Combustion of powdered fuels in entrained flows and fluidised beds
Modern cement plants use precalciners to treat thermally the crude material before introduction in the rotary kiln. These huge reactors use various ground solid fuels as energy supply. This research work was dedicated to understanding and modelling the formation and the reduction of pollutants during their combustion in this part of the plant. Indeed, in the particular case of NO, there are various sources for its formation, and several mechanisms also play a significant role in their reduction. New creteria were establihed to choose the fuels that will burn fast enough and produce as few pollutans as possible.Actions
PhD of L. Van de steene, in French
Institut National Polytechnique de Toulouse, N° 1620, soutenue le 8 décembre 1999
Supervisors : G. Charnay, S. Salvador
Supported by Polysius in Lyon.
Abstract: Cement plant precalciners, which consume large amounts of energy, usually use pulverized coal. They can allow solid fuels, like industrial waste, to develop their energy potential. Industrial waste has particular physical and chemical properties. Operating at low temperatures, 800 to 1000°C, precalciners generate only small quantities of NOx, but need combustion times of several seconds. So as to understand this specific type of combustion and to predict its characteristics, experimental laboratory apparatus was designed with the aim of finely reproducing and controlling the chemical phenomena prevailing in a precalciner. This entrained flow reactor is 2 m long, with a diameter of 80 mm. By means of a fine dosage of solid fuels, the equipment carries out the combustion of isolated monosized particles in a laminar flow that is precisely c ontrolled for temperature and chemical atmosphere. A bituminous coal is characterized for different particle diameters varying the temperature and the oxygen concentration in the environment. We developed a relatively simple numerical model which took account of all the chemical reactions involved : devolatilization, oxydation of volatile matter, and oxydation of carbonaceous residue. Heat and mass transfer phenomena coupled together with these chemical reactions are taken into consideration. The model is validated by comparison with the experimental results. We show that the combustion of particles whose diameter is less than 40 µm is controlled by the chemical reaction alone. For larger diameters, the diffusion of oxygen in the particle and external thermal transfers become limiting factors. Shrinking core models commonly used at high temperatures (1200 to 2000°C) are not suitable. By the combined use of the experimental apparatus and the model, we shall be able, by determining their kinetic parameters, to characterize the solid fuels that precalciners will use.
Formation of nitrogen oxides during combustion of petroleum cokes in cement plant precalciner conditions
PhD of Jean-Michel Commandré, in French
Thèse de l'Institut National Polytechnique de Toulouse,
March the 14th 2002
Supervisors: G. Charnay, S. Salvador
Supported by Lafarge Ciments
Abstract: The precalciner of a cement plant is a reactor in which large quantities of thermal energy are produced by the combustion of pulverised solid fuels at moderate temperatures: 800 to 1100°C. Petroleum cokes are the principal source of energy. This study is concerned with the thermochemical mechanisms which underlie the formation of pollutants such as NO and SO2 in this type of reactor. The first approach was experimental. A total of 22 cokes were burned in an entrained flow reactor at 900°C under conditions typical of those in a precalciner. A statistical analysis of the results did not uncover any simple relationship between the properties of a coke and the amount of pollutant formed, and no prediction of NO emissions was possible. The second approach therefore, was the development of a mathematical model of the system of chemical and thermal process which occur during combustion and NOx formation. Specific experiments were conducted to characterise the species produced by each of the principal reactions and to determine their reaction kinetics. The main mechanisms were therefore distinguished and their relative importance in pollution formation was determined. The quantity of thermal NO is negligible. Gas phase reactions form most of the NO in the flame zone, but contribute only slightly to the final NO emission. Fuel NO is formed principally during the combustion of the carbon residue. In compensation, a significant quantity of NO is reduced by heterogeneous reaction with the residual carbon. Sulphur dioxide is formed in direct proportion to the extent of mass loss by the fuel, both during devolatilisation and the oxidation of residual carbon. It has only a slight effect on the chemistry for the production of nitrogen oxides.
NOx formation and reduction by a coal, a lignite, an anthracite and a petroleum coke in conditions of cement plant calciner
PhD of Julien Cancès Thèse de l'Institut National Polytechnique de Toulouse October 26th 2006 Supervisor: S. Salvador Supported by Polysius in Lyon.
Abstract: The cement industry is a high energy consumer. This energy is mostly provided by the combustion of pulverized carbonaceous solid fuels. Up to 60 % of this fuel income may be injected into the calciner. However, the combustion of solid fuels produces pollutants, particularly NOx. Several reduction technology were developed. The most promising one, in term of cost and efficiency, is the reburning. It consists in a secondary fuel injection, creating a fuel rich zone favorabl to NOx reduction. The fuel injection in cement plant calciners may be compared to this technique. The solids interact with NOx at different levels: . through gas phase reactions with volatile matters released during the pyrolysis, . and through solid-gas heterogeneous reactions: i.e. char oxidation that produces N-species and NO reduction at the char surface. A coupled experimental and modeling protocol was developed to determine the relative contribution of these different phenomena. The used fuels are of four distinct types, commonly used in cement plants: a lignite, a coal, an anthracite and a petcoke. Thus, the elementary heterogeneous reactions - devolatilisation, char oxidation and NO reduction by char - were characterized by specific experiments and modelings. One observes a large disparity between the four fuels in terms of kinetic parameters and quantity of volatile matters released. Finally, the experiments and modelings representative of the reactions occuring simultaneously during reburning in calciner conditions were performed. It appears that the effect of NO reduction in the gas phase is of the same order of magnitude that the effect of reduction by char after a particle residence time of 2 s. The NO reduction by char increases continuously with the temperature, whereas the gas phase reduction presents singularities in function of the temperature for the high volatile fuels: The NO reduction is lower at 900°C than at 800 and 1000_C, in case of lignite and coal. A detailed chemical analysis of these singularities was carried out and enabled to determine the main reaction paths occuring during NOx formation and reduction in the gas phase.
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Publications
Controlling powdered fuel combustion at low temperature in a new entrained flow reactor Van de Steene L., Salvador S., Charnay G. Combustion Science and Technology, 2000, Vol. 159, pp 255-279 The High Temperature Reaction of Carbon with Nitric Oxide Commandre J.M., Stanmore B.R., Salvador S. Combustion and Flame, 128:211-216 (2002) The formation and reduction of NO during the combustion of powdered petroleum coke - The case of cement plant precalciner conditions J-M. Commandré, S. Salvador, R. Gadiou, G. Charnay Combustion Science and Technology, 177 (3): 579-611 March 2005 The Catalytic Effect of Vanadium on the Reactivity of Petroleum cokes with NO S. Salvador, J-M. Commandré, B.R. Stanmore, R. Gadiou Energy and Fuel, 2004, 18, 296-301 The Catalytic Effect of Vanadium on the Reactivity of Petcokes with O2 and NO S. Salvador, J-M. Commandré, B.R. Stanmore CLEAN AIR - International Journal on Energy for a Clean Environment, Vol. 7, N°. 2, 159-173, 2006 Lack of correlation between the properties of a petroleum coke and its behaviour during combustion, J-M. Commandré, S. Salvador Fuel Processing Technology, Vol. 86, issue 7, 25 April 2005, pp 795-808 Interactions Between NO and Volatile Matters Released by a Coal and a Petcoke J. Cances, J. M. Commandré, and S. Salvador, CLEAN AIR - International Journal on Energy for a Clean Environment, Vol. 7, N°. 4,343-361, 2006 NO reduction capacity of four major solid fuels in reburning conditions - experiments and modeling, J. Cances, J-M Commandré, S. Salvador, P. Dagaut FUEL, Volume 87, Issue 3, March 2008, pp. 274-289
Congress
Combustion propre de particules solides : caractérisation expérimentale d'un réacteur modèle Van de Steene L., Charnay G., Salvador S. SFT Thermique et Environnement, Congrès 5-7 mai 1998, Marseille, France, Elsevier - J. Pantaloni and R. Martin, 275-279. The combustion at low temperatures of powdered solid fuels : laboratory experiments and modelling Van de Steene L., Commandré J.M., Salvador S., Charnay G. Poster - 28th International Symposium on Combustion, 30th July to 4th August 2000, Edinburgh, Scotland Investigating the Influence of Sulfur on NO Formation during the Combustion of Highly Sulphurised Petcokes Commandre J.M., Salvador S., Charnay G. 6th International Conference on Technologies and Combustion for a Clean Environment, Clean Air, 9-12 July 2001, New Customs House, Porto Formation et réduction des NOx lors de la combustion de cokes hautement soufrés Commandre J.M., Salvador S., Charnay G. SFGP - 8° Congrès Francophone de Génie des Procédés - Pour la Performance et la Connaissance. 17-19 octobre 2001, Nancy, France Rice husk, straw and bark behaviour during pyrolysis, combustion and gasification : Fundamental study L. Van de steene, A. Napoli, S. Salvador 5th int. Biomass conf. Of the americas, Orlando, 17-21 sept 2001. Congress cancelled; CD-Rom with abstracts publishedv Rice husk, straw and bark behaviour during pyrolysis, combustion and gasification : fundamental study L. Van de steene, A. Napoli, S. Salvador 12th European Conference on Technology Exhibition on Biomass for Energy, Industry and Climate Protection, Amsterdam, The Netherlands, 1721 June 2002 The Catalytic Effect of Vanadium on the Reactivity of Petcokes with O2 and NO S. Salvador, J-M. Commandré, R. Gadiou, B.R. Stanmore 7th Int. Conf. On Energy for a Clean Environment, Lisbon, 7-10 July 2003 Interactions Between NO and Volatile Matters Released by a Coal and a Petcoke J. Cances, J. M. Commandré, and S. Salvador, Eighth International Conference on Energy for a Clean Environment, Lisbon, Portugal, 27-30 June 2005 Flash pyrolysis of four major fuels : released species and kinetics - application to NO reduction in reburning conditions J. Cancès, S. Salvador, J-M. Commandré 17th International Symposium on Analytical & Applied Pyrolysis. May 21 to 26, 2006 - Budapest, Hungary. Poster A Common Methodology for Coals Characterization A-L. Brasseur, I. Flour, W. Rybak, S. Salvador, J-M. Commandré The 24th International Pittsburgh Coal Conference, September 10 - 14, 2007, Johannesburg, South Africa