Pyrolysis, combustion and gasification characteristics of miscanthus and sewage sludge

Energy Conversion and Management 89 (2015) 83–91

Contents lists available at ScienceDirect

Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman

Pyrolysis, combustion and gasification characteristics of miscanthus and sewage sludge Kandasamy Jayaraman ⇑,1, Iskender Gökalp ICARE-CNRS, France

a r t i c l e

i n f o

Article history: Received 21 July 2014 Accepted 20 September 2014

Keywords: Miscanthus Sewage sludge Combustion Gasification Kinetics

a b s t r a c t The energetic conversion of biomass into syngas is considered as reliable energy source. In this context, biomass (miscanthus) and sewage sludge have been investigated. A simultaneous thermal analyzer and mass spectrometer was used for the characterization of samples and identified the volatiles evolved during the heating of the sample up to 1100 °C under combustion and gasification conditions. The TG and DTA results were discussed in argon, oxygen, steam and steam blended gas atmospheres. Different stages of pyrolysis, combustion and gasification of the samples have been examined. It was shown that the combustion and gasification of char were occurred in two different temperature zones. The DTA–MS profile of the sample gives information on combustion and gasification process of the samples (ignition, peak combustion and burnout temperatures) and gases released (H2, O2, CO and CO2). The results showed that the different processes were mainly dependent on temperature. The evolution of the gas species was consistent with the weight loss of the samples during pyrolysis, combustion and gasification process. The effect of the ambiences during pyrolysis, combustion and gasification of the samples were reported. The appropriate temperature range to the sludge and miscanthus gasification was evaluated. The kinetic parameters of the biomass and sewage sludge were estimated for TGA using two models based on first-order reactions with distributed activation energies. The presence of ash in the biomass char was more influential during the gasification process. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Biomass gasification represents one of a number of renewable technologies that intend to alleviate an overdependence on fossil-derived hydrocarbons. Biomass sources exhibit higher hydrogen and oxygen content, but a lower carbon content when compared with coal, hence low level of CO2 emissions. The disposal of sewage sludge in an economic and environmentally compatible manner was a common problem to all communities which has municipal waste water treatment facilities. Research and information are required to change public opinion on sewage sludge and to support the most suitable technological choice in each case. Pyrolysis is a process of producing gas or oil from carbonaceous materials using high temperature thermal cracking via an external heat source without the supply of air or steam. The conventional gasification technology makes use of partial combustion by controlling the amount of air to convert hydrocarbons into carbon ⇑ Corresponding author at: ICARE-CNRS, Orleans 45071, France. Tel.: +33 753183282. E-mail address: [email protected] (K. Jayaraman). 1 Researcher http://dx.doi.org/10.1016/j.enconman.2014.09.058 0196-8904/Ó 2014 Elsevier Ltd. All rights reserved.

monoxide, carbon dioxide, and hydrogen. Due to the high thermochemical reactivity of biomass char, the evolution of biomass gasification technology has been an area of increasing interest over the past few decades [1–4]. Traditionally, biomass is used in combustion for energy related applications. The main application is the use of biomass in utility boilers alone or co-fired with coal. In the recent times, alternative solutions are encouraged to increase the biomass conversion efficiency. Several researchers [5–8] have indicated that co-firing of biomass with coal does not only reduce the emissions of greenhouse gases (CO2, CH4, etc.) per unit of energy produced but may have a positive impact on the emission of other pollutants, such as SO2 and NOx. Biomass can be pyrolyzed or gasified for producing liquid fuel or gaseous fuel such as methane, hydrogen and carbon monoxide. Numerous researchers have pointed out that pyrolysis is one of the suitable technologies with industrial perspectives for biomass valorization, since the process conditions can be optimized to maximize the yields of gas, liquid and char [3,9–12]. Some studies have demonstrated the burning characteristic and gaseous emissions in biomass/coal co-combustion during oxidation process [13,14,8,15]. Thermogravimetry coupled with mass spectrometry (TG–MS) is a well-recognized and suitable technique in the pyrolysis research of solid fuels,

84

K. Jayaraman, I. Gökalp / Energy Conversion and Management 89 (2015) 83–91

particularly coal and biomass for its simultaneous and elaborate information about the weight loss and gas formation behaviors as a function of time and temperature [16–20]. In this regard, most studies have been focused on pyrolysis and combustion processes. However, gasification studies have been less reported in literature, being most of them focused on the study of coals [20,21]. TGA–MS is an excellent tool for determining the kinetics of process. Hence, the kinetics of gasification is essential for modeling gasification processes at an industrial scale. Besides, a knowledge of the process kinetics has great importance for a correct design and product yield control. Magdziarz et al. [17] have examined the combustion products of biomass wood/oats, sewage sludge and coal using thermogravimetric analysis (TGA) coupled with mass spectrometer (MS) methods. The effects of co-combustion of biomass or sewage sludge with coal were assessed and found it to be beneficial. Karampinis et al. [22] have described the combustion properties of miscanthus and poplar. They have concluded that the miscanthus and poplar as reactive fuels with high volatile matter with low ash contents. Özgür et al. [23] have implemented the combustion experiments on miscanthus and poplar, also they have found that the combustion of biomass fuels is highly exothermic and suitable as an appropriate fuel feedstock. Kok and Özgür [24] have characterized the miscanthus, poplar, and rice husk samples using differential scanning calorimeter (DSC) and thermogravimetry (TG–DTG) tests and found that the biomass samples have twostage of combustion and the energy release; due to the combustion of fixed carbon in the later stage. Michel et al. [25] have investigated the miscanthus gasification characteristics in steam ambience to produce syngas using GC/MS method. Recently, Mimmo et al. [26] have examined the effect of pyrolysis temperature on miscanthus char. Thermochemical methods are seen as promising alternatives for sewage sludge disposal when compared to traditional routes, due to its inherent improvement in the reduction of waste volume level and energy production. There are several technologies available, both in the market and under different stage of development, for thermal processing of dried sewage sludge. Typically, these can be grouped into three categories, i.e. mono-incineration, co-combustion, and other thermal processes [27]. Thermogravimetric analysis and mass spectrometry (TGA–MS) have been used to describe the sewage sludge thermal decomposition [28], and estimate the reaction kinetics and also to detect the composition of the non-condensable gases. Some researchers have reported that the composition of the non-condensable gases from thermogravimetric experiments of sewage sludge pyrolysis using gas chromatography–mass-spectrometry (GC–MS) systems [17,29,30]. They have found that the weight loss occurs principally in three stages, centered around the temperature at 250, 350 and 550 °C, producing high quantities of gases such as H2, water, hydrocarbons (C1 ± C4, both saturated and unsaturated), methanol, carbon dioxide and acetic acid. Fonseca et al. [31] have described the kinetics of thermally induced reaction such as the combustion of carbonaceous materials. Liu et al. [32] have found that the sewage sludge released the volatiles around 550 °C and above, char combustion occurred under oxygen-enriched air conditions and also estimated kinetic parameters. Otero et al. [33–35] have investigated the combustion of coal–sludge blends using DTG, DSC and MS analysis and determined the kinetic parameters of the process. Hanmin et al. [36] and Folgueras et al. [37] also studied the co-combustion of coal and sewage sludge using thermo gravimetric analysis. Ischia et al. [38] used clay in a TG–MS study to evaluate the possible advantages of co-pyrolysis of clay and sewage sludge. Su et al. [39] have established the using of TG–FTIR analysis of sewage sludge, in which the released HCl and SO2 decreased with the optimum condition, and also satisfying the environmental requirement. Werther and Ogada [27] have conducted the detailed

study of sewage sludge combustion incorporating various issues. They have stated that the sewage sludge combustion releases low net emissions of NOx along with the conversion ratio of fuel N to NOx being less than 5%. Fonts et al. [40] reviewed the liquid production from sewage sludge pyrolysis, in which recent thermogravimetric pyrolysis mechanism was also discussed. Shao et al. [41] have demonstrated the different stages of pyrolysis and kinetics of sewage sludge using thermogravimetry and FTIR analyzers. Singh et al. [42] have reported the pyrolysis characteristics of waste materials, biomass wood waste, scrap tyre, refuse derived fuel (RDF) and waste plastic materials using TGA–MS and TGA– FTIR analyzers. Soria-Verdugo et al. [43] have reported the biomass and sewage sludge devolatilzation and estimated the kinetic parameters using distributed activation energy model. Since, most of the papers were examined the pyrolysis characteristics of miscanthus and sewage sludge. There is limited research on combustion and gasification characteristics of these materials, this work dealt the gasification of miscanthus and sewage sludge that were selected according to their potential usage in thermochemical conversion processes. This study was performed to understand process along with various factors which influences the combustion and gasification process. Also, it is necessary to understand the pyrolysis mechanism of miscanthus and sewage sludge in order to increase conversion of solids to oil and gases, especially to increase H2 during the gasification. This paper aims at a better understanding of the basic phenomena associated with thermogravimetric analysis of miscanthus and sewage sludge. Particularly, volatile matter evolution and burning characteristics of biomass char during combustion have been estimated. The pyrolysis of biomass samples was carried out to obtain a solid fuel (char), subsequently it was combusted with oxygen and in another test which was gasified using steam/steam mixtures. The gases released during the gasification process were analyzed by MS. Moreover, kinetics of combustion and gasification of miscanthus and sewage sludge have not been documented so well under oxy-fuel atmospheres and steam/steam mixtures, hence the need for the present research. Hence, a preliminary kinetic analysis of the thermo chemical conversion process was performed in order to obtain the apparent reaction rates by using two kinetic models: Friedman method and Coats and Redfern method.

2. Experimental 2.1. Materials and thermogravimetric analysis Miscanthus and sewage sludge samples were originated from France. The size of the miscanthus sample was in the range from 2 to 4 mm, whereas the sewage sludge samples were finer particles, which comprises from few microns to 1 mm size ranges. The dried sewage sludge composition is almost comparable with miscanthus. So, dried sludge is opted for the present study. The ultimate and proximate analyses of the miscanthus and sewage sludge sample are given in Table 1. Thermogravimetric analysis study was performed using a NETZSCH STA 429 thermal analyser at inert and reactive atmospheres containing argon and steam at STP conditions up to 1100 °C. The experimental setup used for the gasification experiments was described in a previous study [20]. A separate steam (water vapor – WV) generator was connected with the STA, in which steam generator and transfer line were maintained at the temperature of 180 °C and 150 °C respectively. During the experiment, about 80 mg of sample was placed in a ceramic crucible and heated up to 1150 °C with a heating rate of 40 °C/min and isothermal sections retained at 950 and 1000 °C in some cases. Argon was used as protective gas with the flow rate of 20 ml/min in pyrolysis and combustion process. In gasification

K. Jayaraman, I. Gökalp / Energy Conversion and Management 89 (2015) 83–91 Table 1 Proximate analysis, ultimate analysis and heating values of miscanthus and sewage sludge samples. Unit

Miscanthus

Sewage sludge

Proximate analysis Moisture Ash V.M Fixed carbon

% % % %

9.8 0.4 69.4 20.4

6.2 15.9 58.9 19

Ultimate analysis C H N O S

% % % % %

53.4 4.4 0.48 41.3 0.3

58.5 9 5 27.45 0.045

Heating value Calorific value

MJ/kg

16.8

20.43

process, argon was used as protective gas (20 ml/min) and carrier gas for the steam (20 ml/min). Steam with the flow rates of 6 g/h was utilized and air with the flow rate of 2 ml/min was used in the gasification process. Oxygen was used with the flow rate of 2 ml/min in steam blended gasification process and 50 ml/min in combustion process. These flow rates were programmed and automatically controlled by the ATG system. The output of the TGA system was connected to the mass spectrometer through a heated line with quartz capillary tube. Mass spectrometric analyzer was used to monitor flue gas composition during the thermal analysis. The TGA–MS runs were carried out in a dynamic gas atmosphere. The sample temperature was measured with a type S (Pt–Rh10/Pt) thermocouple directly which was placed under the sample holder. The low volumes in the thermo balance microfurnace, transfer line, and gas measurement cell permit low carrier gas flow rates was used and allow for good detection of the gases evolved in the pyrolysis, combustion and gasification process. The experimental data were stored as a function of time. The experimental errors were within the acceptable limits.

85

integral method [45] which incorporates the thermal degradation mechanism) were used to study the activation energy of the nonisothermal degradation of the biomass samples. 2.3.1. Friedman method The basic kinetic equation is:

da ¼ kf ðaÞ dt

ð1Þ

where a is the conversion level of the material, defined as following:

a¼

m0  m m0  m1

where m0 is the initial sample weight, m is the sample weight at time t, and m1 is the final sample weight. The temperature dependence of k is expressed by the following Arrhenius equation:

  Ea k ¼ A exp  RT where A is pre-exponential factor (min1); Ea is apparent activation energy (kJ mol1); T is reaction temperature (K); R is gas constant, it equals to 8.314  103 kJ mol1 K1, (1) can be expressed in general terms as written below:

  da da ; ¼b dt dT

where b ¼

  dT ; heating rate ðK=minÞ dt

ð2Þ

Combining the two equations and taking the logarithm

 ln

    da da Ea ¼ ln b ¼ ln ½Af ðaÞ  dt dT RT

It is assumed that the conversion function f(a) remains constant, which implies that biomass degradation is independent of temperature and depends only on the rate of mass loss. A plot of ln[da/dt] versus 1/T yields a straight line, the slope corresponds to Ea/R.

2.2. Mass spectrometric analysis of the gaseous products Online gas analyses were performed for the detection of released gases from thermogravimetry which was fed to quadrupole QMG 511 mass spectrometer. The balance adapter, the transfer line, and the MS gas cell were heated until 250 °C, thus avoiding the condensation of the less volatile compounds. The excitation energy in the mass spectrometer is 1100 eV. The MS was operated under a vacuum and detected the characteristic fragment ion intensity of the volatiles according to their respective mass to charge ratios (m/z). The mass spectrum intensity shows the relative gas release components based on mass to charge ratios qualitatively. A semiqualitative analysis of the gases produced from the biomass samples was performed by comparing the intensity peak areas obtained for each compound. Screening analyses were carried out in the selected-ion monitoring (SIM) mode. The following ions characteristic of each molecules, such as: 2, 16, 18, 28, 32 and 44, for H2, CH4, H2O, CO, O2 and CO2 respectively, were monitored. It is important to notice that the QMS spectrum of mass 16 can represents not only CH4 but also the O fragment of O2 molecules, and 28 can represent CO and N2. 2.3. Kinetic analysis The thermogravimetric analysis was used to determine the kinetic parameters of the thermal degradation of the biomass samples. Two methods, the differential isoconversional technique (Friedman method [44]) and integral method (Coats and Redfern

2.3.2. Coats and Redfern method (CR) The reaction temperature can be expressed with:

T ¼ T 0 þ bt where T0 is the initial temperature (K). Eqs. (1) and (2) is changed as follows:

  da A Ea ¼ f ðaÞ exp  dt b RT

ð3Þ

f(a) is presented as:

f ðaÞ ¼ ð1  aÞn

ð4Þ

where n is the reaction order A combination of Eqs. (3) and (4), with the further integration, it becomes:

Z

a

da n 2

ð1  aÞ T

0

¼

     AR Ea Ea  exp  exp  bEa RT RT 0 

Ea because, 1  exp  RT 0 can be reduced to:

ln

Z

a 0

da n 2

ð1  aÞ T

¼ ln



ð5Þ

in the tested temperature ranges, Eq. (5)

AR Ea  bEa RT

ð6Þ

The left side of Eq. (6) can be plotted against 1/T from which the slope of the straight line is Ea/R. In this work, a reaction order of n = 1.0 is used to calculate the kinetic parameters, since a first order

86

K. Jayaraman, I. Gökalp / Energy Conversion and Management 89 (2015) 83–91

reaction model matches well with the experimental data as reported by other researchers [39,46–50].

3. Results and discussion 3.1. Pyrolysis, combustion and gasification characteristics of miscanthus In this study, the sample was subjected to dynamic heating rate of 40 °C/min until it reached the temperature of 1100 °C in presence of inert and reactive gases. The experiments were carried out in argon, oxygen, steam and steam blended mixture (steam + air + oxygen) ambience to evaluate the pyrolysis, combustion and gasification characteristics of miscanthus respectively. TG and DTG analyses on miscanthus were conducted by measuring the decrease in mass of fuel with the increase in temperature, as shown in Fig. 1. The curves of mass conversion level versus temperature in pyrolysis, combustion and gasification are shown in Fig. 2. The conversion level of the sample is carried out based on ash and char free basis in argon ambience; whereas in oxygen and steam ambience was considered only on ash free basis. The TG–DTG curves of the sample indicate that major pyrolysis occurs in the temperature range from 280 to 400 °C in reactive ambiences, DTGmax is at 315 °C. On the other hand, the DTGmax component in argon ambience is slightly shifted to higher temperature range which is occurred at 350 °C. The presence of oxygen is slightly shifted the maximum pyrolysis phase into lower temperature ranges when compared to argon ambience; also it can be seen that the steam blended ambience shows the early decomposition stage. This may be caused due to surrounding gases diffusional effects over the evolved gases from pyrolysis. It is clearly shown that the combustion and gasification stages are occurred in two different temperature zones. The evolution of gaseous species and products as a result of decomposition of the sample was simultaneously monitored by mass spectrometry during the TGA test. The mass spectra of the gases evolved during pyrolysis are illustrated in Fig. 3. The primary devolatilization stage of miscanthus was observed with major weight loss and the release of organic compounds which leads to the formation of char. This stage releases CO, CO2, H2O, O2, H2 and CH4 as the major gaseous products, in which 70% of total mass loss takes place. The maxima in the DTG curve corresponds to the maximum gas release, as shown in Fig. 3. The first phase change corresponds to devolatilization and volatile matter release due to hemicelluloses, cellulose and lignin decomposition, this is usually occurred in the temperature from 190 to 450 °C (190–320 °C for hemicelluloses, 280–400 °C for celluloses and 320–450 °C for lignin), and can also be split into two smaller peaks if hemicellulose concentration is high [45]. The CH4 and H2 peaks are detected in the secondary pyrolysis of the samples at the temperature ranges from 500 to 750 °C, about 13% of total mass loss. The final pyrolysis

DTG

O2 O2

TG

TG

Fig. 1. TG and DTG curves of miscanthus in different ambiences.

Fig. 2. Curves of conversion of miscanthus for different stages.

Fig. 3. TG and gas evolution profiles of miscanthus in argon ambience.

temperature was assessed in such a way that no further weight loss is appreciated (temperature at which weight loss rate is below 0.01 DTG – %/min). Secondary pyrolysis is the result of decomposition of heavy molecules in the char. The H2 evolved is, in fact, only a part of the hydrogen present due to the higher volatile matter content in the sample. Hence, miscanthus decomposition is a complex process that involves devolatilization and pyrolysis of the sample, essentially based on chemical constituents of the sample. The characteristic temperatures and parameters of Tig (onset, ignition temperature), Tpmax (the temperature corresponding to the peak of the derivative thermogravimetric – DTG curve), Tb (burn out temperature) and |(dm/dt)2max| are investigated to explore the combustion characteristics of samples. The burn out temperature of combustion is also determined as final pyrolysis temperature. Fig. 1 shows the temporal weight loss (mass fraction) and DTG curves of the miscanthus in oxygen ambience. In these tests, the TG/DTG plots clearly suggested that there are two stages of weight loss. The first region on the TG and DTG curve (67% of total mass fraction) is associated the heating up, pyrolysis of miscanthus particles and includes the release, ignition, and release of volatile matter, which is started above 260 °C (Tig). The second region is occurred due to the oxidation of the remaining char after the volatiles are removed from the samples and the gradual diffusion of oxygen to the surface of the samples and subsequent combustion. In this process, fixed carbon (20% total mass fractions) along with un-pyrolyzed compounds (around 13% of total mass fraction) in the char is combusted. The burn out temperature of sample is around 560 °C (Tb) which is in consistent with the reported values of other researchers [24,51] in similar conditions. TG analysis can also give important information about the presence of ash after combustion, as can be determined in which the values are less than 0.5%, also reported in Table 1. More amounts

K. Jayaraman, I. Gökalp / Energy Conversion and Management 89 (2015) 83–91

Fig. 4. TG and gas evolution profiles of miscanthus in oxygen ambience.

of CO and steam is released during devolatilization stage but declined in the later combustion stage as shown in Fig. 4. But, the CO2 and CH4 gases released significantly during the combustion process. As mentioned that the CH4 and oxygen fragment curves coincided with one another, hence, the intensity level of CH4 is relatively more in oxygen ambience when compared in argon ambience. It can be seen that the formation of CH4 and CO is observed even with the oxygen concentration of 70% in the miscanthus char combustion. Hence, it is concluded that the partial oxidation of char (2C + O2 M 2CO) and secondary reactions as methanation (C + H2 M CH4) are also occurred in the tested temperature range when compare the CO and CH4 intensity level from Figs. 3 and 4. In the gasification process, the sample was subjected to dynamic heating rate until 1100 °C in the presence of steam and blended mixture of steam, air and oxygen ambience, as illustrated in Figs. 1, 5 and 6. The char gasification process is more complex than the pyrolysis, as the former is a heterogeneous process where the chemical reactions are occurred over the surface of the material. Also, it can be admitted that the heterogeneous rates of char conversion are ascertained by the fundamental components, represented by surface area, surface accessibility, carbon active sites, catalytic active sites created by indigenous or added inorganic matter, and the gasification agent composition. Additionally, the reactivity depends on three chief characteristics of the sample: chemical structure, porosity and inorganic constituents. After pyrolysis, no significant mass loss or gas evolution is observed over the temperature ranges between 750 and 800 °C. The gasification process of char is started above 800 °C in steam ambience (Fig. 1), which was very close to the reported temperature by Wilson [52] using coal–char. The combustion and gasification

87

Fig. 6. TG and gas evolution profiles of miscanthus in the mixture of steam, air and oxygen ambience.

process started at around 750 °C in steam and air/oxygen blended medium, consequently |(dm/dt)2max| is shifted to lower temperature regions. The complete burn out of miscanthus sample occurs at 1000 °C in blended ambience and, occurred at 1050 °C in steam ambience. The oxygen content influences in the blended ambiences which leads to increase the reaction rate of the sample. The evolution of H2 commenced at around 350 °C in the pyrolysis stage, and reached the peak value at 950 °C due to the char gasification process in the steam blended ambience, and then decreased rapidly, similar trend were observed for CO also (Fig. 5). Hence, the high amount of H2 and CO pointed out that char gasification reactions (C + H2O M CO + H2; C + 2H2O M CO + 2H2) were predominant. The release of the CO2 is reached the peak at 950 °C in the steam blended ambience during gasification process. It can be seen that the H2 flow rate is slightly reduced when the ambience is shifted from pure steam to steam blended oxygen and air, as illustrated in Figs. 5 and 6. This may be either caused due to the H2 combustion with O2 or inhibition of steam reaction with char, because the intensity of CO is almost constant in both the cases. Also, there is slight increment of CO2 level in steam blended ambience. Additionally, there is methanation reaction also taking place in both the tests, which is not shown in the figures due to the overlapping of OH fragments from H2O and oxygen. Fig. 12 shows the time resolved mass loss and temperature characteristics of miscanthus in blended ambiences in isothermal conditions at 950 °C. It is observed that the complete conversion takes place while maintain the temperature of about 5 min. This clearly establishes that the miscanthus gasification can be carried out at around 900–1000 °C using the blended gaseous steam and air/mixtures with an efficient and complete conversion process for syngas production. 3.2. Pyrolysis, combustion and gasification characteristics of sewage sludge

Fig. 5. TG and gas evolution profiles of miscanthus in steam ambience.

In this study, the sewage sludge sample was heated up to 1100 °C in a dynamic heating rate of 40 °C/min. The pyrolysis, combustion and gasification of the dried sludge can be classified as primary pyrolysis and secondary reaction (i.e., pyrolysis and/or gasification), as shown in Fig. 7. The curves of sludge conversion versus temperature in pyrolysis, combustion and gasification are shown in Fig. 8. Combustion and gasification processes are occurred in different temperature ranges which are almost identical to miscanthus samples. The dried sludge is converted into char, tar, and gas during the primary pyrolysis process which is occurred in all the ambiences, but small shift in temperature among argon and other ambiences, which is explained elaborately in miscanthus section. During devolatilization process, the major mass loss starts at 280 °C and

88

K. Jayaraman, I. Gökalp / Energy Conversion and Management 89 (2015) 83–91

DTG

O2 O2

TG

Fig. 7. TG and DTG curves of sewage sludge in different ambiences.

CO, and CO2 in both stages. Decrease in mass at higher temperature under inert condition can be thought due to the cross-linking reactions and the decomposition of functional groups from char. The mass loss of the sample at 1050 °C shows the small amount release of CO, O2 and H2 gases, which accounts 3% of total mass fraction. The TG and DTG profiles in Fig. 7 show that the oxidation of the sewage sludge begins at 380 °C (Tig) and continued until the burn out temperature of 550 °C (Tb), the maximum DTG is occurred at 500 °C (DTGmax). The continuous release of CO, CO2 and H2 gases during this process demonstrated that the burning process occurred at a high oxidation rate as illustrated in Fig. 9. Small amount of CH4 is also released in this stage. The indication of the residual mass of the sample describes ash content which is illustrated in Table 1. As observed in miscanthus char combustion, the formation of CH4 and CO is observed with the oxygen concentration of 70% in the sludge char combustion. Consequently, it is confirmed that the partial oxidation of char is occurred in the tested temperature range when compare the CO magnitudes from Figs. 9 and 10. Figs. 7, 11 and 12 show the mass fraction, residual mass and gas evolution rate as functions of the elapsed temperature in the steam and steam blended oxygen and air ambience. During devolatilization and pyrolysis process, the major mass loss and gas evolution has occurred simultaneously at each temperature during continuous heating of the sample and completed at the temperature of around 600 °C. The results in this study show that the influence of steam on char yield is negligible during pyrolysis stage. After pyrolysis, no significant mass loss or gas evolution is observed at the reactor temperatures between 700 and 750 °C. Hereafter, CO,

Fig. 8. Curves of conversion of sewage sludge for different stages.

completed at 380 °C which is slightly higher with the reported values by Shao et al. [41], but it is lower when compare with the estimation of Magdziarz et al. [17]. This may be due to the differences in mineral and carbonaceous ingredients of tested and reported samples. The gaseous species are released during this process. Sewage sludge is mainly composed of cellulose, a significant amount is converted into tar during the primary pyrolysis (55% of total mass fraction). Then, several residual parts in the tar convert into gas during the secondary reaction [54]. The secondary pyrolysis occurred between the temperature range of 380 and 700 °C (around 19% of total mass fraction). Then, no mass loss is observed until 1000 °C, and some mass loss is detected up to the temperature of 1100 °C. The analysis of the temperature-resolved gas intensity measurements at the exhaust indicates the different profiles of the released gas species along the tested temperature as shown in Fig. 9; earlier release of CH4, H2O and O2, later release of H2 and

Fig. 9. TG and gas evolution profiles of sewage sludge in argon ambience.

Fig. 10. TG and gas evolution profiles of sewage sludge in oxygen ambience.

Fig. 11. TG and gas evolution profiles of sewage sludge in steam ambience.

89

K. Jayaraman, I. Gökalp / Energy Conversion and Management 89 (2015) 83–91

gaseous steam, oxygen and air with efficient carbon conversion. These results will be helpful for researchers to understand gasification, gas evolution process, appropriate gasification temperature, and to develop the utilization technologies by the use of sewage sludge samples. 3.3. Kinetic analysis

Fig. 12. TG and gas evolution profiles of sewage sludge in the mixture of steam, air and oxygen ambience.

Fig. 13. TG analysis in the mixture of steam, oxygen and air ambience.

H2 and CO2 evolutions are observed at the sample temperatures up to 1000 °C in which the gasification process was followed. The maximum mass loss is estimated at the temperature of 940 °C under both the steam and steam blended ambiences. These mass loss and gas evolution are caused by in-situ char reactions, namely char gasification and nascent char carbonization. The evolution of H2 reached its peak at 950 °C due to steam – char gasification in the steam and steam blended ambience, and then decreased rapidly, similar trend observed for CO and CO2 also (Figs. 11 and 12). It can be observed that the H2 flow rate is slightly reduced when the ambience was shifted from pure steam to steam blended oxygen and air. The complete burn out of the char occurs at 950 °C to keep isothermal conditions for longer time, as illustrated in Fig. 13, also the similar statistics reported by Chun et al. [53]. The mass loss is preserved even if the temperature is increased to 1000 °C. These results show that the sewage sludge gasification can be carried out at the temperature of 950 °C using the blended

Kinetic analysis is performed to provide the theoretical basis for the behavior of pyrolysis, combustion and gasification of the samples. Solid state kinetic data are of major interest in technological processes related to energy production from biomass resources using combustion and gasification approach. Furthermore, the combustion of fuels is usually characterized by consecutive steps or zones which can be easily differentiated by TGA. The kinetic parameters can be determined assuming single separate reactions for the different stages of thermal conversion. These stages are characterized for each fuel and, in the case of sewage sludge and miscanthus, it has been seen that, by their respective thermally influential properties, they are obviously different. Biomass ignition began with the release of volatiles. Thus, the ignition temperature of the combustion process is due to the early release of the volatiles from the fermentation residue, which is considered as the reaction mechanism function. A regression analysis with the least square method was used to determine the best straight line. High correlation was obtained for the calculation of the activation energy, thus a reasonable kinetic results were expected in this work. The activation energy (E) of the biomass and sewage sludge samples was determined for low temperature thermal decomposition and combustion and high temperature gasification processes, the results are given in Table 2. Activation energy during the combustion of the miscanthus is 116 kJ/mol using Friedman method, and 88 kJ/mol using CR method which are slightly lower with the reported values of 136 kJ/mol [24] using other methods. The activation energy of the sewage sludge combustion process using Friedman method is 38.9 kJ/mol and CR method is 60.4 kJ/mol which is comparable with the reported values of 49–70 kJ/mol over the tested temperature ranges. The activation energy of the miscanthus sample during pyrolysis is varied from 85 to 96 kJ/mol using two methods. Similarly the activation energies of the sewage sludge are calculated as 33 kJ/mol and 56 kJ/mol using Friedman and CR methods respectively. These values are relatively lower to compare the reported values by Seo et al. [55] in which the activation energy was reported as 78.4 kJ/mol in the temperature range from 224 to 360 °C. The activation energy of the miscanthus during gasification is almost twice when compare to sewage sludge using Friedman method. On the other hand, using CR method, the gasification in steam and steam mixtures activation energy is nearer to 46 kJ/ mol in the tested high temperature ranges. Concerning the comparisons between the kinetic analysis of miscanthus and sewage sludge biomass ones, it can be observed that miscanthus samples

Table 2 Kinetic parameters of miscanthus and sludge samples in different atmospheres. Type of samples and ambient conditions

Miscanthus, oxygen Sewage sludge, oxygen Miscanthus, steam Sewage sludge, steam Miscanthus, steam + oxygen + air Sewage sludge, steam + oxygen + air Miscanthus, argon Sewage sludge, argon

Temperature range (°C)

230–330 200–310 815–1000 800–940 800–900 800–940 250–370 220–375

Activation energy (kJ/mol) Friedman method

Coats and Redfern method

116.2 38.9 107.58 59.39 108.2 51.7 84.8 33.1

88.4 60.4 47.4 45.7 45.9 46.5 96.4 56.6

90

K. Jayaraman, I. Gökalp / Energy Conversion and Management 89 (2015) 83–91

showed higher E values than sewage sludge, which confirmed the higher thermal resistance of their main components. While sewage sludge gasification is dominated by mass-loss at lower temperature, miscanthus loses more mass by mid- to high-temperature char gasification. In general, the char produced from miscanthus contains more lignin component which has high surface area and porous char, facilitates the diffusion of the reactive agent and thus, turning into high gasification rates. On the other hand, cellulose in sewage sludge yields a char with a fibrous structure, lowering the char reactivity. On contrary, the present study reveals that the sewage sludge has higher combustion and gasification rates. This is due to the inorganic matter presented in the sewage sludge ash act as catalyst which promotes the reaction rates. So, the gasification rates of the biomass char is more influenced by the mineral matter in the ash than their initial chemical composition. 4. Conclusions The pyrolysis, combustion, and gasification behaviors of sewage sludge and biomass were investigated using TGA–MS method. The results indicate that the ignition temperature, temperature of maximum mass loss rate and burn out temperatures of miscanthus were occurred at lower temperature range when compare with sewage sludge. The elementary and proximate analysis of the sewage sludge and biomass indicated the differences between these materials. Consequently, the corresponding TG and gas evolution rate were also dissimilar. Different stages of pyrolysis, combustion and gasification of the samples have been analyzed. It is clearly shown that the combustion and gasification stages were occurred in two different temperature zones. The gasification process started at around 750 °C in steam and air/oxygen blended medium for miscanthus samples, while sewage sludge gasification was dominated by mass loss at lower temperature, miscanthus loses more mass by mid- to high-temperature char gasification. The complete burn out of both the sewage sludge char occurs at 950 °C to keep isothermal conditions for longer time. Results indicate that the gasification can be carried out at around 900–1000 °C for miscanthus and 850–950 °C for sludge using the blended gases of steam and air mixtures with an efficient and complete conversion process to produce syngas. The present study has shown that TGA and MS were effective tools for a first and fast assessment of these carbonaceous materials in fundamental research. The kinetic parameters estimated for the combustion and gasification of the biomass and the waste materials employing TGA instruments were in complete agreement using Coats and Redfern method. The activation energy of the miscanthus during gasification is almost twice when compare to sewage sludge. The presence of ash in the biomass char is more influential during the gasification process. Acknowledgement Financial support for this work is provided by the European Commission: Project OPTIMASH, FP7-ENERGY, 2011-1 Project No. 283050. References [1] Puig Arnavat M, Bruno JC, Coronas A. Review and analysis of biomass gasification models. Renew Sustain Energy Rev 2010;14:2841–51. [2] Fatih Demirbas M, Balat M, Balat H. Potential contribution of biomass to the sustainable energy development. Energy Convers Manage 2009;50:1746–60. [3] Kırtay E. Recent advances in production of hydrogen from biomass. Energy Convers Manage 2011;52:1778–89. [4] Chum HL, Overend RP. Biomass and renewable fuels. Fuel Process Technol 2001;71:187–95. [5] Robinson AL, Junker H, Buckley SG, Sclippa G, Baxter LL. Interactions between coal and biomass when cofiring. Proc Combust Inst 1998;27:1351–9.

[6] Demirbas A. Sustainable cofiring of biomass with coal. Energy Convers Manage 2003;44:1465–79. [7] Sami M, Annamalai K, Wooldridge M. Co-firing of coal and biomass fuel blends. Prog Energy Combust Sci 2001;27:171–214. [8] Haykiri-Acma H, Yaman S. Effect of biomass on burnouts of Turkish lignites during co-firing. Energy Convers Manage 2009;50:2422–7. [9] Czernik S, Bridgwater AV. Overview of applications of biomass fast pyrolysis oil. Energy Fuel 2004;18:590–8. [10] Bridgwater AV. Review of fast pyrolysis of biomass and product upgrading. Biomass Bioenergy 2012;38:68–94. [11] Yaman S. Pyrolysis of biomass to produce fuels and chemical feedstocks. Energy Convers Manage 2004;45:651–71. [12] Wiedner K, Rumpel C, Steiner C, Pozzi A, Maas R, Glaser B. Chemical evaluation of chars produced by thermochemical conversion (gasification, pyrolysis and hydrothermal carbonization) of agro-industrial biomass on a commercial scale. Biomass Bioenergy 2013;59:264–78. [13] Kastanaki E, Vamvuka D. A comparative reactivity and kinetic study on the combustion of coal–biomass char blends. Fuel 2006;85:1186–93. [14] Sonobe T, Worasuwannarak N, Pipatmanomai S. Synergies in co-pyrolysis of Thai lignite and corncob. Fuel Process Technol 2008;89:1371–8. [15] Zhang L, Xu C, Champagne P. Overview of recent advances in thermo-chemical conversion of biomass. Energy Convers Manage 2010;51:969–82. [16] Worasuwannarak N, Sonobe T, Tanthapanichakoon W. Pyrolysis behaviors of rich straw, rice husk and corncob by TG–MS technique. J Anal Appl Pyrol 2007;78:265–71. [17] Magdziarz A, Wilk M. Thermogravimetric study of biomass, sewage sludge and coal combustion. Energy Convers Manage 2013;75:425–30. [18] Kebelmann K, Hornung A, Karsten U, Griffiths G. Intermediate pyrolysis and product identification by TGA and Py–GC/MS of green microalgae and their extracted protein and lipid components. Biomass Bioenergy 2013; 49:38–48. [19] Ozveren U, Ozdogan ZS. Investigation of the slow pyrolysis kinetics of olive oil pomace using thermo-gravimetric analysis coupled with mass spectrometry. Biomass Bioenergy 2013;58:168–79. [20] Jayaraman K, Gokalp I. Thermal characterization, gasification and kinetic studies of different sized Indian coal and char particles. Int J Adv Eng Sci Appl Math 2014;6:31–40. [21] Shabbar S, Janajreh I. Thermodynamic equilibrium analysis of coal gasification using Gibbs energy minimization method. Energy Convers Manage 2013;65:755–63. [22] Karampinis E, Vamvuka D, Sfakiotakis S, Grammelis P, Itskos G, Kakaras E. Comparative study of combustion properties of five energy crops and Greek lignite. Energy Fuels 2012;26:869–78. [23] Özgür E, Miller BG, Miller SF, Kok MV. Thermal analysis of co-firing of oil shale and biomass fuels. Oil Shale 2012;29:190–201. [24] Kok MV, Özgür E. Thermal analysis and kinetics of biomass samples. Fuel Process Technol 2013;106:739–43. [25] Michel R, Rapagna S, Burg P, Di Celso GM, Courson C, Zimny T, et al. Steam gasification of miscanthus  giganteus with olivine as catalyst production of syngas and analysis of tars (IR, NMR and GC/MS). Biomass Bioenergy 2011;35:2650–8. [26] Mimmo T, Panzacchi P, Baratieri M, Davies CA, Tonon G. Effect of pyrolysis temperature on miscanthus (miscanthus  giganteus) biochar physical, chemical and functional properties. Biomass Bioenergy 2014;62:149–57. [27] Werther J, Ogada T. Sewage sludge combustion. Prog Energy Combust Sci 1999;25:55–116. [28] Thipkhunthod P, Meeyoo V, Rangsunvigit P, Kitiyanan B, Siemanond K, Rirksomboon T. Pyrolytic characteristics of sewage sludge. Chemosphere 2006;64:955–62. [29] Caballero JA, Front R, Marcilla A, Conesa JA. Characterization of sewage sludges by primary and secondary pyrolysis. J Anal Appl Pyrol 1997;40–41:433–50. [30] Conesa JA, Marcilla A, Moral R, Moreno-Caselles J, Perez-Espinosa A. Evolution of gases in the primary pyrolysis of different sewage sludges. Thermochim Acta 1998;313:63–73. [31] López-Fonseca R, Landa I, Gutiérrez-Ortiz MA, González-Velasco JR. Nonisothermal analysis of the kinetics of the combustion of carbonaceous materials. J Therm Anal Calorim 2005;80:65–9. [32] Liu GH, Ma XQ, Yu Z. Experimental and kinetic modeling of oxygen-enriched air combustion of municipal solid waste. Waste Manage 2009;29:792–6. [33] Otero M, Gomez X, Garcıa AI, Moran A. Effects of sewage sludge blending on the coal combustion. A thermogravimetric assessment. Chemosphere 2007;69:1740–50. [34] Otero M, Diez C, Calvo LF, et al. Analysis of the co-combustion of sewage sludge and coal by TG–MS. Biomass Bioenergy 2002;22:319–29. [35] Otero M, Calvo LF, Gil MV. Co-combustion of different sewage sludge and coal: a non-isothermal thermogravimetric kinetic analysis. Bioresour Technol 2008;99:6311–9. [36] Xiao Hanmin, Ma Xiaoqian, Liu Kai. Co-combustion kinetics of sewage sludge with coal and coal gangue under different atmospheres. Energy Convers Manage 2010;51:1976–80. [37] Folgueras MB, Diaz RM, Xiberta J. Thermo gravimetric analysis of the cocombustion of coal and sewage sludge. Fuel 2003;82:2051–5. [38] Ischia M, Dal Maschio R, Grigiante M, Baratieri M. Clay–sewage sludge copyrolysis. A TG–MS and Py–GC study on potential advantages afforded by the presence of clay in the pyrolysis of wastewater sewage sludge. Waste Manage (Oxford) 2011;31:71–7.

K. Jayaraman, I. Gökalp / Energy Conversion and Management 89 (2015) 83–91 [39] Su W, Ma H, Wang Q, Li J, Ma J. Thermal behavior and gaseous emission analysis during co-combustion of ethanol fermentation residue from food waste and coal using TG–FTIR. J Anal Appl Pyrol 2013;99:79–84. [40] Fonts I, Gea G, Azuara M, Ábrego J, Arauzo J. Sewage sludge pyrolysis for liquid production: a review. Renew Sustain Energy Rev 2012;16:2781–805. [41] Shao J, Yan R, Chen H, Wang B, Lee DH, Liang DT. Pyrolysis characteristics and kinetics of sewage sludge by thermogravimetry Fourier transform infrared analysis. Energy Fuels 2008;22:38–45. [42] Singh S, Wu C, Williams PT. Pyrolysis of waste materials using TGA–MS and TGA–FTIR as complementary characterisation techniques. J Anal Appl Pyrol 2012;94:99–107. [43] Soria-Verdugo A, Garcia-Hernando N, Garcia-Gutierrez LM, Ruiz-Rivas U. Analysis of biomass and sewage sludge devolatilization using the distributed activation energy model. Energy Convers Manage 2013;65:239–44. [44] Friedman HL. Kinetics of thermal degradation of char-forming plastics from thermogravimetry. Application to a phenolic plastic. J Polym Sci Part C: Polym Symp 1964;6:183–95. [45] Eftimie E, Segal E. Basic language programs for automatic processing nonisothermal kinetic data. Thermochim Acta 1987;111:359–67. [46] Cai J, Wang L, Zhou Q, Huang Q. Thermogravimetric analysis and kinetics of coal/plastic blends during co-pyrolysis in nitrogen atmosphere. Fuel Process Technol 2008;89:21–7. [47] Biagini E, Lippu F, Petarca I, Tognotti L. Devolatilization rate of biomasses and coal–biomass blends: an experimental investigation. Fuel 2002;81:1041–50.

91

[48] Zhu L, Wang Y, Huang Q, Cai J. Thermogravimetric characteristics and kinetic of plastic and biomass blends co-pyrolysis. Fuel Process Technol 2006;87:963–9. [49] Kastanaki E, Vamvuka D, Grammelis P. Thermogravimetric studies of the behavior of lignite–biomass blends during de-volatilization. Fuel Process Technol 2002;77:159–66. [50] Pan YG, Velo E, Puigjaner L. Prolysis of blends of biomass with poor coals. Fuel 1996;75:412–8. [51] Fernández RG, García CP, Lavín AG, De las Heras JLB. Study of main combustion characteristics for biomass fuels used in boilers. Fuel Process Technol 2012;103:16–26. [52] Wilson. Method for increasing steam decomposition in a coal gasification process. US Patent No. 4,786,291; 1998. [53] Hosoya T, Kawamoto H, Saka S. Pyrolysis gasification reactivities of primary tar and char fractions from cellulose and lignin as studied with a closed ampoule reactor. J Anal Appl Pyrol 2008;83:71–7. [54] Chun YND, Ji W, Yoshikawa K. Pyrolysis and gasification characterization of sewage sludge for high quality gas and char production. J Mech Sci Technol 2013;27:263–72. [55] Seo MW, Kim SD, Lee SH, Lee JG. Pyrolysis characteristics of coal and RDF blends in non-isothermal and isothermal conditions. J Anal Appl Pyrol 2010;88:160–7.