Kinetic Study of NO Reduction over Biomass Char under Dynamic

Energy & Fuels 2003, 17, 1429-1436

1429

Kinetic Study of NO Reduction over Biomass Char under Dynamic Conditions Elena Garcıá Garijo, Anker Degn Jensen,* and Peter Glarborg CHEC Research Center, Department of Chemical Engineering, Technical University of Denmark, Building 229, DK-2800, Lyngby, Denmark Received November 19, 2002. Revised Manuscript Received July 15, 2003

This paper reports an experimental study of the NO-char reaction using two types of biomasss wheat straw and eucalyptus wood. The experiments were performed in a fixed bed reactor at 1023-1173 K with NO inlet concentrations ranging from 50 to 1500 ppmv. The char was produced in-situ at the reaction temperature, and a special interest of the study was to measure the initial rate constant before significant char thermal deactivation had occurred. Separate experiments were carried out to investigate when the influence of volatiles on NO reduction could be neglected and the observed rate constant was only due to the heterogeneous NO-char interaction. The kinetic expressions for the initial rate obtained under these dynamic conditions are the following: - rNO ) 3.26 × 104 × exp(-11092/T) × CNO0.45 mol/(kg C in char‚s) for wheat straw, and - rNO ) 3.55 × 103 × exp(- 9305.5/T) × CNO0.3 mol/(kg C in char‚s) for eucalyptus wood. For comparison also the reaction rate constant at pseudo steady-state conditions was determined and found to be about a factor of 3 lower than the initial rate. Both chars have reactivities from 1 to 2 orders of magnitude higher than typically reported for coal on a mass basis. This is attributed partly to a generally higher reactivity of biomass compared to coal and partly to significant thermal annealing of the coal chars. The experiments confirm that thermal deactivation is important for the NO-char reaction, but adsorption of NO on the fresh char also contributes to the high initial rate.

Introduction Due to worldwide environmental concern, legislation is becoming stricter to reduce harmful emissions. Nitrogen oxides (NOx) are among the most serious pollutants and participate in the formation of acid rain, photochemical smog, and the destruction of ozone in the stratosphere. Combustion processes constitute an important source of NOx, and much research is directed toward low-emission power plants, including adaptation of existing power plants to the requirements of legislation. One technology currently under consideration for retrofitting on pulverized coal power plants is reburning, using biomass as the reburn fuel. In the reburning process, a secondary fuel is added after the main combustion zone, generating a fuel-rich zone where NO is reduced to N2, and subsequently burnout air is added. Pilot-scale experiments using wood as reburning fuel in coal-fired systems have obtained levels of NOx reduction in the range 50-75%.1,2Besides reducing NOx emissions, this technique has the advantage of lowering the CO2 and SO2 emissions due to the use of the biomass. In reburning using solid fuels, both volatiles and char from the reburn fuel may contribute to NOx reduction, * Corresponding author. Phone: +45 45 25 28 41. Fax: +45 45 88 22 58. E-mail: [email protected]. (1) Zamansky, V. M.; Maly, P. M.; Seeker, Wm. R.; Folsom, B. A. Proceedings of the International Conference: Biomass for Energy and Industry; Wu¨rzburg, Germany, 1998; pp 1537-1540. (2) Harding, N. S.; Adams, B. R. Biomass Bioenergy 2000, 19, 429445.

and for modeling and optimization of the process kinetics for both mechanisms are required. Both the homogeneous reaction chemistry of reburning (see, for example, refs 3-5) and the char + NO reaction for fossil fuel chars (see, for example, refs 6-9) has been studied extensively. However, results for NO reduction over biomass char10-11 are comparatively scarce. The char + NO reaction is described by the following overall reaction:

NO + C f 1/2N2 + CO, CO2 CO and CO2 are identified as the major oxygenated products (e.g., refs 7,12,13). Molecular nitrogen is the dominating nitrogen product observed, although minor (3) Bilbao, R.; Alzueta, M. U.; Millera A.; Duarte M. Ind. Eng. Chem. Res. 1995, 34, 4540-4548. (4) Glarborg, P.; Alzueta, M. U.; Dam-Johansen, K. Combust. Flame 1998, 115, 1-27. (5) Prada, L.; Miller, J. A. Combust. Sci. Technol. 1998, 132, 225250. (6) Furusawa, T.; Kunii, D.; Oguma, A.; Yamada, N. Int. Chem. Eng. 1980, 20, 239-244. (7) Chan, L. K.; Sarofim, A. F.; Beer, J. M. Combust. Flame 1983, 52, 37-45. (8) Aarna, I.; Suuberg, E. M. Proc. Combust. Inst. 1998, 27, 30613068. (9) Jensen, L. S.; Jannerup, H. E.; Glarborg, P.; Jensen, A.; DamJohansen, K. Proc. Combust. Inst. 2000, 28, 2271-2278. (10) Zevenhoven, R.; Hupa, M. Fuel 1998, 77, 1169-1176. (11) Sørensen, C. O.; Johnsson, J. E.; Jensen, A. Energy Fuels 2001, 15, 1359-1368. (12) Suuberg, E. M.; Teng, H.; Calo, J. M. Proc. Combust. Inst. 1990, 23, 1195-1205. (13) Yang, J.; Mestl, G.; Herein, D.; Schlo¨gl, R.; Find, J. Carbon 2000, 38, 715-727.

10.1021/ef020276n CCC: $25.00 © 2003 American Chemical Society Published on Web 10/11/2003

1430 Energy & Fuels, Vol. 17, No. 6, 2003

amounts of N2O have been reported.14 A number of detailed reaction mechanisms have been proposed,7,8,11,14-16 but the complexity of the reaction has excluded a clear conclusion. The reaction order is often found to be unity, but fractional order17 or increasing order with temperature8 have also been observed. The reaction is influenced by several gaseous species that may be present in a reburning process. Carbon monoxide has been shown to enhance the rate of NO reduction over different carbonaceous materials,7,18-21 including wheat straw char.11 The effect is probably due to a reaction between NO and CO catalyzed by carbon. Presence of oxygen also increases the NO reduction rate somewhat, but there is no clear understanding of the mechanism. Suzuki et al.22 and Illań-Go´mez et al.23 suggested that O2 promotes formation of highly active sites that react with NO while Chambrion et al.24 concluded that O2 facilitates the liberation of C(N) complexes as NO which then react with C(N) to form N2. In contrast, the presence of H2O decreases the reaction rate, but the effect diminishes with increasing temperature. Levy et al.21 proposed that the effect of water was to form a chemisorbed oxygen layer, which ties up the active sites at low temperature but decomposes with rising temperature. The mineral content in coal char may have a catalytic effect that results in a higher NO reduction rate.17 This behavior has also been observed with biomass chars.10,11 The catalytic activity is mainly attributed to potassium, which is an active catalyst for the NO-char reaction.23 Potassium, and alkali metals in general, enhance the number of active sites at the carbon surface, thereby increasing the rate of the carbon gasification reactions.25,26 Several studies have reported a fast initial NO reduction rate, which decreased with time to reach a pseudo steady state (e.g., refs 9,20,27,28). The reason for the decrease in rate has been explained by initial adsorption of NO, until a steady state is reached between adsorption and desorption of products, or by thermal deactivation of the char. Thermal deactivation of the char has been shown to significantly impact the rate of the oxygen-char reaction.29-33 Comparatively less attention has been paid to the influence of thermal deactivation on the NO-char reaction. Aarna and Suuberg17 report that thermal deactivation does play an important role. In the studies of Guo and Hecker27 (14) Chambrion, Ph.; Kyotani, T.; Tomita, A. Energy Fuels 1998, 12, 416-421. (15) Smith, R. N.; Swinehart, J.; Lesnini, D. J. Phys. Chem. 1959, 63, 544-547. (16) Teng, H.; Suuberg, E. M.; Calo, J. M. Energy Fuels 1992, 6, 389-406. (17) Aarna, I.; Suuberg, E. M. Fuel 1997, 76, 475-491. (18) Furusawa, T.; Tsunoda, M.; Kunii, D. J. Am. Chem. Soc. 1982, 196, 347-357. (19) Furusawa, T.; Tsunoda, M.; Tsujimura, M.; Adschiri, T. Fuel 1985, 64, 1306-1309. (20) Aarna, I.; Suuberg, E. M. Energy Fuels 1999, 13, 1145-1153. (21) Levy, J. M.; Chan, L. K.; Sarofim, A. F.; Bee´r, J. M. Proc. Combust. Inst. 1981, 18, 111-120. (22) Suzuki, T.; Kyotani, T.; Tomita, A. Ind. Eng. Chem. Res. 1994, 33, 2840-2845. (23) Illań-Go´mez, M. J.; Linares-Solano, A.; Radovic, L. R.; SalinasMartıńez de Lecea, C. Energy Fuels 1996, 10, 158-168. (24) Chambrion, Ph.; Kyotani, T.; Tomita, A. Proc. Combust. Inst. 1998a, 27, 3053-3059. (25) Moulijn, J. A.; Kapteijn, F. Carbon 1995, 33, 1155-1165. (26) Chen, S. G.; Yang, R. T. Energy Fuels 1997, 11, 421-427. (27) Guo, F.; Hecker, W. Proc. Combust. Inst. 1998, 27, 3085-3092.

Garijo et al.

and Aarna and Suuberg,20 the chars were prepared separately at high temperatures and consequently additional deactivation is probably less important in the NO reduction experiments. The most likely explanation for the high initial rate in their experiments is adsorption of NO until a steady state is reached between adsorption and desorption of products. In the experiments by Jensen et al.9 and Molina et al.,28 the chars were prepared in-situ and so both thermal deactivation and adsorption may potentially play a role for the observed decrease from the initially high reaction rate. The purpose of the present work is to measure for two biomasses (wheat straw and eucalyptus wood) both the initial rate of the NO-char reaction, i.e. over freshly formed char generated in-situ, and the pseudo steadystate rate. The initial rate is expected to be relevant in suspension firing/biomass reburning conditions where the time scale of reactivity loss may be similar to the time scale of char conversion.34 The loss of reactivity with time and the relative importance of NO adsorption and thermal deactivation are investigated. Both the initial and the steady-state rate constants for biomass char + NO are compared with values from the literature. Experimental Section Experimental Setup. A control panel connected to mass flow controllers was used to prepare a mixture of gases from gas cylinders. The desired mixture was directed either to the reactor or the bypass. A laboratory-scale fixed bed reactor made of quartz was used for the experiments (Figure 1). This reactor, used previously by Jensen et al.,9 is equipped with a solid feeder device, which allows sample admission into the reactor under inert conditions without disassembling the reactor. The reactor was placed inside an electrically heated oven, with three independent zones for controlling the temperature. The reaction temperature was measured by a thermocouple 0.5 cm below the porous plate where the reaction took place. The uncertainty of the thermocouple measurement was (1.5 °C. A manometer connected to the inlet line measured the reactor pressure. The gases flowing out of the reactor were conveyed to the analysis equipment, passing through two filters and a rotameter. The double filtering had the purpose of preventing the intake of condensed pyrolysis products (tar) to the analyzers. Using a three-way valve, the outlet stream of the reactor could be directed to the flow meter or to the analysis system. The analysis system consisted of three analyzers connected in series for the continuous detection of CO, CO2, and NO. Infrared analyzers were used for CO and CO2 (Hartmann & Braun, Uras 3G) and ultraviolet for NO (Hartmann & Braun, Radas 1G). The UV NO analyzer is crosssensitive to hydrocarbons (present in the biomass volatiles), and consequently a NO/NOx analyzer (Eco Physics, CLD 700 EL) based on chemiluminescense was applied in selected experiments. An estimated uncertainty for the gas analysis is (3%. All data from each experiment (temperature, pressure, and concentrations as a function of time) were logged in a file. Experimental Procedure and Materials. Two types of biomassswheat straw and eucalyptus woodswere used in the (28) Molina, A.; Eddings, E. G.; Pershing, D. W.; Sarotim, A. F. Proc. Comb. Inst. 2002, 29, 2275-2281. (29) Zolin, A.; Jensen, A.; Dam-Johansen, K. Proc.e Combust. Inst. 2000, 28, 2181-2188. (30) Zolin, A.; Jensen, A.; Jensen, P. A.; Frandsen, F.; DamJohansen, K. Energy Fuels 2001, 20, 1110-1122. (31) Zolin, A.; Jensen, A. D.; Jensen, P. A.; Dam-Johansen, K. Fuel 2002, 81, 1065-1075. (32) Shim, H.-S.; Hurt, R. H. Energy Fuels 2000, 14, 340-348. (33) Russell, N. V.; Gibbins, J. R.; Williamson, J. Fuel 1999, 78, 803807.

NO Reduction over Biomass Char

Energy & Fuels, Vol. 17, No. 6, 2003 1431

Figure 1. Schematic representation of the fixed-bed reactor. Table 1. Fuel and Char Analyses. Char Produced at 850 °C % (w/w) dry basis

moisture ash volatiles C H N S Cl Ca Fe K Mg P Si

wheat straw

eucalyptus wood

6.17 6.50 75.11 45.60 5.79 0.62 0.076 0.126 0.396 0.008 0.925 0.062 0.096 1.739

13.24 2.50 73.62 51.23 4.66 0.38 0.00 0.10 0.647 0.030 0.230 0.100 0.034 0.168

wheat straw char

eucalyptus wood char

25.1

6.2

69.4 0.65 1.0 0.22

86.9 0.67 1.4 0.10

experiments, and their composition is shown in Table 1. The NO-char kinetics were determined at four temperatures between 1023 and 1173 K, and at six NO inlet concentrations between 50 and 1500 ppmv. The flow rate in all the experiments was 2 L/min (STP) and the reactor pressure was around 1.2 atm. Samples of milled straw and wood were sieved, and the size range 125-180 µm was used in the experiments as a compromise between mass transfer limitations and problems of feeding very fine particles into the system. To obtain a reasonable conversion of NO, the quantity of biomass introduced in each experiment varied with temperature and NO inlet concentration (between 3 and 64 mg). In each experiment the biomass was mixed with 1 g of sand before admission to the reactor in order to get a bed of sufficient height to approximate plug flow and avoid clogging of the feeder tube. To obtain some knowledge of the chemical composition of the char used in the experiments a larger sample of char of both straw and wood was produced in a muffle furnace under a flow of nitrogen at 1123 K, corresponding to an intermediate temperature in the investigated range. The heating rate was about 200 K/min, and the temperature was kept for about 90 min before cool-down. The char was subsequently analyzed, and the composition is given in Table 1.

An experiment was carried out as follows. The reactor was heated to the desired temperature while the desired NO/N2 mixture was flowing continuously through it. The sample admission into the reactor involved several steps. First, the solid was placed in the feeder chamber (Figure 1), which was flushed free from air with the NO/N2 stream by opening valves number 2 and 4, and closing valve number 1. Then, a slight overpressure in the feeder chamber was generated by closing valve 4 and the solids were fed to the reactor by opening valve 3 a few seconds later. Finally, valve 1 was opened, and the other valves were closed. Once in the reactor, the sample rested on the porous plate where pyrolysis initiated immediately. After each experiment, the sample was taken out of the reactor by removing the bottom and inner part of it. Investigation of the Influence of Volatiles. Hydrocarbons are known to react with nitrogen oxides forming N2 and reduced nitrogen species such as HCN and NH3.3-5 Since the purpose of this work is to measure the initial NO-char reaction kinetics over fresh char, care must be taken that volatiles do not contribute to the observed rate. Separate experiments were carried out to test if the volatiles were able to reduce NO, and if so, at what time this effect disappeared. For that purpose, a new bottom section of the reactor was made in which the thermocouple tube was replaced with a tube equipped with small holes at the end, and used as inlet of NO (Figure 1). Thereby the reaction zone was located below the char bed and NO was only allowed to interact with the volatiles released during pyrolysis. The temperature in this region decreased with a gradient of about 2000 K/s. The NO chemiluminescence analyzer was applied in these experiments to compare with the measurements from the UV analyzer. Both the wheat straw and eucalyptus wood were tested at all reaction temperatures (1023-1173 K) at a fixed NO inlet concentration of 100 ppmv. Data Treatment. The reactor was modeled as a plug flow reactor. Applying the general equation for an nth order reaction rate in the mass balance, and integrating over the reactor length, the NO conversion for a reaction order different from unity is expressed as follows:

[

(n-1)

X ) 1 - 1 - (1 - n)‚k‚CNO,o

W q

‚

1/(1-n)

]

(1)

where CNO,o is the inlet NO concentration (mol‚m-3), W the instantaneous mass of carbon in the char (kg), q the total flow rate (m3‚s-1), and k the rate constant (mol(1-n)‚m3n‚(kg C)-1‚s-1). The reaction rate is based on the instantaneous mass of carbon in the char. Hence, combustion experiments were performed to calculate the carbon contained in the char after pyrolysis. In these experiments the char formed through pyrolysis was completely oxidized and the CO and CO2 signals integrated. This procedure was carried out for both fuels at each temperature. However, the differences in the mass of carbon remaining at each temperature were small and within the experimental uncertainty, and it was decided to average the values obtained at all temperatures for each fuel. The average value, expressed in grams of carbon in the char per 100 g of fuel, was 14.45 for straw and 11.22 for wood. These values were used to calculate the carbon content in the char from the quantity of biomass used in each experiment. Further, from integration of the CO and CO2 time profiles, the carbon consumption due to the reaction with NO could be calculated, and thereby the remaining mass of carbon in the char, W. The amount of char consumed before a measurement of the rate was completed was in general small, less than 10%. The influence of external and internal mass transfer limitations was estimated as follows for straw. The highest temperature of the experiments of 900 °C was used for the calculation. The molecular diffusion coefficient of NO in N2 was estimated to be 2.1 × 10-4 m2/s. The porosity of the straw particles is about 0.6 and consists of straight pores with a size between 1


Figure 2. Comparison and repetitivity of the NO signals from the UV and chemiluminescence analyzers. Volatile influence test with wheat straw char at 1123 K and [NO] ) 100 ppm. and 10 µm.35 Using a tortuosity of 1, this gives an effective diffusion coefficient at 900 °C of 1.26 × 10-4 m2/s. Based on the derived rate expression (see eq 3) for straw this results in an effectiveness factor of about 0.96 at 400 ppmv NO. The external mass transfer coefficient was estimated to 3.18 m/s using the correlation of Dwidewi and Upadhyay,36 which corresponds to a relative influence of external mass transfer resistance of about 6-7%. On the basis of these estimates, it was assumed that internal and external diffusion limitations for the straw could be neglected. Less information was available for the pore structure of the wood, but on the basis of the information for straw it was assumed that mass transfer limitations were small for the wood as well. It is known that CO can enhance the rate of NO reduction as discussed in the Introduction. However, the levels obtained in the present experiments are too low to measurably influence the rate.11

Results and Discussion Influence of Volatiles on NO Reduction. The interaction between biomass volatiles and NO was investigated as described in the Experimental Section. The biomass quantity used varied with temperature, from 28.8 mg at 1023 K to 3.8 mg at 1173 K. Figure 2 shows results from two similar experiments using straw at 1123 K for both the UV and chemiluminescence analyzer. It can be seen that the repeatability is good and that even the chemiluminescense analyzer appears to be cross-sensitive toward the volatiles, although less than the UV analyzer. The NO profiles from both analyzers show almost zero NO conversion at around 30 s. Results obtained for eucalyptus wood were similar to those of wheat straw. The longest time required to be free from the influence of volatiles was 35 s, obtained at the lowest temperature of 1023 K where the largest sample mass is used. On the basis of the results for both fuels at all temperatures, we conclude that after 35 s there will no (34) Feng, B.; Jensen, A.; Bhatia, S. K.; Dam-Johansen, K. Energy Fuels 2003, 17, 399-404. (35) Jensen, P. A. Unpublished results; CHEC Research Centre, Department of Chemical Engineering, Technical University of Denmark, 2002.

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Figure 3. NO conversion from reduction experiments with straw and wood chars at 1073 K and [NO]0 ) 200 ppm.

longer be an influence of volatiles and the observed NO reduction is only due to reaction with char. Kinetics of the Initial Rate. Figure 3 shows a typical example of the raw data for both straw and wood. The initial period influenced by volatiles, which is clearly distinguished, is followed by a longer period with a slowly increasing outlet concentration of NO, caused partly by thermal deactivation, a decreasing rate of NO adsorption and also consumption of the char. The initial NO-char rate constant was calculated at 35 s in all experiments, i.e., when the effect of volatiles was absent, as discussed above. It is uncertain to what extent the char has undergone thermal deactivation during the first 35 s, but the interference of the volatiles prohibits a determination of the rate constant at earlier times with the present experimental system. In the initial data treatment, the reaction between biomass char and NO was assumed to be first-order. However, representing the rate constant k for a firstorder reaction versus the NO inlet concentration resulted in a strong dependence of k on the NO concentration (Figures 4 and 5). Consequently, an nth order model was applied. To determine the value of the kinetic parameters, a graphical method was used. The reactor model eq 1 was expressed in a linear form, y ) kx:

1 - (1 - X)(1-n) (1 - n)‚C

(n-1)

W q

) k‚

(2)

NO,O

At each temperature a range of values of n was tested, and for each a linear regression was carried out. The value of n that provided the best fit to a straight line was chosen as the reaction order at the given temperature. Table 2 shows the obtained reaction order at each temperature. The small change of the reaction order with temperature suggests that an average value for each fuel could be used, 0.45 for straw and 0.3 for wood. The char + NO reaction has often been reported to be first-order in NO.6,7,12 However, the present results are in qualitative agreement with previous results for wheat straw,11 which also indicated a fractional order. From the review by Aarna and Suuberg,17 it appears



Figure 6. Arrhenius plots for NO reduction experiments with the straw and wood chars. Figure 4. Verification of a reaction order different from unity. NO reduction experiments with wheat straw char.

Figure 7. Comparison between the experimental and theoretical data obtained with wheat straw char. Figure 5. Verification of a reaction order different from unity. NO reduction experiments with eucalyptus wood char.

that carbon materials such as graphite and resin-chars have first-order dependency, while many coal chars have reaction orders even lower than 0.6. These observations seem to support the present findings of reaction order less than 1. Using the average reaction orders, the rate constants were recalculated at each temperature. Figure 6 shows an Arrhenius plot of the obtained rate constants along with best-fit lines to the data points. The rate constants for NO reduction over the fresh char were found to be

k ) 3.26 × 104 × exp(-11092/T) for straw n ) 0.45 (3) k ) 3.55 × 103 × exp(-9305.5/T) for wood n ) 0.3 (4) On the basis of the rate constants calculated at each NO inlet concentration, we estimate the uncertainty in the average rate constant at each temperature to be at most (30%.

Table 2. Reaction Orders of the NO-Char Reaction Obtained from the Data Fit 1023 K 1073 K 1123 K 1173 K

wheat straw

eucalyptus wood

0.4 0.5 0.5 0.4

0.4 0.3 0.3 0.2

Figures 7 and 8 compare the NO conversion measured in the experiments with that obtained from the derived rate expressions combined with eq 1. A fair agreement is observed for both fuels. Loss of Char Reactivity. Pyrolysis conditions are known to affect the composition and structure of the char formed, and thereby its reactivity. The effect of temperature and residence time in the heat treatment correlates inversely with char reactivity toward oxygen.29-33,37,38 The decrease in reactivity is caused by changes in the surface area and porosity, loss of active sites in the char,39 and a transformation of the catalytic species by sintering effects. However, for straw, Devi and Kannan37 found that the reactivity toward oxygen increased by a factor of 4 with pyrolysis temperature in the range 823-1173 K. This was attributed to


Figure 8. Comparison between the experimental and theoretical data obtained with eucalyptus wood char.

formation of highly active catalytic potassium species in the biomass chars. This is partly supported by Zolin et al.30 who found that for straw the reactivity toward oxygen remained almost constant up to a pyrolysis temperature of 1173 K. Above this temperature the reactivity decreased rapidly, and at 1673 K the reactivity had decreased by a factor of about 200. It is not yet known to what extent these observations for the char-oxygen reaction apply to the char-NO reaction. Sørensen et al.11 found that the reactivity of a char originally made at 973 K decreased by about a factor of 2 by further heat treatment at 1173 K. However, the original char was prepared at conditions that may have promoted quite severe deactivation despite the rather low temperature. In the present work the char is generated at the reaction temperature, so it is not possible to test how the pyrolysis temperature influences the char reactivity. Instead, it was investigated how an increase in the holding time during the pyrolysis step affected the char reactivity. Figure 9 compares the results from two NO reduction experiments conducted at the same temperature and NO inlet concentration. One of the experiments was a standard experiment; in the other the sample was pyrolyzed about 15 min before NO was admitted. The time for the experiment with extended pyrolysis was shifted 15 min for comparison with the standard experiment. It can be seen that the char with extended pyrolysis has a lower initial reactivity but the difference between the two chars decreases with time. At 35 s the rate constant of the fresh char is about a factor of 1.8 higher than the char pyrolyzed for additional 15 min. However, at about 250 s, the rates over the two chars are similar. If NO adsorption alone was responsible for the higher initial rate and subsequent decrease in reaction rate with time, one would expect the two chars to have identical rates, which they clearly do not. The present (36) Dwidevi, P. N.;Upadhyay, S. N. Ind. Eng. Chem. Process. Des. Dev. 1977, 16, 157. (37) Devi, T. G.; Kannan, M. P. Energy Fuels 2000, 14, 127-130. (38) Illań-Go´mez, M. J.; Salinas-Martıńez de Lecea, C.; LinaresSolano, A. Energy Fuels 1998, 12, 1256-1264.

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Figure 9. Influence of the heat treatment in NO reduction experiments with wheat straw at 1123 K and [NO]o ) 400 ppm.

Figure 10. Deactivation of wheat straw, eucalyptus wood, and bituminous coal chars. Experiments at 1123 K and [NO]o ) 400 ppm. (*): (mol0.55‚m1.35‚kg-1‚s-1) for straw, (mol0.7‚m0.9‚kg-1‚s-1) for wood, and (m3‚kg-1‚s-1) for coal.

results therefore suggest that the initial difference in rate over the two chars is caused by thermal deactivation of the char with extended pyrolysis time. However, it seems likely that NO adsorption does play a role in the subsequent decrease in rate over time for both chars. The loss of reactivity can be quantified by plotting the rate constant as a function of time. Figure 10 compares the rates for the two biomass fuels with that of a bituminous coal also tested by Jensen et al.9 The present data on coal support the results from Jensen et al.9 It can be seen that after 5 min (335 s in the figure) the decrease in char reactivity is a factor of 6.4 for coal char, compared to values of 2.8 and 2.9, respectively, for eucalyptus wood and wheat straw. According to Wornat et al.,40 the higher oxygen content in the biomass fuels may inhibit char graphitization due to the formation of cross-linked rigid carbon structures, which decrease the mobility of the carbon molecules. This may explain the (39) Wang, W. X.; Thomas, K. M. Energy Fuels 1996, 10, 409-416.


Figure 11. Arrhenius plots, expressed in reaction rates, for the eucalyptus wood and wheat straw char used in the present study compared to different fuel chars: (a) wheat straw at steady-state; (b) eucalyptus wood at steady-state; (c) Jensen et al.;9 (d) Jensen et al., at steady-state;9 (e) Sørensen et al.;11 (f) Furusawa et al.;6 (g) Chan et al.;7 (h),(i),(j) de Soete;41 (k),(l) Suzuki et al.42

lower loss of reactivity of the biomass fuels compared to coal at these temperatures. Char Reactivity: Comparison with Other Studies. In this section, the rate constants found in the present study are compared with previously published values. Since rate constants referring to different orders cannot be compared directly, reaction rates at a specified set of conditions will be compared. The conditions chosen are P ) 1,2 atm and [NO] ) 500 ppmv. Furthermore, most available rate constants are based on the instantaneous mass of char, while the present kinetics is based on the mass of carbon in the char. We have therefore recalculated our data to per mass of char basis using the data in Table 1. Figure 11 compares reactivities toward NO reduction of the biomass chars tested in the present work with chars from prior studies. A significantly higher reactivity for the wheat straw and eucalyptus wood chars of the present work is observed compared to most literature values, ranging from 1 to 2 orders of magnitude in difference. The high reactivity of the biomass chars is probably due to their content of catalytic species and highly disordered structure. The bituminous coal tested by Jensen et al.9 (and briefly in this study) is an exception to this general trend. The coal reactivity is of the same order of magnitude as the reactivity of the two biomass fuels investigated, and is even higher than the wheat straw char tested by Sørensen et al.11 This behavior could be explained by differences in both the experimental procedure and the data treatment. The present experimental method (40) Wornat, M. J.; Hurt, R. H.; Yang, N. Y. C.; Headley, T. J. Combust. Flame 1995, 100, 131-143. (41) de Soete, G. G. Proc. Combust. Inst. 1990, 23, 1257-1264. (42) Suzuki, Y.; Moritomi, H.; Kido, N. 4th SCEJ Symposium on Circulating Fluidized Bed Combustion, 1991.


comprises char generation “in situ”, while the general method is to produce the char in a previous step, often at prolonged carbonization times. Such a thermal history of the char could enhance deactivation. The kinetic parameters from both the present study and the work of Jensen et al.9 are obtained for fresh char at the earliest possible time when there is no longer an influence of volatiles. For comparison, Figure 11 also shows values of the rate constants for straw and eucalyptus wood char obtained when a pseudo steady state was reached. These values were determined at 400 s, as a compromise between a low loss of char and the steadiness of the rate constant. At the chosen time, the lowest mass of carbon left in the char was about 50% and changes in the reactivity not accounted for by using the instantaneous mass in calculation of the rate constant were kept to a minimum in this way. In the case of the bituminous coal tested by Jensen et al.,9 only one point is available at steady state, which corresponds to 1123 K and 500 ppmv of NO. The results in Figure 11 show that the reactivity determined at steady-state conditions is reduced by a factor of 2-3 from the initial value in the case of biomass, and a factor of 4-5 for the coal. It is interesting to note that even at steady state the reactivity obtained using the in-situ generated char is significantly higher than the literature values for all fuels investigated. For wheat straw, we speculate that the enhanced rate found in the present study compared to that of Sørensen et al.11 may be attributed to differences in the char preparation step. Sørensen et al.11 employed a slow heating combined with a large sample mass, significantly enhancing thermal annealing. Conclusions An experimental investigation of NO reduction over wheat straw and eucalyptus wood chars, in the temperature range 1023-1173 K, has been carried out. The chars were generated “in situ” to follow the reaction rate as a function of time as the char was formed and aged. The experiments show a fast initial reaction rate, which subsequently decreases over time by a factor of about three for both straw and wood. On the basis of experiments with extended pyrolysis time prior to introduction of NO, it was shown that thermal deactivation is an important reason for the decrease in reaction rate. Initial NO adsorption until the active sites are occupied and a pseudo steady state appears may also contribute to the observed decrease in rate as a function of time. Kinetics of the initial rate showed fractional order in the NO concentration for straw and wood chars of 0.45 and 0.3, respectively. The measured rate constants were up to 2 orders of magnitude faster than values reported in the literature. This difference is attributed mainly to significant thermal deactivation of the chars used in most previous studies. Furthermore, the rate constants for the initial rate in the present study may include some effect of NO adsorption. Moreover, the results indicate that a bituminous coal char deactivates to a greater extent than the biomass char. The results support the conclusions reached by Jensen et al.9 Acknowledgment. The work was funded by the Joule Program under Contract ENK5-CT-2000-00324.


It was carried out as a part of the CHEC (Combustion and Harmful Emission Control) Research Program, which is financially supported by the Danish Ministry of Energy, Elsam (the Jutland-Funen Electricity

Garijo et al.

Consortium), the Danish and Nordic Energy Research Programs, the European Union, and the Danish Technical Research Council. EF020276N

Kinetic Study of NO Reduction over Biomass Char under Dynamic

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