WO2003106591A1

WO2003106591A1 - A method for making a charcoal precursor and a method for making charcoal

Info

Publication number: WO2003106591A1
Application number: PCT/NO2003/000195
Authority: WO
Inventors: Inger Johanne Eikeland; Ola Raaness; Geraint William Rees
Original assignee: Elkem Asa
Priority date: 2002-06-17
Filing date: 2003-06-13
Publication date: 2003-12-24
Also published as: AU2003237722A1

Abstract

Raw wood is heated to a temperature of 170° C and above, for a period of 3 hours and above in the presence of water to pretreat the raw wood and form a charcoal precursor having reduced ash content and increased carbon content. The charcoal precursor is then retorted with a solid carbon material to produce a charcoal having a carbon yield of over 90%.

Description

Title of Invention

A method for making a charcoal precursor and a method for making charcoal.

Background of the Invention

1. Field of the Invention

This invention relates to charcoal and, more particularly, to a method for making charcoal, a precursor used for making charcoal and a method for making the precursor.

2. Art Relating to the Invention

Charcoal is conventionally made in a retort process where raw wood is heated in a furnace to a temperature of about 450 to 800°C in the absence of oxygen for an extended period of time. After heating, the charcoal is discharged from the furnace and cooled. Tar is a by-product of the retort process while the off- gases from the retort process include carbon monoxide (CO), carbon dioxide (C0₂), methane (CH₄) and water (H₂0).

Charcoal has a wide variety of uses to include a reducing agent in the pyrometallurgical production of metals and alloys. The carbon content of charcoal is an important factor because the higher the carbon content, the greater the value of the charcoal as a reducing agent. Typically, charcoal has a carbon content of about 60 to about 90% by weight based on the total weight of charcoal. Charcoal is conventionally made from raw wood or a biomass such as corn cobs, bark of trees, and coconut shells. Raw wood from trees is usually used to make charcoal which is intended for use as a reducing agent in metal and alloy manufacture.

Raw wood is made up of lignin, carbohydrate (cellulose and hemicellulose), minerals and other organic components. For example, Norwegian spruce contains about 24% by weigh lignin, about 43% by weight cellulose, about 27% by weight hemicellulose and about 6% by weight minerals and other organics. In Norwegian spruce, the amount of carbon is about 47% by weight, the amount of oxygen is about 46% by weight, and amount of hydrogen is about 6.5% by weight. The minerals and other organics comprise such elements as aluminum (Al), calcium (Ca), boron (B), phosphorous (P), and titanium (Ti).

The loss of carbon during the retort process is a problem, and attempts have been made to reduce the loss of carbon. Typically, the yield of carbon from the retort process is about 20 to about 30%. The yield of carbon is defined as the weight of carbon in the charcoal divided by the weight of carbon in the raw wood, multiplied by 100, while the amount of carbon in charcoal is about 60% to 90% by weight of charcoal. The amount of carbon in the charcoal varies depending on the maximum heating temperature during the retort process.

One attempt to solve the loss of carbon process is suggested in an article entitled "Attainment of the Theoretical Yield of Carbon from Biomass", M.J.

Antal Jr. et al. in Ind. Eng. Chem. Res. 2000, 394024-4031. In this process, retort is conducted at a temperature of about 400 to about 450°C at a pressure of IMpa. It is reported that the yield of carbon is 27-29% from this process.

Other attempts to decrease the loss of carbon have led to the development of a continuous shaft process where raw material is continuously or substantially continuously supplied to the top of the furnace and charcoal is continuously or substantially continuously discharged from the bottom of the furnace. Such a process has been reported to have a yield of carbon of about 30%.

Since the cost of wood is a substantial cost factor in the production costs of charcoal, it is important to increase the yield of carbon and reduce the loss of carbon to the off-gas and tar by-products.

SUMMARY OF THE INVENTION A method has now been discovered which decreases the loss of carbon during the production of charcoal. The method of the present invention reduces the production of off-gases, especially carbon monoxide (CO), carbon dioxide (C0₂) and methane (CH₄). It has also been found that the process of the present invention reduces the ash content of the charcoal.

Broadly, the present invention entails pretreating raw wood prior to the retort x process. The term raw wood includes saw dust, bark and other materials originating from plant materials. This pretreatment forms the charcoal precursor of the present invention. The charcoal precursor can be subjected to a conventional retort process to produce charcoal or it can be used directly as a reduction material for the production of metals and alloys.

The pretreatment method to form the charcoal precursor can be defined as treating raw wood in a sealed vessel in the presence of water, for a time of about 3 hours or more, and at a temperature of about 170°C or more, and recovering the treated wood as the charcoal precursor. The charcoal precursor can also be used as a carbon reduction material for the production of metals and alloys.

Preferably, the time for the pretreatment is about 6 hours or more. Suitably, the time for pretreatment is about 3 hours to about 8 hours.

The temperature during pretreatment is preferably about 180°C or more and, more preferably, about 210°C or more. Suitably, the temperature range during pretreatment is about 170°C to about 220°C.

The sealed vessel is such that it can operate at high pressure and the atmosphere in the vessel can be controlled during the pretreatment step. Suitably, a conventional digester is used for the sealed vessel.

The atmosphere during the pretreatment should have water vapor at equilibrium pressure for the temperature in the vessel. It is preferred that the atmosphere, during pretreatment, be without non-condensable gases. In other words, only condensable gases, such as water vapor, are present in the vessel.

In order to create an atmosphere absent of non-condensable gases, it is preferred to add water to the vessel and boil the water in the vessel before the vessel is sealed. In this way the atmosphere in the vessel is filled with steam and non-condensable gases are driven out of the vessel. Such boiling is done in a conventional manner in the vessel by simply heating the water in the vessel before sealing the vessel. Steam can also be injected into the vessel to fill the atmosphere of the vessel before closing the vessel. Preferably, the wood is saturated with water prior to pretreatment.

The water present in the vessel during pretreatment can come from the moisture in the raw wood itself. For example, raw wood contains about 20% moisture (free water) based on the weight of wood. Water can also be added to the vessel prior to pretreatment.

It has been found that the pretreatment of the present invention causes the cellulose to undergo a condensation and polymerization process which breakes down the structure of the cellulose whereby the wood looses hydrogen (H) and oxygen (O) in the form of water (H₂0) while leaving behind the carbon. In one example, the raw wood had a water content of 15% (free water) but pretreatment liberated 40.0% by weight water from the wood. It is believed that the additional 25% of water resulted from the decomposition of the cellulose structure of the wood. The formation of water in the raw wood confirms that condensation polymerization has taken place.

It has also been found that pretreatment causes acetic acid and fatty acids which have solvent on chelating properties to form in situ. These acids have been found to transfer organically bound ash-forming elements like aluminum (Al), calcium (Ca), boron (B), phosphorous (P) and titanium (Ti) in a water soluble form and out of the wood. Thus, the pretreated wood results in a charcoal with a lower ash content.

Furthermore, it has been found that the water separated from the wood after pretreatment is high in phosphate, primarily in the form of orthophosphate as well as organically dissolved nitrogen. Tests have shown that these liquid byproducts from the pretreatment phase can be used as growth-promoting agents for plants such as grass.

In order to promote the extraction of ash-forming elements from the wood during pretreatment, the atmosphere in the vessel has condensable gases therein and, after the pretreatment step, a vacuum is applied to the wood in the vessel such that water is extracted from the pores of the wood. This extracted water contains the ash-forming elements as well as the other organics. Preferably, this vacuum is created by cooling the vessel and thereby forming the vacuum in the vessel. It has been found that the ash containing elements can be greatly reduced. It has been found that through the pretreatment and cooling to create a vacuum that the ash content of the wood was decreased from 1 % by weight to 0.1% by weight and in some cases 0.02% by weight. The lowest ash level was obtained when soaking wet wood was treated.

Thus, the present invention pretreatment results in a charcoal precursor which, when subjected to a retort process to make charcoal, yields a charcoal which is substantially higher in carbon that non-pretreated wood, and a coal with substantially reduced ash content compared to non-pretreated wood. Furthermore, the liquid by-product from the pretreatment can be used as a plant growth promoter.

Preferably, the pretreated wood is dried before the retort process to remove water. Drying the pretreated wood is done in a conventional manner with conventional equipment. Suitably, drying is conducted at a temperature up to about 130°C.

Retorting the pretreated wood or charcoal precursor is done in a conventional manner using conventional equipment. Preferably, the temperature during the retort process is about 450°C to about 750°C.

It has also been discovered that the yield of carbon in the charcoal can be increased if the retort process is carried out in the presence of a solid carbon material having a low content of volatile matter. It is believed that when the retort process is carried out in the presence of a solid carbon material having a low content of volatile matters, gaseous hydrocarbon components that form during retort of the pretreated wood are condensed and cracked to solid carbon on the surface of the solid carbon material, thus increasing the overall carbon yield in the produced charcoal.

The solid carbon material having a low content of volatile matter used in the retort process is preferably anthracite or charcoal, but other carbon materials having a low content of volatile matters such as graphite, metallurgical coke and petrol coke can also be used. A particularly suited carbon material having a low content of volatile matter for use in the present invention is charcoal produced by the method of the present invention. Thus, part of the charcoal produced in the present invention can be recycled to the retort process.

It is also preferred that the retort process be carried out in a shaft furnace where the pretreated wood is continuously or substantially continuously supplied to the top of the shaft together with the carbon material having a low content of volatile matter, and charcoal is continuously or substantially continuously discharging from the bottom of the shaft together with the carbon material having a low content of volatile matter. Then, the charcoal is separated from the carbon material having a low volatile matter.

The charcoal is an excellent reducing material for the production of metals and especially silicon because of its high carbon content and low ash content. This is especially true for solar grade silicon. Further, the charcoal has active carbon properties as it is able to decompose methylene blue which is a characteristic of active carbon. The charcoal is thus suitable for use as active carbon.

BRIEF DESCRIPTION OF DRAWING

These and other aspects of the present invention may be more readily understood by referring to the following drawing wherein:

FIG. 1 is a block diagram of an apparatus used in the present invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

FIG. 1 illustrates six digesters (sealed vessels) labelled 1 , 2, 3, 4, 5 and 6. The pretreatment of the raw wood is carried out in these digesters in a continuous or substantially continuous matter by contemporaneously filling digester 1 with wood and water; heating digester 2 to operating temperature (170°C); maintaining digesters 3 and 4 at operating temperature; cooling digester 5; emptying digester 6; then as soon as digester 6 is emptied, digester 6 is refilled with wood and water; digester 1 is heated to operating temperature; digesters 2 and 3 are maintained at operating temperature; digester 4 is cooled; and digester 5 is emptied. By following this cycle, the pretreatment is run continuously or substantially continuously. The number of digesters can be increased and the number of digesters used herein are for illustration only.

During the heating process to bring the raw wood and water up to operating temperature, the water boils and the steam off of the boiling water fills the digester. The steam displaces the non-condensable gases and provides an atmosphere in the digester which is condensable gases, i.e. water vapor. Once the atmosphere in the digester is absent non-condensable gases and the digester is up to operating temperature, it can be sealed.

During the cooling process, a vacuum is formed in the digester which helps extract the ash forming component as well as the phosphates and nitrogens from the pores of the wood.

Acetic acid is formed in the digester during the pretreatment. Acetic acid is a commercial by-product which can be sold. To recover the acetic acid as well as the other liquid by-products formed during the pretreatment, such as the phosphates and dissolved nitrogen, the leftover liquid from the digester is emptied from the digester through pipe 7 and into filtration unit 8. Filtration unit 8 is used to separate the liquid from any solid particles that may be in the liquid. Since the liquid from the pretreatment is primarily water, some of the water is recycled through pipe 9 to be reused in the pretreatment while some of the liquid is bled out in separator 10. Conventional means can be employed to recover primarily acetic acid and the other by-products such as the phosphates and dissolved nitrogen while recycling water to the digesters.

Pretreated wood from the digesters is forwarded to drying unit 11 where the wood is dried, preferably by hot gases from the retort process. The retort process is carried out in vertical shaft furnace 12. Dried, pretreated wood is supplied to vertical shaft furnace 12. Also, anthracite (a solid carbon material having a low content of volatile matter) is recycled by transport line 13 to furnace 12. Some air is added to furnace 12 through air line 14. The air is used to combust some of the carbon in furnace 12 and provide the heat necessary to retort the wood. Such addition of air is conventional. The temperature in furnace 12 gradually increases from top to bottom. Furnace 12 is operated in a conventional manner.

During retort in furnace 12, hydrocarbon compounds (C-H compounds) are released from the wood as gases. This occurs mainly in the lower portion of furnace 12. In the upper portion of furnace 12, these gaseous hydrocarbon compounds condense on the anthracite and on the wood or charcoal. When the condensed hydrocarbons enter the lower portion of furnace 12 they crack to solid carbon.

The charcoal produced by furnace 12 and the anthracite is cooled in cooling unit 15. The cooled anthracite and charcoal is separated from each other in screening unit 16. The anthracite is recycled by transport line 13.

As noted, the charcoal made by the process of the present invention is high in carbon and low in ash and well suited as a carbonaceous reducing material in the carbothermic production of metal and alloys. Further, the charcoal made by the process of the present invention can be used as active carbon. These and other aspects of the present invention may be more readily understood by reference to one or more of the following examples.

EXAMPLE 1

This example illustrates the pretreatment of raw wood using the moisture content of the raw wood to provide water.

Raw wood was dried in dry air for 6 hours. It was then placed in a sealed vessel, heated to 220°C, maintained at 220°C for 3 hours, and cooled to room temperature. Although there was no water in the bottom of the vessel, the wood itself was soaking wet. The wet, raw wood was then dried in a desiccator at 200°C until it reached a constant weight.

This test showed that 16% by weight (on a dry basis) of water was liberated from the treated wood. This water was in excess of the free water or moisture content of the dried, raw wood. Thus, the molar ratio between carbon:hydrogen:oxygen change from 1 :1.7:0.7 to 1 :1.2:0.5. This shows a major chemical change has taken place in the wood. Specifically, that the cellulose structure has broken down during the treatment to remove hydrogen and oxygen in the form of water (H₂0). EXAMPLE 2

This example illustrates the pretreatment of raw wood for a longer period of time than Example 1 , while using the moisture content of the raw wood to provide water for the pretreatment.

Norwegian spruce having a moisture content of 15% was sealed in a digester; heated to 220°C for a period of 12 hours; cooled to room temperature; and dried at 118°C until it obtained a constant weight.

This test showed a weight loss of 25.1 % in the form of water in addition to the 15% of free water or a total of 40.1% by weight of water was liberated from the raw wood. The dried pretreated wood weight 74.9% of the raw wood.

Of the 25.1% water, this came from the chemical changes of the lignin, hemicellulose and cellulose. These components contain 50% of the hydrogen and oxygen in the wood.

EXAMPLE 3

This example illustrates the pretreatment of the present invention where water is added to the sealed vessel.

Norwegian spruce in the form of a normal dried rectangular piece was heated in a sealed vessel together with added water at a temperature of 220°C for 6 hours. Such a treated wood was then dried at 110°C until it reached a constant weight. Tests performed on this treated wood showed that more water was removed from the wood than the free water present in the wood before treatment and that 97% of the carbon in the wood remained in the wood after treatment. The Norwegian spruce supplied to the sealed vessel contained about 20.3% weight moisture (free water).

EXAMPLE 4

This example illustrates the pretreatment process of the present invention wherein the raw wood is boiled in water to provide an atmosphere with no non-condensable gases.

Dry wood having a moisture content of 19.5% was boiled in water for 3 hours to saturate the wood with water.

Then one part by weight of saturated wood (on a dry basis) was placed in a vessel along with 2.5 parts by weight water. As the water was heated to 220°C, the water boiled creating an atmosphere of steam in the vessel. The vessel was then sealed and maintained at 220°C for 12 hours. After 12 hours, the vessel was cooled to room temperature which created a vacuum in the sealed vessel.

When the vessel was opened after cooling, it was found that an acidic smelling, brown liquid was formed in the bottom of the vessel. A test on the brown liquid with ammonium molybdate gave a canary yellow precipitate revealing the presence of phosphate or orthophosphate in the liquid. An ash analysis of the treated wood showed that 94% of the ash content of the raw wood had been removed by the pretreatment.

EXAMPLE 5

This example illustrates the pretreatment process using an atmosphere with condensable gases for a shorter period of time than Example 4 above.

Norwegian spruce and water was added to a vessel. The vessel was heated to above the boiling point of water to remove air from the vessel, then the vessel was sealed. The vessel was continued to be heated to 220°C and maintained at that temperature for 3 hours. The vessel was then cooled to ambient temperature to create a vacuum and facilitate the removal of the ash forming elements from the pores of the wood.

EXAMPLE 6

This example illustrates the retort process of the present invention with anthracite (a solid carbon material having a low content of volatile matter).

One part by weight of the pretreated wood from Example 2 above was mixed with 2 parts by weight anthracite. The anthracite had a particle size of 2-3mm. This mixture was heated in a furnace to 650°C at a rate of 0.5°C per minute for 3 hours. After retort, it was cooled to room temperature. The weight of the charcoal from the retort process was 46.7% by weight of the raw wood used in Example 2. The charcoal was found to contain 0.9% ash and no volatile matter. It was also found that 94% of the carbon in the raw wood was carried through to the charcoal (a carbon yield of 94%).

EXAMPLE 7

This example illustrates the retort process of the present invention using anthracite and the pretreated wood of Example 3 above.

The pretreated wood of Example 3 above was mixed with anthracite having a particle size of 2-3 mm. The weight ratio of pretreated wood to anthracite was 1 :3. The mixture was then retorted at 750°C for a sufficient period of time to produce charcoal.

Test results showed a carbon yield in charcoal of 94%.

EXAMPLE 8

This example illustrates the retort process of the present invention using anthracite and the pretreated wood of Example 5 above.

The pretreated wood of Example 5 above was heated to a temperature of 750°C in the presence of anthracite having a particle size of 2-3 mm and maintained for a period of time to produce charcoal. The charcoal was then analyzed by oxidizing to combust all the carbon. It was found to have an ash content of 0.1 % by weight. This should be compared to the raw wood prior to pretreatment which had an ash content of 1.6% by weight of raw wood. It was also found that both boron and phosphorous had been effectively removed from the wood. This makes the charcoal produced by the present invention especially useful as a carbon reduction material for the production of low boron and low phosphorous silicon which is especially useful in solar cells.

Claims

1. A method for making a charcoal precursor comprising: treating raw wood in a sealed vessel, in the presence of water, for a time of about 3 hours or more, and at a temperature of about 170°C or more; and recovering the treated wood as a charcoal precursor.

2. The method of claim 1 further comprising: boiling said raw wood in water for a period of time sufficient to saturate said raw wood before treating said raw wood in said sealed vessel.

3. The method of claim 1 further comprising: removing non-condensable gases from said vessel before treating said raw wood.

4. The method of claim 1 further comprising: cooling the treated wood in said sealed vessel to create a vacuum and to extract ash components from the treated wood.

5. The method of claim 1 further comprising: adding water to said sealed vessel before treating said raw wood; and removing liquid from said sealed vessel after treating said wood.

6. The method of claim 1 further comprising: drying the treated wood before recovering said charcoal precursor.

7. The method of claim 1 wherein said time of treating is about 6 hours or more.

8. The method of claim 1 wherein said temperature of treating is about 180°C or more.

9. The method of claim 1 wherein said temperature of treating is about 210°C or more.

10. The method of claim 1 wherein said drying is conducted at a temperature of about 130°C or less.

11. A method for making charcoal comprising: treating raw wood in a sealed vessel, in the presence of water, for a time of about 3 hours or more, and at a temperature of about 170°C or more; and heating the treated wood in an atmosphere without oxygen, at a temperature of about 350°C or more to make charcoal.

12. The method of claim 11 further comprising: adding a solid carbonaceous material having a low volatile matter content to said treated wood prior to heating.

13. The method of claim 11 wherein said temperature during heating is about 450°C to about 750°C.

14. The method of claim 12 wherein said solid carbonaceous material is one or more of anthracite, charcoal, graphite, metallurgical coke, petrol coke, and recycled charcoal from the heating.

15. The method of claim 11 further comprising: boiling said raw wood in water for a period of time sufficient to saturate said raw wood before treating said raw wood in said sealed vessel.

16. The method of claim 11 further comprising: removing non-condensable gases from said vessel before treating said raw wood.

17. The method of claim 11 further comprising: cooling the treated wood in said sealed vessel to create a vacuum and to extract ash components from the treated wood.

18. The method of claim 11 further comprising: adding water to said sealed vessel before treating said raw wood; and removing liquid from said sealed vessel after treating said wood.

19. The method of claim 11 further comprising drying the treated wood before heating.

20. The method to claim 11 , wherein the heating of the treated wood is carried out in a shaft furnace where treated wood is continuously or substantially continuously supplied to the top of the shaft together with the carbon material having a low content of volatile matter, continuously or substantially continuously discharging charcoal from the bottom of the shaft together with the carbon material having a low content of volatile matter, and separating the produced charcoal from the carbon material having a low content of volatile matter.