Article pubs.acs.org/EF
Chlorine Release during Fixed-Bed Gasification of Coal Chars with Carbon Dioxide Naoto Tsubouchi,*,† Takeomi Saito,‡ Noriaki Ohtaka,‡ Yoshihiro Nakazato,‡ and Yasuo Ohtsuka‡ †
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
‡
ABSTRACT: CO2 gasification of two kinds of coal chars has been carried out using a fixed-bed quartz reactor mainly at 0.1 MPa and 1000 °C to investigate the behavior of chlorine release during char gasification. For this purpose, an Indonesian subbituminous coal or an American bituminous coal is first pyrolyzed at 400 °C/min up to 1000 °C, and the resulting char is gasified in situ at 1000 °C with 50 vol % CO2. The Cl contents of the sub-bituminous and bituminous coal chars are 310 and 1300 μg/gdry, respectively. Conversion of the Cl to gas species increases with increasing char conversion, and there is an almost 1:1 linear relationship between the two. Chlorine distribution depends strongly on char conversion: yield of water-soluble Cl species increases significantly with increasing conversion up to approximately 50 mass %-daf and then levels off, whereas that of waterinsoluble Cl increases more remarkably at the latter stage of the gasification. No measurable amounts of Cl2 are detectable, irrespective of the extent of char conversion, but HCl can be detected significantly. The temperature-programmed desorption run of an activated carbon sorbent recovered after gasification also shows that most of the water-insoluble Cl is thermally stable even at a high temperature of 1000 °C. The formation of water-insoluble Cl-functional groups is discussed mainly on the basis of the results of some model experiments and subsequent Cl 2p XPS measurements. 10 °C/min with a fixed-bed reactor, about 50−95% of the total Cl is evolved in the form of HCl up to 800−1000 °C in many cases,11,13−15 and the HCl yield at 1000 °C tends to be lower at a higher content of inherent Ca in coal.15 It has also been suggested that most of the HCl evolved in the pyrolysis may come from unstable Cl-containing solid species produced via secondary reactions involving the devolatilizing, nascent char.11,14,15 It is of interest to investigate the fate of the Cl present in pyrolyzed char (denoted as char-Cl) in the gasification process, because the results may help us to understand chlorine chemistry in IGCC/IGFC systems and develop an efficient method for removing Cl species from the raw fuel gas. In this work, therefore, Cl release during CO2 gasification at a constant temperature is studied in detail using a fixed-bed reactor, and several factors determining Cl distribution are clarified.
1. INTRODUCTION It has been estimated that coal-based integrated gasification combined cycle (IGCC) and fuel cell (IGFC) technologies under development can achieve a high power generation efficiency of 45−55% and consequently lead to 20−30% reduction of CO2 emissions, compared with conventional pulverized coal-fired plants.1−3 IGCC or IGFC is thus expected to be one of the most promising clean coal technologies in the future. We have recently been focusing our research interest on examining the fate of halogens in coal utilization. According to previous studies,4,5 the Cl present in coal usually ranged 100− 2000 μg/g-coal and existed in water-soluble Cl forms, such as hydrated inorganic chlorides (NaCl and CaCl2) and/or hydrogen chloride (HCl) complexes on quaternary amines. It has also been shown that most of the Cl is converted to HCl during gasification, irrespective of coal type and reaction conditions.4 The HCl in raw fuel gas from typical gasification processes is generally in the range of 40−700 ppmv,6 and it must be removed prior to a gas combustion process with a gas turbine in the IGCC or fuel cell in the IGFC.1,4,6−8 It is therefore important to examine the behavior of Cl release during gasification. However, only quite limited information on this topic has been given so far.9−12 In a fluidized-bed gasification of two bituminous coals (Cl, 350−370 μg/g-dry) at 800−950 °C with CO2, conversion of the Cl to gas species increased with increasing temperature and reached more than 95% at 950 °C.9 Such a trend was also observed in a fixed-bed gasification of two Chinese coals (C, 83−91 mass %-daf; Cl, 150−190 μg/g-dry) at 800−1100 °C with CO2 and H2O.10 As is well-known, coal gasification takes place in two steps: (i) primary devolatilization (pyrolysis) and (ii) subsequent char gasification. Our earlier studies on Cl release from coal have shown that, when 25 coals with different ranks are pyrolyzed at © 2013 American Chemical Society
2. EXPERIMENTAL SECTION 2.1. Char Gasification. 2.1.1. Coal Sample. Pocahontas coal from the United States and Bontang coal from Indonesia, denoted as POC and BON, respectively, were used throughout this work. The proximate and elemental analyses of the two coals with a size fraction of less than 150 μm are summarized in Table 1, which reveals that the chlorine content is 1500 μg/g-dry with POC coal and 340 μg/g-dry with BON coal. 2.1.2. Gasification. All runs were conducted isothermally using a fixed-bed quartz reactor under ambient pressure. The details of the apparatus have been shown previously.14,16 In a typical run, a quartz cell including about 2.0 g of coal particles was first placed in the center of the reactor. After precautions against leakage, the cell was then Received: March 25, 2013 Revised: August 9, 2013 Published: August 15, 2013 5076
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Table 1. Proximate and Ultimate Analyses of Coals Used ultimate analysis proximate analysis (mass %-dry)
(mass %-daf)
(μg/g-dry)
coal
code
ash
VM
FC
C
H
N
S
O
Cl
Pocahontas No.3 Bontang
POC BON
4.6 5.7
18.6 43.4
76.8 50.9
90.8 77.2
4.8 5.4
1.2 1.8
0.77 0.58
2.4 15.0
1500 340
heated electrically at 400 °C/min up to 1000 °C under flowing ultrapure N2, held for 5 min to remove volatile matter, and finally exposed to a stream of 50 vol % CO2 diluted with the N2 to gasify in situ the resulting char. The height of coal particles in the fixed-bed and the flow rate of 50 vol % CO2/N2 gas were about 5 mm and 100 cm3(STP)/min, respectively. The temperature was controlled with a thermocouple (Pt/Pt87%·Rh13%) inserted at the bottom of the cell. The time required for replacing the dead volume up to the reactor inlet with the gas was approximately 1 min. When the pyrolysis run alone was performed in the above-described manner to determine weight loss in this process, it was comparable to volatile matter (Table 1) measured by the proximate analysis, regardless of the type of coal, indicating the completion of devolatilization in the present pyrolysis. Table 2 presents the C, Cl, and ash contents in pyrolyzed chars. The POC or BON char had 1300 or 310 μg-Cl/g-char (dry), respectively.
2.2. Reaction of HCl with Char and Subsequent TPD Run. A brown coal (ash, 0.6 mass %-dry) from Australia and a homemade activated carbon (ash, 0.07 mass %-dry) were used for this experiment and denoted as BC and AC, respectively. The AC was prepared according to our recent work,15 and it was produced by activating a phenol-resin-derived carbon with 20 vol % O2/He at 500 °C. Reactions of HCl with chars derived from these samples and subsequent TPD runs were performed using a vertical, cylindrical flow-type quartz reactor. The experimental procedure has been provided elsewhere in detail11 and is thus simply explained below. After the BC or AC was heated electrically in ultrapure He up to a predetermined temperature (350−750 °C), 100−130 ppmv HCl/N2 or 100 ppmv HCl/50 vol % CO2/N2 was passed over the resulting char for 30 min. The changes in HCl concentration during the HCl treatment were monitored online at intervals of 1 min by the infrared analyzer. In a TPD run, the HCl-treated char was first quenched to less than 30 °C under flowing ultrapure N2 and then heated at 5 °C/min up to 950 °C in the N2, and HCl desorbed was measured using the online HCl-monitoring method. The Cl 2p XPS measurements of char samples recovered after these runs were performed with a non-monochromatic Mg Kα source operating at 300 W (Kratos Analytical, Inc.). Deconvolution analyses of Cl 2p spectra were also carried out according to the least-squares curve-fitting technique using two binding energies of 198.5 eV for alkali metal chlorides, such as NaCl and KCl, and 200.5 eV for chlorobenzene (C6H5Cl) and 9-chloroanthracene (C14H9Cl).15,17 The N2 adsorption measurements were conducted with an automatic porosimeter (Quantachrome Instruments) at −196 °C, and specific surface area was estimated using the BET method.
Table 2. Analyses of Chars after Pyrolysis at 1000°C char
ash (mass %-dry)
C (mass %-daf)
Cl (μg/g-dry)
POC BON
5.8 10.2
97.3 97.7
1300 310
2.1.3. Chlorine Analysis. HCl or Cl2 evolved during coal pyrolysis and char gasification was determined with a detector tube (Gastec Corp.) after collecting all of the reactor effluent. The accuracy of the HCl analysis was ensured with an infrared spectrometer (Thermo Electron Corp.). The Cl in pyrolyzed or ungasified char was measured in accordance with the American Society for Testing and Materials (ASTM) D 2361 method. The details of these analytical techniques have been reported previously.14 To measure the amounts of gaseous Cl species other than HCl and Cl2, another experiment was conducted separately in the same way as in section 2.1.2; the reactor effluent was first bubbled directly into pure water (specific resistance, ≥ 18.0 MΩ· cm) to dissolve water-soluble Cl species including HCl in it and then passed over an activated carbon (AC) sorbent (Mitsubishi Chemical Corp., DXN-TX3DAC) for the capture of water-insoluble Cl compounds. The water-dissolved Cl− ions were determined with an ion chromatograph equipped with an anion self-regenerating suppressor (Dionex Corp.), whereas the Cl captured by the sorbent was analyzed by the ASTM D 2361 method. The reproducibility of each analysis was ±1% for HCl, ±8% for water-soluble Cl, ±3% for water-insoluble Cl, and ±4 or ±6% for the Cl in pyrolyzed or ungasified char, respectively. In order to examine the stability of the water-insoluble Cl during heating, the temperature-programmed desorption (TPD) run of an AC sorbent recovered after gasification was carried out using the reactor described in section 2.1.2. In the experiment, the sorbent charged into the reactor was heated at 10 °C/min up to 1000 °C under flowing ultrapure N2 to analyze online HCl evolved with the infrared spectrometer. 2.1.4. Data Processing. Char conversion upon gasification was estimated on a dry, ash-free basis from the change in the char weight before and after reaction. On the other had, the yield of each Cl species produced during pyrolysis or gasification was calculated on the basis of total Cl content (Table 1 or 2) in feed coal or pyrolyzed char, respectively. Chlorine mass balances for all runs ranged 95−105%, indicating that all the analytical methods described in section 2.1.3 are reliable.
3. RESULTS 3.1. Fate of Chlorine during Coal Pyrolysis. Chlorine distribution in the pyrolysis of POC and BON coals under different conditions is provided in Figure 1. At a slow heating rate of 10 °C/min,15 about 25−45% of total chlorine was evolved in the form of HCl up to 1000 °C, and the rest was mostly retained as char-Cl, regardless of the coal type, because the Cl in tar (tar-Cl) was negligibly small. No Cl2 was detectable in any cases. When the rate in slow pyrolysis was increased from 10 to 400 °C/min, appreciable amounts of tarCl were observed for both coals, but the yields were as low as less than 5%, and HCl and char-Cl were significantly unchanged. Further, yields of char-Cl observed for BON coal at 10 and 400 °C/min were almost the same as that (Figure 1) in rapid pyrolysis at 1600 °C using a drop-tube reactor.11 These results may indicate that heating rate does not affect char-Cl yield significantly. 3.2. Fate of Chlorine during Char Gasification. 3.2.1. Evolution of Water-Soluble Cl Species. Figure 2 shows char conversion and yield of water-soluble Cl as a function of reaction time during CO2 gasification at 1000 °C of POC or BON char after pyrolysis at 400 °C/min. With POC char, as given in Figure 2a, the conversion increased monotonically when reaction time was increased, and it took about 180 min for the complete gasification. On the other hand, the reactivity of BON char was higher, and the conversion reached more than 95 mass % after 85 min (Figure 2b). 5077
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Figure 3. Influence of reaction time on HCl evolution from POC and BON chars.
The proportion of HCl in the water-soluble Cl, estimated from the results in Figures 2 and 3, is shown against char conversion in Figure 4. The proportion with POC char ranged Figure 1. Influences of heating rate and temperature on chlorine distribution during pyrolysis of POC and BON coals.
As shown in Figure 2a, the yield of water-soluble Cl species with POC char increased with increasing reaction time up to 70 min, and it was about 40% at this time, but it almost leveled off beyond 70 min in spite of the remarkable increase in char conversion. This tendency was also observed with BON char (Figure 2b); the yield of 30% at 20 min did not increase significantly even when reaction time was further prolonged to 85 min, in other words, in spite of increased char conversion of 95 mass %. These observations indicate that the water-soluble Cl may be emitted mainly in the initial stage of char gasification, and the yield may depend on the kind of char. Because part of the water-soluble Cl observed in Figure 2 is likely to exist in the form of HCl, the concentration of HCl in the reactor outlet during the gasification process was measured. The results are illustrated in Figure 3. No Cl2 was detected in any cases. The yield of HCl with POC char increased with increasing reaction time to about 20% at 70 min, but it almost leveled off after this time. Such a trend was also found for BON char, and HCl yield (4.7%) at 20 min was almost the same as that (5.5%) at 85 min. These results suggest that HCl may be released mainly at the beginning step of char gasification.
Figure 4. Proportion of HCl in water-soluble Cl against char conversion during gasification.
approximately 35−55% and tended to level off beyond 50% conversion, whereas the proportion with BON char was
