form

of unreacted carbon, tar and pyrolysis gas were analyzed. The results indicate energy and

11 August 2016

exergy of each component in the pyrolysis gas as well as the total energy and exergy in-

Accepted 21 September 2016

crease with temperature. Energy value and exergy value contributions of the components

Available online xxx

in the pyrolysis gas at 800  C and 900  C are CO > CH4 > H2 > CO2. From 1000  C to 1200  C,

Keywords:

pyrolysis gas are in the range of 64.57e72.68% and 52.93e60.64%, respectively. The

Rice husk

increasing rate of energy value and exergy value of the pyrolysis gas reaches maximum at

High-temperature pyrolysis

1000  C. The energy efficiency and exergy efficiency of unreacted carbon and tar decrease

Exergy analysis

with temperature. The consumption of energy for tar collection and loss of energy and

Energy analysis

exergy carried by tar can be reduced by increasing temperature. The loss exergy efficiency

the rank turns into CH4 > CO > H2 > CO2. The energy efficiency and exergy efficiency of

increases slightly below 900  C and decreases from 38.8% to 34.6% above 900  C. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction As a clean and renewable energy, biomass energy efficiently alleviates the global warming incurred due to constant decrease of overuse of fossil fuel and many other environmental problems [1,2]. Furthermore, biomass obviously decreases the emission of NOx, SOx [3]. Biomass energy includes wood and forest industry wastes, agriculture wastes, water plants, oil plants, municipal refuse, industrial waste and so on [4].

Biomass conversion relies primarily on two methods: biochemical conversion and thermochemical conversion. Thermochemical conversion technology contains technologies of direct combustion, gasification, pyrolysis and liquefaction [5]. Pyrolysis is a complicated thermochemical decomposition process in which biomass organics are heated under the hypoxic condition and decompose into carbon solid and volatile matters [6]. The solid residue of pyrolysis is called bio-char. The condensable part of the volatile matter is bio-oil or tar while the non-condensible part is pyrolysis gas. Bio-oil can be stored for

* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (W. Lv), [email protected] (M. Zhai), [email protected] (P. Dong). http://dx.doi.org/10.1016/j.ijhydene.2016.09.155 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Wang X, et al., Energy and exergy analysis of rice husk high-temperature pyrolysis, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.155

2

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 0

Nomenclature En Ex Cp T P h s xi ex

energy based on 1 kg of biomass, kJ exergy based on 1 kg of biomass, kJ constant pressure specific heat capacity, kJ/ kmol K temperature,  C pressure, Pa specific enthalpy, kJ/kmol specific entropy, kJ/kmol mole fraction of ith species standard specific exergy, kJ/kmol

Greek letters b correlation factor h energy efficiency or percentage j exergy efficiency or percentage Superscripts ki kinetic po potential ph physical ch chemical Subscripts 0 ambient condition heat related to heat T related to temperature gas related to gases tar related to tar uc related to unreacted carbon biomass related to biomass loss related to the loss Abbreviations LHV low heating value, MJ/kg HHV high heating value, MJ/kg

further energy production. Biomass pyrolysis produces pyrolysis gas of the primary decomposition and secondary decomposition of bio-oil or tar. It is evident that biomass pyrolysis has more products than the other thermochemical conversion processes [7,8]. The thermomechanical analysis involves energy analysis and exergy analysis. Energy and exergy analysis are the two essential tools to evaluate energy. In particular, exergy analysis has already been proved as a robust tool to assess and enhance the thermochemical conversion [9,10]. Energy analysis is based on the first law of thermodynamics and the perspective of energy balance and puts thermal efficiency as the basic index of energy evaluation. However, energy is not only about volume, but it also has a discrepancy of quality. Thus, thermal efficiency can only reflex the amount of energy used, instead of an all-around energy utilization status. Exergy analysis, based on the first and the second law of thermodynamics, reflects the theoretical maximum performance of energy. It emphasizes on energy amount and quality at the same time, thus provides a reasonable, scientific and efficient method for energy utilization, and overcomes the limitation of conventional energy analysis and describes efficiency and performance of energy utilization [11].

In the field of pyrolysis, literature are achieved. Broido et al. [12,13] raised three pyrolysis reaction models of cellulose under low-pressure condition. Lin et al. [14] indicated that cellulose converted into left-handed glucan and dehydrated cellulose through pyrolysis, and the dehydrated cellulose eventually turns into carbon and light gas after a series of reactions. Sun et al. [15] pointed out that the component of biomass pyrolysis gas includes CO, CO2, CH4, H2, etc. Zhao et al. [16] pointed out that volatile matter was mostly released in 6 min and the effect of the increase in reaction time on the depth of rice husk pyrolysis is negligible. Dufour et al. [17] and Liu et al. [18] indicated that the evolution of gas composition during pyrolysis is similar. Peters et al. [19,20] using Aspen Plus software conducted exergy analysis on the process of rapid pyrolysis and biomass fuel synthesis with catalytic hydrogenation, and increased the exergy efficiency of the whole system by the improvement. Currently, most studies concentrate on pyrolysis reaction kinetics, energy and exergy analysis for biomass gasification, but few on energy and exergy analysis for biomass pyrolysis. From a perspective of energy and exergy, influence of pyrolysis temperature on the pyrolysis of rice husk (the rice husk is from a farm in Heilongjiang province. In 2015, the rice production in China was 2.08  108 t, and Heilongjiang is China's largest rice-growing area.) in a two-stage fixed bed reactor, the energy value and exergy value of the pyrolysis gas, the total energy value and exergy value of the pyrolysis gas, the energy efficiency and exergy efficiency of pyrolysis gas and unreacted carbon, as well as the energy value and exergy value of tar are investigated. The energy and exergy analysis are useful to show the energy and exergy utilization and balance during pyrolysis, which brings convenience to the energy and exergy management and effective utilization of scale-up pyrolysis systems. The results of the paper can be used to optimize operating conditions and improve the energy and exergy efficiency of the two-stage fixed bed pyrolysis system, as well as to develop models for the two-stage fixed bed pyrolysis system.

Material and methods Material The rice husks were dried, and the size of rice husk required no treatment. Proximate and ultimate analyses are shown Table 1.

Experimental setup A two-stage fixed bed biomass high-temperature pyrolysis system is used for the study, including a primary burner, a secondary burner, a fixed bed reactor, a high-temperature cracking reactor and a tar condenser. Fig. 1 presents the scheme of high-temperature pyrolysis system. Propane is used as fuel for the burners. The outer wall of the burners is made of stainless steel, and the inner wall is welded with stainless steel mesh. Refractory concrete and aggregate are used for interlining. The burners are wrapped by heat insulating material to prevent damage by flame and heat

Please cite this article in press as: Wang X, et al., Energy and exergy analysis of rice husk high-temperature pyrolysis, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.155

3

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 0

loss so that different temperatures for rice husk hightemperature decomposition are realized. Propane and air are consumed by way of swirling combustion and tangential feeding. The high-temperature cracking reactor is for secondary cracking of the condensable gas (tar). The length of the high-temperature cracking reactors is 0.5 m ~ 2 m with a diameter of 448 ~ 4219. The outer wall of the condenser is made of stainless steel.

Experimental procedures and measurements The amount of feedstock was 1.5e2 kg dry rice husk in its original size. Initially, valve 10 was open, and valve 9 was closed. Both of the burners started, and the oxygen content of the flue gas was measured by the flue gas analyzer. The proportion of propane and air was adjusted to ensure O2 was less than 0.2%. As soon as the temperatures at the exit of the burners were stable, valve 9 was opened, and valve 10 was closed. Thus, the flue gas from the primary burner entered the fixed bed reactor for rice husk pyrolysis. At the exit of the primary burner, the temperature was controlled from 400 to 1200  C. When the temperatures in the fixed bed reactor became constant, the pyrolysis at the certain layer finished. After that, volatile matter and flue gas entered into the hightemperature cracking reactor for secondary thermal cracking. At the exit of the secondary burner, the temperature

Table 2 e Influence of temperature on pyrolysis products. Products

Mass fraction at different temperatures%

Rice husk char Pyrolysis gas Tar

800  C

900  C

1000  C

1100  C

1200  C

19.00 79.50 1.50

18.00 80.60 1.40

18.00 81.20 0.80

18.00 81.90 0.10

18.00 82.00 0.02

was controlled from 400 to 1600  C. The temperature at the exit of the secondary burner was set higher than that at the exit of the fixed bed reactor. Product gas exhausted from the high-temperature cracking reactor and entered the condenser to 20e30  C. The operating parameters of rice husk pyrolysis are shown in Table A1 of the Appendix. The condensed gas (tar) flew into the deposition tank and condensed in the condensing tube. The liquid product was dissolved in acetone and collected. Recognizable compounds in the tar at 800  C are shown in Table A2 of the Appendix. The mass fraction of the products is provided in Table 2. Testo350 flue gas analyzer is used to measure the primary components of the flue gas at the exit of the burners. The content of CO2 in pyrolysis gas is a calculated value which excludes the CO2 generated by combustion. The components of gaseous products are shown in Table 3. The primary components of CmHn are C2H2 and C2H4, and the amount of CmHn

Table 1 e The proximate and ultimate analysis of rice husk. Volatileda (%) 69.20 a b

Ashd (%)

Fixed carbond (%)

Cdafb (%)

Hdaf (%)

Odaf (%)

Ndaf (%)

Sdaf (%)

Qnet,d (kJ/kg)

15.90

14.90

46.18

6.08

45.02

2.62

0.10

14,556

d: dry basis. daf: dry, ash free basis.

Fig. 1 e Scheme of rice husk pyrolysis system. Please cite this article in press as: Wang X, et al., Energy and exergy analysis of rice husk high-temperature pyrolysis, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.155

4

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 0

Table 3 e Influence of reaction temperature on components of gaseous products. Gaseous products

Volume fraction at different temperatures % 800  C

900  C

1000  C

1100  C

1200  C

21.5 13.4 46.1 18.9 0.1

23.1 14.2 44.0 18.3 0.4

24.8 15.4 42.7 16.6 0.5

25.2 16.8 41.7 15.5 0.8

25.5 17.6 40.5 15.2 1.2

H2 CH4 CO CO2 CmHn

hgas;T ¼

Theoretical methods Energy analysis The total energy value equals to the sum of all kinds of energy values [21]: (1) ki

where En is the total energy; En is the kinetic energy of pyrolysis gas; Enpo is the potential energy of pyrolysis gas; Enph is the physical energy of pyrolysis gas; Ench is the chemical energy of pyrolysis gas. Kinetic energy and potential energy of pyrolysis gas are slight enough to be neglected. Hence, the equation can be simplified as: En ¼ Enph þ Ench

(2)

Physical energy of pyrolysis gas is: Enph ¼

X

X

huc;T ¼

Enuc;T þ Ench uc;T  100% Enbiomass þ Enheat;T

htar;T ¼

Entar;T þ Ench tar;T  100% Enbiomass þ Enheat;T

(3)

ni HHV

(4)

where ni is the mole number of flue gas, HHV stands for the higher heating value of the gas component, kJ/kmol. Higher heating value and specific enthalpy under standard condition (h0) of some gases are shown in Table 4. For biomass, Equation (2) is denoted as: En ¼ mHHV

(5)

Higher heating value of biomass is calculated by the equation as follows: HHV ¼ LHV þ 21:978H

(7)

(6)

Gas

h0 (kJ/kmol)

s0 (kJ kmo1/K)

HHV (kJ/kmol)

H2 CO CO2 CH4

8468 8669 9364 e

130.574 197.543 213.685 186.16

285,840 282,990 e 890,360

(9)

where, hgas,T, huc,T and htar,T are energy efficiencies of pyrolysis gas, unreacted carbon and tar at T, respectively; EnH2 ;T , EnCH4 ;T , EnCO;T and EnCO2 ;T are energy flow rate of H2, CH4, CO and CO2 at ph T, respectively; Enuc;T and Ench uc;T are unreacted carbon energy ph flow rate of physics and chemistry at T, respectively; Entar;T and ch Entar;T are tar energy flow rate of physics and chemistry at T, respectively; Enbiomass is the total energy value of biomass; Enheat,T is the total energy value of flue gas at T.

Exergy balance Exergy does not obey conservation law due to the unavoidable irreversibility of reaction processes. Under the assumptions: (1) pyrolysis is a control-volume unit; (2) input exergy is from biomass and heat; (3) output exergy is from product gas, tar, and unreacted carbon; (4) the irreversibility (internal exergy loss) is in the lost part, the exergy balance equation is established: (10)

where, Exbiomass,T is the total exergy value of biomass; Exheat,T is the total exergy value of flue gas at T. Exgas,T, Exuc,T and Extar,T are exergy flow rates of pyrolysis gas, unreacted carbon and tar at T, respectively; Exloss,T denotes the exergy loss of pyrolysis at T. The total exergy value equals to the sum of all kinds of exergy values [22]. Ex ¼ Exki þ Expo þ Exph þ Exch

(11)

where, Exki, Expo, Exph and Exch denote the exergy flow rate of kinetic energy, potential energy, physical energy and chemical energy, respectively. Neglecting the kinetic energy exergy (mV2/2) and potential energy exergy (mgZ), the exergy flow rate of gas can be simplified as Ex ¼ Exph þ Exch

(12)

Definition of exergy flow rate of gas is: Exph ¼ n½ðh  h0 Þ  Tðs  s0 Þ

Table 4 e Specific enthalpy, entropy [18].

(8)

ph

Exheat;T þ Exbiomass ¼ Exgas;T þ Extar;T þ Exuc;T þ Exloss;T

ni hi

where, ni is the mole number of flue gas; hi is the specific enthalpy value, kJ kmol1. Chemical energy of pyrolysis gas is: Ench ¼

EnH2 ;T þ EnCH4 ;T þ EnCO;T þ EnCO2 ;T  100% Enbiomass þ Enheat;T ph

is very tiny. Therefore, the effects of CmHn in the following analysis are neglected.

En ¼ Enki þ Enpo þ Enph þ Ench

where HHV and LHV are the higher heating value and lower heating value of biomass, respectively (MJ/kg), H is the proportion of heat quantity in the elemental analysis of fuel (%). The energy efficiencies of pyrolysis gas, unreacted carbon, and tar are:

(13)

where, n is the mole flow rate of gas, h and s are the specific enthalpy and specific entropy of gas under working condition, h0 and s0 are the specific enthalpy and specific entropy of gas under environment condition. Difference of specific enthalpy (h  h0) and difference of specific entropy (s  s0) are denoted as:

Please cite this article in press as: Wang X, et al., Energy and exergy analysis of rice husk high-temperature pyrolysis, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.155

5

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 0

ZT h  h0 ¼

CpdT

(14)

T0

ZT s  s0 ¼ T0

Cp P dT  Rln T P0

(15)

where, R is the general gas constant; Cp is the specific heat capacity at constant pressure, and the empirical equation is Cp ¼ a þ bT þ cT2 þ dT3, where a ~ d is the specific heat capacity at constant pressure coefficient, which is shown in Table 5. The chemical exergy flow rate of gas Exch is [23]: Exch ¼ n

X   xi exch i þ RT0 ln gi xi

Gas

Exch/kJ kmol1

H2 CO CO2 CH4 C2H4

236,100 275,100 19,870 831,650 1,317,680

ph

juc;T ¼

Exuc;T þ Exch uc;T  100% Exbiomass þ Exheat;T

jtar;T ¼

Extar;T þ Exch tar;T  100% Exbiomass þ Exheat;T

(16)

where, xi is the mole fraction of the i-th component in the pyrolysis gas; gi, the activity coefficient, is unity for ideal solutions; exch i is the standard chemical exergy of i-th component in pyrolysis gas, which is shown in Table 6. The physical exergy of reacted carbon can be ignored. The chemical exergy of reacted carbon is 410,260 kJ kmol1 [24], and the exergy value of unreacted carbon during pyrolysis is calculated by: Exuc ¼ 34188:33mεuc

(17)

where, Exuc is the exergy value of unreacted carbon, m is the mass of biomass, εuc is the residual ratio of unreacted carbon. Exergy value of biomass is calculated by using statistical correlation. Ex ¼ bmLHV

(18)

where, LHV is the lower heat value of biomass, MJ/kg, b is the correlation factor, which is calculated by Equation (19) [25,26].



Table 6 e Standard chemical exergy of some gases at 25  C, 0.1 MPa.

  1:044 þ 0:016 HC  0:3493 OC 1 þ 0:0531 HC þ 0:0493 NC (19)

1  0:4124 OC

where, the mass fraction of C, H, O and N (%) are in the ultimate analysis of rice husk.

(21)

ph

  jloss;T ¼ 100%  jgas;T þ juc;T þ jtar;T

(22)

(23)

where, jgas,T, juc,T and jtar,T are exergy efficiencies of pyrolysis gas, unreacted carbon and tar at T, respectively; ExH2 ;T , ExCH4 ;T , ExCO;T and ExCO2 ;T are exergy flow rate of H2, CH4, CO and CO2 at ph T, respectively; Exuc;T and Exch uc;T are unreacted carbon exergy ph flow rate of physics and chemistry at T, respectively; Extar;T and ch Extar;T are tar exergy flow rate of physics and chemistry at T, respectively; jloss,T represents the loss exergy efficiency at T. Exgas,T, Exuc,T and Extar,T are exergy flow rates of pyrolysis gas, unreacted carbon and tar at T, respectively.

Results and discussion All the energy flows of pyrolysis products are shown in Fig. 2. The energy value of each component in the pyrolysis gas increases with temperature, so the energy value of pyrolysis gas increases. The energy value of CH4 reaches a maximum at 1200  C. The energy value of unreacted carbon remains unchanged. The energy value of tar decreases with temperature due to the tar cracking at high temperatures. Table 7 presents the energy and exergy analysis of the twostage fixed bed rice husk high-temperature pyrolysis system.

Exergy efficiency The exergy efficiencies of pyrolysis gas, unreacted carbon and tar are the ratio of the exergy value to the total exergy value input into the pyrolysis. The definition equations are [27]: jgas;T ¼

ExH2 ;T þ ExCH4 ;T þ ExCO;T þ ExCO2 ;T  100% Exbiomass þ Exheat;T

(20)

Table 5 e Coefficients of constant pressure specific heat capacity of some gases [20]. Gas

a

b

c

d

Temperature

H2 CO CO2 CH4

29.11 28.16 22.26 19.89

0.192 0.168 5.981 5.024

0.4 0.533 3.501 1.269

0.87 2.222 7.469 11.01

273e1800 273e1800 273e1800 273e1800

Fig. 2 e The energy flows of pyrolysis products.

Please cite this article in press as: Wang X, et al., Energy and exergy analysis of rice husk high-temperature pyrolysis, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.155

6

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 0

Influence of pyrolysis temperature on total energy and exergy value of pyrolysis gas

Table 7 e Calculated process data for the hightemperature pyrolysis system. State no. Pyrolysis m (kg/kg) T ( C) h (kJ/kg) Ex (kJ/kg) products 1 2 3 4 5 6 7 8 9 10 11 12

Gas Gas Gas Gas Gas Unreacted carbon Unreacted carbon Tar Tar Tar Tar Tar

0.795 0.806 0.812 0.819 0.83 0.19

800 900 1000 1100 1200 700

11 495.47 12,385.03 13,602.59 14,676.45 15,309.91 1343.67

10,337.89 11,079.15 12,114.86 13,038.32 13,569.92 1401.72

0.18

800

1015.95

1059.84

0.15 0.14 0.08 0.01 0.002

800 900 1000 1100 1200

256.51 239.46 136.87 17.14 3.42

251.63 234.85 134.20 16.77 3.35

Temperature plays an active role in the composition of the pyrolysis products [29], and the energy and exergy values are determined by the temperature and yield of the pyrolysis gas. Although different gases have a different contribution to energy value and exergy value of pyrolysis gas, energy value and exergy value of pyrolysis have a good synchronism. Energy value and exergy value of pyrolysis gas at different temperatures are shown in Fig. 3. The energy value increases from 11,495 kJ kg1 at 800  C to 15,309 kJ kg1 at 1200  C. The exergy value rises from 10338 kJ kg1 at 800  C to 13,570 kJ kg1 at 1200  C. Both energy value and exergy value of various components in the pyrolysis gas increase with temperature, and hence the total energy and exergy value increase. From 800  C to 1200  C, the increasing rate of energy value and exergy value reaches a maximum at 1000  C and then decreases above 1000  C. It is because the mass of unreacted carbon remains unchanged, but the tar decomposes into small molecular gas and such reaction dominates the whole process. Along with the rise of temperature, the content of tar decreases but the increasing rate decreases.

The mass flow rate (kg/h), temperature ( C), specific enthalpy (kJ/kg) and specific exergy (kJ/kg) are determined for the each state of the system.

Influence of pyrolysis temperature on the energy value and exergy value of pyrolysis gas

Influence of temperature on energy efficiency and exergy efficiency of pyrolysis gas

Table 8 shows the energy value and exergy value of each component in the pyrolysis gas from 800  C to 1200  C. All the exergy values are less than energy values, which is in compliance with the definition of exergy. These values are determined by the temperature and yield of the gas component. Equations (3) and (13) show that the increase of enthalpy and yield results in the increase of physical energy and exergy of the pyrolysis gas. Besides, Equations (4) and (16) show that the increase of yield leads to the increase of chemical energy and exergy of the pyrolysis gas. The energy value and exergy value of each component in the pyrolysis gas increase with temperature, among which the increases of CH4 and H2 are most obvious because the tar decomposes into CH4 and H2 at high temperatures. The increase of energy and exergy value of CO is because methane reforms at high temperatures with CO production. Energy value and exergy value of CO2 remain unchanged due to the physical energy and exergy of CO2 increase with temperature, but the amount of CO2 decreases [28]. Energy value and exergy value contributions of pyrolysis gases at 800  C and 900  C are in the rank of CO > CH4 > H2 > CO2. From 1000  C to 1200  C, the rank turns into CH4 > CO > H2 > CO2.

Fig. 4 shows the energy efficiency and exergy efficiency of pyrolysis gas at different pyrolysis temperatures. Energy efficiency is higher than exergy efficiency, and energy efficiency and exergy efficiency of pyrolysis gas are nearly synchronized. From 800  C to 900  C, the energy efficiency and exergy efficiency of pyrolysis gas increase from 64.57%, 52.93% to 66.52%, 54.81%. With the rise of temperature, unreacted carbon converts to gas, thus the energy value and exergy value of pyrolysis gas increase. From 900  C to 1100  C, the energy efficiency and exergy efficiency of pyrolysis gas rise from 66.52%, 54.81% to 72.48%, 60.24%, respectively. This trend is mainly determined by the energy and exergy values of the pyrolysis gas. At high temperature, tar decomposes into H2 and CH4, hence the energy value and exergy value increase. From 1100  C to 1200  C, energy efficiency and exergy efficiency of pyrolysis gas increase from 72.48%, 60.24% to 72.68% and 60.64%. It is because the tar seldom decomposes, and the amount of energy and exergy is small, which exerts slight influence on efficiency. Therefore, the energy efficiency and exergy efficiency of pyrolysis gas remain unchanged in general. The exergy efficiency of

Table 8 e Energy value and exergy value of various gases at different temperatures. Gases

Energy and exergy at different temperatures kJ/kg 

900  C

800 C

H2 CH4 CO CO2

1000  C

1100  C

1200  C

En

Ex

En

Ex

En

Ex

En

Ex

En

Ex

2223.22 4245.17 4756.21 270.87

1782.01 3854.48 4438.55 262.86

2521.55 4750.57 4798.43 314.48

2018.86 4300.91 4464.96 294.53

2867.57 5459.00 4938.46 337.56

2293.80 4929.59 4583.34 308.25

3045.06 6223.49 5044.76 363.15

2434.08 5607.50 4671.24 325.63

3168.46 6700.68 5041.24 399.53

2531.46 6026.43 4658.63 353.52

Please cite this article in press as: Wang X, et al., Energy and exergy analysis of rice husk high-temperature pyrolysis, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.155

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 0

Fig. 3 e Energy value and exergy value of pyrolysis gas at different temperatures. pyrolysis gas varies in the range of 52.93e60.64%. There are two reasons for the high exergy efficiencies. Firstly, secondary thermal cracking of the tar occurs at high temperatures, and most of the products are gaseous products, so almost all the exergy is contributed by gas. Secondly, the heat loss is small due to the heat accumulated reactor, so exergy efficiencies are high. Since there is few reference about the exergy efficiency of biomass pyrolysis, the exergy efficiency of pyrolysis gas is compared with that of product gas of steam gasification. In the study of Zhang [30], the exergy efficiency of the product gas of steam gasification varies in the range of 49.31e58.48%, which is smaller than that of pyrolysis gas due to higher exergy input for steam gasification.

Influence of pyrolysis temperature on energy efficiency and exergy efficiency of unreacted carbon

7

Fig. 5 e Energy and exergy efficiency of unreacted carbon at different temperatures. and5.24%at900  C.Itiscausedbytheincreaseofmethane[31]and constantprecipitationofvolatilematters.Theenergyefficiencyand exergyefficiencydecreaseto4.83%and4.74%at1200 C,whichpresentsalineartrend. Although the temperature in the high-temperature cracking reactor is from 900  C to 1200  C, the temperature at the exit of the fixed bed reactor is almost constant. Therefore, the content of unreacted carbon keeps almost constant, while the total energy increases with temperature due to the increase of input heat. Furthermore, the decreasing rate of energy efficiency is a little smaller than that of exergy efficiency, which indicates that the increasing rate of total energy efficiency is higher than that of exergy efficiency.

Influence of pyrolysis temperature on the exergy efficiency of tar

Fig.5showstheenergyandexergyefficiencyofunreactedcarbonat differenttemperatures.Theenergyefficiencyandexergyefficiency ofunreactedcarbondecreasefrom7.55%and7.18%at800 Cto5.46%

Fig. 6 shows the energy and exergy efficiency of tar at different temperatures. The energy efficiency of tar produced decreases

Fig. 4 e Energy value and exergy efficiency of pyrolysis gas at different temperatures.

Fig. 6 e Energy and exergy efficiency of tar at different temperatures.

Please cite this article in press as: Wang X, et al., Energy and exergy analysis of rice husk high-temperature pyrolysis, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.155

8

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 0

efficiencies of tar. Therefore, tar content can be reduced by increasing pyrolysis temperature, which will contribute to the increase of product gas and the exergy efficiency and the decrease the loss exergy efficiency. However, a higher temperature will lead to increased energy consumption and device requirement; it should be optimized.

Conclusions Based on a two-stage fixed bed rice husk pyrolysis system, the influence of temperature on pyrolysis gas composition, total energy and exergy value, as well as exergy and energy efficiency of unreacted carbon, tar, and pyrolysis gas are analyzed. The conclusions, as follows, are reached.

Fig. 7 e The loss exergy efficiency for the different operation conditions.

from 1.44% at 800  C to 0.02% at 1200  C, exergy efficiency of tar produced decrease from 1.28% to 0.01%. This trend is mainly determined by the variation of the tar energy and exergy values. Exergy efficiency is lower than energy efficiency, and exergy efficiency and energy efficiency are nearly synchronized. The main reason is high-temperature drives decomposition of tar so that energy efficiency and exergy efficiency of tar decrease. From 1100  C to 1200  C, energy efficiency is mostly equal to exergy efficiency and accounts for less than 0.1% of the total efficiency. In the experiment, the productivity of energy and exergy carried by tar above 1100  C reaches a mg level, which indicates a significant effect of temperature on the decomposition of condensable liquid products [32]. The tar content of biomass gas is acceptable without post processing. Thus, the increase in temperature will decrease the energy and exergy of the tar and reduce its energy and exergy efficiency, and largely increase the energy and exergy of pyrolysis gas as well as the productivity of the pyrolysis gas. Therefore, the consumption of energy during tar collection and loss of energy and exergy carried by tar can be reduced by increasing temperature.

Influence of pyrolysis temperature on the loss exergy efficiency As shown in Fig. 7, the loss exergy efficiency increases slightly from 38.6% at 800  C to 38.8% at 900  C. From 900  C to 1200  C, the loss exergy efficiency of pyrolysis decreases from 38.8% to 34.6%. According to Equation (23), the loss exergy efficiency is determined by pyrolysis gas, unreacted carbon, and tar. The loss exergy efficiency decreases with temperature due to a higher amount of methane and H2/CO ratio which provides higher exergy output, and the exergy increasing rate of methane is higher than that of the sum of H2 and CO. The exergy values of pyrolysis gas increase, and the exergy values of tar decrease, which corresponds to the increase of exergy efficiencies of pyrolysis gas, and the decrease of exergy

(1) Energy value and exergy value contributions of the components in the pyrolysis gas at 800  C and 900  C are CO > CH4 > H2 > CO2. From 1000  C to 1200  C, the rank turns into CH4 > CO > H2 > CO2. (2) The energy efficiency and exergy efficiency of pyrolysis gas are in the range of 64.57e72.68% and 52.93e60.64%, respectively, and the increasing rates of energy and exergy of pyrolysis gas reach a maximum at 1000  C. (3) The energy efficiency and exergy efficiency of unreacted carbon and tar decrease with temperature, and the consumption of energy for tar collection and loss of energy and exergy carried by tar can be reduced by increasing temperature. (4) The loss exergy efficiency increases slightly below 900  C, and from 900  C to 1200  C it decreases from 38.8% to 34.6%.

Acknowledgment ThisworkwassupportedbyNationalNaturalScienceFoundationof China(GrantNo.:51206032),ChinaPostdoctoralScienceFoundation (GrantNo.:2013M531037)andHeilongjiangPostdoctoralFinancial Assistance(GrantNo.:LBH-Z12101).

Appendix

Table A1 e Pyrolysis operating parameters Pyrolysis operating parameters Feeding amount Particle size Temperature at the outlet of the primary burner Temperature at the outlet of the secondary burner Temperature in the high-temperature cracking reactor Temperature at the outlet of the condenser Retention time Temperature in the fixed bed reactor

1.5e2.0 kg 2e10 mm 400e1200  C 400e1600  C 500e1400  C 20e30  C 0.5e4s 500e800  C

Please cite this article in press as: Wang X, et al., Energy and exergy analysis of rice husk high-temperature pyrolysis, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.155

9

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 0

Table A2 e Recognizable compounds in the tar at the temperature of 800  C No.

Retention time (min)

Name

Molecular formula

Molecular weight

Relative content (%)

9.391 11.022 11.259 12.189 12.525 12.711 12.754 13.105 13.541 13.942 14.736 15.18 15.337 16.060 16.253 16.947 17.055 18.278 18.357 18.872 19.981 20.239 21.269 23.080

Azulene Naphthalene, 2-methylNaphthalene, 1-methylBiphenyl Naphthalene, 1,5-dimethylNaphthalene, 1,6-dimethylNaphthalene, 2,7-dimethylBiphenylene 1-Isopropenylnaphthalene Dibenzofuran Fluorene Dibenzofuran, 4-methyl[1,10 -Biphenyl]-4-carboxaldehyde 9H-Fluorene, 1-methyl3H-Benz[e]indene, 2-methylPhenanthrene Anthracene Pentadecanoic acid, 14-methyl-, methyl ester 4H-Cyclopenta[def]phenanthrene 1,2,4,8-Tetramethylbicyclo[6.3.0]u ndeca-2,4-diene Fluoranthene Pyrene 1-Naphthalenamine, N-phenylCyclopenta[cd]pyrene

C10H8 C11H10 C11H10 C12H10 C12H12 C12H12 C12H12 C12H8 C13H12 C12H8O C13H10 C13H10O C13H10O C14H12 C14H12 C14H10 C14H10 C17H34O12 C15H10 C15H24 C16H10 C16H10 C16H13N C18H10

128 142 142 154 156 156 156 152 168 168 166 182 182 180 180 178 178 270 190 204 202 202 219 226

7.612 5.915 6.547 3.051 1.533 1.338 1.415 9.716 2.009 4.036 5.974 2.304 2.009 1.599 3.648 10.958 3.191 2.033 2.181 1.211 8.342 7.616 1.351 3.448

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

references

[1] Zhang Q, Chang J, Wang T, Xu Y. Review of biomass pyrolysis oil properties and upgrading research. Energy Convers Manag 2007;48:87e92. [2] Zhang B, Liu Q, Li H, Jin H. Performance simulation and analysis of a poly generation system with solar-biomass gasification. Proc CSEE 2015;35:112e8. [3] Quaak P, Knoef H, Stassen H. Energy from biomass: a review of combustion and gasification technologies. World Bank Publications; 1999. [4] Ma L, Wu C, Sun L. Biomass gasification technology and its application. Chemical Industry Press; 2011. [5] Basu P. Biomass gasification and pyrolysis: practical design and theory. Academic Press; 2010. [6] Demirbas A, Arin G. An overview of biomass pyrolysis. Energy sources 2002;24:471e82. [7] Di Blasi C. Modeling chemical and physical processes of wood and biomass pyrolysis. Prog Energy Combust Sci 2008;34:pp.47e90. [8] Sharma A, Pareek V, Zhang D. Biomass pyrolysisda review of modelling, process parameters and catalytic studies. Renew Sustain Energy Rev 2015;50:1081e96. [9] Ometto AR, Roma WNL. Atmospheric impacts of the life cycle emissions of fuel ethanol in Brazil: based on chemical exergy. J Clean Prod 2010;18:71e6. [10] Gourmelon S, Thery-Hetreux R, Floquet P, Baudouin O, Baudet P, Campagnolo L. Exergy analysis in ProSimPlus® simulation software: a focus on exergy efficiency evaluation. Comput Chem Eng 2015;79:91e112. [11] Dincer I. Technical, environmental and exergetic aspects of hydrogen energy systems. Int J Hydrogen Energy 2002;27(2002):265e85. [12] Bradbury AG, Sakai Y, Shafizadeh F. A kinetic model for pyrolysis of cellulose. J Appl Polym Sci 1979;23:3271e80. [13] Broido A, Nelson M. Char yield on pyrolysis of cellulose. Combust Flame 1975;24:263e8.

[14] Lin YC, Cho J, Tompsett GA, Westmoreland PR, Huber GW. Kinetics and mechanism of cellulose pyrolysis. J Phys Chem C 2009;113:20097e107. [15] Sun L, Zhang X. Biomass pyrolysis gasification principle and technology. Chemical Industry Press; 2013. [16] Zhao B, Zhang X, Sun L, Meng G, Chen L, Xiaolu Y. Hydrogen production from biomass combining pyrolysis and the secondary decomposition. Int J Hydrogen Energy 2010;35:2606e11. [17] Dufour A, Girods P, Masson E, Rogaume Y, Zoulalian A. Synthesis gas production by biomass pyrolysis: effect of reactor temperature on product distribution. Int J Hydrogen Energy 2009;34:1726e34. [18] Liu S, Zhu J, Chen M, Wenping X, Zhonglian Y, Lihong K. Hydrogen production via catalytic pyrolysis of biomass in a two-stage fixed bed reactor system. Int J Hydrogen Energy 2014;39:13128e35. [19] Peters JF, Petrakopoulou F, Dufour J. Exergy analysis of synthetic biofuel production via fast pyrolysis and hydroupgrading. Energy 2015;79:325e36. [20] Peters JF, Petrakopoulou F, Dufour J. Exergetic analysis of a fast pyrolysis process for bio-oil production. Fuel Process Technol 2014;119:245e55. [21] Zhang Y, Li B, Li H, Liu H. Thermodynamic evaluation of biomass gasification with air in autothermal gasifiers. Thermochim Acta 2011;519:65e71. [22] Al-Weshahi MA, Anderson A, Tian G. Exergy efficiency enhancement of MSF desalination by heat recovery from hot distillate water stages. Appl Therm Eng 2013;53:226e33. [23] Kotas T. The exergy method of thermal plant analysis. Elsevier; 2013.  lu M, editor. [24] Cengel YA, Boles MA. In: Kanog Thermodynamics: an engineering approach, vol. 5. New York: McGraw-Hill; 2002. [25] Ptasinski K, Prins M, Pierik A. Exergetic evaluation of biomass gasification. Energy 2007;32:568e74. [26] Szargut J, Morris DR, Steward FR. Exergy analysis of thermal, chemical, and metallurgical processes. 1987.

Please cite this article in press as: Wang X, et al., Energy and exergy analysis of rice husk high-temperature pyrolysis, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.155

10

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 0

[27] Parvez AM, Mujtaba IM, Wu T. Energy, exergy and environmental analyses of conventional, steam and CO2enhanced rice straw gasification. Energy 2016;94(2016):579e88. [28] Qi G. Experimental research and numerical simulation of biomass pyrolysis and tar thermal cracking. Harbin: Harbin Institute of Technology; 2010. [29] Zhai M, Wang X, Zhang Y, Dong P, Qi G. Characteristics of rice husk tar pyrolysis by external flue gas. Int J Hydrogen Energy 2015;40:10780e7.

[30] Zhang Y, Li B, Li H, Zhang B. Exergy analysis of biomass utilization via steam gasification and partial oxidation. Thermochim Acta 2012;538:21e8. [31] Zhang Y, Zhao Y, Li B, Sun S. Flow reactor to carry the influence of temperature on sawdust air gasificationeenergy analysis and exergy analysis. Acta Energiae Solaris Sin 2013;3:003. [32] Zhai M, Wang X, Zhang Y, Dong P, Qi G, Huang Y. Characteristics of rice husk tar secondary thermal cracking. Energy 2015;93:1321e7.

Please cite this article in press as: Wang X, et al., Energy and exergy analysis of rice husk high-temperature pyrolysis, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.155