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Formation of Hydrogen Chloride during Temperature-Programmed Pyrolysis of Coals with Different Ranks Naoto Tsubouchi,* Shinya Ohtsuka, Yoshihiro Nakazato, and Yasuo Ohtsuka Research Center for Sustainable Materials Engineering, Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira, Aoba-ku, Sendai 980-8577, Japan Received August 20, 2004. Revised Manuscript Received December 3, 2004
The evolution of HCl during pyrolysis of 16 coals with different ranks at a heating rate of 10 °C/min has been studied with an online monitoring method. Approximately 50%-95% of total chlorine is converted to HCl up to 800 °C, and the remainder is mostly retained in the char, which leads to a strong reverse correlation between the two. As the sum of Na and Ca naturally present in coal increases, the amount of HCl tends to decrease. The temperature dependence of the rate of HCl evolved differs with each coal and shows at least four peaks at 280, 360, 480, and 580 °C. The former two peaks are present for two coals alone, whereas the higher temperature HCl formation at g450 °C is common for almost all of the coals. The HCl peaks at 280 and 360 °C are considerably small by water washing. When model chlorine compounds added to activated carbon, such as hydrated NaCl, hydrated CaCl2, and organic hydrochlorides, are pyrolyzed in the same manner as above, HCl formation occurs dominantly between 250 and 450 °C in every case. The pretreatment of a brown coal char with HCl at 500 °C and subsequent temperatureprogrammed desorption (TPD) measurement up to 950 °C suggest that HCl reacts with the nascent char upon pretreatment to form several types of Cl functional forms, from which the HCl desorption takes place at 450-750 °C upon TPD. The HCl evolved at 99.9995%), the reactor was finally heated at 10 °C/min up to 800 °C in a stream of the N2 (500 cm3(STP)/min). The temperature was measured by a thermocouple (Ni/Cr:Ni/Al) inserted at the bottom of the cell. Tarry materials, liquid hydrocarbons, and water condensed in the tar traps during pyrolysis were recovered by solvent extraction using n-butyl alcohol, and the fraction was denoted as tar throughout this paper. The residue remaining in the cell was recovered as char. Reaction of Char with HCl and Subsequent TPD Run. To examine the possibility of secondary reactions of char with HCl evolved during coal pyrolysis, the following experiments were carried out. An Australian brown coal (C, 66 wt % (daf); Cl, 0.084 wt % (daf); ash, 0.6 wt % (dry)) was first heated in high-purity He at 5 °C/min up to 500 °C, and the He was then replaced by 130 ppmv HCl/N2, which was passed over the nascent char at 500 °C. After 30 min, the char was quenched to room temperature in pure He. In a temperature-programmed desorption (TPD) run, the char sample without exposure to air was heated at 5 °C/min up to 950 °C in a stream of high-purity N2, and HCl evolved was monitored online as mentioned below. Analytical Procedures for HCl and Cl in Coal, Tar, and Char. The HCl evolved during pyrolysis and TPD was determined online at 1 min intervals with an IR analyzer (Thermo Environmental Instruments, Inc.). The chlorine in the tar or char recovered, denoted as tar-Cl or char-Cl, respectively, as well as coal-Cl, was first transformed to chloride ions according to the oxygen bomb method (ASTM D 2361), in which the sample was burned at 2.5 MPa O2 in a pressurized bomb containing an aqueous solution of ammonium carbonate. The chloride ions in the solution were then determined by the absorption spectrophotometry (Hitachi, Ltd.). Yields of HCl, tar-Cl, and char-Cl were expressed in percent of total chlorine in feed sample, the reproducibility of each yield being within (5%, (7%, and (4%, respectively. Analysis of Metal Compositions of Raw Coals. Eight kinds of metals (Na, Mg, Al, Si, K, Ca, Ti, and Fe) inherently present in all of the raw coals used were analyzed as follows. Each coal was first burned up at 815 °C to obtain the ash, which was then subjected to acid leaching, and the metal cations leached were determined by inductively coupled plasma emission spectrometry (ICP-ES; Perkin-Elmer, Inc.).7,8 (7) Wu, Z.; Ohtsuka, Y. Energy Fuels 1997, 11, 902-908. (8) Tsubouchi, N.; Ohtsuka, Y. Fuel 2002, 81, 2335-2342.
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Tsubouchi et al. Table 2. Results for HCl Formation from All Coal Samples Examined rate of HCl formation,a µmol/min/g-coal(daf) coal
Figure 1. Rate of HCl formation and cumulative yield of HCl in the temperature-programmed pyrolysis of YRB, BRU, and CNZ coals.
BRU PSR MBW THY PRM DTN DRT TGH FSL EML ENM MUR CNZ YRB JET HGI a
280 °C
1.63
360 °C
0.98 0.93
480 °C
580 °C
0.19 0.23 0.61 1.01 0.40 0.23 0.62 0.41 0.87 0.33 0.24 0.41
0.069 0.069 0.25 0.21 0.095 0.11 0.12 0.11 0.29 0.13 0.10 0.15 0.30 0.16 0.29 0.15
0.63
At the peak temperature observed in the rate profile.
Figure 2. Rate of HCl formation and cumulative yield of HCl in the temperature-programmed pyrolysis of JET and THY coals.
Results Formation of HCl. The examples for the rate of HCl formation and cumulative yield of HCl against temperature during pyrolysis of YRB, BRU, and CNZ coals are illustrated in Figure 1, where the yield is calculated by integrating each curve. YRB coal provided a large, asymmetric HCl peak at 280 °C and a small, broad one at 580 °C.5 Such a low-temperature HCl peak was not detectable with BRU and CNZ coals. BRU coal exhibited the main and shoulder peaks at 460 and 570 °C, respectively, and CNZ coal showed only a small, broad peak at ∼580 °C. As seen in Figure 1, the cumulative yield was 89%, 61%, and 47% for YRB, CNZ, and BUR coals, respectively. Figure 2 shows the results for JET and THY coals. JET coal exhibited two distinct HCl peaks at 360 and 500 °C, followed by a shoulder one at 580 °C.5 The comparison of the rate profiles with JET (Figure 2) and YRB (Figure 1) coals indicates that the 360 °C peak observed with JET coal should be included in the asymmetric low-temperature peak that appeared with YRB coal. THY coal showed a large, symmetric peak at 480 °C and the small shoulder after 550 °C. The shape of the 480 °C peak was quite similar to that for BRU coal (Figure 1). THY coal gave the highest HCl yield of 95% among all coals examined. The results of HCl formation from all coals are summarized in Table 2. Peak temperatures observed in
Figure 3. Chlorine distribution at 800 °C for all of the coals used.
the rate profiles may roughly be assigned to 280, 360, 480 (460-500), and 580 (560-600) °C, and the presence of such peaks depended strongly on the type of coal. The summaries follow: (1) HCl peak at 280 °C appears for YRB coal only, and the rate is the highest (1.6 µmol/min/g-coal (daf)) among all peaks observed. (2) HCl peak at 360 °C is present with YRB and JET coals alone, and the rate is ∼1.0 µmol/min/g-coal (daf). (3) HCl peak at around 480 °C appears for all coals except CNZ, YRB, and HGI, and the rate ranges 0.21.0 µmol/min/g-coal (daf), depending on coal type. (4) HCl peak at the highest temperature of 580 °C is common with all coals, and the rate is always e0.3 µmol/ min/g-coal (daf). Chlorine Distribution and Coal Type. Figure 3 shows chlorine distribution after pyrolysis at 800 °C of all coals used. The major Cl-containing species was HCl. The yield was in the range of 47%-95%, and the largest yield observed with THY coal was 2 times the smallest one with BRU coal. The yield of char-Cl ranged from
Formation of HCl during Pyrolysis of Coals
Figure 4. Relationship between yields of HCl and char-Cl.
4% to 52%, depending on the coal type. The yield of tarCl was as small as e7% in all cases. It is not clear what Cl-containing species are present as tar-Cl. It may include HCl soluble in condensed water in the tar traps. No Cl2 was detectable by the Gastec standard tube (Gastec Corp.), irrespective of the type of coal. As shown in Figure 3, chlorine mass balances fell within the reasonable range of 90%-108%. These observations mean that more than 90% of coal-Cl is converted predominantly to HCl and char-Cl up to 800 °C. The relationship between yields of HCl and char-Cl is illustrated in Figure 4, where a broken line means parity between the two. As HCl increased, char-Cl decreased linearly, and all of the data were plotted on or around the line. In other words, there was a strong reverse correlation between both species. These observations might indicate that HCl formation from tar-Cl is negligibly small. The following section therefore focuses on making clear the influence of coal type on the distribution of HCl and char-Cl. When yields of HCl and char-Cl were plotted as a function of carbon content (C%), chlorine content (Cl%), or volatile matter (VM) in coal shown in Table 1, no distinct relationships could be deduced. It may thus be reasonable to see that the C%, Cl%, and VM are not important as the factors for determining HCl formation, in other words, char-Cl retention, during coal pyrolysis under the present conditions. Figure 5 illustrates the relationship between the content of Na plus Ca present in coal and yield of HCl or char-Cl. Although some data were scattered, HCl tended to increase with increasing content with a corresponding decrease in char-Cl. These observations may be reasonable, because it has been reported that coal-Cl is present predominantly as inorganic chlorides of NaCl and CaCl2.1,3,9-11 There may be some reasons for the data scattering observed in Figure 5. One reason may be that the inherent Ca exists mainly in the carbonate and sulfate forms as is well known. Another possibility might be the presence of Cl-species other than inorganic chlorides, for example, organic hydro(9) Fynes, G.; Herod, A. A.; Hodges, N. J.; Stokes, B. J.; Ladner, W. R. Fuel 1988, 67, 822-830. (10) Huggins, F. E.; Huffman, G. P. Chlorine in Coal; Coal Science and Technology, Vol. 17; Elsevier: Amsterdam, 1991; pp 43-61. (11) Huggins, F. E.; Huffman, G. P. Fuel 1995, 74, 556-569.
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Figure 5. Relationship between the content of Na plus Ca in coal and yield of HCl or char-Cl.
chlorides (HCl complexes attached to quaternary ammines), which were speculated to be present in bituminous coals.4,10,11 Formation of HCl from Model Chlorine Compounds. The results about HCl formation in the pyrolysis of several Cl-containing compounds are summarized in Table 3, where the choices of NaCl hydrate, CaCl2 hydrate, and organic hydrochlorides, that is, HCl complexes attached to quaternary ammines, are made according to earlier work on the functional forms of coalCl.1,3,9-11 Any HCl from NaCl or CaCl2 anhydride physically mixed with AC could not be detected. On the other hand, NaCl and CaCl2 hydrates impregnated with AC provided the HCl profiles peaking at 350 and 300 °C, respectively.5 It is likely that the HCl observed originates from dehydration reactions of these chlorides. The physical mixture of AC and tetracycline (C22H24N2O8‚HCl) or semicarbazide (CH5N3O‚HCl) hydrochloride showed the maximal rate of HCl formation at 270 or 320 °C, respectively.5 Two organic chlorides, such as 2-naphthoyl chloride (C11H7OCl) and 9-chloroanthracene (C14H9Cl), exhibited the respective HCl peaks at 310 and 390 °C. These observations indicate that HCl formation from these hydrochlorides and organic chlorides mixed with AC readily occurs at low temperatures of e400 °C. Water Washing. Two inorganic chlorides and two hydrochlorides used above are soluble in water. If such water-soluble chlorine forms are actually present as coal-Cl, water washing before pyrolysis may decrease the amount of HCl evolved. To examine this point, YRB and JET coals were used, because their HCl profiles at e450 °C were quite similar to those observed for hydrated NaCl, hydrated CaCl2, tetracycline, and semicarbazide hydrochlorides.5 Figure 6A and B compares rate profiles of HCl formation before and after water washing. With the washed YRB coal, HCl started to evolve at a higher temperature (270 °C) than observed without water washing, and the peak appeared at about 320 °C. Thus, after water washing, HCl formation at e270 °C disappeared almost completely, and the rate at 270-350 °C lowered considerably. On the other hand, the rate at g370 °C did not change significantly. The chlorine content of YRB coal decreased from the original 0.14 to 0.10 wt % (daf) after washing, and the extent of chlorine removal was 28%.
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Figure 7. Profile for HCl desorption from a brown coal char after HCl pretreatment at 500 °C.
Discussion Figure 6. Effect of water washing on HCl formation during temperature-programmed pyrolysis of YRB and JET coals: (A) raw coal and (B) after water washing.
When JET coal with a size fraction that is 55-75 µm lower than the usual 150-250 µm was washed with water, as shown in Figure 6B, no HCl formation took place at e450 °C. In contrast, the HCl profile after 450 °C was almost unchanged before and after washing. The chlorine content decreased from 0.11 to 0.093 wt % (daf) by washing, and the extent of the removal was 15%. Desorption of HCl from Brown Coal Char Pretreated with HCl. The rate profile of HCl evolved in the TPD run of a brown coal char pretreated with 130 ppmv HCl at 500 °C is illustrated in Figure 7, where HCl formation from the char preheated in pure N2 is also shown for comparison. When the preheated char was reheated in N2 flow up to 950 °C, only a small, broad HCl peak appeared at ∼600 °C, and it corresponded to the highest temperature peak observed with all coals examined (Table 2). The HCl came from the chlorine inherently present in the brown coal. On the other hand, the desorption of HCl from the char pretreated with HCl started at ∼450 °C, and the rate profile provided the main and shoulder peaks at 550 and 620 °C, respectively. The rate at the shoulder was larger than the maximal rate for the char preheated in pure N2. These observations mean that HCl reacts readily with the nascent char in the pretreatment process at 500 °C to form several types of Cl functional forms, which provide HCl desorption at different temperatures in the subsequent TPD run.
The rate profiles of HCl evolved during temperatureprogrammed pyrolysis of 16 coals depended strongly on coal type (Figures 1 and 2) and provided at least four distinct peak temperatures at 280, 360, 480, and 580 °C (Table 2). It is thus possible to understand that HCl formation at different temperatures originates from the different sources. This point will be discussed mainly in this section. Sources of HCl Evolved below 450 °C. As seen in Figures 1 and 2, YRB and JET coals showed the HCl peaks at low temperatures of e450 °C, which corresponded to the temperature regions (Table 2) of HCl formation from hydrated inorganic chlorides and organic hydrochlorides. On the other hand, water washing before pyrolysis considerably decreased the amount of HCl evolved either from YRB coal at 210-270 °C or from JET coal at 340-450 °C (Figure 6). Because inorganic chlorides and organic hydrochlorides are water-soluble, such Cl-functional forms may be the main sources of the low-temperature HCl formation. It has been accepted that NaCl exists as hydrated forms and saturated solutions in the coal pores and microcracks.1,3,9-11 The presence of CaCl2 hydrates has also been reported by X-ray absorption near edge structure (XANES) spectra10,11 and may be supported by some correlation between HCl yield and the sum of inherent Na and Ca observed in Figure 5. On the other hand, the existence of organic hydrochlorides as coalCl forms may still be speculative, although HCl peaks observed during pyrolysis of tetracycline and semicarbazide hydrochlorides mixed physically with AC were overlaid with those at e450 °C for YRB and JET coals.5
Table 3. Formation of HCl from Chlorine Compounds Added to Activated Carbon chlorine compound
loading method
appearance temperature (°C)
peak temperature (°C)
NaCl CaCl2 NaCl hydrate CaCl2 hydrate
Inorganic Chloride physical mixing physical mixing impregnation impregnation
310 260
350 300
tetracycline hydrochloride, C22H24N2O8‚HCl semicarbazide hydrochloride, CH5N3O‚HCl
Organic Hydrochloride physical mixing physical mixing
250 280
270 320
2-naphthoyl chloride, C11H7OCl 9-chloroanthracene, C14H9Cl
Organic Chloride physical mixing physical mixing
260 340
310 390
Formation of HCl during Pyrolysis of Coals
Figure 8. Rate of NH3 formation from tetracycline hydrochloride and YRB coal during temperature-programmed pyrolysis.
These hydrochlorides are composed of HCl complexes attached to quaternary ammines, which are readily decomposed to release NH3. It is thus expected that NH3 as well as HCl are formed from the hydrochlorides upon pyrolysis. To confirm this point, the concentration of NH3 gas in the reactor outlet in the pyrolysis of not only tetracycline hydrochloride mixed with AC but YRB coal was monitored with a photo acoustic multi-gas monitor (Innova Air Tech Instruments, Inc.). The results are shown in Figure 8. A significant amount of NH3 was detectable with the hydrochloride and the coal, and NH3 formation from both samples proceeded in the similar temperature region. The increasing rate observed after 400 °C with YRB coal may result from pyridinic and pyrrolic nitrogen forms. The comparison of Figures 1 and 8 indicates the similarity of the release of HCl and NH3 from YRB coal. Such a similarity was also observed in the temperature-programmed pyrolysis of Illinois coal, and it was suggested that coal-Cl might be associated with nitrogen.4 These observations support that part of coal-Cl may be present as organic hydrochlorides (that is, HCl complexes attached to quaternary ammines), which may be the possible sources of HCl evolved at e450 °C. Sources of HCl Evolved above 450 °C. Figure 6 shows that water washing has no significant effect on HCl formation at g450 °C even when JET coal is pulverized. It is likely that the HCl is formed from sources other than water-soluble inorganic chlorides and organic hydrochlorides. As seen in Figure 7, the TPD measurements for the brown coal chars with and without HCl pretreatment reveal that the pretreated char provides the peaks of HCl desorption between 450 and 750 °C. The results strongly suggest that, in the present coal pyrolysis, HCl once-evolved at a low temperature undergoes secondary reactions with the devolatilizing, nascent char to form unstable solid species, which are subsequently decomposed at a high temperature to release HCl. In other words, the HCl observed at g450 °C may originate from such reactions. This possibility will be discussed below. The desorption of HCl in the broad temperature range in Figure 7 suggests the presence of several types of Cl forms as the sources of HCl desorbed. There may thus be at least two sites for secondary reactions: one is some minerals inherently present in the char, and another
Energy & Fuels, Vol. 19, No. 2, 2005 559
Figure 9. Relationship between calcium content in coal and yield of HCl observed at 580 °C.
is active carbon sites on the char surface. In the former case, CaCO3 and FeO are likely to be present, because the ICP-ES measurements revealed the highest contents of Ca and Fe among elements other than Si and Al. Thus, HCl may react with CaCO3 or FeO to form CaCl2 or FeCl2, respectively, according to the following reactions:
2HCl + CaCO3 f CaCl2 + CO2 + H2O
(1)
2HCl + FeO f FeCl2 + H2O
(2)
Thermodynamic calculations show that standard free energy changes (∆G) for reactions 1 and 2 at 500 °C are -18 and -9 kcal/mol, respectively, indicating significant driving forces for the formation of CaCl2 and FeCl2. One may think that, when pyrolysis temperature is increased, these chlorides can react with a slight amount of water evolved from the char to release HCl again. Because ∆G for the reaction of CaCl2 and H2O is estimated to be +29 and +23 kcal/mol at 600 and 900 °C, respectively, HCl formation through this reaction is unfavorable thermodynamically under the present conditions. On the other hand, ∆G for the reverse reaction of (2) is +6, +3, and +1 kcal/mol for 600, 700, and 800 °C, respectively, and the driving force is thus larger at a higher temperature. Thus, HCl may be formed through the reaction of FeCl2 with H2O at a higher temperature. It has been shown that a significant amount of H2O can evolve during coal pyrolysis at >500 °C probably by the dehydration of some minerals.12 As mentioned above, CaCl2 anhydride formed by secondary reactions with HCl, for example through reaction 1, is thermodynamically stable even in the existence of steam and thus unlikely to release HCl again when pyrolysis temperature is raised. One may expect that, because some Ca minerals naturally present in coal can capture HCl to form stable CaCl2, the amount of HCl evolved at g450 °C is lower at a larger content of inherent Ca. Thus, the yield of the HCl peak observed at 580 °C (actually 560-600 °C) (Table 2) was calculated for all coals examined and plotted against the Ca content. This relationship is provided in Figure 9. (12) Aso, H.; Matsuoka, K.; Tomita, A. Energy Fuels 2003, 17, 12441250.
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As the content increased, the yield tended to decrease. Although data scattering was observed, this may be ascribed to not only the existence of Ca species ineffective for HCl capture but the occurrence of secondary reactions of effective Ca species with H2S. It has recently been reported that, when 15 bituminous coals are pyrolyzed up to 900 °C, the yield of char-Cl is larger at a higher content of inherent Ca.13 It is probable that some Ca components as mineral matters react secondarily with HCl to produce CaCl2, which is retained as char-Cl. Active carbon sites of the char formed in the pyrolysis process may work as another site for secondary reactions of HCl. Their importance has been well accepted in the gasification of coal chars and carbons with O2, H2O, and CO2.14-16 When free active site is denoted as C( ), the reaction with HCl may be expressed as follows:
C( ) + HCl f C(HCl)
(3)
C( ) + HCl f C(Cl) + C(H)
(4)
In these reactions, C(HCl) may mean chemisorbed HCl and/or surface HCl species, and HCl may be dissociated to provide two surface species, C(Cl) and C(H). Because it has been reported that NH3 gas can react with activated carbon materials to form pyridinic nitrogen and aromatic ammines,17,18 C(HCl) and C(Cl) may also be transformed to stable aromatic chloride structures. As the temperature increased, HCl may evolve again from these intermediate species. As seen in Table 3, 9-chloroanthracene as one of aromatic chlorides provided the main peak of HCl evolution at 390 °C, which was lower than the temperature region (g450 °C) of the HCl desorption from the brown coal char pretreated with HCl (Figure 7). This difference might be caused by the stronger C-Cl bonds of aromatic chloride structures formed by the reaction of HCl with active carbon sites. On the basis of the above discussion, it can be speculated that the HCl peaks at 480 and 580 °C (Table 2) observed for almost all of the coals examined do not originate from any Cl forms present in the raw coals, but arise mainly from different types of Cl-containing (13) Nomura, S.; Kato, K.; Maeno, Y. J. Jpn. Inst. Energy 2003, 82, 866-873. (14) Laine, N. R.; Vastola, F. J.; Walker, P. L., Jr. J. Phys. Chem. 1963, 67, 2030-2035. (15) Radovic, L. R.; Walker, P. L., Jr.; Jenkins, R. G. Fuel 1983, 62, 849-856. (16) Lahaye, J., Ehrburger, P., Eds. Fundamental Issues in Control of Carbon Gasification Reactivity; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1991. (17) Sto¨hr, B.; Boehm, H. P.; Schlo¨gl, R. Carbon 1991, 29, 707720. (18) Mangun, C. L.; Benak, K. R.; Economy, J.; Foster, K. L. Carbon 2001, 39, 1809-1820.
species which were formed by secondary reactions of HCl once-evolved at
