Fluidized Bed Combustion Waste as a Raw Mix Component for the Manufacture of Calcium Sulphoaluminate Cements

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1 Fluidized Bed Combustion Waste as a Raw Mix Component for the Manufacture of Calcium Sulphoaluminate Cements Giulio Belz 1, Pompilio Caramuscio 1, Milena Marroccoli 2, Fabio Montagnaro 3, Marianna Nobili 2, Antonio Telesca 2, Gian Lorenzo Valenti 2 1 ENEL GEM - Area tecnica Ricerca, Cerano (Brindisi) ITALY 2 Dipartimento di Ingegneria e Fisica dell Ambiente - Università degli Studi della Basilicata, Potenza ITALY 3 Dipartimento di Chimica - Università degli Studi Federico II, Naples ITALY 1. Overview Fluidized bed combustion (FBC) waste, mainly composed by exhausted sulphur sorbent and coal ash, contains CaO, SiO 2, Al 2 O 3 and SO 3 as major oxides [1]. Both disposal in landfill and re-use of FBC waste is generally made difficult by its chemical and mineralogical composition. Upon hydration, exothermal and expansive phenomena occur due to the relatively high content of lime and calcium sulphate [2]. Moreover the utilization of FBC ash in the ordinary cement and concrete industry is hindered by its poor pozzolanic activity due to the reduced glass content related to combustion temperatures which are significantly lower than those of traditional pulverized coal combustors [3]. Calcium sulphoaluminate (CSA) cements are hydraulic binders which can be used in a variety of applications like rapid-hardening as well as shrinkage-compensating and selfstressing cements [4-7]. The key component of these binders is 4CaO 3Al 2 O 3 SO 3 ( C 4 A 3 S, according to the cement chemistry notation under which C=CaO; A=Al 2 O 3 ; S =SO 3 ; S=SiO 2, F=Fe 2 O 3 ; H=H 2 O). C 4 A 3S is able to generate, upon hydration, ettringite ( C 6AS3H32 ), a compound which regulates all the technical properties of calcium sulphoaluminate cements [8-15]. Among the other phases present in C 4A 3S -based cements, dicalcium silicate (C 2 S) can play an important role because it is able to add strength and durability at later ages. Secondary constituents, such as gehlenite (C 2 AS), calcium sulphosilicate ( C 5 S) and various calcium aluminates, have generally a poor hydraulic behaviour and provide a small contribution to the technical properties. Due to its chemical composition, FBC waste represents an excellent raw material for the manufacture of cements. In a previous paper [16] CSA cements obtained from mixtures containing FBC fly ash, calcium carbonate, red mud and/or bauxite were investigated. Quite satisfactory results were achieved in terms of conversion and selectivity. In this work the possibility of using additional sources of alumina and sulphate has been explored. In particular a bottom ash generated within a FBC plant as well as a fly ash (FA) and a flue gas desulphurization (FGD) waste coming from a traditional coal-fired power plant were utilized as raw mix components for the synthesis of CSA cements. IX4.1

2 2. Experimental 29th Meeting on Combustion 2.1 Materials FBC fly- and bottom-ash originated from a CFBC industrial combustor while a traditional fly ash (FA) and a flue gas desulphurization (FGD) waste were generated by a pulverised coal fired plant operating in the same power station. Bauxite came from an aluminium plant based on the Bayer process. The chemical composition of the raw materials employed, in terms of major oxides, evaluated by X-ray fluorescence analysis, is reported in Table 1. FA FGD waste bauxite FBC fly ash FBC bottom ash limestone CaO SO Al 2 O SiO MgO SrO P 2 O TiO Fe 2 O Mn 3 O p.a.f Total Table 1: Chemical composition (dry basis) of traditional fly ash (FA), FGD waste, FBC flyash, FBC botto- ash, bauxite and limestone, mass %. * loss on ignition, according to EN 196 Standard 2.2 Testing procedures Four mixtures (A1, A2, B1, B2), having the composition illustrated in Table 2, were prepared. Mixtures A1 and A2 were prepared with FA, FBC fly-ash, bauxite, FGD waste and limestone. Both B1 and B2 contained FA, a blend of FBC fly- and bottom-ash (flyash/bottom-ash mass ratio equal to 1.5), bauxite and limestone; moreover FGD waste was added to mixture B2. All the mixtures were heated in a laboratory electric oven for 2 hours at, 1250 and 1300 C, then analysed by X-ray diffraction to assess both conversion and selectivity of the reacting systems. Mixture A1 A2 B1 B2 FA FGD waste bauxite FBC fly and bottom ash** FBC fly ash limestone Table 2: Composition of raw mixtures, mass %. ** mass ratio equal to 1.5 IX4.2

3 Italian Section of the Combustion Institute Table 3 shows the potential concentration values of C 4 A 3S and C 2 S in the burning products of the four mixtures. They were calculated assuming that SO 3 and Al 2 O 3 on the one hand, and SiO 2, on the other, react to give only C 4 A 3S and C 2 S, respectively; furthermore, solid solution effects were neglected. Mixture A1 A2 B1 B2 C 4 A 3S C 2 S A S +C 2 S C 3 Table 3: Potential concentration of mass %. 3. Results C 3 4 A S and C 2 S in the burning products of raw mixtures, From the examination of the XRD data concerning the burning products of all the investigated mixtures it can be argued that C 4 A 3S and C 2 S are, in the order, the main mineralogical phases. Unreacted compounds were absent in the burning products of mixtures A1, A2 and B2 while mixture B1, upon heating at all the temperatures investigated, showed an almost negligible presence of CaSO 4. Mixtures A1 and B2, when heated at C, revealed a complete absence of secondary phases; upon burning at 1250 C and 1300 C, they showed the presence of brownmillerite, C 4 AF, and calcium sulphosilicate, C 5 S, respectively, in little amounts. As far as mixtures A2 and B1 are concerned, at every heating temperature, respectively weak peaks of C 5 S and C 4 AF were generally detected. Figs. 1 and 2 as well as Figs. 3 and 4 indicate, for mixtures A1, A2, B1 and B2, respectively, the XRD intensities of the main peaks of C 4 A 3S and C 2 S, in the order, as a function of the burning temperature. It was generally observed a significant influence of the synthesis temperature on the C 4 A 3S and C 2 S concentrations. However, 1250 C seemed to be the optimum temperature for obtaining the maximum amount of both phases Fig. 1 (left) Fig. 2 (right) A1 A C 4 A 3 S -XRD intensity (main peak, counts per second) for the burning products of mixtures A1 (green curve) and A2 (pink curve) vs. synthesis temperature. C 2 S-XRD intensity (main peak, counts per second) for the burning products of mixtures A1 (green curve) and A2 (pink curve) vs. synthesis temperature. A1 A2 IX4.3

4 29th Meeting on Combustion Fig. 3 (left) Fig. 4 (right) B1 B B1 B C 4 A 3 S -XRD intensity (main peak, counts per second) for the burning products of mixtures B1 (blue curve) and B2 (red curve) vs. synthesis temperature. C 2 S-XRD intensity (main peak, counts per second) for the burning products of mixtures B1 (blue curve) and B2 (red curve) vs. synthesis temperature. 4. Concluding remarks It has been found that raw mixes for the manufacture of calcium sulphoaluminate cements are able to contain not only fluidised bed combustion wastes, but also other byproducts, such as fly ash and flue gas desulphurization waste, coming from a traditional coalfired power station. The concentration of FBC waste was comprised between 10 and 20%; the overall concentration of the by-products ranged from 33 to 40%. Very satisfactory results were obtained in terms of conversion and selectivity towards the desired hydraulic compounds, calcium sulphoaluminate and dicalcium silicate, in the range of the temperatures investigated, C. The best synthesis temperature for obtaining the maximum amount of both phases was about 1250 C. 5. References 1. Odler, I., Zhang, H.: 10th International Congress on the Chemistry of Cement, Goteborg, Sweden, 1:1i026, (1997). 2. Bernardo, G., Marroccoli, M., Montagnaro, F., Valenti, G. L.: 8th CANMET/ACI Internationa Conference on Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete, Las Vegas, Nevada, United States, SP 221 ACI:169 (2004). 3. Bernardo, G., Marroccoli, M., Montagnaro, F., Valenti, G.L.: 11th International Congress on the Chemistry of Cement, Durban, South Africa, 3:1227 (2003). 4. Kurdowski, W., George, C.M., Sorrentino, F.P.: 8th International Congress on the Chemistry ofcement, Rio de Janeiro, Brazil, 1:292 (1986). 5. Su, M., Wang, Y., Zhang, L., Li, D.: 10th International Congress on the Chemistry of Cement, Goteborg, Sweden, 4:4iv029 (1997). 6. Muzhen, S., Kurdowski, W., Sorrentino, F.P.: 9th International Congress on the Chemistry of Cement, New Delhi, India, 1:317(1992). 7. Kouznetsova, T.V.: 10th International Congress on the Chemistry of Cement, Goteborg, Sweden, 1:1i001 (1997). IX4.4

5 Italian Section of the Combustion Institute 8. Scrivener, K. L.: 11th International Congress on the Chemistry of Cement, Durban, South Africa, 1:84 (2003). 9. Mehta, P.K.:World Cement Technology, May:166 (1980). 10. Santoro, L., Garofano, L., Valenti, G.L.: 8th International Congress on the Chemistry of Cement, Rio de Janeiro, Brazil, 4:389 (1986). 11. Beretka, J., Santoro, L., Sherman, N., Valenti, G.L.: 9th International Congress on the Chemistry of Cement, New Delhi, India, 3:195 (1992). 12. Ikeda, K., Fukuda, K., Shima, H.: 10th International Congress on the Chemistry of Cement, Goteborg, Sweden, 1:1i025 (1997). 13. Beretka, J., de Vito, B., Santoro, L., Valenti, G.L.: Resources, Conservation and Recycling, 9:179 (1993). 14. Belz, G., Beretka, J. Marroccoli, M., Santoro, L., Sherman, N., Valenti, G.L.: 5th CANMET/ACIInternational Conference on Fly ash, Silica Fume, Slag and Natural Pozzolans in Concrete, Milwaukee, Wisconsin, United States, Special Publication No. 153, 1:513 (1995). 15. Beretka, J., Cioffi, R., Marroccoli, M., Valenti, G.L.: Waste Management, 16: 231, (1996). 16. G. Belz, G. Bernardo, P. Caramuscio, F. Montagnaro, A. Telesca, G. L. Valenti: 28th Meeting of the Italian Section of The Combustion Institute, I-4 (2005). IX4.5