PROCESS

Technical field

The present invention relates to the field of torrefaction of biomass. In particular, it relates to an improvement of the energy efficiency in a

torrefaction process.

Background

To be able to compete with and replace fossil fuel energy carriers such as coal, oil and natural gas, lignocellulosic biomass would benefit from some form of pre-treatment method to overcome inherent drawbacks. The pre- treatment method torrefaction has been shown to improve biomass fuel qualities such as energy density, water content and milling, feeding and hydrophobic properties [1 -4]. These improvements establish torrefaction as a key process in facilitating an expanding market for biomass raw materials. Torrefaction is a thermal pre-treatment method that normally takes place in a substantially inert (oxygen free) atmosphere at a temperature of about 220- 600°C. During the process course a combustible gas comprising different organic compounds is produced from the biomass feedstock in addition to the torrefied biomass.

The process of producing a torrefied material from lignocellulosic biomass can be said to include four stages:

1 ) a drying step, wherein free water retained in the biomass is removed;

2) a heating step in which physically bound water is released and the temperature of the material is elevated to the desired torrefaction

temperature;

3) a torrefaction stage, in which the material is actually torrefied and which starts when the material temperature reaches about 220°C -230°C. During this stage, the biomass partly decomposes and releases different types of volatiles/hydro carbons, such as hydroxy acetone, methanol, propanal, short carboxylic acids etc. In particular, the torrefaction stage is characterised by decomposition of hemicellulose at temperatures from 220°C -230°C, and at higher torrefaction temperatures cellulose and lignin also starts to decompose and release volatiles; cellulose decomposes at a temperature of 305-375°C and lignin gradually decomposes over a temperature range of 250-500°C; 4) a cooling step to terminate the process and facilitate handling. The torrefaction process is terminated as soon as the material is cooled below 220°C -230°C Summary of the present disclosure

During the heating and torrefaction stages in a torrefaction process

combustible gases are released from the biomass. To maximize the efficiency of the torrefaction process these gases are usually collected and combusted in a boiler so that the energy in the gases can be reused as heat in the drying, heating and/or torrefaction stage of the torrefaction process. The present inventors have realized that also during the drying process combustible gases are given off from the biomass. Freshly cut wood contain about 50 % to 80 % moisture, but after storage the moisture content typically fall to about 35-40 %. During the drying step(s) in a torrefaction process, free water retained in the biomass is removed so that the moisture content is reduced close to zero. Therefore the water content in the gas mixture produced during the drying step is high and the gas mixture is thus not suitable for combustion.

Furthermore, the gas mixture released during the drying step comprises different kinds of smelly chemicals, such as terpenes, which needs to be destructed. There is thus a need for a more energy efficiency torrefaction process which utilizes the energy in the gases released during the drying step and for an efficient method of destruction of this smelly gas.

Besides the insight that combustible gases are given off from the biomass during the drying stage, the present invention is based on the finding that the gases given of during the drying step can be collected and efficiently combusted in a combustion chamber together with gases produced later in the torrefaction process, provided that the water content in the biomass is sufficiently low, such as below 15 %. The inventors have also realized that the vast majority of smelly volatiles released during the drying step are released in a late phase of the drying step. The proportion of the smelly volatiles is thus constantly increasing during the drying process and when the biomass is dried to a moister content below 6-7 % (w/w) the amounts are increasing rapidly. The inventors thus solves the problems above by including a pre- drying step in the torrefaction process wherein the biomass initially is dried such as that the water content in the biomass is reduced below 15%, preferably below 10%. The gas released during the first drying step mainly contains water and is therefore not suitable for combustion and does not need to be destructed. Therefore this gas can be released to the surrounding. In the next step of the process the biomass is further dried, e.g. in a drying zone of a torrefaction arrangement. The gases released during this second drying step comprise smelly volatiles and are suitable for combustion. Therefore these gases are collected and preferably combusted in a combustion chamber together with the gases released during the heating and torrefaction stages.

Therefore, in a first aspect the invention relates to a method of torrefaction of biomass and combustion of generated gases, comprising the steps of;

a) drying biomass having an average moisture content of at least 20 %, such as at least 25 % (w/w) to obtain a dried biomass having an average moisture content of 3-15 % (w/w) and a first gas product; b) further drying the biomass from step a) to obtain a dry biomass and a second gas product;

c) heating and torrefying the biomass from step b) to obtain torrefied material and a third gas product; and

d) collectively combusting of the second and the third gas product without the first gas product.

In a second aspect the invention relates to a system for torrefaction of biomass and combustion of generated gases, comprising: a first zone for drying biomass, said first zone having a biomass inlet, a gas outlet and a dried biomass outlet;

at least one additional zone for further drying, heating and torrefying the dried biomass from the first zone, said at least one additional zone having an inlet connected to the dried biomass outlet of the first drying zone and at least one gas outlet; and

a gas combustion chamber being connected to receive gases from the at least one gas outlet of the at least one additional zone but not from the gas outlet of the first zone.

Brief description of the figures

Figure 1 shows a system for torrefaction of biomass and combustion of generated gases. Definitions:

Torrefaction:

A thermal pre-treatment method that takes place in a virtually inert (oxygen- reduced or oxygen free) atmosphere at a temperature above 220 °C but below 600 °C and which produces a torrefied biomass and combustible gases. During a torrefaction stage, parts of the biomass, in particular hemicellulose, decompose and releases different types of organic volatiles. In a torrefaction process starting from raw biomass, the actual torrefaction stage is preceded by a drying stage wherein free water retained in the biomass is removed and by a heating stage wherein the biomass is heated to the desired torrefaction temperature.

Drying zone

A specific region of a compartment in a torrefaction reactor, located upstream of a heating zone in relation to a biomass inlet of a torrefaction reactor, comprising means for regulating the temperature in said specific region and wherein a biomass is dried to a water content below 10 % prior to heating.

Heating zone: A specific region of a compartment in a torrefaction reactor, located upstream of a torrefaction zone in relation to a biomass inlet of a torrefaction reactor, comprising means for specifically regulating the temperature in said specific region and wherein the temperature of a biomass is increased to a

temperature near the desired torrefaction temperature prior to torrefaction.

Torrefaction zone:

A specific region of a compartment in a torrefaction reactor, located downstream of a heating zone in relation to a biomass inlet of a torrefaction reactor, comprising means for specifically regulating the temperature in said specific region and wherein the temperature of a previously heated biomass is kept virtually constant at the desired torrefaction temperature for a desired torrefaction time wherein a desired torrefaction temperature is in a range between 220 °C to 600 °C.

Torrefaction time:

The time the temperature of the material is kept virtually constant at the torrefaction temperature. The residence time of the material in the torrefaction zone may be referred to as the torrefaction time.

Detailed description

In a first aspect the invention relates to a method of torrefaction of biomass and combustion of generated gases, comprising the steps of;

a) drying biomass having an average moisture content of at least 20 %, such as at least 25 % (w/w) to obtain a dried biomass having an average moisture content of 3-15 % (w/w) and a first gas product; b) further drying the biomass from step a) to obtain a dry biomass and a second gas product;

c) heating and torrefying the biomass from step b) to obtain torrefied material and a third gas product; and

d) collectively combusting of the second and the third gas product without the first gas product. In one embodiment, the dry biomass from step b) may have an average moisture content of less than 4 % (w/w), preferably less than 3 % (w/w). According to another embodiment the method comprises a step of determining the moisture content and/or the moisture content distribution between step a) and step b). Preferably the moisture content and/or the moisture content distribution is determined by NIR spectroscopy. A high moisture content between step a) and step b), such as a moisture content above 15 %, indicates that the biomass has not been sufficiently dried in step a) and hence, in order to make the gases released in step b) more suitable for combustion, the amount of energy supplied to the biomass in step a) should be increased. To obtain an efficient combustion, the gas mixture sent to combustion preferably has a theoretical flame temperature of at least 1500 °C. The amount of energy supplied to the biomass in step a) may for example be increased by increasing the residence time of the biomass in the initial drying stage. Conversely, a low moisture content between step a) and step b), such as a moisture content below 3% indicates that the biomass have been over-dried which leads to a unnecessarily high loss of combustible gases is step a). Furthermore this leads to higher amounts of smelly volatiles present in the gas released during step a). In this case the amount of energy supplied in step a) should be decreased. The amount of energy supplied to the biomass in step a) may for example be increased by increasing the residence time of the biomass in the initial drying stage. Therefore, in one embodiment the determined moisture content between step a) and step b) is compared to a reference value and a degree of heating and/or a residence time in step a) is adjusted based on the comparison. In one embodiment a degree of heating and/or a residence time in step a) is increased if the average moisture content of the biomass is above an upper reference value. Further, the degree of heating and/or the residence time in step a) may be decreased if the average moister content of the biomass is below a lower reference value. In one embodiment the upper and the lower reference value is in the water content range of 3-15 % (w/w), 3-10 % (w/w) or 5-10 % (w/w). Preferably, the biomass obtained from step a) has moisture content of 3-12 % (w/w), such as 4-1 1 % (w/w), such as 4-10 %, such as 4-9 % (w/w), such as 4-8 % (w/w), such as 5-8 % (w/w). The optimal moisture content level varies with the type of biomass.

In one embodiment the method further comprises measuring a combustion temperature in step d) and comparing the combustion

temperature to a first reference value and, if the combustion temperature is lower than the first reference value, increasing the degree of drying in step a) or the degree of torrefaction in step c). The first reference value may for example be at least 1000 °C, such as at least 1 100 °C, such as at least 1300 °C, such as in the range of 1300-1500 °C. The degree of drying in step a) may be increased by increasing the temperature or amount of steam supplied in step a). Also, it maybe increased by increasing the residence time of the biomass in step a). Preferably, the degree of torrefaction is increased by increasing the residence time of the biomass in a torrefaction zone (i.e.

torrefaction time) or increasing the temperature of the biomass in the torrefaction zone (i.e. torrefaction temperature). In another embodiment, the combustion temperature in step d) is compared to a second reference value and, if the combustion temperature is higher than the second reference value, the degree of drying in step a) or the degree of torrefaction in step c) is decreased. The degree of drying in step a) may be decreased by decreasing the temperature or amount of steam supplied in step a). Also, it maybe decreased by decreasing the residence time of the biomass in step a).

Preferably, the degree of torrefaction is decreased by decreasing the torrefaction time and/or torrefaction temperature.

In a preferred embodiment the biomass is lignocellulosic biomass, such as a wood material, e.g. wood chips.

A second aspect of the invention relates to a system for torrefaction of biomass and combustion of generated gases, comprising:

a first zone for drying biomass, said first zone having a biomass inlet, a gas outlet and a dried biomass outlet; at least one additional zone for further drying, heating and torrefying the dried biomass from the first zone, said at least one additional zone having an inlet connected to the dried biomass outlet of the first drying zone and at least one gas outlet; and

a gas combustion chamber being connected to receive gases from the at least one gas outlet of the at least one additional zone but not from the gas outlet of the first zone.

In one embodiment a NIR camera is arranged at the dried biomass outlet of the first zone. Thus, the moisture content and/or moisture distribution of the biomass from the first zone may be measured. Further, the NIR camera may be connected to a computer or a programmable logic controller so that the computer may receive a signal containing moisture content information from the NIR camera. The computer may be capable of providing a moisture content value based on the signal. Further, the computer may be capable of comparing such a moisture content value to a reference value, and based on the comparison sending a signal to an arrangement for controlling the degree of drying in the first zone.

In another embodiment a temperature sensor is arranged at the combustion chamber. The temperature sensor may be connected to a computer so that the computer may receive a signal containing temperature information from the sensor. The computer may be capable of providing a temperature value based on the signal. Further, the computer may be capable of comparing such a temperature value to a reference value, and based on the comparison sending a signal to an arrangement for controlling the degree of drying in the first zone or the degree of torrefaction in the at least one additional zone.

Detailed description of exemplary embodiments

Figure 1 a illustrates a system for torrefaction of biomass and

combustion of generated gases (1 ). Said system (1 ) comprises a first zone for drying biomass (2) having an average moisture content of about 40 % (w/w). During this step a dried biomass having an average moisture content of about 3-15 % (w/w) and a first gas product is generated. Said first gas product comprises high amounts of water and is thus not suitable for combustion. Therefore the first gas product is discarded from the system (1 ) through the first drying zone gas outlet (3). The dried biomass having an average moisture content of about 3-15 % (w/w) thereafter enters a zone for further drying (4) wherein a dry biomass, typically having an average moisture content below 3 % (w/w), and a second gas product is generated. In contrast to the first gas product the second gas product comprises lower amount of water and have been shown to be combustible together with other gases. Therefore the second gas product is collected from the zone for further drying (4) via the zone for further drying gas outlet (5). After the biomass have been further dried in the zone for further drying (4) the dry biomass enters a heating zone (6) where the temperature of the biomass is elevated up to a desired torrefaction temperature. Thereafter the biomass enters a torrefaction zone (8) where the temperature is kept at the torrefaction temperature for a desired time (i.e. torrefaction time). Both in the heating zone (6) and the torrefaction zone (8) the biomass is giving of combustible gases which are collected from the said zones via the heating zone gas outlet (7) and the torrefaction zone gas outlet (9). The torrefied biomass is cooled to a temperature below 130 °C in a cooling zone (10) and combustible gases are optionally collected from the cooling zone (10) via the cooling zone gas outlet (1 1 ). The gas collected from the zone for further drying (4), the heating zone (6), the torrefaction zone (8) and optionally the cooling zone (10) are transported and introduced into a combustion chamber (12) of a boiler via a combustion chamber gas inlet (13). The heat generated is used to heat water transported to the boiler via a water inlet (14) to produce steam which exits the boiler via the steam outlet (15). The produced hot steam can be used in the drying steps, heating step and/or torrefaction step in the torrefaction process.

The skilled person understands that the gases from the zones for further drying (4), heating (6), torrefaction (8) and optionally cooling (10) may be collected from a single outlet. Thus, it is not necessary to have four different outlets (5, 7, 9, 1 1 ).

A NIR camera is arranged at the biomass outlet of the first zone (2) for measuring the moisture content of the biomass leaving the first zone (2). The NIR camera may be connected, via a computer, to means for controlling the degree of drying in the first zone (2). Such means may control the residence time of the biomass in the first zone (2). Alternatively, such means may control the amount of steam supplied to the first zone (2). Also, such means may control the temperature of such steam.

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