A novel microalgal system for energy production with nitrogen cycling

T. Minowa*, S. Sawayama

National Institute for Resources and Environment, Onogawa 16-3, Tsukuba, Ibaraki 305-8569, Japan

Accepted 4 March 1999

Abstract

A microalga,Chlorella vulgaris, could grow in the recovered solution from the low temperature catalytic gasification of itself, by whichmethane rich fuel gas was obtained. All nitrogen in the microalga was converted to ammonia during the gasification, and the recoveredsolution, in which ammonia was dissolved, could be used as nitrogen nutrient. The result of the energy evaluation indicated that the novelmicroalgal system for energy production with nitrogen cycling could be created.q 1999 Elsevier Science Ltd. All rights reserved.

Keywords:Microalgae; Biomass; Energy production system; Low temperature catalytic gasification

Global warming has become a serious environmentalissue, and biomass, a renewable and carbon-neutral energysource, has been focused on as an alternative energy source[1]. Since microalgae can grow rapidly (10–30 g dry-cell d21 m22) [2], they are one of the promising biomasssources. However, it is a question whether energy can beproduced from microalgae or not [3], because microalgaerequire a greater amount of nutrient rather than higherplants, that is, energy intensive cultivation. In addition,microalgae have a high moisture content (only 0.5–1 gdry-cell/l), and the conventional thermochemical methods:incineration, pyrolysis, gasification, and so on, require a dryfeedstock. Since drying is an energy intensive process,microalgae are not an attractive feedstock for conventionalthermochemical methods. Recently, Elliott et al., havedeveloped a low temperature catalytic gasification ofbiomass with a high moisture content[4]. We have studiedand proposed a reaction mechanism for this gasificationprocess [5]. Biomass with high moisture is gasified directlyto methane rich fuel gas without drying. In addition, nitro-gen in the biomass is converted to ammonia during thereaction. If microalgae grow in the recovered solution, inwhich ammonia is dissolved, nitrogen, a major nutrient, canbe cycled, and the energy input for nutrients can decrease.Here, we show the low temperature catalytic gasification ofa microalga,Chlorella, and its cultivation in the recoveredsolution to propose a novel microalgal system.

Chlorella vulgaris, a type of commercially available

green microalgae in the market, was used for the gasifica-tion experiments. It had 87.4% of moisture and 21.3 MJ/kgdry-cell of gross calorific value. Its elemental compositionwas carbon 48.9%, hydrogen 6.8%, oxygen 31.3%, nitrogen6.9%, and ash 6.1% on a dry solid basis. The reaction wasperformed in a conventional stainless steel autoclave(120 cm3 capacity) with a magnetic stirrer. About 30 wet-g of C. vulgarisand the desired amount of a commercialnickel catalyst (Engel-hard, NI-3288; 50 wt% on silica–alumina, 74–250mm) were loaded into the autoclave.Nitrogen gas was used to purge the residual air in the auto-clave, and it was added at 3 MPa to avoid vaporization ofwater during the reaction. The reaction was started by heat-ing the autoclave using an electric furnace. When thetemperature in the autoclave reached 3508C, it was imme-diately cooled down to room temperature. It took about35 min to heat the autoclave to 3508C and about 20 min tocool down to lower than 1008C, and the pressure in theautoclave was elevated to about 18 MPa at 3508C. Aftercooling down, the product gas was removed to a samplingbag for analysis. The volume of the gas was measured usinga gas meter (Shinagawa-sheiki, W-NK-0.5), and its compo-sition was determined by gas chromatography (Shimadzu,GC-12A with a TCD detector for inorganic gases and GC-9A with a FID detector for hydrocarbon gases). Table 1shows the gas yield on a carbon basis and the gas composi-tion at different catalyst loadings. With increasing catalystloading, the gas yield increased and the gas compositionapproached the equilibrium composition. Complete conver-sion would be possible at a higher reaction temperature orwith a larger amount of catalyst.

The treated solution was separated from the catalyst by

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* Corresponding author. Tel.:1 81-298-58-8183; fax:1 81-298-58-8158.

E-mail address:minowa@nire.go.jp (T. Minowa)

filtration. It was colorless and transparent, and had littleorganic material (total organic carbon of 258 mg/l). Itsammonia concentration was about 9000 mg/l for Runs 2and 3, and this value shows that all nitrogen in the cells ofC. vulgarisis converted into ammonia. The recovered solu-tion from Run 3, diluted to 300 times, was used for thecultivation of Chlorella vulgaris Beijerinck [6], obtainedfrom the Microbial and Microalgal Research Center, Insti-tute of Applied Microbiology, The University of Tokyo.The cell pellet ofC. vulgaris was added to 60 ml of themedium, and it was cultivated at 258C and 40–60mE s21 m22. Distilled water and a standard culturemedium (Chlorella Ellipsoidea Medium [6]) was also usedas a control. The growth ofC. vulgariswas measured asoptical density at 660 nm (Shimadzu, UV-160A). Table 2shows the growth after 1 and 2 weeks. While it could notgrow in the distilled water,C. vulgariscould grow in therecovered solution: 10 times growth after 1 week and 30times growth after 2 weeks. The growth, however, wasone eighth of that for the standard culture medium. Thiswas due to the effect of lack of nutrients such as phosphoruswith the exception of nitrogen. Therefore, the cultivation ina mixture of the recovered solution and nitrogen-lack stan-dard culture medium was performed. Growth for the mixedmedium was comparable to that for the standard culturemedium. Thus, it is concluded that cultivation in the recov-ered solution is possible and no supplement of nitrogen, atleast, is needed.

Energy balance for the total system was evaluated asshown in Table 3. The simplest conversion method fromalgal biomass to energy is incineration. In case of incinera-tion, the energy required for the production of nutrients ismore than half the total required energy, and the obtained

energy is only 11 MJ/kg dry-cell due to high moisturecontent. In addition, the form of the obtained energy isheat, and, therefore, it is questionable whether the incinera-tion of wet microalgae produces net energy, because theform of the required energy for cultivation and concentra-tion is electric power and the conversion rate (0.3–0.4) fromheat to power should be considered. However, in case of lowtemperature catalytic gasification, the increase in theobtained energy is higher than the heating energy for gasi-fication, and the energy required for nutrients can be cutdown. In addition, both power and heat can be made from

T. Minowa, S. Sawayama / Fuel 78 (1999) 1213–12151214

Table 1Gasification ofC. vulgaris

Catalyst loading Carbon conversion to gas Gas composition (vol%)

(g) (%) CH4 H2 CO2 Others

Run 1 5 35.0 15.6 34.9 46.2 3.3Run 2 10 62.0 27.0 25.5 43.5 4.0Run 3 15 70.1 37.5 10.0 48.8 3.7Equilibrium Calculationa 100 49.7 5.9 44.4 -

a Calculated by a program software for chemical equilibrium using JANAF thermochemical table.

Table 2Growth ofC. vulgarisin different media (optical density at 660 nm)

Cultivation Distilled water Standardculture medium

Recoveredsoultion

Solution1

standard (2 N)

0 day 0.011a (0.002b) 0.011 (0.002) 0.009 (0.001) 0.009 (0.002)1 week 0.025 (0.016) 0.655 (0.061) 0.087 (0.040) 0.649 (0.051)2 weeks 0.022 (0.007) 2.111 (0.109) 0.263 (0.003) 1.796 (0.098)

a Average.b Standard deviation.

Table 3Energy evaluation (MJ/kg dry-cell)

Incineration Low temperaturecatalyticgasification

Energy production 11.09a 17.77b

Energy consumptionNutrientc 4.55 1.54Cultivationd 2.15 2.15Concentratione 0.85 0.85Gasificationf - 5.95Subtotal 7.55 10.48Net energy production 3.55 ? 7.29

a Heat (net calorific value of wet algal cells).b Net calorific value of produced gas (assumption of the completed

gasification).c N, 43.2 kJ/g; P, 15 kJ/g; K, 9 kJ/g [7]; and loss of N during cultivation,

0.33.d Assumption of raceway ponds [8].e Centrifuges, 0.1–2%, 80 kW h/dry-ton, 2–20%, 3.1 kW h/m3 [9].f Gasification at 350(C and 18 MPa; heat recovery, 0.5; and energy

efficiency, 0.6.

the produced gas. These results indicate that the novelmicroalgal system could be created, and this techniquecould be applied to not onlyChlorella, but also other micro-algae and water plants. The development of the process suchas microalgal screening, catalyst modification, optimumreaction and culture conditions, process design, and so onis planned in a future work.

Acknowledgements

The authors thank Ms Tae Kimura of the National Insti-tute for Resources and Environment for her help with thecultivation.

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