- Email: [email protected]
Tar reduction in downdraft biomass gasifier using a primary method
Renewable Energy 78 (2015) 478e483
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Tar reduction in downdraft biomass gasifier using a primary method Luz Silveira a, Einara Blanco Machin a, Daniel Travieso Pedroso a, *, Nestor Proenza b, Jose c a a Leonetto Conti , Lúcia Bollini Braga , Adrian Blanco Machin ~o Paulo State University (UNESP), Guaratingueta , SP, Brazil Energy Department, Sa Mechanical Engineering Department, University of Camagüey, Cuba c Department of Chemistry, University of Sassari, Sassari, Italy a
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 June 2014 Accepted 30 December 2014 Available online
This work present a novel primary method, for tar reduction in downdraft gasification. The principle of this new technology is to change the fluid dynamic behaviour of the mixture, formed by pyrolysis product and gasification agent in combustion zone; allowing a homogeneous temperature distribution in radial direction in this reaction zone. To achieve the change in the fluid dynamic behaviour of the mixture; the entry of gasification agent to combustion zone is oriented by means of wall nozzles in order to form a swirl flow. This modification in combination with the extension of the reduction zone, will allow, to increases the efficiency of the tar thermal cracking inside the gasifier and the extension of the Boudouard reactions. Consequently, the quantity of tar passing through the combustion zone without cracking and the concentration of tar in the final gas, decrease significantly in relation with the common value obtained for this type of reactor, without affecting significantly the heating value of the producer gas. In this work is presented a new design for 15 kW downdraft gasification reactor, with this technology implemented, the tar content obtained in the experiments never overcome 10 mg/Nm3, with a lower heating value of 3.97 MJ/Nm3. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Biomass Downdraft gasifier Gasification Tar Swirl flow
1. Introduction Biomass, mainly in the form of wood, is the oldest form of energy used by humans. Biomass generally means a relatively dry solid of natural matter that has been specifically grown or has originated as waste or residue from handling such materials [1]. The thermochemical conversion of biomass (pyrolysis, gasification, combustion) is one of the most promising non-nuclear forms of future energy. Biomass is a renewable source of energy and has many ecological advantages [2]. Gasification is the key technology of biomass based power generation; is a high-temperature process (873e1273 K) that decomposes complex biomass hydrocarbons into gaseous molecules, primarily hydrogen, carbon monoxide, and carbon dioxide; also are formed some tars, char, methane, water, and other constituents. Several institutions working on biomass gasification have given many definitions of tar. In the EU/IEA/USDOE meeting on tar measurement protocol held in Brussels in the * Corresponding author. E-mail addresses: [email protected] (E.B. Machin), [email protected], [email protected] (D.T. Pedroso). http://dx.doi.org/10.1016/j.renene.2014.12.069 0960-1481/© 2015 Elsevier Ltd. All rights reserved.
year 1998, it was agreed by a number of experts to define tar as all organic contaminants [polycyclic aromatic hydrocarbon (PAH)] with a molecular weight higher than benzene [3]. Tar is undesirable because of various problems associated with its condensation, causing problems in the gasification installations as well as in the equipments that use the producer gas as fuel like internal combustion engines and gas turbines. The required gas quality to fuel internal combustion engines is normally reached easily in the modern downdraft gasifiers, except for the content of dust and tar. Thermal, catalytic or physical processes either within the gasification process (primary methods) or after the process (secondary methods) can be applied to remove tars. Primary methods have the advantage that dispenses the use of an expensive cleaning system for producer gas. In addition, cracking of tars in the reactor could increases the amount of combustible gases in the producer gas and therefore, the overall process efficiency. There are some sophisticated options available, which claimed a significantly reduction of the tar content in the producer gas, however, the method must be efficient in terms of tar removal, economically feasible, but more importantly, it should not affect the formation of useful producer gas components [4].
E.B. Machin et al. / Renewable Energy 78 (2015) 478e483
The catalytic cracking and electrostatic filters are two examples of the options, that claim a significant tar reduction in the producer gas, but they increase the cost of the plants, especially in the small ones. Currently, the preferred option for tar reduction is in the gasifier itself through process control and the use of primary measures such as additives and catalysts which modify gasification conditions [4e12]. Theoretically, producer gas with low tar content can be obtained if a high-temperature zone can be created, where the gaseous products of pyrolysis are forced to reside the necessary time to undergo a secondary gasification. Previous works have been developed in order to design a downdraft gasifier, able to increase the efficiency of tar reduction in the producer gas during gasification process. Bui et al. [13] developed a multi-stage reactor design that separates the flaming-pyrolysis zone from the reduction zone. In that design, the tar vapours generated in the first zone are burned or cracked to simple molecules by high temperature in the second zone, improving the gas quality and conversion efficiency. The minimum content of gravimetric tar obtained with this design was 92 mg/Nm3. Susanto and Beenackers [14] developed a downdraft moving bed gasifier with internal recycle and separate combustion of pyrolysis gas with the aim of reduce a tar content in the producer gas; in their experiments a minimum of 48 mg/Nm3 of tar was obtained. On this background, the main objective of this work is to propose a new downdraft gasifier design, able to generate the producer gas with low tar concentration using a novel primary method without decreasing significantly the heating value of the producer gas.
2. Process principle In the Imbert design of downdraft gasifier, the gasification agent is fed above a constriction (throat) by nozzles uniformly distributed on the wall of the combustion chamber, oriented toward the centre of the circle, that describe the perimeter of the combustion chamber. In this design, some cool zones are created near to the nozzles, where the temperature is not sufficiently higher to permit the thermal cracking of the tar present in the mixture and to undergo its secondary gasification [15]. This is one of the reasons for the presence of tar in the producer gas. If tarry gas is produced from this type of gasifier, is common practice reduce the central constriction area, until a gas with low tar content can be produced. However, this area dimensions also play an important role in the gas production rate. In order to avoid the formation of cool zones, it is proposed in this work to modify the fluid dynamic behaviour of the mixture formed by the pyrolysis gases and the gasification agent in the combustion chamber.
I G¼
Vðr0 ; tÞdl
(1)
L
The circulation of the vector V (ro, t) combined with the downward movement of the fluid, caused by absorption from the base of the chamber through the diaphragm, generates a swirl flow. This fluid dynamic behaviour would allow to increase the mixing of the gasifying agent with the pyrolysis gases [21,22]; homogenizing the temperature inside the combustion chamber, diminishing the formation of cool areas between the nozzles as main result. In addition this modification increase the residence time of the gas inside the combustion chamber; thereby increasing the thermal cracking of the tar in this zone, minimizing its passage to the reduction zone, decreasing the tar concentration in the producer gas. Swirl number S may effectively control the residence time distribution of the gas mixture, which is function of the fluid entry angle [18]. The increase of the residence time has the undesirable effects of decreasing the efficiency and productivity of the gasifier, as described by Susanto [13]. Fig. 1 shows a top view of the combustion chamber of the reactor, illustrating the inclination of the inlet nozzles of gasification agent. 3. Experimental approach 3.1. Investigated samples The gasification tests were performed using three different woody biomasses, supplied by a wood processing factory. The biomasses used were Peach (Prunus persica), Olive (Olea europaea) and Pine (Pinus pinea). The properties of the woody biomass are shown in Table 1. The elemental compositions were determined using a CHNS-O Elementar Vario GmbH EL III and the Higher Heating Value (HHV) using a calorimeter IKA C-5000 (ASTM D3286-91a). The moisture and ash composition were determined using the ASTM E-871-82 and ASTM D-3174-82. The results were similar to literature values. For the experiments, the biomasses were chopped in square-based prism pieces with dimensions of about 2 1 1 cm. The size and shape are very important for the behaviour of biomass in the downdraft gasifier as far as its movement, and bridging and channelling formations. In addition, the height of the oxidation zone and the pressure drop inside the reactor, depend on these characteristics. 3.2. Experimental setup The scheme of the downdraft wood gasifier is show in Fig. 2. The gasifier unit is constituted of two cylindrical coaxial structures constructed using a mild steel sheet. An insulating material coats the external one, while the internal cylinder is provided with additional heat recuperation surfaces to improve the efficiency of
2.1. The combustion chamber Swirl flows are widely used to intensify the process of heat and mass transfer between solid particles and airflow in vortex chambers, the advantages of swirl flows has been deeply studied by several authors [16e20]. The swirl flow of the mixture could be created changing the entry angle of the gasification agent to the combustion chamber. The new angle must be different of the standard 90 in the Imbert design. This modification allow that the circulation G (Equation (1)) of the velocity vector V(ro,t) of any element of the fluid at any position r s 0 in the plane in which the nozzles are located, or any other parallel plane below this until the diaphragm, is different from zero (G s 0).
479
Fig. 1. Nozzles inclination in the combustion chamber.
480
E.B. Machin et al. / Renewable Energy 78 (2015) 478e483
Table 1 Elemental composition and HHV of the studied biomasses. Biomass
C %wt db
H %wt db
N %wt db
O %wt db
Ash %wt db
Moisture %wt
HHV MJ/kg
Peach Olive Pine
48.06 46.43 48.18
5.83 5.63 5.71
0.55 0.55 0.15
44.03 44.91 43.89
1.53 2.48 2.07
9.8 10.6 9.0
18.74 17.80 18.67
the gasification process (Fig. 2). The internal capacity is 0.452 m3, the height of the gasifier is 1.02 m and the internal radius at the drying e pyrolysis zone is 0.30 m. The dimensions of reduction zone are enlarged to boost the rate of the Boudouard and the wateregas reactions, in order to increase the concentration of CO and H2 in the producer gas and also decrease the gas temperature. The gasification agent for the experiments (air) is supplied using an electric blower with control valve, capable of supply the required air for the gasification process. The lines are heated up to 453 K in order to prevent condensation of the producer gas compounds inside the conducts and the measurement device. The producer gas sample is filtered, cooled and drained, before be analysed in the Gasboard-3100P mobile gas analyser. The temperature are measured by mean of six thermocouples (type K) located at different height of the reactor bed. Air and gas flows are measured with an orifice and differential manometer. All the experimental data is recorded by data logger in 5 min intervals. The simplified experimental setup for the test of the modified reactor is presented in Fig. 3. 3.3. Tar sampling principle The principle of the test method for gravimetric tar measurement is based on the continuous sampling of a gas stream,
containing particles and organic compounds (tar) under isokinetic conditions; according to the methodology described in DD CEN/TS 15439:2006 [23]. The determination is carried out in two steps: sampling and analysis. The equipment for sampling shown in Fig. 4, consists of a heated probe (module 1), a heated particle filter (module 2), a condenser, a series of impinger bottles containing a solvent (isopropanol) for tar absorption (module 3), and equipment for pressure and flow rate adjustment and measurement (module 4). Upstream of the condenser, the tubes connecting these parts are heated in order to prevent tar condensation. Temperatures of the condenser and the impingers were properly selected to ensure quantitative collection of the tars (1, 2, and 4 is between 308 and 313 K, and 3, 5 and 6 is between 258 and 253 K). Tar collection occurs both by condensation and by absorption in the condenser, in the impinges, and by capturing of aerosols in glass frits. The analysis of the samples is carried out according to the methodology described in Ref. [23]. 3.4. Process flow description The gasifier system was run nine times, for periods between 2.5 and 4 h. To start the gasifier, initially the fuel biomass is loaded up to the reactor maximum capacity and is closed. Subsequently is introduced a propane gas duct by the air entrance to the reactor, to create a flame inside the combustion chamber, then the vacuum pump was turned on and the propane gas feed is removed. In less than 15 min or when the temperature in pyrolysis zone (TC 2 and TC 3) reaches 573 K the ignition step is completed and the record of the profile of reactor temperatures and the gases flow starts. The producer gas analysis starts when the preset temperature profile in the reactor is reached, due to the high concentration of condensable gases in the producer gas composition during the ignition process. The tar sampling process starts at the same time of the producer gas analysis, with the installation shown in Fig. 4; each tar sampling takes 45 min. 4. Results and discussion
Fig. 2. Reactor's scheme.
Table 2 and Table 3 shown the performance of the biomass gasifier system and the composition of the producer gas during the experiments, at regular intervals of 5 min. Fig. 5 shows a typical behaviour of the temperature profile in the reactor during the experiments. As it is observed, there are an oscillation of the temperature value in all the bed section during all the experiments, with the exception of the temperature of the producer gas, where the temperature remain more stable. The main reason of this variation is biomass movement inside the reactor during the gasification process. The temperature of the producer gas remains in the range of 410e430 K, lower than the typical range of 700e720 K reported for this type of reactor. The HHV of the producer gas is calculated from the concentration of the combustible components. For all the experiments, the HHV obtained was higher to 3.50 MJ/Nm3, and the higher values were obtained in the experiments using Peach as fuel, where the mean value was 3.97 MJ/Nm3. These values are lower than the theoretical and experimental results reported in the literature; Zainal et al. [14] report 4.72 and 4.85 MJ/Nm3 respectively for same capacity and type downdraft gasifier. These results are because the medium content of H2, CO and CH4 in the producer gas obtained in the experiments with the tested reactor was slightly lower than the typical composition of the producer gas reported by several authors [2,3,13,14,24,25]. The O2 concentration has the same behaviour, showing an increase in the
E.B. Machin et al. / Renewable Energy 78 (2015) 478e483
481
Fig. 3. Experimental installation setup.
combustion rate of the fuel gas in the reactor as negative effect of the modifications implemented. The mean tar content of the producer gas obtained in the experiments was 9.10 mg/Nm3 for Olive, 4.07 mg/Nm3 for Peach and 8.73 mg/Nm3 in the case for Pine. Fig. 6 compares the tar content in the producer gas obtained by several authors 19e35 mg/Nm3 [26], 5 mg/Nm3 [25], 97 mg/Nm3 [27], 50 mg/Nm3 [28] and 10 mg/Nm3 [29]; with the content obtained in the studied reactor. The gas quality is comparable with the obtained in experiments with the optimized two stages gasifier, developed by Bentzen [25] (5 mg/ Nm3), but with higher HHV. Burhenne et al. [29] reported similar
gas quality, with a minimum tar content of 10 mg/Nm3 and HHV between 4.85 and 4.48 MJ/m3 using a multi-staged gasification technology. The CO/CO2 and H2/CO ratios are constant; the heating value of the gas is a direct consequence of its chemical composition, which depends on the reaction conditions, rather than the heating value of the entering biomass, equal for all those experienced. The increase of the residence time of the gas mixture in reactor as consequence of the modification in the combustion chamber also has the undesirable effects of decreasing the efficiency and
Table 2 Operating parameters. Biomass
Fig. 4. Modular sampling train of tar.
Mean process time Mean temperature error ± 1.0 K T1 SD T2 SD T3 SD T4 SD T5 SD T6 SD Biomass fed Flows Air Gas
Olive (h) (K)
(kg) (Nm3/h)
3.80
Peach 2.50
513 18 531 49 880 30 1193 60 1123 68 417 7 8.74
473 20 491 21 780 25 1173 65 1153 73 425 9 7.6
5.74 28.9
5.3 18.4
Pine 3.10 503 22 521 18 853 22 1143 61 1103 62 408 5 7.75 5.4 21.3
482
E.B. Machin et al. / Renewable Energy 78 (2015) 478e483
Table 3 Tests results. Biomass Inputs Gasifier conditions Feed (kg/h) Gasifier air (20 C, 1 bar) (kg/h) Outputs Dry gas (kg/h) Water (g/Nm3) Char e ash (kg/h) Tar (mg/Nm3) Error ±0.01 SD Dry gas analysis CO (% vol.) H2 (% vol.) CO2 (% vol.) CH4 (% vol.) O2 (% vol.) N2 (% vol.) Dry gas HHV (MJ/Nm3) Gas density (kg/Nm3) Operating ratios O2/dry biomass CO/CO2 H2/CO Mass balance and energy efficiency Mass in/mass out Cold gas efficiency
Olive
Peach
Pine
3.3 6.79
3.05 6.20
2.5 6.45
9.02 114.5 0.160 9.10
8.60 96.5 0.085 4.07
8.17 102.3 0.128 8.73
0.19
0.19
0.19
17.4 13.2 12.4 0.8 1.3 54.9 3.55 1.183
17.7 15.0 13.5 1.2 0.9 51.7 3.97 1.167
16.0 12.1 11.4 0.2 0.9 59.4 3.65 1.191
0.45 1.40 0.76
0.44 1.31 0.85
0.44 1.40 0.76
1.01 0.61
0.98 0.78
0.99 0.58
productivity of the gasifier; that is why these parameters are lower than in commercial gasifiers. According to this, more experiments are required to determinate the optimum angle to achieve a balance between all these effects in order to obtain a clean gas without diminish significantly the overall efficiency of the gasification process. Furthermore the small size of experimental model and its proportionally higher heat loss, influences in the overall process efficiency. These results have been obtained applying additionally, a cleaning system truly simple and inexpensive, for particles removing. 5. Conclusions A clean producer gas was obtained with a novel downdraft gasifier. A modified combustion chamber that prevents the
Fig. 5. Temperature profile along the reactor height in the 3rd experimental test using Olive.
Fig. 6. Comparison between the gas quality obtained by different authors and the present study.
formation of cool zones inside it and increases the thermal homogenization in this reaction zone was developed. This modification together with an extension of the reduction zone allows diminishing the tar content in the producer gas. The mean values of this parameter in all the experimental tests were lower than 10 mg/ Nm3. The low tar and particle content makes the producer gas obtained in this reactor suitable to the use in cycle Otto engines. Acknowledgement We are grateful to the Coordination for the Improvement of Higher Education Personnel (CAPES) (process 5993105), from the Brazilian Ministry of Education (MEC) and to the National Council for Scientific and Technological Development (CNPq) (process 162633/2013-0) from the Ministry of Science and Technology (MCT) for their generous financing support to this research. References [1] Giltrap DL, McKibbin R, Barnes GRG. A steady state model of gas-char reactions in a downdraft biomass gasifier. Sol Energy 2003;74:85e91. [2] Babu BV, Chaurasia AS. Modeling for pyrolysis of solid particle: kinetics and heat transfer effects. Energy Convers Manag 2003;44:2251e75. [3] Neeft JPA, Knoef HAM, Onaji P. Behaviour of tar in biomass gasification systems. Tar related problems and their solutions. Novem ed. 1999. Nederland. [4] Devi L, Ptasinski KJ, Janssen Frans JJG. A review of the primary measures for tar elimination in biomass gasification processes. Biomass Bioenergy 2003;24: 125e40. [5] Akay G, Dogru M, Calkan OF. Biomass to rescue. Chem Eng Lond 2006;786: 55e7. [6] Huang J, Schmidt KG, Bian Z. Removal and conversion of tar in syngas from woody biomass gasification for power utilization using catalytic hydrocracking. Energies 2011;4:1163e77. [7] Jordan CA, Akay G. Speciation and distribution of alkali, alkali earth metals and major ash forming elements during gasification of fuel cane bagasse. Fuel 2012;91:253e63. [8] Abu El-Rub Z, Bramer E, Brem G. 2004 review of catalysts for tar elimination in biomass gasification. Ind Eng Chem Res 2004;43:6911e9. [9] Miskolczi M, Borsodi N, Buyong F, Angyal A, Williams PT. Production of pyrolytic oils by catalytic pyrolysis of Malaysian refuse-derived fuels in continuously stittred batch reactor. Fuel Process Technol 2011;92:925e32. [10] Elbaba IF, Wu C, Williams PT. Hydrogen production from the pyrolysisgasification of waste tyres with a nickel/cerium catalysts. Int J Hydrogen Energy 2011;36:6628e37. [11] Wu C, Wang L, Williams PT, Shi J, Huang J. Hydrogen production from biomass gasification with Ni/MCM-41 catalysts: influence of Ni content. Appl Catal B Environ 2011;108:6e13. [12] Jordan CA, Akay G. Effect of CaO on tar production and dew point depression during gasification of fuel cane bagasse in a novel downdraft gasifier. Fuel Process Technol 2013;106:654e60. [13] Bui T, Loof R, Bhattacharya SC. Multi-stage reactor for thermal gasification of wood. Energy 1994;19(4):397e404.
E.B. Machin et al. / Renewable Energy 78 (2015) 478e483 [14] Susanto H, Beenackers AACM. A moving-bed gasifier with internal recycle of pyrolysis gas. Fuel 1996;75:1339e47. [15] Reed TB, Das A. Handbook of biomass downdraft gasifier engine systems. The Biomass Energy Fundation Press; 1998. [16] Volchkov EP, Lebedev VP, Lukashov VV. The LDA study of flow gas-dynamics in a vortex chamber. Int J Heat Mass Transf 2004;47:35e42. [17] Guo Hui-Fen, Chen Zhi-Yong, Yu Chong-Wen. 3D numerical simulation of compressible swirling flow induced by means of tangential inlets. Int J Numer Methods Fluids 2009;59:1285e98. [18] Guo Hui-Fen, Chen Zhi-Yong, Yu Chong-Wen. 3D tangentially injected swirling recirculating flow in a nozzle with a slotted-tubedeffects of groove parameters. Int J Numer Methods Fluids 2010;63:1256e69. r M. Combustion in swirling flows: a review. Combust Flame [19] Syred NJ, Bee 1974;23:143e201. [20] Lilley DG. Swirl flow in combustion: a review. AIAA J 1977;15:1063e78. [21] Catrakis H, Aguirre R, Mason J. Physical modeling of turbulent fluid interfaces and flow regions at large Reynolds numbers. World Sci Eng Acad Soc Trans Mech Eng 2005;2:1e19. [22] Tabak EG, Tal FA. Mixing in simple models for turbulent diffusion. Comm Pure Appl Math 2004;57:563e89. http://dx.doi.org/10.1002/cpa.20012.
483
[23] DD CEN/TS 15439. Biomass gasification- tar and particles in products gasessampling and analyze. UE. 2006. [24] Zainal ZA, Ali R, Lean CH, Seetharamu KN. Prediction of performance of a downdraft gasifier using equilibrium modelling for different biomass materials. Energy Convers Manag 2001;42:1499e515. [25] Bentzen JD. Optimized two-stage gasifier. In: Proceedings of first world conference on biomass for energy and industry; 2000. [26] Bhattacharya SC, Mizanur R, Siddique AHM, Pham HL. A study on wood gasification for low-tar gas production. Energy 1999;24:285e96. [27] Walker M, Jackson G, Peacocke GVC. In: Bridgwater AV, editor. Small scale biomass gasification: development of a gas cleaning system for power generation. Progress in thermochemical biomass conversion. Oxford, UK: Blackwell Scientific Publications; 2001. p. 441e51. [28] Reed TB, Levie B. Understanding operating, and testing fixed bed gasifier. In: Bioenergy 0 84, Proceeding of World Conference, Goetborg, Sweden, June 21. Elsevier; 1985. [29] Burhenne L, Rochlitz L, Lintner C, Aicher T. Technical demonstration of the novel Fraunhofer ISE biomass gasification process for the production of a tarfree synthesis gas. Fuel Process Technol 2013;106:751e60.
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Tar reduction in downdraft biomass gasifier using a primary method Luz Silveira a, Einara Blanco Machin a, Daniel Travieso Pedroso a, *, Nestor Proenza b, Jose c a a Leonetto Conti , Lúcia Bollini Braga , Adrian Blanco Machin ~o Paulo State University (UNESP), Guaratingueta , SP, Brazil Energy Department, Sa Mechanical Engineering Department, University of Camagüey, Cuba c Department of Chemistry, University of Sassari, Sassari, Italy a
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 June 2014 Accepted 30 December 2014 Available online
This work present a novel primary method, for tar reduction in downdraft gasification. The principle of this new technology is to change the fluid dynamic behaviour of the mixture, formed by pyrolysis product and gasification agent in combustion zone; allowing a homogeneous temperature distribution in radial direction in this reaction zone. To achieve the change in the fluid dynamic behaviour of the mixture; the entry of gasification agent to combustion zone is oriented by means of wall nozzles in order to form a swirl flow. This modification in combination with the extension of the reduction zone, will allow, to increases the efficiency of the tar thermal cracking inside the gasifier and the extension of the Boudouard reactions. Consequently, the quantity of tar passing through the combustion zone without cracking and the concentration of tar in the final gas, decrease significantly in relation with the common value obtained for this type of reactor, without affecting significantly the heating value of the producer gas. In this work is presented a new design for 15 kW downdraft gasification reactor, with this technology implemented, the tar content obtained in the experiments never overcome 10 mg/Nm3, with a lower heating value of 3.97 MJ/Nm3. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Biomass Downdraft gasifier Gasification Tar Swirl flow
1. Introduction Biomass, mainly in the form of wood, is the oldest form of energy used by humans. Biomass generally means a relatively dry solid of natural matter that has been specifically grown or has originated as waste or residue from handling such materials [1]. The thermochemical conversion of biomass (pyrolysis, gasification, combustion) is one of the most promising non-nuclear forms of future energy. Biomass is a renewable source of energy and has many ecological advantages [2]. Gasification is the key technology of biomass based power generation; is a high-temperature process (873e1273 K) that decomposes complex biomass hydrocarbons into gaseous molecules, primarily hydrogen, carbon monoxide, and carbon dioxide; also are formed some tars, char, methane, water, and other constituents. Several institutions working on biomass gasification have given many definitions of tar. In the EU/IEA/USDOE meeting on tar measurement protocol held in Brussels in the * Corresponding author. E-mail addresses: [email protected] (E.B. Machin), [email protected], [email protected] (D.T. Pedroso). http://dx.doi.org/10.1016/j.renene.2014.12.069 0960-1481/© 2015 Elsevier Ltd. All rights reserved.
year 1998, it was agreed by a number of experts to define tar as all organic contaminants [polycyclic aromatic hydrocarbon (PAH)] with a molecular weight higher than benzene [3]. Tar is undesirable because of various problems associated with its condensation, causing problems in the gasification installations as well as in the equipments that use the producer gas as fuel like internal combustion engines and gas turbines. The required gas quality to fuel internal combustion engines is normally reached easily in the modern downdraft gasifiers, except for the content of dust and tar. Thermal, catalytic or physical processes either within the gasification process (primary methods) or after the process (secondary methods) can be applied to remove tars. Primary methods have the advantage that dispenses the use of an expensive cleaning system for producer gas. In addition, cracking of tars in the reactor could increases the amount of combustible gases in the producer gas and therefore, the overall process efficiency. There are some sophisticated options available, which claimed a significantly reduction of the tar content in the producer gas, however, the method must be efficient in terms of tar removal, economically feasible, but more importantly, it should not affect the formation of useful producer gas components [4].
E.B. Machin et al. / Renewable Energy 78 (2015) 478e483
The catalytic cracking and electrostatic filters are two examples of the options, that claim a significant tar reduction in the producer gas, but they increase the cost of the plants, especially in the small ones. Currently, the preferred option for tar reduction is in the gasifier itself through process control and the use of primary measures such as additives and catalysts which modify gasification conditions [4e12]. Theoretically, producer gas with low tar content can be obtained if a high-temperature zone can be created, where the gaseous products of pyrolysis are forced to reside the necessary time to undergo a secondary gasification. Previous works have been developed in order to design a downdraft gasifier, able to increase the efficiency of tar reduction in the producer gas during gasification process. Bui et al. [13] developed a multi-stage reactor design that separates the flaming-pyrolysis zone from the reduction zone. In that design, the tar vapours generated in the first zone are burned or cracked to simple molecules by high temperature in the second zone, improving the gas quality and conversion efficiency. The minimum content of gravimetric tar obtained with this design was 92 mg/Nm3. Susanto and Beenackers [14] developed a downdraft moving bed gasifier with internal recycle and separate combustion of pyrolysis gas with the aim of reduce a tar content in the producer gas; in their experiments a minimum of 48 mg/Nm3 of tar was obtained. On this background, the main objective of this work is to propose a new downdraft gasifier design, able to generate the producer gas with low tar concentration using a novel primary method without decreasing significantly the heating value of the producer gas.
2. Process principle In the Imbert design of downdraft gasifier, the gasification agent is fed above a constriction (throat) by nozzles uniformly distributed on the wall of the combustion chamber, oriented toward the centre of the circle, that describe the perimeter of the combustion chamber. In this design, some cool zones are created near to the nozzles, where the temperature is not sufficiently higher to permit the thermal cracking of the tar present in the mixture and to undergo its secondary gasification [15]. This is one of the reasons for the presence of tar in the producer gas. If tarry gas is produced from this type of gasifier, is common practice reduce the central constriction area, until a gas with low tar content can be produced. However, this area dimensions also play an important role in the gas production rate. In order to avoid the formation of cool zones, it is proposed in this work to modify the fluid dynamic behaviour of the mixture formed by the pyrolysis gases and the gasification agent in the combustion chamber.
I G¼
Vðr0 ; tÞdl
(1)
L
The circulation of the vector V (ro, t) combined with the downward movement of the fluid, caused by absorption from the base of the chamber through the diaphragm, generates a swirl flow. This fluid dynamic behaviour would allow to increase the mixing of the gasifying agent with the pyrolysis gases [21,22]; homogenizing the temperature inside the combustion chamber, diminishing the formation of cool areas between the nozzles as main result. In addition this modification increase the residence time of the gas inside the combustion chamber; thereby increasing the thermal cracking of the tar in this zone, minimizing its passage to the reduction zone, decreasing the tar concentration in the producer gas. Swirl number S may effectively control the residence time distribution of the gas mixture, which is function of the fluid entry angle [18]. The increase of the residence time has the undesirable effects of decreasing the efficiency and productivity of the gasifier, as described by Susanto [13]. Fig. 1 shows a top view of the combustion chamber of the reactor, illustrating the inclination of the inlet nozzles of gasification agent. 3. Experimental approach 3.1. Investigated samples The gasification tests were performed using three different woody biomasses, supplied by a wood processing factory. The biomasses used were Peach (Prunus persica), Olive (Olea europaea) and Pine (Pinus pinea). The properties of the woody biomass are shown in Table 1. The elemental compositions were determined using a CHNS-O Elementar Vario GmbH EL III and the Higher Heating Value (HHV) using a calorimeter IKA C-5000 (ASTM D3286-91a). The moisture and ash composition were determined using the ASTM E-871-82 and ASTM D-3174-82. The results were similar to literature values. For the experiments, the biomasses were chopped in square-based prism pieces with dimensions of about 2 1 1 cm. The size and shape are very important for the behaviour of biomass in the downdraft gasifier as far as its movement, and bridging and channelling formations. In addition, the height of the oxidation zone and the pressure drop inside the reactor, depend on these characteristics. 3.2. Experimental setup The scheme of the downdraft wood gasifier is show in Fig. 2. The gasifier unit is constituted of two cylindrical coaxial structures constructed using a mild steel sheet. An insulating material coats the external one, while the internal cylinder is provided with additional heat recuperation surfaces to improve the efficiency of
2.1. The combustion chamber Swirl flows are widely used to intensify the process of heat and mass transfer between solid particles and airflow in vortex chambers, the advantages of swirl flows has been deeply studied by several authors [16e20]. The swirl flow of the mixture could be created changing the entry angle of the gasification agent to the combustion chamber. The new angle must be different of the standard 90 in the Imbert design. This modification allow that the circulation G (Equation (1)) of the velocity vector V(ro,t) of any element of the fluid at any position r s 0 in the plane in which the nozzles are located, or any other parallel plane below this until the diaphragm, is different from zero (G s 0).
479
Fig. 1. Nozzles inclination in the combustion chamber.
480
E.B. Machin et al. / Renewable Energy 78 (2015) 478e483
Table 1 Elemental composition and HHV of the studied biomasses. Biomass
C %wt db
H %wt db
N %wt db
O %wt db
Ash %wt db
Moisture %wt
HHV MJ/kg
Peach Olive Pine
48.06 46.43 48.18
5.83 5.63 5.71
0.55 0.55 0.15
44.03 44.91 43.89
1.53 2.48 2.07
9.8 10.6 9.0
18.74 17.80 18.67
the gasification process (Fig. 2). The internal capacity is 0.452 m3, the height of the gasifier is 1.02 m and the internal radius at the drying e pyrolysis zone is 0.30 m. The dimensions of reduction zone are enlarged to boost the rate of the Boudouard and the wateregas reactions, in order to increase the concentration of CO and H2 in the producer gas and also decrease the gas temperature. The gasification agent for the experiments (air) is supplied using an electric blower with control valve, capable of supply the required air for the gasification process. The lines are heated up to 453 K in order to prevent condensation of the producer gas compounds inside the conducts and the measurement device. The producer gas sample is filtered, cooled and drained, before be analysed in the Gasboard-3100P mobile gas analyser. The temperature are measured by mean of six thermocouples (type K) located at different height of the reactor bed. Air and gas flows are measured with an orifice and differential manometer. All the experimental data is recorded by data logger in 5 min intervals. The simplified experimental setup for the test of the modified reactor is presented in Fig. 3. 3.3. Tar sampling principle The principle of the test method for gravimetric tar measurement is based on the continuous sampling of a gas stream,
containing particles and organic compounds (tar) under isokinetic conditions; according to the methodology described in DD CEN/TS 15439:2006 [23]. The determination is carried out in two steps: sampling and analysis. The equipment for sampling shown in Fig. 4, consists of a heated probe (module 1), a heated particle filter (module 2), a condenser, a series of impinger bottles containing a solvent (isopropanol) for tar absorption (module 3), and equipment for pressure and flow rate adjustment and measurement (module 4). Upstream of the condenser, the tubes connecting these parts are heated in order to prevent tar condensation. Temperatures of the condenser and the impingers were properly selected to ensure quantitative collection of the tars (1, 2, and 4 is between 308 and 313 K, and 3, 5 and 6 is between 258 and 253 K). Tar collection occurs both by condensation and by absorption in the condenser, in the impinges, and by capturing of aerosols in glass frits. The analysis of the samples is carried out according to the methodology described in Ref. [23]. 3.4. Process flow description The gasifier system was run nine times, for periods between 2.5 and 4 h. To start the gasifier, initially the fuel biomass is loaded up to the reactor maximum capacity and is closed. Subsequently is introduced a propane gas duct by the air entrance to the reactor, to create a flame inside the combustion chamber, then the vacuum pump was turned on and the propane gas feed is removed. In less than 15 min or when the temperature in pyrolysis zone (TC 2 and TC 3) reaches 573 K the ignition step is completed and the record of the profile of reactor temperatures and the gases flow starts. The producer gas analysis starts when the preset temperature profile in the reactor is reached, due to the high concentration of condensable gases in the producer gas composition during the ignition process. The tar sampling process starts at the same time of the producer gas analysis, with the installation shown in Fig. 4; each tar sampling takes 45 min. 4. Results and discussion
Fig. 2. Reactor's scheme.
Table 2 and Table 3 shown the performance of the biomass gasifier system and the composition of the producer gas during the experiments, at regular intervals of 5 min. Fig. 5 shows a typical behaviour of the temperature profile in the reactor during the experiments. As it is observed, there are an oscillation of the temperature value in all the bed section during all the experiments, with the exception of the temperature of the producer gas, where the temperature remain more stable. The main reason of this variation is biomass movement inside the reactor during the gasification process. The temperature of the producer gas remains in the range of 410e430 K, lower than the typical range of 700e720 K reported for this type of reactor. The HHV of the producer gas is calculated from the concentration of the combustible components. For all the experiments, the HHV obtained was higher to 3.50 MJ/Nm3, and the higher values were obtained in the experiments using Peach as fuel, where the mean value was 3.97 MJ/Nm3. These values are lower than the theoretical and experimental results reported in the literature; Zainal et al. [14] report 4.72 and 4.85 MJ/Nm3 respectively for same capacity and type downdraft gasifier. These results are because the medium content of H2, CO and CH4 in the producer gas obtained in the experiments with the tested reactor was slightly lower than the typical composition of the producer gas reported by several authors [2,3,13,14,24,25]. The O2 concentration has the same behaviour, showing an increase in the
E.B. Machin et al. / Renewable Energy 78 (2015) 478e483
481
Fig. 3. Experimental installation setup.
combustion rate of the fuel gas in the reactor as negative effect of the modifications implemented. The mean tar content of the producer gas obtained in the experiments was 9.10 mg/Nm3 for Olive, 4.07 mg/Nm3 for Peach and 8.73 mg/Nm3 in the case for Pine. Fig. 6 compares the tar content in the producer gas obtained by several authors 19e35 mg/Nm3 [26], 5 mg/Nm3 [25], 97 mg/Nm3 [27], 50 mg/Nm3 [28] and 10 mg/Nm3 [29]; with the content obtained in the studied reactor. The gas quality is comparable with the obtained in experiments with the optimized two stages gasifier, developed by Bentzen [25] (5 mg/ Nm3), but with higher HHV. Burhenne et al. [29] reported similar
gas quality, with a minimum tar content of 10 mg/Nm3 and HHV between 4.85 and 4.48 MJ/m3 using a multi-staged gasification technology. The CO/CO2 and H2/CO ratios are constant; the heating value of the gas is a direct consequence of its chemical composition, which depends on the reaction conditions, rather than the heating value of the entering biomass, equal for all those experienced. The increase of the residence time of the gas mixture in reactor as consequence of the modification in the combustion chamber also has the undesirable effects of decreasing the efficiency and
Table 2 Operating parameters. Biomass
Fig. 4. Modular sampling train of tar.
Mean process time Mean temperature error ± 1.0 K T1 SD T2 SD T3 SD T4 SD T5 SD T6 SD Biomass fed Flows Air Gas
Olive (h) (K)
(kg) (Nm3/h)
3.80
Peach 2.50
513 18 531 49 880 30 1193 60 1123 68 417 7 8.74
473 20 491 21 780 25 1173 65 1153 73 425 9 7.6
5.74 28.9
5.3 18.4
Pine 3.10 503 22 521 18 853 22 1143 61 1103 62 408 5 7.75 5.4 21.3
482
E.B. Machin et al. / Renewable Energy 78 (2015) 478e483
Table 3 Tests results. Biomass Inputs Gasifier conditions Feed (kg/h) Gasifier air (20 C, 1 bar) (kg/h) Outputs Dry gas (kg/h) Water (g/Nm3) Char e ash (kg/h) Tar (mg/Nm3) Error ±0.01 SD Dry gas analysis CO (% vol.) H2 (% vol.) CO2 (% vol.) CH4 (% vol.) O2 (% vol.) N2 (% vol.) Dry gas HHV (MJ/Nm3) Gas density (kg/Nm3) Operating ratios O2/dry biomass CO/CO2 H2/CO Mass balance and energy efficiency Mass in/mass out Cold gas efficiency
Olive
Peach
Pine
3.3 6.79
3.05 6.20
2.5 6.45
9.02 114.5 0.160 9.10
8.60 96.5 0.085 4.07
8.17 102.3 0.128 8.73
0.19
0.19
0.19
17.4 13.2 12.4 0.8 1.3 54.9 3.55 1.183
17.7 15.0 13.5 1.2 0.9 51.7 3.97 1.167
16.0 12.1 11.4 0.2 0.9 59.4 3.65 1.191
0.45 1.40 0.76
0.44 1.31 0.85
0.44 1.40 0.76
1.01 0.61
0.98 0.78
0.99 0.58
productivity of the gasifier; that is why these parameters are lower than in commercial gasifiers. According to this, more experiments are required to determinate the optimum angle to achieve a balance between all these effects in order to obtain a clean gas without diminish significantly the overall efficiency of the gasification process. Furthermore the small size of experimental model and its proportionally higher heat loss, influences in the overall process efficiency. These results have been obtained applying additionally, a cleaning system truly simple and inexpensive, for particles removing. 5. Conclusions A clean producer gas was obtained with a novel downdraft gasifier. A modified combustion chamber that prevents the
Fig. 5. Temperature profile along the reactor height in the 3rd experimental test using Olive.
Fig. 6. Comparison between the gas quality obtained by different authors and the present study.
formation of cool zones inside it and increases the thermal homogenization in this reaction zone was developed. This modification together with an extension of the reduction zone allows diminishing the tar content in the producer gas. The mean values of this parameter in all the experimental tests were lower than 10 mg/ Nm3. The low tar and particle content makes the producer gas obtained in this reactor suitable to the use in cycle Otto engines. Acknowledgement We are grateful to the Coordination for the Improvement of Higher Education Personnel (CAPES) (process 5993105), from the Brazilian Ministry of Education (MEC) and to the National Council for Scientific and Technological Development (CNPq) (process 162633/2013-0) from the Ministry of Science and Technology (MCT) for their generous financing support to this research. References [1] Giltrap DL, McKibbin R, Barnes GRG. A steady state model of gas-char reactions in a downdraft biomass gasifier. Sol Energy 2003;74:85e91. [2] Babu BV, Chaurasia AS. Modeling for pyrolysis of solid particle: kinetics and heat transfer effects. Energy Convers Manag 2003;44:2251e75. [3] Neeft JPA, Knoef HAM, Onaji P. Behaviour of tar in biomass gasification systems. Tar related problems and their solutions. Novem ed. 1999. Nederland. [4] Devi L, Ptasinski KJ, Janssen Frans JJG. A review of the primary measures for tar elimination in biomass gasification processes. Biomass Bioenergy 2003;24: 125e40. [5] Akay G, Dogru M, Calkan OF. Biomass to rescue. Chem Eng Lond 2006;786: 55e7. [6] Huang J, Schmidt KG, Bian Z. Removal and conversion of tar in syngas from woody biomass gasification for power utilization using catalytic hydrocracking. Energies 2011;4:1163e77. [7] Jordan CA, Akay G. Speciation and distribution of alkali, alkali earth metals and major ash forming elements during gasification of fuel cane bagasse. Fuel 2012;91:253e63. [8] Abu El-Rub Z, Bramer E, Brem G. 2004 review of catalysts for tar elimination in biomass gasification. Ind Eng Chem Res 2004;43:6911e9. [9] Miskolczi M, Borsodi N, Buyong F, Angyal A, Williams PT. Production of pyrolytic oils by catalytic pyrolysis of Malaysian refuse-derived fuels in continuously stittred batch reactor. Fuel Process Technol 2011;92:925e32. [10] Elbaba IF, Wu C, Williams PT. Hydrogen production from the pyrolysisgasification of waste tyres with a nickel/cerium catalysts. Int J Hydrogen Energy 2011;36:6628e37. [11] Wu C, Wang L, Williams PT, Shi J, Huang J. Hydrogen production from biomass gasification with Ni/MCM-41 catalysts: influence of Ni content. Appl Catal B Environ 2011;108:6e13. [12] Jordan CA, Akay G. Effect of CaO on tar production and dew point depression during gasification of fuel cane bagasse in a novel downdraft gasifier. Fuel Process Technol 2013;106:654e60. [13] Bui T, Loof R, Bhattacharya SC. Multi-stage reactor for thermal gasification of wood. Energy 1994;19(4):397e404.
E.B. Machin et al. / Renewable Energy 78 (2015) 478e483 [14] Susanto H, Beenackers AACM. A moving-bed gasifier with internal recycle of pyrolysis gas. Fuel 1996;75:1339e47. [15] Reed TB, Das A. Handbook of biomass downdraft gasifier engine systems. The Biomass Energy Fundation Press; 1998. [16] Volchkov EP, Lebedev VP, Lukashov VV. The LDA study of flow gas-dynamics in a vortex chamber. Int J Heat Mass Transf 2004;47:35e42. [17] Guo Hui-Fen, Chen Zhi-Yong, Yu Chong-Wen. 3D numerical simulation of compressible swirling flow induced by means of tangential inlets. Int J Numer Methods Fluids 2009;59:1285e98. [18] Guo Hui-Fen, Chen Zhi-Yong, Yu Chong-Wen. 3D tangentially injected swirling recirculating flow in a nozzle with a slotted-tubedeffects of groove parameters. Int J Numer Methods Fluids 2010;63:1256e69. r M. Combustion in swirling flows: a review. Combust Flame [19] Syred NJ, Bee 1974;23:143e201. [20] Lilley DG. Swirl flow in combustion: a review. AIAA J 1977;15:1063e78. [21] Catrakis H, Aguirre R, Mason J. Physical modeling of turbulent fluid interfaces and flow regions at large Reynolds numbers. World Sci Eng Acad Soc Trans Mech Eng 2005;2:1e19. [22] Tabak EG, Tal FA. Mixing in simple models for turbulent diffusion. Comm Pure Appl Math 2004;57:563e89. http://dx.doi.org/10.1002/cpa.20012.
483
[23] DD CEN/TS 15439. Biomass gasification- tar and particles in products gasessampling and analyze. UE. 2006. [24] Zainal ZA, Ali R, Lean CH, Seetharamu KN. Prediction of performance of a downdraft gasifier using equilibrium modelling for different biomass materials. Energy Convers Manag 2001;42:1499e515. [25] Bentzen JD. Optimized two-stage gasifier. In: Proceedings of first world conference on biomass for energy and industry; 2000. [26] Bhattacharya SC, Mizanur R, Siddique AHM, Pham HL. A study on wood gasification for low-tar gas production. Energy 1999;24:285e96. [27] Walker M, Jackson G, Peacocke GVC. In: Bridgwater AV, editor. Small scale biomass gasification: development of a gas cleaning system for power generation. Progress in thermochemical biomass conversion. Oxford, UK: Blackwell Scientific Publications; 2001. p. 441e51. [28] Reed TB, Levie B. Understanding operating, and testing fixed bed gasifier. In: Bioenergy 0 84, Proceeding of World Conference, Goetborg, Sweden, June 21. Elsevier; 1985. [29] Burhenne L, Rochlitz L, Lintner C, Aicher T. Technical demonstration of the novel Fraunhofer ISE biomass gasification process for the production of a tarfree synthesis gas. Fuel Process Technol 2013;106:751e60.