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Energy & Fuels 2000, 14, 138-141
Oxidation of Coal Char/CaS Mixture at High Temperature S. Ozawa,† Y. Morita,‡ L. Huang,† H. Matsuda,*,† and M. Hasatani‡ Research Center for Advanced Waste and Emission Management and Department of Energy Engineering and Science, Nagoya University, Nagoya 464-8603, Japan Received April 27, 1999
The oxidation of coal char/CaS mixtures was studied with a fixed bed reactor in the temperature range 700∼900 °C. It was found that oxidation of coal char/CaS experienced a process from reducing atmosphere to oxidizing atmosphere according to the change of CO concentration which was caused by partial combustion of coal char. The amount of SO2 released in a reducing atmosphere was more than double that released in an oxidizing atmosphere. It was regarded that the whole process of oxidation of coal char/CaS mixtures was dominated by different reactions. The amount of SO2 released at an oxidation temperature of 900 °C was largely increased compared with that at 800 °C, because the oxidation of CaS was more enhanced at 900 °C. XRD analysis also verified that the amount of CaSO4 produced at 900 °C was more than that at 800 °C. It was revealed that the further oxidation of CaS, especially at low temperature, might be blocked by the reaction product of CaSO4 which prevented the diffusion of O2 into the inside of the unreacted core of CaS particle due to a larger molar volume of CaSO4.
Introduction Integrated gasification combined cycle (IGCC) is widely accepted as a promising solution for thermal power plants to meet more stringent environmental requirements and higher thermal efficiency demands for coal utilization.1 Topping cycle pressurized fluidized bed combustion (TCPFBC) is such a kind of advanced IGCC system, which consists mainly of a pressurized fluidized bed (PFB) gasifier, a PFB char combustor, a topping combustor, and a combined power generation system.2 It is estimated that the new system will realize up to 48% thermal efficiency and dramatically mitigate acid gas emissions. One of the principal advantages of TCPFBC is that it adopts combined partial coal gasification with limestone injection and remained char burning system to avoid an expensive end desulfurization process. The CaS formed in the gasifier at around 680 °C will be further oxidized to a stable form of CaSO4 in a pressurized fluidized bed combustor at 850-1000 °C. Therefore, the best understanding of the performance of simultaneous oxidation of CaS and coal char in the combustor is essential for the better design of the system. The products of the oxidation of CaS can be CaSO4, SO2, and CaO, while the product CaSO4 may be reduced to SO2 and CaO under reducing conditions. It is generally thought that the reaction temperature and atmosphere are the main factors affecting the oxidation of * Author to whom correspondence should be addressed. Tel: +8252-789-3382. Fax: +81-52-789-5619. E-mail:
[email protected]. † Research Center for Advanced Waste and Emission Management. ‡ Department of Energy Engineering and Science. (1) Sunggyu, L. Alternative Fuels; Taylor & Francis Publisher: Philadelphia,1996; Chapter 5.
CaS. Abbasian3 found that the highest conversion of sulfation of partially sulfided calcium-based sorbents can be achieved between 815 and 900 °C. Chen et al.,4 Diaz-Bossio et al.,5 Oh et al.,6 and Kamphuis et al.7 reported that CaSO4 can be reduced to CaO and SO2 by carbon monoxide, hydrogen, or CaS at different operation conditions. Lyngfelt et al.8 observed a re-emission of SO2 by reduction of CaSO4 in a fluidized bed boiler due to reducing conditions in the particle phase of the bed. Dam-Johansen et al.9 and Hansen et al.10 also investigated the influence of the periodically changing oxidizing and reducing atmosphere in CFBC on sulfur capture by limestone, and submitted a reaction path diagram to describe the reaction processes of the system composed of CaO, CaSO4, CaS, SO2, CO, and CO2. The present work is focused on investigating the behavior of simultaneous oxidation of coal char and CaS using a fixed bed reactor in the temperature range 700-900 °C, since it is significant for TCPFBC to convert CaS to CaSO4 with the presence of coal char in a pressurized fluidized bed combustor. The operation conditions such as temperature, O2 concentration, and (2) Mori, S. J. Jpn. Inst. Energy (in Japanese) 1992, 71, 712-717. (3) Abbasian, J.; Rehmat, A. Ind. Eng. Chem. Res. 1991, 30, 19901994. (4) Chen, J. M.; Yang, R. T. Ind. Eng. Chem. Fundam. 1979, 18, 134-138. (5) Diaz-Bossio, L. M.; Squier, S. E.; Pulsifer, A. H. Chem. Eng. Sci. 1985, 40, 319-324. (6) Oh, J. S.; Wheelock, T. D. Ind. Eng. Chem. Res. 1990, 29, 544550. (7) Kamphuis, B.; Potma, A. W.; Prins, W.; Van Swaaij, W. P. M. Chem. Eng. Sci. 1993, 48, 105-116. (8) Lyngfelt, A.; Leckner, B. Chem. Eng. Sci. 1989, 44, 207-213. (9) Dam-Johansen, K.; Østergaard, K. Chem. Eng. Sci. 1991, 46, 855-859. (10) Hansen, P. F. B.; Dam-Johansen, K.; Østergaard, K. Chem. Eng. Sci. 1993, 48, 1325-1341.
10.1021/ef990077c CCC: $19.00 © 2000 American Chemical Society Published on Web 12/09/1999
Oxidation of Coal Char/CaS Mixtures at High Temperature
Energy & Fuels, Vol. 14, No. 1, 2000 139
Table 1. Characteristics of the Employed Coal proximate analysis (wt % db)
ultimate analysis (wt % daf)
FC
VM
ash
moisture
C
H
N
S
O
55.5
27.1
14.6
2.8
84.6
4.9
1.6
0.54
8.4
Table 2. Chemical Composition of the Employed Limestone (wt %) CaCO3
MgCO3
Al2O3
MnO
H2O
98.0
1.67
0.24
0.04
0.04
coal char/CaS mixing ratio are discussed in terms of oxidation efficiency of CaS to CaSO4. Experimental Section A fixed bed reactor system was set up to investigate the oxidation of a coal char/CaS mixture at different operation conditions. It consists of a vertical quartz tube with 30 mm i.d. and 1200 mm length and a quartz distributor which was sintered at the middle of the tube to support the sample layer of coal char/CaS mixture. In the test, the reactor was preheated to the desired temperature by an electric heating element in a heat-insulated cavity with continuous N2 flow. After the reactor reached a stable condition, the samples were set into the reactor and the N2 flow was replaced by an O2-N2 mixture of desired composition from the bottom of tube. All the experiments were performed with a fixed total gas flow rate of 300 mL/min and under atmospheric pressure. Coal char was produced by carbonizing a kind of coal particulate of the diameter range 1000-1410 µm in an electric stove at 850 °C under a N2 atmosphere for 5 min. Table 1 lists the industrial characteristics of the employed coal. A CHNS element analyzer was employed to analyze the change of sulfur content before and after coal carbonization. We found that the original coal contained 0.525 wt % sulfur and the coal char contained 0.426 wt % sulfur. Then it was estimated that about 45% of the total sulfur in original coal was discharged during carbonization and the remaining 55% of the sulfur still remained in coal char. A CaS sample was prepared by sulfidation of CaO with a flowing gas mixture of 1.1 vol % H2S and 6.3 vol % H2 in N2 at 850 °C until 80% of the CaO was converted to CaS. The CaO used in the experiments was produced by calcining a kind of limestone particulate of 710-1000 µm in diameter by means of a thermogravimetric apparatus in a N2 atmosphere at the same temperature. Table 2 shows the chemical composition of the employed limestone. An ultrared spectroscope SO2 analyzer (HORIBA, VIA-500) was employed to measure the outlet concentration of SO2. A TCD gas chromatograph (YANAGIMOTO, G2800) was used for O2, CO, and CO2 online analysis. The chemical compositions of solid products were analyzed by XRD. The gas release rate (R) during the oxidation of coal char/ CaS is described as
R)
C µ [mol/g-1 min-1] 100 22.4 w
where C is the outlet concentration of gas (%), u is the gas flow rate (0.3 L/min), and w is the weight of sample or coal char (g).
Results and Discussion Oxidation of Coal Char/CaS at 800 °C. Figure 1 depicts the time evolution of gas release rate for O2, CO, CO2, and SO2 during the oxidation of coal char/CaS in a N2 stream containing 21 vol % O2 at 800 °C. The solid mixture was composed of 1.0 g of coal char and 20 mg
Figure 1. Time evolution of gas release rate during the oxidation of a coal char/CaS mixture (800 °C, 21 vol % O2, and coal char/CaS ) 1.0 g/20 mg).
Figure 2. Effect of temperature on the total amount of SO2 release.
of CaS sample. It was found that the outlet concentration of O2 was very low during the initial 15 min, and then increased with the time lapse and returned to the initial concentration of 21 vol % at about 30 min when the reaction was actually completed. The outlet concentration of CO showed the highest value at the beginning of the reaction, and then decreased to zero at 15 min. In inverse proportion to CO concentration, the concentration of CO2 increased with the progress of reaction and reached a maximum value at 15 min. It was indicated that the overall process experienced the change of reaction atmosphere from reducing condition (first 15 min) to oxidizing condition (second 15 min). The outlet concentration of SO2 jumped to the highest value at several minutes, and then decreased slowly. Both the outlet concentrations of CO2 and SO2 decreased almost to zero at 30 min. Effect of Temperature. Figure 2 gives the changes of the total amount of SO2 discharged from the oxidation of both coal char/CaS (1.0 g/20 mg) mixture and coal char (1.0 g) at temperatures of 700, 800, and 900 °C (21 vol % O2 in N2). It was found that the total amount of SO2 emitted from the oxidation of coal char was almost the same at three temperatures. In comparison with the oxidation of coal char, the total amount of SO2 discharged from the oxidation of the char/CaS mixture was almost the same as the case when coal char alone was oxidized at 700 °C (17% of total sulfur in the mixture
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Ozawa et al.
Figure 3. Effect of temperature on the amount of SO2 release in two reaction regions.
Figure 4. XRD pattern of solid residue at 800 °C after 30 min of reaction.
was oxidized to SO2). A small increase of SO2 production was found at 800 °C (21% of total sulfur in the mixture was oxidized to SO2). But the total amount of SO2 emission largely increased at 900 °C (over 41% of total sulfur was oxidized to SO2). It reflects that the oxidation of sulfur element in CaS to SO2 was greatly intensified with increasing temperature above 800 °C. The distributions of the amount of SO2 emission between two regions of reducing and oxidizing atmosphere at different temperatures are also shown in Figure 3. It was found that the amount of SO2 emitted in region 1 (reducing atmosphere) was more than double that in region 2 (oxidizing atmosphere). Reaction Mechanism. At the beginning of the oxidation of coal char/CaS mixture, there was a large amount of simultaneous demand of O2 for the oxidation of coal char, and sulfur contained in coal char and CaS. The oxygen was consumed competitively by these demanders. A portion of the coal char could only be oxidized to CO, forming the reducing atmosphere in the reactor. Since all above reactions are exothermic processes, a temperature rise of about 30 °C in the reactor was observed by means of a thermocouple set in the reactor. The oxidation of CaS is generally regarded to proceed according to the following reactions:
Table 3. XRD Analysis of Solid Residue (Intensity, cps)
CaS + 2O2 f CaSO4
(1)
CaS + 3/2O2 f CaO + SO2
(2)
CaO + SO2 + 1/2O2 f CaSO4
(3)
These different reaction paths depend on the reaction atmosphere which changes during the reactions, in accordance with the oxidation of the coal char /CaS mixture as described in Figure 1. At the first half of oxidation under reducing atmosphere, it seems that reaction 2 dominated the process, resulting in much SO2 release. With the progress of the reaction and recovery of O2 concentration, reactions 1 and 3 became to dominate the process. Comparison with the oxidation at 800 °C, a large amount of SO2 which was derived from oxidation of CaS was discharged at 900 °C (Figure 2). It may be ascribed to the fact that reaction 2 was much enhanced with the temperature rising from 800 to 900 °C.4 It was also noticed that the product CaSO4 started to decompose
test condition
CaSO4 (2θ ) 25.4)
CaSO4+CaS (2θ ) 31.5)
CaO (2θ ) 32.3)
CaS (2θ ) 45.1)
800 °C, 10 min 800 °C, 30 min 900 °C, 10 min 900 °C, 30 min
71 64 79 91
232 170 116 123
34 23 25 21
129 115 92 64
by CO around 900 °C according to reactions 4, 5, and 6:8
CaSO4 + CO f CaO + SO2 + CO2
(4)
CaSO4 + 4CO f CaS + 4CO2
(5)
3CaSO4 + CaS f 4CaO + 4SO2
(6)
Furthermore, the molar volumes of CaSO4, CaS, and CaO are largely different as 52.2, 28.9, and 16.9 cm3/ mol, respectively. Therefore, there was a large change in the microstructure of the CaS particle, which was caused by the products of CaSO4, CaS, and CaO accompanied by the above reactions. At 800 °C, it was regarded that the product of CaSO4 might plug the micropores of CaS particle due to its larger molar volume and then retarded further oxidation of CaS by preventing O2 diffusion though micropores into the interior. On the contrary, the oxidation of CaS might access further inside of the particle through micropores at 900 °C, since the molar volumes of CaO and CaS which were produced by the reduction of CaSO4 by reactions 4, 5, and 6 are smaller than that of CaSO4. XRD Analysis. Solid residues after 10 min of reaction in a reducing atmosphere and after 30 min of reaction were analyzed by using XRD to verify the products. Figure 4 gives a typical XRD pattern of the solid residue after 30 min of reaction at 800 °C. It was found that the peaks of CaSO4 and CaS were overlapped at about 31.4°, so we adopted the peaks of CaSO4 at 25.4° and CaS at 45.1° to compare their intensities under different operation conditions (Table 3). It is clear that the intensity of CaS at 900 °C is lower than that at 800 °C, and that the intensity of CaSO4 at 900 °C is higher than that at 800 °C for both reaction times of 10 and 30 min. At the reaction temperature of 900 °C, it is obvious that the intensity of CaS for the reaction time after 30 min is lower than that after 10 min, and the intensity of CaSO4 after 30 min is much greater than that after 10 min. But the intensity changes of both CaSO4 and CaS
Oxidation of Coal Char/CaS Mixtures at High Temperature
Figure 5. Time evolution of gas release rate during the oxidation of a coal char/CaS mixture (800 °C, 30 vol % O2, and coal char/CaS ) 1.0 g/20 mg).
Energy & Fuels, Vol. 14, No. 1, 2000 141
Figure 7. Effect of the CaS proportion in coal char/CaS mixture on the amount of SO2 release (800 °C, char ) 1.0 g, 21 vol % O2, and 30 min of reaction).
found that the coal char was completely burnt in 30 min at all tested ratios of coal char/CaS, and the total amount of SO2 release linearly increased with increasing the fraction of CaS. However, the ratio of sulfur converted to SO2 to the total sulfur contained in the coal char/CaS mixture decreased quickly with increasing CaS proportion in the mixture, and approached a certain constant value. Conclusions
Figure 6. Effect of the inlet O2 concentration on the amount of SO2 release.
are not so remarkable at 800 °C. These results are in accordance with the above experimental results and the present analysis of reaction processes. Effect of O2 Concentration. Figure 5 shows the effect of increasing the inlet concentration of O2 to 30 vol % on the oxidation of coal char/CaS mixture. Comparing with Figure 1, it can be seen that the two reaction zones were maintained although the total reaction time was shortened from 30 to about 20 min. It was also found that the total amount of SO2 release increased from 7.4(×10-5 mol/g-sample) to 8.1(×10-5 mol/g-sample) and 9.7(×10-5 mol/g-sample) with increasing the inlet concentration of O2 from 21 vol % to 30 vol % and 50 vol % at 800 °C, respectively (Figure 6). Although the total amount of SO2 emission was much more at 900 °C, the increase of the O2 concentration seemed to have little influence on the release of SO2. Effect of Coal Char/CaS Ratio. Figure 7 shows the influence of the ratio of coal char/CaS on the total amount of SO2 release at 800 °C after 30 min of reaction (21 vol % O2). The weight of char was constant 1.0 g and the weight of CaS varied from 0 to 500 mg. It was
Oxidation of a coal char/CaS mixture experienced a change of reaction atmosphere from reducing atmosphere to oxidizing atmosphere. Most of SO2 release was found in early stage of oxidation, where the reducing atmosphere appeared with the presence of CO. The release of SO2 at reaction temperature of 900 °C (41% of total sulfur in the mixture of 1 g of coal char and 20 mg of CaS was oxidized to SO2) was greatly increased compared with that at 800 °C(21% of total sulfur in the mixture was oxidized to SO2). This result was attributed to the oxidation of CaS being much enhanced at 900 °C. XRD analysis also verified that the amount of CaSO4 formed at 900 °C was more than that at 800 °C. The successive oxidation of CaS might be blocked by the product of CaSO4 which prevented O2 diffusion into the inside of the unreacted core of CaS particle especially at low temperature. Increasing the inlet concentration of O2 greatly shortened the reaction time, but an increase of SO2 release from 7.4(×10-5 mol/g-sample) to 9.7 (×10-5 mol/gsample) was found at 800 °C with the concentration of O2 increased from 21 vol % to 50 vol % for the coal char/ CaS mixture ratio of 1 g/20 mg. However, the influence of changes of inlet concentration of O2 on SO2 release was not remarkable at 900 °C. Increasing the CaS proportion in the coal char/CaS mixture resulted in a linear increase of the amount of SO2 emission. But the ratio of sulfur converted to SO2 to the total sulfur contained in the coal char/CaS mixture seemed to approach a constant value. EF990077C
