Experimental Investigation of Natural Coke Steam Gasification in a

Jan 5, 2009 - Natural coke was selected as a steam gasification feedstock in a bench-scale fluidized bed (i.d. 50 mm, H 1600 mm). The impacts of the ...
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Energy & Fuels 2009, 23, 805–810

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Experimental Investigation of Natural Coke Steam Gasification in a Bench-Scale Fluidized Bed: Influences of Temperature and Oxygen Flow Rate Wen-guo Xiang* and Chang-sui Zhao School of Energy and EnVironment, Southeast UniVersity, Nanjing 210096, Jiangsu, China

Ke-liang Pang National Power Plant Combustion Engineering Research Center, Shenyang, Liaoning 110034, China ReceiVed August 29, 2008. ReVised Manuscript ReceiVed NoVember 17, 2008

Natural coke was selected as a steam gasification feedstock in a bench-scale fluidized bed (i.d. 50 mm, H 1600 mm). The impacts of the gasification temperature and the oxygen flow rate on product gas composition, product gas yield, and gas heating value were investigated with a steam flow rate of 1050 g/h and a natural coke flow rate of 200 g/h. Experimental results show that gasification temperature has a significant influence on gasification properties. As the temperature goes up, the contents of H2 and CO2 in the product gas decline, whereas the content of CO increases. As the temperature was from 850 to 1000 °C, the volume fractions of H2, CO, and CO2 in the product gas changed from 63.0%, 25.0%, and 9.6% to 59.8%, 20.2%, and 18.5%, respectively. The product gas yield increased by 4.3 times, the carbon conversion rate increased from 10.25% to 47.76%, and the gas heating value increased from 8.87 to 9.33 MJ/m3. Oxygen in the oxidant reagent steam affects the gasification properties. The product gas yield and the carbon conversion increased by 1.76 times and 1.94 times, respectively, when the oxygen flow rate was from 0 to 0.2 L/min. But as it further increased to 1.0 L/min, the product gas yield increased only by 1.16 times and the carbon conversion increased by 1.34 times. The effective composition in the product gas continued to decrease from 76.0% to 54.3%, and the heating value was lowered from 9.01 to 6.35 MJ/m3 as the oxygen flow rate was raised from 0 to 1 L/min. The CO2 composition increased persistently from 23.1% to 37.3%.

1. Introduction Natural coke, a high metamorphic grade coal as a byproduct of the coal mining process, which looks like artificial coke, is a kind of solid mineral fuel from the coal seam subjected to magma intrusion, fast pyrolysis, and carbonization with a heating value of 18-30 MJ/kg.1-3 Natural coke existing in the coal seam in the vertical and horizontal direction is generally discarded in the gob area of coal mines, which is improvident and causes pollution. Natural coke reserves are rich in China; for example, the remaining geological reserves are 3 billion tons in Shandong Province, and the total recoverable natural coke reserves are about 1 billion tons in Huaibei, Anhui Province.4,5 It is helpful for solving energy shortage problems to use natural * To whom correspondence should be addressed. Telephone: 86-2583795545. E-mail: [email protected]. (1) Sanyal, S. P. Petrology of natural coke associated with igneous intrusives in parts of the RANIGANJ coalfield. Mem.sGeol. SurV. India 1984, 117, 111–118. (2) Khorasani, G. K.; Murchison, D. G.; Raymond, A. C. Molecular disordering in natural cokes approaching dyke and still contacts. Fuel 1990, 69, 1037–1046. (3) Kwiecinska, B. K.; Hamburg, G.; Vleeskens, J. M. Formation temperatures of natural coke in the lower Silesian coal basin, Poland. Evidence from pyrite and clays by SEM-EDX. J. Coal Geol. 1992, 21, 217–235. (4) Wang, B. Y.; Wang, Y. H.; Ning, Y. H. The feasibility study on mine and power generation about mine field and natural coke of Yuncheng. Land Resour. Shandong ProVince (in Chinese) 2006, 22, 53–55. (5) Duan, B. Q. Process Study and application of natural coke as synthetic ammonia gasification materials. Nitrogen Fert. Technol. (in Chinese) 2000, 21, 24–29.

coke to generate electricity. However, natural coke was restricted in application and research due to its hot burst, difficult ignition, and abradability. In most cases, natural coke is discarded in nature or is unexploited in the mine. For these reasons, the author and co-workers6-9 chose to use the natural coke as a gasification material in the bench-scale fluidized bed as well as to investigate the apparent morphology, microcrystalline structure, and pyrolysis process by SEM (scanning electron microscopy), XRD (X-ray diffraction), and TG-FTIR (thermogravimetry Fourier transform infrared spectrometry). The hot burst factor of natural coke has no impact on the gasification process due to the dramatic fluidization state of bed material in a fluidized bed. At the same time, natural coke with high abradability is suitable for gasification in the fluidized bed because the fluidized bed has wider adaptability to coal type together with the characteristic of directly utilizing the coal of 10 mm below from coal production. Natural coke will be rapidly (6) Pang, K. L.; Xiang, W. G.; Zhao, C. S. Pyrolysis characteristics of Peicheng natural coke. J. Chem. Ind. Eng. (China) (in Chinese) 2007, 58, 994–1000. (7) Pang, K. L.; Xiang, W. G.; Zhao, C. S. Investigation on pyrolysis and kinetics of natural coke. J. Southeast UniV. (in Chinese) 2006, 36, 751– 754. (8) Pang, K. L.; Zhao, C. S.; Lin, L. S.; Xiang, W. G. XRD and gasification characteristic of natural coke. J. Fuel Chem. Technol. (in Chinese) 2007, 36, 268–272. (9) Pang, K. L.; Xiang, W. G.; Zhao, C. S. Investigation on pyrolysis characteristic of natural coke using thermogravimetric and Fourier-transform infrared. J. Anal. Appl. Pyrolysis 2007, 80, 77–84.

10.1021/ef800724t CCC: $40.75  2009 American Chemical Society Published on Web 01/05/2009

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heated to gasification temperature by higher bed temperature.10,11 Fluidized beds now find wide application in coal or biomass gasification. However, due to their high degree of solids mixing as well as particle entrainment, a single fluidized bed cannot achieve high carbon conversion. Ocampo et al.12 carried out experiments to study coal char gasification characteristics at different steam/coal and air/coal ratios and temperatures of gasifying agent in a fluidized bed. Bayarsaikhan et al.13 studied steam gasification of a Victorian brown coal in an atmospheric bubbling fluidized bed reactor with continuous feeding of the coal. In the study of Chatterjee14 gasification of a high-ash India coal in a laboratory-scale, atmospheric fluidized bed gasifier using steam and air as fluidizing media was studied. The variation of product gas composition, bed temperature, calorific value, and carbon conversion with oxygen and steam feed was discussed, and the experimental data were compared with the predicted. Huang et al.15 studied the gasification characteristics for a sub-bituminous coal with an air-steam mixture in a pressurized fluidized bed gasifier. Australian coal was gasified at atmospheric pressure in an internally circulating fluidized bed with a draft tube by Kim et al.16 The effects of reaction temperature, oxygen/coal mass ratio, coal feed rate, and steam/ coal mass ratio on composition of product gas, carbon conversion, cold gas efficiency and gas yield, and calorific value were determined. Similar work was done also by Kikuchi et al.,17 Gutierrez and Watkinson,18 and Crnomarkovic et al.19 And more, Cousins et al.20 investigated the reactivity of chars formed in fluidized bed gasifiers and found the char reactivity declines rapidly during its formation as part of the pyrolysis of the coal. The effect of pyrolysis time on char reactivity in a fluidized bed was discussed by Liu et al.,21 and they found that a longer pyrolysis time led to lower reactivity of a char, while this effect leveled off as pyrolysis time increased. Guo et al.22 investigated the changes in char structure during the gasification of brown (10) Ma, S. X.; Yang, X. Y. Study on dynamic behavior of the combustion system of a circulating fluidized bed boiler. Proc. CSEE (in Chinese) 2006, 26, 1–16. (11) Jin, Y.; Zhu, J. X.; Wang, Z. W.; Yu, Z. Q. Fluidization Engineering Principles; Tsinghua University Press: Beijing, 2001 (in Chinese). (12) Ocampo, A.; Arenas, E.; Chejne, F.; Espinela, J.; Londonˆo, C.; Aguirrea, J.; Perez, J. D. An experimental study on gasification of Colombian coal in fluidized bed. Fuel 2003, 82, 161–164. (13) Bayarsaikhan, B.; Sonoyama, N.; Hosokai, S. Inhibition of steam gasification of char by volatiles in a fluidized bed under continuous feeding of a brown coal. Fuel 2006, 85, 340–349. (14) Chatterjee, P. K.; Datta, A. B.; Kundu, K. M. Fluidized bed gasification of coal. Can. J. Chem. Eng. 1995, 73, 204–210. (15) Huang, J.; Fang, Y.; Chen, H.; Wang, Y. Coal gasification characteristic in a pressurized fluidized bed. Energy Fuels 2003, 17, 1474– 1479. (16) Kim, Y. J.; Lee, J. M.; Kim, S. D. Coal gasification characteristics in an internally circulating fluidized bed with draught tube. Fuel 1997, 76, 1067–1073. (17) Kikuchi, K.; Suzuki, A.; Mochizuki, T.; Endo, S.; Imai, E. Ashagglomerating gasification of coal in a spouted bed reactor. Fuel 1985, 64, 368–372. (18) Gutierrez, L. A.; Watkinson, P. A. Fluidized-bed gasification of some Western Canadian coals. Fuel 1982, 61, 133–138. (19) Crnomarkovic, N.; Repic, B.; Mladenovic, R.; Neskovic, O.; Veljkovic, M. Experimental investigation of role of steam in entrained flow coal gasification. Fuel 2007, 86, 194–202. (20) Cousins, A.; Paterson, N.; Dugwell, D. R.; Kandiyoti, R. An investigation of the reactivity of chars formed in fluidized bed gasifiers: The effect of reaction conditions and particle size on coal char reactivity. Energy Fuels 2006, 20, 2489–2497. (21) Liu, H.; Kaneko, M.; Luo, C. H.; Kato, S.; Kojima, T. Effect of pyrolysis time on the gasification reactivity of char with CO2 at elevated temperatures. Fuel 2004, 83, 1055–1061. (22) Guo, X.; Tay, H. L.; Zhang, S.; Li, C. Z. Changes in char structure during the gasification of a Victorian brown coal in steam and oxygen at 800 °C. Energy Fuels 2008, 22, 4034–4038.

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coal in a fluidized bed/fixed bed reactor. Li et al.23 performed biomass gasification tests in a pilot-scale air-blown circulating fluidized bed gasifier. Encinar and co-workers24,25 carried out isothermal experiments on steam gasification of Cynara cardunculus between 300 and 800 °C in a fixed bed and found that particle size, nitrogen flow rate, and initial sample weight generally did not exert any influence, whereas temperature was very significant. van Dyk et al.26 studied the effect of temperature and gasification medium with the aim of optimizing cogasification of coal and wastes for Sasol-Lurgi gasifiers. Pohorely et al.27 investigated cogasification of 23 wt % PET and 77 wt % brown coal in an atmospheric fluidized bed gasifier of laboratory scale. The gasification agent was composed of 10 vol % O2 in bulk of nitrogen. The influences of experimental conditions, such as the fluidized bed and freeboard temperatures, on major and minor gas components and tar content, as well as features of the blended fuel gasification in comparison with the single coal gasification, were studied. The study above investigated mostly the gasification of coal char or biomass char. The properties of natural coke are different from those of coal or biomass as discussed in papers.6-9 They should be characterized in natural coke gasification. In this study natural coke from Peicheng coal mine (Xuzhou, Jiangsu, China) was selected as the gasification feedstock, and steam is used as the oxidant reagent. The author investigated the characteristics of natural coke steam gasification in a bench-scale fluidized bed gasifier. The gasifier was operated at an atmospheric pressure to determine the effects of temperatures (850, 900, 950, and 1000 °C) and oxygen flow rates (0-1.0 L/min) in the oxidant reagent steam on the variation of the product gas composition, the product gas yield, the gas heating value, and the carbon conversion. 2. Experimental Section 2.1. Setup. The experimental setup is shown in Figure 1. The setup consists of a fluidized bed reactor (i.d. 50 mm, H 1600 mm), a steam generator, a two-stage steam superheater, natural coke feeding system, gas purification, data acquisition system, and a temperature control section. The preheated steam enters the reactor in the lower conically shaped part of the reactor, and natural coke is blown to the reactor in the lower part of the reactor through nitrogen. Temperature sensors and differential pressure indicators were located along the gasifier reactor. Additional temperature sensors are installed to measure the temperature in the center of the fluidized bed, and the reactor is heated by electrical heaters and is maintained at a given temperature through the control system. Gases from the reactor pass through a small cyclone to separate small particles. The amount of particles collected in the cyclone is measured after each experiment. Then the gas is lead to a water cooler where the steam vapors are condensed. A demister is installed in case the steam vapors are not fully condensed in the cooler. Eventually the product gas is measured by a flow meter and is analyzed by a gas analysis unit (including CO, CO2, CH4, and H2). 2.2. Sample Preparation. Natural coke from Peicheng coal mine (Xuzhou, Jiangsu, China) was used as the sample in this study. (23) Li, X. T.; Grace, J. R.; Lima, C. J. Biomass gasification in a circulating fluidized bed. Biomass Bioenergy 2004, 26, 171–193. (24) Encinar, J. M.; Gonza´lez, J. F.; Gonza´lez, J. Fixed-bed pyrolysis of Cynara cardunculus L. Product yields and compositions. Fuel Process. Technol. 2000, 68, 209–222. (25) Encinar, J. M.; Gonza´lez, J. F.; Gonza´lez, J. Steam gasification of Cynara cardunculus L.: influence of variables. Fuel Process. Technol. 2002, 75, 27–43. (26) van Dyk, J. C.; Keyser, M. J.; Coertzen, M. Syngas production from South African coal sources using Sasol-Lurgi gasifiers. Int. J. Coal Geol. 2006, 65, 243–253. (27) Pohor˘ey´, M.; Vosecky´, M.; Hejdova´, P.; Punc˘ocha´rˇ, M.; Skoblja, S.; Staf, M.; Vos˘ta, J.; Koutsky´, B.; Svoboda, K. Gasification of coal and PET in fluidized bed reactor. Fuel 2006, 85, 2458–2468.

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Figure 3. Gas compositions vs operation temperature.

Figure 1. Experimental setup of fluidized bed reactor: (1) water tank; (2) steam boiler; (3 and 4) steam superheater; (5 and 6) electric heater; (7) slag cans; (8) gasifier body; (9) electric heater; (10) coke feeder; (11) cyclone; (12) ash cans; (13) gas cooler; (14) dry tower; (15) flowmeter; (16) gas analyzer. Table 1. Proximate and Ultimate Analyses of the Natural Coke proximate analysis/% (mass, air-dry)

ultimate analysis/% (mass, daf)

M

A

V

C

0.81

16.15

9.05

fixed 73.99

carbon

93.12

H

O

N

S

1.99

3.21

1.10 0.58

The proximate and ultimate analysis results of the sample are shown in Table 1, which reveals that the natural coke has higher carbon content and less volatile matter with a higher heating value. The quartz sand with an average particle size of 0.23 mm and density of 2400 kg/m3 was used as the bed material of the fluidized bed gasifier. According to the Geldart classification, the sample particle belongs to the B-type particle with a particle size of 100-600 µm and a density of 1400-4000 kg/m3. The fluidization velocity is set to 1.26 m/s, and the fluidization numbers is 2.5. Steam was chosen as the fluidizing medium with a flow rate of 1050 g/h, and the natural coke flow rate was set to 200 g/h at ambient conditions. 2.3. Experiment Schedule. Two sets of experiments, including influences of gasification temperatures and influences of oxygen flow rates, were carried out to explore the steam gasification characteristics of the natural coke, which included product gas compositions, gas heating value, product gas yield, and carbon conversion. Experiments of the natural coke steam gasification under four different temperatures, 850, 900, 950, and 1000 °C were performed. The influences of oxygen flow rate on the gasification properties were performed. The oxygen, mixing with steam as the oxidant reagent, was induced to the fluidized bed in the lower conically shaped part of the gasifier reactor. Six different oxygen flow rates, 0.1, 0.2, 0.4, 0.6, 0.8, and 1.0 L/min at a temperature of 900 °C were included in the experiments. 2.4. Reaction Time. Figure 2 shows the trend of product gas composition versus the reaction time under an experiment condition

(1000 °C, natural coke flow of 200 g/h, steam flow of 1050 g/h, oxygen flow of 0.1 L/min). As can be seen, there was fluctuation in the product gas composition for the first minutes. As the gasification reactions go on, the fluctuation was kept in a narrow range, so the reaction time was kept for 30 min in the experiments.

3. Results and Discussion 3.1. Gasification Characteristics under Different Temperatures. Steam was the fluidization media and the oxidant reagent for the experiments. The reaction temperature is one of the most important operating variables affecting the performance of the fluidized bed natural coke gasifier since the main gasification reactions (C + H2O f CO + H2 and C + CO2 f 2CO) are endothermic. In this section, the influences of gasification temperature on the product gas compositions, heating value, metric volume of product gas, and carbon conversion of natural coke are discussed. Four temperatures, 850, 900, 950, and 1000 °C, were selected to perform the experiments. 3.1.1. Effects of Gasification Temperature on Gas Compositions. The gasification temperature plays an important role in natural coke steam gasification. The effects of the gasification temperature on the compositions of the product gas are shown in Figure 3. As can be seen, compositions of the product gas are H2 (63.0-59.8%), CH4 (2.4-1.5%), CO (9.6-18.5%), and CO2 (25.0-20.2%). The effective gas (H2 + CO + CH4) has slightly increased with the increase of temperature at a constant value of steam/carbon ratio. In this research, the contents of H2 and CH4 decreased by 3.2% and 0.9% from 63.0% and 2.4% to 59.8% and 1.5%, respectively, whereas the volume percentage of CO increased by 8.9% from 9.6% to 18.5% for an increase in gasification temperature from 850 to 1000 °C. The volume percentage of CO2 decreased from 25.0% at 850 °C to 20.2% at 1000 °C. In the steam gasification of a carbonaceous material, several reactions take place simultaneously, reactions 1-5. The extent to which each reaction is involved depends on the operating conditions. Reaction 2 requires high temperature, and reactions 4 and 5 require high pressure. Hence, reactions 1 and 3 are the main reactions of interest in steam gasification when the reaction is carried out at atmospheric pressure and temperatures above 850 °C. C + H2O f H2 + CO C + CO2 f 2CO CO + H2O f CO2 + H2 2CO + 2H2 f CH4 + CO2 C + 2H2 f CH4

Figure 2. Product gas composition vs time.

∆H ) + 130.1 kJ/mol ∆H ) + 170.7 kJ/mol ∆H ) -40.2 kJ/mol

(1) (2) (3)

∆H ) -247.5 kJ/mol (4) ∆H ) -74.9 kJ/mol

(5)

Reaction 1 is endothermic, ∆H ) 130.1 kJ/mol, and reaction 3 is exothermic, ∆H ) -40.2 kJ/mol. Therefore, an increase or

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Figure 4. Gas production vs gasification temperature.

decrease of the temperature displaces reactions 1 and 2 in opposite directions. Consequently, the temperature is an important variable with regard to the final composition of the gases. The thermodynamic expectations are in good accordance with the above experimental results. Reactions 1 and 2 are heterogeneous and strongly endothermic, and thus the rise in temperature is conducive to a shift of equilibrium toward the right side. Reactions 1 and 2 are accompanied by other exothermic reaction 3, which contributes to the formation and depletion of the above-discussed components H2 and CO2. Methane is of a higher heating value and can contribute a lot for gas heating value. However, it can be found that CH4 content, unlike the previously discussed trends for CO, CO2, and H2, exhibits only weak temperature dependence, changing the value from 2.4% at 850 °C to 1.5% at 1000 °C, as shown in Figure 3. The methanation reactions 4 and 5 are exothermic and need higher pressure. In general, the rates of carbon oxidation by steam and carbon dioxide are of the same order of magnitude, whereas the hydrogenation reaction (i.e., reaction 5) is several orders of magnitude slower than the steam-char and CO2-char reactions.28 Huang et al.15 reported that the equilibrium concentration of CH4 should be less than 0.2% at a temperature of 930 °C in their PFB gasifier experiments. In the above experiments, a pyrolysis process and another gasification that produces activated carbons and energy-rich gaseous products take place simultaneously. According to the thermogrametric analysis by Pang et al.,9 the pyrolysis process of the natural coke is different from that of coal with three stages of pyrolysis. Its pyrolysis process can only be divided into two degasification stages. The first stage (room temperature to ∼550 °C) is a drying degasification stage, and its decomposition products are similar to that of coal pyrolysis, of which the oxygenic aroma compounds are in the majority. The second stage (550-1000 °C) is a fast degasification stage, the main stage, and its weigh loss is more than 70% of the total loss. The decomposition gases are mainly CO2 and CO, and less amass double bond and treble bond, C(CH3)3 and -CH(CH3)2 series matter, which may form the light hydrocarbon. That implies that the CH4 would be mainly formed from the volatile cracking, and the higher the temperature is, the less CH4 is released, as reported in the literatures.15,16,25 3.1.2. Effect of Gasification Temperature on the Gas Production Yield. The gasification temperatures of Peicheng natural coke with steam were set to 850, 900, 950, and 1000 °C. The changes of the gas production with the temperature are shown in Figure 4. The gas production increases rapidly with the increase of the gasification temperature, which shows that gasification temperature is the main factor affecting (28) Smoot, L. D.; Smith, P. J. Coal Combustion and Gasification; Plenum Press: New York and London, 1985.

Figure 5. Gas heating value and effective composition content vs gasification temperature.

gasification reaction of Peicheng natural coke. The gas production increased by 56 L/h from 80 L/h at 850 °C to 136 L/h at 900 °C, by 90 L/h from 900 to 950 °C, and 117 L/h from 950 to 1000 °C. The gas production increased by 4.3 times from 80 L/h at 850 °C to 343 L/h at 1000 °C. With increasing temperature there is a major increase in the yield of gases. This increase is due first to the production of gases in the initial period of pyrolysis (increasing with temperature), and second to the cracking of tars which would also be favored by temperature, and third to the increase in reaction rate with temperature. The yield of gases increases sharply about 900 °C. This effect has also been observed by other workers24,25 and seems to be due to the existence of a first period dominated by cracking of the volatile matter and a second period dominated by reactions 1 and 2. In consonance with this, the conversion also increases with temperature. The changes of the product gas components with gasification temperature are also shown in Figure 4. With the increase of gasification temperature, the components of product gas increase along with the increase of the product gas yield. The H2 production increased significantly from 50.4 to 205.1 L/h because of the increase of the total gas production with the increase of gasification temperature, whereas the percent of H2 changed little (as shown in Figure 3) with the increase of the total gas production. 3.1.3. Effect of Gasification Temperature on the Gas Heating Value. From Figure 3, it can be seen that H2 reduces slowly and the CH4 slightly declines, but the content of CO increases gradually with the increase of the gasification temperature. The increase of CO is large enough to offset the decline of H2 and CH4 in composition so that the effective content increases in the gas. CO2 content decreases accordingly. The specific changes are shown in Figure 5, where the heating value of the gas also increases with the increase of gasification temperature resulting from the increase of the effective component production. The gas heating value increased by 5% from 8.87 MJ/m3 at 850 °C to 9.33 MJ/m3 at 1000 °C, while gas heating value per hour increased by 4.5 times from 0.71 to 3.20 MJ, because of the dramatic increase of gas production with the increase of gasification temperature. 3.1.4. Effect of the Gasification Temperature on the Carbon ConVersion. According to the gas production, we estimated that natural coke did not completely react with steam in the fluidized bed. Here carbon conversion rate was used to describe the degree of gasification reaction. Carbon conversion rate was calculated based on total inlet natural coke and outlet product gas data at a given instant, and the gas composition was determined by the gas chromatography at the same instant. Carbon conversion rates of the samples were 10.25%, 17.90%, 30.13%, 47.76% at the gasification temperatures of 850, 900,

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C + 0.5O2 f CO CO + 0.5O2 f CO2 H2 + 0.5O2 f H2O(g)

Figure 6. Carbon conversion rate vs gasification temperature.

Figure 7. Influences of the oxygen flow rate on gas production and carbon conversion.

950, and 1000 °C (as shown in Figure 6), which indicates the gasification temperature is the key factor impacting the carbon conversion. In the experiments, gasification time was kept for 30 min and was relatively short. Because of low gasification reactivity of char, char would be accumulated in the bed during gasification, so the carbon conversion would have been higher if the reaction time was longer. 3.2. Influences of Oxygen Flow on Gasification Characteristics. In the experiments, the natural coke flow and steam flow were set 200 and 1050 g/h, respectively, and the gasification temperature was kept at 900 °C. The oxygen flow was set to zero as in the former experiments and then increased from 0.1 to 1 L/min in the following experiments. 3.2.1. Influences on Gas Production and Carbon ConVersion. The gas production and carbon conversion increase with the rise of the oxygen flow, as shown in Figure 7. The gas production increases quickly with the oxygen flow rate when less than 0.2 L/min, and then increases slowly. When the oxygen flow increased from 0 to 0.2 L/min, the gas production increased 1.8 times from 136 to 239 L/h. And as the oxygen flow increased from 0.2 to 1.0 L/min, the gas production increased only by 1.2 times from 239 to 278 L/h. The carbon conversion rate changes in line with the gas production, which increases quickly when the oxygen flow rate is less than 0.2 L/min, and then increases slowly with the increase of oxygen flow rate. Because of the oxygen in the oxidant reagent, reactions 6-9 occur during the natural coke steam gasification. The reactions are exothermic. The reaction of carbon does not stop at CO, but any free oxygen rapidly reacts with CO in the gas phase to produce CO2. For the fuel-rich fluidized bed gasifier, the much slower endothermic reaction 2 may occur. Because the heat released from the reaction between char and oxygen, reaction 1 is strengthened resulting in the increase of carbon conversion. Because of reactions 8 and 9, CO and H2 are consumed by the existence of oxygen. C + O2 f CO2

∆H ) -393.8 kJ/mol

(6)

∆H ) -283.3 kJ/mol ∆H ) -110.5 kJ/mol ∆H ) -242.0 kJ/mol

(7) (8) (9)

The initial increase of the oxygen flow rate in the steam can strengthen the diffusion of oxygen to the surface of solid carbon, and more oxygen molecules are absorbed in the carbon surfaces, so on the reaction interface natural coke particles (char) contact with oxygen more easily and fully, and char is consumed rapidly with increase of oxygen flow. However, when the oxygen flow rate reaches a certain value, i.e., 0.2 mL/min in the experiments, the reaction surfaces of carbon particles reach a saturated state. Bulk surface diffusion reaction is converted to the pore diffusion reaction or chemical reaction. As the oxygen flow rate increases further, the chemical reaction is rate-determining.28 The increase altitude of reaction rate goes lower, so the influences of oxygen diffusion to the carbon particle surfaces on the gasification reaction are almost weak and the carbon conversion tends to change a little. 3.2.2. Influences on the Gas Composition. With the change of the oxygen flow in the oxidant reagent, gas composition and gas components change, as shown in Figures 8 and 9. Methane shows little correspondence with the oxygen flow because of its relatively lower content. Effective gas compositions (CH4, H2, and CO) are reduced, whereas CO2 content increases because of the reactions 8 and 9. Oxygen was measured unreacted in the outlet gas when oxygen flow was further increased because of the short resident time in the fluidized bed gasifier and quick cooling in the outlet. As described in the former section, the carbon conversion gets increased when oxygen is added to steam as an oxidant reagent. When oxygen was raised from 0 to 0.2 L/min at first, the carbon conversion increased rapidly by 1.94 times and more product gases were generated also. Although the addition of oxygen in the oxidant

Figure 8. Influences of oxygen flow rate on gas compositions.

Figure 9. Influences of oxygen flow rate on gas component production.

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from 0 to 0.2 L/min, the gas heating value per hour increases 1.6 times from 1.23 to 1.96 MJ because of the rapid rise in the carbon conversion rate. The heating value per hour reaches the highest at the oxygen flow rate of 0.2 L/min. 4. Conclusions

Figure 10. Influences of oxygen flow rate on gas heating value and effective components.

steam consumes the combustible gases CO and H2, the CO and H2 yields in the product gas are increased because more effective product gases are produced. When oxygen was raised further from 0.2 to 1.0 L/min, the carbon conversion increased slowly only by 1.34 times and the product gases generated slowed down, and the CO and H2 yields tended to decrease. At the oxygen flow rate of 0.2 L/min, CO and H2 productions reached the peak. 3.2.3. Influences on the Gas Heating Value. Because of reactions 8 and 9, CO2 in the product gas goes up, and effective gases CO and H2 go down, whereas CH4 shows little change. Thus, the gas heating value of the resulting gas gets decreased as the decline of the effective gas contents, as shown in Figure 10. And the gas heating value per hour is also given in the figure. From Figure 10, one observes that the gas heating per hour increases at the oxygen flow rate of 0-0.2 L/min because of the rise of CO and H2 production, and then it declines as the decrease of CO and H2 production when the oxygen flow rate is from 0.2 to 1.0 L/min. When the oxygen flow rate increases

Experiment results of Peicheng natural coke steam gasification show that the gasification temperature and oxygen rate are the major factors affecting the gasification characteristics. With the increase of gasification temperature, the contents of H2 and CO2 in the production gas decline, whereas the content of CO increases. As the gasification temperature rose from 850 to 1000 °C, the volume fractions of H2, CO, and CO2 in the production gas changed from 63.0%, 25.0%, and 9.6% to 59.8%, 20.2%, and 18.5%, respectively. Production gas increased by 4.3 times, carbon conversion rate got increased from 10.25% to 47.76%, and the gas heating value increased from 8.87 to 9.33 MJ/m3. Oxygen in the oxidant reagent steam affects the gasification properties prominently. Product gas yield increased by 1.76 times, and carbon conversion rate increased by 1.94 times, as the oxygen flow rate increased from 0 to 0.2 L/min. But as it further increased to 1.0 L/min, the product gas yield increased only by 1.16 times and carbon conversion rate increased by 1.34 times. Effective composition in the product gas continued to decrease from 76.0% to 54.3%, and the heating value was lowered from 9.01 to 6.35 MJ/m3, as the oxygen flow rate was raised from 0 to 1 L/min. The CO2 composition increased persistently from 23.1% to 37.3%. Acknowledgment. The authors express thanks to the National Natural Science Foundation of China (90410009, 50776018) and the Special Fund of the National Priority Basic Research of China (2007CB210101) for providing financial support of this project. EF800724T