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Influence of coal ash on CO2 gasification reactivity of corn stalk char
Renewable Energy 147 (2020) 2056e2063
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Influence of coal ash on CO2 gasification reactivity of corn stalk char Heng Zhang a, b, Junguo Li c, *, Xin Yang d, Shuangshuang Song a, Zhiqing Wang a, Jiejie Huang a, Yongqi Zhang a, Yitian Fang a, ** a
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, Shanxi, 030001, China University of Chinese Academy of Sciences, Beijing, 100049, China c Academy for Advanced Interdisciplinary Studies, Department of Chemistry, Southern University of Science and Technology, Shenzhen, 518055, China d Department of Engineering Science, University of Oxford, Oxford, OX2 0ES, UK b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 July 2019 Received in revised form 16 September 2019 Accepted 4 October 2019 Available online 5 October 2019
The effect of added coal ash on the gasification behavior of corn stalk char by using the thermalgravimetric analysis and a lab-scale horizontal fixed bed furnace was investigated. For comparison, synthetic ash with different chemical composition was used for studying the effect of difference ash composition on promoting/inhibiting the gasification reactivity. The results show that the slaggingcontrol preferred coal ash additive has a negative impact on the gasification performance. Interestingly, a relatively less slagging-control preferred coal ash additive has a beneficial impact on promoting the gasification performance under low gasification temperature. This could be due to the difference composition between the two coal ash additives, where the former coal ash type has much lower content in the calcium and iron. In addition, the findings in the synthetic ash suggest that iron can have a better performance in promoting gasification than calcium/magnesium. Two types of coal ash show an inhibiting effect under higher temperature due to temperature might be the dominant factor and the restraining effect of additive. These observations are also supported by the evolution of char structure by using the Raman spectroscopy. Further, the modified random pore model can correlate well with the gasification kinetics data. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Biomass Gasification Coal ash additive Metal catalysts Kinetic parameters
1. Introduction Biomass, which is regarded as a kind of low-carbon energy, is attractive in decreasing CO2 emission and reducing the dependency on fossil fuels [1e3]. And biomass gasification is regarded as a feasible and promising technology for the production of syngas and power production [4e6]. However, the efficient and steady operation of biomass gasification is challenging due to the ash related issues [7,8]. This is because biomass generally has a high content of alkali, alkali earth and chlorine species in the inorganic components, especially potassium (K) [9e11]. We have reported the results about using coal ash with high ash fusion temperature to reduce the slagging issues of a biomass from a previous study [12]. It indicates that the coal ash is beneficial not only to fix the potassium species in the biomass, but also to increase the ash fusion temperatures of the biomass. However, as the slagging potential is
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Li), [email protected] (Y. Fang). https://doi.org/10.1016/j.renene.2019.10.009 0960-1481/© 2019 Elsevier Ltd. All rights reserved.
controlled, the gasification rate may be affected owning to the potassium state transformation with the reaction to the coal ash, which needs further to be studied for understanding the effects of the coal ash additives on the gasification rate of the biomass. Therefore, as a continuation of our previous research [12], the effect of additive the coal ash with the high fusion temperatures on the gasification behavior of a biomass with a low fusion temperature should be investigated. Generally, biomass contains relatively higher content of alkaline/alkaline-earth metals (AAEMs) than coal [13,14]. Many studies have carried out the research on the effect of the inorganic components in biomass on co-gasification behavior of biomass and coal. Collot et al. [15] studied the co-gasification behavior of coal and a woody biomass in the bench-scale fixed-bed/fluidized bed reactors, indicating that the woody biomass has a catalytic effect on the co-gasification process. Wei et al. [16] investigated the reactivity characteristics and synergy behaviors of co-gasification between rice straw and Shenfu bituminous coal. They found that the whole gasification reactivity of the mixed fuels was enhanced compared to the bituminous coal while the inhibiting effect was
H. Zhang et al. / Renewable Energy 147 (2020) 2056e2063
observed at the initial stage of co-gasification. These observations had been attributed to the synergy and the K/Ca transformation during the co-gasification procedure. Also, some other literatures reported that the AAEMs in biomass ash can play a significant role in the enhancement of the coal gasification reactivity [16e19]. Although many studies have focused on the effect of biomass ash on coal gasification behavior [20,21], there are much less studies on the influence of coal ash on biomass gasification reactivity, particularly the relationship between the chemical composition of coal ash with the biomass carbon structure evolution and char reactivity. This is important to understand how to match a preferred coal with a specified biomass during co-gasification process. For a desired blending of biomass with coal ash for the purpose of controlling ash related issues (slagging/fouling, corrosion, etc.), the gasification performance should not be significantly reduced by the coal ash additives. The dominant mineral phases in coal ash include quartz, mullite, illite and siderite [22] and the major ash compositions in the form of oxide are SiO2, Al2O3, Fe2O3 and CaO, which can usually account for more than 80 wt % [23]. Lahijani et al. [18] investigated the influence of alkali metal (Na, K), alkaline earth metal (Ca, Mg) and transition metal (Fe) nitrates on CO2 gasification reactivity of the pistachio nut shell char by thermogravimetric analyzer (TGA) and pointed out that the catalytic performance order for the different metal catalysts was Nachar > Ca-char > Fe-char > K-char > Mg-char > raw char. This order was slightly different from the results by Huang et al. [24], in which the order was K > Na > Ca > Fe > Mg. In addition, Habibi et al. [25] observed that Al and Si have an inhibiting effect on co-gasification due to the reaction of K with Al and Si to form the potassium aluminosilicates, which have no catalytic properties [26]. There is a few research is a lack of knowledge on the effects of mineral matter contained in fuel on thermal conversion process between coal and biomass [27,28]. However, the literature about utilizing the coal ash as an additive to explore the effect of its components on biomass gasification is scarce. Therefore, as a continuation of the previous research [12], this study aims to provide a better understanding of the coal ash additives (for reducing the ash related issues of biomass) on the gasification performance of the biomass with a low ash fusion temperature. The biomass fuel and coal additives used in this study are the same as those in the previous research. The biomass gasification was carried out in CO2 atmosphere at different reaction temperatures was performed by using the TGA, and the gasification kinetics and the evolution of the char structure were studied. For comparison, the synthetic ash by mixing pure oxides (SiO2, Al2O3, CaO, MgO and Fe2O3) with the different mass fraction was also used to investigate the effect of metal catalysts or in-catalysts effect on the corn stalk char CO2 gasification reactivity. 2. Materials and methods 2.1. Materials and preparation The biomass used was corn stalk (a type of the agricultural residue with a low ash fusion temperature, soften temperature1373 K), collected from Jilin Province in China. The corn stalk was roughly crushed into chips by a pulverizer first and then ground with a ball mill. Sieved by a sieve shaker, the corn stalk particle size fraction of 48 mme75 mm was used for the experiment. The properties of corn stalks such as proximate and ultimate were measured according to Chinese standards (including GB/T28731-2012 for proximate analysis and GB/T28732-2012; GB/T30727-2014; GB/ T30728-2014 for ultimate analysis) and were presented in Table 1(the repeatability limit within 0.15%). The ash compositions as shown in Table 2 were performed by the semi micro chemical
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analysis method and atomic absorption method according to Chinese standard GB/T1574-2007, and the repeatability limit was <1%. Two different bituminous coals (Ningxia coal and Wuming coal) with high and medium ash fusion temperatures (soften temperature was >1773 K and 1569 K respectively), collected from the Ningxia province and Shanxi province respectively, were ground by a lab jaw crusher and then sieved by a sieve shaker. The coal particle size of 80 mesh was chosen for experimental samples. All the samples (corn stalk and coal) were dried at 378 K for 24 h, and then stored in a desiccator. The preparation of corn stalk char (CS) was carried out in a horizontal tubular furnace and the pyrolysis was performed in N2 (purity > 99.99%, flow rate of 1000 ml/min) at 773 K for 30 min. Ningxia coal and Wuming coal were used to make ashes as additives and the details of the ashing procedure were detailed in the previous publication [12]. The Ningxia coal ash and Wuming coal ash were termed as NA and WA in this study, and the NA was the slagging-control preferred ash while WA was the less slaggingcontrol preferred case according our pervious study. The ash compositions shown in Table 2 were performed according to Chinese standard GB/T1574-2007. The fine char and ashes were stored in a desiccator. The synthetic ashes were prepared from SiO2, Al2O3, CaO, MgO and Fe2O3. The chemical compositions of the synthetic ashes were listed in Table 2 and termed into Groups S, Groups C and Groups F. In Group S, the content of CaO, MgO and Fe2O3 is kept constant as well as the sum of SiO2 and Al2O3. However, the SiO2/ Al2O3 ratio (S/A) is changed in the range of 0.33e3, which covers the S/A range of the NA and WA. In Group C, the content of SiO2, Al2O3 and Fe2O3 is kept constant as well as the sum of CaO and MgO. However, the CaO/MgO ratio (C/M) is changed in the range of 0.2e5. For Group F, the content of Fe2O3 (F) changes from 5 wt % to 20 wt % and the others remain unchanged. All samples were mixed by the ball milling for 2 h and then calcined separately at 1088 K for 2 h. The coal ash and the synthetic ash were mixed with CS by the mechanical mixing method. The blending ratio of the coal ash or synthetic ash is 10 wt %, which is consistent with the adding ratio in our pervious study to reducing the ash deposition behavior of biomass. The mixtures were respectively denoted as CS-10NA, CS10WA and CS-10S1, CS-10S2, CS-10S3, CS-10C1, CS-10C2, CS-10C3, CS-10F1, CS-10F2, CS-10F3 respectively. 2.2. Gasification process Gasification experiments were implemented in a horizontal fixed bed furnace system (as shown in Fig. 1). The experimental setup was made up of a gas feeding unit, a fixed-bed unit and a gas treatment unit. For the gas feeding unit, the continuous and stable CO2 (purity > 99.99%) was controlled by pressure reducing valve and mass flowmeter with the accuracy of 2%. The fixed-bed system was composed of an electrical furnace and a quartz tube (inner diameter of 32 mm and length of 800 mm) with sealing flanges. The total heating length and flat-temperature zone of the furnace were 630 and 100 mm respectively. Measured by K-type thermocouple with the accuracy of 1 K, the temperature of furnace was transmitted and controlled by temperature controller. An alumina boat(60 30 15 mm) housed a certain amount of samples and placed in the quartz tube of furnace the flat-temperature zone, and then heated by the external electric furnace. Prior to gasification, the furnace was heated at a rate of 10 K/min to the preset temperature (1173 K or 1273 K) and maintained for 10 min under CO2 atmosphere with a flow rate of 300 ml/min to remove the air inside the quartz tube. Subsequently, an alumina boat loaded with ca.1.000 g samples was placed inside cold section of the quartz tube for 5 min, then instantly pushed into the flattemperature zone for gasifying. A steep temperature gradient was
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Table 1 Ultimate and proximate analyses of the corn stalk. Proximate analysis wad/%
Ultimate analysis wad/%
M
A
V
FC
C
H
O*
N
St
8.35
10.57
65.74
15.34
40.99
4.72
35.25
0.93
0.12
Note: * by difference; St: total sulfur.
Table 2 Chemical compositions of CS ash, coal ashes and synthetic ashes. Group
Sample
Compositions of ash (%) SiO2
Al2O3
CaO
MgO
Fe2O3
TiO2
SO3
K2O
Na2O
P2O5
Raw Coal ash
CS NA WA S1 S2 S3 C1 C2 C3 F1 F2 F3
54.51 50.11 28.85 20.00 40.00 60.00 38.14 38.14 38.14 44.01 41.07 35.21
5.66 38.33 20.29 60.00 40.00 20.00 26.86 26.86 26.86 30.99 28.93 24.79
9.16 0.61 15.61 10.00 10.00 10.00 5.00 15.00 25.00 15.00 15.00 15.00
5.73 0.42 3.87 5.00 5.00 5.00 25.00 15.00 5.00 5.00 5.00 5.00
2.78 6.30 11.18 5.00 5.00 5.00 5.00 5.00 5.00 5.00 10.00 20.00
0.35 0.84 0.77 / / / / / / / / /
0.34 0.36 14.46 / / / / / / / / /
18.31 0.58 0.83 / / / / / / / / /
1.63 0.16 0.39 / / / / / / / / /
2.53 0.07 0.33 / / / / / / / / /
S
C
F
S/A
C/M
F(wt%)
9.63 1.31 1.42 0.33 1.00 3.00 1.42 1.42 1.42 1.42 1.42 1.42
1.60 1.45 4.03 2.00 2.00 2.00 0.20 1.00 5.00 3.00 3.00 3.00
2.78 6.30 11.18 5.00 5.00 5.00 5.00 5.00 5.00 5.00 10.00 20.00
Note: the content of elements in the ash is represented in the form of oxides.
Fig. 1. Schematic diagram of tube furnace experiment system.
generated between the sample and its surrounding areas, which caused the gasification reaction between sample and CO2 quickly start in a short time. The syngas was immediately carried out by gas flow and collected at the outlet by gasbag with regular intervals (2 or 3 min). The CO concentration of the collected gas was measured by the gas chromatography. Residues with different carbon conversion rate of gasification were obtained according to the changes of CO concentration. After gasification, the gasification residues were collected for the Raman spectroscopy analysis to assess the char structural property.
was placed in a platinum pan and heated at the rate of 10 K/min to a final temperature (with the accuracy of 1 K) under a continuous N2 flow of 100 ml/min (with the accuracy of 2%) and maintained at this condition for 10 min to establish thermal and weight equilibrium. Then, gas flow was switched to CO2 (100 ml/min) and char gasification started. Carbon conversion (xt) and gasification rate (rt, min1) were calculated by the following [24].
2.3. Reactivity measurement
rt ¼
The gasification reactivity of the samples was investigated under isothermal conditions using a thermogravimetric analyzer (TGA, Setaram, France). In each experiment, about 10 mg samples
xt ¼
w0 wt w0 wash 1 dwt dxt ¼ $ w0 wash dt dt
(1)
(2)
where w0 is the sample weight at the start of gasification, wt is the weight at gasification time t, and wash is the weight of ash
H. Zhang et al. / Renewable Energy 147 (2020) 2056e2063
remaining after complete gasification. 2.4. Raman spectroscopy In order to understand the char reactivity from the aspect of the evolution of the carbon structure during the gasification procedure, Raman spectroscopy was adopted to characterize the tested samples due to its advantages in the high sensitivity, high resolution and nondestructive testing [29]. Therefore, a laser Raman spectrometer (HORIBA HR800, Japan) was employed for carbon structure analysis in this study. The wavelength and power of the laser beam were set as 532 nm and 12 mW, respectively. The Raman spectra with the wavenumber in range of 800e2000 cm 1 was analyzed to cover the first-order bands [20]. Different regions of each sample were randomly selected, the average values were used as the final results to evaluate the experimental uncertainty (the relative standard deviation was within 2% for all cases investigated).
three samples was around 50 min at 1123 K, whereas the time was gradually shortened to 27, 20 and 12 min for 1173 K, 1223 K and 1273 K respectively. Moreover, for the three samples, it is obvious that the time of the highest gasification rate was gradually shortened and the rt had a significant improvement with increasing the reaction temperature. In order to investigate the role of two coal ashes played in the 0:9 proposed by Gil gasification procedure, reactivity index R0.9 ðtx¼0:9 Þ et al. [30] was adopted to quantitatively evaluate the whole gasification reactivity of samples in this study [20]. The larger the value of R0.9, the higher the reactivity of char gasification reaction. The reactivity indexes of char samples were exhibited in Fig. 3. It can be found that the R0.9 of three char samples increased with the
3. Results and discussion 3.1. Influence of coal ash additive on gasification reactivity of CS The carbon conversion (xt) and gasification rate (rt) of the sample CS, CS-10NA and CS-10WA were conducted by TGA to investigate the gasification reactivity. Fig. 2 shows the gasification conversion variations of char samples versus time at different gasification temperatures (all cases standard deviations of xt and rt were within 1% and 0.1% respectively). It was found that the carbon conversion was sensitive to the reaction temperature, where high temperatures enhanced the char conversion rate for all samples. The time required to achieve the complete carbon conversion for
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Fig. 3. Gasification reactivity index of samples.
Fig. 2. Carbon conversion and gasification rate of different samples at temperatures of (a) 1123 K, (b) 1173 K, (c) 1223 K, and (d) 1273 K.
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gasification temperature increasing from 1123 K to 1273 K. It is obvious that the two additives had a different effect on the gasification reactivity of CS. Blending WA pronouncedly enhanced the reactivity while the NA had an inhibiting effect on gasification of CS during the reaction temperature from 1123 K to 1173 K. These observations can be explained by two possibilities. One explanation was that due to the complexity of minerals compounds (such as Si, Al, Ca, Mg and Fe-containing), the coal ash additive might improve or inhibit the gasification efficiency during the procedure [18]. As shown in Table 2, although the chemical composition of NA was similar to WA, the content of Ca and Fe in WA was higher than NA, which might enhance the gasification reactivity, especially at low gasification temperature. Therefore, CS-10WA might have higher reactivity than CS-10NA and CS. The influence of mental oxides on gasification behaviors will be detailed discussion in section 3.2. The second explanation was that the NA showed a better capacity to capture potassium in the solid/slag phase than WA in our previous study [12], and the potassium fixed in solid/slag phase was inactive during gasification. Since more KCl (potassium exists mainly in the form of KCl in CS, shown in Fig. S1.) retention in solid/slag phase, which significantly enhanced the gasification reactivity [31]. However, the results from Fig. 3 indicate that the R0.9 of CS increased dramatically as the temperature increasing from 1173 K to 1273 K and even turned to exceed the sample blending WA after 1223 K. This demonstrates that the gasification rate of CS was higher than the char samples with coal ash with the temperature rise. The reason for this finding was that temperature gradually became the dominant factors in determining the gasification reactivity with the increase of temperature, and the reduction of reaction area between CS and CO2 due to a considerable amount of reaction area being occupied by coal ash (restraining effect) resulted in the reactivity decline. Although the reactivity index R0.9 of CS-10NA and CS-10WA is lower than CS due to the restraining effect, the catalysis to gasification of WA is still effectively better than NA. According to our previous study [12], both additives have dramatically enhanced the CS ash melting point from 1273 K to 1473 K when 10 wt % NA or WA is blended, this means that the CS could gasified below 1473 K after coal ash adding. From Fig. 3, it can infer that WA might have the priority to NA when the reaction temperature below is 1223 K due to a higher reactivity of CS-10WA than CS-10NA and the slight difference between CS and CS-10WA in 1223 K. However, there is no slag in the ash of CS-10NA while 13.5 wt % for CS-10WA when the temperature rose to 1273 K [12], which indicates a better ability to inhibit CS ash melting by using NA than WA. On the other hand, our previous work [12] also suggested that NA was a more effective additive than WA to capture potassium to solid phase and therefore avoid the gaseous phase potassium deposit and direct impact on surface corrosion on gasifier. Moreover, it should be noted that the energy barrier will be not effectively reduced by catalytic effects at high temperature (over 1273 K) [32], this means the influence of NA or WA on CS gasification reactivity could be neglected when the temperature is higher than 1273 K. Therefore, NA might be the best choice for CS gasification under higher reaction temperature (over 1273 K).
relative standard deviation was within 1%) of the mixed samples at 1173 K and 1273 K. It can be seen in Table 3 that the reactivity of whole sample at 1273 K was higher than 1173 K, and almost twice than 1173 K. This means that the temperature was the key factor to effect the gasification rate, which was similar to the results of CS with coal ash. The specific reactivity index (i.e., the ratio of R0.9 with to that without additive at the same gasification conditions) was used to quantitatively characterize the influence of synthetic ash additive on CS gasification reactivity [20]. When the value of specific reactivity index is > 1, it means the additive can promote the whole gasification procedure, and vice versa. In addition, the greater specific reactivity index value, the more remarkable positive effect of additive on CS reactivity.
Specific reactivity index ¼
R0:9; CS with additive Ro:9;CS
(3)
Fig. 4 shows the specific reactivity index value of the CS blending with 10 wt % synthetic ash additive at 1173 K and 1273 K. It is obvious that the specific reactivity index value of each sample gasified in 1273 K was much lower than that at 1173 K. The low specific reactivity index value at 1273 K might be attributed to the influence of additive restraining effect gradually became dominant factor to inhibit the gasification rate as the temperature increased rather than the catalysis, which agrees well with the results where the R0.9 of CS was much higher than the coal ash blending samples (as shown in Fig. 3). Fig. 4 also shows the specific reactivity index value of nine samples at 1173 K was greater than 1, which indicates that all the synthetic ashes can promote the gasification reactivity of CS char at the relatively low temperature of 1173 K. In addition, the specific reactivity index value increased with increasing the SiO2/Al2O3 ratio (S/A) and as well as the Fe content. Although the turn point appeared when the CaO/MgO ratio (C/M) achieved 1 and then began to fall, blending CaO and MgO still showed a positive effect on reactivity at 1173 K. It was inferred that with the increase of S/A, C/M and Fe wt % (F), the reactivity trend of CS blending synthetic ash with same ratio gasified at 1173 K was F > C/M > S/A, which was the main reason that the gasification reactivity of CS was higher than CS-10NA and lower than CS-10WA when the reaction temperature was lower than 1173 K. In contrast, there was significant discrepancy of the specific reactivity index of all samples between the reaction temperature 1173 K and 1273 K. As shown in Fig. 4, the increased ratio of C/M and F had a promotion effect on reactivity and the S/A had an obvious inhibiting effect at 1273 K. Unlike the fact that the catalytic effect increases with the increase of C/M, the catalytic effect of Fe turned to decrease when the F achieved 10 wt %. Compared to the chemical composition of NA and WA with the nine synthetic ash, it is remarkable that the samples of C2 and the F2 had a similar composition to NA and WA respectively. As shown in Fig. 4, the specific reactivity index value of CS-10F2 was higher than that of CS-10C2 either in 1173 K or 1273 K, which corresponds well to the fact that the WA has a better gasification reactivity than NA in the temperature from 1123 K to 1273 K. 3.3. Influence of coal ash additive on structural characteristics of CS
3.2. Influence of synthetic ash additive on gasification reactivity of CS In order to investigate the influence of coal ash mineral composition on CS gasification reactivity, the synthetic ash with different chemical composition was used for studying the promoting or inhibiting effect. Blended with nine synthetic ashes with 10 wt % respectively, the mixed samples were tested by TGA with the same condition of section 3.1. Table 3 presents the R0.9 (the
The changes in the char structure can be correlated with char reactivity during the gasification procedure [33,34], a Raman spectroscopy was employed to characterize the microstructures of residual carbons in CS, CS-10NA and CS-10WA which were acquired by the horizontal fixed bed furnace gasification system under the reaction temperature 1173 K or 1273 K with a certain time (ensure the carbon conversion rate of each sample was 0.5). According to the study of Huang et al. [35,36] and Sheng et al. [29,37], the Raman
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Table 3 Gasification reactivity index of the samples. Temperature
1173 K 1273 K
R0.9(min1) CS-10S1
CS-10S2
CS-10S3
CS-10C1
CS-10C2
CS-10C3
CS-10F1
CS-10F2
CS-10F3
0.0658 0.1452
0.0681 0.1513
0.0712 0.1473
0.0697 0.1585
0.0757 0.1657
0.0747 0.1705
0.0701 0.1607
0.0796 0.1721
0.0872 0.1627
Fig. 4. Specific reactivity index of CS with synthetic ash additive.
spectra of carbonaceous materials were commonly consisted of D band (~1350 cm1) and G band (~1600 cm1), which can indicate the amorphous and crystalline structures of carbon, respectively. Each Raman spectrum of the semi-char (carbon conversion, 0.5) was subjected to peak fitting using a curve fitting software, Origin 8.6/Peak Fitting Module, to resolve curve into D band and G band and all curves fitted by the Lorentzian functions (the correlation coefficient R2 of each fitting curve was higher than 0.98). The CS gasified semi-char was used as an example and its original Raman spectrum and the curve-fitted one were shown in Fig. 5. Generally, the changes in the integrated band intensity can provide the quantitative information about structure features of the carbonaceous materials. Therefore, the band area ratios were used as quantitative parameters to (i) represent the evolutions of bands and (ii) reflect the variations of the order degree and amorphous carbon structure with the gasification temperature. In this study, the ratios of band area of the D band to the G band (AD/AG) and the G
band to the integrated area (AG/AAll) were employed to assess the evolution of the carbon structure of the CS and the coal ash blending samples. Table 4 displays the results of Raman band area ratios of gasified semi-chars of CS with or without coal ash additive. As presented in Table 4, the AD/AG value of the three semi-chars at 1173 K were ordered as follows: CS-10NA-1173 < CS-1173 < CS10WA-1173, and the AG/AAll showed an opposite trend. It is indicating that blending WA could reduce the order degree of carbon structure and this can promote the amount of amorphous carbon structure in CS while the NA displayed an opposite effect during the gasification procedure. When the temperature rosed to 1273 K, the value of AD/AG of the tested samples was ranked as follows: CS10NA-1273 < CS-10WA-1273 < CS-1273. It is well known that the amorphous carbon can have a higher reactivity during gasification compared to the graphitic carbon [38], which indicates that the coal ash WA could stimulate the CS gasification reactivity and the NA had an inhibiting effect on gasification rate under the 1173 K while both of the two additive can inhibit the gasification when the temperature rises to 1273 K. The results were in accordance with the R0.9 tested by TGA. In addition, Table 4 illustrates the share of ordered carbon and the graphitization process of CS carbon structure tended to decrease with the increase of temperature evidenced by the higher AD/AG value and lower AG/AAll. For example, AD/AG value of the three samples at temperature of 1173 K was 3.089, 3.036, 3.153 but that at 1273 K was 3.029, 2.848 and 2.907 respectively. Moreover, at the temperature of 1173 K and 1273 K, the difference value of AD/AG between the CS and CS-10NA was 0.053 and 0.181, while this difference value between the CS and CS-10WA was 0.064 and 0.122 respectively, which indicates that the NA showed a more significant inhibition effect on the degree of graphitization of CS than WA with increasing temperature. 3.4. Gasification kinetics of char samples A modified random pore model (MRPM) proposed by Zhang et al. [39] was adopted to describe the CS and mixtures gasification behaviors. The MRPM was indicated as below:
rt ¼
dxt ¼ KRPM ð1 xt Þ dt
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 j lnð1 xt Þð1 þ cxt Þp
(4)
where the c and p present the dimensionless constant and
Table 4 Raman band area ratios of gasified semi-chars of CS with or without coal ash.
Fig. 5. Curve-fitted Raman spectrum of CS semi-char (carbon conversion, 0.5) gasified at 1173 K.
Sample
Band area ratio AD/AG
AG/AAll
CS-1173 CS-10NA-1173 CS-10WA-1173 CS-1273 CS-10NA-1273 CS-10WA-1273
3.089 (±0.019) 3.036(±0.009) 3.153(±0.013) 3.029(±0.01) 2.848(±0.017) 2.907(±0.012)
0.245(±0.002) 0.248(±0.0019) 0.241(±0.0016) 0.246(±0.0011) 0.260(±0.007) 0.250(±0.009)
Note: For example, CS-1173 represents gasified semi-char (conversion, 0.5) of CS gasified at 1173 K.
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Fig. 6. (a) Application of MRPM to fit the gasification rate of samples; (b) Arrhenius plot of different samples.
dimensionless power law constant, respectively. By fitting the experimental results with the MRPM, it can be observed that all samples can be well fitted. The samples gasified at 1123 K was taken as an example and the curve-fitted was shown in Fig. 6a, the fitting parameter results of 1123 K, 1173 K, 1223 K and 1273 K applied MRPM and the correlation coefficients (R2) were summarized in Table 5. As shown in Table 5, the R2 of each sample was higher than 0.97, indicating that the MRPM can well fit the experimental data. In addition, it is obvious that the reaction rate constant (K) increased with the increase in the gasification temperature for all cases and the trend of K were consistent with the order of R0.9 tested by TGA for each samples at the same reaction temperature. Assuming the concentration of CO2 was kept constant in the gasification process, the Arrhenius law could be used to calculate the activation energy in Eq. (5):
and the correlation coefficient R2 of CS, CS-10NA and CS-10WA can reach 0.994, 0.995 and 0.993, respectively. Table 5 indicates that the Ea value of CS was 79.6909 kJ mol1 while the value declined to 53.0114 kJ mol1 and 40.3172 kJ mol1 for CS-10NA and CS-10WA. Compared this results with the results in section 3.1 and 3.3, it can be found that the effect of blending coal ash on the CS activation energy was inconsistent with its effect on the reactivity and the carbon structure evolution of CS. This difference might be attributed to the neglect of the hypothesis for Eq. (5) used, where it is assumed that the reaction can be a single chemical under chemical reaction control [24]. Still, from the results in Table 5, it can be inferred that the WA had a higher ability to reduce the activation energy of CS than NA under the same gasification conditions evidenced by the higher Ea value of CS-10WA than CS-10NA.
Ea K ¼ Aexp RT
4. Conclusions
(5)
where A, R and Ea are pre-exponential factor (min1), the universal gas constant (8.314 J mol1 K1), and the activation energy (kJ$mol1), respectively. Hence, the activation energy and preexponential factor of different samples could be calculated from the slope and intercept of the lnK vs 1/T (as shown in Fig. 6b) and the results are tabulated in Table 5. It was clearly observed in Fig. 6 b that, for the same sample under the different gasification temperatures, the plots for lnK vs 1/T followed a linear variation trend
The effects of two coal ashes, which can be potential to control the slagging behavior of biomass gasification, on the gasification reactivity of the corn stalk char have been investigated. The Ningxia coal ash (NA, slagging-control preferred ash) and Wuming coal ash (WA, less slagging-control preferred ash) had different effects on corn stalk char (CS) gasification reactivity at different gasification temperatures. NA had an inhibiting effect among all the gasification temperatures while WA pronouncedly promoted the reactivity at 1123 K and 1173 K, and then shown an inhibition in the range between 1223 K and 1273 K.
Table 5 Kinetic parameters and regression coefficients of the samples. Sample
Kinetic Parameter T(K)
K(min1)
j
c
p
R2
Ea(kJ$mol1)
A(min1)
CS
1123 1173 1223 1273
0.0161 0.0237 0.0339 0.0435
23.5258 35.2052 61.5863 3.7859
0.9683 1.1396 1.1249 18.0819
2.5975 3.7141 3.5289 0.6855
0.9965 0.9978 0.9966 0.9827
79.6909
83.1054
CS-10NA
1123 1173 1223 1273
0.0155 0.0193 0.0240 0.0304
2.7180 3.1894 3.1651 3.5438
3.4605 10.9238 20.0560 32.9671
0.7319 0.5371 0.5171 0.5469
0.9810 0.9776 0.9860 0.9863
53.0114
4.4783
CS-10WA
1123 1173 1223 1273
0.0218 0.0255 0.0303 0.0362
19.0544 3.0897 3.4456 9.7692
1.1054 3.5958 27.0817 19.3442
4.7599 0.7722 0.4887 0.4031
0.9979 0.9722 0.9847 0.9836
40.3172
1.6135
H. Zhang et al. / Renewable Energy 147 (2020) 2056e2063
The minerals compounds (such as Si, Al, Ca, Mg and Fecontaining) in the coal ash might be the dominant factor and showed different effects on gasification reactivity during 1123 K and 1173 K, while the temperature and the coal ash restraining effect might become the dominant factors when the temperature is higher than 1173 K. WA and NA exhibited different influence on the evolution of the CS structure during gasification. WA could promote the amount of amorphous carbon structure in CS while the NA displayed opposite effect at 1123 K and 1173 K. In addition, both coal ash reduced the order degree of carbon structure when the temperature was higher than 1173 K, which could well explain the function mechanism of the WA and NA on CS gasification reactivity. Using the modified random pore model, the corresponding reaction kinetic parameters were obtained. The results indicate that WA had a higher ability to reduce the activation energy of CS than NA under the same gasification conditions. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No. 21506242) and DNL Cooperation Fund, CAS (DNL180205). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.renene.2019.10.009. References [1] M. Shahbaz, S. Yusup, A. Inayat, D.O. Patrick, M. Ammar, The influence of catalysts in biomass steam gasification and catalytic potential of coal bottom ash in biomass steam gasification: a review, Renew. Sustain. Energy Rev. 73 (2017) 468e476. [2] X. Yang, J. Szuh anszki, Y. Tian, D. Ingham, L. Ma, M. Pourkashanian, Understanding the effects of oxyfuel combustion and furnace scale on biomass ash deposition, Fuel 247 (2019) 36e46. [3] X. Ma, X. Zhao, J. Gu, J. Shi, Co-gasification of coal and biomass blends using dolomite and olivine as catalysts, Renew. Energy 132 (2019) 509e514. [4] J.J. Dai, J. Saayman, J.R. Grace, N. Ellis, Gasification of woody biomass, Annu Rev Chem Biomol Eng 6 (2015) 77e99. [5] D.C. Baruah, D. Baruah, Modeling of biomass gasification: a review, Renew. Sustain. Energy Rev. 39 (2014) 806e815. [6] X. Ku, J. Wang, H. Jin, J. Lin, Effects of operating conditions and reactor structure on biomass entrained-flow gasification, Renew. Energy 139 (2019) 781e795. [7] A. Fuller, Y. Omidiji, T. Viefhaus, J. Maier, G. Scheffknecht, The impact of an additive on fly ash formation/transformation from wood dust combustion in a lab-scale pulverized fuel reactor, Renew. Energy 136 (2019) 732e745. [8] Y. Niu, H. Tan, S. Hui, Ash-related issues during biomass combustion: alkaliinduced slagging, silicate melt-induced slagging (ash fusion), agglomeration, corrosion, ash utilization, and related countermeasures, Prog. Energy Combust. Sci. 52 (2016) 1e61. [9] J. Qi, H. Li, K. Han, Q. Zuo, J. Gao, Q. Wang, C. Lu, Influence of ammonium dihydrogen phosphate on potassium retention and ash melting characteristics during combustion of biomass, Energy 102 (2016) 244e251. [10] Q. Wang, J. Wang, K. Han, J. Gao, C. Lu, Influence of phosphorous based additives on ash melting characteristics during combustion of biomass briquette fuel, Renew. Energy 113 (2017) 428e437. [11] A. Magdziarz, M. Gajek, D. Nowak-Wo zny, M. Wilk, Mineral phase transformation of biomass ashes e experimental and thermochemical calculations, Renew. Energy 128 (2018) 446e459. [12] H. Zhang, Y. Zhang, J. Li, X. Yang, S. Guo, H. Zhan, Y. Fang, Influence of coal ash on potassium retention and ash melting characteristics during gasification of corn stalk coke, Bioresour. Technol. 270 (2018) 416e421. [13] Z. Zhang, S. Pang, T. Levi, Influence of AAEM species in coal and biomass on steam co-gasification of chars of blended coal and biomass, Renew. Energy
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Influence of coal ash on CO2 gasification reactivity of corn stalk char Heng Zhang a, b, Junguo Li c, *, Xin Yang d, Shuangshuang Song a, Zhiqing Wang a, Jiejie Huang a, Yongqi Zhang a, Yitian Fang a, ** a
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, Shanxi, 030001, China University of Chinese Academy of Sciences, Beijing, 100049, China c Academy for Advanced Interdisciplinary Studies, Department of Chemistry, Southern University of Science and Technology, Shenzhen, 518055, China d Department of Engineering Science, University of Oxford, Oxford, OX2 0ES, UK b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 July 2019 Received in revised form 16 September 2019 Accepted 4 October 2019 Available online 5 October 2019
The effect of added coal ash on the gasification behavior of corn stalk char by using the thermalgravimetric analysis and a lab-scale horizontal fixed bed furnace was investigated. For comparison, synthetic ash with different chemical composition was used for studying the effect of difference ash composition on promoting/inhibiting the gasification reactivity. The results show that the slaggingcontrol preferred coal ash additive has a negative impact on the gasification performance. Interestingly, a relatively less slagging-control preferred coal ash additive has a beneficial impact on promoting the gasification performance under low gasification temperature. This could be due to the difference composition between the two coal ash additives, where the former coal ash type has much lower content in the calcium and iron. In addition, the findings in the synthetic ash suggest that iron can have a better performance in promoting gasification than calcium/magnesium. Two types of coal ash show an inhibiting effect under higher temperature due to temperature might be the dominant factor and the restraining effect of additive. These observations are also supported by the evolution of char structure by using the Raman spectroscopy. Further, the modified random pore model can correlate well with the gasification kinetics data. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Biomass Gasification Coal ash additive Metal catalysts Kinetic parameters
1. Introduction Biomass, which is regarded as a kind of low-carbon energy, is attractive in decreasing CO2 emission and reducing the dependency on fossil fuels [1e3]. And biomass gasification is regarded as a feasible and promising technology for the production of syngas and power production [4e6]. However, the efficient and steady operation of biomass gasification is challenging due to the ash related issues [7,8]. This is because biomass generally has a high content of alkali, alkali earth and chlorine species in the inorganic components, especially potassium (K) [9e11]. We have reported the results about using coal ash with high ash fusion temperature to reduce the slagging issues of a biomass from a previous study [12]. It indicates that the coal ash is beneficial not only to fix the potassium species in the biomass, but also to increase the ash fusion temperatures of the biomass. However, as the slagging potential is
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Li), [email protected] (Y. Fang). https://doi.org/10.1016/j.renene.2019.10.009 0960-1481/© 2019 Elsevier Ltd. All rights reserved.
controlled, the gasification rate may be affected owning to the potassium state transformation with the reaction to the coal ash, which needs further to be studied for understanding the effects of the coal ash additives on the gasification rate of the biomass. Therefore, as a continuation of our previous research [12], the effect of additive the coal ash with the high fusion temperatures on the gasification behavior of a biomass with a low fusion temperature should be investigated. Generally, biomass contains relatively higher content of alkaline/alkaline-earth metals (AAEMs) than coal [13,14]. Many studies have carried out the research on the effect of the inorganic components in biomass on co-gasification behavior of biomass and coal. Collot et al. [15] studied the co-gasification behavior of coal and a woody biomass in the bench-scale fixed-bed/fluidized bed reactors, indicating that the woody biomass has a catalytic effect on the co-gasification process. Wei et al. [16] investigated the reactivity characteristics and synergy behaviors of co-gasification between rice straw and Shenfu bituminous coal. They found that the whole gasification reactivity of the mixed fuels was enhanced compared to the bituminous coal while the inhibiting effect was
H. Zhang et al. / Renewable Energy 147 (2020) 2056e2063
observed at the initial stage of co-gasification. These observations had been attributed to the synergy and the K/Ca transformation during the co-gasification procedure. Also, some other literatures reported that the AAEMs in biomass ash can play a significant role in the enhancement of the coal gasification reactivity [16e19]. Although many studies have focused on the effect of biomass ash on coal gasification behavior [20,21], there are much less studies on the influence of coal ash on biomass gasification reactivity, particularly the relationship between the chemical composition of coal ash with the biomass carbon structure evolution and char reactivity. This is important to understand how to match a preferred coal with a specified biomass during co-gasification process. For a desired blending of biomass with coal ash for the purpose of controlling ash related issues (slagging/fouling, corrosion, etc.), the gasification performance should not be significantly reduced by the coal ash additives. The dominant mineral phases in coal ash include quartz, mullite, illite and siderite [22] and the major ash compositions in the form of oxide are SiO2, Al2O3, Fe2O3 and CaO, which can usually account for more than 80 wt % [23]. Lahijani et al. [18] investigated the influence of alkali metal (Na, K), alkaline earth metal (Ca, Mg) and transition metal (Fe) nitrates on CO2 gasification reactivity of the pistachio nut shell char by thermogravimetric analyzer (TGA) and pointed out that the catalytic performance order for the different metal catalysts was Nachar > Ca-char > Fe-char > K-char > Mg-char > raw char. This order was slightly different from the results by Huang et al. [24], in which the order was K > Na > Ca > Fe > Mg. In addition, Habibi et al. [25] observed that Al and Si have an inhibiting effect on co-gasification due to the reaction of K with Al and Si to form the potassium aluminosilicates, which have no catalytic properties [26]. There is a few research is a lack of knowledge on the effects of mineral matter contained in fuel on thermal conversion process between coal and biomass [27,28]. However, the literature about utilizing the coal ash as an additive to explore the effect of its components on biomass gasification is scarce. Therefore, as a continuation of the previous research [12], this study aims to provide a better understanding of the coal ash additives (for reducing the ash related issues of biomass) on the gasification performance of the biomass with a low ash fusion temperature. The biomass fuel and coal additives used in this study are the same as those in the previous research. The biomass gasification was carried out in CO2 atmosphere at different reaction temperatures was performed by using the TGA, and the gasification kinetics and the evolution of the char structure were studied. For comparison, the synthetic ash by mixing pure oxides (SiO2, Al2O3, CaO, MgO and Fe2O3) with the different mass fraction was also used to investigate the effect of metal catalysts or in-catalysts effect on the corn stalk char CO2 gasification reactivity. 2. Materials and methods 2.1. Materials and preparation The biomass used was corn stalk (a type of the agricultural residue with a low ash fusion temperature, soften temperature1373 K), collected from Jilin Province in China. The corn stalk was roughly crushed into chips by a pulverizer first and then ground with a ball mill. Sieved by a sieve shaker, the corn stalk particle size fraction of 48 mme75 mm was used for the experiment. The properties of corn stalks such as proximate and ultimate were measured according to Chinese standards (including GB/T28731-2012 for proximate analysis and GB/T28732-2012; GB/T30727-2014; GB/ T30728-2014 for ultimate analysis) and were presented in Table 1(the repeatability limit within 0.15%). The ash compositions as shown in Table 2 were performed by the semi micro chemical
2057
analysis method and atomic absorption method according to Chinese standard GB/T1574-2007, and the repeatability limit was <1%. Two different bituminous coals (Ningxia coal and Wuming coal) with high and medium ash fusion temperatures (soften temperature was >1773 K and 1569 K respectively), collected from the Ningxia province and Shanxi province respectively, were ground by a lab jaw crusher and then sieved by a sieve shaker. The coal particle size of 80 mesh was chosen for experimental samples. All the samples (corn stalk and coal) were dried at 378 K for 24 h, and then stored in a desiccator. The preparation of corn stalk char (CS) was carried out in a horizontal tubular furnace and the pyrolysis was performed in N2 (purity > 99.99%, flow rate of 1000 ml/min) at 773 K for 30 min. Ningxia coal and Wuming coal were used to make ashes as additives and the details of the ashing procedure were detailed in the previous publication [12]. The Ningxia coal ash and Wuming coal ash were termed as NA and WA in this study, and the NA was the slagging-control preferred ash while WA was the less slaggingcontrol preferred case according our pervious study. The ash compositions shown in Table 2 were performed according to Chinese standard GB/T1574-2007. The fine char and ashes were stored in a desiccator. The synthetic ashes were prepared from SiO2, Al2O3, CaO, MgO and Fe2O3. The chemical compositions of the synthetic ashes were listed in Table 2 and termed into Groups S, Groups C and Groups F. In Group S, the content of CaO, MgO and Fe2O3 is kept constant as well as the sum of SiO2 and Al2O3. However, the SiO2/ Al2O3 ratio (S/A) is changed in the range of 0.33e3, which covers the S/A range of the NA and WA. In Group C, the content of SiO2, Al2O3 and Fe2O3 is kept constant as well as the sum of CaO and MgO. However, the CaO/MgO ratio (C/M) is changed in the range of 0.2e5. For Group F, the content of Fe2O3 (F) changes from 5 wt % to 20 wt % and the others remain unchanged. All samples were mixed by the ball milling for 2 h and then calcined separately at 1088 K for 2 h. The coal ash and the synthetic ash were mixed with CS by the mechanical mixing method. The blending ratio of the coal ash or synthetic ash is 10 wt %, which is consistent with the adding ratio in our pervious study to reducing the ash deposition behavior of biomass. The mixtures were respectively denoted as CS-10NA, CS10WA and CS-10S1, CS-10S2, CS-10S3, CS-10C1, CS-10C2, CS-10C3, CS-10F1, CS-10F2, CS-10F3 respectively. 2.2. Gasification process Gasification experiments were implemented in a horizontal fixed bed furnace system (as shown in Fig. 1). The experimental setup was made up of a gas feeding unit, a fixed-bed unit and a gas treatment unit. For the gas feeding unit, the continuous and stable CO2 (purity > 99.99%) was controlled by pressure reducing valve and mass flowmeter with the accuracy of 2%. The fixed-bed system was composed of an electrical furnace and a quartz tube (inner diameter of 32 mm and length of 800 mm) with sealing flanges. The total heating length and flat-temperature zone of the furnace were 630 and 100 mm respectively. Measured by K-type thermocouple with the accuracy of 1 K, the temperature of furnace was transmitted and controlled by temperature controller. An alumina boat(60 30 15 mm) housed a certain amount of samples and placed in the quartz tube of furnace the flat-temperature zone, and then heated by the external electric furnace. Prior to gasification, the furnace was heated at a rate of 10 K/min to the preset temperature (1173 K or 1273 K) and maintained for 10 min under CO2 atmosphere with a flow rate of 300 ml/min to remove the air inside the quartz tube. Subsequently, an alumina boat loaded with ca.1.000 g samples was placed inside cold section of the quartz tube for 5 min, then instantly pushed into the flattemperature zone for gasifying. A steep temperature gradient was
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H. Zhang et al. / Renewable Energy 147 (2020) 2056e2063
Table 1 Ultimate and proximate analyses of the corn stalk. Proximate analysis wad/%
Ultimate analysis wad/%
M
A
V
FC
C
H
O*
N
St
8.35
10.57
65.74
15.34
40.99
4.72
35.25
0.93
0.12
Note: * by difference; St: total sulfur.
Table 2 Chemical compositions of CS ash, coal ashes and synthetic ashes. Group
Sample
Compositions of ash (%) SiO2
Al2O3
CaO
MgO
Fe2O3
TiO2
SO3
K2O
Na2O
P2O5
Raw Coal ash
CS NA WA S1 S2 S3 C1 C2 C3 F1 F2 F3
54.51 50.11 28.85 20.00 40.00 60.00 38.14 38.14 38.14 44.01 41.07 35.21
5.66 38.33 20.29 60.00 40.00 20.00 26.86 26.86 26.86 30.99 28.93 24.79
9.16 0.61 15.61 10.00 10.00 10.00 5.00 15.00 25.00 15.00 15.00 15.00
5.73 0.42 3.87 5.00 5.00 5.00 25.00 15.00 5.00 5.00 5.00 5.00
2.78 6.30 11.18 5.00 5.00 5.00 5.00 5.00 5.00 5.00 10.00 20.00
0.35 0.84 0.77 / / / / / / / / /
0.34 0.36 14.46 / / / / / / / / /
18.31 0.58 0.83 / / / / / / / / /
1.63 0.16 0.39 / / / / / / / / /
2.53 0.07 0.33 / / / / / / / / /
S
C
F
S/A
C/M
F(wt%)
9.63 1.31 1.42 0.33 1.00 3.00 1.42 1.42 1.42 1.42 1.42 1.42
1.60 1.45 4.03 2.00 2.00 2.00 0.20 1.00 5.00 3.00 3.00 3.00
2.78 6.30 11.18 5.00 5.00 5.00 5.00 5.00 5.00 5.00 10.00 20.00
Note: the content of elements in the ash is represented in the form of oxides.
Fig. 1. Schematic diagram of tube furnace experiment system.
generated between the sample and its surrounding areas, which caused the gasification reaction between sample and CO2 quickly start in a short time. The syngas was immediately carried out by gas flow and collected at the outlet by gasbag with regular intervals (2 or 3 min). The CO concentration of the collected gas was measured by the gas chromatography. Residues with different carbon conversion rate of gasification were obtained according to the changes of CO concentration. After gasification, the gasification residues were collected for the Raman spectroscopy analysis to assess the char structural property.
was placed in a platinum pan and heated at the rate of 10 K/min to a final temperature (with the accuracy of 1 K) under a continuous N2 flow of 100 ml/min (with the accuracy of 2%) and maintained at this condition for 10 min to establish thermal and weight equilibrium. Then, gas flow was switched to CO2 (100 ml/min) and char gasification started. Carbon conversion (xt) and gasification rate (rt, min1) were calculated by the following [24].
2.3. Reactivity measurement
rt ¼
The gasification reactivity of the samples was investigated under isothermal conditions using a thermogravimetric analyzer (TGA, Setaram, France). In each experiment, about 10 mg samples
xt ¼
w0 wt w0 wash 1 dwt dxt ¼ $ w0 wash dt dt
(1)
(2)
where w0 is the sample weight at the start of gasification, wt is the weight at gasification time t, and wash is the weight of ash
H. Zhang et al. / Renewable Energy 147 (2020) 2056e2063
remaining after complete gasification. 2.4. Raman spectroscopy In order to understand the char reactivity from the aspect of the evolution of the carbon structure during the gasification procedure, Raman spectroscopy was adopted to characterize the tested samples due to its advantages in the high sensitivity, high resolution and nondestructive testing [29]. Therefore, a laser Raman spectrometer (HORIBA HR800, Japan) was employed for carbon structure analysis in this study. The wavelength and power of the laser beam were set as 532 nm and 12 mW, respectively. The Raman spectra with the wavenumber in range of 800e2000 cm 1 was analyzed to cover the first-order bands [20]. Different regions of each sample were randomly selected, the average values were used as the final results to evaluate the experimental uncertainty (the relative standard deviation was within 2% for all cases investigated).
three samples was around 50 min at 1123 K, whereas the time was gradually shortened to 27, 20 and 12 min for 1173 K, 1223 K and 1273 K respectively. Moreover, for the three samples, it is obvious that the time of the highest gasification rate was gradually shortened and the rt had a significant improvement with increasing the reaction temperature. In order to investigate the role of two coal ashes played in the 0:9 proposed by Gil gasification procedure, reactivity index R0.9 ðtx¼0:9 Þ et al. [30] was adopted to quantitatively evaluate the whole gasification reactivity of samples in this study [20]. The larger the value of R0.9, the higher the reactivity of char gasification reaction. The reactivity indexes of char samples were exhibited in Fig. 3. It can be found that the R0.9 of three char samples increased with the
3. Results and discussion 3.1. Influence of coal ash additive on gasification reactivity of CS The carbon conversion (xt) and gasification rate (rt) of the sample CS, CS-10NA and CS-10WA were conducted by TGA to investigate the gasification reactivity. Fig. 2 shows the gasification conversion variations of char samples versus time at different gasification temperatures (all cases standard deviations of xt and rt were within 1% and 0.1% respectively). It was found that the carbon conversion was sensitive to the reaction temperature, where high temperatures enhanced the char conversion rate for all samples. The time required to achieve the complete carbon conversion for
2059
Fig. 3. Gasification reactivity index of samples.
Fig. 2. Carbon conversion and gasification rate of different samples at temperatures of (a) 1123 K, (b) 1173 K, (c) 1223 K, and (d) 1273 K.
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H. Zhang et al. / Renewable Energy 147 (2020) 2056e2063
gasification temperature increasing from 1123 K to 1273 K. It is obvious that the two additives had a different effect on the gasification reactivity of CS. Blending WA pronouncedly enhanced the reactivity while the NA had an inhibiting effect on gasification of CS during the reaction temperature from 1123 K to 1173 K. These observations can be explained by two possibilities. One explanation was that due to the complexity of minerals compounds (such as Si, Al, Ca, Mg and Fe-containing), the coal ash additive might improve or inhibit the gasification efficiency during the procedure [18]. As shown in Table 2, although the chemical composition of NA was similar to WA, the content of Ca and Fe in WA was higher than NA, which might enhance the gasification reactivity, especially at low gasification temperature. Therefore, CS-10WA might have higher reactivity than CS-10NA and CS. The influence of mental oxides on gasification behaviors will be detailed discussion in section 3.2. The second explanation was that the NA showed a better capacity to capture potassium in the solid/slag phase than WA in our previous study [12], and the potassium fixed in solid/slag phase was inactive during gasification. Since more KCl (potassium exists mainly in the form of KCl in CS, shown in Fig. S1.) retention in solid/slag phase, which significantly enhanced the gasification reactivity [31]. However, the results from Fig. 3 indicate that the R0.9 of CS increased dramatically as the temperature increasing from 1173 K to 1273 K and even turned to exceed the sample blending WA after 1223 K. This demonstrates that the gasification rate of CS was higher than the char samples with coal ash with the temperature rise. The reason for this finding was that temperature gradually became the dominant factors in determining the gasification reactivity with the increase of temperature, and the reduction of reaction area between CS and CO2 due to a considerable amount of reaction area being occupied by coal ash (restraining effect) resulted in the reactivity decline. Although the reactivity index R0.9 of CS-10NA and CS-10WA is lower than CS due to the restraining effect, the catalysis to gasification of WA is still effectively better than NA. According to our previous study [12], both additives have dramatically enhanced the CS ash melting point from 1273 K to 1473 K when 10 wt % NA or WA is blended, this means that the CS could gasified below 1473 K after coal ash adding. From Fig. 3, it can infer that WA might have the priority to NA when the reaction temperature below is 1223 K due to a higher reactivity of CS-10WA than CS-10NA and the slight difference between CS and CS-10WA in 1223 K. However, there is no slag in the ash of CS-10NA while 13.5 wt % for CS-10WA when the temperature rose to 1273 K [12], which indicates a better ability to inhibit CS ash melting by using NA than WA. On the other hand, our previous work [12] also suggested that NA was a more effective additive than WA to capture potassium to solid phase and therefore avoid the gaseous phase potassium deposit and direct impact on surface corrosion on gasifier. Moreover, it should be noted that the energy barrier will be not effectively reduced by catalytic effects at high temperature (over 1273 K) [32], this means the influence of NA or WA on CS gasification reactivity could be neglected when the temperature is higher than 1273 K. Therefore, NA might be the best choice for CS gasification under higher reaction temperature (over 1273 K).
relative standard deviation was within 1%) of the mixed samples at 1173 K and 1273 K. It can be seen in Table 3 that the reactivity of whole sample at 1273 K was higher than 1173 K, and almost twice than 1173 K. This means that the temperature was the key factor to effect the gasification rate, which was similar to the results of CS with coal ash. The specific reactivity index (i.e., the ratio of R0.9 with to that without additive at the same gasification conditions) was used to quantitatively characterize the influence of synthetic ash additive on CS gasification reactivity [20]. When the value of specific reactivity index is > 1, it means the additive can promote the whole gasification procedure, and vice versa. In addition, the greater specific reactivity index value, the more remarkable positive effect of additive on CS reactivity.
Specific reactivity index ¼
R0:9; CS with additive Ro:9;CS
(3)
Fig. 4 shows the specific reactivity index value of the CS blending with 10 wt % synthetic ash additive at 1173 K and 1273 K. It is obvious that the specific reactivity index value of each sample gasified in 1273 K was much lower than that at 1173 K. The low specific reactivity index value at 1273 K might be attributed to the influence of additive restraining effect gradually became dominant factor to inhibit the gasification rate as the temperature increased rather than the catalysis, which agrees well with the results where the R0.9 of CS was much higher than the coal ash blending samples (as shown in Fig. 3). Fig. 4 also shows the specific reactivity index value of nine samples at 1173 K was greater than 1, which indicates that all the synthetic ashes can promote the gasification reactivity of CS char at the relatively low temperature of 1173 K. In addition, the specific reactivity index value increased with increasing the SiO2/Al2O3 ratio (S/A) and as well as the Fe content. Although the turn point appeared when the CaO/MgO ratio (C/M) achieved 1 and then began to fall, blending CaO and MgO still showed a positive effect on reactivity at 1173 K. It was inferred that with the increase of S/A, C/M and Fe wt % (F), the reactivity trend of CS blending synthetic ash with same ratio gasified at 1173 K was F > C/M > S/A, which was the main reason that the gasification reactivity of CS was higher than CS-10NA and lower than CS-10WA when the reaction temperature was lower than 1173 K. In contrast, there was significant discrepancy of the specific reactivity index of all samples between the reaction temperature 1173 K and 1273 K. As shown in Fig. 4, the increased ratio of C/M and F had a promotion effect on reactivity and the S/A had an obvious inhibiting effect at 1273 K. Unlike the fact that the catalytic effect increases with the increase of C/M, the catalytic effect of Fe turned to decrease when the F achieved 10 wt %. Compared to the chemical composition of NA and WA with the nine synthetic ash, it is remarkable that the samples of C2 and the F2 had a similar composition to NA and WA respectively. As shown in Fig. 4, the specific reactivity index value of CS-10F2 was higher than that of CS-10C2 either in 1173 K or 1273 K, which corresponds well to the fact that the WA has a better gasification reactivity than NA in the temperature from 1123 K to 1273 K. 3.3. Influence of coal ash additive on structural characteristics of CS
3.2. Influence of synthetic ash additive on gasification reactivity of CS In order to investigate the influence of coal ash mineral composition on CS gasification reactivity, the synthetic ash with different chemical composition was used for studying the promoting or inhibiting effect. Blended with nine synthetic ashes with 10 wt % respectively, the mixed samples were tested by TGA with the same condition of section 3.1. Table 3 presents the R0.9 (the
The changes in the char structure can be correlated with char reactivity during the gasification procedure [33,34], a Raman spectroscopy was employed to characterize the microstructures of residual carbons in CS, CS-10NA and CS-10WA which were acquired by the horizontal fixed bed furnace gasification system under the reaction temperature 1173 K or 1273 K with a certain time (ensure the carbon conversion rate of each sample was 0.5). According to the study of Huang et al. [35,36] and Sheng et al. [29,37], the Raman
H. Zhang et al. / Renewable Energy 147 (2020) 2056e2063
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Table 3 Gasification reactivity index of the samples. Temperature
1173 K 1273 K
R0.9(min1) CS-10S1
CS-10S2
CS-10S3
CS-10C1
CS-10C2
CS-10C3
CS-10F1
CS-10F2
CS-10F3
0.0658 0.1452
0.0681 0.1513
0.0712 0.1473
0.0697 0.1585
0.0757 0.1657
0.0747 0.1705
0.0701 0.1607
0.0796 0.1721
0.0872 0.1627
Fig. 4. Specific reactivity index of CS with synthetic ash additive.
spectra of carbonaceous materials were commonly consisted of D band (~1350 cm1) and G band (~1600 cm1), which can indicate the amorphous and crystalline structures of carbon, respectively. Each Raman spectrum of the semi-char (carbon conversion, 0.5) was subjected to peak fitting using a curve fitting software, Origin 8.6/Peak Fitting Module, to resolve curve into D band and G band and all curves fitted by the Lorentzian functions (the correlation coefficient R2 of each fitting curve was higher than 0.98). The CS gasified semi-char was used as an example and its original Raman spectrum and the curve-fitted one were shown in Fig. 5. Generally, the changes in the integrated band intensity can provide the quantitative information about structure features of the carbonaceous materials. Therefore, the band area ratios were used as quantitative parameters to (i) represent the evolutions of bands and (ii) reflect the variations of the order degree and amorphous carbon structure with the gasification temperature. In this study, the ratios of band area of the D band to the G band (AD/AG) and the G
band to the integrated area (AG/AAll) were employed to assess the evolution of the carbon structure of the CS and the coal ash blending samples. Table 4 displays the results of Raman band area ratios of gasified semi-chars of CS with or without coal ash additive. As presented in Table 4, the AD/AG value of the three semi-chars at 1173 K were ordered as follows: CS-10NA-1173 < CS-1173 < CS10WA-1173, and the AG/AAll showed an opposite trend. It is indicating that blending WA could reduce the order degree of carbon structure and this can promote the amount of amorphous carbon structure in CS while the NA displayed an opposite effect during the gasification procedure. When the temperature rosed to 1273 K, the value of AD/AG of the tested samples was ranked as follows: CS10NA-1273 < CS-10WA-1273 < CS-1273. It is well known that the amorphous carbon can have a higher reactivity during gasification compared to the graphitic carbon [38], which indicates that the coal ash WA could stimulate the CS gasification reactivity and the NA had an inhibiting effect on gasification rate under the 1173 K while both of the two additive can inhibit the gasification when the temperature rises to 1273 K. The results were in accordance with the R0.9 tested by TGA. In addition, Table 4 illustrates the share of ordered carbon and the graphitization process of CS carbon structure tended to decrease with the increase of temperature evidenced by the higher AD/AG value and lower AG/AAll. For example, AD/AG value of the three samples at temperature of 1173 K was 3.089, 3.036, 3.153 but that at 1273 K was 3.029, 2.848 and 2.907 respectively. Moreover, at the temperature of 1173 K and 1273 K, the difference value of AD/AG between the CS and CS-10NA was 0.053 and 0.181, while this difference value between the CS and CS-10WA was 0.064 and 0.122 respectively, which indicates that the NA showed a more significant inhibition effect on the degree of graphitization of CS than WA with increasing temperature. 3.4. Gasification kinetics of char samples A modified random pore model (MRPM) proposed by Zhang et al. [39] was adopted to describe the CS and mixtures gasification behaviors. The MRPM was indicated as below:
rt ¼
dxt ¼ KRPM ð1 xt Þ dt
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 j lnð1 xt Þð1 þ cxt Þp
(4)
where the c and p present the dimensionless constant and
Table 4 Raman band area ratios of gasified semi-chars of CS with or without coal ash.
Fig. 5. Curve-fitted Raman spectrum of CS semi-char (carbon conversion, 0.5) gasified at 1173 K.
Sample
Band area ratio AD/AG
AG/AAll
CS-1173 CS-10NA-1173 CS-10WA-1173 CS-1273 CS-10NA-1273 CS-10WA-1273
3.089 (±0.019) 3.036(±0.009) 3.153(±0.013) 3.029(±0.01) 2.848(±0.017) 2.907(±0.012)
0.245(±0.002) 0.248(±0.0019) 0.241(±0.0016) 0.246(±0.0011) 0.260(±0.007) 0.250(±0.009)
Note: For example, CS-1173 represents gasified semi-char (conversion, 0.5) of CS gasified at 1173 K.
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Fig. 6. (a) Application of MRPM to fit the gasification rate of samples; (b) Arrhenius plot of different samples.
dimensionless power law constant, respectively. By fitting the experimental results with the MRPM, it can be observed that all samples can be well fitted. The samples gasified at 1123 K was taken as an example and the curve-fitted was shown in Fig. 6a, the fitting parameter results of 1123 K, 1173 K, 1223 K and 1273 K applied MRPM and the correlation coefficients (R2) were summarized in Table 5. As shown in Table 5, the R2 of each sample was higher than 0.97, indicating that the MRPM can well fit the experimental data. In addition, it is obvious that the reaction rate constant (K) increased with the increase in the gasification temperature for all cases and the trend of K were consistent with the order of R0.9 tested by TGA for each samples at the same reaction temperature. Assuming the concentration of CO2 was kept constant in the gasification process, the Arrhenius law could be used to calculate the activation energy in Eq. (5):
and the correlation coefficient R2 of CS, CS-10NA and CS-10WA can reach 0.994, 0.995 and 0.993, respectively. Table 5 indicates that the Ea value of CS was 79.6909 kJ mol1 while the value declined to 53.0114 kJ mol1 and 40.3172 kJ mol1 for CS-10NA and CS-10WA. Compared this results with the results in section 3.1 and 3.3, it can be found that the effect of blending coal ash on the CS activation energy was inconsistent with its effect on the reactivity and the carbon structure evolution of CS. This difference might be attributed to the neglect of the hypothesis for Eq. (5) used, where it is assumed that the reaction can be a single chemical under chemical reaction control [24]. Still, from the results in Table 5, it can be inferred that the WA had a higher ability to reduce the activation energy of CS than NA under the same gasification conditions evidenced by the higher Ea value of CS-10WA than CS-10NA.
Ea K ¼ Aexp RT
4. Conclusions
(5)
where A, R and Ea are pre-exponential factor (min1), the universal gas constant (8.314 J mol1 K1), and the activation energy (kJ$mol1), respectively. Hence, the activation energy and preexponential factor of different samples could be calculated from the slope and intercept of the lnK vs 1/T (as shown in Fig. 6b) and the results are tabulated in Table 5. It was clearly observed in Fig. 6 b that, for the same sample under the different gasification temperatures, the plots for lnK vs 1/T followed a linear variation trend
The effects of two coal ashes, which can be potential to control the slagging behavior of biomass gasification, on the gasification reactivity of the corn stalk char have been investigated. The Ningxia coal ash (NA, slagging-control preferred ash) and Wuming coal ash (WA, less slagging-control preferred ash) had different effects on corn stalk char (CS) gasification reactivity at different gasification temperatures. NA had an inhibiting effect among all the gasification temperatures while WA pronouncedly promoted the reactivity at 1123 K and 1173 K, and then shown an inhibition in the range between 1223 K and 1273 K.
Table 5 Kinetic parameters and regression coefficients of the samples. Sample
Kinetic Parameter T(K)
K(min1)
j
c
p
R2
Ea(kJ$mol1)
A(min1)
CS
1123 1173 1223 1273
0.0161 0.0237 0.0339 0.0435
23.5258 35.2052 61.5863 3.7859
0.9683 1.1396 1.1249 18.0819
2.5975 3.7141 3.5289 0.6855
0.9965 0.9978 0.9966 0.9827
79.6909
83.1054
CS-10NA
1123 1173 1223 1273
0.0155 0.0193 0.0240 0.0304
2.7180 3.1894 3.1651 3.5438
3.4605 10.9238 20.0560 32.9671
0.7319 0.5371 0.5171 0.5469
0.9810 0.9776 0.9860 0.9863
53.0114
4.4783
CS-10WA
1123 1173 1223 1273
0.0218 0.0255 0.0303 0.0362
19.0544 3.0897 3.4456 9.7692
1.1054 3.5958 27.0817 19.3442
4.7599 0.7722 0.4887 0.4031
0.9979 0.9722 0.9847 0.9836
40.3172
1.6135
H. Zhang et al. / Renewable Energy 147 (2020) 2056e2063
The minerals compounds (such as Si, Al, Ca, Mg and Fecontaining) in the coal ash might be the dominant factor and showed different effects on gasification reactivity during 1123 K and 1173 K, while the temperature and the coal ash restraining effect might become the dominant factors when the temperature is higher than 1173 K. WA and NA exhibited different influence on the evolution of the CS structure during gasification. WA could promote the amount of amorphous carbon structure in CS while the NA displayed opposite effect at 1123 K and 1173 K. In addition, both coal ash reduced the order degree of carbon structure when the temperature was higher than 1173 K, which could well explain the function mechanism of the WA and NA on CS gasification reactivity. Using the modified random pore model, the corresponding reaction kinetic parameters were obtained. The results indicate that WA had a higher ability to reduce the activation energy of CS than NA under the same gasification conditions. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No. 21506242) and DNL Cooperation Fund, CAS (DNL180205). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.renene.2019.10.009. References [1] M. Shahbaz, S. Yusup, A. Inayat, D.O. Patrick, M. Ammar, The influence of catalysts in biomass steam gasification and catalytic potential of coal bottom ash in biomass steam gasification: a review, Renew. Sustain. Energy Rev. 73 (2017) 468e476. [2] X. Yang, J. Szuh anszki, Y. Tian, D. Ingham, L. Ma, M. Pourkashanian, Understanding the effects of oxyfuel combustion and furnace scale on biomass ash deposition, Fuel 247 (2019) 36e46. [3] X. Ma, X. Zhao, J. Gu, J. Shi, Co-gasification of coal and biomass blends using dolomite and olivine as catalysts, Renew. Energy 132 (2019) 509e514. [4] J.J. Dai, J. Saayman, J.R. Grace, N. Ellis, Gasification of woody biomass, Annu Rev Chem Biomol Eng 6 (2015) 77e99. [5] D.C. Baruah, D. Baruah, Modeling of biomass gasification: a review, Renew. Sustain. Energy Rev. 39 (2014) 806e815. [6] X. Ku, J. Wang, H. Jin, J. Lin, Effects of operating conditions and reactor structure on biomass entrained-flow gasification, Renew. Energy 139 (2019) 781e795. [7] A. Fuller, Y. Omidiji, T. Viefhaus, J. Maier, G. Scheffknecht, The impact of an additive on fly ash formation/transformation from wood dust combustion in a lab-scale pulverized fuel reactor, Renew. Energy 136 (2019) 732e745. [8] Y. Niu, H. Tan, S. Hui, Ash-related issues during biomass combustion: alkaliinduced slagging, silicate melt-induced slagging (ash fusion), agglomeration, corrosion, ash utilization, and related countermeasures, Prog. Energy Combust. Sci. 52 (2016) 1e61. [9] J. Qi, H. Li, K. Han, Q. Zuo, J. Gao, Q. Wang, C. Lu, Influence of ammonium dihydrogen phosphate on potassium retention and ash melting characteristics during combustion of biomass, Energy 102 (2016) 244e251. [10] Q. Wang, J. Wang, K. Han, J. Gao, C. Lu, Influence of phosphorous based additives on ash melting characteristics during combustion of biomass briquette fuel, Renew. Energy 113 (2017) 428e437. [11] A. Magdziarz, M. Gajek, D. Nowak-Wo zny, M. Wilk, Mineral phase transformation of biomass ashes e experimental and thermochemical calculations, Renew. Energy 128 (2018) 446e459. [12] H. Zhang, Y. Zhang, J. Li, X. Yang, S. Guo, H. Zhan, Y. Fang, Influence of coal ash on potassium retention and ash melting characteristics during gasification of corn stalk coke, Bioresour. Technol. 270 (2018) 416e421. [13] Z. Zhang, S. Pang, T. Levi, Influence of AAEM species in coal and biomass on steam co-gasification of chars of blended coal and biomass, Renew. Energy
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