Coal Char

Article Cite This: Energy Fuels 2018, 32, 12644−12654

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Study on Cogasification of Food Waste Char with Biomass/Coal Char and Their Interaction Mechanisms Xiaopeng Zou,† Lu Ding,*,‡ and Xin Gong*,† †

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Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, Engineering Research Center of Coal Gasification, East China University of Science and Technology, Shanghai, 200237, People’s Republic of China ‡ School of Environment and Society, Tokyo Institute of Technology G5-8, 4259 Nagatsuta, Midori-Ku, Yokohama 226-8502, Japan ABSTRACT: In this paper, the effects of gasification temperature and blending ratios on cogasification reactivity of food waste char with sawdust/Shenfu coal char were studied via a thermogravimetric analyzer. Moreover, the relationship between synergy and char physical−chemical structure evolution was further studied using scanning electron microscopy equipped with energydispersive X-ray analysis (SEM−EDS), N2 absorption, and Fourier transform (FT) Raman techniques. The results revealed a significant synergetic effect between food waste and Shenfu coal. The synergetic effect always occurred at the mid-later reaction stage and was enhanced with the increase in carbon conversion. However, no synergy was found between food waste and sawdust. In the food waste−Shenfu cogasification process, ash present in food waste (rich in Ca) transferred to the surface of Shenfu char without blocking the pore structure of Shenfu char. Moreover, the FT-Raman analysis revealed that the food waste ash reacted with Shenfu char to form more active sites, resulting in increased gasification reactivity. No ash exchange was found during cogasification of food waste with sawdust.

1. INTRODUCTION Currently, the sharp increase of food waste represents a substantial challenge for environmental protection and economic growth. The Food and Agriculture Organization of the United Nations claimed that about one-third of food was wasted globally in 2009.1 In China, approximately 92.4 million tons of food waste (FW) are generated every year, and the cost of food waste generated in China was estimated to be $32 billion.2,3 How to efficiently and cleanly utilize food waste has received increasing attention. The U.S. Environmental Protection Agency defined the following hierarchy concept in order of preference for FW management: source reduction; feed hungry people; feed animals; industrial uses; composting; incineration or landfilling.4 However, after optimizing the utilization of food waste, 27.18% of food waste still needs to be incinerated or landfilled.5 It is an acknowledged fact that incineration or landfilling can generate greenhouse gases and cause adverse environmental impacts. Therefore, increasing efforts are currently being focused on clean and efficient industrial uses of food waste such as biorefinery, anaerobic digestion, and gasification. In China, food waste from homes and restaurants is a significant challenge for environmental protection and human health. Biorefinery is limited by feedstock type and is not suitable for food waste from households or restaurants.6 Anaerobic digestion is limited by high cost and strict environmental conditions for anaerobions. Gasification seems to be an alternative technology to convert food waste into syngas which can be used for electricity generation or production of chemicals. However, the applicability of FW gasification is also strongly dependent on waste characteristics such as heating values, ash, moisture, and volatile solids content.7,8 Considering that a large number of coal/biomass © 2018 American Chemical Society

gasification plants have operated for decades all over the world, especially in China, food waste could be coprocessed with biomass and coal resources in the current plants to overcome the limitation of waste characteristics (such as heating value and moisture and volatile content) and reduce costs.9 In industrial cases, food waste can be carbonized by exhaust gas from gasification plants to remove the moisture content, increase the energy density, and reduce the crushing cost. Then the carbonized food waste can be further crushed and gasified with coal/biomass. Recently, the closed carbon cycle system was proposed for better utilization of carbon-contained wastes.10 Cogasification of biomass/coal with food waste will be a promising technology within this research field. In the cogasification process, blending ratios and temperature are significant factors that impact the cogasification reactivity of mixtures. It is well-known that gasification rate increases significantly as gasification temperature increases. However, Wei et al.11 reported that a higher waste char proportion and lower gasification temperature were favorable for more a significant overall synergistic effect on waste and petroleum coke cogasification reactivity. Wang et al.12 also found that the synergetic effect decreased with the increase of temperature, which resulted from more significant evaporation and deactivation of active alkali metal happening. Oladejo et al.13 claimed the synergetic effect was enhanced as the blending ratio increased. Hu et al.14 and Howaniec et al.15 both found an optimum mixing ratio between biomass and coal, and the optimum mixing ratios varied for different feedstock types. Therefore, it is necessary to study the effect of blending ratio on cogasification of food waste char with coal/biomass char. Received: October 19, 2018 Revised: November 27, 2018 Published: December 3, 2018 12644

DOI: 10.1021/acs.energyfuels.8b03663 Energy Fuels 2018, 32, 12644−12654

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Energy & Fuels Table 1. Proximate and Ultimate Analyses of Samplesa proximate analysis (d, %)

ultimate analysis (daf, %)

sample

VM

FC

ash

C

H

N

S

O

FW SD SF

39.71 79.80 35.42

50.17 19.61 58.29

10.12 0.59 6.29

71.21 64.52 79.14

4.06 4.64 2.32

3.40 0.29 1.12

1.84 1.57 0.77

19.49 28.98 16.65

VM, volatile matter; FC, fixed carbon; d, dry basis; daf, dry and ash-free basis.

a

heated to 475 °C and maintained for 1 h for carbonization. Details about the carbonizer can be found in the previous study.25,26 2.2. Sample Preparation. Carbonized food waste (FW), sawdust (SD), and Shenfu bituminous coal (SF) were selected as raw material in this work. FW and SD were crushed and sieved to a particle size range of 180−250 μm. SF was sieved to a particle size range from 75 to 95 μm. During char preparation, all three samples were heated at 25 °C/min to 800 °C and held for 30 min in a fixed bed under N2 atmosphere. The proximate and ultimate analyses of samples are summarized in Table 1. In order to prepare ash samples, FW and SD were incinerated in a furnace at 550 °C for 2 h, and SF samples were incinerated in a furnace at 815 °C for 2 h, and the ash was recovered for further study. The ash fusion temperatures and ash compositions of samples are shown in Tables 2 and 3, respectively.

By exploring the reactivity of the mixture char at different ratios, a suitable mixing ratio for high reactivity can be deduced. In this study, the gasification experiments were performed in laboratory scale, and samples were crushed to micrometer scale. The study on mass and heat transfer of larger particle size will be further considered for industrial scale-up on the basis of these laboratory experiments. Numerous studies have been conducted to explore the interaction effect of biomass and coal resources during the cogasification process. Both synergetic and inhibitory effects were found in many cases for biomass−coal cogasification.16−23 Wei et al.16 reported a significant synergetic effect between rice straw and coal, due to the active alkali and alkaline earth metal (AAEM) minerals transformation. Rizkiana et al.20 found inhibiting effects during cogasification of coal with rice straw, whereas synergistic effects were observed during cogasification of coal with brown seaweed/ eelgrass. These findings indicate that the interaction effects are strongly dependent on fuel type and pyrolysis/gasification conditions. To our knowledge, few studies have focused on the effect of ash on both physical and chemical structures during cogasification, especially for the cogasification process of food waste with biomass/coal. The ash performance and AAEM transfer between FW and coal/biomass remain unknown. In the cogasification process, the AAEM may transfer between FW and coal/biomass, and further catalyze cogasification. On the other hand, ash present in FW may transfer to coal/ biomass, block their pore structures, and result in inhibitory effect on cogasification.24 In this study, the physical and chemical structures of semichar were studied to reveal the interaction mechanism between food waste and biomass/coal during cogasification. In this work, the gasification reactivity of food waste and biomass/coal mixtures was tested via thermogravimetric analysis. The ash performance between food waste and biomass/coal was studied via scanning electron microscopy (SEM)−energy-dispersive X-ray analysis (EDS) techniques and N2 adsorption techniques. The various evolution trends of the chemical structures for coal semichar with/without undergoing the cogasification process were further studied via Fourier transform (FT) Raman techniques during gasification. The main goal of this work is to explore the interaction mechanisms of food waste char with sawdust char and Shenfu coal char at different mixing ratios.

Table 2. Ash Fusion Temperatures of Samplesa ash fusion temperature (°C) sample

DT

ST

HT

FT

FW SD SF

>1500 960 1152

>1500 1092 1167

>1500 1152 1175

>1500 1247 1179

a

DT, deformation temperature; ST, soften temperature; HT, hemispherical temperature; FT, fluid temperature.

2.3. Gasification Experiments. The CO2 gasification experiments for samples were carried out with a NETZSCH STA4493F3 thermogravimetric analyzer (TGA). The detailed gasification procedure was described in our previous work.24 2.4. Sample Characterization Tests. The N2 gas adsorption experiments were carried out at −196 °C, which could probe the pore range from 2 to 200 nm, via an ASAP-2020 physisorption apparatus. The specific surface areas of samples were further calculated with the Brunauer−Emmett−Teller (BET) equation. X-ray diffraction (XRD) analyses were performed on powdered samples via a Rigaku Smartlab X-ray diffraction system. Analyses were conducted using a Cu Kα radiation (1.5406 Å, operating at 40 kV/ 100 mA) over a scanning angle 2θ range of 10−80°. The surface morphology and element composition of samples were examined using a Hitachi SU-1510 scanning electron microscope (SEM) equipped with energy-dispersive X-ray spectroscopy (EDS). Multiple areas of different particles were randomly selected for scanning, and average values were calculated to reduce error. The Raman spectra of samples were recorded in the wavenumber range 800−1800 cm−1 covering the first-order bands via Iuvia Reflerx laser Raman spectral analysis. The exciting laser wavelength was 455 nm. Considering the heterogeneity of the char particles, 40 particles from each sample were randomly selected for analysis to determine the mean spectrum and average value. The Raman spectra were further deconvolved into five bands,27,28 as shown in Figure 1. The five bands commonly locate around 1200, 1350, 1530, 1580, and 1620 cm−1. The D1 band (∼1350 cm−1) is attributed to the vibration mode of disordered graphitic lattices with in-plane imperfections.29 In addition, the G band (∼1580 cm−1) is attributed to the stretching vibration mode of the ideal graphitic lattice (E2g symmetry).30 The D4 band (∼1200 cm−1) and D3 band (∼1530 cm−1) are formed from amorphous carbon and poorly organized carbonaceous materials,

2. MATERIALS AND METHODS 2.1. Carbonization of Food Waste. The food waste was obtained from a restaurant in Japan. The food waste composed of 42 wt % meats and bones, 27 wt % rice, and 31 wt % vegetables. Due to the high water content (85 wt %) in raw food waste, drying and carbonization tests were performed separately. Food waste was heated to 250 °C, maintained for 1 h for drying water, and then took out for complete mixing. After mixing homogeneously, the feedstock was 12645

DOI: 10.1021/acs.energyfuels.8b03663 Energy Fuels 2018, 32, 12644−12654

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Energy & Fuels Table 3. Ash Compositions of Samples chemical composition (wt %) sample

SiO2

Al2O3

Fe2O3

CaO

P2O5

Na2O

MgO

K2O

SO3

FW SD SF

1.53 17.11 44.20

0.77 4.08 18.19

1.62 18.84 11.28

40.93 22.32 15.58

24.78 3.60 0.56

15.54 5.37 1.89

5.73 10.79 1.12

3.52 4.01 0.95

1.78 3.81 5.63

where w0 is the initial mass of char, wt is the mass at time t, and wash is the ash mass in the char. Assuming no interaction between blended chars during the cogasification process, the calculated carbon conversion of mixtures (xcal) is expressed as follows: xcal =

MA (wA,0 − wA, t ) + MB(wB,0 − wB, t ) MA (wA,0 − wA,ash) + MB(wB,0 − wB,ash)

(2)

where MA/B is the mass fraction of sample A/B in the mixture, wA/B,0 is the initial mass of sample A/B in independent gasification, wA/B,t is the mass of A/B at time t, and wA/B,ash is the ash mass in sample A/B. R0.5 was defined by Takarada et al. as the comparison parameter between gasification reactivities of different samples.33 In this study, R0.95 was proposed to compare the gasification reactivities of different samples in high conversion stages. R0.5 and R0.95 are expressed as follows:

Figure 1. First-order Raman spectra and curves fitted of SF char.

R 0.5 =

respectively. The two bands are often used to describe the degree of structure. Moreover, the D4 band potentially represents active sitse.31 The D2 band always appears with the D1 band and mainly represents the lattice vibration involving aromatic layers.32 2.5. Experimental Data Processing Methods. The carbon conversion was calculated based on the mass loss obtained by TGA. The carbon conversion x is expressed as follow: w − wt x= 0 w0 − wash (1)

R 0.95 =

0.5 t0.5 0.95 t0.95

(3)

(4)

where t0.5 is the reaction time for 50% carbon conversion, and t0.95 is the reaction time for 95% carbon conversion. r0.5 and r0.95 were proposed to evaluate interaction effects. Cogasification presented synergy when r0.95 > 1 and presented an inhibition effect when r0.95 < 1. r0.5 and r0.95 are expressed as follows:

Figure 2. Carbon conversion vs reaction time for samples at (a) 800, (b) 900, and (c) 1000 °C. 12646

DOI: 10.1021/acs.energyfuels.8b03663 Energy Fuels 2018, 32, 12644−12654

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Energy & Fuels R 0.5,exp

r0.5 =

R 0.5,cal

r0.95 =

(5)

R 0.95,exp R 0.95,cal

(6)

where R0.5,exp and R0.95,exp are the experimental reactivity indexes; R0.5,cal and R0.95,cal are the calculated reactivity indexes. Carbon crystallite structures of samples were described by the stacking heights (Lc) and the spacing between graphitic sheets (d002). Lc and d002 are calculated by the Bragg formula and Scherrer formula as follows:

λ 2 sin θ

(7)

Figure 3. XRD spectra of three char samples.

K1λ B002 cos θ

(8)

Table 4. Crystallite Parameters of Chars with Different Samples

d002 = Lc =

where λ is the wavelength of the incident X-rays, θ is the peak position, and B002 is the corresponding peak width at half-maximum intensity. K1 = 0.89.

3. RESULTS AND DISCUSSION 3.1. Gasification Reactivity and Structures of Three Samples. 3.1.1. Gasification Reactivity of Samples. First, the gasification reactivity of three samples was compared via TGA in the temperature range 800−1000 °C. Figure 2 presents the carbon conversion versus reaction time plots for three char samples. As shown in Figure 2, the gasification reactivity of FW char was lower than that of SD char and higher than that of SF char. The gasification reactivities of the three char samples all increased as the gasification temperature increased. The gasification reactivity of FW char increased more rapidly compared with SD char as the gasification temperature increased. This is attributed to that the gasification reaction of SD and FW chars shifted into the diffusion control regime at high temperature and the diffusion rate restricted the gasification rate, instead of intrinsic reaction.33 Therefore, the gasification curves of FW char were similar to that of SD char at high gasification temperature. Moreover, in Figure 2, FW char and SF char exhibited similar conversion in the early gasification stages. However, as the reaction time increased, the conversion of FW char increased more significantly compared with SF char. This will be verified by study on physical and chemical structures in section 3.1.2. 3.1.2. Initial Physical and Chemical Structures of Char Samples. Gasification reactivity of char is related to the chemical and physical structures. Through studying the char structure, the gasification reactivity of three samples will be understood more clearly and provide a reference for industry enlargement. Figure 3 presents the XRD spectra of three char samples, and Table 4 presents crystallite parameters of char samples. As shown in Table 4, the Lc value of SD char was much smaller than those of FW char and SF char; meanwhile the d002 value of SD char was much larger than those of FW char and SF char. Because the indexes Lc and d002 both reflect the extent of crystalline structure to some extent, the Lc/d002 ratio was used to evaluate the order of crystalline structure. The larger the Lc/d002 ratio is, the more order present in crystalline structure.34 The Lc/d002 values of SD, FW, and SF chars were 1.88, 2.23, and 2.39, respectively; this indicated that the SD char exhibited the most disordered crystalline structure. Meanwhile, the SF char exhibited the most ordered crystalline structure among three char samples.

char

d002

Lc

Lc/d002

SD FW SF

0.3927 0.3758 0.3681

0.7379 0.8385 0.8785

1.88 2.23 2.39

The specific surface areas (SSAs) of three char samples are shown in Table 4. The SD char had the largest SSA at 43.837 m2/g, whereas the FW char had the smallest SSA at 4.375 m2/ g. In combination with the crystallite parameters shown in Table 5, the SD char had the smallest Lc/d002 value and the Table 5. Specific Surface Areas of Samples sample SSA (m2/g)

SD 43.8

FW 4.4

SF 24.6

largest SSA, indicating the highest gasification reactivity among three char samples. Although the FW char had a smaller Lc/ d002 value than the SF char, the SSA of the SF char was larger than that of the FW char; this explained why the gasification rates of the FW char and SF char were similar at the beginning of the gasification process. 3.2. Gasification Reactivity of Independent/Blended Chars. 3.2.1. Influence of Blending Ratio and Gasification Temperature. To study the cogasification process, FW char was further blended with SD/SF at mass ratios of 3:1, 1:1, and 1:3. FW char and SD char mixtures were referred to as FW:SD-3:1, FW:SD-1:1, and FW:SD-1:3, respectively. The mixtures of FW char and SF char were referred to as FW:SF3:1, FW:SF-1:1, and FW:SF-1:3, respectively. The experimental cogasification curves were also compared with calculated cogasification curves in order to compare the interaction effects. Figure 4 presents the carbon conversion versus reaction time plots for char samples and their mixtures. As shown in Figure 4c, the experimental gasification reactivity of mixtures was higher than the calculated gasification reactivity at the same blending ratio. The gasification reactivity of mixtures was improved with the increasing ratio of FW char to SF char. This indicated that FW char and SF char exhibited a synergy effect during cogasification. Table 6 shows the ratios of char reactivity indexes at the gasification temperature of 800 °C. T The r0.5 and r0.95 of SF and FW mixtures were greater than 1. Moreover, the r0.95 of SF and FW mixtures were much higher than r0.5. This indicated that the FW char and SF char presented good synergy, especially in high conversion stages. As shown in Tables 1 and 12647

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Figure 4. Carbon conversion vs reaction time for samples. (a) FW−SD, 800 °C; (b) FW−SD, 900 °C; (c) FW−SD, 1000 °C (d) FW−SF, 800 °C; (e) FW−SF, 900 °C; (f) FW−SF, 1000 °C.

Table 6. Ratios of Char Reactivity Indexes at 800 °C temp (°C)

sample

r0.5

r0.95

sample

r0.5

r0.95

800

FW:SD-3:1 FW:SD-1:1 FW:SD-1:3 FW:SD-3:1 FW:SD-1:1 FW:SD-1:3

0.82 0.88 0.92 1.08 1.09 1.10

1.02 1.02 1.01 1.05 1.06 1.08

FW:SF-3:1 FW:SF-1:1 FW:SF-1:3 FW:SF-3:1 FW:SF-1:1 FW:SF-1:3

1.25 1.31 1.11 1.05 0.92 0.87

1.83 2.07 1.77 1.31 1.30 1.18

1000

values of SD and FW mixtures were all slightly less than 1 and the r0.95 was almost equal to 1; this indicated no synergetic effect between SD char and SF char. The inhibition that occurred at the early reaction stage might be due to the intimate contact between SD char and FW char.16 In addition, comparing the gasification curves in Figure 4a− c or Figure 4d−f, the gasification reactivity was improved as the gasification temperature increased. The experimental cogasification curve gradually approached the calculated cogasification curves with the increase of gasification temperature. Moreover, in Table 6, the r0.95 values of SF and FW mixtures at 800 °C were considerably increased compared with those at 800 °C. This indicated that the synergy effect was weaker as the gasification temperature increased. This was attribute to the inactivation of AAEM at high gasification temperature.11,12 3.2.2. Effects of FW Ash on Gasification Reactivity of Sawdust/Shenfu Char. The catalytic effect of FW ash is a key factor of the synergetic effects on cogasification between SF

3, the ash content of FW was relatively high, and the ash was rich in Ca and Na, which exhibited significant catalysis on char gasification.35 Therefore, the ash derived from FW char (FW ash) might catalyze cogasification and result in the synergetic effects. This hypothesis will be verified later. As shown in Figure 4d−f, the gasification reactivity of SD char was much higher than that of FW char at 800 °C. Moreover, no synergetic effect was found in the cogasification process of FW char and SD char. As shown in Table 6, the r0.5 12648

DOI: 10.1021/acs.energyfuels.8b03663 Energy Fuels 2018, 32, 12644−12654

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Figure 5. Carbon conversion vs reaction time for samples. (a) SF; (b) SD.

Figure 6. SEM images of SF semichar undergoing cogasification (circles, SF ash from SF char; triangles, FW ash; rectangles, coal matrix).

char and FW char. To study the catalytic effect of FW ash on SD char and SF char, 10 wt % FW ash was added to SF char

and SD char and the gasification reactivity of the mixture was tested in the temperature range 800−1000 °C via TGA. The 12649

DOI: 10.1021/acs.energyfuels.8b03663 Energy Fuels 2018, 32, 12644−12654

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Figure 7. SEM images of SD semichar undergoing cogasification (triangles, FW char) .

Table 7. EDS Analysis of SF Semichar Undergoing Independent Gasification conversion (%) 10 25 50 75 90

coal ash coal ash coal ash coal ash coal ash

matrix matrix matrix matrix matrix

C (wt %)

O (wt %)

Si (wt %)

Al (wt %)

Ca (wt %)

P (wt %)

Na (wt %)

79.58 32.71 77.84 27.06 72.1 24.98 70.06 19.83 62.85 12.42

8.28 44.75 12.54 41.01 15.06 38.77 15.19 43.47 22.14 44.52

0.16 6.9 0.53 10.95 1.19 8.48 0.64 7.56 0.21 6.59

0.18 5.92 0.49 6.58 0.4 10.29 0.96 8.79 0.14 8.06

0.4 0.27 0.28 0.54 0.21 3.18 0.96 6.23 0.54 9.57

0.29 1.22 0.3 1.37 0.34 2.8 0.45 4.08 0.49 6.53

1.06 2.51 2.23 3.8 1.48 3.53 2.14 6.5 2.83 8.83

mixtures of SF/SD char with FW ash were referred to as SF− 10% ash and SD−10% ash, respectively. The carbon conversion versus reaction time plots for char samples and mixtures are shown in Figure 5. As shown in Figure 5, the conversion of SF char with 10 wt % FW ash was considerably increased compared with SF char in the temperature range 800−1000 °C; this indicated that the FW ash had a significant catalytic effect on the gasification of SF char. Meanwhile, the gasification rate of SD char was increased due to the addition of FW ash at 800 °C. But no catalytic effect of FW ash on SD char gasification was observed at gasification temperatures of 900 and 1000 °C. The results indicated that FW ash had a significant catalytic effect on the gasification of SF char, but was ineffective for the gasification of SD char in the temperature range 800−1000 °C. This was attributed to the fact that the reactivity of SD char was much higher than that of SF char, and the reaction was limited by diffusion restriction when the gasification temperature was greater than 900 °C.

Based on the above study, carbonized food waste is a suitable gasification feedstock given its relative high gasification reactivity (higher than SF bituminous char). Moreover, given the good synergetic effect between FW char and SF char, cogasification of carbonized food waste with coal char is also a good application for food waste. 3.3. Variation in Surface Morphology and Element Composition. As discussed above, a significant synergistic effect was observed in the FW−SF cogasification process at high conversion stage, and FW ash has a significant catalytic effect on SF char. In order to further investigate the synergistic mechanism, the surface morphology and energy spectra of SF gasification semichar with/without undergoing cogasification at 800 °C were observed and analyzed by the SEM−EDS technique. Figures 6 and 7 present the surface morphologies of SF and SD semichars undergoing cogasification, respectively. In this study, char particles were roughly divided into carbon matrix and coal ash. According to the analysis of the surface element composition, ash on the surface of SF char was 12650

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Energy & Fuels Table 8. EDS Analysis of SF Semichar Undergoing Cogasification conversion (%) 10 25 50 75 90

coal ash coal ash coal ash coal ash coal ash

matrix matrix matrix matrix matrix

C (wt %)

O (wt %)

Si (wt %)

Al (wt %)

Ca (wt %)

P (wt %)

Na (wt %)

82.56 35.24 79.02 32.18 75.75 27.94 68.91 18.90 62.85 10.85

9.42 39.61 8.57 41.46 12.18 43.02 20.07 47.20 22.14 49.62

0.20 5.74 0.36 6.76 0.81 7.9 0.93 8.95 1.21 10.60

0.20 5.58 0.46 6.52 0.40 9.55 0.78 11.98 1.14 12.50

0.44 0.31 0.37 0.89 0.21 2.21 0.38 3.53 0.54 4.02

0.40 0.62 0.19 1.00 0.22 1.03 0.32 1.08 0.49 1.53

1.09 3.01 0.67 2.03 0.80 2.71 1.39 4.35 2.83 6.53

undergoing cogasification and resulted in the increase in Ca, P, and Na levels in ash attached to the surface of SF semichar in cogasification. In the cogasification process, because the FW char reacted faster than SF char, the FW ash was gradually released from FW char particles and attached to SF char particles as carbon conversion increased. Furthermore, the gasification reactivity of SF char was improved significantly by the catalytic effect of FW ash in high conversion stages. Therefore, the FW ash is a key factor of the synergistic effect in the SF−FW cogasification process. As shown in Figure 7, unlike SF semichar undergoing cogasification, neither SD ash nor FW ash particles were present on the surface of SD semichar when carbon conversion was less than 50%. When carbon conversion reached 75%, some FW ash was found on the surface of SD semichar undergoing cogasification. No SD semichar was found in the mixture when carbon conversion was greater than 90%. As shown in Figure 4d, the gasification reactivity of SD char was much higher than that of FW char, and SD char reacted completely when the conversion of the mixture was greater than 90%. In addition, minimal SD ash was found on the surface of FW semichar, because the ash content of SD char was less than 1%. The surface element compositions of SD char and SD char undergoing cogasification are presented in Tables 9 and 10. The C content decreased and Ca, P, Na, Mg, and K contents increased with the increase of carbon conversion for both SD char and SD char undergoing cogasification. This indicated that the SD ash was contained in the coal matrix instead of being exposed on the surface of char during the gasification process. The Ca, P, Na, Mg, and K contents of SD char and SD char undergoing cogasification were similar, indicating that no FW char was transferred into the coal matrix of SD char during cogasification. In the SD−FW cogasification process, SD char was reacted first, and only slight FW ash was transferred from FW char to SD char at the high conversion stage. As studied in section 3.2.2, no significant catalytic effect of FW ash on SD char gasification was observed. Therefore, the synergetic effects of SD char and FW char were limited. 3.4. Variation in Specific Surface Area. In the cogasification process of FW char and SF char, FW ash gradually was transferred from FW char to the surface of SF char as carbon conversion increased. On one hand, the FW ash may block the pore structure of SF char, reducing the specific surface area.24 On the other hand, the AAEM of FW ash on the surface of SF char may diffuse into the carbon matrix and enhance pore growth due to the Kirkendall effect.36,37 Therefore, the SSA evolution of SF−FW mixtures was further

derived from both SF char (rich in Si and Al) and FW char (rich in Ca, P, and Na). In Figures 6 and 7, the circular areas were considered the ash derived from SF char (SF ash); meanwhile the triangle areas were considered the ash derived from FW char (FW ash). The rectangular areas were considered the carbon matrix. The surface element compositions of char particles undergoing gasification were analyzed via the EDS technique. Considering the heterogeneity of semichar particles with different conversions, the surface element compositions of coal matrix and ash were recorded from 40 different particles of each sample. As shown in Figure 6, increasing ash accumulated on the surface of SF char as carbon conversion increased. The ash on SF char particles was divided into FW ash char and SF ash, depending on their element composition. The FW ash is significantly found on the surface of SF char when the carbon conversion is greater than 50%. Tables 7 and 8 present the average element composition of SF semichar with/without undergoing cogasification. Si, Al, Ca, and Na contents were extremely low in the char matrix; meanwhile the ash was rich in Si, Al, and Ca. The carbon contents of char matrix and ash both decreased with the increase in carbon conversion. Figure 8 shows the average Ca,

Figure 8. Average Ca, P, and Na contents in ash attached to SF char with/without undergoing cogasification.

P, and Na contents in ash with conversion. Ca, P, and Na contents of ash increased minimally as carbon conversion increased for SF semichar undergoing gasification without FW. The Ca, P, and Na contents of ash increased more significantly for SF semichar undergoing cogasification with the increase in carbon conversion. This indicated that FW ash, which was rich in Ca, P, and Na, was transferred to the surface of SF semichar 12651

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Energy & Fuels Table 9. EDS Analysis of SD Semichar Undergoing Cogasification conversion (%) 10 25 50 75

coal coal coal coal

matrix matrix matrix matrix

C (wt %)

O (wt %)

Ca (wt %)

P (wt %)

Na (wt %)

Mg (wt %)

K (wt %)

76.65 75.9 53.51 49.11

14.20 13.34 28.33 31.18

0.26 0.24 2.32 3.18

0.55 0.69 1.35 1.80

1.73 0.99 2.57 2.86

0.46 0.38 1.19 2.49

0.36 0.46 1.82 2.10

Table 10. EDS Analysis of SD Semichar Undergoing Independent Gasification conversion (%) 10 25 50 75 90

coal coal coal coal coal

matrix matrix matrix matrix matrix

C (wt %)

O (wt %)

Ca (wt %)

P (wt %)

Na (wt %)

Mg (wt %)

K (wt %)

83.94 79.54 74.39 57.23 47.99

8.57 11.25 16.84 25.45 35.65

0.35 0.33 0.86 1.92 4.10

0.61 0.77 1.20 1.27 0.99

0.73 0.82 0.93 1.02 1.50

0.48 1.04 1.25 1.09 2.12

0.54 0.45 0.80 2.04 2.55

which was higher than that of FW semichar during the cogasification process. The results indicated that the pore structure of FW−SF mixtures developed significantly as carbon conversion increased, instead of being blocked by the ash from FW char during the cogasification process. This might be attributed to the lack of Si and Al in FW ash.24 Therefore, the physical structure of mixtures was not significantly changed by the interaction effect and the variation in the chemical structure is the key to the synergetic effect of cogasification. 3.5. Variation in Chemical Structure Evolution. As discussed above, FW char and SF char presented synergistic effects during cogasification due to the catalytic effect of FW ash on SF char gasification. It is necessary to further investigate the chemical structure evolution of SF char under the effect of FW ash.

studied during the cogasification process. The pore structure parameters of samples are presented in Table 11. Table 11. Pore Structure Parameter of Semichar specific surface area (m2/g) conversion (%)

FW

FW−SF-1:1

SF

0 25 50 75

4.4 190.5 262.1 339.3

12.8 285.2 417.5 478.5

24.6 308.3 412.7 495.4

As shown in Table 11, the SSAs of FW char, SF char, and FW−SF-1:1 char all increased significantly with the increase in carbon conversion, especially in the early reaction stages. The SSA of FW−SF-1:1 semichar was close to that of SF semichar,

Figure 9. Variation of band area ratios with carbon conversion: (a) AG/Aall; (b) AD1/AG; (c) AD4/AG. 12652

DOI: 10.1021/acs.energyfuels.8b03663 Energy Fuels 2018, 32, 12644−12654

Energy & Fuels

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Considering the heterogeneity of the semichar particles, Raman measurements were obtained for 40 different particles from each sample.30 Figure 9 presents the variation of band area ratios for SF semichar and SF semichar undergoing cogasification with carbon conversion. The AD1/AG ratios of SF semichar and SF semichar undergoing cogasification both decreased with the increase in carbon conversion. In addition, the AG/Aall ratios of SF semichar undergoing independent gasification/cogasification increased with carbon conversion. It indicated that the microcrystalline planar size increased with the increase in carbon conversion. The AD4/AG ratios of SF semichar and SF semichar undergoing cogasification both decreased as carbon conversion increased. This finding indicated that the active sites were gradually consumed as the reaction progresses. The AD4/AG variation trends of two char samples are distinct, especially in high conversion stages. The AD4/AG ratio of SF char undergoing cogasification was much higher than that of SF char undergoing independent gasification when the carbon conversion was higher than 50%. This was because more and more FW ash was released to the surface of SF char when carbon conversion was greater than 50%. In addition, the AAEM in FW ash reacted with SF char to generate intermediates (active sites) and maintained the AD4/AG ratio at a relatively high level in the cogasification process.38,39 It was also the key reason for the significant synergetic effect between SF char and FW char.

Article

SYMBOLS USED

w0 = initial mass of char [g] wt = mass at time t [g] wash = ash mass [g] x = carbon conversion FA/B = mass fraction of sample A/B in the mixture wA/B,0 = initial mass of sample A/B in independent gasification [g] wA/B,t = mass of A/B at time t [g] wA/B,ash = ash mass in sample A/B [g] t = reaction time [min] t0.5 = reaction time for 50% carbon conversion [min] t0.95 = reaction time for 95% carbon conversion [min] R0.5 = reactivity index [min−1] R0.95 = reactivity index [min−1] r0.5 = index for evaluating interaction effect at early reaction stage r0.95 = index for evaluating interaction effect at later reaction stage Lc = stacking heights [nm] d002 = spacing between graphitic sheets [nm] B002 = corresponding peak width at half-maximum intensity [rad]

Greek Symbols

θ = peak position [rad] λ = wavelength of incident X-rays [nm] Subscripts

4. CONCLUSION FW char exhibited relatively high gasification reactivity (higher than SF char and lower than SD char) and presented a synergetic effect with SF char. The synergetic effect of SF char and FW char was enhanced with the increasing proportion of FW char. No synergetic effect was found between SD char and FW char. In the SF−FW cogasification process, the FW ash gradually transferred to the surface of SF char and reacted with SF char to generate active sites, resulting in the synergetic effect. Furthermore, the results also demonstrated that the pore structure of SF char was not blocked by FW ash attachment. In the FW−SD cogasification process, no significant ash exchange was observed and the two char samples reacted independently, because SD char reacted much faster than FW char and the ash content of SD char was extremely low.

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exp = experimental data cal = calculated data

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Xiaopeng Zou: 0000-0001-7196-7145 Xin Gong: 0000-0003-3282-9633 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present work was supported by the National Natural Science Foundation of China (21376081), the Sinopec Scientific and Technological Development Projects (415022), and the Fundamental Research Funds for the Central Universities (222201414030) 12653

DOI: 10.1021/acs.energyfuels.8b03663 Energy Fuels 2018, 32, 12644−12654

Article

Energy & Fuels

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DOI: 10.1021/acs.energyfuels.8b03663 Energy Fuels 2018, 32, 12644−12654