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Energy & Fuels 2003, 17, 608-613
Experiments and Kinetic Modeling Study of NO-Reburning by Gases from Biomass Pyrolysis in a JSR Philippe Dagaut* and Franck Lecomte CNRS, Laboratoire de Combustion et Syste` mes Re´ actifs, 1C, Avenue de la Recherche Scientifique, 45071 Orle´ ans Cedex 2, France Received October 27, 2002. Revised Manuscript Received January 22, 2003
A study of the reduction of nitric oxide (NO) by gas mixtures simulating gases obtained by biomass pyrolysis, in simulated conditions of a reburning zone, has been performed in a fused silica jet-stirred reactor at 1 atm. The temperatures ranged from 1100 to 1400 K, the initial mole fraction of NO was 1000 ppm and that of the reburn-fuel (mixture of CO and H2; mixture of methane, ethylene, and acetylene; and mixture of CO, H2, methane, ethylene, and acetylene) was varied. The equivalence ratio was varied from 0.5 to 2.5, corresponding to an excess air of 2 to 0.4. It was shown that the reduction of NO varies as the temperature and that for a given temperature, a maximum reduction of NO occurs, in slightly fuel-rich conditions. Overall, the present results show the same trends as observed in previous studies involving simple hydrocarbons or natural gas as reburn fuel. A detailed chemical kinetic modeling of the present experiments was performed using an updated and improved kinetic scheme. A reasonable agreement between the present data and the modeling was obtained. Furthermore, the proposed kinetic mechanism can be successfully used to model the reduction of NO by ethane, ethylene, a natural gas blend (methane-ethane 10:1), acetylene, propene, propane, and n-butane. According to this study, NO-reduction (i) by a mixture of CO and H2 occurs via H + NO + M ) HNO + M, HNO + H ) NH + OH, NH + NO ) N2O + H, NH + NO ) N2 + OH, N2O + H ) N2 + OH; (ii) by a mixture of methane, ethylene, and acetylene occurs via HCCO + NO ) HCN + CO2 and HCNO + CO; HCNO + H ) HCN + OH; HCN + O and OH )> CN, NCO, and NH; CN + O2 or OH )> NCO; NCO + H )> NH + CO; NH + NO ) N2O + H; N2O + H ) N2 + OH; (iii) by a mixture of CO, H2, methane, ethylene, and acetylene occurs via (ii).
1. Introduction The emission of nitrogen oxides (NOx) from both stationary and mobile combustion systems strongly contributes to the increased amount of nitrogenous species released into the troposphere. These chemicals are important pollutants involved in the formation of smog in urban areas via
NO + RO2 ) NO2 + RO
(a)
NO2 + hν f NO + O
(b)
O + O2 + M ) O3 + M
(c)
and in catalytic ozone-depletion in the stratosphere by perturbation of the Chapman mechanism. The reduction of NOx-emissions from fixed combustion devices is thus of major importance. Reburning, which can be viewed as staged combustion, represents a low-cost and effective technique already used1,2 to reduce NOx. In the primary stage, NOx are produced in fuel-lean to sto* Corresponding author. Tel: (33) 2 38 25 54 66. Fax: (33) 2 38 69 60 04. E-mail:
[email protected]. (1) Wendt, J. O. L.; Sterling, C. V.; Matovich, M. A. In Fourteenth Symposium (International) Symposium on Combustion; The Combustion Institute, Pittsburgh, PA, 1973; pp 897-904. (2) Smoot, L. D.; Hill, S. C.; Xu, H. Prog. Energy Combust. Sci. 1998, 24, 385.
ichiometric conditions; in the secondary stage, the reburning zone, a secondary fuel is added to make the mixture fuel-rich and to allow NOx-reduction; in the last stage, the burnout zone, additional air is introduced to complete the combustion. Among the reburn fuels, one finds natural gas and pulverized coal. Biomass, a carbon dioxide neutral fuel, could also be used as reburn fuel. Its pyrolysis yields gases that have a good potential for reducing NO.3,4 Several studies of the fast pyrolysis of biomass (straw and wood mostly) provided compositions of the gases formed during this process. According to these studies,3-6 increasing the pyrolysis temperature increases the importance of CO and H2. Above 1000 °C, H2 and CO are the most important and potentially the only gaseous products, whereas at lower temperature the other products measured are methane, ethylene, and acetylene in an average ratio of about 4:2:1. The chemistry involved in NO-reburning is complex and still needs both experimental and computational investigations, particularly at moderate temperatures where the oxidation of the reburn fuel and interactions with NOx (3) Storm, C.; Unterberger, S.; Hein, K. R. G. Proc. Conf. (Int.) Thermochemical Biomass Conversion, 5, 2000, Tirol/Austria. (4) Storm, C.; Spliethoff, H.; Hein, K. R. G. Proc. INFUB, 5, 2000, Porto, Portugal. (5) Zanzi, R.; Sjo¨stro¨m, K.; Bjo¨rnbom, E. Fuel 1996, 75, 545. (6) Zanzi, R.; Sjo¨stro¨m, K.; Bjo¨rnbom, E. Proc. Int’l Conf. Biomass for Energy and Industry; Kopetz, Weber, Palz, Chartier, Ferrero, Eds.; C. A. R. M. E. N., Rimpar, Germany, 1998; pp 1581-1584.
10.1021/ef020256l CCC: $25.00 © 2003 American Chemical Society Published on Web 04/11/2003
NO-Reburning
involve a large number of species and reactions. Several recent studies have proposed kinetic schemes for NOreduction using various fuels.7-13 A series of experiments was performed here using several biomass gas pyrolysis blends as reburn-fuel. These new experimental results are presented together with a detailed chemical kinetic modeling. 2. Experimental Section We used a fused-silica jet-stirred reactor (JSR) presented previously.8,14 A 40 mm o.d. spherical sphere equipped with 4 injectors having nozzles of 1 mm i.d., for the admission of the gases achieving the stirring, constitutes the JSR. The JSR is located inside a regulated electrical resistor generating =1.5 kW, surrounded by insulating material. It operates at atmospheric pressure. The reburn fuel, NO, and oxygen flow rates are measured and regulated by thermal mass-flow controllers. These high purity gases (>99.9%) are diluted by a flow of nitrogen (
