Method for producing torrefied biomass solid fuel

Abstract

Provided is a method for producing a semi-carbonized biomass solid fuel, the method comprising subjecting plant biomass to a torrefaction process to produce semi-carbonized plant biomass and then solidifying the semi-carbonized plant biomass by press-molding. The method includes a heating treatment for heating the solidified semi-carbonized plant biomass, which is obtained after the press-molding, at 100-250°C.

Description

[Technical Field]

[0001] The present invention relates to a method for producing torrefied biomass solid fuel from plant-based raw materials. [Background technology]

[0002] In order to solve the global issue of climate change, many countries, including Japan, have declared that they aim to become carbon neutral. Carbon neutrality refers to reducing greenhouse gas emissions to zero overall, and means essentially bringing the balance of human activities to zero by subtracting the amount of carbon dioxide absorbed by plants through afforestation and forest management, and the amount of carbon dioxide fixed by humans, from the amount of carbon dioxide emitted by humans, including carbon dioxide.

[0003] Toward the realization of a carbon-neutral society, coal-fired power plants, which have a large carbon dioxide emission rate, are promoting biomass co-firing, in which plant biomass is mixed with coal as fuel and burned, without substantially increasing the amount of carbon dioxide on the planet. Biomass co-firing requires the transportation of the biomass to the coal-fired power plant or other facility. Unprocessed biomass for co-firing is inefficient in transportation and difficult to crush in pulverized coal boilers, making it difficult to increase the co-firing ratio. Therefore, various methods have been investigated for converting the biomass into a reduced-volume or easily crushed biomass solid fuel by semi-carbonizing and compacting the biomass.

[0004] Torrefaction and steam explosion are two main methods for torrefaction, but the processing methods are very different. The choice between these two methods is based mainly on the properties of the biomass feedstock, the processing volume, the physical properties of the biomass solid fuel required for the usage form, and the production costs.

[0005] Torrefaction is a heat treatment carried out at 250 to 320°C in an oxygen-free or extremely low-oxygen atmosphere (see Non-Patent Document 1). It is known that when plant biomass is heated in an oxygen-free environment, it experiences a rapid weight loss starting at around 250°C, and carbonization progresses through a pyrolysis reaction. Steam explosion is a process in which biomass is steamed in a pressure-resistant, sealed container with high-temperature, high-pressure steam, and then suddenly released to atmospheric pressure to cause adiabatic expansion, followed by rapid cooling and destruction of the plant cell structure of the biomass. During steam steaming, hydrolysis of plant cell fibers occurs, and pyrolysis at high temperature and pressure also occurs, leading to semi-carbonization of the biomass.

[0006] Torrefied biomass solid fuel is usually stored outdoors in the same way as coal. However, problems have often been pointed out, such as organic components leaching out of the fuel due to rainwater, increasing the COD of the wastewater, and reducing the fuel's strength and causing it to pulverize. Therefore, methods for improving these problems have been investigated.

[0007] For example, Patent Documents 1 and 2 disclose methods for producing biomass solid fuel, which involve steam explosion, molding the resulting material into pellets or other lumps, and then heat-treating the lumps at 200 to 260°C (Patent Document 1) or at 180°C or higher for 5 minutes or longer (Patent Document 2).

[0008] Patent Document 3 discloses a method for producing semi-carbonized biomass solid fuel by molding biomass into unheated blocks and then heating the unheated blocks to 150°C to 400°C. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] International Publication No. WO2014 / 087949 [Patent Document 2] International Publication No. WO2022 / 153830 [Patent Document 3] International Publication No. WO2016 / 056608 [Non-patent literature]

[0010] [Non-Patent Document 1] IEA Bioenergy Task 32 (Biomass Combustion and Cofiring): Status overview of torrefaction technologies - A review of the commercialization status of biomass torrefaction. Published in 2015. IEA Bioenergy. Retrieved March 8, 2023. Internet.<URL:https: / / www.ieabioenergy.com / wp-content / uploads / 2015 / 11 / IEA> Summary of the Invention [Problem to be solved by the invention]

[0011] Although steam explosion can destroy plant cell fibers regardless of the type of plant used as raw material, a large amount of water-soluble organic components produced by hydrolysis remains in the processed material after explosion. Therefore, as in the methods described in Patent Documents 1 and 2, by efficiently decomposing or volatilizing the remaining water-soluble organic components through heat treatment at a relatively high temperature, it is possible to reduce COD elution and prevent powdering, along with weight loss.

[0012] However, the methods described in Patent Documents 1 and 2 require not only weight loss due to semi-carbonization during steam cooking, but also the removal of water-soluble organic components produced in the hydrolysis reaction, which reduces the actual yield of biomass solid fuel and leads to increased costs, leaving room for improvement. Furthermore, even when heated at a relatively high temperature, the produced biomass solid fuel still contains 5% or more moisture (see Table 1 in Patent Document 1), making it difficult to say that it is a highly energy-efficient fuel. Furthermore, because the steam explosion process involves repeatedly sealing a pressure-resistant vessel and then suddenly releasing the atmospheric pressure, wear on the seals of the pressure-resistant vessel is severe, making it difficult to increase the vessel size and reduce costs, leaving room for improvement.

[0013] On the other hand, the method described in Patent Document 3 makes it possible to obtain biomass solid fuel that is suppressed from collapsing due to rainwater and reduces the COD of wastewater, without using steam explosion as a semi-carbonization process or a binder to form pellets.

[0014] However, to mold biomass materials without a binder, the biomass must first be highly pulverized to a particle size of less than 1 mm to several hundred microns. Although it depends on the type of biomass material, it is not easy to pulverize biomass materials before torrefaction to the above particle size. This requires a significant amount of energy compared to pulverizing after torrefaction, which increases costs and leaves room for improvement. Furthermore, in the torrefaction process performed after the molding process for torrefaction, the molded pre-torrefaction biomass mass inevitably experiences a temperature difference or heat distribution between the surface and the interior during heating. This results in non-uniform properties of the resulting biomass solid fuel, making stable production difficult. This also leaves room for improvement.

[0015] Therefore, an object of the present invention is to provide a method for industrially and efficiently producing torrefied biomass solid fuel with excellent water resistance. [Means for solving the problem]

[0016] The present inventors have conducted extensive research in light of the above-mentioned problems. As a result of their investigation into the conversion of torrefaction torrefaction biomass into solid fuel, they have discovered that by solidifying (pelletizing) the torrefaction biomass through compaction molding and then heat-treating the solidified torrefaction biomass at a relatively low temperature, it is possible to suppress the elution of COD components when the solidified torrefaction biomass is immersed in water or when water is sprayed on the solidified torrefaction biomass, thereby completing the present invention.

[0017] That is, the present invention is a method for producing torrefaction-treated plant biomass solid fuel, which comprises torrefaction of plant biomass to obtain torrefaction-treated plant biomass, and then solidifying the torrefaction-treated plant biomass by compaction molding, and further comprises a heat treatment of heating the solidified torrefaction-treated plant biomass after compaction molding at 100 to 250°C.

[0018] In the production method of the present invention, the heat treatment is preferably carried out at 100 to 250°C for 1 to 120 minutes. The heat treatment is preferably carried out in an inert gas atmosphere. Furthermore, the moisture content of the semi-carbonized biomass solid fuel produced is preferably 2.0% by mass or less. [Effects of the Invention]

[0019] According to the present invention, a torrefied biomass solid fuel with excellent water resistance can be produced without the need for a steam explosion process, which increases costs, or a molding process that involves advanced pulverization before the torrefaction process. Therefore, the torrefied biomass solid fuel obtained by the production method of the present invention can significantly suppress an increase in COD in wastewater due to rainwater, etc. when stored outdoors, and can withstand long-term storage. DETAILED DESCRIPTION OF THE INVENTION

[0020] The method for producing torrefied biomass solid fuel of the present invention is characterized by heat-treating solidified torrefied plant biomass (hereinafter also referred to as "torrefied pellets") obtained by compacting and molding biomass torrefaction at 100 to 250° C. This production method makes it possible to industrially and efficiently produce torrefied biomass solid fuel that can suppress pulverization due to an increase in COD in drainage water caused by rainwater, etc., and a decrease in strength when stored outdoors.

[0021] Although the details of why the production method of the present invention produces torrefied biomass solid fuel with the above-mentioned effects are unclear, the present inventors speculate as follows. Specifically, lignin remains in the torrefied pellets. The softening temperature of lignin varies depending on the moisture content, but is approximately 100 to 150°C at moisture contents of 5% or less, and the softening temperature tends to decrease as the moisture content increases. The present inventors speculate that when the torrefied pellets are subjected to the heat treatment of the present invention, the lignin in the torrefied pellets softens, filling the gaps between the plant cell fibers that make up the torrefied pellets and rearranging them so that they cover the pellet surface, thereby improving water resistance and suppressing, to some extent, strength loss due to rainwater, etc. This is a mechanism that does not occur in pellets subjected to steam explosion, which destroys the lignin, and is unique to torrefied pellets.

[0022] In this specification, unless otherwise specified, the notation "A to B" for numerical values ​​A and B means "A or more and B or less." In such notation, when a unit is assigned only to numerical value B, the unit also applies to numerical value A. The method for producing torrefied biomass solid fuel of the present invention will be described in detail below.

[0023] (Biomass raw materials) The plant biomass used as a raw material in the present invention is not particularly limited as long as it is a plant that has grown plant cell fibers composed mainly of cellulose, hemicellulose, and lignin by absorbing carbon dioxide from the atmosphere and immobilizing it through photosynthesis, and can be used as a biomass fuel without competing with edible plants. Examples of plant biomass include woody biomass, herbaceous biomass, and agricultural crop residue biomass.

[0024] Specific examples of woody biomass include conifers such as cedar, cypress, Japanese cypress, pine, and fir, and broad-leaved trees such as beech, zelkova, paulownia, chestnut, cherry, birch, and camphor. Woody biomass may also be construction waste (offcuts, chips, sawdust, etc.) and thinned wood.

[0025] Examples of plant biomass include naturally grown plants such as Japanese silver grass, bamboo grass, chrysanthemums, ferns, bamboo, and other weeds, as well as artificially cultivated plants.

[0026] Crop residue biomass is the non-edible parts of crops, such as stems, leaves, roots, and fruit bunches. Specifically, it may be stems, leaves, skins, rice husks, rice straw, palm oil pomace (PKS), empty fruit bunches (EFB), etc. that remain after the edible parts have been harvested from crops such as rice, wheat, corn, potatoes, sugarcane, bananas, and palm trees.

[0027] One of the plant biomasses exemplified above may be used alone as a raw material, or two or more of them may be used in combination.

[0028] (Torrefied pellet manufacturing) In the manufacturing method of the present invention, torrefaction is followed by compaction molding to obtain solidified semi-carbonized plant biomass, i.e., torrefied pellets.

[0029] Known methods can be used for torrefaction. Specifically, the raw biomass is coarsely pulverized to a handleable size and then heat-treated at 250 to 320°C in an oxygen-free or extremely low oxygen atmosphere. Depending on the state of the biomass raw material after coarse pulverization, torrefaction may be carried out after washing with water and drying. Torrefaction can be carried out by feeding the coarsely pulverized biomass raw material into a plug-flow heater such as a rotary kiln, a vibrating fluidized bed, or a screw extruder, and continuously heating it under a flow of inert gas such as nitrogen, argon, or carbon dioxide at a temperature and time appropriately set depending on the type of biomass raw material.

[0030] Because the torrefaction product is at a high temperature, it is cooled and then compacted and molded, for example, into pellets. Any known cooling method can be used as long as it cools the torrefaction product to a temperature at which it can be compacted. Specific examples include spraying water on the high-temperature treated product after torrefaction, passing a cooling gas through the high-temperature treated product, or indirectly cooling the high-temperature treated product by passing it through a heat exchanger through which a refrigerant flows. Prior to compaction, the treated product is typically crushed to a size of approximately 10 mm or less and the moisture content is adjusted to approximately 10-20% to improve moldability. In some cases, a binder may be added to the treated product to improve moldability. When cooling is performed by spraying water on the high-temperature treated product, the amount of water sprayed during this cooling process can be adjusted to ensure that the treated product has a predetermined moisture content before compression molding.

[0031] The main purpose of compaction is to increase the bulk density of the torrefied biomass solid fuel, thereby improving transportation efficiency. Compaction can be performed using a ring die or flat die pelletizer, which extrudes the material through metal through-holes about 5 to 10 mm in diameter and 3 to 50 mm in length, or a briquetting machine, which packs the material into a die and applies pressure.

[0032] Torrefied pellets can be obtained by the above method. The target bulk density of the torrefied pellets can be determined appropriately depending on the type of biomass raw material and the capacity of the production equipment in each process, but the target bulk density of torrefied pellets is generally 600 g / L or more, preferably 700 g / L or more.

[0033] (heat treatment) The production method of the present invention is characterized in that the torrefied pellets are heated at 100 to 250° C. By carrying out the heat treatment, it is possible to produce a semi-carbonized biomass solid fuel that is excellent in water resistance while suppressing weight loss of the torrefied pellets.

[0034] If the heat treatment temperature is less than 100°C, there is almost no effect of improving the water resistance of the torrefied biomass solid fuel, while temperatures above 250°C are undesirable because thermal decomposition progresses, weight loss increases, and product yield decreases. To efficiently improve water resistance, a temperature of 120 to 230°C is preferred, 150 to 230°C is more preferred, 150 to 220°C is even more preferred, and 180 to 220°C is particularly preferred.

[0035] In order to improve water resistance, it is thought that it is necessary to soften the lignin as described above, and when the torrefied pellets are subjected to a heating environment that softens the lignin, some of the organic components volatilize from the torrefied pellets, resulting in a weight loss. If the weight loss is too great, excessive lignin will be removed, and the amount of usable biomass solid fuel obtained will decrease. Therefore, it is preferable to appropriately set the heating conditions by adjusting the heating temperature and heating time so that the weight loss before and after the heating treatment is 5% or less, preferably 3% or less.

[0036] The heating time depends on the state of the torrefied pellets, but is generally 1 to 120 minutes, more preferably 3 to 90 minutes, as the time during which the torrefied pellets are substantially kept at a temperature of 100 to 250°C.

[0037] The heat treatment can be performed by appropriately selecting a method capable of heating to 100 to 250°C, such as placing the torrefied pellets in a heating furnace set to a predetermined temperature, heating the torrefied pellets by irradiating them with microwaves, or heating the torrefied pellets by circulating superheated steam. Further cost reductions can be achieved by adopting a process that uses the exhaust heat from torrefaction as a heat source. During the heat treatment, in case volatile components remain in the torrefied pellets, it is desirable to create a low-oxygen atmosphere by circulating an inert gas such as nitrogen or argon to prevent unintended combustion of the volatile components.

[0038] When torrefaction is performed by steam explosion, hydrolysis of plant biomass progresses, and it is presumed that the pellets obtained by subsequent solidification contain a large amount of water-soluble organic components. Therefore, to improve the water resistance of such pellets, it is presumed that a treatment at a higher temperature than the heat treatment of the present invention is necessary, and it is presumed that weight loss occurs due to decomposition or volatilization of the water-soluble organic components produced by the hydrolysis during such high-temperature heat treatment. On the other hand, the heat treatment of the present invention is performed at a lower temperature than the torrefaction temperature range, which involves thermal decomposition, and therefore it is possible to produce torrefied biomass solid fuel with improved water resistance while suppressing weight loss during heat treatment.

[0039] Furthermore, in the production method of the present invention, the pellets semi-carbonized by torrefaction are heat-treated, so the moisture content of the produced biomass solid fuel is significantly reduced. Therefore, the production method of the present invention produces a biomass solid fuel that is excellent in transportability and storage and has high energy efficiency. The moisture content of the biomass solid fuel produced in the present invention is preferably 2.0% by mass or less, more preferably 1.0% by mass or less, and even more preferably 0.5% by mass or less.

[0040] Note that steam-exploded pellets do not show much reduction in moisture content, as shown in Table 1 of Patent Document 1. Specifically, even when heated at the highest temperature of 300°C for about an hour, 5.4% by mass of moisture remains.

[0041] The biomass solid fuel obtained by the production method of the present invention has high energy efficiency and can be suitably used as a solid fuel for biomass co-firing. [Example]

[0042] The present invention will be described in detail below using examples, but the present invention is not limited to these examples.

[0043] (Comparative Example 1) Palm empty fruit bunches (EFB) were cut into pieces less than 10 mm, crushed, packed into quartz tubes, and torrefied for 50 minutes in a heating furnace set at 270°C under a nitrogen flow. After torrefaction, the EFB was sprayed with water to a moisture content of 10%, packed into a 6 mm diameter mold, and compacted by heat pressing for 1 minute at a set temperature of 125°C and a gauge pressure of 15 MPa to obtain torrefied pellets.

[0044] After heat pressing, 10 g of the torrefied pellets were placed in a glass petri dish, 50 mL of water was added, and the mixture was left for 24 hours. The water turned a deep dark brown color.

[0045] Example 1 Torrefied pellets obtained in the same manner as in Comparative Example 1 were placed in a heating furnace set at 180° C. and collected after 60 minutes. The weight after the heat treatment was 99% on a dry basis.

[0046] The recovered product after the heat treatment was immersed in water in the same manner as in Comparative Example 1, and the water was colorless or pale yellow after 24 hours.

[0047] Examples 2 to 5 Torrefied pellets obtained in the same manner as in Comparative Example 1 were subjected to heat treatment under the conditions shown in Table 1. In Example 4, the pellets were treated at 180°C for 45 minutes and at 220°C for 45 minutes.

[0048] Ten grams of the resulting torrefied biomass solid fuel was weighed into a petri dish, 50 ml of water was added, and the mixture was left to soak for 24 hours at room temperature. After the soaking experiment, the absorbance of the water was measured, and the COD was calculated from the absorbance. The weight loss rate before and after the heat treatment and the moisture content of the pellets after the heat treatment were also determined. The moisture content was determined by heating and drying a 5 g sample at a temperature above the water evaporation temperature using a heat-drying moisture meter (MX-50, manufactured by A&D Co., Ltd.) until the moisture content decreased by 0.05% / min or less over time. The moisture content was calculated from the mass change due to the evaporation of the moisture contained in the sample. The results are shown in Table 1.

[0049] [Table 1]

[0050] As shown in Table 1, the weight loss rate of the pellets after heat treatment was small, and the moisture content was also significantly reduced. The COD was also clearly reduced, confirming the excellent water resistance of the torrefied biomass solid fuel of the present invention.

[0051] Examples 6 to 9 Torrefied pellets obtained in the same manner as in Comparative Example 1 were subjected to heat treatment under the conditions shown in Table 2. In Example 6, the pellets were treated at 180°C for 45 minutes and at 220°C for 45 minutes.

[0052] 1 kg of biomass solid fuel was weighed into a mesh container, and water was sprayed from above using a spray nozzle at a rate of 3 L / h (assuming a rainfall of 72 mm). After 3 hours, the wastewater was sampled and the COD was calculated from the absorbance. In addition, the mechanical durability (DU) and fine particle ratio of the pellets after the water spray test were determined using the following method. The results are shown in Table 2.

[0053] <Mechanical durability> After the water sprinkling test, 100 g of the sample was subjected to a durability test using a durability tester (Tekpro Ligno-Tester) in accordance with ISO17831-1:2015, and the results were calculated using the following formula. DU(%)=(mA / mE)×100 mE: pellet mass before test (g), mA: pellet mass after test (g)

[0054] <Fine powder rate> A metal plate sieve with a circular hole diameter (nominal mesh size) of 3.15 mm was used, and a sample mass of approximately 1 kg was measured before and after sieving, and the fine powder rate (%) was calculated using the following formula. Fine powder rate (%)=(1-m1 / m0)×100 m1: Mass remaining on the sieve after sieving (g) m0: Mass before sieving (g)

[0055] [Table 2]

[0056] As shown in Table 2, the weight loss rate of the pellets after heat treatment was small, and the moisture content also decreased significantly. The COD was also clearly reduced, confirming that the torrefied biomass solid fuel of the present invention exhibits excellent water resistance even when exposed to actual outdoor rain. It was also confirmed that the reduction in strength (pulverization) due to rain can be suppressed.

Claims

1. torrefaction of the plant biomass to obtain torrefied plant biomass; Next, the obtained semi-carbonized plant biomass is solidified by compaction molding, Heat treatment of the compacted and torrefied plant biomass at 180 to 250°C A method for producing torrefied biomass solid fuel, comprising:

2. The method for producing torrefied biomass solid fuel according to claim 1, wherein the heat treatment is carried out at 180 to 250°C for 1 to 120 minutes.

3. The method for producing torrefied biomass solid fuel according to claim 1, wherein the heat treatment is carried out in an inert gas atmosphere.

4. The method for producing torrefied biomass solid fuel according to any one of claims 1 to 3, wherein the moisture content of the torrefied biomass solid fuel produced is 2.0 mass% or less.

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