Bioresource Technology 218 (2016) 631–642

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Bioresource Technology

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Thermodynamics and kinetics parameters of co-combustion betweensewage sludge and water hyacinth in CO2/O2 atmosphere as biomass tosolid biofuel

http://dx.doi.org/10.1016/j.biortech.2016.06.1330960-8524/� 2016 Elsevier Ltd. All rights reserved.

⇑ Corresponding author.E-mail address: www053991@126.com (J. Liu).

Limao Huang, Jingyong Liu ⇑, Yao He, Shuiyu Sun, Jiacong Chen, Jian Sun, KenLin Chang, Jiahong Kuo,Xun’an NingSchool of Environmental Science and Engineering, Institute of Environmental Health and Pollution Control, Guangdong University of Technology, Guangzhou 510006, China

h i g h l i g h t s

� Thermochemical conversion of biomass waste was carried out in oxy-fuel atmosphere.� The performance sewage sludge combustion was improved by blending water hyacinth.� Thermodynamic parameters (4H, 4S, 4G) were calculated through TGA curves.� The lowest activation energy (Ea) of SW was obtained in CO2/O2 = 7/3 atmosphere.

a r t i c l e i n f o

Article history:Received 18 April 2016Received in revised form 28 June 2016Accepted 30 June 2016Available online 2 July 2016

Keywords:Sewage sludgeWater hyacinthBioenergyOxy-fuel combustionThermal analysis

a b s t r a c t

Thermodynamics and kinetics of sewage sludge (SS) and water hyacinth (WH) co-combustion as a blendfuel (SW) for bioenergy production were studied through thermogravimetric analysis. In CO2/O2 atmo-sphere, the combustion performance of SS added with 10–40 wt.% WH was improved 1–1.97 times asrevealed by the comprehensive combustion characteristic index (CCI). The conversion of SW in differentatmospheres was identified and their thermodynamic parameters (DH;DS;DG) were obtained. As theoxygen concentration increased from 20% to 70%, the ignition temperature of SW decreased from243.1 �C to 240.3 �C, and the maximum weight loss rate and CCI increased from 5.70%�min�1 to 7.26%�min�1 and from 4.913%2�K�3�min�2 to 6.327%2�K�3�min�2, respectively, which corresponded to the vari-ation in DS and DG. The lowest activation energy (Ea) of SW was obtained in CO2/O2 = 7/3 atmosphere.

� 2016 Elsevier Ltd. All rights reserved.

1. Introduction

With the growth in the global economy and population, on theone hand, energy demands increase continuously, and fossil fuel isestimated to run out in the next few decades (Gangulya et al.,2012). On the other hand, the level of greenhouse gases in differentscenarios is projected increase in the future by the Intergovern-mental Panel on Climate Change (IPCC, 2011). Atmosphericpollution results in the enforcement of economic policies thatencourage the use of alternative energy sources that are less harm-ful to the environment. Therefore, a new energy source that canreplace fossil fuel is highly necessary. This necessity has resultedin an increased focus on the production of biofuel from biomass.Biomass is expected to contribute to future sustainable energy

systems and sustainable development in both industrialized anddeveloping countries (Vamvuka et al., 2009; Liu et al., 2013).

Biomass fuel can be made of many different substances, such aswood, agricultural waste, algae, sewage sludge and water hyacinth(Santos et al., 2015; Luo et al., 2011).

Sewage sludge is formed during wastewater treatment.The annual output of sewage sludge in China is 3 � 107t (moisturecontent of 80%), 80% of which does not reach the stan-dards of harmlessness and stability (Liu et al., 2015). Sewage sludgeis harmful for humans and the environment if it is dealt with inap-propriately. The perception of sludge being an unwanted waste ischanging to the view that it is a beneficial resource. Thus, develop-ing a suitable technology or using an existing technology to reducethe environmental problem and cost of sludge treatment while uti-lizing it as a source of energy is important (Mountouris et al., 2008).Highly advanced methods of sludge treatment are being developedto address the increasing amounts of sludge being produced. Such

Nomenclature

Symbols/abbreviationsTi ignition temperature (�C)Tp peak temperature (�C)Mr residual mass at 1000 �C (%)a mass conversion degree (%)Ea activation energy (kJ�mol�1)A pre-exponential factor (s�1)4S entropy (J�mol�1)Rp reaction rate at the peaks (%�min�1)

Tb peak temperature of 98% conversion (�C)Tm DTGmax peak temperature (K)Rv average reaction rate during the temperature ranging

from the ignition temperature to the final temperature(%�min�1)

CCI comprehensive combustion index (%2�K�3�min�2)4H enthalpy (kJ�mol�1)4G free Gibbs energy (kJ�mol�1)

632 L. Huang et al. / Bioresource Technology 218 (2016) 631–642

techniques usually include thermal sludge treatment, which ismainly involves sludge incineration, pyrolysis and gasification.Sludge incineration treatment mainly includes combustion andco-combustion. Given that co-combustion entails a low cost andmature equipment and technology, it has become the mainmethodof sludge incineration (Magdziarz and Wilk, 2013).

Water hyacinth (Eichhornia crassipes), one of the world’s ‘‘topten evil grasses”, has a wide geographic distribution and richreserves because of its rapid proliferation and lack of natural con-trol mechanisms (Villamagna and Murphy, 2010). Uncontrolledgrowth of this plant has negatively affected many bodies of water,but the rapid growth rate of water hyacinth makes it a potentialbiomass source (Zimmels et al., 2009). Water hyacinth presentsattributes that are considered ideal for bio-fuel production. Theseattributes include a culture that is composed of natural growthvegetation, preferably ‘‘perennial” (with a fast production rate),that does not compete in terms of space, light and nutrients withcrops; pest-resistance and being easily degradable. Furthermore,it is not prone to genetic pollution by crossbreeding with other cul-tures (Bhattacharya and Kumar, 2010). Current studies on waterhyacinth focus on bioremediation (Gangulya et al., 2012), bio-ethanol and gas production (Aswathy et al., 2010; Mishima et al.,2008), feed production and adsorbent preparation (Guerrero-Coronilla et al., 2015). A few studies have investigated water hya-cinth co-combustion.

Given that traditional combustion results in only approximately15% CO2 in the flue gas, capturing CO2 from the flue gas is difficultand costly (Mondal et al., 2012). Combustion in a CO2/O2 atmo-sphere is one of the several promising new technologies that areassociated with CO2 capture and storage (Li et al., 2009). Notably,a large amount of nitrogen results in minimal radiation in exhaustgas but consumes a large amount of heat, which causes a loss ofenergy in the process of energy transfer. Combustion in a CO2/O2

atmosphere has the following features: (1) reduces the availableenergy loss in the process of converting chemical energy into ther-mal energy, (2) decreases the concentration of organic pollutantsin the exhaust gas, and (3) reduces the thermal energy of theexhaust gas to the minimum (Yu et al., 2008). Implementation ofoxy-fuel atmosphere combustion has been proven to achieve oper-ative and environmental benefits (Habib et al., 2011).

The combustion of sewage sludge in air, pyrolysis in an N2

atmosphere and co-combustion with coal and biomass in air havealready been studied (Liao and Ma, 2010; Xie and Ma, 2013;Magdziarz and Wilk, 2013). However, sewage sludge co-combustion with water hyacinth in an oxygen-enriched atmo-sphere and CO2/O2 atmosphere, especially when thermogravimet-ric analysis is involved, has not been studied in. Therefore,improved understanding of the kinetic characteristics and thermo-dynamic parameters in oxy-fuel combustion needs to be obtained.

Blending water hyacinth with sludge could provide a usefulmeans to solve the environmental problems and could result in a

certain amount of energy supply. The quality and quantity of theobtained bioenergy depend not only on the chemical compositionof the original biomass but also on the reaction conditions. Thus,understanding the chemistry of this alternative biomass is vitalto determine the thermochemical process of conversion intobiomass-derived fuel (Lee et al., 2014).

This paper examines thermochemical conversion of sewagesludge (SS) blended with water hyacinth (WH), including theoxy-fuel combustion characteristics of their blends in differentCO2/O2 atmospheres and the thermodynamic parameters fornon-isothermal analyses using the Ozawa-Flynn-Wall (OFW)kinetic isoconversional model. Then, the values of the apparentactivation energy (Ea), the pre-exponential factor (A) in the Arrhe-nius equation, as well as the changes in the entropy (DS), enthalpy(DH) and free Gibbs energy (DG), were calculated.

2. Methods

2.1. Materials

The biomasses considered in this study were sewage sludge (SS)and water hyacinth (WH). SS samples were collected at intervals of0.5 and 8 h through continuous acquisition from a terminal con-veyor belt in a wastewater treatment plant in Guangzhou, Guang-dong Province, China. WH samples were collected from the canalsthat surround Guangzhou University Mega Center, Guangzhou,Guangdong Province, China. In the laboratory, the biomass wasallowed to dry naturally at room temperature for one week. After-ward, it was milled and sieved with a 74 lm sieve. It was then sub-jected to oven drying at 105 �C to constant weight and stored in adesiccator for testing. The ultimate analysis and proximate analysisof the SS and WH are presented in Table 1.

2.2. Apparatus and procedure

Thermogravimetric analysis was conducted at three differentheating rates, 10, 20 and 40 �C min�1, with a flow rate of50 ml�min�1, using simultaneous DSC–TGA equipment, NETZSCHSTA 409 PC, from 25 to 1000 �C. Approximately (10 ± 0.5 mg) ofthe biomass was used in alumina crucibles in each analysis. Beforethe start of the experiment, several experiments without sampleswere conducted to obtain the baseline, which was deducted whenthe experiments with samples started, to eliminate the systematicerrors of the instrument itself. Furthermore, the samples selectedrandomly in the same batch were repeated three times in anexperiment to confirm the repeatability and authenticity of thegenerated data and the errors were within ±2%. The NETZSCH–T4–Kinetic2 software provided the thermogravimetry (TG) andderivative thermogravimetry (DTG) curves. A Vario EL–II chons ele-mental analyzer (Elementar Analysen Systeme Gmbh, Germany)and a Parr 6300 Oxygen Bomb Calorimeter (Parr Instrument

Table 1The ultimate analyses and proximate analyses of SS and WH on air dried basis.

Sample Ultimate analyses (wt.%) Proximate analyses (wt.%) Qnet,da(MJ�kg�1)

C H O N S Mb Vc Ad FCe

SS 34.04 5.03 23.48 6.09 1.67 5.50 48.80 43.38 2.32 12.02WH 38.96 5.3 35.18 3.51 0.27 8.32 61.57 8.67 21.44 13.51

a Qnet,d, lower heating value on dry basis.b M, moisture.c V, volatile matters.d A, ash.e FC, fixed carbon.

L. Huang et al. / Bioresource Technology 218 (2016) 631–642 633

Company, United States) were used for the ultimate analysis,proximate analysis and lower heating values of the samples.

The apparatuses used were the following: (1) Thermogravime-try analyzer (NETZSCH STA 409 PC); (2) Vario EL–II chons elemen-tal analyzer (Elementar Analysen Systeme Gmbh, Germany); (3)Parr 6300 Oxygen Bomb Calorimeter (Parr Instrument Company,United States); (4) The muffle furnace (SX2–4–13); (5) Electronicbalance (sartorius CAP2P); and (6) The pulverizer (PW177); (7)Electrothermal blowing (DHG–9140A).

2.3. Kinetic theory

2.3.1. Ozawa-Flynn-Wall methodsThe one-step global model assumes that the degradation pro-

cesses result in a single reaction, below.

Biomass!k Volatilesþ Biochar

Here, k is defined as the rate constant of the reaction, whosetemperature dependence is expressed by the Arrhenius equation:

kðTÞ ¼ Aeð�Ea=RTÞ ð1Þwhere a is the conversion degree, t is the time, T is the reaction tem-perature, A is the pre-exponential factor, Ea is the activation energy,and R is the universal gas constant 8.314 J/K�mol�1.

The rate of transformation from a solid-state to volatile productis described by the following expression:

dadt

¼ kðTÞf ðaÞ ð2Þ

The conversion degree a is expressed as (Xie and Ma, 2013)

a ¼ m0 �mt

m0 �m1ð3Þ

wherem0 andm1 are the initial mass and the final mass of the sam-ples, respectively. Here, mt is the mass of the samples at time t.

Eqs. (1) and (2) could be combined into the following equation:

dadt

¼ Aeð�Ea=RTÞf ðaÞ ð4Þ

When the heating rate b (�C�s�1)

b ¼ dT

dtð5Þ

is introduced, Eq. (4) is transformed to

bdadT

¼ Aeð�Ea=RTÞf ðaÞ ð6Þ

Kinetic analysis is essential to design and establish efficient, safeand reasonable processes. The determination of the thermo-kineticbehavior of the biomass allows control of the decompositionmechanism of the biomass. The kinetic parameters of reaction arenecessary for accurate prediction of the reaction behaviors andoptimization of the process toward products during thermal

degradation (Amanda and Leandro, 2016). Non-isothermal thermo-gravimetric analysis is the most popular and simplest method tostudy the kinetics and thermodynamics properties of the biomass.An isoconversional integralmethod appears to be a safer alternativefor the calculation of meaningful activation energy values for cer-tain prerequisites, without knowing the kinetic model of the reac-tion mechanism.

The Ozawa-Flynn-Wall kinetics isoconversional method in Eq.(7) is applied (Kim et al., 2010).

lnðbÞ ¼ Ca � EaR � T ð7Þ

where b is the heating rate; Ea is the activation energy; Ca is thefunction of the conversion degree a; R is the universal gas constant8.314 J/K�mol�1; and T is the reaction temperature.

2.3.2. Kinetic and thermodynamic parametersWe applied at least three values of the heating rate (b) for the

same and different values of the different reaction temperaturesobtained in the thermogravimetric curves. This approach allowsus to investigatewhether themechanism of the conversion changeswith the conversion degree, while estimating the respective activa-tion energy Ea at a conversion degree of a. To calculate the kineticparameters, we used an intermediate value of b (20 �C�min�1).

The thermodynamic parameters using the OFW isoconversionalmethod for analysis of the kinetic and thermodynamic parameters,including the pre-exponential factor (A) in the Arrhenius equationas well as the enthalpy (DH), free Gibbs energy (DG) and thechanges in the entropy (DS), can be expressed by Eqs. (8)–(11)(Kim et al., 2010; Xu and Chen, 2013).

A ¼ b � Eaeð�Ea=R�TmÞ=R � T2m ð8Þ

H ¼ Ea � RT ð9Þ

G ¼ Ea þ R � Tm � ln kB � Tm

h � A� �

ð10Þ

S ¼ DH � DGTm

ð11Þ

where kB is the Boltzmann constant (1.381�10�23 J�K�1); h is thePlank constant (6.626�10�34 J�s); and Tm is the DTG peaktemperature.

To make the research acceptable and applicable in practice withrespect to using SW as a fuel, the combustion characteristicparameters for SW under various oxy-fuel conditions, includingthe ignition temperature (Ti), peak temperature (Tp), burnout tem-perature (Tb), maximum weight loss rate (Rp), and average weightloss rate (Rv), are required. They can be directly determined fromthe TG–DTG curves. Furthermore, the combustion performanceparameters, the comprehensive combustibility index (CCI), are rec-ommended to evaluate the combustion performance of SW underdifferent conditions. The CCI index can be calculated as a function

634 L. Huang et al. / Bioresource Technology 218 (2016) 631–642

of the characteristic temperatures and reaction rates (Liu et al.,2013).

CCI ¼ ð�RpÞ � ð�RvÞT2i � Tb

ð12Þ

3. Result and discussion

3.1. Co-combustion characteristics of blend fuels

The influence of SS blended with WH at the ratio of 10 wt.%,20 wt.%, 30 wt.% and 40 wt.% was investigated. Fig. 1 shows themass loss (TG) and weight loss rate (DTG) curves of the blended

Fig. 1. TG(a)–DTG(b) curves of co-combustio

fuels in a CO2/O2 = 7/3 atmosphere at the heating rate of20 �C�min�1, and the characteristic parameters of SW are given inTable 2.

As shown in Fig. 1, all of the TG and DTG curves of the blends laybetween the individual fuels in the CO2/O2 atmosphere and variedwith the proportion of the individual fuels but, overall, had thecombustion characteristics of the SS and WH. As the blending ratioincreased, the maximum weight loss rate that corresponds to themain decomposition process was gradually reduced (Table 2),the temperature that corresponds to this rate was reduced, andthe trends in the curves shifted to a lower temperature range.The residual weight of the pure SS, their blends of 10 wt.%, 20 wt.%, 30 wt.%, 40 wt.% and pure WH corresponded to 46.36%, 43.74%,39.71%, 37.63%, 34.65%, 6.55%, respectively. This finding could be

n of sludge blends with water hyacinth.

Table 2Characteristic parameters obtained from TG-DTG curves of samples in CO2/O2 atmosphere.

Sample Tia (�C) Tp

b(�C) Rpc (%�min�1) Rv

d (%�min�1) Mre (%) Tb

f(�C) CCIg (10�8)

SS 234.1 298.5 4.01 1.13 46.36 661.14 3.080S/W = 9/1 240.3 295.1 4.72 1.19 43.74 656.08 3.760S/W = 8/2 242.4 292.8 6.03 1.27 39.50 644.59 5.169S/W = 7/3 242.0 292.3 6.65 1.33 37.63 670.86 5.742S/W = 6/4 230.8 291.3 7.45 1.40 34.65 789.38 6.064WH 258.5 289.2 16.57 2.04 6.55 959.90 14.363

a Ti is onset temperature for volatile release.b Tp is the temperature associated to Rp.c Rp is maximum weight loss rate.d Rv is the average mass loss rate.e Mr is residual mass at 1000 �C.f Tb is the temperature of 98% conversion.g CCI is the comprehensive combustion index, unit is %2�K�3�min�2.

L. Huang et al. / Bioresource Technology 218 (2016) 631–642 635

caused by the weight loss rate of the WH being better than that ofthe sludge, which causes some advantageous influence for SS com-bustion. It was worthwhile to note that the reductions in Rp and Tpwere not linearly related to the amount of WH blend and that thecurves of the blends appeared in the third peak, which varied fromthe SS and WH combustion curve. The maximum weight loss rateof both the SS andWH occurred in the stage of volatile combustion,but the maximum weight loss rate of WH (16.57%�min�1) wasgreater than that of SS (4.01%�min�1), which illustrated that theWH in SS can make up for the inadequacy of the SS combustionand made the SS evaporation and combustion phase release morevigorously. After adding 40% WH, the combustion temperature,which corresponding to the mass loss of SS of 75%, 65% and 55%,was lowered by approximately 43.4 �C, 112.7 �C and 82.9 �C,respectively, and the reaction time was advanced. This findingcould arise because the atomic ratio of O/C in the WH was high.The oxygen that contained functional groups such as hydroxyl, car-boxyl, and carbonyl had a higher content and were more activated.Adding WH can cause the SS to contact the oxygen around it anddecompose into relatively easy burning material, enhancing theactivity of the combustion reaction and accelerating the combus-tion of volatiles (Xie and Ma, 2013).

To evaluate the combustion characteristics of SS, WH and SW ina CO2/O2 atmosphere, the comprehensive combustion index (CCI)was calculated. The larger the index CCI was, the more vigorouslythe samples burned and the faster the char burned out. Table 2showed the results of TGA for the samples. It was found that inCO2/O2 atmosphere, the combustion performance of SS added with10–40 wt.% WHwas improved obviously represented as CCI gainedby 1–1.97 times.

3.2. The interaction between sewage sludge and water hyacinth

In order to illustrate the co-combustion mechanism of SS andWH, the interaction of these two fuels was studied. The interactionbetween fuels is a concern in many co-combustion applications. Toinvestigate whether there is an interaction between SS and WH,the theoretical TG–DTG curve of the mixture are calculated bythe average weight of the individuals (Deng et al., 2016).

Ymixture ¼ XSSYSS þ XWHYWH ð13Þwhere XSS and XWH are the percentage of sewage sludge and waterhyacinth in the mixture, respectively, and YSS and YWH are the massloss or weight loss rate of sewage sludge and water hyacinthrespectively.

The experimental thermogravimetry–derivative thermo-gravimetry (TG–DTG), calculated TG–DTG, and deviation curvesof the SW sample are presented in Fig. 2 with the S/W = 8/2mixture as an example. The greater degree of separation of theexperimental and calculated TG–DTG curves was, the stronger

the interaction between the burnt mixtures was. As shown inFig. 2a, the experimental TG curve lagged behind the calculatedTG curve when the temperature was 260 �C–380 �C and 480 �C–580 �C but exceeded it when the temperature was 380 �C–480 �C.This result indicates that some degree of interaction occurredbetween SS and WH during the co-combustion process. The calcu-lated DTG curve almost coincided with the experimental DTGcurve at the temperature range of 170 �C–260 �C. However, in thetemperature ranges of 260 �C–305 �C and 455 �C–500 �C, theexperimental DTG curve lagged behind the calculated DTG curve.The maximum weight loss rates of the experimental DTG curveswere 10.67%�min�1 lower than those of the calculated DTG curves.This result suggests that some degree of interaction occurredbetween the components of SS and WH at low temperatures. Thisinteraction slowed down the combustion process of SW. At 305 �C–455 �C and 500 �C–650 �C, the experimental DTG curves werehigher than the calculated DTG curves. This result confirms thatan interaction occurred between the components of SS and WHat high temperatures, and this interaction accelerated the combus-tion process. The experimental DTG curve shifted at a high temper-ature compared with the calculated DTG curve, and the shapes ofthe curve indicated a significant interaction among the compo-nents of SW during combustion. As shown in Fig. 2a, the peak valueand shape revealed large differences. These differences are attribu-ted to the chemical interactions between the components of SS andWH during decomposition and combustion (Deng et al., 2016). SShad a high ash content of 43.38% as shown in Table 1, and theash inhibited its combustion. Several studies have found that whenthe mineral matter in sludge is removed, the combustion rateincreases significantly (Vamvuka et al., 2009). The interaction ata low temperature range of 260 �C–380 �C might be interpretedas resulting from the release of volatile matter from WH reactingwith SS residues. For this reason, the experimental DTG curvelagged behind the calculated DTG curve. In the high temperaturerange of 305 �C–455 �C and 500 �C–650 �C, the interactionappeared to accelerate the combustion process of SW. After WHwas added to SS, much heat was released to promote the combus-tion reaction. This scenario improved the decomposition depth ofthe sludge residues (Liao and Ma, 2010).

Based on the calculated and the experimental TG value, devia-tion of the SW from the calculated TG values was calculated usingfollowing equation (Haykiri-Acma and Yaman, 2010):

Deviation ð%Þ ¼ TGexp � TGcal

TGcal

� �� 100 ð14Þ

As shown in Fig. 2b, the highest deviation in TG values were upto 5.5%. This value was greater than the statistical significance of5% (Haykiri-Acma and Yaman, 2010). Thus, it can be concluded thatthe synergetic effect on mass loss of the SW was obviously duringco-combustion.

Fig. 2. The comparison of experimental, calculated and deviation curve. (a) TG–DTG; (b) deviation.

636 L. Huang et al. / Bioresource Technology 218 (2016) 631–642

3.3. Oxygen-enriched combustion of blend fuels

3.3.1. Effects of oxygen concentration on the co-combustion of blendfuels

In this work, the effect of the oxygen concentration on the com-bustion characteristics of SW (S/W = 8/2 sample) was evaluated at aheating rate of 20 �C�min�1 in different CO2/O2 atmospheres (CO2/O2 = 8/2, CO2/O2 = 7/3, CO2/O2 = 5/5, CO2/O2 = 3/7 atmosphere).

Fig. 3 shows that three stages could be distinguished for SWduring the oxy-enrich combustion process. Step I (<170 �C) wasmoisture exhalation, and the weight loss rate is approximately7.13%; step II (170–650 �C) was corresponded to the release andcombustion of organic volatile matter and fixed carbon, includingthe decomposition of hemicelluloses, cellulose and lignin, duringwhich the mass of SW was rapidly lost (approximately 86.23%)(Luo et al., 2011); step III (>650 �C) was the residue decomposition.

L. Huang et al. / Bioresource Technology 218 (2016) 631–642 637

In this step, the carbonaceous residuals continue burning at a verylow rate, and a slight mass loss is observed.

As shown in Table 3, in the CO2/O2 atmospheres, with increasingoxygen concentration, ignition became easier, a lower temperature(Tp1) was needed to reach that maximum peak of the DTG curves,and the curves gradually shifted to a lower temperature range.For example, with the increase in the oxygen concentration from20% to 70% in the CO2/O2 atmosphere, Ti decreased from 243.1 to240.3 �C, Tp1 decreased from 293.0 to 289.1 �C, and the maximumweight loss rate (Rp1) increased from 5.70 to 7.26%�min�1. Similarfindings were also reported in previous investigations on the co-combustion between microalgae and textile dyeing sludge (Peng

Fig. 3. TG(a)–DTG(b) curves of S/W = 8/2 in different o

et al., 2015). The increase in the maximum mass loss rate was per-haps due to the force of the fusion layer being reduced in the pres-ence of oxygen and the quick release of volatiles (Liu et al., 2009). Asthe oxygen concentration increased, the peak that corresponds tobiochar combustion (Rp2) also became more apparent, and the cor-responding temperature (Tp2) decreased, while the third peak (Rp3)acted in the opposite fashion. As shown in Table 3, Mr increased inthe following order: CO2/O2 = 7/3＜ CO2/O2 = 8/2＜ CO2/O2 = 3/7＜ CO2/O2 = 5/5.Mr was the best (39.50%) in the CO2/O2 = 7/3 atmo-sphere, while it was the worst (42.10%) in the CO2/O2 = 5/5 atmo-sphere. In the oxy-fuel combustion, the Mr of the SW was notlinearly related to the amount of oxygen concentration.

xygen concentration atmospheres at 20 �C�min�1.

Table 3Characteristic parameters obtained from TG-DTG curves of samples in CO2/O2 atmospheres.

Atmosphere Ti (�C) Tp (�C) Rp (%�min�1) Rv (%�min�1) Mr (%) Tb (�C) CCI (10�8)

Tp1 Tp2 Tp3 Rp1 Rp2 Rp3

CO2/O2 = 8/2 243.1 293.0 498.1 555.1 5.70 2.59 2.68 1.268 39.71 637.86 4.913CO2/O2 = 7/3 242.4 292.8 497.2 560.4 6.03 3.12 2.23 1.274 39.50 644.59 5.169CO2/O2 = 5/5 241.2 289.3 479.5 549.3 6.97 2.94 2.09 1.230 42.10 620.93 6.039CO2/O2 = 3/7 240.3 289.1 477.4 545.9 7.26 2.92 2.08 1.272 40.21 636.78 6.327

Fig. 4. TG–DTG (a) and T–a (b) curves of S/W = 8/2 at different heating rates in oxygen concentration atmosphere.

638 L. Huang et al. / Bioresource Technology 218 (2016) 631–642

L. Huang et al. / Bioresource Technology 218 (2016) 631–642 639

3.3.2. Effect of heating rate on the co-combustion characteristics ofblend fuels

To investigate the effect of the heating rate on the co-combustion characteristics of the SW, the combustion process ofthe SW sample in the CO2/O2 = 7/3 atmosphere was randomlyselected for analysis in this section. Fig. 4(a) and (b) showed theTG–DTG and T–a curves of SW at the heating rates of 10, 20, and40 �C�min�1 in the CO2/O2 = 7/3 atmosphere. It can be seen thatbetween 200 and 650 �C, the TG curves shifted to a higher temper-ature when b increased (Fig. 4(a)), which was due to heat transferlimitations. As b increased, the DTG curves also shifted toward thehigher temperatures, and the values heightened obviously. Thisrelationship occurred because a lower heating rate would inducea higher heat transfer efficiency during heating. A greater temper-ature gradient across the poor heat conductor of the SW particlewould be generated by raising the heating rate, which ultimatelyincreases the thermal oxidative degradation rate (Chen et al.,2013). Similar findings have also been reported from research onthe pyrolysis and oxy-fuel combustion characteristics and thekinetics of petrochemical wastewater sludge (Chen et al., 2015).

For the purpose of designing new combustion facilities or retro-fitting existing plants that employ SW as fuel feedstock under oxy-fuel conditions, data for SW, including the effect of the heating rate(b) on the combustion characteristic parameters, are required. Asshown in Table 4, Ti, Tp (Tp1, Tp2, and Tp3), and Tb shifted to highertemperatures when b increased. The Rp of all of the steps (Rp1,Rp2, and Rp3) and Rv markedly increased as b increased from 10 to40 �C�min�1. As shown in Fig. 4(b), when the temperatureincreased, the mass conversion (a) gradually rose. As b increased,the mass conversion (a) curves moved toward a higher tempera-ture region. Thus, the residual massMr and Tb was better at a lowerb. In addition, the combustion performance of SW was evaluatedby the combustion index of CCI. When b increased from 10 to 40 �-C�min�1, the CCI was increased from 1.325 to 20.197�10�8%2�K�3-�min�2. This relationship illustrates that although increasing theheating rate did not favor burnout, it was beneficial to the separa-tion and combustion of the volatiles. Although the effects that werecaused by increasing b were common for various solid samples inthe TGA experiments, data for SW combustion under oxy-fuel con-ditions have not yet been reported.

3.4. Evaluation of combustion parameters

Ea was an important kinetic parameter for revealing the activa-tion energy that is required to keep the chemical reaction going(Amanda and Leandro, 2016). According to Eq. (7), the plots forthe determination of Ea calculated by OFW at the second stage withb = 10, 20 and 40 �C�min�1 under different oxygen concentrationatmospheres are depicted in Fig. 5(a) and (b), and the thermody-namic parameters of combustion of SW in the CO2/O2 atmosphereswere gathered in Fig. 6(a)–(d). Ea has high linear correlation coef-ficients (R2) in the range of 0.9242–1.0000 at 0.1 < a < 0.90, whichillustrate the calculated value of Ea is reliable. The differentiation ofthe date obtained at different mass conversions in different atmo-spheres could be appropriately reflected at the date obtained forthe maximum weight loss rate in Table 5.

Table 4Co-combustion characteristic parameters of SW at the heating rate of 10, 20, and 40 �C�m

b Ti (�C) Tp (�C) Rp (%�min�1)

Tp1 Tp2 Tp3 Rp1 Rp2

10 234.0 286.1 477.0 551.2 2.66 1.5520 242.4 292.8 497.2 560.4 6.03 3.1240 252.2 304.3 539.0 / 12.03 6.25

As shown in Fig. 6a, the variation in the values of Ea was notsimilar at different constant extents of conversion because thereaction of SW was not simple one-step mechanisms and insteadfollowed a complex multi-step reaction (Xu and Chen, 2013). Itwas clear that the activation energy varied with increasing conver-sion and the Ea value of the SW did not increase all the way withthe mass conversion (a). However, the Ea value calculated in differ-ent atmospheres had almost the same tendency, which was similarto an ‘‘N” shape. Taking the CO2/O2 = 7/3 atmosphere as an exam-ple, the Ea varied with increasing conversions. It increased steeplyfrom approximately 19.24 to 155.16 kJ�mol�1 at the mass conver-sion (a) at 0.1 < a < 0.35, followed by a decline at 0.35 < a < 0.65,and thereafter, it showed a quick increase at a > 0.65 and dryingand thermal decomposition and volatile degradation at approxi-mately a = 0.1–0.6, with fixed carbon combustion at approxi-mately a > 0.65 (Xu and Chen, 2013). The initial activationenergy value was considered to be low due to cleavage of someof the weak bonds and elimination of the volatile components fromthe biomass because at the beginning of the process, all of thestrong bonds are not cleaved (Gašparoviè, 2012). The SW presentedtwo small values of the activation energy in the range a = 0.1 and0.65 between 24.04 kJ�mol�1 and 61.47 kJ�mol�1.

Additionally, the Ea values at maximum weight loss rate shownin Table 5 were 163.54 kJ�mol�1 in CO2/O2 = 8/2, 116.26 kJ�mol�1 inCO2/O2 = 7/3, 165.48 kJ�mol�1 in CO2/O2 = 5/5 and 230.19 kJ�mol�1

in CO2/O2 = 3/7. As shown in Fig. 6a, with an increasing oxygenconcentration from 20 to 30%, the Ea of SW in CO2/O2 atmospherecurves move toward a lower region. However, with an increasingoxygen concentration from 30 to 70%, the Ea of SW curves movetoward the opposite fashion. Similar findings were also reportedin previous investigations (Chen et al., 2013). The Ea of SW wasaffected by a decreased concentration of activated molecules, dif-fusion limitations and organic impurities during the process ofsample combustion. As the oxygen concentration increases, theheat release from semicoke oxidization increases, and thus, thesurface temperature of the semicoke increases. Additionally, thesemicoke structure expands the grain size and increases the ashcontent, which corresponds to an increase in the final temperature(Chen et al., 2011). Therefore, the Ea increased with an increasedoxygen concentration from 30 to 70%, which revealed that it isnot the more oxygen concentration the easier to response in thecombustion. The lowest activation energy of SW was obtained inCO2/O2 = 7/3 atmosphere.

Fig. 6(a) and (b) represents the values of Ea and DH during theprocess. The changes in the enthalpies revealed that the energy dif-ference between the reagent and the activated complex agreedwith the activation energies (Amanda and Leandro, 2016). Thevariation in DH as well as the respective Ea changes representedthe residual carbonaceous material (Turmanova et al., 2008). Thechanges in DH are due to the energy difference between thereagent and the complex activated species (Xu and Chen, 2013).The positive DH showed that an external source of energy isrequired to raise the energy level of the reagents to their transitionstate, which indicated that the co-combustion reaction of SW inthe CO2/O2 atmospheres were all endothermic. In addition, highervalues of the enthalpy indicate a less reactive system. Based on

in�1.

Rv (%�min�1) Mr (%) Tb (�C) CCI (10�8)

Rp3

1.08 0.698 39.42 627.24 1.3252.23 1.274 39.50 644.59 5.169/ 2.620 40.88 652.21 20.197

Fig. 5. Plots for determination of Ea in (a) CO2/O2 = 8/2, in (b) CO2/O2 = 7/3, in (c) CO2/O2 = 5/5, in (d) CO2/O2 = 3/7 at different a.

640 L. Huang et al. / Bioresource Technology 218 (2016) 631–642

Table 5, lower heat energies were required for the CO2/O2 = 7/3atmosphere than for the CO2/O2 = 3/7 atmosphere and CO2/O2 = 5/5 atmosphere to oxidate the reagents. The change in DHshown in Fig. 6b also showed the energy differences between theactivated complex and the reagents. If this difference was small,then the formation of activated complex was favored because thepotential energy barrier is low (Vlaev et al., 2007). The DH variationof SW was between 16.19 and 123.40 kJ�mol�1 in CO2/O2 = 7/3,35.75 and 161.04 kJ�mol�1 in CO2/O2 = 5/5 and 20.95 and225.79 kJ�mol�1 in CO2/O2 = 3/7, respectively. Thus, the formationof activated complex was more favored in CO2/O2 = 7/3.

The entropy (DS) is associated with the formation of complexactivated species, and it is also a measure of disorder. The variedDS showed that the biomass has a high degree of arrangementand had a physical and/or chemical process. Changes in the entro-pies shown in Fig. 6c had positive and negative values, which wasconsistent with the variation in the values of Ea. A positive DSdemonstrates that the reaction will increase the disorder of thesystem and, thus, is favorable. The DS values indicated that the dis-order of the system decreased in the CO2/O2 = 7/3 atmosphere,while those of the CO2/O2 = 5/5 atmosphere and CO2/O2 = 3/7atmosphere increased. In terms of the disorder, the reaction ten-dency order was the following: CO2/O2 = 3/7 atmosphere, CO2/O2 = 5/5 atmosphere, and CO2/O2 = 7/3 atmosphere. In addition, alow DS means that the material has just passed through some typeof physical or chemical aging process, bringing it to a state that isnear to its own thermodynamic equilibrium. In this situation, the

material shows little reactivity, which increases the time taken toform the activated complex. On the other hand, when high DS val-ues are observed, the material is far from its own thermodynamicequilibrium. In this case, the reactivity is high, and the system canreact faster to produce the activated complex, which results in theshort reaction times and fast mass loss rate that were observed(Turmanova et al., 2008). Thus, it was favored when a = 0.3–0.5during the co-combustion process.

The change in the Gibbs free energy ðDGÞ revealed the totalenergy increase of the system at the approach of the reagentsand the formation of the activated complex. It is a comprehensiveevaluation of the heat flow and disorder change, and a higher valueof DG indicates a lower favorability of reaction. The co-combustionreaction of SW in the CO2/O2 = 3/7 atmosphere had a DG of approx-imately 157.18 kJ�mol�1, while the CO2/O2 = 8/2, CO2/O2 = 5/5 andCO2/O2 = 7/3 atmosphere had values that were higher than that.The combustion reaction limit of SW in the CO2/O2 = 3/7 atmo-sphere was the highest. Thus, the favorability order of co-combustion was the following: CO2/O2 = 3/7, CO2/O2 = 5/5, CO2/O2 = 8/2 and CO2/O2 = 7/3 atmosphere. In terms of the DG, co-combustion in the CO2/O2 = 7/3 atmosphere had the lowest energybarrier and absorbed the least amount of heat during the process,and yet, it decreased the disorder of the system, and hence, it hadthe lowest favorability. In contrast, co-combustion in the CO2/O2 = 3/7 atmosphere had the highest energy barrier and requiredthe highest amount of heat, but it had the highest favorability ofreaction because it greatly increased the disorder of the system.

Fig. 6. Changes of (a) Ea, (b) H, (c) S, (d) G during the SW degradation process.

Table 5Thermodynamic and kinetic parameters of SW calculated at the maximum differential mass conversion in combustion stages in oxy-fuel combustion.

Atmosphere a at max. weight loss rate Tm (K) Ea (kJ/mol) Correlation coefficient A (s�1) 4H (kJ/mol) 4G (kJ/mol) 4S (J/mol)

CO2/O2 = 8/2 0.318 516.25 163.54 0.993 3.03 � 1014 158.70 159.89 �1.42CO2/O2 = 7/3 0.341 515.55 116.26 1.000 3.78 � 109 110.90 161.59 �89.86CO2/O2 = 5/5 0.335 514.35 165.48 0.993 6.79 � 1014 161.04 158.76 4.00CO2/O2 = 3/7 0.331 513.45 230.19 0.989 2.35 � 1021 225.79 157.18 129.65

L. Huang et al. / Bioresource Technology 218 (2016) 631–642 641

4. Conclusions

The combustion performance of SS added with 10–40 wt.% WHwas improved 1–1.97 times, as revealed by CCI. As the oxygen con-centration increased from 20% to 70%, the ignition temperature ofSW decreased from 243.1 �C to 240.3 �C, and the maximum weightloss rate and CCI increased from 5.70%�min�1 to 7.26%�min�1 andfrom 4.913%2�K�3�min�2 to 6.327%2�K�3�min�2, respectively,which corresponded to the calculated variation in DS and DG .The lowest activation energy (Ea) of SW was obtained in CO2/O2 = 7/3 atmosphere.

Acknowledgements

This work was supported by the National Natural Science Foun-dation of China (No. 51308132), the Science and Technology Plan-ning Project of Guangdong Province, China (Nos. 2015B020235013;2014A050503063; 2015A020215033), the Scientific and Techno-logical Planning Project of Guangzhou, China (No. 201510010033,

2016201604030058) and Guangdong Special Support Program forTraining High Level Talents (No. 2014TQ01Z248).

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