WO2009001136A2

WO2009001136A2 - Drying particulate biomass

Info

Publication number: WO2009001136A2
Application number: PCT/GB2008/050497
Authority: WO
Inventors: Paul Michael Mason; Frederick Joseph Dumbleton
Original assignee: Biojoule Limited
Priority date: 2007-06-26
Filing date: 2008-06-26
Publication date: 2008-12-31
Also published as: WO2009001136A3; GB0712286D0

Abstract

An apparatus for drying particulate biomass material such as wood chips consists of a container (14) with a floor (16) defined by a multiplicity of longitudinally movable parallel slats (20) shaped to allow air to flow between adjacent slats but so as to inhibit passage of particulate material between them. There is an outlet (35) adjacent to one end of the slats (20), and there are ratchet elements (36) projecting from the upper surface of the slats (20). The slats are moved to-and-fro, and the ratchet elements (36) bring about net movement of the particulate material towards the outlet. The ratchet elements (36) nearer to the outlet (35) may project further from the slat than those further from the outlet.

Description

Drying Particulate Biomass

The invention relates to an apparatus for drying biomass, in particular wood chips and similar materials, and to a method of drying particulate biomass.

Biomass is one of the world's most widely available and green fuels. By-products from forestry and agricultural operations together with energy crops form a huge resource for sustainable energy. In their raw form these biomasses are often inconsistent fuels with high moisture (typically being between 40 and 70% by mass) and low calorific values, as much of the heat energy of combustion is taken up by the latent heat of this moisture. It would therefore be desirable to be able to dry biomass material such as wood chips to improve its calorific value as a fuel. Also if the biomass is to be made into pellets, it must be dried to about 10% moisture before it can be pelletised. However, the material is not easy to dry economically and uniformly.

According to the present invention there is provided an apparatus for drying particulate biomass material such as wood chips, the apparatus comprising a container into which the particulate biomass material may be loaded, the container having a floor defined by a multiplicity of longitudinally movable parallel slats, the slats being shaped to allow air to flow between adjacent slats but so as to inhibit the passage of particulate material between adjacent slats, and an outlet adjacent to one end of the slats, wherein longitudinal to-and-fro movement of the slats moves a bottom layer of particulate biomass material towards the outlet while the particulate biomass material above it moves downwards.

Preferably there are ratchet elements projecting from the upper surface of the slats, the ratchet elements being shaped such that the longitudinal to-and-fro movement of the slats moves the bottom layer of particulate biomass material towards the outlet.

The present invention also provides a method for drying particulate biomass material such as wood chips, using such an apparatus. Such an apparatus can be batch fed while operating substantially continuously; it can achieve substantially uniform drying of the particulate material, to achieve a desired moisture level; and the movement of the slats can be such that the bottom layer of particulate biomass material moves uniformly towards the outlet at a controlled rate with the particulate biomass material above it moving uniformly downwards also at a controlled rate.

The exact shape of the ratchet elements is not critical, but they must be asymmetrical in longitudinal section. They may for example be wedge-shaped. For example the surface remote from the outlet, of each ratchet element, may taper uniformly, or alternatively may taper non-uniformly for example having a part- sinusoidal shape in section. The surface facing the outlet, of each ratchet element, is preferably steeper than the opposite surface, and indeed this surface may be perpendicular to the slat.

The arrangement and sizes of the ratchet elements on the slats will depend on the type of biomass and the desired outfeed rate. The ratchet elements may be of different sizes, for example those nearer to the outlet may project further from the slat than those further from the outlet, and the spacing between successive ratchets along a slat may also vary. The slats are moved in a cycle, being moved together or independently of their neighbours, depending on the type of biomass and the desired outfeed rate. In a preferred embodiment each slat is moved to-and-fro through a distance of about 600 mm.

Preferably the container also comprises baffle plates to inhibit longitudinal movement and compaction of the particulate material that is well above the floor. Preferably, these baffle plates are adjustable, so they may be varied in position and height depending on the biomass being dried, for example they may have their bottom edge between 100 mm and 1000 mm above the floor, more preferably between 200 mm and 400 mm, and extend upwardly. The effect of the position of the baffle plates in combination with the movement cycle of the floor and the arrangement of the ratchets is that particulate material is moved uniformly out from the entire base area of the container through the outlet at a controlled rate and that the particulate material higher in the bed moves downwardly towards the floor.

Preferably hot air is supplied to a plenum below the floor, so that the hot air passes upwardly through the particulate material in the bed, and gradually dries it. The temperature of the hot air supplied to the bed depends upon the drying requirements of the particulate biomass but is typically between about 5O⁰C and HO⁰C, more typically between 7O⁰C and 100⁰C, as such hot air can rapidly evaporate moisture from wood chips; the hotter the air the more rapidly the biomass is dried. If the hot air is at above 100⁰C there is a risk that the biomass will be dried to much less than 10% moisture, which is not satisfactory if the chips are subsequently to be subjected to pelletising. Hence the operations of the dryer, in particular the temperature and rate of flow of hot air, and the rate of removal of biomass from the dryer, may be arranged to ensure that the biomass emerging from the outlet has a desired moisture level.

Preferably all the operations of the apparatus are performed automatically. Typically an operator would be required to fill the dryer with the particulate biomass. Subsequent operations would then be automatic, and substantially continuous. The resulting dried biomass can be discharged to a store or fed directly into another process for example a pelletising apparatus. Thus the initial provision of particulate biomass is in the form of a batch but the operations of the apparatus are substantially continuous. The operator is required only to supply the initial batch of material. Accordingly, the labour costs of running the dryer are minimised.

Controlling moisture content in biomass drying systems is difficult, especially in such a batch-feed, continuous flow process where the feed material can vary considerably and different particles will have had different residence times in the system. However, this apparatus may be fully automated and self regulating. Sensors may be arranged to measure the moisture content of the particulate biomass exiting the dryer and if it falls outside the desired moisture specification it may be diverted and either fed back into the dryer or fed to a biomass burner if the drying heat is so provided. Additionally, the system can automatically regulate the heat input and mass flow of air through the process to correct the aberration and maintain the required throughput of biomass. Thus the dryer may utilise some of the biomass it is drying as fuel, which is likely to be the cheapest fuel available, and can attain a very high level of efficiency so as to further minimise fuel costs. The invention will now be further and more particularly described by way of example only, and with reference to the accompanying drawings, in which: Figure 1 shows a longitudinal cross-sectional view of a drying apparatus; and

Figure 2 shows a cross-sectional view on the line 2-2 of figure 1, showing some of the slats.

Referring to figure 1, an apparatus 10 for drying wood chips 12 includes a large generally rectangular container 14 into which the wood chips 12 may be loaded to form a bed 15 (shown broken away) . The container 14 has a floor 16 above its base, below which is a plenum 18 to which hot air is supplied. Typically the hot air supplied to the drying container 14 is between 5O⁰C and 100⁰C depending on the drying requirements of the biomass .

The floor 16 is somewhat similar to a "moving floor" mechanism in that it consists of a multiplicity of longitudinal slats 20. The slats 20 each extend some 750 mm longer than the length of the floor 16, and can be moved individually to-and-fro longitudinally in different cycles through a distance of 600 mm by a drive mechanism 22 (shown diagrammatically) . Referring to figure 2, there are gaps 24 (about 10 mm wide) between adjacent slats 20, and along the top left-hand edge of each slat 20 (as shown) is a projecting flange 25 bent over to form a cover 26 for the gap 24, while along the top right-hand edge of each slat 20 (as shown) is a shorter diagonally- projecting flange 28 which locates under the edge of the cover 26 of the adjacent slat 20 so as to define a narrow downwardly-inclined slot 27 between the flange 28 and the cover 26. Hence the hot air from the plenum 18 can flow upwardly through the gaps 24 and the slots 27 into the bed 15 of wood chips, but the wood chips can't fall down through the gaps 24.

The slats 20 are of steel sheet, and are welded onto rectangular C-section steel bars 21 of the same length, which are located and supported by T-shaped upward projections 31 on cross-members 30 at each end of the plenum 18, and there are additional cross-members 30 spaced along the plenum 18 to provide support to the bars 21 and the slats 20. Each slat 20 (with the bar 21 to which it is fixed) can be moved independently of the adjacent slats 20, sliding on the T-shaped projections 31.

Referring again to figure 1, the interior of the container 14 is subdivided by vertical baffle plates 32 and smaller baffle plates 34, the lower edges of which are typically about 400 mm above the floor 16 but can be varied to change the outfeed rate from the apparatus 10. The baffle plates 32 and 34 inhibit movement of the wood chips in the bed 15 in directions other than downwardly and prevent compaction of the bed 15. At the left-hand end (as shown) of the container 14 is an outlet aperture 35, of height 100 mm, just above the floor 16, and the floor projects through it. Along the length of each slat 20 are several wedge elements 36 fixed to the top surface of the slat 20, each wedge element 36 having a vertical surface facing the outlet aperture 35 and an inclined surface facing away from it (the wedge elements 36 are not shown in figure 2) . The wedge elements 36 are progressively smaller in height the further from the outlet aperture 35: for example the heights of the wedge elements 36 might vary between 10 mm and 60 mm over the length of a slat 20. Outside the outlet aperture 35 the slats 20 end above a collection trough 38.

Hence in use of the apparatus 10 the wood chips 12 are loaded into the container 14, and hot air is passed upwardly through them. Almost all the drying effect takes place in the bottom 500 mm to 1000 mm of the bed 15, so the driest wood chips are those on or next to the floor 16. When those wood chips are sufficiently dry then the moving floor mechanism is activated, that is to say the slats 20 are moved to-and-fro as indicated by the arrow A, through 600 mm. Because of the wedge elements 36 this to-and-fro movement of the slats 20 moves the lowest layer of wood chips towards the aperture 35, and the dried wood chips fall into the collection trough 38. The chips are removed from substantially the entire area of the floor 16 substantially uniformly, and consequently the wood chips travel downwardly through the bed 15 at a substantially even rate, so that even drying is achieved.

The movement cycle may be different for different types of wood chips, but for example it may involve movement of some individual slats 20 or groups of slats 20 forwards and backwards (i.e. towards and away from the outlet) and then movement of other slats 20 or groups of slats 20 forwards and backwards; or it may involve movement of all of the slats 20 simultaneously forwards and backwards; or it may involve sequential movement of individual slats 20 forwards until all the slats have moved forwards, followed by simultaneous movement of all the slats backwards; etc..

The wood chips would then be removed from the collection trough 38, for example using an auger (not shown), and may be subjected to subsequent treatments. They may for example be subjected to a second drying treatment using cooler air; and they may be formed into pellets for use as fuel. Alternatively the dried wood chips may be used as a fuel without further treatment. Apart from loading a batch of wood chips 12 into the drying container 14, the operation of the apparatus 10 can be substantially continuous, and automated. For example sensors may be provided to monitor the moisture in the chips removed from the trough 38, and to monitor the temperature and its rate of change at one or more places within the bed 15, and to monitor the temperature of the in-flowing hot air below the floor 16. In accordance with these measurements the partially dried chips may be removed from the drying container 14 with a substantially constant moisture level, for example of around 15%.

It will be appreciated that the apparatus described in relation to the drawing can be modified in various ways while remaining within the scope of the present invention. In particular the dimensions quoted above are by way of example only and for example the slats 20 might move to-and-fro through a different distance, for example as little as 150 mm or 300 mm; the wedge elements 36 may have a different size from that described, for example the largest wedge elements 36 might be as large as 100 mm high, or might be smaller for example only 15 mm; and the outlet aperture 35 might be of a different height, for example up to about 250 mm or 200 mm, or indeed of adjustable height for example between 70 mm and 100 mm. The preferential movement of the particulate biomass towards the outlet might also be effected by varying the spacing between wedge elements 36 along the length of a slat 20, rather than the heights of the wedge elements 36; and indeed there might be uniformly-spaced wedge elements of varying heights along part of the length of the slat 20, and wedge elements of constant height at a varied spacing along the remainder of the slat 20. The wedge elements 36 provide a ratchet effect, but it will be appreciated that they might have a non-wedge shape, as long as it is not symmetrical. And they may be attached to the slats 20 by any convenient method, or indeed may be integral with them. Also, the flange 25, cover 26 and flange 28 may have a different geometry, while allowing hot air from the plenum 18 to flow upwardly into the bed 15 of wood chips and preventing the wood chips from falling down through the gaps 24. For example they may be shaped such that the slot 27 is inclined more steeply than in Figure 2, or the slot may even be vertical.

In a further modification the drying container 14 might be provided with two outlet apertures 35, one at each end, each with an associated collection trough 38; in this case the wedge elements 36 would be oriented and shaped so as to bring about movement of the wood chips towards whichever outlet aperture 35 is the closer.

In a further modification the plenum 18 may be subdivided (for example by a valve, a baffle or a partition) so that air may be fed to the different sections of the bed 15 from different sources at different temperatures. For example, if drying wood chips for pellet production using a pellet mill, warm air from the pellet mill might be fed to one end of the plenum 18 and hot air from a burner to the other end.

Claims

Cl aims

1. An apparatus for drying particulate biomass material such as wood chips, the apparatus comprising a container into which the particulate biomass material may be loaded, the container having a floor defined by a multiplicity of longitudinally movable parallel slats, the slats being shaped to allow air to flow between adjacent slats but so as to inhibit the passage of particulate material between adjacent slats, and an outlet adjacent to one end of the slats, wherein longitudinal to-and-fro movement of the slats moves a bottom layer of particulate biomass material towards the outlet while the particulate biomass material above it moves downwards.

2. An apparatus as claimed in claim 1 wherein there are ratchet elements projecting from the upper surface of the slats, the ratchet elements being shaped such that the longitudinal to-and-fro movement of the slats moves the bottom layer of particulate biomass material towards the outlet .

3. An apparatus as claimed in claim 1 or claim 2 wherein the height to which the ratchet elements project and/or the spacing between the ratchet elements vary along the slats.

4. An apparatus as claimed in any one of the preceding claims wherein there are a number of baffles across the width of the container to inhibit longitudinal movement and compaction of the particulate material.

5. An apparatus as claimed in any of the previous claims when dependent on claim 2 wherein the ratchet elements are wedge-shaped.

6. An apparatus as claimed in any of the previous claims when dependent on claim 2 wherein the surface facing the outlet, of each ratchet element, is perpendicular to the slat.

7. An apparatus as claimed in any one of the preceding claims when dependent on claim 2 wherein the ratchet elements nearer to the outlet project further from the slat than the ratchet elements that are further from the outlet.

8. An apparatus as claimed in any one of the preceding claims when dependent on claim 2 wherein there are two outlets, one outlet adjacent to each end of the slats, and the ratchet elements are shaped to cause preferential movement of the particulate material towards whichever outlet is closer.

9. An apparatus as claimed in any of the previous claims wherein sensors are arranged to measure the moisture content of the particulate biomass exiting the drying apparatus, and the apparatus comprises means to divert the particulate biomass if it falls outside a desired moisture specification.

10. An apparatus as claimed in any of the previous claims where sensors are arranged to determine the moisture content of the ambient air, the moisture content of air within the drying apparatus, and the moisture content of particulate biomass exiting the drying apparatus, and wherein automatic control means are arranged to regulate, in response to signals from the sensors, the heat input and mass flow of air through the apparatus to produce a given throughput of biomass dried to a predetermined moisture specification.

11. A method for drying particulate biomass material such as wood chips, by using a drying apparatus comprising a container, the container having a floor defined by a multiplicity of longitudinally movable parallel slats, the slats being shaped to allow air to flow between adjacent slats but so as to inhibit the passage of particulate material between adjacent slats, and an outlet adjacent to one end of the slats, and the method comprises loading the particulate biomass material into the container to form a bed, introducing drying air into the bed from below the slats, and moving the slats to-and-fro longitudinally so as to move a bottom layer of particulate biomass material towards the outlet while the particulate biomass material above it moves downwards.

12. A method as claimed in claim 11 wherein the particulate biomass material is introduced as a batch, the batch being sufficiently large to allow the method to operate for a prolonged period of several hours without being refilled.

13. A method as claimed in claim 11 or claim 12 wherein all the operations of the apparatus apart from filling it are performed automatically.