Energy & Fuels 2004, 18, 1909-1920
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Catalytic Decomposition of N2O over the Bed Material from Circulating Fluidized-Bed (CFB) Boilers Burning Biomass Fuels and Wastes Vesna Barisˇic´,* Ahmad Kalantar Neyestanaki, Fredrik Klingstedt, Pia Kilpinen, Kari Era¨nen, and Mikko Hupa Åbo Akademi Process Chemistry Centre, Biskopsgatan 8, FIN-20500 Åbo, Finland Received April 14, 2004. Revised Manuscript Received August 24, 2004
In this paper, the catalytic decomposition of N2O was studied over the bed materials sampled from the bottom bed of two industrial circulating fluidized-bed (CFB) boilers (a 12 MWth boiler and a 550 MWth boiler) burning biomass fuels and wastes, either alone or as a mixture. The catalytic activity of the bed materials were compared by measuring the conversion of N2O in a laboratory fixed-bed reactor in the temperature range of 600-910 °C. The composition and morphology of the bed material were characterized using X-ray fluorescence and scanning electron microscopy coupled with energy-dispersive X-ray analysis. The activity of bed material was observed to be affected by the sample heterogeneity and tendency to deactivate when exposed to fluidized-bed temperatures. The activity of the bed materials was also influenced by the type of fuel, for which the kinetic expressions are derived. For the bed-material samples from the 12 MWth CFB boiler, it was observed that the higher the amount of the catalytically active oxides (CaO + MgO + Fe2O3 + Al2O3), the higher the activity of the sample toward N2O decomposition. The elemental composition of the surface of the bed material particles changed according to the composition of the ash from the parent fuel. On the other hand, such correlations were not observed for the samples from the 550 MWth CFB boiler.
Introduction Fluidized-bed combustion (FBC) is a well-proven technology for burning a variety of fuels, because it is efficient and reliable and emissions of SO2 and NOx are low. However, a disadvantage of FBC is that the emissions of nitrous oxide (N2O) are much higher, when compared to other combustion technologies: uncontrolled N2O emissions from fluidized-bed boilers that burn coal are in the range of 20-300 ppm, whereas the emissions from pulverized coal boilers are ∼10 ppm.1 Nitrous oxide contributes to the depletion of the stratospheric ozone layer, and its global warming potential over a time scale of 100 years is 296 times higher than that of CO2.2 Emissions of N2O from FBC are the result of the simultaneous formation and destruction of N2O. Focusing here only on the destruction, a complex reaction mechanism is involved with both homogeneous gasphase reactions and heterogeneous gas-solid reactions.3 * Author to whom correspondence should be addressed. Phone: +358 2 215 3513. Fax: +358 2 215 4962. E-mail address: vbarisic@ abo.fi. (1) Bonn, B.; Pelz, G.; Baumann, H. Formation and Decomposition of N2O in Fluidized Bed Boilers. Fuel 1995, 74, 165-171. (2) Climate Change 2001: The Scientific Basis. In Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change; Houghton, J. T., Ding, Y., Griggs, D. J., Noguer, M., van der Linden, P. J.; Dai, X., Maskell, K., Johnson, C. A., Eds.; Cambridge University Press: Cambridge, New York, 2001; p 388. (Available via the Internet at http://www.grida.no/climate/ ipcc_tar/wg1/. ISBN 0521 01495 6.) (3) Johnsson, J. E. Formation and Reduction of Nitrogen Oxides in Fluidized Bed Combustion. Fuel 1994, 73, 1398-1415.
Based on the N2O injection tests at three locations along the combustion chamber (bottom bed, splash zone, and riser), performed in a 12 MWth circulating fluidized-bed boiler (CFB) that was burning bituminous coal without limestone addition, Johnsson et al. estimated that heterogeneous reactions contribute to the total N2O destruction by ∼60%.4 They occur mainly in the bottom portion of the combustion chamber, where the concentration of solids is high. The most important heterogeneous reactions for N2O destruction are reduction by char, reduction by CO on char or bed material, and catalytic decomposition over solids that compose a bed material in a fluidized-bed combustor. Char and ash from fuel, and quartz sand, are usual components of the bed material, and their catalytic activities for the decomposition of N2O are different. Char from coal is a very active catalyst for N2O decomposition and reduction;4-8 its activity increases (4) Johnsson, J. E.; Åmand, L.-E.; Dam-Johansen, K.; Leckner, B. Modeling N2O Reduction and Decomposition in a Circulating Fluidized Bed Boiler. Energy Fuels 1996, 10, 970-979. (5) de Soete, G. G. Heterogeneous N2O and NO Formation from Bound Nitrogen Atoms during Coal Char Combustion. Proc. Symp. (Int.) Combust. 1990, 23rd, 1257-1264. (6) Pels, J. R. Nitrous Oxide in Coal Combustion. Ph.D. Thesis, Technical University of Delft, The Netherlands, 1995. (7) 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. (8) Johnsson, J. E.; Jensen, A.; Nielsen, J. S. Kinetics of Heterogeneous NO and N2O Reduction at FBC Conditions. In Proceedings of the 15th International Conference on Fluidized Bed Combustion; ASME: New York, 1999 (CD-ROM).
10.1021/ef049909j CCC: $27.50 © 2004 American Chemical Society Published on Web 11/02/2004
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as the rank of the parent coal increases. Ash also catalyzes the decomposition of N2O; however, it has been reported that the activity of coal ash is significantly smaller than the activity of the char.3,9 Similarly to ash, the bed material from bituminous coal combustion has a lower activity for N2O decomposition than the char, when measured on a mass basis.1,4 Quartz sand has a very low activity.1,10 When comparing the catalytic activities of solids from a fluidized bed, it is useful to distinguish between the terms ash and bed material. Ash refers to the inorganic solid residue left after burning off a fuel, and it is usually prepared in a laboratory oven from the parent fuel and mixed with quartz sand. Bed material refers to the material sampled from a fluidized-bed combustor; it is also a mixture of ash and sand, but, in this case, the ash has a different composition than the ash prepared in a laboratory oven, and some of the compounds from ash are deposited on sand particles.11 Limestone (CaCO3) added for sulfur capture is another important component of the bed material for the decomposition of N2O. Under fluidized-bed conditions, limestone undergoes calcinations, followed by sulfatation (under oxidizing conditions) or sulfidation (under reducing conditions). The activity of partially calcined/ sulfated limestone for N2O decomposition is dependent on the type of limestone and the degree of conversion. Calcined limestone (CaO) and sulfided limestone (CaS) are very active catalysts,1,6,10,12-15 whereas, for uncalcined or recarbonated limestone (CaCO3), a negligible activity has been reported.14 Sulfated limestone (CaSO4) is a less-active catalyst for N2O decomposition than CaO and CaS.12,14 Many studies have shown that limestone addition reduces emissions of N2O,16-18 which is partially due to an increase of the activity of bed material for N2O decomposition. The activity of bed material when limestone is added is still lower than the activity of char, when compared on a mass basis; however, because the amount of bed material is ∼50 times the (9) Ko¨psel, R. F. W.; Halang, S. Catalytic Influence of Ash Elements on NOx Formation in Char Combustion under Fluidized Bed Conditions. Fuel 1997, 76, 345-351. (10) 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. (11) Zevenhoven-Onderwater, M. Ash-Forming Matter in Biomass Fuels. Ph.D. Thesis, Åbo Akademi Process Chemistry Group, Åbo, Finland, 2001. (12) Hansen, P. F. B.; Dam-Johansen, K.; Johnsson, J. E.; Hulgaard, T. Catalytic Reduction of NO and N2O on Limestone during Sulfur Capture under Fluidized Bed Combustion Conditions. Chem. Eng. Sci. 1992, 47 (9-11), 2419-2424. (13) Shimizu, T.; Inagaki, M. Decomposition of N2O over Limestone under Fluidized Bed Combustion Conditions. Energy Fuels 1993, 7, 648-654. (14) 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. (15) Sasaoka, E.; Sada, N.; Hara, K.; Uddin, A.; Sakata, Y. Catalytic Activity of Lime for N2O Decomposition under Coal Combustion Conditions. Ind. Eng. Chem. Res. 1999, 38, 1335-1340. (16) Åmand, L.-E.; Leckner, B. Formation of N2O in a Circulating Fluidized-Bed Combustor. Energy Fuels 1993, 7, 1097-1107. (17) Gavin, D. G.; Dorrington, M. A. Factors in the Conversion of Fuel Nitrogen to Nitric and Nitrous Oxides during Fluidized Bed Combustion. Fuel 1993, 72, 381-388. (18) Armesto, L.; Boerrigter, H.; Bahillo, A.; Otero, J. N2O Emissions from Fluidised Bed Combustion. The Effect of Fuel Characteristics and Operating Conditions. Fuel 2003, 82, 1845-1850.
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amount of char, both bed material and char are important solids for catalytic decomposition of N2O under inert and oxidizing conditions.8 Under reducing conditions, bed material is the most important catalyst for the reduction of N2O by CO.8 Besides quartz (SiO2), the main compounds of the bed material are oxides and silicates of aluminum, iron, calcium, and magnesium, as well as sulfates of calcium and magnesium. A few studies have examined the decomposition of N2O over pure substances under conditions relevant for FBC. Miettinen studied the N2O decomposition over various oxides (SiO2, CaO, R-Al2O3, MgO, Fe2O3, Fe3O4) and sulfates (CaSO4, MgSO4) in a laboratory fixed-bed reactor in the temperature range of 600-900 °C.19 It was found that R-Al2O3, MgSO4, and SiO2 had no significant activity for N2O decomposition. In that work, CaSO4 had a small activity only in the temperature range of 700-800 °C. Magnetite (Fe3O4) had the greatest decomposing effect on N2O; however, it was consumed during the experiment and, therefore, cannot be regarded as a catalyst. The most active compounds in decomposing N2O catalytically were CaO, Fe2O3, and MgO. Shen et al. reported somewhat contradictory results.20 Experiments were conducted in a laboratory-scale bubbling fluidized bed, and the decomposition of N2O over various compounds was as follows: limestone (or CaO) > Fe2O3 > CaSO4 > Al2O3 > SiO2 > MgSO4 > MgO. Hayhurst and Lawrence studied the reaction between iron, or its oxides, and the oxides of nitrogen (NO and N2O) in an electrically heated bed of sand, fluidized by nitrogen.21 Opposite to that reported by other authors, the oxides of iron were determined to be relatively unreactive in the decomposition of N2O. Although studies with pure substances give important insight into the activity of bed material, note that the elements are not necessarily present in the form of pure substances, but rather combined as complex mixtures of aluminosilicates. Moreover, the correlation of catalytic activity and composition of bed material is rather difficult, because of the uncertainty in attributing the results obtained with the elemental analysis to the surface availability of the active element or compound. Under fluidized-bed conditions, N2O originates from the nitrogen present in the fuel. Most of the studies on catalytic decomposition of N2O in a fluidized bed were conducted when coal was burned. During recent years, because of growing concerns about global warming, replacement or co-combustion of coal with biomass fuels has become increasingly important. Also, the combustion of various wastes has become an option for solving the problem of landfills as well as for producing electricity. Despite advantages such as being a renewable source of sustainable energy, the utilization of biomass fuels and, especially, wastes in a fluidized bed might (19) 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. (20) Shen, B. X.; Mi, T.; Liu, D. C.; Feng, B.; Yao, Q.; Winter, F. N2O Emission under Fluidized Bed Combustion Conditions. Fuel Process. Technol. 2003, 84, 13-21. (21) Hayhurst, A. N.; Lawrence, A. D. The Reduction of the Nitrogen Oxides NO and N2O to Molecular Nitrogen in the Presence of Iron, Its Oxides, and Carbon Monoxide in a Hot Fluidized Bed. Combust. Flame 1997, 110, 351-365.
N2O Decomposition over Bed Material from CFB Boilers
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Figure 1. (a) Experimental setup, and (b) laboratory quartz reactor modified after the design at the Technical University of Denmark.8
lead to higher emissions of N2O than it will be acceptable by the future emission regulations. Several studies indicate that the conversion of nitrogen oxides in a fluidized bed is influenced by the fuel type and the catalytic effect of the fuel ash.3,22,23 Ash composition of biomass fuels and wastes vary significantly.24 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 a reduced nitrogen content. Although 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 are limited data available in the literature regarding the catalytic activity of solids present in a fluidized bed during the combustion of biomass fuels and wastes. The purpose of this work was to shed more light on the catalytic activity of bed materials from the combustion of biomass fuels and wastes toward N2O decomposition. In particular, the purpose was to examine the bed material regarding the heterogeneity and sensitivity toward changes in temperature and gas atmosphere. The decomposition of N2O was studied over the bed materials that were sampled from the bottom bed of two industrial CFB boilers that were burning different biomass fuels and wastes, to examine the influence of fuel type. The kinetic expressions were derived for the (22) Jensen, A. Nitrogen Chemistry in Fluidized Bed Combustion of Coal. Ph.D. Thesis, Department of Chemical Engineering, Technical University of Denmark, 1996. (23) Lo¨ffler, G.; Wargadalam, V. J.; Winter, F. Catalytic Effect of Biomass Ash on CO, CH4 and HCN Oxidation under Fluidized Bed Combustion Conditions. Fuel 2002, 81, 711-717. (24) Phyllis: The Composition of Biomass and Waste; Energy Research Centre of the Netherlands (ECN), Unit Biomass: Petten, The Netherlands (http://www.ecn.nl/phyllis/).
catalytic activities of the bottom bed materials from the combustion of wood pellets, mixtures of wood pellets and two types of municipal sludge, peat, bark, a mixture of bark and peat, and coal. With the intention to connect the catalytic activity of the samples to the fuel composition, the bed materials were characterized in terms of elemental composition (X-ray fluorescence (XRF)), total surface area (N2-physisorption), and morphology and elemental composition of the particle’s surfaces (scanning electron microscopy combined with energy-dispersive X-ray (SEM/EDX) analysis). This paper is the first in a series of papers, the intent of which is to provide kinetic data that could be used as input in the detailed modeling of nitrogen oxide emissions in CFB boilers, where the catalytic N2O decomposition over the bed material is one of the reactions in the complex chemistry of CFB combustion. Experimental Section Experimental Setup. The experimental setup for the investigation of the catalytic activity of solid samples consists of a fixed-bed quartz reactor coupled with heating, feeding, and analyzing devices (see Figure 1a). The reactor is placed in an electrically heated oven. It has a removable bottom and an inner section that has been fitted with a porous quartz plate, which is used to support the solid sample (see Figure 1b). The reaction temperature is measured with a thermocouple (K-type) that is inserted into the sample, passing through a hole in the quartz plate. The thermocouple is covered with a quartz tube, to avoid contact with reactant gases. By changing the position of the thermocouple, this reactor is a modification of a reactor designed at the Technical University of Denmark.8 To minimize gas-phase reactions and reactions catalyzed by the reactor surface, gases are introduced separately through two inlets and mixed just above the sample. Helium (99.996%) is used as a carrier gas, and its flow is
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Barisˇ ic´ et al. Table 1. Fuel Data
12 MWth CFB boiler fuela
sludge A
sludge B
bark and peat
bark
peat
moistureb (wt %, ar) dry matterc (wt %, ar) ash (wt %, dry) volatiles (wt %, dry)
0 99.6 0.4 84.3
0 98.0 46.8 52.4
0 97.2 42.1 59.5
50.0 44.0 6.5 71.1
7.8 88.9 11.5 33.9
52.0 42.6 2.8 76.8
41.0 51.7 6.9 70.0
carbon (wt %, dry) hydrogen (wt %, dry) nitrogen (wt %, dry) oxygen (wt %, dry)
51.1 6.0
