The Effect of Biomass on Fluidity Development in Coking Blends

Article pubs.acs.org/EF

The Effect of Biomass on Fluidity Development in Coking Blends Using High-Temperature SAOS Rheometry Miguel Castro Díaz,*,† Haitao Zhao,† Sylvia Kokonya,† Anthony Dufour,‡ and Colin E. Snape† †

Nottingham Fuel and Energy Centre, Department of Chemical and Environmental Engineering, University of Nottingham, University Park, Nottingham, NG7 2RD, U.K. ‡ CNRS, Nancy Université, Reactions and Processes Engineering Laboratory (LRGP), ENSIC, 1 rue Grandville, B.P. 20451, 54000 Nancy Cedex, France ABSTRACT: The addition of biomass to coking blends has the potential benefits of reducing the amount of expensive coking coals and reducing carbon emissions. However, there is little research in the use of biomass as additive in coking blends. The easily available biomass samples pine wood, sugar beet, and miscanthus have been chosen to study the effect of these additives on the fluidity properties of coal. High-temperature small-amplitude oscillatory-shear (SAOS) rheometry was used to determine the fluidity of the samples as a function of temperature. TGA and solid-state 13C NMR were also used to determine the thermal stability and compositional changes in the samples during pyrolysis. Sugar beet can be added in coking blends up to 5 wt % without altering the viscoelastic properties of the coal, whereas pine wood and miscanthus reduce the fluidity even with 2 wt % additions. No solid−solid interactions were observed in the blends, but there were differences in viscoelastic behavior that were attributed to gas−solid interactions. Fast heating (i.e., 180 °C/min) can facilitate the incorporation of pine wood and miscanthus in coking blends. The use of fast heating increases the fluidity of the coal and preserves the fluid material in the biomass at higher temperatures. As a result, the biomass acts as a regulator of fluid material development in the coal, and the viscoelastic properties of the blend are identical to those of prime coking coals. Although fast heating could also be used to produce good coking blends by combining biomass with high volatile matter, high fluidity coals, the amount of biomass required (>5 wt %) is expected to be detrimental for coke strength.

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the peaks >350 °C are mainly originating from cellulose and lignin decomposition. Caballero et al.9 studied olive stones that were treated with sulfuric acid. They found two overlapping peaks in the DTG curves: the peak at lower temperature due to the decomposition of hemicellulose and the peak at higher temperature due to the decomposition of cellulose. Because lignin is known to decompose slowly over a broad temperature range, its contribution was manifested by the flat tailing section in the DTG curves at higher temperatures. Because prolonged sulfuric acid treatment (>1 h) causes structural changes in the lignin, it was suggested that this component was formed by two independent fractions, the first formed by a “degraded lignin”, which is characterized by a low activation energy, and the second in which the lignin is similar to the native lignin. The degraded lignin would be responsible for a maximum in volatilization rate at around 200 °C. The many models that have been proposed for the devolatilization of lignocellulosic materials consider these materials as blends of cellulose, hemicelluloses, and lignin.10 It has been reported that the interactions among these components have no influence on the biomass pyrolysis, and thus, the devolatilization yield of the biomass can be expressed as the sum of the devolatilization yields of the individual components. Regarding blends of coal and biomass, Kastanaki et al.11 studied the devolatilization behavior of lignite−olive kernel

INTRODUCTION The reduced availability and high cost of prime coking coals has caused great interest in the use of additives in coking blends. The main purpose of the additive is to preserve the coking properties of the blend by either replacing the prime coking coal or modifying the properties of poor coking coals to produce blends that behave the same as prime coking coals. A wide range of carbonaceous materials have been studied as additives in coking blends during carbonization, such as coal tar pitch,1,2 petroleum coke,3,4 and plastics.5−7 However, there is little research related to the application of biomass as an additive in coking blends. A unique benefit of using biomass as an additive compared to other carbonaceous materials is that biomass reduces CO2 emissions. Most of the research carried out to date related to biomass and biomass/coal blends has focused on the characterization of the materials through thermogravimetric analysis (TGA). For instance, Raveendran et al.8 used a thermogravimetric analyzer and a packed-bed pyrolyser to study the pyrolysis characteristics of biomass and biomass components. These authors found that cellulose decomposes at 300−430 °C and that the decomposition rate is the highest while the char yield is the lowest of all the biomass constituents. Lignin decomposes at 250−550 °C, and its char yield is the highest, whereas hemicellulose and xylan are thermally the most unstable and start to decompose at a much lower temperature than the other components. They suggested that the DTG peaks can be identified by the decomposition temperature, where the peaks at 250−350 °C are mainly originating from hemicellulose decomposition and © 2012 American Chemical Society

Received: November 23, 2011 Revised: January 27, 2012 Published: February 3, 2012 1767

dx.doi.org/10.1021/ef2018463 | Energy Fuels 2012, 26, 1767−1775

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Therefore, the aims of this work are to get a better understanding of the effects of biomass on fluidity development in coking blends and explore the benefits of using fast heating rates during carbonization using for the first time hightemperature small-amplitude oscillatory shear (SAOS) rheometry.

blends using TGA and found that there were no significant interactions in the solid phase between the coal and biomass. However, they suggested that possible synergistic effects in the gaseous or gas−solid phase might occur. Vuthaluru12 also found a linear relationship between the char yield and the concentration of biomass in the blend. In this work, the degradation temperature of coal decreased with increasing biomass content in the blend, which was attributed to the decomposition of biofuels. On the contrary, Haykiri-Acma and Yaman13 found that char yields from coal/biomass coprocessing were different than expected. It was suggested that the relatively high reactivity of biomass and its chemical composition could lead to increases in conversion. Moreover, the heat release by secondary reactions was reported to promote the volatilization of primary tars which in turn reduces the char yield. Interestingly, they found that blends with high rank coals such as bituminous coal and anthracite showed relatively low deviations whereas low rank coals showed high deviations. Blesa et al.14 showed that biomass makes easier the release of hydrogen sulfur via pyrolysis in blends with coal, which was explained by the hydrogen donor property of the biomass. Furthermore, Jones et al.15 studied the pyrolysis of blends of three coals with pine wood and found that a significant quantity of C10−C20 aliphatic material was produced from coal during pyrolysis at 500−600 °C. However, this aliphatic material was not observed during copyrolysis of coal/ biomass blends. The product distribution in the blends was predominantly composed of alkyl methoxy benzenes, and it was much simpler than biomass alone indicating synergy. During the low heating rate copyrolysis tests in a TGA, there was no interaction between coal and biomass since the release of volatile material increased linearly with the amount of biomass in the blend. Additivity or absence of synergy was observed when the pyrolysis of the coal did and did not overlap the pyrolysis of the biomass. However, it was pointed out that synergistic effects require overlapping of pyrolysis rates and good particle contact. Indeed, the volatiles from batch pyrolysis tests had a higher contact time and led to synergy in the blends, and the pyrolysis oils from the blends became poor in aromatics and rich in phenols. Altering the analysis conditions by increasing the heating rate can also reduce the intrinsic devolatilization rate, which results in the overlapping of the different devolatilization peaks.10 More importantly, the biomass experiences higher devolatilization onset temperatures, and the volatile release can take place at temperatures where the coal starts to devolatilize. Although these studies provide information about the thermal degradation characteristics of the coal/biomass blends and any synergistic effects, there is a lack of information about the effects on fluidity development that is crucial in the cokemaking process. High-temperature small-amplitude oscillatoryshear (SAOS) rheometry is a powerful technique that can monitor fluidity development in biomass and coal during pyrolysis. This technique has been used in the past to elucidate the effect of different additives in coking blends16 and measures the linear viscoelastic properties of the sample. The elastic (or solid) and viscous (or liquid) character of the sample can be determined by recording the phase angle (δ), which varies between 0° and 90°. For an ideal elastic material, δ = 0°, and for an ideal viscous material, δ = 90°. Another important parameter is the complex viscosity (η*), which decreases as the material becomes more liquidlike in character.

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EXPERIMENTAL SECTION

The pine wood (Scots pine), sugar beet (dried roots), and miscanthus (stems of Miscanthus x giganteus) were ground using a rotor miller and the 80−200 μm fraction was used. Information about the chemical composition of the pine wood and sugar beet samples was not available. However, the chemical composition of the miscanthus was

Table 1. Composition of Scots Pine, Sugar Beet Pulp, and Miscanthus cellulose (wt %, db) hemicellulose (wt %, db) lignin (wt %, db)

Scots pine

sugar beet pulp

miscanthus

40.0 28.5 27.7

24.6 27.9 2.6

45.8 27.6 26.6

determined following the procedure mentioned elsewhere.17 Table 1 presents the chemical characteristics for the miscanthus together with those obtained from the literature for Scots pine18 and sugar beet pulp.19 A prime coking coal (coal A) and a poor coking coal (coal B) with particle sizes of 53−212 μm were used in the blends with biomass. The

Table 2. Characteristics of the Coals volitale matter (wt %, daf) ash (wt %, db) vitrinite (wt %) mean vitrinite reflectance CSR CRI I40 I10 FSI Gieseler max. fluidity (ddpm) dilatation (%)

coal A

coal B

25.2 9.7 58.5 1.13 70 20 49 21 8.0 817 84

31.9 9.8 76.0 1.05 50 30 52 24 7.5 534 88

characteristics of these coals are presented in Table 2. Coal A and coal B are regarded as prime and poor coking coals, respectively, on the basis of their CSR values. The CSR index measures the coke mechanical strength after reaction with carbon dioxide, and it cannot be below 60%, as it results in higher pressure losses and reduced furnace permeability.20 The blends were prepared by weighing and mixing the exact amounts of coal and biomass to ensure that the sample was representative. The blend was mixed mechanically for a few minutes until it became homogeneous. TGA measurements of coal A, biomass samples, and blends were carried out on a TA Q500 instrument. Approximately 10−15 mg of sample was put into the sample cell of the TGA. The samples were pyrolyzed in nitrogen at a constant heating rate of 3 °C/min from room temperature to 600 °C. A nitrogen flow rate of 100 cm3/min was used to sweep out the volatile products. The calculated weight percentage for the blends was determined by the sum of the weight 1768


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percentages of the individual components using the following equation: i W blend 0 W blend

× 100 = Xcoal

i Wcoal 0 W coal

× 100

× 100 + Xbiomass

i W biomass 0 W biomass

(1)

where Wi and W0 are respectively the weight of the sample at time i and time 0, and X is the fraction of the component in the blend. High-resolution solid-state 50 MHz 13C NMR analyses were carried out in a Bruker Avance 200 spectrometer using the cross-polarization (CP) pulse sequence in conjunction with magic angle spinning (MAS). The acquisition time was 0.05 s, the relaxation delay was 1.5 s and the contact time for CP/MAS was 1 ms. The spectra were obtained with 20 000 scans. The samples were packed tight into a cylindrical (7 mm o.d.) zirconia rotor with a cap made of a homopolymer of chlorotrifluoroethene (Kel-F) and spun at the magic angle (54°44′) with a spinning rate of approximately 5 kHz. Tetrakis(trimethylsilyl)silane (TKS) was added to the samples as an internal standard. The FIDs were processed using a line broadening factor of 20 Hz. The small-amplitude oscillatory shear (SAOS) rheometry measurements were performed in a Rheometrics RDA-III high-torque controlled-strain rheometer. Coal, biomass, and coal/biomass mixtures (1.5 g) were compacted under 5 tons of pressure in a 25 mm die to form disks with thickness of approximately 2.6 mm. The tests involved placing the sample disk between two 25 mm parallel plates, which had serrated surfaces to reduce slippage. The biomass samples were heated from room temperature to 400 °C using a heating rate of 3 °C/min. The coal and coal blends were heated in two regimes:

Figure 1. Weight loss as a function of temperature using a heating rate of 3 °C/min for coal A and the biomass samples.

(1) A slow heating regime in which the sample was heated from

room temperature to 330 at 180 °C/min and heated from 330 to 500 °C at a rate of 3 °C/min. This regime was denoted as slow heating because the heating rate in the thermoplastic temperature range of the coal (400−500 °C) is 3 °C/min. (2) A fast heating regime in which the sample was heated from room temperature to the softening temperature of the coal (420−440 °C) at 180 °C/min and from this temperature to 500 °C at 3 °C/min. The furnace surrounding the sample was purged with a constant flow of nitrogen to transfer heat to the sample and remove volatiles. The sample temperature was monitored using a thermocouple inside the furnace. A continuous sinusoidal-varying strain with amplitude of 0.1% and frequency of 1 Hz (6.28 rad/s) was applied to the sample from the bottom plate throughout the heating period. The stress response on the top plate was measured to obtain the complex viscosity (η*), phase angle (δ), and plate gap (ΔL) as a function of temperature.

Figure 2. Rate of weight loss as a function of temperature using a heating rate of 3 °C/min for coal A and the biomass samples.

contain different amounts of pyrolyzed organic constituents (Table 1). In general, the pyrolysis of wood occurs in a stepwise manner with hemicellulose breaking down first at 200−260 °C, cellulose next at 240−350 °C, and lignin at 280−500 °C.21 Therefore, the peak at 320 °C in pine wood can be ascribed to the decomposition of cellulose, the peak at 200 °C and the small shoulder at 260 °C on the left side of the cellulose peak can be assigned to the decomposition of hemicellulose, and the flat tailing section at 360−400 °C to the decomposition of lignin. On the other hand, the peaks at 260 and 320 °C in miscanthus correspond to the decomposition of hemicellulose and cellulose, respectively. Similarly, the peak at 180 °C in sugar beet can mainly be ascribed to hemicellulose decomposition and the peak at 280 °C to cellulose decomposition. Figure 3 shows the complex viscosity as a function of temperature when using a heating rate of 3 °C/min for coal A and the biomass samples. The minimum in complex viscosity for the coal occurs at 460 °C. Sugar beet presents two minima in complex viscosity, one at around 210 °C and the other at 330 °C. Pine wood has one minimum at 350 °C, and the miscanthus has two minima at 260 and 330 °C. The minimum at 330−350 °C in sugar beet, pine wood, and miscanthus seems to be caused by the cellulose and lignin degradation. A comparison of the temperatures of minimum complex viscosity to those for the maximum in weight loss indicates that the temperatures of maximum volatile release are slightly lower than those for minimum complex viscosity in the coal, sugar beet, and miscanthus. Although this finding suggests that the reduction in viscosity is directly related to the thermal

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RESULTS AND DISCUSSION Figure 1 presents the TGA results for coal A and the biomass samples using a heating rate of 3 °C/min under nitrogen. The thermal behavior of pine wood and miscanthus is fairly similar. Sugar beet starts to degrade at lower temperatures than the other two biomass samples, although the amount of residue at 600 °C is higher than in pine wood or miscanthus (i.e., 45% compared to 300 °C. Figure 2 shows the rate of weight loss as a function of temperature in the samples. The coal has a maximum in the rate of weight loss at 450 °C. The biomass samples have two clear maxima in weight loss rate. In the case of sugar beet, the maxima occur at about 180 and 280 °C, whereas the maxima in pine wood and miscanthus occur at around 260 and 320 °C. Although the DTG plots of pine wood and miscanthus look similar they are not identical because they 1769


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cellulose in the original samples is similar, although there is more aromatic carbon (115−150 ppm) in pine wood than in sugar beet and the amount of carboxyl groups from hemicellulose in sugar beet (172−175 ppm) is higher than in pine wood. As the pyrolysis temperature increases to 210 °C, the sugar beet softens, as shown in Figure 3, but the chemical composition in the sample does not change. Hence, the viscoelastic properties seem to be affected by physical rather than chemical processes. At higher temperatures (350 °C), there are chemical transformations mainly due to the pyrolysis of cellulose in pine wood. In this manner, the aromatic carbon content (115−150 ppm) increases at the expense of C-1, C-2, C-3, C-4, C-5, and C-6 in cellulose (60−105 ppm). Further heating to 500 °C leads to the formation of a char, which is highly aromatic in character. Figure 5 shows the weight loss as a function of temperature for coal A, biomass samples, and their blends with 5 wt % and 10 wt % of biomass. The experimental and calculated trends for the blends are also presented to elucidate any synergistic effects. The results show that there are not significant synergistic effects between the coal and biomass, which is in agreement with previous findings.12,23 The viscoelastic behavior of the blends of coal A with 2−20 wt % of pine wood, sugar beet, and miscanthus is shown in Figure 6. None of the additives is able to increase the fluidity in the coal, and an increase in the amount of biomass usually reduces the amount of fluid material in the blend. The extent of this reduction depends on the type of biomass used. For instance, increasing the amount of pine wood and miscanthus in the blend originates a rather gradual increase in the complex viscosity, even with small amounts of additive (2 wt %). On the contrary, the addition of 2−5 wt % sugar beet to coal does not seem to have a significant effect on the fluid characteristics of the coal, and these blends may potentially be used in coking ovens. Further addition of sugar beet reduces the fluidity of the blend but to a lesser extent than the pine wood or the miscanthus. The plots of phase angle against temperature also show similar results. The threshold that defines if the material is predominantly liquidlike or solidlike in character is 45°, where liquidlike materials have phase angle values within 45−90°. The maximum value for phase angle was attained with the coal alone

Figure 3. Complex viscosity as a function of temperature for coal A and the biomass samples.

degradation of the sample, it cannot be extended to the results from pine wood. The pine wood, sugar beet, and miscanthus samples were analyzed through solid-state 13C NMR. The samples analyzed were the initial biomass samples, the samples at the temperature of maximum fluidity (i.e., minimum complex viscosity), and the chars at 500 °C. Since pine wood and miscanthus have shown similar thermal degradation and viscoelastic behavior, only the results of pine wood and sugar beet are presented (Figure 4). The peak position at 3.5 ppm corresponds to the internal standard TKS (tetrakistrimethylsilane). The peak positioned at 22−25 ppm is usually associated to methyl groups of hemicellulose.22 The peak at around 56.8 ppm originates from methoxyl groups from lignin. The peaks between 62 and 89 ppm are associated to C-2, C-3, C-4, C-5, and C-6 in cellulose. The peak at 105 ppm is related to C-1 of cellulose, the broad peak developing at high temperatures in the 115−150 ppm region corresponds to aromatic carbon, and the peak at 175 ppm corresponds to carboxyl groups from hemicellulose. These results show that cellulose and hemicellulose dominate in the biomass samples. The distribution of

Figure 4. Solid-state 13C NMR spectra of pine wood (left) and sugar beet (right) at different temperatures. 1770


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Figure 5. Weight loss as a function of temperature using a heating rate of 3 °C/min for the blends of coal A with pine wood (top), sugar beet (middle), and miscanthus (bottom).

the overlapping of the thermoplastic temperature ranges of both coal and biomass. Initially, the effect of fast heating on the individual components was assessed. The heating rate used was 180 °C/min because this is the maximum rate that can be achieved in the rheometer. Evidently, this is not expected to be the optimum heating rate to increase fluidity development, and thus, a wider range of heating rates will have to be considered in future work. Figure 7 presents the complex viscosity (η*) for coal A when applying fast heating of 180 °C/min to temperatures before and during coal softening (430, 440, and 450 °C), followed by slow heating of 3 °C/min to 500 °C. The softening temperature is defined here as the temperature at which the complex viscosity starts to drop and corresponds to approximately 440 °C for coal A. The results for coal A when using fast heating to 330 °C followed by slow heating of 3 °C/ min to 500 °C are also shown for comparison. The effect of fast heating to temperatures close to the softening temperature of the coal (430 and 440 °C) is an increase in the amount of fluid material (lower viscosity). However, the temperature of maximum fluidity does not change within experimental error, which suggests that the chemical transformations occurring

(ca. 55°). The phase angle in the blends with sugar beet rises above the 45° threshold, even with additions of 20 wt %. The phase angle in the blends with pine wood and miscanthus rises above 45° as long as the concentration of biomass in the blend is below 15 wt % and 10 wt %, respectively. Comparing the blends with pine wood and miscanthus, the latter seems to be the worst additive, as the maximum phase angle in the blends with 10 wt %, 15 wt %, and 20 wt % of miscanthus is lower than the corresponding blends with pine wood. Therefore, the potential use of these biomass samples as additives in coking blends decreases in the order sugar beet > pine wood > miscanthus. Since TGA results showed no interactions in the solid phase, these differences in viscoelastic behavior could be ascribed to interactions of the gas/liquid phases evolving from the biomass with the precursors of fluid material in the coal. The modification of the process conditions may facilitate the introduction of pine wood and miscanthus in coking blends. For instance, it has been shown that an increase in the heating rate causes an increase in the amount of fluid material in the coal and also an increase in the temperature of maximum fluidity.24 The same effect should be expected in the biomass sample in such a way that the ultimate goal would be to achieve 1771


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Figure 6. Complex viscosity (left) and phase angle (right) as a function of temperature using a heating rate of 3 °C/min for coal A and the blends with pine wood, sugar beet, and miscanthus.

miscanthus develop most of their fluid material at 350 °C, and the temperature shift required to overlap the coal is only 100 °C. For this reason, miscanthus was chosen to study the effect of fast heating. Figure 8 presents the rheological results obtained with the miscanthus when the biomass is heated from room temperature to 430 and 440 °C using a heating rate of 180 °C/min followed by heating to 500 at 3 °C/min. The results for the individual miscanthus and coal A with slow heating rate (3 °C/min) from room temperature and 330 °C respectively are also plotted for comparison. It can be seen that fast heating of the miscanthus to 430 and 440 °C increases the fluid material in the sample since the residence time is too low

during softening are not affected. When fast heating is applied well into the softening stage (450 °C), the amount of fluid material increases further but there is also an increase in the temperature of maximum fluidity. Thus, fast heating to temperatures >440 °C is not desirable, and only fast heating to the softening temperature of the coal (430−440 °C) should be used to get overlapping of the thermoplastic temperature ranges of the coal and biomass. In the case of the biomass samples, most of the fluidity evolving from sugar beet occurs at very low temperatures (ca. 200 °C, Figure 3) in comparison to the coal (>440 °C), which could make fast heating impractical. However, pine wood and 1772


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for the volatilization reactions to take place. However, once the heating rate is reduced to 3 °C/min, resolidification takes place immediately, as indicated by the sharp increase in complex viscosity with temperature. As a result, the sample heated to 430 °C has already resolidified when the coal starts to soften. On the other hand, the sample heated to 440 °C still possesses some fluid material during coal softening and could potentially contribute to the fluidity of the blend. Figure 9 shows the complex viscosity and plate gap (ΔL) as a function of temperature for coal A with slow heating and the blend with 5 wt % miscanthus using fast heating to 430 and 440 °C. The plate gap is the difference between the initial position of the top plate of the rheometer and the position of this plate at any given time. The initial position of the top plate corresponds to the thickness of the sample disk at room temperature (ΔL = 0), so that when the sample reaches temperatures above 300 °C the disk has already expanded to certain extent and ΔL > 0. It can be seen that the blends with fast heating produce the same viscoelastic behavior, as indicated by the complex viscosity and expansion and collapse, as indicated by the plate gap, as that in the coal alone with slow heating. It also seems preferable to carry out fast heating to 440 °C, as the fluidity is slightly reduced when heating to 430 °C. Therefore, it is proposed that prime coking coals can be replaced with biomass without affecting the fluid characteristics of the semicoke by carrying out the carbonization process in two stages, whereby the first stage is a fast pyrolysis step (e.g., 180 °C/min) to shift the fluidity development in the biomass to higher temperatures and the second step consists of a slow pyrolysis process (3 °C/min), in which the reduction in fluidity caused by the presence of the biomass is counterbalanced by the increase in fluidity in the coal and leads to a coking blend with optimum fluid properties. Figure 10 compares the rheological behavior of the blends of coal A with 5 wt % of miscanthus, pine wood, and sugar beet when using the two stage process to ascertain whether these results also apply to the other biomass samples. As expected, the trends for complex viscosity in the blends with miscanthus and pine wood are similar within experimental error. However, the blend with sugar beet develops more fluidity, indicating that more sugar beet should be added to the blend or the temperature for fast heating should be reduced to obtain the optimum fluid characteristics.

Figure 7. Complex viscosity as a function of temperature for coal A with slow heating (3 °C/min) and fast heating (180 °C/min) to 430, 440, and 450 °C.

Figure 8. Complex viscosity as a function of temperature for coal A and miscanthus with slow heating (3 °C/min) and fast heating (180 °C/min) to 430 and 440 °C.

Figure 9. Complex viscosity and plate gap as a function of temperature for coal A with slow heating and the blend with miscanthus (5 wt %) using fast heating (180 °C/min) to 430 and 440 °C. 1773


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Figure 10. Complex viscosity as a function of temperature for coal A with slow heating and the blends with 5 wt % miscanthus, pine wood, and sugar beet using fast heating (180 °C/min) to 440 °C.

Figure 12. Complex viscosity as a function of temperature for sugar beet and miscanthus using fast heating (180 °C/min) to 440 °C.

fluid phase concentration in this biomass sample. Therefore, it is suggested that the higher fluidity in the blend with sugar beet could be related to the lower detrimental effect of the interactions between the gas/liquid originating from the sugar beet and the fluid constituents in the coal. If this is the case, the chemical composition of the biomass might play a pivotal role in determining its suitability as an additive in coking blends. As Figure 4 shows that sugar beet contains higher concentration of carboxyl groups (175 ppm) and lower concentration of aromatic carbon (130−150 ppm) than the other biomass samples, the aromatic carbon in the biomass could have a detrimental effect and/or the carboxyl groups from hemicellulose could have a beneficial effect on coal fluidity. It could also be argued that other coals with higher fluidity (η* < 104 Pa·s) and poorer coking properties could potentially be used to produce good coking blends. In this case, not only the amount of fluid material evolving from the coal needs to be reduced but also the temperature of maximum fluidity has to be increased. In order to increase the temperature of maximum fluidity the fast heating regime has to be performed to temperatures well into the softening stage. The increase in temperature for fast heating would lead to an increase in fluidity development in the coal that could be counterbalanced by higher additions of biomass (>5 wt %) to get a minimum in complex viscosity close to 105 Pa·s. For instance, the effect of the addition of 5 wt % miscanthus to a high fluidity coal B, whose softening temperature is around 420 °C, was studied. As previously found with coal A, the use of fast heating to 420 °C in coal B reduced the minimum complex viscosity from 3 × 103 Pa·s to 103 Pa·s without altering the temperature of maximum fluidity (not shown). However, Figure 13 shows that the addition of miscanthus increases the minimum in complex viscosity from 103 Pa·s to 104 Pa·s. Although the addition of biomass significantly reduces fluidity development, the viscoelastic properties of this blend differ to a great extent from those of the prime coking coal A with slow heating (minimum η* = 105 Pa·s, Figure 7). Thus, high amounts of biomass (>5 wt %) must be added to high fluidity, poor coking coals since the reduction in fluidity caused by the addition of 5 wt % biomass is not enough to bring the viscoelastic properties closer to those of prime coking coals.

The higher fluidity in this blend could be attributable to differences in the chemical composition of the blends. Figure 11 shows the 13C NMR spectra for the blends at the

Figure 11. Solid-state 13C NMR spectra of coal A and the blends with miscanthus (5 wt %), pine wood (5 wt %), and sugar beet (5 wt %) at their temperature of maximum fluidity when using fast heating (180 °C/min) to 440 °C.

temperature of maximum fluidity (463 °C). The peak at 230 ppm is a spinning sideband (ssb) originating from the aromatic carbon at 130 ppm, and there is another ssb in the right-hand side of the aromatic peak that overlaps the aliphatic peak. These results show that the different fluid characteristics of the blend with 5 wt % sugar beet cannot be attributable to differences in chemical composition. Another possibility is that fast heating promotes more fluid material in sugar beet than in miscanthus and pine wood. Hence, a test was carried out in the rheometer with sugar beet alone using fast heating to 440 °C. The results in Figure 12 show that the fluidity of sugar beet above 440 °C is lower than the fluidity in miscanthus. Thus, the higher fluidity development in the blend with sugar beet can not be ascribed to the 1774


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AUTHOR INFORMATION

Corresponding Author

*Tel: +44-115-8468874. Fax: +44-115-9514115. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research leading to these results has received funding from the European Union's Research Programme of the Research Fund for Coal and Steel (RFCS) research programme under grant agreement No. [RFCR-CT-2010-00007]. The authors would also like to thank Dr. Paul Pernot from Centre de Pyrolyse de Marienau (CPM) in France and Drazen Gajic from DMT GmbH in Germany for supplying the coals.

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Figure 13. Complex viscosity and plate gap as a function of temperature for coal B and the blend with miscanthus (5 wt %) using fast heating (180 °C/min) to 420 °C.

REFERENCES

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However, the addition of biomass to coking blends is expected to have a deleterious effect on coke strength. Indeed, it has been reported that an increase in the contact surface area between biomass and coal reduces coke strength.25 MacPhee et al.26 found that the addition of 5 wt % of charcoal to a coal blend reduced the CSR index from 56.7 to 35.8, which is well outside the acceptable range for metallurgical coke. Therefore, further investigation would be required to ascertain how higher amounts of biomass (>5 wt %) can be added to the coking blend without reducing coke strength. A promising approach would be the torrefaction of the biomass that increases its bulk density and hence reduces the contact area with the coal. Moreover, the particle sizes used in this study are not realistic in the coke-making industry, and bigger particles need to be considered. In this respect, an increase in the particle size distribution of the biomass might have a beneficial effect on coke strength, as it was found with charcoal.26

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CONCLUSIONS This work has studied the effect of different biomass types on the fluidity of coking blends using high-temperature SAOS rheometry for the first time. It was found that sugar beet can be added in coking blends up to 5 wt % without altering the viscoelastic properties of the coal, whereas the addition of pine wood or miscanthus has a negative effect on fluidity development even with 2 wt % additions. Although TGA showed that there are not solid interactions between the coal and biomass, the different viscoelastic behavior in the blends suggests that there are gas−solid and/or liquid−solid interactions. In addition, fast heating to the softening temperature of the coal (i.e., 180 °C/min instead of 3 °C/ min) has been proven to be a successful methodology to incorporate pine wood and miscanthus in coking blends. The use of fast heating increases the fluidity of the coal and, at the same time, shifts the development of fluid material in the biomass to higher temperatures. As a result, the biomass acts as a regulator of fluid material development in the coal, and the viscoelastic properties of the blend are identical to those of prime coking coals. This methodology could also be used with poor coking coals that possess high volatile matter contents and high fluidity (η* < 104 Pa·s), although the amount of biomass required would be >5 wt % with the consequent negative impact on coke strength. 1775


The Effect of Biomass on Fluidity Development in Coking Blends

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