JP7485904B2 - How charcoal is made - Google Patents

Description

The present invention relates to a method for producing charcoal that can improve the strength of the charcoal.

Carbon dioxide is known as a typical greenhouse gas. The steel industry is known to be an industry that emits a large amount of carbon dioxide, and it is estimated that about 5% of the world's carbon dioxide emissions come from the steel industry. Among the steelmaking processes, the amount of carbon dioxide generated in blast furnaces is large, and it is known that in steelmaking processes that mainly use the blast furnace method, about 80 to 90% of the carbon dioxide is generated as a result of the reduction reaction in the blast furnace. The reduction reaction of iron ore in the blast furnace mainly uses coal and coke as a reducing agent, and a large amount of carbon dioxide derived from fossil fuels is inevitably generated. Therefore, many attempts are being made to reduce the amount of carbon dioxide generated from the reaction in the blast furnace by using non-fossil fuels as a reducing agent.

In order to reduce the amount of fossil fuel used in blast furnaces, attempts have long been made to carbonize biomass such as wood and charge it into the blast furnace as a reducing agent. Several studies have reported that the pulverized coal blown into the blast furnace tuyeres can be entirely replaced with charcoal (Non-Patent Documents 1 and 2), and a method has also been reported in which fine iron ore and charcoal powder are mixed, pelletized, and used as a raw material, and testing has been achieved in an actual blast furnace (Non-Patent Document 3). In Brazil, small blast furnaces have also been operated using charcoal as the only reducing agent.

There have also been reports of attempts to use charcoal in the production of coke, which is charged into the top of modern large blast furnaces. For example, many studies have been conducted on replacing part of the caking coal with charcoal during coke production, and it has been reported that when 5% or less by mass of the coal is replaced with charcoal, strength close to that of regular coke can be obtained (Patent Document 1). In addition, many methods have been disclosed in which biomass powder is mixed with a binder and then carbonized under pressure (Patent Documents 2 to 4), and by carrying out appropriate processing, high strength comparable to that of coke used in steelmaking can be obtained. In addition, some reports have reported that when charcoal is produced by carbonizing wood without carrying out any special processing, strength close to that of coke used in steelmaking can be obtained (Non-Patent Documents 4 and 5).

Wang, Chuan, et al. "Injection of solid biomass products into the blast furnace and its potential effects on an integrated steel plant." Energy Procedia 61 (2014): 2184-2187. Ja de Castro, Jose Adilson, et al. "Analysis of the combined injection of pulverized coal and charcoal into large blast furnaces." Journal of Materials Research and Technology 2.4 (2013): 308-314. Mousa, Elsayed, et al. "Reduced Carbon Consumption and CO 2 Emission at the Blast Furnace by Use of Briquettes Containing Torrefied Sawdust." Journal of Sustainable Metallurgy (2019): 1-11. Kumar, M., B. B. Verma, and R. C. Gupta. "Mechanical properties of acacia and eucalyptus wood chars." Energy sources 21.8 (1999): 675-685. Emmerich, F. G., and C. A. Luengo. "Babassu charcoal: a sulfurless renewable thermo-reducing feedstock for steelmaking." Biomass and Bioenergy 10.1 (1996): 41-44. Characteristics of coal and pine sawdust co-carbonization Weber, Kathrin, and Peter Quicker. "Properties of biochar." Fuel 217 (2018): 240-261.

JP 2004-277452 A JP 2014-231037 A JP 2012-46729 A JP 2009-51985 A

However, all of the previously reported methods for producing coke for steelmaking from charcoal for large blast furnaces have problems. For example, many studies have been conducted on replacing part of the caking coal with charcoal during coke production, but it has been reported that the strength of the product coke is significantly reduced by only replacing about 5% by mass with charcoal (Non-Patent Document 6). Moreover, the degree of strength reduction is lower than the weighted average of coke and charcoal, and it is said that the oxygen-containing functional groups contained in charcoal significantly reduce the quality of the coke. As mentioned above, there are reports that strength is maintained if the amount of biomass added is less than 3% by mass (Patent Document 1), but the effect of reducing carbon dioxide emissions is small when such a small amount is used. In addition, the method of mixing biomass powder and a binder and carbonizing it under pressurized conditions (Patent Documents 2 to 4) is very expensive and is not suitable for producing a huge amount of charcoal to be used in the steelmaking process. In addition, there are many reports of using nuts, which have high strength to begin with, and there are no reports of obtaining high-strength charcoal from wood that is available in large quantities. Furthermore, it has been pointed out that there is a very large variation between samples (Non-Patent Document 7).

Based on the above-mentioned conventional methods, the object of the present invention is to provide a method for producing charcoal that can improve the strength of charcoal using wood that is available in large quantities.

The present invention includes the following aspects .
[1 ]
A method for producing charcoal, comprising the steps of: supplying tar vapor to wood while heating the wood to 800°C to carbonize the wood; and producing charcoal having an apparent specific gravity of 0.55 g/ ^cm3 or more.
［ 2 ］
A method for producing charcoal, comprising the steps of: supplying tar vapor to wood having an air-dry specific gravity of 0.68 g/cm3 or ^more ; and heating the wood to a temperature of 700°C or more and 1200°C or less to carbonize the wood.
［ ３ ］
A method for producing charcoal, comprising the steps of: supplying tar vapor to wood while heating the wood to 1000°C to carbonize the wood, thereby producing charcoal having an apparent specific gravity of 0.48 g/ ^cm3 or more.
［ 4 ］
A method for producing charcoal, comprising the steps of: supplying tar vapor to wood having an air-dry specific gravity of 0.55 g/cm3 or ^more ; and heating the wood to a temperature of 700°C or more and 1200°C or less to carbonize the wood.
［ ５ ］
A method for producing charcoal, comprising the steps of: supplying tar vapor to wood while heating the wood to 1100°C to carbonize the wood; and producing charcoal having an apparent specific gravity of 0.45 g/ ^cm3 or more.
［ ６ ］
The method for producing charcoal according to [ 5 ], characterized in that the air-dry specific gravity of the wood is 0.55 g/ ^cm3 or more.
［ ７ ］
While supplying tar vapor to the wood, the wood is heated to a temperature of 700° C. or more and 1200° C. or less (excluding 700° C.) to carbonize the wood,
A method for producing charcoal, comprising filling the wood on top of a packed bed of coal filled in a carbonization chamber of a coke oven, and using tar components contained in a gas generated by carbonization of the coal as the tar vapor .
[8 ]
While supplying tar vapor to the wood, the wood is heated to a temperature of 700° C. or more and 1200° C. or less (excluding 700° C.) to carbonize the wood,
A method for producing charcoal , characterized in that the charcoal is used as a substitute for blast furnace coke.
[9]
The method for producing charcoal according to any one of [1] to [8], characterized in that the temperature of the wood is increased at a temperature increase rate of 10° C./min or less.

The present invention makes it possible to improve the strength of charcoal.

1 is a graph showing the relationship between apparent specific gravity and crushing strength for Test Result 1 at a carbonization temperature of 800° C. and gas condition 1 (no tar). 1 is a graph showing the relationship between apparent specific gravity and crushing strength for Test Result 1 at a carbonization temperature of 800° C. and gas condition 2 (1x tar). 1 is a graph showing the relationship between apparent specific gravity and crushing strength for Test Result 1 at a carbonization temperature of 800° C. and gas condition 3 (2 times tar). 1 is a graph showing the relationship between apparent specific gravity and crushing strength at a carbonization temperature of 1000° C. and gas condition 1 (no tar) for test result 2. 1 is a graph showing the relationship between apparent specific gravity and crushing strength at a carbonization temperature of 1000° C. and gas condition 2 (1x tar), for test result 2. 1 is a graph showing the relationship between apparent specific gravity and crushing strength for test result 2 at a carbonization temperature of 1000° C. and gas condition 3 (2 times tar). 1 is a graph showing the relationship between apparent specific gravity and crushing strength at a carbonization temperature of 1100° C. and gas condition 1 (no tar) for Test Result 3. 1 is a graph showing the relationship between apparent specific gravity and crushing strength at a carbonization temperature of 1100° C. and gas condition 2 (1x tar), for test result 3.

The following describes a method for producing charcoal, which is an embodiment of the present invention. Charcoal is wood that has been heated and heated without oxygen being supplied, resulting in a higher carbon content. Heating and heating under conditions where the maximum temperature is between 200 and 300°C is called torrefaction (semi-carbonization), while heating and heating at a maximum temperature exceeding 300°C is called carbonization. Regardless of the treatment (semi-carbonization or carbonization) that wood is subjected to, the product is charcoal. First, the performance required for charcoal when using charcoal in the steelmaking process, particularly in blast furnace applications, is shown, and the parameters focused on in order to produce charcoal with these performances are described in detail below. Based on this, an outline of the actual process for producing charcoal is also described below.

(Performance required for charcoal for blast furnace charging)
The coal used as the charge coal for the blast furnace top must have high strength, low reactivity to carbon dioxide, and be inexpensive. Each of these factors is explained below.

First, high strength is required for the coal charged into a blast furnace. The reduction reaction inside a blast furnace mainly proceeds as a reaction between reducing gas and iron oxide (ore). Therefore, lowering the air flow resistance inside the blast furnace and allowing the reducing gas to flow efficiently leads to increased blast furnace productivity. If the strength of the carbonaceous material charged into the blast furnace is low and it quickly becomes pulverized, it will increase the air flow resistance inside the blast furnace, causing a decrease in the productivity of the blast furnace. Therefore, even when charcoal is used as charging coal for a blast furnace, the charcoal needs to have high strength.

The charcoal charged into the blast furnace is exposed to shocks during transportation to the furnace top and loads inside the blast furnace (loads from the charges loaded above), but it needs to have sufficient strength to withstand these. Specifically, when charcoal is used as a substitute for coke, it needs a crushing strength of 8 MPa or more, and when charcoal is used as a substitute for nut coke, it needs a crushing strength of 3 MPa or more. In addition, it needs to maintain its original shape without being pulverized for a while after being charged into the blast furnace. Therefore, it is also necessary to reduce the reactivity with carbon dioxide contained in the atmosphere inside the blast furnace. However, if the effect of inorganic compounds is ignored, the higher the strength of the carbon material, the lower the reactivity with carbon dioxide in general, so in this embodiment, the evaluation is also performed with a particular focus on the strength of the charcoal as a product.

Next, the carbonaceous material used in blast furnaces needs to be cheap and available in large quantities. The price of coal used to manufacture blast furnace coke is approximately 15,000 to 20,000 yen per ton, and the amount of blast furnace coke used in Japan is approximately 40 million tons per year. Therefore, even if part of the blast furnace coke is replaced with charcoal, the price of the charcoal must be equal to or lower than the price of blast furnace coke, and wood must be available that can be produced in an amount of at least 1 million tons per year.

(Things to keep in mind when producing high-strength charcoal)
In order to use charcoal as a replacement for blast furnace coke, it is necessary that the charcoal has high strength, is relatively inexpensive, and is available in large quantities. In order to produce charcoal with these properties, we focused on the species of wood used as the raw material, the carbonization temperature, and the carbonization atmosphere. Each item is described below.

(Air-dry specific gravity of wood species)
The properties of wood, the raw material for charcoal, vary greatly depending on the species. First of all, there is a large difference in specific gravity. Regardless of the species, wood is mainly composed of cellulose, hemicellulose, and lignin, but the composition ratio of these components varies greatly depending on the species, and as a result, the density and strength change greatly before and after carbonization.

When comparing the air-dry specific gravity (also called air-dry density), which is the specific gravity after air-drying in the air, although there are variations depending on the growing environment and the part, the wood with the smallest air-dry specific gravity, balsa, is about 0.15 g/cm ³ , while the wood with the highest air-dry specific gravity, lignum vitae, is about 1.4 g/cm ³ , with a difference of about 10 times depending on the tree species. It has been reported that there is an almost proportional relationship between the air-dry specific gravity and the specific gravity after carbonization, and wood with high density is more likely to become charcoal with a high specific gravity (Byrne, Christopher E., and Dennis C. Nagle. "Carbonization of wood for advanced materials applications." Carbon 35.2 (1997): 259-266.). In addition, although there are no quantitative reports, it is known that charcoal with a high specific gravity tends to have high strength. Therefore, if the apparent specific gravity of the charcoal obtained by carbonization is lower than expected, the apparent specific gravity of the charcoal can be increased by reselecting the wood used as the raw material and choosing wood with a higher air-dry specific gravity. Even for the same tree species, the air-dry specific gravity of wood varies depending on the growing environment and age of the tree, so by obtaining wood with guaranteed traceability, it is possible to produce charcoal with the desired apparent specific gravity.

Therefore, it is preferable to select a tree species with a high specific gravity in order to produce high-strength charcoal, but such wood tends to be expensive and difficult to obtain. For example, balsa (air-dry specific gravity of about 0.15 g/cm ³ ), cedar (air-dry specific gravity of about 0.3 g/cm ³ ), cypress (air-dry specific gravity of about 0.4 g/cm ³ ), and birch (air-dry specific gravity of about 0.7 g/cm ³ ) are widely distributed and can be obtained at home improvement stores. On the other hand, yew (about 0.8 g/cm ³ ), white oak (about 0.9 g/cm ³ ), Japanese ebony (about 1.1 g/cm ³ ), and lignum vitae (about 1.4 g/cm ³ ) are not widely distributed and are only used for some high-end furniture. Lignum vitae is designated as an endangered species (Appendix II of the Convention on International Trade in Endangered Species of Wild Fauna and Flora), and special permission is required for import and export. Therefore, it is necessary to be able to produce charcoal with sufficient strength even when using tree species with a relatively low air-dry specific gravity.

(Inorganic components of wood species)
When selecting a tree species, attention must be paid to its composition as well as its air-dry specific gravity. In particular, care must be taken with the inorganic components contained in the tree, as they remain in the charcoal obtained by carbonization. The inner walls of a blast furnace are covered with carbon bricks, and the time until the blast furnace is refurbished varies depending on the lifespan of the carbon bricks. Therefore, it is undesirable for components that damage the carbon bricks to be contained in charcoal, which is used as a substitute for blast furnace coke.

Specifically, alkali elements from Group 1 and halogen elements from Group 17 should be avoided in charcoal because they accelerate wear on the carbon bricks. Palm trunks contain about 2.5% potassium and about 2% chlorine on a dry basis, so they cannot be used as charcoal for blast furnace coke (S.K. Loh, The potential of the Malaysian oil palm biomass as a renewable energy source, Energy Convers. Manag. 141 (2017) 285-298, K.T. Lee, C. Ofori-Boateng, Sustainability of Biofuel Production from Oil Palm Biomass, 1st ed., Springer Singapore, Singapore, 2013, R. Hashim, N. Saari, O. Sulaiman, T. Sugimoto, S. Hiziroglu, M. Sato, R. Tanaka, Effect of particle geometry on the properties of binderless particleboard manufactured from oil palm trunk, Mater. Des. 31 (2010) 4251-4257).

Bamboo is also known to have a very high potassium content, just like palm, and is therefore not suitable for use as charcoal to replace blast furnace coke. Furthermore, bamboo has hollow stems, which reduces the weight per unit volume, resulting in poor transport efficiency (amount transported in one trip). Generally, palms and bamboo are treated as "trees," but because they do not have a cambium, they are strictly herbaceous plants, and are different from woody plants such as conifers and broad-leaved trees. Plants classified as herbaceous plants often contain a high amount of potassium and chlorine, and often have a higher ash content than woody plants, making them unsuitable for use as charcoal to replace blast furnace coke. Although there have been reports of reducing the content of such elements through wet processing, this is extremely time-consuming and therefore unrealistic to subject a large amount of raw material (herbaceous plants) to similar processing. In woody plants, although there are differences depending on the species and part of the tree, the content of alkali elements and halogen elements is generally low, and the ash content of the charcoal obtained by carbonization is also low, at around 1 to 2% by mass. Therefore, it is a good idea to choose woody plants as a source of charcoal.

(Tree ring direction)
Most woody plants available in large quantities have clear annual rings, with the strength of the winter-eared parts being high and the strength of the spring-eared parts being low. Therefore, the strength varies greatly depending on the relative direction to the annual rings (the three directions described below). The crushing strength of a cylindrical trunk is highest in the longitudinal direction. On the other hand, the crushing strength of a cylindrical trunk is lowest in the circumferential direction. The crushing strength of a cylindrical trunk in the radial direction is lower than the crushing strength in the longitudinal direction and higher than the crushing strength in the circumferential direction. The same is true for charcoal after carbonization, and the strength varies greatly depending on the relative direction to the annual rings. Therefore, when evaluating the strength of charcoal, tests should be carried out taking into account the relative direction to the annual rings. In addition, the shrinkage rate of woody plants due to carbonization also differs depending on the relative direction to the annual rings, with the shrinkage rate increasing in the order of longitudinal, radial, and circumferential. Therefore, if there is a particular size required for the final charcoal product, it may be cut in advance to a size that takes into account the shrinkage rate of the selected tree species. Furthermore, if there is no particular size required, the carbonization process may be carried out on wood of any shape, or the wood may be crushed into pellets and then carbonized.

(Carbonization conditions: temperature)
Regardless of the type of wood used, the properties of the charcoal obtained by carbonization strongly depend on the temperature during the carbonization process. When carbonizing charcoal, both the heating rate and the maximum temperature are important. When wood is heated and pyrolyzed, moisture and hydroxyl groups are released at temperatures below 300°C. In the range of 300 to 600°C, the release of carboxyl groups and methyl groups progresses, carbonization progresses, and the weight decreases significantly. Thereafter, as the temperature rises, the release of hydrogen progresses and graphitization progresses. In this process, a large amount of gas is generated and the volume shrinkage is also large, so if the heating rate is too fast, the temperature in the wood becomes uneven, causing unevenness in the shrinkage rate depending on the location in the wood, which causes cracks to form. Therefore, a slow heating rate is preferable.

Specifically, the heating rate is preferably 10°C/min or less, more preferably 5°C/min or less, and even more preferably 3°C/min or less. In particular, when heating in the range of 300 to 600°C where the carbonization reaction mainly proceeds, the heating rate described above is preferably used. In addition, the higher the maximum temperature reached during the carbonization process, the more graphitization proceeds, which is generally preferable, but it is known that if the maximum temperature is too high, the amount of heat required to reach the maximum temperature increases, which increases costs, and microcracks occur in the charcoal, resulting in a decrease in strength. Therefore, the maximum temperature reached is preferably 700°C or more and 1200°C or less, and more preferably 800°C or more and 1200°C or less. In addition, by maintaining the temperature during the carbonization process near the maximum temperature for a certain period of time, the temperature inside the charcoal particles becomes uniform, and the entire particle can be carbonized uniformly. Specifically, it is preferable to maintain the maximum temperature within a temperature range of ±50°C for 10 minutes or more, more preferably 30 minutes or more, and even more preferably 60 minutes or more.

As shown in the examples described later, by heating wood to 800°C (maximum temperature reached), charcoal with an apparent specific gravity of 0.55 g/ ^cm3 or more can be produced, and wood with an air-dry specific gravity of 0.68 g/cm3 ^or more can be used as the raw material for this charcoal. Also, by heating wood to 1000°C (maximum temperature reached), charcoal with an apparent specific gravity of 0.48 g/cm3 or ^more can be produced, and wood with an air-dry specific gravity of 0.55 g/ ^cm3 or more can be used as the raw material for this charcoal. Furthermore, by heating wood to 1100°C (maximum temperature reached), charcoal with an apparent specific gravity of 0.45 g/ ^cm3 or more can be produced, and wood with an air-dry specific gravity of 0.55 g/cm3 ^or more can be used as the raw material for this charcoal.

(Carbonization conditions: atmosphere)
It is known that the atmosphere during the carbonization process also has a significant effect on the quality of the charcoal. In this embodiment, in order to improve the strength of the charcoal, a carbonization test is performed under tar vapor flow conditions. Below, the technical concept of this method is presented, and the introduction conditions of the tar vapor are explained based on this concept.

There are two main reasons why the strength of charcoal produced by carbonizing wood is lower than that of blast furnace coke. The first reason is that graphitization is difficult to proceed. Wood does not melt at low temperatures, and the molecules that make up wood are huge molecules that contain a large amount of linear chains and ether bonds. Therefore, before the molecular rearrangement that accompanies melting proceeds, the carbon bonds proceed three-dimensionally, and graphitization does not proceed. The second reason is that charcoal has a structure that contains a large number of pores. Charcoal has a porous structure that retains the cell and vascular structures of the raw material wood. Therefore, compared to blast furnace coke, it is thinner, has a smaller specific gravity, and is lower in strength. For the above reasons, in order to increase the strength of charcoal, it is necessary to promote graphitization during the carbonization process and fill the pores to increase the specific gravity.

Supplying tar vapor to wood during the carbonization process promotes graphitization of the charcoal during the carbonization process, and fills the pores, increasing the specific gravity. The mechanism is outlined below. Tar is a liquid that contains many aromatic compounds. Molecules that contain many aromatic rings contain many graphite precursors and have the effect of promoting graphitization. Therefore, the technique of mixing tar with other substances to promote graphitization has long been used. However, it is known that the strength of the charcoal does not improve as expected because the large amount of oxygen contained in wood promotes the three-dimensional reaction of the tar supplied to the wood. In addition, much of the tar supplied to the wood volatilizes and dissipates during the heating process.

Therefore, in this embodiment, tar is continuously supplied even at temperatures of 700°C or higher where the wood has been carbonized, so that graphitization is effectively promoted without being affected by oxygen contained in the wood. This improves the strength of the charcoal obtained by the carbonization process. In addition, the tar itself is carbonized within the pores of the charcoal, which fills the cracks and pores of the charcoal and increases the specific gravity. This eliminates cracks that cause a significant decrease in strength, improves the strength of the charcoal, and suppresses the variation in strength. In particular, in examples under conditions of high tar vapor concentration, the graphitization of the charcoal progressed efficiently, and densification accompanied by a significant change in shape was confirmed. In this embodiment, charcoal with a predetermined apparent specific gravity or more is produced to obtain charcoal with high strength. If the apparent specific gravity of the charcoal obtained by the carbonization process is lower than the target apparent specific gravity, it is effective to increase the amount of tar introduced when performing the carbonization process again using the same wood. It is also effective to change the location of the raw material (wood) in the equipment used to produce charcoal (for example, the carbonization chamber of a coke oven, which will be described later) to increase the partial pressure of the tar that reacts with the wood. These measures increase the apparent specific gravity of the charcoal, and therefore its strength.

Various materials can be used as the tar to be supplied to the charcoal during the carbonization process. Coal tar generated during the thermal decomposition of various ranks of coal, asphaltene, which is a high-boiling fraction of petroleum, and wood vinegar obtained during the thermal decomposition of wood can also be used. However, general wood vinegar contains a large amount of water, and when charcoal is used as a substitute for blast furnace coke as in this embodiment, it is not preferable because it causes gasification of the charcoal by steam and a decrease in strength. Furthermore, most of the organic matter contained in wood vinegar is carboxylic acid or aldehyde (containing oxygen atoms), etc., and does not exhibit the effect of promoting graphitization. Therefore, when wood vinegar is used, it is preferable to reduce the amount of water and oxygen-containing components by devising a recovery method. In addition, the more high-boiling point components in the tar, the more they adhere to the charcoal during the carbonization reaction, and the greater the effect of improving the strength of the charcoal. Therefore, the more high-boiling point components there are, the more preferable it is.

In order to increase the strength of charcoal, it is of course important to avoid the inclusion of gas species that cause a decrease in the strength of charcoal in the atmosphere during the carbonization process as much as possible. In particular, when the charcoal is exposed to oxygen-containing gases such as water vapor or carbon dioxide at a high temperature of 800°C or higher, part of the charcoal will gasify, causing a significant decrease in strength. Therefore, in the temperature range of 800°C or higher, it is preferable to reduce the concentration of water vapor and carbon dioxide as much as possible.

(Outline of carbonization process)
The operating conditions of the carbonization process are outlined below. First, the carbonization process equipment is preferably one that can sufficiently suppress the intrusion of air and can heat the wood at a heating rate of 10° C./min or less. The carbonization process equipment is also provided with a portion for supplying tar steam, such as an inlet that can introduce tar steam into the entire packed bed of wood during the carbonization reaction. Before starting the introduction of tar steam, it is preferable that the wood is dried in advance, and more preferably, the carbon content is increased by a torrefaction process. These pretreatments reduce the endothermic reaction associated with the evaporation of moisture, making it easier to control the temperature of the atmosphere during the carbonization process, and improving the productivity of charcoal.

As mentioned above, it is preferable that the tar vapor to be introduced has a high ratio of high boiling point components. In this case, when the introduction of tar vapor is started when the temperature in the packed bed of wood is low, a large amount of tar vapor may condense in the packed bed, causing blockage. Therefore, it is preferable to start the introduction of tar vapor after the temperature in the packed bed reaches a predetermined temperature or higher. Specifically, it is preferable to start the introduction of tar vapor after the temperature in the packed bed reaches 200°C or higher, and it is more preferable to start the introduction of tar vapor after the temperature in the packed bed reaches 300°C or higher. Here, the temperature in the packed bed of wood can be determined from the temperature of the atmosphere during the carbonization process.

A specific example of a carbonization process that satisfies the above conditions is the production of charcoal using the upper space of a coke oven (carbonization chamber). First, wood that has been dried or torrefied in advance is cut and crushed to a size that can be loaded into the upper space of the carbonization chamber, and then loaded onto the top of the packed bed of coal that has already been loaded into the carbonization chamber. After that, the coke oven is operated as usual to promote the carbonization reaction of the coal, which generates gas containing tar components, and the upper space where the wood is loaded becomes an atmosphere of tar vapor, making it possible to produce charcoal supplied with tar vapor. Here, the tar components contained in the gas generated by the carbonization reaction of the coal are used as the tar vapor introduced into the wood.

The temperature rise rate in the coke oven is slow, and tar is generated by the pyrolysis of coal and moves to the upper space of the carbonization chamber, so the tar vapor reaches the wood layer only after the temperature in the carbonization chamber reaches 300°C or higher. At this time, charcoal is packed intensively in the upper space of the coke oven (carbonization chamber), so tar vapor can be added to the upper space in addition to the tar vapor generated from the coal. In addition, since the gas discharged from the carbonization chamber contains oxygen-containing components derived from wood, the reforming reaction of the gas discharged from the carbonization chamber becomes easier than when wood is not charged. When charging wood into the coke oven, it is important to avoid charging wood near the bottom of the coke oven (carbonization chamber) or mixing wood with coal. This is because, as shown in previous studies, when gas components generated during the pyrolysis of wood come into contact with coal, it reduces the quality of the product coke.

(Method of evaluating charcoal products)
The method for evaluating the quality of charcoal obtained by carbonization is described below. First, the composition of charcoal can be determined by the same method as that usually performed for coal or coke. Standard analysis methods according to JIS standards may be used. When quantifying moisture, ash, and volatile matter by industrial analysis, the method described in JIS M8812 may be used. When quantifying carbon, hydrogen, sulfur, nitrogen, phosphorus, and oxygen by elemental analysis, the method described in JIS M8813 may be used. An example of a specific measurement procedure is described below.

For moisture analysis, an air-dried sample that has been thoroughly dried in the air or in a humidity-controlled space is used. Approximately 1 g of the air-dried sample is weighed out and dried at approximately 107°C for approximately one hour, then cooled in a desiccator before being weighed. At this time, the atmosphere in the drying chamber can be selected from air, nitrogen, helium, etc. For ash analysis, 1 g of the air-dried sample is weighed out and incinerated in the air at approximately 815°C, and the weight of the resulting ash is measured. For volatile matter analysis, 1 g of the weighed air-dried sample is dry-distilled at approximately 900°C, and the change in weight before and after dry-distillation is taken as the volatile matter.

For elemental analysis, 0.06 g of air-dried sample is weighed out and burned at 950°C in an analyzer (e.g., Organic Elemental Analyzer CHN628, manufactured by LECO Japan LLC), the exhaust gas is analyzed, and the carbon, hydrogen, and nitrogen are quantified. For sulfur quantification, 0.6 g of air-dried sample is burned at approximately 1350°C in an oxygen stream, the sulfur oxide in the exhaust gas is absorbed in hydrogen peroxide water, and titrated with NaOH. If phosphorus quantification is required, it is quantified using molybdenum blue absorptiometry or vanadomolybdenum yellow absorptiometry in accordance with Annex 6 of JIS M8813. The amount of oxygen can be roughly estimated by subtracting the masses of carbon, hydrogen, nitrogen, and sulfur from the total mass. Here, according to JIS M8813, the ash content and the sulfur components in the ash can also be taken into account when calculating the amount of oxygen.

In the strength measurement, JIS standard analysis methods for coal and coke (for example, the drop strength test and rotational strength test specified in JIS K2151) require test pieces weighing several tens of kilograms, making it difficult to compare under various carbonization conditions. Therefore, in this example, the evaluation was performed using the crushing strength as an index. A load was applied to the test piece from above, and the load at the time of destruction was recorded. The crushing strength was calculated as the pressure value at the time of destruction using the load value and the dimensions of the test piece. In this case, if the test piece was a rectangular parallelepiped or a similar shape, the crushing strength was calculated by dividing the load at the time of destruction by the base area. When the test specimen had a shape other than a rectangular parallelepiped, the crushing strength was calculated as the indirect tensile strength from the crushing strength according to the following formula (1) based on previous reports (Y. Hiramatsu, Y. Oka, H. Kiyama, Rapid Determination of the Tensile Strength of Rocks with Irregular Test Pieces, J. Min. Metall. Inst. Jpn., 932, 1965, 1024-1030.).

σt=0.9P/ ^d2 (1)
Here, σt is the indirect tensile strength [MPa], P is the crushing strength [N], and d is the diameter [mm] of the test piece. As mentioned above, it is known that the strength of charcoal varies greatly depending on the relative direction to its annual rings, so the strength in multiple load directions should be examined using multiple test pieces. Note that if the shape of the test piece was very distorted and it was determined that it was difficult to perform the crushing strength test, the crushing test was not performed on that test piece.

The apparent specific gravity of the charcoal is preferably 0.55 g/cm ³ or more. The apparent specific gravity of the charcoal can be calculated using the mass and dimensions of the test piece. When the test piece is a rectangular parallelepiped or a shape close to it, the mass [g] of the test piece is measured, the volume [cm ³ ] is calculated based on the lengths of the three sides constituting the test piece (rectangular parallelepiped), and the apparent specific gravity [g/cm ³ ] can be calculated by dividing the mass (measured value) by the volume (calculated value). When the test piece is a shape other than a rectangular parallelepiped, the volume can be estimated by image analysis. In this case, it is not preferable to use the Archimedes method or the like when evaluating the volume. This is because the pores present on the surface of the test piece are not included in the volume of the test piece, so the density will be about the same regardless of the presence or absence of pores on the charcoal surface. In this state, if the pores of the charcoal are blocked by tar vapor and the number of closed pores increases, the void volume included in the volume of the test piece will increase, and the density of the test piece will decrease. If volumetric evaluation is to be performed by an immersion method (such as a water immersion method), it is preferable to vacuum-pack the test piece in thin vinyl and then perform the measurement.

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

(Test 1: Test Piece Cutting)
The wood of the tree species shown in Table 1 below was obtained, cut into 8 mm square blocks with an electric saw, and the crushing strength test described below was performed. For each tree species, more than 150 test pieces were prepared to allow a sufficient amount of tests to be performed. All test pieces were cubes with sides in three directions: longitudinal, radial, and circumferential. For Lignum vitae, the annual rings were curved inside the obtained board, so some test pieces were cut in a direction different from the above directions. It should be noted that the air-dry specific gravity values shown in Table 1 below are actual measurements of wood obtained by the inventor, and do not necessarily take the same value for all wood of the same tree species. It is known that the air-dry specific gravity of the same tree species varies greatly depending on the growing conditions and the part. Therefore, it is important to check the physical properties of the wood obtained in advance.

(Test 2: Carbonization test)
A carbonization test was carried out using the test pieces prepared in the above test 1. Approximately 10 of the prepared test pieces were placed in an alumina tube with an inner diameter of approximately 20 mm and an outer diameter of 25.4 mm, and the test pieces were fixed with silica wool. The alumina tube was placed in a ring furnace, and while passing gas under gas conditions 1 to 4 shown in Table 2 below, the temperature was raised to 800°C, 1000°C, or 1100°C at a heating rate of 3°C/min, and the maximum temperature (800°C, 1000°C, or 1100°C) was held for 1 hour.

As shown in Table 2 above, in gas condition 1, tar vapor was not introduced and nitrogen gas was passed at a flow rate of 100 Ncm ³ /min. In gas condition 2, 1-methylnaphthalene (tar vapor) was passed at a flow rate of 1.7 Ncm ³ /min (reference value for gas conditions 3 and 4), and nitrogen gas was passed at a flow rate of 98.3 Ncm ³ /min. In gas condition 3, 1-methylnaphthalene (tar vapor) was passed at a flow rate of 3.4 Ncm ³ /min (twice the above reference value), and nitrogen gas was passed at a flow rate of 96.6 Ncm ³ /min. In gas condition 4, 1-methylnaphthalene (tar vapor) was passed at a flow rate of 6.8 Ncm ³ /min (four times the above reference value), and nitrogen gas was passed at a flow rate of 93.2 Ncm ³ /min.

In gas conditions 2 to 4, in which tar vapor was introduced, the introduction of tar vapor was started when the temperature exceeded 300°C, which is above the boiling point (approximately 240°C) of 1-methylnaphthalene added as a tar component, in order to prevent condensation of the tar vapor in the piping. In all gas conditions 1 to 4, the temperature near the test piece was measured with a thermocouple held inside the alumina tube, and the temperature of the ring furnace was adjusted based on that temperature. After the carbonization test, the test piece was gradually cooled at a cooling rate of 10°C/min or less, and was allowed to cool when it reached a temperature of 400°C or less. After confirming that the temperature had dropped to near room temperature, the charcoal test piece was collected. It has been confirmed that 1-methylnaphthalene, which was added as a tar component in this test, is also contained as a tar component in actual coke oven gas. In addition to two-membered ring compounds such as 1-methylnaphthalene, actual tar also contains large amounts of three-membered ring compounds, but these are solid at room temperature and difficult to handle, so 1-methylnaphthalene was used.

(Test 3: Crushing Strength Test)
A crushing strength test was carried out on the charcoal test pieces obtained in the above test 2. Before the measurement, each charcoal test piece was photographed, and the lengths in the three directions, the longitudinal direction, the radial direction, and the circumferential direction, were measured, and the volume was calculated. Next, the weight of each charcoal test piece was measured, and the apparent specific gravity [g/cm ³ ] was calculated by dividing the weight by the calculated volume. Then, the charcoal test pieces were divided into two groups, one group was used to measure the crushing strength in the longitudinal direction, and the other group was used to measure the crushing strength in the circumferential direction. As the crushing test device, a uniaxial compression tester (KS-205B (electric type), manufactured by Kansai Machinery Manufacturing Co., Ltd.) was used. Since all of the charcoal test pieces used for measuring the crushing strength had a shape close to a rectangular parallelepiped, one of the faces was placed on the test table of the crushing test device with one of the faces facing down. At this time, the face on which the load was applied in the direction to be measured was selected, and the face was placed facing down. The downward movement of the compression plate installed above the charcoal test piece was started, and the charcoal test piece was compressed from above, thereby applying a load to the charcoal test piece. The compression speed was set to 1000 μm/min or less, and the load at which the charcoal test piece broke was used to calculate the crushing strength.

For all wood species, the longitudinal crushing strength was higher than the circumferential crushing strength. Even for balsa, which had the lowest charcoal strength, the longitudinal crushing strength exceeded 8 MPa in all gas conditions 1 to 4 in the carbonization test at 800°C or higher, confirming sufficient strength. Therefore, in the following explanations (test results 1 to 3), we will focus only on the circumferential crushing strength, which is relatively weak. Note that when the test was conducted under "Gas condition 4: 4 times tar" in Table 2 above, many of the charcoal test pieces were significantly deformed, making it difficult to measure the crushing strength and calculate the volume. This is thought to be because the charcoal test pieces were exposed to a large amount of tar vapor, which caused excessive graphitization. All of the deformed charcoal test pieces withstood a higher load than the "Gas condition 1: no tar" case, but due to the above reasons, it was not possible to calculate the crushing strength.

(Test 4: Statistical processing using Weibull distribution)
In the above test 3, test results were obtained for various tree species, carbonization temperatures, and carbonization atmospheres, but wood is a non-uniform material, and unavoidable variations occur. Therefore, statistical processing was performed to analyze the test results, and the effect of each parameter described below on the crushing strength of charcoal was evaluated. Based on previous research, we assumed that the crushing strength of charcoal is strongly dependent on the apparent specific gravity, and evaluated the relationship between the apparent specific gravity and the crushing strength. For the statistical model, the Weibull distribution, which well describes the strength of materials, was used, and the parameters were determined using maximum likelihood estimation. The Weibull formula is shown in the following formula (2).

f(y; λ, k) = k/λ × (x/λ) k - 1 × exp(-(x/λ) k)
... (2)
Here, y is the crushing strength [MPa] and is a value of 0 or more, and λ and k are both functions that depend on the apparent specific gravity x, and are represented by the following formulas (3) and (4), respectively.

λ = _a1 x + _a2 ^x2 ... (3)
k = _b1 + _b2 × x ... (4)
Here, _a1 , _a2 , _b1 , and _b2 are parameters determined for each carbonization temperature and carbonization atmosphere. The reason for selecting a total of four parameters _a1 , _a2 , _b1 , and _b2 is that when multiple combinations of parameters were compared using Akaike Information Criterion (AIC), it was concluded that the above combinations were optimal. The strength of the charcoal test pieces obtained under each of the gas conditions 1 to 3 shown in Table 2 above was analyzed, and the obtained parameters _a1 , _a2 , _b1 , and _b2 are shown in Table 3 below.

(Test result 1: Carbonization temperature 800°C)
1 to 3 show the relationship between apparent specific gravity [g/cm ³ ] and crushing strength (circumferential direction) [MPa] (results of statistical processing of the above test 4) when the maximum temperature reached in the carbonization test is 800° C. Here, FIG. 1 shows the test results under gas condition 1 in Table 2 above, FIG. 2 shows the test results under gas condition 2 in Table 2 above, and FIG. 3 shows the test results under gas condition 3 in Table 2 above. It can be seen that, compared to the case where tar vapor was not introduced during the carbonization test (FIG. 1), when tar vapor was introduced during the carbonization test (FIGS. 2 and 3), the variation in crushing strength at the same apparent specific gravity was kept small, and particularly the variation at low crushing strength was kept small. Moreover, under gas condition 3 (FIG. 3) where the flow rate of tar vapor is high, the crushing strength showing a peak at each apparent specific gravity is higher than under gas condition 2 (FIG. 2) where the flow rate of tar vapor is low. Based on the test results shown in Figures 1 to 3, an evaluation was conducted to see how the probability of obtaining charcoal with a crushing strength of 8 MPa or more changes with each apparent specific gravity under each of the gas conditions 1 to 3. The results are shown in Table 4 below.

As is clear from the results of Table 4 above, by introducing tar vapor into the carbonization atmosphere, the probability of generating charcoal with a crushing strength of 8 MPa or more is improved compared to the case where tar vapor is not introduced into the carbonization atmosphere, and the variation in charcoal strength during carbonization tests at 800°C is suppressed. In addition, the probability of obtaining charcoal with a crushing strength of 8 MPa or more is dramatically increased even for low-density charcoal with an apparent specific gravity of 0.7 g/cm3 or ^less . Specifically, even for charcoal with an apparent specific gravity of 0.55 g/ ^cm3 , the majority of charcoal has a crushing strength of 8 MPa or more. It was confirmed that the majority of charcoal made from walnut (see Table 1 above), a wood with an air-dry specific gravity of 0.68 g/ ^cm3 , has a crushing strength of 8 MPa or more. This means that this charcoal has a level of strength that can be used in blast furnaces.

(Test result 2: Carbonization temperature 1000°C)
4 to 6 show the relationship between apparent specific gravity [g/cm ³ ] and crushing strength (circumferential direction) [MPa] (the results of statistical processing of the above test 4) when the maximum temperature reached in the carbonization test was 1000° C. Here, FIG. 4 shows the test results under gas condition 1 in Table 2 above, FIG. 5 shows the test results under gas condition 2 in Table 2 above, and FIG. 6 shows the test results under gas condition 3 in Table 2 above.

Comparing the case where tar vapor was not introduced during the carbonization test (Figure 4) with the case where tar vapor was introduced during the carbonization test (Figures 5 and 6), it can be seen that the introduction of tar vapor slightly suppressed the variation in the crushing strength at the same apparent specific gravity, but the suppression of variation was not as significant as in Test Result 1 (800°C) above. The reason for this is thought to be that when the maximum temperature reached in the carbonization test was 1000°C, the carbonization reaction proceeded sufficiently even in the absence of tar vapor. Therefore, when the maximum temperature reached in the carbonization test was 1000°C, it is thought that the same level of crushing strength can be obtained for charcoal with the same apparent specific gravity, regardless of whether tar vapor was introduced or not. On the other hand, when charcoal test pieces of the same tree species were compared at multiple concentrations of tar vapor, a tendency was observed in which the apparent specific gravity and crushing strength increased as the concentration of tar vapor increased. The results are shown in Table 5 below. Table 5 below compares the average apparent specific gravity and average circumferential crushing strength of approximately 10 charcoal test pieces made for each wood species under each gas condition 1 to 3.

The results in Table 5 above show that for all of the results for cypress, American cherry, and walnut, the apparent specific gravity of the charcoal obtained under gas conditions 2 and 3 is higher than the apparent specific gravity of the charcoal obtained under gas condition 1, and the crushing strength also increases accordingly. For white oak, the increase in apparent specific gravity and crushing strength is smaller than for the other three tree species (cypress, American cherry, and walnut), but this is thought to be because the charcoal has a high apparent specific gravity to begin with, and the effect of some of the charcoal's pores being filled by the introduction of tar vapor was not very noticeable.

It was confirmed that the introduction of tar steam caused the apparent specific gravity (gas condition 3) after the carbonization test to exceed 0.50 g/cm ³ and the crushing strength to exceed 8 MPa, even for wood such as American cherry, whose air-dry specific gravity before the carbonization test was not very high at 0.55 g/cm ^3. Also, according to the results of Table 5 above, it was confirmed that when the apparent specific gravity after the carbonization test exceeded 0.48 g/cm ³ , the average crushing strength exceeded 8 MPa. It was also found that, by using wood whose air-dry specific gravity before the carbonization test exceeded 0.55 g/cm ³ as the raw material, charcoal with an average crushing strength exceeding 8 MPa was obtained under gas conditions 2 and 3 in which tar steam was introduced. This means that this charcoal has a level of strength that can be used in a blast furnace.

(Test result 3: Carbonization temperature 1100°C)
7 and 8 show the relationship between apparent specific gravity [g/ ^cm3 ] and crushing strength (circumferential direction) [MPa] (results of statistical processing of the above test 4) when the maximum temperature reached in the carbonization test is 1100°C. Here, FIG. 7 shows the test results under gas condition 1 in Table 2 above, and FIG. 8 shows the test results under gas condition 2 in Table 2 above. It can be seen that the crushing strength, which peaks at the same apparent specific gravity, is higher when tar vapor is introduced during the carbonization test (FIG. 8) compared to when tar vapor is not introduced during the carbonization test (FIG. 7). Based on the test results shown in FIG. 7 and FIG. 8, an evaluation was conducted to see how the probability of obtaining charcoal with a crushing strength of 8 MPa or more changes with each apparent specific gravity under gas conditions 1 and 2, respectively, and the results are shown in Table 6 below.

As is clear from the results in Table 6 above, by introducing tar vapor into the carbonization atmosphere, the crushing strength of charcoal is improved when carbonization tests are conducted at a maximum temperature of 1100°C, and even for low-density charcoal with an apparent specific gravity of 0.45 g/ ^cm3 , the majority of charcoal (gas condition 2) has a crushing strength of 8 MPa or more. It was confirmed that the majority of charcoal made from American cherry, a wood with an air-dry specific gravity of 0.55 g/ ^cm3 before the carbonization test, had a crushing strength of 8 MPa or more.

Claims (9)

A method for producing charcoal, comprising the steps of: supplying tar vapor to wood while heating the wood to 800°C to carbonize the wood; and producing charcoal having an apparent specific gravity of 0.55 g/ ^cm3 or more. A method for producing charcoal, comprising the steps of: supplying tar vapor to wood having an air-dry specific gravity of 0.68 g/cm3 or ^more ; and heating the wood to a temperature of 700°C or more and 1200°C or less to carbonize the wood. A method for producing charcoal, comprising the steps of: supplying tar vapor to wood while heating the wood to 1000°C to carbonize the wood, thereby producing charcoal having an apparent specific gravity of 0.48 g/ ^cm3 or more. A method for producing charcoal, comprising the steps of: supplying tar vapor to wood having an air-dry specific gravity of 0.55 g/cm3 or ^more ; and heating the wood to a temperature of 700°C or more and 1200°C or less to carbonize the wood. A method for producing charcoal, comprising the steps of: supplying tar vapor to wood while heating the wood to 1100°C to carbonize the wood; and producing charcoal having an apparent specific gravity of 0.45 g/ ^cm3 or more. 6. The method for producing charcoal according to claim 5 , wherein the air-dry specific gravity of the wood is 0.55 g/ ^cm3 or more. While supplying tar vapor to the wood, the wood is heated to a temperature of 700° C. or more and 1200° C. or less (excluding 700° C.) to carbonize the wood,
A method for producing charcoal, comprising filling the wood on top of a packed bed of coal filled in a carbonization chamber of a coke oven, and using tar components contained in a gas generated by carbonization of the coal as the tar vapor. While supplying tar vapor to the wood, the wood is heated to a temperature of 700° C. or more and 1200° C. or less (excluding 700° C.) to carbonize the wood,
A method for producing charcoal , characterized in that the charcoal is used as a substitute for blast furnace coke. The method for producing charcoal according to any one of claims 1 to 8, characterized in that the temperature of the wood is increased at a rate of 10°C/min or less.