Chok, Vui

Chua, Han

Gorin, Alexander

Wee, Siaw

Yan, Hong

English

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ISSN: 0128-7680Pertanika J. Sci. & Technol. 17 (2): 343 – 350 (2009) © Universiti Putra Malaysia PressReceived: 6 May 2008Accepted: 20 May 2008*Corresponding AuthorThe Effect of Effective Diameter on Fluidization Quality in Compartmented Fluidized Bed GasifierWee Siaw Khur*, Chok Vui Soon, Alexander Gorin, Chua Han Bing and Yan Hong MingCurtin University of Technology, CDT 250, 98009 Miri, Sarawak, Malaysia*E-mail: wee.siaw.khur@stud.curtin.edu.my ABSTRACTCompartmented Fluidized Bed Gasifier (CFBG), consisting of two compartments - the combustor and gasifier, uses air blown instead of pure oxygen for syngas production in bubbling fluidization mode, eliminating the need of air separation unit, and reducing the capital cost, thus distinguishes it from other traditional ones.  Fluidization quality is a determining factor in the CFBG to guarantee its well-lifted behaviour.  Previous study, without solid circulation at ambient conditions, brought to the fore the necessity of considering the effect of the minimum allowable effective diameter.  The study was then performed in the CFBG cold physical model of 0.66m overall diameter (effective diameter for combustor and gasifier is 0.413m and 0.257m) to investigate the fluidization quality and compare it with the results obtained from the previous cold model of about 1.36 times smaller, but with the same compartmented ratio of 65:35.  Different inert particles (river sand, quartz sand and alumina) were used, over a range of aspect ratios, for the aforementioned objective.  The results showed that the fluidization quality in the gasifier has not been achieved and the degradation of fluidization quality in the combustor is still observed, notwithstanding the fact that the condition of the minimum allowable effective diameter has been met.  The reduction of distributor free area, to increase the distributor pressure drop, showed a marginal effect on the quality.  The effect of the minimum allowable effective diameter on fluidization quality in CFBG as well as the interplay of geometric and operational parameters require further studies be carried out.  The fluidization quality of the binary mixture is also currently under investigation.Keywords: Channelling, Compartmented Fluidized Bed Gasifier (CFBG), effective diameter, fluidization, fluidization qualityINTRODUCTIONIt is of great importance to be able to estimate the quality of fluidization for an efficient and economic use of gas-fluidized beds when the operating parameters are changed.  An idealized fluidization consists of a completely lifted and uniform suspension of inert particles.  Fluidization quality, Q, is used as a distinctive indicator to show the fluidization characteristic in the compartmented fluidized bed gasifier (CFBG).  The CFBG is a unique compartmented reactor which consists of two reactors, namely the combustor and the gasifier, partitioned vertically inside at a ratio of 65:35, based on the heating requirement during the high-temperature operation.  The significance of the CFBG is that it uses air blown instead of pure oxygen for syngas production in bubbling fluidization mode, eliminating the need for air separation unit and thus reducing the capital cost, which distinguishes it from the traditional ones.  The idea of having compartmented reactors arises when the construction of several reactors is not economically viable, owing to the need of solids to be circulated through different Wee Siaw Khur, Chok Vui Soon, Alexander Gorin, Chua Han Bing and Yan Hong Ming344 Pertanika J. Sci. & Technol. Vol. 17 (2) 2009reaction zones in many gas-solid reactions.  In this paper, the combustor or gasifier compartment is neither a full cylindrical nor semi-cylindrical, but rather a segment of a cylindrical, as shown in Fig. 1. A few studies on non-cylindrical fluidized bed have been carried out elsewhere (Bhattacharya et al., 1999; Singh et al., 2005; He, 1993; Yan, 1995), but hitherto none has addressed the type of geometry in the CFBG.  This has thus invited a strong interest in the study of fluidization characteristics in the CFBG. Fig. 1: Isometric view of CFBG A previous study by Wee et al. (2007) brought to the fore the necessity of considering the effect of the minimum allowable effective diameter, without solid circulation at ambient conditions.  The study was carried out (in this paper) at the expanded CFBG cold physical model of 36% increment in the overall diameter, compared to the previous one.  The objectives of this paper were to investigate the fluidization quality, at each compartment, with different inert particles and sizes being used (river sand, quartz sand and alumina) for a range of aspect ratios as well as to compare it with the results obtained from the previous model.THEORYFluidization is defined as a process, in which particles are transformed into a fluid-like state, through contact with either a gas or a liquid.  The fluidization quality indicator, Q, is conventionally defined as the ratio of experimental bed pressure drop to the theoretical bed pressure drop, or in short, Q = Experimental ΔP/ Theoretical ΔP, where theoretical bed pressure drop is simply the weight of the particles over the cross-sectional area of the compartment. It is essential to find the effective diameter for the CFBG reactor of the non-uniform and uncommon shape.  Hence, the effective diameter, De (Nicholas et al., 1984) was representatively used in this study.       mean cross sectional area of flow channels through bed De = 4x  (1)   mean wetted perimeter of flow channelsThe Effect of Effective Diameter on Fluidization Quality in Compartmented Fluidized Bed Gasifier Pertanika J. Sci. & Technol. Vol. 17 (2) 2009 345Distributor free area is the percentage of the area which is occupied by orifices, as described in equation (2).                        Total Orificers Area Free area =   × 100%                           (2)  Distributor Plate AreaEXPERIMENTAL METHODThe experimental set up used in this study is shown in Fig. 2.  The CFBG is custom-made using mild steel as the body structure, and Perspex material as the cover to ease the visual observation during the experiment.  The overall diameter of the CFBG is 0.66m, with a height of 1.8m, and partitioned into two compartments, namely the combustor and the gasifier by a vertical diving wall in a ratio of 65:35, respectively.  Meanwhile, perforated plate distributor, with orifice diameter of 3mm was used to uniformly distribute the fluidizing agent, ambient air into the beds of the particle, at the free area of 0.27% and 0.32% in triangular pitch arrangement for gasifier and combustor accordingly.  The effective diameter for combustor and gasifier is 0.413m and 0.257m correspondingly.  Meanwhile, 100 micron mesh was employed on top of the distributor to avoid the particles from weeping through the orifices.  The air is regulated in the range of 1 – 2 minimum fluidization velocity, Umf, to maintain the bubbling mode of fluidization.  Water manometer is utilized to gauge the pressure drop across the beds and the distributors.Fig. 2: Experimental set up (1: compressor; 2: dryer; 3: pressure regulator; 4: rotameter; 5: plenum; 6: perforated plate distributor; 7: combustor; 8: gasifier; 9: water manometer)   The particles used in the experiment are mostly natural river sand.  However, alumina of different properties, and sized as a perspective heat carrier for pilot plant and quartz sand with free-impurities-surfaces and physical properties were tested on the quality of fluidization.  Their physical properties are tabulated in Table 1. Wee Siaw Khur, Chok Vui Soon, Alexander Gorin, Chua Han Bing and Yan Hong Ming346 Pertanika J. Sci. & Technol. Vol. 17 (2) 2009TABle 1Physical properties of particlesParticle River sand Alumina Quartz sand Quartz sandSize, dp 272 µm 360 µm 256 µm 362.5 µmDensity, ρp 2620 kg/m3 3992 kg/m3 2638 kg/m3 2638 kg/m3Theoretical (Wen & Yu) minimum fluidization velocity, Umf (Kunii, 1991)0.060 m/s 0.154 m/s 0.053 m/s 0.105 m/sRESULTS AND DISCUSSIONThe Effect of Effective DiameterFig. 3 compares the fluidization quality at different effective diameters, using river sand at bed height of 0.34m, over a range of free area from 0.27% to 0.32%.  The results were then compared with the ones obtained from Wee et al. (2007) who obtained De = 0.173m, 0.234m and 0.290m.  As can be observed from the graph, only the diameter of 0.290m was found to achieve good fludization quality, whereas the rest exhibited the opposite quality.  It was also expected that the diameter which was bigger than 0.290m achieved a good fluidization quality (Wee et al., 2007); however, the trend of De = 0.413m was notwithstanding the fact that the condition of the minimum allowable effective diameter had been met.  Further studies need to be done to verify the reasons contributing to it.  The flow behaviour of a gas solid fluidized bed is very complex and highly sensitive to scale (Nicholas et al., 1984).  Hence, the wall effect, which has a strong relationship with De, is a determining factor which leads to the fluidization quality.  Werther (1968) and other researchers showed that the wall effect became progressively more significant as the bed diameter was decreased.  As the diameter was decreased, the friction at the wall was found to support a larger and larger fraction of the weight of the bed, as a result of which the bed became more loosely packed (Srivastava et al., 2002).  This leads to a decrease in the internal friction and a corresponding decrease in the normal stress exerted at the wall, creating a preferential path at wall side, which facilitates the air to flow through it.  This eventually contributes to increasing bed pressure to drop with increasing air velocity which demonstrates the intermediate channelling behaviour (Nicholas et al., 1984). Profiles which are below 1.0 indicate channelling bed.  This offset suggests that the beds are not completely fluidized.The Effect of Effective Diameter on Fluidization Quality in Compartmented Fluidized Bed Gasifier Pertanika J. Sci. & Technol. Vol. 17 (2) 2009 347Fig. 3: Fluidization quality, Q versus U/Umf at different effective diameters (river sand, bed height = 0.34m)The Effect of Free AreaIn order to improve the fluidization quality, the study was done to investigate the effect of the distributor free area.  Fig. 4 shows the fluidization quality of different free areas, e.g. 0.23% and 0.32% at the aspect ratio of 1.09 and the river sand was used.  The higher free area shows a more satisfactory trend in relative to the smaller free area; nevertheless, the desirable quality of fluidization is still unachievable.  The graph shows the inability of the fluidized bed to be fully lifted by the incoming gas rate at Umf and obviously one can observe that channelling occurs in the system studied.  Channelling can be a result of the non-uniformities and inhomogeneities in the bed due to the perturbation, where there is a localized higher velocity through the bed, causing a localized expansion.  Thus, it changes the pressure drop through that portion of the bed.  Channelling becomes more pronounced if the localized pressure drop through the bed-distributor system decreases with the increased velocity (Robert, 1976).Fig.  4: Fluidization quality, Q versus U/Umf  at different distributor free areas (river sand, H/D = 1.09, De = 0.413m)Wee Siaw Khur, Chok Vui Soon, Alexander Gorin, Chua Han Bing and Yan Hong Ming348 Pertanika J. Sci. & Technol. Vol. 17 (2) 2009The Effect of Different ParticlesA comparison was also done between the alumina and quartz sand of identical size in the gasifier compartment, as shown in Fig. 5.  It was observed from the fluidization quality curve that the quartz sand showed a better behaviour as compared to alumina.  According to Wee et al. (2007), the physical properties of the particles play the key effect to the hydrodynamics of the bed. Fig. 5: Fluidization quality, Q versus U/Umf, using different particles (free area = 0.27%, H/D = 1.36, De = 0.257m) The shape and density of the particles are factors which affect channelling (Nicholas et al., 1984).  Channelling always takes place due to the appreciable cohesive force between the particles (Wang et al., 1997).  Thus, it is observed that the quartz sand and alumina shows through and intermediate channelling, respectively.  The through channelling can be distinguished when the Q is less than 1, whereas the intermediate channelling will have an increasing large fluctuation along the Q=1 (Wee et al., 2007).The Effect of Different Particle SizesA comparison of the different particle sizes was made in the gasifier compartment of shallow bed, as can be seen in Fig. 6.  None of the sizes showed a quantitatively good fluidization quality.  It was also observed that smaller size particles had a tendency to show through channelling, while larger ones show intermediate channelling.  Therefore, the increasing of size in diameter would result in a high propensity of intermediate channelling to occur in shallow bed.  This is because smaller size particle diameter leads to a less porous bed, leading to a more complex bed structure, and making it more difficult to fluidize and less bed expansion compared to larger size particles.The Effect of Effective Diameter on Fluidization Quality in Compartmented Fluidized Bed Gasifier Pertanika J. Sci. & Technol. Vol. 17 (2) 2009 349Fig. 6: Fluidization quality, Q versus U/Umf  at different particles size (free area = 0.27%, H/D = 0.78, De = 0.257m)The Effect of Aspect RatioFig. 7 shows the fluidization quality in relation to different aspect ratios, using river sand at the distributor free area of 0.27% and a diameter of 0.257m.  As can be observed from the graph, no distinctive differences are discernible in progression of the aspect ratio.  The average quality which can be obtained over the range of the aspect ratio is approximately 0.8. Fig. 7: Fluidization quality, Q versus U/Umf  at different aspect ratios (river sand, free area = 0.27%, De = 0.257m)Wee Siaw Khur, Chok Vui Soon, Alexander Gorin, Chua Han Bing and Yan Hong Ming350 Pertanika J. Sci. & Technol. Vol. 17 (2) 2009CONCLUSIONThe study was carried out to investigate the fluidization quality in the expanded CFBG cold model. One can summarize the findings as below:The expanded CFBG cold model, which results in a bigger combustor compartment (D1. e = 0.413m), does not achieve the desirable quality of fluidization, even when the diameter is bigger than the minimum allowable effective diameter (De = 0.290m), as obtained in the previous cold model.The effects of different distributor free areas, different particles used, different particle sizes 2. and different aspect ratios to the fluidization quality were also tested; the results showed similar results for all, i.e. the occurrence of channelling was consistent in both compartments. The effect of the minimum allowable effective diameter on fluidization quality in the CFBG and the interplay of geometric and operational parameters invite further study.  In addition, the fluidization quality of the binary mixture is currently under investigation.REFERENCESBhattacharya, B., Sathiyamoorthy, D., Govardhana Rao, V. and Mahajan, S.P. (1999). Solid circulation in a compartmented gas fluidized bed. Power Technology, 101, 191-204.He, Y. (1993). Hydrodynamics of a compartmented dense phase circulating fluidised bed. Ph.D. Dissertation, Dept. of Chemical Engineering, Univ. of Queensland, Australia.Kunii, D. and Levenspiel, O. (1991). Fluidization Engineering. USA: Butterworth-Heinemann.Nicholas, P.C. and Paul, P.C. (1984). Hydrodynamics of Gas-Solids Fluidization. Houston: Gulf Pub. Co. Robert, S. (1976). Effect of distributor plate-to-bed resistance ratio on onset of fluidized-bed channeling. AIChE Journal, 22(3), 590-592.Singh, R.K. and Roy, G.K. (2005). Prediction of minimum bubbling velocity, fluidization index and range of particulate fluidization for gas-solid fluidization in cylindrical and non-cylindrical beds. Powder Technology, 159, 168-172.Srivastava, A. and Sundaresan, S. (2002). Role of wall friction in fluidization and standpipe flow. Powder Technology, 124, 45-54.Wang, Z., Kwauk, M. and Li, H. (1997). Fluidizaton of fine particle. Chemical Engineering Science, 53(3), 377-95. Wee, S.K., Chok, V.S., Srinivasakannan, C., Chua, H.B. and Yan, H.M. (2007). Fluidization Quality Study in Compartmented Fluidized Bed Gasifier. Bio-energy outlook, Singapore.Werther, J. (1968). Influence of the bed diameter on the hydrodynamics of gas fluidized beds. AIChE Symposium Series, 141(70), 53-62.Yan, H.M. (1995). Development and modelling of coal combustion and gasification in a compartmented fluidised bed gasifier. Ph.D. Dissertation, Chemical Engineering Dept., Univ. of Queensland, Australia.

The Effect of Effective Diameter on Fluidization quality in Compartmented Fluidized Bed Gasifier

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The Effect of Effective Diameter on Fluidization quality in Compartmented Fluidized Bed Gasifier

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