Energy & Fuels 2001, 15, 1359-1368
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Reduction of NO over Wheat Straw Char Claus O. Sørensen, Jan E. Johnsson, and Anker Jensen* Department of Chemical Engineering, Technical University of Denmark, Building 229, DK-2800 Kongens Lyngby, Denmark Received October 11, 2000. Revised Manuscript Received August 16, 2001
The kinetics of NO reduction over wheat straw char in the presence and absence of CO was investigated in a laboratory scale fixed bed quartz reactor. Experiments were performed with char from both the raw straw and from washed straw. The wash procedure removed most of the content of potassium and other catalytic species in the straw. The temperature was in the range 600-900 °C, the NO concentration was in the range 50-1000 ppmv, and CO was in the range 0-5 vol %. The char from raw straw was more active than the washed char by up to a factor of 2.5, but both chars were found to be very active compared to the activity of coal chars reported in the literature. At temperatures above about 850 °C the difference in reactivity decreased between the two straw chars, probably because the catalytic species in the char from raw straw was vaporized and/or reacted to form catalytically inactive potassium silicates. The reaction order for NO was about 0.7 independent of temperature for both chars. The rate expressions for NO reduction over chars from raw and washed straw respectively are the following: -rNO ) 2.06 × 0.7 -1 s-1, and -r 4 105 × exp(-16180/T) × C0.7 NO ) 9.53 × 10 × exp(-15950/T) × CNO mol NO mol kg -1 -1 kg s . The addition of CO resulted in an immediate increase in the rate of NO reduction, but the rate decreased over time and could in some cases stabilize at a level similar to that when CO was absent. This effect is not understood in detail. A reaction mechanism for reduction of NO over char is proposed and discussed.
Introduction There is an increasing interest in utilizing biomass as a CO2 neutral fuel in combustion and gasification processes due to concern about the emission of green house gases from fossil fuel combustion. Although biomass is considered CO2 neutral, the emission of other compounds such as SO2 and NOx from biomass utilization may cause concern. For example, the emission of NO from a straw-fired boiler equipped with a grate1,2 was of the order 100-200 ppmv (at 6 vol % O2) indicating the necessity of NOx control. Most biofuels contain significant amounts of minerals (ash) and it has been found that NOx reduction by selective catalytic reduction (SCR) on biomass fired processes is difficult due to accelerated deactivation of the catalysts3 caused by the ash, most probably potassium. Consequently SCR is presently not an option for NOx control on biomass fired processes. This makes NOx reduction by combustion process modifications even more attractive. It is known that the reaction between NO and char in the combustion chamber may contribute to lowering the emission of NOx. This reaction is believed to be of importance for the low NOx emission from fluidized combustors4 and to some extent also in suspension * Corresponding author. (1) van der Lans, R. P. CHEC Report 9913; Department of Chemical Engineering, Technical University of Denmark, 1999. (2) van der Lans, R. P. CHEC Report 9914; Department of Chemical Engineering, Technical University of Denmark, 1999. (3) Andersson, C.; Odenbrand, I.; Andersson, L. H. Va¨ rmeforsk (in Swedish); No. 646, 1998. (4) Johnsson, J. E. In Fluidization; Grace, J. R., Shemilt, L. W., Bergougnou, M. A., Eds.; Engineering Foundation, 1989; pp 435-442.
firing.5 The reduction of NOx by coal chars and graphite has been investigated intensively as summarized in the reviews of Li et al.6 and Aarna and Suuberg,7 but investigations of NOx reduction over biomass char are scarce.8 The minerals present in biomass may enhance the rate of the NO char reaction9-17 indicating a potential of this reaction for NOx control. Wheat straw, which is a typical biomass in Denmark for combustion purposes, contains up to 1.7 wt % potassium which is known as a very active catalyst in combustion and gasification reactions.9,10,12,13,18 The catalytic activity depends on the chemical nature of the catalytic compound. After the initial pyrolysis step, potassium may be associated with the organic matrix or bound in discrete particles as silicates, chlorides, or carbonates, (5) Pedersen, L. S.; Glarborg, P.; Dam-Johansen, K.; Hepburn, P. W.; Hesselmann, G. Comb. Sci. Technol. 1998, 132, 251-314. (6) Li, Y. H.; Lu, G. Q.; Rudolph, V. Chem. Eng. Sci. 1998, 53, 1-26. (7) Aarna, I.; Suuberg, E. M. Fuel 1997, 76, 475-491. (8) Zevenhoven, R.; Hupa, M. Fuel 1998, 77, 1169-1176. (9) Illa`n-Gomez, M. J.; Salinas-Martinez de Lecea, C.; LinaresSolano, A. Energy Fuels 1998, 12, 1256-1264. (10) Garcı´a-Garcı´a, A.; Illa´n-Go´mez., M. J.; Linares-Solano, A.; Salina-Martı´nez de Lecea, C. Fuel 1997, 76, 499-505. (11) Illa´n-Go´mez, M. J.; Linares-Solano, A.; Radovic, L. R.; SalinasMartı´nez de Lecea, C. Energy Fuels 1995, 9, 112-118. (12) Illa´n-Go´mez, M. J.; Linares-Solano, A.; Radovic, L. R.; SalinasMartı´nez de Lecea, C. Energy Fuels 1995, 9, 104-111. (13) Illa´n-Go´mez, M. J.; Linares-Solano, A.; Radovic, L. R.; SalinasMartı´nez de Lecea, C. Energy Fuels 1995, 9, 97-103. (14) Illa´n-Go´mez, M. J.; Linares-Solano, A.; Radovic, L. R.; SalinasMartı´nez de Lecea, C. Energy Fuels 1995, 9, 504-548. (15) Illa´n-Go´mez, M. J.; Linares-Solano, A.; Salinas-Martı´nez de Lecea, C. Energy Fuels 1995, 9, 976-983. (16) Guo, F.; Hecker, W. C. Proc. Combust. Inst. 1996, 2251-2257. (17) Wu, S. L.; Iisa, K. Energy Fuels 1998, 12, 457-463.
10.1021/ef000223a CCC: $20.00 © 2001 American Chemical Society Published on Web 09/26/2001
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or be vaporized during pyrolysis.19 Devi and Kannan20 oxidized wheat straw char in air at 400 °C and found that the reactivity of the char decreased with increasing pyrolysis temperature up to 550 °C. At pyrolysis temperatures above 550 °C the reactivity increased again. It was suggested20 that at the higher pyrolysis temperature potassium is bound to the oxygen-containing aromatic system in the char which is believed to form a species with a high catalytic activity. Zevenhoven and Hupa8 reported a positive influence of potassium on reactivity for NO reduction over different chars including peat and wood at 750 °C whereas at 850 °C there was no effect. It was proposed that at 850 °C the potassium had reacted to form silicates which are believed to be inactive. It appears that the influence of temperature on the effect of potassium is not clear, but may depend on the gasification agent. In practical combustion systems the minerals in biomass may deposit on heat transfer surfaces in the furnace and cause accelerated corrosion.21,22 To avoid this it has been proposed to pretreat the biomass before combustion by washing in order to remove most of the water-soluble minerals,23,24 but this may lower the rate of NO reduction by char because the catalytically active compounds are removed. Carbon monoxide is usually present in the pores of char particles due to the ongoing combustion or gasification process. The presence of CO has been observed to enhance the rate of NO reduction over coal chars25-27 but little is known about the influence of CO on NO reduction over biomass chars.8 In this paper we report an investigation of the reduction of NO by char from both raw and washed wheat straw in the presence or absence of CO. We also propose a reaction mechanism for the reduction of NO including the catalytic effect of the inorganic compounds in the ash. Experimental Section Apparatus. The experiments were performed in a fixed bed laboratory quartz reactor positioned in an electrically heated oven. The chars were held by a porous quartz support plate, and the reaction temperature was measured by a thermocouple positioned 0.5 cm below the quartz plate. A sketch of the reactor can be seen in Figure 1. The temperature profile in the reactor was flat (better than (3 °C) in the zone containing the particles. A gas of well defined composition was mixed from pure gases or gas mixtures from gas cylinders in a panel of precision mass flow controllers. The construction of the reactor (18) Jensen, A.; Dam-Johansen, K.; Wojtowicz, M. A.; Serio, M. A. Energy Fuels 1998, 12, 929-938. (19) Jensen, P. A.; Sander, B.; Dam-Johansen, K. Proceedings of the Fourth Biomass Conference of the Americas, 1999; pp 1169-1174. (20) Devi, T. G.; Kannan, M. P.; Energy Fuels 2000, 14, 127-130. (21) Nielsen, H. P.; Baxter, L. L.; Sclippab, G.; Morey, C.; Frandsen, F. J.; Dam-Johansen, K. Fuel 2000, 79, 131-139. (22) Michelsen, H. P.; Frandsen, F.; Dam-Johansen, K.; Larsen, O. H. Fuel Process. Technol. 1998, 54, 95-108. (23) Knudsen, N. O.; Jensen, P. A.; Sander, B.; Dam-Johansen, K. Proceedings of the International Conference: Biomass for Energy and Industry; Wu¨rzburg, Germany, 8-11 June, 1998; pp 224-228. (24) Jensen, P. A.; Dam-Johansen, K.; Sander, B. Proceedings of the 2nd Olle Lindstro¨ m Symposium on Renewable Energy-Bioenergy; Stockholm, Sweden, 9-11 June, 1999; pp 105-112. (25) Arnaa, I.; Suuberg, E. M. Energy Fuels 1999, 13, 1145-1153. (26) Levy, J. M.; Chan, L. K.; Sarofirm, A. F.; Bee´r, J. M. Proc. Combust. Inst. 1981, 111-120. (27) Furusawa, T.; Tsunoda, M.; Kunii, D. Am. Chem. Soc. Ser. 1982, 196, 347-357.
Sørensen et al.
Figure 1. Sketch of the laboratory reactor. All measures are in millimeters. (1) Bottom gas inlet. (2) Top gas inlet. (3) Thermocouple. (4) Gas outlet. Table 1. Elemental Composition of the Straw (in wt %), As Received (For the chars the data are reported on a dry basis.)
moisture ash volatiles C H N S Cl Si Al Fe Ca Mg Ti Na K P
straw
washed straw
5.8 6.4 70.2 43.2 5.6 1.0 0.18 0.59 1.37 0.03 0.03 0.4 0.09 0.002 0.02 1.7 0.14
5.1 3.2 77.6 45.1 5.9 0.85 0.08 0.04 1.07 0.02 0.02 0.25 0.05 0.002 0.0 0.084 0.07
char (700 °C)
char (washed straw) (700 °C)
23.7 8.6 67.5 1.42 1.07 0.33
26.4 5.6 61.1 1.1 0.99 0.08
offered the possibility of two feed streams (1 and 2 in Figure 1) with the purpose of preventing reaction of the gases, either directly in the gas phase or catalyzed by the reactor walls, before reaching the char particles. The mixing of the reactant streams containing N2/NO and N2/CO occurred about 3 cm above the particles on the quartz plate. The inlet gas mixture may also bypass the reactor to measure the inlet concentrations of NO and CO. The outlet gas composition from the reactor was measured by continuous analyzers for NO, CO, and CO2, and all data were sampled by a computer. Char Preparation and Characterization. The fuel used in the experiments was Danish wheat straw pellets. The composition of the straw is shown in Table 1. It can be seen that the potassium content of the straw was 1.7 wt % and the total ash content was 6.4 wt %. Washing of the straw pellets before pyrolysis was performed in 70 °C hot water and the process repeated 4 times until no more K was released, as
Reduction of NO over Wheat Straw Char verified by ion chromatography on the wash water. The composition of washed straw is also shown in Table 1. Washing reduces the ash content and in particular the K content to less than 5% of the K content in the raw straw. The char was prepared by heating the straw (∼70 g) in a pyrolysis reactor at 30 °C/min for 1 h in a nitrogen flow with a final temperature of approximately 700 °C. The composition of the organic part of the char is also shown in Table 1. Experiments on the release of K from straw during pyrolysis was made by Jensen et al.19 Potassium is not released at temperatures below 700 °C but at higher temperatures an increasing amount of K is released, for example 25-30% at 900 °C. Although K is not released below 700 °C different transformations do occur.19 In raw straw all K is bound to the organic matrix, while in the char K also appears in discrete particles mainly as KCl and K2CO3 and at 800-900 °C about 20% of the K was bound in silicates. The release of K is probably due to evaporation of KCl and decomposition of K2CO3. In this study the initial pyrolysis temperature was 700 °C, which ensures that most of the potassium remains in the char. The char yield of the raw straw pellets was 30 wt % while the corresponding figure for the washed straw pellets was 21 wt %. After pyrolysis, the char was ground and sieved to a size of 180-250 µm. It was attempted to measure the micropore surface area of the prepared chars by CO2 adsorption at 0 °C. However it was not possible to obtain adequate vacuum over the samples. The chars were therefore further heated to 900 °C, the highest temperature of the NO reduction experiments, before the surface area determination. The adsorption data were analyzed using the Dubinin-Astakhov equation. The surface areas were 166 m2/g (raw straw) and 484 m2/g (washed straw). An attempt to perform Hg-porosimetry measurements on the chars was not successful since the chars were too fragile and were crushed in the high pressure and their macropore structure was damaged. Procedure. Char particles were mixed with 1.5 g of quartz sand particles of the same size as the char. The char mass was varied between 50 and 300 mg, depending on the reaction temperature. The purpose of the sand was to give a constant bed height of approximately 1 cm in each experimental run independent of the char mass to ensure plug flow. The activity of the reactor with sand was tested at 900 °C with 750 ppmv NO and 5% CO and no detectable NO reduction was found. The gas flow rate in all experiments was 2 NL/min. The temperature was varied between 600 °C and 900 °C and the reactor pressure was approximately 1.1 bar. The NO concentration was varied between 50 and 1000 ppmv at each temperature. When CO was present the NO concentration was fixed at 250 ppmv. Initially, the mixture of char and sand was kept in the reactor for 1 h at the reaction temperature in a stream of pure nitrogen. The purpose of this treatment was to ensure that any deactivation due to thermal effects28-30 had occurred prior to the experiments so that transient deactivation effects would not influence the results. The effect of heat treating the char at the highest reaction temperature of 900 °C for 1 h before lowering the temperature to the desired reaction temperature was also examined. Another effect of the heat treatment procedure is to remove some of the surface oxides initially present on the char surface. This is discussed further in the Results section. Following the heat treatment, a gas mixture with a well-defined NO concentration was passed through the reactor and the NO outlet concentration was recorded over time and the conversion of NO at steady state was calculated. The time to reach steady state was (28) Garcı´a-Garcı´a, A.; Illa´n-Go´mez, M. J.; Linares-Solano, A.; Salinas-Martı´nez de Lecea, C. Fuel Process. Technol. 1999, 61, 289297. (29) Zolin, A.; Jensen, A.; Dam-Johansen, K. Proc. Combust. Inst. 2000, 28, 2181-2188. (30) Hurt, R.; Sun, J.; Lunden, M. Combust. Flame 1998, 113, 181197.
Energy & Fuels, Vol. 15, No. 6, 2001 1361
Figure 2. Mass-based rate constants against char consumption at 900 °C, chars from washed straw, n ) 0.75, initial char mass ) 0.05 g.
Figure 3. Transient concentration and rate constants at 900 °C, chars from washed straw. (1) 504 ppmv NO, (2) 742 ppmv NO. between 20 min and 1 h and depended on the NO concentration, temperature, and initial conditions. No measures were taken to clean the sample for surface oxides after each measurement before changing to another NO concentration. At 900 °C the amount of char used in the experiments was very low due to the high reaction rate. In these cases the carbon consumption was up to 37 wt % during an experimental run, and the change in char mass had to be accounted for in the data analysis. This was done by integrating the CO and CO2 outlet concentrations. In experiments where the char was completely consumed the mass balance was checked and typically closed within 90% or better. The relatively large char consumption may change the porosity and surface area of the char in a way which is not accounted for by a simple correction of the char mass. Figure 2 shows rate constants based on the current char mass calculated at each inlet NO concentration against char consumption for a single experimental series. It can be seen that the mass-based rate constant is independent of the conversion of the char, and so the conversion of the char was accounted for by using the current char mass in the calculation of the mass based rate constant. The variations in the rate constant in Figure 2 appear to be random and reflects the inaccuracy of the experimental procedure. Data Treatment. In general the NO outlet concentration could be stabilized and the rate constant was calculated on the basis of the instantaneous char mass. However, for the washed straw at 900 °C it was not possible to obtain a steady value of the rate constant even when the current char mass was used, despite the fact that the NO outlet concentration was steady. An example of this is shown in Figure 3. It can be seen that, although the NO concentration becomes constant after about 15 min, the rate constant based on the current char mass increases over time. In such cases the rate constant was taken as the value at the time the NO concentration
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becomes constant. It should be noted that the change in reaction rate constant is of the order of 10% in the period 15 to 25 min, which is not very significant. As explained above, the rate constant is based on the instantaneous char mass, which corresponds to using a volumetric reaction model. However, a random pore model31 was also tested. Using a value of the structural parameter ψ of 1.5 it was possible to obtain a rate constant which was independent of solid conversion for the cases shown in Figure 3. However, since the value of ψ is rather low, the structural changes in the char are small and the random pore model was not used further. The use of a volumetric reaction model also allows easy comparison of the rate constants at different conditions. Reactions of char and NO are much slower than char gasification by oxygen, but external and internal mass transfer may still influence the reaction rate at high temperatures. To test if mass transfer was limiting the rate an order of magnitude calculation was made at the highest temperature of 900 °C. An effective diffusion coefficient for NO in the char of Deff ) 7.9 × 10-6 m2/s was estimated from Johnsson and Jensen32 by correcting for temperature. Based on this the effectiveness factor was estimated to 0.94. By the methods of Prins,33 a mass transfer coefficient to particles in a fixed bed reactor was calculated using a bulk diffusion coefficient for NO of DNO ) 1.84 × 10-4 m2/s. The relative external mass transfer resistance was estimated to be about 1%. It is concluded that internal and external mass transfer limitations are insignificant in this study. The fixed bed reactor was modeled as a plug flow reactor and the NO reduction was assumed to follow nth order kinetics with respect to the NO concentration. Integrating over the reactor length gives the following relation between NO conversion, inlet concentration and char mass (n * 1):
[
X ) 1 - 1 - (1 - n) × k × C(n-1) NO,0 ×
W v
1/(1-n)
]
Figure 4. Influence of experimental procedure on the rate constant. Normal procedure: The char is conditioned at the reaction temperature. Heat-treated char: The char is heat treated at 900 °C for 1 h before lowering the temperature to the reaction temperature. Char from raw straw.
(1)
where CNO,0 is inlet NO concentration in [mol/m3], W is the instantaneous char mass [kg], and v is the gas flow rate [m3/ s]. The unit of the nth order reaction rate constant is [mol(1-n) m3n kg-1 s-1].
Figure 5. NO conversion as a function of inlet concentration. Char from raw straw, 700 °C.
Influence of Heat Treatment. As described in the Experimental section, most experiments were performed by conditioning the char at the reaction temperature. However, to test the influence of thermal treatment some experiments were performed by heat treating the char from raw straw at 900 °C for 1 h before changing the temperature to the desired reaction temperature. A comparison of the rate constants obtained by the two methods is shown in Figure 4. The details of how the rate constant is calculated is discussed below. It can be seen that heat treating the char at 900 °C deactivates the char significantly in comparison to the standard experimental procedure. The rate constant is reduced by up to 50% at 700 °C, where the difference between the two procedures is most pronounced. The rate constants at 900 °C by the two procedures would ideally be identical. The observed difference reflects the experimental uncertainty and sample-to-sample differences. The mechanism behind the influence of the heat treatment is discussed further below.
Reaction Order and Activation Energy. The data for determination of reaction order and activation energy consist of NO outlet concentrations measured at 5-6 different NO inlet concentrations in the range of 50-1000 ppmv. Each experimental series was repeated with a fresh char sample giving a total of 10-12 measurements at each temperature. The rate constant and reaction order were estimated from the experimental data using a least-squares method based on eq 1. An example of an experimental series with a single char sample can be seen in Figure 5. As the temperature is low (700 °C) the char consumption is negligible and so the results at each NO inlet concentration can be compared directly. For a first-order reaction the conversion of NO would be independent of the NO inlet concentration. It can be seen from Figure 5 that the reaction order is below unity. Many investigators have found that NO reduction over chars from a variety of coals and carbons is first order with respect to NO,6,7 but fractional order34-36 and changing order with increasing NO concentration37 has also been reported. Aarna and Suuberg34 noted that the reaction order
(31) Bhatia, S. K.; Perlmutter, D. D. AICHE J. 1981, 27, 379-385. (32) Johnsson, J. E.; Jensen, A. Proc. Combust. Inst. 2000, 28, 23532359. (33) Prins, W. Fluidized Bed Combustion of a Single Carbon Particle. Ph.D. Thesis, University of Twente, 1987.
(34) Aarna, I.; Suuberg, E. M. Proc. Combust. Inst. 1998, 30613068. (35) Johnsson, J. E. Fuel 1994, 73, 1398-1415. (36) Johnsson, J. E.; Dam-Johansen, K. Proc. Int. Conf. Fluidized Bed Combust. 1991, 3, 1389-1396.
Results and Discussion
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Energy & Fuels, Vol. 15, No. 6, 2001 1363
Figure 7. Calculated (with n ) 0.7) versus measured NO outlet concentration. Chars from washed straw, 600-900 °C. Figure 6. Transient experiment with chars from raw straw. Initial concentrated 52 ppmv NO, 745 °C. Table 2. Reaction Orders for the NO-Straw Char Reaction at Various Temperatures
600 °C 650 °C 700 °C 800 °C 850 °C 900 °C
chars from raw straw
chars from washed straw
0.77 not measured 0.67 0.70 0.72 0.79
0.64 0.55 0.59 0.62 0.73 0.75
changed from zero at temperatures below 650 °C to close to first order at 850 °C. Furthermore, cleaning the surface by heating to 900 °C between experiments with different NO inlet concentrations gave higher reduction rates at temperatures lower than 727 °C than in the absence of the surface cleaning step. At temperatures above 727 °C there was no influence of surface cleaning. These experiments were performed in a thermogravimetric analyzer (TGA) at high (900-8200 ppmv) NO concentrations. Parallel experiments in a fixed bed reactor by Aarna and Suuberg34 using lower NO concentrations than in the TGA showed no effect of surface cleaning, and a reaction order in the range 0.7-1.0 that was less sensitive to temperature. The latter experimental method and NO concentration range is similar to those applied in the present study. The increase in NO conversion with decreasing NO inlet concentration is most obvious at low NO levels, see Figure 5. Therefore it may be speculated if under these conditions enough time had elapsed for the population of surface species to reach steady state. During the transient period of reduction over a cleaned surface a higher rate of NO reduction would appear because the adsorption of NO also contributes to NO removal from the gas. Such a behavior has been observed previously by other investigators, e.g., Aarna and Suuberg25 and Guo and Hecker.38 To test this an experiment with an inlet concentration of 52 ppmv NO at 745 °C was therefore made over long time using a fresh char sample. The low NO concentration and fairly low temperature would result in long time for steady state. The results for the first 2 h are shown in Figure 6. First, it is clear that the NO reduction does not begin with an initially higher (37) Yang, J.; Mestl, G.; Herein, D.; Schlo¨gl, R.; Find, J. Carbon 2000, 38, 715-727. (38) Guo, F.; Hecker, W. C. Proc. Comb. Inst. 1998, 3085-3092.
Figure 8. Calculated (with n ) 0.7) versus measured NO outlet concentration. Chars from raw straw, 600-900 °C.
rate followed by a slow decline as would be expected if NO adsorption played a significant role. Furthermore, the reaction has stabilized within the first hour and does not change appreciably within the next hour. In fact, the NO outlet concentration only changed in correspondence to the carbon consumption the next 48 h. The results indicate that the observed fractional reaction order is not caused by slow transients but reflects pseudo steady-state gasification in a regime where both adsorption, surface reaction, and product desorption influence the rate. It should also be mentioned that the experiments were performed in a random way, and sometimes a low NO concentration followed an experiment with a high NO concentration, still resulting in repeatable results. The apparent reaction order for the chars made from both washed and raw straw pellets is shown in Table 2 at each temperature. The values range from 0.55 to 0.79, with a typical standard deviation of (0.025. The reaction order does apparently not depend significantly on temperature, which agrees with the results by Aarna and Suuberg34 under the present conditions. To unify the present results into a single rate constant an average value of the reaction order of 0.7 was assumed and the rate constant at each temperature was refitted using this value. Good agreement between the calculated and measured NO outlet concentrations for all experiments using this reaction order is seen in Figures 7 and 8 for the chars from washed and raw straw, respectively. Using the average reaction order of 0.7 the activation energy was calculated from the Arrhenius plot shown in Figure 9. The error bars at each temperature indicate the minimum and maximum value of the rate constant
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Figure 10. Ratio between rate constants, k/kwash at various temperatures.
Figure 9. Arrhenius plot for chars from washed and raw straw. Lower solid line: Best fit to data for char from washed straw. Upper solid line: Best fit to data for char from raw straw based on all data points. Dashed line: Best fit to data for char from raw straw up to 800 °C.
calculated from the conversion at each NO inlet concentration. For the chars prepared from washed straw the activation energy is estimated to 133 ( 3 kJ/mol and for the chars from the raw straw the activation energy is 135 ( 7 kJ/mol based on the fully drawn line in Figure 9. There is apparently no significant difference between the two values. According to Moulijn and Kapteijn39 and Chen and Yang40 it is expected that the catalyzed and noncatalyzed reactions have the same activation energies. The catalyst increases the number of sites without changing the kinetic network.39 Chan et al.41 on the other hand did observe a lower activation energy for the reduction of NO over a coal char compared to its demineralized counterpart. From Figure 9 it can be argued that the Arrhenius plot for the char from the raw straw only follows a single straight line up to about 800 °C, above which a decrease in the slope occurs. This trend becomes more apparent when comparing the best line through all points (solid line), with the best line obtained using only points up to 800 °C (dashed line). The activation energy up to 800 °C is estimated to 152 ( 2 kJ/mol, which is actually higher than the value for char from washed straw. A break in the Arrhenius plot is often reported42,43 for NO reduction over coal chars, and is commonly explained by a change in reaction mechanism from desorption control at low (39) Moulijn, J. A.; Kapteijn, F. Carbon 1995, 33, 1155-1165. (40) Chen, S. G.; Yang, R. T. Energy Fuels 1997, 11, 421-427. (41) Chan, L. K.; Sarofim, A. F.; Beer, J. M. Combust. Flame 1983, 52, 37-45. (42) Suuberg, E. M.; Teng, H.; Calo, J. M. Proc. Combust. Inst. 1990, 1199-1205. (43) Furusawa, T.; Tsunoda, M.; Tsujimura, M.; Adschiri, T. Fuel 1985, 64, 1306-1309.
temperatures to adsorption control at higher temperatures. In those cases the activation energy is higher in the high-temperature adsorption control regime, in contrast to the results in Figure 9 for the char from raw straw. That a transition from low to high activation energy is not seen in the present results for any of the chars may be due to their high reactivity, as will be shown below. The transition to adsorption control may appear at lower temperatures than those investigated in the present experiments. The change to lower activation energy at higher temperature for the char from raw straw may be explained by mass transfer limitations, but our theoretical calculations suggest that this is not the case. It should also be noticed that the change to lower activation energy at higher temperature only appears for the char from raw straw. A probable explanation for this difference is that the catalytically active components in the raw straw char is partly lost by vaporization and partly transformed into less active components,19 such as potassium silicates, as the temperature is raised above the char preparation temperature of 700 °C. The positive effect on the rate constant of raising the temperature is therefore to some extent counter-balanced by a loss of activity due to loss of catalytic sites. This is also observed from the data in Figure 4. It is clear from Figure 9 that washing of the straw and thereby removing most of the potassium makes the resulting chars less reactive toward NO reduction. This becomes further evident in Figure 10, showing the ratio of the rate constants for chars from washed and raw straw at each temperature. The ratio of the rate constants increases with temperature up to 800 °C, where the rate constant of the char from the raw straw is 2.5 times larger than that of the char from washed straw. Above 800 °C the catalytic effect decreases as a consequence of the loss of catalytic sites for the raw straw. If the rate constants of the two chars are based on CO2 surface area, the catalytic effect of potassium becomes more apparent. The surface area of the chars from washed straw prior to reaction is about 3 times higher than from the raw straw giving a ratio between the surface area-based rate constants of about 4 to 8. It is interesting that the catalytic effect of K on the NOchar reaction appears much less than on the O2-char reaction. Zolin et al.44 investigated the O2 reactivity of straw chars very similar to those in the present study.
Reduction of NO over Wheat Straw Char
Energy & Fuels, Vol. 15, No. 6, 2001 1365
Figure 11. Temperature dependence of the ratio between CO/ CO2 outlet concentrations for both char types.
For chars prepared at 700 °C char from raw straw was about 10 times more reactive than from washed straw (on a mass basis). At a pyrolysis temperature of 1000 °C the difference was about a factor of 200. At even higher temperatures the difference decreased and became insignificant at a pyrolysis temperature of 1673 K. CO/CO2 Ratio. The CO and CO2 concentrations in the outlet gas were measured and used to calculate the CO/CO2 ratio for the NO-char reaction. The CO/CO2 ratios for both chars are shown in Figure 11 using average values of the CO/CO2 ratio for all NO inlet concentrations except the lowest of 50 ppmv where the amounts of CO and CO2 produced were very low thereby increasing the uncertainty. The results did not conform to an Arrhenius plot and are shown simply as a function of temperature. It can be seen that the values for both chars are very similar, increasing from about 0.3 at 600 °C to 2-2.5 at 900 °C, and that a change appears to take place at a temperature around 750 °C. That the CO/CO2 ratio increases with increasing temperature, is in agreement with previous observations7 for the NO-char reaction. Furthermore, we found that the CO/CO2 ratio did decrease with increasing NO concentration. The higher CO2 formation at low temperature and high NO concentration may be explained in terms of a higher oxide surface population at these conditions. It is interesting that there is little difference in the CO/CO2 ratio from raw and washed straw chars indicating little effect of catalysts on this ratio. On the other hand the catalytic effect is not very strong on this reaction, the maximum difference in the rate constant being only up to a factor 2.5 as discussed above. Comparison with Data from Other Chars. Biomass chars are expected to be more reactive than coal chars due to a higher content of catalytic active substances and a less ordered structure of the organic matrix. Figure 12 compares the reactivity of the two straw chars investigated in this study to a range of coal chars and Pet coke. To facilitate the comparison a first-order rate constant was estimated from the experimental data. For this crude comparison the error of assuming first-order kinetics is of minor importance. It is evident from Figure 12 that the reactivity of the char from the (44) Zolin, A.; Jensen, A.; Jensen, P. A.; Frandsen, F.; DamJohansen, K. Energy Fuels, submitted.
Figure 12. Arrhenius plots of first-order rate constants for the straw chars compared to a selection of coal chars. Approximate first-order rate constants for chars from raw and washed straw respectively are: k ) 3.9 × 106 × exp(-17200/ T) m3 kg-1 s-1 and k ) 7.9 × 105 × exp(-16250/T) m3 kg-1 s-1. Taiheiyo Coal,45 (b) Montana Lignite,41 (c) Bituminous Coal,18 (d) Brown Coal,36 (e) Pet Coke,36 (f) Cedar Grove,46 (g) Prosper,46 (h) Eschweiler,46 (i) Datong Coal,47 (j) Pet Coke.47
Figure 13. Influence of CO at 800 °C for chars from washed straw. NO inlet concentration 250 ppmv, 0.06 g char.
raw straw is very high, the rate constant being 5-10 times higher than the rate constant from most of the other chars. Even the chars from the washed straw, which is depleted of catalytic active species, is in the high reactivity range. This indicates the potential of wheat straw char for NO reduction. Effect of CO. The influence of CO on NO conversion was investigated for both char types at 700 °C, 800 °C, and 850 °C. The CO concentration was varied between 0% and 5% while the NO inlet concentration was fixed at 250 ppmv. The experimental procedure allowed for fast switching between the various CO concentrations, thus making it possible to record a transient with no breaks between the measurements. An example of the results is seen in Figure 13. The figure shows an instantaneous and positive effect of increasing the CO concentration and a negative effect of decreasing it. However, there is clearly a secondary effect of adding CO. The NO conversion does not stabilize and the rate continuously decreases over time in the presence of CO. Results for the raw straw showed a similar behavior. In the time scale of the experiments in Figure 13 steady state is obviously not obtained. To investigate if steady
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Figure 15. Initial influence of CO on the NO reaction rate at 700 °C. Figure 14. Long time effect of CO for char from raw straw, 850 °C, 250 ppmv NO, 0.05 g char.
state could be obtained an experiment over a much longer time scale was performed. When CO is present in high concentrations it becomes impossible to calculate the char consumption by measuring the outlet CO and CO2 concentration, and so the experiment was made in the absence of sand in order to allow accurate weighing of the char after the experiment. A parallel experiment in the absence of CO was also performed for comparison. The results are shown in Figure 14. In both experiments CO was initially absent, so the reactivity without CO could be compared. A good agreement between the initial NO conversion in the two experiments is observed. The addition of CO results in a fast increase in the rate of NO reduction, as also seen in Figure 13. The rate then decreases over the next about 800 min. Steady state is not reached even after 800 min, but the rate of decrease is much lower than initially and may be partly due to slow consumption of the char. At the end of the experiment with CO, it was found by weighing that 50% of the char was consumed. Comparing with the experiment without CO, where all char was consumed after about 600 min, shows that the presence of CO greatly reduces the char consumption. Calculating an effective rate constant for NO reduction at 800 min in the presence of CO, accounting for the char loss, shows that the rate constant is very close to the initial rate constant in the absence of CO. In other words, after long reaction time there is little effect of CO on the rate of NO conversion. This was confirmed by removing CO from the inlet gas after 800 min. It can be seen that the NO conversion only decreases from about 52% to 44% corresponding to a decrease in the rate constant of about 20% (assuming first-order kinetics for simplicity). It is not clear why the positive effect of CO is lost over time. For comparison an experiment with a bituminous coal char was performed which did not show this behavior. Clearly more work is needed to understand the mechanism behind these observations. From a practical point of view, however, a substantial enhancement of CO on the rate of NO reduction in a combustion process may be expected since the time scale for the loss of the CO enhancement is much longer than the time scale for the combustion of a char particle by combustion reactions. That the NO conversion does not stabilize in the presence of CO makes it difficult to quantify the effect
Figure 16. Initial influence of CO on the NO reaction rate at 800 °C.
of CO. One way is to calculate the effect that initially appears when CO is added. By assuming a reaction rate expression of the form: 0.7 -r ) k × CnCO × C0.7 NO ) k′ × CNO
(2)
where the effect of CO is incorporated in the rate constant k′, a comparison of the rate constants at different CO concentrations can be made. The consumption of CO is low so that a pseudo 0.7 order reaction in NO can be assumed. Figures 15 and 16 show the influence of CO for both char types at 700 °C and 800 °C, respectively. The values shown are the ratio of the rate constant in the presence of CO to the rate constant without CO. At both temperatures the effect of CO is very similar for the two char types. The rate of NO reduction increases with increasing CO concentration and at about 3 vol % CO the reactivity is increased by a factor 1.5 to 2.5. This is in agreement with the work of Johnsson,48 who reported that the rate of NO reduction increased by 2-3 times at 800 °C for a coal char when 3 vol % CO was introduced. Aarna and Suuberg25 found that the presence of 420 ppmv CO at 800 °C increased the rate of NO reduction over a low-reactivity graphite by an order of magnitude. Such low concentrations of CO did not increase the rate in this study. (45) Furusawa, T.; Kunii, D.; Oguma, A.; Yamada, N. Int. Chem. Eng. 1980, 20, 239-244. (46) de Soete, G. G. Proc. Combust. Inst. 1990, 1257-1264. (47) Suzuki, Y.; Moritomi, H.; Kido, N. 4th SCEJ Symposium on Circulating Fluidized Bed Combustion, 1991. (48) Johnsson, J. E. CHEC Report 9003; Department of Chemical Engineering, Technical University of Denmark, 1990.
Reduction of NO over Wheat Straw Char
Energy & Fuels, Vol. 15, No. 6, 2001 1367
Possibly the effect of CO is higher for less reactive materials such as graphite. NO Reduction Mechanism. We first discuss the mechanism of NO reduction over char including the effect of CO, in the absence of inorganic catalytic species. On the basis of our own observations and the literature we propose the following set of elementary reactions:
-Cf + NO T -C-NO
(R1)
-C-NO + -Cf f -C-N + -C-O
(R2)
-C-N + NO f -C-N-NO
(R3)
-C-N-NO + -Cf f -C-N-N + -C-O
(R4)
-C-N + -C-N f -C-N-N + -Cf
(R5)
-C-NO + CO f -C-N + CO2
(R6)
-C-N-NO + CO f -C-N-N + CO2
(R7)
-C-N-N f -Cf + N2
(R8)
-C-O f CO +-Cf
(R9)
-C-O + -C-O f 2-Cf + CO2
(R10)
-C-O + CO f -Cf + CO2
(R11)
Reactions R1 and R2 are proposed for the steps involved in the adsorption of NO on the carbon surface, instead of the frequently written: 2-Cf + NO f -C-N + -C-O. Only reaction R1 is written as truly reversible. Reactions R3, R4, and R8 are based on the elegant work of Chambrion and co-workers.49-51 From isotopic labeling experiments they found that there is a direct reaction between NO in the gas phase and surface -C-N species to form N2. We suggest that a surface species of the form -C-N-NO is an intermediate in this reaction which may have a significant lifetime at least at low to intermediate temperature, and its decomposition in reaction R4 may limit the rate of NO destruction. Once the species -C-N-N is formed it probably decomposes rather quickly in reaction R8. The -C-N-N species may also be formed through reaction between two -C-N species in reaction R5, but this is a slow reaction, as shown by Chambrion et al.49 It is possible that the -C-N-NO species can desorb directly as N2O, but this appears to be a slow step, since normally only little N2O is formed in NO reduction, at least when O2 is not present in the gas phase. However, Miettinen and Abul-Milh52 found that by gasification of char in very high NO concentrations of 5-50 vol %, N2O yields (based on the nitrogen in char) of 190-430% were obtained. Although they attributed the formation of N2O to a homogeneous mechanism it appears that the proposed heterogeneous mechanism may explain the formation of N2O as well. At the very high NO concen(49) Chambrion, P.; Orikasa, H.; Suzuki, T.; Kyotani, T.; Tomita, A. Fuel 1997, 76, 493-498. (50) Chambrion, P.; Kyotani, T.; Tomita, A. Energy Fuels 1998, 12, 416-421. (51) Chambrion, P.; Kyotani, T.; Tomita, A. Proc. Combust. Inst. 1998, 3053-3059. (52) Miettinen, H.; Abul-Milh, M. Energy Fuels 1996, 10, 421-424.
trations the surface would be mostly covered by the -C-N-NO species with a high possibility of desorbing as N2O. The enhancing effect of CO on the rate of NO reduction is to react with the oxygen atom in the surface species -C-NO and -C-N-NO in reactions R6 and R7, respectively, and not reaction R11 as proposed by Chan et al.41 A reaction similar to reaction R6 between adsorbed NO (with N attached to the surface) and CO in the reduction of NO by CO over calcined limestone was proposed by Dam-Johansen et al.53 This reaction together with reaction R7 explains why there is a fast response to the addition of CO on the reaction rate and why the CO2 concentration immediately increases: CO generates the -C-N2 compound in reaction R7 which decomposes to N2 in reaction R8 releasing a new site. Furthermore, the mechanism explains why the effect of CO diminishes with increasing temperature as often reported. Since the carbon surface becomes more reactive at high temperature the transfer of oxygen to a free carbon site in reactions R2 and R4 becomes fast and CO is not needed to react with the oxygen in the -CNO and -C-N-NO species. Finally, the mechanism explains why the effect of CO depends on the type of carbon. Carbons with a high reactivity participate more easily in the oxygen transfer reactions R2 and R4. Reactions R9 and R10 are the well-known gasification reactions which consume carbon. It is assumed that a new site appears when a carbon atom is gasified. Reaction R11 together with reactions R6 and R7 explain why the consumption of carbon becomes much slower in the presence of CO. It should be mentioned that the recent work of Sibraa et al.54 does not support reaction R11 in the gasification of a highly pure carbon with oxygen and it may possibly be left out of the mechanism. In the present experiments the positive effect of CO was found to decrease over time which the proposed mechanism does not explain. Further work is needed to clarify that observation. NO Reduction in the Presence of Inorganic Catalysts. According to Illa´n-Go´mez and co-workers55 the catalyzed reaction proceeds through dissociative adsorption of NO on a catalyst site, *f :
NO + 2*f f *-N + *-O
(R12)
Formation of N2 follows from reaction between two nitrogen surface species:
2 *-N f N2 + 2-*f
(R13)
Illa´n-Go´mez and co-workers55 suggested that potassium may exist in a reduced state as KxOy and in an oxidized state as KxOy+1. The oxidized state is inactive toward NO adsorption and in order to avoid deactivation of the inorganic catalytic sites, oxygen must be transferred to a nearby carbon site capable of accepting the oxygen:
*-O + -Cf f -C-O + *f
(R14)
(53) Dam-Johansen, K.; Hansen, P. F. B.; Rasmussen, S. Appl. Catal. 1995, 5, 283-304. (54) Sibraa, A.; Newbury, T.; Haynes, B. S. Combust. Flame 2000, 120, 515-525. (55) Illa´n-Go´mez, M. J.; Linares-Solano, A.; Radovic, L. R.; SalinasMartı´nez de Lecea, C. Energy Fuels 1996, 10, 158-168.
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The -C-O species decomposes according to reactions R9-R11. Reaction R14 implies an intimate contact between the catalytic sites and the carbon sites. A slightly different mechanism was suggested by Zevenhoven and Hupa8 in which NO adsorb at a catalyst site in close relation to a carbon site:
NO + -Cf + *f f *-N + -C-O
(R15)
The formation of N2 still follows reaction R13. The two mechanisms do not exclude each other. The catalytic effect is attributed to higher rates of NO adsorption on catalytic sites in reactions R12 and R15 than in the uncatalyzed reaction. The rate-limiting step may be the transfer of oxygen from the catalyst to the carbon surface. In the presence of CO, reduction of the catalyst probably takes place according to
*-O + CO f *f + CO2
(R16)
The mechanisms discussed above do not take the effect of O2 into account. Several investigators have found that the presence of O2 enhances the rate of NO reduction.51,56,57 In our own laboratory we have found58 for a bituminous coal char that the enhancing effect of O2 could not be explained by the formation of CO or increased char particle temperatures due to combustion. At present we tend to believe that the effect of O2 is to generate active sites by opening up the basal planes. (56) Suzuki, T.; Kyotani, T.; Tomita, A. Ind. Eng. Chem. Res. 1994, 33, 2840-2845. (57) Yang, J.; Mestl, G.; Herein, D.; Schlo¨gl, R.; Find, J. Carbon 2000, 38, 729-740. (58) Ravn, N. Reduction of NOx over char in industrial processes. M. Sc. thesis (in Danish), Department of Chemical Engineering, Technical University of Denmark, 1999.
This is in line with the explanation proposed by IllanGomez et al.55 Conclusions The reduction of NO over char from raw and washed wheat straw has been investigated in the temperature range 600-900 °C with and without CO present. The washing process removed the majority of the catalytic inorganic species, mainly potassium, from the straw. The rate of NO reduction for the raw straw char was up to 2.5 times larger than the rate over the char from washed straw, indicating a catalytic effect of potassium. Both chars however, were very active in NO reduction compared to typical coal chars. At temperatures above about 850 °C significant release and/or transformation into inactive species of the catalysts in the char from raw straw took place which narrowed the difference in reactivity between the two chars. The influence of the presence of CO was also investigated. Upon addition of CO there was an immediate increase in the rate for NO reduction by up to a factor four at 5% CO. However for both chars the effect of CO decreased over a time scale of several hours. There is presently no explanation for this behavior. Finally, a reaction mechanism for the reduction of NO over char is suggested which includes the effect of CO on NO reduction over char. Acknowledgment. This work 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 JutlandsFunen Electricity Consortium), Elkraft (the Zealand Electricity Consortium), the Danish and Nordic Energy Research Programs, the European Union and the Danish Technical Research Council. EF000223A
