Post-treatment of Fly Ash by Ozone in a Fixed Bed Reactor Kim Hougaard Pedersen, Merce ` Casanovas Melia `, Anker Degn Jensen,* and Kim Dam-Johansen Department of Chemical and Biochemical Engineering, Technical UniVersity of Denmark, Building 229, DK-2800 Kgs. Lyngby, Denmark ReceiVed July 3, 2008. ReVised Manuscript ReceiVed September 17, 2008 The residual carbon in fly ash produced from pulverized coal combustion can adsorb the air-entraining admixtures (AEAs) added to enhance air entrainment in concrete. This behavior of the ash can be suppressed by exposing the fly ash to oxidizing species, which oxidizes the carbon surface and thus prevents the AEA to be adsorbed. In the present work, two fly ashes have been ozonated in a fixed bed reactor and the results showed that ozonation is a potential post-treatment method that can lower the AEA requirements of a fly ash up to 6 times. The kinetics of the carbon oxidation by ozone was found to be fast. A kinetic model has been formulated, describing the passivation of carbon, and it includes the stoichiometry of the ozone consumption (0.8 mol of O 3 /kg of C) and an ineffective ozone loss caused by catalytic decomposition. The simulated results correlated well with the experimental data. 1. Introduction The generation of heat and electricity from pulverized coal combustion is associated with the production of large amounts of fly ash. The fly ash can be used in the production of concrete, but the residual carbon present in the fly ash is, in certain cases, capable of adsorbing the air-entraining admixtures (AEAs) to an unacceptable extent. 1-3 The AEAs are added to the concrete to enhance the air entrainment, which improves properties, such as increased workability and freeze-thaw resistance. 4 The AEA adsorption of fly ash can be suppressed by post-treatment methods. These methods may be based on a carbon separation technique 5-7 or an oxidizing process, where the latter comprises carbon removal 8 or carbon passivation. The passivation of carbon can be achieved from dry or wet methods that oxidize the carbon surface. The oxidation increases the surface polarity, which prevents the AEAs from being adsorbed. 9 Dry methods are preferred to avoid the costs of drying the treated ashes and loss of pozzolanic activity. 10 Hurt and co-workers 9-11 have successfully applied a dry method based on ozone as a reacting agent. This is a method that further benefits from having a potential to remove ammonia added in post-flame NO x -reduction methods. 12 In the present work, fly ashes acquired from a Danish power plant have been exposed to ozone in a fixed bed reactor. The power plant is known to produce fly ash that negatively affects the air entrainment in concrete, despite the fact that the ash usually has a carbon content significantly below the 5 wt % allowed for a category A fly ash in the European Standard EN- 450. 13 The ozonation was carried out at various concentrations and treatment times, and a kinetic model of the process has been developed. 2. Experimental Section The ozone treatment was performed on fly ashes produced from burning bituminous coal, and their carbon contents and AEA adsorption measured by the foam index (FI, see below) are listed in Table 1. The carbon content in the two fly ashes represents typical values from the investigated power plant. The experimental setup is sketched in Figure 1. The ozone was produced by an ozone * To whom correspondence should be addressed. Telephone: +45- 45252841. Fax: +45-45882258. E-mail: aj@kt.dtu.dk. (1) Gebler, S.; Klieger, P. Effects of fly ash on the air-void stability of concrete. Proceedings of the Canmet/ACI. First International Conference on the Use of Fly Ash, Silica Fume, Slag, and Other Mineral Byproducts in Concrete, 1983; Vol. 1. (2) Freeman, E.; Gao, Y.-M.; Hurt, R.; Suuberg, E. Fuel 1997, 76, 761– 765. (3) Hill, R. L.; Sarkar, S. L.; Rathbone, R. F.; Hower, J. C. Cem. Concr. Res. 1997, 27, 193–204. (4) Paille ´re, A. M. Application of Admixtures in Concrete, 1st ed.; E and FN Spoon: London, U.K., 1995. (5) Ban, H.; Li, T. X.; Hower, J. C.; Schaefer, J. L.; Stencel, J. M. Fuel 1997, 76, 801–805. (6) Gray, M. L.; Champagne, K. J.; Soong, Y.; Killmeyer, R. P.; Maroto- Valer, M. M.; Andre ´sen, J. M. Fuel Process. Technol. 2002, 76, 11–21. (7) Baltrus, J. P.; Diehl, J. R.; Soong, Y.; Sands, W. Fuel 2002, 81, 757–762. (8) Keppeler, J. G. Full scale carbon burn-out and ammonia removal experience. In the 2000 Conference on Unburned Carbon on Utility Fly Ash, 2000. (9) Chen, X.; Farber, M.; Gao, Y.; Kulaots, I.; Suuberg, E. M.; Hurt, R. H. Carbon 2003, 41, 1489–1500. (10) Gao, Y.; Ku ¨laots, I.; Chen, X.; Aggarwal, R.; Mehta, A.; Suuberg, E. M. Fuel 2001, 80, 765–768. (11) Gao, Y.; Ku ¨laots, I.; Chen, X.; Suuberg, E. M.; Hurt, R. H.; Veranth, J. M. The effect of solid fuel type and combustion conditions on residual carbon properties and fly ash quality. Proceedings of the Combustion Institute, 2002; Vol. 29. (12) Gao, Y.; Chen, X.; Fujisaki, G.; Mehta, A.; Suuberg, E.; Hurt, R. Energy Fuels 2002, 16, 1398–1404. (13) European Committee for Standardization. Fly ash for concretesPart 1: Definition, specifications and conformity criteria (EN 450-1), Brussels, Belgium, 2005. Table 1. Carbon Content and FI of Treated Samples a ash carbon (wt %) FI (mL of AEA/2 g of ash) A 2.97 0.24 B 2.57 0.36 a Ash A is produced from 90% Columbian coal and 10% South African coal, while ash B is produced from 90% Polish coal and 10% South African coal. Energy & Fuels 2009, 23, 280–285 280 10.1021/ef800532x CCC: $40.75  2009 American Chemical Society Published on Web 11/14/2008