Influence of Inherently Present Oxygen-Functional ... - ACS Publications

Subscriber access provided by University of Rochester | River Campus & Miner Libraries

Article

Influence of Inherently-Present Oxygen-functional Groups on Coal Fluidity and Coke Strength Yuuki Mochizuki, Ryo Naganuma, and Naoto Tsubouchi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03774 • Publication Date (Web): 15 Jan 2018 Downloaded from http://pubs.acs.org on January 15, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Energy & Fuels is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Influence of Inherently-Present Oxygen-functional Groups on Coal Fluidity and Coke Strength Yuuki Mochizuki, Ryo Naganuma, Naoto Tsubouchi* Center for Advanced Research of Energy and Materials, Hokkaido University, Sapporo 060-8628, Japan.

Abstract The effect of various oxygen-containing compounds added and/or inherent O-species on coal fluidity and coke strength has been investigated in detail. When several O-containing compounds, which have different O-containing groups, are added independently to caking coal, the MF value almost decreases, and the extent of the decrease being ether < ketone < lactone < hydroxyl < acid anhydride < < ether/hydroxyl/lactone < carboxyl group. The COOH content in four coals used increases with decreasing C%, and the MF values decrease with increasing the content. The evolution of gaseous O-containing species (CO, CO2, and H2O) during carbonization at 3°C/min of four coals up to 400°C has been studied mainly with a flow-type quartz-made fixed -bed reactor to clarify clear the effect of the amount of O-containing gases evolved on the Gieseler fluidity of coal particles. A positive correlation is found between the amount of CO, CO2, or H2O evolved up to 400°C and the COOH content in coal. However, a negative correlation between MF and O-containing gases evolved up to 400°C is observed. It is suggested that the COOH amount and/or O-containing gases evolved have adverse effects on the thermoplasticity of coal. When the indirect tensile strength of coke prepared from pelletized samples is plotted against MF values, a positive correlation is found, whereas an inverse correlation is observed between the indirect tensile strength and COOH in coals used or the O-containing gases evolved up to 400°C during carbonization. These observations indicate that some of oxygen-functional groups naturally-present in coal have a negative effect on coal fluidity, and that this effect is particularly strong in carboxyl, which can readily be decomposed into gaseous oxygen-containing species during heating up to the initial softening temperature.

ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 34

Key words: Caking coal, Non- or slightly-caking coal, coke, thermoplasticity, strength, oxygen group

1. Introduction High-strength coke is indispensable for stable operation at low-reducing agent ratio for the reduction of CO2 emissions from blast furnace in the ironmaking process. The used amount of high-quality caking coal as a raw material have been increasing with the increase in steel demand in Asian countries in recent years, and there is a concern regarding the price increases and resource exhaustion. From the viewpoint of securing raw materials for cokemaking, the development of a coke production method from low-quality coal (non- or slightly-caking coal/sub-bituminous coal/lignite) is a very important problem. As is well known, the thermoplastic performance of coal is one of the important factors for determining the coke strength. It is extremely essential to evaluate the coal fluidity to produce high-quality coke. In general, the γ component theory, metaplast theory, hydrogen donation theory, and the continuous self-dissolution model have been proposed for the thermoplastic mechanism of coal 1-5, and it is believed that a large amount of oxygen in low-rank coal has a negative effect on the fluidity: the O in coal is one of the factors dominating for coal fluidity 6-10. According to the a previous report, cross-linking generated by the decomposition/reaction of hydroxyl and carboxyl groups proceeds in the temperature range before the initial softening of coal, and has a negative influence on the coal fluidity performance in the later stage of carbonization

11,12

. In addition, the

Gieseler fluidity of caking-coal oxidized with O2 gas at low temperatures decreased almost linearly with increasing oxidation time

13,14

. These observations strongly suggest that the oxygen

naturally-present in coal has a negative effect on the thermoplasticity. Our group has been investigating the evolution behavior of gaseous O-containing compounds and fluidity during carbonization of ten kinds of caking coals and non- or slightly-caking coals in detail to clarify the effects of O on coal fluidity

9,15

. In our previous work, we found that a strong correlation

exists between the amount of CO, CO2, and H2O evolved until the initial softening temperature and the


Page 3 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Gieseler maximum fluidity value, and that O in the coal has an adverse effect on the thermoplasticity of coal 9. Furthermore, it was found that the addition of oxygen-containing compounds, including has carboxyl, ether, lactone, hydroxyl/lactone/ether groups, to caking-coal decreases the MF value considerably. The effects of carbonyl, acid anhydride and only hydroxyl groups on thermoplasticity of coal have not been cleared in our previous work 9. In addition, the influence of naturally present O-containing species in coal on the fluidity and coke strength have so far been unclear. It is of interest to examine this point in detail, because the results may help us develop a novel technique for producing high-quality coke from low-quality coal, such as lignites and subbituminous coals. In the present study, we first investigate the Gieseler fluidity of O-containing species added to caking coal to identify the adverse effects of O-containing groups on thermoplasticity of coal, following our previous work 9. The naturally present carboxyl group in coal was then measured by Fourier-transform spectrometry and titration methods to clarify the effect of the group on coal fluidity and coke strength. In addition, the release behavior of CO, CO2, and H2O from the coals was also investigated upon the initial step of carbonization, and the relationship among coal fluidity, COOH content, the amounts of O-containing gases evolved, and coke strength was investigated in detail.

2. Experimental 2.1 Coal samples Four kinds coals (caking coal, two non- or slightly caking coals and subbituminous coal) with different fluidity were used in this study. These samples were crushed and sieved with particle size fractions of < 250µm. Table 1 lists the analyses of coals used. The C and O contents in coals ranged at 73-88 and 4.5-19wt%-daf, respectively. Oxygen-containing compounds including acid anhydride, hydroxyl, ketone, ether, lactone, and carboxyl groups, were also added to RW (C, 81.8; H, 5.8; N, 2.0; S, 0.65 Odifference., 9.8 wt%-daf; ash, 8.7; VM, 37.1; FC, 54.2wt%-dry) coal by the physical mixing method 9. Table 2 shows the properties of O-containing compounds used. In this method, each compound was mixed mechanically with the coal at ambient temperature


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 34

(approximately 22°C) in a dry atmosphere. The oxygen loading in the dried sample was 1.0wt%, unless otherwise stated. An O-containing compound, such as phthalic acid anhydride (C8H4O3) and 1,4-napthalenediol (C10H8O2) and 5,7,12,14-pentacenetetron (C22H10O4), were used, following our previous work, which used benzofuran (C8H6O), phthalide (C8H6O2), 2-naphthoic acid (C11H8O2) and fluorescein (C20H12O5) to investigate the influence the O-containing groups, such as acid anhydride, hydroxyl, ketone, ether, lactone, carboxyl, and ether/hydroxyl/lactone, on coal fluidity.

2.2 Carbonization Carbonization experiments were carried out with a flow-type fixed-bed quartz-made reactor. The details of the apparatus have been reported previously

16

. After

approximately 0.50g of the dried sample was charged into a cell in the reactor, the reactor was vacuum pumped. Then, high-purity He (>99.9999%) was introduced at 200cm3(STP)/min into the whole system including the reactor and tar trap, and the effluent was analyzed using a micro gas chromatography (GC) (Agilent, 3000A) using MS-5A and PP-Q columns to ensure that the remaining air was replaced with the He until the N2 concentration in the reactor outlet gas decreased below 30ppm. The reactor was finally heated at 3°C/min to 400°C in a stream of He and quenched to room temperature. The amounts of CO, CO2 and H2O evolved during carbonization were determined with a micro-GC and a multigas monitor (Innova) equipped with photoacoustic infrared detection at an interval of 2-3min. The reproducibility was within ±5%.

2.3 Gieseler plastometer The fluidity properties of the above-described samples were examined with a constant-torque Gieseler plastometer (Yoshida Seisakusho). The analytical principle of the apparatus is based on the Japanese Industrial Standard (JIS) M 8801. In the measurement, a stainless-steel crucible containing approximately 4.5g of coal particles was first placed into a lead solder bath in the plastometer and then heated in laboratory air at 3 °C/min up to 250-550°C. The heating rate value was the same as that during carbonization runs. Every sample was also stirred at a


Page 5 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

constant-torque during the Gieseler tests, and the fluidity was recorded in dial divisions per minute (ddpm) with respect to temperature. The analytical conditions have been reported in more detail previously 16,17. The reproducibility of maximum fluidity value was within ±10%. In the present study, the initial softening temperature, maximum fluidity temperature, resolidification temperature, fluidity range temperature, and maximum fluidity, which were obtained in the Gieseler examination, were abbreviated as IST, MFT, RST, FRT, and MF, respectively.

2.4 Quantitation of carboxyl group Semi-quantitation of COOH in coals used was made by Fourier transform infrared spectroscopy (FT-IR, Jasco). The samples were first crushed and mixed with KBr, and then the mixture was pelletized, and the pellet was used for FT-IR analysis. The FT-IR analysis had a determination range of 5200-400cm-1, resolution of 0.1cm-1, and scan number of 200. The measured spectra were deconvoluted based on the previous work to semi-quantitate the COOH group

18

. The quantitation of the carboxyl group in coal was carried out using titration according

previous work 19. Approximately 0.5g of the sample was first added into 0.1M NaOH solution, and the mixture was then magnetically stirred at 750rpm under N2 at 25°C for 3h. After predetermined time, 0.1M HCl solution was titrated into the mixture in N2 at 25°C, and the pH of mixture was determined by a pH meter in on-line mode. Finally, the pH changing curve with 0.1M HCl addition was described. The same procedure was performed for 0.1M NaOH solution without coal sample addition, and this was defined as a blank. The COOH amount was calculated based on the difference of the amounts of HCl added at pH 8.3 between the pH curves of the blank and coal sample addition.

2.5. Coke strength measurement The pellet first was prepared to obtain the coke. Approximately 0.5g of coal sample was pelletized under 30MPa for 1min, and the pellet was carbonized in the same manner as described above. The reactor was heated up from room temperature to 900°C at 3°C/min in He. After reaching predetermined temperature, the reactor was held for 30min


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 34

and cooled down to room temperature in He. The measurement carried out against two or three coke produced in one condition. The coke compressive load was measured using compression and a crushing machine (Minebea). The indirect tensile strength of the coke was calculated by Eq. (1). f = 2・P/π・d・l

(1)

Here, f is indirect tensile strength of the coke (MPa), P is the failure load (N), d is the sample diameter (mm), and l is the sample thickness (mm).

3. Results and discussion 3.1 Thermoplasticity of coal samples used Fig. 1 shows the Gieseler fluidity profile of the coals used. The softening of both GE and TA began at approximately 395°C, and these samples resolidified at around 460 and 445°C after giving the MF values (2.40 and 1.15 log(ddpm)) at 431 and 426°C, respectively. However, the IST of GA was observed at around 425°C, and the MF value of 2.85 log(ddpm) was measured at approximately 460°C. The RST was approximately 495°C, and the IST, MFT, RST, or MF of GA were higher than those of GE or TA. On the other hand, KP did not show thermoplasticity. The MF values observed for the coal used increased in the following order: KP < TA < GE < GA, and thermoplastic profiles of TA and GE were similar. When the MF values and O contents in the coals used in the present study were investigated, a negative correlation between the above two factors was found. This shows that the inherently present O element in coal adversely affects the fluidity.

3.2 Gieseler fluidity of sample added organic oxygen species To clarify the adverse effect of inherent O-containing groups on coal fluidity, several O-containing compounds were added independently to RW coal by the physical mixing method, and the resulting samples were then subjected to the Gieseler fluidity measurements. The results are provided in Fig. 2 and Table 3. When several O-containing compounds was mixed with the coal, the IST increased from the original 390 to


Page 7 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

410-430°C, and the MF value decreased from 2.2 log(ddpm) without any additives to 0.28 log(ddpm) with 2-naphthoic acid, 0.77 log(ddpm) with fluorescein, 1.00 log(ddpm) with phthalic acid anhydride, 1.41 log(ddpm) with 1,4-naphthalenediol, 1.50 log(ddpm) with phthalide, 1.70 log(ddpm) with 5,7,12,14-pentacenetetron, except for 2.17 log(ddpm) with benzofuran. In addition, the MFT and RST did not change significantly at with the organic O-species additions, whereas the FRT decreased with decreasing MF value. The order of MF values at O-species addition was 2-naphthoic acid < fluorescein < phthalic acid anhydride < 1,4-naphthalenediol < phthalide < 1,4-penetaceneteron < benzofuran ≒ RW. In other words, the fluidity was decreased with the order of O-containing group as follows: ether < ketone < lactone < hydroxyl < acid anhydride < ether/hydroxyl/lactone < carboxyl group. Thus, the O-containing groups, except for ether, may have lowered the fluidity considerably, and it was found that the COOH group gave the largest adverse effect on thermoplasticity of coals. This is very interesting, although melting and/or boiling points of O-containing compounds used in this study are always lower than IST of RW coal, as seen in Table 2, the added O-species almost all decreased the fluidity. These results means that O-containing groups in the compounds added affect the thermoplasticity of coal particle at very low temperature (before the melting/boiling points) until the IST. It has been considered that the amount of transferable hydrogen produced during coal carbonization is one of the important factors controlling the extent of thermoplasticity of coal particles 20,21

. Therefore, it is possible that some of O-functional groups in the compounds used in Fig. 2 and/or

inherently present in coal act as hydrogen acceptors during carbonization to produce H2O and/or immobile components. According to the result in Fig. 2, the accepting ability may be significantly large in carboxyl groups, moreover, it has been reported that the COOH group can readily be converted to CO, CO2, or H2O during carbonization 6,7,11,12, as discussed in detail below, and the resulting cross-linking was formed in semi-coke. The coal with a higher amount of COOH groups may thus give a lower MF value.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

3.3 Influence of carboxyl group on thermoplasticity of coal When 2-naphthoic acid containing COOH group was added to coal, a drastic decrease in fluidity was observed in the Gieseler examination in Section 3.2. Therefore, it is also possible that naturally present COOH in coal is a part of the most important factor against coal fluidity. FT-IR analysis was thus first carried out on the samples used to clarify the effect of naturally present COOH group in raw coals on the fluidity. Fig. 3 shows the deconvoluted results for the spectrum of 1800-1500cm-1 to semi-quantitate the COOH group. In the case of KP, the absorbance band which is attributable to highly conjugated C=O, conjugated C=O, COOH, and ester were observed at 1750-1600cm-1, and KP has many O-containing groups. Similar results were found in TA. However, in GE, the absorbance bands of ester and conjugated C=O were not measured, moreover, in GA, the O-containing groups (conjugated C=O, COOH and ester) observed at other coal samples, except for high conjugated C=O, was not observed. Table 4 shows the semi-quantitative results of FT-IR analyses in the range 1800 to 1500cm-1. The proportion of O-containing groups at 1800-1500cm-1 used in the present work ranged from 11 to 29%, the order was ester < conjugated C=O ≒ COOH < highly conjugated C=O in many cases. In addition, the order of the O-containing groups and the amount of COOH group among coal samples increased as follows: GA < GE < TA < KP, and the tendencies corresponded with inverse of that of the fluidity. The results of pH changing curves by titration for COOH measurement in the coal samples are shown in Fig. 4. When 0.1M HCl solution was added into 0.1M NaOH solution without coal (blank), the pH of the NaOH solution drastically decreased from approximately 45mL of HCl addition and become a constant (pH 2.0) above 50mL addition. However, a drastically decreasing pH was observed at 35-45mL of 0.1M HCl addition in the case of the mixture of 0.1M NaOH solution with coal. The amount of HCl added by the point at which the pH was observed to drastically decrease was ordered as follows: KP < TA < GE < GA, and the pH changing curve depended on the coal samples. Fig. 5 presents the COOH amounts calculated based on the difference between the amounts of 0.1M HCl added in blank and the mixture of 0.1M NaOH and coal samples at pH 8.3 in the pH curves listed in


Page 8 of 34

Page 9 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Fig. 4. The estimated COOH amounts for KP, TA, GE, and GA were 2.00, 1.25, 0.75 and 0.60mmol/g-dry, respectively, and the order was GA < GE < TA < KP. This order observed from the titration examination corresponded with that of the FT-IR measurement and the inverse of that from the fluidity. Although the FT-IR method is semi-quantitation, it is concluded that relative amounts of COOH could be measured by this method because a positive correlation exists between FT-IR and the titration method, as shown in Fig. S1 in Supporting Information. Fig. 6 illustrates the relationship between C% and COOH contents in the coals used. A relatively strong positive correlation was observed between C% and COOH contents. This shows a similar tendency to that of XPS measurement in our previous report 9, which was carried out for ten type of coal (non- or slightly caking coals/caking coals). In other words, if the O content in coal is large, the COOH amount also tends to be high, as seen in Fig. S2 in Supporting Information. Based on the results described above, the relationship between the COOH amounts in a sample and the MF values was investigated to clarify the effect of inherently present COOH group in coal on the fluidity, and as shown in Fig. 7. The MF values tends to decrease with increasing COOH amounts in coal, and a relatively strong correlation was observed. This result suggests that the inherently present COOH group in coals influences decreasing the fluidity during carbonization. Moreover, this result is supported by decreases in fluidity under 2-naphthoic acid addition to RW coal, as shown in Fig. 2.

3.4 Evolution amounts of O-containing gases during carbonization As mentioned above, it is possible that some of the O-functional groups in coal may act as transferable hydrogen accepter, of which the accepting ability may be significantly large in carboxyl groups, resulting in the formation of H2O and inhibition of the coal fluidity. In addition, O-containing groups, especially COOH, easily decompose to promote cross-linking form CO, CO2, and H2O during initial step of carbonization, and it has been believed that the cross-linking inhibits the coal fluidity 6,7. The evolution amounts of O-containing gases are thus used as indicators of the extent of cross-linking formed during


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

carbonization 6,7,11,12. Therefore, the CO, CO2, or H2O evolution until 400°C during carbonization was investigated to clarify the effect of cross-linking on the thermoplasticity of coal, and these evolution profiles are shown in Fig. S3 in Supporting Information. The carbonization temperature of 400°C was selected because it has been found by our research group that the fate of coal-O until the coal thermoplastic range (IST) influences the coal fluidity 9. The evolutions of CO and H2O began above 200°C in each coal, and the latter rate profiles from KP, TA, and GE provided the main or shoulder peaks in the range from 300 to 400°C. On the other hand, the CO and H2O formation rate profiles from GA did not give the main peak up to 400°C. The CO2 evolution from TA and KP started above 100°C, and these rate profiles provided a broad peak at 300-400°C. The starting temperature of each gas evolution was ordered as follows: KP < TA < GE < GA, and the trend was the inverse of that of the MF values. Meanwhile, the CH4 evolution was observed above 250°C in all coals, and the rate profile was almost the same; this was not observed to show a coal type dependency. Based on the results of O-containing gases formation profile, the cumulative amounts until 400°C were calculated by integration of each of the rate profiles. The results are shown in Fig. 8. The formation amounts of CO, CO2, and H2O ranged 10-52, 3-43 and 89-167 µmol/g-dry, respectively, and the order was CO < CO2 < H2O; the amount of H2O evolved was the largest among the O-containing gases evolved until 400oC. In addition, the order of the O-containing gases among the coal types used increased as follows: GA < GE < TA < KP; this order was the inverse of that of fluidity. According previous works 11,12, the evolution of CO during carbonization derives from the reaction between O-containing groups, ie., COOH-COOH and COOH-OH, to form cross-linking and as result, the aromatic-ring carbon structure (ARCS) and ether (-O-) bond are formed, respectively. In addition, the cross-linking reaction between COOH-COOH and COOH-OH leads to the production of ARCS/ketone (-CO-) and ARCS, respectively, accompanied by CO2 formation. However, H2O formation occurs owing to the cross-linking reaction between COOH-COOH, COOH-OH, and OH-OH; the former finally produces the ARCS/-CO-/acid anhydride (-CO-O-CO-), the medium


Page 10 of 34

Page 11 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

results in ARCS/-O-/ester (-CO-O-), and the later forms an -O- bond. The extent of the cross-linking reaction with H2O production may thus be dominant during carbonization of the coals used, because the amount of H2O formation is the greatest, as seen in Fig. 8. As mentioned above, the COOH group in coal is the source of the cross-linking reaction and reacts with itself and/or the OH group, resulting in the production of CO, CO2, H2O, and a thermally stable form of -CO-, -CO-O-, -CO-O-CO, -Oand/or ARCS. Therefore, the relationship between the amounts of COOH group and O-containing gases evolved until 400°C was investigated, and the result is shown in Fig. 9. Although there is some scattering, the amounts of CO, CO2, and H2O evolved up to 400°C during carbonization tended to increase with increasing COOH group in coal. This result suggests that a portion of the CO, CO2, and H2O evolved due to decomposition of the COOH group and cross-linking formation. Fig. 10 presents the relationship between the amounts of O-containing gases evolved up to 400°C and MF values to investigate the influence of cross-linking on coal fluidity. Although there is some scattering in the data, a negative correlation was found between the amounts of O-containing gases evolved up to 400°C from the coals used and the MF values; the MF tended to decrease with increasing gas formation amounts. As mentioned above, the extent of O-containing gas formation during carbonization shows the progress of the cross-linking reaction in coal/semi-coke. Therefore, these results indirectly indicate that the cross-linking produced until 400°C, which is the initial step of carbonization and before IST, adversely affects coal fluidity.

3.5 Influence of MF value or amount of COOH group on coke strength The indirect tensile strength of pelletized coke was measured to investigate the effects of MF values and COOH content on coke strength. Fig. 11 presents the relationship between the indirect tensile strength of pelletized coke and MF values for the four type of coal under study. In the coke preparation, weakly-caking coal (BW) was used because some forming structures developed owing to high coal fluidity, for both Ga and FE which led to low indirect tensile strength of the prepared


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

pelletized coke. Therefore, in this examination, BW coal was added to the four types of coals used to inhibit formation such structures, and the strength was investigated. As seen in Fig. 11, although there is some scattering in the data, the indirect tensile strength of the prepared pelletized coke increased with increasing MF values of coals. Furthermore, the relationship between the indirect tensile strength of the prepared coke and COOH content of the coal types is shown in Fig. 12. The indirect strength of the prepared pelletized coke tended to decrease with increasing COOH content in the coal. Fig. 13 presents the relationship between the indirect tensile strength of the prepared coke and the amounts of CO, CO2, or H2O evolved up to 400°C. As expected, the indirect tensile strength of the prepared cokes decreased with increasing amounts of CO, CO2, and H2O evolved, and a stronger correlation was observed than those with MF seen in Fig. 10. This result suggests that the cross-linking produced derives from naturally present COOH in coal and affects coke strength; furthermore, the inherently present COOH group in coal adversely influence not only the fluidity, but also the indirect tensile strength of coke. These observations indicate that some of the O-functional groups naturally present in coal have a negative effect on coal fluidity, and suggest that this effect is particularly strong for COOH groups, which can readily be decomposed into gaseous oxygen-containing species during heating up to the initial softening temperature. Therefore, the cross-linking reaction mainly occurring from the inherent COOH group in coal is very important for high-strength coke production, and it will be the subject of our future work.

4. CONCLUSIONS Four types of caking coals have been carbonized in high-purity He at 3 °C/min up to 1000 °C with a fixed-bed quartz reactor, and the effect of oxygen-containing compounds and/or inherently-present O-species on coal fluidity and coke strength have been investigated in detail. The principal conclusions are summarized as follows:


Page 12 of 34

Page 13 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

(1) The four coals start softening in the temperature range of 395-400°C, provide a Gieseler maximum fluidity (MF) value of 1.1-2.9 log(ddpm) at 425-460°C, and resolidify until 495°C, except for KP.

(2) When phthalide (C8H6O2), 2-naphthoic acid (C11H8O2), fluorescein (C20H12O5) in our previous work, phthalic acid anhydride (C8H4O3), and 1,4-napthalenediol (C10H8O2), fluorescein (C20H12O5) or 5,7,12,14-pentacenetetron (C22H10O4) are added to an Australian caking coal, the MF value decreases from 2.16 log(ddpm) without any additives to 0.28-1.70 log(ddpm). The order of the decrease is as follows: benzofuran < 5,7,12,14-pentacenetetron < phthalide < 1,4-naphthalenediol < pthalic acid anhydride < fluorescein < 2-naphthoic acid. The fluidity was decreased with the order of O-containing group as follows: ether < ketone < lactone < hydroxyl < acid anhydride < ether/hydroxyl/lactone < carboxyl group, and the extent of the decrease was the largest with 2-naphthoic acid containing -COOH.

(3) The COOH content in the coals used lowered with increasing C%, and the MF values decreased with increasing COOH content. In addition, a positive correlation was found among the amounts of CO, CO2, or H2O evolved up to 400°C, which is before the initial softening temperature upon carbonization, and COOH content in coal. It was suggested that the COOH amount and/or O-containing gases evolved adversely affect the thermoplasticity of coal.

(4) When the indirect tensile strength of the prepared coke from pelletized samples was plotted against the MF values, a positive correlation was found, whereas an inverse correlation was observed between the indirect tensile strength of the prepared coke and the COOH content of the coals or the O-containing gases evolved up to 400°C during carbonization.

■ AUTHOR INFORMATION


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Corresponding Author *N. Tsubouchi. Tel.: +81-11-706-6850. Fax: +81-11-726-0731. E-mail: [email protected]. Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS The present study has been done in the Iron and Steel Institute of Japan (ISIJ) research group on “Development of Cokemaking Technology for Low-Quality Coals and Unused Carbon Resources”. References (1) Gillet, A., Stages in the dissolution of coal, Nature, 167, 1951, 406-407 (2) Krevelen, D.W.van, Heerden, C.van, Huntjens, F.J., Fuel, 30, 1951, 253. (3) Neavel, R.C., Coal Science, Edited by M.L.Gorbaty, J.W.Larsen, I.Wender, 1982, 1. (4) Yokono, Y., The chemistry of coal plasticity, Journal of the Fuel Society of Japan, 66, 1987, 675-686. (5) Takanohashi, T., Yoshida, T., Iino, M., Kato, K., Kumagai, H., Structural changes of coal macromolecules during softening, Nova Science Pub Inc Published, New York 2005, Chapter III. (6) Solomon, P.R., Hamblen, D.G., Carangelo, R.M., Serio, M.A., Deshpande, G.V., General Model of Coal Devolatilization, Energy Fuels, 1, 1988, 405-422. (7) Solomon, P.R., Serio, M.A., Deshpande, G.V., Kroo, E., Cross-Linking Reactions during Coal Conversion, Energy Fuels, 4, 1990, 42-54. (8) Suuberg, E.M., Unger, P.E., Larsen, J.W., Relation between Tar and Extractable Formation and Cross-Linking during Coal Pyrolysis, Energy Fuels, 1, 1987, 305-308. (9) Tsubouchi, N., Mochizuki, Y., Naganuma, R., Kamiya, K., Nishio, M., Ono, Y., Uebo, K., Influence of inherent oxygen species on the fluidity of coal during carbonization, Energy Fuels, 30, 2016, 2095-2101. (10) Goodarzi, F., Murchison, D.G., Optical properties of carbonized vitrinites, Fuel, 51, 1972, 322-328. (11) Sugano, M., Mashimo, K., Effects of Thermal or Steam Pretreatment on the Hydrogenolysis Reactivities of Coals, J. Jpn. Inst. Energy, 80, 2001, 1139-1147. (12) Mae, K., Maki, T., Miura, K., A new method for estimating the cross-linking reaction during the pyrolysis of brown coal, J. Chem. Eng. Jpn., 35, 2002, 778-785. (13) Seki, H.; Ito, O.; Iino, M. Effect of oxidation on caking properties: Oxidation of extract and residue. Fuel 1990, 69, 1047-1051. (14) Cimadevilla, J. L. G.; R. Álvarez, R.; Pis, J. J. Influence of coal forced oxidation on technological properties of cokes produced at laboratory scale. Fuel Process. Technol. 2005, 87, 1-10.


Page 14 of 34

Page 15 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

(15) Mochizuki, Y., Naganuma, R., Uebo K., Tsubouchi, N., Some factors influencing the fluidity of coal blends: Particle size, blend ratio and inherent oxygen species, Fuel Process. Technol., 159, 2017, 67-75. (16) Tsubouchi, N.; Mochizuki, Y.; Ono, Y.; Uebo, K.; Sakimoto, N.; Takanohashi, T. Fate of coal-bound nitrogen during carbonization of caking coals. Energy Fuels 2013, 27, 7330-7335. (17) Tsubouchi, N.; Mochizuki, Y.; Ono, Y.; Uebo, K.; Takanohashi, T.; Sakimoto, N. Sulfur and nitrogen distributions during coal carbonization and the influences of these elements on coal fluidity and coke strength. ISIJ Int. 2014, 54, 2439-2445. (18) Geng,W., Nakajima, T., Takanashi, H., Ohki, A., Analysis of carboxyl group in coal and coal aromaticity by Fourier transform infrared (FT-IR) spectrometry, Fuel, 88, 2009, 139-144. (19) Murakami, K., Kondo, R., Fuda, K., Matsunaga, T., Acidity distribution of carboxyl groups in Loy Yang brown coal: its analysis and the change by heat treatment, J. Colloid and Interface. Sci., 260, 2003, 176-183. (20) Yokono, T.; Obara, T.; Iyama, S.; Yamada, J.; Sanada, Y. Fundamental studies on coking mechanism of coal - Coal plasticity and anisotropic development in terms of transferable hydrogen and free radical. J. Fuel Soc. Jpn. 1984, 63, 239-245. (21) Kidena, K.; Murata, S.; Nomura, M. Studies on the chemical structural change during carbonization process. Energy Fuels 1996, 10, 672-678. Figure Captions Table 1 Analyses of coal samples used in this study Table 2 O-containing compound used in Gieseler fluidity examination Table 3 Summaries of Gieseler fluidity examination of several O-containing compounds added to RW coal. Table 4 Deconvolution of 1800–1500cm-1 zone in the FT-IR spectrum of coal used Fig. 1 Gieseler fluidity profiles of coal samples used. Fig. 2 Gieseler fluidity profiles for RW samples without and with oxygen-containing compounds added. Supplemental our previous results at Ref. [9] of Phthalic acid anhydride, Phthalic acid anhydride, 5,7,12,14-Pentacenetetron. Fig. 3 Deconvolution of FT-IR spectra of coals for a selected zone 1800-1500 cm-1.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig. 4 pH changing curves for 0.1M NaOH solution without and with coals during titration of 0.1M HCl solution. Fig. 5 The amounts of carboxyl group in coals calculated based on the result of Fig. 4. Fig. 6 Relationship between C and COOH contents in coals by titration method. Fig. 7 Relationship between Gieseler maximum fluidity and COOH contents in coals by titration method. Fig. 8 Amounts of CO, CO2, and H2O evolved up to 400oC carbonization of coals. Fig. 9 Relationship between COOH contents in coals determined by titration methods against total amount of CO, CO2, or H2O evolved up to 400°C. Fig. 10 Gieseler maximum fluidity against total amount of CO, CO2, or H2O evolved up to 400°C. Fig. 11 Change in indirect tensile strength of coke prepared from pelletized coal with Gieseler maximum fluidity. Fig. 12 Relationship between indirect tensile strength of coke prepared from pelletized coal and COOH contents in coals determined by titration method. Fig. 13 Indirect tensile strength of cake prepared from pelletized coals mixed with BW coal against total amount of CO, CO2, or H2O evolved up to 400°C.

.


Page 16 of 34

Page 17 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Energy & Fuels

Coal

a

Table 1 Analyses of coal samples used in this study Elemental analysis Proximate analysis wt%-daf wt%-dry C H N S Oa Ash VM b FC a,c

KP GE TA

73.6 82.8 83.5

5.5 5.7 5.1

1.5 1.9 2.0

0.7 0.7 0.4

18.7 8.9 9.0

5.8 9.3 6.2

42.9 37.8 37.5

51.3 52.9 56.3

GA

88.1

5.0

1.8

0.6

4.5

11.3

23.4

65.3

b

c

Estimated by difference. Volatile matter. Fixed carbon.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 18 of 34

Table 2 O-containing compound used in Gieseler fluidity examination Compound

O functionality

Molecular formula

Molecular weight

Melting point, °C

Boiling point °C

Ref.

Benzofuran

Ether

C8H6O

118

-18

173

[9]

Phthalide

Lactone

C8H6O2

134

75

290

[9]

Phthalic acid anhydride

Acid anhydride

C8H4O3

148

131

254

In this work

1,4-Napthalenediol

Hydroxyl

C10H8O2

160

190

-

In this work

2-Naphthoic acid

Carboxyl

C11H8O2

172

185

300

[9]

Fluorescein

Hydroxyl/lactone/ether

C20H12O5

332

315 (decomposition)

-

[9]

5,7,12,14-Pentacenetetron

Ketone

C22H10O4

338

400

-

In this work


Page 19 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Energy & Fuels

Table 3 Summaries of Gieseler fluidity examination of several O-containing compounds added to RW coal. Compounds added

MF e

Temperature, °C

Ref.

IST a

MFT b

RST c

FRT d

None

390

435

460

70

145

2.16

[9]/ In this work

Benzofuran

395

435

460

65

146

2.17

[9]

Phthalide

425

440

455

30

34

1.53

[9]


415

440

455

40

10

1.00

In this work

2-Naphthoic acid

430

440

445

15

0.28

[9]


410

435

460

50

1.41

In this work

Fluorescein

430

440

455

25

0.77

[9]

ddpm

1.9 26 5.9

log(ddpm)

5,7,12,14-Petacenetetron 415 435 460 45 50 1.70 Initial softening temperature. b Maximum fluidity temperature. c Resolidification temperature. d Fluidity range temperature. e Maximum fluidity. a


In this work

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 20 of 34

Table 4 Deconvolution of 1800–1500cm-1 zone in the FT-IR spectrum of coal used Center Proportion of area at 1800-1500cm-1 No. Functionality wavenumber, KP GE TA GA cm-1 1 Ester 1500 1 0 1 0 2

COOH

1540

4

2

3

0

3

Conjugated C=O

1575

3

1

3

0

4

Highly conjugated C=O

1610

21

16

16

11

5

Aromatic C=C

1650

18

20

20

26

6

Aromatic ring stretch

1690

18

18

20

22

7


1695

13

15

11

14

8


1700

14

20

18

18

9

Aromatic C=C

1740

9

9

8

10


Page 21 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Energy & Fuels

Fig. 1 Gieseler fluidity profiles of coal samples used.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Fig. 2 Gieseler fluidity profiles for RW samples without and with oxygen-containing compounds added. Supplemental our previous results at Ref. [9] of Phthalic acid anhydride, Phthalic acid anhydride, 5,7,12,14-Pentacenetetron.


Page 22 of 34

Page 23 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Energy & Fuels

Fig. 3 Deconvolution of FT-IR spectra of coals for a selected zone 1800-1500 cm-1.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Fig. 4 pH changing curves for 0.1M NaOH solution without and with coals during titration of 0.1M HCl solution.


Page 24 of 34

Page 25 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Energy & Fuels

Fig. 5 The amounts of carboxyl group in coals calculated based on the result of Fig. 4.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Fig. 6 Relationship between C and COOH contents in coals by titration method.


Page 26 of 34

Page 27 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Energy & Fuels

Fig. 7 Relationship between Gieseler maximum fluidity and COOH contents in coals by titration method.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Fig. 8 Amounts of CO, CO2, and H2O evolved up to 400oC carbonization of coals.


Page 28 of 34

Page 29 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Energy & Fuels

Fig. 9 Relationship between COOH contents in coals determined by titration methods against total amount of CO, CO2, or H2O evolved up to 400°C.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Fig. 10 Gieseler maximum fluidity against total amount of CO, CO2, or H2O evolved up to 400°C.


Page 30 of 34

Page 31 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Energy & Fuels

Fig. 11 Change in indirect tensile strength of coke prepared from pelletized coal with Gieseler maximum fluidity.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Fig. 12 Relationship between indirect tensile strength of coke prepared from pelletized coal and COOH contents in coals determined by titration method.


Page 32 of 34

Page 33 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Energy & Fuels

Fig. 13 Indirect tensile strength of cake prepared from pelletized coals mixed with BW coal against total amount of CO, CO2, or H2O evolved up to 400°C.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


Page 34 of 34

Influence of Inherently Present Oxygen-Functional ... - ACS Publications

Recommend Documents