Thermal expansion of slag and fly ash from coal gasification in IGCC power plant

Fuel 85 (2006) 2352–2358 www.fuelfirst.com

Thermal expansion of slag and fly ash from coal gasification in IGCC power plant M. Aineto a

a,*

, A. Acosta a, J. Ma. Rinco´n b, M. Romero

b

Laboratory of Applied Mineralogy, University of Castilla La Mancha, C/Camilo Jose´ Cela 7, 13071 Ciudad Real, Spain b Group of Glassy and Ceramic Materials, Instituto de Ciencias de la Construccio´n Eduardo Torroja, CSIC, c/Serrano Galvache 4, 28033 Madrid, Spain Available online 12 June 2006

Abstract Integrated gasification in combined cycle (IGCC) is an electrical power generation system which is characterized to be a clean coal technology different than conventional process in combustible treatment. IGCC process gives rise to inorganic solid wastes in the form of vitreous slag and fly ashes with singular thermal properties. The gasification of the fuel takes place at high temperature and pressure in reducing atmosphere. Under that conditions, gases such as H2, N2 or CO, which are the main components of the gas mixture in the gasifier, show a high solubility in the melt and during the cooling remain enclosed in the vitreous slag. When these wastes are afterward thermal treated in oxidizing conditions, two phenomena occur. The development of a crystalline phase by devitrification of the glassy matrix and the releasing of the enclosed gas, which starts at temperatures nearly to the softening point. At higher temperatures the bubbles with increasing kinetic energy tend to ascent with difficult through the viscous liquid phase and promotes an expansive reaction, giving rise to a foam glass-ceramic product. This paper has been focused on the study of thermal expansion in slag and fly ash samples from the ELCOGAS IGCC power plant located in Puertollano (Spain). Ó 2006 Elsevier Ltd. All rights reserved. Keywords: IGCC power plant wastes; Fly ash; Slag; Coal gasification

1. Introduction The Integrated gasification in combined cycle (IGCC) power plant of ELCOGAS S.A. located in Puertollano, Ciudad Real (Spain) is the first IGCC plant established in Spain and the second one in Europe as result of an European Community project under the THERMIE program [1]. It has been designed to produce electricity by the gasification of a mixture of coal and petroleum coke. Gasification process converts the solid combustible mixture into a gas that is subsequently depurated to separate sulphur and dust particles. The cleaned gas, which is composed by 60% CO, 22% H2, 10% N2, 4% H2O and 4% CO2, is then burned with high efficiency in a combined

*

Corresponding author. Tel.: +3426295300x3438; fax: +3426295318. E-mail address: [email protected] (M. Aineto).

0016-2361/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2006.05.015

cycle electricity-generating unit [2]. The combustible gasification takes place at 1450 °C and reducing atmosphere to get the optimum partial pressure of oxygen to yield CO as synthesis gas. Under these conditions, the inorganic fraction of the coal is fully melted and the resulting melt presents a high capacity to dissolve gases [3]. The melt is then rapidly cooled and solidified by quenching in a water dump giving rise to a vitreous slag. The fly ash comes from small drops of melt and volatile compounds which are carried up by the ascending gases. The mixture of gases is cooled in a refrigerating system where fly ash solidify at temperatures from 950°C to 400 °C. Afterwards, the fly ash is separated and recovered from the filters in the gas cleaning unit. Fly ash is a gray powder of very small particle size, with an average particle diameter below 3 lm. As a consequence of the low pO2 at which fuel is burned, reducing forms of oxides and sulphides are found in its composition [4].

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The pozzolanic activity of this fly ash and its suitability to be used as concrete additive has been yet assessed [5], being at present recycled for that use. The slag from Puertollano plant has been previously characterized [6,7] and several works have investigated its use as grog material in manufacture of construction bricks [8,9] and synthesis of glass-ceramics [10–12]. The aim of this paper is to study the expansive properties at high temperatures of Puertollano IGCC slag and fly ash to produce porous glass-ceramic materials. The increasing development of foam materials for a wide range of applications such as thermal insulation [13], thermal shock resistance [14] or lightweight aggregates [15], represents a promising opportunity for recycling the gasification wastes as high added value products for several applications [16].

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The expansion process was examined by means of hotstage microscopy (HSM) on cylindrical specimens of 2 mm diameter and 4 mm height made from the as received fly ash and slag milled as fine powder of particle size <50 lm. Images of the specimens and data of the specimen height variation were taken at fixed intervals of temperature. The sintering curve displays the change in the specimen height related to the initial height, which is taken as 100%. Consequently shrinkage is measured as values below 100% and expansion as values over 100%. Expansion tests on larger size samples were carried out in an electric furnace. The as received fly ash was hand molding in spherical Table 1 Chemical composition of slag and fly ash from Puertollano IGCC power plant %

2. Materials and methods The fly ash (FA) and slag (SL) here considered come from the ELCOGAS gasification plant (Puertollano, Spain). Mineralogical analysis was carried out by X-ray diffraction (XRD) and chemical analysis by X-ray fluorescence (XRF) and infrared spectroscopy (IR). The thermal properties have been analyzed by differential thermal analysis (DTA) at a heating rate of 10 °C/min, in platinum crucibles and calcined powdered aluminum oxide as reference. For DTA tests, slag samples were milled to particle size below 600 lm whereas the fly ash was analyzed as received.

SiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O TiO2 LOI

(XRF) (XRF) (XRF) (XRF) (XRF) (XRF) (XRF) (XRF) (XRF)

C S

(IR) (IR)

Fig. 1. XRD patterns: (a) IGCC fly ash; (b) IGCC slag.

Slag

Fly ash

55.12 28.20 5.77 0.05 0.93 6.04 0.40 2.38 0.76 0.34

56.47 24.13 3.99 0.03 0.68 3.64 0.70 4.05 0.54 5.78

0.38 0.51

4.95 1.39

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shape (0.90–0.11 mm diameter) by using water (20–22%) as binding, whereas the slag thermal expansion was examined on particles with selected size (2.50–4.75 mm). Thermal expansion were evaluated at increasing temperatures from 900 °C to 1200 °C in steps of 50 °C and a dwell time of 15 min at the maximum temperature. The thermal expansion was measured as the variation of the average diameter of the particles after thermal treatments. The microstructure of the resulting materials were observed by SEM microscopy, and the developed crystalline phases were determined by XRD. The density of heat treated samples was measured by Helium pycnometer on both bulk and powder (<50 lm) samples. The water absorption was measured by immersion of the particles in distilled water for 24 h and calculated as the increment of weight percentage in relation to the dry mass. 3. Results and discussion Table 1 gives the chemical analysis of ELCOGAS IGCC slag and fly ash. Both samples are enriched in glass former oxides (SiO2 + Al2O3 > 80% and SiO2/Al2O3 ratio >1.95) while their modifying or fluxing oxides content is low, which is the main reason for the high viscosity showed by the melted fly ash and slag [17]. Fig. 1 depicts the X-ray diffractograms collected on Puertollano fly ash and slag. The XRD patterns show their mainly amorphous/vitreous character. However, very small diffraction peaks are detected in fly ash, which correspond to sulphide crystalline phases: galene (PbS), sphalerite (ZnS) and pyrrotite (FeS). Concerning DTA analysis, the fly ash trace (Fig. 2) depicts a first small exothermic peak at 540 °C, which is due to the sulphides oxidation. A large exothermic at 720 °C is owing to ferrous oxidation and hematite (Fe2O3) formation. Finally, a broad exothermic effect above 1000 °C is caused by the crystallization of mullite (3Al2O3 Æ 2SiO2), hercynite (FeO Æ Al2O3) and anorthite (CaO Æ Al2O3 Æ SiO2), which are the main crystalline phases

Slag (µV)

SL

FA

140

120

% Sintering

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100

80

60 1000

1100

1200

1300

1400

T ºC Fig. 3. Sintering curve by Hot Stage Microscopy. Fly ash (d) Slag (s).

developed in the heat treated samples, as assessed by XRD analysis (Fig. 5). The DTA trace of the slag displays a similar trend, with an exothermic effect around 700 °C due to hematite formation and a second exothermic peak at higher temperatures due to mullite and anorthite crystallization. The lower intensity of the peak at 700 °C for the slag could be due to its higher particle size. The sintering curve is depicted by the variation of the specimen height on heating at increasing temperatures (Fig. 3). Both traces are different to that showed by a glassy material, which only display a descendent slope from initial shrinkage to melting temperature [18]. Here, after the initial shrinkage at temperatures above 900 °C, the specimen height remains constant for approximately 100 °C and, fol-

Fly ash (µV)

50 80

60

40

40 30 20

20 200

400

600

800

1000

0 1200

T ºC Fig. 2. DTA traces of IGCC fly ash. (

) and IGCC slag (

).

1500

Fig. 4. Hot stage microscopy images: (a) fly ash; (b) slag.

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lowed by and height increase at temperatures over 1130 °C and 1240 °C for fly ash and slag sample respectively. The maximum value of thermal expansion is in the 20–25% interval, which is reached by the fly ash between 1200°C and 1350 °C, and by the slag at 1300–1350 °C. At upper temperatures, the enclosed gases are easily removed from the low viscosity melt and the specimen height value decreases. Several images recorded during heating are shown in Fig. 4, where the variation in specimen dimensions can be observed. Figs. 5 and 6 show the X-ray diffractograms of samples heat treated for 15 min at increasing

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temperatures. At lower temperatures, the patterns show the characteristic amorphous halo of non-crystalline materials and only over 1050 °C, the background decreases as consequence of the crystallization of mineral phases. The formation of mullite and anorthite phases is easily understandable since the fly ash and slag composition is located inside the mullite formation primary field, near to the eutectic with anorthite crystallization in the corresponding CaO–Al2O3–SiO2 phase equilibrium diagram. The development of porosity with increasing temperatures is observed by SEM in Figs. 7 and 8. The release of

Fig. 5. XRD pattern of slag particles heat treated at increasing temperatures from 900 to 1200 °C.

Fig. 6. XRD pattern of fly ash specimens heat treated at increasing temperatures from 900 to 1200 °C.

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Fig. 7. Porosity development by thermal heating at 1150 °C: (a) IGCC fly ash; (b) IGCC slag.

Fig. 8. Fracture face images of slag particles treated at: (a) 900 °C left and 1200 °C right; (b) magnified image of particle at 900 °C; (c) fracture face and surface view (right) of particle at 950 °C; (d) inner view the porosity developed at 1050 °C.

bubbles from the fly ash specimens starts at 1000 °C, giving rise to a porous glass–ceramic. The development of porosity in the slag particles takes place in the same range of temperatures as fly ash, but accounting of the thickness of slag particles, the resultant porosity is less interconnected. The growth of the porosity with the temperature can be observed in a sequential comparison of images obtained at increasing temperatures.

The evolution of density values confirm the SEM observations, as it can be seen from the plots of density vs. heating temperature (Figs. 9 and 10). The density of powder samples are quite regular, with values between 2.69 and 2.75 g/cm3 for fly ash and 2.68–2.85 g/cm3 for slag. In contrast, the evolution of density with temperature of bulk samples show different behavior. Owing to gases releasing, the fly ash density (Fig. 9) shows a progressive decrease

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3.50

Density Kg/l

3.00 2.50 2.00 1.50 1.00 0.50 800

900

1000

1100

1200

1300

1400

T ºC Fig. 9. Fly ash particles density by Helium pycnometry. Bulk sample (d) powdered sample (s).

3.50

Density Kg/l

3.00 2.50 2.00

1075 °C. At higher temperatures, an increase in density values indicates a change from close to open porosity in fly ash specimens. Fig. 11 shows the water absorption of expanded fly ash and slag particles. The lower absorption values are obtained in the 975–1050 °C interval, which correspond to the lower open porosity as result of the sintering process. In the 1050–1150 °C temperature range the decrease in the melt viscosity facilitates the advance of gases enclosed in close pores, in consequence a connection of close pores gives rise to an increase in open porosity and higher absorption values. Above 1150 °C the decrease in water absorption could be due to the sealing of surface voids by a less viscous liquid phase. The density evolution of slag particles depicts a two step curve (Fig. 10). In a first stage the density decreases in the 900–975 °C interval due to the formation of close pores by exolution of gases initially dissolved in the glassy matrix. The density remains constant in the 975–1125 °C interval and in a second step decrease due to bloating as consequence of the higher kinetic energy of gases entrapped in the pores. The minimum value of density (0.80 g/cm3) is reached at 1200 °C. Finally, the water absorption curve of slag particles depicts a similar trend than fly ash specimens but with higher values of absorption (Fig. 11).

1.50

4. Conclusion

1.00 0.50 800

900

1000

1100

1200

1300

1400

T ºC Fig. 10. Slag density by Helium pycnometry. Bulk sample (d) powdered sample (s).

30 25

% Absorption

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20 15 10 5 0 900

950

1000

1050

1100

1150

1200

TºC

The solid wastes originated in ELCOGAS IGCC power plant exhibit expanding properties at high temperatures due to a gas exolution from the glassy phase. This process is simultaneous to the devitrification of crystalline phases, and as consequence porous lightweight glass–ceramic materials are produced. Mullite, hercynite and anorthite are the main crystalline phases developed from fly ash whereas mullite is the only crystalline phase detected in heat treated slag samples. The expansive behavior is due to the release of gases during the thermal treatments. The gases pull up the viscous melt giving as a result the development of porosity that can be modulated by controlling the temperature of thermal treatments. The different behavior on the development of porosity in fly ash and slag can be explained by the finer particle size and higher content of volatile compounds in fly ash that promotes an earlier liquid phase formation and a greater release and coalescence of bubbles with the consequent opening of the porosity. The ability of these wastes as lightweight aggregates by taking advantage of their foaming reaction is now under research.

Fig. 11. Water absorption. Fly ash specimens (d) slag particles (s).

Acknowledgements from a value of 2.74, which is close to that obtained on the powder sample heat treated at 900 °C, followed by descend of density until the minimum value of 0.90 g/cm3 at

The authors thank ELCOGAS and the European Union for the support of this study as a part of the HIVALUE project ECSC 7220-PR145.

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