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Properties of flotation residual carbon from gasification fine slag
Fuel 267 (2020) 117043
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Full Length Article
Properties of flotation residual carbon from gasification fine slag Fanhui Guo a b
a,b
, Zekai Miao
a,b
, Zhenkun Guo
a,b
a
b
a,b,⁎
T
, Jian Li , Yixin Zhang , Jianjun Wu
School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou 221116, PR China Chinese National Engineering Research Center of Coal Preparation and Purification, China University of Mining and Technology, Xuzhou 221116, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Gasification fine slag Froth flotation Residual carbon Waste recycling Fuel properties LOI determine
The purpose of this study was to upgrade the fine slag by a froth flotation kinetics process in order to separate and recycle the residual carbon and tailing ash simultaneously. Fine slag, a kind of solid waste, was obtained from an entrained-flow gasification unit. The carbon content of flotation residual carbon was 64.47% by weight, which is about 3 times of that of gasification fine slag. The results show that when the particle size of fractions is higher (> 75 μm), the carbon content is higher, even > 80%. The fine slag and flotation residual carbon were characterized by elemental and proximate analysis in order to compare the H/C, O/C, and element characteristics. The morphology of fine slag was determined by scanning electron microscopy (SEM). It was found the grinding time of 6 min can meet the crushing requirements of large particle flotation residual carbon. The grinding volume ratio of > 80% is < 75 μm sieving, which is a benefit for further fuel utilization. The nonisothermal thermo-gravimetric analysis (TGA) and loss on ignition (LOI) at 450 °C & 550 °C methods were used to analyze and compare the oxidation of residual carbon. In conclusion, LOI at 550 °C and non-isothermal TGA approaches can be used to determine the carbon content, which provides a reference for the determining LOI of waste-derived fuels in industries.
1. Introduction
However, only a few have systematically studied the separation and recycling of residual carbon from gasification fine slag and then characterized the residual carbon. There are several approaches to recycle residual carbon, including physical and chemical methods [16,17]. Froth flotation is an effective separation method for cleaning fine coal and recycling the residual carbon from coal ash. The difference in hydrophobicity between organic and inorganic minerals is the basic principle of froth flotation technology [18–20]. There are two major approaches to determine the residual carbon. Loss on ignition (LOI) is a simple way to evaluate the residual carbon in the ash, but factors like mineral decomposition may affect the measurement results [21–23]. For the determination of the carbon content, the European standard can be used [24]. However, there is no standard LOI approach yet available for gasification fine slag and residual carbon from the flotation process. In this study, we used the LOI 450 °C and LOI 550 °C approaches for comparison with the non-isothermal TGA method. The main purpose of this paper was to recycle the residual carbon from fine slag by froth flotation kinetic process, and to characterize and describe its properties, so as to provide a reference for industry. The analyses of the gasification fine slag and the flotation residual carbon were carried out by means of the elemental analyzer, particle size analyzer, scanning electron microscopy (SEM), N2 adsorption (-196 °C) technique, LOI determination,
Gasification fine slag, as a by-product from entrained-flow gasification, consists of residual carbon and inorganic matters. In China, hundreds of million tons of gasification slag was produced from coal gasification in 2015 [1]. With the increasing demand for low-cost energy production and high-value chemicals, the application of gasification technology will continue to surge and will bring the problems of waste-based gasification slag to a large extent [2,3]. How to realize the cleaner production and effective resource utilization of gasification slag will become an important research topic. There are two major kinds of slags called “coarse slag” and “fine slag”, which are formed by raw coals minerals at high temperatures. Generally, the carbon content in fine slag is higher than that in coarse slag, while the surface of the residual carbon in the fine slag is rough and porous. The high content of residual carbon in gasification fine slag is the result of incomplete gasification of the coal, which also brings serious environmental safety problems [4,5]. Although the fine slag is a notorious solid waste, the high levels of carbon in the fine slag can be recycled as supplementary energy, which has the resource and environmental benefits [6]. Many researchers have studied ash from power plants and characterized the unburned carbon as fly ash. Some researchers have also focused on the residual carbon in gasification slags [4,5,7–15]. ⁎
Corresponding author at: School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou 221116, PR China. E-mail addresses: [email protected] (F. Guo), [email protected] (J. Wu).
https://doi.org/10.1016/j.fuel.2020.117043 Received 10 September 2019; Received in revised form 16 December 2019; Accepted 7 January 2020 0016-2361/ © 2020 Elsevier Ltd. All rights reserved.
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Fig. 1. The flow chart of gasification slag processing system.
and TGA. 2. Material and methods 2.1. Material preparation Gasification fine slag was obtained from an entrained-flow gasification unit in the commercial operation in Ningxia, China. The flow chart of the gasification slag processing system of the entrained-flow gasifier is shown in Fig. 1. In order to remove the flocculating agent in the sample, 100 g of air-dried fine slag was mixed with 1L of deionized water and stirred at 300r/min for 30 min. The mixture was filtered while rinsing with 1L of deionized water and dried for 24 h at 80 °C to prepare samples for sequent froth flotation. Collector and frother for flotation were obtained from a commercial company in China and were analyzed in the laboratory [25]. As shown in Fig. 2, the oxygen-containing groups such as –OH (3453 cm−1), –COOH (1712 cm−1), -C=O and –C-O- (1084 cm−1 and 1081 cm−1) were the main polar compositions of flotation agents, which can be found in the obtained collector and frother. Under the action of flotation agents, the hydrophobic residual carbon adheres to the bubble and follows the bubble to the surface of the flotation liquid, realizing the process of separating the hydrophobic residual carbon and the hydrophilic tailings.
Fig. 2. Results of FTIR spectra of flotation reagent (collector and frother).
bubbles induced by the air pump were passed through the slurry, forming a froth on the upper layer of the slurry. The flotation process was divided into three steps, each lasting for 7 min, and then the residual carbon and tailings were separated. The residual carbon was rinsed with deionized water, dried overnight at 110 °C and weighed for sequent analysis. The loss-on-ignition and recovery percentage were calculated via methods elsewhere [26]. The element analyses of fine slag and residual carbon were carried out according to GB/T 212-2008 standards. The proximate analysis of samples was performed by TGA. Higher heating value (HHV) analysis was carried out according to the previously reported method based on the element composition [27]. The surface area and pore structures of the samples were analyzed by the N2 adsorption (77 K) technique using a pore analyzer employing the Brunauer-Emmett-Teller (BET) model and Barrett-Joyner-Halenda (BJH) model, respectively.
2.2. Experimental methods The flotation kinetics test of fine slag was conducted on a laboratory-based single tank flotation machine having a capacity of 1L. For each flotation kinetics experiment, 100 g/L of the slurry sample was added into the flotation cell. The impeller rotation, collector dosage, frother dosage, and airflow rate were kept constant at 1800 rpm, 7 kg/t, 14 k g/t, and 0.2 m3/h, respectively. The three-step flotation kinetics experiment is shown in Fig. 3. In brief, the slurry was pre-wetted for 2 min, then collector and frother were added to it with a regulating time of 2 min and 0.5 min, respectively. After the conditioning period, 2
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Fig. 3. A three-step of froth flotation for recycling residual carbon.
Fig. 4. SEM analysis of fine slag, tailings, and flotation residual carbon.
The flotation residual carbon sample was dried overnight and existed in the form of fine powder with a wide particle-size distribution. In order to obtain different particle-size fractions, the mechanical shaker was used to sieve the sample with three mesh screens of 38 μm, 75 μm, and 106 μm. The residual carbon content of each fraction was carried out by TGA and LOI methods. The particle-size fractions over 75 μm were collected for conducting grindability analysis by a laboratory ball mill. The same amount of residual carbon was put into the cell each time and ground with a stainless-steel ball of 1.5 cm at a certain frequency. To compare the grindability, the grinding times were kept at 1.2 min, 2.4 min, 6.0 min, and 12.0 min. The ground residual carbons were then characterized by the laser diffraction particle size distribution analyzer (Microtrac). LOI of fine slag and kinds of particle-size fractions were carried out according to the Irish Standard (EN 13039) [24]. For this purpose, a certain amount of sample was tested in the muffle furnace. In the first process, the sample was heated to 450 °C with a heating rate of 150 °C/
h and a holding time of 6 h. In the second experiment, it was heated to 550 °C with a heating rate of 150 °C/h and a holding time of 2 h. In both experiments, a non-isothermal TGA was used to estimate the residual carbon content of the sample, which was heated from 25 °C to 105 °C in the air/inert atmosphere for 0.5 h. The temperature was then increased to 900 °C at a heating rate of 15 °C/min. Based on the data of TGA, the rate of weight loss called derivative thermo-gravimetric (DTG) was used to describe the reactivity of samples for thermal analysis:
DTG = −
1 dwt × × 100%, %/ min w0 dt
(1)
where, w is the weight of the sample at the time t, w0 is the initial weight of a sample on a dry basis. The TGA data of sample combustion was calculated by CoatsRedfern method [28], and the kinetic parameters of fine slag combustion in the TGA process were analyzed by Arrhenius plot [29,30]: 3
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Fig. 5. The color representation of fine slag, tailing ash and flotation residual carbon.
ln ⎛− ⎝
ln(1 − α ) ⎞ AR E ⎞− = ln ⎛ T2 RT ⎝ τE ⎠ ⎠
As shown in Table 1, it was found that carbon is the dominant element in the flotation residual carbon. The element analysis shows that the carbon content in flotation residual carbon and its mass fractions of different particle-sizes is relatively high. As a fuel with low H/C and O/C ratios, it is beneficial for combustion because it reduces PM and energy loss [36]. The H/C and O/C atomic ratios of flotation residual carbon are lower than that of gasification fine slag. The higher heating value of gasification fine slag is only 6.22 MJ/kg, but when it is converted to a dry-ash-free basis, its higher heating value surge to a relatively high value of about 30 MJ/kg [12]. This means that if residual carbon in fine slag is selected as a recycling fuel, the overall energy efficiency of the gasification process will be significantly improved. Hence, the efficient flotation to separate and enrich the carbon content of fine slag can be considered an important way to achieve raw fine slag upgrading. This was also reported in the coal fly ash field [20,37]. The HHV of flotation residual carbon is elevated to 19.48 MJ/ kg after a three-step froth flotation process. The specific surface area of fine slag (145 m2/g) is about 30% higher than that of residual carbon (111 m2/g). In contrast, the pore volume of residual carbon (0.183286 cm3/g) increases by 42.34% than that of fine slag (0.128765 cm3/g), which is related to lots of small spheres in fine slag and a rich pores in residual carbon, respectively. In other words, the flotation process can partly separate spherical-ash particles from the surface and the pores of carbon matrix, which causes a large decrease of stacked pores of fine slag resulting in decreasing the value of BET specific surface area of flotation residual carbon [26].
(2)
where, α is the carbon conversion rate, R is the universal gas constant (8.314 J/mol/K), τ is the heating rate (15 K/min), A is the pre-exponential factor, E is activation energy of carbon oxidation, and T is the temperature. 3. Results and discussion 3.1. Elementary characteristics of fine slag and residual carbon Fig. 4 presents the surface morphologies of fine slag that composed of rough residual carbon and spherical-smooth ash particles. The Froth flotation process can moderately realize the separation of residual carbon and spherical-smooth ash according to the difference in the physical and chemical properties of hydrophobicity. The removal of the ashes is demonstrated by the changes in color [31]. In Fig. 5, the effect of froth separation on fine slag is qualitatively compared among the color changes of fine slag, ash, and residual carbon. As the carbon content of fine slag and flotation residual carbon is > 20%, they are almost similar in a dark color, while the flotation residual carbon with about 65% of carbon shows a darker color. In contrast, the LOI of lightcolored tailing ash is less 4%, which is much lower than the British standard of coal ash (below 12%) as an addition in the concretes [32–35]. It is clear that froth flotation can efficiently separate and enrich residual carbon, which reflects the classification and utilization of solid-waste fine slag while supplementing the fuel source. Table 1 Element analysis of fine slag and flotation residual carbon. Sample Proximate analysis (wt. %) Water content a Ash a Volatile matter a Fixed carbon a Element analysis (wt. %) Ca Ha Na Ob BET surface area (m2/g) BJH pore volume (cm3/g) HHV (MJ/kg) c O/C H/C a b c
Fine slag
Flotation residual carbon
Tailings
> 106 μm
< 106 μm
75–106 μm
38–75 μm
< 38 μm
1.43 76.50 2.52 19.55
1.78 34.04 3.65 60.53
0.54 96.48 0.34 2.64
1.65 16.31 2.97 79.07
1.80 37.35 2.50 58.35
1.73 19.22 2.58 76.47
1.85 35.41 2.52 60.22
1.79 47.29 2.46 48.46
23.05 0.50 0.11 0.25 144.78 0.13 6.22 0.011 0.022
64.47 0.88 0.26 0.36 111.23 0.18 19.48 0.006 0.013
3.59 0.01 0.07 0.09
/ / / / / / / / /
/ / / / / / / / /
/ / / / / / / / /
/ / / / / / / / /
/ / / / / / / / /
Air dry basis. By difference Higher heating value was determined by calculation. 4
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Fig. 6. Distribution of residual carbon particle size by sieving.
Fig. 8. Grindability of over 75 μm particle size of residual carbon.
3.2. Particle size distribution and grindability of residual carbon
gasification, solid fuels need to reach the required particle-size level [42,43]. The grindability of residual carbon is discussed in this part. The fixed carbon content of > 75 μm fractions from flotation residual carbon is over 75%, which is a potential source as a supply of energy. Fig. 8 presents the grinding ability of > 75 μm fraction from flotation residual carbon, and the size of grindings decreases with increasing grinding time. Grinding time plays a significant role in the particle size distribution of ground flotation residual carbon. Taking the median diameter d50 as an example, it was observed that when the grinding time increased from 2.4 min to 6 min, the d50 value decreased from 95.56 μm to 28.05 μm, and after 12 min to 5.88 μm. As the grinding time increased, the grindability of residual carbon reached a relatively stable level. However, the grinding limit of solid fuel still exists [44]. The grinding time of 6 min achieved the crushing requirement of large particle flotation residual carbon. The volume ratio of grinding over 80% was < 75 μm sieving, which can be further applied in the aspects of slurry fuel preparation (< 75 μm is over 80%) [42,43].
In the process of gasification, the particle size of residual carbon is influenced by the combustion/break and the expansion of char, which is likely to cause the different particles with different characteristics [38,39]. The researchers have studied the relationships of particle size and carbon content/specific surface area/adsorption capacities [40,41]. Hence, it is meaningful to compare the different properties of different particle-size flotation residual carbon. A comparison of the proximate analysis of different particle-size fractions of flotation residual carbon can be seen from Table 1. Fig. 6 presents that the flotation residual carbon fraction of 38–75 μm is the major part and its fixed carbon is still over 60%. The fraction with a particle size > 75 μm accounts for about one-third of the total residual carbon and the fixed carbon is over 75% which is higher than that of the smaller particle-size ones. Although the ash content of fine slag is over 70%, the values of flotation residual carbon and different particle-size fractions are far lower. Especially, relatively low ash values and higher fixed carbon of 75–106 μm and > 106 μm particle-size fractions which are about 20%, which means that large particle-size residual carbon has better fuel properties from the view of caloric value. In Fig. 7, the LOI of gasification fine slag is about 24%, which is quite lower than that in flotation residual carbon and its different particle-size mass fraction, especially the value in > 75 μm particlesize fraction is about 80%. Hence, the flotation residual carbon can be considered as an addition of carbon resource as a fuel [4,12]. The froth flotation of fine slag realizes the upgrading of fine slag and enrich of carbon for further utilization as a fuel. To achieve the utilization of residual carbon in combustion/
3.3. LOI determination and TGA method for residual carbon The determination of carbon content in ashes was carried out by TGA, as it is recommended in previous studies [21,22]. The experimental results to identify the actual carbon content in fine slag are shown in Fig. 9. It is worth noting that, there is little mass loss of gasification fine slag under an inert atmosphere in the temperature range of 200-900 °C, which is far different from the power plant ashes
Fig. 7. LOI of fine slag and flotation residual carbon.
Fig. 9. DTG curves of fine slag in air and inert atmosphere. 5
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Table 2 Combustion reactivity kinetic parameters of fine slag and flotation residual carbon. Sample
E, kJ/mol
R-square
Gasification fine slag Flotation residual carbon
148.36 79.76
0.97 0.98
Fig. 10. DTG curves of tailing ash and residual carbon in air atmosphere.
[13,22]. Huang et al. [45] studied the reaction characteristics of gasification slag during HAc acid treatment and found that there was very small weight loss (< 2.5%) of the sample (pretreat with low concentration weak acid) in TGA (inert atmosphere). In other words, in an inert atmosphere, minerals appear to decompose slightly at 200-800 °C. This is closely related to the fine slag undergoing a high temperature reaction process above 1400 °C in a gasification atmosphere. However, in an air atmosphere, the mass reduction existed at 400–800 °C in TGA, which is due to the oxidation reaction of carbon [13]. Zhao et al. [46] studied the reactivity of residual carbon in coal gasification slag via the time-dependent curves of the weight and weight loss rate (DTG). In our experiment, the DTG results of flotation residual carbon fractions and tailing ash were obtained via TGA in air atmosphere with a heating rate of 15 °C/min (Fig. 10). Although the mass reduction rates of fractions are different, the peak value of curves is almost in the same temperature range. DTG curve of tailing ash with about 4% of carbon shows just one low peak which is similar to reaction temperature interval of residual carbons. On the other hand, it proved that air TGA peaks of fractions in fine slag are strongly associated with residual carbon content. Moreover, the larger particle-size fractions with higher carbon content are more likely to contact with air and the peak rates of mass reduction increase correspondingly. The Arrhenius plots of reactivity kinetics of both the gasification fine slag and the flotation residual carbon were studied, as shown in Fig. 11 and Table 2. Fit linear R2 exceeds 0.96 within a wide residual carbon oxidation range (10–85%). The activation energy value of fine slag is similar to that of chars, which is within the range of the data in other literature of char oxidation
Fig. 12. Comparison of residual carbon content based on TGA method and LOI determined at (a) 450 °C for 6 h and (b) 550 °C for 2 h, respectively.
(128.4–174.2 kJ/mol) [13]. Moreover, the flotation residual carbon has lower activation energy value which is associated with better combustion reactivity. A total of 5 samples were used for determination of the flotation residual carbon, including the one below 38 μm fraction, 38–75 μm fraction, 75–106 μm fraction, over 106 μm fraction and original flotation residual carbon. As shown in Fig. 12a, by comparing the mass reductions of residual carbon and LOI values at 450 °C, it is totally different. However, the mass reductions of LOI values at 550 °C in Fig. 12b are matching well to the TGA results. It means that a better correlation exit between the residual carbon contents by TGA and LOI values obtained at 550 °C. There is such an obvious difference between these two temperature conditions (LOI values obtained at 450 and 550 °C), mainly because the residual carbon of the gasification fine slag has undergone a high temperature process with increasing graphitization degree, which causes a decreasing reactivity of residual carbon, so the higher temperature of 550 °C is more favorable for the oxidation of the residual carbon for the LOI analysis [12,47]. From the view of variance analysis, the variance value of 550 °C LOI is smaller than that of 450 °C LOI, which presents better stability of 550 °C LOI in the aspect of carbon
Fig. 11. Arrhenius plots of oxidation reactivity kinetics of the gasification fine slag and its flotation residual carbon. 6
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content analysis. Besides, the data of 550 °C LOI method is remarkably close to that of the TG analysis method, which shows a much higher accuracy of 550 °C LOI than that of 450 °C LOI for the LOI determine. Both non-isothermal TGA and LOI 550 °C can be trusted methods to estimate carbon content for residual carbon in gasification fine slag.
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4. Conclusions As a considerable amount of solid waste with high carbon content, gasification fine slag cannot be directly used in the concrete industry. Due to high ash content, gasification fine slag is not expected to be a cost-effective fuel. This research shows that froth flotation can recycle residual carbon and obtain relatively high purity tailing ash for other industries. This study draws the following conclusions.
• (1) A three-step froth flotation kinetics experiment can efficiently •
• •
recycle residual carbon to an LOI of 65% and high-purity tailing ash with LOI of below 4%. In flotation residual carbon, the fraction of 38–75 μm is the dominant one and the LOI is about 63.5%. (2)The fixed carbon content increases with the increase of particle size. From the view of carbon content in flotation residual carbon, particle size over 75 μm has better fuel properties and its LOI exceeds 80%. On the other hand, as the grinding time increase to 6 min, the d50 value becomes 28.05 μm. > 80% of the grinding products can be screened by 75 μm sieving, which can be further applied for the slurry fuel preparation. (3) While comparing the DTG curves of fine slag and residual carbon in the air/inert atmosphere, almost carbon oxidation occurs instead of mineral decomposition in the range of 400–800 °C. (4) There is no standard method to determine the LOI of gasification fine slag. Non-isothermal TGA and LOI methods were used to analyze of the oxidation of residual carbon. Comparison to LOI at 450 °C for 6 h, LOI at 550 °C for 2 h matched well with the TGA results, which can be used to evaluate the carbon content in fine slag and its flotation residual carbon.
Declaration of competing interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Fanhui Guo: Conceptualization, Investigation, Writing - original draft. Zekai Miao: Data curation, Investigation. Zhenkun Guo: Investigation. Jian Li: Visualization, Software. Yixin Zhang: Methodology, Writing - review & editing. Jianjun Wu: Conceptualization, Supervision. Acknowledgments This work was supported by the “Fundamental Research Funds for the Central Universities” [grant number 2019XKQYMS31]. References [1] Wang SJ. Development and application of modern coal gasification technology. Chemical Industry and Engineering Progress 2016;35:653–64. [2] Simbeck D, Johnson H. In World gasification survey: Industry trends and developments, Gasification Technologies 2001 Conference. 2001. [3] Dai BQ, Hoadley A, Zhang L. Characteristics of high temperature co-gasification and ash slagging for Victorian brown coal char and bituminous coal blends. Fuel 2018;215:799–812. [4] Xu SQ, Zhou ZJ, Gao XX, Yu GS, Gong X. The gasification reactivity of unburned carbon present in gasification slag from entrained-flow gasifier. Fuel Process Technol 2009;90(9):1062–70. [5] Wu T, Gong M, Lester E, Wang FC, Zhou ZJ, Yu ZH. Characterisation of residual
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Full Length Article
Properties of flotation residual carbon from gasification fine slag Fanhui Guo a b
a,b
, Zekai Miao
a,b
, Zhenkun Guo
a,b
a
b
a,b,⁎
T
, Jian Li , Yixin Zhang , Jianjun Wu
School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou 221116, PR China Chinese National Engineering Research Center of Coal Preparation and Purification, China University of Mining and Technology, Xuzhou 221116, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Gasification fine slag Froth flotation Residual carbon Waste recycling Fuel properties LOI determine
The purpose of this study was to upgrade the fine slag by a froth flotation kinetics process in order to separate and recycle the residual carbon and tailing ash simultaneously. Fine slag, a kind of solid waste, was obtained from an entrained-flow gasification unit. The carbon content of flotation residual carbon was 64.47% by weight, which is about 3 times of that of gasification fine slag. The results show that when the particle size of fractions is higher (> 75 μm), the carbon content is higher, even > 80%. The fine slag and flotation residual carbon were characterized by elemental and proximate analysis in order to compare the H/C, O/C, and element characteristics. The morphology of fine slag was determined by scanning electron microscopy (SEM). It was found the grinding time of 6 min can meet the crushing requirements of large particle flotation residual carbon. The grinding volume ratio of > 80% is < 75 μm sieving, which is a benefit for further fuel utilization. The nonisothermal thermo-gravimetric analysis (TGA) and loss on ignition (LOI) at 450 °C & 550 °C methods were used to analyze and compare the oxidation of residual carbon. In conclusion, LOI at 550 °C and non-isothermal TGA approaches can be used to determine the carbon content, which provides a reference for the determining LOI of waste-derived fuels in industries.
1. Introduction
However, only a few have systematically studied the separation and recycling of residual carbon from gasification fine slag and then characterized the residual carbon. There are several approaches to recycle residual carbon, including physical and chemical methods [16,17]. Froth flotation is an effective separation method for cleaning fine coal and recycling the residual carbon from coal ash. The difference in hydrophobicity between organic and inorganic minerals is the basic principle of froth flotation technology [18–20]. There are two major approaches to determine the residual carbon. Loss on ignition (LOI) is a simple way to evaluate the residual carbon in the ash, but factors like mineral decomposition may affect the measurement results [21–23]. For the determination of the carbon content, the European standard can be used [24]. However, there is no standard LOI approach yet available for gasification fine slag and residual carbon from the flotation process. In this study, we used the LOI 450 °C and LOI 550 °C approaches for comparison with the non-isothermal TGA method. The main purpose of this paper was to recycle the residual carbon from fine slag by froth flotation kinetic process, and to characterize and describe its properties, so as to provide a reference for industry. The analyses of the gasification fine slag and the flotation residual carbon were carried out by means of the elemental analyzer, particle size analyzer, scanning electron microscopy (SEM), N2 adsorption (-196 °C) technique, LOI determination,
Gasification fine slag, as a by-product from entrained-flow gasification, consists of residual carbon and inorganic matters. In China, hundreds of million tons of gasification slag was produced from coal gasification in 2015 [1]. With the increasing demand for low-cost energy production and high-value chemicals, the application of gasification technology will continue to surge and will bring the problems of waste-based gasification slag to a large extent [2,3]. How to realize the cleaner production and effective resource utilization of gasification slag will become an important research topic. There are two major kinds of slags called “coarse slag” and “fine slag”, which are formed by raw coals minerals at high temperatures. Generally, the carbon content in fine slag is higher than that in coarse slag, while the surface of the residual carbon in the fine slag is rough and porous. The high content of residual carbon in gasification fine slag is the result of incomplete gasification of the coal, which also brings serious environmental safety problems [4,5]. Although the fine slag is a notorious solid waste, the high levels of carbon in the fine slag can be recycled as supplementary energy, which has the resource and environmental benefits [6]. Many researchers have studied ash from power plants and characterized the unburned carbon as fly ash. Some researchers have also focused on the residual carbon in gasification slags [4,5,7–15]. ⁎
Corresponding author at: School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou 221116, PR China. E-mail addresses: [email protected] (F. Guo), [email protected] (J. Wu).
https://doi.org/10.1016/j.fuel.2020.117043 Received 10 September 2019; Received in revised form 16 December 2019; Accepted 7 January 2020 0016-2361/ © 2020 Elsevier Ltd. All rights reserved.
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Fig. 1. The flow chart of gasification slag processing system.
and TGA. 2. Material and methods 2.1. Material preparation Gasification fine slag was obtained from an entrained-flow gasification unit in the commercial operation in Ningxia, China. The flow chart of the gasification slag processing system of the entrained-flow gasifier is shown in Fig. 1. In order to remove the flocculating agent in the sample, 100 g of air-dried fine slag was mixed with 1L of deionized water and stirred at 300r/min for 30 min. The mixture was filtered while rinsing with 1L of deionized water and dried for 24 h at 80 °C to prepare samples for sequent froth flotation. Collector and frother for flotation were obtained from a commercial company in China and were analyzed in the laboratory [25]. As shown in Fig. 2, the oxygen-containing groups such as –OH (3453 cm−1), –COOH (1712 cm−1), -C=O and –C-O- (1084 cm−1 and 1081 cm−1) were the main polar compositions of flotation agents, which can be found in the obtained collector and frother. Under the action of flotation agents, the hydrophobic residual carbon adheres to the bubble and follows the bubble to the surface of the flotation liquid, realizing the process of separating the hydrophobic residual carbon and the hydrophilic tailings.
Fig. 2. Results of FTIR spectra of flotation reagent (collector and frother).
bubbles induced by the air pump were passed through the slurry, forming a froth on the upper layer of the slurry. The flotation process was divided into three steps, each lasting for 7 min, and then the residual carbon and tailings were separated. The residual carbon was rinsed with deionized water, dried overnight at 110 °C and weighed for sequent analysis. The loss-on-ignition and recovery percentage were calculated via methods elsewhere [26]. The element analyses of fine slag and residual carbon were carried out according to GB/T 212-2008 standards. The proximate analysis of samples was performed by TGA. Higher heating value (HHV) analysis was carried out according to the previously reported method based on the element composition [27]. The surface area and pore structures of the samples were analyzed by the N2 adsorption (77 K) technique using a pore analyzer employing the Brunauer-Emmett-Teller (BET) model and Barrett-Joyner-Halenda (BJH) model, respectively.
2.2. Experimental methods The flotation kinetics test of fine slag was conducted on a laboratory-based single tank flotation machine having a capacity of 1L. For each flotation kinetics experiment, 100 g/L of the slurry sample was added into the flotation cell. The impeller rotation, collector dosage, frother dosage, and airflow rate were kept constant at 1800 rpm, 7 kg/t, 14 k g/t, and 0.2 m3/h, respectively. The three-step flotation kinetics experiment is shown in Fig. 3. In brief, the slurry was pre-wetted for 2 min, then collector and frother were added to it with a regulating time of 2 min and 0.5 min, respectively. After the conditioning period, 2
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Fig. 3. A three-step of froth flotation for recycling residual carbon.
Fig. 4. SEM analysis of fine slag, tailings, and flotation residual carbon.
The flotation residual carbon sample was dried overnight and existed in the form of fine powder with a wide particle-size distribution. In order to obtain different particle-size fractions, the mechanical shaker was used to sieve the sample with three mesh screens of 38 μm, 75 μm, and 106 μm. The residual carbon content of each fraction was carried out by TGA and LOI methods. The particle-size fractions over 75 μm were collected for conducting grindability analysis by a laboratory ball mill. The same amount of residual carbon was put into the cell each time and ground with a stainless-steel ball of 1.5 cm at a certain frequency. To compare the grindability, the grinding times were kept at 1.2 min, 2.4 min, 6.0 min, and 12.0 min. The ground residual carbons were then characterized by the laser diffraction particle size distribution analyzer (Microtrac). LOI of fine slag and kinds of particle-size fractions were carried out according to the Irish Standard (EN 13039) [24]. For this purpose, a certain amount of sample was tested in the muffle furnace. In the first process, the sample was heated to 450 °C with a heating rate of 150 °C/
h and a holding time of 6 h. In the second experiment, it was heated to 550 °C with a heating rate of 150 °C/h and a holding time of 2 h. In both experiments, a non-isothermal TGA was used to estimate the residual carbon content of the sample, which was heated from 25 °C to 105 °C in the air/inert atmosphere for 0.5 h. The temperature was then increased to 900 °C at a heating rate of 15 °C/min. Based on the data of TGA, the rate of weight loss called derivative thermo-gravimetric (DTG) was used to describe the reactivity of samples for thermal analysis:
DTG = −
1 dwt × × 100%, %/ min w0 dt
(1)
where, w is the weight of the sample at the time t, w0 is the initial weight of a sample on a dry basis. The TGA data of sample combustion was calculated by CoatsRedfern method [28], and the kinetic parameters of fine slag combustion in the TGA process were analyzed by Arrhenius plot [29,30]: 3
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Fig. 5. The color representation of fine slag, tailing ash and flotation residual carbon.
ln ⎛− ⎝
ln(1 − α ) ⎞ AR E ⎞− = ln ⎛ T2 RT ⎝ τE ⎠ ⎠
As shown in Table 1, it was found that carbon is the dominant element in the flotation residual carbon. The element analysis shows that the carbon content in flotation residual carbon and its mass fractions of different particle-sizes is relatively high. As a fuel with low H/C and O/C ratios, it is beneficial for combustion because it reduces PM and energy loss [36]. The H/C and O/C atomic ratios of flotation residual carbon are lower than that of gasification fine slag. The higher heating value of gasification fine slag is only 6.22 MJ/kg, but when it is converted to a dry-ash-free basis, its higher heating value surge to a relatively high value of about 30 MJ/kg [12]. This means that if residual carbon in fine slag is selected as a recycling fuel, the overall energy efficiency of the gasification process will be significantly improved. Hence, the efficient flotation to separate and enrich the carbon content of fine slag can be considered an important way to achieve raw fine slag upgrading. This was also reported in the coal fly ash field [20,37]. The HHV of flotation residual carbon is elevated to 19.48 MJ/ kg after a three-step froth flotation process. The specific surface area of fine slag (145 m2/g) is about 30% higher than that of residual carbon (111 m2/g). In contrast, the pore volume of residual carbon (0.183286 cm3/g) increases by 42.34% than that of fine slag (0.128765 cm3/g), which is related to lots of small spheres in fine slag and a rich pores in residual carbon, respectively. In other words, the flotation process can partly separate spherical-ash particles from the surface and the pores of carbon matrix, which causes a large decrease of stacked pores of fine slag resulting in decreasing the value of BET specific surface area of flotation residual carbon [26].
(2)
where, α is the carbon conversion rate, R is the universal gas constant (8.314 J/mol/K), τ is the heating rate (15 K/min), A is the pre-exponential factor, E is activation energy of carbon oxidation, and T is the temperature. 3. Results and discussion 3.1. Elementary characteristics of fine slag and residual carbon Fig. 4 presents the surface morphologies of fine slag that composed of rough residual carbon and spherical-smooth ash particles. The Froth flotation process can moderately realize the separation of residual carbon and spherical-smooth ash according to the difference in the physical and chemical properties of hydrophobicity. The removal of the ashes is demonstrated by the changes in color [31]. In Fig. 5, the effect of froth separation on fine slag is qualitatively compared among the color changes of fine slag, ash, and residual carbon. As the carbon content of fine slag and flotation residual carbon is > 20%, they are almost similar in a dark color, while the flotation residual carbon with about 65% of carbon shows a darker color. In contrast, the LOI of lightcolored tailing ash is less 4%, which is much lower than the British standard of coal ash (below 12%) as an addition in the concretes [32–35]. It is clear that froth flotation can efficiently separate and enrich residual carbon, which reflects the classification and utilization of solid-waste fine slag while supplementing the fuel source. Table 1 Element analysis of fine slag and flotation residual carbon. Sample Proximate analysis (wt. %) Water content a Ash a Volatile matter a Fixed carbon a Element analysis (wt. %) Ca Ha Na Ob BET surface area (m2/g) BJH pore volume (cm3/g) HHV (MJ/kg) c O/C H/C a b c
Fine slag
Flotation residual carbon
Tailings
> 106 μm
< 106 μm
75–106 μm
38–75 μm
< 38 μm
1.43 76.50 2.52 19.55
1.78 34.04 3.65 60.53
0.54 96.48 0.34 2.64
1.65 16.31 2.97 79.07
1.80 37.35 2.50 58.35
1.73 19.22 2.58 76.47
1.85 35.41 2.52 60.22
1.79 47.29 2.46 48.46
23.05 0.50 0.11 0.25 144.78 0.13 6.22 0.011 0.022
64.47 0.88 0.26 0.36 111.23 0.18 19.48 0.006 0.013
3.59 0.01 0.07 0.09
/ / / / / / / / /
/ / / / / / / / /
/ / / / / / / / /
/ / / / / / / / /
/ / / / / / / / /
Air dry basis. By difference Higher heating value was determined by calculation. 4
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Fig. 6. Distribution of residual carbon particle size by sieving.
Fig. 8. Grindability of over 75 μm particle size of residual carbon.
3.2. Particle size distribution and grindability of residual carbon
gasification, solid fuels need to reach the required particle-size level [42,43]. The grindability of residual carbon is discussed in this part. The fixed carbon content of > 75 μm fractions from flotation residual carbon is over 75%, which is a potential source as a supply of energy. Fig. 8 presents the grinding ability of > 75 μm fraction from flotation residual carbon, and the size of grindings decreases with increasing grinding time. Grinding time plays a significant role in the particle size distribution of ground flotation residual carbon. Taking the median diameter d50 as an example, it was observed that when the grinding time increased from 2.4 min to 6 min, the d50 value decreased from 95.56 μm to 28.05 μm, and after 12 min to 5.88 μm. As the grinding time increased, the grindability of residual carbon reached a relatively stable level. However, the grinding limit of solid fuel still exists [44]. The grinding time of 6 min achieved the crushing requirement of large particle flotation residual carbon. The volume ratio of grinding over 80% was < 75 μm sieving, which can be further applied in the aspects of slurry fuel preparation (< 75 μm is over 80%) [42,43].
In the process of gasification, the particle size of residual carbon is influenced by the combustion/break and the expansion of char, which is likely to cause the different particles with different characteristics [38,39]. The researchers have studied the relationships of particle size and carbon content/specific surface area/adsorption capacities [40,41]. Hence, it is meaningful to compare the different properties of different particle-size flotation residual carbon. A comparison of the proximate analysis of different particle-size fractions of flotation residual carbon can be seen from Table 1. Fig. 6 presents that the flotation residual carbon fraction of 38–75 μm is the major part and its fixed carbon is still over 60%. The fraction with a particle size > 75 μm accounts for about one-third of the total residual carbon and the fixed carbon is over 75% which is higher than that of the smaller particle-size ones. Although the ash content of fine slag is over 70%, the values of flotation residual carbon and different particle-size fractions are far lower. Especially, relatively low ash values and higher fixed carbon of 75–106 μm and > 106 μm particle-size fractions which are about 20%, which means that large particle-size residual carbon has better fuel properties from the view of caloric value. In Fig. 7, the LOI of gasification fine slag is about 24%, which is quite lower than that in flotation residual carbon and its different particle-size mass fraction, especially the value in > 75 μm particlesize fraction is about 80%. Hence, the flotation residual carbon can be considered as an addition of carbon resource as a fuel [4,12]. The froth flotation of fine slag realizes the upgrading of fine slag and enrich of carbon for further utilization as a fuel. To achieve the utilization of residual carbon in combustion/
3.3. LOI determination and TGA method for residual carbon The determination of carbon content in ashes was carried out by TGA, as it is recommended in previous studies [21,22]. The experimental results to identify the actual carbon content in fine slag are shown in Fig. 9. It is worth noting that, there is little mass loss of gasification fine slag under an inert atmosphere in the temperature range of 200-900 °C, which is far different from the power plant ashes
Fig. 7. LOI of fine slag and flotation residual carbon.
Fig. 9. DTG curves of fine slag in air and inert atmosphere. 5
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Table 2 Combustion reactivity kinetic parameters of fine slag and flotation residual carbon. Sample
E, kJ/mol
R-square
Gasification fine slag Flotation residual carbon
148.36 79.76
0.97 0.98
Fig. 10. DTG curves of tailing ash and residual carbon in air atmosphere.
[13,22]. Huang et al. [45] studied the reaction characteristics of gasification slag during HAc acid treatment and found that there was very small weight loss (< 2.5%) of the sample (pretreat with low concentration weak acid) in TGA (inert atmosphere). In other words, in an inert atmosphere, minerals appear to decompose slightly at 200-800 °C. This is closely related to the fine slag undergoing a high temperature reaction process above 1400 °C in a gasification atmosphere. However, in an air atmosphere, the mass reduction existed at 400–800 °C in TGA, which is due to the oxidation reaction of carbon [13]. Zhao et al. [46] studied the reactivity of residual carbon in coal gasification slag via the time-dependent curves of the weight and weight loss rate (DTG). In our experiment, the DTG results of flotation residual carbon fractions and tailing ash were obtained via TGA in air atmosphere with a heating rate of 15 °C/min (Fig. 10). Although the mass reduction rates of fractions are different, the peak value of curves is almost in the same temperature range. DTG curve of tailing ash with about 4% of carbon shows just one low peak which is similar to reaction temperature interval of residual carbons. On the other hand, it proved that air TGA peaks of fractions in fine slag are strongly associated with residual carbon content. Moreover, the larger particle-size fractions with higher carbon content are more likely to contact with air and the peak rates of mass reduction increase correspondingly. The Arrhenius plots of reactivity kinetics of both the gasification fine slag and the flotation residual carbon were studied, as shown in Fig. 11 and Table 2. Fit linear R2 exceeds 0.96 within a wide residual carbon oxidation range (10–85%). The activation energy value of fine slag is similar to that of chars, which is within the range of the data in other literature of char oxidation
Fig. 12. Comparison of residual carbon content based on TGA method and LOI determined at (a) 450 °C for 6 h and (b) 550 °C for 2 h, respectively.
(128.4–174.2 kJ/mol) [13]. Moreover, the flotation residual carbon has lower activation energy value which is associated with better combustion reactivity. A total of 5 samples were used for determination of the flotation residual carbon, including the one below 38 μm fraction, 38–75 μm fraction, 75–106 μm fraction, over 106 μm fraction and original flotation residual carbon. As shown in Fig. 12a, by comparing the mass reductions of residual carbon and LOI values at 450 °C, it is totally different. However, the mass reductions of LOI values at 550 °C in Fig. 12b are matching well to the TGA results. It means that a better correlation exit between the residual carbon contents by TGA and LOI values obtained at 550 °C. There is such an obvious difference between these two temperature conditions (LOI values obtained at 450 and 550 °C), mainly because the residual carbon of the gasification fine slag has undergone a high temperature process with increasing graphitization degree, which causes a decreasing reactivity of residual carbon, so the higher temperature of 550 °C is more favorable for the oxidation of the residual carbon for the LOI analysis [12,47]. From the view of variance analysis, the variance value of 550 °C LOI is smaller than that of 450 °C LOI, which presents better stability of 550 °C LOI in the aspect of carbon
Fig. 11. Arrhenius plots of oxidation reactivity kinetics of the gasification fine slag and its flotation residual carbon. 6
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content analysis. Besides, the data of 550 °C LOI method is remarkably close to that of the TG analysis method, which shows a much higher accuracy of 550 °C LOI than that of 450 °C LOI for the LOI determine. Both non-isothermal TGA and LOI 550 °C can be trusted methods to estimate carbon content for residual carbon in gasification fine slag.
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4. Conclusions As a considerable amount of solid waste with high carbon content, gasification fine slag cannot be directly used in the concrete industry. Due to high ash content, gasification fine slag is not expected to be a cost-effective fuel. This research shows that froth flotation can recycle residual carbon and obtain relatively high purity tailing ash for other industries. This study draws the following conclusions.
• (1) A three-step froth flotation kinetics experiment can efficiently •
• •
recycle residual carbon to an LOI of 65% and high-purity tailing ash with LOI of below 4%. In flotation residual carbon, the fraction of 38–75 μm is the dominant one and the LOI is about 63.5%. (2)The fixed carbon content increases with the increase of particle size. From the view of carbon content in flotation residual carbon, particle size over 75 μm has better fuel properties and its LOI exceeds 80%. On the other hand, as the grinding time increase to 6 min, the d50 value becomes 28.05 μm. > 80% of the grinding products can be screened by 75 μm sieving, which can be further applied for the slurry fuel preparation. (3) While comparing the DTG curves of fine slag and residual carbon in the air/inert atmosphere, almost carbon oxidation occurs instead of mineral decomposition in the range of 400–800 °C. (4) There is no standard method to determine the LOI of gasification fine slag. Non-isothermal TGA and LOI methods were used to analyze of the oxidation of residual carbon. Comparison to LOI at 450 °C for 6 h, LOI at 550 °C for 2 h matched well with the TGA results, which can be used to evaluate the carbon content in fine slag and its flotation residual carbon.
Declaration of competing interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Fanhui Guo: Conceptualization, Investigation, Writing - original draft. Zekai Miao: Data curation, Investigation. Zhenkun Guo: Investigation. Jian Li: Visualization, Software. Yixin Zhang: Methodology, Writing - review & editing. Jianjun Wu: Conceptualization, Supervision. Acknowledgments This work was supported by the “Fundamental Research Funds for the Central Universities” [grant number 2019XKQYMS31]. References [1] Wang SJ. Development and application of modern coal gasification technology. Chemical Industry and Engineering Progress 2016;35:653–64. [2] Simbeck D, Johnson H. In World gasification survey: Industry trends and developments, Gasification Technologies 2001 Conference. 2001. [3] Dai BQ, Hoadley A, Zhang L. Characteristics of high temperature co-gasification and ash slagging for Victorian brown coal char and bituminous coal blends. Fuel 2018;215:799–812. [4] Xu SQ, Zhou ZJ, Gao XX, Yu GS, Gong X. The gasification reactivity of unburned carbon present in gasification slag from entrained-flow gasifier. Fuel Process Technol 2009;90(9):1062–70. [5] Wu T, Gong M, Lester E, Wang FC, Zhou ZJ, Yu ZH. Characterisation of residual
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