Determination of the Effect of Different Additives in Coking Blends

Energy & Fuels 2005, 19, 2423-2431

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Determination of the Effect of Different Additives in Coking Blends Using a Combination of in Situ High-Temperature 1H NMR and Rheometry Miguel C. Dıáz, Karen M. Steel, Trevor C. Drage, John W. Patrick, and Colin E. Snape* Nottingham Fuel and Energy Centre, School of Chemical, Environmental and Mining Engineering, Nottingham University, Nottingham, NG7 2RD, United Kingdom Received April 29, 2005. Revised Manuscript Received August 17, 2005

High-temperature 1H NMR and rheometry measurements were carried out on 4:1 wt/wt blends of a medium volatile bituminous coal with two anthracites, two petroleum cokes, charcoal, wood, a low-temperature coke breeze, tyre crumb, and active carbon to determine the effects on fluidity development to identify the parameters responsible for these effects during pyrolysis and to study possible relationships among the parameters derived from these techniques. Positive, negative, and neutral effects were identified on the concentration of fluid material. Small positive effects (ca. 5-6%) were caused by blending the coal with petroleum cokes. Charcoal, wood, and active carbon all exerted negative effects on concentration (18-27% reduction) and mobility (12-25% reduction in T2) of the fluid phase, which have been associated with the inert character and high surface areas of these additives that adsorb the fluid phase of the coal. One of the anthracites and the low-temperature coke breeze caused deleterious effects to a lesser extent on the concentration (7-12%) and mobility (13-17%) of the fluid material, possibly due to the high concentration of metals in these additives (ca. 11% ash). Despite the high fluid character of tyre crumb at the temperature of maximum fluidity of the coal (73%), the mobility of the fluid phase of the blend was lower than expected. The comparison of 1H NMR and rheometry results indicated that to account for the variations in minimum complex viscosity (η*) for all the blends, both the maximum concentration of fluid phase and the maximum mobility of the fluid material (T2L) had to be considered. For individual blends, two exponential relationships have been found between the complex viscosity and the concentration of solid phase in both the softening and resolidification stages but the parameters are different for each blend.

Introduction The reduction of ferrous and nonferrous oxides in metallurgical processes requires the use of cokes with specific characteristics primarily related to their strength and reactivity. One of the main parameters that determine the suitability of a particular coke for these applications is the thermoplastic character of the coke precursor during carbonization. The reduced availability of prime coking coals has led to the blending of good and poor coking coals.1,2 In addition, different carbonaceous materials or additives have been used with the purpose of modifying the properties of the coke (e.g., lowering ash content) and reducing the environmental problems associated with carbonaceous waste materials. Additives such as pitches,3-5 charcoal,6 plastics,7-9 organic compounds,3,10 semianthracites,11 coke breeze,12 * Corresponding author. Tel: +44-115-9514166. Fax: +44-1159514115. E-mail: [email protected]. (1) Chemistry of coal utilization: Second supplementary volume; Elliot, M. A., Ed.; Wiley-Interscience: New York, 1981. (2) Loison, R.; Foch, P.; Boyer, A. Coke. Quality and Production; Butterworths: Markham, ON, Canada, 1989. (3) Mochida, I.; Matsuoka, H.; Korai, Y.; Fujitsu, H.; Takeshita, K. Fuel 1982, 61, 587-594. (4) Swietlik, U.; Gryglewicz, G.; Machnikowska, H.; Machnikouski, J.; Barriocanal, C.; A Ä lvarez, R.; Dıéz, M. A. J. Anal. Appl. Pyrolysis 1999, 52, 15-31.

and petroleum cokes13,14 have all been employed. The refining of noncoking coals by solvent extraction has also been studied to obtain good coking additives.15 Moreover, blends of different additives have been added to coal to improve the quality of the resultant cokes.16 The degree of interaction between coal and additive depends on the characteristics of the coal (e.g., maceral composition, particle size, ash content), the carbonization conditions (e.g., heating rate), the ratio of mixing, and the characteristics of the additive.10 The hydrogen-transfer (5) Barriocanal, C.; A Ä lvarez, R.; Canga, C. S.; Dıéz, M. A. Energy Fuels 1998, 12, 981-989. (6) Sakurovs, R. Fuel 2000, 79, 379-389. (7) Nomura, S.; Kato, K.; Nakagawa, T.; Komaki, I. Fuel 2003, 82, 1775-1782. (8) Sakurovs, R. Fuel 2003, 82, 1911-1916. (9) Uzumkesici, E. S.; Casal-Barciella, M. D.; McRae, C.; Snape, C. E.; Taylor, D. Fuel 1999, 78, 1697-1702. (10) Mochida, I.; Marsh, H.; Grint, A. Fuel 1979, 58, 633-641. (11) Sakurovs, R. Fuel 1997, 76, 615-621. (12) Collin, G.; Bujnowska, B. Carbon 1994, 32, 547-552. (13) Meneńdez, J. A.; Pis, J. J.; A Ä lvarez, R.; Barriocanal, C.; Canga, C. S.; Dıéz, M. A. Energy Fuels 1997, 11, 379-384. (14) Pis, J. J.; Meneńdez, J. A.; Parra, J. B.; A Ä lvarez, R. Fuel Process. Technol. 2002, 77-78, 199-205. (15) Choudhury, S. B.; Brahmachari, B. B.; Dwivedi, S. R.; Roy, A. K.; Dasgupta, P. K.; Chakraborty, M.; Haque, R. Fuel Process. Technol. 1996, 47, 203-213. (16) Collin, G.; Bujnowska, B.; Polaczek, J. Fuel Process. Technol. 1997, 50, 179-184.

10.1021/ef050126n CCC: $30.25 © 2005 American Chemical Society Published on Web 09/30/2005

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Energy & Fuels, Vol. 19, No. 6, 2005

and -donor abilities of the additive have been related to the stabilization of the plastic phase present during carbonization,17,18 which might explain why additives of similar nature do not necessarily behave in a similar manner.19 To determine the effect of additives on coking blends, several analytical techniques have been used. Proton magnetic resonance thermal analysis (PRMTA), Gieseler plastometer, dilatometer, optical microscopy, and thermogravimetric analysis have all been used to elucidate fluidity development in coals and blends during carbonization. From this suite of techniques, Sakurovs has shown that PRMTA is the only technique capable of predicting thermoplastic properties and quantitatively identifying interactions between the constituents of coking blends.20,21 The parameter used to determine the fluidity of the samples, F, depends on the truncated second moment of the 1H NMR spectrum at a frequency of 16 kHz, and it is a measure of the extent to which the sample is fused. Sakurovs21 found positive and negative interactions between coking coals in blends by comparison of the experimental values with those predicted from the individual components. The magnitude of the interactions was found to depend on the rank, the maceral composition, and the fluidity of the coal, the latter defined by means of the fraction of mobile hydrogen in the sample. The plastic range was decreased in blends of coals with different rank, making these blends behave like coals of intermediate rank. Sakurovs also studied the effect of heating rate, particle size, and sample mass on the thermoplastic behavior of coking coals.6 An increase in heating rate increases the temperature of maximum fluidity and the amount of fluid material without changing the temperature at which softening starts. A linear relationship between the logarithm of the temperature of maximum fluidity and the heating rate was found. Increasing the sample mass increased the extent of fusion of the coal, and the magnitude of interactions between coals decreased with increasing the particle size over the range 0.1-1 mm. Other authors have deconvoluted the spectra into Lorentzian and Gaussian distribution functions that represent the fluid and rigid components in the sample, respectively.22-25 The fraction and mobility of these two phases can be calculated from the areas and the widths at half-height of the spectrum peaks. The major trends found by Maroto-Valer et al.23 using this approach were that particle size below 150 µm suppresses softening at maximum fluidity through a reduction in the mobility of the fluid phase, mild oxidation decreases the concentration of fluid material, an increase in heating rate enhances the mobility of the fluid phase without altering (17) Neavel, R. C. In Coal Science; Gorbaty, M. L., Larsen, J. W., Wender, I., Eds.; Academic Press: New York, 1982; Vol. 1, p 1. (18) Yokono, T.; Obara, T.; Sanada, Y.; Miyazawa, K. Carbon 1984, 22, 169-171. (19) Grint, A.; Marsh, H. Fuel 1981, 60, 513-518. (20) Sakurovs, R. Fuel 1997, 76, 623-624. (21) Sakurovs, R. Fuel 2003, 82, 439-450. (22) Maroto-Valer, M. M.; Andre´sen, J. M.; Snape, C. E. Fuel 1998, 77, 921-926. (23) Maroto-Valer, M. M.; Andre´sen, J. M.; Snape, C. E. Energy Fuels 1997, 11, 236-244. (24) Maroto-Valer, M. M.; Andre´sen, J. M.; Snape, C. E. Fuel 1997, 76, 1301-1308. (25) Andre´sen, J. M.; Martıń, Y.; Moinelo, S. R.; Maroto-Valer, M. M.; Snape, C. E. Carbon 1998, 36, 1043-1050.

Dıáz et al.

the concentration, and fluidity development is a reversible process as long as the sample is quenched rapidly from the temperature of maximum fluidity. The contributions of pyridine extractables and low molecular mass species or metaplast created at low temperatures (ca. 400 °C) to the generation of coal fluidity have also been quantified.23,25 The concentration of fluid material from 200 to 380 °C were mainly attributed to the pyridine extractable material, since very few covalent bonds are expected to break at such low temperatures. The pyridine extractables did not increase the amount of metaplast but increased the mobility of the semifluid phase in the pyridine residue. The metaplast-generated material (pyridine residue) accounted for one-third to one-half of the fluid phase generated. Rheometry is a technique that has been used to measure the linear viscoelastic properties of coal samples. A detailed description of the technique can be found in a previous article by Steel et al.26 Nomura et al.27 showed that small amplitude oscillatory shear rheometry could be used to measure the viscoelastic properties of coal. Further work by Yoshida et al.28 on 10 coals compared results obtained from the rheometer and the Gieseler plastometer and found that the temperatures at which changes occur were in fairly good agreement. Moreover, Hayashi et al.29 found that there is a linear relationship between the percentage of mobile hydrogen obtained through high-temperature 1H NMR and the logarithm of the complex viscosity obtained through rheometry during isothermal and nonisothermal heating of coal. Steel et al.26 have shown that these techniques could be used to gain a greater understanding of the mechanisms behind coking pressure generation. In general, studies have focused on the effect of a particular additive on fluidity of a coal/additive blend using different pyrolysis conditions and a very restricted number of samples. However, little attention has been paid to the effect of a wide range of additives with different characteristics in a similar coking blend and the possible causes for their effects. Furthermore, few techniques can provide information about fluidity development and interactions between different components in a coking blend such as in situ high-temperature 1H NMR and rheometry. Therefore, these techniques have been combined in this study to pursue the following objectives: (1) Elucidate and compare the effects of different additives in coking blends on fluidity development under identical pyrolysis conditions. (2) Identify the parameters that might originate these effects to optimize coking blends. (3) Study possible relationships between 1H NMR and rheometry parameters. Experimental Section 1. Coal and Additives. A prime coking bituminous coal, referred in this work as coal A, with moderate volatile content (26) Steel, K. M.; Castro-Diaz, M.; Patrick, J. W.; Snape, C. E. Energy Fuels 2004, 18, 1250-1256. (27) Nomura, S.; Kato, K.; Komaki, I.; Fujioka, Y.; Saito, K.; Yamaoka, I. Fuel 1999, 78, 1583-1589. (28) Yoshida, T.; Iino, M.; Takanohashi, T.; Katoh, K. Fuel 2000, 79, 399-404. (29) Hayashi, J-I.; Morita, M.; Moriyama, R.; Chiba, T. Fuel 2003, 82, 1735-1741.

Effect of Different Additives in Coking Blends

Energy & Fuels, Vol. 19, No. 6, 2005 2425

Table 1. Characteristics of the Coal

sample

ash content (wt % db)

volatile content (wt % db)

sulfur content (wt % db)

total dilation (%)

maximum fluidity (ddpm)

coal A

5.6

25.2

0.68

248

100

Table 2. Characteristics of the Additives sample petroleum coke 1 petroleum coke 2 anthracite 1 anthracite 2 wood charcoal low T coke breeze tyre crumb

ash content volatile content sulfur content (wt % db) (wt % db) (wt % db) 0.4 0.5 11.8 5.9 0.3 5.1 10.7 5.9

10.7 11.4 6.0 7.8 69.4 17.4 1.7 72.7

4.93 3.57 0.60 1.00 0.07 0.11 0.54 2.21

The spectra were deconvoluted into Gaussian and Lorentzian functions, which enable the fraction of total hydrogen that is mobile (Lorentzian distribution) and rigid (Gaussian distribution) to be calculated. The higher the concentration of mobile hydrogen and the longer the spin-spin relaxation time for the Lorentzian component (T2L), the higher the fluidity. Therefore, fluidity depends on both concentration and mobility of the fluid (mobile) phase. The temperature at which fluidity is maximum is defined here as Tmf. Tmf does not necessarily have to be equal to the temperature of maximum concentration of fluid phase (Tmc). The percentages of fluid phase expected assuming additivity in the blends have been calculated at the Tmf of the coal. The expected percentage of fluid phase is given by:

Predicted % of fluid H ) wt fluid H (A) at Tmf + wt fluid H (X) at Tmf wt H (A) at Tmf + wt H (X) at Tmf

(ca. 25 wt %) and low Gieseler fluidity (100 dppm) was used to prepare 4:1 wt/wt blends with eight different additives. The additives were composed of two anthracites, two petroleum cokes (pet. cokes), wood, a low-temperature coke breeze, charcoal, and tyre crumb. The characteristics of the coal and additives are summarized in Tables 1 and 2, respectively. An active carbon was also used to establish the most detrimental effect that could be attained. 2. In Situ High-Temperature 1H NMR. A Doty 200 MHz 1 H NMR probe was used in conjunction with the Bruker MSL200 instrument for the fluidity development studies. The solid echo pulse sequence (90°-τ-90°) was used to determine the variation in pulse width as a function of the temperature. The pulse length was increased from 3.50 µs at room temperature to 4.75 µs at the final temperature. Typically, 200 scans were accumulated with a recycle delay of 0.3 s. Coal A, the additives, and the 4:1 wt coal/wt additive blends were analyzed. Approximately 140-150 mg of sample was packed lightly into the container. Only particles within the 53-212 µm fraction (>99%) were considered for analysis since particle sizes below 150 µm have been proven to suppress softening at maximum fluidity.23 The container was closed with a screw that had a hole to allow the diffusion of volatiles, and the assembly was placed horizontally in the stator. A flow of 16 dm3 min-1 of dry nitrogen was used to transfer heat to the samples and to remove the volatiles that escaped from the container. Below the sample region, a flow of 50-60 dm3 min-1 of dry air was used to protect the electrical components of the probe. In addition, air was blown at a rate of 20 dm3 min-1 into the region between the top bell Dewar enclosing the sample region and the outer side of the probe to prevent the temperature exceeding 110 °C. The sample temperature was monitored using two thermocouples simultaneously, with one touching the sample container in the stator and the other close to the heater. The probe temperature was calibrated using compounds of known melting point. These compounds were phenanthrene (mp 100 °C), methyl red (mp 179 °C), 2,2′-dithiodibenzoic (salicylic) acid (mp 288 °C), and decacyclene (mp 395 °C). The sample temperature was determined using the temperature indicated by the stator thermocouple corrected by means of the equation derived from the temperature calibration. The temperature was raised in increments of approximately 25 °C from room temperature to 370 °C, and then by 10 °C to the final temperature (ca. 495 or 540 °C). Accordingly, two lag times were employed whereby the lag time in the first and second stages were approximately 3.75 and 1.50 min, respectively. The smaller increments in temperature after 370 °C were used to obtain more detailed information at temperatures close to that of maximum fluidity, and the lag time was introduced to allow the temperature to stabilize. Duplicate analyses were carried out with the blends to ensure reproducibility in the results.

× 100 (1)

where the numerator represents the weight of fluid H in coal (A) and additive (X) at the temperature of maximum fluidity of the blend, and the denominator represents the total H in coal and additive at the temperature of maximum fluidity. The contributions from the coal have been calculated using the following expressions:

wt H (A) at Tmf ) wt A at 120 °C ×

wt A at Tmf

× wt A at 120 °C wt H (A) at Tmf (2) wt A at Tmf

wt fluid H (A) at Tmf ) wt H (A) at Tmf × wt fluid H (A) at Tmf wt H (A) at Tmf

(3)

Correspondingly, the individual contributions of the additive have been calculated using similar expressions (i.e., X instead of A). The ratio of the weight of sample at Tmf and 120 °C was determined by thermal gravimetric analysis (TGA). The ratio of total H weight to sample weight at Tmf was determined by placing the samples in a 20 MHz 1H NMR MARAN instrument previously calibrated with polystyrene. The ratio of fluid H weight to total H weight at Tmf was determined by deconvolution of the peak signal obtained at this temperature in the 200 MHz Bruker instrument. 3. Rheometry. Rheological measurements were performed using a Rheometrics RDA-III high-torque controlled strain rheometer. Coal A and the 4:1 wt coal/wt additive blends with particle sizes within the range 53-212 µm were pressed under 5 tons of pressure in a 25-mm die to form disks with a thickness of approximately 2.6 mm. The experimental procedure has been described elsewhere.26 4. Thermogravimetric Analysis. A Perkin-Elmer Pyris 1 thermogravimetric analyzer was used to quantify the weight loss of coal A and additives during pyrolysis. Approximately 9 mg of the 53-212 µm fraction of sample was loaded into the crucible of the TGA system. The furnace was heated from 25 to 525 °C at a constant heating rate of 3 °C min-1 in nitrogen, and the sample mass in the crucible was recorded every 2 s.

Results and Discussion 1. Behavior of the Coal and Additives during Pyrolysis. To understand the interactions between additives and coal, it is necessary to elucidate the behavior of these samples individually during pyrolysis. In this manner, the rate of mass loss as determined by TGA and percentage of fluid phase (based on mobile H)

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Dıáz et al. Table 3. Proportion of Fluid Phase and T2 Values Determined through High-Temperature 1H NMR for the Additives at the Tmf of Coal A (450 ( 5 °C) properties for additives at 450 ( 5 °Ca additive

% fluid H

petroleum coke 1 petroleum coke 2 anthracite 1 anthracite 2 wood charcoal low T coke breeze tyre crumb

8 7 20%). However, their T2L values are completely different (i.e., 165 µs cf. 35 µs), which points out the significant differences between these two additives. In fact, the low value of T2L for wood suggests that its fluid phase is highly viscous, and it largely disappears by ca. 400 °C (Figure 2). The other additives contain less than 10% fluid material, which should not contribute significantly to the overall fluidity of the blend. The two anthracites and the low T coke breeze are perfectly represented by a single rigid (Gaussian) component. From these results, only tyre crumb and wood might contribute to an increase in the fraction of fluid material in the blend at 450 °C. The other additives are only expected to interact to different extents with the fluid material of coal A. 2. Elucidation of the Effects of the Additives in the Blends by High-Temperature 1H NMR. Prior to presenting the results for the blends, it has to be taken into account that the densities of tyre crumb and wood are lower than those of the other additives, which imply higher volumes of additive in the blends and potential higher degrees of interaction with coal. Bearing this in mind, the peak half-width evolution with temperature for the blends of coal A with 20 wt % additive is shown in Figure 3. Since the effect of additives is primarily in the fluid region, only this region is shown. The temperature of maximum fluidity for the blends is similar to that for the coal despite the high proportion of additive in the blend, which proves that fluid phase formation does not depend on the additive but only on the coal. The minimum peak half-width is similar for coal A and blends with pet. cokes 1 and 2, anthracite 2, and tyre crumb (ca. 3.6 kHz) within experimental error. Blends with charcoal, low T coke breeze, wood, and anthracite 1 all develop a minimum peak half-width of around 4.8 kHz. The blend with active carbon has the highest value for the minimum peak half-width (i.e., 5.4 kHz). Since the peak half-width does not have any physical meaning


Figure 3. Variation of the 1H NMR peak half-width for the coal and the 4:1 wt/wt blends at different pyrolysis temperatures.

Figure 4. Evolution of the fluid phase concentration with temperature for the coal and the 4:1 wt/wt blends.

and depends on both the fluid phase concentration and fluid phase mobility, the effect of the additives on these two parameters will now be studied in more detail. Figure 4 shows the changes in percentage of fluid H for coal A and the 4:1 wt/wt blends with temperature. The fluid H for the coal and blends with petroleum cokes, anthracites, charcoal, and low T coke breeze follows similar nonlinear trends during softening for temperatures below 410 °C. The blends with tyre crumb, active carbon, and wood also follow similar trends in this temperature range, but the increase in fluid H with temperature is slower than that for the coal and the other blends. The differences in the percentage of fluid H among the coal and blends (with the exception of the blend with tyre crumb) at this stage are not significant. However, the differences in the concentration of fluid H in the blends start to vary considerably as the temperature rises from 410 to 450 °C. The resolidification stage seems to mirror the situation of the softening stage. The blend with tyre crumb contains more fluid material than coal A alone, which suggests that the fluidity in the blend mainly originates from the high concentration of fluid material still remaining in the additive (ca. 60%, Table 3). Figure 5 presents the changes in mobility of the fluid phase (T2L) with carbonization temperature. There are small differences in the fluid phase T2L values of the blends at temperatures around 350 °C (i.e., 64 µs for


Figure 5. Evolution of T2 of the fluid phase with temperature for the coal and the 4:1 wt/wt blends.

the blend with pet. cokes and anthracite 2 cf. 56 µs for the blend with low T coke breeze). However, these differences increase slightly as the temperature increases up to temperatures close to 400 °C, where the T2L values for the blends with pet. cokes and anthracite 2 are ca. 80 µs and for the blend with the low T coke breeze it is 64 µs (i.e., 16 µs difference cf. 8 µs at 350 °C). At temperatures higher than ca. 400 °C, there is a drastic increase in the gradients of T2L with temperature for blends with pet. cokes and anthracite 2. On the contrary, the blend with active carbon only shows an increase in T2L similar to that occurring at low softening temperatures (10 wt % db) could be directly related to the deleterious effects in the concentration and mobility of the fluid material possibly catalyzing char/semicoke formation. Since both the concentration and mobility of the fluid phase might control the thermoplastic properties of coking blends, it is important to determine to what extent they affect the viscoelastic properties of the samples. 3. Comparison of High-Temperature 1H NMR and Rheometry Results. Figure 6 shows the complex viscosity for the coal and 4:1 wt/wt blends as a function of temperature. The increase in complex viscosity with temperature between 350 and 400 °C for coal A was

d

n.a. ) not

Figure 6. Evolution of complex viscosity with temperature for the coal and the 4:1 wt/wt blends.

thought to be due to slippage of the coal particles that comprise the sample disk,26 although physical and chemical changes giving an increase in the elastic or storage modulus (G′) should not be ruled out. The temperature at which the complex viscosity of coal A starts to decrease in the softening stage is approximately 415 °C, which is within experimental error for the onset temperature for fast volatilization (Figure 1). The complex viscosities of coal A and the blends remain fairly constant with temperature up to 425 °C. At this temperature, there is a drastic reduction in complex viscosity as the percentage of fluid H in the blends ranges from ca. 35 to 55% (Figure 5). In general, the temperature of minimum complex viscosity from the rheometer is fairly similar to the temperature of maximum fluidity from high-temperature 1H NMR results (i.e., 450 ( 5 °C). Table 4 indicates that coal A develops the lowest minimum complex viscosity (ca. 6 × 103 Pa s) in the whole series. The minimum complex viscosity in the blends increases in the order of tyre crumb (ca. 8 × 103 Pa s) < anthracite 1 (ca. 2.5 × 104 Pa s) < pet. coke 1 (ca. 3 × 104 Pa s) ≈ anthracite 2 (ca. 3 × 104 Pa s) ≈ pet. coke 2 (ca. 3 × 104 Pa s) < low T coke breeze (ca. 6.5 × 104 Pa s) < charcoal (ca. 8 × 104 Pa s) < active carbon (ca. 2 × 105 Pa s) ≈ wood (ca. 2 × 105 Pa s). In the previous section, it was found that significant negative effects in both the concentration and mobility of the fluid phase were caused by charcoal, anthracite 1, low T coke breeze, active carbon, and wood. These additives, with the exception of anthracite 1, also cause


the most deleterious effects in the viscoelastic properties of the blends. To ascertain the possible inter-relation of minimum complex viscosity, fluid phase concentration, and fluid phase mobility, blends with similar rheological or 1H NMR properties will now be compared. Some blends develop similar minimum complex viscosities. For instance, the 4:1 wt/wt blends with charcoal and the low T coke breeze have similar minimum complex viscosities, being approximately 6.5 × 104 Pa s for the blend with coke breeze and 8 × 104 Pa s with charcoal. However, the fluid phase concentration for the blend with coke breeze is significantly higher than the fluid phase concentration with charcoal (60% cf. 52%). However, the T2L in the blend with the low T coke breeze is somewhat lower (85 µs) than the T2L in the blend with charcoal (91 µs), and this might compensate for the lower concentration of fluid material. The minimum complex viscosities in the 4:1 wt/wt blends with wood and active carbon are also fairly similar (ca. 2 × 105 Pa s). However, the amount of fluid phase is inhibited greatly by the addition of wood (45%) in comparison to active carbon (i.e., 51%). Here again, the higher T2L in the blend with wood (91 µs cf. 77 µs with active carbon) might compensate for concentration effects to produce similar viscoelastic characteristics. Other blends give rise to similar concentrations of fluid material. The blend with tyre crumb develops the same amount of fluid material as the coal alone (i.e., 73%). However, the minimum complex viscosity of the blend with tyre crumb (ca. 8 × 103 Pa s) is higher than that of the coal alone (ca. 6 × 103 Pa s). This small difference between the coal and the blend might be ascribed to the small difference in the T2L of the fluid phase (103 µs in coal A cf. 99 µs in the blend). The blend with the low T coke breeze also develops roughly the same amount of fluid phase (60%) as for anthracite 1 (57%), but it has a minimum complex viscosity higher than that of the blend with the anthracite 1 (8 × 104 Pa s cf. 2.5 × 104 Pa s). This difference in viscosity again might be related to the higher T2L of the fluid phase in the blend with anthracite 1 (90 µs) than that in the blend with the low T coke breeze (85 µs). Blends with similar mobilities such as those with charcoal and wood (i.e., T2L ) 91 µs) show an inverse relationship between the minimum complex viscosity (8 × 104 Pa s and 2 × 105 Pa s, respectively) and the percentage of fluid H (51 and 45%). Therefore, the minimum complex viscosity in coking blends seems to be inversely proportional to the maximum fluid phase concentration and the maximum T2L. However, an exception to this rule occurs with the 4:1 wt/wt blend of coal A and anthracite 1. This blend has a minimum complex viscosity slightly lower than those for the blends with pet. coke 1, pet. coke 2, and anthracite 2 (i.e., 2.5 × 104 Pa s ca. 3 × 104 Pa s), but both the maximum fluid phase concentration and T2L in the blend with anthracite 1 (57% and 90 µs, respectively) are also lower than those in the other blends (ca. 64% and 104 µs). Previous studies29,30 have correlated the complex viscosity with the percentage of rigid H in coals. Suspension models were considered for the interpretation of the correlations, assuming that the amount of (30) Hayashi, J-I.; Morita, M.; Chiba, T. Fuel 2003, 82, 1743-1750.


Figure 7. Changes in complex viscosity with percentage of rigid H during softening for coal A and 4:1 wt/wt blends.

Figure 8. Changes in complex viscosity with percentage of rigid H during resolidification for coal A and 4:1 wt/wt blends.

rigid H represents the volume fraction of solid phase. However, no suspension model was found to describe the exponential relationships obtained in the whole fluidity temperature range. The changes in complex viscosity (η*) with the percentage of rigid H (φs) obtained from 1H NMR during softening and resolidification are presented in Figures 7 and 8, respectively. The increase in temperature during the softening and resolidification processes is indicated at the bottom corner of the plots, so that φs decreases with temperature during softening and φs increases with temperature during resolidification. The results for coal A and the 4:1 wt/wt blends with petroleum coke 1, the two anthracites, charcoal, tyre crumb, and active carbon are shown. These correlations have only been made where changes in both complex viscosity and percentage of rigid H have been observed with temperature, and interpolation of data points from high-temperature 1H NMR results has been done using an arithmetic methodology to obtain more reliable information. The results show that multiple exponential functions rather than a single function are necessary to describe the changes in complex viscosity and percentage of rigid H in the whole fluidity temperature range. In addition, the softening and resolidification stages do not superimpose, showing that the physical and chemical transformations in the two fluidity stages are different. Figure 7 indicates that there are two regimes during coal softening, the first one occurring at high φs (i.e.,

2430


Dıáz et al.

Table 5. Fitting Values for the Exponential Relationships between Complex Viscosity and Percentage of Rigid H during Softening and Resolidification for Coal A and 4:1 Wt/wt Blendsa softening η* ) y0 + b × exp(K1φs) sample

φs-min

coal A + petroleum coke 1 + anthracite 1 + anthracite 2 + charcoal + tyre crumb + active carbon

27 33 44 39 n.a. n.a. n.a.

φs-max 36 48 55 53 n.a. n.a. n.a.

η* ) y0 + b × [1 - exp(-K2φs)]

∆φs

b(Pa s)

9 15 11 14 n.a. n.a. n.a.

3.24 × 4.37 × 103 1.92 × 102 0.24 × 102 n.a. n.a. n.a. 105

K1 (-) 0.017 0.086 0.133 0.174 n.a. n.a. n.a.

φs-min 36 48 55 53 49 36 n.a.

φs-max 57 69 74 69 71 45 n.a.

∆φs

b(Pa s)

K2 (-)

21 21 19 16 22 9 n.a.

1.30 × 5.31 × 106 1.10 × 107 1.18 × 107 1.26 × 107 1.14 × 107 n.a.

0.094 0.059 0.065 0.062 0.076 0.121 n.a.

107

resolidification η* ) y0 + b × exp(K1φs) sample

φs-min

coal A + petroleum coke 1 + anthracite 1 + anthracite 2 + charcoal + tyre crumb + active carbon a

33 33 44 39 49 28 49

φs-max 67 55 54 59 59 48 65

∆φs

b(Pa s)

34 22 10 20 10 20 16

1.71 × 0.50 × 102 1.37 × 103 3.01 × 102 1.38 × 102 2.43 × 103 8.94 × 103 103

η* ) y0 + b × [1 - exp(-K2φs)] K1 (-) 0.074 0.164 0.109 0.123 0.128 0.097 0.068

φs-min 67 55 54 59 59 48 65

φs-max 87 73 67 75 88 59 82

∆φs

b(Pa s)

K2 (-)

20 18 13 16 29 11 17

2.93 × 3.80 × 108 4.21 × 108 2.40 × 109 2.70 × 108 1.91 × 108 7.62 × 107

0.140 0.123 0.125 0.146 0.093 0.141 0.073

109

R2 > 0.98 in all cases. n.a. ) not applicable.

beginning of softening) and characterized by an exponential rise to maximum relationship between the complex viscosity and the percentage of solid phase. The second regime during coal softening occurs at low φs (i.e., near maximum fluidity), and it is characterized by an exponential growth relationship between complex viscosity and percentage of solid phase. The same two exponential regimes can be seen during resolidification (Figure 8). Despite T2L effects not being considered, eqs 4 and 5 have been found to fit remarkably well (R2 > 0.98 in all cases) the exponential rise to maximum and exponential growth relationships, respectively.

η* ) y0 + b × [1 - exp(- K2φs)]

(4)

η* ) y0 + b × exp(K1φs)

(5)

where y0, b, K1, and K2 are constants. The values of b, K1, and K2 together with the minimum value of φs (φs-min), the maximum value of φs (φs-max), and the range of φs (∆φs) where the equations can be applied are presented in Table 5 for the coal and blends with petroleum coke 1, the two anthracites, charcoal, tyre crumb, and active carbon during softening and resolidification. The softening stage for the blend of coal A with charcoal is fully represented by a single-exponential rise to maximum relationship. No attempt has been made to fit the data for the blends with active carbon in the whole φs range during softening because there were not enough data points. On the other hand, the data points for the blend with tyre crumb for low φs values during softening (i.e., near maximum fluidity) did not fit any relationship. The values for K1 and K2 are very similar (ca. 0.1) regardless of sample and plastic stage. The values of b in each exponential rise to maximum stage do not change significantly and are very similar to the complex viscosity of the solidlike material before (softening) and after (resolidification) the plastic temperature range. On the contrary, the values of b differ considerably in the exponential growth relationships. The values of φs-min and φs-max also vary significantly

among the samples for each exponential correlation. From these results, the parameter b from the exponential growth relationships and the values of φs-min and φs-max in the exponential correlations during softening could be useful for optimizing coking blends. Despite the omission of mobility effects (T2L) in these relationships, the different stages found here might provide for an alternative and fast way to understand the physical and chemical interactions caused by additives in coking blends. More work is being carried out with a wider range of coking coals to confirm the applicability of these exponential relationships and obtain a clearer understanding of the dependency of the exponential constants. Conclusions The comparison of the experimental and calculated values for the amount of fluid material in the blends showed small positive effects in the blends with the two petroleum cokes (ca. 5% increase in concentration), neutral effects with the addition of tyre crumb and anthracite 2, and negative effects for the rest of the additives (>7% reduction in fluid material and >12% reduction in T2L). In the latter, the destruction of the fluid material is influenced by the additive through a combination of stronger adsorption and chemical reaction mechanisms. The most detrimental effects on fluidity seem to be caused by microporous structures through a combination of adsorption and char/semicoke forming reactions possibly promoted by surface functional groups, as indicated by the low concentration and T2 of the fluid phase and high minimum complex viscosity in blends with wood, charcoal, and active carbon. Additives with high ash contents (e.g., >10% ash on a dry basis) such as anthracite 1 and the low T coke breeze also cause significant negative effects through reactions to increased semicoke/char. The fact that the differences found between the minimum complex viscosity and maximum concentration of fluid H for different blends can be ascribed to the effects caused by the mobility of the fluid phase


suggests that there is inter-relationship among these three parameters. For individual blends, two exponential relationships have been found between the complex viscosity and the concentration of solid phase in both the softening and resolidification stages but the parameters are different for each blend. Acknowledgment. We thank the European Coal and Steel Community (Contract 7220-PR/100) and


Engineering Physical Sciences Research Council (EPSRC, Grant No. GR/R62854/01) for financial support. We also express our gratitude to CPL Industries for supplying the coal and additives and to Dr. Robin G. Graham for providing the software to deconvolute the 1H NMR spectra. EF050126N

Determination of the Effect of Different Additives in Coking Blends

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