Kinetics underpinning the C-CO2 gasification of waste tyre char and its interaction with coal char upon co-gasification

Fuel 256 (2019) 115991

Contents lists available at ScienceDirect

Fuel journal homepage: www.elsevier.com/locate/fuel

Full Length Article

Kinetics underpinning the C-CO2 gasification of waste tyre char and its interaction with coal char upon co-gasification Miriam Issac, Baiqian Dai, Lian Zhang

T

⁎

Department of Chemical Engineering, Monash University, Clayton, Victoria 3800, Australia

G R A P H I C A L A B S T R A C T

A R T I C LE I N FO

A B S T R A C T

Keywords: Waste scrap tyre char C-CO2 gasification Tyre-coal co-gasification Ash effect Kinetic modelling

This study includes a thorough investigation of the char structure, the potential catalytic influence of minerals, the carbon conversion rate, the kinetic behavior and parameters of the CO2 – gasification reaction for waste scrap tyre char and its blends with coal chars. Kinetic modelling was also attempted based on the two classical methods, random pore model (RPM) and grain model (GM) using the isothermal gasification results collected from thermogravimetric analysis (TGA). As has been found, tyre char is characterised with a macroporous structure with a very low reactivity. The catalytic role of the inherent minerals within tyre char is only influential at lower temperatures below ~1373 K. As opposed to the generally assumed first order reaction, the tyre char exhibits a reaction order between 0.62 (for an intrinsic reaction at 1273 K) and 0.92 referring to a dominant diffusion control at 1573 K. The slow intrinsic reaction rate of tyre char was also found to be comparable with the gas diffusion rate, thereby exhibiting a large activation energy of 174.87 kJ/mol that is considerably higher than coal chars. The coal char – tyre char blends displayed declining reactivity with increasing tyre char content, due to the enhanced ash melting and diffusion against the inward gas diffusion towards the unreacted carbon that is encapsulated inside. This was further explained by the experiments of blending tyre char ash with coal chars, and the observation of ash residues collected from the gasification of blends. Furthermore, the grain model was found more suitable to fit the blending conversion data, agreeing with the hypothesised molten ash layer surrounding the unreacted char blend.

⁎

Corresponding author. E-mail address: [email protected] (L. Zhang).

https://doi.org/10.1016/j.fuel.2019.115991 Received 22 June 2019; Received in revised form 7 August 2019; Accepted 8 August 2019 Available online 17 August 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.

Fuel 256 (2019) 115991

M. Issac, et al.

Nomenclature A0 DAB D0 Deff,

CO2

DK dp Ea kp L0 m0 mt mash PCO2, s

PCO2, ∞ Rapp Rin

Rs Rx

Arrhenius equation pre-exponential factor, [g cm−2 s−1/ atm] T molecular diffusion, DAB = D0 ( T )1.75 ( P0 ), [cm2 s−1] P 0 binary diffusion coefficient of gas A in gas B at T0 and P0, 2 −1 [cm s ] effective diffusion coefficient of CO2 in char particle, [cm2 s−1] Knudsen diffusion, [cm2 s−1] char particle diameter [nm] activation energy [kJ mol−1] apparent rate constant, ( dx ), [g g−1 s−1] dt x = 0 initial char pore length per unit volume, [cm] mass of char at time = 0 on a dry-ash-free basis, [g] mass of char at time t on a dry ash free basis, [g] mass of ash, [g] partial pressure of CO2 at the external particle surface, [atm] partial pressure of CO2 in the ambient atmosphere, [atm] apparent gasification reaction rate, [g g−1 s−1] intrinsic reaction rate, [g cm2 atm−1 s−1]

R rp So Tp x

gasification reactivity index, [s−1] gasification reactivity corresponding to conversion x, [g g−1 s−1] gas constant, [8.314 J mol−1 K−1] mean pore radius of the char particle, [m] initial char surface area, [cm2 g−1] particle temperature, [K] carbon conversion, [–]

Greek

εo ψ ηin ηex ν

ρp τ ϕ

1. Introduction

particle porosity of char, [–] structural parameter for random pore model, [–] internal effectiveness factor for gasification reactions, [–] external effectiveness factor for gasification reactions, [–] stoichiometric coefficient (0.0833 mol CO2 consumed per gram carbon reacted), [–] apparent density of char samples, [g cm−3] tortuosity of the pores ( 2 ), [–] Thiele modulus approach, [–]

conducted to date on the co-gasification of lignite with tyre in a commercial scale Lurgi gasification process [9]. The experiments confirmed an improvement in the net calorific value of syngas by 3% compared to lignite alone, due to the abundance of carbon within the scrap tyre. The optimum tyre char content for the blend was also found to be 10 wt%; otherwise the overall reactivity would be decreased. This study aims to reveal the fundamental mechanisms governing the intrinsic C-CO2 gasification reactivity of tyre char alone and its blends with two different coal chars, based on the use of a lab-scale, high-temperature thermogravimetric analyser (TGA). The two coal chars include one from Australian bituminous coal and one from Australian brown coal. The use of char instead of raw coal was considered here, based on the assumption that the initial devolatilisation of coal exerts a negligible effect on tyre char reactivity. By varying the reaction temperature and feedstock blending ratio, this comprehensive study will evaluate the physical properties, the kinetic parameters including the order of reaction, activation energy, pre-exponential factor and the potential catalytic influence of mineral matter on the gasification reactivity of tyre char. In parallel to experimental measurements, kinetic modelling was also attempted based on the two classical methods, random pore model (RPM) and grain model (GM). While the former model has been proven for the single char particle gasification, it is still unknown if it is applicable to the char blend scenarios. Compared to single char particle, the blends of different char could impose extra diffusion resistance on the particle surface, for instance through their mineral interaction. If this is the case, the latter model accounting for an unreacted core covered by molten ash shell could work. Same as the physical structure, the mineralogical properties of tyre char have yet to be explored. This will also be addressed in this study.

Waste tyre disposal is a global problem. Australia alone generates 50 million end-of-life tyres every year [1]. Waste tyres are usually dumped in landfills, which however poses serious risks associated with biological and chemical resistance to degradation [2] and mineral leaching [3]. At present, recovery of energy via thermal valorisation is one of the preferred methods of scrap tyre disposal [2,4]. Waste tyre thermal valorisation techniques include pyrolysis, combustion and gasification. The pyrolysis is an endothermic process that can yield pyrolytic oil, gas and tyre char, all of which are the potential high-value products. In this sense, pyrolysis of scrap tyre and the utilisation of the pyrolysis driven products have been researched intensively. Among all the three products from pyrolysis, the solid char is a promising solid fuel due to its high calorific value of 30–40 MJ/kg [3] and is a suitable precursor for gasification to yield syngas that can be used for advanced power generation via gas turbine or to synthesise chemicals [4]. A recent economic analysis concluded that tyre gasification is attractive due to the lower regulated cost of syngas as compared to the price of natural gas [5]. With regard to the tyre char gasification rate, a broad variety of studies have been conducted on lab and pilot-scales, leading to several findings such as 1) tyre char gasification reactivity is independent of particles size when it is less than 0.65 mm in the CO2 partial pressure of 0.3–1 atm and temperature of 850–1000 °C [6]; 2) a high energy conversion ratio such as 95% has been confirmed based on a pilot-scale gasification study of waste tyre in a rotary-kiln reactor [7]; and 3) tyre char exhibits low gasification reactivity with CO2 and steam. Zabaniotou et al. [8] attribute this stagnant behaviour to the low heteroatom (O and H) contents and inorganic constituents. Nevertheless, the study on the gasification of scrap tyre char is still far from complete, when compared to coal and biomass. Moreover, no study has been conducted to touch base on the co-gasification of scrap tyre char and coal. As a low-volume feedstock, scrap tyre char is very likely to blend with other fuels such as coal which is much more stable and abundant, rather than being fed alone into an existing gasifier. This is beneficial in maintaining a stable feedstock supply but is also expected to increase the stability and sustainability of the gasifier. As far as the authors are aware, only one study has been

2. Experiments and modelling 2.1. Fuel preparation and characterisation Three fuel samples were tested in this study, including scrap tyre char and two coal types, one brown coal char from Loy Yang brown coal found in La Trobe Valley, Victoria, Australia, one bituminous coal char 2

Fuel 256 (2019) 115991

M. Issac, et al.

protected in high-purity argon gas at 100 ml/min and heated up to the required reaction temperature at 50 K/min. Then the isothermal gasification was initiated by switching the gas to CO2 at a flow rate of 100 ml/min and held at the reaction temperature until the carbon conversion was completed. The experiments were repeated at four terminal temperatures of 1273 K, 1373 K, 1473 K and 1573 K. The TGA records mass change with respect to gasification time and temperature continuously. The char conversion can be calculated as [11]:

derived from a Brisbane black coal, Australia. In order to prepare the blended samples, tyre char was blended with each coal at the mass percentages of 10%, 30%, 50% and 80%. All blends were mixed well to ensure the homogeneity of the samples. The char samples were prepared by subjecting to a uniform pyrolysis condition in a fixed – bed pyrolyser that was operated at an average furnace temperature of 1073 K. This pyrolysis step is critical to avoid the interference of the devolatilisation reaction with the CO2 gasification reaction. All the fuel samples were crushed, sieved to the size range of 63–105 µm and dried in a nitrogen-protected oven at 378 K prior to the experimental tests. The proximate and ultimate analysis values of the original samples are summarised in Table 1. Tyre char is characterised by the largest carbon content of 93.77% on a dry ash free (daf) basis, which also contains around 15 wt% ash on dry basis. The Brisbane bituminous coal has a high ash content of 44.4 wt% on dry basis (db) while the Loy Yang brown coal has a minuscule ash content of 2.54 wt%(db). In terms of elemental composition, scrap tyre char is also rich in sulphur, while the other two coals are rich in oxygen. To eliminate the effect of the inherent minerals in tyre char, acid washing was also conducted by washing the tyre char using 1 M HCl up to 2 h. This slurry was filtered using a filter paper and washed several times using Milli-Q water until acidic ions were completely removed. The residue was further dried in the nitrogen-protected oven at 383 K until it was completely dried. The physical properties and mineral composition within a char are influential on its gasification reactivity. The gasification agents tend to penetrate deep into the internal pore structure of the char particle with the progress of the reaction [10], hence it is essential to quantify the pore structure properties of the char samples. The internal surface area of each char was determined by the method of nitrogen porosimetry using Micromeritics 3Flex analyser. Apparent density and mean pore radius was measured using Micrometetics Autopore IV Mercury Porosimeter. Micromeritics AccuPyc 1330 – helium density analysis was used to determine the true density. The unreacted char structure was also observed by JEOL 7001F SEM at 15 kV accelerating voltage. The compositions of ash-forming elements within each char sample were analysed using a pre-calibrated Spectro iQ II X-Ray Fluorescence (XRF) spectroscopy. Note that, the char samples were ashed at 873 K in a muffle furnace prior to the XRF quantification.

Xdaf =

m 0 − mt m 0 − mash

(1)

where m 0 is the initial mass of char, mt is the char mass at time t and mash is the mass of the remaining ash. Gasification reactivity index, Rs can be used to quantify gasification reactivity, defined as [12]:

Rs =

0.5 t0.5

(2)

where t0.5 is the time taken for 50% char conversion. 2.2.2. Model considerations 2.2.2.1. Random pore model and the structural parameter. Random pore model (RPM) is flexible compared to other reaction models such as the grain model (GM) which is more suitable for the case that a reaction rate decreases with an increase in conversion. In comparison, the RPM accounts for pore size distribution within the reacting solid using a structural parameter to explain the solid reactivity. Bhatia et al. [13] show that when an arbitrary pore size distribution exists, the structure can be defined using pore volume, surface area and pore length. As the reaction proceeds, an initial increase is expected for the reaction rate, due to the opening of more pores and the growth of the reacting surfaces of the initial pores. Once reaching its maximum value, the reaction rate, however, is expected to experience a later decrease due to the collapse of the pores and the intersection of growing surfaces. RPM successfully considers a random pore distribution using the structural parameter and the intersection of reaction surfaces as the reaction proceeds, thus avoiding the need to assume a structure of uniformly sized cylindrical pores. The apparent reaction rate (Rapp) can be expressed as:

2.2. Experimental and modelling approach

R app =

As mentioned earlier, both the experimental investigation and modelling approaches were attempted in this study. As shown in the simplified flowchart in Fig. 1, both single and blend char samples were first subjected to experimental measurement in a lab-scale thermogravimetric analyser, at different temperatures in 100% pure CO2 to compare the gasification conversion curves. Afterwards, the conversion curves were first fitted into the RPM model using regression analysis and the determination coefficient R2 was used to indicate the quality of the model fit. When the RPM model was validated by a high R2 value, the corresponding RPM structural parameter for each sample was determined. The apparent order of reaction was determined by performing the TGA experiments at different CO2 partial pressures. The kinetic parameters activation energy and pre-exponential factors were subsequently calculated. However, in case that the regression fitting for the RPM model failed, the GM model was trialled to fit the conversion curve by taking the ash interaction into account. Finally, for the interpretation of the ash interaction, microstructural observation of the ash residues was analysed. Details of each step are provided below.

dx dt

(3)

RPM model developed by Bhatia and Perlmutter [13] described the surface area (S) as:

S = So (1 − x ) 1 − ψln (1 − x )

(4)

where So is the initial surface area, x is conversion ratio of char and ψ is the RPM structural parameter. The apparent reaction rate can be expressed using RPM as [11]: Table 1 Proximate and ultimate analysis of the pre-treatment tyre char and coal samples. Tyre char

Brisbane coal

Loy Yang coal

3.00 3.32 81.44 15.24

5.40 31.40 24.21 44.40

10.10 50.56 46.90 2.54

Ultimate analysis (wt%, daf) Carbon 93.77 Hydrogen 1.04 Oxygen 0.08 Nitrogen 0.42 Sulphur 4.70

75.50 6.84 15.80 1.06 0.80

67.94 4.85 26.08 0.62 0.52

Proximate analysis (wt%) Moisture, ar Volatile matter, db Fixed carbon, db Ash, db

2.2.1. Thermogravimetric analyzer (TGA) experiments A TG-DTA (Shimadzu DTG-60H) was used to perform the gasification experiments using an isothermal method. In each experiment, sample weighing 4 ± 0.5 mg was placed in a platinum crucible and nitrogen gas was used as the purge gas. The samples were initially 3

Fuel 256 (2019) 115991

M. Issac, et al.

Fig. 1. Flowchart to illustrate the overall methodology.

Brunauer–Emmett–Teller (BET) measurements on surface areas and pore volumes [13]. However, these estimates were found to be inaccurate because of the approximations required for non-uniform pore size distribution and micropores size estimation. Studies then used the reaction rate maximum from the conversion curve to determine this parameter [14,15]. However, this is limited by the accuracy of the reaction maxima and also by the narrow carbon conversion range. In this study, regression analysis based on reduced time t/tx using experimental carbon conversion results were used to determine the structural parameter [16]. Experimental results at a lower temperature zone were used where the reactions are kinetically controlled with no mass transfer influences on the measured gasification rate [10]. Structural parameters determined under zone I conditions (chemical reaction controlled) can describe the char conversion under zone II conditions (diffusion controlled). In addition, it has been proven that the use of an upper limit of 90% conversion can help eliminate the uncertainty of the results towards the end of the carbon conversion where the inorganic species are predominant [16]. The overall reaction rate can be defined as [16]:

Table 2 Ash composition of tyre chars and coal chars by XRF analysis. Mineral (%)

Tyre char

Acid washed tyre char

Brisbane char

Loy Yang char

SiO2 Al2O3 CaO MgO SO3 Fe2O3 ZnO Na2O K2O TiO2 Ash content (db. %)

46.18 2.36 7.54 1.39 6.62 1.31 32.36 0.43 1.03 0.12 15.24

58.9 1.8 1.8 0.9 2.9 0.8 30.6 0.1 0.9 – 9.1

63.39 29.43 0.68 0.53 0.44 2.79 0.02 0.31 0.68 1.53 44.40

41.33 24.17 1.51 4.00 10.81 3.42 0.07 4.81 0.21 9.28 2.54

R app = kp (1 − x) 1 − ψln (1 − x )

(5)

where kp is the measured apparent rate constant. The structural parameter is defined as:

4πLo (1 − εo) ψ= So2

rs (1 − x ) So 1 − ψln (1 − x ) dx = dt (1 − εo)

(6)

(7)

where εo is initial porosity and rs is reaction rate. This equation can be expressed in terms of a dimensionless factor :

where Lo is the total pore length per unit volume and εo is the particle porosity. The structural parameter can be evaluated using Table 3 Char properties of tyre chars and coal chars. Properties 3

Apparent density (g/cm ) True density (g/cm3) Porosity Internal surface area (m2/g) Pore radius (m)

Tyre char

Acid washed tyre char

Brisbane bituminous char

Loy Yang brown char

0.706 0.898 0.214 31.26 3.21E−05

1.1245 1.828 0.385 35.75 5.63E−06

1.478 1.8904 0.2181 300 6.00E−08

0.8 1.91 0.581 350 7.20E−08

4

Fuel 256 (2019) 115991

M. Issac, et al.

Tyre char Brisbane char Loy Yang char

(a) At 1373 K

Carbon conversion (daf, %)

100

80

60

40

20

0 0

500

1000

1500

2000

2500

600

800

1000

t (s)

(b) At 1473 K

Carbon conversion (daf, %)

100

80

60

40

20

0 0

200

400

t (s)

(c) At 1573 K

Carbon conversion (daf, %)

100

Fig. 2. SEM images of (a) Tyre char (b) Brisbane char (c) Loy Yang char.

dx = (1 − x ) 1 − ψln (1 − x ) d

(8)

80

60

40

20

rS t

And  = 1s−oε o These equations upon integration can be expressed in terms of t as:

2(1 − εo ) ( 1 − ψln (1 − x ) − 1) rs So ψ

t=

0 0

100

200

300

400

500

t (s) Fig. 3. Carbon conversion curves for tyre char, Brisbane char and Loy Yang char at (a) 1373 K (b) 1473 K (c) 1573 K.

(9)

And in terms of dimensionless  as:

=

2 ( 1 − ψln (1 − x ) − 1) ψ

1 − ψln (1 − x ) − 1

t = t0.9

(10)

1 − ψln (1 − 0.9) − 1 t

 0.9

(11)

where t = 0.9 Advantage of using this expression is that it enables to define an upper limit for reliable experimental conversion value, defined as x (assumed to be 90%) at time tx . Hence, the reduced time t/tx will be

Assuming t0.9 as the time for 90% conversion and all properties for a given char to be the same, structural parameter can be determined using the following equation: 5

Fuel 256 (2019) 115991

M. Issac, et al.

0

80

60

40

20

n = 0.90

-1

Tyre char at 1273 K Aw-tyre char at 1273 K Tyre char at 1373 K Aw-tyre char at 1373 K Tyre char at 1473 K Aw-tyre char at 1473 K Tyre char at 1573 K Aw-tyre char at 1573 K

-2

n = 0.87

ln(Rx)

Carbon conversion (daf, %)

Tyre Char 1273 K 1373 K 1473 K 1573 K

(a)

100

-3

n = 0.72

-4

0 0

2000

4000

6000

8000

n = 0.62

10000 -5 -1.8

t (s)

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

-0.5

independent of parameters outside the square root term in Eq. (9), with the only dependent parameter being ψ .

AW-Tyre Char 1273 K 1373 K 1473 K 1573 K

(b)

-1.0

n = 0.86

-1.5

2.2.2.2. Grain model (GM). GM [17] or commonly known as the shrinking-core model, assumes a char particle to be a uniform assembly of spherical and non-porous individual grains where the inter-particle void among the grains forms the porous structure within the char particle. The reaction happens on each grain surface and as the reaction proceeds, an unreacted solid as a shrinking core which diminishes, eventually leaving an ash layer [18]. The overall reaction rate can be expressed as:

dx = kGM (1 − x )2/3 dt

ln(Rx)

-2.0

n = 0.84

-2.5 -3.0 -3.5

n = 0.68

-4.0

n = 51

-4.5 -5.0 -1.8

(12)

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

ln(PCO2)

2.2.3. Kinetic rate parameters The intrinsic reaction rate of C-CO2 gasification, Rin can be determined using nth order Arrhenius form of the global reaction [10]:

E n Rin = A0 exp ⎛⎜− a ⎞⎟ PCO 2, s RT p⎠ ⎝

(c) 0.5

n = 0.99

0.0

(13)

-0.5

ln(Rx)

-1.5

n = 0.77

-2.5

n = 0.56

-3.0

(14)

-1.8

At higher temperatures, the intrinsic gasification rate needs to account for pore diffusion resistance by incorporating internal and external effectiveness factors. The apparent reaction rate is thus defined as:

Rapp = ηin ηex SRin

n = 0.95

-1.0

-2.0

Rapp S

Brisbane Coal 1273 K 1373 K 1473 K 1573 K

1.0

where in the A0 intrinsic pre-exponential factor, Ea is the activation energy, R is the gas constant, Tp is the particle temperature, PCO2,s is the CO2 partial pressure at external particle surface and n is the reaction order. The intrinsic reaction rate can also be defined as:

Rin =

0.2

ln(PCO2)

Fig. 4. Char conversion curve of tyre char and acid washed tyre char compared between 1273 K and 1573 K.

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

ln(PCO2)

Fig. 5. Linear fitting to determine reaction order for (a) Tyre char (b) Acid washed (AW) tyre char and (c) Brisbane char between 1273 K and 1573 K.

(15)

The internal effectiveness factor, ηin is defined as the actual reaction rate per internal surface area to the ideal reaction rate under no pore Table 4 Calculated RPM structural parameter. Sample

Structural parameter calculated (ψ )

Standard error

Adjusted R-Squared

Structural parameter (ψ )

Tyre char Acid washed Tyre char Brisbane char Loy Yang char

0.32 0.99 1.58 2.42

0.006 0.004 0.06 0.03

0.99 0.99 0.98 0.99

– – 1.2 [1] 3 [1]

6

Fuel 256 (2019) 115991

M. Issac, et al.

(a)

(a)

ln(Rapp) TC ln(Rappd) TC

-4

ln (Rapp) or ln (Rappd)

Gasification reactivity index (s-1)

ln(Rapp) Aw-TC ln(Rappd) Aw-TC

-5

-6

y = -2.10x + 9.29 R2 = 0.99

-7

-8

y = -2.13x + 9.03 R2 = 0.99

-9

-10 6.5

7.0

7.5

8.0

0.009 0.008 0.007 0.006 0.005 0.004 0.003 0.002

8.5

1/T x 10-4

0

20

40

ln(Rappd) BC

Gasification reactivity index (s-1)

y = -2.06x + 10.04 R2 = 0.99

-6

-7

-8 6.5

7.0

7.5

8.0

8.5

1/T x 10-4

0.009 0.008 0.007 0.006 0.005 0.004 0.003

ln(Rappd) LY

-3.0

100

0.002

ln(Rapp) LY

(c)

80

Loy Yang coal blends at 1473 K Loy Yang coal blends at 1573 K

(b) 0.010

-4

-5

60

Tyre char content (%)

ln(Rapp) BC

(b) -3

ln (Rapp) or ln (Rappd)

Brisbane coal blends at 1473 K Brisbane coal blends at 1573 K

0.010

0

20

40

60

80

100

Tyre char content (%)

ln (Rapp) or ln (Rappd)

-3.5

Fig. 7. Gasification reactivity vs tyre char content for (a) Brisbane char blends and (b) Loy Yang char blends at 1473 K and 1573 K.

-4.0 -4.5

y = -1.88x + 9.28 R2 = 0.99

-5.0

diffusion resistance.

-5.5

ηin =

-6.0

1 1 1 ( − ) ϕ tanh (3ϕ) 3ϕ

(16)

Thiele modulus ϕ can be calculated using the equation:

-6.5 -7.0

ϕ= 6.5

7.0

7.5

8.0

8.5

-4

1/T x 10

dp

n−1 (n + 1) νSρp kin Tp PCO 2,s

2

2Deff , CO2

(17)

where dp is the char particle diameter, n is the order of the reaction, ν is the stoichiometric coefficient of the C-CO2 reaction (equal to 0.0833 for moles of CO2 reacted per gram of carbon), ρp is the apparent density of the char particle, Tp is particle temperature, PCO2,s is the particle external surface CO2 partial pressure, and Deff , CO2 is the effective diffusion coefficient. Deff , CO2 can be determined by the equation:

Fig. 6. Plot between apparent reaction rate and 1/T to determine Ea and A0 for (a) Tyre char (TC) and acid washed tyre char (Aw-TC) (b) Brisbane char (BC) (c) Loy Yang char (LY).

Table 5 Activation energy and pre-exponential factor for tyre char and coal char for CO2 gasification reactivity. Sample name

A0 (g/cm2 s−1/atm)

Ea (kJ/mol)

A0 (g/cm2 s−1/atm) in reference [1]

Ea (kJ/mol) in reference [11]

Tyre char Acid washed tyre char Brisbane char Loy Yang char

3.46E−02 2.34E−02 7.65E−03 3.06E−03

174.87 177.03 170.90 156.30

– – 9.75E−02 9.23E−03

– – 189.61 154.5

7

Fuel 256 (2019) 115991

M. Issac, et al.

(a) At 1373 K

RPM GM

RPM linear fitting GM linear fitting

(c) BC + Tyre char 50% at 1373 K 4

4

3

3

(2/Ψ)((1-Ψln(1-X))1/2-1)

R2

0.8

0.6

0.4

R2=0.95

2

2 2

R =0.77 1

3(1-(1-X)1/3)

1.0

1

0.2

0

0.0

80 %

%

1000

0 2000

1500

C

t

LY

LY

LY

500

+T

+T

C

C

30

50

%

% +T

C +T

C

LY

BC +T

10

80 %

50 % C

30 % C

BC +T

BC +T

BC +T

C

10 %

0

(b) At 1573 K

RPM linear fitting GM linear fitting

(d) BC + Tyre char 50% at 1573 K

RPM GM

7

1.0 6

R2

0.8

0.6

0.4

5 4

4

R2=0.82

3 2

3(1-(1-X)1/3)

(2/Ψ)((1-Ψln(1-X))1/2-1)

6

2

R2=0.98

0.2

1 0

0.0

%

150

200

250

300

350

400

450

0 500

C

50

100

LY +T

C LY +T

C LY +T

50

t

80

%

% 30

% C

10

80 % LY +T

50 %

BC +T C

30 %

BC +T C

BC +T C

BC +T C

10 %

0

Fig. 8. R squared value for the linear fitting using RPM and GM for Brisbane char (BC) and Loy Yang char (LY) blends (a) at 1373 K, and (b) at 1573 K. A typical example of fitting Brisbane char (BC) + Tyre char 50% using RPM and GM (c) at 1373 K (d) at 1573 K.

Deff , CO2 =

εo 1 −1 τ 2 DK−1 + DAB

E n Rapp = ηin ηexn SA0 exp ⎜⎛− a ⎟⎞ PCO 2, ∞ RT p ⎝ ⎠

(18)

where τ is the tortuosity of the pores ( 2 ), DK is Knudsen diffusion and DAB is the molecular diffusion. Knudsen diffusion can be determined using the equation:

DK = 97rp

Using this relation, Ea and A0 can be determined by linear regression fitting from the slope and y intercept respectively. The order of gasification reaction can be determined based on its relationship to the reaction rate. However, at high temperature, the order of reaction determined using the measured reaction rate will be the apparent reaction order instead of the true reaction order due to the interference of the diffusion effects [19]. As investigated in detail by Roberts et al. [20], the true reaction order remains a difficult concept to quantify at high-temperature scenarios because it has an intricate relationship with various parameters including the reaction temperature and saturation of available reactive surfaces, based on the LangmuirHinshelwood reaction scheme. Regardless, determination of reaction order measured for each specific process condition (apparent reaction order) is critical for the practical means for quantifying the impacts on the gasification rates. In order to determine the apparent order of C-CO2 gasification reaction, assume that the overall gasification reaction rate is proportional to the nth power of partial pressure of CO2, given as [21]:

Tp MWCO2

(19)

where rp is the mean pore radius of the char particle and MWCO2 is the molecular weight of CO2. The external effectiveness factor, ηex is defined as the ratio of partial pressure on the particle surface (PCO2,s ) to the ambient atmosphere (PCO2, ∞), expressed as:

ηex =

PCO2,s PCO2, ∞

(21)

(20)

The kinetic rate relation by incorporating external and internal diffusion effects can be expressed as [10]:

8

Fuel 256 (2019) 115991

M. Issac, et al.

(a)

2.2.5. Scanning electron microscopy (SEM) analysis After ensuring that the char samples were completely reacted in the TGA at 1573 K, the ash residue was collected from the platinum crucible. The analysis samples were prepared by carefully mounting it to the stub using carbon tape and being carbon-coated prior to the SEM observation. SEM observations were performed using JEOL 7001F at 15 kV

Brisbane char blend at 1473 K Brisbane char blend at 1573 K

100

Liquid slag fraction (%)

90 80 70 60

3. Results and discussion

50

3.1. Physical properties of chars

40

Table 2 summarises the ash compositions determined by a pre-calibrated X-ray fluorescence spectroscopy (XRF). Interestingly, the total ash content of tyre char is 15.24 wt% with a significant amount of ZnO (32.36 wt%) and SiO2 (46.18 wt%) which are the main fillers used during tyre manufacturing [3]. Acid washing of the tyre char removed a significant amount of the minerals, reducing its net ash content down to 9.1 wt%. Brisbane char has a very high content of SiO2 (63.39 wt%) and Al2O3 (29.43 wt%) that are typical in high-rank bituminous coals. It also contains a few impurities such as CaO (0.68%) and Fe2O3 (2.79%) that could catalyse the gasification reactions [23]. Loy Yang char has an ash content of 2.54% with a high content of alkali metal minerals (Na2O and K2O) which are known catalysts for gasification [24]. As shown in Table 3, tyre char interestingly has a very low surface area of 31.26 m2/g, a low porosity of only 0.214 and distinguished with a macroporous structure as evident from the large pore radius of 3.21 × 10−5 m. Acid washing of tyre char resulted in the removal of minerals, which simultaneously improved internal surface area slightly to 35.75 m2/g and porosity to 0.385. Loy Yang coal chars have a welldeveloped porous structure with a high internal surface area of 350 m2/ g, a porosity of 0.581 and an averaged pore radius of 7.2 × 10−8 m. As a bituminous coal char, Brisbane char also has a relatively good porous structure with an internal surface area of 300 m2/g and pore radius of 6 × 10−8 m. However, its porosity is much lower compared to brown coal char. This is due to the abundance of volatiles in the parent brown coal. SEM images shown in Fig. 2 shows a highly dense structure for tyre char as compared to Brisbane char and Loy Yang char.

30 20 10 0

20

40

60

80

100

Tyre char content (%)

(b)

Loy Yang char blend at 1473 K Loy Yang char blend at 1573 K

100

Liquid slag fraction (%)

90 80 70 60 50 40 30 20 10 0

20

40

60

80

100

Tyre char content (%)

3.2. Comparison of gasification reactivity of tyre char and coal char

Fig. 9. Liquid slag fraction against various blending ratios of tyre char with (a) Brisbane char and (b) Loy Yang char at 1473 K and 1573 K. n Rx = kPCO 2

3.2.1. Carbon conversion of the single fuels Fig. 3 shows the carbon conversion rate on a dry-ash-free basis of the three individual chars and their comparisons at three different temperatures in 100% CO2 concentration. To reiterate, the particle size for all samples were kept constant within the size range of 63–105 µm. At 1373 K, tyre char required approximately 2300 s to complete the conversion. Brisbane char is the second most reactive, which is closer to the brown coal char with reference to tyre char. As expected, at 1373 K the Loy Yang brown coal char shows the highest reactivity to CO2 gasification, achieving 100% char conversion in a very short period of less than 400 s. This is broadly consistent with the experimental studies by Dai et al [11] and Tanner et al [25]. On increasing the temperature to 1473 K and 1573 K, the reactivity of all the three have improved, whilst their discrepancy is narrowed considerably. With regard to the reactivity of tyre char at elevated temperatures, an increase in the temperature to 1473 K and 1573 K ensued in a significant improvement in reactivity, as demonstrated in Fig. 3(b) and Fig. 3(c). At 1473 K, 100% carbon conversion was achieved at a much shorter period of ~800 s and it further improved at 1573 K with complete carbon conversion achieved at ~400 s, which is in the same order of magnitude to that of the two coal chars. For Loy Yang coal char, the reactivity further improves at 1473 K and remains comparable at 1573 K. The Brisbane char displays a significant improvement in its reactivity from 1373 K to 1473 K, with 100% conversion achieved at ~900 s and ~225 s, respectively. On further increasing the temperature

(22)

where Rx is the gasification reactivity corresponding to conversion x (assumed as 50% [21,22]), k is the effective rate constant dependent on temperature, PCO2 is the CO2 partial pressure and n is the apparent reaction order. This can be again expressed as:

lnRx = lnk + nlnPCO2

(23)

At a given temperature, gasification reactivity of tyre char and acid washed tyre char was determined at various CO2 partial pressures of 20 vol%, 50 vol%, 75 vol% and 100 vol%, with nitrogen as the balance gas. The experiments are repeated at 1273 K, 1373 K, 1473 K and 1573 K. 2.2.4. Thermodynamic equilibrium calculation FactSage 6.4 was used to make the thermodynamic predictions for the ash slag using the “Equilib” module. For each ash sample, the ash composition determined using the XRF analysis and the corresponding gas environment were used as the input parameters. FToxid-SLAG A was used as the slag phase for the calculations which is suitable for melting of oxides and sulphide. The output predictions were used to calculate the liquid slag fraction at the specified process conditions. 9

Fuel 256 (2019) 115991

M. Issac, et al.

BC+TC ash 1.66% BC+TC ash 6.13% BC+TC ash 13.22% BC+TC ash 37.86%

(a) Bribane char blends at 1473 K

100

80

60

40

100

80

60

40

20

20

0

0 0

500

1000

1500

0

2000

500

1000

1500

2000

t (s)

t (s)

BC+TC ash 1.66% BC+TC ash 6.13% BC+TC ash 13.22% BC+TC ash 37.86%

(b) Bribane char blends at 1573 K 120

LY+TC ash 1.66% LY+TC ash 6.13% LY+TC ash 13.22% LY+TC ash 37.86%

(d) Loy Yang char blends at 1573 K 120

100

Carbon conversion (daf, %)

Carbon conversion (daf, %)

LY+TC ash 1.66% LY+TC ash 6.13% LY+TC ash 13.22% LY+TC ash 37.86%

(c) Loy Yang char blends at 1473 K 120

Carbon conversion (daf, %)

Carbon conversion (daf, %)

120

80

60

40

20

100

80

60

40

20

0 0

500

1000

1500

0

2000

0

t (s)

500

1000

1500

2000

t (s)

Fig. 10. Gasification conversion curves for blends of tyre ash with Brisbane char (BC) (a) at 1473 K and (b) at 1573 K. Gasification reactivity for Loy Yang (LY) char and tyre ash blends (a) at 1473 K and (b) at 1573 K. The gasification reactivity diminished significantly on increasing beyond tyre ash content beyond 6.13 wt% which corresponds to 30% of tyre char in the coal char-tyre char blends.

minimal pore development while a larger value refers to a higher probability of pore development with the progress of the reaction [10]. As summarised in Table 4 (second column), the original tyre char is characterised with a much lower value of only 0.32 for the structural parameter while the Loy Yang coal char has a well-developed pore structure with a ψ value of 2.42, relative to the ψ value of 1.58 for the Brisbane coal char, suggesting a less developed porous structure. The low structural parameter of tyre char is indicative of a low degree of structural imperfections with a high degree of crystallinity [13]. In general, the molecular unit within the tyre char is similar to graphitic materials which are highly crystalline with a lower degree of crosslinking and structural imperfections, such as edges, further reducing the potential active centres for gasification reaction. Moreover, tyre char has a higher number of benzene units and lower content of heteroatoms such as oxygen and hydrogen which are essential to act as active sites to facilitate gasification reaction [8]. On the other hand, coal char features highly disordered small and large aromatic ring systems, typically found in an amorphous structure that is crucial for the gasification conversion [28]. In addition, the low ψ value of tyre char indicates a pore coalescence rather than opening [13] during the reaction. This is in contrast to coal chars where the micro-capillaries enlarges and opens up the pore volume for increased access with the progress of the reaction [29]. Acid washing can remove soluble mineral components such as carbonates and sulphides as well as lead to extraction of certain cations on silicate sheets by ion exchange [30]. Consequently, the acid washing of tyre char has resulted in obvious textural changes as shown in

to 1573 K, the improvement on its reactivity is quite marginal. Considering that the inorganic species such as ZnO has been reported catalytic on the gasification of char [26,27], the acid-washed tyre char was subjected to the same gasification conditions as the tyre char in the TGA between 1273 K and 1573 K. Fig. 4 illustrates the experimentally determined conversion results. As evident, the acid-washed tyre char reactivity is obviously lower than that of the raw char at the two lowest temperatures, 1273 K and 1373 K. This proves the catalytic effect of the acid-soluble species on the char conversion. However, at the two elevated temperatures, the discrepancy between raw char and its acid-washed counterpart diminished, suggestive of a noncatalytic role of the ash-forming elements in tyre char. Instead, these ash-forming elements could react with one another to form a resistance layer against the internal diffusion of gases. Such an inference is somehow similar with the alkali and alkaline earth metals that are mostly catalytic below 1100 K. An increase in the temperature could deactivate these metals due to the vaporisation and immobilisation by refractory alumina and silica within the char matrix [11]. Back to the discrepancy of the three chars in reactivity, it can be further demonstrated by their RPM structural parameter ψ (shown in Table 4), which is indicative of the changes in their porous structure with the progress of the gasification reaction [10]. To reiterate, this parameter was calculated using regression analysis and experimental results at 1273 K using Eq. (11). A lower temperature of 1273 K was chosen to avoid diffusional interferences. The accuracy of the structural parameter determined using this method was evaluated by comparing to similar coal chars in literature [11]. A value closer to zero denotes a 10

Fuel 256 (2019) 115991

M. Issac, et al.

Fig. 11. SEM images of ashes of fuel samples collected from TGA at 1573 K (a) Tyre ash (b) Brisbane char ash (c) Loy Yang char ash (d) Ash of Brisbane char + tyre char 50% (e) Ash of Loy Yang char + tyre char 50%

observed for the acid-washed tyre char shown in Fig. 5 panel (b) and even the bituminous coal char in Fig. 5 panel (c). This indicates that the temperature of 1373 K is the boundary beyond which the entire tyre char C-CO2 reaction experiences diffusion control effects, yet at varying extends between each fuel. The value of n at the lowest temperature of 1273 K refers to the intrinsic reaction order, which is clearly much lower than what has been reported elsewhere [6]. In contrast, usually, the values of n measured at a high temperature 1573 K nearly converges to unity, which is a clear sign of the predominance of diffusion control that has a weak dependence on fuel properties [21]. Next, to determine the Arrhenius parameters, Eqs. (13) and (14) at CO2 partial pressure of 1 atm can be expressed as:

Table 6 Ash Fusion Temperatures (AFT) of various representative fuel samples. (K)

Tyre char

Loy Yang char

Loy Yang char + Tyre char 50%

SST DT HT FT

1513 1603 1673 1703

1433 1653 1653 1693

1443 1513 1533 1583

(SST – Shrinkage Starting Temperature, DT – Deformation Temperature, HT – Hemispherical Temperature, FT – Fusion Temperature).

Table 3, leading to relatively more porosity and development of the surface area, compared to its parent tyre char, which explains the slightly higher ψ of 0.99.

ln (Rapp) = ln (A0 S )−

3.2.1.1. Internal and external diffusion implications at high temperatures. The nth order intrinsic rate equation shown in Eq. (13) is defined in terms of reaction order and the Arrhenius parameters which include activation energy, Ea and pre-exponential factor, A0 . In order to determine the apparent reaction order for CO2 gasification for tyre char and acid washed tyre char, a linear fitting was used as per Eq. (23) using logarithmic values of CO2 partial pressures and the experimentally measured reaction rate. Note that, the term

This expression has the advantage of being mathematically simple for further evaluations. However, at higher temperatures, bulk and pore diffusion effects become prevalent and have to be accounted in by incorporating internal effectiveness factor, ηin and external effectiveness factor, ηex . This is particularly essential while determining the Arrhenius parameters in high-temperature CO2 gasification reaction where, char pore structure and diffusion effects has a significant impact on the reaction rates [10,11]. As such, the apparent reaction rate in Eq. (21) can be expressed as:

⎛ −Ea ⎞

k = A0 e⎝ RTp ⎠ , has a strong dependence on temperature. The results are plotted in Fig. 5 for reference. Note that, the determined reaction orders for Brisbane coal char are consistent with the range reported in the literature [20]. As opposed to the generally assumed first order reaction, the value of n varies significantly across various temperatures and for the type of fuel. For the tyre char shown in panel (a) of Fig. 5, its reaction order varies between 0.62 and 0.92 upon the increase of temperature from 1273 K to 1573 K. In particular, there is a rapid increase in the value of n from 0.72 at 1373 K to 0.87 at 1373 K and a similar trend was

Ea RT

(24)

Rapp Ea ⎞ ln ⎛⎜ ⎟ = ln (A 0 S ) − n n RT ⎝ ηin ηex PCO2, ∞ ⎠ Note that, the term

R app n Pn ηin ηex CO2, ∞

(25) d is expressed as Rapp for simplicity.

Graphical representation by comparing Eqs. (24) and (25) has the advantage to quantify the influence of diffusion resistance on the apparent reaction rate for each sample within the defined temperature range. As shown in Fig. 6(a), Eq. (24) was plotted as ln (Rapp ) vs 1/T and Eq. (25) d ) vs 1/T for tyre char and acid washed tyre char was plotted as ln (Rapp 11

Fuel 256 (2019) 115991

M. Issac, et al.

Fig. 12. Schematic explaining the reduced reactivity of coal char-tyre char blend due to the softening of tyre ash which encapsulates the carbon particles and reduces the carbon conversion rate.

3.3. Gasification reactivity for blends

samples across the entire temperature range. These comparisons were extended to Brisbane coal char and Loy Yang coal char shown in Fig. 6(b) and (c), respectively. Interestingly, the plots of ln (Rapp ) vs 1/T (red squares) for tyre char and acid washed tyre char shows a clear linear fitting and coincides d with ln (Rapp ) vs 1/T plot, as shown in Fig. 6(a). On the other hand for both the coal chars, a turning point at 1273 K for the boundary between different control zones was observed for the plot of ln (Rapp ) vs 1/T, where a clear shift from kinetic controlled reactivity at a lower temperature to a bulk and pore diffusion controlled reactivity at a higher temperature can be confirmed. These results for coal char are consistent with previous observations in [10,11]. When including the diffusion d effects as shown in ln (Rapp ) vs 1/T plot, both coal chars (Fig. 6(b) and Fig. 6(c)) showed a significantly improved linear fitting over the considered temperature range. Back to tyre char and its acid washed counterpart, the boundary between the two control zones is clearly much less blurred in terms of the apparent reaction rate. This means the intrinsic reactivity of tyre char is quite slow and even comparable with the gas diffusion rate, at least on the same magnitude of order. d Based on the linear regression of lnRapp vs 1/ T , Ea and A0 values for tyre char, acid washed tyre char, Brisbane char and Loy Yang char were determined and tabulated, as shown in Table 5. Tyre char is distinguished by its large activation energy of 174.87 kJ/mol and acid washed tyre char showed similar activation energy of 177.03 kJ/mol, along with pre-exponential factors of 3.46 × 10−2 g/cm2 s−1/atm and 2.34 × 10−2 g/cm2 s−1/atm, respectively. These large values of Ea are associated with low reactivity for the carbonaceous matrix of a char [31] while A0 is indicative of the fraction of the molecular collisions that can achieve the activation energy [19]. This result is in line with the previous analysis. On the other hand, Loy Yang brown coal char has the least activation energy of 156.3 kJ/mol with 3.06 × 10−3 g/ cm2 s−1/atm as the pre-exponential factor, relative to the value of 170.9 kJ/mol as activation energy and pre-exponential factor of 7.65 × 10−3 g/cm2 s−1/atm for the Brisbane bituminous coal char. These values are comparable to the values reported in the literature [11].

Fig. 7 shows the gasification reactivity index, Rs (Eq. 2) of the blends of tyre char with coal char at different ratios in 100% CO2 atmosphere. Note that, the x-axis refers to the mass percentage of tyre char in the blends, and the experiments were repeated and error bars are included to indicate the reliability of results. The dotted line indicates the expected reactivity based on the weighted addition of the results from two single fuels. As evident from panel (a) and panel (b) of Fig. 7, the reactivity of the blends is affected by the amount of tyre char present in each blended sample. At both temperatures, the blending with tyre char delayed the gasification reaction with a steady decrease when increasing the tyre char content, irrespective of the coal char sample. This trend was consistent by up to 50 wt% tyre char in the blends. Interestingly, on increasing the tyre char content above 50%, the decrease in gasification reactivity was found to be less significant irrespective of the tyre char content in the blend. The blend reactivity also remains almost unchanged between 50% and 100 wt% of tyre char. Moreover, the experimentally determined reactivity values for blends are far less than the expected reactivity values based on weighted addition. This clearly suggests that there is an auxiliary effect that impedes the CO2-gasification carbon conversion rate when the two different char samples are mixed together. Attempts were first made to check the validity of RPM for the char blends using the linear form, which was obtained by integrating Eq. (5), expressed as:

2 ( 1 − ψln (1 − x ) − 1) = kp t ψ

(26)

This validation step was repeated at a lower temperature of 1373 K and again at a higher temperature of 1573 K. In cases where an acceptable R2 value was not obtained to prove the validity of RPM, further attempts were made to fit the conversion of blends using the linear expression of GM based on Eq. (12), expressed as [18]: 12

Fuel 256 (2019) 115991

M. Issac, et al. 1

3 ⎡1 − (1 − x ) 3 ⎤ = kGM t ⎣ ⎦

brown coal char blended with tyre char at 50 wt% reaches 1513 K, which is far less than both Loy Yang coal char ash only (1653 K) and tyre char ash only (1603 K). The SEM image of Brisbane char – tyre char blend ash (Fig. 11(d)) was observed to be less porous as compared to Brisbane char ash (Fig. 11(b)). Its morphology was not as molten as compared to Loy Yang blend ash (Fig. 11(e)). Moreover, gasification reactivity of Brisbane char + tyre char 50% reduced by only 38% with respect to Brisbane char at 1573 K. This reduction of reactivity is relatively less as compared to Loy Yang char blend which is ~50%. Clearly, the inhibitive effect is less in play for the Brisbane char blends. Regardless, the changed morphology evident from the SEM image in Fig. 11(d) still proves that the ash undergoes softening and fusing, eventually impacting the overall carbon conversion and reactivity of the Brisbane char blend. The mechanism underpinning the impeding effect of tyre char on coal char gasification is finally summarised in Fig. 12. As a single fuel, the coal char yields porous and loose ash residue upon its own gasification, relative to the production of a residue with a relatively dense surface from the tyre char alone. Upon the blending, both the char particles shrink where the coal char breaks to smaller fragments while tyre char shrinks continuously to expose the ash particles. Upon a subsequent flow and inter-particle interaction, the molten eutectics are formed, which encapsulates the unreacted carbon particles, thereby reducing the net carbon conversion rate and gasification reactivity. Although the promoted melting of ashes is beneficial for a slag-tapping gasifier such as entrained bed gasifier, the reduction on the char conversion rate should be concerning, which could reduce the cold efficiency of the entire gasification system.

(27)

The summary of R2 value for RPM fitting and GM fitting for all the blends at 1373 K and at 1573 K are illustrated in Fig. 8(a) and (b) respectively. As shown in Fig. 8(a), RPM fitting yielded a modest R2 value at a lower temperature of 1373 K while GM predicted the conversion poorly with a lower R2 value. However, when the temperature was raised to 1573 K (Fig. 8(b)), a strong deviation from the RPM was observed as lower R2 values, implying the inapplicability of the RPM model to the blends in particular at higher temperatures, where the diffusion control is predominant. Intriguingly, at the higher temperature of 1573 K, the validity of GM was proved to predict the conversion of tyre char blends with high R2 value with reference to RPM (Fig. 8(b)). The difference of the two models are the two temperatures, which is further visualised in Fig. 8(c) and (d) for the case of Brisbane coal char + tyre char 50% at 1373 K and 1573 K, respectively. Considering the classic assumptions of GM and the validity of the model for high ash fuels, the fitting in Fig. 8 for blends clearly indicates the influence of the ash on the conversion of the blends at the higher temperature, which should be mainly related to an enhanced diffusion control caused by ash layer. Based on a theoretical calculation from the thermodynamic equilibrium point of view using FactSage 6.4, Fig. 9 indicates that the formation of molten species is enhanced steadily upon the blending, in particular at 1573 K. The maximum liquid slag formation point is around 80 wt% tyre char in its blends with Brisbane bituminous coal char, relative to merely 10 wt% tyre char for its blends with Loy Yang brown coal char. If these calculations are true, the molten species would cement all the solid ash particles together into a thick and dense layer that is not in favour of the gas diffusion. To prove this hypothesis, the tyre char was ashed at 873 K, and the resultant tyre ash was subsequently blended with either coal char at the mass ratios that are equivalent to the ratios of tyre char in the respective blends. For instance, for the coal char-tyre char blend with 10 wt% tyre char, its corresponding coal char-tyre ash blend requests 1.66 wt% of tyre char ash. Similarly, 6.13 wt% tyre ash is equivalent to 30 wt% tyre char, 13.22 wt% tyre ash for 50 wt% tyre char and 37.88 wt% tyre ash for 80 wt% tyre char blending content. Fig. 10 depicts the gasification reactivity conversion curve for all the coal char-tyre ash blends. As demonstrated, the reactivity reduced with increasing tyre ash content for both coal chars. On increasing the tyre ash content beyond the corresponding tyre char content of 30%, a very significant reduction in gasification reactivity was observed. This interesting observation remained consistent irrespective of the coal char type and the reaction temperature (1473 K and 1573 K). This proves our hypothesis that tyre ash plays a significantly negative role in reducing the gasification reactivity. To investigate further into this phenomenon, the ash samples were collected after the TGA experiments at 1573 K and was subjected to SEM analysis (Fig. 11). Interestingly, the tyre ash has an extremely molten and smooth surface (Fig. 11(a)). On the other hand, pure Brisbane char ash (Fig. 11(b)) and Loy Yang char ash (Fig. 11(c)) have a relatively coarse morphology. With reference to their corresponding parent coal char, the morphology was observed to be significantly different on blending tyre char with Brisbane char and Loy Yang char. On observing the SEM image of Loy Yang – tyre char blend (Fig. 11(e)), it was obvious that the corresponding ash has a highly molten and smooth surface as compared to pure Loy Yang coal ash (Fig. 11(c)). This means that the particles significantly deform and partially melts by the formation of low melting eutectics, resulting in the process of sintering and melting by which ash particles coalesce. Consequently, molten ash inflates the process of encapsulation of carbon particles [32,33], thus reducing both the overall carbon conversion rate and reactivity of the coal char – tyre char blends. This micro-scale observation is in line with the experimentally determined ash fusion temperatures of the blend samples (Table 6). The deformation temperature (DT) of the ash from

4. Conclusion This paper summarises the results from a comprehensive study on the C-CO2 reactivity of tyre char alone and its impact on the blending with two different coal chars, bituminous coal char and brown coal char at various ratios and reaction temperatures. The major findings and conclusions from this study are drawn as below: 1. Comparison of the carbon conversion curve of single fuels revealed that tyre char has a low reactivity as compared to coal char. The catalytic influence of the mineral components within the tyre char was evident only at a lower temperature, below 1373 K. The RPM structural parameter for tyre char was evaluated to be 0.32 which is far less than Brisbane bituminous coal char (ψ = 1.58) and Loy Yang brown coal char (ψ = 2.42). It explains the macroporous structure of tyre char with a low degree of structural imperfections that are the potential active centres for gasification and pore coalescence as the main structural mechanism. 2. As opposed to the generally assumed first order reaction, the tyre char exhibits a reaction order between 0.62 and 0.92 upon the increase of temperature from 1273 K to 1573 K. This indicates that the temperature of 1373 K is the boundary beyond which the entire tyre char C-CO2 reaction experiences diffusion control effects, yet at varying extent when compared to coal chars. In addition, in terms of the apparent reaction rate, the intrinsic reactivity of tyre char is quite slow and even comparable with the gas diffusion rate. Therefore, its reaction activation energy was found to be 174.87 kJ/ mol, which is considerably higher than that from two coal chars. 3. On blending of tyre char with coal char, a consistent trend of decreasing reactivity with increasing tyre char ratio was observed. While the RPM fitted the conversion of all the blends at lower temperature, the GM was proved to better predict the conversion of the blends at a higher temperature. This is due to the enhanced ash melting upon the blending, forming extra resistance against the gas diffusion towards the encapsulated, unreacted carbon in the core. Due to a strong synergetic interaction with tyre char ash, the 13

Fuel 256 (2019) 115991

M. Issac, et al.

reactivity of brown coal char was reduced more remarkably than the bituminous coal char.

[15] Liu G, Benyon P, Benfell KE, Bryant GW, Tate AG, Boyd RK, et al. The porous structure of bituminous coal chars and its influence on combustion and gasification under chemically controlled conditions. Fuel 2000;79:617–26. [16] Everson RC, Neomagus HWJP, Kaitano R, Falcon R, du Cann VM. Properties of high ash coal-char particles derived from inertinite-rich coal: II. Gasification kinetics with carbon dioxide. Fuel 2008;87:3403–8. [17] Szekely J, Evans JW. A structural model for gas—solid reactions with a moving boundary. Chem Eng Sci 1970;25:1091–107. [18] Tomaszewicz M, Łabojko G, Tomaszewicz G, Kotyczka-Morańska M. The kinetics of CO2 gasification of coal chars. J Therm Anal Calorim 2013;113:1327–35. [19] Fogler HS. Elements of chemical reaction engineering. 3rd ed. Upper Saddle River, N.J.: Prentice Hall PTR; 1999. [20] Roberts DG, Hodge EM, Harris DJ, Stubington JF. Kinetics of char gasification with CO2 under regime II conditions: effects of temperature, reactant, and total pressure. Energy Fuels 2010;24:5300–8. [21] Zhou L, Zhang G, Schurz M, Steffen K, Meyer B. Kinetic study on CO2 gasification of brown coal and biomass chars: reaction order. Fuel 2016;173:311–9. [22] Ollero P, Serrera A, Arjona R, Alcantarilla S. The CO2 gasification kinetics of olive residue. Biomass Bioenergy 2003;24:151–61. [23] Nzihou A, Stanmore B, Sharrock P. A review of catalysts for the gasification of biomass char, with some reference to coal. Energy 2013;58:305–17. [24] Irfan MF, Usman MR, Kusakabe K. Coal gasification in CO2 atmosphere and its kinetics since 1948: a brief review. Energy 2011;36:12–40. [25] Tanner J, Bhattacharya S. Kinetics of CO2 and steam gasification of Victorian brown coal chars. Chem Eng J 2016;285:331–40. [26] Tsuji M, Wada Y, Tamaura Y, Steinfeld A, Kuhn P, Palumbo R. Coal gasification by the coal/CH4·ZnO·Zn·H2O solar energy conversion system. Energy Convers Manage 1996;37:1315–20. [27] Gokon N, Hasegawa N, Kaneko H, Aoki H, Tamaura Y, Kitamura M. Photocatalytic effect of ZnO on carbon gasification with CO2 for high temperature solar thermochemistry. Sol Energy Mater Sol Cells 2003;80:335–41. [28] Li X, Hayashi J-I, Li C-Z. Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part VII. Raman spectroscopic study on the changes in char structure during the catalytic gasification in air. Fuel 2006;85:1509–17. [29] Turkdogan ET, Olsson RG, Vinters JV. Pore characteristics of carbons. Carbon 1970;8. 545,IN519,547,IN521,557-546,IN520,556,IN522,564. [30] Calahorro CV, Cano TC, Serrano VG. Effect of acid and heat treatments on surface area and porosity of a Spanish coal with high mineral matter content. Fuel 1987;66:479–85. [31] Satterfield CN. Chemical reaction engineering, Octave Levenspiel, Wiley, New York (1972). 578 pages. $16.95. AIChE J 1973;19:206–7. [32] Matjie RH, Li Z, Ward CR, French D. Chemical composition of glass and crystalline phases in coarse coal gasification ash. Fuel 2008;87:857–69. [33] Li J, Zhu M, Zhang Z, Zhang D. A new criterion for determination of coal ash sintering temperature using the pressure-drop technique and the effect of ash mineralogy and geochemistry. Fuel 2016;179:71–8.

Acknowledgements The authors are grateful for the financial support from the Australian Research Council (ARC) industrial transformation research hub for energy-efficient separation (IH170100009) and the joint project between Monash University and Shanghai Boiler Works Co Ltd. References [1] Review WM. A rubbery problem – the Australian scrap tyre management challenge. Waste management review. Prime Creative Media; 2016. [2] Oboirien BO, North BC. A review of waste tyre gasification. J Environ Chem Eng 2017;5:5169–78. [3] Rowhani A, Rainey T. Scrap tyre management pathways and their use as a fuel—a review; 2016. [4] López FA, Centeno TA, Alguacil FJ, Lobato B, López-Delgado A, Fermoso J. Gasification of the char derived from distillation of granulated scrap tyres. Waste Manage 2012;32:743–52. [5] Zang G, Jia J, Shi Y, Sharma T, Ratner A. Modeling and economic analysis of waste tire gasification in fluidized and fixed bed gasifiers. Waste Manage 2019;89:201–11. [6] Lee JS, Kim SD. Gasification kinetics of waste tire-char with CO2 in a thermobalance reactor. Energy 1996;21:343–52. [7] Saito I, Sakae K, Ogiri T, Ueda Y. Effective use of waste tyres by gasification in cement plant; 1987. [8] Zabaniotou AA, Stavropoulos G. Pyrolysis of used automobile tires and residual char utilization. J Anal Appl Pyrol 2003;70:711–22. [9] Straka P, Bučko Z. Co-gasification of a lignite/waste-tyre mixture in a moving bed. Fuel Process Technol 2009;90:1202–6. [10] Kim R-G, Hwang C-W, Jeon C-H. Kinetics of coal char gasification with CO2: impact of internal/external diffusion at high temperature and elevated pressure. Appl Energy 2014;129:299–307. [11] Dai B, Hoadley A, Zhang L. Characteristics of high temperature C-CO2 gasification reactivity of Victorian brown coal char and its blends with high ash fusion temperature bituminous coal. Fuel 2017;202:352–65. [12] Ma Z, Bai J, Bai Z, Kong L, Guo Z, Yan J, et al. Mineral transformation in char and its effect on coal char gasification reactivity at high temperatures, part 2: char gasification. Energy Fuels 2014;28:1846–53. [13] Bhatia SK, Perlmutter DD. A random pore model for fluid-solid reactions: I. Isothermal, kinetic control. AIChE J. 1980;26:379–86. [14] Lu GQ, Do DD. A kinetic study of coal reject-derived char activation with CO2, H2O, and air. Carbon 1992;30:21–9.

14