Method for producing fuel

Abstract

To provide a method for producing fuel by utilizing carbon dioxide separated and recovered from various commercial facilities, industrial facilities, or incineration facilities as a carbon source, while ensuring rapidity, simplicity, and reduction of energy cost.SOLUTION: A method for producing fuel is provided, where carbon dioxide separated and recovered from commercial facilities, industrial facilities, or incineration facilities is supplied into a culture tank containing microalgae and a culture solution for culturing the microalgae, and the microalgae are irradiated with light to culture the microalgae, while generating sugar through photosynthesis by the microalgae. When the concentration of sugar in the culture tank increases over time, with the maximum value of the rate of increase in the concentration of sugar generated in the culture tank being defined as vmax, the culturing of the microalgae is stopped during a period from a point in time when the rate of increase in the concentration of sugar generated in the culture tank reaches 0.8×vmax in the course of rising over time until a point in time when the concentration of sugar generated in the culture tank reaches a maximum value.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing fuel using recovered carbon dioxide as a carbon source. [Background technology]

[0002] Carbon dioxide has traditionally been emitted from various furnaces and boilers, including blast furnaces, lime kilns, heating furnaces, reactors, incinerators, boilers, etc. installed in various industrial facilities such as power plants, steel mills, cement plants, oil refineries, and chemical plants, incinerators, boilers, etc. installed in various commercial facilities such as hotels, commercial buildings, hot spring facilities, and heated swimming pools, and incinerators installed in incineration facilities of local governments, etc.

[0003] On the other hand, in response to the recent demand for reducing carbon dioxide emissions to combat global warming, there is a growing demand for carbon recycling, which involves separating and capturing the carbon dioxide, and then using the captured carbon dioxide as a carbon source to produce fuel oil and reuse (recycle) it.

[0004] Specifically, currently proposed carbon recycling methods include: (1) a carbon dioxide separation and capture step in which carbon dioxide is separated and captured from various commercial facilities, industrial facilities, or incineration facilities; (2) a microbial culture step in which lipids including various fats and oils (triglycerides) are produced by culturing microorganisms using the captured carbon dioxide as a carbon source; and (3) a fuel oil base material production step in which the obtained lipids are subjected to hydrocracking or hydrotreating to produce a fuel oil base material, thereby reusing (recycling) the recovered carbon dioxide as a carbon source as a fuel oil base material.

[0005] As an embodiment of the above (1) carbon dioxide separation and recovery step, for example, a method can be considered which carries out an absorption step in which exhaust gas emitted from various commercial facilities, industrial facilities, or incineration facilities is brought into contact with an absorbent made of an aqueous solution of an alkanol (amine compound) to cause absorption and reaction, thereby obtaining a carbon dioxide-rich absorption liquid in which carbon dioxide has reacted with the absorbent, and a regeneration step in which carbon dioxide is obtained by desorbing carbon dioxide from the carbon dioxide-rich absorption liquid obtained by carrying out the absorption step, and at the same time, the absorbent is regenerated (see Patent Document 1 or Patent Document 2, etc.).

[0006] Furthermore, as for the above-mentioned (2) microbial culture step, currently, various methods for producing lipids including various oils and fats by culturing microorganisms using carbon dioxide as a carbon source are being investigated (see, for example, Non-Patent Document 1 or Non-Patent Document 2).

[0007] Furthermore, in the above-mentioned (3) fuel oil base material production step, the lipids obtained by the above-mentioned microbial culture are subjected to hydrocracking or hydrogenation treatment, thereby converting the fats and oils contained in the lipids into paraffins (chain saturated hydrocarbons), and a fuel oil base material containing paraffins as the main component can be produced.The obtained fuel oil base material is obtained by reusing the recovered carbon dioxide as a carbon source, and does not contain ester compounds such as fats and oils, but is composed mainly of paraffins like petroleum-based base materials, so it is expected to be suitable for use as a fuel oil base material for jet fuel, etc. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-225537 [Patent Document 2] International Publication No. 2018 / 190104 [Non-patent literature]

[0009] [Non-Patent Document 1] Bioresource Technology 129(2013), pp. 150-155, “Sequential accumulation of starch and lipid induced by sulfur deficiency in Chlorella and Parachlorella species” [Non-patent document 2] Biotechnology for Biofuels(2016)9:13, “Highly effective lipid production in the green alga Parachlorella kessleri:genome and transcriptome endorsed by whole-cell 3D ultrastructure” DISCLOSURE OF THE INVENTION [Problem to be solved by the invention]

[0010] However, first, in the (2) microbial culture step, when lipids containing various fats and oils (triglycerides) are produced by culturing microorganisms using the recovered carbon dioxide as a carbon source, a long culture period of about three weeks to several months is usually required to produce the lipids. Furthermore, in the (2) microbial culture step, when the lipids are produced by culturing microorganisms using the recovered carbon dioxide as a carbon source, the resulting culture solution must be recovered and sequentially subjected to solid-liquid separation and drying treatment, after which the lipids accumulated within the microbial cells must be extracted with an organic solvent or the like. For this reason, in the carbon recycling methods proposed so far, (2) the microbial culture step requires a long period of time and is complicated.

[0011] Furthermore, in the above method, when a fuel oil basestock is produced by hydrocracking or hydrotreating lipids containing various oils and fats in the (3) fuel oil basestock production step, an expensive hydrocracking or hydrotreatment device that can be used under high temperature and high pressure is required. It is also possible to repurpose existing oil refining facilities as the above-mentioned hydrocracking treatment unit or hydrotreatment unit. In this case, however, it would be necessary to transport the recovered carbon dioxide to a remote oil refinery (oil refinery) and perform the (2) microbial culture step and (3) fuel oil base material production step within the oil refinery, or to transport the culture solution obtained in the (2) microbial culture step to a remote oil refinery (oil refinery) and perform the (3) fuel oil base material production step. This would require a lot of time and effort to recover or transport the carbon dioxide or culture solution to be treated. In addition, in the first place, the step (3) of producing fuel oil base stock requires considerable energy and production costs for hydrocracking or hydrotreating.

[0012] Under these circumstances, an object of the present invention is to provide a method for producing fuel quickly, simply, and at reduced production costs, using carbon dioxide separated and recovered from various commercial facilities, industrial facilities, or incineration facilities as a carbon source. [Means for solving the problem]

[0013] In order to solve the above problems, the present inventors have focused on the step (2) of culturing microorganisms in the above method. That is, in the above-mentioned (2) microbial culture step, (a) in an environment where there is sufficient light, water, carbon dioxide, and nutrients, microorganisms grow and their population increases rapidly, and sugars such as starch are produced within the microbial cells by photosynthesis; then, (b) as the population of the microorganisms increases, the culture conditions change (causing a deficiency of nitrogen or sulfur components, etc.), the growth rate of the microorganisms slows, and the sugars produced within the microbial cells are converted into various lipids.

[0014] In this case, the inventors came up with the idea of ​​significantly simplifying or speeding up the entire production process consisting of production steps (1) to (3) in the conventionally proposed carbon recycling methods, by stopping the culture of the microorganisms during the process of producing sugars by photosynthesis in the (2) microorganism culture step, thereby limiting the conversion of the produced sugars to lipids, thereby shortening the culture period and eliminating the need for lipid extraction. Furthermore, by limiting the conversion of the sugars to lipids, the (3) fuel oil base material production step, in which the obtained lipids are hydrocracking or hydrotreating, is also unnecessary.

[0015] The inventors then discovered that by stopping the cultivation of the microorganisms during the process of producing sugar through photosynthesis, and appropriately drying the resulting culture solution, and recovering the microorganisms containing sugars, and using them as fuel for power generation or boilers, for example, it is possible to quickly, simply, and at low production costs, while also effectively reusing carbon dioxide separated and recovered from various commercial facilities, industrial facilities, or incineration facilities and utilizing it as fuel, and based on this finding, the present invention has been completed.

[0016] That is, the present invention is (1) Carbon dioxide separated and recovered from a commercial facility, industrial facility, or incineration facility is supplied to a culture tank containing microalgae and a culture solution for cultivating the microalgae, and light is irradiated onto the microalgae to cultivate the microalgae and produce sugars through photosynthesis by the microalgae; When the sugar concentration in the culture tank increases over time, The maximum rate of increase in the concentration of sugar produced in the culture tank is v max When the rate of increase in the concentration of sugar produced in the culture tank increases over time, the rate of increase is 0.8 × v max The cultivation of the microalgae is stopped between the time when the concentration of the sugar produced in the culture tank reaches a maximum value and the time when the concentration of the sugar produced in the culture tank reaches a maximum value. A method for producing a fuel, (2) When the sugar concentration in the culture tank increases over time, the maximum rate of increase in the sugar concentration produced in the culture tank is defined as vmax When the sugar concentration is increased over time, the rate of increase is 0.9 × v max The method for producing fuel according to (1) above, wherein the cultivation of the microalgae is stopped between the time when the concentration of sugar produced in the culture tank reaches a maximum value and the time when the concentration of sugar produced in the culture tank reaches a maximum value. (3) The method for producing fuel according to (1) or (2) above, wherein the microalgae is one or more selected from Chlorella, Nannochloropsis, Neochloris, Euglena, Spirulina, Dunaliella, Haematococcus, Pseudococomyxa, Chlamydomonas, Botryococcus, Scutellaria, and marine diatoms. (4) The method for producing a fuel according to any one of (1) to (3) above, further comprising supplying oxygen into the culture tank. (5) The method for producing fuel according to any one of (1) to (4) above, wherein after the cultivation of the microalgae is stopped, the obtained culture solution is subjected to solid-liquid separation and then dried. This provides: [Effects of the Invention]

[0017] According to the present invention, when culturing microalgae while supplying carbon dioxide separated and recovered from a commercial facility, industrial facility, or incineration facility, the cultivation of the microalgae is stopped during the process of producing sugars by photosynthesis, thereby limiting the conversion of the produced sugars to lipids, shortening the cultivation period, and eliminating the need for lipid extraction procedures. Furthermore, according to the present invention, by limiting the conversion of the sugars to lipids, the step of hydrocracking or hydrogenating the obtained lipids is also unnecessary, which can significantly simplify the entire production process, increase the processing speed, and significantly reduce production costs. Therefore, according to the present invention, it is possible to provide a method for producing fuel quickly, simply, and at reduced energy costs, using carbon dioxide separated and recovered from various commercial facilities, industrial facilities, or incineration facilities as a carbon source. Therefore, according to the present invention, it is possible to easily achieve so-called carbon recycling, in which the recovered carbon dioxide is used as a carbon source to produce fuel and reuse (recycle). [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is a schematic diagram showing the change over time in the concentration of sugars and lipids produced in a culture tank in an embodiment in which carbon dioxide is supplied to a culture tank containing chlorella and a culture solution, and light is irradiated onto the chlorella to culture the chlorella and cause photosynthesis by the chlorella. DETAILED DESCRIPTION OF THE INVENTION

[0019] Hereinafter, embodiments of the present invention will be described in detail. In this specification, the term "to" used to express a range of values ​​indicates a range that includes the values ​​stated as the upper and lower limits. When a unit is stated for only the upper limit of a range of values ​​expressed as "to," this also means that the lower limit is expressed in the same unit. In the numerical ranges described in stages in this specification, the upper or lower limit value described in one numerical range may be replaced with the upper or lower limit value of another numerical range described in stages. As used herein, combinations of preferred embodiments are more preferred embodiments.

[0020] The method for producing fuel according to the present invention comprises: Carbon dioxide separated and recovered from a commercial facility, industrial facility, or incineration facility is supplied to a culture tank containing microalgae and a culture solution for cultivating the microalgae, and light is irradiated onto the microalgae to cultivate the microalgae and produce sugars through photosynthesis by the microalgae; When the sugar concentration in the culture tank increases over time, The maximum rate of increase in the concentration of sugar produced in the culture tank is v max When the rate of increase in the concentration of sugar produced in the culture tank increases over time, the rate of increase is 0.8 × v max The cultivation of the microalgae is stopped between the time when the concentration of the sugar produced in the culture tank reaches a maximum value and the time when the concentration of the sugar produced in the culture tank reaches a maximum value. It is characterized by the following.

[0021] In the fuel production method of the present invention, microalgae are single-celled organisms with photosynthetic function, and mean tiny algae with a body length (longest diameter of the cell) of 100 μm or less, whose individual existence cannot be recognized or is difficult to recognize with the naked eye. The body length (long diameter of the cell) means the long axis diameter of the cell as observed using an optical microscope.

[0022] Microalgae have the ability to convert carbon dioxide into organic matter through photosynthesis, and because they absorb carbon dioxide and convert it into organic matter at a much faster rate than other biomass, they can efficiently produce fuel from the captured carbon dioxide.

[0023] In the fuel production method of the present invention, the microalgae can be one or more species selected from Chlorella (chlorophyta), Nannochloropsis, Neochloris, Euglena (Euglena, Euglena plant), Spirulina (cyanobacteria), Dunaliella (chlorophyta), Haematococcus (chlorophyta), Pseudococcomyxa (chlorophyta), Chlamydomonas (chlorophyta), Botryococcus (chlorophyta), Frassula gracilis (chlorophyta), and marine diatoms (ochrophyta). Among these, the microalgae is preferably one or more species selected from Chlorella and Nannochloropsis.

[0024] In the fuel production method according to the present invention, the culture tank containing the culture solution for culturing the microalgae can be a closed-system culture tank when the microalgae are cultured in a closed system, or an open-system culture tank when the microalgae are cultured in an open system.

[0025] In this application, the term "closed system" means that the inside of the culture tank is isolated from the outside air.

[0026] It is preferable to culture microalgae in a closed system culture tank, since the culture of microalgae is less affected by the environment outside the culture tank and is less affected by other microorganisms (other microalgae, predatory organisms, etc.).

[0027] The closed culture tank may be, for example, a closed photobioreactor tank. Examples of the photobioreactor tank include flat-plate tanks, tube (cylindrical) tanks, sunlight-collecting tanks, internally irradiated tanks, and artificial light-utilizing tanks.

[0028] In addition, in the present application, the term "open system" means that the inside of the culture tank is not isolated from the outside air. Examples of the open culture tank include circular or raceway tanks, etc. The open culture tank may also be an outdoor open pond such as an artificial pond or a natural pond.

[0029] The culture medium contained in the culture tank may be appropriately determined depending on the type of microalgae to be cultured. The culture solution contained in the culture tank may be, for example, a diluted solution obtained by diluting a known culture medium capable of increasing the concentration of microalgae through culture with water or the like.

[0030] Examples of the medium include general inorganic media such as CHU medium, JM medium, and MDM medium. Examples of the medium include Gamborg B5 medium, BG11 medium, and HSM medium.

[0031] The medium may contain, as a nitrogen source, one or more selected from yeast extract, corn steep licorice, sodium glutamate, ammonium sulfate, Ca(NO3)2·4H2O, KNO3, NH4Cl, etc., and may also contain, as other major nutrients, one or more selected from KH2PO4, MgSO4·7H2O, FeSO4·7H2O, etc. The medium may also contain, for example, glucose, fructose, etc. as a carbon source. Furthermore, the medium may contain antibiotics, vitamins, etc. that do not affect the growth of microorganisms.

[0032] The pH of the culture solution contained in the culture tank may be determined appropriately depending on the type of microalgae to be cultured, etc.

[0033] In the fuel production method of the present invention, carbon dioxide separated and recovered from a commercial facility, industrial facility, or incineration facility is supplied to a culture tank containing microalgae and a culture solution for cultivating the microalgae. The carbon dioxide may be separated and recovered from various furnaces and boilers.

[0034] Examples of carbon dioxide separated and recovered from industrial facilities include carbon dioxide recovered from exhaust gases from blast furnaces, lime kilns, heating furnaces, reactors, incinerators, boilers, etc. installed in various industrial facilities such as power plants, steel mills, cement plants, refineries, and chemical plants.

[0035] Examples of carbon dioxide separated and recovered from commercial facilities include carbon dioxide recovered from exhaust gases from incinerators or boilers installed in various commercial facilities such as hotels, commercial buildings, hot spring facilities, and heated swimming pools.

[0036] Furthermore, examples of carbon dioxide separated and recovered from incineration facilities include carbon dioxide recovered from the exhaust gas of waste incinerators installed in waste incineration facilities of each local government.

[0037] In the method for producing fuel according to the present invention, there are no particular limitations on the method for separating and capturing carbon dioxide from commercial facilities, industrial facilities, or incineration facilities.

[0038] In the method for producing fuel according to the present invention, examples of a method for separating and recovering carbon dioxide from a commercial facility, industrial facility, or incineration facility include a method that includes an absorption step in which exhaust gas emitted from various commercial facilities, industrial facilities, or incineration facilities is brought into contact with an absorbent consisting of an aqueous solution of an alkanol (amine compound) to cause absorption and reaction, thereby obtaining a carbon dioxide-rich absorption liquid in which the carbon dioxide has reacted with the absorbent, and a regeneration step in which carbon dioxide is obtained by desorbing carbon dioxide from the carbon dioxide-rich absorption liquid obtained by the absorption step, and at the same time, the absorbent is regenerated.

[0039] In the fuel production method according to the present invention, the method for supplying the carbon dioxide to the culture tank containing the microalgae and the culture solution for culturing the microalgae is not particularly limited, and examples thereof include a method in which a carbon dioxide supply pipe is provided to supply carbon dioxide from the outside of the culture tank to the culture tank, and carbon dioxide is supplied from this carbon dioxide supply pipe. The carbon dioxide supply pipe may be provided with a pump for supplying carbon dioxide at one end of the carbon dioxide supply pipe.

[0040] The carbon dioxide supply pipe is preferably equipped with a bubble generating means for generating bubbles from the carbon dioxide supplied into the pipe, and examples of the bubble generating means include an air stone. For example, by attaching an air stone to the end of the carbon dioxide supply pipe, and then positioning the end where the air stone is attached at a desired position in the culture tank, and supplying carbon dioxide using a pump from the opposite end of the carbon dioxide supply pipe, it is possible to suitably supply carbon dioxide in the form of bubbles to a desired range in the culture tank.

[0041] In the method for producing fuel according to the present invention, the carbon dioxide supply pipe is provided with a bubble generating means, so that carbon dioxide can be supplied uniformly to the microalgae in the culture tank.

[0042] In the fuel production method according to the present invention, carbon dioxide may be supplied to the culture tank continuously or intermittently depending on the progress of the culture of microalgae in the culture tank.

[0043] In the method for producing fuel according to the present invention, oxygen may be further supplied into the culture tank, and by supplying oxygen into the culture tank, the microalgae can respire suitably. As a form of supplying oxygen into the culture tank, for example, an oxygen supply pipe may be provided in addition to the carbon dioxide supply pipe, and oxygen (preferably air) may be supplied from this oxygen supply pipe. Furthermore, examples of a form of supplying oxygen into the culture tank include a form in which carbon dioxide is intermittently supplied to the culture tank from the carbon dioxide supply pipe, and oxygen (preferably air) is supplied from the carbon dioxide supply pipe during times when carbon dioxide is not being supplied. Furthermore, examples of a form of supplying oxygen into the culture tank include a form in which a mixed gas of carbon dioxide and oxygen (preferably air) is supplied to the culture tank from the carbon dioxide supply pipe or oxygen supply pipe.

[0044] In the fuel production method of the present invention, carbon dioxide separated and recovered from a commercial facility, industrial facility, or incineration facility is supplied to a culture tank containing microalgae and a culture solution for cultivating the microalgae, and light is irradiated onto the microalgae to cultivate them and cause the microalgae to produce sugar through photosynthesis.

[0045] In the method for producing fuel according to the present invention, the light irradiated onto the microalgae may be artificial light or sunlight. When the light irradiated onto the microalgae is artificial light, the light may be irradiated by a known illumination means. The illumination means may be provided in advance in the culture tank, or may be provided separately from the culture tank. The illumination means may be, for example, an LED (light emitting diode) illumination device.

[0046] In the method for producing fuel according to the present invention, the microalgae may be irradiated with light continuously or intermittently.

[0047] In the method for producing fuel according to the present invention, the temperature at which the microalgae are cultured by supplying carbon dioxide into the culture tank and irradiating it with light is preferably 4 to 40°C, more preferably 15 to 35°C.

[0048] In the fuel production method according to the present invention, when the microalgae are cultured by supplying the carbon dioxide into the culture tank and irradiating it with light, it is preferable to culture the culture solution while rotating (circulating) it or vibrating or stirring it.

[0049] In the fuel production method of the present invention, carbon dioxide separated and recovered from a commercial facility, industrial facility, or incineration facility is supplied to a culture tank containing microalgae and a culture solution for cultivating the microalgae, and the microalgae are cultivated by irradiating them with light, allowing the microalgae to grow by photosynthesis using the culture solution as nutrients.

[0050] In the fuel production method according to the present invention, an embodiment in which microalgae are cultured while photosynthesizing can be exemplified by a mode in which carbon dioxide is supplied and light is irradiated to perform photosynthesis for 12 hours each day, and oxygen (preferably air) is supplied and light is blocked for the remaining 12 hours. The time for light irradiation and the time for blocking light can be selected appropriately. Furthermore, in the method for producing fuel according to the present invention, the concentrations of carbon dioxide and oxygen supplied during the cultivation of microalgae can also be selected appropriately.

[0051] In the fuel production method of the present invention, carbon dioxide separated and recovered from a commercial facility, industrial facility, or incineration facility is supplied to a culture tank containing microalgae and a culture solution for cultivating the microalgae, and the microalgae are irradiated with light to cultivate the microalgae and produce sugars through photosynthesis by the microalgae. The sugar may be one or more selected from monosaccharides (glucose, fructose, etc.), disaccharides (sucrose, maltose, lactose, etc.), oligosaccharides, and polysaccharides (starch, etc.).

[0052] In the fuel production method according to the present invention, when the sugar concentration in the culture tank increases over time, the maximum rate of increase in the sugar concentration produced in the culture tank is defined as v max When the concentration of sugars produced in the culture tank increases over time, the rate of increase is 0.8 × v max The cultivation of the microalgae is stopped between the time when the concentration of the sugar produced in the culture tank reaches a maximum value and the time when the concentration of the sugar produced in the culture tank reaches a maximum value.

[0053] In the fuel production method according to the present invention, when the sugar concentration in the culture tank increases over time, the maximum rate of increase in the sugar concentration produced in the culture tank is defined as v max When the concentration of sugars produced in the culture tank increases over time, the rate of increase is 0.9 × v max It is preferable to stop the cultivation of the microalgae between the time when the concentration of sugars produced in the culture tank reaches a maximum value and the time when the rate of increase in the concentration of sugars produced in the culture tank reaches a maximum value. max It is more preferable to stop the cultivation of the microalgae between the time when the concentration of the sugar produced in the culture tank reaches the maximum value and the time when the concentration of the sugar produced in the culture tank reaches the maximum value.

[0054] In the fuel production method of the present invention, when the sugar concentration in the culture tank increases over time, stopping the cultivation of microalgae within the above period makes it possible to produce high concentrations of sugar in a short period of time, thereby enabling the efficient production of the desired fuel.

[0055] FIG. 1 is a schematic diagram showing the change over time in the concentration of sugars and lipids formed in a culture tank in an embodiment in which carbon dioxide is supplied to a culture tank containing chlorella and a culture solution for culturing the chlorella, and light is irradiated onto the chlorella to culture the chlorella and cause photosynthesis by the chlorella. In Figure 1, curve a shows the change in sugar concentration over time in the culture tank, curve b shows the change in lipid concentration over time in the culture tank, and curve c shows the change in the combined sugar and lipid concentration over time in the culture tank.

[0056] As shown by curve a in Figure 1, when carbon dioxide is supplied to microalgae such as chlorella and they are cultivated in a culture tank while photosynthesis is taking place, the microalgae first multiply and form sugars inside, causing the sugar concentration in the culture tank to increase rapidly over time, reaching a maximum value X at time t3 in the figure. On the other hand, as shown by curve a in Figure 1, after the sugar concentration in the culture tank reaches a maximum value X at time t3 in the figure, the growth rate of the microalgae decreases due to a change in the culture conditions (such as a deficiency of nitrogen or sulfur components), and the sugar formed in the cells gradually converts to lipids, so the sugar concentration in the culture tank begins to decrease over time.Instead, as shown by curve b in Figure 1, the lipid concentration in the culture tank increases over time.

[0057] Changes over time in sugar concentration and lipid concentration similar to those shown in Figure 1 are described, for example, in Figures 2 and 3 of the above-mentioned Non-Patent Document 1 and Figures 1b and 1d of Non-Patent Document 2. From these descriptions, it is generally believed that when photosynthetic microalgae are cultured, the sugar concentration and lipid concentration will show changes over time similar to those described above.

[0058] As shown by curve a in Figure 1, the sugar concentration in the fermentor increases rapidly over time at the beginning of the cultivation, and at time t2 in the figure, the rate of increase (the slope of the tangent to curve a) reaches a maximum value of v max After reaching this value, the sugar concentration reaches a maximum value X at time t3 in the figure, and then the sugar concentration decreases over time.

[0059] In the fuel production method according to the present invention, when the sugar concentration in the culture tank increases over time, the maximum rate of increase in the sugar concentration produced in the culture tank is defined as v maxWhen the concentration of sugars produced in the culture tank increases over time, the rate of increase is 0.8 × v max The cultivation of the microalgae is stopped between the time when the concentration of the sugar produced in the culture tank reaches a maximum value and the time when the concentration of the sugar produced in the culture tank reaches a maximum value.

[0060] That is, based on the curve a shown in FIG. 1, in the initial stage of the culture in which the sugar concentration in the culture tank increases over time, the rate of increase in the sugar concentration (the slope of the tangent to the curve a) reaches a maximum value v max However, in the fuel production method according to the present invention, the rate of increase in sugar concentration increases over time to 0.8 × v max The cultivation of the microalgae is stopped between time t1 in the figure when the sugar concentration reaches the maximum value X and time t3 in the figure when the sugar concentration reaches the maximum value X.

[0061] In the fuel production method according to the present invention, the timing for stopping the cultivation of microalgae can be determined by culturing microalgae in advance under the same conditions as those under which the cultivation is to be carried out, obtaining information on the change in sugar concentration over time as shown in FIG. 1, and determining whether the rate of increase in the sugar concentration produced in the cultivation tank increases over time at an initial stage of cultivation where the sugar concentration increases over time is 0.8 × v max and the time when the concentration of sugar produced in the culture tank reaches maximum value X, any time between these points can be determined as the timing to stop the cultivation of the microalgae.

[0062] More specifically, in the fuel production method according to the present invention, the sugar concentration in the culture solution obtained in the culture tank can be identified by the following method, and the timing for stopping the cultivation of microalgae can be determined.

[0063] <Method for determining when to stop culturing microalgae> In this method, first, the concentration of sugars in a sample is quantified using the glucose concentration as a representative value, according to the method described in Section 2.3. Starch analysis of the aforementioned Non-Patent Document 1 (Bioresource Technology 129 (2013), pp. 150-155), by the following steps (1) to (10). (1) A 1 mL sample of the culture medium is centrifuged for solid-liquid separation, and the microalgae cells (solid phase) are separated and collected. (2) The cells obtained in (1) are mixed with an equal volume of glass beads (0.1 mm diameter) and 0.25 mL of distilled water to form a mixture, and the mixture is stirred in a vortex mixer (Vortex Genie 2, Scientific Industries) to disrupt the cells. (3) To the mixture containing the disrupted cells, 80% ethanol heated to 50°C is added to extract the pigments in the cells, and then the cell disruption material (solid phase) containing sugars is separated and recovered by solid-liquid separation using a centrifuge. The resulting cell disruption material containing sugars is mixed with 0.15 mL of distilled water to obtain a cell suspension. (4) The cell suspension obtained in (3) above is kept in a water bath at 100°C for 15 minutes, cooled, and then mixed with 0.25 mL of 60% perchloric acid and stirred for 15 minutes to hydrolyze the sugars in the cells that make up the cell suspension. (5) The cell suspension hydrolyzed in (4) above is mixed with 0.6 mL of distilled water and centrifuged in a centrifuge. (6) 0.4 mL of the supernatant (liquid phase) of the cell suspension centrifuged in (5) above is mixed with 2 mL of anthrone solution (0.2 g of anthrone dissolved in 100 mL of 75% sulfuric acid) to obtain a mixed solution. (7) The mixture obtained in (6) above is heated in a water bath at 100°C for 8 minutes. (8) Cool the mixture heated in (7) to room temperature and measure the absorbance at a wavelength of 625 nm using a spectrophotometer. At the same time, perform calibration using glucose as a standard substance (to determine the glucose concentration (mg / mL) with the corresponding absorbance). Furthermore, multiply the glucose concentration (mg / mL) obtained by calibration by the volume (mL) of the mixture to determine the mass (mg) of glucose in the mixture. (9) The above steps (1) to (8) are repeated three times, and the arithmetic mean value of the mass (mg) of glucose obtained in each measurement is calculated. (10) Divide the arithmetic mean value of the glucose mass (mg) obtained in (9) above by the sample volume (1 mL) to obtain the glucose concentration (mg / mL) in the culture device, and use this as the sugar concentration (mg / mL) in the culture device.

[0064] (11) Using the above method, data on the change in the concentration (mg / mL) of sugars (monosaccharides) in the culture solution over time for each culture time is obtained every two hours. By performing natural cubic spline interpolation on the data on the change in the concentration (mg / mL) of sugars (monosaccharides) in the culture solution over time for each culture time, a graph showing the change in the concentration of sugars in the culture solution over time for each culture time is obtained, such as curve a shown in Figure 1. Then, the maximum value v of the rate of increase in the concentration of sugars produced in the culture tank on the obtained graph is calculated. max (the maximum value of the slope of the tangent line of the obtained graph) is calculated, and then, when the sugar concentration in the culture tank increases over time, the "rate of increase in the sugar concentration produced in the culture tank increases over time" is calculated by 0.8 × v max Calculate the "culture time when the gradient of the tangent to the graph obtained first reaches its maximum value x 0.8" and "culture time when the concentration of sugar produced in the culture tank reaches its maximum value."

[0065] (12) In addition, the turbidity (transmitted light turbidity) of the culture solution at a wavelength of 730 nm for each of the above culture times is measured using a spectrophotometer (AS ONE Corporation, model ASV11D-H), and the correspondence between the sugar concentration in the resulting culture solution and the turbidity of the culture solution is obtained. From the above correspondence, when the sugar concentration in the fermenter increases over time, "the rate of increase in the sugar concentration produced in the fermenter increases over time as 0.8 × v max The turbidity of the culture solution corresponding to the "sugar concentration (mg / mL) in the culture solution at the culture time when the sugar concentration (mg / mL) produced in the culture tank is at its maximum" and the "sugar concentration (mg / mL) in the culture solution at the culture time when the sugar concentration (mg / mL) produced in the culture tank is at its maximum" are obtained.

[0066] (13) Furthermore, in actual cultivation, when the sugar concentration in the culture tank increases over time from the turbidity of the culture solution that changes over time, "the rate of increase in the sugar concentration produced in the culture tank increases over time and is 0.8 × v max By identifying the "culture time at which the concentration of sugars produced in the culture tank reaches its maximum value" and the "culture time at which the concentration of sugars produced in the culture tank reaches its maximum value," it is possible to determine any point between these two as the timing to stop the cultivation of microalgae.

[0067] In the fuel production method according to the present invention, the lipid concentration in the culture medium can be determined by the following method. <Method for determining lipid concentration in culture medium> (1) A sample of the culture medium is subjected to solid-liquid separation using a centrifuge, and the microalgae (solid phase) are separated and collected. (2) The microalgae are frozen in liquid nitrogen, n-hexane is added, and the algae are disrupted using an ultrasonic homogenizer. (3) The cell debris is centrifuged and the solvent is completely evaporated. The remaining material is the lipids. (4) The obtained lipids are measured on an analytical balance to determine the mass of lipids in the sample. (5) The lipid concentration (mg / mL) in the culture device is calculated by dividing the measured mass (mg) of lipid in the sample by the volume (mL) of the sample.

[0068] In the fuel production method of the present invention, when the sugar concentration in the culture tank increases over time, by stopping the cultivation of microalgae within the specific period described above, carbon dioxide can be effectively absorbed by the microalgae and cultivated, while producing high concentrations of sugar in a short period of time.By repeating this cultivation operation, the desired fuel can be efficiently produced.

[0069] In the present application, "stopping the cultivation of microalgae" means removing the culture medium from the culture tank.

[0070] In the method for producing fuel according to the present invention, examples of a method for stopping the cultivation of microalgae include the following methods. (1) After determining the correlation between the sugar concentration in the culture medium and the turbidity of the culture medium in advance, the turbidity of the target culture medium is measured. (2) When the measured turbidity of the culture medium is within a predetermined range, the culture is stopped by withdrawing the culture medium from the culture tank.

[0071] In the fuel production method of the present invention, by culturing microalgae as described above and stopping the cultivation of the microalgae at a specific stage in which sugars are produced by photosynthesis, it is possible to effectively produce a sufficient amount of sugar while limiting the conversion of the produced sugars to lipids.As a result, the cultivation period of the microalgae can be shortened and the operation of extracting lipids from the microalgae can be eliminated.

[0072] Furthermore, in the fuel production method of the present invention, by limiting the conversion of the sugars to lipids, the step of hydrocracking or hydrogenating the fats and oils (triglycerides) in the obtained lipids is also unnecessary, which can significantly simplify the entire production process, increase the processing speed, and significantly reduce production costs.

[0073] In the method for producing fuel according to the present invention, after the cultivation of the microalgae is stopped, it is preferable to further subject the obtained culture solution to solid-liquid separation as appropriate and to a drying treatment.

[0074] The method for solid-liquid separation of the culture solution can be filtration, flotation / sedimentation separation or centrifugation. The filtration method may be a method using a microscreen, a drum screen or a belt filter. Examples of the flotation / sedimentation separation method include a method of solid-liquid separation of microalgae that float near the surface of the culture solution or that sink in the culture solution, in which the microalgae are in a floating or sinking state. Other examples of the flotation / sedimentation separation method include a method of supplying bubbles containing air or the like into the culture solution, which adsorb to the microalgae particles and cause them to float, and then recovering the foam particles on the water surface, and a method of adding a flocculant (KAl(SO4)2·12H2O, FeCl3, chitosan, etc.) to the culture solution to cause the microalgae to settle and recover the sediment.

[0075] The drying method may be one or more selected from spray drying (drying with a spray dryer), drum drying, freeze drying, sun drying, and the like.

[0076] In the fuel production method according to the present invention, by carrying out the drying treatment, microalgae containing sugars therein can be recovered as a dried material, and the obtained dried material can be suitably handled as a solid fuel.

[0077] An example of an embodiment of the fuel production method according to the present invention is one in which carbon dioxide separated and recovered from a thermal power plant or a large factory equipped with a boiler is supplied to a microalgae culture device installed within such a facility, the microalgae culture is stopped during the process of producing sugar by photosynthesis, and the resulting culture solution is appropriately dried and reused as fuel for thermal power generation or the boiler.

[0078] Furthermore, an embodiment of the fuel production method according to the present invention may include, for example, accumulating carbon dioxide separated and recovered from factories equipped with boilers in industrial parks established in local municipalities, supplying the collected carbon dioxide to a microalgae culture device established in the industrial park, stopping the microalgae culture during the process of producing sugar through photosynthesis, appropriately drying the resulting culture solution, and reusing it as fuel for the boilers established in the factories in the industrial park.

[0079] Thus, the present invention can provide a method for producing fuel quickly, simply, and at reduced production costs, using carbon dioxide separated and recovered from various commercial facilities, industrial facilities, or incineration facilities as a carbon source. Therefore, according to the present invention, it is possible to easily achieve so-called carbon recycling, in which the recovered carbon dioxide is used as a carbon source to produce fuel and reuse (recycle). [Industrial Applicability]

[0080] According to the present invention, it is possible to provide a method for producing fuel quickly, simply, and at reduced energy costs, using carbon dioxide separated and recovered from various commercial facilities, industrial facilities, or incineration facilities as a carbon source.

Claims

1. Carbon dioxide separated and recovered from a commercial facility, industrial facility, or incineration facility is supplied to a culture tank containing microalgae and a culture solution for cultivating the microalgae, and light is irradiated onto the microalgae to cultivate the microalgae and produce sugars through photosynthesis by the microalgae. When the sugar concentration in the culture tank increases over time, The maximum rate of increase in the concentration of sugar produced in the culture tank is defined as v ｍａｘ When the concentration of sugar produced in the culture tank increases over time, the rate of increase in the concentration of sugar produced in the culture tank increases to 0.8 × v ｍａｘ The cultivation of the microalgae is stopped between the time when the concentration of the sugar produced in the culture tank reaches a maximum value and the time when the concentration of the sugar produced in the culture tank reaches a maximum value. A method for producing fuel comprising the steps of:

2. When the sugar concentration in the culture tank increases over time, the maximum rate of increase in the sugar concentration produced in the culture tank is defined as v ｍａｘ When the concentration of sugar produced in the culture tank increases over time, the rate of increase in the concentration of sugar produced in the culture tank increases to 0.9 × v ｍａｘ 2. The method for producing fuel according to claim 1, wherein the cultivation of the microalgae is stopped between the time when the concentration of the sugar produced in the culture tank reaches a maximum value and the time when the concentration of the sugar produced in the culture tank reaches a maximum value.

3. 2. The method for producing fuel according to claim 1, wherein the microalgae is one or more selected from Chlorella, Nannochloropsis, Neochloris, Euglena, Spirulina, Dunaliella, Haematococcus, Pseudococomyxa, Chlamydomonas, Botryococcus, Fructus gracilis, and marine diatoms.

4. The method for producing fuel according to claim 1 , further comprising supplying oxygen into the culture tank.

5. The method for producing fuel according to claim 1, wherein after the cultivation of the microalgae is stopped, the resulting culture solution is subjected to solid-liquid separation and then dried.

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