WO2016204610A1

WO2016204610A1 - Improved process for the thermo-chemical treatment of biomass using controlled application of oxygen

Info

Publication number: WO2016204610A1
Application number: PCT/NL2016/050424
Authority: WO
Original assignee: VAN EXEL, Roland Alexander; DE GROOT, Klaas Pieter Hendrik; VAN MEIJEL, Henricus Peter Marie
Priority date: 2015-06-15
Filing date: 2016-06-13
Publication date: 2016-12-22
Also published as: EP3280786A1

Abstract

The present invention relates to a process for thermo-chemical treatment of biomass, said process comprising the following steps: Step 1) supplying biomass;Step 2) heating said biomass obtained in step 1) to a predetermined temperature T _hto obtain a heated biomass; and Step3) the thermo-chemically treatment of said heated biomass obtained in step 2) at a temperature T _t between a minimal treatment temperature T _tmin and a maximum treatment temperature T _tmax wherein T _h < T _tmin by supplying a controlled flow of oxygen to said biomass during at least part of said treatment to obtain a thermo-chemically treated biomass; Step 4) optionally cooling said thermo-chemically treated biomass obtained in step 3) to a cooled temperature T _c in order to obtain a solid fuel and to the solid fuel thus obtained.

Description

Improved process for the thermo-chemical treatment of biomass using controlled application of oxygen.

The present invention relates to a process for thermo-chemical treatment of biomass. Moreover, the present invention relates to the use of oxygen for heating biomass, the use of oxygen during thermo-chemical treatment of biomass, and the use of oxygen during a step of cooling biomass that has been thermo-chemically treated. The present invention also relates to a solid fuel product thus obtained. Background

The use of biomass as fuel has increasingly received attention because fossil fuels are being depleted and because it is more friendly to the environment. The burning of biomass in comparison to fossil fuels is a C0₂-neutral process. Although C0₂ is released during burning of biomass; the same amount was extracted from the atmosphere when the biomass was formed (e.g. when the trees grew). This is a very short life cycle for this biomass (few (tens of) years). Fossil fuels have been stored beneath the surface of the earth for so long that they add C0₂ to the atmosphere that was not present, at least for a very long time. There is strong political movement regarding the decrease of C0₂ emission and the preparation of a solid fuel based on biomass may contribute to reaching these goals.

To be able to effectively and efficiently use biomass as an energy source that is carbon neutral or at least has a reduced carbon footprint relative to fossil fuels, it requires certain pre-treatment to overcome inherent drawbacks. Examples of such drawbacks are a high moisture level reducing the net energy per unit of weight or volume of the energy source; the fibrous nature of the biomass requiring vast amounts of energy to grind into a suitable energy source; the relatively low energy density of biomass; the hydrophilic properties of biomass making it sensitive to rot and decay and moreover making shaped forms of pressed biomass such as pellets and briquettes sensitive to disintegration.

When thermo-chemical treatment of biomass is mentioned, several different process can be cited. Torrefaction is a process that is usually carried out at a temperature between 220°C and 300 °C or between 220 °C and 270°C during which the hemi-cellulose decomposes and some of the lignin. Carbonization is a process that is carried out at a higher temperature, usually between 300°C and 400°C during which hemi-cellulose, lignin, and cellulose all decompose and almost all oxygen atoms are removed from the chemical structures in the form of e.g. C0₂. Pyrolysis is a process carried out at an even higher temperature, usually between 500°C and 600°C and is known to be either executed very fast (fast or flash pyrolysis) or slow. Pyrolysis leads to gasification of the majority of the biomass. Thermo-chemical treatment of biomass and especially torrefaction and carbonization have shown to have the following positive effects. It increases the specific (particle) weight and volumetric (bulk) weight of the solid fuel when compared to the starting material. It increases the energy density of the product per unit of weight and even more per volumetric unit at the cost of a fraction of the energy contained. It decreases sensitivity to decay/rot, to environmental moist or submersion in water. It decreases the energy required to mill and the energy required to press into transportable shapes (pellets, briquettes, etc.). When a thermo-chemically treated biomass is burned, it produces less smoke than when biomass as such is burned. Most torrefaction systems available to date cannot produce a consistent torrefied product. Most material produced in continuous processes varies in degree of torrefaction and even far into carbonization. The material often shows a solid black outside but especially with larger particles of biomass, the inside is still raw or unprocessed. The only consistent product is a fully carbonized product which is inefficient for torrefaction as much of the caloric value is lost in the large amounts of gas produced from the decomposing biomass. As a result, there is little mass left and little energy.

Prior art

WO 2005/056723 discloses the production of a solid fuel by a continuous torrefaction treatment of biomass. Oxygen is used in the combustion of the gases liberated during torrefaction. US 2003/0221363 discloses a process for producing a solid fuel from biomass by first torrefying the biomass and then densifying it to make pellets. There is a large volume of patent publications related to the torrefaction of biomass that almost exclusively relate to the exclusion of oxygen from the process; which is understandable due to the highly pyrophoric behaviour of the biomass which could lead to spontaneous combustion.

WO 2012/1581 18 relates to a torrefaction process wherein an oxygen-containing gas is supplied in countercurrent to the torrefaction gases inside the torrefaction chamber. As the oxygen is supplied to the torrefaction gases which are thereby burned the heat is generated in the gaseous envelope and not in the biomass itself and is therefore far less efficient. The amount of heat that is generated in the gaseous envelope and the part of that heat that is transferred on to the biomass is very difficult to control. This method only addresses a way to accelerate the heating of the biomass, not controlling the torrefaction or carbonization phase in terms of temperature and or time.

Such known methods have the disadvantage that the reaction is difficult to control in an efficient manner. During the thermo-chemical treatment it is preferred that the temperature remains within a certain predetermined bandwidth. With the methods according to the prior art, there is a significant risk of a thermal runaway wherein the temperature increases to a value beyond the desired bandwidth, leading to pyrolysis or carbonisation of the biomass, which leads to a decrease in energy content of the solid fuel obtained.

An aim of the present invention is to allow for accelerated heating of biomass prior to a thermo-chemical treatment. Another aim of the present invention is to have an accurate control over the temperature of the biomass during thermo-chemical treatment. Another aim of the present invention is to have accurate control over the duration of the process of thermo-chemical treatment of biomass. Another aim of the present invention is to reduce or even eliminate the pyrophoricity of thermo-chemically treated biomass. Another aim of the present invention is to provide a solid fuel that has homogeneous properties between particles (viz. particles compared to each other) and/or within the particles (viz. outside of particle compared to inside of particle).

Summary of the present invention

The present invention relates in a first aspect to a process for thermo-chemical treatment of biomass, said process comprising the following steps:

Step 1) supplying biomass;

Step 2) heating said biomass obtained in step 1 ) to a predetermined temperature T_h to obtain a heated biomass; and

Step 3) the thermo-chemically treatment of said heated biomass obtained in step 2) at a temperature T_t between a minimal treatment temperature T_tmin and a maximum treatment temperature T_tmax wherein T_{h <} T_tmj_n by supplying a controlled flow of oxygen to said biomass during at least part of said treatment to obtain a thermo-chemically treated biomass;

Step 4) optionally cooling said thermo-chemically treated biomass obtained in step 3) to a cooled temperature T_c in order to obtain a solid fuel.

In an embodiment, said process comprises steps 1 ), 2), 3) and 4) and wherein steps 2), 3) , and 4) are carried out in separate reactors or reaction chambers. In an embodiment, one or more additional controlled flows of oxygen are supplied to the biomass during one or more of 2) and/or 4), preferably the (additional) controlled flow of oxygen is a controlled flow of oxygen gas or oxygen-containing gas. In an embodiment, the oxygen is supplied during the total duration of the treatment step 3). In an embodiment, wherein the treatment step 3) is stopped by stopping the controlled flow of oxygen. In an embodiment, oxygen is supplied during at least part of heating step 2) and during at least part of cooling step 4). In an embodiment, an additional step 1 b) is present after step 1) and prior to step 2), said step 1 b) being a step of drying the biomass supplied in step 1). In an embodiment, T_tmj_n is at least 10 °C, preferably at least 20 °C higher than T_h. In an embodiment, the thermo-chemical treatment is torrefaction and wherein said (cooled) thermo-chemically treated biomass is (cooled) torrefied biomass. In an embodiment, the contact time of oxygen in the biomass is at least 0.5 second, preferably at least 1 second. In an embodiment, during at least part of step 4) oxygen is supplied and wherein the flow of oxygen during step 4) is increased when the temperature of the thermo-chemically treated biomass tends to decrease to a value below T_tmin and the flow of oxygen is decreased or stopped when the temperature of the thermo-chemically treated biomass tends to increase to a value above T_tmax. In an embodiment, an additional step 5) is present in which the thermo-chemically treated biomass obtained in step 3) or the cooled biomass obtained in step 4) is (reduced in size and) densified to a plurality of shaped particles, preferably in the form of pellets or briquettes, at a cooled biomass temperature of at least 100 °C.

In a second aspect, the present invention relates to the use of oxygen for heating biomass, wherein said oxygen is supplied in a controlled flow to said biomass in order to expedite the heating of said biomass by reaction of said biomass with said oxygen.

In a third aspect, the present invention relates to the use of oxygen during thermo-chemical treatment of biomass, wherein said biomass has a temperature between a minimal treatment temperature and a maximum treatment temperature; wherein said oxygen is supplied in a controlled flow to said biomass in order to control the temperature of said biomass between the minimal treatment temperature and the maximum treatment temperature.

In a fourth aspect, the present invention relates to the use of oxygen to reduce the pyrophoric nature of a thermo-chemically treated biomass during the cooling thereof; wherein said oxygen is supplied in a controlled flow to said thermo-chemically treated biomass during the cooling thereof. In this aspect, the invention relates to the use of oxygen during the cooling of the thermo-chemically treated biomass to reduce the pyrophoric nature thereof; wherein said oxygen is supplied in a controlled flow to said thermo-chemically treated biomass during the cooling thereof

In a fifth aspect, the present invention relates to a solid fuel obtained by or obtainable by a process according to the process of the first, second, third and/or fourth aspects of the invention. In a sixth aspect, the present invention relates to a solid fuel prepared by the torrefaction of biomass having a visual score of 3 or 4 for the core and a score of 4 for the outer surface of the biomass particle prior to any optional densification, according to the following scores: Score 1 : characteristic of biomass prior to any heat treatment; Score 2: initially torrefied observable by slightly darkening of the material but still fibrous when scratching the surface; Score 3: partially torrefied observable by darkening (mid brown) of the material which is becoming brittle when scratching the surface; Score 4: fully torrefied observable by dark brown, shrunk material being brittle; Score 5: fully carbonized and black. Short description of Drawings

Figure 1 shows a graphical representation of torrefaction temperatures for several biomass sources.

Figure 2 discloses a flow diagram of the steps in the present process.

Figure 3 discloses a specific embodiment of the equipment suitable for carrying out a process of the present invention.

Figure 4 discloses the flow of the biomass through the embodiment in Figure 3.

Figure 5 discloses the flow of heating gases through the embodiment in Figure 3.

Figure 6 discloses the locations for application of oxygen in the embodiment in Figure 3.

Figure 7 discloses the flow of torrefaction gas though the embodiment in Figure 3.

Figure 8 discloses the cross section of the application of oxygen.

Detailed description of the present invention

The present invention, its aspects and embodiments will be discussed in more detail below. It should be noted that embodiments described for one aspect may also be applicable for other aspects, unless specified otherwise. Moreover, one or more embodiments may be combined.

Definitions

The following definitions are used in the present description and claims to define the stated subject matter. Other terms not cited below are meant to have the generally accepted meaning in the field.

"Biomass" as used in the present description means: organic matter, especially plant matter. Biomass may be used as a fuel or may be used as a feedstock for thermochemical treatment for preparing a solid fuel.

"Torrefaction" as used in the present description means: a process in which a biomass is subjected to a certain heat treatment; usually at temperatures between 220 and 300 °C; wherein the moisture content is reduced and the energy content is increased.

Torrefaction is carried out at temperatures that are lower than pyrolysis temperatures.

Torrefaction is carried out at temperatures that are lower than carbonization temperatures.

Without wishing to be bound by any theory, the inventors propose that only hemicellulose and a portion of the lignin is broken down due to removal of oxygen from the molecules, allowing the material to become brittle while a large amount of the volatiles are remaining.

"Carbonization" as used in the present description means: a process in which a starting material is subjected to a certain heat treatment, usually between 300°C and 500°C.

Carbonization is carried out at temperatures that are higher than torrefaction temperatures.

Carbonization is carried out at temperatures that are lower than pyrolysis temperatures. Without wishing to be bound by any theory, the inventors propose that hemicellulose, lignin and cellulose is broken down due to removal of oxygen from the molecules, allowing the material to become brittle while hardly any volatiles are remaining.

"Pyrolysis" as used in the present description means: a process in which a starting material is subjected to a certain high temperature heat treatment, usually between 500°C and 1000°C. Pyrolysis is carried out at temperatures that are higher than torrefaction temperatures. Pyrolysis is carried out at temperatures that are higher than carbonization temperatures. Without wishing to be bound by any theory, the inventors propose that all organic material is gasified.

"Controlled flow" as used in the present description means: that the amount of oxygen added to the biomass is controlled in order to achieve the desired effect. The amount of oxygen added may be increased, decreased, stopped or started in order to achieve the desired effect.

"Reactor or reaction chamber" as used in the present description means: the place where a step of the process is carried out, this may be any type of reactor or reaction chamber that is suitable for this purpose. A reaction chamber may also be a separate section of a larger enclosure. "Contact time" as used in the present description means: the time during which the oxygen is in direct contact with the biomass. In other words, the time from the moment that the oxygen is supplied to the biomass and the moment that the oxygen has either reacted with or has passed through the biomass into the gases that are present in the reactor. With "direct contact" is meant that the oxygen is present within the biomass not merely present in the gas phase outside of the biomass.

"Contact surface" as used in the present description means: the surface of biomass that the oxygen contacts.

"Residence time" as used in the present description means: the time during which the biomass is present in a specific process step (e.g. drying, heating, treatment, cooling etc.). "Total residence time" as used in the present description means: the total time that the biomass is present in the process according to the present invention, starting from the biomass starting material to the final product. Present process

The present invention relates to a process of thermo-chemical treatment of optionally pre- dried and/or pre-treated biomass in which the biomass is heated either directly or indirectly and the biomass partially or completely decomposes into gases because of such heating. Such a thermo-chemical treatment of biomass as such is known and is usually referred to as torrefaction. However, it may also be referred to as (mild) pyrolysis, roasting, wood cooking and high-temperature drying and even gasification.

During torrefaction the biomass is typically heated at a temperature within a certain range or bandwidth around the temperature of the heat treatment. The main chemical effect of torrefaction is typically believed to be the removal of oxygen out of the biomass. Roughly speaking torrefaction typically reduces the weight of the material by 30% whereas the energy content reduces by 10%; meaning an energy density by weight increase of 30%. During torrefaction typically the hemi-cellulose and some of the lignin decomposes. During carbonization - which is carried out at a higher temperature- typically hemi-cellulose, lignin, and cellulose all decompose and almost all oxygen atoms are removed from the chemical structures in the form of e.g. C0₂. Pyrolysis is typically known to be either executed very fast (fast or flash pyrolysis) or slow. Pyrolysis typically leads to gasification of the majority of the biomass.

Up to now there seems to have been a lack of understanding of the process of thermo- chemically treating of biomass, especially towards the complexity of reactions of the elements in the biomass to varying temperatures and residence times. For example, it was known that the process of torrefaction can be sped up by increasing the temperature to which the biomass is exposed; however, it was not known that this can only be done to a limited extend before reaching the point where the outside of the biomass is carbonized and the inside of the biomass is still raw.

The present inventors have carried out an extensive research program and have arrived at the present invention, based on their novel insights into some important reasons for the prior art processes not succeeding in producing a consistently thermo-chemically treated product.

The key to the present invention is the controlled supply of oxygen to biomass during one or more steps of this process. The oxygen may be supplied in the form of an oxygen gas or oxygen-containing gas, e.g. air. The oxygen is supplied in such a manner that it comes into direct contact with the biomass. This can for example be the injection directly into the biomass or the addition thereof below the biomass so that it may rise through the biomass. It does not encompass the addition of oxygen to the gases above the biomass in the reaction chamber.

The present invention allows for the use of controlled addition of oxygen in one or more steps of the process. Depending on the step(s) in which controlled flow of oxygen is used, the present invention allows for i) accelerated heating of the biomass, ii) selective starting of the thermo-chemical treatment, iii) accurate temperature control of the biomass during thermo- chemical treatment, iv) accurate control over the duration of the process of thermo-chemical treatment of biomass, v) the possibility of reducing or even eliminating the pyrophoricity of the product resulting from thermo-chemical treatment of biomass; and/or vi) increasing the homogeneity of the solid fuel that is formed. Oxygen also is beneficial for the energy balance since it is a very efficient means to heat biomass.

First aspect of the present invention

In a first aspect, the present invention relates to a process for thermo-chemical treatment of biomass, said process comprising the following steps: Step 1) supplying biomass; Step 2) heating said biomass obtained in step 1 ) to a predetermined temperature T_h to obtain a heated biomass; and Step 3) thermo-chemically treating of said heated biomass obtained in step 2) at a temperature T_t between a minimal treatment temperature T_tmin and a maximum treatment temperature T_tmax wherein T_{h <} T_tmin by supplying a controlled flow of oxygen to said biomass during at least part of said treatment to obtain a thermo-chemically treated biomass; step 4) optionally cooling said thermo-chemically treated biomass obtained in step 3) to a cooled temperature T_c in order to obtain a solid fuel. The present inventors have found out that oxygen may provide two essential effects to the solid fuel product: i) oxygen was found to reduce the pyrophoricity of the resulting solid fuel and; ii) oxygen was moreover found to increase the in-particle homogeneity of the solid fuel; the fifth aspect of the present invention relates to this. The process for thermo-chemical treatment of biomass can be divided into several distinct steps or phases; not all of these are required for the process according to the present invention. Each of these steps or phases is discussed in more detail below. The preferred embodiments for each of these steps is also discussed with these steps. In an embodiment, said process comprises steps 1), 2), and 3) and wherein steps 2), and 3) are carried out in separate reactors or reaction chambers. In this embodiment, the process comprises at least the steps of supplying a biomass; heating the biomass; and thermo- chemically treating the biomass. More preferably, these steps are carried out separately. In case more steps are present in the process each of these steps is preferably carried out separately. In an embodiment, said process comprises steps 1 ), 2), 3) and 4) and wherein steps 2), 3) , and 4) are carried out in separate reactors or reaction chambers. In this embodiment, the process comprises at least the steps of supplying a biomass; heating the biomass; thermo-chemically treating the biomass and cooling the biomass. More preferably, these steps are carried out separately. In case more steps are present in the process each of these steps is preferably carried out separately. In an embodiment, said process comprises steps 1), 1 b), 2), and 3) and wherein steps 1 b), 2), and 3) are carried out in separate reactors or reaction chambers. In this embodiment, the process comprises at least the steps of supplying a biomass; drying the biomass, heating the biomass; and thermo-chemically treating the biomass More preferably, these steps are carried out separately. In case more steps are present in the process each of these steps is preferably carried out separately. In an embodiment, said process comprises steps 1), 1 b), 2), 3) and 4) and wherein steps 1 b), 2), 3) , and 4) are carried out in separate reactors or reaction chambers. In this embodiment, the process comprises at least the steps of supplying a biomass; drying the biomass, heating the biomass; thermo-chemically treating the biomass and cooling the biomass. More preferably, these steps are carried out separately. In case more steps are present in the process each of these steps is preferably carried out separately.

Thus, in a preferred embodiment of the first aspect of the present invention, the steps of drying, heating, and thermo-chemical treatment are separated from each other. In other words, they are carried out sequentially and not (partly) simultaneously. In prior art methods, the biomass is often directly heated to a temperature that is well above a drying temperature. The systems developed in the prior art use a single process-step for heating and torrefying, allowing smaller particles to heat faster and subsequently torrefy much longer. Since there is only one single way of applying heat, it is unknown at which point in time the particles have which temperatures and at which point they start torrefying. In some cases, even drying is included in the same process step, decreasing both the control and the efficiency further as it is unknown where and when in the process the biomass goes from one phase into the other.

Moreover, when drying is included the torrefaction gas will hold a large amount of water vapor which decreases its use for heating the reaction by combustion. However, since the biomass comprises moisture, first a drying process will start. But because of the high temperature, when e.g. smaller particles have been dried, they will begin to heat up and possibly even torrify whereas e.g. larger particles are still in the drying phase. This will lead to a inhomogeneous processing since particles in the same batch of biomass will be in different stages of the process at the same time. The present inventors have found that the combination of the steps of drying, heating and torrefying is not an optimal way to carry out the process. The present inventors have found that drying and torrefying are time driven processes whereas heating is a power driven process. Drying is a time driven phase with a limited effect of increasing the temperature. Water molecules require energy to evaporate but especially require time to travel through the biomass in order to be eliminated to the outside. Heating is a power driven phase wherein an increase in the temperature will increase the rate of heating. In order to save time it is possible to apply a larger amount of heat in which case the biomass will heat up more quickly. Torrefying is a time driven phase benefiting from an accurate temperature control. The present inventors have observed that the duration of torrefaction is mainly dictated by the heat transfer inside the material; the outer areas of the biomass particles see a fast temperature rise but the inner areas are lagging behind. The material to be treated needs to be torrefied for a certain amount of time within a temperature bandwidth, which will be discussed in more detail below. The process according to the present invention comprises the following steps - however additional steps not cited here may be present before, in between or after these steps: Step of supplying a biomass (Step 1); Optional step of size reduction of biomass (Step 1 a); Optional step of drying the biomass (Step 1 b); Step of heating the biomass (Step 2); Step of thermo-chemically treating the biomass (Step 3); Optional step of cooling the biomass (Step 4); Optional step of (reducing the size of and) densifying the biomass (Step 5). Each of these steps is discussed in more detail below.

Supply of biomass

Several types of biomass may be used in the process according to the present invention. Examples of biomass streams are woody remains from any type of tree, grass types like grass, straw, reed, and miscanthus, bushes and plants like Eucalyptus, Agave, and (pseudo) Acacia, and other plant, fruit, or shell remains such as Palm Kernel Shells (PKS), rice husk, and saw dust either from agricultural waste streams, industrial waste streams, forestry, landscaping, or purposely grown energy crops. Biomass that is suitable for use in the present invention may be obtained from industrial waste streams such as saw dust, sugar cane bagasse, agave, palm kernel shells, or eucalyptus. Biomass may also be obtained from agricultural waste streams such as straw or corn stems. Biomass may also be obtained from forest and land management such as wood, (pseudo) acacia, reed, miscanthus or mesquite. Biomass may also be obtained from dedicated energy crops. Other sources of biomass may also be used.

Thus biomass may for example be wood-based, grass-based and plant-based biomass. Each of these biomasses has a specific composition (including a specific moisture content) as well as a specific size. Mixture of one or more types and/or batches of biomass may also be used in the present invention. However, it should be noted that in order for the process to work optimally, it is very important that the feedstock is as homogeneous as possible, in respect to size of the particles, moisture content and chemical constitution. The more homogeneous the feedstock, the more homogeneous the end product will be. The biomass that is supplied may be directly obtained from the source or it may be subjection to prior passive drying, e.g. drying without the addition of energy, or drying with forced circulation of air (e.g. by fans).

The optimal temperature for the torrefaction treatment depends on the type of biomass used. Figure 1 provides a graphical representation of a thermo-gravimetric analysis (TGA) for four different types of biomass (viz. willow, bamboo, coconut shell and wood) taken from a publication by Wei-Hsin Chen (" ¾ study on torrefaction of various biomass materials and its impact on lignocellulosic structure simulated by a thermogravimetry", Energy, 35, 2010, pages 2580-2586). This is non-limiting list of biomasses that may be used and optimal temperature ranges for the thermo-chemical treatment.

It is important to understand that biomass processing, unlike chemical processes, is inherently not stable and cannot be calculated exactly. The biomass not only differs from one type to another, even within one particular type of biomass and even within one batch of one particular biomass there are fluctuations in mass density, moist level, etc. This implies that temperatures mentioned e.g. torrefaction temperature are not exact and cannot be exact and do vary significantly with the biomass processed. In a preferred embodiment of the process this process is carried out near the site where the biomass is produced in order to reduce or eliminate costs for transportation of biomass and the associated carbon footprint. Due to the high volume, low bulk density and low energy density it is not economical to transport biomass. It is much more economical to transport the solid fuel obtained by the present invention.

Size reduction of the biomass

As discussed above, biomass is available in all types and sizes. In order for the thermo- chemical treatment to be most effective, it is desired not to have too large particles. When the biomass as starting material has a size that is too large, e.g. above 10 cm, it might be preferably to first reduce the size of the biomass prior to the following steps. It should be noted that the size that is optimal for use in the present process depends largely on the material used as feedstock. For example, when straw is used as feedstock, straws having a length of 10 cm may be easily used in the present process, however when wood chips are used, a piece of wood of 10 cm x 10 cm will most likely be too large to provide optimal results due to the solid structure and insulating properties of the wood. A person skilled in the art of biomass and torrefaction will be able to determine easily up front or after a few test runs what the optimal size is for a given biomass feedstock.

This step is an optional step depending on the feedstock used. The size reduction may for example be carried out by known methods, such as crushing, grinding, milling. The manner of size reduction depends on the type of starting material (biomass) used and on the size of the biomass starting material. A person skilled in the art will know if size reduction is required and if it is required which apparatus and process to use. The temperature of the biomass that is used as a starting material optionally after size reduction is denoted as temperature of the starting material (T_s). The starting material T_s may for example be between 15 and 35 °C, more preferably between 20 and 25 °C, such as room temperature but is in no way limiting. Drying of the biomass

This optional step may be carried out when the biomass has a moisture level that is above the desired moisture level. Biomass as supplied may for example comprising between 10 and 55 wt.% moisture (e.g. water), that is structurally, physically and/or chemically bound. The process according to the present invention does not require a step of drying; a wet biomass, e.g. having up to 40 wt.% moist may be used in the process according to the present invention. However, this seriously reduces the efficiency since the heating step will take much longer since first moisture needs to be removed. Hence in order to increase the efficiency of the current process, it is desired to have a certain maximum moisture level upon starting the heating process.

Passive drying may also be used for biomass, meaning drying without the addition of any energy. Biomass may for example be spread on a surface (e.g. the ground) and left to dry for a longer period of time in open air or with fans to increase the drying speed. This type of passive drying is not part of the present step of drying of the biomass, when passive drying is used, a passively dried biomass will be supplied in step 1). Active drying with the addition of heat energy may also be part of the present invention as optional step 1 b).

A preferred maximum moisture level of the biomass for feeding into the heating step 2) is 15 wt.%, more preferably 12 wt.%, even more preferably 10 wt.%. The moisture content is determined by the measuring method as described in ISO standard 18134-2:2015 "Determination of moisture content - Oven dry method - Part 2: Total moisture - simplified procedure". The step of drying is often carried out during prior art process together with the step of heating the biomass. In other words, a heating step is carried out during which heating process the biomass is also dried. This is not preferred according to the present invention because it lowers the efficiency.

According to an embodiment of the process of the present invention the step of drying is separate from the step of heating. In a preferred embodiment, the step of drying is carried out in a separate reactor or reaction chamber as the step of heating; preferably the biomass is first dried in a drying reactor/reaction chamber set to a temperature of drying and then the dried biomass is transferred to a heating reactor/reaction chamber. Preferably, the step of drying is carried out at a different temperature than the steps of heating and/or treatment.

Before biomass can be treated it needs to be heated to the treatment temperature or a temperature within the range of treatment temperatures (see next step of the process below). It was found by the present inventors that in order for biomass to be heated above 100°C, it needs to be sufficiently dry (viz. have a moisture content that is below the desired moisture content). In an embodiment of the present invention, the step of drying is carried out under an atmosphere of air. In an embodiment of the present invention, hot gas is used to indirectly dry said biomass preferably hot gas having a temperature of between 150 and 350 °C. Preferably, the step of drying is carried out at a temperature of between 100 and 150 °C, preferably between 120 and 140 °C. Preferably, the step of drying is carried out during a time sufficient to reduce the moisture content of the biomass to a value of below 15 wt.%, preferably to a value of between 5 and 12 wt.%.

The temperature of the dried biomass at the end of the drying step is denoted as the dried biomass temperature or T_d. Preferably, the dried biomass to be used in the following step has a T_d of between 60 and 80 °C, such as approximately 70 °C, depending on the dew point of the biomass in the given drying process. This ensures the material being dry and allows the next phase to be more efficient as the energy captured by the biomass to heat up is not lost. A higher temperature is undesirable as the biomass may already start gasifying and cause pollution and loss of energy. Preferably, the dried biomass is directly transferred to the heating step in order to be more energy efficient; the temperature of the dried biomass is then the starting temperature for the heating step. The residence time in the step of drying is highly dependent on the moist level of the biomass supplied, the type of dryer used and the available heating gas which may be available in different volumes and temperatures. The heat energy required for drying is preferably (at least partially) derived from the torrefaction gases. This makes the process autarkic. Preferably, at the end of this process step at least 80 %, preferably at least 90%, more preferably at least 95%, or even at least 98 % or even at least 99% or all of the biomass particles have a moisture content of at most 15 wt.%, preferably at most 12 wt.%, more preferably at most 10 wt.%.

The drying may be carried out in any suitable reactor/reaction chamber that is known to a person skilled in the art, for example belt dryers, rotary drum dryers, flash dryers or superheated steam dryers which are known to a skilled person. The gas that is emitted from this drying contains large amounts of water vapor and should be preferably treated differently than and separate from the torrefaction gases that are combusted (see below). Heating of biomass

This step relates to the heating of a biomass either as supplied or as previously reduced in size and/or dried. This step is an essential step in the process according to the present invention. This step relates to the increasing of the temperature of the biomass from a supplied biomass temperature T_s or a dried biomass temperature T_d or a temperature in between to a heated biomass temperature T_h. Without wishing to be bound to any theory, the present inventors believe that during the step of heating any remaining water in the biomass (either as supplied or as pre-dried) that is either structurally or physically absorbed will be removed. This first phase of the heating process may also be referred to as final drying. The temperature at the beginning of the heating step may vary according to possible pre- drying and/or pre-treatment. When a step of drying is carried out directly prior to the step of heating, the starting temperature will be the temperature of the dried biomass (T_d). However, when the biomass supplied is used without a drying step or with an intermediate storage of dried biomass, the temperature at the start of this heating step is the temperature of the supplied biomass (T_s) and may e.g. be room temperature. The temperature at the end of the heating step is important considered to be the heated biomass temperature (T_h). This temperature is predetermined prior to the heating step depending on the desired temperature for the thermo-chemical treatment (T_t). The heated biomass temperature should be below the minimum temperature for the thermo-chemical treatment such that T_h < T_t. The reason for this is that according to the present invention the thermo-chemical treatment step is started and tuned by the addition of oxygen (see more detailed explanation in step of thermo- chemical treatment below). The temperature of the biomass may be increased by adding (more) oxygen and may be decreased by adding no or less oxygen. When the biomass has a temperature at the start of the thermo-chemical treatment that is within the treatment range when no oxygen is present, it is not possible to accurate control (e.g. decrease) the temperature. Thus according to the present invention the temperature of the biomass when entering the step of the thermo-chemical treatment is required to be below the desired temperature (range), viz. T_h < T_tmin < T_t < T_tmax.

In some prior art process there is no clear differentiation between the step of heating and the step of thermo-chemical treatment but the present inventors have observed that by separating these two steps the process may be better and more efficiently controlled; it is hence preferred to separate the step of heating and the step of treatment. The step of heating is a power driven phase as higher temperatures that are applied will increase the heating speed; but the heating needs to be limited to a temperature below the treatment temperature for preferably all biomass particles - big and small - as otherwise some particles will already start to be thermo-chemically treated before others which will lead to an inhomogeneous end product. That is why the present invention preferably relates to separating the steps of heating and treatment. In an embodiment, the step of heating and the step of treatment are carried out in separate reactors or reaction chambers.

The present process is preferably carried out using indirect heating, preferably using hot gas e.g. having a temperature of between 400 and 550 °C. Even though direct heating is more energy efficient, the indirect heating process allows for a better process control and lower dust production. However, as discussed above for the drying step a combination of direct and indirect heating may be applied. The biomass to be heated is generally a loose stacking of biomass particles that have a imprecisely defined shape and size, leading to a low heat conductivity. The slope of the temperature profile during the heating step depends e.g. on the power and the surface area of heating. The present inventors have observed that the heat produced from the burning of the torrefaction gases is sufficient for drying, heating, and torrefaction and, depending on the moisture level of the biomass supplied, at least partially, drying. The torrefaction gas produced may be converted into thermal energy by combustion. The flue gases themselves or other gases heated by the flue gases through a heat exchanger may be used in the drying and/or heating steps as indirect heat source or used directly (e.g. in counter current) to heat the biomass. The heating rate in the step of heating is preferably between 10 and 40 °C/minute, more preferably between 20 and 30 °C/minute. A higher heating rate delays the thermal degradation process of the biomass thereby shifting it to the well-controlled thermo- chemical treatment step. A too high heating rate, however, creates undesired hot-spots, especially in the larger particles of the biomass. As heat conductivity varies with different types of biomass and different shapes of particle, the heating rate also varies with these characteristics. The residence time in the step of heating is determined by the desired temperature T_h and the heat transfer capacity of the biomass being used. The heat energy required for heating is preferably (at least partially) derived from the torrefaction gases. This can make the process autarkic. The gas formed in the heating step is removed from the heater and typically comprises water and hydrocarbon compounds. It may be combusted but has a low calorific content due to the water that is present.

A preferred T_h is above 160°C, preferably above 180°C; however a T_h of above 200° may be envisaged. The preferred T_h depends on the type of biomass used and is preferably at least 10 °C, such as at least 15 °C or even 20 °C below the T_tmin of that specific biomass used. The temperature T_h is preferably measured by a temperature sensor in the biomass at the end of the heater, e.g. when exiting the heater. Said temperature sensor may be embedded in a conductive material, e.g. copper. Preferably, additional temperature sensors are present within the heater to measure the temperature of the biomass, e.g. at least one at the beginning of the heater and at least one between the beginning and the end of the heater, the readings of which can be used to tune the process as it is in operation. In addition to biomass-temperature sensors one or more temperature sensors to measure the temperature of the gas envelop above the biomass may be present. Preferably, at the end of this process step at least 80 %, preferably at least 90%, more preferably at least 95%, or even at least 98 % or even at least 99% or all of the biomass particles have a temperature of T_h. In an embodiment, an additional controlled flow of oxygen is supplied to the biomass during step 2), viz. during the heating step. The flow of oxygen may be oxygen gas or oxygen- containing gas, such as air. This controlled flow of oxygen is additional to the controlled flow of oxygen during the step of thermo-chemical treatment. The present inventors have observed that by the controlled presence of oxygen during the heating process, the heating rate may be increased and the amount of heat energy needed to heat the biomass to the desired temperature decreases. As discussed above oxygen was found to increase the in-particle homogeneity. This means that when oxygen is used during the step of heating of the biomass, the particle is heated more homogeneously over the thickness or diameter thereof. In other words, the difference between the temperature of the core of the particle and the temperature of the outer surface of the particle is smaller when oxygen is used then when no oxygen is used. - resulting in a more homogeneous treatment in the following step and hence in a more homogeneous solid fuel product. At this time there is no formal method for determining the degree of torrefaction of biomass. The major physical changes are hydrophobicity and grindability as the biomass becomes more brittle when hemicellulose and some lignin are decomposed. Unfortunately the ISO standard for the determination of grindability is still under construction. The major chemical changes are the reduction of the amount of volatiles as some material is gasified for which determination ISO 18123 "Solid Biofuels - Determination of the content of volatile matter" could be used and the relatively increased amount of fixed carbon as with the gasification more H and O atoms are used for which determination ISO 16948 "Solid biofuels - Determination of content of carbon hydrogen and nitrogen" could be used. As there is no standard to determine the degree of torrefaction it is not possible to standardize the homogeneity of torrefaction across larger particles. The present inventors have therefore devised a method to visually determining the in-particle homogeneity. They have devised a set of 5 possible visually observable effects that can be scored. A particle of biomass prior to the process and after the process is taken and a cross section is made, e.g. by a doctors blade or knife, the outer surface of said particle as well as the core is scored along the below indicators. The status of a particle prior to the process is taken as the starting point (score 1). Score 1 : characteristic of biomass prior to any heat treatment; Score 2: initially torrefied observable by slightly darkening of the material but still fibrous when scratching the surface; Score 3: partially torrefied observable by darkening (mid brown) of the material which is becoming brittle when scratching the surface; Score 4: fully torrefied observable by dark brown, shrunk material being brittle; Score 5: fully carbonized and black.

It was observed by the present inventors that with prior art methods the outside surface of the solid fuel obtained was usually either a score 4 or 5 whereas the core was a score 1 or 2. The present invention allows for the preparation of particles having a score of 3 or 4 for the core and a score of 4 for the outside surface.

In an embodiment, oxygen is supplied during at least part of heating step 2). In other words, only during part of the heating step the oxygen is added. In a specific embodiment, oxygen is supplied (only) in the final part of the heater(s), e.g. in the last 50 to 200 cm, for example the last 80 to 120 cm of the heater; this depends on the temperature of the biomass which is preferably at least 160 °C or even 180°C for optimal reactivity. The oxygen may also be added - e.g. in varying amounts - during the complete heating step, in a specific embodiment. In an embodiment, oxygen is supplied during at least part of step 2) and during at least part of step 4), the cooling step. In this specific embodiment, oxygen is added during drying, treatment and cooling. In this way all of the desired effects of the controlled flow of oxygen according to the present invention are obtained.

Applying oxygen, either as oxygen gas or through air; will lead to combustion of the biomass it encounters. As a non-limiting calculation example, the following is observed. Under optimal conditions and in stoichiometric ratios applying 0,01 Nm³ of pure oxygen to 1 kg of wood would lead to the burning of 1.125% of that wood, which generates sufficient energy (viz. 203kJ) to heat the 1 kg of wood by approximately 170K. Applying these ratios of oxygen during the heating or thermo-chemical treatment steps show a raise of 80 to 100K. The remainder of the energy is believed to be conducted to the gaseous environment and apparatus. These numbers are used to calculate the amount of air necessary to obtain the desired rise in temperature. Thus to raise the temperature of 1 kg of wood with 10K one would need to inject approximately 0.0015 kg of oxygen or 0.0065 kg of air.

The flow of oxygen in the step of heating is determined by the amount of biomass being processed per hour and the increase in temperature that is required for optimal processing at that point. The contact time of oxygen with the biomass in the step of heating is preferably between 0.5 and 1 seconds; this is influenced by the thickness of the biomass bed at the place of oxygen injection or release and by the speed at which the oxygen is injected. Although the exact temperature and therefore the amount of oxygen and the contact time of oxygen during heating is of less importance than during the step of thermo-chemical treatment, neither a surplus of oxygen in the gaseous environment nor biomass heated over the required heating temperature is desirable.

Thermo-chemical treatment of the heated biomass

This process step is key in the present aspect. The heated biomass is treatment in order to produce a solid fuel. The treatment phase of prior art processes is often not sufficiently controlled in terms of temperature and/or in terms of time. Most prior art systems use a too high temperature to increase the speed of torrefaction and thus efficiency of the system. This merely leads to carbonization on the outside and raw (untreated) material on the inside. This is due to the prior art's inability to separate torrefaction from carbonization, in order words to prevent the torrefaction to proceed to carbonization. This is related to the inability to control the biomass temperature and possibly also the duration of the torrefaction. It has proven to be very difficult to control the biomass temperature as the process can become exothermic even in small hot-spots in the process.

Thus, a key problem during this step that the present inventors have identified and solved is the accurate control of the temperature. As the treatment process becomes exothermic at a certain point this will accelerate the process which will increase the energy produced which will accelerate the process and so on. There is thus a significant chance of creating a runaway process. This will lead to a temperature that is well above the treatment temperature and this may lead to undesirable pyrolysis of (part) of the biomass. The present inventors have observed, that in the process according to the first aspect oxygen plays three intimately linked roles: oxygen starts the process, oxygen keeps process running, and oxygen stops the process.

Oxygen starts the process: Oxygen was found to start the treatment reaction when the temperature (T_h) is somewhat below the actual treatment temperature (T_t). In other words, oxygen has a temperature increasing effect. This effect is used by the present inventors to start the reaction during the treatment step. It should be noted that it is not possible to completely replace the addition of heat by the addition of oxygen, only the last/final several (tens) of degrees Celsius can be effected by oxygen. According to this step in the reaction, a heated biomass having a temperature T_h is supplied. Oxygen is then added in a controlled matter to increase the temperature from T_h to a temperature of at least T_tmin which is below the treatment temperature T_t. At a certain temperature the treatment (e.g. torrefaction) starts and torrefaction gas will be emitted. Torrefaction gases may comprise carbon monoxide, hydrogen, methane, carbon dioxide, acetic acid and other tar components. Oxygen keeps process running: It was found that as long as oxygen is being supplied the thermo-chemical treatment continues; oxygen reduced the energy consumption during the treatment step.

The flow of oxygen is adjusted during the residence time in order to keep the temperature of the biomass between T_tmj_n and T_tmax , preferably during the entire step and hence the flow of oxygen at the various places in the reactor may fluctuate in time. At the beginning of the reactor, oxygen is supplied to quickly initiate the torrefaction of the biomass; in other words, the addition of oxygen increases the temperature of the biomass by directly heating said biomass by reaction between the biomass and the oxygen. This increases the temperature from T_h to Ttmin and starts the torrefaction process. Over the length of the reactor oxygen can be supplied in order to allow an accurate control of the temperature of the biomass. When the temperature increases and approaches T_tmax, there is a risk of a run away, the supply of oxygen may be decreased to immediately decrease the temperature. This is a far more effective and quick way to decrease the temperature than by tuning the temperature of the heated gases passing through the mantle of the reactor since the latter is an indirect process and the former is a direct process and the steel mantle with its large heat capacity is relatively inert and will only slowly respond. When the temperature decreases and approaches T_tmj_n, there is a risk that the torrefaction process will become more slow or will stop; at that moment the flow of oxygen can be increased in order to increase the temperature. This makes it preferred to the measure the temperature of the biomass after each application of oxygen to determine how much oxygen needs to be applied through the subsequent oxygen inlet. The temperature of the process can e.g. be determined by measuring the temperature of the torrefaction gas (e.g. by a thermocouple). It should be noted that another level of control that can be used in additional to oxygen-based control is to vary the indirect heating of the reactor/reaction chamber; which has a slow response time in comparison with the fast response time of the oxygen-based control. It should be noted that yet another level of control that can be used in additional to oxygen-based control and/or indirect heating variation is to vary the residence time in the reactor/reaction chamber; this controls the time of treatment and hence the properties of the final product. These three levels of control together may be used to finely tune the process according to the needs.

Oxygen stops the process: When the biomass has reached the desired degree of torrefaction, the torrefaction process can be stopped by stopping the supply of oxygen, since this will lead to a decrease of the temperature to below T_tmin. Thus when the oxygen supply is stopped, the process stops gradually, as long as the temperature rise in the material does not significantly exceed the oxygen free torrefaction temperature. Thus oxygen itself does not stop the process but the process may be stopped by decreasing or stopping the supply of oxygen.

Hence, preferably during the complete reaction process oxygen is supplied, possibly with different flow rates over the process length. The supply of oxygen is preferably only stopped when torrefaction has reached the desired result. ***

The treatment according to the present invention may be torrefaction or carbonization. Most preferably, the present invention relates to a torrefaction treatment. In this embodiment, said thermo-chemical treatment is torrefaction and the (cooled) thermo-chemically treated biomass is (cooled) torrefied biomass. In this specific embodiment, the T_t may be denoted as torrefaction temperature instead of treatment temperature.

The temperature of thermo-chemical treatment is denoted as T_t and depends on the type of biomass used. This is the optimal temperature for effecting thermo-chemical treatment of said specific biomass. The thermo-chemical treatment is preferably carried out in a range of temperature between T_tmin and T_tmax, wherein T_tmin < T_t < T_tmax. Preferably, the difference between T_tmin and T_t is the same or substantially the same as the difference between T_t and Ttmax, in other words, T_t is in the middle of the range of T_tmj_n and T_tmax. In an embodiment, T_tmj_n is at least 10 °C, preferably at least 20 °C higher than T_h. In a specific embodiment, T_t - T_tmj_n is between 5 and 20 °C, e.g. between 8 and 15 °C, e.g. 10 °C. In a specific embodiment, T_tmax - T_t is between 5 and 20 °C, e.g. between 8 and 15 °C, e.g. 10 °C. In other words, T_t + 5- 20°C, preferably T_t + 8-15 °C, more preferably T_t + 10 °C. The residence time in the step of treatment is preferably between 2 and 10 minutes, more preferably between 4 and 7 minutes; it depends on the biomass being used. The heat energy required for torrefaction is preferably (at least partially) derived from the torrefaction gases. This makes the process autarkic. The present process is preferably carried out using indirect heating, preferably using hot gas e.g. having a temperature of between 300 and 500 °C, preferably between 400 and 450 °C which heat should preferably be distributed evenly along the reaction chamber e.g. by using multiple inlets to ensure minimal temperature gradient.

As discussed above oxygen was found to reduce the pyrophoricity of the resulting solid fuel when oxygen is added during the thermo-chemical treatment and/or the cooling step(s). The present inventors have observed that the pyrophoric characteristic of the treated biomass is reduced significantly by the use of oxygen during either the treatment step and/or the cooling step. Without wishing to be bound to any theory, the inventors assume that this is due to oxygen being absorbed by the most reactive elements in the biomass thereby reducing the reactivity of the product (solid fuel).

Oxygen was moreover found to increase the in-particle homogeneity. This means that when oxygen is used during the step of thermo-chemical treatment that the thermo-chemically treated particle obtained has a more even treatment over the particle, in other words both the outer surface as well as the inner core have been treated. In method according to the prior art without the controlled use of oxygen, when a particle is examined after the treatment, the core is often raw (Score 1) or only partially treated (Score 2 or 3) whereas the outer surface is already carbonized (Score 5). This is inherent to the use of biomass, which mainly consist of carbon-containing material which has an insulating effect that is also visible when a log of wood is e.g. burned in a fire, the outside is blackened whereas the inside is not affected. The present inventors have observed that oxygen has the capacity to enter into cracks that are formed in the surface of biomass particles during the thermo-chemical treatment and oxygen is capable of "introducing" the heat into the center or core of the biomass particle, hence effecting heating from the inside which has a very positive effect on the homogeneity, viz. a core having a Score 3 or 4 and a outer surface of Score 4.

In an embodiment, oxygen is supplied during the total duration of step 4). Oxygen may also be added during the complete treatment process until the treatment is stopped. In an embodiment, the treatment step 4) is stopped by stopping the controlled flow of oxygen. In an embodiment, the flow of oxygen is applied in such volume and airspeed that the flow will not cause a fluidized bed nor create chimneys through the biomass bed. Preferably, the residence time of oxygen in the biomass is at least 0.5 second, more preferably at least 1 second before being exposed to the gaseous environment. In an embodiment, the flow of oxygen during step 4) is increased to increase the temperature of the biomass being torrefied when the temperature tends to decrease to a value below T_tmin and the flow of oxygen is decreased or stopped to decrease the temperature of the biomass being torrefied when the temperature tends to increase to a value above T_tmax. In a specific embodiment, oxygen is supplied in a larger dose in part of the reactor, e.g. in the first 50 to 200 cm, for example the first 80 to 120 cm of the reactor, than in the remaining part of the reactor in order to quickly increase the biomass temperature from T_h to at least T_tmin in order to initiate the thermo-chemical treatment.

Step of cooling treated biomass

The treated biomass is preferably cooled to a temperature (T_c) allowing densification or storage of the solid fuel obtained. The treated biomass may also be stored upon which it will automatically cool to the surrounding temperature. However, it is preferred that the treated biomass is actively cooled, in order to have a more controlled cooling. In an embodiment, an additional controlled flow of oxygen (additional to the flow during the treatment) is supplied to the biomass during step 4), viz. the step of cooling. The flow of oxygen may be oxygen gas or oxygen-containing gas. It is preferred to supply a controlled flow of oxygen during the step of cooling since the present inventors have observed that this will decrease or eliminate the pyrophoric behavior of the solid fuel obtained. In an embodiment, oxygen is supplied during at least part of the cooling step. Oxygen may also be added during the complete duration of the cooling step, in other words over the complete residence time. Without wishing to be bound by any theory, the present inventors believe that during the cooling the tar elements in the treated biomass will make the biomass sticky and ultimately clunk together. Preferably, the biomass is cooled to a temperature T_c of between 100 and 140 °C, preferably between 1 10 and 130 °C, such as approx. 120 °C depending on the biomass used, the present inventors have observed that the flow properties of the treated biomass depend significantly on the temperature. This selection of temperature allows for the remaining of some lubricating effect of the tar components which will allow easy further processing, e.g. densification, and at the same time allow for sufficient deactivation of the hot treated biomass. This high-temperature densification is not possible with any of the prior art process due to the pyrophoric behavior of the solid fuel which would lead to serious safety issues during densification process. In the case that no further treatment step is applied after the cooling step, the biomass is preferably cooled to a temperature T_c of between 50 and 100 °C, preferably between 60 and 80 °C, such as approx. 70 °C. This high-temperature densification is not possible with any of the prior art process due to the pyrophoric behavior of the solid fuel which would lead to serious safety issues during densification process. As torrefied biomass has a low heat capacity, cooling to the desired temperature can be achieved easily and quickly. To avoid potentially remaining hot-spots within the biomass the cooling rate in the step of cooling is preferably between 10 and 50 °C/minute, more preferably between 30 and 40 °C/minute. The residence time in the step of cooling is determined by the desired temperature T_c. The flow of oxygen in the step of cooling is preferably applied in the final part of the cooling reactor where potential reactiveness of the torrefied biomass is already reduced. The contact time of oxygen with the biomass in the step of cooling is preferably between 0.5 and 1 second.

Size reduction and/or densification of the (cooled) treated biomass

In an embodiment, an additional step 6) is present in which the thermo-chemically treated biomass obtained in step 3) or the cooled biomass obtained in step 4) is e.g. reduced in size by milling and/or densified to a plurality of shaped particles (step 5), preferably in the form of pellets or briquettes, at a biomass temperature of at least 100 °C. This optional step relates to the creating of shaped objects from the product obtained, thus creating e.g. pellets or briquettes from the processed material for easier and safer transportation, storing and handling and burning. Prior art processes are not able to densify at elevated temperatures because due to the pyrophoric behavior the material may combust spontaneously. Densification may be carried out by any suitable prior art method and is not particular limiting. Densification may be preceded by a size reduction step if this is required. Also that step may be carried out by any suitable prior art method and is not particular limiting.

*** Second aspect

In a second aspect of the present invention, the invention relates to the use of oxygen for heating biomass, wherein said oxygen is supplied in a controlled flow to said biomass in order to expedite the heating of said biomass by reaction of said biomass with said oxygen. The present inventors have observed that a controlled flow of oxygen may be used to increase the heating of biomass. A lower amount of energy needs to be used in order to obtain the same final temperature of the biomass. Without wishing to be bound by any theory, the present inventors believe that the addition of oxygen in the biomass leads to an exothermal reaction of the oxygen with components of the biomass leading to an increase in heat in the biomass.

The oxygen is added into the biomass to be in direct contact with the biomass. The oxygen is not added in the gases above the biomass. In this second aspect of the present invention, the oxygen is added to the biomass during the step of heating of said biomass. The step of heating the biomass may be a step of heating the biomass from room temperature to a temperature for thermo-chemical treatment. Preferably, however the step of heating is carried out after a step of drying, most preferably the dried biomass still has an elevated temperature from the drying step (T_d). This decreases the amount of energy (and time) that is need for the biomass to be heated to the temperature for the thermo-chemical treatment. In an embodiment, the dried biomass obtained from step 1 b) that is used as the starting material for step 2) has a temperature of at least 60 °C, more preferably at least at least 70 °C, most preferably approximately 120 °C. All of the embodiments presented above for the first aspect, especially for the step of heating, are also applicable to this aspect and vice versa.

***

Third aspect

In a third aspect of the present invention, the invention relates to the use of oxygen during thermo-chemical treatment of biomass, wherein said biomass has a temperature between a minimal predetermined temperature and a maximum predetermined temperature; wherein said oxygen is supplied in a controlled flow to said biomass in order to control the temperature of said biomass between a minimal predetermined temperature and a maximum predetermined temperature. In an embodiment of this aspect, oxygen is added during part of the treatment step. In another more preferred embodiment, oxygen is added during the complete duration of the treatment step. With controlled flow of oxygen is meant that the amount of oxygen added to the biomass is controlled in order to achieve the desired effect. In this third aspect, the controlled flow of oxygen means varying the amount of oxygen supplied in order to control the temperature of the biomass for the duration of this phase and reducing or stopping the supply of oxygen to stop (quench) the process at the required degree of treatment of the biomass. One effect of controlled supplying oxygen or oxygen containing gases to the biomass during the treatment is that pyrophoricity of the material produced is reduced. All of the embodiments presented above for the first aspect, especially for the step of thermo-chemical treatment, are also applicable to this aspect and vice versa.

*** Fourth aspect

In a fourth aspect of the present invention, the invention relates to the use of oxygen during a step of cooling biomass that has been thermo-chemically treatment; wherein said oxygen is supplied in a controlled flow to said treated biomass in order to reduce the pyrophoric nature of the thermo-chemically treated biomass. The present inventors have observed that when supplying oxygen or oxygen containing gases to the biomass during cooling largely reduces or even fully eliminates the pyrophoricity of the material produced. This improves the stability of the final product as well as the process ability. All of the embodiments presented above for the first aspect, especially for the step of cooling, are also applicable to this aspect and vice versa.

***

Fifth aspect

In a fifth aspect, the present invention relates to a solid fuel obtained by or obtainable by any of the processes according to the aspects of the present invention. The degree of torrefaction or level of torrefaction cannot be easily stated since it depends on the biomass used. To what extend biomass is or can be torrefied can only be described from the end product in terms of biological, physical and chemical characteristics. The present invention relates to a solid fuel having a degree of homogeneity such that the outer surface of the torrefied particle - prior to any densification step - has a score of 4 and wherein the core of said particle has a score of 3 or 4, based on the scores 1-5 as discussed above. In an embodiment, the solid fuel has a calorific value of at least 21 MJ/kg wherein the calorific value is measured according to ISO

18125:2015, "Solid biofuels - Determination of calorific value". In an embodiment, the solid fuel has a bulk density of at least 800 kg/m3 wherein the bulk density is measured according to ISO 17828:2015, "Solid biofuels - Determination of bulk density". In an embodiment, the solid fuel has a bulk energy density of at least 16,8 GJ/m³

*** Description of the embodiments shown in the drawings

In the drawings, the following reference numbers are used:

1) feeder, used to continuously supply biomass; 2) heater, used to heat biomass to T_h; 3) reactor for thermo-chemical treatment; 4) reactor for cooling treated biomass; 5) combustor, used to combust torrefaction gases; 6) stack, used to release excess flue gases into the atmosphere; 7) gas lock, used to ensure ambient air cannot penetrate the process or process gases escape; 8) connecting tubing between reactors for biomass and torrefaction gases to pass freely; 9) ventilator, used to suck hot flue gases through the heat exchangers (mantles) of the reactors for heating and thermo-chemical treatment; 10) outer mantle of reactors for heating and for thermo-chemical treating; 11) inner mantle of reactors for heating and for thermo-chemical treating; 12) axis of auger in reactors for heating and for thermo-chemical treating; 13) Heat exchanger in reactors for heating and for thermo-chemical treatment through which hot flue gases are lead; 14) biomass bed in reactors; 15) oxygen being supplied to biomass bed; 16) unit for application of oxygen/air in biomass bed; 17) connection with oxygen/oxygen containing gas supply ; 18) (removable) dust collection.

Figure 2 shows a flow diagram showing an embodiment of the process flow with the following (optional) process steps: step 1 : supply of biomass; [optional] step 1 a: reducing in size of biomass; [optional] step 1 b:drying of biomass ; step 2: heating of biomass; step 3: thermo- chemically treating of biomass; [optional] step 4: cooling of torrefied biomass and [optional] step 5: densifying of torrefied biomass.

Figure 3 shows an embodiment of a system that may be used to carry out the present process. Said system consisting of a feeder 1 that continuously supplies biomass according to step 1 through a gas lock (also called air lock) 7 which allows the biomass to pass while preventing ambient air to enter the system or process gas to exit the system. Then, the biomass proceeds to a heater 2 that according to step 2 heats the biomass to a temperature of Th after which the biomass drops through a connecting tube 8 into the reactor for thermo- chemical treatment 3 that according to step 3 thermo-chemically treats the biomass after which the torrefied biomass drops through another connecting tube 8 into the cooler 4 which, according to the optional step 4 cools the treated biomass to a desired temperature for optional further processing for which it is released from the cooler trough another gas lock 7. This figure also shows - top left - a combustor 5 that is connected through - preferably the shortest possible - connecting tube 8 to the heater 2 through which torrefaction gases can pass into the combustor; flue gases will leave the combustor through chimney 6.

Figure 4 shows the embodiment as described for Figure 1 with arrows indicating the flow of the biomass through the system.

Figure 5 shows a more detailed diagram of the embodiment as described in Figure 1. It includes arrows indicating the flow of the heating gases. The reactor for thermo-chemical treatment 3 is provided with a segmented mantle. At the upper part of the combustor 5 flue gases flow out having an approximate temperature of 800 °C and are provided to both the mantle of the heater 2 as well as to the segmented mantle of the reactor for thermo-chemical treatment 3. These mantles function as a heat exchanger with the inside of the respective reactors. The gases passing through the heating reactor 2 will have a starting temperature of approximately 550 °C obtained by mixing in ambient air into the glue gases. The gases will leave the reactor at a temperature of approximately 250°C. The gases passing through the reactor for thermo-chemical treatment 3 will have a starting temperature of approximately 450°C obtained by mixing in ambient air with the hot flue gases and will leave the mantle segments at a temperature of approximately 400 °C. The flow of hot gases through all heat exchangers i.e. all mantel segments, can be regulated using valves in order to control power and subsequent temperature of the reactors. The gases are further cooled if necessary to a temperature of approximately 250°C to allow for safe passing through the ventilator 9 which distributes the hot gases throughout the system at the required capacity and subsequently blows the gases into the chimney 6 to be released into the atmosphere.

Figure 6 shows another more detailed diagram of the embodiment as described in Figure 1. Figure 6 shows possible locations (indicated by arrows) to supply oxygen or oxygen containing gas into the system, either into or below the biomass bed. In the heater 2 the application of oxygen takes place towards the end; viz. where the biomass is more reactive and is applied only and to such extend that the temperature of the biomass reaches the desired Th. The oxygen or oxygen containing gas in the reactor for thermo-chemical treatment 3 can independently be supplied though multiple inlets over the full length whose number is dependent on the length of the system. The effect of each application of oxygen or oxygen containing gas is subsequently measured by measuring the temperature of the biomass and determines the amount of oxygen or oxygen containing gas that will be supplied though the next inlet. In the cooler 4 oxygen or oxygen containing gas can be supplied towards the end of the cooler if required to reduce the pyrophoricity of the torrefied biomass being produced. Figure 7 shows another more detailed diagram of the embodiment as described in Figure 1. Figure 7 shows the flow of torrefaction gases and water vapor through the system, indicated by an arrow. Approximately one third of the total volume of gases that is produced originate from the heater 2 and consist of low calorific gas due to a relatively large amount of water vapor that is present. The remaining two thirds of the total volume of gases that is produced are the torrefaction gases originating from the reactor for thermo-chemical treatment 3 combined with a small amount of gas originating from the cooler 4. These remaining gases have a somewhat better though still low calorific value of approximately 4.5 MJ/m³.

Figure 8 shows a cross section of a reactor (either heating or treatment) including a device suitable for the application of oxygen or oxygen containing gas into a cylindrical reactor. Between the outer mantle 10 of the reactor and inner mantle 1 1 of the reactor there is a flow of heating gases 13. Within the inner mantle 11 an auger with axis 12 rotates (arrow) to steadily move the biomass 14 through the reactor. The device 16 is mounted gastight to the inner mantle which has a number small holes (3 mm diameter) to allow for the oxygen 15 to be fed into the biomass bed via a connection to an oxygen supply 17. The holes will, when no oxygen is flowing, allow for dust and small particle to enter into the device; this has to be accounted for e.g. by a trap or collection device 18. The number of holes is determined by the amount of oxygen that (potentially) needs to be fed into the system at that point and the maximum airspeed to allow for required residence time and prevent air pockets, chimneys and fluidized bed. Supply of oxygen

As discussed, the supply of oxygen in a controlled manner is a key feature of the present invention. Oxygen may be supplied during one or more steps as discussed above. More information regarding the supply of oxygen is provided here.

The supply of oxygen is for example carried out by injection of oxygen, e.g. by one or more injection points or nozzles within the reactor/reaction chamber. In an embodiment, a pipe having a plurality of holes is used for supplying the oxygen. Said injection points may be position to be at several stages of the reaction; e.g. when the reactor is a screw extruder oxygen may be added at several points along the length of the path the biomass has to travel from the start of the process step to the end of the process step. Other means of oxygen addition may be contemplated by a skilled person as long as it may be added in a controlled manner. The oxygen is added directly into the biomass. It is thus not added to the gas phase that is present surround or above the biomass. With direct application is meant that oxygen passes through the biomass and does not only contact the biomass at the boundary between biomass and gas phase; this allows oxygen to react with the biomass itself instead of the gaseous environment above the biomass. When the oxygen is injected into the biomass, it travels through the biomass from the point of injection towards the gas phase due to the inherent nature of oxygen to rise.

The supply of oxygen by means of injection may lead to the formation of bubbles, chimneys or channels within the biomass in case the pressure and subsequent velocity of the oxygen injected exceeded a certain value. This may lead to the oxygen to travel in a short path through the biomass in a short amount of time, thereby decreasing the contact time and contact surface and hence the efficacy. In an embodiment, sufficient oxygen is applied to the biomass to achieve the purpose but at the same time care is taken to use a limited injection speed as to circumvent creating gas pockets in the biomass bed, chimneys or channels through the biomass bed (e.g. when the bed is fluidized); the contact time should be sufficient to allow the biomass to react with the oxygen supplied and to prevent the oxygen (or gas containing oxygen) to mix with the gaseous envelope above the biomass bed. Moreover, care should be taken that the pressure should be such that no biomass may enter into the oxygen supply system. It is preferred that as much oxygen as possible reacts with the biomass and that as little oxygen as possible ends up in the gaseous phase. In case the oxygen is blown through the biomass package (in other words if the contact time is very short), a combustible gas envelope above the biomass bed could develop. Even though this is not considered very harmful, the oxygen level in such a gas composition should be kept below the explosive limit, preferably the Lower Oxygen Concentration (LOC) will be 5 vol.%. Preferably, the oxygen supply should be such that no more than 3 wt.%, preferably at most 2 wt.%, of the biomass is combusted away. Preferably, the maximum oxygen concentration in the gas above/surrounding the biomass in the treatment reactor/reaction chamber is 5 vol.%, preferably maximally 4 vol.%, more preferably maximally 4 vol.%.

Oxygen or oxygen containing gas such as air may be supplied by any suitable means provided the application meets the demands of supplying the oxygen in the required volume directly in or under the biomass and with the maximum speed to guarantee the required contact time and to avoid the creation of chimneys, channels, pockets or fluidized bed. An oxygen or oxygen containing gas dosing device will be mounted gastight to the mantle of the reactor within which the biomass is contained at a location directly below the biomass and is connected to an oxygen source. The mantle is perforated with a plurality of small orifices or openings (e.g. between 1 and 5 mm, such as 3 mm in diameter). This allows for oxygen to be entered into the biomass. A trap for dust or small biomass particles may be provided to avoid contamination of the oxygen supply system through the orifices when no oxygen flows. The number and size of the orifices depends on the amount of oxygen that (potentially) needs to be fed into the system at that point and the maximum airspeed to allow for required residence time and prevent air pockets, chimneys and fluidized bed. This may be determined on a case by case basis by a person skilled in the art. If indirect heating is used in the reactors, the reactor preferably comprises an outer mantle and an inner mantle which are spaced apart. Through the spacing between the outer and inner mantle heating gases may be provided The oxygen dosing device is mounted gastight on the inner mantle and the pipe or hose protrudes gastight through the outer mantle which, due to the heating of the system and differences of expansion of the mantles will lead to mechanical tension that needs to be catered for. Equipment suitable for use in present invention

The present invention is not limited to a specific apparatus for carrying out the process. Commercially available equipment for use in torrefaction may be used for carrying out the present invention. There are several torrefaction methods known using indirect heat, based on various equipment. In an embodiment, the present process is carried out in a screw or auger based system since the present inventors have observed that this provides the best control of the process. By changing the auger rotation speed the (total) residence time may be tuned directly, having an effect on the process at a specific chosen temperature. In an embodiment, the process according to the present invention is carried out in a system, preferably comprising the following components: a feeder, optionally at least one dryer, at least one heater, at least one reactor, a combustor and a heat distribution system. Each of these components is discussed in more detail below. Preferably, the process is carried out in a continuous manner wherein e.g. auger (screw) based reactors are connected in series, each having its own purpose. However, other types of reaction systems, e.g. based on drums or a multi-hearth furnace may also be used in the process according to the present invention. The present inventors have observed that due to the removal of water and partially gasification there is a significant reduction in volume going from the biomass as starting material to the solid fuel as end product. This reduction in volume should be taken into account in the design and set up of the different process stages.

Implementing the process of the invention described in a system can be carried out by a person skilled in the area of mechanical engineering and process engineering. Based on the volume of biomass to be processed, a mass balance and energy balance need to be prepared, resulting in requirements for the heating capacity, cooling capacity, dimensions of components and piping, etc. In addition such a system will produce flammable and potentially hazardous torrefaction gases and therefore will need to comply with local regulations in these areas, a person skilled in the art will be able to take the desired measures to comply with local regulations. Although not described in detail, the burning of torrefaction gases before releasing them in the environment might requires a purposely build combustor or other means known by a person skilled in the art. Feeding system

A biomass feeding system may be used that allow for continuous feeding of biomass starting material through a screw-plug, preferably with a weight loaded lid to provide a gas lock that ensures that air is kept out and torrefaction gas is kept in the system. Said feeding system or feeder feeds the biomass (step 1) of the present process) into either a drying system or a heating system.

Drying system

A biomass drying system may be used that allows for continuous drying of the biomass obtained in step 1) during drying step 1 b). One or more drying systems may be used sequentially operation at the same or different temperatures. A drying system of any readily available type (belt-dryer, drum dryer, etc.) aimed at the removal of water from the biomass may be used. In an embodiment, heat that is generated in the treatment process is used to feed the drying system. Any suitable drying system known in the art may be used in the process according to the present invention. When the torrefaction gases hold insufficient energy to heat, treat and dry the biomass, e.g. when a high-moisture biomass is used - it may be preferred to use direct heating in the drying step additional to the indirect heating using hot gases. Direct heat may be applied by using hot gases (preferably that which are removed from the indirect heating process) that are injected (e.g. in counter current) directly in the biomass. The present inventors have observed that the combination of direct and indirect heating provides better drying performances that the sum of the two separately. Direct heating using e.g. thermal oil or steam may also be contemplated. From the drying system the dried biomass is transported to a heating system comprising or one or more heaters. Preferably, the drying system is separated from the heating system, e.g. by a lock, to prevent gas exchange. Wet gas is removed from this system comprising the moisture that is removed from the biomass.

Heating system

One or more (e.g. parallel placed) augers rotating within a housing (such as a cylindrical pipe or trough) may be used as heaters. Said augers may be heated to provide a direct heating method. Otherwise or in addition, said housing may contain a mantle through which heated gases are lead may be used to indirectly heat the cylinder(s) and thus the biomass contained therein. Preferably, the heated gases for heating are obtained from combustion of the torrefaction gases, making this process very energy-efficient. However, other heated gases or even heated oil or other liquids or molten salts may be used to heat. Direct heating using e.g. thermal oil or steam may also be contemplated. From the heating system the heated biomass is transported to a reactor system comprising or one or more reactors. One or more temperature sensors for measuring the biomass temperature may be present. One or more temperature sensors for measuring the gas temperature may be present. A mantle may surround the heating reactor; said mantle functioning as a heat exchanger with the inside of the reactor when hot gases are transported through said mantle.

Treatment system

One or more (e.g. parallel placed) augers rotating within a housing (such as a cylindrical pipe or trough) may be used as reactors/reaction chambers for the treatment step. Said augers may be heated to provide a direct heating method. Otherwise or in addition, said housing may contain a mantle through which heated gases are lead may be used to indirectly heat the cylinder(s) and thus the biomass contained therein. Direct heating using e.g. thermal oil or steam may also be contemplated. The reaction system also comprises one or more oxygen supplies in order to keep the biomass at the required temperature specific for the biomass being processed. In an embodiment, the rotating speed of the auger is set in such a manner -depending on the biomass type used, that when the biomass has reached the desired degree of torrefaction, the biomass has reached the end of the reactor. Optionally, the treated biomass is transported to a cooling system. One or more temperature sensors for measuring the biomass temperature may be present. One or more temperature sensors for measuring the gas temperature may be present. One or more oxygen sensors for measuring the oxygen content in the gas may be present. A mantle may surround the treatment reactor; said mantle functioning as a heat exchanger with the inside of the reactor when hot gases are transported through said mantle.

Cooling system A cooling system may include an indirectly cooled auger rotating in a cylindrical pipe containing a mantle through which cold liquid - preferably cold water - is led to cool the biomass to the required temperature (T_c). The cooling system may comprise one or more oxygen suppliers through which oxygen gas or gas containing oxygen can be supplied to treated biomass. One or more temperature sensors for measuring the biomass temperature may be present. One or more temperature sensors for measuring the gas temperature may be present.

Combustor and Heat distribution system

A combustor is preferably present in the system according to the present invention. Said combustor is fed either with gases derived from the heaters and/or the reactors and/or with natural gas and/or other gases. Preferably, the system is autarkic using only torrefaction gases, but at least at the startup of the reaction additional gas is required. These gases are burned in said combustor using oxygen (preferably air) and by igniting said mixture, e.g. through a natural gas fed pilot flame. Preferably, the temperature of the gases is between 800 and 1200 °C to burn all components of the torrefaction gas and to avoid the production of No_x compounds. In an embodiment, said combustor is connected to a heat distribution system. The combustor supplies said heat and that heat is distributed using said heat distribution system.

The heat distribution system is designed to distribute heat to all system components. The hot flue gases exiting the combustor are used to provide heat for the (external) drying system, the heating reactor, and the torrefaction reactor. The heat distribution system is an arrangement of valves, ventilators, optionally water injection nozzles to decrease the heat of the gas if required, and heat exchangers aimed to ensure that every component in the system that requires heat (e.g. dryers, heaters and/or reactors) is supplied with the adequate amount of heat at the right temperature.

The gases passing through the heating reactor will preferably have a starting temperature of approximately 550 °C obtained by mixing in ambient air into the hot flue gases - which preferably have a temperature of 800 °C. The gases will leave the heating reactor at a temperature of approximately 250°C.

The gases passing through the reactor for thermo-chemical treatment 3 will have a starting temperature of approximately 450°C obtained by mixing in ambient air with the hot flue gases - which preferably have a temperature of 800 °C and will leave the mantle (segments) at a temperature of approximately 400 °C.

The flow of hot gases through all heat exchangers i.e. all mantel segments, can be regulated using valves in order to control power and subsequent temperature of the reactors. The gases are further cooled if necessary to a temperature of approximately 250°C to allow for safe passing through the ventilator 9 which distributes the hot gases throughout the system at the required capacity and subsequently blows the gases into the chimney 6 to be released into the atmosphere.

Densification system

All the above components (dryers, heaters, reactors, coolers, combustor, heat distribution system) as such are known. However, the specific set up including oxygen suppliers in order to carry out the present process is novel and inventive. The separate components should be set in such a manner that the amount of biomass leaving one component can be introduced in the next component in order to have a good process flow.

The present process may however be used in any system suitable for thermo-chemical treatment of biomass whether heated directly or indirectly and in which biomass is maintained in one particular enclosure as is the case in a batch oriented process or propelled through a system in a (semi-)continuous process via an auger or screw, rotating drum, oscillating bed, rotating arm or other mechanical means or even by gravity as in e.g. a moving bed principle.

The invention will be further elucidated with the following examples without being limited hereto. Example

In an example, approximately 50-60 liters of air were used to torrefy 1 kg of biomass. The oxygen was fully absorbed by the biomass and the nitrogen mixed in with the torrefaction gas. The torrefied biomass with particles up to 30 mm in diameter was homogeneously torrefied.

It is noted that the invention relates to all possible combinations of features recited in the claims. Features described in the description may further be combined. Although the invention has been described in detail for purposes of illustration, it is understood that such detail is solely for that purpose and variations can be made therein by those skilled in the art without departing from the spirit and scope of the invention as defined in the claims. It is further noted that the term 'comprising' does not exclude the presence of other elements. However, it is also to be understood that a description on a process comprising certain steps also discloses a process consisting of these steps. The present invention is defined by the appended claims

Claims

1. Process for thermo-chemical treatment of biomass, said process comprising the following steps:

Step 1) supplying biomass;

Step 3) the thermo-chemically treatment of said heated biomass obtained in step 2) at a temperature T_t between a minimal treatment temperature T_tmin and a maximum treatment temperature T_tmax wherein T_{h <} T_tmin by supplying a controlled flow of oxygen to said biomass during at least part of said treatment to obtain a thermo-chemically treated biomass;

2. Process according to claim 1 , wherein said process comprises steps 1), 2), 3) and 4) and wherein steps 2), 3) , and 4) are carried out in separate reactors or reaction chambers.

3. Process according to any one of the preceding claims, wherein one or more additional controlled flows of oxygen are supplied to the biomass during one or more of 2) and/or 4), preferably the (additional) controlled flow of oxygen is a controlled flow of oxygen gas or oxygen-containing gas.

4. Process according to any one of the preceding claims, wherein the oxygen is supplied during the total duration of the treatment step 3).

5. Process according to any one of the preceding claims, wherein the treatment step 3) is stopped by stopping the controlled flow of oxygen.

6. Process according to any one of the preceding claims, wherein oxygen is supplied during at least part of heating step 2) and during at least part of cooling step 4).

7. Process according to any one of the preceding claims, wherein an additional step 1 b) is present after step 1 ) and prior to step 2), said step 1 b) being a step of drying the biomass supplied in step 1).

8. Process according to any one of the preceding claims, wherein T_tmin is at least

10 °C, preferably at least 20 °C higher than T_h.

9. Process according to any one of the preceding claims, wherein said thermo- chemical treatment is torrefaction and wherein said (cooled) thermo-chemically treated biomass is (cooled) torrefied biomass.

10. Process according to any one of the preceding claims, wherein the contact time of oxygen in the biomass is at least 0.5 second, preferably at least 1 second.

1 1. Process according to any one of the preceding claims, wherein during at least part of step 4) oxygen is supplied and wherein the flow of oxygen during step 4) is increased when the temperature of the thermo-chemically treated biomass tends to decrease to a value below Ttmin and the flow of oxygen is decreased or stopped when the temperature of the thermo-chemically treated biomass tends to increase to a value above T_tmax.

12. Process according to any one of the preceding claims, wherein an additional step 5) is present in which the thermo-chemically treated biomass obtained in step 3) or the cooled biomass obtained in step 4) is reduced in size and densified to a plurality of shaped particles, preferably in the form of pellets or briquettes, at a biomass temperature of at least 100 °C.

13. Use of oxygen for heating biomass, wherein said oxygen is supplied in a controlled flow to said biomass in order to expedite the heating of said biomass by reaction of said biomass with said oxygen.

14. Use of oxygen during thermo-chemical treatment of biomass, wherein said biomass has a temperature between a minimal treatment temperature and a maximum treatment temperature; wherein said oxygen is supplied in a controlled flow to said biomass in order to control the temperature of said biomass between the minimal treatment temperature and the maximum treatment temperature.

15. Use of oxygen to reduce the pyrophoric nature of a thermo-chemically treated biomass during the cooling thereof; wherein said oxygen is supplied in a controlled flow to said thermo-chemically treated biomass during the cooling thereof.

16. Solid fuel obtained by or obtainable by a process according to any one of claims 1-11.

17. Solid fuel prepared by the torrefaction of biomass having a visual score of 3 or 4 for the core and a score of 4 for the outer surface of the biomass particle prior to any optional densification, according to the following scores: Score 1 : characteristic of biomass prior to any heat treatment; Score 2: initially torrefied observable by slightly darkening of the material but still fibrous when scratching the surface; Score 3: partially torrefied observable by darkening (mid brown) of the material which is becoming brittle when scratching the surface; Score 4: fully torrefied observable by dark brown, shrunk material being brittle; Score 5: fully carbonized and black.