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Kinetics of the Catalytic Decomposition of N2O over Bed Materials from Industrial Circulating Fluidized-Bed Boilers Burning Biomass Fuels and Wastes Vesna Barisˇic´,* Fredrik Klingstedt, Pia Kilpinen, and Mikko Hupa Åbo Akademi Process Chemistry Centre, Biskopsgatan 8, FIN-20500 Åbo, Finland Received April 19, 2005. Revised Manuscript Received September 12, 2005
This paper reports the kinetic expressions for the catalytic activities toward N2O decomposition on three particle size fractions of the bottom bed materials, sampled from two industrial circulating fluidized-bed (CFB) boilers, a 12 MWth and a 550 MWth, burning biomass fuels and wastes. In addition, the kinetic expressions are derived for the activity of bed materials in the presence of 15 vol % of water vapor. The catalytic activity of the bed materials was determined by measuring the conversion of N2O in a laboratory fixed bed quartz reactor in the temperature range from 600 to 910 °C. The composition and morphology of the bed material were characterized using N2-physisorption, an X-ray fluorescence (XRF), and a scanning electron microscope coupled with an energy-dispersive X-ray analyzer (SEM/EDXA). For bed-material samples from the 12 MWth CFB boiler, it was found that the activity of different size fractions decreased when the particle size increased from 125-297 to 355-500 µm. The effect was more pronounced for the sample with highly reactive ash originating from municipal sludge. For bed-material samples from the 550 MWth CFB boiler, activities of different size fractions were mostly dependent on calcined limestone concentration. The presence of water vapor (0-30 vol %) was shown to inhibit the activity of bed materials considerably. Decrease in the activity of bed materials was significantly related to an increase of water to 15 vol %, but from 15 to 30 vol % there was only a minor further decrease.
Introduction Modeling emissions of nitrogen oxides from CFB boilers requires, among others, knowledge about kinetics of the most important catalytic nitrogen oxide reactions. One of the catalytic reactions that has extensively been studied in relation to relatively high emission of nitrous oxide is the decomposition of nitrous oxide over solids that compose a bed material in a combustion chamber. Most of these studies were focused on coal combustion, with or without limestone addition, showing that individual components of the bed material have different catalytic activities. Broadly, char, calcined, and sulfided limestone show higher catalytic activity toward N2O decomposition than ash, limestone, and sulfated limestone, while the activity of quartz sand is low or insignificant.1-6 * Corresponding author. Phone: +358 2 215 3513. Fax: +358 2 215 4962. E-mail:
[email protected]. (1) Iisa, K.; Salokoski, P.; Hupa, M. Heterogeneous Formation and Destruction of Nitrous Oxide under Fluidized Bed Combustion Conditions. In Proceedings of the 11th International Conference on Fluidized Bed Combustion; Anthony, E. J., Ed.; ASME: New York, 1991; pp 1027-1033. (2) Johnsson, J. E.; Dam-Johansen, K. Reduction of N2O over Char and Bed Material from CFBC. In Proceedings of the 13th International Conference on Fluidized Bed Combustion; Heinschel, K. J., Ed.; ASME: New York, 1995; pp 859-869. (3) Johnsson, J. E.; A° mand, L.-E.; Dam-Johansen, K.; Leckner, B. Modeling N2O Reduction and Decomposition in a Circulating Fluidized Bed Boiler. Energy Fuels 1996, 10, 970-979. (4) Johnsson, J. E. Formation and Reduction of Nitrogen Oxides in Fluidized Bed Combustion. Fuel 1994, 73, 1398-1415.
The reported activation energy values for the decomposition of N2O over bed material from coal/coke combustion without limestone addition are in the range of 177.9-184.6 kJ/mol,1-3 and for the bed material from coal combustion with limestone addition 57.0-169.6 kJ/ mol.5,6 The reported activation energy values for the decomposition of N2O over calcined limestone are in the range 59.0-165.4 kJ/mol.1,7,8 Miettinen has reported the activation energy value of 100.4 kJ/mol for the decomposition of N2O over CaO (pro analysi), and 138.1-144.3 kJ/mol over MgO (pro analysi).9 During boiler operation, the constituting particles of the bed material are going through continuous transformation. Besides the coating of sand particles by ash elements,10-13 composition of the bed material changes due to an increase in the amount of ash particles, (5) Bonn, B.; Pelz, G.; Baumann, H. Formation and Decomposition of N2O in Fluidized Bed Boilers. Fuel 1995, 74, 165-171. (6) Johnsson, J. E.; Jensen, A.; Nielsen, J. S. Kinetics of Heterogeneous NO and N2O Reduction at FBC Conditions. Proceedings of the 15th International Conference on Fluidized Bed Combustion; ASME: New York, 1999 (CD-ROM). (7) Johnsson, J. E.; Jensen, A.; Vaaben, R.; Dam-Johansen, K. Decomposition and Reduction of N2O over Limestone under FBC Conditions. In Proceedings of the 14th International Conference on Fluidized Bed Combustion; Preto, F. D. S., Ed.; ASME: New York, 1997; pp 953-964. (8) Shimizu, T.; Inagaki, M. Decomposition of N2O over Limestone under Fluidized Bed Combustion Conditions. Energy Fuels 1993, 7, 648-654. (9) Miettinen, H. Formation and Decomposition of Nitrous Oxide at Fluidized Bed Conditions (and included papers). Ph.D. Thesis, Department of Inorganic Chemistry, Chalmers University of Technology and Go¨teborg University, Sweden, 1995.
10.1021/ef050110x CCC: $30.25 © 2005 American Chemical Society Published on Web 10/05/2005
Kinetics of the Catalytic Decomposition of N2O in CFB
calcinations, sulfation or sulfidation of limestone, and addition of fresh sand. In CFB boilers, a part of the bed material is carried out with the flue gases and recirculated back to the combustion chamber. The activity of bed material as a catalyst for N2O destruction depends on many factors, such as type of fuel, particle size, hydrodynamic conditions, presence of oxidizing or reducing atmosphere, temperature, and presence of other gases in the reaction mixture. During recent years, replacement of coal with biomass and biomass-derived fuels has become increasingly important due to growing concerns about global warming. Also, the combustion of various wastes has become an option for solving the problem of landfills as well as for producing electricity. In contrast to the combustion of coals, during the combustion of plant-derived biomass, N2O emissions are usually lower. This is often attributed to differences in nitrogen functional groups and lower nitrogen content. Even though plant-derived biomass will not be a source of N2O emissions in the case of co-combustion with coal, the catalytic effect of their ash components can be important. However, there is limited data available in the literature regarding the kinetics of reactions catalyzed by the solids from fluidized bed during combustion of biomass fuels and wastes. Also, there is little information available about the influence of flue gases, for example, water vapor, on the kinetics of catalytic nitrogen oxide reactions. The work presented in this paper is a part of an extensive study on the catalytic activity of bed materials from combustion of biomass fuels and wastes toward N2O decomposition. Our previous work showed that the catalytic activity of the bed material was affected by fuel type.14 The finding was attributed to the change in the elemental composition of the coating layer of sand particles according to the composition of the ash from parent fuel.15 In this paper, the bed materials were further examined with the focus on the change in catalytic activity of bed material with particle size, and the presence of water vapor. In particular, the aim of the paper is to provide the kinetic data that could be used as input in detailed modeling of nitrogen oxide emissions in CFB boilers, where the catalytic N2O decomposition over bed material is one of the reactions in the complex chemistry of CFB combustion. The (10) Brus, E.; O ¨ hman, M.; Nordin, A.; Skrifvars, B.-J.; Backman, R. Bed Material Consumption in Biomass Fired Fluidized Bed Boilers due to Risk of Bed Agglomeration - Coating Formation and Possibilities for Regeneration. IFRF Combustion Journal 2003, ISSN 1562479X. (11) Vuthaluru, H. B.; Zhang, D.-k.; Linjewile, T. M. Behavior of inorganic constituents and ash characteristics during fluidised-bed combustion of several Australian low-rank coals. Fuel Process. Technol. 2000, 67, 165-176. (12) Tiainen, M.; Daavitsainen, J.; Laitinen, R. S. The Role of Amorphous Material in Ash on the Agglomeration Problems in FB Boilers. A Powder XRD and SEM-EDS Study. Energy Fuels 2002, 16, 871-877. (13) Nuutinen, L. H.; Tiainen, M. S.; Virtanen, M. E.; Enestam, S. H.; Laitinen, R. S. Coating Layers on Bed Particles during Biomass Fuel Combustion in Fluidized-Bed Boilers. Energy Fuels 2004, 18, 127-139. (14) Barisˇic´, V.; Kalantar Neyestanaki, A.; Klingstedt, F.; Kilpinen, P.; Hupa, M. Catalytic Decomposition of N2O over the Bed Material from Circulating Fluidized-Bed (CFB) Boilers Burning Biomass Fuels and Wastes. Energy Fuels 2004, 18, 1909-1920. (15) Barisˇic´, V.; Klingstedt, F.; Naydenov, A.; Stefanov, P.; Kilpinen, P.; Hupa, M. Catalytic Activity of Bed Materials from Industrial CFB Boilers for the Decomposition of N2O. Catal. Today 2005, 100, 337342.
Energy & Fuels, Vol. 19, No. 6, 2005 2341
kinetic expressions were derived for the catalytic activities of three particle size fractions of the bottom bed materials from two industrial CFB boilers (a 12 MWth and a 550 MWth) burning biomass fuels and wastes, with and without the presence of water vapor. With the intention to connect the catalytic activity of the particle size fractions of the selected bed-material samples to the fuel composition, the bed materials were characterized in terms of elemental composition (X-ray fluorescence), BET-surface area, pore specific volume, poresize distribution (N2-physisorption), morphology, and elemental composition of the particle’s surfaces (scanning electron microscope combined with an energydispersive X-ray analyzer). Experimental Section The solids tested for the catalytic activity were sampled from the bottom bed of two industrial-scale circulated fluidized bed (CFB) boilers while burning different fuels, or fuel mixtures.14 Three bed-material samples were collected from a 12 MWth CFB boiler at Chalmers University of Technology, Go¨teborg, Sweden: (1) Ch1, in which the fuel was wood pellets produced from pine and birch trees; (2) Ch2, in which the fuel was a mixture of the wood pellets and mechanically dewatered municipal sewage sludge A; and (3) Ch3, in which the fuel was a mixture of the wood pellets and predried municipal sewage sludge B. Details about the full-scale experiments are published by Åmand et al.,16 and the fuel data are given in Table 1. Four bed-material samples were collected from the second boiler, a 550 MWth CFB boiler at Oy Alholmens Kraft Ab in Pietarsaari, Finland: (1) A1, in which the fuel was a mixture of 25 wt % bark and 75 wt % peat; (2) A2, in which the fuel was coal; (3) A3, in which the fuel was bark; and (4) A4, in which the fuel was peat. The full-scale experiments were run within six sequential days, and limestone was added for sulfur capture together with the fuel in the samples A1, A2, and A4. Fuel data are given in Table 1. There were two main reasons for the choice of the bottom bed material for this study. First, several studies indicate that in the bottom bed heterogeneous reactions play an important role in the destruction of N2O contributing to the low N2O concentrations measured in the bottom part of the combustion chamber.3,17 Second, we found that the elemental composition of the bed-material samples from the 12 MWth CFB boiler did not change significantly with the height of the boiler (results for the elemental composition of samples from transport zone are not shown here), and thus we could avoid the technical difficulties connected with sampling the bed material from transport and splash zone in case of the 550 MWth CFB boiler. In addition, for the catalytic decomposition of N2O, Johnsson et al. reported that the activity of bed materials sampled at three locations along the combustion chamber was almost independent of the height in the combustion chamber.7 Nevertheless, and taking into account that highest concentrations of nitrogen oxides are measured above the bottom bed, the variation in catalytic activity of bed materials with height in the combustion chamber will be tested in the future. The sample of the bottom bed material, obtained while burning particular fuel, is a mixture of ash, sand, char, and partly calcined/sulfated limestone in case of the second boiler. The samples were sieved to a narrow size fractions before testing, and the particle sizes of 125-710, 125-297, and 355(16) A° mand, L.-E.; Leckner, B.; Hansson, L.; Norrlo¨w, O. CoCombustion of Municipal Sludge with Wood/Coal an CFB-Enrichment of Phosphorous and Cadmium in Ashes. Proceedings of the 17th International Conference on Fluidized Bed Combustion; ASME: New York, 2003 (CD-ROM). (17) A° mand, L.-E.; Leckner, B. Formation of N2O in a Circulating Fluidized-Bed Combustor. Energy Fuels 1993, 7, 1097-1107.
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Barisˇ ic´ et al. Table 1. Fuel Data14 a
CFB boiler:
12 MWth
550 MWth
fuel:
wood pellets
sludge A
sludge B
bark and peat
coal
bark
peat
moistureb (wt %, ar) dry matterc (wt %, ar) ash (wt %, dry) volatiles (wt %, dry) carbon (wt %, dry) hydrogen (wt %, dry) nitrogen (wt %, dry) oxygen (wt %, dry) SiO2 (wt % of ash) Al2O3 (wt % of ash) Fe2O3 (wt % of ash) CaO (wt % of ash) MgO (wt % of ash) P2O5 (wt % of ash) Na2O (wt % of ash) K2O (wt % of ash) SO3 (wt % of ash) Cl- (wt % of ash) CO2 (wt % of ash) sum
0 99.6 0.4 84.3 51.1 6.0
