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Understanding fusibility characteristics and flow properties of the biomass and biomass-coal ash samples
Renewable Energy 147 (2020) 1352e1357
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Understanding fusibility characteristics and flow properties of the biomass and biomass-coal ash samples Jie Xu a, *, Ju Wang a, Chunhua Du a, Shuaidan Li b, Xia Liu c, ** a
Laboratory of Reaction and Separation Technology, Qingdao Agricultural University, 266109, Qingdao, China China Huadian Electric power research institute Co., LTD., 310030, Hangzhou, China c Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, Shanghai Engineering Research Center of Coal Gasification, East China University of Science and Technology, 200237, Shanghai, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 September 2018 Received in revised form 31 January 2019 Accepted 15 September 2019 Available online 17 September 2019
The fusibility characteristics and flow properties of the biomass and biomass-coal ash samples were investigated in this paper. Ash fusibility of the biomass-coal blended samples was evaluated systematically. The mineral matters variation and liquidus temperature of the biomass-coal blended samples were demonstrated by FactSage. The results show that the ash fusion temperature of biomass-coal blended samples decreases and then increases with the increasing content of biomass (straw). The ash fusion temperature reaches the minimum when straw content is 50%. Moreover, the liquidus temperature reflected in the phase diagram can predict the variation of ash fusion temperature well. Furthermore, the flow properties of high temperature ash (HTA) and low temperature ash (LTA) straw samples at different atmosphere were observed using the heating stage microscope. In the heating process, the HTA and LTA straw samples show the similar variation: shrinkage, fusion and spreading. However, the volume changes of the HTA and LTA straw samples are different considerably attributing to the mineral matters variation. It was revealed that the volume change of HTA and LTA straw samples is not obvious at the lower temperature. For the LTA straw sample in air atmosphere, the maximal volume change is obtained. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Straw ash Fusibility Flow properties Heating stage microscope Volume change
1. Introduction The serious crisis exists in the area of the fast fossil fuels consumption and the resulting greenhouse gases and air pollutants [1]. With the environmentally friendly characteristics, biomass is regarded as a potentially significant source of renewable energy [2]. However, the high transportation cost and seasonal nature of biomass may make it difficult to operate the biomass-only power plants [3]. With the increasing labor cost in China, biomass price rises sharply. Thus, the rising cost of biomass reduces the benefit of the biomass-only power plants. Gasification technology is an important clean technology for the high utilization efficiency and the contribution to environment protection [4,5]. Co-gasification of biomass and coal could use the high parameter units in power plant and reduce the investment of equipment [6]. The biomass-coal ratio could be regulated in co-gasification when raw material
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Xu), [email protected] (X. Liu). https://doi.org/10.1016/j.renene.2019.09.066 0960-1481/© 2019 Elsevier Ltd. All rights reserved.
price changes rapidly. On the other hand, the high H/C of biomass could adjust the parameter of the raw material. Therefore, a higher flexibility and efficiency is achieved in the co-gasification of biomass and coal [7]. The feedstocks such as biomass are converted into synthesis gas in the gasifier. The mineral matters are converted into ash and the ash forms slag which flows down under the gravity at high temperature [8]. The ash fusibility characteristics and flow properties are essential for the operation of the gasifier [9]. Generally, the operating temperature is 50e200 C above the flow temperature (FT) [10]. Based on Chinese standard GB/T 212-2008 and GB/T 28731-2012, coal ash and biomass ash are prepared at 815 C and 550 C respectively. During the heating process, the mineral matters in the high temperature ash (HTA) are transformed and the alkali metals volatilize. The HTA could not estimate the real transformation of mineral matter and volatilization of alkali metals in the gasifier [11]. However, the low temperature ash (LTA) is prepared at the temperature lower than 150 C and the mineral matters in the LTA is relatively unchanged [12,13]. Certainly, the phase transformation has not occurred in the whole process. The alkali metals would not volatilize at the low temperature. The reaction
J. Xu et al. / Renewable Energy 147 (2020) 1352e1357
2.2. Fusion temperature test
Table 1 Proximate analysis and ultimate analysis of Guizhou coal and straw. Sample
Guizhou coal Straw
Proximate analysis wt/%
1353
Ultimate analysis wt/%
Mad
Aad
Vad
FCad
Cd
Hd
Od
Nd
Sd
0.89 7.81
16.80 8.85
10.46 69.19
71.85 14.15
74.63 44.08
2.87 6.24
1.41 38.57
0.98 1.13
3.16 0.38
ad-air dry basis; d-dry basis.
and transformation of mineral matters are not only affected by its contents, but also affected by the atmosphere. At the air atmosphere, the mineral matters in the ashes are oxidized. In order to avoid the oxidization of ashes, vacuum atmosphere was studied in this paper. The investigations on fusibility characteristics and flow properties of biomass ash have been conducted [14,15]. Niu et al. [16] found that the ash flow behavior is related to the hightemperature molten material in biomass ash. Li et al. [17] found that ash fusion temperature of biomass ashes and simulated ashes can be used to guide the boiler design. Qin et al. [18] studied the ash fusion behavior of biomass-coal blended ashes under the CO2 atmosphere. However, in these studies, the flow behavior of biomass ash samples and the visual observation of the biomass HTA and LTA samples have little been explored. Therefore, it is necessary to investigate the flow behavior of HTA and LTA samples at different atmosphere and discuss the diverseness. The fusibility characteristics and flow properties of biomass and biomass-coal ash samples were investigated in this work. The ash fusibility of the biomass-coal blended samples were studied. With the variation of the biomass to coal blended ratio, the mineral matters transformation and the liquidus temperature of the samples were obtained by the software FactSage. In addition, the differences of biomass HTA and LTA samples in the heating process were analyzed systematically. The volume changes of biomass HTA and LTA samples were calculated. The study will provide the available information for the full utilization of biomass in the cogasification technology.
2. Experimental 2.1. Sample characterization Straw is the representative biomass in China. Guizhou coal is commonly used in the gasification. Thus, straw and Guizhou coal are chosen in this study. The Guizhou coal and straw samples were milled to 0.180e0.250 mm. The Guizhou coal and straw samples were blended thoroughly at different mass ratios. The mass ratios of straw were 0%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% and 100% (wt%, dry basis), respectively. The blended ash samples were prepared in a muffle furnace at 815 C based on the Chinese standard GB/T212-2008. Proximate analysis and ultimate analysis of Guizhou coal and straw are shown in Table 1. As shown in Table 1, the H/C of Guizhou is 0.04 and H/C of straw is 0.14. Therefore, straw and Guizhou coal are suitable for co-gasification. The compositions of the samples were carried out by X-ray fluorescence (XRF). The chemical compositions of samples are displayed in Table 2.
The fusion temperature tests include four temperatures, which represents the softening and melting behavior of ash sample [19]. The four temperatures are: deformation temperature (DT), softening temperature (ST), hemispherical temperature (HT), and flow temperature (FT). According to the Chinese standard procedure (GB/T219-2008), the fusion temperature tests were proceeded in 5E-MACIII intelligent ash fusion determination meter. The specific geometry ash cone was heated and the reducing atmosphere was provided by activated carbon and graphite in the experiment procedure. The ash fusion temperature of Guizhou coal and straw are shown in Table 2, respectively. From Table 2, it can be seen that the flow temperatures of Guizhou coal and straw are all above 1350 C.The sample with high flow temperature such as Guizhou coal or straw is unsuitable for entrained-flow gasifier directly. Therefore, co-gasification of coal and biomass is maybe helpful for improving fusion temperature and promoting the effectively use of biomass. 2.3. Thermodynamic calculations FactSage has been widely used for prediction of mineral transformation, fusibility of ash sample and the property of slag sample [20,21]. The thermodynamic software package FactSage was used to predict the liquidus temperature (Tliquidus), proportion of solids and mineral formation for the multi-component system in this work. Moreover, the FToxid database in thermodynamic software package FactSage was chosen for the equilibrium calculation [22]. Calculations were carried out at the given temperature under reducing atmosphere with 60% CO and 40% CO2 in volume fraction at 1atm pressure. 2.4. Heating stage microscope The flow behavior of ash samples in the heating process can be investigated visually by heating stage microscope [23,24]. The detailed information of ash samples is displayed in the formation of images, which is based on the variety of shape and dimensions in the heating process. 2.4.1. The instrument of heating stage microscope The heating stage microscope is consisted of microscope, heating unit and image analysis software. The apparatus applied in the experiment is TS1500 heating stage microscope, which is manufactured by the Linkam Scientific Instruments. The diagram of the heating stage microscope is displayed in Fig. 1. The HTA and LTA straw samples were placed into the holder of the heating unit. The samples were heated from the room temperature to 1350 C and the heating temperature was regulated by the Linksys32 software package. The experiments were carried out in the air atmosphere and vacuum atmosphere, respectively. With the Olympus light microscope (Olympus Optical Corporation, Tokyo, Japan), the superficial characterizations of the samples were obtained and the images of samples were collected. With an accuracy of ±0.01% per 1 C,the volume changes of samples at different temperature were observed.
Table 2 Ash chemical composition and ash fusion temperature of samples. Sample
Guizhou coal Straw
Ash fusion temperature/ C
Ash chemical composition wt/% SiO2
Al2O3
Fe2O3
CaO
MgO
Na2O
K2O
SO3
P2O5
TiO2
Others
DT
ST
HT
FT
49.07 67.40
32.37 0.73
7.83 0.61
1.94 5.47
1.32 5.02
0.95 2.46
1.46 11.52
2.16 1.81
0.31 3.32
2.38 0.04
0.21 1.62
1210 1198
1311 1257
1342 1290
1379 1380
1354
J. Xu et al. / Renewable Energy 147 (2020) 1352e1357
range of 800 Ce1100 C, the heating rate of 20 C/min is chosen. With the increasing temperature, the change of volume is obvious and the heating rate of 10 C/min is selected. 3. Results and discussion 3.1. Effect of straw content on ash fusion temperature
Fig. 1. The diagram of the heating stage microscope.
2.4.2. Measurement method and heating rate selection With the heating microscope webcam, the sample images were obtained for analyzing the volume change. Meanwhile, the ImageJ software was used to calculate the surface area of samples at different temperature and atmosphere. The Depth of Field (DOF: The height over which a sample can be clearly focused is called the Depth of Field.) of the microscope is very small, which is only a few microns. Whole of the sample is clearly observed during the experiment, so we assume that the height is a constant value “h". The volume change is expressed as the following equation: Volume change (%) ¼ 100% (At Ao)/Ao where, Ao - initial volume of the sample At - the volume at each time interval. For the detailed information of flow behavior, the straw samples were heated to 1350 C at different heating rate. The volume change vs temperature curve of the sample at different heating rates is shown in the Fig. 2. When the temperature is lower than 800 C, the 50 C/min heating rate is selected. In the temperature
10
10 20 50
0
/min /min /min
Volume Change%
-10 -20 -30 -40 -50
The ash fusion characteristics of straw are different from that of coal. The contents of alkali metal and some other elements in straw ash are the critical influences [25]. The ash fusion temperature of straw and coal in co-gasification process cannot be reflected by each single sample. Thus, it is necessary to study the ash fusion temperature variation with the biomass content and to find appropriate ratio of straw and coal for co-gasification. Guizhou coal and straw were mixed thoroughly at various ratios and the ash fusion temperatures of those mixtures were measured. Fig. 3(a) shows the ash fusion temperature variation with the content of straw. It indicates from Fig. 3(a) that ash fusion temperature decreases with the increasing content of straw until the straw content achieves 50%. At higher content of straw, the ash fusion temperature increases. Moreover, the liquidus temperature calculated by FactSage can also reflect the variation of ash fusion temperature. This prediction trend is similar to the experimental variation trend. The linear relationship between flow temperature and liquidus temperature is expressed as FT ¼ aþb$Tliquidus [26]. Fig. 3(b) presents the flow temperature as a function of the liquidus temperature. Good correlations between flow temperature and liquidus temperature can be seen from Fig. 3(b), and the relationship can be represented as the following equation: FT ¼ 0.64Tliquidþ314.39. Besides, the measured flow temperatures are in the experimental errors of ±20 C and the correlation coefficient R is 0.92. The literature [27] reported that the prediction is excellent when the value of coefficient is 0.9 or higher. Hence, the prediction of liquidus temperature is accurate and reliable. This finding provides a guide for the suitable ratio of straw to coal in the co-gasification. It is well known that FeO and CaO are the mainly basic oxides which are responsible for various fusibility [28]. The compositions of blended samples are different and thus the average mole ratio of FeO to CaO is selected as 1.75. In this work, the blended samples are normalized to SiO2-Al2O3-CaO-FeO system, and the ternary system is drawn by FactSage. Fig. 4 illustrates the influence of straw content on the liquidus temperature of the SiO2-Al2O3-CaO-FeO system with a FeO/CaO mole ratio of 1.75. It can be seen from Fig. 4 that the sample with straw content of 10% lies in the mullite primary phase region. The liquidus temperature is above 1650 C. With the increasing straw content, the liquidus temperatures of these samples decrease. When the straw content is 50%, the liquidus temperature decreases to 1550 C. With further increasing of straw content, the samples are located in the quartz primary phase field and the liquidus temperatures increase. When the straw content is 90%, the liquidus temperature increases to 1770 C. The effect of straw content on the liquidus temperature of the SiO2-Al2O3-CaOFeO system is similar to that on the fusion temperatures of samples. It suggests that the prediction of fusion temperature can be reflected by the liquidus temperature and clarified by the ternary phase diagram.
-60 -70
3.2. The flow behavior of the HTA and LTA straw samples 0
200
400
600
800
1000
1200
1400
1600
Temperature/ Fig. 2. The volume change vs temperature curve of the sample at different heating rates.
3.2.1. The HTA straw sample in the air atmosphere The flow behavior of straw HTA sample was investigated by the heating stage microscope. In the temperature range of 400 Ce1350 C, the visual images of the HTA straw sample in the
J. Xu et al. / Renewable Energy 147 (2020) 1352e1357
(a)
1355
(b)
Fig. 3. Ash fusion temperature variation with the content of straw (a) and the correlation between flow temperature and liquidus temperature (b).
Fig. 5. The flow behavior of HTA straw sample in the air atmosphere.
Fig. 4. The liquidus temperature in the SiO2-Al2O3-CaO-FeO system with a FeO/CaO mole ratio of 1.75.
air atmosphere are shown in Fig. 5. When the temperature is lower than 600 C, the change of HTA sample could not be observed. Moreover, a large quantities of solids exist in the sample. Obviously, the reactions between the solid particles at the lower temperature are occurred slowly. From 600 C to 800 C, the shrinkage of HTA straw sample appears and the volume decreases. The fusion behavior and further shrinkage of the HTA straw sample could be observed at 1000 C. With the increasing temperature, the flowability of HTA straw sample enhances. At 1100 C, the volume of HTA straw sample increase sharply. When the temperature reaches to 1200 C, the amorphous materials are identified in the HTA straw sample. Furthermore, the refractory materials decrease and the eutectic materials form in the HTA straw sample. When the temperature is 1350 C, a large number of molten materials and the spreading behavior are manifested.
3.2.2. The HTA straw sample in the vacuum atmosphere Fig. 6 shows the flow behavior of HTA straw sample in the vacuum atmosphere. When the temperature is lower than 800 C, the volume change of HTA straw sample is not evident in the heating process. Obviously,the volume change of HTA straw sample in the vacuum atmosphere is different from that in the air atmosphere. In the temperature range of 1000 Ce1100 C, the fusibility of HTA straw sample enhances. Moreover, the HTA straw sample reveals a degree of shrinkage. With the increasing temperature, the refractory substances are reduced considerably and the reinforced flow behavior is performed. At 1350 C, the HTA sample has covered
Fig. 6. The flow behavior of HTA straw sample in the vacuum atmosphere.
1356
J. Xu et al. / Renewable Energy 147 (2020) 1352e1357
the whole sample holder. 3.2.3. The LTA straw sample in the air atmosphere The flow behavior of LTA straw sample in the air atmosphere is displayed in the Fig. 7. At the temperature range of 400 Ce600 C, the obvious change of LTA straw sample has not been observed. At 800 C, the LTA straw sample shows marked shrinkage. During 1000 Ce1100 C, the LTA straw sample melts. With the increasing temperature, the LTA straw sample shows the intensified fusibility. 3.2.4. The LTA straw sample in the vacuum atmosphere The change of LTA straw sample in the vacuum atmosphere is indicated in the Fig. 8. During the temperature range of 400 Ce800 C, the LTA straw sample represents the tiny change. When the temperature is reached to 1100 C, the LTA straw sample fuses and the volume exhibits the considerable decrease. At 1200 Ce1350 C, the spreadability of LTA straw sample enhances and the increasing volume is presented.
Fig. 8. The flow behavior of LTA straw sample in the vacuum atmosphere.
20
Comparing the HTA and LTA straw samples (Figs. 5e8) in the heating process, the samples shows the similar change. With the increasing temperature, the three processes of the samples are presented. The shrinkage appear in the first stage at lower temperature. Then the melting phenomenon is shown in the second stage. The samples spread at last. Based on the different sample and atmosphere, the volume changes of samples are varied. The volume changes of HTA and LTA straw samples at different atmosphere are calculated, which are displayed in Fig. 9. It is indicated from Fig. 9 that the volume change of LTA sample in air atmosphere is the most remarkable. The maximal volume change are in order of LTA in air atmosphere (81.5%), LTA in vacuum atmosphere (73.2%), HTA in air atmosphere (54.1%) and HTA in vacuum atmosphere (15.5%). The mineral matters in the LTA exist with the original state, which contributes to the higher volume change. In the same atmosphere, the temperature of LTA straw sample that the maximal volume change appears is higher than that of HTA straw sample. For example, the temperatures of LTA and HTA in air atmosphere are 1000 C and 950 C, respectively. Furthermore, the effect of atmosphere on the volume change is significant. In the vacuum atmosphere, the volume of LTA and HTA samples has little change with the temperature lower than 800 C. In the vacuum atmosphere, the duration time of fusion process is longer. For the LTA straw sample, the volume change arrives the maximum value at the 1000 C in the air atmosphere. However, the volume change arrives the maximum value at the 1230 C under the vacuum atmosphere. Moreover, the atmosphere has greater influence on the HTA straw sample than that on the LTA straw sample.
The volume change %
0
3.3. The volume change comparison of HTA and LTA in different atmosphere
-20 -40
HTA in air atmosphere LTA in air atmosphere LTA in vacuum atmosphere HTA in vacuum atmosphere
-60 -80 0
200
400
600
800
1000
1200
1400
Temperature/ Fig. 9. The volume changes of HTA and LTA straw samples in different atmosphere.
4. Conclusions The fusibility characteristics and flow properties of the biomass and biomass-coal ash samples were researched in this paper. Ash fusibility of the biomass-coal blended samples was investigated. When the content of straw increases, the ash fusion temperature of straw-coal blended samples decreases firstly and then increases. For the 50% content of straw sample, the minimum value of ash fusion temperature occurs. Moreover, the liquidus temperature calculated by the software FactSage shows the consistent tendency with the variation of ash fusion temperature. The relationship between flow temperature and liquidus temperature can be expressed as: FT ¼ 0.64Tliquidusþ314.39 and the correlation coefficient R is 0.92. By means of the heating stage microscope, the flow properties of HTA and LTA straw samples at the air and vacuum atmosphere were studied. The maximal volume change of 81.5% is achieved with LTA straw sample in air atmosphere. In addition, the influence of atmosphere on the volume change was observed. The duration time of fusion process in the vacuum atmosphere is longer than that in the air atmosphere. Furthermore, the atmosphere has greater influence on the HTA straw sample than that on the LTA straw sample. In the vacuum atmosphere, the volume of HTA and LTA straw samples has little change with the temperature lower than 800 C.
Acknowledgments
Fig. 7. The flow behavior of LTA straw sample in the air atmosphere.
This work is supported by National Natural Science Foundation of China (Grant 21706142).
J. Xu et al. / Renewable Energy 147 (2020) 1352e1357
Nomenclature DT ST HT FT Tliquidus
deformation temperature [ C] softening temperature [ C] hemispherical temperature [ C] flow temperature [ C] liuquidus temperature [ C]
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Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Understanding fusibility characteristics and flow properties of the biomass and biomass-coal ash samples Jie Xu a, *, Ju Wang a, Chunhua Du a, Shuaidan Li b, Xia Liu c, ** a
Laboratory of Reaction and Separation Technology, Qingdao Agricultural University, 266109, Qingdao, China China Huadian Electric power research institute Co., LTD., 310030, Hangzhou, China c Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, Shanghai Engineering Research Center of Coal Gasification, East China University of Science and Technology, 200237, Shanghai, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 September 2018 Received in revised form 31 January 2019 Accepted 15 September 2019 Available online 17 September 2019
The fusibility characteristics and flow properties of the biomass and biomass-coal ash samples were investigated in this paper. Ash fusibility of the biomass-coal blended samples was evaluated systematically. The mineral matters variation and liquidus temperature of the biomass-coal blended samples were demonstrated by FactSage. The results show that the ash fusion temperature of biomass-coal blended samples decreases and then increases with the increasing content of biomass (straw). The ash fusion temperature reaches the minimum when straw content is 50%. Moreover, the liquidus temperature reflected in the phase diagram can predict the variation of ash fusion temperature well. Furthermore, the flow properties of high temperature ash (HTA) and low temperature ash (LTA) straw samples at different atmosphere were observed using the heating stage microscope. In the heating process, the HTA and LTA straw samples show the similar variation: shrinkage, fusion and spreading. However, the volume changes of the HTA and LTA straw samples are different considerably attributing to the mineral matters variation. It was revealed that the volume change of HTA and LTA straw samples is not obvious at the lower temperature. For the LTA straw sample in air atmosphere, the maximal volume change is obtained. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Straw ash Fusibility Flow properties Heating stage microscope Volume change
1. Introduction The serious crisis exists in the area of the fast fossil fuels consumption and the resulting greenhouse gases and air pollutants [1]. With the environmentally friendly characteristics, biomass is regarded as a potentially significant source of renewable energy [2]. However, the high transportation cost and seasonal nature of biomass may make it difficult to operate the biomass-only power plants [3]. With the increasing labor cost in China, biomass price rises sharply. Thus, the rising cost of biomass reduces the benefit of the biomass-only power plants. Gasification technology is an important clean technology for the high utilization efficiency and the contribution to environment protection [4,5]. Co-gasification of biomass and coal could use the high parameter units in power plant and reduce the investment of equipment [6]. The biomass-coal ratio could be regulated in co-gasification when raw material
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Xu), [email protected] (X. Liu). https://doi.org/10.1016/j.renene.2019.09.066 0960-1481/© 2019 Elsevier Ltd. All rights reserved.
price changes rapidly. On the other hand, the high H/C of biomass could adjust the parameter of the raw material. Therefore, a higher flexibility and efficiency is achieved in the co-gasification of biomass and coal [7]. The feedstocks such as biomass are converted into synthesis gas in the gasifier. The mineral matters are converted into ash and the ash forms slag which flows down under the gravity at high temperature [8]. The ash fusibility characteristics and flow properties are essential for the operation of the gasifier [9]. Generally, the operating temperature is 50e200 C above the flow temperature (FT) [10]. Based on Chinese standard GB/T 212-2008 and GB/T 28731-2012, coal ash and biomass ash are prepared at 815 C and 550 C respectively. During the heating process, the mineral matters in the high temperature ash (HTA) are transformed and the alkali metals volatilize. The HTA could not estimate the real transformation of mineral matter and volatilization of alkali metals in the gasifier [11]. However, the low temperature ash (LTA) is prepared at the temperature lower than 150 C and the mineral matters in the LTA is relatively unchanged [12,13]. Certainly, the phase transformation has not occurred in the whole process. The alkali metals would not volatilize at the low temperature. The reaction
J. Xu et al. / Renewable Energy 147 (2020) 1352e1357
2.2. Fusion temperature test
Table 1 Proximate analysis and ultimate analysis of Guizhou coal and straw. Sample
Guizhou coal Straw
Proximate analysis wt/%
1353
Ultimate analysis wt/%
Mad
Aad
Vad
FCad
Cd
Hd
Od
Nd
Sd
0.89 7.81
16.80 8.85
10.46 69.19
71.85 14.15
74.63 44.08
2.87 6.24
1.41 38.57
0.98 1.13
3.16 0.38
ad-air dry basis; d-dry basis.
and transformation of mineral matters are not only affected by its contents, but also affected by the atmosphere. At the air atmosphere, the mineral matters in the ashes are oxidized. In order to avoid the oxidization of ashes, vacuum atmosphere was studied in this paper. The investigations on fusibility characteristics and flow properties of biomass ash have been conducted [14,15]. Niu et al. [16] found that the ash flow behavior is related to the hightemperature molten material in biomass ash. Li et al. [17] found that ash fusion temperature of biomass ashes and simulated ashes can be used to guide the boiler design. Qin et al. [18] studied the ash fusion behavior of biomass-coal blended ashes under the CO2 atmosphere. However, in these studies, the flow behavior of biomass ash samples and the visual observation of the biomass HTA and LTA samples have little been explored. Therefore, it is necessary to investigate the flow behavior of HTA and LTA samples at different atmosphere and discuss the diverseness. The fusibility characteristics and flow properties of biomass and biomass-coal ash samples were investigated in this work. The ash fusibility of the biomass-coal blended samples were studied. With the variation of the biomass to coal blended ratio, the mineral matters transformation and the liquidus temperature of the samples were obtained by the software FactSage. In addition, the differences of biomass HTA and LTA samples in the heating process were analyzed systematically. The volume changes of biomass HTA and LTA samples were calculated. The study will provide the available information for the full utilization of biomass in the cogasification technology.
2. Experimental 2.1. Sample characterization Straw is the representative biomass in China. Guizhou coal is commonly used in the gasification. Thus, straw and Guizhou coal are chosen in this study. The Guizhou coal and straw samples were milled to 0.180e0.250 mm. The Guizhou coal and straw samples were blended thoroughly at different mass ratios. The mass ratios of straw were 0%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% and 100% (wt%, dry basis), respectively. The blended ash samples were prepared in a muffle furnace at 815 C based on the Chinese standard GB/T212-2008. Proximate analysis and ultimate analysis of Guizhou coal and straw are shown in Table 1. As shown in Table 1, the H/C of Guizhou is 0.04 and H/C of straw is 0.14. Therefore, straw and Guizhou coal are suitable for co-gasification. The compositions of the samples were carried out by X-ray fluorescence (XRF). The chemical compositions of samples are displayed in Table 2.
The fusion temperature tests include four temperatures, which represents the softening and melting behavior of ash sample [19]. The four temperatures are: deformation temperature (DT), softening temperature (ST), hemispherical temperature (HT), and flow temperature (FT). According to the Chinese standard procedure (GB/T219-2008), the fusion temperature tests were proceeded in 5E-MACIII intelligent ash fusion determination meter. The specific geometry ash cone was heated and the reducing atmosphere was provided by activated carbon and graphite in the experiment procedure. The ash fusion temperature of Guizhou coal and straw are shown in Table 2, respectively. From Table 2, it can be seen that the flow temperatures of Guizhou coal and straw are all above 1350 C.The sample with high flow temperature such as Guizhou coal or straw is unsuitable for entrained-flow gasifier directly. Therefore, co-gasification of coal and biomass is maybe helpful for improving fusion temperature and promoting the effectively use of biomass. 2.3. Thermodynamic calculations FactSage has been widely used for prediction of mineral transformation, fusibility of ash sample and the property of slag sample [20,21]. The thermodynamic software package FactSage was used to predict the liquidus temperature (Tliquidus), proportion of solids and mineral formation for the multi-component system in this work. Moreover, the FToxid database in thermodynamic software package FactSage was chosen for the equilibrium calculation [22]. Calculations were carried out at the given temperature under reducing atmosphere with 60% CO and 40% CO2 in volume fraction at 1atm pressure. 2.4. Heating stage microscope The flow behavior of ash samples in the heating process can be investigated visually by heating stage microscope [23,24]. The detailed information of ash samples is displayed in the formation of images, which is based on the variety of shape and dimensions in the heating process. 2.4.1. The instrument of heating stage microscope The heating stage microscope is consisted of microscope, heating unit and image analysis software. The apparatus applied in the experiment is TS1500 heating stage microscope, which is manufactured by the Linkam Scientific Instruments. The diagram of the heating stage microscope is displayed in Fig. 1. The HTA and LTA straw samples were placed into the holder of the heating unit. The samples were heated from the room temperature to 1350 C and the heating temperature was regulated by the Linksys32 software package. The experiments were carried out in the air atmosphere and vacuum atmosphere, respectively. With the Olympus light microscope (Olympus Optical Corporation, Tokyo, Japan), the superficial characterizations of the samples were obtained and the images of samples were collected. With an accuracy of ±0.01% per 1 C,the volume changes of samples at different temperature were observed.
Table 2 Ash chemical composition and ash fusion temperature of samples. Sample
Guizhou coal Straw
Ash fusion temperature/ C
Ash chemical composition wt/% SiO2
Al2O3
Fe2O3
CaO
MgO
Na2O
K2O
SO3
P2O5
TiO2
Others
DT
ST
HT
FT
49.07 67.40
32.37 0.73
7.83 0.61
1.94 5.47
1.32 5.02
0.95 2.46
1.46 11.52
2.16 1.81
0.31 3.32
2.38 0.04
0.21 1.62
1210 1198
1311 1257
1342 1290
1379 1380
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range of 800 Ce1100 C, the heating rate of 20 C/min is chosen. With the increasing temperature, the change of volume is obvious and the heating rate of 10 C/min is selected. 3. Results and discussion 3.1. Effect of straw content on ash fusion temperature
Fig. 1. The diagram of the heating stage microscope.
2.4.2. Measurement method and heating rate selection With the heating microscope webcam, the sample images were obtained for analyzing the volume change. Meanwhile, the ImageJ software was used to calculate the surface area of samples at different temperature and atmosphere. The Depth of Field (DOF: The height over which a sample can be clearly focused is called the Depth of Field.) of the microscope is very small, which is only a few microns. Whole of the sample is clearly observed during the experiment, so we assume that the height is a constant value “h". The volume change is expressed as the following equation: Volume change (%) ¼ 100% (At Ao)/Ao where, Ao - initial volume of the sample At - the volume at each time interval. For the detailed information of flow behavior, the straw samples were heated to 1350 C at different heating rate. The volume change vs temperature curve of the sample at different heating rates is shown in the Fig. 2. When the temperature is lower than 800 C, the 50 C/min heating rate is selected. In the temperature
10
10 20 50
0
/min /min /min
Volume Change%
-10 -20 -30 -40 -50
The ash fusion characteristics of straw are different from that of coal. The contents of alkali metal and some other elements in straw ash are the critical influences [25]. The ash fusion temperature of straw and coal in co-gasification process cannot be reflected by each single sample. Thus, it is necessary to study the ash fusion temperature variation with the biomass content and to find appropriate ratio of straw and coal for co-gasification. Guizhou coal and straw were mixed thoroughly at various ratios and the ash fusion temperatures of those mixtures were measured. Fig. 3(a) shows the ash fusion temperature variation with the content of straw. It indicates from Fig. 3(a) that ash fusion temperature decreases with the increasing content of straw until the straw content achieves 50%. At higher content of straw, the ash fusion temperature increases. Moreover, the liquidus temperature calculated by FactSage can also reflect the variation of ash fusion temperature. This prediction trend is similar to the experimental variation trend. The linear relationship between flow temperature and liquidus temperature is expressed as FT ¼ aþb$Tliquidus [26]. Fig. 3(b) presents the flow temperature as a function of the liquidus temperature. Good correlations between flow temperature and liquidus temperature can be seen from Fig. 3(b), and the relationship can be represented as the following equation: FT ¼ 0.64Tliquidþ314.39. Besides, the measured flow temperatures are in the experimental errors of ±20 C and the correlation coefficient R is 0.92. The literature [27] reported that the prediction is excellent when the value of coefficient is 0.9 or higher. Hence, the prediction of liquidus temperature is accurate and reliable. This finding provides a guide for the suitable ratio of straw to coal in the co-gasification. It is well known that FeO and CaO are the mainly basic oxides which are responsible for various fusibility [28]. The compositions of blended samples are different and thus the average mole ratio of FeO to CaO is selected as 1.75. In this work, the blended samples are normalized to SiO2-Al2O3-CaO-FeO system, and the ternary system is drawn by FactSage. Fig. 4 illustrates the influence of straw content on the liquidus temperature of the SiO2-Al2O3-CaO-FeO system with a FeO/CaO mole ratio of 1.75. It can be seen from Fig. 4 that the sample with straw content of 10% lies in the mullite primary phase region. The liquidus temperature is above 1650 C. With the increasing straw content, the liquidus temperatures of these samples decrease. When the straw content is 50%, the liquidus temperature decreases to 1550 C. With further increasing of straw content, the samples are located in the quartz primary phase field and the liquidus temperatures increase. When the straw content is 90%, the liquidus temperature increases to 1770 C. The effect of straw content on the liquidus temperature of the SiO2-Al2O3-CaOFeO system is similar to that on the fusion temperatures of samples. It suggests that the prediction of fusion temperature can be reflected by the liquidus temperature and clarified by the ternary phase diagram.
-60 -70
3.2. The flow behavior of the HTA and LTA straw samples 0
200
400
600
800
1000
1200
1400
1600
Temperature/ Fig. 2. The volume change vs temperature curve of the sample at different heating rates.
3.2.1. The HTA straw sample in the air atmosphere The flow behavior of straw HTA sample was investigated by the heating stage microscope. In the temperature range of 400 Ce1350 C, the visual images of the HTA straw sample in the
J. Xu et al. / Renewable Energy 147 (2020) 1352e1357
(a)
1355
(b)
Fig. 3. Ash fusion temperature variation with the content of straw (a) and the correlation between flow temperature and liquidus temperature (b).
Fig. 5. The flow behavior of HTA straw sample in the air atmosphere.
Fig. 4. The liquidus temperature in the SiO2-Al2O3-CaO-FeO system with a FeO/CaO mole ratio of 1.75.
air atmosphere are shown in Fig. 5. When the temperature is lower than 600 C, the change of HTA sample could not be observed. Moreover, a large quantities of solids exist in the sample. Obviously, the reactions between the solid particles at the lower temperature are occurred slowly. From 600 C to 800 C, the shrinkage of HTA straw sample appears and the volume decreases. The fusion behavior and further shrinkage of the HTA straw sample could be observed at 1000 C. With the increasing temperature, the flowability of HTA straw sample enhances. At 1100 C, the volume of HTA straw sample increase sharply. When the temperature reaches to 1200 C, the amorphous materials are identified in the HTA straw sample. Furthermore, the refractory materials decrease and the eutectic materials form in the HTA straw sample. When the temperature is 1350 C, a large number of molten materials and the spreading behavior are manifested.
3.2.2. The HTA straw sample in the vacuum atmosphere Fig. 6 shows the flow behavior of HTA straw sample in the vacuum atmosphere. When the temperature is lower than 800 C, the volume change of HTA straw sample is not evident in the heating process. Obviously,the volume change of HTA straw sample in the vacuum atmosphere is different from that in the air atmosphere. In the temperature range of 1000 Ce1100 C, the fusibility of HTA straw sample enhances. Moreover, the HTA straw sample reveals a degree of shrinkage. With the increasing temperature, the refractory substances are reduced considerably and the reinforced flow behavior is performed. At 1350 C, the HTA sample has covered
Fig. 6. The flow behavior of HTA straw sample in the vacuum atmosphere.
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J. Xu et al. / Renewable Energy 147 (2020) 1352e1357
the whole sample holder. 3.2.3. The LTA straw sample in the air atmosphere The flow behavior of LTA straw sample in the air atmosphere is displayed in the Fig. 7. At the temperature range of 400 Ce600 C, the obvious change of LTA straw sample has not been observed. At 800 C, the LTA straw sample shows marked shrinkage. During 1000 Ce1100 C, the LTA straw sample melts. With the increasing temperature, the LTA straw sample shows the intensified fusibility. 3.2.4. The LTA straw sample in the vacuum atmosphere The change of LTA straw sample in the vacuum atmosphere is indicated in the Fig. 8. During the temperature range of 400 Ce800 C, the LTA straw sample represents the tiny change. When the temperature is reached to 1100 C, the LTA straw sample fuses and the volume exhibits the considerable decrease. At 1200 Ce1350 C, the spreadability of LTA straw sample enhances and the increasing volume is presented.
Fig. 8. The flow behavior of LTA straw sample in the vacuum atmosphere.
20
Comparing the HTA and LTA straw samples (Figs. 5e8) in the heating process, the samples shows the similar change. With the increasing temperature, the three processes of the samples are presented. The shrinkage appear in the first stage at lower temperature. Then the melting phenomenon is shown in the second stage. The samples spread at last. Based on the different sample and atmosphere, the volume changes of samples are varied. The volume changes of HTA and LTA straw samples at different atmosphere are calculated, which are displayed in Fig. 9. It is indicated from Fig. 9 that the volume change of LTA sample in air atmosphere is the most remarkable. The maximal volume change are in order of LTA in air atmosphere (81.5%), LTA in vacuum atmosphere (73.2%), HTA in air atmosphere (54.1%) and HTA in vacuum atmosphere (15.5%). The mineral matters in the LTA exist with the original state, which contributes to the higher volume change. In the same atmosphere, the temperature of LTA straw sample that the maximal volume change appears is higher than that of HTA straw sample. For example, the temperatures of LTA and HTA in air atmosphere are 1000 C and 950 C, respectively. Furthermore, the effect of atmosphere on the volume change is significant. In the vacuum atmosphere, the volume of LTA and HTA samples has little change with the temperature lower than 800 C. In the vacuum atmosphere, the duration time of fusion process is longer. For the LTA straw sample, the volume change arrives the maximum value at the 1000 C in the air atmosphere. However, the volume change arrives the maximum value at the 1230 C under the vacuum atmosphere. Moreover, the atmosphere has greater influence on the HTA straw sample than that on the LTA straw sample.
The volume change %
0
3.3. The volume change comparison of HTA and LTA in different atmosphere
-20 -40
HTA in air atmosphere LTA in air atmosphere LTA in vacuum atmosphere HTA in vacuum atmosphere
-60 -80 0
200
400
600
800
1000
1200
1400
Temperature/ Fig. 9. The volume changes of HTA and LTA straw samples in different atmosphere.
4. Conclusions The fusibility characteristics and flow properties of the biomass and biomass-coal ash samples were researched in this paper. Ash fusibility of the biomass-coal blended samples was investigated. When the content of straw increases, the ash fusion temperature of straw-coal blended samples decreases firstly and then increases. For the 50% content of straw sample, the minimum value of ash fusion temperature occurs. Moreover, the liquidus temperature calculated by the software FactSage shows the consistent tendency with the variation of ash fusion temperature. The relationship between flow temperature and liquidus temperature can be expressed as: FT ¼ 0.64Tliquidusþ314.39 and the correlation coefficient R is 0.92. By means of the heating stage microscope, the flow properties of HTA and LTA straw samples at the air and vacuum atmosphere were studied. The maximal volume change of 81.5% is achieved with LTA straw sample in air atmosphere. In addition, the influence of atmosphere on the volume change was observed. The duration time of fusion process in the vacuum atmosphere is longer than that in the air atmosphere. Furthermore, the atmosphere has greater influence on the HTA straw sample than that on the LTA straw sample. In the vacuum atmosphere, the volume of HTA and LTA straw samples has little change with the temperature lower than 800 C.
Acknowledgments
Fig. 7. The flow behavior of LTA straw sample in the air atmosphere.
This work is supported by National Natural Science Foundation of China (Grant 21706142).
J. Xu et al. / Renewable Energy 147 (2020) 1352e1357
Nomenclature DT ST HT FT Tliquidus
deformation temperature [ C] softening temperature [ C] hemispherical temperature [ C] flow temperature [ C] liuquidus temperature [ C]
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