3826
Energy & Fuels 2009, 23, 3826–3830
Investigation on Coal Pyrolysis in CO2 Atmosphere Lunbo Duan, Changsui Zhao,* Wu Zhou, Chengrui Qu, and Xiaoping Chen Institute for Thermal Power Engineering of Southeast UniVersity, Nanjing 210096, China ReceiVed March 23, 2009. ReVised Manuscript ReceiVed May 10, 2009
Considerable studies have been reported on the coal pyrolysis process and the formation of SO2 and NOx processors such as H2S, COS, SO2, HCN, and NH3 in inert atmospheres. Similar studies in CO2 atmosphere also need to be accomplished for better understanding of the combustion characteristics and the SO2/NOx formation mechanism of oxy-fuel combustion, which is one of the most important technologies for CO2 capture. In this study, thermogravimetry coupled with Fourier Transform Infrared (TG-FTIR) analysis was employed to measure the volatile yield and gas evolution features during coal pyrolysis process in CO2 atmosphere. Results show that replacing N2 with CO2 does not influence the starting temperature of volatile release but seems to enhance the volatile releasing rate even at 480 °C. At about 760 °C, CO2 prevents the calcite from decomposing. In CO2 atmosphere, the volatile yield increases as the temperature increases and decreases as the heating rate increases. COS is monitored during coal pyrolysis in CO2 atmosphere while there are only H2S and SO2 formed in N2 atmosphere. The COS is most likely formed by the reaction between CO2 and H2S. No NH3 was monitored in this study. In CO2 atmosphere, the gasification of char elevates the conversion of char-N to HCN. The HCN yield increases as the temperature increases and decreases as the heating rate increases.
1. Introduction Pyrolysis, as the preliminary process of coal combustion, plays a crucial role in determining flame stability, ignition, and product distributions. Pyrolysis is an extremely complex process. It generally goes through a series of reactions and can be influenced by many factors, such as coal type, particle size, heating rate, temperature, pressure, atmosphere, and so on. Extensive studies have been done on coal pyrolysis in Ar, He, and N2 atmospheres.1 In oxy-fuel combustion, which is considered as one of the most promising technologies for CO2 reduction,2-4 a combination of pure oxygen and recycled flue gas (CO2 concentration higher than 95%) is used for combustion of the fuel. The recycled flue gas is used to control flame temperature and make up the volume of the missing N2 to ensure there is enough gas to carry the heat through the boiler. The high proportions of CO2 in the furnace gases result in great differences with the conventional air combustion. Predictably, CO2 affects the coal pyrolysis process in two main ways: (1) CO2 is a product of coal pyrolysis, which may affect the volatile composition and yield; and (2) CO2 is a reactant in the char gasification reaction, which may bring differences to the formation of SO2/NOx precursors. Messenbck et al.5 studied flash pyrolysis of a bituminous coal under three different atmospheres (He, H2O, and CO2) of 1 MPa * Corresponding author. Phone and fax: +86 25 83793453; E-mail:
[email protected]. (1) Solomon, P. R.; Serio, M. A.; Suuberg, E. M. Prog. Energy Combust. Sci. 1992, 18, 133–220. (2) Buhre, B. J. P.; Elliott, L. K.; Sheng, C. D.; Gupta, R. P.; Wall, T. F. Prog. Energy Combust. Sci. 2005, 31, 283–307. (3) Krishnamoorthy, G.; Veranth, J. M. Energy Fuels. 2003, 17, 1367– 1371. (4) Zheng, L.; Furimsky, E. Fuel Process. Technol. 2003, 81, 23–34. (5) Messenbck, R. C.; Dugwell, D. R.; Kandiyoti, R. Energy Fuels. 1999, 13, 122–129.
at a peak temperature, heating rate, and holding time of 1000 °C, 1000 °C s-1, and 0-60 s, respectively. There were no significant effects of changing the atmosphere from He to CO2 on the volatile yield until the beginning of the temperature holding. But afterward, the reactive gases caused the gasification rates much greater than expected from previous reports on the char gasification. Thus, simultaneous occurrence of the thermal cracking and CO2 gasification of the nascent char has been evidenced, whereas the rates of the gasification seem to be much higher than those reported previously. Jamil et al.6 studied the pyrolysis of a Victorian brown coal under He and CO2 atmospheres at both fast heating rate (1000 °C s-1) and slow heating rate (1 °C s-1). Changing the atmosphere from He to CO2 influences neither the yield nor the composition of the tar. Even under heating at 1000 °C s-1, the tar evolution was completed before temperature reached 600 °C. After completion of the tar evolution, CO2 participated in the formation of light gases from the nascent char. Initial CO2 gasification of the nascent char occurred at a considerably high rate simultaneously with its thermal cracking. It seems the rate of such rapid CO2 gasification strongly depends on the rate of thermal cracking. The nascent char from the pyrolysis at 1 °C s-1 up to 700 °C has thermal cracking and gasification reactivities very similar to those of the nascent char from pyrolysis at 1000 °C s-1 up to the same temperature. However, knowledge of the pyrolysis of coal under CO2 atmosphere is still rare, and it is the basis and thus essentially important for a better understanding to oxy-fuel combustion. Also, the effects of CO2 on the formation of the SO2/NOx precursors are still unclear. In this study, effects of CO2 on pyrolysis of a Chinese bituminous coal and the formation of SO2/NOx precursors were investigated by TG-FTIR system. Also, effects of temperature and heating rate on the yields of (6) Jamil, K.; Hayashi, J.; Li, C. Z. Fuel. 2004, 83, 833–843.
10.1021/ef9002473 CCC: $40.75 2009 American Chemical Society Published on Web 06/08/2009
Coal Pyrolysis in CO2 Atmosphere
Energy & Fuels, Vol. 23, 2009 3827
Table 1. Proximate and Ultimate Analyses of Xuzhou Bituminous Coal Samples ultimate analysis wtdaf %
proximate analysis wt %
C
H
O
N
S
Mad
Ad
Vdaf
81.51
5.04
10.09
0.93
2.43
2.10
26.10
34.58
Table 2. Forms of Sulfur in the Sample (%) total sulfur
pyrite sulfur
organic sulfur
sulfate sulfur
2.43
0.42
1.53
0.48
Table 3. Ash Constituent of the Sample (wt %) CaO
SO3
Fe2O3
SiO2
MnO2
Al2O3
MgO
K2O
4.77
1.54
7.55
44.51
1.12
37.15
2.38
0.76
Figure 2. DTG curves of coal pyrolysis.
volatile and SO2/NOx precursors in CO2 atmosphere were taken into consideration. 2. Experimental Section Samples. The coal samples studied in this work were Xuzhou bituminous coal in China. All the samples were sieved to less than 100 µm. The proximate analysis and ultimate analysis of the coal sample were performed and listed in Table 1, where “ad” means air day basis, “d” means dry basis, and “daf” means dry and ash free basis. The forms of sulfur in the parent coal were listed in Table 2. The ash constituent of the parent coal was listed in Table 3. Apparatus and Procedure. The TG-FTIR system employed in this study was the SETARAM TGA 92 and BRUKER VECTOR 22 FTIR spectrometer. The programmed temperature pyrolysis is as follow: a sample of about 10 mg was weighted into an open alumina crucible for each run. The samples were heated from ambient temperature to 150 °C to dry for 5 min in N2 and CO2 atmospheres and then to an end temperature at a fixed heating rate for pyrolysis. Upon reaching the end temperature and holding the temperature for 30 s, the samples were quickly cooled by the carrier gas. The end temperature is fixed to 700, 800, 900, and 1000 °C, respectively. The heating rate is fixed to 10, 30, 50, and 70 °C min-1. The flow rates of N2 and CO2 were 80 mL min-1. Evolved gases from the TG analyzer were connected to the FTIR spectrometer by a heated line (about 180 °C) to avoid the condensation of the less volatile gas compounds. The spectra were collected at a resolution of 1 cm-1. Spectra were recorded with a temporal resolution of about 6s. The IR absorption band was in the range of 4000-400 cm-1.
3. Experimental Results Volatile Yield Analysis. The TG and DTG curves of coal samples heated to 900 at 30 °C min-1 in N2 and CO2 atmospheres are presented in Figures 1 and 2. From the TG curves, replacing N2 with CO2 does not influence the beginning temperature of volatile release significantly, but will enhance the volatile releasing rate. The weight loss of coal sample pyrolyzed in CO2 atmosphere from 150 to 900 °C is 25.73%, higher
Figure 1. TG curves of coal pyrolysis.
Figure 3. FTIR spectra in N2 atmosphere at 760 °C.
than 22.85% in N2 atmosphere, which is certainly caused by the gasification of nascent char by CO2. It can be noticed from Figure 2 that the DTG curves in CO2 atmosphere and that in N2 atmosphere are almost the same before the temperature reaches 450 °C, indicating that CO2 behaves as an inert gas on coal pyrolysis in low temperature zone. There are three main differences between the two DTG curves, marked as 1, 2, and 3 in Figure 2. In CO2 atmosphere, the mass loss rate peak at 480 °C is a little bigger than that in N2 atmosphere. The difference was well repeated by experiment and seems to be out of the range of the experimental error. At 480 °C, the tar evolution process is almost completed, and thermal cracking is mostly in progress and CO2, CO, light aliphatic gases (main CH4 and C2H6 in this study), and H2O are produced. In CO2 atmosphere, the evolution of light gases at the temperature seems to be caused by simultaneous thermal cracking and CO2 gasification, as suggested by Jamil et al.6 Although in the work of Jamil et al., the temperature of the initial CO2 gasification seems to be higher than 600 °C, the temperature in this study is lower with a smaller gasification rate. At about 760 °C, there is a peak of DTG curves in N2 atmosphere, but no peak in CO2 atmosphere. The FTIR spectra of gas species evolved in N2 atmosphere at 760 °C are shown in Figure 3. CO2 is the main gas product at the temperature. As confirmed in ref,7 calcite will decompose and release CO2 at about 760 °C. So the CO2 peak at the temperature is likely caused by the decomposition of calcite in the coal sample. In CO2 atmosphere, the high partial pressure of CO2 will prevent the calcite from decomposing. When the temperature gets close to 900 °C, another peak of DTG curve appears in CO2 atmosphere, which can be attributed to the gasification of the nascent char. Also, the decomposition of calcite may be partly attributed to this peak, because the decomposing temperature of calcium carbonate in pure CO2 is about 900 °C.8 The DTG curves of coal pyrolysis in CO2 atmosphere from 150 °C to different end temperatures at a heating rate of 30 °C min-1 are presented in Figure (7) Charland, J. P.; Macphee, J. A.; Giroux, L.; Price, J. T.; Khan, M. A. Fuel Process. Technol. 2003, 81, 211–221. (8) Hughes, R. W.; Lu, D.; Anthony, E. J.; Wu, Y. H. Ind. Eng. Chem. Res. 2004, 43, 5529–5539.
3828
Energy & Fuels, Vol. 23, 2009
Figure 4. Effects of temperature on coal pyrolysis in CO2 atmospheres.
Figure 5. Effects of temperature and heating rate on the volatile yields.
Duan et al.
Figure 7. Sulfur release profiles in N2 and CO2 atmospheres.
uniformly from the inside to the outside, the sample decomposes promptly, and the mass is lost quickly. The maximum value of mass loss rate is also enhanced. The volatile yield decreases as the heating rate increases due to the delay effects of the particle temperature at high heating rates, as shown in Figure 5. Sulfur Release Characteristics. The release profiles of sulfurcontaining gases evolved in N2 and CO2 atmosphere are shown in Figure 7. There is a H2S release peak and a SO2 release peak in N2 atmosphere, and only a negligible amount of COS is formed in N2 atmosphere. In CO2 atmosphere, the peak values of H2S and SO2 are a little smaller than that in N2 atmosphere and another COS peak is formed. During the coal pyrolysis process, both pyrite (FeS2) and the organic sulfur compounds such as thiols may generate H2S. When heated in an inert atmosphere, pyrite will decompose to release sulfur gas and form iron sulfide of lower sulfur contents pyrrhotite FeSx (here 1 e x e 2).11 Then the sulfur will be hydrogenated to form H2S in the hydrogen radical existing atmosphere like coal pyrolysis. COS will be formed by both gas phase secondary reactions:12
H2S + CO f COS + H2
(1)
H2S + CO2 f COS + H2O
(2)
and gas-solid reaction:
FeS2 + CO f FeS + COS
Figure 6. Effects of heating rate on coal pyrolysis in CO2 atmospheres.
4. When the end temperature is 700 and 800 °C, the peak of DTG curves at the last stage of pyrolysis is not obvious for the holding time is 30 s in this study, indicating that the rate of CO2 gasification is much slower than the rate of thermal cracking. As the temperature goes up to 1000 °C, the peak of the mass loss rate is much bigger, indicating that the gasification rate becomes much higher. The volatile yields of different temperatures are shown in Figure 5. The increase of volatile yields from 700 to 1000 °C in CO2 atmosphere is much higher than that in N2 atmosphere, confirming the simultaneous occurrence of thermal cracking and CO2 gasification.6 The same magnitude of the thermal cracking and CO2 gasification can also be observed. The DTG curves of different heating rates to 900 °C are shown in Figure 6. As the heating rate increases from 10 to 70 °C min-1, the temperature of the maximum mass loss rate is delayed, and its maximum value also decreased. This is in agreement with results of pyrolysis in inert atmospheres.9,10 The changes in the temperature of different heating rates are most likely associated with the differences of heat and mass transfer of the sample particles internally or externally. At lower heating rates, samples are heated
(3)
In CO2 atmosphere, CO is more easily formed due to the gasification of char, as presented in Figure 8. High concentration of CO2 together with CO enhances the formation of COS in CO2 atmosphere. SO2 release is associated with certain sulfur groups in the coal, such as sulfoxide, sulfone, and sulfate. In CO2 atmosphere, when the temperature gets over 800 °C, the SO2 releasing rate increases again. This is attributed to the decomposition of sulfate, which is detected by X-ray photoelectron spectroscopy (XPS) in the parent coal sample. The probable reaction is as follow:
MSO4 + CO f MO + SO2 + CO2
(4)
where M represents metal, such as iron, calcium, magnesium, etc. The reaction rate increases as the CO partial pressure and temperature increase.13,14 The total yield of gas species was (9) Yang, H. P.; Yan, R.; Chin, T.; Liang, T. D.; Chen, H. P.; Zheng, C. G. Energy Fuels 2004, 18, 1814–1821. (10) Biagini, E.; Fantei, A.; Tognotti, L. Thermochim. Acta 2008, 472, 55–63. (11) Hu, G. L.; Dam, K.; Wedel, S.; Hansen, J. P. Prog. Energy Combust. Sci. 2006, 32, 295–314. (12) Shao, D. K.; Hutchinson, E. J.; Heidbrink, J.; Pan, W. P.; Chou, C. L. J. Anal. Appl. Pyrol. 1994, 30, 91–100. (13) Anthony, E. J.; Granatstein, D. L. Prog. Energy Combust. Sci. 2001, 2, 215–236. (14) Hoteit, A.; Bouquet, E.; Schonnenbeck, C.; Gilot, P. Chem. Eng. Sci. 2007, 62, 6827–6835.
Coal Pyrolysis in CO2 Atmosphere
Figure 8. CO release profiles in N2 and CO2 atmospheres.
Figure 9. Effects of temperature on yields of sulfur-containing species in CO2 atmosphere.
Figure 10. Effects of heating rate on yields of sulfur-containing species in CO2 atmosphere.
estimated by carrying out the integration over the whole period of the experimental time. The yields of the sulfur-containing species at different temperatures are presented in Figure 9. At the same heating rate of 30 °C min-1, the SO2 yield increases as the temperature increases. It appears that the SO2 formation from the thermal cracking of sulfoxide and sulfone ceased at about 800 °C, and then the reduction of sulfate by CO plays a main role, as presented in Figure 7. The yields of H2S and COS also increase as the temperature increases, but the increasing amount from 900 to 1000 °C is very small, indicating that the formation of H2S and COS ended at about 900 °C in the study. The yields of sulfurcontaining species at 900 °C with different heating rates are shown in Figure 10. The yield of SO2 decreases as the heating rate increases from 10 to 70 °C min-1, which is partly because of the difference of heat and mass transfer in the temperature-programmed procedure at different heating rates. The H2S yield increases as the heating rate increases, and the COS yield decreases as the heat rate increases. The thiols and pyrite would be easily decomposed to form H2S, and would be oxidized to form SO2 and COS by H2O
Energy & Fuels, Vol. 23, 2009 3829
Figure 11. HCN release profiles in N2 and CO2 atmospheres.
and CO2 during the slow pyrolysis process,15 particularly by CO2 in the study. At small heating rate, the secondary reaction between H2S and CO2 are more likely to occur and form COS. Nitrogen Release Characteristics. It is known that the majority of coal-N exists in coal in the N-containing heteroaromatic ring systems. Pyridines, pyrrole, and quaternary nitrogen are the most common N-compounds in the coal. At low temperature or residence time nitrogen is preferentially retained in the char, whereas at high temperatures it is depleted.16 For coal, the nitrogen volatiles evolve comparatively late in the primary pyrolysis. For bituminous coals, the amount of nitrogen released as condensable products during pyrolysis appears to be proportional to the tar yield and the nitrogen content in the fuel. During secondary pyrolysis of char and tar, the heteroaromatic rings may decompose, releasing the nitrogen mostly as HCN to the gas-phase.17 There are strong indications that NH3 is formed both by direct release from the solid matrix and from hydrogenation of HCN on the surface of the solid matrix.It is well established, that the nitrogen volatiles partitioning depends on fuel type, pyrolysis temperature and heating rate. In the study, HCN seems to be volatilized easily in both atmospheres, whereas no NH3 is detected. The HCN releasing profiles in N2 and CO2 at 900 °C and heating rate of 30 °C min-1 are shown in Figure 11. The HCN starts to release at about 350 °C with quite minor amount. There is a HCN peak at 540 °C in CO2 atmosphere, and there is a HCN peak at 580 °C in N2 atmosphere. This peak is probably associated with the decomposition of quaternary nitrogen, which is proven to decompose at a comparatively low temperature.16 The second peak in both atmospheres (620 °C in N2 atmosphere and 660 °C in CO2 atmosphere) may relate to pyrrole nitrogen in the sample. Then, while the HCN yield increases only very slightly in N2 atmosphere, the HCN yield further increases largely from 700 to 900 °C in the CO2 atmosphere. This indicates that the gasification effect of CO2 is favorable for the fuel-N conversion (most the char-N) to HCN. It can be assumed that when CO2 reacts with char, the -CN bonds will more easily be opened from the matrix and HCN would be formed if H2 exists. Large amount of H2 could only form at temperatures much higher than 600 °C, according to the fact that the cleavage of the bonds (mainly C-H) to form H2 requires high temperatures.17,18 Figure 12 presents the effects of temperature on the HCN yield at the heating rate of 30 °C min-1. As temperature increases from 700 to 1000 °C in CO2 atmosphere, the HCN yield increases a lot, in agreement with the tendency in the literatures.19,20 Effects of heating rate on the HCN yield at 900 °C are presented in Figure 13. At low heating rate, the (15) Miura, K.; Mae, K.; Shimada, M.; Minami, H. Energy Fuels. 2001, 15, 629–636. (16) Kambara, S.; Takarada, T.; Yamamoto, Y.; Kato, K. Energy Fuels. 1993, 7, 1013–1020. (17) Glarborg, P.; Jensen, A. D.; Johnsson, J. E. Prog. Energy Combust. Sci. 2003, 29, 89–113. (18) Porada, D. Fuel 2004, 83, 1191–1196. (19) Tan, L. L.; Li, C. Z. Fuel 2000, 79, 1891–1897. (20) Ledesma, E. B.; Li, C. Z.; Nelson, P. F.; Mackie, J. C. Energy Fuels. 1998, 12, 536–541.
3830
Energy & Fuels, Vol. 23, 2009
Figure 12. Effects of temperature on HCN yield in CO2 atmospheres.
gasification of char and the cracking of C-CN bonds are sufficiently reacted in the experimental condition due to the longer reaction time. So as the heating rate increases, the HCN yield decreases.
4. Conclusions Before temperature reaches 400 °C, CO2 behaves as an inert atmosphere during the coal pyrolysis process. CO2 seems to enhance the maximum volatile releasing rate at about 480 °C. At about 760 °C, CO2 prevents the calcite from decomposing. In CO2 atmosphere, the volatile yield increases as the temperature increases and decreases as the heating rate increases.
Duan et al.
Figure 13. Effects of heating rate on HCN yield CO2 atmospheres.
COS is monitored during coal pyrolysis in CO2 atmosphere, whereas there are only H2S and SO2 formed in N2 atmosphere. The COS is most likely formed by the reaction between CO2 and H2S. No NH3 was monitored in this study. In CO2 atmosphere, the gasification of char enhances the conversion of fuel-N to HCN. The HCN yield increases as the temperature increases and decreases as the heating rate increases. Acknowledgment. Financial support from the Special Funds for Major State Basic Research Projects of China (2006CB705806), the National Key Program of Basic research of China (No. 2006CB705806), and the Foundation for Excellent Doctoral Dissertation of Southeast University, China. EF9002473
