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Gasification of char from dried sewage sludge in fluidized bed: Reaction rate in mixtures of CO2 and H2O
Fuel 105 (2013) 764–768
Contents lists available at SciVerse ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Short communication
Gasification of char from dried sewage sludge in fluidized bed: Reaction rate in mixtures of CO2 and H2O Susanna Nilsson ⇑, Alberto Gómez-Barea, Pedro Ollero Chemical and Environmental Engineering Department, Escuela Superior de Ingenieros, University of Seville, Camino de los Descubrimientos s/n. 41092 Seville, Spain
a r t i c l e
i n f o
Article history: Received 9 July 2012 Received in revised form 4 September 2012 Accepted 5 September 2012 Available online 24 September 2012 Keywords: Gasification Kinetics Sewage sludge Fluidized bed Modeling
a b s t r a c t The rate of gasification of char from dried sewage sludge (DSS) with a gas containing CO2 and H2O was studied. The tests were conducted in an atmospheric laboratory fluidized bed (FB), using CO2–H2O–N2 mixtures as fluidizing gas and temperatures in the range of 800–900 °C. The gasification rates were compared with those previously obtained for DSS char using one single reactant (either CO2 or H2O) in the gas, i.e. in mixtures of CO2 and N2 or H2O and N2. It was found that the char gasification rates measured in a mixture containing both reactants, i.e. CO2 and H2O, were well approximated as the sum of the individual rates measured separately with CO2 and H2O. Besides this result, an alternative method to accurately measure the char gasification in an FB using CO2–H2O–N2 mixtures was employed, based on determining the char remaining in the reactor up to a certain time of gasification from the CO and CO2 concentrations in the gas during the combustion of this char. It was shown that this method compares well with the traditional method (based on tracking the CO and CO2 concentrations during gasification) when using a single reactant, but improves significantly the reliability of measurements for tests where both H2O and CO2 react simultaneously with char. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction The rate of char gasification is one of the key factors to analyze the performance of gasifiers. Fluidized bed gasifiers (FBGs) offer advantages compared to other types of gasifiers for the gasification of biomass and waste fuels. Various FBG prototypes have been developed for biomass species and wastes at low or medium pressure (from 0.1 to 3 MPa). At moderate or low pressure in the gasifier, the char particles are mainly converted by H2O and CO2, having less significance the reaction of char with H2. In an FBG a distribution of char particles with different sizes and state of conversion is gasified in a complex mixture containing CO2, H2O, CO, H2, light hydrocarbons (and N2 for air gasification) and other minor species. These effects have been studied in TGA [1,2] and fluidized bed (FB) [3]. A great deal of works have measured the gasification rate of a variety of chars using different gas mixtures [4,5]. Most kinetics studies have been conducted using gasification agents comprising CO2 or H2O (typically diluted in a mixture containing some inert gas like N2 and sometimes with addition of H2 and CO to assess inhibition effects). Less work exists regarding the gasification of char in a reacting gas containing both CO2 or H2O in the same mixture. Simultaneous gasification of char with CO2 and H2O has been ⇑ Corresponding author. Tel.: +34 954 482163; fax: +34 954 461775. E-mail address: [email protected] (S. Nilsson). 0016-2361/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2012.09.008
studied for coal char [1,2,6–9], mostly in TGA [1,7,8]. There is little or no data available regarding the gasification of char in fluidized bed (FB) using CO2 and H2O simultaneously. When carrying out char gasification experiments in FB, attrition of the char needs to be considered and elutriation of fines should be avoided by adjusting the gas velocity [10]. Previous findings have shown that attrition of DSS char during combustion in FB is not significant [11]. A variety of kinetics models have been employed to describe the simultaneous gasification of char with CO2 and H2O [6,7,9]. Based on the higher diffusivity of H2O compared to CO2 and on the existence of inorganic compounds in the char having catalytic activity, it has been argued that active sites may have more affinity for one of the two reactants. Then kinetic models assuming that CO2 and H2O react at different active sites have been developed. In this type of model the rate of char gasification (r) in a mixture of CO2 and H2O is calculated as the sum of the individual reaction rates, i.e. rC-CO2 + rC-H2O, where rC-CO2 and rC-H2O are determined in tests conducted using gas mixtures containing only one of the two reactive gases (the rest being nitrogen or other inert gas). Other models assume that the two reactants compete for the same active sites. For most chars, the gasification with H2O is faster than the reaction with CO2 [6,9,10]. Inhibition effect of CO2 has been observed at high pressure, leading to a decrease in reaction rate when adding CO2 to an H2O–N2 mixture [6]. In some studies carried out at atmospheric pressure the assumption that the reaction rate in mixtures of CO2 and H2O is the sum of the individual reaction rates
S. Nilsson et al. / Fuel 105 (2013) 764–768
765
Nomenclature Ea F(x) k0 mC mC0 n pCO2 pH2O R r rC-CO2 rC-H2O
activation energy, kJ/mol function that expresses the variation of reaction rate with char conversion, – preexponential factor, barns1 mass of carbon in char at any time, g initial mass of carbon in char produced after devolatilization, g reaction order, – partial pressure of CO2 in the feed gas, bar partial pressure of H2O in the feed gas, bar ideal gas constant, 8.314 J/(mol K) conversion rate, s1 rate of gasification of char in CO2–N2 mixture, s1 rate of gasification of char in H2O–N2 mixture, s1
measured with only CO2 or H2O has given good agreement with experiments [7,8], while in another study, the reaction rates measured with different CO2–H2O–N2 mixtures were close to the rates measured without CO2 in the mixture [1]. Therefore, further research is necessary to make this issue clearer. To obtain results representative for fluidized bed gasification (FBG) the char gasification measurements should be carried out in FB and the char should be generated in situ in the reactor [12]. In this work, the gasification of char from dried sewage sludge (DSS) in CO2–H2O–N2 mixtures was measured in a laboratory FB between 800 and 900 °C at atmospheric pressure. Measurements carried out in a previous work with the same char (obtained from the same fuel using the same method) and in the same FB, but conducting the tests in CO2–N2 and H2O–N2 mixtures [12], were employed to compare the results with those obtained here with CO2–H2O–N2 mixtures. In the previous experiments the char conversion was determined from the concentrations of CO and CO2 in the exit gas during gasification. However, in FB experiments with in situ generation of char, when both gasification reactions take place simultaneously, the accurate determination of the amount of reacted char from measurements of the exit gas concentration is more complicated, as will be discussed below. In most studies of simultaneous gasification of char with CO2 and H2O the char conversion has been determined from the mass loss of the sample [1,2,6–9], using TGA. In FB it is possible to determine the char conversion by extracting the char from the reactor after a certain time of gasification [13,14]. This method enables to study the properties of the char produced, but it is complex and time consuming. In this work, 1–3 mm size particles have been employed to ensure measurements under kinetic regime [12] and fuel batches with more than 100 particles were employed, so to extract all these particles from the reactor and separate them from the bed material is not feasible. Instead another method has been applied where the char conversion achieved up to a certain time was measured from the CO and CO2 concentrations in the gas during the combustion of the remaining char. This method has been employed to measure the carbon load in the bed for gasification, combustion and devolatilization experiments [4], but it has not been applied up to date to measure the rate of gasification of char with CO2 and H2O. 2. Experimental Experiments have been carried out in a laboratory FB reactor. The setup enabled to fluidize the reactor with mixtures of N2, CO2, H2O and air and the concentrations of CO, CO2, CH4 and H2 in the exit gas were measured using a Siemens gas analyzer. A de-
T t x
temperature, K time, s char conversion, –
Greek letters d variable for integration, – Abbreviations DSS dried sewage sludge FB fluidized bed FBG fluidized bed gasification or fluidized bed gasifier
tailed description of the experimental setup, as well as the properties of DSS and DSS char have been presented elsewhere [12]. The bed material employed was bauxite with particle size of 250– 500 lm and minimum fluidization velocity of 0.20 m/s. 2.1. Experimental procedure The DSS was first devolatilized in situ under N2-flow at the test temperature. As soon as the CO, CO2, CH4 and H2 concentrations measured in the outlet gas were nearly zero, devolatilization was considered to be complete. Then the CO2 and H2O flows were initiated, adjusting the flow of N2 to set the desired composition of the gasification mixture and fluidization velocity. The char gasification stage was maintained for a certain time, set to 3, 4, 5, 6, 10, 15 and 20 min for different tests. After the gasification step the feed gas was changed to air to burn the remaining char. This method was applied to accurately track the rate of char gasification from measurements of CO and CO2 in the outlet gas during combustion of char, as will be discussed below. 2.2. Operating conditions Tests were conducted at three temperatures: 800, 850 and 900 °C, at fixed partial pressure of H2O in the mixture, pH2O, of 0.20 bar. Three partial pressures of CO2, pCO2, were employed: 0, 0.20 and 0.40 bar. The pressure inside the reactor during the experiments was below 1.05 bar, i.e. atmospheric conditions. The gas velocity, during both fuel devolatilization and gasification of char, was set to three times the minimum fluidizing velocity of the bed material. With this value, entrainment was avoided, no slug flow was detected and the char particles were assumed to be sufficiently well mixed with the bed material [15]. 2.3. Data treatment During char gasification tests with CO2–H2O–N2 mixtures, the following reactions occur:
C þ CO2 ¢ 2CO
ð1Þ
C þ H2 O ¢ CO þ H2
ð2Þ
CO þ H2 O ¢ CO2 þ H2
ð3Þ
In order to determine the amount of carbon in the char that is gasified at any time, continuous measurements of the concentrations of at least two of the four gases taking part in reactions (1)–(3) (H2O, CO2, H2 and CO) are required. It is known that
S. Nilsson et al. / Fuel 105 (2013) 764–768
continuous measurement of H2O is not easy to carry out. On the other hand, continuous determination of the H2 concentration is usually based on the thermal conductivity of the gas, which is strongly affected by the presence of other gas species. In particular, during the experiments carried out in this work, the high concentration of CO2 in the exit gas affected the H2 measurement. The H2 concentration can be more accurately determined using a micro GC, but this equipment can only carry out one measurement every 3–5 min. The char conversion can also be determined from the CO2, which is a product of reaction (3) and a reactive in reaction (1). Since the reactor is differential (to assure uniform CO2 and H2O concentrations along the reactor) the net production or consumption of CO2 is very small compared to the flow of CO2 fed to the reactor, so it is difficult to measure accurately. In addition, at the beginning of the experiment, the CO2 concentration measured by the gas analyzer is strongly influenced by the dispersion in the exit line and these effects need to be properly addressed, even in a laboratory FB. Due to these limitations, an alternative method for measuring the char conversion was employed in this work. After a certain time of gasification, t, the amount of carbon in char remaining in the reactor was calculated from the total moles of CO2 and CO measured during combustion of the char. 3. Results and discussion In a previous DSS gasification study [12] the kinetics expressions for DSS char gasification in CO2–N2 and H2O–N2 mixtures were obtained. For both reactions, experimental conversion rates (dx/dt), r, were well represented by the following expressions:
Ea;1 n1 PCO2 FðxÞ RT Ea;2 n2 ¼ k0;2 exp PH2 O FðxÞ RT
r CCO2 ¼ k0;1 exp r CH2 O
ð4Þ
being F(x) an empirical function accounting for the variation of the reaction rate with conversion [12], given by:
FðxÞ ¼ ð1 xÞð30:8x þ 3:6Þ expð4:0x1=2 Þ
ð5Þ
and x the degree of conversion of char at time t, defined as:
xðtÞ ¼
mc0 mc ðtÞ mc0
ð6Þ
This expression for F(x) has shown to give good agreement with experiments both with CO2–N2 and H2O–N2 mixtures [12]. The kinetic parameters, k0i, Eai and ni for the two reactions C–CO2 (i = 1) and C–H2O (i = 2) are given in Table 1. In the following, the kinetic expression given by Eqs. (4)–(6) will be used to analyze the experimental data obtained in this work with CO2–H2O–N2 mixtures, as explained below. In order to check the ability of the alternative method developed in Section 2 to measure the char conversion, experiments were carried out using mixtures containing only H2O and N2 to compare the results with those obtained in [12] (obtained with the method based on gas measurements during gasification). The comparison was only conducted for the gasification reaction with H2O, because this reaction is faster than that with CO2.
Table 1 Values of kinetic parameters: activation energy, Ea, frequency factor, k0, and order of reaction, n, for the char gasification reactions with CO2 and H2O.
CO2 H2O
n
Ea (kJ/mol)
k0 (barn s1)
0.41 0.33
163.5 171.0
6.33 104 3.90 105
1
900 ºC 0.9 0.8
850 ºC
0.7
Conversion, x
766
0.6
800 ºC 0.5 0.4 0.3 0.2
800 ºC 850 ºC 900 ºC
0.1 0 0
200
400
600
800
1000
1200
Time, s Fig. 1. Comparison between the two methods for measuring the char conversion. Experiments carried out using H2O–N2 mixture with pH2O = 0.20 bar at three different temperatures. The curves represent the char conversion calculated using kinetics from [12] (Eqs. (4)–(6) and kinetic parameters from Table 1) and the points represent the conversions measured in this work.
Fig. 1 compares the experimental data measured in this work with the conversion versus time curves calculated using the kinetics given in [12] (Eqs. (4)–(6) and kinetic parameters from Table 1) at the three temperatures studied. The calculated curves in Fig. 1 show good agreement with experimental data measured from the CO and CO2 concentrations during gasification with H2O–N2 mixture [12]. The results in Fig. 1 enable to conclude that both methods give very similar results at all temperatures and degrees of conversion studied, indicating that they are both valid. In order to study the gasification in mixtures containing both CO2 and H2O, experiments were carried out using CO2–H2O–N2 mixtures with partial pressure of H2O of 0.20 bar and two different partial pressures of CO2: 0.20 and 0.40 bar. The partial pressure of 0.20 bar for CO2 and H2O was chosen because it is close to the typical values found in atmospheric air-blown gasifiers, while the higher value pCO2 = 0.40, was chosen to assess if high concentration of CO2 in the mixture produces inhibition effects. The results have been compared to the conversion versus time curves calculated using the kinetics from [12], assuming that the overall reaction rate is the sum of the individual reaction rates with CO2 and H2O. In this case, the time required to reach a certain char conversion can be calculated as:
tðxÞ ¼
Z 0
x
1 dd rCCO2 ðdÞ þ r CH2 O ðdÞ
ð7Þ
with rC-CO2 and rC-H2O given by Eqs. (4)–(6), and using kinetics parameters from Table 1. The comparison is shown in Fig. 2 at three different temperatures: 800, 850 and 900 °C. Also the experimental results from Fig. 1 obtained for pCO2 = 0 are shown in Fig. 2 for comparison (note that the calculated curve for pCO2 = 0 is not included in Fig. 2 but only the measurements). The results show that using Eq. (7) for calculating the conversion reached after a certain time of gasification gives good agreement with most of the experimental values, so the gasification rate in a CO2–H2O–N2 mixture can be estimated as the sum of the individual reaction rates with CO2 and H2O. It is observed that the addition of CO2 to the H2O–N2 mixture increases slightly the char gasification rate and no inhibition effect was observed in such a way that the char reaction rate decreases at high CO2 concentration in the mixture. The contribution of CO2 to the overall gasification rate is seen to be small, especially at high
767
S. Nilsson et al. / Fuel 105 (2013) 764–768
1
1
pCO2=0
0.9
pCO2=0.2
0.9
0.8
pCO2=0.4
0.8
pCO2 =0.4
0.7
T=800 ºC
Conversion, x
Conversion, x
0.7
pCO2 =0 pCO2 =0.2
0.6 0.5 0.4
0.5 0.4
0.3
0.3
0.2
0.2
0.1
(a)
0 0
200
400
600
800
1000
1200
T=850 ºC
0.6
0.1
(b)
0 0
200
400
Time, s
600
800
1000
Time, s 1
pCO2=0
0.9
pCO2=0.2
0.8
Conversion, x
0.7
T=900 ºC
0.6 0.5 0.4 0.3 0.2 0.1
(c)
0 0
100
200
300
400
500
600
Time, s Fig. 2. Char conversion as a function of time at three different temperatures: (a) 800 °C; (b) 850 °C; (c) 900 °C. The points are the values measured in this work using CO2– H2O–N2 mixtures at fixed pH2O = 0.20 bar for the indicated values of pCO2. The x vs t curves are calculated using Eq. (7): pCO2 = 0.20 bar (solid lines) and pCO2 = 0.40 bar (dashed lines).
temperature. This result was expected because the reaction with steam is three times faster than that with CO2 [12]. For other chars where the rate of gasification with CO2 is more similar to the rate with H2O the conclusions made in this work may not be valid: in such a case an hypothetical competition of CO2 for carbon sites could decrease the rate of gasification with H2O to such extent that the overall rate of char gasification was smaller than that expected by considering the superposition of the two individual rates. Such a situation is, however, not often found in practice [3–5], because most of biomass chars exhibit much higher gasification rate with H2O than with CO2. Therefore the findings from this work are expected to be applied for most practical atmospheric FBG. 4. Conclusions The rate of gasification of char from dried sewage sludge in fluidized bed (FB) was measured using CO2–H2O–N2 mixtures as fluidizing gas. For a fixed partial pressure of H2O in the mixture the conversion rate generally increased slightly when increasing the partial pressure of CO2, so no inhibition effect was detected in such a way that the char reaction rate decreases at high CO2 concentra-
tion in the mixture. Since the reaction of the investigated char with CO2 is slow compared to the reaction with H2O, the contribution of CO2 to the overall gasification rate was small. The measurements were compared with kinetics data obtained in a previous work (using the same experimental setup) with mixtures containing one single reactant, i.e. H2O–N2 and CO2–N2 mixtures. This comparison showed that the overall gasification rate was well approximated by the sum of the individual reaction rates with CO2 and H2O. Besides this result, an alternative method for measuring the char conversion, based on calculating the char conversion achieved up to certain time by burning the remaining char, was introduced. This method was more reliable for measuring the kinetics when both H2O and CO2 react simultaneously with carbon in the char in an FB. This method gave good agreement with the traditional method, based on tracking the CO and CO2 concentrations during gasification, when using a single reactant. Acknowledgements The authors acknowledge the Junta de Andalucía for its financial support in the project FLETGAS. They also wish to express their
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S. Nilsson et al. / Fuel 105 (2013) 764–768
appreciation to Elisa López and Verónica Hidalgo for their help in the experimental work.
References [1] Umemoto S, Kajitani S, Hara S. Modeling of coal char gasification in coexistence of CO2 and H2O considering sharing of active sites. Fuel. in press, http:// dx.doi.org/10.1016/j.fuel.2011.11.030. [2] Mühlen HJ, Heek KH, Juntgen H. Kinetic studies of steam gasification of char in the presence of H2, CO, and CO2. Fuel 1985;64:944–9. [3] Gómez-Barea A. Modeling of diffusional effects during gasification of biomass particles in fluidized-bed. PhD thesis. University of Seville, May 2006. [4] Di Blasi C. Combustion and gasification rates of lignocellulosic chars. Prog Energ Combust 2009;35:121–40. [5] Gómez-Barea A, Leckner B. Modeling of biomass gasification in fluidized bed. Prog Energ Combust 2010;36:444–509. [6] Roberts DG, Harris DJ. Char gasification in mixtures of CO2 and H2O: competition and inhibition. Fuel 2007;86:2672–8.
[7] Everson RC, Neomagus HWJP, Kasaini H, Njapha D. Reaction kinetics of pulverized coal-chars derived from inertinite-rich coal discards: gasification with carbon dioxide and steam. Fuel 2006;85:1072–82. [8] Huang Z, Zhang J, Zhao Y, Zhang H, Yue G, Suda T, et al. Kinetic studies of char gasification by steam and CO2 in the presence of H2 and CO. Fuel Process Technol 2010;91:843–7. [9] Koba K, Ida S. Gasification reactivities of metallurgical cokes with carbon dioxide, steam and their mixtures. Fuel 1980;59:59–63. [10] Scala F, Chirone R, Salatino P. Combustion and attrition of biomass chars in a fluidized bed. Energy Fuels 2006;20:91–102. [11] Gomez-Barea A, Nilsson S, Vidal-Barrero F, Campoy M. Devolatilization of biomass and waste in fluidized bed. Fuel Process Technol 2010;91:1624–33. [12] Nilsson S, Gómez-Barea A, Fuentes-Cano D. Gasification reactivity of char from dried sewage sludge in a fluidized bed. Fuel 2012;92:346–53. [13] Kajitani S, Tay HL, Zhang S, Li CZ. Mechanisms and kinetic modelling of steam gasification of brown coal in the presence of volatile–char interactions. Fuel, in press. http://dx.doi.org/10.1016/j.fuel.2011.09.059. [14] Jand N, Foscolo PU. Decomposition of wood particles in fluidized beds. Ind Eng Chem Res 2005;44:5079–89. [15] Nilsson S, Modeling and simulation of a three-stage gasification technology for waste and biomass. PhD thesis. University of Seville; 2012.
Contents lists available at SciVerse ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Short communication
Gasification of char from dried sewage sludge in fluidized bed: Reaction rate in mixtures of CO2 and H2O Susanna Nilsson ⇑, Alberto Gómez-Barea, Pedro Ollero Chemical and Environmental Engineering Department, Escuela Superior de Ingenieros, University of Seville, Camino de los Descubrimientos s/n. 41092 Seville, Spain
a r t i c l e
i n f o
Article history: Received 9 July 2012 Received in revised form 4 September 2012 Accepted 5 September 2012 Available online 24 September 2012 Keywords: Gasification Kinetics Sewage sludge Fluidized bed Modeling
a b s t r a c t The rate of gasification of char from dried sewage sludge (DSS) with a gas containing CO2 and H2O was studied. The tests were conducted in an atmospheric laboratory fluidized bed (FB), using CO2–H2O–N2 mixtures as fluidizing gas and temperatures in the range of 800–900 °C. The gasification rates were compared with those previously obtained for DSS char using one single reactant (either CO2 or H2O) in the gas, i.e. in mixtures of CO2 and N2 or H2O and N2. It was found that the char gasification rates measured in a mixture containing both reactants, i.e. CO2 and H2O, were well approximated as the sum of the individual rates measured separately with CO2 and H2O. Besides this result, an alternative method to accurately measure the char gasification in an FB using CO2–H2O–N2 mixtures was employed, based on determining the char remaining in the reactor up to a certain time of gasification from the CO and CO2 concentrations in the gas during the combustion of this char. It was shown that this method compares well with the traditional method (based on tracking the CO and CO2 concentrations during gasification) when using a single reactant, but improves significantly the reliability of measurements for tests where both H2O and CO2 react simultaneously with char. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction The rate of char gasification is one of the key factors to analyze the performance of gasifiers. Fluidized bed gasifiers (FBGs) offer advantages compared to other types of gasifiers for the gasification of biomass and waste fuels. Various FBG prototypes have been developed for biomass species and wastes at low or medium pressure (from 0.1 to 3 MPa). At moderate or low pressure in the gasifier, the char particles are mainly converted by H2O and CO2, having less significance the reaction of char with H2. In an FBG a distribution of char particles with different sizes and state of conversion is gasified in a complex mixture containing CO2, H2O, CO, H2, light hydrocarbons (and N2 for air gasification) and other minor species. These effects have been studied in TGA [1,2] and fluidized bed (FB) [3]. A great deal of works have measured the gasification rate of a variety of chars using different gas mixtures [4,5]. Most kinetics studies have been conducted using gasification agents comprising CO2 or H2O (typically diluted in a mixture containing some inert gas like N2 and sometimes with addition of H2 and CO to assess inhibition effects). Less work exists regarding the gasification of char in a reacting gas containing both CO2 or H2O in the same mixture. Simultaneous gasification of char with CO2 and H2O has been ⇑ Corresponding author. Tel.: +34 954 482163; fax: +34 954 461775. E-mail address: [email protected] (S. Nilsson). 0016-2361/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2012.09.008
studied for coal char [1,2,6–9], mostly in TGA [1,7,8]. There is little or no data available regarding the gasification of char in fluidized bed (FB) using CO2 and H2O simultaneously. When carrying out char gasification experiments in FB, attrition of the char needs to be considered and elutriation of fines should be avoided by adjusting the gas velocity [10]. Previous findings have shown that attrition of DSS char during combustion in FB is not significant [11]. A variety of kinetics models have been employed to describe the simultaneous gasification of char with CO2 and H2O [6,7,9]. Based on the higher diffusivity of H2O compared to CO2 and on the existence of inorganic compounds in the char having catalytic activity, it has been argued that active sites may have more affinity for one of the two reactants. Then kinetic models assuming that CO2 and H2O react at different active sites have been developed. In this type of model the rate of char gasification (r) in a mixture of CO2 and H2O is calculated as the sum of the individual reaction rates, i.e. rC-CO2 + rC-H2O, where rC-CO2 and rC-H2O are determined in tests conducted using gas mixtures containing only one of the two reactive gases (the rest being nitrogen or other inert gas). Other models assume that the two reactants compete for the same active sites. For most chars, the gasification with H2O is faster than the reaction with CO2 [6,9,10]. Inhibition effect of CO2 has been observed at high pressure, leading to a decrease in reaction rate when adding CO2 to an H2O–N2 mixture [6]. In some studies carried out at atmospheric pressure the assumption that the reaction rate in mixtures of CO2 and H2O is the sum of the individual reaction rates
S. Nilsson et al. / Fuel 105 (2013) 764–768
765
Nomenclature Ea F(x) k0 mC mC0 n pCO2 pH2O R r rC-CO2 rC-H2O
activation energy, kJ/mol function that expresses the variation of reaction rate with char conversion, – preexponential factor, barns1 mass of carbon in char at any time, g initial mass of carbon in char produced after devolatilization, g reaction order, – partial pressure of CO2 in the feed gas, bar partial pressure of H2O in the feed gas, bar ideal gas constant, 8.314 J/(mol K) conversion rate, s1 rate of gasification of char in CO2–N2 mixture, s1 rate of gasification of char in H2O–N2 mixture, s1
measured with only CO2 or H2O has given good agreement with experiments [7,8], while in another study, the reaction rates measured with different CO2–H2O–N2 mixtures were close to the rates measured without CO2 in the mixture [1]. Therefore, further research is necessary to make this issue clearer. To obtain results representative for fluidized bed gasification (FBG) the char gasification measurements should be carried out in FB and the char should be generated in situ in the reactor [12]. In this work, the gasification of char from dried sewage sludge (DSS) in CO2–H2O–N2 mixtures was measured in a laboratory FB between 800 and 900 °C at atmospheric pressure. Measurements carried out in a previous work with the same char (obtained from the same fuel using the same method) and in the same FB, but conducting the tests in CO2–N2 and H2O–N2 mixtures [12], were employed to compare the results with those obtained here with CO2–H2O–N2 mixtures. In the previous experiments the char conversion was determined from the concentrations of CO and CO2 in the exit gas during gasification. However, in FB experiments with in situ generation of char, when both gasification reactions take place simultaneously, the accurate determination of the amount of reacted char from measurements of the exit gas concentration is more complicated, as will be discussed below. In most studies of simultaneous gasification of char with CO2 and H2O the char conversion has been determined from the mass loss of the sample [1,2,6–9], using TGA. In FB it is possible to determine the char conversion by extracting the char from the reactor after a certain time of gasification [13,14]. This method enables to study the properties of the char produced, but it is complex and time consuming. In this work, 1–3 mm size particles have been employed to ensure measurements under kinetic regime [12] and fuel batches with more than 100 particles were employed, so to extract all these particles from the reactor and separate them from the bed material is not feasible. Instead another method has been applied where the char conversion achieved up to a certain time was measured from the CO and CO2 concentrations in the gas during the combustion of the remaining char. This method has been employed to measure the carbon load in the bed for gasification, combustion and devolatilization experiments [4], but it has not been applied up to date to measure the rate of gasification of char with CO2 and H2O. 2. Experimental Experiments have been carried out in a laboratory FB reactor. The setup enabled to fluidize the reactor with mixtures of N2, CO2, H2O and air and the concentrations of CO, CO2, CH4 and H2 in the exit gas were measured using a Siemens gas analyzer. A de-
T t x
temperature, K time, s char conversion, –
Greek letters d variable for integration, – Abbreviations DSS dried sewage sludge FB fluidized bed FBG fluidized bed gasification or fluidized bed gasifier
tailed description of the experimental setup, as well as the properties of DSS and DSS char have been presented elsewhere [12]. The bed material employed was bauxite with particle size of 250– 500 lm and minimum fluidization velocity of 0.20 m/s. 2.1. Experimental procedure The DSS was first devolatilized in situ under N2-flow at the test temperature. As soon as the CO, CO2, CH4 and H2 concentrations measured in the outlet gas were nearly zero, devolatilization was considered to be complete. Then the CO2 and H2O flows were initiated, adjusting the flow of N2 to set the desired composition of the gasification mixture and fluidization velocity. The char gasification stage was maintained for a certain time, set to 3, 4, 5, 6, 10, 15 and 20 min for different tests. After the gasification step the feed gas was changed to air to burn the remaining char. This method was applied to accurately track the rate of char gasification from measurements of CO and CO2 in the outlet gas during combustion of char, as will be discussed below. 2.2. Operating conditions Tests were conducted at three temperatures: 800, 850 and 900 °C, at fixed partial pressure of H2O in the mixture, pH2O, of 0.20 bar. Three partial pressures of CO2, pCO2, were employed: 0, 0.20 and 0.40 bar. The pressure inside the reactor during the experiments was below 1.05 bar, i.e. atmospheric conditions. The gas velocity, during both fuel devolatilization and gasification of char, was set to three times the minimum fluidizing velocity of the bed material. With this value, entrainment was avoided, no slug flow was detected and the char particles were assumed to be sufficiently well mixed with the bed material [15]. 2.3. Data treatment During char gasification tests with CO2–H2O–N2 mixtures, the following reactions occur:
C þ CO2 ¢ 2CO
ð1Þ
C þ H2 O ¢ CO þ H2
ð2Þ
CO þ H2 O ¢ CO2 þ H2
ð3Þ
In order to determine the amount of carbon in the char that is gasified at any time, continuous measurements of the concentrations of at least two of the four gases taking part in reactions (1)–(3) (H2O, CO2, H2 and CO) are required. It is known that
S. Nilsson et al. / Fuel 105 (2013) 764–768
continuous measurement of H2O is not easy to carry out. On the other hand, continuous determination of the H2 concentration is usually based on the thermal conductivity of the gas, which is strongly affected by the presence of other gas species. In particular, during the experiments carried out in this work, the high concentration of CO2 in the exit gas affected the H2 measurement. The H2 concentration can be more accurately determined using a micro GC, but this equipment can only carry out one measurement every 3–5 min. The char conversion can also be determined from the CO2, which is a product of reaction (3) and a reactive in reaction (1). Since the reactor is differential (to assure uniform CO2 and H2O concentrations along the reactor) the net production or consumption of CO2 is very small compared to the flow of CO2 fed to the reactor, so it is difficult to measure accurately. In addition, at the beginning of the experiment, the CO2 concentration measured by the gas analyzer is strongly influenced by the dispersion in the exit line and these effects need to be properly addressed, even in a laboratory FB. Due to these limitations, an alternative method for measuring the char conversion was employed in this work. After a certain time of gasification, t, the amount of carbon in char remaining in the reactor was calculated from the total moles of CO2 and CO measured during combustion of the char. 3. Results and discussion In a previous DSS gasification study [12] the kinetics expressions for DSS char gasification in CO2–N2 and H2O–N2 mixtures were obtained. For both reactions, experimental conversion rates (dx/dt), r, were well represented by the following expressions:
Ea;1 n1 PCO2 FðxÞ RT Ea;2 n2 ¼ k0;2 exp PH2 O FðxÞ RT
r CCO2 ¼ k0;1 exp r CH2 O
ð4Þ
being F(x) an empirical function accounting for the variation of the reaction rate with conversion [12], given by:
FðxÞ ¼ ð1 xÞð30:8x þ 3:6Þ expð4:0x1=2 Þ
ð5Þ
and x the degree of conversion of char at time t, defined as:
xðtÞ ¼
mc0 mc ðtÞ mc0
ð6Þ
This expression for F(x) has shown to give good agreement with experiments both with CO2–N2 and H2O–N2 mixtures [12]. The kinetic parameters, k0i, Eai and ni for the two reactions C–CO2 (i = 1) and C–H2O (i = 2) are given in Table 1. In the following, the kinetic expression given by Eqs. (4)–(6) will be used to analyze the experimental data obtained in this work with CO2–H2O–N2 mixtures, as explained below. In order to check the ability of the alternative method developed in Section 2 to measure the char conversion, experiments were carried out using mixtures containing only H2O and N2 to compare the results with those obtained in [12] (obtained with the method based on gas measurements during gasification). The comparison was only conducted for the gasification reaction with H2O, because this reaction is faster than that with CO2.
Table 1 Values of kinetic parameters: activation energy, Ea, frequency factor, k0, and order of reaction, n, for the char gasification reactions with CO2 and H2O.
CO2 H2O
n
Ea (kJ/mol)
k0 (barn s1)
0.41 0.33
163.5 171.0
6.33 104 3.90 105
1
900 ºC 0.9 0.8
850 ºC
0.7
Conversion, x
766
0.6
800 ºC 0.5 0.4 0.3 0.2
800 ºC 850 ºC 900 ºC
0.1 0 0
200
400
600
800
1000
1200
Time, s Fig. 1. Comparison between the two methods for measuring the char conversion. Experiments carried out using H2O–N2 mixture with pH2O = 0.20 bar at three different temperatures. The curves represent the char conversion calculated using kinetics from [12] (Eqs. (4)–(6) and kinetic parameters from Table 1) and the points represent the conversions measured in this work.
Fig. 1 compares the experimental data measured in this work with the conversion versus time curves calculated using the kinetics given in [12] (Eqs. (4)–(6) and kinetic parameters from Table 1) at the three temperatures studied. The calculated curves in Fig. 1 show good agreement with experimental data measured from the CO and CO2 concentrations during gasification with H2O–N2 mixture [12]. The results in Fig. 1 enable to conclude that both methods give very similar results at all temperatures and degrees of conversion studied, indicating that they are both valid. In order to study the gasification in mixtures containing both CO2 and H2O, experiments were carried out using CO2–H2O–N2 mixtures with partial pressure of H2O of 0.20 bar and two different partial pressures of CO2: 0.20 and 0.40 bar. The partial pressure of 0.20 bar for CO2 and H2O was chosen because it is close to the typical values found in atmospheric air-blown gasifiers, while the higher value pCO2 = 0.40, was chosen to assess if high concentration of CO2 in the mixture produces inhibition effects. The results have been compared to the conversion versus time curves calculated using the kinetics from [12], assuming that the overall reaction rate is the sum of the individual reaction rates with CO2 and H2O. In this case, the time required to reach a certain char conversion can be calculated as:
tðxÞ ¼
Z 0
x
1 dd rCCO2 ðdÞ þ r CH2 O ðdÞ
ð7Þ
with rC-CO2 and rC-H2O given by Eqs. (4)–(6), and using kinetics parameters from Table 1. The comparison is shown in Fig. 2 at three different temperatures: 800, 850 and 900 °C. Also the experimental results from Fig. 1 obtained for pCO2 = 0 are shown in Fig. 2 for comparison (note that the calculated curve for pCO2 = 0 is not included in Fig. 2 but only the measurements). The results show that using Eq. (7) for calculating the conversion reached after a certain time of gasification gives good agreement with most of the experimental values, so the gasification rate in a CO2–H2O–N2 mixture can be estimated as the sum of the individual reaction rates with CO2 and H2O. It is observed that the addition of CO2 to the H2O–N2 mixture increases slightly the char gasification rate and no inhibition effect was observed in such a way that the char reaction rate decreases at high CO2 concentration in the mixture. The contribution of CO2 to the overall gasification rate is seen to be small, especially at high
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1
1
pCO2=0
0.9
pCO2=0.2
0.9
0.8
pCO2=0.4
0.8
pCO2 =0.4
0.7
T=800 ºC
Conversion, x
Conversion, x
0.7
pCO2 =0 pCO2 =0.2
0.6 0.5 0.4
0.5 0.4
0.3
0.3
0.2
0.2
0.1
(a)
0 0
200
400
600
800
1000
1200
T=850 ºC
0.6
0.1
(b)
0 0
200
400
Time, s
600
800
1000
Time, s 1
pCO2=0
0.9
pCO2=0.2
0.8
Conversion, x
0.7
T=900 ºC
0.6 0.5 0.4 0.3 0.2 0.1
(c)
0 0
100
200
300
400
500
600
Time, s Fig. 2. Char conversion as a function of time at three different temperatures: (a) 800 °C; (b) 850 °C; (c) 900 °C. The points are the values measured in this work using CO2– H2O–N2 mixtures at fixed pH2O = 0.20 bar for the indicated values of pCO2. The x vs t curves are calculated using Eq. (7): pCO2 = 0.20 bar (solid lines) and pCO2 = 0.40 bar (dashed lines).
temperature. This result was expected because the reaction with steam is three times faster than that with CO2 [12]. For other chars where the rate of gasification with CO2 is more similar to the rate with H2O the conclusions made in this work may not be valid: in such a case an hypothetical competition of CO2 for carbon sites could decrease the rate of gasification with H2O to such extent that the overall rate of char gasification was smaller than that expected by considering the superposition of the two individual rates. Such a situation is, however, not often found in practice [3–5], because most of biomass chars exhibit much higher gasification rate with H2O than with CO2. Therefore the findings from this work are expected to be applied for most practical atmospheric FBG. 4. Conclusions The rate of gasification of char from dried sewage sludge in fluidized bed (FB) was measured using CO2–H2O–N2 mixtures as fluidizing gas. For a fixed partial pressure of H2O in the mixture the conversion rate generally increased slightly when increasing the partial pressure of CO2, so no inhibition effect was detected in such a way that the char reaction rate decreases at high CO2 concentra-
tion in the mixture. Since the reaction of the investigated char with CO2 is slow compared to the reaction with H2O, the contribution of CO2 to the overall gasification rate was small. The measurements were compared with kinetics data obtained in a previous work (using the same experimental setup) with mixtures containing one single reactant, i.e. H2O–N2 and CO2–N2 mixtures. This comparison showed that the overall gasification rate was well approximated by the sum of the individual reaction rates with CO2 and H2O. Besides this result, an alternative method for measuring the char conversion, based on calculating the char conversion achieved up to certain time by burning the remaining char, was introduced. This method was more reliable for measuring the kinetics when both H2O and CO2 react simultaneously with carbon in the char in an FB. This method gave good agreement with the traditional method, based on tracking the CO and CO2 concentrations during gasification, when using a single reactant. Acknowledgements The authors acknowledge the Junta de Andalucía for its financial support in the project FLETGAS. They also wish to express their
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appreciation to Elisa López and Verónica Hidalgo for their help in the experimental work.
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