Technical field

The present invention relates to the field of torrefaction of biomass. In

particular, it relates to the control of the quality of the torrefied material during the torrefaction process.

5

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

10 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.

15 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.

20 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

25 temperature;

3) a torrefaction stage, in which the material is actually torrified and which starts when the material temperature reaches about 220°C -230°C. During this stage, the biomass partly decomposes and gives off different types of volatiles, such as hydroxy acetone, methanol, propanal, short carboxylic acids etc. In particular, the torrefaction stage is characterized by decomposition of hemicellulose at temperatures from 220°C -230°C, and at higher torrefaction temperatures cellulose and lignin also starts to decompose and give off 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.

Rousset et al. ("Characterisation of the torrefaction of beech wood using NIRS: Combined effects of temperature and duration", part of Biomass and Bioenergy, 201 1 , vol. 35, no. 3, pp. 1219-1226) presents a study of torrified wood using spectral analysis in the NIR range. The study shows i.a.

compensatory effects of temperature and duration. Summary of the present disclosure

The requirements for quality and properties of torrefied products differ considerably depending of the intended use of the product. The inventors have realized that it is important to be able to precisely control the torrefaction temperature and the torrefaction time to generate a torrefied product having the desired characteristics (e.g. a specified dry matter content,

hydrophobicity, brittleness, heating value and/or grade of torrefaction).

Furthermore, the inventors have realized that the different kinds of

lignocellulosic raw materials may need different torrefaction times and temperatures to generate a product of a desired quality and property.

Therefore the torrefaction process is preferably adjusted based on the properties of the biomass used. In many cases it is difficult or even impossible to know which the optimal torrefaction time and torrefaction temperature is for a specific biomass before the process has started. Therefore there is a demand for an on-site analysis of a torrefaction process wherein the analysis can be used for product quality control and for online control of the

torrefaction process.

The present invention solves the problem above by a method for on- site analysis of at least one quality parameter of a material in or from a torrefaction process. In said method the absorbance of the torrefied material is analyzed by a camera (e.g. a NIR-camera) to generate a data set which is processed in a computer using a multivariate statistical method. The invention is partially based on the discovery that the data generated from the camera could be correlated to several product qualities of the torrefied material such as dry matter content, hydrophobicity, brittleness, heating value and grade of torrefaction. The invention is also partly based on the insight that the torrefied material is preferably disintegrated prior to hyperspectral analysis to ensure high performance of the method. The disintergration exposes the interior of the individual pieces in the material such that the information about the relevant properties provided by the image is not only derived from the surfaces of the pieces but also from their insides. Thus, the disintegration in combination with the hyperspectral image provides for representative analysis of the properties in the material taken as a whole. The invention also provides a method for online control of the torrefaction process. In said method the absorbance of the torrefied material is analysed by a camera (e.g. a NIR- camera) to generate a data set which is processed in a computer using a multivariate statistical method. Surprisingly, the inventors have demonstrated that the method can be used to determine how the torrefaction temperature and the torrefaction time should be changed in a torrefaction process in order to control the process so that a torrefied material with a desired characteristic is obtained.

In contrast to the method of the present disclosure which generates and analyses a hyperspectral image, the method disclosed in Rousset et al. generates and analyses simple spectra. Thus, the method in Rousset et al. gives no information about the distribution of properties in the analysed material resulting in a less accurate quality control.

It is, for example, generally known that the temperature and the residence time of a torrefaction reaction affect the grade to which the material in the reaction is torrefied. However, if the material is found to be insufficiently torrefied, it has in the prior art not been possible to determine whether it is preferred to primarily increase the temperature or the residence time in order to obtain a material with the desired torrefaction grade. For example, in Rousset et al. it is simply concluded that there is a compensatory effect of treatment temperature and duration/residence time. Thus, Rousset et al. do not acknowledge the problem of deciding whether to adjust the temperature or the residence time. Rather, Rousser et al. indicate that there is no such problem due to the compensatory effect. However, the present disclosure provides a solution to this problem.

Accordingly in a first aspect the present invention relates to a method of on- site analysis of at least one quality parameter of material in or from a torrefaction process, comprising the steps of:

a) diverting material from the torrefaction process;

b) disintegrating the material from step a) to expose cross-sections of the material;

c) obtaining a hyperspectral image in the region from 400 nm to 25000 nm of the disintegrated material from step b) using an on-site camera, such as an on-site NIR-camera; and

d) correlating the hyperspectral image from step c) to the at least one quality parameter of the material. In another aspect the invention relates to a method of controlling a torrefaction process having an operation temperature and a residence time, comprising the steps of:

a) obtaining of a hyperspectral image in the region from 400 nm to 25000 nm of optionally disintegrated at least partly torrefied biomass from the torrefaction process;

b) analyzing the hyperspectral image using multivariate analysis to obtain a data set;

c) comparing the data set from step b) with a reference data set;

d) based on the comparison of step c), determining an optimal operation temperature and an optimal residence time; and

e) controlling the torrefaction process based on the optimal operation temperature and the optimal residence time. Brief description of the figures

Figure 1 a shows the torrefaction reactor used in the experiment described in the examples

Figure 1 b shows a schematic description of a method for quality

determination and on-site analysis of material from a continuous torrefaction process. Figure 2 is a schematic illustration of hyperspectal processing of the signals detected from a torrefied biomass by a NIR-camera.

Figure 3 shows the correlation between the hydrophobicity of a torrefied product and the process parameters torrefaction time and torrefaction temperature.

Figure 4 shows a data set generated from NIR-camera signals from torrefied disintegrated wood chips. Figure 5 shows images of quantified untorrefied material in three different batches 5a) the average amount of untorrefied material in batch 1 was 7,6% 5b) the average amount of untorrefied material in batch 2 was 9,5% 5c) the average amount of untorrefied material in batch 3 was 26%.

Detailed description

In a first aspect the invention relates to a method of on-site analysis of at least one quality parameter of material in or from a torrefaction process, comprising the steps of:

a) diverting material from the torrefaction process;

b) disintegrating the material from step a) to expose cross-sections of the material;

c) obtaining a hyperspectral image in the region from 400 nm to 25000 nm of the disintegrated material from step b) using an on-site NIR camera; and d) correlating the hyperspectral image from step c) to the at least one quality parameter of the material.

The hyperspectral image may for example be in the region from 400 nm to 10000 nm, such as in the region from 400 nm to 5000 nm, such as in the region from 400 nm to 2500 nm, such as in the region from 800 nm to 2500 nm. Thus, the hyperspectral image may be a NIR image.

In one embodiment the at least one quality parameter is selected from dry matter content, hydrophobicity, brittleness, heating value and grade of torrefaction.

In another embodiment the torrefaction process comprises separate steps of heating and torrefaction, and the material is diverted between the step of heating and the step of torrefaction or after the step of torrefaction. In one embodiment the torrefaction process comprises separate steps of drying and heating, and the material is diverted between the step of drying and the step of heating.

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

Sometimes, the quality of a batch of torrefied material need to be determined before it is decided whether or not the batch can be used for the intended application. Such a determination may be particularly important when the quality requirements for the application are high. Thus, in one embodiment, the method further comprises the additional step of:

e) based on the at least one quality parameter obtained in step d), determining whether or not to waste a batch or stream of torrefied material from which the diverted material was obtained.

In a second aspect the invention relates to a method of controlling a torrefaction process having an operation temperature and a residence time, comprising the steps of:

a) obtaining of a hyperspectral image in the region from 400 nm to 25000 nm of optionally disintegrated at least partly torrefied biomass from the torrefaction process;

b) analyzing the hyperspectral image using multivariate analysis to obtain a data set;

c) comparing the data set from step b) with a reference data set;

d) based on the comparison of step c), determining an optimal operation temperature and an optimal residence time; and

e) controlling the torrefaction process based on the optimal operation temperature and the optimal residence time.

The hyperspectral image may for example be in the region from 400 nm to 10000 nm, such as in the region from 400 nm to 5000 nm, such as in the region from 400 nm to 2500 nm, such as in the region from 800 nm to 2500 nm. Thus, the hyperspectral image may be a NIR image.

In one embodiment of the second aspect step e) is based on the difference between the actual operation temperature and the optimal operation temperature and the difference between the actual residence time and the optimal residence time. Thus, the operation temperature and/or the residence time may be independently modified in response to the detected difference(s).

According to another embodiment of the second aspect the torrefaction process is carried out in a torrefaction reactor. The residence time may for example be controlled by controlling the speed by which biomass is transported through the torrefaction reactor. The speed may in turn be controlled by the controlling the rotational speed of a transport screw arranged in the torrefaction reactor. The transport screw may for example be fixed to a drum which constitutes the torrefaction reactor, and in such case the whole drum rotates to transport the material through it.

According to another embodiment of the second aspect, the operation temperature is controlled/modiefied by controlling/modifying a degree of a heating of the biomass carried out before the biomass enters the torrefaction reactor or by controlling/modifying a degree of a cooling of the biomass carried out in the torrefaction reactor.

In one embodiment the biomass is lignocellulosic biomass, such as a wodd material. In a preferred embodiment the biomass is wood chips.

In one embodiment the multivariate analysis includes at least one of the following statistical methods: Partial least squares regression (PLS); PLS Discriminant Analysis (PLS-DA); Ordinary Least Squares (OLS) regression; MLR (multiple linear regression); OPLS (Orthogonal-PLS); SVM (support vector machines); GLD (general discriminant analysis); GLMC (generalized linear model); GLZ (generalized linear and non-linear model); LDA (Linear Discriminant Analysis); classification trees; cluster analysis; neural networks; and Pearson correlation.

Definitions: Near-infrared (NIR) spectroscopy:

A spectroscopic method that uses the near-infrared region of the

electromagnetic spectrum (from about 800 nm to 2500 nm).

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 combustional gases. During a torrefaction stage, parts of the biomass, in particular hemicellulose, decompose and give off 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. Heating zone:

A specific region of a compartment in a torrefaction arrangement, located upstream of a torrefaction zone in relation to a biomass inlet of a torrefaction arrangement, 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 arrangement, located downstream of a heating zone in relation to a biomass inlet of a torrefaction arrangement, 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.

Drying zone:

A specific region of a compartment in a torrefaction arrangement, located upstream of a heating zone in relation to a biomass inlet of a torrefaction arrangement, 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.

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 of exemplary embodiments Figure 1 a shows a pilot scale torrefaction arrangement having a biomass inlet (1 ) wherein the biomass is introduced in the torrefaction reactor by means of a feeding screw (2). The biomass is dried in a drying zone (3) ) wherein heat is supplied to the drying zone (3) by means of a heating media (e.g. hot gases) through a drying zone heating media inlet (4) and wherein the heating media leaves the drying zone through the drying zone heating media outlet (5). Dried biomass is transported through the drying zone (3) at a speed regulated by the feeding speed in the biomass inlet (1 ) and enters the heating zone (6) where the temperature of the biomass is elevated to a temperature near the desired torrefaction temperature. The heat is supplied to the heating zone (6) by means of a heating media through a heating zone heating media inlet (7) which leaves the heating zone through a heating zone heating media outlet (8). The heated material enters a first torrefaction zone (9) in which the temperature can be controlled by introducing heating media and/or cooling media in the first torrefaction zone heating/cooling media inlet (10) wherein said heating/cooling media exits the first torrefaction zone through the torrefaction zone heating/cooling media outlets (1 1 ). The biomass thereafter enters a second torrefaction zone (12). Heating/cooling media can be supplied to the second torrefaction zone (12) via the torrefaction zone heating/cooling media inlet (13) and said heating/cooling media exits the torrefaction zone via a torrefaction zone heating/cooling media outlet (14). The material transport in the heating zone (6) and torrefaction zones (9, 12) is driven by a common transport screw which is attached to a drum enclosing the heating zone (6) and torrefaction zones (9, 12).

Figure 1 b shows a schematic description of a method for quality determination and on-site analysis of material from a continuous torrefaction process. A biomass, such as wood chips, is transported by a first transporting means (21 ) and are introduced into a torrefaction arrangement (22) where the biomass is dried in a drying zone, heated to a desired torrefaction

temperature in a heating zone and torrefied in torrefaction zones for a desired torrefaction time. Next, the biomass is cooled in a cooling zone, comprising a screw cooler and thereafter, the torrefied biomass exits the torrefaction arrangement by falling down on a first conveyor belt (23). A fraction of the torrefied biomass is diverted from the first conveyor belt (23) onto a second conveyor belt (24) where said diverted biomass is transported to a

disintegrator (25). The said diverted biomass is disintegrated in the

disintegrator (25) such that cross-sections of the said diverted biomass appear. The disintegrated biomass is transported on the second transport belt (24) such that it passes under a NIR-camera (26) and is thereafter collected in a collection vessel (27). The NIR-camera is emitting light. Part of the light is reflected from the disintegrated biomass and is detected by the NIR-camera. (26). The detected signals (pictures), are conveyed via a first cable (28) to a computer (29) attached to a monitor (30). The signals from the NIR-camera are processed in the computer using multivariate analysis and multivariate statistic programs to obtain a data set; which is presented on the monitor (30). The acquired data set is compared with a reference data set; and based on the comparison it is possible to determining an optimal operation temperature (i.e. torrefaction temperature) and an optimal residence time (i.e. torrefaction time). Unless the comparison shows that operation temperature and/or operation time needs to be changed, the parameters is kept constant during the whole run. If the comparison shows that operation temperature and/or operation time needs to be changed, a signal is sent from the computer (29) via a second cable (32) to the torrefaction arrangement (22), wherein said signal leads to that the torrefaction temperature and/or torrefaction time is changed accordingly. If the comparison further shows that the torrefied biomass does not fulfil a predetermined quality, a signal is sent from the computer (29) via a third cable (31 ) which leads to the opening of a disposal trap door along the first conveyor belt (23). The biomass on the first conveyor belt (23) is then transported via a transporting means (32) to the collection vessel (27). The disposed biomass from the collection vessel (27) may used as a fuel in another application than the one indented for torrefied material of appropriate quality.

Figure 2 is a schematic illustration of hyperspectal processing of the signals detected by the NIR-camera. A picture of torrefied disintegrated wood chips lying on the second transport belt (24) is shown to the left in the figure. The surface of the said picture is divided into square shaped pixels covering the said wood chips, and from every pixel light of a wave-length of 1000 nm to 2500 nm (i.e. in the near-infrared region) is emitted. In reality the size of the pixels are very small. Therefore, in order to make the figure more

perspicuous, the pixels shown in the picture is much larger that in reality. The intensity of the reflected light (i.e. the absorbance) from the pixels, at different wave length within the near-infrared region, is shown in the right part of the figure. The vast numbers of generated curves are processed by a computer using multivariate statistic analysis such as partial least squares regression (PLS) or multiple linear regressions (MLR).

Examples

All the torrefaction experiments described below have been performed in the pilot scale torrefaction reactor shown in figure 1 a. Example 1

Figure 4 shows a data set generated from NIR-camera signals from torrefied disintegrated wood chips. The signal from the NIR-camera was processed in a computer using partial least squares regression (PLS). The generated data set was compared to reference data sets to determine the properties of the torrefied wood chips and to which class said torrefied wood chips belonged to. The absorbance at different wavelength correlates with properties of the torrefied wood and could for example be described with an equation such as:

Propertyl = bX = b ₀ + b^ + b ₂ X ₂ + b ₃ X ₃ ^{+ +} b _n X _n wherein the X-variables contain information from every wavelength in the NIR data, n is the number of spectral wavelengths used in the NIR data and every b-coefficient is determined using partial least squares regression (PLS) for the quantification and PLS Discriminant Analysis (PLS-DA) for classification using historical known measured values.

Measured values are given as principal component 1 and principal

component 2 representing the X- and Y-axis in the diagram shown in figure 3. The measured values are plotted in the said diagram so that separate cloudlike surfaces are formed within the diagram. Every cloud-like surface can be correlated to a particular property of the torrefied product, such as dry matter content, hydrophobicity, britleness, heating value and grade of torrefaction. More than two principal component are often used but in the present case only two principal component are illustrated.

Thus, it is demonstated that is is possible, using a NIR-based method, to determine the properties of torrefied biomass produced at different process parameter settings.

Example 2

Figure 3 shows the correlation between the hydrophobicity of the torrefied product and the process parameters torrefaction time and torrefaction temperature. The numbers given in the figure (105, 1 10 and 1 15) are values of the hydrophobicity. Two different batches of spruce chips were torrefied separately using a torrefaction temperature of 255 °C and a torrefaction time of 13 minutes. The hydrophobicity of the torrefied products were determined using the method described in example 1. The torrefied product from the first batch had a hydrophobicity according to the surface A plotted in figure 3. The second batch was torrefied using the same torrefaction temperature and torrefaction time as the first batch but the torrefied product from the second batch had a hydrophobicity according to the surface B plotted in figure 3. Using the diagram in figure 3 it was possible to determine how to change the

torrefaction time and torrefaction temperature in order to get a hydrophobicity corresponding to the value in the origo of the diagram. Therefore in a second run the torrefaction time was increased with 5 minutes (At) and the

torrefaction temperature was increased with 10 °C (ΔΤ). As predicted, the hydrophobicity from this run ended up in the surface A, i.e. in the origo of the diagram. This demonstrates that the present invention could be used to control a torrefaction process and to determining an optimal operation temperature and an optimal residence time for a torrefaction process in order to generate a product of a desired quality. Using a vast number of torrefaction experiments the present inventors have determined the correlation between several product quality parameters and the process parameters torrefaction time and torrefaction temperature. Two examples of discovered correlations are shown below, wherein t is the time in minutes and T is the temperature in Celsius:

Product yield = Y _PU = 132.9 - 0.16T - 0.13t

Hydrophobicity = Y _FU = 80 +33T +10t +15Tt + 10T ² + 5t ²

Thus, it is demonstated that is is possible, using a NIR-based method, to predict the torrefied quality and also to adjust process parameters to maintain the desired quality.

Example 3

This example demonstrates the possibility to detect if the biofuel is completely torrefied. Three different batches with different amount of untorrefied material is used in the demonstration. Torrefied material was diverted from the torrefaction process and grinded to fine powder. The diverted samples were measured on-line with a NIR camera (800-2500nm). The data was pre-processed in a computer and a PLS calibration model was used to determine the amount of untorrefied material in each batch.

The result demonstrates that in a batch with 5% untorrefied material, the model will predict an average quantity of 7.6% untorrefied material (figure 5a). In a batch with 10% untorrefied material, the model predicts 9,6% as untorrefied material (figure 5b) and in a batch with 30% untorrefied material the model predicts 26% untorrefied material (figure 5c). Furthermore, a critical distance of acceptance must be set in order to determine if the material is acceptable for further refinery. The step of measuring that the material is completely torrefied is importat to ensure the right product quality are obtained.

The results are visualized in figure 5a, 5b, 5c.