An enhanced approach for biochar preparation using fluidized bed and its application for H2S removal

Chemical Engineering and Processing 104 (2016) 1–12

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Chemical Engineering and Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep

An enhanced approach for biochar preparation using fluidized bed and its application for H2S removal Yong Suna,b,* , Jing Ping Zhangc, Chao Wend , Lian Zhange a

Edith Cowan University School of Engineering, 270 Joondalup Drive Joondalup WA 6027, Australia Commonwealth Science and Industrial Research Organization (CSIRO), Earth Science and Resources Engineering, 26 Dick Perry Avenue, Kensington, WA 6151, Australia c National Engineering Laboratory of Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China d School of Information Science and Technology, Northwest University, Xi’an 710069, China e Monash University Department of Chemical Engineering, VIC 3800, Australia b

A R T I C L E I N F O

A B S T R A C T

Article history: Received 27 July 2015 Received in revised form 4 October 2015 Accepted 16 February 2016 Available online 18 February 2016

Fluidized bed efficiently intensifies the carbonization step for biochar preparation by significantly reducing carbonization duration down to 6 min at 450
C. The BET specific surface area of the char carbonized at 6 min can reach 60 m2/g. The produced porous biochar possesses both micropores and mesopores. The produced biochars are found to be alkaline. The degree of heterogeneity on the surface of biochar is far more than that of commercial activated carbon. The maximum monolayer adsorption capacity from Langmuir model can reach 25 mg/g at room temperature and ambient pressure by using equilibrium isotherm approximation. The maximum sulfur removal can reach 70 mg/g using column dynamic adsorption. The moisture shows a beneficial effect on the adsorption of H2S on biochar. The maximum removal rate and saturation constants were obtained using Mechael’s—Menten type equation. The mass transfer diffusivities data indicate the surface diffusion being significant to the effective diffusivity at experimental conditions. The dynamic experimental results indicate a good performance of resultant biochar in H2S removal at room temperature. ã 2016 Elsevier B.V. All rights reserved.

Keywords: Biochar Fluidized bed H2S removal Black liquor

1. Introduction Due to the current challenging goal of CO2 emission reduction from 2020 onward [1], the practical solutions of employing biorenewable resources have drawn many attentions recently [2]. Among current technologies of utilization of bio-renewable resources, production of biochar is found to have huge mitigation effect on climate change, the relevant researches for preparation of biochar and its applications into the environmental areas have been growing steadily [3]. The properties and functions of biochars are highly depending on the feedstock materials and preparation conditions. In terms of preparation conditions, the conventional approaches such as slow pyrolysis [4], hydrothermal carbonization [5], have drawn many attentions due to advantages of relatively higher solid yields, easiness of processing wide range of feedstock etc. However, these processes are generally conducted at fixed bed

* Corresponding author at: Edith Cowan University School of Engineering, 270 Joondalup Drive Joondalup WA 6027 Australia. E-mail address: [email protected] (Y. Sun). http://dx.doi.org/10.1016/j.cep.2016.02.006 0255-2701/ ã 2016 Elsevier B.V. All rights reserved.

reactors or batch reactors, and the obvious drawbacks of relatively longer processing time (usually a few hours or a few days), relatively poor mass and heat transfer limit their large scale utilization of feedstock. The fluidized-bed reactor is thought to yield a uniform product due to its efficient heat and mass transfer that minimizes temperature variation and ensures good mixing. These advantages are so compelling that the application of a fluidized bed to biomass fast carbonization can be very attractive for producing biochar. The reports concerning with preparation of biochars by fast carbonization using fluidized bed, to our best knowledge, are still very limited. This is one of our motivations for this work. With growing demand of paper pulp worldwide, production of paper pulp from agricultural wastes and by-products are regarded as one of the most profitable approaches for high-value conversion of biomass, and the pulping of non-wood biomass is a technical alternative to mitigate the crisis for the shortage of domestic wood and forestry products in Australia [6]. However, the major technical hurdle of non-wood pulping is its severe environmental pollution of the BL (Black liquor) due to poor performance of its alkali recovery and separation of biomass from the BL [7]. In our

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Y. Sun et al. / Chemical Engineering and Processing 104 (2016) 1–12

Nomenclature c cp cb c0 De Dk Dm Dp Ds DZ DH koverall KL l ms M ns Nm* P Q q qs qm* R rp S t tb T u U

vA

vB V

Concentration of adsorbent in the gas phase (mmol g1) Average concentration in the pores of the adsorbent (mmol g1) Breakthrough concentration (ppm) Initial concentration (ppm) Effective diffusivity (m s1) Knudsen diffusivity (m s1) Molecular diffusivity (m s1) Pore diffusivity (m s1) Surface diffusivity (m s1) Axial diffusivity (m s1) Isosteric enthalpy (kJ mol1) Overall mass transfer coefficient (s1) Langmuir constant (L mg1) Length of bed (m) Mass of adsorbent (kg) Gas molecular weights Average adsorbent loading (mmol g1) Adsorption saturation capacity (mmol g1) Pressure (kPa) Volume flow rate (m3 s1) Concentration of adsorbent in the adsorbed phase (kg mol1) The average concentration of adsorbent in the adsorbed phase reaching equilibrium (kg mol1) Maximum monolayer adsorption capacity from Langmuir (kg mol1) Pressure gases constant (0.00831 kJ mol1 K1) Radius of adsorbent (m) Mass balance source term (mol s1) Time (s) Breakthrough time (min) Temperature (K) Superficial fluid velocity (m s1) Kinetic analysis constant Molecular volume of A (m3) Molecular volume of B (m3) Volume of packing (m3)

Greek characters a Specific surface area per unit bed volume (m1) b Conversion coefficient eb Porosity of the bed rs Adsorbent density (kg/m3) t Tortuosity factor m Viscosity of the fluid (Pa s1)

recent work, biomass was separated by flue gas CO2 precipitation together with continuous hydrothermal process on a very costeffective and energy-efficient manner, which shows a promising separation approach comparing with the conventional approaches such as multi-stage evaporation and subsequent alkali recovery [8]. In this work, the biochar being prepared from BL using fluidized bed fast carbonization, to the best of our knowledge, has been rarely published before. Hydrogen sulfide is one of the most common compounds that can be found in the petrochemical processing plants, feedstock of syngas [9], wastewater treatment plants [10]. The efficient removal of H2S has been a strategic issue in relation to many industrial processes such as gas to liquid (GTL) process [11]. Taking the FTS

(Fischer–Tropsch Synthesis) for example, the presences of hydrogen sulfide in the syngas in a few ppm, the deactivation of catalyst, especially for Cobalt based catalyst, will occur in a few days, sometimes even in a few hours. Among various sulfur removal techniques, the physisorption using carbon based adsorbents at ambient temperature and pressure is found to be very economical and cost-effective for medium and small-scale sulfur removal units such as for compact catalytic process and residential fuel cell system [12]. Although there are many reports concerning with the application of biochars to the environmental researches such as water pollutant removal, soil slow-release fertilizer [13], reports of using biochar for sulfur removal are very limited. This is another objective of this work. In this work, the fluidized bed is employed to intensify the carbonization step. The effects of carbonization duration and temperature in fluidized bed upon physiochemical characteristic properties (in terms of specific surface area and acidity) of biochar, dynamic adsorption performance of biochar by varying the operational parameters (such as initial H2S concentration, relative humidity in feed gas), and kinetic modeling of resultant biochar for H2S removal in the adsorption column were closely investigated. 2. Experimental 2.1. Preparation of biochar The detailed procedures for precipitation of biomass from black liquor by flue gas could be found in our previous work [8]. The characterization of the BL is shown in Table 1. The obtained BL presents relatively high carbon content, relatively low sulfur and ash content indicating a good precursor for the subsequent thermal treatment. BL (approximately 10 g) was carbonized at 450
C for different duration in fluidized bed reactor using 8 SLPM (in order to fully fluidize feedstock) hot simulated flue gas (preheated at 450
C). The detailed configurations, dimensions and operational parameters of fluidized bed reactor can be found in our previous works [14]. The biochar carbonized at 450
C for 1, 2, 4, 6 min are denoted as BL-450-1, BL-450-2, BL-450-4, and BL-450-6, respectively. The commercial pelletized activated carbon is employed for performance comparison and denoted as AC, which was purchased from Lusefeng commercial activated carbon Co Ltd. The biochar carbonized at 450
C for 6 min in fixed bed reactor is denoted as BLF-450-6. 2.2. Equilibrium and kinetic adsorption of H2S The isotherm of H2S is measured by gravimetric methods using B.E.T apparatus with the spring balance at 298 K, 308 K and 318 K respectively at ambient pressure.

Table 1 Physicochemical characteristic of BL. Property

BL

Moisture (%) volatile matter (%) Fix carbon (%) Ash N (%) C (%) H (%) S (%) O (%) (estimate by difference) Cellulose Hemicellulose Lignin

15 35 25 5 0.7 45 5 1 48.3 10 28 55

Y. Sun et al. / Chemical Engineering and Processing 104 (2016) 1–12

The detailed information of kinetic adsorption rig could be found in our previous works [15]. The dimension of the adsorption bed is the 12.7 mm ID and 200 mm in length immersed in a thermostatic bath. The simulated H2S gas was passed through the reactor for H2S removal. The cylinder gas of H2S (8000 ppm) and 5.0 Nitrogen (are supplied by Scott Gas Company) are used for simulated gas feed and pressurized at 3 bar. The humidity was controlled by choosing the corresponding water vapor saturation temperature. The H2S concentration was varied from 50–1000 ppm in the adsorption system. The H2S stream constitutes were analyzed by the gas chromatograph (HP 6890) equipped with a pulsed flame photometric detector (PFPD). The internal reference and Quality Control (QC) were conducted to ensure acceptable experimental data performance. The biochar sample was pelletized and seized with average diameter of 1 mm (0.85–1.8 mm) for column adsorption test. The total flow ranged from 0.2–1 SLPM. The packing length of the bed is 100 mm. The adsorption capacity could be calculated by integrating the area above the breakthrough curve for a given inlet H2S concentration, mass of adsorbents and flow rate as the followings:  Z tb  MC0 Q C 1  b dt ð1aÞ qm ¼ 1000ms 0 C0

3

Content of lignin, cellulose, and hemicellulose were analyzed by standard Van Soest and Klason lignin analysis. All experiments were conducted with three replicas with experimental uncertainties being less than 5%. 2.4. Mathematical calculation The kinetic adsorption by using porous adsorbent is a system that fluid flow through the porous media, which involves the mass balance in both gas-phase and pore phase. The following assumptions were made to construct the model equations: a The system is an isothermal system. b The reactor is plug flow reactor. c The mass transfer resistance across the boundary layer of the solid particle is controlled by external film mass transfer. d Intraparticle mass transport is determined by effective pore diffusion. e Instantaneous equilibrium between bulk and pellet concentrations. Mass balances for gases in the bed of porosity eb resulted in the following equation:

@c @c @2 c þ u  Dz 2 ¼ R @t @z @z

2.3. Characterization of biochar

eb

The specific surface area and porosities of the biochar samples were determined by nitrogen gas adsorption at 77 K at a saturation pressure of 106.65 kPa using a Micromeritics ASAP 2020 Automated Gas Sorption System. The sample was degassed (106 Torr) overnight at 350
C. The BET surface area was assessed within the range of relative pressures from 0.05 to 0.3. The total pore volume was calculated by measuring the N2 adsorbed at a relative pressure of 0.99. The pore size distribution of sample is analyzed by DFT (density function theory) by assuming slit pore geometry.

where c is concentration of adsorbent in the gas phase, Dz is the axial dispersion coefficient, u is the superficial fluid velocity. The terms on the left hand side are transient, flux and diffusion flux. The term on the right hand side is the source term, which represents the diffusion of the gas from bulk phase to the surface of the pores and is calculated as:

2.3.1. FT-IR analysis The Spectrum GX (Tensor-27) infrared spectrometer was used for the study of the surface functional groups. Disc was prepared by mixing the 0.5 mg sample with 200 mg of KBr (Merck, for spectroscopy) in an agate mortar and then pressing the result mixture at 2 Mpa for 1 min. The samples were scanned in the spectra range of 4000–370 cm1. 2.3.2. Raman spectroscopy The Raman Spectra (Spectrum GX) Perkin Elmer, Laser: Nd: YAG 1064 nm equipped with an air cooled CCD (charge coupled device) detector for signal recording. The samples were scanned in the spectra range of 100–3600 cm1 SEM morphology Surface morphology was examined using a JSM-7001F + INCA X-MAX Field emission electron microscope.

R¼½

ð1bÞ

1  eb akoverall  ðc  cp Þ

eb

rs

ð1cÞ

where c and cp are the concentration of adsorbent in the gas phase and the average concentration in the pores of the adsorbent, respectively, a is the specific surface area per unit bed volume, rs is the adsorbent density and koverall is the overall mass transfer coefficient. The initial and boundary conditions appropriate for the breakthrough curve simulation are: cðt ¼ 0Þ ¼ 0; cp ðt ¼ 0Þ ¼ 0 c0 þ

@c j ¼0 @z z¼0

@c j ¼0 @z z¼1

koverall can be calculated as the sum of film and pore mass transfer coefficients. The Sherwood number Sh is employed to estimate the film coefficient as follows [16]:     2k rp m 1=3 2ra u 1=2 ð1eÞ Sh ¼ overall 2 þ 0:6 Dm m rDm

2.3.3. pH measurement The pH was measured in deionized water (Milli-Q water) with liquor to solid ratio of 4. The sample is thoroughly mixed and then equilibrates for about 1 h. The pH was measured by pH meter.

where rp is radius of adsorbent, Dm is molecular diffusivity, u is superficial gas velocity, m is viscosity of the fluid. The correlation between pore and molecular diffusivity can be calculated as the followings:

2.3.4. Elemental analysis Flash EA 1112 (Thermo Scientific) elemental analyzer to analyze sample which is decomposed at 950
C with helium as carrier gas.

Dp ¼

2.3.5. Thermogravimetric analysis TG was performed on a Shimadzu TGA-50 under a nitrogen (Q = 20 ml/min) atmosphere at ramping rate of 10
C min1.

Dm

t

ð1fÞ

where Dp is pore diffusivity, t is the tortuosity factor. And Dm could be calculated as the followings:   1:0  103  T 1:75 1 1 1=2 þ ð1gÞ Dm ¼ h i P 1=3 P 1=3 2 MA MB P ð vÞA þ vÞB

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Y. Sun et al. / Chemical Engineering and Processing 104 (2016) 1–12

P P where ð vÞA and ð vÞB are molecular volume of the gases A and B, respectively, MA and MB are the gas molecular weights. Then Knudsen diffusivity DK is used to calculate the diffusivity per unit cross-sectional area of the pores: rffiffiffiffiffi T Dk ¼ 9700rp ð1hÞ M And the surface diffusivity can be obtained from the followings: koverall ¼ Dk þ

1  ep

ep

ð1iÞ

KDs

The mass balance within a differential radial section of an adsorbent particle is the following [17]:

@ cp 1 ¼ @t ½1 þ rs ð1e ep Þ@q=@cp De ð@2 cp =@r2 þ 2=r@cp =@rÞ

ð1jÞ

following:

@c Nm c Sa ¼  b @z K s þ c F

ð1nÞ

where Nm is maximum removal rate (mg g1h1), Ks is the saturation constant (ppm), Sa is cross section of the bed surface area (m2), F is flow rate, and the corresponding conversion coefficient b is calculated as the followings:

b¼

½22:4 þ ðT=273Þ  103 ms  32 V

By integrating (1n) together with the boundary conditions, c ¼ c0 ; z ¼ 0 c ¼ ce ; z ¼ l

ð1pÞ

One can get the following form:

p

Sa b Ks 1 1 ¼ þ  Fðc0  ce Þ Nm ðc0 þ ce Þ=lnðc0 =ce Þ Nm

The boundary conditions are the following:

@cp j ¼0 @r r¼0 De ep @cp jRp ¼ koverall ðc  cp jr ¼ Rp Þ @r

ð1oÞ

ð1qÞ

By setting U and cin as the followings: Flðc0  ce Þ Sa b ðc0  ce Þ ¼ lnðc0 =ce Þ

U¼ cin

The initial conditions are the followings: c ¼ 0; 0  z  l ðt  0Þ

ð1rÞ

The linearized form can be achieved as the followings:

c ¼ c0 ; z ¼ 0 ðt > 0Þ

cin Ks c ¼ þ in U Nm Nm

cp ¼ 0; 0  z  l ðt  0Þ

ð1sÞ

q ¼ 0; 0  z  l ðt  0Þ 3. Results and discussion The MOL (method of line) is applied for numerical solving the coupled PDEs of Eqs. (1b) and (1j). The finite volume method was applied to discretize the space l and r of PDEs of Eqs. (1b) and (1j), then the resulting ODEs were integrated using backward differentiation formulas method. The GA (Genetic Algorithm) is employed to optimize the unconstrained nonlinear optimization problems. Then De was obtained by fitting the mathematical model to the experimental breakthrough curves. These approximations were estimated by minimizing an objective function defined in Eq. (1m): " #2 Nexp  exp X mi  mcal i ð1mÞ F obj ¼ mexp i i¼1 The Michael–Menten-type equation [18] was employed for kinetic analysis of H2S adsorption on the produced biochars as

3.1. Characterization and optimization of producing biochars during fluidized bed carbonization The TG analysis curve of BL was shown in Fig. 1. It indicates that the weight loss is divided into two general stages. These stages correspond to the elimination of fixed water around 200
C (stage 1), and the degradation and elimination of organic polymers (hemicellulose, cellulose and lignin) around 350–450
C (stage 2) [19]. In this work, based upon the TG analysis of BL, the 450
C was chosen as carbonization temperature in fluidized bed. Fig. 2 shows pressure drop and burn-off rate of BL as a function of carbonization time in the fluidized bed at 450
C. It shows that the burn-off rate does not change appreciably after the first 6 min of carbonization, neither does the pressure drop. This means that after the volatile matter had been removed during the first 6 min of carbonization,

-0.0004 o

400 C

100

-0.0006

Weight loss

Weight/%

-0.0010

o

200 C -0.0012

60

-0.0014 40

-0.0016 -0.0018

20

-0.0020

dW/dT 0

0

200

400

600

800

-0.0022 1000

o

Temperature/ C Fig. 1. Thermogravimetric differential analysis curve of BL.

dW/dT (mg/K)

-0.0008

80

Y. Sun et al. / Chemical Engineering and Processing 104 (2016) 1–12

5

5000 Pressure drop Burn off rate

70

4500

4000

50 40

3500

Burn off/%

Pressure drop /Pa

60

30 3000 1

2

3 4 5 Carbonization time/min

6

20

Fig. 2. Pressure drop and burn off as function of carbonization time.

characteristic adsorption peaks of the valence vibration of CH groups, the valence vibration of C O bond, deformation vibrations of C H groups [23]. As carbonization proceeds, the intensity of those bands decreased. This is due to the organic volatile material removal during carbonization process. The band at 1600 cm1 represents stretching vibration of aromatic groups, this band significantly decrease as the carbonization progress. Raman spectroscopy of the resultant biochars was shown in Fig. 6. Clearly, both graphite band (G) at about 1600 cm1 and disordered carbon band at about 1350 cm1 [24] appear in spectroscopy. The ratio of the intensity of D to G displays the crystallographic structure of the chars. As carbonization proceeds, the ratio slightly increases indicating the slightly increase of the fraction of graphite carbon in the chars. This result also suggests that the intensive carbonization in fluidized bed during the first 6 min can effectively remove the volatile and amorphous carbon material from the precursor. The SEM morphology of BL-450-1 and BL-450-6 is shown in Fig. 7a and b, respectively. Both of the produced biochars present pore formation. As carbonization proceeds and more volatile material being removed, the BL-450-6 shows a better pore formation than that of BL-450-1. This was also confirmed from the BET specific surface area analysis, which shows that the surface of BL-450-6 is around 60 m2/g, while surface area of BL-450-6 is less than 10 m2/g.

the volatilization largely reduced. The effect of carbonization time upon the BET specific surface area of biochar and their corresponding N2 adsorption isotherms are shown in Fig. 3. The result indicates that the carbonization treatment increases the surface area when the carbonization duration exceeds 2 min, the maximum specific surface area of the char reaches around 60 m2/g. In terms of nitrogen adsorption isotherms, according to Brunauer–Deming–Deming–Teller (BDDT) classification [20], BL-450-6 exhibits a type IV isotherm. The adsorption happens at entire pressure range indicating the existence of both micropores and mesopores. The nitrogen adsorption isotherm of BL-450-6 is greater than that of BL-450-1 indicating the increased surface area is mainly from removal of volatile materials during the carbonization of BL in fluidized bed reactor. The NLDFT (non-linear density functional theory) pore size distribution of BL-450-6 is shown in Fig. 4. The results indicate that BL-450-6 possesses wide pore size distribution, which distributes in the microporous region (0.6–2 nm) and mesoporous region (2–50 nm). The FT-IR spectroscopy of different biochars is shown in Fig. 5. The reduction of the peak intensity of 1070 cm1 (characteristic C O stretching of carbohydrate substance) and 1470 cm1 (CO phenolic, carboxylic, and alcoholic groups) [21,22] can be observed. The spectra of BL-450-1 present bands at 2800, 1070, 840 cm1, which are

20

15 40

BL-450-6 BL-450-4 BL-450-2 BL-450-1

3

-1

Volume (cm g )

2

BET Specific Area/(m /g)

60

20

10

5

0 1

2

3

4

5

6

7

Carbonization time/min

0

0.2 0.4

0.6

0.8

1.0

(P/Po)

Fig. 3. BET Specific surface area as a function of carbonization time and N2 adsorption isotherms of different biochar.

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Y. Sun et al. / Chemical Engineering and Processing 104 (2016) 1–12

BL-450-6

3

Incremental pore volume/(cm /g )

0.008

0.006

0.004

0.002

0.000 4

8

12

16

20

24

28

32

36

40

44

48

Pore width/nm Fig. 4. DFT pore size distribution in microporous and mesoporous region of BL-450-6.

830

1070

1460

3400

1600

Adsorbance/%

BL-450-1 BL-450-2 BL-450-4 BL-450-6

500

1000

1500

2000 2500 3000 -1 Wavelength/cm

3500

4000

Fig. 5. FTIR spectroscopy of biochars at different carbonization time.

The physiochemical characterization of prepared biochars, commercial AC, and BLF-450-6 (BL was carbonized at 450 for 6 min in fixed bed reactor) are summarized in Table 2. The pH value and fixed carbon of biochar produced from fluidized bed all presents a

gradual increased pattern as carbonization proceeds. This is mainly due to the removal of volatile organic compounds and the surface acidic functional groups (such as  COOH and  OH), which were confirmed by TG and FT-IR analysis. In addition, the corresponding

1800 D Band 1600

BL-450-6 BL-450-4 BL-450-2 BL-450-1

G Band

1400

Intensity

1200 1000 800 600 400 200 0

500

1000

1500

2000

2500

3000

3500

4000

-1

Raman Shift/cm

Fig. 6. Raman spectroscopy of biochar at different carbonization time.

Y. Sun et al. / Chemical Engineering and Processing 104 (2016) 1–12

7

Fig. 7. Scanning electron microscopy images of biochar, where (a) refers to BL-450-1; (b) refers to BL-450-6.

gradual increase of sulfur content is also observed indicating the increased sulfur uptake by the prepared biochar when sulfur was adsorbed by the prepared biochar in adsorption column test. Overall, the specific surface area of BL-450-6 (60 m2/g) is larger than that of BLF-450-6 (<10 m2/g). The pH value of BL-450-6 (10) is higher than that of BLF-450-6 (8.5), and the carbonization duration (6 min) is shorter, comparing with the conventional slow pyrolysis. These improved performances all indicate the efficient intensification of carbonization by fluidized bed. However, one needs to keep in mind that this improved efficiency is based upon the compromise of the significant increase of burn-off rate of the resultant char. In this work, the optimal biochar preparation condition was chosen to be at 450
C for 6 min in fluidized bed. The BL-450-6 will be used for further H2S equilibrium and dynamic adsorption test. 3.2. Equilibrium adsorption of H2S on biochars In order to estimate isosteric enthalpy of biochar for H2S adsorption, the adsorption isotherms for H2S onto BL-450-6 and AC

are obtained and shown in Fig. 8. The Langmuir model is used to correlate our experimental low pressure equilibrium data. q¼

qm K L P ð1 þ K L PÞ

ð2Þ

where qm is the adsorption saturation capacity (mmol/g), KL is the constant in Langmuir equation. The constants obtained from the Langmuir model are listed in Table 3. The maximum monolayer adsorption capacity of BL-450-6 at 293 K could reach 25 mg/g. Although the specific surface area of AC is larger than that of BL450-6, the H2S equilibrium adsorption amount of BL-450-6 is much larger than that of AC in all experimental temperature range. This indicates that the removal of hydrogen sulfide by biochar is not dominated by the simple pore filling physisorption such as inert N2 adsorption on the adsorbent. There are more complicated adsorption mechanisms occurring on the surface of biochar during adsorption. The enthalpy of adsorption is a significant property for characterization of the type of adsorption and degree of heterogeneity of adsorbent surface. The isosteric enthalpies of adsorption were not measured experimentally in this paper but are

Table 2 Physicochemical characteristic of the different biochars and AC, SSA refers to BET Specific surface area. O* content is calculated by difference. BLF-450-6 refers to biochar prepared from carbonization of BL in fixed bed reactor for 6 min. Property

Commercial AC

BLF-450-6

BL-450-1

BL-450-2

BL-450-4

BL-450-6

pH Weight loss (%) Ash content (%) N (%) C (%) H (%) S (%) O* (%) SSA/m2/g

7 – 50 0.6 55 4.5 3 36.9 810

8.5 10 48 0.5 49 6.5 2 42 8

9.1 30 42 0.5 45 5 3 46.5 10

9.5 50 42 0.4 50 5 4 40.6 20

10 60 43 0.4 55 5 6 33.6 40

10 75 46 0.4 60 5 7 27.6 60

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Y. Sun et al. / Chemical Engineering and Processing 104 (2016) 1–12

10

298 K 328 K 358 K

3.0

8

Adsorption/mg.g

-1

2.5 2.0

6

1.5 4 1.0 2

0.5

BL-450-6

AC 0.0

0

40

0

80

120 0 40 H2S pressure/kPa

80

120

Fig. 8. Equilibrium adsorption of H2S on AC and BL-450-6.

estimated from the low pressure adsorption equilibrium data by using the Clausius–Clapeyron equation:   dlnP DH ¼ RT 2 ð3Þ dT n By using the low pressure adsorption equilibrium data, we calculated the isosteric enthalpy of adsorption of BL-400-6 and AC for H2S, which is shown in Fig. 9. The isosteric enthalpy of adsorption of both BL-400-6 and AC decrease with increase of loading below 0.04 mmol/g. Comparing with AC, a much more steeper isosteric enthalpy drop of BL-400-6 with low loading is observed, this suggests that there are more energetic sites on BL450-6 at low loading being filled preferentially by H2S than that on AC surface during the initial adsorption. The different isosteric enthalpy drop pattern could be possibly due to different pH value on BL-450-6 and AC, of which the BL-450-6 is around 10 while it is almost neutral on AC. As adsorption proceeds, less active sites are filled. The steep decrease of isosteric enthalpy with increase of loading of BL-450-6 in adsorbing H2S indicates that the degree of heterogeneity on the surface of BL-450-6 is far more than that of AC. The isosteric enthalpies of BL-450-6 for H2S adsorption is approximately from 30 to 50 kJ/mol, which is slightly higher than that of physisorption of H2S on carbonaceous adsorbent (10–45 kJ/mol) [25] but much lower than that of chemisorption (over 100 kJ/mol) [26]. 3.3. Dynamic adsorption of H2S on biochar 3.3.1. H2S breakthrough test in dry conditions The H2S breakthrough curves for different biochars and AC are shown in Fig. 10 under dry conditions with inlet H2S concentration maintained at 600 ppm. The breakthrough time for AC was short, indicating its relative poor performance in H2S removal. This also

Table 3 Constants of Langmuir model from adsorption of H2S on BL-450-6. BL-450-6 gas

Temperapure (K)

KL (m3/mg)

nm (mg/g)

R2

H2S

293 303 313

1.2 1.0 0.9

25 15 9

0.99 0.98 0.99

agrees well with the equilibrium adsorption data. The adsorption capacity of BL-450-6 could reach 70 mg/g, while the AC could only reach around 20 mg/g. It has been widely accepted that the local pH within the pore system of biochar dominates the rate of initiation of molecular H2S on the surface of chars. With neutral or acidic pH, the initiation and dissociation of H2S is significantly inhibited, which consequently suppresses its rate of oxidative reaction from H2S to sulfur [27]. Since the surfaces on the biochar are caustic, it catalyzes the oxidative reaction, which in turns increases its H2S removal. This could explain the discrepancy of AC with large specific surface area but possesses relatively poor H2S removal performance. The kinetic analysis of biochar (BL-450-6) and AC were performed after 1 h by increasing H2S concentration from 50 to 800 ppm at the constant 400 h1 GHSV (gas hour space velocity). The relationship between cin/U and cin is shown Fig. 11. The obtained parameters are listed in Table 4. The overall removal rate was determined by both Ks and Nm, of which was dependent on adsorbent. The removal rate of BL-450-6 is clearly superior to that of the commercial AC indicating the advantages of this fast carbonization approach. 3.3.2. H2S breakthrough test in wet conditions One of the advantages of using carbonaceous adsorbent system for sulfur removal is its enhanced capability in presence of water, whereas in other system such as ZnO system, the presence of water in the system will undermine the H2S removal capability [28]. In this work, the experiments were carried out to investigate the effect of the RH (relative humidity) on the dynamic adsorption on BL-450-6 and AC. In Fig. 12, the breakthrough tests were performed with RH = 20% for BL-450-6 and AC. The BL-450-6 also shows longer breakthrough time. The effects of RH (varied from 0 to 60% at 298 K) upon dynamic adsorption of BL-450-6 were investigated and the results were shown in Fig. 13. The rapid breakthrough is observed at dry condition when compare with the BL-450-6 at different RH. By increasing RH, the extended breakthrough time is observed. This indicates that the existence of moisture is beneficial to the adsorption of H2S on BL-450-6. These results therefore, imply that in the presence of water, different mechanisms might affect the adsorption and reaction path simultaneously as following: (i) the catalytic reaction may take place in the water phase (faster than on the catalyst surface) within the pores of biochar by dissolving O2 and H2S in a water film [29]; (ii) the presence of water slows down the deactivation process [30].

Y. Sun et al. / Chemical Engineering and Processing 104 (2016) 1–12

9

55000

Isosteric enthalpy/ (J/mol)

50000 45000 40000 35000

AC

30000

BL-450-6

25000 20000 0.04

0.06

0.08

0.10

0.12

0.14

Amount absorbed/mmol/g Fig. 9. Isosteric enthalpies of adsorption with respect to surface loading on AC and BL-450-6.

1.0

0.8

Ct/C0

0.6

0.4 AC BL-450-1 BL-450-2 BL-450-4 BL-450-6

0.2

0.0

0

50

100

150

t/min Fig. 10. Breakthrough curves of H2S on different biochars at 298 K with 600 ppm at dry condition.

4500

BL-450-6 AC

4000 3500 3000 Cin/U

2500 2000 1500 1000 500 0

0

100

200

300

400

500

Cin Fig. 11. Kinetic analysis of H2S removal by BL-450-6 and AC at 298 K at dry condition.

10

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Table 4 Removal capacity of H2S by stationary bed and kinetic analysis parameters for AC and BL-450-6. Adsorbent

Removal capacity (mg g1)

Nm (mg g1 h1)

Ks (ppm)

R2

AC BL-450-6

19 73

1 101 3  101

7  101 9  101

0.98 0.98

Table 5 Calculated diffusivity of H2S on BL-450-6 system at 298 K. Diffusivity (cm2/s)

3.4. Mass transfer analysis and simulation study At ambient temperature, the different calculated diffusivities of H2S on BL-450-6 are summarized in Table 5. The De does not follow the proportional increase pattern as inlet H2S concentration increases. In Fig. 14, the experimental and the calculated results generally in good agreement at the initial zone of the breakthrough curves, with slight deviation in the tailing zone, this is especially obvious in high H2S inlet concentration. The small slope at the end of the breakthrough curve indicates the adsorbent is approaching equilibrium. The calculated surface diffusion at experimental condition is slow, thus the surface diffusion has significant effect on the effective diffusivity during mass transfer. This result agrees with our assumption that both the adsorption and chemical reaction rate are faster than that of intraparticle diffusion. The

Concentration of H2S (ppm)

De

Dm

Dp

Ds

300 500 700 1000

5  108 6  108 7  108 1.5  107

2  101 2  101 2  101 2  101

1 104 1.6  104 1.9  104 2.2  104

5  108 5  108 6  108 1 107

assumption of adsorption and reaction being quasi-equilibrium are reasonable and appropriate. 3.5. Evaluation of the adsorption system using biochars Due to limited numbers of reports and description of detailed dynamic adsorption conditions on the hydrogen sulfide removal by biochar adsorption, we only make a very limited number of comparisons of H2S dynamic adsorption among different carbonaceous adsorbents and the results are shown in Table 6. It shows that the physical porosity does not play important role in H2S removal, though the specific surface of produced BL-450-6 is much lower than that of commercial ACs. The H2S removal capacity in the

1.0 BL-450-6 AC 0.8 RH=20% Ct/C0

0.6

0.4

0.2

0.0

0

100

200

300

400

500

600

700

t/min Fig. 12. Breakthrough curves of H2S on AC and biochars at 308 K with 200 ppm with RH = 20%.

1.0 RH=0 RH=20% RH=40% RH=60% RH=80%

Ct/C0

0.8

0.6

0.4

0.2

0.0

0

100

200

300

400

500

600

700

t/min Fig. 13. Breakthrough curves of H2S on BL-450-6 at 308 K with 200 ppm.

Y. Sun et al. / Chemical Engineering and Processing 104 (2016) 1–12

1.0

11

1000 ppm 700 ppm 500 ppm 300 ppm

0.8

----Calculation

Ct/C0

0.6

0.4

0.2

0.0

0

50

100 150 200 250 300 350 400 450 500 t/min

Fig. 14. Simulation of the breakthrough curve of BL-450-6 at different H2S concentration (temperatures 293 K, under dry conditions).

Table 6 Comparison of adsorption capacity, where CP8PA refers to Ac prepared from coffee waste by using KOH as activator, CP8PC refers to AC prepared from coffee waste by using CO2 as activator, BL-900 refers to AC by using steam as activator. Sample

SSA (m2/g)

GHSV (h1)

Inlet H2S concentration (ppm)

Adsorption capacity (mgg1)

Carbonization duration

Reference

BL-900 Biochar Commerical Ac CP8PA CP8CA Pig manure biochar Sewage slug char Commerical Ac BLF-450-6 BL-450-6

1010 110 732 2000 20 47 105 810 10 60

1000 380 7200 9000 9000 3400 – 2000 2000 2000

900 50 200 100 425 10000 1000 1000 1000 1000

120 40 4 13.8 20 60 20 20 10 70

4h 5h – 2h 2h 48 h 0.5 – 6 min 6 min

[15] [27] [31] [32] [32] [33] [34] This work This work This work

column adsorption of our works is comparable to the other literature reports and superior to commercial ACs and BLF-450-6. Considering the much reduced thermal treatment time and superior performance of the H2S removal both in dry and wet to the commercial ACs, the advantage of using fluidized bed is very obvious. 4. Conclusions The fluidized bed shows efficient process intensification during carbonization step for biochar preparation. The optimal conditions for preparing biochar with the largest sulfur removal are: carbonization at 450
C for duration of 6 min in fluidized bed. The BET specific surface area of the char carbonized at 6 min can reach 60 m2/g. The maximum monolayer adsorption capacity can reach 25 mg/g at room temperature by using equilibrium isotherm approximation. The degree of heterogeneity on the surface of biochar is far more than that of commercial AC. The moisture shows a beneficial effect on the adsorption of H2S. The maximum removal rate and saturation constants were obtained using Mechael’s—Menten-type equation. The surface diffusion is significant to the effective diffusivity during mass transfer at our experimental condition. The dynamic experimental results indicate a good performance in H2S removal by the prepared biochar. Acknowledgements The National High Technology Research and Development Program 863 (2011AA060703) and Innovation funds of institute of

processes engineering of Chinese Academy of Sciences (No: 062702) are highly appreciated for financial support for PhD scholarship, domestic and international conferences attendance. The critical comments from two anonymous reviewers are highly appreciated. Authors would like to thanks for Commonwealth Science and Industrial Research Organization (CSIRO), Australian Resource Research Center of Earth Science and Resources Engineering. References [1] R.S. Haszeldine, Carbon capture and storage: how green can black be, Science 325 (2009) 1647–1652. [2] Y. Sun, V. Parikh, L. Zhang, Sequestration of carbon dioxide by indirect mineralization using Victorian brown coal fly ash, J. Hazard. Mater. 209–210 (2012) 458–466. [3] X.F. Tan, Y.G. Liu, G.M. Zeng, X. Wang, X.J. Hu, Y.L. Gu, Z.Z. Yang, Application of biochar for the removal of pollutants from aqueous solutions, Chemosphere 125 (2015) 70–85. [4] X. Liu, Z.F. Li, Y.Z. Zhang, R. Feng, I.B. Mahmood, Characterization of human manure-derived biochar and energy-balance analysis of slow pyrolysis process, Waste Manage. 34 (2014) 1619–1626. [5] Z.G. Liu, A. Quek, S.K. Hoekman, R. Balasubramanian, Production of solid biochar fuel from waste biomass by hydrothermal carbonization, Fuel 103 (2013) 943–949. [6] Y. Sun, J. Wei, Y.S. Wang, G. Yang, J.P. Zhang, tion of activated carbon by K2CO3 activation treatment of cornstalk lignin and its performance in removing phenol and subsequent bioregeneration, Environ. Technol. 31 (2010) 53–61. [7] Y. Sun, G. Yang, J.P. Zhang, M.S. Yao, Clean production of corn stover pulp using KOH + NH4OH solution and its kinetics during delignification, Chem. Ind. Chem. Eng. Q. 18 (2012) 137–145. [8] J.P. Zhang, Y.S., Wang, G. Yang, An energy-effecient process for straw lignin separation from black liquor by continuous hydro-thermal flue gas precipitation. China Patent. 201210030298, Publication number: CN 102587187 A.

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