Gasification of pyrolysis chars from sewage sludge

Fuel 143 (2015) 476–483

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Gasification of pyrolysis chars from sewage sludge Lech Nowicki, Maciej Markowski ⇑ Lodz University of Technology, Faculty of Process and Environmental Engineering, Wolczanska 213, 90-924 Lodz, Poland

h i g h l i g h t s  TG–MS study of the gasification of sewage sludge chars was performed.  Three reaction rate models for gas–solid reaction with CO2 and H2O were identified.  The original quasi-quantitative analysis of TG–MS data was proposed.

a r t i c l e

i n f o

Article history: Received 5 June 2013 Received in revised form 16 October 2014 Accepted 21 October 2014 Available online 6 November 2014 Keywords: Steam reforming Gasification Sewage sludge char Kinetic modelling

a b s t r a c t Gasification of char derived from sewage sludge was studied by using the TG–MS system. Experiments were carried out at different temperatures and steam concentrations. The temperatures of 700–900 °C were necessary to complete the conversion in reasonable time. Three reaction rate models for gas–solid reaction were applied to describe the effect of char conversion on the reaction rate. In comparison to the shrinking core model for reaction-controlled regime and random pore distribution models, simple pseudo-homogeneous first-order kinetics was found to be the best for predicting the rate of char steam gasification. Kinetic parameters estimated from the experimental data are in accordance with the literature for lignocellulosic char gasification and are the first ones published for sewage sludge char gasification. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Sewage sludge, the inevitable by-product of wastewater treatment, can be considered a valuable raw material for energy generation. Carbon-containing sewage sludge can be used to obtain heat or electricity from direct combustion or co-combustion with coal [1]. However, as a rather low energy–density fuel, it could be convenient to convert it into a synthesis gas (mixture of CO and H2) by means of gasification [2]. The production of gaseous fuels from wastes such as sewage sludge seems to be a very appealing idea because the use of such materials to produce clean energy could significantly contribute to sustainable development. Gasification consists of two main chemical steps: pyrolysis and conversion of a solid devolatilization product—char composed of carbon and ash. Pyrolysis is a relatively fast thermal decomposition of organic matter in solid fuel, whereas char gasification is a slow gas–solid heterogeneous reaction [3]. Thus, the kinetics of char gasification is crucial for the design and development purposes [3]. Therefore, knowledge of the reaction kinetics of char ⇑ Corresponding author. E-mail addresses: [email protected] (L. Nowicki), markowski.m.p@gmail. com (M. Markowski). http://dx.doi.org/10.1016/j.fuel.2014.10.056 0016-2361/Ó 2014 Elsevier Ltd. All rights reserved.

gasification is essential for proper design and operation of the gasification reactor [3]. The reaction of char with the gasification reagent occurs at higher temperatures when most thermal decomposition processes are finished. The type of the gasification agent used has a major impact on the composition of the resulting gas product. Gasification is usually carried out with oxygen and steam or the mixtures thereof. The gasification process is very often performed with steam because the product of steam gasification of chars, which is rich in hydrogen, can be considered not only a fuel but also a useful raw material for chemical industries, not to mention its use as clean fuel [4]. Recently, we have witnessed a growing interest in the so-called dry gasification by means of CO2 [5,6]. The presence of CO2 can improve char conversion and decrease the volume of gasification residue [7]. The rate of gasification is affected by char reactivity, type and concentration of the gasifying agent used and the operating conditions, such as pressure and temperature [8,9]. On the other hand, the reactivity of the char depends on the raw material, from which the char was produced, as well as on pyrolysis conditions [8,10]. Most publications available in the literature on the gasification of sewage sludge relate to the process in which dewatered and dried sewage sludge is used as a starting material [9,11–19].

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Nomenclature

a Ct E k m

conversion (–) concentration of active size (mol g1) activation energy (kJ mol1) reaction rate constant (s1) reaction order in respect to H2O partial pressure (–)

Studies on the steam or carbon dioxide gasification of chars from sewage sludge are fairly limited. The influence of different pyrolysis conditions (temperature and heating rate) on the reactivity of sewage sludge char in air and CO2 gasification was studied by Inguanzo et al. [20]. Scott et al. [10] measured the rate of CO2 gasification of three chars, including one derived from sewage sludge. They found that the sewage sludge char was the most reactive; as a result, the char contained large amounts of inorganic material, components of which catalyse the reaction of carbon with CO2. The effect of inorganic constituents of waste biomass chars (from municipal solid wastes, sewage sludge and waste paper) on CO2 gasification was studied by Vamvuka et al. [21]. Based on thermogravimetric measurements, they found that sewage sludge char was the most reactive due to alkaline and alkaline earth carbonates and sulphates contained in the ash. The CO2 and steam gasification kinetics of char from dried sewage sludge was measured in a laboratory-scale fluidized bed by Nillson et al. [22]. It was found that the rate of both reactions depends on temperature, partial pressure of gas reactant and degree of conversion. For the whole range of conversion, the char reactivity in steam/N2 was roughly three times higher than that in a mixture with the corresponding partial pressure of CO2. In tests with steam at 900 °C, reactivity was influenced only by particle size greater than 1.2 mm. In our previous study [23], we analysed gasification of char that originated from anaerobic sewage sludge produced in urban waste water treatment plants using thermogravimetric analysis to obtain rate equations and kinetic parameters for different oxidizing agents (O2, H2O, CO2). The purpose of this work is to study the CO2 and steam gasification of pyrolysis char derived from two sewage sludge samples of different origin and composition in order to compare gas product yields, product composition and carbon conversion kinetics. 2. Material and methods A thermobalance, Netzsch STA 409 PG, coupled with a mass spectrometer, Balzers ThermoStar QMS 200, were was used for measurements. The thermobalance was equipped with a water vapour furnace, enabling measurements in the atmosphere containing a controlled amount of steam. The mass spectrometer was connected with the thermobalance by means of a heated quartz capillary with an internal diameter of 0.2 mm maintained at 200 °C. The intensity of selected ions was measured at different times, together with sample temperature and mass. Gasification tests were performed in an alumina crucible with a diameter of 5 mm and height of 3 mm. Two types of sewage sludge were used in this study to obtain the pyrolysis char: stabilized sewage sludge from an urban wastewater treatment plant (WWTP) in a big city (designated as SS1) and row sludge from a small WWTP in rural area (designated as SS2). Both types of sludge differ mainly with ash (mineral fraction) content, which was about 41 and 27.9 wt.%, respectively (Table 1). Proximate analysis of moisture, volatiles (including fixed carbon) and ash was performed in preliminary experiments in which the samples were heated to 1000 °C in inert gas (argon) and then kept at this temperature in air atmosphere to burn out the char

n R t T yg

number of moles (mol) universal gas constant (kJ mol1 K1) time (s) temperature (K) concentration of oxidizing agent (mole frac.)

formed during the pyrolysis. The contents of C, H, N and S elements were determined by an elemental analyser (CE Instruments NA 2500). The char was obtained by devolatilization of sewage sludge or of an oil cake in a horizontal quartz fixed-bed reactor (with a 20 mm internal diameter) heated by an electric furnace. The sample of dried and ground material was heated under the stream of argon at a constant heating rate of 100 °C/h, held in the final conditions (1000 °C) for about 60 min and then cooled to room temperature. The fraction of the char particles with a diameter of 70–125 lm was used for gasification tests. Characteristics of the pyrolysis char are given in Table 2. 2.1. Evaluation of TG/MS data Each chemical compound present in the gas phase can be represented by characteristic ions (charged molecules and molecule fragments) formed from its ionization and fragmentation process occurring during the mass spectrometric analysis. Individual ions are indicated by their mass-to-charge (m/z) ratio. Table 3 shows normalized (the largest peak corresponding to 1) intensities of the peaks on mass spectrum for the ions that can be formed from selected chemical species important for this study. These values are assumed according to the Spectra library supplied with QUADSTAR 422 apparatus. As can be seen from the table, some ions can be formed from the fragmentation of different molecules, which makes mass spectrometry data analysis difficult. However, only a limited number of main species evolved during the char gasification can be chosen to make further analysis possible. To obtain an approximate composition of gas from MS measurements, the following procedure was applied. Evolution profiles (ion current values for selected m/z ratios recorded during the whole analysis) were normalized to the intensity of the m/z = 40 signal corresponding to the carrier gas (argon) in order to minimize errors caused by the shift in the mass spectrometer sensitivity. Normalized signals, Sj(t), were integrated during the whole process to obtain an integrated evolution profile, Q 0j , for all considered (m/z)j ratios

Q 0j ¼

1 tk

Z

tk

Sj ðtÞdt;

j ¼ 1 . . . 5;

ð1Þ

0

where tk is the duration of the process. Generally, the signal measured for some (m/z)j ratios can involve the contributions arising from the fragmentation of different components. According to the data in Table 3, the problem concerns CO and CH4. The values of integrated profiles for these two components were corrected using the following formulas:

Q 3 ¼ Q 03  0:11Q 04

ð2Þ

Q 5 ¼ Q 05  0:08Q 04

ð3Þ Q 0j .

For other components, Q j equals Then, a molar fraction of each component evolved from the sample was calculated from

Q =RCF i yi ¼ PN i ; k¼1 Q i =RCF k

ð4Þ

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Table 1 Physical and chemical properties of materials used for char production. Sample

SS1 SS2

Proximate analysis (wt.%)

Ultimate analysis (wt.%)

Moisture

Volatile matter + fixed carbon

Ash

N

C

H

S

5.5 4.7

53.8 67.4

40.7 27.9

3.5 4.9

27.2 34.4

4.5 5.1

0.9 0.8

Table 2 Characteristics of pyrolysis chars. Yield (%)a

Char sample

SC1 SC2 a

Ash (wt.%)

54.0 45.7

HV (MJ/kg)

85.6 69.1

Chemical composition (wt.%)

5.6 9.8

N

C

H

S

0.39 1.20

15.9 28.1

0.13 0.31

1.12 0.54

% of initial mass of dried sludge remaining after pyrolysis at 1000 °C.

Table 3 Normalized intensities of ions that can be formed from gasification products.

2 1. 2. 3. 4. 5. 6.

Hydrogen Water Carbon oxide Carbon dioxide Methane Argon

100

(m/z)i 16

18

28

40

44

1 1 0.08 1

1 0.11

1 1

RCFi 1.00 1.05 1.52 1.33 1.00 –

Sample mass (TG), %

i/Component

90

80

SC1, 17% H2O

70

SC2, 50% CO2

SC2, 17% H2O

60

where RCFi denotes its calibration factor that has to be determined experimentally. To determine calibration factors of the main components, the reaction of copper oxide (CuO) with carbon monoxide or hydrogen was performed in the TG–MS system.

CuOðsÞ þ COðgÞ ¼ CuðsÞ þ CO2 ðgÞ

ð5Þ

CuOðsÞ þ H2 ðgÞ ¼ CuðsÞ þ H2 OðgÞ

ð6Þ

The amount of evolved (CO2 or H2O) and consumed gases (H2 or CO) can be easily calculated on the basis of sample mass loss measured by TG and reaction stoichiometry. These numbers can be compared with integrated and corrected (for CO) MS signals to calculate RCF (Table 3). The RCF for methane was assumed to be equal to 1. Calibration experiments were carried out by heating the 20 mg sample of CuO at 10 °C/min in the atmosphere of argon containing 5 mol.% of CO or H2. More details on calibration can be found in [24]. 3. Results and discussion 3.1. Gas evolution Evolution of various gases produced from the char during gasification was analysed by the coupled thermogravimetry/mass spectroscopy system. A sample weighing approximately 50 mg was heated in an argon flow from room temperature to 1050 °C with a heating rate of 10 °C/min. When the oven temperature reached about 120 °C, water in an amount of 0.5 or 1.0 g/h was fed into the gas stream to obtain a proper atmosphere for sample gasification. As can be seen from TG profiles presented in Fig. 1, the reaction with steam starts at about 620 °C and occurs before the sample has reached the temperature of 1000 °C. It is worth noting that the mass of samples obtained from sewage sludge increased noticeably after the introduction of steam into the

200

400

600

800

1000

Temperature, oC Fig. 1. TG profiles of steam gasification of char derived from sewage sludge.

carrier gas, probably due to the sorption of the component. For the purpose of comparison, Fig. 1 also shows the thermogram of the reaction of char with CO2. This reaction requires higher temperatures; the process can be observed only above 700 °C. As we can see in Fig. 1, there are differences in final mass depending on the oxidizing reagent used; however, the total mass loss is almost the same. The mass of the sample heated in the atmosphere of CO2 drops soon after the start of heating, probably due to the loss of water adsorbed on the char. The m/z signals of less than 50 were recorded during the whole heating up and then used to obtain the composition of evolved gases according to the procedure described above. Fig. 2 shows the ion intensities (normalized to m/z = 40 for Ar), which represent typical evolution profiles of main gas products during steam gasification of two chars. Steam gasification of char is a complex process comprising several chemical reactions. During the process, the following main reactions may be considered [25]:

CðsÞ þ H2 O ¼ 2CO þ H2

ð7Þ

H2 O þ CO ¼ H2 þ CO2 CðsÞ þ CO2 ¼ 2CO

ð8Þ ð9Þ

CðsÞ þ 2H2 ¼ CH4

ð10Þ

Reaction (7) describes the main reaction occurring between carbon and steam to produce carbon monoxide and hydrogen. The primary products are involved in the reversible water gas shift reaction (reaction 8), which becomes the source of carbon dioxide observed in significant amounts in the evolved gases. These reactions usually proceed very slowly in the absence of catalysts or inorganic matter in the char [26]. The reaction between carbon

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L. Nowicki, M. Markowski / Fuel 143 (2015) 476–483

0.00

0.035

(a) 0.021 0.014

m/z = 2 m/z = 16 m/z = 28 m/z = 44

-0.01

DTG exp. DTG calc.

-0.03

-0.02

-0.04

0.007 0.000 600

DTG, %/s

MS, Rel. units

0.028

(b)

-0.05 700

800

900

1000

600

o

700

800

900

1000

o

Temperature, C

Temperature, C

Fig. 2. DTG curves and MS evolution profiles of steam gasification products for sewage sludge chars: (a) SC1 sample, 1 g/h of water; (b) SC2, 1 g/h of water.

and carbon dioxide (Boudouard reaction, reaction 9) occurs at higher temperatures than the reaction of carbon with steam (see Fig. 1). Thus, at higher temperatures, the effect of CO2 on the overall rate of gasification process can be expected. This can be seen in Fig. 2, which shows DTG plots recorded in experiments with deconvolution into two single peaks. The first peak with the maximum at 830 °C may be attributed to the primary steam gasification, whereas the second one represents a secondary reaction between CO2 and char carbon. Based on deconvolution, which is a purely mathematical operation, the second reaction reaches the maximum at 880–980 °C and the mass loss caused by this reaction was about 35% of the total mass of carbon in sample SC1 and 42% in sample SC2. The composition of gaseous products formed in the gasification of three types of pyrolysis char are given in Table 4. In addition to the concentrations, the table shows the yields of reaction products obtained from combined TG/MS data. The stoichiometric model of the gasification process (Eqs. (7)– (10)) leads to the following mass balance equation

nH2 z¼ ¼ 1; nCO þ 2nCO2  2nCH4

ð11Þ

which can be used to assess the compatibility of the results of composition analysis. The values of the parameter for individual experiments given in Table 4 show good agreement of the results, at least from the point of view of the test. The mixture of gaseous products evolved during gasification consists essentially of hydrogen (61–64 mol.%), a significant amount of carbon dioxide (27–28%) and carbon monoxide (7–12%). Methane concentration was very low (less than 1%). This result could be expected as a formation of methane in the side reaction between carbon and hydrogen, which, however, requires a higher pressure [8]. The composition of gaseous products was very similar for the two tested samples of char. It is also difficult to see the effect of steam concentration in the gas mixture on the composition of the product. However, it should be noted that the steam concentration in the experiments was very high, much higher than that of

other components, so changing the concentration of water from 0.5 to 1.0 g/h could not be relevant in this case. 3.2. Overall kinetics of char gasification The rates of char gasification were measured under isothermal conditions at temperatures 750, 800, and 850 °C for steam and 800, 850, and 900 °C when CO2 was present. Three different concentrations of gasifying reagent in the mixture with argon were used. Concentrations of gasifying agent were as follows: 17, 32, and 45 mol.% for H2O and 10, 50, and 100 mol.% in the case of CO2. The sewage sludge char conversion (defined as the ratio of the gasified carbon to the total mass of carbon in the char sample) vs. time data for different reaction conditions are shown in Fig. 3. Assuming that the loss of mass caused by gasification is a onestep reaction, the conversion, a, in an isothermal batch reactor can be expressed by the following equation:

da ¼ kðyg ; TÞf ðaÞ; dt

ð12Þ

where k is the reaction rate constant including the effect of gasifying agent concentration in the gas phase, yg, and the effect of temperature and f(a) is a structural term describing the changes in available internal surface and structural properties of the char as the reaction proceeds. The gasification of char particles can be represented through different f(a) terms. In this work, two models were applied to study the reactivity of sewage sludge chars, those that we used in our previous study [23]: the pseudo-homogeneous volume reaction model (VRM) and the shrinking core model (SCM). Specific equations arising from the use of these models are shown in Table 5. The usefulness of the model for the description of the rate of gasification process can be evaluated using the plots of experimental data shown in Fig. 4. The plots represent integral forms of rate equations given in Table 5. The relationship between the left-hand side of the equation versus time should be a straight line to prove its validity, and the slope of the line determines the reaction rate

Table 4 The composition and yield of the products evolved during gasification of char. Sample ID

Water conc.

Gas product composition H2

CH4

z CO

Yield

LHV

Nm3/kg

MJ/Nm3

CO2

wt.%

mol.%

SC1

17 29

64.1 63.3

0.6 0.9

6.6 7.3

28.4 28.5

0.99 1.01

1.84 1.87

8.7 8.8

SC2

17 29

61.5 61.1

0.4 0.6

11.5 10.4

26.6 27.9

0.96 0.94

3.29 3.27

8.6 8.6

L. Nowicki, M. Markowski / Fuel 143 (2015) 476–483

Conversion

480

1.0

1.0

0.8

0.8

0.6

0.6 o

o

0.4

800 C o 850 C o 900 C

0.2

850 C o 800 C o 750 C

0.4 0.2

(a) 0.0 0

Conversion

(b) 0.0

4000

8000

12000

16000

0

1.0

1.0

0.8

0.8

0.6

4000

8000

12000

16000

0.6

45% H2O

100% CO2 0.4

32% H2O

0.4

50% CO2

17% H2O

10% CO2 0.2

(c)

0.0

0.2

(d)

0.0 0

500

1000

1500

2000

2500

Time, s

0

2000

4000

6000

8000

Time, s

Fig. 3. Conversion versus time for gasification of different sewage sludge chars (symbols are used for experimental data and lines for model predictions; solid symbols for SC1 sample and open symbols for SC2); (a) effect of temperature on CO2 gasification (50 mol.% of CO2 in Ar); (b) effect of temperature on steam gasification (17 mol.% of H2O in Ar); (c) effect of CO2 concentration (900 °C); (d) effect of steam concentration (800 °C).

Table 5 Equations of the reaction rate models for char gasification. Volume reaction model   da k T; yg f ðaÞ dt i

Shrinking core model

f(a) term

ð1  aÞ

Rate constant, s1 Integral form of rate equation, T, yg = const Effect of temperature, yg = const

kV (T, yg) kV t ¼  ln ð1  aÞ

ð1  aÞ2=3 kS (T, yg)

Reaction rate

kS t ¼ 1  ð1  aÞ1=3

  Ei ki ðT Þ ¼ Ai exp  RT ; i ¼ V; S

constant kVRM or kSCM. A range of conversions varying from 0.05 to 0.95 is presented on the plots. The analysis of data presented in Fig. 4 leads to the conclusion that the shrinking core model should be applied for the CO2 gasification, whereas in the case of steam, the first-order reaction model fits the experimental data much better. The same models were found to be the most suitable for other studied CO2 and H2O concentrations (not presented in Fig. 4). The observation may suggest that steam gasification of char proceeds according to a different mechanism than that for gasification with carbon dioxide. The volumetric model and the shrinking core model are particular cases of the general model of gas–solid reaction (see e.g. [27]). The first model represents the case for which chemical reaction is rate controlling and there are no gradients of solid components inside the particle. The reaction rate constants were determined from the slope of straight lines drawn for the better-fitting models. Then, by using these values calculated at different temperatures but for the same gasifying agent concentration, the activation energy (E) and the pre-exponential factor (A) were estimated from the Arrhenius plot

(ln k vs. 1/T). Table 6 shows the values of parameters A and E, obtained for two gasifying reagents. Generally, activation energies reported in Table 6 remain within a wide range of values determined for different waste and biomass char gasification reactions [8,21]. The analysis of a large number of experimental data made by Di Blasi [8] shows that activation energy values vary between 88 and 250 kJ/mol (typically 200– 250 kJ/mol) and 143–237 kJ/mol (typically around 180–200 kJ/ mol) for CO2 and H2O gasification, respectively. The values greater than 180 kJ/mol for CO2 gasification indicate a chemically controlled process [4]. It should be noted that the sample SC1 comes from the same WWTP as the sample of sewage sludge studied in our previous work [23]. Despite that, the values are now lower than the previous ones by about 10% (179 kJ/mol instead of 193 kJ/mol for steam gasification or 202 kJ/mol and 223 kJ/mol for 50% CO2 mixture). However, samples of sewage sludge were collected at an interval of two years, which could explain observed differences. These differences may be due to changes in the composition of the char ash [21]. It can be seen from Table 6 that the activation energy values are practically not dependent on steam partial pressure, while in the case of CO2, a gasifying agent concentration effect is evident. Thus, to describe the effect of steam concentration in the gas phase on the reaction rate, a simple exponential equation (Eq. (13)) can be used.

    E0 0 kV T; yH2 O ¼ kV ðT ÞynH2 O ¼ A0V exp  V ynH2 O RT

ð13Þ

To estimate the parameters appearing in this equation, first the reaction order with respect to steam mole fraction (n) and rate 0 constant (kV ) were obtained, by plotting ln(kV) as a function of ln(yH2O) for different temperatures (the results are given in

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L. Nowicki, M. Markowski / Fuel 143 (2015) 476–483

-1-ln(1-α)

3.5

3.5

(a)

3.0

3.0

2.5

2.5

2.0

2.0

1.5

1.5

1.0

1.0

0.5

0.5

0.0 0

2500

5000

7500

10000

0.0

12500

0.7

1/3

1-(1-α)

0

5000

10000

15000

0.7

(c)

0.6

(b)

20000

(d)

0.6

0.5

0.5

0.4

0.4

0.3

0.3

0.2

0.2

0.1

0.1

0.0

o

750 C o

800 C o 850 C o 900 C

0.0 0

2500

5000

7500

10000

12500

0

5000

Time, s

10000

15000

20000

Time, s

Fig. 4. Plots of the linearized VRM and SCM equations for sewage sludge char gasification under different conditions. (solid symbols – SC1 sample and open symbols – for SC2): (a) VRM for 50 mol.% of CO2 in Ar; (b) VRM for 17 mol.% of H2O in Ar; (c) SCM for 50 mol.% of CO2 in Ar; (d) SCM for 17 mol.% of H2O in Ar.

Table 6 Comparison of kinetic parameters of char during gasification with different oxidizers. Oxidizing reagent

yg (mol.%)

AV or AS (1/s)

EV or ES (kJ/mol)

AV or AS (1/s)

SC1 Carbon dioxide

100 50 10

2.65  106 6.40  105 3.56  104

211 202 183

1.353  107 2.01  106 7.04  104

234 218 196

Steam

45 32 17

5.62  105

180.0

3.02  105 1.41  106 1.15  105

177 173 174

3.28  10

5

178

Table 7). Averaged for all temperatures, the value of parameter n (together with the parameters A0V and E0V ) was obtained by minimizing the objective function below:

J ðnÞ ¼

X

EV or ES (kJ/mol)

SC2

0 0 2 kV  kVm ;

ð14Þ

temp: 0

where kVm is the reaction rate constant calculated from Eq. (13) for given values of the parameters n, A0V and E0V . The simple optimization algorithm was as follows: 1. A reaction order n was assumed.

2. The parameters A0V and E0V were determined from the Arrhenius 0 plot ln(kV =ynH2O ) vs. (1/T) created for all experimental values of rate constants. 0 3. For assumed n, A0V and E0V (calculated in step 2), the kVm and the objective function were established. 4. Steps 1–3 were repeated to find the minimum value of the objective function. Estimated values of parameters are also reported in Table 7, and a parity plot for the raw reaction rate constants against calculated

Table 7 Parameters of the rate model for steam gasification of sewage sludge. Sample

T (°C)

Double-log plots 0 kV

Averaged values n (–)

(1/s) 3

n

A0V (1/s)

E0V (kJ/mol) 5

SC1

850 800 750

2.84  10 1.28  103 4.24  104

0.279 0.246 0.251

0.259

5.99  10

178.8

SC2

850 800 750

2.74  103 1.10  103 4.24  104

0.619 0.566 0.588

0.591

3.52  105

174.6

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L. Nowicki, M. Markowski / Fuel 143 (2015) 476–483

based on averaged values of the kinetic parameters is shown in Fig. 5. Furthermore, the kinetic model predictions of conversion vs. time profiles for steam gasification of pyrolysis chars are compared with the experimental data in Fig. 3. The exponent of the gaseous reactant mole fraction for biomass chars is around 0.4–1 for steam gasification [9]. The value 0.59 obtained for sample SC2 is in good agreement with these data, while the reaction order for SC1 sample is out of the range. But it is close to the value of 0.3 reported in our previous paper [23] and the result obtained by Nielson et al. [22], who reported the value of 0.33. A strong dependence of the apparent activation energy on the concentration of CO2 (see Table 6) indicates that the simple global model in the form of Eq. (13) may be not sufficient to describe the effects of temperature and gasifying agent concentration on the gasification rate. For this process the rate equation derived from the most widely used oxygen exchange mechanism [8] can be applied. According to the simplified version of the mechanism suitable for the process occurring at low partial pressure of CO, the reaction of CO2 with active surface carbon, Cs, involves the following steps [10]: 0.0020

k'Vm, 1/s

0.0010

SC1 SC2

0.0000 0.00 00

0.00 05

0.0010

0.0015

ð15Þ

k2

CðOÞ ! CO

ð16Þ

and the rate equation for steady-state conditions can be expressed as follows:

r¼

k1 C t pCO2 ; 1 þ ðk1 =k2 ÞpCO2

ð17Þ

where k1 and k2 are the Arrhenius rate constants and Ct is the concentration of active carbon sites (per unit mass of sample). A similar rate equation was used to describe the effect of temperature and CO2 mole fraction on the reaction rate under the experimental conditions used in this work (unsteady-state conditions). This equation can be expressed as follows:

  kS T; yCO2 ¼

kS1 yCO2 1 þ kS2 yCO2

ð18Þ

or in an alternative form:

1 1 1 kS2 ¼ þ ; kS kS1 yCO2 kS1

ð19Þ

which was used for the estimation of parameters kS1 and kS2. For each temperature, the plot 1/kS versus 1/yCO2 should be a straight line with a slope of 1/kS1 and an intercept of 1/kS1, from which kS1 and kS2 can be calculated. Then, by using the Arrhenius plot for each parameter, the pre-exponential factors and activation energies were derived. These values are reported in Table 8. As can be seen from Fig. 3, Eq. (18), with the parameters given in the Table 8, was found to give a reasonable fit to the measured conversions. Negative values of the parameter Es2 obtained from the calculations should not be questionable because it relates to ks2 in Eq. (18), which is the adsorption equilibrium constant. Therefore, the parameter Es2 is in fact the enthalpy of adsorption and should have a negative value as the adsorption process is exothermic.

0.0015

0.0005

k1

Cs þ CO2 ! CðOÞs þ CO

0.0020

k'V, 1/s

3.3. Char reactivity

Fig. 5. Parity plot for the rate constant predicted by averaging procedure.

Table 8 Parameters of the rate model for CO2 gasification of sewage sludge. Sample

AS1 (1/s)

ES1 (kJ/mol)

AS2 (1/s)

ES2 kJ/mol

SC1 SC2

2.22  102 2.11  102

109.0 146.7

2.31  102 2.22  102

33.1 43.9

Differences in reactivity of two sewage sludge chars as a function of conversion are shown in Fig. 5. Reactivity was expressed as a gasification rate and was calculated from an appropriate rate model with parameters given in Tables 7 and 8. As can be seen from Fig. 6, the rate of gasification reaction with steam seems to be higher than with carbon dioxide, especially at lower conversions. It should be noted that the data presented in the figure refer to different temperatures, 950 °C for CO2 and 900 °C in the case of steam gasification.

1.6x10 -3

SC1, 45% H2O

dα/dt, 1/s

1.2x10

SC1, 100% CO2

SSC2, 45% H2O

-3

SC2, 100% CO2

SC1, 17% H2O

SC1, 10% CO2

SC2, 17% H2O

8.0x10 -4

SC2, 10% CO2

4.0x10 -4

0.0 0.0

0.2

0.4

0.6

Conversion α

0.8

1.0

0.0

0.2

0.4

0.6

Conversion α

Fig. 6. Reactivity of the sewage sludge chars in H2O and CO2 gasification reaction.

0.8

1.0

L. Nowicki, M. Markowski / Fuel 143 (2015) 476–483

Under the same operating conditions, the reactivity of the char labelled as SC1 was always higher than the second sample. Higher reactivity of the char containing large amounts of ash was probably attributable to the catalytic activity of inorganic components of the ash and may be due to char textural properties, such as a specific surface, pore structure, and pore structure and volume, which have not been measured. 4. Conclusions In this study, the steam and carbon dioxide gasification of two samples of sewage sludge char of different ash content were studied in a thermobalance/mass spectrometer system at atmospheric pressure. The major achievements of the work can be summarized as follows: 1. Thermogravimetric experiments performed in a dynamic mode with constant heating rate showed that steam reacts with char at relatively lower temperatures than carbon dioxide. In this case, the reaction occurs with a noticeable rate from the temperature of 600 °C with the maximum at about 650 °C, whereas the beginning of CO2 reaction with char and the maximum rate are shifted to temperatures higher by about 50 °C. 2. The main product of steam gasification is hydrogen (about 60 mol.% of the total product). Hydrogen is produced in the reaction of water with carbon and partly in the water gas shift reaction together with CO2, which at higher temperatures is converted back to CO in Boudouard reaction. Steam conversion of fixed carbon was greater for the sample containing the larger amount of ash. 3. The composition of gaseous products evolved and was not affected by steam concentration. 4. By fitting experimental data with different kinetic models, it was found that the volumetric reaction model is suitable for modelling steam gasification of char, whereas the shrinking core model predicts well the experimental data for the CO2 gasification process. 5. Various models allowed corresponding kinetic parameters to be obtained. 6. The reactivity of char containing more minerals is greater, which may account for their catalytic activity in the gasification process.

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