EP3247467A1

EP3247467A1 - Prevention of combustion in storage silos

Info

Publication number: EP3247467A1
Application number: EP16701348.1A
Authority: EP
Inventors: Simon Mills
Original assignee: Linde GmbH
Current assignee: Linde GmbH
Priority date: 2015-01-22
Filing date: 2016-01-22
Publication date: 2017-11-29
Also published as: GB201501076D0; WO2016116617A1

Abstract

An apparatus for inerting a container headspace comprises a gas conduit configured to feed inert gas from an inert gas source at a pressure greater than atmospheric pressure to a container configured to receive flammable material, and at least one nozzle assembly operably connected to the gas conduit. The assembly comprises gas pressure reduction means configured to reduce the pressure of the inert gas to approximately atmospheric pressure, and at least one gas outlet configured to release the inert gas into the container's headspace at a velocity of less than 20 m/s.

Description

Prevention of combustion in storage silos

The present invention relates to an apparatus and method for preventing unwanted combustion in a container for storing a flammable material. In particular, the invention relates to preventing fires and/or explosions in a silo in which biomass fuel is stored prior to combustion.

The burning of biomass as a fuel in power stations has become more prevalent in recent years, and the volume of biomass used and stored at power stations has

correspondingly increased. In general terms, biomass comprises plant matter, which may be in the form of wood, a fluff material or pellets formed from material which has been shredded and compacted. The biomass material is stored in large silos to keep the material dry and reduce loss of the material prior to being conveyed for use in boilers. Such silos can range from hundreds to thousands of cubic meters in volume. Biomass dust may be generated from the biomass during storage and handling. The dust is drawn off in an air stream which is filtered to remove the dust.

Fires may occur in both biomass storage silos and dust storage silos, and the factors which cause fires in both cases are broadly the same. Fires in biomass storage silos can come about as a result of bacterial and fungal activity which generate heat and produce methane, carbon monoxide and carbon dioxide. Heat accumulates to over 50°C leading to thermal oxidation of the biomass. Due to the thermal insulating properties of the biomass, the rate of heat generation may exceed the rate of heat loss, leading to a temperature rise, and may eventually lead to ignition. Fires may also be imported into silos, for example through hot product, or from hot bearings within the conveying system. Although water is the best medium for removing heat from smouldering fires, the use of water sprinklers would destroy the biomass product and cause significant damage to the storage silos due to the expansion of the wet pellets.

It is known in the art that smouldering fires can be controlled and extinguished by providing an inert atmosphere within the silo. This is commonly achieved by providing a carbon dioxide and/or nitrogen atmosphere within the silo, although other gases such as argon can also be used. Accordingly, inert gas can be injected into the headspace to minimise the risk of self heating, to inert the headspace in the event of a surface fire or to provide an inert atmosphere in the event of a high risk of imminent explosion.

Particularly in the last two situations, it might be necessary to add a significant volume of gas in a relatively short period of time. In large silos, the top may be about 40 to 60 metres above ground level. Accordingly, to minimise the use of materials, the most efficient way of piping an inert gas into the headspace of a large silo is to pipe the gas in a relatively small bore pipe at a relatively high pressure (approximately 5-10 barg) and at a velocity of approximately 10-30 m/s, and then pass the gas through an orifice on a nozzle into the silo. As the gas passes through the nozzle, a critical pressure drop takes place, reducing the gas to effectively atmospheric pressure, thereby causing the velocity of the gas to increase further. A problem with high velocity gas is that it can disturb any dust present in the silo, thereby creating a dust cloud. A problem resulting from the dust cloud is that it can cause the nozzle to become blocked, and sudden dust cloud movement can lead to static electricity which can create sparks, which increase the risk of fires and/or explosions in the silo.

There is therefore a need to provide improved systems for preventing unwanted combustion and fires in silos used for storing flammable materials. The invention arises from the inventor's work in trying to overcome the problems associated with the prior art.

In accordance with a first aspect of the present invention, there is provided an apparatus for inerting a container headspace, the apparatus comprising:

a gas conduit configured to feed inert gas from an inert gas source at a pressure greater than atmospheric pressure to a container configured to receive flammable material; and

at least one nozzle assembly operably connected to the gas conduit, wherein the assembly comprises (i) gas pressure reduction means configured to reduce the pressure of the inert gas to approximately atmospheric pressure, and (ii) at least one gas outlet configured to release the inert gas into the container's headspace at a velocity of less than 20 m/s.

Advantageously, the apparatus enables the inert gas to be fed along a relatively small bore pipe (e.g. approximately 80mm) at a relatively high pressure (e.g. approximately 5 barg), thereby minimising the use of materials, while allowing the gas to be released into the container's headspace at a low velocity (i.e. less than 20 m/s). This reduces the disturbance experienced by the flammable material in the container and prevents the formation of a dust cloud therein, which could otherwise result in an explosion, an ignition event or fire.

Preferably, the container is a storage silo. The flammable material preferably comprises a biomass substance, for example plant material. The flammable material may be in the form of pellets and/or dust. The flammable material may be a fuel source.

The inert gas source may comprise a liquid gas store, a Pressure Swing Adsorption (PSA) unit, a membrane gas generation plant, or any other appropriate inert gas source. However, in a preferred embodiment, the inert gas source comprises a liquid gas store. Preferably, the inert gas comprises carbon dioxide and/or nitrogen gas and/or argon.

Preferably, the apparatus is configured to maintain the inert gas upstream of the gas pressure reduction means at a pressure of greater than 3 barg, more preferably greater than 4 barg, and most preferably greater than 5 barg. Preferably, the apparatus is configured to maintain the inert gas upstream of the gas pressure reduction means at a pressure of between 3 and 20 barg, more preferably between 4 and 15 barg, and most preferably between 5 and 10 barg.

Preferably, the gas pressure reduction means comprises a restriction orifice configured to reduce the pressure of the gas. The restriction orifice may be disposed in the conduit and have a diameter which is at least 30% less than the diameter of the conduit upstream thereof, i.e. between the inert gas source and the orifice. Preferably, the restriction orifice has a diameter which is at least 40% less, 50% less, 60% less, 70% less, 80% less or 90% less than the diameter of the conduit upstream thereof.

Preferably, the gas pressure reduction means comprises a widened section of conduit which has a diameter that is greater than the diameter of the conduit upstream thereof. Preferably, the diameter of the widened section of conduit is at least 10% greater than the diameter upstream thereof, and more preferably by at least 15%, 20% or 25% greater than the diameter upstream thereof.

In a preferred embodiment, however, the gas pressure reduction means comprises both a restriction orifice and a widened section of conduit. Preferably, the widened section of conduit is disposed downstream of the restriction orifice. Advantageously, this arrangement allows the majority of the conduit upstream of the gas pressure reduction means to comprise a relatively small bore pipe which leads to a significant cost saving in materials and installation. The apparatus may further comprise a turbuliser disposed in the conduit downstream of the gas pressure reduction means, which turbuliser is configured to create turbulent gas flow. The purpose of this is to ensure that the inert gas flow downstream of the turbuliser and out of the outlet is well mixed and so that its pressure and speed is uniform across the width of the conduit leading to and through the outlet. This further reduces the risk of a dust cloud or static forming in the container, which could lead to ignition or fire.

Preferably, the or each nozzle assembly comprises a mesh or filter extending across the gas conduit, preferably downstream of the gas pressure reduction means. The mesh preferably comprises apertures of such a dimension that gas flows therethrough to the outlet, but which prevent dust and particles from passing from inside the headspace of the container and back up into the nozzle assembly, and thereby prevents the restriction orifice from becoming blocked. Preferably, the mesh has a sieve size of imm or less. More preferably, the mesh has a sieve size of 0.8 mm or less, 0.7 mm or less, 0.5 mm or less, or 0.4 mm or less. Most preferably, the mesh has a sieve size of 0.3 mm or less, 0.25 mm or less, or 0.2mm or less.

The at least one nozzle assembly may comprise a plurality of spaced apart gas outlets. For example, the at least one nozzle assembly may comprise at least 2, 3, 4 or 5 outlets. Preferably, each nozzle assembly comprises 4 outlets

Preferably, the or each outlet is configured to release the inert gas into the container headspace at a velocity of less than 15 m/s, more preferably at a velocity of less than 12 m/ s, and most preferably less than 10 m/ s.

Preferably, the or each outlet comprises guide means configured to guide the inert gas into the container's headspace at an angle substantially away from flammable material stored within the container. Preferably, the guide means comprises a cone. Preferably, the cone tapers outwardly and thereby diffuses the inert gas being fed therethrough. In one embodiment, the gas conduit may extend through a sidewall of the container, preferably at least adjacent to the top or roof of the container. This position would correspond to the headspace of the container. In this embodiment, the or each outlet is preferably arranged, in use, to release the inert gas at an angle of at least 20° above the horizontal, and preferably at least 30⁰ above the horizontal. Such angles substantially correspond to the angle of repose of material inside the container, and so the released gas is not guided directly at the material, thereby preventing the formation of a dust cloud. In an alternative embodiment, however, the gas conduit may extend through the top or roof of the container. In this embodiment, the or each outlet is preferably arranged, in use, to release the inert gas at an angle of no more than 40⁰ below the horizontal, and preferably no more than 30⁰ below the horizontal. Again, such angles correspond to the angle of repose of material inside the container, such that the released gas is not guided directly at the material.

It will be appreciated that the container comprises a fill point via which the flammable material is fed into the container. The fill point may be disposed substantially centrally in the container, or towards or adjacent to a sidewall thereof. Preferably, in an embodiment in which the or each outlet is configured to release the inert gas away from the fill point, the or each outlet is arranged, in use, to release the inert gas at an angle of no more than 40⁰ below the horizontal, and preferably no more than 30⁰ below the horizontal. Alternatively, in an embodiment in which the or each outlet is configured to release the inert gas towards the fill point, the or each outlet is arranged, in use, to release the inert gas at an angle of at least 20⁰ above the horizontal, and preferably at least 30⁰ above the horizontal.

Advantageously, by angling the inert gas away from the flammable material, it further reduces the disturbance experienced by the flammable material, thereby helping to prevent the formation of a dust cloud and reduce the risk of ignition or fire.

The apparatus may comprise one nozzle assembly. Accordingly, in this embodiment, the apparatus comprises a gas pressure reduction means, and one or more outlet disposed downstream therefrom. However, in a preferred embodiment, the apparatus comprises a plurality of nozzle assemblies. Accordingly, the apparatus preferably comprises at least 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 15, 16, 17, 18, 19 or 20 nozzle assemblies. It will be appreciated that in an embodiment in which the apparatus comprises four nozzle assemblies, and the plurality of outlets comprises 16 outlets, then each nozzle assembly comprises four outlets. Alternatively, it will be appreciated that in an embodiment in which the apparatus comprises 16 nozzle assemblies and the plurality of outlets comprises 16 outlets, then each nozzle assembly comprises one outlet, and so on. It will be appreciated that the apparatus of the invention has several important uses.

Thus, in a second aspect, there is provided use of the apparatus of the first aspect to:

(i) inert the atmosphere in a container's headspace to minimise the risk of its contents from self-heating,

(ii) inert a container's headspace in the event of a fire occurring at the surface of the container's contents, and/or

(iii) provide an inert atmosphere in a container in the event of a high risk of imminent explosion therein. Furthermore, in accordance with a third aspect, there is provided a method of preventing combustion and/or an explosion, in a container, of a flammable material contained therein, the method comprising:

a) feeding an inert gas, at a pressure greater than atmospheric pressure, from an inert gas source to a container of flammable material;

b) reducing the pressure of the inert gas to approximately atmospheric

pressure; and

c) releasing the inert gas into the container's headspace at a velocity of less than 20 m/s. The method of the third aspect comprises the use of the apparatus of the first aspect.

Preferably, therefore, the method comprises feeding the inert gas through a gas conduit to at least adjacent to the top or roof of the container. Preferably, the method comprises feeding the inert gas to at least adjacent to the container's headspace. Preferably, the method comprises feeding the gas at a pressure of greater than 3 barg, more preferably greater than 4barg, and most preferably greater than 5 barg. Preferably, the method comprises reducing the pressure of the inert gas by passing it through a restriction orifice disposed in the gas conduit. Preferably, the method comprises reducing the pressure of the inert gas by passing it through a widened section of the gas conduit. Most preferably, the method comprises reducing the pressure of the inert gas by passing it through a restriction orifice and a widened section of the gas conduit. Preferably, the method comprises feeding the gas through a tubuliser in order to create turbulent gas flow. Preferably, after the pressure of the inert gas has been reduced, the method comprises passing the inert gas through a mesh or filter.

In one embodiment, the method preferably comprises feeding the inert gas through a side wall of the container, preferably adjacent to the top or roof of the container. In an alternative embodiment, the method preferably comprises feeding the inert gas into the container through its top or roof.

Preferably, the method comprises releasing the inert gas into the container's headspace at an angle substantially away from the flammable material stored in the container. Preferably, the method comprises guiding the inert gas using guide means. Preferably, the method comprises releasing the inert gas into the headspace at an angle of at least 20° above horizontal, more preferably at least 30⁰ above horizontal.

Preferably, prior to releasing the inert gas into the container, the method comprises dividing the inert gas into a plurality of streams. The inert gas may be divided into a plurality of streams prior to the step of reducing the pressure of the inert gas.

Alternatively, or additionally, the inert gas may be divided into a plurality of streams after the step of reducing the pressure of the inert gas. Preferably, the inert gas is released into the headspace of the container at a velocity of less than 15 m/s, more preferably at a velocity of less than 12 m/s, and most preferably at a velocity of less than 10 m/s.

Preferably, the inert gas comprises carbon dioxide and/or nitrogen gas and/or argon. The inventor believes that the nozzle assembly in the apparatus of the first aspect is novel per se.

Thus, in accordance with a fourth aspect, there is provided a gas nozzle assembly comprising:

a gas inlet and at least one gas outlet defining a conduit therebetween; gas pressure reduction means disposed in the conduit and configured to reduce the pressure of gas flowing therethrough; and

a mesh disposed in the conduit between the gas pressure reduction means and the or each outlet; and/ or

guide means disposed adjacent to the or each gas outlet, and configured to guide the direction of the gas released therefrom.

The gas nozzle assembly is preferably configured to feed inert gas therethrough, for example carbon dioxide, nitrogen or argon. Preferably, the assembly is configured to maintain the pressure of gas upstream of the pressure reduction means at greater than 3 barg, more preferably greater than 4 barg, and most preferably more than 5 barg.

Preferably, the gas pressure reduction means comprises a restriction orifice having a diameter which is at least 30% less than the diameter of the conduit upstream thereof. More preferably, the restriction orifice has a diameter which is at least 40% less, 50% less, 60% less, 70% less, 80% less or 90% less than the diameter of the conduit upstream thereof. Preferably, the gas pressure reduction means comprises a widened section of conduit which has a diameter that is greater than the diameter of the conduit upstream thereof. Preferably, the diameter of the widened section is at least 10% greater than the diameter of conduit upstream thereof, and more preferably by at least 15%, 20% or 25% greater than the diameter of conduit upstream thereof.

In a preferred embodiment, the gas pressure reduction means comprises both a restriction orifice and a widened section of conduit. Preferably, the widened section is downstream of the restriction orifice.

The assembly may further comprise a turbuliser disposed downstream of the gas pressure reduction means, and configured to create turbulent gas flow. Preferably, the mesh extends across the gas conduit, and comprises apertures of such a dimension that gas flows therethrough to the or each outlet, but which prevent dust and particles from passing from inside the headspace of the container and back up into the nozzle assembly, and thereby prevents the restriction orifice from becoming blocked. Preferably, the mesh has a sieve size of ι mm, 0.9mm, 0.8mm, 0.7mm, 0.6mm, 0.5mm, 0.4mm, 0.3mm, 0.2mm or less.

The at least one nozzle assembly may comprise a plurality of gas outlets. The at least one nozzle assembly may comprise 2, 3 or 4 outlets.

Preferably, the guide means comprises a cone. Preferably, the cone tapers outwardly and thereby diffuses the inert gas being fed therethrough.

Preferably, the or each outlet is configured to release inert gas at a velocity of less than 15 m/ s, more preferably at a velocity of less than 12 m/s, and most preferably less than 10 m/s.

The nozzle assembly of the fourth aspect is preferably attached to a container in which flammable material is stored, preferably at least adjacent to the top or roof of the container. The assembly is preferably used for feeding inert gas to at least adjacent to the container's headspace to prevent combustion or ignition of flammable contents in the container.

Hence, in a fifth aspect, there is provided use of the gas nozzle assembly according to the fourth aspect for feeding inert gas into the headspace of a container in which flammable material is stored.

All features described herein (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined with any of the above aspects in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying Figures, in which:- Figure 1 is a schematic diagram of a first embodiment of an apparatus according to the invention, which is used for piping inert gas into the headspace of a storage silo (not shown);

Figure 2 is a side view of one embodiment of a storage silo fitted with the apparatus for piping inert gas into the headspace;

Figure 3 is a side view of another embodiment of a storage silo fitted with the apparatus for piping inert gas into the headspace;

Figure 4a is a plan view from above of another embodiment of a storage silo which has been fitted with the apparatus for piping inert gas through the wall of the silo and into its headspace;

Figure 4b is a plan view from above of another storage silo which has been fitted with the apparatus for piping inert gas through the top of the silo and into its headspace; Figure 5 is a schematic side view of a silo showing two ways in which the apparatus can be connected to the silo for piping the inert gas into the silo headspace;

Figure 6 is a schematic side view of a nozzle assembly which forms part of the apparatus for piping inert gas into the silo headspace; and

Figure 7 is a schematic diagram of a second embodiment of the apparatus for piping inert gas into the headspace of a silo (not shown). Example 1

Figure 1 shows a first embodiment of an apparatus 2 for injecting inert gas 4 (e.g.

nitrogen, carbon dioxide or argon) into the headspace 6 of a storage silo 8. The purpose of the gas 4 is:- (i) to inert the atmosphere in the headspace 6 to minimise the risk of the contents of the silo 8 from self-heating, (ii) to inert the headspace 6 in the event of a fire occurring at the surface of the silo's contents, or (iii) to provide an inert atmosphere in the event of a high risk of imminent explosion in the silo 8. The inert gas 4 is fed along pipe 12 under the control of a valve 10 from an inert gas source (not shown), which may be a liquid gas store, a Pressure Swing Adsorption (PSA) plant or a membrane nitrogen generator plant, or any other commercially available means of inert gas generation. In the present example, the valve 10 is a manual initiation valve but it will be understood that an actuated valve could also be used. Inert gas 4 from the gas source is generally provided at a pressure of between about 5 and 10 barg. In one embodiment of the apparatus 2, the inert gas 4 is provided at a pressure of 6.5 barg. A vertical section of the pipe 12 includes a riser 14 through which the inert gas 4 is fed up a vertical side wall 16 of the silo 8, as is best illustrated in Figures 2 and 3. In order to minimise the use of materials and cost, a relatively small bore pipe 12 is used for the riser 14, for example a diameter of 80 mm. An upper section of the pipe 12 forming the riser 14 is connected to a horizontal header 18 by which the inert gas 4 is then fed around either a section of the silo wall 16 (as shown in Figures 3, 4a and 5) or along the top 20 of the silo 8 (as shown in Figures 4b and 5).

A number of spaced apart nozzle assemblies 22 are connected to, and extend out of, the header 18. In the embodiment illustrated in Figure 1, the apparatus 2 has four spaced apart nozzle assemblies 22 disposed along the header 18, but the skilled person will understand that any number of nozzle assemblies 22 can be provided depending on the size and configuration of the silo 8 to which the apparatus 2 is attached.

The structure of one of the nozzle assemblies 22 is shown in detail in Figure 6. The gas pipe 12 of the header section 18 is connected to a restriction orifice 24 having a diameter of 21 mm which restricts the flow of the inert gas 4 as it passes therethrough. The orifice 24 causes a critical pressure drop where the gas outlet is at silo pressure (plus sufficient pressure to flow through the nozzle outlet). Because the gas pressure has dropped from about 6 barg to o barg, it is now 7 times less dense that it was at 6 barg, meaning that for the same size pipe and flow, the gas velocity is now about 7 times greater than its velocity upstream of the orifice 24. In order to achieve even velocity downstream of the orifice 24, sufficient length of pipe is required to ensure distributed turbulent flow within the pipe. Alternatively, in another embodiment, a turbuliser (not shown) can be fitted, which creates turbulent gas flow immediately downstream of the orifice 24.

In order to further reduce the pressure of the inert gas 4 flowing therethrough, downstream of the restriction orifice 24, the pipe 12 tapers outwards into a widening point 26. The widening point 26 is a section of the pipe 12 where it widens from a diameter of 80 mm to a diameter of 100 mm. The pressure of the inert gas 4 that has passed through the restriction orifice 24 and the widening point 26 is therefore at about atmospheric pressure.

Disposed inside the lumen of the pipe 12 at a position downstream of the widening point 26, there is provided a mesh covering 28, for example one which is available from The Mesh Company. The mesh 28 is metallic, and has apertures of such a dimension that gas flows therethrough, but which prevent dust and particles from passing from inside the headspace 6 of the silo 8 and back up into the nozzle assembly 22, and thereby prevents the restriction orifice 24 from becoming blocked.In the present example, a #40 mesh, with a 0.41mm aperture and 0.22mm wire OD was used. In the event that the mesh covering 28 were to become partially blocked by dust, the pressure upstream of the mesh covering 28 increases automatically to such a point where there is sufficient back pressure to dislodge the blockage.

With reference to Figures 1 and 6, downstream of the mesh covering 28, the pipe 12 forms a first tee junction 34 which divides into two arms 42. Each arm 42 extends through an angled elbow joint 38 to a second tee junction 36, which divides each arm 42 into two distal limbs 44. A gas outlet consisting of a nozzle 30 is disposed at the end of each of the four distal limbs 44, and includes an aperture in the shape of a cone 40 by which the inert gas is dispersed into the silo's 8 headspace 6. Accordingly, the illustrated nozzle assembly 22 includes four nozzles 30, but it will be appreciated that more or less nozzles 30 could be provided. In the illustrated assembly 22, one pair of gas nozzles 30 is disposed vertically above the second pair 30.

Referring to Figure 4a, there is shown an embodiment of the apparatus 2 in which the header 18 of the pipe 12 extends along a side 16 of the silo 8. Nozzle assemblies 22, as described above, extend from the header 18 through the side wall 16 of the silo 8. Each nozzle assembly 22 has four gas nozzles 30. However, as in Figure 6, each assembly 22 comprises one pair of gas nozzles 30 which is disposed vertically above a second pair.

Figure 4b shows an alternative arrangement of the apparatus 22 attached to the silo 8. The header 18 of the pipe 12 extends along the top 20 of the silo 8, and nozzle assemblies 22 extend down from the header 18 through the top 20 and into the silo 8.

Referring now to Figures 2 and 5, there is shown the apparatus 2 and corresponding nozzle assemblies 22 attached to a silo 8 in which biomass 32, or the like, can be stored. The biomass 32 material can include plant matter, which may be in the form of wood, a fluff material, or pellets formed from material which has been shredded and

compacted. The biomass 32 can be quite dusty. The nozzles 30 are arranged in such a way that they are angled away from the biomass 32 in order to prevent dust from being disturbed or distributed around the headspace 6 of the silo 8. As shown in Figures 2 and 5, the biomass 32 is poured into the silo 8 through a fill point 46. In Figures 2 and 5 the fill point 46 is provided in the centre of the top 20 of the silo. When the biomass 32 is poured into the silo 8 through the fill point 46 the angle of repose is 30⁰ above horizontal (shown as Θ in the Figures). Accordingly, in the embodiment where the nozzle assemblies 22 extend though the side 16 of the silo 8 (as illustrated in Figure 4a), the angle of the nozzles 30 is at least 30⁰ above the horizontal, as shown on the right- hand side of Figure 5. Alternatively, in the embodiment where the nozzle assemblies 22 extend though the top 20 of the silo 8 (as illustrated in Figure 4b), the angle of the nozzles 30 is no more than 30⁰ below the horizontal when the nozzles 30 are directed away from the fill point 46. Alternatively, when the nozzles 30 are directed towards the fill point 46 then the angle of the nozzles 30 is at least 30⁰ above the horizontal, as shown on the left-hand side of Figure 5. The outlets 30 can be angled due to the orientation of the elbow joint 38 between each arm 42 and distal limb 44, and due to the rolled steel cone outlet 34 which is welded to the nozzle outlet 30 and which directs the inert gas 4 into the headspace 6. Accordingly, Figure 5 shows the nozzles assemblies extending downwards (left) or horizontally (right) into the silo but in each case the angle does not expel inert gas directly at the biomass.

Referring to Figure 1, the inventor has calculated the pressure and velocity at the different points A to F in the apparatus 2 shown in the Figure, and these are set out below in Table 1.

Table 1: Flow, pressure, pipe 12 diameter and inert gas 4 velocity at points A to F as shown on Figure 1

As can be seen from Table 1, as the gas 4 flows up the riser 14, it is at a pressure of 6.50 barg and at a velocity of 32.8 m/ s. The gas 4 continues to flow along the header 18 and the pressure decreases slightly and the velocity decreases in direct proportion to the proportion of the gas 4 which is diverted from the header 18 as each nozzle assembly 22 splits off. Accordingly, the pressure and velocity of the gas 4 in the header 18 upstream of the first nozzle assembly 22a and downstream of the second nozzle assembly 22b is 6.15 barg and 25.8 m/s, i.e. the velocity has decreased by approximately a quarter, as about a quarter of the gas has been diverted in the first nozzle assembly 22a whilst the upstream pressure (and thus gas density) remains constant. Similarly, the pressure and velocity of the gas 4 in the header 18 upstream of the second nozzle assembly 22b and downstream of the third nozzle assembly 22c is 6.10 barg and 17.3 m/s and the pressure and velocity of the gas 4 in the header 18 upstream of the third nozzle assembly 22c and downstream of the fourth nozzle assembly 22d is 6.05 barg and 8.7 m/s.

Accordingly, it will be understood that the gas 4 will enter each nozzle assembly 22 as a pressure of about 6.05 barg and a velocity of 8.7 m/s. In each nozzle assembly 22 the gas 4 then passes through the restriction orifice 24, widening point 26 and mesh 28. As explained above, the restriction orifice 24 causes the pressure to drop to about atmospheric pressure, or 0.05 barg. While the pressure has dropped by a factor of 7 the velocity has not increased by this factor but instead has increased to 37.1 m/s due to the increase in the diameter of the pipe from 80 mm to 100 mm. Accordingly, the ratio by which you would expect the velocity to increase by can be approximated by the following formula:

(Pressure Drop Factor) (Diameter of Pipe Upstream)²

(Diameter of Pipe Downstream)²

In the present example, this gives:

7 x 8o² = 4.48

100²

This corresponds with the observed values as the velocity has increased by a factor of approximately 4.48 from 8.7 m/s to 37.1 m/s.

Finally, the presence of the first tee junction 34 and second tee junction 36 causes the inert gas 4 stream in each nozzle assembly to split into four. Accordingly, while the pressure of the inert gas 4 flowing out of the nozzles 30 is at about atmospheric pressure (0.02 barg) the velocity has decreased by a factor of four and is at 9.54 m/ s. Example 2

Referring to Figure 7, there is shown a second embodiment of the apparatus 2' for injecting inert gas 4 into the headspace 6 of a silo 8. The second embodiment of the apparatus 2' is similar to the first embodiment, but has several differences which are described below.

The apparatus 2' includes a manual initiation valve 10', a pipe 12', which includes a riser 14' and a header 18', and a number of spaced apart nozzle assemblies 22' disposed along the header 18'. As shown in Figure 7, 16 nozzle assemblies 22' are provided, each one having a restriction orifice 24', a widening point 26' and a mesh covering 28' disposed within the pipe 12'. However, unlike the first embodiment of the apparatus 2 described in Example 1, the nozzle assemblies 22' in the second embodiment of the apparatus 2' do not have any tee junctions. Instead, each nozzle assembly 22' has one gas outlet nozzle 30'. Each of the outlet nozzles 30' are angled away from the biomass 32 to prevent any dust from being disturbed in the silo 8. As in the first embodiment, the angle of the outlet nozzles 30' can be varied due to the orientation of an elbow 38' (not shown) and due to a rolled steel cone 34' welded to the outlet nozzle 30' which directs the inert gas 4 into the headspace 6. The inventor has calculated the pressure and velocity at the different points A' to H' in the apparatus 2' shown in Figure 7, and these are set out below in Table 2.

Table 2: Flow, pressure, pipe 12' diameter and inert gas 4 velocity at points A' to H' as shown on Figure 7

As can be seen from Table 2, as the gas flows up the riser 14', it is at a pressure of 6.50 barg and at a velocity of 32.8 m/ s. The gas 4 continues to flow along the header 18' and the pressure decrease slightly and the velocity decreases in direct proportion to the proportion of the gas 4 which is diverted from the header 18' as each nozzle assembly 22' splits off. Accordingly, the pressure and velocity of the gas 4 in the header 18' downstream of the first nozzle assembly 22'a and upstream of the second nozzle assembly 22'b is 6.48 barg and 30.8 m/s, i.e. the velocity has decreased by

approximately a sixteenth as about a sixteenth of the gas has been diverted in the first nozzle assembly 22'a. Similarly, the pressure and velocity of the gas 4 in the header 18' downstream of the second nozzle assembly 22'b and upstream of the third nozzle assembly 22'c is 6.47 barg and 28.8 m/ s, the pressure and velocity of the gas 4 in the header 18' upstream of the third nozzle assembly 22'c and downstream of the fourth nozzle assembly 22'd is 6.45 barg and 26.8 m/ s. This trend continues until the sixteenth and final nozzle assembly 22'q. Accordingly, the pressure and velocity of the gas 4 in the header 18' downstream of the fifteenth nozzle assembly 22'p and upstream of the sixteenth nozzle assembly 22'q is 6.37 barg and 2.1 m/s Accordingly, it will be understood that the gas 4 will enter each nozzle assembly 22' as a pressure of about 6.3-6.4 barg and a velocity of 2.1 m/ s. In each nozzle assembly 22' the gas 4 then passes through the restriction orifice 24', widening point 26' and mesh 28'. As explained above, the restriction orifice 24' causes the pressure to drop to about atmospheric pressure, or 0.05 barg, and, due to the restriction orifice and widening point, the velocity of the inert gas increases by a factor of about 4.48, as explained above. Since no tee junctions are provided in this embodiment, the inert gas 4 will vent from the nozzle 30' at a pressure of approximately 0.02 barg and a velocity of 9.5 m/s.

It will be noted that while each nozzle assembly 22' only comprises one nozzle outlet 30', the velocity of the inert gas 4 as it flows through the nozzle 30' (at point H') is the same as the velocity of the inert gas 4 as it flows through the outlet 30 (at point F) in the apparatus 2 described in Example 1. This is because, although the number of outlet nozzles 30, 30' varies between each nozzle assembly 22, 22' in each embodiment of the apparatus 2, 2', the total number of outlet nozzles 30, 30' is unchanged, i.e. 16.

Summary

The ability to reduce the velocity of an inert gas piped into a silo 8, without reducing the flow of the gas, is advantageous as it allows a significant volume of gas to be piped into a headspace 6 in a relatively short period of time without increasing the risk of fires or explosions. Additionally, by reducing the pressure and velocity of the inert gas in a region which is close to the headspace 6, it means that small bore pipes can be used to transport the inert gas from the inert gas store which will reduce the amount of materials needed. It will be appreciated that the silo 8 can be used to contain biomass 32, or other flammable materials, such as combustible powders.

Claims

1. An apparatus for inerting a container headspace, the apparatus comprising:

2. An apparatus according to claim 1, wherein the container is a storage silo.

3. An apparatus according to either claim 1 or claim 2, wherein the flammable material comprises a biomass substance, and may be in the form of pellets and/or dust.

4. An apparatus according to any preceding claim, wherein the inert gas source comprises a liquid gas store, a Pressure Swing Adsorption (PSA) unit, a membrane nitrogen generator plant, or any other commercially available means of on-site inert gas generation.

5. An apparatus according to any preceding claim, wherein the inert gas comprises carbon dioxide and/ or nitrogen gas and/ or argon.

6. An apparatus according to any preceding claim, wherein the apparatus is configured to maintain the inert gas upstream of the gas pressure reduction means at a pressure of greater than 3 barg, 4 barg, or 5 barg.

7. An apparatus according to any preceding claim, wherein the gas pressure reduction means comprises a restriction orifice configured to reduce the pressure of the gas.

8. An apparatus according to claim 7, wherein the restriction orifice is disposed in the conduit and has a diameter which is at least 30% less than the diameter of the conduit upstream thereof.

9. An apparatus according to any preceding claim, wherein the gas pressure reduction means comprises a widened section of conduit which has a diameter that is greater than the diameter of the conduit upstream thereof.

10. An apparatus according to claim 9, wherein the diameter of the widened section of conduit is at least 10%, 15%, 20% or 25% greater than the diameter upstream thereof.

11. An apparatus according to any one of claims 7-10, wherein the gas pressure reduction means comprises both a restriction orifice and a widened section of conduit.

12. An apparatus according to any preceding claim, wherein the apparatus comprises a turbuliser disposed in the conduit downstream of the gas pressure reduction means, which turbuliser is configured to create turbulent gas flow.

13. An apparatus according to any preceding claim, wherein the or each nozzle assembly comprises a mesh or filter extending across the gas conduit, preferably downstream of the gas pressure reduction means.

14. An apparatus according to any preceding claim, wherein the at least one nozzle assembly comprises a plurality of spaced apart gas outlets.

15. An apparatus according to any preceding claim, wherein the or each outlet is configured to release the inert gas into the container headspace at a velocity of less than 15 m/s, 12 m/s, or 10 m/s.

16. An apparatus according to any preceding claim, wherein the or each outlet comprises guide means configured to guide the inert gas into the container's headspace at an angle substantially away from flammable material stored within the container.

17. An apparatus according to claim 16, wherein the guide means comprises a cone which tapers outwardly and thereby diffuses the inert gas being fed therethrough.

18. An apparatus according to any preceding claim, wherein the gas conduit extends through a sidewall of the container, preferably at least adjacent to the top or roof of the container.

19. An apparatus according to claim 18, wherein the or each outlet is arranged, in use, to release the inert gas at an angle of at least 20⁰ above the horizontal, or at least 30⁰ above the horizontal.

20. An apparatus according to any preceding claim, wherein the gas conduit extends through the top or roof of the container.

21. An apparatus according to claim 20, wherein the or each outlet is arranged, in use, to release the inert gas at an angle of no more than 40⁰ below the horizontal, or no more than 30⁰ below the horizontal.

22. An apparatus according to any preceding claim, wherein the apparatus comprises a plurality of nozzle assemblies.

23. Use of the apparatus according to any one of claims 1-22 to:

(iii) provide an inert atmosphere in a container in the event of a high risk of imminent explosion therein.

24. A method of preventing combustion and/or an explosion, in a container, of a flammable material contained therein, the method comprising:

b) reducing the pressure of the inert gas to approximately atmospheric pressure; and

c) releasing the inert gas into the container's headspace at a velocity of less than 20 m/s.

25. The method according to claim 24, wherein the method comprises the use of the apparatus according to any one of claims 1-22.

26. The method according to either claim 24 or claim 25, wherein the method comprises feeding the inert gas through a gas conduit to at least adjacent to the top or roof of the container.

27. The method according to any one of claims 24-26, wherein the method comprises feeding the gas at a pressure of greater than 3 barg, 4barg, or 5 barg.

28. The method according to any one of claims 24-27, wherein the method comprises reducing the pressure of the inert gas by passing it through a restriction orifice disposed in the gas conduit and/or a widened section of the gas conduit.

29. The method according to any one of claims 24-28, wherein the method comprises feeding the gas through a tubuliser in order to create turbulent gas flow.

30. The method according to any one of claims 24-29, wherein after the pressure of the inert gas has been reduced, the method comprises passing the inert gas through a mesh or filter.

31. The method according to any one of claims 24-30, wherein the method comprises releasing the inert gas into the container's headspace at an angle

substantially away from the flammable material stored in the container.

32. The method according to any one of claims 24-31, wherein the method comprises guiding the inert gas away from the material using guide means.

33. The method according to any one of claims 24-32, wherein the method comprises releasing the inert gas into the headspace at an angle of at least 20⁰ above horizontal, or at least 30⁰ above horizontal.

34. The method according to any one of claims 24-33, wherein the inert gas is released into the headspace of the container at a velocity of less than 15 m/s, 12 m/s, or 10 m/s.

35. A gas nozzle assembly comprising:

a mesh disposed in the conduit between the gas pressure reduction means and the or each outlet; and/or

36. A gas nozzle assembly according to claim 35, wherein the gas nozzle assembly is as defined in any one of claims 1-22.

37. Use of the gas nozzle assembly according to either claim 35 or claim 36, for feeding inert gas into the headspace of a container in which flammable material is stored.