GB2534916A

GB2534916A - Improved grate assembly for a gasifier

Info

Publication number: GB2534916A
Application number: GB1501944.1A
Authority: GB
Inventors: Atkins John
Original assignee: Individual
Current assignee: Individual
Priority date: 2015-02-05
Filing date: 2015-02-05
Publication date: 2016-08-10
Also published as: GB201501944D0

Abstract

A grate assembly for use in a gasifier is described. The grate assembly comprises a first and second grate having respective first and second grate elements. The grate assembly also comprises a driving mechanism arranged to enable relative motion between the two grates. Moreover, the driving mechanism is arranged to drive the first and second grate elements relative to one another through a primary movement. The primary movement causes the first and second grates to move alongside one another so that a spacing therebetween is substantially maintained. Moreover, the primary movement causes material trapped within the spacing to be ground between the grate elements. The grate elements 20, 30 may each be made of concentric rings 22 joined by spokes 26, with two elements rotated relative to each other.

Description

Improved grate assembly for a gasifier

Field of invention

The present invention relates to gasifiers, and more specifically relates to an improved grate assembly for use in downdraft gasifiers.

Background of invention

Biomass is a non-fossilized and biodegradable organic material originating from plants, animals and micro-organisms; it also includes residue and waste from agriculture, forestry and related industries as well as biodegradable organic fractions of industrial and municipal waste. Over recent years, biomass has become a very important source of energy in many countries around the world.

In order to generate energy, biomass materials undergo a number of thermo-chemical processes: heating the material to around 350-500°C in the absence of oxygen; carrying out a process called pyrolysis which produces pyrogas; combusting and reacting the resultant material at high temperatures (>700°C); and carrying out gasification in the presence of a controlled amount of oxygen or steam to produce synthetic gas (syngas). This syngas is itself a fuel and undergoes combustion to generate power (for example, in internal combustion engines).

The gasification process is carried out within a gasifier, which is a large well-sealed container filled with biomass materials. Inlets are provided at certain points to allow air to enter the gasifier and aid combustion, and gas outlets are also present to allow the syngas produced to exit the gasifier for collection. The biomass materials are supported on a grate assembly, which is typically located near the base of the gasifier.

The grate assembly for a gasifier typically comprises one or more grates, which each comprise a plurality of grate elements. These are arranged to maintain a space or gap between adjacent grate elements that is large enough to allow small pieces of ash and char to pass through, along with the syngas, but small enough to retain the majority of the biomass. Typically, each grate is generally planar, having a 'main' face which can support the majority of the biomass material and another 'main' face on the reverse side. The grate assembly provides a means of separating the biomass undergoing gasification from waste products such as ash or char, which are collected in an ash pit at the base of the gasifier.

Currently, there are two commonly-used gasifier designs, the updraft gasifier and the downdraft gasifier, defined according to the direction of airflow within the gasifier. An example of a downdraft gasifier is shown in Figure 1, and illustrates the different processes occurring in the gasifier.

In a downdraft gasifier, the following processes occur in order, and with reference to Figure 1, from top to bottom: drying, pyrolysis, combustion and reduction. During the drying stage, the high moisture content of the biomass material is reduced; the pyrolysis stage then follows, where the temperature is increased while the oxygen level is maintained at a minimum, causing large quantities of the biomass to be decomposed into gases (such as hydrogen and methane) and solids (char). During the combustion stage, air is introduced into the gasifier and reacts with the gases and char (produced in the previous stage) to generate heat for the subsequent reduction process. In the final reduction stage, the char reacts with carbon dioxide and steam to produce syngas (which comprises a mixture of gases, including but not limited to any one or more of carbon dioxide, hydrogen, carbon monoxide and steam). This syngas passes through the gaps in the grate assembly and exits the gasifier, where it is collected.

As carbon is absorbed from the char, the char is reduced in size until it is sufficiently small to pass through the gaps in the grate assembly and into the ash pit below. However, pieces of char often get stuck in the gaps and smaller ash particles collect around them, effectively plugging the gaps and blocking the grate. This blockage can disrupt the flow of gas through the grate, and ultimately decreases the efficiency of the gasifier. As the gasification reactions take place under controlled conditions (low oxygen, high temperatures), the gasifier must be shut down to allow grate blockages to be cleared, which is undesirable. Once cleared, the gasifier is powered up and the controlled conditions re-established before proceedings with gasification. This process is time and energy consuming.

The problems relating to grate blockage are well-known in the art, and there have been several attempts to solve them. An early example can be found in US Patent Application No. US 4764185 (Mayer, 1988) which discloses a concentric conical grate assembly, comprising conical baffles installed below the grate and arranged to direct the flow of air and waste material through the gaps between adjacent grate elements, and into an ash removal container below. However, this arrangement is still prone to blockage by pieces of ash and waste too large to pass fully through the gaps, as the pressure exerted by the airflow alone is often insufficient to remove the blockage.

Other known solutions to this problem comprise the use of additional moving parts that sweep the ash and waste off the surface of the grate elements, and through the gaps between adjacent grate elements, into the ash pit below.

One such known solution disclosed in US Patent Application No. US 6981455 B2 (Lefcort, 2006) describes a grate assembly comprising a series of grate elements, with plates mounted on the surface of each grate element. Each plate is connected to a series of driving means, and in operation moves across the surface of the grate element and sweeps off any ash that has collected there.

A further known solution disclosed in Patent Application No. US 201 2/001 751 0 Al (Leveson, 2012) describes an actuated sliding grate arrangement, comprising lower flat plate sections and upper cone-shaped canopy sections. Sliding paddles are mounted on each of the lower plate sections, and move laterally across the plates' width to sweep the ash and waste through the gaps between the plate sections and into an ash removal container below. The size and quantity of waste that can pass through the gaps is controlled by adjusting the size of the gaps and the stroke length of the paddles.

Both above cited solutions involve numerous moving parts which increases the likelihood of equipment malfunctions occurring. Due to their complexity, these grate assemblies require frequent maintenance. Furthermore, it is still possible for waste particles to be swept off the surfaces of the grate elements and get stuck in (and block) the gaps between the grate elements. Hence, this solution is of limited use in preventing and removing blockages.

Therefore, there is a need to provide a mechanism for preventing waste build-up in the grate, which mechanism does not suffer from the maintenance problems and limitations of

the prior art.

It is an object of the present invention to address at least some of the above-highlighted shortcomings of the prior art.

Summary of invention

According to a first aspect of the invention, there is provided a grate assembly for a gasifier comprising: a first and a second grate having a respective first and second grate element; and a driving mechanism. Ideally, the driving mechanism is arranged to drive the first and second grate elements relative to one another through a primary movement. Ideally, the primary movement causes the first and second grates to move alongside one another so that a spacing therebetween is substantially maintained. Moreover, the primary movement causes material trapped within the spacing to be ground between the grate elements.

Advantageously, the relative movement between the first and second grate elements prevents material becoming permanently trapped in the space between them. Moreover, any such material that does become trapped between the first and second grate elements is subject to a grinding action that is particularly effective in reducing the size of the material sufficiently to allow it to fall through the spacing between the first and second elements.

Thus, the grate assembly enables efficient removal of materials such as ash and waste particles that fall into the spacing between the first and second grate elements, preventing these materials from collecting in the spacing and blocking the grate assembly. In turn, this ensures that the flow rate of syngas across the grate assembly is maintained.

In use, the relative motion between the two grates and/or grate elements also redistributes biomass material more evenly across the grate assembly thereby improving gasification efficiency and/or performance. This redistribution causes waste particles, which tend to be smaller than the biomass material, to fall through the spacing between the grate elements, thus removing the unreactive waste particles from the relatively more reactive biomass, and hence maximising the effective reactive surface area of the biomass material.

Ideally, the driving mechanism is arranged to drive the first and second grate elements relative to one another through a primary movement and a secondary movement. Ideally, the secondary movement alters the spacing between the first and second grate elements. Preferably, the extent of the primary movement is greater than the extent of the secondary movement. This further enhances the grinding effect. The grate assembly may comprise a camming mechanism which translates at least a part of the primary movement into the secondary movement.

Preferably, the driving mechanism is arranged to drive the grate elements of the first and second grates relative to one another about a common axis. This configuration reduces the number of separate moving parts that are required (compared to the prior art), and results in a simple, compact grate assembly that is easy to maintain. In certain embodiments, the relative motion may be achieved by displacing one of the grates relative to the other grate. For example, the first grate may be fixed in place, whilst the second grate is displaceable relative to the first grate, or vice versa. This implementation as previously implied requires, advantageously, fewer moving parts. A further advantage of this arrangement is that the grate assembly is subject to less fatigue in use, which reduces maintenance requirements.

The grate that is displaced is subject to the most fatigue in such embodiments.

Preferably, the grate elements are shaped substantially in the form of rings. Preferably, the rings are centred on the common axis. Preferably, the grate elements are arranged concentrically to one another about the common axis, with each grate element having a different circumference.

As many commonly-used gasifiers have internal cross-sections that are substantially circular in shape, this arrangement and shape of grate elements has the advantage of ensuring that the grates can span, at the very least, the majority of the internal cross-sectional area of the gasifier, as the shape of the grate elements is complementary to the internal cross-sectional shape of the gasifier. This results in a more efficient use of the internal space within the gasifier, by ensuring that the maximum possible amount of biomass material is supportable within the gasifier. This helps to improve the output performance (i.e. the amount of syngas that the gasifier is able to output per unit time) of the gasifier, by maximising the reactive surface area of the biomass.

The extent of the primary movement may be over thirty times greater than the extent of the secondary movement. Preferably, the extent of the primary movement is over fifty times greater than the extent of the secondary movement. More preferably, the extent of the primary movement is over one-hundred times greater than the extent of the secondary movement. Most preferably, the extent of the primary movement is over two-hundred times greater than the extent of the secondary movement.

By way of example, where the grate elements are in the form of concentric rings centred on a common axis, and having an annular spacing therebetween, the primary movement may be a relative counter-rotating of those rings about the common axis, and the secondary movement may be a relative displacement of those rings in opposite directions along the common axis. In such a case, the angular displacement between the first and second grate elements is greater than the axial displacement. In other words, over the extent of the primary and secondary movements, the circular distance travelled between two points on respective grate elements over a certain time period will be greater than the axial distance travelled by the grate elements in the same time period. For example, lithe diameter of the first grate elements is 256mm, then the circular distance travelled by a first point on the first grate element (relative to a second point on an adjacent second grate element) will be around 804mm. Compared with an axial displacement between the first and second grate elements of 3mm, the circular movement is over 268 times greater than the axial movement. Advantageously, this lame ratio between the extent of the primary movement and the secondary movement imparts a relatively large grinding force to material trapped between the first and second grate elements, making the grinding effect far more effective than prior known grate clearing mechanisms.

Preferably, the first grate comprises a plurality of first grate elements. Preferably, the second grate comprises a plurality of second grate elements. Preferably, the first and second grates are arranged relative to one another so that the plurality of first grate elements are interleaved between the plurality of second grate elements. Thus, adjacent grate elements ideally belong to different grates. Due to the relative motion between first grate elements and second grate elements, as driven by the drive mechanism, this arrangement further promotes the redistribution of biomass material supported on the grate assembly.

Preferably, grate elements of a respective grate are affixed to one another.

Preferably, grate elements of a respective grate are cross-linked to one another via one or more spokes.

Preferably, the spokes extend radially outward from a centre of a respective grate.

The spokes provide a means of connection between adjacent elements of a grate, and enable a distance of separation between adjacent grate elements to be maintained. The spokes also provide support for the grate elements, and help to reduce any deformation that may occur in use within the gasifier as a result of the high operational temperatures.

Preferably, the cross-sectional profile of the one or more spokes is wedge-shaped.

Preferably, the driving mechanism comprises a drive shaft to which the one or more spokes are connected, the drive shaft being coincident with a common axis about which the driving mechanism is arranged to drive the grate elements of the first and second grates relative to one another.

Preferably, the one or more spokes of one of the first and second grates and one or more grate elements of the other of the first and second grates together define confronting camming surfaces which interact during at least a part of the primary movement to translate at least a part of the primary movement into a secondary movement that alters the spacing between the first and second grate elements.

Preferably, the cross-sectional profile of one or more grate elements is tapered, narrowing in a direction away from a material support area of the one or more grate elements.

Preferably, the space defined between adjacent grate elements is tapered in cross-section, narrowing in a direction towards material support areas defined by said adjacent grate elements.

Preferably, the first grate has at least two first grate elements, the at least two first grate elements being arranged in spaced relation to define a receiving space therebetween; and the second grate element is arranged, in use, to fit at least partly within the receiving space when the first and second grates are arranged in complementary relation; and the driving mechanism is arranged to enable relative motion between the first and second grates, such that, in use, the second grate element is displaceable within the receiving space.

According to a second aspect of the present invention, there is provided a grate assembly for a gasifier comprising a first grate having at least two first grate elements, the at least two first grate elements being arranged in spaced relation to define a receiving space therebetween (i.e. a space between the at least two first grate elements). The grate assembly additionally comprises a second grate having at least one second grate element, the second grate element being arranged in use to fit at least partly within the receiving space when the first and second grates are arranged in complementary relation. The grate assembly additionally comprises a driving mechanism arranged to enable relative motion between the first and second grates, such that in use the second grate element is displaceable within the receiving space.

The first and second grates may share a common axis. In certain embodiments, the at least two first grate elements and the second grate element may be shaped in the form of rings arranged substantially perpendicular to the common axis. Additionally, the at least two first grate elements and the second grate element may be arranged concentrically about the common axis, such that each one of the grate elements has a different diameter.

In certain embodiments, the grate elements may be affixed to one or more spokes extending radially outward from the centre of each grate.

In some embodiments, the cross-section of the one or more spokes, taken in a plane comprising or parallel to the common axis is wedge shaped in profile. For example, the cross-sectional profile may be triangular, such as a right-angled triangle. An advantage of a wedge-shaped profile is that this facilitates redistribution of the biomass material across the main face of the grate in use, and helps to separate waste material from reactive biomass material.

The driving mechanism may comprise a drive shaft arranged to be coincident with the common axis of the first and second grates, and the one or more spokes of at least one of the grates is connected to the drive shaft. This configuration operatively couples the spokes to the drive shaft, and thus enables the grate elements to which the spokes are affixed, to be moved by the drive shaft.

Where the grate elements are shaped in the form of rings, the driving mechanism may be arranged to rotate at least one of the grate elements about the common axis. In certain embodiments, the driving mechanism, via the drive shaft, may be arranged to rotate the first grate relative to the second grate.

The degree of rotation of the grate elements and hence the degree of rotation of the first grate relative to the second grate, may vary from small reciprocating oscillations of the grate elements and hence of the first grate about the common axis, to complete rotations around the common axis. In both scenarios, the relative rotational motion of the first grate relative to the second grate in use has the advantage of effecting a grinding action on waste particles trapped within the receiving spaces. This grinding action scrapes the particles against the grate elements and reduces their size, allowing them to pass through the receiving spaces more easily, thereby preventing grate blockages.

In use, deformities may arise in the grate elements due to the high operational temperatures within the gasifier, which may change the shape and/or size of the grate elements, and hence of the receiving space. A further advantage of driving complete rotations of the first grate relative to the second grate is that this ensures that the grinding action is applied uniformly around the entirety of the grate, and prevents isolated pockets of waste particles collecting in the receiving space.

Each one of the grate elements may comprise a tapered cross-sectional profile, when viewed in a plane perpendicular to their diameter. Accordingly, the cross-sectional profile of the receiving spaces, when viewed in the same plane perpendicular to the diameter of the adjacent grate elements, is also tapered in profile.

In certain embodiments where the first and second grates share a common axis, the common axis is substantially parallel to a vertical axis, and the cross-section of the receiving space, taken in a plane comprising the common axis, is tapered in profile in an axially-upwards direction.

This direction of taper causes the size of the receiving space to increase in a direction that is parallel to the flow of waste material through the grate, reducing the probability of particles entering the receiving spaces and becoming stuck. This configuration hence aids the exit of particles from the grate assembly once they have entered the receiving spaces, and prevents grate blockage.

In certain embodiments of the present invention, one or more of the first grate elements comprises at least one protrusion that is affixed to the surface of the first grate, and oriented to face the second grate, when the first and second grate are arranged in complementary relation. The one or more protrusions may be arranged at regular intervals on the surface of the one or more first grate elements. Or, the protrusions may be affixed to at least the radially-outermost element of the first grate.

In addition, one or more of the spokes affixed to the second grate are dimensioned and arranged in use to come into contact with the protrusions affixed to the first grate, when the first and second grates are arranged in complementary relation, the contact with the protrusions displacing the first grate relative to the second grate in a direction substantially parallel to the common axis.

For example, one or more of the spokes of the second grate may be elongated to protrude beyond the radially-outermost element of the second grate. In use, when the second grate is rotated relative to the first grate, the elongated spokes of the second grate come into contact with the protrusions of the first grate, resulting in a displacement of the second grate relative to the first grate, in a direction substantially parallel to the common axis, by an amount substantially equal to the height of the protrusions. This displacement results in a corresponding displacement of the at least one second grate element within its associated receiving space in a direction substantially parallel to the common axis, and by an amount substantially equal to the height of the protrusions.

The duration of this displacement lasts as long as the spokes of the second grate remain in contact with the protrusions of the first grate. Subsequently, the at least one second grate element is returned to its original position within its respective receiving space.

The synergy between the protrusions on one grate and the elongated spokes on the opposing grate, in combination with the relative rotational motion between the grates, results in the grate elements of the two grates being at least partly withdrawn and subsequently reinserted into their respective receiving spaces with a frequency dependent on the number of protrusions. In use, these temporary displacements improve the grinding action and further help to dislodge any waste particles trapped in the receiving spaces. This further improves the efficacy with which waste particles and material such as char, are expelled from the grate assembly, to prevent grate blockage.

According to a third aspect of the present invention there is provided a gasifier, such as a downdraft gasifier. The gasifier may comprise at least one of a body, an air intake, a grate assembly, a fuel feed inlet, a gas exit port and an ash pit. The air intake may be arranged to introduce air into the body above the grate assembly. The fuel feed inlet may be arranged to introduce fuel into the body above the grate assembly. The gas exit port may be arranged to expel gas from the body below the grate assembly. The ash pit may be situated within the body below the grate assembly, and arranged for collecting waste products.

The features and advantages of the different aspects of the present invention may be combined or substituted where context allows.

For example, the gasifier of the third aspect of the present invention may comprise a grate assembly according to the first or second aspect of the present invention.

Additionally, features and advantages of the different aspects of the present invention may constitute further aspects of the present invention.

For example, a further aspect of the present invention may comprise a grate assembly comprising a plurality of grate elements, each grate element defining a material support area, wherein the cross sectional profile of at least one of the grate elements is tapered, narrowing in a direction away from the material support area. As intimated above, such a taper increases the size of the spacing between adjacent grate elements in a direction parallel to the flow of waste material through the grate assembly. This reduces the probability of waste particles becoming stuck between adjacent grate elements, and so ameliorates blockage.

Brief description of the drawings

Preferred embodiments of the present invention are described below with reference to the appended figures, in which: Figure 1 is a schematic diagram of a downdraft gasifier, which has already been referenced

in the background section of the description;

Figure 2 is an exploded, perspective view of a grate assembly in accordance with an embodiment of the present invention; Figure 3 is a plan view of the grate assembly illustrated in Figure 2; Figure 4 is a plan view of the lower grate comprised in the assembly of Figures 2 and 3; Figure 5 is a plan view of the upper grate comprised in the assembly of Figures 2 and 3; Figure 6a is a cross-sectional profile view of the grate assembly of Figures 2 and 3, and taken in a plane perpendicular to the main faces of the grate assembly (e.g. along the line labelled B-B in Figure 3); and Figure 6b is a corresponding perspective cross-sectional view of the grate assembly shown in Figure 6a.

The figures laid out herein illustrate embodiments of the present invention but should not be construed as limiting to the scope of the invention. Where appropriate, like reference numerals will be used in different figures to relate to the same structural features of the illustrated embodiments.

Detailed description of the invention

Figures 2 and 3 show an exploded, perspective view and a plan view respectively, of the grate assembly 5 in accordance with an embodiment of the present invention. The grate assembly 5 comprises two grates, a first grate 20 and a second grate 30, which share a common axis 10 that is arranged perpendicular to the main faces of the grates and passes through the centres of both grates. A drive shaft 40 is arranged to be coincident with the common axis 10, and passes through the centres of both grates.

Each grate comprises a plurality of broadly circular grate elements 22 and 32, in the form of rings, arranged concentrically about the common axis 10, and each ring having a different radius. The grate elements 22, 32 are arranged such that a receiving space is defined between the adjacent elements, which receiving space 24 of the first grate 20 is sized to receive a grate element 32 of the second grate 30 within it, and vice versa. The first grate 20 and second grate 30 are arranged in use to fit together in complementary relation, where the grate elements 22 of the first grate 20 are arranged to be received in the receiving spaces 34 of the second grate 30, and the grate elements 32 of the second grate 30 are arranged to be received by the receiving spaces 24 of the first grate 20. In other words, the first and second grates 20, 30 are arranged, in use, relative to one another so that the plurality of first grate elements 22 are interleaved between the plurality of second grate elements 32 (and vice-versa).

The grate elements 22 of the first grate 20 are affixed to spokes 26 that extend radially outwards from the centre of the first grate 20, and provide the means for connecting the grate elements 22 together. Similarly, the grate elements 32 of the second grate 30 are affixed to spokes 36, 38 that extend radially outwards from the centre of the second grate 30, and provide the means for connecting the grate elements 32 together. Thus, the grate elements of a respective grate are cross-linked to one another via the spokes of that grate.

The spokes 26 of the first grate 20, are affixed to the outwardly-facing main face 2 of the first grate, and the one or more spokes 36, 38 of the second grate 30 are affixed to the outwardly-facing main face 3 of the second grate 30. In the specific example illustrated in Figure 2, the outwardly facing main faces 2, 3 correspond to the downward-facing surface of the first grate 20 and the upward-facing surface of the second grate 30 respectively.

The spokes 36, 38 of the second grate 30 are connected to the central drive shaft 40 by a sleeve. The spokes 26 of the first grate 20 are configured to extend radially-inwards such that their innermost ends surround the drive shaft 40, but are not connected to it.

In use, the first grate 20 is affixed to the internal sides of the gasifier body via its spokes 26 (for example, by welding) and remains substantially fixed in place. The first grate 20 may altematively be affixed to the lower inner surface of the gasifier by means of vertical rods. The second grate 30 is arranged to rotate relative to the first grate 20, at a rate and frequency determined by the drive shaft 40, about the common axis 10 and in a plane perpendicular to it. The relative rotational motion between the first 20 and second 30 grates redistributes the biomass across the main face 3 of the second grate 30, on which the biomass is supported. This relative rotational motion constitutes a primary movement of the first and second grates relative to one another.

The relative rotational motion between the first grate 20 and the second grate 30 results in the relative rotation of the plurality of grate elements 32 of the second grate 30, within the receiving spaces 24 of the first grate 20, and relative to the plurality of grate elements 22 of the first grate 20.

This relative rotational motion results in a grinding action being applied to any waste particles trapped in the spacing between adjacent grate elements. Specifically, waste particles in the receiving spaces 24, 34 of the grates, are ground so that their physical size is reduced. This advantageously prevents the waste particles from being retained within the receiving spaces 24, 34, thus preventing blockage of the grate assembly 5.

It is to be appreciated that the number of grates, grate elements and spokes disclosed in the illustrated embodiments is for illustrative purposes only and is not intended to be limiting. Alternative embodiments are envisaged comprising more than two grates and/or a plurality 15 of grate elements and/or spokes. The length of each spoke may also be varied.

In some embodiments, the one or more spokes 36, 28 that are affixed to the second grate 30 may be wedge-shaped in cross-sectional profile, the cross-sectional profile taken in a plane comprising or parallel to the common axis 10. For example, each of the spokes 36, 38 may have a cross-sectional profile that is shaped as a right-angled triangle, or any other triangular shape.

In general, the use of spokes 36, 38 with wedge-shaped cross-sectional profiles, where the wedge tapers in the direction of rotation of the second grate 30, facilitates the redistribution of biomass material across the main face 3 of the second grate 30. In use, as the grate rotates and the biomass material comes into contact with the spokes 36, 38, the direction of taper urges at least some of the biomass material over the spokes 36, 38, and prevents localised accumulation of biomass material.

Where the spokes 36, 38 have a right-angled cross-sectional profile, the spokes 36, 38 are arranged such that the adjacent side is substantially parallel to the main face 3 of the second grate 30, and in contact with the grate elements 32. The hypotenuse side is arranged to come into contact with at least some of the biomass material during rotation of the second grate 30. As the grate rotates, the hypotenuse side pushes the biomass material, such that the biomass material rotates with the grate. During this rotation, as the biomass material comes into contact with the hypotenuse side at least some of the biomass material is urged across it and over the spokes 36, 38. This urging action helps to redistribute the biomass across the main face 3 of the second grate 30, and also prevents localised concentrated accumulations of biomass material forming on the grate when in use. Redistribution of the biomass material also improves separation of the waste particles from the remaining reactive biomass material, thereby improving the efficiency with which waste particles are removed from the gasifier, and reducing the likelihood of grate blockage.

In certain embodiments, the drive shaft 40 may comprise a single rod shaft, having a first end and a second end. The single rod shaft being arranged such that the first end is located within the gasifier and is connected to the spokes 36, 38 of the second grate 30, whilst the second end is located outside the gasifier, and is operatively connected to a drive sprocket arranged to rotate the drive shaft 40. Operation of the drive sprocket may be controlled by a PLC (Programmable Logic Computer), which is operatively connected to the drive sprocket. The PLC is configured to control the rate and/or frequency of rotation of the drive shaft 40. In this way, the rate and/or frequency of rotation of the second grate 30 with respect to the first grate 20, may be controlled.

The PLC may be arranged to allow real-time updates to the rate and/or frequency of rotation, on the basis of control inputs programmed by an operator. Alternative embodiments are also envisaged, wherein the gasifier comprises one or more sensors arranged to measure operational properties within the gasifier (e.g. the bed height, temperature and pressure), and wherein the PLC is configured to vary the rate and/or frequency of rotation on the basis of the received sensor inputs. For example, and for illustrative purposes only, the PLC may be configured such that when a temperature increase within the gasifier above a predetermined threshold level is measured, which may be indicative of a build-up of waste material in the grate assembly, the PLC increases the rate of rotation of the grate assembly, in order to increase the grinding action to increase the rate of removal of waste products located in the grate assembly 5.

Embodiments are also envisaged, wherein the second grate 30 is affixed to the gasifier via welding, and the first grate 20 is operatively connected to the drive shaft 40 and is arranged to rotate relative to the second grate 30. Further alternative embodiments are also envisaged, wherein both grates are free to rotate at different speeds or in opposite directions. These embodiments may comprise the use of a telescopic driving shaft, wherein the first grate 20 and the second grate 30 are connected to different parts of the telescopic drive shaft, which parts are independently rotatable relative to each other. In this way, it is possible to rotate the grates independently of each other at different rates, or in different directions.

Figure 4 is a plan view of the first grate 20 comprised in the grate assembly 5 illustrated in Figures 2 and 3. The radially-outermost grate element 29 comprises one or more protrusions 27 affixed to the surface of the grate element 29, and oriented to face the second grate 30. The protrusions 27 are evenly distributed across the surface of the grate element 29.

Figure 5 is a plan view of the second grate 30 comprised in the grate assembly 5 illustrated in Figures 2 and 3. A subset of the spokes 38 that are affixed to the second grate 30 are radially elongated, and are arranged to protrude beyond the diameter of the outermost grate element 32. The spokes 38 are sized and arranged so as to come into contact with the protrusions 27 affixed to the surface of the first grate 20, when the first 20 and second 30 grates are brought together in complementary relation.

In use, as the second grate is rotated relative to the first grate 20, the elongated spokes 38 of the second grate 30 come into contact with the protrusions 27 located on the first grate 20, and the second grate 30 is displaced in a direction substantially parallel to the common axis 10. As the second grate 30 is displaced substantially parallel to the common axis 10, the grate elements 32 of the second grate 30 are also displaced within their associated receiving spaces 24 in a direction substantially parallel to the common axis.

The displacement of each grate element 32 is substantially equal to the height of the protrusions 27. Thus, by varying the height of the protrusions, it is possible to vary the amount by which each grate element 32 is displaced within its associated receiving space 24. For example, if the height of the protrusions 27 is selected to be larger than the cross-sectional height of the grate elements 32, then in use as the second grate 30 is rotated relative to the first grate 20, the grate elements 32 are entirely withdrawn from their associated receiving spaces 34, when the elongated spokes 38 of the second grate 30 come into contact with the protrusions 27 of the first grate 20. This displacement of the grate elements 32 in a direction substantially parallel to the common axis 10 is temporary and lasts only whilst the elongated spokes 38 of the second grate 30 are in contact with the protrusions 27 as the second grate 30 rotates relative to the first grate 20, after which the grate elements 32 are returned to their original position within their respective receiving spaces 34.

In effect, this axial displacement constitutes a secondary movement that alters the spacing between the first and second grate elements. By contrast, the primary movement (wherein adjacent first and second grate elements rotate alongside one another) substantially maintains the spacing between those adjacent grating elements. It will be appreciated that the extent of the primary movement is generally greater than the extent of the secondary movement.

This temporary displacement improves the grinding action and helps to dislodge any pieces of waste material that are trapped in the receiving spaces 34, and helps to further prevent blockage of the grate assembly 5.

It is to be appreciated that whilst Figure 4 illustrates two protrusions 27, this is not limiting and it is to be appreciated that alternative embodiments are envisaged comprising a single protrusion or more than two protrusions, and such alternatives fall within the scope of the present invention. Similarly, the number of illustrated elongated spokes 38 is also not intended to be limiting, and alternative embodiments are envisaged comprising a different number of elongated spokes. Similarly, the grate element to which the one or more protrusions are affixed may also be varied.

In general terms, certain embodiments of the grate assembly 3 comprise a camming mechanism that includes camming surfaces which interact during at least a part of the primary movement to translate at least a part of the primary movement into the secondary that alters the spacing between the first and second grate elements. In the embodiment shown in Figure 4, the elongated spokes 38 and the protrusions of the radially-outermost grate element 29 together define those confronting camming surfaces, and a part of the relative rotational motion between the grates is translated into a relative axial displacement of the grates.

Figure 6a is a cross-sectional profile view of the grate assembly 5 of Figures 2 and 3, taken in a plane perpendicular to the main faces of the assembly 5, which bisects the main faces. Figure 6b is a corresponding perspective cross-sectional view. Referring to Figures 6a and 6b, in accordance with the illustrated embodiment, each one of the plurality of grate elements 22, 32 has a tapered profile. As a result of the tapered profile of the grate elements 22, 32, the receiving spaces 24, 34 also comprise a tapered cross-sectional profile. The tapered cross-sectional profiles of the receiving spaces 24, 34, are tapered in a direction away from the main face of each of the grates 20, 30 that are in intersection with one another, so as to facilitate movement of waste particles through the grate assembly 5, and help to prevent grate blockage. In other words, the cross-sectional profile of each grate element is tapered, narrowing in a direction away from a material support area of the grate elements, the material support areas together defining the upwardly-facing main face of the grate assembly. Viewed another way, the space defined between adjacent grate elements is tapered in cross-section, narrowing in a direction towards the material support areas defined by said adjacent grate elements. Generally, the cross-section of the receiving spaces tapers in a direction that is parallel to the direction of flow of the waste particles.

Figure 6a includes a magnified cross-sectional profile view of one of the grate elements 22, 32 that clearly highlights the tapered cross-sectional profile.

Alternative grate element cross-sectional profiles are envisaged. For example, the grate elements 22, 32 may comprise a chamfered cross-sectional profile.

Alternative embodiments of the invention It is to be appreciated that the afore-described shape of the grate elements is not intended to be limiting and is for illustrative purposes only.

For example, alternatively shaped grates, such as oval shaped grates, or rectangular shaped grates are also envisaged, although in such alternative embodiments the driving means is configured to move the second grate relative to the first grate in a reciprocating motion (e.g. backwards and forwards). Similarly, alternatively shaped grating element profiles are also envisaged, and fall within the scope of the present invention. For example, the grate elements may comprise a triangular or rectangular cross-sectional profile, or an oval shaped profile.

It is also to be appreciated that for the purposes of the present invention, the number of grate elements may also be varied, and additionally, each grate may comprise a different number of elements than that shown in the illustrated embodiments, and such alternatives also fall within the scope of the present invention.

It is also to be appreciated that the grate elements may be formed from any material that is durable and can withstand the potentially high velocities and temperatures within a gasifier without incurring too great a degree of distortion or deformation. A possible material that would be suitable would be steel, but it will be appreciated by the skilled person that altemative embodiments utilising any other materials that fulfil the above requirements will

be suitable for use.

All herein described embodiments are for illustrative purposes only and are not to be deemed limiting to the present invention. Minor amendments of the herein described embodiments may be made without departing from the scope of the present invention.

Claims

Claims 1. A grate assembly for a gasifier comprising: a first and second grate having a respective first and second grate element; and a driving mechanism arranged to drive the first and second grate elements relative to one another through a primary movement, wherein the first and second grate elements move alongside one another so that a spacing therebetween is substantially maintained and material trapped within the spacing is ground between the grate elements.
2. The grate assembly of claim 1, wherein the driving mechanism is arranged to drive the first and second grate elements relative to one another through a secondary movement that alters the spacing between the first and second grate elements, and wherein the extent of the primary movement is greater than the extent of the secondary movement.
3. The grate assembly of claim 2, further comprising a camming mechanism which translates at least a part of the primary movement into the secondary movement.
4. The grate assembly of any preceding claim, wherein the driving mechanism is arranged to drive the grate elements of the first and second grates relative to one another about a common axis.
5. The grate assembly of any preceding claim, wherein the grate elements are shaped substantially in the form of rings.
6. The grate assembly of claim 5 when dependent on claim 4, wherein the rings are centred on the common axis.
7. The grate assembly of claim 5 when dependent on claim 4 or claim 6, wherein the grate elements are arranged concentrically to one another about the common axis, with each grate element having a different circumference.
8. The grate assembly of any preceding claim, wherein the first grate comprises a plurality of first grate elements, and the second grate comprises a plurality of second grate elements.
9. The grate assembly of claim 8, wherein the first and second grates are arranged relative to one another so that the plurality of first grate elements are interleaved between the plurality of second grate elements.
10. The grate assembly of claim 8 or claim 9, wherein the grate elements of a respective grate are affixed to one another.
11. The grate assembly of claim 10, wherein the grate elements of a respective grate are cross-linked to one another via one or more spokes.
12. The grate assembly of claim 11, wherein the spokes extend radially outward from a centre of a respective grate.
13. The grate assembly of claim 11 or 12, wherein the cross-sectional profile of the one or more spokes is wedge-shaped.
14. The grate assembly of any one of claims 11 to 13, wherein the driving mechanism comprises a drive shaft to which the one or more spokes are connected, the drive shaft being coincident with a common axis about which the driving mechanism is arranged to drive the grate elements of the first and second grates relative to one another.
15. The grate assembly of any one of claims 11 to 14, wherein the one or more spokes of one of the first and second grates and one or more grate elements of the other of the first and second grates together define confronting camming surfaces which interact during at least a part of the primary movement to translate at least a part of the primary movement into a secondary movement that alters the spacing between the first and second grate elements.
16. The grate assembly of any preceding claim, wherein the cross-sectional profile of one or more grate elements is tapered, narrowing in a direction away from a material support area of the one or more grate elements.
17. The grate assembly of any preceding claim, wherein the space defined between adjacent grate elements is tapered in cross-section, narrowing in a direction towards material support areas defined by said adjacent grate elements.
18. The grate assembly of any preceding claim, wherein: the first grate has at least two first grate elements, the at least two first grate elements being arranged in spaced relation to define a receiving space therebetween; and the second grate element is arranged, in use, to fit at least partly within the receiving space when the first and second grates are arranged in complementary relation; and the driving mechanism is arranged to enable relative motion between the first and second grates, such that, in use, the second grate element is displaceable within the receiving space.
19. A grate assembly comprising a plurality of grate elements, each grate element defining a material support area, wherein the cross sectional profile of at least one of the grate elements is tapered, narrowing in a direction away from the material support area.
20. A grate assembly for a gasifier substantially as described herein and/or as illustrated in any one of Figures 2, 3, 4, 5, 6a and 6b.
21. A gasifier comprising the grate assembly of any one of claims 1 to 20.
22. A downdraft gasifier according to claim 21.
23. A gasifier according to claim 21 or claim 22 comprising a body within which the grate assembly is located, and at least one of: a. an air intake for introducing air into the body above the grate assembly; b. a fuel feed inlet for introducing fuel into the body above the grate assembly; c. a gas exit port for expelling gas from the body below the grate assembly d. an ash pit for collecting waste products, the ash pit being situated below the grate assembly.