Kinetics of Sewage Sludge Pyrolysis and Air Gasification of Its Chars

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Kinetics of sewage sludge pyrolysis and air gasification of its chars Beata Barbara Urych, and Adam Smoli#ski Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00332 • Publication Date (Web): 05 May 2016 Downloaded from http://pubs.acs.org on May 7, 2016

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Kinetics of sewage sludge pyrolysis and air gasification of its chars

Beata Urych1, Adam Smoliński1* 1

Central Mining Institute, Department of Energy Saving and Air Protection, Plac Gwarków 1, 40-166 Katowice,

Poland

*Corresponding author E-mail: [email protected]

Abstract An optimal management and utilization of sewage sludge requires innovative technological solutions. One of the prospective and novel concepts for the utilization of this waste, targeted at energy production is the two-stage process combining pyrolysis and gasification. The paper presents the kinetic analysis of sewage sludge pyrolysis and air gasification of "hot" chars with the application of the thermogravimetric analyzer SDT Q600. The pyrolysis was performed below 1173 K in the inert gas atmosphere and with various heating rates - 1, 10, 50 and 100 K/min. The air gasification of chars was performed at 973, 1073 or 1173 K, respectively. The lumped model, which encompasses six first order parallel reactions was indicated as the most appropriate for the interpretation of the kinetics of sewage sludge pyrolysis. For the description of the char gasification stage the Avrami–Erofeev model (A3) was proposed. The kinetic parameters estimated based on the experimental data are the first published for sewage sludge "hot" char air gasification. The impact of the sewage sludge pyrolysis process parameters on the reactivity of chars in the in-situ air gasification was also analyzed.

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Keywords: sewage sludge, kinetics, gasification, pyrolysis

Nomenclature A

Pre-exponential factor, 1/min

E

Activation Energy, kJ/mol

i

Index of reactions

j

Index of data points

k

Reaction rate, 1/min

m0

Initial mass, kg

mf

Final mass, kg

N

Total number of data points

n

Total number of reactions

R

Universal gas constant, kJ/(mol·K)

rα

Reactivity index, 1/min

t

Time, min

T

Temperature, K

wi

Mass fraction of reaction i, -

α

Degree of conversion, -

β

Heating rate, K/min

σi

Standard deviation of each fitted peak, K

2


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INTRODUCTION The operation of the existing and opening of new wastewater treatment plants as well as the introduction of more restrictive environmental protection regulations is accompanied with the increasing amount of sewage sludge. Due to the content of organic matter, microelements, as well as biogenic elements (among other nitrogen and phosphorus) in sewage sludge the best neutralization method seems to be its utilization in agriculture. However, it often does not meet the sanitary, chemical and environmental safety standards due to, among other, excessive content of heavy metals and pathogenic organisms which, in consequence, prevents their recycling in the natural environment. It seems that the promising management method for this type of sewage sludge is its utilization for energy production. EU regulations1 on the renewables define sewage sludge as a raw material which comprises renewable and zero-CO2-emission energy source. The possibility to use the sewage sludge as a fuel depends on its heating value which in turn is impacted by the content of organic matter in the raw material and its moisture. Currently, the main methods of thermal conversion of sewage sludge being developed include combustion, cocombustion, pyrolysis and gasification, as well as combination of these processes.2-4 Among the thermal conversion methods gasification of the biomass/biowaste is regarded as clean energy production technology which complies with the sustainable development rules. Gasification is also considered to be an effective utilization method for sewage sludge, simultaneously providing the possibility of recovering the valuable elements, such as phosphorus from sewage sludge ash.5 Another significant benefit of application of this technology is the possibility to locate the gasification installation on the premises of the sewage treatment plant and use the energy generated from pyrolysis and gasification products for the highly energy intensive sewage drying process.

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The laboratory tests of sewage sludge gasification have been conducted for twenty years, now.6 Numerous sewage sludge gasification tests in a semi-technical scale in fixed and fluidized beds are known.7-9 The disadvantage of such systems is a significant contamination of the produced gas with dust, tars, light and heavy hydrocarbons and their derivatives. Currently, sewage sludge air gasification tests in pilot and demonstration scale are conducted on the premises of the sewage treatment plant in Balingen and Mannheim in Baden-Wurtemberg (Germany).10 Lack of a broad application of this technology is the result of the absence of technologically mature sewage sludge gasification solutions. The primary obstacle to the commercial use of produced gas in electricity generation is the requirement in terms of gas purity. Therefore, the new, advanced concepts of biomass gasification are developed, such as the multi-stage gasification with the separated pyrolysis and gasification zones. It allows to optimize the process efficiency and the quality of produced gas at each stage of the process. Furthermore, a higher char conversion rate is achieved and higher gasification efficiency in comparison with the one-stage gasification process. On the other hand, the complexity of the process is increased by combining different reactors. An example of the multi-stage gasification technology is a two-zone reactor Viking at the Technical University in Copenhagen11. For the purpose of the design of a sewage sludge gasification reactor a kinetic analysis of the chemical processes occurring within is indispensable. The char gasification rate depends on the type of the gasification agent applied, the operating conditions (temperature and pressure) adopted and the char’s reactivity. The latter one is influenced by the fuel properties and pyrolysis conditions.12,13 The practical application of the kinetic models is to define the fuel conversion degree at each of the stages of the thermochemical process. In the field of the thermal decomposition of solid-state, Khawam and Flanagan14 have provided a comprehensive review of the kinetic models. In the most frequently proposed models of this process the Arrhenius equation are applied. Studies of the kinetics of sewage sludge pyrolysis and 4


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gasification are not as common as those of biomass and coal. The actual reaction scheme of pyrolysis of sewage sludge in non-isothermal conditions is very complex. Consequently, most researchers propose models based on the grouping reactions occurring at the same temperature. These models differ in the order and the amount of decomposition reactions.15-17 There are several empirical models for interpretation of gasification of chars derived from sewage sludge. In most of the works reported in the literature the feed material for gasification kinetic studies is the dried sewage sludge or “cold” sewage sludge char. Nowicki et al.12 suggest that the volumetric and shrinking core models are the best for predicting the “cold” char ex-situ gasification process in O2, CO2 or H2O atmosphere. Studies with the application of “hot” sewage sludge chars are scarce, though they are considered to be of key importance in case of multi-stage gasification with separated pyrolysis and gasification zones. Recently, Sun et al.18 have suggested that the Avrami–Erofeev model (A2) could best describe the stage of “hot” char in in-situ gasification in CO2 atmosphere. However, little is known about the mechanism and kinetics of “hot” char gasification with air. The research presented contributes to the limited studies on kinetics required for modeling and simulation studies. Furthermore, the knowledge on the effects of the sewage sludge pyrolysis conditions on the sewage sludge char reactivity in the in-situ gasification process is limited. The paper defines the kinetic parameters of sewage sludge pyrolysis and air gasification. The assessment of the sewage sludge chars suitability to thermochemical conversion in the gasification process applying the fuel reactivity index rα is also presented. Both, the kinetic parameters of the thermal decomposition of the raw material and the fuel reactivity index rα were determined experimentally for the process parameters applied in the study.

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EXPERIMENTAL SECTION Materials The tested material was a stabilized, dried (95% dry weight) municipal sewage sludge from large urban agglomeration in Poland. The selected sewage sludge was produced in the following technological series: the excess sewage sludge from the secondary settling tank was transported to the gravity thickener from where it was further directed, together with the primary sludge, to the mesophilic fermentation process, where it was stabilized and where biogas was produced. The fermented sewage sludge was gravitationally thickened, mechanically dewatered and, finally, dried at the temperature of approximately 573 K. The product of the process is a granulate with the dry mass of 90-95%. The proximate and ultimate analyses of the tested fuels were performed in the accredited laboratory of the Department of Solid Fuel Quality Assessment of the Central Mining Institute according to the relevant standards in force and testing procedures of the Department with the application of automatic thermogravimetric analyzers LECO: TGA 701 or MAC 500 (contents of moisture, ash, volatiles acc. to PN-G-04560:1998 and PN-G-04516:1998), calorimeters LECO: AC-600 and AC-350 (heat of combustion acc. to PN-G-04513:1981), TruSpecCHN analyzer (contents of carbon, hydrogen, nitrogen acc. to PN-G-04571:1998) and TruSpecS analyzer (sulfur acc. to PN-G-04584:2001). Oxygen content was calculated as: 100% – Wa – Aa – Cat – Hat – Sac (PN-G-04510:1991), and fixed carbon as: 100% – Wa – Aa – Va (PN-G-04516:1998). The characteristics of the sewage sludge is presented in Table 1.

Table 1

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Methods The experiments on sewage sludge pyrolysis and air gasification of chars were conducted applying the thermogravimetric analyser SDT Q600, TA Instruments. Ground samples (particle size below 0.1 mm, mass of 35 mg) were placed in a crucible made of corundum ceramics. The sewage sludge sample was heated in the inert gas atmosphere (nitrogen) with the selected heating rate to the set air gasification temperature. Next, the nitrogen was replaced with air (flow rate of 0.1 dm3/min), and the sample was kept in the set temperature until constant weight was obtained (40 min). The thermogravimetric (TG) and thermogravimetric derivative (DTG) curves were recorded. The kinetics of the pyrolysis was tested for the sewage sludge sample heated up to 1173 K in non-isothermal conditions and at linear temperature increase with the set heating rate i.e. 1, 10, 50 or 100 K/min. The kinetics of the air gasification process was conducted in two variants – in the first one for various gasification temperatures (973, 1073 and 1173 K) for the chars obtained in the sewage sludge pyrolysis process conducted at constant heating rate of 10 K/min, and in the other for the temperature of 1173 K for chars obtained in the sewage sludge pyrolysis process conducted at different heating rates of 1, 10, 50 and 100 K/min. For heterogeneous reactions of the thermal decomposition of solids (αA(s)→bB(s)+cC(g) ) and non-catalytic heterogeneous reactions of solids with a gasification agent (αA(s)+ bB(s) →cC(g)) the rate of decomposition of a solid substance (A) can be expressed as:

dα = k ⋅ f (α ) dt

(1)

where α is the degree of conversion of the reaction in time

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α(t ) =

m0 − m(t ) , α ∈ 0,1 m0 − m f

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(2)

where, m0 is the initial mass, m(t) is the mass in time t, and mf is the final mass of the sample, f(α) is the reaction model, k(T) is the reaction rate coefficient, which generally obeys the Arrhenius equation: k (T ) = Ae − ( E / RT )

(3)

and A is the pre-exponential factor, E is the activation energy, T is the absolute temperature, R is the gas constant. Kinetic parameters (reaction model, A, E) can be estimated from thermogravimetric data under isothermal conditions by utilizing Eq. 1. Alternatively, the following relationship (Eq. 1) can be defined for non-isothermal experiments:

dα dα dt = dT dt dT

(4)

where dα/dT is the non-isothermal reaction rate, dα/dt is the isothermal reaction rate, and dT/dt is the constant heating rate (β). Substituting Eq. 1 into Eq. 4 gives: d α A − ( E / RT ) = e f (α ) dT β

(5)

The integral form of the isothermal and non-isothermal reaction rate is obtained by separating variables and integrating Eq. 1 and Eq. 5, respectively14: g (α ) = A ⋅ e − ( E / RT ) ⋅ t

(6)

and A T − ( E / RT ) e dt β ∫0

g (α ) =

(7)

where g(α) is the integral reaction model, defined by: g (α ) = ∫

α

0

dα f (α )

(8)

8


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The integral in Eq. 7 does not have any analytical solution. The most common kinetic model forms of f(α) and g(α) are listed in Table 2.

Table 2

RESULTS AND DISCUSSION The TG and DTG curves were recorded and analyzed in order to identify the chemical reactions occurring in the pyrolysis of sewage sludge and air gasification of chars. The results of the sewage sludge pyrolysis in the temperature range from ambient to 1173 K with heating rate of 50 K/min and the air gasification of chars at a constant temperature of 1173 K are presented in Figure 1.

Figure 1.

The results of the sewage sludge pyrolysis for various heating rates are presented in Figure 2.I. The analysis of the results obtained indicates that the onset and offset temperatures of the main decomposition stage of the reaction rate curves obviously shift toward the high-temperature range as the heating rate rises. Analogously, the maximum reaction rate is shifted to higher temperature when the chars are produced at high heating rates. The offset temperature of decomposition is defined as the latest relative minimum of the reaction rate curve.

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Figure 2.

Figure 2.II presents the curves of conversion and conversion rate of the sewage sludge chars for various temperatures of the gasification process. Based on the analysis of the results it has been determined that full conversion of char is achieved in a shorter time for higher temperature values of the gasification process (30% less time at 1173 K versus at 973 K). The results of the tests targeted at defining the impact of the heating rate applied in pyrolysis on the reactivity of the char in air gasification process are presented in Figure 3. For the applied pyrolysis rates the loss of reactivity in the gasification process was observed in the case of chars obtained at the heating rates of 50 and 100 K/min.

Figure 3.

Kinetics of sewage sludge pyrolysis Based on the DT curves it has been found that the sewage sludge pyrolysis process in nonisothermal conditions comprises of many independent parallel reactions. For estimating the kinetic parameters of the pyrolysis process the lumped reactions method for the reactions occurring at the same temperature and characterized by the release of the same gas products was applied.19-21 Nowicki et al.15 distinguished six reaction groups – i.e. in terms of the temperatures, starting with the ambient temperature up to 453 K – drying reaction, from 453 to 873 K – three basic biomass decomposition groups, from 873 to 1173 K – two reaction groups related to the 10


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degradation of the inorganic residues such as calcium carbonate. The kinetic model and Eq. 5 can be expressed as the following set of six ordinary differential equations: 6 dα  dα  = ∑ wi  i  ; dT i =1  dT 

6

∑w =1 i

(9)

i =1

where the lower index i – refers to the given elementary reaction; wi is the mass fraction for each main component. The rate of a single biomass pyrolysis reaction (dαi/dT) is generally presented in the first order reaction kinetics (Table 2), and thus: dα i A = i e − ( E i / RT ) (1 − α i ) dT β

(10)

where αi is the conversion degree of i-th reactions at temperature (from the ambient temperature to 1173K) defined as: α i (T ) =

m0,i − mi (T ) , αi ∈ 0,1 m0,i − m f ,i

(11)

Since the pyrolysis process comprises of many parallel reactions, the DTG curve represents an overlap of the peaks. Therefore the procedure for defining the process kinetics requires deconvolution of the DTG curve into single peaks which correspond to the respective reactions or reaction groups.22 The DTG curve’s deconvolution also enables to define temperature at which the rate of the process reaches the maximum value – Tmax,i and the fraction wi of the reactions in the total weight loss. The results of the DTG curve’s deconvolution are presented in Figure 4.

Figure 4.

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The multiple peaks were fitted as the accumulation of Gaussian function. The fitting was based on the non-linear optimization (Levenberg-Marquardt algorithm23,24), minimalizing the sum of the squares of the residual errors between the proposed function and the measured data points. For Gaussian function the coefficient of determination R2 values were higher than 0.99, indicating that the fit was good. The single peaks were described by adequate curve equation (Eq. 12), which comprises the mathematical description of the shape of the peak: dα Gauss ,i dT

=

1 σi

 − (T − Tmax,i )2  exp   2 2σ i 2π  

(12)

where temperature T is the independent variable, while Tmax,i corresponds to the temperature at the maximum weight loss rate of i-th reaction, σi represents standard deviation of each fitted peak. The kinetic parameters of the Arrhenius equation (Eq. 10), for the respective pyrolysis reactions (peaks), were calculated based on the data obtained from the deconvolution of the DTG curve, applying the Gaussian function. In this work, two models were implemented in order to describe the kinetics of pyrolysis in non-isothermal conditions. The first model is based on the method of Kissinger25 and the second one on the method of non-linear regression. The Kissinger method is based on the study of the reaction rate equation (Eq. 10) under the condition of the maximum reaction rate.26 This method uses the Tmax values from at least three heating rates for a single decomposition reaction. The Kissinger equation is expressed as:  β ln 2 T  max,i

    = − Ei + ln  Ai R     RTmax,i  Ei  

12


(13)

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The values of E and A for a given stage were calculated based on the Eq. 13, plotting a straight line in the system in ln(β/Tmax,i2) = f(1/Tmax,i). Kinetic parameters of the sewage sludge pyrolysis reactions are summarized in Table 3.

Table 3

The advantage of the Kissinger method is fast and easily estimation of kinetic parameters in the entire heating rate range. Moreover, one common set of the kinetic parameters for the whole range of the heating rate shortens the number of calculations while increasing the scale of the reactors. However, the method has a number of important limitations discussed by Vyazovkin et. al.27 and the fitting of the model to experimental data is poorer than in the non-linear regression method. For the non-linear regression method, determination of the activation energy Ei and the preexponential factor Ai was conducted applying the non-linear optimization procedure by Levenberg-Marquardt, through minimizing the following dependence: = ∑ ∑ (α Gauss ,i , j − α cal ,i , j ) n

min Ei , Ai

N

2

(14)

i =1 j =1

where αGauss,i,k are the values calculated by the Gauss model, αcal,i,k are the values calculated by the proposed model, N is total number of data points, and n is total number of reactions, j is the index of data points, i is the index of reactions, Ei and Ai are the parameters to be estimated. The kinetic parameters for the sewage sludge pyrolysis (Eq. 10) for the respective reactions (peaks), at selected heating rate (1, 10, 50 or 100 K/min) are presented in Table 4. The 13


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comparison of the model TG curve based on six first order parallel reactions with the experimental curve for different heating rates is presented in Figure 5. A satisfactory fit for all the conducted experiments was obtained, R2 values were higher than 0.99 and the mean squared error (MSE) values were below 1E-04.

Table 4

Figure 5.

Determination of kinetic parameters of gasification of char derived from sewage sludge The sewage sludge char air gasification process comprises of several homogenous and heterogeneous reactions: C+O2→CO2

(r1)

C+0,5O2→CO

(r2)

CO+0.5O2→CO2

(r3)

C+CO2→2CO

(r4)

C+H2O→CO+H2

(r5)

C+2H2→CH4

(r6)

CO+H2O→CO2+H2

(r7)

The primary reaction of the conversion of carbonaceous material in the presence of oxygen is the two-stage reaction r1. First, the heterogeneous reaction r2 takes place, and next the product of the incomplete combustion is oxidized to carbon dioxide in the exothermic reaction r3. Under the 14


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conditions of quantitative dominance of carbon, the thermodynamically stable ratio of CO:CO2 is reached as the results of the Boudouard reaction (r4). The remaining reactions of the gasification process are hydrogenation reaction (r5), methanation reaction (r6) and water gas shift reaction (r7). Assuming that the isothermal air gasification of sewage sludge char is the one-stage process, the conversion rate can be expressed by Eq. 1. Many models have been proposed in solid-state kinetics, and these models have been developed based on certain mechanistic assumptions (Table 2). For the purpose of the description of the biomass and coal gasification most often the following models are proposed: volumetric model (VM), based on the first-order reaction equation (F1), unreacted shrinking core model (URCM), corresponding to the contracting volume equation (R3)12,28, 29 and accidental nucleation model, specifically, the Avrami–Erofeev equations (Table 2).18,30 The VM and URCM models showed an unsatisfactory fitting to the char gasification experiments, R2 values were lower than 0.5. This can be attributed to the lack of compliance of the sewage sludge char air gasification experiments, gasified immediately after the pyrolysis process with the literature data on char gasification12 which had previously been cooled to the ambient temperature. Łabojko et al.30 suggested that the stabilized and chemically deactivated char of the same conversion degree is achieved in longer time than the char which is gasified without cooling. For the description of the decomposition reaction for the gasification of char immediately after the sewage sludge pyrolysis process the Avrami–Erofeev model (A3) was proposed: 2 dα = k ⋅ 3(1 − α )[− ln(1 − α)]3 dt

(15)

or in the integrated form: 1

[− ln(1 − α)]3 = kt

(16) 15


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after transformation:

α(t ) = 1 − e −( k ⋅t )

3

(17)

where the reaction rate coefficient k is defined by the Arrhenius equation (Eq. 3). The value of the chemical reaction rate coefficient, k, for the given temperature of the gasification process can be calculated based on the Eq. 16, plotting a straight line in the system [-ln(1α)]^(1/3)=f(t), whose slope is k. Figure 6 shows the application of this equation to the experimental results obtained for the air gasification of char at different temperatures. The kinetic analysis was conducted in the conversion degree range of 0.05–0.95.

Figure 6.

The values of the reaction rate coefficient, k, determined from the slope of straight lines drawn are given in Table 5. At all temperatures, the Avrami–Erofeev (A3) model had good fit to the empirical data, R2 values were greater than 0.99. Determination of the activation energy E and the pre-exponential factor A was conducted applying the non-linear optimization procedure, by Levenberg-Marquardt algorithm, minimizing the sum squared error between the experimental data and the proposed function described by the Eq. 17. The kinetic parameters of the sewage sludge char gasification reaction are presented in Table 5.

Table 5

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Char reactivity Char reactivity index rα which allows to assess the ability of a fuel to react with the gasifying agent,31-33 may be calculated applying the following relation:  dα  rα =    dt  α

(18)

 dα  where rα – char reactivity,   – the rate of decomposition of a solid substance for a specific  dt  α

degree of char conversion α. Based on the experimental results the value of the reactivity at given conversion degree was determined. The reactivity tests were conducted for chars obtained in the pyrolysis process at various heating rates 1, 10, 50 or 100 K/min. The relation between the char reactivity and the conversion degree is presented in Figure 7.

Figure 7.

The reactivity values from the start of the gasification reaction until around 0.4 of the conversion degree did not differ despite various heating rates of pyrolysis. Together with the progress of the reaction (conversion degrees between 0.4 and 0.95) for the char obtained at the heating rate of 50 and 100 K/min a smaller value of reactivity was observed than that of the chars subjected to a slower heating rate. Chars obtained in the pyrolysis process conducted at heating rates of 1 and 10 K/min showed similar reactivity in the entire range of the conversion degree. Maximum reactivity for these chars was observed at the conversion degree of around 0.75. For chars 17


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obtained at the higher heating rates (50 and 100 K/min) the maximal reactivity was observed at the conversion degree of around 0.8. In Figure 8 the relations between the reactivity: maximal (rmax) and at 0.5 char conversion degree (r0.5) in air gasification and the heating rate in the pyrolysis is presented. The average value of the maximal reactivity of the analyzed chars equals 0.25 1/min (±5%), and the average reactivity at 0.5 char conversion degree equals 0.23 1/min (±3%). Literature devoted to the impact of the conditions of pyrolysis on the reactivity of the “cold” chars of biomass in the gasification process describes the increase of the reactivity together with the heating rate during pyrolysis34. In the case of the conducted tests no such correlation has been found. Most probably lack of such, follows from the different reactivity of the chars by quenching at an ambient temperature before the gasification process and the reactivity of „hot” char derived from pyrolysis.

Figure 8.

CONCLUSIONS The paper analyses the kinetics of pyrolysis of sewage sludge and air gasification of the resulting chars. The classical Arrhenius equation was proposed for the mathematical description of these processes. Based on the obtained results it has been determined that the sewage sludge pyrolysis process in the temperature range from ambient to 1173 K, under a dynamic heating rate, with low heating 18


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rates comprises of many elementary reactions. When simplified, the process can be described using six first order reaction groups, which were identified based on the DTG curve according to the literature data. In the case of the air gasification stage of the sewage sludge char the obtained results suggest that for the description of the kinetics of the process the Avrami–Erofeev model (A3) can be applied. The models used in the paper are well chosen for the description of the experiments. The estimated kinetic parameters may be used in modeling and simulation of a twostage gasification of sewage sludge “hot” chars. The results indicate that the reaction peaks of sewage sludge pyrolysis shift to higher temperatures as the heating rate rises. Still the change of the heating rate from 1−100 K/min, at the pyrolysis stage had a minor impact on the char reactivity in air gasification. A significant impact, however, on the isothermal air gasification rate of the char had the process temperature.

ACKNOWLEDGEMENTS This work was supported by the Ministry of Science and Higher Education, Poland, under Grant No. 11310046.

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Energy & Fuels

Table captions Table 1. Proximate and ultimate analyses of sewage sludge tested Table 2. The comparison of the most common kinetic model forms, f(α) and g(α), used for the description of solid state decomposition reactions11 Table 3. Parameters of the Eqs. 9 and 10 for the pyrolysis reactions with the heating rate range 1100 K/min Table 4. The kinetic parameters for the pyrolysis reactions Table 5. Reaction rate and the formal kinetics parameters of decomposition reaction for gasification process of sewage sludge char

Figure captions Figure 1. TG and DTG profiles of pyrolysis (β=50 K/min, T≤1173 K) and air gasification (T=1173 K). Figure 2. Experimental conversion (a) and conversion rate curves (b) of: (I) sewage sludge pyrolysis at different heating rates and (II) chars air gasification, where the chars obtained by the pyrolysis of sewage sludge at 10 K/min heating rate and different final pyrolysis temperatures (T= 973, 1073 or 1173 K). Figure 3. Experimental conversion (a) and reaction rate curves (b) of chars in air gasification, where the chars obtained by the pyrolysis of sewage sludge at a given temperature (T=1173 K) and different heating rates (1, 10, 50 or 100 K/min). Figure 4. Deconvolution of the DTG curve according to the Gauss profile; pyrolysis with a heating rate of 50 K/min. Figure 5. Relationships between experimental curves and calculated curves.

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Figure 6. Plots of [-ln(1-α)]^(1/3) versus t for determining the values of reaction rate coefficient k for gasification process of sewage sludge char. Figure 7. Relation between the sewage sludge char reactivity and the conversion degree in the isothermal air gasification process (T=1173 K). Figure 8. Reactivity of the sewage sludge chars obtained at different pyrolysis heating rate, for the isothermal air gasification process (T=1173 K) (a) rmax (b) r0.5.

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Energy & Fuels

Table 1. Proximate analysis, wt%

Ultimate analysis, wt% HHV,

Volatile

Fixed

matter

carbon

45.94

7.96

Moisture

10.52

Ash

N

C

H

35.58

4.08

25.28 5.3

S

O

MJ/Kg

1.62

15.8

12.39

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Table 2. Symbol and name of model

differential form f(α)

Integral form g(α)

A2 Avrami–Erofeev

2(1-α)[-ln(1-α)]1/2

[-ln(1-α)]1/2

A3 Avrami–Erofeev

3(1-α)[-ln(1-α)]2/3

[-ln(1-α)]1/3

3(1-α) 2/3

1-(1-α) 1/3

F1 first-order

(1-α)

-ln(1-α)

F2 second-order

(1-α) 2

[1/(1-α)]-1

nucleation model

geometrical contraction models R3 contracting volume reaction-order models

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Energy & Fuels

Table 3. Ei, Peak No.

Ai, 1/min R2

wi, −

kJ/mol 1

38

1.5E+05 0.959 0.18

2

200

1.0E+18 0.978 0.41

3

179

4.1E+14 0.996 0.09

4

257

1.3E+19 0.977 0.22

5

237

4.4E+12 0.982 0.04

6

280

2.1E+13 0.988 0.06

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Table 4. Ei, β, K/min

Reaction No.

2 Ai, 1/min R

MSE

kJ/mol 1

16

1.35E+01 0.99997 4.9E-07

2

68

7.50E+04 0.99961 6.9E-05

3

152

1.30E+12 0.99969 6.4E-05

4

91

3.20E+05 0.99961 8.4E-05

5

468

4.30E+26 0.99972 6.1E-05

6

468

1.50E+23 0.99951 7.6E-05

1

29

4.90E+03 0.99973 1.1E-05

2

69

4.30E+05 0.99962 7.2E-05

3

153

2.00E+12 0.99968 7.0E-05

4

99

4.60E+06 0.99960 9.0E-05

5

328

4.40E+17 0.99951 8.8E-05

6

369

6.30E+17 0.99908 9.7E-05

1

37

7.62E+04 0.99955 3.7E-05

2

67

6.12E+05 0.99962 7.4E-05

3

180

4.10E+14 0.99970 7.0E-05

4

83

7.80E+05 0.99958 9.1E-05

5

384

2.40E+20 0.99942 8.7E-05

6

323

2.60E+15 0.99861 8.4E-05

1

32

1.80E+04 0.99965 3.0E-05

2

67

8.52E+05 0.99962 7.5E-05

1

10

50

100

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Energy & Fuels

3

201

1.50E+16 0.99971 6.9E-05

4

73

2.20E+05 0.99959 8.7E-05

5

379

7.08E+19 0.99930 9.2E-05

6

362

2.20E+17 0.99889 5.1E-05

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Table 5. Temperature, K k, 1/min R2

MSE

E, kJ/mol A, 1/min R2

MSE

973

1.6E-01

0.9964 2.4E-04 17

1.4

0.9923 1.1E-03

1073

1.8E-01

0.9936 4.3E-04 15

1.0

0.9937 9.1E-04

1173

2.1E-01

0.9936 4.1E-04 12

0.7

0.9965 5.5E-04

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Figure 1.

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Figure 2. I) a)

b)

II) a)

b)

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Energy & Fuels

Figure 3. a)

b)

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Figure 4.

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Figure 5.

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Figure 6.

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Figure 7.

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Figure 8. a)

b)

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Kinetics of Sewage Sludge Pyrolysis and Air Gasification of Its Chars

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