Bioresource Technology 181 (2015) 174–182

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Integrated carbon dioxide/sludge gasification using waste heat from hot slags: Syngas production and sulfur dioxide fixation Yongqi Sun a, Zuotai Zhang a,b,⇑, Lili Liu a, Xidong Wang a,b a b

Department of Energy and Resources Engineering, College of Engineering, Peking University, Beijing 100871, PR China Beijing Key Laboratory for Solid Waste Utilization and Management, College of Engineering, Peking University, Beijing 100871, PR China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Integrated CO2/sludge gasification

Syngas production was achieved using an integrated method of CO2/sludge gasification using hot slags from the steel industry. This method mainly involved a rotary cup atomizer (RCA) system and a gasifying system.

was explored using the waste heat from slags.  The characteristics and the mechanism of syngas release were identified.  The hot slags acted as not only good heat carrier but also effective SO2 fixation.  A conceptual model of CO2/sludge gasification with multi-system was designed.

a r t i c l e

i n f o

Article history: Received 17 November 2014 Received in revised form 13 January 2015 Accepted 14 January 2015 Available online 22 January 2015 Keywords: Sludge gasification Waste heat recovery Hot slag Syngas production SO2 fixation

a b s t r a c t The integrated CO2/sludge gasification using the waste heat in hot slags, was explored with the aim of syngas production, waste heat recovery and sewage sludge disposal. The results demonstrated that hot slags presented multiple roles on sludge gasification, i.e., not only a good heat carrier (500–950 °C) but also an effective desulfurizer (800–900 °C). The total gas yields increased from 0.022 kg/kgsludge at 500 °C to 0.422 kg/kgsludge at 900 °C; meanwhile, the SO2 concentration at 900 °C remarkably reduced from 164 ppm to 114 ppm by blast furnace slags (BFS) and 93 ppm by steel slags (SS), respectively. A three-stage reaction was clarified including volatile release, char transformation and fixed carbon using Gaussian fittings and the kinetic model was analyzed. Accordingly, a decline process using the integrated method was designed and the optimum slag/sludge ratio was deduced. These deciphered results appealed potential ways of reasonable disposal of sewage sludge and efficient recovery of waste heat from hot slags. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Nowadays, numerous advanced routes have been exploited to foster clean and sustainable energy sources and reduce the energy ⇑ Corresponding author at: Department of Energy and Resources Engineering, College of Engineering, Peking University, Beijing 100871, PR China. E-mail address: [email protected] (Z. Zhang). http://dx.doi.org/10.1016/j.biortech.2015.01.061 0960-8524/Ó 2015 Elsevier Ltd. All rights reserved.

consumption in traditional industrial sectors, which is significant not only for energy savings but also for greenhouse gas (GHG) reduction. Amongst the traditional sectors, the steel industry contributes to a large part of energy consumption and GHG emission. Currently, the energy consumption per ton crude steel in China is 15% higher than the international advanced level (Li and Zhu, 2014) and it has been evaluated that the heat recovery from hot slags represents a great potential to reduce energy consumption

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in the steel industry (Barati et al., 2011; Zhang et al., 2013). The untapped molten slags at 1550–1650 °C, carry a high quality thermal energy, and blast furnace slags (BFS) and steel slags (SS) are two kinds of byproducts generated from iron and steel making processes, which account for more than 90% of the waste heat in slags (Barati et al., 2011). In China, the substantial waste heat of BFS and SS (200 million tons and 70 million tons in 2012) is more than the heat of 16 million tons of standard coal; however, the recovery ratio was less than 2% (Cai et al., 2007). Therefore, this study was motivated with the aim of heat recovery from BFS and SS. The thermal conductivity is ranged from 1–3 Wm1K1 for solid slags to 0.1–0.3 Wm1K1 for liquid slags (Barati et al., 2011), which accounted for the fundamental constraint of heat extraction. To meet this challenge, we have analyzed the cooling path of BFS (Sun et al., 2014a) and proposed a multi-stage control method (Sun et al., 2014b), based on which a chemical method at temperatures lower than 950 °C was theoretically reasonable. Amongst the chemical methods, coal gasification and methane reforming have been extensively investigated recently. Li et al. (2012a, 2013) found that BFS acted as an active catalyst for CO2/coal gasification at 1300–1400 °C because of a remarkable increase of the reactivity index. A series of studies (Maruoka et al., 2004; Purwanto and Akiyama, 2006; Shimada et al., 2000) on methane reforming using hot slags were performed and the catalytic effect was also identified at 700–1000 °C and the economic feasibility was evaluated. Recently, Nakano and Bennett (2014) made a pioneering effort to produce CO and H2 by utilizing reaction between CO2/H2O, CaO-rich slags and V2O3-riched slags at 1405–1460 °C. Additionally, Malvoisin et al. (2013) even designed an emerging method to produce H2 from water using the heat at 200–400 °C by means of the Fe2O3 formation from FeO in slags. Meanwhile, more than 6.55 million tons of dry sewage sludge was discharged by urban wastewater treatment plant in China, the timely and effective disposal of which has been a severe environmental issue. Currently, pyrolysis and gasification of dry sewage sludge are considered as the waste-to-clean strategies and have been extensively studied. De Andres et al. (2011a) investigated the air–steam gasification of sludge at 750–850 °C using dolomite, olivine and alumina as catalysts and a syngas composed of CO, H2, CH4 and CO2 was obtained. Roche et al. (2014) studied the air and air–steam sludge gasification and they found that dolomite enhanced the tar decomposition and increased the syngas yield at 800 °C. Nipattummakul et al. (2010) explored the evolutionary behavior of sludge–steam gasification at 700–1000 °C and discovered that sludge yielded more hydrogen than paper and food wastes. Recently, supercritical water gasification of sludge has been intensively investigated because of the high yield of hydrogen in spite of the accompanied operational difficulties (Acelas et al., 2014; Li et al., 2012b; Wilkinson et al., 2012). Besides, Liu et al. (2013) discovered that the calcium in lime-conditioned sludge encouraged the cleavages of CAC and CAH bonds and greatly improved the gaseous production.

These studies provided increasing possibilities of sludge disposal using novel routes. In views of heat recovery from slags, the waste heat could act as heat carrier for gasification and an integrated system was therefore designed. Although the advantageous role of the thermal energy in the slag is obvious, it is not known whether the presence of slag could exhibit a possible catalytic effect or even change the kinetic mechanism of sludge gasification, and therefore the present study was motivated. In this study, both BFS and SS were utilized to perform the sludge gasification; meanwhile, CO2 was used as gasifying agent rather than steam from point of view of carbon capture and storage (CCS). Both the overall characteristics of the sludge gasification and the possible variation of reaction mechanism due to the hot slags were analyzed. 2. Methods 2.1. Sample preparations The sewage sludge sample was supplied from a municipal wastewater treatment plant located in Beijing, China. The results of the proximate and ultimate analyses of the sludge samples are shown in Table 1 in addition with the chemical compositions of the BFS and SS collected from Shougang Corporation, China, analyzed by X-ray fluoroscopy (XRF, S4-Explorer, Bruker). The sludge and slag samples were dried in air at 105 °C for 24 h, crushed and ground to 300 meshes, and then thoroughly mixed using a ball mill. Three samples containing 0.05 g dry sludge were used to conduct the gasification, i.e., the dry sewage sludge (S1), the mixture of sludge and BFS with the mass ratio of 1:1 (S2), and the mixture of sludge and steel SS with the mass ratio of 1:1 (S3). In addition, the chemical composition of the sludge ashes were determined by XRF, as listed in Table 1. 2.2. Experimental apparatus and procedure A series of isothermal gasification experiments were conducted using a fixed bed system, which was composed of a control part of gasifying agent, a tube furnace reactor, a gas condenser and purifier part and a gas analyzer (depicted in Supplemental Fig. S1). A quartz boat was used to hold the samples and placed into a quartz tube reactor, which was externally heated by an electric furnace. Pure CO2 was used to perform the gasification and the flow rate was accurately controlled by a mass flow meter, i.e., 200 ml/min; the gasification temperature was selected as 500–950 °C, which was widely employed for sludge gasification (De Andres et al., 2011a; Nipattummakul et al., 2010; Roche et al., 2014). The tube reactor was first heated with the gasifying agent pumped into the system to fully expel the air inside and as the temperature reached the prescribed point, the quartz boat filled with samples was then placed on the right side of the tube for 20 min to stabilize the temperature and the atmosphere. Then the sample boat was rapidly

Table 1 Characteristics of sewage sludge, blast furnace slags and steel slags. Dry sludge

Proximate analysis (%)

Ultimate analysis (%)

HHV (MJ/kg)

Moisture 1.97

Volatile 36.53

Ash 54.34

Fixed carbon 9.13

C 22.11

H 3.37

O 22.92

N 4.97

S 1.07

9.35

Sludge ash (XRF)

SiO2 34.44

Al2O3 18.14

CaO 13.77

P2O5 13.02

Fe2O3 8.01

MgO 5.69

Na2O 2.43

K2O 2.34

TiO2 0.661

S 0.186

BFS (XRF)

CaO 37.88

SiO2 34.61

MgO 15.62

Al2O3 8.56

S 0.92

Fe2O3 0.6

Na2O 0.54

TiO2 0.45

MnO 0.36

K2O 0.26

SS (XRF)

CaO 41.98

Fe2O3 18.89

SiO2 15.85

Al2O3 8.59

MgO 7.79

MnO 3.15

P2O5 1.19

TiO2 1.17

V 0.453

S 0.283

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pushed into the middle of the tube to make the gasification reaction occur and meanwhile, the volume and compositions of the syngas (CO, H2 and CH4) versus time were detected by a gas analyzer (Testo pro350, Testo).

the FT-IR spectroscopy absorption spectra were recorded in the range of 4000–400 cm1 with a resolution of 2 cm1.

2.3. Characteristics of the sludge ashes after gasification

3.1. Transient behavior of syngas release

To clarify the mechanism of sludge gasification and connect it to the structure of the sludge, the solid wastes after gasification were analyzed by X-ray diffraction (XRD, D/Max 2500, Rigaku) and Fourier transformation infrared (FT-IR) spectroscopy. FT-IR measures were performed by a spectrophotometer (Tensor 27, Bruker), equipped with a KBr detector. To prepare the samples, 2.0 mg solid wastes and 200 mg pure KBr was mixed, grinded in an agate motor and pressed into a disc with 13.0 mm in diameter. Besides,

3.1.1. Characteristics of syngas release The isothermal gasification experiments were performed in the temperature range of 500–950 °C, during which the compositions of the syngas were detected and accordingly the transient behavior of the syngas release was identified. Fig. 1(a)–(d) depicts the transient syngas compositions versus time for sample S1 gasifying at 600–900 °C. Before interpretation, it should be pointed out the overall curve shape of syngas release did not present a significant

3. Results and discussion

Fig. 1. Syngas release in terms of time for sample S1 gasifying at (a) 600 °C, (b) 700 °C, (c) 800 °C, (d) 900 °C, (e) CO gas and (f) Gaussian fittings.

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variation for samples S1, S2 and S3, and therefore only that of sample S1 was presented here. According to Fig. 1, several characteristics related to syngas release could be clarified. First, the dominant gases released during gasification were CO, H2 and CH4 and the increasing temperature enhanced the gas releases. However, no pronounced CH4 release was detected at 500–600 °C, whereas substantial CH4 gas was released at 700–950 °C. This trend was in agreement with a previous study (De Andres et al., 2011b) to some extent and this could originate from two factors. Firstly, the produced CH4 was less during the decomposition of the sludge at low temperatures, while more CH4 was yielded at higher temperature because of the enhanced breakdown of functional groups in the organics. Secondly, the main reactions for CH4 formation were the homogeneous reaction of CO and H2 and the heterogeneous reaction of C and H2, as shown in Eqs. (1) and (2). Since the formed CH4 was taken away by the continuous CO2 flow under a non-equilibrium condition, the positive reaction rate of these two reactions was dominantly enhanced with increasing temperature; thus, profound release of CH4 gas was observed.

2CO þ 2H2 ¼ CH4 þ CO2

ð1Þ

C þ 2H2 ¼ CH4

ð2Þ

Generally, the sludge gasification process could be divided into volatile release stage and char reaction stage (Magdziarz and Werle, 2014; Shao et al., 2013), which was clearly demonstrated by the variation of transient curve of CO release. At 500–600 °C, the transient curve of CO was mainly composed of one peak, which could be attributed to the volatile release stage comprising the decomposition of organics and the formation of char (Eq. (3)). As the temperature increased from 700 °C to 800 °C, a shoulder on the right side of the CO curve became more pronounced and finally evolved into the main peak of the CO curve, which could be due to the char reaction stage comprising the transformation of char into fixed carbon (FC) and the reaction between FC and reactive agent (Eq. (4)). The two-stage reaction style of sludge gasification observed in this study was in agreement with some previous studies (Magdziarz and Werle, 2014; Shao et al., 2013) and Fig. 1(e) further presents the change of the transient evolution of CO prominently. As can be observed, with the increasing temperature, the time with the maximum CO concentration reached was shortened for the both stages. In Addition, the reaction time at 700 °C showed a minimum value because of the gradual occurrence of char reaction and the improving reaction rate of both stages with increasing temperature.

CH1:82 O0:77 ! xCHm On þ ð0:77  nxÞCO þ ð0:45 þ 2x  2nx  0:5mxÞH2 þ ð0:23 þ nx  xÞCH4

ð3Þ

CHm On þ xCO2 ! ðn þ 2xÞCO þ ð0:5m þ 2n þ 2x  2ÞH2 þ ð1  n  xÞCH4

ð4Þ

To further clarify the syngas release process during gasification, we used several Gaussian functions to fit the syngas yield curves (Cai et al., 2013; Chen et al., 2014), especially those of CO gas because of the dominant trend of multiple stages. Fig. 1(f) presents the fitting results for the sample S1 at 850 °C as an example. As can be seen, the transient curve of CO was exactly composed of three Gaussian functions, which indicated that the sludge gasification process could be divided into three stages. The first peak was assigned to volatile release stage; the latter two peaks were attributed to char transformation into FC stage and FC reaction stage, which made up the aforementioned char reaction stage (Chen et al., 2014; Magdziarz and Werle, 2014). Correspondingly, the

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equation of char reaction (Eq. (4)) could be divided into two equations attributed to the two individual stages, as described by Eqs. (5) and (6).

CHm On ! xC þ nCO þ ð0:5m þ 2n þ 2x  2ÞH2 þ ð1  n  xÞCH4 ð5Þ CO2 þ C ! 2CO

ð6Þ

3.1.2. Mechanism of sludge gasification In order to further identify the mechanism of syngas release, especially the variation of the basic functional groups during gasification, the sludge ashes after gasification at different temperatures were analyzed by FT-IR spectra, as shown in Supplemental Fig. S2(a)–(b). The results show some remarkable variation trend including the position and relative intensity of different bands, peaks and shoulders, indicating the variation of the functional groups during gasification (Barroso-Bogeat et al., 2014; Chen et al., 2002; Prokopowicz et al., 2014; Shan et al., 2011). The detail assignments are displayed in Supplemental Table S1. The functional groups could be categorized into three types, overall. The groups related to the organic matters gradually became less pronounced because of the sludge/CO2 gasification while those related to the inorganic matters almost remained constant; meanwhile some new functional groups gradually became more profound. Firstly, for the spectra of the ashes gasified at 500 °C, the peaks associated with the CAH stretching of pyrone and chromene (2923 cm1, 2854 cm1), the C@O stretching of pyrone and carboxylic acid (1730 cm1, 532 cm1), and the C@C stretching of aromatic ring and pyrone (1539 cm1, 1439 cm1) first disappeared, which contributed to the syngas yield during volatile release and the char formation; then the formed char reacted with CO2 and the syngas was further produced. Subsequently, with increasing gasifying temperature, the signals associated with some individual groups in organic matters weakened, especially the band originating from the OAH stretching in polymeric compounds centralized at 3400 cm1 and the band originating from the C@O stretching in 4-pyrone, quinone and sketone centralized at 1650 cm1. Meanwhile, several structural groups finally disappeared at 700 °C because of the relative stability, i.e., the OAH stretching in quinone oximes (2954 cm1 and 2516 cm1), the OAC@O stretching (2353 cm1) and the CAH bending in benzene ring (876 cm1). At the relatively high temperatures, the reaction rate was improved especially when the char/CO2 reaction was vigorously occurring because of the high temperatures and therefore more syngas was released. Secondly, the band located at 800– 1200 cm1 and two peaks centralized at 798 cm1 and 778 cm1 did not showed obvious change with varying temperature, which could be assigned to the stretching of SiO4 tetrahedral in inorganics and the stretching of quartz, as demonstrated by the XRD (Supplemental Fig. S2(c)) and XRF (Table 1) results. Moreover, compared with the foregoing two structural groups, two peaks at 602 cm1 and 564 cm1, became more profound with higher temperature, especially over 700 °C. These two peaks were generally attributed to the bending of PO4 tetrahedral in the inorganic matters (Prokopowicz et al., 2014), which were formed during sludge gasification; the XRD results, shown in Supplemental Fig. S2(c), also identified the presence of some phosphates (KH2PO4 and Ca9Fe(PO4)7). Additionally, Supplemental Fig. S2(c) also displays the existence of CaCO3 in the sludge, which completely disappeared at 900 °C. Summarily, the organic structures related to CAH, C@C and C@O were first gradually broken down during the stages of volatile release and char/CO2 reactions and thereby part of CO and H2 were primarily released in the temperature range 500–700 °C; while at 700–950, the OAH were consumingly broken

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Table 2 Common gas–solid reaction mechanism functions. No Am A1 A2 A3 A4 Sm S1/2 S1/3 S1/4 S2 S3 Dm D1 D2 D3 D4 D5 D6 D7 D8 Cn C2 C3/2

Reaction mechanism Avrami–Erofeev m=1 m=2 m=3 m=4 Shrinking core m = 1/2 m = 1/3 m = 1/4 m=2 m=3 Diffusion model One-dimensional Two-dimensional Three-dimensional Three-dimensional 3-D (anti-Jander) 3-D (ZLT) 3-D (Jander) 2-D (Jander) Chemical reaction n=2 n = 3/2

Differential function: f(x) m1/m

Integral function: F(x)

m(1  x)[ln(1  x)] 1–x 2(1  x)[ln(1  x)]1/2 3(1  x)[ln(1  x)]2/3 4(1  x)[ln(1  x)]3/4 m(1  x)m1/m (1/2)(1  x)1 (1/3)(1  x)2 (1/4)(1  x)3 2(1  x)1/2 3(1  x)2/3

[ln(1  x)]1/m ln(1  x) [ln(1  x)]1/2 [ln(1  x)]1/3 [ln(1  x)]1/3 1(1  x)1/m 1(1  x)2 1(1  x)3 1(1  x)4 1(1  x)1/2 1(1  x)1/3

1/2x1 [ln(1  x)]1 (3/2)(1  x)2/3[1(1  x)1/3]1 (3/2)[(1  x)1/31]1 (3/2)(1 + x)2/3[(1 + x)1/31]1 (3/2)(1  x)4/3[(1  x)1/31]1 6(1  x)2/3[1(1  x)1/3]1/2 (1  x)1/2[1(1  x)1/2]2 (1  x)n (1  x)2 2(1  x)(3/2)

x2 x+(1  x) ln(1  x) [1(1  x)1/3]2 1–2/3x(1  x)2/3 [(1 + x)1/31]2 [(1  x)1/31]2 [1(1  x)1/3]1/2 [1(1  x)1/2]2 (1(1  x)1n)/(1  n) (1  x)11 (1  x)1/21

Fig. 2. Kinetic models of syngas release of various stages for different samples (a) Stage 1 of S1, (b) Stage 2 of S1, (c) Stage 3 of S1, (d) Stage 1 of S2, (e) Stage 2 of S2, (f) Stage 3 of S2, (g) Stage 1 of S3, (h) Stage 2 of S3 and (i) Stage 3 of S3.

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Fig. 3. SO2 release during sludge gasification (a) SO2 release at different temperatures for sample S1, (b) SO2 release at 800 °C and (c) SO2 release at 900 °C.

down with the CAH, C@C and C@O groups, especially that the char/ CO2 reaction occurred vigorously, resulting in a substantial production of CO, H2 and CH4. Furthermore, the variation trend of structures of the sludge ashes agreed well with that of syngas release, especially the drastic change at 700 °C. 3.2. Kinetic mechanism of sludge gasification 3.2.1. Kinetic study of sludge gasification process In this study, the apparent kinetics of sludge gasification was interpreted and generally, the kinetic equation of common type can be described as follows (Tanaka, 1995; Vyazovkin et al., 2011; Xie et al., 2012):

dx ¼ kðTÞf ðxÞ dt

ð7Þ

where x is the conversion degree of syngas, t is time, k is apparent gasification rate constant, T is the absolute temperature, and f(x) is the differential function of reaction mechanism. Alternatively, by rearranging Eq. (7) and integrating, the integral mechanism function F(x) can be deduced as follows (Vyazovkin et al., 2011; Xie et al., 2012):

FðxÞ ¼

Z

x 0

dx ¼ kðTÞt f ðxÞ

ð8Þ

By analyzing the linear relationship between F(x) and t using different mechanism functions, as listed in Table 2 (Li et al., 2013; Tanaka, 1995), the most probable reaction mechanism could be verified.

3.2.2. Models of sludge gasification process After reasonably Gaussian fitting the transient curves of syngas yield, the kinetic mechanism of various stages during gasification could be further identified. The plots of F(x) versus t with the correlation coefficients (R2) are presented in Fig. 2 using various mechanism functions including nucleation growth, chemical reaction and mass diffusions. The results show that the gasification process for these samples could be reproduced by a series of Avrami–Erofeev models. Generally Avrami–Erofeev models were employed to interpret the gas–solid reactions with high content of volatile (Tanaka, 1995; Xie et al., 2012) where the nucleation process was the controlling step of the gasification reaction; the sewage sludge in this study contained 37% volatile, and therefore it was scientific to employ an Avrami–Erofeev model to interpret the sludge gasification process. The experimental data of samples S1 could be appropriately reproduced by an A3 model (m = 3) (Fig. 2a), described by means of Eq. (9). The presence of SS did not change the kinetic model of the sludge gasification for sample S3 (Fig. 2c); however, the kinetic model of gasification for sample S2 changed from the A3 model to the A2 model (Avrami–Erofeev, m = 2) because of the existence of BFS (Fig. 2b), as described by Eq. (10). The mechanism variation by different Avrami–Erofeev models presented the prominent influence of slags on gasification, which was in consistent with the studies of Li et al. (2012a, 2013) on the coal gasification using hot slags. 1=3

ð9Þ

1=2

ð10Þ

FðxÞ ¼ ½ lnð1  xÞ FðxÞ ¼ ½ lnð1  xÞ

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Fig. 4. Effect of reactive CO2 and gasifying temperature on gas yield (a) CO, (b) H2, (c) CH4, (d) total gases, (e) HHV of the yielded gas and (f) experimental gas yield for sample S1.

3.3. Release of SO2 during sludge gasification During the incineration and thermal-chemical process of sludge, the release of hazardous gases gives rise to an important environmental issue, especially that of SO2, which was detected in this study. Fig. 3(a) presents the evolution of SO2 gas for sample S1 with varying gasifying temperature and time. As can be observed, the concentration curve of SO2 was in consistent with that of CO, i.e., with increasing temperature, the SO2 concentration remarkably increased and the maximum value increased from 5 ppm at 500 °C to 164 ppm at 900 °C. This indicated that the break and decomposition of the chemical bonds (C@S and S@O bonds at 1033 cm3) were promoted by the increasing temperature, as

demonstrated by the FT-IR measurements (Supplemental Fig. S2(a)–(b)). The most important and meaningful phenomenon about SO2 release was the substantial reduction of SO2 emission by the hot slags, especially those at high temperatures (800–900 °C). As shown in Fig. 3(b) and (c), the maximum SO2 concentration was pronouncedly reduced from 81 ppm, 164 ppm to 70 ppm, 114 ppm, by the BFS and to 57 ppm, 93 ppm by the SS gasifying at 800 °C and 900 °C, respectively; thus, the SS showed a stronger sulfur fixation because of the relatively higher content of CaO. In fact, the utilization of SS as desulfurizer in wet process has been explored in a previous study (Liu and Shih, 2004), which showed a predominant performance to reduce the SO2 content in the flue

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Fig. 5. Conceptual model of CO2/sludge gasification using hot slags.

gas. Generally 3CaOSiO2 and 2CaOSiO2 are the main mineral phases in the SS, as demonstrated by XRD results in Supplemental Fig. S2(c). The XRD results also demonstrated that these mineral phases could react with the released SO2 to form CaSO3, as described by Eqs. (11) and (12).

ð3CaO  SiO2 Þ þ 3SO2 ¼ 3CaSO3 þ SiO2

ð11Þ

ð2CaO  SiO2 Þ þ 2SO2 ¼ 2CaSO3 þ SiO2

ð12Þ

3.4. Thermodynamics calculation of syngas yield The cumulative syngas yield could be calculated according to the transient concentration and volume of syngas. In order to further theoretically predict the syngas yield, the equilibrium syngas production was calculated by the FactSage software (FactSage 6.3) (Bale et al., 2009) and compared with the experimentally cumulative syngas yield. To simplify the calculation process, the amount of sludge were assumed to be 1 kg, then the reactive organic matter could be deduced as 0.4566 kg with the composition of CH1.83O0.78 according to Table 1; meanwhile, the amount of reactive CO2 and gasifying temperature were employed as two variables. Based on the individual gas yields (CO, H2 and CH4), the higher heating value (HHV) of the product gas per mass of sludge was calculated by means of Eq. (13) (Louw et al., 2014; Xie et al., 2012).

HHV ¼ ð12:63½CO þ 12:75½H2  þ 39:82½CH4 ÞMJ=kgsludge

ð13Þ

where [CO], [H2] and [CH4] are the individual gas yields. Fig. 4 presents the calculated results of yielded gases. It can be noted that with the increase of temperature, the produced CH4 monotonically decreased (Fig. 4c) and the produced H2 first increased and then decreased (Fig. 4b); while the amount of CO remarkably increased (Fig. 4a). Thus, an increasing temperature enhanced the syngas yield monotonically (Fig. 4d), which was in agreement with the experimental date, for example the total gas yield increased from 0.022 kg/kgsludge at 500 °C to 0.422 kg/kgsludge at 900 °C (Fig. 4f) and therefore the HHV of the total gases correspondingly increased (Fig. 4e). It should be pointed out that the amount of syngas did not show visible change with the presence of BFS and SS, and thereby only that of sample S1 was shown here (Fig. 4f). In addition, it can be seen that the amount of CH4 and H2 decreased with increasing reactive CO2 (Fig. 4b and c), while that of CO remarkably increased (Fig. 4a); thus the total gas yield and the HHV of syngas increased with the increase of reactive CO2 (Fig. 4d

and e). However, the increasing CO2 amount enhanced the content of CO2 in syngas, which could cause more difficulties of syngas separation; hence the reactive CO2 should be reasonably adjusted. It should be pointed out that not all the calculated results by FactSage agreed with the experimental data including the relative content of the individual gases because of the simplified assumption by FactSage; the calculated results, however, provided important clues of sludge gasification in terms of the prediction of gas yield and HHV of syngas. First, an increasing CO2 amount contributed to an increase of CO yield, a decrease of CH4 yield and a non-monotonic variation of H2 yield; therefore the reactive CO2 should be adjusted for the purposes of selective products. Moreover, the gasifying temperature and reactive CO2 could be reasonably designed and controlled according to the FactSage results for different kinds of sludge because of the individual compositions. 3.5. Design of decline industrial process and conceptual industrial model The objective of this study was to provide a fundamental clue of sludge gasification using hot slags by employing isothermal experiments; accordingly a non-isothermal industrial process with declining temperature could be further designed and most importantly, an optimum sludge/slag mass ratio could be deduced. As temperature was below 900 °C, the HHV of syngas was lower than that of sludge, and thereby the gasification process could be an auto-thermal process; while as temperature was above 900 °C, the heat for gasification should be externally supplied. Provided a CO2/sludge gasification process was performed in the range of 1000–800 °C, then thermal heat required to be externally supplied could be approximately calculated by the following equation:

Q ext ¼ HHVsyngas  HHVsludge

ð14Þ

Assuming the HHVsyngas was employed of the syngas gasifying at 900 °C, then 2.18 MJ/kgsludge thermal energy should be externally supplied for gasification ignoring the heat to operate the gasifying reactor. Additionally, the heat capacity of hot slags was 1.15 kJ kg1 K1 (Barati et al., 2011; Sun et al., 2014b), and consequently the sensible heat of hot slags was 0.23 MJ/kgslag (denoted as QDT) at 1000–800 °C, which supplied the heat for sludge gasification (Qext). Accordingly the mass ratio of sludge to slags (Rsludge/slag) could be derived as follows:

Rsludge=slag ¼

Q DT Q ext

ð15Þ

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Therefore the calculated optimum mass ratio of sludge/slag was 1:9 for gasification in the temperature range of 800–1000 °C. Furthermore, this ratio could be reasonably adjusted according to the different sludge compositions and gasifying temperature ranges; for example, if the gasifying temperature decreases, the amount of slags supplied can be correspondingly decreased. Based on CO2/sludge gasification and our previous studies (Sun et al., 2014a,b), a conceptual industrial model could be proposed, as presented in Fig. 5, which was composed of a rotary cup atomizer (RCA) system where the molten slags were granulated into small particles in the temperature range of 1550–1000 °C (Barati et al., 2011; Zhang et al., 2013) and a sludge gasification system where CO2/sludge gasification occurred using hot slags in the temperature range of 1000–500 °C. Moreover, it was expectative that a real industrial throughput was quite larger than those tested here, which could result in considerable differences of syngas production and required operations; whatever, the fundamental regularity of CO2/sludge gasification using hot slags was provided in this study, which may pave an integrated method of both waste heat recovery from hot slags and disposal of sewage sludge. 4. Conclusions The thermal heat tapped in blast furnace slags (BFS) and steel slags (SS) could supply the heat for sludge gasification, the feasibility of which was explored in this study. At 500–950 °C, the hot slags could act as not only a good heat carrier but also an effective SO2 fixer. A three-stage reaction was identified during sludge gasification including volatile release, char transformation and FC reaction; meanwhile a kinetic mechanism variation was clarified because of the added hot slags. Additionally, a gas yield of 0.42 kg/kgsludge composed of CO, H2 and CH4 could be achieved by CO2/sludge gasification at 900 °C. Acknowledgement Supports by the National High Technology Research and Development Program of China (863 Program, 2012AA06A114) and Key Projects in the National Science & Technology Pillar Program (2013BAC14B07) are acknowledged. The authors also acknowledge financial support by the Common Development Fund of Beijing and the National Natural Science Foundation of China (51472006, 51272005 and 51172001). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.biortech.2015.01. 061. References Acelas, N.Y., López, D.P., Brilman, D.W., Kersten, S.R., Kootstra, A.M.J., 2014. Supercritical water gasification of sewage sludge: gas production and phosphorus recovery. Bioresour. Technol. 174, 167–175. Bale, C.W., Bélisle, E., et al., 2009. FactSage thermochemical software and databasesrecent developments. Calphad 33, 295–311. Barati, M., Esfahani, S., Utigard, T.A., 2011. Energy recovery from high temperature slags. Energy 36, 5440–5449. Barroso-Bogeat, A., Alexandre-Franco, M., Fernández-González, C., Gomez-Serrano, V., 2014. FT-IR analysis of pyrone and chromene structures in activated carbon. Energy Fuels 28, 4096–4103. Cai, J., Wu, W., Liu, R., 2013. Sensitivity analysis of three-parallel-DAEM reaction model for describing rice straw pyrolysis. Bioresour. Technol. 132, 423–426. Cai, J.J., Wang, J.J., Chen, C.X., Lu, Z.W., 2007. Recovery of residual heat integrated steelworks. Iron Steel 42, 1–6.

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