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Influence of Biomass Ash Additive on Reactivity Characteristic and Structure Evolution of Coal Char-CO2 Gasification Juntao Wei, Yan Gong, Lu Ding, Junqin Yu, and Guangsuo Yu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02028 • Publication Date (Web): 11 Sep 2018 Downloaded from http://pubs.acs.org on September 12, 2018
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Influence of Biomass Ash Additive on Reactivity Characteristic and Structure Evolution of Coal Char-CO2 Gasification Juntao Wei a, Yan Gong a, Lu Ding c,*, Junqin Yu a, Guangsuo Yu a,b,* a Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, East China University of Science and Technology, Shanghai 200237, PR China b State Key Laboratory of High-efficiency Coal Utilization and Green Chemical Engineering, College of Chemistry and Chemical Engineering, Ningxia University, Yinchuan 750021, PR China c Division of Energy Science, Department of Engineering Sciences and Mathematics, Luleå University of Technology, Luleå 97187, Sweden
Abstract: In this study, the influence of biomass ash (rice straw ash, RSA) additive on char gasification reactivity of different rank coals (Shenfu bituminous coal and Zunyi anthracite) was investigated by thermogravimetric analysis. Moreover, the structure characteristics (i.e., the chemical forms and concentrations of AAEM species and the order degree of carbon structure) of gasified semi-chars were quantitatively studied to evaluate the effect of RSA additive on coal char structure evolution during gasification. Specific reactivity index was proposed as a quantitative index and showed that RSA additive facilitated coal char gasification, especially at lower gasification temperature and for high-rank coal char. Additionally, the results from the active AAEM concentrations and the Raman band area ratios of gasified semi-chars indicate that RSA additive was conducive to the increase of active AAEM concentrations in coal char and the decrease of the graphitization degree of coal char carbon structure during gasification, which were more significant at lower temperature and for high-rank coal 1
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char. It could be concluded from these results that the function mechanism of RSA additive on coal char gasification reactivity had a close relationship with char structure evolution during gasification. Kinetics analysis using isoconversional method demonstrated that the gasification activation energy of Shenfu and Zunyi coal char with RSA additive were respectively lower than those of the corresponding coal chars by 8.33 kJ·mol-1 and 22.32 kJ·mol-1, indicating that RSA additive was favorable for activation energy reduction of coal char gasification, especially for high-rank coal char. This work verified the possibility of promoting coal gasification using biomass ash as a natural catalyst and revealed the function mechanism of biomass ash additive on coal char gasification. Keywords ： Biomass ash; Gasification reactivity; Coal char; Structure evolution; Kinetics
1. Introduction Coal gasification technology is an important means for the clean and high-efficiency utilization of coal resource, and the key technology for coal-based chemicals, liquid fuel, hydrogen production, reduction iron-making, IGCC power generation and other process industries as well [1]. Thus, promoting the research and development of coal gasification is of great significance for the enhancement of energy utilization efficiency, the reduction of energy shortage pressure and the improvement of environment quality. Nowadays, advanced large-scale coal gasification has achieved widespread industrial application, whereas its harsh operation condition of high temperature and pressure would result in relatively high equipment investment and operation cost. 2
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Catalytic coal gasification technology shows great advantages in enhancing coal gasification reactivity and decreasing the reaction temperature for the complete coal conversion, and has received increasing attentions from researchers and industry. There were many studies focusing on the gasification reactivity of coal char with catalyst loading. Huang et al.
[2]
pointed out that the catalytic performance order for different
metal catalysts (including alkali metals, alkali earth metals and transition metals) was K>Na>Ca>Fe>Mg. Wang et al. [3] indicated that Na2CO3 showed a better catalytic effect on high-aluminum coal char-steam gasification than K2CO3. Ding et al. [4] and Popa et al. [5]
found that there was an optimal catalyst loading amount for the best catalytic
performance. Additionally, the influence of catalyst loading on product distribution of coal gasification was explored. Domazetis et al.
[6]
demonstrated that iron catalyst
stimulated H2 production during coal char-steam gasification. Fan et al.
[7]
showed that
compared with single catalyst, K2CO3-CaO composite-catalyst was more favorable for the H2 yield enhancement in low-rank coal gasification process. Zubek et al. [8] found that the addition of metal catalysts led to the activation energy reduction of CO and H2 formation reaction during coal gasification. Some researchers also investigated the char structural changes during catalytic coal gasification
[9,10,11]
. Although catalytic coal
gasification technology has positive effects on coal gasification reactivity and product yields, its industrialization process is limited mainly due to the high costs of industrial catalyst design, manufacture and recovery [12,13]. Biomass ash, the residue of biomass combustion, is a promising alternative for industrial catalyst [13,14], mainly due to the following superiorities. (1) Large production. 3
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Biomass is an important clean energy and the annual production increment of the global biomass is 112-220 billion tonnes. About 95 percentage of global biomass resource is utilized by direct combustion and the predicted value for global biomass ash production per year is ~476 million tonnes
[15]
; (2) High content of alkaline and alkaline earth
metals (AAEMs) [16,17]. Hence, the application of biomass ash as natural catalyst to coal gasification is not only helpful to realize the effective treatment of biomass ash and decrease the environment risks from biomass ash, but also favorable to enhance coal gasification reactivity and decrease the above-mentioned costs of industrial catalyst. At present, biomass ash is mainly employed as soil conditioner, agricultural fertilizer, building material and adsorbent
[18]
, but the literatures about utilizing it for enhancing
coal gasification reactivity are almost a gap [19]. Gasification reactivity is the crucial influence factor for the overall operation efficiency of coal gasification units
[20]
. Thus, in this study a Thermogravimetric
Analyzer (TGA) was used to evaluate the influence of biomass ash additive on char gasification reactivity of different rank coals at different gasification temperatures. Char structural characteristics are the main reasons for gasification reactivity difference
[21]
,
so the influence of biomass ash additive on char structural property (i.e., the chemical forms and concentrations of AAEM species and the characteristic of carbon structure) evolution during gasification was also quantitatively investigated in this work. Furthermore, gasification kinetics analysis of char samples was conducted. This work was helpful to verify the possibility of promoting coal gasification using biomass ash as a natural catalyst and reveal the function mechanism of biomass ash additive on coal 4
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char gasification.
2. Experimental section 2.1. Samples. Two different rank coals from China, Shenfu bituminous coal (SF) and Zunyi anthracite (ZY), were chosen as raw materials. Prior to pyrolysis experiments, pulverized coal particles were ground and sieved to the particle size of 80-120µm, then dried at 105℃ for 12 hours. The proximate analysis and ultimate analysis of SF and ZY are listed in Table 1, and the ash fusion temperature and ash composition of SF and ZY are illustrated in Table 2. Rice straw (RS), a kind of typical agricultural wastes and the largest source of crop straw in China, was selected for biomass ash preparation. Biomass ash was prepared at 550℃ using a muffle furnace and the detailed operating procedures were based on the National Standards of PRC, GB/T 30725-2014. The ash fusion temperature and ash composition of RS are shown in Table 2. Table 1 Proximate and ultimate analysis of tested samples. Proximate analysis/d,%
Ultimate analysis/d,%
Samples VM
FC
Ash
C
H
N
O
S
SF
35.42
58.29
6.29
79.14
2.32
1.12
10.36
0.77
ZY
7.59
73.46
18.95
76.57
2.13
1.10
0.83
0.42
Note: Proximate analysis and ultimate analysis are determined according to the National Standards of PRC, GB/T 212-2008 and GB/T 31391-2015, respectively. VM and FC represent volatile matter
5
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and fixed carbon, respectively.
Table 2 Ash fusion temperature and ash composition of tested samples. Ash composition/%
Ash fusion temperature/℃ Samples DT
ST
HT
FT
SiO2
Al2O3
K2O
Na2O
CaO
Fe2O3
MgO
SF
1152
1167
1175
1179
33.36
12.44
0.67
1.73
27.78
9.11
1.34
ZY
1345
1370
1395
1463
55.67
30.63
1.04
1.64
0.95
4.45
0.59
RS
1198
1257
1290
1380
58.88
0.18
21.97
1.13
4.20
0.26
2.73
Note: Ash fusion temperature, coal ash composition and biomass ash composition are determined according to the National Standards of PRC, GB/T 219-2008, GB/T 1574-2007 and GB/T 30725-2014, respectively. DT, ST, HT and FT represent deformation temperature, softening temperature, hemispherical temperature and flow temperature, respectively.
Pyrolysis experiments of coal samples were conducted in a fixed bed reactor, the details of which were shown elsewhere
[12]
. The specific experimental steps were as
follows: The furnace was heated up to 800℃ by temperature programming under the continuous purge of high purity nitrogen (0.5L·min-1). Then the basket loaded with ~10g raw coal particles was dropped from the water-cooling region at the top of reactor to reaction region, and kept for 30 min to remove the most of volatile matter in coal particles. Afterwards, the basket was pulled to the water-cooling region for char sample cooling. The char yields of SF and ZY were 65.99% and 93.43%, respectively. The results indicate that the most of volatile matter in coal samples has been removed during pyrolysis. In order to avoid the particle size difference of SF char and ZY char and its effect on 6
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char gasification, coal char samples were sieved to the particle size of 80-120µm for subsequent analysis. SF char and ZY char were denoted as SF-800P and ZY-800P, respectively. RS ash (RSA) was thoroughly mixed with coal char samples by mechanical mixing method (the biomass ash proportion in blends, 10 wt. %), and the blends were denoted as SF-800P-RSA and ZY-800P-RSA. 2.2. Analysis of char-CO2 isothermal gasification reactivity. Char isothermal gasification reactivity was investigated using a TGA (Netzsch STA449F3, Germany). The target gasification temperatures were set as 850℃, 900℃, 950℃ and 1000℃ in order to ensure that char gasification was controlled by chemical reaction
[22]
. CO2 was usually used as gasification agent for laboratory-scale studies on
gasification reactivity and kinetics analysis
[14]
. Therefore, CO2 was employed as
gasification agent in this work. The specific sequences for TGA experiments were as follows: ~10mg char particles was loaded in an alumina crucible and placed inside the reactor. Under the continuous purge of high purity nitrogen (40 mL·min-1), the temperature was heated up to the target gasification temperature at heating rate of 25K/min. Then high purity nitrogen was cut off and switched to CO2 for starting isothermal char gasification. The flow rate of CO2 was set as 120 mL·min-1 to eliminate the effect of external diffusion on gasification
[22]
. Each TGA test was repeated three
times to evaluate data reliability and a good reproducibility was verified. The gasification conversion of char samples versus gasification time could be calculated using the following formula:
7
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X =
w0 − wt w0 − wa
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(1)
Where w0, wt and wa denote the initial sample mass, sample mass at the gasification time of t and sample mass when gasification completed, respectively.
2.3. Measurement of active AAEM concentration in samples. The chemical forms and concentrations of AAEM species in gasification feedstock directly determined the amount of catalytic active sites in char particles. Numerous researches have confirmed the catalytic effect of water-soluble AAEM (mainly including water-soluble inorganic salts) and ion-exchanged AAEM (mainly including AAEM combined with oxygen-containing functional groups) on coal gasification
[2,3,5]
.
Thus, the concentrations of these two chemical forms of AAEM (defined as active AAEM) was determined in this study. Active AAEM could be transferred from solid phase into liquid phase using chemical fraction analysis
[23]
. The detailed sequences were as follows: ~0.2g sample was
dissolved in NH4Ac solution with the concentration of 1mol·L-1 and the mixture was stirred at room temperature for 72 hours. Then the liquid-solid separation was conducted using a centrifugal machine and the filtrate was gained by filtration treatment. Afterwards, the filtrate was diluted with ultrapure water to 100 mL for further analysis. An inductively coupled plasma optical emission spectrometer (ICP-OES, Agilent 725, USA) was used to measure the active AAEM concentrations in tested samples, and the standard deviation was less than 3%. 2.4. Characterization of carbon structure of samples. 8
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The carbon structure of gasification materials affected the resistance of gasifying agent permeating into the internal carbon layer during gasification and decided the amount of carbon active sites in char samples. Raman spectroscopy is widely adopted to characterize carbon structure of tested samples because of its advantages of simple sample preparation, high resolution, high sensitivity and nondestructive testing. Thus, a laser micro-Raman spectrometer (Renishaw inVia Reflex, England) was used for carbon structure analysis in this study. The wavelength and power of the laser beam were set as 514 nm and 2 mW, respectively. The Raman spectra with the wavenumber range of 800-2000 cm-1 was analyzed to cover the first-order bands. During Raman analysis, multiple particles of each tested sample were randomly selected and tested, and the average values were used as the final results to ensure the data accuracy.
3. Results and discussion 3.1. Influence of biomass ash additive on gasification reactivity of coal chars. Figure 1 shows the gasification conversion variations of char samples versus time at different gasification temperatures. It can be seen in Figure 1 that when reached the same gasification conversion, the gasification time for coal char with RSA additive was less than that for the corresponding coal char. Moreover, the gasification reactivity curve difference between coal char with and without RSA additive was more obvious for ZY coal char.
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1.0
0.8
0.8
C arbon conversion X
1.0
0.6
0.4
0.2
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0.6
0.4
0.2
(a) 0.0
0
180
360
540
(b)
720
0.0
0
180
360
540
1.0
0.8
0.8
C arbon conversion X
1.0
0.6
0.4
0.2
0.6
0.4
0.2
(d)
(c) 0.0
0
70
140 210 Gasification time t/min
SF-800P
720
Gasification time t/min
Gasification time t/min
C arbon conversion X
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
C arbon con version X
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280
SF-800P-RSA
0.0
0
ZY-800P
40
80 120 Gasification time t/min
160
ZY-800P-RSA
Figure 1 Curves of carbon conversion variations versus gasification time during char sample gasification at temperatures of (a) 850℃, (b) 900℃, (c) 950℃ and (d) 1000℃.
In previous research, particle life time was employed to assess combustion characteristics [24], corresponding to both 90% and 95% conversion of raw carbonaceous material or char. Based on this viewpoint, reactivity index R0.9 proposed by Gil et al. was adopted to quantitatively evaluate the overall gasification reactivity of char samples in this study [25]. R0.9 was defined as below:
R0.9 =
0.9
(2)
t X =0.9 10
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Where tX=0.9 represent gasification time when carbon conversion reaches 0.9, min. 0.20
Reactivity index R0.9/min-1
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SF-800P SF-800P-RSA ZY-800P ZY-800P-RSA
0.15
0.10
0.05
0.00
850
900
950
Gasification temperature T/oC
1000
Figure 2 Gasification reactivity index of char samples.
The greater R0.9 value meant the higher char gasification reactivity. Figure 2 gives the R0.9 of different char samples. R0.9 values of ZY-800P and ZY-800P-RSA gasified at 850℃ were absent because the gasification time required for the conversion of 0.9 was too long for these two chars. It could be found in Figure 2 that R0.9 increased with the increment of gasification temperature. Moreover, the R0.9 order for different char samples
gasified
at
the
same
ZY-800P
