Usability of PFBC Coal Ash/Glass Wastes for Heat Insulators Production

Transcription

1 International Journal of Engineering & Technology IJET-IJENS Vol:12 No:04 34 Usability of PFBC Coal Ash/Glass Wastes for Heat Insulators Production Mohammed Al-Naafa Abstract Thermal insulating bricks were produced from treated fly ash/glass pastes. The pastes were prepared by mixing dried fly ash wastes with water at different water to ash weight ratios. The fly ash samples utilized were derived from a pressurized fluidized-bed combustor, PFBC, and were used asreceived (9.7 µm mass median diameter) and after grinding (7.1 µm mass median diameter). The effect of adding car-window glass waste fragments at 0.1, 0.2 and 0.3 weight ratios to the wet ash was also investigated. Tests of the density, thermal conductivity and mechanical properties of the resulting brick were done to evaluate the produced heat insulator properties. The insulators prepared from water to ground ash weight ratio (W/A) of 0.5 showed good specifications in terms of low density and thermal conductivity, and high mechanical strengths. The addition of 10-wt% glass to the wet ground ash enhanced the mechanical properties of the insulating material produced but had no significant effect on its density and thermal conductivity. Index Term Heat Insulator, Pozzolan, PFBC, Coal, Fly Ash, Calcium Silicate Hydrate. I. INTRODUCTION COAL combustion results in a residue consisting of the inorganic mineral constituents (Si, Al, Fe and Ca) in the coal and organic matter which is not fully burned. Ash, the inorganic mineral constituents residue, makes up from 3 to 30 wt% of the coal. Fly ash, the ash collected by the air pollution devices, makes up from 10 to 85 wt% of the ash residue [1]. Despite the more improved power-generation efficiency and less air pollution of the PFBC, compared to other conventional combustion technologies, the huge amounts of the discharged fly ash residue are still causing environmental problems and additional disposal cost. As of 2006, over 72 million tons of coal-combustion derived fly ash were produced in the USA each year, with almost 45 percent of that amount was used in 12 of 15 applications tracked by American Coal Ash Association [2]. Owing to its pozzolanic properties, fly ash is used as a replacement for some of the Portland cement content of concrete [3]. Fly ash exhibits cementitious properties when wetted, and therefore millions of tons of it can be disposed of M. Al-Naafa is with the Chemical Engineering Dept., College of Engineering, Hail University, P.O. Box 2440, Hail 81451, Kingdom of Saudi Arabia Tel manaafa@uoh.edu.sa annually in an environmentally acceptable manner through, for instance, a well-developed technology using fly ash and lime in pavements, buildings and bridge construction [4]. Others [5], [6] have, moreover, showed that active sorbents for sulfur dioxide capture could be prepared by special treatment of fly ashes from both atmospheric fluidized-bed combustors (AFBC) and pulverized coal combustors (PCC). Jozewicz, Chang, Brna, and Sedman [7] reported that the CaO-Al 2 O 3 - SiO 2 -CaSO 4 -H 2 O yielded cementitious phases, which originated from the pozzolanic reaction of lime with fly ash or clay. A pozzolan is defined as a siliceous or a siliceous and aluminous material that in itself possesses little or no cementitious value but that will, in finely divided form and in the presence of moisture, chemically react with calcium hydroxides (slaked lime) at ordinary temperatures to form compounds possessing cementitious properties [8], [9]. The pozzolanic reactions are lime reactions with siliceous and/or with aluminous pozzolanic materials according to the reactions 1 and 2, respectively: 3 Ca( OH ) + 2SiO = 3( CaO) 2( SiO ) 3( H O) (1) Ca( OH ) 2 + Al2O3 + 3H2O = 3( CaO) Al2O3 6( H2O) (2) Fly ash can be also utilized for the production of thermal insulation bricks in a process involving wet mixing of the ingredients of 77 wt% fly ash, 20 wt% lime, 3 wt% gypsum and aluminum powder [1]. The ways of fly ash utilization include concrete production, embankments and other structural fills (usually for road construction), waste stabilization and solidification, cement clinkers production (as a substitute material for clay), mine reclamation, as aggregate substitute material (e.g. for brick production), mineral filler in asphaltic concrete, and various agricultural uses. The current study was aimed at manufacturing thermal insulation bricks only from sub-bituminous or lignitic coalfired PFBC fly ash containing considerable amounts of anhydrite and lime (12.8 wt% CaSO 4 and 24.6 wt% CaO). The density, thermal conductivity and mechanical properties of the manufactured brick were measured for the sake of evaluating its suitability as a thermal insulator. The effect of glass wastes addition to fly ash was also investigated. I. PROCEDURE FOR PAPER SUBMISSION A. Fly Ash and Glass Samples The fly ashes employed in the present study were collected

2 International Journal of Engineering & Technology IJET-IJENS Vol:12 No:04 35 from the cyclone of a 71-MW pilot PFBC (Wakamatsu City, Go Island, Japan). These cyclone ash samples were oven dried for 24 hrs at 200 C to ensure the transformation of gypsum hydrates to anhydrous gypsum. Part of the ash was ground and sieved to be less than 58 µm in diameter. Based on the mass equivalent and using a laser granulometer (Granulometer HR 850B, Cilas Alcatel, Marcoussis RC, France), the particle-size distributions in both the as-received (unground) ash and the ground ash are shown in Fig. 1. Mass% of particles Ground ash Unground ash Particle diameter (μm) Fig. 1. Particle-size distributions in unground, and ground Wakamatsu fly ashes.. The particle diameter ranged from 1 to 247 µm and from 1 to 58 µm for the as-received and ground ashes, respectively. The mass median particle diameter was 9.7 µm for the as-received ash and 7.1 µm for the ground ash. Table 1 presents the chemical composition of an arbitrary sample of the dried ash. TABLE I CHEMICAL COMPOSITION OF WAKAMATSU FLY ASH Component wt% Unburned C 1.5 CaSO CaO 24.6 SiO Al 2 O Fe 2 O Difference 23.8 The presence of silica (24.1 wt %) and alumina (12.3 wt%) is expected to be beneficial for the pozzolanic reactions and strength gain. The mass ratios of slaked lime to silica (1.89) and slaked lime to alumina (3.7) are higher than what is required according to reactions 1 and 2. The glass samples were obtained as wastes from Toyota Car Corporation (Toyota, Japan), crushed and sieved to particles smaller than 40 µm. Their chemical composition is shown in Table 2. Component TABLE 2 CHEMICAL COMPOSITION OF TOYOTA CAR GLASS wt% SiO Al 2 O CaO 9.2 MgO 2.0 Na 2 O 14.1 B. Manufacturing Procedure Cyclone Ash Paste The fly ash paste was prepared by slurrying the ash with deionized water inside a vessel agitated at 170 rpm for 5 min. Initial hydration of the ash resulted in mix temperatures over 100 C caused by the exothermic reactions of water with ash CaO and, to a lesser extent, with CaSO 4. The 5-min hydration period was considered to minimize this undesirable temperature rise. The water to ash (W/A) weight ratios varied from 0.5 to 1.0. The paste was poured into a wooden rectangular mold of length, width and depth of 6.0, 4.5 and 2.0 cm, respectively, and it was then placed in a humiditycontrolled chamber (Tabai Labostar, ESPEC LH-112T) for curing at 80% relative humidity and at 80 C for 3 days. Cyclone Ash-Glass Paste The ash-glass paste was prepared by adding glass fragments to the cyclone ash in deionized water. Three sets of different glass to ash weight ratios (G/A) of 0.1, 0.2 and 0.3 were slurried with water at 0.5 W/A ratio for 5 min and at 170-rpm agitation speed. The curing method and conditions follow the same procedure described in the previous section. Characterization of the Produced Insulator For the produced insulating brick, the external surface and microstructure were observed by a scanning electron microscope (SEM), and the crystalline and glassy phases were determined using an X-ray powder diffractometer (Rigaku XRD). The density was determined by measuring the brick s weight and volume, the thermal conductivity was measured by a Quick Thermal Conductivity Meter (QTM-500, Kyoto Denshi Kyogyoo Kabushikigaisha), and the mechanical properties (bending and compressive strengths) were measured with a tensile-compressive tester (Imada Factory, Load Tester Type SV55, Toyohashi, Japan). II. RESULTS AND DISCUSSION A. Effect of W/A on Insulator Characteristics Figs. 2 5 show the effect of the W/A mass ratio on the density, thermal conductivity (k), and bending and compressive strengths of the FA insulating specimens produced from unground (as-received) ash and on other specimens produced from ground ash.

3 International Journal of Engineering & Technology IJET-IJENS Vol:12 No:04 36 Density (kg/m 3 ) Fig. 2. Effect of W/A ratio on density of produced unground and ground FA insulators. and thermal conductivities, appreciable losses in their mechanical properties were experienced. Notice that only slight improvement in their conductivities was observed at W/A ratios greater than 0.6. The main cause for the drop in these properties with increasing the W/A ratio was attributed to the more porous structure developed through water evaporation during the 3 day curing. Bending Strength (MPa) Unround FA insulator Fig. 4. Effect of W/A ratio on bending strength of produced unground and ground FA insulators. k (W/m.K) Fig. 3. Effect of W/S ratio on thermal conductivity of produced unground and ground FA insulators. As can be seen in this figure, the insulators prepared from the ground ash possessed enhanced properties in terms of lower densities ( kg/m 3 ), lower conductivities ( W/m K), larger bending strengths ( MPa) and larger compressive strengths ( MPa) compared to those of insulators from unground ash. The obtained minimum thermal conductivity (0.128 W/m K) is lower compared to the minimum W/m K of the thermal insulator prepared from coal fly ash, clay, perlite and epoxidized linseed oil reported by Balo, Ucar, and Yücel [10]. Although the increase in W/A ratio from 0.5 to 1.0 created insulators with lower densities Compressive Strength (MPa) Fig. 5. Effect of W/A ratio on compressive strength of produced unground and ground FA insulators. B. SEM and XRD Findings Typical SEM micrographs showed that the produced ash insulator was composed of rough particles with the presence

4 International Journal of Engineering & Technology IJET-IJENS Vol:12 No:04 37 of networks of acicular (needle-shaped) crystals. The porous microstructure is believed to be responsible for the lower density and conductivity for the produced insulator, while its developed mechanical properties are most likely due to the networks of the needle-shaped crystals inside the available pore space. Based on XRD analysis, cementitious phases such as calcium silicate hydrates, C-S-H (tobermorite; 5CaO 6SiO 2 5H 2 O), calcium aluminate sulfate hydrate (ettringite; 3CaO Al 2 O 3 3CaSO 4 32H 2 O), and hemihydrate (CaSO 4 0.5H 2 O) were detected in the ash insulator. Tsuchiai, Ishizuka, Ueno, Hattori, and Kita [11] reported that the hydration of a mix of coal fly ash, lime and anhydrite resulted in the formation of C-S-H and ettringite. Graham, Sutterer, and Robl [4] related the strength gain of the hydrated fly ash to the formation of C-S-H and partly to the formed ettringite network. Ettringite needles form in the early hydration time when the ash anhydrite (12.8 wt%, Table 1) reacts with aluminate and lime. The C-S-H accounting for a long-term increase in the compressive strength. The hydrated calcium silicates and/or aluminates were found to be mainly responsible for strength development in both cementhardening reactions and pozzolanic reactions as referred by Pollard, Montgomery, and Sollar [12]. C. Effect of Glass Addition New thermal insulators were prepared from ground ash and glass fragments at W/A ratio of 0.5 while other slurrying and curing conditions remained unaltered (slurrying at 170 rpm for 5 min, and curing at 80 C, and 80% relative humidity for 3 days). Three sets of insulating specimens of different glass to ash (G/A) weight ratios of 0.1, 0.2 and 0.3 were produced. The density, conductivity and mechanical properties of these new glass/ash insulators were measured and compared with the previously made ash insulators as shown in Fig. 6. Addition of glass up to G/A ratio of 0.3 was found to have no significant effect on the density and thermal conductivity of the insulator as can be observed in Figs. 6a and 6b. On the contrast, there was an increase in the measured bending and compressive strengths of about 30% and 10%, respectively, at G/A ratio of 0.1 (Figs. dc and 6d). Mixes with G/A ratios of 0.2 and 0.3, however, experienced a significant fall in their mechanical properties. This fall might be attributed to depletion in the formation of C-S-H and/or ettringite due to insufficient alumina and lime in glass (2.3 wt% Al 2 O 3, and 9.2 wt% CaO). Additional studies on the effect of glass on the produced insulators will be conducted in the near future to draw more conclusive results. found to develop in a network of crystals that reinforced and strengthened the structure of produced insulation brick. The addition of 10% of glass to the ash paste enhanced the mechanical properties of the insulator produced but had no significant effect on its density and thermal conductivity. Further research work may be done to investigate the specific surface area and pore volume of ash before and after synthesis. REFERENCES [1] S. Torrey, Coal ash utilization: fly ash, bottom ash and slag, Noyes Data Corporation, NJ, USA., [2] American Coal Ash Association 2006, Coal Combustion Products Production & Use Statistics. [3] A. N. Scott, and M. D. A. Thomas, Evaluation of fly ash from cocombustion of coal and petroleum coke for use in concrete, ACI Materials Journal (American Concrete Institute), 104 (1): 62 70, [4] U. Graham, K. G. Sutterer, and T. L. Robl, The relationship of mineralogy and strength development in dry FGD derived cements, Proceedings of the 8 th International Conference on Coal Science (8 th ICCS), September 10-15, Oviedo, Spain, p , [5] A. Al-Shawabkeh, H. Matsuda, and M. Hasatani, Utilization of highly improved fly ash for SO 2 capture, J. Chem. Eng. Japan, 28(1), 53-58, 1995a. [6] A. Al-Shawabkeh, H. Matsuda, and M. Hasatani, Comparative reactivity of treated FBC- and PCC-fly ashes for SO 2 removal, Canad. J. Chem. Eng., 73, , 1995b. [7] W. Jozewicz, J. C. S. Chang, T. G. Brna, and C. B. Sedman, Reactivation of solids from furnace injection of limestone for SO 2 control, Environ. Sci. Technol., 21(7), , [8] ASTM Specification C311-92, Standard test methods for sampling and testing fly ash or natural pozzolans for use as a mineral admixture in Portland cement concrete, American Society of Testing Materials, Philadelphia, PA., [9] ACI, ACI Concrete Terminology, American Concrete Institute, Farmington Hills, MI, (accessed June 1, 2012), [10] F. Balo, A. Ucar, and H. L. Yücel, Development of the insulation materials from coal fly ash, perlite, clay and linseed oil, Ceramics Silikáty, 54 (2) , [11] H. Tsuchiai, T. Ishizuka, T. Ueno, H. Hattori, and H. Kita, Highly active absorbent for SO 2 removal prepared from coal fly ash, Ind. Eng. Chem. Res., 34(4), , [12] T. J. S. Pollard, M. D. Montgomery, and J. C. Sollar, Organic compounds in the cement-based stabilization/solidification of hazardous mixed waste mechanistic and process consideration, Imperial College Center for Toxic Waste Management, Imperial College of Science, Technology, and Medicine, London, UK, SW7 2BU., III. CONCLUSIONS Thermal insulation bricks can be manufactured from PFBCdischarged fly ash treated in a controlled hydration and curing environment. The pozzolanic reaction between the hydrated fly ash constituents resulted in a developed porous microstructure in the produced insulator specimen to which the lower conductivity and density were attributed. The strength gain of the produced insulation brick was related to the C-S-H and ettringite crystals formation. Ettringite was

5 International Journal of Engineering & Technology IJET-IJENS Vol:12 No: a b Density (kg/m 3 ) 1200 k (W/m.K) c 4.0 6d Bending Strength (MPa) Compressive Strength (MPa) Fig. 6. Effect of G/A ratio on density (6a), thermal conductivity (6b), bending (6c) and compressive strengths (6d) of FA insulators.