Energy Fuels 2009, 23, 4502–4506 Published on Web 07/20/2009
: DOI:10.1021/ef900271d
Effect of Pretreatment and Additives on Boron Release during Pyrolysis and Gasification of Coal Yuuki Mochizuki,† Katsuyasu Sugawara,*,† and Yukio Enda‡ †
Faculty of Engineering and Resources Science, Akita University, 1-1 Tegata Gakuen-cho, Akita City, Akita Prefecture 010-8502 Japan, and ‡Environmental System Department, Akita Research and Development Center, 4-11 Sanuki, Araya, Akita City 010-1623, Japan Received March 28, 2009. Revised Manuscript Received June 26, 2009
Boron is one of the most toxic and highly volatile elements present in coal. As part of a series of studies carried out on coal cleaning to prevent environmental problems and to promote efficient coal utilization processes, the removal of boron by leaching with water and acetic acid has been investigated. The effects of the addition of ash components, that is, SiO2, Al2O3, and CaO on the control of boron release during pyrolysis and gasification were investigated. Here, 20-70% of boron in coal was removed by leaching the coal with water and acetic acid. Boron leached by water and acetic acid was related to the volatiles released from coal in pyrolysis below 1173 K. The addition of ash components such as SiO2 and Al2O3 was found to be effective in suppressing the release of boron during pyrolysis at temperatures below and above 1173 K, respectively. The addition of CaO to coal was effective in suppressing the release of boron during gasification at 1173 K.
and tourmaline19-22 is in the inorganic form. Most previous studies on boron in coal focused on the distribution of boron in fly ash and bottom ash during combustion. Clemens et al.23 reported that Ca11Si14B2O22 was formed in the bottom ash when coal containing considerable amounts of boron and calcium was combusted in a stoker furnace. Reed et al.24 showed that boron was not detected in the exhaust gas when coal containing dolomite and limestone was gasified. In other studies, the chemical form of boron in coal has been investigated by using various solvents. It has been reported that the boron in coal exists in various forms that are soluble in water, hydrochloric acid, acetic acid, and nitric acid.8-15 We have reported that the release behavior of boron varies with the type of coal used during pyrolysis.25 The content of watersoluble boron decreases from 45 to 0% in the raw coal with the increase in the carbon content from 60 to 80%. The boron of the raw coal separated into three group with different specific gravities;GI (below 1.36), GII (1.36-1.46), and GIII (above 1.46);to distinguish the distribution of boron in coal. 44, 27, and 27% of boron were distributed in GI, GII, and GIII groups, respectively, where the GI fraction has the high boron content.25 A few studies have reported the distribution of boron during the combustion of coal. However, no studies have
Introduction Boron is one of the hazardous trace elements and highly volatile elements present in coal. Boron easily volatilizes during combustion and condenses on the surface of fly ashes in the exhaust gas.1,2 The enriched boron on the ash surface is readily soluble in water.3-5 In the absence of a desulfurization system and high-temperature dust collection system in power plants, boron is released as exhaust gas into the atmosphere, which poses a risk to environmental and human health.6 It has been reported that a large part of boron present in coal is in the organic form,7-16 and boron present in Illite17,18 *To whom correspondence should be addressed. Telephone: þ81-18889-2750. Fax: þ81-18-889-2750. E-mail:
[email protected]. (1) Swaine, D. J.; Goodarzi, F. Environmental Aspects of Trace Elements in Coal; 1995; p 128. (2) Clark, L. B.; Sloss, L. L. IEA Coal Research 49; 1992 (3) Querol, X.; Juan, R.; Lopez-Soler, A.; Fernandez-Turiel, J. L.; Ruiz, R. C. Fuel 1996, 75, 821. (4) Querol, X; Umana, J. C.; Alastuey, A.; Ayora, C.; Lopez-Soler, A.; Plana, F. Fuel 2001, 80, 801. (5) Iwashita, A.; Sakaguchi, Y.; Nakajima, T.; Takahashi, H.; Ohki, A.; Kambara, S. Fuel 2005, 84, 479. (6) Clark, R. B.; Zeto, S. K.; Ritchey, K. D.; Baliqgar, V. C. Fuel 1999, 78, 179. (7) Newman, N. A.; Moore, T. A.; Esterle, J. S. Int. J. Coal. Geol. 1997, 33, 103. (8) Querol, X.; Klika, Z.; Weiss, Z.; Finkelman, R. B.; Alastuey, A.; Juan, R.; Lopez-Soler, A.; Plana, F.; Koker, A.; Chenery, S. R. N. Fuel 2001, 80, 83. (9) Kojima,T.; Furusawa, T. Nennryou kyoukaishi 1986, 65. (10) Swaine, D. J. Org. Geochem. 1992, 18, 259. (11) Solari, J. A.; Fiedler, H.; Schneider, C. L. Fuel 1989, 68, 536. (12) Eskenazy, G.; Delibatova, D.; Mincheva, E Int. J. Coal Geol. 1994, 25, 93. (13) Kimura, T. Res. Geol. 1993, 43, 187. (14) Querol, X.; Fernandez-Turiel, J. L.; Angel, L.-S. Fuel 1995, 47, 331. (15) Zhang, L.; Kawashima, H.; Takanohashi, T.; Nakazato, T.; Saito, I.; Tao, H. Energy Fuels 2008, 22, 1183–1190. (16) Goodarzi, F.; Swaine, D. J. Chem. Geol. 1994, 118, 301. (17) Bohor, B. F.; Gluskoter, H.J. J Sedim. Petro. 1973, 43, 945. (18) Boyd, R. J. Int. J. Coal Geol. 2002, 53, 43. r 2009 American Chemical Society
(19) Querol, X.; Whately, M. K. G.; Fernandez-Turiel, J. L.; Tuncali, E. Int. J. Coal Geol. 1997, 33, 255. (20) Querol, X.; Cabrera, L. I.; Pickel, W.; Lopez-Soler, A.; Hagemann, H. W.; Fernandez-Turiel, J. L. Int. J. Coal Geol. 1996, 29, 67. (21) Burchill, P.; Howarth, O. W.; Richards, D. G.; Sword, B. Fuel 1990, 69, 421. (22) Burchill, P.; Howarth, O. W.; Sword, B. Fuel Process. Technol. 1990, 24, 375. (23) Clemens, A. H.; Gong, D.; Damiano, L. F.; Matheson, T. W. Fuel 1999, 78, 1379. (24) Reed, G. P.; Dugwell, D. R.; Kandiyoti, R. Energy Fuels 2001, 15, 794. (25) Mochizuki,Y.; Kato,T.; Sugawara,K.; Enda,Y. Energy Fuels, submitted.
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Energy Fuels 2009, 23, 4502–4506
: DOI:10.1021/ef900271d
Mochizuki et al. Table 1. Analyses of Sample Coals
ultimate [wt %, daf] sample coals (abbrev.) Prima (PR) Ebenezer (EB) Illinois No. 6 (IL) Barau (BA) Samca (SA)
[ppmw, db]
proximate [wt %, db]
C
H
N
S
Odiff.
B
ash
V.M.
75.4 74.4 72.8 68.2 59.7
4.6 5.6 6.6 4.8 4.8
1.5 1.5 1.7 1.7 1.3
0.7 0.5 3.3 0.6 7.0
17.8 18.0 15.6 24.7 27.2
128 52 266 165 150
2.9 12.3 9.9 3.5 15.7
39.6 38.4 35.3 46.2 41.1
Table 2. Ash Composition of Sample Coals [wt %] sample coals (abbrev.)
SiO2
A12O3
Fe2O3
CaO
SO3
MgO
TiO2
Na2O
K2O
Prima (PR) Ebenezer (EB) Illinois No. 6 (IL) Barau (BA) Samca (SA)
53.2 65.4 57.0 25.5 28.2
15.9 27.5 18.5 11.3 19.0
19.8 2.0 9.5 24.4 30.8
3.2 1.6 3.8 19.1 10.2
1.3 1.5 6.7 15.3 9.1
1.3
1.9 2.0 1.0
1.0
2.4
1.0 2.2 1.7
P2O5
2.5 2.2 1.0
Table 3. Analyses of Coals before and after Leaching ultimate [wt %, daf] sample coals (abbrev.)
pretreatment
Prima (PR)
non water acetic acid non water acetic acid non water acetic acid non water acetic acid
Illinois No. 6 (IL) Barau (BA) Samca (SA)
yield [wt %]
C
H
N
S
100 99.5 99.0 100 95.5 96.1 100 88.2 87.8 100 78.7 75.4
75.4 76.2 76.8 72.8 74.1 74.6 68.2 63.6 63.5 59.7 71.0 71.5
4.6 4.7 4.8 6.6 5.9 5.8 4.8 4.8 4.9 4.8 5.2 5.2
1.5 1.7 1.6 1.7 1.5 1.5 107 2.0 2.0 1.3 1.6 1.6
0.7 0.7 0.7 3.3 3.3 3.3 0.6 0.7 0.7 7.0 9.0 9.0
been carried out on the precleaning of boron and its effect on the release of boron during pyrolysis and gasification. The present study aims to separate boron from coal by pretreatment and to suppress the volatilization of boron during pyrolysis and gasification of coal by means of additives. First, the boron in coal was leached by distilled water and acetic acid. We found that most of the coals contained water-soluble and hydrochloric acid-soluble boron.25 It was confirmed in the preliminary experiment that boron dissolved by acetic acid contained one dissolved by hydrochloride acid. The coal leached by water and acetic acid was pyrolyzed and the release behavior of boron was observed during pyrolysis. SiO2, Al2O3, and CaO were added to coal in order to investigate the release control of boron during the pyrolysis and gasification of coal.
[ppmw, db] diff
O
17.8 16.7 16.1 15.6 15.2 14.8 24.7 28.9 28.9 27.2 13.2 12.7
proximate [wt %, db]
B
ash
V.M.
128 102 64 266 217 140 165 87 62 150 81 49
2.9 2.8 2.6 9.9 8.8 7.8 3.5 3.3 2.3 15.7 15.6 15.5
39.6 35.9 37.3 35.3 37.4 36.9 46.2 41.9 43.2 41.1 40.3 43.2
hydrofluoric acid. The boron content in the solution was analyzed by using an inductively coupled plasma spectrometer (Seiko Instruments, SPS4000). Ash Composition. The ash composition of the coal samples was analyzed by using a wavelength-dispersive X-ray fluorescence spectrometer (Shimadzu, XRF-1700). According to the Japan Industrial Standard JIS M8812 for ash analysis, the coal samples were heated to 773 K at a heating rate of 8.3 K/min. Then, they were heated to 1088 K at a heating rate of 10.5 K/ min, and the temperature was maintained constant for 1 h in air. Glass bead samples were prepared from the obtained ash using a glass-bead-preparing apparatus (Takeda Rikakogaku, TR Auto-Bead-1000-S) and supplied to a wavelength-dispersive X-ray fluorescence spectrometer. The ash compositions of the coal samples are listed in Table 2. Leaching by Solvents. The coal samples were leached by water and a 3 mol/L acetic acid solution in order to separate the boron from the coal. A 1 g portion of the coal sample was added to 100 mL of water and acetic acid aqueous solution, and the mixture was agitated for 3 h at room temperature. The leached coal samples were pyrolyzed at the temperatures ranging from 573 to 1173 K in a nitrogen stream. The boron content of pyrolyzed chars was determined by the method described in the previous section. Pyrolysis. The pyrolysis of the coal samples was carried out in a fixed-bed reactor (alumina tube, i.d. = 20 mm). An alumina boat loaded with dried coal samples was placed at the center of the reactor. The coal samples were heated to a final temperature of 473-1473 K at a heating rate of 10 K/min in a nitrogen stream having a flow rate of 420 cm3 NTP/min. When the samples were heated to the desired final temperature, the reactor was cooled by removing it from the electric furnace. Gasification. The coal samples were gasified in a carbon dioxide stream at 1173 K in the same fixed-bed reactor. They were heated to 1173 K at a heating rate of 10 K/min in a nitrogen
Experimental Section Coal Sample. Five coal samples were crushed and sieved to -65 þ 200 mesh. The results of the proximate and ultimate analyses of the coal samples with their boron contents are listed in Table 1. The coal samples were obtained from Prima (Indonesia), Ebenrezer (Australia), Illinois No. 6 (USA), Barau (Indonesia), and Samca (Spain). The ultimate analysis of the coal samples was carried out using an HCN coder (Yanaco, MT-700HCN) and a CS analyzer (Horiba, EMIA-220 V). Analysis of Boron Content. The coals and chars were crushed to -200 mesh and dried at 380 K for 1 h. A 0.2 g portion of each sample was placed in a Teflon container and completely submerged in a solution containing 10 mL of 90-94% fuming nitric acid, 5 mL of 70% perchloric acid, and 1 mL of 48% hydrofluoric acid. The container was placed in a heating vessel and heated at 393 K for 12 h. Then, the solution in the container was filtered and heated again at 353 K for 3 h in order to remove the 4503
Energy Fuels 2009, 23, 4502–4506
: DOI:10.1021/ef900271d
Mochizuki et al.
Figure 2. Effect of leaching on release behavior of boron during pyrolysis for PR.
Figure 1. Effect of leaching on release behavior of boron during pyrolysis for BA.
stream. The nitrogen stream was then switched to a carbon dioxide stream when the temperature reached 1173 K. On the basis of preliminary experiments, the gas flow rate of carbon dioxide was set to 420 cm3 NTP/min in order to negate the effect of film diffusion on the gasification rate. After the desired reaction times of 1.5, 3.0, and 6.0 h, the nitrogen stream was again switched on, and the reactor tube was cooled to room temperature.
Results and Discussion Leaching of Boron. Table 3 shows ultimate and proximate analyses of coals conducted before and after the leaching treatment. A weight change of less than 5 wt % was observed for PA and IL after leaching, whereas a larger weight loss of 12-25 wt % was observed for lignite BA and SA. The water and acetic acid pretreatments leached 18-47 and 50-67% of boron from coals, respectively. The removal extents of boron for low-rank coals, BA and SA, are larger than those of IL and PR. In the previous work, phenol, polycyclic aromatic compounds, and humic substrates were observed in the wastewater from washing and hydrothermal treatment of the lignite.26 The weight loss of lignite BA and SA are estimated to be these organic compounds. The form of boron dissolved by water is estimated to be organic-associated boron, because the extent of water-soluble boron corresponded with the release extent of boron below 573 of 873 K during pyrolsyis (Figures 1-4). The forms of boron dissolved by water may be assumed to be sodium borates and partly organic associated boron. The acetic-acid-soluble forms of boron may consist of water-insoluble organic boron and borates. After the leaching, no significant changes were observed in the ultimate and proximate analyses. Therefore, it can be concluded that the coal structure remained unaffected during leaching. Furthermore, leaching is effective for boron removal, especially in the case of low-rank coals.
Figure 3. Effect of leaching on release behavior of boron during pyrolysis for IL.
Release Behavior during Pyrolysis of Leached Coals. Figure 1 shows the changes in char yield and boron content in the solid phase at pyrolysis temperature for BA with and without leaching. A weight loss of 10 wt % was observed for the coal sample after leaching. 75 ppm of boron was released from the nontreated coal at the pylolysis temperature up to 573 K. In contrast, no boron release was observed for the water-treated coal pyrolyzed at temperatures up to 573 K. With a further increase in the pyrolysis temperature, 20% of the total boron was released from the water-treated coal. However, no boron release was observed for the acetic acid treated coal for temperatures ranging from 273 to 1173 K.
(26) Berrueta, A.; Fernandez, L. A.; Vicente, F. Anal. Chem. Acta 1991, 243, 115.
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: DOI:10.1021/ef900271d
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Figure 6. Effect of SiO2 and A12O3 addition on boron release during pyrolysis of IL.
Figure 4. Effect of leaching on release behavior of boron during pyrolysis for SA.
Figure 7. Effects of SiO2 and A12O3 addition on boron release during pyrolysis of PR.
Figure 4 shows the changes in char yield and release behavior of boron for SA. Both water and acetic-acidtreated coal did not release boron during pyrolysis for temperatures ranging from 473 to 1173 K. The boron released from SA is a water-soluble form that is similar to one present in IL. Release Control of Boron during Pyrolysis. Clemens et al.23,27 reported that boron has a high affinity for a silicate matrix comprising SiO2, CaO, and Al2O3. The formation process of the silicate matrix affects the distribution of boron in coal. We have found that the release extent of boron during pyrolysis is inversely correlated with the contents of Al2O3 and SiO2.25 To clarify the effect of ash components on the release of boron from coal, chemical reagents of Al2O3 and SiO2 were mixed with coal and the mixture was pyrolyzed in a nitrogen stream. Figure 5 shows the effect of the addition of SiO2 and Al2O3 on boron release during the pyrolysis of BA. The amounts of additives were; 1.0 and 4.0 wt % of SiO2 and 1.0 and 1.5 wt % of Al2O3. When SiO2 was added to BA at 1473 K, the release extent of boron was decreased from 70 to 40%. The release extent of boron at 1473 K was effectively suppressed from 70 to 15% with the addition of Al2O3. The effect of the amount of additives on the inhibition of boron release is negligible; in fact, even 1.0 wt % of an additive is enough to suppress the boron release. Figure 6 shows the effect of additives on boron release from IL during pyrolysis. 1.0 wt % of SiO2 and Al2O3 were
Figure 5. Effects of SiO2 and A12O3 addition on boron release during pyrolysis of BA.
Figure 2 shows changes in the char yield and boron content in the solid phase at pyrolysis temperature for PR. Leaching had no significant effect on the weight change of PR. Acetic-acid-treated PR did not release boron over the entire temperature range; in contrast, nontreated PR releases an increasing amount of boron with an increase in the pyrolysis temperature. From the above results for BA and PR, it can be concluded that the forms of boron released at temperatures below 573 K are soluble in water, whereas those released at temperatures ranging from 573 to 1173 K are soluble in acetic acid. The pretreatment of coal by water and acetic acid is effective for the removal of boron volatilizing by 1173 K. The effects of leaching on char yield and release behavior of boron during pyrolysis for IL are shown in Figure 3. The boron content of water-soluble IL was constant during pyrolysis. No significant release of boron was observed in the case of acetic-acid-treated coal as well as water-treated coal. The untreated raw coal released boron at 873 K, which is attributable to the water-soluble form.
(27) Clemens, A. H.; Deely, J. M.; Gong, D.; Moore, T. A.; Shearer, J. C. Fuel 2000, 79, 1781.
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: DOI:10.1021/ef900271d
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were present in coal during gasification. Boyd described that the calcium present in siderite and crandallite mineral might adsorb boron during the combustion condition of coal. These results suggest that calcium affects the release behavior of boron from coal. In the present study, 1.0 wt % CaO was mixed physically with EB. The resultant mixture was gasified at 1173 K for 3-6 h in a CO2 gas stream. As shown in Figure 8, boron release was minimal during pyrolysis at temperatures below 1173 K. However, boron began to volatilize when gasification commenced, and the release extent of boron increased with the gasification time. No significant release of boron was observed during gasification when CaO was added to coal. Thus, the release behavior of boron during gasification is successfully controlled by the addition of CaO. Figure 8. Effect of CaO addition on boron release during gasification of EB.
Conclusion
added to coal. No boron release was observed below 1173 K in the case of SiO2 addition, whereas Al2O3 addition was not effective in suppressing the release of boron. However, Al2O3 addition suppressed the boron release from 55 to 20% at 1473 K. Figure 7 shows the release behavior of boron from PR with and without the additives. It was observed that SiO2 addition is effective in suppressing the release of boron below 1173 K. Release Control of Boron during Gasification. We have investigated the release behavior of boron during the gasification of 7 types of coals.25 Boron release during gasification was observed only for EB coal. The ash composition of the coal samples are listed in Table 2. The CaO content of EB is less than that of other coals. Reed et al.24 reported that boron was not detected in flue gas when limestone and dolomite
In a series of studies conducted on coal cleaning, boron in coal was leached by water and acetic acid aqueous solution. 20-70% of boron could be removed from coal by leaching. No significant release of boron was observed during pyrolysis of the acetic-acid treated coal. To control the boron release during pyrolysis, ash components were added to coal. SiO2 was found to be effective in the suppression of boron release from coal at temperatures below 1173 K, whereas Al2O3 addition was effective in the suppression of boron release at temperatures above 1173 K. The addition of CaO to coal was effective in the suppression of boron release during the gasification of coal at 1173K. Acknowledgment. The authors are grateful to Professor Takuo Sugawara of University of Akita for his comments.
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