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Applied Thermal Engineering 61 (2013) 123e127

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Applied Thermal Engineering

journal homepage: www.elsevier .com/locate/apthermeng

Optimization of multi-stage pyrolysis

Adetoyese Olajire Oyedun, Ka Leung Lam, Tesfaldet Gebreegziabher, Chi Wai Hui*

Department of Chemical and Biomolecular Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong

a r t i c l e i n f o

Article history:Received 23 November 2012Accepted 22 March 2013Available online 2 April 2013

Keywords:PyrolysisMulti-stage pyrolysisWaste-tireEnergyOptimization

* Corresponding author. Tel.: þ852 2358 7137; fax:E-mail addresses: keoyedun@ust.hk (A.O. Oyedun

(K.L. Lam), tgaa@ust.hk (T. Gebreegziabher), kehui@u

1359-4311/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.applthermaleng.2013.03.04

a b s t r a c t

Pyrolysis process is considered as a beneficial option in waste treatment largely due to the productsgenerated and the energy recovery when compared to other methods. In the conventional pyrolysisprocess, heat is continually supplied to the reactor until the final pyrolysis temperature is attained. Thereactor is then maintained isothermally at this temperature until the pyrolysis is completed. Thistechnique does not take into consideration the mechanism of the pyrolysis which involves bothexothermic and endothermic reaction and the opportunity of gaining some processing benefits is oftenignored. Multi-stage pyrolysis which is an approach to carry out pyrolysis with multiple heating stages inorder to gain certain processing benefits has been introduced in our earlier works. 22.5% energyreduction was achieved in our past work with a 100% increase in completion time. This work thereforeproposes the optimization of the operating parameters in multi-stage pyrolysis in order to limit theincrease in completion time and also reduces the overall energy. This innovative approach can achieve arange of 24.7%e37.9% reduction in energy usage with 37%e50% increase in completion time dependingon the heating rate for each heating stages. This approach has also been used for charcoal production.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

One of the thermochemical processes used inwaste treatment isthe pyrolysis process. This is largely due to the products generatedand the opportunity to recover energy when compared to othermethods. Pyrolysis process is an overall endothermic process whichrequires net energy input to complete the process [1]. Yang and Royfurther explained that pyrolysis involves series of both exothermicand endothermic processes [2]. During the pyrolysis process,organic matters are cracked down into smaller fractions and thisresults in the given out of energy (exothermic reactions) [2]. Asthe temperature increases, secondary products are formed as wellas volatile during the vaporization reaction (endothermic reaction)[2,3].

Conventional pyrolysis process does not take into considerationthe opportunity to gain certain processing benefits during theheating of the reactor resulting in excess amount of heat beingapplied into the process. This contributed to the limitation of py-rolysis as an energy intensive process [3]. Furthermore, duringpyrolysis, there exists significant heat barrier since pyrolysis is

þ852 2358 0054.), kaleung.lam@family.ust.hkst.hk (C.W. Hui).

All rights reserved.3

completed earlier at the surface of the particle and some heats arewasted to heat up the pyrolyzed part [4].

Multi-stage pyrolysis however, is a proposed operational strat-egy with multiple heating stages in order to gain certain processingbenefits [5]. Multi-stage pyrolysis approach can be applied to solvethe critical issue related to conventional pyrolysis. The concept ofmulti-stage approach in thermochemical methods is not entirelynew. Recently, Gómez-Barea et al. [6] proposed a 3-stage (multi-stage) gasification system to increase the conversion of char and tarin fluidized bed gasifiers. They declared their new process aspromising since it helped to increase the flexibility and capacity ofexisting staged gasification developments.

The feasibility of the multi-stage process as a method ofreducing the overall energy usage in pyrolysis has been demon-strated in previous modeling work [5] and also experimentally inour work on charcoal production via multi-stage pyrolysis [7]. Inwaste tire multi-stage pyrolysis [5], we proposed four stages: (1)heating (2) adiabatic (3) heating and (4) adiabatic. Only timeduration was used as the basis for switching between the differentstages and also same heating rate was applied for the two heatingstages. An energy reduction of 22.5% was achieved in earlier work[5] but with up to 100% increase in completion time.

The switching temperature between different stages is veryimportant in multi-stage pyrolysis and the extent of it can deter-mine both the completion time and the overall heat requirement.For example, in waste tire pyrolysis, if the temperature to start the

A.O. Oyedun et al. / Applied Thermal Engineering 61 (2013) 123e127124

first adiabatic stage is too low (<523 K), cracking may not beinitialized within the expected time and if the temperature is toohigh (>673 K), the exothermic heat may have not been fully uti-lized. Also as noted in Lam et al. [8], a higher temperature can alsopromotes secondary reactions which tends to increase the energyused during pyrolysis. Therefore it is important to optimize thetemperature to start each stage for full usage of heat and optimalprocess efficiency.

Heating rate for the different heating stages is anotherimportant consideration in multi-stage pyrolysis. This can affectthe efficiency of the process since feedstocks like waste tire ex-hibits different kinetics behaviors at different heating rates [9].The use of different heating rates for the second heating stage canhelp to reduce the amount of energy and time used duringpyrolysis.

The objective of this work therefore, is to optimize the operatingparameters in multi-stage pyrolysis so as to further reduce theoverall energy usage without drastically increasing the completiontime. Optimal temperature and time duration for the differentstages were determinedwhich result in good process efficiency andreduced overall energy consumption. The use of different heatingrates for the second heating stage was also considered and differentoptimization case studies were employed. The success of this workis expected to make multi-stage pyrolysis a new consideration as apyrolysis technique and improve the overall economic advantagesof slow pyrolysis processes.

2. Pyrolysis model

The pyrolysis kinetics and heat flow parameters were obtainedfrom the experimental results of the thermogravimetric and dif-ferential thermal analyses (TG/DTA). The parameters were thenused to build a pyrolysis model which described the mass loss andheat transfer phenomena of each feedstock pyrolysis.

The details of the kinetics framework for the modeling of thepyrolysis process have been explained extensively in previous work[5]. The waste-tire pyrolysis model was based on the pyrolysismechanism proposed by Senneca et al. [10]. In recent work [11], theheating trend of the pyrolysis progress was modeled to use thedynamic heating approach which makes the heat of reactions andthe kinetics governed by the local heating rates at each time steplayer of the pyrolysis process. To avoid a detailed repetition, asummary of the kinetics and heat transfer model are representedby Eqs. (1)e(17). MATLAB� platform with the use of finite differ-ence method (FDM) was used to build the kinetic model and theheat equation, Eq. (11) was solved using the CrankeNicolson im-plicit scheme numerical method [12].

The pyrolysis kinetic is governed by:

daidt

¼ Aiexp�� EiRT

�ð1� aiÞni (1)

dgjdt

¼ Ajexp�� EjRT

��1� gj

�nj(2)

aT ¼Xi

uiai (3)

ui ¼ aib2 þ bibþ ci (4)

The heat flow kinetics is given as;

qr ¼ �4p2lvTvr

(5)

qrþDr ¼ qr þ vqrvr

Dr (6)

The overall heat of mass loss reaction is given by

qmlr ¼ 4pr2rDrXi

�Hivaivt

wi

�(7)

and the overall heat of exothermic reaction is given by

qexo ¼ 4pr2rDrXj

�Hjvgivt

�(8)

The sensible heat (qs) in each layer is

qs ¼ 4pr2rDr CpvTvt

(9)

The overall heat balance in the layer with radius r is

qs ¼ qr � qrþDr þ qmlr þ qexo (10)

The energy conservation equation is therefore expressed as;

vTvt

¼ l

rCp

v2Tvr2

þ 2r

l

rCp

vTvr

þ 1Cp

Xi

�Hivaivt

ui

�þ 1Cp

Xj

�Hjvgjvt

�

(11)

The variation of the heat capacity and the thermal conductivityare given by;

l ¼�aT ;f � aT

��1� aT ;f

� lf þaT ;f�

1� aT ;f

� lc (12)

Cp ¼�aT ;f � aT

��1� aT;f

� Cp;f þaT ;f�

1� aT ;f

�Cp;c (13)

While the heats of mass loss reaction and the heats ofexothermic reaction are given by;

Hi ¼ aH;ib2 þ bH;ibþ cH;i (14)

Hj ¼ aH;jb2 þ bH;jbþ cH;j (15)

At the particle surface, the heat transfer is defined by theboundary condition;

�lvTvr

����r¼R

¼ hðTR � TbulkÞ (16)

While the heat transfer at the particle centre assuming a sym-metric geometry is defined by the boundary condition;

vTvr

����r¼0

¼ 0 (17)

3. Multi-stage pyrolysis and optimization studies

Previous work on multi-stage pyrolysis have shown that theapproach can reduce the overall energy usage during pyrolysis bytaking advantage of the thermal behavior of each pyrolysis feed-stock though with a very high increase in completion time [5].Reduction in energy consumed during pyrolysis is worthwhile but

Table 1Overall energy usage and completion time for 10 K/min.

Approach Overall energyusage (J/g-tire)

Completiontime (h)

No ofstages

Conventional 673.74 1.47Multi-stage (case 1) 484.69 2.02 4Multi-stage (Case 2) 457.74 2.00 4

A.O. Oyedun et al. / Applied Thermal Engineering 61 (2013) 123e127 125

not at the expense of the completion time. The efficiency of thepyrolysis process is determined by both the processing time andthe energy usage.

In multi-stage pyrolysis three parameters are very importantand these can subsequently determine the amount of energy usedand the extent of the increase in completion time. These parame-ters are; target temperature to start the first adiabatic stage, thetime duration of the first adiabatic stage and the temperature tostart the second adiabatic stage. There are various possibilities andcombinations with these three parameters and therefore usingoptimization techniques can determine the optimal values for theseparameters at different heating rates.

A detailed study of the differential thermal analysis (DTA) forwaste tire as shown in Fig. 1 suggested that heat supplied to thepyrolysis reaction at the early stage was used to initialize theexothermic reaction. The process is more exothermic at tempera-tures between 553 and 653 K and the heat generated can be usedup in the endothermic reaction between 673 and 783 K.

The optimization model was developed to utilize the kineticmodel and reduce the energy used. It is subject to several con-straints. The pyrolysis model consists of non-smooth function withboundary constraints, minimizing the overall energy used was setas the objective function while limiting the completion time.

Particle size was not optimized in this work since it has alreadybeen studied and optimized in a previous work [13]. Based on theresult obtained, we chose 35 mm as the desired particle size.

The objective function of the optimization process is to mini-mize the energy used during the multi-stage pyrolysis and it isrepresented by Eq. (18).

min ET (18)

The objective function is subject to the following inequalityconstraints i.e. Eqs. (19)e(23).

From the previous work [5], any reduction in energy usage mustbe compensated with an increase in completion time. We thereforeset a maximum of 50% increase in completion time with referenceto the time used for conventional pyrolysis. This inequalityconstraint is represented by Eq. (19).

tcomplete ðmulti� stageÞ � 1:50*tcompleteðconventionalÞ (19)

where ET and tcomplete represent the overall energy usage (J/g) andthe completion time (h).

The constraints for the temperature to start the first adiabaticstage Tadia1 (K), and the temperature to start the second adiabaticstage Tadia2 (K) were set based on heat flow profile analyzed in Fig. 1

Fig. 1. Heat flow of waste tire pyrolysis under different heating rate.

and represented by Eqs. (20) and (21) while Eq. (22) is theinequality constraint for the time duration for the first adiabaticstage tadia1 (min).

553 � Tadia1 � 623 (20)

723 � Tadia2 � 783 (21)

5 � tadia1 � 30 (22)

If the temperature to start the second adiabatic stage calculatedin Eq. (22) is greater than 773 K, then the number of stages equalsthree.

In this work, the concept of using a lower or higher heating ratefor the second heating stage was introduced. Considering theexperimental heat flow plot for the waste tire in Fig. 1, the behaviorof the different heating rate after 653 K was independent of theinitial starting heating rate. Therefore Eqn. (23) was added to theoptimization model for the case two scenario.

5 � b2 � 20 (23)

where b2 is the heating rate for the second heating stage (K/min).The optimization problem was solved using the optimization

toolbox in MATLAB�. The fmincon solver was used to initiate thestarting point and the simulannealbnd solver was then used toachieve the global optimumwhich was obtained after at least 4000iterations.

4. Results and discussion

The optimization results are stated and analyzed in this sectionbased on the scenario of using same heating rate or differentheating rate for the second heating stage. The results of the opti-mum operating parameters are discussed accordingly.

4.1. Case one: same heating rate approach

The results for the overall energy usage and completion time forboth heating rate of 10 K/min and 20 K/min are presented inTables 1 and 2. For 10 K/min, the overall energy usage decreased by24.7% while the completion time increased by just 37% whereas for20 K/min, the overall energy usage decreased by 25.6% while thecompletion time increased by just 40%.

The optimization results of case one scenario are presented inTable 3. The temperature to start the first adiabatic stage for heatingrate of 10 K/min was 582 K and the process was at this mode for15 min. At this stage, processing benefits can be gained from the

Table 2Overall energy usage and completion time for 20 K/min.

Approach Overall energyusage (J/g-tire)

Completiontime (h)

No ofstages

Conventional 693.91 1.24Multi-stage (case 1) 516.38 1.74 3Multi-stage (Case 2) 431.29 1.86 4

Table 3Optimization results for same heating rate approach.

b1 (K/min) b2 (K/min) Tadia1 (K) tadia1 (min) Tadia2 (K)

10 10 582 15 76420 20 606 30 773

Table 4Optimization results for different heating rate approach.

b1 (K/min) b2 (K/min) Tadia1 (K) tadia1 (min) Tadia2 (K)

10 5 553 15.13 75420 5 582 25 754

A.O. Oyedun et al. / Applied Thermal Engineering 61 (2013) 123e127126

first heating stage and all excess heat in the process was used tofacilitate the pyrolysis process. The second heating starts after15 min and raised the temperature of the reactor to 764 K as shownin the temperature profile illustrated in Fig. 2. After the secondheating stage, second adiabatic stage starts and the remainingexcess heat was used to complete the pyrolysis process.

The temperature to start the first adiabatic stage for heating rateof 20 K/min was higher than that of 10 K/min and this can beexplained based on the heat flow at the two heating rates. As notedin Fig. 1, the heat flow of 10 K/min tends to be more exothermic attemperature ranges below 650 K than that of 20 K/min. The timeduration for the first adiabatic stage for 20 K/min was longer thanthat of 10 K/min and as shown in the result in Table 2, the numberof stages for this case scenario was three unlike that of 10 K/minwhich was four stages. A similar result was obtained in our earlierexperimental work when multi-stage approach was used forcharcoal production [7], a reduction of 29% in energy used wasachieved with three-stage pyrolysis.

Therefore using the scenario of same heating rate for the twoheating stages, multi-stage pyrolysis approach can achieve an en-ergy usage reduction of 24.7e25.6% with increment in completiontime of 37e40% irrespective of the heating rate used. Comparatively,the results are far better than the result obtained in earlier work [5].

4.2. Case two: different heating rate approach

Multi-stage pyrolysis consists of more than one heating stageand therefore different heating rate can be used for the two heatingstages. The overall results for this case scenario for both 10 K/minand 20 K/min are also presented in Tables 1 and 2. The result showsthat for 10 K/min, the overall energy usage decreased by 29% whilethe completion time increased by 36% by using a lower heating rateof 5 K/min for the second heating stage. The result here is far betterthan that of case one and the optimization result in Table 4 in-dicates that the temperature to start the first adiabatic stage re-duces to 553 K while increasing the time duration slightly.

For heating rate of 20 K/min, same heating rate of 5 K/min wasachieved for the second heating stage with energy reduction of37.9% though with an increase in completion time of 50%. As ex-pected the temperature to start the first adiabatic stage was higher(582 K) than that of 10 K/min (553 K) and also a longer duration of

Fig. 2. Temperature profile for the multi-stage pyrolysis for case 1 (10 K/min).

25 min for the first adiabatic stage. The number of stages forheating rate of 20 K/min for this case was four unlike case onewhere the number of stages was three. This can be as a result ofusing a lower heating rate for the second heating stage. Cases withlower heating rate of 5 K/min for the first heating stage was testedbut because of the longer time it takes to reach the target tem-perature, this case does not yield a competitive and preferableresult and was therefore ignored in the overall consideration.

Therefore based on the scenario of using a lower heating rate forthe second heating stage, multi-stage pyrolysis approach can ach-ieve an energy usage reduction of 29e37.9% with an increase incompletion time of 36e50% irrespective of the heating rate used.Comparatively, the results are far better than those obtained in caseone i.e. same heating rate approach.

5. Conclusion

The multi-stage pyrolysis process can be used to obtain certainprocessing benefits during pyrolysis process. In this study, optimi-zation has been used to determine the important operating param-eters in multi-stage pyrolysis, which are the target temperature tostart the first adiabatic stage, the time duration of the first adiabaticstage and the temperature to start the second adiabatic stage. The useof different heating rates for the second heating stage was alsoconsidered in this work and the result shows that using a lowerheating rate for the second heating stage can further reduce theoverall energyusagewith a considerable increase in completion time.The optimizedmulti-stage pyrolysis approach can reduce the energyusage to a maximum of 37.9% while increasing the completion timeby 50% which is far better compared to results in previous works.

Acknowledgements

The authors gratefully acknowledge the support from HongKong RGC in form of PhD Fellowship to Oyedun Adetoyese Olajire(PF09-05997).

Nomenclature

ai extent of reaction for mass loss reaction iaT overall mass loss fractionaT,f final total mass loss fractiongj extent of reaction for exothermic reaction jt reaction time [s]T temperature [K]n order of reactionA pre-exponential factor [s�1]E activation energy [J/mol]R universal gas constant [J/mol K]b heating rate [K/min]ui mass loss contribution by mass loss reaction ia quadratic coefficient for the mass loss contribution

[min2/K2]b linear coefficient for the mass loss contribution [min/K]c constant term for the mass loss contributionaH quadratic coefficient for the heat of reaction [J min2/kg K2]bH linear coefficient for the heat of reaction [J min/kg K]cH constant term for the heat of reaction [J/kg]

A.O. Oyedun et al. / Applied Thermal Engineering 61 (2013) 123e127 127

H heat of reaction [J/kg]Cp specific heat capacity [J/kg K]l thermal conductivity [W/m K]r particle radius [m]r density [kg/m3]TR particle surface temperature [K]Tbulk bulk temperature [K]ET overall energy usage [J/g]tcomplete pyrolysis completion time [h]Tadia1 target temperature to start the first adiabatic stage [K]tadia1 time duration for the first adiabatic stage [min]Tadia2 target temperature to start the second adiabatic stage [K]

Subscripti mass loss reaction ij mass loss reaction jf feedstock (tire)c char

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