Steam Gasification of Low Rank Fuel – Biomass, Coal, and Sludge Mixture in A Small Scale Fluidized Bed K.H. Ji1, B.H. Song*1, Y.J. Kim1, B.S. Kim1, W. Yang2, Y.T. Choi2, S.D. Kim3 1
Department of Chemical Engineering, Kunsan National university, Gunsan 573-701, Korea Manufacturing System Division, Korean Institute of Industrial Technology, Cheonan, 330-825, Korea 3 Department of Chemical and Biomolecular Engineering, KAIST, Daejeon 305-701, Korea
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Abstract The steam gasification of low rank fuels has been carried out in a fluidized bed of 0.02 m i.d. and 0.6 m height to study their product gas composition as an additional gaseous fuel for boiler application. The highest content of hydrogen and carbon monoxide were observed at 900 C and at partial pressure of steam of 0.95 atm within the studied range. The LHV of product gas were between 6.1 and 9.2 MJ/m3 with steam pressure of 85%. And LHV of 13.4 MJ/m3 (sludge mixture), 10.1 MJ/m3(woodchip), and 6.9 MJ/m3 (lignite) were obtained at 900’C and 95%H2O condition. In this study, specially made sludge mixture shows a great potential as fuel for syngas production. Introduction Although biomass has been widely used to obtain heat by direct combustion, it represents a rather low energy-density source in modern, highly efficient heat and power production systems. It has been found convenient, however, to convert it into a gaseous fuel by means of gasification. As a result of rather complex thermo-chemical processes [1], the biomass is thus transformed into permanent gases, such as hydrogen carbon monoxide, carbon dioxide and methane, together with organic vapours which condense under ambient conditions and are known collectively as tar, and a solid residue consisting of char and ash. Sewage sludge produced in large quantities has carbon content of around 50%[2], thus it can be a new resource for gasification processes. The sludge from waste water treatment plant contains water content of above 80% and therefore they need to be dried before its use as a fuel. Furthermore a bad smell is inevitable during the above processing. One way to use wet sewage sludge without the above problems is to use sludge as a component for making a fuel mixture, e.g. sludge-oil-coal agglomerate (SOCA)[3], the manufacture of SOCA does not need drying process and no smell released. The coal, sludge, and also waste oil can be converted together to valuable fuel gas or oil through pyrolysis and gasification. In addition, the unnecessary inorganic components in sludge can be separated through the gasification. The heating value of SOCA is around 7,500 kcal/kg. Both devolatilization and gasification of carbonaceous material are occurred in gasification processes. Devolatilization is very fast process and thus gasification is limiting process. Main reaction of gasification is C + H2O = CO + H2 ΔH = +131.4 kJ/mol (1) Some of carbon can be used to generate heat by combustion, which releases CO, CO2. Also, water-gas ------------------------------------*Corresponding author:
[email protected] Proceedings of the European Combustion Meeting 2009
shift reaction may occur to produce more hydrogen. It is well known that mineral metters in the ash may act as catalyst for gasification process. The performance of gasifier depends on many variables like fuel type, reaction temperature, pressure, catalyst etc. In the present work, steam gasification of several low rank fuel has been carried out in a small scale fluidized bed to explore the effects of operating parameters on gasification performance and to evaluate their fuel performance for gasification process. Experimental study A schematic diagram of the lab-scale fluidized-bed gasifier used in the experiments appears in Fig. 1. The gasifier employs a riser of 20 mm i.d. and 600 mm height. With silica sand of average diameter of 0.3 mm as bed material, the activated SOCA obtained from KIER and woodchip were used as the feedstock. Various size ranges of feed were selected for tests. The proximate and ultimate analyses of the feedstock were shown in Table 1. Table 1. Ultimate analyzes and procimate analyzes of the fuel. wt% Carbon Hydrogen Nitrogen Sulfur Oxygen*
Activated SOCA 79.70 6.38 1.19 0.46 5.60
Moisture 8.74 Volatile 44.57 Fixed Carbon 40.02 Ash 6.67 HHV**, kcal/kg 7530 *by difference : 100 – (C+H+N+S) ** higher heating value of fuel
Woodchip 45.96 5.81 0.00 0.00 48.23 19.20 47.28 10.87 22.65 3638
and various steam partial pressure (15, 55, 85, 95%) were adapted for the gasification experiment. Low heating value of dry product gas can be determined as [4] : LHV = (30.0 × CO + 25.7 × H 2 + 85.4 × CH 4 + 151.3 ×
The test facility consists of the following components: the fluidized bed reactor, temperature control, fluidization gas, fuel delivery, data acquisition, and gas sampling section. The gasifier is a bubbing fluidized bed of sand, and nitrogen and steam mixture are gasifying agent. The minimum fluidization velocity for sand material at atmospheric pressure was measured to be 0.034 m/s, practically it was observed that about twice the minimum fluidization velocity of sand particles is needed to maintain good fluidization for sand material. Activated SOCA is fed inside the reactor at 0.22 kg/h. The feeding system is maintained at the top of the reactor. Some amount of nitrogen is diverted to the screw feeder to prevent feeding difficulty. The steam from steam generator and nitrogen enter the reactor through the distributor plate. The flow rate of water to steam generator was controlled by micro pump.
[
C n H m ) × 4.2 kJ/m 3
]
(2)
where CnHm is assumed to be zero, and CO, H2, CH4 are the gas concentrations of the product gas. Boudard reaction is as ΔH = +172 kJ/mol (3) C + CO2 = 2 CO Results and discussion Temperature is crucial for the overall gasification process. In the present study, reactor temperature was varied from 750 to 900C. The gasification results are presented in Fig. 2(a)-(d). And the calculated calorific values of the product gas with variation of steam partial pressure were shown in Table 2. From Fig. 2, it can be seen that CO and H2 concentration increased with increasing temperature, however, the dependence of the content of CH4 on gasification temperature are different at different partial pressure. According to Le Chatelier’s principle, higher temperatures favor the reactants in exothermic reactions and favor the products in endothermic reactions. Therefore the endothermic reactions (1) and (4) were strengthened with increasing temperature, partial pressure, which resulted in an increase of H2 concentration and a decrease of CH4 concentration. As shown in Fig. 2., CO content was higher than H2 content when the temperature is lower than 850 and steam partial pressure is 85%. Nevertheless, H2 content exceeded CO content under higher temperature in the present study. According to Equation (3), hydrogen content should decrease if LHV decrease. However, it is noticed that LHV value increase with temperature, LHV reduced a little at 850’C, and finally LHV increase again. It seems that the dependency of hydrogen content on LHV is small. Fig.3 shows the effect of temperature on the product gas composition from gasification of activated SOCA. the product gases of the gasification reaction in a fluidized bed reactor were composed of H2(12.127.8%), CO(16.9-30.0%), CO2(2.8-6.8%) and CH4(6.512.6%). The contents of H2 and CO tend to increase, but the contents of CH4, CO2 decrease with an increase in reaction temperature and in partial pressure of steam. High temperature enhances the rate of steam gasification and thus enhance the composition of H2 and CO. The content of CH4 increase with temperature because devolatilization is also enhanced, however CH4 tend to somewhat decrease because of the increase in the product gas flow. CO2 seems to decrease because of the effect of water-gas shift reaction [5]. Fig.4 shows the effect of steam partial pressure on gas composition in woodchip gasification at 900’C. The concentrations of all the gas components tend to increase with steam partial pressure. The carbon monoxide content is higher than others.
Fig. 1. Schematic diagram of a small fluidized bed reactor. 1: biomass feeder, 2: screw feeder, 3: quartz main reactor, 4: furnace, 5: cyclone, 6: tar condenser, 7: gas sample, 8: gas analyzer, 9: PC, 10: MFC, 11: air source, 12: nitrogen, 13: water, 14: water pump, 15: steam generator, 16. thermocouple, 17: celullose thimble filter, 18 : sample collector Experiment was carried at atmospheric pressure. Initially the fluidized bed reactor above the gas distributor was loaded with sand at a ratio of depth to radius of around 1:1. When the desired temperature that is about 900C, is reached, the biomass screw feeder was turned on to feed the biomass material inside the reactor. In general, it took 40 minutes to make the system stable. At a steady state condition, all experiment parameters were kept constant for about 30 minutes for gas sampling and analysis. After the fine particles were separated in the cyclone, the part of product gas flow was passed through a cooling water trap and a cotton thimble filter and housing filter for drying and cleaning. Then the dry and clean gas was sent to on-line gas analyzer (Model: ABB inc.) to detect H2, O2, CH4, CO and CO2. The lower heating value (LHV) of product gas was calculated by Equation (3). Four temperatures (750, 800, 850, 900℃)
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Table 2. LHV (kJ/m3) of produced gas from the gasification of SOCA and woodchip. PH2O (atm) 0.15 0.55 0.85 0.95
750C 2853 4598 6143 7632
800C 6315 8070 8417 11026
850C 4682 6931 9637 10652
900C 5160 7714 9204 13384
900C(woodchip) 3336 8224 9136 10064
Fig.2. Effect of temperature on the product gas composition from the gasification of activated SOCA (feed rate= 0.23kg/h; steam feed rate= 0.11kg/h).
Fig. 3. Effect of partial pressure of steam on the gas composition for the gasification of SOCA (feed rate= 0.22 kg/h, steam feed rate= 0.11 kg/h, S/C=0.5).
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It was found through the experiment that it is very important to supply steam to the reactor very stably. Otherwise not only the gas composition data will scatter a lot but also the state of fluidization will become poor. Thus the steam generator was made of 1/4 inch tube and the inside of tube was filled with inert powder, and after all very stable feed of steam could be obtained.
The effect of gasification temperature on the product gas yields at the gasification of SOCA was presented in Fig. 6. The yield of permanent gas increased with temperature up to 800℃ regardless of steam partial pressure. However, at the higher temperature range, the total gas yield exhibit somewhat different trend. At higher steam partial pressure, the total product gas yield increased at the all the temperatures used.
Fig. 4. Effect of temperature on gas composition in woodchip gasification (PH2O=0.15-0.95 atm, T=900’C) The effect of partial pressure of steam on the production of hydrogen and carbon monoxide is shown in Fig. 5 for SOCA and woodchip. It can be seen that the content of CO is relatively low and CO does not change much with steam pressure in case of SOCA gasification.
Fig.6. Effect of temperature on the product gas composition for the gasification of activated SOCA. Conclusions Gasification experiment with woodchip and sludge mixture provided the following results:. The product gases of the steam gasification in a fluidized bed reactor were composed of H2(12.1-27.8%), CO(16.9-30.0%), CO2(2.8-6.8%) and CH4(6.5-12.6%). Temperature and steam partial pressure were the most important factor in this process. High temperature favored hydrogen production and gas yield but did not always favor gas heating value. The obtained LHV of product gas were between 6,143 and 9,204 kJ/m3 with steam pressure of 85%. And LHV of SOCA(13,384 kJ/m3) and woodchip (10,063 kJ/m3) was obtained at 900’C and 95%H2O. It was found that the total flow rate of (N2+steam) should be carefully controlled in order to maintain good fluidization state with keeping a given steam partial pressure. SOCA fuel shows a great potential in syngas production. As reaction temperature and steam partial pressure increased, the heating value of the product gas increased. The calorific value of syngas produced from sludge mixture, woodchip, and lignite were found to be 13, 10, 6.9, and 5.7 MJ/m3, respectively. Acknowledgements This work has been supported by KESRI(R-2005-7072) and by New and Renewable Energy Technology Development Project (2005-N-WA02-P-02-3-010-2005) which are funded by MOCIE (Ministry of Commerce, Industry and Energy). Also supported by the Korea Research Foundation Grant (KRF-2007-041-D0019) funded by the Korean Government (MOEHRD)
Fig. 5. Hydrogen and carbon monoxide vs. partial pressure of steam at the gasification of SOCA and woodchip (T = 900C)
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