Experimental and Kinetic Study at High Temperatures of the NO

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Experimental and Kinetic Study at High Temperatures of the NO Reduction over Eucalyptus Char Produced at Different Heating Rates ngela Millera, María U. Alzueta, and Rafael Bilbao Marta Guerrero,* A Aragon Institute of Engineering Research (I3A), University of Zaragoza, Campus Río Ebro, C/Mariano Esquillor s/n, 50018 Zaragoza, Spain ABSTRACT: The NO reduction process over eucalyptus char has been studied in a laboratory-scale quartz reactor. Experiments were performed with char obtained from the pyrolysis of eucalyptus at a low heating rate (LHR) and high heating rate (HHR) and a final temperature of 900 C. The LHR char sample was prepared in a fixed-bed reactor using a pyrolysis heating rate of 10 C min-1, while a fluidized-bed reactor was employed to obtain chars at a HHR. The parameters considered in the study of the char/NO interaction comprise the inlet NO concentration, temperature, and pyrolysis heating rate. The inlet NO concentration was varied, ranging from 300 to 1100 ppm at a temperature of 900 C. To analyze the temperature influence, char/NO interaction tests were also performed in the 750-900 C temperature range for a given NO concentration of 900 ppm. It has been found that the NO reduction ability is significant for the eucalyptus chars considered, showing that temperature has a marked effect on the NO reduction process. Results have also revealed that HHR char is more active toward NO reduction compared to LHR char. The differences in NO reactivity between LHR and HHR chars are further discussed in terms of the chemical and physical char properties. Kinetic parameters of the NO reduction process over eucalyptus char have also been determined from the experimental data using an intrinsic rate equation defined as a function of the available active site concentration, which fitted properly the random pore model.

1. INTRODUCTION Concerning the depletion of fossil fuels worldwide and the increasing environmental pollution, numerous endeavors have been attempted to find other renewable and environmentally friendly energy sources. With regard to the power generation, the coal-biomass co-combustion is one of the most promising short-term options for the use of renewable fuels.1-3 This process allows for the reduction of the dependence upon fossil fuels and also offers additional environmental and economic advantages, among which are the mitigation of fossil CO2 emissions, low emissions of SO2 and heavy metals, and the development of a market for biomass materials that have a renewable energy potential. Likewise, the co-combustion in existing boilers is potentially the least costly way for coal-burning utilities using renewable fuels. Although different co-combustion configurations can be handled, one of the most attractive options to decrease NO emissions into the atmosphere is reburning with biomass as a reductive fuel.4-8 Within this frame, pilot-scale experiments using biomass as the reburning fuel in coal-combustion systems9,10 have allowed for levels of NO reduction in the range of 50-75% to be obtained, similar to those of coal and natural gas. Using a solid reburning fuel, both volatiles and char from the reburning fuel may contribute to NO reduction. The homogeneous reaction chemistry of reburning has been extensively studied5,11,12 and, nowadays, is known with certain confidence. Besides, many investigations have been reported during the last few decades on NO reduction for different coal chars and summarized in the reviews by Li et al.13 and Aarna and Suuberg.14 However, few studies have been reported on the interaction r 2011 American Chemical Society

between NO and biomass chars of diverse origin and thermal history.7,15-18 Results from these studies indicated that the kinetics of the reaction of biomass chars with NO was of fractional order with respect to NO, varying between 0.3 and 0.7,7,16,17 and that biomass chars were more reactive than coal chars because of a less ordered structure of the organic matrix and, in some cases, their higher content of mineral impurities. The heterogeneous char/NO interaction is quite complex, involving several physical and chemical processes, such as chemisorption, desorption of surface complexes, and release of products. The overall biomass char/NO interaction can be described by the following reactions:7 C þ NO f CO þ 1=2N2

ðR1Þ

C þ 2NO f CO2 þ N2

ðR2Þ

where CO and CO2 are identified as the major oxygenated products and molecular nitrogen is the dominating nitrogen product observed. It is worthwhile to mention that, in some cases, minor amounts of N2O were detected from the char/NO interaction in the absence of O2 by Chambrion et al.19 These authors observed that the maximum N2O formation was reached at 800 C, although this amount was quite small. Contrary to this fact, there are many reports concluding that N2O formation is only possible in the presence of O2.20-24 An oxidizing atmosphere Received: September 15, 2010 Published: February 18, 2011 1024

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is needed to release the cyano compounds from the char particles and to produce the NO reduction to N2O.20 The main factors influencing the char reactivity toward NO are the chemical and physical properties of char, which are strongly affected by the parent material properties and the pyrolysis conditions used25 (mainly the heating rate of the fuel sample, the maximum heat treatment temperature, and the holding time at this temperature), together with the operating parameters, such as temperature and NO concentration. The reaction is also influenced by several gaseous species that may be present in a reburning process, such as CO and O2. It was found that char can be an effective catalyst for NO reduction in the presence of CO.16,26-30 As different authors29,31 suggested, the enhancement of NO reduction in the presence of CO may be due to a direct reaction between CO and NO catalyzed by the char surface (R3). -C

CO þ NO f CO2 þ 1=2N2

ðR3Þ

As well as the char surface, the mineral content in biomass char may have a catalytic effect on the direct reaction between CO and NO15,32-35 (R3), resulting in a higher NO reduction rate. This catalytic activity is mainly attributed to potassium.36 The relative importance of the carbon-consuming reactions R1 and R2 and the catalytic reaction R3 in the presence of CO was studied by Furusawa et al.28 for coal char. Their results showed that, at low temperatures (800 C) unless the concentration of CO was much higher than that of NO (i.e., CO/ NO ratio . 1). In this context, the aim of the present work is to carry out an experimental and kinetic study of the NO reduction over biomass char at high temperatures to acquire a deeper knowledge of this process, analyzing the influence of the inlet NO concentration, temperature, and pyrolysis heating rate. This study will allow us to contribute to the improvement of the performance of combustors and the development of effective technologies for minimizing NO emissions. To accomplish the objective considered, eucalyptus has been selected as the starting material to obtain the chars, because this biomass material has a very fast growth rate and can therefore be used as a regular supply of fuel and is low in nitrogen and ash content. Because of the relationship between heat treatment and char reactivity, eucalyptus chars were prepared at 900 C under low heating rate (LHR) and high heating rate (HHR) conditions. Therefore, the influence of the heating rate on char/NO reactivity is also analyzed. It is worth pointing out that the present work complements earlier work by our group on the characterization of LHR and HHR eucalyptus chars37 and their reactivity toward oxygen.38

2. EXPERIMENTAL SECTION 2.1. Biomass Samples and Analysis. The biomass chars selected stem from the pyrolysis in a nitrogen atmosphere of eucalyptus

Table 1. Elemental Composition (wt %, Dry Basis), O*/C Ratio (Molar Basis), and Surface Area by CO2 Adsorption, of LHR and HHR Eucalyptus Chars Obtained at 900 C LHR chara

HHR charb

C

89.54

81.22

H

0.52

1.25

N

0.95

0.64

S

0.02

H/C ratio (molar basis) O*/C ratio (molar basis)

0.0697 0.0628

0.1847 0.1057

surface area (m2 g-1)

362

539

Char obtained in a fixed-bed reactor at 10 C min-1. b Char obtained in a fluidized-bed reactor. a

(Eucalyptus globulus Labill. from a forest station located in Cedeira, La Coru~ na, Spain) performed at LHR and HHR and a final temperature of 900 C. The experimental facilities used for preparing LHR and HHR chars, as well as the experimental procedures followed and the detailed results from char characterization, have been reported in a previous work.37 Pyrolysis experiments at a LHR (approximately 10 C min-1) were carried out in a fixed-bed reactor, while a fluidized-bed reactor was employed to prepare chars at HHR. In this work, char comprises the carbonaceous solid and ashes remaining after thermal decomposition. Eucalyptus ashes are mainly formed by CaO (55.73 wt %), MgO (14.35 wt %), SiO2 (10.95 wt %), Na2O (2.60 wt %), K2O (2.01 wt %), Al2O3 (1.58 wt %), Fe2O3 (0.38 wt %), and TiO2 (0.07 wt %). Assuming that ashes from eucalyptus remain in char, the ash content of LHR and HHR chars can be calculated from the values of eucalyptus ash content (0.98 wt %) and char yields obtained from the LHR and HHR pyrolysis experiments (21.4 and 18.3 wt %, respectively).37 When these values are taken into account, the ash contents of LHR and HHR chars are found to be 4.5 and 5.4 wt %, respectively. Table 1 shows the main results regarding the ultimate analysis and oxygen functional group content (O*) of LHR and HHR chars and the values of their surface area determined by CO2 adsorption at 0 C using the Dubinin-Radushkevich method. As can be seen, the H/C and O*/C ratios and the specific surface area values corresponding to HHR chars are higher than those from LHR chars. Concerning the char structural characteristics, HHR chars have a more disordered structure than LHR chars, as was observed in the Raman spectroscopy analyses.37 2.2. NO Reduction Experiments. Taking into account the literature addressing the biomass char/NO interaction,7,16,17 the experimental conditions used for the study of the NO reduction process over eucalyptus char have been selected such that char/NO interaction takes place under chemical reaction control without mass-transfer limitations. This fact is further checked in the Kinetic Parameters. To study the influence of the inlet NO concentration, experiments were performed with an inlet NO concentration ranging from 300 to 1100 ppm in a nitrogen flow, a temperature of 900 C, and a flow rate of 1000 mL/min [standard temperature and pressure (STP)]. The influence of the temperature on the char/NO interaction was also evaluated in the 750-900 C temperature range for a given NO concentration of 900 ppm. The experimental apparatus employed (Figure 1) has been described elsewhere.38 For each run, the amount of char put into the reactor was approximately 27 mg and was always previously mixed with 500 mg of silica sand with an average particle size of 150 μm. The sand is necessary to facilitate the introduction of the sample into the reactor and to prevent agglomeration of the char particles. The mixture was placed on a quartz wool plug located in a bottleneck in the middle of a quartz reactor 1025

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Figure 1. Experimental setup used for the char-NO interaction tests: (1) N2 and NO cylinders, (2) mass flow meters, (3) control unit, (4) bubble flow meters, (5) fixed-bed reactor, (6) electric furnace, (7) temperature controller, (8) condenser, (9) particle filter, (10) NO analyzer, (11) CO/CO2 analyzer, and (12) vent. of 550 mm in length and 15 mm in external diameter, resulting in a very thin layer. An inert flow of N2 was fed while the sample was heated up to the reaction temperature. Once the required temperature was reached, it was held for 30 min in nitrogen before the char sample was exposed to the reactant gas mixture used (NO/N2). The experimental setup was equipped with two online Uras14/IR gas analyzers, which allow for measuring NO, CO, and CO2 concentrations, main reaction products during the course of a NO reduction experiment. From blank runs conducted, the quartz wool and silica sand were found to be inert in the reaction with NO under the conditions considered. Several tests were also repeated to account for repeatability, and it was found to be good, with standard deviation values for the NO concentration lower than 15 ppm.

3. RESULTS AND DISCUSSION The key variables affecting the interaction between NO and carbon solids are temperature and nitric oxide concentration.14 The influence of these operating parameters and the pyrolysis heating rate on the NO reduction, concentrations of the main reaction products (CO and CO2), and the CO/CO2 ratio are examined below. Besides, the reaction order and activation energy values of the char/NO interaction for both char samples (LHR and HHR chars) have also been determined. 3.1. Influence of the Operating Conditions and Char Characteristics. During the process of NO reduction over char,

carbon is principally evolved from the particles in the form of CO and CO2. The carbon weight in the reactor at any time (WC) is calculated from the measured time variation of CO and CO2 concentrations, CCO and CCO2, in parts per million (ppm) of the outlet gas. In this way, the initial amount of carbon (in milligrams) in the reactor, WC0, was determined as Z ¥ ðCCO þ CCO2 Þdt ð1Þ WC0 ¼ MC FT 10-3 0

MC is the atomic weight of carbon, and FT is the outing flow expressed in moles per unit time and is expressed by eq 2 FT ¼

QP Rg T

ð2Þ

Figure 2. Evolution of carbon conversion as a function of time for LHR char. Influence of the inlet NO concentration at 900 C.

where Q is the feeding flow rate, P is the reactor pressure, Rg is the universal gas constant in appropriate units, and T is the reactor temperature. The amount of carbon (in milligrams) in the reactor at any time, WC, is calculated as Z t WC ¼ WC0 - MC FT 10-3 ðCCO þ CCO2 Þdt ð3Þ 0

NO reduction, CO and CO2 concentrations, and the CO/CO2 ratio are mainly dependent upon the inlet NO concentration, temperature, and thermal history of char preparation. The influence of these parameters is discussed below. 3.1.1. Influence of the Inlet NO Concentration. The inlet NO concentration affects the adsorption rate of NO and the formation of surface complexes on the char surface. Figure 2 shows the influence of the inlet NO concentration on the evolution of carbon conversion as a function of time for LHR char/NO interaction tests at a given temperature of 900 C. A similar behavior is observed for the HHR char experiments (not shown). The carbon conversion at any time, XC, is calculated from eq 4 XC ¼

WC0 - WC WC0

ð4Þ

It can be observed that carbon conversion increases when increasing the inlet NO concentration at any time, indicating that the carbon-consuming rate is enhanced with the increased reactant gas concentration. Figure 3 illustrates the influence of the inlet NO concentration on the evolution of NO reduced amount, CO and CO2 concentrations, and the CO/CO2 ratio as a function of the remaining carbon weight in the reactor at any time (WC) for the LHR char experiments at 900 C. Similar observations are attained for HHR char (not shown). The amount of NO reduced is determined by the difference between the NO concentration in the inlet gas and the NO concentration in the outlet gas. As shown in Figure 3, a general increase in the amount of NO reduced and CO and CO2 concentrations can be observed when increasing the inlet NO concentration, although the percentage of NO reduction is not greatly affected. These trends are expected and consistent with char/NO mechanisms reported in the literature.13,16,39 The first step of NO reduction, which is generally assumed, is a dissociative adsorption of NO on two adjacent 1026


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Figure 3. Evolution of NO reduced amount, CO and CO2 concentrations, and the CO/CO2 ratio (molar basis) as a function of the carbon weight (WC) in the reactor at any time, for LHR char. Influence of the inlet NO concentration at 900 C.

free carbon sites. The chemisorption of NO on free carbon sites (-C) goes through N-down orientation with dissociation of the N-O bond to yield carbon-oxygen complexes, -C(O), and carbon-nitrogen complexes, -C(N), through reaction R4. Oxygen species, -C(O), existing on the char surface can also be active sites for the gas chemisorption, leading to the formation of unstable oxygen complexes -C(O2) (R5). The desorption of the carbon surface complexes, -C(O), -C(O2), and -C(N), give CO, CO2, and N2, as well as nascent carbon active sites, -C* (R6-R8), which can further react with NO (R9 and R10). It should be noticed that these new carbon sites, -C*, may be more active toward the adsorption and dissociation of NO than -C.39 - C þ - C þ NO f - CðOÞ þ - CðNÞ

ðR4Þ

-C þ - CðOÞ þ NO f - CðO2 Þ þ - CðNÞ

ðR5Þ

-CðOÞ f CO þ a - C

ðR6Þ

-CðO2 Þ f CO2 þ b - C

ðR7Þ

-CðNÞ þ - CðNÞ f N2 þ d - C

ðR8Þ

ð1 þ xÞ - C þ - CðOÞ þ NO f CO2 þ - CðNÞ þ x - C ðR9Þ ð2 þ yÞ - C þ NO f CO þ - CðNÞ þ y - C

ðR10Þ

Therefore, the enhancement in the CO and CO2 concentrations with increasing inlet NO concentration is due to the increase in the NO adsorption rate and the amount of carbon-oxygen complexes created (R4 and R5), which in turn lead to both CO and CO2. These products are removed from not only the char surface by desorption of carbon-oxygen complexes (R6 and R7) but also the NO direct attack on the nascent active sites, -C* (R9 and R10). It is worth noting that the contribution of the reactions involving -C* (R9 and R10) is remarkable at the temperature studied, 900 C, because high temperatures induce the thermal desorption of the surface complexes.40 On the other hand, Figure 3d shows a decrease in the CO/CO2 ratio with the increased inlet NO concentration. These results are in good agreement with observations reported by Sorensen et al.16 for biomass chars. For both char samples (LHR and HHR chars), the CO/CO2 ratio values are higher than unity in the NO concentration range studied. It is known that both char surface and mineral impurities in biomass char may enhance the rate of NO reduction in the presence of CO, through the catalytic reaction R3. In the experiments performed, CO is only present as a result of NO reduction (R1). As shown in Figure 3b, the amount of CO produced is lower than 500 ppm. According to the literature,28 it can be expected that the catalytic reduction of NO by CO was not a major process under the present reaction conditions (high temperatures and CO/NO ratio < 1). To examine the effect of mineral content on the catalytic reaction R3, different tests were carried out at 900 C using the remaining ashes in the reactor obtained from the LHR and HHR 1027


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Figure 4. Evolution of the amounts of CO produced and NO reduced and the CO2 concentration as a function of time, for LHR char. Influence of the presence of different initial concentrations of CO on the char/NO interaction at 900 C and for the inlet NO concentration of 300 ppm.

char/NO experiments (1.2 and 1.5 mg, respectively). In these tests, different concentrations of CO (150-1000 ppm) were added to the NO (300-1100 ppm)/N2 mixtures. The experimental results showed no further reduction of NO, concluding that the catalytic reaction between CO and NO (R3) does not contribute to NO reduction in the experimental conditions studied. This finding is consistent with the literature,28 in which it was found

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that, at high temperatures, reasonably high concentrations of CO (i.e., CO/NO ratio . 1) are required to observe a contribution of this catalytic reaction R3. To elucidate the global role of the catalytic reaction R3 on the NO reduction process, experiments of char/NO interaction in the presence of CO were also performed. At 900 C and for a fixed NO concentration of 300 ppm, different concentrations of CO were introduced, corresponding to CO/NO ratios varying between 0 and 40. Figure 4 shows for LHR chars the effect of CO addition on the amounts of CO produced and NO reduced and the CO2 concentration as a function of time. Similar trends are found for HHR chars (not shown). As can be seen in Figure 4a, CO is greatly consumed when the inlet CO concentration is much higher than that of NO, increasing both the amount of NO reduced (Figure 4b) and the CO2 concentration (Figure 4c), with the carbon consumption rate being significantly low. These results show that the catalytic reaction between CO and NO (R3) is present and its contribution becomes relevant when the CO/NO ratio is higher than 15. This fact is in agreement with the findings by Sorensen et al.16 These authors observed that CO influenced the NO reduction over wheat straw char when the CO/NO ratio was higher than 12 and that this positive effect decreased with time, which coincides with that observed in this work (Figure 4b). 3.1.2. Influence of the Temperature. The temperature is a key factor influencing the NO reduction process. This operating parameter determines the desorption rate of the surface complexes (R6-R8) and, subsequently, the rate of generation of active sites, -C*, more active toward the adsorption and dissociation of NO than the original char surface. Figure 5 shows the effect of the temperature on the percentage of NO reduction, CO and CO2 concentrations, and the CO/CO2 ratio as a function of the remaining carbon weight for HHR char at a given inlet NO concentration of 900 ppm. Similar results have been found for LHR char (not shown). It is worth noting that the amount of NO reduced was negligible in the experiments carried out at temperatures lower than 800 C, and therefore, these results are not shown. As can be observed in Figure 5, an increase in the reaction temperature leads to an increase in NO reduction, resulting in the enhancement of CO and CO2 concentrations and the CO/CO2 ratio. This can be due to the fact that thermal desorption is an activated phenomenon and increases with increasing temperature, promoting the reactions R6-R10. Therefore, high temperatures cause an enhanced rate of desorption of -C(O) and -C(O2) complexes (R6 and R7), creating a higher concentration of active sites, -C*, on the remaining carbon solid, which can reduce NO molecules. These observations are consistent with the findings found in the related biomass literature.16,17 Furthermore, the significant contribution of reactions involving -C* at high temperatures (>800 C) was also emphasized by Aarna and Suuberg.40 Results from Figure 5 also reveal that the NO reduction process over char strongly depends upon the temperature. As an example, for a remaining carbon weight of 10 mg, NO reduction is observed to increase from 25% at 800 C to 70% when the temperature is increasing to 900 C. 3.1.3. Influence of the Pyrolysis Heating Rate. The pyrolysis heating rate influences char properties and, thereby, its reactivity with gases usually present in combustion systems.41,42 A comparison between the chemical and physical properties of eucalyptus chars produced at LHR and HHR has been reported elsewhere.37 Figure 6 shows a comparison between the behavior of LHR and HHR chars for an inlet NO concentration of 900 ppm and a 1028


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Figure 5. Evolution of the percentage of NO reduction, CO and CO2 concentrations, and the CO/CO2 ratio (molar basis) as a function of the carbon weight (WC) in the reactor at any time, for HHR char. Influence of the temperature for the inlet NO concentration of 900 ppm.

temperature of 900 C. As can be observed, NO is more readily reduced on HHR char than on LHR char (Figure 6a), leading to higher CO and CO2 concentrations (Figure 6b). A significant NO reduction occurred at 900 C, reaching values up to 55% for LHR char and 75% for HHR char (Figure 6a). The ratio of the amount of NO reduced over HHR and LHR chars in similar experimental conditions has been determined. The values of this ratio are nearly 1.35, as can be deduced from data in Figure 6a. Similar values are obtained with other operating conditions. If this value is compared to the ratio of char surface areas [SgHHR char (539 m2/g)/SgLHR char (362 m2/g) = 1.49], it can be concluded that the rates of NO conversion on the two sorts of char are nearly proportional to the surface areas of the chars. Therefore, the greater reactivity toward NO corresponding to HHR char is mainly due to its higher surface area, as compared to that obtained for LHR char (539 and 362 m2 g-1 for HHR and LHR chars, respectively), and then a higher availability of active sites. This fact is in agreement with results found in the literature.43,44 However, the enhancement in the reactivity of char produced at HHR compared to LHR char could also be related to its higher degree of structural disorder37 and content in the oxygen functional group (Table 1). An increase in the structural order removes edge atoms and other imperfections that serve as active sites.14 Besides, the content in the oxygen functional group is related to the surface carbon-oxygen complexes. The presence of these oxygen species enhances char reactivity toward NO because of the high activity of the new active sites, -C*, created by -C(O) and -C(O2) desorption (R6 and R7).

With regard to the CO and CO2 formation (Figure 6b), the differences between LHR and HHR chars are more significant for CO and the CO/CO2 ratio is higher for the HHR char (Figure 6c). 3.2. Kinetic Parameters. Experimental data of the LHR and HHR char/NO interaction tests have been analyzed to determine the kinetic parameters. When the results reported in the literature about the biomass char/NO interaction are taken into account,7,16,17 it could be expected that the NO reduction process over eucalyptus char takes place under chemical reaction control in all of the conditions studied. To check this assumption for the results of this study, the effectiveness factors were calculated using the macroscopic model of type I in regime II (kinetics is controlled by the chemical reaction and pore diffusion). For both char samples (LHR and HHR chars), these values turned out to be above 0.7. According to Sch€onenbeck et al.,45 this fact means that the process is mainly controlled by the chemical reaction (regime I). For a heterogeneous gas-porous solid reaction in regime I, the intrinsic reaction rate (rC) is usually defined as a function of the available active site concentration13 (eq 5) rC ¼ -

1 dWC dXC ¼ WC CC dt CC ð1 - XC Þdt

ð5Þ

where CC represents the available active site concentration at any time (amount of available active sites per amount of carbon). For a given temperature, the intrinsic reaction rate (rC) depends upon the reactant gas concentration.13,46,47 Assuming a nth order 1029


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surface active site concentration. The surface area of the carbon solid has usually been used as an approximation. Moreover, in gas-solid reactions, the surface and pore structure change with carbon conversion. Therefore, CC should also represent this evolution. In general, CC may be expressed as a function of carbon conversion,48 CC = f(XC), where f(XC) represents a structural factor. If the carbon active site concentration is a function of carbon conversion, the specific reaction rate (r0 C) must change with carbon conversion for a given temperature and inlet NO concentration. To check over this assumption in the experimental conditions studied, the specific reaction rate (r0 C) has been plotted versus carbon conversion for the LHR and HHR char/ NO tests. As an example, Figure 7 shows this evolution for the LHR and HHR char experiments at different temperatures and a given inlet NO concentration of 900 ppm. As can be seen, the specific reaction rate (r0 C) increases with increasing the carbon conversion. This indicates that the carbon active site concentration, CC, also increases with the enhancement of carbon conversion. A similar behavior has been observed in all conditions studied. The expressions of the function CC = f(XC) and its assumptions with appropriate mathematical approaches form a variety of structural models for gas-solid reactions.49-53 In this study, the random pore model, developed by Bhatia and Perlmutter,49 has been used for a quantitative interpretation of rate data versus carbon conversion evolution, because of its wide applicability in the prediction of char reaction rates. According to the random pore model, the specific reaction rate (r0 C) under chemical reaction control can be expressed as ks CNO n Sv0 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 - ψ lnð1 - XC Þ ð1 - εp0 Þ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ A0 1 - ψ lnð1 - XC Þ

r0 C ¼

Figure 6. Evolution of the percentage of NO reduction, CO and CO2 concentrations, and the CO/CO2 ratio (molar basis) as a function of the carbon weight (WC) in the reactor at any time, at 900 C for the inlet NO concentration of 900 ppm. Comparison between LHR and HHR chars.

with respect to the NO concentration (CNO), the specific reaction rate (r0 C) can be expressed as48 r0 C ¼ -

1 dWC dXC ¼ ¼ ks CC CNO n WC dt ð1 - XC Þdt

ð6Þ

where ks is the intrinsic kinetic constant. The determination of carbon active sites is complex, and it is necessary to introduce certain approximations to quantify the

ð7Þ

where Sv0 is the initial char reaction surface area per unit volume, εp0 is the initial particle porosity, and ψ is a structural parameter based on the initial properties of char. For both LHR and HHR chars, the random pore model parameters have been calculated by fitting the experimental data of r0 C and XC to eq 7 using the Marquardt-Levenberg method. Table 2 summarizes the random pore model parameters obtained in the experimental conditions studied. As can be seen in Table 2, similar values of the structural parameter ψ have been obtained for each of the chars considered at all temperatures studied. This fact supports the validity of the random pore model. The model predictions for the LHR char/NO experiments at different temperatures are also included in Figure 7. As can be observed, the random pore model describes quite successfully the reaction rate data for the NO reduction process over eucalyptus char. Therefore, it can be concluded that the monotonically increasing rate observed is due to the evolution of the carbon active site concentration during the reaction. 3.2.1. Evaluation of the Reaction Order and Activation Energy. Kinetic parameters of the NO reduction process over eucalyptus char have been determined using eq 6, as in the kinetic study for the NO-carbon reaction performed by Li et al.48 The reaction order is calculated from the experimental data of the char/NO tests at 900 C and different inlet NO concentrations (300-1100 ppm). For a given carbon conversion (XC), the available active site concentration is supposed to be the same. 1030


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Figure 7. Specific reaction rate (r0 C) versus carbon conversion for the char/NO interaction for an inlet NO concentration of 900 ppm and different temperatures: (a) LHR char and (b) HHR char. Comparison between experimental data (symbols) and model fitting (eq 7) (lines).

Table 2. Values of the Structural Parameter, ψ, for the Experiments of LHR and HHR Char/NO T = 900 C CNO (ppm) ψ

LHR chara

HHR charb

300 500 700 900 1100 300 500 700 900 1100

6.0 6.1 6.1 6.2 6.0 4.5 4.5 4.4 4.5 4.6

CNO = 900 ppm

A0 (s-1)

R2

1.05 10-4 1.33 10-4 1.62 10-4 1.99 10-4 2.16 10-4 1.52 10-4 2.20 10-4 3.20 10-4 3.70 10-4 3.11 10-4

0.9927 0.9919 0.9909 0.9921 0.9963 0.9780 0.9827 0.9900 0.9785 0.9840

T (C) ψ 800 825 850 875 900 800 825 850 875 900

6.1 6.0 6.2 6.0 6.2 4.3 4.5 4.5 4.4 4.5

A0 (s-1)

R2

0.54 10-4 0.77 10-4 1.18 10-4 1.54 10-4 1.99 10-4 0.98 10-4 1.45 10-4 2.01 10-4 2.57 10-4 3.70 10-4

0.9963 0.9969 0.9950 0.9959 0.9921 0.9809 0.9776 0.9823 0.9823 0.9785

Char obtained in a fixed-bed reactor at 10 C min-1. b Char obtained in a fluidized-bed reactor. a

Therefore, the reaction order with respect to the NO concentration can be obtained from the slope of the logarithmic plot of eq 6, ln(r0 C) versus ln(CNO). Figure 8 shows the results obtained at three carbon conversion levels (XC = 0.25, 0.50, and 0.75). As shown, the slope is unchanged with the carbon conversion, revealing that the reaction is under chemical control. Table 3 illustrates the values of the reaction order, n, obtained in the experimental data fitting to the logarithmic plot of eq 6, with the regression coefficient (R2). Fractional orders with respect to the NO concentration have been obtained for the NO reduction process over both chars (0.61 and 0.81 for the LHR and HHR chars, respectively). These results are in agreement with those reported in the literature.7,16,17 Garijo et al.7 obtained reaction orders of 0.45 and 0.3 for wheat straw char and eucalyptus char, in the 50-1000 ppm inlet NO concentration range and temperatures between 750 and 900 C. Sorensen et al.16 found a fractional order of 0.7 for NO reduction over wheat straw char in the 50-1000 ppm range of the inlet NO concentration and temperatures between 600 and 900 C. Dong et al.17 also obtained fractional reaction orders (0.65, 0.73, and 0.66) with respect to the NO concentration for three types of biomass chars (sawdust, rice husk, and corn straw, respectively) in the temperature range of 700-900 C. The activation energies for the LHR and HHR char/NO interaction, Ea, are determined using data obtained from the

Figure 8. Evaluation of the reaction order, n, for the char/NO interaction. Plot of ln(r0 C) versus ln(CNO) at three carbon conversion levels: (a) LHR char and (b) HHR char.

experiments performed through a range of temperatures (800900 C) with a given inlet NO concentration of 900 ppm. For each char, the values of the specific reaction rate (r0 C) are obtained and treated by the Arrhenius plot. The Arrhenius plots, ln (r0 C) versus (1000/T), of LHR and HHR chars at different carbon conversion levels are presented in Figure 9. From the slope of the 1031


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Table 3. Values of the Reaction Order, n, and Activation Energy, Ea, of the LHR and HHR Char/NO Interaction Ea XC

a

LHR char

b

HHR char

n

R2

0.25 0.60 0.9939 0.50 0.63 0.9923

Ea,average R2

naverage (kJ/mol) 174 168

0.9989 0.9976

0.75 0.60 0.9946

137

0.9972

0.25 0.79 0.9934

129

0.9966

0.50 0.80 0.9938

137

0.9980

123

0.9983

0.75 0.84 0.9930

0.61

0.81

(kJ/mol)

160

130

-1 b

Char obtained in a fixed-bed reactor at 10 C min . Char obtained in a fluidized-bed reactor. a

(160 kJ mol-1). This is indicative of a major influence of the temperature on the LHR char/NO interaction. The values of activation energy found in the present study are in good agreement with the literature data in regime I.16,17,45,54,55 As an example, Sorensen et al.16 reported an activation energy of 152 kJ mol-1 for the NO reduction process over wheat straw char. Therefore, it can be concluded that the eucalyptus char/NO interaction occurs under chemical reaction control in the conditions studied.

4. CONCLUSIONS Experimental and kinetic research on NO reduction over eucalyptus chars obtained from LHR and HHR pyrolysis at 900 C has been performed. The influences of the inlet NO concentration (300-1100 ppm), temperature (750-900 C), and pyrolysis heating rate on NO reduction, CO and CO2 concentrations, and the CO/CO2 ratio have been analyzed. Results from this study revealed that substantial NO reduction is achieved by the use of chars from eucalyptus pyrolysis, showing that the temperature has a marked effect on the NO reduction process. Furthermore, HHR char exhibited a higher reactivity toward NO in the conditions studied. This is attributed to the higher surface area and oxygen functional group content, a more disordered structure, and thus, a higher availability of active sites. To evaluate the reaction order and activation energy of the char/ NO interaction, an intrinsic rate equation defined as a function of the available active site concentration has successfully been applied, which fitted properly the random pore model. Fractional reaction orders with respect to the NO concentration have been found for LHR and HHR chars, 0.61 and 0.81, respectively. An average activation energy of 130 kJ mol-1 was obtained for the HHR char/NO interaction, whereas the NO reduction process over LHR char was characterized by a higher activation energy (160 kJ mol-1). ’ AUTHOR INFORMATION Corresponding Author

*Fax: þ34-976-761879. E-mail: [email protected].

Figure 9. Arrhenius plots of the char/NO interaction at three carbon conversion levels: (a) LHR char and (b) HHR char.

Arrhenius plot, the values of activation energy for the LHR and HHR char/NO interaction are obtained. These values and their corresponding regression coefficients are summarized in Table 3. Considering the activation energy values obtained at different conversion levels, it can be confirmed that the NO reduction process was carried out under chemical reaction control. As shown in Table 3, an average activation energy of 130 kJ mol-1 is obtained for the HHR char/NO experiments in the temperature range studied (800-900 C), whereas the NO reduction process over LHR char is characterized by a higher activation energy

’ ACKNOWLEDGMENT The authors express their gratitude to the Ministry of Science and Technology (MCYT) (Project PPQ2000-1207) and Ministry of Science and Innovation (MICINN) (Project CTQ200912205) for financial support and to the Instituto de Carboquímica (ICB), Consejo Superior de Investigaciones Científicas (CSIC) for allowing us the use of the fast pyrolysis facility. Dr. Marta Guerrero acknowledges the Government of Aragon (DGA) for the awarded predoctoral grant (B179/2003). ’ REFERENCES (1) Munir, S.; Daood, S. S.; Nimmo, W.; Cunliffe, A. M.; Gibbs, B. M. Bioresour. Technol. 2009, 100, 1413–1418. (2) Gil, M. V.; Casal, D.; Pevida, C.; Pis, J. J.; Rubiera, F. Bioresour. Technol. 2010, 101, 5601–5608. (3) Sahu, S. G.; Sarkar, P.; Chakraborty, N.; Adak, A. K. Fuel Process. Technol. 2010, 91, 369–378. (4) Smoot, L. D.; Hill, S. C.; Xu, H. Prog. Energy Combust. Sci. 1998, 24, 385–408. (5) Glarborg, P.; Kristensen, P. G.; Dam-Johansen, K.; Alzueta, M. U.; Millera, A.; Bilbao, R. Energy Fuels 2000, 14, 828–838. 1032


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Experimental and Kinetic Study at High Temperatures of the NO

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