FIELD OF THE INVENTION

The invention relates to a method of producing fuel pellets from biomass materials, and in particular lignocellulose materials such as agricultural waste, sawdust, wood waste from forest industries etc, using lignin contained in the raw material as a binder in the pellets. The invention also relates to an arrangement for carrying out the method, and pellets obtainable by means of said method.

BACKGROUND OF THE INVENTION

Biomass is a clean, abundant and renewable energy source. In many countries, great ef- forts are made to the reduce the dependency on fossil fuel, and to reduce CO2 emissions which is one of the main factors leading to global warming. It has a big potential due to the fact that the biomass fuel market is very small at present.

Pelletization is a kind of pre-treatment of biomass, and it can significantly enhance the bulk density of biomass, so as to reduce the storage and transport costs of biomass, and is consequently widely used in bio-fuel manufacture. The aims of pelletization are to provide a high volumetric energy density.

It is known that biological materials, such as agricultural waste, wood and bark, mainly contain cellulose fibres, lignin, hemicelluloses and extractives. Lignin is the "glue" which holds the fibres together, it is more highly concentrated in the middle lamella.

WO 2006/006863 discloses a method for producing fuel pellets from biological materials using a steam explosion process. According to this process, the raw materials are pre- conditioned by heating in a steam compression reactor in the range of 200-300 ⁰ C. After a certain exposure time the pressure in the reactor is reduced, causing the biological materials contained in the reactor to "explode". According to WO 2006/006863, the process will cause defibration and will release a portion of lignin which will act as a binding agent in the subsequent pelletizing. According to said reference it is therefore not necessary to supply additional binding agent. In the process described in WO 2006/006863, the pressure vessel(s) is being operated batch-wise in a sequence which is repeated during the process. The principal process described in WO 2006/006863 is consequently not a continuous process.

US 4 211 740 discloses a method for producing fuel pellets from botanical materials, wherein the materials are being heated and subsequently pelletized. US 4 211 740 does not specifically mention lignin, but generally mentions resins, natural thermoplastics and other binders which are being brought to a plasticized state during the heating. In case of wood bark, heating to 93°C has been found sufficient to initiate the transition of the resins and thermoplastics for adequate binding of the final pellets. For wood fibers, the preferred temperature is about 121°C. According to teachings of US 4 211 740, higher temperatures may very well initiate a greater plasticizing of the resin and thermoplastic materials; however, as the compression and friction experienced in the pelletizing mill adds further heat to the material, the preheating temperature must not be so great as to cause the materials to reach a flash point in the mill.

In the art, the softening or glass transition temperature of lignin is usually stated to occur within a temperature range having, according to some references, its lower end only slightly below the preferred temperature (121°C) of US 4 21 1 740, or, according to other references, somewhat above said preferred temperature. Accordingly, the process of US 4 211 740 will not be effective to fully benefit from the binder properties of lignin.

Accordingly, it would be desirable to provide a process for pelletizing lignocellulose raw material, which process is able to more fully benefit from the binder properties of lignin, and which process readily can be operated continuously.

Also, it would be desirable to provide binder-free, i.e. self-binding, pellets having improved properties in terms of e.g. physical integrity, and storage life.

For a general process such as that disclosed by WO 2006/006863, the former object has been accomplished by means of separating the combined heating and defibration step, i.e. the steam explosion step of WO 2006/006863, into two separate steps, i.e. into a heating step and a defibration step, respectively, wherein the defibration is carried out using me- chanical means, and by carrying out the pelletization step at a temperature within the glass transition or softening temperature interval of the lignin contained in the raw material in an atmosphere essentially free from free oxygen. Thereby the material can be heated safely to a temperature above the glass transition temperature of the lignin con- tained in the raw material, and the binder properties of lignin can be more fully utilized.

The latter object has been accomplished by means of carrying out the pelletization step of the inventive method at a temperature of at least about 150 ⁰ C.

SUMMARY OF THE INVENTION

The present inventors have found that the steam explosion step of WO 2006/006863 can be divided into two separate steps, i.e. into a heating step and a defibration step. By doing so, no pressurizing and depressurizing cycles of the reactor will be necessary in order to cause defibration and bursting of the particles of raw material fed into the reactor (or heater). The process can therefore be made continuous. According to the inventive process, the corresponding defibration and bursting of the particles of raw material, which is accomplished by means of the steam explosion step of WO 2006/006863, is instead accomplished by a refining step using mechanical means for the refining. Consequently, in the inventive process, no defibration or refining is performed relying solely on heating as in WO 2006/006863. According to the invention, the heating, defibration and pelletization steps are carried out in an atmosphere essentially free from free oxygen. Thereby the pelletization step can be carried out at a sufficiently high temperature to be able to more fully benefit from the binder properties of the lignin contained in the raw material.

The glass transition or softening temperature of dry lignin seems not have been exactly specified in the literature, but it can be generally stated to occur within the temperature range of 120-200 ⁰ C, depending on the source of the lignin. The present inventors have found that softened lignin will serve as a binder during the pelletizing step of the inven- tive process. It is also very beneficial in the densification of biomass. By using the lignin contained in the raw material as a binder no added binders are necessary.

In its most generic embodiment the inventive method comprises the following steps: A providing a biological lignocellulose raw material; - A -

B heating the lignocellulose raw material in an atmosphere essentially free from free oxygen; bl defibrating the raw material in an atmosphere essentially free from free oxygen; and C pelletizing the heated and defibrated lignocellulose raw material, wherein heating step B and defibrating step bl are carried out as separate steps, and, in step B, the lignocellulose raw material is heated to a temperature within the glass transition or softening temperature interval of the lignin contained in the raw material, and the pelletizing step C is performed at a temperature within the glass transition or softening temperature interval of the lignin contained in the raw material in an atmosphere essentially free from free oxygen. That is to say, the lignin should be in a glassy or softened state during steps B and C. As pointed out above, this process can be operated continuously.

The glass transition or softening temperature interval of the lignin contained in a specific lignocellulose raw material to be used in the method can be established by the skilled person with reference to literature, or by means of performing routine experimentation.

Pellets produced by means of the inventive method, when using a temperature of at least about 150 ⁰ C in the pelletization step, have been found to have a markedly improved hy- drophobicity, physical integrity, storage life, and grindability.

The heating step B of the inventive process is generally carried out at a temperature within the interval of 120 ⁰ C to 210 ⁰ C, and is thus energetically favourable due to the low degree of heating required, as compared for example to the process of WO 2006/006863.

In one embodiment the heating in step B is performed by means of a flow of a heated gaseous heating medium essentially free from free oxygen. Thereby heating can be closely regulated at a specific temperature, and the desired heat transfer can also be closely con- trolled.

In a preferred embodiment the gaseous flowing heating medium is steam. By using steam, a clean biomass with low content of mineral matters, such as alkali, e.g. potassium, can be obtained. Thus, the ash content will be markedly reduced. Notably, chlorine contained in the raw material can thereby be reduced by 20-50%. Thus, corrosion problems in boilers can also be substantially reduced.

In yet an embodiment the method includes a pre-heating step B', wherein physical, i.e. free, water is removed at a lower temperature than used in heating step B, preferably at a temperature of about 90 ⁰ C. Pre-heating step B' precedes heating step B.

In one embodiment the heating temperature in step B is maximum 210 ⁰ C. Thereby, maximum benefit from the lignin can be derived, while any devolatilization of the material, which would otherwise lead to a reduced energetic yield, can be kept to a minimum. Pellets can thereby be produced with a high energetic yield from the raw material.

In a further embodiment the heating step B is carried out under high pressure, as is preferably also the pelletizing step C, and as is also more preferably the intermediate re- fining step b 1.

In one aspect the invention relates to binder-free fuel pellets exhibiting a hydrophobic surface of lignin obtainable by means of the inventive process when using a temperature of at least about 150 ⁰ C in the pelletizing step.

In another aspect the invention relates to an arrangement, such as shown in Figure 1 , for carrying out the inventive method, comprising, in its most generic embodiment, a heater 2, into which a biological lignocellulose raw material is being fed, mechanical refining means 3, and a pelletizer unit 7 capable of pelletizing the heated and refined material in an atmosphere essentially free from free oxygen. The heater 2 is preferably provided with an inlet for a heated gaseous heating medium.

By means of the inventive method fuel pellets can be obtained having a good grinding ability into pulverized powder, which can be used for existing pulverized coal boilers..

By suitable grinding in refining step b 1 , pellets can be produced comprising a microstruc- ture homogeneous biological material for biomass thermal conversions, such as combustion, pyrolysis, and gasification, to energy. The fuel pellets can be produced with a minimal energy consumption for larger scale industrial applications.

Further embodiments and advantages will become apparent from the detailed description and appended claims.

The terms "glass transition temperature interval" and "softening temperature interval" have been used herein interchangeably to denote a temperature range wherein the lignin is in a softened or glassy state. Similarly, the terms "glassy state" and "softened state" have been used herein interchangeably to denote same physical state.

The terms "glass transition temperature" and "softening temperature" have been used herein interchangeably to denote the temperature at which the lignin becomes softened or glassy.

The terms "defibrating" and "refining" have been used herein interchangeably to denote the process of defibrating, using mechanical means, the heated raw material before the pelletization step.

The term "refiner" has been used to denote mechanical means for defibrating or refining the heated raw material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. l is a generalized flow diagram illustrating the various steps of a preferred embodiment of the method according to the invention, wherein 1 is a dryer, 2 is a pre-heater, 3 is a means for refining, 4 is a separator, 5 is a storage means, 6 is a dryer, 7 is a pelletiz- ing reactor, 8 is a heat recovery unit, and 9 is a water treatment unit.

FIG. 2 shows the relationship between the pressure of saturated steam and the temperature according to a preferred embodiment, wherein the preferred working range of "glassy pellets process" is marked. DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS THEREOF

The core idea of this invention is using a glassy lignin as a coating material in a pelletiz- ing process for producing pellets from biological lignocellulose raw materials. The process is apt to produce high density, self-binding, and hydrophobic pellets, having improved storage life, and physical integrity. The inventive pellets have improved resistance to penetration by water into the pellet when exposed humid conditions, such as water or moisture.

According to the invention any lignocellulose material can be used as the raw material of the process, such as, wood, straw, husk, hull, bagass etc.

The present inventors have found that in order to develop new pelletizing processes and technologies, it is extremely important to understand the physical structure, major organic components, and chemical structure of biological wood.

In the inventive process, more particularly in step B, the lignin is softened and released from the raw materials by heating to a temperature above the glass transition tempera- ture of the lignin contained in the raw material. While a temperature of 120 ⁰ C may produce some softening of the lignin, depending on the origin of the biological material, this temperature will only rarely be of any practical use. Preferably, the heating temperature in step B should be 30-40 ⁰ C above the glass transition temperature of the lignin contained in the raw material used. The lignin will thereby be sufficiently softened, and sepa- ration will occur at the middle lamella, leaving the fibres with a hard lignin surface. This glassy lignin-coated material is quite easy to adhere to each other without any binding agent. In practice this requirement will lead to a preferred minimum temperature of about 150 ⁰ C. As pointed out above, at this temperature pellets will be obtained having improved storage life, grindability, resistance to penetration by water into the pellet, which pellets exhibit a dark, hydrophobic surface of lignin. Generally, the lignin's glass transition temperature is about 140 ⁰ C for wood. Accordingly, in the case of such glass transition temperature, the lower limit temperature of the inventive process is preferably in the range of 170-180 ⁰ C. If on the other side the temperature is too high, energy will be lost from the material due to an increasing degree of carbonization and devolatilization of the thus heated material. Carbonization and devolatilization, and associated temperature intervals during heating of lignocellulosic materials at pre-pyrolytic conditions (torrefaction) have been described in more detail by Bergman et al. in "Development of torrefaction for bio- mass co-firing in existing coal-fired power stations", ECN report, ECN-C — 05-013, 2005. The heating temperature in step B should be lower than that of devolatilization of the material in order to provide maximum yield, and minimum energy consumption for the heating process. In order to keep any losses due to carbonization and devolatilization acceptably low, the temperature in heating step B should generally not exceed 210 ⁰ C. The heating in step B is carried out in an atmosphere essentially free from free oxygen, such as carbon dioxide, nitrogen and/or steam, preferably by means of a flow of a heated gaseous heating medium essentially free from free oxygen, such as carbon dioxide, nitrogen and/or steam, and more preferably with a flow of saturated steam of a specified temperature and pressure. For obvious reasons, any heating of the raw material above the glass transition or softening temperature interval of lignin must not be used, especially as the binding properties of the lignin may be lost, partly of fully. The duration time of step B should preferably be less than 2 minutes depending of the type of materials, and moisture content. Step B is preferably carried out under high pressure, especially when steam is used as the heating medium. Figure 2 shows the function of saturated steam temperature versus pressure. The preferred high pressure working range of the invention in the case of steam is marked in the graph, corresponding to a temperature of from about 170 to 210 ⁰ C. This preferred working range of temperature and pressure also applies to the refining and pelletizing steps bl and C, respectively, which steps will be described below.

The biological raw materials used typically contain a humidity of 40-90% weight percent. In the inventive process, the biological materials are therefore preferably first dried in a drying step B', which precedes the heating step B. The temperature of step B' is kept lower than the temperature of step B. During the drying phase, the physical water is released from the heated materials. All physical water should preferably be recovered in this stage. The material that exits this step typically has a humidity content of 50%. The heating in step B' is preferably carried out with steam. As an example, a suitable temperature and duration of step B' is 90 ⁰ C for about 30 minutes. The pre-heating temperature should preferably not exceed 105 ⁰ C. A corresponding arrangement for carrying out the method includes also a dryer 1 , wherein the dryer 1 and heater 2 both preferably are provided with a steam inlet. If a temperature lower than the lignin glass transition temperature is used during heating in step B, the processes will not be effective to produce sufficient self-binding property from the lignin contained in the raw material, and will result in a lower density of the pel- lets. Conventionally prepared pellets using such low temperature have been found to be associated with a number of drawbacks. During storage and transport, the pellets will give off gases like CO, CO2 and CH4. At the same time, they also give off heat, which may lead to the self- ignition of the pellets. When brought into contact with water, they will quickly expand and disintegrate. Moreover, these kinds of pellets are hard to grind into powder due to the fibre structure in the biological materials, which means that more energy is needed during milling. A further drawback is that raw materials which have been subjected to a simple pre-treatment, such as only drying, of the biological materials before the pelletizing process have poor cohesion ability. This causes the pellets to disintegrate before use, and thus a binder may be required. The combustion of the pellets with added binder produces a large amount of ash dust. The dust may have enormous environmental impact, and a treatment is required in order to prevent the dust to pollute the air and the working environment. Such drawbacks are avoided by means of the present invention. For example, the glassy surface of the pellets obtainable by means the invention will seal the pellets, which will markedly reduce the above-mentioned storage and transporting problems associated with the prior art pellets. The glassy lignin surface will also reduce the water tolerance problems by preventing water from penetrating into the pellet. Moreover, the glassy lignin will also provide a hydrophobic surface to the pellets, making them less prone to adsorb water.

The material fed to the process should preferably be in a form allowing for maximum heat transfer from the heating medium to the raw material in step B, and also in step B' when used, and avoiding irregular heating. For example, in the case of wood, chips of a suitable size may be used.

In a preferred embodiment the heating in step B is carried out using steam obtained from the process. Accordingly, steam obtained downstream in the process can be provided to heating step B. The process includes a refining step bl, wherein the heated materials from step B are refined in a reactor by means of a refiner, which operates by either grinding or refining. Due to shear forces and compression in the reactor, electrical energy will be transformed into heat energy, and steam be generated. Steam thus generated can be separated in a separation step b2 from which separation step steam can be led back to step B and/or B', preferably to both. Accordingly, in a preferred embodiment, and as depicted in Figure 1, the arrangement of the invention includes a separator 4 for separating steam from the biological material exiting refiner 3, and conventional means for passing steam generated in refiner 3 back into heater 2, and preferably also back into pre-heater 1, which heater(s) is provided with an inlet for steam thus generated, and also with an outlet for steam.

The aim of the refining step bl is to separate the fibres from each other by mechanical forces, so that the fibres become soft. Thereby the surface of the fibres will also acquire good binding properties. Generally, the fibre length before pelletization, i.e. the resulting fibre length after grinding, should be as short as possible, depending on the intended end use.

It is preferred to carry out step bl under high pressure, since the time required for grinding thereby will be markedly reduced, such as to e.g. merely 2-3 seconds. The preferred working range in terms of temperature and pressure of step bl corresponds to the range marked in Figure 2, i.e. corresponding to temperature of from about 170 to 210 ⁰ C, especially when steam is used as the heating medium in step B.

Finally, the pre-treated material exiting from heating step B, or preferably from separa- tion step b2 is fed into a pelletizing step C for fuel pellets generation. It is preferred to carry out the step C under high pressure. The preferred working range in terms of temperature and pressure of step C corresponds to the working range marked in Figure 2, i.e. corresponding to temperature of from about 170 to 210 ⁰ C, especially when steam is used as the heating medium in step B. The means for pelletization are not critical, and any conventional means for pelletization could be used, as long as the means selected is suitable for the specific conditions of temperature and pressure used. An additional drying step b4, optionally via an intermediate storage step b3, may be included in the process preceding the pelletizing step C. A corresponding arrangement accordingly includes an additional dryer 6, and, optionally, a storage chamber 5, as shown in Figure 1.

When steam is used as the heating medium in drying step B' and/or heating step B, most of the mineral matter, such as alkali metals, e.g. potassium, will be dissolved in to the high temperature and high pressure steam. The steam from drying step B' and/or heating step B can be led to a heat recovery step D, wherein steam is condensed into water. The water obtained from the heat recovery step D can be further passed to a water treatment step E, wherein the water can be purified.

By means of the inventive method pellets having a glassy lignin surface can be obtained. Thereby, the surface of pellets will be hydrophobic. Due to the higher temperature used in the pelletization step, the appearance (i.e. colour) of the inventive pellets will be darker than the pellets of the prior art. The colour may be even darker due to the colour of the fibres, and any presence of bark.

By means of the inventive method a higher productivity (over 97% of feedstock on dry base) can be achieved, and volatiles can be retained in the pellets.

Example

The process and arrangement for carrying out same will now be described in greater de- tail by reference to Figure 1.

The lignocellulosic raw material enters the pre-heater 1 , wherein step B' is carried out in order to remove physical water from the raw material. As an example, a suitable temperature is 90 ⁰ C, and a suitable residence time is 30 minutes. The preheated material is then fed into a heater 2 by means of e.g. a plug screw (not shown), wherein step B is carried out. In the heater 2 the material is heated to a temperature within the softening temperature interval of the lignin of the raw material. In the case of a softening temperature of about 140 ⁰ C, such as for wood, heating to a temperature of 170-180 ⁰ C is preferred. The heating is carried out in an atmosphere essentially free from free oxygen, preferably by means of a heated flow of a gaseous heating medium essentially free from free oxygen which is being fed into heater 2. Preferably the heating source medium is saturated steam. As a suitable example saturated steam of 0.8 to 2 MPa at 170-210 ⁰ C can be used. The residence time of the lignocellulosic material in heater 2 should typically be less than 2 minutes, depending on the type of raw material, and moisture content of the material fed into heater 2.

The heated material exiting heater 2 is fed into a refiner, such as a grinder 3, wherein step bl is carried out. In 3 the material is refined by a refiner, which preferably operates at the same temperature and pressure as in the heating chamber 2. The aim of this process is to further release and make available the softened lignin using a mechanical treatment for refining, such as a grinder, consisting of rotating discs with patterned surface, between which the biological material is processed. The thermo-mechanical process in 3 comprising either a grinding or refining process is a very rapid process, and typically takes only 2-3 seconds.

A separator 4, wherein separation step b2 is carried out, is preferably provided into which the material exiting refiner 3, is fed. Separator 4 is used to separate steam generated from the above refining process in 3 from the biological material. Separator 4 preferably comprises a cyclone. The steam is preferably led back into dryer 1 and/ or into heater 2 as the heating medium, and preferably to both dryer 1 and heater 2.

When steam is used as the heating medium in dryer 1 and/or heater 2, most of the min- eral matter, such as alkali metals, e.g. potassium, will be dissolved in to the high temperature and high pressure steam. The steam exiting dryer 1 and/or dryer can be led to a heat recovery unit 8, wherein steam is condensed into water. The water exiting heat recovery unit 8 can be further passed to a means for water treatment 9, wherein the water can be purified.

Finally, the pre-treated material exiting from heater 2, or preferably separator 4 is fed into a pelletizing means 7, such as a pelletizing press for fuel pellets generation. Any conven- tionally known means for pelletization, which can be used under the selected conditions of temperature and pressure, can be used in the inventive arrangement and process.