Article pubs.acs.org/EF
Experimental Investigation of Carbonate Formation Characteristics during Coal and Biomass Pyrolysis under CO2 Hirotatsu Watanabe,* Kiyomi Shimomura, and Ken Okazaki Department of Mechanical and Control Engineering, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1-I6-7, Ookayama, Meguro-ku, Tokyo 152-8552, Japan ABSTRACT: In this study, carbonate formation characteristics during solid fuel pyrolysis in CO2 were experimentally studied. Chars derived from coal or rice straw in Ar or CO2 were characterized by Fourier transform infrared spectroscopy (FTIR) and water extraction and electrophoretic procedures. Inorganic element concentrations in chars were measured by inductively coupled plasma−atomic emission spectrometry (ICP−AES). When coal with a K/Si ratio of 0.04 was heated in CO2, carbonate was not formed. In contrast, when rice straw with a K/Si ratio of 2.97 was heated in CO2, carbonate was formed in chars at 973 K under a heating rate of 1 or 30 K/s. The main carbonates formed through the reaction with CO2 were assumed to be potassium carbonates (K2CO3). It was also shown that carbonate in the rice straw chars prepared in CO2 was almost twice of that in the chars prepared in Ar, whereas the total concentration of K in the rice straw chars prepared in CO2 was almost the same as that in the rice straw chars prepared in Ar. Metal oxides or hydroxides in the chars prepared in CO2 decreased significantly. These results suggest that carbonates formed as a result of the reaction of basic oxides or hydroxides with CO2. In fact, the chemical form of potassium species in the chars was altered by CO2 during rice straw pyrolysis.
1. INTRODUCTION Oxy-fuel combustion is an emerging technology for postcombustion CO2 capture.1 Oxy-fuel combustion indicates that fuel is burned with blended oxygen, produced in an air separation unit, and recirculated flue gas. When biomass cofiring is included in the oxy-fuel combustion, it can be used as a sink for CO2.2,3 One of the most important features of oxy-fuel combustion is the high CO2 concentration. CO2 is not inert. Rather, as many researchers have reported, it participates in chemical reactions in the gas phase4−7 and char gasification.8,9 Thus, flue gas cleaning prior to recycling or storage, ash deposition, and burner adjustments still need to be investigated from the viewpoint of CO2 reactivity if oxy-fuel combustion is to be successfully implemented. One of the chemical compounds produced through the reaction with CO2 is carbonate. Some researchers have expected the carbonate formation during oxy-fuel combustion (CO2-rich environments).3 It is important to determine the chemical form of inorganic species because they might have significant effects on corrosion, agglomeration, and hot-gas cleaning and catalyst poisoning during combustion or gasification.10 From the viewpoint of slag recovery, understanding the chemical form of ash is also important. In addition, if carbonate is formed during pyrolysis in CO2, char properties might be altered because the mineral component works as a catalyst. It is wellknown that char gasification with oxygen and CO2 is catalyzed by small concentrations of alkali metals, and the catalyst effect of alkali metals depends upon their chemical form.11 Meanwhile, some researchers have demonstrated carbonate formation in O2/CO2 mixtures through CO2 reaction with silicates by equilibrium calculation.12,13 Fryda et al. have observed that the particle size distribution of oxy-fired ash shifted to a larger size and stated that the size difference might arise from the carbonates. 14 Although theoretical approaches12,13 and inference based on experiments14 indicate © 2014 American Chemical Society
carbonate formation in CO2 -rich environments during combustion, little effort has been made to show carbonate formation directly by experiments. Our recent study has shown that carbonate formed during alkali lignin pyrolysis under CO2.15 Lignin has often been used to study biomass as the model compound.16 However, further investigation is required to study carbonate formation in solid fuel during pyrolysis in CO2 because sodium is the main alkali metal element in the alkali lignin,15 whereas biomass generally contains more potassium than sodium.17 This study aims to study carbonate formation in coal or biomass during pyrolysis in CO2 experimentally. Chars derived from coal or rice straw in Ar or CO2 were characterized by Fourier transform infrared spectroscopy (FTIR) and water extraction and electrophoretic procedures to study carbonate formation characteristics.
2. EXPERIMENTAL SECTION 2.1. Pyrolysis Experiment. Coal and rice straw were used as solid fuels in this study. Tables 1 and 2 show the ultimate and proximate analyses and the inorganic composition of the samples, respectively. An important difference in the inorganic compositions of coal and rice straw is potassium. K/Si molar ratios, which influence the release and water solubility of K,18,19 are 0.04 and 2.97 for coal and rice straw, respectively. According to Zheng et al.,18 water-soluble K in fly ash generally increases with the increasing K/Si molar ratio of the fuel. Water solubility of K is important when considering carbonate formation, as described later in this paper. A thermobalance used in this study has been described in ref 15. A small quantity of raw sample (about 40 mg) was used. Our previous studies showed that carbonate was formed during lignin pyrolysis in CO2 at 1 K s−1.15 Thus, 1 K s−1 was also used in this experiment. In Received: April 14, 2014 Revised: June 8, 2014 Published: June 18, 2014 4795
dx.doi.org/10.1021/ef500832t | Energy Fuels 2014, 28, 4795−4800
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Article
When the total K, Ca, and Na concentrations in chars were measured, all chars were dissolved in strong acid. During the process, the fact that all chars were dissolved was visually confirmed. The total inorganic species concentrations were measured using inductively coupled plasma−atomic emission spectroscopy (ICP−AES).
Table 1. Ultimate and Proximate Analyses (wt %, Dry Basis) coal
rice straw
Ultimate Analysis (wt %, Dry Basis) C 68.0 H 4.79 N 0.66 S 0.10 Cl 0.27 O (difference) 22.8 Proximate Analysis (wt %, Air-Dried Basis) volatile matter 47.0 fixed carbon 42.9 ash 3.4 moisture 6.7 fuel ratio 0.91
40.4 5.41 0.40 0.00 0.52 42.2
3. RESULTS AND DISCUSSION Figure 1 shows the weight fractions and temperature histories of coal and rice straw during pyrolysis in Ar and CO2. The
65.5 16.6 11.1 6.8 0.25
Table 2. Inorganic Composition (wt %, Dry Basis) coal Si K Na Ca Mg Fe
1.26 0.0682 0.0685 0.141 0.0625 0.954 rice straw
Si K Na Ca Mg Fe
0.578 2.44 0.0913 0.113 0.0355 0.0117
addition to the low heating rate of 1 K s−1, 30 K s−1 was used to investigate the effect of the heating rate on carbonate formation. In the experiment, after drying at about 383 K for 12 min, the temperature of the sample was increased to 973 K at a heating rate of 1 or 30 K s−1 in Ar or CO2. The flow rate of Ar or CO2 was set to 0.8 L/min. The main purpose of this study is to study oxy-fuel combustion; however, for simplification, 100% CO2 was used to study carbonate formation. When the temperature reached 473, 673, and 973 K, heating was terminated to avoid CO2 gasification. After char preparation, char samples were preserved under an Ar atmosphere immediately because a trace of CO2 in air might react with chars, resulting in carbonate formation. The surface chemistry of the char was investigated by FTIR (FT/IR-4200, Jasco) to study carbonate formation characteristics. The samples were first powdered in an agate mortar and then mixed with KBr at an approximate ratio of 1:400 to prepare wafers. The infrared (IR) spectra were obtained using FTIR with 4 cm−1 resolution. 2.2. Char Characterization: Water Extraction and Electrophoretic Procedures. Water extraction is a convenient method for studying the chemical form of inorganic species. Batch water washing was used to quantify water-soluble inorganic species. First, the mass ratios of sample/water were set at 1:100, and water extracts from chars were prepared by mixing samples in water for 3 min at room temperature. The pH values of the water extracts were measured with a pH meter (twin pH, AS-212, Horiba). A capillary electrophoresis (CE) system (Agilent G1600A, Agilent Technologies) was used to record electropherograms for the water extracts. Ions, such as CO32−, in the water extracts were separated using fused-silica capillaries. The electrolyte was 2,6-pyridine dicarboxylic acid and cetyltrimethylammonium hydroxide. The electrolyte pH was adjusted to 12.0. Ion detection was performed by monitoring indirect ultraviolet (UV) absorbance at a signal wavelength of 350 nm with 275 nm as a reference. The CE was 5% accurate.
Figure 1. Weight fractions and temperature histories of coal and rice straw in Ar and CO2.
temperature at which pyrolysis starts for rice straw is lower than that for coal. In coal and rice straw, no significant differences between CO2 and Ar appear in the weight fraction histories. These results indicate that CO2 gasification does not advance at 973 K in chars prepared in this study. Meanwhile, the small undershoot appeared at around 12 s in the rice straw pyrolysis 4796
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Figure 2. FTIR spectra of chars derived from coal and rice straw at two heating rates (973 K).
at 30 K s−1 (Figure 1c). It is caused not by pyrolysis physics but by rapid heating termination at a high heating rate. This small undershoot does not influence any findings here. Figure 2 shows FTIR spectra of chars prepared in Ar and CO2 at 973 K. IR spectra ranging from 800 to 2000 cm−1 are magnified to focus the peak at 1450 cm−1, which is associated with CH deformation, aromatic ring vibrations, and ionic carbonate.20 Although the IR spectra of coal chars produced in CO2 are almost the same as those produced in Ar (Figure 2a), a notable difference between Ar and CO2 appears in rice straw chars at 1450 cm−1. A clear peak is seen at 1450 cm−1 only in chars derived from rice straw in CO2, regardless of the heating rate. One possible reason is carbonate formation because 1450 cm−1 is a typical IR peak related to carbonate. On the other hand, weight fraction histories do not show any differences between Ar and CO2, as shown in Figure 1. This implies that CO2 changes the chemical form of inorganic species in rice straw chars. Figure 3 shows an electropherogram for water extracts of chars derived from coal at 973 K. In water extracts of chars derived from coal, only small peaks appear in the electropherogram. This indicates that chars derived from coal contain less water-soluble salts, such as carbonate. No significant differences appear between chars prepared in Ar and CO2. The results show that carbonate is not formed during coal pyrolysis in CO2. Figure 4 shows electropherograms for water extracts of chars derived from rice straw at 973 K. Unlike water extracts of coal, several peaks appear. The OH− peak is negative because the pH of the electrolyte is larger than that of the water extract. An important finding here is that a notable difference in the carbonate peak between water extracts is seen. The carbonate peak in water extracts of chars produced in CO2 is higher than that in water extracts of chars prepared in Ar. This result is consistent with IR results shown in Figure 2. The carbonate ion concentration is determined using electropherograms of reference solutions, as shown in Figure 5. Figure 5 shows K, Ca, Na, and carbonate ion (CO32−) concentrations for rice straw chars prepared at 973 K. Inorganic cations, particularly alkali and alkaline earth metals, such as Na,
Figure 3. Electropherogram of the water extract from chars derived from coal at 973 K under 1 K s−1.
K, Ca, Mg, and Fe, can form carbonate; however, Mg and Fe concentrations in rice straw are much lower, as shown in Table 2. Thus, K, Ca, and Na concentrations, which are relatively high, are shown. The CO32− concentration in chars prepared in CO2 is almost twice that of chars prepared in Ar, and it is much higher than Ca and Na concentrations. Therefore, the main carbonates formed through the reaction with CO2 are assumed to be potassium carbonates. Figure 6 shows potassium composition in rice straw chars prepared in Ar and CO2 at 973 K under 1 K s−1. Assuming that detected anion species (CO32−, Cl−, and OH−) must come from potassium salts, concentrations of K2CO3 and KCl are calculated. Considering the weak basicity of K2CO3, the KOH concentration is calculated from the measured OH− concentration. However, KOH may in fact be K2O because K2O easily reacts with H2O and forms KOH. It is difficult to distinguish KOH and K2O in the water leachate process. In this study, the 4797
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mass balance for K compounds calculated from Cl−, CO32−, and OH− is about 50−60% for the chars prepared under Ar or CO 2 . An important finding here is that the K 2 CO 3 concentration increases significantly in chars prepared in CO 2 , whereas the sum of KCl, K 2 CO 3 , and KOH concentrations in chars prepared in CO2 is almost the same as that in chars prepared in Ar. This suggests that a portion of KOH or K2O is altered to K2CO3 by CO2 during rice straw pyrolysis. In fact, carbonates form as a result of the reaction of basic oxides or hydroxides with CO2. A basic potassium species in chars is highly likely to form carbonate during heating under CO2 from the viewpoint of the equilibrium calculation.12,13 Zheng et al. said that there is a significant driving force for the reaction of CO2 with potassium oxides and hydroxides as reactions R1 and R2.12 K 2O + CO2 → K 2CO3
(R1)
2KOH + CO2 → K 2CO3 + H 2O
(R2)
Other than K2O and KOH, potassium mainly appears in components, such as phosphates (K3 PO 4 ), carbonates (K2CO3), chlorides (KCl), K−silicates (K2O·SiO2), and oxides (K2O).18,19,21,22 Sulfates (K2SO4) are not included because the rice straw used in this study does not contain sulfur, as shown in Table 1. In addition to reactions R1 and R2, CO2 interacts with the ash-forming compounds, leading to the formation of carbonates as indicated in reaction R3.13
Figure 4. Electropherograms for water extracts of chars derived from rice straw at 973 K.
K 2O·SiO2 + CO2 → K 2CO3 + SiO2
(R3)
Contreras et al. showed that K2O·SiO2 silicate exhibited a higher tendency to form carbonates than Na2O·SiO2, and equilibrium calculation revealed an increase in carbonate formation at T < 973 K for O2/CO2 environments. Silicate formation is enhanced at T > 1073 K.13 They also stated that, between 773 and 873 K, carbonation is the dominant reaction for O2/CO2 mixtures. During pyrolysis of rice straw in CO2, CO2 enhances reactions R1−R3, leading to carbonate formation. Meanwhile, K2CO3 is also included in chars prepared in Ar. K2CO3 formation mechanisms without CO2 are described in ref 23. Briefly, char-bonded K might be transformed into K2CO3, which is a stable compound at lower temperatures (
