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Energy & Fuels 2004, 18, 1442-1450
Experimental and Modeling Study of Biomass Reburning Eduardo Vilas,† Ulrik Skifter,† Anker Degn Jensen,† Christian Lo´pez,§ Jo¨rg Maier,§ and Peter Glarborg*,† Department of Chemical Engineering, Technical University of Denmark, DK-2800 Lyngby, Denmark, and Institute of Process Engineering and Power Plant Technology, University of Stuttgart, D-70569 Stuttgart, Germany Received February 2, 2004. Revised Manuscript Received May 28, 2004
An experimental and modeling study of reburning with biomass is presented. The experiments are conducted in an entrained flow reactor and in a pilot scale facility. The effects of reburn fuel (biomass char, wheat straw, and poplar wood) and process conditions (i.e., stoichiometry, temperature, particle size) are investigated. The model is a chemical reaction engineering type of model with a detailed reaction mechanism for the gas-phase chemistry, together with a simplified description of the mixing of flows, heating and devolatilization of particles, and gassolid reactions. The different subsets and the complete model are validated against experimental results from the present work, as well as from the literature. The model provides a good description of reburning with both gaseous and solid fuels. However, further experimental work is needed for a more complete validation of the model.
Introduction Reburning is an in-furnace technique for reducing nitrogen oxides (NOx) in large-scale utility boilers and other combustion facilities. In the reburning process, a secondary fuel is injected downstream from the main combustion zone, to establish a fuel-rich combustion region where the nitric oxide is reduced mainly to molecular nitrogen (N2) and other species such as ammonia (NH3) and hydrogen cyanide (HCN). To complete the process, burnout air is injected further downstream, to secure complete combustion of the unburned fuel; here, remaining NH3 and HCN are oxidized to N2 or NO. Reburning has been demonstrated as a practical NOx reduction method in several full-scale boilers (50-200 MW).1 Natural gas has been used as the reburning fuel in most tests, but coal also has been shown to yield high NOx reductions.2 Typical reduction efficiencies in these large-scale facilities are in the range of 50%-75%. Recently, there has been an increasing interest in biomass fuels, partly due to the reasonable cost level of biomass, in comparison to other renewable energy sources, and partly due to its advantage of reducing the carbon dioxide (CO2) and sulfur dioxide (SO2) emissions, compared to fossil fuels. An effective way of using biomass could be in the reburning process. Pilot-scale experiments using biomass as the reburning fuel in coalfired systems have obtained levels of NOx reduction in the range of 50%-75% (similar to those of coal and * Author to whom correspondence should be addressed. Telephone: 45 4525 2840. E-mail address:
[email protected]. † Technical University of Denmark. § University of Stuttgart. (1) Harding, N. S.; Adams, B. R. Biomass Bioenergy 2000, 19, 429445. (2) Maly, P. M.; Zamansky, V. M.; Ho, L.; Payne, R. Fuel 1999, 78, 327-334.
natural gas).1-4 However, no full-scale tests on biomass reburning have been reported, and the effect of process parameters such as biomass type requires further examination. Based on the experience of using natural gas and coal as reburning fuels, it is clear that volatiles are important for efficient NOx reduction. Indeed, pyrolysis gas from biomass has been shown to reduce NOx effectively.3,5-7 However, the influence of the volatiles composition in the NOx reduction potential of the biomass and the relative importance of char in the overall NOx reduction has not been assessed. Reduction of NO over coal char has been reported to be an effective mechanism for the reduction of NOx emissions.8-10 Results for NO reduction over biomass char are comparatively scarce but indicate similar11 or greater12,13 reactivity toward NO than coal chars. (3) Ru¨diger, H.; Kicherer, A.; Greul, U.; Spliethoff, H.; Hein, K. R. G. Energy Fuels 1996, 10, 789-796. (4) Lo´pez, Ch.; Maier, J.; Hein, K. R. G. NOx Reduction by Solid Fuel Reburning: Pilot Scale Investigations. In Proceedings of the IEA Conference: Clean Coal Technologies for Our Future; IEA Clean Coal Center: London, U.K., 2002. (5) Ru¨diger, H.; Greul, U.; Spliethoff, H.; Hein, K. R. G. Pyrolysis Gas of Biomass and Coal as a NOx-Reductive in a Coal Fired Test Facility. In Third International Conference on Combustion Technologies for a Clean Environment; Centro Cultural De Belem: Lisbon, Portugal, 1995. (6) Storm, C.; Spliethoff, H.; Hein, K. R. G. Generation of a Gaseous Fuel by Pyrolysis of Biomass and Sewage Sludge for Use as Reburn Gas in Coal-Fired Boilers. In Fifth International Conference on Combustion Technologies for a Clean Environment; Centro Cultural De Belem: Lisbon, Portugal, 1999. (7) Dagaut, P.; Lecomte, F. Energy Fuels 2003, 17, 608-613. (8) Levy, J. M.; Chan, L. K.; Sarofim, A. F.; Bee´r, J. M. Proc. Combust. Inst. 1981, 18, 111-120. (9) Johnsson, J. E. Fuel 1994, 73, 1398-1415. (10) Wendt, J. O. L. Combust. Sci. Technol. 1995, 108, 323-344. (11) Zevenhoven, R.; Hupa, M. Fuel 1998, 77, 1169-1176. (12) Sørensen, C. O.; Johnson, J. E.; Jensen, A. Energy Fuels 2001, 15, 1359-1368. (13) Garijo, E. G.; Jensen, A. D.; Glarborg, P. Energy Fuels 2003, 17, 1429-1436.
10.1021/ef049965k CCC: $27.50 © 2004 American Chemical Society Published on Web 07/22/2004
Experiment and Modeling of Biomass Reburning
Despite the potential importance of biomass reburning, little modeling work has been reported in the literature. This can be attributed both to the fact that it is a fairly recent technology, and to the chemical and physical complexity of the process. Biomass reburning involves interaction between mixing of the reburn fuel with the bulk flow, heating and devolatilization of particles, gas-phase chemistry, and gas-solid reactions. In this work, a model for the biomass reburning process is proposed that comprises a detailed description of the gas-phase chemistry, together with a simplified description of the remaining phenomena noted previously. The intent of the model is to assess the effect of the fuel and process parameters on the reburning efficiency without relying on a detailed flow description. To facilitate the model development and allow for validation, experiments on biomass reburning are conducted in an entrained flow reactor and in a pilot-scale facility. Modeling predictions are compared to these experiments, as well as results from the literature. Experimental Section Experimental Setup. The reburning experiments with straw char were performed in an entrained flow reactor at DTU.14,15 The main portion of the reactor consisted of a ceramic tube with a length of 2 m and an inner diameter of 80 mm. It was electrically heated using seven zones to allow for good temperature control. The facility included equipment for data acquisition, gas supply, solid fuel feeding, gas preheating, and controlled extraction of gas and particles. The gas preheater section was mounted on top of the reactor, in which the main inlet gas for the reactor may be heated in an annular space with dimensions of 500 mm (length) and 8 mm (width). Fuel was fed into the reactor by a central water-cooled injection probe that passed through the gas preheater, with the fuel entering the reactor at room temperature. The fuel and the preheated gas were then mixed at the top of the reactor. The gas and particles reacted while flowing down the central ceramic reactor tube. Samples of particles and gas could be extracted after different residence times by a moveable watercooled bottom probe. The flue gas was analyzed for CO, CO2, O2, and NO. The straw char was prepared in a small electrically heated fluidized bed reactor operated at 800 °C in which the gas and volatiles flowed upward. The straw was fed into the reactor from the side, together with a small stream of N2. A preheated gas stream of N2 was fed through the gas distributor to fluidize the material. Air was introduced through a tertiary port to burn the volatiles before leaving the reactor. Straw and poplar wood reburning experiments were conducted in a pilot-scale test rig at IVD.4 It is a 500 kW pulverized fuel combustion chamber with various burner configurations for different solid and gaseous fuels. The core of the test facility is a vertical combustion chamber with a total length of 7 m and a diameter ranging between 0.75 m and 0.85 m. The main burner is installed in the center at the top of the furnace. Three rows of access ports allowed measurements of the flame temperature and composition of the flue gas in the combustion chamber. The flue gas passed through an air preheater, cyclone ash separator, bag house filter, and ID fan and finally flowed to the stack. The composition of the flue gas when leaving the combustion chamber was registered by continuous measurement of NOx, SO2, O2, CO2, and CO. A coal preparation rig allowed coal mass flows up to 120 kg/h for bituminous coals and up to 300 kg/h for lignites. The dosage (14) Zolin, A.; Jensen, A.; Jensen, P. A.; Dam-Johansen, K. Fuel 2002, 81, 1065-1075. (15) Feng, B.; Jensen, A.; Bhatia, S. K.; Dam-Johansen, K. Energy Fuels 2003, 17, 399-404.
Energy & Fuels, Vol. 18, No. 5, 2004 1443 Table 1. Elemental Composition and Particle Size Distribution of the Reburning Fuels Used wheat straw wheat straw poplar wood (DTU)a (IVD) (IVD) composition (wt %, db) moisture ash volatiles C H N S Cl O sieve (µm)
