Structure characteristics and gasification activity of residual carbon from updraft fixed-bed biomass gasification ash

Energy Conversion and Management 136 (2017) 108–118

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Structure characteristics and gasification activity of residual carbon from updraft fixed-bed biomass gasification ash Sheng Huang a,b, Shiyong Wu a,b,⇑, Youqing Wu a,⇑, Jinsheng Gao a,b a b

Department of Chemical Engineering for Energy Resources, East China University of Science and Technology, Shanghai 200237, China Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, East China University of Science and Technology, Shanghai 200237, China

a r t i c l e

i n f o

Article history: Received 27 August 2016 Received in revised form 29 December 2016 Accepted 30 December 2016 Available online 11 January 2017 Keywords: Updraft fixed-bed gasifier Biomass gasification ash Residual carbon Structure characteristics Gasification activity

a b s t r a c t The structure characteristics and gasification activity of residual carbon in biomass ash from a 5 MW commercial updraft fixed-bed gasification power plant were investigated using a Raman spectroscopy, a pore structure analyzer and a thermo-gravimetric analyzer. In the investigated biomass gasification ashes, some fractions with a relatively high residual carbon contents could be probably recycled as fuel of gasifier. The crystalline structure of residual carbons in biomass gasification ashes were poorly organized, and the total active sites (sp2 and mixed sp2-sp3 bond forms) of residual carbons with were ordered as: fly ash > rice straw > bottom ash. The residual carbons in gasification ashes had a higher surface area and a higher gasification activity than those of rice straw. The residual carbons in bottom ashes had a lower surface area, more ordered carbon crystalline structures and less total active sites than those of fly ash. Consequently, the residual carbons in bottom ashes presented a lower gasification activity. The residual carbon which was generally considered as ‘‘unburnt carbon” in gasification ashes were probably originated from partly-gasified carbons and unreacted pyrolytic carbons. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction The depletion of fossil fuel and concerns over environmental pollution have motivated the exploration of renewable and clean energy. Because of its large reserves and environmentally friendly nature, biomass which currently provides about 10% of global primary energy demand, is considered a potential source of sustainable energy [1–3]. Several thermochemical conversion technologies can be used for the production of energy from biomass, and gasification technology can convert the biomass into valuable syngas with nearly zero pollution emissions. Therefore, gasification is considered to be a promising technology to utilize biomass for the production of syngas [4–6]. Biomass gasification ash is a by-product of biomass gasification process, and the output of biomass gasification ash increased substantially over the past two decades [7–9]. Previous investigations [7–12] indicated that biomass gasification ash differed considerably from biomass combustion ash and coal gasification/combustion ash. For instance, the unburned carbon of biomass ⇑ Corresponding authors at: Department of Chemical Engineering for Energy Resources, East China University of Science and Technology, Shanghai 200237, China (S. Wu). E-mail addresses: [email protected] (S. Wu), [email protected] (Y. Wu). http://dx.doi.org/10.1016/j.enconman.2016.12.091 0196-8904/Ó 2016 Elsevier Ltd. All rights reserved.

gasification ash is generally present in large amounts, typically 10–60% of the ash mass, which is usually larger than that of biomass combustion ash and coal gasification/combustion ash. This is a major issue of biomass gasification technology because it has a direct impact on gasification efficiency and ash recycling. In order to fully understand the essential characteristics of biomass gasification ash, the detailed characterization on the physicochemical properties of biomass gasification ash is highly imperative and significant. Typically, a standard updraft fixed-bed gasification process produces two types of ashes, bottom ash sampled from the outlet of lock-hopper and fly ash sampled from the syngas scrubber [4]. Generally, the biomass gasification bottom ash is a dense and abrasive solid with a relatively low carbon content, can be used for many applications such as cement additive, lightweight bricks and roofing granules [13,14]. The biomass gasification fly ash is irregularly–shaped particles with a highly developed pore structure and a high carbon content, can be beneficiated and recycled as construction materials and sorbents, synthesis and production of minerals, ceramics and other materials [13,15,16]. Besides, biomass ashes may be used directly as a fertilizer or soil improver or may be used as a raw material in production of mineral fertilizer due to the abundant of K [9]. In recent years, biomass gasification ashes have also been developed as the heterogeneous base

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catalysts for production of biodiesel, because CaO in biomass gasification ashes can promote the transesterification of vegetable oils with methanol [11]. Thus far, there are extensive literatures concerned the physicochemical properties characterization and utilization of biomass combustion ashes and coal gasification/combustion ashes, whereas special reports on the characteristics of biomass gasification ashes are limited. GómezBarea et al. [7] and Leiva et al. [16] found that the residual carbon contents in biomass gasification ashes from a fluidized bed gasifier varied in the range of 0.44–37.54%, depending on ash sampled locations and operation parameters of the gasifier, and the possibilities of biomass ashes used in agriculture (as soil conditioner, fertilizer and neutralising agent), as fuel (used in rotary kiln and boiler) and in other construction applications (cement and concrete) were evaluated. Hoff et al. [17] observed that the total carbon content in biomass gasification fly ash from a circulating fluidized bed (31.6%) was much higher than that from a gratefiring process (4.5%) and a bubbling fluidized bed (17.7%), especially organic carbon content. Eberhardt and Pan [12] found that the carbon content of pine wood chips gasification fly ash sampled from a pilot-scale downdraft gasifier was 47.0%, and the carbon contents of fly ash with different particle sizes varied in the range of 11.8–62.4%. Obviously, these results are quite dispersive and are not enough to deeply understand its essential characteristics, which can provide some fundamental supports for its efficient and reasonable utilization. Therefore, it is very necessary to do more studies on this topic. Normally, biomass gasification ash is roughly composed of residual carbons and inorganic minerals. Residual carbon in biomass ash is not only the major determinant of gasification efficiency, but also an important parameter determining the productive reuse and disposal methods of biomass gasification ash. For example, to recycle biomass gasification ash as fuel, high carbon content is beneficial. Conversely, low carbon content is preferable when gasification ash is used in cement and concrete sectors [15–17]. In order to minimize the carbon loss, thus reduce the biomass ashes produced as well as improve the biomass gasification efficiency, a greater understanding to the physicochemical properties of residual carbons in biomass ashes are required. The objectives of this work are to gain a comprehensive understanding to the physicochemical properties of biomass gasification ashes sampled from a 5 MW commercial updraft fixed-bed gasification power plant with focus on the structure characteristics and gasification activity of residual carbons in biomass ashes. It is favorable for the development of advanced biomass gasification technology. 2. Experimental 2.1. Materials Two biomass gasification ashes, fly ash and bottom ash (termed as FA and BA) were obtained from a 5 MW updraft fixed-bed

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gasification power plant located in Yangzhou city, Jiangsu province, China. Rice straw (termed as RS) briquettes were used as raw materials of the gasifier, and the schematic of updraft fixedbed biomass gasification power plant can be seen in Fig. 1. Briefly, RS briquettes were batch-fed automatically into the reactor through the hopper at the top of the gasifier, and gasifying agents (mixture of air and steam) were sucked into the gasifier through the inlet valve at the bottom of the gasifier. The RS was first introduced into the drying zone at the top of the gasifier, followed by the pyrolysis and reduction zones and finally unconverted solid passed through the oxidation zone. In reduction zone, the descending RS char was gasified with rising gasifying agents, and in oxidation zone, RS char was combusted. The residence time of RS briquettes in the gasifier was about 20 min under the normal operating conditions. The internal diameter of the gasifier was 3 m and height of 9 m, RS briquettes consumption rate of 2.5–3.0 t/h, gasification temperature of around 1273–1473 K, syngas output of about 5000 m3/h. The typical dry gas composition and lower heating value of biogas from RS gasification under the conditions of gasification temperature of 1353 K, ambient pressure, rice straw consumption rate of about 2.65 t/h and air consumption rate of about 2600 m3/h are shown in Table 1 [18]. BA was sampled from the outlet of lock-hopper at the bottom of gasifier and FA was collected from the bottom of gas scrubber, as shown in Fig. 1. The mixture of fly ash, tars and water were stored in a wastewater pool, and fly ash in the pool was collected using grab bucket regularly. The obtained fly ash mixed with tars was treated by tetrahydrofuran extraction to remove tars. Due to the heterogeneity of BA (as shown in Fig. 2), BA was separated into the following three fractions using sieving combined with hand-picking. Firstly, BA with diameter smaller than 4.75 mm (4 mesh) was collected using stainless steel sieve and termed as BAP. Secondly, the obviously molten clinker in BA with diameter larger than 4.75 mm was hand-picked and termed as BAC, and the remaining fraction of BA kept the original appearance of RS briquettes was collected and termed as BAO (as depicted in Fig. 2). The pictures of RS, FA and BA fractions (BAP, BAC and BAO) are shown in Fig. 2, and weight percentages of BAC, BAO and BAP fractions in BA are 37.06%, 27.65% and 35.29%, respectively. Some degree of bias may have introduced during handpicking, but sieving combined with hand-picking was deemed a suitable technique for the separation of BA. In order to obtain the residual carbons in RS, FA and BA fractions, RS, FA and BA fractions after being crushed to a size lower than 74 lm were subjected to 40 wt% HCl and 36% HF at temperature of 353 K and reaction time of 72 h to remove inorganic minerals, and finally washed with excess demineralized water. The ash contents of RS, FA and BA fractions after demineralization are less than 2.0%, indicating that the demineralized biomass ashes can be regarded as their corresponding residual carbons. In order to detect the inorganic elements in biomass ashes, the ash samples from RS, FA and BA fractions were prepared according

Fig. 1. Schematic of the 5 MW commercial updraft fixed-bed biomass gasification power plant.

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Table 1 Typical dry gas composition (vol.%) and lower heating value of biogas from rice straw.

a b

H2

CO

CH4

CO2

CnHma

N2

O2

LHVb (MJ/m3)

18.4

24.5

3.8

12.0

0.5

40.4

0.4

6.59

CnHm: unsaturated hydrocarbon. LHV: lower heating value.

RS briquettes

FA

BA

BAC

BAO

BAP

Fig. 2. Pictures of RS, FA and BA fractions.

to the standard of GB/T28731-2012. Briefly, the samples (RS, FA and BA fractions) were heated up to 523 K at a heating rate of 5 K/min and held at this temperature for 1 h in the atmosphere. Subsequently, the samples were heated up to 823 K further at a heating rate of 5 K/min and held at this temperature for 2 h in the atmosphere, and then the residues were stored as the corresponding ash samples.

2.2. Experimental method The calorific value testing of samples (RS, FA and BA fractions) were performed according to the Chinese standard of GB/T 2132008. Dry samples (FA and BA fractions) were placed in a muffle with an air atmosphere at 823 K for 2 h, and the fired samples were cooled to room temperature in a desiccator and then weighed. The

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weight loss of sample after thermal treatment is known as the losson-ignition (LOI). All element contents presented in ash samples from RS, FA and BA fractions were measured by a sequential X-ray fluorescence spectrometer (XRF, XRF-1800). The measurements of surface area and pore volume of residual carbons in RS, FA and BA fractions were performed with N2 and CO2 by a Micromeritics pore structure analyzer (ASAP 2020). Prior to analysis, the samples were degassed at 523 K for 12 h in a helium stream. N2 adsorption at 77 K was measured for the relative pressure (P/P0) range from 0.01 to 0.99. The Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods were adopted to calculate the surface area and pore size distribution of samples when N2 was used as adsorptive. CO2 adsorption at 273 K was performed for the relative pressure (P/P0) of less than 0.03, and the isotherms were analyzed using the Micromeritics non-local density functional theory (NLDFT) software package to obtain surface area and pore size distribution of samples. Raman spectroscopy analysis was performed for microstructures of residual carbons in RS, FA and BA fractions with a Jobin Yvon Labram HR800 spectrometer. In this study, the Raman experimental conditions and procedures were similar to those reported in literatures [19,20]. In order to obtain the spectral parameters such as peak position, full width at half maximum (FWHM), intensity (I) and integrated area (A) of each band, a linear baseline correction was used, and Raman spectra in the first-order region was resolved into 4 Lorentzian bands (respectively designated for the G, D1, D2 and D4 bands) and 1 Guassian band (designated for the D3 band) using a curve fitting software of Origin 8.0/Peak Fitting Module [19–24]. Due to the heterogeneity nature of residual carbon particles, three spectra were collected for each sample to obtain three sets of Raman spectral parameters. Here, the mean values of spectral parameters are used as the indicators of residual carbon microstructure parameters. The CO2 gasification activity of residual carbon was performed using a thermo-gravimetric analyzer (SETARAM TG-DTG/DSC) at the constant temperature of 1273 K and atmosphere pressure. The gasification rate (dX/dt) was used to characterize the gasification activity of samples. The details of thermo-gravimetric analyzer and calculations of dX/dt could be seen in our previous articles [25,26]. The gasification of residual carbons were carried out at least three times in order to determine the variability of the results and to assess the experimental errors. The gasification experiments showed an acceptable standard deviation of 2.0% with three replicates.

3. Results and discussion 3.1. Elementary characteristics of FA and BA fractions Table 2 displays the elementary properties of original rice straw (RS), gasification fly ash (FA) and bottom ash (BA) fractions. The ratio of BA to FA discharged by the updraft fixed-bed gasification unit of this study is 2.9:1, which is mainly dependent on biomass type, operation conditions and structure of the gasifier, etc. As presented in Table 2, the residual carbon contents are relatively high in FA, BAO and BAP fractions, respectively with a loss on ignition (LOI, dry basis) of 47.38%, 28.72% and 18.07%. It is noteworthy that residual carbon in FA is up to 32.31%. This is probably ascribed to that FA is entrained out of the gasifier by rising producer gas mainly undergo severe devolatilization process, and the gasification of FA is insufficient. Besides, BAO and BAP fractions present the relatively high residual carbon contents, which are mainly due to short residence time of RS in the gasifier and low heat and mass transfer efficiency between big biomass briquettes and gasifying agents. Straka et al. [27] observed that residual carbon

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contents in ten biomass combustion ashes were in the range of 0.15–22.63%, which are lower than that of biomass gasification ashes investigated in this study. The relatively high residual carbon contents in biomass gasification ashes, on the one hand, resulting in a lower gasification efficiency and a direct negative influence on the power generation efficiency. On the other hand, high residual carbon contents of gasification ashes (such as FA, BAO and BAP fractions) would hinder their utilization as an additive in cement, concrete, lightweight bricks, etc. [7,16]. Besides, Table 2 shows the calorific values of FA and BAO are 10.12 MJ/kg and 5.63 MJ/kg, on the dry basis. This means that if FA and BAO are beneficiated to be recycled as fuel of gasifier, the gasification efficiency will be probably elevated. Residual carbon in biomass gasification ash has a direct impact on process efficiency and ash recycling, as more carbon is converted the higher efficiency and the lower volume of ash generated. The caloric value of biomass ash is most important, but the behavior of the inorganic fraction in the gasifier is also of importance when considering using biomass ash as fuel. The FC/V ratio (a ratio between the content of fixed carbons and that of volatile matters) of RS is 0.10, whereas for FA and BA fractions, FC/V ratios respectively increases to 0.79 and 0.98–1.62, indicates that FA is more suitable to be recycled as fuel [14]. Utilization as fuel is a viable option for carbonrich biomass ash, as this option is valuable for a biomass-toenergy scenario. Demirbas [28] found that fly ash could be reburned to remove residual carbon and 1–2% ash with high residual carbon has replaced the fuel input to a boiler, reducing fuel costs and NOx emissions by about 20–30% depending on the amount of ash used. However, Gomez-Barea et al. [7] concluded that the recycling of fly ash in gasifier would be technically impossible, and regarded the reuse of ash as a fuel source was limited by heavy metals, Cl and K in biomass ash. Therefore, whether the biomass ash can be used as fuel still need further research, such as the critical analysis about the behaviors of inorganic matters in biomass ash, the volatilization of chlorine and potassium during recycling, and effective ways of capturing energy in residual carbon are of importance when using biomass ash as fuel. In addition, according to British standards on fly ash to be used as an addition in concrete [29], the LOI should not exceed 12%. Therefore, only BAC fraction can be directly used as a concrete additive. Carbon conversion efficiency is one of the most important parameter to evaluate the gasification technology. Despite the relatively high residual carbon contents in FA, BAO and BAP fractions, the total carbon conversion is still as high as 90.4%. The biomass gasification technology with a 90.4% carbon conversion efficiency is viewed as commercially acceptable [2,30,31]. For biomass gasification, the preferred technologies are fixed and fluidized bed gasifiers, developed and already commercially operated by many companies [30,31]. The fixed and fluidized bed gasification technologies both present the relatively high carbon conversion efficiency of about 85–95%, while fixed bed gasifiers are used mainly in power plant smaller than 10 MW and fluidized bed gasifiers are used mainly in power plant larger than 10 MW [2,30,31]. However, compared with the coal gasification technologies, such as Texaco [32], Shell [32,33] and opposed multi-burner gasifier [34], the relatively low carbon conversion efficiency is one of the main problems that slow down the commercial exploitation of biomass gasification technology [2,30,31]. Therefore, much more efforts should be made to improve the biomass gasification efficiency. In addition, Table 2 indicates that the H/C and O/C mole ratios of FA and BA fractions are much lower than those of RS, especially BAC and BAP fractions. These results suggest that RS had experienced the severe thermo-chemical conversion in the gasifier. Obviously, BA fractions (BAC, BAO and BAP) present completely different ash contents, residual carbon contents and LOI values, indicates that BA fractions investigated in this study are highly

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Table 2 Elementary properties of original RS, FA and BA fractions. Samples

Proximate analysis (wt%, dry basis) A

RS BA BAC BAO BAP FA

c d e f g

b

FC

72.81 8.84 1.75 13.77 9.19 26.60

FC/V

LOId (wt%)

LHVe (MJ/kg)

0.10 1.05 1.62 1.10 0.98 0.79

– 14.59 4.47 28.72 18.07 47.38

15.03 3.02 0.57 5.63 4.34 10.12

c

7.05 9.24 2.84 15.21 9.05 21.03

Ultimate analysis (wt%, dry basis)

RS BA BAC BAO BAP FA a

V

20.14 81.92 95.41 71.02 81.76 52.37

Samples

b

a

C

H

Stf

43.91 13.89 3.02 20.52 13.97 32.31

5.75 0.61 0.11 1.17 0.58 2.42

1.31 1.14 1.05 1.61 1.20 1.58

H/C

O/C

1.57 0.53 0.44 0.68 0.50 0.90

0.49 0.12 0.05 0.16 0.12 0.26

g

N

O

0.23 0.21 0.20 0.21 0.18 0.22

28.66 2.23 0.21 4.47 2.31 11.10

Content of ashes. Content of volatile matters. Content of fixed carbons. A weight loss percentage on ignition (823 K, 2 h). Lower heating value. Content of total sulfur. By difference.

uneven (as shown in Table 2). The completely different surface morphologies of BA fractions in Fig. 2 also confirmed the above conclusions. This is probably ascribed to the non-uniform heat and mass transfer between the big size of RS briquettes and gasifying agent in the updraft fixed-bed gasifier [6,31]. Table 3 shows the proximate and ultimate analysis results of residual carbons in RS, FA and BA fractions. As presented in Table 3, it can be observed that after being treated by HCl/HF, the fixed carbon and volatiles of residual carbons in RS, FA and BA fractions increase, especially fixed carbon. From ultimate analysis results, it can be noted that acid treatment have reduced the contents of hydrogen and sulfur and meanwhile increased the content of carbon. The substantially decrease of sulfur contents indicated that the acid washing liberated some of sulfur in biomass ashes [35,36]. Clearly, the behaviors of inorganic matters during gasification significantly affect the conversion of organic matters in rice straw, and ultimately determined the physicochemical properties of gasification residues. Therefore, the elemental compositions of ashes in RS, FA and BA fractions (normalized to 100%) were analyzed and shown in Table 4. Table 4 shows the rice straw contains silicon, potassium, calcium, magnesium and aluminum as its principal ash-forming components. The ash of rice straw contains 67.18% SiO2, and SiO2 enrichment is evident in all of the rice straw gasification ashes (in the range of 71.70–79.65%). It has been reported that the reaction of alkali with silica to form silicates that melt

or soften at temperatures lower than 973 K, depending on the ash compositions [37]. Therefore, the behaviors of inorganics, especially silica, should be paid more attention due to the slagging of silica compounds, resulting in the poor performance or failure of the gasifier. Besides, it can be found that the elemental compositions of FA and BA are completely different, mainly due to different thermal history of FA and BA in the gasifier. 3.2. Carbon microstructures of residual carbon in FA and BA fractions Understanding of carbon structural features change during gasification is essential for better understand the gasification behaviors of carbonaceous materials under high temperature, and Raman spectroscopy is the most powerful technique to characterize the structural features of carbonaceous materials, because it is sensitive not only to the crystalline structures but also to the amorphous structures [19–24]. Therefore, a Raman spectroscopy was employed to characterize the microstructures of residual carbons in RS, FA and BA fractions, and the first-order region Raman spectra (800–2000 cm
1) of residual carbons in RS, FA and BA fractions are presented in Fig. 3. Generally, the first-order region Raman spectra of highly disordered carbon present two overlapping bands, which are called the D and G bands. In order to obtain the Raman parameters, each spectrum in Fig. 3 was subjected to peak fitting using Origin

Table 3 Proximate and ultimate analysis of residual carbons in RS, FA and BA fractions. Proximate analysis (wt%, dry basis)

RS BA BAC BAO BAP FA a b c d

Content Content Content Content

of of of of

Ultimate analysis (wt%, dry basis)

Aa

Vb

FCc

C

H

N

Std

1.08 1.81 1.93 0.89 1.02 1.95

84.16 20.47 12.34 27.56 21.30 36.49

14.76 77.72 85.73 71.55 77.68 61.56

55.38 75.58 79.02 73.59 77.65 68.19

4.62 0.62 0.23 0.87 0.54 1.85

0.26 0.21 0.22 0.19 0.24 0.20

0.43 0.51 0.58 0.41 0.63 0.39

ashes. volatile matters. fixed carbons. total sulfur.

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S. Huang et al. / Energy Conversion and Management 136 (2017) 108–118 Table 4 Elemental compositions of ashes in RS, FA and BA fractions at the ashing temperature of 823 K. Chemical composition

RS (wt%)

BA (wt%)

BAC (wt%)

BAO (wt%)

BAP (wt%)

FA (wt%)

SiO2 K2O CaO MgO Al2O3 Na2O Fe2O3 TiO2 P2O5 Cl SO3

67.18 10.89 5.18 3.40 2.31 1.93 1.31 0.12 2.66 2.63 2.38

72.50 8.81 6.02 3.31 2.25 1.45 1.65 0.10 2.84 0.25 0.82

71.70 10.92 5.63 3.21 2.18 1.38 1.98 0.15 2.36 0.27 0.20

73.55 7.31 5.69 3.90 2.40 1.17 1.55 0.15 2.61 0.31 1.34

72.69 9.52 5.55 3.63 1.92 1.33 1.51 0.14 2.69 0.31 0.70

79.65 4.54 4.39 2.92 1.29 0.74 1.57 0.08 2.69 0.42 1.72

G band

Intensity (×10 3 a.u)

5

D band

4 BAP 3 BAC 2 BAO 1 0 800

FA RS 1000

1200

1400

1600

1800

2000

Wavenumber (cm-1) Fig. 3. Typical first-order region Raman spectra of residual carbons in RS, FA and BA fractions.

8.0/Peak Fitting Module, and the fitting bands of residual carbons in RS and BAP are presented in Fig. 4 as the representatives, indicating excellent agreements between the fitting curves and Raman data. As shown in Fig. 4, except the G band at about 1580 cm
1, additional bands appear in the first-order region at about 1200, 1350, 1530 and 1620 cm
1 due to the defects in carbon crystalline lattices of biomass [19,20]. The G band at about 1580 cm
1 is corresponds to the stretching vibration mode with E2g symmetry in the aromatic layers of the graphite crystalline. The band at 1350 cm
1 (D1 band) is ascribed to the vibration mode of disordered graphitic lattices with the in-plane imperfections such as defects and heteroatoms. The band at 1620 cm
1 (D2 band) is attributed to the vibration mode of disordered graphitic lattices with E2g symmetry (surface graphene layers) [19–22]. The band at 1530 cm
1 (D3 band) originates from the vibration of

10

Intensity ( 10 2a.u)

8 D1 band D3 band 4

35

Fitting Curve

(a) RS

6

sp2-bond form of amorphous carbon, such as organic molecules, fragments or functional groups, in poorly organized carbonaceous materials [21–23]. The band at 1200 cm
1 (D4 band) is ascribed to the mixed sp2–sp3 bond form at the periphery of crystallites or to CAC and C@C stretching vibration of polyene-like structure [21–24]. Qualitatively, the D3 and D4 bands are lower in intensity but wider in FWHM (full width at half maximum) compared with G, D1 and D2 bands, indicating considerable amounts of amorphous carbons in residual carbons of biomass ashes. The spectrum deconvolution with curve-fitting technique provided quantitative information about the structural features of residual carbons in FA and BA fractions. Table 5 displays the microstructure parameters of residual carbons in RS, FA and BA fractions. As presented in Table 5, the FWHM of the G bands range from 50.3 to 55.8 cm
1, which is far greater than that of the highly oriented pyrolytic graphite of about 15–23 cm
1 [38]. This feature indicates a low degree of crystalline order of residual carbons after high temperature gasification. Besides, the FWHM of D1 and G bands of RS are larger than those of BA fractions (especially BAC and BAP fractions) and smaller than those of FA. These results suggest that the crystalline structure of residual carbons in BA fractions are more ordered than that in RS, which are in accordance with the conclusions found by previous investigations that disordered carbon was gasified preferentially during coal gasification process, and relatively ordered carbon was retained in biomass ash [39,40]. However, the above results also indicate that the crystalline structure of residual carbon in FA is less ordered than that of RS, which is different from the structural features of coal gasification fly ash found by Wu et al. [20]. It is generally accepted that the dependence between the ID1/IG ratio and the microcrystalline planar size shows an inversely proportional behavior, that is ID1/IG / 1/La (La is carbon crystallite diameter). Theoretically, ID1/IG ratio should decrease after gasification. However, there is an increase for ID1/IG ratio of FA, and this is

G band D2 band

D4 band

2

Intensity ( 10 2a.u)

6

30

(b) BAP

25 20 15

1000 1200 1400 1600 1800 2000

Wavenumber (cm-1)

D3 band D1 band

10 5

0 800

Fitting Curve

0 800

G band

D2 band

D4 band

1000 1200 1400 1600 1800 2000

Wavenumber (cm-1)

Fig. 4. Typical first-order region Raman spectra and fitting bands of residual carbons in RS and BAP.

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Table 5 Carbon microstructure parameters of residual carbons in FA and BA fractions. Parameters a

D1-FWHM (cm) G-FWHM (cm)b G-D separation (cm)c ID1/IG AD3/AAll (%) AD4/AAll (%) AD3+AD4/AAll (%)

c

FA

BAO

BAP

BAC

149.4 54.5 255 1.16 13.14 7.44 20.58

150.3 55.8 253 1.22 14.07 6.94 21.01

141.6 53.1 235 1.10 11.85 6.12 17.97

127.8 50.3 216 1.01 10.18 5.09 15.27

132.2 51.4 220 1.04 11.34 5.27 16.61

The full width at half maximum of the D1 band. The full width at half maximum of the G band. The peak position between the G and D bands; I represent the peak intensity of each band; A represent the area of each band.

mainly due to the crystallite of residual carbon in FA is too small to be detected by Raman signal [41,42]. That is to say, the crystallite of residual carbon in FA is too small to effectively couple with the incident laser beam, and thus has a small contribution to Raman scattering signal. After sufficient gasification, the crystallites of residual carbons in BA fractions grow and reach a size that contributes to Raman spectrum. The decrease of ID1/IG ratios of residual carbons in BA fractions, therefore, implies the increase of the average planar size of carbon crystalline structures of BA fractions. Similar behaviors have also been found by Zaida et al. [41] and Zhu and Sheng [42] when considering the effects of heat treatment on microstructures of char from coal and biomass. Zaida et al. [41] observed that ID1/IG ratio of char from cellulose fibers increased up to 2173 K and then decreased gradually with the rose of pyrolysis temperature. Zhu and Sheng [42] found that the ID1/IG ratio of lignite char increased gradually until the pyrolysis temperature of 1173 K and then decreased with increasing pyrolysis temperature. The above behaviors of ID1/IG ratio of residual carbons are mainly ascribed to the severity of thermal treatment. The D3 band originates from the amorphous sp2-bonded forms of carbon in carbonaceous materials with a poorly organized structure [19–23]. Previous literatures reported that D3 band is responsible for the reacting active sites and consequently the sample reactivity [19,20,23]. Theoretically, AD3/AAll ratio should decreases after gasification, because annealing decreases the number of active sites. However, Table 5 shows that AD3/AAll ratio of RS is larger than that of BA fractions (BAO, BAC and BAP) and smaller than that of FA. The increases of AD3/AAll ratio of FA can attribute to the intensification of interstitial defects between the aromatic layers [41]. The decrease of AD3/AAll ratios of residual carbons in BA fractions can be explained by the preferentially gasification of disordered carbon in original RS [43]. Occurrence of the D4 band is typical for very poorly organized materials, such as soot, coal char of low degree of structural organization. The D4 band is associated with mixed sp2–sp3 bonded forms at the periphery of crystallites or to CAC and C@C stretching vibration of polyene-like structures, and it may also be responsible for the reacting sites [19,20,23]. As shown in Table 5, the decrease of AD4/AAll ratios of residual carbons in FA and BA fractions imply that the structural defects and imperfections of carbon crystalline were gradually eliminated when undergoing heat treatment, such as the decomposition of mixed sp2–sp3 carbon structure (e.g. alkylaryl CAC structure and ethers). Besides, it is noted that the AD3/AAll and AD4/AAll ratios of BAC are larger than those of BAP. This is probably ascribed to the residual carbon in BAC was wrapped in molten minerals at elevated temperature and cannot fully contact with gasifying agents, resulting in the insufficient gasification of carbon in BAC. In addition, it is found that AD3+D4/AAll ratio of RS is slightly smaller than that of FA and distinctly larger than that of BA fractions. This indicates that the total active sites of residual carbons with sp2 and mixed sp2-sp3 bond forms are ordered as FA > RS > BAO > BAC > BAP.

3.3. Pore structures of residual carbon in FA and BA fractions Generally, it is accepted that carbon crystalline structure and surface area are two main factors which affect the gasification activity of carbonaceous materials [25,26]. Therefore, surface areas and pore structures of residual carbons in RS, FA and BA fractions were analyzed by N2 and CO2 adsorption. As presented in Fig. 5, the adsorption isotherms of residual carbons in RS, FA and BA fractions present a reversible ‘‘S” shape. Therefore, the isotherms in Fig. 5 can be classified to type II according to the classification of BET adsorption isotherm of IUPAC [44,45]. The forepart of the isotherms presents a slow ascending trend, indicating that the N2 adsorption transfers from monolayer to multilayer. The rearward part of the isotherms rise rapidly when the relative pressure approaches to 1.0, suggesting that capillary coacervation occurs during the N2 adsorption of residual carbons. This indicates that residual carbons in RS, FA and BA fractions have a relatively complete porous structure, including micro-, meso- and macro-pores, which are in accordance with the pore size distributions of residual carbons shown in Figs. 6 and 8. Besides, it can be also noted that the adsorption isotherms are inconsistent with the desorption isotherms at the relative pressure of larger than 0.4 because of the capillary condensation in mesopores, resulting in the presence of hysteresis loops. Useful information about pore structure can be obtained from the shape of hysteresis loop [46]. It is observed that the residual carbon of RS has a hysteresis loop which cannot be fitted by any proposed standards, suggesting that the pore shape of residual carbon in RS is diversiform, probably including inkbottle-shaped pores, parallelplate pores, cylindrical pores, slit-shaped pores, etc. [44–46]. However, Fig. 5 also indicates that the hysteresis loops of residual carbons in FA and BA fractions can be classified as type H4 according

75

Absorbed quantity (cm3/g STP)

a b

RS

60

FA

BAC

BAO

BAP

RS

45 30 15 0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (p/p0 ) Fig. 5. N2 adsorption/desorption isotherms of residual carbons in RS, FA and BA fractions.

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0.12

dV/d(logD) (cm3/g)

FA BAC

0.09

BAO

0.06

RS

BAP

0.03

0.00 1

10

100

D (nm) Fig. 6. Pore size distributions of residual carbons in RS, FA and BA fractions, determined by N2 adsorption at 77 K. (V: pore volume; dV: differential of pore volume; D: pore diameter).

to IUPAC [44,45]. The H4 hysteresis is often associated with aggregates of plate-like particles giving rise to narrow slit-shaped pores [46]. The above results implying that the residual carbons in FA and BA fractions mainly present slit-shaped pores. Therefore, it is suggested that the complex pore shapes of RS transformed to slit-shaped pores in residual carbons of FA and BA fractions during gasification process. Fig. 6 shows the pore size distributions of residual carbons in RS, FA and BA fractions, determined by N2 adsorption at 77 K. From Fig. 6, it can be noted that the pore size determined by N2 adsorption are in the range of 1.8–55.0 nm. Besides, it is clearly presented that there is a significant change in pore size distribution of carbon in RS after gasification. The pore size distribution curve of RS is lower than that of FA and BA fractions, especially FA and BAC. This illustrates that the porosity of residual carbons in FA and BA fractions are more abundant than that in RS, which can be proved by their surface area and pore volume distributions shown in Fig. 7. Furthermore, the pore size distributions determined by N2 adsorption clearly indicates that the pores of residual carbons in RS, FA and BA fractions can be divided into pores below 10 nm and pores above 10 nm due to their respectively contributions to surface areas and pore volumes. It can be observed from Fig. 7 that the surface areas of pores below 10 nm are larger than those above 10 nm, while the volumes of pores below 10 nm are obviously smaller than those above 10 nm, especially FA and BAC. Hence, it can be inferred that pores below and above 10 nm are separately make a main contribution to surface areas and pore volumes of residual carbons in RS, FA and BA fractions.

(a)

6

S (m 2/g)

18

FA BAC BAO BAP RS

4

V ( 10-3 cm3/g)

8

It is well accepted that N2 adsorption has limitations in determining the porosity of micro-porous carbons, and this have been confirmed by the pore size distributions in Figs. 6 and 8 [47,48]. CO2 is often considered as the preferred adsorptive for micropore determination, since CO2 adsorption are mostly performed at temperatures near ambient, which will enhance CO2 diffusion properties in micro-pores compared with the low temperatures used in N2 [47,48]. Therefore, CO2 adsorption was conducted at 273 K for determining the micro-pores in residual carbons of RS, FA and BA fractions, and the results are shown in Fig. 8. From Fig. 8, it can be observed that residual carbons of RS, FA and BA fractions are abundant in micro-pores with pore size of 0.4–1.0 nm, demonstrating that CO2 is a good adsorptive for micro-pore determinations. Besides, it can be noted that the porosity of micro-pore in residual carbons followed the order of BAO > FA > BAC > BAP > RS, which is different from that of mesoand macro-pore determined by N2 (as shown in Fig. 6). The above results indicate that micro-pore increased greatly after gasification, especially FA and BAO. It is well-known that the gasification of carbonaceous materials is a typical gas-solid reaction, and pore size has a significant effect on the mode of gas diffusion in a porous matrix with Knudsen diffusion dominating at smaller pore size and molecular diffusion dominating at larger pore size [26,49]. Sadhukhan et al. [49] concluded that Knudsen and molecular diffusivities become comparable for a pore size of about 250 nm. As presented in Figs. 6 and 8, the pores in residual carbons of RS, FA and BA fractions are in the range of 0.4–55.0 nm, which are much smaller than 250 nm. Therefore, it is inferred that gaseous Knudsen diffusion is dominated during the gasification process of RS. This means that the pore size distribution of residual carbons in FA and BA fractions is a very significant factor which can affect their gasification activity. As depicted in Fig. 9, it can be noted that the surface areas determined by N2 adsorption are obviously smaller than that determined by CO2 adsorption, indicating that the micro-pore surface areas of residual carbons in RS, FA and BA fractions are larger than that of meso- and macro-pores. The above results show that the residual carbons in RS gasification ashes contain a large number of micro-pores and a certain amount of meso- and macropores. Due to the main contribution of meso- and macro-pores to pore volumes, the meso- and macro-pore volumes are larger than that of micro-pores (except BAO fraction), as shown in Fig. 9b. Besides, it can be observed that the surface areas and pore volumes of residual carbons in FA and BA fractions are far higher than those of RS, especially FA and BAC fraction. This is probably ascribed to the greatly increasing number of pores after gasification, especially meso- and macro-pores. In general, the gasification of biomass

2

(b)

15 12 9

FA BAC BAO BAP RS

6 3

0 1

10

D (nm)

100

0 1

10

100

D (nm)

Fig. 7. Surface area and pore volume distribution of residual carbons in RS, FA and BA fractions, determined from N2 adsorption isotherms at 77 K. (D: pore diameter; DS: incremental surface area; DV: incremental pore volume).

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dX/dt (×10-2min-1)

dV/d(logD) (cm3/g)

7.5

BAO

FA

6.0 4.5 3.0 1.5

BAC BAP

0.4

0.5

0.6

0.7

0.8

0.9

1.0

BAO

BAC

RS

BAP

0.0 0.0

RS

FA

0.2

1.1

0.4

0.6

0.8

1.0

Carbon conversion

D (nm) Fig. 10. CO2 gasification rates of residual carbons in RS, FA and BA fractions.

comprise two steps, the severe devolatilization of biomass and subsequent the gasification of the resultant biomass char, results in the opening of some blocked pores and enlarging of some relatively small pores during devolatilization and gasification processes [48]. Consequently, the surface areas and pore volumes of residual carbons in FA and BA fractions are substantially larger than those of RS. Besides, it can be noted that the micro-pore surface areas and volumes of residual carbons followed the order of BAO > FA > BAC > BAP > RS, while those of meso- and macro-pores were ordered as FA > BAC > BAO > BAP > RS. 3.4. CO2 gasification activity of residual carbon in FA and BA fractions The CO2 gasification rates of residual carbons in RS, FA and BA fractions at the constant gasification temperature of 1273 K are plotted against the carbon conversion and shown in Fig. 10. From Fig. 10, it can be noted that in the whole carbon conversion range, the gasification rates of residual carbons follow the trend of FA > BAO > BAC > RS > BAP. It is interesting to note that the gasification activity of residual carbons in FA and BA fractions (except BAP fraction) is obviously higher than that of RS. Common belief is that a smaller surface area, more ordered carbon crystalline structure and/or less active sites of carbonaceous materials results in its lower gasification activity [20,25,43]. Fig. 9 indicates that the surface areas and pore volumes of residual carbons in FA and BA fractions are far higher than those of RS. Thereby, it can be inferred that the higher gasification activity of residual carbons in FA and BA fractions should be mainly ascribed to their more abundant porosity, especially BA fractions with more ordered carbon crystalline structure and fewer active sites (as presented in Table 5).

Surface area ( m2.g-1)

160 120

(a)

N2 CO2

80 40 0 RS

FA

BAC

BAO

BAP

Besides, it can be observed that the gasification activity of RS is higher than that of BAP. The obtained results indicate that the surface area of residual carbon in BAP is higher than that of RS, and the carbon crystalline structure of BAP is more ordered than that of RS. Therefore, it is probably inferred that the lower gasification activity of residual carbon in BAP should be mainly attributed to its more ordered carbon structures and/or less total active sites (as shown in Table 5). Furthermore, Fig. 10 also shows that the gasification activity of residual carbon in FA is higher than that of BA fractions, which is probably ascribed to the larger surface area and relatively disordered microstructures of residual carbon in FA, as shown in Table 5 and Fig. 9. However, Wu et al. [20] and Xu et al. [50] found that the residual carbon in coal gasification coarse slag presented a higher gasification activity compared with that of fly ash, which is different from the conclusions obtained in this study. The above differences probably attributed to the different physicochemical properties and thermal history of raw materials in the gasifier. It has been accepted that the preferentially gasification of disordered carbon during the whole gasification process of carbonaceous materials, and the relatively ordered carbon was retained in the gasification residue, which should presents relatively low gasification activity [19,39,50]. Therefore, it is concluded that the residual carbon (are generally regarded as the relatively organized carbon after sufficient gasification of biomass) in biomass ash may be partly derived from the carbon packaged by molten minerals during gasification process [20,50]. In other words, the residual carbon which are generally considered as ‘‘unburnt carbon” in biomass gasification ash probably composed of partly-gasified carbons (due to a quite short residence time in the gasifier) and unreacted pyrolytic carbons (those pyrolytic carbons cannot be timely gasified due to the package of molten minerals). In summary, one of the main problems that slow down the commercial exploitation of biomass gasification technology is the

Pore volume (10-2cm3.g-1)

Fig. 8. Pore size distribution of residual carbons in RS, FA and BA fractions, determined from CO2 adsorption at 273 K. (V: pore volume; dV: differential of pore volume; D: pore diameter).

9.0

(b)

N2 CO2

7.5 6.0 4.5 3.0 1.5 0.0

RS

FA

BAC

BAO

BAP

Fig. 9. Surface areas and pore volumes of residual carbons in RS, FA and BA fractions, determined by N2 and CO2 adsorption.

S. Huang et al. / Energy Conversion and Management 136 (2017) 108–118

relatively low energy efficiency [2,6,30], which are mainly ascribed to the relatively low gasification temperature, short residence time of raw materials in the gasifier, and non-uniform heat and mass transfer between the big biomass briquettes and gasifying agents [6,38]. Therefore, measures should be taken to enhance the performance of the gasifier. The equivalent ratio and residence time are two important factors which can significantly affect the performance of biomass gasification. At relatively low equivalent ratio, a small amount of biomass is combusted to produce heat, and temperature in the gasifier is relatively low, which would lead to a lower carbon conversion efficiency and a higher concentration of tars in the syngas. The gasification temperature increases with increasing equivalent ratio due to more biomass is combusted to produce heat, and carbon conversion of biomass would increase to some extent. However, as the equivalent ratio exceed a threshold value, excess biomass is combusted and CO2 content in biogas would increase, and the content of syngas (H2 + CO) would decrease. Therefore, an appropriate equivalent ratio needs to be controlled. With the increase of particle residence time, the conversion of rice straw would increase, while the capacity of the gasifier would decrease to some extent. Therefore, an appropriate residence time should be adopted to balance between the carbon conversion of rice straw and capacity of the gasifier. Furthermore, the heat and mass transfer between the biomass briquettes and gasifying agents can be intensified by decrease the briquettes size, because smaller briquettes can facilitate the faster mass and heat transfer. Besides, it has been generally accepted that the pressurized biomass gasification can enhance the biomass gasification intensity and efficiency greatly [30,31]. However, increasing the pressure of gasifier comes with inherent feeding problems. Therefore, how to improve the efficiency of biomass gasification technology is a very complicated issue, and it is necessary to do more studies on this topic to realize the clean and efficient utilization of biomass. Besides, in order to achieve the steady, continuous and efficient operating of the gasifier, attention should also be paid on the operating problems of the gasifier. The pelletization of biomass is a common pre-treatment in the fixed-bed gasifiers which allows to increase the biomass density and to produce a shape suitable to form the bed without packing it. The usage of rice straw pellet with a uniform structure can facilitate breathability and make less of a biomass bridge than the usage of rice straw. However, the wide range of pellet size, especially the presence of large pellets compromise the feeding system and form bridges inside the reactor or presence of by-pass channels. Hence, the pressure drops across the gasifier is low, and the gasifier permeability would increases [51,52]. However, rice straw pellet is known to be extremely fragile during gasification process, producing fine dust and reducing the bed permeability to air (giving a high resistance to the gas flow and the pressure drops across the gasifier is high) with a consequent reduction of the syngas production or even obstruction of the gasifer [51–53]. Therefore, the pressure drops across the gasifier as a measure of the acceptable flowability should be monitored.

4. Conclusions Some fractions of the investigated biomass gasification ashes presented relatively high residual carbon contents, such as FA and BAO, could be recycled as fuel of the gasifier. Conversely, BAC fraction can be directly used as a concrete additive due to its extremely low carbon content. The crystalline structure of residual carbons in FA and BA fractions were quite poor, and the residual carbons in BA fractions were obviously more ordered than that in RS, while the residual carbon in FA was relatively disordered than

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that in RS. The total active sites of residual carbons in RS, FA and BA fractions with sp2 and mixed sp2-sp3 bond forms were ordered as: FA > RS > BAO > BAC > BAP. The residual carbons in FA and BA fractions contained a large number of micro-pores and a certain amount of meso- and macro-pores with mainly slit-shaped pores. The gasification activity of residual carbons in FA and BA fractions were higher than that of RS, mainly attributed to their quite more abundant porosity. The residual carbon in FA presented higher gasification activity than that in BA fractions, mainly ascribed to its larger surface area and more disordered carbon microstructures. The residual carbon which was generally considered as ‘‘unburnt carbon” in biomass gasification ash probably composed of partly-gasified carbons (probably due to a quite short residence time in the gasifier) and unreacted pyrolytic carbons (those pyrolytic carbons cannot be timely gasified due to the package of molten minerals).

Acknowledgements The authors acknowledge the financial support provided by the National Natural Science Foundation of China (Grand No. 21506060) and the National High Technology Research and Development Program of China (No. 2012AA101810).

References [1] Abbasi T, Abbasi SA. Biomass energy and the environmental impacts associated with its production and utilization. Renew Sust Energy Rev 2010;14:919–37. [2] Moret S, Peduzzi E, Gerber L, Maréchal F. Integration of deep geothermal energy and woody biomass conversion pathways in urban systems. Energy Convers Manage 2016;129:305–18. [3] Maglinao Jr AL, Capareda SC, Nam Hyungseok. Fluidized bed gasification of high tonnage sorghum, cotton gin trash and beef cattle manure: evaluation of synthesis gas production. Energy Convers Manage 2015;105:578–87. [4] Zheng XY, Chen C, Ying Z, Wang B. Experimental study on gasification performance of bamboo and PE from municipal solid waste in a bench-scale fixed bed reaction. Energy Convers Manage 2016;117:393–9. [5] Tremel A, Becherer D, Fendt S, Gaderer M, Spliethoff H. Performance of entrained flow and fluidised bed biomass gasifiers on different scales. Energy Convers Manage 2013;69:95–106. [6] Sharma S, Sheth PN. Air–steam biomass gasification: experiments, modeling and simulation. Energy Convers Manage 2016;110:307–18. [7] Gómez-Barea A, Vilches LF, Leiva C, Campoy M, Fernández-Pereira C. Plant optimisation and ash recycling in fluidised bed waste gasification. Chem Eng J 2009;146:227–36. [8] Maneerung T, Kawi S, Wang CH. Biomass gasification bottom ash as a source of CaO catalyst for biodiesel production via transesterification of palm oil. Energy Convers Manage 2015;92:234–43. [9] Stanislav VV, David B, Lars KA, Christina GV. An overview of the composition and application of biomass ash. Part 2. Potential utilisation, technological and ecological advantages and challenges. Fuel 2013;105:19–39. [10] Girón RP, Ruiz B, Fuente E, Gil RR, Suárez-Ruiz I. Properties of fly ash from forest biomass combustion. Fuel 2013;114:71–7. [11] Maneerung T, Kawi S, Wang CH. Biomass gasification bottom ash as a source of CaO catalyst for biodiesel production via transesterification of palm oil. Energy Convers Manage 2015;92:234–43. [12] Eberhardt TL, Pan H. Elemental analyses of chars isolated from a biomass gasifier fly ash. Fuel 2012;96:600–3. [13] Nortey Yeboah NN, Shearer CR, Burns SE, Kurtis KE. Characterization of biomass and high carbon content coal ash for productive reuse applications. Fuel 2014;116:438–47. [14] Wu T, Gong M, Lester E, Wang FC, Zhou ZJ, Yu ZH. Characterisation of residual carbon from entrained-bed coal water slurry gasifiers. Fuel 2007;86:972–82. [15] Ahmaruzzaman M. A review on the utilization of fly ash. Prog Energ Combust 2010;36:327–63. [16] Leiva C, Gómez-Barea A, Vilches LF, Ollero P, Vale J, Fernández-Pereira C. Use of biomass gasification fly ash in lightweight plasterboard. Energy Fuels 2007;21:361–7. [17] Hoff D, Meyer J, Kasper G. Influence of the residual carbon content of biomass fly ashes on the filtration performance of ceramic surface filters at high temperatures. Powder Technol 2008;180:172–7. [18] Li L, Huang S, Wu SY, Wu YQ, Gao JS, Gu JW, et al. Fuel properties and chemic al compositions of the tar produced from a 5 MW industrial biomass gasification power generation plant. J Energy Inst 2015;88:126–35. [19] Sheng CD. Char structure characterized by Raman spectroscopy and its correlations with combustion reactivity. Fuel 2007;86:2316–24.

118

S. Huang et al. / Energy Conversion and Management 136 (2017) 108–118

[20] Wu SY, Huang S, Ji LY, Wu YQ, Gao JS. Structure characteristics and gasification activity of residual carbon from entrained-flow coal gasification slag. Fuel 2014;122:67–75. [21] Sadezky A, Muckenhuber H, Grothe H, Niessner R, Poschl U. Raman microspectroscopy of soot and related carbonaceous materials: spectral analysis and structure information. Carbon 2005;43:1731–42. [22] Oboirien BO, Engelbrecht AD, North BC, Cann VMD, Verryn S, Falcon R. Study on the structure and gasification characteristics of selected South African bituminous coals in fluidised bed gasification. Fuel Process Technol 2011;92:735–42. [23] Chabalala VP, Wagner N, Potgieter-Vermaak S. Investigation into the evolution of char structure using Raman spectroscopy in conjunction with coal petrography; Part 1. Fuel Process Technol 2011;92:750–6. [24] Morga R. Micro-Raman spectroscopy of carbonized semifusinite and fusinite. Int J Coal Geol 2011;87:253–67. [25] Huang S, Wu SY, Ping YM, Wu YQ, Gao JS. Effect of CS2 extraction on the physical properties and gasification activity of liquid-phase carbonization cokes. J Anal Appl Pyrol 2012;93:33–40. [26] Wu YQ, Wu SY, Huang S, Gao JS. Physicochemical properties and structural evolutions of gas-phase carbonization chars at high temperatures. Fuel Process Technol 2010;9:1662–9. [27] Straka P, Náhunková J, Zaloudková M. Analysis of unburned carbon in industrial ashes from biomass combustion by thermogravimetric method using Boudouard reaction. Thermochim Acta 2014;575:188–94. [28] Demirbas A. Potential applications of renewable energy sources, biomass combustion problems in boiler power systems and combustion related environmental issues. Prog Energy Combust Sci 2005;31:171–92. [29] Brithish Standard. Pulverized-fuel ash Part 2. Specification for pulverized-fuel ash to be used as a Type I addition; In British Standards Institution; 1996 (BS 3892–2). [30] Peter MK. Energy production from biomass (part 3): gasification technologies. Bioresour Technol 2002;83:55–63. [31] Warnecke R. Gasification of biomass: comparison of fixed bed and fluidized bed gasifier. Biomass Bioenergy 2000;18:489–97. [32] Zheng LG, Furinsky E. Comparison of Shell, Texaco, BGL and KRW gasifiers as part of IGCC plant computer simulations. Energy Convers Manage 2005;46 (11–12):1767–79. [33] Lee HH, Lee JC, Joo YJ, Oh M, Lee CH. Dynamic modeling of Shell entrained flow gasifier in an integrated gasification combined cycle process. Appl Energy 2014;131:425–40. [34] Guo QH, Zhou ZJ, Wang FC, Yu GS. Slag properties of blending coal in an industrial OMB coal water slurry entrained-flow gasifier. Energy Convers Manage 2014;86:683–8. [35] Zhang LJ, Li ZH, Yang YL, Zhou YB, Kong B, Li JH, et al. Effect of acid treatment on the characteristics and structures of high-sulfur bituminous coal. Fuel 2016;184:418–29.

[36] Roets L, Strydom CA, Bunt JR, Neomagus HWJP, Niekerk DV. The effect of acid washing on the pyrolysis products derived from a vitrinite-rich bituminous coal. J Anal Appl Pyrol 2015;116:142–51. [37] Vassilev Stanislav V, Baxter David, Andersen Lars K, Vassileva Christina G. An overview of the chemical composition of biomass. Fuel 2010;89:913–33. [38] Tsachi L, Ezra BZ, Osvalda S, Piero S. Evolution of reactivity of highly porous chars from Raman microscopy. Combust Sci Technol 2000;153:65–82. [39] Tay HL, Li CZ. Changes in char reactivity and structure during the gasification of a Victorian brown coal: comparison between gasification in O2 and CO2. Fuel Process Technol 2010;91:800–4. [40] Wu SY, Gu J, Zhang X, Wu YQ, Gao JS. Variation of carbon crystalline structures and CO2 gasification reactivity of Shenfu coal chars at elevated temperatures. Energy Fuels 2008;22:199–206. [41] Zaida A, Bar-Ziv E, Radovic LR, Lee YJ. Further development of Raman Microprobe spectroscopy for characterization of char reactivity. Proc Combust Inst 2007;31:1881–7. [42] Zhu XL, Sheng CD. Influences of carbon structure on the reactivates of lignite char reacting with CO2 and NO. Fuel Process Technol 2010;91:837–42. [43] Asadullah M, Zhang S, Min ZH, Yimsiri P, Li CZ. Effects of biomass char structure on its gasification reactivity. Bioresour Technol 2010;101:7935–43. [44] Gregg SJ, Sing KSW. Adsorption, surface area and porosity. London: Academic Press; 1982. [45] Yan J, Zhang Q, Gao J. Adsorption and coacervation. Beijing: Science Press; 1986 (in Chinese). [46] Sing KSW, Everett DH, Haul RAW, Moscou L, Pierotti RA, Rouquerol J, et al. Peporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity. Pure Appl Chem 1985;57:603–19. [47] Groen JC, Peffer LAA, Perez-Ramırez J. Pore size determination in modified micro- and mesoporous materials. Pitfalls and limitations in gas adsorption data analysis. Microporous Mesoporous Mater 2003;60:1–17. [48] Feng B, Bhatia SK. Variation of the pore structure of coal chars during gasification. Carbon 2003;41:507–23. [49] Sadhukhan AK, Gupta P, Saha RK. Characterization of porous structure of coal char from a single devolatilized coal particle: coal combustion in a fluidized bed. Fuel Process Technol 2009;90:692–700. [50] Xu SQ, Zhou ZJ, Gao XX, Yu GS, Gong X. The gasification reactivity of unburned carbon present in gasification slag from entrained-flow gasifier. Fuel Process Technol 2009;90:1062–70. [51] Biagini E, Barontini F, Tognotti L. Gasification of agricultural residues in a demonstrative plant: vine pruning and rice husks. Bioresour Technol 2015;194:36–42. [52] Biagini E, Barontini F, Tognotti L. Gasification of agricultural residues in a demonstrative plant: corn cobs. Bioresour Technol 2014;173:110–6. [53] Simone M, Guerrazzi E, Biagini E, Nicolella C, Tognotti L. Technological barriers of biomass gasification. Int J Heat Technol 2009;27:127–32.