The present invention relates in general to production of biofuel pellets and the like, and more particularly to an improved method wherein intermittent

production and complete interruptions in the production process are eliminated or at least minimized, due to an improved control of feed layer formation.

Background of the Invention

A pelletizer for production of fuel pellets or animal feed pellets has at least one die with at least one press channel and a device to apply force to press material of mainly organic (e.g. biomass) origin through the press channels. There are three main techniques for such pellet production systems.

1) a die where at least one roller applies pressure when it passes a press

channel;

2) at least two matched cylinders where each cylinder may act both as a

and a pressure applying roller;

3) a die and at least one piston where either the die or piston applies

pressure.

Pelletizers using a system with at least one die and at least one roller, as in 1) above, are mostly based on free rolling rollers. For matched cylinder systems, as in 2) above, at least one cylinder may be free rotating. Such free rolling devices are dependent on friction against a raw material feed layer to rotate.

Free rolling press rollers, as in 1) above, are mounted with a gap (e.g. fractions of millimetres) with respect to the die. This is also the case for free rolling cylinders as in 2) above. The distance to the free rolling device (e.g. roller) is required to enhance the formation of a compressed raw material film, feed layer, e.g. between rollers and die. The free rolling device needs friction against the feed layer to rotate. When this occurs the feed layer is successively pressed into die channels for each turnover by the roller and the material is

successively fed into the pelletizer to obtain continuous pellet production. Therefore, a stable feed layer formation is essential to sustain continuous pellet production and to obtain consistently high pellet quality. Feed layer formation depends on material specific characteristics, such as compressibility, relaxation and binding properties between particles as well as between particles and die surface, preventing the feed layer from being swept off from the die surface.

Due to occasionally occurring poor feed layer formation, pelletizing by roller- die-channel techniques, such as described above, suffers from serious problems with discontinuous production e.g. intermittent production and, in severe cases, complete interruptions in the production of pellets. Intermittent pellet production causes tremendous stress on the pelletizer and produced pellets are of variable quality. Especially prone to this kind of behaviour are straw-type biomass materials, e.g. rice husks, straw of wheat, oat, barley and the like, grasses like switchgrass, Miscanthus sp., reed canary grass etc., but has also been experienced with other feedstock, e.g. woody materials. Problems are caused by in-homogeneities in the development of a continuous feed-layer and/or by feed layer breakages. Larsson et al. (vide infra) defined continuous pellet production as a production pattern when the coefficient of variation (CV) of the pelletizer motor current was lower or equal to 0.2 over a 2 to 3 minute period. They found when pelletizing reed canary grass, that continuous pellet production (CV < 0.2) could only be obtained using pre-densified material of reed canary grass with densities above c. 260 up to c. 350 kg/m ³ if also one of the following settings within the investigated variable range was fulfilled:

moisture content of the raw material >13.8%; die temperature <83 °C; raw pre- compacted material density > 324 kg/m ³ .

Manufacture of pellets from biological material is the subject of a large number of patents. As mentioned above, one problem that is commonly occurring is that the so called feed layer, which is the layer of material present on the extrusion die, is not maintained in the desired continuous condition, which frequently leads to interruptions in the production.

In US-4,529,407 (Johnston) an extruder is used to produce pellets with 1 - 3 % thermoplastic material where the injection temperature of at least 95 °C. US- 4,834,777 (Endebrock) is based on a piston technique and comprises a reciprocating punch press having a movable punch plate and a stationary die holder and formation of pellets done under controlled temperature conditions at preferable 121-177°C.

US-5,643,342 and 5,980,595 (Andrews) focus on cooling of already formatted pellets containing of least 1% thermoplastic material in a mix with coal and cellulosic material.

US-6, 165,238 (Parkinson et. al.) have invented an improved palletized water resistant compacted solid fuel produced preferable at 121-177°C from a mixture of 70-98% coal and 2-30% thermoplastic polymeric materials preheated to 204 and 100 °C, respectively.

In US patent application 2009/0064569 (Khater) temperature is controlled to about 100-110 °C at pelletizing. A desired temperature of the fixed flat die is maintained by circulating hot or cold water. Temperature control may be achieved by directly heating or cooling the die, or by controlling temperature of not only the die but additional adjacent equipment as well, or by relying entirely upon thermal conduction and thermally coupling of a heating and cooling source such as a temperature-controlled water line to the die.

In an article by Larsson et al, " High quality biofuel pellet production from pre- compacted low density raw materials", Bioresource Technology 99 (2008) 7176- 7182, the authors found that pre-compaction is a way to reach conditions for continuous pellet production provided a number of conditions are met.

Temperature control is required in the above cited patents, but none of them focuses explicitly on the feed layer formation nor do they give any suggestions how to tackle the problem of feed layer disturbances.

The article presents ways to alleviate the problem of discontinuous production, but die temperature is specifically mentioned as an uncontrolled variable. Summary of the Invention

Thus, in order to overcome the drawbacks of prior art systems and methods that suffer from frequently occurring intermittent production and often complete interruptions in the production due to the inability to maintain a continuous feed layer, the inventors have devised a method and a system for controlling the feed layer formation in a method of producing pellets from biomaterial of numerous various kinds. The method according to the invention is defined in claim 1, and a system according to the invention is defined in claim 15.

The method provides continuous feed-layers of materials of mainly organic origin for the production of biofuel pellets, feed pellets and the like in a pelletizer, said pelletizer comprising a die, means for pressing feed material through said die, and a motor, preferably electric, coupled in driving

engagement with the pressing means, and is characterised by continuously monitoring the status of the feed-layer in the pelletizer by measuring at least one process and/ or equipment variable; in response to said measurement, performing a comparison with preset target values and threshold values for said variable; depending on the result of said comparison, sending signals to at least one regulator adapted to adjust process variables so as to enhance the binding properties of the feed layer to the die surface. In a preferred embodiment the method makes use of a regulator, which is a device regulating the infusion of cooling media for cooling the feed layer, the die surface and/ or the die.

In a still more preferred embodiment the method is implemented in a pelletizer in which the motor is an electric motor, and the measurement performed is a measurement of the variation in motor current of the pelletizer to provided motor current data. The present invention makes it possible to use raw materials for the production of biofuel pellets, feed pellets and of the like. Possible raw materials include straw-type raw materials without any costly pre-densification or other pre- treatments to improve stable feed-layer formation. Thus, materials having natural bulk densities lower than 260 kg/m ³ , (e.g. reed canary grass has at a moisture content of 15-20% (wet basis) a density of about 140-160 kg/m ³ when milled over a sieve size of 4 mm), when milled to particle sizes sufficient for pellet production can be utilised. This simplifies machinery equipment and lower the costs of material handling prior pelletizing. The invention also makes pelletizers using free rolling devices more general concerning different raw materials, especially concerning straw-type materials for biofuel production. Furthermore, the invention lowers stresses on the pelletizer and lower quality variations caused by intermittent pellet production. The system according to the invention provides continuous feed-layers of materials of mainly organic origin for the production of biofuel pellets, feed pellets and the like in a pelletizer. The pelletizer comprises an extrusion die, die heating and/ or cooling means, means for pressing feed material through said die, and a motor coupled in driving engagement with the pressing means. The system is characterised by means for continuously monitoring the status of the feed-layer in the pelletizer by measuring at least one process and/ or equipment variable; means for performing a comparison with preset target values and threshold values for said variable, in response to said measurement; means for sending signals to at least one regulator adapted to adjust process variables so as to enhance the binding properties of the feed layer to the die surface, depending on the result of said comparison.

In a preferred embodiment the monitoring means comprises means for on-line collection of electrical data of the pelletizer motor.

The invention is especially useful for materials that have showed poor pelletizing characteristics like this in terms of uneven pellet production when die temperature is rising. Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter and the accompanying drawings which are given by way of illustration only, and thus not to be considered limiting on the present invention, and wherein

Fig. la is a block diagram for schematically representing a system to enhance binding properties of the feed layer to the die surface and to control feed layer formation according to the embodiment of the present invention. Fig. lb schematically illustrates the core components of a pelletizer;

Fig. 2 is a flowchart for schematically representing the general principles for enhancing binding properties of the feed layer to the die surface and control of feed layer formation according to the embodiment of the present invention.

Fig. 3 shows pelletizer motor current (A) (grey thin line) at three different die temperatures (°C) (black thick line);

Fig. 4 shows pelletizer motor current (A) (grey thin line) when die temperature (°C) (black thick line) is first increased from about 30 °C to 50°C and further to 60°C and then lowered from 60 to about 30°C.

Detailed Description of Preferred Embodiments For the purpose of the present application the term "process variable" shall be taken to mean any measurable quantity that is relevant for carrying out the process, and in particular that is related to the feed-layer formation.

Non-limiting examples are raw material feed rate, material properties (density, particle size etc.), thickness of the feed-layer.

The term "equipment variable" shall be taken to mean any measurable property of the components of the equipment, i.e. the pelletizer, that affects or is influenced by the process that is being carried out, and in particular that can be correlated with the formation of the feed-layer.

Non-limiting examples are motor current, die temperature, die surface

temperature, cooling media temperature, cooling media flow rate.

The inventors have identified that die surface temperature is the most critical factor for binding of the feed layer to the die surface and thus to maintain continuous pelletizing conditions. However, the reason for this is not fully clear. Without wishing to be bound by theory, it is believed that lower evaporation rates of moisture in the raw material at lower die surface temperatures already at feed layer formation can be of importance.

One observation is that the mechanical work during pelletizing generates heat. The heat is mainly absorbed by the produced pellets, by vaporizing moisture in the pelletizing material or by the machinery. At continuous and even process conditions, a particular steady state process temperature is reached when radiation and convection of heat from the goods of the pelletizer reach a stable level. When die temperature is increased from a primary level in the range of 20-50°C to 50-80°C and further to 80-120°C during pelletizing of some materials the production is going from even to uneven and further to extremely uneven production and sometime no production at all.

An embodiment of the method according to the present invention will now be described in detail with reference to Figs. 1 and 2.

Fig. la schematically illustrates a system according to the present invention. No details of the pelletizer as such are shown since the apparatus can be generic. Instead the actual apparatus is represented by the motor 1 driving the apparatus. A computer 2 is coupled to the apparatus via suitable sensors for registering relevant process and/or equipment related variables. The signals from said sensors are processed in the computer 2 and instructions based on the calculations are fed to device 3 regulating the infusion of cooling media that cool the feed layer, the die surface and/or the die, or into channels in the die or onto flanges connected to the die. There are also provided back-up systems 4, 5 that are activated if the normal operation according to the invention

nevertheless should fail.

Fig. lb is a schematic illustration showing the main functional components of a pelletizer in which the invention suitably is implemented.

It comprises a die 10 in the form of a circular drum, inside of which there are free rolling devices 12 exerting a pressure on a raw material feed-layer 14 that forms inside the drum on its walls.

Fig. 2 is a block diagram for schematically representing a process according to one embodiment of the present invention, for enhancing binding properties of the feed layer to the die surface and to control feed layer formation in a pelletizer for making pellets from biomaterials.

A pelletizer with which the present invention can be implemented in very general terms comprises feeding means for feeding raw material to the actual pelletizing device. The pelletizing device comprises a die having a plurality of apertures through which the feed material is pressed. The feed material forms a layer on the die, referred to as a "feed layer". The pressing is suitably achieved by utilizing free rolling devices, thus forming elongated material strings. There is also provide a cutting means for chopping up the material strings to shorter segments, referred to as pellets.

There are also provided means for heating and cooling the die, since

temperature is a critical variable in the pelletizing process.

The method according to a first embodiment of the present invention comprises continuously collecting actual electric current data from the pelletizer motor by means of a measurement device 1 in Fig.1. In particular the variation in electric motor current is measured. Data is sent to a computer system 2 that in real time stores data and registration time, and further continuously processes (i.e. compares) the data according to target values and threshold values. Target values are within a desired interval and threshold values are outside the smallest of such an interval but both values are used for regulation. Depending on the results of these continuous calculations, signals are sent to a device 3 regulating the infusion of cooling media that cool the feed layer, the die surface and/ or the die, or into channels in the die or onto flanges connected to the die.

During operation of the pelletizer calculations are also performed in the computer system 2 to optimise the target values and the threshold values in order to minimise cooling while at the same time lower feed layer disturbances. In one embodiment there is a backup system 4 (which is an optional feature) based on at least one temperature sensor, which is a much slower reacting signal of feed layer disturbances then the electric motor current. This sensor is adapted to send actual temperature data to the computer system 2. In case of a failure of the device 1, i.e. the device that reads or measures the actual current, these temperature data signals are stored, processed (in the same manner as for the motor current data) according to target values and threshold values for temperature and optimisations of those values are done in the computerised system 2. Resulting signals are sent to the regulator device 3. Another back-up system 5 is provided for the purpose of adding stabilisers to the feed layer if the cooling provided by the regulator device 3 is not enough to stabilise feed-layer formation. Such stabilisers can be e.g. natural binders like starch, lignin, proteins, sugars, water etc. or synthetic binders like plastics etc. Such binders are only used until the feed layer is recovered into a continuous feed layer formation in pelletizing.

In a situation where cooling of the feed layer, the die surface and/ or the die alone is not sufficient to cancel disturbances in the feed-layer formation, the method preferably comprises adding feed-layer stabilizers, such as powder or granules of lignin, starch, plastics, sugar, oils, fats, rape seed cake, water and mixtures of one or more components thereof.

The system for providing continuous feed-layers of materials of mainly organic origin for the production of biofuel pellets, feed pellets and the like according to the present invention comprises a pelletizer. The pelletizer comprises an extrusion die, die heating and/ or cooling means, means for pressing feed material through said die, and a motor, preferably an electric motor although any other type of motor is usable, coupled in driving engagement with the pressing means.

It further comprises means for continuously monitoring the status of the feed- layer in the pelletizer by measuring at least one process and/or equipment variable. There are also provided means for performing a comparison with preset target values and threshold values for said variable, in response to said measurement. Also the system comprises means for sending signals to at least one regulator adapted to adjust process variables so as to enhance the binding properties of the feed layer to the die surface, depending on the result of said comparison.

The monitoring means suitably comprises means for on-line collection of electrical data of the pelletizer motor, which preferably is a computer assisted device for continuously testing if said motor current is within set targeted values regarding the actual read-out of electrical data and regarding variability measures within a successive moving window of actual and preceding observed read-outs of electrical data.

The at least one regulator is adapted to control infusion of cooling media either to the feed layer or the die surface or to both, or into channels or onto flanges connected to the die.

The computer assisted device is suitably adapted to generate regulation signals to regulators controlling infusion of cooling media, either to the feed layer or to the die surface or to both, or in channels or on flanges connected to the die when the actual current and/ or the actual variability measurement pass set targeted values.

Suitably there is provided at least one sensor for recording feed layer temperature, die surface temperature and/or die temperature. The cooling media that is used for the die can be any of water aerosols, liquids and/or gases (e.g. air). The system for enhancing binding properties of the feed layer to the die surface and control of feed layer formation according to the embodiment of the present invention, as indicated in Fig 1 , may also contain algorithms to analyse and present trends e.g. of average number of disturbances over a time period, but also to minimise cooling while at the same time minimise number of feed layer disturbances per time period.

In Fig. 2 a flowchart which schematically represents a method according to an embodiment of the present innovation, is shown. The different steps Sl-Sl l in Fig. 2 are presented in the direction (indicated by arrows) of the signal and actions according to one embodiment. This setup is suitable for enhancing binding properties of the feed layer to the die surface and control of feed layer formation according to the embodiment of the present invention. First, the method comprises a step S 1 of more or less continuously measuring the electric current of the pelletizer motor. These data are checked in S3 and S5 according to set target values and threshold values in S2. An example of such target values is values inside an interval covering mean current (μ) at the ongoing production rate at continuous production plus/minus one standard deviation (±σ) of the current at said continuous production, i.e. μ ± o, preferably an asymmetrical interval from j xlidie to μ ± kxo where Iidie is idle current, j is a value between 1 and 1,5 and k is a value between 1 and 3.

Examples of disturbances in feed layer formation are shown in Figures 3 and 4 and it should be noticed that a breakage of the feed layer is manifested in a sudden drop of electric current of the pelletizer motor close to idle current. Outside each end of the interval there is at least one threshold value. When the actual electric current data passes such a threshold at least one signal is changed and sent S4 to a regulator device that dependent on the type of signal may start/ stop or increase/decrease the cooling of the feed layer, the die surface and/ or the die. The regulator device is influenced by signals from the check S5 of electrical data done regarding variability of the electrical current of the pelletizer motor. Here at S2, there are set target values inside a lower interval based on some variability measure of the motor current. This variability of the current is calculated e.g. within a moving window that may have mathematical weights on the observations in relation to passed time since the actual observation. Example of a variability measurement is coefficient of variation (CV) and a CV interval starting from zero and the upper end e.g. at

0.1. Example of window size is shortest time of one cycle of said disturbance,

1. e. from stating of disturbance to recovering of continuous feed layer formation. Outside the interval there is at least one set threshold value and a signal is changed when such a threshold value is passed by the actual electrical variability and sent S4 to the regulator device to e.g. increase cooling if the CV is higher than e.g. 0.2 or decrease cooling if the CV is lower than e.g. 0.2. In particular targets for the electrical variability measurement are set at several coefficients of variation in the range of 0-3, preferably between 0 and 0,75, or still more preferred between 0 and 0,2.

The preset target value for basic cooling preferably should equal the heat remaining after heating of the raw material, evaporating moisture etc, e.g. about 25-80% of the mechanical work done by the motor at continuous pellet production minus that of idle running at no production at all plus possible heat originating from steam addition. Furthermore, said preset value for basic cooling should be evaluated at every major disturbance of the feed layer formation, e.g. after an intermittent pellet production cycle, to calculate if said preset value should be changed.

The feed layer temperature, die surface temperature and / or die temperature is/are suitably in preferred embodiments controlled to between 0 to 80°C and preferably 0-70°C, more preferably 0-60°C, and most preferably 0-50°C.

Feed layer disturbances manifested as electric current variation and resulting in initiated cooling actions in S4 are recorded and the number of such disturbances is calculated per time unit within at least one moving window, e.g. covering at least one cycle of said disturbance, i.e. from stating of disturbance to recovering of continuous feed layer formation. Another moving window over a longer time period register only events of feed layer breakage, i.e. when about idle current is reached followed by a clear peak value (see Figs. 3 and 4). Also these moving windows over a time period contains the actual electrical data and preceding electrical data. Also these windows may have mathematical weights on the observations in relation to passed time since the actual observation. Also at S6 and S7 there are target values within a suitable interval and threshold values outside this interval. Example of an interval for S6 is 0 to 2 events of said breakages over the last hour, but of caorse a lower number of events is preferable.

If the checks within S6 and S7 show higher numbers of said disturbances per time unit and/or of said events over a time interval than set target values, new target values of basic cooling is set to decrease the number of feed layer disturbances. Thus, this is an iterative process.

If cooling is not enough (checked in S8) to recover a continuous feed layer formation a back-up system S10, indicated by the dashed lines, may be used for adding a stabilizer to the feed layer.

Another back-up, if the system to collect electric current data fails, is to register temperature data of the feed-layer, the die surface or the die, box S9. Then intervals, target values, threshold values and variability measures at S2-S7 are done, set and calculated for temperature data.

A still further alternative embodiment of carrying out the method is to use non- contact monitoring of the thickness of the feed-layer itself. Suitably, radar type monitoring could be used. In this way immediate changes in the status of the feed-layer can be registered and fed to the control system and the regulators can be activated accordingly. The invention will now be further illustrated by the following non-limiting examples.

Example experiments - manufacturing of pellets

Examples of die temperature influence on motor current variability are taken from pelletizing runs using reed canary grass as a model species for materials with poor feed layer sustainability and with natural powder bulk densities of about 140-160 kg/m ³ (Figures 1 and 2). Motor current curves (grey thin lines) at three different die temperatures (black thick lines) are shown in Figure 1. At a die temperature of about 30°C, motor current is continuous (CV < 0.2) at a low, steady level. At higher die temperatures, motor current becomes

intermittent (CV > 0.2), with periods of idle running (c. 17 A) in between short, high peak loads. The effect of die surface cooling during on-going operation is shown in Figure 2. The intermittent (CV > 0.2) production pattern developed at high die

temperatures is interrupted and replaced by a continuous (CV < 0.2) production pattern when cooling the die from 60 to 35°C.