Effect of Blending Carbon-Bearing Waste with Coal on Mineralogy and

Sep 30, 2013 - Effect of Blending Carbon-Bearing Waste with Coal on Mineralogy ... noted to be the most notable change in the mineralogy of the coke b...
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Effect of Blending Carbon-Bearing Waste with Coal on Mineralogy and Reactivity of Cokes Ana M. Fernández,† Carmen Barriocanal,† Sushil Gupta,*,‡ and David French§ †

Instituto Nacional del Carbón (INCAR), Consejo Superior de Investigaciones Científicas (CSIC), Post Office Box 73, 33080 Oviedo, Spain ‡ School of Materials Science and Engineering, Centre of Sustainable Materials Research and Technology, University of New South Wales, Sydney, New South Wales 2052, Australia § CSIRO Energy Technology, North Ryde, New South Wales 2113, Australia ABSTRACT: Because of rising costs, there is a constant demand to minimize coking coal use in favor of cheaper alternatives for ironmaking. A range of carbon-bearing additives were carbonized with a typical medium-rank Australian coking coal with the aims of minimizing the deterioration of coke quality while increasing the recycling of waste materials for ironmaking. Additives included two types of tire-recycling residues, coke breeze and a bituminous residue from the distillation column of benzol. Coke blends were prepared in a semi-pilot-scale coke oven, while a horizontal furnace was used to prepare char/coke samples of additives. The percentage of minerals in raw additive samples as well as coke blends was quantified by X-ray diffraction (XRD). Scanning electron microscopy (SEM) was used to determine the mode of occurrence of the minerals, while a fixed-bed reactor was used to measure the apparent reaction rate with CO2. After pyrolysis, recycling tire chars indicated the highest reactivity, which was attributed to zinc dispersion in the carbon matrix. Coke breeze showed the least reactivity but still greater than the base coke reactivity. The bituminous pitch residue char reactivity was significantly higher than the base coke reactivity. Under the test conditions, magnetite formation was noted to be the most notable change in the mineralogy of the coke blends. The higher reaction rate of the coke blends is related to the increase in the magnetite content as well as an enlargement of the micropore size. The apparent reaction rate of the coke blends is shown to be related to the coke strength after reaction (CSR). The study has implications for the use of various carbon-bearing wastes in a cokemaking process. waste products “in situ”, and also to improve coke properties or to reduce costs. Therefore, the interaction of such additives with coal and their influence on coke quality as well as related environmental concerns have been of continuing interest.7−12 Recently, we reported the effect of such carbonaceous additives on modification of the thermoplastic properties of coals.13−15 However, there is limited understanding of the implications of such additives on coal minerals during carbonization and their subsequent impact on coke reactivity, which still requires clarification. In this paper, we have examined the influence of a few such additives, mainly tire waste and pitch, on the modification of minerals that were formed in coke and their impact on coke reactivity. The implication of coke mineralogy as a consequence of blending a small amount of carbonaceous waste materials on coke quality is also discussed.

1. INTRODUCTION Coke is essential to produce iron via the blast furnace route. Because of rising costs and environmental concerns, there is a constant demand to minimize its use in favor of cheaper alternatives. For these reasons, the availability of carbon-rich materials that can be included in coal blends appears to be a matter of increasing importance. The use of high-quality coke in a blast furnace is essential to achieve low coke rate operation (lower CO2 emissions), high productivity, and cheaper hot metal production.1−3 Consequently, strict control of coke quality is needed to ensure the smooth operation of the blast furnace. Coke reactivity to CO2 has been regarded as an important quality indicator since the 1970s, when the Nippon Steel Corporation introduced a test that has been generally accepted by many as a reasonable approach to assess hightemperature coke reactivity and strength. In this test, about 200 g of 19−22.4 mm size coke particles are reacted with CO2 at 1100 °C. The percentage of weight loss of coke after 2 h of the reaction is known as the coke reaction index (CRI), while the coke strength after reaction (CSR) is defined as the weight percentage of coke retained on a +9.5 mm sieve after tumbling for 600 revolutions in a standard drum configuration. Coke behavior in a blast furnace can be affected by several factors, including the nature of minerals formed in coke during carbonization.4,5 Moreover, during carbonization, the majority of feed coal minerals undergo complex transformations.6 A variety of additives have a potential for partial substitution in the blends to decrease coking coal consumption, to recycle © XXXX American Chemical Society

2. MATERIALS AND METHODS 2.1. Sample Selection. Usually at industrial scale, blends of coal of different rank and quality are used. In the present research work, a single coal of high quality has been studied. This will have the advantage of the repeatability of the results obtained, and the results could be comparable to those obtained for an industrial coal blend, Special Issue: 4th (2013) Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies Received: July 4, 2013 Revised: September 27, 2013

A

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which will have a high CSR. Therefore, a typical medium-rank Australian coking coal with high CSR (65) was selected in this study. Two residues obtained from a tire recycling plant (Ty) and the textile fibers (TxTy) used as reinforcement were selected. Coke fines (CF) was selected on the basis of common use as an inert additive in industrial coke ovens. A bituminous residue pitch (RP) from a coke oven gas treatment installation was also used as one of the additives. Table 1 provides proximate and ultimate analyses of the reference coal

the process of mechanical grinding of the tires once the metallic parts and the rubber have been removed. Coal blends were carbonized in a 17 kg movable wall electrically heating INCAR semi-pilot coke oven of 250 mm in length, 790 mm in height, and 165 mm in width. Coal and additives were physically mixed to prepare the feed charge for the pilot-coke oven. Moisture of the charge was maintained around 5 wt %, providing a typical bulk density (756 kg/m3, dry basis) used in a large-scale industrial coke oven. The oven wall temperature was maintained at 1100 °C throughout the test duration. The coke was soaked for 15 min after the center temperature reached 950 °C. The total coking time interval was around 3.5 h. The CRI/CSR values of the tested cokes were measured as per ASTM standards (D5341) and are presented in Table 3. The repeatability limit for the CRI under this

Table 1. Main Characteristics of Base Coal and the Raw Additives Studied property

coal A

moisture (wt %, db) ash yield (wt %, db) volatile matter (wt %, db)

tire fiber (TxTys)

Proximate Analysis 0.7 0.9

0.9 9.8

C (wt %, daf) H (wt %, daf) N (wt %, daf) S (wt %, daf) O (wt %, daf) a

tire crumbs (Ty)

9.3

8.4

22.3

63.0

65.7

88.7 5.0 2.1 0.55 3.6

Ultimate Analysis 87.1 83.1 7.6 7.2 0.3 0.3 2.00 1.74 3.1 7.8

coke fines (CF)

bituminous residue pitch (RP)

Table 3. CSR/CRI Values of Pilot Cokes and the Description of Blend Components

1.0

1.1

69.7a

1 2

A A5Ty

96.2 0.6 1.5 0.53 1.2

87.6 5.1 3.6 1.72 2.0

3

A5TxTys

95

4 5

A10CF A5RP

90 95

Table 2. Ash Composition of Base Coal and the Raw Additives Expressed as Oxides (wt %) coal A

tire crumbs (Ty)

tire fiber (TxTys)

coke fines (CF)

bituminous residue pitch (RP)

8.93

9.36

8.06

10.27

0.88

61.96 21.39 1.59 4.09 4.86 0.50 0.92 0.28 4.16 0.13 0.03 0.07

52.49 1.26 0.11 0.75 6.33 0.84 0.67 0.84 7.54 0.54 28.56 0.05

22.94 2.29 0.15 22.74 9.76 1.45 0.97 0.69 8.55 0.57 29.88 0.00

52.00 28.38 1.53 9.73 3.23 0.89 1.74 0.57 1.11 0.70 0.03 0.08

42.85 12.86 0.80 30.49 3.78 1.22 0.95 3.05 1.03 0.25 0.05 2.66

100 95

additive content (wt %)

CSR

CRI

nil 5 wt % tire recycling scrap residue (Ty) 5 wt % tire recycling fiber residue (TxTys) 10 wt % coke fines (CF) 5 wt % benzol distillation residue (RP)

69 58

24 28

60

28

39 67

32 24

standard procedure is 2.8 units. To examine the effect of minerals on reactivity, another batch of samples, often referred to as coke or char, were also prepared, being pyrolyzed in a horizontal oven at 1000 °C using Gray king apparatus. These samples were sufficient to provide information on changes in chemistry or mineralogy, even though the physical structure was not well-developed. The apparent reaction rate and mineralogy of both sets of pilot cokes and cokes or char were measured using a fixed-bed reactor. 2.3. Fixed-Bed Reactor. A custom-made fixed-bed reactor was used to measure the CO2 reactivity of the coke specimens. The coke samples were dried at 378 K overnight, and then 1.2 g of sample was supported on a sintered glass frit inside a quartz tube placed in an electrically heated furnace. A mass-flow controller was used to pass 100% CO2 from the top to the bottom flowing at the rate of 0.75 L/ min. A thermocouple monitored the sample bed temperature. Carbon dioxide was passed through an oxygen and moisture trap prior to injection into the furnace. The CO concentration of the exhaust gas was continuously monitored via an infrared analyzer to calculate the apparent reaction rate and activation energy. The reaction rate of the cokes was normalized to the same reaction temperature of 1173 K using the apparent reaction rate and activation energy as reported in previous studies.4−6 The reaction rate of the coke blend made of tar pitch could not be measured because of insufficient sample. The apparent reaction rate after 10% conversion was used for comparing the reaction rate of different samples. Under the tested conditions, the apparent reaction rate of cokes can be reproduced within an accuracy of 10% of the measured value.6 2.4. Mineral Analysis. The elemental composition of the inorganic matter present in each sample was measured using X-ray fluorescence (XRF) in a SRS 3000 Bruker spectrometer as per ASTM standard (D4326-04). Mineral phases were quantified using X-ray diffraction of mineral-rich specimens of each sample using a Philips PW1050 goniometer located at CSIRO, Sydney, New South Wales, Australia. Mineral-rich samples were prepared using radio frequency oxygen plasma ashing at a low temperature of 393 K. The X-ray diffraction (XRD) patterns were collected using Co Kα radiation at 45 kV and 30 mA, with step scans over the 2θ range from 3° to 90°, a step interval of 0.04°, and a 10 s count time per step. The XRD diffractogram was processed using SIROQUANT, a PC-based program that uses the full-profile fitting Rietveld method for quantitative mineral analysis.

and the additives. Proximate analyses were performed following ISO 562 and ISO 1171 standards. The elemental analysis was measured as per ASTM D5773 and ASTM D5016 standards. Table 2 provides the

oxide

coal A (wt %)

number

On the basis of thermogravimetric analysis.

ash yield (wt %) SiO2 Al2O3 TiO2 Fe2O3 CaO MgO K2O Na2O SO3 P2O5 ZnO V2O5

pilot-coke code

0.9

10.7

chemical composition of the coal and additive samples. The ashes were subjected to a fusion step using lithium tetraborate in a Philips Perl’X3 automatic fused bead machine to obtain sample beads for analysis. The concentrations of oxides were determined using X-ray fluorescence spectrometry. 2.2. Coke and Semicoke Preparation. The coal sample was crushed to the typical particle size range of a commercial coke plant, i.e., 85 wt % passing through a 3 mm sieve. Coke fines were ground to less than 1 mm size particles. The reinforcing fiber and tire crumb samples were used as received. A total of 58% of tire crumbs presents a particle size between 1 and 2 mm; a total of 37% of tire crumbs presents a particle size between 2 and 3 mm; and the remainder of the particle size was less than 1 mm. Textile fibers were obtained during B

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2.5. Scanning Electron Microscopy (SEM)/Energy-Dispersive Spectrometry (EDS) Analysis. Coke samples were examined using the field emission scanning electron microscope (Hitachi S3400-I) facility of the University of New South Wales. The specimen piece was mounted in a slow-setting epoxy resin in plastic molds and polished. Elemental distribution in the coke matrix was mapped using a LINK ISIS 200 energy-dispersive X-ray (EDX) microanalysis system by mapping selected regions of the specimens at SEM operating accelerating voltage of 20 kV. 2.6. Surface Area Measurements. Microporosity of the coke blend samples was measured by CO2 adsorption at 273 K using a Nova Quantachrome 4200e model. Specimens were degassed for 24 h at 200 °C in a vacuum prior to CO2 adsorption. The Dubinin− Radushkevich (D−R) equation was applied to the CO2 adsorption isotherms to obtain the volume of micropores (W0), the surface area (SDR), and the characteristic adsorption energy (E0).16 The adsorption energy can be related to the average width of the micropores (L).17,18

Panels a, b, and c of Figure 2 show the surface of the carbon matrix of the coke A, the blend coke A5Ty, and the char Ty particles, respectively. The coke A surface appears smooth and uniform, consisting of mainly the carbon phase (Figure 2a). The surface of char Ty particles shows the presence of tiny bright spots, being more visible along the edges of the pores as well as the carbon matrix (Figure 2c). The EDS analysis of the bright spots in the char Ty particles confirmed that they consist of Zn and S. The blend coke A5Ty surface also shows bright spots in the carbon matrix (Figure 2b), which appears more extensive because of different magnification. The EDS analysis of the bright dots in blend coke A5Ty showed that they also consist of Zn and S, and hence, their origin is clearly attributed to recycling tire material. The association of Zn and S in char Ty is further clarified using a magnified view of a typical Ty char particle (Figure 3a). This image shows a contour of bright dots along the edges of the pore as pointed out by arrow 2. The EDS spectrum of a typical bright dot shows that these bright dots are made of Zn and S (Figure 3b). Few bright spots in the carbon matrix of char Ty (spot 1) also indicated the presence of calcium peaks in the EDS spectrum (Figure 3b). Careful examination of the EDS data of tiny bright spots of several char particles showed that Zn and S are clearly visible along the edges of the char pores. A comparison of Zn mapping (Figure 3c) and S mapping (Figure 3d) of char particles illustrates the strong association between Zn and S in the bright spots along the boundary of the pore, which can be attributed to ZnS formation. Zinc occurs in the tire residues mainly as ZnO,19 which reacts with S during devolatilization to form ZnS.20 The high reactivity of Ty char can be attributed to the catalytic effect of ZnS on char reactivity.21,22 A comparison of the mineralogy of coke A and coke A5Ty clearly indicates that blending of recycling tire does not significantly change the nature as well as proportion of mineral phases of coke A, except for the occurrence of new Zn-bearing phases and increased magnetite levels (Table 5). The absence of ZnS peaks in the XRD diffractograms of Ty char and coke blend ash samples can be attributed to transformation of ZnS to a range of Zn-bearing phases during low-temperature ashing (LTA). Therefore, it can be inferred that, as far as mineralogy is concerned, ZnS is the primary phase responsible for increasing the reactivity of coke blends. The textural data show that the average micropore size of coke blends A5Ty increases to 0.93 from 0.83 nm of that of base coke A (Table 6). It seems that the interaction of Zn and S during coking is contributing to enhance the micropore size. The presence of ZnS around pores further suggests that ZnO present in recycling tire increases the pore area during transformation to ZnS during coking.

3. RESULTS AND DISCUSSION 3.1. Effect of the Blending Recycling Tire Residue. A comparison of the apparent reaction rates of tire char, base coke A, and coke blend ATy, containing 5 wt % of recycling tire, is shown in Figure 1. The reaction rate of coke A at 10%

Figure 1. Variation of the apparent reaction rates of base coke A, coke blend A5Ty, and char Ty with carbon conversion at 1173 K because of the CO2 reaction.

conversion was 7.02 × 10−6 g g−1 s−1 and is slightly less than the reaction rate of coke blend A5Ty (7.48 × 10−6 g g−1 s−1). In both cases, the reaction rates are well below the similar reaction rate of typical high CSR cokes.5 The intensity of recycling tire blending on the reactivity of coke blend can be understood by examining the characteristics of the individual char. The final reaction rate of recycling tire char Ty is 30 × 10−6 g g−1 s−1, which is more than 4 times the reaction rate of coke A. Despite the high reactivity of recycling tire char, a modest increase in the reactivity of the coke blend is mainly due to the small amount of the recycling tire blended. Several factors, including mineral matter, could affect the reactivity of the coke blend.

Figure 2. SEM images of (a) base coke A and (b) coke blend A5Ty containing 5% recycling tire and (c) char Ty. C

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Figure 3. (a) SEM image of a char Ty particle, (b) EDS spectra of bright spots, and (c and d) elemental maps illustrating Zn and S association in the char particle.

(ZnO),23 and iron oxide (Fe2O3) in the case of TxTys (Table 2). During LTA, the common zinc-bearing phases are observed as sulfates and complex zinc-bearing phases, including zincite (Table 4). The TxTys residue contains relatively high calcite levels. Calcite is believed to be the result of carbonation of calcium oxide, which is formed during gasification. Figure 5a illustrates the surface of the A5TxTys coke blend. The SEM/EDS analysis shows that zinc is present in the cokes occurring in association with sulfur, indicating that either a zinc sulfide or sulfate is present as an amorphous phase (Figure 5b). Figure 6 illustrates the presence of ZnS in recycling tire residue char TxTys also. The effect of tire residue addition on coke quality at the semi-pilot scale is noticeable, although to a lesser extent than for the coke fines. As shown in Table 3, the addition of tire recycling residues increases the CRI of the base coke, from 24% for coke A to 28% for A5Ty and A5TxTys coke blends. 3.2. Effect of the Blending Coke Fines. Figure 7 compares the apparent reaction rate of base coke A and blend coke A10CF with carbon conversion. The final reaction rate of coke fines is about twice the reaction rate of coke A. Consequently, the reaction rate of blend coke A10CF was increased. The differences in the reaction rate of coke fines and the coke can be explained on the basis of mineralogy and pore structure. The EDS analysis of mineral grains in the coke blend is shown in Figure 8. The ash and mineral composition (Tables 2 and 4, respectively) of the coke fines appears quite similar to that of the coal base, except that the coke fines contain higher iron levels. Accordingly, coke A10CF has a higher magnetite content in comparison to coke A, which also contains hematite (Table 5). Both are believed to catalyze the gasification reaction. At a higher conversion level, reactivity of coke is increasingly influenced by the surface area compared to minerals, most likely because of the diminishing contact surface between catalytic mineral phases and coke matrix as a

Interestingly, mullite, a common coke mineral, was not detected in any of the tested coke samples of this study. The exact reason for this is not clear. It is possible that, during cocarbonization, volatiles released from the recycling tire material may have a catalytic impact on clay decomposition and subsequent mullite transformation to an amorphous phase and, hence, could not be detected in the XRD patterns. Bassanite and jarosite are the artifacts of the ashing process. Their presence indicates the existence of sufficient amounts of sulfur, calcium, and iron phases, such as oldhamite and pyrrhotite, which are known to transform during low-temperature plasma ashing.6 The apparent reaction rates of coke A5TxTys containing another type of recycling tire residue is higher than that of base coke A, as shown in Figure 4. The effect of 5% blending TxTys tire residue did not increase the reaction rate of the coke blend significantly. The reaction rates of recycling tire fiber char TxTys and char Ty are of a similar range but significantly greater than the reaction rate of the base coke A (Figures 1 and 4). In both types of recycling tire residues, the most prevalent compounds detected are silicon dioxide (SiO2), zinc oxide

Figure 4. Variation of the apparent reaction rates of base coke A, coke blend A5TxTys, and char TxTys with carbon conversion at 1173 K because of the CO2 reaction. D

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Table 4. SIROQUANT Analysis of Minerals of the LTA Specimens of Base Coal and the Additives mineral phase in the LTA sample (wt %) minerals

formula

coal A

quartz kaolinite mullite illite mixed layer illite sanidine pyrite ankerite magnetite siderite szmolnockite ferricopiapite coquimbite jarosite calcite dolomite gypsum bassanite zincite bianchite zinc oxide salt fluorapatite anatase rutile amorphous

SiO2 Al2Si2O5(OH)4 Al6Si2O13 (K,H3O)(Al,Mg,Fe)2 (Si,Al)4O10[(OH)2,(H2O)] (K,H3O)(Al,Mg,Fe)2 (Si,Al)4O10[(OH)2,(H2O)] K[AlSi3O8] FeS2 Ca(Mg,Fe)(CO3)2 Fe3O4 FeCO3 Fe2·SO4·H2O Fe2/3·Fe4·(SO4)6·(OH)2·(H2O)20 Fe2·(SO4)3·(H2O)9 (KH3O)Fe3(SO4)2(OH)6 CaCO3 Ca Mg(CO3)2 CaSO4·(H2O)2 2CaSO4(H2O) Zn, Mn, O (Zn,Fe2+)(SO4)·6(H2O) Zn(OH)2)3(ZnSO4)(H2O)5 Ca5(PO4)3F TiO2 TiO2

30.5 43.6 0.0 2.6 8.9 1.1 1.0 0.0 0.0 2.1 0.0 0.0 0.0 0.0 7.4 0.5 0.0 0.8 0.0 0.0 0.0 0.0 0.7 0.7

tire crumbs (Ty)

tire fiber (TxTys)

coke fines (CF)

bituminous residue pitch (RP)

0.3 0.8 0.0 0.0 0.0 0.0 0.0 2.1 0.0 0.0 0.0 0.0 0.0 0.0 1.5 0.0 1.7 0.6 0.0 17.5 4.0 0.0 0.0 0.9 70.5

1.3 2.0 0.0 0.0 0.0 0.0 0.0 3.3 0.0 0.0 0.0 0.0 0.0 0.0 19.9 0.0 0.0 1.8 4.5 21.8 1.3 0.0 0.0 0.0 44.2

14.5 0.0 15.0 0.0 0.0 0.0 0.0 0.0 1.2 0.0 0.0 0.0 0.0 1.6 0.0 0.0 0.0 1.4 0.0 0.0 0.0 2.0 0.5 0.7 63.1

1.5 0.0 0.0 0.0 0.0 0.0 0.0 2.0 0.0 2.0 2.6 8.8 5.2 5.7 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 72.3

Figure 5. (a) SEM image of coke blend A5TxTys containing 5% recycling tire residue and (b) illustration of ZnS dispersion in the carbon matrix of coke A5TxTys.

Figure 6. (a) Illustration of ZnS dispersion in the carbon matrix of char TxTys and (b) EDS spectra of char TxTys.

coke fines increases the reactivity of coke as a consequence of the increased surface area as well as because of magnetite also. Although coke fines char is less reactive than tire residues chars, their addition to the base coke produces a coke blend A10CF

consequence of the consumption of carbon around mineral grains.24 The surface area of coke blend A10CF (156 m2 g−1; Table 6) is higher than coke A (120 m2 g−1), which could be attributed to a smaller grain size. Therefore, the addition of E

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Table 6. Microporosity of Pilot-Coke Blends Based on CO2 Adsorption Isotherms microtexture data of coke blends parameter

A

A5Ty

ATxTys

ACF

A5RP

surface area, SDR (m2 g−1) volume of micropores, W0 (cm3 g−1) adsorption energy, E0 (kJ mol−1) mean width of the micropores, L (nm)

120 0.050

72 0.030

98 0.041

156 0.065

83 0.035

24.4

23.0

21.9

21.6

23.9

0.83

0.93

1.03

1.09

0.87

Figure 7. Variation of apparent reaction rates of base coke A, coke blend A10CF, and char coke fines with carbon conversion at 1173 K because of the CO2 reaction.

Figure 9. Variation of apparent reaction rates of base coke A and coke RP with carbon conversion at 1173 K because of the CO2 reaction. Figure 8. (a) SEM image of coke blend A10CF and (b) EDS spectra illustrating aluminosilicate grain (particle 1) and iron rich (particle 2) in coke matrix.

reason, the final product is referred to as RP coke rather than RP char. The ash yield of the residual tar pitch (RP) is significantly low and contains high iron and low silica levels. It also contains abundant amorphous material and iron sulfates (Table 4). The major iron form present in the residue tar pitch is ferricopiapite. Other sulfate minerals observed in this pitch, such as coquimbite and jarosite, are mainly related to the LTA process, formed most likely from the oxidation of oldhamite and pyrrhotite, respectively. Of all of the mineral phases identified in the cokes, iron oxides, oldhamite, and pyrrhotite were the only minerals considered to be potential catalysts. Oldhamite is potentially a catalyst because carbon dioxide can oxidize it to calcium oxide, which is a known gasification catalyst.24 The morphology of coke A5RP is shown in Figure 10. Because of the significantly lower surface area of coke A5RP

with the highest CRI, as shown in Table 3. This can be attributed to the particle size effect.25 3.3. Effect of the Blending Bituminous Pitch Residue. A pilot-coke blend A5RP containing 5% residual pitch was not available for the reactivity measurements because of insufficient sample. However, the implications of blending pitch on the expected reactivity of the coke blend can be commented upon the basis of the contrast between the reactivity of their coke samples and mineralogy. Figure 9 shows that the apparent reaction rate of RP coke is more than 4 times the reaction rate of coke A. Unlike the other residues, the bituminous residue passes through a plastic stage during heating, and for that

Table 5. SIROQUANT Analysis of Minerals of the LTA Specimens of the Coke Blends mineral phases in the LTA sample of coke blends (wt %) mineral name

chemical formula

LTA yield (wt %) quartz mullite hematite magnetite jarosite szmolnockite bassanite anatase rutile illite sanidine mixed layer illite others, calcite amorphous

SiO2 Al6Si2O13 Fe2O3 Fe3O4 (KH3O)Fe3(SO4)2(OH)6 Fe2·SO4.H2O 2CaSO4(H2O) TiO2 TiO2 (K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2,(H2O)] K[AlSi3O8] (K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2,(H2O)] CaCO3

F

A

A5Ty

A5TxTys

A10CF

14.4 27.4

15.6 28.0 0.0 0.5 2.3 0.6 0.4 0.9 0.4 0.4 3.7 1.0 2.3 0.5 59.1

14.8 28.2 0.0 0.0 2.3 0.6 0.0 1.1 0.4 0.4 3.6 1.2 2.2 0.0 60.0

15.0 23.0 0.0 0.0 2.7 0.6 0.0 0.8 0.3 0.3 4.6 1.0 2.4 0.0 64.4

0.5 1.1 0.0 0.3 0.0 0.4 0.3 4.0 1.2 2.2 0.0 62.8

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apparent reaction rate of coke blends increases with a mean diameter of the micropores (Figure 12a) as well with an increase in the magnetite content (Figure 12b). This means that the presence of an additive is responsible for increasing the size of the micropore during carbonization, leading to increased reactivity and, hence, decreased CSR values. The study suggests that, as far as coke strength is concerned, a small amount of the additives may not have a significant adverse effect on coke strength. However, further application of the additives, particularly tire scrap, would require optimization of acceptable zinc and sulfur levels possibly through some pretreatment to address associated environmental impact. Figure 10. SEM image of coke blend A5RP, illustrating homogenization of the carbon matrix.

4. CONCLUSION A suite of carbon-bearing wastes of different origins was used to prepare the char samples and the coke blends. The mineralogy and CO2 reaction rate of char and the coke blends were measured. The reaction rate of the char samples of all of the tested additives was found to be greater than the reaction rate of the coke. The reactivity of pitch and tire residue chars was almost 4 times the reaction rate of the base coke, while coke fines showed the least reactivity. The additives did not cause significant modification of the original minerals of the base coke, except an increase in the magnetite formation and dispersion of inorganic derivatives, such as Zn. The effect of recycling tire residues upon a coke reactivity increase is mainly attributed to Zn and S dispersion in the carbon matrix of coke as well as an increased pore size. The coke fines addition showed a modest increase in the reactivity of the coke blend but a significant drop in the CSR value, which could be attributed to poor assimilation of inert fines into the carbon matrix. On the other hand, despite having the highest reactivity, the pitch residues showed the least modification of coke reactivity and strength because of improved assimilation within the coke matrix. The reaction rate of coke blends is shown to increase with an increase in the micropore size and an increased magnetite formation. The apparent reaction rates were also related to the CSR such that high CSR cokes indicated a low reaction rate. The study suggests that, despite high reactivity of the additives, low amounts may be blended with coal without compromising coke strength.

(83 m2/g; Table 6), the increased reactivity of the pitch cokes cannot be attributed to the surface area. The presence of a variety of iron-bearing phases appears to be responsible for the high reactivity of the RP coke. On the basis of similar CRI values of coke A and coke A5RP, as shown in Table 3, the apparent reaction rate of A5RP coke is expected to be less than 10 units and much closer to the base coke reaction rate. 3.4. Implications of Blending on CSR. Figure 11 shows that an increasing apparent reaction rate of cokes at 10 wt %

Figure 11. Correlation between the apparent reaction rate of coke blends and the CSR value.

carbon conversion correlates with decreasing CSR values of the coke blends. It may be noted that the CSR value of coke containing 5 wt % recycling tire residue is 60%. With 10 wt % coke fines, the CSR of the coke blend decreases to 39%, while the apparent reaction rate increases only by 2 units. Figure 11 further shows that the apparent reaction rate of the majority of coke A blends is less than 10 units, which is of the order of the apparent reaction rate value of typical high CSR cokes.5 The

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

Figure 12. Correlation between the apparent reaction rate of cokes with the (a) mean micropore size and (b) magnetite content. G

dx.doi.org/10.1021/ef401268f | Energy Fuels XXXX, XXX, XXX−XXX

Energy & Fuels

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Article

ACKNOWLEDGMENTS The research leading to these results has received funding from the European Union’s Research Fund for Coal and Steel (RFCS) Research Programme under Grant Agreement RFCRCT-2010-00007. The authors also thank Yun Lin, Dong-Min Jang, Prof. Veena Sahajwalla, and staff from the Mark Wainwright Analytical Centre, University of New South Wales, for the assistance with the SEM measurements.

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dx.doi.org/10.1021/ef401268f | Energy Fuels XXXX, XXX, XXX−XXX