Latent Heat of Amorphous Slags and Their Utilization as a High Temperature PCM

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1 , pp Latet Heat of Amorphous Slags ad Their Utilizatio as a High Temperature PCM Yoshiaki KASHIWAYA, 1) Tomohiro AKIYAMA 2) ad Yutaro IN-NAMI 3) Formerly Graduate School of Egieerig, Hokkaido Uiversity. Now at Departmet of Eergy Sciece ad Techology, Kyoto Uiversity, Sakyo-ku, Yoshida Homachi, Kyoto Japa. 2) Ceter for Advaced Research of Eergy Coversio Materials, Hokkaido Uiversity, 3) Formerly Studet of Graduate School of Egieerig, Hokkaido Uiversity. Now at Sumitomo Metal Idustries, Ltd. (Received o February 16, 2010; accepted o April 30, 2010) I a previous study, a rotary cylider atomizig (RCLA) method for slag was developed ad the slag spoutig behavior from a ozzle was ivestigated. The theoretical approach of the previous study agreed well with the observed result. I this study, the potetial of slag particles obtaied by RCLA method as a phase chagig material (PCM) was ivestigated. The latet heats ad the temperature rages of heat geeratio were measured for two kids of amorphous slags (slag particles ad water graulated-slag from blast furace) usig DTA (differetial thermal aalysis). Mathematical simulatio of a coutercurret heat exchager was performed ad the effect of slag latet heat o gas temperature was estimated. The temperature rage of the latet heat of amorphous slag was estimated to rage from 800 to 950 C. The values of latet heat for slag particles ad the water-graulated slag were 294 kj/kg ad 235 kj/kg, respectively. Simulatio results idicated that the latet heat of slag ca decrease the temperature of waste gas which might icreased the usable kid of waste gas. KEY WORDS: slag recycle; amorphous slag; rotary cylider atomizig; PCM. 1. Itroductio Fidig ew ways to decrease CO 2 emissio i order to preserve the eviromet is a importat challege i the Japaese steel idustry. I a previous paper, 1) two methods of rotary cylider atomizatio (RCLA) of blast furace (BF) slag were developed, ad the slag particles obtaied from these processes were aalyzed. Usig a high-speed camera, the spoutig behaviors of slag from ozzles were observed ad compared to a theoretical explaatio of slag atomizatio i aother paper. 2) The relatioship betwee slag particle diameter (d) ad ozzle diameter (2a) was derived as follows: 2 2 π ρlz 3 2a d...(1) 5 400γ where r is desity, L is the distace from the ceter of rotatio to the tip of the slag strig, Z is rotatio speed ad g is surface tesio of slag. The flow rate of slag was evaluated usig Hage Poiseuille s equatio, ad the relatioship betwee particle diameter ad slag strig diameter was obtaied usig Weber s equatio. From a compariso of experimetal results ad theoretical calculatios, it was cocluded that a strig diameter of 0.2 mm for a ozzle with a diameter of 1.3 mm was adequate i this experimet. O the other had, it is importat to develop a way of utilizatio for the obtaied slag particles. The slag particles have two characteristic features, oe is a small sphere aroud 1 mm i diameter ad the other is a amorphous phase. The later feature meas that the material has a latet heat of crystallizatio, which is a huge ethalpy, because about 38 Mto of slag per year are produced i Japa. I the preset study, we ivestigated the possibility of a amorphous slag as a PCM (Phase Chage Material). The geeral recogitio to PCM may be that the material itself is used for may times. However, i the case of slag, as the huge amout ad high temperature of slags are exhausted, the reuse of slag for PCM is ot ecessary. This is a quite large advatage of slag as a PCM. I this study, the utilizatio of slag particles by the RCLA method was ivestigated. Particularly, the amorphous phase of slag has a latet heat of crystallizatio. The temperature rage of crystallizatio ad the amout of latet heat of slag particles were measured ad compared to those of water-graulated slag. Because there is a huge amout of readily available water-graulated slag, it would be highly desirable as a high temperature phase chage material (PCM) if its latet heat ca be utilized. Oe example of the utilizatio as the slag PCM was show usig mathematical simulatio. The subject of simulatio is a waste heat recovery from high temperature gas more tha 900 C ISIJ

2 such as a LD coverter gas. Usig a coutercurret heat exchager, amorphous slag is heated by the waste gas ad the lower temperature gas ca be used for other process. This simulatio is supposig a cascade utilizatio of waste gas, because, some time, it is difficult to utilize the gas i high temperature. The utilizatio of latet heat of slag crystallizatio ca decrease the temperature of waste gas, or i other words, eve a lower grade waste heat ca be used. 2. Experimetal The experimetal apparatus ad procedure for makig the slag particles are the same as that described i a previous paper. 1) The slag sample used i this study cosists of BF slag, wherei its chemical compositio is summarized i Table 1. The slag particles used i this study were fabricated usig a multi-ozzle rotary cylider at a rotatioal speed of rpm ad with a 1.3-mm diameter ozzle. The obtaied slag particles were served for differetial thermal aalysis (DTA) ad differetial scaig calorimetry (DSC), i the both measuremet, samples (20 mg, mmf) were set i a Pt pa (5 mmf, 2.5 mm i height). The temperature rage of latet heat formatio ad the latet heat were determied. I additio, XRD (X-ray diffractio) aalysis ad optical microscope observatio before ad after experimet were performed. 3. Results ad Discussio 3.1. Measuremet of the Temperature Rage of Latet Heat Geeratio I this experimet, amorphous phase slag particles obtaied from the multi-ozzle rotary cylider were used. If the latet heat of crystallizatio could be utilized, eergy could be saved ad CO 2 reductio is possible; however, there is o data i the literature cocerig the latet heat of amorphous slag. Figure 1 depicts the TTT (time temperature-trasformatio) diagram of BF slag measured by Kashiwaya et al. 3) It Table 1. Chemical compositio of BF slag. was observed that two kid of crystals, Gehleite (2CaO Al 2 O 3 SiO 2 ) ad Merwiite (3CaO MgO 2SiO 2 ), precipitate i BF slag. The ose of the TTT diagram depicted i Fig. 1 occurs at C ad 4 s, which correspods to the area of Merwiite, whereas the area of Gehleite is covered by the area of Merwiite. I Fig. 1, the glassy area is expressed util s. Whe the multi-ozzle rotary cylider was operated at rpm with a 1.3-mm diameter ozzle, the slag spouted from the ozzle cosisted of a strig shape that quickly separated i particles. Durig this fabricatio process, if a slag particle hit the wall of the cotaier, its shape was ot spherical. I a previous paper, 1) it was observed that the temperature of slag particles decreased from to 800 C withi s, which is ot hot eough to deform the slag sphere. The estimated coolig rate is also plotted o the Fig. 1, wherei it ca be observed that the slag has eough time to stay amorphous phase. The temperature profiles depicted i Fig. 1 resulted whe the amorphous slag was heated to temperatures greater tha 800 C at heatig rates of 10 C/mi or 5 C/mi. These heatig curves itersected the crystallizatio area at approximately 900 C i Fig. 1. If the temperature had bee iitially icreased from room temperature istead of from 800 C, which could ot have bee plotted i Fig. 1, the aforemetioed heatig curves might have itersected the crystallizatio area at approximately 800 C due to a gradual decrease of the low temperature side of the crystallizatio area over time. The temperature rage of amorphous slag latet heat geeratio was measured usig DTA at a heatig ramp of 5 C/mi up to C. The obtaied result of this measuremet is depicted i Fig. 2. It is difficult to determie the baselie of DTA because it chages with heatig rate ad is affected by heat geeratio i the sample. It is importat to compare collected DTA data to the baselie of a empty crucible. Noetheless, the baselie was sometimes moved by ukow causes. A edothermic dehydratio reactio at less tha 100 C ca be observed i Fig. 2, o the other had, the baselie aroud C quickly icreased because the furace cotrol was chaged for a maximum temperature of C. Except for these temperature rages, a heat evolutio was observed from 550 to C. A dehydratio reactio observed at a temperature less tha 100 C is most likely due to adsorbed water o the surface of the Fig. 1. TTT diagram of blast furace slag obtaied by Kashiwaya Fig. 2. DTA curve of blast furace slag particles obtaied from a et al. 3) multi-ozzle rotary cylider ISIJ 1260

3 slag while it was kept i a ambiet atmosphere. As metioed i a previous paper, 1) the slag particles used i this study typically have may small holes o the surface. I additio, may traces of gas bubbles iside the particle ear surface which precipitated durig solidificatio were foud. It was cosidered that the holes o the surface was coected to the gas bubbles ear surface. These holes also provide a additioal surface area that icreases moisture adsorptio. Figure 2 depicts the possible two baselie chages i the temperature rage from 550 to C. To make clear the itrisic baselie, the samples were queched from 600 ad 950 C, as depicted i Fig. 3 (the slag particles used here cosisted of mixtures of glassy but relatively large ad stickig particles). Figure 3(a) depicts the appearaces of slag particles before ad after heat treatmet at 600 C. There was o chage i slag particle appearace at 600 C, whereas the slag particles were observed to have bee crystallized by the heat treatmet at 950 C (Fig. 3(b)). XRD patters after heat treatmets at 600 C ad 950 C are depicted i Figs. 4(a) ad 4(b), respectively, wherei it ca be see that the origially amorphous slag particles aealed at 600 C were amorphous (XRD patter was the same as before heatig) ad those at 900 C were crystallie. The crystallie phases i the slag particle aealed at 950 C cosisted of Merwiite ad Gehleite, as idicated by the TTT diagram (Fig. 1). From this result, the baselie of the DTA was determied ad is depicted by the solid lie i Fig Measuremet of the Latet Heat of Amorphous Water-graulated Slag ad Slag Particles The latet heat of amorphous slag was measured usig DTA ad DSC. Although it is difficult to determie the baselie of the peak of heat geeratio i the case of DTA, a reliable baselie was idetified via quechig at several temperatures, as previously metioed. I this study, the latet heat of water-graulated slag was measured such that it could be a poit of compariso for the slag particles. Sice there are substatial amouts of water-graulated slag available, it would be potetially valuable to use these materials as high temperature PCMs if their latet heats could be utilized. Ufortuately, it was observed that water-graulated slag degraded with time. Withi oe year, the amorphous phase was preserved; however, the slag gradually crystallized over periods of more tha oe year (Fig. 5). I the preset study, the latet heats of three kids of slag, ew water-graulated slag (withi 1 moth of productio), old water-graulated slag (more tha 1 year old), ad slag particles, were measured usig DTA ad DSC. DTA measuremet was coducted usig a heatig ramp of 5 C/mi util C uder a Ar atmosphere, ad the results are summarized i Fig. 6. The old water-graulated slag exhibited o exothermic reactio peak i the temperature rage of 800 to 950 C. I cotrast, the ew water-graulated slag ad the slag particles obtaied i the preset study exhibited exothermic reactios of crystallizatio i this same temperature rage. The maximum peak for the ew water-graulated slag was observed at 930 C, whereas the slag particles exhibited two exothermic reactio peaks, Fig. 3. Compariso of slag particles before ad after aealig at differet temperatures (600 C ad 950 C). Fig. 5. Compariso of XRD patters of ew water-graulated slag to water-graulated slag more tha oe year old. Fig. 4. Compariso of XRD patters of amorphous ad crystallized slag particles aealed at 600 C ad 950 C, respectively. Fig. 6. Compariso of slag particle, ew water-graulated slag, ad old water-graulated slag DTA curves ISIJ

4 most likely correspodig to the crystallizatios of Merwiite ad Gehleite phases. The area of the exothermic reactio peak correspods to the ethalpy chage of crystallizatio (latet heat), DH cry. The area of the heat geeratio, A S, ca be related to the latet heat of crystallizatio (DH cry ) usig R S, which is a istrumetal costat ad represets the thermal resistace betwee the sample ad the sample-temperature measurig poit. 4) AS RSDHcry...(2) Because the value of R S for DTA is a istrumetal costat, it was estimated i advace by comparig the DTA results to the DSC results; however, the usage of DSC was limited i the authors laboratory, ad therefore, DTA was primarily used for thermal aalysis. The obtaied latet heats of slag particles ad ew watergraulated slag were 294 kj/kg ad 235 kj/kg, respectively, as show i Fig. 6. This differece i latet heat results from the respective slag treatmets, wherei the former slag was made usig a dry quechig method (slag particle) ad the latter by a water quechig method. The lower latet heat of the water-graulated slag (235 kj/kg) i compariso to the slag particles (294 kj/kg) is most likely due to some degradatio caused by a reactio with water. Figure 7 compares XRD patters of ew water-graulated slag ad slag particles after DTA measuremet coducted with a heatig ramp of 5 C/mi to a maximum temperature of 950 C uder a Ar atmosphere. From this figure, it ca be observed that both slags were completely crystallized ad cosisted of Gehleite ad Merwiite crystal phases. Table 2 depicts the results of slag latet heat measuremet ad the correspodig estimated decrease i CO 2 per year. Approximately 25 Mto of BF slag is produced per Fig. 7. Table 2. Compariso of ew water-graulated slag ad slag particle XRD patters after DTA measuremet (maximum temperature: 950 C). Latet heats of slags ad the correspodig estimated decrease i CO 2 emissio. year, ad the latet heats of water-graulated slag ad slag particles were idetified to be 235 kj/kg ad 294 kj/kg, respectively. The total eergy of the latet heat is MJ/year for water-graulated slag ad MJ/year for slag particles. Whe a coversio factor of kg-co 2 /MJ is assumed for the estimatio of CO 2 emissio from heat, the amout of CO 2 emitted was Mto for water-graulated slag ad Mto for slag particles, which correspods to 0.02% ad 0.025% decreases i CO 2 emissio, respectively, by the steel idustry i Although this is a small percetage relative to the etire CO 2 emissio of the steel idustry, the absolute value of CO 2 emissio reductio is substatial. The latet heats ad meltig poits of PCMs are summarized i Table 3 ad Fig ) I Fig. 8, the latet heat of BF slag obtaied i this study is also plotted agaist its meltig poit (about C). The order of the amorphous slag s latet heat is ot very low, ad BF slag melts at a very high temperature as a PCM. Therei, amorphous slag could act as a high temperature PCM, such that high temperature waste gas, such as coverter exhaust, which is typically at a temperature of C, could utilize it. Oe example for the utilizatio of slag as a PCM is cosidered i the ext sectio Process Image ad Simulatio of the Utilizatio of the Latet Heat of Slag as a PCM Process Image I the preset study, a high temperature waste coverter exhaust gas cosistig of 100% CO ad a temperature of at least 800 C (maximum C) was assumed. The process image is show i Fig. 9. The process cosists of two parts. The first part cosists of a coutercurret heat exchager betwee high temperature CO gas ad low temperature amorphous slag (Fig. 9(a)), which is the primary part of the simulatio i this study. Sice the slag is exhausted from the bottom of the exchager, it is ecessary to develop a heat exchager to utilize the residual heat of the slag. The waste gas from the coverter is batch type, ad therefore, it works as a time adjustig system. The image depicted i Fig. 9(a) ca also work as a time-adjustig process, ad furthermore, it is importat to use the residual heat of the slag, as show i Fig. 9(b). This process is a cascade-like utilizatio of slag heat depedig o the temperature rage. 12,13) Simulatio Model of the Coutercurret Heat Exchager The simulatio model was developed o the basis of a blast furace simulatio. 14) The differetial equatio is expressed by mass balace (Eq. (3)) ad heat balace (Eq. (4)): d(c S V S )/dz 0...(3) d(v S C PCS T S )dz H P A P (T S T G ) Q...(4) subscript S, G: solid ad gas C S : Cocetratio of solid (kmol/m 3 -bed) A P : Specific surface area of solid (m 2 /m 3 -bed) C P : Specific heat (kj/kmol C) H P : Heat trasfer coefficiet betwee solid ad gas (kj/m 2 mi C) Q : Rate of heat evolutio (kj/mi m 3 -bed) 2010 ISIJ 1262

5 Table 3. Latet heats ad meltig poits of possible PCM cadidates. 5 11) Fig. 8. Relatioship betwee latet heat ad meltig poit of a PCM listed i Table 3. Fig. 9. Process image of the utilizatio of slag as a PCM. T : Temperature ( C) V : Descedig velocity (m/mi) Z : Distace from top (m) The temperatures of the solid ad gas at step- T 1 S, T 1 G ca be derived from T S, T G. T S 1 T S QΔZ W ( 1 S 1 Q ( TG TS ) ( 1 ΔZ 1 exp...(5) WG ISIJ

6 T G 1 QΔZ TG WS ( 1 β Q ( TG TS ) ( 1 ΔZ 1 exp...(6) WG where W G ad W S are the heat fluxes of a gas ad solid (W V C Ci P, i=s or G), respectively, ad b W S /W G. Calculatios were performed assumig that the latet heat of slag was 0%, 50%, ad 100% of the latet heat, Q cry, (Q 0, Q Q cry /2, ad Q Q cry, ad Q cry is assumed to be 230 kj/kg). The temperature rage of latet heat geeratio was fixed from 800 to 950 C, ad the rate of heat geeratio was assumed to be 96.6 kj/mi (estimated from the result of DTA). The heat geeratio at each step was calculated util the maximum heat reached 230 kj/kg. I this simulatio, the CO flow rate was 0.78 Nm 3 /mi, the solid descedig speed was 0.6 m/mi, ad the heat exchager was cylidrical with a 1-m diameter. The height of the heat exchager was a ucertai value that was determied from the term of H P A P i Eqs. (5) ad (6). Specifically, this value was used with some factor ( ) i order to fit the empirical value. 14) I the preset study, H P A P was assumed to be 600 (J/m); however, a more accurate value should be obtaied empirically. The results of the simulatio are show i Fig. 10. The temperatures of the top were assumed to 695 K (422 C) ad 300 K (27 C) for the iitial temperatures of the gas ad solid, respectively. Nie meters from the top, the temperature profiles of the solid ad gas (T S ad T G ) were the same because o heat geeratio occurred. Whe heat geeratio occurred, the icrease i temperature was largest at Q Q cry, while oly a moderate icrease i temperature occurred for Q Q cry /2. Whe the latet heat geeratio occurred, the solid temperature (T S ) icreased at first, but the icrease of the gas temperature (T G ) delayed a little. The positio becomig the same temperature betwee T S ad T G is about 13 m from the top. Fially, the solid ad gas temperatures became the same value, which meas that the slag temperature became the same temperature as the gas exhausted from the bottom. As the solid temperature decreased with icreasig latet heat, Q, the iitial gas temperature ca be lower or a lower temperature waste gas ca be used. Although the distace i this simulatio icludes a ucertai factor as metio above, the height about 13 m would be the most efficiet legth of the heat exchager uder the preset simulatio coditios. If adiabatic reservatio of hot slag could be established, high temperature exhaust slag could be used as a heat trasportatio material (Fig. 9(b)). Fig. 10. Results of simulatio of heat exchage betwee waste gas ad slag particles with or without latet heat. 4. Coclusios The latet heats ad temperature rages of heat geeratio for two kids of amorphous slags (slag particles obtaied by RCLA ad water-graulated slag from a steel plat) were measured usig DTA. I additio, mathematical simulatio for the coutercurret heat exchager was performed ad the effect of the latet heat of slag was estimated. (1) The crystallizatio temperature of amorphous slags raged from 800 to 950 C. (2) The latet heats of slag particles ad the watergraulated slag were 294 kj/kg ad 235 kj/kg, respectively. (3) Accordig to simulatio, the latet heat of slag ca be used to lower the temperature of waste gas. REFERENCES 1) Y. Kashiwaya, Y. I-ami ad T. Akiyama: ISIJ It., 50 (2010), ) Y. Kashiwaya, Y. I-ami ad T. Akiyama: ISIJ It., 50 (2010), ) Y. Kashiwaya, T. Nakauchi, P. Khah So, S. Akiyama ad K. Ishii: ISIJ It., 47 (2007), 44. 4) Y. Saito, K. Saito ad T. Atake: Thermochim. Acta, 99 (1986), ) A. Kaizawa, N. Maruoka, A. Kawai, H. Kamao, T. Jozuka, T. Seda ad T. Akiyama: Heat Mass Tras., 44 (2008), ) R. Tamme, T. Bauer, J. Buschle, D. Laig, H. M. Steihage ad W. D. Steima: It. J. Eergy Res., 32 (2008), ) Japa Society of Thermophysical Properties: Sipe Netubutsusei Hadbook, Yokedou, Tokyo, (2008), ) S. D. Sharma ad K. Sagara: It. J. Gree Eergy, 2 (2005), 1. 9) S. Picemi, R. Olives, X. Py ad M. Christ: Solar Eergy Mater. Solar. Cells, 92 (2008), ) N. Seki: Tikuetsu Kogaku, Morikita, Publ., (1995), ) N. Maruoka ad T. Akiyama: Eergy, 31 (2006), ) L. Zhag ad T. Akiyama: It. J. Exergy, 6 (2009), ) T. Nomura, N. Okiaka ad T. Akiyama: J. Jp. Ist. Eergy, 88 (2009), ) K. Ishii, Y. Kashiwaya, H. Yamaguchi ad S. Kodo: Tetsu-to- Hagaé, 72 (1986), ISIJ 1264