Co-gasification Reactivity of Coal and Woody Biomass in High

Sep 24, 2009 - cedar bark, and the mixtures of coal and cedar bark were gasified with carbon ... dioxide from power stations.2 Although biomass is exp...
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Energy Fuels 2010, 24, 145–151 Published on Web 09/24/2009

: DOI:10.1021/ef900526h

Co-gasification Reactivity of Coal and Woody Biomass in High-Temperature Gasification† Shiro Kajitani,* Yan Zhang, Satoshi Umemoto, Masami Ashizawa, and Saburo Hara Energy Engineering Research Laboratory, Central Research Institute of Electric Power Industry (CRIEPI), 2-6-1, Nagasaka, Yokosuka, Kanagawa 240-0196, Japan Received May 25, 2009. Revised Manuscript Received August 22, 2009

While biomass has been co-fired at pulverized coal boilers in power stations for a high-efficiency use of biomass, there have been few reports on the co-gasification using entrained flow gasifiers, which are adopted for the major integrated gasification combined cycle plants. In this work, two bituminous coals, cedar bark, and the mixtures of coal and cedar bark were gasified with carbon dioxide at high temperature using a drop tube furnace. As a result, distinguished synergy between coal and cedar bark to improve their gasification reactivity was not observed through the high-temperature co-gasification. The product yields during the co-pyrolysis in nitrogen gas at high temperature agreed with the equilibrium yields, and the char reactivity of the mixtures of coal and biomass was almost the same as that of single coals at the hightemperature gasification. In the case of low-temperature gasification, however, a little improvement of the char gasification reactivity of the mixtures was found because of the catalysis of alkaline and alkaline-earth metal species in biomass.

not yet been experienced in Japan. Some papers on cogasification3-9 and co-pyrolysis10-13 of coal and woody biomass have been reported for the past decade. Collot et al.3 reported on a series of the co-gasification tests with carbon dioxide using fluidized- and fixed-bed reactors, and “no evidence of synergy was found with the fluidized bed reactor”. Sj€ ostr€ om et al.4 and McLendon et al.5 reported on the co-gasification using pressurized fluidized-bed reactors. The reactivity of the mixture fuel seemed to have increased in the co-gasification process at the tests of Sj€ ostr€ om et al., while synergies were not readily apparent at the pilot-scale tests of McLendon et al. Fixed- or fluidized-bed reactors were used in most previous works, and there have been few reports on the co-gasification using entrained flow gasifiers, which are adopted for the major IGCC plants. In this work, the mixtures of bituminous coal and woody biomass were pyrolyzed and gasified with carbon dioxide at high temperate using drop tube furnaces (DTFs). In addition, the gasification reactivity of char was analyzed using a pressurized drop tube furnace (PDTF) and a thermogravimetric analyzer (TGA) to clarify their co-gasification reactivity and synergy in the entrained flow gasifiers.

1. Introduction Use of biomass is indispensable for sustainable energy secureness. The biomass gasification technologies are under development for the dispersed power sources, and the cocombustion technology of biomass in large-scale pulverized coal boilers for power generation is put to practical use. Because co-combustion technology has an advantage in the point of thermal efficiency because of the scale merits, it has been adopted in several power stations in Japan. However, the mixing ratio of biomass is 3 wt % or less because of the limited supply, poor grind ability, and high moisture of woody biomass.1 Co-gasification of biomass in coal-based integrated gasification combined cycle (IGCC) plants, which are promising technologies for advanced coal-fired power generation, should also be effective in reducing the emission of carbon dioxide from power stations.2 Although biomass is expected to be suitable for the coal-based IGCC technology because of its high reactivity, the co-gasification in an IGCC plant has † Presented at the 2009 Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies. *To whom correspondence should be addressed. Telephone: þ81-46856-2121. Fax: þ81-46-856-3346. E-mail: [email protected]. (1) Kiga, T. IEEJ Trans. Electr. Electron. Eng. 2008, 3, 43. (2) Valero, A.; Us on, S. Energy 2006, 31, 1643. (3) Collot, A.-G.; Zhuo, Y.; Dugwell, D. R.; Kandiyoti, R. Fuel 1999, 78, 667. (4) Sj€ ostr€ om, K.; Chen, G.; Yu, Q.; Brage, C.; Rosen, C. Fuel 1999, 78, 1189. (5) McLendon, T. R.; Lui, A. P.; Pineault, R. L.; Beer, S. K.; Richardson, S. W. Biomass Bioenergy 2004, 26, 377. (6) Kumabe, K.; Hanaoka, T.; Fujimoto, S.; Minowa, T.; Sakanishi, K. Fuel 2007, 86, 684. (7) Lapuerta, M.; Hernandez, J. J.; Pazo, A.; L opez, J. Fuel Process. Technol. 2008, 89, 829. (8) Alzate, C. A.; Chejne, F.; Valdes, C. F.; Berrio, A.; de la Cruz, J.; Londo~ no, C. A. Fuel 2009, 88, 437. (9) Fermoso, J.; Arias, B.; Plaza, M. G.; Pevida, C.; Rubiera, F.; Pis, J. J.; Garcı´ a-Pe~ na, F.; Casero, P. Fuel Process. Technol. 2009, 90, 926.

r 2009 American Chemical Society

2. Experimental Section 2.1. Test Coals and Biomass. Two bituminous coals and biomass were used for pyrolysis, char gasification, and cogasification tests. Their properties are shown in Table 1. Coal A is an Australian coal with a high fuel ratio of 2.1 and used for pulverized coal boilers in Japan. Coal F is a Chinese coal with the fuel of 1.8 and suitable for gasification because of its lower ash melting point. Cedar is one of the major woody biomasses (10) Meesri, C.; Moghtaderi, M. Biomass Bioenergy 2002, 23, 55. (11) Zhang, L.; Xu, S.; Zhao, W.; Liu, S. Fuel 2007, 86, 359. (12) Jones, J. M.; Kubacki, M.; Kubica, K.; Ross, A. B.; Williams, A. J. Anal. Appl. Pyrolysis 2005, 74, 502. (13) Haykiri-Acma, H.; Yaman, S. Fuel 2007, 86, 373.

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Table 1. Properties of Coal and Biomass coal A

ash volatile matter fixed carbon C (wt %, daf) H (wt %, daf) N (wt %, daf) S (wt %, daf) O (wt %, daf) Na (mg/kg, db) K (mg/kg, db) SiO2 Al2O3 Fe2O3 CaO

coal F

cedar bark (CB)

Proximate Analysis (wt %, dry basis) 13.8 6.0 2.3 28.7 36.1 75.1 57.5 57.9 22.6 Ultimate Analysis 82.5 80.8 5.1 4.8 1.4 1.0 0.4 0.1 10.6 13.3 280 470 620 220 Ash Composition (wt %) 49.9 23.5 37.6 8.8 4.4 7.9 1.8 45.8

53.1 5.7 0.6 0.0 40.6 100 2100 9.2 3.0 3.2 58.0

harvested from the Japanese planted forest. Each of the coals and cedar bark was pulverized and dried at 107 °C before the blend of coal and biomass. The mixture samples of coal and cedar bark were named as A-C30 or F-C30 in this paper. “A” or “F” means mixture with coal A or coal F, and the number shows the dry-base weight percentage of cedar bark in the mixture sample. The average particle size of the pulverized samples at the cumulative frequency of 50% was 29 μm for coal A, 38 μm for coal F, and 85 μm for cedar bark. 2.2. High-Temperature Rapid Pyrolysis Tests. Rapid pyrolysis including primary pyrolysis and secondary vapor phase cracking is the first stage of the gasification reaction in the entrained flow gasifiers that are adopted for major IGCC systems. In addition, pyrolysis and char gasification reaction are often discussed separately to clarify coal reactivity. In this work, the preparations of char and gasification rate analysis of char were conducted following the method in the previous paper.14 Conditions of char preparations, i.e., the heat history, are important because the temperature and the heating rate at pyrolysis have a large influence on the gasification reactivity. Although the pressure also has an influence on the pyrolysis reaction,15,16 the pressure at pyrolysis had little influence on the gasification reactivity of char.17 Then, the pulverized and dried samples were pyrolyzed rapidly at atmospheric pressure. A DTF and PDTF shown in Figure 1 were used in this work. They are similar facilities; however, the DTF facility is not available for pressurized conditions, and the PDTF facility has been designed for pressurized conditions. Rapid heating was realized by dropping samples into the alumina tube reactor with nitrogen as a carrier gas, and the heating rate was estimated to reach several thousand K/s. Pyrolysis tests for coals, cedar bark, and the mixture samples were carried out at 1400 °C using the DTF facility, considered the conditions in entrained flow gasifiers. The feeding rate of the samples was 100 g/h; the length of the heating zone was 900 mm; and the residence time was 2.5 s. Product gas was analyzed continuously with micro gas chromatography with thermal conductivity detectors (micro GC-TCDs) (3000 Micro GC, Agilent Technologies), and solid carbon was collected from the bottom of the furnace and the dust filter. The particle size

Figure 1. Schematic of the PDTF facility.

distribution of the solid carbon was measured with a laser diffraction method (LMS-30, Seishin Enterprise); the particle surface was observed with scanning electron microscopy (SEM) (S-3500N, Hitachi); and the X-ray diffraction (XRD) pattern was obtained with a diffractometer (RINT-TTR III, Rigaku; 50 kV, 300 mA, Cu KR radiation). 2.3. Char Gasification Tests. A TGA (TG-DTA2000S, MAC Science) was used to estimate the gasification reactivity of solid carbon collected in rapid pyrolysis tests. After the isothermal reaction technique, the flow gas was switched from argon to carbon dioxide, while the temperature was kept stable at the measurement temperature, and the weight loss caused by the gasification reaction was measured. It has been confirmed that the influence of the mass transfer, which is both the diffusion inside the bed of char particles and the diffusion inside the pores on the single particle, is negligible and the gasification rate is controlled (zone I18) by chemical reactions when the reaction rate is less than 0.003 s-1 in our TGA system.14,19 Therefore, the gasification tests were conducted mainly at 850 °C for the char of coal F and at 950 °C for the char of coal A with carbon dioxide. Because it is difficult to measure the valid reaction rate at high temperature using the TGA, owing to mass transfer inside the bed of char particles,19 the PDTF facility was used for the char gasification test with carbon dioxide at high temperature when the reaction rate of char was controlled by pore diffusion (zone II18), the same as in entrained flow gasifiers. The partial pressure of carbon dioxide was 0.2 MPa, simulating the reductor of an air-blown two-stage entrained flow coal gasifier.20 Although the total pressure was 0.5 MPa and lower than the usual conditions of the gasifier, it has been confirmed that the total pressure has little influence on the gasification rate of char when the partial pressure of the gasifying agent is the same and the total pressure is less than 2 MPa.19 The char particles were fed with the micro screw feeder at the rate of about 10 g/h and dropped with carrier nitrogen gas, and the gasifying agent, i.e., carbon dioxide, was conditioned to be far richer compared to the carbon in char to keep its concentration constant in the furnace. The mole ratio of carbon in char to carbon dioxide (C/CO2) was about 0.06.

(14) Kajitani, S.; Suzuki, N.; Ashizawa, M.; Hara, S. Fuel 2006, 85, 163. (15) Miura, K.; Nakagawa, H.; Nakai, S.; Kajitani, S. Chem. Eng. Sci. 2004, 59, 526. (16) Kajitani, S.; Nakagawa, H.; Miura, K.; Hara, S. 2005 International Conference on Coal Science and Technology, Okinawa, Japan, 2005. (17) Kajitani, S.; Suzuki, N. 12th International Conference on Coal Science, Cairns, Australia, 2003.

(18) Smoot, L. D.; Smith, P. J. Coal Combustion and Gasification; Plenum Press: New York, 1985. (19) Kajitani, S.; Hara, S.; Matsuda, H. Fuel 2002, 81, 539. (20) Ishibashi, Y. Gasification Technologies Conference 2008, Gasification Technologies Council, Washington, D.C., 2008.

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Figure 3. Product yields during rapid pyrolysis of coal A, cedar bark, and the mixture sample at 1400 °C. (;) Approximations of the experiment. (- - -) Predictions from equilibrium calculation.

Figure 2. Product yields during rapid pyrolysis of coal F, cedar bark, and the mixture samples at 1400 °C. (;) Approximations of the experiment. (- - -) Predictions from equilibrium calculation.

The particle residence time was assumed to be equal to the gas residence time because the estimated free-fall velocity of the char particle under 100 μm was much lower than the gas flow. The validity of this assumption on the residence time and the dispersibility of fed particles have been confirmed with the computational fluid dynamics (CFD) numerical simulation. The residence time was controlled by vertically traversing a water-cooled sampling probe inserted upward from the bottom of the furnace. The conversion ratio was estimated from the ultimate analysis of sampled char with the ash balance method. 2.4. High-Temperature Co-gasification Tests. While the pyrolysis and the char gasification were tested separately in the above experiments, raw samples of coals, cedar bark, and the mixtures were gasified with carbon dioxide at high temperature using the PDTF facility in this section, the same as the reductor in the air-blown two-stage entrained flow coal gasifier. The temperature was 1200 or 1300 °C; the furnace pressure was 0.5 MPa; and the partial pressure of carbon dioxide was 0.05 MPa. Although the pressure was lower than the usual conditions of the actual gasifier, it should be sufficient to confirm the gasification model obtained from the above experiments. The feeding rates of the samples were from 80 to 100 g/h depending upon the carbon content in the sample. It is preferable that the weight ratio of sample to carbon dioxide (fuel/CO2) and the mole ratio of total supplied oxygen to total supplied carbon in both the sample and carbon dioxide (O/C) are as constant as possible. The ratio of fuel/CO2 was 0.20, 0.21, 0.21, 0.23, and 0.24 for coal A, coal F, A-C30, F-C30, and cedar bark, respectively. The ratio of O/C was 1.36, 1.33, 1.41, 1.38, and 1.57 for coal A, coal F, A-C30, F-C30, and cedar bark, respectively. The products were collected by the water-cooled sampling probe, and the product gas composition was analyzed with the micro GC-TCDs. The carbon conversion was estimated from the composition of the product gas and solid.

Figure 4. Particle size distributions before and after pyrolysis of F-C30. Coal, char, and soot was tested with the laser diffraction method, and biomass was classified with sieves.

cedar bark, and the mixture samples of coal and cedar bark in nitrogen gas flow at 1400 °C. The carbon balance shown in the figures was defined as the percentage of the total amount of carbon in the products divided by the total amount of carbon in the fed sample. The primary pyrolysis and the secondary vapor phase cracking were completed rapidly at a high temperature of 1400 °C, and the volatile gas was decomposed almost to CO, H2, and solid carbon. No or a very small amount of CO2, CH4, and H2O were detected. Every yield of the gases and solid carbon had linear relationships with the mixing ratio of coal and biomass and agreed with chemical equilibrium yields, which were estimated with a chemical thermodynamics software package (FactSage 5.2,21 Thermfact/CRCT and GTT Technologies). As a result, distinct synergy between

3. Results and Discussion

(21) Balea, C. W.; Chartranda, P.; Degterova, S. A.; Erikssonb, G.; Hackb, K.; Mahfouda, R. B.; Melanc-ona, J.; Peltona, A. D.; Petersen, S. CALPHAD: Comput. Coupling Phase Diagrams Thermochem. 2002, 26, 189.

3.1. Rapid Pyrolysis Properties at High Temperature. Figures 2 and 3 show the product yields of coal F, coal A, 147

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Figure 5. SEM images of (a) char and (b) soot, which are produced during the rapid pyrolysis of F-C30.

Figure 6. XRD patterns of char and soot, which are produced during the rapid pyrolysis of the mixture samples of coal and cedar bark.

Figure 7. TGA curves of char gasification with CO2 (coal F and cedar bark). (;) Experimental results. (- - -) Prediction from experimental results of single chars.

coal and cedar bark could not be observed in the rapid pyrolysis properties at high temperature. Two kinds of solid carbon are formed at high-temperature pyrolysis: one is “char” originated from fixed carbon, and the other is “soot” (or coke) produced from volatiles.15,16 Although the yields of char and soot are not able to be distinguished with the equilibrium calculation, char and soot were separated roughly by their sampled positions in these experiments, the same as in the previous work.22 Char was collected from the bottom of the furnace, while soot was fine particulates passed through the bottom trap and collected mainly from the dust filter. Obvious synergy was not also observed in the yield of char and soot. The characterizations of char and soot are shown in Figures 4-6. Sub-micrometer particles of soot seemed to aggregate and was difficult to be dispersed for the analysis. Although most of the soot particles must be smaller than the measuring limit of the laser diffraction method, the particle size distributions and the SEM images show that the soot particle was much smaller than the char particle and raw sample particle. The (002) diffraction pattern of carbon was shown in Figure 6, and the peak of soot is significantly sharper than that of the char. It means that the carbon structure of soot was developed more than that of the char, and it suggests that the reactivity of soot must be lower than that of the char. 3.2. Char Gasification Reactivity. Figures 7 and 8 show the gasification reactivity of the char, which was formed in the

Figure 8. TGA curves of char gasification with CO2 (coal A and cedar bark). (;) Experimental results. (- - -) Predictions.

above pyrolysis tests of coal F, coal A, cedar bark, and the mixture samples. Coal F had high reactivity compared to many types of bituminous coal because of the catalysis of calcium in coal,14 while cedar bark had much higher reactivity than coal F because of the catalysis of the inherent alkaline and alkaline-earth metal (AAEM) species,23 especially calcium and potassium24 (see Table 1). Broken lines “calculated” for mixture samples in the figures show the predictions obtained from the results of single coal and single (23) Abu El-Rub, Z.; Bramer, E. A.; BremZhang, G. Ind. Eng. Chem. Res. 2004, 43, 6911. (24) Zhang, Y.; Ashizawa, M.; Kajitani, S.; Miura, K. Fuel 2008, 87, 475.

(22) Zhang, Y.; Kajitani, S.; Ashizawa, M.; Miura, K. Energy Fuels 2006, 20, 2705.

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Figure 11. Char gasification with CO2 at 1200 and 1400 °C using PDTF.

Figure 9. TGA curves of soot gasification with CO2 (coal F and cedar bark). (;) Experimental results. (- - -) Prediction from experimental results of single chars.

Figure 12. Arrhenius plots of the char gasification rate with CO2. Plots of TGA at 0.2 MPa were extrapolated from the gasification rate measured with TGA at 0.1 MPa with the analyzed reaction order.

Figure 10. TGA curves of soot gasification with CO2 (coal A and cedar bark). (;) Experimental results. (- - -) Predictions.

cedar bark as a parallel reaction and were estimated from the equation Xmix ¼ ð1 - FCB ÞXcoal þ FCB Xcedar

was also used to predict the conversion of soot of the mixture samples. In the case of soot, X means a conversion of soot and FCB means a fraction of cedar bark origin in soot of the mixture sample. The value of FCB was 0.129 for soot of F-C30 and 0.114 for soot of A-C30, respectively. However, the results should be treated with caution, for much more potassium might have been deposited on the surface of soot at the dust filter and not in the furnace, because the dust filter was not heated up and product gas passed though the soot bed on the surface of the dust filter. Because the gasification temperature in the entrained flow gasifier is estimated in the range from 1000 to 1600 °C, the gasification rate of the char of coal F, F-C30, and cedar bark was also measured at high temperature using the PDTF. The plots in Figure 11 show the results of the char gasification tests of coal F and F-C30 at 1200 and 1400 °C, and the lines show the fitting with the random pore model25 (RPM). The gasification rate of coal char was slightly improved by mixing cedar bark at the gasification temperature of 1200 °C, the same at low temperature using the TGA. It is suggested that the improvement of gasification reactivity at low temperature in the above experiments was not caused by the measurement methods using the TGA. However, the gasification rate of char of single coal and the mixture sample became almost the same at 1400 °C. Figure 12 shows the Arrhenius plots of the initial gasification rate dX/dt (at X = 0) obtained from the TGA and PDTF together. The plots of the TGA were extrapolated from the gasification rate at 0.1 MPa with the analyzed reaction order n. The values of n were 0.43, 0.35, and 0.49 for coal F, F-C30, and

ð1Þ

where Xmix, Xcoal, and Xcedar are conversions of the char of the mixture sample, single coal, and single cedar bark, respectively, at time t and FCB is a fraction of cedar bark origin in the char of the mixture sample. The value of FCB, which is obtained from Figures 2 and 3 with the least-squares method, was 0.116 for the char of F-C30 and 0.108 for the char of A-C30, respectively. In the case of cedar bark at 950 °C in Figure 8, the conversion was predicted with the activation energy analyzed later, because the estimated gasification rate was too high to be measured with the TGA without the influence of the mass transfer, as mentioned above. In comparison to the predictions, the experimental results showed that char of the mixture samples took a shorter time to complete the gasification reaction and slightly improved its reactivity. This suggests that some amount of potassium, which was inherent in cedar bark, had an effect on the gasification reactivity of coal char. Two kinds of behaviors of potassium can be considered. One is that some amount of potassium was volatilized from cedar bark and condensed on the coal char particle through the rapid pyrolysis. The other is that potassium on the cedar char particles was transferred to the coal char particles during the gasification tests using the TGA because char particles were close or touched each other in the bed of char. On the other side, as shown in Figures 9 and 10, the improvement of the gasification reactivity of soot of the mixture samples was much larger than that of char. Equation 1

(25) Bhatia, S. K.; Perlmutter, D. D. AIChE J. 1980, 26, 379.

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Figure 13. Temperature profiles in the PDTF before gasification tests.

Figure 15. Results of co-gasification with CO2 in PDTF (coal A and cedar bark).

temperature profiles measured using a thermocouple before the sample is fed in the furnace. It can be said that furnace temperature became flat after 0.8 s, and it has been confirmed that the temperature in the furnace hardly dropped while the sample was fed in the furnace. Figures 14 and 15 show the results of the gasification tests. The carbon conversion was defined as the percentage of the amount of carbon in the product gas divided by the total amount of carbon in the fed sample. The carbon balance shown in the figure was defined as the percentage of the total amount of carbon in the products divided by the total amount of carbon in the fed sample. Cedar bark reached a higher carbon conversion than coal at the same residence time, because of its lower fuel ratio and higher reactivity of char. However, the reactivity of soot is much lower than char, and the carbon conversion did not seem to approach 100% in several seconds because of soot. Solid lines in the figures show the simulation using the results of the above-mentioned gasification rate analysis of solid carbon and the equilibrium calculation in the gas phase. While the char yields obtained from the pyrolysis tests in nitrogen gas were also used, yields of soot and gas were not suitable to be used for these simulations because the volatile should react with the gasifying agent, so that a lower soot yield should be expected in the gasification atmosphere than in the nitrogen atmosphere. The yields of gas and soot during the pyrolysis of single coals and cedar bark were treated as fitting parameters in these simulations, and those of the mixture sample F-C30 were estimated from its mixing ratio. The behavior of soot formation in the gasification atmosphere needs further discussion. Broken lines in the figures show the predictions about the mixture obtained from the proportional distribution of the simulations of single coal and cedar bark. The carbon conversions of F-C30 agreed well with the simulation and the prediction, and those of A-C30 also agreed well with the prediction. As a result of co-gasification tests, it can be said that distinguished synergy between coal and cedar bark to improve the gasification reactivity or to inhibit the gasification was not observed through the high-temperature gasification. The behavior of pyrolysis in the gasification atmosphere needs further discussion. Although the AAEM species in woody biomass have an advantage in the reactivity of biomass, they may also become the cause of the fouling in the syngas cooler, the corrosion downstream, or other problems. The behavior of volatilization of the AAEM species also needs further discussion.

Figure 14. Results of co-gasification with CO2 in PDTF (coal F and cedar bark).

cedar bark, respectively. The correlation between the conversion and the gasification rate agreed well with the RPM or extended RPM,24,26 whose equation is pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi dX=dt ¼ kp ð1 -XÞ 1 - Ψ lnð1 -XÞð1 þ ðcXÞp Þ

ð2Þ

where kp is a reaction rate constant, Ψ is a initial pore structure parameter, and c and p are empirical constants. The values of parameters were Ψ = 0 and c = 0 for coal F and F-C30, and Ψ = 1, c = 1.22, and p = 14 for cedar bark. Although the slight improvement of the gasification reactivity of coal char was observed at 1200 °C or lower temperature, the gasification rate of char of F-C30 was almost the same as that of the single coal char at a high temperature of 1400 °C, as mentioned above. Two reasons are considered for the deference in the char reactivity of single coal and the mixture sample at high temperature: One is the influence of the pore diffusion, because the temperature dependency of the gasification rate became lower significantly at 1400 °C and it is obviously in zone II. The other must be the loss of the catalytic activity of potassium because of its vaporization.27 3.3. Co-gasification Properties at High Temperature. Finally, the raw samples were gasified with carbon dioxide using the PDTF. These experiments simulated the behavior of the fuel, which was pyrolyzed and gasified with the combustion gas in the reductor of the air-blown two-stage entrained flow coal gasifier. The origin of the residence time at the gasification tests was defined as the tip of the water-cooled feeding tube, because the primary pyrolysis of the sample fed into the furnace should start immediately. Figure 13 shows the (26) Zhang, Y.; Hara, S.; Kajitani, S.; Ashizawa, M. Fuel 2009, doi: 10.1016/j.fuel.2009.06.004. (27) Lizzio, A. A.; Radovic, L. R. Ind. Eng. Chem. Res. 1991, 30, 1735.

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and the analyzed gasification rate of char seemed to be appropriate for the prediction of the carbon conversion. As a whole, it can be said that distinguished synergy between coal and cedar bark to improve the gasification reactivity was not observed through the high-temperature co-gasification. However, an improvement of the gasification reactivity of char and soot formed through rapid pyrolysis was observed at lowtemperature gasification, owing to the inherent AAEM species in cedar bark. This result suggests that the gasification reactivity might be able to be improved at low temperature through the co-gasification technology.

4. Conclusions Two bituminous coals, cedar bark, and the mixture samples of coal and cedar bark were pyrolyzed in nitrogen gas or gasified with carbon dioxide at high temperature. As a result of the rapid co-pyrolysis tests, the product yields at high temperature of 1400 °C agreed with the equilibrium yields and the distinct synergy between coal and cedar bark could not be observed. The gasification reactivity of the char formed through rapid pyrolysis was discussed next. While the reactivity of the mixture sample became higher than that of single coal at 1200 °C or lower temperature, their reactivity became almost the same at a high temperature of 1400 °C. Finally, the carbon conversion during the high-temperature co-gasification of the raw samples with carbon dioxide was discussed,

Acknowledgment. The authors gratefully acknowledge the Ministry of Economy, Trade, and Industry of Japan (METI) who supported a part of this work financially.

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