Energy & Fuels 1998, 12, 945-948
945
Apparent Activation Energies for the Reduction of NO by CO and H2 over Calcined Limestone and CaO Surfaces Filip Acke* Department of Inorganic Chemistry, Chalmers University of Technology and Go¨ teborg University, S-412 96 Go¨ teborg, Sweden
Dan Stro¨mberg Department of Applied Environmental Science, Go¨ teborg University, Medicinaregatan 20 A, S-413 90 Go¨ teborg, Sweden Received February 5, 1998
Apparent activation energies for the reduction of NO by CO and H2 are determined for calcined limestone and CaO surfaces. For calcined limestone, the importance of particle size on the apparent activation energy is shown. For small particle sizes, activation energies of about 20 kcal/mol are observed for both CO and H2 as the reducing agent. For coarse particles, an apparent activation energy of 32 kcal/mol is found for the reduction of NO by CO. For H2 as the reducing agent, a low-temperature value of 35 kcal/mol (550-650 °C) and a high-temperature value of 16 kcal/mol (650-800 °C) are observed. These values are compared to the apparent activation energies for the same reactions determined over three different CaO surfaces. The differences seen are ascribed to the presence of catalytically active second phases in the calcined limestone.
I. Introduction Fluidized bed combustion gives low emissions of nitrogen oxides compared to other combustion techniques. This is due to several factors: a low combustion temperature (800-900 °C), reducing zones, and the presence of numerous catalytic surfaces. Calcined limestone,1-5 sulfated limestone,4,7 CaO,1-4,6 MgO,6 calcined dolomite,6 CaSO4,4,8 quartz sand,8-10 ash,9 and carbon1,3,4,9 surfaces are all catalytically active toward the reduction of NO by CO. Lime and dolomite surfaces are also active in the conversion of fuel nitrogen to NO.11-13 These are added to the bed to lower SO2 * Corresponding author. Tel: +46 (0)31 772 28 86. Fax: +46 (0)31 772 28 53. E-mail:
[email protected]. (1) Moritomi, H.; Suzuki, Y.; Kido, N.; Ogisu, Y. Proceedings of the 11th International Conference on Fluidized Bed Combustion, Montreal, 1991, p 1005. (2) Tsujimura, M.; Furusawa, T.; Kunii, D. J. Chem. Eng. Jpn. 1983, 16, 132. (3) Chan, L. K.; Sarofim, A. F.; Beer, J. M. Comb. Flame 1983, 52, 37. (4) Furusawa, T.; Koyama, M.; Tsujimura, M. Fuel 1985, 64, 413. (5) Dam-Johansen, K.; A° mand, L. E.; Leckner, B. Fuel 1993, 72, 565. (6) Olanders, B.; Stro¨mberg, D. Energy Fuels 1995, 9, 680. (7) Hansen, P. F. B.; Dam-Johansen, K.; Johnsson, J. E.; Hulgaard, T. Chem. Eng. Sci. 1992, 47, 2419. (8) Allen, D.; Hayhurst, A. N. Conference on Fluidized Bed Combustion, London, December 1991, p 221. (9) Johnsson, J. E.; Dam-Johansen, K. Proceedings of the 11th International Conference on Fluidized Bed Combustion, Montreal 1991, p 1389. (10) Wittler, W.; Schu¨tte, K.; Rotzoll, G.; Schu¨gerl, K. Fuel 1988, 67, 438. (11) Jensen, A.; Johnsson, J. E.; Dam-Johansen, K. Proceedings of the 12th International Conference on Fluidized Bed Combustion, San Diego, CA, 1993, p 447.
emissions from the oxidation of fuel sulfur. The addition of limestone and dolomite to a fluidized bed produces both calcined and sulfated limestone and dolomite in the combustor. This article investigates the catalytic activity of calcined limestone and CaO surfaces toward the reduction of NO, and apparent activation energies are determined. Effectiveness factors are calculated for each experiment. Apparent activation energies for NO reduction by CO or H2 are found to depend on particle size for the calcined limestone. However, no significant difference in apparent activation energies was observed for the CaO substrates investigated. The difference between calcined limestone and CaO is attributed to catalytically active second phases in the former. II. Experimental Section A fixed bed reactor connected to a photo acoustic Fourier transform infrared gas analyzer (Bru¨el and Kjaer) was used. The quartz reactor (i.d. 22 mm, length 480 mm) had an asymmetric construction to avoid heating the upper metal fitting and the vacuum tight Viton O-ring. The temperature was measured with a K-type thermocouple positioned 5 mm under the sintered quartz filter and 4 mm under the exit of the reactor. The positioning of the thermocouple in this way avoids any interference of the results due to its catalytic activity. (12) de Soete, G. G.; Natsoll, W. Report IFP (Institute Francais du Petrole), No. 39362, 1991. (13) Gavin, D. G.; Dorrington, M. A. Fuel 1993, 72, 381.
S0887-0624(98)00024-3 CCC: $15.00 © 1998 American Chemical Society Published on Web 08/08/1998
946 Energy & Fuels, Vol. 12, No. 5, 1998
Acke and Stro¨ mberg
Table 1. (a) Bed Compositions as Well as (b) Gas Mixtures and Corresponding Flow Rates Used To Determine Kinetical Parameters (a) Bed Compositions
calcined limestone (Ignaberga) calcined Ca acetate
active material (g)
quartz sand (g)
0.060 0.500 0.500 0.500 0.500 0.500 0.500 0.500 0.500 0.500 0.500
1.000 10.000 4.000 6.000 10.000 4.000 6.000 10.000 4.000 6.000 10.000
fine coarse
untreated CaO Na promoted CaO
(b) Gas Mixtures and Corresponding Flow Rates
NO (ppm)
gas mixture reducing agent (H2 or CO) (ppm)
flow rate (ml/min)
862 862 844 635
918 918 424 890
693 433 525 525
The bed materials investigated consisted of a mixture of active material and quartz sand (quartz fine granular, pro analysi from Merck). Table 1a shows bed compositions. The quartz sand was added both to reduce the pressure drop and to control the residence time. The active materials tested were a calcined limestone (Ignaberga), a calcined calcium acetate (Fisher Scientific), an untreated CaO (Fisher Scientific), and a Na promoted CaO. The latter was prepared by dissolving calcium acetate and Na2CO3 in double-distilled water. By evaporation of the water, a promoted calcium acetate was obtained which was then calcined. All calcination was done in an oxygen flow at 950 °C for 2.5 h. Table 1b gives the investigated gas mixtures and flow rates, all flows referring to ambient temperature and pressure. The gases used were 5000 ppm H2, 5000 ppm CO, 5000 ppm CO2, and 5000 ppm NO, all in Ar. The flows were regulated with mass flow controllers of Brookes, type 5850E. Table 2 gives the chemical compositions of the active materials examined. Two different particle sizes were investigated for the calcined limestone, i.e., one coarse and one fine. Table 3a gives the particle size distribution for the calcined limestone and Table 3b the mean particle diameter of the three CaO materials investigated. The results are free from interference effects of the metal parts in the reactor. This was checked by testing a bed consisting of quartz sand only.
the feed concentration of NO, and τ for the mean residence time defined as the ratio of the void volume of the bed and the volumetric flow rate. At a given temperature, the rate constant can be evaluated from the slope of a plot of the absolute value of ln((R - XNO)/ R(1 - XNO)) versus (R - 1)CNOfτ. In Figure 1, such a plot is shown for the NO reduction by H2 over CaO (Fisher Scientific) at 600, 700, and 800 °C. The straight lines, observed for all temperatures, confirm that both reactants show first-order behavior. Similar plots were obtained for all materials. Applying the Arrhenius equation then gives the apparent activation energies and the preexponential factor of the reactions investigated. Arrhenius plots are shown for the reduction of NO by H2 for all four materials investigated and for the reduction by CO for the calcined limestone (Figure 2). Table 4 summarizes the apparent activation energies, included is also the 95% confidence interval. The effectiveness factors, η, when taking pore and film diffusion into account were calculated using Thiele’s modulus and Yoshida’s number, respectively. Values above 0.998 were found for all trials, proving the determined apparent activation energies to be unaffected by mass-transfer limitations. For the calcined limestone, the apparent activation energies clearly depend on particle size distribution. Different activation energies in different temperature intervals were found for the reduction of NO by H2 over the coarse particles, i.e., 16 and 35 kcal/mol. However, for the reduction of NO by CO, only one apparent activation energy was found for the whole temperature range investigated, i.e., 32 kcal/mol. Grinding the Ignaberga limestone to smaller particle sizes resulted in a single apparent activation energy for the temperature range investigated, about 21 kcal/mol for both examined reactions. For H2 as the reducing agent, the apparent activation energies presented here can be connected to the actual reaction
NO + H2 S 1/2N2 + H2O
k ) ln[(R - XNO)/R(1 - XNO)]/[(R - 1)CNOfτ] (2)
since the effectiveness factors, η, were systematically higher than 0.998. For CO as the reducing agent, the effect of the CO2 formation on the apparent activation energy must be accounted for. Poisoning by CO2 has been shown for the oxidative coupling of methane over alkaline earth oxides14,15 and can be expected for the reduction of NO by CO over calcined Ignaberga limestone. The poisoning effect was investigated by addition of CO2 to the feed reaction mixture, and the apparent activation energy was remeasured for the fine particle size. The same temperature interval and gas compositions were investigated, this time with the addition of CO2, 500 and 1000 ppm, respectively. An increase in apparent activation energy with the amount of CO2 present in the reactor is observed in Figure 3. Extrapolating these results to zero CO2 gives a corrected value of 19 kcal/mol for the fine particles. The presence of CO2 causes an increased blocking of sites with decreas-
for the rate constant k, where R is defined as the ratio of the inlet concentration of CO or H2 to that of NO, XNO stands for the fractional conversion of NO, CNOf for
(14) Xu, M.; Shi, C.; Yang, X.; Rosynek, M. P.; Lunsford, J. H. J. Phys. Chem. 1992, 96, 6395. (15) Coulter, K.; Goodman, D. W. Catal. Lett. 1993, 20, 169.
III. Results and Discussion Kinetic studies of the reduction of NO by CO or H2 have been previously carried out for calcined limestone2,4 and calcined dolomite6 surfaces. All these materials showed an equimolecular consumption of the reactants and first-order behavior in NO and CO (or NO and H2), i.e.
-d[NO]/dt ) k[NO] [CO]
(1)
Solving differential eq 1 results in
Apparent Activation Energies for the Reduction of NO
Energy & Fuels, Vol. 12, No. 5, 1998 947
Table 2. (a) Chemical Composition of the Ignaberga Limestone as Well as (b) the Na and Fe Content for the Calcined Calcium Acetate, Untreated CaO, and the Na Promoted CaO (a) Chemical Composition Ignaberga limestone
CaCO3
MgCO3
91.1
1.0
SiO2
Fe
Na
K
Al
Zn
Ba
Sr
7.3
0.39
4.5
1.3
