Coprocessing of Pyrolytic Nitrogen Removal of Low-Rank Coals and

Mar 21, 2017 - Temperature-programmed pyrolysis of low-rank coals physically mixed with goethite-rich limonite ore has been studied with a fixed-bed q...
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Coprocessing of Pyrolytic Nitrogen Removal of Low-Rank Coals and Reduction of Limonite Ore Tsubouchi Naoto,*,† Mikawa Yusuke,† Mochizuki Yuuki,† Kikuchi Takemitsu,‡ and Ohtsuka Yasuo‡ †

Center for Advanced Research of Energy and Materials, Hokkaido University, Sapporo 060-8628, Japan Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan

‡

S Supporting Information *

ABSTRACT: Temperature-programmed pyrolysis of low-rank coals physically mixed with goethite-rich limonite ore has been studied with a fixed-bed quartz reactor for the purpose of developing a novel coprocessing method for the removal of coal-bound nitrogen and the reduction of goethite. Limonite at loadings of 5−40 mass % promotes the formation of N2 from brown coal at 650−1000 °C and the consequent nitrogen removal, although the effect levels off beyond 20 mass % and thus seems to be limited. X-ray diffraction (XRD) measurements after pyrolysis showed that goethite can readily be reduced to metallic iron (αFe), irrespective of the limonite loading and the type of coexisting coal, and that part of the α-Fe reacts with a small amount of carbon to form cementite, a solid solution of iron and carbon. This method might have potential for the simultaneous production of low-nitrogen chars and reduced iron materials by utilizing low-value coals and limonite ores.

1. INTRODUCTION

2. EXPERIMENTAL SECTION

During the sintering of iron ore, the use of low-nitrogen coking coal/char is important to reduce the amount of NOx that must be removed downstream. Low-rank coals, which are both abundant and inexpensive, show promise as fuels for the sintering of iron ore, provided that their nitrogen content can be reduced. It has been reported that the majority of the nitrogen contained in coal (coal-N) is present in thermally stable pyrrolic and pyridinic forms and that the fate of coal-N during pyrolysis is mainly dependent on the temperature and heating rate.1−8 We have been developing a method to remove coal-N through the use of inexpensive catalytic materials. During this endeavor, we have determined that FeOOH nanoparticles precipitated/supported on low-rank coals (in particular, brown coal) can significantly promote the conversion of coal-N to N2 during pyrolysis in a fluidized bed at 750−1000 °C.9−12 Limonite is a low-grade iron ore whose main component is goethite (α-FeOOH). This material has been considered for applications as a raw material in blast furnaces, where it might be an alternative to more expensive high-grade ores, providing that it can be reduced by a simple process. However, because α-FeOOH displays excellent catalytic performance during the degradation of ammonia and pyridine to N2,13−19 it is anticipated that this material might have applications as a catalyst for the pyrolytic removal of coalN. In this study, we performed a detailed investigation of the formation of gaseous nitrogen-containing compounds during the pyrolysis of mixtures of low-rank coals and limonite. We also elucidated the chemical states of the iron in these samples before and after pyrolysis. The main objective of this work was to assess the feasibility of nitrogen removal from low-rank coals with the simultaneous reduction of limonite.

2.1. Coal, Limonite, and Their Mixture. Brown coal from Australia and Adaro sub-bituminous coal from Indonesia, denoted as LY and AD, respectively, were used in the present study. Although the coal samples were distributed from the Japanese boiler maker company Loy Yang, it is unknown how and when they were recovered. These samples were supplied in vacuum-sealed plastic bags to avoid air oxidation. These samples were air-dried at room temperature, ground, and sieved to coal particles with the size fraction of 75−150 μm. The ultimate and proximate analyses of the samples are reported in Table 1 and reveal that the total nitrogen contents of LY and AD were 0.60 and 1.0 mass %, respectively, on a dry, ash-free (daf) basis. The metal composition of each coal sample employed in this study was analyzed by inductively coupled plasma emission spectrometry (ICP-ES) and atomic absorption spectrophotometry (AAS) after acid leaching of the ashes obtained by heating the coal samples to 815 °C.20 These ash composition are listed in Table S1 of the Supporting Information. As expected, Si was the main component for LY, followed by Ca, Fe, Al, and Mg. On the other hand, for AD, the order of these metals was Al < Fe ≈ Ca < Mg < Si, and these metal contents was smaller than those of LY. Robe river (RR) limonite ore from Australia was employed as a lowvalue iron ore, because it contains a large amount of α-FeOOH, as mentioned below. Table 2 lists the physical and chemical properties of RR limonite with the same size fraction (75−150 μm) as for the coal samples. The metal content was measured by ICP-ES and AAS after acid leaching of the limonite.20 As expected, Fe was the predominant metal element in the limonite, and the amount was 59 mass % on a dry basis, followed by Si. XRD measurements also showed that the limonite gave very strong diffraction peaks attributable to α-FeOOH (Figure S1, Supporting Information), the content of which was estimated to be approximately 90 mass % (dry). The average crystalline size of α-FeOOH was as small as 30 nm, and the Brunauer−Emmett−Teller (BET) surface area of the limonite was 50 m2/g.

© 2017 American Chemical Society

Received: January 12, 2017 Revised: March 17, 2017 Published: March 21, 2017 3885

DOI: 10.1021/acs.energyfuels.7b00123 Energy Fuels 2017, 31, 3885−3891

Article

Energy & Fuels Table 1. Ultimate and Proximate Analyses of Coals Used ultimate analysis [mass % (daf)]

a

proximate analysis [mass % (dry)]

coal

countrya

C

H

N

S

Ob

ash

VMc

FCb,d

LY AD

AUS IDN

70.9 76.9

7.8 8.0

0.60 1.0

0.36 0.14

20.3 14.0

1.4 1.7

53.9 51.8

44.7 46.5

AUS, Australia; IDN, Indonesia. bEstimated by difference. cVM, volatile matter. dFC, fixed carbon. X-ray photoelectron spectroscopy (XPS) measurements were also conducted with a nonmonochromatic Mg Kα source operating at 240 W to investigate the chemical forms and surface functionalities of the char nitrogen and the α-FeOOH-derived iron species. The analytical conditions were described in detail previously.17,22,24,25 Each specimen was first pressed onto an indium plate, then kept into the vacuum chamber for about 1 h to remove adsorbed water and gas, and finally introduced into the detector chamber under a high vacuum to start the XPS measurement. A long acquisition time of several hours was used to ensure good resolution for the N 1s and Fe 2p3/2 spectra. To examine the depth profile, Ar sputtering for 1 min was also carried out. Binding energies of all of the N 1s and Fe 2p spectra observed were referred to the In 3d5/2 peak of In2O3 at 444.9 eV,24,25 and the former spectra were deconvoluted into pyridinic, pyrrolic, and quaternary nitrogen forms by the least-squares curve-fitting method using Gaussian peak shapes in a manner similar to that reported previously.12,17,22 The reproducibility of the present deconvolution analysis was within ±5% in every case.

Table 2. Chemical and Physical Properties of Limonite Used property content [mass % (dry)] Mg Al Si Ca Fe crystalline speciesa α-FeOOH content [mass % (dry)] α-FeOOH sizeb (nm) surface areac (m2/g)

value 0.04 0.87 2.2 0.04 58.5 α-FeOOH (s), SiO2 (w) 93 30 50

a

Identified by XRD: w, weak; s, strong in intensity. bAverage crystalline size determined by the Debye−Scherrer method. c Measured by the BET method.

3. RESULTS AND DISCUSSION 3.1. Nitrogen Mass Balance. Figure 1 shows the nitrogen distributions after pyrolysis at 1000 °C of LY and AD coals

Limonite/coal mixtures were prepared in the following manner: LY or AD coal was first mixed with limonite in high-purity water at ambient temperature for 1.5 h with a rotary evaporator, and the resulting mixture was then dried under a vacuum at 40 °C for 15−16 h to remove water. The limonite loading in the dried samples was 20 mass %, unless otherwise stated. 2.2. Pyrolysis Run and Product Analysis. All experiments were carried out in a fixed-bed quartz reactor. In a typical run, about 0.50 g of the as-prepared mixture was first charged into a quartz cell in the center of the reactor, which was then heated at 10 °C/min in a stream of high-purity He (>99.99995%) at a flow rate of 200 mL (STP)/min with an electric furnace with infrared image lamps attached and soaked for 2 min at a predetermined temperature (550−1000 °C). Special care was taken to ensure that the whole reaction system was free from any leakage before each run. Measures were taken to ensure that the system was leak-proof, and measurements were carried out to check the leakage of the reactor system. Details of the apparatus and procedure were described previously.21 The amount of N2 evolved during pyrolysis was determined online at 2-min intervals with a high-speed micro gas chromatograph (GC) equipped with a thermal conductivity detector, whereas HCN and NH3 were analyzed at 3-min intervals with a multigas monitor employing the photoacoustic infrared detection method. The amounts of CO, CO2, CH4, and H2O evolved were also measured by the GC. The nitrogen in the tar or solid residue recovered after pyrolysis was determined with a conventional, combustion-type nitrogen analyzer. The analytical procedure has been reported in more detail elsewhere.22,23 Yields of N2, HCN, NH3, tar nitrogen, and char nitrogen are expressed in terms of percentages on a coal nitrogen basis, where the reproducibility of each yield was within ±5% in every case. The extent of nitrogen removal was estimated on a dry, ash- and limonitefree (dalf) basis from the change in the nitrogen content of the feed sample before and after pyrolysis. The extent of removal of N was calculated based on the amount of N in the char and the amount of N in the raw coal. 2.3. Characterization. XRD analyses of limonite/coal mixtures pyrolyzed at 550−1000 °C were performed with an X-ray diffractometer using Mn-filtered Fe Kα radiation to follow the changes in the crystalline forms of the iron species naturally present in the added limonite. The average crystalline sizes of α-FeOOH, magnetite (Fe3O4), wustite (FeO), α-Fe, and cementite (Fe3C) were estimated by the Debye−Scherrer method.

Figure 1. Effect of limonite addition on nitrogen distribution after pyrolysis of LY and AD coals at 1000 °C.

without and with 20 mass % limonite. The weight loss of each coal upon pyrolysis was almost unchanged by limonite addition, and the value was nearly equal to content of volatile matter (Table 1) in the corresponding coal determined by proximate analysis. As shown in Figure 1, the nitrogen mass balance fell within the range of 95−103% in every case, and N2 was the predominant product among nitrogen-containing species evolved up to 1000 °C for all samples. Interestingly, the presence of limonite changed the nitrogen distribution, mainly in terms of the yields of N2 and char nitrogen, irrespective of the type of coal: It increased N2 but decreased char nitrogen. The limonite also tended to lower the yields of tar nitrogen, HCN, and NH3 from both coals. It has been reported that fine particles of FeOOH precipitated onto an Australian brown coal can catalyze N2 formation from char nitrogen during fluidizedbed pyrolysis.9−12 We have also shown that an Australian αFeOOH-rich limonite promotes the decomposition of NH3 and pyridine (C5H5N) as a model compound of tar nitrogen and that it provides high NH3 and C5H5N conversions of more than 3886

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a main peak at 440 °C followed by a shoulder at about 650 °C (dashed line in Figure 3b). As shown by the solid lines in Figure 3a,b, the limonite added to the coal decreased the rates of both HCN evolution at 300−1000 °C and NH3 evolution at 300−800 °C, and the extent of the decrease was slightly larger for HCN. These tendencies were also observed with AD coal. Every data point in Figure 1 was normalized on the basis of a nitrogen balance of 100%, and the changes in the yields of N2, volatile nitrogen (sum of tar nitrogen, HCN, and NH3), and char nitrogen upon limonite addition, denoted as ΔN2, ΔVolatile-N, and ΔChar-N, respectively, were calculated. With LY coal, ΔN2, ΔVolatile-N, and ΔChar-N were estimated to be +21%, −6%, and −15%, respectively. In other words, the absolute value of ΔN2 corresponded to approximately 70% of the absolute value of ΔChar-N, which means that the N2 formed upon limonite addition comes mainly from char nitrogen and/or precursors. ΔN2, ΔVolatile-N, and ΔChar-N for AD coal were +10%, −3%, and −7%, respectively, and thus, the extent of the contribution of char nitrogen to the N2 formed in the presence of limonite was almost the same as that observed for LY coal. It is also likely that about 30% of the N2 arises from secondary decomposition reactions of volatile nitrogen.28 Figure 4 shows the effects of the limonite loading on the N2 yield and nitrogen removal at 1000 °C for LY coal. The yields

90% at 750 °C in fuel gas components produced in an airblown coal gasifier.13−19 3.2. Fate of Coal-Bound Nitrogen and Extent of Nitrogen Removal. Figure 2 presents the rate of N2

Figure 2. Rate of N2 formation in the temperature-programmed pyrolysis of LY coal without and with 20 mass % limonite.

formation as a function of temperature during the pyrolysis of LY coal without and with 20 mass % limonite. In the absence of limonite, N2 started to evolve at about 550 °C, and the rate increased with increasing temperature, but it decreased after reaching its maximal value at about 680 °C. When limonite was added to the coal, it promoted N2 formation predominantly in the temperature range of 650−1000 °C, and the catalytic effect was largest at 800−820 °C, as the rate of N2 formation with 20 mass % limonite in this region was about 6.5 times higher than that without limonite. A similar trend was also observed with AD coal. However, the effects of limonite on N2 formation differed significantly for the two coals, and it was larger for the LY coal (Figure 1). The rates at which HCN and NH3 were evolved during pyrolysis of the same samples as in Figure 2 are provided in Figure 3, where the vertical scale of each rate is expressed as

Figure 4. N2 yield and nitrogen removal at 1000 °C for LY coal as a function of limonite loading.

of tar nitrogen, HCN, and NH3 at 5−40 mass % limonite were 5−6%, 2−4%, and 2−3%, respectively, and were thus almost constant, irrespective of the type of nitrogen. On the other hand, as seen in Figure 4, the N2 yield increased with increasing limonite loading and tended to level off beyond 20 mass % limonite, being about 40% at 20−40 mass % limonite. The extent of nitrogen removal was also larger at higher loadings, and it exhibited almost the same loading dependence as N2 and became approximately 55% at 20−40 mass % limonite (Figure 4). On the basis of the results in Figures 2 and 4, it is likely that the limonite at loadings of 5−20 mass % can efficiently catalyze the formation of N2 from coal at temperatures of ≥650 °C and the consequent nitrogen removal. It has been reported that nanoscale particles of FeOOH precipitated onto an Australian brown coal can catalyze conversion reactions of char nitrogen to N2 during fluidized-bed pyrolysis and that the N2 yield at 1000 °C reaches about 50%.9−12 Although the catalytic effect of limonite is thus lower than that of the precipitated FeOOH

Figure 3. Rates of evolution of (a) HCN and (b) NH3 in the two experiments shown in Figure 2.

twice that for N2 in Figure 2. In the absence of limonite, the evolution of HCN from LY coal started at about 320 °C, and the rate gave an asymmetric, broad peak at 520 °C and tended to decrease gradually with increasing temperature beyond the maximal value at 520 °C (dashed line in Figure 3a). On the other hand, NH3 was evolved predominantly between 300 and 800 °C, and the profile for the rate of NH3 evolution exhibited 3887

DOI: 10.1021/acs.energyfuels.7b00123 Energy Fuels 2017, 31, 3885−3891

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the sizes of α-Fe and Fe3C were in the ranges of 85−90 and 70−85 nm, respectively, whereas FeO was too large (more than 100 nm) to be determined by the Debye−Scherrer method. At 1000 °C, FeO seemed to disappear completely, and only the two species of α-Fe and Fe3C remained as crystalline iron forms (Figure 5f), with sizes of 90 and 65 nm, respectively (Table 3). Similar results were also observed for all limonite/coal mixtures examined. It is thus evident that the α-FeOOH in limonite can readily be reduced to α-Fe under the conditions applied, regardless of the limonite loading or the type of coexisting coal. In addition, the presence of Fe3C observed in Figure 5d−f points out the occurrence of a reaction between α-Fe and the LY char substrate at temperatures of 785−1000 °C. Figure 6 shows the Fe 2p3/2 XPS spectra of 20 mass % limonite/LY samples after pyrolysis at 710 and 810 °C, where

catalyst, it is interesting that limonite catalyzes N2 formation from char nitrogen in spite of their simple physical mixing. AD coal showed a loading dependence of N2 yield and nitrogen removal similar to that of LY coal, although the degree of the dependence was smaller. Improvement of the catalytic effect for sub-bituminous coal will be the subject of future work, because the removal of nitrogen from such coal might be of practical significance. 3.3. Characterization Results of Limonite/Coal Mixtures before and after Pyrolysis. Figure 5 presents the XRD

Figure 5. XRD profiles of 20 mass % limonite/LY samples: (a) asprepared and (b−f) after pyrolysis at (b) 550, (c) 710, (d) 785, (e) 810, and (f) 1000 °C. Figure 6. Fe 2p3/2 XPS spectra (a) before and (b) after Ar sputtering for 20 mass % limonite/LY samples after pyrolysis at 710 and 810 °C.

a

Table 3. Average Crystalline Sizes (nm) of All of the Iron Species Observed in Figure 5 temperature (°C) − 550 710 785 810 1000 b

α-FeOOH 30 ndc ndc ndc ndc ndc

Fe3O4 c

nd 25 ndc ndc ndc ndc

FeO c

nd ndc 30 >100 >100 ndc

α-Fe c

nd ndc 90 85 90 90

significant formation of N2 from the mixture takes place (Figure 2). The sample without Ar sputtering provided a broad spectrum in the binding energy range of 707−716 eV, regardless of pyrolysis temperature (Figure 6a). Given that it has been reported that iron oxides, such as Fe3O4, Fe1−xO, and Fe2O3, give 2p3/2 signals at 708−712, 709−711, and 710−712 eV, respectively,26,27 such oxides are likely to be the main iron forms on the sample surface. Part or all of the oxides could be the FeO identified by the XRD analyses (Figure 5 and Table 3), and the remainder could be formed by the oxidation of α-Fe upon exposure to laboratory air for sample recovery. When each sample in Figure 6a was exposed to Ar-ion bombardment to remove surface oxides, the 2p3/2 shoulder peak attributable to α-Fe and/or iron carbides appeared clearly at 707 eV in every case,26,27 and the peak intensity was larger at a higher temperature (Figure 6b). These observations indicate the transformation of the α-FeOOH in limonite into α-Fe and/or the carbides upon pyrolysis and thus agree with the XRD results (Figure 5 and Table 3). Figure 7 shows the N 1s XPS spectra of the same samples as in Figure 6. Without Ar sputtering, the observed N 1s spectrum was broad in the binding energy range of 396−403 eV, irrespective of pyrolysis temperature (solid lines in Figure 7a). The least-squares curve-fitting analysis also found that three Gaussian components existed at 398.6, 400.3, and 401.5 eV (broken lines in Figure 7a), corresponding to pyridinic,

Fe3C c

nd ndc ndc 70 85 65

a

Calculated by the Debye−Scherrer method. bBefore pyrolysis. cNot detectable.

profiles of 20 mass % limonite/LY samples, and Table 3 summarizes the average crystalline sizes of all of the iron species identified by the XRD analyses. As can be expected from the results in Table 2, the as-prepared mixture provided the diffraction signals attributable to α-FeOOH and SiO2 (Figure 5a), and the crystalline size of the former species was 30 nm (Table 3). When the mixture was heated to 550 °C, as shown in Figure 5b and Table 3, the initial α-FeOOH was transformed into Fe3O4 with a size of 25 nm. At 710 °C, Fe3O4 disappeared almost completely, and instead, FeO and α-Fe appeared (Figure 5c), with sizes calculated to be 30 and 90 nm, respectively (Table 3). When the temperature was increased further to 785−810 °C, the XRD intensity of α-Fe increased, accompanied by a corresponding decrease in the intensity of FeO, and Fe3C appeared (Figure 5d,e). As reported in Table 3, 3888

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analogy with the catalysis of N2 formation from char nitrogen by iron nanoparticles.11,12,30 Clarification of the mechanism of limonite-catalyzed N2 formation in detail should be the subject of future study. 3.4. Behavior of Oxygen Release and Extent of Reduction. Figure 8 presents the rates of evolution of H2O,

Figure 7. N 1s XPS spectra (a) before and (b) after Ar sputtering for 20 mass % limonite/LY samples after pyrolysis at 710 and 810 °C.

pyrrolic, and quaternary nitrogen forms, respectively.2−4 The former two species were the main nitrogen forms in the char at 710 °C, whereas quaternary nitrogen was minor. The N 1s XPS measurement of 20 mass % limonite/AD after pyrolysis at 700 °C showed that pyridinic nitrogen was the predominant nitrogen form, as seen in Figure S2 of the Supporting Information. In other words, the proportions of nitrogen forms in the AD char were different from those in the LY char. When the temperature was raised to 810 °C, where the catalytic effect of limonite on N2 formation was largest (Figure 2), pyridinic-N decreased significantly, and pyrrolic-N became the dominant form, whereas the trend for quaternary-N was unclear as a result of the slight change of