FLUIDIZED BED GASIFICATION OF MARINE MICROALGAE IN A SPOUTED BED REACTOR

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1 FLUIDIZED BED GASIFICATION OF MARINE MICROALGAE IN A SPOUTED BED REACTOR Israa K. Alghurabie a,b, Brent Jackson a, Basim O. Hasan b, Adam Kosminski a, and Peter J. Ashman a a School of Chemical Engineering, University of Adelaide, AUSTRALIA b Department of Chemical Engineering, Nahrain University, IRAQ peter.ashman@adelaide.edu.au ABSTRACT The sustainable production of biofuels from marine microalgae shows enormous potential. While current research is focused on the cheap and sustainable production of algal lipids, another key technical question is the optimum process option for the residual biomass. One possibility is gasification of the biomass in a fluidized-bed to produce syn-gas which can be used for energy directly, converted to liquid hydrocarbons or converted to other valuable bulk chemicals such as methanol, etc. Possible impediments to this technology include the high concentration of alkali metals in the biomass, high moisture content and the small particle size. This paper presents preliminary attempts to gasify a marine microalga that has been sundried. The dried biomass was sieved to recover a size fraction in the range mm and this was gasified with air and steam in a spouted fluidized bed reactor. Early experiments, utilising the as-received biomass, proved unsuccessful due to rapid bed sintering and blockage of the downstream product lines. Leaching of the algal biomass to remove the extracellular salt was performed, however gasification of the resultant biomass also proved unsuccessful. Co-gasification of the leached biomass with Kingston coal, a low-rank coal from South Australia, at a biomass-to-coal ratio of 1:9 was also unsuccessful. In all cases it appears that rapid attrition of the biomass feed within the screw-feeder led to the almost complete elutriation of biomass from the reactor which resulted in the downstream blockages. INTRODUCTION Biomass has long been considered an attractive alternative energy source due to increasing fossil fuel prices and the effects of global warming. Biomass can produce electricity with the same type of equipment and power plants that now burn fossil fuels, unlike other renewable energy sources that need expensive new technology. However, low thermal efficiency and high cost, amongst other issues, has delayed the widespread implementation of biomass energy and the major challenge now is to develop low-cost high-efficiency systems (Lim and Alimuddin, 2008). Marine microalgae have recently received a lot of interest as a new and potentially sustainable biomass source for the production of renewable energy using non-arable land and non-potable water. Microalgae utilise carbon dioxide and nutrients to convert solar energy to chemical energy via photosynthesis (Wagner, 2007; Wen and Michael, 2009). Some of the major characteristics which set microalgae apart from other biomass are that algae can have high areal productivities, and thus high biomass yield per unit land area, and high lipid content (Chisti, 2007; Rodolfi, et al., 2008). Many of the current technologies being pursued focus on the production of lipids, for the manufacture of biodiesel or aviation fuel, with residual microalgae biomass as a byproduct that must be dealt with. One option is to gasify the residual biomass to produce syn-gas which can be used directly for energy production, converted to liquids fuels, via a Fischer-Tropsch synthesis, or converted to other relatively high-value chemicals. The technology of choice for biomass gasification is fluidized bed gasification. However, the gasification of marine microalgae biomass in fluidized beds is

2 expected to be challenging due to its high salt content leading to operational problems such as fouling, and also to agglomeration and defluidisation of the fluidized bed gasifiers. In the present work gasification experiments were first attempted using marine microalgal biomass in a fluidized bed. Further experiments were also conducted to investigate the cogasification of a blend of 10 wt% microalgal biomass with South Australian low-rank coal. All of these experiments were performed in a spouted fluidized bed gasifier. Producer gas compositions were analysed and compared to those from coal gasification alone. Samples of bed material were also examined to identify the major ash components. MATERIALS AND METHODS Reactor description For the gasification experiments, a fluidized spout bed reactor was used. A schematic of the reactor is shown in Fig. 1. Air was pre-heated to o C and fed to the base of the reactor, while the fuel was admitted using lock hoppers at the top of the reactor. The product gas was combusted using a flare burner prior to discharge. Water, metered at a rate of 0.33 kg.hr -1, was vaporized in the hot air stream (35 l.min -1 at NTP) before the mixture was passed through a bed of 0.6-mm silica sand (200 g). N 2 K J L I P H M N Fluid -Bed- Furnace control panel T G PC C A F P E D Compressed Air H 2 O/ Steam Fig. 1: Fluidized bed gasifier rig setup. Legend: A: air flow meter; C: air heater; D: water flow meter; E: bed material discharge cylinder; F: distributor; G: temperature controllers; H: pressure transmitters; I: sampling port; J: spouted column; K: ash collector; L: fuel hopper; M: motor feeder; N: furnace. Rig setup The gasifier is constructed of stainless steel. At the base it has a conical section with an inside diameter of 12 mm which expands to a cylindrical section of 77 mm inside diameter. The gasifier is 1.2 m high. The bed material was added from the top of the gasifier, after removing a movable plate, and the fuel was added, via a set of lock hoppers, using a screw feeder. 2

3 A mixture of air and steam was used as the gasification agent; compressed air was passed through a Leister electric hot air tool (Type 5000) and water (flow rate measured using a rotameter) was fed directly into the hot air stream where it vaporized. The air/steam mixture was passed through a stainless steel mesh into the gasifier; this mesh was attached to a removable pipe, which can be used to recover the bed material after the run. The heat loss to the surroundings was controlled using ceramic brick insulation which enclosed both the gasifier and heating elements. Once the reactor reached a stable temperature, two of the heating elements were switched off and the reactor temperature was controlled using the air and steam flow rates. The gas feed to the reactor was also heated to the required temperature, which can be manually controlled to vary the heat input to the bed. The fuel is fed, at a location approx. 550 mm above the entrance to the conical base, by a pair of lock-hoppers and a screw feeder. The coal hoppers are back-pressured using nitrogen gas to provide an inert atmosphere within each of the hoppers. The fuel feed line was water cooled near the entrance to the reactor to prevent the fuel heating up in the screw feeder. Thermocouples type-k (3 mm outer diameter) are attached to the reactor to measure the temperature at various locations across the bed. Thermocouples are labeled T 1 through to T 4 with T 1 located just below the conical distributer and T 2, T 3 and T 4 are located 35, 65, and 105 mm above the gas inlet respectively. The bed pressure drop is measured using pressure tappings located at 15 mm below (P 1 ) and 190 mm above (P 2 ) the gas inlet. Experimental procedure Algae gasification Experiments were attempted with a fuel mass rate of 1.43 kg/h, steam-to-fuel (S/F) ratio of 0.5 and bed temperature of 850 C. The steam-to-fuel ratio was calculated using Eq. (1). This accounts for both the water in the fuel and the water added as steam. where, F S is the flow rate of steam into the bed (kg/h), m c is the mass flow rate of fuel (wet basis) into the bed (kg/h) and M C is the moisture content of the fuel (kg H 2 O/kg wet fuel). Algae biomass was dried and sieved before each experiment. An amount of sand (200 g) was added to the gasifier before the gasifier was heated to 450 C (via the combined actions of the external heating elements and the air pre-heater) with an air flow rate of 70 L/min. The air flow rate was then reduced to 35 L/min and the fuel feed started. Initially the bed temperature increased to 750 C and then, after 10 minutes, it decreased to between 620 and 600 C and stabilized. Based on this abnormal operation, it was suspected that defluidisation had and so the gasifier was shut down and left to cool. Visual inspection of the bed material confirmed the presence of agglomerates. Samples of agglomerate and bed material were analyzed by SEM to determine their structure and composition. Co-gasification Co-gasification experiments were completed using 90% coal and 10% algae to study the mechanism of gasification reactions present for this fuel composition, and to investigate whether or not agglomeration would occur with a lower flow of biomass into the gasifier. The gasifier was operated with air-to-fuel and steam-to-fuel ratios of 2 and 0.5, respectively, at a temperature of 820 C. After beginning the fuel feed the temperature increased rapidly to 820 C, gas samples were drawn at different periods (30, 45, 60, 75, and 90 minutes). The bed temperature continued to rise steadily to 885 o C and the gasifier was then shut down to (1) 3

4 prevent damage to the reactor. Note that it is a feature of this reactor that during defluidisation events the measured bed temperature increases rapidly. Materials A sample of marine microalgae, Tetraselmis sp., was obtained from the Muradel Pty Ltd pilot plant located in Karratha. The biomass was grown in 200m 2 raceway ponds, under a range of hypersaline conditions, during commissioning work at the pilot plant. The biomass was harvested and placed in the sun to dry before being frozen and shipped to Adelaide. The biomass was thawed and dried further prior to sieving. Two size fractions of dried algal biomass were used: mm and mm. Low-rank coal from the Kingston deposit in South Australia was mixed with 10% algae biomass for the co-gasification experiments. The coal composition and ash analysis results are shown in Table 1. The coal was dried and sieved to a particle diameter of mm. Silica sand (200 g) was sieved to 0.6 mm diameter and used as the bed material for each run. Gas analysis Gas samples were collected in sealed bags from the gas sample valve shown in Fig. 1. each sample was then analysed used a twin-column Agilent 3000 Micro GC (PoraPLOT Q on channel A and Molecular Sieve 5A with PoraPLOT U pre-column on channel B). Samples are preheated to 90 C by the GC prior to column injection. Carrier gases are helium (Channel A) and argon (Channel B). Cerity NDS for Chemical QA/QC software was used to analyze the samples and calculate molar concentrations from the peaks obtained. The concentration of nitrogen in the gas sample is used as a tracer to quantify the molar flows of all products. In this calculation, the molar flow of fuel nitrogen is negligible as compared to the flow of molecular nitrogen introduced with the air reactant and thus is ignored. The water vapor in the product stream was calculated using an oxygen balance over the reactor, assuming that all of the oxygen present in the air reactant is consumed within the reactor. Table 1: Coal compositional information (daf = dry, ash-free) Proximate Analysis (wt%) Composition Moisture (as received) 12-18% Volatile matter (dry basis) 45.7 Fixed carbon (dry basis) 38.7 Ash yield (dry basis) 15.6 Ultimate Analysis (wt%, daf) C 56.4 H 4.3 S 2.9 Na 0.93 Cl 0.16 N 0.6 O 23.4 Ash Analysis (wt% in ash) SiO Al 2 O Fe 2 O TiO K 2 O 0.5 MgO 14.6 Na 2 O 5.7 CaO 12.4 SO

5 RESULTS AND DISCUSSION Algal gasification Immediately upon commencement of the fuel feed, the bed temperature increased to 750 C and then, after about 10 minutes, was observed to decrease to between 620 and 600 C and stabilize. The bed material was found to have agglomerated in this case and so the gasifier was shut down and left to cool. Samples of the agglomerates were analysed using SEM and typical images are shown in Fig. 2 and Fig. 3. The composition of the agglomerate at the point marked in Fig. 2 and Fig. 3 is reported in Table 2 and Table 3, respectively. Images of agglomerates were also collected using the SEM in backscattered electron (BSE) mode and these are shown in Fig. 4 and Fig. 5 with the composition at the various locations in these figures reported in Table 4 and Table 5, respectively. A sample of the bed material for this run was also analyzed by SEM-EDS to determine the composition of this material. SEM images of the bed material are shown in Fig. 6 with the composition of the sample at the marked points reported in Table 6. Element O Na 4.97 Mg 1 Al Si 3.11 S 1.89 Cl K 1.61 Ca 2.24 Fe 0.97 Total 100 Fig. 2: Micrograph of SEM secondary electron image for a section of the agglomerated bed Table 2: Agglomerate composition at the spot marked in Fig. 2 Fig. 3: Micrograph of SEM secondary electron image for a section of the agglomerated bed material Element O Na Mg 7.42 Al 6.12 Si 2.06 P 1.3 S 2.84 Cl K 3.97 Ca 2.76 Fe 3.29 Table 3: Agglomerate Total 100 composition at the spot marked in Fig. 3 5

6 Fig. 4: Micrograph of a SEM backscattered image of a section of agglomerate Element Sec.1 Sec.2 Sec.3 O Na Mg Al Si P S Cl K Ca Total Table 4: Composition of the sample at points marked 1, 2, and 3 in Fig. 4. Element Sec.1 Sec.2 O F 0 0 Na Mg Al Si P S Cl K Ca Total Fig. 5: Micrograph of a SEM backscattered image of a section of agglomerate Table 5: Composition of the sample at points marked 1 and 2 in Fig. 5. A B Fig. 6: Micrograph of SEM secondary electron image of the bed material A, B: Sections in the bed material 6

7 Element Fig. 6A (wt%) Fig. 6B (wt%) O Na Mg Al Si P S Cl K Ca Fe Total The previous Figures show an excess amount of sodium, aluminium and magnesium Table 6: Compositions of the materials at the points marked in Fig. 6A and Fig. 6B, respectively These data show an excess amount of sodium, aluminium, magnesium, chloride ion present in the examined samples. These would be chloride salts present definitely due to the seawater being used to grow the algae. The salt is likely to have crystallized during the process of drying the algae. Relatively pure silica oxide is also observed in examined samples, what reflects the presence of the sand used for the bed material. Agglomeration of the bed material was therefore caused by the excess salt present in the algae. To test for the level of salt present in the algae a sample was immersed in de-mineralized water (conductivity = 0) and agitated. After 3 hours the conductivity of the microalgae solution was measured giving a value of 13.3 ms. The water was replaced three times to insure all the salt was dissolved and then the solution was left to settle after which the supernatant was removed and the remaining material left to dry by sun. The algae was then put into the oven to further dry to a moisture content less than 20% (for further analysis and for using in co-gasification experiments). The dried algae was using SEM with the Secondary Electron (SE) detector, both before (Fig. 7) and after (Fig. 8) leaching. The corresponding spot analyses (Table 7 and Table 8, respectively) show that the NaCl content, relative to the other elements present, decreased substantially due to leaching. A basic analysis was completed to calculate the volatile, salt, and ash components in algae. This was done by placing a known weight of dried leached algae into a furnace at 840 C for three hours. This combusted all volatile matter leaving behind any non-volatiles, the crucible was then taken out of the oven and then weighed. The sample was then added to de-mineralized water and the conductivity and salinity were measured to calculate the salt content of the non-volatile matter (which was found to be 3.59 %). From this the ash content was found to be approximately 38%. 7

8 Fig. 7: Micrograph of SEM secondary electron image of raw algae before leaching Element Sec.1 Sec.2 O F Na Mg 0 0 Al Si P S Cl K Ca Total Table 7: Composition data of the contents in raw algae before leaching, section 1, and 2 in Fig. 7 Element Wt% O 45.8 Na 3.01 Mg 6.38 Al Si 1.41 P 1.91 S Cl 8 K 1.55 Ca 9.48 Total 100 Fig. 8: Micrograph of SEM secondary electron image of raw algae after leaching Table 8: Composition data of the contents in raw algae after Co-gasification results The leached algae was co-gasified with Kingston coal in the ratio of 10 wt% algae to 90 wt% coal. After beginning the fuel feeding the temperature increased rapidly to 820 C, gas samples were drawn at different periods (30, 45, 60, 75, and 90 minutes) and during these periods the bed temperature was increased from 830 to 840, 860, 870, and 880 C over 15 minute intervals, the bed temperature continued to rise steadily until the gasifier had to be shut down due to excessive temperature. Figure 9 reports the mole fraction of H 2, CO 2, CH 4 and CO in the producer gas, as a function of bed temperature, during this test. 8

9 Fig. 9: Producer gas composition (vol%) measured from the co-gasification experiment Agglomeration did not occur however the bed pressure drop and bed temperature increased to high values of approx. 500 Pa and more than 900 o C, respectively, after 90 minutes. This might have resulted from decreased bed fluidization, or it could have been due to blockages of the gasifier downstream which prevent part of the produced gas from leaving the gasifier and cause high levels of temperature and pressure inside the gasifier. After the gasifier was cooled down, the bed material was recovered and there were no agglomerates found. The tubes for passing the producer gas were disconnected and found to be blocked with grey/black ash (Fig. 10). A sample of the gray material blocking the pipes was taken and the salt content was tested. The salt level in the blockage was the same in the raw algae which suggests that blockage could be attributed to the presence of raw algae. It is inferred that the dried algae, which was relatively soft and friable, may have been crushed in the screw feeder and entered the gasifier as fine dust, rather than as discrete particles with size of mm. In which case, the fine dust would have been elutriated from the bed and carried over into the gas outlet of the gasifier where they became trapped. This led to a high pressure drop across the gasifier and failure of the bed to fluidize. CONCLUSIONS Figure 10: Blockage in the outer tubes of the gasifier Gasification of microalgae was attempted in a 77 mm I.D. spouted bed gasifier. Agglomeration of the bed occurred with dried algae as the feedstock, apparently due to the high salt content of the raw algae. To avoid this leaching of the raw algae was done to 9

10 dissolve the salt content. Co-gasification of coal and the leached algae was then performed. The temperature of the bed was unstable and increased rapidly with a high pressure drop across the gasifier due to blockages of the downstream gas outlet which caused by the elutriation of biomass which had been crushed in the screw feeder. In future work, to avoid this problem the feeder could be relocated to within the bed to decrease the possibility of elutriation. Further work is underway to resolve these operational problems. ACKNOWLEDGEMENTS This research was supported under Australian Research Council's Linkage Projects funding scheme (project number LP ) with our industry partner SQC Pty Ltd. Ms Alghurabie acknowledges financial support from the Ministry of Higher Education and Scientific Research, Iraq. REFERENCES Chisti, Y. (2007). Biodiesel from microalgae, Biotechnology Advances, 25, Choi, Y. C., Li, X. Y., Park, T. J., Kim, J. H., and Lee, J. G. (2001). Numerical study on the coal gasification characteristics in an entrained flow coal gasifier, Fuel, 80, Lim, M. T., and Alimuddin, Z. (2008). Bubbling fluidized bed biomass gasification performance, process findings and energy analysis, Renewable Energy, 33, Rodolfi, L., Zittelli, C., G., Bassi, N., Padovani, G., Biondi, N., Bonini, G., and Tredici, M., R. (2008). Microalgae for oil: strain selection, induction of lipid synthesis and outdoor mass cultivation in a low-cost photobioreactor. Biotechnology & Bioengineering, 102, Sharma, A.K., Equilibrium modeling of global reduction reactions for a downdraft (biomass) gasifier. Department of Mechanical Engineering, Deen Bandhu Chhotu Ram University of Science and Technology: Murthal, Haryana, India, (2007). Wagner, L., Biodiesel from algae oil, research report, July (2007). Wen, Z., and Michael B., J., Microalgae as a Feedstock for Biofuel Production, College of Agriculture and Life Sciences, Virginia Polytechnic Institute and State University, Publication , (2009). 10