HIGH TEMPERATURE AIR AND STEAM GASIFICATION OF DENSIFIED BIOFUELS

Transcription

1 1 ST WORLD PELLETS NFERENCE SEPTEMBER 2-7, 2002, STOCKHOLM, SWEDEN HIGH TEMPERATURE AIR AND STEAM GASIFICATION OF DENSIFIED BIOFUELS C. Lucas*, D. Szewczyk, W. Blasiak Royal Institute of Technology (KTH) Division of Energy and Furnace Technology Brinellvägen 23, S Stockholm, Sweden Telephone: , Fax: ABSTRACT: Experimental study was carried out to investigate gasification of densified biofuels using highly preheated air and steam as a gasifying agent. Preheat of air and steam is realised by means of the newly developed high-cycle regenerative air/steam preheater. Use of highly preheated feed gas provides additional energy into the gasification process, what enhances the thermal decomposition of the gasified solids. For the same type of the feedstock the operating parameters, temperature, composition and amount of gasifying agent, were varied over a wide range. Results of experiments conducted in a high temperature air/steam fixed bed updraft gasifier show the capability of this technology of maximising the gaseous product yield as a result of the high heating rates involved, and the efficient tar reduction. Increase of the feed gas temperature reduces production of tars, soot and char residue as well as increases heating value of the fuel gas produced. Overall, it has been seen that the yield and the lower heating value of the dry fuel gas increase with increasing temperature. Keywords: High temperature gasification, Fixed bed, Updraft, Pellets 1 INTRODUCTION Research and Development carried out in Japan under leadership of Ryoichi Tanaka [1] on High Temperature Air Combustion (HiTAC) of gas and liquid fuels was followed by development of a new coal-fired power generation system [2]. Specific progress has been made in the areas of High Temperature Air Combustion [3] and the designing of highly efficient regenerative air and steam preheaters [4]. This progress has created the possibility to begin research on gasification processes with high temperature air or steam called High Temperature Air/Steam Gasification (HTAG). Gasification is carried out with air or steam or a mixture of these as a gasification agent [5,6]. Gasification produces a gaseous mixture of hydrogen, carbon monoxide, steam, methane, nitrogen and light hydrocarbons with other undesired effluents, such as organic aerosols, inorganic particulates, condensable organic vapours (tars), sodium, potassium and chlorine compounds, ammonia, and hydrocyanic acid. Particularly problematic is the behaviour of tars, whose content [7] in the gas varies much from one process to another, from about 1 to 180 g/nm 3. The energy content of the gas produced through gasification depends on numerous factors, such as the feed gas, reactor type, fuel type and form, etc [8]. Apart from fluidised bed [9] the fixed bed gasifiers have been tested [10,11,12]. Fixed-bed, counter-current (updraft), and concurrent (downdraft) gasifiers are, in general, of very simple construction and operation. They also present high carbon conversion, long solid residence times, and low ash carry-over. On one hand, the updraft process is more thermally efficient than the downdraft process but the tar content of the gas is very high [8]. Performance of the air gasification depends on initial air temperature supplied to the gasifier. In general we can distinguish gasification where cold or slightly preheated air is used or the new High Temperature Air Gasification (HTAG) process in which high temperature air or steam is used. Initial temperature of feed gas decides mainly about heating value of the fuel gas produced. Simply for higher the air temperature the higher heating value of the fuel gas can be obtained. The purpose of this work is to study the influence of the high temperature feed gas on gasification of densified biomass fuels in an updraft fixed bed gasifier. Trials were performed for the characterisation of tests for gasification of wood pellets on the updraft gasification with simultaneous measurements of temperature profiles and gas composition for different operating conditions. 2 EXPERIMENTAL SET-UP 2.1 Gasification System High Temperature Air/Steam Gasification (HTAG) test facility has been built at Royal Institute of Technology (KTH). HTAG test facility as described in detail elsewhere [13] consists of a counter-current (updraft) fixed bed gasifier, a combustion chamber, electrical preheater, highcycle regenerative air/steam preheater, air blowers, temperature and pressure sensors as well as data 1

2 acquisition equipments. Fuel gas produced in the gasifier is completely burnt on the combustion chamber. Electrical steam boiler is used to produce slightly preheated steam (180 o C, 2.5 bar). 2.2 Methodology of gasification tests Wood pellets 12 mm in diameter was the feedstock used for the present investigation. The wood pellets used had a composition (ultimate analysis dry weight basis) of C, 5%; H, 6.2%; O, 42.8%; S, 4%. Other properties (in dry weight basis) were ash, %; total moisture, 8.22; fixed C, 15.7%; volatiles, 83.9%; (), MJ/kg daf. Feeding of the feedstock was carried out manually from top of the gasifier. Experiments were conducted at different feed gas temperature. Gasification run started when preset temperature conditions of the feed gas were achieved and temperature of the gasifier stabilized. In both low and high temperature gasification experiments the preheated feed gas is injected into the gasifier coming in to the charge through bottom. The composition of the fuel gas was analysed using a micro gas chromatograph (micro GC) equipped with Solid State Detector SSD. The SSD is a micro machined version of the Thermal Conductivity Detector (TCD) [14]. This micro GC is configured with four different GC modules (A, B, C and D). Argon was the carrier gas used on column A operating at 115 o C. Due to the poor thermal conductivity of argon, helium is used as the carrier gas on columns B, C and D that were operating respectively at 70, 95 and 90 o C. Helium is used as carrier gas to offer the highest sensitivity for detecting low concentrations. The following gas components of the fuel gas were measured in this work: hydrogen (H 2 ), carbon monoxide (), carbon dioxide ( 2 ), nitrogen (N 2 ), oxygen (O 2 ), methane (CH 4 ), and higher hydrocarbons (, x>1). The composition of fuel gas was determined from gas samples taken at the gasifier outlet. All measurements were performed using a sampling frequency of one sample per 115 seconds. The run time of 115 seconds is built up in a sampling time of 15 seconds and an analysis time of 100 seconds. 3 RESULTS AND DISCUSSION 3.1 Gasification profiles Gas composition (normalized values) obtained by air gasification of wood pellets is presented in Figure 1 and 2. Temperature of the feed gas is increased from 350 o C in case 1 (C1), further increased for case 2 (C2) and finally higher temperature is applied in case 3 (C3). The composition is given in dry basis. As also shown in Figure 1 (C1) and Figure 2 (C3), this gas contained H 2,, 2, N 2, and CH 4 as the main constituents. Almost no amount of O 2 were detected whereas hydrocarbons such as C 2 H 4, C 2 H 6, C 3 H 6, and C 3 H 8 were detected in small amounts and grouped as (x>1) and called higher hydrocarbons. Normalized value of H 2,,,, and [-] Figure 1: Air gasification profile for Case 1 Normalized value of,,,, and [-] Figure 2: Air gasification profile for Case 3 The maximum increases and mass flow rate of fuel gas increases from 98 to 125 kg/h on increasing feed gas temperature. As is already reported by different authors, for instance [15], the gas yield increases and the char and the tar yield decrease with the rise in gasification temperature. Mass balances at the end of the run presented in our previous work [13] suggests that at high temperature gasification the steam would be reacted in the gasifier bed, or the steam conversion, increases with the gasifier temperature. Normalized mass flow of fuel gas [-] m (FG_D), C1 m (FG_D), C2 m (FG_D), C3 m (FeG_D), C1-3, C1, C2, C3 Figure 3: Variations of the mass flow rate and lower heating value () of dry fuel gas (FG_D) produced during gasification runs at various temperature of feed gas (air) Normalized value of [-] 2

3 Figure 3 shows the mass flow rate and lower heating value of the fuel gas produced in three different runs. For the same volume flow rate of air used as feed gas (50 Nm 3 /h), maximum value of mass flow rate increases from 98 up to 125 kg/h and maximum value of increase when the temperature of the air increase. 3.2 Air and steam gasification Figure 4 presents the case of experiments conducted at high temperature (C5) and at highest content of steam in the composition of the feed gas. The normalized composition (main components) of the fuel gas at different molar fraction of steam in the feed gas is shown in Figure 5. Maximum flow rate of fuel gas decreases from 125 kg/h down to 31 kg/h on increasing molar fraction of steam in the feed gas. Gasification with use of steam is a highly endothermic process. Also in this case the heat needed for thermal decomposition of the feedstock can be supplied by partial oxidation of the feedstock or from outside. Partial oxidation can be realized by supplying air or oxygen. Alternatively heat can be supplied through gasifier walls or with preheated steam. In this work steam was mixed with air and preheated. In this way partial oxidation of the feedstock also takes place. Normalized value of H 2,,,, and [-] Figure 4: Air and steam gasification profile for Case 5 Normalized mass flow of fuel gas [-] m(fg_d), C3 m(fg_d), C4 m(fg_d), C5 m(feg_d), C3 m(feg_d), C4 m(feg_d), C5, C3, C4, C5 Figure 5: Mass flow rate and lower heating value of fuel gas produced during high temperature gasification runs at various molar fraction of steam in the feed gas Normalize valued of [-] High temperature gasification using highly preheated air or steam changes energy balance of the gasifier. Less feedstock is burned thus less flue gases is produced what results in less diluted fuel gas. Heating value of the fuel gas from high temperature air and mixture of air and steam gasification is slightly the same with that resulting from gasification of wood pellets at high temperature air gasification. Since pyrolysis and char residue gasification is carried out at much higher temperature the tar is already decomposed in much higher degree. It results thus in much lower contamination of the fuel gas with tar and dust. Figure 5 shows the profiles of maximum mass flow rate and maximum of the fuel gas produced during gasification at high temperature with use of a mixture of air and steam as a feed gas. The three cases shown in Figure 5 correspond to three different cases of content of molar fraction of steam in the feed gas. Case 3 represents 0% of steam and case 5 (C5) the highest content of steam used. When comparing profiles of mass flow rate and of maximum lower heating value presented in Figure 3 and Figure 5, important differences are seen. It could be seen that temperature of the feed gas has positive effects on the lower heating value of the produced gas, whereas increase of molar fraction of steam in the feed gas has little effect. Maximum flow rate of produced gas increases with increase of temperature (Figure 3) and decreases with increase of molar fraction of steam in the feed gas (Figure 5). 3.3 Gasification temperature profile Figure 6 and 7 show a normal operation of an updraft fixed bed gasifier [11]. Immediately above the grate the solid char (the residual solid remaining after the release of volatiles) formed higher up the gasifier is combusted and the temperature reaches about 1000 o C. Higher up the gasifier again, the biomass is pyrolysed and in the top zone, the feed is dried, cooling the gases to around o C. In the pyrolysis zone, where the volatile compounds are released, considerable quantities of tar are formed which condenses partly on the biomass higher up and partly leaves the gasifier with the product gas. As shown in Figure 6, the use of high temperature air as feed gas results in increase of temperature above the bed. Moreover, Figure 6 shows that temperature of fuel gas (T og1 ) at the outlet of the gasifier is higher than 300 o C. The range of temperature of the exiting gas ( o C) achieved during the gasification runs with high temperature preheated air very much agrees with the reported value in the range o C [16]. In the work of Brammer [16], however, the gasifier is based in a configuration based in two stage cracking of the tar-laden gas, first thermally 3

4 in an air-fired secondary air oxidation reactor and finally in a catalytic cracker. Adding steam in the feed gas results in a significant decrease of the process temperature, reaching the lowest value of about 700 o C when molar fraction of steam in the feed gas was the highest (case 5). Figure 7 shows that fuel gas leaves the gasifier with temperature in the range of o C. The above discussion agrees with the expected results about the increase of quality of fuel gas produced when using high temperature technology as this described in this work. As shown above, lower heating value and mass flow rate of fuel gas both increase with increase of air temperature. Air and steam is added causing oxidation (combustion) of a fraction of the pyrolysis gas at high temperature. As also stated in the literature [17,18], at these high temperatures, the pyrolysis tars decompose into simple gases significantly. Thus even the very short exposure time for the pyrolysis tars eliminate nearly all the tars from the gas. The feedstock reacts with steam and oxygen from the air producing, 2, H 2, CH 4. Since gasification is endothermic, the process temperature decreases from high temperature to about 700 o C through the bed (Figure 7). Figure 6 and 7 also show that HTAG provides high temperature tar cracking zone (T b1 temperature of the bed)., C3, C2, C1, C3, C2, C1 Figure 6: Temperature profile for air gasification This occurs because several actions occur simultaneously in the region of the gasifier itself: the thorough mixing of the fuel particles with high temperature feed gas, the rapid heating of this mixture to an elevated temperature, and the stabilization of the volatiles evolved from the fuel particles. Moreover, the yield increases also because higher heating value of the fuel gas can be obtained from wood pellets when high temperature preheated air and steam are used as feed gas., C3, C4, C5, C3, C4, C5 Figure 7: Temperature profile for air and steam gasification It is well known that the tar concentrations and its composition depend on the gasifier type and design, on the operation conditions, feedstock, etc. [12]. In general, as the gas leaves the gasifier near the pyrolysis zone, the gas generated in updraft gasifiers exhibit a high level of organic components (tar). 4 NCLUSIONS The effect of the feed gas composition (air/steam) on the lower heating value and on the composition of the fuel gas produced in the updraft fixed bed gasifier is discussed. It shows that the higher the molar fractions of steam in the feed gas, the higher the content of hydrogen on the produced gas. Comparing the effect of the feed gas (air or mixture of air and steam) it could be seen that increase of the lower heating value was due to the increases on the molar fraction of the combustible gases (CH 4, H 2 and ) caused by the pyrolysis process and cracking of hydrocarbons when high temperature gasification is applied. The gasification temperature response to the change of the feed gas composition and temperature of the feed gas is also discussed. The results showed that the use of steam in the feed gas decreases the process temperature sharply. Furthermore the higher the molar fractions of steam in the fuel gas the lower the temperature in the bed (T b1 ) and that of the off gas (T og1 ) as well. For the same type of feedstock the operating parameters, temperature, composition and amount of gasifying agent, were varied over a wide range. Results of experiments conducted in a high temperature air/steam fixed bed updraft gasifier show the capability of this technology of maximising the gaseous product yield as a result of the high heating rates involved, and the efficient tar reduction. Increase of the feed gas temperature reduces production of tars, soot and char residue as well as increases heating value of the fuel gas produced. Overall, it has been seen that the yield and the lower heating value of the dry fuel gas increase with increasing temperature. Lower heating value above 5 MJ/Nm 3 has been reached during experiments described in this work. ACKNOWLEDGMENTS Authors would like to acknowledge financial support of the grant "High Exergy Rate Gas aided 4

5 Low Grade Fuel Utilization Technology" from the New Energy and Industrial Technology Development Organization, NEDO (Japan) and Swedish National Energy Administration, STEM (Sweden). The fruitful collaboration with Nippon Furnace Kogyo Kaisha Ltd, (NFK) Japan is very much acknowledged. REFERENCES [1] Tanaka, R., New progress of energy saving technology toward the 21st century; Frontier of combustion & heat transfer technology, Proceedings of 11th IFRF Members Conference, 10-12th May [2] Yoshikawa K, Ootsuka T, Katsushima H, Hasegawa T, Tanaka R, Kiga T, Makino K, High temperature air coal combustion utilizing Multi-staged Enthalpy Extraction Technology, Proceedings of 1997 International Joint Power Generation Conference, Vol. 1, pp , 1997 [3] Tsuji, H., Gupta, A. K., Hasegawa, T., Katsuki, M., Kishimoto, K., Morita, M.: High Temperature Air Combustion: From Energy Conservation to Pollution reduction, CRC Press, [4] Mochida S., Hasegawa T., Development of highly preheated turbulent jet generator, 3rd International Symposium on Advanced Energy Conversion Systems and Related Technologies, RAN2001, Nagoya, Japan, December 15-17, 2001 [5] Bridgwater, A. V. The technical and economic feasibility of biomass gasification for power generation. Fuel 1995, 74, 631. [6] Beenackers A and K Maniatis, Gasification technologies for heat and power from biomass, Proceedings 5th European Bioenergy Conference, [7] Delgado, J.; Aznar, M. P.; Corella, J. Biomass gasification with steam in fluidized bed: effectiveness of CaO, MgO, and Ca-MgO for hot raw gas cleaning. Ind. Eng. Chem. Res. 1997, 36, [8] Di Blasi, C. Gabriella Signorelli, and Giuseppe Portoricco. Countercurrent Fixed-Bed Gasification of Biomass at Laboratory Scale. Ind. Eng. Chem. Res. 1999, 38, [9] Rapngà, S., Provendier, H., Petit C, Kiennemann, A. and Foscolo, P. U. Development of catalysts suitable for hydrogen or syn-gas production from biomass gasification, Biomass and Bioenergy 22 (2002) [10] De Bari, I., Barisano, D., Cardinale, M., Matera, D., Nanna, F., and Viggiano D. Air Gasification of Biomass in a Downdraft Fixed Bed: A Comparative Study of the Inorganic and Organic Products Distribution Energy & Fuels 2000, 14, [11] McKendry, P. Energy production from biomass (part 3): gasification technologies. Bioresource Technology. 2002; 83 (55-63). [12] Beenackers, A. A. C. M. Biomass gasification in moving beds, a review of European technologies. Renewable Energy 16 (1999) [13] Blasiak, W., Szewczyk, D., Lucas, C., High Temperature Air/Steam Gasification of biomass wastes, 21st International Conference on Incineration and Thermal Treatment Technologies, IT3 Conference, New Orleans, USA, May [14] Klingenberg, T. H., J.; Breithaupt P.P., Combustion process analysis by micro gas chromatograph measurement techniques, 2001 International Gas Research Conference; Amsterdam, November [15] Corella, J., Aznar M. P., Delgado, J., and Aldea, E. Steam Gasification of Cellulosic Wastes in a Fluidized Bed with Downstream Vessels. Ind. Eng. Chem. Res. 1991,30, [16] Brammer, J. G. and Bridgwater, A. V. The influence of feedstock drying on the performance and economics of a biomass gasifier engine CHP system, Biomass and Bioenergy 22 (2002) [17] Hindsgaul C., Schramm J., Gratz L., Henriksen U., Bentzen J.B. Physical and chemical characterization of particles in producer gas from wood chips. Bioresource Technology 73 (2000) [18] Babu, S. P., Thermal gasification of biomass technology developments: end of task report for 1992 t Biomass and Bioenergy 9 (1995)