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Production of porous material from coal ash discharged from fluidized bed combustor
Coal Science
J.A. Pajares and J.M.D. Tasc6n (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
PRODUCTION
1983
OF P O R O U S M A T E R I A L F R O M C O A L A S H
DISCHARGED FROM FLUIDIZED
BED COMBUSTOR
T e r u y u k i O k a j i m a , Y o n g Chen and S h i g e k a t s u M o r i * Dept. o f C h e m . E n g . , N a g o y a U n i v e r s i t y F u r o - C h o , C h i k u s a - Ku, N a g o y a 4 6 4 - 0 1 , J a p a n (*Corresponding author)
INTRODUCTION Above 50% of fly ash discharged from pulverized coal combustor (PCC-ash) are used by cement industry in Japan. Since the limestone is fed as the in-bed desulferization material into the pressurized fluidized bed coal combustor(PFBC), PFBC ash contains considerable amount of CaSO 4 and unreacted CaO. PFBC-ash is difficult to feed into the cement kiln like as PCC-ash, since CaSO 4 is easily decomposed and then the sulfer is accumulated in the kiln . In case that PFBC-ash is used as the landfill materials, CaO contained in PFBC-ash is dissolved in water, consequently it is difficult to treat the PFBCash. To utilize the PFBC-ash, various materials have been tried to be produced. However, most of them could not economically succeeded to develop the industrial process. PFBC ash consists of the fly ash collected from the cyclones and the filter, and of the boiler ash discharged from the combustor. The fly ash contains mainly SiO 2, A120 3 and Fe203 like as the clay. The boiler ash contains more calcium compounds such as CaSO 4 and unreacted CaO, CaCO3.These calcium compounds have the self-hardening property. 1) In this paper, a new process to produce a heat insulator from PFBC-ash is proposed.
1.EXPERIMENTAL METH OD 1.1 Samples Figure 1 shows a schematic diagram of Wakamatsu PFBC pilot plant in Japan. This P F B C - a s h consists of filter fly ash, cyclone fly ash, bed ash and bottom ash. Since the properties of the filter fly ash is similar to that of the cyclone fly ash, and the properties of the bottom ash is similar to that of the bed ash, the filter fly ash and bottom ash are used here as the raw materials. Their chemical compositions are shown in Table 1. The boiler ash contains more calcium compounds such as CaSO 4 and also Ca(OH) 2, CaCO 3. Ca(OH) 2 and CaCO 3 are considered to be formed from CaO after discharged from
1984 the PFBC plant.The particle sizes of the fly ash is under 100/1 m and the boiler ash are 37210~tm. 1.2 Produ ction Procedure At first, the slurries are prepared by mixing the fly ash and the boiler ash with water in a stirred tank.Stirr time is 5 minutes and the rate is 169 r.p.m.. The mixed ratio of the fly ash to the boiler ash (F/B) is changed from 0.6 to 2.0, and also the ratio of water to the total solid (W/S) is changed from 0.6 to 1.4. The slurries are fed into a woodwn rectangular mold which size is 6.0 x 4.5cm and its height is 2.0cm and then this mold is settled in a curing box during 24h.The curing temparature is 353K and its relative humidity is 50%. 1.3 Char acterization of Prod uced Material The density of the produced heat insulator is determined by measuring its weight and volume. Thermal conductivity is measured by the hot-wire method of the unsteady-state heat flow. Its bending and compressive strength are measured by the tensile-compressive testing
2. EXPERIMENTAL RESULTS Figure 2 shows the external surface of the produced heat insulator, and it is found that the porous material can be produced. The porous material are formed by the evaporation of the water during its curing process. Micro structure of the insulator observed by the scanning electron microscope(SEM) is shown in Figure 3. The network structure in the insulator is formed by acicular growth of the crystals. Figure 4 shows X-ray diffraction(XRD) patterns of the insulator, and crystals of calcium silicate hydrates (to bermo rite) and of calcium aluminate sulfate (ettringite) are detected. These crystals are produced by the pozzolanic reaction . Figure 5 shows the effects of the ratios of (F/B) and (W/S) on the density, the thermal conductivity and the bending and compressive strength of the insulator. It can be found from these figures that the density is 920-1280kg/m3, the thermal conductivity is 0. 04-0. 08 W/m " K, the bending strength is 0.3-1.4MPa and the compressive strength is 1.0-3.6MPa. These values decrease with increasingly the ratio of W/S until 1.0 and the effect of F/B is not clearly found. The density, the thermal conductivity and the compressive strength of the insulator is compared with these determined values of the brick as B 1 by Japan Industril Standard. It can be demonstrated from this table that although the density of the insulator produced in this work is larger than that of B 1 brick, the thermal conductivity is considerably smaller and also the compressive strength is better than that of B 1 brick.
3. C OCLUS ION To utilize PFBC-ash, a new process to produce a heat insulator made from PFBC-ash is proposed, and it can be demonstrated that although the density of the heat insulator
1985 produced in this work is larger than that of B1 brick, its thermal conductivity is considerably smaller and also the compressive strength is better than that of B 1 brick.
ACKNOWLEDGMENT This study is supported by GENERAL S EKIYU RESEARCH & DEVELOPMENT ENCOURAGEMENT & AS SISTANCE FOUNDATION.
REFERENCE 1) H. Ohhara, N. H osoda and M. Ohhara ;Hardening Character of Fluidized Bed Combustion (E B.C.) Ash, Proc. 46th Annual Conference, JSCE, 1991 2) T.Murakami, K. Miwa and K. Suda;Production test of ALC utilizing Coal ash, 4th conference on Coal Utilization Technology, 1994 3) K. Nyuuraku;Sekitan Riyou Gijutu Jouhou, 16-2, Feb, 1995 4) H. Ohhara, H. Niijima and N. Hosoda;R&D on Utilization of Coal Ash General from F.B. C for Civil Material, 1st Conference on Coal Utilization Technology (CCUJ) Sept. 1991
Table 1 Composition of filter ash and bottom ash unbumed CaSO4 carbon filter
Ca(OH)2
CaCO3
SiO2
Al203
5.9
40.5
21.3
19.4
11.3
4.3
ash[%]
4.6
10.0
2.6
bottom ash[%]
0.0
27.1
29.3 ,
Fe203
diff.
1.8
13.3
3.6
5.0
,,
Fig.2 The external surface of the produced heat insulator
1986
Fig.3 SEI~ photograph of the produced heat insulator
Fig.4 Xl~ pattern of heat insulator
Figure 5 The effectsof the ratio of waterto solid and the ratio of fly ash to boilerash on the density(a), the thermalconductivity(b),the flexualstrength (c) and the compressivestrength(d). Table 2 Comparison h e a t insulator(F/B=2.0,W/S=0.6) with B1 b r i c k dens i ty
[kg/m3 ]
[W/m" K]
compressive strength [~a]
thermal conduct i v i ty
heat insulator
1080
0.053
2.73
B1 b r i c k
700
0.13
2.53
J.A. Pajares and J.M.D. Tasc6n (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
PRODUCTION
1983
OF P O R O U S M A T E R I A L F R O M C O A L A S H
DISCHARGED FROM FLUIDIZED
BED COMBUSTOR
T e r u y u k i O k a j i m a , Y o n g Chen and S h i g e k a t s u M o r i * Dept. o f C h e m . E n g . , N a g o y a U n i v e r s i t y F u r o - C h o , C h i k u s a - Ku, N a g o y a 4 6 4 - 0 1 , J a p a n (*Corresponding author)
INTRODUCTION Above 50% of fly ash discharged from pulverized coal combustor (PCC-ash) are used by cement industry in Japan. Since the limestone is fed as the in-bed desulferization material into the pressurized fluidized bed coal combustor(PFBC), PFBC ash contains considerable amount of CaSO 4 and unreacted CaO. PFBC-ash is difficult to feed into the cement kiln like as PCC-ash, since CaSO 4 is easily decomposed and then the sulfer is accumulated in the kiln . In case that PFBC-ash is used as the landfill materials, CaO contained in PFBC-ash is dissolved in water, consequently it is difficult to treat the PFBCash. To utilize the PFBC-ash, various materials have been tried to be produced. However, most of them could not economically succeeded to develop the industrial process. PFBC ash consists of the fly ash collected from the cyclones and the filter, and of the boiler ash discharged from the combustor. The fly ash contains mainly SiO 2, A120 3 and Fe203 like as the clay. The boiler ash contains more calcium compounds such as CaSO 4 and unreacted CaO, CaCO3.These calcium compounds have the self-hardening property. 1) In this paper, a new process to produce a heat insulator from PFBC-ash is proposed.
1.EXPERIMENTAL METH OD 1.1 Samples Figure 1 shows a schematic diagram of Wakamatsu PFBC pilot plant in Japan. This P F B C - a s h consists of filter fly ash, cyclone fly ash, bed ash and bottom ash. Since the properties of the filter fly ash is similar to that of the cyclone fly ash, and the properties of the bottom ash is similar to that of the bed ash, the filter fly ash and bottom ash are used here as the raw materials. Their chemical compositions are shown in Table 1. The boiler ash contains more calcium compounds such as CaSO 4 and also Ca(OH) 2, CaCO 3. Ca(OH) 2 and CaCO 3 are considered to be formed from CaO after discharged from
1984 the PFBC plant.The particle sizes of the fly ash is under 100/1 m and the boiler ash are 37210~tm. 1.2 Produ ction Procedure At first, the slurries are prepared by mixing the fly ash and the boiler ash with water in a stirred tank.Stirr time is 5 minutes and the rate is 169 r.p.m.. The mixed ratio of the fly ash to the boiler ash (F/B) is changed from 0.6 to 2.0, and also the ratio of water to the total solid (W/S) is changed from 0.6 to 1.4. The slurries are fed into a woodwn rectangular mold which size is 6.0 x 4.5cm and its height is 2.0cm and then this mold is settled in a curing box during 24h.The curing temparature is 353K and its relative humidity is 50%. 1.3 Char acterization of Prod uced Material The density of the produced heat insulator is determined by measuring its weight and volume. Thermal conductivity is measured by the hot-wire method of the unsteady-state heat flow. Its bending and compressive strength are measured by the tensile-compressive testing
2. EXPERIMENTAL RESULTS Figure 2 shows the external surface of the produced heat insulator, and it is found that the porous material can be produced. The porous material are formed by the evaporation of the water during its curing process. Micro structure of the insulator observed by the scanning electron microscope(SEM) is shown in Figure 3. The network structure in the insulator is formed by acicular growth of the crystals. Figure 4 shows X-ray diffraction(XRD) patterns of the insulator, and crystals of calcium silicate hydrates (to bermo rite) and of calcium aluminate sulfate (ettringite) are detected. These crystals are produced by the pozzolanic reaction . Figure 5 shows the effects of the ratios of (F/B) and (W/S) on the density, the thermal conductivity and the bending and compressive strength of the insulator. It can be found from these figures that the density is 920-1280kg/m3, the thermal conductivity is 0. 04-0. 08 W/m " K, the bending strength is 0.3-1.4MPa and the compressive strength is 1.0-3.6MPa. These values decrease with increasingly the ratio of W/S until 1.0 and the effect of F/B is not clearly found. The density, the thermal conductivity and the compressive strength of the insulator is compared with these determined values of the brick as B 1 by Japan Industril Standard. It can be demonstrated from this table that although the density of the insulator produced in this work is larger than that of B 1 brick, the thermal conductivity is considerably smaller and also the compressive strength is better than that of B 1 brick.
3. C OCLUS ION To utilize PFBC-ash, a new process to produce a heat insulator made from PFBC-ash is proposed, and it can be demonstrated that although the density of the heat insulator
1985 produced in this work is larger than that of B1 brick, its thermal conductivity is considerably smaller and also the compressive strength is better than that of B 1 brick.
ACKNOWLEDGMENT This study is supported by GENERAL S EKIYU RESEARCH & DEVELOPMENT ENCOURAGEMENT & AS SISTANCE FOUNDATION.
REFERENCE 1) H. Ohhara, N. H osoda and M. Ohhara ;Hardening Character of Fluidized Bed Combustion (E B.C.) Ash, Proc. 46th Annual Conference, JSCE, 1991 2) T.Murakami, K. Miwa and K. Suda;Production test of ALC utilizing Coal ash, 4th conference on Coal Utilization Technology, 1994 3) K. Nyuuraku;Sekitan Riyou Gijutu Jouhou, 16-2, Feb, 1995 4) H. Ohhara, H. Niijima and N. Hosoda;R&D on Utilization of Coal Ash General from F.B. C for Civil Material, 1st Conference on Coal Utilization Technology (CCUJ) Sept. 1991
Table 1 Composition of filter ash and bottom ash unbumed CaSO4 carbon filter
Ca(OH)2
CaCO3
SiO2
Al203
5.9
40.5
21.3
19.4
11.3
4.3
ash[%]
4.6
10.0
2.6
bottom ash[%]
0.0
27.1
29.3 ,
Fe203
diff.
1.8
13.3
3.6
5.0
,,
Fig.2 The external surface of the produced heat insulator
1986
Fig.3 SEI~ photograph of the produced heat insulator
Fig.4 Xl~ pattern of heat insulator
Figure 5 The effectsof the ratio of waterto solid and the ratio of fly ash to boilerash on the density(a), the thermalconductivity(b),the flexualstrength (c) and the compressivestrength(d). Table 2 Comparison h e a t insulator(F/B=2.0,W/S=0.6) with B1 b r i c k dens i ty
[kg/m3 ]
[W/m" K]
compressive strength [~a]
thermal conduct i v i ty
heat insulator
1080
0.053
2.73
B1 b r i c k
700
0.13
2.53