As part of biomass energy utilization, attempts have been made to hydrolyze cellulose or hemicellulose, which are major components of plants, to produce a saccharified solution, and then obtain ethanol from the saccharified solution through alcoholic fermentation of sugar. Ethanol thus obtained is planned to be utilized mainly as a fuel to be mixed into an automobile fuel or as an alternative fuel for gasoline.
In recent years, industrially, a saccharified solution obtained by hydrolytically saccharifying cellulose or hemicellulose is used for lactic acid fermentation to produce L-lactic acid, which is then polymerized to produce polylactic acid, which is one kind of biomass-based polymer. Polylactic acid has been drawing attention as a biodegradable plastic.
Major components of plants include cellulose (a polymer of glucose which is a C6 monosaccharide composed of six carbon atoms), hemicellulose (a polymer of C5 and C6 monosaccharides; a C5 monosaccharide is composed of five carbon atoms), lignin, and starch. Ethanol is produced by using saccharides as raw materials, such as a C5 monosaccharide, a C6 monosaccharide, and an oligosaccharide which is a complex of these saccharides. Ethanol is produced through fermentation of microorganisms such as yeast.
For hydrolyzing cellulosic biomass containing cellulose or hemicellulose into saccharides, there are the following three possible methods to be industrially applied: 1) a method of hydrolyzing such biomass by utilizing oxidizing power of a strong acid such as sulfuric acid; 2) a method of hydrolyzing such biomass by utilizing an enzyme; and 3) a method utilizing oxidizing power of supercritical water or subcritical water. However, the acidolysis method 1) indispensably requires a treatment for neutralizing the added acid after hydrolysis of cellulose or hemicellulose into saccharides and before fermentation of the saccharides, because the added acid acts as an inhibitor against fermentation bacteria, such as yeast or lactic acid bacteria. The cost of such treatment makes it difficult to put this method into practical use from an economic standpoint. Although the enzymolysis method 2) can be realized by a treatment under a normal temperature and constant pressure, no effective enzyme for the method has been found yet, and even if an effective enzyme is found, the outlook for industrial-scale realization of the method is still unclear in terms of cost efficiency, because such an enzyme is expected to incur a high production cost thereof.
As examples of the method 3) of hydrolyzing cellulosic biomass into saccharides by using supercritical water or subcritical water, there are disclosed methods as described below. Patent Literature 1 discloses a method of producing water-insoluble polysaccharides, which is characterized by hydrolysis of cellulose powder that is performed by bringing the powder into contact with pressurized hot water of 240 to 340° C. Patent Literature 2 discloses a method including: hydrolyzing biomass chips for a predetermined time with hot water pressurized to a saturated vapor pressure or higher at 140 to 230° C., thereby extracting hemicellulose; and then hydrolyzing the biomass chips with pressurized hot water heated to a temperature not lower than a cellulose hydrolyzing temperature, thereby extracting cellulose. Patent Literature 3 discloses a method of producing glucose and/or water-soluble cello-oligosaccharides, which is characterized in that cellulose with a mean polymerization degree of not less than 100 is hydrolyzed by: bringing the cellulose into contact reaction with supercritical water or subcritical water at a temperature of not lower than 250° C. and not higher than 450° C. and at a pressure of not lower than 15 MPa and not higher than 450 MPa for a time period of not less than 0.01 second and not more than 5 seconds; then cooling down the cellulose; and thereafter bringing the cellulose into contact with subcritical water at a temperature of not lower than 250° C. and not higher than 350° C. and at a pressure of not lower than 15 MPa and not higher than 450 MPa for a time period of not less than 1 second and not more than 10 minutes.
Patent Literature 4 discloses a method of producing saccharides, by which not only are the saccharides obtained from ligneous biomass with high yield and high efficiency, but also saccharides containing C5 and C6 saccharides and saccharides containing C6 saccharides can be separately collected. The method of producing saccharides disclosed in Patent Literature 4 includes: a first slurry heating step (S1) of heat-treating slurry that is prepared by adding high-temperature and high-pressure water to ligneous biomass; a first separating step (S2) of separating the heat-treated slurry into a liquid component and a solid component; a second slurry heating step (S3) of adding water to the separated solid component to form slurry and heat-treating the slurry; a second separating step (S4) of separating the heat-treated slurry into a liquid component and a solid component; and a useful component obtaining step (S5) of removing water from the separated liquid component to obtain saccharides. The method is characterized by obtaining saccharides by removing water from the liquid component separated in the first separating step (S2) in addition to obtaining saccharides in the useful component obtaining step (S5).
Patent Literature 5 discloses a method of producing L-lactic acid, the method including a step of culturing lactic acid bacteria from which L-lactic acid is producible (but excluding Enterococcus mundtii NITEBP-965) in an environment (medium) containing, as a substrate, any one selected from the group consisting of cellobiose, cell-oligosaccharide, xylose, arabinose, and glucose that are derived from cellulose and/or hemicellulose, thereby obtaining L-lactic acid.
Patent Literature 6 discloses liquid-containing solid treatment equipment, in which a liquid-containing solid discharged through a treatment passage of a treatment apparatus is fed through piping to a solid-liquid separator where solid-liquid separation is performed. The treatment equipment is characterized by being configured to supply part of a liquid generated by the solid-liquid separation into the treatment passage of the treatment apparatus. In the case of hydrolyzing biomass under a high temperature and a high pressure to continuously obtain a saccharified solution, generally speaking, a continuous reactor including therein conveying means such as a screw feeder is used. Patent Literature 6 uses such a continuous reactor.
Patent Literature 7 discloses a biomass treatment apparatus similar to the treatment equipment of Patent Literature 6. In the biomass treatment apparatus of Patent Literature 7, water or a saccharified solution is sprayed in a reactor, and thereby carbonization of a raw material and blockage caused by carbide are reduced.
Patent Literature 8 discloses a method of and an apparatus for hydrolytically saccharifying cellulosic biomass. In the method and the apparatus, high-temperature and high-pressure slurry taken out of a pressure container is flash-evaporated in order to prevent excessive hydrolysis of a saccharified solution.